Photochemical Properties, Composition, and Structure in Molecular Beam Epitaxy Grown Fe "Doped" and (Fe,N) Codoped Rutile TiO2(110)

We have investigated the surface photochemical properties of Fe "doped" and (Fe,N) codoped homoepitaxial rutile TiO2(110) films grown by plasma-assisted molecular beam epitaxy. Fe does not incorporate as an electronic dopant in the rutile lattice but rather segregates to the film surface. However, codeposition of Fe with N enhances the solubility of Fe, and DFT calculations suggest that codopant complex formation is the driving force behind the enhanced solubility. The codoped films, in which a few atomic percent of Ti (O) are replaced with Fe (N), exhibit significant disorder compared to undoped films grown under the same conditions, presumably due to dopant-induced strain. Codoping redshifts the rutile bandgap into the visible. However, the film surfaces are photochemically inert with respect to hole-mediated decomposition of adsorbed trimethyl acetate. The absence of photochemical activity may result from dopant-induced trap and/or recombination sites within the film. This study indicates that enhanced visible light absorptivity in TiO2 does not necessarily result in visible light initiated surface photochemistry.