Understanding the Intrinsic Chemical Activity of Anatase TiO2(001)-(1 x 4) Surface

We report our investigation on the intrinsic chemical activity of the anatase TiO2(001)-(1 X 4) reconstructed surface, using epitaxially grown anatase TiO2(001) thin films and using methanol molecules as a probe, characterized by combining scanning tunneling microscopy and temperature-programmed desorption. Our results provide direct evidence that the perfect (1 x 4) lattice sites of the surface are intrinsically quite inert for the reaction of methanol. We obtain that the activation energy for desorption of molecular methanol is about 0.55-0.64 eV, which is in good agreement with our first-principle calculations based on the structural model with 5-fold coordinated Ti atoms at the ridges of (1 x 4) reconstruction. We find that two types of defect sites, that is, reduced Ti pairs and partially oxidized Ti pairs, are responsible for the chemical activity of the surface, evidenced by the desorption of water due to the dehydrogenation of methanol at the defect sites. The methoxy left at the reduced Ti-pair sites further produced CH3 radical, and the methoxy near the partially oxidized Ti-pair sites produced formaldehyde arid methanol through disproportionation reaction. The determination of these intrinsic properties can be important to understanding the conflicting results from this surface in the literature and thus to reveal the actual reaction mechanisms.