Effects of Morphology on Stability, Electronic, and Optical Properties of Rutile TiO₂ Nanowires

By means of first principles calculations we show that morphology of TiO 2 nanowires in the rutile phase turns out to be important in their stability and band gap engineering. The most stable ⟨001⟩-oriented TiO 2 nanowires are found to display the {110} facets while the most thermodynamically favorable nanowires with the ⟨110⟩ axis are characterized by the {110} and {001} facets. Our results clearly demonstrate that appearance of the {100} facets in the shape of TiO 2 nanowires in the ⟨001⟩ orientation leads to a sizable reduction of band gaps, however, it does not cause the shift of the absorption edge to the lower energy range. Presence of flat bands similar to localized band gap states in the gap region of the ⟨110⟩-oriented TiO 2 nanowires is predicted to be an intrinsic feature, which is not connected with O vacancies. We also discuss how our findings can be used for optimization of photovoltaic and photocatalytic cells.