Article pubs.acs.org/Langmuir
Electrochemical Generation of a Hydrogen Bubble at a Recessed Platinum Nanopore Electrode Qianjin Chen, Long Luo, and Henry S. White* Department of Chemistry, University of Utah, 315 S 1400 E, Salt Lake City, Utah 84112, United States S Supporting Information *
ABSTRACT: 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-togas 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.
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copy,10 and attenuated total internal reflection infrared spectroscopy11,15 as well as the theoretical investigations of nucleation and stabilization20,21 are providing new insights into nanobubble phenomena. For example, the saturation level of the dissolved gas in the surrounding solution,16 the pinning of the three-phase boundary,16,20,22 dynamic equilibrium,21 and the possible presence of water-insoluble contaminants on the bubble surface23 have been proposed to understand the long lifetime for interfacial nanobubble stability. Electrochemistry is well-suited for studies of nano- and microbubble generation,13,14,19,24,25 as high local gas concentrations at an electrode surface can be achieved by electrolysis of a redox species, including the solvent. Zhang et al.13 observed the electrochemically controlled formation and growth of hydrogen nanobubbles on bare highly oriented pyrolytic graphite (HOPG) surface via in situ tapping mode atomic force microscopy (TMAFM). More recently, Fernandez et al.19 studied the electrochemical growth of larger hydrogen bubbles and their detachment from a Pt microelectrode by using a combination of high-speed photography and frequency spectrum analysis.
INTRODUCTION Observations of the remarkable long-term stability of nanoscale bubbles at the solid/water interfaces has generated significant recent fundamental interest in their formation and equilibrium properties. Gas bubbles at the nanometer scale regime are assumed to possess very high internal Laplace pressures (e.g., a 10 nm radius bubble in water would have a Laplace pressure of ∼140 atm) and are observed in solutions in which the dissolved gas is present at concentrations below that based on Henry’s law, the observed bubble radius, and Laplace’s equation.1,2 The lifetimes of such bubbles are expected to be very short (almost always less than 1 s) due to rapid dissolution by diffusion. Despite the inconsistency between experimental evidence and theoretical expectations, the observation of gas nanobubbles has led investigators to explore and develop applications of nanobubbles in friction reduction in micro- or nanofluidics,3,4 colloidal particle stabilization,5 nanobubbles cleaning,6,7 and ultrasound imaging.8,9 Research advances on various physical aspects of interfacial nanobubbles in the past decade include new methods of nanobubble generation based on solvent exchange,10,11 temperature fluctuation,12 and water electrolysis.13,14 Experimental investigations using atomic force microscopy (AFM),12−16 light scattering,17 quartz crystal microbalance (QCM),18 high-speed photography,19 total internal reflection fluorescence micros© XXXX American Chemical Society
Received: January 20, 2015 Revised: March 24, 2015
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DOI: 10.1021/acs.langmuir.5b00234 Langmuir XXXX, XXX, XXX−XXX
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Scheme 1. Schematic Drawing of the Electrochemical Formation of a Single H2 Bubble from H+ Reduction and the Subsequent H2 Oxidation within a Recessed Pt Nanopore Electrodea
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The radius of the nanopore opening, a, is drawn disproportionately large for clarity. i.d., softening point 700 °C) using a H2/air flame. The capillary was then polished on silicon carbide polishing papers (400 grit/p800 to 1200 grit/p4000) until a Pt nanodisk was exposed, as indicated by the use of electronic feedback circuitry. The radii of the nanodisk electrodes, a, were determined from the voltammetric steady-state diffusion-limited current, ilim, for the oxidation of Fc (Fc → Fc+ + e−) dissolved in acetonitrile (CH3CN) containing 0.10 M tetrabutylammonium hexafluorophosphate (TBAPF6). The radii were calculated using the equation
Recently, we reported the electrogeneration of individual H2 nanobubbles at Pt nanodisk electrodes (radii less than ∼50 nm) via reduction of H+ in aqueous solutions containing different proton sources.26,27 We find that H2 nanobubble nucleation at a Pt nanodisk electrode occurs at a constant H2 supersaturation concentration, independent of the electrode size and proton source. The supersaturation concentration corresponding to nanobubble nucleation (∼0.25 M H2) is ∼310 times larger than the saturation value of H2 at room temperature and pressure (0.8 mM). After a nanobubble is formed, it is kinetically stable if the voltage is held sufficiently negative for electrogeneration of H2 to balance the diffusive outflux of H2 across the bubble/ solution interface. In the current report, we have extended these studies to larger individual H2 bubbles generated within the cavity of a recessed nanopore electrode, as shown in Scheme 1. The recessed nanopore electrodes, with geometries well characterized by a voltammetric method developed in our group,28−30 are fabricated by etching a Pt nanodisk electrode to create a cavity of depth that can be varied over micrometer dimensions. The nanopore electrode provides a unique platform to study gas generation and bubble formation, as diffusion of gas molecules generated at the electrode located at the pore base is strongly restricted by orifice of the nanopore; thus, H2 that accumulates within the nanopore cavity is trapped for significant periods, allowing quantitative measurements of H2 bubble nucleation relative to H2 diffusion from the electrode surface. We demonstrate that oxidation of the H2 originating from within the gas bubble can be readily observed at moderate voltammetric time scales (2 V/s), the voltammetric response displays “thin-layer” behavior, with a symmetrical peak shape and minimal splitting between the anodic and cathodic peak caused by residual ohmic drop across the pore orifice. The significant capacitive current corresponds to a capacitance of ∼200 μF/cm2 and is due to charging of the Pt electrode at the base of the pore, which, as shown below, has a micrometer scale radius. The measured capacitance is larger than that expected for a smooth Pt/CH3CN interface and suggests that the Pt disk is rough (as discussed below, this is consistent with an independent measurement of the true Pt surface area). At low scan rates (
