Formation of Free-Standing Supercrystals from the Assembly of Polyhedral Gold Nanocrystals by Surfactant Diffusion in the Solution

Gold supercrystals with polyhedral morphologies can be prepared from the ordered packing of octahedral and rhombic dodecahedral nanocrystals in the presence of a sufficient amount of surfactant by slow water droplet evaporation. The whole supercrystal formation process has been video-recorded using a specially designed chamber to enclose a substrate containing the nanocrystal droplet in a moist environment. Supercrystal growth from the assembly of octahedra is completed within a shorter time. The presence of cetyltrimethylammonium chloride (CTAC) within the supercrystals has been confirmed by small-angle X-ray diffraction analysis. Transmission electron microscopy examination reveals the tendency of two gold octahedra with face contact to fuse, a process frequently observed in the formation of octahedron-assembled supercrystals. Remarkably, we have developed a diffusional surfactant transport approach to make free-standing supercrystals in bulk aqueous solution by adding a concentrated CTAC solution to a concentrated particle solution with a lower CTAC concentration in an Eppendorf tube. Gradual diffusion of CTAC to the lower nanocrystal solution promotes the growth of polyhedral supercrystals. A solution with a sufficiently high surfactant concentration has been shown to be necessary for particle aggregation and supercrystal formation. This method allows the deposition of dense but evenly distributed supercrystals on a substrate. Supercrystals were also used to make a modified electrode for electro-oxidation of glucose. This simple and organic solvent-free approach to making a large quantity of supercrystals allows an ample supply of supercrystals for studies of densely assembled nanocrystal systems and for biomedical applications

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

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

Laterally Stitched Heterostructures of Transition Metal Dichalcogenide: Chemical Vapor Deposition Growth on Lithographically Patterned Area

Two-dimensional transition metal dichalcogenides (TMDCs) have shown great promise in electronics and optoelectronics due to their unique electrical and optical properties. Heterostructured TMDC layers such as the laterally stitched TMDCs offer the advantages of better electronic contact and easier band offset tuning. Here, we demonstrate a photoresist-free focused ion beam (FIB) method to pattern as-grown TMDC monolayers by chemical vapor deposition, where the exposed edges from FIB etching serve as the seeds for growing a second TMDC material to form desired lateral heterostructures with arbitrary layouts. The proposed lithographic and growth processes offer better controllability for fabrication of the TMDC heterostrucuture, which enables the construction of devices based on heterostructural monolayers

Surfactant-Directed Fabrication of Supercrystals from the Assembly of Polyhedral Au–Pd Core–Shell Nanocrystals and Their Electrical and Optical Properties

Au–Pd core–shell nanocrystals with cubic, truncated cubic, cuboctahedral, truncated octahedral, and octahedral structures have been employed to form micrometer-sized polyhedral supercrystals by both the droplet evaporation method and novel surfactant diffusion methods. Observation of cross-sectional samples indicates shape preservation of interior nanocrystals within a supercrystal. Low-angle X-ray diffraction techniques and electron microscopy have been used to confirm the presence of surfactant between contacting nanocrystals. By diluting the nanocrystal concentration or increasing the solution temperature, supercrystal size can be tuned gradually to well below 1 μm using the surfactant diffusion method. Rectangular supercrystal microbars were obtained by increasing the amounts of cubic nanocrystals and surfactant used. Au–Ag core–shell cubes and PbS cubes with sizes of 30–40 nm have also been fabricated into supercrystals, showing the generality of the surfactant diffusion approach to form supercrystals with diverse composition. Electrical conductivity measurements on single Au–Pd supercrystals reveal loss of metallic conductivity due to the presence of insulating surfactant. Cubic Au–Pd supercrystals show infrared absorption at 3.2 μm due to extensive plasmon coupling. Mie-type resonances centered at 9.8 μm for the Au–Pd supercrystals disappear once the Pd shells are converted into PdH after hydrogen absorption

Functional Two-Dimensional Coordination Polymeric Layer as a Charge Barrier in Li–S Batteries

Ultrathin two-dimensional (2D) polymeric layers are capable of separating gases and molecules based on the reported size exclusion mechanism. What is equally important but missing today is an exploration of the 2D layers with charge functionality, which enables applications using the charge exclusion principle. This work demonstrates a simple and scalable method of synthesizing a free-standing 2D coordination polymer Zn₂(benzimidazolate)₂(OH)₂ at the air–water interface. The hydroxyl (−OH) groups are stoichiometrically coordinated and implement electrostatic charges in the 2D structures, providing powerful functionality as a charge barrier. Electrochemical performance of the Li–S battery shows that the Zn₂(benzimidazolate)₂(OH)₂ coordination polymer layers efficiently mitigate the polysulfide shuttling effects and largely enhance the battery capacity and cycle performance. The synthesis of the proposed coordination polymeric layers is simple, scalable, cost saving, and promising for practical use in batteries