Electron spin resonance observations of oxygen chemisorption on zinc oxide

Abstract

Oxygen chemisorption at 20°C has been examined in ZnO samples derived by vacuum decomposition of zinc carbonate. Evidence is given that an e.s.r. spectrum with features at g-values of 2.051, 2.042, 2.0085 and 2.0036 results from the super-position of triplet spectra characterized by different low field features at g-values of 2.051 and 2.042. A further signal at g≃ 1.96 also observed prior to chemisorption is assigned to free electrons from donors. Spin density measurements for the chemisorption spectrum and the signal at g≃ 1.96 provide evidence that the two triplet spectra result from different processes. The triplet spectrum with the low field feature at g≃ 2.051 is assigned to O^–₂ radicals resulting from trapping of free electrons. The other spectrum is attributed to O^–₂ or O^– radicals arising from oxygen interaction with electrons in surface defects. A model is discussed in which the radicals from both processes are adsorbed on cation sites neighbouring an oxygen ion vacancy.