Revisiting Co 2p core‐level photoemission in LiCoO2 by in‐lab soft and hard X‐ray photoelectron spectroscopy: A depth‐dependent study of cobalt electronic structure

Abstract

Addressing the oxidation state of functional materials such as transition metal oxides is a current critical challenge and requires new methodologies to characterize their electronic structure within surface to bulk resolution at the nanometer scale. One approach to this issue is the combination of co‐localized soft and hard X‐ray photoelectron spectroscopies for a non‐destructive depth profiling. In this work, we demonstrate the capability to characterize the oxidation state of cobalt in LiCoO2 thin films, a model electrode material in the Li‐ion battery field, within the first 15 nm. The capability of the methodology to address the surface evolution of the cobalt oxidation state is tested through a proof of concept surface modification introduced by Ar‐ion sputtering. To address the cobalt local electronic structure at different depths from the extreme surface, we exploited Auger‐free Hard X‐ray photoelectron spectroscopy (HAXPES) spectra to fit the Co 2p core‐level features in the X‐ray photoelectron spectroscopy (XPS) spectra. This approach may pave the way for a better understanding of the surface electronic structure changes in transition metal oxides driven by their applications in a broad range of technologies.