Mechanism of photocatalytic production of active oxygens on highly crystalline TiO2 particles by means of chemiluminescent probing and ESR spectroscopy

The effect of calcination on photocatalytic activities was studied for highly crystalline TiO2 powder, which was synthesized by the method of hydrothermal crystallization in organic media (HyCOM). The productions Of O(2)(.-)and H2O2 under ambient condition were monitored by means of luminol chemiluminescence probing. The amount of produced O-2(.-) was decreased with the calcination at lower temperatures in parallel to the decrease of inner Ti3+ signal observed at 77 K by ESR spectroscopy. The formation route for H2O2 under aerated condition is mainly two-step O-2 reduction in the presence of oxidizable organic molecules such as luminol. Oxidization activity was significantly high when HyCOM-TiO2 was calcined at higher temperatures, 973-1173 K, as monitored by the amount of oxidized luminol. The observation in ESR measurements and other evidences indicated that the luminol is oxidized via hydroxyl radicals rather than the direct oxidation by photoinduced surface trapped holes.