Enhanced visible-light absorption of mesoporous TiO₂ by co-doping with transition-metal/nitrogen ions

Titanium (IV) oxide, TiO₂, has been the object of intense scrutiny for energy applications. TiO₂ is inexpensive, non-toxic, and has excellent corrosion resistance when exposed to electrolytes. A major drawback preventing the widespread use TiO₂ for photolysis is its relatively large band gap of ∼3eV. Only light with wavelengths shorter than 400 nm, which is in the ultraviolet portion of the spectrum, has sufficient energy to be absorbed. Less than 14 percent of the solar irradiation reaching the earth's surface has energy exceeding this band gap. Adding dopants such as transition metals has long been used to reduce the gap and increase photocatalytic activity by accessing the visible part of the solar spectrum. The degree to which the band gap is reduced using transition metals depends in part on the overlap of the d-orbitals of the transition metals with the oxygen p-orbitals. Therefore, doping with anions such as nitrogen to modify the cation-anion orbital overlap is another approach to reduce the gap. Recent studies suggest that using a combination of transition metals and nitrogen as dopants is more effective at introducing intermediate states within the band gap, effectively narrowing it. Here we report the synthesis of mesoporous TiO₂ spheres, co-doped with transition metals and nitrogen that exhibit a nearly flat absorbance response across the visible spectrum extending into the near infrared.