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.