Fabrication and Properties of a Free-Standing Two-Dimensional Titania

The synthesis of free-standing two-dimensional titania (2-D TiO<sub>2</sub>) with a reduced band gap presents complex challenges to synthetic chemists. Here, we report a free-standing 2-D TiO<sub>2</sub> sheet synthesized via a one-step solvothermal methodology, with a measured optical onset at ∼1.84 eV. Using first-principles calculations in combination with experiment, we propose that the as-formed 2-D TiO<sub>2</sub> sheets are layers of the lepidocrocite TiO<sub>2</sub> structure, but with large nonuniform strains consistent with its crumpled morphology. These strains cause a significant change in the quasiparticle band structure and optical absorption spectra, resulting in large absorption in the visible-light region. This narrow band gap 2-D TiO<sub>2</sub> can catalyze the formation of singlet oxygen and the degradation of dye pollutants with low-energy photons of solar light. Our work demonstrates that lattice strains intrinsic to 2-D materials, especially its crumpled, free-standing forms, can result in new and useful properties.