Adsorption Features of Formaldehyde on TiO2(110) Surface Probed by High-Resolution Scanning Tunnelling Microscopy.

We report a real-space imaging of formaldehyde (HCHO) adsorption on a TiO2(110) surface probed by high-resolution scanning tunnelling microscopy (STM). Density functional theory calculations (DFT) were carried out to assign the observed features. The adsorptions occur exclusively on 5-fold coordinated Ti (Ti5c) sites and oxygen vacancies (OVs). The well-resolved configurations on the Ti5c sites feature the overlapping of the two "dumbbell" structures which are originated from the empty orbitals of HCHO. The STM images for the physical adsorption of HCHO on the OV sites appear fuzzy because of the rapid switching of HCHO among the three stable orientations, while those for the chemical adsorption are much clearer, revealing a distinctive difference between chemical and physical adsorptions. This work presents a systematic characterization of the topological features of HCHO/TiO2(110) and provides useful information for mechanical understanding of the reaction mechanism of HCHO on the surfaces.