Mercury(II) sulphide: a photo-stable semiconductor

Abstract

THERE is much interest in photo-electrochemical cells which use a semiconductor–electrolyte junction to transduce light into electrical and chemical energy¹. The semiconductors investigated to date fall into two main groups, the first of which contains photostable material but which shows response in the UV rather than the visible region of the spectrum, for example, n-titanium dioxide², n-strontium titanate³, n-potassium tantalate⁴, n-tin oxide⁵ and n-barium titanate⁶. The second group comprises compounds which respond to the visible region of the spectrum but are photochemically unstable such as n-gallium phosphide⁷, n-indium phosphide⁸, n-cadmium sulphide⁹, and n-cadmium selenide¹⁰. Attempts have been made to extend the spectral response of the materials belonging to the first group, by doping¹¹ and chemically affixing dyes to the surface of the semiconductor¹². Some success has been attained in stabilising some of the members of the second group; for example, cadmium sulphide and selenide are stable in poly-chalcogenide solutions¹³. If cells containing a semiconductor–electrolyte junction are to be of any practical use in the harnessing of solar energy then the following criteria for the semiconductor should be met: (1) they should absorb visible radiation; and (2) be photostable and capable of use in powder form. Clearly, if the conduction and valence bands of the semiconductor have energies that facilitate proton reduction and oxidation of hydroxyl ions respectively this would be an added bonus as the cells could be used to either photo-assist or photo-electrolyse the water¹⁴. We report here on mercury(II) sulphide (cinnabar) band gap = 2.1 eV¹⁵ which fulfils most of the above criteria¹⁶.