Imaging the Anisotropic Reactivity of a Tungsten Diselenide Photocathode

The reported activity of a photoelectrode reflects all kinds of reactive sites, good or bad. To understand localized behavior, such as catalytic sites or recombination centers, methods for mapping the anisotropic surface photoreactivity are constantly sought. The layered crystal structure of tungsten diselenide, a small bandgap semiconductor of great importance to solar fuels research, makes it especially prone to step-edge defects. Such steps are detrimental to photoelectrochemical performance. The first direct mapping of the photoreactivity of p-doped WSe₂ single crystals in solution by using scanning electrochemical microscopy under illumination (photo-SECM) is presented herein. This nondestructive method reveals the anisotropic reactivity of step edges of p-WSe₂ photocathodes, which serve as sites for both dark oxidation and lowered photoreduction in the dark and under illumination.