Article pubs.acs.org/JPCC
Study on the Absorption Coefficient of Reduced Graphene Oxide Dispersion Rui Su, Shao Fen Lin, Dan Qing Chen, and Guo Hua Chen* Department of Polymer Science & Engineering Huaqiao University, Xiamen 361021, People’s Republic of China S Supporting Information *
ABSTRACT: Light absorption of graphene plays an important role in optoelectronic applications. In this work, a series of reduced graphene oxide (RGO) dispersions containing flakes with various configurations are prepared, and their optical absorption coefficients are investigated. Our results suggest that the lateral size distribution, the mean number of layers per flake and the functional groups on RGO are all important factors influencing the absorption coefficient. We find the dispersion with a larger amount of small flakes (≤600 nm), as well as less layers per flake, gives a smaller absorption coefficient at 660 nm. Essentially, functional groups grafted on graphene flakes promote an eminent role in the absorption coefficient.
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INTRODUCTION Graphene, a prototype two-dimensional and one-atomic carbon layer with a honeycomb structure, has shown unique optical properties1−3 and holds great promise in optoelectronic devices.4 Many ground-breaking experiments have been carried out to study optical properties of single- or multilayer graphene. Geim et al. reported that single layer graphene on an oxidized Si wafer absorbed 2.3% of white light, while the reflectivity was almost negligible ( 10), the reflectivity cannot be neglected, where the reflectivity of 10 layers of graphene was about 2%.8,9 Even though the optical absorption of monolayer or multilayer graphene on a solid substrate has been well confirmed, the optical absorption in liquid is much more intriguing. Coleman’s team determined the absorption coefficient of graphene dispersion using spectroscopy (UV/vis).10−14 Briefly, according to the Lambert−Beer law, A = αcl, absorption coefficient of graphene can be determined by preparing a serious of dispersions at given concentrations. While straightforward, different values of α have been found. For example, Coleman et al. reported a constant value of 2460 mL·mg−1·m−1 in different solvents.10 Later on, they expanded the values from 1390 to 6600 mL·mg−1·m−1.11−14 So far, the exact reason for such a wide range of α values has not been well understood. Herein, we describe four reduced graphene oxide (RGO) dispersions with different values of absorption coefficient in N-methyl-2-pyrrolidone (NMP). We reveal their absorption © 2014 American Chemical Society
coefficients depend on multiple factors including lateral size, the number of layers per flake, and the surface functional groups.
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EXPERIMENTAL SECTION Preparation of our samples started from synthesis of graphite oxide (GO). The GO was prepared according to the reported procedure15 from graphite powder (500 mesh, Sinopharm). Then, the obtained brown dispersion was washed with 10 wt % HCl and distilled water several times (assisted by vacuum filtration), followed by drying at 45 °C for 48 h. After that dried, GO was placed in a microwave oven (Midea, China) for 15 s at a power of 700 W to give us reduced graphite oxide. The above reduced graphite oxide was further ball-milled to yield reduced graphene oxide (RGO) in NMP (N-methyl-2pyrrolidone, content > 98%, Xilong) in a planetary ball-miller. We obtained four dispersions with different absorption coefficients by adjusting processing parameters. In a typical ball-milling, 75 mg of reduced graphite oxide and 30 mL NMP were placed in a polytetrafluoroethylene (PTFE) jar containing three types of zirconia balls (diameter of 0.4, 1.0, 2.0 mm, respectively). The jar was then fixed in our planetary ball-miller, and agitated at 40 rpm for 100 h. It is to be noted that, during the ball-milling the weight ratio of three sized balls was kept at 1:1:1 for S1. The resultant product was centrifuged at 6000 rpm for 10 min to give us a clear liquid on the top. Then the asobtained liquid was heated to 170 °C and washed with water repeatedly to remove physically adsorbed NMP. The other three samples were prepared in a similar fashion by varying processing parameters such as centrifugation rate, ball-milling Received: January 16, 2014 Revised: May 13, 2014 Published: May 20, 2014 12520
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Figure 1. (A) Absorbance at 660 nm as a function of concentration for the four samples, respectively. Note that the slopes of four lines represent different α values of the four samples. (B) The absorbance of the four samples between 200 nm ∼250 nm. Different peak positions of S1, S2, S3, and S4 at 230, 229, 231, and 234 nm indicate different areas of the aromatic system. (C) Photos of the four dispersion samples at a concentration of 0.2 mg/mL. From left to right, the color gradually deepens to black, suggesting light absorption gradually increases.
Figure 2. (A) TEM images of flakes in the four dispersion samples. The difference in lateral size of flakes has been observed. The specific lateral size distribution (B) is corresponding to each sample and the fractions of small flakes (≤600 nm) are 77, 62, 66, and 44% from S1 to S4, respectively. Moreover, an increase in stacking degree has also been observed from S1 to S4. In histogram of number of layers per flake (C), we find the mean numbers of layers per flake are 4.38, 4.96, 5.22, and 6.20 from S1 to S4.
time and proportion of three sized balls. Particularly, S2 was ball-milled for the same period of time and with the same weight ratio of balls as S1, but was centrifuged at a slower speed (4000 rpm) for 10 min. S3 was ball-milled for 48 h, and other
parameters are the same as for S1. S4 was ball-milled with different weight ratio of balls, where the ratio was kept at 2:1:1. In addition, the ball-milling time for S4 was 100 h, the centrifugation speed was 8000 rpm and the centrifugation time was 10 min. 12521
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Table 1. Fraction of Flakes (Length ≤ 500 nm), Fraction of Flakes (width ≤ 500 nm), and Mean Number of Layers per Flake, as Well as the α Values of the Two Samples in the Coleman Team’s Work12 samples S5 S6
a
Lamber-Beer law, as mentioned in the Introduction. As shown in Figure 1A, the value of α at 660 nm increases from sample 1 (S1) to sample 4 (S4). For each given sample, the absorbance increases linearly with increasing concentration, indicating that RGO dispersion follows the Lambert−Beer law. The approximate α values of S1, S2, S3 and S4, determined by the slopes of the four lines, are 2.67 × 106, 4.82 × 106, 5.04 × 106, and 6.72 × 106 mL·mg−1·m−1, respectively. These α values are almost 3 orders of magnitude compared to Coleman’s work.10 In addition, it is clear to the naked eye that those samples of the same concentration have different light absorptions, as shown in Figure 1C. Maximum absorption shows around 230 nm, with a slightly different profile for these four samples, as shown in Figure 1B. (Herein, the RGO was dispersed in water to avoid solvent effect of NMP.) These peaks are attributed to the π−π* transitions of the aromatic CC bonds. The shift of peak position reflects the area of an aromatic system.17 Obviously, S4 has a larger area of aromatic system compared with other three samples. The first factor to be investigated and interpreted for absorption coefficient is lateral size distribution. Approximately 100 flakes are counted for each sample during TEM imaging and then we measure the length along the diagonal direction of the flakes to plot a statistical map as shown in Figure 2B. (We notice that length and width of flakes are usually measured by others,11 but such measurement is not appropriate due to the irregular shape of our samples.) For S1, S2, S3, and S4, the fractions of small flakes (≤600 nm) are 77, 62, 66, and 44%, respectively. A striking feature is that, with the decrease of small
fraction of flakes fraction of mean numbers (length ≤ flakes (width ≤ of layers per values of αb 500 nm) 500 nm) flake (L g−1 m−1) 37% 70%
57% 79%
5 3.5
∼6600 ∼6300
a
Here samples S5 and S6 are prepared at the 500 and 5000 rpm in ref 12. bTheir values of α are measured at 660 nm.
UV−vis absorption spectra were recorded using a UV-1600 spectrophotometer (Beijing Rayleigh Analytical Instruments). Quartz cuvettes (3.5 mL) with a 10 mm optical path were used. Morphology of the solid RGO was characterized by transmission electron microscopy (TEM, JEM-2010 JEOL). For high-resolution TEM (HR-TEM, Hitachi, H-7650), the sample dispersions were mixed with epoxy resin to form a composite. Then this composite was microtomed to form thin slices before loading on TEM grids for HR-TEM imaging.16 X-ray photoelectron spectroscopy (XPS) was performed on a VG Escalab MK II spectrometer (Scientific Ltd.). Raman spectra were recorded with a He−Ne laser (532 nm) as the excitation source by using Labram spectrometer (Super LabRam II system).
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RESULTS AND DISCUSSION Stable RGO dispersions were obtained in NMP and there were no indication of coagulation even after several months (Figure 1C). The values of α were calculated according to the
Figure 3. XPS spectra of the four samples. The O/C ratios of S1, S2, S3, and S4 are 0.127, 0.149, 0.149, and 0.185 (peak area). The N/C ratios of S1, S2, S3, and S4 are 0.037, 0.040, 0.042, and 0.048 (peak area). 12522
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Figure 4. (A) Raman spectra of the four samples and (B) the intensity ratio of the D and G peaks from Raman spectra.
flakes, the absorption coefficient increases from 2.67 × 106 to 6.72 × 106 mL·mg−1·m−1. The number of layers per flake is also one of the important factors on absorption coefficient. In our experiment, with the value of α increasing, an increase in the stacking degree was also observed (Figure 2A). Most of the flakes of S1 are monolayers. The flakes of S2 and S3 consist of few layers. While for S4, heavier stacks composed of large flakes are clearly observed. The mean numbers of layers per flake are 4.38, 4.96, 5.22, and 6.20 for S1, S2, S3, and S4, respectively, according to the statistics data (Figure 2C) collected from HR-TEM images.16 It has been proposed that when the number of layers (N) is equal
or fewer than 5, and its opacity is proportional to N.5 But for thicker graphene flakes (N > 10), the samples behave not just as a simple superposition of single layers, because closer interaction between the layers changes the optical path.8 In short, at the same weight concentration, the value of α increases from S1 to S4 with the increasing of the mean number of layers per flake. Now we explain why different flake sizes and stacking layers can affect absorption coefficient. Since optical absorption overwhelms light scattering for our flakes in such a lowconcentration (0.02−0.2 mg/mL).18 The extension of a π-conjugated system will be a primary factor to enhance the 12523
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absorption intensity.19 Our conjugate system is composed of flakes with different sizes and stacking layers. So, the dispersion with a larger amount of small flakes and less layers per flake gives a smaller absorption coefficient. We can further extend our conclusion above to Coleman’s work. We include two graphene samples from them (numbered as S5 and S6, Table 1).12 This table indicates that the sample with a higher fraction of small flakes (both length and width are smaller than 500 nm) as well as a lower mean number of layers per flake will give a lower value of α at 660 nm. Clearly, the earlier analysis on size and thickness effect to the absorption coefficient is successfully extended to other graphene dispersions. Unlike Coleman’s one-step liquid exfoliation of graphite, it is worthwhile to note that we used reduced graphite oxide and exfoliated them into RGO. As a consequence, our sample has many more structural defects and noncarbon (O and N) elements on surfaces. Both of these are beneficial to the increase of the optical absorption. Next we will discuss the presence of noncarbon groups grafted onto the graphene flakes and their contribution to the light absorption. The inevitable hydrocarbon contamination when the sample is exposed to air will make monolayer and bilayer graphene a smaller transmittance when wavelength (λ) is shorter than 500 nm.5 The oxygen-containing groups (−COOH, −OH, and −C−O−C−) decorated on carbon flakes also make GO20−23 a much smaller transmittance or higher absorbance at 660 nm.24 In our work, a small part of −OH and −COOH groups remains on the flake after the microwave process. NMP molecules will be introduced to the RGO sheets during the ball milling process, as can be seen in Supporting Information. We find that with the coefficient increasing, both oxygen and nitrogen contents are increasing. The O/C ratio of S1, S2, S3, and S4 are 0.127, 0.149, 0.149, and 0.185. The N/C ratio of S1, S2, S3, and S4 are 0.037, 0.040, 0.042, and 0.048 (Figure 3). This can be ascribed to the increase of auxochromic groups such as −OH, −COOH, −NR2. We argue the abundant functional groups as the primary factor to increase α by almost 3 orders of magnitude. Additionally, these samples are probed by Raman spectroscopy to reveal surface defects. The intensity ratio of the D peak at ∼1346 cm−1 and G peak at ∼1585 cm−1, I(D)/I(G), allows one to tell the amount of defects (Figure 4A).25−27 Approximately 10 specimens were collected for each sample (S1, S2, S3, and S4) to make a plot of distribution of I(D)/I(G). As shown in Figure 4B, the four samples all have high defect contents, likely due to abundant edges produced by small graphene flakes. Surprisingly, S1 has lower defect content than the other three samples, though it has more small flakes. This may be ascribed to the fact that defects localize mainly along the edges in S1, while they distribute both in the bulk and at the edges for the other three samples. We then discuss how the defects affect the optical absorption. As indicated by our XPS data, a major source of defects is related to oxygen, with the other as topological or vacancies from our thermal processing.28−30 For samples with oxygenrich defects, higher content will lead to a larger auxochrome effect or a larger optical absorption coefficient. However, the topological defects will lead to an increase in the optical bandgap in RGO. The presence of hydrogen at the defect sites and big holes will damage the π-conjugated system. So the topological defects will lead to a smaller absorption coefficient. In our work, oxygen-related defects dominate because the absorption coefficient increases with defects increasing.
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CONCLUSIONS In this work, the light absorption characteristics of four RGO dispersion samples have been studied, and the dependence of absorption coefficient on lateral size distribution, numbers of layers per flake, and noncarbon (O and N) contents are investigated. The results show that the higher content of small flakes (≤600 nm) as well as the lower mean number of layers per flake will give a lower value of absorption coefficient at 660 nm. This was ascribed to the shrinkage of the π-conjugated system. Moreover, the higher content of functional groups will lead to a higher absorption coefficient because of the increase in the auxochromic effect. This study of the optical absorption properties of graphene dispersion is important in its own right and also useful for understanding the absorption of other submicrometer two-dimensional particles.
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ASSOCIATED CONTENT
S Supporting Information *
Additional supporting material. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by Natural Science Foundation of China (51373059, 50373015) and Science Foundation of Fujian Province (2013H6014).
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REFERENCES
(1) Geim, A. K. Graphene: Status and Prospects. Science 2009, 324, 1530−1534. (2) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (3) Singh, V.; Joung, D.; Zhai, L.; Das, S.; Khondaker, S. I.; Seal, S. Graphene Based Materials: Past, Present and Future. Prog. Mater. Sci. 2011, 56, 1178−1271. (4) Torrisi, F.; Hasan, T.; Wu, W.; Sun, Z.; Lombardo, A.; Kulmala, T.; Hshieh, G. W.; Jung, S. J.; Bonaccorso, F.; Paul, P. J.; et al. InkjetPrinted Graphene Electronics. ACS Nano 2012, 6, 2992−3006. (5) Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.; Peres, N. M. R.; Geim, A. K. Fine Structure Constant Defines Visual Transparency of Graphene. Science 2008, 320, 1308. (6) Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J. S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. I.; et al. Roll-to-Roll Production of 30 in. Graphene Films for Transparent Electrodes. Nat. Nanotechnol. 2010, 5, 574−578. (7) Kravets, V. G.; Grigorenko, A. N.; Nair, R. R.; Blake, P.; Anissimova, S.; Novoselov, K. S.; Geim, A. K. Spectroscopic Ellipsometry of Graphene and an Exciton-Shifted Van Hove Peak in Absorption. Phys. Rev. B 2010, 81, 155413. (8) Casiraghi, C.; Hartschuh, A.; Lidorikis, E.; Qian, H.; Harutyunyan, H.; Gokus, T.; Novoselov, K. S.; Ferrari, A. C. Rayleigh Imaging of Graphene and Graphene Layers. Nano Lett. 2007, 7, 2711− 2717. (9) Bonaccorso, F.; Sun, Z.; Hasan, T.; Ferrari, A. C. Graphene Photonics and Optoelectronics. Nat. Photonics 2010, 4, 611−622. (10) Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z. Y.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun’ko, Y.; et al. High-Yield Production of Graphene by Liquid-Phase Exfoliation of Graphite. Nat. Nanotechnol. 2008, 3, 563−568.
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(11) Khan, U.; O’Neill, A.; Lotya, M.; De, S.; Coleman, J. N. High Concentration Solvent Exfoliation of Graphene. Small 2010, 6, 864− 871. (12) Lotya, M.; King, P. J.; Khan, U.; De, S.; Coleman, J. N. HighConcentration, Surfactant-Stabilized Graphene Dispersions. ACS Nano 2010, 4, 3155−3162. (13) Lotya, M.; Hernandez, Y.; King, P. J.; Smith, R. J.; Nicolosi, V.; Karlsson, L. S.; Blighe, F. M.; De, S.; Wang, Z. M.; McGovern, I. T.; et al. Liquid Phase Production of Graphene by Exfoliation of Graphite in Surfactant/Water Solutions. J. Am. Chem. Soc. 2009, 131, 3611− 3620. (14) Khan, U.; Porwal, H.; O’Neill, A.; Nawaz, K.; May, P.; Coleman, J. N. Solvent-Exfoliated Graphene at Extremely High Concentration. Langmuir 2011, 27, 9077−9082. (15) Hummers, W. S., Jr; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339. (16) Chen, J.; Duan, M.; Chen, G. Continuous Mechanical Exfoliation of Graphene Sheets via Three-Roll Mill. J. Mater. Chem. 2012, 22, 19625−19628. (17) Tölle, F. J.; Fabritius, M.; Mülhaupt, R. Emulsifier-Free Graphene Dispersions with High Graphene Content for Printed Electronics and Freestanding Graphene Films. Adv. Funct. Mater. 2012, 22, 1136−1144. (18) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; John Wiley and Sons: New York, 1983; Vol. 11, p 323. (19) Yan, Z. Anal. Chem.; Southeast University Press: Nanjing, 2005; Vol. 9, p 121. (20) He, H.; Riedl, T.; Lerf, A.; Klinowski, J. Solid-State NMR Studies of the Structure of Graphite Oxide. J. Phys. Chem. 1996, 100, 19954−19958. (21) He, H.; Klinowski, J.; Forster, M.; Lerf, A. A New Structural Model for Graphite Oxide. Chem. Phys. Lett. 1998, 287, 53−56. (22) Lerf, A.; He, H.; Forster, M.; Klinowski, J. Structure of Graphite Oxide Revisited. J. Phys. Chem. B 1998, 102, 4477−4482. (23) Hontoria-Lucas, C.; Lopez-Peinado, A. J.; López-González, J. D. D; Rojas-Cervantes, M. L.; Martin-Aranda, R. M. Study of OxygenContaining Groups in a Series of Graphite Oxides: Physical and Chemical Characterization. Carbon 1995, 33, 1585−1592. (24) Park, O. K.; Kim, S. G.; You, N. H.; Ku, B. C.; Hui, D.; Lee, J. H. Synthesis and Properties of Iodo Functionalized Graphene Oxide/ Polyimide Nanocomposites. Composites, Part B 2014, 56, 365−371. (25) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; et al. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97, 187401. (26) Ferrari, A. C.; Robertson, J. Interpretation of Raman Spectra of Disordered and Amorphous Carbon. Phys. Rev. B 2000, 61, 14095− 14107. (27) Tuinstra, F.; Koenig, J. L. Raman Spectrum of Graphite. J. Chem. Phys. 2003, 53, 1126−1130. (28) Loh, K. P.; Bao, Q.; Eda, G.; Chhowalla, M. Graphene Oxide as a Chemically Tunable Platform for Optical Applications. Nat. Chem. 2010, 2, 1015−1024. (29) 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. (30) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The Chemistry of Graphene Oxide. Chem. Soc. Rev. 2010, 39, 228−240.
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