The kinetics of particle nucleation and growth are critical to a wide variety of electrochemical systems. While studies carried out at the single particle level are promising for improving our understanding of nucleation and growth processes, conventional analytical frameworks commonly employed in bulk studies may not be appropriate for single particle experiments. Here, we present scanning electrochemical cell microscopy (SECCM) studies of Ag nucleation and growth on carbon and indium tin oxide (ITO) electrodes. Statistical analyses of the data from these experiments reveal significant discrepancies with traditional, quasi-equilibrium kinetic models commonly employed in the analysis of particle nucleation in electrochemical systems. Time-dependent kinetic models are presented capable of appropriately analysing the data generated via SECCM to extract meaningful chemical quantities such as surface energies and kinetic rate constants. These results demonstrate a powerful new approach to the analysis of single particle nucleation and growth data which could be leveraged in differentiating behavior within spatially heterogeneous systems.

Rare earth elements (REEs) are a class of critical materials vital to an array of applications such as electronics, batteries, and defense weapons systems. These elements coexist in minerals and ores, and due to their similar chemical properties, the challenge remains to find an effective means of separation. Current large-scale separation processes rely on slight differences in size and acidity between REEs, however these separations are inefficient and pose detrimental risks to the environment due to the large volumes of volatile organics and acids required to achieve separations. REEs can be separated on the basis of their migration under an applied electric field, i.e., their electrophoretic mobility, which depends on differences in size and charge and may be modified through coordination with ligands. The degree to which coordination with ligands may influence the transport properties of these metal-ligand systems remains, for the most part, unknown. Here, we investigate the fundamental transport properties of a series of REE-carboxylate ligand complexes in aqueous solution via capillary electrophoresis and molecular dynamics (MD) simulations are carried out to validate metal-ligand coordination geometry and diffusion properties. Iminodiacetic acid (IDA) is identified as a ligand that greatly enhances separation between light and heavy lanthanides by selectively coordinating with light lanthanides in a tridentate mode and heavy lanthanides in a bidentate mode.

Colloidal nanoparticles are inherently heterogeneous, exhibiting variations in size, shape, or composition that will impact their catalytic behavior. Understanding these particle-to-particle variations in catalytic behavior will be critical to realizing more stable, selective, and efficient catalyst systems, but it remains difficult to generate this understanding using conventional characterization techniques. Here, we demonstrate how targeted electrochemical cell microscopy (TECCM) can be utilized to rigorously evaluate the electrocatalytic behavior of hundreds of individual metal nanoparticle catalysts, enabling statistically meaningful insights into these systems to be generated. The electrocatalytic oxidation of hydrazine was studied in a series of Au nanoparticle systems (nanorods, nanospheres, triangular nanoprisms, and nanocubes), directly revealing particle-to-particle variations in key kinetic parameters and catalyst stability. On average, "sharper" nanoparticle geometries were found to exhibit higher initial activities but quickly degraded upon potential cycling. Interestingly, our single particle studies also reveal that while the smoother nanorod and nanosphere geometries exhibit stable behavior in an ensemble sense, this is in fact due to a complicated balance of populations which exhibit increasing or decreasing catalytic behavior over typical experimental time scales. Results from correlated optical spectroscopy and electron microscopy experiments suggest that these observed changes in catalytic behavior are not associated with significant changes in particle structure. Together, these results demonstrate the extensive heterogeneity present in common colloidal nanoparticle systems and the utility of single particle analytical techniques for studying these systems.

Two-dimensional semiconductors (2DSCs) are attractive materials for a variety of applications in electronics, photovoltaics, and catalysis. Despite their promise, it is often unclear how the performance of 2DSCs is influenced by structural defects present in these materials such as atomic vacancies or step-edges. A better fundamental understanding of how such structural features influence the generation and transport of charge carriers in 2DSCs will be critical in the pursuit of improved practical devices moving forward. In this Opinion, we highlight how electrochemistry can be leveraged to reveal fascinating insights into the behavior of 2DSCs. Recent advancements in techniques for mapping the rate of photoelectrochemical processes at 2DSCs are outlined and salient experiments employing these techniques are discussed. We conclude with sharing our perspective on opportunities within this field moving forward.

Organometallic halide perovskites have garnered significant attention in various fields of material science, particularly solar energy conversion, due to their desirable optoelectronic properties and compatibility with scalable fabrication techniques. It is often unclear, however, how carrier generation and transport within complex polycrystalline films are influenced by variations in local structure. Elucidating how distinct structural motifs within these heterogeneous systems affect behavior could help guide the continued improvement of perovskite-based solar cells. Here, we present studies applying scanning electron microscopy (SECCM) to map solar energy harvesting within well-defined model systems of organometallic halide perovskites. Methylammonium lead bromide (MAPbBr3) single crystals were prepared via a low-temperature solution-based route, and their photoelectrochemical properties were mapped via SECCM using p-benzoquinone (BQ) in dichloromethane as a redox mediator. Correlated SECCM mapping and electron microscopy studies enabled facet-to-facet variations in photoelectrochemical performance to be revealed and carrier transport lengths to be evaluated. The photoelectrochemical behavior observed within individual single crystals was quite heterogeneous, attributable to local variations in crystal structure/orientations, intrafacet junctions, and the presence of other structural defects. These observations underscore the significance of controlling the microstructure of single perovskite crystals, presenting a promising avenue for further enhancement of perovskite-based solar cells.

Transition metal dichalcogenide (TMD) heterostructures are promising for a variety of applications in photovoltaics and photosensing. Successfully exploiting these heterostructures will require an understanding of their layer-dependent electronic structures. However, there is no experimental data demonstrating the layer-number dependence of photovoltaic effects (PVEs) in vertical TMD heterojunctions. Here, by combining scanning electrochemical cell microscopy (SECCM) with optical probes, we report the first layer-dependence of photocurrents in WSe2/WS2 vertical heterostructures as well as in pristine WS2 and WSe2 layers. For WS2, we find that photocurrents increase with increasing layer thickness, whereas for WSe2 the layer dependence is more complex and depends on both the layer number and applied bias (Vb ). We further find that photocurrents in the WSe2/WS2 heterostructures exhibit anomalous layer and material-type dependent behaviors. Our results advance the understanding of photoresponse in atomically thin WSe2/WS2 heterostructures and pave the way to novel nanoelectronic and optoelectronic devices.

Two-dimensional semiconductors (2DSCs) are attractive for a variety of optoelectronic and catalytic applications due to their ability to be fabricated as wide-area, monolayer-thick films and their unique optical and electronic properties which emerge at this scale. One important class of 2DSCs are the transition metal dichalcogenides (TMDs), which are of particular interest as absorbing layers in ultrathin optoelectronic devices. While TMDs are known to exhibit excellent photovoltaic properties at the bulk level, it is not yet clear how carriers are transported in these materials at thicknesses approaching the monolayer limit, where distinct changes in band structure and the nature of photogenerated carriers occur. Here, it is demonstrated that electrochemical microscopy techniques can be employed as powerful tools for visualizing these processes in 2DSCs, even within individual monolayers. Carrier generation-tip collection scanning electrochemical cell microscopy (CG-TC SECCM), which utilizes spatially-offset optical and pipet-based electrochemical probes to locally generate and detect photogenerated carriers, was applied to visualize carrier generation and transport within well-defined n-WSe2 samples prepared via mechanical exfoliation. Data from these experiments directly reveal how carrier transport varies within complex 2DSC structures as layer thicknesses approach the monolayer limit. These results not only provide valuable new insights into carrier transport within monolayer TMD materials, but also demonstrate electrochemical imaging to be a powerful, yet underutilized approach for visualizing solid-state processes in semiconducting materials.

Nanostructured materials are frequently employed as active components in electrochemical devices for energy conversion and storage. Unfortunately, the complexity of nanostructured materials, which can exhibit significant heterogeneities in morphology and/or composition within a macroscopic sample, makes it difficult to generate fundamental insights into their operation using traditional experimental techniques. Analytical methods that can probe the behavior of individual, discrete reactive entities, such as nanoparticles (NPs), may serve as powerful tools for the study of complex, heterogeneous systems, but remain experimentally challenging. Here, the application of probe-based electroanalytical methods is demonstrated to be a powerful, high-throughput strategy for the characterization of electrocatalytic systems. A pipet-based approach, Targeted Electrochemical Cell Microscopy (TECCM), was applied to characterize the electrocatalytic properties of individual, shape-controlled Au NPs toward the borohydride oxidation reaction (BOR), a model fuel cell reaction. Using TECCM, the BOR could be quantitatively interrogated at individual NPs in a high-throughput fashion, directly revealing significant NP-to-NP variations in reactivity and stability. BOR kinetics were found to exhibit a significant shape dependence, generally increasing in the order Triangles < Spheres approximate to Octahedra < Rods, and prominent voltammetric features were observed that could be attributed to surface deactivation/reactivation process occurring at individual NPs. Together, these results demonstrate the large degree to which catalytic behavior varies at the single NP level and the power of applying single NP analytical techniques to the study of these systems.

Well-ordered nanoparticle arrays are attractive platforms for a variety of analytical applications, but the fabrication of such arrays is generally challenging. Here, it is demonstrated that scanning electrochemical cell microscopy (SECCM) can be used as a powerful, instantly reconfigurable tool for the fabrication of ordered nanoparticle arrays. Using SECCM, Ag nanoparticle arrays were straightforwardly fabricated via electrodeposition at the interface between a substrate electrode and an electrolyte-filled pipet. By dynamically monitoring the currents flowing in an SECCM cell, individual nucleation and growth events could be detected and controlled to yield individual nanoparticles of controlled size. Characterization of the resulting arrays demonstrate that this SECCM-based approach enables spatial control of nanoparticle location comparable with the terminal diameter of the pipet employed and straightforward control over the volume of material deposited at each site within an array. These results provide further evidence for the utility of probe-based electrochemical techniques such as SECCM as tools for surface modification in addition to analysis.

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Semiconducting materials play a central role in photocatalytic systems which utilize solar energy to create new chemical bonds. While photocatalytic systems could generate a clean, renewable source of energy to meet the world's ever-rising energy demands, semiconducting materials possessing the necessary combination of efficiency, stability, and cost have yet to be identified. In the search for improved materials for these applications, scanning electrochemical microscopy (SECM) has proven to be a valuable tool for providing fundamental insights into photocatalytic reactions at semiconductor-liquid interfaces and screening the performance of novel semiconductor compositions. This chapter discusses the basics of how SECM techniques can be applied to the study of photocatalytic systems, different SECM experimental configurations which have been demonstrated, and their application in materials discovery and fundamental investigations. Specific topics include the optical fiber-based screening of oxide and non-oxide photocatalysts, studies of photocatalytic reaction kinetics using the substrate generation-tip collection (SG-TC) and surface interrogation modes, and emerging experimental methods such as dual-function SECM probes for correlated high resolution optical-electrochemical studies and scanning electrochemical cell microscopy (SECCM) techniques. The strengths and weaknesses of each configuration are discussed with selected examples from the literature. Challenges, new experimental strategies, and future applications are provided at the outset which may help advance this field moving forward.

Two-dimensional (2D) semiconductors are attractive materials for a variety of electronic and photonic applications. This is due in large part to the ability to fabricate films of these materials with thicknesses approaching the monolayer limit and the emergent optical and electronic properties displayed at this extreme scale. Despite these fundamental advantages, active layers based on these materials exhibit a high density of edge/step terminations which are detrimental to carrier transport, limiting their utility in practical applications. Unambiguously evaluating the impact of different types of defects on carrier transport would help to guide the design of improved practical devices, but such information is impossible to generate using existing characterization techniques. Here, a novel mode of Scanning Electrochemical Cell Microscopy (SECCM) will be described which is capable of probing carrier transport across well-defined pathways within individual, well-defined 2D nanosheets. In this “Carrier Generation-Tip Collection” (CG-TC) mode, a focused light source is employed to locally generate carriers and an SECCM tip is used to collect carriers at another point within the same structure, allowing carrier transport within an individual nanosheet to be directly visualized. Analysis of CG-TC data obtained within pristine, defect-free regions of a material allows carrier diffusion lengths to be directly generated. CG-TC data obtained spanning step/edge defects then allows for recombination at individual, nanoscale defects to be rigorously evaluated. Quantitative studies applying CG-TC SECCM to visualize carrier transport within Transition Metal Dichalcogenide (TMD) materials will be presented to demonstrate the efficacy of this approach.

Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Feringa Nobel Prize Scientist Joint Research Center, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai, China, Department of Chemistry, University of Nevada, Reno, NV, United States, State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai, China, Department of Chemistry, University of Wyoming, Laramie, WY, United States

Control over photophysical and chemical properties of two-dimensional (2D) transition metal dichalcogenides (TMDs) is the key to advance their applications in next-generation optoelectronics. Although chemical doping and surface modification with plasmonic metals have been reported to tune the photophysical and catalytic properties of 2D TMDs, there have been few reports of tuning optical properties using dynamic electrochemical control of electrode potential. Herein, we report (1) the photoluminescence (PL) enhancement and red-shift in the PL spectrum of 2D MoS2, synthesized by chemical vapor deposition and subsequent transfer onto an indium tin oxide electrode, upon electrochemical anodization and (2) spatial heterogeneities in its photoelectrochemical (PEC) activities. Spectroelectrochemistry shows that positive electrochemical bias causes an initial ten-fold increase in the PL intensity followed by a quick decrease in the enhancement. The PL enhancement and spectrum red-shift are associated with the decrease in nonradiative decay rates of excitons formed upon electrochemical anodization of 2D MoS2. Additionally, scanning electrochemical cell microscopy (SECCM) study shows that the 2D MoS2 crystal is spatially sensitive to PEC oxidation at positive potentials. SECCM also shows a photocurrent increase caused by spatially heterogeneous edge-type defect sites of the crystal.

Transition metal dichalcogenides (TMDs) are attractive materials for a variety of applications in solar energy conversion and electrocatalysis, due to their favorable optical and electrical properties and their unique two-dimensional structures which facilitate the fabrication of wide-area, ultrathin layers. Unfortunately, the basal planes which make up the majority of these materials are relatively inert, and thus a great deal of effort has been directed to engineering favorable, catalytically active defects into these materials. Here, we demonstrate how probe-based electrochemical techniques can be employed as multifunctional tools for locally modifying TMD materials and probing the electrochemical behavior of the resulting defects. Scanning Electrochemical Cell Microscopy (SECCM) was employed to locally anodize exfoliated p-type WSe2 nanosheets, creating hole-like defects within individual basal planes in a highly controllable fashion. Photoelectrochemical SECCM imaging was then employed to characterize the chemical behavior of these engineered defects, revealing significantly enhanced activity toward the Hydrogen Evolution Reaction (HER). Atomic force microscopy studies are presented which suggest these enhancements result from an increased density of monolayer-high step features within the anodized defects. Analysis of the SECCM data in the context of finite element simulations revealed that these enhancements increased with increasing anodization time, with local kinetic rates over 2 orders of magnitude higher than unaltered basal planes.

The progression of nanoscience necessitates quantitative tools to understand reactivity at the nanoscale. Here, we report a quantitative model that describes both the electro-kinetic and diffusion-limited growth of a single nanoparticle (NP within sub-femtoliter reactors and apply the model to quantify growth kinetics of platinum NPs within single aqueous nanodroplets (r(drop) approximate to 500 nm). The time-resolved growth mechanism of the platinum NPs can be observed by studying the collisions of chloroplatinate-filled aqueous nanodroplets with ultramicroelectrodes (UMEs). If the potential of the UME is biased sufficiently negative to drive the reduction of chloroplatinate to platinum metal, transients in the amperometric trace can be observed for individual nanodroplet collision events. At low overpotentials, a parabolic increase in current following t(2), indicative of electrokinetic growth, is observed, and at high overpotentials, an instantaneous rise in current following t(1/2), indicative of diffusion-controlled growth, is observed. Each transient is followed by a rounded peak and a slow decay to baseline, which can be explained by the competition between the rate of NP growth and the rate of metal precursor consumption (electrolysis). We demonstrate that the model couples electrocrystallization and nanodroplet electrolysis to quantitatively predict the collision transient shape, amplitude, and duration. Importantly, the only adjustable parameter in the developed model for electrokinetic growth is the heterogeneous rate constant, k, allowing the growth kinetics of single NPs to be directly evaluated. We measured k for 100 single platinum NPs and found k = 0.003 +/- 0.001 cm.s(-1) for the growth of platinum NPs on a platinum UME. This platform permits rapid data acquisition for the high-throughput study of the growth kinetics of single NPs, and the model can be generalized to elucidate kinetic and mass-transfer rates under nanoscale conditions of high spatial confinement.