Hexagonal TiO2 for Photoelectrochemical Applications

Band gap narrowing is crucial for applications of TiO2, especially for photoelectrochemical water splitting. In this article, a novel hexagonal TiO2 phase is predicted by means of the first-principles calculations with the hybrid density functional. The crystal structure of this hexagonal phase of TiO2 consists of metastable TiO2 layers (space group P63/MMC), similar to that of graphite. The calculated electronic structure reveals that bulk hexagonal TiO2 is a semiconductor with a much smaller band gap of 1.7 eV compared to the rutile and anatase phases, while that of a single layer of TiO2 is 2.1 eV, which are in excellent agreement with available experimental results. This hexagonal phase of TiO2 would be a good candidate for photo catalyst in hydrogen production using sun light which is considered to be the ultimate clean fuel.