Visible light photocatalysis of pristine anatase TiO2 mesocrystals induced by largely exposed and stepped {001} surface

Currently, many strategies have been developed to improve the visible light photocatalytic ability of TiO2. Most of the strategies, such as doping, dye sensitization, formation of heterojunctions, use of a co-catalyst, and preparation of black TiO2, involve the introduction of new substances into pristine TiO2, which may cause new concerns about biosecurity of the new substances. Additionally, most researchers are focusing on the nano-sized TiO2 materials. However, nano-sized materials may affect the recovery process at the end of the reaction. In this work, we developed an easy and efficient method suitable for large scale production to prepare square brick-like anatase TiO2 mesocrystals with micrometer sizes and stepped structures on exposed {001} facets. First, square brick-like NH4TiOF3 particles were synthesized and sintered at a relatively low temperature and then converted to relatively less crystallized TiO2 mesocrystals with exposed {001} facets via topotactic transformation. Then, a sintering process was applied to promote crystal growth and fusion between the elemental building units of the less crystallized TiO2 mesocrystals, which finally led to the formation of stepped structures on the exposed {001} facets. We demonstrated that the unique structure can largely improve the visible light photocatalytic activity of TiO2. This strategy is interesting and different from the current widely used strategies because the visible light photocatalytic activity comes from the change in surface morphology instead of the change in the composition of a photocatalyst, which indicates the already well-proven safety. Also, our materials are mesocrystals, which provides more ease of recovery.