Synthetic Roadmap to a Large Library of Colloidal High-Entropy Rare Earth Oxyhalide Nanoparticles Containing up to Thirteen Metals.

Nanoparticles of high-entropy materials that incorporate five or more elements randomized on a crystalline lattice often exhibit synergistic properties that can be influenced by both the identity and number of elements combined. These considerations are especially important for structurally and compositionally complex materials such as multimetal multianion compounds, where cation and anion mixing can influence properties in competitive and contradictory ways. Here, we demonstrate the synthesis of a large library of colloidal high-entropy rare earth oxyhalide (REOX) nanoparticles. We begin with the synthesis of (LaCePrNdSmEuGdDyHoErYbScY)OCl, which homogeneously incorporates 13 distinct rare earth elements. Through time point studies, we find that (LaNdSmGdDy)OCl, a 5-metal analogue, forms through in situ generation of compositionally segregated core@shell@shell intermediates that convert to homogeneously mixed products through apparent core-shell interdiffusion. Assuming that all possible combinations of 5 through 13 rare earth metals are synthetically accessible, we propose the existence of a 7099-member REOCl nanoparticle library, of which we synthesize and characterize 40 distinct members. We experimentally validate the incorporation of a large number of rare earth elements using energy dispersive X-ray spectra, despite closely spaced and overlapping X-ray energy lines, using several fingerprint matching strategies to uniquely correlate experimental and simulated spectra. We confirm homogeneous mixing by analyzing elemental distributions in high-entropy nanoparticles versus physical mixtures of their constituent compounds. Finally, we characterize the band gaps of the 5- and 13-metal REOCl nanoparticles and find a significantly narrowed band gap, relative to the constituent REOCl phases, in (LaCePrNdSmEuGdDyHoErYbScY)OCl but not in (LaNdSmGdDy)OCl.

Disentangling Competitive and Synergistic Chemical Reactivities During the Seeded Growth of High-Entropy Alloys on High-Entropy Metal Sulfide Nanoparticles.

The seeded growth of one type of nanoparticle on the surface of another is foundational to synthesizing many multifunctional nanostructures. High-entropy nanoparticles that randomly incorporate five or more elements offer enhanced properties due to synergistic interactions. Incorporating high-entropy nanoparticles into seeded growth platforms is essential for merging their unique properties with the functional enhancements that arise from particle-particle interactions. However, the complex compositions of high-entropy materials complicate the seeded growth process due to competing particle growth and chemical reactivity pathways. Here, we design and synthesize a 36-member nanoparticle library to identify and disentangle these competitive interactions, ultimately defining chemical characteristics that underpin the seeded growth of high-entropy alloys on high-entropy metal sulfide nanoparticles. As a model system, we focus on (Cu,Zn,Co,In,Ga)S-SnPdPtRhIr, which combines a high-entropy metal sulfide semiconductor with a high-entropy alloy catalyst. We study the seeded growth of all possible pairwise combinations of Sn, Pd, Pt, Rh, Ir, and SnPdPtRhIr on the metal sulfides Cu1.8S, ZnS, Co9S8, CuInS2, CuGaS2, and (Cu,Zn,Co,In,Ga)S, which have comparable morphologies and sizes. Through these studies, we uncover unexpected chemical reactivities, including cation exchange, redox reactions, and diffusion. Reaction temperature, threshold reduction potentials, metal/sulfide chemical reactivity, and the relative strengths of the various bonds that could be formed during particle growth emerge as the primary factors that underpin seeded growth. Finally, we disentangle these competitive and synergistic chemical reactivities to generate a reactivity map that provides practical guidelines for achieving seeded growth in compositionally complex systems.

Soft Chemistry of Hard Materials: Low-Temperature Pathways to Bulk and Nanostructured Layered Metal Borides.

Conspectus"Synthesis by design" is often considered to be the primary goal of chemists who make molecules and materials. Synthetic chemists usually have in mind a target they want to make, and they want to be able to design a pathway that can get them to that target as quickly and efficiently as possible. Chemists who synthesize refractory solids, which have melting points above 1000 °C and are often chemically inert at these high temperatures, have access to only a small number of synthetic strategies due to the need to overcome solid-state diffusion, which is the rate-limiting step in such reactions. The use of extremely high temperatures to facilitate diffusion among two or more refractory solids, which precedes any chemical reaction that must occur, generally drives the system to form only the product that is the most thermodynamically stable-the global minimum on an energy landscape-for a certain combination of elements. When trying to target a different product in the same system, one generally cannot rely on thermally driven reactions. Lower-temperature reactions that side step this diffusion limitation can succeed where high temperatures fail by providing access to local minima on an energy landscape. These local minima represent metastable phases that are primed for synthesis, but only if an appropriate pathway and set of reactions can be identified. It is therefore important to develop and understand low-temperature, or "soft", chemical reactions in "hard" refractory systems. These reactions allow us to apply the retrosynthetic framework that molecular chemists rely on to systems where chemists have not previously had such control over reactions, reactivities, and metastable product formation.In this Account, we discuss the development of soft chemical reactions of hard materials in the context of a class of layered, refractory metal borides that are precursors to an emerging family of two-dimensional nanomaterials. Layered ternary metal boride phases such as MoAlB have layers of metal borides, which are chemically unreactive, interleaved with layers of aluminum, which are reactive. Some of the interlayer aluminum can be deintercalated at room temperature in dilute aqueous sodium hydroxide, transforming stable MoAlB into destabilized MoAl1-xB. Mild thermal treatment of submicrometer grains of this destabilized MoAl1-xB sample allows it to traverse the energy landscape and crystallize as Mo2AlB2, a metastable compound. Further thermal treatment transforms Mo2AlB2 into a Mo2AlB2-alumina nanolaminate and ultimately mesoporous MoB, all through continued traversing of the energy landscape using mild chemical and thermal treatments. Similar topochemical manipulations, which maintain structure but change composition, are emerging for other MAB phases and are opening the door to new types of metastable compounds and nanostructured materials in traditionally refractory systems.

Topochemical Anionic Subunit Insertion Reaction for Constructing Nanoparticles of Layered Oxychalcogenide Intergrowth Structures.

Intergrowth compounds contain alternating layers of chemically distinct subunits that yield composition-tunable synergistic properties. Synthesizing nanoparticles of intergrowth structures requires atomic-level intermixing of the subunits rather than segregation into stable constituent phases. Here we introduce an anionic subunit insertion reaction for nanoparticles that installs metal chalcogenide layers between metal oxide sheets. Anionic [CuS]- subunits from solution replace interlayer chloride anions from LaOCl to form LaOCuS topochemically with retention of crystal structure and morphology. Sodium acetylacetonate helps extract Cl- concomitant with the insertion of S2- and Cu+ and is generalized to other oxychalcogenides. This topochemical reaction produces nanoparticles of ordered mixed-anion intergrowth compounds and expands nanoparticle ion exchange chemistry to anionic subunits.

Thermally Induced Domain Migration and Interfacial Restructuring in Cation Exchanged ZnS-Cu1.8S Heterostructured Nanorods.

Partial cation exchange reactions can be used to rationally design and synthesize heterostructured nanoparticles that are useful targets for applications in photocatalysis, nanophotonics, thermoelectrics, and medicine. Such reactions introduce intraparticle frameworks that define the spatial arrangements of different materials within a heterostructured nanoparticle, as well as the orientations and locations of their interfaces. Here, we show that upon heating to temperatures relevant to their synthesis and applications, the ZnS regions and Cu1.8S/ZnS interfaces of heterostructured ZnS-Cu1.8S nanorods migrate and restructure. We first use partial cation exchange reactions to synthesize a library of seven distinct samples containing various patches, bands, and tips of ZnS embedded within Cu1.8S nanorods. Upon annealing in solution or in air, ex situ TEM analysis shows evidence that the ZnS domains migrate in different ways, depending upon their sizes and locations. Using differential scanning calorimetry, we correlate the threshold temperature for ZnS migration to the superionic transition temperature of Cu1.8S, which facilitates rapid diffusion throughout the nanorods. We then use in situ thermal TEM to study the evolution of individual ZnS-Cu1.8S nanorods upon heating. We find that ZnS domain migration occurs through a ripening process that minimizes small patches with higher-energy interfaces in favor of larger bands and tips having lower-energy interfaces, as well as through restructuring of higher-energy Cu1.8S/ZnS interfaces. Notably, Cu1.8S nanorods containing multiple patches of ZnS thermally transform into ZnS-Cu1.8S heterostructured nanorods having ZnS tips and/or central bands, which provides mechanistic insights into how these commonly observed products form during synthesis.

Band Gap Narrowing in a High-Entropy Spinel Oxide Semiconductor for Enhanced Oxygen Evolution Catalysis.

High-entropy oxides (HEOs), which contain five or more metal cations that are generally thought to be randomly mixed in a crystalline oxide lattice, can exhibit unique and enhanced properties, including improved catalytic performance, due to synergistic effects. Here, we show that band gap narrowing emerges in a high-entropy aluminate spinel oxide, (Fe0.2Co0.2Ni0.2Cu0.2Zn0.2)Al2O4 (A5Al2O4). The 0.9 eV band gap of A5Al2O4 is narrower than the band gaps of all parent spinel oxides. First-principles calculations for multicomponent AAl2O4 spinels indicate that the band gap narrowing arises from the broadening of the energy distribution of the 3d states due to variations in the electronegativities and crystal field splitting across the 3d transition-metal series. As a catalyst for the oxygen evolution reaction in an alkaline electrolyte, A5Al2O4 reaches a current density of 10 mA/cm2 at an overpotential of 400 mV, outperforming all of the single-metal end members at an applied potential of 1.7 V vs RHE. Catalyst deactivation occurs after 5 h at 10 mA/cm2 and is attributed, based on elemental analysis and grazing-incidence X-ray diffraction, to the formation of a passivating layer that blocks the high-entropy oxide surface. This result helps to validate that the HEO is the active catalyst. The observation of band gap narrowing in A5Al2O4 expands the scope of synergistic properties exhibited by high-entropy materials and offers insight into the question of how the electronic structure of multicomponent oxide materials can be engineered via a high-entropy approach to achieve enhanced catalytic properties.

Introducing Porosity into Refractory Molybdenum Boride through Controlled Decomposition of a Metastable Mo-Al-B Precursor.

The high temperatures typically required to synthesize refractory compounds preclude the formation of high-energy morphological features, including nanoscopic pores that are beneficial for applications, such as catalysis, that require higher surface areas. Here, we demonstrate a low-temperature multistep pathway to engineer mesoporosity into a catalytic refractory material. Mesoporous molybdenum boride, α-MoB, forms through the controlled thermal decomposition of nanolaminate-containing sheets of the metastable MAB (metal-aluminum-boron) phase Mo2AlB2 and amorphous alumina. Upon heating, the Mo2AlB2 layers of the Mo2AlB2-AlOx nanolaminate, which is derived from MoAlB, begin to bridge and decompose, forming inclusions of alumina in a framework of α-MoB. The alumina can be dissolved in aqueous sodium hydroxide in an autoclave, forming α-MoB with empty and accessible pores. Statistical analysis of the morphologies and dimensions of the pores reveals a correlation with grain size, which relates to the pathway by which the alumina inclusions form. The transformation of Mo2AlB2 to α-MoB is topotactic due to crystal structure relationships, resulting in a high density of stacking faults that can be modeled to account for the observed experimental diffraction data. Porosity was validated by comparing surface areas and demonstrating catalytic viability for the hydrogen evolution reaction.

Formation and Transformation of Cu2-xSe1-yTey Nanoparticles Synthesized by Tellurium Anion Exchange of Copper Selenide.

Ion exchange reactions of colloidal nanoparticles post-synthetically modify the composition while maintaining the morphology and crystal structure and therefore are important for tuning properties and producing otherwise inaccessible and/or metastable materials. Reactions involving anion exchange of metal chalcogenides are particularly interesting, as they involve the replacement of the sublattice that defines the structure while also requiring high temperatures that can be disruptive. Here, we show that the tellurium anion exchange of weissite Cu2-xSe nanoparticles using a trioctylphosphine-tellurium complex (TOP═Te) yields weissite Cu2-xSe1-yTey solid solutions, rather than complete exchange to weissite Cu2-xTe, with compositions that are tunable based on the amount of TOP═Te used. Upon storage at room temperature in either solvent or air, tellurium-rich Cu2-xSe1-yTey solid solution nanoparticles transform, over the span of several days, to a selenium-rich Cu2-xSe1-yTey composition. The tellurium that is expelled from the solid solution during this process migrates to the surface and forms a tellurium oxide shell, which correlates with the onset of particle agglomeration due to the change in surface chemistry. Collectively, this study demonstrates tunable composition during tellurium anion exchange of copper selenide nanoparticles along with unusual post-exchange reactivity that transforms the composition, surface chemistry, and colloidal dispersibility due to the apparent metastable nature of the solid solution product.

Unexpected Multi-Step Transformation of AgCuS to AgAuS During Nanoparticle Cation Exchange.

Cation exchange reactions can modify the compositions of colloidal nanoparticles, providing easy access to compounds or nanoparticles that may not be accessible directly. The most common nanoparticle cation exchange reactions replace monovalent cations with divalent cations or vice versa, but some monovalent-to-monovalent exchanges have been reported. Here, we dissect the reaction of as-synthesized AgCuS nanocrystals with Au+ to form AgAuS, initially hypothesizing that Au+ could be selective for Cu+ (rather than for Ag+) based on a known Au+-for-Cu+ exchange and the stability of the targeted AgAuS product. Unexpectedly, we found this system and the putative cation exchange reaction to be much more complex than anticipated. First, the starting AgCuS nanoparticles, which match literature reports, are more accurately described as a hybrid of Ag and a variant of AgCuS that is structurally related to mckinstryite Ag5Cu3S4. Second, the initial reaction of Ag-AgCuS with Au+ results in a galvanic replacement to transform the Ag component to a AuyAg1-y alloy. Third, continued reaction with Au+ initiates cation exchange with Cu+ in AuyAg1-y-AgCuS to form AuyAg1-y-Ag3CuxAu1-xS2 and then AuyAg1-y-AgAuS, which is the final product. Crystal structure relationships among mckinstryite-type AgCuS, Ag3CuxAu1-xS2, and AgAuS help to rationalize the transformation pathway. These insights into the reaction of AgCuS with Au+ reveal the potential complexity of seemingly simple nanoparticle reactions and highlight the importance of thorough compositional, structural, and morphological characterization before, during, and after such reactions.

Synthesis and Magnetic, Optical, and Electrocatalytic Properties of High-Entropy Mixed-Metal Tungsten and Molybdenum Oxides.

High-entropy oxides (HEOs) are of interest for their unique physical and chemical properties. Significant lattice distortions, strain, and tolerance for high-vacancy concentrations set HEOs apart from single-metal or mixed-metal oxides. Herein, we synthesized and characterized the structures and compositions, along with the optical, magnetic, and electrocatalytic properties, of two families of high-entropy mixed-metal tungsten and molybdenum oxides, AWO4 and B2Mo3O8, where A and B are 3d transition metals. The HEOs A6WO4 (A = Mn, Fe, Co, Ni, Cu, and Zn) and B25Mo3O8 (B = Mn, Fe, Co, Ni, and Zn), as well as all accessible single-metal AWO4 and B2Mo3O8 parent compounds, were synthesized using high-temperature solid-state methods. X-ray photoelectron spectroscopy analysis of the surfaces revealed that the HEOs largely had the metal oxidation states expected from the bulk chemical formulas, but in some cases they were different than in the parent compounds. A6WO4 exhibited antiferromagnetic (AFM) ordering with a Néel temperature of 30 K, which is less than the average of its AFM parent compounds, and had a narrow band gap of 0.24 eV, which is much lower than all of its parent compounds. B25Mo3O8 was paramagnetic, despite the existence of AFM and ferromagnetic ordering in several of its parent compounds and had no observable band gap, which is analogous to its parent compounds. Both A6WO4 and B25Mo3O8 exhibited superior catalytic activity relative to the parent compounds for the oxygen evolution reaction, the oxidation half reaction of overall water splitting, under alkaline conditions, based on the overpotential required to reach the benchmark surface area normalized current density. Consistent with literature predictions of OER durability for ternary tungsten and molybdenum oxides, A6WO4 and B25Mo3O8 also exhibited stable performance without significant dissolution during 10 h stability experiments at a constant current.

Colloidal Nanostructures of Transition-Metal Dichalcogenides.

ConspectusLayered transition-metal dichalcogenides (TMDs) are intriguing two-dimensional (2D) compounds where metal and chalcogen atoms are covalently bonded in each monolayer, and the monolayers are held together by weak van der Waals forces. Distinct from graphene, which is chemically inert, layered TMDs exhibit a wide range of electronic, optical, catalytic, and magnetic properties dependent upon their compositions, crystal structures, and thicknesses, which make them fundamentally and technologically important. TMD nanostructures are traditionally synthesized using gas-phase chemical deposition methods, which are typically limited to small-scale samples of substrate-bound planar materials. Colloidal synthesis has emerged as an alternative synthesis approach to enable the scalable synthesis of free-standing TMDs. The judicious selection of precursors, solvents, and capping ligands together with the optimization of synthesis parameters such as concentrations and temperatures leads to the fabrication of colloidal TMD nanostructures exhibiting tunable properties. In addition, understanding the formation and transformation of TMD nanostructures in solution contributes to the discovery of important structure-function relationships, which may be extendable to other anisotropic systems.In this Account, we summarize recent progress in the colloidal synthesis, characterization, and applications of TMD nanostructures with tunable compositions, structures, and thicknesses. On the basis of the preparation of Mo- and W-based disulfide, diselenide, and ditelluride nanostructures, we discuss examples of phase engineering where various metastable TMD compounds can be directly accessed at low temperatures in solution. We also analyze the chemistry involved in broadly tuning the composition across the MoSe2-WSe2, WS2-WSe2, and MoTe2-WTe2 solid solutions as well as atomic-level microscopic characterization and the resulting composition-tunable properties. We then highlight how the high densities of defects in the colloidally synthesized TMD nanostructures enable unique catalytic properties, including their ability to facilitate the selective hydrogenation of substituted nitroarenes using molecular hydrogen. Finally, using this library of colloidal TMD nanostructures as substrates, we discuss the pathways by which noble metals deposit onto them in solution. We highlight the importance of the relative strengths of the interfacial metal-chalcogen bonds in determining the sizes and morphologies of the deposited noble metal components. These synthesis capabilities for colloidal TMD nanostructures, which have been generalized to a library of composition-tunable phases, enable new systematic studies of structure-property relationships and chemical reactivity in this important class of 2D materials.

Simultaneous Multication Exchange Pathway to High-Entropy Metal Sulfide Nanoparticles.

High entropy materials, which contain a large number of randomly distributed elements, have unique catalytic, electrochemical, and mechanical properties. The high configurational entropy of the randomized elements drives the formation of high entropy materials; therefore, high temperatures and quenching are typically required to stabilize them. Because of this, colloidal nanoparticles of high entropy materials are difficult to synthesize and remain rare, despite their desirable high surface areas and solution dispersibilities. Here, we introduce simultaneous multication exchange as an alternative low-temperature pathway to colloidal nanoparticles of high entropy materials. Roxbyite Cu1.8S nanoparticles react with a substoichiometric mixture of Zn2+, Co2+, In3+, and Ga3+ to produce nanoparticles of the high entropy metal sulfide Zn0.25Co0.22Cu0.28In0.16Ga0.11S. The Zn0.25Co0.22Cu0.28In0.16Ga0.11S nanoparticles are thermally stable, and exchange reactions using fewer cations do not produce the high entropy phase. The use of colloidal nanoparticle cation exchange as a synthetic platform provides both entropic and enthalpic driving forces that, in addition to configurational entropy, enable the formation of high entropy phases at solution-accessible temperatures.

Emergence and Control of Stacking Fault Formation during Nanoparticle Cation Exchange Reactions.

Cation exchange reactions modify the composition of a nanocrystal while retaining other features, including the crystal structure and morphology. In many cases, the anion sublattice is considered to be locked in place as cations rapidly shuttle in and out. Here we provide evidence that the anion sublattice can shift significantly during nanocrystal cation exchange reactions. When the Cu+ cations of roxbyite Cu1.8S nanorods exchange with Zn2+ to form ZnS nanorods, a high density of stacking faults emerges. During cation exchange, the stacking sequence of the close-packed anion sublattice shifts at many locations to generate a nanorod product containing a mixture of wurtzite, zincblende, and a wurtzite/zincblende polytype that contains an ordered arrangement of stacking faults. The reagent concentration and reaction temperature, which control the cation exchange rate, serve as synthetic levers that can tune the stacking fault density from high to low, which is important because once introduced, the stacking faults could not be modified through thermal annealing. This level of synthetic control through nanocrystal cation exchange is important for controlling properties that depend on the presence and density of stacking faults.

Morphology-Dependent Phase Selectivity of Cobalt Sulfide during Nanoparticle Cation Exchange Reactions.

Achieving phase selectivity during nanoparticle synthesis is important because crystal structure and composition influence reactivity, growth, and properties. Cation exchange provides a pathway for targeting desired phases by modifying composition while maintaining crystal structure. However, our understanding of how to selectively target different phases in the same system is limited. Here, we demonstrate morphology-dependent phase selectivity for wurtzite (wz) CoS, which is hcp, vs pentlandite Co9S8, which is ccp, during Co2+ exchange of roxbyite Cu1.8S plates, spheres, and rods. The plates form wz-CoS, the spheres form both wz-CoS and Co9S8, and the rods form Co9S8. The plates, spheres, and rods have nearly identical widths but increase in length in the direction that the close-packed planes stack, which influences the ability of the anions to shift from hcp to ccp during cation exchange. This morphology-dependent behavior, which correlates with the number of stacked close-packed planes, relies on an anion sublattice rearrangement that is concomitant with cation exchange, thereby providing a unique pathway by which crystal structure can be controlled and phase selectivity can be achieved during nanocrystal cation exchange.

Made-to-Order Heterostructured Nanoparticle Libraries.

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 (Cu1.8S) that have high cation mobilities and a high density of vacancies, cations from solution replace cations in the nanoparticle. Replacing only a fraction of the cations can produce phase-segregated products having internal interfaces, i.e., heterostructured nanoparticles. By the use of multiple partial cation exchange reactions, multicomponent heterostructured nanoparticles can be synthesized.In this Account, we discuss the use of multiple sequential partial cation exchange reactions to rationally construct complex heterostructured nanoparticles toward the goal of made-to-order synthesis. Sequential partial exchange of the Cu+ cations in roxbyite Cu1.8S spheres, rods, and plates produces a library of 47 derivatives that maintain the size, shape, and uniformity defined by the roxbyite templates while introducing various types of interfaces and different materials into the resulting heterostructured nanoparticles. When an excess of the metal salt reagent is used, the reaction time controls the extent of partial cation exchange. When a substoichiometric amount of metal salt reagent is used instead, the extent of partial cation exchange can be precisely controlled by the cation concentration. This approach allows significant control over the number, order, and location of partial cation exchange reactions. Up to seven sequential partial cation exchange reactions can be applied to roxbyite Cu1.8S nanorods to produce derivative heterostructured nanorods containing as many as six different materials, eight internal interfaces, and 11 segments, i.e. ZnS-CuInS2-CuGaS2-CoS-[CdS-(ZnS-CuInS2)]-Cu1.8S. We considered all possible injection sequences of five cations (Zn2+, Cd2+, Co2+, In3+, Ga3+) applied to all accessible Cu1.8S-derived nanorod precursors along with simple design criteria based on preferred cation exchange locations and crystal structure relationships. Using these guidelines, we mapped out synthetically feasible pathways to 65 520 distinct heterostructured nanorods, experimentally observed 113 members of this heterostructured nanorod megalibrary, and then made three of these in high yield and in isolatable quantities. By expansion of these capabilities into a broader scope of materials and identification of additional design guidelines, it should be possible to move beyond model systems and access functional targets rationally and retrosynthetically. Overall, the ability to access large libraries of complex heterostructured nanoparticles in a made-to-order manner is an important step toward bridging the gap between design and synthesis.

Incorporation of Metal Phosphide Domains into Colloidal Hybrid Nanoparticles.

Colloidal hybrid nanoparticles have generated considerable attention in the inorganic nanomaterials community. The combination of different materials within a single nanoparticle can lead to synergistic properties that can enable new properties, new applications, and the discovery of new phenomena. As such, methodologies for the synthesis of hybrid nanoparticles that integrate metal-metal, metal chalcogenide, metal oxide, and oxide-chalcogenide domains have been extensively reported in the literature. However, colloidal hybrid nanoparticles containing metal phosphide domains are rare, despite being attractive systems for their potentially unique catalytic, photocatalytic, and optoelectronic properties. In this Forum Article, we report a study of the synthesis of colloidal hybrid nanoparticles that couple the metal phosphides Ni2P and CoxPy with Au, Ag, PbS, and CdS using heterogeneous seeded-growth reactions. We also investigate the transformation of Au-Ni heterodimers to Au-Ni2P, where phosphidation of preformed metal-metal hybrid nanoparticles offers an alternative route to metal phosphide systems. We also study sequential cation-exchange reactions to target specific metal phosphide hybrids, i.e., the transformation of Ni2P-PbS into Ni2P-Ag2S and then Ni2P-CdS. Throughout all of these pathways, the accompanying discussion emphasizes the synthetic rationale, as well as the challenges in synthesis and characterization that are unique to these systems. In particular, the observation of oxide shells that surround the phosphide domains has implications for the potential photocatalytic applications of these hybrid nanoparticles.

Phosphine-Induced Phase Transition in Copper Sulfide Nanoparticles Prior to Initiation of a Cation Exchange Reaction.

Cation exchange reactions of colloidal copper sulfide nanoparticles are widely used to produce derivative nanoparticles having unique compositions, metastable crystal structures, and complex heterostructures. The copper sulfide crystal structure plays a key role in the mechanism by which cation exchange occurs and the product that forms. Here, we show that digenite copper sulfide nanoparticles undergo a spontaneous phase transition to tetragonal chalcocite in situ, prior to the onset of cation exchange. Room-temperature sonication of digenite (Cu1.8S) in trioctylphosphine, a Lewis base that drives cation exchange, extracts sulfur to produce tetragonal chalcocite (Cu2S). The subtle structural differences between digenite and tetragonal chalcocite are believed to influence the accessibility of cation diffusion channels and concomitantly the mechanism of cation exchange. Structural relationships in nanocrystal cation exchange are therefore dynamic, and intermediates generated in situ must be considered.

Phase-Selective Solution Synthesis of Perovskite-Related Cesium Cadmium Chloride Nanoparticles.

All-inorganic metal halide perovskite-related phases are semiconducting materials that are of significant interest for a wide range of applications. Nanoparticles of these materials are particularly useful because they permit solution processing while offering unique and tunable properties. Of the many metal halide systems that have been studied extensively, cesium cadmium chlorides remain underexplored, and synthetic routes to access them as nanoscale materials have not been established. Here we demonstrate that a simple solution-phase reaction involving the injection of a cesium oleate solution into a cadmium chloride solution produces three distinct cesium cadmium chlorides: hexagonal CsCdCl3 and the Ruddlesden-Popper layered perovskites Cs2CdCl4 and Cs3Cd2Cl7. The phase-selective synthesis emerges from differences in reagent concentrations, temperature, and injection rates. A key variable is the rate at which the cesium oleate solution is injected into the cadmium chloride solution, which is believed to influence the local Cs:Cd concentration during precipitation, leading to control over the phase that forms. Band structure calculations indicate that hexagonal CsCdCl3 is a direct band gap semiconductor while Cs2CdCl4 and Cs3Cd2Cl7 have indirect band gaps. The experimentally determined band gap values for CsCdCl3, Cs2CdCl4, and Cs3Cd2Cl7 are 5.13, 4.91, and 4.70 eV, respectively, which places them in a rare category of ultrawide-band-gap semiconductors.

Multi-Step Topochemical Pathway to Metastable Mo2AlB2 and Related Two-Dimensional Nanosheet Heterostructures.

The rational synthesis of metastable inorganic solids, which is a grand challenge in solid-state chemistry, requires the development of kinetically controlled reaction pathways. Topotactic strategies can achieve this goal by chemically modifying reactive components of a parent structure under mild conditions to produce a closely related analogue that has otherwise inaccessible structures and/or compositions. Refractory materials, such as transition metal borides, are difficult to structurally manipulate at low temperatures because they generally are chemically inert and held together by strong covalent bonds. Here, we report a multistep low-temperature topotactic pathway to bulk-scale Mo2AlB2, which is a metastable phase that has been predicted to be the precursor needed to access a synthetically elusive family of 2-D metal boride (MBene) nanosheets. Room-temperature chemical deintercalation of Al from the stable compound MoAlB (synthesized as a bulk powder at 1400 °C) formed highly strained and destabilized MoAl1-xB, which was size-selectively precipitated to isolate the most reactive submicron grains and then annealed at 600 °C to deintercalate additional Al and crystallize Mo2AlB2. Further heating resulted in topotactic decomposition into bulk-scale Mo2AlB2-AlOx nanolaminates that contain Mo2AlB2 nanosheets with thickness of 1-3 nm interleaved by 1-3 nm of amorphous aluminum oxide. The combination of chemical destabilization, size-selective precipitation, and low-temperature annealing provides a potentially generalizable kinetic pathway to metastable variants of refractory compounds, including bulk Mo2AlB2 and Mo2AlB2-AlOx nanosheet heterostructures, and opens the door to other previously elusive 2-D materials such as 2-D MoB (MBene).

Structure-Selective Synthesis of Wurtzite and Zincblende ZnS, CdS, and CuInS2 Using Nanoparticle Cation Exchange Reactions.

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) Cu2- xS nanoparticles transform upon cation exchange with Cd2+, Zn2+, and In3+ to wurtzite (hexagonal) and zincblende (cubic) CdS, ZnS, and CuInS2, respectively. These products retain the anion and cation sublattice features programmed into the copper sulfide template, and each phase forms to the exclusion of other known crystal structures. These results significantly expand the scope of structure-selective cation exchange reactions in polymorphic systems.