366798 Design Principles for Quantum Dot-Based Photocatalysts for Water Splitting
Semiconductor nanocrystals of size in the strong quantum confinement regime, or quantum dots, exhibit size-dependent energy gap. The size-tunable shift of band-edge states can be a powerful tool in the control of exciton dynamics. For example, heterostructure nanocrystals with mere changes in geometric dimensions can lead to the band alignment from type I to type II, which could dramatically increase the exciton recombination lifetime. 
In this presentation, we describe the design of heterostructure nanocrystals based on Pb and Cd chalcogenides showing ultralong recombination lifetime. For the study, we synthesized PbSe/CdSe core/shell nanocrystals and introduced CdS outer-most shell with varying thickness and morphology. Spectroscopic study revealed that as the outer CdS shell becomes thicker, the quantum confinement in the shell region is relieved and thus the heterostructure has quasi-type-II energy alignment. Furthermore, photoluminescence lifetime of the sample also increased when the CdS shell became thicker: nearly 2-orders-of-magnitude increase was observed. Notably, slight change in CdS deposition condition resulted in varying morphology of the CdS shell. Not only could we design heterostructure nanocrystals with enhanced electron-hole separation, the structure could turn into “open” geometry, such as tetrapods. In the reduction of methylene blue, we observed noticeable increase in photocatalytic activity as the geometry became open. For instance, tetrapod-shaped PbSe/CdSe/CdS core/shell/shell nanocrystals exhibited enhanced photocatalysis compared to pyramid-shaped PbSe/CdSe/CdS nanocrystals, let alone spherical structures. Our study of photocatalysis using PbSe-based heterostructure nanocrystals suggests that the geometric nature of nanocrystals should be considered in the context of the efficient conversion of photon energy into chemical reactions. 
We extended our design principle into CdSe quantum dot / TiO2 composite photocatalysts. The new photocatalytic system shows controlled and improved photocatalytic efficiency of water splitting. Overall water splitting using heterostructure nanocrystals will be discussed.
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