Formation of Diverse Supercrystals from Self-Assembly of a Variety of Polyhedral Gold Nanocrystals

Cubic, rhombic dodecahedral, octahedral, and corner-truncated octahedral gold nanocrystals with sizes of tens of nanometers have been used as building blocks to form micrometer-sized supercrystals by slowly evaporating a water droplet on a substrate placed in a moist environment. Drying the droplet at 90 °C was found to yield the best supercrystals. Supercrystals were evenly distributed throughout the entire substrate surface originally covered by the droplet. Diverse supercrystal morphologies have been observed. Nanocubes formed roughly cubic supercrystals. Rhombic dodecahedra were assembled into truncated triangular pyramidal supercrystals. Rhombic dodecahedral, octahedral, and hexapod-shaped supercrystals were generated through the assembly of octahedra. Corner-truncated octahedra formed tetrapod-shaped supercrystals at room temperature, but octahedral, truncated triangular pyramidal, and square pyramidal supercrystals at 90 °C. Nanocrystal assembly was found to be strongly shape-guided. Expulsion of excess surfactant to the surfaces of supercrystals suggests that responsive adjustment of surfactant concentration during particle assembly mediates supercrystal formation. Transmission X-ray microscopy and optical microscopy have been employed to follow the supercrystal formation process. Surprising rotational water current near the droplet perimeter carrying the initially formed supercrystals has been observed. Supercrystals appear to grow from the edge of the droplet toward the central region. Supercrystals assembled from octahedra inherently contain void spaces and possibly connected channels. The mesoporosity of these supercrystals was confirmed by infiltrating H₂PdCl₄ into the supercrystal interior and reducing the precursor to form Pd nanoparticles. The embedded Pd particles can still catalyze a Suzuki coupling reaction, demonstrating the application of these supercrystals for molecular transport, sensing, and catalysis

Insights into Electrocatalytic Oxygen Evolution over Hierarchical FeCo₂S₄ Nanospheres

In this work, a system of spinel FeCo2S4-catalyzed oxygen evolution reaction (OER) is studied. The hierarchical FeCo2S4 nanospheres (HNSs) are synthesized by a two-step hydrothermal method on Ni foam (NF) by vulcanizing Fe–Co precursors with different concentrations of Na2S. Since FeCo2S4 HNSs have the advantages of a large exposed surface area and high dispersity on the Ni foam, the FeCo2S4/NF heterostructure is used as a working electrode for OER in an electrochemical system. The catalysis results represent that the performance of FeCo2S4/NF in OER correlates with the vulcanized HNS surfaces made with different concentrations of Na2S, where the 0.1 M FeCo2S4/NF catalyst is the optimized condition to exhibit the lowest OER overpotential. Besides, the structures of FeCo2S4 HNSs are stable after a 12 h OER durability test. In the results of in situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) measurements with the FeCo2S4/NF electrode, the mechanism of FeCo2S4-catalyzed water splitting is through the formation of MOOH (M = metal) followed by the release of O2, the so-called indirect pathway, in the alkaline condition. Co has the major role to play in the OER reaction, while Fe holds its electronic state. Moreover, the reason for sulfide-assisted OER is the reduced charge-transfer barrier in the HNS structure that benefits MOOH generation

Fabrication of Au–Pd Core–Shell Heterostructures with Systematic Shape Evolution Using Octahedral Nanocrystal Cores and Their Catalytic Activity

By using octahedral gold nanocrystals with sizes of approximately 50 nm as the structure-directing cores for the overgrowth of Pd shells, Au–Pd core–shell heterostructures with systematic shape evolution can be directly synthesized. Core–shell octahedra, truncated octahedra, cuboctahedra, truncated cubes, and concave cubes were produced by progressively decreasing the amount of the gold nanocrystal solution introduced into the reaction mixture containing cetyltrimethylammonium bromide (CTAB), H₂PdCl₄, and ascorbic acid. The core–shell structure and composition of these nanocrystals has been confirmed. Only the concave cubes are bounded by a variety of high-index facets. This may be a manifestation of the release of lattice strain with their thick shells at the corners. Formation of the [CTA]₂[PdBr₄] complex species has been identified spectroscopically. Time-dependent UV–vis absorption spectra showed faster Pd source consumption rates in the growth of truncated cubes and concave cubes, while a much slower reduction rate was observed in the generation of octahedra. The concave cubes and octahedra were used as catalysts for a Suzuki coupling reaction. They can all serve as effective and recyclable catalysts, but the concave cubes gave higher product yields with a shorter reaction time attributed to their high-index surface facets. The concave cubes can also catalyze a wide range of Suzuki coupling reactions using aryl iodides and arylboronic acids with electron-donating and -withdrawing substituents

Facet-Dependent Catalytic Activity of Gold Nanocubes, Octahedra, and Rhombic Dodecahedra toward 4‑Nitroaniline Reduction

In this study, cubic, octahedral, and rhombic dodecahedral gold nanocrystals synthesized by a seed-mediated growth method were employed as catalysts for the examination of facet-dependent catalytic activity toward NaBH₄ reduction of <i>p</i>-nitroaniline to <i>p</i>-phenylenediamine at different temperatures. Different amounts of the nanocrystal solutions were used so that all samples contain particles with the same total surface area. UV–vis absorption spectra monitored the reaction progress. Rhombic dodecahedra showed the best catalytic efficiency at all the temperatures examined. Nanocubes have higher reaction rates than those of octahedra from 25 to 36 °C, so the catalytic activity for the reduction reaction follows the order of {110} > {100} > {111}. However, the reaction rates for octahedra increase rapidly with rising temperature; their reaction rate surpasses that for the nanocubes at 40 °C. Rate constants and activation energies were determined, again showing that the activation energy is lowest for rhombic dodecahedra. Density functional theory (DFT) calculations indicate highest binding energy between <i>p</i>-nitroaniline and the Au(110) plane. The results reveal rhombic dodecahedral gold nanocrystals as highly efficient catalysts

A Comparative Study of Gold Nanocubes, Octahedra, and Rhombic Dodecahedra as Highly Sensitive SERS Substrates

Gold nanocubes, octahedra, and rhombic dodecahedra with roughly two sets of particle sizes have been successfully synthesized via a seed-mediated growth approach. All six samples were analyzed for comparative surface-enhanced Raman scattering (SERS) activity. All of these Au nanostructures were found to yield strong enhancement at a thiophenol concentration of 10–7 M and are excellent SERS substrates. Rhombic dodecahedra with a rhombus edge length of 32 nm showed significantly better enhancement than the other samples and can reach a detection limit of 10–8 M. Simulations of the binding energies of thiophenol on the different faces of gold and electric near-field intensities of these nanocrystals have been performed to evaluate the experimental results. Superior SERS activity of these nanocrystals can be expected toward the detection of many other molecules