High Coverage Formation of CdS Quantum Dots on TiO₂ by the Photocatalytic Growth of Preformed Seeds

Photoelectrochemical experiments and density functional theory calculations indicated that visible-light irradiation of the CdS quantum dots (QDs)–TiO 2 direct coupling system (CdS/TiO 2 ) causes the electron injection from the valence band (VB) of CdS into the conduction band (CB) of TiO 2 (path 2) in addition to the inter-CB electron transfer from CdS to TiO 2 (path 1). Path 2 can be induced by the sub-bandgap excitation of CdS QDs to extend the spectral response of the CdS/TiO 2 system. For path 2 as well as path 1 to effectively work, CdS QDs should be directly deposited on the TiO 2 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/TiO 2 prepared by the PCGS technique indicated that the TiO 2 surface is highly covered by CdS QDs. The technique was applied to mesoporous TiO 2 nanocrystalline films (mp-TiO 2 ) to yield CdS/mp-TiO 2 . CdS QD-sensitized photoelectrochemical (QD-SPEC) cells with a structure of CdS/mp-TiO 2 (photoanode)|aqueous sulfide solution|Ag/AgCl (reference electrode)|Pt (cathode) were fabricated. The rate of hydrogen (H 2 ) 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 TiO 2 -surface coverage by CdS QDs.