Synergetic control of band gap and structural transformation for optimizing TiO₂ photocatalysts

Impurity doping and synthesizing polymorphic particles are the common strategies to improve activity of TiO₂ photocatalyst by lowering the band gap and enhancing electron-hole separation rate. However, these two approaches have side effects. Doping of impurities make space charge region (SCR) thinner near the surface, which requires smaller sized particles than undoped TiO₂ for the optimal performance. Polymorphic TiO₂ particles, in which rutile and anatase phases coexist in a particle, are usually large due to energetic unstability of the rutile phase in a fine particle. For this contradiction that one needs small size while the other needs large size, two effects are not easy to be combined. In this study, we suggest a dual-doping strategy to solve the contradictory problem of SCR reduction by donor doping and inevitable size growth in polymorphic particles. We successfully dope W, a band gap narrower, into fine size of polymorphic particles by Sn-codoping, a promoter of the anatase-to-rutile transformation (ART), and demonstrate greatly improved photocatalytic activity. The accelerated ART by Sn-doping could keep the size of polymorph junctioned TiO₂ small (∼10 nm) as lower temperature annealing become able to induce the ART. The concept of dual doping with a band gap narrower and an ART promoter provides a way to synthesize highly active photocatalysts by overcoming the drawback from shortened SCR length.