Evaluation Criteria for Reduced Graphene Oxide - The Journal of

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Evaluation Criteria for Reduced Graphene Oxide Dachao Luo, Guoxin Zhang, Junfeng Liu,* and Xiaoming Sun* State Key Laboratory of Chemical Resource Engineering, P.O. Box 98, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: Reduced graphene oxide (RGO) is an intriguing nanomaterial with tremendous potential for many applications. Although considerable efforts have been devoted to develop the reduction methods, it still needs further improvement, and how to choose an appropriate one for a specific application is a troublesome problem. In this study, RGOs were prepared by six typical reduction methods: N2H4 3 H2O, NaOH, NaBH4, solvothermal, high-temperature, and two-step. The samples were systematic compared by four aspects: dispersibility, reduction degree, defect repair degree, and electrical conductivity. On the basis of the comparison, a simple evaluation criterion was proposed for qualitatively judging the quality of RGO. This evaluation criterion would be helpful to understand the mechanism of reduction and design more ideal reduction methods.

’ INTRODUCTION Graphene, a single-layer graphite with unusual electronic properties18 and potential applications in various fields,927 has attracted tremendous attention from researchers in recent years. Current methods for preparation of graphene sheets include chemical vapor deposition (CVD),2832 micromechanical exfoliation of graphite,5,3335 epitaxial growth on electrically insulating surface,3639 and the creation of colloidal suspension.1,4052 Graphene obtained by the first three methods showed limited uniformity or unsuitable single layer selectivity. Moreover, the low productivity of these methods makes them unsuitable for large-scale applications.42 The most promising route for the bulk production of graphene sheet is the chemical oxidation and exfoliation of graphite in the liquid phase and followed by reduction, due to its simplicity, reliability, ability for large-scale production, relatively low material cost, and versatile in terms of being well-suited to chemical functionalization.42,44 However, the oxidation in this chemical method induces a variety of defects and oxygen-containing functional groups such as hydroxyl and epoxide, resulting in degradation of the electronic properties of graphene. Meanwhile, the reduction process can lead to the occurrence of aggregate, ion doping, and so on. That is, the reduction methods were gradually verified to play a vital role in the chemical exfoliation method. Up to now, considerable efforts have been devoted to develop the reduction methods. Here we present a simple classification as followed: using reducing agents (hydroquinone,53 dimethylhydrazine,41 hydrazine,1,2,42,48,52,5460 sodium borohydride,6163 sulfurcontaining compounds,6466 aluminum powder,67 vitamin C,6871 hexamethylenetetramine,72 ethylenediamine (EDA),73 polyelectrolyte,74 reducing sugar,75 protein,76 sodium citrate,77 carbon monoxide,78 Fe,79 and norepinephrine,80), performing under various conditions (acid/alkali,8185 thermal treatment,44,45,8696 and others like microwave,9799 photocatalytic,100106 sonochemical,107 laser,108112 plasmas,113 bacterial respiration,114 lysozyme,115 tea r 2011 American Chemical Society

solution116), electrochemical,15,117121 electric current,122 two-step reduction,123,124 and so on. These different reduction methods result in RGO with different properties. For instance, large-scale production of aqueous graphene dispersion can be easily realized by hydrazine-reduced graphene oxide without the surfactant stabilizers.52 However, hydrazine is toxic and explosive, and it makes difficult in an actual process. Solvothermal reduction methods can decrease the defects and oxygen content as well as the resistivity.86 Recently, two or more types of reduction methods were combined to further improve the electrical conductivity or other performances.89,123127 For instance, hydrazine-reduced RGO film after thermal treatment usually showed an enhanced electrical conductivity.59,124 As we know, the unique properties of graphene are associated with individual sheet. However, some of present reduction methods tend to form irreversible agglomerates through van der Waals interaction, making further processing difficult.41 There have been reported that stable suspension of RGO could be obtained by application of sonochemical and stabilizing agent.48,107,115,128131 Other cases mostly focused on the selection of reducing agents or environments instead of properties of final products, especially not caring whether they can be used for industry. Besides, it also should be noted that reduction methods for GO solution are not all suitable for the reduction of GO films. Because the substrates and GO film itself should meet the reduction methods.82,84,132 A simple and quick standard is needed for evaluating the quality of RGO and choosing a suitable one for special application. Meanwhile, as the reduction mechanism (most focused on the removal of epoxy)42,64,70,73,82,96,133 remains ambiguous and the detailed structure of GO or RGO is still unclear,123,134139 it is very difficult to establish a Received: October 19, 2010 Revised: May 2, 2011 Published: May 19, 2011 11327

dx.doi.org/10.1021/jp110001y | J. Phys. Chem. C 2011, 115, 11327–11335

The Journal of Physical Chemistry C straightforward and operator-friendly standard for different eduction methods. There have been reported that electrical/ quantum resistance was used as standard for pristine/epitaxial graphene,140,141 but problems still exist in distinguishing between reduction degree and defect repair degree on the influence of conductivity if applied in measuring RGO. To avoid followup researchers’ confusion, giving a systemic comparison for common reduction methods would be interesting and of great meaning. Herein, six commonly used reduction methods were selected as candidates for preparation of RGO in this paper. Systematic comparisons were carried out on several aspects including dispersibility, reduction degree, defect repair degree, and electrical conductivity by means of atomic force microscopy (AFM), UVvis absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and four-point probe conductivity measurement. The systematic comparison should be helpful to understand the mechanism of reduction and further develop more ideal reduction method.

’ EXPERIMENTAL SECTION GO Synthesis and Purification. GO was made by a modified Hummers method.40,86 Nature graphite (5 g, 50 mesh, 99.95% in purity, HuaDong Inc.) was grounded with NaCl (100 g) for 30 min. Afterward, NaCl was washed away using water with vacuum filtration. The remaining graphite was heated at 60 °C in oven for 24 h to remove any water. The dried solid was then mixed with 115 mL of concentrated sulfuric acid in a 1000 mL round-bottom flask and stirred at room temperature for 24 h. Next, 0.5 g of NaNO3 was added to the mixture and allowed to dissolve for 20 min. The flask was then placed in an ice bath, and 15 g of KMnO4 was slowly added while the temperature was kept below 20 °C for 1.5 h. The solution was then changed into the oil bath and heated at 3540 °C for 1 h. Increase the temperature to 70 °C and keep for 30 min; afterward, 15 mL of water was added to the flask, and this could increase the temperature to 100 °C. Keep the temperature for 20 min. Twenty minutes later another 15 mL of water was added. After 30 min, 200 mL of water was added. Then add 500 mL of ice water into the resultant suspension; this step can dilute and cool down the system to ∼50 °C. After 15 min, 50 mL of 30% H2O2 was added to the flask under vigorous stirring. This suspension was stirred at room temperature for 1 h. The suspension was centrifuged at low speed for 3 times (4500 rpm, 15 min) and washed with 5% HCl solution for 2 rounds and then centrifuged at high speed for 3 times (12000 rpm, 60 min) with distilled water (first step we add 0.1 g of NH4Cl). Finally, the GO was kept in a vacuum desiccator with self-indicating silica gel for 1 week. Reduction 1. Hydrazine hydrate42,52,54,55 reduction: GO dispersion (25.0 mL, 0.05 wt %) was mixed with 25.0 mL of water, 11.0 μL of hydrazine solution (80 wt %), and 175.0 μL of ammonia solution (28 wt %) in a glass vial. After being vigorously shaken or stirred for a few minutes, the vial was put in a water bath (95 °C) for 1 h. 2. NaOH81 reduction: GO suspension (0.5 mg/mL, 75 mL) and 1 mL of NaOH solution (8 M) were loaded into a vessel with constant temperature water bath. The vessel was kept at 70 °C and mild sonicated for a few minutes. 3. NaBH461,62 reduction: In a typical procedure, 75 mg of graphite oxide was dispersed in 75 g of water with sonication. 600 mg of sodium borohydride in 15 g of water was added into the GO dispersion after pH being

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adjusted to 910 with 5 wt % sodium carbonate solution. The mixture was then kept at 80 °C for 1 h under constant stirring. During reduction, the dispersion turned from dark brown to black accompanied by outgassing. 4. Solvothermal86,87 reduction: A total of 35 mL of 0.5 mg/mL GO aqueous solution was transferred to a Teflon-lined autoclave and heated at 180 °C for 6 h. 5. High-temperature44,45 reduction: Put the GO powder in porcelain boat, and then place it into the edge of long quartz tube. Argon was then inserted into the long quartz tube. The sample was flushed with argon for 10 min, and the porcelain boat was quickly inserted into the middle of long quartz tube furnace preheated to 900 °C and held in the furnace for 30 s. 6. Two-step123 reduction: 100 mg of dry GO sample was dispersed in deionized water to give a 1.0 mg/mL colloidal solution. The pH value of this solution was adjusted to 910 by 5 wt % sodium carbonate solution. Sodium borohydride (800 mg) was directly added into 100 mL of GO dispersion under magnetic stirring, and the mixture was kept at 80 °C for 1 h with constant stirring.61 Reduced product was separated by filtration and washed with large amount of water several times to remove most residual ions. This partially reduced GO was kept in a vacuum desiccator with phosphorus pentoxide for 2 days and redispersed in concentrated sulfuric acid and heated to 120 °C with stirring for 12 h. After cooling down, the dispersion was diluted with deionized water. The final product was separated by filtration and thoroughly rinsed with water to remove most impurities. The product powder was compressed into a pellet and further annealed at 900 °C under gas flow of Ar for 15 min. Preparation of the GO and RGO Films. GO and RGO films were prepared by the vacuum filtration method. A cellulose ester membrane (50 mm in diameter, 220 nm pore size, Shenghemo) was used. After being dried at 60 °C in a vacuum desiccator for 3 days, a paper like GO and RGO films were obtained. Characterization. AFM (Multimode Nanoscope IIIa, Veeco Instruments) was used to measure GO sheet size and thickness. Silicon substrate was cleaned with acetone, methanol, and isopropanol and then soaked in an aqueous solution of 3-aminopropyltriethoxysilane (APTES; 12 μL of APTES in 20 mL of H2O) for 15 min. After thoroughly rinsing with deionized H2O and blow-drying with Ar, the substrate was then soaked in a solution of GO for 10 min or longer. Optical properties of GO were characterized by absorbance spectroscopy (UV-2501PC, Shimadzu, working in 200900 nm range) at room temperature. Raman spectra were recorded from 800 to 3600 cm1 on a Renishaw 1000 confocal Raman microprobe (Renishaw Instruments) using a 514 nm argon ion laser. XPS measurements were performed using an ESCALAB 250 instrument (Thermo Electron) with Al KR radiation. The electrical conductivity of the GO and RGO films was measured by fourpoint probe method (SB100A/2, Qianfeng). Measurements were repeated on three different areas of the films to ensure sample uniformity and their geometric averages.

’ RESULTS AND DISCUSSION Any assessment method should be fair, comprehensive, simple, fast, acceptable, and promote the development of this industry. RGO is no exception. Considering RGO for large-scale applications, the dispersion stability of RGO become a top 11328

dx.doi.org/10.1021/jp110001y |J. Phys. Chem. C 2011, 115, 11327–11335

The Journal of Physical Chemistry C

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Figure 1. Digital pictures of GO and different methods reduced RGO dispersed in water, DMF, and CCl4 through ultrasonication. Left: dispersions immediately after sonication. Right: dispersions 1 week after sonication.

priority because the unique properties of graphene are associated with individual sheet and most of the applications require RGO first to be dispersed in solvent. As mentioned in many papers, graphene oxide (GO) can be dispersed in water and many organic solvents through ultrasonication and remains stable for several weeks without any visible precipitation. However, the dispersibility of RGO varies with different reduction methods and has not been compared before. Therefore, the dispersibility was selected as an important parameter in evaluation criteria. In our work, we compared the dispersibility of RGO by different methods in three kinds of commonly used solvents: water, typical polar organic (DMF, dimethylformide), and typical nonpolar solvent (CCl4). The as-prepared RGO were dispersed in the three kinds of solvents to form a nominal concentration (0.2 mg mL1) solution through ultrasonication, and then the dispersions were placed without disturbance for a week. Figure 1 shows digital pictures of GO and RGO dispersed immediately after sonication and 1 week after sonication, respectively. For the just sonicated samples, it can be noticed that GO and RGO could be well dispersed in water and DMF, while only RGO reduced through solvothermal method, high-temperature method and two-step reduced method can be dispersed in CCl4 solvent. However, many of these dispersions displayed only short-term stability and precipitated completely in a couple of hours or few days. It also indicated that the GO had been changed after reduced. As well-known, the suspension stability is mainly determined by the solvation degree and the colloid size. Decrease in dispersion stability implies weaker interaction between solvents and RGO, and consequent worse solvation degree. To further study their dispersibilities, we took AFM characterizations for GO and RGO. AFM is one of the most direct methods for quantifying the degree of exfoliation of graphene sheet level or agglomeration of the sheets after being dispersed in a solvent. To investigate the degree of dispersibility of RGO reduced by different methods, the dispersions were deposited onto silicon substrate and analyzed by AFM. Representative results are shown in Figure 2. For samples reduced from GO dispersions in water, the AFM images reveal the presence of irregularly shaped sheets of nonuniform thickness and their lateral dimension ranging from a few nanometers to micrometers. As illustrated in Figure 2H, for the sheet marked by the yellow line in Figure 2D,G, their thicknesses lie between 1.0 and 3.6 nm, which assigned to single-layer or few-

Figure 2. (AG) Tapping-mode AFM images (2 2 μm2, scale bar: 400 nm) of GO (A) and different methods reduced RGO (from B to G: N2H4, NaOH, NaBH4, solvothermal, high-temperature, two-step). (H) Tapping-mode height images of RGO.

layer GO/RGO sheets.142,143 Some of the reduction methods can lead to a certain number of multilayer RGO sheets. The second criterion is the reduction degree. More and more researchers are beginning to realize the importance of the reduction degree on the performance of RGO. UVvis absorption spectroscopy and XPS are the two major characterization techniques. UVvis absorption spectroscopy was utilized to gain further insight into the reduction method’s effects on GO. The UVvis spectra of GO exhibit an obvious characteristic feature that can be used as an identification tool: maximum absorption peak at about 230 nm, corresponding to π f π* transition of aromatic CC bonds. Figure 3 shows UVvis absorption spectra of GO and RGO obtained by different methods in solution. Compared with GO, the maximum absorption peaks 11329



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Figure 3. (A) UVvis absorption spectra of GO and different methods reduced RGO. (B) Variation in the wavelength of the absorption maximum of GO (sample 1) and different methods reduced RGO (from sample 2 to 7: N2H4, NaOH, NaBH4, solvothermal, high-temperature, two-step).

of all of RGO had a clear trend of red shift. The maximum absorption peak of RGO is always >270 nm with the involvement of high temperature. When the reducing agent (e.g., hydrazine or sodium borohydride) was used, the maximum absorption peak of RGO located between 250 and 270 nm. While adjust the environment like pH, the maximum absorption peak showed little red shift (240250 nm). This phenomenon of red shift has also been previously reported when GO being reduced144,145 and used as a monitoring tool for the reduction of GO.52 UVvis absorption spectroscopy gives us an average reflection of reduction degree of RGO by different reduction methods. To further reveal the details, XPS was employed to analyze the GO and RGO. The C(1s) XPS spectra of the GO and RGO are shown in Figure 4. The binding energy of 284.6 eV is attributed to the CC bonds, and the ones of 286.5 and 288.5 eV are typically assigned to the CO and CdO functional groups, respectively. Here the CO bonds include epoxide (O) and hydroxyl (OH); CdO bonds include carbonyl (CdO), carboxyl (COOH), and carboxylate (COOR). The C(1s) XPS spectra of RGO exhibit the same oxygen functionalities that have been assigned for GO, while some peak intensities of these components in RGO samples are much smaller than that in GO, indicating considerable deoxygenation by the reduction process. In addition, the relative C/O atomic ratios of GO and RGO were calculated from the XPS spectra (by using ratio of area of the C(1s) peak to area of the O(1s) peak) for a better comparison (see Figure 4H). The changes of C/O ratio of RGO indicate the different reduction degree. Notably, the high-temperature process can significantly increase the C/O ratio. In order to get clear analysis on reduction methods, the relative ratios of peak area of oxygen-containing bonds (CO and CdO bonds) to the CC bonds for GO and different methods reduced RGO are summarized in Table 1. (The rough error estimate of semiquantitative analysis of element contents by XPS characterization is about 10%.) The primary role of reducing agent (N2H4 or NaBH4) is turning CdO into CO. That is why the CO/CC ratio had increased after being reduced. While the solvothermal reduction can remove the CO function groups obviously. The hightemperature process is quite a powerful tool to remove most of the CdO groups on the GO flakes, which is possibly by forming CO or CO2.44 While the amount of CO being decreased or steady comparable with samples reduced by other methods, which indicated that CdO had not converted into CO. It has also confirmed by Akhavan’ s work,146 when the temperature

increased, the XPS peak of CdO would sharply decline, but the CO peak’s area had no significant reduction. On the basis of the above results, we can rationally design the reaction process for transforming the functional groups of GO to improve the reduction degree, for example, by adjusting the environment (e.g., pH) to convert some functional groups into carboxyl or hydroxyl groups and then using a reducing agent to further restore the structure. The third criterion is based on degree of defect repair. Up to now, there are only few reports on the restoration of GO.123,147149 Developing an effective method to qualitatively analyze the defect repair degree of RGO is an important challenge.150 Raman spectroscopy plays a significant role in characterization of graphitization. In our systemic comparison, as shown in Figure 5A, almost all the D bands of RGO reduced by various methods show obvious enhancement compared to pristine graphene. Relative to GO, the D band of RGO did not show monotonic change and no obvious Raman shift in our comparison. The G band of GO and wet-chemically reduced RGO usually red shift relative to graphite (1580 cm1). While G band of RGO obtained by high-temperature-related (high-temperature and two-step) method has smaller Raman shift. However, how to quantitatively analyze the Raman spectra of RGO is still a problem. The ratio between the intensity of D and G bands has been used as a parameter on single GO monolayer before and after chemical reduction, but it does not always reflect the oxidation or reduction degree, because the D/G ratio can be influenced by edges, charge puddles, ripples, or many other defects.151 For instance, Ruoff has reported that reduced GO has an increased D/G ratio than GO.42 Marko reported that reduced GO did not show any noticeable change in D/G ratio.1 In the case of hydrothermal reduction, the D/G ratio varied with the reduction time and temperature.86 That is why it is questionable for now to use D/G ratio as a measure of the degree of oxidation or reduction. On the contrary, XPS is a more powerful technique to monitor the chemical changes of graphene oxide sheets in a reducing process than Raman spectroscopy has been realized gradually.146,152 The D/G ratio is a measure of the disorder in graphene,86,153 but how to quantify it is still a problem. Here we borrow the D/G ratio of single-layer graphene prepared by mechanical exfoliation of graphite (about 1.0, HOPG is about zero) as a reference to solve this problem. In our systemic comparison, as shown in Figure 5B, the D/G ratio has no obvious variation compared to initial GO. When the reference value of 1.0 was 11330



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Figure 4. (AG) XPS spectra of GO (A) and different methods reduced RGO (from B to G: N2H4, NaOH, NaBH4, solvothermal, high-temperature, two-step). (H) Values of C/O atomic ratios obtained by the XPS analysis for GO (sample 1) and different methods reduced RGO (from sample 2 to 7: N2H4, NaOH, NaBH4, solvothermal, high-temperature, two-step).

Table 1. Peak Area Ratios (Error of 10%) of the OxygenContaining Bonds to the CC Bonds for GO and Different Methods Reduced RGO high GO N2H4 NaOH NaBH4 solvothermal temperature two-step CO 0.31 0.39

0.28

0.45

0.17

0.36

0.30

CdO 0.23 0.08

0.13

0.16

0.26

0.01

0.05

used, we can find that the ID/IG ratios of GO and RGO are increasing, which means that the GO or RGO reduced by all present methods contains more defects. RGO reduced by NaOH and two-step method occupy lower ID/IG ratio that indicates less defects containing on the RGO. While for the case of GO treated by high-temperature and two-step method, they all contain a high temperature process, the two-step method is an exception with

relative lower ID/IG ratio. The explanation is possibly due to the two-step method containing defect repair process, which would be helpful for decreasing defects amount. It has been reported that using thermochemical nanolithography (TCNL) to converse the GO’ s sp3 carbon bonds into sp2 carbon bonds makes the RGO more conductive.148 Current reduction methods are most focused on the loss of oxygen-functional groups from the GO flake surface; the process of defect repair is usually ignored. Using the value of ID/IG, we can coarsely evaluate one reduction method by the rule: the lower ID/IG value, the better defect repairs. Applied in the specific case, one reduction method can be qualified as good or not in defect repair degree. And further, the ID/IG value can also direct how to modify reaction conditions in order to obtain well reduced GO with high quality. The last criterion is the electrical conductivity, which reflects the reduction degree and defect repair degree more directly. 11331



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Figure 5. (A) Raman spectra of GO and different methods reduced RGO. (B) D/G intensity ratios of GO (sample 1) and different methods reduced RGO (from sample 2 to 7: N2H4, NaOH, NaBH4, solvothermal, high-temperature, two-step).

Table 2. Electrical Conductivity Dataa of GO and Different Methods Reduced RGO 1

electrical conductivity [S m ] a

GO

N2H4

NaOH

NaBH4

solvothermal

high temperature

two-step

insulator

156.2

3.6

0.006

4.8

232.1

267.8

A range of less than 1 order of magnitude.

Electrical conductivity is mainly tested by preparing RGO films or making devices. Very recently, plenty of papers92,154156 have summarized the conductivity of RGO obtained by various methods. However, during the layer effect, area-selecting, measurement method, test equipment, and other random operations can strongly influence the final signal; conductivity data provided by different measurement methods vary from each other in orders of magnitude with the same reduction method. Here we use the vacuum filtration method to form RGO films and use the same condition in the electrical conductivity test to eliminate the systematic errors. The average values of different method reduced RGO are listed in Table 2. We can conclude that all the electrical conductivity has been improved compared with GO after performing reduction in various conditions. For wet chemical reduction, hydrazine hydrate is much better than sodium borohydride as the reducing agents. The high-temperature process gives greater impact on the electrical conductivity than wet chemical treatment, implying better graphitization on GO sheets. According to the XPS peak-fitting data and electrical conductivity, we found that removing some of the CdO groups could help to enhance the electrical conductivity. Possibly, the degree of destroying the structure of graphene depends on types of oxygen-containing groups (e.g., CdO and OH). Moreover, the defect repair process can further improve the electrical conductivity. This might be the reason why present reduction methods for GO films are always followed by an additional annealing step. On the basis of the above results, we can see currently used reduction methods have their own advantages in one or two aspects, but not in all four aspects mentioned before. For instance, the high-temperature process can increase the maximum UVvis absorption peak and the values of C/O atomic ratios, but it can also increase D/G intensity ratios and the agglomeration, indicating higher reduction degree, but simultaneous higher defect degree and weak dispersibility. The reducing agents play a major role in the transformation of oxygen-containing functional groups, while it does not mean a higher reduction degree. The environmental factors like pH change only take

obvious effect on the restoration of GO. All these showed that evaluation for reduction methods should be done by combining all the four aspects. Naturally, an ideal reduction process is a course of removing oxygen containing functional groups and providing repair to obtain the nearly pristine graphene.

’ CONCLUSION In summary, systematic studies were carried out on evaluation the reduction method of GO by several factors including dispersibility, reduction degree, defect repair degree, and electrical conductivity. On the basis of the systemic comparison for the reduction methods, we provide a systemic method for half quanlitively judging the reduction method. The exfoliation/ aggregation degree of RGOs can be reflected from AFM, which associated with their dispersion stability. Combining XPS with absorbance and Raman spectra, information about reduction degree and defect can be gained. The electrical conductivity test would be the best and final choice to assess the quality of RGO. An ideal reduction method is not only to remove the oxygencontaining functional groups but also to repair the defect to obtain high-quality RGO. Under this evaluation criterion, the two-step method appears the best one for better RGO synthesis in the six reduction methods compared. RGO reduced by the two-step method has better reduction degree, defect repair degree and electrical conductivity, but the relatively weak dispersibility and tedious preparation process still need to improve. In practice, the reduction degree and the defect repair degree are the two key factors for effective reduction of GO. The combination of these two factors will thus give the criterion to choose the best reduction method. In addition, we can rationally design the reaction process for transforming the functional groups of GO before reduction or repair and also helped to develop one-pot synthesis of RGO according to XPS and Raman data. The systemic comparison would also be helpful to understand the mechanism of reduction further and then help to select or develop more effective reduction method. 11332



’ AUTHOR INFORMATION Corresponding Author

*Phone: þ86-10-64448751. E-mail: [email protected] (J.L.) and [email protected] (X.S.).

’ ACKNOWLEDGMENT This work was supported by NSFC, the Foundation for Authors of National Excellent Doctoral Dissertations of P. R. China, the Program for New Century Excellent Talents in University, the 973 Program (2011CBA00503 and 2011CB932403), and Beijing Natural Science Foundation (2102033). ’ REFERENCES (1) Gomez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Nano Lett. 2007, 7, 3499–3503. (2) Gilje, S.; Han, S.; Wang, M.; Wang, K. L.; Kaner, R. B. Nano Lett. 2007, 7, 3394–3398. (3) Geim, A. K.; Novoselov, K. S. Nature Mater. 2007, 6, 183–191. (4) Liang, X.; Fu, Z.; Chou, S. Y. Nano Lett. 2007, 7, 3840–3844. (5) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666–669. (6) Watcharotone, S.; Dikin, D. A.; Stankovich, S.; Piner, R.; Jung, I.; Dommett, G. H. B.; Evmenenko, G.; Wu, S.-E.; Chen, S.-F.; Liu, C.-P.; Nguyen, S. T.; Ruoff, R. S. Nano Lett. 2007, 7, 1888–1892. (7) Eda, G.; Fanchini, G.; Chhowalla, M. Nature Nanotechnol. 2008, 3, 270–274. (8) Blake, P.; Brimicombe, P. D.; Nair, R. R.; Booth, T. J.; Jiang, D.; Schedin, F.; Ponomarenko, L. A.; Morozov, S. V.; Gleeson, H. F.; Hill, E. W.; Geim, A. K.; Novoselov, K. S. Nano Lett. 2008, 8, 1704–1708. (9) Zhou, X.; Wei, Y.; He, Q.; Boey, F.; Zhang, Q.; Zhang, H. Chem. Commun. 2010, 46, 6974–6976. (10) Li, B.; Cao, X.; Ong, H. G.; Cheah, J. W.; Zhou, X.; Yin, Z.; Li, H.; Wang, J.; Boey, F.; Huang, W.; Zhang, H. Adv. Mater. 2010, 22, 3058–3061. (11) Liu, J.; Yin, Z.; Cao, X.; Zhao, F.; Lin, A.; Xie, L.; Fan, Q.; Boey, F.; Zhang, H.; Huang, W. ACS Nano 2010, 4, 3987–3992. (12) Yin, Z.; Sun, S.; Salim, T.; Wu, S.; Huang, X.; He, Q.; Lam, Y. M.; Zhang, H. ACS Nano 2010, 4, 5263–5268. (13) He, Q.; Sudibya, H. G.; Yin, Z.; Wu, S.; Li, H.; Boey, F.; Huang, W.; Chen, P.; Zhang, H. ACS Nano 2010, 4, 3201–3208. (14) Yin, Z.; Wu, S.; Zhou, X.; Huang, X.; Zhang, Q.; Boey, F.; Zhang, H. Small 2010, 6, 307–312. (15) Wang, Z.; Zhou, X.; Zhang, J.; Boey, F.; Zhang, H. J. Phys. Chem. C 2009, 113, 14071–14075. (16) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Nature Mater. 2007, 6, 652–655. (17) Robinson, J. T.; Perkins, F. K.; Snow, E. S.; Wei, Z.; Sheehan, P. E. Nano Lett. 2008, 8, 3137–3140. (18) Arsat, R.; Breedon, M.; Shafiei, M.; Spizziri, P. G.; Gilje, S.; Kaner, R. B.; Kalantar-Zadeh, K.; Wlodarski, W. Chem. Phys. Lett. 2009, 467, 344–347. (19) Agarwal, S.; Zhou, X.; Ye, F.; He, Q.; Chen, G. C. K.; Soo, J.; Boey, F.; Zhang, H.; Chen, P. Langmuir 2010, 26, 2244–2247. (20) Liu, Z.; Liu, Q.; Huang, Y.; Ma, Y.; Yin, S.; Zhang, X.; Sun, W.; Chen, Y. Adv. Mater. 2008, 20, 3924–3930. (21) Yang, X.; Zhang, X.; Liu, Z.; Ma, Y.; Huang, Y.; Chen, Y. J. Phys. Chem. C 2008, 112, 17554–17558. (22) Liang, J.; Xu, Y.; Huang, Y.; Zhang, L.; Wang, Y.; Ma, Y.; Li, F.; Guo, T.; Chen, Y. J. Phys. Chem. C 2009, 113, 9921–9927. (23) Ghosh, A.; Subrahmanyam, K. S.; Krishna, K. S.; Datta, S.; Govindaraj, A.; Pati, S. K.; Rao, C. N. R. J. Phys. Chem. C 2008, 112, 15704–15707.

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