The XPS measurements were performed using a custom-built SPECS XPS spectrometer equipped with a monochromatized Al-Kα X-ray source (μFocus 350). The samples were loaded as a powder onto the sample holder using double-sided carbon tape. Charge correction was applied so the adventitious carbon peak for binding energy calibration (284.8 eV). XPS data evaluation was carried out with CasaXPS software, employing Scofield sensitivity factors and the Shirly background type.