TEM image of a Ti02 nanocrystal after hydrogenation reveals engineered disorder on the crystal’s surface, a change that enables the photocatalyst to absorb infrared light. Credit S. Mao et al; Science Express.
By tinkering with the outer layer of titanium oxide nanocrystals, researchers at Lawrence Berkeley National Lab have figured out a way to turn the material into a tough and more effective photocatalyst for environmental and energy applications. They claim this is the first time durability and efficiency have been combined in a photocatalyst.
Samuel Mao, an investigator with the Advanced Energy Technologies Department of the lab’s Environmental Energy Technologies Division, says they were trying to improve hydrogen production from organic materials in water when they had the idea to introduce disorder in nanophase TiO2 and hopefully expand its light-absorption ability.
The groups work (doi:10.1126/science.1200448), “Increasing solar absorption for photocatalysis with black, hydrogenated titanium dioxide nanocrystals” is published in Science Express, and may offer a path for generating hydrogen from organic compounds found in natural and polluted water sources.
Mao shows how the disorder-engineered titanium dioxide nanocrystals turned the material from white to black. Credit: Roy Kaltschmidt, Berkeley Lab Public AffairsMao’s group used hydrogenation to engineer disorder in the TiO2. They had a hint the nanocrystals might be effective over a wider spectrum of light when they saw that the material had turned from white to black post hydrogenation.
After 22 days of lab test using a full-spectrum solar light simulator with methanol serving as a sacrificial reagent, they report that, “We found that one hour of solar irradiation generated 0.2 
0.02 mmol of H2 using 0.02 g of disorder-engineered black TiO2 nanocrystals (10 mmol hour–1 g–1 of photocatalysts). This H2 production rate is about two orders of magnitude greater than the yields of most semiconductor photocatalysts. The energy conversion efficiency for solar hydrogen production, defined as the ratio between the energy of solar-produced hydrogen and the energy of the incident sunlight, reached 24% for disorder-engineered black TiO2 nanocrystals,” which they attribute to the nanocrystals new ability to absorb light from the infrared part of the spectrum.
The group also demonstrated similar effects when they substituted phenol and methylene blue for the methanol.
According to an LBL news release, the group says this is the first time a TiO2-based photocatalyst is able to convert infrared, visible and ultraviolet light. “The more energy from the sun that can be absorbed by a photocatalyst, the more electrons can be supplied to a chemical reaction, which makes black titanium dioxide a very attractive material,” says Mao in the release.
Theoretical physicist Peter Yu explains in the release that, “by introducing a specific kind of disorder, mid-gap electronic states are created accompanied by a reduced band gap,” says Yu, who also is a professor in the University of California at Berkeley’s Physics Department. “This makes it possible for the infrared part of the solar spectrum to be absorbed and contribute to the photocatalysis.”
Mao and his group say they are now tackling how to reach similar energy conversion levels in water containing more commonplace organic compounds.