7032 Phys. Chem. Chem. Phys., 2012, 14, 7032–7039 This journal is c the Owner Societies 2012 Cite this: Phys. Chem. Chem. Phys., 2012, 14, 7032–7039 Light induced water oxidation on cobalt-phosphate (Co–Pi) catalyst modified semi-transparent, porous SiO 2 –BiVO 4 electrodesw Satyananda Kishore Pilli,* a Todd G. Deutsch, c Thomas E. Furtak, b John A. Turner, c Logan D. Brown a and Andrew M. Herring* a Received 2nd March 2012, Accepted 6th March 2012 DOI: 10.1039/c2cp40673j A facile and simple procedure for the synthesis of semi-transparent and porous SiO 2 –BiVO 4 electrodes is reported. The method involves a surfactant assisted metal–organic decomposition at 500 1C. An earth abundant oxygen evolution catalyst (OEC), cobalt phosphate (Co–Pi), has been used to modify the SiO 2 –BiVO 4 electrode by electrodeposition (ED) and photoassisted electrodeposition (PED) methods. Modified electrodes by these two methods have been examined for light induced water oxidation and compared to the unmodified SiO 2 –BiVO 4 electrodes by various photoelectrochemical techniques. The PED method was a more effective method of OEC preparation than the ED method as evidenced by an increased photocurrent magnitude during photocurrent-potential (I–V) characterizations. Electrode surfaces catalyzed by PED exhibited a very large cathodic shift (B420 mV) in the onset potential for water oxidation. The chopped-light I–V measurements performed at different intervals over 24-hour extended testing under illumination and applied bias conditions show a fair photostability for PED Co–Pi modified SiO 2 –BiVO 4 . Introduction Over the past few decades, photoelectrochemical (PEC) water splitting through semiconductor photoelectrodes has been of great interest as it could be a cost-effective and direct method for the production of renewable fuel and energy storage from solar energy. 1–5 Several metal oxide based semiconductor electrodes have been extensively explored as photoelectrodes. The ease of manufacture and stability compared to non-oxide based semiconductor electrodes makes them particularly suitable for the oxygen evolution reaction. The revival of interest in the development of photoelectrodes is based on production of low cost visible light active semiconductor electrodes with high surface area, high-energy conversion efficiency, and stability. Recently, BiVO 4 having hybridized O-2p and Bi-6s orbitals contributing to the formation of the valence band has attracted much attention in developing visible light driven water oxidation electrodes. 6,7 It has a moderate band gap (2.4–2.6 eV) 8–10 and a conduction band edge at 0 V vs. NHE at pH = 0. 8,11 Though it has shown activity towards water oxidation under visible light irradiation, the efficiencies are found to be low due to the difficult transfer of the photogenerated electron-hole pairs. 12 This can be accomplished, either by integration of more effective adsorption sites on the surface so that holes can be rapidly scavenged, or by promoting the rate of transfer of h + to OH À ions present in the electrolyte using a charge-transfer catalyst. 13 Extensive studies have been commenced to improve the photo- response and efficiency of BiVO 4 based photoelectrodes. Incor- poration of Mo, 14,15 W, 16,17 Si, 18 Na, 19 Eu, 20 Dy, 21 and Pd 22 into BiVO 4 shows enhanced PEC performance. The development of semi-transparent electrodes is very impor- tant, for the fabrication of tandem cells with high efficiency, as they can maximize the transmission of photons. 23 Recently, it has been reported that the synthesis of semiconductor electrodes with the presence of Si precursors resulted in transparent composite electro- des containing SiO 2 . 24–26 The presence of SiO 2 has also been found to enhance the hole transfer process and PEC characteristics of semiconductor electrodes such as TiO 2 by introducing new active sites through improved hydroxyl ion adsorption. 27 Further, water oxidation generally occurs at significant overpotentials as it involves removal of a total of four electrons and four protons from two water molecules to form one O 2 molecule. The integration of an effective catalyst is essential to improve the solar conversion efficiency and also to reduce the overpotential required for water oxidation. Recently, Kanan and Nocera have reported a heterogeneous oxygen evolution catalyst (OEC) consisting of the earth-abundant elements cobalt and phosphate (Co–Pi) prepared by a simple anodic electrodeposition which a Department of Chemical and Biological Engineering, Colorado School Mines, Golden, CO 80401, USA. E-mail: aherring@mines.edu; Fax: (+1) 303 273 3730; Tel: (+1) 303 384 2082 b Department of Physics, Colorado School Mines, Golden, CO 80401, USA c Energy Sciences, National Renewable Energy Laboratory, Golden, CO 80401, USA w Electronic supplementary information (ESI) available: EDX mapping, linear sweep voltammograms under illuminated, dark conditions, SEM image, IPCE, and chronoamperometry. See DOI: 10.1039/c2cp40673j PCCP Dynamic Article Links www.rsc.org/pccp PAPER