Synthesis of fluorinated graphene with tunable degree of fluorination

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Synthesis of fluorinated graphene with tunable degree of fluorination Zhaofeng Wang a, Jinqing Wang a,*, Zhangpeng Li a,b, Peiwei Gong a,b, Xiaohong Liu a, Libin Zhang a,b, Junfang Ren a, Honggang Wang a, Shengrong Yang a a b

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Graduate University of Chinese Academy of Sciences, Beijing 100080, PR China

A R T I C L E I N F O

A B S T R A C T

Article history:

An easy, low-cost and effective synthesis of fluorinated graphene with tunable C/F atomic

Received 22 May 2012

ratio (RC/F) has been realized by the reaction between dispersed graphene oxide and hydro-

Accepted 15 July 2012

fluoric acid. The results show that fluorine is grafted onto the basal plane of graphene, and

Available online 22 July 2012

the RC/F can be easily adjusted by controlling the reaction conditions. The as-synthesized fluorinated graphene exhibits a sheet-like morphology with 1–2 layered thickness and tunable bandgap energy from 1.82 to 2.99 eV, which has potential applications in optoelectronic and photonic devices.  2012 Elsevier Ltd. All rights reserved.

1.

Introduction

Bandgap engineering of graphene is important and attracts a great deal of attention both in experimental and theoretical research [1,2]. Opening the bandgap of this material can expand the applications of graphene-based materials into optoelectronics and photonics. Several methods, such as confinement of the physical dimensions (i.e., graphene nanoribbon), control of the graphene–substrate interaction, and construction of bilayer graphene, have recently been attempted to open and tune the bandgap of graphene [3–6]. However, these methods offer limited bandgap opening and are difficult to apply in large scale production. A novel and effective approach to tune the bandgap of this material was recently developed by XeF2 fluorination, in which fluorine (F) atoms are introduced in the form of CAF covalent bond accompanied by the structural transformation of the CAC bonds from sp2 to sp3 configuration [7]. Theoretical studies have indicated that the bandgap of fluorinated graphene sheets (FGS) can be varied from 0 eV to 3 eV with changing degrees of fluorination, and the full FGS acts as an insulator with the highest band gap of approximately 3.1 eV [8–10].

Similar results have been validated through actual experiments [11,12]. At present, the experimental synthesis of FGS, which has been cited in a few reports, can be divided into two systems: (1) directly fluorinating graphene sheets (GS) utilizing XeF2 and plasma (CF4 and SF6) as fluorinating agents [7,8,12–14]; and (2) exfoliating graphite fluoride (FGR) by mechanical cleavage or ultrasonication in sulfolane, isopropanol, or ionic liquid [10,11,15]. However, intrinsic disadvantages are inevitable in these systems. The use of XeF2 requires high thermal treatment temperature (350 C) during the fluorination process, which can result in inevitable aggregation. The prepared FGS by the reaction of GS and plasma has low F content, whereas the yield is very low for the FGR. Moreover, the XeF2 and FGR are too expensive to be generalized, especially for XeF2. Therefore, the development of an easy, low-cost and efficient synthesis route of FGS with tunable C/F atomic ratio (RC/F) is highly desirable. The fluorination difficulty of GS mainly results from its low surface activity and high chemical stability in contrast with the graphene oxide (GO) because of the connected oxygencontaining groups in the latter [16]. A previous study has

* Corresponding author: Fax: +86 931 4968076. E-mail address: [email protected] (J. Wang). 0008-6223/$ - see front matter  2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.07.026

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reported that the oxygen-containing functional groups on carbon framework in GO can be removed through the hydrothermal process (HP) [17], which provides the possibility of the reaction between GO and fluorinating agents. Drawn by this idea, a novel, feasible, and effective route is exploited for the first time in the current study to synthesize FGS through a simple hydrothermal reaction between homogeneously dispersed GO and hydrofluoric acid (HF). The obtained FGS has tunable degree of fluorination. The chemical composition, morphology, fluorination mechanism and bandgap opening of the obtained FGS are investigated and discussed. Some interesting achievements are presented.

2.

Experimental

2.1.

Preparation of GO dispersion

Details are presented in Supplementary material.

2.2. Synthesis of FGS graphene sheets (HGS)

and

hydrothermally

reduced

FGS was synthesized by a simple hydrothermal reaction between GO dispersion and HF. In a typical procedure, 40 mL GO dispersion (2 mg/mL) and 0. 5 mL HF (40 wt%) was mixed by ultrasonication for 1 min. Then, the mixture was transferred into a 50 mL Teflon-lined autoclave and maintained at 180 C for 30 h. The autoclave was naturally cooled to room temperature. At last, the product was filtered using microporous membrane and throughout washed with ultrapure water, followed being dried through freeze drying. Thusly, FGS was obtained. For comparison, HGS were also prepared. The synthesis process is almost the same with that of FGS except the addition of HF.

2.3.

Materials characterizations

Crystal phase of the obtained materials were determined by powder X-ray diffraction (XRD, Rigaku D/MAX – 2400 X-ray diffractometer with Ni-filtered Cu Ka radiation). Raman spectroscopy (Renishaw in Via Raman microscope with 633 nm line of an Ar ion laser as an excitation source) was employed to characterize the microstructure of the samples. Fourier transformation infrared (FTIR) spectra of the prepared samples were recorded on an IFS 66V/S FTIR spectrometer (Bruker, Germany). X-ray photoelectron spectroscopy (XPS, PHI-5702, Physical Electronics) was performed using a monochromated Al Ka irradiation. The chamber pressure was 3 · 108 Torr under testing conditions. Peak deconvolution and quantification of elements were accomplished using Origin 7.0. The morphology and thickness of different sheet-like samples were observed by atomic force microscopy (AFM, VEECO Nanoscope IIIa). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai F30, operated at 300 kV) were also employed to investigate the microstructure and morphology of the samples.

The optical absorption of the samples was measured by an UV–vis spectrophotometer (Analytic Jena AG, Specord 50). The electrical conductivity was operated in a four-probe instrument (RTS-4).

3.

Results and discussion

HRTEM and AFM observations were employed to explore the microstructure quality of the as-synthesized FGS (Fig. 1). Prior to measurements, the FGS column was dispersed in ethanol by ultrasonication. A typical two-dimensional sheet-like structure is presented in Fig. 1a. The FGS nanosheets are transparent and exhibit a very stable nature under the electron beam. In contrast to the smooth and regular surface of HGS (Supplementary material, Fig. 1S), the FGS sheets show irregular atomic arrangement (Fig. 1c), which may be caused by variations in thickness, density, or composition. The edge micrograph in Fig. 1b indicates that the FGS is of few layers, which is further confirmed by the AFM image (Fig. 1e), and its thickness is determined to be 1.240 or 2.008 nm suggesting that the FGS should be of 1–2 layers (Fig. 1f). The selected area electron diffraction pattern in Fig. 1d is clearly visible, and the annular structure reveals that the achieved FGS is polycrystal. Fig. 2 shows the XPS characterization of GO, HGS and FGS samples to determine the elemental composition and nature of the chemical bonds in FGS. As shown in Fig. 2a, the contents of oxygen in HGS and FGS decrease evidently due to the occurrence of reduction and an evident F1s peak appears in FGS sample, indicating that F has been grafted onto the carbon framework through the hydrothermal reaction between GO and HF. For comparison, we also measured the F1s curve of FGS sample which has residual HF (washing step was omitted during the preparation of FGS as shown in Supplementary material, Fig. 2S). Obviously, there are three peaks appeared at about 686.2, 690.1 and 695.9 eV, which can be ascribed to the F1s of CAF, F1s of HF and ionization energy of HF, respectively [18]. However, in the F1s spectrum of FGS after being throughout washed, the F1s and ionization energy of HF peaks all disappear (Supplementary material, Fig. 2S). These results reveal that the F in the final product is not originated from the residual HF. The reduction and fluorination of GO also can be observed from their C1s core lines as shown in Fig. 2b–d. The C1s spectrum of GO can be fitted into four peaks centered at about 284.8, 286.4, 287.1 and 288.8 eV, corresponding to CAC, CAO, AC@O and ACOOA groups, respectively [19,20]. When GO is converted into HGS, the content of oxygen clearly decreases (especially for AC@O) and the C1s spectrum presents nearly a single peak, implying that GO can be effectively reduced by hydrothermal treatment without adding any reducing agent. Upon the addition of HF during HP, a new band appears at approximately 290 eV which can be attributed to the CAF covalent bonds [8,18]. Majority of the bonds in the FGS samples are CAF (288.2 eV) and CFACF2 (289.9 eV), with smaller fractions of CACF (285.8 eV), CACF2 (286.9 eV), CAF2 (292.1 eV), and CAF3 (293.7 eV) (the details are presented in Table 1). The fluorination degree of FGS can be recognized by estimating the relative content of F to C (RF/C) from highresolution C1s spectra by the sum of the F content in each

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Fig. 1 – (a) TEM and (b) HRTEM micrographs of the synthesized FGS. (c) Magnified image of the red section in panel b. (d) Selected area electron diffraction pattern of FGS sample. (e–f) The AFM image and thickness of FGS. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 2 – (a) Survey XPS spectra of the as-prepared GO, HGS, and FGS samples. (b–d) Gaussian fitting results of the C1s core lines of GO, HGS, and FGS.

F-containing group (the area of the peak divided by the total area of C1s, multiplied by 1 for CF and CACF2, 1/2 for CACF, 3/2 for CFACF2, 2 for CF2, and 3 for CF3) [21]. By calculation and conversion to RC/F, the RC/F of FGS is estimated to be approximately 2.1. The structures of the as-prepared samples were measured by XRD, as shown in Fig. 3. Raw graphite (GR) exhibits a typical

(0 0 2) peak at 26.44 with a d-spacing of 0.34 nm. As GR is converted into GO, the GR peak shifts downward to 11 with a corresponding d-spacing of 0.8 nm, revealing that many of the oxygen atoms are intercalated into the interlayer space [22]. In the XRD curve of GO, a broad peak with weak intensity is still observed at approximate 23.6, suggesting that a fraction of GO is not fully intercalated by oxygen-containing functional

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Table 1 – Location, ascription and content of fluorine-containing groups in the sample of FGS that synthesized at the condition of 180 C, 30 h and 0.5 mL HF. Location (eV)

285.8

286.9

288.2

289.9

292.1

293.7

Ascription Content (%)

CACF 4.3

CACF2 3.3

CF 8.2

CFACF2 6.2

CF2 2.8

CF3 1.4

Fig. 3 – Typical XRD patterns of GR, GO, HGS and FGS samples.

groups. When the GO dispersion is treated by HP (180 C, 30 h), the peak at 11 disappears and a broad peak at around 22.04 with a d-spacing of 0.4 nm appears, implying that the oxygen-containing functional groups of GO can be removed and graphene is successfully synthesized [23]. With the further addition of a certain amount of HF in the HP, the obtained FGS presents an additional peak at approximately 2h = 15.8 with a d-spacing of 0.56 nm. The novel peak could be indexed as the (0 0 1) reflection in a hexagonal system particular for compounds exhibiting very high F levels [24]. The XRD results are consistent well with those obtained from XPS and further confirm the generation of FGS. The as-prepared FGS is difficult to be dispersed in water even after long-time ultrasonic treatment (Fig. 4). When the solvent is replaced by ethanol, dimethyl formamide (DMF) or N-methyl-2-pyrrolidone (NMP), FGS can be dispersed. However, upon the dispersions being stood for 24 h, the ethanol dispersive powder almost fully precipitates on the bottom of the bottle and the one dispersed in DMF partly precipitates. While the one dispersed in NMP exhibits the best dispersibility, which remains well after being placed for 1 day. It is found that the fluorination degree of FGS can be adjusted by varying the reaction temperature, times and amounts of HF, and their XPS spectra are shown in Supplementary material, Fig. 3S. The values of RC/F are estimated and presented in Table 2. The optimum FGS sample is obtained as prepared by HP at 180 C for 30 h with the addition of 0.5 mL of HF. The decrease in F content along with an increase in HF from 0.5 to 1 mL is probably caused by the variation of the pH value which could have strong effect on

Fig. 4 – Optical photographs of FGS dispersed in (a) water, (b) ethanol, (c) DMF and (d) NMP. The left photos are FGS just dispersed by ultrasonic, and the right ones in (b–d) are the FGS dispersions after being stayed for 24 h.

reaction kinetics as reported in the Refs. [17,25]. The optimum FGS suggests that a pH value of 1.5 of the solution containing 40 mL GO (2 mg/mL) and 0.5 mL HF (40 wt%) is the most beneficial for fluorination. The contents of each F-containing group (area percentage of the peak multiplied by 1/2 for CACF, CACF2, and CFACF2, or 1 for CF, CF2, and CF3) dependent on the reaction temperature, time, and amount of HF are estimated in Fig. 5. The enhancement in the F coverage with an increase in temperature is mainly due to the formation of the CFACF2 group while the content of CF is almost constant. The variation in the F-containing groups, except CF2 and CF3, as a result of the varied reaction time and amount of HF are considered as the main factors for tunable fluorination. The fluorination mechanism of the as-prepared FGS was explored according to the Raman spectra of GO, HGS, and two typical FGS samples (FGS-150 and FGS-180 are the samples synthesized at 150 and 180 C for 30 h with the addition

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Table 2 – RC/F values of FGS dependent on the reaction temperature, time and amount of HF. Reaction condition

RC/F

Variation of temperature (0.5 mL HF, 30 h)

Variation of reaction time (180 C, 0.5 mL HF)

Variation of the amount of HF (180 C, 30 h)

150 C

180 C

10 h

20 h

30 h

0.25 mL

0.5 mL

0.75 mL

1 mL

3.0

2.1

9.5

6.5

2.1

6.6

2.1

5.5

6.2

Fig. 5 – Contents of individual fluorine-containing groups (CACF, CACF2, CF, CFACF2, CF2 and CF3) under different (a) temperatures, (b) reaction times, and (c) amounts of HF.

of 0.5 mL HF, respectively) under the excitation of 633 nm (Fig. 6a). Generally, the intensity ratio of ID/IG is a measure of disorder [26]. The ID/IG values of HGS (1.06), FGS-150 (1.05), and FGS-180 (1.09) are very close. The ID/IG value of FGS should be higher than that of HGS, provided that F reacts with sp2 C in GO or HGS by forming CAF covalent bonds during HP. Therefore, the nearly similar values of HGS and FGS suggest that fluorination occurs at the original sp3 C sites in

GO, that is, the C sites connect with the oxygen-containing groups. Hence, a possible fluorination mechanism of FGS is proposed, as shown in Fig. 6b. The six F-containing groups in FGS can be obtained by a combination of Fig. 6b (ii–iv) because the oxygen-containing groups distribute randomly in GO. The XPS spectra of samples directly synthesized by graphene and HF through HP are also prepared and shown in Supplementary material, Fig. 4S, in which no F peak is

Fig. 6 – (a) Raman spectra of GO, HGS, FGS-150 and FGS-180 under the excitation of 633 nm. (b) Possible fluorination mechanisms: (i) Three main oxygen-containing groups in GO; (ii–iv) Fluorination reactions between GO and HF.

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found. This phenomenon indicates that the oxygen-containing groups in GO play a major role in the formation of FGS and support our fluorination mechanism. It is worthy noting that deoxygenation (removal of oxygencontaining groups) in GO also could occur during HP [17]. Therefore, the RC/O of the final products should be utilized to evaluate both the deoxygenation and fluorination degrees. The RC/O values of GO and FGS series are presented in Supplementary material, Table 1S. Obviously, the contents of oxygen in the FGS products are nearly same, which suggest that most of the residual oxygen-containing groups that are not substituted by F atoms could be removed by HP. The most significant attribute of the as-synthesized FGS is its semiconducting property, and UV–vis spectroscopy has been proven to be an effective optical characterization tool for understanding the electronic structure of several semiconductor materials [27]. The optical absorption spectra of GO, HGS, FGS-150, and FGS-180 were recorded, and the results are shown in Fig. 7a. The optical absorbance feature of FGS is changed remarkably by fluorination, which causes the appearance of an additional peak at 204 nm and the characteristic p ! p* electron transition of C@C in HGS downward shift to the short wavelength. The appearance of the new peak is most likely related to p ! p* electron transitions in the polyene-type structures, which are increasingly formed

along with improving extent of fluorination [28]. The blue shift of the characteristic p ! p* electron transition of C@C (from 269 nm to 251 nm and 247 nm) demonstrates that the bandgap is opened by the introduction of CAF covalent bonds in graphene. Given that the ideal graphene possesses excellent conductivity, Fan et al. reported that GS showed an electrical conductivity as high as 20200 S/m after being annealed at 1100 C in Ar/H2 [29]. However, the electrical conductivities of the FGS samples are very low (Fig. 7b), suggesting that the fluorination of GS reduces the electrical conductivities of the samples. The electrical conductivity gradually decreases along with a decrease in RC/F. Such phenomenon is also attributed to the graft of F onto GS, resulting in the bandgap opening. To estimate the bandgap, the activation-energy method was used [30,31]. Fig. 7c shows the Arrhenius plot (logarithm of conductivity versus inverse of temperature) of FGS series with different F coverage. The Arrhenius plot is a straight line and the bandgap energy of the samples can be obtained from the slope of the line (2303kÆkB, where k is the value of slope and kB presents the Boltzmann constant with the value of 8.62 · 105 eV/K) as shown in Fig. 7d. The result confirms the widening of the bandgap along with increasing F coverage, which is caused by the interaction of the p orbits of F with the p orbits of C that modify the charge densities and

Fig. 7 – (a) UV–vis absorption spectra of GO, HGS and two typical FGS samples (FGS-150 and FGS-180). (b) Electrical conductivities of FGS samples accompanying the variation in RC/F; the inset is the model of four-probe instrument. (c) Arrhenius plot for the logarithm of conductivities of FGS series versus the inverse of temperature. (d) The experimental band gap energies of fluorinated graphene for several F coverages.

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introduce scattering centers. In particular, the bandgap is estimated to be about 2.99 eV and the p bands are largely disrupted in the case of C2.1F. The hydrothermally synthesized FGS would effectively modify the transport properties of graphene and expand its application in optoelectronics and photonics because of the tunability of bandgap.

4.

Conclusion

An easy, low-cost and effective synthesis route of FGS by HP through the reaction between dispersed GO and HF was developed. The results suggest that the oxygen-containing groups in GO play a major role in the FGS formation and the fluorination degree can be easily controlled by varying the reaction temperature, times and amounts of HF. Along with various RC/F values, the FGS samples present different bandgap energies, exhibiting potential applications in optoelectronic and photonic devices. The proposed technique for the preparation of FGS is reported for the first time and has guiding significance in fluorinating other carbon compounds for wider applications in various areas.

Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant Nos. 20823008 and 51075384), ‘‘Top Hundred Talents Program’’ of Chinese Academy of Sciences and the ‘‘Funds for Distinguished Young Scientists of Gansu’’.

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2012.07.026.

R E F E R E N C E S

[1] Elias DC, Nair RR, Mohiuddin TMG, Morozov SV, Blake P, Halsall MP, et al. Control of graphene’s properties by reversible hydrogenation. Science 2009;323(5914):610–3. [2] Eda G, Chhowalla M. Chemically derived graphene oxide: towards large-area thin-film electronics and optoelectronics. Adv Mater 2010;22(22):2392–415. [3] Li X, Wang X, Zhang L, Lee S, Dai H. Chemically derived, ultrasmooth graphene nanoribbon semiconductors. Science 2008;319(5867):1229–32. [4] Zhu L, Wang J, Zhang T, Ma L, Lim CW, Ding F, et al. Mechanically robust tri-wing graphene nanoribbons with tunable electronic and magnetic properties. Nano Lett 2010;10(2):494–8. [5] Zhou SY, Gweon GH, Fedorov AV, First PN, de Heer WA, Lee DH, et al. Substrate-induced band gap opening in epitaxial graphene. Nat Mater 2007;6(10):770–5. [6] Zhang Y, Tang TT, Girit C, Hao Z, Martin MC, Zettl A, et al. Direct observation of a widely tunable bandgap in bilayer graphene. Nature 2009;459(7248):820–3. [7] Nair RR, Ren W, Jalil R, Riaz I, Kravets VG, Britnell L, et al. Fluorographene: a two-dimensional counterpart of teflon. Small 2010;6(24):2877–84.

5409

[8] Robinson JT, Burgess JS, Junkermeier CE, Badescu SC, Reinecke TL, Perkins FK, et al. Properties of fluorinated graphene films. Nano Lett 2010;10(8):3001–5. [9] Samarakoon DK, Chen Z, Nicolas C, Wang XQ. Structural and electronic properties of fluorographene. Small 2011;7(7):965–9. [10] Zborˇil R, Karlicky´ F, Bourlinos AB, Steriotis TA, Stubos AK, Georgakilas V, et al. Graphene fluoride: a stable stoichiometric graphene derivative and its chemical conversion to graphene. Small 2010;6(24):2885–91. [11] Cheng SH, Zou K, Okino F, Gutierrez HR, Gupta A, Shen N, et al. Reversible fluorination of graphene: evidence of a twodimensional wide bandgap semiconductor. Phys Rev B 2010;81(20):205435. [12] Jeon KJ, Lee Z, Pollak E, Moreschini L, Bostwick A, Park CM, et al. Fluorographene: a wide bandgap semiconductor with ultraviolet luminescence. ACS Nano 2011;5(2):1042–6. [13] Bon SB, Valentini L, Verdejo R, Garcia Fierro JL, Peponi L, Lopez-Manchado MA, et al. Plasma fluorination of chemically derived graphene sheets and subsequent modification with butylamine. Chem Mater 2009;21(14):3433–8. [14] Yang H, Chen M, Zhou H, Qiu C, Hu L, Yu F, et al. Preferential and reversible fluorination of monolayer graphene. J Phys Chem C 2011;115(34):16844–8. [15] Chang H, Cheng J, Liu X, Gao J, Li M, Li J, et al. Facile synthesis of wide-bandgap fluorinated graphene semiconductors. Chem Eur J 2011;17(32):8896–903. [16] Loh KP, Bao Q, Ang PK, Yang J. The chemistry of graphene. J Mater Chem 2010;20(12):2277. [17] Zhou Y, Bao Q, Tang LAL, Zhong Y, Loh KP. Hydrothermal dehydration for the ‘‘green’’ reduction of exfoliated graphene oxide to graphene and demonstration of tunable optical limiting properties. Chem Mater 2009;21(13):2950–6. [18] Lee JM, Kim SJ, Kim JW, Kang PH, Nho YC, Lee YS. A high resolution XPS study of sidewall functionalized MWCNTs by fluorination. J Ind Eng Chem 2009;15(1):66–71. [19] Wu ZS, Ren W, Gao L, Liu B, Jiang C, Cheng HM. Synthesis of high-quality graphene with a pre-determined number of layers. Carbon 2009;47(2):493–9. [20] Peng XY, Liu XX, Diamond D, Lau KT. Synthesis of electrochemically-reduced graphene oxide film with controllable size and thickness and its use in supercapacitor. Carbon 2011;49(11):3488–96. [21] Ren PG, Yan DX, Ji X, Chen T, Li ZM. Temperature dependence of graphene oxide reduced by hydrazine hydrate. Nanotechnology 2011;22(5):055705. [22] Kuila T, Khanra P, Bose S, Kim NH, Ku BC, Moon B, et al. Preparation of water-dispersible graphene by facile surface modification of graphite oxide. Nanotechnology 2011;22(30):305710. [23] Shen J, Hu Y, Shi M, Lu X, Qin C, Li C, et al. Fast and facile preparation of graphene oxide and reduced graphene oxide nanoplatelets. Chem Mater 2009;21(15):3514–20. [24] Gue´rin K, Pinheiro JP, Dubois M, Fawal Z, Masin F, Yazami R, et al. Synthesis and characterization of highly fluorinated graphite containing sp2 and sp3 carbon. Chem Mater 2004;16(9):1786–92. [25] Bosch-Navarro C, Coronado E, Marti-Gastaldo C, SanchezRoyo JF, Gomez MG. Influence of the pH on the synthesis of reduced graphene oxide under hydrothermal conditions. Nanoscale 2012;4(13):3977–82. [26] Wang H, Robinson JT, Li X, Dai H. Solvothermal reduction of chemically exfoliated graphene sheets. J Am Chem Soc 2009;131(29):9910–1. [27] Chang H, Sun Z, Yuan Q, Ding F, Tao X, Yan F, et al. Thin film field-effect phototransistors from bandgap-tunable, solution-

5410

CARBON

5 0 ( 2 0 1 2 ) 5 4 0 3 –5 4 1 0

processed, few-layer reduced graphene oxide films. Adv Mater 2010;22(43):4872–6. [28] Liu Y, Vander Wal RL, Khabashesku VN. Functionalization of carbon nano-onions by direct fluorination. Chem Mater 2007;19(4):778–86. [29] Fan ZJ, Kai W, Yan J, Wei T, Zhi LJ, Feng J, et al. Facile synthesis of graphene nanosheets via Fe reduction of exfoliated graphite oxide. ACS Nano 2011;5(1):91–8.

[30] Yavari F, Kritzinger C, Gaire C, Song L, Gulapalli H, BorcaTasciuc T, et al. Tunable bandgap in graphene by the controlled adsorption of water molecules. Small 2010;6(22):2535–8. [31] Singh V, Joung D, Zhai L, Das S, Khondaker SI, Seal S. Graphene based materials: past, present and future. Prog Mater Sci 2011;56(8):1178–271.