Chemical functionalization of the $TiO_2$(110)-(1 x 1) surface by deposition of terephthalic acid molecules : a density functional theory and scanning tunneling microscopy study

Periodic DFT calculations were used to explore structural properties of terephthalic acid (TPA) deposited on the rutile TiO2(110)-(1 x 1) surface at low and high coverage. Theoretical results were compared with scanning tunneling microscopy imaging data. At low loading the TPA molecules adsorb dissociatively as discrete entities adopting a flat-lying plane-on geometry. The resultant terephthalic anion is attached to the surface by two covalent bonds between the carboxylic oxygens and the 5-fold coordinated Ti atoms with an additional stabilization due to the hydrogen bond formation with the adjacent surface hydroxyls. When the saturation coverage is achieved, a well-ordered monolayer of the vertically oriented molecules is formed. In both cases the TPA admolecules bind to the surface via carboxylic groups as terephthalic anions. Formation of dimers results from the formation of hydrogen bonds between the adjacent TPA molecules. To elucidate the reactivity of the functionalized surface, we deposited zinc formate ions on top of the compact TPA monolayer. Calculations showed that the anchoring properties of the TiO2/TPA system are not perturbed by the dimer formation, auguring well for its prospective application as a promising chemically functionalized surface for on-top growth of metal organic frameworks.