Structural and Electronic Properties of Selected Rutile and Anatase TiO2 Surfaces:  An ab Initio Investigation

Five low-index stoichiometric TiO2 rutile and anatase surfaces, i.e., rutile (110), (100), and (001) as well as anatase (101) and (100), have been investigated using different Hamiltonians with all-electron Gaussian basis sets, within a periodic approach. Full-relaxations of the aforementioned surfaces have been essentially carried out at the Hartree-Fock (HF) level, but selected surfaces were treated also using pure and hybrid Density Functional Theory (DFT) models. Mulliken charges, band structures, and total and projected-densities of states have been computed both at the HF and the hybrid DFT (B3LYP and PBE0) levels. As regards DFT, the local density (LDA) and generalized gradient approximations (GGA) have been used. No matter which Hamiltonian is considered, as long as sufficiently thick slabs are taken into account, computed atomic relaxations show an overall excellent agreement with the most recent experimental reports. This is especially true when using hybrid functionals which enable the clarification of some conflicting results. Moreover, both at the LDA and HF levels, we were able to classify the surface relative energies in the following sequence:  anatase (101) < rutile (110) < anatase (100) < rutile (100) ≪ rutile (001). Instead, when using PBE, B3LYP, or PBE0, the two most stable surfaces are reversed.