XPS surface study of nanocrystalline Ti-Ru-Fe materials

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 oxidi zing solution. The O surface concentrations of both sets of samples are alm ost identical, indicating that the O-free samples readily react with air. A ll samples in their as-milled state have an elemental Ti, Ru and Fe surface contents that closely follow that expected from their bulk composition, in dicating 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 possi ble oxidation state, while Ru is in the metallic state. Following immersion of the samples in an oxidizing chlorate electrolyte, the Ru surface concen tration decreases by a factor of two. This is also accompanied by an increa se in the oxidation state of the Ru atoms left at the surface from 0 to +4. From a comparison between the Ru 3d(5/2,3/2) core level spectra of the ele ctrodes 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 the se findings for the electrocatalytic activity of the electrodes in chlorate electrolyte are finally discussed. (C) 2000 Elsevier Science B.V. All righ ts reserved.