A two-dimensional phase of TiO₂ with a reduced bandgap.

Titanium dioxide is the prototypical transition metal oxide photocatalyst. However, the larger than 3 eV bandgap of common bulk phases of TiO₂ limits its light absorption to UV light, making it inefficient for solar energy conversion. Attempts at increasing visible light activity by narrowing the bandgap of TiO₂ through doping have proven difficult, because of defect-induced charge trapping and recombination sites of photo-excited charge carriers. Here, we report the existence of a dopant-free, pure TiO₂ phase with a narrow bandgap. This new pure TiO₂ phase forms on the surface of rutile TiO₂(011) by oxidation of bulk titanium interstitials. We measure a bandgap of only ~2.1 eV for this new phase, matching it closely with the energy of visible light.