Localization of Conduction Band Electrons in Polycrystalline TiO2 Studied by ESR

Abstract

Adsorption of electron acceptors on partially reduced TiO2 leads to the localization of mobile conduction band electrons which is indicated by the appearance of an ESR signal with g = 2.003. The localization process is accompanied by an electron transfer from donor centers in reduced TiO2 (Ti3+ ions) to adsorbed molecules. The ESR spectra show a decrease of the Ti3+ signal intensity at g = 1.96. Additionally, in the case of O2, SO2 and C6H5NO2 adsorption, the ESR signals of the respective anion radicals are observed. Illumination of samples with an electron acceptor adsorbed enhances the g = 2.003 signal. The activation energy corresponding to the temperature dependence of the localization process has been determined to be ~ 1 kcal/mole in the temperature range -140 to -40°C. In the case of O2 and N2O the signal at g = 2.003 could be annihilated chemically by using propylene which reacted with the adsorbed electron acceptor and thus removed the species which reduced the electron mobil ity. © 1981, Walter de Gruyter. All rights reserved.