Accessing the Electrochemical Activity of Single Nanoparticles by Eliminating the Heterogeneous Electrical Contacts

Platinum Nanoparticles on Metalloid Antimony Functionalized Graphitic Nanoplatelets for Enhanced Water Electrolysis

Platinum (Pt) nanoparticles are considered to be the most efficient catalyst for acidic hydrogen evolution reaction (HER). However, they are expensive and unstable, because of agglomeration and Ostwald ripening. It is critically necessary for developing a better catalytic support to stabilize the Pt nanoparticles at low loading amounts. One efficient route to improving both catalytic activity and durability is metal catalysts stably anchored on heteroatom functionalized carbon supports via their strong interactions. Nevertheless, the interactions between "metallic" catalysts and "nonmetallic" heteroatom functionalized carbon supports are still unsatisfactory. Here, "metalloid" antimony (Sb) functionalized graphitic nanoplatelets (SbGnP) are reported to stably anchor Pt nanoparticles. The resulting Pt@SbGnP catalyst shows a record high acidic HER performance, attributable to the unique nature of Sb functional groups on SbGnP. Unlike typical low-period nonmetallic heteroatoms on carbon supports, high-period metalloid Sb with various oxidation states of SbOx provided strong binding sites to stably anchor Pt nanoparticles, suppressing particle aggregation, and thus sustaining catalytic activity and stability.

Seeing nanoscale electrocatalytic reactions at individual MoS2 particles under an optical microscope: probing sub-mM oxygen reduction reaction

MoS2 is a promising electrocatalytic material for replacing noble metals. Nanoelectrochemistry studies, such as using nanoelectrochemical cell confinement, have particularly helped in demonstrating the preferential electrocatalytic activity of MoS2 edges. These findings have been accompanied by considerable research efforts to synthesize edge-abundant nanomaterials. However, to fully apprehend their electrocatalytic performance, at the single particle level, new instrumental developments are also needed. Here, we feature a highly sensitive refractive index based optical microscopy technique, namely interferometric scattering microscopy (iSCAT), for monitoring local electrochemistry at single MoS2 petal-like sub-microparticles. This work focuses on the oxygen reduction reaction (ORR), which operates at low current densities and thus requires high-sensitivity imaging techniques. By employing a precipitation reaction to reveal the ORR activity and utilizing the high spatial resolution and contrast of iSCAT, we achieve the sensitivity required to evaluate the ORR activity at single MoS2 particles.

Scanning Electrochemical Microscopy Meets Optical Microscopy: Probing the Local Paths of Charge Transfer Operando in Booster-Microparticles for Flow Batteries

Understanding the oxidation/reduction dynamics of secondary microparticles formed from agglomerated nanoscale primary particles is crucial for advancing electrochemical energy storage technologies. In this study, the behavior of individual copper hexacyanoferrate (CuHCF) microparticles is explored at both global and local scales combining scanning electrochemical microscopy (SECM), for electrochemical interrogation of a single, but global-scale microparticle, and optical microscopy monitoring to obtain a higher resolution dynamic image of the local electrochemistry within the same particle. Chronoamperometric experiments unveil a multistep oxidation/reduction process with varying dynamics. On the one hand, the global SECM analysis enables quantifying the charge transfer as well as its dynamics at the single microparticle level during the oxidation/reduction cycles by a redox mediator in solution. These conditions allow mimicking the charge storage processes in these particles when they are used as solid boosters in redox flow batteries. On the other hand, optical imaging with sub-particle resolution allows the mapping of local conversion rates and state-of-charge within individual CuHCF particles. These maps reveal that regions of different material loadings exhibit varying charge storage capacities and conversion rates. The findings highlight the significance of porous nanostructures and provide valuable insights for designing more efficient energy storage materials.

Impedimetric Measurement of Exchange Currents and Ionic Diffusion Coefficients in Individual Pseudocapacitive Nanoparticles

Among electroanalytical techniques, electrochemical impedance spectroscopy (EIS) offers the unique advantage of a high degree of frequency resolution. This enables EIS to readily deconvolute between the capacitive, resistive, and diffusional processes that underlie electrochemical devices. Here, we report the measurement of impedance spectra of individual, pseudocapacitive nanoparticles. We chose Prussian blue as our model system, as it couples an electron-transfer reaction with sodium ion intercalation-processes which, while intrinsically convoluted, can be readily resolved using EIS. We used a scanning electrochemical cell microscope (SECCM) to isolate single Prussian blue particles in a microdroplet and measured their impedance spectra using the multi-sine, fast Fourier transform technique. In doing so, we were able to extract the exchange current density and sodium ion diffusivity for each particle, which respectively inform on their electronic and ionic conductivities. Surprisingly, these parameters vary by over an order of magnitude between particles and are not correlated to particle size nor to each other. The implication of this apparent heterogeneity is that in a hypothetical battery cathode, one active particle may transfer electrons 10 times faster than its neighbor; another may suffer from sluggish sodium ion transport and have restricted charging rate capabilities compared to a better-performing particle elsewhere in the same electrode. Our results inform on this intrinsic heterogeneity while demonstrating the utility of EIS in future single-particle studies.

Imaging Single Prussian Blue Nanoparticles with Extraordinary Low-Spin Iron Capacity

In attempts to obtain high-capacity Prussian blue nanomaterials, current efforts are predominantly focused on the particle-ensemble-level understanding of their structure-activity relationships. Complementarily, it would be insightful to screen out extraordinary individuals from the nanoparticle population. Using a simple and efficient technique of bright-field microscopy, this work enables, for the first time, quantitative characterization of the overall two-redox-center electrochemistry of single Prussian blue nanoparticles many at a time. Quantitative optical voltammograms with little interference from solvent breakdown and non-Faradaic electrode charging/discharging are extracted for each single nanoparticle, revealing clear heterogeneity among them. On this basis, the microscopic method allows a detailed comparative analysis between the two redox-active sites. It is found that while the synthesized nanoparticles show a similar specific capacity of the high-spin (HS-Fe) sites with STD/mean = 30%, most individual nanoparticles exhibit monodispersedly small capacities of the low-spin iron (LS-Fe) sites, only about 1 7 ± 1 of the HS-Fe capacity. Most importantly, it is discovered that there is always a small fraction (∼8%) of the single nanoparticles showing an impressively tripled LS-Fe capacity. Facilitated by optical imaging, the discovery of this easily overlooked extraordinary subpopulation confers alternative opportunities for targeted efforts for material chemists to improve synthesis and material design based on these unusual individuals, which in turn implies the general significance of nanoparticle screening.

Investigation of the Single-Particle Scale Structure-Activity Relationship Providing New Insights for the Development of High-Performance Batteries

As electric vehicles, portable electronic devices, and tools have increasingly high requirements for battery energy density and power density, constantly improving battery performance is a research focus. Accurate measurement of the structure-activity relationship of active materials is key to advancing the research of high-performance batteries. However, conventional performance tests of active materials are based on the electrochemical measurement of porous composite electrodes containing active materials, polymer binders, and conductive carbon additives, which cannot establish an accurate structure-activity relationship with the physical characterization of microregions. In this review, in order to promote the accurate measurement and understanding of the structure-activity relationship of materials, the electrochemical measurement and physical characterization of energy storage materials at single-particle scale are reviewed. The potential problems and possible improvement schemes of the single particle electrochemical measurement and physical characterization are proposed. Their potential applications in single particle electrochemical simulation and machine learning are prospected. This review aims to promote the further application of single particle electrochemical measurement and physical characterization in energy storage materials, hoping to achieve 3D unified evaluation of physical characterization, electrochemical measurement, and theoretical simulation at the single particle scale to provide new inspiration for the development of high-performance batteries.

Near-infrared visualisation of single microparticle electrochemistry for batteries

While optical microscopy of single particle electrochemistry has proven insightful for future nanoparticle-based batteries, little is explored for micron-sized particles of more practical interest. This is largely hindered by the currently limited methodology. Accordingly, we report transmission optical microscopy using near-infrared light for accessing intra-particle electrochemistry in virtue of strong light penetration as compared to visible light. Using near-infrared (λ > 730 nm) bright-field microscopy, the redox electrochemistry of single LiCoO2 microparticles can be readily measured based on the measurements of optical contrast changes during electrochemical cycling. Further using the established methodology, we discover that the solid-state diffusion inside most single microparticles is distinctly directional, instead of in an isotropic manner from outer to inner as observed for the other particles. This phenomenon is also observed using dark field scattering microscopy with near-infrared light, suggesting non-uniform crystal inner structures responsible for the geometrically asymmetric heterogeneity of charge transfer kinetics within each single particle. These results indicate potential opportunities offered by the near-infrared optical methodology for operando studying practical battery materials.

Optical Approach for Mapping the Intercalation Capacity of Porous Electrodes

The intercalation capacity of a porous electrode in real batteries is not uniform spatially due to the inevitable structural and compositional inhomogeneity and site-dependent ion and electron transport features. Reliable methods to quantify the capacity distribution are highly desirable but absent so far in battery research. In this paper, a novel optical technique, oblique incident reflection difference (OIRD), was employed to monitor in situ the electrochemical ion (de)intercalation behavior of Prussian blue analogue (PBA) porous films. The OIRD signal responded synchronously to the ion (de)intercalation, and the change in the OIRD signal (ΔI) was positively correlated with the local electrochemical capacity, thereby enabling mapping of the spatially resolved ion storage capacity of the films. Optical analysis further showed that the OIRD response originated from the ion (de)intercalation-induced dielectric constant change of PBA films. This work therefore offers an intriguing in situ and spatially resolved tool for the study of rechargeable batteries.

Multiscale Analysis of Electrocatalytic Particle Activities: Linking Nanoscale Measurements and Ensemble Behavior

Nanostructured electrocatalysts exhibit variations in electrochemical properties across different length scales, and the intrinsic catalytic characteristics measured at the nanoscale often differ from those at the macro-level due to complexity in electrode structure and/or composition. This aspect of electrocatalysis is addressed herein, where the oxygen evolution reaction (OER) activity of β-Co(OH)2 platelet particles of well-defined structure is investigated in alkaline media using multiscale scanning electrochemical cell microscopy (SECCM). Microscale SECCM probes of ∼50 μm diameter provide voltammograms from small particle ensembles (ca. 40-250 particles) and reveal increasing dispersion in the OER rates for samples of the same size as the particle population within the sample decreases. This suggests the underlying significance of heterogeneous activity at the single-particle level that is confirmed through single-particle measurements with SECCM probes of ∼5 μm diameter. These measurements of multiple individual particles directly reveal significant variability in the OER activity at the single-particle level that do not simply correlate with the particle size, basal plane roughness, or exposed edge plane area. In combination, these measurements demarcate a transition from an "individual particle" to an "ensemble average" response at a population size of ca. 130 particles, above which the OER current density closely reflects that measured in bulk at conventional macroscopic particle-modified electrodes. Nanoscale SECCM probes (ca. 120 and 440 nm in diameter) enable measurements at the subparticle level, revealing that there is selective OER activity at the edges of particles and highlighting the importance of the three-phase boundary where the catalyst, electrolyte, and supporting carbon electrode meet, for efficient electrocatalysis. Furthermore, subparticle measurements unveil heterogeneity in the OER activity among particles that appear superficially similar, attributable to differences in defect density within the individual particles, as well as to variations in electrical and physical contact with the support material. Overall this study provides a roadmap for the multiscale analysis of nanostructured electrocatalysts, directly demonstrating the importance of multilength scale factors, including particle structure, particle-support interaction, presence of defects, etc., in governing the electrochemical activities of β-Co(OH)2 platelet particles and ultimately guiding the rational design and optimization of these materials for alkaline water electrolysis.

Spatiotemporally and Chemically Resolved Imaging of Electrocatalytic Oxygen Evolution on Single Nanoplates of Cobalt-Layered Hydroxide

Transition metal-layered hydroxides have been extensively studied in order to address the key challenge of slow kinetics of the oxygen evolution reaction (OER). However, how the catalytically active sites are evolved and the corresponding heterogeneous structure-property relationship remain unclear. Herein, using cobalt-layered hydroxide as a representative catalyst, we report a strategy for the comprehensive in situ investigation of the electrocatalytic OER process at the single electrocatalyst level using combined electrochemiluminescence (ECL) and vis-absorption microscopy. The stepwise heterogeneous electrocatalytic responses of single-cobalt hydroxide nanoplates are unveiled with ECL imaging, and the corresponding valence state changes are revealed by vis-absorption imaging. The correlated in situ and ex situ multimode analyses indicate that, during the oxidation process, the Co2+ cations in the tetrahedral sites (CoTd2+) turned into CoTd3+ and even the highly unstable CoTd4+, assisted by the interlayer water in a metastable CoOOH·xH2O phase. Crucially, the CoTd4+ sites are mainly distributed in the inner part of the nanoplates and show superior electrocatalytic properties. The correlative single-particle imaging approach for electrocatalytic process analysis with high spatiotemporal and chemical resolution enables in-depth mechanistic insights to be generated and, in turn, will benefit the rational design of electrocatalysts with enhanced performance.

Measuring the Pseudocapacitive Behavior of Individual V2O5 Particles by Scanning Electrochemical Cell Microscopy

V₂O₅ is a promising pseudocapacitive material for electrochemical energy storage with balanced power and energy density. Understanding the charge-storage mechanism is of significance to further improve the rate performance. Here, we report an electrochemical study of individual V₂O₅ particles using scanning electrochemical cell microscopy with colocalized electron microscopy. A carbon sputtering procedure is proposed for the pristine V₂O₅ particles to improve their structure stability and electronic conductivity. The achieved high-quality electrochemical cyclic voltammetry results, structural integrity, and high oxidation to reduction charge ratio (as high as 97.74%) assured further quantitative analysis of the pseudocapacitive behavior of single particles and correlation with local particle structures. A broad range of capacitive contribution is revealed, with an average ratio of 76% at 1.0 V/s. This study provides new opportunities for quantitative analysis of the electrochemical charge-storage process at single particles, especially for electrode materials with electrolyte-induced instability.

Probing and Leveraging the Structural Heterogeneity of Nanomaterials for Enhanced Catalysis

The marriage between nanoscience and heterogeneous catalysis has introduced transformative opportunities for accessing better nanocatalysts. However, the structural heterogeneity of nanoscale solids stemming from distinct atomic configurations makes it challenging to realize atomic-level engineering of nanocatalysts in the way that is attained for homogeneous catalysis. Here, we discuss recent efforts in unveiling and exploiting the structural heterogeneity of nanomaterials for enhanced catalysis. Size and facet control of nanoscale domains produce well-defined nanostructures that facilitate mechanistic studies. Differentiation of surface and bulk characteristics for ceria-based nanocatalysts guides new thoughts toward lattice oxygen activation. Manipulating the compositional and species heterogeneity between local and average structures allows regulation of catalytically active sites via the ensemble effect. Studies on catalyst restructurings further highlight the necessity to assess the reactivity and stability of nanocatalysts under reaction conditions. These advances promote the development of novel nanocatalysts with expanded functionalities and bring atomistic insights into heterogeneous catalysis.

Merging Platinum Single Atoms to Achieve Ultrahigh Mass Activity and Low Hydrogen Production Cost

Single atom catalysts (SACs) with isolated active sites exhibit the highest reported mass activity for hydrogen evolution catalysis, which is crucial for practical applications. Here, we demonstrate that ultrahigh mass activity can also be achieved by rationally merging the isolated platinum (Pt) active sites in SAC. The catalyst was obtained by the thermodynamically driven diffusing and merging phosphorus-doped carbon (PC) supported Pt single atoms (Pt₁@PC) into Pt nanoclusters (Pt_M@PC). X-ray absorption spectroscopy analysis revealed that the merged nanoclusters exhibit much stronger interactions with the support than the traditional method, enabling more efficient electron transfer. The optimized Pt_M@PC exhibited an order of magnitude higher mass activity (12.7 A mg_Pt^-1) than Pt₁@PC (0.9 A mg_Pt^-1) at an overpotential of 10 mV in acidic media, which is the highest record to date, far exceeding reports for other outstanding SACs. Theoretical study revealed that the collective active sites in Pt_M@PC exhibit both favorable hydrogen binding energy and fast reaction kinetics, leading to the significantly enhanced mass activity. Despite its low Pt content (2.2 wt %), a low hydrogen production cost of ∼3 USD kg^-1 was finally achieved in the full-water splitting at a laboratory scale.

Operando Imaging of Crystallinity-Dependent Multicolor Thermochromic Processes for Single Hydrated Hybrid Perovskite Particles

The thermochromic properties of hydrated metal halide perovskites (MHPs) are promising for applications in smart windows, solar cells, optical sensors, and information storage. Traditional ensemble characterization methods always study the averaged thermochromic activity, lacking the accurate structure-activity correlation. Here we utilize dark-field microscopy (DFM) to in situ image the thermochromic processes of single isolated hydrated hybrid perovskite (CH₃NH₃)₄PbI_6-xCl_x·2H₂O (MA₄PbI_6-xCl_x·2H₂O) microparticles. The thermal-induced dehydration transition is demonstrated to alter the color of single MA₄PbI_6-xCl_x·2H₂O particles. Operando single-particle mapping results reveal the significant intra- and interparticle variations of thermochromic behaviors, yielding unexpected single or multistep multicolor thermochromic processes. These phenomena are confirmed to be governed by the crystallinity of single MA₄PbI_6-xCl_x·2H₂O particles that results in distinct composition-dependent bandgaps and thermal decomposition pathways. The present work highlights the important role of single-particle imaging for resolving the intrinsic thermochromic characteristic of hydrated MHPs, therefore opening a way for rational design of stimuli-responsive materials.

Emerging Optical Microscopy Techniques for Electrochemistry

An optical microscope is probably the most intuitive, simple, and commonly used instrument to observe objects and discuss behaviors through images. Although the idea of imaging electrochemical processes operando by optical microscopy was initiated 40 years ago, it was not until significant progress was made in the last two decades in advanced optical microscopy or plasmonics that it could become a mainstream electroanalytical strategy. This review illustrates the potential of different optical microscopies to visualize and quantify local electrochemical processes with unprecedented temporal and spatial resolution (below the diffraction limit), up to the single object level with subnanoparticle or single-molecule sensitivity. Developed through optically and electrochemically active model systems, optical microscopy is now shifting to materials and configurations focused on real-world electrochemical applications.

Real-Time Plasmonic Imaging of the Compositional Evolution of Single Nanoparticles in Electrochemical Reactions

Real-time probing of the compositional evolution of single nanoparticles during an electrochemical reaction is crucial for understanding the structure-performance relationship and rationally designing nanomaterials for desirable applications; however, it is consistently challenging to achieve high-throughput real-time tracking. Here, we present an optical imaging method, termed plasmonic scattering interferometry microscopy (PSIM), which is capable of imaging the compositional evolution of single nanoparticles during an aqueous electrochemical reaction in real time. By quantifying the plasmonic scattering interferometric pattern of nanoparticles, we establish the relationship between the pattern and composition of single nanoparticles. Using PSIM, we have successfully probed the compositional transformation dynamics of multiple individual nanoparticles during electrochemical reactions. PSIM could be used as a universal platform for exploring the compositional evolution of nanomaterials at the single-nanoparticle level and offers great potentials for addressing the extensive fundamental questions in nanoscience and nanotechnology.

Concluding remarks: next generation nanoelectrochemistry - next generation nanoelectrochemists

The aim of this paper is to describe the scientific journey taken to arrive at present-day nanoelectrochemistry and consider how the area might develop in the future, particularly in light of papers presented at this Faraday Discussion. By adopting a generational approach, this brief contribution traces the story of the nanoelectrochemistry family within the broader electrochemistry field, with a focus on scientific capability and themes that were important to each generation. I shall consider research questions and the impact of technology that was developed or available in each period. Nanoelectrochemistry is still somewhat niche, but is attracting increasing numbers of researchers. It is set to become a major part of electrochemistry and interfacial science. It is studied by people with a fairly unique skillset, and I shall speculate on the skills and expertise that will be needed by nanoelectrochemists to address the challenges and opportunities that lie ahead. I conclude by asking: who will be the nanoelectrochemists of the future and what will they do?

Operando Imaging of Chemical Activity on Gold Plates with Single-Molecule Electrochemiluminescence Microscopy

Classical electrochemical characterization tools cannot avoid averaging between the active reaction sites and their support, thus obscuring their intrinsic roles. Single-molecule electrochemical techniques are thus in high demand. Here, we demonstrate super-resolution imaging of Ru(bpy)₃ ²⁺ based reactions on Au plates using single-molecule electrochemiluminescence microscopy. By converting electrochemical signals into optical signals, we manage to achieve the ultimate sensitivity of single-entity chemistry, that is directly resolving the single photons from individual electrochemical reactions. High spatial resolution, up to 37 nm, further enables mapping Au chemical activity and the reaction kinetics. The spatiotemporally resolved dynamic structure-activity relationship on Au plates shows that the restructuring of catalysts plays an important role in determining the reactivity. Our approach may lead to gaining new insights towards evaluating and designing electrocatalytic systems.

Enhanced electrocatalytic activity of Cu-modified, high-index single Pt NPs for formic acid oxidation

A key goal of nanoparticle-based catalysis research is to correlate the structure of nanoparticles (NPs) to their catalytic function. The most common approach for achieving this goal is to synthesize ensembles of NPs, characterize the ensemble, and then evaluate its catalytic properties. This approach is effective, but it excludes the certainty of structural heterogeneity in the NP ensemble. One means of addressing this shortcoming is to carry out analyses on individual NPs. This approach makes it possible to establish direct correlations between structures of single NPs and, in the case reported here, their electrocatalytic properties. Accordingly, we report on enhanced electrocatalytic formic acid oxidation (FAO) activity using individual Cu-modified, high-indexed Pt NPs. The results show that the Cu-modified Pt NPs exhibit significantly higher currents for FAO than the Pt-only analogs. The increased activity is enabled by the Cu submonolayer on the highly stepped Pt surface, which enhances the direct FAO pathway but not the indirect pathway which proceeds via surface-absorbed CO*.

Single-crystal Pt nanoparticles with a diameter of ∼200 nm were electrosynthesized, covered with a single monolayer of Cu, and then fully characterized. The resulting materials exhibit excellent electrocatalytic properties for formic acid oxidation.

Collision, Adhesion, and Oxidation of Single Ag Nanoparticles on a Polysulfide-Modified Microelectrode

We report the collision, adhesion, and oxidation behavior of single silver nanoparticles (Ag NPs) on a polysulfide-modified gold microelectrode. Despite its remarkable success in volume analysis for smaller Ag NPs, the method of NP-collision electrochemistry has failed to analyze particles greater than 50 nm due to uncontrollable collision behavior and incomplete NP oxidation. Herein, we describe the unique capability of an ultrathin polysulfide layer in controlling the collision behavior of Ag NPs by drastically improving their sticking probability on the electrode. The ultrathin sulfurous layer is formed on gold by sodium thiosulfate electro-oxidation and serves both as an adhesive interface for colliding NPs and as a preconcentrated reactive medium to chemically oxidize Ag to form Ag₂S. Rapid particle dissolution is further promoted by the presence of bulk sodium thiosulfate serving as a Lewis base, which drastically improves the solubility of generated Ag₂S by a factor of 10¹³. The combined use of polysulfide and sodium thiosulfate allows us to observe a 25× increase in NP detection frequency, a 3× increase in peak amplitude, and more complete oxidation for larger Ag NPs. By recognizing how volumetric analysis using transmission electron microscopy (TEM) may overestimate quasi-spherical NPs, we believe we can have full NP oxidation for particles up to 100 nm. By focusing on the electrode/solution interface for more effective NP-electrode contact, we expect that the knowledge learned from this study will greatly benefit future NP collision systems for mechanistic studies in single-entity electrochemistry as well as designing ultrasensitive biochemical sensors.

A Novel Approach to the Facile Growth and Organization of Photothermal Prussian Blue Nanocrystals on Different Surfaces

We report here a novel "one-pot" approach for the controlled growth and organization of Prussian blue nanostructures on three different surfaces: pure Au0, cysteamine-functionalized Au0, and SiO2-supported lipid bilayers with different natures of lipids. We demonstrate that fine control over the size, morphology, and the degree and homogeneity of the surface coverage by Prussian Blue (PB) nanostructures may be achieved by manipulating different parameters, which are the precursor concentration, the nature of the functional groups or the nature of lipids on the surfaces. This allows the growth of isolated PB nanopyramids and nanocubes or the design of thin dense films over centimeter square surfaces. The formation of unusual Prussian blue nanopyramids is discussed. Finally, we demonstrate, by using experimental techniques and theoretical modeling, that PB nanoparticles deposited on the gold surface exhibit strong photothermal properties, permitting a rapid temperature increase up to 90 °C with a conversion of the laser power of almost 50% for power source heat.

Characterization of Ligand Adsorption at Individual Gold Nanocubes

Cetyltrimethylammonium bromide (CTAB) is a widely used surfactant that aids the aqueous synthesis of colloidal nanoparticles. However, the presence of residual CTAB on nanoparticle surfaces can significantly impact nanoparticle applications, such as catalysis and sensing, under hydrated conditions. As such, consideration of the presence and quantity of CTAB on nanoparticle surfaces under hydrated conditions is of significance. Herein, as part of an integrated material characterization framework, we demonstrate the feasibility of in situ atomic force microscopy (AFM) to detect CTAB on the surface of Au nanocubes (Au NCs) under hydrated conditions, which enabled superior characterization compared to conventional spectroscopic methods. In situ force-distance (FD) spectroscopy and Kelvin probe force microscopy (KPFM) measurements support additional characterization of adsorbed CTAB, while correlative in situ AFM and scanning electron microscopy (SEM) measurements were used to evaluate sequential steps of CTAB removal from Au NCs across hydrated and dehydrated environments, respectively. Notably, a substantial quantity of CTAB remained on the Au NC surface after methanol washing, which was detected in AFM measurements but was not detected in infrared spectroscopy measurements. Subsequent electrochemical cleaning was found to be critically important to remove CTAB from the Au NC surface. Correlative measurements were also performed on individual nanoparticles, which further validate the method described here as a powerful tool to determine the extent and degree of CTAB removal from nanoparticle surfaces. This AFM-based method is broadly applicable to characterize the presence and removal of ligands from nanomaterial surfaces under hydrated conditions.

Tracking the optical mass centroid of single electroactive nanoparticles reveals the electrochemically inactive zone

The inevitable microstructural defects, including cracks, grain boundaries and cavities, make a portion of the material inaccessible to electrons and ions, becoming the incentives for electrochemically inactive zones in single entity. Herein, we introduced dark field microscopy to study the variation of scattering spectrum and optical mass centroid (OMC) of single Prussian blue nanoparticles during electrochemical reaction. The "dark zone" embedded in a single electroactive nanoparticle resulted in the incomplete reaction, and consequently led to the misalignment of OMC for different electrochemical intermediate states. We further revealed the dark zones such as lattice defects in the same entity, which were externally manifested as the fixed pathway for OMC for the migration of potassium ions. This method opens up enormous potentiality to optically access the heterogeneous intraparticle dark zones, with implications for evaluating the crystallinity and electrochemical recyclability of single electroactive nano-objects.

Vertical Diffusion of Ions within Single Particles during Electrochemical Charging

Determining the trajectory of ionic transport and diffusion within single electroactive nanomaterials is critical for understanding the charging kinetics and capacity fading associated with ion batteries, with implications for rational design of excellent-performance electrode materials. While the horizontal pathway of mass transport has been feasibly investigated by optical superlocalization methods and electron microscopes, determination on the vertical trajectory has proven a more challenging task. Herein, we developed dual-angle total internal reflection microscopy by simultaneously introducing different angle-dependent illumination depths to trace the optical centroid shifts of nano-objects in the vertical dimension. We first demonstrated the proof of concept by resolving the vertical moving trails of a nanosphere doing Brownian motion and subsequently explored the picture of mass transport in the interior of single Prussian blue (PB) particles during electrochemical cycling. The results indicated that the vertical centroids of single PB particles remained unchanged when ions were inserted or extracted, suggesting an outside-in ionic transport pathway instead of bottom-up trajectory that one would intuitively expect.

Imaging and Quantifying the Formation of Single Nanobubbles at Single Platinum Nanoparticles during the Hydrogen Evolution Reaction

While numerous efforts have been made toward the design of sustainable and efficient nanocatalysts of the hydrogen evolution reaction, there is a need for the operando observation and quantification of the formation of gas nanobubbles (NBs) involved in this electrochemical reaction. It is achieved herein through interference reflection microscopy coupled to electrochemistry and optical modeling. In addition to analyzing the geometry and growth rate of individual NBs at single nanocatalysts, the toolbox offered by superlocalization and quantitative label-free optical microscopy allows analyzing the geometry (contact angle and footprint with surface) of individual NBs and their growth rate. It turns out that, after a few seconds, NBs are steadily growing while they are fully covering the Pt nanoparticles that allowed their nucleation and their pinning on the electrode surface. It then raises relevant questions related to gas evolution catalysts, such as, for example, does the evaluation of NB growth at the single nanocatalyst really reflect its electrochemical activity?

High-resolution imaging of catalytic activity of a single graphene sheet using electrochemiluminescence microscopy

Here, the electrocatalytic activity of a single graphene sheet is mapped using electrochemiluminescence (ECL) microscopy with a nanometer resolution. The achievement of this high-spatial imaging relies on the varied adsorption of hydrogen peroxide at different sites on the graphene surface, leading to unsynchronized ECL emission. By shortening the exposure time to 0.2 ms, scattered ECL spots are observed in the ECL image that are not overlaid with the spots in the consecutive images. Accordingly, after stacking all the images into a graph, the ECL intensity of each pixel could be used to reflect the electrocatalytic features of the graphene surface with a resolution of 400 nm. This novel ECL method efficiently avoids the long-standing problem of classic ECL microscopy regarding the overlap of ECL emissions from adjacent regions and enables the nanometer spatial resolution of ECL microscopy for the first time.