A Density Functional Tight Binding Study of Acetic Acid Adsorption on Crystalline and Amorphous Surfaces of Titania

We present a comparative density functional tight binding study of an organic molecule attachment to TiO(2) via a carboxylic group, with the example of acetic acid. For the first time, binding to low-energy surfaces of crystalline anatase (101), rutile (110) and (B)-TiO(2) (001), as well as to the surface of amorphous (a-) TiO(2) is compared with the same computational setup. On all surfaces, bidentate configurations are identified as providing the strongest adsorption energy, E(ads) = −1.93, −2.49 and −1.09 eV for anatase, rutile and (B)-TiO(2), respectively. For monodentate configurations, the strongest E(ads) = −1.06, −1.11 and −0.86 eV for anatase, rutile and (B)-TiO(2), respectively. Multiple monodentate and bidentate configurations are identified on a-TiO(2) with a distribution of adsorption energies and with the lowest energy configuration having stronger bonding than that of the crystalline counterparts, with E(ads) up to −4.92 eV for bidentate and −1.83 eV for monodentate adsorption. Amorphous TiO(2) can therefore be used to achieve strong anchoring of organic molecules, such as dyes, that bind via a -COOH group. While the presence of the surface leads to a contraction of the band gap vs. the bulk, molecular adsorption caused no appreciable effect on the band structure around the gap in any of the systems.