Charge transfer in Cr adsorption and reaction at the rutile TiO ₂(110) surface

The rutile TiO₂(110) surface has been doped with sub-monolayer metallic Cr, which oxidises and donates charge to specific surface Ti ions. X-Ray and ultra violet photoemission spectroscopy and first principles density functional theory with Hubbard U are used to assign the oxidation states of Cr and surface Ti and we find that Cr²⁺ forms on bridging oxygen ions and a 5-fold coordinated surface Ti atom is reduced to Ti³⁺ and the Cr ions readily react with oxygen (to Cr³⁺), which leads to depletion of surface Ti³⁺ 3d electrons.