ESR INVESTIGATION INTO THE EFFECTS OF HEAT-TREATMENT AND CRYSTAL-STRUCTURE ON RADICALS PRODUCED OVER IRRADIATED TIO2 POWDER

Electron spin resonance measurements were carried out at 77 K under ir radiation for anatase TiO2 powders treated by heating at various tempe ratures in the air. For the untreated powder photoproduced holes were trapped at the surface forming Ti4+O-.Ti4+OH- radicals, while, for the heated powder, they were trapped as Ti4+O2-Ti4+O-. radicals at the su rface. Photoproduced electrons were trapped as Ti3+ at the surface of the unheated powder, while they trapped at the inner part of the heat- treated powder. These differences could be explained by the desorption of surface hydroxyl groups and the change in the surface structure ac companied by the crystalline growth. No photoproduced radicals were de tected for rutile TiO2 powder, which may explain the low photocatalyti c activity of this crystalline structure. (C) 1997 Elsevier Science S. A.