Linker-Assisted Assembly of Ligand-Bridged CdS/MoS2 Heterostructures: Tunable Light-Harvesting Properties and Ligand-Dependent Control of Charge-Transfer Dynamics and Photocatalytic Hydrogen Evolution.

Abstract

We used linker-assisted assembly (LAA) to tether CdS quantum dots (QDs) to MoS2 nanosheets via L-cysteine (cys) or mercaptoalkanoic acids (MAAs) of varying lengths, yielding ligand-bridged CdS/MoS2 heterostructures for redox photocatalysis. LAA afforded precise control over the light-harvesting properties of QDs within heterostructures. Photoexcited CdS QDs transferred electrons to molecularly linked MoS2 nanosheets from both band-edge and trap states; the electron-transfer dynamics was tunable with the properties of bridging ligands. Rate constants of electron transfer, estimated from time-correlated single photon counting (TCSPC) measurements, ranged from (9.8 ± 3.8) × 106 s-1 for the extraction of electrons from trap states within heterostructures incorporating the longest MAAs to >5 × 109 s-1 for the extraction of electrons from band-edge or trap states in heterostructures with cys or 3-mercaptopropionic acid (3MPA) linkers. Ultrafast transient absorption measurements revealed that electrons were transferred within 0.5-2 ps or less for CdS-cys-MoS2 and CdS-3MPA-MoS2 heterostructures, corresponding to rate constants ≥5 × 109 s-1. Photoinduced CdS-to-MoS2 electron transfer could be exploited in photocatalytic hydrogen evolution reaction (HER) via the reduction of H+ to H2 in concert with the oxidation of lactic acid. CdS-L-MoS2-functionalized FTO electrodes promoted HER under oxidative conditions wherein H2 was evolved at a Pt counter electrode with Faradaic efficiencies of 90% or higher and under reductive conditions wherein H2 was evolved at the CdS-L-MoS2-heterostructure-functionalized working electrode with Faradaic efficiencies of 25-40%. Dispersed CdS-L-MoS2 heterostructures promoted photocatalytic HER (15.1 μmol h-1) under white-light illumination, whereas free cys-capped CdS QDs produced threefold less H2 and unfunctionalized MoS2 nanosheets produced no measurable H2. Charge separation across the CdS/MoS2 interface is thus pivotal for redox photocatalysis. Our results reveal that LAA affords tunability of the properties of constituent CdS QDs and MoS2 nanosheets and precise, programmable, ligand-dependent control over the assembly, interfacial structure, charge-transfer dynamics, and photocatalytic reactivity of CdS-L-MoS2 heterostructures.