Adsorption And Diffusion Of Small Organic Molecules Across Rutile TiO₂ Surfaces

We studied adsorption and diffusion of organic molecules on oxide surfaces by destiny functional theory (DFT).We investigated the diffusion behavior of catechol on the rutile TiO2(110) surfaces under different conditions. It has been found that the degree of hydroxylation of the surface is essential for the facile diffusion of catechol at the surface. The diffusion of catecholate adsorbed on Ti02(110) surface can be possible only with the existence of surface hydroxyls. The so-called Hydrogenated Rotation Route is energetically the most feasible at room temperature, where the transfer of hydrogen from surface hydroxyls to the molecule and its interaction with surface hydroxyls substantially lowered the activation barrier for rotational motion across the surface. However, a heavily hydroxylated surface is not favorable to the diffusion of catechol. This work illustrates the essential role of hydrogen bonding in controlling dynamics during the initial stage of molecular assembly.We also investigated the adsorption behavior of acetic acid on the rutile Ti02(110) and (011)-2×1 surfaces, and the diffusion at TiO2(110) surface. DFT calculations showed that the initial sticking of adsorbed acetic acid at room temperature is low on the (011)-2×1 surface with initial chemisorption occurring at surface defects only, and for high acetic acid exposures we determined adsorption of quasi-one dimensional ordered acetate-clusters. Pre-adsorbed acetates act as nuclei for further adsorption, causing the formation of acetate islands. Furthermore, only monodendate adsorption is possible for the acetate in the clusters on TiO2(011)-2×1 surface, very different from the established bidentate bridging adsorption on TiO2(110). At room temperature, the adsorption kinetics on the (110) surface is randomly at low coverage and follows a Langmuirian adsorption, and adsorbed acetate arrange in an ordered 2×1 superstructure at saturation coverage. While on the (011)-2×1 surface acetate adsorbs to pre-adsorbed molecules in an island growth mechanism. Moreover, the most feasible diffusion route of acetic acid across TiO2(110) shows that the catecholate assumes a bidentate chelating intermediate state. Calculation results indicate that different molecular configurations can critically influence the way of diffusive motion, and different sites of the surface hydroxyl groups will affect the diffusion barrier.