Metal Doping of BiVO 4 by Composite Electrodeposition with Improved Photoelectrochemical Water Oxidation

We report that oxide composite electrodeposition can be used for the facile preparation of metal-doped BiVO4 photoelectrodes for photoelectrochemical water oxidation. The photoactivity of electrodeposition film was improved by the addition of a small amount of tungstic acid particles during the electrodeposition. These particles are incorporated in the deposit and finally generate tungstendoped bismuth vanadate. The suspended particles in the plating solution were electrostatically attracted to the cathode and accordingly incorporated into the deposit (electrostatic deposition). WO3 nanoparticles (NPs) can be used instead of tungstic acid, to yield a BiVO4 with different properties. Enhanced photoelectrochemical (PEC) water oxidation was confirmed via scanning electrochemical microscopy (SECM) by detecting increased oxygen evolution with using optical fiber incorporating a ring electrode. ■ INTRODUCTION BiVO4 was reported to be an n-type photocatalyst by Kudo et al. in 1999, and many studies have been reported using this material for water oxidation and the PEC decomposition of organic materials. Photocatalysts for water oxidation have been extensively studied as a component in systems for water splitting to produce hydrogen as a solar fuel. Among the many candidates for water oxidation photocatalysts, metal oxides have been extensively investigated because of their good physical and chemical stability. For example, extensive studies of TiO2 and attempts to modify its large band gap have been reported since Fujishima and Honda suggested the possible photolysis of water using TiO2 electrodes. 7−10 Other metal oxides, e.g., WO3, 11−13 Fe2O3, 14−16 or BiVO4 as mentioned above, have also been widely studied. However, no simple oxide has yet been discovered with sufficient efficiency and stability as a photocatalyst to achieve practical water splitting. Monoclinic BiVO4, which has been considered as the highly active photocatalyst among its many polymorphs, has a band gap size of 2.4−2.5 eV, so it can absorb the visible portion of the solar energy so as to have a theoretical efficiency of 9% for solar-to-chemical conversion. However, the short carrier diffusion lengths and significant recombination of photongenerated electron−hole pairs limit the photoactivity of BiVO4. 20 To enhance the activity of BiVO4 for water oxidation, there have been many studies to reduce electron−hole recombination: (a) metal dopants added into BiVO4 to increase the donor density and increase carrier mobility, e.g., W-, Mo-, or P-doped BiVO4; 21−23 (b) semiconductor layers added, e.g., at the FTO/BiVO4 interface, decrease surface recombination of an electron with a surface trapped-hole, e.g., with WO3 and SnO2 as the barrier layers; 24−27 (c) treatments of BiVO4 at the liquid surface, e.g., the addition of electrocatalysts, to increase the rate of water oxidation reactions. Further, the relationship between photocatalytic activity and many different preparation methods of the metal oxide has been reported. Among various preparation methods such as chemical bath deposition, precipitation, hydrothermal, spray pyrolysis, metal−organic decomposition, and electrochemical approaches (e.g., electrodeposition), electrodeposition has the advantage of being a simple, low cost process, that is compatible with various size surfaces, but the precise control of adding effective dopant elements to the film is challenging. In the case of electrochemically grown hematite, which is formed by the reduction of H2O2 in the presence of Fe , its doping with metals such as Pt, Mo, and Cr was accomplished by the reduction of the metal ion with consequent codeposition of metal in the film; these improve the photoactivity of hematite. However, the doping of semiconductors via metal codeposition cannot be simply applied to the preparation of semiconductors such as BiVO4, TiO2, and WO3, which are generally synthesized by an electrochemical oxidation reaction where the metal would not codeposit simultaneously unless the oxidized metal ion formed a precipitate on the deposit surface. Received: August 28, 2013 Revised: October 7, 2013 Published: October 11, 2013 Article