Influence of the Ion Coordination Number on Cation Exchange Reactions with Copper Telluride Nanocrystals

Highly Reversible Tin Film Anode Guided via Interfacial Coordination Effect for High Energy Aqueous Acidic Batteries

Sn metal is a preferable choice as anode material for aqueous acidic batteries due to its acid-tolerance, non-toxicity, and ease of recycling. However, the large size and irregular deposition morphology of polyhedral Sn particles are bad for constructing stable and high-capacity Sn metal anode because of severe hydrogen evolution and metal shedding. To tackle this critical issue, 4-tert-octylphenol pentaethoxylate (POPE) is used as an electrolyte additive to generate a thin-film Sn anode with reversible stripping/plating behavior. POPE can not only induce homogeneous surface chemistry by adsorbing on the Sn surface via coordination bonds but also inhibit hydrogen evolution by modulating the solvation shell of Sn2+. The Sn film anode delivers improved electrochemical stability over 480 h with satisfactory rate performance and low polarization. Moreover, the as-assembled PbO2//Sn battery can also provide outstanding durability at 10 mAh cm-2. This work offers new inspiration for developing a reversible Sn metal film anode.

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.

Synthesis and Modulation of Low-Dimensional Transition Metal Chalcogenide Materials via Atomic Substitution

In recent years, low-dimensional transition metal chalcogenide (TMC) materials have garnered growing research attention due to their superior electronic, optical, and catalytic properties compared to their bulk counterparts. The controllable synthesis and manipulation of these materials are crucial for tailoring their properties and unlocking their full potential in various applications. In this context, the atomic substitution method has emerged as a favorable approach. It involves the replacement of specific atoms within TMC structures with other elements and possesses the capability to regulate the compositions finely, crystal structures, and inherent properties of the resulting materials. In this review, we present a comprehensive overview on various strategies of atomic substitution employed in the synthesis of zero-dimensional, one-dimensional and two-dimensional TMC materials. The effects of substituting elements, substitution ratios, and substitution positions on the structures and morphologies of resulting material are discussed. The enhanced electrocatalytic performance and photovoltaic properties of the obtained materials are also provided, emphasizing the role of atomic substitution in achieving these advancements. Finally, challenges and future prospects in the field of atomic substitution for fabricating low-dimensional TMC materials are summarized.

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.

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.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

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.

Multimetallic post-synthetic modifications of copper selenide nanoparticles

In this report, we investigate the addition of two metal cations, simultaneously and sequentially to Cu_2-xSe nanoparticles. The metal combinations (Ag-Au, Ag-Pt, Hg-Au and Hg-Pt) are chosen such that one metal adds to the structure via cation exchange and the other adds to the structure via metal deposition when added individually to Cu_2-xSe nanoparticles. Surprisingly, we find that for each metal combination, across all three synthesis routes, cation exchange and metal deposition products are obtained without deviation from the outcomes seen in the binary metal systems. However, within those outcomes the data show several types of heterogeneities in the morphologies formed including extent and composition of cation exchange products as well as the extent and composition of the metal deposited products. Taken together, these results suggest a hierarchical control for nanoheterostructure morphologies where the pathways of cation exchange or metal deposition in post-synthetic modification of Cu_2-xSe exhibit relatively general outcomes as a function of metal, regardless of synthetic approach or metal combination. However, the detailed composition and interface populations of the resulting materials are more sensitive to both metal identities and synthetic procedure (e.g. order of reagent addition), suggesting that certain principles of metal chalcogenide post-synthetic modification are excitingly robust, while also revealing new avenues for both mechanistic discovery and structural control.

Anisotropic Heavy-Metal-Free Semiconductor Nanocrystals: Synthesis, Properties, and Applications

Heavy-metal (Cd, Hg, and Pb)-containing semiconductor nanocrystals (NCs) have been explored widely due to their unique optical and electrical properties. However, the toxicity risks of heavy metals can be a drawback of heavy-metal-containing NCs in some applications. Anisotropic heavy-metal-free semiconductor NCs are desirable replacements and can be realized following the establishment of anisotropic growth mechanisms. These anisotropic heavy-metal-free semiconductor NCs can possess lower toxicity risks, while still exhibiting unique optical and electrical properties originating from both the morphological and compositional anisotropy. As a result, they are promising light-emitting materials in use various applications. In this review, we provide an overview on the syntheses, properties, and applications of anisotropic heavy-metal-free semiconductor NCs. In the first section, we discuss hazards of heavy metals and introduce the typical heavy-metal-containing and heavy-metal-free NCs. In the next section, we discuss anisotropic growth mechanisms, including solution-liquid-solid (SLS), oriented attachment, ripening, templated-assisted growth, and others. We discuss mechanisms leading both to morphological anisotropy and to compositional anisotropy. Examples of morphological anisotropy include growth of nanorods (NRs)/nanowires (NWs), nanotubes, nanoplatelets (NPLs)/nanosheets, nanocubes, and branched structures. Examples of compositional anisotropy, including heterostructures and core/shell structures, are summarized. Third, we provide insights into the properties of anisotropic heavy-metal-free NCs including optical polarization, fast electron transfer, localized surface plasmon resonances (LSPR), and so on, which originate from the NCs' anisotropic morphologies and compositions. Finally, we summarize some applications of anisotropic heavy-metal-free NCs including catalysis, solar cells, photodetectors, lighting-emitting diodes (LEDs), and biological applications. Despite the huge progress on the syntheses and applications of anisotropic heavy-metal-free NCs, some issues still exist in the novel anisotropic heavy-metal-free NCs and the corresponding energy conversion applications. Therefore, we also discuss the challenges of this field and provide possible solutions to tackle these challenges in the future.

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.

Potent intrinsic bactericidal activity of novel copper telluride nano-grape clusters with facile preparation

Bactericidal nanomedicines often suffer from a complicated design and insufficient intrinsic inhibitory efficacy. Herein, novel anti-bacterial copper telluride (CuTe) nano-clusters are reported, featuring superior bactericidal efficiency, facile preparation, and unique mechanism. These nanoparticles, well dispersable in water, resembled grape clusters with rough surfaces. The CuTe nano-grape clusters exhibited ultra-high sterilization efficacy at ultra-low concentration, particularly for Gram-negative bacteria, and were more potent than conventional anti-microbial nanoparticles. Also, the grape clusters effectively inhibited the bacterial biofilm development. Further investigation revealed the synergized mechanisms of reactive oxygen species (ROS) generation and glutathione (GSH) depletion. Interestingly, electron microscopy revealed that the grape clusters served as bacterial hunters by tightly adhering to bacterial surfaces. The bacteria subsequently suffered from the leakage of various intracellular components including nucleic acid, proteins, and potassium. Most encouragingly, CuTe drastically reduced bacterial number in a mouse model with lethal intraperitoneal infection and increased the mouse survival rate to 90%. This finding could inspire the development of highly potent bactericidal inorganic formulations with simplified structure, multiple antibacterial mechanisms, and promising application potential.

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.

Tuning Luminescence of Lanthanide-Doped Upconversion Nanoparticles through Simultaneous Binary Cation Exchange

Dual-mode luminescent nanomaterials have outstanding performance in biosensing and multistage anticounterfeiting. Herein, we report the tuning of optical attributes of lanthanide-doped nanoparticles (NPs) via simultaneous binary cation exchange. We show that cation exchange of NaYF₄:Yb/Er (18/2 mol %)@NaLnF₄ (Ln = Y and Gd) NPs with a combination of Ce³⁺ and Tb³⁺ enables the resultant nanoparticles to exhibit both upconversion and downshifting emissions upon excitation at 980 and 254 nm, respectively. We find that in addition to introducing downshifting emission attributes, the use of Tb³⁺ ions allows conservation of the integrity of the parent core@shell NPs by decreasing the dissociation tendency caused by Ce³⁺ ions during the cation exchange. The upconversion color output can be tuned from green to red and blue by changing lanthanide combinations in the core NPs. This work not only provides an effective strategy for the optical tuning of lanthanide-doped NPs but also builds a platform for probing the difference in the reactivity nature of lanthanides.

Telluride Nanocrystals with Adjustable Amorphous Shell Thickness and Core-Shell Structure Modulation by Aqueous Cation Exchange

Engineering the structure of core-shell colloidal semiconductor nanoparticles (CSNPs) is attractive due to the potential to enhance photo-induced charge transfer and induce favorable optical and electronic properties. Nonetheless, the sensitivity of telluride CSNPs to high temperatures makes it challenging to precisely modulate their surface crystallinity. Herein, we have developed an efficient strategy for synthesizing telluride CSNPs with thin amorphous shells using aqueous cation exchange (ACE). By changing the synthesis temperature in the range of 40-110 °C, the crystallinity of the CdTe nanoparticles was controllable from perfect crystals with no detectable amorphous shell (c-CdTe) to a core-shell structure with a crystalline CdTe NP core covered by an amorphous shell of tunable thickness up to 7-8 nm (c@a-CdTe). A second ACE step transformed c@a-CdTe to crystalline CdTe@HgTe core-shell NPs. The c@a-CdTe nanoparticles synthesized at 60 °C and having a 4-5 nm thick amorphous shell exhibited the highest surface-enhanced Raman scattering activity with a high enhancement factor around 8.82 × 10⁵, attributed to the coupling between the amorphous shell and the crystalline core.

Plasmon-induced ultrafast charge transfer in single-particulate Cu1.94S-ZnS nanoheterostructures

Recombination centers generated from structural and interfacial defects in nanoheterostructures (NHs) prevent effective photo-induced charge transfer and have blocked the advance of many photoresponsive applications. Strategies to construct high-quality interfaces in NHs are emerging but are limited in the release of interfacial strain and the integrality of the sublattice. Herein, we synthesize single-particulate Cu_1.94S-ZnS NHs with a continuous sublattice using a nanoscale cation exchange reaction (CE). Under near-infrared (NIR) radiation (λ = 1500 nm), femtosecond open-aperture (OA) Z-scan measurements are applied to investigate the nonlinear optical features of samples and verify the existence of plasma-induced charge transfer in the Cu_1.94S-ZnS NHs system. The resulting charge transfer time (τ _CT) of ∼0.091 picoseconds (ps) was confirmed by the femtosecond time-resolved pump-probe technique. Such an ultrafast charge transfer process has been rarely reported in semiconductor-semiconductor NHs. The results suggest that CE can be used as a promising tool to construct well-ordered interfacial structures, which are significant for the performance enhancement of NHs for photon utilization.

Connecting Cation Exchange and Metal Deposition Outcomes via Hume-Rothery-Like Design Rules Using Copper Selenide Nanoparticles

Heterogenous nanomaterials containing various inorganic phases have far-reaching impacts both from the physical phenomena they reveal and the technologies they enable. While the variety and impact of these materials has been demonstrated in many reports, there is critical ambiguity in the factors that lead to major bifurcations in developing these heterostructures, for example, the formation of either mixed metal semiconductors or segregated metal-semiconductor phases. Here, we compare outcomes of independently introducing 5 different metal cations (Au³⁺, Ag⁺, Hg²⁺, Pd²⁺, and Pt²⁺) to antifluorite copper selenide (Cu_2-xSe) nanoparticles (diameter = 52 ± 5 nm). This suite of metal cations allowed us to control for and evaluate a variety of potentially competing intrinsic system parameters including metal cation size, valency, and reduction potential as well as lattice volume change, lattice formation energy, and lattice mismatch. Upon secondary metal addition, we determined that the transformation of a cubic Cu_2-xSe lattice will occur via cation exchange reaction when the change in symmetry to the resulting metal selenide phase(s) preserves mutually orthogonal lattice vectors. However, if the new lattice symmetry would be disrupted further, metal deposition is the likely outcome of secondary metal cation addition, forming metal-semiconductor heterostructures. These results suggest a synthesis design rule that relies on an intrinsic property of the material, not the reaction pathway, and indicates that more such factors may be found in other particle and synthetic systems.

Binary and Ternary Colloidal Cu-Sn-Te Nanocrystals for Thermoelectric Thin Films

Recent advances in copper chalcogenide-based nanocrystals (NCs), copper sulfide, and copper selenide derived nanostructures, have drawn considerable attention. However, reports of crystal phase and shape engineering of binary or ternary copper telluride NCs remain rare. Here, a colloidal hot-injection approach for producing binary copper/tin telluride, and ternary copper tin telluride NCs with controllable compositions, crystal structures, and morphologies is reported. The crystal phase and growth behavior of these tellurides are systematically studied from both experimental and theoretical perspectives. The morphology of Cu_1.29 Te NCs is modified from 1D nanorods with different aspect ratios to 2D nanosheets and 3D nanocubes, by controlling the preferential growth of specific crystalline facets. A controllable phase transition from Cu_1.29 Te to Cu_1.43 Te NCs is also demonstrated. The latter can be further converted into Cu₂ SnTe₃ and SnTe through Sn incorporation. Temperature dependent thermoelectric properties of metal (Cu and Sn) telluride nanostructure thin films are also studied, including Cu_1.29 Te, Cu_1.43 Te, Cu₂ SnTe₃ , and SnTe. Cu₂ SnTe₃ is a low carrier density semimetal with compensating electron and hole Fermi surface pockets. The engineering of crystal phase and morphology control of colloidal copper tin telluride NCs opens a path to explore and design new classes of copper telluride-based nanomaterials for thermoelectrics and other applications.

Colloidal Synthesis Path to 2D Crystalline Quantum Dot Superlattices

By combining colloidal nanocrystal synthesis, self-assembly, and solution phase epitaxial growth techniques, we developed a general method for preparing single dot thick atomically attached quantum dot (QD) superlattices with high-quality translational and crystallographic orientational order along with state-of-the-art uniformity in the attachment thickness. The procedure begins with colloidal synthesis of hexagonal prism shaped core/shell QDs (e.g., CdSe/CdS), followed by liquid subphase self-assembly and immobilization of superlattices on a substrate. Solution phase epitaxial growth of additional semiconductor material fills in the voids between the particles, resulting in a QD-in-matrix structure. The photoluminescence emission spectra of the QD-in-matrix structure retains characteristic 0D electronic confinement. Importantly, annealing of the resulting structures removes inhomogeneities in the QD-QD inorganic bridges, which our atomistic electronic structure calculations demonstrate would otherwise lead to Anderson-type localization. The piecewise nature of this procedure allows one to independently tune the size and material of the QD core, shell, QD-QD distance, and the matrix material. These four choices can be tuned to control many properties (degree of quantum confinement, quantum coupling, band alignments, etc.) depending on the specific applications. Finally, cation exchange reactions can be performed on the final QD-in-matrix, as demonstrated herein with a CdSe/CdS to HgSe/HgS conversion.

Synthesis of vulcanite (CuTe) and metastable Cu1.5Te nanocrystals using a dialkyl ditelluride precursor

This study demonstrates that a dialkyl ditelluride reagent can produce metastable and difficult-to-achieve metal telluride phases in nanocrystal syntheses. Using didodecyl ditelluride and without the need for phosphine precursors, nanocubes of the pseudo-cubic phase (Cu1.5Te) were synthesized at the moderate temperature of 135 °C. At the higher temperature of 155 °C, 2-D nanosheets of vulcanite (CuTe) resulted, a nanomaterial in a phase that has not been previously achieved through thermal decomposition methods. Materials were characterized with TEM, powder XRD and UV-Vis-NIR absorbance spectroscopy.

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 (Cu_1.8S) 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 Cu_1.8S 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 Cu_1.8S 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)]-Cu_1.8S. We considered all possible injection sequences of five cations (Zn²⁺, Cd²⁺, Co²⁺, In³⁺, Ga³⁺) applied to all accessible Cu_1.8S-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.

Metastable CdTe@HgTe Core@Shell Nanostructures Obtained by Partial Cation Exchange Evolve into Sintered CdTe Films Upon Annealing

Partial Hg²⁺ → Cd²⁺ cation exchange (CE) reactions were exploited to transform colloidal CdTe nanocrystals (NCs, 4-6 nm in size) into CdTe@HgTe core@shell nanostructures. This was achieved by working under a slow CE rate, which limited the exchange to the surface of the CdTe NCs. In such nanostructures, when annealed at mild temperatures (as low as 200 °C), the HgTe shell sublimated or melted and the NCs sintered together, with the concomitant desorption of their surface ligands. At the end of this process, the annealed samples consisted of ligand-free CdTe sintered films containing an amount of Hg²⁺ that was much lower than that of the starting CdTe@HgTe NCs. For example, the CdTe@HgTe NCs that initially contained 10% of Hg²⁺, after being annealed at 200 °C were transformed to CdTe sintered films containing only traces of Hg²⁺ (less than 1%). This procedure was then used to fabricate a proof-of-concept CdTe-based photodetector exhibiting a photoresponse of up to 0.5 A/W and a detectivity of ca. 9 × 10⁴ Jones under blue light illumination. Our strategy suggests that CE protocols might be exploited to lower the overall costs of production of CdTe thin films employed in photovoltaic technology, which are currently fabricated at high temperatures (above 350 °C), using post-process ligand-stripping steps.

Phase segregated Cu2-x Se/Ni3Se4 bimetallic selenide nanocrystals formed through the cation exchange reaction for active water oxidation precatalysts

Control over the composition and nanostructure of solid electrocatalysts is quite important for drastic improvement of their performance. The cation exchange reaction of nanocrystals (NCs) has been reported as the way to provide metastable crystal structures and complicated functional nanostructures that are not accessible by conventional synthetic methods. Herein we demonstrate the cation exchange-derived formation of metastable spinel Ni₃Se₄ NCs (sp-Ni₃Se₄) and phase segregated berzelianite Cu_2−xSe (ber-Cu_2−xSe)/sp-Ni₃Se₄ heterostructured NCs as active oxygen evolution reaction (OER) catalysts. A rare sp-Ni₃Se₄ phase was formed by cation exchange of ber-Cu_2−xSe NCs with Ni²⁺ ions, because both phases have the face-centered cubic (fcc) Se anion sublattice. Tuning the Ni : Cu molar ratio leads to the formation of Janus-type ber-Cu_2−xSe/sp-Ni₃Se₄ heterostructured NCs. The NCs of sp-Ni₃Se₄ and ber-Cu_2−xSe/sp-Ni₃Se₄ heterostructures exhibited high catalytic activities in the OER with small overpotentials of 250 and 230 mV at 10 mA cm⁻² in 0.1 M KOH, respectively. They were electrochemically oxidized during the OER to give hydroxides as the real active species. We anticipate that the cation exchange reaction could have enormous potential for the creation of novel heterostructured NCs showing superior catalytic performance.

Bimetallic selenide nanocrystals formed by cation exchange reaction work as a precursor of efficient water oxidation electrocatalyst.

Janus to Core-Shell to Janus: Facile Cation Movement in Cu2-xS/Ag2S Hexagonal Nanoplates Induced by Surface Strain Control

Nanocrystals with multiple compositions and heterointerfaces have received great attention due to promising multifunctional and synergistic physicochemical properties. In particular, heterointerfaces have been at the focal point of nanocatalyst research because the strain caused by lattice mismatches between different phases is the dominant determinant of surface energy and catalytic activity. The ensemble effects of different material phases have also contributed to the interest in heterointerfaced multicomponent materials. Until now, heterointerfaces have largely been regarded as static, and the dynamic movement of components within the multicomponent material phases has received little attention, although the dynamic movement of individual components within multicomponent materials can revise the interpretation of the catalytic behaviors of these materials and lead to fascinating opportunities for nanostructure synthesis. In this study, we demonstrate unprecedented cation migrations within a sulfide matrix induced by surface strain modulation initiated by cation exchange. Specifically, Cu and Ag cations in the sulfide matrix were initially segregated to form a Janus structure. This Janus configuration was then transformed into a core-shell Cu_2-xS@Ag₂S structure via surface Pt doping. When the surface strain was relieved by a reduced Pt concentration at the nanoparticle surface, the core-shell transitioned back into a Janus structure. We expect that the facile composition fluctuations in multiphasic nanostructures will expand synthetic methodologies for the design and synthesis of intricate nanostructures with useful physicochemical properties.

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) Cu_{2- x}S 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.

Janus Nanoparticle Structural Motif Control via Asymmetric Cation Exchange in Edge-Protected Cu1.81S@Ir xS y Hexagonal Nanoplates

Post-synthetic transformation of nanoparticles has received great attention, because this approach can provide an unusual route to elaborately composition-controlled nanostructures while maintaining the overall structure of the template. In principle, anisotropic heteronanoparticles of semiconductor materials can be synthesized via localized, that is, single site, cation exchange in symmetric nanoparticles. However, the differentiation of multiple identical cation exchange sites in symmetric nanoparticles can be difficult to achieve, especially for semiconductor systems with very fast cation exchange kinetics. We posited that single-site cation exchange in semiconductor nanoparticles might be realized by imposing a significant kinetic hurdle to the cation exchange reaction. The different atomic arrangements of the core and crown in core-crown structures might further differentiate the surface energies of originally identical cation exchange sites, leading to different reactivities of these sites. The first cation exchange site would be highly reactive due to the presence of a formed interface, thereby continuing to act as a site for cation exchange propagation. Herein, we present the proof-of-concept synthesis of Janus nanoparticles by using edge-protected Cu_1.81S@Ir _xS _y hexagonal nanoplates. The Janus nanoparticles comprising {Au₂S-Cu_1.81S}@Ir _xS _y or {PdS-Cu_1.81S}@Ir _xS _y exhibited dissimilar structural motifs due to the disparate cation exchange directions. This synthetic methodology exploiting cation exchange of surface-passivated semiconductor nanoparticles could fabricate the numerous symmetry-controlled Janus heterostructures.

Ab Initio Structure Determination of Cu2- xTe Plasmonic Nanocrystals by Precession-Assisted Electron Diffraction Tomography and HAADF-STEM Imaging

We investigated pseudo-cubic Cu_{2- x}Te nanosheets using electron diffraction tomography and high-resolution HAADF-STEM imaging. The structure of this metastable nanomaterial, which has a strong localized surface plasmon resonance in the near-infrared region, was determined ab initio by 3D electron diffraction data recorded in low-dose nanobeam precession mode, using a new generation background-free single-electron detector. The presence of two different, crystallographically defined modulations creates a 3D connected vacancy channel system, which may account for the strong plasmonic response of this material. Moreover, a pervasive rotational twinning is observed for nanosheets as thin as 40 nm, resulting in a tetragonal pseudo-symmetry.

Generating plasmonic heterostructures by cation exchange and redox reactions of covellite CuS nanocrystals with Au3+ ions

We demonstrate the fabrication of various types of heterostructures, including core-shells and dimers. This is achieved by reacting platelet-shaped covellite (CuS) nanocrystals (NCs) with Au³⁺ ions under various reaction conditions: the exposure of CuS NCs to Au³⁺ ions, in the presence or in the absence of ascorbic acid (AA), leads to the formation of CuS@Au core-shell nanostructures; the reaction of CuS NCs with Au³⁺ ions in the presence of oleylamine (OM) leads to the formation of CuS@Au₂S; the presence of both OM and AA leads to the formation of Au/CuS dimers. Depending on which condition is chosen, either cation exchange (CE) between gold and copper ions is predominant (leading to amorphous Au₂S) or the reduction of Au³⁺ leads to the nucleation of metallic Au domains (which are operated by the AA). In the heterostructures achieved by CE, the Au₂S shell is almost entirely amorphous, and can be converted to polycrystalline upon electron beam irradiation. Finally, when both oleylamine and AA are present in the reaction environment, Au/CuS dimers are formed due to the reduction of Au³⁺ to metallic Au domains which nucleate on top of the CuS seeds. The experimental dual plasmonic bands of the CuS@Au core-shells and Au/CuS dimers are in agreement with the theoretical optical simulations. The procedures described here enable the synthesis of core-shell nanostructures with tunable localized surface plasmon resonances (LSPRs) in the near-infrared (NIR) region, and of plasmonic metal/semiconductor heterostructures with LSPRs in both the NIR and the visible regions.

On the growth morphology and crystallography of the epitaxial Cu7Te4/CdTe interface

The manipulation of interface structures in the epitaxial growth of thin films is very essential, especially for some special heterogeneous structures. Crystallographic calculation method is a powerful tool when analyzing and predicting the structures of the coherent interfaces.

Role of the Crystal Structure in Cation Exchange Reactions Involving Colloidal Cu2Se Nanocrystals

Stoichiometric Cu₂Se nanocrystals were synthesized in either cubic or hexagonal (metastable) crystal structures and used as the host material in cation exchange reactions with Pb²⁺ ions. Even if the final product of the exchange, in both cases, was rock-salt PbSe nanocrystals, we show here that the crystal structure of the starting nanocrystals has a strong influence on the exchange pathway. The exposure of cubic Cu₂Se nanocrystals to Pb²⁺ cations led to the initial formation of PbSe unselectively on the overall surface of the host nanocrystals, generating Cu₂Se@PbSe core@shell nanoheterostructures. The formation of such intermediates was attributed to the low diffusivity of Pb²⁺ ions inside the host lattice and to the absence of preferred entry points in cubic Cu₂Se. On the other hand, in hexagonal Cu₂Se nanocrystals, the entrance of Pb²⁺ ions generated PbSe stripes "sandwiched" in between hexagonal Cu₂Se domains. These peculiar heterostructures formed as a consequence of the preferential diffusion of Pb²⁺ ions through specific (a, b) planes of the hexagonal Cu₂Se structure, which are characterized by almost empty octahedral sites. Our findings suggest that the morphology of the nanoheterostructures, formed upon partial cation exchange reactions, is intimately connected not only to the NC host material, but also to its crystal structure.

Structure and vacancy distribution in copper telluride nanoparticles influence plasmonic activity in the near-infrared

Copper chalcogenides find applications in different domains including photonics, photothermal therapy and photovoltaics. CuTe nanocrystals have been proposed as an alternative to noble metal particles for plasmonics. Although it is known that deviations from stoichiometry are a prerequisite for plasmonic activity in the near-infrared, an accurate description of the material and its (optical) properties is hindered by an insufficient understanding of the atomic structure and the influence of defects, especially for materials in their nanocrystalline form. We demonstrate that the structure of Cu_1.5±xTe nanocrystals can be determined using electron diffraction tomography. Real-space high-resolution electron tomography directly reveals the three-dimensional distribution of vacancies in the structure. Through first-principles density functional theory, we furthermore demonstrate that the influence of these vacancies on the optical properties of the nanocrystals is determined. Since our methodology is applicable to a variety of crystalline nanostructured materials, it is expected to provide unique insights concerning structure–property correlations.

CuTe nanocrystals may be used as an alternative to noble metals for plasmonics but requires understanding of the atomic structure and the influence of defects. Here Willhammar et al. use electron tomography to reveal the distribution of vacancies in the nanocrystals and their effect on the optical properties.