http://physicsweb.org/article/news/5/12/2
<<Solar cell edges towards endless energy
7 December 2001
A limitless source of clean-burning fuel is a step closer following the
discovery of a material that can extract hydrogen from water using energy
from visible light. Zhigang Zou of the National Institute of Advanced
Industrial Science and Technology in Japan and colleagues have developed a
photocatalyst that uses optical radiation – which makes up 43% of solar
energy. A reliable source of hydrogen is one of the ‘holy grails’ of energy
production – hydrogen releases lots of energy when it burns and the only
by-product is water. Previous catalysts have only responded to ultraviolet
radiation, which accounts for just 4% of the Sun’s energy (Z Zou et al 2001
Nature 414 625).
Photocatalysis is the use of energy from absorbed light to initiate chemical
reactions. Semiconductors are useful in such reactions because they can be
designed so that their electronic characteristics change when they absorb
radiation. But their energy bandgaps are often large, and this means that
only photons with short-wavelengths and high-energies – such as ultraviolet
photons – can promote electrons from the valence band to the conduction band.
To create a suitable material, Zou and co-workers added nickel to the
semiconductor indium tantalum oxide. This reduced its energy bandgap from 2.6
to 2.3 electronvolts, which means that visible photons carry enough energy to
make electrons jump the bandgap. They immersed this semiconductor in water
and illuminated it with an arc lamp. As the semiconductor absorbs energy from
the photons, electrons jump from the valence band to the conduction band,
leaving positive holes in the valence band. Provided the conduction band is
at a higher energy than the ‘reduction potential’ of hydrogen, the ‘promoted’
electrons drift to the surface of the semiconductor where they combine with
hydrogen ions in the water to make hydrogen gas. To balance this reaction,
the valence band must be at a lower energy than the ‘oxidation potential’ of
oxygen – this allows the positive holes to surface and accept electrons from
oxygen ions in the water, creating oxygen gas. The new semiconductor is also
resilient – existing semiconductors that use visible light either corrode or
become inert when they come into contact with water. Zou and colleagues point
out that although their set-up is only 0.66% efficient, they are confident
that this will improve when they increase the surface area of the
semiconductor, and adjust its layout.>>
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