Photoelectrochemistry of mercury (II) sulphide

Abstract

Some photoelectrochemical reactions of red mercury(II) sulphide powder coated on a platinum mesh are presented. The sulphide is an n-type semiconductor stable to photocorrosion in suitable electrolytes, but in aqueous halides can undergo darkening. In certain electrolytes (halides, thiocynate) light-induced decompostion also occurs. The blackened sulphide, when used in an electrolyte that does not induce solubilisation, is more sensitive than the red form, in that larger photocurrents can be generated. The role of the electrolyte is emphasised, with respect to potential-determining ions, those which influence the semiconductor space-charge layer, and those which “block” surface states used in electron transfer. The blackening is discussed. Photoacoustic spectra show metacinnabar to be present, and the red–black transformation is treated in terms of rehybridisation of mercury under the influence of illumination and strongly interacting solution species. Band edges for red mercury(II) sulphide are presented, and the potential of the material to harness solar energy evaluated.