Electroniic Structure of TiO2 Surfaces and Effect of Molecular Adsorbates Using Different DFT Implementations

We model TiO2 rutile (110) and anatase (101) surfaces and investigate the effect of adsorption of benzoic acid on the electronic structure of these systems. The surface-adsorbate electronic structure determines the rates of electron-transfer processes, which are important for dye-sensitized solar cells, in particular. We use density functional theory (DFT) and test the accuracy and efficiency of several DFT implementations (plane waves vs localized basis sets, all-electron vs pseudopotential calculations) as applied to the TiO2-adsorbate system. We explore the variation in the band gap, surface energies, and benzoic acid adsorption energies as a function of slab thickness. Our results show that a two-layer slab is sufficient to model the adsorption of benzoic acid on anatase, whereas for rutile, convergence is much more slow and not yet achieved in five-layer slabs. The effect of the adsorbate on the electronic structure is small in the case of anatase, but noticeable for rutile, where adsorbate states appear near the valence band edge, and TiO2 states near the conduction band edge are shifted upward. The TiO2 surface layers make a small contribution to the density of states near the conduction band edge, but a prominent contribution deeper in the conduction band, at energies where electron injection from sensitizing chromophores is most likely to occur.