Nanoimpact Electrochemistry to Quantify the Transformation and Electrocatalytic Activity of Ni(OH)2 Nanoparticles: Toward the Size-Activity Relationship at High Throughput.

The fast establishment of structure-reactivity relationships is crucial to identifying the most appropriate nanoparticles (NPs) for a given application. This requires the development of methodologies allowing, simultaneously, the unraveling of the NPs geometry and the screening of their reactivity. Herein, nanoimpact electrochemistry (NIE) allows for quantifying the transformation and measuring the electrocatalytic activity for the oxygen evolution reaction (OER) of >100 Ni(OH)2 NPs of a wide range of size (NP radii from 25 to 100 nm). This is achieved by scanning electrochemical microscopy in a generation/collection-like mode, with one electrode being used to electrogenerate by local precipitation colloidal Ni(OH)2 NPs and the second one being used to collect them by NIE. It allows (i) quantifying the reductive and oxidative conversion of the Ni(OH)2 NPs and (ii) separating the electrochemical conversion and the OER electrocatalysis, leading to the evaluation of a structure-activity relationship.

Deciphering Competitive Routes for Nickel-Based Nanoparticle Electrodeposition by an Operando Optical Monitoring.

Electrodeposition of earth-abundant iron group metals such as nickel is difficult to characterize by simple electrochemical analyses since the reduction of their metal salts often competes with inhibiting reactions. This makes the mechanistic interpretation sometimes contradictory, preventing unambiguous predictions about the nature and structure of the electrodeposited material. Herein, the complexity of Ni nanoparticles (NPs) electrodeposition on indium tin oxide (ITO) is unraveled operando and at a single entity NP level by optical microscopy correlated to ex situ SEM imaging. Our correlative approach allows differentiating the dynamics of formation of two different NP populations, metallic Ni and Ni(OH)2 with a <25 nm limit of detection, their formation being ruled by the competition between Ni2+ and water reduction. At the single NP level this results in a self-terminated growth, an information which is most often hidden in ensemble averaged measurements.

Direct vs Indirect Grafting of Alkyl and Aryl Halides.

The direct and indirect electrochemical grafting of alkyl and aryl halides (RX, ArX) on carbon, metal and polymer surfaces is examined. Their electrochemical reduction occurs at highly negative potential in organic solvents and very often produces carbanions because the reduction potentials of RX and ArX are more negative than those of their corresponding radicals. Therefore, direct electrografting of alkyl and aryl radicals generated from RX and ArX is not easy to perform. This obstacle is overcome using aryl radicals derived from the 2,6-dimethylbenzenediazonium salt (2,6-DMBD), which do not react on the electrode surface due to their steric hindrance but react in solution by abstracting an iodine or bromine atom from RX (X=I, Br) or ArI to give alkyl or aryl radicals. As a consequence, alkyl and aryl radicals are generated at very low driving force by diverting the reactivity of aryl radicals derived from an aryl diazonium salt; they attack the electrode surface and form strongly attached organic layers. This strategy applies to the chemical modification of polymers (polyethylene, polymethylmethacrylate) by alkyl halides under heating.

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?

Revealing the sub-50 ms electrochemical conversion of silver halide nanocolloids by stochastic electrochemistry and optical microscopy.

Silver based ionic crystal nanoparticles (NPs) are interesting nanomaterials for energy storage and conversion, e.g. their colloidal solutions could be used as a reversible redox nanofluid in semi-solid redox flow cells. In this context, the reductive transformation of Brownian silver halide, AgX, NPs into silver NPs is probed by single NP electrochemistry, complemented by operando high resolution monitoring. However, their light sensitivity and poor conductivity make the operando monitoring of their chemical activity challenging. The electrochemical collisions of single AgX NPs onto a negatively biased electrode evidence a full conversion through multiple reduction steps within 3-10 ms. This is further corroborated by simulation of the conversion process and operando through a high resolution optical microscopy technique (Backside Absorbing Layer Microscopy, BALM). Both techniques are interesting strategies to infer at the single NP level the intrinsic charge capacity and charging rate of redox active Brownian nanomaterials, demonstrating the interest of the fast and reversible AgX/Ag system as a redox nanofluid.

In Situ Optical Monitoring of the Electrochemical Conversion of Dielectric Nanoparticles: From Multistep Charge Injection to Nanoparticle Motion.

By shortening solid-state diffusion times, the nanoscale size reduction of dielectric materials-such as ionic crystals-has fueled synthetic efforts toward their use as nanoparticles, NPs, in electrochemical storage and conversion cells. Meanwhile, there is a lack of strategies able to image the dynamics of such conversion, operando and at the single NP level. It is achieved here by optical microscopy for a model dielectric ionic nanocrystal, a silver halide NP. Rather than the classical core-shrinking mechanism often used to rationalize the complete electrochemical conversion and charge storage in NPs, an alternative mechanism is proposed here. Owing to its poor conductivity, the NP conversion proceeds to completion through the formation of multiple inclusions. The superlocalization of NP during such heterogeneous multiple-step conversion suggests the local release of ions, which propels the NP toward reacting sites enabling its full conversion.

Effect of the driving force on nanoparticles growth and shape: an opto-electrochemical study.

Most protocols developed to synthesize nanoparticles (NPs) and to control their shape are inspired from nucleation and growth theories. However, to rationalize the mechanisms of the shape-selective synthesis of NPs, experimental strategies allowing to probe in situ the growth of NPs are needed. Herein, metal Au or Ag nanoparticles (NPs) are produced by reaction of a metallic ion precursor with a reversible redox reducer. The process is explored by an oxidative electrosynthesis strategy using a sacrificial Au or Ag ultramicroelectrode to both trigger the metallic ion generation and control the local concentrations of the different reactants. The effect of the driving force for the metallic ion reduction over metal NP growth dynamics is inspected in situ and in real time at the single NP level by high-resolution optical microscopy from the tracking of the Brownian trajectories of the growing NPs in solution. The NP reductive growth/oxidative etching thermodynamics, and consequently the NP shape, are shown to be controlled electrochemically by the reversible redox couple, while the intervention of an Au(i) intermediate ion is suggested to account for the formation of gold nanocubes.

Light Driven Design of Dynamical Thermosensitive Plasmonic Superstructures: A Bottom-Up Approach Using Silver Supercrystals.

When narrowly distributed silver nanoparticles (NPs) are functionalized by dodecanethiol, they acquire the ability to self-organize in organic solvents into 3D supercrystals (SCs). The NP surface chemistry is shown to introduce a light-driven thermomigration effect, thermophoresis. Using a laser beam to heat the NPs and generate steep thermal gradients, the migration effect is triggered dynamically, leading to tailored structures with high density of plasmonic hot spots. This work describes how to manipulate the hot spots and monitor the effect by holography, thus providing a complete characterization of the migration process on a single object basis. Extensive single object tracking strategies are employed to measure the SCs trajectories, evaluate their size, drift velocity magnitude and direction, allowing the identification of the physical chemical origins of the migration. The phenomenon is shown to happen as a result of the combination of thermophoresis (at short length scales) and convection (long-range), and does not require a metallic substrate. This constitutes a fully optical method to dynamically generate plasmonic platforms in situ and on demand, without requiring substrate nanostructuration and with minimal interference on the chemistry of the system. The importance of the proof-of-concept herein described stems from the numerous potential applications, spanning over a variety of fields such as microfluidics and biosensing.

Alkyl-Modified Gold Surfaces: Characterization of the Au-C Bond.

The surface of gold can be modified with alkyl groups through a radical crossover reaction involving alkyliodides or bromides in the presence of a sterically hindered diazonium salt. In this paper, we characterize the Au-C(alkyl) bond by surface-enhanced Raman spectroscopy (SERS); the corresponding peak appears at 387 cm-1 close to the value obtained by theoretical modeling. The Au-C(alkyl) bond energy is also calculated, it reaches -36.9 kcal mol-1 similar to that of an Au-S-alkyl bond but also of an Au-C(aryl) bond. In agreement with the similar energies of Au-C(alkyl) and Au-S-(alkyl), we demonstrate experimentally that these groups can be exchanged on the surface of gold.

Combining Electrodeposition and Optical Microscopy for Probing Size-Dependent Single-Nanoparticle Electrochemistry.

Electrodeposition of nanoparticles (NPs) is a promising route for the preparation of highly electroactive nanostructured electrodes. By taking advantage of progressive electrodeposition, disordered arrays with a wide size distribution of Ag NPs are produced. Combined with surface-reaction monitoring by using highly sensitive backside absorbing-layer optical microscopy (BALM), such arrays offer a platform for screening size-dependent electrochemistry at the single NP level. In particular, this strategy allows rationalizing the electrodeposition dynamics at the single-NP level (>10 nm), up to the point of quantifying the presence of metal nanoclusters (<2 nm), and probing easier NP oxidation with size decrease, either through electrochemical or galvanic reactions.

The promise of antireflective gold electrodes for optically monitoring the electro-deposition of single silver nanoparticles.

The interest in nano-objects has recently dramatically increased in all fields of science, and electrochemistry is no exception. As a consequence, in situ and operando visualization of electrochemical processes is needed at the nanoscale. Herein, we propose a new interferometric microscopy based on an antireflective thin metal electrode layer. The technique is coupled to electrochemistry in a model example: the electro-deposition of Ag metallic nanoparticles (NPs). This challenges the current opto-electrochemical methods and even those relying on nano-impact detection. Indeed, the sensitivity allows the dynamic in situ visualization of the electrochemical growth and dissolution of individual Ag NPs, whose size was tracked dynamically down to 15 nm in diameter. The use of microelectrodes provides interesting quantitative analysis of the NPs, from optically resolved arrays of single NPs to condensed arrays of (unresolved) NPs. Particularly, the optical analysis of all the individual NPs allows the reconstruction of optical voltammograms similar to the electrochemical ones. Finally, the NP dissolution-redeposition is also investigated.

Monitoring Cobalt-Oxide Single Particle Electrochemistry with Subdiffraction Accuracy.

By partially overcoming the diffraction limit, superlocalization techniques have extended the applicability of optical techniques down to the nanometer size-range. Herein, cobalt oxide-based nanoparticles are electrochemically grown onto carbon nanoelectrodes and their individual catalytic properties are evaluated through a combined electrochemical-optical approach. Using dark-field white light illumination, edges superlocalization techniques are applied to quantify changes in particle size during electrochemical activation with down to 20 nm precision. It allows the monitoring of (i) the anodic electrodeposition of cobalt hydroxide material and (ii) the large and reversible volume expansion experienced by the cobalt hydroxide particle during its oxidation. Meanwhile, the particle light scattering provides chemical information such as the Co redox state transformation, which complements both the particle size and the recorded electrochemical current and provides in operando mechanistic information on particle electrocatalytic properties.

Platinum Nanoparticle Impacts at a Liquid|Liquid Interface.

Single nanoparticle (NP) electrochemistry detection at a micro liquid|liquid interface (LLI) is exploited using the catalyzed oxygen reduction reaction (ORR). In this way, current spikes reminiscent of nanoimpacts were recorded, which corresponded to electrocatalytic enhancement of the ORR by Pt NPs. The nature of the LLI allows exploration of new phenomena in single NP electrochemistry. The recorded impacts result from a bipolar reaction occurring at the Pt NP straddling the LLI. O2 reduction takes place in the aqueous phase, while ferrocene hydride (Fc-H+ ; a complex generated upon facilitated interfacial proton transfer by Fc) is oxidized in the organic phase. Ultimately, the role of reactant partitioning, NP bouncing, or the ability of NPs to induce Marangoni effects, is demonstrated.

Some Theoretical and Experimental Insights on the Mechanistic Routes Leading to the Spontaneous Grafting of Gold Surfaces by Diazonium Salts.

The spontaneous grafting of diazonium salts on gold may involve the carbocation obtained by heterolytic dediazonation and not necessarily the radical, as usually observed on reducing surfaces. The mechanism is addressed on the basis of DFT calculations and experiments carried out under conditions where the carbocation and the radical are produced selectively. The calculations indicate that the driving force of the reaction leading from a gold cluster, used as a gold model surface, and the carbocation to the modified cluster is higher than that of the analogous reaction starting from the radical. The experiments performed under conditions of heterolytic dediazonation show the formation of thin films on the surface of gold. The grafting of a carbocation is therefore possible, but a mechanism where the cleavage of the Ar-N bond is catalyzed by the surface of gold cannot be excluded.

Electrochemical transformation of individual nanoparticles revealed by coupling microscopy and spectroscopy.

Although extremely sensitive, electrical measurements are essentially unable to discriminate complex chemical events involving individual nanoparticles. The coupling of electrochemistry to dark field imaging and spectroscopy allows the triggering of the electrodissolution of an ensemble of Ag nanoparticles (by electrochemistry) and the inference of both oxidation and dissolution processes (by spectroscopy) at the level of a single nanoparticle. Besides the inspection of the dissolution process from optical scattering intensity, adding optical spectroscopy reveals chemical changes through drastic spectral changes. The behaviours of single NPs and NP agglomerates are differentiated: in the presence of thiocyanate ions, the transformation of Ag single nanoparticles to AgSCN is investigated in the context of plasmonic coupling with the electrode; tentative interpretations for optically unresolved groups of nanoparticles are proposed.

Electrochemistry of Single Nanodomains Revealed by Three-Dimensional Holographic Microscopy.

Interest in nanoparticles has vigorously increased over the last 20 years as more and more studies show how their use can potentially revolutionize science and technology. Their applications span many different academically and industrially relevant fields such as catalysis, materials science, health, etc. Until the past decade, however, nanoparticle studies mostly relied on ensemble studies, thus leaving aside their chemical heterogeneity at the single particle level. Over the past few years, powerful new tools appeared to probe nanoparticles individually and in situ. This Account describes how we drew inspiration from the emerging fields of nanoelectrochemistry and plasmonics-based high resolution holographic microscopy to develop a coupled approach capable of analyzing in operando (electro)chemical reaction over one single nanoparticle. A brief overview of selected optical strategies to image NPs in situ with emphasis on scattering based methods is presented. In an electrochemical context, it is necessary to track particle behavior both in solution and near a polarized electrode, which is why 3D optical observation is particularly appealing. These approaches are discussed together with strategies to track NPs beyond the diffraction limit, allowing a much finer description of their trajectories. Then, the holographic setup is used to study electrochemically triggered Ag NP oxidation reaction in the presence of different electrolytes. Holography is shown to be a powerful technique to track and analyze the trajectory of individual NPs in situ, which further sheds light on in operando behaviors such as electrogenerated NP transport, aggregation, or adsorption. We then show that spectroscopy and scattering-based optical methods are reliable and sensitive to the point of being used to investigate and quantify NP (electro)chemical reactions in model cases. However, since real chemical reactions usually take place in an inherently complex environment, approaches based exclusively on optical imaging only reach their limitations. The strategy is then taken one step further by merging together electrochemical nanoimpact experiments with 3D optical monitoring. Previous strategies are validated by showing that in simple cases, these two independent ways of probing NP size and reactivity yield the same results. For more complicated reactions (e.g., multistep reactions), one must go beyond either technique by showing that the two approaches are perfectly complementary and that the two signals contain information of different natures, thus providing a much better characterization of the reaction. This point is illustrated by studying Ag NP oxidation (single or agglomerates) in the presence of a precipitating agent, where the actual oxidation is uncoupled from the dissolution of the particle, thus proving the point of our symbiotic approach.

Electron Transfer to a Phosphomolybdate Monolayer on Glassy Carbon: Ambivalent Effect of Protonation.

The polyoxomolybdate hybrid TBA3[PMo11O39{Sn(C6H4)C≡C(C6H4)N2}] K(Mo)Sn[N2(+)] was prepared through Sonogashira-type coupling between TBA4[PMo11O39{Sn(C6H4)I}] K(Mo)Sn[I] and an excess of 3,3-diethyl-1-(4-ethynylphenyl)triaz-1-ene bearing a protected diazonium function, followed by its deprotection by the addition of trifluoroacetic acid (TFA). This enlarges the family of organic-inorganic polyoxomolybdate-based hybrids, which has been far less investigated than their related polyoxotungstates. The diazonium function allows for the electrochemical grafting on glassy carbon, and the K(Mo)Sn-modified electrode was further probed by cyclic voltammetry. The PMo11Sn core was found to be highly sensitive to protonation, and five bielectronic proton-coupled electron transfer processes were detected in the presence of an excess of TFA, thus corresponding to the injection of up to 10 electrons in the potential range between 0.15 and -0.45 V/SCE. The gain observed in the thermodynamic potentials is however detrimental to the apparent kinetics of the electron transfer, which drops from 500 s(-1) in the absence of acid to 12 s(-1) in the presence of an excess of TFA.

Surface Functionalization of Metals by Alkyl Chains through a Radical Crossover Reaction.

Alkyl chains are covalently attached onto metal surfaces by indirect reduction of the bromoalkyl derivative (RBr). This indirect reaction involves the formation (by spontaneous or electrochemical reduction of the 2,6-dimethylbenzenediazonium salt) of a sterically hindered aryl radical that abstracts a Br atom from RBr but does not react with the surface. This crossover reaction furnishes an alkyl radical that reacts with the surface. Starting from 6-bromohexanoic acid, carboxylic functionalized gold surfaces are prepared. "Layer-by-layer" assemblies are built from these surfaces and present some ionic selectivity.

Correlated Electrochemical and Optical Detection Reveals the Chemical Reactivity of Individual Silver Nanoparticles.

Electrochemical (EC) impacts of single nanoparticles (NPs) on an ultramicroelectrode are coupled with optics to identify chemical processes at the level of individual NPs. While the EC signals characterize the charge transfer process, the optical monitoring gives a complementary picture of the transport and chemical transformation of the NPs. This is illustrated in the case of electrodissolution of Ag NPs. In the simplest case, the optically monitored dissolution of individual NPs is synchronized with individual EC spikes. Optics then validates in situ the concept of EC nanoimpacts for sizing and counting of NPs. Chemical complexity is introduced by using a precipitating agent, SCN(-), which tunes the overall electrodissolution kinetics. Particularly, the charge transfer and dissolution steps occur sequentially as the synchronicity between the EC and optical signals is lost. This demonstrates the level of complexity that can be revealed from such electrochemistry/optics coupling.

Surface Modification of Polymers by Reaction of Alkyl Radicals.

The surfaces of poly(methyl methacrylate) and polyethylene are modified either (i) by a two-step process including the thermal reaction of alkyl radicals derived from bromohexanoic acid in a mixture of 2,6-dimethylbenzene diazonium salt and neat isopentyl nitrite at 60 °C, followed by reaction with p-nitroaniline, anthraquinone, neutral red, and polyethylene glycol moieties, or (ii) by reaction of a previously anthraquinone-modified bromohexanoic acid. The modified surfaces are characterized by IR, XPS, UV, and water contact angles. A mechanism is proposed to rationalize the results. This approach is an efficient way to modify and pattern polymer surfaces with different organic groups and chemical functionalities under mild conditions.