Formic acid adsorption on anatase TiO2(001)-(1x4) thin films studied by NC-AFM and STM

Atomic resolution scanning tunneling microscopy (STM), noncontact atomic force microscopy (NC-AFM), X-ray photoemission spectroscopy (XPS), low-energy electron diffraction (LEED), and formic acid adsorption experiments were used to characterize the (001) surface of anatase TiO2. A combination of NC-AFM and STM imaging was used to distinguish features due to geometrical and electronic effects. The contrast in images of the bare (1 x 4) surface and the formate-covered surface is dominated by the surface topography in both NC-AFM and STM, although electronic effects in the troughs contribute features to the STM images that are absent from NC-AFM images. High-resolution imaging by STM and NC-AFM revealed that the highest point of the ridge of the (1 x 4) structure consists of a single row of atoms. Formate adsorbs at under-coordinated Ti sites in the added rows with a minimum separation of 2a(0) and never adsorbs in the trenches even though the trenches also likely expose under-coordinated Ti atoms. The sticking probability of formate on the anatase TiO2(001)-(1 x 4) surface is near unity initially, and falls dramatically once the saturation coverage of (1)/(8) ML is reached. We show that of the current models for the (1 x 4) reconstruction, an added row plus surface oxygen vacancy model and the recently proposed add-molecule model are in best agreement with all of the data