The development of nanomaterials for next generation photonic, optoelectronic, and catalytic applications requires a robust synthetic toolkit for systematically tuning composition, phase, and morphology at nanometer length scales. While de novo synthetic methods for preparing nanomaterials from molecular precursors have advanced considerably in recent years, postsynthetic modifications of these preformed nanostructures have enabled the stepwise construction of complex nanomaterials. Among these postsynthetic transformations, cation exchange reactions, in which the cations ligated within a nanocrystal host lattice are substituted with those in solution, have emerged as particularly powerful tools for fine-grained control over nanocrystal composition and phase. In this feature article, we review the fundamental thermodynamic and kinetic basis for cation exchange reactions in colloidal semiconductor nanocrystals and highlight its synthetic versatility for accessing nanomaterials intractable by direct synthetic methods from molecular precursors. Unlike analogous ion substitution reactions in extended solids, cation exchange reactions at the nanoscale benefit from rapid reaction rates and facile modulation of reaction thermodynamics via selective ion coordination in solution. The preservation of the morphology of the initial nanocrystal template upon exchange, coupled with stoichiometric control over the extent of reaction, enables the formation of nanocrystals with compositions, morphologies, and crystal phases that are not readily accessible by conventional synthetic methods.

Although the vast majority of hydrocarbon fuels and products are presently derived from petroleum, there is much interest in the development of routes for synthesizing these same products by hydrogenating CO 2 . The simplest hydrocarbon target is methane, which can utilize existing infrastructure for natural gas storage, distribution, and consumption. Electrochemical methods for methanizing CO 2 currently suffer from a combination of low activities and poor selectivities. We demonstrate that copper nanoparticles supported on glassy carbon (n-Cu/C) achieve up to 4 times greater methanation current densities compared to high-purity copper foil electrodes. The n-Cu/ C electrocatalyst also exhibits an average Faradaic efficiency for methanation of 80% during extended electrolysis, the highest Faradaic efficiency for room-temperature methanation reported to date. We find that the level of copper catalyst loading on the glassy carbon support has an enormous impact on the morphology of the copper under catalytic conditions and the resulting Faradaic efficiency for methane. The improved activity and Faradaic efficiency for methanation involves a mechanism that is distinct from what is generally thought to occur on copper foils. Electrochemical data indicate that the early steps of methanation on n-Cu/C involve a pre-equilibrium one-electron transfer to CO 2 to form an adsorbed radical, followed by a rate-limiting nonelectrochemical step in which the adsorbed CO 2 radical reacts with a second CO 2 molecule from solution. These nanoscale copper electrocatalysts represent a first step toward the preparation of practical methanation catalysts that can be incorporated into membrane-electrode assemblies in electrolyzers.

In situ soft X-ray absorption spectroscopy (XAS) was employed to study the adsorption and dissociation of carbon monoxide molecules on cobalt nanoparticles with sizes ranging from 4 to 15 nm. The majority of CO molecules adsorb molecularly on the surface of the nanoparticles, but some undergo dissociative adsorption, leading to oxide species on the surface of the nanoparticles. We found that the tendency of CO to undergo dissociation depends critically on the size of the Co nanoparticles. Indeed, CO molecules dissociate much more efficiently on the larger nanoparticles (15 nm) than on the smaller particles (4 nm). We further observed a strong increase in the dissociation rate of adsorbed CO upon exposure to hydrogen, clearly demonstrating that the CO dissociation on cobalt nanoparticles is assisted by hydrogen. Our results suggest that the ability of cobalt nanoparticles to dissociate hydrogen is the main parameter determining the reactivity of cobalt nanoparticles in Fischer-Tropsch synthesis.

III-V nanocrystals displaying high crystallinity and low size dispersity are difficult to access by direct synthesis from molecular precursors. Here, we demonstrate that cation exchange of cadmium pnictide nanocrystals with group 13 ions yields monodisperse, crystalline III-V nanocrystals, including GaAs, InAs, GaP, and InP. This report highlights the versatility of cation exchange for accessing nanocrystals with covalent lattices.

As a cation-deficient, p-type semiconductor, copper sulfide (Cu 2−x S) shows promise for applications such as photovoltaics, memristors, and plasmonics. However, these applications demand precise tuning of the crystal phase as well as the stoichiometry of Cu 2−x S, an ongoing challenge in the synthesis of Cu 2−x S materials for a specific application. Here, a detailed transformation diagram of cation-exchange (CE) chemistry from cadmium sulfide (CdS) into Cu 2−x S nanowires (NWs) is reported. By varying the reaction time and the reactants' concentration ratio, the progression of the CE process was captured, and tunable crystal phases of the Cu 2−x S were achieved. It is proposed that the evolution of Cu 2−x S phases in a NW system is dependent on both kinetic and thermodynamic factors. The reported data demonstrate that CE can be used to precisely control the structure, composition, and crystal phases of NWs, and such control may be generalized to other material systems for a variety of practical applications.

Ion exchange of nanocrystals has the potential to emerge as an alternative to conventional routes for synthesis of ionic nanocrystals. [1][2][3][4][5][6][7][8][9] The facile ability to replace all cations of a nanocrystal with another cation, while preserving size and shape, allows us to employ nanocrystals as templates for the fabrication of other nanocrystals of interest.[6] Such a templated synthesis strategy is especially useful when the chemistry or crystallographic phase of the target nanocrystals is difficult to access via hot-injection methods. For instance, we recently showed [10] that Cu I sulfide quantum dots prepared by hot injection mostly result in the highly Cu-deficient djurleite phase. [11,12] The stoichiometric chalcocite phase is achievable, however, by room-temperature cation exchange of template CdS quantum dots with Cu + ions. Cation exchange holds particular promise for the fabrication of multicomponent heterostructured nanocrystals, [6,9] which allow independent tunability of electron and hole wavefunctions, but present potential synthetic challenges due to their greater structural complexity. Here, it is advantageous that the anionic framework of the heterostructure is maintained during cation exchange, allowing structural preservation of interfaces and junctions that define the electronic band alignment within the heterostructure. This has made possible the design and templated fabrication of novel semiconductor heterostructures that can range [13,14] from type-I, with high quantum yield emission useful for imaging and light-emitting diodes, [15,16] to type-II, which allow charge separation for photovoltaic and photocatalytic applications. [9,17] However, the ion exchange technique has been found to present a severe drawback: it results in nanocrystals with poor optoelectronic properties.[1] This is clear from a quantitative comparison of the optical properties of nanocrystals obtained from cation exchange with those prepared by standard hot injection for the model CdSe/CdS dot/rod heterostructure. [13][14][15][16] In this work, we trace the cause of the poor optical properties of cation-exchange-obtained nanocrystals to chemical impurities on the few atom per nanocrystal level. We have also found a method to purify the nanocrystals of these detrimental impurities post exchange and achieve optical properties comparable to those of hot-injection synthesized nanocrystals.Hot-injection synthesis of CdSe/CdS dot/rods with a 3.9 nm dot yields highly photoluminescent nanorods with a quantum yield (QY) of over 55 %, enabled by the type-I band alignment. On the other hand, CdSe/CdS dot/rods obtained from room-temperature exchange of Cu 2 Se/Cu 2 S dot/rods with Cd 2+ (see the Supporting Information) show relatively negligible emission, that is, a quantum yield (QY) of 0.07 %, almost three orders of magnitude smaller. This is despite the fact that the nanorods prepared by the two methods possess similar heterostructure morphologies, especially seed sizes, and consequently identical excitonic stru...

For the practical application of nanocatalysts, it is desirable to understand the spatiotemporal fluctuations of nanocatalytic activity at the single-nanoparticle level. Here we use time-lapsed superresolution mapping of single-molecule catalysis events on individual nanoparticles to observe time-varying changes in the spatial distribution of catalysis events on Sb-doped TiO 2 nanorods and Au triangle nanoplates. Compared with the active sites on well-defined surface facets, the defects of the nanoparticle catalysts possess higher intrinsic reactivity but lower stability. Corners and ends are more reactive but also less stable than flat surfaces. Averaged over time, the most stable sites dominate the total apparent activity of single nanocatalysts. However, the active sites with higher intrinsic activity but lower stability show activity at earlier time points before deactivating. Unexpectedly, some active sites are found to recover their activity ("selfhealing") after deactivation, which is probably due to desorption of the adsorbate. Our superresolution measurement of different types of active catalytic sites, over both space and time, leads to a more comprehensive understanding of reactivity patterns and may enable the design of new and more productive heterogeneous catalysts.single-molecule nanocatalysis | optical superresolution imaging | photocatalysis | surface restructuring A ll kinds of nanoparticles, such as metals, metal oxides, and even nonmetals, are used as catalysts in a variety of chemical processes (1). In support of developing better and less-expensive nanoparticle catalysts, single-molecule mapping of nanocatalyst activity using superresolution imaging at the single-nanoparticle level can determine active site locations on nanoparticles and which structures are most reactive (2-5). For example, defects, corners, and edges on nanoparticle surfaces have been shown by singlemolecule, superresolution imaging of single nanocatalysts to be more active than other sites. The relative abundance of these sites on nanoparticle catalysts is important for explaining their observed reactivity (2, 4, 6-13).However, prior reports are based on longtime integrations of catalytic activity and thus do not provide information about any dynamic changes in catalytic activity, such as the time-dependent evolution of different active sites. Studies of time-varying catalytic activity are thus arguably more informative than time-averaged observations.The temporal fluctuation of catalytic activity on nanocatalysts or electrodes has been previously observed in situ at both the ensemble and single-nanoparticle level (14-17). More recently, superresolution imaging techniques have been applied to identify the spatial distribution of catalytic activity on a few different nanocatalysts including Au@SiO 2 nanorods, Au@SiO 2 nanotriangles (2, 4) and Au-CdS nanohybrids (6). The observed spatial variations in catalytic activity between different catalysts were merely attributed to possible reaction-driven surface reconstruction of...

We used a fluorogenic reaction to study in conjunction the photocatalytic properties for both active sites (trapped photogenerated electrons and holes) on individual Sb-doped TiO(2) nanorods with single-molecule fluorescence microscopy. It was found that active sites around trapped holes show higher activity, stronger binding ability, and a different dissociation mechanism for the same substrate and product molecules in comparison with the active sites around trapped electrons. These differences could be elucidated by a model involving the charged microenvironments around the active sites.