Chemical Physics Letters 595–596 (2014) 1–5
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Fast electron transfer kinetics on electrodes composed of graphene oxide ‘patched’ with direct exfoliated pristine graphene nanosheets Wencheng Du, Bo Zhou, Xiaoqing Jiang ⇑ Jiangsu Key Laboratory of New Power Batteries, Laboratory of Electrochemistry, College of Chemistry and Materials Science, Nanjing Normal University, 122 Ninghai Road, Nanjing 210097, PR China
a r t i c l e
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Article history: Received 6 December 2013 In ﬁnal form 21 January 2014 Available online 3 February 2014
a b s t r a c t A new electrode material composed of graphene oxide (GO) and pristine graphene (PG) was simply fabricated, and a very fast electron transfer kinetics on GO/PG electrodes was exhibited. This improved electrochemical performance of the GO/PG composites should be ascribed to a unique structure formed when GO combined with PG nanosheets serving as patches. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In electrochemistry, various carbon-based materials such as carbon nanotubes (CNT), activated carbons, highly oriented pyrolytic graphite (HOPG), glassy carbon, graphite ﬁbers, etc., have been widely studied and used as electrodes [1–4]. The rate of charge transfer at these carbon-based electrodes is diverse due to different structures as well as morphologies of these carbonaceous matter used in the electrodes. In many practical applications, surface oxides and the intrinsic charge transfer capability of electrode materials are essential for achieving excellent electrochemical behaviours in aqueous electrolytes [5,6]. However, introducing oxides into sp2-hybridized carbon sheets will destroy p-conjugated network, causing decline in conductivity and electrochemical performance of electrode. It has been known that the newest two dimensional carbon layer — pristine graphene (PG) has superior conductivity and can be used as electrode materials [7,8]. On the other hand, although graphene oxide (GO) — highly oxidized graphene sheet, has poor conductivity, it exhibited importance not only as precursor for large-scale production of chemically converted graphene , but also in many practical applications due to its abundant oxygen-containing functional groups . Integration of the conductivity of PG and active oxides of GO may generate good electrochemical performances. In this Letter, we report fast charge transfer behaviour in a GO/ PG material. The water-processable GO/PG composites were simply prepared by mixing dispersions of GO and PG obtained respectively via well-established modiﬁed Hummer method  and
⇑ Corresponding author. Fax: +86 25 85891767. E-mail address: [email protected]
(X. Jiang). http://dx.doi.org/10.1016/j.cplett.2014.01.027 0009-2614/Ó 2014 Elsevier B.V. All rights reserved.
direct liquid-phase exfoliation method [12,13], using easily-available graphite powder as the same precursor. 2. Experimental 2.1. Materials Natural graphite powders (Gr) with a particle size of 630 lm, dimethyl sulfoxide (DMSO), sodium citrate (C6H5Na3O72H2O), potassium ferricyanide and potassium hexacyanoferrate were purchased from Sinopharm Chemical Reagent Co. Ltd. and Shanghai Lingfeng Chemical Reagent Co., Ltd. All the chemicals were used as received without further treatments. Ultrapure water with resistivity of 18.25 MX cm was used throughout the work. 2.2. Preparation of GO/PG composites GO was synthesized from natural graphite powder by a wellestablished modiﬁed Hummer’s method . PG was prepared by direct sonication of the graphite powder in DMSO with assistant of C6H5Na3O72H2O as reported in our previous work . Homogeneous composites of GO and PG were prepared by ﬁrstly mixing colloidal dispersions of GO/H2O (0.5 mg mL 1) and PG/DMSO (0.5 mg mL 1) with controlled volume ratios. After mixing, additional sonication for 15 min is given. The resulting homogeneous composites dispersions were centrifuged at 9000 rpm for 10 min (sometimes adding drops of electrolyte solution such as KCl (aq) into these dispersions is needed before centrifugation, which is helpful for sedimentation of these nanocomposites). The resulting precipitates were washed with water and then re-dispersed using 2.00 ml H2O by sonication for about 1 h. The weight ratios (feed ratios) of GO to PG varied as 80:20, 60:40, 50:50, and the
W. Du et al. / Chemical Physics Letters 595–596 (2014) 1–5
resulting composites were designated as GO/PG20, GO/PG40, GO/ PG50, respectively. 2.3. Characterizations Transmission electron microscopy (TEM) measurements were carried out on a Hitachi-7650 transmission electron microscope operated at an accelerated voltage of 80 kV. The sample for TEM test was prepared by drop casting the dispersion of GO/PG onto carbon-coated copper grid (230 mesh size). UV–visible spectrum was recorded on a Varian Cary 50 spectrometer using a quartz cell with a 1 cm optical path. Fourier transform infrared spectra (FT-IR) were recorded on a Bruker tensor 27 Fourier transform infrared spectrometer. The spectra were measured in the 4000–400 cm 1 wave number range. 2.4. Electrochemical measurements For the electrochemical measurements, 5.0 lL of each GO/PG dispersion (0.5 mg mL 1) was deposited on a polished glassy carbon (GC) electrode (diameter: 3 mm) and dried in the air at room temperature (for PG/DMSO case, the dropped PG/DMSO on GC electrode was dried in a vacuum oven at 40 °C for more than 5 h, because DMSO has a high boiling point of 189 °C). All electrochemical measurements were carried out in a three-electrode setup: GO, PG or GO/PG composites modiﬁed GC electrodes as the working electrode, platinum wire and Ag/AgCl (saturated KCl) electrodes as the counter and reference electrodes respectively. All tests were measured by a CHI 660C electrochemical workstation. The electron transfer kinetics tests were carried out using cyclic voltammograms (CV) and elecctrochemical impedance spectroscopy (EIS). CVs of GO, PG, and GO/PG modiﬁed glassy carbon electrode were conducted in a solution of 5 mM Fe(CN)63 /4 in 0.1 M KCl with different scan rates. Nyquist plot of the three types of electrodes were measured in the same electrolyte solution. The frequency range is from 1 Hz to 100 kHz.
3. Results and discussion The process for preparation of GO/PG composites is shown in Figure 1a, in which the structural representations of PG, GO, and GO/PG composites are also shown. FT-IR spectra (Figure 1b) demonstrate the successful preparation of PG and GO through route 1 and 2 in Figure 1a, respectively. The spectrum of GO illustrates strong [email protected]
(t(carboxylic acid and carbonyl)) at 1735 cm 1, [email protected]
(skeletal vibrations of unoxidized graphitic domains) at 1625 cm 1, C–O (t(epoxy or alkoxy)) at 1054 cm 1 and C–OH stretching peak or C–O vibrations at 1225 cm 1, which is complete consistent with those reported for GO . The spectrum of PG shows that no signals from C–O, C–OH or [email protected]
groups are observed, except a weak [email protected]
at 1580 cm 1, suggesting the PG can be considered as pristine as original graphite. The as-made GO/ PG composites with different weight ratios can be stably dispersed in water for certain time, forming dark colloidal suspensions (Figure 1c). CVs were conducted in a solution of 5 mM Fe(CN)63 /4 in 0.1 M KCl at a scan rate of 100 mV s 1. In order to simplify the comparison of peak potential separation, all current values in Figure 2a–c were normalized at oxidation peak current of each CV. As shown in the typical CVs in Figure 2a–c, peak potential separation is 108 and 162 mV for GO- and PG-based electrodes respectively, which approaches those reported values . Interestingly, GO/PG based electrode exhibited the lowest peak-to-peak potential separation (DEP) of ca. 56 mV, indicating a fast electron transfer behavior. It should be noted that the value of DEP is slightly lower than the expected theoretical value of 59 mV at 25 °C, which might be caused by the possible chemical adsorption or bonding between electrode materials and iron ion species . The charge transport resistance (Rct) was analyzed by EIS as shown in Figure 2d. The Rct between the electrolyte and electrode is about 2000, 350, and 250 X for GO, PG, and GO/PG40 (PG: 40 wt%) electrodes respectively, indicating an enhanced electrical contact between GO/PG material and electrolyte, owing to a facilitated interfacial electron transfer .
Figure 1. (a) Illustration of the process for preparation of GO, PG and water-processable GO/PG composites. (b) FT-IR spectra of GO, PG, and graphite precursor. (c) Picture of bottles containing aqueous dispersions of 0.5 mg mL 1 GO, GO/PG20, GO/PG40, and GO/PG50 respectively.
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Figure 2. CVs of (a) PG, (b) GO and (c) GO/PG (PG: 40 wt%) modiﬁed glassy carbon electrode in a solution of 5 mM Fe(CN)63 /4 in 0.1 M KCl with scan rate of 100 mV/s Nyquist plot of the three electrodes (inset on the top left corner is the equivalent circuit with a Warburg impedance). The frequency range is from 1 Hz to 100 kHz.
The DEP is nearly the same at various sweep rates from 30 to 200 mV s 1 (Figure 3a–c) for all GO/PG electrodes. (In Figure 3a– c, in order to simplify the comparison of peak potential separation, all current values in each ﬁgure were normalized at the maximum oxidation peak current of each ﬁgure.) DEP is found to only increase slightly as sweep rate beyond 300 mV s 1 for GO/PG40 electrode in a much broader scan rate range (Figure 3d). In addition,
the anodic and cathodic peak currents almost have the same absolute value under each potential scan rate and exhibited good linear relationship with square root of scan rates, almost intercepting the origin by extrapolation (as shown in Figure 3d), suggesting a nearly ideal reversibility or single electron Nernstian behavior at the GO/ PG electrode . This good electron transfer performance is comparable to that of multiwalled CNT .
Figure 3. CVs of (a) GO/PG20, (b) GO/PG50, (c) GO/PG40 electrode in a solution of 5 mM Fe(CN)63 /4 in 0.1 M KCl with different scan rates. (d) CVs of GO/PG40 electrode in a broader scan rate range (10–1000 mV s 1), the inset presents the linear dependences of ipa and ipc on square root of scan rate (m) at GO/PG40 electrode.
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Figure 4. (a) Acetylacetone-like structures for inner sphere catalysis reactions of electron transfer in electrode materials; (b) similar structures in GO/PG composites; (c) aqueous solution phase UV–Vis spectra of the GO/PG50 composite and the individual GO, PG constituent; (d) representative TEM images of GO/PG50.
This improvement of charge transfer in the GO/PG composite can be attributed to surface oxides-induced inner sphere catalysis charge transfer mechanism resulted from special structure of GO/ PG composite. It has been demonstrated that acetylacetone-like structure (as shown in Figure 4a) introduced by laser activation in HOPG electrode surface can greatly increase the charge transfer rate of certain metal ions . The GO/PG composite should have the similar acetylacetone-like structure (as shown in Figure 4b). According to the reported models of GO structures, GO itself contains acetylacetone-like structures , however, the continuous and large-area p conjugated domains were seriously destroyed and become scattered and small-sized aromatic systems, which could result in interruption of charge transfer. PG nanosheets can be used as patches to heal such p conjugated structures and make electron transfer smoothly. This healing role should be by virtue of p–p interaction between sp2-carbon domains in PG and GO, similar to the case of GO and CNT . UV–Vis spectra of GO and GO/PG (as shown in Figure 4c) exhibited that the peak associated with p–p⁄ transition have red-shifted 29 nm (232–261 nm) when PG was composited with GO, suggesting the enlarged sp2-carbon domains due to this patching effect. TEM also shows small-sized PG overlapped on large GO sheet (Figure 4d). It should be mentioned that PG ﬂakes obtained by direct sonicated exfoliation in organic solvent often have rather small lateral dimensions (typically less than 1 lm) due to sonication-induced cleavage effect and high centrifugation speed . So, small PG ﬂakes decorated on largesized GO look like ‘patches’. Thus, for GO electrode, charge transfer process will be strongly hindered by its scattered sp2-carbon domains; while for GO/PG electrode, electron can smoothly transfer in the electrode system owing to extended and completed sp2-carbon domains resulted from PG patches. In regard of PG electrode, instead of the innersphere electron transfer, an out-sphere electron transfer for Fe(CN)63 /4 mostly occurs due to the oxide-deﬁcient and high hydrophobic properties of PG, implying no strong interactions between electrode surface and iron ions surrounded by water .
4. Conclusions In summary, a simple and excellent carbon-based electrode material with very fast charge transfer performance based on common graphene oxide and easily obtained direct liquid exfoliated graphene was prepared. The electrodes made of such GO/PG composites show ideal Nernstien behavior and very fast charge-transfer kinetics for electrochemical reactions of Fe(CN)63 /4 . This electrochemical performance is comparable to that of multiwalled CNT reported in literature. The enhancement of charge transfer in GO/PG can be explained by the unique inner sphere reaction mechanism originated from acetylacetone-like structures of GO along with PG patched p conjugation systems of GO. The GO/PG composites can be used as electrode materials for potential applications in organic optoelectronic devices and electrochemical sensor platform, etc. Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant No. 20773066 and 21175068), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Jiangsu Collaborative Innovation Center of Biomedical Functional Materials. References  K. Kinoshita, Carbon: Electrochemical and Physiochemical Properties, Wiley, New York, 1988.  S. Ranganathan, T. Kuo, R.L. McCreery, Anal. Chem. 71 (1999) 3574.  J.M. Nugent, K.S.V. Santhanam, A. Rubio, P.M. Ajayan, Nano Lett. 1 (2001) 87.  R.L. McCreery, Chem. Rev. 108 (2008) 2646.  L. Kavan, Chem. Rev. 97 (1997) 3061.  C.A. McDermott, K.R. Kneten, R.L. McCreery, J. Electrochem. Soc. 140 (1993) 2593.  X.M. Chen, G.H. Wu, Y.Q. Jiang, Y.R. Wang, X. Chen, Analyst 136 (2011) 4631.  W.C. Du, X.Q. Jiang, L.H. Zhu, J. Mater. Chem. A 1 (2013) 10592.  S. Stankovich et al., Carbon 45 (2007) 1558.  D.R. Dreyer, S.J. Park, C.W. Bielawski, R.S. Ruoff, Chem. Soc. Rev. 39 (2010) 228.
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