Photocatalytic Hydrogen Production with Visible Light over Pt-Interlinked Hybrid Composites of Cubic-Phase and Hexagonal- Phase CdS

A hybrid photocatalytic system, which is based on a mixed-phase cadmium sulfide matrix composed of nano-particulate cubic-phase CdS (c-CdS) with average particle diameters of 13 nm and a bandgap energy of 2.6 eV, is coupled with bulk-phase hexagonal CdS (hex-CdS) that has a bandgap energy of 2.3 eV and is interlinked with elemental platinum deposits. The resulting hybrid nano-composite catalysts are photocatalytically efficient with respect to hydrogen gas production from water with visible light irradiation at  > 420 nm. Rates of H2 production approaching 1.0 mmol-H2 g h are obtained with a c-CdS/Pt/hex-CdS composite photocatalyst, in the presence of a mixed sodium sulfide and sodium sulfite background electrolyte system at pH 14. In contrast, the same composite produces H2 at a rate of 0.15 mmol g h at pH 7 in a water-isopropanol solvent system. The relative order of reactivity for the synthesized hybrid catalysts was found to be: c-CdS/Pt/hex-CdS > Pt/c-CdS/hex-CdS > c-CdS/hexCdS > Pt/hex-CdS > hex-CdS > quantum-sized c-CdS. A mechanism involving enhanced lifetimes of electron-hole trapping states that are dependent on the surface chemistry of hydrated CdS involving surface hydroxyl (>CdOH) and sulfhydryl groups (>CdSH) are invoked.