Iodide-Mediated Control of Rhodium Epitaxial Growth on Well-Defined Noble Metal Nanocrystals: Synthesis, Characterization, and Structure-Dependent Catalytic Properties

Metal nanocrystals (NCs) comprising rhodium are heterogeneous catalysts for CO oxidation, NO reduction, hydrogenations, electro-oxidations, and hydroformylation reactions. It has been demonstrated that control of structure at the nanoscale can enhance the performance of a heterogeneous metal catalyst, such as Rh, but molecular-level control of NCs comprising this metal is less studied compared to gold, silver, platinum, and palladium. We report an iodide-mediated epitaxial overgrowth of Rh by using the surfaces of well-defined foreign metal crystals as substrates to direct the Rh surface structures. The epigrowth can be accomplished on different sizes, morphologies, and identities of metal substrates. The surface structures of the resulting bimetallic NCs were studied using electron microscopy, and their distinct catalytic behaviors were examined in CO stripping and the electro-oxidation of formic acid. Iodide was found to play a crucial role in the overgrowth mechanism. With the addition of iodide, the Rh epigrowth can even be achieved on gold substrates despite the rather large lattice mismatch of ∼7%. Hollow Rh nanostructures have also been generated by selective etching of the core substrates. The new role of iodide in the overgrowth and the high level of control for Rh could hold the key to future nanoscale control of this important metal’s architecture for use in heterogeneous catalysis

Optimized Metal–Organic-Framework Nanospheres for Drug Delivery: Evaluation of Small-Molecule Encapsulation

We have developed a general synthetic route to encapsulate small molecules in monodisperse zeolitic imid-azolate framework-8 (ZIF-8) nanospheres for drug delivery. Electron microscopy, powder X-ray diffraction, and elemental analysis show that the small-molecule-encapsulated ZIF-8 nanospheres are uniform 70 nm particles with single-crystalline structure. Several small molecules, including fluorescein and the anticancer drug camptothecin, were encapsulated inside of the ZIF-8 framework. Evaluation of fluorescein-encapsulated ZIF-8 nanospheres in the MCF-7 breast cancer cell line demonstrated cell internalization and minimal cytotoxicity. The 70 nm particle size facilitates cellular uptake, and the pH-responsive dissociation of the ZIF-8 framework likely results in endosomal release of the small-molecule cargo, thereby rendering the ZIF-8 scaffold an ideal drug delivery vehicle. To confirm this, we demonstrate that camptothecin encapsulated ZIF-8 particles show enhanced cell death, indicative of internalization and intracellular release of the drug. To demonstrate the versatility of this ZIF-8 system, iron oxide nanoparticles were also encapsulated into the ZIF-8 nanospheres, thereby endowing magnetic features to these nanospheres

Size-Dependent Sulfur Poisoning of Silica-Supported Monodisperse Pt Nanoparticle Hydrogenation Catalysts

Colloidal techniques were used to synthesize monodisperse Pt nanoparticles of four distinct sizes between 2 and 7 nm before immobilization onto silica. Ethylene hydrogenation demonstrated structure-insensitive behavior with TOFs of ∼12 s^–1 before poisoning. With thiophene being a strong binding adsorbate, TOFs decreased by orders of magnitude, and the poisoning-induced antipathetic structure sensitivity because thiophene adsorbed more strongly to the coordinatively unsaturated, as compared with coordinatively saturated, surfaces, and the degree of saturation increased with decreasing Pt size. This effort is part of a broader study in which structure sensitivity is analyzed for adsorbates in complex reaction networks

Serial Morphological Transformations of Au Nanocrystals via Post-Synthetic Galvanic Dissolution and Recursive Growth

Geometric modification of Au nanostructures is typically achieved in multistep reactions, where synthesis parameters need to be well-controlled. In this work, we report a facile method using IrCl₃ to refine morphologically diverse Au nanostructures and trigger their morphological transformations. The synthesis is accomplished at room temperature by an iterative process of galvanic dissolution and recursive growth. Seeds retrieved after the dissolution of different Au nanostructure archetypes served in the structural recovery and morphological transformation via rapid and slow regrowth, respectively. The rapid regrowth was accomplished by adding ascorbic acid (AA), while the slow regrowth occurred spontaneously. In the structural recovery, the nanostructures regrew back to their original morphologies. Improvements in the shape quality and size distributions were observed for the rapid regrowth case. In the spontaneous slow regrowth transformation, the resulting nanostructures were encased by {111} facets, minimizing total surface energy through the more closely packed planes. Transformation of the four nanostructure archetypes showed correlation, trending toward these lower indexed facets and to twinned structures (from RDs to OCTs, OCTs to TPs, and TPs to PSs). Surveying all observations, our work of the metal cation-mediated geometric modulation of Au nanostructures delivers important clues in understanding nanoparticle synthesis and provides a new path for the fabrication of nanocrystals with high-quality size and shape distribution

Spiny Rhombic Dodecahedral CuPt Nanoframes with Enhanced Catalytic Performance Synthesized from Cu Nanocube Templates

Platinum was coated on the surfaces of copper nanocubes to form Cu–CuPt core–alloy–frame nanocrystals with a rhombic dodecahedral (RD) shape. Co-reduction of Pt²⁺ ions and residual Cu⁺ ions in the supernatant of the Cu nanocube solution followed by the interdiffusion of Cu and Pt atoms over the core–shell interface allowed their formation. Growth in the ⟨100⟩ directions of the {100}-terminated Cu nanocubes resulted in the {110}-faceted rhombic dodecahedra. By the introduction of additional Pt precursor, the {100} vertices of the Cu–CuPt RD nanocrystals could be selectively extended to form spiny CuPt RD nanocrystals. After removing the Cu core template, both CuPt alloy RD and spiny CuPt alloy RD nanoframes (NFs) were obtained with Pt/Cu ratios of 26/74 and 41/59, respectively. Abundant surface defects render them highly active catalysts due to the open frame structure of both sets of NFs. The spiny RD NFs showed superior specific activity toward the oxygen reduction reaction, 1.3 and 3 times to those of the RD NFs and the commercial Pt/C catalysts, respectively. In 4-nitrophenol reduction, both NFs displayed better activity compared to commercial Pt NPs in the dark. Their activities were improved ∼1.3 times under irradiation of visible light, attributed to the effect of LSPR enhancement by the Cu-rich skeleton

Turning the Halide Switch in the Synthesis of Au–Pd Alloy and Core–Shell Nanoicosahedra with Terraced Shells: Performance in Electrochemical and Plasmon-Enhanced Catalysis

Au–Pd nanocrystals are an intriguing system to study the integrated functions of localized surface plasmon resonance (LSPR) and heterogeneous catalysis. Gold is both durable and can harness incident light energy to enhance the catalytic activity of another metal, such as Pd, via the SPR effect in bimetallic nanocrystals. Despite the superior catalytic performance of icosahedral (IH) nanocrystals compared to alternate morphologies, the controlled synthesis of alloy and core–shell IH is still greatly challenged by the disparate reduction rates of metal precursors and lack of continuous epigrowth on multiply twinned boundaries of such surfaces. Herein, we demonstrate a one-step strategy for the controlled growth of monodisperse Au–Pd alloy and core–shell IH with terraced shells by turning an ionic switch between [Br^–]/[Cl^–] in the coreduction process. The core–shell IH nanocrystals contain AuPd alloy cores and ultrathin Pd shells (<2 nm). They not only display more than double the activity of the commercial Pd catalysts in ethanol electrooxidation attributed to monatomic step terraces but also show SPR-enhanced conversion of 4-nitrophenol. This strategy holds promise toward the development of alternate bimetallic IH nanocrystals for electrochemical and plasmon-enhanced catalysis

Mesoporous Nickel Ferrites with Spinel Structure Prepared by an Aerosol Spray Pyrolysis Method for Photocatalytic Hydrogen Evolution

Submicron-sized mesoporous nickel ferrite (NiFe₂O₄) spheres were prepared by an aerosol spray pyrolysis method using Pluronic F127 as a structure-directing agent, and their photocatalytic performance for hydrogen (H₂) evolution was examined in an aqueous MeOH solution by visible light irradiation (λ > 420 nm). The structure of the spherical mesoporous nickel ferrites was studied by transmission electron microscopy, powder X-ray diffraction, and N₂ adsorption–desorption isotherm measurements. Mesoporous NiFe₂O₄ spheres of high specific surface area (278 m² g^–1) with a highly crystalline framework were prepared by adjusting the amount of structure-directing agent and the calcining condition. High photocatalytic activity of mesoporous NiFe₂O₄ for H₂ evolution from water with methanol was achieved due to the combination of high surface area and high crystallinity of the nickel ferrites

Aqueous Synthesis of Concave Rh Nanotetrahedra with Defect-Rich Surfaces: Insights into Growth‑, Defect‑, and Plasmon-Enhanced Catalytic Energy Conversion

The control of morphology in the synthesis of Rh nanocrystals can be used to precisely tailor the electronic surface structure; this in turn directly influences their performance in catalysis applications. Many works have brought attention to the development of Rh nanostructures with low-index surfaces, but limited effort has been devoted to the study of high-index and surface defect-enriched nanocrystals as they are not favored by thermodynamics because of the involvement of high-energy surfaces and increased surface-to-volume ratios. In this work, we demonstrate an aqueous synthesis of concave Rh nanotetrahedra (CTDs) serving as efficient catalysts for energy conversion reactions. CTDs are surface defect-rich structures that form through a slow growth rate and follow the four-step model of metallic nanoparticle growth. Via the tuning of the surfactant concentration, the morphology of Rh CTDs evolved into highly excavated nanotetrahedra (HETDs) and twinned nanoparticles (TWs). Unlike the CTD surfaces with abundant adatoms and vacancies, HETDs and TWs have more regular surfaces with layered terraces. Each nanocrystal type was evaluated for methanol electrooxidation and hydrogen evolution from hydrolysis of ammonia borane, and the CTDs significantly showed the best catalytic performance because of defect enrichment, which benefits the surface reactivity of adsorbates. In addition, both CTDs and HETDs have strong absorption near the visible light region (382 and 396 nm), for which they show plasmon-enhanced performance in photocatalytic hydrogen evolution under visible light illumination. CTDs are more photoactive than HETDs, likely because of more pronounced localized surface plasmon resonance hot spots. This facile aqueous synthesis of large-surface-area, defect-rich Rh nanotetrahedra is exciting for the fields of nanosynthesis and catalysis

Yolk–Shell Nanocrystal@ZIF‑8 Nanostructures for Gas-Phase Heterogeneous Catalysis with Selectivity Control

A general synthetic strategy for yolk–shell nanocrystal@ZIF-8 nanostructures has been developed. The yolk–shell nanostructures possess the functions of nanoparticle cores, microporous shells, and a cavity in between, which offer great potential in heterogeneous catalysis. The synthetic strategy involved first coating the nanocrystal cores with a layer of Cu₂O as the sacrificial template and then a layer of polycrystalline ZIF-8. The clean Cu₂O surface assists in the formation of the ZIF-8 coating layer and is etched off spontaneously and simultaneously during this process. The yolk–shell nanostructures were characterized by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and nitrogen adsorption. To study the catalytic behavior, hydrogenations of ethylene, cyclohexene, and cyclooctene as model reactions were carried out over the Pd@ZIF-8 catalysts. The microporous ZIF-8 shell provides excellent molecular-size selectivity. The results show high activity for the ethylene and cyclohexene hydrogenations but not in the cyclooctene hydrogenation. Different activation energies for cyclohexene hydrogenation were obtained for nanostructures with and without the cavity in between the core and the shell. This demonstrates the importance of controlling the cavity because of its influence on the catalysis

Nanoscale-Phase-Separated Pd–Rh Boxes Synthesized via Metal Migration: An Archetype for Studying Lattice Strain and Composition Effects in Electrocatalysis

Developing syntheses of more sophisticated nanostructures comprising late transition metals broadens the tools to rationally design suitable heterogeneous catalysts for chemical transformations. Herein, we report a synthesis of Pd–Rh nanoboxes by controlling the migration of metals in a core–shell nanoparticle. The Pd–Rh nanobox structure is a grid-like arrangement of two distinct metal phases, and the surfaces of these boxes are {100} dominant Pd and Rh. The catalytic behaviors of the particles were examined in electrochemistry to investigate strain effects arising from this structure. It was found that the trends in activity of model fuel cell reactions cannot be explained solely by the surface composition. The lattice strain emerging from the nanoscale separation of metal phases at the surface also plays an important role