To produce “green” fuels, some scientists are looking for a little help from above. Sunlight is the key ingredient in photocatalytic water splitting, a process that breaks down water into oxygen and, most importantly, hydrogen, which could be used in future energy technologies like fuel cells. The problem is that the most effective photocatalysts, like pure titanium dioxide, are only activated by ultraviolet light.
Recently, scientists have found a way alter titanium dioxide to react to visible light, harnessing the power of the entire solar spectrum. They do this by adding, or “doping,” a small amount of nitrogen to titanium dioxide.
At NSLS, scientists from Brookhaven, the National Institute of Standards and Technology (NIST), and the University of Delaware set out to reveal what’s so special about the doped form of the material.
The group analyzed samples made at the University of Delaware with hard x-ray photoelectron spectroscopy (HAXPES) studies at NSLS beamline X24A — the only facility in the United States that performs this advanced technique. HAXPES allows researchers to probe the electronic structure of materials deeper than regular photoemission studies, which are usually limited to surfaces. By combining these data with calculations carried out by the theory group at NIST, the researchers discovered that the addition of nitrogen changed the electronic structure both directly and indirectly.
Their findings: by adding nitrogen to the material, a certain amount of oxygen is forced out. The resulting oxygen vacancy, combined with the presence of nitrogen itself, explains the observed electronic structure
A.K. Rumaiz, J.C. Woicik, E. Cockayne, H.Y. Lin, G. H. Jaffari, and S.I. Shah, “Oxygen Vacancies in N Doped Anatase TiO2: Experiment and First-principles Calculations,” Applied Physics Letters, 95, 262111 (2009).
Left: (a) Structure of pure anatase, (b) N/Ti
3+/O vacancy defect complex with O opposite N, (c) “Merging” of
two O into central position. In (b) and (c), the Ti3+ ion is
obscured from this viewpoint.
Right: Researchers Abdul Rumaiz (left) and Joe Woicik.