Single-Entity Electrocatalysis of Individual "Picked-and-Dropped" Co3 O4 Nanoparticles on the Tip of a Carbon Nanoelectrode

Correlating Thickness and Phase of Single Co(OH)2 Micro-Platelets to the Intrinsic Activity of Oxygen Evolution Electrocatalysis

Morphology, crystal phase, and its transformation are important structures that frequently determine electrocatalytic activity, but the correlations of intrinsic activity with them are not completely understood. Herein, using Co(OH)2 micro-platelets with well-defined structures (phase, thickness, area, and volume) as model electrocatalysts of oxygen evolution reaction, multiple in situ microscopy is combined to correlate the electrocatalytic activity with morphology, phase, and its transformation. Single-entity morphology and electrochemistry characterized by atomic force microscopy and scanning electrochemical cell microscopy reveal a thickness-dependent turnover frequency (TOF) of α-Co(OH)2. The TOF (≈9.5 s-1) of α-Co(OH)2 with ≈14 nm thickness is ≈95-fold higher than that (≈0.1 s-1) with ≈80 nm. Moreover, this thickness-dependent activity has a critical thickness of ≈30 nm, above which no thickness-dependence is observed. Contrarily, β-Co(OH)2 reveals a lower TOF (≈0.1 s-1) having no significant correlation with thickness. Combining single-entity electrochemistry with in situ Raman microspectroscopy, this thickness-dependent activity is explained by more reversible Co3+/Co2+ kinetics and larger ratio of active Co sites of thinner α-Co(OH)2, accompanied with faster phase transformation and more extensive surface restructuration. The findings highlight the interactions among thickness, ratio of active sites, kinetics of active sites, and phase transformation, and offer new insights into structure-activity relationships at single-entity level.

In-situ electrochemical reconstruction and modulation of adsorbed hydrogen coverage in cobalt/ruthenium-based catalyst boost electroreduction of nitrate to ammonia

The electroreduction of nitrate offers a promising, sustainable, and decentralized route to generate valuable ammonia. However, a key challenge in the nitrate reduction reaction is the energy efficiency of the reaction, which requires both a high ammonia yield rate and a high Faradaic efficiency of ammonia at a low working potential (≥-0.2 V versus reversible hydrogen electrode). We propose a bimetallic Co-B/Ru12 electrocatalyst which utilizes complementary effects of Co-B and Ru to modulate the quantity of adsorbed hydrogen and to favor the specific hydrogenation for initiating nitrate reduction reaction at a low overpotential. This effect enables the catalyst to achieve a Faradaic efficiency for ammonia of 90.4 ± 9.2% and a remarkable half-cell energy efficiency of 40.9 ± 4% at 0 V versus reversible hydrogen electrode. The in-situ electrochemical reconstruction of the catalyst contributes to boosting the ammonia yield rate to a high level of 15.0 ± 0.7 mg h-1 cm-2 at -0.2 V versus reversible hydrogen electrode. More importantly, by employing single-entity electrochemistry coupled with identical location transmission electron microscopy, we gain systematic insights into the correlation between the increase in the catalyst's active sites and its structural transformations during the nitrate reduction reaction.

Single Entity Electrocatalysis

Making a measurement over millions of nanoparticles or exposed crystal facets seldom reports on reactivity of a single nanoparticle or facet, which may depart drastically from ensemble measurements. Within the past 30 years, science has moved toward studying the reactivity of single atoms, molecules, and nanoparticles, one at a time. This shift has been fueled by the realization that everything changes at the nanoscale, especially important industrially relevant properties like those important to electrocatalysis. Studying single nanoscale entities, however, is not trivial and has required the development of new measurement tools. This review explores a tale of the clever use of old and new measurement tools to study electrocatalysis at the single entity level. We explore in detail the complex interrelationship between measurement method, electrocatalytic material, and reaction of interest (e.g., carbon dioxide reduction, oxygen reduction, hydrazine oxidation, etc.). We end with our perspective on the future of single entity electrocatalysis with a key focus on what types of measurements present the greatest opportunity for fundamental discovery.

Optical in situ deciphering of the surface reconstruction-assistant multielectron transfer event of single Co3O4 nanoparticles

Surface reconstruction determines the fate of catalytic sites on the near-surface during the oxygen evolution reaction. However, deciphering the conversion mechanism of various intermediate-states during surface reconstruction remains a challenge. Herein, we employed an optical imaging technique to draw the landscape of dynamic surface reconstruction on individual Co3O4 nanoparticles. By regulating the surface states of Co3O4 nanoparticles, we explored dynamic growth of the CoOx(OH)y sublayer on single Co3O4 nanoparticles and directly identified the conversion between two dynamics. Rich oxygen vacancies induced more active sites on the surface and prolonged surface reconstruction, which enhanced electrochemical redox and oxygen evolution. These results were further verified by in situ electrochemical extinction spectroscopy of single Co3O4 nanoparticles. We elucidate the heterogeneous evolution of surface reconstruction on individual Co3O4 nanoparticles and present a unique perspective to understand the fate of catalytic species on the nanosurface, which is of enduring significance for investigating the heterogeneity of multielectron-transfer events.

Benchmarking the Intrinsic Activity of Transition Metal Oxides for the Oxygen Evolution Reaction with Advanced Nanoelectrodes

The intrinsic activity assessment of transition metal oxides (TMOs) as key electrocatalysts for the oxygen evolution reaction (OER) has not been standardized due to uncertainties regarding their structure and composition, difficulties in accurately measuring their electrochemically active surface area (ECSA), and deficiencies in mass-transfer (MT) rates in conventional measurements. To address these issues, we utilized an electrodeposition-thermal annealing method to precisely synthesize single-particle TMOs with well-defined structure and composition. Concurrently, we engineered low roughness, spherical surfaces for individual particles, enabling precise measurement of their ECSA. Furthermore, by constructing a conductor-core semiconductor-shell structure, we evaluated the inherent OER activity of perovskite-type semiconductor materials, broadening the scope beyond just conductive TMOs. Finally, using single-particle nanoelectrode technique, we systematically measured individual TMO particles of various sizes for OER, overcoming MT limitations seen in conventional approaches. These improvements have led us to propose a precise and reliable approach to evaluating the intrinsic activity of TMOs, not only validating the accuracy of theoretical calculations but also revealing a strong correlation of OER activity on the melting point of TMOs. This discovery holds significant importance for future high-throughput material research and applications, offering valuable insights in electrocatalysis.

Recent Trends and Perspectives in Single-Entity Electrochemistry: A Review with Focus on a Water Splitting Reaction

Electrochemical measurements involving single nanoparticles have attracted considerable research attention. In recent years, various studies have been conducted on single-entity electrochemistry (SEE) for the in-depth analyses of catalytic reactions. Although, several electrocatalysts have been developed for H2 energy production, designing innovative electrocatalysts for this purpose remains a challenging task. Stochastic collision electrochemistry is gaining increased attention because it has led to new findings in the SEE field. Importantly, it facilitates establishing structure activity relationships for electrocatalysts by monitoring transient signals. This article reviews the recent achievements related to hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) using different electrocatalysts at the nanoscale level. In particular, it discusses the electrocatalytic activities of noble metal nanoparticles, including Ag, Au, Pt, and Pd nanoparticles, at the single-particle level. Because heterogeneity is a key factor affecting the catalytic activity of nanostructures, our work focuses on the influence of heterogeneities in catalytic materials on the OER and HER activities. These results may help to achieve a better understanding of the fundamental processes involved in the water splitting reaction.

Role of Fe decoration on the oxygen evolving state of Co3O4 nanocatalysts

The production of green hydrogen through alkaline water electrolysis is the key technology for the future carbon-neutral industry. Nanocrystalline Co3O4 catalysts are highly promising electrocatalysts for the oxygen evolution reaction and their activity strongly benefits from Fe surface decoration. However, limited knowledge of decisive catalyst motifs at the atomic level during oxygen evolution prevents their knowledge-driven optimization. Here, we employ a variety of operando spectroscopic methods to unveil how Fe decoration increases the catalytic activity of Co3O4 nanocatalysts as well as steer the (near-surface) active state formation. Our study shows a link of the termination-dependent Fe decoration to the activity enhancement and a significantly stronger Co3O4 near-surface (structural) adaptation under the reaction conditions. The near-surface Fe- and Co-O species accumulate an oxidative charge and undergo a reversible bond contraction during the catalytic process. Moreover, our work demonstrates the importance of low coordination surface sites on the Co3O4 host to ensure an efficient Fe-induced activity enhancement, providing another puzzle piece to facilitate optimized catalyst design.

Stability Investigations on a Pt@HGS Catalyst as a Model Material for Fuel Cell Applications: The Role of the Local pH

Increasing the resistance of catalysts against electrochemical degradation is one of the key requirements for the wider use of Proton Exchange Membrane Fuel Cells (PEMFCs). Here, we study the degradation of one entity of a highly stable catalyst, Pt@HGS, on a nanoelectrode under accelerated mass transport conditions. We find that the catalyst degrades more rapidly than expected based on previous ensemble measurements. Corroborated by identical location transmission electron microscopy and catalyst layer experiments, we deduce that locally different pH values are likely the reason for this difference in stability. Ultimately, this work provides insights into the actual conditions present in a PEMFC and raises questions about the applicability of accelerated stress tests usually performed to evaluate catalyst stability, particularly when they are performed in half-cell setups under inert gas.

Electrocatalytic Mechanism of Defect in Spinels for Water and Organics Oxidation

Spinels display promising electrocatalytic ability for oxygen evolution reaction (OER) and organics oxidation reaction because of flexible structure, tunable component, and multifold valence. Unfortunately, limited exposure of active sites, poor electronic conductivity, and low intrinsic ability make the electrocatalytic performance of spinels unsatisfactory. Defect engineering is an effective method to enhance the intrinsic ability of electrocatalysts. Herein, the recent advances in defect spinels for OER and organics electrooxidation are reviewed. The defect types that exist in spinels are first introduced. Then the catalytic mechanism and dynamic evolution of defect spinels during the electrochemical process are summarized in detail. Finally, the challenges of defect spinel electrocatalysts are brought up. This review aims to deepen the understanding about the role and evolution of defects in spinel for electrochemical water/organics oxidation and provide a significant reference for the design of efficient defect spinel electrocatalysts.

High-Spatiotemporal-Resolution Electrochemical Measurements of Electrocatalytic Reactivity

The real-time measurement of the individual or local electrocatalytic reactivity of catalyst particles instead of ensemble behavior is considerably challenging but very critical to uncover fundamental insights into catalytic mechanisms. Recent remarkable efforts have been made to the development of high-spatiotemporal-resolution electrochemical techniques, which allow the imaging of the topography and reactivity of fast electron-transfer processes at the nanoscale. This Perspective summarizes emerging powerful electrochemical measurement techniques for studying various electrocatalytic reactions on different types of catalysts. Principles of scanning electrochemical microscopy, scanning electrochemical cell microscopy, single-entity measurement, and molecular probing technique have been discussed for the purpose of measuring important parameters in electrocatalysis. We further demonstrate recent advances in these techniques that reveal quantitative information about the thermodynamic and kinetic properties of catalysts for various electrocatalytic reactions associated with our perspectives. Future research on the next-generation electrochemical techniques is anticipated to be focused on the development of instrumentation, correlative multimodal techniques, and new applications, thus enabling new opportunities for elucidating structure-reactivity relationships and dynamic information at the single active-site level.

Synthesis of Co,Ce Oxide Nanoparticles Using an Aerosol Method and Their Deposition on Different Structured Substrates for Catalytic Removal of Diesel Particulate Matter

The synthesis of Co and Ce oxide nanoparticles using precipitation of precursor salt solutions in the form of microdroplets generated with a nebulizer proved to be an efficient, fast and inexpensive method. Different morphologies of single oxides particles were obtained. Ceria nanoparticles were almost cube-shaped of 8 nm average size, forming 1.3–1.5 μm aggregates, whereas cobalt oxide appeared as rounded-edged particles of 37 nm average size, mainly forming nanorods 50–500 nm. Co3O4 and CeO2 nanoparticles were used to generate structured catalysts from both metallic (stainless steel wire mesh monoliths) and ceramic (cordierite honeycombs) substrates. Ceria Nyacol was used as a binder to favor the anchoring of catalytic particles thus enhancing the adhesion of the coating. The resulting structured catalysts were tested for the combustion of diesel soot with the aim of being used in the regeneration of particulate filters (DPFs). The performance of these structured catalysts was similar to or even better than that exhibited by the catalysts prepared using commercial nanoparticles. Among the catalysts tested, the structured systems using ceramic substrates were more efficient, showing lower values of the maximum combustion rate temperatures (TM = 410 °C).

Crystal Plane-Related Oxygen-Evolution Activity of Single Hexagonal Co3 O4 Spinel Particles

The electrocatalytic activity for the oxygen evolution reaction in alkaline electrolyte of hexagonal spinel Co₃ O₄ nanoparticles derived using scanning electrochemical cell microscopy (SECCM) is correlated with scanning electron microscopy and atomic force microscopy images of the droplet landing sites. A unique way to deconvolute the intrinsic catalytic activity of individual crystal facets of the hexagonal Co₃ O₄ spinel particle is demonstrated in terms of the turnover frequency (TOF) of surface Co atoms. The top surface exposing 111 crystal planes displayed a thickness-dependent TOF with a TOF of about 100 s^-1 at a potential of 1.8 V vs. RHE and a particle thickness of 100 nm. The edge of the particle exposing (110) planes, however, showed an average TOF of 270±68 s^-1 at 1.8 V vs. RHE and no correlation with particle thickness. The higher atomic density of Co atoms on the edge surface (2.5 times of the top) renders the overall catalytic activity of the edge planes significantly higher than that of the top planes. The use of a free-diffusing Os complex in the alkaline electrolyte revealed the low electrical conductivity through individual particles, which explains the thickness-dependent TOF of the top planes and could be a reason for the low activity of the top (111) planes.

Single-entity Electrochemistry Unveils Dynamic Transformation during Tandem Catalysis of Cu2 O and Co3 O4 for Converting NO3 - to NH3

Electrochemically converting nitrate to ammonia is an essential and sustainable approach to restoring the globally perturbed nitrogen cycle. The rational design of catalysts for the nitrate reduction reaction (NO₃ RR) based on a detailed understanding of the reaction mechanism is of high significance. We report a Cu₂ O+Co₃ O₄ tandem catalyst which enhances the NH₃ production rate by ≈2.7-fold compared to Co₃ O₄ and ≈7.5-fold compared with Cu₂ O, respectively, however, most importantly, we precisely place single Cu₂ O and Co₃ O₄ cube-shaped nanoparticles individually and together on carbon nanoelectrodes provide insight into the mechanism of the tandem catalysis. The structural and phase evolution of the individual Cu₂ O+Co₃ O₄ nanocubes during NO₃ RR is unveiled using identical location transmission electron microscopy. Combining single-entity electrochemistry with precise nano-placement sheds light on the dynamic transformation of single catalyst particles during tandem catalysis in a direct way.

A nanoelectrode-based study of water splitting electrocatalysts

The development of low-cost and efficient catalytic materials for key reactions like water splitting, CO₂ reduction and N₂ reduction is crucial for fulfilling the growing energy consumption demands and the pursuit of renewable and sustainable energy. Conventional electrochemical measurements at the macroscale lack the potential to characterize single catalytic entities and nanoscale surface features on the surface of a catalytic material. Recently, promising results have been obtained using nanoelectrodes as ultra-small platforms for the study of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) on innovative catalytic materials at the nanoscale. In this minireview, we summarize the recent progress in the nanoelectrode-based studies on the HER and OER on various nanostructured catalytic materials. These electrocatalysts can be generally categorized into two groups: 0-dimensional (0D) single atom/molecule/cluster/nanoparticles and 2-dimensional (2D) nanomaterials. Controlled growth as well as the electrochemical characterization of single isolated atoms, molecules, clusters and nanoparticles has been achieved on nanoelectrodes. Moreover, nanoelectrodes greatly enhanced the spatial resolution of scanning probe techniques, which enable studies at the surface features of 2D nanomaterials, including surface defects, edges and nanofacets at the boundary of a phase. Nanoelectrode-based studies on the catalytic materials can provide new insights into the reaction mechanisms and catalytic properties, which will facilitate the pursuit of sustainable energy and help to solve CO₂ release issues.

Precise Polishing and Electrochemical Applications of Quartz Nanopipette-Based Carbon Nanoelectrodes

Quartz nanopipette-based carbon nanoelectrodes (CNEs) have attracted extensive attention in nanoscale electrochemistry due to their simple and efficient fabrication, chemically inert materials, flexible size (down to a few nanometers), and ultrathin insulating encapsulation. However, these pristine CNEs usually have significantly irregular morphology on the surface, which greatly limits the applications where inlaid nanodisks are urgently needed. To address this critical issue, we have developed a new precise polishing strategy using paraffin coating protection (i.e., avoiding breakage of quartz materials) and real-time monitoring with a high impedance meter (i.e., indicating electrode exposure) to produce flat carbon nanodisk electrodes. The surface flatness of polished CNEs has been confirmed by a combination of scanning electron microscopy, fast-scan cyclic voltammetry, and scanning electrochemical microscopy. As compared to the expensive focused ion beam processing, this strategy is competitive in terms of the low cost and availability of the equipment and enables the preparation of polished CNEs with sufficiently small size. The flattened CNEs have been exemplified for grafting molecular catalysts to achieve the durable catalysis of reactive molecules or for immobilizing single-particle electrocatalysts to measure the intrinsic activity under sufficient mass-transfer rates.

Single gold nanowire-based nanosensor for adenosine triphosphate sensing by using in-situ surface-enhanced Raman scattering technique

Development of new hot spots for surface enhanced Raman scattering (SERS) technique is of great significance recently. Herein, we developed a single Au nanowire (NW)-based nanosensor for adenosine triphosphate (ATP) sensing by using in-situ SERS technique. Single Au NWs, fabricated by laser-assisted pulling method and hydrofluoric acid (HF) etching process, were linked with single-stranded HS-terminated DNA. After that, gold-silver bimetallic nanoparticles (Au/Ag NPs), attached with thiol-containing Raman dyes and ATP aptamer, were immobilized on DNA-modified single AuNW due to the designed affinity between ATP aptamer and single-stranded DNA. This single AuNW-based device exhibited strong SERS signals. In the presence of adenosine triphosphate (ATP), due to the strong specific affinity between the aptamer and the target, the Au/Ag NPs will be separated from the AuNW, resulting in the obvious decrease of the Raman signals, which can be used for ATP sensing with high sensitivity, selectivity and stability. This nanosensor can be used as an ideal platform for real applications, especially at some confined-space samples, such as trace detection, single cell and in vivo analysis.

In Situ Carbon Corrosion and Cu Leaching as a Strategy for Boosting Oxygen Evolution Reaction in Multimetal Electrocatalysts

The number of active sites and their intrinsic activity are key factors in designing high-performance catalysts for the oxygen evolution reaction (OER). The synthesis, properties, and in-depth characterization of a homogeneous CoNiFeCu catalyst are reported, demonstrating that multimetal synergistic effects improve the OER kinetics and the intrinsic activity. In situ carbon corrosion and Cu leaching during the OER lead to an enhanced electrochemically active surface area, providing favorable conditions for improved electronic interaction between the constituent metals. After activation, the catalyst exhibits excellent activity with a low overpotential of 291.5 ± 0.5 mV at 10 mA cm^-2 and a Tafel slope of 43.9 mV dec^-1 . It shows superior stability compared to RuO₂ in 1 m KOH, which is even preserved for 120 h at 500 mA cm^-2 in 7 m KOH at 50 °C. Single particles of this CoNiFeCu after their placement on nanoelectrodes combined with identical location transmission electron microscopy before and after applying cyclic voltammetry are investigated. The improved catalytic performance is due to surface carbon corrosion and Cu leaching. The proposed catalyst design strategy combined with the unique single-nanoparticle technique contributes to the development and characterization of high-performance catalysts for electrochemical energy conversion.

Enhanced Electrochemistry of Single Plasmonic Nanoparticles

The structure-function relationship of plasmon enhanced electrochemistry (PEEC) is of great importance for the design of efficient PEEC catalyst, but is rarely investigated at single nanoparticle level for the lack of efficient nanoscale methodology. Herein, we report the utilization of nanoparticle impact electrochemistry to allow single nanoparticle PEEC, where the effect of incident light on the plasmonic Ag/Au nanoparticles for accelerating Co-MOFNs catalyzed hydrogen evolution reaction (HER) is systematically explored. It is found that the plasmon excited hot carrier injection can lower the reaction activation energy, resulting in a much promoted reaction probability and the integral charge generated from individual collisions. Besides, a plasmonic nanoparticle filtering method is established to effectively distinguish different plasmonic nanoparticles. This work provides a unique view in understanding the intrinsic physicochemical properties for PEEC at the nano-confined domains.

Single Co3O4 Nanocubes Electrocatalyzing the Oxygen Evolution Reaction: Nano-Impact Insights into Intrinsic Activity and Support Effects

Single-entity electrochemistry allows for assessing electrocatalytic activities of individual material entities such as nanoparticles (NPs). Thus, it becomes possible to consider intrinsic electrochemical properties of nanocatalysts when researching how activity relates to physical and structural material properties. Conversely, conventional electrochemical techniques provide a normalized sum current referring to a huge ensemble of NPs constituting, along with additives (e.g., binders), a complete catalyst-coated electrode. Accordingly, recording electrocatalytic responses of single NPs avoids interferences of ensemble effects and reduces the complexity of electrocatalytic processes, thus enabling detailed description and modelling. Herein, we present insights into the oxygen evolution catalysis at individual cubic Co₃O₄ NPs impacting microelectrodes of different support materials. Simulating diffusion at supported nanocubes, measured step current signals can be analyzed, providing edge lengths, corresponding size distributions, and interference-free turnover frequencies. The provided nano-impact investigation of (electro-)catalyst-support effects contradicts assumptions on a low number of highly active sites.

Electrochemical synthesis of core-shell nanoparticles by seed-mediated selective deposition

Conventional solvothermal synthesis of core-shell nanoparticles results in them being covered with surfactant molecules for size control and stabilization, undermining their practicality as electrocatalysts. Here, we report an electrochemical method for the synthesis of core-shell nanoparticles directly on electrodes, free of surfactants. By implementation of selective electrodeposition on gold cores, 1st-row transition metal shells were constructed with facile and precise thickness control. This type of metal-on-metal core-shell synthesis by purely electrochemical means is the first of its kind. The applicability of the nanoparticle decorated electrodes was demonstrated by alkaline oxygen evolution catalysis, during which the Au-Ni example displayed stable catalysis with low overpotential.

Direct Probing of the Oxygen Evolution Reaction at Single NiFe2O4 Nanocrystal Superparticles with Tunable Structures

Due to the precisely controllable size, shape, and composition, self-assembled nanocrystal superlattices exhibit unique collective properties and find wide applications in catalysis and energy conversion. Identifying their intrinsic electrocatalytic activity is challenging, as their averaged properties on ensembles can hardly be dissected from binders or additives. We here report the direct measurement of the oxygen evolution reaction at single superparticles self-assembled from ∼8 nm NiFe₂O₄ and/or ∼4 nm Au nanocrystals using scanning electrochemical cell microscopy. Combined with coordinated scanning electron microscopy, it is found that the turnover frequency (TOF) estimated from single NiFe₂O₄ superparticles at 1.92 V vs RHE ranges from 0.2 to 11 s^-1 and is sensitive to size only when it is smaller than ∼800 nm in diameter. After the incorporation of Au nanocrystals, the TOF increases by ∼6-fold and levels off with further increasing Au content. Our study demonstrates the first direct single entity electrochemical study on individual nanocrystal superlattices with tunable structures and unravels the intrinsic structure-activity relationship that is not accessible by other methods.

Single Particle Nanoelectrochemistry Reveals the Catalytic Oxygen Evolution Reaction Activity of Co3 O4 Nanocubes

Co₃O₄ nanocubes are evaluated concerning their intrinsic electrocatalytic activity towards the oxygen evolution reaction (OER) by means of single‐entity electrochemistry. Scanning electrochemical cell microscopy (SECCM) provides data on the electrocatalytic OER activity from several individual measurement areas covering one Co₃O₄ nanocube of a comparatively high number of individual particles with sufficient statistical reproducibility. Single‐particle‐on‐nanoelectrode measurements of Co₃O₄ nanocubes provide an accelerated stress test at highly alkaline conditions with current densities of up to 5.5 A cm⁻², and allows to derive TOF values of up to 2.8×10⁴ s⁻¹ at 1.92 V vs. RHE for surface Co atoms of a single cubic nanoparticle. Obtaining such high current densities combined with identical‐location transmission electron microscopy allows monitoring the formation of an oxy(hydroxide) surface layer during electrocatalysis. Combining two independent single‐entity electrochemistry techniques provides the basis for elucidating structure–activity relations of single electrocatalyst nanoparticles with well‐defined surface structure.

Electrospray deposition for single nanoparticle studies

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