XPS surface study of nanocrystalline Ti–Ru–Fe materials

Abstract

The surface properties of Ti:Ru:Fe (2−x:1+x/2:1+x/2) (with x=0, 0.25, 0.5, 0.75, and 1) and Ti:Ru:Fe:O (2:1:1:w) (with w=0.0, 0.5, 1.0, 1.5, and 2.0) have been determined by X-ray photoelectron spectroscopy (XPS) in both their as-milled state and after being in contact with a chlorate oxidizing solution. The O surface concentrations of both sets of samples are almost identical, indicating that the O-free samples readily react with air. All samples in their as-milled state have an elemental Ti, Ru and Fe surface contents that closely follow that expected from their bulk composition, indicating that there is no surface enrichment in any of the elements. In the as-milled state, more than 90% of Ti and Fe atoms are in the highest possible oxidation state, while Ru is in the metallic state. Following immersion of the samples in an oxidizing chlorate electrolyte, the Ru surface concentration decreases by a factor of two. This is also accompanied by an increase in the oxidation state of the Ru atoms left at the surface from 0 to +4. From a comparison between the Ru 3d5/2,3/2 core level spectra of the electrodes with those of crystalline and hydrated RuO2, it is postulated that dissolution and re-deposition of Ru in the form of hydrated RuO2 occurs at open circuit potential in the chlorate electrolyte. The consequences of these findings for the electrocatalytic activity of the electrodes in chlorate electrolyte are finally discussed.