ISSN 00181439, High Energy Chemistry, 2013, Vol. 47, No. 6, pp. 331–338. © Pleiades Publishing, Ltd., 2013. Original Russian Text © Yu.M. Shul’ga, A.S. Lobach, S.A. Baskakov, N.G. Spitsyna, V.M. Martynenko, A.V. Ryzhkov, V.B. Sokolov, K.I. Maslakov, A.P. Dement’ev, A.V. Eletskii, V.A. Kazakov, S.K. Sigalaev, R.N. Rizakhanov, N.Yu. Shul’ga, 2013, published in Khimiya Vysokikh Energii, 2013, Vol. 47, No. 6, pp. 481–489.
NANOSTRUCTURED SYSTEMS AND MATERIALS
A Comparative Study of Graphene Materials Formed by Thermal Exfoliation of Graphite Oxide and Chlorine TrifluorideIntercalated Graphite Yu. M. Shul’gaa, A. S. Lobacha, S. A. Baskakova, N. G. Spitsynaa, V. M. Martynenkoa, A. V. Ryzhkovb, V. B. Sokolovb, K. I. Maslakovc, A. P. Dement’evb, A. V. Eletskiib, V. A. Kazakovd, S. K. Sigalaevd, R. N. Rizakhanovd, and N. Yu. Shul’gae a Institute of Problems of Chemical Physics, Russian Academy of Sciences, pr. Akademika Semenova 1, Chernogolovka, Moscow oblast, 142432 Russia email:
[email protected] b Kurchatov Institute Russian Research Center, pl. Kurchatova 1, Moscow, 123182 Russia c Moscow State University, Moscow, 119991 Russia d Keldysh Research Center, ul. Onezhskaya 8, Moscow, 125438 Russia e MISiS National University of Science and Technology, Leninskii pr. 4, Moscow, 119049 Russia
Received April 29, 2013; in final form, June 24, 2013
Abstract—Graphene 3D materials GM1 and GM2 obtained by explosive exfoliation of graphite oxide and graphite intercalated with chlorine trifluoride, respectively, have been studied by elemental analysis, Xray photoelectron spectroscopy, mass spectrometry, infrared and Raman spectroscopy, and scanning electron microscopy. The specific surface area, the pore size, and electrical conductivity of the materials have been measured. A comparative study has shown that the gas mixture produced during the preparation of GM1 is less hazardous than that in the case of GM2. However, GM2 exhibits a higher conductivity and a larger size of graphene crystallites. The feasibility of isolation of a suspension of graphene nanosheets from the test 3D materials has been demonstrated. Possible applications of these materials are discussed. DOI: 10.1134/S001814391306009X
The urgent need to obtain a porous material with a low density, a high conductivity, and a large specific surface area is caused by the development and produc tion of supercapacitors. An ideal candidate for this role is a 3D graphene (expanded graphene), which is prepared by hightemperature treatment of a metal foam with hydrocarbon vapor followed by removal of the metal substrate [1–3]. Unfortunately, this is quite an expensive material, whose fabrication requires the use of sophisticated CVD equipment. For this reason, materials that are much simpler to manufacture and are supposed to acquire properties approaching those of expanded graphite are actively explored. Among these there are a material formed by explosive exfolia tion of graphite oxide [4] and a group of materials that can be obtained by thermal exfoliation of graphite intercalated with reactive molecules [5]. In this paper, we report the results of a comparative study of graphene materials produced by explosive exfoliation of graphite oxide (GO) and graphite inter calated with chlorine trifluoride (ClF3). Of particular interest for technology was to determine the composi tion of gases that are produced by the explosion and may be harmful if inhaled by the experimenter.
EXPERIMENTAL Graphite oxide was synthesized by the modified Hummers method according to the procedure detailed in [6]. A typical procedure for preparing sus pensions consisted of mixing GO (100 mg) with water (100 mL) in a glass flask. Graphite oxide films of a 200–300 µm thickness were prepared by deposition from the layer of aqueous suspensions. The films were separated from the glass substrate mechanically. A GO film with an area of about 1 cm2 was placed in a deep fusedsilica vessel, which had a cotton fabric filter mounted on the mouth to capture the explosion products. Then, the fusedsilica vessel was placed in a microwave oven (2450 MHz, 900 W) and heated until the explosion after which heating was stopped. The resulting sample was designated as graphene material 1 (GM1). The procedure for preparing another graphene material (GM2) involved several steps: in the first step, highly oriented pyrolytic graphite (HOPG) was inter calated with liquid ClF3 at room temperature; in the second step, the graphite intercalation product (GIP) was subjected to rapid heating until explosion. The first step was carried out in accordance with the proce dure described in [7]. A Teflon reactor (F4MB mate
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rial) was charged with HOPG (110 mg) and filled with gaseous ClF3 by gradually raising the pressure from 0 to 1.5 atm for 4 h and holding over 2 h at this pressure. Then, the gas was condensed at –196°С and the sam ple was held in the liquid ClF3 for 8 days at room tem perature. The GIP was a layered material golden in color, having the volume in the ClF3 atmosphere 70– 100 times that of the original HOPG sample. The weight of the sample increased to 190 mg as a result of intercalation. For exfoliation, the GIP was loaded into a long fusedsilica ampoule, which was then intro duced for 10⎯20 s into the orifice of a muffle furnace heated to Т = +750°C. The explosion turned the GIP into a black powder. The yield of GM2 was ~70% of the graphite charged. Suspensions of the graphene materials were pre pared by dispersing the material in an aqueous solu tion of the surfactant sodium dodecylbenzene sulfonate (0.5% w/v) using ultrasound (UZDN1 ultrasonic disperser, frequency 35 kHz, power 500 W, treatment time 30 min) and subsequent ultracentri fuging (10000 g, 30 min). Optical absorption spectra of multilayer graphene suspensions were obtained with a Shimadzu UV3101PC UV–Vis–NIR scanning spectrophotometer in the spectral range of 200– 1400 nm. To determine the composition of the gases released during the explosion, GM1 and GM2 samples were placed in a fusedsilica vessel with a vacuum valve through which to pump down to a pressure of 3 × 10–7 Torr. After evacuation, the vessel with the sample was placed in a microwave or a muffle furnace and heated up to explosion. After cooling to room temper ature, the vacuum vessel was connected via a valve with the inlet of the mass spectrometer for analysis. The composition of the gases produced by the explo sion of the samples in a vacuum was analyzed using an MI 1201V mass spectrometer in the electron ioniza tion mode with an electron energy of 70 eV. Mass spectra were recorded in the range of 1 < m/z < 105 (m is the atomic weight and z is the ion charge). The C, H, and O contents were determined with the use of an Elementar Vario Sube analyzer. The BET specific surface area was measured by the lowtemper ature nitrogen adsorption technique with an Autosorb1 instrument (Quntachrome Corp.). The total pore volume was determined by the amount of nitrogen adsorbed at p/p0 ~ 1. IR spectra were measured on a PerkinElmer Spec trum 100 FTIR spectrometer with an UATR attach ment in the range of 4000–670 cm–1. The Xray photoelectron spectra of GM1 samples were recorded with a PHI5500 spectrometer. Photo emission was excited with Mg Kα radiation at a 300 W power. The analysis area was 1.2 mm2. The residual pressure in the spectrometer chamber did not exceed 1 × 10–9 Torr. The Xray photoelectron spectra of GM2 samples were recorded with a Quantera SXI
spectrometer. Photoemission was excited with mono chromatic Al Kα radiation at a power of 25 W. The analysis area was 100 µm2. The residual pressure in the spectrometer chamber did not exceed 1 × 10–8 Torr. Photomicrographs of the samples were taken using a Zeiss LEO SUPRA 25 scanning electron micro scope. Raman spectra were obtained in the laser excita tion mode at λ = 514.5 nm and recorded with a Horiba Jobin Yvon T64000 instrument. The conductivity of the samples (as both films and pressed tablets) was measured by the standard four probe technique using an automated resistance and temperature measurement device [8]. RESULTS AND DISCUSSION Morphology of the graphene materials, which is characterized by the size and shape of their flakes, was studied by scanning electron microscopy. Figure 1 presents SEM images of the surface of GM1 and GM2 films. As can be seen, the materials differ in morphol ogy, which is determined by the graphene preparation procedure. The linear dimensions of flakes of the both materials are several micrometers, whereas the volume of flakes in GM1 is much greater. GM1 is composed of flakes containing many graphene sheets, with the sheets being highly defective and jagged. GM2 consists of flakes with a smaller number of graphene sheets as indicated by the transparency of some of the flakes, and the flakes have smooth edges and are less defec tive. This morphology imparts a higher specific surface area and a larger pore volume to GM1 compared with GM2 (Table 3). Figure 2a shows the IR spectra of the samples. The spectrum of graphite (curve 3) is also given there for comparison. We believe that the low transmittance of the test samples is due to their marked conductivity. Of course, their conductivity is less than that of graphite. The spectrum of GM2 exhibits an absorption band at 1060 cm–1, which is close in position to the C–F stretching vibration frequency. However, we do not associate its nature directly with residual fluorine in the sample, since the spectrum of graphite displays the same lowintensity band. Raman spectra are often attracted to certify various carbon structures. Diamond in the Raman spectrum is shown in the form of a narrow peak at 1332 cm–1 (sometimes this peak is deemed to appear at 1333 cm–1 [9]). In the Raman spectrum of “good” graphite (e.g., HOPG), the most narrow intense peak usually referred to as peak G [10] occurs at 1580 cm–1. In the case of graphite with a large number of defects, peak D (“disorder” peak) at approximately 1350 cm–1 appears in the spectrum. The ratio ID/IG is associated with the size of the graphene crystallites in the basal plane, La [9]: HIGH ENERGY CHEMISTRY
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(а)
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Fig. 1. SEM image of graphene materials.
La = (2.4 × 10–10)λ 4laser (ID/IG)–1,
(1)
where λ is the wavelength of the excitation laser, nm. Figure 2b collates the measured Raman spectra of the samples and HOPG. Table 1 lists the parameters of individual peaks in the spectra measured in this work in comparison with published data. The designation of the peaks is the same as that in [17]. The position of the maximum of the G band in the spectrum of GM2 is close to that of graphite. In the case of GM1, the position of this band is shifted by almost 10 cm–1. Published data for graphite oxide range from 1582 to 1603 cm–1. We believe that if the halfwidth of the band is large, the position of its max imum determined accurate to within 1 cm–1 is hardly a reliable parameter for certification of the material. The halfwidth of the G band of graphite is 13 cm–1. A large halfwidth of the G band in the case of GM1, in our opinion, is due to high defectiveness of the sam ple. The high defectiveness of the sample is also responsible for the large ID/IG ratio. Calculation of the
size of graphene crystallites by the above formula gave values of La = 6.9 and 10 nm for GM1 and GM2, respectively. The Raman spectra of GM1 and GM2 also differ in the overtone region above 2700 cm–1 (see Fig. 2b). In our opinion, the larger size of graphene crystal lites in GM2 is due to the fact that the carbon matrix loses one carbon atom when four fluorine atoms are removed, whereas the removal of four oxygen atoms is accompanied by the loss of two to four carbon atoms. Thus, GM1 is more defective compared to GM2. XPS spectroscopy is widely used for studying the composition and state of elements of a solid. Figure 3a presents survey XPS spectra of ClF3intercalated graphite before and after the explosive treatment. It is seen that the F1s peak is the most intense in the initial sample. After the explosion, the intensity of this peak is dramatically reduced. The composition of the test samples as calculated from the measured integral intensities of photoelectron peaks with allowance for the photoionization cross sections [18] is shown in
Table 1. Position, halfwidth, and ratio between integrated intensities of the main bands in the Raman spectra of the sam ples and published data D band Sample peak, GM1 GM2 HOPG GO GO GO GO GO GO
cm–1
1348 1350 – 1334 1350 1347 1363 1375
G band
Δ,
cm–1
190 59 –
peak,
cm–1
1589 1579.5 1579.8 1582 1595 1603 1594 1588 1593
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Δ, cm–1 110 44 13
Intensity ratio, ID/IG 2.45 1.67 – 1.16 1.2
83
λ, nm
Reference
514
This work
633 633 532 514 514 514
[11] [12] [13] [14] [15] [16]
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Table 2. Concentrations of elements according to XPS data (from survey spectra), at % Sample
C
F
Cl
O
Si
GIP GIP* GM2 GM2**
62.3 64.8 95.6 93.6
33.4 25.5 2.5 0.9
3.6 3.4 0.5 0.6
0.8 5 1.4 4.5
– 1.3 – 0.3
* After storage in air for 1 year. ** Prepared by explosive treatment of GIP.
Table 2. The presented data suggest that a significant portion of ClF3 molecules undergo degradation in the graphite interlayer space. This conclusion follows from the fact that the [F/Cl]at ratio in the probed GIP layer is significantly greater than 3. The oxygen con tent in GIP increases during its storage. Thermal explosion reduces the concentration of fluorine and chlorine in the sample. The oxygen concentration in the sample increases if the explosion is performed in air. If the sample is exploded under vacuum and subse quently cooled in a vacuum as well, the oxygen con tent even decreases. The F1s spectrum of GIP (Fig. 3b) in form is a sin gle peak with a halfwidth of 1.7 eV. After the explo sion, the peak intensity decreases, as has been noted above, but its position remains almost unchanged. In principle, the F1s peak can be used for calibration of spectra. The peak position (686.8 eV) in the test sam ples is close to that of other samples of fluorinated car
bon materials containing the ≡C–F bond and corre sponds to the position of the covalent C–F bond [19]. The highresolution Cl2p spectra of the test sam ples appeared interesting (Fig. 3c). It turned out that chlorine in the GIP occurs in two charged states cor responding to the binding energies of 200.4 and 205.0 eV. The majority of the chlorine atoms (80%) has a negative charge corresponding to the >C–Cl bond [20]. The rest has a positive charge that the chlo rine atom bears in the ClF3 molecule. Thus, the ClF3 molecules in the GIP almost completely degrade. As a result of degradation, the >C–F and >C–Cl bonds are formed. Taking into account the elemental analysis data, we can conclude that chlorine molecules are released to the gas phase during the intercalation and only a small portion of ClF3 molecules occur in the initial state in freshly prepared samples. No undissoci ated ClF3 molecules were detectable in the sample after storing at room temperature for a year. The C1s spectrum with a high energy resolution consists of two peaks (286.0 and 288.5 eV) in the case of GIP (Fig. 3d). According to published data [21], the peak at 288.5 eV is attributed to carbon atoms with one C–F bond and the peak at 286.0 eV is due to carbon atoms that lack such a bond but are closely surrounded by carbon atoms with a C—F bond. The pattern of the C1s spectrum for the products of explosive exfoliation of GIP resembles that of the XPS spectra of graphite. The main peak has asymmetry on the high binding energy side characteristic of conductive materials and a satellite due to excitation of plasmon oscillations of πelectrons (πplasmon).
Table 3. Comparative characteristics of graphene materials Graphene material Parameter Specific surface area, m2/g Total pore volume, cm3/g Conductivity, S/cm Oxygen content, wt % Fluorine content, wt % Hydrogen content, wt % Plasmon energy, eV Composition of the gas generated during exfoliation Size of the graphene crystallites, nm Temperature kink in the TGA curve for GO and GIP, °C Mass loss near temperature kink, % GO and GIP exfoliation temperature, °C (this work) Yield of GM, % relative to graphite taken
GM1
GM2
485 1.76 0.5* 4.2 0 0.7 4.75 CO, CO2, H2O, SO2 6.9 220 30 500 55
208 0.42 14** 3.78 1.4 0 4.59 CO2, CO, CF4, O2 10 585 50 750 70
* For the tablet pressed from the powder. ** For the film obtained from the suspension of GM2. HIGH ENERGY CHEMISTRY
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(а)
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2 1060
Transmittance, %
1
50
40
3
30 4000
3500
3000 2500 2000 1500 Wavenumber, cm–1
1000
(b) G
Intensity, arb. units
D
2D 2 1
3 500
1000
1500 2000 2500 Raman shift, cm–1
3000
3500
Fig. 2. (a) IR and (b) Raman spectra of graphene materials (1) GM2 and (2) GM1 and (3) graphite.
The C1s XPS spectra of GO and GM1 are shown in Fig. 4a. The C1s spectrum of GO is well approximated by three Gaussians. According to [22], the peak at Eb = 284.6 eV is due to carbon atoms having only other carbon atoms in the immediate environment. The sec ond peak (287 eV) is associated by the majority of authors with carbon atoms having at least one bond with an oxygen atom, i.e., with epoxy or hydroxyl groups. The appearance of the third peak is attributed to carbon atoms bonded with two oxygen atoms. Com paring the intensities of individual peaks, we can state that 57 and 8% of carbon atoms in initial GO are bonded with one and two oxygen atoms, respectively. The C1s spectrum of GM1 differs from that of ini tial GO. Its deconvolution into individual components showed that as low amounts as 15 and 5% of carbon atoms in GM1 are bonded with one and two oxygen atoms, respectively. Thus, the treatment of the GO film involving microwave heating described above is accompanied by a significant increase in the propor tion of carbon atoms that are not bound with oxygen HIGH ENERGY CHEMISTRY
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atoms. Moreover, the ratio of the integral intensities I(O1s)/I(C1s) decreased by a factor of 4.5 on passing from GO to GM1. It is also noteworthy that the survey XPS spectra of some of the samples display sulfur peaks. The presence of sulfur in the samples is due to the graphite oxide manufacturing technology according to the Hummers method. Intense washing of graphite oxide with water does not seem to be sufficient for the complete removal of sulfuric acid residues, some of which can occur in closed pores. The highresolution S2p spectra of GM1 samples appeared surprising (Fig. 4b). It turned out that along with the expected signals from SO 24−, the spectrum exhibits a signal due to zero charged sulfur. The reduced sulfur is assumed to result from reactions occurring during the microwave treat ment of graphite oxide. Optical absorption spectra of the samples in a sus pension are shown in Fig. 5. The spectra have one characteristic absorption peak each in the UV region
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(b)
CKW
Intensity, arb. units
Intensity, arb. units
F1s F1s O1s
2
FKW
C1s
1
1 2
Cl2p 1200 1000 800 600 400 200 Binding energy, eV
Intensity, arb. units
1
690
688 686 684 Binding energy, eV
682
(d)
CCl
C1s Intensity, arb. units
(c) CI2p
0
CIF3
πplasmon 2 CF
2
CC
1
210 208 206 204 202 200 198 196 Binding energy, eV
294 292 290 288 286 284 282 Binding energy, eV
Fig. 3. (a) Survey, (b) F1s, (c) Cl2p, and (d) C1s XPS spectra of (1) GIP and (2) GM2.
(at 261 and 270 nm for GM1 and GM2, respectively), which is attributed to the πplasmon of graphene [23], and a long gently sloping absorption band, which extends to the red and nearinfrared region. The cor
responding values of the plasmon energy in eV are given in Table 3. Dispersion of the graphene materials in various solutions or the socalled liquidphase exfo liation is used to prepare a suspension of nanosheets (b)
(а)
S6+
Intensity, arb. units
Intensity, arb. units
S2p
1
S0
2
291
288 285 Binding energy, eV
282
180
175
170 165 Binding energy, eV
160
Fig. 4. XPS spectra of starting graphite oxide and GM1: (a) the C1s spectra of (1) GM1 and (2) initial graphite oxide and (b) the S2p spectrum of GM1. HIGH ENERGY CHEMISTRY
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In the case of explosion of the graphite oxide film, the main components of the gas mixture are the car bon oxides CO (m/z 28) and CO2 (m/z 44) (Fig. 6b). Since the abundance of the peak at m/z 28 ([СО]+ ions) in the mass spectrum of pure CO2 is ~40% of that at m/z 44 ([СО2]+ ions), it can be said that there are 0.6 7 CO molecules per 10 СО2 molecules in the gas phase. In addition to the carbon oxides, the GO sam ple releases water into the gas phase (peak at m/z 18). 0.4 Molecular oxygen is hardly detectable in the gas 2 phase. Of the surprising results obtained by examining the mass spectra, the firmly established release of SO2 1 molecules (peaks at m/z 48 ([SО]+ ions) and m/z 64 0.2 ]+ ions)) to the gas phase is noteworthy. We asso 300 450 600 750 900 1050 1200 1350 ([SO2 ciate the formation of these molecules with the resid Wavelength, nm ual sulfuric acid, which was used to prepare graphite oxide. From the massspectrometric data, it follows Fig. 5. Optical absorption spectra of (1) GM1 and (2) that the microwavestimulated explosion leads to not GM2 suspensions in an aqueous solution of the surfactant only reduction, but also removal of sulfuric acid resi sodium dodecylbenzenesulfonate. dues. 270 nm 0.8 261 nm
composed of one to ten layers in ratios depending on the experimental conditions. Mass spectrometric study of the composition of gases produced by the explosion of graphite interca lated with ClF3 (Fig. 6a) showed the presence of CO (m/z = 28), CO2 (m/z = 44), and oxygen O2 (m/z = 32). We attribute this to the fact that the sample was stored in air for a long time before the explosion. In addition to these molecules, SiF4, CF4, and COF2 were found in the gas phase, with their most intense peaks appearing at m/z 85, 69, and 47, respectively. The presence of SiF4 in the gas phase is due to the fact that the explosion accompanied by the release of reac tive fluorinecontaining molecules was carried out in a fusedsilica ampoule.
The main characteristics of the compared graphene 3D materials are shown in Table 3. Note a few techno logical aspects and possible applications of these materials. The manufacturing process for GM1 is much simpler than that for GM2. The materials and equipment used to produce GM1 are more available than those for GM2. The gases released during the exfoliation of GO do not contain halogen derivatives and are therefore less harmful than those formed dur ing the exfoliation of GIP. However, GM2 has a higher conductivity and a larger graphene crystallite size. We believe that the set of the revealed properties of GM1 makes it suitable for use as a component in the manufacture of supercapacitor electrodes, a sorbent, and a filler in certain polymer reactions. In [24], we showed GM1 to act as an accelerator of lowtempera
(а)
(b) 28 44
28
Intensity, arb. units
Intensity, arb. units
44
85 32
69 16 12
47
20
18 12 16
40
60
10
80
m/z
48
20
30
40
50 60 m/z
64
70
80
90
Fig. 6. Mass spectra of gases formed during the thermal explosion of the (a) GIP and (b) GO film samples in vacuum. HIGH ENERGY CHEMISTRY
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ture radiation polymerization of tetrafluoroethylene. The GM1–PTFE composite produced by this process holds much promise for manufacturing articles in which the use of pure PTFE is limited by its high flowability under pressure. It was shown [25] that films made from polymer composites based on GM2 and carboxymethylcellulose exhibit nonlinear optical properties and can be used in optical devices for mod ulating laser radiation. GM2 is more appropriate for making optically transparent films and coatings with high conductivity. REFERENCES 1. Chen, Z., Ren, W., Gao, L., Liu, B., Pei, S., and Cheng, H.M., Nat. Mater., 2011, vol. 10, no. 6, p. 424. 2. Cao, X., Shi, Y., Shi, W., Lu, G., Huang, X., Yan, Q., Zhang, Q., and Zhang, H., Small, 2011, vol. 7, no. 22, p. 3163. 3. Pettes, M.T., Ji, H., Ruoff, R.S., and Shi, L., Nano Lett., 2012, vol. 12, no. 6, p. 2959. 4. Zhu, Y., Murali, S., Stoller, M.D., Ganesh, K.J., Cai, W., Ferreira, P.J., Pirkle, A., Wallace, R.M., Cychosz, K.A., Thommes, M., Su, D., Stach, E.A., and Ruoff, R.S., Science, 2011, vol. 332, no. 6062, p. 1537. 5. Makotchenko, V.G., Grayfer, E.D., Nazarov, A.S., Kim, S.J., and Fedorov, V.E., Carbon, 2011, vol. 49, p. 3233. 6. Muradyan, V.E., Ezerskaya, M.G., Smirnova, V.I., Kabaeva, N.M., Novikov, Yu.N., Parnes, Z.N., and Vol’pin, M.E., Zh. Obshch. Khim., 1991, vol. 61, no. 12, p. 2626. 7. Selig, H., Sander, W.A., Vasile, M.J., Stevie, F.A., Gal lagher, P.K., and Ebert, L.B., J. Fluorine Chem., 1978, vol. 12, no. 5, p. 397. 8. Buravov, L.I., Zverev, V.N., Kazakova, A.V., Kushch, N.D., and Manakov, A.I., Instrum. Exp. Tech., 2008, vol. 51, no. 1, p. 156. 9. Pimenta, M.A., Dresselhaus, G., Dresselhaus, M.S., Cancado, L.G., Jorio, A., and Saito, R., Phys. Chem. Chem. Phys., 2007, vol. 9, p. 1276. 10. Cancado, L.G., Pimenta, M.A., Neves, B.R.A., Dan tas, M.S.S., and Jorio, A., Phys. Rev. Lett., 2004, vol. 93, p. 247401. 11. Hong, S., Jung, S., Kang, S., Kim, Y., Chen, X., Stank ovich, S., Ruoff, S.R., and Baik, S., J. Nanosci. Nano technol., 2008, vol. 8, no. 1, p. 424.
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