TRAPPED HOLES ON TIO2 COLLOIDS STUDIED BY ELECTRON-PARAMAGNETIC-RESONANCE

An electron paramagnetic resonance (EPR) study of paramagnetic species formed on band gap irradiation of TiO2 colloids, aqueous suspensions of anatase (Degussa P-25), and rutile (Aldrich) powders is reported. The EPR signals (observed at 6-200 K) of the trapped hole in all of these systems exhibit similar properties. Holes produced by band gap irradiation of a TiO2 colloid move from the oxygen lattice to the surface and are trapped directly on oxygen atoms bound to surface Ti(IV) atoms. The results obtained with TiO2 colloids prepared with oxygen-17 enriched water support the identification of trapped holes as an oxygen surface anion radical covalently bound to titanium atoms, Ti(IV)-0-Ti(IV)-O.. The intensity of the EPR signal from the hole trap is very sensitive to hydration and the total surface area available for chemical reactions. The EPR signal disappears with the addition of hole scavengers that are strongly bound to the surface, such as polyvinyl alcohol and KI. The signal obtained for trapped holes at 6-240 K is not the same as that of surface bound or free OH. radicals, since no change in the EPR line width was observed when D2O was used for preparation of colloids instead of H2O. In addition, spectra obtained at Q-band microwave frequency show that the spectral splittings are not due to hyperfine coupling. The EPR signal remains the same with increasing temperature up to 150 K, indicating that primary radicals do not convert into other radicals at higher temperatures. The EPR signal from trapped holes detected in aqueous suspensions of ZnO particles show a similar pattern to that from the TiO2 systems.