Metal cocatalysts dictate electron transfer in Ag-decorated MoS2 nanosheets

Two-dimensional transition-metal dichalcogenides such as atomically thin MoS2 nanosheets are useful as low-cost solar energy conversion materials. Colloidally stable few-layer and monolayer MoS2 nanosheets in dimethylformamide were prepared via an electrochemically assisted liquid phase exfoliation approach. These nanosheets were further modified with Ag nanoparticles by using photocatalytic reduction of Ag+ ions. An MV2+/MV•+ redox couple was employed as a probe to determine the rate constants of photoinduced forward electron transfer (kf) and dark back electron transfer (kb) processes to establish the role of Ag cocatalyst in promoting photocatalytic reduction. The competition between these two processes dictates the buildup of a steady-state concentration of the reduction product (MV•+) under visible-light irradiation. The kf and kb rate constants increase with increasing Ag nanoparticle loading but with different dependencies, yielding a maximum reduction efficiency of 4.4%. At higher Ag loadings, the reduction yield decreases as back electron transfer dominates over the forward electron transfer process. Establishing the role of noble metal cocatalyst in the photoinduced charge transfer processes of Ag-MoS2 hybrid composites offers design strategies to maximize the photocatalytic performance of semiconductor–metal heterostructures.