Self-Limiting Shell Formation in Cu@Ag Core-Shell Nanocrystals during Galvanic Replacement

Atomic Au3Cu Palisade Interlayer in Core@Shell Nanostructures for Efficient Kirkendall Effect Mediation

Plasmonic Cu@semiconductor heteronanocrystals (HNCs) have many favorable properties, but the synthesis of solid structures is often hindered by the nanoscale Kirkendall effect. Herein, we present the use of an atomically thin Au3Cu palisade interlayer to reduce lattice mismatch and mediate the Kirkendall effect, enabling the successive topological synthesis of Cu@Au3Cu@Ag, Cu@Au3Cu@Ag2S, and further transformed solid Cu@Au3Cu@CdS core-shell HNCs via cation exchange. The atomically thin and intact Au3Cu palisade interlayer effectively modulates the diffusion kinetics of Cu atoms as demonstrated by experimental and theoretical investigations and simultaneously alleviates the lattice mismatch between Cu and Ag as well as Cu and CdS. The Cu@Au3Cu@CdS HNCs feature exceptional crystallinity and atomically organized heterointerfaces between the plasmonic metal and the semiconductor. This results in the efficient plasmon-induced injection of hot electrons from Cu@Au3Cu into the CdS shell, enabling the Cu@Au3Cu@CdS HNCs to achieve high activity and selectivity for the photocatalytic reduction of CO2 to CO.

Probing the atomically diffuse interfaces in Pd@Pt core-shell nanoparticles in three dimensions

Deciphering the three-dimensional atomic structure of solid-solid interfaces in core-shell nanomaterials is the key to understand their catalytical, optical and electronic properties. Here, we probe the three-dimensional atomic structures of palladium-platinum core-shell nanoparticles at the single-atom level using atomic resolution electron tomography. We quantify the rich structural variety of core-shell nanoparticles with heteroepitaxy in 3D at atomic resolution. Instead of forming an atomically-sharp boundary, the core-shell interface is found to be atomically diffuse with an average thickness of 4.2 Å, irrespective of the particle's morphology or crystallographic texture. The high concentration of Pd in the diffusive interface is highly related to the free Pd atoms dissolved from the Pd seeds, which is confirmed by atomic images of Pd and Pt single atoms and sub-nanometer clusters using cryogenic electron microscopy. These results advance our understanding of core-shell structures at the fundamental level, providing potential strategies into precise nanomaterial manipulation and chemical property regulation.

Galvanic Replacement Synthesis of Metal Nanostructures: Bridging the Gap between Chemical and Electrochemical Approaches

ConspectusGalvanic replacement synthesis involves oxidation and dissolution of atoms from a substrate while the salt precursor to another material with a higher reduction potential is reduced and deposited on the substrate. The driving force or spontaneity of such a synthesis comes from the difference in reduction potential between the redox pairs involved. Both bulk and micro/nanostructured materials have been explored as substrates for galvanic replacement synthesis. The use of micro/nanostructured materials can significantly increase the surface area, offering immediate advantages over the conventional electrosynthesis. The micro/nanostructured materials can also be intimately mixed with the salt precursor in a solution phase, resembling the setting of a typical chemical synthesis. The reduced material tends to be directly deposited on the surface of the substrate, just like the situation in an electrosynthesis. Different from an electrosynthesis where the two electrodes are spatially separated by an electrolyte solution, the cathodes and anodes are situated on the same surface, albeit at different sites, even for a micro/nanostructured substrate. Since the oxidation and dissolution reactions occur at sites different from those for reduction and deposition reactions, one can control the growth pattern of the newly deposited atoms on the same surface of a substrate to access nanostructured materials with diverse and controllable compositions, shapes, and morphologies in a single step. Galvanic replacement synthesis has been successfully applied to different types of substrates, including those made of crystalline and amorphous materials, as well as metallic and nonmetallic materials. Depending on the substrate involved, the deposited material can take different nucleation and growth patterns, resulting in diverse but well-controlled nanomaterials sought for a variety of studies and applications.In this Account, we recapitulate our efforts over the past two decades in fabricating metal nanostructures for a broad range of applications by leveraging the unique capability of galvanic replacement synthesis. We begin with a brief introduction to the fundamentals of galvanic replacement between metal nanocrystals and salt precursors, followed by a discussion of the roles played by surface capping agents in achieving site-selected carving and deposition for the fabrication of various bimetallic nanostructures. Two examples based on the Ag-Au and Pd-Pt systems are selected to illustrate the concept and mechanism. We then highlight our recent work on the galvanic replacement synthesis involving nonmetallic substrates, with a focus on the protocol, mechanistic understanding, and experimental control for the fabrication of Au- and Pt-based nanostructures with tunable morphologies. Finally, we showcase the unique properties and applications of nanostructured materials derived from galvanic replacement reactions for biomedicine and catalysis. We also offer some perspectives on the challenges and opportunities in this emerging field of research.

Plasmonic Hybrid Nanostructures in Photocatalysis: Structures, Mechanisms, and Applications

(Sun)Light is an abundantly available sustainable source of energy that has been used in catalyzing chemical reactions for several decades now. In particular, studies related to the interaction of light with plasmonic nanostructures have been receiving increased attention. These structures display the unique property of localized surface plasmon resonance, which converts light of a specific wavelength range into hot charge carriers, along with strong local electromagnetic fields, and/or heat, which may all enhance the reaction efficiency in their own way. These unique properties of plasmonic nanoparticles can be conveniently tuned by varying the metal type, size, shape, and dielectric environment, thus prompting a research focus on rationally designed plasmonic hybrid nanostructures. In this review, the term "hybrid" implies nanomaterials that consist of multiple plasmonic or non-plasmonic materials, forming complex configurations in the geometry and/or at the atomic level. We discuss the synthetic techniques and evolution of such hybrid plasmonic nanostructures giving rise to a wide variety of material and geometric configurations. Bimetallic alloys, which result in a new set of opto-physical parameters, are compared with core-shell configurations. For the latter, the use of metal, semiconductor, and polymer shells is reviewed. Also, more complex structures such as Janus and antenna reactor composites are discussed. This review further summarizes the studies exploiting plasmonic hybrids to elucidate the plasmonic-photocatalytic mechanism. Finally, we review the implementation of these plasmonic hybrids in different photocatalytic application domains such as H₂ generation, CO₂ reduction, water purification, air purification, and disinfection.

Catalytic Membrane Nanoreactor with Cu-Agx Bimetallic Nanoparticles Immobilized in Membrane Pores for Enhanced Catalytic Performance

A catalytic membrane nanoreactor (CMNR) with Cu-Ag_x (where x is the millimolar concentration of AgNO₃) bimetallic catalysts immobilized in membrane pores has been fabricated via coupling flowing synthesis and replacement reaction. Surface characterization by transmission electron microscopy (TEM) gives obvious evidence of the formation of Cu-Ag bimetallic core-shell nanostructures with Ag islands deposited on the Cu core metal. An apparent high shift phenomenon for the Cu element and a low shift phenomenon for the Ag element was determined by X-ray photoelectron spectroscopy (XPS), indicating a close interaction with the transfer of electron density from the Cu atom to the Ag atom. The hydrogenation catalysis of p-nitrophenol (p-NP) was tested to evaluate the catalytic performance. During the catalytic process, the Cu core acts as an electron-deficient site to adsorb and activate the -NO₂ group for p-NP, and the Ag shell is beneficial for enhancing active H spilling to the Cu surface and then performing hydrogenation. A volcano-shaped apparent reaction rate constant can be achieved, which rises initially with the increasing Ag content and subsequently drops with a further increase in the Ag content. The highest value of 1071 min^-1 can be achieved for CMNR immobilized with Cu-Ag₂ owing to the suitable adsorption activation behavior and the best hydrogen spillover behavior.

Mapping Composition-Selectivity Relationships of Supported Sub-10 nm Cu-Ag Nanocrystals for High-Rate CO2 Electroreduction

Colloidal Cu-Ag nanocrystals measuring less than 10 nm across are promising candidates for integration in hybrid CO₂ reduction reaction (CO₂RR) interfaces, especially in the context of tandem catalysis and selective multicarbon (C₂-C₃) product formation. In this work, we vary the synthetic-ligand/copper molar ratio from 0.1 to 1.0 and the silver/copper atomic ratio from 0 to 0.7 and study the variations in the nanocrystals' size distribution, morphology and reactivity at rates of ≥100 mA cm^-2 in a gas-fed recycle electrolyzer operating under neutral to mildly basic conditions (0.1-1.0 M KHCO₃). High-resolution electron microscopy and spectroscopy are used in order to characterize the morphology of sub-10 nm Cu-Ag nanodimers and core-shells and to elucidate trends in Ag coverage and surface composition. It is shown that Cu-Ag nanocrystals can be densely dispersed onto a carbon black support without the need for immediate ligand removal or binder addition, which considerably facilitates their application. Although CO₂RR product distribution remains an intricate function of time, (kinetic) overpotential and processing conditions, we nevertheless conclude that the ratio of oxygenates to hydrocarbons (which depends primarily on the initial dispersion of the nanocrystals and their composition) rises 3-fold at moderate Ag atom % relative to Cu NCs-based electrodes. Finally, the merits of this particular Cu-Ag/C system and the recycling reactor employed are utilized to obtain maximum C₂-C₃ partial current densities of 92-140 mA cm^-2 at -1.15 V_RHE and liquid product concentrations in excess of 0.05 wt % in 1 M KHCO₃ after short electrolysis periods.

Cu-Ag Bimetallic Core-shell Nanoparticles in Pores of a Membrane Microreactor for Enhanced Synergistic Catalysis

A bimetallic catalytic membrane microreactor (CMMR) with bimetallic nanoparticles in membrane pores has been fabricated via flowing synthesis. The bimetallic nanoparticle is successfully immobilized in membrane pores along its thickness direction. Enhanced synergistic catalysis can be expected in this CMMR. As a concept-of-proof, Cu-Ag core-shell nanoparticles have been fabricated and immobilized in membrane pores for p-nitrophenol (p-NP) hydrogenation. Transmission electron microscopy (TEM) for the characterization of the bimetallic core-shell nanostructure and X-ray photoelectron spectroscopy (XPS) for the characterization of the electron transfer behavior between Cu-Ag bimetal have been performed. The Ag shell on the core of Cu can improve the utilization of Ag atoms, and electron transfer between bimetallic components can promote the formation of high electron density active sites as well as active hydrogen with strong reducing properties on the Ag surface. The dispersed membrane pore can prevent nanoparticle aggregation, and the contact between the reaction fluid and catalyst is enhanced. The enhanced mass transfer can be achieved by the plug-flow mode during the process of hydrogenation catalysis. The p-NP conversion rate being over 95% can be obtained under the condition of a membrane flux of 1.59 mL·cm^-2·min^-1. This Cu-Ag/PES CMMR has good stability and has a potential application in industry.