Surface compositional modification of Weyl semimetal Co3Sn2S2(0001) from vacuum to electrochemical water splitting conditions

Abstract

This study investigates the surface compositional evolution of the topological Weyl semimetal Co₃Sn₂S₂, proposed to be an active catalyst for water splitting, from ultra high vacuum (UHV) to catalytic reaction conditions, using synchrotron X-ray spectroscopy (XPS). We found that the surface cleaved in ambient air is completely oxidized, forming Co oxides and Sn oxides/hydroxides. This surface exhibits an overpotential of 503 mV to achieve a current density of 10 mA cm⁻² for OER. However, the UHV-cleaved surface maintains Co₃Sn₂S₂ pristine features, showing remarkably slow kinetics compared to the air-cleaved sample. Systematically tracking the oxidation process under various exposure conditions reveals that a small amount of Sn oxides forms upon exposures to O₂ and H₂O under UHV, while a strong interaction between Sn and liquid water occurs at atmosphere conditions. In contrast, higher exposed pressure promotes Co oxidation, resulting in the formation of Co oxides in atmospheric air, which correlates with enhanced catalytic activity. Despite unfavorable evidence regarding the surface robustness, this work provides crucial insights into Co₃Sn₂S₂ surface reactivity.