Electronic structure study of TiO2 polymorphs, evaluation of formic acid adsorption on dry (001) and (100) TiO2(B) facets by DFT calculations

Abstract: Structural and electronic properties comparison between anatase, rutile and monoclinic TiO2(B) titania polymorphs in bulk and related nanostructures simulated by two-dimensional slabs has been performed by means of density functional theory calculations. TiO2(B) was included due to experimental results where it appears as a metastable phase during thermal annealing of low content sodium layered titanate nanostructures obtained via hydrothermal synthesis. Band gaps and surface energies are discussed confronting results obtained by Plane Waves (PW) and Localized Basis Set methods (LCNAO), observing an excellent agreement between both methodologies when considering floating basis to calculate surfaces energies using LCAO. The computed formation energy for the TiO2(B) (001) slab, as has been previously reported by Vittadini A. et al., J. Phys. Chem. C 2009, 113 (44), 18973-18977, is in the order of the one calculated for anatasa (101) which represents one of the most stable TiO2 surfaces. The adsorption of formic acid on (001) and (100) - less stable - TiO2(B) dry facets was investigated as the first step for further evaluation of TiO2(B) nanostructures for dye sensitized solar cells (DSSC) applications. The results show that the bidentate dissociated configuration is the most stable for both surfaces being the computed adsorption energy 0.76 eV and 1.35 eV for (001) and (100) respectively. These results differ from what was previously found for dry anatasa (101) where monodentate coordination prevails. The evaluated reactivity of TiO2(B) slabs is comparable with anatase and indicates that it could be a good adsorbent for common dyes used for Dye Sensitized Solar Cells purposes.