Preventing the Galvanic Replacement Reaction toward Unconventional Bimetallic Core-Shell Nanostructures.

A bimetallic core-shell nanostructure is a versatile platform for achieving intriguing optical and catalytic properties. For a long time, this core-shell nanostructure has been limited to ones with noble metal cores. Otherwise, a galvanic replacement reaction easily occurs, leading to hollow nanostructures or completely disintegrated ones. In the past few years, great efforts have been devoted to preventing the galvanic replacement reaction, thus creating an unconventional class of core-shell nanostructures, each containing a less-stable-metal core and a noble metal shell. These new nanostructures have been demonstrated to show unique optical and catalytic properties. In this work, we first briefly summarize the strategies for synthesizing this type of unconventional core-shell nanostructures, such as the delicately designed thermodynamic control and kinetic control methods. Then, we discuss the effects of the core-shell nanostructure on the stabilization of the core nanocrystals and the emerging optical and catalytic properties. The use of the nanostructure for creating hollow/porous nanostructures is also discussed. At the end of this review, we discuss the remaining challenges associated with this unique core-shell nanostructure and provide our perspectives on the future development of the field.

Coherent hexagonal platinum skin on nickel nanocrystals for enhanced hydrogen evolution activity.

Metastable noble metal nanocrystals may exhibit distinctive catalytic properties to address the sluggish kinetics of many important processes, including the hydrogen evolution reaction under alkaline conditions for water-electrolysis hydrogen production. However, the exploration of metastable noble metal nanocrystals is still in its infancy and suffers from a lack of sufficient synthesis and electronic engineering strategies to fully stimulate their potential in catalysis. In this paper, we report a synthesis of metastable hexagonal Pt nanostructures by coherent growth on 3d transition metal nanocrystals such as Ni without involving galvanic replacement reaction, which expands the frontier of the phase-replication synthesis. Unlike noble metal substrates, the 3d transition metal substrate owns more crystal phases and lower cost and endows the hexagonal Pt skin with substantial compressive strains and programmable charge density, making the electronic properties particularly preferred for the alkaline hydrogen evolution reaction. The energy barriers are greatly reduced, pushing the activity to 133 mA cmgeo-2 and 17.4 mA µgPt-1 at -70 mV with 1.5 µg of Pt in 1 M KOH. Our strategy paves the way for metastable noble metal catalysts with tailored electronic properties for highly efficient and cost-effective energy conversion.

Seed-Mediated Synthesis of Thin Gold Nanoplates with Tunable Edge Lengths and Optical Properties.

Thin Au nanoplates show intriguing localized surface plasmon resonance (LSPR) properties with potential applications in various fields. The conventional synthesis of Au nanoplates usually involves the formation of spherical nanoparticles or produces nanoplates with large thicknesses. Herein, we demonstrate a synthesis of uniform thin Au nanoplates by using Au-Ag alloy nanoframes obtained by the galvanic replacement of Ag nanoplates with HAuCl4 as the seeds and a sulfite (SO32-) as a ligand. The SO32- ligand not only complexes with the Au salt for the controlled reduction kinetics but also strongly adsorbs on Au {111} facets for effectively constraining the crystal growth on both basal sides of the Au nanoplates for controlled shape and reduced thicknesses. This seed-mediated synthesis affords Au nanoplates with a thickness of only 7.5 nm, although the thickness increases with the edge length. The edge length can be customizable in a range of 48-167 nm, leading to tunable LSPR bands in the range of 600-1000 nm. These thin Au nanoplates are applicable not only to surface-enhanced Raman spectroscopy with enhanced sensitivity and reliability but also to a broader range of LSPR-based applications.

Lattice Mismatch-Induced Formation of Copper Nanoplates with Embedded Ultrasmall Platinum or Palladium Cores for Tunable Optical Properties.

Although noble metal nanocrystals have been studied extensively in the past decades, the shape-controlled synthesis of non-noble metal nanocrystals has remained challenging with limited success, which is a grand obstacle to their wide applications. Herein, a novel lattice mismatch-involved shape-control mechanism of Cu nanocrystals in a seed-mediated synthesis is reported, which can produce Cu nanoplates in high yield with tailored sizes (28-130 nm), holding great potential in optical and catalytic applications. The lattice mismatch between Cu and the seed is found effective in inducing crystallographic defects for symmetry breaking toward anisotropic nanocrystals. While a too-large lattice mismatch (11.7% for Au seeds) leads to multiple twin defects to form quasi-spherical Cu nanocrystals, an appropriately large lattice mismatch (7.7% for Pt and 6.9% for Pd seeds) successfully induces planar defects for the formation of Cu nanoplates. The size of the Cu nanoplates is customizable by controlling the concentration of the seeds, leading to tunable optical properties. A prototype of a colorimetric indicator with Cu nanoplates, potentially applicable to the safety control of foods and drugs is demonstrated. This mechanism paves a new way for the shape-controlled synthesis of Cu and other metal nanocrystals for a broad range of applications.

Highly Strained Au-Ag-Pd Alloy Nanowires for Boosted Electrooxidation of Biomass-Derived Alcohols.

Although strain engineering is effective in boosting the activities of noble metal catalysts, it remains desirable to construct fully strained catalysts to push the activity to even higher levels. Herein, we report a novel route to strong lattice strains of a Pd-based catalyst by radial growth of a Pd-rich phase on Au-Ag alloy nanowires that are no thicker than 1.5 nm. It creates not only tensile strains in the Pd-rich sheath due to the core-sheath lattice mismatch but also distortion and twinning of the lattice, producing nonhomogeneous local strains as hotspots for the catalysis. Toward the electrochemical oxidation of biomass-derived alcohols including ethanol, ethylene glycol, and glycerol, the highly strained nanowires outperformed their less strained counterparts and reached up to 13.6, 18.2, and 11.1 A mgPd-1, respectively. This strain engineering strategy may open new avenues to highly efficient catalysts for direct alcohol fuel cells and many other applications.

Ultrasensitive determination of ascorbic acid by using cobalt oxyhydroxide nanosheets to enhance the chemiluminescence of the luminol-H2O2 system.

Ascorbic acid (AA) as an essential vitamin in the human body participates in various physiological reactions and plays a key role in many biochemical processes. Therefore, it is of vital importance to monitor and quantify AA in commercial tablets, beverages and food. In this work, a rapid and ultrasensitive chemiluminescence (CL) system for the detection of AA was developed, in which ultrathin cobalt oxyhydroxide (CoOOH) nanosheets were applied in the conventional luminol-H2O2 CL system. The results showed that ultrathin CoOOH nanosheets as a catalyzer remarkably improved the CL intensity of the CoOOH-luminol-H2O2 system, up to about 1400-fold. Under the optimized conditions, the CL inhibition efficiencies increase linearly with the concentrations of AA in the range of 1-500 pmol L-1, and the limit of detection was 39 fmol L-1. Moreover, the proposed CL system was successfully applied in the determination of AA in medicinal tablets with satisfactory results.