In what they're calling a breakthrough discovery, UCL researchers studying the properties of titanium dioxide catalysts, which are widely used in self-cleaning products and materials, claim to have challenged the accepted view of how mixed-phase samples of the material actually behave.
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.
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.
"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," says UCL chemist Dr. David Scanlon.
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).
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. Gizmag has covered the self-cleaning applications of titania over the years from garden furniture, to architectural cladding to clothing. "All [uses] could be improved by better understanding of the photophysics that is occurring," Dunnill told Gizmag. "Quite a few of them already use multi phase materials but usually in the form of separate particles rather than composite particles."
The team's research was published today in the journal Nature Materials.