UCL Scientists Solve Titanic Puzzle Of Popular Photocatalyst: A Self-Cleaning Material

A titania nanofiber viewed under electron microscope

UCL researchers studying the properties of titanium dioxide catalysts, which are widely used in self-cleaning products and materials,  uncovering a decades old misunderstanding that has clouded our knowledge of how mixed phase titania catalysts operate.

Mixed-phase titania (another name for titanium dioxide) combines both rutile and anatase titania, the two most abundant forms of the material, both of which can break down water in the presence of UV light. Though anatase titania is more effective than rutile, the combination of the two is more effective still.

anatase titania

rutile

By carrying out cutting-edge computational simulations alongside precise experimental measurements of physical samples of the mineral, scientists at UCL found that the widely accepted explanation for how mixed phase titania catalysts operate was misguided. Their discovery, published today in Nature Materials, will help scientists and engineers develop improved photocatalysts, which have applications in clean energy technologies, self-cleaning coatings and a number of other fields.

In an attempt to explain this property, a 1996 study concluded that electron energy levels in rutile are 0.2 eV lower than in anatase. The precise band gap has significant implications for the properties of the material, and it was this difference that was thought to account for the superior properties when mixing the two. However, a problem with this idea is that electrons have since been observed flowing from rutile to anatase titania when combined, which should not be the case given the lower energy levels in rutile.

“There were about 12,000 scientific articles written about titania last year,” says lead author David Scanlon (UCL Chemistry), “so this is one of the most heavily studied catalysts out there. Despite the mountains of research into this material, the explanation for the observed performance increase when mixed-phase samples were employed instead of single phase materials had remained a mystery for decades.”

By synthesizing high quality anatase-rutile junctions and analyzing the material using UCL's Legion Cluster distributed memory supercomputer, the researchers have actually arrived at the opposite conclusion, the electrons in anatase having less energy, and to the tune of about 0.4 eV. The team say that their calculations are conversant with recent X-ray photoelectron spectroscopy alignment of rutile, putting the energy levels (specifically, the ionization energy) in rutile at 7.83 eV, where previous studies have taken it to be 7.1 eV. (The ionization energy of anatase is 8.3 eV).

Better understanding how titania breaks down water and other molecules has implications for the design of equipment for producing green fuels, as well as self-cleaning coatings for sterilisation.

The researchers say that research of this type could help in the development of improved photocatalysts which in turn could lead to greener fuels and self-sterilizing material coatings. 

The UCL-led team, which also includes researchers from the University of Bath, Trinity College Dublin and the STFC Daresbury Laboratory, published their results in the journal Nature Materials on 7 July.

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Write by: RC - Monday, July 8, 2013