Adsorption of acetylene on a rutile TiO2(110) surface: a low temperature STM study

Abstract

Low temperature scanning tunneling microscopy (LT-STM) has been applied to investigate the adsorption of acetylene (C₂H₂) on the rutile TiO₂(110) surface at 77 K. Through in situ exposing and ex situ annealing experiments we unambiguously demonstrate that C₂H₂ molecules adsorb weakly on top of the Ti_5c ions. Moreover, our data clearly reveal that the oxygen vacancy (O_v) cannot directly adsorb C₂H₂ molecules but can activate its next-nearest Ti_5c ions which register as a binding site for C₂H₂ molecules with enhanced stability. Further increasing the C₂H₂ coverage leads to the stepwise formation of c-(4 × 2) and c-(2 × 2) ordered structures before final saturation at around 0.68 monolayer. The saturated film is characterized by dominating C₂H₂ dimers and other minority linear clusters whose intermolecular distances are slightly larger than the intervals between Ti_5c ions yet significantly shorter than the C₂H₂ dimers in the gas phase. We propose that such clustered C₂H₂ may serve as potential precursors for the polymerization reactions on metal oxide surfaces.