Photoelectrochemical behaviour of layer-type transition metal dichalcogenides

Abstract

The photoelectrochemical properties of semiconducting layer-type disulphides and diselenides of transition metals belonging to Groups IV (zirconium, hafnium), VI (molybdenum, tungsten) and VIII (platinum), which have energy gaps between 1 and 2 eV, have been studied systematically. Characteristic reaction mechanisms and differences between compounds were found which can be interpreted in terms of the contribution of d-orbitals to the valence band in which photoreactive holes are generated. Evidence is provided that the behaviour of d-band semiconductors is critically influenced by a specific light-induced interaction of the interface with the redox system. This interaction leads to surface states which are apparently charged to an electric potential which is proportional or equivalent to the difference between the valence band edge and the redox potential of the electron donor (ΔE). It is the source of a photoelectrochemical reaction enthalpy which is useful for energy conversion. d-Band semiconductors thus have behaviour which is characteristically different from that of classical semiconducting compounds where hold formation is equivalent to the destruction of existing chemical bonds and the energy difference ΔE is dissipated as heat. The particular photoelectrochemical properties of transition metal dichalcogenides enable a larger variety of new applications to be explored. While the compounds of Mo and W can be used as reasonably stable electrodes for regenerative and fuel producing solar cells, those of Zr and Hf could be developed for the combined conversion and storage of solar energy by means of light-induced intercalation reactions. PtS₂, finally, could be used as a sensitive and highly selective photoelectroanalytical probe for electron-exchanging ions and molecules. Various theoretical aspects ranging from photoelectrode stability to water decomposition with visible light are considered in this context.