AN ABSTRACT OF THE DISSERTATION OF
Watcharee Katinonkul for the degree of Doctor of Philosophy in Chemistry presented
on August 27, 2007.
Title: Graphite Intercalation Compounds containing Fluoroanions.
Abstract approved:
Michael M. Lerner
New chemical synthesis strategies have been investigated for the preparation
of acceptor-type graphite intercalation compounds (GIC’s) with fluoroanions in order
to obtain new materials and to develop a better understanding of the synthetic
approaches and properties of the products. New GIC’s containing borate chelate anion
such as bis(oxalato)borate, CxB[OC(O)C(O)O]2·δF, are prepared by the chemical
oxidation of graphite with fluorine gas in the presence of a solution containing the
intercalate anion in anhydrous hydrofluoric acid. Due to the chemical method
employed (F2/AHF as the oxidizing agent), fluoride co-intercalates are present in the
galleries. The GIC’s compositions are determined using a newly-developed digestion
method with an ion-selective electrode and potentiometer. The composition
parameters x and δ are obtained using both elemental and thermogravimetric analyses
(TGA). Elemental analyses of B and F for a stage 1 GIC indicate x ≅ 41 and δ ≅ 4.
Powder XRD data and the structural refinement of these GIC’s show that the
intercalate anions “stand up” in the galleries, with the longer anion dimension oriented
perpendicular to the graphene sheets, leading to the relatively large gallery height of
di ≅ 1.42 nm.
The same chemical synthesis strategy is also used for chemical preparations of
other
two
borate
chelate
CxB[OC(CF3)2C(CF3)2O]2·δF,
and
GIC’s;
bis(perfluoropinacolato)borate,
bis(hexafluorohydroxyisobutyrato)borate,
CxB[OC(CF3)2C(O)O]2·δF. Stage 1 GIC’s of these anions are obtained with the gallery
height, di, of 1.43-1.45 nm. The fluoride co-intercalate contents in these GIC samples
are also determined. The composition parameters x and δ are obtained using elemental
analyses
for
stage
1
CxB[OC(CF3)2C(CF3)2O]2·δF
and
for
stage
1
CxB[OC(CF3)2C(O)O]2·δF. TGA cannot be used to confirm these compositions due to
the incomplete degradation of intercalates. Most likely, anions and their degradation
products are trapped in the bulk of the large flaky graphite particles.
The detailed compositions of GIC’s containing bis(trifluoromethanesulfonyl)
imide, CxN(SO2CF3)2·δF, and perfluorooctanesulfonate, CxC8F17SO3·δF and fluoride
co-intercalates are determined for the first time. These GIC’s are prepared in aqueous
HF, using K2MnF6 as the oxidizing agent. The fluoride co-intercalate content is
approximately equimolar to anion content in both GIC’s. For CxN(SO2CF3)2·δF, the
fluoride content increases steadily with reaction time while the anion content is
relatively constant. In contrast, CxC8F17SO3·δF shows little change in the fluoride
content over time.
©
Copyright by Watcharee Katinonkul
August 27, 2007
All Rights Reserved
Graphite Intercalation Compounds containing Fluoroanions
by
Watcharee Katinonkul
A DISSERTATION
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Doctor of Philosophy
Presented August 27, 2007
Commencement June 2008
Doctor of Philosophy dissertation of Watcharee Katinonkul presented on August 27,
2007
APPROVED:
Major Professor, representing Chemistry
Chair of the Department of Chemistry
Dean of the Graduate School
I understand that my dissertation will become part of the permanent collection of
Oregon State University libraries. My signature below authorizes release of my
dissertation to any reader upon request.
Watcharee Katinonkul, Author
ACKNOWLEDGMENTS
I would like to thank Drs. Kevin Gable, Douglas Keszler, Philip Watson, and
Jim McManus for serving on my doctoral dissertation committee and for their helpful
suggestions and comments.
I thank to Chris Pastorek, Joe Magner, and Ted Hinke for their help with
equipment throughout many years, to Alan Richardson for his suggestions about
structural model calculation, and past members of the Lerner group; Wei Yan and
Lioubov Kabalnova, for their help and guidance.
I thank to my parents for giving me an opportunity to get the highest education
mankind can acquire, especially my beloved mother, Sa-ngium Katinonkul, for her
inspiration.
I also would like to acknowledge the Royal Thai Government for financial
support and thank all the people at the Royal Thai Embassy in Washington DC for
taking care of me during my stay in the United States of America.
Finally, this thesis would never be successfully completed without the
excellent advice from my thesis advisor, Dr. Michael Lerner, who always provides me
useful guidance, suggestions, encouragement, understanding and also patiently
practices my technical skill during the whole research.
CONTRIBUTION OF AUTHORS
Dr. Michael M. Lerner was involved in the design, analysis, and writing of
each manuscript.
TABLE OF CONTENTS
Page
1. INTRODUCTION..........................................................................................
1
1.1 GENERAL INTRODUCTION…….....................................................
1
1.2
GRAPHITE INTERCALATION COMPOUNDS…............................
8
1.2.1
Types of GIC’s…………………………….. ………………....
13
1.2.1.1
Donor-type compounds………………………. …….
13
1.2.1.2
Acceptor-type compounds.....……………………….
15
1.2.2 Preparation of acceptor-type GIC’s…………………….……..
21
1.2.2.1
Electrochemical oxidation method………………….
21
1.2.2.2
Chemical oxidation method…………………………
23
1.3 GENERAL APPARATUS AND PROCEDURES…………...…….....
26
1.3.1
Stainless steel vacuum line……………….……………………
26
1.3.1.1 Handling anhydrous HF (AHF)…………………......
28
1.3.1.2
Assembled reaction apparatus for use with AHF.......
28
1.3.2
Graphite……………………..…………..……….……………
29
1.3.3
Elemental analyses ……………………………………………
29
1.3.4
Powder X-ray diffraction (Powder XRD) …………………….
30
1.3.5
Thermal gravimetric analysis (TGA) …………………………
30
TABLE OF CONTENTS (Continued)
Page
1.4 FLUORINE ANALYSIS IN GIC’s …………………………..............
1.4.1
31
Control of solution parameters………………………………..
31
1.4.2 Test of digestion procedure …………………………………..
31
1.4.3
Test for container reactions …………………………………...
34
POWDER X-RAY DIFFRACTION ANALYSIS OF GIC’S ……......
36
1.6 STRUCTURAL REFINEMENT MODELING……… ………………
39
1.5
1.6.1
Overview………………………………………………………
39
1.6.2
Structural model calculations………………………….. ……..
40
1.6.3
One-dimensional electron density map…………………..……
41
1.6.3.1 Optimization of intercalate gallery structure…………
46
THESIS OVERVIEW..……………………………………………….
47
1.8 REFERENCES ……………………………………………………….
49
2. EFFECT OF REACTION TIME ON COMPOSITION OF GRAPHITE
BIS(TRIFLUOROMETHANESULFONYL)IMIDE ……………………....
55
2.1 ABSTRACT…………………………………………………………..
56
2.2
INTRODUCTION…………………………………………………….
57
2.3
EXPERIMENTAL ………………………………………………........
61
2.4 RESULTS AND DISCUSSION ……………………………………...
64
2.5 REFERENCES………………………………………………………..
75
1.7
TABLE OF CONTENTS (Continued)
Page
3. A STUDY OF FLUORIDE CONTENT IN GRAPHITE
PERFLUOROOCTANESULFONATE INTERCALATION
COMPOUNDS………………………………………………………………
77
3.1 ABSTRACT…………………………………………………………..
78
3.2
INTRODUCTION…………………………………………………….
79
3.3
EXPERIMENTAL ………………………………………………........
82
3.4 RESULTS AND DISCUSSION ……………………………………...
85
3.5 REFERENCES………………………………………………………..
94
4. THE FIRST SYNTHESIS OF A GRAPHITE BIS(OXALATO)BORATE
INTERCALATION COMPOUND…………………………………………
96
4.1 ABSTRACT…………………………………………………………..
97
4.2
INTRODUCTION…………………………………………………….
98
4.3
EXPERIMENTAL ………………………………………………........
102
4.4 RESULTS AND DISCUSSION ……………………………………...
107
4.5 REFERENCES………………………………………………………..
118
5. CHEMICAL SYNTHESIS OF GRAPHITE BIS(PERFLUOROPINACOLATO)BORATE AND GRAPHITE BIS(HEXAFLUOROHYDROXYISOBUTYRATO)BORATE INTERCALATION
COMPOUNDS……………………………………………………………...
120
5.1 ABSTRACT…………………………………………………………..
121
5.2
INTRODUCTION…………………………………………………….
122
5.3
EXPERIMENTAL ………………………………………………........
124
TABLE OF CONTENTS (Continued)
Page
5.4 RESULTS AND DISCUSSION ……………………………………...
128
5.5 REFERENCES………………………………………………………..
140
6. CONCLUSION ……………………………………………………………..
142
BIBLIOGRAPHY …………………………………………………………........
144
LIST OF FIGURES
Figure
Page
1.1
Dimensional classification of intercalation hosts…………………………
4
1.2
Schematic drawing of a layered intercalation compound…………………
5
1.3
Schematic view of the graphite structure, formed by the ABAB stacking
of graphene sheets…………………………………………………………
7
1.4 Schematic view of GIC’s showing staging, or ordering, along the
stacking direction……………………………………………………..…...
10
1.5
Daumas-Hérold model ……………………………………………............
11
1.6
Galvanostatic potential-charge curve of LiN(SO2CF3)2 in nitromethane…
22
1.7
Schematic of stainless steel vacuum line ……………………………........
27
1.8
Powder XRD pattern of stage 1 products of CxB[OC(O)C(O)O]2·δF.
Assigned (00l) indices are indicated. ……………………………………
37
(a) A geometry-optimized structural model of isolated bis(oxalato)borate
anion, (b) structural model of stage 2 CxB[OC(O)C(O)O]2·δF, with four
fluoride co-intercalates per borate anion. Axes and some atomic labels
are included…………………………………………………………. ……
43
1.10 One-dimensional electron density profiles of (a) graphene sheets and
(b) bis(oxalato)borate anion intercalate using (i) 9, (ii) 25, and
(iii) 50 terms in the calculation……………………………………………
44
1.11 One-dimensional electron density profiles of stage 2
CxB[OC(O)C(O)O]2·δF derived from calculated structural model using
(a) 9 terms, (b) 25 terms, and (c) derived from observed powder XRD
data using 9 terms…………………………………………………………
45
1.9
2.1
Powder XRD patterns of the products obtained, with reaction times
indicated. For each powder XRD trace, there is a break in intensity data
as indicated by hashes on the y-axis………………………………………
66
LIST OF FIGURES (Continued)
Figure
2.2
Page
Structural model for CxN(SO2CF3)2·δF. The fluoride positions are
partially occupied …………………………………………………………
68
2.3 TGA data for products described in Table 2.1 ……………………………
69
2.4
Increase in fluoride content, δ, in CxN(SO2CF3)2·δF vs. reaction time …...
71
2.5
One-dimensional electron density profiles derived from calculated
structural model (i) and observed powder XRD data (ii)…………………
74
Powder XRD patterns of the products obtained in a mixed acid solution
(HF/HNO3:66/33), using K2MnF6 as an oxidizing reagent, with reaction
times indicated. Impurity peaks of CxF are marked *. ……………………
88
Structural model for CxC8F17SO3·δF. The fluoride positions are partially
occupied.…………………………………………………………………..
90
3.3
TGA data for products described in Table 3.1.……………………………
91
4.1
Scheme of GIC showing staging, or ordering along the stacking
direction, for different products… ……….………….................................
100
General design for reactor employed in syntheses using anhydrous HF
And F2.........……………………………………………………….............
105
Powder XRD pattern of stage 1-3 products of CxB[OC(O)C(O)O]2·δF.
Assigned (00l) indices are indicated.……………………………………...
111
TGA mass loss profiles for LiBOB salt and stage 2
GIC CxB[OC(O)C(O)O]2·δF. ……………………………………………..
116
(a) Energy-minimized BOB anion conformation, (b) GIC structural
model, and (c) one-dimensional electron density profiles derived from
calculated structural model (i) and observed powder XRD data (ii).……..
117
3.1
3.2
4.2
4.3
4.4
4.5
LIST OF FIGURES (Continued)
Figure
5.1
5.2
5.3
Page
Powder XRD pattern of stage 1 products of
(a) CxB[OC(CF3)2C(CF3)2O]2·δF and (b) CxB[OC(CF3)2C(O)O]2·δF.
Assigned (00l) indices are indicated; impurity peaks are marked*.……...
131
Structural models of stage 1 GIC’s of (a) CxB[OC(CF3)2C(CF3)2O]2·δF
and (b) CxB[OC(CF3)2C(O)O]2·δF. ………………………………………
133
TGA mass loss data for (a) NaB[OC(CF3)2C(CF3)2O]2 salt (⋅⋅⋅) and
stage 1 CxB[OC(CF3)2C(CF3)2O]2·δF (⎯), (b) NaB[OC(CF3)2C(O)O]2
salt (⋅⋅⋅) and stage 1 CxB[OC(CF3)2C(O)O]2·δF (⎯)…………… ………..
139
LIST OF TABLES
Table
Page
1.1
Donor-type GIC’s with gallery height (di) and n (stage)…………...……..
14
1.2
Fluoride GIC’s with gallery height (di) …………………………………..
18
1.3
Stage-1 acid GIC’s with c-axis repeat distances (Ic) ……………………..
19
1.4
Synthetic reactions and results using different oxidizing reagents …….…
25
1.5
Fluoride content of digested control samples …………………………….
33
1.6
Comparison of fluoride content between digested and undigested
control samples…….……………………………………………………...
34
Comparison of fluoride analyses between digested samples using glass
and plastic labware ………………………………………………………
35
Data obtained from powder XRD of stage 1 CxB[OC(O)C(O)O]2·δF……
38
1.7
1.8
2.1 Gallery height (di) and composition for stage 2 graphite bis(trifluoromethanesulfonyl)imide, CxN(SO2CF3)2·δF, at different reaction times as
determined by powder XRD, elemental analyses, and thermogravimetry...
3.1
67
Gallery height (di) and composition for stage 2 graphite perfluorooctanesulfonate, CxC8F17SO3·δF, at different reaction times as determined by
powder XRD, elemental analyses, and thermogravimetry.……………….
89
4.1
Powder XRD data for CxB[OC(O)C(O)O]2·δF products………………….
112
4.2
Compositional data for CxB[OC(O)C(O)O]2·δF from elemental analyses..
113
5.1 Powder XRD structural data for (a) CxB[OC(CF3)2C(CF3)2O]2·δF and
(b) CxB[OC(CF3)2C(O)O]2·δF products obtained after 24 h reaction..……
132
Compositional data for CxB[OC(CF3)2C(CF3)2O]2·δF and
CxB[OC(CF3)2C(O)O]2·δF products….…………………………...............
135
5.2
GRAPHITE INTERCALATION COMPOUNDS CONTAINING
FLUOROANIONS
CHAPTER 1
INTRODUCTION
1.1 GENERAL INTRODUCTION
The term intercalation is derived from the Latin verb intercalare and refers to
(in the original Latin) the insertion of an additional day in special years to synchronize
the calendar with the solar year. [1] In host-guest chemistry, intercalation is defined as
the insertion of guest species (atoms, ions or molecules) into a crystalline host lattice
generally with an increase in some dimensions but also retention of major host
structure. The term intercalation, however, does not indicate anything about the
specific nature of the chemical reaction underlying this process. It describes only the
topological relationship between the host and the final product of the reaction.
The particular requirement for forming intercalation compounds is anisotropic
bonding in the host leading to unchangeable structural elements such as layers, chains,
or three-dimensional frameworks as illustrated in Figure 1.1. If electrically neutral
structural elements are formed, they are held together by weak van der Waals forces or
they contain empty one-dimensional channels (running parallel through the structure,
or intersecting each other) or interconnected cavities. If these structural elements carry
2
electrical charges, these are compensated by small counterions occupying the channels
and cavities. [2, 3-8] In most of these compounds there are many more available sites
than can be occupied by additional atoms or molecules. Therefore the mobility of the
interstitial species can be very high if their interaction with the host structure is not too
strong. [5] Consequently, this can lead to the reversible uptake of atoms, ions, or
molecules while the structure of the host lattice is conserved.
Whereas in framework structures only those compounds can be taken up that fit
into the channels or cavities, in layered or chain host lattices expansion can occur in
one or two dimensions without serious restriction. This allows the accommodation of
guest species other than simple ions or atoms, such as large metal complexes, or
organic/ inorganic cations or anions. For two dimensional hosts, layer expansions up
to 5 nm have been observed (Figures 1.1 and 1.2).
In channel compounds, defects can completely block diffusion and it can
therefore be difficult to achieve homogeneous filling of all possible lattice sites. In
chain and layer compounds, the species to be intercalated has access from all, or at
least two, dimensions. However, in chain structures the insertion of additional species,
especially if they are bulky, weakens the interaction between the chains, thus leading
to the easy disintegration of the solid. In layered hosts, the two-dimensional structural
element provides enough stability to prevent host disintegration, at least for moderate
layer separations. This ideal combination of easy access to the interlayer galleries and
the stability of the final products is the reason that layered compounds are preferred
3
for reversible and facile intercalation reactions, and why most of the known
intercalation hosts have layered structures. [2, 3-8, 10-16]
Layered hosts may be neutral, positively-charged, or negatively-charged, and
range in electrical properties from metallic conductors to semiconductors or insulators.
[2] Clay materials, such as montmorillonite or hectorite, are probably the most widely
studied hosts in this group because they are naturally-occurring and readily form a
wide range of intercalation compounds. Some other important layered hosts that have
been studied include layered silicates, MoO3, MPS3 (M=Mn, Cd, Fe, Co, Ni, Zn, V),
protonic
layered
titanate
(HxTi2-x
x/4O4·H2O),
protonic
layered
tetratitanate
(H2Ti4O9·xH2O), and also a number of layered dioxides and layered dichalcogenides.
Layered host surfaces can also be modified with organic groups, so that they become
compatible with hydrophobic intercalates. For instance, a variety of molten styrenederivative polymers can intercalate into alkylammonium-functionalized layered
silicates. [17,18] Another example is a nanocomposite of poly(ether imide) and an
organically modified layered silicate, which is prepared by cation exchange between
Li fluorohectorite or Na montmorillonite and protonated primary amines. [19] The
intercalated polymer shows improved fire retardancy relative to the pure polymer.
4
Type of lattice
frame work structures
schematic
representation
model
compound
dimensions of
accessibility
Zeolite
3
other
examples
Mo6S8,
WoO3
a) 3-dimensional
network of channels
Nb3S4
b) uni-directional
channels
layer structures
+
1
Ti3S4, WO3
2
graphite,
+
TaS2
FeOCl, MoO3
layered dichalcogenides
chain structures
3
NbSe3
KFeS2,
AMo3X3
Se
Nb
Figure 1.1 Dimensional classification of intercalation hosts. [1]
(A= alkali metal,
In, Tl; X=S, Se)
5
the unit repeat along
the stacking direction
gallery expansion
host layer
thickness
Figure 1.2 Schematic drawing of a layered intercalation compound.
6
Graphite is one of many layered materials that can form intercalation
compounds. Graphite is composed of planar sheets (graphene sheets) with carbon
atoms bonded to three others by a trigonal sp2 interaction to form a hexagonal
network. The remaining non-hybridized 2p electrons are delocalized in an extended π
bonding interaction. Adjacent sheets are thus bound only by van der Waals forces.
Figure 1.3 shows the structure of graphite indicating the in-plane carbon-carbon bond
length (0.142 nm) and the layer separation (0.335 nm). Graphite has two types of
stacking structures, known as the hexagonal and rhombohedral phases. The planes are
stacked ABAB for the hexagonal phase as shown in Figure 1.3 and ABCABC for the
rhombohedral phase. The rhombohedral structure is metastable and transforms into
hexagonal graphite at high temperature or by the formation of an intercalation
compound followed by its dissociation. [4] The direction parallel to the graphene
sheets exhibits metallic properties, but in the perpendicular direction, graphite is a
semiconductor. Graphite is a unique layered material that has excellent thermal and
electrical conductivity. Additionally, graphite has distinctive chemical and physical
properties, including a high sheet flexibility that leads to the staging phenomenon,
which will be explained in Section 1.2.
7
0.142 nm
A
B
0.335 nm
A
Figure 1.3 Schematic view of the graphite structure, formed by the ABAB stacking of
graphene sheets.
8
1.2 GRAPHITE INTERCALATION COMPOUNDS
Graphite can undergo intercalation reactions because of the relatively weak
bonding between graphene sheets. The intercalation reaction always involves a redox
process, electrons can be removed from the graphite valence band by oxidation, and
anions inserted between sheets to satisfy the neutrality condition. This produces
acceptor-type graphite intercalation compounds (GIC’s). Alternatively, donor-type
compounds with cationic intercalates can be synthesized using strong reducing agents
that donate electrons into the graphite conduction band. These reactions are illustrated
in equations 1.1 and 1.2.
xC + An¯
→
CxAn + e¯
(1.1)
xC + M+ + e¯
→
MCx
(1.2)
where C represents graphene sheets and An¯ and M+ represent intercalate anions and
cations, respectively.
In a GIC, not every pair of adjacent sheets are necessarily separated to form a
gallery occupied by intercalate guests. The appearance of an ordered sequence of
occupied galleries and adjacent graphene sheets, the so-called staging phenomenon, is
one of the many fascinating properties of GIC’s. Stage n refers to n graphene sheets
between intercalate galleries. Therefore, in stage 1 compounds, graphene sheets and
9
intercalate alternate, and in stage 2 compounds two adjacent graphene sheets alternate
with intercalate as is illustrated in Figure 1.4. The symbol Ic denotes the unit cell
repeat distance along the stacking direction, and di is the gallery height or distance
along the c axis between graphene sheet centers encasing an intercalate gallery. The
relationship between Ic, di and n is indicated by equation 1.3
Ic
=
di
+
(n-1) 0.335 nm
(1.3)
The actual compositions vary with synthetic conditions and therefore GIC’s are
examples of non-stoichiometric compounds. It is common to specify the composition
together with the stage, for example stage-4 NaC24 or NaC6n (n = 4-8). [20]
To describe staging in graphite intercalation, the Daumas-Hérold model has been
employed. This model describes intercalate volumes as domains of intercalate islands.
GIC’s are composed of a number of domains with regularly stacked islands. Stage
disorder is common in GIC’s and exists not only between domains but also within
each domain as shown in Figure 1.5. [21] This model was derived from the flexibility
and presence of defects in graphene sheets. The movement of islands over domain
boundaries allows facile staging transitions. [21] The Daumas-Hérold model has been
supported by a large number of experiments, and the domains have been observed
directly in CxFeCl4·δFeCl3 by lattice imaging with high resolution TEM. [22]
10
di
Ic
di
Ic
Ic = di
0.335 nm
graphite
stage 3
stage 2
stage 1
graphene sheet
intercalates
Figure 1.4 Schematic view of GIC’s showing staging, or ordering, along the stacking direction.
10
11
stage 1
stage 2
graphene sheet
intercalate
Figure 1.5 Daumas-Hérold model.
stage 3
12
In some GIC’s, there are neutral molecules present in the galleries along with
intercalate ions. There is no convincing evidence that graphite ever intercalates neutral
molecules without simultaneously being oxidized or reduced. However, GIC’s
commonly take up neutral molecules along with intercalate ions, this is also observed
with other layered hosts, such as transition metal chalcogenides. These neutral
molecules generally do not require significant additional separation of the graphene
sheets, and their uptake is reversible. Repulsive forces between the intercalate ions
contribute to decreased stability of the lattice. Neutral co-intercalates are usually
positioned between the ions, thereby acting as dielectric spacers and stabilizing the
GIC lattice.
Sources of neutral co-intercalates can include (1) excess neutral reactants, (2) byproducts of the intercalation reaction, and (3) solvents used in the intercalation
reaction. For examples, FeCl3 dissolved in nitromethane (CH3NO2) yields the cointercalation compound C24FeCl3·0.73CH3NO2 through direct reaction with graphite
[23], the electrochemical intercalation of PF6¯ and AsF6¯ anions dissolved in
nitromethane provides stage-2 C48PF6·2-4CH3NO2 with Ic = 1.1 nm and stage-1
C24AsF6·2CH3NO2 with Ic = 0.80 nm, respectively. [24,25] Similarly, an
electrochemical oxidation in solutions of complex anion salts in dry propylene
carbonate (PC) yields C24An·δPC with An = BF4¯, PF6¯, or SbF6¯. [26]
13
1.2.1
Types of GIC’s
1.2.1.1
Donor-type compounds
The most common cationic intercalates are alkali metal cations, and these
result in relatively small intercalate gallery dimensions (di ≤ 0.7 nm). These GIC’s are
readily prepared and their brilliant gold color and high conductivities for stage 1 has
attracted research interest for decades.
There are several intercalation methods for these GIC’s, including direct
reactions of graphite with the alkali metal, high pressure reactions, electrochemical
reactions, and solution-phase reactions. The most common intercalation method used
is the direct reaction of graphite with the metal in the temperature range 200-550°C. A
Pyrex glass tube can be used as a reaction chamber, and alkali metal GIC’s with welldefined single stage phases can be obtained. An example is shown in the following
reaction.
xC + K(m)
150-400°C
KCx
(1.4)
The most studied and technologically important donor-type GIC is LiCx, which is used
as the active anodic material in commercial Li-ion batteries. [5] KCx has also been
widely
studied
for
its
interesting
properties,
including
low
resistivity,
14
magnetoresistivity, and high hydrogen sorption capacity. [27] Other donor-type GIC’s
are shown in Table 1.1.
Alkali metal cations can also intercalate into graphite when the alkali metal is
solvated in liquid ammonia or an organic solvent such as dimethylsulfoxide (DMSO),
(CH3)2SO, dimethylformamide (DMF), C3H7NO, or furan, C4H4O. In this case, the
solvent is often included within galleries. Electrochemical reduction can also be used
for alkali metal intercalation. Using KI dissolved in DMSO solution, CxK·(CH3)2SO
compounds have been prepared by electrochemical reduction. [28]
Table 1.1 Donor-type GIC’s with gallery height (di) and n (stage).
compound
di (nm)
Reference
NaC6n (n = 4-8)
0.46
[20]
LiC6n (n ≥ 1)
0.37
[29]
KC8
0.54
[29]
RbC8
0.56
[30]
CaC6
0.46
[31]
SrC6
0.49
[31]
BaC6
0.52
[31]
NaC2-3
0.70
[32, 33]
RbC24·6(CH3)2SO
1.2
[34]
KC24·1.44C4H4O
0.88
[35]
15
1.2.1.2
Acceptor-type compounds
A wide range of anions have been intercalated into graphite, including
tetrahedral or octahedral fluoro-, chloro-, bromo- or oxo-metallates, and perfluorinated
organic anions. The interlayer spacing of acceptor-type GIC’s ranges from 0.67 nm
(for CxHF2) to 3.4 nm (for the bilayer gallery structure observed in CxC10F21SO3).
[36,37]
Following early work by Rüdorff et al. since 1963 [38], transition metal
chloride GIC’s have been extensively investigated, both in the synthetic chemistry and
structure as well as materials properties, including electrical and magnetic properties,
and application in batteries and catalysts. [20, 39-41] Either vapor-phase or solutionphase syntheses can be used. Metal chlorides can be reacted with graphite in the
presence of chlorine gas in the appropriate temperature range. [20] Cl2 is involved in
the oxidation process as shown in the following reaction.
xC + MClm + (δ/2) Cl2
CxMClm+δ
(1.5)
However, the presence of Cl2 is not required when using oxidizing metal chlorides
such as FeCl3, AuCl3, and SbCl5. In these cases, auto-dissociation is responsible for
graphite oxidation, as shown in the following reaction:
e¯ + 2FeCl3
FeCl2 + FeCl4¯
(1.6)
16
A large number of fluoride intercalation compounds have been prepared,
including metal fluorides, halogen fluorides, boron fluorides, hydrogen fluoride, and
rare gas fluorides as summarized in Table 1.2. [42-49] The intercalation of
hexafluorides or pentafluorides has been studied largely because of their high vapor
pressure and the strong covalent character of the metal-fluorine bond. Among these
species, AsF5 and to a lesser extent SbF5, have been particularly well investigated. [45,
48, 50-51] The catalytic activity of metal fluoride-GIC’s has also been widely
investigated for organic fluorine chemistry and is now of great importance for
perchlorofluorocarbon substitution reactions. [52-54] Various preparation procedures
have been described. Some fluorometallate anions are intercalated spontaneously by
direct reaction of graphite with the metal fluoride. For example, intercalation of some
hexafluorides, MF6, into graphite occurs as shown in the following reaction scheme:
[55-56]
xC + MF6
CxMF6
(1.7)
where M = Re, Os, Ir, Pt, U
Spontaneous interaction of some metal pentafluorides, MF5, into graphite also occurs,
involving the disproportionation:
2e¯ + 3MF5
2MF6¯ + MF3
(1.8)
17
where M = As [57], Sb [58], Ru [59], Os [59]
Non-oxidizing metal fluorides require the addition of oxidizing reagents such as
fluorine, chlorine, CrO3, or chromyl fluoride [56-57]. For example, some
pentafluorides MF5 intercalate into graphite only in the presence of F2 [58] or Cl2. [60]
e¯ +
PF5 + 1/2 Cl2 + HF
PF6¯ + HCl
(1.9)
The intercalation of fluorides in graphite can be performed in liquid
anhydrous HF (AHF). [61] The intercalation rate is generally increased by the addition
of elemental fluorine which can be added by bubbling the gas through the liquid. The
intercalation of HF2¯ under these conditions is rapid, a phase-pure stage-2 CxHF2 can
be produced within minutes at ambient temperature. [62] Following this, a slower
exchange between intercalated HF2¯ and dissolved fluorometallate species takes place,
[42] as shown in the following reaction scheme:
(i)
(ii)
xC + (1+δ)HF + (1/2)F2
CxHF2·δHF + MFn¯
CxHF2·HF
CxMFn + HF2¯ + δHF
(1.10)
(1.11)
A wide range of alternate chemical and electrochemical oxidation reactions have also
been studied. [63-64] For example, the oxidant K2MnF6 has been used in aqueous HF.
18
[63] MF6¯ anions (M = P,As,Sb) can be intercalated into graphite by an electrochemical method in oxidatively-stable solvents such as propylene carbonate (PC) [64]
or nitromethane. [65-67]
Table 1.2 Fluoride GIC’s with gallery height (di)
anion (An)
composition x
as CxAn
di (nm)
reference
HF2¯
C2-3(HF2)0.2-0.5
0.55-0.64
[42]
RhF3¯
C28RhF3.3·1.3HF2
0.79
[42]
SiF6 ¯
C12SiF6
0.79
[43]
PF6¯
C12PF6
0.76
[43]
MoF6¯
C11MoF6
0.84
[43]
BF4¯
C7BF4
0.77
[44]
AsF6¯
C8AsF6
0.81
[45]
WF6¯
C14WF6·δHF
0.84
[46]
OsF6¯
C8OsF6
0.81
[46]
PtF6¯
C12PtF6
0.76
[46]
TaF6¯
C5-8.8TaF 6
0.84
[47]
SbF6¯
C6SbF6
0.85
[48]
NbF6¯
C8.3-8.6NbF6
0.84
[49]
2
19
The conjugate anions of strong oxo-acids can also form acceptor-type
GIC’s, as summarized in Table 1.3. HNO3 and H2SO4 GIC’s have been most
intensively investigated among the acid GIC’s. [68-71] These compounds are prepared
by oxidant-assisted intercalation or electrochemical methods. Several oxidants have
been used in the intercalation reactions, including HNO3 [72], CrO3 [73], KMnO4 [74],
(NH4)2S2O8 [75], PbO2 [76], HClO4 [76], MnO2 [77], Na2O2 [78], H2O2 [79], and N2O5
[80]. In highly oxidative conditions with acid, the graphite host is itself converted to
graphite oxide, where oxygen bonds covalently to graphite, resulting in the loss of
π-bonding and formation of an oxygen-bridged corrugated sheet structure. [81]
Table 1.3 Stage-1 acid GIC’s with c-axis repeat distances (Ic).
Acid
composition
Ic (nm)
Reference
HNO3
C24 NO3·3HNO3
0.78
[82]
H2SO4
C24 HSO4·2H2SO4
0.80
[83]
HClO4
C24 ClO4·2HClO4
0.77-0.80
[84]
HSO3CF3
C26 CF3SO3·1.63HSO3CF3
0.80
[85]
CF3COOH
C26 CF3COO·δCF3COOH
0.82
[86]
Graphite forms intercalation compounds with all the halides except iodine. In
addition, mixed halogen species such as ICl and IBr can be intercalated. Some specific
features in the synthesis of halogen GIC’s are that the reaction temperature is quite
20
low, ranging around room temperature, and it is difficult to obtain stage-1 GIC’s even
at high reactant vapor pressures. The intercalation of fluorine is considerably different
from that of the other halogens, however. When graphite reacts with fluorine gas, two
types of product can be obtained, classified as fluorinated and fluorine-intercalated
graphite. The former is produced at high temperature ~ 380-600°C, comprises only sp3
hybridized carbon, and is gray or white in color. [43, 47, 87-88] In the latter type,
fluoride or bifluoride ions are intercalated in planar-sheet graphitic galleries under
milder conditions. [43,89]
Many other acceptor-type GIC’s have been reported. Among these, CrO3 GIC’s
which obtained from graphite and CrO3 in refluxing glacial acetic acid have been
studied extensively. [90]
When GIC’s are heated rapidly, a dramatic thermal expansion can occur along
the c-axis, resulting in extremely large volume increases, a process called exfoliation.
Exfoliation is generally irreversible and leads to a spongy or foamy, low-density, highsurface area carbon, which is advantageous for some applications because of its
flexibility, chemical inertness, low transverse thermal conductivity, and ability to
withstand high temperatures. [91] Examples of applications are in gas or oil adsorption
or, when exfoliated graphite is pressed into sheets, the product (Grafoil®) is widely
used as a high-temperature gasket or packing material.
21
1.2.2
Preparation of acceptor-type GIC’s
1.2.2.1
Electrochemical oxidation method
By evaluation of galvanostatic potential-charge curves and coulometry
when oxidizing a graphite electrode, intercalation reactions can be continuously
monitored and controlled. Intermediate phases (higher stages) can therefore be readily
obtained. The oxidation potential required for graphite intercalation is greater than
4.0 V vs. Li/Li+, requiring very oxidatively-stable anions for intercalation. This
requirement also limits the list of appropriate solvents, current collectors, and binders.
As the applied potential is increased, higher intercalate content (lower stage) GIC’s are
obtained as illustrated in Figure 1.6. The curve displayed was obtained with graphite
electrode oxidized in the LiN(SO2CF3)2/CH3NO2 electrolyte and shows a series of
transition points (Figure 1.6, labeled a-e) that indicate intermediate stages formed by
intercalation of the graphite electrode. A stage-3 GIC is obtained at point a and a
lower stage obtained at higher potential (stage 1 at point e). Beyond point d, the
overpotential increases rapidly and the final plateau at 5.5 V corresponds to the
oxidative stability limit for the electrolyte.
22
6.0
d
5.0
E, V
b
e
c
a
4.0
3.0
0
50
100
150
200
250
Charge, mAh/g
Figure 1.6 Galvanostatic potential-charge curve of LiN(SO2CF3)2 in nitromethane.
23
1.2.2.2
Chemical oxidation method
This method allows the preparation of bulk quantities of the GIC in a
shorter time, and requires no binders or additives. The key step in this method is the
selection of an appropriate oxidizing agent for graphite and, if required, a suitable
oxidatively-stable solvent. In aqueous HF solution, our research group has found
K2MnF6 to be a convenient and effective oxidizing agent. [63] It is highly soluble,
stable at room temperature, and can be easily prepared. However, due to the limited
oxidative stability of aqueous HF, only stage-2 or higher GIC’s can be formed. [92]
KMnO4 and K2Cr2O7 are both strong oxidizing agents that are used in preparation of
low-stage graphite sulfate and graphite nitrate in concentrated acids, [74] but they are
not stable in hydrofluoric acid. PbO2 and NaBiO3 can oxidize graphite and form lowstage GIC’s of perfluorooctanesulfonate (PFOS) in aqueous HF, but are relatively
insoluble, resulting in slow reactions and an admixture of solid reagents and products.
The oxidation of graphite in anhydrous hydrogen fluoride (AHF) using
F2 gas in a metal-vacuum line is another approach. F2 is a powerful oxidizing reagent
and can generate stage-1 GIC’s. However, the handling of F2 or AHF is relatively
difficult, and the scale for such reactions in the laboratory is typically limited to < 1 g.
Also, anions must dissolve in AHF and be stable to the strong oxidizing reagent F2.
However, in cases where no route can be found in aqueous acid, the anhydrous
method can be successful. For example, the GIC containing bis(oxalato)borate anion
was first prepared this way, as will be described in Chapter 3. [93] However, in many
24
cases, the large fluoroanion GIC’s that have been prepared using other methods cannot
be obtained using F2/AHF (Table 1.4). Most likely, these anions are either not stable
or sufficiently soluble under these conditions for the desired reaction to proceed. Some
GIC compounds described later in this thesis were prepared using electrochemical
intercalation when chemical methods failed. For examples, GIC containing
bis(perfluoropinacolato)borate anion, and GIC containing bis(hexafluorohydroxyisobutyrato)borate anion cannot be obtained using K2MnF6 in aqueous HF, but can be
prepared using electrochemical method in nitromethane.
25
Table 1.4 Synthetic reactions and results using different oxidizing reagents.
anion
oxidizing reagent / solution
product
N(SO2CF3)2¯
K2MnF6 / aq.HF
F2 / AHF
stage 2 GIC
NR
C(SO2CF3)3¯
K2MnF6 / aq.HF
F2 / AHF
high stage GIC
NR
N(SO2CF2CF3)2¯
K2MnF6 / aq.HF
F2 / AHF
high stage GIC
NR
N(SO2CF3)(SO2(CF2)3CF3)¯
K2MnF6 / aq.HF
F2 / AHF
high stage GIC
NR
Al(OR)4¯ *
(R=C(CF3)3)
(Ag salt)
K2MnF6 / aq.HF
PbO2 / aq.HF
NaBiO3 / aq.HF
F2 / AHF
NR
NR
NR
NR
Al(OR)4¯ *
(R=C(CF3)3)
(Li salt)
K2MnF6 / aq.HF
PbO2 / aq.HF
NaBiO3 / aq.HF
F2 / AHF
NR
NR
NR
NR
B[OC(O)C(O)O]2¯
K2MnF6 / aq.HF
F2 / AHF
NR
stage 1/2 GIC
B[OC(CF3)2C(O)O]2¯
K2MnF6 / aq.HF
F2 / AHF
NR
stage 1 GIC
B[OC(CF3)2C(CF3)2O]2¯
K2MnF6 / aq.HF
F2 / AHF
NR
stage 1 GIC
* salt provided courtesy of Dr.Ingo Krossing, Anorganische Chemie, Universität Karlsruhe, Institut Für
Anorganische Chemie, Karlsruhe.
26
1.3 GENERAL APPARATUS AND PROCEDURES
1.3.1
Stainless steel vacuum line
Anhydrous hydrogen fluoride (AHF) (Matheson, Pure grade) and F2 (Air
Products and Chemicals, Inc., > 97 %) were handled on a stainless steel vacuum line
equipped with high-pressure Autoclave Engineering SW-4074 valves and Whitey
1KS4 valves. The vacuum line was fitted with a Kel-F trap (approximately 20 mL
volume) into which AHF could be dynamically condensed (Figure 1.7). Un-reacted
volatile fluorides were destroyed by passing through a soda-lime tower placed before a
N2 cold trap. A Helicoid gauge (0-1500 torr) and Varian thermocouple gauge (0-2000
μm) were attached to the vacuum line in order to monitor the pressure. Reaction
vessels were constructed with 316 stainless steel Swagelok compression fittings
equipped with Teflon ferrules and connected to the line. The vacuum line and reaction
vessels were periodically passivated with F2 under appropriate conditions.
27
autoclave
valves
Helicoid gauge
F2 cylinder
to vacuum pump
to reactors
AHF trap
soda lime
trap
AHF cylinder
autoclave valves
Whitey valves
Figure 1.7 Schematic of stainless steel vacuum line.
N2 (liq) trap
28
1.3.1.1
Handling anhydrous HF (AHF)
AHF (Matheson, pure grade) was condensed into a Kel-F trap, and then
subjected to two freeze/pump/thaw cycles at 0°C to remove H2 before use. The trap
was fitted with Whitey valves at both the inlet and outlet ends of the stainless steel
head. The inlet of the trap was connected to the metal vacuum line while the outlet
was connected to the HF reagent cylinder. The trap was held at 77 K while HF was
condensed in under dynamic vacuum. After collection, the trap was slowly warmed to
room temperature. Very dry AHF may be transferred into reaction vessels when the
temperature is held at 0°C to reduce the vapor pressure of any H2O impurity. The trap
may contain a small amount of SbF5 (to react with water and form involatile oxides).
1.3.1.2
Assembled reaction apparatus for use with AHF
Reactions utilizing AHF were carried out in 3/8‫ ״‬FEP Teflon tubing with
one end heat sealed and the other connected via a 316 stainless steel tee to
accommodate two Teflon reaction tubes (as illustrated in Figure 3.1). The reaction
tubes were preloaded with involatile reactants in an Ar/He glove box and then attached
to the metal vacuum line via a Swagelok fitting with Teflon ferrules.
29
1.3.2
Graphite
Graphite flake (Graftech Inc., GTA grade, average particle diameter ≅ 250 μm)
was employed and used as received in reactions except where otherwise noted. SP-1
grade powder (Union Carbide, average particle diameter ≅ 100 μm) and natural
graphite flake (Aldrich, 1-2 μm) were also used in some reactions.
1.3.3
Elemental analyses
GIC samples to be analyzed for F and B content were first digested using
microwave heating (CEM Corporation, MDS 2000) at 100, 150, and then 200 psi for 5
min each in Teflon advanced composite vessels containing excess HNO3 (70%, Fisher
Scientific) in order to form an acid solution. The solutions were then diluted with
milli-Q water. Elemental analyses for F in GIC products were performed using an ionselective fluoride combination electrode (VWR International, Inc.) with a standard
single-junction sleeve-type reference electrode and pH meter with an expanded mV
scale (VWR 8005). Elemental analyses for B in GIC product were performed using
inductively coupled plasma - atomic emission spectroscopy, ICP-AES (S.A. Inc,
JY2000U analyzer).
30
1.3.4
Powder X-ray diffraction (Powder XRD)
Powder XRD data were collected on a Siemens D5000 powder diffractometer
with Ni-filtered Cu Kα radiation (1.5418 Å) using a variable slit mode and a detector
slit width of 0.2 mm. Data were collected at 0.02° 2Ө steps, between 2° and 65°.
Collection times varied from 0.1 s/step for routine analyses to 1 s/step for data used in
structural modeling. The materials dealt with mostly were layered structures, and the
platy particles tend to lie flat on the sample holder (the preferred orientation effect).
Therefore, the structural information obtained often relates primarily to the direction
perpendicular to the layers. The diffracted peak positions directly provide the repeat
distances of layers (basal spacing) and the change of interlayer distance due to an
intercalation reaction can be readily obtained.
1.3.5
Thermal gravimetric analysis (TGA)
Thermal analyses were used to observe physical and chemical changes as a
function of temperature. TGA data shows the mass of samples as the temperature
increases linearly. In this research, TGA data were obtained using a Shimadzu, Inc.
TGA-50 thermogravimetric analyzer. Samples were loaded into a platinum pan; the
sample chamber was flushed with nitrogen. Thermal scans from ambient to 800°C
were performed.
31
1.4
FLUORINE ANALYSIS IN GIC’s
1.4.1
Control of solution parameters
In this thesis research, a new sample digestion protocol/method was developed to
allow the determination of free fluoride for GIC products using analyses based on ionselective fluoride electrodes. HF is a weak acid in aqueous solutions (pKa = 3.18). For
pH < 5, the formation of HF or HF2¯ (which cannot be detected by the fluoride
electrode) interferes with accurate F analyses. In addition, the total ionic strength of
sample solutions must be maintained with the specification range of the electrode. In
order to address these issues, we used a combination of both samples and standards
with an equal volume of low level–total ionic strength adjustment buffer (LL-TISAB)
to provide control over both pH and ionic strength.
1.4.2
Test of digestion procedure
In order to determine the effectiveness of our digestion protocol, and eliminate
the possibility of the presence of fluoride from other sources, control experiments
were performed. The other sources of fluoride could be from HNO3 or other reagents
used in digestion, the anions, graphite flake, or even milli-Q water used in dilution of
samples. All dilutions were performed using standard silica glassware at ambient
32
temperature. Sodium fluoride was used as a standard fluoride sample to prepare stock
and standard fluoride solutions. Control samples of milli-Q water, graphite flake,
70%HNO3, and salts were dissolved with 70%HNO3 using the microwave digester
and diluted to 0.2%v/v acid solutions due to the limited pH range of fluoride electrode.
Equal volumes of LL-TISAB were added into sample solutions prior to analysis in
order to adjust total ionic strength. Less than 0.1 ppm F of all samples was detected as
shown in Table 1.5. It was therefore shown that no other significant source of fluoride
is introduced in this digestion method.
Additionally, in order to check the accuracy of fluoride measurement
determined by this method, a comparison study between digested and undigested
samples with known fluoride content was performed. Sodium fluoride was used as
fluoride source. One set of samples was digested according to the previously described
procedure, and another was only dissolved into milli-Q water. Results are shown in
Table 1.6.
33
Table 1.5 Fluoride content of digested control samples.
Control samples
fluoride (ppm)
milli-Q water
0.009
graphite flake
0.011
HNO3
0.024
lithium(trifluoromethanesulfonyl)imide, LiN(SO2CF3)
0.021
potassium perfluorooctanesulfonate, KC8F17SO3
0.029
lithium(oxalato)borate,
LiB[OC(O)C(O)O]2
0.026
sodium bis(perfluoropinacolato)
borate, NaB[OC(CF3)2C(CF3)2O]2
0.027
sodium bis(hexafluorohydroxyisobutyrato)borate,
NaB[OC(CF3)2C(O)O]2
0.023
34
Table 1.6 Comparison of fluoride content between digested and undigested control
samples.
glassware
fluoride (ppm)
detected
calculated
%error
digested
1
2
3
3.277
3.408
3.042
3.225
3.507
3.037
mean
1.6
-2.8
-0.2
-0.5
mean
1.9
1.2
1.4
1.5
undigested
1
2
3
3.767
4.613
3.521
3.696
4.556
3.472
Both sample sets gave average errors <2%. Results obtained from digested
samples are slightly lower than those from undigested samples; this might be
explained by the sample loss during the additional transfer step.
1.4.3
Test for container reactions
One concern in handling hydrofluoric acid for quantitative analysis is the error
associated with HF reaction with silica. Because it is convenient to perform dilutions
using standard silica glassware at ambient temperature, control experiments were
performed using only Nalgene® plastic labware in order to evaluate any effect on
35
fluoride content resulting from HF reaction with silica. Data obtained are in Table 1.7.
As can be seen, there is no significant difference between the sample sets. In both
cases, detected fluoride is consistently lower than the calculated value, by
approximately 2%. This is most likely explained by a slight loss of sample during
transfer from microwave vessel to volumetric flask.
Table 1.7 Comparison of fluoride analyses between digested samples using glass and
plastic labware.
glassware
fluoride (ppm)
detected
calculated
%error
Pyrex®
1
2
3
Nalgene
1
2
3
2.084
1.769
1.441
2.114
1.802
1.472
®
1.630
1.468
1.843
mean
-1.4
-1.8
-2.1
-1.8
mean
-1.7
-2.2
-1.8
-1.9
1.658
1.502
1.877
As can be seen from all control experiments, the digestion method developed
with standard glassware can be used for quantitative F analyses with errors of < 2%.
Additional work in the future should be performed to reduce the small systematic error
found in these control experiments.
36
1.5 POWDER X-RAY DIFFRACTION ANALYSIS OF GIC’S
Powder XRD data provided a series of (00l) reflections, as shown in the Figure
1.8 for stage 1 CxB[OC(O)C(O)O]2·δF, the data from this Figure are summarized in
Table 1.8. Integrated peak areas are determined by DIFFRAC AT software from
intensities in counts per second (cps) multiplied by the step time (s); a linear
background was assumed in all cases. [94] As can be seen, the major reflections of the
powder XRD patterns in Figure 1.8 agree well for stage 1 GIC in both observed and
calculated gallery heights, di. However, there is some evidence for a mixture of stage 1
and 2 GIC’s, as observed from asymmetrical peaks and a shoulder on the (005) peak.
In the case of flaky graphite, which has a very large particle diameter (≅ 250 μm), it is
often difficult to obtain homogeneous single-stage GIC products. It is likely that the
higher stage product is located near the particle centers, and lower stage GIC’s are
near the edges.
0
10
30
40
50
(009)
(006)
20
(008)
(005)
(002)
(001)
intensity
(004)
37
60
2 Ө / degree
Figure 1.8 Powder XRD pattern of stage 1 products of CxB[OC(O)C(O)O]2·δF.
Assigned (00l) indices are indicated.
38
Table 1.8 Data obtained from powder XRD of stage 1 CxB[OC(O)C(O)O]2·δF
peak #
2Ө
dobs
/ nm
dcalc
/ nm
index
(hkl)
integrated
peak area
1
6.163
1.43
1.42
(001)
9.9
2
12.43
0.713
0.712
(002)
11.3
3
25.19
0.354
0.350
(004)
385
4
31.43
0.284
0.285
(005)
51
5
37.94
0.237
0.237
(006)
3.5
6
51.46
0.177
0.178
(008)
10.8
7
58.32
0.158
0.158
(009)
8.8
39
1.6 STRUCTURAL REFINEMENT MODELING
1.6.1 Overview
The energy-minimized structure of large anions such as C8F17SO3¯ [95],
N(SO2CF3)2¯
[92],
B[OC(CF3)2C(CF3)2O]2¯
B[OC(O)C(O)O]2¯
[93],
B[OC(O)C(CF3)2O]2¯
[96],
[97] are calculated using the hybrid density functional
method (B3LYP) with a 6-31G* basis set and GAUSSIAN software. One-dimensional
electron density maps are calculated from a structural model and also obtained from
observed powder XRD reflection intensities.
Observed intensity data are used to generate structure factors after correction by
a Lorentz-polarization factor:
Fobs
⎡ ⎛ sin 2 θ cosθ
= ± ⎢ I × ⎜⎜
2
⎣ ⎝ 1 + cos 2θ
⎞⎤
⎟⎟⎥
⎠⎦
1/ 2
(1.12)
where I is the integrated peak intensity. Calculated structure factors, F00l, are obtained
using the angle-dependent atomic scattering factors. [98] Electron density maps are
then generated for z = 0-1, at increments of 0.004, using:
1⎡
⎛ 2π
ρ ( z ) = ⎢ F0 + 2∑ F00l cos⎜
c⎣
⎝ z
⎞⎤
⎟⎥
⎠⎦
(1.13)
40
where z is the fractional coordinate of atoms along the c axis, c is the unit cell
dimension, and Fo is the zero-order structure factor. Structures are refined by
minimizing the crystallographic R factor:
R=
∑ k F (00l ) − F (00l )
∑ k F (00l )
00 l
cal
(1.14)
obs
In order to illustrate the method used to fit structural models from a onedimensional electron density map, the GIC containing bis(oxalato)borate anion
(CxB[OC(O)C(O)O]2·δF) with the powder XRD data shown in the previous section
will be used as an example below.
1.6.2 Structural model calculations
All of the molecular structures generated from this research are determined
using Gaussian 03W software. The standard orientation geometry displays atomic
coordinates for atoms internally in Cartesian coordinates. This orientation is chosen
for maximum calculation efficiency, and corresponds to placing the center of nuclear
charge for the molecule at the origin.
Figure 1.9 (a), shows the optimized geometry determined for an isolated
bis(oxalato)borate anion. Figure 1.9 (b) indicates that this relatively large anion is
oriented with the long axis perpendicular to the graphene surfaces, having the B atom
41
at the gallery center, with four O atoms coordinated in tetrahedral geometry. This
orientation is also employed for the other borate chelate anions. [96,97]
In the
structural model shown in Figure 1.9 (b), the GIC containing bis(oxalato)borate is
prepared using F2/AHF, leading to additional fluoride ions being co-intercalated into
the galleries. The position of these fluoride sites is indicated; these sites are only
partially filled.
1.6.3 One-dimensional electron density map
One-dimensional electron density maps of graphene sheets and bis(oxalato)
borate anion intercalate for the structural model were calculated and are shown in
Figure 1.10 (a) and (b), respectively. The number of the terms used in the calculation
the one-dimensional electron density map affects the intensity and width of calculated
peaks, and also the presence higher-order (ringing) peaks.
The dimensions of the bis(oxalato)borate anion in Figure 1.9 (a) indicate
that this relatively large anion must be oriented with the long axis perpendicular to the
graphene surfaces. As illustrated in Figure 1.10 (b), this arrangement results in the
electron density peak due to the B atom at the center (z = 0.5) with two adjacent O
atom peaks (z ≅ 0.45 and 0.55), two C atom peaks at z ≅ 0.35 and 0.65, and another
two O atom peaks appear at z ≅ 0.25 and 0.75. When fluoride co-intercalates are
42
included in the galleries by positioning on the graphene surfaces, these electron
density peaks appear at z ≅ 0.28 and 0.72.
One-dimensional electron density profiles of stage 2 CxB[OC(O)C(O)O]2·δF
derived from the calculated structural model using 9 terms, 25 terms, and derived from
observed powder XRD data using 9 terms are shown in Figure 1.11 (a), (b), and (c),
respectively. As can be seen, using more terms in the one-dimensional electron density
profile calculation provides a more detailed and sharper calculated density map.
However, the number of terms for calculated maps should not be much greater than
the number of available diffraction peak intensities in observed data. Due to the
limited diffraction data, the correspondence of these maps is sufficient to show the
orientation of the chelate borate anions; however, they cannot be used to quantify the
compositional parameters x and δ in CxAn·δF.
43
B
C
O
F
x
z
(a)
y
(b)
Figure 1.9 (a) A geometry-optimized structural model of isolated bis(oxalato)borate
anion, (b) structural model of stage 2 CxB[OC(O)C(O)O]2·δF, with four fluoride cointercalates per borate anion. Axes and some atomic labels are included.
44
(a)
e¯ density
(i)
(ii)
(iii)
0.0
0.2
0.4
O
C
O
0.6
B
O
0.8
1.0
C O
(b)
e¯ density
(i)
(ii)
(iii)
0.0
0.2
0.4
0.6
0.8
1.0
z
Figure 1.10 One-dimensional electron density profiles of (a) graphene sheets and
(b) bis(oxalato)borate anion intercalate using (i) 9, (ii) 25, and (iii) 50 terms in the
calculation.
45
density
(a)
0.0
0.2
0.4
0.6
0.8
1.0
z
density
(b)
0.0
0.2
0.4
0.6
0.8
1.0
z
density
(c)
0.0
0.2
0.4
0.6
0.8
1.0
z
Figure 1.11
One-dimensional electron density profiles of stage 2
CxB[OC(O)C(O)O]2·δF derived from calculated structural model using (a) 9 terms,
(b) 25 terms, and (c) derived from observed powder XRD data using 9 terms.
46
1.6.3.1 Optimization of intercalate gallery structure
In order to obtain the best fit of one-dimensional electron density
maps for the structural model with the observed data, the orientation and position of
borate chelate anion and fluoride co-intercalates can be optimized. In these studies, the
borate chelate anions are oriented with long z-axis fixed to be perpendicular to the
graphene sheets, with the anion centered between graphene sheets. The following
parameters were considered in fitting the one-dimensional electron density:
The distance of graphene sheet
to fluoride co-intercalate plane
fixed at 0.31 nm
Mole ratio of graphene
sheet and intercalate, x
optimized parameter
Mole ratio of borate chelate anion
to fluoride co-intercalate, δ
optimized parameter
The distance of graphene sheet to fluoride co-intercalate plane was at first set to be
0.32 nm from the value known from stage 2 CxHF2. Starting with this value, a better
fit was obtained by slightly adjusting the value to 0.31 nm. The fluoride co-intercalates
are fixed to lie in equal concentration on both encasing graphene surfaces. The
optimized values obtained for x and δ by this method can subsequently be compared to
those determined by elemental analyses.
47
1.7
THESIS OVERVIEW
Further investigation into new graphite intercalation compounds can provide
new materials and a better understanding of their properties. In this research, the goal
has been to develop new chemical synthesis strategies, and to prepare acceptor-type
GIC’s with new anions.
Chapter 2 and 3 are studies on the fluoride contents of GIC’s containing both
large fluoroanions (bis(trifluoromethanesulfonyl)imide and perfluoroctanesulfonate)
and fluoride co-intercalate. A new digestion method is developed using an ionselective electrode and potentiometer that can quantitatively determine the fluoride cointercalate content in the solid sample. Before these studies, these GIC’s had been
reported without detailed analyses of fluoride co-intercalate content.
Chapter 4 describes the preparation of a new GIC containing bis(oxalato) borate
by chemical intercalation using F2(g)/AHF. This is the first time that this GIC was
synthesized. GIC’s up to stage 1 have been prepared, with a gallery height of 1.42 nm.
The composition of this GIC is determined from thermogravimetric and elemental
analyses.
Chapter 5 describes the preparation of GIC’s containing the borate chelate
anions bis(perfluoropinacolato)borate and bis(hexafluorohydroxyisobutyrato)borate by
chemical intercalation using F2(g)/AHF. This is the first time that these GIC’s were
prepared by chemical intercalation; previously they had been prepared using
electrochemical oxidation. By these chemical syntheses, GIC’s up to stage 1 can be
48
obtained, with gallery heights of 1.45 and 1.43 nm, respectively. The composition of
these GIC’s are determined using thermogravimetric and elemental analyses.
49
1.8 REFERENCES
1.
Lerf, A. Intercalation Compounds in Layered Host Lattices: Supramolecular
Chemistry in Nanodimensions. In Handbook of Nanostructured Materials and
Nanotechnology; Nalwa, H., Ed.; Acad. Press: New York, 2000; Vol. 5, 1.
2.
Barrer, R. M. Inorganic Inclusion Complex. In Nonstoichiometric Compounds;
Mandelcorn, L., Ed.; Academic Press: New York, 1964; Chapter 6.
3.
Gamble, F. R.; Geballe, T. H. Inclusion Compounds. In Treatise on Solid State
Chemistry; Hannay, N. B. Ed.; Plenum: New York, 1976; Vol. 3, Chapter 2.
4.
Whittingham, M. S.; Jacobson, A. J. Intercalation Chemistry; Academic Press:
New York, 1982.
5.
Schöllhorn, R. Intercalation Compounds. In Inclusion Compounds; Atwood, J.
L.; Davies, J. E. D.; MacNicol, D. D., Eds.; Academic Press: London, 1984;
Vol. 1, Chapter 7.
6.
Legrand, A. P.; Flandrois, S. Chemical Physics of Intercalation; NATO ASI
Series B; Plenum: New York, 1987; p 172.
7.
Müller-Warmuth, W.; Schöllhorn, R. Progress in Intercalation Research;
Kluwer Academic: Dordrecht, the Netherlands, 1994.
8.
O’Hare, D. Inorganic Intercalation Compounds. In Inorganic Materials;
Bruce, D. W.; O’Hare, D., Eds.; Wiley: Chichester, 1992; Chapter 4.
9.
Whittingham, M. S. Prog. Solid State Chem. 1978, 12, 41.
10.
Albeti, G. Layered Solids and Their Intercalation Chemistry. In Comprehensive
Supramolecular Chemistry; Alberti, G.; Bein, T., Eds.; Pergamon: Oxford, 1996;
Vol. 7, Chapter 1.
11.
Jacobson, A. J. Intercalation Reactions of Layered Compounds. In Solid State
Chemistry — Compounds; Cheetham, A. K.; Day, P., Eds.; Clarendon: Oxford,
1992: Chapter 6.
12. Levy, F. Intercalated Layered Materials; Reidel: Dordrecht, the Netherlands,
1979.
13. Dresselhaus, M. S. Intercalation in Layered Materials; NATO ASI Series B;
Plenum: New York, 1986, Vol. 148.
50
14.
Grim, R. E. Clay Mineralogy, 2nd ed.; McGraw-Hill: New York, 1968.
15. Theng, B. K. G. The Chemistry of Clay-Organic Reactions; Hilger: London,
1974.
16.
Lagaly, G. Smectitic Clays as Ionic Macromolecules. In Developments in Ionic
Polymers; Wilson, A. D.; Prosser, J., Eds.; Elsevier Applied Science Publishers:
London, 1986; Vol. 2, Chapter 2.
17.
Vaia, R. A.; Giannelis, E. P. Macromolecules 1997, 30, 8000.
18. Biswas, M.; Sinha Ray, S. Adv. Polym. Sci. 2001, 155, 167.
19.
Lee, J.; Takekoshi, T.; Giannelis, E. P. Mat. Res. Soc. Symp. Proc. 1997, 457,
513.
20. Métrot, A.; Guerard, D.; Billaud, D.; Herold, A. Synth. Met. 1979/1980, 1, 363.
21.
Dresselhaus, M. S.; Dresselhaus, G. Adv.Phys. 1981, 30, 139.
22. Thomas, J. M.; Millward, G. R.; Schlögl, R.; Boehm, H. P. Mater. Res. Bull.
1980, 15, 671.
23.
Hooley, J. G. Carbon 1972, 10, 155.
24.
Billaud, D.; Metrot, A.; Willann, P.; Herold, A. Proceedings of International
Carbon Conference, Baden-Baden, 1980, p 140.
25.
Billaud, D.; Chenite, A.; Metrot, A. Carbon 1982, 20, 493.
26.
Besenhard, J. O.; Fritz, H. P. Naturforsch. 1972, 27B, 1294.
27.
Akuzawa, N.; Kamoshita, T.; Tsuchiya, K.; Matsumoto, R. Tanso 2006, 222,
107.
28. Okuyama, N.; Takahashi, T.; Kanayama, S.; Yasunaga, H. Physica 1981, 105B,
298.
29. Parry, G. S.; Nixon, D. E.; Lester, K. M.; Levene, B. C. J. Phys. C. 1969, 2,
2156.
30.
Underhill, C.; Krapchev, T.; Dresselhaus, M. S. Synth. Met. 1980, 2, 47.
51
31. Guérard, D.; Chaabouni, M.; Lagrange, P.; El Makrini, S.; Herold, A. Carbon
1980, 18, 257.
32. Avdeev, V. V.; Nalimova, V. A.; Semenenko, K. N. High Pressure Res. 1990,
6, 11.
33. Avdeev, V. V.; Nalimova, V. A.; Semenenko, K. N. Synth. Met. 1990, 38, 363.
34.
Besenhard, J. O.; Möhwald, H.; Nickl, J. J. Carbon 1980, 18, 399.
35. Jegoudez, J.; Mazieres, C.; Setton, R. Carbon 1986, 24, 747.
36.
Takenaka, H.; Kawaguchi, M.; Lerner, M. M.; Bartlett, N. J. Chem. Soc., Chem.
Commun. 1987, 19, 1431.
37. Zhang, Z.; Lerner, M. M. Chem. Mater. 1996, 8, 257.
38.
Rüdorff, W.; Stumpp, E.; Spriesseler, W.; Siecke, F. W. Angew. Chem. Int. Ed.
Engl. 1963, 2, 67.
39.
Dresselhaus, G.; Nicholls, J. T.; Dresselhaus, M. S. In Graphite Intercalation
Compounds II; Zabel, H., Solin, S. A., Eds.; Springer-Verlag: Berlin, 1990;
p 247.
40.
Flandrois, S. Synth. Met. 1982, 4, 255.
41.
Hérold, A. Crystallo-Chemistry of Carbon Intercalation Compounds. In Physics
and Chemistry of Materials with Layered Structures; Levy, F., Ed.; Reidel:
Dordrecht, 1979, vol. 6, p 323.
42.
Amine, K.; Tressaud, A.; Imoto, H.; Fargin, E.; Hagenmuller, P.; Touhara, H.
Mater. Res. Bull. 1991, 26, 337.
43.
Nakajima, T. Fluorine-Carbon and Fluoride-Carbon Materials—Chemistry,
Physics and Applications. Marcel Dekker: NewYork, 1995.
44.
Billaud, D.; Pron, A.; Vogel, F.; Herold, A. Mater. Res. Bull. 1980, 15, 1627.
45. Foley, G. M. T.; Zeller, C.; Vogel, F. L. Solid State Commun. 1977, 24, 371.
46.
Tressaud, A.; Amine, K.; Mayorga, S. G.; Fargin, E.; Grannec, J.;
Hagenmuller, P.; Nakajima, T. Eur. J. Solid State Inorg. Chem. 1992, 29, 947.
52
47.
Nakajima, T.; Watanabe, N. Graphite Fluorides and Carbon-Fluorine
Compounds; CRC Press: Boca Raton, FL, 1991.
48.
Vogel, F. L.; Foley, G. M. T.; Zeller, C.; Falardeau, E. R.; Gan, J. Mater. Sci.
Eng. 1977, 31, 261.
49. Hamwi, A.; Touzain, P.; Bonnetain, L. Rev. Chim. Minér. 1982, 19, 651.
50.
Interrante, L. V.; Markiewiz, R. S.; McKee, D. W. Synth. Met. 1979, 1, 287.
51.
Thompson, T. E.; McCarron, E. M.; Barlette, N. Synth. Met. 1981, 3, 255.
52.
Du Pont de Nemours, E. I. and Co., Res. Discl. 1976, 146, 13.
53. Preisegger, E.; Henrici, R. Int. J. Refrig. 1992, 15, 326.
54.
Nader, F. W. Beitr. Dechema-Fachgespraechs Umweltschutz, 8th 1990, 373.
55.
Vaknin, D.; Davidov, D.; Zevin, V.; Selig, H. Phys. Rev. B 1987, 35, 6423.
56. Hamwi, A.; Touzain, P.; Bonnetain, L. Mater. Sci. Eng. 1977, 31, 95.
57.
Mouras, S.; Hamwi, A.; Djurado, D.; Cousseins, J. C. Rev. Chem. Miner.
1987, 24, 572.
58. McCarron, E. M.; Grannec, Y. J.; Bartlett, N. J. Chem. Soc. Chem. Commu.
1980, 890.
59.
Flandrois, S.; Grannec, J.; Hauw, C.; Hun, B.; Lozano, L.; Tressaud, A.
J. Solid State Chem. 1988, 77, 264.
60. Lerner, M.; Hagiwara, R.; Bartlett, N. J. Fluorine Chem. 1992, 57, 1.
61.
Kadono, K. Ph. D. Thesis, Kyoto University (1986).
62.
Touhara, H.; Kadono, K.; Imoto, H.; Watanabe, N.; Tressaud, A.; Grannec, J.
Synth. Met. 1987, 18, 549.
63.
Zhang, X.; Lerner, M. M. Chem. Mater. 1999, 11, 1100.
64. Jobert, A.; Touzain, P.: Bonnetain, L. Carbon 1981, 19, 193.
65. Billaud, D.: Pron, A.: Vogel, L. V. Synth. Met. 1980, 2, 177.
66. Billaud, D.; Flanders, P. J.; Pron, A.; Fischer, J. E. Mater. Sci. Eng. 1982, 54, 31.
53
67.
Moran, M. J.; Miller, G. R.; De Marco, R. A.; Resing, H. A. J. Phys. Chem.
1984, 88, 1580.
68.
Aronson, S.;Lermont, S.; Weiner, J. Inorg. Chem. 1971, 10, 1296.
69.
Bindl, M. Thesis. TU München, 1986
70.
Aronson, S.; Frishberg, C.; Frank, G. Carbon 1971, 9, 715.
71.
Bottomley, M. J.; Parry, G. S.; Ubbelohde, A. R. Proc. R. Soc. London Ser.A
1964, A 279, 291.
72.
Avdeev, V. V.; Sorokina, N. E.; Nikol'skaya, I. V.; Monyakina, L. A.;
Voronkina, A. V. Inorg. Mater. (Translation of Neorganicheskie
Materialy) 1997, 33, 584.
73.
Mittal, J.; Konno, H.; Inagaki, M. Synth. Met. 1998, 96, 103.
74.
Sorokina, N. E.; Khaskov, M. A.; Avdeev, V. V.; Nikol'skaya, I. V. Russian
Journal of General Chemistry 2005, 75, 162.
75.
Yang, X.; Makita, Y.; Liu, Z.; Sakane, K.; Ooi, K. Chem. Mater. 2004, 16, 5581.
76. Barsukov, I.; Zaleski, P. L. U.S. Patent 6,756,027, June 29, 2004.
77.
Amriou, T.; Khelifa, B.; Aourag, H.; Aouadi, S. M.; Mathieu, C. Mater. Chem.
Phys. 2005, 92, 499.
78. Essaddek, A.; Herold, C.; Lagrange, P. Comptes Rendus de l'Academie des
Sciences, Serie IIb: Mecanique,Physique, Chimie, Astronomie 1994, 319, 1009.
79. Jagtap, N.; Ramaswamy, V. Applied Clay Science 2006, 33, 89.
80. Fuzellier, H.; Lelaurain, M.; Mareche, J. F. Synth. Met. 1990, 34, 115.
81.
De la Cruz, F. A.; Cowley, J. M. Acta Crystallog. 1963, 16, 531.
82.
Nixon, D. E.; Parry, G. S.; Ubbelohde, A. R. Proc. R. Soc. London 1966,
291A, 32.
83. Lopez-Gonzalez, J. D.; Rodriguez, A. M.; Vega, F. D. Carbon 1969, 7, 583.
84.
Nixon, D. E.; Parry, G. S.; Ubbelohde, A. R. Proc. R. Soc. London 1964,
A291, 324.
54
85.
Horn, D.; Boehm, H. P. Mater. Sci. Eng. 1977, 31, 87.
86.
Rüdorff, W.; Siecke, W. F. Chem. Ber. 1958, 91, 1348.
87.
Touhara, H.; Kadano, K.; Fujii, Y.; Watanabe, N. Z. Anorg. Allg. Chem. 1987,
544, 7.
88. Watanabe, N.; Takashima, M. Kogyo Kagaku Zasshi 1971, 74, 1788.
89.
Nakajima, T.; Kameda, I.; Endo, M.; Watanabe, N. Carbon 1986, 24, 343.
90.
Platzer, N.; de la Martiniére, B. Bull. Soc. Chim. Fr. 1961, 1961, 177.
91.
Enoki, T.; Suzuki, M.; Endo, M. Graphite Intercalation Compounds and
Applications; Oxford University Press: New York, 2003.
92. Zhang, X.; Sukpirom, N.; Lerner, M. M. Mater. Res. Bull. 1999, 34, 363.
93. Katinonkul, W.; Lerner, M. M. J. Phys. Chem. Sol. 2007, 68, 394.
94. SIEMENS, DIFFRAC AT software; Version 3.1, Socabim, 1993.
95.
Zhang, X.; Lerner, M. M. Phys. Chem. Chem. Phys. 1999, 1, 5065.
96.
Yan, W.; Lerner, M. M. J. Electrochem. Soc. 2004, 151, 1.
97.
Yan, W.; Lerner, M. M. J. Electrochem. Soc. 2003, 150, D169.
98.
Su, Z.; Coppens, P. Acta Crystallogr. 1997, A53, 749.
55
CHAPTER 2
EFFECT OF REACTION TIME ON THE COMPOSITION OF
GRAPHITE BIS(TRIFLUOROMETHANESULFONYL)IMIDE
Watcharee Katinonkul and Michael M. Lerner
Department of Chemistry
Oregon State University,
Corvallis, OR 97331-4003
USA
Carbon 2007, 45(3), 499-504.
56
2.1 ABSTRACT
Graphite intercalation compounds (GIC’s) of composition CxN(SO2CF3)2·δF
are prepared under ambient conditions in 48% hydrofluoric acid, using K2MnF6 as an
oxidizing reagent. The stage 2 GIC product structures are determined using powder
XRD and modeled by fitting one dimensional electron density profiles. A new
digestion method followed by selective fluoride electrode elemental analyses allows
the determination of free fluoride within products, and the compositional x and δ
parameters are determined for reaction times from 0.25 to 500 h.
Keywords: Graphite; Intercalation; X-ray diffraction; Modeling
57
2.2 INTRODUCTION
Transition metal fluoroanions can be used as oxidants in the formation of
graphite intercalation compounds (GIC’s) in anhydrous HF (AHF) or hydrofluoric
acid. An initial study reported the formation of graphite bifluoride and planar-sheet
graphite fluoride by this approach: [1]
xC
where M
=
AMFn / AHF
25 °C
CxHF2·δHF (x > 28) or CxF (x > 2)
(2.1)
Mn (IV), Ni (IV), Co (III), or Fe (III). In these GIC products, x
corresponds to the ratio of graphene carbon to anion, and δ to the ratio of neutral
intercalate to anion intercalate.
Reactions using these same oxidants in combination with fluoroanion salts
have also produced a range of new GIC’s. The oxidant Mn(IV), in the form of MnF62-,
has been particularly suitable for obtaining new GIC’s because the fluoromanganate
anion does not itself intercalate into graphite under these conditions. For example, the
addition of soluble perfluoroalkanesulfonate salts leads to intercalation of these
anions, such as in the following reaction where a stage 2 GIC is obtained with x≅18
and δ≅4 [2]:
aqu. HF
xC + KC8F17SO3
K2MnF6
CxC8F17SO3·δF
(2.2)
58
Stage 2 GIC products of the intercalates C10F21SO3¯, C2F5OC2F4SO3¯, and
C2F5(C6F10)SO3¯ have been similarly prepared in hydrofluoric / nitric acid solutions,
[3] and mixed-stage GIC’s with large fluoroanions such as bis(pentafluoroethanesulfonyl)imide,
sulfonylimide,
(CF3CF2SO2)2N¯,
trifluoromethanesulfonyl-n-nonafluorobutane-
(CF3SO2)(CF3(CF2)3SO2)N¯,
and
tris(trifluoromethanesulfonyl)
methide, (CF3SO2)3C ¯, have also been obtained by similar reactions. [4]
GIC’s containing the bis(trifluoromethanesulfonyl)imide (TMSI) anion,
CxN(SO2CF3)2, were prepared for the first time under ambient conditions in 48%
hydrofluoric acid containing K2MnF6. Within 15 minutes, a stable, stage 2 GIC
product with x=37 and gallery height, di, of 0.81 nm was obtained. The anion
monolayers are oriented within the galleries such that the pseudo-cylindrical anions
have their long axes parallel to the graphene sheets, with the N oriented towards either
encasing sheet. [4]
It is well known that the oxidation of graphite and intercalation of anions can
also lead to reversible co-intercalation of neutral species. These can act as dielectric
spacers (i.e. solvents) within galleries. Examples include the co-intercalation of parent
acid with conjugate anions:
xC + HSO4¯
xC + HF2¯
H2SO4
HF
CxHSO4·δH2SO4
CxHF2·δHF
+
+
e¯
e¯
(2.3)
(2.4)
59
where typical x and δ values are 24 and 2-3, respectively [5]. A chemical method to
obtain a similar product to that from reaction (2.4) used elemental fluorine in the
presence of liquid or gaseous HF at 20°C to produce a stage 2 salt, CxF·δHF (stage 2:
x=16; 1≤δ≤4.7). This GIC evolves HF and increases in stage (from stage 2 to 3, or 1 to
2) after evacuation to remove neutrals. [6-8] Another common co-intercalation
involves organic solvent molecules, for example:
xC + LiAsF6
- e¯ / CH3NO2
CxLiAsF6·δ CH3NO2
+
e¯
(2.5)
where typical x and δ values are 24 and 2, respectively. [9]
Aside from the co-intercalation of neutral species, the intercalation reaction
can be complicated by the incorporation of more than one species of anionic
intercalate. Important examples of this complexity are the presence of oxide or
hydroxide in highly-oxidized GIC’s containing oxoanion intercalates, and the presence
of fluoride in GIC’s containing fluoroanion intercalates. The presence of oxide or
fluoride intercalate within graphene galleries can have significant impact on physical
or chemical properties. [10]
During graphite oxidation in an aqueous H2SO4 electrolyte, graphite sulfate
salts are formed initially. Graphite oxide, which, unlike the graphite salts, involves
directed C-O bonding, is produced when oxidation is continued on the 1st-stage salt
beyond a charge on carbon corresponding to C21+. [11] This is reinforced by longknown chemical preparations of graphite oxide which involve the action of strong
60
oxidizing agent (KMnO4, KClO3) on the graphite salts of oxoanions (ClO4¯, HSO4¯,
NO3¯). [12] The mechanism for the conversion of the graphite salt to graphite oxide is
not completely understood. Indeed, the structure for graphite oxide itself is still being
explored: the oxide or hydroxide within galleries may result from conversion of
oxoanions of neutral co-intercalate species. [13] Similar reactions are known for
fluoroanions to generate fluoride intercalate within graphene galleries. For example,
the electrochemical conversion of the graphite bifluoride salt, CxHF2(HF)δ (x=16), to
the planar-sheet graphite fluoride, CxF(HF)δ (x=5.5), by oxidation in anhydrous
HF/1M NaF has been demonstrated. [6] In this study, we explore the detailed
composition of the GIC obtained by oxidation of graphite in hydrofluoric acid
containing the TMSI anion.
After evacuation of the product to remove any neutral HF co-intercalate, a
quantitative approach to determining the fluoride intercalate content is developed and
employed. Several quantitative analytical methods for fluoride are known, including
spectrophotometry [14] and chromatography; [15] however, the most common and
convenient approach is the electroanalytical method based on potentiometry with ionselective electrodes. [16, 17] Aqueous matrices are generally the easiest to analyze,
and typical applications include the determination of fluoride ion in drinking water,
[18] rainwater, [19] serum, [20] etc. The accuracy of the analyses depends on the
calibration procedure (direct reading potentiometry, single or double standard
addition, bracketing solutions, Gran’s method). [21] In this work, a new digestion
method is developed for GIC samples containing both fluoroanions and fluoride co-
61
intercalate that can quantitatively determine the fluoride intercalate content in the solid
sample.
2.3 EXPERIMENTAL
Graphite flake (Graftech Inc., GTA grade, 250 µm average particle diameter),
hydrofluoric acid (Mallinckrodt AR, 48% w/o), hexane (Fischer Chem, certified
grade), lithium trifluoromethanesulfonimide, LiN(SO2CF3)2, (3M Corp), nitric acid
(Fischer Scientific, certified ACS PLUS, 68.0-70.0%), glacial acetic acid (EM
Science, 99.7%), and sodium chloride (Mallinckrodt, AR grade) were used as
received. Sodium fluoride (Mallinckrodt, AR grade) was baked at 105°C for 1 h
before use. Potassium hexafluoromanganate, K2MnF6, was synthesized as a bright
yellow powder according to a literature method. [22] Ultrapure water (resistivity = 18
MΩ·cm at 25°C) from a Milli-Q Labo system (Millipore, Milford, MA), was used
throughout the experiments. Low Level Total Ionic Strength Adjustment Buffer (LLTISAB) was prepared by addition of glacial acetic acid (57 mL) and reagent-grade
NaCl (58 g) to 500 mL water, slow addition of 5 M NaOH until the pH was between
5.0-5.5, and then dilution to 1.00 L total volume. [23]
In these reactions, LiN(SO2CF3)2 (200 mg, 0.70 mmol) and K2MnF6 (180 mg,
0.70 mmol) were first dissolved into hydrofluoric acid (15 mL) at ambient
temperature, and then graphite (100 mg, 8 mmol) was added to provide mole ratios of
62
12:1:1 for graphene C : Mn (IV) : N(SO2CF3)2¯ anion. The solutions were stirred in air
for the specified reaction times, then filtered and rinsed with 15 mL hydrofluoric acid.
The filtrate was briefly rinsed with 5 mL hexane and dried overnight under vacuum.
Products were subsequently stored under an inert atmosphere.
Powder X-ray diffraction (powder XRD) data were collected on a Siemens
D5000 diffractometer with Ni-filtered Cu Kα radiation using a variable slit mode and
a detector slit width of 0.2 mm. Data were collected at 0.02 °2Ө steps, between 2˚ and
65˚. Collection times varied from 0.1 s / step for routine analyses to 1 s / step for data
used in structural modeling.
TGA data were obtained using a Shimadzu, Inc. TGA-50 thermogravimetric
analyzer. Samples were loaded into a platinum pan; the sample chamber was flushed
with nitrogen. Thermal scans from ambient to 800 °C were performed under flowing
N2 at 5°C / min.
A new sample digestion protocol was developed for the GIC products.
Samples (30 mg) were digested with 70% HNO3 (1 mL) using a microwave digester
(CEM Corporation, MDS 2000) at 100, 150, and then 200 psi for 5 min each; this
method completely decomposed the solid product and generates a homogeneous
solution. The solutions were diluted to 50.0 mL and stored in Nalgene® plastic
bottles. Sample solutions were added to equal volumes of LL-TISAB prior to analysis.
Fluoride analyses on the products were performed using an ion-selective
fluoride combination electrode with a standard single-junction sleeve-type reference
electrode and pH meter with an expanded mV scale (VWR International, Inc.). The
63
mean of three replicate data are reported. A calibration curve was obtained using
fluoride standards prepared by diluting a standard solution (10 µg F / 1.00 mL) to
0.0500, 0.100, 0.200, 0.400, 0.600, 1.00, 2.00, 3.00, and 4.00 mg F / L.
All dilutions were performed using standard silica glassware at ambient
temperature. Below pH 5, the formation of HF or HF2¯, which cannot be detected by
the fluoride electrode, can interfere with accurate F analyses. [23]. In addition, the
total ionic strength of the samples solutions must be maintained with the specification
range of the electrode. Finally, under some conditions, hydrofluoric acid can react
with silica. In order to address these issues, combination of both samples and
standards with an equal volume of LL-TISAB, provided the required control over both
pH and ionic strength. In addition, control experiments performed using only
Nalgene® labware showed less than 1% difference in detected fluoride content from
data obtained using standard glassware.
The energy-minimized structure of the N(SO2CF3)2¯ anion was initially
calculated using the hybrid density functional method (B3LYP) with a 6-31G* basis
set and GAUSSIAN 03W software. One-dimensional electron density maps were
calculated from a structural model and also obtained from observed powder XRD
(00l) reflection intensities (corrected for Lorentz-polarization). The mole ratio of
graphene carbon to N(SO2CF3)2¯ anions (x) and to intercalated fluoride (δ) were
refined to minimize the crystallographic R factor for observed and calculated maps.
Additional details of this method have been described previously. [2,4]
64
2.4 RESULTS AND DISCUSSION
As reported previously, intercalation of the bis(trifluoromethanesulfonyl)imide anion (TMSI) by chemical oxidation in hydrofluoric acid is remarkably rapid
and forms a stage 2 GIC product, which is obtained within 0.25 h at ambient
temperature. However, due to the limited chemical oxidation potential in 48%
hydrofluoric acid, stage 1 GIC’s cannot be obtained under these conditions. [4] Over
the range of reaction times explored here (0.25–500 h), all powder XRD reflections
observed can be indexed with (00l) indices for stage 2 GICs with gallery heights, di,
close to 0.8 nm. Powder XRD and compositional data for products are provided in
Figure 2.1 and Table 2.1. After the longest reaction time (500 h), a shoulder on the
largest diffraction peak (at approximately 24 °2θ) indicates a minor impurity phase
that can be ascribed to decomposition of K2MnF6 in aqueous HF to form an insoluble
fluoromanganate salt.
The gallery heights of these GIC products are similar to those for graphite
sulfate [24] or graphite nitrate [25] anion intercalate. This places a steric requirement
that the long axis of the TMSI anion be parallel to the encasing graphene sheets.
Figure 2.2 indicates the structure model employed for the graphite TMSI product; this
will be discussed in greater detail below.
Because the GIC’s were dried in vacuo prior to analyses, low temperature
losses that can be ascribed to surface-adsorbed or intercalated H2O or HF are almost
entirely absent (less than 0.5% mass loss to 100°C). TGA data shows three mass loss
65
major regions from ambient to 800°C (Figure 2.3). The first loss occurs at
approximately 100-120°C, the next at 150-300°C and the third above 550°C. Using
DSC and powder XRD on thermolyzed samples, the first two mass loss regions have
been ascribed to intercalate anion decomposition and volatilization. [4] Powder XRD
patterns of the GIC products heated briefly at 300°C showed an amorphous structure,
with only a broad reflection at 0.34 nm. After annealing under an inert atmosphere at
300°C, a crystalline graphite structure was reformed, indicating that the graphene
sheets remain largely intact after heating at this temperature. The final mass loss above
550°C is ascribed to decomposition of the graphene sheets. Under the same
conditions, graphite itself starts to decompose at 700°C. In these analyses, mass loss
below 500°C is therefore ascribed to intercalate loss only.
It is notable that the TMSI anion is far less thermally stable in the GIC product
than in the lithium salt, where decomposition occurs as a single sharp event with an
onset at 400-500°C. (Figure 2.3)
In order to evaluate the fluoride ion intercalate content in these products
separately from that of the TMSI anion, a new digestion procedure was developed.
Using control experiments, conditions were developed that allowed quantitative
dissolution of the graphene sheets and dissolution of the fluoride co-intercalate
without significant decomposition of the TMSI anion, which would itself form
additional fluoride co-intercalate. Control experiments using LiN(SO3CF3)2 and NaF
were run and indicated that under these conditions < 1% of fluoride intercalate content
determined by analysis were due to TMSI anion decomposition.
(006)
(007)
500 h
(004)
(002)
(003)
66
intensity
190 h
100 h
50 h
25 h
0.25 h
0
10
20
30
40
50
60
2Ө / degree
Figure 2.1 Powder XRD patterns of the products obtained, with reaction times
indicated. For each powder XRD trace, there is a break in intensity data as indicated
by hashes on the y-axis.
67
Table 2.1 Gallery height (di) and composition for stage 2 graphite bis(trifluoromethanesulfonyl)imide, CxN(SO2CF3)2·δF, at different reaction times as determined by
powder XRD, elemental analyses, and thermogravimetry.
reaction time
/h
di
/ nm
mass loss
/ pct
x
δ
0.25
0.793
29.4
56
0.08
25
0.799
31.6
51
0.14
50
0.799
32.3
50
0.27
100
0.796
32.4
50
0.30
190
0.793
31.5
53
0.54
500
0.802
8.5
62
0.84
68
y
z
x
N
B
S
C
C
O
O
F
F
Figure 2.2 Structural model for CxN(SO2CF3)2·δF. The fluoride positions are partially
occupied.
69
100
mass loss / pct
80
25 h
60
40
20
0
LiN(SO2CF3)2
0
200
400
600
800
temperature / °C
100
0.25 h
mass loss/ pct
25 h
100 h
80
60
40
0
200
400
600
temperature /°C
Figure 2.3 TGA data for products described in Table 2.1.
800
70
d powder For the composition of CxN(SO2CF3)2·δF, both x and δ need to be
determined. From the data obtained above, mass loss below 500°C was ascribed to
total intercalate content, and the fluoride anion content was separately determined
using F analyses as described above. Therefore, the compositional parameters were
determined by simultaneously solving the following relations:
mass loss pct
=
280.1 + 19.0δ
× 100
(2.6)
12.0x + 280.1 + 19.0δ
mass fraction of fluoride
=
19.0δ
(2.7)
12.0x + 280.1 + 19.0δ
Figure 2.4 and Table 2.1 indicate the values determined for x and δ vs. reaction
time. These data represent an unusually detailed understanding of the intercalate
composition during chemical synthesis in hydrofluoric acid, which is typically limited
to estimate of the fluoroanion content. As can be seen, fluoride content increases
steadily over long reaction times, but does not exceed a value of 1 (which would
indicate equimolar content of fluoride and the imide anions). From the powder XRD
data for the sample obtained at longest reaction time, the value obtained, δ=0.84, is
likely to be slightly higher than actually present in the GIC phase due to a small
impurity of a fluoromanganate salt byproduct in the sample .
71
1.0
0.8
δ
0.6
0.4
0.2
0.0
0
100
200
300
400
500
time / h
Figure 2.4 Increase in fluoride content, δ, in CxN(SO2CF3)2·δF vs. reaction time.
72
This result contrasts the fluoride content determined for graphite
perfluorooctanesulfonate from partial refinement of crystallographic data. In that
study, δ was determined to be approximately 4 after a 72 h reaction. The large
difference in the fluoride content is likely related to the very different gallery
dimensions, the perfluorooctanesulfonate anion forms GIC’s containing intercalate
bilayers with a gallery height of 3 nm. [2]
Because the sulfonate oxygen and nitrogen bear the largest negative charge
(Mulliken charges calculated are N = -0.71, O = -0.54), the anion orientation was
restricted in our structure model to provide close contact of these atoms with the
positively charged graphene sheets. This is accommodated by having the N and 2
sulfonate O’s atoms directed towards a single graphene sheet. This orientation allows
for two possibilities, the anions may face towards either of the two encasing positive
graphene surfaces. The structure model employed assumes equal populations of both
“up” and “down” orientations. The graphene C to N / O anion atom separations were
fixed at approximately 0.30 nm, which is comparable to the graphene plane-oxygen
distance of 0.32 nm known for graphite nitrate. [25] The fluoride anions present in the
intercalate galleries were fixed to lie in a plane 0.31 nm from the graphene carbon
plane. This distance was determined by comparison to stage 1 planar-sheet graphite
fluorides, CxF, where the distance is in the range of 0.28–0.30 nm (assuming a central
position of the fluoride in the galleries). [26] Due to the bilayer arrangement of
fluoride in CxF, and the semi-covalent nature of the C-F intercalation, these values are
likely to represent a lower limit for this distance. [27,28] Stage 2 graphite bifluoride
73
exhibits Ic=0.98 nm, which gives a maximum graphene plane to fluoride plane
distance of 0.32 nm. The structure model uses equal populations of fluoride intercalate
on both encasing graphene sheets.
Figure 2.5 shows one-dimensional electron density profiles derived from
refined structural model and observed powder XRD data for the product obtained after
50 h reaction. The x value determined in Table 2.1 was used in the structural model.
The agreement between profiles shows, at least qualitatively, that the structural model
employed correctly describes the GIC products obtained.
74
1.0
0.8
(i)
(ii)
0.6
z
0.4
0.2
0
electron density
Figure 2.5 One-dimensional electron density profiles derived from calculated
structural model (i) and observeXRD data (ii).
75
2.5 REFERENCES
1. Lemmon, J.; Lerner, M. M. Carbon 1993, 31, 437.
2. Zhang, X.; Lerner, M. M. Chem. Mater. 1999, 11, 1100.
3. Yan, W.; Lerner, M. M. Carbon 2004, 42, 2981.
4. Zhang, X.; Sukpirom, N.; Lerner, M. M. Mater. Res. Bull. 1999, 34, 363.
5. Aronson, S.; Lemont, S.; Weiner, J. Inorg. Chem. 1971, 10, 1296.
6. Takenaka, H.; Kawaguchi, M.; Lerner, M. M.; Bartlett, N. J. Chem. Soc. Chem.
Commun. 1987, 19, 1431.
7. Watanabe, N.; Touhara, H.; Nakajima, T.; Bartlett, N.; Mallouk, T.; Selig, H.
Fluorine Intercalation Compounds of Graphite. In Inorganic Solid Fluorides;
Hagenmuller, P., Ed.; Academic Press: New York, 1985; 332.
8. Hagiwara, R.; Bartlett, N. Thermodynamic Aspects of the Intercalation of Graphite
by Fluoroanions. In Fluorine-Carbon and Fluoride Carbon Materials;
Nakajima, T., Ed.; New York: Marcel Dekker, 1995; 67.
9. Billaud, D.; Chenite, A.; Metrot, A. Carbon 1982, 20, 493.
10. Dresselhaus, M. S.; Endo, M.; Issi, J. Physical Properties of Fluorine- and
Fluoride-Graphite Intercalation Compounds. In Fluorine-Carbon and Fluoride
Carbon Materials; Nakajima, T., Ed.; Marcel Dekker : New York, 1995; 95.
11. Metrot, A.; Fischer, J. E. Synth. Met. 1981, 3, 201.
12. Hummers, W. S., Jr.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339.
13. Nakajima, T.; Mabuchi, A.; Hagiwara, R. Carbon 1988, 26, 357.
14. Greenberg, A. E.; Clesceri, L. S.; Eaton, A. D. Standard Methods for the
Examination of Water and Waste Water 18th Edition. American Public Health
Assn: Washington, DC, 1992; 4.
15. Michigami, Y.; Kuroda, Y.; Ueda, K.; Yamoto, Y. Anal. Chim. Acta. 1993, 274,
299.
16. Wen, M.; Li, Q.; Wang, C. Fluoride 1996, 29, 82.
76
17. Wen, M.; Shi, N.; Qin, Y.; Wang, C. Fluoride 1998, 31, 74.
18. Ortiz, D.; Castro, L.; Turrubiartes, F.; Milan, J.; Diaz-Barriga, F. Fluoride 1998,
31, 183.
19. Hara, H.; Huang, C. C. Anal. Chim. Acta. 1997, 338, 141.
20. Torra, M.; Rodamilans, M.; Corbella, J. Sci. Total Environ. 1998, 220, 81.
21. Commann, K. Application of Ion Selective Electrodes. Wydawnictwa NaukowoTechniczne. Scientific publishers: Warsaw, 1977.
22. Bode, H.; Jenssen, H.; Bandte, F. Angew. Chem. 1953, 65, 304.
23. VWR International, Inc. Combination Fluoride Electrodes Instruction Manual.
VWR International, Inc., Westchester, PA, USA, 2002.
24. Lopez-Gonzalez, J. D.; Rodriguez, A. M.; Vega, F. D. Carbon 1969, 7, 583.
25. Zhang, X.; Lerner, M. M.; Gotoh, H.; Kawaguchi, M. Carbon 2000, 38, 1775.
26. Mallouk, T.; Bartlett, N. J. Chem. Soc. Chem. Commun. 1983, 3, 103.
27. Mallouk, T.; Hawkins, B. L.; Conrad, M. P.; Zilm, K.; Maciel, G. E.; Bartlett, N.
Phil. Trans. R. Soc. London 1985, A314, 179.
28. Matsuo, Y.; Segawa, M.; Mitani, J.; Sugie, M. J. Fluorine Chem. 1998, 87,
145.
77
CHAPTER 3
A STUDY OF FLUORIDE CONTENT IN GRAPHITE
PERFLUOROOCTANESULFONATE INTERCALATION COMPOUNDS
Watcharee Katinonkul and Michael M. Lerner
Department of Chemistry
Oregon State University,
Corvallis, OR 97331-4003
USA
78
3.1 ABSTRACT
Graphite intercalation compounds (GIC) containing perfluorooctanesulfonate,
CxC8F17SO3·δF are prepared under ambient conditions in 48% hydrofluoric acid, using
K2MnF6 as an oxidizing reagent. Powder X-ray diffraction indicates a gallery height,
di, of 3.0 nm. The composition (x and δ parameters) is evaluated by thermogravimetric
and elemental analyses.
79
3.2 INTRODUCTION
The intercalation of perfluoroalkanesulfonate anions into graphite was first
described by Boehm and co-workers. They prepared these compounds by the
electrochemical oxidation of graphite in the corresponding acids at elevated
temperature. [1] The first and smallest perfluoroalkylsulfonate intercalate, CF3SO3¯,
shows a gallery expansion of 0.47 nm, which is consistent with a monolayer
arrangement of intercalate anions. [2] These studies included the significant
observation that GIC’s were formed with greatly expanded galleries when n > 4 in
CnF2n+1SO3¯. In subsequent work on the electrochemical oxidation of graphite in neat
CnF2n+1SO3H
(n = 4, 6, 8) at elevated temperature, or at ambient temperature in an
acid/propylene carbonate electrolyte, the first GIC’s with a bilayer arrangement of
anion intercalates were obtained. [1,3-4].
The bilayer arrangement is a common intercalate orientation upon exchange of
smectite clays [5] or other layered metal oxides or sulfides for surfactant-like
intercalates. [6] This bilayer arrangement is favorable as it allows increased interaction
between the charged sheet surfaces and oppositely-charged headgroups, while at the
same time providing close interaction between nonpolar chains at the gallery centers.
These reports demonstrated for the first time that graphite could form intercalation
compounds with the galleries larger than 1.0 nm, and provided the first example of an
amphoteric bilayer intercalate structure in a GIC. For example, the distance between
graphene carbon planes encasing intercalated galleries, di, was found to be ~ 3.4 nm
80
for the perfluorooctanesulfonate compound, CxC8F17SO3 or CxPFOS, which agreed
well with the expected dimensions for a bilayer intercalate structure.
Previously in our group, CxPFOS was prepared by the electrochemical
oxidation of graphite in LiC8F17SO3 (sat.)/CH3NO2 at ambient temperature. [7]
Compounds of stages 3-7 were obtained, with di ≅ 2.5 nm for all stages, and x ≅ 17 for
the stage 2 product with di ≅ 2.7 nm. However, the electrosynthetic reactions are
inherently limited by the low charge density required to avoid a large overpotential at
the carbon surface, which would result in electrolyte decomposition and passivation of
the graphite surface. In addition, the high potential required for graphite oxidation
limits the selection of binders, electrolytes, and current collectors. For these reasons,
scalable chemical oxidation reactions are more likely to be realized. The first chemical
route for the bulk preparation of this GIC was successfully found in 48% hydrofluoric
acid, using the oxidant K2MnF6. Within 15 minutes, a stable, stage 2 GIC product with
x=22 and gallery height, di, of 3.0 nm was obtained. [8] The same chemical method
has also been employed to produce new GIC’s containing N(SO2CF3)2¯,
N(SO2CF2CF3)2¯, N(SO2CF3)(SO2C4F9)¯, and C(SO2CF3)3¯. [9-10]
It is well known that the oxidation of graphite and intercalation of anions can
also lead to reversible co-intercalation of neutral species, a common case is the parent
acid co-intercalates with the conjugate anions (H2SO4/HSO4¯ or HF/HF2¯). Another
common case is the co-intercalation of organic solvent molecules. Also, the
intercalation reaction can be complicated by the incorporation of more than one
species of anionic intercalate. Important examples of this complexity are the presence
81
of oxide or hydroxide in highly-oxidized GIC’s containing oxoanion intercalates, and
the presence of additional fluoride in GIC’s containing fluoroanion intercalates. The
cointercalation of oxide or fluoride within graphene galleries can have significant
impact on the products’ physical or chemical properties. [11]
In previous studies, there are few, if any, detailed compositional analyses on
fluoride co-intercalate in GIC’s. Recently, a GIC containing bis(trifluoromethanesulfonyl)imide, TMSI, was studied in this regard and it showed approximately
equimolar content of fluoride and the imide anions. [12] For the larger anions such as
borate chelate anions; bis(oxalato)borate, B[OC(O)C(O)O]2¯, the fluoride content
tends to increase accordingly. [13] In this chapter, we determine the fluoride cointercate contents in GIC’s containing perfluorooctanesulfonate, CxPFOS, prepared in
hydrofluoric acid with K2MnF6 as an oxidant.
82
3.3 EXPERIMENTAL
SP-1 grade graphite (Union Carbide, 100 µm average particle diameter),
hydrofluoric acid (Mallinckrodt AR, 48% w/o), hexane (Fischer Chem, certified
grade), nitric acid (Fischer Scientific, certified ACS PLUS, 68.0-70.0%), glacial acetic
acid (EM Science, 99.7%), and sodium chloride (Mallinckrodt, AR grade) were used
as received. Sodium fluoride (Mallinckrodt, AR grade) was baked at 105°C for 1 h
before use. C8F17SO2F (3M, experimental product) was treated with excess 20% KOH
solution and refluxed at 120 °C overnight. The white precipitate potassium perfluorooctanesulfonate, KC8F17SO3, was washed with distilled water and dried under vacuum
at room temperature. Potassium hexafluoromanganate, K2MnF6, was synthesized as a
bright yellow powder according to a literature method. [14] Ultrapure water
(resistivity = 18 MΩ·cm at 25°C) from a Milli-Q Labo system (Millipore, Milford,
MA), was used throughout the experiments. Low Level Total Ionic Strength
Adjustment Buffer (LL-TISAB) was prepared by addition of glacial acetic acid (57
mL) and reagent-grade NaCl (58 g) to 500 mL water, then slow addition of 5 M NaOH
until the pH was between 5.0-5.5, and then dilution to 1.00 L total volume. [15]
To prepare the GIC’s, KC8F17SO3 (200 mg, 0.37 mmol) and K2MnF6 (200 mg,
0.81 mmol) were first dissolved into hydrofluoric acid (15 mL) at ambient
temperature, and then graphite (50 mg, 4.2 mmol) was added with mole ratios of
11:2:1 for graphene C:Mn (IV):C8F17SO3¯ anion. The solutions were stirred in air for
the specified reaction times, then filtered and rinsed with 15 mL hydrofluoric acid.
83
The filtrate was briefly rinsed with 5 mL hexane and dried overnight under vacuum.
Products were subsequently stored under an inert atmosphere.
Powder X-ray diffraction (powder XRD) data were collected on a Siemens
D5000 diffractometer with Ni-filtered Cu Kα radiation using a variable slit mode and
a detector slit width of 0.2 mm. Data were collected at 0.02 °2Ө steps, between 7˚ and
60˚. Collection times varied from 0.1 s / step for routine analyses to 1 s / step for data
used in structural modeling.
TGA data were obtained using a Shimadzu, Inc. TGA-50 thermogravimetric
analyzer. Samples were loaded into a platinum pan; the sample chamber was flushed
with nitrogen. Thermal scans from ambient to 800°C were performed under flowing
N2 at 5°C / min.
A new sample digestion protocol was developed for the GIC products.
Samples (30 mg) were digested with 70% HNO3 (1 mL) using a microwave digester
(CEM Corporation, MDS 2000) at 100, 150, and then 200 psi for 5 min each; this
method completely decomposed the solid product and generates a homogeneous
solution. The solutions were diluted to 50.0 mL and stored in Nalgene® plastic
bottles. Sample solutions were added to equal volumes of LL-TISAB prior to analysis.
Fluoride analyses on the products were performed using an ion-selective
fluoride combination electrode with a standard single-junction sleeve-type reference
electrode and pH meter with an expanded mV scale (VWR International, Inc.). The
mean of three replicate data are reported. A calibration curve was obtained using
84
fluoride standards prepared by diluting a standard solution (10 µg F / 1.00 mL) to
0.0500, 0.100, 0.200, 0.400, 0.600, 1.00, 2.00, 3.00, and 4.00 mg F / L.
All dilutions were performed using standard silica glassware at ambient
temperature. Below pH 5, the formation of HF or HF2¯, which cannot be detected by
the fluoride electrode, can interfere with accurate F analyses. [15]. In addition, the
total ionic strength of the sample solutions must be maintained with the specification
range of the electrode. Finally, under some conditions, hydrofluoric acid can react
with silica. In order to address these issues, combination of both samples and
standards with an equal volume of LL-TISAB, provided the required control over both
pH and ionic strength. In addition, control experiments performed using only
Nalgene® labware showed less than 1% difference in detected fluoride content from
data obtained using standard glassware.
The energy-minimized structure of the C8F17SO3¯ anion was initially
calculated using the hybrid density functional method (B3LYP) with a 6-31G* basis
set and GAUSSIAN 03W software.
85
3.4 RESULTS AND DISCUSSION
Stage 2 GIC’s containing perfluorooctanesulfonate anion, CxPFOS, prepared
by chemical oxidation in a mixture of hydrofluoric acid and nitric acid (66/33) can be
obtained within 24 h at ambient temperature. Because of the limited chemical
oxidation potential in 48% hydrofluoric acid, stage 1 GIC’s cannot be obtained under
these conditions. [9] In Figure 3.1, powder XRD data are presented for various
reaction times from 3 to 96 h. A relatively constant pattern is obtained after 12 h. The
powder XRD patterns obtained are similar in general appearance to those previously
reported. [8] Most reflections relate to an Ic ≅ 3.3-3.4 nm, with notable set of five
strong peaks centered at d ≅ 0.34 nm. Figure 3.2 indicates the structural model
employed for stage 2 CxPFOS product. As the anion length is ~ 1.5 nm, the gallery
dimensions indicate that a bilayer intercalate structure is most reasonable. All the
powder XRD patterns show additional reflections at d = 0.58 and 0.55 nm, which
occur in the region typical of (001) reflections from layered graphite fluorides. These
impurity peaks are somewhat variable in intensity, but are only a few percent of the
total diffracted intensity after 12 h.
The use of aqueous media for graphite intercalation is clearly restricted to
acidic solutions due to the high potential required for graphite oxidation. Concentrated
acids such as fuming H2SO4 and 97%HNO3, have long been used to prepare GIC’s of
their conjugate anions. [16-18]
86
Table 3.1 shows compositional data for these products obtained from
thermogravimetric and elemental analyses. TGA mass loss of KPFOS salt and GIC
products at various reaction times are shown in Figure 3.3 a-b, respectively. The TGA
curve of the KPFOS salt shows a sharp mass decrease beginning at 420°C. The
remaining carbonaceous material is completely volatilized during the final mass loss
beginning at 600°C. For GIC products, four mass loss regions between 25 and 900°C
are shown in the TGA traces. The first loss, below 120°C, ranges from 0.5 to 3 mass
percent, with longer reaction times associated with a greater mass loss. This mass loss
is ascribed to intercalated or surface-adsorbed H2O or HF. The next two losses occur
between 150-200°C and 400-500°C, which have been ascribed to intercalate anion
decomposition and volatilization. The final loss above 600°C is ascribed to
decomposition of the graphene sheets. Under the same thermolysis conditions,
graphite itself starts to decompose at 700°C. It is notable that the PFOS anion is far
less thermally stable in the GIC product than in the potassium salts, where
decomposition occurs as a single sharp event with an onset at 420-600°C.
(Figure 3.3 a)
In order to evaluate the fluoride co-intercalate content in these products
separately from that of the PFOS anion, a new digestion procedure was developed.
Using control experiments, conditions were developed that allowed quantitative
dissolution of the graphene sheets and dissolution of the fluoride anions but avoiding
significant decomposition of the PFOS anion, which could produce additional
fluoride. Control experiments using KPFOS and NaF were run and indicated that
87
under these conditions < 1% of fluoride content determined by analysis are due to
PFOS anion decomposition.
88
96 h
intensity / arb unit
24 h
12 h
*
6h
*
3h
10
20
30
40
50
2θ / deg
Figure 3.1 Powder XRD patterns of the products obtained in a mixed acid solution
(HF/HNO3:66/33), using K2MnF6 as an oxidizing reagent, with reaction times
indicated. Impurity peaks of CxF are marked *.
89
Table 3.1 Gallery height (di) and composition for stage 2 graphite perfluorooctanesulfonate, CxC8F17SO3·δF, at different reaction times as determined by powder XRD,
elemental analyses, and thermogravimetry.
reaction time
/h
di
/ nm
TGA mass mass fraction x
loss/ pct
of fluoride
δ
x/δ
3
3.04
54
0.021
37
1.1
33.6
6
3.04
60
0.021
29
0.93
31.2
12
3.05
66
0.016
22
0.66
33.3
24
3.02
65
0.010
23
0.42
54.8
96
3.05
73
0.016
16
0.58
27.6
90
N
S
C
O
F
di ≅ 3.0 nm
Figure 3.2 Structural model for CxC8F17SO3·δF. The fluoride positions are partially
occupied.
91
100
KPFOS
mass loss / pct
80
60
24 h
40
20
0
0
200
400
600
800
temperature / °C
mass loss / pct
100
80
60
3h
6h
40
12 h
24 h
20
96 h
0
0
200
400
600
temperature / °C
Figure 3.3 TGA data for products described in Table 3.1.
800
92
Table 3.1 shows compositional data of CxC8F17SO3·δF determined from
thermogravimetric and elemental analyses. The compositional parameters x and δ were
obtained by ascribing mass loss below 450°C to total intercalate content, along with
the elemental analysis of fluoride anion content as described above. Therefore, the
composition can be determined by simultaneously solving the following relations:
mass loss pct
=
280.1 + 19.0δ
× 100
(3.1)
12.0x + 499.1 + 19.0δ
mass fraction of fluoride
19.0 δ
=
(3.2)
12.0 x + 499.1 + 19.0 δ
Table 3.1 indicates the values determined for x and δ vs. reaction time. These
data represent a detailed understanding of the intercalate composition during chemical
synthesis in hydrofluoric acid, which is typically limited to an estimate of the
fluoroanion content. As can be seen, fluoride co-intercalate content decreases slightly
over long reaction times, and δ does not exceed a value of which indicates
approximately equimolar content of fluoride and the PFOS anions. As can be seen
from the powder XRD data in Figure 3.1, the sample obtained at the shortest reaction
time has the greatest content of impurity of fluorine-rich phases CxF (x ≅ 3-4), which
may account in part for the higher δ value.
93
The result obtained in this study does not agree well with those obtained from
previous study (δ≅4 after 72 h reaction), which was determined both from refinement
of crystallographic data and elemental analyses. [8]
One possible explanation lies in different methods of the elemental analysis
used to determine fluoride content. The method used in this study is specific to
fluoride co-intercalate and likely to be more accurate than a digestion method that
counts all F in the GIC, because the anion has 17 fluoride atoms and small errors in
determination of x lead to large errors in δ. Also, the previous refinement of
crystallographic data, assumed that the entire additional electron density observed
above the graphene sheets was due to fluoride only. It is also possible that an oxide cointercalate forms during synthesis in aqueous HF, to form CxC8F17SO3·δ(F or O)
To test this theory, the synthesis was also attempted in anhydrous HF as
indicated below.
xC + C8F17SO3¯ (sol)
F2/ AHF
CxC8F17SO3·δF + e¯
(3.3)
After 24 h reaction, the GIC product obtained was a mixture of stages, with
values of δ and x are 1.9 and 91, respectively. It was therefore not possible to prepare
comparable GIC’s of single phase to test the latter theory.
94
3.5 REFERENCES
1. Boehm, H.; Helle, W.; Ruisinger, B. Synth. Met .1988, 23, 395.
2. Horn D.; Boehm, H. P. Mater. Sci. Eng. 1977, 31, 87.
3. Ruisinger, B.; Boehm, H. P. Angew. Chem. Int. Ed. Engl. 1987, 26, 253.
4. Ruisinger, B.; Boehm, H. P. Carbon 1993, 31, 1131.
5. Pinnavaia, T. Science 1983, 220, 365.
6. Jacobson, A. Organic and Organometallic Intercalation Compounds of the
Transition Metal Dichalcogenides. In Intercalation Chemistry; Whittingham, S.;
Jacobsen, A., Eds.; Academic Press: New York, 1982; Chapter 7.
7. Zhang, Z.; Lerner, M. M. Chem. Mater. 1996, 8, 257.
8. Zhang, X.; Lerner, M. M. Chem. Mater. 1999, 11, 1100.
9. Zhang, X.; Sukpirom, N.; Lerner, M. M. Mater. Res. Bull. 1999, 34, 363.
10. Zhang, X.; Lerner, M. M. Mol. Cryst. Liq. Cryst. Sci. Tech., Section A 2000, 340,
37.
11. Dresselhaus, M. S.; Endo, M.; Issi, J. Physical Properties of Fluorine- and
Fluoride-Graphite Intercalation Compounds. In Fluorine-Carbon and Fluoride
Carbon Materials; Nakajima, T., Ed.; Marcel Dekker : New York, 1995; 95.
12. Katinonkul, W.; Lerner, M. M. Carbon 2007, 45, 499.
13. Katinonkul, W.; Lerner, M. M. J. Phys. Chem. Sol. 2007, 68, 394.
14. Bode, H.; Jenssen, H.; Bandte, F. Angew. Chem. 1953, 65, 304.
15. VWR International, Inc. Combination Fluoride Electrodes Instruction Manual.
VWR International, Inc., Westchester, PA, USA, 2002.
16. Bartlett, N.; McQuillan, B. Graphite Chemistry. In Intercalation Chemistry;
Whittingham, S.; Jacobson, A., Eds.; Academic Press: New York, 1982;
Chapter 2.
95
17. Watanabe, N.; Touhara, H.; Nakajima, T.; Bartlett, N.; Mallouk, T.; Selig, H.
Fluorine Intercalation Compounds of Graphite. In Inorganic Solid Fluorides;
Hagenmuller, P., Ed.; Academic Press: New York, 1985; 332.
18. Ubbelohde, A. Proc. R. Soc. London A 1968, 304, 25.
96
CHAPTER 4
THE FIRST SYNTHESIS OF A
GRAPHITE BIS(OXALATO)BORATE INTERCALATION COMPOUND
Watcharee Katinonkul and Michael M. Lerner
Department of Chemistry and
Oregon State University,
Corvallis, OR 97331-4003
USA
J. Phys. Chem. Sol. 2007, 68, 394-399.
97
4.1 ABSTRACT
A new graphite intercalation compound containing the bis(oxalato)borate
anion, B[OC(O)C(O)O]2¯, is prepared for the first time by chemical oxidation of
graphite with fluorine gas in the presence of a solution containing the intercalate anion
in anhydrous hydrofluoric acid. The products of stage 1-3 compounds are
characterized by powder X-ray diffraction, thermogravimetric analysis, and elemental
analyses. X-ray diffraction data indicate a standing-up orientation for the borate anion,
with long axis perpendicular to the graphene sheets. Elemental analyses provide x and
δ for the nominal composition of CxB[OC(O)C(O)O]2·δF, where the chelate borate
and fluoride are co-intercalates, and indicate a low borate, and high fluoride,
intercalate content as compared to anion packing in other graphite intercalation
compounds.
Keywords: A. inorganic compounds, B. chemical synthesis, C. x-ray diffraction
98
4.2 INTRODUCTION
Acceptor-type graphite intercalation compounds (GIC’s), in which the planar
graphene sheets are oxidized and anions intercalate, have been prepared with a wide
range of intercalate anions, including octahedral or tetrahedral chloro-, fluoro-, or oxometallates. GIC’s show staging, or ordering, in the introduction of intercalate, for
example a stage 2 GIC has intercalate between alternate sheets, stage 3 has intercalate
between every third set of sheets (see Figure 1). For GIC’s containing tetrahedral or
octahedral anions, an x value (a mole ratio of graphene carbon / intercalate anions) of
approximately 24n, where n=stage, is consistently observed at a very distinctive
transition in potential-charge curves. [1] The relative independence of this transition
on the nature of the intercalate anion suggests that it is the graphene-sheet charge
density, rather than the intercalate distribution, that governs stage transitions. Anions
such as MF4¯, MF6¯ or MCl6¯ require “footprints” (i.e., areas above the graphene
surfaces) that are far less than the areas available within a C24n+ gallery. These GIC’s
therefore have significant gallery void volumes. It is well known that neutral
intercalates can be accommodated into these voids with little or no expansion in the
gallery dimension. [2-3]
For larger intercalates, it may be important to consider the effects of sterics in
the packing within graphene galleries. This can helps to explain the “standing-up”
orientation of larger anisotropic anions. This effect has also been described previously
by Boehm and co-workers. [4] With the longer z-axis parallel to graphene sheets, i.e.,
99
in a “lying down” orientation where di is minimized, the anions would require a larger
footprint than that available for stage 2 with x=48. An example of this “standing-up”
orientation is found for the long chain alkylsulfonate intercalate, CxC8F17SO3, which
forms a bilayer intercalate structure with anionic sulfonate headgroups oriented
towards the graphene sheets and the hydrophobic fluorocarbon chains oriented toward
the gallery centers. The gallery height for this GIC is 3.3 nm. [5] The even longer
anion, C10F21SO3¯, has a gallery height of 3.5 nm and a similar gallery structure [6].
The chelate borate anions, as will be discussed below, provide another example of this
anion orientation.
The most significant technological application to date has probably been their
use in the preparation of exfoliated graphite microparticles that form self-supporting
structures used in high-temperature seals. [7] In pillared clays and other pillared
layered compounds, the generation of open gallery structures allows for catalytic
activity, and the incorporation of organics and polymers within galleries has been
widely studied as a means to prepare porous materials for various applications. [8-11]
Similar materials prepared from GIC’s would offer interesting new properties because
graphene sheets have very different electronic, acid/base, and electrochemical
properties than clays or other layered inorganic hosts. A larger anion intercalate
dimension, relatively isotropic geometry, and monolayer intercalate arrangement
could lead to steric constraints for anion packing and therefore hold promise for the
generation of novel properties, such as facile intragallery chemistry for GIC’s with
lower graphene sheet charge densities.
100
di Ic
di
Ic
Ic= di
stage 1
stage 2
stage 3
Figure 4.1. Scheme of GIC showing staging, or ordering along the stacking direction,
for different products.
101
Generally, either chemical or electrochemical oxidation methods can be used
to prepare acceptor-type GIC’s. The electrochemical method may provide additional
information on products and allow greater control over the reaction progress, but a
chemical method can be more scalable for commercial production. Both methods have
been applied to the intercalation of large fluoroalkylanions. Bis(trifluoromethanesulfonyl)imide, N(SO2CF3)2¯, was prepared for the first time by electrochemical
oxidation. [12] Electrolytes with nitromethane and ethyl methyl sulfone were also
used to prepare GIC’s of this intercalate up to stage 1 (with all galleries occupied).
Larger
imide
or
methide
derivatives,
including
N(SO2CF2CF3)2¯
and
N(SO2CF3)(SO2C4F9)¯, have also been prepared by a chemical method. [13] These
GIC’s containing large perfluoroanions are remarkably more air stable than those
containing smaller anions. [13]
Acceptor-type GIC’s containing borate chelate anions have been prepared
recently. [14,15] Aside from BF4¯, the electrochemical intercalation of the chelate
borate anions B[OC(CF3)2C(CF3)2O]2¯ and B[OC(CF3)2C(O)O]2¯ provided the first
GIC’s containing boroanions. GIC’s from stage n=1-4 were obtained with gallery
heights, di, of 1.40-1.45 nm, indicating a single intercalate layer between graphene
sheets with the intercalate anions orientated with long axis perpendicular to the
graphene sheet surface. The large anion dimension and unusual anion orientation (with
long axis perpendicular to graphene sheets) result in an unprecedented large gallery
height for a single intercalate layer. [15] These GIC’s also demonstrated
102
comparatively high ambient stabilities; stage 2 is stable for several days and even the
stage 1 product can persist for several hours in air.
The B[OC(O)C(O)O]2¯ (BOB) anion has been the subject of several reports
recently, principally due to the interest in new electrolyte salts for lithium-ion
batteries. [16-19] It is reported to have an oxidative stability limit close to 4.5 V vs.
Li/Li+ in organic electrolytes. Since this anion contains no labile fluorine and is
thermally stable its electrolyte solutions demonstrate stable performance in cells even
at 60°C, where LiPF6 based electrolytes would usually fail. [18] Its solubility (>1.0 M
in some solvents), high ambient temperature ion conductivities (8-9 mS/cm), low cost,
thermal stability, and the stability of its decomposition products all make this an
attractive candidate for Li-ion battery electrolytes.
The electrochemical reactions described above to prepare borate chelate
anions, did not succeed in preparing any GIC’s containing the BOB anion. In this
paper, the first preparation of GIC’s containing the BOB anion intercalate is described
by the chemical oxidation of graphite with F2 (g) in an anhydrous HF /LiBOB solution.
4.3 EXPERIMENTAL
Graphite flake (Graftech Inc., GTA grade, 250 µm average particle diameter),
graphite powder (Union Carbide, SP-1 grade, 100 µm), lithium hydroxide (Lancaster
Synthesis, Inc., anhydrous 98+%), oxalic acid dihydrate (Sigma-Aldrich, 99%), and
103
boric acid (Aldrich, 99.999%) were used as received. Sodium fluoride (Mallinckrodt,
AR grade) was baked at 105°C for 1 h before use. Lithium bis(oxalato)borate salt
(LiBOB) was synthesized according to a previous literature method. [20] Anhydrous
hydrofluoric
acid
(AHF)
(Matheson,
pure grade) was subjected to two
freeze/pump/thaw cycles at 0°C to remove H2 before use. Fluorine gas (Air Products,
> 97 %) was connected directly to a stainless steel vacuum line fitted with a Helicoid
gauge (0-1500 torr) and used as received. Ultrapure water (resistivity = 18 MΩcm at
25°C) from a Milli-Q Labo system was used throughout the experiments. LL-TISAB
(low level- total ionic strength adjustment buffer) was prepared by addition of glacial
acetic acid (57 mL) and reagent-grade NaCl (58 g) to 500 mL water, slowly addition
of 5 M NaOH until the pH was between 5.0-5.5, and then dilution to 1.00 L total
volume. [21]
To prepare GIC’s, graphite (30 mg) and the desired mass of LiBOB were
loaded into separate 3/8″ FEP Teflon tubes that were heat-sealed at one end. The tubes
were connected via 316 stainless-steel Swagelok fittings to a 1KS4 Whitey valve (see
Figure 4.2). The assembled reactor was connected to a metal vacuum line, evacuated,
and AHF (approximately 2 mL) condensed onto the LiBOB salt to produce a paleyellow solution. The solution was poured onto the graphite powder in approximately
0.25 mL aliquots. Fluorine gas was then slowly and incrementally added into the
reactor over a period of several minutes to an overpressure of approximately 1200 torr.
A pressure of 1000-1200 torr was maintained by adding additional F2 for the entire
104
reaction. For reactions where the desired reaction stoichiometries were determined in
advance, the graphite and LiBOB powders were loaded into the same tube.
After the reaction, fluorine gas was removed by evacuation through a soda
lime tower, and the AHF solution was carefully poured into the second reaction tube.
The product was rinsed at least three times by re-condensing AHF onto the product,
allowing soluble components to dissolve, and again pouring the solution to the second
tube. In this way, soluble byproducts and unreacted LiBOB were removed from the
GIC product. After rinsing, AHF was evacuated through a soda lime tower and the
solid product dried under vacuum for several hours at ambient temperature. The GIC
product was subsequently maintained under an inert atmosphere.
Powder X-ray diffraction (PXRD) data were collected on a Siemens D5000
diffractometer with Ni-filtered Cu Kα radiation using a variable slit mode and a
detector slit width of 0.2 mm. Data were collected at 0.02°2Ө steps, between 2˚ and
65˚. Collection times varied from 0.1 s/step for routine analyses to 1 s/step for data
used in structural modeling.
TGA data were obtained using a Shimadzu, Inc. TGA-50 thermogravimetric
analyzer. Samples were loaded into a platinum pan; the sample chamber was flushed
with nitrogen. Thermal scans from ambient to 800°C were performed at 2°C/ min.
A new sample digestion protocol was developed for the GIC products.
Samples (30 mg) were digested into 70% HNO3 (1 mL) using a microwave digestor
(CEM Corporation, MDS 2000) at 100, 150, and then 200 psi for 5 min each; this
105
To metal vacuum line
1KS4 Whitey valves
3/8″ OD FEP Teflon tube
heat seals
Figure 4.2. General design for reactor employed in syntheses using anhydrous HF
and F2.
106
method completely decomposed the solid products and generated homogeneous
solutions. The solutions were diluted to 50.0 mL and stored in Nalgene plastic bottles.
Sample solutions were added to equal volumes of LL-TISAB prior to analysis.
Fluoride analyses on the products were performed using an ion-selective
fluoride combination electrode (VWR International, Inc.) with a standard singlejunction sleeve-type reference electrode and pH meter with an expanded mV scale
(VWR 8005). A calibration curve was obtained using fluoride standards prepared by
diluting a standard solution (10 µg F / 1.00 mL) to 0.0500, 0.100, 0.200, 0.400, 0.600,
1.00, 2.00, 3.00, and 4.00 mg F/L.
All dilutions were performed using standard silica glassware at ambient
temperature. Below pH 5, the presence of HF or HF2¯, which cannot be detected by
the fluoride electrode, can interfere with accurate F analyses. [21] In addition, the total
ionic strength of the samples solutions must be maintained with the specification range
of the electrode. Finally, under some conditions, hydrofluoric acid can react with
silica. In order to address these issues, the combination of both samples and standards
with an equal volume of LL-TISAB provided the required control over both pH and
ionic strength. In addition, control experiments were performed using only Nalgene
labware and showed less than 1% difference in detected fluoride content from data
obtained using standard glassware.
Boron analyses used the same digestion procedure as described above.
Samples were subsequently diluted to form a 2% v/v HNO3 solution, and B content
determined using ICP-AES (S.A. Inc, JY2000U analyzer) calibrated using a boron
107
standard (1.0-10 mg B/L) that was prepared from anhydrous boric acid according to a
literature procedure. [22] A 2 % v/v HNO3 acid solution was used as control blank
solution for calibration.
The energy-minimized structure of the B[OC(O)C(O)O]2¯ anion was
calculated using the hybrid density functional method (B3LYP) with a 6-31G* basis
set and GAUSSIAN 03W software. One-dimensional electron density maps were
calculated from a structural model and also obtained from observed powder XRD
(00l) reflection intensities (corrected for Lorentz-polarization). Additional details of
this method have been described previously. [5,6]
4.4 RESULTS AND DISCUSSION
In the synthesis of these graphite intercalation compounds (GIC’s), F2 serves
as the oxidant and bis(oxalato)borate (BOB) and fluoride anions are co-intercalates
according to the following overall reaction:
xC + [(δ+1)/2]F2 + B[OC(O)C(O)O]2¯
CxB[OC(O)C(O)O]2·δF + F¯
(4.1)
where is x is defined as the mole ratio of graphene carbon to borate anion and δ is the
mole ratio of intercalate fluoride to BOB. To our knowledge, this work represents the
first preparation of any graphite BOB intercalation compound. When the borate anion
108
is not included under these reaction conditions, graphite bifluoride (CxHF2·δHF) is
formed within minutes of the introduction of fluorine gas. It is therefore likely the
bifluoride anion also intercalates first in these reactions, but is subsequently replaced
by BOB and also converted to fluoride.
Initial experiments, using aliquot additions of the LiBOB solution to graphite,
provided an estimate of reaction stoichiometries to within approximately 10%. It was
determined that an initial ratio of graphene carbon to anion of 24:1 (mol:mol) was
sufficient to produce the first-stage GIC at longer reaction times. After the initial tests,
this stoichiometry was employed for all reactions.
Graphite flake is a highly crystalline material with a large particle diameter,
that results in higher diffraction intensity for (00l) reflections in the GIC’s obtained
(about 10-fold greater diffraction peak intensities than for SP-1 graphite). Due to the
strong preferred orientation of the platy particles, only (00l) reflections are observed in
powder XRD data. The large particle diameter in graphite flake also results in slower
intercalation reactions, these typically required 2-3 times longer to reach similar stages
compared with the SP-1 graphite. Also, many GIC products using graphite flake show
a mixture of stages, presumably arising from a lower intercalate content in the
crystallite particle centers. Because higher quality diffraction data were desired in this
study, graphite flake was employed exclusively after initial tests.
As shown in Figure 4.3, the products obtained showed a series of reflections
that can be ascribed to the GIC CxB[OC(O)C(O)O]2·δF. Table 4.1 provides the basal
repeat, Ic, and gallery height di, obtained for the GIC products at different reaction
109
times. In all products, di is close to 1.4 nm. This gallery dimension is similar to that
obtained previously for GIC’s with the bis(hexafluorohydroxyisobutyrato) and
bis(perfluoropinacolato)borate chelate anions. [14,15] A stage 1 GIC (with a small
shoulder on the main PXRD peak indicating a minor stage 2 component) is obtained
within 24 h, while shorter reaction times resulted in higher-stage products.
A simple structural model, based on the dimensions of the energy-minimized
isolated anion, indicates that the BOB anion is oriented with long axis perpendicular to
the graphene surfaces. This unusual intercalate orientation, which maximizes rather
than minimizes the gallery height, di, can likely be ascribed to a packing effect for
anions with large footprints. This orientation was observed for the other borate chelate
anions [14,15] and allows all these large intercalate anions to pack more densely in the
galleries.
Elemental analyses for B and F in the GIC’s obtained (Table 4.2) were
determined using ICP-AES and the digestion protocol described above. The observed
mass percents for these elements were converted to the compositional parameters x
and δ in CxB[OC(O)C(O)O]2·δF products by using the following relations:
mass % B
=
10.8
× 100
(4.2)
× 100
(4.3)
12.0x + 186.8 + 19.0δ
mass % F
=
19.0 δ
12.0x + 186.8 + 19.0δ
110
Previous studies on borate chelate GIC’s prepared by electrochemical
oxidation in nitromethane employed both coulometry and the modeling of diffraction
data to determine the graphene carbon to anion ratio, x. Due to the unknown efficiency
for the electrochemical reaction, coulometry could only provide a lower limit for x.
Results obtained were as follows: for the stage 2 CxB[OC(CF3)2C(O)O]2, x>28
(coulometry) and x=44 (diffraction); [14] for the stage 2 CxB[OC(CF3)2C(CF3)2O]2,
x>41 (coulometry) and x=49 (diffraction). [15] These x values are similar to those
generally obtained with smaller intercalate anions, and approximately follow the
relation of C24nAn where n=stage. [1] There are numerous examples of smaller anion
intercalates that are close to x=48 for stage 2, including CxPF6 (x=48) [23], CxAsF6
(x=45) [23], CxSbF6 (x=48) [24], CxHSO4 (x=48) [25], and CxSO3CF3 (x=52) [26].
The present study found much larger x values for stages 1-3 than those for the
compounds just described. For example,x=110 for the stage 2 CxB[OC(O)C(O)O]2·δF.
These results indicate a much lower packing density for the BOB anions within
galleries. On the other hand, relatively high concentrations of the fluoride cointercalate (δ≈3-4) were observed. (Table 4.2) These two results are likely to be
related, because the fluoride anion requires space on graphene sheet surface, and also
must be associated with some charge transfer with the host sheets. Thus the presence
of relatively large concentration of fluoride co-intercalate will tend to decrease the
packing density of the larger BOB anions within the galleries. In previous studies,
intercalation of borate chelate anions was obtained using electrochemical oxidation in
an organic solvent; therefore, no fluoride anions were present to co-intercalate.
111
(009)
(006)
(008)
(005)
(002)
(001)
(004)
stage 1
10
30
40
50
(0011)
(0010)
(007)
20
(0011)
(007)
(006)
(005)
stage 3
(002)
(001)
0
(0010)
(006)
(002)
(001)
intensity
(005)
stage 2
60
2 Ө / degree
Figure 4.3 Powder XRD pattern of stage 1-3 products of CxB[OC(O)C(O)O]2·δF.
Assigned (00l) indices are indicated.
112
Table 4.1 Powder XRD data for CxB[OC(O)C(O)O]2·δF products.
Sample
reaction time
/h
stage
Ic
/ nm
di
/ nm
1
3
3
2.10
1.43
2
12
2
1.77
1.43
3
24
1/2*
1.42
1.42
* the product is predominantly stage 1 with a small stage 2 component
113
Table 4.2 Compositional data for CxB[OC(O)C(O)O]2·δF from elemental analyses.
Sample
B
/ mass pct
F
/ mass pct
x
δ
1
1.422
10.0
41
4.0
2
0.677
3.40
110
2.9
3
0.649
3.49
120
3.1
114
Another study has determined the composition of CxN(SO2CF3)2·δF prepared using
chemical oxidation in hydrofluoric acid, and found for stage 2, x=50-60 and δ=0-1.
[27] For this imide anion intercalate, the gallery height is only about 0.8 nm, similar to
that in graphite sulfate or nitrate, and much less than for the borate chelate anions. The
amount of fluoride co-intercalate appears to be highly dependent on both anion size
and synthetic method.
The composition obtained was also compared to TGA mass loss data (Figure
4.4). The decomposition of the BOB anion is known from common salts such as
LiBOB, which completely decomposes to Li2CO3 and B2O3 by 650°C via reaction
(4.4):
LiB[OC(O)C(O)O]2
→
½Li2CO3
+
½B2O3 +
volatiles
(4.4)
For reaction (4), the mass losses are: 63.0 % (calc), 68.7 % (obs), it may be that some
Li2O is formed rather than complete conversion to Li2CO3. The loss of graphene
carbon due to sheet degradation is not expected below 650°C, and planar-sheet
graphite fluorides lose all fluoride content above 450°C, [17] therefore the thermolysis
reaction for CxB[OC(O)C(O)O]2·δF to 650°C was ascribed to reaction (4.5):
CxB[OC(O)C(O)O]2·δF
→
Cx
+
½B2O3 + volatiles
(4.5)
115
Using the compositional parameters x and δ from Table 1, the stage 2 GIC
mass loss was 13.2% (calc) and 13.1% (obs). The TGA analysis for total intercalate
content therefore agrees well with the compositional data presented.
Due to the large gallery height, the structural model employed for
CxB[OC(O)C(O)O]2·δF places the borate chelate anion with long axis oriented
perpendicular to the graphene sheets. The known anion and gallery dimensions result
in a graphene C to anion O distance (along the direction perpendicular to graphene
sheets) of 0.39 nm, somewhat longer than that for graphite nitrate (0.32 nm). The
greater separation of the intercalate anion from host sheet surface may be related to the
high concentration of fluoride co-intercalate. Figure 4.5 provides one-dimensional
electron density profiles derived from the refined structure model and from observed
powder XRD data for a stage 2 product. The fluoride co-intercalate anions were fixed
to lie in equal concentration on both encasing graphene surfaces at 0.31 nm above the
surfaces (for comparison, stage 2 CxHF2 has a graphene C–F distance of 0.32 nm).
Although the observed profile shows a general correspondence to the structural model,
the electron density peaks for intercalates near z=0.3, 0.5, and 0.7 show differences
from the model. It seems most likely that arises due to a minor phase impurity (the
presence of a higher stage GIC or graphite) in the diffraction pattern.
116
mass loss / pct
100
80
60
LiBOB
40
0
200
400
600
800
temp / ° C
Figure 4.4 TGA mass loss profiles for LiBOB salt and stage 2 GIC
CxB[OC(O)C(O)O]2·δF.
117
B
C
O
F
1.0
z
x
y
0.8
0.6
z
(i)
(ii)
0.4
0.2
(b)
0
(a)
electron density
(c)
Figure 4.5 (a) Energy-minimized BOB anion conformation, (b) GIC structural model,
and (c) one-dimensional electron density profiles derived from calculated structural
model (i) and observed powder XRD data (ii).
118
4.5 REFERENCES
1. Besenhard, J.; Fritz, H. Angew. Chem. Int. Ed. Engl. 1983, 22, 950.
2. Billaud, D.; Pron, A.; Vogel, F. Synth. Met. 1980, 2, 177.
3. Billaud, D.; El Haouari, A.; Gerardin, R. Synth. Met. 1989, 29, F241.
4. Boehm, H.; Helle, W.; Ruisinger, B. Synth. Met. 1988, 23, 395.
5. Zhang, X.; Lerner, M. M. Chem. Mater. 1999, 11, 1100.
6. Yan, W.; Lerner, M. M. Carbon 2004, 42, 2981.
7. Buckley, J.; Edie, D. Carbon-Carbon Materials and Composites; Noyes Publ.:
Park Ridge, NJ, 1993.
8. Pinnavaia, T. Science 1983, 220, 365.
9. Rebbah, H.; Borel, M.; Raveau, B. Mater. Res. Bull. 1980, 15, 317.
10. Yang, D.; Sandoval, S.; Divigalpitiya, M.; Irwin, J.; Frindt, R. Phys. Rev. B 1991,
43, 12053.
11. Lemmon, J.; Wu, J.; Lerner, M. M. Preparation and Characterization of
Nanocomposites of Poly(Ethylene Oxide) with Layered Solids. In Hybrid
Organic/Inorganic Composites; Mark, J.; Lee, C.; Bianconi, P., Eds.; ACS Symp.
Series 585; ACS: Washington DC, 1995; Chapter 5.
12. Yan, W.; Lerner, M. M. J. Electrochem. Soc. 2001, 148, D83.
13. Zhang, X.; Sukpirom, N.; Lerner, M. M. Mater. Res. Bull. 1999, 34, 363.
14. Yan, W.; Lerner, M. M. J. Electrochem. Soc. 2004, 151, J15.
15. Yan, W.; Lerner, M. M. J. Electrochem. Soc. 2003, 150, D169.
16. Ue, M.; Shima, K.; Mori, S. Electrochim. Acta. 1994, 39, 2751.
17. Xu, W.; Angell, C. Electrochim. Solid-State Lett. 2001, 4, E1.
18. Xu, K.; Zhang, S.; Jow, T.; Xu, W.; Angell, C. A. Electrochim. Solid-State Lett.
2002, 5, A26.
119
19. Xu, W.; Shusterman, A. J.; Videa, M.; Velikov, V.; Marzke, R.; Angell, C.
J. Electrochem. Soc. 2003, 50, E74.
20. Lishka, U.; Wietelmann, U.; Wegner, M. German Pat. DE19829030 C1, 1999.
21. VWR International, Inc. Combination Fluoride Electrodes Instruction Manual.
VWR International, Inc., Westchester, PA, USA, 2002.
22. Greenberg, A. E.; Clesceri, L. S.; Eaton, A. D. Standard Methods for the
Examination of Water and Waste Water; 18th Edition; American Public Health
Assn: Wash, DC, 1992.
23. Billaud, D.; Chenite, A. J. Power Sources 1984, 13, 1.
24. Jobert, A.; Touzain, P.; Bonnain, L. Carbon 1981, 19, 193.
25. Besenhard, J.; Wudy, E.; Moehwald, H.; Nickl, J.; Biberacher, W.; Foag, W.
Synth. Met. 1983, 7, 185.
26. Horn, D.; Boehm, H. Mater. Sci. Eng. 1977, 31, 87.
27. Katinonkul, W.; Lerner, M. M. Carbon 2007, 45, 499.
120
CHAPTER 5
CHEMICAL SYNTHESIS OF GRAPHITE
BIS(PERFLUOROPINACOLATO)BORATE AND GRAPHITE
BIS(HEXAFLUOROHYDROXYISOBUTYRATO)BORATE
INTERCALATION COMPOUNDS
Watcharee Katinonkul and Michael M. Lerner
Department of Chemistry
Oregon State University,
Corvallis, OR 97331-4003
USA
Carbon 2007, in press.
121
5.1 ABSTRACT
Graphite chelate borate intercalation compounds, CxB[OC(CF3)2C(CF3)2O]2·δF
and CxB[OC(CF3)2C(O)O]2·δF, are prepared for the first time by chemical oxidation
of flaky graphite with fluorine gas in the presence of a solution of the intercalate anion
in anhydrous hydrofluoric acid. Powder XRD data indicate that products up to stage 1
with gallery heights, di =1.43-1.45 nm are formed. The compositions of
CxB[OC(CF3)2C(CF3)2O]2·δF and CxB[OC(CF3)2 C(O)O]2·δF are determined using B
and F elemental analyses. Thermogravimetric analyses indicate the graphene sheets
begin to decompose before intercalate thermolysis is complete.
122
5.2 INTRODUCTION
Graphite intercalation compounds (GIC’s) are layered compounds comprising
graphene sheets and intercalate guests. Both chemical and electrochemical oxidation
methods have been used to prepare GIC’s. In general, the electrochemical method may
provide additional information on products and allow greater control over the reaction
progress, but chemical methods can produce purer products and may be more readily
scaled to commercial production.
GIC’s are also unusual in the common occurrence of several well-defined
stages, i.e., phases with ordered arrangements of occupied and unoccupied galleries,
where the integral stage number (n) indicates the number of galleries contained in a
repeat sequence where only the first is occupied by intercalate. The gallery height (di)
indicates the separation of graphene sheet centers in an intercalated gallery, and the
basal repeat length (Ic) is the repeating unit cell along the stacking direction. The
relationship between these parameters can also be expressed as:
Ic
=
di
+
(n-1) 0.335 nm
(5.1)
GIC’s with oxidized graphene sheet (acceptor-type) have been studied for a
wide range of intercalate anions such as octahedral or tetrahedral chloro-, fluoro-, or
oxo-metallates. GIC’s containing fluoroborate anions have been prepared using both
chemical and electrochemical methods. [1-5] As an example, CxBF4 was prepared by
123
electrochemical oxidation in nitromethane, with a gallery height di ≅ 0.78 nm
obtained: [1,2]
xC
+
BF4¯(sol)
CH3NO2
CxBF4·δCH3NO2
+
e¯
(5.2)
This electrochemical method produced materials with compositions near
x = 24n, where n is the stage of the product. [5] The intercalation of oxidatively-stable
larger anions such as perfluoroalkylsulfonates (CyF2y+1SO3¯, y = 1, 4, 6, 8, or 10)
[6-12],
trifluoroacetate
sulfonylimide
or
(CF3COO¯)
methide
[13],
anions
and
the
perfluoroalkyl-substituted
(N(SO2CF3)2¯,
N(SO2CF2CF3)2¯,
N(SO2CF3)(SO2C4F9)¯, and C(SO2CF3)3¯) [14-15] have also been reported. For the
perfluoroalkylsulfonate anions, a bilayer intercalate arrangement of intercalate results
in di as large as 3.4 nm. [12] Such highly expanded gallery in GIC’s may lead to a
broader subsequent chemistry and new applications. By analogy to pillared clays, the
generation of open gallery structures could allow for size-selective catalysis [16-18],
and the incorporation of organics and polymers within galleries has been widely
studied as a means to prepare nanocomposite materials for various applications. [1923] Analogous materials prepared from expanded GIC’s would offer interesting new
properties because graphene sheets have very different electronic, acid/base, and
electrochemical properties than clays or other layered inorganic hosts.
Our group has previously reported the first synthesis of acceptor-type GIC’s
containing relatively large borate chelate anions; bis(perfluoropinacolato)borate,
124
B[OC(CF3)2C(CF3)2O]2¯,
and
bis(hexafluorohydroxyisobutyrato)borate,
B[OC(CF3)2C(O)O]2¯. These GIC’s were prepared by an electrochemical method.
Both GIC’s have relatively large gallery heights, di, of 1.40-1.45 nm, that indicate an
anion monolayer with the intercalate anions orientated with long axis perpendicular to
the graphene sheet surface. [24,25] Recently the first chemical preparation of GIC
containing the bis(oxalato) anion, B[OC(O)C(O)O]2¯ (BOB) was successfully
accomplished by the chemical oxidation of graphite with F2 in an anhydrous HF
solution. [26] In this chapter, the first chemical preparations of GIC’s containing
B[OC(CF3)2C(CF3)2O]2¯ and B[OC(CF3)2C(O)O]2¯ are described.
5.3 EXPERIMENTAL
Graphite flake (Graftech Inc., GTA grade, 250 µm average particle diameter),
perfluoropinacol (Indofine Chemical Company, Inc., 95%), sodium borate (EM
Science, >99%), hexafluoro-2-hydroxyisobutyric acid (SynQuest Laboratories, Inc.,
95+%), and boric acid (Aldrich, 99.999%) were used as supplied. Sodium fluoride
(Mallinckrodt, AR grade) was baked at 105°C for 1 h before use. Anhydrous
hydrofluoric acid (AHF) (Matheson, pure grade) was subjected to two freeze/pump/
thaw cycles at 0°C to remove H2 before use. Fluorine gas (Air Products, > 97%) was
connected directly to a stainless steel vacuum line fitted with a Helicoid gauge (0-1500
torr) and used as received. Ultrapure water (resistivity = 18 MΩcm at 25°C) from a
125
Milli-Q Labo system was used throughout the experiments. LL-TISAB (low leveltotal ionic strength adjustment buffer) was prepared by dissolving glacial acetic acid
(57 mL) and reagent-grade NaCl (58 g) in 500 mL water, slow addition of 5 M NaOH
until the pH was between 5.0-5.5, and then dilution to 1.00 L total volume. [27]
Sodium
bis(perfluoropinacolato)borate,
NaB[OC(CF3)2C(CF3)2O]2,
was
synthesized according to the following literature method. [28] Perfluoropinacol
(12.026 g, 36.0 mmol) was added to 10 mL of a hot aqueous solution containing
sodium borate (0.763 g, 2.0 mmol). After concentration, the colorless gel obtained was
crystallized from ethanol to yield a white solid, then dried in vacuo at 100°C for 48 h.
Sodium
bis(hexafluorohydroxyisobutyrato)borate,
NaB[OC(CF3)2C(O)O]2,
was
synthesized according to the following literature method. [25] Hexafluoro-2hydroxyisobutyric acid (4.44 g, 21.0 mmol) was added to 10.0 mL of a hot aqueous
solution containing sodium borate (1.00 g, 2.62 mmol). After heating at 80°C for 10
min, the solution was concentrated and cooled to yield a white solid. The solid was
recrystallized from ethanol and then dried in vacuo at 100°C for 48 h.
To prepare GIC’s, graphite (30 mg) and the desired mass of borate salts were
loaded into separate 3/8″ FEP Teflon tubes that were heat-sealed at one end. The tubes
were connected via 316 stainless-steel Swagelok fittings to a 1KS4 Whitey valve.
[26]. The assembled reactor was connected to a metal vacuum line, evacuated, and
AHF (approximately 2 mL) condensed onto the borate chelate salt to produce a paleyellow solution. The solution was poured onto the graphite powder in approximately
0.25 mL aliquots. Fluorine gas was then slowly and incrementally added into the
126
reactor over a period of several minutes to an overpressure of approximately 1200 torr.
A pressure of 1000-1200 torr was maintained by adding additional F2 during the entire
reaction. For reactions where the desired reaction stoichiometries were determined in
advance, the graphite and borate salt powders were loaded into the same Teflon tube.
After the reaction, fluorine gas was removed by evacuation through a soda
lime tower, and the AHF solution was carefully poured into the second reaction tube.
The product was rinsed at least three times by re-condensing AHF onto the product,
allowing soluble components to dissolve, and again pouring the solution into the
second tube. In this way, soluble byproducts and unreacted borate salts were removed
from the desired GIC product. After rinsing, AHF was evacuated through a soda lime
tower and the solid product dried under vacuum for several hours at ambient
temperature. The product was subsequently stored under an inert atmosphere.
Powder X-ray diffraction (PXRD) data were collected on a Siemens D5000
diffractometer with Ni-filtered Cu Kα radiation using a variable slit mode and a
detector slit width of 0.2 mm. Data were collected at 0.02°2Ө steps, between 2° and
65°. Collection times varied from 0.1 s/step for routine analyses to 1 s/step for data
used in structural modeling.
TGA data were obtained using a Shimadzu, Inc. TGA-50 thermogravimetric
analyzer. Samples were loaded into a platinum pan; the sample chamber was flushed
with nitrogen. Thermal scans from ambient to 800°C were performed at 2°C/min.
A new sample digestion protocol was developed for the GIC products.
Samples (30 mg) were digested into 70% HNO3 (1 mL) using a microwave digestor
127
(CEM Corporation, MDS 2000) at 100, 150, and then 200 psi for 5 min each; this
method completely decomposed the solid products and generated homogeneous
solutions. The solutions were diluted to 50.0 mL and stored in Nalgene plastic bottles.
Sample solutions were added to equal volumes of LL-TISAB prior to analysis.
Fluoride analyses on the products were performed using an ion-selective
fluoride combination electrode (VWR# 14002-788 F) with a standard single-junction
sleeve-type reference electrode and pH meter with an expanded mV scale (VWR
8005). A calibration curve was obtained using fluoride standards prepared by diluting
a standard solution (10 µg F / 1.00 mL) to 0.0500, 0.100, 0.200, 0.400, 0.600, 1.00,
2.00, 3.00, and 4.00 mg F/L.
All dilutions were performed using standard silica glassware at ambient
temperature. Below a pH of ≈5, the presence of HF or HF2¯, which cannot be detected
by the fluoride electrode, can interfere with accurate F analyses. [27] In addition, the
total ionic strength of sample solutions must be maintained within the specification
range of the electrode. Finally, hydrofluoric acid can react with silica. In order to
address these issues, the combination of both samples and standards with an equal
volume of LL-TISAB provided the required control over both pH and ionic strength.
In addition, control experiments were performed using only Nalgene labware and
these showed less than 1% difference in detected fluoride content from data obtained
using standard glassware.
Boron analyses used the same digestion procedure as described above.
Samples were subsequently diluted to form a 2% v/v HNO3 solution, and the B
128
content determined using ICP-AES (S.A. Inc, JY2000U analyzer) calibrated using a
boron standard (1.0-10 mg B/L) prepared from anhydrous boric acid according to a
literature procedure. [29] A 2% v/v HNO3 acid solution was used as control blank
solution for calibration.
Energy-minimized structural models for the B[OC(CF3)2C(CF3)2O]2¯ and
B[OC(CF3)2C(O)O]2¯ anions were calculated using Gaussian 03W software, and full
geometry optimizations were carried out using density functional method (B3LYP)
with a 6-31G* basis set.
5.4 RESULTS AND DISCUSSION
In these syntheses, F2 serves as the oxidant, and B[OC(CF3)2C(CF3)2O]2¯, or
B[OC(CF3)2C(O)O]2¯, and fluoride anions are co-intercalates according to the
following overall reaction:
xC + [(δ+1)/2]F2 (g) + An¯ (sol)
CxAn·δF + F ¯ (sol)
(5.3)
where An¯ is the borate anion, x is the ratio of graphene carbon to borate intercalate,
and fluoride is co-intercalated into the galleries. Although the GIC products obtained
were placed in vacuo at ambient temperature for at least 8 h, the formulation of cointercalate as fluoride should also allow that F(HF)n¯ species such as bifluoride may
129
also be present. This work therefore represents the first chemical preparation of GIC’s
containing borate chelate anions.
No reaction occurs between graphite and elemental fluorine at room
temperature even with a high fluorine pressure (5 kbar) and several days reaction time,
unless a carrier such as gaseous or liquid HF is present. [30] The compositions,
structures and physical properties of graphite fluoride (CxF) and graphite bifluoride
(CxHF2·δHF) are strongly dependent on the preparative conditions employed. Under
the conditions used in this study, CxHF2·δHF is formed within minutes. It is therefore
likely that bifluoride anions intercalate first in these reactions, but are replaced over
time by borate chelate anions and are also converted to fluoride.
In initial reactions, additions of aliquots of AHF solutions containing
NaB[OC(CF3)2C(CF3)2O]2 or NaB[OC(CF3)2C(O)O]2 indicated that a mole ratio of
graphene carbon to anion of 24:1 were sufficient to produce first-stage GIC’s. For all
subsequent reactions, this stoichiometry was employed. The stability of these borate
chelate anion salts in AHF was confirmed separately.
Powder XRD for the products obtained are shown in Figure 5.1 (a-b). All
major reflections can be ascribed to a single phase of CxB[OC(CF3)2C(CF3)2O]2·δF or
CxB[OC(CF3)2C(O)O]2·δF. Table 5.1 provides the list of indexed reflections and the
gallery height, di, for the GIC products obtained after 24 h reaction. Both products
have gallery heights close to 1.4 nm. This gallery dimension is similar to those
obtained previously for borate chelate GIC’s using electrochemical oxidation. [24,25]
A stage 1 CxB[OC(CF3)2C(O)O]2·δF can be obtained with this chemical synthesis, but
130
not with the electrochemical method employed previously, where the maximum extent
of intercalation was stage 2. [25] However, some minor impurities are also present in
the GIC products obtained from the chemical syntheses. As can be seen in Figure 5.1
(a), there is a peak ascribed to CxF at 14.9° 2θ (d = 0.60 nm), and very small peaks
ascribed to residue anion salt. Small impurity peaks from residue anion salt are also
seen in Figure 5.1 (b).
Figure
5.2
(a-b)
shows
the
structural
models
of
stage
1
CxB[OC(CF3)2C(CF3)2O]2·δF and CxB[OC(CF3)2C(O)O]2·δF, respectively, including
fluoride co-intercalates positioned on the graphene surfaces in the gallery. The
observed gallery heights agree well with those calculated when the chelate borate
anions adopt a “standing up” orientation in the intercalate galleries, with the long
anion z-axis aligned perpendicular to the graphene sheets.
*
*
10
(009)
CxF
(008)
(005)
(007)
a
intensity
(004)
131
20
30
40
50
60
70
60
70
(009)
(006)
(008)
(005)
intensity
b
(004)
2θ/ degree
*
10
20
30
40
50
2θ/ degree
Figure 5.1 Powder XRD pattern of stage 1 products of (a) CxB[OC(CF3)2C(CF3)2O]2
·δF and (b) CxB[OC(CF3)2C(O)O]2·δF. Assigned (00l) indices are indicated; impurity
peaks are marked *.
132
Table 5.1 Powder XRD structural data for (a) CxB[OC(CF3)2C(CF3)2O]2·δF and
(b) CxB[OC(CF3)2C(O)O]2·δF products obtained after 24 h reaction.
peak #
2Ө
dobs
/ nm
(a) CxB[OC(CF3)2C(CF3)2O]2·δF
dcalc
/ nm
index
(hkl)
(di = 1.45 nm)
1
14.86
0.607
0.600
2
24.55
0.362
0.362
(004)
3
31.16
0.287
0.290
(005)
4
45.38
0.200
0.207
(007)
5
50.87
0.180
0.181
(008)
6
58.28
0.159
0.161
(009)
(b) CxB[OC(CF3)2C(O)O]2·δF
*
(di = 1.44 nm)
1
24.72
0.360
0.360
(004)
2
30.96
0.289
0.288
(005)
3
37.60
0.239
0.240
(006)
4
50.95
0.179
0.180
(008)
5
57.63
0.160
0.160
(009)
* this peak belongs to graphite fluoride, CxF
133
z
B
C
O
x
F
y
a
1.45 nm
b
1.44 nm
Figure 5.2 Structural models of stage 1 GIC’s of (a) CxB[OC(CF3)2C(CF3)2O]2·δF
and (b) CxB[OC(CF3)2C(O)O]2·δF.
134
Table 5.2 provides the compositional data for these GIC products as
determined by elemental analyses for B and F in the solid samples using ICP-AES and
the digestion protocol described above. The observed mass fractions were converted to
the compositional parameters x and δ in CxB[OC(CF3)2C(CF3)2O]2·δF and
CxB[OC(CF3)2C(O)O]2·δF using the following relations:
mass fraction B
=
10.8
(5.4)
12.0x + Mw of anion + 19.0δ
mass fraction F
=
19.0 δ
(5.5)
12.0x + Mw of anion + 19.0δ
Previous studies on borate chelate GIC’s prepared by electrochemical
oxidation in nitromethane used both coulometric data and structural modeling of
diffraction data to determine the graphene carbon to anion mol/mol ratio, x. However,
due to the inefficient electrochemical oxidation, coulometry could determine only a
lower limit for this parameter. Results obtained were as follows: for stage 2
CxB[OC(CF3)2C(O)O]2, x≥28 (coulometry) and x=44 (diffraction); [25] for stage 2
CxB[OC(CF3)2C(CF3)2O]2, x≥41 (coulometry) and x=49 (diffraction). [24] These x
values are similar to those obtained with smaller intercalate anions, and approximately
follow the general composition of C24nAn where n = stage. [31] Examples of smaller
anion GIC’s that are close to x=48 for stage 2 include CxPF6·2CH3NO2 (x=48) [5],
135
Table 5.2 Compositional data for CxB[OC(CF3)2C(CF3)2O]2·δF and
CxB[OC(CF3)2C(O)O]2·δF products.
Sample
B
F
/ mass fraction / mass fraction
x
δ
CxB[OC(CF3)2C(CF3)2O]2·δF
0.149
0.0503
58
3.8
CxB[OC(CF3)2C(O)O]2·δF
0.193
0.0692
51
4.1
136
CxAsF6·2CH3NO2 (x=45) [5], CxSbF6·4C4H6O3 (x=48) [32], CxHSO4·2H2SO4 (x=48)
[33], and CxSO3CF3·2CF3SO3H (x=52) [6].
The present study found x values for stage 1 that are much larger than those
noted above (x=58 for the stage 1 CxB[OC(CF3)2C(CF3)2O]2·δF and x=51 for the
stage 1 CxB[OC(CF3)2C(O)O]2·δF). Note that these are for stage 1, not stage 2,
products. The GIC for the smaller anion, CxB[OC(O)C(O)O]2·δF, prepared by
chemical oxidation had x=41 for the mixed stage 1/2 CxB[OC(O)C(O)O]2·δF. [26]
These results indicate a much lower packing density for the large borate chelate anions
within GIC galleries. In addition, these products show relatively high concentrations
of fluoride co-intercalate (δ≈3-4). Both the large size of these anions, and the steric
and charge effect of the fluoride co-intercalate, can explain the low packing density of
borate chelate anions within the galleries. In previous electrochemical syntheses
studies, no fluoride co-intercalate was present because those borate chelate GIC’s
were prepared using electrochemical oxidation in an organic solvent.
Another previous study of fluoride co-intercalates in GIC’s described the
preparation of CxN(SO2CF3)2·δF using chemical oxidation in hydrofluoric acid. It
found a composition for stage 2 of x=50-60 and δ=0-1. [34] With this imide anion
intercalate, the gallery height was about 0.8 nm, similar to that in GIC’s containing
sulfate or nitrate, and much less than that for the GIC’s containing borate chelate
anions. The relative concentration of fluoride co-intercalate therefore appears to be
strongly dependent on anion and gallery dimensions; however, more examples are
needed to understand this relationship in detail.
137
The thermal degradation of these borate chelate GIC’s was studied using
thermogravimetric analysis (TGA). Mass loss data for the GIC’s and for
NaB[OC(CF3)2C(CF3)2O]2 and NaB[OC(CF3)2C(O)O]2 salts are shown in Figure 5.3.
The decomposition of borate chelate anions has been investigated for common salts
such as lithium bis(oxalato)borate which completely decomposes to Li2CO3 and B2O3
by 650°C via reaction (5.6):
LiB[OC(O)C(O)O]2
½Li2CO3 + ½B2O3 + volatiles
(5.6)
However, the volatile decomposition products were not identified in this study.
Therefore, for NaB[OC(CF3)2C(CF3)2O]2 and NaB[OC(CF3)2C(O)O]2 salts, the
expected thermal decomposition reactions are:
NaB[OC(CF3)2C(CF3)2O]2
NaB[OC(CF3)2C(O)O]2
½Na2CO3 + ½B2O3 + volatiles (5.7)
½Na2CO3 + ½B2O3 + volatiles
(5.8)
Using reactions (5.7) and (5.8), the calculated mass losses agree reasonably well with
the observed values: 95.0 % (calc), 99.0 % (obs), and 92.3 % (calc), 90.0 % (obs),
respectively.
The loss of graphene carbon due to sheet degradation is not generally observed
below 650°C, and planar-sheet graphite fluorides have been shown to lose essentially
138
all fluoride content above 450°C [17], therefore the thermolysis reaction for
CxB[OC(CF3)2C(CF3)2O]2·δF and CxB[OC(CF3)2C(O)O]2·δF might be expected to
follow equations (5.9) and (5.10):
CxB[OC(CF3)2C(CF3)2O]2·δF
CxB[OC(CF3)2C(O)O]2·δF
Cx + ½B2O3 + volatiles
(5.9)
Cx + ½B2O3 + volatiles
(5.10)
and therefore be useful in compositional analysis for x and δ. However, using the x
and δ from Table 5.2, the stage 1 CxB[OC(CF3)2C(CF3)2O]2·δF mass losses are 53.2 %
(calc) and 21.5 % (obs), and 61.6 % (calc) and 15.0 % (obs) for stage 1
CxB[OC(CF3)2C(O)O]2·δF. Thus, TGA analysis for total intercalate content does not
match with results from the elemental analysis. Powder XRD of the GIC’s after
heating to 450°C (not shown) indicate that the GIC’s are not fully converted to
graphite at this temperature. The product appears to be a mix of high-stage GIC’s,
most likely, the anions are trapped in the bulk of the large flaky graphite particles.
Therefore, observed mass losses are much less than those calculated assuming the full
conversion back to graphite.
139
100
a
mass pct
80
60
40
20
0
0
200
400
600
800
0
200
400
600
800
100
b
mass pct
80
60
40
20
0
temperature, °C
Figure 5.3 TGA mass loss data for (a) NaB[OC(CF3)2C(CF3)2O]2 salt (⋅⋅⋅) and stage 1
CxB[OC(CF3)2C(CF3)2O]2·δF (⎯), (b) NaB[OC(CF3)2C(O)O]2 salt (⋅⋅⋅) and stage 1
CxB[OC(CF3)2C(O)O]2·δF (⎯).
140
5.5 REFERENCES
1.
Billaud, D.; Pron, A.; Vogel, F.; Herold, A. Mater. Res. Bull. 1980, 15, 1627.
2.
Billaud, D.; Pron, A.; Vogel, F. Synth. Met. 1980, 2, 177.
3.
Forsman, W.; Mertwoy, H. Carbon 1982, 20, 255.
4.
Chenite, A.; Billaud, D. Carbon 1982, 20, 120.
5.
Billaud, D.; Chenite, A. J. Power Sources 1984, 13, 1.
6.
Horn, D.; Boehm, H. Mater. Sci. Eng. 1977, 31, 87.
7.
Ruisinger, B.; Boehm, H. Carbon 1993, 31, 1131.
8.
Boehm, H.; Helle, W.; Ruisinger, B. Synth. Met. 1988, 23, 395.
9.
Ruisinger, B.; Boehm, H. Angew. Chem. Int. Ed. Engl. 1987, 26, 253.
10. Zhang, Z.; Lerner, M. M. Chem. Mater. 1996, 8, 257.
11.
Zhang, X.; Lerner, M. M. Chem. Mater. 1999, 11, 1100.
12.
Yan, W.; Lerner, M. M. Carbon 2004, 42, 2981.
13.
Bourelle, E.; Douglade, J.; Metrot, A. Mol. Cryst. Liq. Cryst. Sci. Technol.,
Sect.A 1994, 244, 227.
14.
Zhang, X.; Sukpirom, N.; Lerner, M. M. Mater. Res. Bull. 1999, 34, 363.
15.
Zhang, X.; Lerner, M. M. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 2000,
340, 37.
16.
Davies, J.; Eric, D. J. Inclusion Phenom. Mol. Recognit. Chem. 1996, 24, 133.
17.
Kloprogge, J. T. J. Porous Mater. 1998, 5, 5.
18.
Ding, Z.; Kloprogge, J. T.; Frost, R. L.; Lu, G. Q.; Zhu, H. Y. J. Porous Mater.
2001, 8, 273.
19.
Vaia, R. A.; Vasudevan, S.; Krawiec, W.; Scanlon, L. G.; Giannelis, E. P. Adv.
Mater. (Weinheim, Ger.) 1995, 7, 154.
141
20.
Kim, D. W.; Blumstein, A.; Kumar, J.; Tripathy, S. K. Chem. Mater. 2001, 13,
243.
21.
Lee, T. W.; Park, O. O.; Kim, J. J.; Hong, J. M.; Kim, Y. C. Chem. Mater. 2001,
13, 2217.
22.
Carrado, K. A. Appl. Clay Sci. 2000, 17, 1.
23. Alexandre, M.; Dubois, P. Mater. Sci. Eng., R. 2000, 28, 1.
24.
Yan, W.; Lerner, M. M. J. Electrochem. Soc. 2003, 150, D169.
25.
Yan, W.; Lerner, M. M. J. Electrochem. Soc. 2004, 151, J15.
26. Katinonkul, W.; Lerner, M. M. J. Phys. Chem. Sol. 2007, 68, 394.
27.
VWR International, Inc. Combination Fluoride Electrodes Instruction Manual.
VWR International, Inc., Westchester, PA, USA, 2002.
28.
Allen, M.; Janzen, A. F.; Willis, C. J. Can. J. Chem. 1968, 46, 3671.
29. Greenberg, A. E.; Clesceri, L. S.; Eaton, A. D. Standard Methods for the
Examination of Water and Waste Water; 18th Edition; American Public Health
Assn: Wash, DC, 1992.
30.
Nakajima, T. Fluorine-Carbon and Fluoride-Carbon Materials—Chemistry,
Physics and Applications. Marcel Dekker: NewYork, 1995; Chapter 1.
31.
Besenhard, J.; Fritz, H. Angew. Chem. Int. Ed. Engl. 1983, 22, 950.
32. Jobert, A.; Touzain, P.; Bonnain, L. Carbon 1981, 19, 193.
33.
Besenhard, J.; Wudy, E.; Moehwald, H.; Nickl, J.; Biberacher, W.; Foag, W.
Synth. Met. 1983, 7, 185.
34. Katinonkul, W.; Lerner, M. M. Carbon 2007, 45, 499.
142
CHAPTER 6
CONCLUSION
A new acceptor-type graphite intercalation compound (GIC) containing the
borate chelate anion bis(oxalato)borate, CxB[OC(O)C(O)O]2·δF is obtained for the
first time by chemical oxidation of graphite with fluorine gas in the presence of a
solution containing the intercalate anion in anhydrous hydrofluoric acid. GIC’s up to
stage 1 with a gallery height, di ≅ 1.42 nm, can be obtained. The structural model and
one-dimensional electron density map indicates that the intercalate anions adopt a
“standing up” orientation in the gallery. The composition (x and δ parameters) is
determined using both thermogravimetric and elemental analyses. The concentration
of co-intercalated fluoride anions from this chemical synthesis is studied quantitatively
with a newly-developed digestion method using an ion-selective electrode and
potentiometer. Results indicate that there are approximately four fluoride cointercalate per borate chelate anion in the galleries.
Stage 1 GIC’s of two other borate chelate anions, bis(perfluoropinacolato)
borate, CxB[OC(CF3)2C(CF3)2O]2·δF, and bis(hexafluorohydroxyisobutyrato)borate,
CxB[OC(CF3)2C(O)O]2·δF, are obtained for the first time using chemical oxidation.
The gallery heights, di, for these GIC’s are 1.43-1.45 nm. The chemical syntheses
used to prepare these GIC’s are the same as described above. The fluoride
co-intercalate content for both these GIC products is determined to be approximately
four fluoride co-intercalates per borate chelate anion. The composition (x and δ
143
parameters) is determined using elemental analyses, but is not confirmed by
thermogravimetric analyses due to the incomplete degradation of intercalates.
The
concentration
of
fluoride
co-intercalate
in
GIC’s
containing
bis(trifluoromethanesulfonyl)imide, Cx(SO2CF3)2·δF, and perfluorooctanesulfonate,
CxC8F17SO3·δF, prepared in aqueous HF with K2MnF6 as an oxidizing agent, are
determined. In both GIC’s, approximately equimolar of fluoride co-intercalates to
anion are observed. For CxN(SO2CF3)2·δF, the fluoride content increases steadily with
reaction time, while the imide anion content is relatively constant. In contrast,
CxC8F17SO3·δF shows little change in the fluoride content over reaction time.
However, these two GIC’s have different gallery dimensions and structures, so in
order to understand more about this, other GIC samples will need to be studied.
144
BIBLIOGRAPHY
Akuzawa, N.; Kamoshita, T.; Tsuchiya, K.; Matsumoto, R. Tanso 2006, 222, 107.
Albeti, G. Layered Solids and Their Intercalation Chemistry. In Comprehensive
Supramolecular Chemistry; Alberti, G.; Bein, T., Eds.; Pergamon: Oxford, 1996;
Vol. 7, Chapter 1.
Alexandre, M.; Dubois, P. Mater. Sci. Eng., R. 2000, 28, 1.
Allen, M.; Janzen, A. F.; Willis, C. J. Can. J. Chem. 1968, 46, 3671.
Amine, K.; Tressaud, A.; Imoto, H.; Fargin, E.; Hagenmuller, P.; Touhara, H.
Mater. Res. Bull. 1991, 26, 337.
Amriou, T.; Khelifa, B.; Aourag, H.; Aouadi, S. M.; Mathieu, C. Mater. Chem. Phys.
2005, 92, 499.
Aronson, S.; Frishberg, C.; Frank, G. Carbon 1971, 9, 715.
Aronson, S.;Lermont, S.; Weiner, J. Inorg. Chem. 1971, 10, 1296.
Avdeev, V. V.; Nalimova, V. A.; Semenenko, K. N. High Pressure Res. 1990, 6, 11.
Avdeev, V. V.; Nalimova, V. A.; Semenenko, K. N. Synth. Met. 1990, 38, 363.
Avdeev, V. V.; Sorokina, N. E.; Nikol'skaya, I. V.; Monyakina, L. A.; Voronkina, A.
V. Inorg. Mater. (Translation of Neorganicheskie Materialy) 1997, 33, 584.
Barrer, R. M. Inorganic Inclusion Complex. In Nonstoichiometric Compounds;
Mandelcorn, L., Ed.; Academic Press: New York, 1964; Chapter 6.
Barsukov, I.; Zaleski, P. L. U.S. Patent 6,756,027, June 29, 2004.
Bartlett, N.; McQuillan, B. Graphite Chemistry. In Intercalation Chemistry;
Whittingham, S.; Jacobson, A., Eds.; Academic Press: New York, 1982; Chapter 2.
Besenhard, J. O.; Fritz, H. P. Naturforsch. 1972, 27B, 1294.
Besenhard, J. O.; Möhwald, H.; Nickl, J. J. Carbon 1980, 18, 399.
Besenhard, J.; Fritz, H. Angew. Chem. Int. Ed. Engl. 1983, 22, 950.
145
Besenhard, J.; Wudy, E.; Moehwald, H.; Nickl, J.; Biberacher, W.; Foag, W.
Synth. Met. 1983, 7, 185.
Billaud, D.: Pron, A.: Vogel, L. V. Synth. Met. 1980, 2, 177.
Billaud, D.; Chenite, A.; Metrot, A. Carbon 1982, 20, 493.
Billaud, D.; Chenite, A. J. Power Sources 1984, 13, 1.
Billaud, D.; El Haouari, A.; Gerardin, R. Synth. Met. 1989, 29, F241.
Billaud, D.; Flanders, P. J.; Pron, A.; Fischer, J. E. Mater. Sci. Eng. 1982, 54, 31.
Billaud, D.; Metrot, A.; Willann, P.; Herold, A. Proceedings of International Carbon
Conference, Baden-Baden, 1980, p 140.
Billaud, D.; Pron, A.; Vogel, F. Synth. Met. 1980, 2, 177.
Billaud, D.; Pron, A.; Vogel, F.; Herold, A. Mater. Res. Bull. 1980, 15, 1627.
Bindl, M. Thesis. TU München, 1986.
Biswas, M.; Sinha Ray, S. Adv. Polym. Sci. 2001, 155, 167.
Bode, H.; Jenssen, H.; Bandte, F. Angew. Chem. 1953, 65, 304.
Boehm, H.; Helle, W.; Ruisinger, B. Synth. Met .1988, 23, 395.
Bottomley, M. J.; Parry, G. S.; Ubbelohde, A. R. Proc. R. Soc. London Ser.A
1964, A 279, 291.
Bourelle, E.; Douglade, J.; Metrot, A. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect.A
1994, 244, 227.
Buckley, J.; Edie, D. Carbon-Carbon Materials and Composites; Noyes Publ.: Park
Ridge, NJ, 1993.
Carrado, K. A. Appl. Clay Sci. 2000, 17, 1.
Chenite, A.; Billaud, D. Carbon 1982, 20, 120.
Commann, K. Application of Ion Selective Electrodes. Wydawnictwa NaukowoTechniczne. Scientific publishers: Warsaw, 1977.
146
Davies, J.; Eric, D. J. Inclusion Phenom. Mol. Recognit. Chem. 1996, 24, 133.
De la Cruz, F. A.; Cowley, J. M. Acta Crystallog. 1963, 16, 531.
Ding, Z.; Kloprogge, J. T.; Frost, R. L.; Lu, G. Q.; Zhu, H. Y. J. Porous Mater. 2001,
8, 273.
Dresselhaus, G.; Nicholls, J. T.; Dresselhaus, M. S. In Graphite Intercalation
Compounds II; Zabel, H., Solin, S. A., Eds.; Springer-Verlag: Berlin, 1990; p 247.
Dresselhaus, M. S.; Dresselhaus, G. Adv.Phys. 1981, 30, 139.
Dresselhaus, M. S. Intercalation in Layered Materials; NATO ASI Series B;
Plenum: New York, 1986, Vol. 148.
Dresselhaus, M. S.; Endo, M.; Issi, J. Physical Properties of Fluorine- and FluorideGraphite Intercalation Compounds. In Fluorine-Carbon and Fluoride Carbon
Materials; Nakajima, T., Ed.; New York: Marcel Dekker, 1995; 95.
Du Pont de Nemours, E. I. and Co., Res. Discl. 1976, 146, 13.
Enoki, T.; Suzuki, M.; Endo, M. Graphite Intercalation Compounds and
Applications; Oxford University Press: New York, 2003.
Essaddek, A.; Herold, C.; Lagrange, P. Comptes Rendus de l'Academie des Sciences,
Serie IIb: Mecanique,Physique, Chimie, Astronomie 1994, 319, 1009.
Flandrois, S. Synth. Met. 1982, 4, 255.
Flandrois, S.; Grannec, J.; Hauw, C.; Hun, B.; Lozano, L.; Tressaud, A.
J. Solid State Chem. 1988, 77, 264.
Foley, G. M. T.; Zeller, C.; Vogel, F. L. Solid State Commun. 1977, 24, 371.
Forsman, W.; Mertwoy, H. Carbon 1982, 20, 255.
Fuzellier, H.; Lelaurain, M.; Mareche, J. F. Synth. Met. 1990, 34, 115.
Gamble, F. R.; Geballe, T. H. Inclusion Compounds. In Treatise on Solid State
Chemistry; Hannay, N. B. Ed.; Plenum: New York, 1976; Vol. 3, Chapter 2.
Greenberg, A. E.; Clesceri, L. S.; Eaton, A. D. Standard Methods for the Examination
of Water and Waste Water 18th Edition. American Public Health Assn: Washington,
DC, 1992; 4.
147
Grim, R. E. Clay Mineralogy, 2nd ed.; McGraw-Hill: New York, 1968.
Guérard, D.; Chaabouni, M.; Lagrange, P.; El Makrini, S.; Herold, A. Carbon 1980,
18, 257.
Hagiwara, R.; Bartlett, N. Thermodynamic Aspects of the Intercalation of Graphite by
Fluoroanions. In Fluorine-Carbon and Fluoride Carbon Materials; Nakajima, T., Ed.;
New York: Marcel Dekker, 1995; 67.
Hamwi, A.; Touzain, P.; Bonnetain, L. Mater. Sci. Eng. 1977, 31, 95.
Hamwi, A.; Touzain, P.; Bonnetain, L. Rev. Chim. Minér. 1982, 19, 651.
Hara, H.; Huang, C. C. Anal. Chim. Acta. 1997, 338, 141.
Hérold, A. Crystallo-Chemistry of Carbon Intercalation Compounds. In Physics and
Chemistry of Materials with Layered Structures; Levy, F., Ed.; Reidel: Dordrecht,
1979, vol. 6, p 323.
Hooley, J. G. Carbon 1972, 10, 155.
Horn D.; Boehm, H. P. Mater. Sci. Eng. 1977, 31, 87.
Hummers, W. S., Jr.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339.
Interrante, L. V.; Markiewiz, R. S.; McKee, D. W. Synth. Met. 1979, 1, 287.
Jacobson, A. J. Intercalation Reactions of Layered Compounds. In Solid State
Chemistry — Compounds; Cheetham, A. K.; Day, P., Eds.; Clarendon: Oxford, 1992:
Chapter 6.
Jacobson, A. Organic and Organometallic Intercalation Compounds of the Transition
Metal Dichalcogenides. In Intercalation Chemistry; Whittingham, S.; Jacobsen, A.,
Eds.; Academic Press: New York, 1982; chapter 7.
Jagtap, N.; Ramaswamy, V. Applied Clay Science 2006, 33, 89.
Jegoudez, J.; Mazieres, C.; Setton, R. Carbon 1986, 24, 747.
Jobert, A.; Touzain, P.: Bonnetain, L. Carbon 1981, 19, 193.
Kadono, K. Ph. D. Thesis, Kyoto University (1986).
Katinonkul, W.; Lerner, M. M. Carbon 2007, 45, 499.
148
Katinonkul, W.; Lerner, M. M. Carbon 2007, in press.
Katinonkul, W.; Lerner, M. M. J. Phys. Chem. Sol. 2007, 68, 394.
Kim, D. W.; Blumstein, A.; Kumar, J.; Tripathy, S. K. Chem. Mater. 2001, 13, 243.
Kloprogge, J. T. J. Porous Mater. 1998, 5, 5.
Lagaly, G. Smectitic Clays as Ionic Macromolecules. In Developments in Ionic
Polymers; Wilson, A. D.; Prosser, J., Eds.; Elsevier Applied Science Publishers:
London, 1986; Vol. 2, Chapter 2.
Lee, J.; Takekoshi, T.; Giannelis, E. P. Mat. Res. Soc. Symp. Proc. 1997, 457, 513.
Lee, T. W.; Park, O. O.; Kim, J. J.; Hong, J. M.; Kim, Y. C. Chem. Mater. 2001, 13,
2217.
Legrand, A. P.; Flandrois, S. Chemical Physics of Intercalation; NATO ASI
Series B; Plenum: New York, 1987; p 172.
Lemmon, J.; Lerner, M. M. Carbon 1993, 31, 437.
Lemmon, J.; Wu, J.; Lerner, M. M. Preparation and Characterization of
Nanocomposites of Poly(Ethylene Oxide) with Layered Solids. In Hybrid
Organic/Inorganic Composites; Mark, J.; Lee, C.; Bianconi, P., Eds.; ACS Symp.
Series 585; ACS: Washington DC, 1995; Chapter 5.
Lerf, A. Intercalation Compounds in Layered Host Lattices: Supramolecular
Chemistry in Nanodimensions. In Handbook of Nanostructured Materials and
Nanotechnology; Nalwa, H., Ed.; Acad. Press: New York, 2000; Vol. 5, 1.
Lerner, M.; Hagiwara, R.; Bartlett, N. J. Fluorine Chem. 1992, 57, 1.
Levy, F. Intercalated Layered Materials; Reidel: Dordrecht, the Netherlands, 1979.
Lishka, U.; Wietelmann, U.; Wegner, M. German Pat. DE19829030 C1, 1999.
Lopez-Gonzalez, J. D.; Rodriguez, A. M.; Vega, F. D. Carbon 1969, 7, 583.
Mallouk, T.; Bartlett, N. J. Chem. Soc. Chem. Commun. 1983, 3, 103.
Mallouk, T.; Hawkins, B. L.; Conrad, M. P.; Zilm, K.; Maciel, G. E.; Bartlett, N.
Phil. Trans. R. Soc. London 1985, A314, 179.
149
Matsuo, Y.; Segawa, M.; Mitani, J.; Sugie, M. J. Fluorine Chem. 1998, 87,145.
McCarron, E. M.; Grannec, Y. J.; Bartlett, N. J. Chem. Soc. Chem. Commu. 1980, 890.
Metrot, A.; Fischer, J. E. Synth. Met. 1981, 3, 201.
Métrot, A.; Guerard, D.; Billaud, D.; Herold, A. Synth. Met. 1979/1980, 1, 363.
Michigami, Y.; Kuroda, Y.; Ueda, K.; Yamoto, Y. Anal. Chim. Acta. 1993, 274, 299.
Mittal, J.; Konno, H.; Inagaki, M. Synth. Met. 1998, 96, 103.
Moran, M. J.; Miller, G. R.; De Marco, R. A.; Resing, H. A. J. Phys. Chem. 1984, 88,
1580.
Mouras, S.; Hamwi, A.; Djurado, D.; Cousseins, J. C. Rev. Chem. Miner. 1987, 24,
572.
Müller-Warmuth, W.; Schöllhorn, R. Progress in Intercalation Research;
Kluwer Academic: Dordrecht, the Netherlands, 1994.
Nader, F. W. Beitr. Dechema-Fachgespraechs Umweltschutz, 8th 1990, 373.
Nakajima, T. Fluorine-Carbon and Fluoride-Carbon Materials—Chemistry, Physics
and Applications. Marcel Dekker: NewYork, 1995.
Nakajima, T.; Kameda, I.; Endo, M.; Watanabe, N. Carbon 1986, 24, 343.
Nakajima, T.; Mabuchi, A.; Hagiwara, R. Carbon 1988, 26, 357.
Nakajima, T.; Watanabe, N. Graphite Fluorides and Carbon-Fluorine Compounds;
CRC Press: Boca Raton, FL, 1991.
Nixon, D. E.; Parry, G. S.; Ubbelohde, A. R. Proc. R. Soc. London 1964, A291, 324.
Nixon, D. E.; Parry, G. S.; Ubbelohde, A. R. Proc. R. Soc. London 1966, 291A, 32.
O’Hare, D. Inorganic Intercalation Compounds. In Inorganic Materials; Bruce, D. W.;
O’Hare, D., Eds.; Wiley: Chichester, 1992; Chapter 4.
Okuyama, N.; Takahashi, T.; Kanayama, S.; Yasunaga, H. Physica 1981, 105B, 298.
Ortiz, D.; Castro, L.; Turrubiartes, F.; Milan, J.; Diaz-Barriga, F. Fluoride 1998, 31,
183.
150
Parry, G. S.; Nixon, D. E.; Lester, K. M.; Levene, B. C. J. Phys. C. 1969, 2, 2156.
Pinnavaia, T. Science 1983, 220, 365.
Platzer, N.; de la Martiniére, B. Bull. Soc. Chim. Fr. 1961, 1961, 177.
Preisegger, E.; Henrici, R. Int. J. Refrig. 1992, 15, 326.
Rebbah, H.; Borel, M.; Raveau, B. Mater. Res. Bull. 1980, 15, 317.
Rüdorff, W.; Siecke, W. F. Chem. Ber. 1958, 91, 1348.
Rüdorff, W.; Stumpp, E.; Spriesseler, W.; Siecke, F. W. Angew. Chem. Int. Ed. Engl.
1963, 2, 67.
Ruisinger, B.; Boehm, H. P. Angew. Chem. Int. Ed. Engl. 1987, 26, 253.
Ruisinger, B.; Boehm, H. P. Carbon 1993, 31, 1131.
Schöllhorn, R. Intercalation Compounds. In Inclusion Compounds; Atwood, J. L.;
Davies, J. E. D.; MacNicol, D. D., Eds.; Academic Press: London, 1984; Vol. 1,
Chapter 7.
SIEMENS, DIFFRAC AT software; Version 3.1, Socabim, 1993.
Sorokina, N. E.; Khaskov, M. A.; Avdeev, V. V.; Nikol'skaya, I. V. Russian Journal
of General Chemistry 2005, 75, 162.
Su, Z.; Coppens, P. Acta Crystallogr. 1997, A53, 749.
Takenaka, H.; Kawaguchi, M.; Lerner, M. M.; Bartlett, N. J. Chem. Soc., Chem.
Commun. 1987, 19, 1431.
Theng, B. K. G. The Chemistry of Clay-Organic Reactions; Hilger: London, 1974.
Thomas, J. M.; Millward, G. R.; Schlögl, R.; Boehm, H. P. Mater. Res. Bull. 1980, 15,
671.
Thompson, T. E.; McCarron, E. M.; Bartlett, N. Synth. Met. 1981, 3, 255.
Torra, M.; Rodamilans, M.; Corbella, J. Sci. Total Environ. 1998, 220, 81.
Touhara, H.; Kadano, K.; Fujii, Y.; Watanabe, N. Z. Anorg. Allg. Chem. 1987, 544, 7.
151
Touhara, H.; Kadono, K.; Imoto, H.; Watanabe, N.; Tressaud, A.; Grannec, J.
Synth. Met. 1987, 18, 549.
Tressaud, A.; Amine, K.; Mayorga, S. G.; Fargin, E.; Grannec, J.; Hagenmuller, P.;
Nakajima, T. Eur. J. Solid State Inorg. Chem. 1992, 29, 947.
Ubbelohde, A. Proc. R. Soc. London A 1968, 304, 25.
Ue, M.; Shima, K.; Mori, S. Electrochim. Acta. 1994, 39, 2751.
Underhill, C.; Krapchev, T.; Dresselhaus, M. S. Synth. Met. 1980, 2, 47.
Vaia, R. A.; Giannelis, E. P. Macromolecules 1997, 30, 8000.
Vaia, R. A.; Vasudevan, S.; Krawiec, W.; Scanlon, L. G.; Giannelis, E. P. Adv. Mater.
(Weinheim, Ger.) 1995, 7, 154.
Vaknin, D.; Davidov, D.; Zevin, V.; Selig, H. Phys. Rev. B 1987, 35, 6423.
Vogel, F. L.; Foley, G. M. T.; Zeller, C.; Falardeau, E. R.; Gan, J. Mater. Sci. Eng.
1977, 31, 261.
VWR International, Inc. Combination Fluoride Electrodes Instruction Manual. VWR
International, Inc., Westchester, PA, USA, 2002.
Watanabe, N.; Takashima, M. Kogyo Kagaku Zasshi 1971, 74, 1788.
Watanabe, N.; Touhara, H.; Nakajima, T.; Bartlett, N.; Mallouk, T.; Selig, H. Fluorine
Intercalation Compounds of Graphite. In Inorganic Solid Fluorides; Hagenmuller, P.,
Ed.; Academic Press: New York, 1985; 332.
Wen, M.; Shi, N.; Qin, Y.; Wang, C. Fluoride 1998, 31, 74.
Wen, M.; Li, Q.; Wang, C. Fluoride 1996, 29, 82.
Whittingham, M. S. Prog. Solid State Chem. 1978, 12, 41.
Whittingham, M. S.; Jacobson, A. J. Intercalation Chemistry; Academic Press:
New York, 1982.
Xu, K.; Zhang, S.; Jow, T.; Xu, W.; Angell, C. A. Electrochim. Solid-State Lett. 2002,
5, A26.
Xu, W.; Angell, C. Electrochim. Solid-State Lett. 2001, 4, E1.
152
Xu, W.; Shusterman, A. J.; Videa, M.; Velikov, V.; Marzke, R.; Angell, C.
J. Electrochem. Soc. 2003, 50, E74.
Yan, W.; Lerner, M. M. Carbon 2004, 42, 2981.
Yan, W.; Lerner, M. M. J. Electrochem. Soc. 2001, 148, D83.
Yan, W.; Lerner, M. M. J. Electrochem. Soc. 2003, 150, D169.
Yan, W.; Lerner, M. M. J. Electrochem. Soc. 2004, 151, J15.
Yang, D.; Sandoval, S.; Divigalpitiya, M.; Irwin, J.; Frindt, R. Phys. Rev. B 1991, 43,
12053.
Yang, X.; Makita, Y.; Liu, Z.; Sakane, K.; Ooi, K. Chem. Mater. 2004, 16, 5581.
Zhang, X.; Lerner, M. M. Chem. Mater. 1999, 11, 1100.
Zhang, X.; Lerner, M. M. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 2000, 340, 37.
Zhang, X.; Lerner, M. M. Phys. Chem. Chem. Phys. 1999, 1, 5065.
Zhang, X.; Lerner, M. M.; Gotoh, H.; Kawaguchi, M. Carbon 2000, 38, 1775.
Zhang, X.; Sukpirom, N.; Lerner, M. M. Mater. Res. Bull. 1999, 34, 363.
Zhang, Z.; Lerner, M. M. Chem. Mater. 1996, 8, 257.