Heterogeneity of surface colour centres on alkaline earth metal oxides as revealed through EPR/ENDOR spectroscopy

A variety of surface anion vacancies, or point defects, are created by high-temperature activation of a series of polycrystalline alkaline earth metal oxides (MgO, CaO and SrO). Subsequent UV irradiation of the activated oxide under a hydrogen atmosphere results in the generation of surface colour centres [F-S+(H)], by electron trapping at these anion vacancies. The paramagnetic properties of these colour centres were studied by EPR and ENDOR spectroscopy. H-1 ENDOR spectroscopy revealed that a well defined heterogeneity of trapped electron species exists on each oxide surface, as characterized by the different superhyperfine couplings between the trapped electron and the nearby proton of the F-S(+) (H) centre. On MgO and CaO two dominant F-S(+) (H) centres were identified (labelled sites I and II) whereas on SrO three F-S(+) (H) species were found (sites I, II and III). The possible surface sites responsible for electron stabilization are discussed, and include a 3C corner mono-vacancy, a 4C mono-vacancy and an anion-cation di-vacancy. The results indicate that regardless of the oxide used, a common degree of morphological similarities exists on each oxide. Copyright (C) 2002 John Wiley Sons, Ltd.