Electrocatalytic reduction reactions (i.e., the hydrogen evolution reaction (HER) and oxygen reduction reaction) at individual, faceted Au nanocubes (NCs) and nano-octahedra (ODs) expressing predominantly {100} and {111} crystal planes on the surface, respectively, were studied by nanoscale voltammetric mapping. Cyclic voltammograms were collected at individual nanoparticles (NPs) with scanning electrochemical cell microscopy (SECCM) and correlated with particle morphology imaged by electron microscopy. Nanoscale measurements from a statistically informative set of individual NPs revealed that Au NCs have superior HER electrocatalytic activity compared to that of Au ODs, in good agreement with macroscale cyclic voltammetry measurements. Au NCs exhibited more particle-to-particle variation in catalytic activity compared to that with Au ODs. The approach of single-particle SECCM imaging coupled with macroscale CV on well-defined NPs provides a powerful toolset for the design and activity assessment of nanoscale electrocatalysts.

The subphase pH effect on the surface micelles of polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymers was investigated by the π−A isotherm and the morphology of the Langmuir−Blodgett (LB) film. Ionization of P2VP block significantly affects the π−A isotherm of the surface micelles at the air−water interface. At high pH, where the degree of ionization is low, the π−A isotherm shows a more expanded form and high transition surface pressure reflecting the strong tendency of P2VP blocks spreading on the subphase surface. As the subphase pH is lowered, the transition surface pressure decreases while the transition region is more extended, indicating the facile equilibrium between the flotation and submergence of the P2VP blocks upon surface pressure change. At pH 1.8 at which the 2VP units are completely ionized, the ionized P2VP blocks submerge into the subphase even at low surface pressure, and the transition behavior is not observed. The π−A isotherm behavior can be understood considering the balance of the solubility and the electrostatic repulsion of the ionized P2VP chains. Atomic force microscopy images of the LB films of the PS-b-P2VP surface micelles show isolated circular micelles at high pH. As the subphase pH decreases, the intermicellar distance becomes shorter and the micelles eventually contact each other to form a laced network of circular micelles. The association behavior of surface micelles and the π−A isotherms at low pH are strongly dependent on the ionic strength of the subphase. The linear association of the surface micelles at low pH appears to result from the balance of the hydrophobic attraction among the floated PS cores and the electrostatic repulsion among the submerged P2VP chains. On the other hand, the average aggregation number of each surface micelle is independent of subphase pH, which indicates that the aggregation of the block copolymers to form surface micelles is likely to take place before the P2VP blocks are ionized.

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The conversion of CO 2 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 CO 2 reduction reaction (CO 2 RR) 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 CO 2 RR 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 CO 2 RR 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.

We further describe a protocol for the investigation of surface charge with scanning ion conductance microscopy. The protocol measures current-voltage curves at positions close to and far from the surface of interest and reports the differential response. The data can be interpreted in terms of rectification ratios, an intuitive quantity for such studies. With this protocol, we further investigate the effect of electrolyte concentration and study the influence of scan potential on surface charge measurement on chemically modified surfaces.Charge is a fundamental interfacial property that governs physical and chemical interactions at surfaces. The workings of catalysts, [1] sensors, [2] separation devices, [3] biological interfaces, [4] and colloidal systems, [5] are well known to be strongly influenced by surface charge, typically present in the form of protonated or deprotonated chemical moieties. Directly measuring charge in situ, especially for small (micro/nanoscale), heterogenous charge distributions presents an interesting and important challenge for electroanalytical chemistry. Here, we communicate studies in mapping interfacial charge with scanning ion conductance microscopy (SICM) [6][7][8] and the influence of electrolyte concentration on the charge sensing mechanism.When immersed in electrolyte, a charged substrate attracts counter ions and forms an electrical double layer (EDL), a key process in the consideration of nearly all electrochemical systems. With SICM, a small pipette, typically made of quartz or borosilicate, is brought near a surface of interest. The pipette is filled with electrolyte and an electrode (Ag/AgCl, WE) is placed inside the pipette, with a second electrode (RE) placed in the electrolyte solution (bath) surrounding the pipette and surface. Application of a potential between these two electrodes generates an ion current, with the dimensions of the pipette tip serving as a resistive element to the ion current. As the tip of the pipette is moved towards the surface, a distance dependent access resistance (R ac ) develops. With proper feedback methods, R ac can be used to control the vertical position of the pipette. If the tip of the pipette is small (e. g. a nanopipette), then the feedback regime occurs at distances where the EDL of the tip and the surface interact (typically on the order of the radius of the pipette opening), [9] and this interaction forms the basis for measuring or detecting the charge presented at the surface, as reflected in the EDL.Interactions between the ion current flowing through the tip of the nanopipette and the charge of a surface have been reported previously by our group [10][11][12] and others, [13][14][15][16][17][18][19][20] with charge mapping initially reported by Unwin and coworkers. [13] Charge mapping has been applied to chemically modified surfaces, [15] cell interfaces, [14,[16][17] chromosomes, [18] and supported lipid bilayers. [19,21] In previous studies, phase [13,15,17] or changes in apparent imaging height [19,21] have been used to in...

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...

Nanoelectrode ensembles (NEEs), prepared by Au template synthesis, are presented as a proof-of-concept sample platform to study individual electrodeposited materials by scanning electrochemical cell microscopy (SECCM). With this platform, the non-conductive membrane support does not contribute to the electrocatalytic activity recorded at each electrode. Use of low-density template membranes results in electrodes that are isolated because initial membrane pores are typically separated by significant (microscale) distances. Electrodeposition of catalytic nanoparticles onto the electrodes of the array and observation of electrocatalytic activity are demonstrated to be suitable for correlative SECCM voltammetric mapping and electron microscopy. Suitability of NEEs for studies of surface Au oxidation, hydrazine oxidation, and hydrogen evolution (hydrogen evolution reaction, HER), and at Pt particles on NEEs (Pt-NEEs) for HER is demonstrated.

Nanoscale fluctuations on the apical surfaces of epithelial cells connected to neighboring cells were investigated by scanning ion conductance microscopy. Mapping the ion current as a function of the tip–surface distance revealed that in untreated cells, the apparent fluctuation amplitude increased towards the cell center. We found that the spatial dependence was less correlated with the heterogeneities of cell stiffness but was significantly reduced when actin filaments were disrupted. The results indicate that apical surface fluctuations are highly constrained at the cell–cell interface, in the vertical direction to the surface and by the underlying actin filaments.