A Trialkylphosphine-Driven Chemical Transformation Route to Ag- and Bi-Based Chalcogenides

Kirkendall Effect-Driven Reversible Chemical Transformation for Reconfigurable Nanocrystals

The potential universality of chemical transformation principles makes it a powerful tool for nanocrystal (NC) synthesis. An example is the nanoscale Kirkendall effect, which serves as a guideline for the construction of hollow structures with different properties compared to their solid counterparts. However, even this general process is still limited in material scope, structural complexity, and, in particular, transformations beyond the conventional solid-to-hollow process. We demonstrate in this work an extension of the Kirkendall effect that drives reversible structural and phase transformations between metastable metal chalcogenides (MCs) and metal phosphides (MPs). Starting from Ni3S4/Cu1.94S NCs as the initial frameworks, ligand-regulated sequential extractions and diffusion of host/guest (S2-/P3-) anions between Ni3S4/Cu1.94S and Ni2P/Cu3P phases enable solid-to-hollow-to-solid structural motif evolution while retaining the overall morphology of the NC. An in-depth mechanistic study reveals that the transformation between metastable MCs and MPs occurs through a combination of ligand-dependent kinetic control and anion mixing-induced thermodynamic control. This strategy provides a robust platform for creating a library of reconfigurable NCs with tunable compositions, structures, and interfaces.

Chalcogen-Based Precursors for Transition Metal (Co, Ni) Phosphides: (Di)chalcogenide-to-Phosphide Transformation via Chemical Extraction of Chalcogenides

The chemical properties of polymorphic compounds are highly dependent on their stoichiometry and atomic arrangements, making certain phases technologically more important. Selective development of these phases is challenging. This study introduces a method where chalcogenide atoms from metal chalcogenides are chemically extracted by trioctylphosphine (TOP) and substituted with phosphide. Using this approach, dithiocarbamate/xanthate complexes of cobalt and nickel were employed for the selective synthesis of pure metal sulfides or phosphides. Optimization yielded either sulfur-deficient phases (Ni3S2, Co9S8) or a complete transformation to phosphides (Ni2P, CoP). Likewise, for the first time, selenobenzoate complexes of Ni and Co were used for the synthesis of transition metal diselenides (NiSe2, CoSe2), which could then be converted to metal phosphides (Ni2P, Co2P). The synthesis used solution-phase thermal decomposition with various precursors, surfactants, and temperatures. With TOP, different phases of metal chalcogenides, metal phosphides, and mixed metal phosphide nanomaterials (NiS, Ni3S2, Co9S8, NiSe2, CoSe2, Ni2P, Ni5P4, Co2P, CoP, and Ni2-xCoxP) were obtained by varying reaction conditions. The formation mechanism of nickel and cobalt phosphide nanoparticles from precursors is proposed, demonstrating that presynthesized metal chalcogenides can be transformed into phosphides, opening a new research avenue for dimensionally controlled metal chalcogenides as templates for metal phosphides.

Epitaxial growth of highly symmetrical branched noble metal-semiconductor heterostructures with efficient plasmon-induced hot-electron transfer

Epitaxial growth is one of the most commonly used strategies to precisely tailor heterostructures with well-defined compositions, morphologies, crystal phases, and interfaces for various applications. However, as epitaxial growth requires a small interfacial lattice mismatch between the components, it remains a challenge for the epitaxial synthesis of heterostructures constructed by materials with large lattice mismatch and/or different chemical bonding, especially the noble metal-semiconductor heterostructures. Here, we develop a noble metal-seeded epitaxial growth strategy to prepare highly symmetrical noble metal-semiconductor branched heterostructures with desired spatial configurations, i.e., twenty CdS (or CdSe) nanorods epitaxially grown on twenty exposed (111) facets of Ag icosahedral nanocrystal, albeit a large lattice mismatch (more than 40%). Importantly, a high quantum yield (QY) of plasmon-induced hot-electron transferred from Ag to CdS was observed in epitaxial Ag-CdS icosapods (18.1%). This work demonstrates that epitaxial growth can be achieved in heterostructures composed of materials with large lattice mismatches. The constructed epitaxial noble metal-semiconductor interfaces could be an ideal platform for investigating the role of interfaces in various physicochemical processes.

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.

Viscoelasticity Investigation of Semiconductor NP (CdS and PbS) Controlled Biomimetic Nanoparticle Hydrogels

The viscoelastic properties of colloidal nanoparticles (NPs) make opportunities to construct novel compounds in many different fields. The interparticle forces of inorganic particles on colloidal NPs are important for forming a mechanically stable particulate network especially the NP-based soft matter in the self-assembly process. Here, by capping with the same surface ligand L-glutathione (GSH), two semiconductor NP (CdS and PbS) controlled biomimetic nanoparticle hydrogels were obtained, namely, CdS@GSH and PbS@GSH. The dependence of viscoelasticity of colloidal suspensions on NP sizes, concentrations, and pH value has been investigated. The results show that viscoelastic properties of CdS@GSH are stronger than those of PbS@GSH because of stronger surface bonding ability of inorganic particles and GSH. The hydrogels formed by the smaller NPs demonstrate the higher stiffness due to the drastic change of GSH configurations. Unlike the CdS@GSH hydrogel system, the changes of NP concentrations and pH value had great influence on the PbS@GSH hydrogel system. The higher the proportion of water in the small particle size PbS@GSH hydrogel system, the greater the mechanical properties. The stronger the alkalinity in the large particle size PbS@GSH hydrogel system, the greater the hardness and storage modulus. Solution˗state nuclear magnetic resonance (NMR) indicated that the ligand GSH forms surface layers with different thickness varying from different coordination modes which are induced by different semiconductor NPs. Moreover, increasing the pH value of the PbS@GSH hydrogel system will dissociate the surface GSH molecules to form Pb²⁺ and GSH complexes which could enhance the viscoelastic properties.

Soft chemistry of metastable metal chalcogenide nanomaterials

The metastable nature of metal chalcogenide nanomaterials (MCNs) provides us with fresh perspectives and plentiful grounds in the search of new strategies for physicochemical tuning. In the past decade, numerous efforts have been devoted to synthesizing and modifying diverse emerging MCNs based on their "soft chemistry", that is, gently regulating the composition, structure, phase, and interface while not entirely disrupting the original features. This tutorial review focuses on design principles based on the metastability of MCNs, such as ion mobility and vacancy, thermal and structural instability, chemical reactivity, and phase transition, together with corresponding soft chemical approaches, including ion-exchange, catalytic growth, segregation or coupling, template grafting or transformation, and crystal-phase engineering, and summarizes recent advances in their preparation and modification. Finally, prospects for the future development of soft chemistry-directed synthetic guidelines and metastable metal chalcogenide-derived nanomaterials are proposed and highlighted.

Regioselective Construction of Chemically Transformed Phosphide-Metal Nanoheterostructures for Enhanced Hydrogen Evolution Catalysis

Engineering nanoheterostructures (NHs) plays a key role in exploring novel or enhanced physicochemical properties of nanocrystals. Despite previously reported synthetic methodologies, selective synthesis of NHs to achieve the anticipated composition and interface is still challenging. Herein, we presented a colloidal strategy for the regioselective construction of typical Ag-Co₂P NHs with precisely controlled location of Ag nanoparticles (NPs) on unique chemically transformed Co₂P nanorods (NRs) by simply changing the ratio of different surfactants. As a proof-of-concept study, the constructed heterointerface-dependent hydrogen evolution reaction (HER) catalysis was demonstrated. The multiple Ag NP-tipped Co₂P NRs exhibited the best HER performance, due to their more exposed active sites and the synergistic effect at the interfaces. Our results open up new avenues in rational design and fabrication of NHs with delicate control over the spatial distribution and interfaces between different components.

Chemical conversion synthesis of magnetic Fe1-xCox alloy nanosheets with controlled composition

Chemical conversion provides a versatile platform for the synthesis of advanced nanomaterials with targeted phase, composition, and architecture. Here, we report a trioctylphosphine (TOP)-driven chemical conversion route to transform lamellar Fe_1-xCo_xS_1.2-DETA (x = 0.1, 0.3, 0.5, 0.7, DETA = diethylenetriamine) inorganic-organic hybrid solid solutions into two-dimensional (2D) single-crystal Fe_1-xCo_x alloy with controllable composition and dimensionality. Synergetic transformation coupled with DETA removal and sulfur extraction of lamellar Fe_0.9Co_0.1S_1.2-DETA hybrids was examined in detail. The highest magnetization of 175 emu g^-1 was recorded for the prepared Fe_0.7Co_0.3. Our results not only provide a new lamellar inorganic-organic hybrid solid solution but also extend the chemical conversion strategy to the synthesis of previously unavailable magnetic alloy nanosheets.

Topochemical synthesis of low-dimensional nanomaterials

Over the past several decades, nanomaterials have been extensively studied owing to having a series of unique physical and chemical properties that exceed those of conventional bulk materials. Researchers have developed a lot of strategies for the synthesis of low-dimensional nanomaterials. Among them, topochemical synthesis has attracted increasing attention because it can provide more new nanomaterials by improving and upgrading inexpensive and accessible nanomaterials. In this review, we summarize and analyze many existing topochemical synthesis methods, including selective etching, liquid phase reactions, high-temperature atmosphere reactions, electrochemically assisted methods, etc. The future direction of topochemical synthesis is also proposed.

Soft-Chemical Method for Synthesizing Intermetallic Antimonide Nanocrystals from Ternary Chalcogenide

The synthesis of intermetallic antimonides usually depends on either the high-temperature alloying technique from high-purity metals or the flux method in highly poisonous Pb-melt. In this paper, we introduced a soft-chemical method to synthesize intermetallic antimonides from ternary chalcogenide precursors under an argon atmosphere below 200 °C. Powder X-ray diffraction and compositional analysis clearly indicate that a new phase of the Ag₃Sb nanocrystal was synthesized from the Ag₃SbS₃ precursors. Three types of trialkylphosphines (TAPs) were applied as desulfurization agents, and the transformation mechanism was elucidated. The capability of the desulfurization agent follows the sequence of triphenylphosphine (TPP) > tributylphosphine (TBP) > trioctylphosphine (TOP). Besides, this TAP-driven desulfurization route to synthesize the intermetallic phase could also be possible for AgSbSe₂ and Sb₂S₃. Therefore, this paper provides an efficient and mild technique for the fabrication of intermetallic nanocrystals.

Ligand-Solvent Compatibility: The Unsung Hero in the Digestive Ripening Story

Digestive ripening (DR) is a process where a polydisperse nanocrystal (NC) system is converted into a monodisperse one with the aid of thermal heating of NCs in the presence of an excess surface-active organic ligand called digestive ripening agent (DRA) and a solvent. Here, we demonstrate that the solvent-DRA compatibility influences the final size and size distribution of the NCs in a significant manner. Accordingly, in this study, using the DR of gold NCs as the test case with alkanethiol (decanethiol/C10HT) and fluorinated thiol (1 H,1 H,2 H,2 H-perfluorodecanethiol/C10FT) as DRA's and toluene and α,α,α-trifluoro-toluene (TFT) and their combination as solvents, we clearly establish that alkanethiols result in best-quality NCs after DR in toluene while the fluorinated thiols provide reasonably monodispersed NCs in TFT. Our results also ascertain that even when DR is carried out in a mixture of solvents, as long as the compatible solvent is the major component, the DR process results in reasonably monodisperse NCs. As soon as the amount of uncompatible solvent exceeds a threshold limit, there is perceptible increase in the polydispersity of the NCs. We conclude that the polarity of the solvent, which affects the buildup of ligated atoms/clusters, plays a key role in controlling the size distributions of the NCs.

Phase-Controlled Synthesis of High-Bi-Ratio Ternary Sulfide Nanocrystals of Cu1.57 Bi4.57 S8 and Cu2.93 Bi4.89 S9

High Bi-ratio ternary sulfides have been recently reported as superior thermoelectric materials. However, the synthesis of high Bi-ratio Cu-Bi-S nanocrystal remains a challenge. Reported here are the synthesis and characterization of three-phase Cu-Bi-S nanocrystals with the nominal chemical formulae of Cu_1.57 Bi_4.57 S₈ , Cu_2.93 Bi_4.89 S₉ and Cu₃ BiS₃ . The samples were prepared using a Bi₂ S₃ precursor by varying the amount and type of Cu_2-x S (i. e. Cu₂ S or Cu_7.2 S₄ ) reactants. TEM images reveal that two new samples crystalized having nanorod morphology with radii of approximately 50 nm and lengths of 200 nm. XPS results indicate that the valence states of Bi in both the two new phases are +3 with viable oxidation states for Cu. UV-Vis-NIR absorption spectroscopy reveals that narrow direct bandgaps are 1.12 and 1.27 eV for Cu_1.57 Bi_4.57 S₈ and Cu_2.93 Bi_4.89 S₉ , respectively. Besides, this method could also be applied to synthesize the Cu₃ BiS₃ phase with a new nanoplate morphology. The as-synthesized Cu-Bi-S samples show Cu/Bi ratio-dependent photoresponsive properties. This study not only reports the structure and bandgap of two ternary sulfides, which have only been discovered in the mineral previously, but also provides an efficient method for synthesizing Bi-rich ternary chalcogenide nanocrystals.

Modulated Triple-Material Nano-Heterostructures: Where Gold Influenced the Chemical Activity of Silver in Nanocrystals

For efficient charge separations, multimaterial hetero-nanostructures are being extensively studied as photocatalysts. While materials with one heterojunction are widely established, the chemistry of formation of multijunction heterostructures is not explored. This needs a more sophisticated approach and modulations. To achieve these, a generic multistep seed mediated growth following controlled ion diffusion and ion exchange is reported which successfully leads to triple-material hetero-nanostructures with bimetallic-binary alloy-binary/ternary semiconductors arrangements. Ag2 S nanocrystals are used as primary seeds for obtaining AuAg-AuAgS bimetallic-binary alloyed metal-semiconductor heterostructures via partial reduction of Ag(I) using Au(III) ions. These are again explored as secondary seeds for obtaining a series of triple-materials heterostructures, AuAg-AuAgS-CdS (or ZnS or AgInS2 ), with introduction of different divalent and trivalent ions. Chemistry of each step of the gold ion-induced changes in the rate of diffusion and/or ion exchanges are investigated and the formation mechanism for these nearly monodisperse triple material heterostructures are proposed. Reactions without gold are also performed, and the change in the reaction chemistry and growth mechanism in presence of Au is also discussed.

Generic and Scalable Method for the Preparation of Monodispersed Metal Sulfide Nanocrystals with Tunable Optical Properties

A rational synthetic method that produces monodisperse and air-stable metal sulfide colloidal quantum dots (CQDs) in organic nonpolar solvents using octyl dithiocarbamic acid (C₈DTCA) as a sulfur source, is reported. The fast decomposition of metal-C₈DTCA complexes in presence of primary amines is exploited to achieve this purpose. This novel technique is generic and can be applied to prepare diverse CQDs, like CdS, MnS, ZnS, SnS, and In₂S₃, including more useful and in-demand PbS CQDs and plasmonic nanocrystals of Cu₂S. Based on several control reactions, it is postulated that the reaction involves the in situ formation of a metal-C₈DTCA complex, which then reacts in situ with oleylamine at slightly elevated temperature to decompose into metal sulfide CQDs at a controlled rate, leading to the formation of the materials with good optical characteristics. Controlled sulfur precursor's reactivity and stoichiometric reaction between C₈DTCA and metal salts affords high conversion yield and large-scale production of monodisperse CQDs. Tunable and desired crystal size could be achieved by controlling the precursor reactivity by changing the reaction temperature and reagent ratios. Finally, the photovoltaic devices fabricated from PbS CQDs displayed a power conversion efficiency of 4.64% that is comparable with the reported values of devices prepared with PbS CQDs synthesized by the standard methods.

Multifunctional Cu-Ag2S nanoparticles with high photothermal conversion efficiency for photoacoustic imaging-guided photothermal therapy in vivo

Photothermal therapy (PTT) has attracted increasing interest and become widely used in cancer therapy owing to its noninvasiveness and low level of systemic adverse effects. However, there is an urgent need to develop biocompatible and multifunctional PTT agents with high photothermal conversion efficiency. Herein, biocompatible Cu-Ag₂S/PVP nanoparticles (NPs) with strong near-infrared absorption and high photothermal conversion efficiency were successfully synthesized for high-performance photoacoustic (PA) imaging-guided PTT in vivo. The novel Cu-Ag₂S/PVP NPs feature high photothermal conversion efficiency (58.2%) under 808 nm light irradiation, noticeably higher than those of most reported PTT agents. Because of their good dispersibility, Cu-Ag₂S/PVP NPs passively accumulate within tumors via the enhanced permeability and retention effect, which can be confirmed by PA imaging, photothermal performance, and biodistribution in vivo. Furthermore, Cu-Ag₂S/PVP NPs are thoroughly cleared through feces and urine within seven days, indicating a high level of biosafety for further potential clinical translation.

AgInS2-ZnS Quantum Dots: Excited State Interactions with TiO2 and Photovoltaic Performance

Multinary quantum dots such as AgInS₂ and alloyed AgInS₂-ZnS are an emerging class of semiconductor materials for applications in photovoltaic and display devices. The nanocrystals of (AgInS₂)_x-(ZnS)_1-x (for x = 0.67) exhibit a broad emission with a maximum at 623 nm and interact strongly with TiO₂ nanostructures by injecting electrons from the excited state. The electron transfer rate constant as determined from transient absorption spectroscopy was 1.8 × 10¹⁰ s^-1. The photovoltaic performance was evaluated over a period of a few weeks to demonstrate the stability of AgInS₂-ZnS when utilized as sensitizers in solar cells. We report a power conversion efficiency of 2.25% of our champion cell 1 month after its fabrication. The limitations of AgInS₂-ZnS nanocrystals in achieving greater solar cell efficiency are discussed.

Synthesis of WOn -WX2 (n=2.7, 2.9; X=S, Se) Heterostructures for Highly Efficient Green Quantum Dot Light-Emitting Diodes

Preparation of two-dimensional (2D) heterostructures is important not only fundamentally, but also technologically for applications in electronics and optoelectronics. Herein, we report a facile colloidal method for the synthesis of WO_n -WX₂ (n=2.7, 2.9; X=S, Se) heterostructures by sulfurization or selenization of WO_n nanomaterials. The WO_n -WX₂ heterostructures are composed of WO_2.9 nanoparticles (NPs) or WO_2.7 nanowires (NWs) grown together with single- or few-layer WX₂ nanosheets (NSs). As a proof-of-concept application, the WO_n -WX₂ heterostructures are used as the anode interfacial buffer layer for green quantum dot light-emitting diodes (QLEDs). The QLED prepared with WO_2.9 NP-WSe₂ NS heterostructures achieves external quantum efficiency (EQE) of 8.53 %. To our knowledge, this is the highest efficiency in the reported green QLEDs using inorganic materials as the hole injection layer.

Sub-1.1 nm ultrathin porous CoP nanosheets with dominant reactive {200} facets: a high mass activity and efficient electrocatalyst for the hydrogen evolution reaction

The exploration of a facile strategy to synthesize porous ultrathin nanosheets of non-layered materials, especially with exposed reactive facets, as highly efficient electrocatalysts for the hydrogen evolution reaction (HER), remains challenging. Herein we demonstrate a chemical transformation strategy to synthesize porous CoP ultrathin nanosheets with sub-1.1 nm thickness and exposed {200} facets via phosphidation of Co3O4 precursors. The resultant samples exhibit outstanding electrochemical HER performance: a low overpotential (only 56 and 131 mV are required for current densities of 10 and 100 mA cm-2, respectively), a small Tafel slope of 44 mV per decade, a good stability of over 20 h, and a high mass activity of 151 A g-1 at an overpotential of 100 mV. The latter is about 80 times higher than that of CoP nanoparticles. Experimental data and density functional theory calculations reveal that a high proportion of reactive {200} facets, high utilization efficiency of active sites, metallic nature, appropriate structural disorder, facile electron/mass transfer and rich active sites benefiting from the unique ultrathin and porous structure are the key factors for the greatly improved activity. Additionally, this facile chemical conversion strategy can be developed to a generalized method for preparing porous ultrathin nanosheets of CoSe2 and CoS that cannot be obtained using other methods.

1D Colloidal Hetero-Nanomaterials with Programmed Semiconductor Morphology and Metal Location for Enhancing Solar Energy Conversion

A new kind of multitetrahedron sheath ternary ZnS-(CdS/Au) hetero-nanorod is prepared, in which one 1D ultrathin ZnS nanorod is integrated with segmented tetrahedron sheaths made of CdS, and more importantly, Au nanoparticles can be decorated in a targeted manner onto the vertexes and edges of CdS tetrahedron sheaths solely, for achieving performance improvement in photoelectric and photochemical conversion applications.

Converting 2D inorganic-organic ZnSe-DETA hybrid nanosheets into 3D hierarchical nanosheet-based ZnSe microspheres with enhanced visible-light-driven photocatalytic performances

Engineering two-dimensional (2D) nanosheets into three-dimensional (3D) hierarchical structures is one of the great challenges in nanochemistry and materials science. We report a facile and simple chemical conversion route to fabricate 3D hierarchical nanosheet-based ZnSe microspheres by using 2D inorganic-organic hybrid ZnSe-DETA (DETA = diethylenetriamine) nanosheets as the starting precursors. The conversion mechanism involves the controlled depletion of the organic-component (DETA) from the hybrid precursors and the subsequent self-assembly of the remnant inorganic-component (ZnSe). The transformation reaction of ZnSe-DETA nanosheets is mainly influenced by the concentration of DETA in the reaction solution. We demonstrated that this organic-component depletion method could be extended to the synthesis of other hierarchical structures of metal sulfides. In addition, the obtained hierarchical nanosheet-based ZnSe microspheres exhibited outstanding performance in visible light photocatalytic degradation of methyl orange and were highly active for photocatalytic H2 production.