Defect Mediated Improvements in the Photoelectrochemical Activity of MoS 2 /SnS 2 Ultrathin Sheets on Si Photocathode for Hydrogen Evolution.

Solar-driven water electrolysis to produce hydrogen is one of the clean energy options for the current energy-related challenges. Si as a photocathode exhibits a large overpotential due to the slow hydrogen evolution reaction (HER) kinetics and hence needs to be modified with a cocatalyst layer. MoS 2 is a poor HER cocatalyst due to its inert basal plane. Activation of the MoS 2 basal plane will facilitate HER kinetics. In this study, we have incorporated SnS 2 into MoS 2 ultrathin sheets to induce defect formation and phase transformation. MoS 2 /SnS 2 composite ultrathin sheets with a Sn 2+ state create a large number of S vacancies on the basal sites. The optimized defect-rich MoS 2 /SnS 2 ultrathin sheets decorated on surface-modified Si micro pyramids as photocathodes show a current density of -23.8 mA/cm 2 at 0 V with an onset potential of 0.23 V under acidic conditions, which is higher than that of the pristine MoS 2 . The incorporation of SnS 2 into 2H-MoS 2 ultrathin sheets not only induces a phase but also can alter the local atomic arrangement, which in turn is verified by their magnetic response. The diamagnetic SnS 2 phase causes a decrease in symmetry and an increase in magnetic anisotropy of the Mo 3+ ions.