  Thin films of amorphous (a)-TiO2 are ubiquitous as photocatalysts, protective
coatings, photoanodes and in memory application, where they are exposed to
excess electrons and holes. We investigate trapping of excess electrons and
holes in the bulk of pure amorphous titanium dioxide, a-TiO2, using hybrid
density functional theory (h-DFT) calculations. Fifty 270-atom a-TiO2
structures were produced using classical molecular dynamics and their
geometries fully optimised using h-DFT simulations. They have the density,
distribution of atomic coordination numbers and radial pair-distribution
functions in agreement with experiment. The calculated average a-TiO2 band gap
is 3.25 eV with no states splitting into the band gap. Trapping of excess
electrons and holes in a-TiO2 is predicted at precursor sites, such as
elongated Ti-O bonds. Single electron and hole polarons have average trapping
energies (ET) of -0.4 eV and -0.8 eV, respectively. We also identify several
types of electron and hole bipolaron states and discuss their stability. These
results can be used for understanding the mechanisms of photo-catalysis and
improving the performance of electronic devices employing a-TiO2 films.
