Synthesis of graphene from natural and industrial carbonaceous wastes

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Synthesis of graphene from natural and industrial carbonaceous wastes

Omid Akhavan,*âbKeyvan Bijanzadâand Ali Mirsepahâ

Graphene oxide (GO) and reduced graphene oxide (rGO) sheets have usually been synthesized through Hummers' method by using highly pure graphite (HPG) as the main starting material. However, HPG can be relatively expensive for mass production of high-quality graphene. In this work, a general method for synthesis of high-quality GO and rGO sheets from various natural and industrial carbonaceous wastes such as vegetation wastes (wood, leaf, bagasse, and fruit wastes), animal wastes (bone and cow dung), a semi-industrial waste (newspaper), and an industrial waste (soot powders produced in exhaust of diesel vehicles) was developed. Based on atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and current–voltage characteristics of the synthesized sheets, the single- and multi-layer properties, chemical state, carbonaceous structure, and electrical properties of the graphene sheets synthesized from various waste materials (with#4-monolayer thicknesses and electrical sheet resistance of10⁵MUsq¹for GO and1 MUsq¹for rGO sheets) were found to be nearly independent of the starting materials used; moreover, they were comparable to those of the high-quality graphene sheets achieved using HPG. These results provide a possible route for inexpensive mass production of high- quality graphene sheets from natural and industrial carbonaceous wastes.

1. Introduction

In recent years, graphene (the thinnest two-dimensional carbonaceous nanomaterial with unique properties) has attracted much attention in various scientic^1,2and theoretical^3–5 areas.

Although there are various methods for fabrication of graphene (such as mechanical exfoliation,² chemical vapor deposition (CVD),⁶ and unzipping of carbon nanotubes),^7,8 the chemical exfoliation of graphite, resulting in production of graphene oxide (GO), is known as one of the most inexpensive and easy methods with mass production capability.^9,10Of course, in this method, an eﬀective chemical or thermal reduction is also required for conversion of GO into the reduced graphene oxide (rGO). The GO and rGO sheets produced by the chemical exfoliation method have been promisingly applied in various potential applications, such as bactericidal,^11–14 nematocidal¹⁵ and antiviral¹⁶ nanomaterials; wastewater purication;^17,18 exible and transparent conductors;¹⁹high-performance polymer–matrix composites;^20,21 ultrasensitive biosensing,²²drug delivery;^23–25cancer cell imaging, targeting and therapy;^26–31 tissue engineering;^32,33 and neural network regeneration,^34,35due to good dispersion of GO in water (good hydrophilicity) and easy fabrication and functionalization of GO.

One of the drawbacks in mass production of the chemically exfoliated rGO sheets (which is not the subject of this investi- gation) is the necessity of using a strong chemical reductant (such as hydrazine), which is usually highly corrosive, explosive and toxic.³⁶ Thus, many environmentally friendly reducing agents such as vitamin C,³⁷ melatonin,³⁸ sugar,³⁹ glucose,⁴⁰ polyphenols of green tea,^41,42 ginseng,⁴³ and protein bovine serum albumin⁴⁴have been proposed as eﬀective substitutes for hydrazine. In addition, replacing the usual chemical reduction methods by some physical and environmentally friendly methods has been recently suggested for the eﬀective reduction of GO.^45–47

Another drawback in mass production of chemically exfoliated graphene (the subject of our study in this work) is that the main starting material used in this method (i.e., highly pure graphite (HPG)) is expensive (at least about one order in magnitude higher than the industrial graphite). To overcome this problem, one can think of production of appropriate and inexpensive graphitic materials from carbonaceous wastes (as free materials). However, no investigations on the production of chemically exfoliated GO and rGO sheets by using carbonaceous waste materials have been reported yet. There is only one recent report on the fabrication of graphene from materials such as cookies, chocolate, grass, plastics, roaches, and dog feces by CVD at 1050C in H2/Arow.⁴⁸

In this work, we concentrated on developing the chemical exfoliation method for synthesis of GO from inexpensive carbonaceous materials (i.e., overcoming the second drawback).

aDepartment of Physics, Sharif University of Technology, P.O. Box 11155-9161, Tehran, Iran. E-mail: [email protected]; Fax: +98-21-66022711; Tel: +98-21-66164566

bInstitute for Nanoscience and Nanotechnology, Sharif University of Technology, P.O.

Box 14588-89694, Tehran, Iran Cite this:RSC Adv., 2014,4, 20441

Received 22nd February 2014 Accepted 3rd April 2014 DOI: 10.1039/c4ra01550a www.rsc.org/advances

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For this purpose, some low-cost natural and industrial carbonaceous wastes such as vegetation wastes (wood, leaf, bagasse, and fruit wastes), animal wastes (bone and cow dung), a semi- industrial waste (newspaper), and an industrial waste (soot powders produced in exhaust of diesel vehicles) were used to synthesize high-quality GO and rGO sheets through the chemical exfoliation method (as one of the competitors of the CVD method). The single- and multi-layer properties, chemical state, carbonaceous structure, and electrical properties of the graphene sheets synthesized by the various waste materials were compared with those of the sheets synthesized from HPG.

2. Experimental

Natural charcoal and industrial soot preparation

Seven diﬀerent natural carbonaceous waste materials such as wood (from black mulberry tree of the garden of a house in Tehran), leaf (from plane trees grown in Tehran), bagasse (waste of Khoozestan Sugarcane Factory), fruit (rind of orange from Bam, Kerman), newspaper (Hamshahri newspaper published in Tehran, Iran), bone (bone of chicken produced by Zarbal Toyour Co.), and cow dung (from the cows living in Anzali, north of Iran) were selected for charcoal preparation (carbonization).

The material was wrapped in an aluminum foil with limited access to air and transferred to a chimney for imperfect burning at400–500C for 5 days. The obtained charcoal materials were ground by a mortar into powders, which were then wrapped in an aluminum foil and heated at 450C for 24 h.

The industrial soot powder (as another starting material) was gathered from the exhaust of diesel vehicles (Mercedes-Benz- Khavar, L 2624, IKCO) using Euro II gas oil. Highly pure natural graphite powder (Sigma-Aldrich, with 99.99% purity) was used as a standard starting material and as a benchmark in this study.

Graphitization of the carbonized materials

Atrst, 1.0 g of the carbonized materials and 0.5 g FeCl3$6H2O were added to 100 mL distilled water. In addition, the pH of the solution was adjusted 2 by adding HCl. The mixture was stirred at 60 C for 5 h, then le for one week for gradual evaporation of water at room temperature, andnally dried at 100C for 5 h to achieve a black solid material. The obtained graphitized materials were ground by a mortar to obtain powders that were used in the next step.

Synthesis of GO and rGO suspensions

The prepared charcoal or soot powders were used as raw materials to synthesize GO by using a modied Hummers' method.

For this purpose, 1.0 g of each graphitized powder was separately dissolved in 50 mL H₂SO₄at 80C for 24 h. Then, 1.0 g NaNO₃ was added into the solution and stirred in an ice bath for 1 h.

Then, 6.0 g KMnO₄ was slowly added into the solution and vigorously stirred for 4 h. Then, it was warmed to room temperature while being stirred continuously in a water bath at 35C for 1 h. The prepared solution was diluted by 100 mL distilled water.

During the dilution, the temperature of the solution was controlled to be <60C. In addition, 6 mL H2O2(30%) diluted by

200 mL distilled water was added into the solution in order to reduce residual permanganate to soluble manganese ions and stop the gas evolution from the solution. The residual acids and dissolved impurities of the solution were removed by centrifuging the solution at 8000 rpm for 30 min, using an Eppendorf 5702 centrifuge with a rotor radius of 10 cm. The centrifugation step was performed10 times to completely remove the super- natant solution containing the acids and impurities. Then, further purication was done by repeated (2–5 times, depend- ing on the starting materials used)ltering of the centrifuged solution through an anodic membranelter (47 mm in diameter, 0.2 mm pore size, Whatman) and washing with deionized (DI) water. The materials obtained byltering were redispersed in DI water to obtain an aqueous graphite oxide suspension with yellowish-brown color. Then, the aqueous suspension was centrifuged at 2000 rpm for 30 min and 8000 rpm for 60 min to remove any unexfoliated graphitic plates and small graphite particles, respectively. Finally, a GO suspension was achieved by ultrasonication of the centrifuged graphite oxide suspension at a frequency of 40 kHz and power of 150 W for 30 min. For chemical reduction of the GO aqueous suspension by hydrazine (as a standard reductant), the pH of 100 mL GO suspension with a concentration of 0.5 mg mL¹was adjusted to9.0 by adding a diluted ammonia solution. Then, 100 mL hydrazine solution (35%) was added to the suspension while it was stirred at room temperature. Finally, the suspension was reuxed at 90C for 3 h in an oil bath.

Material characterizations

Surface topography and height prole of the graphene sheets were studied by atomic force microscopy (AFM, Park Scientic CP-Research model (VEECO)) in tapping mode. X-ray photoelectron spectroscopy (XPS) was utilized to monitor the presence of residual elemental impurities in thenal products and to study the chemical state of the GO sheets. The data were gathered using a hemispherical analyzer supplied by an Al KaX- ray source (hn¼1486.6 eV) operating at a vacuum higher than 10⁷ Pa. For more quantitative analyses, the XPS peaks were deconvoluted by using Gaussian components aer the Shirley background subtraction. The relative concentrations of various elements in the graphene samples were evaluated by using peak area ratio of the core levels and considering the sensitivity factor of each element in XPS. The carbon structures of the graphene samples were examined by Raman spectroscopy (HR-800 Jobin- Yvon) at room temperature using an Nd-YAG laser operating at wavelength of 532 nm. Each sample for AFM, XPS and Raman spectroscopy was prepared by drop-casting the desired graphene suspension onto a cleaned SiO2/Si(100) substrate followed by drying at 80C in vacuum (with pressure of0.8 Pa) for 30 min. For better AFM imaging, a diluted graphene suspension (0.1 mg mL¹) was utilized.

To study the current–voltage (I–V) characteristics of the graphene sheets, they were randomly deposited between two Au electrodes (coated on a SiO₂(300 nm)/Si(100) substrate by using electron-beam evaporation) through drop-casting the prepared graphene suspensions. The thickness and width of the

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electrodes were about 200 nm and 120mm, respectively; moreover, the average distance between the electrodes was0.5mm.

TheI–Vcharacteristics of the rGO sheets was measured aer deposition of the GO sheets between the Au electrodes, followed by reduction of the deposited sheets by hydrazine vapor in a

ask. Before any I–Vmeasurement, all the electrode samples deposited by the graphene sheets were dried in vacuum (0.8 Pa) at 80C for 30 min. The number of graphene sheets randomly deposited between the two Au electrodes was counted by using an optical microscope. The data ofI–Vcurves were obtained by using a Keithley 485 Autoranging Picoammeter.

The surface morphology of the rGO sheets deposited on the Au electrodes (especially, those that were deposited on the gap of the two electrodes) was studied by scanning electron microscopy using a Philips XL30 scanning electron microscope.

3. Results and discussion

Fig. 1A shows AFM images of the GO sheets produced from highly pure graphite, wood, leaf, bagasse, fruit, newspaper, bone, cow dung, and soot aer deposition on SiO2/Si substrates. The overlapped sheets with $1 mm lateral dimensions are clearly distinguishable in the images. The height prole distributions of the sheets produced from the various materials are also presented in Fig. 1B. It is well known that the typical thickness of monolayer (ML) GO sheets is0.8 nm (i.e.,0.44 nm thicker than the typical thickness of graphene with thickness of0.36

nm) because of the presence of the oxygen groups on either side of the sheets.^49–51In the height prole distribution diagrams, the minimum roughness was adjusted to zero; hence, the position of therst peak appearing at 0.8 nm can be assigned to 1 ML sheets. The position of the second, third and forth peaks at1.6, 2.4 and 3.1 nm correspond to the presence of 2, 3 and 4 ML sheets on the surface, respectively. No peaks related to greater thicknesses were observed for any of the tested materials. Fig. 1B also shows that the highly pure graphite, wood, and bagasse could yield 1 ML sheets with signicantly higher abundances. In contrast, the abundance of 3 ML sheets was higher for the sheets produced from the materials such as leaf, newspaper, bone, and cow dung. The higher full width at half maximum (FWHM) of some samples (such as the samples produced from newspaper, bone, and soot) can be assigned to the presence of some residual impurities originating from the starting materials (see the XPS results given in Table 1). These results mean that, although the quality of the graphene sheets produced from the various carbonaceous materials can be slightly diﬀerent (here, abundance of the single-layer sheets and/or residual impurities), synthesis of GO sheets with thicknesses#4 MLs is feasible, and it is independent of the starting carbonaceous materials.

Raman spectroscopy was utilized to examine the carbon structure of the GO sheets, as presented in Fig. 2A, and the peak intensity ratios ofI_D/I_GandI_2D/I_Gare shown in Fig. 2B. It was found that theID/IGratios of the GO sheets synthesized from leaf, bone and soot materials were slightly higher than the

Fig. 1 (A) AFM images and (B) height proﬁle distributions of GO sheets produced from (a) highly pure graphite, (b) wood, (c) leaf, (d) bagasse, (e) fruit, (f) newspaper, (g) bone, (h) cow dung, and (i) soot.

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ratios of the sheets produced from the other materials. More- over, the GO sheets synthesized from the HPG showed the lowest I_D/I_G ratio. Higher I_D/I_G ratio can be assigned to decreasing the graphitic domain size and/or increasing the defects/disorders in the graphene sheets.

It is known that the 2D band of Raman spectra is more sensitive to stacking of graphene sheets.⁵² For instance, the position of the 2D band of single-layer graphene is at 2679 cm¹, while for multi-layer graphene (containing 2–4 layers), the position of the 2D band shis into larger wave- numbers by 19 cm¹along with a peak broadening.⁵³Further- more, theI2D/IGintensity ratios for single-, double-, triple- and multi- (>4) layer graphene are typically >1.6,0.8, 0.30 and 0.07, respectively (see, for example, ref. 12, 54 and 55). In this work, all the GO samples synthesized from the various starting materials showed a 2D band located at 2680–2700 cm¹, indicating the presence of single- and multi-layer GO sheets in the samples. Moreover, although none of the samples showed anI_2D/I_G > 0.5, this ratio was found to be >0.3 for the whole samples. Therefore, all the GO sheets synthesized by this proposed method (independent of the starting material used in

the synthesis) showed multilayer structures with <4 ML sheets, consistent with the AFM results.

Fig. 3 shows XPS peak deconvolution of C(1s) core levels of the GO and rGO sheets obtained from the various starting carbonaceous materials. In the peak deconvolution, the peak centered at 285.0 eV was assigned to the C–C and C]C bonds.

The other deconvoluted peaks located at the binding energies of 286.6, 287.3, 288.3 and 289.4 eV were attributed to the C–

OH, C–O–C, C]O, and O]C–OH oxygen-containing functional groups, respectively (see, for example, ref. 44 and 56).

Fig. 3A indicates that the C(1s) chemical states of all synthesized GO sheets were approximately the same, independent of the starting materials used in the procedure. Fig. 3B shows not only the eﬀective deoxygenation of the GO sheets by hydrazine, but also nearly the same chemical state for all the reduced samples (once again, independent of the starting materials used). It should be noted that reduction by hydrazine resulted in the appearance of another peak component at 286.0 eV, associated with formation of C–N bonds on the surface of the reduced sheets, as reported in more detail elsewhere (see, for example, ref. 41).

Table 1 The residual elemental impurities (at% with respect to carbon found by XPS) in the GO sheets synthesized from the various starting materials

Element material N Na Mg P S K Ca Fe Cu Zn Pb Total

HPG — — — — — — — — — — — 0.01

Wood 0.06 — 0.01 — 0.11 0.11 0.09 0.03 — — — 0.38

Leaf 0.04 — — — 0.03 0.05 — 0.02 — — — 0.12

Bagasse 0.09 — 0.09 — 0.08 0.17 0.12 0.03 — 0.01 — 0.56

Fruit 0.13 — 0.06 — 0.05 0.24 0.16 0.05 — 0.02 — 0.66

Newspaper 0.04 — — — 0.02 — — 0.09 0.01 — 2.9 3.06

Bone — 0.01 0.03 0.38 — 0.19 1.90 0.03 — — — 2.51

Cow dung 0.84 0.61 0.01 0.15 0.07 0.09 0.01 0.04 — — — 1.78

Soot 2.1 — — — 2.6 — — 0.07 0.03 — 0.02 4.82

Fig. 2 (A) Raman spectra of GO sheets synthesized from (a) highly pure graphite, (b) wood, (c) leaf, (d) bagasse, (e) fruit, (f) newspaper, (g) bone, (h) cow dung, and (i) soot as the starting materials, and (B)ID/IGandI2D/IGratios of the samples.

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Although all the samples exhibited nearly the same C(1s) core level spectra, the samples synthesized with the various materials showed some diﬀerent residual elemental impurities in the XPS survey spectra. Table 1 summarizes the relative concentrations of various elements (such as N, Na, Mg, P, S, K, Ca, Fe, Cu, Zn, and Pb) with respect to carbon content of the

graphene samples. All the materials showed total residual impurities <5 at%, due to the eﬀectiveness of the centrifuging,

ltering and washing processes used in the synthesis. The vegetation substances (such as the natural carbonaceous materials) yielded the lower residual impurities (<0.7 at%), while the soot (as one of the industrial carbonaceous materials) resulted in a relatively high impurity level (5 at%), substantially related to the residual S and N impurities. The animal carbonaceous materials (i.e., bone and cow dung) presented 2 at% residual impurity level, substantially related to the residual Ca for the former and residual N and Na for the latter.

The newspaper (as a semi-industrial starting material) yielded a relatively high residual impurity level of3 at%, substantially due to application of Pb in printing the newspaper. Now, the lower quality of the graphene materials obtained by using newspaper, bone and soot can be assigned to the high levels of impurities of these raw materials. It is worth noting that, although the GO sheets synthesized from the diﬀerent wastes exhibited various level of impurities, they typically showed the same plots on thermogravimetric analysis (TGA) as the HPG- synthesized GO ones (the TGA of the HPG-synthesized GO sheets was reported elsewhere).⁵⁷

To investigate the electronic properties of the graphene sheets synthesized from the various starting materials, theI–V characteristics of the sheets were studied. Fig. 4a shows theI–V curves of the rGO sheets obtained by using the various starting

Fig. 4 (a) TheI–Vdiagram of the rGO sheets synthesized from highly pure graphite (2 sheets), wood (2 sheets), leaf (3 sheets), bagasse (2 sheets), fruit (4 sheets), newspaper (4 sheets), bone (5 sheets), cow dung (3 sheets), and soot (2 sheets) as compared to that of the GO sheets (74 sheets) synthesized from graphite, and theRsvalues of the (b) GO and (c) rGO sheets obtained from the various materials. The inset of (c) presents an SEM image of some rGO sheets on Au electrodes deposited on a SiO2/Si substrate. (d) Shows (b) and (c) in one diagram with a logarithmic scale forRs

for better comparison.

Fig. 3 XPS peak deconvolution of C(1s) core level of (A) GO and (B) rGO sheets produced from (a) highly pure graphite, (b) wood, (c) leaf, (d) bagasse, (e) fruit, (f) newspaper, (g) bone, (h) cow dung, and (i) soot as the starting materials.

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materials as compared to the I–V curves of the GO sheets synthesized from the HPG. The linear form of theI–Vcurves indicated the metallic property of the sheets and formation of Ohmic contacts between the graphene sheets and the Au electrodes. Using the slope of the curves and by considering the number of graphene sheets connected the two electrodes, the electrical sheet resistance (R_s) of the sheets was evaluated, as presented in Fig. 4b and c for the GO and rGO sheets synthesized from the various materials. The graphene sheets fabricated from HPG exhibited the lowestR_svalues (by factors >1/6) among the corresponding sheets fabricated by the other starting materials, due to the high purity of the HPG. Furthermore, the starting materials resulting in higher defects and residual impurities such as bone and soot (see Fig. 2 and Table 1) yielded higher Rs values either before or aer hydrazine reduction.

Nevertheless, Fig. 4d shows that the order of magnitude of the Rsvalues of the GO or rGO sheets are nearly the same, independent of the starting materials used (10⁵MUsq¹for GO and 1 MU sq¹ for rGO sheets). In fact, rGO sheets with suitable electronic properties (comparable with the properties of the sheets achieved using HPG) are obtainable from various natural and industrial carbonaceous materials.

4. Conclusions

A general method for synthesis of high-quality GO and rGO sheets from various natural and industrial carbonaceous wastes (as starting raw materials) was developed. In this method, the vegetation wastes (wood, leaf, bagasse, and fruit wastes), animal wastes (bone and cow dung), and a semi-industrial waste (newspaper) were initially carbonized at400–500C through imperfect burning. Then, the charcoal materials and an industrial waste (exhaust soot of diesel vehicles) were used in a Hummers' method for chemical exfoliation of the graphitized materials. The surface morphology (including thickness of the sheets), chemical state, carbonaceous structure, and electrical properties of the sheets synthesized by the various feedstocks were found to be nearly the same and also comparable to those of the graphene sheets obtained by HPG. These results indicate that many kinds of solid carbonaceous wastes with the capability of graphitization can be applied in the production of high- quality graphene sheets. Moreover, this method provides successful recycling of low-value and sometimes pollutant and/

or hazardous wastes into valuable and high-quality graphene nanomaterials.

Acknowledgements

The authors would like to thank the Research Council of Sharif University of Technology and Iran Nanotechnology Initiative Council for thenancial support for this work.

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37 J. Gao, F. Liu, Y. Liu, N. Ma, Z. Wang and X. Zhang, Environment-Friendly Method to Produce Graphene that Employs Vitamin C and Amino Acid,Chem. Mater., 2010,22, 2213–2218.

38 A. Esfandiar, O. Akhavan and A. Irajizad, Melatonin as a Powerful Bio-Antioxidant for Reduction of Graphene Oxide, J. Mater. Chem., 2011,21, 10907–10914.

39 C. Zhu, S. Guo, Y. Fang and S. Dong, Reducing Sugar: New Functional Molecules for the Green Synthesis of Graphene Nanosheets,ACS Nano, 2010,4, 2429–2437.

40 O. Akhavan, E. Ghaderi, S. Aghayee, Y. Fereydooni and A. Talebi, The Use of a Glucose-Reduced Graphene Oxide Suspension for Photothermal Cancer Therapy, J. Mater.

Chem., 2012,22, 13773–13781.

41 O. Akhavan, M. Kalaee, Z. S. Alavi, S. M. A. Ghiasi and A. Esfandiar, Increasing the Antioxidant Activity of Green Tea Polyphenols in the Presence of Iron for the Reduction of Graphene Oxide,Carbon, 2012,50, 3015–3025.

42 Y. Wang, Z. X. Shi and J. Yin, Facile Synthesis of Soluble Grapheneviaa Green Reduction of Graphene Oxide in Tea Solution and Its Biocomposites,ACS Appl. Mater. Interfaces, 2011,3, 1127–1133.

43 O. Akhavan, E. Ghaderi, E. Abouei, S. Hatamie and E. Ghasemi, Accelerated Diﬀerentiation of Neural Stem Cells into Neurons on Ginseng-Reduced Graphene Oxide Sheets,Carbon, 2014,66, 395–406.

44 J. Liu, S. Fu, B. Yuan, Y. Li and Z. Deng, Toward a Universal

“Adhesive Nanosheet” for the Assembly of Multiple Nanoparticles Based on a Protein-Induced Reduction/

Decoration of Graphene Oxide,J. Am. Chem. Soc., 2010,132, 7279–7281.

45 Y. Zhou, Q. Bao, L. A. L. Tang, Y. Zhong and K. P. Loh, Hydrothermal Dehydration for the “Green” Reduction of Exfoliated Graphene Oxide to Graphene and Demonstration of Tunable Optical Limiting Properties, Chem. Mater., 2009,21, 2950–2956.

46 O. Akhavan, M. Abdolahad, A. Esfandiar and M. Mohatashamifar, Photodegradation of Graphene Oxide Sheets by TiO₂ Nanoparticles aer a Photocatalytic Reduction,J. Phys. Chem. C, 2010,114, 12955–12959.

47 O. Akhavan, Photocatalytic Reduction of Graphene Oxides Hybridized by ZnO Nanoparticles in Ethanol,Carbon, 2011, 49, 11–18.

48 G. Ruan, Z. Sun, Z. Peng and J. M. Tour, Growth of Graphene from Food, Insects, and Waste, ACS Nano, 2011,5, 7601–

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49 H. C. Schniepp, J. L. Li, M. J. McAllister, H. Sai, M. Herrera- Alonso, D. H. Adamson, R. K. Prud'homme, R. Car, D. A. Saville and I. A. Aksay, Functionalized single graphene sheets derived from splitting graphite oxide, J.

Phys. Chem. B, 2006,110, 8535–8539.

50 M. J. McAllister, J. L. LiO, D. H. Adamson, H. C. Schniepp, A. A. Abdala, J. Liu, M. Herrera-Alonso, D. L. Milius, R. Car, R. K. Prud'homme and I. A. Aksay, Single sheet functionalized graphene by oxidation and thermal expansion of graphite,Chem. Mater., 2007,19, 4396–4404.

51 O. Akhavan, The Eﬀect of Heat Treatment on Formation of Graphene Thin Films from Graphene Oxide Nanosheets, Carbon, 2010,48, 509–519.

52 L. M. Malard, M. A. Pimenta, G. Dresselhaus and M. S. Dresselhaus, Raman Spectroscopy in Graphene,Phys.

Rep., 2009,473, 51–87.

53 D. Graf, F. Molitor, K. Ensslin, C. Stampfer, A. Jungen, C. Hierold and L. Wirtz, Spatially Resolved Raman

Spectroscopy of Single- and Few-Layer Graphene, Nano Lett., 2007,7, 238–242.

54 I. Calizo, A. A. Balandin, W. Bao, F. Miao and C. N. Lau, Temperature Dependence of the Raman Spectra of Graphene and Graphene Multilayers, Nano Lett., 2007, 7, 2645–2649.

55 L. Liu, S. Ryu, M. R. Tomasik, E. Stolyarova, N. Jung, M. S. Hybertsen, M. L. Steigerwald, L. E. Brus and G. W. Flynn, Graphene Oxidation: Thickness-Dependent Etching and Strong Chemical Doping, Nano Lett., 2008, 8, 1965–1970.

56 F. Liu and T. S. Seo, A Controllable Self-Assembly Method for Large-Scale Synthesis of Graphene Sponges and Free- Standing Graphene Films,Adv. Funct. Mater., 2010,20, 1–7.

57 E. Ezzatpour Ghadim, N. Rashidi, S. Kimiagar, O. Akhavan, F. Manouchehri and E. Ghaderi, Pulsed laser irradiation for environment friendly reduction of graphene oxide suspensions,Appl. Surf. Sci., 2014,301, 183–188.