Structural and chemical properties of NiO x thin films: the role of oxygen vacancies in NiOOH formation in a H 2 O atmosphere.

NiO films grown from 50 nm thick Ni on Si(111) were put in contact with oxygen and subsequently water vapor at elevated temperatures. Near ambient pressure (NAP)-XPS and -XAS reveal the formation of oxygen vacancies at elevated temperatures, followed by HO dissociation and saturation of the oxygen vacancies with chemisorbing OH. Through repeated heating and cooling, OH-saturated oxygen vacancies act as precursors for the formation of thermally stable NiOOH on the sample surface. This is accompanied by a significant restructuring of the surface which increases the probability of NiOOH formation. Exposure of a thin NiO film to HO can lead to a partial reduction of NiO to metallic Ni accompanied by a distinct shift of the NiO spectra with respect to the Fermi edge. DFT calculations show that the formation of oxygen vacancies and subsequently Ni leads to a state within the band gap of NiO which pins the Fermi edge.