Density functional theory study of rutile VO2 surfaces.

We present the results of a density functional theory investigation of the surfaces of rutile-like vanadium dioxide, VO(2)(R). We calculate the surface energies of low Miller index planes and find that the most stable surface orientation is the (110). The equilibrium morphology of a VO(2)(R) particle has an acicular shape, laterally confined by (110) planes and topped by (011) planes. The redox properties of the (110) surface are investigated by calculating the relative surface free energies of the non-stoichiometric compositions as a function of oxygen chemical potential. It is found that the VO(2)(110) surface is oxidized with respect to the stoichiometric composition, not only at ambient conditions but also at the more reducing conditions under which bulk VO(2) is stable in comparison with bulk V(2)O(5). The adsorbed oxygen forms surface vanadyl species much more favorably than surface peroxo species.