An electron spin resonance study of charge-carrier stabilization in ZnO

Abstract

The effect of u.v. irradiation on ZnO at 77 K in the presence of different reducing and oxidizing molecules has been studied by e.s.r. spectroscopy. In all cases the stabilization of one type of charge carrier (electron or hole) in the bulk is due to the trapping of the other type by molecules adsorbed on the surface. Adsorption of H₂, SO₂ and CO leads to the stabilization of electrons, mainly as Zn⁺ centres, while O₂ leads to the stabilization of trapped holes, both paramagnetic centres stabilized in subsurface regions. Generally the reaction of the adsorbed molecules with the photo-generated holes produces diamagnetic species at the surface, except for oxygen, which leads to the stabilization of various paramagnetic oxygen species (O₂^–, O^– and O₃^– and/or O₃^3–). The degree of surface hydroxylation greatly influences the type of species formed during irradiation, owing to the capture of holes by the hydroxyl groups.