(640g) Tailored Coupling of Biomineralized CdS Quantum Dots to Reduced Graphene Oxide to Realize Ambient Synthesis of a High-Performance Hydrogen Evolution Photocatalyst

Nanocomposite photocatalysts offer a promising route to efficient and clean hydrogen production. However, the multi-step, high temperature, solvent based syntheses typically utilized to prepare these photocatalysts can limit their scalability and sustainability. Bio-synthetic routes for photocatalyst production offer an attractive room temperature and aqueous phase alternative, but often result in poorly performing catalysts. We have developed a method to produce CdS quantum dots (QDs) supported on reduced graphene oxide (rGO) using enzyme-based syntheses and a ligand exchange mediated self-assembly method for a highly efficient hydrogen evolution photocatalyst. All preparation steps are carried out in an aqueous environment at ambient temperature. Size-controlled CdS QDs are prepared through a biomineralization process involving the enzyme-mediated turnover of L-cysteine to H2S in aqueous solutions of Cd-acetate. Graphene oxide is reduced by enzymatically produced H2S to form rGO. Exchange of cysteamine (CA) for the native L-cysteine ligand capping the CdS QDs, as confirmed by zeta potential measurements and ATR-FTIR spectroscopy, enables exploitation of electrostatic interactions to drive assembly of the positively charged CA-capped CdS (CdS/CA) onto negatively charged rGO. UV-vis spectroscopy and TEM imaging offer insight into the CdS loading and spatial distribution on the rGO. The short CA linker molecule allows for efficient charge transfer from CdS to rGO, increasing exciton lifetime and, subsequently, photocatalytic activity. The visible light hydrogen evolution rate of the resulting CdS/CA/rGO photocatalyst was measured at 3300 μmol hr-1 g-1, which is nearly double the rate of the unsupported CdS/CA. This represents, to our knowledge, one of the highest reported rates for a CdS/rGO nanocomposite photocatalyst. Beyond advancing the state-of-the-art in benign, scalable synthesis of high-performance photocatalysts, the ligand exchange strategy developed here offers a potentially generalizable approach for assembling other high-performing, nanocomposite photocatalysts under ambient conditions.