In Situ Visualization of Self-Assembly of Charged Gold Nanoparticles

Self-assembly of Au nanoparticles (NPs) coated with positively charged cetyltrimethylammonium ions (CTA⁺) and negatively charged citrate ions in aqueous liquid cell was investigated by in situ transmission electron microscopy (TEM). Under electron illumination in TEM, the hydrated electrons will reduce the overall positive charges of the CTA⁺ covered Au NPs and decrease the repulsive electrostatic forces among NPs, leading to assembly of individual NPs into one-dimensional structures. On the contrary, the negatively charged Au NPs coated with citrate ions are steady in liquid cell regardless of electron beam intensity

Solvent-Mediated End-to-End Assembly of Gold Nanorods

We demonstrate a new method for the bottom-up assembly of anisotropic nanoparticles, showing that alkanethiol molecules can induce controlled end-to-end assembly of gold nanorods in mixed water/acetonitrile solutions. The assembly is driven by solvent-mediated interactions among hydrophobic alkanethiol ligands selectively bound to the ends of the nanorods and among hydrophilic cetyltrimethylammonium bromide (CTAB) surfactants on the sides of the rods. It occurs only when the gold-nanorod samples have been aged for approximately two weeks. We compare the kinetics of solvent-mediated assembly using undecanethiol ligands to assembly processes driven by covalent bonding using α,ω-undecanedithiol ligands and processes driven by hydrogen bonding using 11-mercaptoundecanoic acid ligands. Our experiments demonstrate the different assembly mechanisms involved as well as the conditions needed to obtain selective end-to-end assembly

Strong Resistance to Bending Observed for Nanoparticle Membranes

We demonstrate how gold nanoparticle monolayers can be curled up into hollow scrolls that make it possible to extract both bending and stretching moduli from indentation by atomic force microscopy. We find a bending modulus that is 2 orders of magnitude larger than predicted by standard continuum elasticity, an enhancement we associate with nonlocal microstructural constraints. This finding opens up new opportunities for independent control of resistance to bending and stretching at the nanoscale

Size-Dependent Coherent-Phonon Plasmon Modulation and Deformation Characterization in Gold Bipyramids and Nanojavelins

Localized surface plasmon resonances (LSPRs) arising from metallic nanoparticles offer an array of prospective applications that range from chemical sensing to biotherapies. Bipyramidal particles exhibit particularly narrow ensemble LSPR resonances that reflect small dispersity of size and shape but until recently were only synthetically accessible over a limited range of sizes with corresponding aspect ratios. Narrow size dispersion offers the opportunity to examine ensemble dynamical phenomena such as coherent phonons that induce periodic oscillations of the LSPR energy. Here, we characterize transient optical behavior of a large range of gold bipyramid sizes, as well as higher aspect ratio nanojavelin ensembles with specific attention to the lowest-order acoustic phonon mode of these nanoparticles. We report coherent phonon-driven oscillations of the LSPR position for particles with resonances spanning 670 to 1330 nm. Nanojavelins were shown to behave similarly to bipyramids but offer the prospect of separate control over LSPR energy and coherent phonon oscillation period. We develop a new methodology for quantitatively measuring mechanical expansion caused by photogenerated coherent phonons. Using this method, we find an elongation of approximately 1% per photon absorbed per unit cell and that particle expansion along the lowest frequency acoustic phonon mode is linearly proportional to excitation fluence for the fluence range studied. These characterizations provide insight regarding means to manipulate phonon period and transient mechanical deformation

Self-Assembled Nanoparticle Drumhead Resonators

The self-assembly of nanoscale structures from functional nanoparticles has provided a powerful path to developing devices with emergent properties from the bottom-up. Here we demonstrate that freestanding sheets self-assembled from various nanoparticles form versatile nanomechanical resonators in the megahertz frequency range. Using spatially resolved laser-interferometry to measure thermal vibrational spectra and image vibration modes, we show that their dynamic behavior is in excellent agreement with linear elastic response for prestressed drumheads of negligible bending stiffness. Fabricated in a simple one-step drying-mediated process, these resonators are highly robust and their inorganic–organic hybrid nature offers an extremely low mass, low stiffness, and the potential to couple the intrinsic functionality of the nanoparticle building blocks to nanomechanical motion

Kinetic Pathway of Palladium Nanoparticle Sulfidation Process at High Temperatures

A significant issue related to Palladium (Pd) based catalysts is that sulfur-containing species, such as alkanethiols, can form a PdS_<i>x</i> underlayer on nanoparticle surface and subsequently poison the catalysts. Understanding the exact reaction pathway, the degree of sulfidation, the chemical stoichiometry, and the temperature dependence of this process is critically important. Combining energy-filtered transmission electron microscopy (EFTEM), X-ray diffraction (XRD), and X-ray absorption spectroscopy experiments at the S <i>K-</i>, Pd <i>K</i>-, and <i>L</i>_2,3-edges, we show the kinetic pathway of Pd nanoparticle sulfidation process with the addition of excess amount of octadecanethiol at different temperatures, up to 250 °C. We demonstrate that the initial polycrystalline Pd-oleylamine nanoparticles gradually become amorphous PdS_<i>x</i> nanoparticles, with the sulfur atomic concentration eventually saturating at Pd/S = 66:34 at 200 °C. This final chemical stoichiometry of the sulfurized nanoparticles closely matches that of the crystalline P₁₆S₇ phase (30.4% S), albeit being structurally amorphous. Sulfur diffusion into the nanoparticle depends strongly on the temperature. At 90 °C, sulfidation remains limited at the surface of nanoparticles even with extended heating time; whereas at higher temperatures beyond 125 °C, sulfidation occurs rapidly in the interior of the particles, far beyond what can be described as a core–shell model. This indicates sulfur diffusion from the surface to the interior of the particle is subject to a diffusion barrier and likely first go through the grain boundaries of the nanoparticle

Electrooxidative Tandem Cyclization of Activated Alkynes with Sulfinic Acids To Access Sulfonated Indenones

An electrooxidative direct arylsulfonlylation of ynones with sulfinic acids via a radical tandem cyclization strategy has been developed for the construction of sulfonated indenones under oxidant-free conditions. This method provides a simple and efficient approach to prepare various sulfonylindenones in good to excellent yields, demonstrating the tremendous prospect of utilizing electrocatalysis in oxidative coupling. Notably, this reaction could be easily scaled up with good efficiency

Low-Pressure Flow Chemistry of CuAAC Click Reaction Catalyzed by Nanoporous AuCu Membrane

Click chemistry has been widely used in bioconjugation, polymer synthesis, and the development of new anticancer drugs. Here, we report a nanoporous membrane made of AuCu alloy nanowires, which can effectively catalyze copper­(I)-catalyzed 1,3-dipolar cycloaddition between azide and terminal alkyne (CuAAC) in flow condition with pressure less than one bar. Comparison studies of the nanowires before and after the reaction using X-ray photoelectron spectroscopy reveal Cu(0) and Cu­(I) are main species that promote the reaction. This simple strategy can be used to synthesize a variety of compounds with triazole linkage and extended to gram level chemical production

Phonon-Driven Oscillatory Plasmonic Excitonic Nanomaterials

We demonstrate that coherent acoustic phonons derived from plasmonic nanoparticles can modulate electronic interactions with proximal excitonic molecular species. A series of gold bipyramids with systematically varied aspect ratios and corresponding localized surface plasmon resonance energies, functionalized with a J-aggregated thiacarbocyanine dye molecule, produces two hybridized states that exhibit clear anticrossing behavior with a Rabi splitting energy of 120 meV. In metal nanoparticles, photoexcitation generates coherent acoustic phonons that cause oscillations in the plasmon resonance energy. In the coupled system, these photogenerated oscillations alter the metal nanoparticle’s energetic contribution to the hybridized system and, as a result, change the coupling between the plasmon and exciton. We demonstrate that such modulations in the hybridization are consistent across a wide range of bipyramid ensembles. We also use finite-difference time domain calculations to develop a simple model describing this behavior. Such oscillatory plasmonic-excitonic nanomaterials offer a route to manipulate and dynamically tune the interactions of plasmonic/excitonic systems and unlock a range of potential applications

Binary Transition-Metal Oxide Hollow Nanoparticles for Oxygen Evolution Reaction

Low-cost transition metal oxides are actively explored as alternative materials to precious metal-based electrocatalysts for the challenging multistep oxygen evolution reaction (OER). We utilized the Kirkendall effect allowing the formation of hollow polycrystalline, highly disordered nanoparticles (NPs) to synthesize highly active binary metal oxide OER electrocatalysts in alkali media. Two synthetic strategies were applied to achieve compositional control in binary transition metal oxide hollow NPs. The first strategy is capitalized on the oxidation of transition-metal NP seeds in the presence of other transition-metal cations. Oxidation of Fe NPs treated with Ni (+2) cations allowed the synthesis of hollow oxide NPs with a 1–4.7 Ni-to-Fe ratio via an oxidation-induced doping mechanism. Hollow Fe–Ni oxide NPs also reached a current density of 10 mA/cm² at 0.30 V overpotential. The second strategy is based on the direct oxidation of iron–cobalt alloy NPs which allows the synthesis of hollow Fe_<i>x</i>Co_{100–<i>x</i>}-oxide NPs where <i>x</i> can be tuned in the range between 36 and 100. Hollow Fe₃₆Co₆₄-oxide NPs also revealed the current density of 10 mA/cm² at 0.30 V overpotential in 0.1 M KOH