Resolving the Valence of Iron Oxides by Resonant Photoemission Spectroscopy

TL;DR

This paper demonstrates how Resonant Photoemission Spectroscopy can accurately determine the mixed valence states of iron in ultrathin FeO2 films, overcoming limitations of conventional XPS.

Contribution

It introduces the application of ResPES to quantify iron cation valence states in complex oxides, revealing a mixed Fe2+/Fe3+ valence in ultrathin FeO2.

Findings

FeO2 film contains equal Fe2+ and Fe3+ cations, averaging +2.5 valence.

ResPES effectively distinguishes valence states where XPS overlaps.

FeO2 likely derived from Fe3O4 sublattice structure.

Abstract

Precisely determining the oxidation states of metal cations within variable-valence transition metal oxides remains a significant challenge, yet it is crucial for understanding and predicting the properties of these technologically important materials. Iron oxides, in particular, exhibit a remarkable diversity of electronic structures due to the variable valence states of iron (Fe2+ and Fe3+), however, quantitative analysis using conventional X-ray photoelectron spectroscopy (XPS) is challenging because of significant overlapping of the Fe2p spectra among different oxidation states. In this study, we leverage the intriguing case of Pt supported FeO2 phase of monolayer thickness (ML) as a model system and employ Resonant Photoemission Spectroscopy (ResPES) to directly quantify the cation valence states and compositional ratios in this complex Fe oxide. Our results reveal that this ultrathin FeO2 film (Pt-O-Fe-O), contrary to the +3 valence predicted by density functional theory (DFT), consists of an equal mixture of Fe2+ and Fe3+ cations, yielding an average valence of +2.5. Structurally, FeO2 is likely derived from the Fe3O4 sublattice, featuring an octahedral Fe layer (50% Fe3+ and 50% Fe2+) bonded to upper and lower oxygen layers.