Transformations of Ionic Nanocrystals via Full and Partial Ion Exchange Reactions

Nanomaterials Research at a Primarily Undergraduate Institution: Transforming Nanorods, Undergraduate Research Communities, and Infrastructure

Undergraduate research transforms student's conceptions of themselves as scientists and encourages participation and retention in science, technology, engineering, and mathematics (STEM) fields. Many barriers exist to carrying out scientifically impactful undergraduate research in nanomaterials at primarily undergraduate institutions (PUIs). Here, we share several practices and design principles that demonstrate pathways to overcome these barriers. Design of modular research projects with low entry barriers is essential. Postsynthetic transformation of nanoparticles is a field that enables such design and has been used successfully to advance nanoscience research while being achievable within undergraduate laboratories. Relatively large, inclusive research communities can be supported through the creation of opportunities with peer- and near-peer mentoring. We also share emerging strategies for enabling routine undergraduate access to transmission electron microscopy, which is one of the most mainstream characterization techniques in nanoscience yet is frequently absent from the infrastructure at undergraduate-focused institutions.

Waning-and-waxing shape changes in ionic nanoplates upon cation exchange

Flexible control of the composition and morphology of nanocrystals (NCs) over a wide range is an essential technology for the creation of functional nanomaterials. Cation exchange (CE) is a facile method by which to finely tune the compositions of ionic NCs, providing an opportunity to obtain complex nanostructures that are difficult to form using conventional chemical synthesis procedures. However, due to their robust anion frameworks, CE cannot typically be used to modify the original morphology of the host NCs. In this study, we report an anisotropic morphological transformation of Cu1.8S NCs during CE. Upon partial CE of Cu1.8S nanoplates (NPLs) with Mn2+, the hexagonal NPLs are transformed into crescent-shaped Cu1.8S-MnS NPLs. Upon further CE, these crescent-shaped NPLs evolve back into completely hexagonal MnS NPLs. Comprehensive characterization of the intermediates reveals that this waxing-and-waning shape-evolution process is due to dissolution, redeposition, and intraparticle migration of Cu+ and S2-. Furthermore, in addition to Mn2+, this CE-induced transformation process occurs with Zn2+, Cd2+ and Fe3+. This finding presents a strategy by which to create heterostructured NCs with various morphologies and compositions under mild conditions.

Cation Exchange in Colloidal Transition Metal Nitride Nanocrystals

Transition metal nitride (TMN)-based nanostructures have emerged as promising materials for diverse applications in electronics, photonics, energy storage, and catalysis due to their highly desirable physicochemical properties. However, synthesizing TMN-based nanostructures with designed compositions and morphologies poses challenges, especially in the solution phase. The cation exchange reaction (CER) stands out as a versatile postsynthetic strategy for preparing nanostructures that are otherwise inaccessible through direct synthesis. Nevertheless, exploration of the CER in TMNs lags behind that in metal chalcogenides and metal phosphides. Here, we demonstrate cation exchange in colloidal metal nitride nanocrystals, employing Cu3N nanocrystals as starting materials to synthesize Ni4N and CoN nanocrystals. By controlling the reaction conditions, Cu3N@Ni4N and Cu3N@CoN core@shell heterostructures with tunable compositions can also be obtained. The Ni4N and CoN nanocrystals are evaluated as catalysts for the electrochemical oxygen evolution reaction (OER). Remarkably, CoN nanocrystals demonstrate superior OER performance with a low overpotential of 286 mV at 10 mA·cm-2, a small Tafel slope of 89 mV·dec-1, and long-term stability. Our CER approach in colloidal TMNs offers a new strategy for preparing other metal nitride nanocrystals and their heterostructures, paving the way for prospective applications.

Structural and optical control through anion and cation exchange processes for Sn-halide perovskite nanostructures

Metal halide perovskite nanostructures, characterized by their ionic nature, present a compelling avenue for the tunability of dimensions and band gaps through facile compositional transformations involving both cationic and anionic exchange reactions. While post-synthetic ion-exchange processes have been extensively explored in Pb-halide perovskite nanocrystals, the inherent instability of Sn2+ has limited the exploration of such processes in Sn-halide perovskite nanostructures. In this study, we present a straightforward cation exchange process wherein 2D [R-NH3]2SnX4 Ruddlesden-Popper (RP) nanostructures with n = 1 transition to 3D ASnX3 nanocrystals at room temperature with the addition of A-cation oleate. In addition, anion exchange processes have been demonstrated for both 2D [R-NH3]2SnX4 RP nanostructures and 3D nanocrystals, showcasing transitions between iodide and bromide counterparts. Furthermore, we have fabricated a thin film of 2D [R-NH3]2SnX4 RP nanostructures for cation exchange, wherein A-cation diffusion through a liquid-solid interface facilitates the transformation into a 3D ASnX3 crystal. This investigation underscores the versatility of ion exchange processes in engineering the composition of Sn-halide perovskite nanostructures and, consequently, modulating their optical properties.

Anion Exchange in Semiconductor Magic-Size Clusters

Ion exchange is an effective postsynthesis strategy for the design of colloidal nanomaterials with unique structures and properties. In contrast to the rapid development of cation exchange (CE), the study of anion exchange is still in its infancy and requires an in-depth understanding. Magic-size clusters (MSCs) are important reaction intermediates in quantum dot (QD) synthesis, and studying the ion exchange processes can provide valuable insights into the transformations of QDs. Here, we achieved anion exchange in Cd-based MSCs and elucidated the reaction pathways. We demonstrated that the anion exchange was a stepwise intermolecular transition mediated by covalent inorganic complexes (CICs). We proposed that this transition involved three essential steps: the disassembly of CdE1-MSCs into CdE1-CICs (step 1), an anion exchange reaction from CdE1-CICs to CdE2-CICs (step 2), and assembly of CdE2-CICs to CdE2-MSCs (step 3). Step 3 was the rate-determining step and followed first-order reaction kinetics (kobs = 0.01 min-1; from CdSe-MSCs to CdS-MSCs). Further studies revealed that the activity of foreign anions only affected the reaction kinetics without altering the reaction pathway. The present study provides a deeper insight into the anion exchange mechanisms of MSCs and will further shed light on the synthesis of QDs.

Pseudomorphic amorphization of three-dimensional superlattices through morphological transformation of nanocrystal building blocks

Nanocrystal (NC) superlattices (SLs) have been widely studied as a new class of functional mesoscopic materials with collective physical properties. The arrangement of NCs in SLs governs the collective properties of SLs, and thus investigations of phenomena that can change the assembly of NC constituents are important. In this study, we investigated the dynamic evolution of NC arrangements in three-dimensional (3D) SLs, specifically the morphological transformation of NC constituents during the direct liquid-phase synthesis of 3D NC SLs. Electron microscopy and synchrotron-based in situ small angle X-ray scattering experiments revealed that the transformation of spherical Cu2S NCs in face-centred-cubic 3D NC SLs into anisotropic disk-shaped NCs collapsed the original ordered close-packed structure. The random crystallographic orientation of spherical Cu2S NCs in starting SLs also contributed to the complete disordering of the NC array via random-direction anisotropic growth of NCs. This work demonstrates that an understanding of the anisotropic growth kinetics of NCs in the post-synthesis modulation of NC SLs is important for tuning NC array structures.

Chemical and Structural Stability of CsPbX3 Nanorods during Postsynthetic Anion-Exchange: Implications for Optoelectronic Functionality

We examine halide anion-exchange reactions on CsPbX3 nanorods (NRs), and we identify reaction conditions that provide complete anion exchange while retaining both the highly quantum-confined 1-D morphology and metastable crystal lattice configurations that span a range between tetragonal structures and thermodynamically preferred orthorhombic structures. We find that the chemical stability of CsPbBr3 NRs is degraded by the presence of alkyl amines that etch CsPbBr3 and result in the formation of Cs4PbBr6 and 2-D bromoplumbates. Our study outlines strategies for maintaining metastable states of the soft lattices of perovskite nanocrystals undergoing exchange reactions, despite the thermodynamic driving force toward more stable lattice configurations during this disruptive chemical transformation. These strategies can be used to fine-tune the band gap of LHP-based nanostructures while preserving structure-property relationships that are contingent on metastable shapes and crystal configurations, aiding optoelectronic applications of these materials.

Pre-phase transition of a Cu2-xS template enables polymorph selective synthesis of MS (M = Zn, Cd, Mn) nanocrystals via cation exchange reactions

Utilization of copper-deficient Cu2-xS nanocrystals (NCs) with diverse crystal phases and stoichiometries as cation exchange (CE) templates is a potential route to overcome the current limitations in the polymorph selective synthesis of desired nanomaterials. Among the Cu2-xS NCs, covellite CuS is emerging as an attractive CE template to produce complicated and metastable metal sulfide NCs. The presence of a reducing agent is essential to induce a phase transition of CuS into other Cu2-xS phases prior to the CE reactions. Nevertheless, the effect of the reducing agent on the phase transition of CuS, especially into the hexagonal close packing (hcp) phase and the cubic close packing (ccp) phase, has been scarcely exploited, but it is highly important for the polymorphic production of metal sulfides with the wurtzite phase and zinc blende phase. Herein, we report a reducing agent dependent pre-phase transition of CuS nanodisks (NDs) into hcp and ccp Cu2-xS NCs. 1-Dodecanethiol molecules and oleylamine molecules selectively reduced CuS NDs into hcp djurleite Cu1.94S NDs and ccp digenite Cu1.8S NCs. Afterward, the hcp Cu1.94S NDs and ccp Cu1.8S NCs were exchanged by Zn2+/Cd2+/Mn2+, and the wurtzite phase and the zinc blende phase of ZnS, CdS, and MnS NCs were produced. Without the pre-phase transition, direct CE reactions of CuS NDs are incapable of synthesizing the above wurtzite and zinc blende metal sulfide NCs. Therefore, our findings suggest the importance of the pre-phase transition of the CE template in polymorphic syntheses, holding great promise in the fabrication of other polymorphic nanomaterials with novel physical and chemical properties.

Synthesis of oriented J type ZnIn2S4@CdIn2S4 heterojunction by controllable cation exchange for enhancing photocatalytic hydrogen evolution

Construction of semiconductor heterojunctions which promote the separation and transport of photogenerated carriers is an effective strategy for improving photocatalytic reaction efficiency. Based on the anisotropic electrical conductivity of layered ZnIn2S4 (ZIS) photocatalyst, an efficient heterojunction should be constructed along the layer plane of ZIS, that is, a J type heterojunction. However, achieving controllable synthesis of the oriented heterojunction of ZIS faces challenges. Herein, we develop a facile, cost-effective and spatially-selective cation exchange synthesis approach to construct J type ZnIn2S4@CdIn2S4 (J-ZIS@CIS) heterojunction using a flower-like hexagonal ZIS as the parent material. The developed synthesis approach can also control crystal structure of the heterojunction component CIS. This work presents a facile and controllable synthesis strategy to construct oriented anisotropic heterojunctions that are otherwise inaccessible. The as-prepared J-ZIS@CIS heterojunction displays a greatly enhanced photocatalytic hydrogen evolution activity with a rate of 183 μmol h-1, 2.77 times higher than that of pristine ZIS. Furthermore, the possible photocatalytic reaction mechanism is presented for the heterojunction.

Temperature-Dependent Selection of Reaction Pathways, Reactive Species, and Products during Postsynthetic Selenization of Copper Sulfide Nanoparticles

Rational design of elaborate, multicomponent nanomaterials is important for the development of many technologies such as optoelectronic devices, photocatalysts, and ion batteries. Combination of metal chalcogenides with different anions, such as in CdS/CdSe structures, is particularly effective for creating heterojunctions with valence band offsets. Seeded growth, often coupled with cation exchange, is commonly used to create various core/shell, dot-in-rod, or multipod geometries. To augment this library of multichalcogenide structures with new geometries, we have developed a method for postsynthetic transformation of copper sulfide nanorods into several different classes of nanoheterostructures containing both copper sulfide and copper selenide. Two distinct temperature-dependent pathways allow us to select from several outcomes-rectangular, faceted Cu2-xS/Cu2-xSe core/shell structures, nanorhombuses with a Cu2-xS core, and triangular deposits of Cu2-xSe or Cu2-x(S,Se) solid solutions. These different outcomes arise due to the evolution of the molecular components in solution. At lower temperatures, slow Cu2-xS dissolution leads to concerted morphology change and Cu2-xSe deposition, while Se-anion exchange dominates at higher temperatures. We present detailed characterization of these Cu2-xS-Cu2-xSe nanoheterostructures by transmission electron microscopy (TEM), powder X-ray diffraction, energy-dispersive X-ray spectroscopy, and scanning TEM-energy-dispersive spectroscopy. Furthermore, we correlate the selenium species present in solution with the roles they play in the temperature dependence of nanoheterostructure formation by comparing the outcomes of the established reaction conditions to use of didecyl diselenide as a transformation precursor.

Dicarboxylic Acids Induced Tandem Transformation of Silver Nanocluster

Structural transformation of metal nanoclusters (NCs) is of great ongoing interest regarding their synthesis, stability, and reactivity. Although sporadic examples of cluster transformations have been reported, neither the underlying transformation mechanism nor the intermediates are unambiguous. Herein, we have synthesized a flexible 54-nuclei silver cluster (Ag54) by combining soft (tBuC≡C-) and hard (nPrCOO-) ligands. The existence of weakly coordinated nPrCOO- enhances the reactivity of Ag54, thus facilitating the dicarboxylic acid to induce structural transformation. X-ray structural analyses reveal that Ag54 transforms to Ag28 cluster-based 2D networks (Ag28a and Ag28b) induced by H2suc (succinic acid) and H2glu (glutaric acid), whereas with H2pda (2,2'-(1,2-phenylene)diacetic acid), a discrete Ag28 cluster (Ag28c) is isolated. The key intermediate Ag17 that emerges during the self-dissociation of Ag54 was isolated by using cryogenic recrystallization and characterized by X-ray crystallography. The "tandem transformation" mechanism for the structure evolution from Ag54 to Ag28a is established by time-dependent electrospray ionization mass spectrometry (ESI-MS) and UV-vis spectroscopy. In addition, the catalytic activity in the 4-nitrophenol reduction follows the sequence Ag28c > Ag28b > Ag28a > Ag54 due to more bare silver sites on the surface of the Ag28 cluster unit. Our findings not only open new avenues to the synthesis of silver NCs but also shed light on a better understanding of the structural transformation mechanism from one cluster to another or cluster-based metal-organic networks induced by dicarboxylates.

Quantifiable Regulation of Chemical Kinetics Barriers for Creation of Single-Atom Metal Sites on Photocatalytic Atomic Layers

Cation exchange (CE) under mild conditions promises a facile strategy to anchor single metal sites on colloidal chalcogenides toward catalytic applications, which however has seldom been demonstrated. The dilemma behind this is the rapid kinetics and high efficiency of the reaction disfavoring atomic dispersion of the metal species. Here we report that a fine-tuning of the affinity between the incoming metal cations and the deliberately introduced ligands can be exploited to manipulate the kinetics of the CE reaction, in a quantitative and systematic manner defined by the Tolman electronic parameter of the ligands used. Moreover, the steric effect of metal-ligand complexes offers thermodynamic preference for spatial isolation of the metal atoms. These thereby allow the rational construction of single atom catalysts (SACs) via simple one-step CE reactions, as exemplified by the CE-derived incorporation of single metal atoms (M = Cu, Ag, Au, Pd) on SnS₂ two-unit-cell layers through M-S coordination.

Region-Controlled Framework Interface Mediated Anion Exchange Chemical Transformation to Designed Metal Phosphosulfide Heteronanostructures

Postsynthetic chemical transformation provides a powerful platform for creating heteronanostructures (HNs) with well-defined materials and interfaces that generate synergy or enhancement. However, it remains a synthetic bottleneck for the precise construction of HNs with increased degrees of complexity and more elaborate functions in a predictable manner. Herein, we define a general transformative protocol for metal phosphosulfide HNs based on tunable hexagonal Cu_1.81S frameworks with corner-, edge- and face-controlled growth of Co₂P domains. The region-controlled Cu_1.81S-Co₂P framework interfaces can serve as "kinetic barriers" in mediating the direction and rate between P and S anion exchange reactions, thus leading to a family of morphology and phase designed Cu₃P_1-xS_x-Co₂P HNs with hollow (branched, dotted and crown), porous and core-shell architectures. This study reveals the internal transformation mechanism between metal sulfide and phosphide nanocrystals, and opens up a new way for the rational synthesis of metastable HNs that are otherwise inaccessible.

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 Cu_2-xSe nanoparticles using a trioctylphosphine-tellurium complex (TOP═Te) yields weissite Cu_2-xSe_1-yTe_y solid solutions, rather than complete exchange to weissite Cu_2-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 Cu_2-xSe_1-yTe_y solid solution nanoparticles transform, over the span of several days, to a selenium-rich Cu_2-xSe_1-yTe_y 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.

Skeleton-coated CoCu-Based bimetal hollow nanoprisms as High-Performance electrocatalysts for oxygen evolution reaction

CoSx materials with high catalytic activity are considered as promising HER electrocatalysts, but their inherent low electrical conductivity and easy loss of active sites have greatly limited their applications in OER electrocatalysis. Herein, we present a convenient method to synthesize Co-Cu hollow nanoprisms after wrapping and calcining with trithiocyanuric acid (C3H3N3S3) (denoted N-Co-Cu-S-x HNs). The results showed that Cu doping modified the charge density of Co center, leading to the enhancement of the intrinsic activity of the Co3S4 active center, meanwhile wrapping trithiocyanuric acid on the surfaces and calcinating to form N-containing C skeleton as a flexible substrate to encapsulate the catalysts, which effectively protected the active sites inside the catalysts. Notably, the OER catalyst that was optimized by adjusting the metal ratio and controlling the trithiocyanuric acid incorporation exhibited a low overpotential of 306 mV under a current density of 10 mA cm-2 and showed a superior durability of more than 27 h. This work may provide some insights into the preparation of oxygen evolution reaction catalysts with excellent performance through doping transition metals and protecting the internal active sites strategies.

Crystallization of SrAl12O19 Nanocrystals from Amorphous Submicrometer Particles

Advanced instrumentation and modern analysis tools such as transmission electron microscopy (TEM) have led to phenomenal progress in understanding crystallization, in particular from solution, which is a prerequisite for the design-based preparation of a target crystal. Nevertheless, little has been understood about the crystallization pathway under high-temperature annealing (HTA) conditions. Metal oxide crystals are prominent materials that are usually obtained via HTA. Despite the widespread application of hydro-/solvothermal methods on the laboratory scale, HTA is the preferred method in many industries for the mass production of metal oxide crystals. However, poor control over the morphology and grain sizes of these crystals under extreme HTA conditions limits their applications. Here, applying ex-situ TEM, the transformation of a single amorphous spherical submicrometer precursor particle of SrAl12O19 (SA6) at 1150 °C toward a nanosized thermodynamically favored hexagonal crystal is explored. It is illustrated in real space, step by step, how both kinetic and thermodynamic factors contribute to this faceting and morphology evolution. These results demonstrate a nonclassical nucleation and growth process consisting of densification, crystallite domain formation, oriented attachment, surface nucleation, 2-dimensional (2D) growth, and surface diffusion of the atoms to eventually result in the formation of a hexagonal platelet crystal. The TEM images further delineate a parent crystal driving the crystal lattice and morphological orientation of a network of interconnected platelets.

Recent progress and future prospects on halide perovskite nanocrystals for optoelectronics and beyond

As an emerging new class of semiconductor nanomaterials, halide perovskite (ABX₃, X = Cl, Br, or I) nanocrystals (NCs) are attracting increasing attention owing to their great potential in optoelectronics and beyond. This field has experienced rapid breakthroughs over the past few years. In this comprehensive review, halide perovskite NCs that are either freestanding or embedded in a matrix (e.g., perovskites, metal-organic frameworks, glass) will be discussed. We will summarize recent progress on the synthesis and post-synthesis methods of halide perovskite NCs. Characterizations of halide perovskite NCs by using a variety of techniques will be present. Tremendous efforts to tailor the optical and electronic properties of halide perovskite NCs in terms of manipulating their size, surface, and component will be highlighted. Physical insights gained on the unique optical and charge-carrier transport properties will be provided. Importantly, the growing potential of halide perovskite NCs for advancing optoelectronic applications and beyond including light-emitting devices (LEDs), solar cells, scintillators and X-ray imaging, lasers, thin-film transistors (TFTs), artificial synapses, and light communication will be extensively discussed, along with prospecting their development in the future.

3D Spectroscopic Tracking of Individual Brownian Nanoparticles during Galvanic Exchange

Monitoring chemical reactions in solutions at the scale of individual entities is challenging: single-particle detection requires small confocal volumes, which are hardly compatible with Brownian motion, particularly when long integration times are necessary. Here, we propose a real-time (10 Hz) holography-based nm-precision 3D tracking of single moving nanoparticles. Using this localization, the confocal collection volume is dynamically adjusted to follow the moving nanoparticle and allow continuous spectroscopic monitoring. This concept is applied to study galvanic exchange in freely moving colloidal silver nanoparticles with gold ions generated in situ. While the Brownian trajectory reveals particle size, spectral shifts dynamically reveal composition changes and transformation kinetics at the single-object level, pointing at different transformation kinetics for free and tethered particles.

Renaissance of Topotactic Ion-Exchange for Functional Solids with Close Packed Structures

Recently, many new, complex, functional oxides have been discovered with the surprising use of topotactic ion-exchange reactions on close-packed structures, such as found for wurtzite, rutile, perovskite, and other structure types. Despite a lack of apparent cation-diffusion pathways in these structure types, synthetic low-temperature transformations are possible with the interdiffusion and exchange of functional cations possessing ns² stereoactive lone pairs (e. g., Sn(II)) or unpaired nd^x electrons (e. g., Co(II)), targeting new and favorable modulations of their electronic, magnetic, or catalytic properties. This enables a synergistic blending of new functionality to an underlying three-dimensional connectivity, i. e., [-M-O-M-O-]_n , that is maintained during the transformation. In many cases, this tactic represents the only known pathway to prepare thermodynamically unstable solids that otherwise would commonly decompose by phase segregation, such as that recently applied to the discovery of many new small bandgap semiconductors.

Manipulating Cation Exchange Reactions in Cu2-xS Nanoparticles via Crystal Structure Transformation

Copper-deficient Cu_2-xS nanoparticles (NPs) are extensively exploited as a superior cation exchange (CE) template to yield sophisticated nanostructures. Recently, it has been discovered that their CE reactions can be facilely manipulated by copper vacancy density, morphology, and NP size. However, the structural similarity of usually utilized Cu_2-xS somewhat limits the manipulation of the CE reactions through the factor of crystal structure because it can strongly influence the process of the reaction. Herein, we report a methodology of crystal structure transformation to manipulate the CE reactions. Particularly, roxbyite Cu_1.8S nanodisks (NDs) were converted into solid wurtzite CdS NDs and Janus-type Cu_1.94S/CdS NDs by a "full"/partial CE reaction with Cd²⁺. Afterward, the roxbyite Cu_1.8S were pseudomorphically transformed into covellite CuS NDs. Unlike Cu_1.8S, the CuS was scarcely exchanged because of the unique disulfide (S-S) bonds and converted into hollow wurtzite CdS under a more reactive condition. The S-S bonds were gradually split and CuS@CdS core@shell-type NDs were generated. Therefore, our findings in the present study provide not only a versatile technique to manipulate CE reactions in Cu_2-xS NPs but also a better comprehension of their reaction dynamics and pathways.

Successive Anion/Cation Exchange Enables the Fabrication of Hollow CuCo2S4 Nanorods for Advanced Oxygen Evolution Reaction Electrocatalysis

Hollow CuCo₂S₄ nanorods (H-CCS-Ns) have been successfully developed via a facile successive anion/cation-exchange method. The outstanding electrocatalytic performance of H-CCS-Ns is mainly attributed to its distinctive hollow structure, which accelerates the electron transfer rate and provides abundant active sites. Moreover, a mechanism study indicates that H-CCS-Ns has highly active octahedral Co³⁺, and the existence of Co³⁺ cations optimizes the adsorption of oxygen-involved intermediates, making H-CCS-Ns a promising OER electrocatalyst. Optimized H-CCS-Ns only need an ultralow overpotential of 220 mV to drive a current density of 10 mA·cm^-2 and exhibit distinguished cycling stability with a negligible fluctuation for 30 h. More impressively, when H-CCS-Ns are assembled with Pt/C for overall water splitting, a voltage as low as 1.545 V is required at a current density of 10 mA·cm^-2, and the catalyst shows outstanding stability for as long as 38 h. This study offers a feasible strategy to design hollow spinel catalysts for efficient OER catalysis.

Two-Dimensional CdSe-PbSe Heterostructures and PbSe Nanoplatelets: Formation, Atomic Structure, and Optical Properties

Cation exchange enables the preparation of nanocrystals (NCs), which are not reachable by direct synthesis methods. In this work, we applied Pb²⁺-for-Cd²⁺ cation exchange on CdSe nanoplatelets (NPLs) to prepare two-dimensional CdSe-PbSe heterostructures and PbSe NPLs. Lowering the reaction temperature slowed down the rate of cation exchange, making it possible to characterize the intermediary NCs ex situ with atomically resolved high-angle annular dark-field scanning transmission electron microscopy and optical spectroscopy. We observe that the Pb²⁺-for-Cd²⁺ cation exchange starts from the vertices of the NPLs and grows into the zinc blende CdSe (zb-CdSe) lattice as a rock salt PbSe phase (rs-PbSe), while the anion (selenium) sublattice is being preserved. In agreement with previous works on CdTe-PbTe films, the interfaces between zb-CdSe and rs-PbSe consist of shared {001} and {011} planes. The final PbSe NPLs are highly crystalline and contain protrusions at the edges, which are slightly rotated, indicating an atomic reconfiguration of material. The growth of PbSe domains into CdSe NPLs could also be monitored by the emission peak shift as a function of the exchange time. Temperature-dependent emission measurements confirm a size-dependent change of the band gap energy with temperature and reveal a strong influence of the anisotropic shape. Time-resolved photoluminescence measurements between 4 and 30 K show a dark-bright exciton-state splitting different from PbSe QDs with three-dimensional quantum confinement.

Rapid, Energy-Efficient and Pseudomorphic Microwave-Induced-Metal-Plasma (MIMP) Synthesis of Mg2Si and Mg2Ge

Polycrystalline magnesium silicide, Mg2Si and magnesium germanide, Mg2Ge were synthesised from the elemental powders via the microwave-induced-metal-plasma (MIMP) approach at 200 W within 1 min in vacuo for the first...

Elastic forces drive nonequilibrium pattern formation in a model of nanocrystal ion exchange

Chemical transformations, such as ion exchange, are commonly employed to modify nanocrystal compositions. Yet the mechanisms of these transformations, which often operate far from equilibrium and entail mixing diverse chemical species, remain poorly understood. Here we explore an idealized model for ion exchange in which a chemical potential drives compositional defects to accumulate at a crystal's surface. These impurities subsequently diffuse inward. We find that the nature of interactions between sites in a compositionally impure crystal strongly impacts exchange trajectories. In particular, elastic deformations which accompany lattice-mismatched species promote spatially modulated patterns in the composition. These same patterns can be produced at equilibrium in core/shell nanocrystals, whose structure mimics transient motifs observed in nonequilibrium trajectories. Moreover, the core of such nanocrystals undergoes a phase transition-from modulated to unstructured-as the thickness or stiffness of the shell is decreased. Our results help explain the varied patterns observed in heterostructured nanocrystals produced by ion exchange and suggest principles for the rational design of compositionally patterned nanomaterials.

Versatile Synthesis of Hollow Metal Sulfides via Reverse Cation Exchange Reactions for Photocatalytic CO2 Reduction

Herein, we explore a general Cu_2-x S nanocube template-assisted and reverse cation exchange-mediated growth strategy for fabricating hollow multinary metal sulfide. Unlike the traditional cation exchange method controlled by the metal sulfide constant, the introduction of tri-n-butylphosphine (TBP) can reverse cation exchange to give a series of hollow metal sulfides. A variety of hollow multinary metal sulfide cubic nanostructures has been demonstrated while preserving anisotropic shapes to the as-synthesized templates, including binary compounds (CdS, ZnS, Ag₂ S, PbS, SnS), ternary compound (CuInS₂ , Zn_x Cd_1-x S), and quaternary compound (single-atom platinum anchored Zn_x Cd_1-x S; Zn_x Cd_1-x S-Pt₁ ). Experimental and density functional theory (DFT) calculations show that the hollow metal sulfide semiconductors obtained could significantly improve the separation and migration of photogenerated electron-hole pairs. Owing to the efficient charge transfer, the Zn_x Cd_1-x S-Pt₁ exhibited outstanding photocatalytic performance of CO₂ to CO, with the highest CO generation rate of 75.31 μmol h^-1 .

Nanoarchitectonics for Hierarchical Fullerene Nanomaterials

Nanoarchitectonics is a universal concept to fabricate functional materials from nanoscale building units. Based on this concept, fabrications of functional materials with hierarchical structural motifs from simple nano units of fullerenes (C60 and C70 molecules) are described in this review article. Because fullerenes can be regarded as simple and fundamental building blocks with mono-elemental and zero-dimensional natures, these demonstrations for hierarchical functional structures impress the high capability of the nanoarchitectonics approaches. In fact, various hierarchical structures such as cubes with nanorods, hole-in-cube assemblies, face-selectively etched assemblies, and microstructures with mesoporous frameworks are fabricated by easy fabrication protocols. The fabricated fullerene assemblies have been used for various applications including volatile organic compound sensing, microparticle catching, supercapacitors, and photoluminescence systems.

Determinants of crystal structure transformation of ionic nanocrystals in cation exchange reactions

Changes in the crystal system of an ionic nanocrystal during a cation exchange reaction are unusual yet remain to be systematically investigated. In this study, chemical synthesis and computational modeling demonstrated that the height of hexagonal-prism roxbyite (Cu_1.8S) nanocrystals with a distorted hexagonal close-packed sulfide anion (S^2-) sublattice determines the final crystal phase of the cation-exchanged products with Co²⁺ [wurtzite cobalt sulfide (CoS) with hexagonal close-packed S^2- and/or cobalt pentlandite (Co₉S₈) with cubic close-packed S^2-]. Thermodynamic instability of exposed planes drives reconstruction of anion frameworks under mild reaction conditions. Other incoming cations (Mn²⁺, Zn²⁺, and Ni²⁺) modulate crystal structure transformation during cation exchange reactions by various means, such as volume, thermodynamic stability, and coordination environment.