Visible light-responsive radial TiO2 mesocrystal photocatalysts for the oxidation of organics

Abstract

Radial TiO₂ mesocrystals (rad-TiO₂ MCs), the so-called “sea urchin-like microspheres”, usually consisting of rutile TiO₂ nanorods with a length of several microns and a diameter of several tens of nanometers, are very promising photocatalyst materials due to the efficient light harvesting ability, large surface area, and easy separation from reaction solution. The key to actually making use of rad-TiO₂ MCs with a high potential as the base material of practical photocatalysts is the reduction of synthesis temperature and time and the simultaneous impartation of the high activity for multiple electron-oxygen reduction reaction (ORR) and visible-light responsiveness to them. This minireview highlights recent progress on the hot topics through the surface modification by Au nanoparticles (Au/rad-TiO₂ MCs) and extremely small iron oxide clusters (FeO_x/rad-TiO₂ MCs) acting as both photosensitizer (or oxidation sites) and electrocatalyst for multiple electron-ORR (or reduction sites). Subsequent to the introduction, the synthesis of rad-TiO₂ MCs by a seed-assisted hydrothermal method is described. The fundamental parts deal with the optical and multiple electron-ORR properties of Au/rad-TiO₂ MCs and FeO_x/rad-TiO₂ MCs and the redox reaction site separation mechanism unique to the 3D structure. Then, the applications of the rad-TiO₂ MC-based photocatalysts to the oxidation of organics and other important reactions are explored. Finally, the conclusions are summarized with the advantages of rad-TiO₂ MCs over the conventional TiO₂ particles clarified, and some future subjects are described.