Disorder-induced electron and hole trapping in amorphous TiO2
D Mora-Fonz and M Kaviani and AL Shluger, PHYSICAL REVIEW B, 102, 054205 (2020).
Thin films of amorphous (a)-TiO2 are ubiquitous as photocatalysts, protective coatings, photo-anodes, and in memory applications, 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 optimized using h-DFT simulations. They have the density, distribution of atomic coordination numbers, and radial pair-distribution functions in agreement with experiments. 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 (E-T) 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.
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