Porous materials provide a plethora of technologically important applications that encompass molecular separations, catalysis, and adsorption. The majority of research in this field involves network solids constructed from multitopic constituents that, when assembled either covalently or ionically, afford macromolecular arrangements with micro- or meso-porous apertures. Recently, porous solids fabricated from discrete organic cages have garnered much interest due to their ease of handling and solution processability. Although this class of materials is a promising alternative to network solids, fundamental studies are still required to elucidate critical structure-function relationships that govern microporosity. Here, we report a systematic investigation of the effects of building block shape-persistence on the porosity of molecular cages. Alkyne metathesis and edge-specific postsynthetic modifications afforded three organic cages with alkynyl, alkenyl, and alkyl edges, respectively. Nitrogen adsorption experiments conducted on rapidly crystallized and slowly crystallized solids illustrated a general trend in porosity: alkynyl > alkenyl > alkyl. To understand the molecular-scale origin of this trend, we investigated the short and long time scale molecular motions of the molecular cages using ab initio molecular dynamics (AIMD) and classical molecular dynamics (MD) simulations. Our combined experimental and computational results demonstrate that the microporosity of molecular cages directly correlates with shape persistence. These findings discern fundamental molecular requirements for rationally designing porous molecular solids.
Recent applications of ultrasound to the production of nanostructured materials are reviewed. Sonochemistry permits the production of novel materials or provides a route to known materials without the need for high bulk temperatures, pressures, or long reaction times. Both chemical and physical phenomena associated with high-intensity ultrasound are responsible for the production or modification of nanomaterials. Most notable are the consequences of acoustic cavitation: the formation, growth, and implosive collapse of bubbles, and can be categorized as primary sonochemistry (gas-phase chemistry occurring inside collapsing bubbles), secondary sonochemistry (solution-phase chemistry occurring outside the bubbles), and physical modifications (caused by high-speed jets, shockwaves, or inter-particle collisions in slurries).
Spray-coating using ultrasonic nebulization is reported for depositing nanoparticles on a TEM grid without many of the drying artifacts that are often associated with dropcasting. Spray-coating is suitable for preparing TEM samples on fragile support materials, such as suspended single-layer graphene, that rupture when samples are prepared by dropcasting. Additionally, because ultrasonic nebulization produces uniform droplets, nanoparticles deposited by spray-coating occur on the TEM grid in clusters, whose size is dependent on the concentration of the nanoparticle dispersion, which may allow the concentration of nanoparticle dispersions to be estimated using TEM.
Household items coated with various metals or titanium compounds can be heated to produce colorful films with nanoscale thicknesses.
Soft-hard interfaces at the surface of nanoparticles determine interaction potentials, including the mechanisms of growth, spatial reactivity, colloidal stability, and nanoparticle functionality [1]. For example, soft molecular ligands are thought to guide growth and symmetry breaking in anisotropic nanoparticles. These ligands can also act as soft templates for site-selective deposition of functional coatings [2][3][4]. Quantitative details regarding the local attachment, distribution, and structure of these softhard interfaces would enable the development of methods for high-yield, monodisperse nanoparticle synthesis for applications ranging from catalysis [5] to drug delivery [6].Conventional techniques to characterize soft-hard interfaces-such as nuclear magnetic resonance, small angle x-ray scattering, and other bulk methods-lack the spatial resolution necessary to probe key details of the soft-hard interface, including how the surface structure and chemistry varies within and between individual particles [1]. Such limitations are critical in studying nanoparticles, where polydispersity is a defining characteristic and where surface energies and interactions vary widely based on local facet, curvature, and composition. While electron microscopy (EM) can in principle address this challenge, an ideal EM approach requires a combination of: low-background substrates, the ability to quantify elemental distributions of small molecules, and the speed and efficiency necessary to probe multiple nanoparticles. Here we report a particle-by-particle analysis of soft-hard interfaces on gold nanorods (AuNRs) using aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) spectrum imaging on graphene substrates.In order to demonstrate the ability of EM methods to probe soft-hard interfaces, we investigate anisotropic mesoporous silica functionalization of AuNRs. Mesoporous silica can be deposited on AuNRs with site selectivity to either the ends or the sides. Such growth is thought to be templated by the anisotropic distribution of capping ligand cetyl trimethylammonium bromide (CTAB) [2, 3]. We spray deposited these mesoporous silica-coated AuNRs onto suspended graphene substrates. Using EELS spectral imaging, we map the presence of carbon, silicon, and oxygen. We are able to directly observe a mesoporous silica frame with carbon containing pores. On end silica-coated AuNRs, we identify the radial orientation of the pores and a 2.5 nm average pore size (Figure 1). The side coated AuNR pores are predominantly oriented parallel to the transverse axis with a 1.6 nm pore size (Figure 2). Furthermore, we observe an increased carbon signal on the surface of the nanoparticles, indicating the presence of a residual CTAB shell directly surrounding the particle (Figure 1d, 2d). Using graphene as a reference for the carbon inelastic scattering cross section, we are able to quantify the CTAB present before and after silica deposition. These results indicate that before deposit...
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