Structural, electronic, and surface properties of anatase TiO2 nanocrystals from first principles

The structural and electronic properties of anatase TiO2 nanocrystals (NCs) are investigated through first-principles calculations. 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 findings. The band-gap blueshift 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 confinement. 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 finding that the truncated bipyramidal morphology is typical of the moderate acidic environment. Oxygen desorption from the bare NC is unfavorable, 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.