Hydriodic acid photodecomposition on layered-type transition metal dichalcogenides

Abstract

The photodecomposition of hydriodic acid on platinized n-WSe 2 single crystals immersed in an aqueous 1 M HI solution was studied. A special in situ technique was followed in which only a small region of the sample surface was illuminated and simultaneously observed under an optical microscope. During the photodecomposition process, hydrogen evolution only occurred on the microscopic defects of the sample surface, whereas iodine was produced on the smooth areas where a diffuse orange-red colouring appeared. For polycrystalline specimens, however, hydrogen gas bubbles were formed over the entire surface, the rate of the process being markedly slower than on single crystals. The results are discussed with the assumption that the n-WSe 2 single crystals behave as Schottky-type photochemical diodes, that the cathodic reaction takes place on the stepped platinum-covered areas and that the anodic reaction occurs on the smooth unplatinized areas.