Electronic structures and current conductivities of B, C, N and F defects in amorphous titanium dioxide.

Although titanium dioxide (TiO2) has been extensively studied and widely used in energy and environmental areas, the amorphous form and its related defect properties are poorly understood. Recent studies, however, have emphasized the crucial role of amorphousness in producing competitively good performances in photochemical applications. In this work we have investigated for the first time the effects of various dopants (B, C, N and F) on charge carrier transport in amorphous titanium dioxide (a-TiO2), given that doping is a common technique used to tune the electronic properties of semiconductors, and that the existence of these impurities could also be unintentionally introduced during the synthesis process. The a-TiO2 model was obtained using a classical molecular dynamics method, followed by density-functional theory calculations (DFT + U, with Hubbard correction term U) on electronic structures and defect states. The formation of these impurity defects in a-TiO2 was found to be energetically more favorable by several eV than their crystal counterparts (in rutile). The contributions of these defect states to the charge transfer processes were examined by means of Marcus theory.