Jun 1, 2010
Bandgap engineering produces visible light response in WO3
Transition metal-oxide semiconductors, such as TiO2 or WO3, have some outstanding electronic and ionic properties making them premier-choice materials for photocatalytic applications such as self-cleaning or wettability adjustable surfaces, electrochromic devices and drug-release systems. However, due to their wide bandgap (~3 eV) the materials only show considerable absorption in the UV range of the spectrum. Therefore, they are not very efficient when it comes to the direct use of solar illumination (which consists of only ~7% UV light).
In 2001 Asahi et al.  introduced a co-sputtering process to produce N-doped TiO2. This led to a modification of the band structure, which activated a visible light photo response in TiO2. The modified material immediately stimulated broad interest and was explored widely for photocatalytic applications. Usually nanoscale powders or nanotubes  are used to achieve a high surface area and thus to increase the reaction rate.
Recently, researchers in Germany have shown that porous WO3 – a material that in many respects has an even higher potential for wide bandgap applications than TiO2 – can be modified by a comparably simple heat treatment in NH3 gas to drastically increase (18 times) its visible light photo response. The combination of ordered nanoscale morphology obtained by the self-ordering anodization of W-sheet and the now introduced visible light activity makes the material a promising candidate for enhanced solar cells, electrochromic materials and specific photocatalytic applications.
The researchers presented their work in the journal Nanotechnology.
About the author
This work was performed at the Materials Science Department of the University of Erlangen-Nürnberg, Germany, where Prof. Schmuki heads the Chair of Surface Science. A key competence is the formation and application of self-organized nanostructures on various metal and semiconductor surfaces. In particular, transition metal oxide nanotubes are explored for DSSCs, electrochromic devices, biomedical applications and photocatalysis. Dr Yoon-Chae Nah, is a postdoctoral fellow whose research focus is the investigation of WO3–TiO2 composite materials for electrochromic devices, and bandgap engineering by doping. Indhumati Paramasivam and Robert Hahn are PhD students in the Schmuki lab, who explore novel hierarchical structures on TiO2 and WO3 and their use in photocatalysis. Nabeen Shrestha is a postdoctoral researcher funded by the Alexander von Humboldt Foundation and his major areas of research are advanced electrochemical techniques for materials synthesis.