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

The titanium dioxide (TiO2) complexes are widely investigated for their striking and multipurpose capabilities. The TiO2 key feature lies in its photocatalytic activity for several reactions of social (bioengineering, environmental and artistic protection, pollution containment) and commercial (photovoltaic, alternative-energy, gas sensing) interests. The possibility to enhance specific reactions at the nanoscale by a fine tuning of the nano-sized single crystals properties boosted in the last decade the scientific research. Thus a theoretical understanding of the fundamental properties of TiO2 nanocrystals became necessary to predict and expedite the experimental effords. We present here a characterization of TiO2 0D nanoclusters and 1D nanowires in the framework of ab initio DFT calculations. Based on both theoretical and experimental evidences we defined a stoichiometric TiO2 NC by modifying a perfect bipyramidal morphology and then used this NC as a chain repetition unit in the NW. We analyzed the effect of the surface coverage by functionalizing dangling bonds with simple adsorbates (dissociated water and hydrogens) modeling two acidic environments. These terminations are important to model the basical interactions of TiO2 nanosystems with the hydration sphere, which is always found to surround the nanosamples and toaffect their photocatalytic activity. We thus address the electronic reorganization and the surface weight in determining the global features 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. Quantum confinement effects in the electronic properties are observed through the bandgap widening and the discretization of the energy distribution, but the surface competes to determine the energy dispersion of the electronic levels. The hydrogenated nanocrystals do show occupied levels at the bottom of the coduction bands, thus leading to metallic nanowires in one dimension. Whereas in the hydrogenated cluster such levels present a localized charge distribution with respect to the whole structure and they are also similar for the atomic orbital character and energy position to the defect states obtained by 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. Formation energy calculations reveal that surface hydration leads to the most stable nanocrystals, in agreement with the experimental findings that water coverage stabilizes the surface.