(704h) Biomineralization of Hydrogen Evolution Photocatalysts

While photocatalysts offer a promising route to efficient and clean hydrogen production, 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—occurring at room temperature and in aqueous conditions—address these problems, but the as-synthesized materials are often not high-performing due to lack of control over product size and inability to form composites and heterojunctions. Herein, we demonstrate that composite photocatalysts synthesized through bio-inspired room temperature, aqueous routes can achieve hydrogen evolution rates comparable to those made by traditional means. We have investigated the enzyme-based biomineralization of photocatalysts and supports as well as the assembly of these materials to form high-performing composite photocatalysts. We have isolated an enzyme, cystathionine γ-lyase, that turns over L-cysteine to HS^-, which further reacts with metal ions in solution to form metal sulfide nanocrystals. Through this biomineralization route, we have demonstrated the size-controlled synthesis of CdS nanocrystals that are active for photocatalytic hydrogen evolution. Additionally, the biomineralization approach reduces graphene oxide to produce reduced graphene oxide (rGO), a conductive support for photocatalysts. Ligand exchange, involving replacement of the negatively charged native L-cysteine ligands on CdS for positively charged cysteamine ligands, mediates self-assembly of CdS nanocrystals on rGO in solution at room temperature. The visible light hydrogen evolution rate of the resulting CdS/rGO photocatalyst was measured at 3300 μmol hr^-1 g^-1, which is nearly double the rate of the unsupported CdS. This represents, to our knowledge, one of the highest reported rates for a CdS/rGO nanocomposite photocatalyst. Furthermore, the biomineralization of a heterojunction CdS/ZnS photocatalyst was also investigated by Zn addition through post-synthetic addition of zinc acetate to biomineralized CdS nanocrystals. Adding zinc acetate to CdS nanocrystals was shown to controllably increase the radiative recombination rate of the nanocrystals and to, thereby, improve the photocatalytic hydrogen evolution rate from 1300 μmol hr^-1 g^-1 to 3200 μmol hr^-1 g^-1.