Anisotropic 2D Cu2-x Se Nanocrystals from Dodecaneselenol and Their Conversion to CdSe and CuInSe2 Nanoparticles

Molybdenum Disulfide Induced Phase Control Synthesis of Multi-dimensional Co3S4-MoS2 Heteronanostructures via Cation Exchange

Phase control over cation exchange (CE) reactions has emerged as an important approach for the synthesis of nanomaterials (NMs), enabling precise determination of their reactivity and properties. Although factors such as crystal structure and morphology have been studied for the phase engineering of CE reactions in NMs, there remains a lack of systematic investigation to reveal the impact for the factors in heterogeneous materials. Herein, we report a molybdenum disulfide induced phase control method for synthesizing multidimensional Co3S4-MoS2 heteronanostructures (HNs) via cation exchange. MoS2 in parent Cu1.94S-MoS2 HNs are proved to affect the thermodynamics and kinetics of CE reactions, and facilitate the formation of Co3S4-MoS2 HNs with controlled phase. This MoS2 induced phase control method can be extended to other parent HNs with multiple dimensions, which shows its diversity. Further, theoretical calculations demonstrate that Co3S4 (111)/MoS2 (001) exhibits a higher adhesion work, providing further evidence that MoS2 enables phase control in the HNs CE reactions, inducing the generation of novel Co3S4-MoS2 HNs. As a proof-of-concept application for crystal phase- and dimensionality-dependent of cobalt sulfide based HNs, the obtained Co3S4-MoS2 heteronanoplates (HNPls) show remarkable performance in hydrogen evolution reactions (HER) under alkaline media. This synthetic methodology provides a unique design strategy to control the crystal structure and fills the gap in the study of heterogeneous materials on CE reaction over phase engineering that are otherwise inaccessible.

Single-step synthesis of ternary metal chalcogenides (sf-CuInS2 and sf-CuInSe2) stripped off the organic cover and their use as a catalyst for symmetric Glaser-Hay coupling reactions

Generally, inorganic nano/microparticles produced by chemical routes are covered by organic surfactants or polymers to control their agglomeration during their synthesis. However, these surfactants and polymers negatively affect their catalytic activity because these molecules mask the surface. This work presents the synthesis of surfactant-free CuInS2 and CuInSe2 (sf-CuInS2 and sf-CuInSe2) nano/microparticles through simple reactions without surfactant or polymer coatings using LiBH4 under a thermodynamically favourable condition. These reactions are rare observations of a single-step process to produce ternary metal chalcogenides without any template assistance. We have also demonstrated efficient catalysis by sf-CuInS2 nanoparticles in the coupling reaction of substituted phenylacetylenes. We tested it as catalysts in dimerizing 1,3-diyne derivatives while using 8-diazabicyclo[5.4.0]undec-7-ene (DBU) as the base. These Glassar-Hay coupling reactions are conducted at room temperature in acetonitrile (4-7 h, depending on the substrate) using 10 mg of sf-CuInS2. The maximum yield obtained in these reactions is 97%, while the catalyst is reusable for five cycles with little difference in its ability to catalyse. The effectiveness of the catalyst is credited to the availability of a free catalytic surface.

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.

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.

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.

Remarkable Electrical Conductivity Increase and Pure Metallic Properties from Semiconducting Colloidal Nanocrystals by Cation Exchange for Solution-Processable Optoelectronic Applications

The authors report a strategic approach to achieve metallic properties from semiconducting CuFeS colloidal nanocrystal (NC) solids through cation exchange method. An unprecedentedly high electrical conductivity is realized by the efficient generation of charge carriers onto a semiconducting CuS NC template via minimal Fe exchange. An electrical conductivity exceeding 10 500 S cm^-1 (13 400 S cm^-1 at 2 K) and a sheet resistance of 17 Ω/sq at room temperature, which are among the highest values for solution-processable semiconducting NCs, are achieved successfully from bornite-phase CuFeS NC films possessing 10% Fe atom. The temperature dependence of the corresponding films exhibits pure metallic characteristics. Highly conducting NCs are demonstrated for a thermoelectric layer exhibiting a high power factor over 1.2 mW m^-1 K^-2 at room temperature, electrical wires for switching on light emitting diods (LEDs), and source-drain electrodes for p- and n-type organic field-effect transistors. Ambient stability, eco-friendly composition, and solution-processability further validate their sustainable and practical applicability. The present study provides a simple but very effective method for significantly increasing charge carrier concentrations in semiconducting colloidal NCs to achieve metallic properties, which is applicable to various optoelectronic devices.

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.

2D II-VI Semiconductor Nanoplatelets: From Material Synthesis to Optoelectronic Integration

The field of colloidal synthesis of semiconductors emerged 40 years ago and has reached a certain level of maturity thanks to the use of nanocrystals as phosphors in commercial displays. In particular, II-VI semiconductors based on cadmium, zinc, or mercury chalcogenides can now be synthesized with tailored shapes, composition by alloying, and even as nanocrystal heterostructures. Fifteen years ago, II-VI semiconductor nanoplatelets injected new ideas into this field. Indeed, despite the emergence of other promising semiconductors such as halide perovskites or 2D transition metal dichalcogenides, colloidal II-VI semiconductor nanoplatelets remain among the narrowest room-temperature emitters that can be synthesized over a wide spectral range, and they exhibit good material stability over time. Such nanoplatelets are scientifically and technologically interesting because they exhibit optical features and production advantages at the intersection of those expected from colloidal quantum dots and epitaxial quantum wells. In organic solvents, gram-scale syntheses can produce nanoparticles with the same thicknesses and optical properties without inhomogeneous broadening. In such nanoplatelets, quantum confinement is limited to one dimension, defined at the atomic scale, which allows them to be treated as quantum wells. In this review, we discuss the synthetic developments, spectroscopic properties, and applications of such nanoplatelets. Covering growth mechanisms, we explain how a thorough understanding of nanoplatelet growth has enabled the development of nanoplatelets and heterostructured nanoplatelets with multiple emission colors, spatially localized excitations, narrow emission, and high quantum yields over a wide spectral range. Moreover, nanoplatelets, with their large lateral extension and their thin short axis and low dielectric surroundings, can support one or several electron-hole pairs with large exciton binding energies. Thus, we also discuss how the relaxation processes and lifetime of the carriers and excitons are modified in nanoplatelets compared to both spherical quantum dots and epitaxial quantum wells. Finally, we explore how nanoplatelets, with their strong and narrow emission, can be considered as ideal candidates for pure-color light emitting diodes (LEDs), strong gain media for lasers, or for use in luminescent light concentrators.

Room-Temperature Interconversion Between Ultrathin CdTe Magic-Size Nanowires Induced by Ligand Shell Dynamics

The formation mechanisms of colloidal magic-size semiconductor nanostructures have remained obscure. Herein, we report the room temperature synthesis of three species of ultrathin CdTe magic-size nanowires (MSNWs) with diameters of 0.7 ± 0.1 nm, 0.9 ± 0.2 nm, and 1.1 ± 0.2 nm, and lowest energy exciton transitions at 373, 418, and 450 nm, respectively. The MSNWs are obtained from Cd(oleate)₂ and TOP-Te, provided diphenylphosphine and a primary alkylamine (RNH₂) are present at sufficiently high concentrations, and exhibit sequential, discontinuous growth. The population of each MSNW species is entirely determined by the RNH₂ concentration [RNH₂] so that single species are only obtained at specific concentrations, while mixtures are obtained at concentrations intermediate between the specific ones. Moreover, the MSNWs remain responsive to [RNH₂], interconverting from thinner to thicker upon [RNH₂] decrease and from thicker to thinner upon [RNH₂] increase. Our results allow us to propose a mechanism for the formation and interconversion of CdTe MSNWs and demonstrate that primary alkylamines play crucial roles in all four elementary kinetic steps (viz., monomer formation, nucleation, growth in length, and interconversion between species), thus being the decisive element in the creation of a reaction pathway that leads exclusively to CdTe MSNWs. The insights provided by our work thus contribute toward unravelling the mechanisms behind the formation of shape-controlled and atomically precise magic-size semiconductor nanostructures.

Demand-oriented construction of Mo3S13-LDH: A versatile scavenger for highly selective and efficient removal of toxic Ag(I), Hg(II), As(III), and Cr(VI) from water

Inspired by the classic ion-exchange reaction, a single phase material of Mg_0.66Al_0.34(OH)₂(Mo₃S₁₃)_0.03(NO₃)_0.14(CO₃)_0.07·H₂O (Mo₃S₁₃-LDH) was masterly constructed by intercalating Mo₃S₁₃^2- into the MgAl-LDH gallery. Prepared Mo₃S₁₃-LDH displays excellent binding affinity and high selectivity for Ag(I) and Hg(II) in a mixed solution, in which an apparent selectivity order of Hg(II) > Ag(I) ≫ Pb(II), Cu(II), Ni(II), Co(II), Cd(II), and Mn(II) is observed. Enormous capture capacities (q_m^Ag = 446.4 mg/g, q_m^Hg = 354.6 mg/g) and fast equilibration time (within 60 min) place Mo₃S₁₃-LDH in the upper ranks of materials for such removal. For oxoanions, As(III) (HAsO₃^2-) and Cr(VI) (CrO₄^2-) can be specifically trapped by Mo₃S₁₃-LDH with comparable loading ability (q_m^As = 61.8 mg/g, q_m^Cr = 90.6 mg/g) in the coexistence of multiple interfering anions. Notably, high Hg(II) and Cr(VI) concentrations are finally reduced below the safe limit of drinking water. The excellent capture capacity of Mo₃S₁₃-LDH benefits from the rational design by following two aspects: (i) the multiple sulfur ligands in Mo₃S₁₃^2- groups give place to various capture modes and different affinity orders for target ions, and (ii) large-sized Mo₃S₁₃^2- groups widen the interlayer spacing of LDH, thereby accelerating the mass transfer process. Furthermore, the satisfactory structural stability of Mo₃S₁₃-LDH is also reflected through the unchanged hexagonal prismatic shape after adsorption. All of these highlight the great potential of Mo₃S₁₃-LDH for the application in water remediation.

Two-dimensional copper based colloidal nanocrystals: synthesis and applications

Two-dimensional (2D) semiconductor nanocrystals display unconventional physical and opto-electronic properties due to their ultrathin and unique electronic structures. Since the success of Cd-based photoemissive nanocrystals, the development of sustainable and low-cost nanocrystals with enhanced electronic and physical properties has become a central research theme. In this context, copper-based semiconductor 2D nanocrystals, the cost-effective and eco-friendly alternative, exhibit unique plasmonic resonance, transport properties, and high ionic conductivity beneficial for sensing, energy storage, conversion, and catalytic applications. This review summarizes recent progress in the colloidal synthesis, growth mechanisms, properties, and applications of 2D copper-based nanostructures with tunable compositions, dimensions, and crystal phases. We highlight the growth mechanisms concerning their shape evolution in two dimensions. We analyse the effectiveness of cation exchange as a tool to synthesize multinary nanocrystals. Based on the preparation of Cu-based chalcogenide and non-chalcogenide compositions, we discuss synthesis control achieved via colloidal approaches to allow dimension tunability, phase engineering, and plasmonic and thermoelectric property optimization. Furthermore, their potential in various applications of catalysis, energy storage, and sensing is reviewed. Finally, we address the current challenges associated with 2D Cu-based nanocrystal development and provide an outlook pertaining to unexplored research areas.

Alkyl selenol reactivity with common solvents and ligands: influences on phase control in nanocrystal synthesis

This study develops mechanistic understanding of the factors which control the phase in syntheses of copper selenide nanocrystals by investigating how the chemistry of the dodecylselenol reactant is altered by the ligand and solvent environment. ¹H NMR and ⁷⁷Se NMR were used to study how commonly used solvents (octadecene and dioctylether) and ligands (oleylamine, oleic acid, stearylamine, stearic acid and trioctyl phosphine) change the nature of the dodecylselenol reactant at 25 °C, 155 °C and 220 °C. Unsaturations were prone to selenol additons, carboxylates underwent selenoesterification, amines caused the release of H₂Se gas, and the phosphine formed phosphine selenide. Adventitious water caused oxidation to didodecyldiselenide. NMR studies were correlated with the phases that resulted in syntheses of nanocrystalline copper selenides, in which berzalianite, umangite or a metastable hexagonal phase were produced as identified by X-ray diffraction, depending on the ligand and solvent environemnts. Formation of the rare hexagonal Cu_2-xSe phase could be assigned to cases that included DD₂Se₂ as a reactive intermediate, or strong L-type ligation of amines which was dependant on alkyl chain length.

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.

Metal Cation Valency Dependence in Morphology Evolution of Cu2-x S Nanodisk Seeds and Their Pseudomorphic Cation Exchanges

A crucial parameter in the design of semiconductor nanoparticles (NPs) with controllable optical, magnetic, electronic, and catalytic properties is the morphology. Herein, we demonstrate the potential of additive metal cations with variable valency to direct the morphology evolution of copper-deficient Cu_2-x S nanoparticles in the process of seed-mediated growth. In particular, the djurleite Cu_1.94 S seed could evolve from disk into tetradecahedron in the presence of tin(IV) cations, whereas they merely formed sharp hexagonal nanodisks with tin(II) cations. In addition to djurleite Cu_1.94 S, the tin(IV) cations could be generalized to direct the growth of roxbyite Cu_1.8 S and covellite CuS nanodisk seeds into tetradecahedra. We further perform pseudomorphic cation exchanges of Cu_1.94 S tetradecahedra with Zn²⁺ and Cd²⁺ to produce polyhedral zinc sulfide (ZnS) and cadmium sulfide (CdS) NPs. Moreover, we achieve Cu_1.8 S/ZnS and Cu_1.94 S/CdS tetradecahedral heterostructures via partial cation exchange, which are otherwise inaccessible by traditional synthetic approaches.

Controllable synthesis of non-layered two-dimensional plate-like CuGaSe2 materials for optoelectronic devices

CuGaSe2 semiconductor materials, as an important member of the I-III-VI2 family, have sparked tremendous attention due to their fascinating structure-related properties and promising applications in solar energy storage and conversion. Nevertheless, the controllable preparation of two-dimensional (2D) CuGaSe2 structures is still a daunting challenge owing to the intrinsic non-layered crystal structure and inaccessible reactivity-matching of multiple reaction precursors, which will seriously impede the much deeper research progress on their properties and applications. Herein, non-layered 2D CuGaSe2 plates possessing high crystallinity, and uniform size and morphology have been first synthesized by a feasible cation exchange strategy. Because the fabrication of 2D CuGaSe2 crystals is rarely reported, a particular highlight is laid on the compositional analysis, structural characterization, and formation mechanism. Furthermore, the optical absorption and optoelectronic measurements reveal that the as-synthesized CuGaSe2 plates exhibit high light harvesting capacity and excellent photoelectric performance. This study opens up a new avenue for the feasible fabrication of non-layered CuGaSe2 plates possessing a high-quality crystalline structure and provides a promising candidate for the development of novel solar energy conversion and storage devices.

Multinary copper-based chalcogenide nanocrystal systems from the perspective of device applications

Multinary chalcogenide semiconductor nanocrystals are a unique class of materials as they offer flexibility in composition, structure, and morphology for controlled band gap and optical properties. They offer a vast selection of materials for energy conversion, storage, and harvesting applications. Among the multinary chalcogenides, Cu-based compounds are the most attractive in terms of sustainability as many of them consist of earth-abundant elements. There has been immense progress in the field of Cu-based chalcogenides for device applications in the recent years. This paper reviews the state of the art synthetic strategies and application of multinary Cu-chalcogenide nanocrystals in photovoltaics, photocatalysis, light emitting diodes, supercapacitors, and luminescent solar concentrators. This includes the synthesis of ternary, quaternary, and quinary Cu-chalcogenide nanocrystals. The review also highlights some emerging experimental and computational characterization approaches for multinary Cu-chalcogenide semiconductor nanocrystals. It discusses the use of different multinary Cu-chalcogenide compounds, achievements in device performance, and the recent progress made with multinary Cu-chalcogenide nanocrystals in various energy conversion and energy storage devices. The review concludes with an outlook on some emerging and future device applications for multinary Cu-chalcogenides, such as scalable luminescent solar concentrators and wearable biomedical electronics.

Sonochemical synthesis of graphitic carbon nitrides-wrapped bimetal oxide nanoparticles hybrid materials and their electrocatalytic activity for xanthine electro-oxidation

A novel network-like magnetic nanoparticle was fabricated on a graphitic carbon nitride through a facile sonochemical route at frequency 20 kHz and power 70 W. To enhance the electrocatalytic activity of the modified materials, the graphitic carbon nitrides (g-C₃N₄) was prepared from melamine. Monitoring of xanthine concentration level in biological fluids is more important for clinical diagnosis and medical applications. As modified CuFe₂O₄/g-C₃N₄ nanocomposite exhibits better electrochemical activity towards the oxidation of xanthine with higher anodic current compared to other modified and unmodified electrode for the detection of xanthine with larger linear range (0.03-695 µM) and lower limit of detection (13.2 nM). To compare with these methods, the electrochemical techniques may be an alternative high sensitive method due to their simplicity and rapid detection time. In addition, the practical feasibility of the sensor was inspected with biological samples, reveals the acceptable recovery of the sensor in real samples.

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.

Tailoring Cu+ for Ga3+ Cation Exchange in Cu2-xS and CuInS2 Nanocrystals by Controlling the Ga Precursor Chemistry

Nanoscale cation exchange (CE) has resulted in colloidal nanomaterials that are unattainable by direct synthesis methods. Aliovalent CE is complex and synthetically challenging because the exchange of an unequal number of host and guest cations is required to maintain charge balance. An approach to control aliovalent CE reactions is the use of a single reactant to both supply the guest cation and extract the host cation. Here, we study the application of GaCl₃-L complexes [L = trioctylphosphine (TOP), triphenylphosphite (TPP), diphenylphosphine (DPP)] as reactants in the exchange of Cu⁺ for Ga³⁺ in Cu_2-xS nanocrystals. We find that noncomplexed GaCl₃ etches the nanocrystals by S^2- extraction, whereas GaCl₃-TOP is unreactive. Successful exchange of Cu⁺ for Ga³⁺ is only possible when GaCl₃ is complexed with either TPP or DPP. This is attributed to the pivotal role of the Cu_2-xS-GaCl₃-L activated complex that forms at the surface of the nanocrystal at the onset of the CE reaction, which must be such that simultaneous Ga³⁺ insertion and Cu⁺ extraction can occur. This requisite is only met if GaCl₃ is bound to a phosphine ligand, with a moderate bond strength, to allow facile dissociation of the complex at the nanocrystal surface. The general validity of this mechanism is demonstrated by using GaCl₃-DPP to convert CuInS₂ into (Cu,Ga,In)S₂ nanocrystals, which increases the photoluminescence quantum yield 10-fold, while blue-shifting the photoluminescence into the NIR biological window. This highlights the general applicability of the mechanistic insights provided by our work.