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The Importance of Interbands on the Interpretation of the Raman Spectrum of Graphene Oxide Sergi Claramunt,†,‡ Aïda Varea,*,† David López-Díaz,§ M. Mercedes Velázquez,§ Albert Cornet,† and Albert Cirera† †

MIND/IN2UB, Departament d’Electrònica, Facultat de Física, Universitat de Barcelona, 08028 Barcelona, Spain Departamento de Química Física, Facultad de Ciencias Químicas, Universidad de Salamanca, E37008 Salamanca, Spain

§

S Supporting Information *

ABSTRACT: Raman spectra of graphene oxide and thermally reduced graphene oxide were analyzed in order to relate spectral parameters with the structural properties. The chemical composition of different graphene oxides was determined by organic elemental analysis, and the microstructure of nanocrystals was analyzed by X-ray diffraction. We find five reported bands (D, D′, G, D″, and D*) in the region between 1000 and 1800 cm−1 in all spectra. The band parameters such as position, intensity ratio, and width have been related with structural properties such as oxygen content, crystallinity, and disorder degree of GO and rGO platelets. An assessment of the validity of the Tuinstra−Koenig and Cuesta models was carried out by using the results obtained from the fit of the first-order spectra of graphene oxide derivatives at five functions: two Gaussian and three pseudoVoigt peaks.

1. INTRODUCTION Since it was discovered by A. Geim and K. Novoselov,1 graphene has been widely studied as a potential material to be implemented in advanced technological applications. Due to its amazing properties, it has been suggested as a promising candidate for fabrication of transparent conducting electrodes,2 transistors,3 hydrogen storage devices,4 and gas sensors.5 However, each application requires a different set of properties. Thus, graphene synthesized by chemical vapor deposition (CVD) or micromechanical exfoliation provides high quality materials suitable for electronic applications, while they are unsuitable for fabrication of inks or composites because they do not present functionalized groups. In these cases, chemical graphene6 is preferred, because it contains a range of reactive oxygen functional groups (O-groups) that can attach polymers or nanoparticles on the graphitic surfaces for potential use in polymer composites,7 gas sensor,8 or photovoltaic applications.9 Another advantage of chemical graphene is that it can be fabricated in large scale and in a low-cost way. The first synthesis of graphene oxide (GO) was reported by B. C. Brodie10 and further modified by L. Staudenmaier11 and W. S. Hummers.12 These authors proposed the chemical structure of GO as a carbon compound where oxygen functional groups are attached at carbon atoms within the hexagonal plane.13 According to their proposed structure, GO consists of two main regions constituted by hydrophobic πconjugated C-sp2 and the C-sp3 domains. The latter are mainly constituted by alcohol and epoxy groups located at the basal plane and carboxylic acids groups at the structure edges.14 The electronic and mechanical properties of GO can be efficiently © 2015 American Chemical Society

modified by controlling the oxygen functionalities attached to its carbon structure, thus, GO is often reduced by chemical agents15−18 or thermal annealing,19 obtaining reduced graphene oxide (rGO). The reduction process by using reducing agents such as hydrazine or vitamin C often introduces functional groups such as amino nitrogen atoms attached to the sheets in the case of reduction by hydrazine18,20 and some O-groups in the case of chemical reduction by vitamin C.16,18 These functional groups bonded to basal planes and edges of carbon structures make the GO an electrical insulator.21 Hence, to study the evolution of the chemical composition of GO and its reduction products in a systematic way, chemical reduction must be avoided. Alternatively, GO of different oxidation grade can be obtained by thermal reduction. In this sense, thermal reduction seems to be an excellent process for industrial applications because it is a cheap and scalable method; however, the mechanism of thermal reduction is a complex process which is still an object of discussion.13 On the other hand, chemical and thermal reductions introduce defects on the network which also modify and determine the physical and chemical properties of graphene-based materials. Then, taking into account the important role of defects in the properties of these materials, it becomes necessary to develop an accurate methodology to study them. Raman spectroscopy is one of the most powerful techniques for this purpose and has been widely used to study graphite and graphene.22 Results showed that Received: February 16, 2015 Revised: April 13, 2015 Published: April 13, 2015 10123

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The Journal of Physical Chemistry C while the Raman peaks in crystalline graphene are the G (∼1585 cm−1) and the 2D bands (∼2700 cm−1), associated with the first- and second-order allowed Raman mode E2g, respectively,23 changes in the width of these bands are observed in the spectrum of the GO.24 These changes were attributed to high defect concentration related to the oxidation and are accompanied by the appearance of an intense peak centered at ∼1350 cm−1, called the D band. The D band is related to the A1g breathing mode, and it is observed because graphite oxidation and the subsequent reduction of GO seriously alter the basal plane structure of graphene.25 Also, some weak peaks centered between 1100 and 1800 cm−1 were observed in GO flakes and powders,18,26,27 while they have not been detected in pristine graphene or graphite. Theoretical calculations have estimated Raman modes for graphene with different kinds of defects such as rings with different numbers of C atoms and configurations of C−O bonds.25,28 In these cases, a peak centered at ∼1620 cm−1, D′ band, has been attributed to the disorder-induced phonon mode due to crystal defects29 although other authors have attributed this band to double vacancy corresponding to pentagonal and octagonal rings, usually referred as 5−8−5 defects.30 Furthermore, recent works reported a small peak located between 1150 and 1200 cm−1 (D*) and a broad peak between 1500 and 1550 cm−1 (referred to here and in another work as D″,31 but also referred to as D332−34 or T2 bands35), indicating the need of two extra peaks to better fit the 1000−1800 cm−1 spectral region. In fact, some examples of these bands can be found in the Raman spectral analysis of carbon-based materials such as soot and carbon black.32,33,36−38 Previous work related the D* with disordered graphitic lattice provided by sp2−sp3 bonds at the edges of networks,32 while the D″ band allocation is controversial. Some authors attributed the D″ band to contributions from the phonon density of states in finite-size crystals of graphite.39 S. Vollebregt et al. related the band to amorphous lattices since they observed a decrease of the intensity of the D″ peak with the increase of the crystallinity.35 A similar ascription is the endorsement of the D″ peak to interstitial defects associated with amorphous sp2-bonded forms that may include functionalized small molecules32,34,36 while R. Saito et al. assigned this peak to a double resonance process with the iTO phonon branch.40 Thus, the aim of this work is to study the correlation between the structure and the first-order Raman spectra of the graphene oxide and reduced graphene oxides with different reduction grade. Therefore, we analyze the evolution of the Raman spectrum of graphene oxides obtained by thermal annealing using the temperature range between 100 and 800 °C under vacuum pressure. Because we are interested in maintaining a certain grade of structural order inside the network, annealing temperatures beyond 800 °C were avoided, since it is well established that thermal annealing at temperatures above 800 °C cause considerable damage to the network structure. To minimize heterogeneity effects, we have selected GANF carbon nanofibers as starting material because they render GO sheets more homogeneous than those obtained by graphite oxidation.27,41 The oxygen content and the microstructure of different-grade thermally reduced samples have been correlated with 5-peak analysis of Raman spectra.

Eight samples with different reduction grades were prepared by thermal annealing. The thermal reduction of GO powder was made in a tubular furnace under vacuum conditions (p < 10−5 mbar) and annealing temperatures (Tann) ranging from 100 to 800 °C. To obtain reduced GO at a given temperature, 10.0 ± 0.1 mg of GO powder was introduced in a crucible and heated at 5 °C/min to reach the selected temperature. Then, samples were cooled down to room temperature after being kept at Tann for 1 h to guarantee homogeneous and reproducible results. Oxygen content was determined by organic elemental analysis (OEA) with a Thermo Flash 2000 analyzer. The microstructure of the samples was studied by X-ray diffraction (XRD). X-ray diffraction patterns were recorded on PANalytical X’Pert PRO MPD diffractometer using the Cu Kα radiation (1.540598 Å) in the 5−50° range (0.026° step size, 100 s holding time). The operating tube voltage and current were 45 kV and 40 mA, respectively. Interlayer spacing, d002, values were obtained using Bragg’s law for (002) reflection. The crystallite size along basal planes, La, was obtained from the (100) reflection using Scherrer’s equation. Raman measurements were performed in a Jobin Yvon HR800 LabRam using laser excitation wavelength of 532 nm with a 50× objective at room temperature. We chose 532 nm as excitation wavelength to guarantee a good signal/noise ratio and because our samples do not present fluorescence emission. Laser power was maintained at 0.5 mW in all cases to avoid laser-induced heating. To ensure the reliability of the results, five Raman spectra were acquired and analyzed for each GO and rGO sample. Prior to the data analysis, the baseline of the spectrum was extracted using the same software used for the data acquisition (NGSLabSpec). All spectra were corrected using the Si band at 520 cm−1. Experimental data were fitted to a sum of five functions using the Origin 8.0 software. Lorentzian, Gaussian, pseudo-Voigt and Breit-Wigner-Fano functions are widely used to fit the Raman spectra of carbonaceous and graphene-based materials, with any wellestablished rule to choose one particular function.28,31,32,42,43 During the analysis it was found that Gaussian functions better fit D* and D″ bands while, for D, G and D′ bands as pseudoVoigt functions render the best fit. Thus, two Gaussian functions and three pseudo-Voigt functions have been used to suit the Raman spectra in the range of 1000−1800 cm−1.

3. RESULTS AND DISCUSSION 3.1. Structural Characterization of Graphene Oxide Powders. Figure 1a shows the XRD patterns in the 5° to 50° region for GO and the annealed samples (rGO) from 100 to 800 °C. Detailed plots of the XRD patterns are collected in Figure S1 of the Supporting Information and allow observing details of the XRD peaks. From our results it is possible to conclude that the temperature dependence of the XRD spectrum is quite similar to that observed for GO obtained by graphite oxidation. Consequently, two peaks corresponding to (100) and (002) reflections can be identified. The (100) peak located at 2θ = ∼43° and related to basal-spacing (La) is not notoriously shifted in angle while changes in width can be detected. Moreover, the major feature is the presence of the (002) peak that corresponds to interlayer distance (d002) between sp2 carbon layers. This reflection greatly varied across the different annealed samples yielding two kinds of (002) peaks.44 The first one, referred to as peak I, is centered at 2θ = ∼12° and is observed in nonannealed GO and rGO annealed at 100 °C. The d002 distance values calculated from the 2θ

2. EXPERIMENTAL DETAILS GO powder was synthesized from carbon nanofibers (GANF) using a modified Hummers method previously reported.27 10124

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in the literature as peak III, has been related with amorphouslike carbon comprising impurities, defects, or folding wrinkles.44 In our XRD spectra, the large width of peak III did not allow observing peak II. This peak (II) centered at 22−23°, Figure 1a, was observed in the XRD spectrum of GO synthesized from graphite44 and in our samples could be related with oxidative impurities,27 nonoxidized nanofibers, and different kinds of defects. Our results show that peak III appears when the annealing temperature increases above 200 °C and it gradually moves to the right with the increase of the annealing temperature until it reaches a position close to that corresponding to pristine graphene (d002 ∼ 0.34 nm)15 at 800 °C. As can be observed in Figure 1b, the most accentuated decrease of d002 distance (circles) is located between 100 and 200 °C; this behavior was previously observed for GO synthesized by graphite oxidation and attributed to the drastic evaporation of water intercalated between sheets.44 Figure 1b also shows the variation of the peak width (squares) with the annealing temperature. Results show that peak III is broader than peak I and the width of peak III decreases when the annealing temperature increases. Both results are similar to data published for GO obtained from graphite, and they are related to an increase of crystallinity during the thermal annealing.44 To analyze the dependence of d002 with the chemical composition we plot the d002 distance values against the oxygen amount measured by OEA in Figure 1c. Results show that the interlayer distance decreases when the O-groups attached to sheets are removed and the most accentuated decrease of the d002 distance is related to the greatest loss of O-groups of the GO. The inset of Figure 1c shows a detailed magnification for low oxygen content values; a continuous decrease of d002 is revealed as the oxygen content decreases. 3.2. Raman Spectra. After microstructural characterization, the Raman spectra of GO and annealed GO were recorded (Figure 2a). At first glance no significant differences are found between them. All the spectra present D and G peaks centered at ∼1350 cm−1 and ∼1585 cm−1 respectively, and the splitting of 2D Raman-active bands centered at ∼2900 cm−1. Recent reports show that the 2D band of graphene with high disorder degree reduces its intensity and is replaced by a bump.24 The Raman spectra of our samples agree very well with these findings. However, an important feature is the presence of a broad shoulder between D and G peaks. The Raman spectral analysis of graphene derivatives often neglected this shoulder because it is very weak;48,49 nevertheless, it has been described in the Raman spectra of some carbon-based materials.32,36,37 Some authors fitted this shoulder using five functions which were ascribed to G, D, and D′ bands and two poorly understood peaks, referred to as D* (∼1150−1200 cm−1)32 and D″ (∼1500−1550 cm−1).32,34−36,39,40 Accordingly, we have fitted Raman spectra of GO and the different rGO to five functions: three pseudo-Voigt and two Gaussian. Figure 2b shows an example for the Raman spectrum of rGO annealed at 700 °C. Section 2 of the Supporting Information presents details of fittings for the rest of the Raman spectra as well as the band parameters obtained from the fit process (see Figure S2 and Table S1 of the Supporting Information). Our results show good agreement between the experimental spectrum and that calculated as a sum of the five proposed functions.28 The band position of both D″ and D* bands against the oxygen content is plotted in Figure 3a. The position of the D″ and D* peaks is shifted to lower and higher wavenumber

Figure 1. (a) XRD patterns for GO and thermally reduced rGO. (b) Dependence of d002 (circles) and FWHM002 (squares) with the annealing temperature (Tann). (c) Decrease of interplanar distance (d002), as a function of the oxygen content (%). Error in d002 values represents the standard deviation determined from at least 5 measurements and is less than 3%. The error value of the oxygen content was of 1%.

position by using Bragg’s law were 0.77 and 0.67 nm, for GO at room temperature and annealed at 100 °C, respectively. The interlayer distance value of 0.77 nm is quite similar to that found for GO samples prepared by oxidation of graphite,45 nanotubes,46 and nanofibers.47 The change on the interlayer distance after annealing at 100 °C is attributed to a mild vaporization of intercalated H2O molecules. A difference with respect to XRD measurements carried out in GO powders obtained by graphite is the presence of a broad band centered at 2θ between 24° and 25°. The width of this peak, referred to 10125

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Figure 2. (a) Raman spectra for all the samples analyzed in this work. For the sake of clarity, spectra have been vertically displaced. (b) An example of five functions (D*, D, D″, G, and D′ bands) deconvolution for the rGO obtained at 700 °C.

values, respectively, when oxygen content decreases. Therefore, these behaviors suggest that it is possible to consider the position of these bands as a good parameter to estimate the reduction degree of GO. In an attempt to clarify the role of the D″ band we have considered the interpretation given by Vollebregt et al. about the origin of the band.35 According to this interpretation, the D″ band is related to amorphous phases since its intensity decreases with the increase of the crystallinity. To analyze this point, the D″ band intensity normalized to the intensity of the G band is related to the La value as indicator of the crystallinity of the GO and rGO samples, Figure 3b (see more details in Figure S3 of the Supporting Information). As can be clearly seen in Figure S3 of the Supporting Information, two different trends can be observed. For nonannealed GO and for annealed rGO at 100 °C, the ID″/IG ratio is almost independent of the sample and the averaged value calculated was 1.1. This is the highest value found in this work. Conversely, for samples annealed above 200 °C (Figure 3b and Figure S3 in the Supporting Information), the ID″/IG ratio decreases as the crystallinity increases. We have also represented the variation of the FWHM of the D″ band with the crystallinity in Figure 3b. Results show a similar trend to that observed for the ID″/IG value, i.e., the FWMH scales very well with crystallinity of rGOs. According to our results, it is not possible to compare results of samples annealed above and below 200 °C. This can be due to the influence of water confined between sheets on the nanocrystal structure under the annealing. Results reported in a recent work50 demonstrated that the presence of water between

Figure 3. (a) Dependence of the position of the D″ (squares) and D* (dots) bands with the oxygen content. (b) Variation of the ID″/IG values (triangles) and the FWHM of D″ band (triangles) versus the crystallite size of basal planes for GO and rGOs. (c) ID*/IG value evolution with the oxygen content (%). Error in peak position values represents the standard deviation determined from at least 5 measurements and is less than 1%. The error values for ID″/IG and FWHM are 1% and 2% respectively.

sheets during the annealing process leads to different products than those obtained when the annealing process is carried out in the absence of water. Our results are consistent with this behavior, since the annealing process drives to different structures when the water molecules remain between sheets, Tann < 200 °C, and when water molecules are completely removed from the intersheet space, Tann > 200 °C. Finally, results corresponding to samples annealed at temperature values above 200 °C, Figure 3b, allow us to demonstrate that the intensity and width of the D″ band can be related to the crystallinity of rGO, in agreement with the interpretation given by Vollebregt.35 The analysis of the intensity evolution of the D* band during the annealing process was also carried out. Figure 3c shows the 10126

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The Journal of Physical Chemistry C plot of the D* band intensity normalized to the intensity of the G band against the oxygen content. To interpret data in Figure 3c it is necessary to take into account that the D* band has been related to disordered graphitic lattice of soot provided by the existence of sp3 bonds.32 Accordingly, nonannealed GO presents an elevated number of these groups due to the Ogroups attached at the edges of sheets; as a consequence, the D* intensity is relatively high. Since the O-groups decrease when the annealing temperature increases, the intensity of the D* band decreases until it reaches a minimum value at the annealing temperature of 600 °C. Above 600 °C, once COOH groups have been mainly removed, the elimination of the hydroxyl molecules starts and an additional source of carbon becomes necessary to yield CO gas formation. If the carbon source comes from ordered aromatic structure,51 the cleavage of the network drives to the increase of polyene groups and, consequently, the ID*/IG ratio increases, as seems to happen in the Raman spectra of our rGOs. All results presented in our study demonstrate the importance of the analysis of the D−G interbands of the GO Raman spectrum to properly interpret the GO and rGO structure. It is well established that the relative intensity of the D band with respect to the G band (ID/IG) is a good parameter to estimate the degree of defects in the graphene network and has been related to the inverse of the crystallite size on basal planes (1/La) through the Tuinstra−Koenig model.48 However, even though the model acceptably interprets results for graphene obtained by micromechanical exfoliation25 and graphitic materials,49 in the case of thermal and chemical rGO the ID/ IG ratio does not present significant changes either with the annealing temperature between 100 and 800 °C42 or with the reducing agent.18,27,41,52 We think that this fact can be due to the influence of the D″ and D* bands on the relative intensity of G and D bands, since they were not taken into account in the previous works. To confirm this assumption we analyze the ID/IG values obtained by the raw Raman spectra and results obtained by fitting the Raman spectra to five Lorentzian− Gaussian functions according to the Tuinstra−Koenig model. Figure 4a shows plots of the inverse of the crystallite size along basal planes (1/La) determined from the XRD measurements and Scherrer’s equation against the ID/IG ratio. According to the Tuinstra−Koenig model, the ID/IG ratio presents a linear dependence of 1/La; calculating the ID/IG ratio values from the raw spectrum, they showed a vertical tendency instead of the linear dependence predicted by the Tuinstra−Koenig model. In contrast, the ID/IG values calculated from the five-peak fitting show an almost linear dependence of 1/La, although the slope of the line is quite different from the value reported by Tuinstra−Koenig (line in Figure 4a). Recently, Ferrari et al.28 reported that the Tuinstra−Koenig correlation is not valid at small values of La (≤2 nm); however, this is not the situation for our samples because the smallest La value found was 3.11 nm. We think that deviation observed in our case could be attributed to the high disorder degree of the GO and rGO sheets. In this situation the Cuesta model proposed to interpret Raman measurements of disordered carbon-based materials53 could be preferred. To confirm this issue we plot in Figure 4b the variation of 1/La with the ID/(ID + IG) ratio. The solid line in Figure 4b is calculated from the Cuesta model. Experimental ID/(ID + IG) values calculated from the raw spectra cannot be interpreted from the Cuesta model, however they agree very well when calculated from the five-peak-fit values.

Figure 4. (a) Variation of 1/La with the ID/IG ratio. The dotted and dashed lines are a visual guide, and the solid line is calculated according to the Tuinstra−Koenig model.48 In all figures, triangles represent results obtained from the raw spectrum while squares correspond to data calculated using five-peak functions, see text. (b) Plot of 1/La against ID/(IG + ID) ratio; the line is calculated from the Cuesta model (line).53 (c) Variation of interlayer distance calculated from XRD with ID/(IG + ID) ratio. The gray line is represented according to the Cuesta empirical relationship.53 Error in d002 values represents the standard deviation determined from at least 5 measurements and is less than 3%; for 1/La, error values are less than 4%. The error values for ID/IG and ID/(ID + IG) are less than 5%.

We also represent our data in terms of the second Cuesta relationship (Figure 4c), which correlates the d002 values with the defect percentage expressed as the ID/(ID + IG) ratio. The shadow region in Figure 4c represents the ID/(ID + IG) values obtained for disordered carbon samples according to the model, and the solid line is also calculated from the Cuesta model. As observed in the 1/La dependence of ID/(ID + IG), the 10127

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intensity ratio values calculated from the experimental spectra fitted to five peaks agree better than the raw spectral data with values calculated from the model.

REFERENCES

(1) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (2) Wang, X.; Zhi, L.; Mullen, K. Transparent, Conductive Graphene Electrodes for Dye-Sensitized Solar Cells. Nano Lett. 2007, 8, 323− 327. (3) Wang, X.; Ouyang, Y.; Li, X.; Wang, H.; Guo, J.; Dai, H. RoomTemperature All-Semiconducting Sub-10-nm Graphene Nanoribbon Field-Effect Transistors. Phys. Rev. Lett. 2008, 100, 206803-1−2068034 DOI: 10.1103/PhysRevLett.100.206803. (4) Dimitrakakis, G. K.; Tylianakis, E.; Froudakis, G. E. Pillared Graphene: A New 3-D Network Nanostructure for Enhanced Hydrogen Storage. Nano Lett. 2008, 8, 3166−3170. (5) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Detection of Individual Gas Molecules Adsorbed on Graphene. Nat. Mater. 2007, 6, 652−655. (6) Park, S.; Ruoff, R. S. Chemical Methods for the Production of Graphenes. Nat. Nanotechnol. 2009, 4, 217−224. (7) Potts, J. R.; Dreyer, D. R.; Bielawski, C. W.; Ruoff, R. S. Graphene-Based Polymer Nanocomposites. Polymer 2011, 52, 5−25. (8) Prezioso, S.; Perrozzi, F.; Giancaterini, L.; Cantalini, C.; Treossi, E.; Palermo, V.; Nardone, M.; Santucci, S.; Ottaviano, L. Graphene Oxide as a Practical Solution to High Sensitivity Gas Sensing. J. Phys. Chem. C 2013, 117, 10683−10690. (9) Eda, G.; Chhowalla, M. Chemically Derived Graphene Oxide: Towards Large-Area Thin-Film Electronics and Optoelectronics. Adv. Mater. 2010, 22, 2392−2415. (10) Brodie, B. C. On the Atomic Weight of Graphite. Philos. Trans. R. Soc. London 1859, 149, 249−259. (11) Staudenmaier, L. Verfahren zur Darstellung der Graphitsäure. Ber. Dtsch. Chem. Ges. 1898, 31, 1481−1487. (12) Hummers, W. S., Jr.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339−1339. (13) Acik, M.; Lee, G.; Mattevi, C.; Pirkle, A.; Wallace, R. M.; Chhowalla, M.; Cho, K.; Chabal, Y. The Role of Oxygen during Thermal Reduction of Graphene Oxide Studied by Infrared Absorption Spectroscopy. J. Phys. Chem. C 2011, 115, 19761−19781. (14) Lerf, A.; He, H.; Forster, M.; Klinowski, J. Structure of Graphite Oxide Revisited. J. Phys. Chem. B 1998, 102, 4477−4482. (15) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Synthesis of Graphene-Based Nanosheets Via Chemical Reduction of Exfoliated Graphite Oxide. Carbon 2007, 45, 1558−1565. (16) Fernández-Merino, M. J.; Guardia, L.; Paredes, J. I.; Villar-Rodil, S.; Solís-Fernández, P.; Martínez-Alonso, A.; Tascón, J. M. D. Vitamin C Is an Ideal Substitute for Hydrazine in the Reduction of Graphene Oxide Suspensions. J. Phys. Chem. C 2010, 114, 6426−6432. (17) Paredes, J. I.; Villar-Rodil, S.; Fernandez-Merino, M. J.; Guardia, L.; Martinez-Alonso, A.; Tascon, J. M. D. Environmentally Friendly Approaches toward the Mass Production of Processable Graphene from Graphite Oxide. J. Mater. Chem. 2011, 21, 298−306. (18) Martín-García, B.; Velázquez, M. M.; Rossella, F.; Bellani, V.; Diez, E.; García Fierro, J. L.; Pérez-Hernández, J. A.; Hernández-Toro, J.; Claramunt, S.; Cirera, A. Functionalization of Reduced Graphite Oxide Sheets with a Zwitterionic Surfactant. ChemPhysChem 2012, 13, 3682−3690. (19) Bagri, A.; Mattevi, C.; Acik, M.; Chabal, Y. J.; Chhowalla, M.; Shenoy, V. B. Structural Evolution during the Reduction of Chemically Derived Graphene Oxide. Nat. Chem. 2010, 2, 581−587. (20) Panchakarla, L. S.; Subrahmanyam, K. S.; Saha, S. K.; Govindaraj, A.; Krishnamurthy, H. R.; Waghmare, U. V.; Rao, C. N. R. Synthesis, Structure, and Properties of Boron- and Nitrogen-Doped Graphene. Adv. Mater. 2009, 21, 4726−4730.

ASSOCIATED CONTENT

S Supporting Information *

Figure S1showing the XRD patterns for GO and rGO annealed at a temperature range from 100 to 800 °C. Figure S2 showing the deconvolution of Raman spectra in the 1000−1800 cm−1 region for all the samples analyzed in this work. Table S1 displaying the calculated parameters for the five used Raman bands. Figure S3 presenting the evolution of the ID″/IG versus the crystallite size of the basal planes for GO and rGOs. This material is available free of charge via the Internet at http:// pubs.acs.org.

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ABBREVIATIONS

GO, graphene oxide; rGO, reduced graphene oxide

4. CONCLUSIONS The spectral analysis by fitting the Raman spectra of different graphene oxides evidenced the existence of five Raman bands (D, D′, G, D″, and D*) previously reported for soot and functionalized graphene. Our results demonstrated that the peak positions of the D″ and D* bands exhibit a pronounced dependence of the oxygen content of graphene oxides and, therefore, can be used as good parameters to estimate the oxygen content. We also demonstrated that both the ID″/IG intensity ratio and the width of the D″ band decrease when the crystallinity of sheets increases, while the ID*/IG ratio decreases when the number of sp3 bonds of sheets decreases. We observe discrepancies between the experimental ID/IG values and those calculated by both the Tuinstra−Koenig and Cuesta models when the intensity ratio was calculated from the raw Raman spectra. However, the intensity ratio values obtained from spectra fitted to 5 peaks agree very well with those calculated according to the Cuesta model. Summarizing, our results demonstrated that the five reported bands have to be taken into account to properly interpret the Raman spectrum of graphene oxides. Moreover, the dependence proved in this work between the spectral parameters of the interbands, D″ and D*, and the structural properties, such as oxygen content, crystallinity, or disorder degree, can be used to evaluate these properties.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +34 (0)934 034 804. Present Address ‡

Electronic Engineering Department, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Spain.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank European Regional Development Fund, ERDF, Ministerio de Educación y Ciencia (MAT 2010-19727), and Centro para el Desarrollo Tecnológico e Industrial (IDI20111312) for financial support. Dr. Jawhari is acknowledged for helpful discussions. A. Cirera acknowledges support from ́ ICREA Academia program. A. Varea thanks Grup de Fisica de Materials II from Universitat Autònoma de Barcelona for the oven facility. Grupo Antoliń Ingenieriá is also acknowledged for the generous gift of graphene oxide. 10128

DOI: 10.1021/acs.jpcc.5b01590 J. Phys. Chem. C 2015, 119, 10123−10129

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DOI: 10.1021/acs.jpcc.5b01590 J. Phys. Chem. C 2015, 119, 10123−10129