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Separation of photogenerated electron-hole pairs in semiconductor is one of the key processes in photocatalysis. Heterostructured nanocrystals of semiconductors with a type-II bandgap offset facilitate the dissociation of excitons. Photocatalytic activity improves when the photogenerated electrons and holes become available to participate in reduction and oxidation on the surface, respectively. Therefore, prolonged electron-hole recombination time, or exciton lifetime, and heterostructures with both reduction and oxidation sites available on the surface are deemed to enhance photocatalytic activity. In this presentation, we describe the design of heterostructure nanocrystals based on Pb and Cd chalcogenides showing ultralong recombination lifetime, amorphous oxide shells that protect the nanocrystals from oxidation and enhance separation of photogenerated charge carriers. A principle that underlies the design strategy is to synthesize nano-sized junctions with electron electronhole separation, with the structure of “open” geometry, such as tetrapods. Also, the surface photochemistry will be discussed in the context of defect passivation and photocatalysis using the semiconductor nanocrystal quantum dots. The hydrogen production rate is comparable to the record values reported thus far, and we will discuss the possibility of expanding the nanocrystal design strategy for CO<sub>2</sub> reduction and other photocatalysis.
PbSe nanocrystals (NCs) exhibit intriguing properties, e.g., size-dependent exciton energy levels and controllably exciton-exciton interactions, which could be uniquely suited to various applications. Of particular interest is the use of PbSe NCs in solar cells. Because of their over-unity multi-exciton generation efficiency, PbSe NCs are considered to be an active layer in solar cells for high photocurrent density. However, their rapid oxidation and poor instability hamper the widespread use of PbSe NCs in devices. In this study, we describe a simple halide treatment of colloidal PbSe NCs which become extraordinarily stable both in solution and in devices. Halide salts (e.g., NH<sub>4</sub>Cl, KCl, InCl, NH<sub>4</sub>Br, and NH<sub>4</sub>I) in methanol solution are injected into the crude synthesis solution of PbSe NCs, and the simple step does the magic. In this talk, the effect of halide treatment, the passivation mechanism, and the implications in transistors or solar cells will be discussed.