X-ray photoelectron spectroscopy studies of reactions on chromium metal and chromium oxide surfaces

This paper reports several interrelated XPS investigations of the Cr-O system that provide additional interpretation of the Cr 2p(3/)2 spectra, along with more detail on the reaction mechanisms between O(2) gas and H(2)O vapour with metallic Cr. The Cr 2p3/2 spectra of polycrystalline Cr(2)O(3) contain multiplet structures that bear a strong resemblance to those calculated by Gupta and Sen for the free Cr(3+) ion. The O/Cr ratio derived from the corrected Cr(3+) and substitutional O(2-) intensities for all polycrystalline Cr(2)O(3) samples was 1.5. Annealing of freshly cleaved polycrystalline Cr(2)O(3) surfaces above 550 C resulted in a sharpening of the multiplet structures that may be caused by the increased prevalence of the (0 0 0 1) Cr(2)O(3) orientation on the surface.The slight difference in binding energies between the low energy (0 0 0 1) plane and the other major crystallographic orientations could contribute to the spectral broadening that is observed for the Cr 2p3/2 spectral line shapes. The Cr 2p(3/2) spectrum for Cr(OH)(3) was isolated from that produced by Cr(2)O(3) and the multiplet structure was qualitatively re-assembled for that of the hydroxide. The Cr 2p3/2 and O is spectra of reaction products on polycrystalline metal surfaces were analysed using the above information. The oxidic portion of the Cr 2p(3/2) spectra could be cleanly separated from the metallic substrate using spectral subtraction. Reactions of either O(2) gas or H(2)O vapour both produced a thin Cr(2)O(3) film that was deficient in Cr(3+) and, depending on the reactant, showed varying concentrations of hypo-stoichiometric Cr moieties. The surface corrected O/Cr ratios calculated for all the oxidised surfaces were 1.7. There was little evidence to support the formation of Cr(OH)(3) on these surfaces. For comparison, an aqueous reaction of a Ni-Cr (20%) alloy was studied where both Cr(OH)(3) and Cr(2)O(3) components could be analysed. For this example no sub-stoichiometric Cr oxide species were produced. (C) 2010 Elsevier B.V. All rights reserved.