Electron-transfer dependent photocatalytic hydrogen generation over cross-linked CdSe/TiO₂type-II heterostructure

Abstract

Developing type-II heterostructures with a spatial separation of photoexcited electrons and holes is a useful route to promote photocatalytic hydrogen generation. However, few investigations on the charge transfer process across the heterojunction have been carried out, which can allow us to uncover the reaction mechanism. Herein, CdSe quantum dots (QDs) and TiO₂ nanocrystals were synthesized and combined in water yielding CdSe/TiO₂ type II heterostructures. It was found that mercaptopropionic acid as bifunctional molecules could bind with CdSe and TiO₂ to form a cross-linked morphology. The charge carrier dynamics of bare CdSe and CdSe/TiO₂ were detected using femtosecond transient absorption spectroscopy. In the presence of TiO₂, the average exciton lifetime of CdSe QDs was apparently decreased, owing to the electron transfer from photoexcited CdSe to TiO₂. Particularly, the electron-transfer rate from small CdSe QDs (3.0 nm) was much faster than that from big CdSe QDs (4.2 nm). The improved photocatalytic hydrogen generation was observed for CdSe/TiO₂ compared to bare CdSe QDs. The enhancement factor for small CdSe QDs was higher than that for big CdSe QDs, which was in good agreement with the electron-transfer rates. This result indicated that the electron transfer between CdSe and TiO₂ played an important role in photocatalytic hydrogen generation on CdSe/TiO₂ type-II heterostructure. Our study provides a fundamental guidance to construct efficient heterostructured photocatalysts by delicate control of the band alignment.