Dynamic Processes of Formaldehyde at Terminal Ti Sites on the Rutile TiO₂(110) Surface

We investigate the dynamic processes of formaldehyde (HCHO) molecules on 5-fold-coordinated titanium (Ti 5c ) sites of rutile TiO 2 (110) surface using scanning tunneling microscopy (STM) together with density functional theory simulations. Our results show that the adsorbed HCHO molecules at Ti 5c sites are present as two types of protrusions, either centered at Ti 5c rows or centered at bridging oxygen (O b ) rows in the STM images, corresponding to the monodentate adsorption configuration through a O–Ti 5c bond and to the bidentate adsorption configuration through both O–Ti 5c and C–O b bonds, respectively, which can be well supported by the simulated images. It is also observed that the monodentate adsorption tends to spontaneously switch to bidentate adsorption. Our results confirm the existence of the energetically more favored bidentate adsorption for HCHO at Ti 5c sites. We obtain that the energy barriers are approximately 0.28 and 0.75 eV for the adsorbed HCHO molecules switching from monodentate adsorption to bidentate adsorption and reversely switching from bidentate adsorption to monodentate adsorption, respectively, from measurements of their dynamic processes. Our findings can well elucidate the missing signature of the energetically more favored bidentate configuration in some previous experiments and provide insightful understanding of formaldehyde on TiO 2 (110) surface.