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1. Galvanic replacement of intermetallic nanocrystals as a route toward complex heterostructures

   https://doi.org/10.1039/D0NR08255D  

   Chen, Alexander N. ; Endres, Emma J. ; Ashberry, Hannah M. ; Bueno, Sandra L. ; Chen, Yifan ; Skrabalak, Sara E. ( February 2021 , Nanoscale)

   null (Ed.)

   Galvanic replacement reactions are a reliable method for transforming monometallic nanotemplates into bimetallic products with complex nanoscale architectures. When replacing bimetallic nanotemplates, even more complex multimetallic products can be made, with final nanocrystal shapes and architectures depending on multiple processes, including Ostwald ripening and the Kirkendall effect. Galvanic replacement, therefore, is a promising tool in increasing the architectural complexity of multimetallic templates, especially if we can identify and control the relevant processes in a given system and apply them more broadly. Here, we study the transformation of intermetallic PdCu nanoparticles in the presence of HAuCl 4 and H 2 PtCl 6 , both of which are capable of oxidizing both Pd and Cu. Replacement products consistently lost Cu more quickly than Pd, preserved the crystal structure of the original intermetallic template, and grew a new phase on the sacrificial template. In this way, atomic and nanometer-scale architectures are integrated within individual nanocrystals. Product morphologies included faceting of the original spherical particles as well as formation of core@shell and Janus-style particles. These variations are rationalized in terms of differing diffusion behaviors. Overall, galvanic replacement of multimetallic templates is shown to be a route toward increasingly exotic particle architectures with control exerted on both Angstrom and nanometer-scale features, while inviting further consideration of template and oxidant choices. 

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2. Open and Close‐Packed, Shape‐Engineered Polygonal Nanoparticle Metamolecules with Tailorable Fano Resonances

   https://doi.org/10.1002/adma.202301323  

   Cai, Yi‐Yu ; Fallah, Asma ; Yang, Shengsong ; Choi, Yun Chang ; Xu, Jun ; Stein, Aaron ; Kikkawa, James M. ; Murray, Christopher B. ; Engheta, Nader ; Kagan, Cherie R. ( August 2023 , Advanced Materials)

   A top‐down lithographic patterning and deposition process is reported for producing nanoparticles (NPs) with well‐defined sizes, shapes, and compositions that are often not accessible by wet‐chemical synthetic methods. These NPs are ligated and harvested from the substrate surface to prepare colloidal NP dispersions. Using a template‐assisted assembly technique, fabricated NPs are driven by capillary forces to assemble into size‐ and shape‐engineered templates and organize into open or close‐packed multi‐NP structures or NP metamolecules. The sizes and shapes of the NPs and of the templates control the NP number, coordination, interparticle gap size, disorder, and location of defects such as voids in the NP metamolecules. The plasmonic resonances of polygonal‐shaped Au NPs are exploited to correlate the structure and optical properties of assembled NP metamolecules. Comparing open and close‐packed architectures highlights that introduction of a center NP to form close‐packed assemblies supports collective interactions, altering magnetic optical modes and multipolar interactions in Fano resonances. Decreasing the distance between NPs strengthens the plasmonic coupling, and the structural symmetries of the NP metamolecules determine the orientation‐dependent scattering response.

    

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3. Integration of Sequential Reactions in a Continuous Flow Droplet Reactor: A Route to Architecturally Defined Bimetallic Nanostructures

   https://doi.org/10.1002/ppsc.201900142  

   Santana, Joshua S. ; Gamler, Jocelyn T. L. ; Skrabalak, Sara E. ( August 2019 , Particle & Particle Systems Characterization)

   Microreactors for nanoparticle (NP) synthesis offer advantages over batch reactions in terms of scale‐up and integration with online analyses. Herein, two microreactors (i.e., a duo‐microreactor) are integrated to achieve sequential reactions for the synthesis of bimetallic NPs with architectural control. The generality of the duo‐microreactor is shown with the synthesis of branched Pd‐Pt NPs and core@shell Pd@Au NPs, both achieved by synthesizing Pd nanocubes in the first part of the duo‐microreactor and then using those nanocubes downstream as seeds for Pt or Au deposition. Control of the dimensions of these NPs is further demonstrated and achieved by tailoring metal precursor concentrations inline. This microreactor methodology is anticipated to be applicable to other bimetallic NP systems.

    

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4. Controllably Hollow AgAu Nanoparticles via Nonaqueous, Reduction Agent‐Assisted Galvanic Replacement

   https://doi.org/10.1002/ppsc.201700381  

   Rehn, Sarah M. ; Ringe, Emilie ( January 2018 , Particle & Particle Systems Characterization)

   The galvanic replacement reaction is a robust tool for the controlled synthesis of hollow and semihollow bimetallic nanostructures, which have applications in a range of science, engineering, and medical fields due to the tunability of their localized surface plasmon resonances (LSPRs) and surface chemistry. Here, a controllable galvanic replacement of Ag by Au coupled with coreduction is described, performed in nonaqueous solvents including methanol, ethanol, and anN,N‐dimethylformamide:toluene mixture and yielding hollow and semihollow alloyed nanoparticles. Structural control, from semihollow to nanoshell, and plasmon tunability are demonstrated via control of the Au:Ag stoichiometry. The high structural dependence on temperature is shown, with striking changes in nanoparticle surface smoothness and pinhole density, and reveals the optimal reaction temperature to be 65 °C in alcohols. Through optimizing this reaction, smooth closed shell AgAu alloy nanoparticles with LSPRs tunable from 494 to 567 nm are obtained. This work provides a framework for galvanic replacement of large anisotropic Ag nanoparticles with Au in nonaqueous media, which can be extended to other solvent systems suitable for air‐sensitive metals and precursors.

    

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5. Copper Template Design for the Synthesis of Bimetallic Copper–Rhodium Nanoshells through Galvanic Replacement

   https://doi.org/10.1002/ppsc.201700420  

   Preston, Arin S. ; Hughes, Robert A. ; Demille, Trevor B. ; Neretina, Svetlana ( February 2018 , Particle & Particle Systems Characterization)

   Galvanic replacement reactions are widely used in the synthesis of bimetallic nanoshells. Essential to these syntheses is the design of template materials with electrochemical potentials that are low enough to facilitate the replacement of a wide variety of metals. While Cu is an attractive template from this standpoint, it has only rarely been used due to its propensity for oxidation and the associated difficulties in achieving chemically stable colloids. Here, a synthetic scheme is demonstrated for the design of supported Cu templates and their subsequent replacement with Rh where the detrimental influences of oxidation are not only mitigated but used to place shape and compositional controls on the reaction product. It is shown that the CuRh nanoshells can be produced that are shaped as substrate‐truncated nanocubes or cuboctahedrons depending upon the degree of exposure that the Cu templates have to dissolved oxygen. Moreover, it is demonstrated that the intentional surface oxidation of the Cu template followed by Cu₂O removal results in galvanic replacement reactions yielding porous nanoshells with far greater Rh replacement. The study forwards the design of Cu templates for galvanic replacement reactions and presents opportunities for their use in other template‐mediated syntheses.

    

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