Novel three-dimensional TiO₂ 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 TiO₂: δ-, ε-, and ζ-TiO₂. Our results indicate that the band gaps of TiO₂ decrease almost linearly with an increase in the coordination number of Ti. Moreover, δ-TiO₂ and ζ-TiO₂ are semiconductors, whereas ε-TiO₂ is a metal, and the lowest energy of ζ-TiO₂ 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-TiO₂ may be a good photocatalyst candidate. Importantly, ζ-TiO₂ with the lowest energy is dynamically stable, and phase diagrams based on total energies at a specific pressure indicate that ζ-TiO₂ can be synthesized from rutile TiO₂ at high-pressure conditions.