Novel three-dimensional TiO 2 structure with a unique quasi-direct band gap for photocatalysts.

Motivated by fundamental interests and practical applications, three-dimensional (3D) photocatalysts are a fascinating area of research in clean energy. Based on first-principles calculations, we predicted three new 3D polymorphs of TiO 2 : δ-, ε-, and ζ-TiO 2 . Our results indicate that the band gaps of TiO 2 decrease almost linearly with an increase in the coordination number of Ti. Moreover, δ-TiO 2 and ζ-TiO 2 are semiconductors, whereas ε-TiO 2 is a metal, and the lowest energy of ζ-TiO 2 is a quasi-direct band gap semiconductor with a distinctive band gap of 2.69 eV, calculated by the HSE06 level. In addition, the calculated imaginary part of the dielectric function indicates that the optical absorption edge is located in the visible light region, suggesting that the proposed ζ-TiO 2 may be a good photocatalyst candidate. Importantly, ζ-TiO 2 with the lowest energy is dynamically stable, and phase diagrams based on total energies at a specific pressure indicate that ζ-TiO 2 can be synthesized from rutile TiO 2 at high-pressure conditions.