Challenges in Co-Alloyed Titanium Oxynitrides, a Promising Class of Photochemically Active Materials

We present a perspective on recent developments in modified TiO2 photo catalysts for visible light-driven photochemistry with an emphasis on water splitting. We focus on doped and alloyed TiO2 and in particular address the synergistic effects observed in materials with both transition metal cations and nonmetal anions. Several reports have demonstrated absorption of longer wavelengths (lambda = 500-600 nm) by codoped materials compared to the absorption edge of TiO2. We review these advances against the backdrop of well-established doped TiO2 research, suggesting on the basis of compositional analysis and wavelength-resolved measurements of photon conversion efficiency that the increase in visible light absorption is likely due to absorption between defect states rather than true band gap narrowing. We draw a distinction between codoped and co-alloyed materials, stressing the attractive electronic structure of the latter. In highlighting recent literature, data examining the rate of photochemical water splitting or magnitude of anodic current as they depend on the wavelength of incident light are emphasized. Finally, areas for further research are highlighted, particularly in the synthesis of co-alloyed compositions of TiO2.