Particle Synthesis in Condensed Phases
Heinrich Hofmann
Swiss Federal Institute of Technology,EPFL
Lausanne, Switerland
heinrich.hofmann@epfl.ch
“Nanochemistry and Nanophysics”
Nanochemistry can be described as a special discipline of inorganic or solid
state chemistry. It focuses on the synthesis of nanoparticulate systems.
The nanochemist can be considered to work towards this goal from the
atom „up“, whereas the nanophysicist tends to operate from the bulk
„down“:
G. A. Ozin Adv. Mater. 4/10 (1992) 612ff.
Goals, Problems, and Methods in
nanoparticle synthesis
The goal is to elaborate a method of synthesis, which
a) is reproduceable
b) yields monodisperse nanoparticles
c) produces „perfect“ particles
d) may control the shape of the particles
e) is easy, cheap
The chemical methods are either based on the kinetic control of nucleation and
growth of the particles, on electrostatic stabilization in (aqueous) suspension,
or on the introducion of spatial constraints. The latter include particle
formation within or at the interface of micelles, vesicles, or bilayer lipid
membranes, surface monolayers and Langmuir Blodgett films, within the
channels of zeolites, in interlayers of clay, in peptides, or in biological cells.
2D. Myers Surfaces and Interfaces, VCH Publishers Inc., New York, 1991.
3J. P. Spatz, A. Roescher, M. Möller Adv. Mater. 8/4 (1996)
Nucleation and Growth (La Mer)
The stages of nucleation and growth for the preparation
of monodisperse NCs in the framework of the La Mer model. As NCs grow with time,
a size series of NCs may be isolated by periodically removing aliquots from the reaction
vessel.
C. B. Murray and C. R. Kagan and M. G. Bawendi Annu. Rev. Mater. Sci. 2000. 30:545–610
Kinetics:
•
Most of the time, the reactions are so fast, that they can’t be controlled easily.
In some cases, better results can be obtained using a precipitation method,
which is called „precipitation from homogeneous solution“:
Example: Synthesis of ZnS nanoparticles:
Zn2+ + S2- ⇔ ZnS↓
A „regular“ method of synthesis for zincsulfide particles involves the reaction of
Zn2+-ions with a sulfide (S2-) source i.e. H2S (Hydrogensulfide) or Na2S
(Sodiumsulfide). ZnS forms instantenously at a certain pH value. The obtained
ZnS powder consists of particles with a very large shape and size distribution,
ZnS-regularly precipitated particles
Precipitation from homogeneous solution:
Thiacetamide is used as a sulfide source. It hydrolyses according to:
S
H3C
C
+
2 H 2O
+
H2S + CH3COO- + NH4
NH2
The equilibrium is shifted by a change in temperature, concentration, and
pressure. Only when the available sulfide ions are no longer present in the
reaction solution, (which means that they have reacted with Zn2+ to form
zincsulfide,) new sulfide ions are released by the thioacetamide.
ZnS-precipitation from homogeneous solution
Influence of Counter-ions
• Sometimes the size as well as the morphology can be
influenced by the counter-ions.
ZinctfMS
Zincacetate
Zincacetylacetonate
Morphology
Spherica Spherical Spherical
l
Size in nm
(TEM)
800
40
30
The influence of the counter-ion on the particle size is again a kinetic one. The
Zn2+-ion forms complexes with the above mentioned acetate, acetylacetonate, or
trifluoromethane-sulfonate anions. Depending on the complexation, less „free“
cations are available for reaction, the crystallite growth is suppressed.
Lit.: R. Vacassy, S. M. Scholz, J. Dutta, H. Hofmann et al., J. Am. Chem. Soc. 81/10 (1998)
2669ff.
Quantum-dots
Size- and material-dependent emission spectra of
several surfactant -coated semiconductor nanocrystals
in a variety of sizes (A).
Blue series: different sizes of CdSe (Diameter :
2.1, 2.4, 3.1, 3.6, 4.6 nm)
Green series: InP nanocrystals (Diameter: 3.0, 3.5, and
4.6 nm)
Red series: InAs nanocrystals (Diameter:
2.8, 3.6, 4.6, 6.0 nm)
(B) A true-color image of a series of silica-coated core
(CdSe)-shell (ZnS or CdS) nanocrystal probes in
aqueous buffer, all illuminated simultaneously with a
handheld ultraviolet lamp
Synthesis: There are many wet chemical methods of synthesis for semiconductor nanoparticles, a organic and an inorganic method are
presented here:
Cd2+ + Se2Stabilizer Ln
[Me2Cd] + [(TMS)2Se]
CdSeLn
Optimized synthesis parameters:
9<pH<12.5
Surfactant: Thioalcohols/Thioacids
Atmosphere: inert gas
Transmission electron micrograph
of CdSe
A. P. Alivisatos J.Phys.Chem. 100/31 (1996) 13226ff.
TOP/TOPO
Optimized synthesis
parameters:
230 < T < 260°C
Surfactant: TOP/TOPO
Atmosphere: inert gas
CdSe
Brus and co-workers suggested that sulfur
vacancies, located at the surface of the
material, might be important in mediating
low-energy emissions. There are several
reasons for this, one of which is the
considerable size of such shallow traps.
Moreover, as the size of these traps
approaches that of the nanoparticle, the
wave functions of the trap and excited state
overlap. Transfer to these levels, in the form
of a separate event, should subsequently
be minimized, and the possibility of electronhole recombination, with emission close to
the absorption peak of the bound exciton,
can become the predominant
event.
Nanocrystalline Semiconductors: Synthesis, Properties,
and Perspectives (review) Tito Trindade et al. Chem. Mater. 2001, 13,
3843-3858
Arrested Precipitation in Solution
Controlled precipitation reactions can yield dilute suspensions of quasi
monodispersed particles. This synthetic method sometimes involves the
use of seeds of very small particles for the subsequent growth of larger
ones. The stability of the initially small crystallites formed is influenced by
the dynamic equilibrium illustrated in
Small crystallites are less stable than larger ones and tend to dissolve into their
respective ions. Subsequently, the dissolved ions can recrystallize on larger
crystallites, which are thermodynamically more stable (Ostwald ripening). The
use of acetonitrile, as a solvent, or the addition of styrene/maleic anhydride
copolymer allowed the preparation of stable CdS nanoparticles, with an
average size of 34 and 43 Å, respectively.Cubic ZnS and CdS nanocrystallites
were synthesized in aqueous and methanolic solutions without organic
surfactant (capping agent).
Nanocrystalline Semiconductors: Synthesis, Properties,
and Perspectives (review) Tito Trindade et al. Chem. Mater. 2001, 13, 3843-3858
Example CdSe
Preparation of semiconductor nanocrystallites:
Solutions of (CH3)2Cd and tri-n-octylphosphine selenide (TOPSe) are
injected into hot tri-n-octylphosphine oxide (TOPO) in the temperature
range 120-300 °C. This produced TOPO capped nanocrystallites of
CdSe.
C. B. Murray and C. R. Kagan and M. G. Bawendi Annu. Rev. Mater. Sci. 2000. 30:545–610
ZnS:Mn (Me) Photoluminescence
ZnSO4 + Na2S + (MnCl2)
L-cysteine
ZnS(:Mn2+) + 2 Na+ + SO42- + (Cl-)
Absorption Spectrum
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
260.0
30 min
1h
1h 30 min
2h
280.0
300.0
320.0
wavelength (nm)
340.0
ZnS:Mn (Me) Photoluminescence
wavenumber (cm-1)
3400.0
O
2400.0
1400.0
O
O
NH2
O
20.0
H2N
2090 cm -1
S
H2N
S
400.0
0.0
40.0
60.0
O
2559 cm -1
S
1581 cm -1 1531 cm -1
O
Cysteine
80.0
100.0
Cysteine on particles
O
120.0
S
S
O
NH2
H2N
O
O
2559 cm-1: S-H stretch band
• 2090 cm-1: NH3+ stretch band
• 1581 cm-1: COO- stretch band
• 1531 cm-1: NH3+ deformation band
Attachment of cysteine on the ZnS surface occurs
via mercapto-group. Free carboxylic groups at the
particle surface
SiO2
Silica powders were prepared by precipitation in ethanol
according to the St¨ober et al. method, which is a precipitation
technique based on controlled hydrolysis of a silicon alkoxide
(tetraethylorthosilicate, TEOS) in a mixture of ethanol, aqueous
ammonia (21%), and water. The porogen used, 3aminopropyltriethoxysilane (APTES) can be mixed to the TEOS
prior to adding to the solvent mixture. Five TEOS/APTES ratios
were investigated (100/0, 90/10, 80/20, 50/50, and 20/80), the
total alkoxide concentration being kept constant and equal to
0.2 M and the water concentration being maintained at 3.2 M.
The use of glycerol (Aldrich, reagent ACS, 99.5%) as a
porogen was investigated since this organic compound is
known to adsorb very well to cations and oxide surfaces under
basic conditions.
SEM micrographs of SiO2 nanoparticles formed
by the hydrolysis
of TEOS in ethanol containing ammonia after 24
h reaction time (SiO2 100/0).
R. Vacassy, R. J. Flatt, H. Hofmann, K. S. Choi, and R. K. Singh ,Journal of Colloid and Interface
Science 227, 302–315 (2000)
SiO2 nanoparticles formed by the
co-hydrolysis of TEOS in ethanol
containing ammonia at an
intermediate stage of the synthesis
(4 h reaction time, SiO2 100/0).
Evolution of the particle size and particle size distribution of silica during the
hydrolysis precipitation of TEOS (SiO2 100/0). The results were determined using
PCS, and error bars indicate the spread of the particle size distribution.
The total pair interaction energy V for
particles of masses i and j and for the
center-to-center separation R is the sum of
Van der Waals,VA, electrostatic, VE, and
solvation, VS, interactions.
Total particle interaction energy (potential barrier)
after 24 h reaction as a function of center to center
separation for two particles of the same size (a) and
of different sizes (b)
R-ri-rj (nm)
Evolution of the maximum of the total particle interaction energy
(potential barrier) during the early stage of the silica nanoparticle synthesis,
considering the interaction between primary and growing particles. Dashed
lines present linear corrections due to particle density variations.
Side Reactions
Very often, syntheses, which seem straight forward are in fact very complicated
and result in various reaction products. A very good example is the well known
iron oxide Fe2O3
.
Overall reaction:
2Fe3+ + 6OH- ⇔ Fe2O3 + 3H2O
In detail:
[Fe(H2O)6]3+
[Fe(OH)(H2O)5]2+ + H+
2 [Fe(OH)(H2O)5]2+
pH: 0-2
[(H2O)4 Fe(OH)2Fe(H2O)4]4+ + 2H2O
[(H2O)4 Fe(OH)2Fe(H2O)4]4+
Isopolyoxo-cations
Fe2O3·xH2O
Jean-Pierre Jolivet et al. C h e m . C o m m u n . , 2 0 0 4 , 4 8 1 – 4 8 7
pH: 3-5
pH: 2-3
Nanosized particles in a biological environment
are complex systems
Inorganic or Organic
bead with nanoparticles
Nanoparticle
Cells Bacteria
Parts of DNA, Proteins Virus
1 nm
10 nm
Nanoparticles
102 nm
103 nm
Beads
104 nm
105 nm
Typical Functionalisation and Derivatisation
Chemistry
Physics
Colloidal chemistry
Core Biocompatible Fuctionalisation Spacer
Coating
Carboxyl
Mn:ZnS
PVA,
Fe2O3
Silica
O
Derivatisation
Drug, Proteine,..
Amino
Thiol
CH2
CH
O
(CH2)3
Biotin
O
CH2
OH
2 – 30 nm
CH
CH2
NH
OH
Iron oxide-PVA-Linker-Transferrin
8 nm 10 nm
Biology
Avidin
Synthesis of
Maghemite
Wet chemical coprecipitation
Base
Sedimentation
Fe2+
Fe3+
30 nm
HRTEM
Fe3O4
Oxidation/Redispersion
γFe2O3
Bare
particles
2 nm
+ Polymer
PRODUCT
+
NH2
NH2
NH2
Bare particle with
double layer + PVA
NH2
PVA coated particle PVA coated & functionalized PARTICLE & BEAD
Silica beads showing well separated iron oxide particles
Synthesis in Templates
Example for nanostructure tailoring by precursor entrapping:
The high porosity of the gels/xero gels enables the substitution of the water logged in the
pores by a designed liquid precurser. The densification of the the host (xero) gel matrix will
entrap the precursor which will be transformed. The low temperature densifiction prevents
in most cases an uncontrolled reaction between the matrix and the entrapped particles.
TEM micrograph of a Cobalt-Al2O3·2SiO2
composite prepared by infiltrating a porous
host matrix with a cobalt-nitrate precursor
solution, and a thermal treatment at 900
°C under H2 atmosphere.
R. Nayak, J. Galsworthy, P. Dobson, J. Hutchison J. Mater. Res. 3/4 (1998) 905ff.
Gold particles in micelles
Synthesis: A-B diblock copolymer is used for micelle formation
Polymer: Poly(styrene-block-2-vinyl-pyridine)
Idea:
An inorganic compound such as HAuCl4 is bound selectively to the
Polyvinylpyridine block of the polymer and thus solubilized within the
core of the micelle. Afterwards, the compound is transformed by
chemical reaction to the metal.
J. P. Spatz, A. Roescher, M. Möller Adv. Mater. 8/4 (1996)
Synthesis in a Structured Medium
A number of matrices have been used for the preparation of semiconductor nanoparticles
including: zeolites, layered solids, molecular sieves, micelles/microemulsions, gels,
polymers, and glasses. These matrices can be viewed as nano-chambers which limit the
size to which crystallites can grow. The properties of the nanocrystallites are determined,
not only by the confinements of the host material but also by the properties of the system,
which include the internal/external surface properties of the zeolite and the lability of
micelles.
Nanocrystalline Semiconductors: Synthesis, Properties, and Perspectives (review) Tito Trindade et al. Chem. Mater.
2001, 13, 3843-3858
Segmented Flow Tubular Reactor
Reactant 1
Mixer
-segmenter
Tubular section
Immiscible Fluid
Reactant 2
Film on tube wall
Well mixed
reactants Immiscible limits fouling
Segmentation – plug flow not parabolic
Parobolic flow
Fluid
Quasi - Plug Flow
•
•
•
Temperatures
- 95°C
Flow rates
Residence times
1.4 L/hr
1-60 mins
•
pH
1-14
30 m long
Segmenting
Fluid
Dodecane
5
Perfect Segmentation –no fouling
In
10 cm
Out
Previous Results- Narrower size distributions
Copper Oxalate 25°C – self assembled nanocyrstals
geometrical standard deviation σg
SFTR
σg = 4.3
log(particle size)
Frequency
Frequency
Batch
σg = 1.7
log(particle size)
Continuous Production - 25 hrs - CaCO3
- Ca0 = 0.02 M,
Conditions: - PAA = 0.01 %
- C/Ca = 1.01
- S = 46
dv50
span
= 0.39 µm
= 1.06
Freq vol %
15
SFTR 1h
SFTR 9h
SFTR 25h
Mini-batch
10
5
0
0.01
0.1
d [µm]
1
10
Crystallographic Control Seeding(25°C)- CaCO3
Seed
Powder
Calcite
H
Calcite
Vaterite
H
Vaterite
Aragonite
H
Calcite
BaTiO3 synthesis – Batch vs SFTR
Low Temperature Aqueous Synthesis (LTAS) developed at Genoa
Reactants 0.6 M pH 12-14
Ba(OH) 2 + TiCl4 + 4NaOH → BaTiO3 + 4NaCl + 3H 2 O
•
•
•
•
•
•
Batch 6 litre reactor
85°C
Nitrogen atmosphere
5hrs ageing
Washing
Freeze drying
•
•
•
•
•
•
•
SFTR
95°C
Nitrogen segmenting fluid
4mm φ tube PTFE
Residence time 10 mins
Washing
Freeze Drying
A.Testino, M.Viviani, M.T.Buscaglia, V.Buscaglia, P.Nanni
Institute for Physical Chemistry of Materials - CRN, Genoa, Italy
Chemical and Process Engineering Department - University of Genoa, Italy
Powder Characterisation (1)
Powder
Ba/Ti (±1%)
nominal
experim.
BaCO3
%
SFTR
1.12
1.11
0.5%
Batch
1.025
1.01
1-3%
• Stoichiometry well controlled –
batch and SFTR
• Secondary phases – lower Ba
CO3 with SFTR
• SFTR Finer
Powder Characterisation (2) Granulometry
100
•
•
•
SFTR powder
Finer,
High surface area,
% Volume
80
60
40
Batch
20
SFTR
0
0
0.05
0.1
0.15
0.2
Diameter (µm)
0.25
Powder
SSA
m2/g
dBET
nm
SFTR
50.3
23.7
49.7
67.5
111
2.8
Batch
37.6
30.9
54.1
86.3
328
2.8
PSD (nm)
dv16 dv50 dv84
Fag
(dv50 / dBET)
0.3
Sintering of Nanometer BaTiO3
• Initial powders
primary particles
22-40 nm
•Sintering SPS
- 50 MPa,
- vacuum – N2
- 800-1000°C
Dr.Zhao Zhe,
Prof. Mats Nygren,
Dr. Zhijian Shen
Dept. of Inorg. Chem.,
Arrhenius Lab.,
Stockholm Univ. S106
91, Sweden
B10 Paper 659 Tuesday
14.50, Dolmabahce C
Batch
- 50% < 100 nm
- density - 96%
- grain size 150 nm
SFTR
- 90% < 100 nm
- density – 97 %
- grain size 80 nm