Formic Acid Adsorption on Anatase TiO₂(001)−(1 × 4) Thin Films Studied by NC-AFM and STM^†

Abstract

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 TiO 2 . 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 × 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 × 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 2 a 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 TiO 2 (001)−(1 × 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 × 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.