Insights into the surface of mesoporous nickel oxide and its interaction with oxygen and water

The surface chemistry of mesoporous nickel oxide (NiO_x) plays a pivotal role in its functionality across various technological applications. Herein, we present a detailed study of NiO_x surface states using multiple spectroscopic techniques, including ambient pressure X-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV-vis) spectroscopy, and Fourier-transform infrared (FTIR) spectroscopy, to observe the removal and reformation of surface species during gas dosing. Our findings reveal the presence of both surface oxygen- and hydroxide-species on the NiO_x surface. Furthermore, the results suggest that NiO_x surface states consist of approximately 50% hydroxides and the remainder comprising adsorbed oxygen species, likely predominantly diatomic oxygen ions (O₂^-), that are associated with higher valence Ni states (Ni³⁺). In situ experiments demonstrate that the formation and stability of hydroxides and diatomic oxygen ions depend on temperature and are significantly influenced by interactions with atmospheric oxygen and water. Our insights into the NiO_x surface state composition and reactivity offer a nuanced understanding of its surface chemistry, with implications for enhancing its performance in catalysis, sensing, energy harvesting and energy storage devices where the surface states of nickel oxide are known to dictate electronic and chemical properties of the material.