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2009, 113, 7985–7989 Published on Web 04/17/2009
Electrochemical Reduction of Oriented Graphene Oxide Films: An in Situ Raman Spectroelectrochemical Study Ganganahalli K. Ramesha and Srinivasan Sampath* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India ReceiVed: December 24, 2008; ReVised Manuscript ReceiVed: March 11, 2009
Graphene oxide (GO) is assembled on a gold substrate by a layer-by-layer technique using a self-assembled cystamine monolayer. The negatively charged GO platelets are attached to the positively charged cystamine monolayer through electrostatic interactions. Subsequently, it is shown that the GO can be reduced electrochemically using applied DC bias by scanning the potential from 0 to -1 V vs a saturated calomel electrode in an aqueous electrolyte. The GO and reduced graphene oxide (RGO) are characterized by Raman spectroscopy and atomic force microscopy (AFM). A clear shift of the G band from 1610 cm-1 of GO to 1585 cm-1 of RGO is observed. The electrochemical reduction is followed in situ by micro Raman spectroscopy by carrying out Raman spectroscopic studies during the application of DC bias. The GO and RGO films have been characterized by conductive AFM that shows an increase in the current flow by at least 3 orders of magnitude after reduction. The electrochemical method of reducing GO may open up another way of controlling the reduction of GO and the extent of reduction to obtain highly conducting graphene on electrode materials. Introduction 1
Graphene is a flat monolayer of a hexagonal network of carbon atoms that has received considerable attention in the last several years due to its extraordinary electronic, thermal, and mechanical properties.2,3 Graphene has been shown to exhibit excellent characteristics in various applications such as transistors in nanoelectronics,4 nanoelectromechanical devices,5 nanocomposites,6 sensors,7 ultracapacitors,8 solar cells,9 liquid crystal devices,10 etc. Until recently, the only method known in the literature to obtain graphene involved the micromechanical cleavage of highly oriented pyrolytic graphite.11 Similar to the early days of carbon nanotube research, the preparation of graphene has also been going through various stages of improvement. There have been reports on the solution phase exfoliation of graphite through intercalation of surfactant in aqueous12,13 as well as in nonaqueous media.14 In the past few years, efforts have been made to obtain graphene via graphene oxide (GO), an oxidized form of graphene, decorated by hydroxyl and epoxy functional groups on the hexagonal network of carbon atoms with carboxyl groups at the edges. The GO is highly hydrophilic and forms stable aqueous colloids due to the large number of oxygen-containing functional groups as well as the repulsive electrostatic interactions at the edges of the platelets. There have been reports on reducing GO15-19 in the solution phase using various reducing agents, such as hydrazine or sodium borohydride, and in the vapor phase using hydrazine/ hydrogen or just by thermal annealing. The present study deals with the electrochemical reduction of graphene oxide assembled on a conducting substrate through a self-assembled monolayer using a layer-by-layer (L-b-L) method. Further, in situ measurements have been carried out to * Corresponding author. Phone: +91 80 22933315. Fax: +91 80 23601552. E-mail:
[email protected].
10.1021/jp811377n CCC: $40.75
follow the electrochemical reduction of GO to RGO by combining Raman spectroscopy with electrochemistry. Raman spectroscopy is a widely used nondestructive method to characterize carbon-based materials to locate single-layer graphene20 and also to determine the diameter of single-walled carbon nanotubes.21 The changes observed in the G band of GO from 1610 to 1585 cm-1 reveals that the electrochemical reduction of GO, indeed, takes place. This indirectly shows that GO starts to gain electronic conjugation in the hexagonal network at low applied potentials. Conductive atomic force microscopy (C-AFM) has been used to qualitatively understand the reduction and subsequent changes in the conductivity of the sample. Experimental Section The self-assembled monolayers (SAMs) were prepared on polycrystalline gold surfaces on the basis of a previous report.22 The gold electrodes were cleaned by immersing them in two successive baths of freshly prepared piranha solution for a few minutes each (Caution: piranha reacts violently when comes in contact with organic compounds). The electrodes were further cleaned by cycling between 0.5 and 1.4 V vs SCE, in 0.5 M H2SO4 until reproducible voltammograms for gold oxide formation and its reduction were observed. The scheme of formation of GO assembly on gold is given in the Supporting Information (Supporting Information, scheme S1). The base monolayer of cystamine was assembled by immersing the clean gold electrode in a 5 mM aqueous solution of cystamine · 2HCl for 12 h. The substrate was taken out and washed well with deionized water to remove any physisorbed material. The electrode with a cystamine monolayer is represented as Au/Cys. The preparation of GO was based on the oxidation of exfoliated graphite using a modified Hummers method reported earlier from our group.23 The GO colloid was prepared by 2009 American Chemical Society
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Letters The C-AFM experiments were performed using a (Digital Instruments, Santa Barbara, CA) nanoscope IVA multimode AFM equipped with a J-scanner and current-sensitive attachment. Contact mode etched silicon probes with cobalt-chrome coating were used for C-AFM measurements. The I-V curves were recorded using the same tip at different places of the GO nanoplatelets on the gold substrate. The gold substrates for AFM measurements were prepared by a template lift-off technique.24 Briefly, gold was coated on mica substrates without a chromium underlayer. The gold surface was then attached to polished silicon through epoxy. The assembly was heated to 130 °C for about 2 h. Subsequently, it was immersed in tetrahydrofuran that delaminates mica from the deposited gold, thus exposing the nearly atomically smooth gold surface. This surface is represented as template stripped gold. Results and Discussion
Figure 1. Cyclic voltammogram Au/Cyst/GO film in deaerated 0.1 M KNO3 at a scan rate of 10 mV/s. The scan is carried out from 0 V to -1.0 V, and the potentials are given with respect to SCE. The area of the gold disk electrode used is 0.12 cm2. 1 and 2 correspond to the first and second scans.
dispersing it in Millipore water by ultrasonication and used for further studies. This colloid was found to be very stable for several months, and the pH of the as-prepared aqueous colloid was found to be 2.5. The cystamine-modified electrodes were immersed in the as-prepared GO colloid (1 mg/mL) for 4 h. The positively charged cystamine monolayer at a pH of 2.5 attracts the GO platelets, forming an organized GO layer. The electrodes were further washed well with double-distilled water and used for electrochemical studies. The GO modified electrode is represented as Au/Cyst/GO. The electrochemical measurements were carried out in a three-electrode cell using a CHI 660A electrochemical analyzer (CH Instruments, Austin, TX). Cyclic voltammograms were carried out in 0.1 M KNO3 at a scan rate of 10 mV/s. The modified gold electrode served as the working electrode. The counter electrode was a platinum foil of area 2 cm2. A saturated calomel electrode (SCE) {Hg, Hg2Cl2/Cl- (saturated)} was used as the reference electrode. Raman spectroscopic measurements were carried out using a Witec Confocal (50× objective) spectrometer with 600 lines/mm grating and 514.5 nm excitation wavelength. (The schematics for in situ spectroelectrochemical study is shown in scheme S2 of the Supporting Information.) All the experiments were carried out at 25 °C.
The thin film of GO has been assembled by a L-b-L method using electrostatic interactions. Initially, stable cystamine SAM is formed on the gold surface. The cystamine monolayer on gold is known to be a very disorganized structure25 due to the small number of methylene groups available for lateral stabilization. The pKa of the -NH2 groups of the cystamine monolayer has been reported to be 7.6.26 Hence, the amine groups are in the protonated form at pH 2.5. The driving force for the formation of the GO layer is the attraction of the negatively charged GO platelets to the positively charged cystamine monolayer. Once the GO monolayer is formed on the surface, the large negative charges associated with the GO platelets possibly repel each other. This might help in the control of the GO thickness on the surface. Figure 1 shows the cyclic voltammogram of the GO-modified electrode (Au/Cyst/GO) in a deaerated aqueous 0.1 M KNO3 solution. The reduction has been carried out by scanning the potential from 0 to -1.0 V with respect to SCE at a scan rate of 10 mV/s. The peak observed at -0.75 V shows that the electrochemical reduction, indeed, takes place. The second scan does not result in a peak at the same potential, revealing that the reduced GO does not get oxidized in the potential range studied, and hence, the reduction of GO to RGO is an electrochemically irreversible process.16 The morphology of GO before and after electrochemical reduction has been characterized using AFM. Figure 2 shows the surface morphology of GO. The thickness of the self-assembled GO sheet is found to be ∼1.2 nm before reduction and is similar to the previously reported values of GO thickness that vary between 1.1 and 2 nm.15,27 The size of the self-assembled GO flakes is fairly high
Figure 2. AFM images showing the Au/Cyst/GO monolayer before (a) and after (b) reduction. The height profiles are also shown.
Letters
Figure 3. Raman spectra of Au/Cyst/GO (a) before and (b) after electrochemical reduction.
Figure 4. Raman spectra of Au/Cyst/GO as a function of applied dc bias in an electrochemical cell containing deaerated 0.1 M KNO3. The reference electrode used is SCE. The spectra are taken by keeping the electrode at each potential for 5 min.
and is on the order of several square micrometers in area. However, it is observed that the GO does not completely cover the surface, and this might be due to the disorganized cystamine monolayer on the gold surface. The thickness of GO film
J. Phys. Chem. C, Vol. 113, No. 19, 2009 7987 remains nearly the same after electrochemical reduction (figure 2). It varies in the range of ∼1.2 to ∼2 nm. The observed small increase might be due to the intercalated water molecules during the electrochemical treatment. As observed from figure 2b, there are changes in the morphology of reduced GO, and the sheets seem to be fractured after electrochemical treatment. However, the coverage remains almost the same as that of the coverage before reduction. The reduction is further confirmed using spectroelectrochemistry where a combination of Raman spectroscopy and electrochemistry is used. The Raman spectrum of graphite has been reported to show bands at ∼1370, ∼1583, ∼1620, ∼2700, and ∼3200 cm-1, designated as D, G, D′, G′, and D′′ bands respectively.28 Figure 3 shows the Raman spectrum of GO, and the bands associated at ∼1350 and ∼1610 cm-1 correspond to D and G modes respectively.29 The spectrum shows a shift in the G band to higher wave numbers as compared to graphite. This is attributed to three different reasons: (i) overlap of the G band with the D′ band that becomes active due to defects;30 (ii) a reduced number of sheets, resulting in the blue shift as observed from graphite to graphene;20,31,32 and (iii) the presence of isolated double bonds separated by functional groups on the carbon network of GO.33 The reduction of GO restores the G band to 1581 cm-1, corresponding to the recovery of hexagonal network of carbon atoms with defects.29 This is very clearly observed in the present case (Figure 3). The intensity of the G band is known to be higher than that of the D band for GO, whereas it is vice versa for the RGO.15 This change in the intensity ratio of the D to G bands is attributed to the increased defect concentration present in RGO relative to that in GO.15 Spectroelectrochemical characterization is expected to yield additional information on the conversion of GO to RGO under DC bias. The schematic diagram of the experimental setup for in situ spectroelectrochemical measurements is given in the Supporting Information (scheme S2). Figure 4 shows the in situ Raman spectra of the Au/Cyst/GO surface as a function of the applied DC bias. The reduction is observed at -0.75 for the Au/Cyst/GO system, as described earlier in the voltammetric studies. The presence of electrolyte in the spectroelectrochemical cell leads to attenuation in the intensities of the bands as compared to that obtained in the dry state. The intensity of the second order bands of GO and RGO in the presence of the electrolyte are very weak (not shown). The positions of the bands at ∼1350 and 1610 cm-1 corresponding to GO changes when the potential is scanned in the negative direction and the D/G intensity ratio varies, as well. The variations in the band positions as a function of
Figure 5. Band positions of the G (a) and D bands (b) as a function of the applied DC potential.
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Letters clearly reveal the increase in conductivity, though the absolute conductance is not quantified. Conclusions It has been shown that GO can be electrostatically assembled onto conducting substrates and subsequently reduced electrochemically. The reduction leads to RGO based on in situ Raman measurements. Further studies are in progress to obtain large flakes of GO by assembling other positively charged molecules/polymers and also by electrophoretic deposition. Acknowledgment. We thank DST, New Delhi, for financial assistance.
Figure 6. I-V curves for Au/Cyst/GO in the dry state (a) before and (b) after electrochemical reduction.
applied DC bias values are given in Figure 5. The band at ∼1610 cm-1 starts shifting toward lower wave numbers, indicating the beginning of the reduction process when the potential is changed from -0.2 to -0.8 V. The band positions start to change even at -0.2 V and are in line with the small currents flowing in the voltammograms even at low potentials. The G band is observed around ∼1585 cm-1 at -1.0 V. The D band also shows a similar variation in its position, from ∼1350 to 1340 cm-1 in the same potential range. This is similar to the data reported by Stankovich et al. for the chemical reduction of GO to RGO,15 where a difference of 10 cm-1 is observed in the band positions before and after reduction. While reversing the scan direction (moving from negative to positive direction of the applied potential), there is no change in the band positions observed (spectra not shown), confirming the irreversible reduction of GO observed electrochemically. The changes observed in the relative intensity of D/G bands are shown in the Supporting Information (Figure S1). The ratio of intensities of D/G increases as the reduction proceeds, suggesting a decrease in the size of the sp2 domains. This is confirmed by the AFM morphology pictures (Figure 2) that reveal a decrease in size after reduction. The change in conductivity of the film is followed by conductive AFM in the dry state. Figure 6 shows the I-V curves for the as-prepared GO and electrochemically reduced GO platelets. The domains of reduced GO are fairly large (Figure 2), and it is possible to carry out the I-V measurements without any ambiguity. We have carried out I-V measurements at 15 different points on each sample (after making sure that the position of the tip is at the correct place on the basis of the morphology data), and five different samples before and after reduction have been analyzed and the data are reproducible. Additionally, the conductivity mapping of the samples before and after reduction (Supporting Information, Figure S2) also shows the corresponding changes, as expected, wherein the reduced GO shows larger conductivity than the unreduced GO domains. Navarro et al. observed a three-order increase in conductivity when GO is chemically reduced using hydrazine to RGO.27 The present studies show a three-order increase in the current flow when GO is electrochemically reduced to RGO. The C-AFM data
Supporting Information Available: Schematics of monolayer formation, experimental setup for in situ spectroelectrochemical study, relative variation of intensities of D and G bands as a function of applied potential, and the conductive AFM images are given. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Geim, A. K.; MacDonald, A. H. Phys. Today 2007, 60, 35. (2) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (3) Katneson, M. I. Mater. Today 2007, 10, 20. (4) Ponomarenko, L. A.; Schedin, F.; Katsnelson, M. I.; Yang, R.; Hill, E. W.; Novoselov, K. S.; Geim, A. K. Science 2008, 320, 356. (5) Bunch, J. S.; van der Zande, A. M.; Verbridge, S. S.; Frank, I. W.; Tanenbaum, D. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L. Science 2007, 315, 490. (6) Stankovich, S.; Dikin, D. A.; Geoffrey Dommett, H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442, 282. (7) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Nat. Mater. 2007, 6, 652. (8) Stoller, M. D.; Park, S.; Zhu, Y.; An, J.; Ruoff, R. S. Nano Lett. 2008, 8, 3498. (9) Wang, X.; Zhi, L.; Mullen, K. Nano Lett. 2008, 8, 323. (10) 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. (11) 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. (12) Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun’ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.; Krishnamurthy, S.; Goodhue, R.; Hutchison, J.; Scardaci, V.; Ferrari, A. C.; Coleman, J. N. Nat. Nanotechnol. 2008, 3, 563. (13) Stankovich, S.; Piner, R.; Chen, X.; Wu, N.; Nguyen, S. T.; Ruoff, R. S. J. Mater. Chem. 2006, 16, 155. (14) Paredes, J. I.; Villar-Rodil, S.; Martın´ez-Alonso, A.; Tascon´, J. M. D. Langmuir 2008, 24, 10560. (15) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Carbon 2007, 45, 1558. (16) Kotov, N. A.; Dekany, I.; Fendler, J. H. AdV. Mater. 1996, 8, 637. (17) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nat. Nanotechnol. 2008, 3, 101. (18) Si, Y.; Samulski, E. T. Nano Lett. 2008, 8, 1679. (19) Yang, D.; Velamakanni, A.; Bozoklu, G.; Park, S.; Stoller, M.; Piner, R. D.; Stankovich, S.; Jung, I.; Field, D. A.; Ventrice, C. A., Jr.; Ruoff, R. S. Carbon 2009, 47, 145. (20) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Phys. ReV. Lett. 2006, 97, 187401. (21) Rao, A. M.; Richter, E.; Bandow, S.; Chase, B.; Eklund, P. C.; Williams, K. A.; Fang, S.; Subbaswamy, K. R.; Menon, M.; Thess, A.; Smalley, R. E.; Dresselhaus, G.; Dresselhaus, M. S. Science 1997, 275, 187. (22) Sarkar, S.; Sampath, S. Langmuir 2006, 22, 3388. (23) Ramesh, P.; Bhagyalakshmi, V.; Sampath, S. J. Colloid Interface Sci. 2004, 95, 274. (24) Wagner, P.; Hegner, M.; Guntherodt, H. J.; Semenza, G. Langmuir 1995, 11, 3867.
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