Monday, April 22, 2013
Designing Chemically Robust Metal Oxides for Visible-Light Photocatalysis: toward Solar Z-Scheme Water Splitting
Bart M. Bartlett, Assistant Professor, Chemistry and Chemical Biology, University of Michigan
Abstract: Solar driven water splitting for large-scale hydrogen fuel production from semiconductor photo-electrodes has the potential to provide energy on large scale from renewable, sustainable sources. Our research focuses on the kinetically more demanding oxygen-evolution reaction, and we prepare thin film metal oxide photoanodes by low-temperature, solution-based processes. One promising light absorber is TiO2:(Nb,N) where Nb and N substitute for Ti and O on their respective lattice sites in anatase. These materials are prepared by sol-gel processing followed by annealing in flowing ammonia. We observe a band-gap energy as low as 2.0 eV at 25% Nb and 2% N. In conjunction with a RuO2 catalyst, powdered TiO2:(Nb,N) evolves O2. A second class of materials we study is the transition-metal tungstates, and we have prepared our most promising candidate, CuWO4, by several routes: electrochemical deposition, sol-gel processing, and spray pyrolysis. These methods afford highly reproducible and robust CuWO4 thin-film electrodes on transparent conducting substrates. CuWO4 is an n–type semiconductor with a band-gap energy of ~2.4 eV. CuWO4 thin films photooxidize water at with simulated solar radiation with a nearly quantitative Faradaic efficiency for O2 evolution at no applied bias in the presence of the sacrificial electron acceptor, [Fe(CN)6]3–. Most important, these thin-film electrodes are stable against photocorrosion when illuminated with visible light at neutral pH, a significant improvement to the more commonly studied photoanode, WO3. Current efforts are aimed at preparing complex tungstates that absorb lower energy light to improve the quantum yield.