Characterization of Ligand Adsorption at Individual Gold Nanocubes

Optically Transparent Carbon Electrodes for Single Entity Electrochemistry

We demonstrate the application and benefit of optically transparent carbon electrodes (OTCEs) for single entity nanoelectrochemistry. OTCEs are prepared by pyrolyzing thin photoresist films on fused quartz coverslips to create conductive, transparent, thin films. Optical, electrical, topographical, and electrochemical properties of OTCEs are characterized to evaluate their suitability for single entity electrochemistry. Nanoscale electrochemical imaging of the OTCEs using scanning electrochemical cell microscopy (SECCM) revealed uniform electrochemical activity for reduction of the hexaammineruthenium(III) redox complex, that was comparable to Au-coated glass, and in contrast to the heterogeneity observed with commonly used indium tin oxide (ITO) substrates. Additionally, we demonstrate the utility of the prepared OTCEs for correlative SECCM-scanning electron microscopy studies of the hydrogen evolution reaction at the surface of Au nanocubes. Lastly, we demonstrate the benefit of OTCEs for optoelectrochemical experiments by optically monitoring the electrodissolution of Au nanocrystals. These results establish OTCE as a viable transparent support electrode for multimode electrochemical and optical microscopy of nanocrystals and other entities.

Characterization of Surface Patterning on Polymer-Grafted Nanocubes Using Atomic Force Microscopy and Force Volume Mapping

Atomic force microscopy (AFM), in particular force spectroscopy, is a powerful tool for understanding the supramolecular structures associated with polymers grafted to surfaces, especially in regimes of low polymer density where different morphological structures are expected. In this study, we utilize force volume mapping to characterize the nanoscale surfaces of Ag nanocubes (AgNCs) grafted with a monolayer of polyethylene glycol (PEG) chains. Spatially resolved force-distance curves taken for a single AgNC were used to map surface properties, such as adhesion energy and deformation. We confirm the presence of surface octopus micelles that are localized on the corners of the AgNC, using force curves to resolve structural differences between the micelle "bodies" and "legs". Furthermore, we observe unique features of this system including a polymer corona stemming from AgNC-substrate interactions and polymer bridging stemming from particle-particle interactions.

Direct visualization of ligands on gold nanoparticles in a liquid environment

The interactions between gold nanoparticles, their surface ligands and the solvent critically influence the properties of these nanoparticles. Although spectroscopic and scattering techniques have been used to investigate their ensemble structure, a comprehensive understanding of these processes at the nanoscale remains challenging. Electron microscopy makes it possible to characterize the local structure and composition but is limited by insufficient contrast, electron beam sensitivity and the requirement for ultrahigh-vacuum conditions, which prevent the investigation of dynamic aspects. Here we show that, by exploiting high-quality graphene liquid cells, we can overcome these limitations and investigate the structure of the ligand shell around gold nanoparticles and at the ligand-gold interface in a liquid environment. Using this graphene liquid cell, we visualize the anisotropy, composition and dynamics of ligand distribution on gold nanorod surfaces. Our results indicate a micellar model for surfactant organization. This work provides a reliable and direct visualization of ligand distribution around colloidal nanoparticles.

Bridging colloidal and electrochemical syntheses of metal nanocrystals with seeded electrodeposition for tracking single nanocrystal growth

Metal nanocrystals (NCs) produced by colloidal synthesis have a variety of structural features, such as different planes, edges, and defects. Even from the best colloidal syntheses, these characteristics are distributed differently in each NC. This inherent heterogeneity can play a significant role in the properties displayed by NCs. This manuscript reports the use of electrochemistry to synthesize Au NCs in a system evaluated to track individual NC growth trajectories as a first step toward rapid identification of NCs with different structural features. Au nanocubes were prepared colloidally and deposited onto a glassy carbon electrode using either electrospray or an airbrush, resulting in well-spaced Au nanocubes. The Au nanocubes then served as seeds as gold salt was reduced to deposit metal at constant potential. Deposition at constant potential facilitates overgrowth on the Au nanocubes to achieve new NC shapes. The effects of applied potential, deposition time, precursor concentration, and capping agents on NC shape evolution were studied. The outcomes are correlated to results from traditional colloidal syntheses, providing a bridge between the two synthetic strategies. Moreover, scanning electron microscopy was used to image the same NCs before and after deposition, linking individual seed features to differences in deposition. This ability is anticipated to enable tracking of individual growth trajectories of NCs to elucidate sources of heterogeneity in NC syntheses.

Molecular-scale Insights into Cooperativity Switching of xTAB Adsorption on Gold Nanoparticles

Quantifying adsorption behaviors is crucial for various applications such as catalysis, separation, and sensing, yet it is generally challenging to access in solution. Here, we report a combined experimental and computational study of the adsorption behaviors of alkyl-trimethylammonium bromides (xTAB), a class of ligands important for colloidal nanoparticle stabilization and shape control, with various alkyl chain lengths x on Au nanoparticles. We use density functional theory (DFT) to calculate xTAB binding energies on Au{111} and Au{110} surfaces with standing-up and lying-down configurations, which provides insights into the adsorption affinity and cooperativity differences of xTAB on these two facets. We demonstrate the key role of van der Waals interactions in determining the xTAB adsorption behavior. These computational results predict and explain the experimental discovery of xTAB's adsorption behavior switch from stronger affinity, negative cooperativity to weaker affinity, positive cooperativity when the concentration of xTAB increases in solution. We also show that in the standing-up configuration, bilayer adsorption may occur on both facets, which can lead to different differential binding energies and consequently adsorption crossover between the two facets when the ligand concentration increases. Our combined experimental and computational approaches demonstrate a paradigm for gaining molecular-scale insights into adsorbate-surface interactions.

Chemical and Physical Properties of Photonic Noble-Metal Nanomaterials

Colloidal noble metal nanoparticles (NPs) are composed of metal cores and organic or inorganic ligand shells. These NPs support size- and shape-dependent plasmonic resonances. They can be assembled from dispersions into artificial metamolecules which have collective plasmonic resonances originating from coupled bright and dark optical electric and magnetic modes that form depending on the size and shape of the constituent NPs and their number, arrangement, and interparticle distance. NPs can also be assembled into extended 2D and 3D metamaterials that are glassy thin films or ordered thin films or crystals, also known as superlattices and supercrystals. The metamaterials have tunable optical properties that depend on the size, shape, and composition of the NPs, and on the number of NP layers and their interparticle distance. Interestingly, strong light-matter interactions in superlattices form plasmon polaritons. Tunable interparticle distances allow designer materials with dielectric functions tailorable from that characteristic of an insulator to that of a metal, and serve as strong optical absorbers or scatterers, respectively. In combination with lithography techniques, these extended assemblies can be patterned to create subwavelength NP superstructures and form large-area 2D and 3D metamaterials that manipulate the amplitude, phase, and polarization of transmitted or reflected light.

Unraveling the Structural Sensitivity of CO2 Electroreduction at Facet-Defined Nanocrystals via Correlative Single-Entity and Macroelectrode Measurements

The conversion of CO₂ into value-added products is a compelling way of storing energy derived from intermittent renewable sources and can bring us closer to a closed-loop anthropogenic carbon cycle. The ability to synthesize nanocrystals of well-defined structure and composition has invigorated catalysis science with the promise of nanocrystals that selectively express the most favorable sites for efficient catalysis. The performance of nanocrystal catalysts for the CO₂ reduction reaction (CO₂RR) is typically evaluated with nanocrystal ensembles, which returns an averaged system-level response of complex catalyst-modified electrodes with each nanocrystal likely contributing a different (unknown) amount. Measurements at single nanocrystals, taken in the context of statistical analysis of a population, and comparison to macroscale measurements are necessary to untangle the complexity of the ever-present heterogeneity in nanocrystal catalysts, achieve true structure-property correlation, and potentially identify nanocrystals with outlier performance. Here, we employ environment-controlled scanning electrochemical cell microscopy to isolate and investigate the electrocatalytic CO₂RR response of individual facet-defined gold nanocrystals. Using correlative microscopy approaches, we conclusively demonstrate that {110}-terminated gold rhombohedra possess superior activity and selectivity for CO₂RR compared with {111}-terminated octahedra and high-index {310}-terminated truncated ditetragonal prisms, especially at low overpotentials where electrode kinetics is anticipated to dominate the current response. The methodology framework described here could inform future studies of complex electrocatalytic processes through correlative single-entity and macroscale measurement techniques.

Electrospray deposition for single nanoparticle studies

Single entity electrochemical (SEE) studies that can probe activities and heterogeneity in activities at nanoscale require samples that contain single and isolated particles. Single, isolated nanoparticles are achieved here with electrospray deposition of colloidal nanoparticle solutions, with simple instrumentation. Role of three electrospray (ES) parameters, viz. spray distance (emitter tip-to-substrate distance), ES current and emitter tip diameter, in the ES deposition of single Au nano-octahedra (Au ODs) is examined. The ES deposition of single, isolated Au ODs are analyzed in terms of percentage of single NPs and local surface density of deposition. The local surface density of ES deposition of single Au ODs was found to increase with decrease in spray distance and emitter tip diameter, and increase in ES current. While the percentage of single particle ES deposition increased with increase in spray distance and decrease in emitter tip size. No significant change in the single Au ODs ES deposition percentage was observed with change in ES current values included in this study. The most favourable conditions in the ES deposition of Au ODs in this study resulted in the local surface density of 0.26 ± 0.05 single particles per μm² and observation of 96.3% single Au OD deposition.