Thin Amorphous TiO2 Shell on CdSe Nanocrystal Quantum Dots Enhances Photocatalysis of Hydrogen Evolution from Water

Abstract

In this study, we designed and synthesized photocatalysts for hydrogen evolution from water by coating a thin layer of amorphous TiO2 (a-TiO2) on CdSe nanocrystals (NCs). The thin shell of a-TiO2 serves as a channel for charge carriers otherwise unutilized. Albeit a previous notion that a-TiO2 is a poor photocatalyst, the enhanced photocatalytic activity in the presence of a-TiO2 suggests that the material helps utilize the photogenerated charge carriers when it is in a form of thin shell on CdSe NCs. Type II band offset in CdSe/a-TiO2 appears to allow the electron in the conduction band of CdSe NCs to migrate over to that of a-TiO2, and the electron participates in the hydrogen production from water. Size of CdSe NCs influences the photocatalytic hydrogen evolution rate as the energy difference between the conduction bands of semiconductors becomes larger. Electron transfer from CdSe NCs to a-TiO2 layer is influenced by the level of the conduction-band edge of CdSe NCs: the size dependence indicates that electron injection to TiO2 is facilitated with energy level offset between CdSe and TiO2, while smaller NCs have larger band gap and thus narrower spectral range of absorption. The interplay between charge-transfer rate and absorption cross-section should be considered in designing heterostructure NC-based photocatalysts for water splitting.