Architectural design of heterogeneous metallic nanocrystals--principles and processes

Intermetallic Nanocrystals for Fuel-Cells-Based Electrocatalysis

Electrocatalysis underpins the renewable electrochemical conversions for sustainability, which further replies on metallic nanocrystals as vital electrocatalysts. Intermetallic nanocrystals have been known to show distinct properties compared to their disordered counterparts, and been long explored for functional improvements. Tremendous progresses have been made in the past few years, with notable trend of more precise engineering down to an atomic level and the investigation transferring into more practical membrane electrode assembly (MEA), which motivates this timely review. After addressing the basic thermodynamic and kinetic fundamentals, we discuss classic and latest synthetic strategies that enable not only the formation of intermetallic phase but also the rational control of other catalysis-determinant structural parameters, such as size and morphology. We also demonstrate the emerging intermetallic nanomaterials for potentially further advancement in energy electrocatalysis. Then, we discuss the state-of-the-art characterizations and representative intermetallic electrocatalysts with emphasis on oxygen reduction reaction evaluated in a MEA setup. We summarize this review by laying out existing challenges and offering perspective on future research directions toward practicing intermetallic electrocatalysts for energy conversions.

Plasmonic Au-Cu nanostructures: Synthesis and applications

Plasmonic Au-Cu nanostructures composed of Au and Cu metals, have demonstrated advantages over their monolithic counterparts, which have recently attracted considerable attention. Au-Cu nanostructures are currently used in various research fields, including catalysis, light harvesting, optoelectronics, and biotechnologies. Herein, recent developments in Au-Cu nanostructures are summarized. The development of three types of Au-Cu nanostructures is reviewed, including alloys, core-shell structures, and Janus structures. Afterwards, we discuss the peculiar plasmonic properties of Au-Cu nanostructures as well as their potential applications. The excellent properties of Au-Cu nanostructures enable applications in catalysis, plasmon-enhanced spectroscopy, photothermal conversion and therapy. Lastly, we present our thoughts on the current status and future prospects of the Au-Cu nanostructures research field. This review is intended to contribute to the development of fabrication strategies and applications relating to Au-Cu nanostructures.

Ultrathin Nanosheet-Supported Ag@Ag2O Core-Shell Nanoparticles with Vastly Enhanced Photothermal Conversion Efficiency for NIR-II-Triggered Photothermal Therapy

Photothermal therapy (PTT) working in the second near-infrared (NIR-II) region has aroused a huge interest due to its potential application in terms of clinical cancer treatment. However, owing to the lack of photothermal nanoagents with high photothermal conversion efficiency, NIR-II-driven PTT still suffers from poor efficiency and subsequent cancer recurrence. In this work, we show a new and highly efficient preparation approach for NIR-II photothermal nanoagents and tailor ultrathin layered double hydroxide (LDH)-supported Ag@Ag₂O core-shell nanoparticles (Ag@Ag₂O/LDHs-U), vastly improving NIR-II photothermal performance. A combination study (high-resolution transmission electron microscopy (HRTEM), extended X-ray absorption fine structure spectroscopy (EXAFS), and X-ray photoelectron spectroscopy (XPS)) verifies that ultrafine Ag@Ag₂O core-shell nanoparticles (∼3.8 nm) are highly dispersed and firmly immobilized within ultrathin LDH nanosheets, and their Ag₂O shell possesses abundant vacancy-type defects. These unique Ag@Ag₂O/LDHs-U display an impressive photothermal conversion efficiency as high as 76.9% at 1064 nm. Such an excellent photothermal performance is likely attributed to the enhanced localized surface plasmon resonance (LSPR) coupling effect between Ag and Ag₂O and the reduced band gap caused by vacancy-type defects in the Ag₂O shell. Meanwhile, Ag@Ag₂O/LDHs-U also show prominent photothermal stability, due to the unique supported core-shell nanostructure. Moreover, both in vitro and in vivo studies further confirm that Ag@Ag₂O/LDHs-U possess good biocompatible properties and outstanding PTT therapeutic efficacy in the NIR-II region. This research shows a new strategy in the rational design and preparation of an efficient photothermal agent, which is helpful to achieve more accurate and effective cancer theranostics.

Bimetallic Janus Nanocrystals: Syntheses and Applications

Bimetallic Janus nanocrystals have received considerable interest in recent years owing to their unique properties and niche applications. The side-by-side distribution of two distinct metals provides a flexible platform for tailoring the optical and catalytic properties of nanocrystals. First, a brief introduction to the structural features of bimetallic Janus nanocrystals, followed by an extensive discussion of the synthetic approaches, is given. The strategies and experimental controls for achieving the Janus structure, as well as the mechanistic understandings, are specifically discussed. Then, a number of intriguing properties and applications enabled by the Janus nanocrystals are highlighted. Finally, this article is concluded with future directions and outlooks with respect to both syntheses and applications of this new class of functional nanomaterials.

Synthesis of Pd3 Sn and PdCuSn Nanorods with L12 Phase for Highly Efficient Electrocatalytic Ethanol Oxidation

The crystal phase of nanomaterials is one of the key parameters determining their physicochemical properties and performance in various applications. However, it still remains a great challenge to synthesize nanomaterials with different crystal phases while maintaining the same composition, size, and morphology. Here, a facile, one-pot, wet-chemical method is reported to synthesize Pd₃ Sn nanorods with comparable size and morphology but different crystal phases, that is, an ordered intermetallic and a disordered alloy with L1₂ and face-centered cubic (fcc) phases, respectively. The crystal phase of the as-synthesized Pd₃ Sn nanorods is easily tuned by altering the types of tin precursors and solvents. Moreover, the approach can also be used to synthesize ternary PdCuSn nanorods with the L1₂ crystal phase. When used as electrocatalysts, the L1₂ Pd₃ Sn nanorods exhibit superior electrocatalytic performance toward the ethanol oxidation reaction (EOR) compared to their fcc counterpart. Impressively, compared to the L1₂ Pd₃ Sn nanorods, the ternary L1₂ PdCuSn nanorods exhibit more enhanced electrocatalytic performance toward the EOR, yielding a high mass current density up to 6.22 A mg_Pd ^-1 , which is superior to the commercial Pd/C catalyst and among the best reported Pd-based EOR electrocatalysts.

Temperature-Selective Self-Assembled Superlattices of Gold Nanoparticles Driven by Block Copolymer Template Guidance

Self-assembly of nanoparticles (NPs) into highly ordered structure can enhance their electronic and optical properties that provide great potential applications such as nanoelectronics and nanophotonics. However, the self-assembly of NPs upon external stimuli was still mainly continuous and irreversible, making various potential applications of NPs difficult. Herein, the self-assembled superlattices of gold nanoparticles (AuNPs) with a temperature-selective response had been investigated by using the amphiphilic block copolymer as a template. The AuNPs in the block copolymer template, which has the closed looplike phase behavior upon heating, self-assembled into the highly ordered body centered cubic (BCC) or face centered cubic (FCC) structures at a specific temperature region that means a temperature-selective responsiveness. The formation of highly ordered self-assembled superlattices (BCC or FCC symmetries) of AuNPs with the closed looplike phase behavior was controlled by the additive and temperature. This study is the first demonstration for temperature-selective response of the cooperative self-assembly of AuNPs in the block copolymer template.

The Facile Dissociation of Carbon-Oxygen bonds in CO2 and CO on the Surface of LaCoSiHx Intermetallic Compound

In catalysis science, the electronic structure of the active site determines the structure-activity relationship of the catalyst to a large extent. Therefore, modulating the electronic structure has become a main route for the rational design of metal-based catalyst materials. In this work, we prepared a LaCoSiH_x material that has more electronegativity and a lower workfunction than traditional supported Co-based catalysts. Using CO₂ methanation as a model catalytic reaction, the facile dissociation of CO₂ and CO (a key reaction intermediate) on the surface of the LaCoSiH_x catalyst is observed by various experimental methods (e.g., in situ Raman and FTIR) at room temperature. Moreover, theoretical calculation results further show that LaCoSiH_x has a much stronger capacity for carbon-oxygen bond activation than the Co surface. The intrinsic mechanism is attributed to the marked electron transfer from catalysts into the antibonding orbital of CO₂ and CO.

PdAgPt Corner-Satellite Nanocrystals in Well-Controlled Morphologies and the Structure-Related Electrocatalytic Properties

The functions of heterogeneous metallic nanocrystals (HMNCs) can be undoubtedly tuned by controlling their morphologies and compositions. As a less-studied kind of HMNCs, corner-satellite multi-metallic nanocrystals (CSMNCs) have great research value in structure-related electrocatalytic performance. In this work, PdAgPt corner-satellite nanocrystals with well-controlled morphologies and compositions have been developed by temperature regulation of a seed-mediated growth process. Through the seed-mediated growth, the morphology of PdAgPt products evolves from Pd@Ag cubes to PdAgPt corner-satellite cubes, and eventually to truncated hollow octahedra, as a result of the expansion of {111} facets in AgPt satellites. The growth of AgPt satellites exclusively on the corners of central cubes is realized with the joint help of Ag shell and moderate bromide, and hollow structures form only at higher reaction temperatures on account of galvanic displacement promoted by the Pd core. In view of the different performances of Pd and Pt toward formic acid oxidation (FAO), this structure-sensitive reaction is chosen to measure electrocatalytic properties of PdAgPt HMNCs. It is proven that PdAgPt CSMNCs display greatly improved activity toward FAO in direct oxidation pathway. In addition, with the help of AgPt heterogeneous shells, all PdAgPt HMNCs exhibit better durability than Pd cubes and commercial Pt.

Glutathione protected bimetallic gold-platinum nanoclusters with near-infrared emission for ratiometric determination of silver ions

A controlled method to prepare glutathione-protected bimetallic gold-platinum nanoclusters (Au-PtNCs) has been established. The Au-PtNCs show either strong red (625 nm) or near-infrared (NIR, 805 nm) emission. Further characterizations indicated that the average particle size grows from 1.42 to 1.78 nm, the larger particles being responsible for the redshift of emission. The NIR emitted Au-PtNCs are applied as a novel ratiometric probe of Ag(I), which induces a new emission peak at ~635 nm and quenches the initial emission gradually. The determination shows very high selectivity toward Ag(I) among other metal ions. A limit of determination (10 nM) and the linear range (0.10 to 15 μM) are achieved, which is much lower than the EPA mandate of 0.46 μM for Ag(I) in drinking water. The response mechanism is attributed to the fact that the added Ag(I) has been reduced by the core of Au-PtNCs and deposited on the surface, which induces new fluorescence emission around 635 nm. In addition, the ratiometric method is feasible for Ag(I) determination in serum serum with good recovery (between 98.3% and 102.0%, n = 3), showing very high application potential. The present study provides a controlled method to prepare Au-PtNCs with strong red and NIR emission and supplies a novel NIR ratiometric probe of Ag(I). Schematic presentation of the controlled preparation of glutathione-protected bimetallic gold-platinum nanoclusters (Au-PtNCs) with either red or near-infrared (NIR) emission, and application in ratiometric detection of Ag(I) with high selectivity and sensitivity.

The positional isomerism in bimetal nanoclusters

Enriching the variety of isomerism in the nanocluster field is exciting but challenging.

Engineering Functional Metal Materials at the Atomic Level

With continuous research efforts devoted into synthesis and characterization chemistry of functional nanomaterials in the past decades, the development of metal materials is stepping into a new era, where atom-by-atom customization of property-dictating structural attributes is expected. Herein, the state-of-the-art modulation of functional metal nanomaterials at the atomic level, by size- and structure-controlled synthesis of thiolate-protected metal (e.g., Au and Ag) nanoclusters (NCs), is exemplified. Metal NCs are ultrasmall (<3 nm) particles with hierarchical primary, secondary, and tertiary structures, reminiscent of natural proteins or enzymes. Given the proven dependence of their physicochemical properties on their size and structure, documented synthetic methodologies delivering NCs with atomic-level monodispersity and tailorable size and structural attributes at individual hierarchical levels are categorized and discussed. Such assured atomic-level modulation could confer metal NCs with novel application opportunities in diverse fields, which are also exemplified by their size- and structure-dictated catalytic and biomedical performance. The precise synthesis and application chemistry developed based on the hierarchical structure scheme of metal NCs could increase the acceptance of metal NCs as a new family of functional materials.

Novel Au Catalysis Strategy for the Synthesis of Au@Pt Core-Shell Nanoelectrocatalyst with Self-Controlled Quasi-Monolayer Pt Skin

Design of catalytically active Pt-based catalysts with minimizing the usage of Pt is a major issue in fuel cells. Herein, for the first time, we have developed a Au catalytic reduction strategy to synthesize a Au@Pt core-shell electrocatalyst with a quasi-monolayer Pt skin spontaneously formed from the gold surface catalysis. In the presence of presynthesized gold nanocrystals (used as the catalyst and Au seeds) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid buffer (used as reductant), under the Au surface catalysis, platinum ions can be reduced and deposited on the gold nanocrystals to form a Pt skin surface with a quasi-monatomic thickness. In the present strategy, Pt ions can be reduced only under the catalysis of gold surface and thus when the surface of Au NPs is covered by a monatomic Pt layer, the reduction reaction of Pt ions will be spontaneously turned off. Therefore, the significant advantage of this synthesis strategy is that the formation of quasi-monolayer Pt skin surface can be self-controlled and is completely free of controlling the dosage of platinum ions and the size distribution of Au cores. The synthesized Au@Pt core@shell structure exhibited enhanced electrocatalytic activities for oxygen reduction reaction and methanol oxidation reaction, which are 6.87 and 10.17 times greater than those of Pt/C catalyst, respectively. The present study provides a new strategy for obtaining high-performance bimetallic/multimetallic electrocatalysts with high utilization of precious metals.

High-Yield Synthesis of Crystal-Phase-Heterostructured 4H/fcc Au@Pd Core-Shell Nanorods for Electrocatalytic Ethanol Oxidation

Noble-metal nanomaterials are attracting increasing research interest due to their promising applications in electrochemical catalysis, for example. Although great efforts have been devoted to the size-, shape-, and architecture-controlled synthesis of noble-metal nanomaterials, their crystal-phase-controlled synthesis is still in its infancy. Here, for the first time, this study reports high-yield synthesis of Au nanorods (NRs) with alternating 4H/face-centered cubic (fcc) crystal-phase heterostructures via a one-pot wet-chemical method. The coexistence of 4H and fcc phases is relatively stable, and the 4H/fcc Au NRs can serve as templates for crystal-phase-controlled epitaxial growth of other metals. As an example, bimetallic 4H/fcc Au@Pd core-shell NRs are synthesized via the epitaxial growth of Pd on 4H/fcc Au NRs. Significantly, the 4H/fcc Au@Pd NRs show superior mass activity toward the ethanol oxidation reaction, i.e., 6.2 and 4.9 times those of commercial Pd black and Pt/C catalysts, respectively. It is believed that this new synthetic strategy can be used to prepare other novel catalysts for various promising applications.

Robust raspberry-like metallo-dielectric nanoclusters of critical sizes as SERS substrates

Raspberry-like nano-objects made of large plasmonic satellites (>10 nm) covering a central dielectric particle have many potential applications as photonic materials, superlenses and (bio-) sensors, but their synthesis remains challenging. Herein, we show how to build stable and robust raspberry-like nano-systems with close-packed satellites, by combining monodisperse silica particles (80 or 100 nm diameter) and oppositely charged noble metal nanoparticles (Au or Ag) with well-defined sizes (10-50 nm). The spectral characteristics of their associated plasmonic resonances (wavelength, linewidth, extinction cross-section) and the electromagnetic coupling between satellites were observed using the spatial modulation spectroscopy technique and interpreted through a numerical model. The composite nano-objects exhibit numerous hot spots at satellite junctions, resulting in excellent surface-enhanced Raman scattering (SERS) performance. The SERS efficiency of the raspberry-like clusters is highly dependent on their structure.

A highly selective and sensitive fluorescent probe for lactate dehydrogenase based on ultrabright adenosine monophosphate capped gold nanoclusters

The ultrabright AuNCs@AMP are used as fluorescence probe to detect lactate dehydrogenase (LDH) with high sensitivity and selectivity, showing an extremely low detection limit of 0.2 nM (26 pg μL⁻¹, 0.8 U L⁻¹).

Thermally prepared ultrabright adenosine monophosphate capped gold nanoclusters and the intrinsic mechanism

AMP capped gold nanoclusters, AuNC@AMP, have been prepared in a fast and cost-effective manner by using the heating and citrate reduction procedure, and have been found to show a strong and stable luminescence emission at 480 nm with a high quantum yield (QY, 14.52%).

Red-emitting p53-protected gold nanoclusters and their screening of anti-tumor agents from Chinese medicine

The synthesis route of red-emitting p53–AuNCs is straightforward and the products can be used to test for the anti-tumor agent myricetin.

Seed-Mediated Growth of Colloidal Metal Nanocrystals

Seed-mediated growth is a powerful and versatile approach for the synthesis of colloidal metal nanocrystals. The vast allure of this approach mainly stems from the staggering degree of control one can achieve over the size, shape, composition, and structure of nanocrystals. These parameters not only control the properties of nanocrystals but also determine their relevance to, and performance in, various applications. The ingenuity and artistry inherent to seed-mediated growth offer extensive promise, enhancing a number of existing applications and opening the door to new developments. This Review demonstrates how the diversity of metal nanocrystals can be expanded with endless opportunities by using seeds with well-defined and controllable internal structures in conjunction with a proper combination of capping agent and reduction kinetics. New capabilities and future directions are also highlighted.

Galvanic Exchange in Colloidal Metal/Metal-Oxide Core/Shell Nanocrystals

While galvanic exchange is commonly applied to metallic nanoparticles, recently its applicability was expanded to metal-oxides. Here the galvanic exchange is studied in metal/metal-oxide core/shell nanocrystals. In particular Sn/SnO₂ is treated by Ag⁺, Pt²⁺, Pt⁴⁺, and Pd²⁺. The conversion dynamics is monitored by in situ synchrotron X-ray diffraction. The Ag⁺ treatment converts the Sn cores to the intermetallic Ag _x Sn (x ∼ 4) phase, by changing the core's crystal structure. For the analogous treatment by Pt²⁺, Pt⁴⁺, and Pd²⁺, such a galvanic exchange is not observed. This different behavior is caused by the semipermeability of the naturally formed SnO₂ shell, which allows diffusion of Ag⁺ but protects the nanocrystal cores from oxidation by Pt and Pd ions.

Preparation and Catalytic Activity for Aerobic Glucose Oxidation of Crown Jewel Structured Pt/Au Bimetallic Nanoclusters

Understanding of the “structure-activity” relations for catalysts at an atomic level has been regarded as one of the most important objectives in catalysis studies. Bimetallic nanoclusters (NCs) in its many types, such as core/shell, random alloy, cluster-in-cluster, bi-hemisphere, and crown jewel (one kind of atom locating at the top position of another kind of NC), attract significant attention owing to their excellent optical, electronic, and catalytic properties. PVP-protected crown jewel-structured Pt/Au (CJ-Pt/Au) bimetallic nanoclusters (BNCs) with Au atoms located at active top sites were synthesized via a replacement reaction using 1.4-nm Pt NCs as mother clusters even considering the fact that the replacement reaction between Pt and Au³⁺ ions is difficult to be occurred. The prepared CJ-Pt/Au colloidal catalysts characterized by UV-Vis, TEM, HR-TEM and HAADF-STEM-EELS showed a high catalytic activity for aerobic glucose oxidation, and the top Au atoms decorating the Pt NCs were about 15 times more active than the Au atoms of Au NCs with similar particle size.

Luminescent Metal Nanoclusters with Aggregation-Induced Emission

Thiolate-protected metal nanoclusters (or thiolated metal NCs) have recently emerged as a promising class of functional materials because of their well-defined molecular structures and intriguing molecular-like properties. Recent developments in the NC field have aimed at exploring metal NCs as novel luminescent materials in the biomedical field because of their inherent biocompatibility and good photoluminescence (PL) properties. From the fundamental perspective, recent advances in the field have also aimed at addressing the fundamental aspects of PL properties of metal NCs, shedding some light on developing efficient strategies to prepare highly luminescent metal NCs. In this Perspective, we discuss the physical chemistry of a recently discovered aggregation-induced emission (AIE) phenomenon and show the significance of AIE in understanding the PL properties of thiolated metal NCs. We then explore the unique physicochemical properties of thiolated metal NCs with AIE characteristics and highlight some recent developments in synthesizing the AIE-type luminescent metal NCs. We finally discuss perspectives and directions for future development of the AIE-type luminescent metal NCs.

Solvothermal fabrication of thin Ag nanowires assisted with AAO

Silver nanowires were synthesized using solvothermal method assisted with AAO. AAO here is playing a role as a heterogeneous medium that can promote PVP molecules to form into one dimensional template and thus guiding the growth of Ag nanowires.

Metal-enhanced fluorescence-based multilayer core–shell Ag-nanocube@SiO2@PMOs nanocomposite sensor for Cu2+ detection

The metal-enhanced fluorescence-based core–shell Ag-nanocube@SiO₂@PMOs nanocomposite functionalized with rhodamine derived probe for Cu²⁺ detection was prepared.

The alloying effect and AgCl-directing growth for synthesizing a trimetallic nanoring with improved SERS

We report the synthesis of high quality trimetallic Au/Ag/Pt nanorings (TAAPNs) by using Au/Ag alloy decahedra (AAAD) as templates. The alloying effect and AgCl-directing growth have been investigated in detail during the formation of TAAPN. It was found that the doping of Ag in AAAD changes the surrounding environment of Au atoms and decreases the oxidization reduction potential (ORP) of [AuCl(2)](-)/Au because of the alloying effect, resulting in the dissolved O(2) molecules that serve as an effective etchant for oxidizing Au to Au(I). Ascorbic acid (AA) and chloroplatinic acid (H(2)PtCl(6)) are weak acids which can accelerate the etching by increasing the concentration of H(+). The AgCl selectively absorbs on {100} of the decahedra and induces the preferential deposition of H(2)PtCl(6) here via their complexing interaction. AA reduces Pt(IV) and Ag(I) to atoms which grow on {100} facets. The formed Pt/Ag layer changes the etching direction from along [100] to [111] and generates the TAAPN. Besides, it has been noted that the TAAPNs exhibit good Surface Enhanced Raman Scattering (SERS) performance.

Universal chitosan-assisted synthesis of Ag-including heterostructured nanocrystals for label-free in situ SERS monitoring

A universal chitosan-assisted method was developed to synthesize various Ag-including heterostructured nanocrystals, in which chelation probably plays a vital role. The as-prepared Ag/Pd heterostructured nanocrystals show outstanding properties when used as bifunctional nanocomposites in label-free in situ SERS monitoring of Pd-catalyzed reaction.

Heterostructured Au/Pd-M (M = Au, Pd, Pt) nanoparticles with compartmentalized composition, morphology, and electrocatalytic activity

The synthesis, processing, and galvanic exchange of three heterostructured nanoparticle systems is described. The surface accessibility and redox potential of a Au/Pd-Ag dumbbell nanoparticle, where a Au/Pd core/shell region, and a silver region make up the domains, was used to prepare the new nanostructures with controlled composition, morphology, and microstructure. Results indicate that the silver domain was particularly susceptible to galvanic displacement, and was exchanged to Au/Pd-M (M = Au, Pd, Pt). Interestingly, the dumbbell morphology remained after exchange, and the silver region was transformed to hollow, parachute, or concentric domains respectively. The morphology and microstructure change was visualized via TEM and HRTEM, and the composition changes were probed via STEM-EDS imaging and XPS. The electrocatalytic activity of the Au/Pd-M towards methanol oxidation was studied, with results indicating that the Au/Pd-Pt nanoparticles had high activity attributed to the porous nature of the platinum domains.