The Role of the Surface Coverage on the Structural and the Electronic Properties of TiO2 Nanocrystals

We present here a characterization of TiO2 0D nanoclusters and 1D nanowires in the framework of ab initio density functional theory (DFT) calculations. We analyze the effect of the surface coverage by functionalizing dangling bonds with simple adsorbates modeling the basical interactions of TiO2 nanosystems with the hydration sphere. We thus address the electronic reorganization and the surface role in determining the overall properties of the nanostructures. The structural reconstruction is found to depend on the surface coverage and the experimental evidences on the structural variations can be explained by a topological analysis of the Ti-O bonds. Q-size effects are observed through the bandgap widening, but the surface competes to determine the energy distribution of the electronic levels. The hydrogenated nanocrystals do show occupied levels at the bottom of the conduction bands, which can enhance the conductive properties of the nanowires. In the hydrogenated cluster such levels present a localized charge distribution with strong similarities (orbital character, energy position) to the defect states arising after oxygens desorption. From the analysis of the electronic density of states we found that Ti-H bonds induce in-gap states above the valence bands, whereas hydration leads to occupied states that shift the valence bands to lower binding energies.