Ab initio investigation of the structural and electronic properties of tio2 nanostructures

Abstract

In the first part of this thesis, we have investigated the structural and electronic properties of anatase TiO2 nanocrystals (NCs) through ab initio calculations based on the Density Functional Theory (DFT). The dependence of the structural properties (e.g., NC volume variations) on the surface chemistry is discussed by considering two different surface coverages (dissociated water and hydrogens). Both prevent a pronounced reconstruction of the surface, thus ensuring a better crystalline organization of the atoms with respect to the bare NC. In particular, the results for the hydrated NC do show the largest overlap with the experimental ndings. The band-gap blue shift with respect to the bulk shows up for both the bare and the hydrated NC, whereas hydrogen coverage or oxygen desorption from the bare NCs induce occupied electronic states below the conduction levels thus hindering the gap opening due to quantum connement. These states are spatially localized in a restricted region and can be progressively annihilated by oxygen adsorption on undercoordinated surface titanium atoms. Formation energy calculations reveal that surface hydration leads to the most stable NC, in agreement with the experimental ndings that the first layer of the water coverage is important in stabilizing the NCs surfaces. Oxygen desorption from the bare NC is unfavourable, thus highlighting the stabilizing role of surface oxygen stoichiometry for TiO2 . Available experimental data on the electronic and structural properties of TiO2 NCs are summarized and compared with our results. In the second part of this thesis, we have performed DFT calculations on anatase TiO2 nanowires (NWs) to investigate the dependence of their structural and electronic properties on the size, the surface coverage, and the morphology. We have found that the overall crystallinity of the NWs increases on increasing the diameter size or upon surface coverage with simple water-derived adsorbates. The NWs grown along the [010] direction are found to be more ordered with respect to the NWs in the [001] direction, thus highlighting the dependence of the crystallinity on the choice of the morphology. The bare and hydrated NWs do show the band-gap blue shift due to the size connement, but deviations from an ideal trend with the size are found and ascribed to the morphology and the crystallinity. Through the analysis of the valence band maximum and conduction band minimum energies we found that the TiO2 NWs photocatalytic ability prots from the conned size, for example

2 for the water splitting process. Moreover, the availability of internal channels for an efcient charge transport can be tuned by the surface coverage. The terminal hydroxyl groups of the hydrated NWs cannot be considered as deep hole traps since their related electronic states have binding energies in the same range of the NW oxygen states. The hydrogenated NWs grown along the [001] direction show occupied states at the bottom of the conduction bands, thus we expect that TiO2 NWs can be used as efcient hydrogen sensors. Finally, the surface hydration leads to the most stable NWs with formation energies that are even close to the bulk limit.