Visible-light activation of TiO2 photocatalysts: Advances in theory and experiments

The remarkable achievement by Fujishima and Honda (1972) in the photo-electrochemical water splitting results in the extensive use of TiO2 nanomaterials for environmental purification and energy storage/conversion applications. Though there are many advantages for the TiO2 compared to other semiconductor photocatalysts, its band gap of 3.2eV restrains application to the UV-region of the electromagnetic spectrum (λ≤387.5nm). As a result, development of visible-light active titanium dioxide is one of the key challenges in the field of semiconductor photocatalysis. In this review, advances in the strategies for the visible light activation, origin of visible-light activity, and electronic structure of various visible-light active TiO2 photocatalysts are discussed in detail. It has also been shown that if appropriate models are used, the theoretical insights can successfully be employed to develop novel catalysts to enhance the photocatalytic performance in the visible region. Recent developments in theory and experiments in visible-light induced water splitting, degradation of environmental pollutants, water and air purification and antibacterial applications are also reviewed. Various strategies to identify appropriate dopants for improved visible-light absorption and electron-hole separation to enhance the photocatalytic activity are discussed in detail, and a number of recommendations are also presented.