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. T he EPR signals (observed at 6-200 K) of the trapped hole in all of the se systems exhibit similar properties. Holes produced by band gap irra diation 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 enri ched water support the identification of trapped holes as an oxygen su rface 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 sensi tive to hydration and the total surface area available for chemical re actions. The EPR signal disappears with the addition of hole scavenger s that are strongly bound to the surface, such as polyvinyl alcohol an d 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 coll oids 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 othe r radicals at higher temperatures. The EPR signal from trapped holes d etected in aqueous suspensions of ZnO particles show a similar pattern to that from the TiO2 systems.