Adsorption states and mobility of trimethylacetic acid molecules on reduced TiO₂(110) surface

Combined scanning tunneling microscopy (STM), X-rays photoelectron spectroscopy (XPS) and density functional theory (DFT) studies have probed the bonding configurations and mobility of trimethylacetic acid (TMAA) molecules on the TiO₂(110) surface at RT. Upon TMAA dissociation through deprotonation, two distinctly different types of stable chemisorption configurations of the carboxylate group (TMA) have been identified according to their position and appearance in STM images. In configuration A, two carboxylate O atoms bond to two Ti⁴⁺ cations, while in configuration B one O atom fills the bridging oxygen vacancy (V_O) with the other O bounded at an adjacent regular Ti⁴⁺ site. Calculated adsorption energies for the configurations A and B are comparable at 1.28 and 1.36 eV, respectively. DFT results also show that TMA may rotate at RT about its O atom that filled the V_O (in configuration B), with a rotation barrier of ∼0.65 eV. Both the observation of the constant initial sticking coefficient and preference for TMAA molecules to dissociate at selective sites indicate that TMAA adsorption is mediated by a mobile precursor state. Several possible molecular (physisorbed) states of TMAA have indeed been identified by DFT, all being highly mobile at RT. In contrast, the TMA diffusion in the chemisorbed (dissociative) state is a very slow with a calculated barrier of 1.09 eV for diffusion along the Ti row.