Engineering Titanium Dioxide To Respond To Visible Light

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 one snag: The material catalyzes reactions only in response to ultraviolet light. Now, researchers in Singapore have found a way to dope TiO₂ with nitrogen so that it responds to visible light, drastically increasing its activity in sunlight (J. Phys. Chem. C 2013, DOI: 10.1021/jp408798f).

A material’s photoactivity depends in part on its band gap, the energy needed to kick electrons from a nonconductive state to a conductive one. TiO₂ has a band gap of 3 eV, which corresponds to energies of photons in the UV range. Doping TiO₂ with nitrogen lowers the band gap below ₂ eV, making the material photoactive with visible light. However, up to now, the doping methods used, such as magnetron sputtering and high-energy ion bombardment, have created defects in the bulk TiO₂, which reduced the photocatalytic efficiency of the material.

A team of researchers from the Institute of Materials Research & Engineering (IMRE) and the National University of Singapore used a different source of nitrogen atoms that sent low-speed beams of atoms at the TiO₂. Because these nitrogen atoms did not penetrate beyond the TiO₂ surface, the researchers could replace the top layer of the material with nitrogen and create a defect-free surface, says Junguang Tao, a physicist at IMRE.

The team measured the photocatalytic activity of TiO₂ samples with and without a nitrogen-doped surface. The doped TiO₂ 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, unlike TiO₂ doped using previous methods.