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Thin films of amorphous (a)-TiO2 are ubiquitous as photocatalysts, protectivecoatings, photoanodes and in memory application, where they are exposed toexcess electrons and holes. We investigate trapping of excess electrons andholes in the bulk of pure amorphous titanium dioxide, a-TiO2, using hybriddensity functional theory (h-DFT) calculations. Fifty 270-atom a-TiO2structures were produced using classical molecular dynamics and theirgeometries fully optimised using h-DFT simulations. They have the density,distribution of atomic coordination numbers and radial pair-distributionfunctions in agreement with experiment. The calculated average a-TiO2 band gapis 3.25 eV with no states splitting into the band gap. Trapping of excesselectrons and holes in a-TiO2 is predicted at precursor sites, such aselongated Ti-O bonds. Single electron and hole polarons have average trappingenergies (ET) of -0.4 eV and -0.8 eV, respectively. We also identify severaltypes of electron and hole bipolaron states and discuss their stability. Theseresults can be used for understanding the mechanisms of photo-catalysis andimproving the performance of electronic devices employing a-TiO2 films.

Disorder-induced electron and hole trapping in amorphous TiO2