Influence of electronic structures of doped TiO2 on their photocatalysis

The modification of bandgap of TiO2 was intensively studied for decades to improve its visible light absorbance efficiency. The practical application potential of TiO2 as photocatalysts for water splitting and water purification has motivated enduring experimental and theoretical research of the doping effects in bulk and nanosized TiO2 using transition metals, rear earths, p-block metals and metalloids, and non-metal elments as dopants to decrease the bandgap of TiO2. This review summarized the typical theoretical results of the dopant induced variation in electronic structure, bandgap, and density of states of TiO2. The codoping effects of metal/metal, metal/non-metal combinations were also introduced briefly to display the modification of electronic structures. Some results were accompanied by experimental results to demonstrate the influence of improved light absorbance efficiency on the photocatalytic performance. The doping effects on the density of states of surface were also summarized briefly. The metal dopants show clear influences on the 3d electrons of titanium to elevate or depress the minimum of conduction band, while the non-mental dopants mainly interact with the 2p electrons of oxygen to change the position of the maximum of the valence band. The review also noticed the theoretical development of the doping effect with the establishment of novel models, such as the water-TiO(2)surface interaction. It should be noted that the theoretical models rarely consider the doping induced variation of defect types and concentration, Fermi level position, surface active sites, and charge transport due to the ground state simulation and shortcoming of density functional theory (DFT). The phenomenological explanations of the experimental results are arbitrary in most of the reports. A universal model is required to explain the complex dependence of the process of photocatalysis on the semiconducting properties, such as bandgap, Fermi level, charge transport, and surface states. ((c) 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)