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7Статья поступила в редакцию 19.10.14. Ред. per. № 2120
The article has entered in publishing office 19.10.14. Ed. reg. No. 2120
УДК 541.145+541.141.3
SYNTHESIS AND CHARACTERIZATION OF GRAPHENE-BASED
MATERIALS PRODUCED VIA THERMAL EXFOLIATION OF GRAPHENE OXIDE AND OF ClF3 INTERCALATED GRAPHITE
Oleksandr B. Bondarchuk1*, Anatoly S. Lobach2, Sergey A. Baskakov2, Natalia G. Spitsyna2, Aleksandr V. Ryzhkov3, Valery A. Kazakov4, Alexander Michtchenko5, Alexander L. Gusev6, Roman D. Mysyk1,
Yury M. Shulga27
'CIC energiGUNE, Parque Tecnológico c/ Albert Einstein 48, 01510 Miñano, Alava, Spain 2Institute of Problems of Chemical Physics RAS 1 Ac. Semenov Av., Moscow Region, Chernogolovka, 142432 Russian Federation 3National Research Center "Kurchatov Institute" 1 Kurchatov sq., Moscow, 123182 Russian Federation
4Keldysh Research Center 8 Onezhskaya, Moscow, 125438 Russian Federation
5
5Instituto Politecnico Nacional, SEPI-ESIME-Zacatenco
C.P. 07738, D.F., Mexico
Scientific and Technical Center" TATA"" LLC
Post Box Office 683, Sarov, Nizhny Novgorod, 607183 Russian Federation i-
i
ph./fax: (83130)6-31-07, e-mail: gusev@hydrogen.ru National University of Science and Technology MISIS 4 Leninsky pr., Moscow, 119049 Russian Federation
doi: 10.15518/isjaee. 2014.19.001
ti
Referred 21 October 2014 Received in revised from 24 October 2014 Accepted 27 October 2014
£
Graphene-based materials GM1 and GM2 have been synthesized by explosive exfoliating two different precursors: graphite oxide and graphite intercalated with chlorine trifluoride respectively. Compositional and structural transformations of the precursors into final graphene-based materials have been followed by using combination of X-ray photoelectron spectroscopy, FTIR and Raman spectroscopy, and Scanning Electron Microscopy. Specific surface area, pore size and electrical conductivity of the synthesized materials have also been measured.
Comparative mass spectrometry analysis of the gas co-products emitted during synthesis has revealed that synthesis of GM1 from graphite oxide is more environmentally viable. However, synthesized GM2 materials possess higher electrical conductivity and are characterized by larger size of graphene sheets.
We have demonstrated that the graphene nanosheets can be produced from suspensions of the GM1 and GM2 materials in the aqueous solution of a surfactant dodecylbenzenesulfonate.
The potential applications areas for the synthesized materials have been discussed.
Key words: Graphen-based materials, XPS, Raman spectroscopy.
International Scientific Journal for JM, n r- ——, №19(159) Международный научный журнал
Alternative Energy and Ecology -tSi- Г^)М,С\ H г-> ?ni4 «Альтернативная энергетика и экология»
©ScientificTechnical Centre «ТАТА», 2014 "^T^ —J<-— —'—> © Научно-технический центр «ТАТА», 2014
Бондарчук Александр
Борисович Oleksandr Bondarchuk
Сведения об авторе: кандидат физико-математических наук, заведующий лабораторией научно-исследовательского центра CIC energiGUNE, Миняно, Испания.
Образование: инженер-физик, радиофизический факультет Киевского университета им. Т.Г. Шевченко.
Область научных интересов: физика поверхности, рентгеновская спектроскопия, сканирующая зондовая микроскопия Публикации: около 40.
Information about the author: PhD
(physics), Head of Surface Science Laboratory of research center CIC energiGUNE, Minano, Spain.
Education: engineer-physicist, Faculty of Radiophysics, Taras Shevchenko National University of Kyiv, Ukraine.
Area of researches: surface science, X-ray photoelectron spectroscopy, scanning probe microscopy.
Publications: about 40.
Лобач Анатолий
Степанович Lobach Anatoly Stepanovich
Сведения об авторе: кандидат химических наук, старший научный сотрудник Института проблем химической физики РАН.
Образование: инженер-технолог, Московский химико-технологический институт им. Д.И. Менделеева.
Область научных интересов: химия углеродных наноматериалов, композиционные наноматериалы.
Публикации: более 100 работ в рецензируемых журналах; индекс Хирша 18.
Information about the author: Ph.D. (Chemistry), Senior research scientist of Institute of Problems of Chemical Physics Russian Academy of Science.
Education: engineer-technologist, Moscow D. Mendeleev Institute of Chemical Technology.
Area of researches: chemistry of carbon nanomaterials, composite nanomaterials.
Publications: more 100 peer-reviewed articles; h-index is 18.
M, - С -'м1
с о
Баскаков Сергей
Алексеевич Baskakov Sergey Alekseevich
Сведения об авторе: кандидат химических наук, старший научный сотрудник Института проблем химической физики РАН.
Образование: химик, Ивановский
государственный университет, биолого-химический факультет.
Область научных интересов: углеродные наноматериалы, композиционные материалы на основе графена, суперконденсаторы.
Публикации: автор более 30 научных работ в рецензируемых журналах.
Information about the author: Ph.D. (Chemistry), Senior research scientist of Institute of Problems of Chemical Physics Russian Academy of Science.
Education: chemist, Ivanovo State University, Faculty of Biology and Chemistry
Area of researches: carbon nanomaterials, composite materials based on graphene, supercapacitors.
Publications: more than 30 scientific publications.
МысыкРоман Дмитривич Roman Mysyk
Сведения об авторе: кандидат химических наук, заведующий группой суперкондесаторов научно-исследовательского центра С1С energiGUNE, Миняно, Испания.
Образование: инженер-химик, факультет экологии и химической технологии, Донецкий технический университет.
Область научных интересов: углеродные наноматериалы, суперкондесаторы (ионисторы).
Публикации: около 30 научных работ.
Information about the author:
Candidate of Science (PhD in Chemistry), Leader of Capacitor Group, research center CIC energiGUNE, Minano, Spain.
Education: engineer-chemist, Faculty of Ecology and Chemical Technology, Donetsk Technical University, Ukraine.
Area of researches: carbon nanomaterials, supercapacitors.
Publications: about 30 scientific publications.
-O
N
1. Introduction
Porous materials possessing low density in combination with high electrical conductivity and high specific surface area are demanded for various applications, particularly as electrode materials for the electrical energy storage devices. Graphene foam could be an idea candidate for this role. Graphene foam is synthesized by high temperature treatment of metal foam in atmosphere of hydrocarbons - CVD deposition, after which the metal is processed away [1-12]. The result is 3D mesh but constructed of graphene. Unfortunately, large scale production of graphene foam in this way is hindered by high production cost. Therefore several others strategies have been explored recently. Among them are explosive exfoliation of graphite oxide (GO) [13-15] or thermal exfoliation of graphite intercalated by reactive molecules [16-18]. The materials produced in these ways are believed to have properties similar to graphene foam.
In this work we have carried out a comparative study of the graphene-based materials synthesized by using explosive exfoliation of two different precursors: GO and graphite intercalated with ClF3. Special attention in this study has been given to identification of the potentially hazardous gaseous substances generated during explosive exfoliation of the precursors.
Q.
2. Experimental
c
Graphite oxide was prepared using modified Hammerss method [19]. The details of the preparation procedure can be found elsewhere [20]. The suspensions were made by mixing up GO (100 mg) with water (100mg) in a glass vial. GO films with thickness of 200300 wm were prepared by precipitating top layer of water suspension on a cover glass and then the GO films were mechanically removed from the support.
Samples of GO film with area about 1 cm2 were introduced into a deep glass bottle, to collect the explosion products the open part of the bottle was covered with a filter (cotton tissue). Then the bottle was heated up in a microwave oven (2450 MHz, 900 W) until the content of the bottle has exploded. The heating was stopped immediately with the onset of explosion The material prepared in the above described wa will be referred as graphene-based material of type 1 or GM1.
Graphene-based material of 2d type (GM2) was synthesized in several steps. Intercalation of highly oriented pyrolytic graphite (HOPG) at room temperature with liquid ClF3 was the first step. In the second step the product of graphite intercalation (PIG) underwent fast heating until onset of explosion. The intercalation procedure was similar to the one described elsewhere [21]. A 110 mg HOPG sample was put in a PTFE reactor and further treated with ClF3 gas. The pressure of ClF3 was gradually increased from 0 to 15 bar over 4 hours and then it was dwelling at the maximum pressure for
another 2 hours. In the next step the gas was condensed at -196 °C, and the sample was held in liquid ClF3 at room temperature for 8 days. Produced PIG is a layered material with golden colour, its volume is ~70-100 times of the initially pristine HOPG sample. Weight of the intercalated sample increased up to 190 mg. PIG sample was exfoliated by sealing it off in a long quartz ampoule which then was introduced for 10-20 s into a muffle furnace heated up to + 750 °C. Thermally stimulated explosion rendered PIG into black powder - GM2 sample. Weight of the GM2 yield was about ~70 mass % of the HOPG load.
GM suspensions were prepared by ultrasonic-assisted dispersion of the material in an aqueous surfactant solution, dodecylbenzenesulfonate (0.5 % w/v), using an ultrasonic dispenser UZDN-1 (frequency 35 kHz, power 500 W, treatment time 30 min), which was followed by ultracentrifugation (10000 g, 30 min). Optical absorption spectra of the suspensions were recorded using a UV-Vis-NIR Scanning Spectrophotometer (Shimadzu UV-3101PC) in the wavelength range from 200 to 1400 nm.
For identification of the gaseous co-products of the explosive exfoliation GO and PIG samples were sealed off in a quartz vessel with a vacuum valve. Via the valve the vessel was pumped down to 3x10-7 torr, sealed off, ^ disconnected from the pumping line and heated in the muffle furnace up to explosion. After cooling down the vessel was hooked up via the vacuum valve to the inlet line of the mass spectrometer (MI 1201B, operated at 70 V ionization voltage) for mass spectroscopy analysis. The mass spectra were recorded in the rage from 1 m/z up to 105 m/z.
Concentrations of C, H and O were measured by means of elemental analyzer Elementar Vario Cube" (Elementar Analysensysteme).
BET specific surface area was determined using instrument Autosorb-1 (Quntachrome Corp.). Total pore volume was evaluated from the amount of nitrogen adsorbed at p/p0~1.
Raman spectra were collected using excitation sources of several different wavelengths: 514 nm (T64000, Horiba Jobin Yvon), 532 nm and 785nm (inVia, Ranishaw). Radiation power was 0.1, 0.1 mW and 0.05 mW for the 514 nm, 532 nm and 785 nm wavelengths respectively. Spectral lines in the Raman spectra were fitted assuming that the components could be described by the mixed Gaussian/Lorentzian line shape.
XPS spectra of the GM1 samples were measured by using UHV systems PHI-5500 with acceptance area 1.2 mm2. Photoelectron spectra were excited by means of non-monochromated Mg Ka radiation source with power 300W. For measuring XPS spectra from GM2 samples we have used an UHV spectrometer Quantera SX equipped by an electron analyzer with acceptance area of 100 wm2 and with monochromated Al Ka radiation source operated at power of 25W. The XPS spectra were peak-fitted using CasaXPS data processing software.
Quantification has been done using sensitivity factors provided by the elemental library of CasaXPS.
Microphotography images of the studied samples were taken by scanning electron microscope LEO SUPRA 25 (Zeiss).
Electrical conductivity of the film samples and samples compressed in tablets was measured by standard 4-contact method using automated system for temperature and resistivity measurements described elsewhere [22].
3. Results and discussion
3.1 Scanning Electron Microscopy (SEM)
Morphology of the synthesized graphene-based materials can be described as a conglomeration of flakes with different shape and size. SEM images of GM1 and GM2 materials are shown in Fig. 1. One can see from Fig. 1 that the morphology strongly depends on the preparation method. While the lateral size of the flakes is about couple microns for both materials, the flakes in GM1 are thicker, containing much more graphene layers, than the flakes in the GM2 material. Layers in the GM1samples appear more defective with rough edges of the flakes. The GM2 samples are composed of transparent flakes which is indication of small number of layers. Flakes in the GM2 are more ordered, having more regular shape.
Fig. 1. SEM images of GM1 (A) and GM2 (B) materials
Such morphology of GM1 explains higher specific area and larger volume of pores in comparison with GM2 (see Table 4).
3.2 Raman spectroscopy
Raman spectroscopy is a technique of choice for structure characterization of carbon-based materials: diamond, graphite, nanotubes and graphene [23]. Raman spectrum of HOPG (see an example in Fig. 2) consists of distinct bands at 1580 cm-1 and ~ 2700 cm-1. The former one is labeled "G-band" and is related to the bond stretching of the sp2 C-C bonds. The band at ~ 2700 cm-1 is called 2D and it is an overtone of the so-called D-band. D-band centered at 1360 cm-1 is due to the breathing modes of six carbon atom rings and it requires a defect to be activated. That is why it is usually referred to as "defect" mode. The D mode is dispersive with excitation energy. The ratio of the D peak intensity (ID) to that of the G peak (IG) is routinely used to quantify disorder in the graphitic sample [24]:
10~10 for X
where X is the laser's wavelength, C=2,4 x =514 nm, La is the dimension or the effective size of the crystallites.
350C
Raman shift, cm"
¡o/¡G = C(X) X4(La Г1
(1) Fig. 2. Raman spectra of the GM1, GM2 and HOPG samples. Excitation radiation wavelength was 532nm. Peak assignments were made according to [23]
№ 19 (159) 2014
For heavily distorted graphitic materials, like amorphous carbon, characterized by the loss of sp2 rings, intensity of the D peak (ID) will be proportional to the number of the six-fold rings. Thus Eq.1 doesn't hold anymore and it leads to a new relation [25]:
Id/IG = C'(X) (La )2 (2)
where C'(514 nm) „ 0.0055.
Therefore Eq.1 and Eq.2 apply for two different types of graphitic materials. To distinguish between these two types one has to look into the dispersion of the G and D bands. The G peak does not disperse in graphite and nanocrystalline graphite. Dispersion of the G peak is
indicative of highly amorphous state of the graphitic material. Position of the D peak always disperses with excitation energy in all carbons but in contrast to the G peak the dispersion of the D peak is less pronounced for more defective species [23, 25].
Fig. 2 shows Raman spectra for GM1 and GM2 materials taken with 532 nm laser. Raman spectrum of a HOPG crystal is shown in Fig. 2 as a reference. Parameters of the D and G peaks obtained for GM1 and GM2 samples using lasers with various excitation wavelengths: 514 nm, 532 nm and 785 nm are listed in Table 1. Eq.1 was applied to estimate the effective size of the crystallites for both materials.
Table 1
Peak positions, full width at half maximum (FWHM) of the peaks and the ratio of integral intensities of the D- and G- bands in Raman spectra of the studied samples in comparison with correspondent literature data
Sample D peak D peak G peak G peak Ratio La, Excitation light
position, FWHM, position, FWHM, id/iG nm wavelength,
см-1 см-1 см-1 см-1 nm
GM1 1348 190 1589 110 2.45 6.8 514
GM2 1350 59 1579.5 44 1.67 10.0
HOPG - - 1579.8 13 -
GM1 1374 229 1596 97 2.35 8.2 This
GM2 1352 51 1589 48 1.5 12.8 532 work
HOPG - - 1583 16 - -
GM1 1335 209 1582 109 5.3 17.2
GM2 1314 64 1597 67 4.0 22.8 785
HOPG - - 1583 15 - -
RGO1 1337 81 1591 50 1.3 14,8* 532 [27]
RGO2 1341 1582 2.1 9.2* 532 [28]
RGO3 1346 1582 0.82 20.4* 514 [29]
M, - С -'м1
с о
i-, to I
s
1GO reduced by hydrazine, 2GO reduced by L-Lysine, 3GO reduced by NaBH4 *Calculated by using Formula (1)
As can be seen in Table 1, the FWHMs of the D and G peaks for all used excitation wavelengths are much greater for GM1 samples than that for GM2. The later observation points out to the fact that the GM1 samples are more defective than the GM2 ones and this is in accord with the SEM data shown in Fig.1. Dependence of the peak positions on the excitation wavelengths for D- and G- bands of different materials is shown in Fig. 3. Position of the G band for both types of samples and for all used excitation wavelengths demonstrates some tendency to blue shift ~10 cm-1 in comparison with G band of graphite.
Laser wavelength, nm
Fig. 3. Dependence of peak positions of the D- and G- bands in Raman spectra of the GM1, GM2 and HOPG samples on the excitation wavelengths. Lasers used in this work operated at 514, 532 and 785 nm wavelengths
# ISMEE 18
In [26] authors based on the first principal calculations have argued that the observed blue shift of the G-band in the Raman spectra of GO could be explained within a GO model containing areas of sp2 carbons with an alternating pattern of single-double carbon bonds and Stone-Wales defects and 5-8-5 defects. For GM2 sample dispersion of the D peak with radiation energy was found to be close to 50 cm-1/eV and it is consistent with previously reported dependence [25 and references therein]. In the same time dispersion of the D peak for GM1 is smaller by factor of 1.5-2 than that of GM2 which is consistent with the above conclusion on higher defective status of GM1. Position of the G peak fluctuates with changing of the excitation energy for about 20 cm-1 for both types of samples. Estimated crystallite size for all studied samples was in the range of
6-20 nm while for GM2 sample this size was about 1.5 times larger than for GM1. Overall, the Raman spectroscopy data could be explained by high level of structural disordering in the GM1 and GM2 samples characterized by small crystallite size and by notable contribution from amorphous components for which Eq.1 is not valid.
3.3 X-ray photoelectron spectroscopy (XPS)
Chemical composition of the samples and oxidation state of the elements were examined by X-ray photoelectron spectroscopy (XPS). Elemental compositions of the GO, PIG, GM1GM2 samples determined by XPS are listed in Table 2.
Table 2
Elemental composition (in at%) of PIG, GM2, GO and GM1 samples determined by XPS
Sample C F Cl O Si S
PIG 62.3 33.4 3.6 0.8 - -
PIG* 64.8 25.5 3.4 5 1.3 -
GM2 95.6 2.5 0.5 1.4 - -
GM2** 93.6 0.9 0.6 4.5 0.3 -
GO 69.8 - 0.6 27.4 - 2.1
GM1 91 - 0 8.7 - 0
M, луV.
- С -
'ДО
с О
* PIG sample after storing for one year under ambient conditions ** GM2 sample produced by explosive exfoliation of the PIG* material
Fig. 4 shows wide range (survey) XPS spectra of drastically. Carbon and oxygen with some traces of
graphite intercalated by ClF3 before (sample PIG) and after explosive treatment (sample GM2). XPS spectrum of the PIG sample reveals presence of carbon, oxygen, chlorine, silicon and large amount of fluorine. After thermal treatment, fluorine concentration dropped
sulfur were main detected elements. From the data listed in Table 2 it is clear that the ratio of fluorine concentration to that of chlorine (F/Cl) in PIG sample is much higher than the stoichiometric number of 3.
Fig. 4. Survey XPS spectra of PIG sample before (bottom curve) and after (top curve) explosive exfoliation. PIG sample after explosive exfoliation referred to as GM2 (see text)
-1-1-1-1-■-1-■-г
1200 1000 800 600 400 200
-О
N
Binding energy, eV
0
№ 19 (159) 2014
That evidences decomposition of the ClF3 molecules upon intercalation in between graphite layers. It has been found that the long-term storage of the PIG sample leads to substantial accumulation of oxygen. Thermal explosion reduces concentrations of fluorine and chlorine. At the same time oxygen content is higher if the explosive treatment is carried out on air. On the other hand, explosive treatment of the PIG samples is carried out in vacuum and followed by cooling down also in vacuum results in oxygen poor GM2 samples. F1s
spectrum of the PIG sample (Fig. 5a) is represented by single peak at 686.8 eV with FWHM of 1.7 eV. Intensity of the peak decreases upon explosive treatment while the position of the peak remains unchanged. The F1s peak was found centered at 686.8 eV for all studied samples. This binding energy of the F1s core level is typical for C-F bonding [30-33] in carbon fluorinated materials. The F1s peak position can be used as a reference for energy calibration of the XPS spectra.
q
(Л
с
Ф
690
688 686 Binding energy, eV
684
Fig. 5. F1s XPS spectra of PIG material before (black upper curve) and after (red lower curve) explosive exfoliation (a), Cl2p XPS spectra of PIG material before (black upper curve) and after (red lower curve) explosive exfoliation (b)
210
208
206
204
202
Г
200
198
196
Binding energy, eV
Interestingly, high resolution Cl 2p XPS spectrum of the PIG sample (Fig. 5b) reveals two oxidation states of chlorine represented by peaks at 200.4 and 205 eV. Most of the chlorine atoms (80%) are negatively charged and as though were assigned to C-Cl bonding [34-35]. The rest of Cl atoms are positively charged as they belong to
the preserved ClF3 molecules. To summarize, majority of the ClF3 molecules decompose upon intercalating into HOPG. Products of the decomposition interact with host material forming C-F and C-Cl bonds. Based on the data of elemental analysis for the intercalated samples one can speculate that chlorine is another decomposition co-
product released during graphite intercalation with ClF3. Only small portion of intercalating ClF3 molecules can be found intact in freshly prepared samples. Upon one year storing at room temperature no ClF3 molecules have been detected in the PIG samples (see Table 2).
Fig. 6 shows high resolution C1s XPS spectra of the PIG and GM2 samples. The C1s spectrum of PIG sample exhibits two peaks at 286 and 288.5 eV. The later peak's position is characteristic of carbon atoms bound to fluorine atoms (C-F bond) [36-37]. The former one, at 286 eV, can be assigned to the carbon atoms which do not bind to fluorine atoms directly but they are bound to another carbon atom which, in turn, has fluorine atoms among their nearest neighbors (C-C-F). Shape of the C1s peak in the XPS spectra of GM2 sample (Fig. 6b red curve) is very similar to that of pure graphite. That gives a reason to fit the C1s peak line of GM2 sample using the line shape (%HOPG) derived from experimentally measured (under identical conditions) C1s peak of HOPG sample which centered at 284.5 eV and is usually
assigned to carbon atoms with sp hybridization. Asymmetry observed on the high binding energy side of the C1s peak of the GM2 sample is typical for conductive materials. At the same time this asymmetry can be well fitted with a fairly wide Gaussian/Lorenzian line (component C II in Fig. 6b) centered at 285.9 eV. Component CII (Fig. 6b) given its large FWHM of 3.5 eV can be attributed to C-O bonds and to the presence of carbon atoms with sp3 hybridization [38]. Satellite peak at ~6 eV from the main 1s peak also on the high binding energy side can be well fitted using %HOPG line shape (see inset in Fig. 6b) and therefore we attribute this peak to energy loss for excitation of plasmon oscillations of the n- electrons (n-plasmon). n-plasmon is characteristic of sp2 hybridization. The later observation points to 2D ordering of carbon in GM2 samples - the conclusion is in accord with the above mentioned Raman spectroscopy results.
C-F
(a)
C-C-F
pig
292
290
288
Binding Ene rgy (eV)
286
284
282
Name Pos. FWHM L.Sh. %Area CI 284.53 1.019 %HOPG 86.699 CII 285.90 3.503 GL(30) 13.301
CI
Fig. 6. C 1s XPS spectra of PIG material before (a) and after (b) explosive exfoliation. Peak fitting for the GM2 sample was done by using a line shape derived from a C1s spectrum of the HOPG sample (component C I with line shape labeled as %HOPG, for details see "Peak fitting" in CasaXPS) and Gaussian/Lorentzian line shape (component C II). Inset in (b) shows fitting of the C1s spectrum with %HOPG line shape in the range of rc-plasmon peak
282
Binding Ene rgy (eV)
№ 19 (159) 2014
Fig. 7 shows C 1s XPS spectra of GO and GM1 samples. The C1s spectrum of GO (Fig. 7a) can be well fitted with three peaks centered at 284.6 eV, ~287 eV and ~289 eV. Component I at 284.6 eV (Fig. 7a) corresponds to C-C bonds as it is widely accepted in the
literature [39-44]. Most of the authors assign the component II at 287 eV (Fig. 7a) to the bonding with oxygen (C-O), i.e. to hydroxyl and epoxide and/or ether groups. Finally, component III at ~289 eV (Fig. 6a) can be ascribed to carbon-oxygen double bonds (C=O).
Name C I C II C III
Pos. FWHM "/«Area 284.99 1.613 42.168 1.456 1.982
287.08 288.84
44.314 13.518
I
296
I
292
(a)
GO
280
I
276
Name Pos. FWHM L.Sh. /Are a
CI 284.64 1.406 %CSP2 58.066
C II 285.15 2.377 GL(30) 24.957
C III 287.10 2.377 GL(30) 11.468
C IV 290.19 2.377 GL(30) 5.509
296
292
T—r
288
Binding Energy (eV)
284
280
Fig. 7. C1s XPS spectra for GO (upper spectrum) (a) and GM1 (bottom spectrum) samples (b). GO sample after explosive exfoliation is referred to as GM1 (see text). Peak fitting for the GM1 sample was done by using a line shape derived from a C1s spectrum of the HOPG sample (component C I, for details see "Peak fitting" in CasaXPS) and Gaussian/Lorentzian line shape
(components C II, CIII and C IV)
The GO sample C1s spectrum deconvolution estimates relative contributions of the C-O and C=O species into the total intensity of C1s line as ~44% and ~13% respectively (Table 3). It is instructive to estimate concentrations of functional groups contributing into the
intensity of the C1s line component II (C-O bondings) by using the later values. Let us assume that there are Xi mol% of C-OH groups, X2 mol% of C-O-C (epoxy and/or ether) groups and X3 mol% of C=O groups.
Table 3
Concentrations of oxygen (Otot) and carbon (Ctot) and partial concentrations (C II and C III) of the C 1s line in the XPS spectrum of the GO sample
O , at % tot C , at% tot C II/C , % tot CIII/C , % tot
27 70 44 13
Then for the component II of C1s peak one can write the following equation:
X + 2X2= 0.44 Cot,
(3)
where Ctot=70 at% is a total amount of carbon atoms in at%. At the same time for the O1s peak shall be true the following:
X + X2+ Хз= Otot,
X3= °.l3 Ctot,
(5)
(6)
where Otot=27 at% is a total amount of oxygen atoms. Values of Ctot and Otot were obtained from the respective survey XPS spectrum. Finally, we will arrive to a system of linear equations:
X1 + 2 X 2 = 31,
X1 + X 2 + X 3 = 27, (7)
X 3 = 9.
The solution set to the above system is given by:
XI = 5,
X 2 = 13, (8)
X 3 = 9.
Notably, any other combination of functional groups different from that used in the above calculations: missing one of the above mentioned functional groups or having some other kinds of functionals (for example: peroxide, etc), leads to a system of linear equations with at least one negative solution, which is meaningless.
Upon explosive treatment the intensities ratio I(O1s)/I(C1s) drops by factor of ~4 thus reflecting substantial but not complete reduction of oxygen content. Interestingly, major loss of oxygen content occurred by the cost of functional groups characterized by C-O bonding. C1s spectrum of GM1 sample (Fig. 7b) is different from that of the GO sample. Indeed, the GM1 sample C1s spectrum deconvolution revealed concentrations of the C-O and C=O species to be 11.5% and 5.5% correspondently. Thus the concentration of the C-O groups is reduced stronger than that of C=O. All attempts to set a system of linear equations like (7), even assuming different combinations of functional groups,
ended up with negative solutions for Xl variables. This finding hints at an idea that the more complex and energetically favorable functional groups (semiquinone, lactone etc [45, 46]) could form upon explosive treatment of the precursor materials.
Noteworthy to mention that traces of sulfur have been detected in the XPS spectra for some samples. Presence of sulfur in the studied samples is related to the technology of graphite oxide synthesis by Hammers method. Thorough rinsing with water of the produced GO doesn't remove completely leftovers of the sulfuric acid which can get partially trapped in the closed pores. It has been already reported in [47] that the high resolution S2p spectrum of the GM1 samples reveals two oxidation states of sulfur: positively charged sulfur from the SO2~ groups and zero charged sulfur. The totally reduced sulfur can originate in the reactions occurring throughout microwave treatment of the GO samples.
3.4 Optical absorption
Optical absorption spectra measured for suspensions of the GM1 and GM2 samples are presented in Fig. 8. Each spectrum shows only one characteristic peak in the UV range (at 261 nm and 270 nm for GM1 and GM2 respectively) which is due to excitation of the n-plasmon in graphene layers [48]. The plasmon peak is followed by almost flat absorption band extending through infrared and up to near infra-red region.
с о
о
1Л <
300
■ISO
600
750
900 lOSO 1200 1350
Wave length, nm
Fig. 8. Optical absorption spectra for suspensions of GM1 (curve 1) and GM2 (curve 2) in aqueous solution of a surfactant dodecylbenzenesulfonate. A step observed at ~900 nm is an artifact which occurs when the spectrometer switches from one wavelength range to another.
3.5 Mass spectrometry
Composition of gaseous co-products of the explosive heating of the PIG and GO precursors has been investigated by mass spectroscopy. Fig. 9a shows mass spectrum of gas evolved during conversion of PIG into GM2 samples. Peaks at m/z: 28, 32 and 44 correspond to CO, O2, and CO2 molecules respectively which were
ISMEE № 12901(Г
23
accumulated in the sample during long term storing on air. Other detected molecules SiF4, CF4, and COF2 are represented by their most intensive peaks at 85 m/z, 69 m/z and 47 m/z respectively. SiF4 component in the gaseous yield originates from the quartz walls of the reactor attacked by the aggressive fluorine. The fluorine contains compounds formed during explosive treatment carrying out in the reactor. Mass spectrum of the gases produced from a GO sample is shown in Fig. 9b. The main components here are CO (28 m/z), CO2 (44 m/z). Since the measured fragmentation pattern of pure CO2 gives the (28 m/z/44 m/z) intensity ratio about 0.4, we conclude that in gas phase concentration ratio CO:CO2 is 7:10. Water (peak at 18 m/z in the mass spectrum in Fig. 8b) is another co-product of the explosive thermal processing of the GM1 samples. The signal of molecular oxygen (32 m/z) in the released gas products was below detection limit of the technique. Unexpectedly, well reproducible signals at 48 m/z and 64 m/z have been observed in the gas evolved from the GO samples. These signals represent SO2 molecules which fragmentation pattern contains ions of [SO]+ (48 m/z) and [SO2]+ (64 m/z). We ascribe formation of SO2 molecules to the leftovers of sulfuric acid used in synthesis of GO samples. Overall, mass spectroscopic tracking of the gas phase products has demonstrated that the explosive heating of the GO samples leads not only to the reduction of precursor material but also effectively removes sulfuric contaminations in the product material. Analysis of the gas yielded during thermal exfoliation of GO and PIG precursors shed light on the origin of the observed different level of ordering in GM1 and GM2 samples. Indeed, releasing of one CF4 molecule during exfoliation of PIG material costs for the graphite matrix only one carbon atom per 4 fluorine atoms. In contrast, releasing one CO molecule or one CO2 molecule during exfoliation of GO material requires 4 or 2 carbons atoms removal per 4 oxygen atoms correspondingly. Therefore GM1 materials come out more defective with smaller size of the crystallites in comparison with the GM2.
m/z
ro
о Ш
28
18
12
16,
44
(b)
48
64 1
ax/A - с -
с о
10 20 30 m/z 80 90
Fig. 9. Mass spectrum of gas evolved during explosive heating of PIG (a) and GO (b) in vacuum
4. Conclusions
In this paper, two types of graphene-based materials (GM1 and GM2) have been synthesized from two different precursors: Hummers graphite oxide and ClF3 intercalated graphite (PIG), by explosive thermal heating in microwave oven In Table 4, we have summarized main characteristics of the compared GM1 and GM2 materials. The listed numbers are self- explanatory.
-O
N
Table 4
Characteristics of the GM1 and GM2 materials
40
60
Parameter Graphene based material
GM1 GM2
Specific surface area, m2/g 485 208
Total pore volume, cm3/g 1.76 0.42
Specific conductance, S/m 50* 1400**
Oxygen content, wt.% 4.2 3.78
Fluorine content, wt.% 0 1.4
Hydrogen content, wt.% 0.7 0
Plasmon energy, eV 4.75 4.59
Gas products of exfoliation CO, CO2, H2O, SO2 CO2, CO, cf4, o2
Parameter Graphene based material
GM1 GM2
Crystallite size, nm 6.9 10
Inflection point on TGA curve for GO and PIG, 0C 220 585
Weight loss near inflection point, % 30 50
Exfoliation temperature for GO and PIG, 0C (this work) 500 750
Yield of GM material (%) 55 70
* for the powders compressed into tablets
** for the film obtained by precipitation of GM2 suspension
It is worthy of mentioning some issues related to the technology of synthesis and to the possible applications of these materials. Firstly, synthesis of GM1 type of materials is much easier and cheaper than GM2. Gas co-product evolved during explosive exfoliation of GO is not as hazardous as in the case of explosive treatment of PIG - precursor for GM2 material. On the other hand, the GM2 materials are characterized by higher electrical conductivity and larger size of graphene crystallites.
One can assume that the combination of the established properties of GM1 type of samples presents the GM1 as a promising material for the electrodes in supercapacitors, for sorbents, and to be used as a filler in some polymer reactions. It has been shown recently [44] that GM1 increases the rate of low-temperature radiation induced polymerization of tetrafluoroethylene. We believe that formed in the latter synthesis composite material could be a suitable alternative for pure PTFE in the applications where low creeping under high pressure is required. It has also been recently demonstrated [45] that the films of polymer-based composites made of GM2 and carboxymethylcellulose have nonlinear optical properties and thus can be used in optical devices for modulation of laser radiation. GM2 is more suited for creating optically transparent films and coatings with high electrical conductivity.
Acknowledgements:
The work has been partially supported by Russian Foundation for Basic Researcl (research projects Nos. 14-03-00428 and 14-03-00133) and RF Ministry of Education and Science (State Contract No. 14.594.21.0007, RFMEFI59414X0007), State Assignment No. 11.1797.2014/K
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Транслитерация no ISO 9:1995
5 International Conference on Nanotek and Expo
Dear Friends and Colleagues,
It gives us immense delight to welcome you to join us at the 5th International Conference on Nanotek and Expo during November 16-18, 2015 at San Antonio, USA.
Join us to share the advancements in the field of Nanomaterials and technology at Nanotek-2014 where we can extend your opportunities by providing interdisciplinary research to industrial and commercial breakthroughs. The meeting will address, identify and focus Nanobiotechnology, Biomedical engineering, Applications of Nanotechnology etc and showcase of current research in Nanomaterials and Nanocomposites. The gathering will highlight the challenges and opportunities in both medical and commercial usage of nanotubes and fibers.
The meeting will be an evidence of success stories in nanotechnology and developments of existed approaches in nanostructures. In the other hand, the meeting focus on the dark side of the technology and discuss nanotoxicology, and personalised medicine, which are not fully understood yet. For emerging researchers and industries, development of ecofriendly pharmaceutical products and host friendly drug delivery systems are major provocations. Hopefully, this expert gathering of academicians, public and private agencies will provide spotlight and new insights on these critical areas.
Since the meeting aimed to cover multiple applications of nanostructures such Nanoplasmonics, Carbon Nanotubes, Nanoelectronics,Quantum dots etc, the participating agencies will accelerate the advancements of nanoscience and technology. The conversations, talks and presentations of young and expert researchers will enhance the understanding of nanoparticles and improve the characterizations, clinical, medical, physical and chemical properties as well. The meeting ensures that sharing the ideas and visions will empowers and establishes you by satisfy the theme "Challenging Aspects and Frontiers in Nanoscience & Nanotechnology".
Presently the 4th International Conference on Nanotek and Expo is going to be held at San Francisco, USA that provides a perfect symposium for scientists, engineers, directors of companies and students in the field of Nanotech-nology to meet and share their knowledge. The scientific program paves a way to gather visionaries through the research talks and presentations and put forward many thought provoking strategies.
The previous three conferences on Nanotechnology which were held at USA have gained interest from eminent scientists all over the world in Nanotechnology R&D. Now it is the time for Nanotek-2015 to share novel approaches in all those research areas where Nanoscience and Nanotechnology. These conferences were marked with the presence of renowned scientists, talented young researchers, students and business delegates representing more than 30 different countries ultimately driving the events into the path of success.
Conference Highlights
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