J. Phys. Chem. C 2008, 112, 6099-6102
6099
Structural and Photoelectrochemical Properties of BiVO4 Thin Films Hongmei Luo,*,† Alex H. Mueller,‡ T. Mark McCleskey,† Anthony K. Burrell,† Eve Bauer,† and Q. X. Jia*,† Materials Physics and Applications DiVision and Chemistry DiVision, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 ReceiVed: NoVember 29, 2007; In Final Form: January 25, 2008
Monoclinic scheelite BiVO4 thin films have been successfully prepared by a chemical solution approach of polymer-assisted deposition. The films are transparent yellow with granular morphology and show a strong absorption in the visible light region. The band gap of BiVO4 films is estimated to be 2.54 eV. The photoresponse of the films on conducting oxide electrode substrates is observed, and the photocurrent increases with the higher energy absorption bands.
Introduction Photoelectrochemical water splitting using semiconductor photocatalysts has been considered as an attractive route to convert solar energy directly into hydrogen (solar hydrogen) for future renewable energy applications.1-3 The efficient utilization of the visible portion of the solar spectrum is essential to both solar hydrogen generation and the photocatalytic decomposition of organic pollutants. Bismuth vanadate (BiVO4) has been recognized as a visible-light-driven photocatalyst for these applications.4-17 BiVO4 exists in three phases, monoclinic scheelite, tetragonal zircon, and tetragonal scheelite.4,5 Experimental results have shown that the photocatalytic properties of BiVO4 are strongly related to its crystalline phase and morphology. For example, monoclinic scheelite-type BiVO4 has been reported to show higher photocatalytic activity for O2 evolution compared to the other two phases.4,5 Several methods have been reported to prepare BiVO4 in either bulk or thin film forms. For instance, solid-state reaction,6 solution coprecipitation,5,7,8 hydrothermal9,10 and sonochemical11 reactions, metalorganic decomposition,12-14 the sol-gel process,15 chemical bath deposition,16 and the hybrid organicinorganic precursor route17 have been used to synthesize BiVO4. Recently, it has been reported that porous BiVO4 thin films on a conducting oxide electrode deposited by metalorganic decomposition exhibit enhanced incident photon-to-current conversion efficiency (IPCE) for photoelectrochemical water decomposition under visible light.13,14 The much improved IPCE by using a porous BiVO4 film has been attributed to the enhanced penetration of the electrolyte solution into the pore space of the nanocrystalline films.13 It is well-known that the large surface area of a photocatalyst is one of the most important factors in achieving high efficiency in the photocatalytic reaction from studies of TiO2 systems.18,19 Due to its large surface area and easy process, mesoporous silica is often chosen as the matrix to support semiconductor nanoparticles for higher photocatalyst activity.18-21 Herein we report on the preparation and the photoresponse of porous BiVO4 films on indium tin oxide (ITO) coated glass (ITO-glass) and mesoporous silica covered ITO* To whom correspondence should be addressed. E-mail:
[email protected] (H.L.),
[email protected] (Q.X.J.). † Materials Physics and Applications Division. ‡ Chemistry Division.
glass by a polymer-assisted deposition (PAD) approach.22 The major distinction for this process compared to other chemical solution approaches lies in the fact that the soluble polymer plays a significant role: the polymer not only controls the desired viscosity for the process, but also binds the metal ions to prevent premature precipitation and formation of metal oxide oligomers. Experimental Section The precursor solution for the deposition of BiVO4 films was obtained from a mixture of separate solutions of Bi and V bound to polymers. Specifically, 2 g of Bi(NO3)3 was dissolved in 40 mL of H2O, followed by addition of 2 g of ethylenediaminetetraacetic acid (EDTA) and 2 g of poly(ethyleneimine) (PEI) into the solution. To prepare the V solution, 2 g of NaVO3 was dissolved in 40 mL of H2O followed by addition of 2 g of PEI. The two solutions were separately purified by repeated Amicon filtration that retains the high molecular weight polymer with bound metal atoms while allowing any low molecular weight (
