DFT study of various tungstates for photocatalytic water splitting

Tungsten oxide (WO₃) is a promising photocatalytic material, but it has some limitations on its optoelectronic properties. Compared with binary materials, ternary compounds provide a much greater variety of compositions and hence properties, which can be tuned to suit particular applications. In this work, the effect of introducing a second metal cation into tungsten oxide is studied by Density Functional Theory (DFT) calculations. The compounds investigated include AWO₄ tungstates (A = Sn, Fe), M₂WO₆ tungstates (M = Bi, Sb), tungstite (WO₃·H₂O) and hydrotungstite (WO₃·2H₂O). The tungstates studied are found to have either a small band gap (SnWO₄, FeWO₄, WO₃·H₂O and WO₃·2H₂O), and thus potentially improved visible-light activity compared with WO₃, or a more negative conduction band edge than WO₃ (Bi₂WO₆, Sb₂WO₆), which means they may be able to achieve overall water splitting, in contrast to WO₃. The band gap narrowing and the band edge changes are attributed to the introduction of new electronic states due to the second metal cation, as well as structural changes, particularly a larger spacing between layers of WO₆ octahedra. All the materials studied have a relative high static dielectric constant (ε_r > 10), allowing for exciton dissociation, and a small enough electron effective mass (m_e* < 0.5m₀) along at least one direction for carrier diffusion. The performance of all the compounds is likely to be limited by poor hole mobility, except for Sb₂WO₆ and the hydrated compounds which also have a relatively small hole effective mass (m_h* < 0.5m₀). Through this comparative study, the key trends in properties as a function of composition for a family of complex materials have been identified, allowing appropriate compositions to be selected and tuned for specific applications.