John C. Eklund

Dual activation: coupling ultrasound to electrochemistry—an overview

By introducing a powerful immersion horn probe as a source of intense ultrasound into a thermostatted conventional three-electrode cell, dual activation experiments by simultaneously passing a current and applying ultrasound may be undertaken. These sono-voltammetric experiments may be used in order to analyse and quantify the processes induced by ultrasound at the electrode/solution interface. Different effects have been described. First, by applying intense sound fields direct effects of ultrasound on electrode surfaces such as depassivation and erosion can be induced in cavitation events, violent collapses of oscillating bubbles. Second, the huge effect of ultrasound on the mass transport at the electrode surface detected by various voltammetric techniques may be described by the model of an extremely thinned diffusion layer of uniform accessibility. This experimentally verified model may then be used in voltammetric experiments in order to separate pure mass transport from other effects induced by sound waves. Several working electrode geometries have been employed and particularly the use of an electrode embedded in the tip of the ultrasound transducer, a so-called sonotrode, allows extreme conditions to be studied. In aqueous media and under these conditions voltammetry parallel to “classical” hydrodynamic techniques based on the effect of “acoustic streaming” was observed. A wide range of systems including the reduction of a metalloprotein, cytochrome c, are described. In this overview the current state-of-the-art is critically reviewed and the information that has been derived from sonovoltammetric measurements illustrated.

Voltammetry in the presence of ultrasound: mass transport effects

The voltammetry of various well-defined systems in acetonitrile solution has been studied using both micro and macroelectrodes in the presence of power ultrasound. A simple model is established which quantifies the mass transport observed under these conditions; this assumes that the effect of ultrasound is to promote mixing within the bulk of the solution up to within a certain distance of the electrode surface. Thus the ultrasound serves to thin the diffusion layer which would exist at the corresponding electrode under silent conditions. The relative enhancement of transport limited currents by ultrasound is dependent on the size of the electrode; for micrometre-sized electrodes the steady state limiting current observed tends to that predicted under silent conditions whereas for large electrodes a thin, steady-state diffusion layer is seen with a thickness which is power dependent. In addition to steady-state experiments, a.c. impedance measurements and potentials steps are used to verify the model proposed.

Electrochemistry at boron doped diamond films grown on graphite substrates: redox-, adsorption and deposition processes

Highly boron-doped (atomic concentration ∼1021 cm−3) conducting diamond films were grown on graphite substrates by microwave assisted vapor deposition from a gaseous feed of hydrogen and methane and solid boron. These diamond films of ca. 5 to 10 μm thickness composed of crystals of up to 10 μm size were characterized by both surface analytical techniques such as Raman spectroscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM), and electrochemical techniques. The reduction of Ru(NH3)63+, the oxidation of chlorpromazine, and the reduction of Pb2+ in aqueous media were studied in order to investigate the processes involving oxidation and reduction as well as adsorption and deposition. The one-electron oxidation of chlorpromazine in aqueous 0.1 M KCl and the one electron reduction of Ru(NH3)63+ in aqueous 0.1 M KCl gave well defined cyclic voltammetric responses at diamond electrodes with peak currents proportional to the square root of the scan rate consistent with diffusion control at a macroscopically uniformly active electrode. For the oxidation of chlorpromazine at concentrations less than 4 mM adsorption of the neutral chlorpromazine at the diamond electrode was additionally detected. At very low scan rates a transition to sigmoidally shaped responses occurred which may be attributed partly to the presence of areas of low or no conductivity on a microscopic level. The reduction of Pb2+ in aqueous 0.1 M HClO4 allowed the deposition of metallic lead on the diamond surface. Studies by ex-situ SEM and in-situ AFM show that Pb nuclei are formed and distributed inhomogeneously over the polycrystalline diamond film in the active surface areas, irrespective of grain boundaries. However, little or no Pb deposition occurred in some less active areas with dimensions of 2 to 100 μm. This effect may be attributed to regions of poor conductivity between the graphite substrate and the diamond film coupled to low lateral conductivity of the film itself. The size of the regions of lower activity decreases with higher applied over-potential. A characteristic delay in the re-dissolution process of the Pb deposit was observed and attributed to both poor adhesion and a resistive contact between deposit and electrode surface.

Voltammetry at C60-modified electrodes

Abstract The reduction of electrodes coated with C 60 -fullerene is examined in acetonitrile solution containing a wide variety of supporting electrolytes (MClO 4 ; M = Li, Na, Ba 0.5 , NR 4 ). Electrochemical intercalation is observed with the formation of fulleride salts. Intercalation is typically irreversible except for the case of R = n -butyl where stable coats of fullerides may be formed into which charge can be passed with near chemical reversibility. Electron-transfer reactions at electrodes modified with C 60 coatings were investigated for diverse substrates. With unreduced coats impeded diffusion to the metal surface was seen, whereas with films reduced in the presence of tetrabutylammonium perchlorate coat-mediated electron transfer to the substrate was possible.

Electrode processes at the surfaces of sonotrodes

Abstract Two designs of sonotrode were investigated in which the tip of a 20kHz ultrasonic horn was adapted for use as the working electrode, in a conventional three-electrode system, for sonoelectrochemical studies. The first consisted of a platinum disc embedded in the titanium alloy tip of the horn probe and the second merely of an untreated Ti alloy tip, used as suggested by Reisse et al.[14] Their performances were compared. The former showed high mass transport conditions and clean, reversible electrochemistry for a variety of model systems. The latter gave behaviour characteristic of the n-type semiconducting layer of TiO2 known to be present on the surface of metallic titanium.

Voltammetry in the presence of ultrasound: A novel sono-electrode geometry

A new sono-electrode geometry is discussed in this paper, in which a square electrode of millimetre dimensions is placed perpendicular to a 20 KHz ultrasonic horn probe, its centre being some 30–40 mm distant from the surface of the tip of the ultrasonic horn. It is shown that the mass transport of electroactive material to the electrode can be approximately described by that of a convective flow over a stationary plate through comparison of the experimental results with theoretical data obtained via the analytical solution of the mass transport equations describing this model. This novel sono-electrode geometry is compared with other more popular geometries introduced in the recent sonoelectrochemical literature and the relative merits of these sonotrodes are discussed with respect to their mass transport characteristics.

In situ electrochemical ESR studies of reactive radicals: the reductions of bromo-anthraquinone and methyl viologen

The application of channel electrodes for quantitative in situ electrochemical ESR studies of unstable radicals is discussed with reference to two systems. First, the reduction of 1-bromo-anthraquinone in acetonitrile solution in the presence of laser light of wavelength 488 nm is shown to lead to the loss of bromide and the formation of the anthraquinone radical anion via a photo-ECE process. The kinetics of this reaction as determined by channel electrode ESR studies are found to be in good agreement with those measured by independent methods. Second, the one-electron reduction of the methyl viologen dication in aqueous solution is shown to lead to the formation of viologen radicals which are known to dimerise. However, it is shown that since this reaction is rapid and reversible, measurements of the ESR signal as a function of electrolyte flow rate and current are essentially blind to this reaction. Lower estimates for the dimerisation rate constant are deduced.

Mercury-electroplated platinum electrodes and microelectrodes for sonoelectrochemistry

Abstract The preparation of mercury-electroplated platinum disc electrodes of both micrometre and conventional dimensions from an aqueous solution of mercury(I) nitrate and potassium cyanide is described. The mercury film electrodes so produced led to very reproducible voltammetric results, as observed by the cathodic reduction of fluorescein to semifluorescein at high pH, both under silent conditions and under ultrasonic irradiation. The electroreduction in the absence of ultrasound is a simple reversible one-electron transfer process (Tafel gradient ∼ 65 mV decade −1 ). On ultrasonic irradiation an increase in the mass transport-limited current was observed in excess of that expected for a simple one-electron transfer. Possible mechanisms for the electro-reduction in ultrasound are postulated.

The application of microelectrodes to the study of photochemical reactions

Abstract The use of microelectrodes for the study of the kinetics and mechanism of photoinduced processes is described with reference to two examples. First, the rate of light-induced electron transfer between anthraquinone and the tetraphenylborate anion is quantified using phototransient experiments at microband electrodes and the resulting data are shown to be in good agreement with independent measurements. Second, microdisc experiments are used to characterize the rate of photofragmentation of electrogenerated radical anions of 1-bromoanthraquinone. Again, comparison with independent measurements shows excellent agreement. Finally the ability of using microelectrodes for quantitative voltammetry in the near absence of supporting electrolyte is exploited to demonstrate that in the photoelectrochemical reduction of 1-bromoanthraquinone to anthraquinone the source of the required H atoms is the supporting electrolyte.

Channel electrode voltammetry and electrodimerization processes. The electro-oxidation of tris(dithiocarbamato)cobalt(III) complexes

Abstract Channel electrode waveshape analysis is used to demonstrate that the electro-oxidation of the tris(dithiocarbamato)cobalt(III) complexes, Co(R 2 dtc) 3 , where R = Me and Et, at platinum electrodes in acetonitrile solution proceeds via an EC 2 (dimerization) mechanism in which the one-electron oxidation product, Co(R 2 dtc) + 3 , undergoes irreversible dimerization resulting in the formation of Co 2 (R 2 dtc) + 5 . Both the shift in half-wave potential and in the voltammetric shape as revealed by Tafel analysis are used to derive kinetic parameters for the dimerization reaction, which are found to be in excellent agreement with independently determined values.