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 similar to 2.1 eV for this new phase, matching it closely with the energy of visible light.