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1.
Artigo em Inglês | MEDLINE | ID: mdl-32880446

RESUMO

Thin film electrodes, produced by coating a conductive support with a thin layer (nm to µm) of active material, retain the unique properties of nanomaterials (e.g., activity, surface area, conductivity etc.) while being economically scalable, making them highly desirable as electrocatalysts. Despite the ever-increasing methods of thin film deposition (e.g., wet chemical synthesis, electrodeposition, chemical vapor deposition etc.), there is a lack of understanding on the nanoscale electrochemical activity of these materials in relation to structure/composition, particularly for those that lack long-range order (i.e., amorphous thin film materials). In this work, scanning electrochemical cell microscopy (SECCM) is deployed in tandem with complementary, co-located compositional/structural analysis to understand the microscopic factors governing the electrochemical activity of amorphous molybdenum sulfide (a-MoSx) thin films, a promising class of hydrogen evolution reaction (HER) catalyst. The a-MoSx thin films, produced under ambient conditions by electrodeposition, possess spatially-heterogeneous electrocatalytic activity on the tens-of-micron scale, which is not attributable to microscopic variations in elemental composition or chemical structure (i.e., Mo and/or S bonding environments), shown through co-located, local energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analysis. A new SECCM protocol is implemented to directly correlate electrochemical activity to the electrochemical surface area (ECSA) in a single measurement, revealing that the spatially-heterogeneous HER response of a-MoSx is predominantly attributable to variations in the nanoscale roughness/porosity of the thin film. As microscopic composition, structure and porosity (ECSA) are all critical factors dictating the functional properties of nanostructured materials in electrocatalysis and beyond (e.g., battery materials, electrochemical sensors etc.), this work further cements SECCM as a premier tool for structure-function studies in (electro)materials science.

2.
Anal Chem ; 2020 Aug 24.
Artigo em Inglês | MEDLINE | ID: mdl-32786472

RESUMO

Electrochemical impedance spectroscopy (EIS) is a versatile tool for electrochemistry, particularly when applied locally to reveal the properties and dynamics of heterogeneous interfaces. A new method to generate local electrochemical impedance spectra is outlined, by applying a harmonic bias between a quasi-reference counter electrode (QRCE) placed in a nanopipet tip of a scanning ion conductance microscope (SICM) and a conductive (working electrode) substrate (two-electrode setup). The AC frequency can be tuned so that the magnitude of the impedance is sensitive to the tip-to-substrate distance, whereas the phase angle is broadly defined by the local capacitive response of the electrical double layer (EDL) of the working electrode. This development enables the surface topography and the local capacitance to be sensed reliably, and separately, in a single measurement. Further, self-referencing the probe impedance near the surface to that in the bulk solution allows the local capacitive response of the working electrode substrate in the overall AC signal to be determined, establishing a quantitative footing for the methodology. The spatial resolution of AC-SICM is an order of magnitude larger than the tip size (100 nm radius), for the studies herein, due to frequency dispersion. Comprehensive finite element method (FEM) modeling is undertaken to optimize the experimental conditions and minimize the experimental artifacts originating from the frequency dispersion phenomenon, and provides an avenue to explore the means by which the spatial resolution could be further improved.

3.
Artigo em Inglês | MEDLINE | ID: mdl-32633454

RESUMO

Achieving control over the size distribution of metal-organic-framework (MOF) nanoparticles is key to biomedical applications and seeding techniques. Electrochemical control over the nanoparticle synthesis of the MOF, HKUST-1, is achieved using a nanopipette injection method to locally mix Cu2+ salt precursor and benzene-1,3,5-tricarboxylate (BTC3- ) ligand reagents, to form MOF nanocrystals, and collect and characterise them on a TEM grid. In situ analysis of the size and translocation frequency of HKUST-1 nanoparticles is demonstrated, using the nanopipette to detect resistive pulses as nanoparticles form. Complementary modelling of mass transport in the electric field, enables particle size to be estimated and explains the feasibility of particular reaction conditions, including inhibitory effects of excess BTC3- . These new methods should be applicable to a variety of MOFs, and scaling up synthesis possible via arrays of nanoscale reaction centres, for example using nanopore membranes.

4.
Anal Chem ; 92(17): 11673-11680, 2020 Sep 01.
Artigo em Inglês | MEDLINE | ID: mdl-32521997

RESUMO

Many applications in modern electrochemistry, notably electrosynthesis and energy storage/conversion take advantage of the "tunable" physicochemical properties (e.g., proton availability and/or electrochemical stability) of nonaqueous (e.g., aprotic) electrolyte media. This work develops general guidelines pertaining to the use of scanning electrochemical cell microscopy (SECCM) in aprotic solvent electrolyte media to address contemporary structure-electrochemical activity problems. Using the simple outer-sphere Fc0/+ process (Fc = ferrocene) as a model system, high boiling point (low vapor pressure) solvents give rise to highly robust and reproducible electrochemistry, whereas volatile (low boiling point) solvents need to be mixed with suitable low melting point supporting electrolytes (e.g., ionic liquids) or high boiling point solvents to avoid complications associated with salt precipitation/crystallization on the scanning (minutes to hours) time scale. When applied to perform microfabrication-specifically the electrosynthesis of the conductive polymer, polypyrrole-the optimized SECCM set up produces highly reproducible arrays of synthesized (electrodeposited) material on a commensurate scale to the employed pipet probe. Applying SECCM to map electrocatalytic activity-specifically the electro-oxidation of iodide at polycrystalline platinum-reveals unique (i.e., structure-dependent) patterns of surface activity, with grains of specific crystallographic orientation, grain boundaries and areas of high local surface misorientation identified as potential electrocatalytic "hot spots". The work herein further cements SECCM as a premier technique for structure-function-activity studies in (electro)materials science and will open up exciting new possibilities through the use of aprotic solvents for rational analysis/design in electrosynthesis, microfabrication, electrochemical energy storage/conversion, and beyond.

6.
J R Soc Interface ; 17(166): 20200013, 2020 May.
Artigo em Inglês | MEDLINE | ID: mdl-32429828

RESUMO

The last five decades of molecular and systems biology research have provided unprecedented insights into the molecular and genetic basis of many cellular processes. Despite these insights, however, it is arguable that there is still only limited predictive understanding of cell behaviours. In particular, the basis of heterogeneity in single-cell behaviour and the initiation of many different metabolic, transcriptional or mechanical responses to environmental stimuli remain largely unexplained. To go beyond the status quo, the understanding of cell behaviours emerging from molecular genetics must be complemented with physical and physiological ones, focusing on the intracellular and extracellular conditions within and around cells. Here, we argue that such a combination of genetics, physics and physiology can be grounded on a bioelectrical conceptualization of cells. We motivate the reasoning behind such a proposal and describe examples where a bioelectrical view has been shown to, or can, provide predictive biological understanding. In addition, we discuss how this view opens up novel ways to control cell behaviours by electrical and electrochemical means, setting the stage for the emergence of bioelectrical engineering.

7.
ACS Nano ; 13(11): 13271-13284, 2019 Nov 26.
Artigo em Inglês | MEDLINE | ID: mdl-31674763

RESUMO

Conductive polymers are exceptionally promising for modular electrochemical applications including chemical sensors, bioelectronics, redox-flow batteries, and photoelectrochemical systems due to considerable synthetic tunability and ease of processing. Despite well-established structural heterogeneity in these systems, conventional macroscopic electroanalytical methods-specifically cyclic voltammetry-are typically used as the primary tool for structure-property elucidation. This work presents an alternative correlative multimicroscopy strategy. Data from laboratory and synchrotron-based microspectroscopies, including conducting-atomic force microscopy and synchrotron nanoscale infrared spectroscopy, are combined with potentiodynamic movies of electrochemical fluxes from scanning electrochemical cell microscopy (SECCM) to reveal the relationship between electrode structure and activity. A model conductive polymer electrode system of tailored heterogeneity is investigated, consisting of phase-segregated domains of poly(3-hexylthiophene) (P3HT) surrounded by contiguous regions of insulating poly(methyl methacrylate) (PMMA), representing an ultramicroelectrode array. Isolated domains of P3HT are shown to retain bulk-like chemical and electronic structure when blended with PMMA and possess approximately equivalent electron-transfer rate constants compared to pure P3HT electrodes. The nanoscale electrochemical data are used to model and predict multiscale electrochemical behavior, revealing that macroscopic cyclic voltammograms should be much more kinetically facile than observed experimentally. This indicates that parasitic resistances rather than redox kinetics play a dominant role in macroscopic measurements in these conductive polymer systems. SECCM further demonstrates that the ambient degradation of the P3HT electroactivity within P3HT/PMMA blends is spatially heterogeneous. This work serves as a roadmap for benchmarking the quality of conductive polymer films as electrodes, emphasizing the importance of nanoscale electrochemical measurements in understanding macroscopic properties.

8.
Anal Chem ; 91(23): 14854-14859, 2019 Dec 03.
Artigo em Inglês | MEDLINE | ID: mdl-31674764

RESUMO

As part of the revolution in electrochemical nanoscience, there is growing interest in using electrochemistry to create nanostructured materials and to assess properties at the nanoscale. Herein, we present a platform that combines scanning electrochemical cell microscopy with ex situ scanning transmission electron microscopy to allow the ready creation of an array of nanostructures coupled with atomic-scale analysis. As an illustrative example, we explore the electrodeposition of Pt at carbon-coated transmission electron microscopy (TEM) grid supports, where in a single high-throughput experiment it is shown that Pt nanoparticle (PtNP) density increases and size polydispersity decreases with increasing overpotential (i.e., driving force). Furthermore, the coexistence of a range of nanostructures, from single atoms to aggregates of crystalline PtNPs, during the early stages of electrochemical nucleation and growth supports a nonclassical aggregative growth mechanism. Beyond this exemplary system, the presented correlative electrochemistry-microscopy approach is generally applicable to solve ubiquitous structure-function problems in electrochemical science and beyond, positioning it as a powerful platform for the rational design of functional nanomaterials.

9.
ACS Sens ; 4(8): 2173-2180, 2019 08 23.
Artigo em Inglês | MEDLINE | ID: mdl-31353890

RESUMO

Screen-printed carbon electrodes (SPCEs) are widely used for electrochemical sensors. However, little is known about their electrochemical behavior at the microscopic level. In this work, we use voltammetric scanning electrochemical cell microscopy (SECCM), with dual-channel probes, to determine the microscopic factors governing the electrochemical response of SPCEs. SECCM cyclic voltammetry (CV) measurements are performed directly in hundreds of different locations of SPCEs, with high spatial resolution, using a submicrometer sized probe. Further, the localized electrode activity is spatially correlated to colocated surface structure information from scanning electron microscopy and micro-Raman spectroscopy. This approach is applied to two model electrochemical processes: hexaammineruthenium(III/II) ([Ru(NH3)6]3+/2+), a well-known outer-sphere redox couple, and dopamine (DA), which undergoes a more complex electron-proton coupled electro-oxidation, with complications from adsorption of both DA and side-products. The electrochemical reduction of [Ru(NH3)6]3+ proceeds fairly uniformly across the surface of SPCEs on the submicrometer scale. In contrast, DA electro-oxidation shows a strong dependence on the microstructure of the SPCE. By studying this process at different concentrations of DA, the relative contributions of (i) intrinsic electrode kinetics and (ii) adsorption of DA are elucidated in detail, as a function of local electrode character and surface structure. These studies provide major new insights on the electrochemical activity of SPCEs and further position voltammetric SECCM as a powerful technique for the electrochemical imaging of complex, heterogeneous, and topographically rough electrode surfaces.

10.
Anal Chem ; 91(14): 9229-9237, 2019 Jul 16.
Artigo em Inglês | MEDLINE | ID: mdl-31251561

RESUMO

Scanning electrochemical cell microscopy (SECCM) has been applied for nanoscale (electro)activity mapping in a range of electrochemical systems but so far has almost exclusively been performed in controlled-potential (amperometric/voltammetric) modes. Herein, we consider the use of SECCM operated in a controlled-current (galvanostatic or chronopotentiometric) mode, to synchronously obtain spatially resolved electrode potential (i.e., electrochemical activity) and topographical "maps". This technique is first applied, as proof of concept, to study the electrochemically reversible [Ru(NH3)6]3+/2+ electron transfer process at a glassy carbon electrode surface, where the experimental data are in good agreement with well-established chronopotentiometric theory under quasi-radial diffusion conditions. The [Ru(NH3)6]3+/2+ process has also been imaged at "aged" highly ordered pyrolytic graphite (HOPG), where apparently enhanced electrochemical activity is measured at the edge plane relative to the basal plane surface, consistent with potentiostatic measurements. Finally, chronopotentiometric SECCM has been employed to benchmark a promising electrocatalytic system, the hydrogen evolution reaction (HER) at molybdenum disulfide (MoS2), where higher electrocatalytic activity (i.e., lower overpotential at a current density of 2 mA cm-2) is observed at the edge plane compared to the basal plane surface. These results are in excellent agreement with previous controlled-potential SECCM studies, confirming the viability of the technique and thereby opening up new possibilities for the use of chronopotentiometric methods for quantitative electroanalysis at the nanoscale, with promising applications in energy storage (battery) studies, electrocatalyst benchmarking, and corrosion research.

11.
Angew Chem Int Ed Engl ; 58(14): 4606-4611, 2019 Mar 26.
Artigo em Inglês | MEDLINE | ID: mdl-30724004

RESUMO

The redox activity (Li-ion intercalation/deintercalation) of a series of individual LiMn2 O4 particles of known geometry and (nano)structure, within an array, is determined using a correlative electrochemical microscopy strategy. Cyclic voltammetry (current-voltage curve, I-E) and galvanostatic charge/discharge (voltage-time curve, E-t) are applied at the single particle level, using scanning electrochemical cell microscopy (SECCM), together with co-location scanning electron microscopy that enables the corresponding particle size, morphology, crystallinity, and other factors to be visualized. This study identifies a wide spectrum of activity of nominally similar particles and highlights how subtle changes in particle form can greatly impact electrochemical properties. SECCM is well-suited for assessing single particles and constitutes a combinatorial method that will enable the rational design and optimization of battery electrode materials.

12.
Anal Chem ; 91(7): 4632-4639, 2019 04 02.
Artigo em Inglês | MEDLINE | ID: mdl-30807113

RESUMO

The surface charge and topography of human hair are visualized synchronously at the nanoscale using scanning ion conductance microscopy (SICM), a scanning nanopipette probe technique that uses local ion conductance currents to image the physicochemical properties of interfaces. By combining SICM data with finite element method (FEM) simulations that solve for ion transport at the nanopipette under bias, one is able to quantitatively correlate colocated surface charge and topography. The hair samples studied herein, from a 25-year-old Caucasian male with light hair (as an exemplar), reveal that untreated hair, in areas ca. 1 cm from the root, has a fairly uniform negative charge density of ca. -15 mC/cm-2 (in pH 6.8 aqueous solution), with some higher magnitude negative values localized near the boundaries between hair cuticles. Common chemical treatments result in varying degrees of charge heterogeneity. A bleach treatment produces some highly negatively charged localized regions (-80 to -100 mC/cm-2 at pH 6.8), due to hair damage, while a chemical conditioner treatment causes an overall increase in the homogeneity of the surface charge, together with a shift in the surface charge to positive values. Bleached surfaces are temporarily repaired to some extent through the use of a conditioner, as judged by the surface charge values. Finally, SICM is able to detect differences in the surface charge density of hair at different distances from the root (equivalent to hair age). Presently, the assessment of hair surface charge mainly relies on zeta-potential measurements which lack spatial resolution, among other drawbacks. In contrast, SICM enables quantitative surface charge mapping that should be beneficial in deepening understanding of the physicochemical properties of hair and lead to the rational development of new treatments and the assessment of their efficacy at the nanoscale. Given the widespread interest in the surface charge properties of interfaces, this work further demonstrates that SICM should generally become an important characterization tool for surface analytical chemists.

13.
Anal Chem ; 91(3): 2516-2524, 2019 02 05.
Artigo em Inglês | MEDLINE | ID: mdl-30608117

RESUMO

Scanning ion conductance microscopy (SICM) is becoming a powerful multifunctional tool for probing and analyzing surfaces and interfaces. This work outlines methodology for the quantitative controlled delivery of ionic redox-active molecules from a nanopipette to a substrate electrode, with a high degree of spatial and temporal precision. Through control of the SICM bias applied between a quasi-reference counter electrode (QRCE) in the SICM nanopipette probe and a similar electrode in bulk solution, it is shown that ionic redox species can be held inside the nanopipette, and then pulse-delivered to a defined region of a substrate positioned beneath the nanopipette. A self-referencing hopping mode imaging protocol is implemented, where reagent is released in bulk solution (reference measurement) and near the substrate surface at each pixel in an image, with the tip and substrate currents measured throughout. Analysis of the tip and substrate current data provides an improved understanding of mass transport and nanoscale delivery in SICM and a new means of synchronously mapping electrode reactivity, surface topography, and charge. Experiments on Ru(NH3)63+ reduction to Ru(NH3)62+ and dopamine oxidation in aqueous solution at a carbon fiber ultramicroelectrode (UME), used as the substrate, illustrate these aspects. Finite element method (FEM) modeling provides quantitative understanding of molecular delivery in SICM. The approach outlined constitutes a new methodology for electrode mapping and provides improved insights on the use of SICM for controlled delivery to interfaces generally.

15.
J Am Chem Soc ; 141(6): 2179-2193, 2019 02 13.
Artigo em Inglês | MEDLINE | ID: mdl-30485739

RESUMO

Nanostructured electrochemical interfaces (electrodes) are found in diverse applications ranging from electrocatalysis and energy storage to biomedical and environmental sensing. These functional materials, which possess compositional and structural heterogeneity over a wide range of length scales, are usually characterized by classical macroscopic or "bulk" electrochemical techniques that are not well-suited to analyzing the nonuniform fluxes that govern the electrochemical response at complex interfaces. In this Perspective, we highlight new directions to studying fundamental electrochemical and electrocatalytic phenomena, whereby nanoscale-resolved information on activity is related to electrode structure and properties colocated and at a commensurate scale by using complementary high-resolution microscopy techniques. This correlative electrochemical multimicroscopy strategy aims to unambiguously resolve structure and activity by identifying and characterizing the structural features that constitute an active surface, ultimately facilitating the rational design of functional electromaterials. The discussion encompasses high-resolution correlative structure-activity investigations at well-defined surfaces such as metal single crystals and layered materials, extended structurally/compositionally heterogeneous surfaces such as polycrystalline metals, and ensemble-type electrodes exemplified by nanoparticles on an electrode support surface. This Perspective provides a roadmap for next-generation studies in electrochemistry and electrocatalysis, advocating that complex electrode surfaces and interfaces be broken down and studied as a set of simpler "single entities" (e.g., steps, terraces, defects, crystal facets, grain boundaries, single particles), from which the resulting nanoscale understanding of reactivity can be used to create rational models, underpinned by theory and surface physics, that are self-consistent across broader length scales and time scales.

16.
Chem Commun (Camb) ; 55(5): 628-631, 2019 Jan 10.
Artigo em Inglês | MEDLINE | ID: mdl-30556069

RESUMO

A scanning electrochemical droplet cell technique has been employed to screen the intrinsic electrocatalytic hydrogen evolution reaction (HER) activity of hexagonal boron nitride (h-BN) nanosheets supported on different metal substrates (Cu and Au). Local (spatially-resolved) voltammetry and Tafel analysis reveal that electronic interaction with the underlying metal substrate plays a significant role in modulating the electrocatalytic activity of h-BN, with Au-supported h-BN exhibiting significantly enhanced HER charge-transfer kinetics (exchange current is ca. two orders of magnitude larger) compared to Cu-supported h-BN, making the former material the superior support in a catalytic sense.

17.
Faraday Discuss ; 210(0): 365-379, 2018 10 01.
Artigo em Inglês | MEDLINE | ID: mdl-29999075

RESUMO

Techniques in the scanning electrochemical probe microscopy (SEPM) family have shown great promise for resolving nanoscale structure-function (e.g., catalytic activity) at complex (electro)chemical interfaces, which is a long-term aspiration in (electro)materials science. In this work, we explore how a simple meniscus imaging probe, based on an easily-fabricated, single-channeled nanopipette (inner diameter ≈ 30 nm) can be deployed in the scanning electrochemical cell microscopy (SECCM) platform as a fast, versatile and robust method for the direct, synchronous electrochemical/topographical imaging of electrocatalytic materials at the nanoscale. Topographical and voltammetric data are acquired synchronously at a spatial resolution of 50 nm to construct maps that resolve particular surface features on the sub-10 nm scale and create electrochemical activity movies composed of hundreds of potential-resolved images on the minutes timescale. Using the hydrogen evolution reaction (HER) at molybdenite (MoS2) as an exemplar system, the experimental parameters critical to achieving a robust scanning protocol (e.g., approach voltage, reference potential calibration) with high resolution (e.g., hopping distance) and optimal scan times (e.g., voltammetric scan rate, approach rate etc.) are considered and discussed. Furthermore, sub-nanoentity reactivity mapping is demonstrated with glassy carbon (GC) supported single-crystalline {111}-oriented two-dimensional Au nanocrystals (AuNCs), which exhibit uniform catalytic activity at the single-entity and sub-single entity level. The approach outlined herein signposts a future in (electro)materials science in which the activity of electroactive nanomaterials can be viewed directly and related to structure through electrochemical movies, revealing active sites unambiguously.

18.
Anal Chem ; 90(12): 7700-7707, 2018 06 19.
Artigo em Inglês | MEDLINE | ID: mdl-29808685

RESUMO

Nanoelectrochemistry is an important and growing branch of electrochemistry that encompasses a number of key research areas, including (electro)catalysis, energy storage, biomedical/environmental sensing, and electrochemical imaging. Nanoscale electrochemical measurements are often performed in confined environments over prolonged experimental time scales with nonisolated quasi-reference counter electrodes (QRCEs) in a simplified two-electrode format. Herein, we consider the stability of commonly used Ag/AgCl QRCEs, comprising an AgCl-coated wire, in a nanopipet configuration, which simulates the confined electrochemical cell arrangement commonly encountered in nanoelectrochemical systems. Ag/AgCl QRCEs possess a very stable reference potential even when used immediately after preparation and, when deployed in Cl- free electrolyte media (e.g., 0.1 M HClO4) in the scanning ion conductance microscopy (SICM) format, drift by only ca. 1 mV h-1 on the several hours time scale. Furthermore, contrary to some previous reports, when employed in a scanning electrochemical cell microscopy (SECCM) format (meniscus contact with a working electrode surface), Ag/AgCl QRCEs do not cause fouling of the surface (i.e., with soluble redox byproducts, such as Ag+) on at least the 6 h time scale, as long as suitable precautions with respect to electrode handling and placement within the nanopipet are observed. These experimental observations are validated through finite element method (FEM) simulations, which consider Ag+ transport within a nanopipet probe in the SECCM and SICM configurations. These results confirm that Ag/AgCl is a stable and robust QRCE in confined electrochemical environments, such as in nanopipets used in SICM, for nanopore measurements, for printing and patterning, and in SECCM, justifying the widespread use of this electrode in the field of nanoelectrochemistry and beyond.

19.
Chem Commun (Camb) ; 54(24): 3053-3056, 2018 Mar 25.
Artigo em Inglês | MEDLINE | ID: mdl-29513314

RESUMO

A strong relationship between the surface structure and the redox activity of Li2O2 is visualized directly using scanning electrochemical cell microscopy, employing a dual-barrel nanopipette containing a unique gel polymer electrolyte. These measurements reveal considerable local heterogeneity with significantly enhanced electrochemical activity at toroidal Li2O2 structures when compared to the conformal layer that is usually formed on the cathode of Li-O2 batteries.

20.
Angew Chem Int Ed Engl ; 57(15): 4093-4097, 2018 04 03.
Artigo em Inglês | MEDLINE | ID: mdl-29377499

RESUMO

In order to design more powerful electrocatalysts, developing our understanding of the role of the surface structure and composition of widely abundant bulk materials is crucial. This is particularly true in the search for alternative hydrogen evolution reaction (HER) catalysts to replace platinum. We report scanning electrochemical cell microscopy (SECCM) measurements of the (111)-crystal planes of Fe4.5 Ni4.5 S8 , a highly active HER catalyst. In combination with structural characterization methods, we show that this technique can reveal differences in activity arising from even the slightest compositional changes. By probing electrochemical properties at the nanoscale, in conjunction with complementary structural information, novel design principles are revealed for application to rational material synthesis.

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