DFT Investigations of Formic Acid Adsorption on Single-Wall TiO₂ Nanotubes: Effect of the Surface Curvature

We carried out a theoretical study based on DFT calculations to provide a detailed characterization of the structural, electronic, and adsorption properties of single-walled TiO 2 anatase nanotubes. We investigated nanotube models of increasing diameter, formally obtained by rolling a TiO 2 anatase monolayer around the [1̅01] and [010] directions, giving rise to ( n,0) and (0, m ) nanotubes, respectively. We considered finite cluster models for both ( n,0) and ( m,0) TiO 2 nanotubes, with diameters ranging from 5 to 30 Å, thus approaching realistic nanotube dimensions. Our results show that ( n,0) tubes are lower in energy with respect to (0, m ) tubes. For ( n,0) tubes with diameters greater than 23 Å, the electronic energy and the band gap are almost converged with respect to the diameter length. We then investigated the adsorption of formic acid on the TiO 2 nanotube sidewalls, as the simplest model of photosensitizers binding to the TiO 2 surface, relevant to dye-sensitized solar cells. Adsorption of formic acid was investigated on (12,0) and (0,4) TiO 2 nanotubes, optimizing two monodentate modes and one bidentate adsorption mode, and comparing the results to those obtained for a planar TiO 2 surface. We find that while for a planar surface a bridged bidentate configuration is the more stable, the effect of the curvature in TiO 2 nanotubes leads a monodentate configuration to be the more stable structure. These results are interpreted in terms of the peculiar electronic properties of TiO 2 nanotubes and their implications for use of nanotubes in dye-sensitized solar cells are discussed.