Donghwan Kim Every element on the surface (5 - 10 atomic layer) with XPS is likely to be in its highest oxidation state if the system is in air on on an oxide support. Carbon makes up a large proportion of the surface and is used as the reference point (285 eV) to correct for charge and provide 'accurate' oxidation states. In a mixture one component will always be pulled preferentially to the surface with chemisorption induced segregation. Use argon ion etching to get well below the surface and look at the bulk, not surface, composition.

Artur Braun Interesting. My experience is with alloys on oxide supports (SiO2, Al2O3, etc) for catalysis. As well as chemisorption induced segregation pulling one component preferentially to the surface, the metals were invariably in their highest oxidation state. This includes Au where in PdAu and Au alloys on silica the gold was found (very surprisingly) in the +3 oxidation state (via XPS). Perhaps oxygen from the support is affecting these values rather than in air. In Ag alloys the Ag was always in the +1 oxidation state and obviously this must be the basis for the bactericide properties of 'silver' colloids. Ag2O is slightly soluble in water; Ag metal is not.... We could only recover the bulk composition by argon ion etching down to 10 - 15 atomic layers. The surface oxide was very easily reduced by hydrogen in all cases.

The picture shown shows the Ta(4f) peaks, the peak assigned as Ta3+ is the O(2s) peak as the sample without Ta shows.

The reason you get a lower energy peak in the Ti(2p) spectra is due to the TiO2 lattice becoming defective, hence reduced states

the Ta 4f peak is overlapped with O 2s peaks.