How to increase the photocatalytic activity of titanium dioxide for hydrogen production

Photocatalytic hydrogen production by the splitting of water is a sustainable, renewable and promising technology. Being able to meet future energy needs by producing green and clean hydrogen fuel will have a significant impact on the environment. By using a TiO2 photocatalyst with sunlight, water can be separated into H2 and O2 and hydrogen fuel production can be supported. However, TiO2 has a wide band-gap (~3.2 eV) and is only excited by UV-light: it is inactive under visible light irradiation. Thus, much of the research developed in recent years has been focused on extending the optical absorption of TiO2 to the visible region of the spectrum in order to substitute UV-light by sunlight to make use of solar energy for practical applications. It has been known that doped cations induce visible-light absorption and influence the photoreactivity of TiO2. Despite extensive research on cation-doped TiO2, there is still no unifying result or explanation for the effect of cations on the photocatalytic activity of TiO2 under solar light. In this study, the non-defective anatase (001) surface was modelled with finite, neutral, stoichiometric cluster models cut from the anatase bulk structure in order to determine the location and the bonding status of the dopant cations. In the doped models, substitutional locations of cations were analyzed. The structures of the doped models were constructed by replacing one titanium atom by one cation. All the calculations were carried out using the Density Functional Theory (DFT) method. The DFT calculations were performed by the hybrid B3LYP functional. 6-31 G(d) basis set was used. The dopant positions were optimized by changing their locations in the clusters to find the lowest energy configuration. All of the investigated cations reduced the band gap energy of TiO2, thus increasing visible-light activity. In addition, it was calculated that metal doping affected the decomposition of water positively.