Molecular oxygen-mediated vacancy diffusion on TiO2(1 1 0)-new studies of the proposed mechanism

The recent report [Science 299 (2003) 377], in which the scanning tunneling microscope (STM) shows that adsorbed molecular oxygen locally induces anisotropic surface vacancy diffusion by dissociation near the vacancy, is inconsistent with negative observations of isotopic mixing between adsorbed 18O2 and the 16O atoms in the TiO2 lattice. Using adsorbed 18O2 on TiO2(1 1 0), prepared identically to the surface studied by STM, both the photodesorption and thermal desorption of adsorbed O2 fail to detect any isotopic exchange with lattice O. These results suggest that the mechanism proposed for the O2-mediated vacancy diffusion must be modified.