Electrogeneration of single nanobubbles at sub-50-nm-radius platinum nanodisk electrodes

Electrochemistry of single nanobubbles. Estimating the critical size of bubble-forming nuclei for gas-evolving electrode reactions

In this article, we address the fundamental question: “What is the critical size of a single cluster of gas molecules that grows and becomes a stable (or continuously growing) gas bubble during gas evolving reactions?” Electrochemical reactions that produce dissolved gas molecules are ubiquitous in electrochemical technologies, e.g., water electrolysis, photoelectrochemistry, chlorine production, corrosion, and often lead to the formation of gaseous bubbles. Herein, we demonstrate that electrochemical measurements of the dissolved gas concentration, at the instant prior to nucleation of an individual nanobubble of H₂, N₂, or O₂ at a Pt nanodisk electrode, can be analyzed using classical thermodynamic relationships (Henry's law and the Young–Laplace equation – including non-ideal corrections) to provide an estimate of the size of the gas bubble nucleus that grows into a stable bubble. We further demonstrate that this critical nucleus size is independent of the radius of the Pt nanodisk employed (<100 nm radius), and weakly dependent on the nature of the gas. For example, the measured critical surface concentration of H₂ of ∼0.23 M at the instant of bubble formation corresponds to a critical H₂ nucleus that has a radius of ∼3.6 nm, an internal pressure of ∼350 atm, and contains ∼1700 H₂ molecules. The data are consistent with stochastic fluctuations in the density of dissolved gas, at or near the Pt/solution interface, controlling the rate of bubble nucleation. We discuss the growth of the nucleus as a diffusion-limited process and how that process is affected by proximity to an electrode producing ∼10¹¹ gas molecules per second. Our study demonstrates the advantages of studying a single-entity, i.e., an individual nanobubble, in understanding and quantifying complex physicochemical phenomena.

Electrochemical Nucleation of Stable N2 Nanobubbles at Pt Nanoelectrodes

Exploring the nucleation of gas bubbles at interfaces is of fundamental interest. Herein, we report the nucleation of individual N2 nanobubbles at Pt nanodisk electrodes (6–90 nm) via the irreversible electrooxidation of hydrazine (N2H4 → N2 + 4H(+) + 4e(–)). The nucleation and growth of a stable N2 nanobubble at the Pt electrode is indicated by a sudden drop in voltammetric current, a consequence of restricted mass transport of N2H4 to the electrode surface following the liquid-to-gas phase transition. The critical surface concentration of dissolved N2 required for nanobubble nucleation, CN2,critical(s), obtained from the faradaic current at the moment just prior to bubble formation, is measured to be ∼0.11 M and is independent of the electrode radius and the bulk N2H4 concentration. Our results suggest that the size of stable gas bubble nuclei depends only on the local concentration of N2 near the electrode surface, consistent with previously reported studies of the electrogeneration of H2 nanobubbles. CN2,critical(s) is ∼160 times larger than the N2 saturation concentration at room temperature and atmospheric pressure. The residual current for N2H4 oxidation after formation of a stable N2 nanobubble at the electrode surface is proportional to the N2H4 concentration as well as the nanoelectrode radius, indicating that the dynamic equilibrium required for the existence of a stable N2 nanobubble is determined by N2H4 electrooxidation at the three phase contact line.

Time-Resolved Detection and Analysis of Single Nanoparticle Electrocatalytic Impacts

There is considerable interest in understanding the interaction and activity of single entities, such as (electro)catalytic nanoparticles (NPs), with (electrode) surfaces. Through the use of a high bandwidth, high signal/noise measurement system, NP impacts on an electrode surface are detected and analyzed in unprecedented detail, revealing considerable new mechanistic information on the process. Taking the electrocatalytic oxidation of H2O2 at ruthenium oxide (RuOx) NPs as an example, the rise time of current-time transients for NP impacts is consistent with a hydrodynamic trapping model for the arrival of a NP with a distance-dependent NP diffusion-coefficient. NP release from the electrode appears to be aided by propulsion from the electrocatalytic reaction at the NP. High-frequency NP impacts, orders of magnitude larger than can be accounted for by a single pass diffusive flux of NPs, are observed that indicate the repetitive trapping and release of an individual NP that has not been previously recognized. The experiments and models described could readily be applied to other systems and serve as a powerful platform for detailed analysis of NP impacts.

Dynamics of Single Hydrogen Bubbles at a Platinum Microelectrode

Bubble dynamics, including the formation, growth, and detachment, of single H2 bubbles was studied at a platinum microelectrode during the electrolysis of 1 M H2SO4 electrolyte. The bubbles were visualized through a microscope by a high-speed camera. Electrochemical measurements were conducted in parallel to measure the transient current. The periodic current oscillations, resulting from the periodic formation and detachment of single bubbles, allow the bubble lifetime and size to be predicted from the transient current. A comparison of the bubble volume calculated from the current and from the recorded bubble image shows a gas evolution efficiency increasing continuously with the growth of the bubble until it reaches 100%. Two different substrates, glass and epoxy, were used to embed the Pt wire. While nearly no difference was found with respect to the growth law for the bubble radius, the contact angle differs strongly for the two types of cell. Data provided for the contact point evolution further complete the image of single hydrogen bubble growth. Finally, the velocity field driven by the detached bubble was measured by means of PIV, and the effects of the convection on the subsequent bubble were evaluated.

Effect of tetraalkylammonium cations on gas coalescence at a hydrogen-evolving microelectrode

Hydrogen gas evolution at the surface of a microelectrode may result in periodic release of single bubbles larger than the electrode diameter. Bubbles often grow by incorporating smaller bubbles that coalesce with them. To explore the coalescence, we investigate how a series of six tetralkylammonium cations (TXA(+)), where the number of carbons on the alkyl chain varies from 1 to 6, affects the oscillatory behavior of the gas-evolving microcathode. Different concentrations of TXA(+) bromide salts ranging from a few micromolar up to 1 M were added in the acid electrolyte. The frequency of bubble release and the transition from periodic to aperiodic release are related to the inhibition of bubble coalescence and gas streaming. The concentration range where this transition occurs depends strongly on the cation hydrophobicity and it ranges from very small values for the hydrophobic cations to over 1 M for the most hydrophilic one. For some of the TXA(+) cations, the transition shows a smooth increase in release frequency before switching completely to bubble-stream behavior, while for others the transition is abrupt. A smooth increase in the gas oscillator frequency with concentration indicates that the adsorption of TXA(+) cations on the bubble surface is mass transport-limited. The inhibition of bubble coalescence by the smallest cations is electrochemically driven, facilitated by specific interactions established between the ions and the electrode surface.

Electrochemical Generation of a Hydrogen Bubble at a Recessed Platinum Nanopore Electrode

We report the electrochemical generation of a single hydrogen bubble within the cavity of a recessed Pt nanopore electrode. The recessed Pt electrode is a conical pore in glass that contains a micrometer-scale Pt disk (1-10 μm radius) at the nanopore base and a nanometer-scale orifice (10-100 nm radius) that restricts diffusion of electroactive molecules and dissolved gas between the nanopore cavity and bulk solution. The formation of a H2 bubble at the Pt disk electrode in voltammetric experiments results from the reduction of H(+) in a 0.25 M H2SO4 solution; the liquid-to-gas phase transformation is indicated in the voltammetric response by a precipitous decrease in the cathodic current due to rapid bubble nucleation and growth within the nanopore cavity. Finite element simulations of the concentration distribution of dissolved H2 within the nanopore cavity, as a function of the H(+) reduction current, indicate that H2 bubble nucleation at the recessed Pt electrode surface occurs at a critical supersaturation concentration of ∼0.22 M, in agreement with the value previously obtained at (nonrecessed) Pt disk electrodes (∼0.25 M). Because the nanopore orifice limits the diffusion of H2 out of the nanopore cavity, an anodic peak corresponding to the oxidation of gaseous and dissolved H2 trapped in the recessed cavity is readily observed on the reverse voltammetric scan. Integration of the charge associated with the H2 oxidation peak is found to approach that of the H(+) reduction peak at high scan rates, confirming the assignment of the anodic peak to H2 oxidation. Preliminary results for the electrochemical generation of O2 bubbles from water oxidation at a recessed nanopore electrode are consistent with the electrogeneration of H2 bubbles.

Cleaning nanoelectrodes with air plasma

Unlike macroscopic and micrometer-sized solid electrodes whose surface can be reproducibly cleaned by mechanical polishing, cleaning the nanoelectrode surface is challenging because of its small size and extreme fragility. Even very gentle polishing typically changes the nanoelectrode size and geometry, thus, complicating the replication of nanoelectrochemical experiments. In this letter, we show the possibility of cleaning nanoelectrode surfaces nondestructively by using an air plasma cleaner. The effects of plasma cleaning have been investigated by atomic force microscopy (AFM) imaging, voltammetry, and scanning electrochemical microscopy (SECM). A related issue, the removal of an insoluble organic film from the nanoelectrode by plasma cleaning, is also discussed.

Potential-controlled current responses from staircase to blip in single Pt nanoparticle collisions on a Ni ultramicroelectrode

Collisions of electrocatalytic platinum (Pt) single nanoparticles (NPs) with a less electrocatalytic nickel (Ni) ultramicroelectrode (UME) surface were detected by amplification of the current by electrocatalysis of NPs. Two typical types of current responses, a current staircase or blip (or spike), in single NP collision experiments were observed at a time with a new system consisting of Pt NP/Ni UME/hydrazine oxidation. The staircase current response was obtained when the collided NPs were attached to the electrode and continued to produce electrocatalytic current. On the other hand, the blip current response was believed to be obtained when the NP attached but was deactivated. The different current responses depend on the different electrocatalytic reaction mechanism, characteristics of the NP, or the electrode material. How the deactivation of the electrocatalytic process affects on the current response of NP collision was investigated using the Ni UME. The current response of a single Pt NP collision is controllable from staircase to blip by changing the applied potential. The current response of the Pt NP was observed as a staircase response with 0 V (vs Ag/AgCl) and as a blip response with 0.1 V (vs Ag/AgCl) applied to the Ni UME.

Bubble formation at a gas-evolving microelectrode

The electrolytic production of gas bubbles involves three steps--nucleation, growth, and detachment. Here the growth of hydrogen bubbles and their detachment from a platinum microelectrode of diameter 125 μm are studied using high-speed photography and overpotential frequency spectrum (noise) analysis. The periodic release of large <800 μm bubbles--gas oscillator behavior--was often observed, with a corresponding periodic oscillation of the overpotential which is reflected as a main peak and a series of harmonics in the power spectral density. The release frequency is inversely correlated with the bubble size and hydrogen production rate. When the coalescence of bubbles at the electrode surface is inhibited, either chemically with a surfactant or ethylene glycol or hydrodynamically by magnetically induced convection, swarms of small ∼50 μm bubbles are released in an aperiodic stream. The abrupt transition from periodic to aperiodic release occurs when the surface tension falls below 70 mN m(-1). Hydrogen bubble growth is also studied on a transparent platinum thin-film electrode, where the bubble coalescence can be observed directly. It leaves sessile droplets of electrolyte within the footprint of the growing bubble, showing that the growth involves scavenging smaller bubbles from solution due to hydrogen generated directly at the electrode. A possible role of nanobubbles in the lift-off process is discussed.

Nanobubble formation on a warmer substrate

The solvent exchange procedure is an often-used protocol to produce surface nanobubbles. In this procedure, the substrate is exposed to a good solvent for gas which is then mixed and rinsed with a poor solvent for gas and the nanobubbles form on the solid-liquid interface. Here we study the effects of temperatures of the substrate and the first solvent on nanobubble formation. Atomic force microscopy with temperature control was used to examine the formation of nanobubbles at temperatures between 37 °C and 54 °C. It was found that the probability of nanobubble formation was larger on substrates at higher temperatures. Moreover, on warmer substrates we found nanobubbles with lateral extensions up to 8 μm. A morphologic analysis shows that all nanobubbles, including giant nanobubbles, have a similar aspect ratio, independent of the substrate temperature, and that this aspect ratio corresponds to a contact angle between 13° and 22° (on the gas side), much smaller than the macroscopic counterparts. We finally discuss the implications of our results for various theories on nanobubble stability.

Electrochemical Measurements of Single H2 Nanobubble Nucleation and Stability at Pt Nanoelectrodes

Single H2 nanobubble nucleation is studied at Pt nanodisk electrodes of radii less than 50 nm, where H2 is produced through electrochemical reduction of protons in a strong acid solution. The critical concentration of dissolved H2 required for nanobubble nucleation is measured to be ∼0.25 M. This value is ∼310 times larger than the saturation concentration at room temperature and pressure and was found to be independent of acid type (e.g., H2SO4, HCl, and H3PO4) and nanoelectrode size. The effects of different surfactants on H2 nanobubble nucleation are consistent with the classic nucleation theory. As the surfactant concentration in H2SO4 solution increases, the solution surface tension decreases, resulting in a lower nucleation energy barrier and consequently a lower supersaturation concentration required for H2 nanobubble nucleation. Furthermore, amphiphilic surfactant molecules accumulate at the H2/solution interface, hindering interfacial H2 transfer from the nanobubble into the solution; consequently, the residual current decreases with increasing surfactant concentration.

Influence of the redox indicator reaction on single-nanoparticle collisions at mercury- and bismuth-modified Pt ultramicroelectrodes

Single-Pt nanoparticles (NPs) can be detected electrochemically by measuring the current-time (i-t) response associated with both hydrazine oxidation and proton reduction during individual Pt NP collisions with noncatalytic Hg- and Bi-modified Pt ultramicroelectrodes (Hg/Pt and Bi/Pt UMEs, respectively). At Hg/Pt UMEs, the i-t response for both hydrazine oxidation and proton reduction consists of repeated current "spikes" that return to the background level as Hg poisons the Pt NP after collision with the Hg/Pt UME due to amalgamation and deactivation of the redox reaction. Furthermore, at a Hg/Pt UME, the applied potential directly influences the interfacial surface tension (electrocapillarity) that also impacts the observed i-t response for single-Pt NP collisions for proton reduction that exhibits a faster decay of current (0.7-4 ms) to background levels than hydrazine oxidation (2-5 s). Because the surface tension of Hg is lower (-0.9 V), Pt NPs possibly react faster with Hg (amalgamate at a faster rate), resulting in sharp current spikes for proton reduction compared to hydrazine oxidation. In contrast, a stepwise "staircase" i-t response is observed for proton reduction for single-Pt NP collisions at a Bi/Pt UME. This different response suggests that electrostatic forces of negatively charged citrate-capped Pt NPs also influence the i-t response at more negative applied potentials, but the Pt NPs do not poison the electrochemical activity at Bi/Pt UMEs.