Observation of Ir 5d Orbitals in Epitaxial IrO₂ Thin Films Using Resonant Inelastic X-Ray Scattering

Transition metal oxides have been considered one of the alternative material systems for energy conversion and storage systems due to their intriguing physical and chemical properties. Specifically, IrO₂ exhibits high activity and better stability under oxygen evolution reaction (OER) than other 4d transition metal oxides, e.g., ruthenates [1]. Ir 5d orbital states in IrO₂ can play an important role in the OER process because of interactions between d orbitals near Fermi level and adsorbates. However, understanding of the process of the model system is yet incomplete due to experimental limitations, specifically spectroscopic techniques.<br/><br/>Here, we measured 5d orbital states in IrO2epitaxial thin films grown on TiO₂ substrates using Ir L₃-edge resonant inelastic x-ray scattering (RIXS). Using the RIXS technique, we systematically investigated 5d orbital states near Fermi level and understood the transition between specific orbitals coupled with orientations and strain. For instance, our RIXS measurement demonstrated that the 5d d_x2-_y2 orbital in the plane of edge-sharing of octahedra along [001] dominantly contributed to about 2eV energy loss signal. We propose that the in-situ RIXS techniques combined with an electric field can understand the OER process and explore emergent physical properties.<br/><br/>[1] G. Kwon et al., ACS Catalysis 11 10084 (2021).