Sunlight turns water to fuel

By Kimberly Patch, Technology Research News

Hydrogen is the ultimate green fuel because it burns in air without producing pollution. It is also abundant because it is one of the atoms that makes up water.

Hydrogen can be extracted from water using electricity, but today's electricity-generating methods usually produce some pollution. The key to using hydrogen as a clean fuel is finding a non-polluting way to get water to give up its hydrogen.

Water can also be split using photocatalysts, which use the energy of light to break the molecular bonds. This is how plants gain their energy.

The difficulty has been finding a stable photocatalyst that can harness enough light energy to split the molecular bonds of water. Visible light photocatalysts tend to either break down too soon to be used in practical products or are not powerful enough to split the water bonds. And although stable ultraviolet photocatalysts exist, there is much less energy in ultraviolet light than in the visible light spectrum. Ultraviolet light represents about four percent of the energy the sun throws off, while visible light accounts for 43 percent.

A group of researchers in Japan has taken a large step toward producing clean hydrogen by finding a stable photocatalyst that can use the energy in visible light to split water. The group's research shows that it is possible to "photocatalyticly decompose water using solar energy and an oxide semiconductor to generate clean energy hydrogen like green plants," said Zhigang Zou, a researcher at the National Institute of Advanced Industrial Science and Technology in Japan.

The researchers doped indium-tantalum oxide semiconductor material with nickel to produce the photocatalyst.

The photocatalyst splits water by absorbing a photon of light, which provides enough energy to separate the negatively-charged electrons from the positively-charged holes in the material. These charges then move to the surface of the semiconductor particle where they react with the water, splitting it into its two gases.

The key to the researchers' success is that their material has an energy gap, or bandgap, that is low enough that the energy provided by photons of visible light is sufficient to move its electrons to a higher energy band.

Electrons whiz around an atom's nucleus in certain orbits, or bands, similar to the way planets orbit the sun. Unlike planets, however, electrons can change orbits. When a material gains energy, its electrons hop to a higher band, and when it gives up energy by fueling a chemical reaction like splitting water, the electrons fall to a lower energy band. The difference in energy between the first, or valence band and, the second, or conduction band, is the energy gap.

The researchers tested the material by suspending doped indium tantalum oxide powder in water in a closed glass circulation system under a lamp, then measured the ensuing gases. The researchers material is not very efficient, using only 0.66 percent of the energy in light to split the water, but this can be improved by increasing the surface area of the photocatalyst and by changing its chemical composition, said Zou. "It is necessary to enhance the efficiency up to about 50 percent for commercial [use]," he said.

Researchers have been working on finding catalysts that split water since the first paper on the subject appeared in 1972, said Thomas Mallouk, a professor of materials chemistry at Pennsylvania State University. "This is the first bona fide report of such a photocatalyst, which carries out the reaction using visible light," he said.

There is still work to be done before the method can be made practical, he added. The paper doesn't contain much detail on exactly how the reaction works. "I think it is safe to say that neither the authors nor anyone else understands in detail how this photocatalyst really works. This does not take away from the discovery, which is quite important, it just means that more work needs to be done," he said.

The yield must also be increased for the photocatalyst to the practical. "If it could be improved by a factor of ten or so then it would become quite interesting for solar energy conversion. I expect that this discovery will stimulate more work in this area," Malick said.

The researchers are now working on increasing the efficiency of the photocatalyst; they're also looking for similar substances that may be more efficient, said Zou. It will probably be 20 years before clean water splitting devices are practical, he added.

Zou's research colleagues were Jinhua Ye, Kazuhiro Sayama and Hironori Arakawa. They published the research in the December 6, 2001 issue of the journal Nature. The research was funded by the Japanese Ministry of Education and Science.

Timeline:   20 years
Funding:   Government
TRN Categories:  Energy; Materials Science and Engineering
Story Type:   News
Related Elements:  Technical paper, "Direct Splitting of Water under Visible Light Irradiation with an Oxide Semiconductor Photocatalyst," Nature, December 6, 2001.


January 9, 2002

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