Interface-Sensitive Soft X-ray Absorption Spectroscopy by Electron/Ion-Yield Methods and Its Application to Solid-Liquid Interfaces for SrTiO₃ and CoOOH

The understanding of solid-liquid interfaces is crucial for numerous chemical processes, although it presents significant challenges. We employed soft X-ray absorption spectroscopy (sXAS) in electron and ion detection modes to explore various interfaces. It was observed that the electron yield mode provides interface-sensitive results, while the ion yield mode captures ionic currents from the surface to the bulk liquid, which results in bulk sensitivity. Interestingly, differences in the currents in the ion mode for the two electrodes replicate the interface sensitivity. We applied these interface-sensitive modes to strontium titanate (SrTiO₃) and cobalt oxyhydroxide (CoOOH) systems to characterize structures of interface water and chemical states of the solid surfaces. Notably, surface cobalt atoms in CoOOH exhibit a high spin state when interacting with liquid water, which is a sharp contrast to its low spin state in a vacuum. The interface water on the SrTiO₃ surface displayed an ice-like structure. These findings provide insights into the sensitivities of different sXAS detection modes, which enhance our understanding of dynamic behaviors at the liquid-solid interfaces under various conditions.