Splitting water with the aid of sunlight

Water and sunlight are renewable sources, are available in sufficient quantities and are cheap. Up to now, however, there has been no economical technology for producing hydrogen. Japanese researchers have now imitated plants by utilising a semiconducting mixture of metal oxides which absorb sunlight and use the acquired energy to split water into hydrogen and oxygen. At the present time, however, this method only works with 0.66% efficiency, but the researchers foresee a whole range of possibilities to increase this efficiency. Furthermore, the new semiconductor is elastic and does not corrode.

Systems which convert and save solar energy efficiently have to fulfil three requirements. The sunlight has to be absorbed efficiently in order to have excited electron states in the photo-catalyst. The electrons excited by the light and the accompanying ‘holes’ have to be separated spatially so that they are unable to recombine. These requirements are fulfilled by semiconductors. However, the excited electrons have to be capable both energetically and kinetically to split water efficiently. And that is the difficulty.

Japanese scientists have developed a semiconducting mixture of metal oxides which absorb in the range of visible light and utilise 43 % of the sunlight. This mixture consists of InTa04 doped with nickel, to be exact: In(1-x)NixTa04 (x=0-0.2). This reduces the band gap from 2.6 to 2.3 ev, i.e. photons of the visible light are sufficient for the excitation of the electrons. The scientists placed this semiconductor in water and illuminated it with an arc lamp. As the conduction band has a higher energy than the reduction potential of the hydrogen, these electrons drift to the surface of the semiconductor, combine with the hydrogen ions of the water and form hydrogen gas. In order to compensate for this reaction, the valence band has to be energetically lower than the oxidation potential of the oxygen. In this way, the positive holes on the surface are able to receive electrons from the oxygen and therefore form oxygen gas.

Zhigang Zou, Photoreaction Control Research Center (PCRC), (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
tel: +81-298-61-4750, fax: +81-298-61-4750
email: z.zou@aist.go.jp