turns water to fuel
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
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
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
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.
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