Direct measuring of single-heterogeneous bubble nucleation mediated by surface topology.

Heterogeneous bubble nucleation is one of the most fundamental interfacial processes ranging from nature to technology. There is excellent evidence that surface topology is important in directing heterogeneous nucleation; however, deep understanding of the energetics by which nanoscale architectures promote nucleation is still challenging. Herein, we report a direct and quantitative measurement of single-bubble nucleation on a single silica nanoparticle within a microsized droplet using scanning electrochemical cell microscopy. Local gas concentration at nucleation is determined from finite element simulation at the corresponding faradaic current of the peak-featured voltammogram. It is demonstrated that the criteria gas concentration for nucleation first drops and then rises with increasing nanoparticle radius. An optimum nanoparticle radius around 10 nm prominently expedites the nucleation by facilitating the special topological nanoconfinements that consequently catalyze the nucleation. Moreover, the experimental result is corroborated by our theoretical calculations of free energy change based on the classic nucleation theory. This study offers insights into the impact of surface topology on heterogenous nucleation that have not been previously observed.

Revealing the Heterogeneous Bubble Nucleation at Individual Silica Nanoparticles.

Deep understanding of the bubble nucleation process is universally important in systems, from chemical engineering to materials. However, due to its nanoscale and transient nature, effective probing of nucleation behavior with a high spatiotemporal resolution is prohibitively challenging. We previously reported the measurement of a single nanobubble nucleation at a nanoparticle using scanning electrochemical cell microscopy, where the bubble nucleation and formation were inferred from the voltammetric responses. Here, we continue the study of heterogeneous bubble nucleation at interfaces by regulating the local nanostructures using silica nanoparticles with a distinct surface morphology. It is demonstrated that, compared to the smooth spherical silica nanoparticles, the raspberry-like nanoparticles can further significantly reduce the nucleation energy barrier, with a critical peak current about 23% of the bare carbon surfaces. This study advances our understanding of how surface nanostructures direct the heterogeneous nucleation process and may offer a new strategy for surface engineering in gas involved energy conversion systems.

Ultrasensitive detection of dopamine using Au microelectrodes integrated with mesoporous silica thin films.

An electrochemical method was developed for ultrasensitive and selective detection of dopamine in human serum using mesoporous silica thin film modified gold microelectrodes. Vertically aligned mesoporous silica thin films were deposited onto Au microelectrodes by electrochemically assisted self-assembly (EASA). The mesochannels have uniform pore sizes of 2.1 nm in diameter and a negatively charged wall surface. Cyclic voltammetry reveals effective charge permselectivity through the negatively charged mesoporous channels. By using differential pulse voltammetry, the mesoporous silica thin film modified Au microelectrode can be employed for the ultrasensitive detection of dopamine with a detection limit as low as 0.084 µM. In addition, thanks to the electrostatic and steric effects of the silica mesochannels, excellent anti-interference and anti-fouling properties of the electrochemical sensors are demonstrated.

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.

Elucidating the Geometric Active Sites for Oxygen Evolution Reaction on Crystalline Iron-Substituted Cobalt Hydroxide Nanoplates.

Transition-metal (oxy)hydroxides are among the most active and studied catalysts for the oxygen evolution reaction in alkaline electrolytes. However, the geometric distribution of active sites is still elusive. Here, using the well-defined crystalline iron-substituted cobalt hydroxide as a model catalyst, we reported the scanning electrochemical cell microscopy (SECCM) study of single-crystalline nanoplates, where the oxygen evolution reaction at individual nanoplates was isolated and evaluated independently. With integrated prior- and post-SECCM scanning electron microscopy of the catalyst morphology, correlated structure-activity information of individual electrocatalysts was obtained. Our result reveals that while the active sites are largely located at the edges of the pristine Co(OH)2 nanoplates, the Fe lattice incorporation significantly promotes the basal plane activities. Our approach of correlative imaging provides new insights into the effect of iron incorporation on active site distribution across nano-electrocatalysts.

Exploring the Strain Effect in Single Particle Electrochemistry using Pd Nanocrystals.

Tuning the surface strain of heterogeneous catalysts is recognized as a powerful strategy for tailoring their catalytic activity. However, a clear understanding of the strain effect in electrocatalysis at single-particle resolution is still lacking. Here, we explore the electrochemical hydrogen evolution reaction (HER) of single Pd octahedra and icosahedra with the same surface bounded {111} crystal facet and similar sizes using scanning electrochemical cell microscopy (SECCM). It is revealed that tensilely strained Pd icosahedra display significantly superior HER electrocatalytic activity. The estimated turnover frequency at -0.87 V vs RHE on Pd icosahedra is about two times higher than that on Pd octahedra. Our single-particle electrochemistry study using SECCM at Pd nanocrystals unambiguously highlights the importance of tensile strain on electrocatalytic activity and may offer new strategy for understanding the fundamental relationship between surface strain and reactivity.

Electrochemical Sulfonylation-Induced Lactonization of Alkenes: Synthesis of Sulfonyl Phthalides.

An electrochemical cascade sulfonylation and lactonization process of alkenes and a most widely used arylsulfonylation reagent─sulfonyl hydrazines─was developed for the first time. This electrochemical sulfonyl lactonization avoided the use of toxic metal catalysts or stoichiometric oxidants and was carried out under mild conditions. The target product γ-sulfonylated phthalides with broad and excellent substrate tolerance were achieved.

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.

Electrochemical Visualization of Gas Bubbles on Superaerophobic Electrodes Using Scanning Electrochemical Cell Microscopy.

Electrocatalytic gas evolution reactions, where gaseous molecules are electrogenerated by reduction or oxidation of a species, play a central role in many energy conversion systems. Superaerophobic electrodes, usually constructed by their surface microstructures, have demonstrated excellent performance for electrochemical gas evolution reactions due to their bubble-repellent properties. Understanding and quantification of the gas bubble behavior including nucleation and dynamics on such microstructured electrodes is an important but underexplored issue. In this study, we reported a scanning electrochemical cell microscopy (SECCM) investigation of individual gas bubble nucleation and dynamics on nanoscale electrodes. A classic Pt film and a nonconventional transition-metal dichalcogenide MoS2 film with different surface topologies were employed as model substrates for both H2 and N2 bubble electrochemical studies. Interestingly, the nanostructured catalyst surface exhibit significantly less supersaturation for gas bubble nucleation and a notable increase of bubble detachment compared to its flat counterpart. Electrochemical mapping results reveal that there is no clear correlation between bubble nucleation and hydrogen evolution reaction (HER) activity, regardless of local electrode surface microstructures. Our results also indicate that while the hydrophobicity of the nanostructured MoS2 surface promotes bubble nucleation, it has little effect on bubble dynamics. This work introduces a new method for nanobubble electrochemistry on broadly interesting catalysts and suggests that the deliberate microstructure on a catalyst surface is a promising strategy for improving electrocatalytic gas evolution both in terms of bubble nucleation and elimination.

Dynamic Equilibrium Model for Surface Nanobubbles in Electrochemistry.

Gas bubbles are ubiquitous in electrochemical processes, particularly in water electrolysis. Due to the development of gas-evolving electrocatalysis and energy conversion technology, a deep understanding of gas bubble behaviors at the electrode surface is highly desirable. In this work, by combining theoretical analysis and molecular simulations, we study the behaviors of a single nanobubble electrogenerated at a nanoelectrode. With the dynamic equilibrium model, the stability criteria for stationary surface nanobubbles are established. We show theoretically that a slight change in either the gas solubility or solute concentration results in various nanobubble dynamic states at a nanoelectrode: contact line pinning in aqueous and ethylene glycol solutions, oscillation of pinning states in dimethyl sulfoxide, and mobile nanobubbles in methanol. The above complex nanobubble behavior at the electrode/electrolyte interface is explained by the competition between gas influx into the nanobubble and outflux from the nanobubble.

Electrochemical Oxidative Cross-Coupling between Vinyl Azides and Thiophenols: Synthesis of gem-Bisarylthio Enamines.

An electrochemical radical strategy involving alkene substrates provides a powerful approach for alkene functionalization. Herein, we described the first electrochemical synthesis of gem-bisarylthio enamines from vinyl azides and thiophenols through the C-H/S-H cross-coupling. This electrochemical oxidative cross-coupling is characterized by good functional group tolerance, affording a series of gem-bisarylthio enamines in excellent yields, and was carried out at room temperature without additional oxidant, transition-metal catalyst, or base. Notably, the reaction could be easily performed on a gram scale with good efficiency.

Electrochemical Oxidative Halogenation of N-Aryl Alkynamides for the Synthesis of Spiro[4.5]trienones.

We developed a green method for the synthesis of spiro[4.5]trienones through an electrochemical oxidative halocyclization with N-aryl alkynamides. This reaction was conducted under metal-catalyst- and exogenous-oxidant-free conditions at room temperature. Using readily available LiCl, LiBr, and LiI as the halogen source, a variety of dearomative halo-spirocyclization products were obtained in good to excellent yields with a broad scope and functional group tolerance.

Nitrogen Bubbles at Pt Nanoelectrodes in a Nonaqueous Medium: Oscillating Behavior and Geometry of Critical Nuclei.

Gas bubble evolution is present in many electrochemical and photoelectrochemical processes. We previously reported the formation of individual H2, N2, and O2 nanobubbles generated from electrocatalytic reduction of H+ and oxidation of N2H4 and H2O2, respectively, at Pt nanodisk electrodes in an aqueous solution. All the nanobubbles formed display a dynamic stationary state of a three-phase boundary with an invariant residual current. Here, we test the hypothesis that gas nanobubbles can also be electrogenerated in a nonaqueous medium. Interestingly, we found oscillating bubble behavior corresponding to nucleation, growth, and dissolution in dimethyl sulfoxide and methanol. One possible explanation of the oscillation mechanism is provided by the instable dynamic equilibrium between the gas influx due to supersaturation and outflux due to Laplace pressure. Furthermore, the critical gas concentrations for N2 nanobubble nucleation are estimated to be 148, 386, 200, and 16 times supersaturation and the contact angles of the critical nuclei to be 164°, 151°, 160°, and 174° in water, dimethyl sulfoxide, ethylene glycol, and methanol, respectively. This is the first report on electrochemical nucleation of gas bubbles in nonaqueous solvents. Our electrochemical gas bubble study based on a nanoelectrode platform has proven to be a prototypical example of single-entity electrochemistry.

Electrochemical Tandem Fluoroalkylation-Cyclization of Vinyl Azides: Access to Trifluoroethylated and Difluoroethylated N-Heterocycles.

A transition-metal- and oxidant-free electrochemical strategy for radical fluoroalkylation of vinyl azides was developed. The reaction was carried out under mild conditions by using inexpensive and bench-stable RfSO2Na (Rf = CF3, CF2H) as fluorination reagents. Depending on the starting material, both the electrochemical radical cyclization and dearomatization products could be obtained. This method provides a green and safe approach to synthesize fluorinated nitrogen heterocycles.

Correlation between Gas Bubble Formation and Hydrogen Evolution Reaction Kinetics at Nanoelectrodes.

We report the correlation between H2 gas bubble formation potential and hydrogen evolution reaction (HER) activity for Au and Pt nanodisk electrodes (NEs). Microkinetic models were formulated to obtain the HER kinetic information for individual Au and Pt NEs. We found that the rate-determining steps for the HER at Au and Pt NEs were the Volmer step and the Heyrovsky step, respectively. More interestingly, the standard rate constant ( k0) of the rate-determining step was found to vary over 2 orders of magnitude for the same type of NEs. The observed variations indicate the HER activity heterogeneity at the nanoscale. Furthermore, we discovered a linear relationship between bubble formation potential ( Ebubble) and log( k0) with a slope of 125 mV/decade for both Au and Pt NEs. As log ( k0) increases, Ebubble shifts linearly to more positive potentials, meaning NEs with higher HER activities form H2 bubbles at less negative potentials. Our theoretical model suggests that such linear relationship is caused by the similar critical bubble formation condition for Au and Pt NEs with varied sizes. Our results have potential implications for using gas bubble formation to evaluate the HER activity distribution of nanoparticles in an ensemble.

Electrochemical Size Measurement and Characterization of Electrodeposited Platinum Nanoparticles at Nanometer Resolution with Scanning Electrochemical Microscopy.

The properties of nanoparticles (NPs) are determined by their size and geometric structures. A reliable determination of NP dimension is critical for understanding their physical and chemical properties, but sizing ultrasmall particles on the order of nanometer (nm) scale in the solution is still challenging. Here, we report the size measurement of PtNP at nanometer resolution by in situ scanning electrochemical microscopy (SECM), performed with the electrochemical generation and removal of H2 bubble at a reasonably small distance between tip and substrate electrodes in 200 or 500 mM HClO4 solution. A series of different PtNPs or nanoclusters were electrodeposited and in situ measured in the solution, proving the concept of sizing ultrasmall particles using tip generation/substrate collection mode of SECM. This technique could be also used for investigations of other supported ultrasmall metal nanocluster systems.

Single Nanochannel Platform for Detecting Chiral Drugs.

Chiral drugs play an essential role in medical and biochemical systems, and thus enantioselective analysis of chiral molecules has become a central focus in chemical, biological, medical, and pharmaceutical research. The design of chiral drug-detecting systems is a long-term and challenging task. Here we report the use of a modification-free nanochannel method for enantioselective recognition of S-naproxen from R-naproxen using N-acetyl-l-cysteine-capped gold nanoparticles as a chiral selector. The chiral discrimination is based on a drug-induced nanoparticle diastereoselective aggregation mechanism that blocks ion transport through the nanochannel. We demonstrated that high S-Npx selectivity in both water and biological samples can be achieved. This simple method has potential applications as a general platform for the detection of chiral molecules.

The Dynamic Steady State of an Electrochemically Generated Nanobubble.

This article describes the dynamic steady state of individual H2 nanobubbles generated by H+ reduction at inlaid and recessed Pt nanodisk electrodes. Electrochemical measurements coupled with finite element simulations allow analysis of the nanobubble geometry at dynamic equilibrium. We demonstrate that a bubble is sustainable at Pt nanodisks due to the balance of nanobubble shrinkage due to H2 dissolution and growth due to H2 electrogeneration. Specifically, simulations are used to predict stable geometries of the H2/Pt/solution three-phase interface and the width of exposed Pt at the disk circumference required to sustain the nanobubble via steady-state H2 electrogeneration. Experimentally measured currents, iss, corresponding to the electrogeneration of H2, at or near the three-phase interface, needed to sustain the nanobubble are between 0.2 and 2.4 nA for Pt nanodisk electrodes with radii between 2.5 and 40 nm. However, simple theoretical analysis shows that the diffusion-limited currents required to sustain such a single nanobubble at an inlaid Pt nanodisk are 1-2 orders larger than the observed values. Finite element simulation of the dynamic steady state of a nanobubble at an inlaid disk also demonstrates that the expected steady-state currents are much larger than the experimental currents. Better agreement between the simulated and experimental values of iss is obtained by considering recession of the Pt disk nanoelectrode below the plane of the insulating surface, which reduces the outward flux of H2 from the nanobubble and results in smaller values of iss.