The use of photoexcited electrons and holes in semiconductor nanocrystals as reduction and oxidation reagents is an intriguing way of harvesting photon energy to drive chemical reactions. This review focuses on recent research efforts to understand and control the photocatalytic processes mediated by colloidal II-VI nanocrystalline materials, such as cadmium and zinc chalcogenides. First, we highlight how nanocrystal properties govern the rates and efficiencies of charge-transfer processes relevant to photocatalysis. We then describe the use of nanocrystal catalyst heterostructures for fuel-forming reactions, most commonly H2 generation. Finally, we review the use of nanocrystal photocatalysis as a synthetic tool for metal–semiconductor nano-heterostructures.

Bulk oxy(nitride) (Ga(1-x)Zn(x))(N(1-x)O(x)) is a promising photocatalyst for water splitting under visible illumination. To realize its solar harvesting potential, it is desirable to minimize its band gap through synthetic control of the value of x. Furthermore, improved photochemical quantum yields may be achievable with nanocrystalline forms of this material. We report the synthesis, structural, and optical characterization of nanocrystals of (Ga(1-x)Zn(x))(N(1-x)O(x)) with the values of x tunable from 0.30 to 0.87. Band gaps decreased from 2.7 to 2.2 eV over this composition range, which corresponded to a 260% increase in the fraction of solar photons that could be absorbed by the material. We achieved nanoscale morphology and compositional control by employing mixtures of ZnGa(2)O(4) and ZnO nanocrystals as synthetic precursors that could be converted to (Ga(1-x)Zn(x))(N(1-x)O(x)) under NH(3). The high quality of the resulting nanocrystals is encouraging for achieving photochemical water-splitting rates that are competitive with internal carrier recombination pathways.

The ligands that passivate the surfaces of semiconductor nanocrystals play an important role in excited state relaxation and charge transfer. Replacement of native long-chain organic ligands with chalcogenides has been shown to improve charge transfer in nanocrystal-based devices. In this report, we examine how surface-capping with S 2-, Se 2-, and Te 2-impacts the excited state relaxation in CdSe quantum dots (QDs). We use transient absorption spectroscopy with state-specific pumping to reveal the kinetics of electron and hole cooling, band edge electron relaxation, hole trapping, and trapped hole relaxation, all as a function of surface-capping ligand. We find that carrier cooling is not strongly dependent on the ligand. In contrast, band edge relaxation exhibits strong ligand dependence, with enhanced electron trapping in chalcogenide-capped QDs. This effect is the weakest with the S 2-ligand, but is very strong with and Se 2-and Te 2-, such that the average band edge electron lifetimes for QDs capped with those ligands are under 100 ps. We conclude that, unlike the case of S 2-, improvements in electron transfer rates with Se 2-and Te 2-ligands may be overshadowed by the extreme electron lifetime shortening that may lead to low quantum yields of electron transfer.

We make direct observations of localized light absorption in a single nanostructure irradiated by a strong femtosecond laser field, by developing and applying a technique that we refer to as plasma explosion imaging. By imaging the photoion momentum distribution resulting from plasma formation in a laser-irradiated nanostructure, we map the spatial location of the highly localized plasma and thereby image the nanoscale light absorption. Our method probes individual, isolated nanoparticles in vacuum, which allows us to observe how small variations in the composition, shape, and orientation of the nanostructures lead to vastly different light absorption. Here, we study four different nanoparticle samples with overall dimensions of ∼100 nm and find that each sample exhibits distinct light absorption mechanisms despite their similar size. Specifically, we observe subwavelength focusing in single NaCl crystals, symmetric absorption in TiO2 aggregates, surface enhancement in dielectric particles containing a single gold nanoparticle, and interparticle hot spots in dielectric particles containing multiple smaller gold nanoparticles. These observations demonstrate how plasma explosion imaging directly reveals the diverse ways in which nanoparticles respond to strong laser fields, a process that is notoriously challenging to model because of the rapid evolution of materials properties that takes place on the femtosecond time scale as a solid nanostructure is transformed into a dense plasma.

Chalcogenide ligands (S 2-, Se 2-, Te 2-) are attractive candidates for passivation of surfaces of colloidal quantum dots (QDs) because they can enhance inter-particle or particleadsorbate electronic coupling. Devices made with QDs in which insulating long-chain aliphatic ligands were replaced with chalcogenide ligands have exhibited improved charge transfer and transport characteristics. While these ligands enable promising device performance, their impact on the electronic structure of the QDs that they passivate is not understood. In this work, we describe significant (up to 250 meV) changes in band-gap energies of CdTe QDs that occur when native aliphatic ligands are replaced with chalcogenides. These changes are dependent on the ligand and the particle size. To explain the observed changes in band-gap energies, we used the single band effective mass approximation (EMA) to model the ligand layer as a thin shell of Cd-chalcogenide formed by the bonding of chalcogenide ligands to partially coordinated Cd surface atoms. The model correctly predicted the observed trends in CdTe QD band-gap energies. The model also predicts that electrons and holes in chalcogenide-capped QDs can be significantly delocalized outside the core/shell structure, enhancing electronic coupling between QDs and adjacent species. Our work provides a simple description of the electronic structure of chalcogenide-capped QDs and may prove useful for the design of QD-based devices.