Photoassisted Preparation of Cobalt Phosphate ... - Wiley Online Library

Graphene Oxide

Photoassisted Preparation of Cobalt Phosphate/ Graphene Oxide Composites: A Novel Oxygen-Evolving Catalyst with High Efﬁciency Jingqi Tian, Haiyan Li, Abdullah M. Asiri, Abdulrahman O. Al-Youbi, and Xuping Sun* Fossil fuels, such as coal, oil, and natural gas, play a critical role in the development of our modern society; however, they are nonrenewable energy sources and emit vast quantities of greenhouse gases into the atmosphere upon combustion. Therefore, developing renewable clean energy sources is vitally important and has become one of the most profound challenges of the 21st century.[1] Hydrogen, as a storable, clean, and environmentally friendly fuel, is an ideal candidate for the replacement of fossil fuels in the future. Photocatalytic water splitting into hydrogen and oxygen by using solar energy, the most attractive and abundant renewable source meeting mankind’s future energy needs,[2] in the presence of semiconductor photocatalysts is envisioned as a promising and attractive approach for hydrogen fuel production, and indeed, recent years have witnessed great achievements and progress in this field since the pioneering work by Honda and Fujishima in 1972.[3,4] The water oxidation half-reaction involves a four-electron transfer and is a critically challenging step in artificial photosynthesis for hydrogen production from water.[5–7] Oxygen-evolving catalysts (OECs) constitute the bottleneck for the development of energy-conversion schemes based on sunlight.[8] As a result, considerable effort has been devoted to developing efficient OECs.[3,9–14] To date, a wealth of engineering photocatalysts for water oxidation have been developed, including photosystem II of the natural

J. Tian, Dr. H. Li, Prof. X. Sun State Key Laboratory of Electroanalytical Chemistry Changchun Institute of Applied Chemistry Changchun 130022, Jilin, PR China E-mail: [email protected] J. Tian Graduate School of the Chinese Academy of Sciences Beijing 100039, PR China Prof. A. M. Asiri, A. O. Al-Youbi, Prof. X. Sun Chemistry Department, Faculty of Science King Abdulaziz University Jeddah 21589, Saudi Arabia Prof. A. M. Asiri, A. O. Al-Youbi, Prof. X. Sun Center of Excellence for Advanced Materials Research King Abdulaziz University Jeddah 21589, Saudi Arabia DOI: 10.1002/smll.201203202 small 2013, 9, No. 16, 2709–2714

photosynthesis system,[15] dye-coupled molecular catalysts,[16] molecular catalysts integrated with a dye-sensitized TiO2 photoanode,[17–19] and all-inorganic catalysts.[20–22] In 2008, Nocera and co-workers discovered that cobalt phosphate (CoPi) film generated by in situ electrodeposition on indium tin oxide (ITO) or fluorine tin oxide (FTO) electrodes is a robust electrocatalyst for water oxidation.[23,24] This Nocera’s catalyst is inexpensive and self-healing, in analogy with the oxygen-evolving center in photosystem II enzyme, and works in neutral pH and salt water with a moderate overvoltage and high turnover frequencies. Moreover, integrating CoPi co-catalyst with other semiconductors, including ZnO, mesostructured α-Fe2O3, and WO3 semiconductors, by photochemical or electrochemical deposition can accelerate the use of light-generated holes for water oxidation and decrease the electron–hole recombination, thereby enhancing the photocurrent and O2 evolution rate as well as reducing the overvoltage.[25–28] Graphene oxide (GO), an atomically two-dimensional sheet, is a new type of solution-processable macromolecule that can complex with many organic and inorganic systems.[29–31] GO can serve as an electron-transfer medium in GO/semiconductor composite photocatalysts[32] and can be tuned from an insulator to a semiconductor with different bandgaps.[33] In addition, GO can form transparent thin film, thus leading to enhanced light absorption.[34] Putting all these points together, it is rational to believe that GO can be a promising material for CoPi-based oxygen-evolving composite photocatalysts. However, there has been no report on the synthesis of CoPi/GO composites for water oxidation. Herein, for the first time, we demonstrate that CoPi nanoplates can be integrated with GO by photochemical deposition from an aqueous solution under visible-light illumination. We further demonstrate that the resulting CoPi/GO composites can be used as a novel highly efficient OEC toward water oxidation. The findings suggest that integrating CoPi with GO improves the interfacial hole transfer and decreases the electron–hole recombination, thereby leading to reduction of the overvoltage and enhancement of the photocurrent. The CoPi was photochemically deposited on the GO surface by using Co2+ in phosphate solution at pH 7.0 as a precursor and generating holes from visible-light-irradiated GO as the oxidizing driving force in the presence of oxygen as an electron scavenger. Figure 1a shows a typical transmission

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Figure 1. TEM images of a) GO and b) the composites; c) EDS of the composites. Inset in (b): TEM image of a single plate.

electron microscopy (TEM) image of the GO sheet used for CoPi deposition, which indicates that the sheet is several micrometers in size. Figure 1b shows the TEM image of the products obtained after the photodeposition process, and indicates the formation of nanoplates with size below 1 μm on GO. The inset in Figure 1b shows the TEM image of a single plate. The chemical composition of the composites was determined by energy-dispersive X-ray spectroscopy (EDS) and X-ray mapping analysis of the products coated on an ITO glass substrate. EDS (Figure 1c) shows peaks corresponding to C, Co, and P elements (other peaks originated from the substrate), which indicates that these nanoplates are CoPi. The corresponding X-ray maps (Figure S1, Supporting Information) reveal that both Co and P elements are relatively uniformly distributed in these nanoplates. The surface composition and elemental analysis of the composites were further characterized by X-ray photoelectron spectroscopy (XPS). Figure S2a shows the Co 2p XPS peaks of the composites. The characteristic peaks of Co 2p3/2 and Co 2p1/2 at around 781 and 797 eV, originating from Co2+ and Co3+, respectively, are consistent with the reported CoPi XPS peaks.[35,36] McDonald et al. have reported that CoPi generated by photodeposition contains Co2+ ions, which results in the appearance of a satellite peak located at approximately 802.5 eV.[37] In our present study, we also observe a satellite peak at around 802.5 eV. The P 2p XPS spectrum (Figure S2b) shows a single peak at 133 eV, which indicates the presence of phosphate in the composites.[25] All these observations provide further evidence to support the formation of CoPi/GO composites. The 1.4:1 Co/P ratio in the CoPi/GO composites, estimated by integrating the P 2p and Co 2p XPS spectra, is different from the 2:1 Co/P in an electrochemically deposited thick CoPi layer[23] and the 1:1 Co/Pi in photochemically deposited CoPi nanoparticles on ZnO,[25] which could be attributed to a different size or surface area

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to volume ratio of the CoPi particles.[37] From the EDS measurement, the weight percentage for Co and P is 4.68 and 2.05%, respectively, corresponding to 1.2:1 Co/P ratio. Note that keeping the mixture in the dark or visible-light irradiation of Co2+ in phosphate solution without the presence of GO fails to yield CoPi, thus demonstrating that GO acts as a visible-light photocatalyst for CoPi deposition. Figure 2a shows a schematic diagram to illustrate the photochemical deposition of the CoPi nanoplates on GO. Because of the oxygen functional groups, GO is highly soluble in water and exhibits a light brown color in suspension, which reflects the visible-light-absorbing ability of GO. Figure S3a shows the UV/Vis absorption spectrum for GO with an absorption onset at approximately 500 nm. Theoretical band structure analyses for graphene show that the gap nature of graphene changes from direct to indirect with increasing oxidation level.[38] Note that the oxygen content in GO could be varied by changing the synthesis conditions, such as oxidation time, and has a significant influence on the catalytic performance of GO.[39] We plotted the square and square root of the absorption energy (αE, where α is the absorbance) against the photon energy (E) to determine the energies for the direct and indirect gap transitions, as shown in Figures S3b and S3c, respectively. The approximately linear extrapolation gives apparent energies of 3.4–4.1 eV for the direct transition and 2.2–2.6 eV for the indirect transition in the GO specimen. The bandgap energy is sufficiently large to overcome the endothermic characteristics (1.23 eV theoretically) of the water-splitting reaction and reflects the visiblelight-absorbing ability of GO. However, to generate O2, the valence band (VB) edge of GO has to be lower (or more positive) than the energy level for water oxidation. To determine the conduction band (CB) edge position of GO, a linear potential scan was applied. Figure S4 presents the results of the scan from 0.4 to −1.0 V (vs. Ag/AgCl). The observation of a sharp current increase due to formation of an inversion layer at approximately −0.75 V (vs. Ag/AgCl) indicates that the GO specimen has a CB edge level at −0.75 V (vs. Ag/ AgCl). Based on the above data, the VB edge energy level of the GO is calculated to be 1.6 eV. Figure 2b shows the relevant energy diagram. We adopt the lower limits of the bandgap energies (Eg) to construct the energy level diagram. As previously reported, the oxidation potential of Co2+ to form Co2O3 is 0.70 eV versus the normal hydrogen electrode

Figure 2. Schematic representation of a) photochemical deposition of the CoPi nanoplates on GO and b) the relevant energy diagram. e− = electron, h+ = hole.


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voltammetry (DPV) of these electrodes was used to gain further insight into the large catalytic oxidation wave. As shown in Figure 3c, the CoPi/GO/GCE shows a very strong peak of the catalytic wave at 1.21 V, exhibiting a 60 mV negative shift of the oxidation peak potential relative to a previously reported result[43] and 220 mV negative shift relative to the electrodeposited CoPi-modified GCE. However, no catalytic peak is observed in this region for the other two electrodes. All these results indicate that the CoPi/GO composites can serve as a highly efficient OEC. Moreover, the stability of the CoPi/GCE and CoPi/ GO/GCE was investigated. As shown in Figure S5, after 50 cycles the CoPi exhibits a sharp decrease (48.3%) of current density; however, the CoPi/GO composites only show a slight decrease (7.6%) of current density. Such observations indiFigure 3. a) SEM image of the CoPi/GO/GCE. b)CV and c)DPV of bare GCE (curve 1), GO/GCE cate that the composite-modified elec(curve 2), CoPi/GO/GCE (curve 3), and CoPi/GCE (curve 4) in a standard three-compartment trode exhibits excellent stability. Previous cell with working electrode GCEs along with a Pt wire counter electrode and a reference works revealed that there are cracks in the electrode (Ag/AgCl). Electrolyte: 0.1 M potassium phosphate (pH = 7). electrodeposited film upon drying of the sample, which may lead to the poor sta(NHE) when [Co2+] = 0.5 mm at pH 7.0. Formation of the bility of the electrode. The improved oxygen-evolving activity CoPi, which is also based on Co2+/Co3+ oxidation, is expected of the composites can be attributed to the good conductivity to require a similar potential.[23,40] The VB of the GO has a of GO,[44] thereby leading to the increased charge transfer in potential level high enough for the oxidation of Co2+ to form the composites and better stability. Co3+. During this deposition, oxygen scavenges photogenWe also used rotating-disk electrode (RDE) measureerated electrons, leaving holes behind. These holes react with ments to reveal the oxygen evolution reaction (OER) CoPi/ Co2+, which leads to the in situ formation of CoPi on the GO GO composites in 0.1 m potassium phosphate (Figure 4a). surface. The current density increases along with the increased To ascertain the catalytic properties of the CoPi/GO com- sweeping rate. The linearity of the Koutecky–Levich plots posites for electrochemical oxidation of water to oxygen, and near parallelism of the fitting lines suggests first-order the composites were deposited on a glassy carbon electrode reaction kinetics toward the OER. Figure 4b shows a plot (GCE). Figure 3a shows the scanning electron microscopy of the current density as a function of potential (Tafel plot). (SEM) image of the CoPi/GO-modified GCE electrode The linear regime of the Tafel plot shows a similar slope for (CoPi/GO/GCE), which reveals the formation of a thin film the CoPi/GO to that for the CoPi electrodeposited on FTO consisting of transparent GO sheets and a large amount of (CoPi/FTO),[45] thus indicating that the mechanism of the CoPi nanoplates. Figure 3b shows the cyclic voltammetry water oxidation is similar for both electrodes. The slope of (CV) of bare GCE, GO-modified GCE (GO/GCE), and the Tafel plot for the CoPi/GO-modified GCE is 102 mV per CoPi/GO/GCE. The CoPi/GO/GCE shows the onset of a log unit, and 110 mV per log unit for CoPi/FTO. Both values large anodic current upon reaching the potential of water oxi- are larger than the 59 mV per log unit expected for a onedation (for which the thermodynamic limit is 0.62 V vs. Ag/ electron pre-equilibrium step to the reaction, as is implicated AgCl at pH 7) near 1.1 V (Figure 3b, curve 3), which is com- in the mechanism for CoPi catalysis.[46] It is demonstrated parable with previous studies.[23,41,42] In contrast, both bare that the significant thickness of the porous and amorphous GCE (curve 1) and GO/GCE (curve 2) exhibit poor cata- film would lead to a larger slope. Compared with CoPi/FTO, lytic activities even when the potential was extended to 1.4 V. the CoPi/GO-modified GCE has a smaller slope, which can Moreover, compared with the bare GCE and GO/GCE, the be attributed to the fact that the conductive 3D network CoPi/GO/GCE exhibits significant enhancement of the cur- formed in CoPi/GO composites can facilitate mass transport rent intensity. Because CoPi itself has oxygen-evolving ability, through the pores and decrease resistance to the electron we compared the electrocatalytic performance of the CoPi/ transport throughout the film. GO composites with pure CoPi fabricated by electrodeposiThe photoelectrochemical properties of the GO-modition on GCE, as shown in Figure 3b (curve 4). It is seen that fied ITO electrode (GO/ITO) and CoPi/GO-modified ITO the electrodeposited CoPi shows a large anodic current upon electrode (CoPi/GO/ITO) were compared by measuring reaching the potential of water oxidation near 1.15 V, which the photocurrent in potassium phosphate electrolyte under is in accordance with the literature.[23] Differential pulse white-light illumination (λ > 400 nm). Figure 5a shows that small 2013, 9, No. 16, 2709–2714


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to the literature[28] over Fe2O3 is 1.6 times at 0 V, as shown in Figure S6. So, the CoPi/GO exhibits superior photocurrent enhancement performance over other CoPi/semiconductor composites. All these results indicate that GO functions as a very effective visible-light photosensitizer, and integrating CoPi with GO can efficiently enhance the photocurrent by improving the interfacial hole transfer and suppressing electron–hole recombination. Furthermore, the photocurrent of the CoPi/GO-modified electrode after 2 h of illumination remained similar without any significant sign of catalyst dissolution or degradation. In summary, visible-light irradiation of Co2+ in phosphate solution in the presence of GO has been proven to be an effective strategy toward CoPi/GO composites with the use of GO as a photocatalyst for in situ photodeposition of CoPi. We have further demonstrated that the composites can act as a novel OEC with high efficiency. The GO not only serves as substrate for CoPi growth, but also increases charge transfer in the composites. Our present study is significant for the following two reasons: 1) it is the first example demonstrating the use of GO for preparing CoPi-based oxygen-evolving composite photocatalysts; and 2) it provides a general methodology for green and economic synthesis of GO-based visible-light OECs for renewable clean energy applications.

Experimental Section

Figure 4. a) Rotating-disk voltammograms of CoPi/GO composites in 0.1 M potassium phosphate (pH = 7.0) with a sweep rate of 10 mV s−1 at the different rotation rates indicated. The inset shows the corresponding Koutecky–Levich plot (J−1 versus ω−0.5). b) Tafel plot of the CoPi/GO composite OER currents in (a). Electrolyte: 0.1 M potassium phosphate (pH = 7.0) in a standard three-compartment cell with working GCE along with a Pt wire counter electrode and a reference electrode (Ag/AgCl).

the photocurrent of the CoPi/GO/ITO (curve 2) is considerably enhanced compared to that of the GO/ITO (curve 4) in this region. Both CoPi/GO composites (Figure 5a, curve 1) and GO (curve 3) show decreased dark currents. Integrating CoPi with GO causes a shift of onset potential from 0.3 V (curve 4) to −0.05 V (curve 2). Such negative shift of the onset potential suggests a larger accumulation of electrons and decreased charge-carrier recombination in the composites, and therefore, improved photocatalytic activity was achieved. Figure 5b shows the photocurrent of GO/ITO and CoPi/ GO/ITO measured at a bias of 0.0 V. A cathodic photocurrent is observed for GO upon illumination (curve 1), which means the photogenerated holes move to the ITO electrode due to the p-type nature of GO.[47] However, compared with GO, the CoPi/GO composites exhibit a 3.6-fold enhancement in the photocurrent (Figure 5b, curve 2). The use of a bias of −0.2 V leads to a 4.4-fold photocurrent enhancement (Figure 5c). Previous works reported that at 0 V, the CoPi/ ZnO catalyst shows a 2.6 times enhancement over ZnO in the intensity of the photocurrent,[25] and the enhancement of CoPi/Fe2O3 synthesized using a modified method according


Materials: Potassium phosphate was purchased from Beijing Chemical Corp. All other chemicals were purchased from Aladdin Ltd. (Shanghai, China) and used as received without further purification. The water used throughout all experiments was purified through a Millipore system. Preparation of CoPi/GO: GO was prepared from natural graphite powder through a modified Hummer’s method[48] using graphite powder, H2SO4, NaNO3, and H2O2 (30%) as the starting materials. As-synthesized GO was dispersed into individual sheets in distilled water at a concentration of 0.5 mg mL−1 with the aid of ultrasound for further use. Photochemical deposition of CoPi on GO was carried out by adding GO (1.0 mg) to a 0.5 mM CoCl2/0.1 M potassium phosphate electrolyte (pH = 7), which was purged with oxygen, followed by visible-light irradiation (λ > 400 nm) for 30 min with a 500 W xenon lamp (CHFXQ500W, Beijing). The obtained CoPi/GO composites were collected by centrifugation, washed with water by centrifugation twice, and then redispersed in water (4 mL) for characterization and further use. An electrodeposited CoPi-modified GCE was prepared according to the reported method.[23] In brief, a GCE was used as the working electrode with application of +1.2 V against Ag/AgCl in 4 M KCl in an electrolyte containing 0.5 mM CoCl2 and 0.1 M potassium phosphate buffer (pH 7). After 10 min of deposition, the working electrode was washed with water thoroughly and blow dried with N2 before use. Characterization: Scanning electron microscopy (SEM) measurements were made on an XL30 ESEM FEG scanning electron microscope at an accelerating voltage of 20 kV. For characterization by SEM, the suspension (20 μL) was placed on an ITO glass slide and air-dried at room temperature. An energy-dispersive X-ray spectroscopy (EDS) detecting unit was used to collect spectra for elemental analysis. The UV/Vis spectrum was obtained on a


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the CoPi/GO/GCE, an aliquot of the CoPi/GO composite solution (5 μL) was pipetted onto the GCE surface, and the solution was allowed to dry in air. For GO/GCE preparation, the same method was followed. The potentials were measured with an Ag/AgCl electrode as the reference electrode and converted to NHE potentials by using E(NHE) = E(Ag/AgCl) + 0.197 V. For rotating-disk electrode (RDE) measurements, the catalyst was prepared by the same method as for CV. The CoPi/GO composite solution (5 μL) was loaded on a glassy carbon RDE 5 mm in diameter (Pine Instruments). The working electrode was scanned cathodically at a rate of 10 mV S−1 with varying rotation speed from 400 to 2500 rpm. Photoelectrochemical Measurements: Photoelectrochemical test systems were composed of a CHI 660D electrochemical analyzer (CH Instruments, Inc., Shanghai), a 500 W xenon lamp (CHFXQ500W, Beijing) with a UV cutoff filter (λ > 400 nm, input power: 100 mW cm−2), and a homemade three-electrode cell with a KCl-saturated Ag/AgCl electrode, a platinum wire, and CoPi/GO as the reference, counter, and working electrodes, respectively. The CoPi/GO-modified ITO elecFigure 5. a) Current–voltage characteristics in a standard three-compartment cell with working electrode GO/ITO (curves 3 and 4) and CoPi/GO/ITO (curves 1 and 2) along with a trode was prepared by the dip-coating method: Pt wire counter electrode and a reference electrode (Ag/AgCl). b,c) Photocurrent of GO/ITO typically, the CoPi/GO suspension (100 μL) was (curve 1) and CoPi/GO/ITO (curve 2) measured at a bias of b) 0.0 and c)−0.2 V against the dip-coated onto a 0.5 cm × 2 cm ITO glass elecAg/AgCl reference electrode under white-light illumination (λ > 400 nm). Electrolyte: 0.1 M trode. The electrode was then air-dried in the potassium phosphate (pH = 7.0). dark at room temperature. For the GO-modified ITO electrode, the same method was followed. UV-1800 spectrophotometer. Transmission electron microscopy All solutions were purged with high-purity nitrogen for 10 min before (TEM) measurements were made on an H-8100 electron micro- photocurrent measurements to purge the dissolved oxygen. scope (Hitachi, Tokyo, Japan) with an accelerating voltage of 200 kV. The sample for TEM characterization was prepared by placing a drop of sample solution on a carbon-coated copper grid and drying at room temperature. X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALABMK II X-ray photoelec- Supporting Information tron spectrometer using Mg as the excitation source. To determine the amount of electrodeposited CoPi, the total electric charge was Supporting Information is available from the Wiley Online Library measured and the number of electrons transferred was calculated or from the author. according to Faraday’s Law: Q = nF (n: the number of moles of electrons; F: the Faraday constant). The amount of CoPi electrodeposited on the electrode was calculated to be 2.3 × 10−8 mol based on Acknowledgements a Q value of 0.218 × 10−3 C. The amount of CoPi in CoPi/GO was determined by the UV/Vis absorbance difference before and after This work was supported by the National Natural Science Foundation of the photochemical deposition, which is represented by C0 and C, China (No. 21175129), the National Basic Research Program of China respectively. According to the equation: n = (C0−C)·V (in which V is (No. 2011CB935800), and the Scientiﬁc and Technological Developthe volume of the reaction solution), the amount of CoPi photode- ment Plan Project of Jilin Province (Nos. 20100534 and 20110448). posited on GO was calculated to be 1.6 × 10−5 mol. [1] J. Chow, R. J. Kopp, P. R. Portney, Science 2003, 302, 1528. Electrochemical Measurements: Electrochemical measure[2] L. Hammarström, S. Hammes-Schiffer, Acc. Chem. Res. 2009, 42, ments were performed with a CHI 660D electrochemical analyzer 1859. (CH Instruments, Inc., Shanghai). A conventional three-electrode [3] H. Zhou, Y. Qu, T. Zeid, X. Duan, Energy Environ. Sci. 2012, 5, 2 cell was used, including a GCE (geometric area = 0.07 cm ) as the 6732. working electrode, an Ag/AgCl (saturated KCl) electrode as the ref[4] A. Fujishima, K. Honda, Nature 1972, 238, 37. erence electrode, and platinum foil as the counter electrode. All the [5] F. M. Toma, A. Sartorel, M. Lurlo, M. Carraro, P. Parisse, C. Maccato, S. Rapino, B. R. Gonzalez, H. Amenitsch, T. D. Ros, L. Calalis, experiments were carried out at ambient temperature. To prepare small 2013, 9, No. 16, 2709–2714


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communications [6] [7]

[8] [9] [10] [11]

[12] [13] [14] [15] [16] [17] [18] [19]

[20] [21] [22] [23] [24] [25] [26] [27] [28] [29]

J. Tian et al.

A. Goldoni, M. Marcaccio, G. Scorrano, G. Scoles, F. Paolucci, M. Prato, M. Bonchio, Nat. Chem. 2010, 2, 826. J. Barber, Chem. Soc. Rev. 2009, 28, 185. M. Carraro, A. Sartorel, F. M. Toma, F. Puntoriero, F. Scandola, S. Campagna, M. Prato, M. Bonchio, Top. Curr. Chem. 2011, 303, 121. J. L. Fillol, Z. Codolà, I. Garcia-Bosch, L. Gómez, J. J. Pla, M. Costas, Nat. Chem. 2011, 3, 807. S. Y. Reece, J. A. Hamel, K. Sung, T. D. Jarvi, A. J. Esswein, J. J. H. Pijpers, D. G. Nocera, Science 2011, 334, 645. L. Duan, L. Tong, Y. Xu, L. Sun, Energy Environ. Sci. 2011, 4, 3296. R. K. Hocking, R. Brimblecombe, L.-Y. Chang, A. Singh, M. H. Cheah, C. Lover, W. H. Casey, L. Spiccia, Nat. Chem. 2011, 3, 461. V. Artero, M. Chavarot-Kerlidou, M. Fontecave, Angew. Chem. Int. Ed. 2011, 50, 7238. P. D. Tran, L. H. Wong, J. Barber, J. S. C. Loo, Energy Environ. Sci. 2012, 5, 5902. J. Chen, P. Wagner, L. Tong, G. G. Wallace, D. L. Officer, G. F. Swiegers, Angew. Chem. Int. Ed. 2012, 51, 1907. M. Kato, T. Cardona, A. W. Rutherford, E. Reisner, J. Am. Chem. Soc. 2012, 134, 8332. M. Yagi, M. Toda, S. Yamada, H. Yamazaki, Chem. Commun. 2010, 46, 8594. R. Brimblecombe, G. F. Swiegers, G. C. Dismukes, L. Spiccia, Angew. Chem. Int. Ed. 2008, 47, 7335. R. Brimblecombe, A. Koo, G. C. Dismukes, G. F. Swiegers, L. Spiccia, J. Am. Chem. Soc. 2010, 132, 2892. G. F. Moore, J. D. Blakemore, R. L. Milot, J. F. Hull, H. Song, L. Cai, C. A. Schmuttenmaer, R. H. Crabtree, G. W. Brudvig, Energy Environ. Sci. 2011, 4, 2389. Y. V. Geletii, Z. Huang, Y. Hou, D. G. Musaev, T. Lian, C. L. Hill, J. Am. Chem. Soc. 2009, 131, 7522. M. M. Najafpour, T. Ehrenberg, M. Wiechen, P. Kurz, Angew. Chem. Int. Ed. 2010, 49, 2233. F. Jiao, H. Frei, Angew. Chem. Int. Ed. 2009, 48, 1841. M. W. Kanan, D. G. Nocera, Science 2008, 312, 1072. M. W. Kanan, Y. Surendranath, D. G. Nocera, Chem. Soc. Rev. 2009, 38, 109. E. M. P. Steinmiller, K. S. Choi, Proc. Natl. Acad. Sci. USA 2009, 106, 20633. D. K. Zong, D. R. Gamelin, J. Am. Chem. Soc. 2010, 132, 4202. D. K. Zhong, M. Cornuz, K. Sivula, M. Grätzel, D. R. Gamelin, Energy Environ. Sci. 2011, 4, 1759. J. A. Seabold, K.-S. Choi, Chem. Mater. 2011, 23, 1105. X. Huang, Z. Yin, S. Wu, X. Qi, Q. He, Q. Zhang, Q. Yan, F. Boey, H. Zhang, Small 2011, 7, 1876.


[30] [31] [32] [33] [34] [35] [36] [37] [38] [39]

[40] [41] [42] [43] [44] [45] [46] [47] [48]

X. Huang, Y. Qi, F. Boey, H. Zhang, Chem. Soc. Rev. 2012, 41, 666. K. P. Loh, Q. Bao, G. Eda, M. Chhowalla, Nat. Chem. 2012, 2, 1015. Q. Xiang, J. Yu, M. Jaroniec, Chem. Soc. Rev. 2011, 41, 782. H. K. Jeong, M. H. Jin, K. P. So, S. C. Lim, Y. H. Lee, J. Phys. D: Appl. Phys. 2009, 42, 065418. G. Eda, G. Fanchini, M. Chhowalla, Nat. Nanotechnol. 2008, 3, 270. S. K. Pilli, T. G. Deutscch, T. E. Furtak, J. A. Turner, L. D. Brown, A. M. Herring, Phys. Chem. Chem. Phys. 2012, 14, 7032. Y. S. Zhu, M. H. Cao, X. Y. Ma, C. Y. Xu, X. Wang, L. Ren. C. W. Hu, Dalton Trans. 2012, 41, 2935. K. J. McDonald, K. S. Choi, Chem. Mater. 2011, 23, 1686. J. Ito, J. Nakamura, A. Natori, J. Appl. Phys. 2008, 103, 113712. The oxygen content in GO could be varied by changing the synthesis conditions, such as oxidation time, and the GO oxygen content has a significant influence on the catalytic performance of GO. Previous works have demonstrated that for GO species with different oxygen content, the top energy level of the valence band is different, resulting in the inability or ability for O2 evolution [T.-F. Yeh, J.-M. Syu, C. Cheng, T.-H. Chang, H. Teng, Adv. Funct. Mater. 2010, 20, 2252]. Because the redox potential of the Co2+/Co3+ couple is similar to that of H2O/O2, the semiconductors have a proper valence band position to evolve O2 and can ideally also oxidize Co2+ ions to photodeposit CoPi OEC.[37] We also tried GO species with low oxygen content as the photosensitizer but failed to obtain CoPi on GO by photodeposition, which can be attributed to the fact that its valence band level is not low enough to oxidize Co2+ to Co3+. Y. Surendranath, M. Dinca, D. G. Nocera, J. Am. Chem. Soc. 2008, 131, 2615. T. L. Wee, B. D. Sherman, D. Gust, A. L. Moore, T. A. Moore, Y. Liu, J. T. Scaiano, J. Am. Chem. Soc. 2011, 133, 16742. E. R. Young, R. Costi, S. Paydavos, D. G. Nocera, V. Bulovic´, Energy Environ. Sci. 2011, 4, 2058. D. Shevchenko, M. F. Anderlund, A. Thapper, S. Styring, Energy Environ. Sci. 2011, 4, 1284. A. Pendashteh, M. F. Mousavi, M. S. Rahmanifar, Electrochim. Acta 2013, 88, 347. E. R. Young, D. G. Nocera, V. Bulovic´, Energy Environ. Sci. 2010, 3, 1726. Y. Surendranath, M. W. Kanan, D. G. Nocera, J. Am. Chem. Soc. 2010, 132, 16501. T. F. Yeh, F. F. Chan, C. T. Hsieh, H. Teng, J. Phys. Chem. C 2011, 115, 22587. W. S. Hummers, Jr., R. Offeman, J. Am. Chem. Soc. 1958, 80, 1339.


Received: December 20, 2012 Published online: February 18, 2013

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