Photocatalytic Activity Enhancement of Anatase/Rutile-Mixed Phase TiO2 Nanoparticles Annealed with Low-Temperature O-2 Plasma

Photodecomposition and photobactericidal activities of anatase/rutile-mixed phase TiO2 nanoparticles annealed with low-temperature O-2 plasma are clarified by comparing them with those annealed in ambient air. The photocatalytic activities of plasma-assisted-annealed sample greatly enhance as compared with the untreated sample, under not only UV irradiation but also visible-light irradiation. The photocatalytic activities of air-annealed samples do not enhance under UV irradiation but enhance under visible-light irradiation. The enhanced photocatalytic activities due to the plasma-assisted annealing (PAA) originate from the increased photoexcited carrier concentration. This enhancement is discussed from PAA-induced characteristic factors. PAA facilitates the phase transformation to anatase, contributing directly to extending the photoexcited carrier lifetime. PAA introduces more oxygen vacancies, contributing to trapping more photogenerated electrons. PAA also introduces more bridging/terminal oxygen groups adsorbed on the surface, increasing the upward band-bending, the depletion layer width at the surface, and the charge transfer from rutile to anatase. These two introductions contribute to facilitating the separation of photoexcited carriers. Furthermore, PAA reduces the aggregate size of TiO2 nanoparticles formed on the surface, contributing to increasing optical absorptions. More reactive oxygen species produced from the bridging/terminal oxygen groups by the photoexcited carriers will also enhance the photocatalytic activities.