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ISSN 0020-1685, Inorganic Materials, 2008, Vol. 44, No. 7, pp. 697–704. © Pleiades Publishing, Ltd., 2008.Original Russian Text © L.V. Dubrovina, V.M. Ogenko, O.V. Naboka, V.A. Dimarchuk, Ya.V. Zaulichnyi, O.Yu. Khizhun, 2008, published in Neorganicheskie Materialy, 2008,Vol. 44, No. 7, pp. 799–807.
697
INTRODUCTION
Carbon-loaded porous materials are widely used ashigh-performance sorbents, catalyst supports, andmembrane systems for solution filtration and gas sepa-ration. To produce carbon in porous materials, use isoften made of the carbonization of polymers andorganic substances. Thermal decomposition of a poly-meric precursor leads to the formation of nanoporouscarbon structures, such as activated carbon, molecularsieves, and fibers. The formation of carbon structuresdepends on carbonization conditions, in particular, onthe process rate, precursor concentration, and the pres-ence of a catalyst. The structure of carbon is also influ-enced by the orientation of polymer chains. Using tem-plates with different shapes of structural voids, one cantailor the morphology and anisotropy of resulting nano-structures [1–4].
In this paper, we report the electronic structure andproperties of carbon-loaded porous materials producedby the carbonization of poly(vinylidene fluoride)(PVDF) in fine-particle silica and its mixtures with nat-ural graphite.
EXPERIMENTAL
As a structure-forming component of composites,we used fine-particle silica (particle size,
≤
80
nm; spe-cific surface
S
= 50 g/m
2
) or its mixture with S-00 nat-ural graphite (particle size, 1–5
µ
m). Silica particles in
a liquid dispersion medium form stable structures—gels [5]. During subsequent drying, they convert toporous materials, which can be used as matrices.
The carbon precursor used was commercially avail-able PVDF with a characteristic viscosity [
η
] = 1.2 dl/gas determined at
25°
C in dimethylformamide (DMFA).A PVDF solution in acetone with a concentration of2.0 g/100 ml (at
50°
C) or in DMFA with a concentra-tion of 2.0 or 4.0 g/100 ml (at room temperature) wasused as a dispersion medium for gel preparation. Driedsamples were carbonized at
750°
C in flowing argon.To dissolve away the silica, the samples were etched
in a mixture of concentrated sulfuric and hydrofluoricacids (5 drops of H
2
SO
4
and 1 ml of HF), dried, andwashed with water. The precipitate was dispersed inacetone, and the suspensions were used to determinethe particle size and shape by transmission electronmicroscopy (TEM) on a JEOL JEM-100CX-II usingstandard techniques. To visualize the structure of thecarbon-loaded composites and determine their compo-sition, we used a Carl Zeiss ULTRA55 FESEM instru-ment equipped with an Oxford InstrumentsINCAx-sight energy dispersive x-ray (EDX) spectrom-eter.
X-ray photoelectron spectra were measured on aKratos Analytical SERIES 800 XPS spectrometer using anonmonochromatic Mg
K
α
x-ray source (
h
ν
= 1253.6 eV).Powder samples for examination were applied to anadhesive tape.
Carbon-Loaded Porous Composites Produced by Matrix Carbonization of Poly(vinylidene fluoride)
L. V. Dubrovina
a
, V. M. Ogenko
a
, O. V. Naboka
a
, V. O. Dymarchuk
b
, Ya. V. Zaulychnyy
b
, and O. Yu. Khyzhun
b
a
Vernadsky Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine, pr. Akademika Palladina 32/34, Kiev, 03680 Ukraine
b
Frantsevich Institute of Materials Science Problems, National Academy of Sciences of Ukraine, ul. Krzhizhanovskogo 3, Kiev, 03680 Ukraine
e-mail: [email protected]
Received June 18, 2007
Abstract
—Using poly(vinylidene fluoride) (PVDF) carbonization at 750
°
C in fine-particle silica and its mix-tures with graphite, we have prepared carbon-loaded porous composites which offer benzene absorption from0.90 to 1.52 ml/g, compressive strength of 6 MPa, and Brinell hardness of up to 18 MPa. We observed the for-mation of various nanostructures (spheres, spherical segments, and layered platelets) and sizes (several to hun-dred nanometers). X-ray photoelectron and energy dispersive x-ray spectroscopy data indicate the presence ofC
–
C
,
C
=
C, CO, COO, and CHF groups on the carbon surface. X-ray emission spectroscopy data show that thesilica matrix composite prepared via PVDF carbonization contains small carbon clusters weakly bonded to thematrix. The silica/graphite matrix composite contains multilayer carbon films strongly bonded to the matrix.The O
K
α
spectra of both composites are similar to the spectrum of pure SiO
2
.
DOI:
10.1134/S0020168508070054
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Ultrasoft x-ray emission spectra were recorded onan RSM-500 x-ray spectrometer/monochromatorequipped with an oil-free pumping system. The resid-ual pressure in the test chamber was
5
×
10
–6
Pa. X-rayswere dispersed by a diffraction grating (600 lines/mm)with a 6026-mm radius of curvature. The acceleratingvoltage and anode current of the x-ray tube were 5 kVand 1.5 mA, respectively. The x-rays were detected bya VEU-6 secondary electron multiplier with a CsI pho-tocathode. The samples were rubbed into the copperanode, cooled by running water.
The absorption of organic solvents in the sampleswas determined by weighing after holding for a day inan appropriate solvent at room temperature. The physi-cal and mechanical properties of the materials werestudied using samples
1
×
1
×
0.5
cm in dimensions.
RESULTS AND DISCUSSION
Fabrication and electron-microscopic character-ization of porous composites.
Ordered modifyingnanostructures cannot be prepared using disorderedporous oxide matrices (SiO
2
,
Al
2
O
3
), in contrast to
ordered matrices (mesoporous silica and zeolites). Theuse of a strictly determined pore structure limits, ormakes impossible, control over the shape and size ofpyrolytic carbon particles through changes in thesupramolecular structure of the polymeric precursorbecause, when penetrating into pores, macromoleculesunfold [6, 7]. The use of fine-particle SiO
2
matrices thatare formed concurrently with the introduction of apolymeric precursor and are labile at this stage makes itpossible to tune the distribution of the polymer in thematrix and, hence, the structure of the pyrolytic carbonowing to the interaction between the components [8].
The use of PVDF as the carbonization precursorallows one to examine the effect of the supramolecularstructure of the polymer on the structure of the resultingpyrolytic carbon because crystalline PVDF exists infour polymorphs, depending on the crystallization con-ditions [9]. Moreover, during PVDF carbonization theremoval of the HF molecules may be followed by theformation of carbon structures containing
ë=ë
and
ë
≡
ë
groups [10, 11].
Figure 1 presents micrographs of a carbon-loadedmaterial (Fig. 1a) and carbon structures produced by
(c) 100 nm 100 nm
100 nm100 nm
(d)
(‡) (b)
Fig. 1.
Micrographs of a carbon-loaded silica matrix composite produced via PVDF carbonization (a) and carbon structures obtained fromPVDF introduced as a solution in acetone (b) or DMFA (a, c, d); PVDF concentration of (b, c) 2.0 and (a, d) 4.0 g/100 ml.
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CARBON-LOADED POROUS COMPOSITES PRODUCED BY MATRIX CARBONIZATION 699
carbonizing PVDF introduced into SiO
2
as a solution inacetone (Fig. 1b) or DMFA (Figs. 1a, 1c, 1d). The com-posite is seen to consist of connected spherical particles
~100
nm in size.During the formation of gels in PVDF as a disper-
sion medium, the self-organization of silica particles isinfluenced by the adsorption of polymer macromole-cules on their surfaces. Acetone is a poor solvent forPVDF but readily adsorbs on the surface of SiO
2
parti-cles. The competing adsorption of solvent moleculesprevents the transition of polymer molecules to thesolid surface, causing them to desorb into spacesbetween silica particles and aggregates owing to thehigher energy of the interaction between polymer mac-romolecules compared to the interaction with acetonemolecules and the surface of SiO
2
particles [9, 12]. Thesize and shape of the carbon particles resulting fromcarbonization correspond to the structure of such poly-mer layers: the particles have the form of spherical seg-ments up to 100 nm in size (Fig. 1b).
When a PVDF solution in DMFA is used, weobserve the formation of similar particles (Fig. 1c) or
~100
-nm spherical particles, their aggregates, and lay-ered platelike (fibrillar) structures several nanometersin thickness (Fig. 1d). These results can be rationalizedin terms of changes in the supramolecular structure ofPVDF macromolecules in solution and a molecularaggregation mechanism of adsorption from concen-trated polymer solutions. DMFA is a good solvent forPVDF: the energy of the interaction between polymerand solvent molecules is higher compared to the inter-action between polymer molecules. During adsorptionfrom a solution with
ë
= 2.0 g/100 ml, PVDF macro-molecules adsorb not only on the surface of silicaaggregates but also on separate particles. Some of thepolymer macromolecules do not adsorb and residebetween silica particles [12–14]. Carbonization con-verts adsorbed PVDF macromolecules to spherical car-bon structures close in size (
~100
nm) to the SiO
2
parti-cles (Fig. 1c).
PVDF is a partially crystalline polymer. It exists infour crystalline polymorphs, which differ in chain con-formation and macromolecule arrangement. Crystalli-zation from a solution in DMFA yields a mixture of the
α
,
β
, and
γ
polymorphs. PVDF macromolecules are in
TGTG
-
,
T
3
GT
3
G
-
, and planar zigzag conformations inthe
α
,
γ
, and
β
conformations, respectively. Heating to
160–170°
C leads to the
α
γ
polymorphic transfor-mation [9, 10]. The likely reason for the formation oflayered platelike (fibrillar) carbon structures (Fig. 1d) isthe presence of trans-sequences in the PVDF macro-molecules located outside the adlayer.
During adsorption from a solution with
ë
= 4 g/100ml, aggregates of PVDF macromolecules adsorb on thesurface of SiO
2
particles. The bonding to the surface isconsiderably weaker compared to the adsorption fromless concentrated solutions. The adlayers undergo gela-tion, and their structure becomes similar to that of layersproduced by adsorption from a poor solvent [12–14]. Itis probably for this reason that PVDF carbonizationunder such conditions yields spherical segments up to100 nm in size, like in the carbonization of PVDF intro-duced as a solution in acetone (Figs. 1b, 1c).
Figure 2a presents micrographs of a carbon-loadedcomposite prepared through PVDF carbonization in amixture of silica and graphite. The pyrolytic carboncoating formed in the composite consists, for the mostpart, of thin (3–5 nm) filmlike particles. Also presentare spherical particles up to 100 nm in size (Fig. 2b).The formation of carbon structures of these size andshape can be accounted for by the fact that PVDF mac-romolecules adsorb predominantly on the surface of thegraphite particles, while the SiO
2
particles remain in theform of aggregates between graphite–adsorbed PVDFblocks [12–14].
Surface composition of the composites.
The com-posites had the form of monolithic samples with metal-lic luster on their surface.
100 nm500 nm(‡) (b)
Fig. 2.
Micrographs of a carbon-loaded silica/graphite matrix composite produced via PVDF carbonization (a) and carbon structuresobtained from PVDF introduced as a solution in DMFA (b); PVDF concentration of 4.0 g/100 ml.
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Figure 3 shows the EDX spectrum of the surface ofa silica matrix composite prepared via PVDF carbon-ization. The corresponding elemental composition is asfollows (wt %/at %): carbon, 7.11/11.03; oxygen,53.45/62.23; fluorine, 1.82/1.79; silicon, 37.62/24.95.It follows from these data that the oxygen content of thecomposite exceeds that of SiO
2
, which implies that asignificant fraction of the oxygen is bonded to carbon.
The x-ray photoelectron spectra of the compositeswere measured in the
ë1
s
region. The observed asym-metry and significant broadening of the lines suggestthat these consist of several components. To separateindividual components, we used the method of curvesynthesis. Measured curves were reconstructed usingan increasing number of individual peaks. The recon-structed spectrum of a composite produced by carbon-izing PVDF introduced into SiO
2
as a solution inDMFA consists of five individual peaks. The peaks at284.0 and 285.0 eV are due to
ë–ë
and
ë=ë
groups.The peak at 286.2 eV corresponds to the carbon atomsof CO groups. The peaks at 288.2 and 290.3 eV arisefrom COO and CHF groups, respectively [15].
Electronic structure of the composites studied byx-ray emission spectroscopy.
Ultrasoft x-ray emissionspectroscopy in the
ë
K
α
region provides direct infor-mation about the energy distribution of
ë 2
states incarbides and carbon materials [16, 17]. To identifyPVDF carbonization products, we first measured the
ë
K
α
x-ray emission spectra of graphite, diamond,fullerenes, and nanotubes (Fig. 4).
As seen in Fig. 4, the shape, energy position, andrelative intensity of the fine structure of the
ë
K
α
bandare very sensitive to the nature of hybridization (
sp
2
ingraphite and
sp
3
in diamond), overlap of the
ë
z
orbit-als over and in fullerene molecules (
ë
60
and
ë
70
), andnanotube diameter (20 and 70 nm).
0
Energy, keV
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
ë
F
O Si
Fig. 3.
EDX spectrum of the surface of a carbon-loaded silica matrix composite prepared via PVDF carbonization.
265 270 275 280 285
1
260 Energy, eV
2
3
4
5
6
Inte
nsity
Fig. 4. ë Kα x-ray emission spectra of (1) graphite, (2) dia-mond, (3) ë60 fullerene, (4) ë70fullerene, and (5, 6) carbonnanotubes 20 and 70 nm in diameter, respectively.
INORGANIC MATERIALS Vol. 44 No. 7 2008
CARBON-LOADED POROUS COMPOSITES PRODUCED BY MATRIX CARBONIZATION 701
Figure 5 illustrates the evolution (several consecu-tive scans) of the ë Kα x-ray emission spectrum of acarbon-loaded silica matrix composite produced viaPVDF carbonization. The ëKα spectrum was recordedconcurrently with the éKα. Since the intensity of theéKα band was found to remain unchanged in repeatedmeasurements, all of the ë Kα bands in Fig. 5 were nor-malized by the same intensity of the éKα band (the sec-ond-order reflection overlaps with the low-energy tailof the ë Kα band). It is seen in Fig. 5 that, in the firstscan, the ë Kα spectrum of the carbon-loaded silicamatrix composite produced via PVDF carbonization issimilar to that of ë60 fullerene (Fig. 4). In subsequentscans, however, the intensity of the ë Kα emissiondecreases, and its shape varies. This effect can beaccounted for by the fact that the carbon coating con-sists of small clusters (Fig. 1). The electron beamremoves them via diffusion or sublimation [18].
In the case of the silica/graphite matrix composite(Fig. 6), the intensity and shape of the ë Kα emissionremain unchanged un several consecutive scans, andthe average spectrum is almost identical to the spectrumof graphite (Fig. 4). The pyrolytic carbon in this com-posite resides predominantly on graphite (Fig. 2). It
seems, therefore, likely that the carbon coating of thematrix consists of thin multilayer films, which are sta-ble and are not subject to diffusion or sublimation underthe action of the electron beam [18]. The relative inten-sity of the éKα band in the second-order reflection (rel-ative to the intensity of the ë Kα emission) for the sil-ica/graphite matrix composite is substantially lowerthan that for the silica matrix composite, which atteststo a reduction in the amount of oxygenated carbongroups in the composite (Figs. 5, 6).
Figure 7 shows the éKα spectra of the silica (Fig. 7,spectrum 1) and silica/graphite (Fig. 7, spectrum 2)matrix composites. The éKα spectra of the two com-posites are identical in shape to the spectrum of purequartz (α-SiO2) [19, 20]. As seen in Fig. 7, the éKαspectra of the composites show, in addition to maxi-mum d, two features (a and c) and a well-defined shoul-der (b) on the low-energy side of the major peak. Asshown earlier [21], features a and b in the éKα spec-trum of α-quartz are due to σ-bonds: O 2p–Si 3s andO 2p–Si 3p, respectively. Feature c, which is locatedmost closely to the major peak d, in the éKα spectrumof α-SiO2 is due to the O 2p states weakly bonded to the
256 261 266 271 276
1
251 Energy, eV
2
3
4
5
281 286 291
Inte
nsity
Fig. 5. Evolution of the shape and relative intensity of the ë Kα and éKα (second-order reflection) x-ray emission spectra of a car-bon-loaded silica matrix composite produced via PVDF carbonization: (1–5) first through fifth scans.
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INORGANIC MATERIALS Vol. 44 No. 7 2008
DUBROVINA et al.
258 263 268 273 2781
253Energy, eV
2
3
4
283 288
Inte
nsity
Fig. 6. ëKα x-ray emission spectra of a carbon-loaded silica/graphite matrix composite produced via PVDF carbonization: (1–3)first through third scans, (4) average spectrum.
515 520 525 530
1
510Energy, eV
2
a
d
535 540
Inte
nsity
c
b
Fig. 7. éKα x-ray emission spectra of carbon-loaded (1) silica and (2) silica/graphite matrix composites produced via PVDF car-bonization.
INORGANIC MATERIALS Vol. 44 No. 7 2008
CARBON-LOADED POROUS COMPOSITES PRODUCED BY MATRIX CARBONIZATION 703
Si s,d valence states. The region of the major peak cor-responds to the é 2p nonbonding states [21].
It also follows from Fig. 7 that the full width at halfmaximum (FWHM) of the éKα band of the silica-matrixcomposite substantially exceeds (by 0.6 ± 0.1 eV) that ofthe silica/graphite matrix composite. The broadeningoccurs in the energy range corresponding to the majorpeak d. Since, in the energy range of peak d, the éKαband of silica is only due to the O 2p nonbonding states,one possible reason for the broadening is that the O 2pstates, nonbonding in pure SiO2, are involved in chem-ical bonds with the products of PVDF carbonization inthe silica matrix. The bonding between the SiO2 matrixand PVDF carbonization products is very weak, and theelectron beam removes the carbonization products dur-ing measurements, as evidenced by the reduction in theFWHM of the éKα band in subsequent scans (Fig. 5)and the systematic reduction in the intensity of the ë Kαband relative to that of the éKα band in the spectrum ofthe silica matrix composite.
No fluorine was detected by x-ray emission spec-troscopy in the bulk of the composites. This seems to berelated to the combustion behavior of polymer materi-als: their surface is much cooler than their bulk, and thefluorine-containing fragments of PVDF remain in thesurface layer [22].
Properties of carbon-loaded porous composites.The table summarizes the properties of the carbon-loaded composites. The materials are seen to have a lowapparent density (high total porosity) and sufficientstrength. Some of the composites exhibit a molecularsieve effect: smaller absorption of hexane compared tobenzene [23]. The best performance parameters areoffered by the carbon-loaded silica matrix compositesproduced by carbonizing PVDF introduced as a solu-tion in DMFA with a concentration of 4.0 g/100 ml.
CONCLUSIONS
We prepared monolithic carbon-loaded porousmaterials.
Our results demonstrate that carbon coatings pro-duced using PVDF solutions consist of nanostructureswhose size and shape depend on the matrix composi-tion and supramolecular structure of the PVDF macro-
molecules. The resultant pyrolytic carbon containsë−ë, ë=ë, CO, COO, and CHF groups.
The ë Kα x-ray emission data indicate that, in thesilica matrix composite produced via PVDF carboniza-tion, the pyrolytic carbon is present in the form of smallclusters and is weakly bonded to the matrix. In the sil-ica/graphite matrix composite, the pyrolytic carbon hasthe form of thin multilayer films and is more stronglybonded to the matrix. The éKα x-ray emission spectraof both the silica and silica/graphite matrix compositesare similar to the spectrum of pure SiO2.
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PVDF Matrix C, g/100 ml
Apparent density, g/cm3
Compressive strength, MPa
Brinell hardness, MPa
Benzene absorption, ml/g
Hexane absorption, ml/g
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In DMFA SiO2 2 0.65 1.08 1.05
SiO2 4 0.45 6 18 1.15 1.06
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INORGANIC MATERIALS Vol. 44 No. 7 2008
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