New anode materials for photoelectrolysis
The design strategy for a successful oxide anode for use in photoelectrolysis cells is reviewed. Conduction bands based on Ti4+ and Nb5+ can be made to possess sufficiently small electron affinities to allow the cell to operate without external bias. For example, data on SrTi0.75Zr0.25O3 are presented to show that the Ti4+ conduction band can be fine-tuned to the required electron affinity using Zr4+ as dopant. The Zr4+ dopant also enhances remarkably oxygen evolution in the dark, and a mechanism for this observation is advanced. In order to reduce the bandgap in oxide titanates, incorporation of a second metal-ion sublattice appears to be essential. Three types of substituent cation that can, in principle, give rise to a valence band at the right energy are discussed: (i) transition metals giving rise to ndm bands, (ii) lanthanides giving rise to 4fm bands and (iii) reduced B-metals giving rise to ns2 bands. Optical absorption studies of MnTiO3 and NaCeTi2O6 indicate that the bulk Mn2+ 3d5 and Ce3+ 4f1 levels are at about the right energy, but holes photogenerated in these levels become trapped in surface–polaron sites, as they only oxidise water slowly. Analysis of the transient photocurrents for MnTiO3 shows that holes arrive at the surface in both the O2–:2p6 band and the Mn2+:3d5 levels and the former oxidise water rapidly.