Many scientists view titanium dioxide as an attractive, low-cost photocatalyst for a variety of applications, including water purification, water splitting, and solar power. But there is a snag: The material catalyzes reactions mainly in response to ultraviolet light. Now, researchers in Singapore have found a way to dope the surface of TiO2 with nitrogen so that the material responds to visible light, drastically increasing its photocatalytic activity (J. Phys. Chem. C 2013, DOI: 10.1021/jp408798f). Previously used doping methods, such as magnetron sputtering and high-energy ion bombardment, create defects in the bulk TiO2 that reduce the photocatalytic efficiency of the material. But directing a low-energy beam of nitrogen atoms at TiO2 deposits nitrogen on the surface only and keeps the material free of defects, says Junguang Tao, a physicist at the Institute of Materials Research & Engineering, in Singapore. He and his team found that TiO2 doped in this way showed photoactivity when illuminated with visible light, unlike the undoped TiO2. What’s more, the surface-doped TiO2 showed greatly enhanced photoactivity under UV illumination, compared with TiO2 doped via other methods.