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: delta-, epsilon-, and zeta-TiO2. Our results indicate that the band gaps of TiO2 decrease almost linearly with an increase in the coordination number of Ti. Moreover, delta-TiO2 and zeta-TiO2 are semiconductors, whereas epsilon-TiO2 is a metal, and the lowest energy of zeta-TiO2 is a quasi-direct band gap semiconductor with a distinctive band gap of 2.69 eV, ...
