Dehydration Pathway Study of Ultrathin Ni (OH) ₂ Probed via In Situ X‐Ray Photoelectron Spectromicroscopy

ABSTRACT In this work, the conversion of ultrathin 2D‐Ni (OH) 2 to NiO is studied using in situ X‐ray absorption near edge spectroscopy at the Ni L 3 and O K edges and using variable photon energy X‐ray photoelectron spectroscopy at the Ni 2p 3/2 and O 1s edges. The Ni L 3 X‐ray absorption spectroscopy and corresponding 2p 3/2 X‐ray photoelectron spectroscopy show the oxidation state of the Ni atom to be Ni 2+ in the course of the reaction; however, the film surface and oxygen defect structure show the presence of a large metallic Ni component before the complete disappearance of the hydroxide phase indicating oxygen vacancy formation before the completion of water desorption. O 1s spectral changes during heat treatment under ultrahigh vacuum, O 2 dosing, and H 2 O dosing suggest the spectral features are due to water desorption and adsorption. By comparing the relative probe depths of X‐ray photoelectron spectroscopy and X‐ray absorption spectroscopy, it is determined that the surface, at each step in the dehydration process, consists of a combination of Ni, NiO, and Ni (OH) 2 phases, which each contain a mixture of adsorbed hydroxyl groups. Repeated dosing with H 2 O, studied with O 1s X‐ray photoelectron spectroscopy, suggests that the dissociation of water facilitates the formation of oxygen vacancies.