Local Structure of Nanocrystalline Ru1−xNixO2−δ Dioxide and Its Implications for Electrocatalytic Behavior—An XPS and XAS Study

Chemical composition, crystal structure, as well as short-range atomic arrangement of nanocrystalline Ru 1− x Ni x O 2−δ oxides with x ranging between 0 and 0.3 were studied using energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS). The prepared materials form single-phase nanocrystals with rutile structure. Regardless of the chemical composition, the surface of Ru 1− x Ni x O 2−δ oxides is Ni-enriched with respect to overall chemical composition. According to both XPS and XANES, the oxidation state of Ru remains +4 in the studied materials. Ni ions are present in both divalent and trivalent states with the fraction of trivalent ions decreasing with increasing Ni content. The refinement of local structure using EXAFS data based on Ru−K and Ni−K edge absorption spectra shows that Ru preserves local arrangement characteristic for ruthenium dioxide. The incorporated Ni shows a tendency to form clusters within a rutile structure for low Ni concentration. At high Ni content, the architecture of the Ni-rich defects resembles architecture of shear planes in oxygen-deficient rutile. These Ni-rich regions likely manifest themselves on the surface as line or plane defects, which are the most likely structural features active in the electrocatalytic processes.