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

We used linker-assisted assembly (LAA) to tether CdSquantum dots(QDs) to MoS2 nanosheets via L-cysteine(cys) or mercaptoalkanoic acids (MAAs) of varyinglengths, yielding ligand-bridged CdS/MoS2 heterostructuresfor redox photocatalysis. LAA afforded precise control over the light-harvestingproperties of QDs within heterostructures. Photoexcited CdS QDs transferredelectrons to molecularly linked MoS2 nanosheets from bothband-edge and trap states; the electron-transfer dynamics was tunablewith the properties of bridging ligands. Rate constants of electrontransfer, estimated from time-correlated single photon counting (TCSPC)measurements, ranged from (9.8 & PLUSMN; 3.8) x 10(6) s(-1) for the extraction of electrons from trap stateswithin heterostructures incorporating the longest MAAs to >5 x10(9) s(-1) for the extraction of electronsfrom band-edge or trap states in heterostructures with cys or 3-mercaptopropionic acid (3MPA) linkers. Ultrafast transientabsorption measurements revealed that electrons were transferred within0.5-2 ps or less for CdS-cys-MoS2 and CdS-3MPA-MoS2 heterostructures, corresponding torate constants & GE;5 x 10(9) s(-1). Photoinduced CdS-to-MoS2 electron transfer could beexploited in photocatalytic hydrogen evolution reaction (HER) viathe reduction of H+ to H-2 in concert with theoxidation of lactic acid. CdS-L-MoS2-functionalizedFTO electrodes promoted HER under oxidative conditions wherein H-2 was evolved at a Pt counter electrode with Faradaic efficienciesof 90% or higher and under reductive conditions wherein H-2 was evolved at the CdS-L-MoS2-heterostructure-functionalizedworking electrode with Faradaic efficiencies of 25-40%. DispersedCdS-L-MoS2 heterostructures promoted photocatalyticHER (15.1 & mu;mol h(-1)) under white-light illumination,whereas free cys-capped CdS QDs produced threefoldless H-2 and unfunctionalized MoS2 nanosheetsproduced no measurable H-2. Charge separation across theCdS/MoS2 interface is thus pivotal for redox photocatalysis.Our results reveal that LAA affords tunability of the properties ofconstituent 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.