Extending ambient pressure x-ray photoelectron spectroscopy to plasma studies: A custom-built plasma source apparatus.

The characterization of the electronic structure and chemical states at heterogeneous interfaces (e.g., gas-solid or liquid-solid) can be effectively achieved using ambient pressure x-ray photoelectron spectroscopy (AP-XPS). However, acquiring these properties under plasma conditions remains a significant challenge. In this study, we introduce an AP-XPS system equipped with a custom-built spatially confined plasma source. This configuration confines the plasma to a controlled region, with the plasma intensity precisely regulated by adjusting either the working distance or the applied voltage. This system enables the direct detection of plasma electrons and the real-time characterization of surface states during plasma-surface interactions. To validate its efficacy and versatility, we conducted two proof-of-concept experiments: argon (Ar) plasma etching of graphene and oxygen (O2) plasma oxidation of platinum (Pt). The results demonstrate that the Ar plasma effectively etches graphene, as evidenced by distinct changes in the XPS spectra. Similarly, the exposure of a Pt surface to the oxygen plasma induces pronounced surface oxidation. This system significantly extends the operando capabilities of AP-XPS for in situ studies of plasma-enhanced reactions. Its application holds considerable promise for advancing fundamental and applied research in materials science and chemical engineering, particularly in the areas of surface modification and catalytic processes.