Structure and Optical Properties of Small (TiO₂)_<i>n</i> Nanoparticles, <i>n</i> = 21–24

Recently, nanostructured TiO₂ (“black TiO₂”) has been discovered to absorb visible light, which makes it an efficient material for water splitting. Hydrogenization has been proposed to be at the origin of this beneficial electronic structure of black TiO₂. Here, we investigate, using ab initio methods, alternative mechanisms related to structure modifications in nanoclusters that could be responsible for absorption in the visible range. To that end, we apply a combination of computational structure prediction using simulated annealing and minima-hopping methods based on density-functional theory to predict low-energy configurations and time-dependent density-functional theory (TDDFT) using a hybrid functional with optimized Hartree–Fock content to obtain optical absorption edges