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Nanoparticles that contain multiple materials connected through interfaces, often called heterostructured nanoparticles, are important constructs for many current and emerging applications. Such particles combine semiconductors, metals, insulators, catalysts, magnets, and other functional components that interact synergistically to enable applications in areas that include energy, nanomedicine, nanophotonics, photocatalysis, and active matter. To synthesize heterostructured nanoparticles, it is important to control all of the property-defining features of individual nanoparticles-size, shape, uniformity, crystal structure, composition, surface chemistry, and dispersibility-in addition to interfaces, asymmetry, and spatial organization, which facilitate communication among the constituent materials and enable their synergistic functions. While it is challenging to control all of these nanoscale features simultaneously, nanoparticle cation exchange reactions offer powerful capabilities that overcome many of the synthetic bottlenecks. In these reactions, which are often carried out on metal chalcogenide materials such as roxbyite copper sulfide (Cu

Heterostructured nanorod libraries The synthesis of nanostructures with well-defined interfaces between different materials can enable applications in areas such as catalysis and solar energy harvesting. Nanomaterials containing several different materials are usually synthesized through top-down approaches, such as surface growth or templating techniques, which usually produce small quantities of particles. Steimle et al. performed up to seven cation-exchange reactions (with ions such as Zn 2+ and Co 2+ ) on solution-synthesized copper sulfide nanorods. In principle, more than 65,000 different variations of materials and their interfaces can be made, depending on the order and extent of cation exchange. Science , this issue p. 418

Colloidal heterostructured nanoparticles that integrate multiple materials through direct solid–solid interfaces are desirable across a wide range of applications. However, chemical, structural, in...

Covalent organic framework (COF) membranes are of great promise for energy-efficient separations. Thick, polycrystalline COF films have been reported to separate dyes, salts, bacteria, and nanoparticles on the basis of size-selective transport through ordered pores. Here, we show that these materials function as adsorbents, not as size-sieving membranes. Binding isotherms of several dyes typical of the COF membrane literature to three COF powder samples illustrate that COFs are high-capacity adsorbents with affinities that span a range of 3 orders of magnitude, trends which map onto previously reported separation behavior. Computational results suggest that observed differences in adsorption can be correlated to variable entropic gains driving the adsorption process. Polycrystalline COF pellets show volume-dependent and flow-rate dependent "rejection" of dyes, consistent with an adsorption-based removal mechanism. Previous reports of thick, polycrystalline COF membranes used low flow rates and small dye volumes to probe rejection capabilities, where membrane and adsorbent behavior is not distinguishable. A mixed dye separation experiment in flow shows affinity-dependent performance. These results necessitate a careful reexamination of the COF membrane literature, as separations based on differential transport through 2D COF pores remain an important yet unrealized frontier.

The precise placement of different materials in specific regions of a nanocrystal is important for many applications, but this remains difficult to achieve synthetically. Here we show that regioselectivity during partial cation exchange reactions of metal chalcogenide nanocrystals emerges from crystallographic relationships between the precursor and product phases. By maximizing the formation of low-strain interfaces, it is possible to rationally integrate three distinct materials within uniform spherical and rod-shaped colloidal nanoparticles to produce complex asymmetric heterostructured isomers. Through sequential partial exchange of Cu+ in Cu1.8S nanocrystals with Zn2+ and Cd2+, five distinct ZnS/CdS/Cu1.8S nanosphere and nanorod isomers are accessible.

Interfaces between nanoscale solids can facilitate coupling between dissimilar materials, leading to emergent and synergistic properties as well as mix-and-match multifunctionality. Seeded-growth methods, whereby one material is grown directly off of the surface of another, can lead to the formation of hybrid nanoparticles containing such solid-state heterojunctions. Successfully applying seeded-growth methods to the synthesis of hybrid nanoparticles, however, requires a precise balance of competing reaction variables, which limits the scope of materials that can be routinely incorporated into them and, accordingly, the types of achievable interfaces. Here, we describe an alternate pathway that overcomes key limitations of seeded-growth methods by synthetically deconvoluting the formation of particle–particle interfaces and the incorporation of desired materials components. Readily accessible hybrid nanoparticles can be rationally modified using sequential anion and cation exchange reactions which transfo...

The ability to selectively form one crystal structure among several options in a polymorphic system is an important goal in solid-state synthesis. Nanocrystal cation exchange, which proceeds rapidly under mild conditions, can retain key structural features and yield otherwise inaccessible phases, but the extent to which crystal structure can be retained and therefore selectively targeted during such reactions has been limited. Here, we show that nanocrystals of digenite Cu2-x S transform to zincblende MnS and CoS upon cation exchange. Zincblende MnS and CoS, which are metastable in bulk, retain both the tetrahedral cation coordination and cubic close packed anion sublattice of digenite Cu2-x S. Comparison with wurtzite MnS and CoS, which have been accessed previously through analogous cation exchange of roxbyite Cu2-x S, demonstrates the selective formation of the related zincblende vs. wurtzite polymorphs by cation exchange of structurally distinct templates.

Colloidal hybrid nanoparticles integrate two or more nanocrystal domains into a single architecture that can have properties not found in, or enhanced relative to those of, the individual components. These hybrid nanomaterials are typically constructed using multistep seeded-growth reaction sequences, which are conceptually analogous to the total synthesis approaches used in molecular synthesis. Here, we discuss in detail the synthetic protocols that lead to the formation of three-component Ag–Pt–Fe3O4 and Au–Pt–Fe3O4 heterotrimers. These instructive model systems highlight the important synthetic details that underpin successful hybrid nanoparticle reactions. We provide detailed, step-by-step protocols for generating these materials, focusing on describing and rationalizing the key reaction parameters that need to be rigorously controlled to minimize unwanted nanoparticle byproducts. The importance of comprehensive analysis using a suite of materials characterization tools is highlighted, as such efforts...

For polymorphic solid-state systems containing multiple distinct crystal structures of the same composition, identifying rational pathways to selectively target one particular structure is an important synthetic capability. Cation exchange reactions can transform a growing library of metal chalcogenide nanocrystals into different phases by replacing the cation sublattice, often while retaining morphology and crystal structure. However, only a few examples have been demonstrated where multiple distinct phases in a polymorphic system could be selectively accessed using nanocrystal cation exchange reactions. Here, we show that roxbyite (hexagonal) and digenite (cubic) Cu

Retrosynthesizing complex nanostructures The solution synthesis of complex and asymmetric nanostructures is still challenging. For many applications, it will be important to gain simultaneous control over particle size and morphology, constituent materials, and internal interfaces. Fenton et al. have developed a strategy that mimics chemical retrosynthesis, starting with simple nanoparticle synthons—in this case, Cu 1.8 S nanoparticles, nanorods, and nanosheets. Various types of interfaces and junctions can be introduced, for example, by cation substitution. This intervention breaks the symmetry of the synthons and assembles them into higher-order structures. The nanostructures can thus be formed with asymmetric, patchy, porous, or sculpted regions. Science , this issue p. 513

We report the synthesis and structural characterisation of two titanium(IV) complexes. A new titanium(IV) complex bearing four picolinato ligands, denoted as Ti(pic)4, has been prepared under solvothermal conditions. This complex identified by single-crystal diffraction is a new example of an octacoordinated environment for titanium(IV) and is close to an ideal triangular dodecahedron. A second synthesis of a hexaprismatric carboxylato titanium complex formulated as [Ti6(μ3-O)6(OiPr)6(μ-O2CC5H3NCOOiPr)6] starting from a dissymmetric anhydride pyridine carboxylic precursor is described. The solid-state analysis shows the presence of a single compound among 13 possible species. Calculations gave evidence of a highly selective mechanism leading to the formation of the final product.

Tetrapod-shaped ZnO nanocrystals exhibit exceptional optoelectronic properties, including intense ultraviolet photoluminescence emission, that make them attractive for applications that include lasers, sensors, and photocatalysts. However, synthetic methods that produce ZnO tetrapods typically include high-temperature vapor-deposition approaches that do not readily achieve characteristic dimensions of less than 100 nm or colloidal methods that require added metal dopants, which modify the inherent properties of ZnO. Here, we report a robust, modified solution-phase synthetic protocol for generating colloidal ZnO tetrapods that does not require the use of metal dopants. The ZnO tetrapod arm lengths can be tuned from 10 to 25 nm by adjusting the amount of Zn reagent used in the reaction. Subsequent seeded-growth produced even larger colloidal ZnO tetrapods with 62 nm arms. Photoluminescence (PL) measurements confirm that the tetrapods are of high crystalline quality, and the ultraviolet PL emission wavelengths that are observed fall between those of previously reported metal-doped colloidal ZnO tetrapods, which exhibit dopant-induced red- or blue-shifts. Furthermore, the reaction strategy can be modified to produce cobalt-substituted ZnO, offering a chemical pathway to tetrapod-shaped Zn1−xCoxO nanocrystals.

Ion exchange reactions of colloidal nanocrystals provide access to complex products that are synthetically challenging using traditional hot-injection methods. However, such reactions typically achieve only partial material transformations by employing either cation or anion exchange processes. It is now shown that anion and cation exchange reactions can be coupled together and applied sequentially in one integrated pathway that leads to complete material transformations of nanocrystal templates. Although the product nanocrystals do not contain any of the original constituent elements, the original morphology is retained, thereby fully decoupling morphology and composition control. The sequential anion/cation exchange process was applied to pseudo-spherical CdO nanocrystals and ZnO tetrapods, producing fully transformed and shape-controlled nanocrystals of copper and silver sulfides and selenides. Furthermore, hollow core-shell tetrapod ZnS@CdS heterostructures were readily accessible.