The defect structure of nickel oxide surfaces as revealed by photoelectron spectroscopy

Abstract

Nickel oxides characterised by electron microscopy and with different defect concentrations have been studied by X-ray photoelectron spectroscopy. Oxides preannealed in air at 700, 1100 and 1450 °C were examined after heating in vacuo in the spectrometer between 20 and 500 °C. High-binding-energy components of O(1s) spectra at 531.4 eV and Ni(2p_3/2) spectra at 856.1 eV can be correlated with the oxide defect structure. Loss of electron-acceptor surface hydroxyl groups as water during evacuation and heating results in the development of surface charge owing to increased band bending at the surface. Above 300 °C, carbon impurity present at < 0.15 of a monolayer is removed predominantly as CO. With the defective ‘700 °C annealed’ nickel oxide this results in a major increase in the intensity of the O(1s) peak at 531.4 eV, but the other two samples show a continuous decrease in this peak's intensity. The O(1s) peak at 531.4 eV is attributed initially to OH, and after evacuation at 500 °C to an O^– surface species. O/Ni atom ratios > 1.0 are found on all surfaces, even after evacuation at 500 °C, but higher values, up to 2.5, are found after evacuation at 25 °C. Spectra from a NiO(100) single-crystal surface evacuated at 600 °C and heated in oxygen at 450 °C show enhancement of the high-binding-energy O(1s) and Ni(2p_3/2) peaks at low electron take-off angles, indicating stabilisation of defect O^– and Ni³⁺ species at the solid/vacuum interface.