Interpretation of Oxygen 1s X-ray Photoelectron Spectroscopy of ZnO

X-ray photoelectron spectroscopy (XPS) is widely used to determine the chemical and electronic states of atoms within a material. However, it is often complex to interpret the O 1s region in metal oxides, where an ∼531 eV binding energy feature appears between lattice oxygen (∼530 eV) and oxygen-containing surface species (∼532 eV). This feature has been vaguely ascribed to oxygen vacancies or oxygen deficient regions for many decades. This work employs full-potential density functional theory to calculate the binding energies of the O 1s electrons under two- and three-dimensional periodic boundary conditions as a probe of expected XPS spectra. ZnO is used as an example system. Both bulk crystal regions containing a range of oxygen defects and slabs with a range of surface terminations and functionalizations have been considered. The slabs considered are mostly {1010} and {1120} surfaces that are not expected to be reconstructed from the cleaved bulk structure. The resulting O 1s binding energies show no signature for oxygen defects in bulk regions. Furthermore, the 531 eV binding energy feature often ascribed to oxygen vacancies or oxygen deficient regions can instead be readily explained by the O 1s electrons from water molecules strongly bound to the exposed ZnO surface (i.e., chemisorbed, as distinct from more loosely bound water) or surface oxygen passivated with hydrogen. This work will rectify many misinterpretations of XPS data of the O 1s region in metal oxides, provide guidance for precisely understanding the oxygen states of a material, and subsequently enable the real origin of material properties to be revealed.