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In the gold nanoparticle (Au NP)-loaded CdS film on fluorine-doped tin oxide electrode (Au/CdS/FTO), the localized plasmonic resonance excitation-induced electron injection from Au NP to CdS has been proven by photoelectrochemical measurements. Formation of ZnS thin films between the Au NP and CdS film leads to a drastic increase of the photocurrent under visible-light irradiation (λ > 610 nm) in a 0.1 M NaClO aqueous electrolyte solution due to the electron filtering effect. The photocurrent strongly depends on the thickness of the ZnS film, and the maximum value is obtained at a thickness as thin as 2.1 nm. Furthermore, the ZnS overlayer significantly stabilizes the photocurrent of the CdS/FTO electrode in a polysulfide/sulfide electrolyte solution even under the excitation of CdS (λ > 430 nm). This work presents important information about the design for new plasmonic photocatalysts consisting of plasmonic metal NPs and chalcogenide semiconductors with high conduction band edge.

Photoelectrochemical experiments and density functional theory calculations indicated that visible-light irradiation of the CdS quantum dots (QDs)–TiO2 direct coupling system (CdS/TiO2) causes the electron injection from the valence band (VB) of CdS into the conduction band (CB) of TiO2 (path 2) in addition to the inter-CB electron transfer from CdS to TiO2 (path 1). Path 2 can be induced by the sub-bandgap excitation of CdS QDs to extend the spectral response of the CdS/TiO2 system. For path 2 as well as path 1 to effectively work, CdS QDs should be directly deposited on the TiO2 surface with high coverage. According to the guideline, a photocatalytic growth of the preformed seed (PCGS) technique has been developed. Transmission electron microscopy observation and X-ray photoelectron spectroscopy measurements of the CdS/TiO2 prepared by the PCGS technique indicated that the TiO2 surface is highly covered by CdS QDs. The technique was applied to mesoporous TiO2 nanocrystalline films (mp-TiO2) to yield CdS/mp-TiO2. CdS QD-sensitized photoelectrochemical (QD-SPEC) cells with a structure of CdS/mp-TiO2 (photoanode)|aqueous sulfide solution|Ag/AgCl (reference electrode)|Pt (cathode) were fabricated. The rate of hydrogen (H2) generation in the QD-SPEC cell under illumination of simulated sunlight (AM 1.5, 1 sun, λ > 430 nm) increases with an increase in the TiO2-surface coverage by CdS QDs.

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