Oxygen 1s X-ray Photoelectron Spectra of Iridium Oxides as a Descriptor of the Amorphous–Rutile Character of the Surface

Characterization of the surface of iridium oxide (IrO_x) materials is of crucial importance to understand catalysts for the oxygen evolution reaction (OER) in low-temperature water electrolysis. While much of our current knowledge is based on well-defined single-crystal surfaces, surface-sensitive techniques like X-ray photoelectronic spectroscopy (XPS) are relevant to characterize the nanostructures considered. In this work, we describe a simple approach to use oxygen 1s spectra as an identifier of the amorphous/crystalline characteristics of iridium oxide structures from purely amorphous to purely crystalline. This conceptual approach was validated on seven commercially available materials. The presence of oxygen-associated defects in the surface moieties/species is shown even for purely crystalline materials with defect concentration increasing with greater amorphous character. This methodology provides us with an accessible ex situ descriptor of the catalyst surface as a baseline for further studies of the impact on catalytic properties.