Mechanisms of Nucleation and Stationary States of Electrochemically Generated Nanobubbles

Effect of dissolved oxygen on the peroxymonosulfate activation pathway in an electrochemical Co/P/CA cathode system

Although dissolved oxygen plays an important role in electro-Fenton-like processes, few investigations have revealed its underlying effects in such processes. Herein, the effect of dissolved oxygen on peroxide activation in an electro-Fenton-like system comprising electrochemical cells and peroxymonosulfate (PMS) was investigated. Cobalt phosphide-modified carbon aerogel (Co/P/CA) was used as the cathode material owing to the high conductivity and catalytic activity of Co/P/CA. Several free radicals and their effects on organic pollutant removal were observed using electron paramagnetic resonance spectrometry and quenching experiments, respectively. The observations revealed that in the presence of O2, hydroxyl radical (·OH), superoxide (O2-·), and singlet oxygen (1O2) served as the primary active species in the PMS activation process, while in the presence of N2, ·OH and sulfate radical (SO4-·) served as the dominant active species in this process. The factor responsible for the difference in the PMS activation pathways available under O2 and N2 conditions was investigated using rotating disk electrode tests and free energy calculations. The tests indicated that O2 facilitates PMS activation to form ·OH instead of SO4-·. The dissolved oxygen subsequently underwent a single-electron-reduction reaction and was converted into O2-·, which could serve as a source of 1O2. When N2 was introduced, Co species, particularly Co(II), played a key role in activating PMS. The free radicals ·OH and SO4-· were generated during the PMS activation process. This study clearly demonstrates the mediating catalysis role of dissolved oxygen in electro-Fenton-like system through experimental data and theoretical calculations, thereby positively contributing to future studies regarding the continuous activation of peroxides in composite systems and improvement of the efficiency of waterbody remediation.

Dual-Mode Imaging of Dynamic Interaction between Bubbles and Single Nanoplates during the Electrocatalytic Hydrogen Evolution Process

Gas bubble formation at electrochemical interfaces can significantly affect the efficiency and durability of electrocatalysts. However, obtaining comprehensive details on bubble evolution dynamics, particularly their dynamic interaction with high-performance structured electrocatalysts, poses a considerable challenge. Herein, dual-mode interference/total internal reflection fluorescence microscopy is introduced, which allows for the simultaneous capture of the evolution pathway of bubbles and the 3D motion of nanoplate electrocatalysts, providing high-resolution and accurate spatiotemporal information. During the hydrogen evolution reaction, the dynamics of hydrogen bubble generation and their interactions with single nanoplate electrocatalysts at the electrochemical interface are observed. The results unveiled that, under constant potential, bubbles initially manifest as fast-moving nanobubbles, transforming into stationary microbubbles subsequently. The morphology of stationary nanoplates regulates the trajectories of these moving nanobubbles while the pinned microbubbles induce the motion of the electrocatalysts. The dual-mode microscopy can be employed to scrutinize numerous multiphase electrochemical interactions with high spatiotemporal resolution, which can facilitate the rational design of high-performance electrocatalysts.

Sulfur occupancy-induced construction of ant-nest-like NiMo/CF(N) electrode for highly efficient hydrogen evolution

The microstructure of the electrocatalyst plays a critical role in the reaction efficiency and stability during electrochemical water splitting. Designing an efficient and stable electrocatalyst, further clarifying the synthesis mechanism, is still an important problem to be solved urgently. Inspired by the copper pyrometallurgy theory, an exceptionally active NiMo/CF(N) electrode, consisting of an ant-nest-like copper foam substrate (defined as CF(N)) and deposited NiMo layer, was fabricated for the alkaline hydrogen evolution reaction (HER). Our findings expounded the structure construction mechanism and highlighted the pivotal role of the spatial occupancy of sulfur atoms in the construction of the ant-nest-like structure. The NiMo/CF(N) composite, characterized by channels with a 2 μm diameter, showcases strong electronic interactions, increased catalytic active sites, enhanced electron/ion transport, and facilitated gas release during HER. Remarkably, NiMo/CF(N) demonstrates ultralow overpotentials of 21 mV to deliver a current density of 10 mA cm-2 in 1 M KOH. This electrode also exhibits outstanding durability, maintaining a current density of 200 mA cm-2 for 110 h, attributed to the chemical and structural integrity of its catalytic surface and the excellent mechanical properties of the electrode. This work advances the fundamental understanding of constructing micro/nano-structured electrocatalysts for highly efficient water splitting.

Electrochemically reactive colloidal nanobubbles by water splitting

HYPOTHESIS

The existing literature reports have conflicting views on reactive oxygen species (ROS) generation by bulk nanobubbles. Consequently, we propose the hypothesis that (i) ROS may be generated during the process of nanobubble generation through water splitting, and (ii) bulk nanobubbles possess electrochemical reactivity, which could potentially lead to continuous ROS generation even after the cessation of nanobubble production.

EXPERIMENTS

A comprehensive set of experiments was conducted to generate nanobubbles in pure water using the water-splitting method. The primary aims of this study are as follows: (i) nanobubble generation by electrolysis and its characterization; (ii) to provide conclusive evidence that the nano-entities are indeed nanobubbles; (iii) to quantify the production of reactive oxygen species during the process of nanobubble generation and (iv) to establish evidence for the presence of electrochemically reactive nanobubbles. The findings of our experiment suggest that bulk nanobubbles possess the ability to generate reactive oxygen species (ROS) during the process of nanobubble nucleation. Additionally, our results indicate that bulk nanobubbles are electrochemically reactive after the cessation of nanobubble production. The electron spin spectroscopy (ESR) response and degradation of the dye compound over time confirm the electrochemical reactivity of bulk nanobubbles.

Threshold current density for diffusion-controlled stability of electrolytic surface nanobubbles

Understanding the stability mechanism of surface micro/nanobubbles adhered to gas-evolving electrodes is essential for improving the efficiency of water electrolysis, which is known to be hindered by the bubble coverage on electrodes. Using molecular simulations, the diffusion-controlled evolution of single electrolytic nanobubbles on wettability-patterned nanoelectrodes is investigated. These nanoelectrodes feature hydrophobic islands as preferential nucleation sites and allow the growth of nanobubbles in the pinning mode. In these simulations, a threshold current density distinguishing stable nanobubbles from unstable nanobubbles is found. When the current density remains below the threshold value, nucleated nanobubbles grow to their equilibrium states, maintaining their nanoscopic size. However, for the current density above the threshold value, nanobubbles undergo unlimited growth and can eventually detach due to buoyancy. Increasing the pinning length of nanobubbles increases the degree of nanobubble instability. By connecting the current density with the local gas oversaturation, an extension of the stability theory for surface nanobubbles [Lohse and Zhang, Phys. Rev. E 91, 031003(R) (2015)] accurately predicts the nanobubble behavior found in molecular simulations, including equilibrium contact angles and the threshold current density. For larger systems that are not accessible to molecular simulations, continuum numerical simulations with the finite difference method combined with the immersed boundary method are performed, again demonstrating good agreement between numerics and theories.

Negative Differential Resistance in Conical Nanopore Iontronic Memristors

Emerging ion transport dynamics with memory effects at nanoscale solution-substrate interfaces offers a unique opportunity to overcome the bottlenecks in traditional computational architectures, trade-offs in selectivity and throughput in separation, and electrochemical energy conversions. Negative differential resistance (NDR), a decrease in conductance with increasing potential, constitutes a new function from the perspective of time-dependent instead of steady-state nanoscale electrokinetic ion transport but remains unexplored in ionotronics to develop higher-order complexity and advanced capabilities. Herein, NDR is introduced in hysteretic and rectified ion transport through single conical nanopipettes (NPs) as ionic memristors. Deterministic and chaotic behaviors are controlled via an electric field as the sole stimulus. The NDR arises fundamentally from the availability and redistribution of the ionic charges during the hysteretic and rectified transport at asymmetric nanointerfaces. The elucidated mechanism is generalizable, and the drastically simplified operations enable tunable state-switching dynamics with higher-order complexity besides the first-order synaptic functions in multiple excitatory and inhibitory states.

Performance Enhancement of Electrocatalytic Hydrogen Evolution through Coalescence-Induced Bubble Dynamics

The evolution of electrogenerated gas bubbles during water electrolysis can significantly hamper the overall process efficiency. Promoting the departure of electrochemically generated bubbles during (water) electrolysis is therefore beneficial. For a single bubble, a departure from the electrode surface occurs when buoyancy wins over the downward-acting forces (e.g., contact, Marangoni, and electric forces). In this work, the dynamics of a pair of H2 bubbles produced during the hydrogen evolution reaction in 0.5 M H2SO4 using a dual platinum microelectrode system is systematically studied by varying the electrode distance and the cathodic potential. By combining high-speed imaging and electrochemical analysis, we demonstrate the importance of bubble-bubble interactions in the departure process. We show that bubble coalescence may lead to substantially earlier bubble departure as compared to buoyancy effects alone, resulting in considerably higher reaction rates at a constant potential. However, due to continued mass input and conservation of momentum, repeated coalescence events with bubbles close to the electrode may drive departed bubbles back to the surface beyond a critical current, which increases with the electrode spacing. The latter leads to the resumption of bubble growth near the electrode surface, followed by buoyancy-driven departure. While less favorable at small electrode spacing, this configuration proves to be very beneficial at larger separations, increasing the mean current up to 2.4 times compared to a single electrode under the conditions explored in this study.

Coupling Curvature and Hydrophobicity: A Counterintuitive Strategy for Efficient Electroreduction of Nitrate into Ammonia

The electrochemical upcycling of nitrate (NO3-) to ammonia (NH3) holds promise for synergizing both wastewater treatment and NH3 synthesis. Efficient stripping of gaseous products (NH3, H2, and N2) from electrocatalysts is crucial for continuous and stable electrochemical reactions. This study evaluated a layered electrocatalyst structure using copper (Cu) dendrites to enable a high curvature and hydrophobicity and achieve a stratified liquid contact at the gas-liquid interface of the electrocatalyst layer. As such, gaseous product desorption or displacement from electrocatalysts was enhanced due to the separation of a wetted reaction zone and a nonwetted zone for gas transfer. Consequently, this electrocatalyst structure yielded a 2.9-fold boost in per-active-site activity compared with that with a low curvature and high hydrophilic counterpart. Moreover, a NH3 Faradaic efficiency of 90.9 ± 2.3% was achieved with nearly 100% NO3- conversion. This high-curvature hydrophobic Cu dendrite was further integrated with a gas-extraction membrane, which demonstrated a comparable NH3 yield from the real reverse osmosis retentate brine.

Solutal Marangoni force controls lateral motion of electrolytic gas bubbles

Electrochemical gas-evolving reactions have been widely used for industrial energy conversion and storage processes. Gas bubbles form frequently at the electrode surface due to a small gas solubility, thereby reducing the effective reaction area and increasing the over-potential and ohmic resistance. However, the growth and motion mechanisms for tiny gas bubbles on the electrode remains elusive. Combining molecular dynamics (MD) and fluid dynamics simulations (CFD), we show that there exists a lateral solutal Marangoni force originating from an asymmetric distribution of dissolved gas near the bubble. Both MD and CFD simulations deliver a similar magnitude of the Marangoni force of ∼0.01 nN acting on the bubble. We demonstrate that this force may lead to lateral bubble oscillations and analyze the phenomenon of dynamic self-pinning of bubbles at the electrode boundary.

Does a Higher Density of Active Sites Indicate a Higher Reaction Rate?

A consensus view in catalysis is that a higher density of catalytically active sites indicates a higher reaction rate. Using molecular dynamics simulations capable of mimicking the electrochemical formation of gas molecules, we herein demonstrate that this view is problematic for electrocatalytic gas production. Our simulation results show that a higher density of catalytic active sites does not necessarily indicate a higher reaction rate─a high density of active sites could lead to a reduction in the rate of reaction. Further analysis reveals that this abnormal phenomenon is ascribed to aggregation of the produced gas molecules near catalytic sites. This work challenges the consensus view and lays the groundwork for better developing gas-producing reaction electrocatalysts.

Molecular Mechanisms of the Generation and Accumulation of Gas at the Interface

Gas-evolving reactions are widespread in chemical and energy fields. However, the generated gas will accumulate at the interface, which reduces the rate of gas generation. Understanding the microscopic processes of the generation and accumulation of gas at the interface is crucial for improving the efficiency of gas generation. Here, we develop an algorithm to reproduce the process of catalytic gas generation at the molecular scale based on the all-atom molecular dynamics simulations and obtain the quantitative evolution of the gas generation, which agrees well with the experimental results. In addition, we demonstrate that under an external electric field, the generated gas molecules do not accumulate at the electrode surface, which implies that the electric field can significantly increase the rate of the gas generation. The results suggest that the external electric field changes the structure of the water molecules near the electrode surface, making it difficult for gas molecules to accumulate on the electrode surface. Furthermore, it is found that gas desorption from the electrode surface is an entropy-driven process, and its accumulation at the electrode surface depends mainly on the competition between the entropy and the enthalpy of the water molecules under the influence of the electric field. These results provide deep insight into gas generation and inhibition of gas accumulation.

Bubble Management for Electrolytic Water Splitting by Surface Engineering: A Review

During electrocatalytic water splitting, the management of bubbles possesses great importance to reduce the overpotential and improve the stability of the electrode. Bubble evolution is accomplished by nucleation, growth, and detachment. The expanding nucleation sites, decreasing bubble size, and timely detachment of bubbles from the electrode surface are key factors in bubble management. Recently, the surface engineering of electrodes has emerged as a promising strategy for bubble management in practical water splitting due to its reliability and efficiency. In this review, we start with a discussion of the bubble behavior on the electrodes during water splitting. Then we summarize recent progress in the management of bubbles from the perspective of surface physical (electrocatalytic surface morphology) and surface chemical (surface composition) considerations, focusing on the surface texture design, three-dimensional construction, wettability coating technology, and functional group modification. Finally, we present the principles of bubble management, followed by an insightful perspective and critical challenges for further development.

The Secret of Nanoarrays toward Efficient Electrochemical Water Splitting: A Vision of Self-Dynamic Electrolyte

Nanoarray electrocatalysts with unique advantage of facilitating gas bubble detachment have garnered significant interest in gas evolution reactions (GERs). Existing research is largely based on a static hypothesis, assuming that buoyancy is the only driving force for the release of bubbles during GERs. However, this hypothesis overlooks the effect of the self-dynamic electrolyte flow, which is induced by the release of mature bubbles and helps destabilize and release the smaller, immature bubbles nearby. Herein, the enhancing effect of self-dynamic electrolyte flow on nanoarray structures is examined. Phase-field simulations demonstrate that the flow field of electrode with arrayed surface focuses shear force directly onto the gas bubble for efficient detachment, due to the flow could pass through voids and channels to bypass the shielding effect. The flow field therefore has a more substantial impact on the arrayed surface than the nanoscale smooth surface in terms of reducing the critical bubble size. To validate this, superaerophobic ferrous-nickel sulfide nanoarrays are fabricated and employed for water splitting, which display improved efficiency for GERs. This study contributes to understanding the influence of self-dynamic electrolyte on GERs and emphasizes that it should be considered when designing and evaluating nanoarray electrocatalysts.

Imaging Single H2 Nanobubbles Using Off-Axis Dark-Field Microscopy

A robust and detailed physicochemical description of electrochemically generated surface nanobubbles and their effects on electrochemical systems remains at large. Herein, we report the development and utilization of an off-axis, dark-field microscopy imaging tool for probing the dynamic process of generating single H2 nanobubbles at the surface of a carbon nanoelectrode. A change in the direction of the incident light is made to significantly reduce the intensity of the background light, which enables us to image both the nanoelectrode and nanobubble on the electrode surface or the metal nanoparticles in the vicinity of the electrode. The correlated electrochemical and optical response provides novel insights regarding bubble nucleation and dissolution on a nanoelectrode previously unattainable solely from its current-voltage response.

Nanoelectrochemistry in electrochemical phase transition reactions

Electrochemical phase transition is important in a range of processes, including gas generation in fuel cells and electrolyzers, as well as in electrodeposition in battery and metal production. Nucleation is the first step in these phase transition reactions. A deep understanding of the kinetics, and mechanism of the nucleation and the structure of the nuclei and nucleation sites is fundamentally important. In this perspective, theories and methods for studying electrochemical nucleation are briefly reviewed, with an emphasis on nanoelectrochemistry and single-entity electrochemistry approaches. Perspectives on open questions and potential future approaches are also discussed.

Toward Precision Deposition of Conductive Charge-Transfer Complex Crystals Using Nanoelectrochemistry

The lack of understanding for precise synthesis and assembly of nano-entities remains a major challenge for nanofabrication. Electrocrystallization of a charge-transfer complex (CTC), tetrathiafulvalene bromide (TTF)Br, is studied on micro/nanoelectrodes for precision deposition of functional materials. The study reveals new insights into the entire CTC electrocrystallization process from the initial nanocluster nucleation to the final elongated crystals with hollow ends grown from the working electrode to the neighboring receiving electrode. On microelectrodes, the number of nucleation sites is reduced to one by lowering the applied overpotential or precursor concentration. Certain current-time transients exhibit significant induction periods prior to stable nucleus growth. The induction regime contains small fluctuating current spikes consistent with stochastic formation of precritical nanoclusters with lifetimes of 0.1-30 s and sizes of 20-160 nm. Electrochemical analyses further reveal rate, size distribution, and formation/dissipation dynamics of the nanoclusters. Crystal growth of (TTF)Br is further studied on triangular nanoelectrode patterns with thickness of 5-500 nm, which shows a mass-transfer-controlled process applicable for precision deposition of functional (TTF)Br crystals. This study, for the first time, establishes CTC nanoelectrochemistry as a platform technology for precise deposition of conductive crystal assemblies spanning the source and drain electrode for sensing applications.

Current oscillations from bipolar nanopores for statistical monitoring of hydrogen evolution on a confined electrochemical catalyst

Taking advantage of bipolar electrochemistry and a glass nanopipette, continuous single bubbles can be controlled which are generated and detached from a nanometer-sized area of confined electrochemical catalysts. The observed current oscillations offer opportunities to rapidly collect data for the statistical analysis of single-bubble generation on and departure from the catalysts.

Confined Electrochemical Behaviors of Single Platinum Nanoparticles Revealing Ultrahigh Density of Gas Molecules inside a Nanobubble

Understanding the basic physicochemical properties of gas molecules confined within nanobubbles is of fundamental importance for chemical and biological processes. Here, we successfully monitored the nanobubble-confined electrochemical behaviors of single platinum nanoparticles (PtNPs) at a carbon fiber ultramicroelectrode in HClO₄ and H₂O₂ solution. Due to the catalytic decomposition of H₂O₂, a single oxygen nanobubble was formed on individual PtNPs to block the active surface of particles for proton reduction and to suppress their stochastic motion, resulting in significantly distinguished current traces. Furthermore, the combination of theoretical calculations and high-resolution electrochemical measurements allowed the nanobubble size and the oxygen gas density inside a single nanobubble to be quantified. Moreover, the ultrahigh oxygen density inside (1046 kg/m³) was revealed, indicating that gas molecules in a nanosized space existed with a high state of aggregation. Our approach sheds light on the gas aggregation behaviors of nanoscale bubbles using single-entity electrochemical measurements.

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.

Single Particle Hopping as an Indicator for Evaluating Electrocatalysts

The design and screening of electrocatalysts for gas evolution reactions suffer from little understanding of multiphase processes at the electrode-electrolyte interface. Due to the complexity of the multiphase interface, it is still a great challenge to capture gas evolution dynamics under operando conditions to precisely portray the intrinsic catalytic performance of the interface. Here, we establish a single particle imaging method to real-time monitor a potential-dependent vertical motion or hopping of electrocatalysts induced by electrogenerated gas nanobubbles. The hopping feature of a single particle is closely correlated with intrinsic activities of electrocatalysts and thus is developed as an indicator to evaluate gas evolution performance of various electrocatalysts. This optical indicator diminishes interference from heterogeneous morphologies, non-Faradaic processes, and parasitic side reactions that are unavoidable in conventional electrochemical measurements, therefore enabling precise evaluation and high-throughput screening of catalysts for gas evolution systems.

Single-Entity Electrochemistry of Nano- and Microbubbles in Electrolytic Gas Evolution

Gas bubbles are found in diverse electrochemical processes, ranging from electrolytic water splitting to chlor-alkali electrolysis, as well as photoelectrochemical processes. Understanding the intricate influence of bubble evolution on the electrode processes and mass transport is key to the rational design of efficient devices for electrolytic energy conversion and thus requires precise measurement and analysis of individual gas bubbles. In this Perspective, we review the latest advances in single-entity measurement of gas bubbles on electrodes, covering the approaches of voltammetric and galvanostatic studies based on nanoelectrodes, probing bubble evolution using scanning probe electrochemistry with spatial information, and monitoring the transient nature of nanobubble formation and dynamics with opto-electrochemical imaging. We emphasize the intrinsic and quantitative physicochemical interpretation of single gas bubbles from electrochemical data, highlighting the fundamental understanding of the heterogeneous nucleation, dynamic state of the three-phase boundary, and the correlation between electrolytic bubble dynamics and nanocatalyst activities. In addition, a brief discussion of future perspectives is presented.

Unraveling the effects of gas species and surface wettability on the morphology of interfacial nanobubbles

The morphology of interfacial nanobubbles (INBs) is a crucial but controversial topic in nanobubble research. We carried out atomistic molecular dynamics (MD) simulations to comprehensively study the morphology of INBs controlled by several determinant factors, including gas species, surface wettability, and bubble size. The simulations show that H₂, O₂ and N₂ can all form stable INBs, with the contact angles (CAs, on the liquid side) following the order CA(H₂) < CA(N₂) < CA(O₂), while CO₂ prefers to form a gas film (pancake) structure on the substrate. The CA of INBs demonstrates a linear relation with the strength of interfacial interaction; however, a limited bubble CA of ∼25° is found on superhydrophilic surfaces. The high gas density and high internal pressure of the INBs are further confirmed, accompanied by strong interfacial gas enrichment (IGE) behavior. The morphology study of differently sized INBs shows that the internal density of the gas is drastically decreased with the bubble size at the initial stage of bubble nucleation, while the CA remains almost constant. Based on the simulation results, a modified Young's equation is presented for describing the extraordinary morphology of INBs.

Interaction Mechanisms and Application of Ozone Micro/Nanobubbles and Nanoparticles: A Review and Perspective

Ozone micro/nanobubbles with catalytic processes are widely used in the treatment of refractory organic wastewater. Micro/nanobubble technology overcomes the limitations of ozone mass transfer and ozone utilization in the application of ozone oxidation, and effectively improves the oxidation efficiency of ozone. The presence of micro/nanobubbles keeps the catalyst particles in a dynamic discrete state, which effectively increases the contact frequency between the catalyst and refractory organic matter and greatly improves the mineralization efficiency of refractory organic matter. This paper expounds on the characteristics and advantages of micro/nanobubble technology and summarizes the synergistic mechanism of microbubble nanoparticles and the mechanism of catalyst ozone micro/nanobubble systems in the treatment of refractory organics. An interaction mechanism of nanoparticles and ozone microbubbles is suggested, and the proposed theories on ozone microbubble systems are discussed with suggestions for future studies on systems of nanoparticles and ozone microbubbles.

Dynamic Visualization of Free Radicals at Single Oxygen Bubbles using Chemiluminescence

The generation of free radicals is a key process in the formation and the collapse of the bubbles in water, however, the direct and dynamic observation of the radicals in this process at single bubbles has never been achieved. Here, the hydroxyl (OH^. ) and oxygen (O₂ ^.- ) radicals at single oxygen bubbles are continuously traced using chemiluminescence (CL), in which these radicals at the bubble react with the surrounding luminol in the solution emitting the light. Varied increase trends of luminescence are observed in the generation of a bubble, floating, short parking at the water/air interface and the final explosion, revealing the complexity in the distribution of radicals at the bubble unprecedentedly. Despite more radicals are observed at the bubble generated at a deep position under the water for the stabilization, almost the same amount of radicals are included in the bubbles that is independent on the water pressure during the production of the bubble. This rich information collected from the dynamic study of bubbles illustrates the complicated generation and distribution process of radicals at the bubbles, and will facilitate the understanding of the function about the bubbles.

Single-Molecule Interactions at a Surfactant-Modified H2 Surface Nanobubble

In schematics and cartoons, the gas-liquid interface is often drawn as solid lines that aid in distinguishing the separation of the two phases. However, on the molecular level, the structure, shape, and size of the gas-liquid interface remain elusive. Furthermore, the interactions of molecules at gas-liquid interfaces must be considered in various contexts, including atmospheric chemical reactions, wettability of surfaces, and numerous other relevant phenomena. Hence, understanding the structure and interactions of molecules at the gas-liquid interface is critical for further improving technologies that operate between the two phases. Electrochemically generated surface nanobubbles provide a stable, reproducible, and high-throughput platform for the generation of a nanoscale gas-liquid boundary. We use total internal reflection fluorescence microscopy to image single-fluorophore labeling of surface nanobubbles in the presence of a surfactant. The accumulation of a surfactant on the nanobubble surface changes the interfacial properties of the gas-liquid interface. The single-molecule approach reveals that the fluorophore adsorption and residence lifetime at the interface is greatly impacted by the charge of the surfactant layer at the bubble surface. We demonstrate that the fluorescence readout is either short- or long-lived depending on the repulsive or attractive environment, respectively, between fluorophores and surfactants. Additionally, we investigated the effect of surfactant chain length and salt type and concentration on the fluorophore lifetime at the nanobubble surface.

Theranostic nanobubbles towards smart nanomedicines

Targeted therapy and early accurate detection of malignant lesions are essential for the effectiveness of treatment and prognosis in cancer patients. The development of gaseous system as a versatile platform for the fabricated nanobubbles, has attracted much interest in improving the efficacy of ultrasound therapeutic, diagnostic, and theranostic platforms. Nano-sized bubble, as an ultrasound contrast agent, with spherical gas-filled structures exhibited contrast enhancement capability due to their inherent EPR effect. Additionally, nanobubbles exhibited good stability with extended retention time in the blood stream. The current review summarized various nanobubbles and discussed about the crucial parameters affecting the stability of ultrafine bubbles. Furthermore, therapeutic and theranostic gaseous systems for fighting against cancer were described.

Ultrasound-boosted selectivity of CO in CO2 electrochemical reduction

Among the possible products of CO₂ electrochemical reduction, CO plays a unique and vital role, which can be an ideal feedstock for further reduction to C₂₊ products, and also the important component of syngas that can be used as feedstock for value-added chemicals and fuels. However, it is still a challenge to tune the CO selectivity on Cu electrode. Here we newly construct an ultrasound-assisted electrochemical method for CO₂ reduction, which can tune the selectivity of CO₂ to CO from less than 10% to >80% at -1.18 V versus (vs.) reversible hydrogen electrode (RHE). The partial current density of CO production is significantly improved by 15 times. By in-situ Raman study, the dominating factor for the improved CO production is attributed to the accelerated desorption of *CO intermediate. This work provides a facile method to tune the product selectivity in CO₂ electrochemical reduction.

Stimuli-responsive nanobubbles for biomedical applications

Stimuli-responsive nanobubbles have received increased attention for their application in spatial and temporal resolution of diagnostic techniques and therapies, particularly in multiple imaging methods, and they thus have significant potential for applications in the field of biomedicine. This review presents an overview of the recent advances in the development of stimuli-responsive nanobubbles and their novel applications. Properties of both internal- and external-stimuli responsive nanobubbles are highlighted and discussed considering the potential features required for biomedical applications. Furthermore, the methods used for synthesis and characterization of nanobubbles are outlined. Finally, novel biomedical applications are proposed alongside the advantages and shortcomings inherent to stimuli-responsive nanobubbles.

Oxygen-Selective Diffusion-Bubbling Membranes with Core-Shell Structure: Bubble Dynamics and Unsteady Effects

Oxygen is the second-largest-volume industrial gas that is mainly produced using cryogenic air separation. However, the state-of-the-art cryogenic technology thermodynamic efficiency has approached a theoretical limit as near as is practicable. Therefore, there is stimulus to develop an alternative technology for efficient oxygen separation from air. Mixed ionic electronic-conducting (MIEC) ceramic membrane-based oxygen separation technology could become this alternative, but commercialization aspects, including cost, have revealed inadequacies in ceramic membrane materials. Currently, diffusion-bubbling molten oxide membrane-based oxygen separation technology is being developed. It is a potentially disruptive technology that would propose an improvement in oxygen purity and a reduction in capital costs. Bubbles play an important role in ensuring the oxygen mass transfer in diffusion-bubbling membranes. However, there is not sufficient understanding of the bubble dynamics. This understanding is important to be able to control transport properties of these membranes and assess their potential for technological application. The aim of this feature article is to highlight the progress made in developing this understanding and specify the directions for future research.

Electrocatalytic generation and tuning of ultra-stable and ultra-dense nanometre bubbles: an in situ molecular dynamics study

Electrocatalytic generation of nanometre gas bubbles (nanobubbles) and their tuning are important for many energy and chemical processes. Studies have sought to use indirect or ex situ methods to investigate the dynamics and properties of nanobubbles, which are of fundamental interest. Alternatively, we present a molecular dynamics simulation method, which features in situ and high spatial resolution, to directly address these fundamentals. Particularly, our simulations can quantitatively reproduce the generation of ultra-stable and ultra-dense nanobubbles observed in electrochemical experiments. More importantly, our results demonstrate that the classical nucleation theory is still valid even for the scale down to several nanometres, to predict the dynamics and properties of nanobubbles. This provides general guidelines to design efficient nanocatalysts and nanoelectrodes. In our specific case, nanoelectrodes with wetting angles below 71° can suppress the generation of surface nanobubbles.

Effect of Surfactant on Electrochemically Generated Surface Nanobubbles

Surfactants, mimics of contamination, play an important role in nanobubble nucleation, stability, and growth at the electrode surface. Herein, we utilize single-molecule fluorescence microscopy as a sensitive imaging tool to monitor nanobubble dynamics in the presence of a surfactant. Our results show that the presence of anionic and nonionic surfactants increase the rate of nanobubble nucleation at all potentials in a voltage scan. The fluorescence and electrochemical responses indicate the successful lowering of the critical gas concentration needed for nanobubble nucleation across all voltages. Furthermore, we demonstrate that the accumulation of surfactants at the gas-liquid interface changes the interaction of fluorophores with the nanobubble surface. Specifically, differences in fluorophore intensity and residence lifetime at the nanobubble surface suggest that the labeling of nanobubbles is affected by the nature of the nanobubble (size, shape, etc.) and the structure of the gas-liquid interface (surfactant charge, hydrophobicity, etc.).

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.

Stochasticity in Single-Entity Electrochemistry

Most electrochemical processes are stochastic and discrete in nature. Yet experimental observables, e.g., i vs E, are typically smooth and deterministic, due to many events/processes, e.g., electron transfers, being averaged together. However, when the number of entities measured approaches a few or even one, stochasticity frequently emerges. Yet all is not lost! Probabilistic and statistical interpretation can generate insights matching or superseding those from macroscale/ensemble measurements, revealing phenomena that were hitherto averaged over. Herein, we review recent literature examples of stochastic processes in single-entity electrochemistry, highlighting strategies for interpreting stochasticity, contrasting them with macroscale measurements, and describing the insights generated.

Enhancing carbon dioxide gas-diffusion electrolysis by creating a hydrophobic catalyst microenvironment

Electroreduction of carbon dioxide (CO2) over copper-based catalysts provides an attractive approach for sustainable fuel production. While efforts are focused on developing catalytic materials, it is also critical to understand and control the microenvironment around catalytic sites, which can mediate the transport of reaction species and influence reaction pathways. Here, we show that a hydrophobic microenvironment can significantly enhance CO2 gas-diffusion electrolysis. For proof-of-concept, we use commercial copper nanoparticles and disperse hydrophobic polytetrafluoroethylene (PTFE) nanoparticles inside the catalyst layer. Consequently, the PTFE-added electrode achieves a greatly improved activity and Faradaic efficiency for CO2 reduction, with a partial current density >250 mA cm-2 and a single-pass conversion of 14% at moderate potentials, which are around twice that of a regular electrode without added PTFE. The improvement is attributed to a balanced gas/liquid microenvironment that reduces the diffusion layer thickness, accelerates CO2 mass transport, and increases CO2 local concentration for the electrolysis.

Harnessing bubble behaviors for developing new analytical strategies

Gas bubbles are easily accessible and offer many unique characteristic properties of a gas/liquid two-phase system for developing new analytical methods. In this minireview, we discuss the newly developed analytical strategies that harness the behaviors of bubbles. Recent advancements include the utilization of the gas/liquid interfacial activity of bubbles for detection and preconcentration of surface-active compounds; the employment of the gas phase properties of bubbles for acoustic imaging and detection, microfluidic analysis, electrochemical sensing, and emission spectroscopy; and the application of the mass transport behaviors at the gas/liquid interface in gas sensing, biosensing, and nanofluidics. These studies have demonstrated the versatility of gas bubbles as a platform for developing new analytical strategies.

Construction of Position-Controllable Graphene Bubbles in Liquid Nitrogen with Assistance of Low-Power Laser

Graphene bubbles (GBs) are of significant interest owing to their distinguished electrical, optical, and magnetic properties. GBs can also serve as high-pressure reaction vessels to numerous chemical reactions. However, previous strategies to produce GBs are relatively elaborate and random. Therefore, their potential applications are severely restricted. Here, a facile and effective protocol is proposed to construct position-controllable GBs in liquid nitrogen (LN) with the assistance of laser and graphene wrinkles. Specifically, a film of graphene mounted on a SiO₂ substrate (G@SiO₂) is subjected to irradiation by a low-power laser in LN and then many GBs emerge from the surface of G@SiO₂. Most impressively, the domain where GBs arise is the position of the laser beam spot. Hence, we demonstrated that the high collimation of laser facilitates the position definition of GBs. The microscopic results indicate that some GBs split into three parts when they were subjected to irradiation by an electron. Meanwhile, some GBs degenerate into pores with a diameter of 500 nm when they are exposed to air. To grasp the properties of GBs in depth, the molecular dynamics (MD) simulations are performed, and the corresponding results indicate that temperature has very little impact on the GBs' shape. A phase transition process of the substance inside GBs is also revealed. Moreover, a two-dimensional (2D) solid nitrogen is discovered by MD simulations. The simplicity of our protocol paves the way to engineer high-pressure microreaction vessels and fabricate porous graphene membranes.

Concentration profile of dissolved gas during hydrogen gas evolution: an optical approach

We develop an optical image tracking technique for the simultaneous observation of a wide area in proximity to the electrode, and study the growth of bubbles during hydrogen gas evolution in alkaline water electrolysis. Using a diffusion model we can successfully extract the concentration profile of dissolved hydrogen gas as a function of distance from the electrode. The obtained concentrations agree well with the values by the electrochemical method.

Electrochemically Generated Nanobubbles: Invariance of the Current with Respect to Electrode Size and Potential

Gas-producing electrochemical reactions are key to energy conversion and generation technologies. Bubble formation dramatically decreases gas-production rates on nanoelectrodes, by confining the reaction to the electrode boundary. This results in the collapse of the current to a stationary value independent of the potential. Startlingly, these residual currents also appear to be insensitive to the nanoelectrode diameter in the 5 to 500 nm range. These results are counterintuitive, as it may be expected that the current be proportional to the circumference of the electrode, i.e., the length of the three-phase line where the reaction occurs. Here, we use molecular simulations and a kinetic model to elucidate the origin of current insensitivity with respect to the potential and establish its relationship to the size of nanoelectrodes. We provide critical insights for the design and operation of nanoscale electrochemical devices and demonstrate that nanoelectrode arrays maximize conversion rates compared to macroscopic electrodes with same total area.

Stability of pinned surface nanobubbles against expansion: Insights from theory and simulation

While growth and dissolution of surface nanobubbles have been widely studied in recent years, their stability under pressure changes or a temperature increase has not received the same level of scrutiny. Here, we present theoretical predictions based on classical theory for pressure and temperature thresholds (p_c and T_c) at which unstable growth occurs for the case of air nanobubbles on a solid surface in water. We show that bubbles subjected to pinning have much lower p_c and higher T_c compared to both unpinned and bulk bubbles of similar size, indicating that pinned bubbles can withstand a larger tensile stress (negative pressure) and higher temperatures. The values of p_c and T_c obtained from many-body dissipative particle dynamics simulations of quasi-two-dimensional (quasi-2D) surface nanobubbles are consistent with the theoretical predictions, provided that the lateral expansion during growth is taken into account. This suggests that the modified classical thermodynamic description is valid for pinned bubbles as small as several nanometers. While some discrepancies still exist between our theoretical results and previous experiments, further experimental data are needed before a comprehensive understanding of the stability of surface nanobubbles can be achieved.

Electrochemical Generation of Individual Nanobubbles Comprising H2, D2, and HD

The electrochemical reduction of deuterons (2D⁺ + 2e^- → D₂) at Pt nanodisk electrodes (radius = 15-100 nm) in D₂O solutions containing deuterium chloride (DCl) results in the formation of a single gas nanobubble at the electrode surface. Analogous to that previously observed for the electrochemical generation of H₂ nanobubbles, the nucleation and growth of a stable D₂ nanobubble is characterized in voltammetric experiments by a highly reproducible and well-resolved sudden drop in the faradaic current, a consequence of restricted mass transport of D⁺ to the electrode surface following the liquid-to-gas phase transition. D₂ nanobubbles are stable under potential control due to a dynamic equilibrium existing between D₂ gas dissolution and electrochemical generation of D₂ at the circumference of the Pt nanodisk electrode. Remarkably, within the error of the experimental measurement (<6%), the electrochemical current required to nucleate a D₂ gas phase in a D₂O solution is identical to that for the H₂ gas phase in a H₂O solutions, indicating that the concentration required for nucleating a D₂ nanobubble in D₂O (0.29 M) is ∼1.25 times larger than that for a H₂ nanobubble (0.23 M), while the supersaturation is ∼300 in each case. We further demonstrate that individual nanobubbles can be electrogenerated in mixed D₂O/H₂O solutions containing both D⁺ and H⁺ at respective individual concentrations well below those required to nucleate a gas phase containing either pure D₂ or H₂. This latter finding indicates that the resulting nanobubbles comprise a mixture of D₂, H₂, and HD molecules with the chemical composition of a nanobubble determined by the concentrations and diffusivities of D⁺ and H⁺ in the mixed D₂O/H₂O solutions.

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.