Paper AS-TuP9
In Situ XPS Depth-Profiling of Hydrogen Storage Material VCrTi during Thermal Annealing

Tuesday, November 10, 2009, 6:00 pm, Room Hall 3

VCrTi is a candidate material for hydrogen-storage. The presence of a surface oxide layer, its thickness, and its composition can be expected to effect the properties of charging and release of stored hydrogen, and as such it is of interest to characterise the native oxide, and to investigate whether more desirable properties can be obtained by artificially modifying the surface composition. In this study we have used synchrotron radiation photoelectron spectroscopy to study the chemical composition of the native oxide layer of a commercial sample of V₂₅Cr₄₀Ti₃₅ alloy. The total XPS energy resolution is sufficient to provide chemical state-specific information, and spectra recorded at a range of different take-off angles provide information on the depth-profiles of the composition. The maximum entropy method is used to generate element-specific (and chemical state-specific) depth profiles from the spectra. The following conclusions are drawn:-

• The thickness of the native oxide layer as determined from the XPS data is consistent with a TEM analysis, at approximately 5 nm, and consists of oxides of vanadium, chromium, and titanium, in amounts roughly corresponding to the composition of the alloy. It also contains carbon. Depth-profiling suggests that the carbon forms a sub-layer.
• Thermal annealing gradually reduces the oxide coverage with increasing temperature. After heating to 800 degrees C, the dominant surface component appears to be titanium oxides, with vanadium and chromium oxides only remaining below the surface.