Hisayoshi Kobayashi
Kyoto Institute of Technology ,Japan
Title: DFT study of electronic structures on TiO2/CdS and TiO2/CdSe quantum dots
Biography
Biography: Hisayoshi Kobayashi
Abstract
Photoelectrochemical experiments and DFT calculations indicated that visible-light irradiation of the CdS quantum dots (QDs)−TiO2 direct coupling system (CdS/TiO2) causes the electron injection in two paths: The inter-CB electron transfer from CdS to TiO2 (path 1) and that the electron transfer from the valence band (VB) of CdS into the conduction band (CB) of TiO2 (path 2). 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. We also show that the sub-bandgap excitation of a directly coupled CdSe quantum dot–TiO2 system induces electron injection from CdSe levels to the conduction band of TiO2.