Many potential semiconductors for photocatalysis of water splitting, including TiO2, have band gaps that are too large for absorption of visible light and hence have low efficiencies under sunlight. Solid solution formation is one approach to reducing band gaps and enhancing visible-light absorption. In this work, we use density functional theory calculations to investigate the electronic properties of ZnO-AlN and ZnS-GaP solid solutions and multi-layered structures. The solid solutions are found to have small energies of mixing from the two constituents, and tunable band gaps that depend on both composition and atomic ordering. The band gaps of the solid solutions are often smaller than those of either constituent on their own. Addition of only a small amount of AlN to ZnO and GaP to ZnS reduces the band gap into the correct energy range for absorption of visible light and close to the optimum for photocatalysis of water splitting under sunlight. Similar band gap tunability can be achieved with multi-layered structures. Preliminary experimental results show high photocatalytic actvitiy for ZnS-GaP multi-layered thin films under visible light, in agreement with the computational predictions.
Dr Judy Hart joined the School of Materials Science and Engineering at UNSW as a Lecturer in July, 2013. She was previously a Ramsay Memorial Fellow and lecturer in the Centre for Computational Chemistry, University of Bristol, U.K. Her research interests are in the combined use of computational and experimental approaches to design new semiconductor materials, particularly for applications in photocatalysis.