Functionalization
of
Carbon Nanotubes
Den Naturwissenschaftlichen Fakultäten
der Friedrich-Alexander-Universität Erlangen-Nürnberg
zur
Erlangung des Doktorgrades
vorgelegt von
Jürgen Abraham
aus Neumarkt i. d. Opf.
Als Dissertation genehmigt von den Naturwissenschaftlichen Fakultäten der Universität
Erlangen-Nürnberg.
Tag der mündlichen Prüfung: 13.10.2005
Vorsitzender der Promotionskommission:
Prof. Dr. D.-P. Häder
Erstberichterstatter:
Prof. Dr. A. Hirsch
Zweitberichterstatter:
Prof. Dr. L. Ley
Die hier vorliegende Arbeit entstand in der Zeit von März 2002 bis Juni 2005 am Institut für
Organische Chemie der Friedrich-Alexander-Universität Erlangen-Nürnberg.
Im Allgemeinen sprechen die Leute, welche wenig wissen, viel, während die Leute, welche
viel wissen, wenig reden. Es hängt sehr einfach zusammen, dass ein unwissender Mensch alles,
was er weiß, für höchst wichtig hält und es vor aller Welt ausposaunt. Allein ein unterrichteter
Mann öffnet nicht leicht die Fundgrube seines Wissens; er hätte zu viel zu sagen und weiß nur
zu wohl, dass auch nach ihm noch weit mehr zu sagen wäre. So schweigt er denn.
Jean-Jacques Rousseau
Index of Abbreviations
AFM
Atomic Force Microscopy
CNT
Carbon Nanotube
CPP
Chlorinated Polypropylene
CVD
Chemical Vapor Deposition
DCC
Dicyclohexylcarbodiimide
DCE
1,2-Dichloroethane
DMA
N,N-Dimethylacetamide
DMF
Dimethylformamide
DMSO
Dimethylsulfoxide
DOS
Density of States
EA
Element Analysis
EI
Electron Impact
EtOAc
Ethyl acetate
FAB
Fast Atom Bombardment
FT
Fourier-Transformation
IR
Infrared
ISC
Intersystem Crossing
LDS
Lithium Dodecyl Sulfate
MS
Mass Spectrometry
MWCNT
Multi Walled Carbon Nanotube
NMR
Nuclear Magnetic Resonance
ODA
Octadecylamine
ODCB
ortho-Dichlorobenzene
PAN
Polyacrylonitrile
PMMA
Poly(methyl methacrylate)
ppm
Parts per Million
PSS
Polystyrenesulfonate
PTFE
Polytetrafluoroethylene
PVP
Polyvinylpyrrolidone
VI
RBM
Raman Breathing Mode
RT
Room temperature
SDS
Sodium Dodecyl Sulfate
SEM
Scanning Electron Microscopy
STM
Scanning Tunneling Microscopy
SWCNT
Single Walled Carbon Nanotube
TEM
Transmission Electron Microscopy
TGA
Thermal Gravimetric Analysis
THF
Tetrahydrofurane
UHV
Ultra High Vacuum
UV/Vis
Ultraviolet/Visible
VHS
Van Hove Singularities
XPS
X-ray Photoelectron Spectroscopy
δ
chemical shift
λ
wavelength
VII
Table of Contents
1 Introduction
1
1.1
History of Carbon Nanotubes
. . . . . . . . . . . . . . . . . . . . . . . . . .
1
1.2
Structure of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . .
2
1.3
Properties of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . .
5
1.4
Synthesis of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . .
7
1.5
Reactivity of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . .
12
1.6
Purification of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . .
14
1.7
Chemical Functionalization of CNTs . . . . . . . . . . . . . . . . . . . . . . .
16
1.8
Characterization Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
1.8.1
Microscopic Techniques . . . . . . . . . . . . . . . . . . . . . . . . .
25
1.8.2
Optical Absorption Spectroscopy . . . . . . . . . . . . . . . . . . . .
30
1.8.3
Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . .
31
1.8.4
X-ray Photoelectron Spectroscopy . . . . . . . . . . . . . . . . . . . .
33
Carbon Nanotube Polymer Composites . . . . . . . . . . . . . . . . . . . . . .
35
1.9
2 Proposal
37
3 Results
39
3.1
Oxidation of Single Walled Carbon Nanotubes . . . . . . . . . . . . . . . . . .
39
3.1.1
Oxidation in Air . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
3.1.2
Oxidation in Solution . . . . . . . . . . . . . . . . . . . . . . . . . . .
47
3.2
Reactivity of Different Types of Single Walled Carbon Nanotubes . . . . . . .
53
3.3
Functionalization of Carbon Nanotubes with Organo-Lithium Compounds . . .
63
3.3.1
Reaction of SWCNTs with Organo-Lithium Compounds . . . . . . . .
65
3.3.2
Enhancement of the Degree of Functionalization . . . . . . . . . . . .
71
VIII
Table of Contents
3.3.3
Functionalization of the Charged Intermediate . . . . . . . . . . . . . .
77
3.3.4
In situ Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . .
80
Integration of Carbon Nanotubes into Polymers . . . . . . . . . . . . . . . . .
86
3.4.1
Characterization of the starting materials . . . . . . . . . . . . . . . .
86
3.4.2
Polymer composites from MWCNTs . . . . . . . . . . . . . . . . . .
89
3.5
Reductive Charging of Single Walled Carbon Nanotubes . . . . . . . . . . . .
96
3.6
Reductive Alkylation of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . 103
3.4
3.6.1
Variation of the Reaction Conditions for the Reductive Alkylation of
Single Walled Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . 107
3.7
3.6.2
Functionalization of SWCNTs with a Variety of Functional Groups . . 120
3.6.3
Functionalization of Multi Walled Carbon Nanotubes . . . . . . . . . . 135
Further Reactions on Functionalized SWCNTs . . . . . . . . . . . . . . . . . 141
3.7.1
Ether Cleavage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
3.7.2
Esterification of Hydroxy-SWCNTs . . . . . . . . . . . . . . . . . . . 144
4 Summary
147
4 Zusammenfassung
150
5 Experimental Part
155
5.1
Instruments and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
5.2
Chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
5.3
Experimental Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
6 References
216
7 Appendix
229
IX
1 Introduction
1.1 History of Carbon Nanotubes
Carbon nanotubes have been synthesized for a long time as byproducts in many production
processes involving the appearance of atomic carbon such as chemical vapor deposition or arc
discharge techniques, but unfortunately nobody had realized the high potential which lies in
this new carbon species. This situation changed in 1991, when Iijima reported the occurrence
of nearly perfect concentric multi walled carbon nanotubes as byproducts of the formation of
fullerenes by the electric arc technique.[1] The publication initiated worldwide enthusiasm
when the high potential which lies in this new material was soon realized.
Already two years later another major breakthrough was achieved simultaneously by
Bethune[2] and Iijima.[3] While trying to fill the carbon nanotubes in situ with various metals
they observed the formation of single walled carbon nanotubes. The existence of this carbon
allotrope was already proposed as a homologous to the known fullerenes C60 , C70 and C76 .[4]
Further studies carried out on the synthesized carbon nanotubes revealed the astonishing
properties of this novel material. Carbon nanotubes are one of the most exciting molecular
systems showing a range of unique properties such as superconductivity,[5] high mechanical
strength,[4] and high thermal conductivity.[6] Therefore, a lot of applications can be imagined
which would benefit from the incorporation of carbon nanotubes. Over the past few years, a
highly interdisciplinary field of research was developed to exploit the outstanding properties of
carbon nanotubes.
1
1 Introduction
1.2 Structure of Carbon Nanotubes
Carbon nanotubes are divided into two sorts, the single walled carbon nanotubes (SWCNTs)
which consist only of one wall and the multi walled carbon nanotubes (MWCNTs) which
consist of two to 70 concentric walls.
Single walled carbon nanotubes
Single walled carbon nanotubes can be imagined as a perfect graphene sheet rolled up into a
cylinder, with the hexagonal rings put in contact to join seamlessly (figure 1.1). Theoretically,
Figure 1.1: The schematic way to make a single wall carbon nanotube. Starting from a 2D graphene
sheet with the graphene lattice vectors a1 , a2 , and the roll-up vector Ch =na1 + na2 a
SWCNT is formed. The translation vector T defines the periodicity along the tube axis.
The shaded blue and green areas represent the unit cell formed by T and Ch to give a zigzag (green) or an armchair (blue) SWCNT.
2
1.2 Structure of Carbon Nanotubes
there is no limitation for either the nanotube diameter or the nanotube length. However, calculations have shown that a collapse of the tube into a flattened two layer ribbon is energetically
more favorable than maintaining the tubular morphology beyond a diameter of ∼2.5 nm.[7] Experimental data are consistent with this statement, since SWCNTs with a diameter larger than
2.5 nm are rarely reported in literature. A decrease in the diameter of the SWCNTs leads to
an increase of the curvature and therefore to higher strain, followed by a lower stability of the
SWCNTs. Although SWCNTs with a diameter of 0.4 nm have been successfully synthesized
using a template technique,[8] the most common diameter range encountered, regardless of the
synthesis techniques, was determined to be around 1.2 nm. This gives rise to the conclusion
that this diameter is a suitable energetic compromise. In contrast to these limitations of the
diameter, the length of carbon nanotubes can reach up to several centimeters,[9] limited only by
the production technique.
As illustrated in figure 1.1, there is a third variable besides length and the diameter, which is
the orientation of the so called roll-up vector Ch . The roll-up vector Ch is a vector along the
circumference of the SWCNT, expressed as multiples of the unit vectors of graphene, a1 and a2 :
Ch = na1 + ma2 .
(1.1)
The vector of helicity Ch is perpendicular to the tube axis (figure 1.1), and the helical angle
Θ is defined as the angle between zig-zag direction, parallel to a1 and the roll-up vector. The
integers n and m are sufficient to define a SWCNT by noting them as (n,m).[10]
Figure 1.2 shows the three different types of SWCNTs, the achiral zig-zag (Ch =(9,0), Θ=0◦ ) (a),
the armchair (Ch =(10,0), Θ=30◦ ) (b) and the chiral (5,5)-SWCNT (c). The expressions zig-zag
and armchair nanotubes refer to the way carbon atoms are arranged at the edge of the nanotube
cross section. It is clear from figure 1.2 that having the vector of helicity perpendicular to
any of the three overall C=C bond directions will provide zig-zag-type SWCNTs, noted (n,0),
while having the vector of helicity parallel to one of the three C=C bond directions will provide
armchair-type SWCNTs, noted (n,n). Because of the sixfold symmetry of the graphene sheet,
the angle of helicity for the chiral (n,m) nanotubes is in between 0◦ < Θ <30◦ .
3
1 Introduction
Figure 1.2: Schematic representation of three different SWCNT structures as examples for a zig-zagtype nanotube (a), an armchair-type nanotube (b), and a helical nanotube (c).[11]
Multi walled carbon nanotubes
The easiest way to imagine a multi walled carbon nanotube is to start with a SWCNT and
subsequently add other SWCNTs around it with regularly increasing diameters to get a
Russian-doll-model like arrangement of the nanotubes into a multi walled carbon nanotube
(figure 1.3). The number of walls varies, starting from two (double walled carbon nanotube)
Figure 1.3: Schematic representation of a multi walled carbon nanotube.
with no upper limit. The intertube distance (0.34 nm) is approximately that of the intergraphene
distance in graphite.[12, 13]
4
1.3 Properties of Carbon Nanotubes
1.3 Properties of Carbon Nanotubes
The properties of carbon nanotubes change drastically depending on whether SWCNTs or
MWCNTs are considered. In the following some of the outstanding properties of both types
are presented.
Electronic Properties
Studying the electronic properties of carbon nanotubes one can start with the simplest model, a
graphene sheet and treat the curvature as a perturbation. Graphene is a zero-band gap semiconductor, the highest valence band being formed by the occupied π-states, the lowest conduction
band by the empty π ∗ -states, π- and π ∗ -bands are degenerated at special points in reciprocal space, the K-points.[4] By rolling-up a graphene sheet into a cylinder, periodic boundary
conditions are imposed for the allowed wavevectors in directions parallel to Ch . These allowed wavevectors form parallel lines in reciprocal space. Whenever one or more of these lines
touches the K-points of graphene, the SWCNT will be metallic and semiconducting with an
energy gap inversely proportional to the diameter otherwise. For a semiconducting nanotube
with a diameter of 3 nm the gap falls in the range of the thermal energy at room temperature.
Knowing (n,m) allows to predict whether the tube exhibits a metallic behavior or not via the
equation:
(n − m) = 3q.
(1.2)
Therefore, all nanotubes which fulfill this equation with q being an integer, are metallic. This
implies that one third of the synthesized SWCNTs are metallic, whereas the remaining two
third are semiconducting (figure 1.1: red dots represent metallic SWCNTs, blue dots the
semiconducting SWCNTs).
To summarize, we can say that the electronic structure of SWCNTs depends both on the
orientation of the honeycomb lattice with respect to the tube axis and the radius of curvature
imposed to the bent graphene sheet. The conduction of charge carriers in metallic SWCNTs
is thought to be ballistic.[5] Because of the reduced scattering, metallic SWCNTs can sustain
huge current densities (max. 109 A/cm2 ) without being damaged, i.e. about three orders of
magnitude higher than copper.
5
1 Introduction
As the number of walls increases, the properties of MWCNTs at the same time become similar
to those of regular polyaromatic solids, since the radius of curvature is increasing. The conduction in MWCNTs occurs essentially through the outermost nanotube, but the interactions with
the internal coaxial nanotubes may lead to variations in the electronic properties.
In metals or semiconductors, significant features appear in the electronic density of states
(DOS) whenever the electronic bands in reciprocal space are flat. They are commonly referred
as van Hove singularities (VHS).[14] The form of these singularities is a characteristic of the
dimension of a system (figure 1.4). In one dimensional systems, the VHS form peaks. SWCNTs
Figure 1.4: Schematic representation of the calculated DOS of a metallic (7,7) SWCNT (a), and a semiconducting (7,6) SWCNT (b).[14]
exhibit such peaks in the DOS due to the 1D nature of their band structure (Figure 1.4). The
VHS of typical SWCNTs fall in the energy range of visible light. As a consequence, visible
light is strongly absorbed and a mixture of various carbon nanotubes appears black.
Mechanical Properties
The covalent carbon-carbon bond within a graphene sheet is one of the strongest chemical bonds
known. However, weak interplanar interactions in graphite hinder its application as mechanical robust materials. Since this effect is not found in individual carbon nanotubes they should
show outstanding mechanical properties. To study these properties, early measurements were
carried out using atomic force microscopy. These measurements revealed the enormous tensile strength of SWCNTs being up to 100 times that of steel[4] reaching up to 45 GPa[15] for
SWCNTs. Even higher values were measured for defect free multi walled carbon nanotubes
reaching a tensile strength of 150 GPa.[16] It has to be noted here that using a non capped
6
1.4 Synthesis of Carbon Nanotubes
MWCNT leads to lower values since the inner nanotubes can slide relative to each other by
inducing high strain.
Furthermore, the carbon nanotubes show a high flexibility which decreases as the number of
walls increases. Single walled carbon nanotubes for example can be twisted, flattened and bent.
This deformation does not lead to any structural damage at the carbon nanotube. Measurements of the flexibility gave values for the tensile modulus of 1 TPa for multi walled carbon
nanotubes[17] and 1.3 TPa for single walled carbon nanotubes.[18]
1.4 Synthesis of Carbon Nanotubes
Carbon nanotubes can be produced by various techniques. These can be divided into two main
groups:
a) solid carbon based methods, such as arc discharge and laser ablation, and
b) gaseous carbon based methods (CVD).
All production techniques are capable for the production of SWCNTs or MWCNTs in up to
several grams per hour. Nevertheless, it is not possible to control the configuration (chirality)
or the electronic character by using these techniques.
The arc discharge and the laser ablation process for the synthesis of carbon nanotubes
originate from the synthesis of fullerenes. As opposed to the formation of fullerenes, which
also requires the presence of carbon atoms in high temperature media and the absence of
oxygen, the utilization of these techniques for the synthesis of nanotubes requires the presence
of a catalyst. The techniques afford high temperatures (1000 ◦ C - 6000 ◦ C) for the sublimation
of the graphite. The introduced energy is necessary for the dissociation of the graphene sheets
of a graphite rod. The recombination process of these atoms takes place at the outer regions
of the reactor. The energy necessary for the sublimation of graphite is supplied either by the
interaction between the target material and an external radiation (laser beam or solar energy) or
the electrode and the plasma (in case of electric arc).
Besides these similarities, the carbon nanotubes obtained can vary in terms of morphology
according to the applied reaction conditions. Furthermore, both techniques produce carbon
nanotubes contaminated with other carbon phases and catalyst remnants.
7
1 Introduction
Electric Arc Discharge
Iijima discovered MWCNTs while studying the carbon nanostructures which form along
with the fullerenes in the solid carbon deposit at the cathode.[1] He found a process for the
catalyst-free synthesis of MWCNTs. Later the catalyst-promoted formation of SWCNTs was
incidentally discovered after transition metals were introduced into the anode in an attempt to
fill the MWCNTs with metals during growth.[2, 19] Since then a lot of work has been carried
out by many groups using this technique to understand the mechanisms of nanotube growth as
well as the role of the catalysts for the synthesis of MWCNTs and SWCNTs.[20, 21]
The electric arc discharge process is based on the vaporization of carbon in the presence of
catalysts (iron, nickel, cobalt, yttrium, boron or gandolinium) under reduced pressure, and
inert gas (argon or helium), by an electric arc. After the triggering of the arc between two
electrodes, a plasma is formed consisting of the mixture of carbon vapor and the vapors of
catalysts. The vaporization is caused by the energy transfer from the arc to the anode made
of graphite with small amounts of catalyst. As the evaporation occurs, the anode has to be
translated continuously towards the cathode to keep a constant distance between the electrodes
(figure 1.5). The reactor setup typically consists of a cylinder of about 30 cm in diameter
Figure 1.5: Schematic representation of the electric arc apparatus.
equipped with a sapphire window to observe the anode and the arc. The reactor is further
equipped with a valve to control inert gas pressure while the evaporation takes place. The
carbon nanotube raw soot deposits on different places in the apparatus:
1. the collaret, which is formed around the cathode,
2. a web-like deposit, found above the cathode, and
8
1.4 Synthesis of Carbon Nanotubes
3. as soot, deposited all around the reactor walls and bottom.
The nanotube content at the different positions varies with the setup, the catalyst, and the
reaction conditions applied.[22, 23]
The highest yields for the production of SWCNTs have been obtained using a pressure value of
about 600 mbar of helium at a current of 80 A in the arc, and an electrode gap of 1 mm while
using Ni/Y as catalysts. The SWCNT content was described to be about 70% in the collaret and
less than 50% in the web.[3, 23] The formed SWCNTs show lengths of several microns and a
typical outer diameter of about 1.4 nm. Nevertheless, it should be noted that the products do
not consist solely of carbon nanotubes, but also carbon nanoparticles, fullerene-like structures,
polyaromatic carbons, amorphous nanofibers, multi wall shells, and amorphous carbon. In
addition, catalyst remnants in various concentrations are found in the product. Despite of these
problems, the electric arc method is one of three methods currently used for the commercial
production of carbon nanotubes.
Laser Ablation
The synthesis of carbon nanotubes by the laser ablation process was discovered in 1995.[24]
This technique is based on the evaporation of graphite by a laser beam. The reactor setup
consists of a quartz tube placed in an oven heated to 1200 ◦ C.[25] The quartz tube is equipped
with a rotating graphite rod and a gas inlet and outlet to allow the flow of an inert gas while
the synthesis takes place. The laser beam which is focused on the graphite rod vaporizes the
graphite and the incorporated catalyst. The carbon species are subsequently swept away by
the gas flow and collected outside the oven (figure 1.6). In order to improve the production
efficiency, Thess et al.[26] employed a second pulsed laser that follows the initial one. This
second laser pulse with a different frequency ensures a more complete and efficient irradiation
of the graphite rod.
The synthesis of SWCNTs requires the use of transition metal catalysts such as nickel, cobalt
or iron in the graphite rod. If they are absent, MWCNTs are formed. The length of the
SWCNTs obtained by this method reaches several microns with a diameter of 1.2-1.4 nm. The
SWCNTs tend to conglomerate during the production process leading to the formation of ropes
with a diameter of about 5 nm. The ends of the SWCNTs appear to be perfectly closed with
9
1 Introduction
Figure 1.6: Schematic representation of the laser ablation apparatus.
hemispherical end caps and the tubes appear to be remarkably purer than those obtained from
the arc discharge method.
However, since the laser ablation process does not allow a continuous production of carbon
nanotubes it is not competitive in the long term for the low-cost production of SWCNTs
compared to CVD based methods.
Chemical Vapor Deposition (CVD)
In the 1970s, Endo et al.[27, 28] already reported the discovery of carbon nanotubes obtained
by the CVD process. The synthesis of carbon nanotubes by the chemical vapor deposition
process involves the thermal dissociation of gaseous carbon sources (hydrocarbons or CO) on
the surface of a catalyst. The transition metals used for the CVD process are typically iron,
cobalt and nickel. The synthesis is carried out at relative low temperatures (600 ◦ C - 1000 ◦ C)
compared to the arc discharge and the laser ablation process. Due to the low temperature the
selectivity of the method for the synthesis of carbon nanotubes is generally higher than for
amorphous carbon. However, the lower temperature applied for the CVD process has one
major disadvantage. The carbon nanotubes obtained show more structural defects than carbon
nanotubes from arc discharge or laser ablation techniques since, the lower temperature does
not allow structural rearrangements. In order to overcome this problem, the carbon nanotubes
yielded are subsequently annealed under vacuum or inert gas atmosphere to increase the level
of graphitization. SWCNTs produced by the CVD method are generally gathered into bundles
10
1.4 Synthesis of Carbon Nanotubes
of a diameter of up to 40 nm.
The basic setup for the CVD process is simple: it consists of a quartz tube placed in an oven
heated to the desired temperature. The quartz tube is equipped with a gas inlet on one side
and a gas outlet on the other side. While the synthesis takes place, a gas flow containing
the carbon source (CH4 , C2 H2 , C2 H4 , or C6 H6 ) and hydrogen or an inert gas passes through
the tube (figure 1.7). When the feeding gas comes into contact with the catalyst at the
Figure 1.7: Schematic representation of the CVD apparatus.
increased temperature, the hydrocarbons undergo a catalysis-enhanced thermal cracking into
carbon atoms and hydrogen leading to the subsequent formation of carbon nanotubes. If CO
is used instead of hydrocarbons, the reaction is chemically referred as catalysis-enhanced
disproportionation 2 CO → C + CO2 .
The nanotube formation takes place on the surface of the metal particles, by a mass-transport
of the freshly produced carbon either by surface or volume diffusion until the carbon concentration reaches the solubility limit. Since the carbon nanotubes are formed on the surface of the
catalyst particles a very small size of the particles is essential. The catalytic metal particles are
prepared mainly by the reduction of transition metals compounds by H2 prior to the nanotube
formation step. It is possible, to produce the catalytic metal particles in situ in the presence of
the carbon source, allowing a one-step process.[29] As the control of the metal particle size
is important, their coalescence is generally avoided by supporting them on an inert support
substrate such as Al2 O3 , SiO2 , zeolites, MgAl2 O4 and MgO.
Another method is the direct formation of the catalyst particles in the reactor. For this purpose,
a metal precursor such as Fe(CO)5 ,[30] or a metallocene[31] is added to the gas flow. The
metal-organic compound decomposes in the reactor to generate the nanoscale catalyst particles
which subsequently catalyze the nanotube formation. The control of the particle-size remains
11
1 Introduction
an unsolved problem, since the formation of large particles leads to their encapsulation with
graphitic carbon which hinders the nanotube growth.
A significant breakthrough concerning this technique was the so-called HiPCO process[32]
developed at Rice University. The process requires high pressure (30-50 atm), high temperature
(900 ◦ C - 1100◦ C), carbon monoxide and Fe(CO)5 for the synthesis. The yielded SWCNTs
show a high purity and a diameter of ∼0.7 nm. Furthermore, the process was scaled up recently
to produce SWCNTs in large amounts.
1.5 Reactivity of Carbon Nanotubes
The chemical reactivity of carbon nanotubes exhibits some common features with the other
sp2 -hybridized carbon allotropes, such as graphite and fullerenes. In contrast to other regular
polyaromatic carbon materials, the chemistry of carbon nanotubes differs due to their unique
structure. Compared to graphite, a perfect infinite SWCNT has no chemically active dangling
bonds. However, the curvature applied to the sp2 -bonded carbon atoms of the nanotube induces
pyramidalization and misalignment of the π-orbitals resulting in local strain.[33, 34] Therefore,
the carbon nanotube is expected to be more reactive than a flat graphene sheet.[35] Since the end
caps are even curved in 2D, their reactivity should be comparable to the reactivity of fullerenes.
It is conventionally useful to divide the carbon nanotube into two reaction sites: the end caps
and the sidewall.[33]
The end caps of carbon nanotubes resemble a hemispherical fullerene. Therefore, their reactivity is comparable with the reactivity of the corresponding fullerene. The reactivity of
fullerenes originates primarily from the enormous strain introduced by their 2D curvature which
reflects the pyramidalization angles of the carbon atoms. For a sp2 -hybridized carbon atom,
planarity is strongly preferred, which implies a pyramidalization angle of ΘP = 0◦ , whereas
a sp3 -hybridized carbon atom requires ΘP = 19.5◦ (figure 1.8). In the case of C60 all carbon
atoms have ΘP = 11.6◦ , concluding that the chemical conversion of the trivalent carbon into
tetravalent carbon releases strain at the point of attachment and migrates the strain at the other
59 remaining carbon atoms. Thus, it is possible to reduce the maximum pyramidalization angle
of any fullerene below ΘP = 9.7◦ . This ensures a quite high reactivity at the end caps of the
12
1.5 Reactivity of Carbon Nanotubes
Figure 1.8: Diagrams of pyramidalization angle (ΘP ) (a), and the π-orbital misalignment angles (Φ)
along the C1-C4 in the (5,5) SWCNT and its capping fullerene, C60 (b).[33]
carbon nanotubes.[33]
The sidewalls of the carbon nanotube are bent in 1D in contrast to the 2D curvature of the end
caps, resulting in a lower pyramidalization angle ΘP of the carbon atoms and therefore, a lower
strain. For example, the carbon atoms of the sidewall of a (5,5) SWCNT show a ΘP = 6.0◦
whereas the carbon atoms of the end caps show a ΘP = 11.6◦ . The strain energy of pyramidalization is roughly proportional to ΘP 2 , so in this case the fullerene stores about 10 times
the strain energy of pyramidalization per carbon atom, compared to the "equivalent" CNT.[33]
Only the smallest (and probably unstable) nanotube with (2,2) dimension has a higher degree
of pyramidalization than C60 , although the C60 diameter (7.10 Å) is much larger than that of a
(2,2) SWCNT (2.75 Å).[36]
In conclusion, carbon nanotubes in general display a lower reactivity than fullerenes and the
end caps of carbon nanotubes are always more reactive than the sidewalls, irrespective of the
diameter of the carbon nanotube.[33, 36]
However, while the π-orbital alignment in fullerenes is almost perfect, this is not the case for
all bonds in carbon nanotubes (figure 1.8). Although all carbon atoms are equivalent, there are
13
1 Introduction
two types of bonds: those that lay perpendicular to the nanotube axis and those that have an
angle to the nanotube axis with π-orbital misalignment angles (Φ) of 0◦ and 21.3◦ in the case of
a (5,5) SWCNT, respectively. On the basis of previous calculations of torsional strain energies
in conjugated organic molecules, the π-orbital misalignment is likely to be the main source of
strain in the carbon nanotubes. Since the pyramidalization angles and the π-orbital misalignment angles of SWCNTs scale inversely with the diameter of the nanotubes, a higher reactivity
of nanotubes with a lower diameter is expected.[33]
Furthermore, the overall chemical reactivity of carbon nanotubes might strongly depend on the
structural perfection of the sidewall and therefore on the way they are synthesized.
1.6 Purification of Carbon Nanotubes
Since all production techniques yield carbon nanotubes contaminated with a variety of impurities, an important prerequisite for the application of carbon nanotubes is their purification. The
purification processes reported up to now involve both oxidative and non-oxidative processes.
The non-oxidative techniques such as filtration,[37] centrifugation[38] or chromatographic
methods[39, 40] are limited to a lab scale so far. The most efficient purification processes to
remove the undesirable phases are based on oxidation processes that greatly affect the structure
of SWCNTs.[41]
The aim of the oxidative treatment, which is inspired from the well known graphite
chemistry,[42, 43] is the oxidative removal of metallic catalyst particles, used in the synthesis
of the tubes, and of amorphous carbon.[44] It is considered that amorphous carbon is oxidized
first, followed by the SWCNTs and then the MWCNTs. SWCNTs are known to be stable up to
750 ◦ C in air and up to 1800 ◦ C in inert atmosphere.[45] Therefore, choosing the right oxidation
conditions allows only the pure carbon nanotubes to pass the procedure. Nevertheless, the
oxidative treatment leads to the opening of the nanotubes’ caps introducing carboxylic groups
and other groups bearing oxygen atoms at the ends and at the carbon nanotube defect sites
(figure 1.9).
Subsequent thermal treatment at 1200 ◦ C under inert gas atmosphere is able to recover the former structural quality of the SWCNTs by removing the oxygen bearing groups. Furthermore,
the high temperature allows the aromatic rings of the SWCNT to rearrange.[46]
14
1.6 Purification of Carbon Nanotubes
Figure 1.9: Schematic representation of the defect sites generated by the oxidative treatment.
Chemical oxidation of nanotubes is mainly achieved by using either wet chemistry or gaseous
oxidants. The oxidation is achieved by applying boiling nitric acid,[47, 48] sulfuric acid,[48]
mixtures of both,[46] "piranha" (sulfuric acid-hydrogen peroxide)[44] at elevated temperatures.
Less common reagents for the oxidative purifications are HF/BF3 , OsO4 , RuO4 , OsO4 −NaIO4 ,
H2 O2 , K2 Cr2 O7 , and KMnO4 combined with phase transfer catalysts.[49, 50, 51, 52]
The gaseous approach is based on the treatment of carbon nanotubes with oxygen,[53, 54]
ozone[55, 56, 57, 58] or air as oxidant,[59, 60, 61] at elevated temperatures. Furthermore,
combinations of both approaches have been reported utilizing nitric acid followed by air
oxidation.[62]
More detailed studies on the influence of the oxidative treatment on the carbon nanotubes
showed that their stability depends on the diameter of the nanotubes.[63] The smaller nanotubes
revealed a lower stability towards the oxidative treatment since the reactivity is a function of
curvature.[64] Yates et al.[55] showed that tubes with relatively high diameters of ∼1.4 nm
undergo no structural damage of their sidewall when treated at room temperature with ozone.
15
1 Introduction
However, the end caps which display a higher reactivity, were destroyed by this procedure.
Bahr and Tour[65] reported in their review on the covalent chemistry of SWCNTs that smaller
diameter SWCNTs produced by the HiPCO process (ca. 0.7 nm) were found to be more
reactive towards ozone than larger diameter SWCNTs produced by laser ablation. Further
studies on the diameter dependent stability by Fischer et al.[66] have recently confirmed the
direct relationship between diameter and reactivity. Using the intensities of the resonance
enhanced Raman radial breathing modes (RBM) as an indication, the authors clearly showed
that smaller diameter tubes are more rapidly air-oxidized than larger diameter tubes.[66] This
provided a simple way to enrich large-diameter single walled carbon nanotubes by using a
mixture of concentrated H2 SO4 /HNO3 .[67]
Recent results reported by An et al.[68] demonstrated the higher reactivity of metallic SWCNTs
towards oxidation with NO2 SbF6 leading to their complete disintegration. The authors claimed
that the treatment leaves the semiconducting SWCNTs unharmed and therefore, allows the
separation of the semiconducting nanotubes from the metallic ones.
However, the oxidation resistance of the nanotubes can vary depending on the production
process, the presence of catalyst remnants, the type of nanotubes (SWCNTs, MWCNTs) and
the oxidizing reagent. Due to these parameters, the comparison of published results is difficult.
1.7 Chemical Functionalization of CNTs
It became soon obvious that in order to benefit from the outstanding properties of carbon nanotubes their functionalization would be inevitable, as the application in materials and devices is
hindered by processing and manipulation difficulties. Therefore, the attachment of appropriate
chemical functionalities on the carbon nanotube allows the tailoring of their properties for the
respective application. The enhancement of the solubility is one of the major tasks since, the
pristine carbon nanotubes are insoluble in both water and organic solvents. The solubility measured for pristine SWCNTs was not higher than 95 mgl−1 in ODCB and even less than 31 mgl−1
for all other solvents tested.[69] Thus, the improvement of the solubility by chemical functionalization, became the most promising concept for synthetic chemists and materials scientists.
Tailoring of the chemical bonds might as well lead to an optimized interaction of the carbon
16
1.7 Chemical Functionalization of CNTs
nanotubes with entities such as, solvent or polymer matrices. Furthermore, the functionalized
nanotubes might reveal mechanical or electrical properties that differ from those of the pristine
nanotubes.
In the last few years four different approaches evolved for the functionalization of carbon nanotubes such as, non covalent functionalization, endohedral functionalization, defect functionalization and sidewall functionalization (figure 1.10).
Figure 1.10: Schematic representation of the different possibilities for the functionalization of carbon
nanotubes.
Since a large number of papers has been published on the functionalization of carbon
nanotubes, we do not attempt to review all of these works.
We chose to cite the pio-
neering works and the ones related with the following studies. For further details we want
to refer to several review articles which have been published in the past years.[69, 70, 71, 72, 73]
The non covalent functionalization is based on the ability of the extended π-system of
the carbon nanotubes sidewall to bind guest molecules via π-π-stacking interactions. Other
approaches take only advantage of the adsorption via van der Waals interactions between the
adsorbates and the nanotube. The non covalent functionalization has attracted considerable
interest in terms of non destructive purification of carbon nanotubes. It has been shown that
surface active molecules, such as LDS, SDS, and Triton X-100, benzylalkonium chloride
17
1 Introduction
can interact with the hydrophobic surface of the carbon nanotubes and therefore give them
a hydrophilic character which makes the carbon nanotubes water soluble.[74, 75, 76, 77]
Pronounced non-covalent interactions were also found between SWCNTs and anilines[78] and
several types of alkyl amines,[79] leading to an increased solubility of the carbon nanotubes in
various solvents.
Furthermore, it was demonstrated that polymer ropes wrap around the nanotube lattice in
a well-ordered periodic fashion.[80] The polymer intercalated between the nanotubes was
found to unravel the ropes by a decrease of the interactions between the individual SWCNTs.
Moreover, Raman and absorption studies suggested that the polymer interacts preferentially
with nanotubes of specific diameters or a range of diameters.[81] The coating of carbon
nanotubes with a conjugate polymer was also shown to be a suitable technique for the nondestructive purification and quantification of MWCNTs.[82] By choosing a polymer carrying
polar side-chains such as polyvinylpyrrolidone [PVP] or polystyrenesulfonate [PSS], stable
solutions of the corresponding SWCNT/polymer complexes in water[83] were obtained. In
this case, the wrapping of the polymer is driven by the avoidance of unfavorable interactions
between the apolar nanotube sidewalls and the solvent.
Studies using carbon nanotubes for the immobilization of biomolecules showed that a wide
range of proteins and peptides can be bound on the nanotubes surface.[84, 85, 86] For example,
streptavidin was found to adsorb on MWCNTs presumably via hydrophobic interactions
between the nanotubes and hydrophobic domains of the protein.[87]
In 2001, Chen et al.[88] described the attachment of N-succinimidyl-1-pyrenebutanoate on the
nanotube sidewall via the effective π-stacking interactions between the graphitic sidewall and
the pyrene-group. The N-succinimidyl-1-pyrenebutanoate was irreversibly adsorbed onto the
inherently hydrophobic surface of the SWCNT. Subsequent substitution of the succinimidyl
group by primary or secondary amino groups from proteins such as, ferritin or streptavidin
caused the immobilization of the biopolymers on the surface of the nanotubes (figure 1.11). The
same π-stacking interactions of SWCNTs with 17-(1-pyrenyl)-13-oxa-heptadecanethiol [PHT]
allowed the decoration of carbon nanotubes with colloidal gold particles. By self-assembling
of additional gold nanoparticles on the thiol group a dense coverage of the nanotube surface
was observed.[89] In further experiment, a pyrene-carrying ammonium ion was used to achieve
a transparent solution of carbon nanotubes in aqueous media.[90]
18
1.7 Chemical Functionalization of CNTs
NH2
NH2
H2N
O
H2N
NH2
O N
O
N
O
O
H2N
O
O
O
O
N
O
O
O
NH2
O
N
O
O
O
H2N
O NH2
N
O
O
O
Figure 1.11: Schematic representation of the N-succinimidyl-1-pyrenebutanoate irreversibly bound to
the sidewall of the SWCNT via π-stacking interactions. Amine groups on a protein react with the anchored succinimidyl ester to form an amide bond resulting in the protein
immobilization.
A different approach is the endohedral functionalization of carbon nanotubes, i.e. the filling of the nanotube with various atoms or small molecules. The small inner cavity of nanotubes
gives an amazing tool to study the properties of confined nanostructures of any nature, such
as salts, metals, oxides, gases or even discrete molecules like C60 . The encapsulated materials
exhibit changed physical and chemical properties due to the surrounding carbon nanotube.
The filling of the carbon nanotubes can be achieved either during their growth or as a separated
step via wet chemistry, capillarity effects or by sublimation of materials.[91]
The incorporation of fullerenes such as C60 [92, 93] or metallofullerenes such as Sm@C82 [94]
are examples of the endohedral chemistry of SWCNTs and may enable the synthesis of new
materials for molecular scale devices. The encapsulated fullerenes tend to form chains that are
coupled by van der Waals forces. Upon annealing, the encapsulated fullerenes coalesce in the
interior of the SWCNTs, which results in a new, concentric, endohedral tube with a diameter of
0.7 nm. The progress of such reactions inside the tubes could be monitored in real time by the
use of high-resolution transmission electron microscopy (HR-TEM)(figure 1.12).[94]
19
1 Introduction
Figure 1.12: HR-TEM image of five regular C60 molecules encapsulated together with two higher
fullerenes (C120 and C180 ) as distorted capsules (on the right) within a regular 1.4 nm
diameter SWCNT.[91]
The defect functionalization of carbon nanotubes is based on the conversion of carboxylic
groups and other oxygenated sites formed through oxidative purification. The carboxylic
groups, located mainly at the ends of the nanotube, can be coupled with different chemical
groups. The oxidized carbon nanotubes are usually reacted with thionyl chloride to activate
the carboxylic group for a later reaction with amines or alcohols (figure 1.13).[95, 44] Similarly, carboxamide nanotubes have been prepared using dicyclohexylcarbodiimide (DCC) as
dehydrating agent and allowing the direct coupling of amines and carboxylic functions under
neutral conditions.[96]
In 1998, Haddon and co-workers reported the functionalization of carbon nanotubes with
COOH
COOH
HNO 3
H
CO N R
COCl
H
CO N R
COCl
SOCl2
COOH
H2 N R
COCl
H
CO N R
DCC
H2N R
Figure 1.13: Schematic representation of the oxidative treatment of SWCNTs followed by the treatment
with thionyl chloride and subsequent amidation. The reaction with dicyclohexylcarbodiimide (DCC) is shown as an alternative route for the amidation.
octadecylamine (ODA) resulting in a substantial solubility of the functionalized SWCNTs
in chloroform, dichloromethane, and aromatic solvents.[95] The analysis of octadecylamido
functionalized SWCNTs by solution phase mid-IR showed that the weight percentage of the
acylamido functionality was about 50%.[97]
20
1.7 Chemical Functionalization of CNTs
Functionalization with glucosamine using similar procedures allowed the water solubilization
of SWCNTs reaching a solubility of 0.1 to 0.3 mg/mL. This increased solubility in water is of
special interest considering biological applications of functionalized nanotubes.[98]
In 2003, Lim et al. succeeded in a direct thiolation of the open ends of SWCNTs via successive oxydation (H2 SO4 /HNO3 ; H2 O2 /H2 SO4 ; sonication), reduction (NaBH4 ), chlorination
(SOCl2 ), and thiolation (Na2 S/NaOH).[99] The thiolated CNTs were adsorbed on micron-sized
silver and gold particles as well as on gold surfaces to study the interaction between the thiol
groups of the nanotube and the noble metals. The authors claimed that they found a bow type
arrangement of the carbon nanotubes, with the two ends strongly attached to the metal surface.
A last example is the possible interconnection of nanotubes via chemical functionalization.
This has been recently achieved by Chiu et al.[100] upon reacting SWCNT-acyl chlorides with
diamines as molecular linkers and subsequent diamide formation. The reaction led to the formation of end-to-end and end-to-side nanotube interconnections which were observed by AFM.
Statistical analysis of the AFM images showed around 30% junctions in functionalized material.
The sidewall functionalization is based on covalent linkage of functional groups onto the
sidewall of the nanotube. The covalent sidewall functionalization is associated with the change
of hybridization from sp2 to sp3 and a simultaneous loss of conjugation. In contrast to the
well-developed chemistry of fullerenes, the covalent functionalization chemistry of carbon
nanotubes has only been achieved recently.[65] This might be either related to the lack of
availability of SWCNTs in the 1990s or to the significantly lower reactivity of the commercial
available carbon nanotubes[101] compared to fullerenes (see chapter 1.5).
Not surprisingly, most of the reported sidewall functionalization reactions of carbon nanotubes
require very reactive reagents. Nevertheless, within the last few years a diverse chemistry was
developed to covalently modify the surface of carbon nanotubes (figure 1.14).[73]
The covalent functionalization of the sidewalls of carbon nanotubes was firstly achieved by
Mickelson et al.[102] in 1998, by the treatment of carbon nanotubes with elemental fluorine.
The degree of addition ranges from 0.1 % to the complete combustion of the carbon nanotubes
under these drastic conditions.
Subsequent treatment of the fluoronanotubes with N2 H4
or LiBH4 /LiAlH4 leads to the restoration of the carbon nanotubes.[103, 104] Fluorinated
carbon nanotubes are now commercially available and therefore, give rise to a widespread
21
1 Introduction
Figure 1.14: Schematic
describing
SWCNTs.[35]
22
various covalent sidewall functionalization
reactions
of
1.7 Chemical Functionalization of CNTs
chemistry by using them as starting material to carry out subsequent derivatization reactions.
Thus, sidewall-alkylation of the nanotubes was achieved by the nucleophilic substitution with
Grignard-reagents or the reaction with alkyllithium precursors.[105] Oxidation in air or heat
treatment leads to the removal of the alkyl groups and therefore the defunctionalization of the
SWCNTs. Electrochemical addition of aryl radicals to the carbon nanotubes has also been
reported by Bahr et al..[105]
The addition of radicals on carbon nanotubes was first introduced by Holzinger et al. who
used heptadecafluorooctyl iodide to photoinduce the generation of the perfluorinated radicals
followed by their addition to the SWCNTs.[106] Later studies used organic peroxides as
precursors to achieve covalent sidewall functionalization.[107]
Chen et al.
reported the addition of carbenes by the reaction of SWCNTs with
dichlorocarbenes.[101] The carbene was first generated from chloroform with potassium
hydroxide,[101] and later from phenyl(bromodichloromethyl)mercury.[95] However, the
degree of functionalization was low, a chlorine atom amount of only 1.6 at % was determined
by XPS.[108]
The addition of nucleophilic carbenes on the electrophilic SWCNT resulted in the formation of
a zwitterionic polyadduct.[106] The covalently bound imidazolidene addend bears a positive
charge whereas, the nanotube bears one negative charge for each addend attached to the
sidewall causing the n-doping of the carbon nanotubes. This offered a new way for the
controlled modification of the electronic properties of SWCNTs.
The addition of nitrenes on the sidewall of carbon nanotubes was achieved via reactive
alkyloxycarbonyl nitrenes obtained from alkoxycarbonyl azides.[106, 109] The driving force
for this reaction is the thermally-induced N2 -extrusion. The generated nitrenes attack the
nanotube sidewall in a [2+1]-cycloaddition forming an aziridine ring on the nanotube sidewall.
With this technique, a broad range of adducts was obtained by cycloadding addends like alkyl
chains, aromatic groups, crown ethers, and oligoethylene glycol units. Nitrene additions led to
a considerable increase of the solubility in organic solvents such as 1,1,2,2-tetrachloroethane,
DMSO, and 1,2-dichlorobenzene.
Nucleophilic cyclopropanation was achieved via the Bingel reaction for the functionalization of
carbon nanotubes. Coleman et al.[110] used purified SWCNTs and diethyl bromomalonate as
23
1 Introduction
addend. To study the distribution of the addends along the nanotube the authors have developed
a chemical tagging technique, including the transesterification with 2-(methylthio)ethanol for
the complexation of gold nanoparticles, which allows the visualization of the functional groups
by AFM.
Prato and coworkers succeeded in the development of one of the most powerful techniques for
the functionalization of CNTs using the 1,3-dipolar cycloaddition of azomethine ylides.[111]
The treatment of pristine SWCNTs with an aldehyde and a N-substituted glycine derivative
resulted in the formation of substituted pyrrolidine moieties on the SWCNT surface. The
approach works effectively with both SWCNTs, prepared by several different methods, and
MWCNTs. The authors claim a dramatically increased solubility of the pyrrolidino functionalized CNTs. Water soluble carbon nanotube derivatives synthesized via this method bearing
a free amino-terminal oligoethylene glycol moiety attached to the nitrogen atom formed
supramolecular associates with plasmid DNA through ionic interactions. The complexes were
found to be able to penetrate cell membranes.[112]
Recently the first Diels-Alder [4+2] cycloaddition on carbon nanotubes has been reported.[113]
Ester functionalized SWCNTs were reacted with o-quinodimethane, generated in situ from
benzo-1,2-oxathiin-2-oxide, under microwave irradiation.
Electrochemistry has become an useful tool for the functionalization of CNTs. The functionalization was achieved by applying a constant potential or a constant current to a CNT electrode,
immersed in a suitable reagent solution. Under this conditions highly reactive species are
generated in situ which readily react with the nanotubes or self-polymerize, resulting in a
polymer coating of the nanotubes. In 2001, Bahr et al. grafted a series of phenyl residues
by electrochemical coupling of aryl diazonium salts onto a SWCNT bucky paper as working
electrode.[105] The reaction mechanism has been interpreted as an one electron reduction
of the diazonium salt and the subsequent addition of the reactive aryl radicals. These basic
studies on diazonium nanotube chemistry led to two very efficient techniques of derivatization
of SWCNTs, the "solvent-free functionalization"[114, 115], and the "functionalization of
individual nanotubes".[116] With the latter method, aryl diazonium salts react efficiently with
individual (unbundled) HiPCO produced and SDS-coated SWCNTs in water. The resulting
functionalized nanotubes remained unbundled throughout their entire lengths and were incapable of re-bundling.[116] Later studies revealed that this reaction is capable for the selective
24
1.8 Characterization Techniques
functionalization of metallic SWCNTs, if carried out under controlled conditions.[117]
Ryu and co-workers introduced reactions with alkyllithium on the SWCNT’s surface.[118]
The reaction was carried out by treating the carbon nanotubes with sec-butyllithium, providing
initiating sites for the polymerization of styrene. Very recently, Chen et al. reported that by
treating SWCNTs with sec-butyllithium and reacting the generated intermediate with carbon
dioxide under strictly oxygen free and anhydrous conditions, the SWCNTs can be alkylated and
carboxylated. The derivatized SWCNTs could be solubilized in water to a nearly transparent
solution of 0.5 mg/ml.[119]
The reductive alkylation of carbon nanotubes was reported recently by Liang et al. who used
lithium in liquid ammonia and alkyl halides to yield sidewall functionalized nanotubes. The
obtained SWCNT-derivatives were soluble in common organic solvents due to the occurrence
of extensive debundling in the functionalized nanotubes material.[120]
1.8 Characterization Techniques
The characterization of chemically modified carbon nanotubes represents a challenge for
an organic chemist. The synthesized carbon nanotube derivatives consist of a polydisperse
mixture of carbon nanotubes with different lengths, diameters, and helicities. Furthermore,
the functionalization leads to an increase of the inhomogeneity of the samples due to a wide
range of different addition patterns on the carbon nanotubes. In order to prove the success of
the reaction in the proposed way, the comparison of a series of data obtained from different
characterization techniques is inevitable.
1.8.1 Microscopic Techniques
Several microscopic techniques have become available over the last few years allowing the
direct imaging of the carbon nanotubes. In the following, we give a brief description of some
of the techniques used for the characterization of carbon nanotubes.
25
1 Introduction
Scanning Electron Microscopy (SEM)
The electron microscopic techniques are based on the irradiation of the sample by an electron
source. The scattered electrons are used to extract structural information of the substrate.
With SEM an electron beam is scanned across the specimen and the back scattered electrons
are detected to generate an image of the morphology or topography of the sample.[121] The
obtained images (figure 1.15) are often used to study multi walled carbon nanotubes since the
resolution of single walled carbon nanotubes is complicated. The resolution of this method is
commonly limited to objects larger than 1 nm.
Figure 1.15: SEM image of densely packed and aligned MWCNTs.[122]
Transmission Electron Microscopy (TEM)
The design of a transmission electron microscope is analogous to that of an optical microscope
with the difference that in a TEM, high-energy (100-300 kV) electrons are used instead of
photons and electromagnetic lenses instead of glass lenses. The electron beam passes an
electron-transparent sample and a magnified image is formed using a set of lenses. This
image is projected onto a fluorescent screen or a photographic plate. The samples for TEM
measurements have to be prepared in a special way since the sample has to be thin enough to
26
1.8 Characterization Techniques
allow the electrons to transmit the sample.[121] Therefore, the carbon nanotubes are usually
placed on a carbon covered copper grid.
The small de Broglie wavelength of electrons allows a resolution of 0.2 nm by this technique.
An image contrast is obtained by interaction of the electron beam with the sample. In the
resulting TEM image denser areas and areas containing heavier elements appear darker due
to scattering of the electrons at the atom cores. Magnifications of 350,000 times can be
routinely obtained for many materials, while in special circumstances atoms can be imaged at
magnifications greater than 15 million times.
In contrast to SEM, this technique is capable for the resolution of the SWCNT sidewall or
the walls of a multi walled carbon nanotube (figure 1.16). Furthermore, the imaging of metal
Figure 1.16: TEM images of a multi walled carbon nanotube[41] (left) and a bundle of single walled
carbon nanotubes[3] (right).
clusters on the nanotubes is possible. The resolution of single carbon atoms or individual small
addends is not possible.
Scanning Tunneling Microscopy (STM)
The scanning tunneling microscope was developed by Binnig and his colleagues in 1981. It
was the first technique capable of directly obtaining three-dimensional images of solid surfaces
with atomic resolution.[123]
The operation of a scanning tunneling microscope is based on the so-called tunneling current,
which starts to flow when a sharp tip approaches a biased conducting surface at a distance
of approximately 1 nm (figure 1.17 (left)).[124] The tip is mounted on a piezoelectric tube,
27
1 Introduction
which allows movements in all directions. The STM can be used in any environment such as
20
nm
15
10
5
0
0
5
10
15
20
nm
Figure 1.17: Left: Principle of STM operation, a sharp tip attached to a scanner is scanned on a sample.
Right: STM image of pristine laser ablation SWCNTs.
ambient air, various gases, liquid, vacuum, as well as at low and high temperatures. STM can
only be used to study surfaces which are electrically conductive to some degree. The principle
of STM is straightforward. A sharp metal tip is brought close enough to the surface to be
investigated, at a operating voltage of 10 mV to 1 V. The tunneling current varies from 1 pA to
10 nA. The tip is scanned over the surface at a distance of 0.3-1 nm, while the tunneling current
between it and the surface is measured. A feedback loop changes the distance between the tip
and the surface to keep the current constant. The displacement of the tip given by the voltage
applied to the piezoelectric drives yields a topographic map of the surface. Alternatively, in
the constant height mode, a metal tip can be scanned across the surface at constant height
and constant voltage while the current is monitored. Constant current mode is generally used
for atomic-scale images. This mode is not practical for rough surfaces. A three-dimensional
picture [z(x,y)] of a surface consists of multiple scans [z(x)] displaced laterally from each other
in the y direction. It should be noted that if different atomic species are present in a sample,
they may produce different tunneling currents for a given bias voltage. Thus the height data
may not be a direct representation of the topography of the surface of the sample.
STM can not only be used for imaging of carbon nanotubes on an atomic level which allows
28
1.8 Characterization Techniques
the resolution of the single graphene unit cells of a carbon nanotubes (figure 1.17 (right)). More
important is its spectroscopical application, which allows the measurement of the electronic
density of states of a carbon nanotube at a desired position.
Atomic Force Microscopy (AFM)
Based on the design of the STM, Binnig et al. developed in 1985 the Atomic Force Microscope
(AFM) to measure ultra small forces (less than 1µN) present between the AFM tip and the
sample surface.[125] While STM requires that the surface to be measured has to be electrically
conductive, AFM is capable for investigating surfaces of both conductors and insulators. AFM
is also used for the manipulation of a surface on nanometer scale and the measurement of
elastic/plastic mechanical properties (such as hardness and the modulus of elasticity).[126]
The AFM works by scanning a fine tip over the sample surface. The tip is positioned at the end
of a cantilever mounted on a substrate. As the tip is repelled or attracted by the surface, the
cantilever beam deflects. The magnitude of the deflection is captured by a laser that reflects at
an oblique angle from the very end of the cantilever (figure 1.18). A plot of the laser deflection
versus tip position on the sample surface provides an image of the topography of the surface.
In close contact, the atoms at the end of the tip experience a repulsive force due to electronic
10
µm
0
0
µm
10
Figure 1.18: Left: Principle of AFM operation: a sharp tip mounted on a substrate is scanned on a
sample. Right: AFM image of functionalized arc discharge SWCNTs.
29
1 Introduction
orbital overlap with the atoms of the sample surface while, at slightly larger distances attractive
forces dominate. The force acting on the tip causes a cantilever deflection which is measured
by optical detectors. The AFM can be operated in two modes, the contact mode and the
noncontact mode. In contact mode, the tip is moved towards the sample until the cantilever is
deflected by repulsive forces. During the measurements the deflection and therefore, the force
is kept constant by adjusting the distance to the sample. In the noncontact imaging mode, the
tip is brought in close proximity with the sample where the attractive forces dominate while,
the cantilever is deliberately vibrated and the change in vibration frequency is recorded. In
both modes the surface topography is measured by laterally scanning the sample under the tip
while keeping the distance dependent forces between the tip and the surface constant. AFM
relies strongly on the shape of the tip which has to be as sharp as possible. Tips with a radius
ranging from 5 to 50 nm are commonly available, unfortunately those tips are not sharp enough
to achieve atomic resolution. Atomic structures have been obtained from imaging the crystal
periodicity. Reported data show either perfectly ordered periodic atomic structures or defects
on a larger lateral scale, but no well-defined, laterally resolved atomic-scale defects like those
seen in images routinely obtained with a STM.
The atomic force microscopy is widely used for the characterization of carbon nanotubes,
not only for the observation of the carbon nanotubes’ purity but also for the study of their
mechanical properties. Furthermore it is used for the preparation of nanotube based devices
since it is capable for the manipulation of nano-structures.
1.8.2 Optical Absorption Spectroscopy
Solution phase absorbtion spectroscopy is a technique that offers qualitative information
about the electronic state of single walled carbon nanotubes. As seen in figure 1.19 the
absorption spectrum of SWCNTs shows three major bands located at 650, 950 and 1600 nm
along with substructures. The bands are superposed on a broad exponential absorption due to
π-π ∗ transitions.[127] The three bands arise from electron transitions between the van Hove
singularities of the SWCNTs. Since wave-vector conserving optical transitions should only
occur between mirror image VHS spikes, the occurring bands could be assigned as followed:
30
1.8 Characterization Techniques
Figure 1.19: Electronic transitions between the energy bands of SWCNTs, observed by UV/Vis spectroscopy, together with a schematic of the nomenclature used to designate the interband
transitions.[34]
The bands centered at 950 and 1600 nm are attributed to transitions between the first (S11 ) and
second (S22 ) singularities in semiconducting nanotubes respectively, while the bands centered
at 650 nm (M11 ) are attributed to the first transition in metallic nanotubes. The broadening
of the bands is due to the diameter distribution in the samples, since every nanotube of a
distinct (n,m) gives rise to three bands at distinct wavelengths which for a mixture of different
nanotubes collapse to a broad band.[128]
Intense oxidation or sidewall functionalization leads to the loss of structure in the UV/Vis/nIR
spectrum as a consequence of the conversion of significant number of sp2 -hybridized carbons
to sp3 which leads to the distortion of the nanotubes electronic structure.[65]
1.8.3 Raman Spectroscopy
Raman spectroscopy is a powerful tool for the investigation of the vibrational and electronic
properties of carbon nanotubes. A typical Raman spectrum of carbon nanotubes can be
divided into the low-energy (100 - 300 cm−1 ) and the high-energy region (1200 - 1700 cm−1 )
31
1 Introduction
(figure 1.20). In the low-energy region the radial breathing modes (RBM) are observed. These
Figure 1.20: Raman spectra for laser ablation SWCNTs excited at five different laser frequencies.[129]
modes appear due to a vibration of all atoms in phase in the radial direction. The RBM
frequency is approximately proportional to the inverse tube diameter and can therefore be
used for the determination of the diameter distribution in the sample.[130, 131] Furthermore,
the Kataura-plot allows the assignment of the RBM-modes at a known excitation wavelength
to metallic or semiconducting nanotubes.[127] The Raman peaks at ∼1600 cm−1 (G-modes)
stem from tangential C-C bond motions. The peak at ∼1350 cm−1 is the D-mode, which is
also present in disordered graphite. The D-mode is induced by the presence of defects.[132]
A number of additional Raman peaks is observed in the intermediate frequency range, which
are due to combination modes of optical and acoustic phonons.
The intensity of the Raman modes of carbon nanotubes modes depend on the wavelength of the
exciting laser light as different SWCNTs become resonant for different excitation wavelengths.
Besides the intensity, the position and shape of the peaks is influenced (figure 1.20). The
position and intensity of the D-mode depends strongly on the laser energy. The G-modes shift
32
1.8 Characterization Techniques
as well, although much less than the D-mode whereas, the line shape changes significantly
with excitation energy. The asymmetric shoulder at the lower energy side of the G-band is
usually attributed to metallic nanotubes.[133, 134] At an excitation wavelength of 647 nm
the intensity of this shoulder is significantly increased. One has to take into account that the
recorded Raman spectra represents all nanotubes in the sample. Therefore, the spectra changes
due to differences in the diameter distribution. Raman scattering from a nanotube sample will
dominate the spectrum when the laser photon energy matches the energy difference between
the VHS. The center of each Raman mode then shifts to the frequency of the respective
vibrational mode being resonantly-enhanced.[129]
Raman spectroscopy offers a great deal of information concerning functionalization of carbon
nanotubes, especially in cases of significant sidewall functionalization. The RBM modes give
information on the diameter distribution in a SWCNT sample and therefore, an indication
for the reactivity. The intensity of the D-mode is an indication of disorder in the hexagonal
framework which can be introduced by the functional groups on the sidewalls. A change
of intensity of this mode can therefore be taken as a measure for the amount of covalent
modifications on the nanotube sidewall.
1.8.4 X-ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (XPS) is a useful tool for the investigation of the atomic composition of a sample and the nature of an atom’s environment. The sample is irradiated with
x-rays of an energy h̄ω, resulting in the ejection of electrons from the sample when photons
impinge upon it.[135] The experiments are usually carried out using the characteristic Kα emission line of an Al anode (Al Kα , (h̄ω=1486.6eV)). The XPS technique is highly surface sensitive
(2-5 nm) due to the short inelastic mean free path of the photoelectrons that are excited from
the solid. The kinetic energy (Ekin ) of the photoelectrons leaving the sample are determined
using a hemispherical analyzer to give a spectrum with a series of photoelectron peaks from the
specific core levels of the elements. The binding energy (EB ) of a core line can be calculated
by applying the following equation:
EB = h̄ω - Ekin - Eφ , where Eφ is the work function.
33
1 Introduction
Since each element has a characteristic binding energy, the signals in the spectrum can be
assigned to distinct elements. Note that XPS is not sensitive to hydrogen or helium, but
can detect all other elements. The peak areas can be used, together with the calculated
photoionization cross section, to determine the composition of the sample. The XPS gives also
a deeper insight in the chemical environment of an atom. The core level spectra, recorded with
a better resolution show small shifts (chemical shift) of the binding energies. These shifts can
be attributed to differences in the chemical bonding of the measured atoms. Figure 1.21 shows
the C 1s core level spectrum of C 1s poly(vinyl triflouroacetate). The compound gives rise to
Figure 1.21: C 1s care level spectrum of Poly(vinyl triflouroacetate).[136]
four distinct signals in the spectrum. These signals can be attributed to the four different carbon
atoms present in poly(vinyl triflouroacetate). Three of these signals are shifted towards higher
binding energies, since the corresponding carbon atoms are bound to atoms bearing a higher
electronegativity. The differing electronegativity leads to a decreased electron density at the
carbon atom resulting in a slightly changed binding energy of the electrons.
XPS of carbon nanotube derivatives provides useful data on the atomic composition of the
sample. Furthermore, it allows the assignment of the present atoms to different functional
groups.
34
1.9 Carbon Nanotube Polymer Composites
1.9 Carbon Nanotube Polymer Composites
The exceptional morphological, electrical, thermal, and mechanical characteristics make carbon
nanotubes particularly promising materials as reinforcement in composite materials.[137] The
challenge for the production of carbon nanotube polymer composites is to achieve a good
dispersion or solution of the nanotubes in order to control the nanotube/matrix bonding, the
densification of bulk composites and thin films, and the possibility of aligning the nanotubes.
In addition, the selection of the suitable type of nanotubes (SWCNT, MWCNT) and their
origin (arc discharge, laser ablation, CVD, etc.) is also of great importance since the structural
perfection, surface reactivity, and aspect ratio and therefore, their possible application differ.
In the following, we give an overview of the current work on polymer-matrix composites
reinforced with nanotubes.
Since the first report on nanotube polymer composites in 1994 by Ajayan et al.,[138] this
field was intensively studied. Regarding the mechanical properties of the nanotube polymer
composites three key issues were determined: the strength and toughness of the fibrous
reinforcement, the orientation, and the interfacial bonding, crucial for the load transfer to
occur.[139] The main mechanisms of load transfer are micromechanical interlocking, chemical
bonding, and van der Waals bonding between the nanotubes and the matrix. A high interfacial
shear stress between the fiber and the matrix will transfer the applied load to the fiber over
a short distance.[140] SWCNTs longer than 10 to 100 µm would be needed for a significant
load-bearing ability in the case of nonbonded SWCNT-matrix interactions, whereas the critical
length for SWCNTs cross-linked to the matrix is only 1 µm.[141] Furthermore, the use of
individual SWCNTs might be advantageous compared to MWCNTs or bundles for dispersion
in a matrix, since the weak interfacial interactions between the layers of MWCNTs and between
SWCNTs in bundles might weaken the composite.[139] Mechanical tests performed on 5 wt.%
SWCNT-epoxy composites[142] showed that SWCNTs bundles were pulled out of the matrix
during the deformation of the material. Figure 1.22 shows a SEM image of a SWCNT-epoxy
composite, demonstrating the pull out of the SWCNTs from the polymer matrix at the fracture
surface.
Further studies have focused on the electrical characteristics of SWCNT- and MWCNT-epoxy
composites which are described with the percolation theory. Very low percolation threshold
35
1 Introduction
Figure 1.22: SEM image of SWCNT ropes, observed bridging a fatigue fracture surface in an epoxy
matrix.[143]
at 0.0025 wt.% CNTs and a conductivity at 2 S/m at 1.0 wt.% in epoxy matrices has been
reported.[144] Similarly, epoxy loaded with 1 wt.% unpurified CVD-prepared SWCNTs
showed an increase in thermal conductivity of 70% and 125% at 40 K and at room temperature,
respectively.[145]
Polymer composites with other matrices include MWCNT-polyvinyl alcohol,[146] MWCNTpolyurethane
acrylate,[147]
MWCNT-polyacrylonitrile,[150]
MWCNT-polyaniline,[148]
MWCNT-polystyrene,[149]
MWCNT-poly(3-octylthiophene),[151]
SWCNT-poly(3-
octylthiophene),[152] etc..
In conclusion, two critical issues have to be considered for the application of nanotubes as
reinforcement for advanced composites. One is to choose between SWCNTs and MWCNTs
and the other issue is to tailor the nanotube/matrix interface with respect to the matrix.
First attempts on the optimization of the nanotube/matrix interaction focused on the wetting
between MWCNTs and PMMA. The wetting of the MWCNTs was significantly improved by
coating the MWCNTs with poly(vinylidene fluoride) prior to melt-blending with PMMA.[153]
Recently Blake et al. reported that covalent linkage between CPP and previously with nbutyllithium functionalized carbon nanotube leads to excellent interfacial stress transfer.[154]
Therefore the chemical modification of carbon nanotubes might be a promising approach
towards the optimization of the nanotube/matrix interface.
36
2 Proposal
The first aim of this work is to study the effect of different oxidative techniques on laser
ablation nanotubes to obtain purified carbon nanotubes for further sidewall functionalizations.
The major goal is to develop a procedure which introduces a as low as possible amount of
defects on the carbon nanotubes to ensure that the occurrence of side reactions with the defect
groups during the side wall functionalization does not significantly influence the reaction
course. Furthermore, the nature of the defect groups on the SWCNTs has to be investigated.
In the second part of this work the reactivity of SWCNTs, synthesized via different production
techniques, towards sidewall functionalization has to be studied. For this purpose, we decided
to take advantage of the well-known addition of nitrenes in order to obtain comparable data on
the degree of functionalization.
Based on the results of these first two tasks, it is intended to develop an efficient process for
the sidewall functionalization of carbon nanotubes. This process should result in the covalent
sidewall functionalization of individual SWCNTs instead of bundles. Therefore the potential
of the reaction of SWCNTs with organo-lithium compounds and the reductive alkylation of
SWCNTs should be exploited.
Further studies should focus on the specific chemical modification of the surface of carbon
nanotubes via the in situ polymerization of acrylnitrile to afford an optimized interaction with
a PAN polymer matrix. The yielded CNT derivatives should be applied for the production of
compostite materials to study their potential for the reinforcement of polymers.
Due to the polydisperse nature of the carbon nanotube derivatives, the characterization is
challenging. As a consequence, a reliable characterization procedure has to be established. This
procedure should combine already known characterization methods such as optical absorption
37
2 Proposal
spectroscopy, Raman spectroscopy, TG analysis, and microscopic techniques. Furthermore, a
method for the determination of the degree of functionalization should be developed to allow
the comparison of the results.
38
3 Results
3.1 Oxidation of Single Walled Carbon Nanotubes
The optimization of the production techniques for the synthesis of carbon nanotubes made them
accessible in larger quantities. However, the pristine materials obtained via the different production techniques still contain impurities, such as catalyst particles and amorphous carbon.[3, 23]
These impurities interfere with most applications and therefore the purification of the pristine
material is inevitable. In order to study the efficiency of different oxidative treatments on the
purity of pristine SWCNTs we applied several processes for the purification of laser ablation
SWCNTs. The SWCNT material used in this study was supplied by Frank Hennrich at the
University of Karlsruhe and SWCNTs were synthesized via the laser ablation process using a
mixture of cobalt and nickel as catalysts.[155]
Up to now, a wide range of different purification techniques have been reported. The purification processes can be separated into two categories:
1.
physical techniques, such as filtration,[37] centrifugation[38] or chromatographic
methods[39, 40] which are based on the different size and weight of the impurities compared to
the CNTs, and
2. chemical treatments,[47, 48] which are based on the different reactivity of the CNTs compared to the impurities.
In contrast to the physical processes, the chemical techniques lead to the partial destruction of
the single walled carbon nanotubes followed by the conversion of sp2 carbon atoms from the
graphitic shell into sp3 carbon atoms bearing either carboxylic, carboxy or alcohol functionalities. Despite of this, the chemical purification has one major advantage: this techniques can be
easily scaled up.
In the following study we applied several oxidative procedures, such as oxidation in air, with
H2 O2 , and with HNO3 for the purification of the pristine SWCNT material.
39
3 Results
Since the oxidation in air does not lead to the removal of the catalyst particles, the samples Ox09
and Ox10 were etched with hydrochloric acid to dissolve the cobalt/nickel particles. The variety
of oxidized SWCNT samples (table 3.1) were subsequently characterized by TEM, XPS, and
Raman spectroscopy.
Sample
treatment
temperature [◦ C]
reaction time [h]
pristine SWCNTs
-
-
-
Ox01
HClconc.
RT
12
Ox02
air
200
10
Ox03
air
250
1
Ox04
air
250
3
Ox05
air
250
10
Ox06
air
300
0.5
Ox07
air
300
1
Ox08
air
300
1.5
Ox09
air/HClconc.
300
1
Ox10
air/HClconc. /air
300
1
Ox11
air
400
1
Ox12
3M HNO3
100
1
Ox13
3M HNO3
100
6
Ox14
3M HNO3
100
12
Ox15
3M HNO3
100
24
Ox16
3M HNO3
100
48
Ox17
3M HNO3
100
72
Ox18
10M HNO3
100
1
Ox19
10M HNO3
100
2
Ox20
10M HNO3
100
24
Table 3.1: Variety of oxidation techniques applied for the purification.
40
3.1 Oxidation of Single Walled Carbon Nanotubes
3.1.1 Oxidation in Air
Carbon nanotubes are considered to be chemically more stable than amorphous carbon. Therefore, oxygen is expected to react faster with the amorphous carbon than the carbon nanotubes,
followed by to the combustion of the amorphous material. In order to study the effect of temperature and reaction time on the oxidation of carbon nanotubes, we carried out several experiments
and obtained the samples Ox01 - Ox11. The pristine SWCNTs were placed in an oven and the
resulting oxidized SWCNTs were subsequently dispersed in water. The suspension was filtered
through a 0.2 µm cellulose nitrate membrane filter to give a bucky paper.
To study the effect of the treatment on the SWCNTs we recorded Raman spectra of the so
obtained oxidized SWCNTs. Table 3.2 shows the line position determined from the Raman
measurements. The data reveal minor shifts of the line positions.
G-band
Sample
D-band
ID /IG
RBM 1
RBM 2
Position Position
Position Position
[cm−1 ]
[cm−1 ]
[cm−1 ]
[cm−1 ]
pristine SWCNTs
1594
1277
0.029
179
164
Ox01
1597
1277
0.056
178
165
Ox02
1595
1275
0.039
178
163
Ox03
1596
1279
0.035
182
165
Ox04
1595
1271
0.044
179
163
Ox05
1595
1271
0.050
179
163
Ox06
1594
1273
0.027
182
167
Ox07
1595
1278
0.034
180
162
Ox08
1597
1274
0.048
181
164
Ox09
1596
1274
0.026
182
169
Ox10
1595
1279
0.034
181
165
Ox11
1594
1277
0.118
183
170
Table 3.2: Position of the different Raman lines and intensity ratios of the D-band and the G-band
(λex =1064 nm).
The relative D-band intensity indicates the disorder of the graphitic lattice and therefore,
41
3 Results
gives an indication for the amount of damage on the SWCNTs due to oxidation. Figure 3.1
shows the relationship between the increase of the ID /IG ratios and treatment time. Only minor
Figure 3.1: D- to G-line intensity ratio determined from Raman spectra recorded with a λex =1064 nm
vs. treatment time in hot air at different temperatures.
changes for the oxidation up to 300 ◦ C and no significant changes with the reaction time are visible. This leads to the conclusion that the SWCNTs remain unaffected under these conditions.
In contrast to this, the oxidation at 400 ◦ C leads to a dramatic increase of the D-band intensity.
This indicates the destruction of the SWCNTs at this temperature.
To obtain a more detailed picture on the effect of the applied procedures on the SWCNTs, we
recorded XPS spectra of the samples. Figure 3.2 displays the XPS survey spectrum of the pristine SWCNTs. The spectrum shows only the presence of carbon and oxygen in the sample.
The absence of nickel and cobalt in the spectrum indicates, that the catalyst particles are covered with carbon. On the contrary, the XPS survey spectra of the oxidized samples revealed the
presence of the catalyst. This finding demonstrates the combustion of the amorphous carbon
coating on the catalyst particles, leaving the particles uncovered and therefore detectable for the
measurements. The calculation of the samples’ compositions demonstrated the highest amount
of nickel and cobalt in the samples Ox07 and Ox08. Figure 3.3 shows the XPS survey spectrum
of Ox08 displaying the upcoming signals due to nickel and cobalt. Furthermore, an increase in
42
3.1 Oxidation of Single Walled Carbon Nanotubes
Figure 3.2: XPS survey spectrum of the pristine SWCNTs.
Figure 3.3: XPS survey spectrum of Ox08.
the amount of oxygen is observed. The XPS C 1s core level reveals the formation of carboxylic,
carbonyl, and alcohol functionalities on the SWCNTs due to oxidation (figure 3.4).
To remove the exposed catalyst particles, the sample was treated with concentrated hydrochloric acid leading to a green suspension of the SWCNTs indicative for the dissolution of the
catalyst. After filtration and washing with water, Ox09 was obtained as a bucky paper. The
43
3 Results
Pristine SWCNTs
08
Count Rate (arb. units)
Ox
290
288
286
284
282
Binding Energy (eV)
Figure 3.4: C 1s core level spectra pristine SWCNTs and of Ox08.
Raman data (table 3.2) of Ox09 indicate that the SWCNTs were not affected by the etching
procedure. The XPS survey spectrum of Ox09 (figure 3.5) proves the successful removal of the
catalyst as the signals of nickel and cobalt disappear.
Table 3.3 shows the relative concentrations of selected elements of the pristine SWCNTs,
Figure 3.5: XPS survey spectrum of Ox09.
Ox07, and Ox09. The amount of nickel and cobalt (∼1.8 at.%) seems to be too high compared
44
3.1 Oxidation of Single Walled Carbon Nanotubes
Compound
Element
at.%
pristine SWCNTs
C
94.0
O
6.0
C
86.2
O
10.3
Ni
1.8
Co
1.7
C
89.6
O
10.4
Ox07
Ox09
Table 3.3: Relative concentrations of selected elements determined from XPS survey spectra of pristine
SWCNTs, Ox07 and Ox09.
to the amount used for the synthesis of the SWCNTs. However, it should be taken into account,
that carbon has been removed from the sample due to oxidation. The amount of oxygen remained the same after the HCl etching process.
To get a more detailed picture on the purity of the samples we studied the samples with TEM.
The TEM image of the pristine SWCNTs displays the presence of SWCNT bundles with diameters up to 20 nm (figure 3.6). Furthermore, the images show the presence of catalyst particles
Figure 3.6: TEM image of the pristine SWCNTs.
45
3 Results
which appear as black dots. The dots appear to be covered with carbon indicated by the grayish
regions. This gave further evidence to our findings derived from the XPS experiments.
The TEM images of Ox07 prove the enhanced purity of the sample due to the oxidative treatment (figure 3.7(left)). Furthermore, partial loss of the amorphous coating on the catalyst parti-
Figure 3.7: TEM image of Ox07 (left), and Ox09 (right).
cles can be observed explaining the appearance of cobalt and nickel in the XPS spectra.
The TEM images of Ox09 show the removal of the uncovered catalyst particles (figure
3.7(right)). However, the images still show the presence of impurities within the sample. The
impurities present in the sample appear to be either amorphous carbon or coated catalyst particles both undetectable for XPS. We attribute the presence of remaining impurities to the method
applied for the oxidation. Since the bulk material was placed in an oven, the inner particles of
the bulk were not exposed sufficiently to air and therefore, were not oxidized to the same extend. Subsequent dispersion of the material in water lead to the homogenization of the sample.
In order to prove this speculation we re-oxidized to Ox09 to yield Ox10. XPS spectra of the
sample demonstrated the reappearance of cobalt and nickel.
The oxidation in air at elaborated temperatures turned out to be a mild method for the purification of SWCNTs. The SWCNTs were not significantly damaged by this oxidative treatment,
further the C 1s core level spectra demonstrate that, compared to the HNO3 treatment discussed
below, no notable oxidation of the SWCNTs occurred. However, we have to note that the pro-
46
3.1 Oxidation of Single Walled Carbon Nanotubes
cedure did not lead to a complete removal of the impurities.
3.1.2 Oxidation in Solution
To overcome the problems caused by the aforementioned inaccessibility of the inner material,
we carried out further experiments in solution. We applied the oxidative treatment of SWCNTs
with HNO3 , which is a well known procedure for the purification of SWCNTs.[47, 48] The
oxidation leads to the removal of the amorphous carbon and catalyst particles in a one-step
process.
This purification process is known to damage the carbon nanotubes leading to the formation
of carboxylic, carboxy, and alcohol functionalities which are the target for defect-group
functionalization.[41] Since our goal is the covalent sidewall functionalization these functionalities are not desirable. Since the published results can not be applied to our SWCNT material
we decided to study the effect of various treatments on the SWCNTs to give us a customized
starting material for the subsequent sidewall functionalization.
Table 3.4 shows a summary of the Raman results displaying a dramatic increase of the D-band
intensity after treating the sample with 3 M nitric acid for one hour. However, the oxidation
in concentrated nitric acid seems to affect the SWCNTs only after prolonged reaction periods.
The Raman spectra of the oxidized samples Ox12 - Ox14 show a change in the intensity of the
D-band (figure 3.8). The D-band increases with the reaction time concluding that an elongation
of the treatment time leads to the destruction of the SWCNTs. The loss of intensity at the low
energy side of the G-band indicates that preferable the metallic SWCNTs were oxidized by the
nitric acid.
As the treatment time increases further, the D-band intensity increases (figure 3.9) and after
48 hours the D-band intensity reaches the intensity of the G-band indicating the occurrence of
extensive disorder within the graphitic lattice of the SWCNTs. Further oxidation leads to the
loss of intensity in the RBM-modes in the spectra giving further evidence for the destruction of
the SWCNTs. The G-band displays a loss of the fine structure leading to a broad band in the
most oxidized material Ox17.
To get more information on the desired enhancement of the purity within the SWCNT material
we carried out TEM experiments on the oxidized samples as well. Figure 3.10 (left) shows a
47
3 Results
G-band
Sample
D-band
ID /IG
RBM 1
RBM 2
Position Position
Position Position
[cm−1 ]
[cm−1 ]
[cm−1 ]
[cm−1 ]
pristine SWCNTs
1594
1277
0.029
179
164
Ox12
1600
1312
0.063
182
172
Ox13
1604
1319
0.324
181
163
Ox14
1603
1318
0.419
182
172
Ox15
1606
1322
0.651
182
-
Ox16
1603
1319
0.915
182
-
Ox17
1603
1322
1.500
182
-
Ox18
1597
1305
0.070
180
162
Ox19
1599
1303
0.077
181
164
Ox20
1606
1285
0.243
183
174
Table 3.4: Position of the different Raman lines and intensity ratios of the D-band and the G-band
(λex =1064 nm).
12
Ox13
Ox14
Intensity (a. u.)
Ox
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman shift (cm )
Figure 3.8: Raman spectra (λex =1064 nm) of Ox12, Ox13 and Ox14.
48
3.1 Oxidation of Single Walled Carbon Nanotubes
Figure 3.9: Raman spectra (λex =1064 nm) of the pristine SWCNTs, Ox15, Ox16 and Ox17.
TEM image of Ox12. The image reveals the removal of impurities leading to a higher purity of
the sample. However, the image still displays the presence of catalyst particles covered with
Figure 3.10: TEM image of Ox12 (left), and Ox14 (right).
amorphous material as well as the presence of hollow spheres of amorphous material. This
indicates that the removal of the carbon covered catalyst particles starts with the oxidation of
49
3 Results
the amorphous shell. Once the catalyst is partially uncovered the removal continues with the
dissolution of the catalyst. The catalyst dissolution seems to be a faster process compared with
the oxidation of the amorphous shell. Therefore some of the hollow shells are still present after
one hour of treatment.
Elongation of the treatment time to twelve hours (Ox14) leads to the removal of the impurities
and yields SWCNTs with a high purity (figure 3.10(right)). Further elongation of the treatment
time did not lead to a further improvement of the purity but facilitated the oxidation of the
SWCNTs. The increased purity of Ox14 makes this to a suitable product for further sidewall
functionalization of SWCNTs.
In order to get a deeper insight in the nature of the defects introduced by the oxidative
treatment, XPS spectra of the samples were recorded. The survey spectra of the nitric acid
treated SWCNTs show that, besides carbon and oxygen (figure 3.11), nitrogen is also present
in the sample. The sodium can be caused by the neutralization of the nitric acid with sodium
hydroxide.
The C 1s core level spectra of the nitric acid treated SWCNTs appears to be considerably
C 1s
Na a
Na 1s
Count Rate (arb. units)
O 1s
Oa
1000
800
600
N 1s
400
200
0
Binding Energy (eV)
Figure 3.11: XPS survey spectra of Ox16 (top) and Ox19 (bottom).
broader than the samples subjected to air oxidation (figure 3.12). Furthermore, more intense
50
3.1 Oxidation of Single Walled Carbon Nanotubes
Figure 3.12: C1s core level spectra of Ox11 and Ox18.
chemically shifted components at higher binding energies are observed. These components
can be attributed to carboxylic, carbonyl, and hydroxyl functionalities on the SWCNTs. A
comparison of the spectra leads to the conclusion that the nitric acid treatment results in a more
intense oxidation of the SWCNTs than the treatment in air. Subsequent annealing in UHV
leads to the disappearance of the carboxyl groups and the decrease of the amount of carbonyl
and hydroxyl functionalities.
The N 1s spectra of Ox19 and Ox16 (figure 3.13(bottom)) display the presence of two main
lines in the samples. We attribute the component at 400 eV binding energy to nitroso (-NO)
groups, whereas the peak at 406 eV corresponds to nitro (-NO2 ) groups. The comparison
of the intensity reveals a lower amount of nitrogen in the samples treated with 3 M nitric
acid compared with the concentrated nitric acid. Annealing these samples at temperatures
above 350 ◦ C causes the decrease of the amount of -NO groups while the -NO2 peaks vanish.
Moreover, a new peak at a binding energy of 399 eV (figure 3.13 (top)) appears. This peak
can be attributed to the formation of nitrogen species bound to several carbon atoms. Since no
Raman data of the annealed samples are available, it remains unclear whether the annealing led
only to the removal of the nitrogen and the oxygen atoms present in the sample or even to the
restoring of the graphitization of the SWCNTs.
In conclusion, the nitric acid treatment (applied for Ox12) gave purified SWCNTs with a low
51
Count Rate/Scan (arb. units)
3 Results
annealed at 350°C
annealed at 430°C
as prepared
as prepared
410
405
400
395
410
405
Binding Energy (eV)
400
395
Figure 3.13: N 1s core level spectra of Ox19 (blue) and Ox16 (red) as prepared (bottom) and after
annealing (top) in UHV at the indicated temperatures.
amount of impurities, furthermore the Raman data prove that the SWCNTs show only a low
amount of distortions in the graphitic lattice. However, if a higher purity of the sample is
required (Ox14), subsequent annealing is necessary to the remove the introduced functionalities
to yield suitable SWCNTs for further sidewall functionalization.
52
3.2 Reactivity of Different Types of Single Walled Carbon Nanotubes
3.2 Reactivity of Different Types of Single Walled
Carbon Nanotubes
We studied three types of single walled carbon nanotubes obtained from different production
techniques in order to analyze possible differences in their behavior towards chemical functionalization. Specifically, we used samples synthesized by the three most common production techniques, namely arc discharge, laser ablation, and from the HiPCO process. The arc
discharge and the laser ablation material were processed as pristine material without any further purification. The HiPCO material was used as purchased. To study possible differences
in the reactivity we applied the well known addition of nitrenes to yield aziridino-SWCNTs
(scheme 3.1).[109]
At elaborated temperatures, azidocarbonates generate singlet and triplet nitrenes by N2 extruO
O
N
- N2
N
N
SWCNTs
170° C
O
O
O
N
N
N
O
O
O
N
O
O
Scheme 3.1: Reaction scheme for the functionalization of the SWCNTs with ethylazidocarbonate to give
ethoxycarbonylaziridino-SWCNTs.
sion. The singlet nitrenes are transformed into triplet nitrenes by intersystem crossing (ISC) and
can cycloadd on the sidewall of the nanotube sidewall in a [2+1] fashion.[109] The triplet state
nitrene reacts with the π-system of the nanotube sidewall via a biradical intermediate which
53
3 Results
collapses to the aziridine ring product (scheme 3.2).
The reaction was carried out with three types of nanotubes under the same experimental conO
O
N
+ SWCNT
O
N
O
- N2
O
O
ISC
+ ∆T
N3
- N2
O
O
O
NH
+ SWCNT
N
O
Scheme 3.2: Schematic representation for the attack of nitrenes on SWCNTs.
ditions to study possible differences. In a typical experiment, SWCNTs were sonicated for
24 hours in o-dichlorobenzene under nitrogen atmosphere to give a stable black suspension.
The resulting suspension was subsequently heated to 160 ◦ C followed by the dropwise addition of ethylazidocarbonate. The reaction mixture was stirred at this temperature for two hours
and then allowed to reach room temperature. To obtain the pure SWCNT derivative, the solvent
was evaporated and the black residue was resuspended in acetone and washed several times with
acetone to remove byproducts. The resulting solid was dried in an oven at 80 ◦ C to yield the
SWCNT-derivatives as black solids (table 3.5). Preliminary experiments using 40 mg SWCNTs
Starting material
SWCNT-derivative
Laser ablation-SWCNTs (A SWCNTs)
Ethoxycarbonylaziridino-A SWCNTs (1)
Arc discharge-SWCNTs (B SWCNTs)
Ethoxycarbonylaziridino-B SWCNTs (2)
HiPCO-SWCNTs (C SWCNTs)
Ethoxycarbonylaziridino-C SWCNTs (3)
Table 3.5: Synthesized aziridino-SWCNT-derivatives.
(3.3 mMol carbon) and 43.5 mMol (26 equiv.) ethylazidoformate led to the complete destruction of B SWCNTs and C SWCNTs. Thus, the amount of ethylazidoformate was reduced to
3.5 equiv. leading to the formation of the anticipated SWCNT-derivatives 1, 2, and 3.
In order to get a first overview of the functionalized SWCNTs 1, 2, and 3 were analyzed by
54
3.2 Reactivity of Different Types of Single Walled Carbon Nanotubes
using TEM microscopy (figures 3.14-3.15).
The TEM images revealed a general increase
in the purity of all functionalized samples compared to the starting materials. Furthermore, a
general decrease of the bundle diameter was observed. The TEM images shown in figure 3.15
revealed that the functionalization process leads to highly pure samples for the HiPCO and
the arc discharge material. The functionalized laser ablation sample still showed the presence
(a) Laser ablation-SWCNTs
(b) Ethoxycarbonylaziridino-
(A SWCNTs)
A SWCNTs
(c) Arc discharge-SWCNTs
(d) Ethoxycarbonylaziridino-
(B SWCNTs)
B SWCNTs
Figure 3.14: TEM images of the starting material and the respective functionalized SWCNTs.
55
3 Results
(a)
HiPCO-SWCNTs
(C SWCNTs)
(b) EthoxycarbonylaziridinoC SWCNTs
Figure 3.15: TEM images of the starting materials and the respective functionalized SWCNTs.
of impurities that might be due to the higher amount of impurities and the larger size of the
particles in the starting material compared to the other samples. It seems that the purification
process, which is a sequence of sonication, centrifugation and decantation after the functionalization step, is not capable to remove the larger particles due to a similar solubilization behavior
compared with the nanotubes.
Spectroscopic studies via XPS revealed that the reaction succeeded in the way we proposed.
The spectra shown in figure 3.16 show an increased amount of oxygen in the sample along
with an upcoming signal due to nitrogen. These two features can be attributed to the addends
attached on the SWCNTs.
56
3.2 Reactivity of Different Types of Single Walled Carbon Nanotubes
Count Rate (arb. units)
C 1s
O 1s
1000
800
600
400
200
0
Binding Energy (eV)
(a) Arc discharge-SWCNTs (B SWCNTs)
1000
Cl 2s
800
600
400
Cl 2p
O 1s
O a
N 1s
Count Rate (arb. units)
C 1s
200
0
Binding Energy (eV)
(b) Ethoxycarbonylaziridino- B SWCNTs
Figure 3.16: XPS survey spectra of B SWCNTs (a) and ethoxycarbonylaziridino- B SWCNTs (b).
57
3 Results
A careful analysis of the O 1s core level spectrum of the samples showed the presence of two
different oxygen components. Figure 3.17 displays as an example the O 1s core level spectrum
of ethoxycarbonylaziridino- B SWCNTs. The two components in the spectrum are attributed to
the NCOO-group within the addend bearing two different kinds of oxygen, one bound only to
one carbon atom and another one bound to two carbon atoms. The spectrum reveals that the
amount of both oxygen components in the sample is the same, as proposed.
Count Rate (arb. units)
The comparison of the atomic compositions determined by XPS survey scans (table 3.6) shows
540
535
530
525
Binding Energy (eV)
Figure 3.17: O 1s core level spectrum of ethoxycarbonylaziridino- B SWCNTs.
Compound
C (%)
O (%)
N (%)
Laser ablation-SWCNTs (A SWCNTs)
94.0
6.0
0
Ethoxycarbonylaziridino-A SWCNTs (1)
76.6
18.6
4.8
Arc discharge-SWCNTs (B SWCNTs)
99.0
1.0
0
Ethoxycarbonylaziridino-B SWCNTs (2)
90.5
6.6
2.9
HiPCO-SWCNTs (C SWCNTs)
93.0
6.0
1.0
Ethoxycarbonylaziridino-C SWCNTs (3)
78.0
16.0
6.0
Table 3.6: Carbon to oxygen to nitrogen ratio as determined by XPS survey spectra.
a dramatic increase in the amount of oxygen and nitrogen for all the functionalized compounds.
58
3.2 Reactivity of Different Types of Single Walled Carbon Nanotubes
This can be explained by the presence of the addends and therefore, is an indication for the degree of functionalization. The atomic ratio of the change in the oxygen concentration compared
to the upcoming nitrogen peak fits well with our prediction of a 2:1 ratio (oxygen to nitrogen)
(compare scheme 3.1). The introduction of these heteroatoms allowed us to calculate the degree
of functionalization by applying the following equation:
ch
zh
Df unct.(XP S) [%] =
× 100%,
zca
cc − (
× ch )
zh
(3.1)
Df unct.(XP S) [%] = percentage of carbon atoms bearing an addend, which is defined by:
ch [at.%]
= amount of considered heteroatoms atoms in the sample,
zh
= number of considered hetroatoms in the addend,
cc [at.%]
= amount of carbon atoms in the sample, and
zca
= number of addend carbon atoms.
According to the calculations shown in table 3.7 the HiPCO sample revealed the highest degree
of functionalization. This leads to the conclusion that because of their small diameter, the
HiPCO SWCNTs show the highest reactivity, followed by the laser ablation tubes. The fact
that the arc discharge material has the lowest degree of functionalization, combined with the
TEM images, testifies that this starting material has the most pronounced graphitization level
between the studied samples and therefore the lowest reactivity.
The Raman spectra recorded with an excitation wavelength of 514.5 nm (figure 3.18) reveals
Compound
Df unct.(XP S) [%]
Ethoxycarbonylaziridino-A SWCNTs (1)
7.7
Ethoxycarbonylaziridino-B SWCNTs (2)
3.5
Ethoxycarbonylaziridino-C SWCNTs (3)
7.9
Table 3.7: Degree of functionalization achieved for the different SWCNT samples.
an increase of the D-band intensity for all functionalized samples compared to the starting
materials. Furthermore, a dramatic change of intensity of the RBM-modes for B SWCNTs and
C
SWCNTs was observed. The dramatically increased intensity of the ethoxycarbonylaziridino-
B
SWCNTs RBM-modes could be attributed to the destruction of the defective nanotubes within
59
3 Results
the samples and the subsequent removal from the sample. In order to verify this interpretation,
A
SWCNTs
A
1
B
2
C
3
Ethoxycarbonylaziridino- SWCNTs ( )
B
SWCNTs
Ethoxycarbonylaziridino- SWCNTs ( )
C
Intensity (a. u.)
SWCNTs
Ethoxycarbonylaziridino- SWCNTs ( )
100
200
300 1200
1300
R
1400
1500
1600
1700
1800
-1
aman Shift (cm )
Figure 3.18: Raman spectra (λex =514.5 nm) of the ethoxycarbonylaziridino-SWCNTs and the corresponding starting materials.
we recorded Raman spectra with an excitation wavelength of 1064 nm (figure 3.19). Due to
the higher sensitivity of this excitation wavelength to the D-band, it is more intense compared
to the spectra recorded with λex =514.5 nm. The spectrum for A SWCNTs shows the same
behavior as discussed previously. The spectrum of B SWCNTs shows a broad D-band which
indicates the high amount of impurities within this starting material. The sharper D-band in the
spectrum of the corresponding functionalized material indicates an enhancement of the purity
during the functionalization process, leading to a more homogeneous sample. Furthermore, the
RBM-modes within this sample display significant changes in their intensity, in contrast to the
general decrease of the RBM intensities the RBM modes located at higher wavenumbers gain
intensity. Finally the spectra of the ethoxycarbonylaziridino-C SWCNTs shows a significant
increase of the D-band and therefore, a decrease of the RBM-modes caused by the distortion of
the SWCNTs electronic structure due to the functionalization.
Our results show for the first time a comparison of a variety of single walled carbon nanotubes
60
3.2 Reactivity of Different Types of Single Walled Carbon Nanotubes
A
SWCNTs
A
1
B
2
C
3
Ethoxycarbonylaziridino- SWCNTs ( )
B
SWCNTs
Ethoxycarbonylaziridino- SWCNTs ( )
Intensity (a. u.)
C
SWCNTs
Ethoxycarbonylaziridino- SWCNTs ( )
100 200 300
1100
1200
R
1300
1400
1500
1600
1700
1800
-1
aman Shift (cm )
Figure 3.19: Raman spectra (λex =1064 nm) of the ethoxycarbonylaziridino-SWCNTs and the starting
materials.
and their behavior towards chemical functionalization. We obtained information on the deviant
reactivity of laser ablation, arc discharge and HiPCO single walled carbon nanotubes towards
chemical functionalization via reactive nitrenes. The functionalization process leads to a remarkable increase in the purity of all functionalized samples compared to the starting materials.
Furthermore, we have shown that the change in the RBM intensities of the functionalized
SWCNTs are strongly SWCNT-type dependent. The comparison of the XPS spectra reveals
that the HiPCO SWCNTs are the most reactive ones in the studied range proving that HiPCO
SWCNTs are the most appropriate SWCNTs for covalent sidewall functionalizaton.
However, the TEM images recorded at higher resolution indicate that the functionalization
via nitrenes is not capable of splitting up all the bundles into individual tubes (figure 3.20).
Moreover, the images show an amorphous coating surrounding the bundles. The amorphous
material can be attributed to the addends on the SWCNTs sidewalls. Therefore, we conclude
that the functionalization of entire bundles occurs during the functionalization process. This
leads to an inhomogeneity within the products consisting of functionalized individual tubes
and bundles. The low efficiency in terms of separation into individual tubes might be due to the
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3 Results
(a) Arc discharge-SWCNTs (B SWCNTs)
(b) Ethoxycarbonyl-aziridino- B SWCNTs
Figure 3.20: TEM images of B SWCNTs and 2.
high reaction velocity of the nitrenes.
Achieving a more homogenous and efficient functionalization in a slower reaction would be
favorable. Furthermore, this new reaction on the SWCNTs should lead to the solubilization of
the SWCNT bundles into individual SWCNTs.
62
3.3 Functionalization of Carbon Nanotubes with Organo-Lithium Compounds
3.3 Functionalization of Carbon Nanotubes with
Organo-Lithium Compounds
It has been reported by Hirsch et al.[156] that C60 can react with organo-lithium compounds,
giving alkylated metal fullerides such as RC60 − Li+ . The reactivity of single walled carbon
nanotubes is known to be an intermediate between the low reactivity of graphite and the higher
reactivity of fullerenes. The increased reactivity of carbon nanotubes relative to graphite can
be explained in terms of structural deformation. Due to the roll-up of the graphene sheet into a
carbon nanotube, the sp2 -carbon atoms are forced out of the favored planar conformation.
Early studies described the reaction of fluorinated single walled carbon nanotubes with organolithium compounds resulting in the replacement of the fluorine atom on the carbon nanotubes
by alkyl groups.[157] However, the pronounced reactivity of organo-lithium compounds should
also be capable for a nucleophilic attack on the SWCNT sidewall resulting in the formation of
SWCNT anions.
We used purified SWCNTs obtained from the HiPCO process for the covalent sidewall functionalization with organo-lithium compounds. Our studies (chapter 3.2) revealed that this type of
single walled carbon nanotubes shows the highest reactivity towards sidewall functionalization
compared to other nanotubes. Furthermore, the commercial availability of HiPCO SWCNTs
makes them a standard material in most workgroups dealing with the functionalization of
carbon nanotubes. Therefore the utilization of this type of nanotubes allows the comparison of
our results with the results of other groups.
For the reaction with organo-lithium compounds, the SWCNTs were dispersed in anhydrous
benzene in an ultrasonic bath (scheme 3.3). The resulting suspension was unstable, since the
removal from the ultrasonic bath lead to the reagglomeration of the SWCNTs in the solvent.
A solution of the lithium-organyl was added slowly to this suspension and the mixture was
stirred for one hour at room temperature. While stirring, the SWCNT dispersion turned into a
black homogenous solution. This observation can be explained by the nucleophilic attack of
the organo-lithium compound on the SWCNTs sidewall leading to a negative charge on the
SWCNTs. We attribute the dissolution of the SWCNTs to this transferred charge leading to
a repulsive interaction between the negatively charged nanotubes. The resulting solution was
stable for approximately one day under inert gas atmosphere. The reaction was accomplished
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3 Results
Scheme 3.3: Schematic of the functionalization process for the reaction of SWCNTs with lithiumorganyls.
by quenching the charged intermediate with diluted hydrochloric acid accompanied by the
precipitation of the functionalized carbon nanotubes. The resulting mixture was filtered through
a 0.2 µm PTFE membrane filter and washed with ethanol and THF. The yielded black solid
was dried under vacuum at 50 ◦ C overnight.
Scheme 3.4 demonstrates the proposed mechanism of the nucleophilic attack on the SWCNTs
Scheme 3.4: Schematic of the attack of lithium-organyls on a benzene ring at the SWCNTs sidewall.
sidewall. We believe that the organo-lithium compound attacks a double bond of a benzene ring
64
3.3 Functionalization of Carbon Nanotubes with Organo-Lithium Compounds
of the carbon nanotube. This attack leads to the formation of a C-C bond with the alkyl-group
attached and an anionic charge on the carbon nanotube. The charge should be delocalized
over the whole π-system and therefore leads to the negative charging of the SWCNT. Since
the completion of the reaction takes about one hour, we expect a homogeneous distribution of
the functional groups along the tube. Using an excess of alkyl lithium should also lead to the
functionalization of the SWCNTs within the bundles due to the debundling during the reaction
course.
3.3.1 Reaction of SWCNTs with Organo-Lithium Compounds
We investigated the functionalization of SWCNTs by using a variety of different organolithium compounds, namely t-butyllithium, n-butyllithium, and phenyllithium to yield
t-butyl-H-SWCNTs (4), n-butyl-H-SWCNTs (5) and phenyl-H-SWCNTs (6) (figure 3.21).
This allowed us to study possible difference in the reactivity of the different organo-lithium
compounds towards the addition on SWCNTs.
All the reactions described here were performed as described previously and gave the
H
H
H
H
H
(a) t-Butyl-H-SWCNTs (4)
H
(b) n-Butyl-H-SWCNTs
(c) Phenyl-H-SWCNTs (6)
(5)
Figure 3.21: Schematic representation of the functionalized SWCNTs achieved by the reaction with
Lithium organyls.
functionalized SWCNTs as black solids. The recorded TEM images (figure 3.22) reveal a
general increase of the purity of the functionalized SWCNTs compared to the starting material.
This observation can be attributed to the charging of the carbon nanotubes which leads to the
65
3 Results
(a) HiPCO-SWCNTs
(b) t-Butyl-H-SWCNTs (4)
(c) n-Butyl-H-SWCNTs (5)
(d) Phenyl-H-SWCNTs (6)
Figure 3.22: TEM images of HiPCO SWCNTs and functionalized SWCNTs.
debundling of the SWCNTs and therefore to the release of intercalated catalyst particles. As
the functionalization process proceeds, the catalyst particles are removed from the material by
intense washing and filtration. By using a filter with a pore size of 0.2 µm the catalyst particles
with a diameter of about 10 nm are washed out.
The TEM images recorded with higher resolution show a significant decrease of the
66
3.3 Functionalization of Carbon Nanotubes with Organo-Lithium Compounds
bundle diameter from ∼20 nm in the starting material to ∼5 nm within the functionalized
samples (figure 3.23). Furthermore, the images reveal an amorphous coating on the tubes
(a) t-Butyl-H-SWCNTs (4)
(b) n-Butyl-H-SWCNTs (5)
Figure 3.23: TEM images of samples 4 and 5 recorded with higher resolution.
which can be attributed to the addends on the sidewalls. The resolution of single addends was
not possible due to their irregular distribution on the sidewalls.
The use of XPS-spectroscopy for this systems is limited due to the absence of heteroatoms in
the addends. Figure 3.24 shows the XPS survey spectrum of t-butyl-H-SWCNTs (4), as an
example. The spectrum shows, beside carbon, the presence of impurities such as silicon and
oxygen. However, since no iron is present in the functionalized material the spectrum gives
further prove for the purification of the SWCNTs by the functionalization process. The C
1s core level spectrum of t-butyl-H-SWCNTs (4) (figure 3.25) shows a minor shift to higher
binding energy. This indicates, that the quenching of the negatively charged intermediate did
not lead to the complete removal of the electron excess on the SWCNTs. The fact that no
chemically shifted components on the C 1s core level are found indicates that the oxygen
present in the survey spectrum is not attached on the carbon nanotubes.
The increased solubility of the functionalized SWCNTs enabled us to perform NMR-
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3 Results
Figure 3.24: XPS survey spectra of HiPCO SWCNTs and t-butyl-H-SWCNTs (4).
Figure 3.25: XPS C 1s core level spectra of HiPCO SWCNTs and t-butyl-H-SWCNTs (4).
spectroscopic measurements of the functionalized SWCNTs. Figure 3.26 shows the 1 H-NMR
spectra of t-butyl-H-SWCNTs (4) and n-butyl-H-SWCNTs (5) measured in TCE-d2 . The 1 HNMR spectrum of 4 (figure 3.26 (top)) gives rise to a signal at δ=7.28 which corresponds to
the proton directly bound to the SWCNT and a signal ascribed to the t-butyl-group at δ= 1.47.
In contrast to this, the 1 H-NMR spectrum of 5 gives rise to three signals with the first signal at
δ=7.29, which can be assigned to the proton directly bound to the SWCNT. The n-butyl-group
68
3.3 Functionalization of Carbon Nanotubes with Organo-Lithium Compounds
Figure 3.26: 1 H-NMR spectra of t-butyl-H-SWCNTs (4)(blue), and n-butyl-H-SWCNTs (5) (green) in
TCE-d2 .
includes two signals, one for the CH2 -group directly bond onto the SWCNTs at δ= 1.99 and a
second broad signal for the methyl and methylene protons at δ= 1.51. The integration of the
signals gave a ratio of 1:9 for proton to the butyl-group. The chemical shifts are in agreement
with chemical shifts found for the corresponding fullerene derivatives.[158]
The nucleophilic attack of organo-lithium compounds forms sp3 carbon atoms in the nanotubes
sidewalls. This should lead to a change in the ratio of the intensities of the D-band and the
G-band in the Raman spectrum. Figure 3.27 shows the Raman spectra of the starting material in comparison with the functionalized SWCNTs. The spectra display a shift of the whole
spectrum for the t-butyl-H-SWCNTs and n-butyl-H-SWCNTs to lower wavenumbers and for
the phenyl-H-SWCNTs to higher wavernumbers compared to the starting material. The G-band
appears to be narrower in all functionalized materials indicating an increase in purity of the
samples. Furthermore, a decrease at the lower energy side of the G-band is observed. This re-
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3 Results
HiPCO SWCNTs
Intensity (a. u.)
t-Butyl-H-SWCNTs (4)
n-Butyl-H-SWCNTs (5)
Phenyl-H-SWCNTs (6)
200
300
1200
1300
R
1400
1500
1600
1700
1800
1
-
aman Shift (cm )
Figure 3.27: Raman spectra (λex =514.5 nm) of the SWCNT-derivatives and the starting materials.
gion is characteristic for metallic SWCNTs and therefore, indicates that preferably the metallic
tubes were attacked by the organo-lithium compounds. Table 3.8 shows a comparison of the
D-band intensities of the studied compounds.
Compound
ID /IG (λex = 514.5 nm) ID /IG (λex = 1064 nm)
HiPCO SWCNTs
0.051
0.06
t-Butyl-H-SWCNTs (4)
0.064
0.19
n-Butyl-H-SWCNTs (5)
0.079
0.23
Phenyl-H-SWCNTs (6)
0.089
-
Table 3.8: Relative intensities of the D- to G-band achieved for the different SWCNT samples.
70
3.3 Functionalization of Carbon Nanotubes with Organo-Lithium Compounds
3.3.2 Enhancement of the Degree of Functionalization
The results discussed so far already demonstrate the high potential of the use of organo-lithium
compounds for the covalent sidewall functionalization of single walled carbon nanotubes.
However, the relative degree of functionalization determined by the ID /IG was relatively low.
Attempts to increase the degree of functionalization by increasing the amount of organo-lithium
compound added to the SWCNTs did not lead to an increase of the number of functionalities
on the nanotubes. This lead us to the conclusion that after the addition of a certain number of
functional groups the further reaction is hindered.
A closer look at the reaction mechanism reveals two possible reasons for the hinderance
of a further addition on the carbon nanotube. The first reason might be the steric crowding
on the nanotubes surface caused by a number of functional groups on the sidewalls which
prevent the access of additional reagent. The second reason might be due to the amount of
negative charge on the SWCNTs generated after the nucleophilic addition of butyllithium.
Therefore the question arises whether the negative charge is located on the carbon atom
next to the functionalized one or if it is distributed over the whole π-system of the SWCNT.
Delocalization over the π-system would lead to a homogenous charging of the nanotubes to a
certain amount of negative charge on the whole surface. The homogeneous distribution of the
negative charge on the SWCNT would render the whole SWCNT unreactive towards a further
nucleophilic attack of butyllithium. To visualize the charge distribution on the SWCNTs we
calculated the electron density distribution on a pristine SWCNT, the charged intermediate
(t-butyl-SWCNT- ), and the the protonated product (t-butyl-H-SWCNT) (figure 3.28). The
calculations visualize the changes in the charge distribution on the SWCNTs surface as the
reaction proceeds. The reaction course starts with an uncharged SWCNT (I) followed by the
addition of the organo-lithium compound. The addition of the butylanion on the SWCNT leads
to the charging of the SWCNT resulting in a t-butyl-SWCNT- (II). As the reaction proceeds
the anion is protonated by the addition of HCl to give the discharged t-butyl-H-SWCNT (III).
The calculations for I show no areas of enhanced electron density whereas in the SWCNT
anion II the negative charge is indicated by the green and yellow areas which represent areas
with high electron density. The calculation indicates that the negative charge is not localized
next to the addend but delocalized over the whole π-system of the SWCNT. Furthermore the
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3 Results
t-BuLi
I
II
HCl
- LiCl
III
Figure 3.28: Electron density distribution on carbon nanotubes (yellow areas indicate high electron density).
calculation reveals that an increased electron density on the SWCNT after the protonation (III)
remains, which we attribute to the +I effect of the t-butyl group.
We applied a second functionalization process to the t-butyl-H-SWCNTs, where the charge
is removed, to figure out whether the degree of functionalization is limited due steric or
charging effects. The Raman spectra of the resulting t-butyl-H-SWCNTsa (figure 3.29) show
a significant increase of ID /IG , and therefore prove the occurrence of further functionalization
of the SWCNTs. This indicates that the limitation of the functionalization was not due to steric
effects but due to the charging of the SWCNTs.
The significant increase in D-band intensity gives rise to the question whether a change of the
reaction conditions would lead to an even further increase of the degree of functionalization.
For this purpose we varied the reaction time and the applied temperature (table 3.9) for the
synthesis of t-butyl-H-SWCNTsb , and t-butyl-H-SWCNTsc (9).
The Raman spectra reveal significant changes in the spectra of 7-9 compared to the starting
material and the t-butyl-H-SWCNTs (figure 3.29). The second functionalization step leads to
a decrease of the RBM-modes intensities. Furthermore, an increase of the D-Band intensity
was detected due to the nucleophilic attack of organo-lithium compounds which forms further
72
3.3 Functionalization of Carbon Nanotubes with Organo-Lithium Compounds
HiPCO SWCNTs
4
t-Butyl-H-SWCNTs (7)
b
t-Butyl-H-SWCNTs (8)
c
t-Butyl-H-SWCNTs (9)
t-Butyl-H-SWCNTs ( )
Intensity (a. u.)
a
200
300
1200
1300
R
1400
1500
1600
1700
1800
1
-
aman Shift (cm )
Figure 3.29: Raman spectra (λex =514.5 nm) of the SWCNT-derivatives and the starting materials.
Compound
reaction conditions
t-Butyl-H-SWCNTsa (7)
1 h | RT
t-Butyl-H-SWCNTsb (8)
72 h | RT
t-Butyl-H-SWCNTsc (9)
1 h | 80 ◦ C
Table 3.9: Applied reaction conditions to enhance the degree of functionalization.
sp3 -carbon atoms on the nanotubes sidewalls. The G-band appears to be narrower in all functionalized materials indicating the increased purity of the samples. Furthermore, a decrease
at the lower energy side of the G-band was observed. This region is commonly attributed to
metallic SWCNTs and therefore indicates that preferable the metallic tubes were attacked by
the organo-lithium compound.
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3 Results
Table 3.10 shows the ID /IG values for the SWCNT-derivatives synthesized in this study. The
comparison shows only minor effects of the variation of the reaction conditions on the relative
degree of functionalization. However the relatively low value for t-butyl-H-SWCNTsb indicates
that the prolonged reaction time lead to the decomposition of the functionalized SWCNTs.
We recorded the Raman spectra of the RBM-region with higher resolution to further estabCompound
ID /IG
t-Butyl-H-SWCNTs (4)
0.064
t-Butyl-H-SWCNTsa (7)
0.302
t-Butyl-H-SWCNTsb (8)
0.274
t-Butyl-H-SWCNTsc (9)
0.346
Table 3.10: Relative intensities of the D- to G-band achieved for the different SWCNT samples determined from Raman spectra recorded with λex = 514.5 nm.
lish the preferential attack of metallic tubes indicated by the changes of the G-band shape.
Figure 3.30 shows the spectra of t-Butyl-H-SWCNTsa , t-butyl-H-SWCNTs and the starting
material. The t-butyl-H-SWCNTsa reveal a significant decrease in intensity loss of the modes
Figure 3.30: Raman spectra (λex =532 nm) of t-butyl-H-SWCNTsa , t-butyl-H-SWCNTs, and the starting material. The different curves for one set are taken on different locations of one sample.
located at 270 cm−1 whereas no major changes are visible in the spectra of the t-butyl-HSWCNTs compared to the starting material. The bands appearing in this region of the spectrum
74
3.3 Functionalization of Carbon Nanotubes with Organo-Lithium Compounds
can be assigned to metallic nanotubes.[127] The decrease in intensity indicates that the electronic structure of the related tubes has been distorted due to the functionalization process.
TEM images revealed the further decrease in bundle diameter of the functionalized SWCNTs.
Furthermore, an amorphous coating on the surface of the SWCNTs is visible (figure 3.31). The
observation of an individual addend itself is not possible by this microscopy method, therefore we carried out scanning tunnelling microscopy (STM) experiments on the functionalized
Figure 3.31: TEM image of t-butyl-H-SWCNTsa .
SWCNTs in order to achieve further prove for the successful functionalization. STM should
be capable to resolve single addends on the sidewall of the SWCNT. Due to the rigidity of the
t-butyl group compared to the other addends studied here we chose this system for these experiments.
Figure 3.32(left) shows a bundle of three SWCNTs with atomic resolution of the SWCNTs sidewalls. The three single walled carbon nanotubes shown in the image exhibit different chiralities.
Furthermore the image revealed a decoration of the sidewalls on the surface of the SWCNTs.
A close-up of the lower region in the middle of figure 3.32 (left), shown in figure 3.32 (right),
reveals a threefold symmetry within the addends which led us to identify these addends with
the t-butyl-groups on the sidewall of SWCNTs. The increased size of the t-butyl group (7 Å)
arises from the technique, since not the real size is detected but an excited molecular orbital.
Therefore the measured size of the addend changes, depending on the measurement conditions.
Thus, allowing us for the first time the direct visualization of an addend on a SWCNT.
Further scanning on the surface of this functionalized SWCNT leads to the removal of the ad-
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3 Results
5Å
Figure 3.32: STM images of a small bundle of t-butyl-H-SWCNTs (4).
dend leaving a disorder on the SWCNTs surface. Figure 3.33 shows the same SWCNT as
5Å
Figure 3.33: STM image of t-butyl-H-SWCNTs (4).
shown in figure 3.32 after intense scanning. The disturbed areas located at the regions of the
SWCNTs which former displayed the addend indicate the distortion of the graphitic lattice due
to the STM tip induced removal of the addend. Therefore, the tearing of the t-butyl group leaves
a defect site on the surface of the SWCNT.
76
3.3 Functionalization of Carbon Nanotubes with Organo-Lithium Compounds
3.3.3 Functionalization of the Charged Intermediate
The functionalization of SWCNTs with organo-lithium compounds as shown in scheme 3.3
proceeds via the formation of a charged intermediate in the first step, followed by protonation
with diluted hydrochloric acid. To introduce further functionalities on the SWCNTs we treated
the intermediate with different electrophiles. Electrophilic reagents should attack the negatively
charged SWCNT leading to a bond formation and subsequently to the introduction of further
functionalities on the carbon nanotubes.
We chose bis(2,3,6-trimethylpyridine)iodine(I)hexafluorophosphate[159] and p-toluenesulfonyl cyanide as electrophiles to target the synthesis of t-butyl-iodo-SWCNTs (10) and
t-butyl-cyano-SWCNTs (11) (scheme 3.5). The t-butyl-cyano-SWCNTs yielded also t-butylcarboxyl-SWCNTs (12) after a saponification reaction was carried out.
The recorded TEM images (figure 3.34) reveal a general increase of the purity of the
Scheme 3.5: Schematic representation of the synthesis of t-butyl-iodo-SWCNTs (10) and t-butyl-cyanoSWCNTs (11).
functionalized SWCNTs compared to the starting material. The functionalization process leads
to the removal of the catalyst particles due to the intense washing and filtration steps. The
usage of a filter with a pore size of 0.2 µm allowed the catalyst particles which have a diameter
of about 10 nm to pass through. Furthermore, the presence of an amorphous coating on the
SWCNTs sidewalls was observed in TEM images recorded with a higher resolution.
The Raman spectra of all functionalized materials show an increase of the D-band intensity due
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3 Results
(a) HiPCO-SWCNTs
(b) t-Butyl-iodo-SWCNTs
(c) t-Butyl-cyano-SWCNTs
(d) t-Butyl-carboxyl-SWCNTs
Figure 3.34: TEMimages of HiPCO SWCNTs and the functionalized SWCNTs 10, 11, and 12.
to the formation of sp3 -carbon atoms within the SWCNTs sidewalls (figure 3.35). The G-band
remained unaffected. The comparison of the D-band intensity reveals higher values compared
to the products formed by protonation of the charged intermediate with HCl (table 3.11). The
values for 10, 11 and 12 show no major variations and thus the two reactions lead to SWCNTs
with a comparable degree of functionalization. The high value for the saponified SWCNTs 12
indicates that the applied reaction procedure did not lead to the defunctionalization of the
78
3.3 Functionalization of Carbon Nanotubes with Organo-Lithium Compounds
HiPCO SWCNTs
10)
11)
t-Butyl-Carboxyl-SWCNTs (12)
t-Butyl-Iodo-SWCNTs (
Intensity (a. u.)
t-Butyl-Cyano-SWCNTs (
200
300
1200
1300
R
1400
1500
1600
1700
1800
1
-
aman Shift (cm )
Figure 3.35: Raman spectra (λex =514.5 nm) of the starting material and the functionalized SWCNTs.
Compound
ID /IG (λex =514.5 nm)
t-Butyl-iodo-SWCNTs(10)
0.104
t-Butyl-cyano-SWCNTs (11)
0.109
t-Butyl-carboxyl-SWCNTs (12)
0.120
Table 3.11: Relative degree of functionalization achieved for the different SWCNT samples.
SWCNTs.
The composition of the samples determined from XPS survey spectra show only a low amount
of iodine in the t-butyl-iodo-SWCNTs which is not consistent with the results of the Raman
spectroscopy (table 3.12). We attribute this deviation to the high reactivity of the iodine on
the SWCNTs that favors secondary reactions followed by its replacement. The XPS survey
spectrum of t-butyl-carboxyl-SWCNTs reveals that the reaction conditions applied for the
saponification reaction did not lead to a complete conversion of the nitrile species into a
carboxyl functionality.
Very recently Chen et al. [119] reported on the reaction of sec-butyl-SWCNT− with carbon
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3 Results
Compound
Element
at.%
t-Butyl-iodo-SWCNTs (10)
C
99.5
I
0.5
C
84.1
O
12.2
N
3.7
C
79.1
O
18.7
N
2.1
t-Butyl-cyano-SWCNTs (11)
t-Butyl-carboxyl-SWCNTs (12)
Table 3.12: Relative concentrations of selected elements determined from XPS survey spectra of the
SWCNT derivatives.
dioxide to yield carboxyl-alkyl-SWCNTs (c-a-SWCNTs). The comparison of the ID /IG ratios
determined for 10, 11, and 12 with the data given for the c-a-SWCNTs is limited by the
different excitation wavelengths (514.5 nm↔632.8 nm) used for the Raman studies. Since
a lower the excitation wavelength results in a lower ID /IG ratio. However, the ID /IG ratios
determined for 10, 11, and 12 are still greater than for the c-a-SWCNTs indicating a more
pronounced functionalization of the SWCNTs synthesized here.
3.3.4 In situ Polymerization
The results obtained from the functionalization of carbon nanotubes with organo-lithium
compounds have already shown the high potential of this reaction. However, we have to note
one major disadvantage of the reaction. The protonation of the reactive intermediate leads to
the subsequent conglomeration of the SWCNTs, forming small bundles of the functionalized
material. To overcome this problem, we decided to apply a polymerization process leading
to a coating of the SWCNTs and therefore hinder the rebundling of the SWCNTs after the
protonation step. In doing so we take advantage of the formed charged intermediate to initiate
an anionic polymerization during the reaction course.
However, during the course of our studies Viswanathan et al.[118] reported on the reaction of
80
3.3 Functionalization of Carbon Nanotubes with Organo-Lithium Compounds
sec-butyllithium with SWCNTs and the subsequent in situ polymerization of styrene.
We decided that t-butyl acrylate and acrylonitrile might be the most promising candidates
for the polymerization to yield t-butyl-poly(t-butyl acrylate)-SWCNTs (13) and t-butylpolyacrylnitrile-SWCNTs (14). The reaction proceeds as described in scheme 3.3. The charged
intermediate is treated with t-butyl acrylate or acrylonitrile and the reaction mixture is stirred
for twelve hours at room temperature to complete the polymerization (scheme 3.6). As the
Scheme 3.6: Schematic of the in situ polymerization on SWCNTs.
reaction proceeds, the viscosity of the mixture increased due to polymerization to give a stable
solution of the product.
To reduce the polymerization initiated by the excess of butyllithium, the amount of butyllithium
added was reduced to favor the addition on the SWCNTs.
The functionalized SWCNTs showed increased solubility in common organic solvents allowing
the 1 H-NMR spectroscopic study of the functionalized tubes. Figure 3.36 shows the 1 H-NMR
spectrum of t-butyl-poly(t-butyl acrylate)-SWCNTs. The spectrum gives rise to signals which
can be attributed to the polymer. The extraction of the signal due to the t-butyl-group directly attached to the SWCNT was not possible since it overlaps with the signals of the polymer.
The XPS spectra of the functionalized SWCNTs showed all the expected atoms present in
the samples (figure 3.37). The calculation of the atomic composition from the XPS survey
spectra gave the composition of the polymer with a slight excess of carbon (table 3.13). The
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3 Results
Figure 3.36: 1 H-NMR spectrum of t-butyl-poly(t-butyl acrylate)-SWCNTs.
Figure 3.37: XPS survey spectrum of t-Butyl-Poly(t-butyl acrylate)-SWCNTs.
excess of carbon can be attributed to the SWCNTs present in the samples.
The collection of Raman spectra of the functionalized SWCNTs was complicated, since the
spectra displayed a very low Raman intensity due to the low content of SWCNTs. This finding
is in contrast to the results of Viswanathan et al.[118] We attribute this to the more pronounced
82
3.3 Functionalization of Carbon Nanotubes with Organo-Lithium Compounds
Compound
Element
at.%
t-Butyl-poly(t-butyl acrylate)-SWCNTs (13)
C
75.4
O
24.6
C
83.1
N
16.9
t-Butyl-polyacrylnitrile-SWCNTs (14)
Table 3.13: Relative concentrations of selected elements determined from XPS survey spectra of the
SWCNT-derivatives.
polymer coating on the SWCNTs in our work. Unfortunately Viswanathan did not report on
the thickness of the polymer coating. However the low polymer content of 10 % in contrast to
70 % in our studies demonstrates the higher efficiency of the approach developed in this work.
To overcome this problem we increased the laser power which results to heating of the sample
and therefore to the defunctionalization of the SWCNT derivatives. Nevertheless, the recorded
Raman spectra (figure 3.38) still show a minor increase of the relative D-band intensity.
Furthermore, a dramatic decrease at the lower energy side of the G-band was observed. This
HiPCO SWCNTs
t-Butyl-Poly(t-butyl acrylate)-SWCNTs (
14)
13)
Intensity (a. u.)
t-Butyl-Polyacrylnitrile-SWCNTs (
200
300
1200
1300
R
1400
1500
1600
1700
1800
1
-
aman Shift (cm )
Figure 3.38: Raman spectra (λex =514.5 nm) of t-butyl-poly(t-butyl acrylate)-SWCNTs, t-butylpolyacrylnitrile-SWCNTs, and HiPCO SWCNTs.
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3 Results
region is characteristic for metallic SWCNTs and therefore indicates that the metallic tubes
were preferentially attacked by the reaction.
TEM images of the functionalized SWCNTs revealed the presence of a thick amorphous
coating surrounding the SWCNTs (figure 3.39). This amorphous coating can be attributed to
(a) t-Butyl-poly(t-butyl acrylate)-SWCNTs
(b) t-Butyl-polyacrylnitrile- SWCNTs
(13)
(14)
Figure 3.39: TEM images of the functionalized SWCNTs.
the polymer on the SWCNTs surface. A more careful investigation of the SWCNTs reveals
that the polymerization did not lead to a homogeneous coating. This observation is in good
agreement with our expectations, since the limited charges would also lead to a limited degree
of polymerization on the SWCNTs.
TEM images recorded with a higher resolution demonstrate that mainly individual and small
bundles were covered with the polymer (figure 3.40). This gives further evidence for the
separation of the SWCNT bundles due to the charging. The presence of functionalized bundles
can be explained by the reduced amount of butyllithium which was added.
84
3.3 Functionalization of Carbon Nanotubes with Organo-Lithium Compounds
(a) t-Butyl-poly(t-butyl acrylate)-SWCNTs
(b) t-Butyl-polyacrylnitrile- SWCNTs
(13)
(14)
Figure 3.40: TEM images with a higher resolution of the functionalized SWCNTs.
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3 Results
3.4 Integration of Carbon Nanotubes into Polymers
As carbon nanotubes reveal outstanding mechanical properties their integration into plastics is
a field of great interest.[137] However, mechanical tests performed on 5 wt.% SWCNT-epoxy
composites[142] showed that SWCNTs bundles were pulled out of the matrix during the deformation of the material. This indicates that the optimization of the nanotube/matrix interaction
is inevitable for the production of composite materials. First attempts on the optimization
of this interaction focused on the wetting between MWCNTs and PMMA. The wetting of
the MWCNTs was significantly improved by coating the MWCNTs with poly(vinylidene
fluoride) prior to melt-blending with PMMA.[153] Recently Blake et al. reported that covalent
linkage between CPP and previously with n-butyllithium functionalized carbon nanotube leads
to excellent interfacial stress transfer.[154] Therefore the chemical modification of carbon
nanotubes might be a promising approach towards the optimization of the nanotube/matrix
interface.
The increased solubility of the SWCNT derivatives synthesized so far encouraged us to apply
our functionalization process for the integration of carbon nanotubes into polymers.
Since the work reported here was carried out in collaboration with SGL Carbon we decided to
use multiwalled carbon nanotubes due to their availability in larger quantities.
3.4.1 Characterization of the starting materials
Multiwalled carbon nanotubes show a reduced reactivity towards covalent sidewall functionalization compared to single walled carbon nanotubes due to the lower pyramidalization angle of
the carbon atoms (chapter 1.5). This reduced reactivity can be attributed to the larger diameter
of the outermost shell and the corresponding smaller deformation of the graphene sheet.
To determine the most promising candidates for a sidewall functionalization we obtained a
variety of MWCNT samples from five sources, synthesized via different production techniques
(table 3.14). The comparison of the recorded TEM images revealed remarkable differences
of the materials (figure 3.41). The MWCNTs produced via the CVD process appeared to
be much longer than the nanotubes synthesized via the arc discharge process. Furthermore,
the
86
A
MWCNTs -
C
MWCNTs showed higher purity.
Regarding the following chemical
3.4 Integration of Carbon Nanotubes into Polymers
(a) A MWCNTs
(b)
B MWCNTs
(c)
C MWCNTs
(d)
D MWCNTs
(e)
E MWCNTs
Figure 3.41: TEM images of the MWCNT samples.
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3 Results
MWCNT Sample
Producer
production technique
A
MWCNTs
NanoCyl
CVD
B
MWCNTs
Thomas Swan & Co
CVD
C
MWCNTs
ILJIN Nanotech
CVD
D
MWCNTs
n-TEC
arc discharge
E
MWCNTs
Rosseter Holdings
arc discharge
Table 3.14: Overview of the different MWCNTs studied.
functionalization we also studied the diameter distribution within the samples determining a
lower diameter for the CVD tubes.
However, in contrast to this, the Raman spectra of the samples (figure 3.42) show a very
A
MWCNTs
B
C
Intensity (a. u.)
D
E
MWCNTs
MWCNTs
MWCNTs
MWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 3.42: Raman spectra (λex =514.5 nm) of the different MWCNT samples.
high D-band intensity for the CVD MWCNTs indicating the high amount of defects within
the structure. In order to determine the origin of the high D-band intensity we recorded TEM
images with a better resolution.
88
3.4 Integration of Carbon Nanotubes into Polymers
The TEM-images revealed the origin of the dramatically increased D-band intensity of the
CVD grown nanotubes compared to the arc discharge MWCNTs (figure 3.43). The sidewalls
of
A
MWCNTs -
C
MWCNTs appeared to be not straight but winded, indicating the poor
graphitization of these materials. In contrast, the D MWCNTs and E MWCNTs appeared to be
very well graphitisized.
According to these results we decided to apply our functionalization process to one material
(a) A MWCNTs
(b)
E MWCNTs
Figure 3.43: TEM images of two MWCNT samples, taken at higher resolution.
of each production technique. From the CVD grown tubes we chose the A MWCNTs due to the
lowest median diameter of this material. For the arc discharge grown material we chose the
D
MWCNTs due to the higher purity compared to the E MWCNTs.
3.4.2 Polymer composites from MWCNTs
For the fabrication of MWCNT polymer composites we used polyacrylnitrile (PAN) provided
from SGL Carbon. The choice of PAN was based on the later application of this process by
SGL Carbon, to the production of MWCNT/PAN fibres.
In order to study the effect of the functionalization on the properties of the resulting composite,
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3 Results
we prepared MWCNT/PAN composites using the MWCNT starting material (table 3.15). The
compound
concentration of MWCNTs
resulting composite
within the polymer [wt.%]
A
MWCNTs
1
Ia)
A
MWCNTs
2.5
Ib)
D
MWCNTs
1
IIa)
D
MWCNTs
2.5
IIb)
Table 3.15: Produced MWCNT/PAN composites.
preparation of the composites starts with the dissolution of the 1 g polyacrylnitrile in 10 ml
N,N-dimethylacetamide (DMA). To this solution, the desired amount of MWCNTs was added.
The resulting mixture was sonicated for twelve hours to obtain a stable black dispersion of the
MWCNTs. To maintain a film of this dispersion 5 ml were filled into a mold and dried for two
days at 40 ◦ C in a vacuum oven.
The resulting composites (figure 3.44) showed an inhomogeneous distribution of the MWCNTs
within the polymer. Furthermore, the sample showed shrinking of about 20 % along with
(a) Ia)
(b) IIb)
Figure 3.44: Photographs of composite Ia) and IIb).
drastic deformation. The incorporation of the MWCNTs led in all samples to the embrittlement
of the polymer leaving a drastically reduced flexibility to the material.
90
3.4 Integration of Carbon Nanotubes into Polymers
To achieve better solubilization of the MWCNTs within the PAN matrix we applied the
functionalization process developed for the synthesis of n-butyl-H-SWCNTs (5) to yield
n-butyl-H-A MWCNTs. Attempts to functionalize B MWCNTs in the same way failed, leaving
the starting material unchanged. We attribute the lack of reactivity of the arc discharge tubes
on the well pronounced graphitization of this material. Therefore, the distortions present in the
graphitic lattice of the A MWCNTs seem to represent the reactive carbon atoms.
The synthesized n-butyl-H-A MWCNTs were used to produce the polymer films Ic) and Id)
(table 3.16). The shrinking of the two composites was less pronounced compared to the
compound
concentration of MWCNTs
resulting composite
within the polymer [wt.%]
n-Butyl-H-A MWCNTs
1
Ic)
n-Butyl-H-A MWCNTs
2.5
Id)
Table 3.16: Produced n-butyl-H-A MWCNT/PAN composites.
composites with untreated MWCNTs. Furthermore, the composites revealed a high flexibility.
Figure 3.45 shows photographs of composite Ic) demonstrating the reduced deformation of the
material. A closer look at the edges of the composite reveals the occurrence of reconglom-
(b) Ic)
(a) Ic)
Figure 3.45: Photographs of the composite Ic).
eration during the drying process leading to the formation of MWCNT particles within the
polymer matrix. This demonstrated the requirement of an improvement of the solubility of the
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3 Results
MWCNTs in the polymer.
In order to achieve this goal we decided to synthesize n-butyl-polyacrylnitrile-A MWCNT
according to the functionalization process described for the functionalization of SWCNTs
in section 3.3.4. The polyacrylnitrile coating on these CNTs should lead to an optimized
compatibility with the PAN matrix.
Subsequent incorporation in the PAN matrix led to the composites Ie) and If) (table 3.17).
These materials displayed no shrinking and no deformation. Furthermore, the composites
compound
concentration of MWCNTs
resulting composite
within the polymer [wt.%]
n-Butyl-Polyacryl-
1
Ie)
2.5
If)
nitrile-A MWCNTs
n-Butyl-Polyacrylnitrile-A MWCNTs
Table 3.17: Produced n-Butyl-Polyacrylnitrile-A MWCNT/PAN composites.
appeared to be flexible and transparent. A closer look at the edges of the composite Ie) (figure
3.46 (right)) demonstrates the homogeneity of the material. No indications for reconglomeration of the MWCNTs were observed in the sample.
In order to study the effect of the incorporation of MWCNTs in the PAN matrix, we carried out
(b) Ie)
(a) Ie)
Figure 3.46: Photographs of composite Ie).
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3.4 Integration of Carbon Nanotubes into Polymers
tensile tests on the composites produced. Figure 3.47 shows the results of these measurements.
The data reveal an increase of the tensile strength of all composites compared to PAN. The
Figure 3.47: Tensile strength of the produced composites.
n-butyl-polyacrylnitrile-A MWCNT/PAN shows the highest values indicating the homogenous
distribution of the MWCNTs in the polymer matrix. The lower tensile strength of the materials
with the higher loading indicates overloading of these composites. The tensile strength increase
for Ie) of about 70% is two times higher compared to the results obtained by Weisenberger
et al.[150] who reported on MWCNT/PAN composites.
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3 Results
More pronounced improvements were found on the elongation of the sample (figure 3.48).
The results show an increase by a factor of 7 for the composite Ie) compared to the starting
material.
The ripping of the composites during the tensile strength tests allowed us to get an insight into
Figure 3.48: Elongation at break of the produced composites.
the polymer matrix. SEM images were taken at the fracture surface of the composites. Those
of the samples containing the untreated MWCNTs displayed a flat edge showing only a few
CNTs (figure 3.49). This gives further evidence for the inhomogeneity of the sample due to the
conglomeration of the MWCNTs.
In contrast to this, the composites with the functionalized MWCNTs shows at the fracture surface many MWCNTs sticking out of the polymer (figure 3.50). The image demonstrates the
homogeneous distribution of the MWCNTs in the PAN matrix. The SEM image reveals that the
MWCNTs are not significantly pulled out of the matrix, this indicates a good polymer/matrix
interaction.
94
3.4 Integration of Carbon Nanotubes into Polymers
Figure 3.49: SEM image of the fracture surface of Ia).
Figure 3.50: SEM image of the fracture surface of Ie).
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3 Results
3.5 Reductive Charging of Single Walled Carbon
Nanotubes
Previous studies have shown that the dipping of a bucky paper into a Broenstedt acid leads to
a shift of the C 1s core level towards lower binding energy.[160] This shift indicates a p-type
doping of the SWCNTs caused by the protonation of the SWCNTs. In the case of a reaction
of SWCNTs with organo-lithium compounds the resulting charging of the SWCNTs should
lead to a dramatic shift of the C 1s core level towards higher binding energies. However, our
XPS-experiments carried out so far on this systems did not show any significant changes in the
position of the C 1s core level. In order to obtain a more detailed picture of the reaction course
for the reaction of SWCNTs with organo-lithium compounds we have to study the intermediate
(scheme 3.3) instead of the protonated product. Since our previous experiments showed that
the charged intermediate is not stable in air we had to develop an in situ technique for the XPS
measurements.
The samples for the XPS measurements were prepared by the dispersion of SWCNTs in
anhydrous benzene under nitrogen atmosphere for two hours in a sonication bath followed
by the addition of the reducing agent at room temperature and subsequent sonication for one
hour to give the charged intermediate as a black solution. The solution was transferred with
a syringe to a gold covered sample holder which was placed into a nitrogen purged vacuum
chamber (figure 3.51). The solvent was removed by pumping down the chamber which resulted
in the formation of a closed, black coating on the sample holder. The resulting samples were
transferred into the XPS chamber under UHV conditions to carry out the measurements.
We studied three different approaches for the charging of SWCNTs:
I:
treatment of SWCNTs with t-Butyllithium,
II:
reduction of SWCNTs with sodium naphthalite, and
III:
reduction of SWCNTs with sodium.
96
3.5 Reductive Charging of Single Walled Carbon Nanotubes
Figure 3.51: Schematic of the load-lock chamber used for the in situ preparation of the samples for XPS
measurements (I vacuum pump, II analysis chamber).
Although great care was taken to avoid contamination of the samples, a considerable amount
of oxygen was found in the samples. We attribute this to the contact with the stainless steel
needle of the syringe. Moreover, the concentration of sodium and lithium on the surface exceeded the concentration in solution. We attribute this to the drying process which caused the
sedimentation of the SWCNTs and therefore led to the enrichment of the lithium and sodium
concentration on the surface as the solvent evaporated.
The C 1s core level spectrum of the untreated SWCNTs (figure 3.52) shows the typical asymmetric lineshape, characteristic for sp2 -bonded carbon systems. The measured binding energy
for the graphitic carbon was 284.42 eV.
In contrast to this, the C 1s core levels of the treated SWCNTs are all shifted towards higher
binding energies compared to the untreated sample. The sodium treated SWCNTs show only a
small change in binding energy (∆E=0.3 eV), whereas the sodium naphthalite treated SWCNTs
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3 Results
Count Rate (arb. units)
I
II
III
Pristine
292
290
288
286
284
282
Binding Energy (eV)
Figure 3.52: C 1s core level of untreated, pristine SWCNTs compared to the sodium(III) , sodium naphthalite (II), and the t-butyllithium (I) treated SWCNTs. The dashed line marks the position
of the C 1s core level of the untreated SWCNTs.
show a shift of ∆E=1.5 eV and a significant broadening of the signal. The most pronounced
change in binding energy was measured for the t-butyllithium treated sample which shows a
shift of ∆E=2.7 eV for the main C 1s component. The shift of the C 1s core level is caused by
the reduction which leads to the filling of the previously unoccupied π ∗ -orbitals (figure 3.53).
This results in the shift of the Fermi-level and therefore in a change of the C 1s core level
towards higher binding energies. The shift towards higher binding energies indicates the delocalization of the additional electrons over the π-system of the SWCNT since a localized charge
should cause a change in binding energy of the C 1s core level of the corresponding carbon atom
to lower binding energies (chemical shift). Therefore the shift of the binding energy gives an
indication for the amount of charge transferred and therefore for the efficiency of the reduction.
A more detailed investigation of the reduction of SWCNTs with sodium reveals that the change
in binding energy increases upon heating the sample at 400 ◦ C (∆E=0.71 eV) whereas the signal
is shifted back to the position of the as prepared SWCNTs if heating was continued at 800 ◦ C
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3.5 Reductive Charging of Single Walled Carbon Nanotubes
Figure 3.53: A change in the position of the Fermi-level (EF ) results in the shift of the binding energy
of all spectral features in photoelectron spectroscopy.
(figure 3.54). We attribute the comparatively low changes in binding energy of the C 1s core
Sodium
Count Rate (arb. units)
ann. at
292
800°C
ann. at
ann. at
400°C
200°C
as prepared
290
288
286
284
282
Binding Energy (eV)
Figure 3.54: C 1s core level of the sample prepared using metallic sodium (bottom) and after annealing
to the indicated temperatures. The dashed line marks the position of the C 1s core level of
the untreated SWCNTs.
level in the as prepared SWCNTs to the formation of large clusters of sodium in the mixture.
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3 Results
Therefore the accessibility of the sodium and the inhomogeneous distribution in the sample hinders an efficient interaction with the nanotubes. Successive annealing of the sample at 400 ◦ C
leads to a more pronounced shift of the C 1s core level to higher binding energies. This can be
explained by a melting of the sodium at elevated temperature and therefore to a decrease of the
cluster size allowing a more sufficient reduction of the SWCNTs followed by a further increase
of the C 1s binding energy to 285.13 eV. Subsequent annealing at 800 ◦ C led a position of the
C 1s core level as in the pristine samples. The shift to lower binding energies indicates the
reoxidation of the SWCNTs and therefore the removal of electrons from the previously filled
π ∗ -orbital. This interpretation is confirmed by the survey spectrum which shows that the sodium
disappeared. We conclude that the annealing at 800 ◦ C led to the evaporation of metallic sodium
which includes the oxidation of the previously charged SWCNTs.
The treatment with sodium naphthalide led to a higher binding energy compared to the sodium
treated sample, increasing even further at 200 ◦ C, whereas annealing at 410 ◦ C and 800 ◦ C
again led to a decrease of the binding energy (figure 3.55). We attribute the more pronounced
Na Naphthlite
Count Rate (arb. units)
ann. at
800°C
ann. at
ann. at
410°C
200°C
as prepared
292
290
288
286
284
282
Binding Energy (eV)
Figure 3.55: C 1s core level of the sample prepared using sodium naphthalide (bottom) and after annealing to the indicated temperatures. The dashed line marks the position of the C 1s core level
of the untreated SWCNTs.
shift in the as prepared sample to the dissolution of sodium naphthalide in the reaction mixture
in contrast to the formation of sodium clusters in the previous experiment. The dissolution of
100
3.5 Reductive Charging of Single Walled Carbon Nanotubes
the reducing agent allows a homogeneous interaction with the carbon nanotubes and therefore
results in a more intense reduction of the SWCNTs. Heating of the sample at 200 ◦ C leads to a
further shift towards higher binding energies, a narrowing of the main peak and gives rise to a
new component at a lower binding energy. The shift combined with the decrease of the peak is
caused by an increase in transferred charge from the naphthalide to the SWCNT. The upcoming
signal at lower binding energies can be assigned to the naphthalene. Annealing to 400 ◦ C leads
to a shift to lower binding energies and a narrowing of the C 1s core level. We attribute the
lower width of the signal to the evaporation of naphthalene and the shift of the signal, combined
with the decrease in the amount of sodium in the survey spectrum, to the evaporation of metallic
sodium followed by the oxidation of the SWCNTs. The result that in this case the evaporation
of sodium starts at lower temperatures compared to the case when metallic sodium was used
might be due to the presence of atomic sodium in atomic form instead of small clusters.
The treatment with t-butyllithium (figure 3.56) caused the most intense shift of the binding en-
Count Rate (arb. units)
t-Butyllithium
294
ann. at
400°C
as prepared
292
290
288
286
284
282
Binding Energy (eV)
Figure 3.56: C 1s core level of the sample prepared using t-butyllithium (bottom) and after annealing at
400 ◦ C. The dashed line marks the position of the C 1s core level of the untreated SWCNTs.
ergy to 287.16 eV detected in this study. Annealing of the sample to 400 ◦ C again leads to
the shift of the C 1s core level peak to lower binding energy. Furthermore, a peak appears at
291.5 eV. This peak can be attributed to the formation of lithiumcarbonate under these drastic
conditions.
101
3 Results
Taking the measured binding energy shifts as an indication for the reduction of the carbon
nanotubes, the treatment with t-butyllithium turns out to be the most effective, followed by the
treatment with sodium naphthalide and sodium. We attribute the lower reductive potential of the
metallic sodium to the heterogenous character of this reaction. In conclusion the dissolution of
sodium has a dramatic effect on the efficiency of the reduction, and therefore the use of sodium
in liquid ammonia might be a promising approach, which will be discussed in the next chapter.
102
3.6 Reductive Alkylation of Carbon Nanotubes
3.6 Reductive Alkylation of Carbon Nanotubes
Our previous results on the reduction of single walled carbon nanotubes proved that the
reductive alkylation might be a promising approach for the derivatization of carbon nanotubes.
In a new approach, we used sodium and naphthalene in absolute THF to achieve the reduction
of the carbon nanotubes followed by alkylation. As discussed in chapter 3.5, the dissolved
naphthalene is supposed to act as a charge transfer reagent to enhance the efficiency of
the reduction of the solid SWCNTs by the solid sodium. The assumed mechanism for this
reduction is shown in scheme 3.7. To obtain a homogeneous charging of the SWCNT material
Na(solid)
Na+
(solid)
Scheme 3.7: Charging of carbon nanotubes via sodium naphthalide.
sonication was applied during the reaction. After one hour, the mixture was removed from the
sonication bath and stirred at room temperature. Due to the charging of the carbon nanotubes
a homogenous stable black solution was formed. To this solution the alkyl halide was added
dropwise and the reaction mixture was stirred for twelve hours. In order to compensate the
consumption of alkyl halide by the side reaction with naphthalene, the alkyl halide was added
in a large excess. The mechanism of the addition of the alkyl group on the carbon nanotube is
not known so far.
Scheme 3.8 shows two possible pathways for the reaction of SWCNTn− with alkyliodide.
Path (A) starts with an electron transfer from the charged SWCNT to the alkyl halide to form
a radical anion that dissociates readily to yield halide and the alkyl radical. The alkyl radical
attacks the carbon nanotube via a radical addition. Path (B) proceeds via a SN 2-mechanism.
103
3 Results
A
B
SWCNTn- + IC8H16I
SWCNTn- + IC8H16I
SWCNT(n-1)- + IC8H16I
- I-
-I
SWCNT(n-1)- +
C6H12I
I
(n-1)-
Scheme 3.8: Alkylation of the charged SWCNTs with alkyl halides.
Fukuzumi et al.[161] reported on the mechanism for the comparable reaction of C60 2− with
alkyl halides, For this purpose they compared two possible mechanism:
1. via electron transfer to the alkyl halide to give a radical anion. Subsequent cleavage of the
R-X bond leads to the formation of an alkyl radical which adds on the C60 (path A), and
2. via a SN 2 pathway (B) initiated by the nucleophilic attack of the fullerene anion on the alkyl
halide.
The authors reported that the reaction mechanism depends strongly on the negative charge
on the carbon atoms. Therefore a low negative charge of about -0.06 (like in C60 2− ) at the
carbon atoms favors an electron transfer mechanism (path A). Whereas the second addition on
R-C60 1− follows the SN 2 pathway due to the localization of the negative charge on the carbon
atoms next to the first addend R which results in a calculated charge of these atoms of about
-0.33.[161]
Feng Liang et al.[120] has reported recently a similar reaction supporting that the reaction
follows path A. However, our results lead us to the conclusion that path B is more likely. The
104
3.6 Reductive Alkylation of Carbon Nanotubes
results from the XPS measurements indicate an intense charging of the SWCNTs resulting in a
negative charge on the atoms of < -0.12. This charge leads to an increased nucleophilic reactivity of the carbon nanotubes. Since the charge on the SWCNTs reduces as the reaction proceeds
the question may arise whether the reaction path changes, but we assume in accordance with
R-C60 1− that the addition of R on the SWCNT leads to a increased electron density on the
neighboring carbon atoms and therefore retains the nucleophilic reactivity.
Furthermore, in contrast to Feng Liang et al.[120] we did not observe the complete conversion
of the alkyl halide into the corresponding radical followed by the polymerization of 1,8diiodooctane. We found that the synthesized SWCNT-derivative still bares an iodide atom on
the alkyl chain. Evidence for this was obtained by the XPS spectrum (figure 3.57) which shows
the presence of iodine along with the expected elements at the anticipated binding energies
in the sample.
A closer look at the I 3d core level (figure 3.58) shows that all of the iodine
C 1s
85% C
12.3% O
Count Rate (arb. units)
2.6% N
0.17% I
O 1s
I 3d
O a
1000
800
600
N 1s
400
200
0
Binding Energy (eV)
Figure 3.57: XPS survey spectrum of 8-iodooctyl-SWCNTsa (15).
present in the sample is bound to carbon and that no I− is present.
Unfortunately, the 8-Iodooctyl-SWCNTsa are only barely soluble in organic solvents. This
indicates a low degree of functionalization which can be verified by the comparison of the
Raman spectra of the functionalized SWCNTs and the starting material shown in figure 3.59.
The spectra showed the broadening of the D-band but no significant increase of the intensity.
However, the G-band shows a decrease of the left shoulder of the main signal. This decrease
105
3 Results
3d
Count Rate (arb. units)
I
635
630
625
620
615
d
Bin ing Energy (eV)
Figure 3.58: XPS I3d spectrum of 8-iodooctyl-SWCNTsa (15).
can be explained by a loss of metallic tubes due to the functionalization. This low degree of
HiPCO SWCNTs
8-Iodooctyl-SWCNTs (15)
Intensity (a. u.)
a
1200
1300
1400
R
1500
1600
1700
1800
-1
aman Shift (cm )
Figure 3.59: Raman spectrum (λex =514.5 nm) of 8-iodooctyl-SWCNTsa (15) and the starting material.
functionalization was far beyond our expectations, and thus we decided to modify the reaction
conditions in order to increase the degree of functionalization.
106
3.6 Reductive Alkylation of Carbon Nanotubes
3.6.1 Variation of the Reaction Conditions for the Reductive
Alkylation of Single Walled Carbon Nanotubes
3.6.1.1 Optimization of the Reaction Conditions for the Synthesis of
8-Iodooctyl-SWCNTs
As the low degree of functionalization achieved during the synthesis of 15 was quite unsatisfying, we modified the functionalization process to obtain better results. We believe that the
key to a higher degree of functionalization lies in the optimization of the reduction step of
the SWCNTs. In order to improve the electron transfer on the carbon nanotubes we applied
a method discovered by Wooster, Godfrey and Birch[162, 163] for the reduction of aromatic
rings. The reaction process starts with the condensation of ammonia into a nitrogen purged
flask. Sodium was added to the liquid ammonia leading to a dark blue color associated with
the solvated electrons. To this solution the SWCNTs were added. The SWCNTs were rapidly
dissolved in the liquid ammonia, leading to a homogenous black solution of the reduced
SWCNTs. As mentioned earlier, the SWCNTs dissolve readily due to the repulsive interaction
of the negative charge on the carbon nanotubes. This shows that an extensive negative charging
of the SWCNTs helps to overcome the π-π-stacking interactions. This effect was also recently
reported by Pénicaud et al.[164] who was able to synthesize a sodiumTHFSWCNT salt which
instantly dissolves in THF.
This solution was stirred for one hour at -70 ◦ C while this time the color was monitored to
ensure the presence of an excess of sodium in the solution. To this solution the alkyl halide was
added dropwise and the reaction mixture was stirred overnight with the slow evaporation of the
ammonia. After about one hour, the SWCNTs started to precipitate again due to the alkylation
reaction which leads to the loss of the negative charge on the SWCNTs as discussed in chapter
3.6. The work up of the crude product gave 8-iodooctyl-SWCNTsb (16) as a black solid. The
use of lithium instead of sodium as reducing agent gave 8-iodooctyl-SWCNTsc (15).
To evaluate the influence of the reducing agent on the degree of functionalization of the
product, the experiment was repeated by using lithium instead of sodium to give 8-iodooctylSWCNTsc (17).
The comparison of the Raman spectra of 15, 16 and 17 with the starting material (figure 3.60)
shows a significant increase of the D-band for the compounds 16 and 17 compared to 15
107
3 Results
and the starting material. This increase of the D-band gave the first evidence for a higher
HiPCO SWCNTs
Intensity (a. u.)
8-Iodooctyl-SWCNTsa (15)
8-Iodooctyl-SWCNTsb (16)
8-Iodooctyl-SWCNTsc (17)
100
200
300
1200
1300
R
1400
1500
1600
1700
1800
-1
aman Shift (cm )
Figure 3.60: Raman spectra (λex =514.5 nm) of HiPCO-SWCNTs, 8-iodooctyl-SWCNTsa (15), 8iodooctyl-SWCNTsb (16) and 8-iodooctyl-SWCNTsc (17).
degree of functionalization due to the modified reaction conditions used. Furthermore, a
decrease in the intensity of the RBM-modes of the samples 16 and 17 can be detected. The
origin of this decrease lies in the destruction of the electronic structure of the SWCNTs due
to covalent functionalization. The RBM-modes and the G-bands of all of the functionalized
materials are upshifted by about 4 cm−1 .
This upshift was also detected by McNeil et
al. [165] who attributed that upshift to the absence of metal particles in the sample, since
a residue of metal catalyst transfers charge to the SWCNTs, leading to a downshift of the modes.
The comparison of the XPS survey spectra of 16 (figure 3.61) and 17 (figure 3.62) shows a
higher iodine content in 16. By using the composition determined for the samples via XPS we
were able to estimate the degree of functionalization by applying equation 3.1.
The thermogravimetric analysis of the synthesized 8-iodooctyl-SWCNTs indicates the decomposition of the samples at elevated temperatures. The thermogravimetric analysis were carried
out under nitrogen atmosphere to ensure that the carbon nanotubes were not burned during the
108
3.6 Reductive Alkylation of Carbon Nanotubes
82.1%
14.2% O
2.7% N
0.2% I
0.8%
C
1s
Si
3d
O
N 1s
O 1s
a
I
Count Rate (arb
.
s
unit )
C
1000
800
600
400
200
0
Binding Energy (eV)
Figure 3.61: XPS survey spectrum of 8-iodooctyl-SWCNTsb (16).
84.7% C,
10.2% ,
4. % Si,
0.77%
C 1s
O
I
O
I 3p3/
d
1s
I 3
I
p
Si 2s
Si 2
4d
O a
I 3p /
12
2
Ra
(a
Count te rb. units)
3
1000
800
600
400
inding nergy eV)
B
E
200
0
(
Figure 3.62: XPS survey spectrum of 8-iodooctyl-SWCNTsc (17).
measurement. The detected weight loss due to the removal of the addend and the weight of the
residue which is ascribed to the SWCNTs allowed us to calculate the degree of functionalization
(Df unct.(T GA) [%]) by applying the following equation:
109
3 Results
Df unct.(T GA) [%] =
m(addend) [g] × M(addend) [g/mol]
× 100%
m(SW CN T s) [g] × M(C) [g/mol]
Df unct.(T GA) [%]
= percentage of carbon atoms bearing an addend,
m(addend) [g]
= weight loss determined by TGA,
M(addend) [g/mol]
= 239,
m(SW CN T s) [g]
= mass of the TGA residue, and
M(C) [g/mol]
= 12.011.
(3.2)
Table 3.18 gives a summary of all the obtained data on the degree of functionalization for the
8-iodooctanyl-SWCNTs. The values given were determined by the interpretation of the data we
got from the XPS-, Raman- and TGA-measurements. None of the values shown here is affected
by one of the other measurements.
compound
ID /IG
Df unct.(XP S) [%]
Df unct.(T GA) [%]
8-Iodooctyl-SWCNTsa (15)
0.016
0.2
2.2
8-Iodooctyl-SWCNTsb (16)
0.078
0.2
1.0
8-Iodooctyl-SWCNTsc (17)
0.067
1.0
1.6
Table 3.18: Degree of functionalization achieved via the different functionalization processes.
The comparison of the data in table 3.18 reveals an inconsistency in the degree of functionalization determined. We attribute this to the formation of a variety of different byproducts
(figure 3.63) during the functionalization process.
The possible formation of different products leads to inaccurate values for Df unct.(XP S) and
Df unct.(T GA) as the molecular masses were used for the calculations. Therefore, the data we
obtained from Raman measurements are the most reliable results concerning the efficiency of
the reaction conditions, proving that the procedure used for the synthesis of 16 is the most
effective.
However, the comparison of the data shown in table 3.18 allows us to determine the main
product of each procedure. For the conditions used for the synthesis of 15, the low ID /IG
110
3.6 Reductive Alkylation of Carbon Nanotubes
I
I
n
(I)
(II)
(III)
Figure 3.63: Possible reaction products formed by the reductive alkylation of SWCNTs with 1,8diiodooctane.
indicates a low degree of functionalization on the SWCNTs. The large value of Df unct.(T GA)
combined with the low intensity of the D-band shows that the addend has a much higher
molecular mass than estimated and therefore, that mainly the product III was formed during
the reaction.
For the procedure used for the synthesis of 16 the high value of ID /IG in contrast to the low
value of Df unct.(T GA) and Df unct.(XP S) reveals that both halides of the 1,8-diiodooctane reacted
with the carbon nanotube leading to II, as the main product. The method used for the synthesis
of 17 gave relatively consistent data revealing that mainly product I was formed. The high
value for Df unct.(T GA) indicates that the formation of III occurred to some extend.
Furthermore, it should be noted here that the samples 16 and 17 showed a decreased solubility
in organic solvents, leading to the conclusion that due to the existence of two functionalities on
the 1,8-diiodooctane, the formation of interconnects between the nanotubes took place.
111
3 Results
3.6.1.2 Optimization of the Reaction Conditions for the Synthesis of
Dodecyl-SWCNTs
In order to overcome the problems caused by addition of an alkyl-chain bearing two iodine
functionalities, we utilized a monoiodo alkane. For the following experiments we chose 1iodododecane as reagent to obtain dodecylated SWCNTs (figure 3.64).
Encouraged by the impressive results of the applied method for the reductive alkylation of
Figure 3.64: Schematic representation of dodecyl-SWCNTs.
aromatic rings in terms of the degree of functionalization for the compounds 16 and 17, we repeated those reactions with 1-iodododecane. Furthermore, we decided to apply other conditions
known for the reductive alkylation of aromatic rings[166, 167, 168] to achieve an increase in
the efficiency of the functionalization process.
Table 3.19 shows the reaction conditions applied for the functionalization of single walled
compound
reducing agent
solvent
temperature
Dodecyl-SWCNTsa (18)
sodium
ammonia
-70 ◦ C
Dodecyl-SWCNTsb (19)
lithium
ammonia
-70 ◦ C
Dodecyl-SWCNTsc (20)
sodium
amonia/THF (1:2)
-70 ◦ C
Dodecyl-SWCNTsd (21)
sodium
1,2-diaminoethane
RT
Dodecyl-SWCNTse (22)
sodium
1,2-diaminoethane/ morpholine
RT
Dodecyl-SWCNTsf (23)
4x sodium
ammonia
-70 ◦ C
Table 3.19: Variation of the reaction conditions for the synthesis of dodecyl-SWCNTs.
carbon nanotubes. To remove water, all solvents were distilled over sodium prior to use.
112
3.6 Reductive Alkylation of Carbon Nanotubes
In contrast to the reaction for the synthesis of 18, 19, and 23, in the reactions for the synthesis
of 20, 21, and 22 the addition of sodium did not lead to a dark blue color of the solution associated with the solvated electrons. The majority of sodium remained undissolved in the solvent
as well as the carbon nanotubes yielded from these experiments remained insoluble during the
reaction process. The insolubility of the SWCNTs might be explained by the lack of charge
transfer on the nanotubes due to the insolubility of the sodium under the reaction conditions.
The observation that the addition of sodium to a mixture of liquid ammonia and THF does not
result in the complete dissolution of the sodium leads us to the conclusion that there might be a
critical concentration of ammonia to allow the solvation or uptake of electrons into the solution.
In order to compare the efficiency of the different conditions for the functionalization we
recorded Raman spectra of the synthesized SWCNT derivatives 18-23. The results of these
measurements are shown in figure 3.65.
HiPCO SWCNTs
a
18)
Dodecyl-SWCNTs (19)
c
Dodecyl-SWCNTs (20)
d
Dodecyl-SWCNTs (21)
e
Dodecyl-SWCNTs (22)
f
Dodecyl-SWCNTs (23)
Dodecyl-SWCNTs
(
Intensity (a. u.)
b
200
300
1100
1200
R
1300
1400
1500
1600
1700
1
-
aman Shift (cm )
Figure 3.65: Raman spectra (λex =514.5 nm) of HiPCO-SWCNTs,
dodecyl-SWCNTsa (18),
dodecyl-SWCNTsb (19), dodecyl-SWCNTsc (20), dodecyl-SWCNTsd (21), dodecylSWCNTse (22), and dodecyl-SWCNTsf (23.)
The Raman spectra confirmed the observations made during the reaction process. The samples 18, 19, and 23 show a significant increase of the D-band compared to the G-band, whereas,
113
3 Results
the samples 20, 21, and 22 show no significant change in the intensity of the D-band. This
testifies that the approaches for the synthesis of 20, 21, and 22 are not suitable for an efficient
functionalization of single walled carbon nanotubes. A closer look at the G-band reveals an upshift of about 5 cm−1 for the samples 20, 21 and, 22 and an even weaker upshift of about 2 cm−1
for the samples 18, 19, and 23. The spectra also show that the intensities of the RBM-modes
decrease with the increase of the intensity of the D-band. The spectrum for dodecyl-SWCNTsa
shows the complete loss of all RBM-modes leading to the conclusion that the intense degree
of functionalization leads to the distortion of the electronic structure of the SWCNTs. Furthermore, the Raman spectra reveal a higher efficiency for sodium compared to lithium as reducing agent. This supports the results shown for the functionalization with 1,8-diiodooctan
(figure 3.60). The higher efficiency of sodium compared to lithium was also proved for the reductive alkylation of aromatic rings. Lindow et al.[169] gave also an explanation for the lower
degree of functionalization of 23, where we used a higher amount of sodium compared to the
other experiments. They claimed that in a side reaction the excess of sodium reacts with the
alkyl halide:
RX + N a + N H3 → RH + N aN H2 + N aX.
This also demonstrates that the control over the amount of sodium added to the reaction is crucial for a sufficient functionalization process. As a consequence, we added the sodium in small
portions during the reduction of the SWCNTs to ensure that only a slight excess of sodium is
present in the reaction mixture indicated by the blue color of the solution.
In order to find further evidence for the distortion of the electronic structure of the SWCNTs due
to the functionalization process, we carried out UV/Vis measurements. The collected UV/Vis
spectra confirmed the data obtained from the interpretation of the RBM-modes in the Raman
spectra (figure 3.65). The decrease of the intensity of the RBM-modes is accompanied by a
decrease of the intensity of the bands in the UV/Vis spectrum (figure 3.66). The absorption
maxima in the UV/Vis spectrum originate from the transition of electrons between the van
Hove singularity in the valence band to the corresponding van Hove singularity in the conduc1
). The absence of these features in sample 18 shows that the
tion band (Vs2 → Cs2 and Vm1 → Cm
functionalization gave rise to a dramatic distortion of the electronic structure of the SWCNTs.
The thermogravimetric analysis of the synthesized dodecyl-SWCNTs indicate the decomposi-
114
3.6 Reductive Alkylation of Carbon Nanotubes
W
W
HiPCO S
CNTs
CNTs
a
(
18)
Absorbance (a. u.)
Dodecyl-S
200
300
400
W
500
600
700
800
900
avelength (nm)
Figure 3.66: UV/Vis spectra (D2 O, LDS) of dodecyl-SWCNTsa (18) and pristine SWCNTs.
Figure 3.67: Thermogravimetric analysis of dodecyl-SWCNTsa (18).
tion of the samples at elevated temperatures. Figure 3.67 shows exemplarily the thermogravimetric analysis of 18. The spectrum shows that the total weight loss is divided into two steps.
We attribute the first step to remaining solvent. The second step shows a dramatic weight loss
of 27.24 wt.% beginning at about 170 ◦ C. This can be attributed to the decomposition of the
compound followed by the evaporation of the addend at this temperature.
To study the influence of heating at 600 ◦ C on the carbon nanotubes, we recorded Raman spectra
115
3 Results
of the annealed SWCNT-derivatives. Figure 3.68 shows the Raman spectra of the functionalized
SWCNTs in comparison with the annealed and the starting material. The spectra gave evidence
for the defunctionalization of the SWCNT derivative under elaborated temperatures. Furthermore, the significantly decreased D-band indicates that the decomposition led to the healing or
regraphitization of the SWCNTs. The intensity of the D-band is even lower in the annealed
material compared to the starting material, indicating that the defects present in the starting material were removed during the functionalization or the annealing process. A closer look at the
RBM-modes shows the reappearance of the intensity in the annealed material. The intensity of
the RBM-modes nearly reaches the intensity of the starting material proving the reversibility of
the functionalization process.
The TEM images of these materials also revealed dramatic changes in the morphology of the
functionalized material compared to the starting material. Images at lower resolution (figure
3.69) showed a decrease of the bundle diameter in the functionalized material. Furthermore,
an increase of the purity of the material was detected by the absence of catalyst particles in
HiPCO SWCNTs
Dodecyl-SWCNTs
18
(
18)
Intensity (a. u.)
annealed
a
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 3.68: Raman spectra (λex =514.5 nm) of dodecyl-SWCNTsa (18), annealed dodecyl-SWCNTsa ,
and pristine SWCNTs.
116
3.6 Reductive Alkylation of Carbon Nanotubes
Figure 3.69: TEM images of SWCNT starting material (left) and dodecyl-SWCNTsa (18) (right).
Figure 3.70: TEM images with a higher resolution of SWCNT starting material (left) and dodecylSWCNTsa (18) (right).
the functionalized material in contrast to the starting material. A more detailed look at the
SWCNTs at higher resolution (figure 3.70) displayed the presence of amorphous coating around
the SWCNTs, rendering the resolution of the nanotube sidewalls impossible. We attribute this
coating to the functional groups on the sidewalls of the carbon nanotubes.
The XPS survey spectra of the functionalized materials studied here did not give any indications for the functionalization due to the absence of heteroatoms in the addend attached to the
117
3 Results
Figure 3.71: XPS survey spectrum of Dodecyl-SWCNTsa (18).
SWCNT. The spectrum (figure 3.71) shows only the presence of carbon. The oxygen present
in the sample can be attributed to adsorbates on the surface and is therefore not related to the
sample.
118
3.6 Reductive Alkylation of Carbon Nanotubes
The C 1s loss spectrum (figure 3.72) of the functionalized material confirms the distortion
of the electronic structure of the SWCNTs via the decrease in the intensity of the π-plasmon
located at about 290 eV. This effect arises from a decreased π-electron-density on the SWCNTs
Count Rate (arb. units)
HiPCO SWCNTs
functionalized SWCNTs
320
310
300
290
280
Binding Energy (eV)
Figure 3.72: XPS C 1s electron loss spectra of the starting material and of Dodecyl-SWCNTsa (18).
which goes along with the rehybridization from sp2 to sp3 of the carbon atoms in the carbon
nanotubes sidewall which bear an addend.
In summary, it can be ascertained that the reductive alkylation method is a powerful tool for the
functionalization of single walled carbon nanotubes. The variation of the reaction conditions
showed that the alkylation with liquid ammonia as solvent and sodium as reducing agent gave
the most promising results.
During the course of our studies Liang et al.[120] reported on the reductive alkylation of
SWCNTs in liquid ammonia with lithium as reducing agent. The degrees of functionalization reported by Liang are in agreement with our results, unfortunately the comparison of the
Raman data is hindered due to the different excitation wavelengths used.
119
3 Results
3.6.2 Functionalization of SWCNTs with a Variety of Functional
Groups
To study the variability of the reductive alkylation we decided to synthesize SWCNT derivatives
with a variety of functional groups on the sidewall.
3.6.2.1 Alkylation of SWCNTs
We investigated the influence of different addends on the properties of the SWCNTs and the
functionalization process by the use of a variety alkyl halides.
For the synthesis of the alkyl-SWCNTs shown in figure 3.73 we applied the procedure
we established for the synthesis of dodecyl-SWCNTsa . To prove the advantage of sodium
(a) Dodecyl-SWCNTs
(b) Octyl-SWCNTs
(c) t-Butyl-SWCNTs
Figure 3.73: Schematic representation of the variety of alkylated SWCNTs synthesized.
compared with lithium, we synthesized the octyl-SWCNTs using both reducing agents giving
octyl-SWCNTsa (Na) and octyl-SWCNTsb (Li).
120
3.6 Reductive Alkylation of Carbon Nanotubes
The comparison of the intensity of the D-band in the Raman spectrum of octyl-SWCNTsa
and octyl-SWCNTsb (figure 3.74) confirmed the advantage of sodium compared to the use of
lithium as reducing agent. Furthermore, the comparison of the D-band intensities revealed a
Dodecyl-SWCNTs
a
a
(
18)
24)
Octyl-SWCNTs (25)
t-Butyl-SWCNTs (26)
Octyl-SWCNTs
(
Intensity (a. u.)
b
200
1100
1200
1300
R
1400
1500
1600
1700
1800
1
-
aman Shift (cm )
Figure 3.74: Raman spectra (λex =514.5 nm) of dodecyl-SWCNTsa (18), octyl-SWCNTsa (24), octylSWCNTsb (25), and t-butyl-SWCNTs (26).
higher degree of functionalization for the dodecyl-SWCNTsa compared to the octyl-SWCNTs
and the t-Butyl-SWCNTs.
This effect is also confirmed by the lower intensities of the
RBM-modes in the Raman spectrum of dodecyl-SWCNTsa .
To estimate of the degree of functionalization, we carried out TGA measurements to calculate
Df unct.(T GA) from the measured weight loss due to the decomposition of the compounds. By
applying equation 3.2 for the functional groups attached, we calculated the values shown
in table 3.20 for the four different synthesized SWCNT derivatives. The comparison of
Df unct.(T GA) and ID /IG shows a good correlation between the two indicators for the degree of
functionalization. However, Df unct.(T GA) for the dodecyl-SWCNTsa is lower than expected
from the ID /IG ratio. This might indicate that the dependence of Df unct.(T GA) and ID /IG is not
linear.
All the alkylated SWCNT derivatives show an increased solubility in common organic solvents
due to the functionalization. The increased solubility of the SWCNTs arises from the reduction
121
3 Results
compound
ID /IG
Df unct.(T GA) [%]
Dodecyl-SWCNTsa (18)
0.358
5.2
Octyl-SWCNTsa (24)
0.087
4.2
Octyl-SWCNTsb (25)
0.072
4.0
t-Butyl-SWCNTs (26)
0.083
4.2
Table 3.20: Estimated degree of functionalization of the alkyl-SWCNTs.
of the π-π-stacking interactions between the nanotubes caused by the presence of the functional
groups. This effect plays also a prominent role for the AFM sample preparation due to the
hindrance of bundling during the evaporation of the solvent. Figure 3.75 shows a 5x5 µm AFM
image of t-Butyl-SWCNTs. The image displays a carpet of very thin bundles and individual
Figure 3.75: 3D visualization of a 5x5 µm AFM image of t-butyl-SWCNTs (26).
functionalized SWCNTs deposited on a silicon wafer. Furthermore, the image proves the high
purity of the samples obtained after the functionalization process.
High resolution TEM images proved the distortion of the π-π-stacking interactions between
the SWCNTs due to the presence of the addend molecules. Figure 3.76 shows that the three
122
3.6 Reductive Alkylation of Carbon Nanotubes
individual SWCNTs lying next to each other are separated by a gap introduced by the steric
demand of the addends. The measured gap between the SWCNTs is about 1 nm and therefore
Figure 3.76: TEM image of octyl-SWCNTsa (24), the black bars represent the width of the three individual SWCNTs.
exceeds the distance of unfunctionalized SWCNTs three times.
Further studies on the morphology of the alkylated SWCNTs with TEM confirmed the findings
of the AFM images. The TG-analysis combined with the results from the Raman measurements
proved the reversibility of the reaction. The UV/Vis and XPS-measurements were in complete
agreement with our expectations and the previous experiments. These additional data are
summarized in section 5.3.
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3 Results
3.6.2.2 Ether-SWCNTs
Previous studies showed that the presence of heteroatoms within the addend attached on the
single walled carbon nanotubes have a dramatic effect on their solubility in common organic
solvents. Therefore we applied the functionalization method with alkyl halides bearing ether
functionalities targeting the SWCNT derivatives, shown in figure 3.77.
In order to study the efficiency of the functionalization process for these systems, we recorded
O
O
O
O
O
O
(b) 2-(Methoxy(a) 3-(Tetrahydropyran-2-
methyl)-SWCNTs (28)
yloxy)propyl-SWCNTs
(27)
O
O
O
O
O
O
n
O
O
(c) 2-(2-(2-Methoxyethoxy)ethyl)-SWCNTs
(d) Poly(ethylene
(29)
glycol)-SWCNTs (30)
n
Figure 3.77: Schematic representation of the variety of ether-SWCNTs synthesized.
124
3.6 Reductive Alkylation of Carbon Nanotubes
Raman spectra of the functionalized SWCNTs. The Raman spectra (figure 3.78) showed a
significant increase of the D-band intensity compared to the G-band. However, the derivatives
HiPCO SWCNTs
3-(Tetrahydrofuran-2-yloxy)propyl-SWCNTs (27)
2-(Methoxy- methyl)-SWCNTs (
28)
Intensity (a. u.)
2-(2-(2-Methoxyethoxy)ethyl)-SWCNTs (
Poly(ethyleneglycol)-SWCNTs (
200
900
30)
29)
1000 1100 1200 1300 1400 1500 1600 1700
R
1
-
aman Shift (cm )
Figure 3.78: Raman spectra (λex =514.5 nm) of 3-(tetrahydropyran-2-yloxy)propyl-SWCNTs (27),
2-(methoxymethyl)-SWCNTs
(28),
2-(2-(2-methoxyethoxy)ethyl)-SWCNTs
(29),
poly(ethylene glycol)-SWCNTs (30), and SWCNT starting material.
28 and 29 show a smaller increase of the D-band intensity. We attribute this to the lower purity
of the reagents. The bromo(methoxy)methane and 1-bromo-2-(2-methoxyethoxy)ethane used
for the synthesis were only available with a purity of 90% and 95%, respectively. The present
impurities might have caused side reactions under the applied conditions and therefore lowered
the concentration of the halide for the functionalization. The lower degree of functionalization
of 30 might be due to steric effects caused by the large polyethyleneglycol-chain attached to
the halide. The RBM-region shows a minor decrease for the modes present in 28 and 29. In
contrast to the D- and G-band which are upshifted, the RBM-modes remain at the position of
the starting material. The complete spectrum of 30 is downshifted by 2 cm−1 . The spectra
of the 3-(tetrahydropyran-2-yloxy)propyl-SWCNTs display the largest increase of the D-band
intensity accompanied by a complete loss of the intensity of the RBM-modes due to the
distortion of the electronic structure of the SWCNTs caused by the sidewall functionalization.
125
3 Results
The TEM image of 27 (figure 3.79) gave further evidence for the tremendous effect of this
high degree of functionalization on the morphology of the SWCNTs. The SWCNT bundle
Figure 3.79: TEM
images
of
3-(tetrahydropyran-2-yloxy)propyl-SWCNTs
(27)
(left)
and
poly(ethyleneglycol)-SWCNTs (30) (right).
displayed in the image shows a completely amorphous surface.
The individual carbon
nanotubes present in the image prove the increased solubility of the functionalized carbon
nanotubes. Unfortunately, the TEM images showed the presence of bundles due to the sample
preparation including the evaporation of the solvent. The slow evaporation of the solvent allows
the agglomeration of the dissolved SWCNTs and therefore a rebundling of the SWCNTs.
In contrast to this, the TEM images of the poly(ethyleneglycol)-SWCNTs are only partially
covered with an amorphous coating. However, large amorphous particles attached to the
SWCNTs are visible. These particles arise from the large polymer molecules (M≈20000
g/mol) attached to the nanotubes.
The improved solubility influences also the AFM images. Figure 3.80 shows a 10x10 µm
AFM image of 2-(methoxymethyl)-SWCNTs. The image displays plenty of thin bundles and
individual functionalized SWCNTs deposited on a silicon wafer. The image gives evidence
that the functionalization process did not lead to the shortening of the SWCNTs. Furthermore,
the image proves the high purity of the samples obtained after the functionalization process.
For the further characterization of the functionalized SWCNTs we recorded XPS spectra of
all synthesized SWCNT derivatives. The survey spectrum of 2-(methoxymethyl)-SWCNTs
(28) (figure 3.81) displays the presence of traces of sodium and nitrogen remained from the
126
3.6 Reductive Alkylation of Carbon Nanotubes
Figure 3.80: 10x10 µm AFM image of 2-(methoxymethyl)-SWCNTs (28).
functionalization process. The iron might be due to carbon cleared catalyst particles. More
800
600
400
Na 2s
N 1s
O 1s
1000
Na a
Fe 2p
Na 1s
O a
Cou
nt Rate (arb.
u
nits)
C 1s
200
0
Binding Energy (eV)
Figure 3.81: XPS survey spectrum of 2-(methoxymethyl)-SWCNTs (28).
importantly, we detected a significant increase in the amount of oxygen present in the sample
due to the ether functionalities in the addends.
Further evidence for the successful functionalization can be obtained by a closer look at the
C 1s core level (figure 3.82). The core level spectrum displays a major component attributed
127
Count Rate (arb. units)
3 Results
292
290
288
286
284
282
280
Binding Energy (eV)
Figure 3.82: XPS C 1s spectrum of 2-(methoxymethyl)-SWCNTs (28).
to carbon atoms exclusively bound to other carbon atoms. In addition, the spectrum gives rise
to an upcoming shoulder shifted to higher binding energy. This shoulder can be attributed
to a carbon atom covalently connected with an oxygen atom and therefore, to carbon atoms
introduced by the functionalization.
Count Rate (arb. units)
The O 1s core level (figure 3.83) of the 2-(methoxymethyl)-SWCNTs is also in accordance
540
535
530
525
Binding Energy (eV)
Figure 3.83: XPS O 1s spectrum of 2-(methoxymethyl)-SWCNTs (28).
with the predictions for this system. The spectrum shows the presence of only one oxygen
species at a binding energy related to oxygen within ethers.
Table 3.21 shows the calculated degrees of functionalization for the variety of SWCNT
derivatives synthesized here. The ID /IG -ratio is in good agreement with the values calculated
128
3.6 Reductive Alkylation of Carbon Nanotubes
compound
ID /IG
Df unct.(XP S) [%]
Df unct.(T GA) [%]
3-(Tetrahydropyran-2-yloxy)
0.516
7.3
5.3
0.076
5.1
3.3
0.093
4.1
2.6
0.086
0.1
0.2
propyl-SWCNTs (27)
2-(Methoxymethyl)SWCNTs (28)
2-(2-(2-Methoxyethoxy)
ethyl)-SWCNTs (29)
Poly(ethyleneglycol)
-SWCNTs (30)
Table 3.21: Degrees of functionalization achieved for the variety of SWCNT derivatives.
from the XPS- and TGA- experiments. However, the results from XPS-measurements gave a
slightly higher degree of functionalization for 27, 28, and 29. The calculated values are in good
agreement with the increase of the D-band intensity.
3.6.2.3 Nitrile-SWCNTs
The SWCNT derivatives synthesized so far all revealed increased solubility in organic solvents, but none of the compounds synthesized was soluble in water. Nevertheless, the solubility
in water is of great interest for some biological applications. Therefore, the attachment of a
hydrophilic moiety to the SWCNTs surface should overcome the hydrophobic nature of the
SWCNTs and enhance the solubility in water. To achieve this, we investigated the functionalization of SWCNTs with carboxylic groups. Unfortunately, the reaction conditions applied for
the functionalization of the SWCNTs do not allow the direct introduction of carboxylic groups.
Therefore, we decided to synthesize SWCNTs bearing a nitrile group and subsequently convert
the nitrile group into a carboxylic group.
To achieve this goal, 2-(bromomethyl)benzonitrile was reacted with SWCNTs to afford (2cyanobenzyl)-SWCNTs (31) (figure 3.84). Applying the reaction to this functionalization, product 31 was obtained as a black solid with dramatically increased solubility in organic solvents.
The increased solubility of the functionalized SWCNTs indicates a high degree of functiona-
129
3 Results
NC
NC
Figure 3.84: Schematic representation of (2-cyanobenzyl)-SWCNTs (31).
lization, followed by the separation of the bundles into individual SWCNTs. Despite of this,
the AFM images (figure 3.85) taken from this product show only the presence of thick strands.
Furthermore, the image revealed that the diameter of the strands varies along the nanotube axis.
Figure 3.85: AFM image of (2-cyanobenzyl)-SWCNTs (31).
130
3.6 Reductive Alkylation of Carbon Nanotubes
The red arrow shown in the image points at a strand which arises probably from an individual
SWCNT (diameter ≈ 1.1 nm). Following the nanotube to the blue arrow, the diameter considerably increases to ≈ 5 nm. This large increase of the diameter can not be explained by
the addition of 2-cyanobenzyl-groups on the SWCNTs surface, but by the occurrence of a side
reaction leading to the formation of a polymer coating around the SWCNTs.
Due to the high amount of functional groups within the sample we were able to record mass
spectra of the derivatized SWCNTs. The mass spectrum of the functionalized SWCNTs (figure 3.86) gave further evidence on the occurrence of a side reaction, leading to the polymerization of 2-(bromomethyl)benzonitrile. The spectrum gives rise to a variety of fragments
Figure 3.86: Mass spectrum of (2-cyanobenzyl)-SWCNTs (31).
(MS (FAB, NBA): [m/z] = 232, 347, 462, 578, 693 and 808). These fragments can be attributed
to oligomers with the empirical formula (C8 H5 N)n H (n = 2-7). This empirical formula leads to
two possible oligomers which might have been formed due to the polymerization (figure 3.87).
The attempt to distinguish which of the oligomers was formed during the reaction failed. The
*
N
CN
*
*
*
n
I
n
II
Figure 3.87: Schematic representation of the oligomers formed during the synthesis of (31).
131
3 Results
recorded IR-spectra did not show any bands which can be attributed to the cyano-functionality,
as due to the black-body behavior of the SWCNTs which gives rise to a strong continuous
absorption especially in the region of low energy excitation, the spectra did not show any significant absorption bands. Furthermore, the recorded XPS N 1s core level spectrum reveals the
presence of two nitrogen species. This leads us to the conclusion that both products were formed
during the reaction process.
In order to distinguish which of the two oligomers was formed during the functionalization process, we used 4-bromobutanenitrile for the functionalization to yield (3-cyanopropyl)SWCNTs (32) (figure 3.88). The 4-bromobutanenitrile bears no aromatic ring and would there-
CN
CN
Figure 3.88: Schmatic representation of (3-cyanopropyl)-SWCNTs (32).
fore not undergo a reductive alkylation reaction to form a polymer type II (figure 3.87), whereas
the formation of the type I polymer is still possible.
Studies of the (3-cyanopropyl)-SWCNTs via AFM and TEM (figure 3.89) did not show any
hints for the occurrence of polymerization along the nanotubes. The AFM image displays the
high purity of the functionalized SWCNTs. Furthermore, no coating surrounding the SWCNTs
was found. This was confirmed by the TEM images which showed individual SWCNTs and
small bundles. A closer look at the TEM image reveals that the sidewalls of the SWCNTs are
covered by some amorphous parts due to the functionalization. The SWCNTs within the bundles are separated by the addends on the SWCNTs sidewalls as shown earlier.
The XPS spectra of both compounds synthesized show the presence of nitrogen in the sample which is attributed to the cyano functionality within the addend. The XPS N 1s core level
spectra (figure 3.90) shows only the presence of one nitrogen species for the (3-cyano-propyl)-
132
3.6 Reductive Alkylation of Carbon Nanotubes
Figure 3.89: AFM and TEM image of (3-cyanopropyl)-SWCNTs (32).
(2-Cyanobenzyl)-SWCNTs
3
Count Rate (arb. units)
( -Cyanopropyl)-SWCNTs
410
405
400
395
Binding Energy (eV)
Figure 3.90: XPS N 1s core level spectra of (2-cyanobenzyl)-SWCNTs (31) and (3-cyano-propyl)SWCNTs (32).
SWCNTs. This gives further evidence that no polymerization occurred during the functionalization process. The position of the N 1s peak indicates that the nitrogen species present in the
sample can be attributed to the nitrogen atom of nitrile groups.
The Raman spectra (figure 3.91) showed an increase of the D-band intensity for both compounds
due to functionalization. Furthermore, a decrease of the RBM-mode intensity was detected for
the (3-cyanopropyl)-SWCNTs indicating the distortion of the electronic structure due to functionalization.
133
3 Results
HiPCO SWCNTs
31)
3-Cyanopropyl)-SWCNTs (32)
(2-Cyanobenzyl)-SWCNTs (
Intensity (a. u.)
(
200
1000 1100 1200 1300 1400 1500 1600 1700 1800
R
1
-
aman Shift (cm )
Figure 3.91: Raman spectra (λex =514.5 nm) of (2-cyanobenzyl)-SWCNTs (31) and (3-cyanopropyl)SWCNTs (32).
Both compounds exhibit an increased solubility in organic solvents, most notably the (2cyanobenzyl)-SWCNTs that revealed a dramatically enhanced solubility. We attribute this behavior to the polymer coating on the SWCNTs, hindering the rebundling of the SWCNTs.
134
3.6 Reductive Alkylation of Carbon Nanotubes
3.6.3 Functionalization of Multi Walled Carbon Nanotubes
The covalent sidewall functionalization of MWCNTs is more difficult than the functionalization
of single walled carbon nanotubes due to the larger diameter of the nanotubes. This increase
in diameter of the outer shell leads to a decrease of the reactivity of the nanotubes towards an
attack on the sidewalls. However, the degrees of functionalization achieved for the reductive
alkylation of SWCNTs gave rise to the prediction that a respectable degree of functionalization
might be achieved for the functionalization of multiwalled carbon nanotubes, too. We decided to
apply the method to the two different types of MWCNTs namely A MWCNTs and D MWCNTs
that were studied previously (section 3.4).
Due to the advantages of the oxygen atoms within the addends in terms of solubility and estimation of the degree of functionalization, we used 1-bromo-2-(2-methoxyethoxy)ethane as reagent
to yield 2-(2-(2-methoxyethoxy)ethyl)-A MWCNTs (33), and 2-(2-(2-methoxyethoxy)ethyl)D
MWCNTs (34) (figure 3.92). Because of the earlier experiments performed on the func-
O
O
O
O
Figure 3.92: Schematic representation of 2-(2-(2-methoxyethoxy)ethyl)-A MWCNTs (33), and 2-(2-(2methoxyethoxy)ethyl)-D MWCNTs (34).
tionalization of MWCNTs, we did not expect a significant change in the Raman spectra of the
functionalized samples. However, the recorded spectra revealed the complete opposite. Figure
3.93 displays the Raman spectrum of the A MWCNT starting material revealing the distortion of
the MWCNTs sidewalls because of the high intensity of the D-band compared to the G-band.
The comparison of the starting material with 33 demonstrates that the functionalization leads to
a further increase of the D-band intensity. The Raman spectra of the annealed material demonstrate the reversibility of the functionalization by a subsequent decrease of the D-band reaching
135
3 Results
A
MWCNTs
funct.
A
MWCNTs (
33
33)
Intensity (a. u.)
annealed
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 3.93: Raman spectra (λex =514.5 nm) of 2-(2-(2-methoxyethoxy)ethyl)-A MWCNTs (33),
A MWCNT
starting material, and the annealed material.
B
MWCNTs
funct.
B
MWCNTs (
34
34)
Intensity (a. u.)
annealed
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 3.94: Raman spectra (λex =514.5 nm) of 2-(2-(2-methoxyethoxy)ethyl)-D MWCNTs (34),
D MWCNT
136
starting material, and the annealed material.
3.6 Reductive Alkylation of Carbon Nanotubes
the intensity of the starting material.
Figure 3.94 shows the Raman spectra of the D MWCNT starting material. The lower intensity
of the D-band in the spectrum of the D MWCNT starting material compared to the A MWCNT
starting material demonstrates the superior graphitization of the D MWCNTs. In contrast to our
findings from the functionalization of D MWCNTs with organo lithium compounds the method
applied here seems to yield D MWCNTs with a high degree of functionalization. The increase
of the D-band observed here equals the results for the SWCNTs.
The TEM image of 33 (figure 3.95) shows no significant changes. The purity of the sample
Figure 3.95: TEM images of
A MWCNTs
A MWCNT
starting material (left) and 2-(2-(2-methoxyethoxy) ethyl)-
(33) (right).
remains the same due to the high purity of the starting material. The MWCNTs are all individual as MWCNTs have no tendency to conglomerate to bundles. TEM images recorded with
a higher resolution (figure 3.96) revealed a change in the morphology of the MWCNTs due to
functionalization. Whereas the starting material still shows the disturbed MWCNT sidewalls,
the functionalized MWCNTs do not allow the resolution of the sidewalls due to the presence
of the addends on the sidewalls and the additional disturbance introduced by the covalent functionalization. The TEM images of the D MWCNTs (figure 3.97) display a significant increase
of the purity in the functionalized sample due to the functionalization process. The majority
of amorphous material present in the untreated sample was removed, leaving the functionalized
137
3 Results
Figure 3.96: TEM images of
A MWCNTs
starting material (left) and 2-(2-(2-methoxyethoxy) ethyl)-
(33) (right).
Figure 3.97: TEM images of
D MWCNTs
A MWCNT
D MWCNT
starting material (left) and 2-(2-(2-methoxyethoxy) ethyl)-
(34) (right).
MWCNTs with a high purity. TEM images of 34 (figure 3.98) recorded with a higher resolution
revealed a change of the MWCNTs structure. The sidewalls shown in the starting material with
a fairly good resolution are completely coated with amorphous material in the functionalized
MWCNTs. This shows the achievement of a high degree of functionalization within this sam-
138
3.6 Reductive Alkylation of Carbon Nanotubes
Figure 3.98: TEM images of
D MWCNTs
D MWCNT
starting material (left) and 2-(2-(2-methoxyethoxy) ethyl)-
(34) (right).
ple.
The XPS spectra (figure 3.99) of the functionalized MWCNTs revealed a significant increase
A
MW
C 1s
CNTs
2-(2-(2-Methoxyethoxy)-
A
CNTs
C
O 1s
ount Rate (arb. units)
ethyl)- MW
1000
800
600
400
200
0
Binding Energy (eV)
Figure 3.99: XPS survey spectra of A MWCNTs and 2-(2-(2-methoxyethoxy)ethyl)-A MWCNTs (33).
of the amount of oxygen present in functionalized MWCNT samples due to the oxygen in the
addend, compared to the starting material.
139
3 Results
Table 3.22 summarizes the changes in the carbon to oxygen ratio of the functionalized
MWCNTs in comparison to the starting materials. Contributions due to SiO2 originating from
the sonication in glassware were subtracted from the oxygen content.
Figure 3.100 shows the C 1s core level of 2-(2-(2-methoxyethoxy)ethyl)-A MWCNTs and the
compound
C [at.%]
O [at.%]
98.6
1.4
2-(2-(2-Methoxyethoxy)ethyl)-A MWCNTs (33)
97.2
2.8
D
98.4
1.6
96.5
3.5
A
MWCNTs
MWCNTs
2-(2-(2-Methoxyethoxy)ethyl)-D MWCNTs (34)
Table 3.22: Carbon to oxygen ratio of the samples.
starting material. The spectra show the broadening of the signal due to the introduction of
the CHx carbon atoms of the addend. The carbon atoms of the addend bond to an oxygen
atom (-CH2 -O-) give rise to the difference in intensity at a binding energy of 286 eV. By ap-
C 1s
A
MWCNTs
2-(2-(2-Methoxyethoxy)-
s
A
Count Rate (arb. units)
ethyl)- MWCNT
292
290
288
286
284
282
280
Binding Energy (eV)
Figure 3.100: XPS C 1s core level spectra of
A MWCNTs
A MWCNTs
and 2-(2-(2-methoxyethoxy)ethyl)-
(33).
plying the equations 3.1 and 3.2 to the acquired data we were able to calculate Df unct.(XP S)
and Df unct.(T GA) . The values given here were calculated for SWCNTs since the ratio of carbon
140
3.7 Further Reactions on Functionalized SWCNTs
atoms of the outermost shell to the carbon atoms of the inner shells is unknown. Due to the large
difference within the D-band intensity for the two starting materials we compared the changes
of the values of ID /IG for the two samples:
∆ID /IG = (ID /IG )f unct.M W CN T s - (ID /IG )startingmaterial .
compound
2-(2-(2-Methoxyethoxy)ethyl)A
∆ID /IG
Df unct.(XP S)
Df unct.(T GA)
(SWCNT) [%]
(SWCNT) [%]
0.404
0.7
1.0
0.776
1.0
1.1
MWCNTs (33)
2-(2-(2-Methoxyethoxy)ethyl)D
MWCNTs (34)
Table 3.23: Calculated degrees of functionalization for 33 and 34
For the calculations, all carbon atoms were considered to be part of the outermost shell and
thus, the achieved values for Df unct.(XP S) and Df unct.(T GA) are far to low. Estimating that only
one fourth of the carbon atoms are part of the outermost shell and therefore accessible for the
functionalization, the degree of functionalization will rise to about 4%. The higher increase
of ID /IG for the 2-(2-(2-methoxyethoxy)ethyl)-D MWCNTs might be explained in terms of
a more efficient electron uptake due to the less disturbed electronic structure of these nanotubes.
3.7 Further Reactions on Functionalized SWCNTs
Since the drastic reaction conditions applied for the reductive alkylation of the SWCNTs impede the introduction of some functionalities or more elaborated addend systems, we decided
to utilize the ether functionalities for those purposes. We have chosen the ether-SWCNTs due
to the readily accessible hydroxy functionality which offers the possibility of further chemical
transformations.
141
3 Results
3.7.1 Ether Cleavage
In order to cleave the ether moieties we had to apply methods leading to their complete conversion to hydroxy functionalities since, the separation of a reaction mixture is impossible
because of the presence of multiple addends on a single SWCNT. For the conversion of 27
O
O
OH
HCl, THF
O
OH
O
27
35
OH
O
1. BBr3, benzene
2. H2O
O
OH
28
36
Scheme 3.9: Two methods for the cleavage of the ether functionality on the SWCNTs to yield hydroxySWCNTs.
into 35, 3-(tetrahydropyran-2-yloxy)propyl-SWCNTs were dissolved in a mixture of THF and
concentrated hydrochloric acid to cleave the THP-ether. For the conversion of 28 into 36 2(methoxymethyl)-SWCNTs were dissolved in anhydrous benzene followed by the dropwise
addition of boron tribromide. In both cases, the reaction mixture was purified and dried to yield
the products as black solids. Both compounds exhibit a decreased solubility due to the reduced
steric demand of the smaller addends.
In order to prove that the applied reaction conditions did no facilitate the defunctionalization of
the SWCNTs we recorded Raman spectra of the reaction products (figure 3.101). The Raman
spectra of the products show a decrease in the D-band intensity compared to the corresponding
142
3.7 Further Reactions on Functionalized SWCNTs
27)
3-(Tetrahydrofuran-2-yloxy)propyl-SWCNTs (
35)
28)
Hydroxymethyl-SWCNTs (36)
3-Hydroxypropyl-SWCNTs (
Intensity (a. u.)
2-(Methoxymethyl)-SWCNTs (
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 3.101: Raman spectra (λex =514.5 nm) of the ether cleaved compounds and the starting materials.
starting materials thus, indicating the removal of addends from the SWCNTs during the reaction. This result is confirmed by the calculated Df unct. (table 3.24) which showed a decrease in
the amount of addends for 35 and 36.
compound
ID /IG
Df unct.(XP S)
(SWCNT)
[at.%]
3-(Tetrahydropyran-2-yloxy)propyl-
0.669
7.3
3-Hydroxypropyl-SWCNTs (35)
0.249
5.0
2-(Methoxymethyl)-SWCNTs (28)
0.076
5.1
Hydroxymethyl-SWCNTs (36)
0.032
3.0
SWCNTs (27)
Table 3.24: Calculated degrees of functionalization for 27, 35, 28, and 36.
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3 Results
3.7.2 Esterification of Hydroxy-SWCNTs
The synthesized SWCNT derivatives bearing hydroxy functionalities were subjected to an esterification reaction for further derivatization of the SWCNTs. In order to obtain 37 and 38
(figure 3.102) 3-hydroxypropyl-SWCNTs (35) were reacted with the desired acyl chlorides.
The XPS spectra showed the presence of sulfur in the survey spectrum of 37 and a significant
O
S
O
O
O
O
C3H6 O
O
O
C3H6
O
O
S
(a) 3-(Thiophene-2carboxylate)propyl-SWCNTs
(37)
(b) Fullerene-SWCNTs (38)
Figure 3.102: Schematic representation of the synthesized compounds.
increase of the amount of oxygen atoms in the survey spectrum of 38. These changes in the
spectra can be attributed to the presence of the introduced functionalities. The recorded Raman spectra showed no changes in the D-band intensity of the functionalized SWCNTs compared to the 3-hydroxypropyl-SWCNTs. However, the Raman spectrum of 3-(thiophene-2carboxylate)propyl-SWCNTs showed a new signal at 1419 cm−1 due to the introduction of the
thiophene-group (figure 3.103). Recorded TEM images of the fullerene-SWCNTs allowed the
direct proof of the presence of fullerene molecules on the sidewalls of the SWCNTs. Figure
3.104 shows a comparison of the starting material and the fullerene-SWCNTs demonstrating
the "grape-like" shape of the new compound.
These two novel compounds may show interesting electron transfer properties and therefore,
attract considerable interest in the future.
144
3.7 Further Reactions on Functionalized SWCNTs
HiPCO SWCNTs
35
37
37
Intensity (a. u.)
annealed
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 3.103: Raman spectra (λex =514.5 nm) of 3-(thiophene-2-carboxylate)propyl-SWCNTs (37), the
annealed 37, 3-hydroxypropyl-SWCNTs (35), and HiPCO SWCNTs.
In conclusion, the new approach presented her, for the further chemical modification of
SWCNTs offers a convenient route to attach a wide range of functionalities on the core of
carbon nanotubes and therefore, might be the was of choice to access functionalized carbon
nanotubes for further applications.
145
3 Results
Figure 3.104: TEM images of 3-hydroxypropyl-SWCNTs (35) (top) and fullerene-SWCNTs (38) (bottom).
146
4 Summary
Carbon nanotubes have an enormous potential due to their outstanding electronic, optical,
and mechanical properties. However, any technological application is still hindered due to
problems regarding the processibility of the pristine carbon nanotubes. In the past few years,
it has been shown that the chemical modification of the carbon nanotubes is an inevitable step
prior to their application.
The first part of this work (chapter 3.1) was focused on the purification of pristine laser
ablation carbon nanotubes. In the course of this work we developed a customized process
to obtain purified carbon nanotubes with a very low amount of oxygenated species on the
nanotube core in order to avoid possible side reactions during the sidewall functionalization.
A solid based process involving the oxidation of the carbon nanotubes in hot air was proved
to be very mild and therefore, and yielded carbon nanotubes with insignificant distortions
as determined by Raman spectroscopy. However, the samples still contained a considerable
amount of amorphous material, due to the heterogenous character of this method. In a second
approach, we attempted the purification with nitric acid in different concentrations and various
reaction times. The purified carbon nanotubes were characterized by Raman spectroscopy,
TEM, and XPS. The characterization of the purified carbon nanotubes revealed a significantly
increased purity even after one hour of treatment in the acidic media. The Raman spectra
proved that this process led to a minor increase of the disorder in the graphitic lattice. However,
the XPS data showed the presence of functional groups such as carboxylic, carbonyl, hydroxyl,
nitroso, and nitro groups. Further experiments showed that the functionalities can be removed
by subsequent annealing of the sample in vacuum to afford the carbon nanotubes in the desired
shape.
In the second part of this work (chapter 3.2), we studied the influence of the production
147
4 Summary
technique of carbon nanotubes on the reactivity towards sidewall functionalization. It was
still an open question whether the production technique influences the reactivity of carbon
nanotubes, but up to now no experimental data was available. We were able to show the deviant
behavior of laser ablation, arc discharge and HiPCO single walled carbon nanotubes towards
chemical sidewall functionalization. In this study we investigated the addition of nitrenes on
the sidewall of carbon nanotubes to achieve comparable data. The functionalization process
leads to a remarkable increase in the purity of the functionalized samples compared to the
starting materials. The analysis of the XPS spectra revealed that the HiPCO SWCNTs are the
most reactive in the studied range. We attribute this observation to the lower diameter of the
HiPCO SWCNTs and to the inferior graphitization of the sidewalls.
Further work focused on the reaction of carbon nanotubes with organo lithium compounds
(chapter 3.3). The reaction is a two step procedure, in the first step the organo lithium compound
attacks the carbon nanotube to afford the charged intermediate R-SWCNT− . The negative
charges on the SWCNTs lead to their dissolution in the solvent due to the repulsive interaction.
In a second step the intermediate is protonated by the addition of diluted hydrochloric acid to
give R-H-SWCNTs. A more detailed study of this reaction showed that the negative charges
delocalize over the whole π-system of the SWCNT and therefore, hinder the further addition of
the organo lithium compound, thus resulting in a limitation of the degree of functionalization.
In order to overcome this limitation we introduced a second functionalization procedure
resulting in a further increase of the number of functionalities on the SWCNTs. STM images
of the functionalized SWCNTs enabled us to visualize for the first time the functional groups
on the SWCNTs. Raman measurements revealed the selective reaction with the metallic carbon
nanotubes and therefore, might offer a way for the separation of metallic and semiconductive
carbon nanotubes.
In a second approach (chapter 3.3.3), we introduced a new way for the further functionalization, for this purpose we treated the charged intermediate with an electrophile to target
R-R’-SWCNT derivatives. Furthermore, we utilized the charged intermediate to initiate an
anionic polymerization. TEM experiments showed the presence of a polymer coating on
the SWCNTs. The polymer covered SWCNTs showed significantly increased solubility in
common organic solvents like, CHCl3 , MeOH, EtOH, DCE and CH2 Cl2 .
148
In further experiments, we tested the reaction of organo lithium compounds with multi walled
carbon nanotubes (chapter 3.4). The synthesized CNT derivatives were used to produce carbon
nanotube PAN composites. In order to optimize the interaction between the carbon nanotubes
and the polymer, we developed a new approach for the functionalization of carbon nanotubes
that gave CNT/polymer films with significantly increased mechanical properties.
To obtain a deeper insight in the nature of SWCNT anions, we synthesized either by the addition of R(−) or by metal assisted reduction of the SWCNTs, and developed a new procedure for
in situ XPS measurements of the charged SWCNTs (chapter 3.5). The experiments revealed
for the first time a significant shift of the C 1s core level towards higher binding energy. This
shift was attributed to the filling of the previously unoccupied π ∗ -orbitals of the SWCNTs.
In the final part of this work (chapter 3.6), we focused our interest on the reductive alkylation
of carbon nanotubes. This reaction provides a convenient approach for the functionalization of
individual carbon nanotubes since, the reaction procedure involves the charging and therefore
the dissolution of the carbon nanotubes followed by the addition of alkyl halides. After
optimization of the reaction conditions, nanotube derivatives with a high degree of functionalization were obtained. Furthermore, we applied this method for the functionalization of
MWCNTs to synthesize new multi walled carbon nanotubes derivatives with a high degree of
functionalization. Further transformation of the functional groups introduced via the reductive
alkylation of carbon nanotubes afforded carbon nanotubes decorated with fullerene molecules
on the surface. This novel material might reveal interesting new properties.
149
4 Zusammenfassung
Kohlenstoffnanoröhren weisen aufgrund ihrer herausragenden elektronischen, optischen und
mechanischen Eigenschaften ein enormes Potential auf. Bisher scheitern jedoch die meisten
technologischen Anwendungen an Problemen, die auf die schlechte Prozessierbarkeit der
unbehandelten Nanoröhren zurückzuführen sind. In den letzten Jahren wurde gezeigt, dass die
Funktionalisierung von Kohlenstoffnanoröhren für viele Anwendungen unumgänglich ist.
Der erste Teil dieser Arbeit (Kapitel 3.1) beschäftigt sich mit der Reinigung von Kohlenstoffnanoröhren, die durch das Laser-Ablations-Verfahren hergestellt wurden. Im Verlauf
dieser Arbeiten wurde ein optimierter Prozess für die oxidative Reinigung von Kohlenstoffnanoröhren erarbeitet. Das Hauptaugenmerk lag hierbei auf der Minimierung des Anteils
der Defektgruppen im oxidierten Material, die an Sauerstoff gebunden sind, da diese im
Verlauf der kovalenten Seitenwandfunktionalisierung störende Nebenreaktionen eingehen
können.
Bei der Untersuchung der oxidativen Reinigung von Kohlenstoffnanoröhren in
heißer Luft zeigte sich, dass es sich hierbei um ein sehr mildes Verfahren handelt. Des
Weiteren zeigten ramanspektroskopische Untersuchungen, dass die aus diesem Verfahren
hervorgehenden gereinigten Nanoröhren nur eine sehr geringe Anzahl an Defekten aufweisen.
Der heterogene Charakter dieses Verfahrens führt somit zu Einschränkungen hinsichtlich der
Reinheit des erhaltenen Materials. Um diese Einschränkung zu überwinden, wurde in einem
zweiten Ansatz die Reinigung von Kohlenstoffnanoröhren durch Oxidation mit unterschiedlich
konzentrierter Salpetersäure in Abhängigkeit von der Reaktionszeit untersucht. Die gereinigten
Kohlenstoffnanoröhren wurden anschließend mittels Ramanspektroskopie, TEM und XPS
charakterisiert. Diese Untersuchungen an den gereinigten Nanoröhren zeigten, dass bereits
nach kurzer Reaktionszeit Kohlenstoffnanoröhren in einer hohen Reinheit erhalten werden.
Die Ramanspektren dieses Materials (Ox 12) weisen auf eine geringe Anzahl von Störungen in
der graphitischen Struktur hin und unterstreichen somit ihre Eignung für die weitere Funktion-
150
alisierung. Die Erhöhung der Reaktionszeit (Ox13 - Ox17) führt zur weiteren Verbesserung
der Reinheit. Die Ramanspektren dieser Proben deuten jedoch auf signifikante Störungen im
graphitischen Gitter der Nanoröhren hin. XPS Untersuchungen dieser Proben deuten auf die
Entstehung von funktionellen Gruppen wie z. B. Carboxyl-, Carbonyl-, Hydroxyl-, Nitroso-,
und Nitro-Gruppen hin. Anschließende Experimente zeigten, dass die funktionellen Gruppen
durch anschließendes Erhitzen der Proben im Vakuum entfernt werden können, um schließlich
die Kohlenstoffnanoröhren in der gewünschten Reinheit zu erhalten.
Im zweiten Teil dieser Arbeit (Kapitel 3.2) wurde der Einfluss des Herstellungsverfahrens
(Arc Discharge, Laser Ablation und HiPCO) auf die Reaktivität der Kohlenstoffnanoröhren
hinsichtlich einer Seitenwandfunktionalisierung untersucht. Um vergleichbare Ergebnisse zu
erhalten, griffen wir bei der Funktionalisierung auf die in unserem Arbeitskreis entwickelte
Umsetzung mit Nitrenen zurück.
Der Funktionalisierungsprozess führt bei allen Proben
zu einer signifikanten Erhöhung der Reinheit. Die Charakterisierung der funktionalisierten
Proben zeigt, dass sich die durch unterschiedliche Herstellungsverfahren synthetisierten
Kohlenstoffnanoröhren hinsichtlich ihrer Reaktivität deutlich voneinander unterscheiden. So
konnte durch den Funktionalisierungsgrad, der aus XPS-Daten ermittelt wurde gezeigt werden,
dass die nach dem HiPCO Verfahren hergestellten Nanoröhren die höchste Reaktivität aus der
untersuchten Reihe besitzen. Wir führen dieses Ergebnis auf den geringeren Durchmesser der
HiPCO SWCNTs zurück.
Im weiteren Verlauf dieser Arbeit (Kapitel 3.3) wurde die Funktionalisierung von Kohlenstoffnanoröhren mit Organo-Lithium Verbindungen untersucht. Dieser Funktionalisierungsprozess verläuft in zwei Schritten. Im ersten Schritt addiert die Organo-Lithium-Verbindung
an die Nanoröhre unter Bildung des geladenen Ziwschenprodukts R-SWCNT− . Die repulsiven Wechselwirkungen zwischen den negativ geladenen Kohlenstoffnanoröhren führen
hierbei zur Ausbildung einer homogenen Lösung der SWCNTs in der Reaktionsmischung.
In einem zweiten Schritt wird das Zwischenprodukt (R-SWCNT-) durch die Zugabe von
verdünnter Salzsäure protoniert, um das Endprodukt (R-H-SWCNT) zu erhalten. Die genauere
Untersuchung dieser Reaktion zeigte, dass die negativen Ladungen über das gesamte πSystem der Kohlenstoffnanoröhren delokalisiert sind und somit die weitere Umsetzung mit
151
4 Zusammenfassung
Organo-Lithium Verbindungen verhindern.
Um die daraus resultierende Limitierung des
Funktionalisierungsgrades zu überwinden, wurde erstmalig eine Zweitfunktionalisierung
durchgeführt.
Die aus der Zweifunktionalisierung resultierende weitere Steigerung des
Funktionalisierungsgrades zeigte, dass die hier vorliegende Limitierung ausschließlich von
der negativen Ladung auf den Nanoröhren beruht und sterische Effekte ausgeschlossen
werden können. Durch STM Untersuchungen an den funktionalisierten Kohlenstoffnanoröhren
konnten erstmals direkt die Funktionalitäten an den Seitenwänden der Kohlenstoffnanoröhren
nachgewiesen werden. Vertiefende ramanspektroskopische Untersuchungen der funktionalisierten Kohlenstoffnanoröhren zeigten, dass die Addition von Organo-Lithium Verbindungen
an Kohlenstoffnanoröhren bevorzugt an metallischen Nanoröhren stattfindet.
Die hieraus
hervorgehende Selektivität dieser Reaktion bietet vielleicht eine Möglichkeit zur Trennung von
metallischen und halbleitenden Nanoröhren.
Ausgehend von dem geladenen Zwischenprodukt R-SWCNT− wurde in einem weiteren
Abschnitt die Umsetzung mit elektrophilen Verbindungen untersucht (Kapitel 3.3.3), um
R-R’-SWCNTs zu erhalten. Hieraus konnte ein neues Verfahren zur einfachen Synthese von
bis-funktionellen Kohlenstoffnanoröhren-Derivaten entwickelt werden.
In weiteren Experimenten (Kapitel 3.3.4) konnte gezeigt werden, dass sich das oben
vorgestellte Verfahren auch für die Initiierung einer anionischen Polymerisationsreaktion
eignet, um auf diesem Weg polymerbeschichtete Nanoröhren zu synthetisieren. Mit TEM
Untersuchungen konnte die Polymerschicht nachgewiesen werden, die Nanoröhren umgibt.
Die polymeren Addenden an den hier synthetisierten Polymer-SWCNTs führen zu einer sehr
guten Löslichkeit in gängigen organischen Lösungsmitteln.
Die Anwendung der hier gesammelten Erfahrungen ermöglichte es uns in einem weiteren Schritt Komposite aus nach diesem Verfahren funktionalisierten MWCNTs und PAN
herzustellen (Kapitel 3.4). Hierbei zeigte sich, dass die Einbringung von bereits mit polyacrylnitril vorfunktionalisierten Nanoröhren in eine PAN-Matrix die Eigenschaften des
resultierenden Kompositmaterials entscheidend verbessert. Untersuchungen der mechanischen
Eigenschaften des auf diese Weise erhaltenen MWCNT/PAN Kompositematerials demonstri-
152
erten die herausragenden Eigenschaften dieses neuen Materials und damit das hohe Potential
dieses neuen Ansatzes zur Integration von CNTs in Polymere.
Durch die intensive Kooperation mit dem Lehrstuhl für Technische Physik II (Prof. Dr. Ley)
ist es uns in einem weiteren Abschnitt dieser Arbeit (Kapitel 3.5) gelungen die Eigenschaften
von geladenen Kohlenstoffnanoröhren zu untersuchen. Im Verlauf dieser Arbeiten wurde
ein neues Verfahren zur in situ XPS Messung von SWCNT− entwickelt und angewendet.
Die hieraus resultierenden Ergebnisse zeigten erstmalig eine signifikante Verschiebung des
C 1s Rumpfniveaus zu höherer Bindungsenergie. Diese Verschiebung der Bindungsenergie
steht in direktem Zusammenhang mit dem Auffüllen der vorher ungefüllten π ∗ -Orbitale der
Kohlenstoffnanoröhren und stellt damit ein Maß für die Aufladung der Nanoröhren dar.
Im letzten Teil der vorliegenden Arbeit wurde die reduktive Alkylierung von Kohlenstoffnanoröhren untersucht (Kapitel 3.6). Die reduktive Alkylierung von Kohlenstoffnanoröhren
bietet einen einfachen Weg zur Funktionalisierung von vereinzelten Nanoröhren. Hierbei
werden die Nanoröhren in einem ersten Schritt reduziert. Die damit einhergehende negative
Aufladung der Nanoröhren führt zur Vereinzelung der Nanoröhren und damit zur Ausbildung
einer homogenen Lösung. In einem zweiten Schritt werden die geladenen Nanoröhren durch
die Zugabe eines Alkylhalogenides funktionalisiert. Durch die Untersuchung unterschiedlicher
Verfahren konnte gezeigt werden, dass die Umsetzung der SWCNTs in flüssigem Ammoniak
mit Natrium als Reduktionsmittel zu SWCNT-derivaten führt, die sich durch einen sehr hohen
Funktionalisierungsgrad auszeichnen. Des Weiteren konnte gezeigt werden, dass sich das
hier entwickelte Verfahren auch zur effektiven Funktionalisierung von MWCNTs eignet. Die
erhaltenen SWCNT-Derivate konnten in Folgereaktionen weiter modifiziert werden. Auf diese
Weise wurden SWCNT-Derivate dargestellt, an deren Seitenwände Fullerenmoleküle kovalent
gebunden sind.
Des Weiteren wurden im Rahmen dieser Arbeit erstmalig zwei Methoden (a) aus den XPS
Ergebnissen und b) aus den TGA Untersuchungen) zur Bestimmung des Funktionalisierungsgrades erarbeitet, die somit den direkten Vergleich unterschiedlicher Verfahren erlauben.
153
4 Zusammenfassung
Von größerer Bedeutung als die Fortschritte in den unterschiedlichen hier bearbeiteten Teilgebieten ist jedoch der hier erstmalig mögliche Vergleich einer Vielzahl detailliert untersuchter
Methoden zur kovalenten Seitenwandfunktionalisierung von sowohl einwandigen als auch
mehrwandigen Kohlenstoffnanoröhren.
154
5 Experimental Part
5.1 Instruments and Methods
UV/Vis Spectra were recorded on a UV-3102 PC UV-VIS-NIR Scanning Spectrophotometer,
Shimadzu Corporation, Analytical Instruments Division, Kyoto, Japan.
NMR Spectra were recorded on a Bruker Avance 300. The chemical shifts are given in ppm
relative to the appropriate solvent peak as standard reference. Processing of the raw data was
carried out using MestRE-C 2.3.
TEM Images were taken an a CM30 or CM300, Philips. The CNT material was adsorbed on a
carbon coated copper grid.
AFM Images were recorded on a SOLVER PRO (Scanning Probe Microscope), NT-MDT Co.,
Zelenograd. The CNT material was adsorbed on oxidized silicon wafers.
STM Images were recorded on a Omicron LT-STM.
Raman Spectra were recorded on a Renishaw Ramanscope 2000 (λex 514.5 nm).
XPS Spectra were recorded on a Surface Science Instruments, txpe M-Probe. X-ray gun: small
spot, monochromatic Al Kα -radiation (hν=1486.6 eV). Detection system: angle integrating
hemispherical analyser multichannel detection system.
Elemental Analysis: CE Instrument EA 1110 CHNS.
155
5 Experimental Part
Differential Scanning Calorimetry (DSC): MicroCal MCII (Northhampton, MA, USA).
Thermogravimetric Analysis (TGA): SDTA 811e, Mettler Toledo.
Tensile Tests: were carried out by applying 3 cm broad test specimens on a Zwick Typ 1445.
The results were normalized according to the thickness of the samples.
5.2 Chemicals
Reagents were purchased from Aldrich, Fluka or Acros Organics. Unless otherwise stated,
solvents were not further purified prior to use.
The Carbon Nanotube materials were obtained from:
GDPC, Universite de Montpellier II, France (Arc Discharge SWCNTs, pristine SWCNTs);
Forschungszentrum Karlsruhe, Germany (Laser Ablation SWCNTs, pristine SWCNTs);
Carbon Nanotechnologies Inc., US (HiPCO SWCNTs, purified SWCNTs); and
SGL Carbon Bonn Germany (a variety of MWCNT samples from different sources).
All the carbon nanotube samples were not further purified prior to use unless otherwise stated.
5.3 Experimental Details
Note: The compound names given in this section do not necessarily comply with the IUPAC
nomenclature rules, especially where the use of those names would be impractical.
156
5.3 Experimental Details
General procedure for the synthesis of Ethoxycarbonylaziridino-SWCNTs
In a round bottom flask, equipped with gas inlet,
O
40 mg SWCNTs (3.3 mmol carbon) were dispersed
O
N
in ODCB using an ultra sonic bath. The dispersion was degassed in high vacuum, purged with
nitrogen and preheated to 160 ◦ C. A solution of
0.5 g (4.3 mmol, 3.5 equiv.) ethylazidiformate in
N
O
5 ml ODCB was added dropwise over a period of
O
20 min. The temperature was maintained at 160 ◦ C
for another hour. The reaction mixture was allowed to reach room temperature and diluted
with ethanol. The mixture was filtered through a 0.2 µm PTFE membrane filter and washed
subsequently with ethanol and THF. The resulting black solid was dried in the oven at 80 ◦ C
overnight.
Ethoxycarbonylaziridino-A SWCNTs (1)
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
76.6
C 1s
285
O
18.6
O 1s
532
O KVV Auger
978
N 1s
400
N
4.8
• Raman shift (λex =514.5 nm): 1595.8, 1567.6, 1553.0, 1340.8, 186.7 cm−1 .
• ID /IG : 0.130.
• Raman shift (λex =1064 nm): 1593.5, 1565.5, 1549.9, 1278.5, 179.2 cm−1 .
• ID /IG : 0.082.
• Estimated degree of functionalization (XPS): 7.7 %
• UV/Vis (D2 O/LDS): λmax = 1.48, 1.66, 1.88, 2.37, 2.55 eV.
157
5 Experimental Part
Ethoxycarbonylaziridino-B SWCNTs (2)
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
90.5
C 1s
285
O
6.6
O 1s
533
O KVV Auger
978
N 1s
400
N
2.9
• Raman shift (λex =514.5 nm): 1591.8, 1556.4, 1512.0, 1335.2, 270.3, 263.8, 249.3 cm−1 .
• ID /IG : 0.043.
• Raman shift (λex =1064 nm): 1590.3, 1545.7, 1281.7, 268.7 cm−1 .
• ID /IG : 0.119.
• Estimated degree of functionalization (XPS): 3.5%.
• UV/Vis (D2 O/LDS): λmax = 1.37, 1.48, 1.66, 1.86, 2.04, 2.21, 2.43, 2.73, 3.26 eV.
Ethoxycarbonylaziridino-C SWCNTs (3)
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
78.0
C 1s
286
O
16.0
O 1s
532
O KVV Auger
979
N 1s
400
N
6.0
• Raman shift (λex =514.5 nm): 1597.8, 1561.2, 1559.2, 1336.7, 186.7 cm−1 .
• ID /IG : 0.198.
• Raman shift (λex =1064 nm): 1593.0, 1549.0, 1284.1, 270.2 cm−1 .
• ID /IG : 0.283.
• Estimated degree of functionalization (XPS): 7.9%
• UV/Vis (D2 O/LDS): λmax = 1.43, 1.86 eV.
158
5.3 Experimental Details
t-Butyl-H-SWCNTs (4)
In a 100 ml nitrogen-purged and heat dried round bottom
H
flask, equipped with a gas inlet and a pressure compensation, 20 mg (1.7 mmol of carbon) of HiPCO SWCNTs
were dispersed in 50 ml anhydrous benzene. To this
dispersion 2.5 ml of a 1.7 molar solution of t-butyllithium
(4.25 mmol) in hexane were added dropwise over a
H
period of 10 min. After the completion of the addition
the resulting suspension was stirred for one hour at room temperature. As stirring continues
the SWCNTs dispersion turns into a black homogeneous solution. The solution was stirred
for another hour and subsequently quenched by the addition of diluted hydrochloric acid. The
resulting dispersion was diluted with 100 ml acetone and filtered through a 0.2 µm PTFE
membrane filter and washed with ethanol and THF. The resulting black solid was dried in a
vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
core level
Binding energy (eV)
C
C 1s
284.4
• Raman shift (λex =514.5 nm): 1586.0, 1566.0, 1526.1, 1328.7, 266.6, 258.6, 242.5 cm−1 .
• ID /IG : 0.064.
• Raman shift (λex =1064 nm): 1593, 1279, 269 cm−1 .
• ID /IG : 0.190.
• 1 H NMR (300 MHz, TCE d2, 25◦ C): 1.47, 7.28 ppm.
159
5 Experimental Part
n-Butyl-H-SWCNTs (5)
In a 100 ml nitrogen-purged and heat dried round bottom
H
flask, equipped with a gas inlet and a pressure compensation, 20 mg (1.7 mmol of carbon) of HiPCO SWCNTs
were dispersed in 50 ml anhydrous benzene. To this dispersion 2.5 ml of a 1.7 molar solution of n-butyllithium
(4.25 mmol) in pentane were added dropwise over a
H
period of 10 min. After the completion of the addition
the resulting suspension was stirred for one hour at room temperature. As stirring continues
the SWCNTs dispersion turns into a black homogeneous solution. The solution was stirred
for another hour and subsequently quenched by the addition of diluted hydrochloric acid. The
resulting dispersion was diluted with 100 ml acetone and filtered through a 0.2 µm PTFE
membrane filter and washed with ethanol and THF. The black solid was dried in a vacuum
oven at 50 ◦ C overnight.
• XPS (solid film):
Element
core level
Binding energy (eV)
C
C 1s
285
• Raman shift (λex =514.5 nm): 1586.5, 1562.6, 1520.2, 1327.6, 265.0, 257.0, 242.5 cm−1 .
• ID /IG : 0.079.
• Raman shift (λex =1064 nm): 1593, 1283, 268 cm−1 .
• ID /IG : 0.230.
• 1 H NMR (300 MHz, TCE d2, 25◦ C): 1.51, 1.99, 7.29 ppm.
160
5.3 Experimental Details
Phenyl-H-SWCNTs (6)
In a 100 ml nitrogen-purged and heat dried round bottom
H
flask, equipped with a gas inlet and a pressure compensation, 20 mg (1.7 mmol of carbon) of HiPCO SWCNTs
were dispersed in 50 ml anhydrous benzene. To this
dispersion 2.3 ml of a 1.9 molar solution of phenyllithium
(4.25 mmol) in butyl ether were added dropwise over a
H
period of 10 min. After the completion of the addition
the resulting suspension was stirred for one hour at room
temperature. As stirring continues the SWCNTs dispersion turns into a black homogeneous
solution. The solution was stirred for another hour and subsequently quenched by the addition
of diluted hydrochloric acid. The resulting dispersion was diluted with 100 ml acetone and
filtered through a 0.2 µm PTFE membrane filter and washed with ethanol and THF. The black
solid was dried in a vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
core level
Binding energy (eV)
C
C 1s
284.5
• Raman shift (λex =514.5 nm): 1587.4, 1563.2, 1526.1, 1330.2, 268.3, 260.2, 245.7 cm−1 .
• ID /IG : 0.089.
• 1 H NMR (300 MHz, TCE d2, 25◦ C): 0.70, 3.02, 6.46 ppm.
161
5 Experimental Part
t-Butyl-H-SWCNTsa−c (7-9)
In a 100 ml nitrogen-purged and heat dried round bottom
H
H
flask, equipped with a gas inlet and a pressure compensation, 20 mg (1.7 mmol of carbon) of t-Butyl-H-SWCNTs
(4) were dispersed in 50 ml anhydrous benzene. To this
dispersion 2.5 ml of a 1.7 molar solution of t-butyllithium
H
(4.25 mmol) in hexane were added dropwise over a
H
period of 10 min. After the completion of the addition
the resulting suspension was stirred for one hour at room temperature. As stirring continues the
SWCNTs dispersion turns into a black homogeneous solution.
t-Butyl-H-SWCNTsa (8): Stirring was continued for 72 hours at RT.
t-Butyl-H-SWCNTsb (9): The mixture was stirred for three hours at 80 ◦ C.
t-Butyl-H-SWCNTsc (7): Stirring was continued for another hour at RT.
The mixture was subsequently quenched by the addition of diluted hydrochloric acid. The
resulting dispersion was diluted with 100 ml acetone and filtered through a 0.2 µm PTFE
membrane filter and washed with ethanol and THF. The black solid was dried in a vacuum
oven at 50 ◦ C overnight.
t-Butyl-H-SWCNTsa (7)
• XPS (solid film):
Element
core level
Binding energy (eV)
C
C 1s
284
• Raman shift (λex =514.5 nm): 1587.5, 1559.6, 1528.5, 1330.1, 269.9, 260.2, 249.0,
244.1 cm−1 .
• ID /IG : 0.302.
162
5.3 Experimental Details
t-Butyl-H-SWCNTsb (8)
• XPS (solid film):
Element
core level
Binding energy (eV)
C
C 1s
284.5
• Raman shift (λex =514.5 nm): 1589.6, 1561.9, 1520.5, 1330.1, 269.9, 261.8, 245.7 cm−1 .
• ID /IG : 0.274.
t-Butyl-H-SWCNTsc (9)
• XPS (solid film):
Element
core level
Binding energy (eV)
C
C 1s
284.5
• Raman shift (λex =514.5 nm): 1587.4, 1560.6, 1531.3, 1330.3, 271.5, 260.2, 245.7 cm−1 .
• ID /IG : 0.346.
t-Butyl-iodo-SWCNTs (10)
In a 100 ml nitrogen-purged and heat dried round bottom
I
flask, equipped with a gas inlet and a pressure compensation, 15 mg (1.3 mmol of carbon) of HiPCO SWCNTs
were dispersed in 50 ml anhydrous benzene. To this
dispersion 4.5 ml of a 1.7 molar solution of t-butyllithium
I
(7.65 mmol) in hexane were added dropwise over a
period of 10 min. After the completion of the addition
the resulting suspension was stirred for one hour at room temperature. As stirring continues
the SWCNTs dispersion turns into a black homogeneous solution. To this solution 3.9 g
(7.6 mmol) Bis(2,3,6-trimethylpyridine)iodine(I) hexafluorophosphate dissolved in 10 ml
anhydrous benzene were added dropwise. The solution was stirred for another hour and
163
5 Experimental Part
subsequently quenched by the addition of i-propanol. The resulting dispersion was diluted with
100 ml acetone and filtered through a 0.2 µm PTFE membrane filter and washed with ethanol
and THF. The black solid was dried in a vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
99.5
C 1s
-
I
0.5
I 3d
-
• Raman shift (λex =514.5 nm): 1589.9, 1554.2, 1515.5, 1331.1, 271.7, 263.7, 249.3 cm−1 .
• ID /IG : 0.104.
• Estimated degree of functionalization (XPS): 1.0%.
t-Butyl-cyano-SWCNTs (11)
In a 100 ml nitrogen-purged and heat dried round bottom
CN
flask, equipped with a gas inlet and a pressure compensation, 15 mg (1.3 mmol of carbon) of HiPCO SWCNTs
were dispersed in 50 ml anhydrous benzene. To this
dispersion 1.5 ml of a 1.7 molar solution of t-butyllithium
CN
(2.6 mmol) in hexane were added dropwise over a period
of 10 min.
After the completion of the addition the
resulting suspension was stirred for one hour at room temperature. As stirring continues
the SWCNTs dispersion turns into a black homogeneous solution. To this solution 0.46 g
(2.5 mmol) p-toluenesulfonyl cyanide dissolved in 5 ml anhydrous benzene were added
dropwise. The resulting suspension was stirred for another two hours to complete the reaction
and subsequently diluted with 100 ml acetone. The mixture was filtered through a 0.2 µm
PTFE membrane filter and washed with ethanol and THF. The resulting black solid was dried
in a vacuum oven at 50 ◦ C overnight.
164
5.3 Experimental Details
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
84.1
C 1s
285
O
12.2
O 1s
533
O KVV Auger
977
N 1s
400
N
3.7
• Raman shift (λex =514.5 nm): 1590.6, 1566.6, 1526.0, 1332.1, 286.1, 271.7, 263.7,
247.8 cm−1 .
• ID /IG : 0.109.
• Estimated degree of functionalization (XPS): 5.7%.
t-Butyl-carboxyl-SWCNTs (12)
In a 100 ml round bottom flask, equipped with a gas inlet
COOH
and a pressure compensation, 5 mg (0.4 mmol of carbon)
of t-butyl-cyano-SWCNTs (11) were dispersed in 20 ml
of a 1 molar sodium hydroxide solution. The resulting
suspension was heated to 80 ◦ C and stirred for 24 hours.
COOH
The reaction mixture was cooled to room temperature
and diluted with 100 ml ethanol. The mixture was filtered
through a 0.2 µm PTFE membrane filter and washed with ethanol and THF. The resulting black
solid was dried in a vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
79.1
C 1s
285
O
18.7
O 1s
532
O KVV Auger
979
N 1s
400
N
2.3
• Raman shift (λex =514.5 nm): 1590.5, 1568.0, 1523.5, 1331.2, 270.1, 262.1, 247.8 cm−1 .
• ID /IG : 0.120.
165
5 Experimental Part
• Estimated degree of functionalization (XPS): 5.6%.
t-Butyl-poly(t-butyl acrylate)-SWCNTs (13)
In a 250 ml nitrogen-purged and heat dried round
COOtBu
COOtBu
n
bottom flask, equipped with a gas inlet and a pressure compensation, 50 mg (4.2 mmol of carbon)
of HiPCO SWCNTs were dispersed in 100 ml
anhydrous benzene. To this dispersion 1 ml of a
1.7 molar solution of t-butyllithium (1.7 mmol)
in hexane was added dropwise over a period of
COOtBu
10 min. After the completion of the addition the
COOtBu
n
resulting suspension was stirred for one hour at
room temperature. As stirring continues the SWCNTs dispersion turns into a black homogeneous solution. To this solution 13.6 ml (94 mmol) t-butyl acrylate were added dropwise. The
resulting mixture was stirred for one hour as the viscosity of the solution increases yielding to
a black solution of the SWCNTs. The solution was dried at 50 ◦ C under reduced pressure to
give the product as a black solid.
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
75.4
C 1s
283
O
24.6
O 1s
530
O KVV Auger
978
• Raman shift (λex =514.5 nm): 1589.8, 1558.1, 1500.8, 1331.8, 267.0, 260.6, 246.1 cm−1 .
• ID /IG : 0.073.
• Estimated degree of functionalization (XPS): −.
• 1 H NMR (300 MHz, CDCl3 , 25◦ C): 1.28, 1.66, 2.10 ppm.
• TGA (weight loss): 70.4%.
• Estimated degree of functionalization (TGA): −.
166
5.3 Experimental Details
t-Butyl-polyacrylnitrile-SWCNTs (14)
In a 250 ml nitrogen-purged and heat dried round bottom
CN
CN
n
flask, equipped with a gas inlet and a pressure compensation, 50 mg (4.2 mmol of carbon) of HiPCO SWCNTs
were dispersed in 100 ml anhydrous benzene. To this
dispersion 1 ml of a 1.7 molar solution of t-butyllithium
(1.7 mmol) in hexane was added dropwise over a period
of 10 min.
CN
CN
After the completion of the addition the
resulting suspension was stirred for one hour at room
n
temperature. As stirring continues the SWCNTs disper-
sion turns into a black homogeneous solution. To this solution 6.25 ml (94 mmol) acrylonitrile
were added dropwise. The resulting mixture was stirred for one hour as the viscosity of the
solution increases to give a black solution of the SWCNTs. The solution was dried at 50 ◦ C
under reduced pressure to give the product as a black solid.
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
83.1
C 1s
286
N
16.9
N 1s
400
• Raman shift (λex =514.5 nm): 1589.0, 1566.2, 1517.1, 1328.1, 266.9, 260.5, 246.2 cm−1 .
• ID /IG : 0.080.
• Estimated degree of functionalization (XPS): −.
• TGA (weight loss): 72.6%.
• Estimated degree of functionalization (TGA): −.
167
5 Experimental Part
8-Iodooctyl-SWCNTsa (15)
I
In a 250 ml nitrogen-purged two necked flask, equipped
with a gas inlet and a pressure compensation, 10 mg
(0.0078 mmol) naphthalene and 25 mg (2.1 mmol of
carbon) of HiPCO SWCNTs were dissolved in 100 ml
anhydrous THF. To this solution 150 mg (6.5 mmol)
sodium were added leading to a dark green color of
the solution. After 1 h sonication 2 ml (10.0 mmol)
I
1,8-diiodooctane were added and the sonication was
continued for another hour. To complete the reaction the mixture was stirred for 12 h at room
temperature. The excess of sodium was quenched by the careful addition of 20 ml water. The
solution was diluted with ethanol and filtered through a 0.2 µm PTFE membrane filter and
washed with ethanol and THF. The resulting black solid was dried in a vacuum oven at 50 ◦ C
overnight.
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
99.8
C 1s
284.6
I
0.2
I 3d
631.9; 620.4
• Estimated degree of functionalization (XPS): 0.2%.
• UV/Vis (D2 O/LDS): λmax = 816, 738, 653, 602, 560, 509, 452, 415, 379, 270 nm.
• TGA (weight loss): 29.7%.
• Raman shift (λex =514.5 nm): 1593.7, 1570.9, 1546.2, 1507.5, 1334.8, 273.2, 265.1,
250.7 cm−1 .
• ID /IG : 0.016.
• Raman shift (λex =514.5 nm), (annealed): 1595.9, 1557.3, 1516.8, 1340.7, 271.8, 267.0,
250.9 cm−1 .
• Estimated degree of functionalization (TGA): 2.2%.
168
5.3 Experimental Details
Figure 5.1: TEM images of 8-iodooctyl-SWCNTsa (15).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.2: Raman spectra of 8-iodooctyl-SWCNTsa (15), annealed 8-iodooctyl-SWCNTsa and pristine
SWCNTs.
W
W
HiPCO S
CNTs
700
800
Absorbance (a. u.)
200
300
400
W
500
600
CNTs
funct. S
900
avelength (nm)
Figure 5.3: UV/Vis spectra of 8-iodooctyl-SWCNTsa (15) and pristine SWCNTs.
169
5 Experimental Part
8-Iodooctyl-SWCNTsb (16)
I
Into a dry, 1 l nitrogen-purged three necked flask,
equipped with a gas inlet and a high-efficiency condenser, 300 ml anhydrous ammonia were condensed at
-70 ◦ C. 200 mg (8.7 mmol) sodium were added to the
liquid ammonia resulting in a dark blue solution. To this
mixture 50 mg (4.1 mmol of carbon) of HiPCO SWCNTs
were added leading to a black solution of the SWCNTs
I
in liquid ammonia. Stirring was continued for 1 h. 2 ml
(10 mmol) 1,8-diiodooctane were then added dropwise and the reaction mixture was stirred
overnight with the slow evaporation of the ammonia. 100 ml water were added carefully to
the black solid. After acidification with 20 ml HCl (10%) the nanotubes were extracted with
200 ml hexane and washed twice with 100 ml water. The organic phase was filtered through
a 0.2 µm PTFE membrane filter and washed with ethanol and THF. The resulting black solid
was dried in a vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
99.8
C 1s
284.8
I
0.2
I 3d
631.6; 620.2
• Estimated degree of functionalization (XPS): 0.2%.
• UV/Vis (D2 O/LDS): λmax = 820, 740, 652, 603, 557, 449, 412, 378, 266 nm.
• TGA (weight loss): 24.8%.
• Raman shift (λex =514.5 nm): 1588.3, 1564.6, 1541.1, 1508.8, 1327.0, 263.7, 257.3,
242.9 cm−1 .
• ID /IG : 0.078.
• Raman shift (λex =514.5 nm), (annealed): 1591.9, 1577.0, 1553.3, 1522.7, 1339.7, 271.6,
268.3, 263.5, 249.1 cm−1 .
• Estimated degree of functionalization (TGA): 1.0%.
170
5.3 Experimental Details
Figure 5.4: TEM images of 8-iodooctyl-SWCNTsb (16).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.5: Raman spectra (λex 514.5 nm) of 8-iodooctyl-SWCNTsb (16), annealed 8-iodooctylSWCNTsb and pristine SWCNTs.
W
W
HiPCO S
CNTs
700
800
Absorbance (a. u.)
200
300
400
W
500
600
CNTs
funct. S
900
avelength (nm)
Figure 5.6: UV/Vis spectra of 8-iodooctyl-SWCNTsb (16) and pristine SWCNTs.
171
5 Experimental Part
8-Iodooctyl-SWCNTsc (17)
I
Into a dry, 1 l argon-purged three necked flask, equipped
with a gas inlet and a high-efficiency condenser, 300 ml
anhydrous ammonia were condensed at -70 ◦ C. 160 mg
(23 mmol) lithium were added to the liquid ammonia
resulting in a dark blue solution. To this mixture 50 mg
(4.2 mmol of carbon) of HiPCO SWCNTs were added
leading to a black solution of the SWCNTs in liquid
I
ammonia. Stirring was continued for 1 h. 2 ml (10 mmol)
1,8-diiodooctane were then added dropwise and the reaction mixture was stirred overnight with
the slow evaporation of the ammonia. 100 ml water were added carefully to the black solid.
After acidification with 20 ml HCl (10 %) the nanotubes were extracted with 200 ml hexane
and washed twice with 100 ml water. The organic phase was filtered through a 0.2 µm PTFE
membrane filter and washed with ethanol and THF. The resulting black solid was dried in a
vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
99.6
C 1s
284.9
I
0.4
I 3d
632.0; 620.5
• Estimated degree of functionalization (XPS): 1.0%.
• UV/Vis (D2 O/LDS): λmax = 815, 742, 650, 606, 560, 451, 415, 379, 277, 224 nm.
• TGA (weight loss): 62.5%.
• Raman shift (λex =514 nm): 1594.9, 1571.4, 1553.8, 1513.6, 1333.7, 273.2, 265.1,
250.7 cm−1 .
• ID /IG : 0.067.
• Raman shift (λex =514 nm), (annealed): 1596.2, 1577.2, 1566.7, 1514.5, 1338.3, 273.4,
265.4, 250.9, 189.8 cm−1 .
• Estimated degree of functionalization (TGA): 1.6%.
172
5.3 Experimental Details
Figure 5.7: TEM images of 8-iodooctyl-SWCNTsc (17).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.8: Raman spectra (λex 514.5 nm) of 8-iodooctyl-SWCNTsc (17), annealed 8-iodooctylSWCNTsc and pristine SWCNTs.
W
W
HiPCO S
CNTs
700
800
Absorbance (a. u.)
200
300
400
W
500
600
CNTs
funct. S
900
avelength (nm)
Figure 5.9: UV/Vis spectra of 8-iodooctyl-SWCNTs5 (17) and pristine SWCNTs.
173
5 Experimental Part
Dodecyl-SWCNTsa (18)
Into a dry, 1 l nitrogen-purged three necked flask,
equipped with a gas inlet and a high-efficiency
condenser, 300 ml anhydrous ammonia were
condensed at -70 ◦ C. 140 mg (6 mmol) sodium
were added to the liquid ammonia resulting in
a dark blue solution.
To this mixture 30 mg
(2.5 mmol of carbon) of HiPCO SWCNTs were
added leading to a black solution of the SWCNTs
in liquid ammonia. Stirring was continued for 1 h. 3 ml (12 mmol) 1-iodododecane were then
added dropwise and the reaction mixture was stirred overnight with the slow evaporation of the
ammonia. 100 ml water were added carefully to the black solid. After acidification with 20 ml
HCl (10%) the nanotubes were extracted with 200 ml hexane and washed twice with 100 ml
water. The organic phase was filtered through a 0.2 µm PTFE membrane filter and washed with
ethanol and THF. The resulting black solid was dried in a vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
core level
Binding energy (eV)
C
C 1s
284.3
• UV/Vis (D2 O/LDS): λmax = −.
• TGA (weight loss): 27.2%.
• Raman shift (λex =514 nm): 1591.4, 1571.7, 1562.5, 1521.7, 1330.8, 262.1, 246.1 cm−1 .
• ID /IG : 0.358.
• Raman shift (λex =514 nm), (annealed): 1593.4, 1581.7, 1532.8, 1500.2, 1332.9, 269.9,
265.1, 261.9, 247.4 cm−1 .
• Estimated degree of functionalization (TGA): 5.2%.
174
5.3 Experimental Details
Figure 5.10: TEM images of dodecyl-SWCNTsa (18).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.11: Raman spectra (λex 514.5 nm) of dodecyl-SWCNTsa (18), annealed dodecyl-SWCNTsa
and pristine SWCNTs.
W
W
Absorbance (a. u.)
HiPCO S
200
300
400
W
500
600
CNTs
funct. S
CNTs
700
800
900
avelength (nm)
Figure 5.12: UV/Vis spectra of dodecyl-SWCNTsa (18) and pristine SWCNTs.
175
5 Experimental Part
Dodecyl-SWCNTsb (19)
Into a dry, 1 l argon-purged three necked flask,
equipped with a gas inlet and a high-efficiency
condenser, 300 ml anhydrous ammonia were condensed at -70 ◦ C. 180 mg (26 mmol) lithium were
added to the liquid ammonia resulting in a dark
blue solution. To this mixture 30 mg (2.5 mmol of
carbon) of HiPCO SWCNTs were added leading
to a black solution of the SWCNTs. Stirring was
continued for 1 h. 3 ml (12 mmol) 1-iodododecane were then added dropwise and the reaction
mixture was stirred overnight with the slow evaporation of the ammonia. 100 ml water were
added carefully to the black solid. After acidification with 20 ml HCl (10%) the nanotubes
were extracted with 200 ml hexane and washed twice with 100 ml water. The organic phase
was filtered through a 0.2 µm PTFE membrane filter and washed with ethanol and THF. The
resulting black solid was dried in a vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
core level
Binding energy (eV)
C
C 1s
284.5
• UV/Vis (D2 O/LDS): λmax = -.
• TGA (weight loss): 30.8%.
• Raman shift (λex =514.5 nm): 1590.1, 1580.1, 1555.5, 1524.7, 1331.8, 270.1, 262.1,
247.7 cm−1 .
• ID /IG : 0.235.
• Raman shift (λex =514.5 nm), (annealed): 1594.7, 1565.9, 1532.5, 1515.2, 1334.3, 268.3,
261.9, 249.1 cm−1 .
• Estimated degree of functionalization (TGA): 3.2%.
176
5.3 Experimental Details
Figure 5.13: TEM images of dodecyl-SWCNTsb (19).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.14: Raman spectra (λex 514.5 nm) of dodecyl-SWCNTsb (19), annealed dodecyl-SWCNTsb
and pristine SWCNTs.
W
W
HiPCO S
CNTs
700
800
Absorbance (a. u.)
200
300
400
W
500
600
CNTs
funct. S
900
avelength (nm)
Figure 5.15: UV/Vis spectra of dodecyl-SWCNTsb (19) and pristine SWCNTs.
177
5 Experimental Part
Dodecyl-SWCNTsc (20)
In a dry, 1 l nitrogen-purged three necked flask,
equipped with a gas inlet and a high-efficiency
condenser, 100 ml anhydrous ammonia were
dissolved in 200 ml anhydrous THF at -70 ◦ C.
140 mg (6 mmol) sodium were added to the
liquid ammonia resulting in a dark blue solution.
To this mixture 30 mg (2.5 mmol of carbon) of
HiPCO SWCNTs were added leading to a black
solution of the SWCNTs in liquid ammonia. Stirring was continued for 1 h. 3 ml (12 mmol)
1-iodododecane were then added dropwise and the reaction mixture was stirred overnight with
the slow evaporation of the ammonia. 100 ml water were added carefully to the black solid.
After acidification with 20 ml HCl (10%) the nanotubes were extracted with 200 ml hexane
and washed twice with 100 ml water. The organic phase was filtered through a 0.2 µm PTFE
membrane filter and washed with ethanol and CH2 Cl2 . The resulting black solid was dried in a
vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
core level
Binding energy (eV)
C
C 1s
284.6
• UV/Vis (D2 O/LDS): λmax = 819, 739, 655, 605, 562, 454, 410, 380, 272, 236 nm.
• TGA (weight loss): 12.6%.
• Raman shift (λex =514.5 nm): 1595.7, 1568.6, 1556.0, 1517.8, 1330.5, 271.6, 266.7,
250.7 cm−1 .
• ID /IG : 0.046.
• Raman shift (λex =514.5 nm), (annealed): 1593.1, 1575.3, 1522.1, 1494.7, 1331.0, 269.9,
263.5, 247.4 cm−1 .
• Estimated degree of functionalization (TGA): 1.0%.
178
5.3 Experimental Details
Figure 5.16: TEM images of dodecyl-SWCNTsc (20).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.17: Raman spectra (λex 514.5 nm) of dodecyl-SWCNTsc (20), annealed dodecyl-SWCNTsc
and pristine SWCNTs.
W
W
HiPCO S
CNTs
700
800
Absorbance (a. u.)
200
300
400
W
500
600
CNTs
funct. S
900
avelength (nm)
Figure 5.18: UV/Vis spectra of dodecyl-SWCNTsc (20) and pristine SWCNTs.
179
5 Experimental Part
Dodecyl-SWCNTsd (21)
In a dry, 1 l nitrogen-purged three necked flask,
equipped with a gas inlet and a reflux condenser,
140 mg (6 mmol) sodium were added to 100 ml
anhydrous 1,2-diaminoethane at room temperature.
To this colorless suspension 30 mg (2.5 mmol of
carbon) of HiPCO SWCNTs were added leading
to a black suspension of the SWCNTs. After 2 h
at room temperature the suspension was heated to
130 ◦ C for 4 h. At the elaborated temperature the sodium melts to small droplets. When the
reaction mixture has reached room temperature 3 ml (12 mmol) 1-iodododecane were added
dropwise to the reaction mixture. Stirring was continued overnight at room temperature. To
quench the excess of sodium, the mixture was cooled on an ice bath and 100 ml water were
added carefully. After acidification with 20 ml HCl (10%) the nanotubes were extracted with
200 ml hexane and washed twice with 100 ml water. The organic phase was filtered through a
0.2 µm PTFE membrane filter and washed with acetone. The resulting black solid was dried in
a vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
core level
Binding energy (eV)
C
C 1s
284.8
• UV/Vis (D2 O/LDS): λmax = 896, 829, 747, 659, 606, 562, 513, 454, 417 nm.
• TGA (weight loss): 19.4%.
• Raman shift (λex =514.5 nm): 1592.2, 1569.8, 1512.7, 1330.8, 268.3, 263.5, 249.1 cm−1 .
• ID /IG : 0.017.
• Raman shift (λex =514.5 nm), (annealed): 1594.0, 1579.8, 1543.0, 1511.7, 1333.6, 271.6,
265.1, 252.3 cm−1 .
• Estimated degree of functionalization (TGA): 1.7%.
180
5.3 Experimental Details
Figure 5.19: TEM images of dodecyl-SWCNTsd (21).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.20: Raman spectra (λex 514.5 nm) of dodecyl-SWCNTsd (21), annealed dodecyl-SWCNTsd
and pristine SWCNTs.
W
W
HiPCO S
Absorbance (a. u.)
funct. S
200
300
400
W
500
600
700
CNTs
CNTs
800
900
avelength (nm)
Figure 5.21: UV/Vis spectra of dodecyl-SWCNTsd (21) and pristine SWCNTs.
181
5 Experimental Part
Dodecyl-SWCNTse (22)
In a dry, 1 l nitrogen-purged three necked flask,
equipped with a gas inlet and a reflux condenser,
140 mg (6 mmol) sodium were added to a mixture
of 100 ml anhydrous 1,2-diaminoethane and 200 ml
anhydrous morpholine at room temperature. To this
colorless suspension 30 mg (2.5 mmol of carbon)
of HiPCO SWCNTs were added leading to a black
suspension of the SWCNTs. After 3 d stirring at
room temperature the suspension turned into a black solution. 3 ml (12 mmol) 1-iodododecane
were added dropwise to the reaction mixture and stirring was continued overnight at room
temperature. To quench the excess of sodium, the mixture was cooled on an ice bath and 20 ml
water were added carefully. The solvent was then removed in vacuo and the black residue was
redissolved in a 100 ml hexane. The suspension was washed twice with 100 ml water. The
organic phase was filtered through a 0.2 µm PTFE membrane filter and washed with acetone.
The resulting black solid was dried in a vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
core level
Binding energy (eV)
C
C 1s
284.7
• UV/Vis (D2 O/LDS): λmax = 830, 752, 664, 610, 562, 516, 456, 415 nm.
• TGA (weight loss): 6.0%.
• Raman shift (λex =514.5 nm): 1590.8, 1579.2, 1553.7, 1513.8, 1328.4, 265.3, 258.9,
246.0 cm−1 .
• ID /IG : 0.028.
• Raman shift (λex =514.5 nm), (annealed): 1595.9, 1587.6, 1584.5, 1506.8, 1336,1, 277.4,
271.3, 265.2 cm−1 .
• Estimated degree of functionalization (TGA): 0.5%.
182
5.3 Experimental Details
Figure 5.22: TEM images of dodecyl-SWCNTse (22).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.23: Raman spectra (λex 514.5 nm) of dodecyl-SWCNTse (22), annealed dodecyl-SWCNTse
and pristine SWCNTs.
W
W
HiPCO S
CNTs
700
800
Absorbance (a. u.)
200
300
400
W
500
600
CNTs
funct. S
900
avelength (nm)
Figure 5.24: UV/Vis spectra of dodecyl-SWCNTse (22) and pristine SWCNTs.
183
5 Experimental Part
Dodecyl-SWCNTsf (23)
Into a dry, 1 l nitrogen-purged three necked flask,
equipped with a gas inlet and a high-efficiency
condenser, 300 ml anhydrous ammonia were
condensed at -70 ◦ C . 500 mg (21.7 mmol) sodium
were added to the liquid ammonia resulting in
a dark blue solution.
To this mixture 30 mg
(2.5 mmol of carbon) of HiPCO SWCNTs were
added leading to a black solution of the SWCNTs
in liquid ammonia. Stirring was continued for 1 h. 3 ml (12 mmol) 1-iodododecane were then
added dropwise and the reaction mixture was stirred overnight with the slow evaporation of the
ammonia. 100 ml water were added carefully to the black solid. After acidification with 20 ml
HCl (10%) the nanotubes were extracted with 200 ml hexane and washed twice with 100 ml
water. The organic phase was filtered through a 0.2 µm PTFE membrane filter and washed with
ethanol and THF. The resulting black solid was dried in a vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
core level
Binding energy (eV)
C
C 1s
284.7
• UV/Vis (D2 O/LDS): λmax = 602, 559, 414 nm.
• TGA (weight loss): 23.0%.
• Raman shift (λex =514.5 nm): 1590.9, 1563.2, 1551.4, 1515.8, 1329.1, 268.5, 262.1,
246.2 cm−1 .
• ID /IG : 0.184.
• Raman shift (λex =514.5 nm), (annealed): 1591.9, 1567.3, 1533.2, 1500.8, 1330.5, 266.9,
260.5, 244.4, 181.7 cm−1 .
• Estimated degree of functionalization (TGA): 2.2%.
184
5.3 Experimental Details
Figure 5.25: TEM images of dodecyl-SWCNTsf (23).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.26: Raman spectra (λex 514.5 nm) of dodecyl-SWCNTsf (23), annealed dodecyl-SWCNTsf
and pristine SWCNTs.
W
W
Absorbance (a. u.)
HiPCO S
200
300
400
W
500
600
CNTs
funct. S
CNTs
700
800
900
avelength (nm)
Figure 5.27: UV/Vis spectra of dodecyl-SWCNTsf (23) and pristine SWCNTs.
185
5 Experimental Part
Octyl-SWCNTsa (24)
Into a dry, 1 l nitrogen-purged three necked flask,
equipped with a gas inlet and a high-efficiency condenser, 300 ml anhydrous ammonia were condensed at
-70 ◦ C. 140 mg (6 mmol) sodium were added to the
liquid ammonia resulting in a dark blue solution. To this
mixture 30 mg (2.5 mmol of carbon) of HiPCO SWCNTs
were added leading to a black solution of the SWCNTs
in liquid ammonia. Stirring was continued for 1 h. 2.2 ml (12 mmol) 1-iodooctane were then
added dropwise and the reaction mixture was stirred overnight with the slow evaporation of the
ammonia. 100 ml water were added carefully to the black solid. After acidification with 20 ml
HCl (10%) the nanotubes were extracted with 200 ml hexane and washed twice with 100 ml
water. The organic phase was filtered through a 0.2 µm PTFE membrane filter and washed with
ethanol and THF. The resulting black solid was dried in a vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
core level
Binding energy (eV)
C
C 1s
284.6
• UV/Vis (D2 O/LDS): λmax = 743, 655, 605, 560, 453, 414, 383 nm.
• TGA (weight loss): 12.5%.
• Raman shift (λex =514.5 nm): 1590.9, 1565.8, 1549.6, 1519.3, 1330.5, 270.1, 262.1,
246.1 cm−1 .
• ID /IG : 0.087.
• Raman shift (λex =514.5 nm), (annealed): 1592.9, 1568.3, 1549.4, 1511.0, 1337.7, 268.3,
261.9, 247.4 cm−1 .
• Estimated degree of functionalization (TGA): 4.2%.
186
5.3 Experimental Details
Figure 5.28: TEM images of octyl-SWCNTsa (24).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.29: Raman spectra (λex 514.5 nm) of octyl-SWCNTsa (24), annealed octyl-SWCNTsa and
pristine SWCNTs.
W
W
HiPCO S
CNTs
700
800
Absorbance (a. u.)
200
300
400
W
500
600
CNTs
funct. S
900
avelength (nm)
Figure 5.30: UV/Vis spectra of octyl-SWCNTsa (24) and pristine SWCNTs.
187
5 Experimental Part
Octyl-SWCNTsb (25)
Into a dry, 1 l nitrogen-purged three necked flask,
equipped with a gas inlet and a high-efficiency condenser, 300 ml anhydrous ammonia were condensed at
-70 ◦ C. 180 mg (26 mmol) lithium were added to the
liquid ammonia resulting in a dark blue solution. To this
mixture 30 mg (2.5 mmol of carbon) of HiPCO SWCNTs
were added leading to a black solution of the SWCNTs
in liquid ammonia. Stirring was continued for 1 h. 3 ml (12 mmol) 1-iodododecane were then
added dropwise and the reaction mixture was stirred overnight with the slow evaporation of the
ammonia. 100 ml water were added carefully to the black solid. After acidification with 20 ml
HCl (10%) the nanotubes were extracted with 200 ml hexane and washed twice with 100 ml
water. The organic phase was filtered through a 0.2 µm PTFE membrane filter and washed with
ethanol and THF. The resulting black solid was dried in a vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
core level
Binding energy (eV)
C
C 1s
284.6
• UV/Vis (D2 O/LDS): λmax = 748, 653, 606, 560, 452, 416, 381 nm.
• TGA (weight loss): 12.4%.
• Raman shift (λex =514.5 nm): 1592.4, 1569.6, 1515.6, 1332.5, 270.1, 263.7, 247.7 cm−1 .
• ID /IG : 0.072.
• Raman shift (λex =514.5 nm), (annealed): 1593.6, 1566.1, 1539.2, 1499.9, 1333.0, 269.9,
266.7, 261.9, 247.4 cm−1 .
• Estimated degree of functionalization (TGA): 4.0%.
188
5.3 Experimental Details
Figure 5.31: TEM images of octyl-SWCNTsb (25).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.32: Raman spectra (λex 514.5 nm) of octyl-SWCNTsb (25), annealed octyl-SWCNTsb and
pristine SWCNTs.
W
W
HiPCO S
CNTS
700
800
Absorbance (a. u.)
200
300
400
W
500
600
CNTs
funct. S
900
avelength (nm)
Figure 5.33: UV/Vis spectra of octyl-SWCNTsb (25) and pristine SWCNTs.
189
5 Experimental Part
t-Butyl-SWCNTs (26)
Into a dry, 1 l nitrogen-purged three necked flask,
equipped with a gas inlet and a high-efficiency condenser, 300 ml anhydrous ammonia were condensed at
-70 ◦ C. 300 mg (13 mmol) sodium were added to the
liquid ammonia resulting in a dark blue solution. To this
mixture 30 mg (2.5 mmol of carbon) of HiPCO SWCNTs
were added leading to a black solution of the SWCNTs in
liquid ammonia. Stirring was continued for 1 h. 1.4 ml (12 mmol) 2-bromo-2-methylpropane
were then added dropwise and the reaction mixture was stirred overnight with the slow
evaporation of the ammonia. 100 ml water were added carefully to the black solid. After
acidification with 20 ml HCl (10%) the nanotubes were extracted with 200 ml hexane and
washed twice with 100 ml water. The organic phase was filtered through a 0.2 µm PTFE
membrane filter and washed with ethanol and THF. The resulting black solid was dried in a
vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
core level
Binding energy (eV)
C
C 1s
284.7
• UV/Vis (D2 O/LDS): λmax = 657, 615, 562, 455, 416, 371 nm.
• TGA (weight loss): 15.2%.
• Raman shift (λex =514.5 nm): 1593.8, 1565.1, 1536.0, 1496.6, 1330.4, 268.5, 260.5, 244.4,
184.9 cm−1 .
• ID /IG : 0.083.
• Raman shift (λex =514.5 nm), (annealed): 1595.4, 1569.0, 1536.7, 1510.5, 1331.1, 270.2,
262.2, 249.3, 188.2 cm−1 .
• Estimated degree of functionalization (TGA): 4.2%.
190
5.3 Experimental Details
Figure 5.34: TEM images of t-butyl-SWCNTs (26).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.35: Raman spectra (λex 514.5 nm) of t-butyl-SWCNTs (26), annealed t-butyl-SWCNTs and
pristine SWCNTs.
W
W
HiPCO S
CNTs
700
800
Absorbance (a. u.)
200
300
400
W
500
600
CNTs
funct. S
900
avelength (nm)
Figure 5.36: UV/Vis spectra of t-butyl-SWCNTs (26) and pristine SWCNTs.
191
5 Experimental Part
3-(Tetrahydropyran-2-yloxy)propyl-SWCNTs (27)
Into a dry, 1 l nitrogen-purged three necked flask,
O
equipped with a gas inlet and a high-efficiency con-
O
denser, 300 ml anhydrous ammonia were condensed at
-70 ◦ C. 500 mg (22 mmol) sodium were added to the
liquid ammonia resulting in a dark blue solution. To
this mixture 30 mg (2,.5 mmol of carbon) of HiPCO
SWCNTs were added leading to a black solution of the
O
SWCNTs in liquid ammonia. Stirring was continued
O
for 1 h.
2.04 ml (12 mmol) 2-(3-bromopropoxy)-
tetrahydropyran were then added dropwise and the reaction mixture was stirred overnight with
the slow evaporation of the ammonia. 100 ml water were added carefully to the black solid.
The nanotubes were extracted with 200 ml hexane and washed twice with 100 ml water. The
organic phase was filtered through a 0.2 µm PTFE membrane filter and washed with ethanol
and THF. The resulting black solid was dried in a vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
86.5
C 1s
284.7
O
13.5
O 1s
532.7
• Estimated degree of functionalization (XPS): 7.3%.
• UV/Vis (D2 O/LDS): λmax = −.
• TGA (weight loss): 22.0%.
• Raman shift (λex =514.5 nm): 1593.9, 1574.2, 1560.8, 1529.1, 1330.8, 184.9 cm−1 .
• ID /IG : 0.516.
• Raman shift (λex =514.5 nm), (annealed): 1591.8, 1574.2, 1533.3, 1504.6, 1329.6, 265.3,
258.9, 244.4, 183.3 cm−1 .
• Estimated degree of functionalization (TGA): 5.3%.
192
5.3 Experimental Details
Figure 5.37: TEM images of 3-(tetrahydrofuran-2-yloxy)propyl-SWCNTs (27).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.38: Raman spectra (λex 514.5 nm) of 3-(tetrahydrofuran-2-yloxy)propyl-SWCNTs (27), annealed 3-(tetrahydrofuran-2-yloxy)propyl-SWCNTs and pristine SWCNTs.
W
W
HiPCO S
CNTs
700
800
Absorbance (a. u.)
200
300
400
W
500
600
CNTs
funct. S
900
avelength (nm)
Figure 5.39: UV/Vis spectra of 3-(tetrahydrofuran-2-yloxy)propyl-SWCNTs (27) and pristine
SWCNTs.
193
5 Experimental Part
2-(Methoxymethyl)-SWCNTs (28)
Into a dry, 1 l nitrogen-purged three necked flask,
O
equipped with a gas inlet and a high-efficiency condenser, 500 ml anhydrous ammonia were condensed at
-70 ◦ C. 800 mg (35 mmol) sodium were added to the
liquid ammonia resulting in a dark blue solution. To this
mixture 80 mg (6.7 mmol of carbon) of HiPCO SWCNTs
O
were added leading to a black solution of the SWCNTs
in liquid ammonia. Stirring was continued for 1 h. 2.6 ml (32 mmol) bromo(methoxy)methane
were then added dropwise and the reaction mixture was stirred overnight with the slow
evaporation of the ammonia. 100 ml water were added carefully to the black solid. After
acidification with 20 ml HCl (10%) the nanotubes were extracted with 200 ml hexane and
washed twice with 100 ml water. The organic phase was filtered through a 0.2 µm PTFE
membrane filter and washed with ethanol and THF. The resulting black solid was dried in a
vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
95.1
C 1s
284.7
O
4.9
O 1s
532.6
• Estimated degree of functionalization (XPS): 5.1%.
• UV/Vis (D2 O/LDS): λmax = 746, 659, 608, 561, 455, 415, 380 nm.
• TGA (weight loss): 6.2%.
• Raman shift (λex =514.5 nm): 1593.2, 1567.1, 1529.9, 1492.9, 1331.0, 268.5, 260.5, 244.4,
184.9 cm−1 .
• ID /IG : 0.076.
• Raman shift (λex =514.5 nm), (annealed): 1591.6, 1574.8, 1530.6, 1500.1, 1329.0, 265.3,
246.0, 181.7 cm−1 .
• Estimated degree of functionalization (TGA): 3.3%.
194
5.3 Experimental Details
Figure 5.40: TEM images of 2-(methoxymethyl)-SWCNTs (28).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.41: Raman spectra (λex 514.5 nm) of 2-(methoxymethyl)-SWCNTs (28), annealed 2(methoxymethyl)-SWCNTs and pristine SWCNTs.
W
W
HiPCO S
CNTs
700
800
Absorbance (a. u.)
200
300
400
W
500
600
CNTs
funct. S
900
avelength (nm)
Figure 5.42: UV/Vis spectra of 2-(methoxymethyl)-SWCNTs (28) and pristine SWCNTs.
195
5 Experimental Part
2-(2-(2-Methoxyethoxy)ethyl)-SWCNTs (29)
O
O
Into a dry, 1 l nitrogen-purged three necked flask,
equipped with a gas inlet and a high-efficiency condenser, 300 ml anhydrous ammonia were condensed at
-70 ◦ C. 500 mg (21.7 mmol) sodium were added to the
liquid ammonia resulting in a dark blue solution. To
this mixture 30 mg (2.5 mmol of carbon) of HiPCO
O
O
SWCNTs were added leading to a black solution of the
SWCNTs in liquid ammonia. Stirring was continued for 1 h. 1.63 ml (12 mmol) 1-bromo2-(2-methoxyethoxy)ethane were then added dropwise and the reaction mixture was stirred
overnight with the slow evaporation of the ammonia. 100 ml water were added carefully to
the black solid. After acidification with 20 ml HCl (10%) the nanotubes were extracted with
200 ml hexane and washed twice with 100 ml water. The organic phase was filtered through
a 0.2 µm PTFE membrane filter and washed with ethanol and THF. The resulting black solid
was dried in a vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
98.4
C 1s
284.8
O
1.6
O 1s
533.1
• Estimated degree of functionalization (XPS): 4.1%.
• UV/Vis (D2 O/LDS): λmax = 748, 660, 607, 559, 453, 417, 381 nm.
• TGA (weight loss): 17.8%.
• Raman shift (λex =514.5 nm): 1592.6, 1566.9, 1543.8, 1508.1, 1329.2, 264.9, 245.9,
184.0 cm−1 .
• ID /IG : 0.093.
• Raman shift (λex =514.5 nm), (annealed): 1593.6, 1566.1, 1560.8, 1499.6, 1330.0, 265.3,
260.5, 244.4, 183.3 cm−1 .
• Estimated degree of functionalization (TGA): 2.6%.
196
5.3 Experimental Details
Figure 5.43: TEM images of 2-(2-(2-methoxyethoxy)ethyl)-SWCNTs (29).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.44: Raman spectra (λex 514.5 nm) of 2-(2-(2-methoxyethoxy)ethyl)-SWCNTs (29), annealed
2-(2-(2-methoxyethoxy)ethyl)-SWCNTs and pristine SWCNTs.
W
W
HiPCO S
CNTs
700
800
Absorbance (a. u.)
200
300
400
W
500
600
CNTs
funct. S
900
avelength (nm)
Figure 5.45: UV/Vis spectra of 2-(2-(2-methoxyethoxy)ethyl)-SWCNTs (29) and pristine SWCNTs.
197
5 Experimental Part
Poly(ethylene glycol)-SWCNTs (30)
Into a dry, 1 l nitrogen-purged three necked flask,
O
O
n
equipped with a gas inlet and a high-efficiency
condenser, 300 ml anhydrous ammonia were condensed at -70 ◦ C. 140 mg (6 mmol) sodium were
added to the liquid ammonia resulting in a dark
blue solution. To this mixture 30 mg (2.5 mmol of
carbon) of HiPCO SWCNTs were added leading to
O
O
n
a black solution of the SWCNTs in liquid ammonia.
Stirring was continued for 1 h. 2 g (0.1 mmol) O-(2-bromoethyl)-O’methylpolyethylenglycol
(M≈20000 g/mol) dissolved in 50 ml THF(anhydrous) were then added dropwise and the
reaction mixture was stirred overnight with the slow evaporation of the ammonia. 100 ml water
were added carefully to the black solid. After acidification with 20 ml HCl (10%) the nanotubes
were extracted with 200 ml hexane and washed twice with 100 ml water. The organic phase
was filtered through a 0.2 µm PTFE membrane filter and washed with ethanol and THF. The
resulting black solid was dried in a vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
76.7
C 1s
284.6
O
23.3
O 1s
532.3
• Estimated degree of functionalization (XPS): 0.1%.
• UV/Vis (D2 O/LDS): λmax = 609, 560, 517, 453, 415, 381 nm.
• TGA (weight loss): 27.7%.
• Raman shift (λex =514.5 nm): 1589.2, 1564.8, 1530.1, 1496.4, 1327.6, 264.9, 258.6, 244.3,
183.9 cm−1 .
• ID /IG : 0.086.
• Raman shift (λex =514.5 nm), (annealed): 1592.3, 1570.1, 1547.8, 1509.9, 1331.8, 266.9,
260.5, 246.0 cm−1 .
• Estimated degree of functionalization (TGA): 0.2%.
198
5.3 Experimental Details
Figure 5.46: TEM images of poly(ethylene glycol)-SWCNTs (30).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.47: Raman spectra (λex 514.5 nm) of poly(ethylene glycol)-SWCNTs (30), annealed
poly(ethylene glycol)-SWCNTs and pristine SWCNTs.
W
W
HiPCO S
CNTs
700
800
Absorbance (a. u.)
200
300
400
W
500
600
CNTs
funct. S
900
avelength (nm)
Figure 5.48: UV/Vis spectra of poly(ethylene glycol)-SWCNTs (30) and pristine SWCNTs.
199
5 Experimental Part
(2-Cyanobenzyl)-SWCNTs (31)
Into a dry, 1 l nitrogen-purged three necked flask,
NC
equipped with a gas inlet and a high-efficiency condenser, 400 ml anhydrous ammonia were condensed at
-70 ◦ C. 500 mg (21.7 mmol) sodium were added to the
liquid ammonia resulting in a dark blue solution. To this
mixture 50 mg (4.2 mmol of carbon) of HiPCO SWCNTs
were added leading to a black solution of the SWCNTs in
liquid ammonia. Stirring was continued for 1 h. 3.92 mg
NC
(20 mmol) 2-(bromomethyl)benzonitrile dissolved in
10 ml THF(anhydrous) were then added dropwise and the reaction mixture was stirred overnight
with the slow evaporation of the ammonia. 100 ml water were added carefully to the black
solid. After acidification with 20 ml HCl (10%) the nanotubes were extracted with 200 ml
hexane and washed twice with 100 ml water. The organic phase was filtered through a 0.2 µm
PTFE membrane filter and washed with ethanol and THF. The resulting black solid was dried
in a vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
95.6
C 1s
284.5
N
4.4
N 1s
399.0
• UV/Vis (D2 O/LDS): λmax = 815, 740, 657, 603, 559, 512, 453, 415, 380, 270 nm.
• TGA (weight loss): 19.2%.
• Raman shift (λex =514.5 nm): 1588.9, 1562.6, 1526.1, 1494.0, 1333.1, 267.0, 260.5,
246.1 cm−1 .
• ID /IG : 0.056.
• Raman shift (λex =514.5 nm), (annealed): 1589.3, 1563.8, 1512.3, 1335.3, 263.4, 257.0,
242.5 cm−1 .
• Estimated degree of functionalization (TGA): 2.5%.
200
5.3 Experimental Details
Figure 5.49: TEM images of (2-cyanobenzyl)-SWCNTs (31).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.50: Raman spectra (λex 514.5 nm) of (2-cyanobenzyl)-SWCNTs (31), annealed (2cyanobenzyl)-SWCNTs and pristine SWCNTs.
W
W
HiPCO S
CNTs
700
800
Absorbance (a. u.)
200
300
400
W
500
600
CNTs
funct. S
900
avelength (nm)
Figure 5.51: UV/Vis spectra of (2-cyanobenzyl)-SWCNTs (31) and pristine SWCNTs.
201
5 Experimental Part
(3-Cyanopropyl)-SWCNTs (32)
CN
Into a dry, 1 l nitrogen-purged three necked flask,
equipped with a gas inlet and a high-efficiency condenser, 400 ml anhydrous ammonia were condensed at
-70 ◦ C. 500 mg (21.7 mmol) sodium were added to the
liquid ammonia resulting in a dark blue solution. To this
mixture 50 mg (4.2 mmol of carbon) of HiPCO SWCNTs
were added leading to a black solution of the SWCNTs
in liquid ammonia. Stirring was continued for 1 h. 2 ml
CN
(20 mmol) 4-bromobutanenitrile were then added dropwise and the reaction mixture was stirred
overnight with the slow evaporation of the ammonia. 100 ml water were added carefully to
the black solid. After acidification with 20 ml HCl (10 %) the nanotubes were extracted with
200 ml hexane and washed twice with 100 ml water. The organic phase was filtered through
a 0.2 µm PTFE membrane filter and washed with ethanol and THF. The resulting black solid
was dried in a vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
98.6
C 1s
284.6
N
1.4
N 1s
399.8
• UV/Vis (D2 O/LDS): λmax = 814, 735, 605, 560, 413, 379 nm.
• TGA (weight loss): 17.3%.
• Raman shift (λex =514.5 nm): 1589.4, 1564.8, 1529.5, 1500.1, 1328.8, 265.1, 258.6, 242.5,
183.1 cm−1 .
• ID /IG : 0.115.
• Raman shift (λex =514.5 nm), (annealed): 1592.9, 1568.3, 1546.5, 1509.2, 1334.8, 268.6,
262.2, 246.1, 185.0 cm−1 .
• Estimated degree of functionalization (TGA): 4.0%.
202
5.3 Experimental Details
Figure 5.52: TEM images of (3-cyanopropyl)-SWCNTs (32).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.53: Raman spectra (λex 514.5 nm) of (3-cyanopropyl)-SWCNTs (32), annealed (3cyanopropyl)-SWCNTs and pristine SWCNTs.
W
W
HiPCO S
CNTs
700
800
Absorbance (a. u.)
200
300
400
W
500
600
CNTs
funct. S
900
avelength (nm)
Figure 5.54: UV/Vis spectra of (3-cyanopropyl)-SWCNTs (32) and pristine SWCNTs.
203
5 Experimental Part
2-(2-(2-Methoxyethoxy)ethyl)-A MWCNTs (33)
O
Into a dry, 1 l nitrogen-purged three necked flask,
O
equipped with a gas inlet and a high-efficiency
condenser, 500 ml anhydrous ammonia were
condensed at -70 ◦ C . 2 g (87 mmol) sodium were
added to the liquid ammonia resulting in a dark
blue solution. To this mixture 50 mg (4.2 mmol
of carbon) of A MWCNTs were added leading to a
O
O
black solution of the SWCNTs in liquid ammonia.
Stirring was continued for 1 h. 1.63 ml (12 mmol) 1-bromo-2-(2-methoxyethoxy)ethane were
then added dropwise and the reaction mixture was stirred overnight with the slow evaporation
of the ammonia. 100 ml water were added carefully to the black solid. After acidification with
20 ml HCl (10%) the nanotubes were extracted with 200 ml hexane and washed twice with
100 ml water. The organic phase was filtered through a 0.2 µm PTFE membrane filter and
washed with ethanol and THF. The resulting black solid was dried in a vacuum oven at 50 ◦ C
overnight.
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
93.8
C 1s
284.6
O
6.2
O 1s
533
• Estimated degree of functionalization (XPS, SWCNT): 0.7%.
• UV/Vis (D2 O/LDS): λmax = 260 nm.
• TGA (weight loss): 7.6%.
• Raman shift (λex =514.5 nm): 1585.8, 1346.1 cm−1 .
• ID /IG : 1.380.
• Raman shift (λex =514.5 nm), (annealed): 1599.5, 1352.0 cm−1 .
• Estimated degree of functionalization (TGA, SWCNT): 1.0%.
204
5.3 Experimental Details
Figure 5.55: TEM images of 2-(2-(2-methoxyethoxy)ethyl)-A MWCNTs (33).
A
MWCNTs
Intensity (a. u.)
funct.
A
MWCNTs
annealed
200
400
600
A
MWCNTs
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.56: Raman spectra (λex 514.5 nm) of 2-(2-(2-methoxyethoxy)ethyl)-A MWCNTs (33), annealed 2-(2-(2-methoxyethoxy)ethyl)-A MWCNTs and pristine A MWCNTs.
A
WCNT
M
s
Absorbance (a. u.)
funct.
200
300
400
W
500
600
700
800
900
avelength (nm)
Figure 5.57: UV/Vis spectra of 2-(2-(2-Methoxyethoxy)ethyl)-MWCNTsa (33).
205
5 Experimental Part
2-(2-(2-Methoxyethoxy)ethyl)-D MWCNTs (34)
O
Into a dry, 1 l nitrogen-purged three necked flask,
O
equipped with a gas inlet and a high-efficiency
condenser, 500 ml anhydrous ammonia were condensed at -70 ◦ C. 2.5 g (109 mmol) sodium were
added to the liquid ammonia resulting in a dark
blue solution. To this mixture 50 mg (4.2 mmol
of carbon) of D MWCNTs were added leading to a
O
O
black solution of the SWCNTs in liquid ammonia.
Stirring was continued for 1 h. 1.63 ml (12 mmol) 1-bromo-2-(2-methoxyethoxy)ethane were
then added dropwise and the reaction mixture was stirred overnight with the slow evaporation
of the ammonia. 100 ml water were added carefully to the black solid. After acidification with
20 ml HCl (10%) the nanotubes were extracted with 200 ml hexane and washed twice with
100 ml water. The organic phase was filtered through a 0.2 µm PTFE membrane filter and
washed with ethanol and THF. The resulting black solid was dried in a vacuum oven at 50 ◦ C
overnight.
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
92.9
C 1s
284.6
O
7.1
O 1s
533
• Estimated degree of functionalization (XPS, SWCNT): 1.0%.
• UV/Vis (D2 O/LDS): λmax = 265.
• TGA (weight loss: 8.8%.
• Raman shift (λex =514.5 nm): 1576.3, 1334.9 cm−1 .
• ID /IG : 0.903.
• Raman shift (λex =514.5 nm), (annealed): 1583.4, 1355.2 cm−1 .
• Estimated degree of functionalization (TGA, SWCNT): 1.1%.
206
5.3 Experimental Details
Figure 5.58: TEM images of 2-(2-(2-methoxyethoxy)ethyl)-D MWCNTs (34).
B
MWCNTs
funct.
B
MWCNTs
B
MWCNTs
Intensity (a. u.)
annealed
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.59: Raman spectra (λex 514.5 nm) of 2-(2-(2-methoxyethoxy)ethyl)-D MWCNTs (34), annealed 2-(2-(2-methoxyethoxy)ethyl)-D MWCNTs and pristine B MWCNTs.
D
WCNT
M
s
Absorbance (a. u.)
funct.
200
300
400
W
500
600
700
800
900
avelength (nm)
Figure 5.60: UV/Vis spectra of 2-(2-(2-methoxyethoxy)ethyl)-D MWCNTs (34).
207
5 Experimental Part
3-Hydroxypropyl-SWCNTs (35)
In a 250 ml flask with a mixture of 50 ml THF and
OH
10 ml HCl (conc.), 10 mg 3-(tetrahydrofuran-2yloxy)propyl-SWCNTs (27) were dissolved. The
suspension was sonicated for 3 h and stirred at
room temperature overnight. 100 ml hexane were
added to the solution to precipitate the SWCNTs.
OH
The solution was filtered through a 0.2 µm PTFE membrane filter and washed with ethanol and
THF. The resulting black solid was dried in a vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
93.6
C 1s
284.6
O
6.4
O 1s
532.7
• Estimated degree of functionalization (XPS): 7.3%.
• UV/Vis (D2 O/LDS): λmax = −.
• TGA (weight loss): 20.5%.
• Raman shift (λex =514.5 nm): 1592.9, 1565.7, 1548.8, 1523.9, 1331.5, 260.2, 184.0 cm−1 .
• ID /IG : 0.249.
• Raman shift (λex =514.5 nm), (annealed): 1593.4, 1570.4, 1523.9, 1495.5, 1331.0, 266.9,
260.5, 247.6, 185.0 cm−1 .
• Estimated degree of functionalization (TGA): 5.7%.
208
5.3 Experimental Details
Figure 5.61: TEM images of 3-hydroxypropyl-SWCNTs (35).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.62: Raman spectra (λex 514.5 nm) of 3-hydroxypropyl-SWCNTs (35), annealed 3hydroxypropyl-SWCNTs and pristine SWCNTs.
W
W
HiPCO S
CNTs
700
800
Absorbance (a. u.)
200
300
400
W
500
600
CNTs
funct. S
900
avelength (nm)
Figure 5.63: UV/Vis spectra of 3-hydroxypropyl-SWCNTs-SWCNTs (35) and pristine SWCNTs.
209
5 Experimental Part
Hydroxymethyl-SWCNTs (36)
OH
Into a 250 ml nitrogen-purged round bottom flask,
equipped with a pressure compensation, 20 mg
2-(methoxymethyl)-SWCNTs (28) were dissolved
in 40 ml anhydrous benzene. The suspension was
sonicated for 1h to give a dark black solution.
OH
0.2 ml (2.1 mmol) boron tribromide were added
dropwise to the solution leading to the precipitation of the SWCNTs. The suspension was
sonicated for 1 h to complete the reaction. After quenching the reaction by the addtition of
50 ml water the SWCNTs were extracted with 100 ml hexane and washed twice with 100 ml
water. The organic phase was filtered through a 0.2 µm PTFE membrane filter and washed with
ethanol and THF. The resulting black solid was dried in a vacuum oven at 50 ◦ C overnight.
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
92.5
C 1s
284.6
O
7.5
O 1s
532.9
• Estimated degree of functionalization (XPS): 3.0%.
• UV/Vis (D2 O/LDS): 745, 654, 610, 562, 506, 412 nm.
• TGA (weight loss): −.
• Raman shift (λex =514.5 nm): 1595.1, 1569.0, 1527.5, 1493.9, 1333.0, 266.9, 249.3,
186.6 cm−1 .
• ID /IG : 0.032.
• Raman shift (λex =514.5 nm), (annealed): 1591.4, 1334.2, 266.8, 260.5, 244.4 cm−1 .
• Estimated degree of functionalization (TGA): −.
210
5.3 Experimental Details
Figure 5.64: TEM images of hydroxymethyl-SWCNTs (36).
HiPCO SWCNTs
funct. SWCNTs
Intensity (a. u.)
annealed SWCNTs
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.65: Raman spectra (λex 514.5 nm) of hydroxymethyl-SWCNTs (36), annealed hydroxymethylSWCNTs and pristine SWCNTs.
W
W
HiPCO S
CNTs
700
800
Absorbance (a. u.)
200
300
400
W
500
600
CNTs
funct. S
900
avelength (nm)
Figure 5.66: UV/Vis spectra of hydroxymethyl-SWCNTs (36) and pristine SWCNTs.
211
5 Experimental Part
3-(Thiophene-2-carboxylate)propyl-SWCNTs (37)
O
S
O
In a 250 ml flask, 30 mg of 35 were dissolved in
30 ml anhydrous THF. To this solution 3 ml of
2-thiophenecarbonylchloride were added and the
resulting reaction mixture was subsequently stirred
at room temperature for 2 d. After this time the
solvent was evaporated under reduced pressure,
the solid was redissolved in a mixture of 50 ml
O
S
O
water and 50 ml ethanol.
The suspension was
filtered through a 0.2 µm PTFE membrane filter
and washed with ethanol and THF. The resulting black solid was dried in a vacuum oven at
50 ◦ C overnight.
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
87.4
C 1s
284.5
O
9.1
O 1s
532.5
S
3.5
S 2p
164
• Estimated degree of functionalization (XPS): 5.8%.
• TGA (weight loss): 21.66%.
• Raman shift (λex =514.5 nm): 1593.1, 1422.7, 1339.1, 271.8, 263.8, 249.3 cm−1 .
• ID /IG : 0.106.
• Raman shift (λex =514.5 nm), (annealed): 1588.8, 1333.1, 263.6, 257.2, 241.2 cm−1 .
• Estimated degree of functionalization (TGA): 2.6%.
• EA: found C 61.63; H 1.46; S 9.36.
212
5.3 Experimental Details
Figure 5.67: TEM images of 3-(thiophene-2-carboxylate)propyl-SWCNTs (37).
HiPCO SWCNTs
35
37
37
Intensity (a. u.)
annealed
200
400
600
800
R
1000
1200
1400
1600
1800
1
-
aman Shift (cm )
Figure 5.68: Raman spectra (λex 514.5 nm) of 3-(thiophene-2-carboxylate)propyl-SWCNTs (37), annealed 3-(thiophene-2-carboxylate)propyl-SWCNTs and pristine SWCNTs.
213
5 Experimental Part
Fullerene-SWCNTs (38)
In
O
O
O
C3H6 O
a
250
ml
flask
with
a
mixture
of
10 ml TCE and 20 ml of CHCl3 , 100 mg
(0.105 mmol) of 1,2-((6-hydroxy-6-oxyhexyloxycarbonyl)acetatmethylester)methano-1,2-
O
dihydro[60]fulleren were dissolved under nitrogen
O
C3H6
atmosphere. To this solution 2 ml SOCl2 were
added and the resulting reaction mixture was subsequently heated to 60 ◦ C and stirred for one hour.
After the heating the mixture was allowed to reach
room temperature. To this solution a suspension of
16 mg 35 in TCE was added and the mixture was stirred for 4 d. After this time the SOCl2 was
evaporated under reduced pressure, the resulting solid was redissolved in a mixture of 50 ml
water and 50 ml ethanol. The suspension was filtered through a 0.2 µm PTFE membrane filter
and washed with ethanol and THF. The resulting black solid was dried in a vacuum oven at
50 ◦ C overnight.
• XPS (solid film):
Element
at.%
core level
Binding energy (eV)
C
77.8
C 1s
284.4
O
22.2
O 1s
535.2
• Vis/nIR (D2 O/LDS): 615, 564, 457, 382, 261 nm.
• TGA (weight loss): 18.5%.
• Raman shift (λex =514.5 nm): 1595.9, 1580.5, 1544.3, 1529.4, 1336.5, 267.0, 251.0 cm−1 .
• ID /IG : 0.063.
• Raman shift (λex =514.5 nm), (annealed): 1590.2, 1567.2, 1511.5, 1331.9, 257.0,
244.4 cm−1 .
• Estimated degree of functionalization (TGA): 1.0%.
214
5.3 Experimental Details
Figure 5.69: TEM images of fullerene-SWCNTs (38).
Figure 5.70: Raman spectra (λex 514.5 nm) of fullerene-SWCNTs (38), annealed fullerene-SWCNTsSWCNTs and pristine SWCNTs.
W
W
HiPCO S
CNTs
700
800
Absorbance (a. u.)
200
300
400
W
500
600
CNTs
funct. S
900
avelength (nm)
Figure 5.71: UV/Vis spectra of fullerene-SWCNTs (38) and pristine SWCNTs.
215
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228
7 Appendix
229
Compound numbers
Compound numbers
1 Ethoxycarbonylaziridino-A SWCNTs
2 Ethoxycarbonylaziridino-B SWCNTs
3 Ethoxycarbonylaziridino-C SWCNTs
4 t-Butyl-H-SWCNTs
5 n-Butyl-H-SWCNTs
6 Phenyl-H-SWCNTs
7 t-Butyl-H-SWCNTsa
8 t-Butyl-H-SWCNTsb
9 t-Butyl-H-SWCNTsc
10 t-Butyl-iodo-SWCNTs
11 t-Butyl-cyano-SWCNTs
12 t-Butyl-carboxyl-SWCNTs
13 t-Butyl-poly(t-butyl acrylate)-SWCNTs
14 t-Butyl-polyacrylnitrile-SWCNTs
15 8-Iodooctyl-SWCNTsa
16 8-Iodooctyl-SWCNTsb
17 8-Iodooctyl-SWCNTsc
18 Dodecyl-SWCNTsa
19 Dodecyl-SWCNTsb
20 Dodecyl-SWCNTsc
21 Dodecyl-SWCNTsd
22 Dodecyl-SWCNTse
23 Dodecyl-SWCNTsf
24 Octyl-SWCNTsa
25 Octyl-SWCNTsb
26 t-Butyl-SWCNTs
27 3-(Tetrahydropyran-2-yloxy)propyl-SWCNTs
28 2-(Methoxymethyl)-SWCNTs
29 2-(2-(2-Methoxyethoxy)ethyl)-SWCNTs
230
Compound numbers
30 Poly(ethylene glycol)-SWCNTs
31 (2-Cyanobenzyl)-SWCNTs
32 (3-Cyanopropyl)-SWCNTs
33 2-(2-(2-Methoxyethoxy)ethyl)-A MWCNTs
34 2-(2-(2-Methoxyethoxy)ethyl)-D MWCNTs
35 3-Hydroxypropyl-SWCNTs
36 Hydroxymethyl-SWCNTs
37 3-(Thiophene-2-carboxylate)propyl-SWCNTs
38 Fullerene-SWCNTs
231
Danksagung
Mein besonderer Dank gilt an dieser Stelle meinem Doktorvater Prof. Dr. Andreas Hirsch
für sein Interesse am Fortgang dieser Arbeit und für die Möglichkeit die Themenstellung
weitgehend selbstständig ausgestalten zu können.
Des Weiteren möchte ich mich für die tatkräftige und freundliche Unterstützung zahlreicher
Institutsangehöriger bedanken, ohne deren Hilfe diese Arbeit in der hier vorliegenden Form
nicht möglich gewesen wäre. Hervorzuheben sind hierbei: Frau E. Erhardt, Dr. G. Joachim,
Dr. R. Zimmermann, Dr. O. Vostrowsky, W. Donaubauer, dem Werkstatt-Team Herrn
Schreier und Herrn Ruprecht (für die Erfüllung vieler Sonderwünsche), Frau Hergenröder
(für unzählige TGA-Messungen)und den Glasbläsern Herrn Fronius und Herrn Saberi.
Mein besonderer Dank gilt hier auch meinen beiden "Jung"räten Dr. M. Brettreich (für das
Asyl in seinem Büro) und Dr. M. T. Speck (für die zahllosen Verträge und die vergeblichen
Versuche mein Deutsch aufzupolieren!!).
Ferner möchte ich mich bei vielen Angehörigen anderer Institute bedanken die mich bei
der Charakterisierung meiner Verbindungen tatkräftig unterstützt haben. Hervorzuheben
sind hier: Dr. Ralf Graupner (für die vielen Mess-Sessions, hätten ruhig noch ein paar mehr
sein können!!), Andrea Vencelova (für die schönen STM-Bilder), Peter Laufer (für die
nervigsten Diskussionen), Jonas Röhrl (für seine Begeisterung bei den Ramanmessungen),
Dr. G. Frank, H. Mahler und I. Brauer (für Ihre Geduld bei der TEM-Einweisung und den
zahlreichen Fragen) und dem Raman-Team bei WTM.
232
Mein besonderer Dank gilt meinen Kollegen des Arbeitskreises für unvergessliche vier
Jahre: Domenico Balbinot (für die spaßige Studienzeit), Adrian Jung (für viele Diskussionen), Dr. Michael Holzinger (Jesus, für die tolle Vorarbeit), Dr. Nikos Chronakis (für
die Korrektur dieser Arbeit), Dr. Michael Kellermann, Dr. Christian Klinger, Dr. Stephan
Burghardt, Dr. Jürgen Schmidt, Dr. Alexander Franz, Dr. Boris Buschhaus, Dr. Liam
Sutton, Dr. Norbert Jux, Christian Kovacs, Jörg Dannhäuser, Uwe Hartnagel, Patrick Witte,
Stefan Jasinski, Siggi Eigler, Jörg Dannhäuser, Torsten Brandmüller, Daniela Jannasch,
Jutta Siebenkees, Katja Maurer, Kristine Hager, Miriam Becherer, Florian Beuerle, Torsten
Schunk, Nicolai Mooren, und dem ganzen Rest!!!
Mein größter Dank gilt jedoch meiner Freundin Kordelia Kolk, die mich während der
gesamten Schreibarbeiten unglaublich unterstützt hat und auch noch an mich geglaubt hat,
als ich schon am Boden war.
233
Index of Publications
Functionalization of Single-Walled Carbon Nanotubes with (R-)Oxycarbonyl Nitrenes
Holzinger, Michael; Abraham, Juergen; Whelan, Paul; Graupner, Ralf; Ley, Lothar;
Hennrich, Frank; Kappes, Manfred; Hirsch, Andreas. J. Am. Chem. Soc., 2003, 125, 8566.
Doping of single-walled carbon nanotube bundles by Bronsted acids
Graupner, Ralf; Abraham, Juergen; Vencelova, Andrea; Seyller, Thomas; Hennrich, Frank;
Kappes, Manfred M.; Hirsch, Andreas; Ley, Lothar. Phys. Chem. Chem. Phys., 2003, 5,
5472.
The effect of addend variation on the solubility of single-wall carbon nanotubes
Abraham, Juergen; Hirsch, Andreas; Hennrich, Frank; Kappes, Manfred; Dziakova,
Andrea; Graupner, Ralf; Ley, Lothar. Institute of Organic Chemistry, Erlangen, Germany.
AIP Conference Proceedings 633 (Structural and Electronic Properties of Molecular
Nanostructures), 2002, 92.
Covalent functionalization of arc discharge, laser ablation and HiPCO single-walled
carbon nanotubes
Abraham, Juergen; Whelan, Paul; Hirsch, Andreas; Hennrich, Frank; Kappes, Manfred;
Samaille, Damien; Bernier, Patrick; Vencelova, Andrea; Graupner, Ralf; Ley, Lothar. AIP
Conference Proceedings 685 (Molecular Nanostructures), 2003, 291.
Vibrational spectroscopy studies of single-walled carbon nanotubes subjected to different levels of nitric acid oxidation treatment
Whelan, Paul; Abraham, Juergen; Hirsch, Andreas; Graupner, Ralph; Dizakova, Andrea;
234
Hennrich, Frank; Kappes, Manfred; Forsyth, Jeffery. AIP Conference Proceedings 685
(Molecular Nanostructures), 2003, 189.
The purification of single-walled carbon nanotubes studied by x-ray induced photoelectron spectroscopy
Graupner, Ralf; Vencelova, Andrea; Ley, Lothar; Abraham, Juergen; Hirsch, Andreas;
Hennrich, Frank; Kappes, Manfred. AIP Conference Proceedings 685 (Molecular Nanostructures), 2003, 120.
Purification of single-walled carbon nanotubes studied by STM and STS
Vencelova, Andrea; Graupner, Ralf; Ley, Lothar; Abraham, Juergen; Holzinger, Michael;
Whelan, Paul; Hirsch, Andreas; Hennrich, Frank; Kappes, Manfred. AIP Conference
Proceedings 685 (Molecular Nanostructures), 2003, 112.
235
Curriculum Vitae
Personal Data
Name
Jürgen Leonhard Abraham
Date of birth
02.10.1973
Place of birth
Neumarkt i. d. Opf.
Nationality
german
Marital status
unmarried
Education
09/1980 - 08/1984
Elementary school Heng
09/1984 - 08/1985
Secondary school Postbauer-Heng
09/1985 - 06/1994
Willibald Gluck grammar school Neumarkt i. d. Opf.
06/1994
School leaving examination (Abitur) in Chemistry (main),
Physics (main), Geography, and English
Military Service
07/1994 - 06/1995
Transportbataillon 133, Erfurt
Higher Education
10/1995 - 02/1998
Basic studies: Chemistry (Diplom), Friedrich-AlexanderUniversity Erlangen-Nuremberg
02/1998
Vordiplom examination in Organic Chemistry, Inorganic
Chemistry, Physical Chemistry, and Physics
236
03/1998 - 02/2002
Main study period: Chemistry (Diplom), FriedrichAlexander-University Erlangen-Nuremberg
Subsidiary subject: Biochemistry
09/1998 - 04/1999
University of Loughborough, Loughborough/UK
Department of Chemistry, Supervisor: Dr. S. Christie
Project: "Chiral Bidentate Ligands in
Asymmetric Catalysis"
07/2001
Diploma examination in Organic Chemistry, Inorganic
Chemistry, and Physical Chemistry
09/2001 - 02/2002
Diploma thesis at the Institute of Organic Chemistry,
Friedrich-Alexander-University Erlangen-Nuremberg
Supervisor: Prof. Dr. A. Hirsch
Title: "Funktionalisierung und Charakterisierung von
einwandigen Kohlenstoffnanoröhren"
(Functionalization and Characterization of Single Walled
Carbon Nanotubes)
03/2002 - 06/2005
PhD studies at the Institute of Organic Chemistry,
Friedrich-Alexander-University Erlangen-Nuremberg
Supervisor: Prof. Dr. A. Hirsch
Title: "Functionalization of Carbon Nanotubes"
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