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Nanoscale heterostructures that interface with multiple distinct materials provide opportunities to engineer functional complexity into single-particle constructs. However, existing synthetic pathways to such hybrid nanoparticles emphasize surface-seeded growth, which limits the scope of accessible systems. Here, we introduce an alternative approach that transforms isotropic nanocrystals into asymmetric, multicomponent Janus particles through sequential deposition, reactive phase segregation, and cation exchange processes that are mediated by an unusual class of reactive synthons. After Ag-Au seed particles had formed and had reacted with sulfur, a series of segregated Au1-xAgx-AgAuS and Au1-xAgx-Ag3AuS2 hybrid nanoparticles form. The AgAuS and Ag3AuS2 domains provide a synthetic entryway into solution-mediated cation exchange reactions, with the compositions of the Ag-Au-S synthons defining the components, morphologies, and interfaces of the hybrid nanoparticle products. Upon cation exchange with Pb2+, Au1-xAgx-AgAuS forms Ag1-xAux-PbS heterodimers while Au1-xAgx-Ag3AuS2 forms Ag1-xAux-Ag2S-PbS heterotrimers. The process by which isotropic metal nanoparticles transform into asymmetric hybrid nanoparticles through reactive Ag-Au-S synthons provides important insights that will be applicable to the retrosynthetic design of complex nanoscale heterostructures having expanded multifunctionality and synergistic properties. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Hybrid nanoparticles composed ofmultiple material systems providea platform for studying the coupling between nanoparticles with distinctproperties. Here, we describe a nontraditional synthetic pathway toAu-In2O3hybrid nanoparticles that contain twodistinct plasmonic domains: Au, with a localized surface plasmon resonance(LSPR) in the visible, and In2O3, with a LSPRin the mid-infrared. The hybrid nanocrystals are produced by slowlyintroduce In(III) stock solution to Au nanoparticle seeds using asyringe pump. Rather than forming through a traditional heterogeneousseeded-growth process, a series of in situand ex situstudies reveal an alternate multistep pathway. TheAu nanoparticles first combine with In to form an alloy of Au andIn, which is colloidally stable up to 300 °C in 1-octadecene.The Au-In alloy nanoparticles then transform into intermetallic AuIn2nanoparticles that are surrounded by a shell of amorphousindium oxide (AuIn2@InOx),followed by the final Au-In2O3heterodimersupon complete phase segregation. [ABSTRACT FROM AUTHOR]

Abstract: A solution precursor route has been used to synthesize a series of nanocrystalline rare-earth borates. Amorphous precursor powders are precipitated during an aqueous reaction between RE 3+ and NaBH4, and the isolated powders can be annealed in air at 700°C to form YBO3, NdBO3, SmBO3, EuBO3, GdBO3, and HoBO3. YBO3:Eu formed using this strategy shows red-orange emission properties that are similar to high-quality nanocrystals prepared by other methods. The materials have been characterized by FTIR spectroscopy, powder XRD, SEM, DSC, UV–Vis fluorimetry, and TEM with EDS and element mapping. [Copyright &y& Elsevier]

When reacted with trioctylphosphine at ~360 °C, rhodium nanocrystals convert to rhodium phosphide (Rh2P) nanocrystals. Careful control over synthetic variables, such as temperature, stabilizing ligands, and cosolvents, can result in Rh2P nanocrystals with shapes that reflect the Rh nanocrystal templates. Accordingly, Rh nanocrystals with multipod, cube- and triangle-derived shapes convert to Rh2P nanocrystals that maintain the shape of their Rh precursors. Both dense and hollow Rh2P nanocrystals can be generated using a single unified chemical conversion strategy. These empirical guidelines for generating a morphologically diverse library of Rh2P nanocrystals provide important insights into shape conservation using nanocrystal templates and will likely be portable to other multielement systems for which rigorous shape-controlled synthesis remains challenging. [ABSTRACT FROM AUTHOR]

The ability to control the shape of metal nanocrystals is critical to applications such as catalysis, magnetism, and plasmonics. Despite significant advances in controlling the shapes of single-metal nanocrystals, rigorous shape control of multimetal nanocrystals remains challenging, and has been limited largely to alloy systems of similar metals. Here we describe a robust strategy that produces shape-controlled intermetallic nanocrystals involving elements of notably different reduction potentials, reduction kinetics, and reactivity. The approach utilizes shape- and size-controlled β-Sn nanocrystals as reactive templates that can be converted into binary M-Sn (M = Fe, Co, Ni, Pd) intermetallic compounds by reaction with appropriate metal salt solutions under reducing conditions. The result, demonstrated in detail for the FeSn2 system, is a variety of nanostructures with morphologies that include spheres, cubes, hollow squares, U-shaped structures, nanorods, and nanorod dimers. Our experiments demonstrate a size- and shape-dependent reactivity toward the formation of hollow FeSn2 nanostructures and provide empirical guidelines for the formation of other intermetallic nanocrystals. In addition to those of FeSn2, nanocrystals of intermetallic PdSn, CoSn3, and NiSn3 can be formed using this same chemical conversion strategy. [ABSTRACT FROM AUTHOR]

Understanding how solids form is a challenging task, and few strategies allow for the elucidation of reaction pathways that are useful for designing new solids. Here, we describe an unusual multistep reaction pathway that leads to the formation of AuCuSn2, a new ternary intermetallic compound that was discovered as nanocrystals using a low-temperature solution route. The formation of AuCuSn2 using a modified polyol process occurs through a multistep pathway that was elucidated by taking aliquots throughout the course of the reaction and studying the products using a variety of techniques. The reaction proceeds through four distinct steps: (a) formation of Au nanoparticles at or near room temperature, mediated by a galvanic reaction between Au3+ and Sn2+ (forming Au0 and Sn4+, precipitated as SnO2 that forms a shell around the nanoparticles), (b) formation of NiAs-type AuSn nanoparticles, along with Cu and Sn, upon addition of NaBH4, (c) aggregation and thermal interdiffusion to form AuCuxSny alloy nanoparticles, and (d) nucleation of intermetallic AuCuSn2, which has an ordered NiAs-derived structure. The proposed mechanism was tested by starting the reaction with the AuSn intermediate. AuSn nanoparticles were synthesized separately and reacted with Cu and Sn nanoparticles, and ordered AuCuSn2 formed as expected. Elucidation of this reaction pathway has important implications for guiding the design of new intermetallic solids, as well as for controlling the synthesis of complex multimetal nanocrystals. [ABSTRACT FROM AUTHOR]

A new multistep approach was developed to synthesize atomically ordered intermetallic nanocrystals, using AuCu and AuCu3 as model systems. Bimetallic nanoparticle aggregates are used as precursors to atomically ordered nanocrystals, both to precisely define the stoichiometry of the final product and to ensure that atomic-scale diffusion distances lower the reaction temperatures to prevent sintering. In a typical synthesis, PVP-stabilized Au-Cu nanoparticle aggregates synthesized by borohydride reduction are collected by centrifugation and annealed in powder form. At temperatures below 175 °C, diffusion of Cu into Au occurs, and the atomically disordered solid solution CuxAu1-x exists. For AuCu3, nucleation occurs by 200 °C, and atomically ordered AuCu exists between 200 and 400 °C. For AuCu3, an AuCu intermediate nucleates at 200 °C, and further diffusion of Cu into the AuCu intermediate at 300 °C nucleates AuCu3. Atomically ordered AuCu and AuCu3 nanocrystals can be redispersed as discrete colloids in solution after annealing between 200 and 300 °C. [ABSTRACT FROM AUTHOR]

Palladium is converted into palladium hydride, β-PdHx, in polyol solution using NaBH4 as a hydrogen source; nanocrystalline Pd reacts to form PdHx faster and at lower temperatures than bulk Pd, and can be made to release hydrogen to regenerate Pd. [ABSTRACT FROM AUTHOR]

Palladium is converted into palladium hydride, β-PdHx, in polyol solution using NaBH4as a hydrogen source; nanocrystalline Pd reacts to form PdHxfaster and at lower temperatures than bulk Pd, and can be made to release hydrogen to regenerate Pd. [ABSTRACT FROM AUTHOR]

The article reports on the ZnO-templated Synthesis of Wurtzite-Type ZnS and ZnSe nanoparticles. It states that significant attention was given to wide bandgap semiconductor nanocrystals because of their size-dependent properties and diverse applications. The article shows that ZnO nanoparticles, which adopt the WZ structure, can serve as structural templates for the controllable formation of WZ-type ZnS and ZnSe.

The article offers information about a study conducted to explore the implications of the discovery that nanocrystalline intermetallics undergo facile solution-mediated reaction to form derivative phases. The discovery provides a robust toolkit for systematically modifying the compositions and structures of multi-metal nanocrystals. Along with the discovery is the uncovering of the conversion strategy which produces important compounds that are difficult to make by other methods.

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 nano-crystals 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. [ABSTRACT FROM AUTHOR]

The ability to selectively synthesize one particular polymorph in a solid-state system having multiple crystal structures with the same composition is important for accessing desired properties. Solutionmediated reactions, including anion and cation exchange, that chemically transform colloidal nanoparticles with preprogrammed structural features into targeted products have emerged as a powerful platform for predictably accessing metastable polymorphs. While nanocrystal ionexchange reactions that retain anion sublattice features are well known, analogous reactions that preserve cation sublattice features are much less common, and guidelines for predictably targeting such sublattice features are not well established. Here, we report that both anion and cation sublattice features--hexagonal close-packed anions and tetrahedrally coordinated cations-can be preserved during cation exchange of roxbyite-type Cu2-xS nanocrystals to selectively produce wurtzite-type CoS and MnS. These polymorphs, which are metastable in bulk systems, form relative to other accessible structures having cubic close-packed anions and/or octahedrally coordinated cations. To facilitate these transformations, the scope of existing nanocrystal cation-exchange reactions was expanded to include 3d transition metal systems that previously have not been investigated in depth. [ABSTRACT FROM AUTHOR]

Tin sulfide, SnS, is a narrow band gap semiconductor comprised of inexpensive, earth abundant, and environmentally benign elements that is emerging as an important material for a diverse range of applications in solar energy conversion, energy storage, and electronics. Relative to many comparable systems, much less is known about the factors that influence the synthesis or morphology-dependent properties of SnS nanostructures. Here, we report the synthesis of colloidal SnS cubes, spherical polyhedra, and sheets and demonstrate their activity for the photocatalytic degradation of methylene blue. We also study their morphology-dependent polymorphism using an in-depth crystallographic analysis that correlates high-resolution TEM data of individual nanocrystals with ensemble-based electron diffraction and powder XRD data. These studies reveal that the crystal structure adopted by the SnS cubes and spherical polyhedra is expanded along the a and b axes and contracted along c, converging on a pseudotetragonal cell that is distinct from that of orthorhombic a-SnS, the most stable polymorph. All of the peaks observed in powder XRD patterns that are often interpreted as originating from a mixture of metastable zincblende-type SnS and a-SnS can instead be accounted for by this single-phase pseudotetragonal modification, and this helps to rationalize discrepancies that exist between theoretical predictions of SnS polymorph stability and interpretations of experimental diffraction data. This same crystallographic analysis also indicates the morphologies of the nanocrystals and the facets by which they are bound, and it reveals that the SnS cubes form through selective overgrowth of spherical polyhedral seeds. [ABSTRACT FROM AUTHOR]

Abstract: Au–Zn was used as a prototype binary system for exploring intermetallic phase accessibility in nanoparticle systems using low-temperature solution chemistry methods. By reacting Au nanoparticles with diethylzinc (Et2Zn) in oleylamine at temperatures of 250–300°C, nanoparticles of five distinct binary intermetallic compounds were accessible: Au3Zn, Au5Zn3, AuZn, Cu5Zn8-type γ-(Au,Zn), and Mg-type ɛ-(Au,Zn). A variety of nanoparticle shapes, including spheres, triangles, hexagons, and rods, are accessible in the Au–Zn system via a pseudomorphic reaction of Au nanocrystal shapes with Et2Zn. The Au–Zn nanoparticles have optical properties that vary with Zn content, ranging from a surface plasmon resonance peak at ∼495nm for Au3Zn to absorption in the ultraviolet region for the Zn-rich phases. [Copyright &y& Elsevier]

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 are useful for diagnosing subtle chemical and morphological features that can lead to synthetic bottlenecks throughout the course of the reaction sequences. Finally, we offer strategies for circumventing these commonly encountered problems as well as insights that can lead to increased hybrid nanoparticle yields and improved sample-to-sample reproducibility. Although this work specifically details the synthesis of Ag-Pt-Fe3O4 and Au-Pt-Fe3O4 heterotrimers, these synthetic strategies and protocol guidelines are generally applicable to many other hybrid nanoparticle systems. [ABSTRACT FROM AUTHOR]

The article reports on the low-temperature solution synthesis of the non-equilibrium ordered intermetallic compounds with iron, golf, copper and nickel as nanocrystals. The intermetallic compounds have been observed via interfacial diffusion, coevaporation and ion beam manipulation. The synthesis was modified and the product isolated by adding ethanol and then extracting into the toluene layer of a toluene/water mixture.

The article cites a study which states that low-temperature solution routes originally developed for the synthesis of nanocrystals and nanocrystalline powders can be exploited to access new ordered intermetallics with ternary compositions. While ternary phase diagrams are difficult to map out, the gold-copper-tin system has been studied in detail. Intermetallics that form at the solder/metal interface are responsible for the brittleness and fracture of solder joints, and the gold-copper-tin system is among the most important for understanding interfacial phase formation in microelectronic devices.

Germanium telluride (GeTe) nanostructuresare a demonstrated platformfor studying the effects of scaling on reversible, amorphous-to-crystallinephase transitions that are important for data storage and computingapplications, and for understanding ferroelectric behavior at thenanometer scale. Despite the interest in GeTe, and the apparent advantagesof solution-phase processing, there is a dearth of information relatedto the synthesis of high-quality, morphology-controlled, colloidalGeTe. This paper describes the preparation of colloidal GeTe nanostructuresin the presence of surface-stabilizing polymers, which mediate particle–particleinteractions and prevent aggregation of GeTe crystallites more effectivelythan conventional molecular stabilizers. As a result, several novelGeTe nanostructures are formed, including faceted octahedral nanoparticles,amorphous GexTe1–xalloy nanospheres and single-crystal two-dimensional (2D)GeTe nanosheets. The colloidal stability conferred by the polymermay provide the key experimental degree of freedom necessary to achievehigher-order morphology control for GeTe and related materials. [ABSTRACT FROM AUTHOR]

Abstract: During the in-situ order–disorder transition of intermetallic L12-type Au3Fe1−x nanocrystals, structural information has been retrieved from their electron diffraction patterns based on the Pawley refinement that is unrelated to the electron kinematical or dynamical scattering nature as well as the Rietveld refinement using a kinematical approximation. At room temperature, it was found that the nanocrystals contain approximately x=40% vacancies at the Fe site. Based on in-situ heating this phase displayed an irreversible order–disorder transition, with the transition temperature between 553 and 593K. A sudden increase in lattice parameter was detected during the first heating from the ordered phase, while the second heating of the disordered phase showed only a linear relationship with temperature. From the lattice parameter measurement of the disordered phase, the coefficient of thermal expansion was estimated as 1.462×10−5 K−1. The long-range order parameter S was determined by the refined site occupancies, as well as the integrated intensities of the superlattice (100) and fundamental (220) reflections using the Pawley and Rietveld refinements during the order–disorder transition. Considering the dynamical scattering effect, Blackman two-beam approximation theory was applied to corrected S, which slightly attenuated after the correction. A comparison of the electron diffraction with X-ray diffraction data was made. It was demonstrated that elemental and structural information could be retrieved through quantitative electron diffraction studies of the nanomaterials. Since the Pawley refinement algorithm does not include the electron scattering event, it is especially useful to refine the electron diffraction data regardless of the sample thickness. [Copyright &y& Elsevier]

Abstract: The concept of nanocrystal conversion chemistry, which involves the use of pre-formed nanoparticles as templates for chemical transformation into derivative solids, has emerged as a powerful approach for designing the synthesis of complex nanocrystalline solids. The general strategy exploits established synthetic capabilities in simple nanocrystal systems and uses these nanocrystals as templates that help to define the composition, crystal structure, and morphology of product nanocrystals. This article highlights key examples of “conversion chemistry” approaches to the synthesis of nanocrystalline solids using a variety of techniques, including galvanic replacement, diffusion, oxidation, and ion exchange. The discussion is organized according to classes of solids, highlighting the diverse target systems that are accessible using similar chemical concepts: metals, oxides, chalcogenides, phosphides, alloys, intermetallic compounds, sulfides, and nitrides. [Copyright &y& Elsevier]