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Titania Photocatalysis

February 16, 2011

Titanium is an abundant element . The Earth's crust is about a half percent titanium by weight, which makes it about a tenth as plentiful as iron , and that's a lot. Titanium metal is difficult to refine from its minerals , so it comes at a high price (presently about twelve dollars per pound); but its oxide (titanium dioxide , TiO₂, also called titania) is an abundant material that's used as a pigment in opaque white paint . Titania is relatively inexpensive, and it would be nice if you can get it do something more useful than reflect white light.

Fortunately, titania is a photocatalyst ; that is, it will promote some chemical reactions in the presence of light, usually the ultraviolet portion of the spectrum . As I've reported in a previous article (The High Road, September 22, 2010), there are attempts to use titania in paving blocks in combination with the ultraviolet contained in sunlight to mitigate vehicle exhaust .

The photocatalytic properties of titania were first published in 1972 by Akira Fujishima and Kenichi Honda, who described a photoelectrochemical cell that used titania as a photoanode .[1-2] This cell was capable of water photolysis ; that is splitting water into hydrogen and oxygen . This was big news in my research circles, since we were investigating materials for hydrogen storage as portable energy sources, and the Fujishima-Honda cell was a potentially important piece in a hydrogen economy . Unfortunately, the bandgap of titania as rutile is 3.0 eV , which means it will only split water when illuminated by light shorter in wavelength than 413.75 nm . Less than 10% of solar energy appears in those wavelengths. The other crystal form of titania, anatase , is even worse at 3.2 eV (388 nm). In short, titania was not a practical solar energy collector.

If you're trying to decrease the bandgap of titania, doping is one route. The material is difficult to dope, owing to the small atomic size of the titanium cation , although nitrogen has been somewhat effective. One clue to another way of bandgap modification is the difference in bandgap between the rutile and anatase structures. They're both titania, but the arrangement of atoms is slightly different, and this leads to a different bandgap. Nowadays, nanotechnology is applied to everything, often with good results, and that appears to be the case for titania, as well. A research group from Lawrence Berkeley National Laboratory and the University of California at Berkeley has just published results of a new study on titania for photocatalysis.[4-5]

Ordered/disordered titania powders

White on the left, black on the right. The results of hydrogen-induced disorder in titania nanocrystals. The gloved hand is that of principal investigator, Samuel Mao (Click for full view. Photo by Roy Kaltschmidt, Lawrence Berkeley Laboratory).

The Berkeley team enhanced the solar absorption of titania by introducing surface disorder in TiO₂ nanocrystals by hydrogenation. The process turned the titania from white to black, a change indicative of increased optical absorption . The TiO₂ nanocrystals performed photo-oxidation of organic molecules in water, a process that's important for remediation of contaminated water. Introducing this disorder enabled absorption of infrared light . Computer calculations show that the disorder created mid-gap electronic states, similar to doping, but it also reduced the intrinsic bandgap . This work appears in the February 11, 2011, issue of Science .[4]

Experiments showed that nearly a quarter of the solar spectrum was absorbed by the disordered titania. In photolysis, the titania had about a hundred-fold better hydrogen yield than other photocatalysts, and it showed no sign of degradation after 22 days. The research was funded by the Energy Efficiency and Renewable Energy program of the United States Department of Energy .

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