Doped and undoped titania nanocrystals : theoretical bottom-up approach vs. experimental flatband potential studies

A synergistic theoretical and experimental approach has been developed in order to determinate the electronic band structure in pure and nitrogen doped titania nanocrystals. Theoretical ab initio Density Functional Theory (DFT) calculations were employed using the state of the art DFT functionals available within the VASP suite of codes. On one hand, these studies allowed us to calculate the density of the electronic states (DOS), including the Fermi Energy location at the bandgap and the eventual presence of intra-gap states. On the other hand, electrochemical characterizations were carried out on samples of titania nanocrystals with doping concentrations comparable to the theoretical models. These brought us to the determination of fundamental parameters, such as the effective donor density (ND), the flat-band potential (FB), the position of the conduction and valence band edges at the interface. The FB potentials of pure and nitrogen-doped nanotitania thin films, prepared in our laboratory, have been determined by the capacitance method, based on Mott-Schottky plots. Merging these results with the theoretical ones provided us with a unison physical picture. This allowed us to better understand the N-doping effects on titania nanocrystals, such as the formations of intraband gap states, and to elucidate a long standing diatriba about the possible bandgap narrowing induced by N doping of titania.