Abstract
We report on the fabrication of CdSe quantum dot (QD) sensitized electrodes by direct adsorption of colloidal QDs on mesoporous TiO2 followed by 3-mercaptopropionic acid (MPA) ligand exchange. High efficiency photoelectrochemical hydrogen generation is demonstrated by means of these electrodes. The deposition of ZnS on TiO2/CdSe further improves the external quantum efficiency from 63% to 85% at 440 nm under -0.5 V vs. SCE. Using the same photoelectrodes, solar cells with the internal quantum efficiency approaching 100% are fabricated. The ZnS deposition increases the photocurrent and chemical stability of the electrodes. Investigation of the carrier dynamics of the solar cells shows that ZnS enhances the exciton separation rate in CdSe nanocrystals, which we ascribe to the formation of a type II heterojunction between ZnS and CdSe QDs. This finding is confirmed by the dynamics of the CdSe photoluminescence, which in the presence of ZnS becomes noticeably faster.
Highlights
Narrow band gap nanoparticles seem ideal due to their tunable band gap, high extinction coefficient and stability
The photocatalytic water splitting device is composed of a quantum dot (QD) sensitized mesoporous TiO2 photoelectrode (details in Fig. 1(b)), the electrolyte, a saturated calomel electrode (SCE) as reference, and a Pt coil as a counter electrode
We demonstrate highly efficient water splitting and sensitized solar cells by direct adsorption of colloidal CdSe QDs on mesoporous TiO2 followed by 3-mercaptopropionic acid ligand exchange
Summary
Narrow band gap nanoparticles seem ideal due to their tunable band gap, high extinction coefficient and stability. Among the many heterostructures proposed, CdSe and CdS sensitized electrodes are the ones more systematically studied.[15,21,22] In particular a CdS interlayer deposited between TiO2 and CdSe has been found to help charge separation.[15] The alignment of the Fermi level after contact between CdS and CdSe results in a downward and upward shift of the CdS and CdSe band gap, respectively. This has been reported to allow the formation of a type II heterostructure, which drives the charge separation upon illumination.[15,22]. These data are confirmed by impedance spectroscopy, which shows that ZnS enhances the charge injection (separation) efficiency
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