Peter R. Birkin

Electrochemical deposition of macroporous platinum,palladium and cobalt films using polystyrene latex sphere templates

Highly ordered macroporous films of platinum, palladium and cobalt with regular arrays of spherical pores with diameters of 0.40, 0.70 or 1 μm are prepared by electrochemical deposition into the interstitial spaces of a template formed by polystyrene latex spheres self-assembled on gold electrodes; after deposition of platinum, palladium or cobalt, the polystyrene spheres are fully removed by washing in toluene to leave a highly periodic, hexagonal close packed, interconnected network of monodisperse spherical pores within the metal film, the size of which is determined by the diameter of the polystyrene latex particles used to prepare the template.

A study of the effect of ultrasound on mass transport to a microelectrode

Abstract Microelectrodes (Pt, Au 25 μm, 10 μm diameter and C 9 μm diameter) have been employed to record individual cavitation events produced as a consequence of high intensity ultrasonic irradiation of a solution containing a redox probe. Cavitation events are detected at a microelectrode as a series of current-time transients. The potential dependence of the observed events in relation to the redox chemistry of the particular probe under investigation is demonstrated. Ferrocene and hexaamine ruthenium(III) chloride have been employed as model redox systems. The effects of temperature, solvent, electrode/ultrasonic source separation, position of the electrode with respect to the ultrasonic source and ultrasonic intensity on the frequency and magnitude of the observed cavitation events are demonstrated. The shapes of the current-time transients recorded are analysed with comparison to conventional chronoamperometry at microelectrodes. Mass transport coefficients of up to 1.6 cm s −1 are reported.

TEMPO‐Mediated Electrooxidation of Primary and Secondary Alcohols in a Microfluidic Electrolytic Cell

A general procedure for the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated electrooxidation of primary and secondary alcohols modified for application in a microfluidic electrolytic cell is described. The electrocatalytic system utilises a buffered aqueous tert-butanol reaction medium, which operates effectively without the requirement for additional electrolyte, providing a mild protocol for the oxidation of alcohols to aldehydes and ketones at ambient temperature on a laboratory scale. Optimisation of the process is discussed along with the oxidation of 15 representative alcohols.

Electrochemical measurement of erosion from individual cavitation events generated from continuous ultrasound

Passivated electrode surfaces of aluminium and lead have been employed to investigate individual erosion corrosion events produced by ultrasound. Individual events are recorded with a resolution within the sub-millisecond time scale. The probability of the recorded events was found to fit with the expected physical model of the system. Distance dependence of the erosion corrosion currents recorded are reported showing that the active transient cavitation zone of the sound field employed is within the first ca. 5 mm of the sound emitting surface. The electrode material is also shown to have a marked effect on the distance dependence of surface erosion. The extent of surface damage, as the result of individual cavitation events, are calculated from the current time transients recorded on passivated lead electrodes.

A study investigating the sonoelectrochemical degradation of an organic compound employing Fenton's reagent

The degradation of an organic dye molecule (specifically meldola blue, MDB) is reported under the influence of power ultrasound in combination with electrochemically-generated hydrogen peroxide. A novel flow system is employed to measure the degradation as a function of time while minimising the disturbance to the acoustics of the sonoelectrochemical reactor employed. The effect of adding Fe2+ to the rate of dye degradation is measured and demonstrated to be significant. Under optimum conditions the rate constant for dye degradation was found to reach a maximum value of (23.7±0.35)×10−3 min−1 assuming pseudo-first order kinetics. The rate constant for the complete destruction of MDB, determined by chemical oxygen demand, was found to be significantly slower at (10.2±2.6)×10−3 min−1.

Multiple observations of cavitation cluster dynamics close to an ultrasonic horn tip

Bubble dynamics in water close to the tip of an ultrasonic horn (∼23 kHz, 3 mm diameter) have been studied using electrochemistry, luminescence, acoustics, light scattering, and high-speed imaging. It is found that, under the conditions employed, a large bubble cluster (∼1.5 mm radius) exists at the tip of the horn. This cluster collapses periodically every three to four cycles of the fundamental frequency of the horn. Following the collapse of the cluster, a short-lived cloud of small bubbles (each tens of microns in diameter) was observed in the solution. Large amplitude pressure emissions are also recorded, which correlate temporally with the cluster collapse. Bursts of surface erosion (measured in real time using an electrochemical technique) and multibubble sonoluminescence emission both also occur at a subharmonic of the fundamental frequency of the horn and are temporally correlated with the bubble cluster collapse and the associated pressure wave emission.

Determination of heterogeneous electron transfer kinetics in the presence of ultrasound at microelectrodes employing sampled voltammetry.

The technique of sampled voltammetry at microelectrodes irradiated with ultrasound is demonstrated for the first time. This technique is used to determine the heterogeneous electron transfer rate constants for the redox couples ferrocene/ferrocenium, Ru(NH(3))(6)(3+/2+), and IrCl(6)(3)(-)/IrCl(6)(2)(-). Determination of the heterogeneous rate constants is also achieved for comparison purposes by analysis of fast sweep rate voltammetry of the redox systems studied at microelectrodes and comparison of the results obtained to the theory developed by Nicholson and Shain. The heterogeneous rate constants determined using sampled voltammetry were 1.0, 0.6, 1.23, and 0.18 cm s(-)(1) for the ferrocene/ferrocenium (0.1 mol dm(-)(3) TEATFB, CH(3)CN), Ru(NH(3))(6)(3+/2+) (0.1 mol dm(-)(3) KCl), IrCl(6)(3)(-) (1 mol dm(-)(3) KCl), and IrCl(6)(3)(-)/IrCl(6)(2)(-) (1 mol dm(-)(3) NaCl), respectively, in agreement with those obtained in the absence of ultrasound.

A study on the effect of ultrasound on electrochemical phenomena.

We have employed microelectrodes in order to resolve single transient cavitation events. High time resolution studies of individual current time transients were shown. Distance and temperature dependencies proved to be in agreement with both the acoustic pressure field generated by the ultrasonic horn and cavitation bubble collapse violence with decreasing vapour pressure within the bubble, respectively. Measurements of electrode kinetics at the point of impact of the transient events were shown for the first time by the employment of sampled voltammetry.

Experimental and theoretical characterisation of sonochemical cells.

Cavitation theory is used to predict the acoustic pressure at the boundary of the inertial/non inertial threshold for a range of bubble sizes. The sound field generated from a commonly employed sonoelectrochemical cell is modelled. The model is tested with a calibrated hydrophone far from the transducer to avoid spatial averaging. This allows the model to provide the absolute pressure amplitude as a function of axial distance from the source. An electrochemical technique for detecting both inertial and non-inertial cavitation within the solution is employed. This technique uses a dual microelectrode to map the boundary between the regions where inertial cavitation occurs (associated with surface erosion), and where it does not. This zone occurs close to the transducer for the microelectrode employed (<1.5 mm). Further characterisation of the inertial cavitation zone is achieved by imaging of multibubble sonoluminescence (MBSL). The pressures at the boundary between inertial and non inertial cavitation that are determined from the electrochemical and imaging experiments are compared to a sound field model and cavitation theory. Qualitative arguments for the invasive nature of the electrode into the sound field are proposed. Evidence for cavity cluster collapse and shock wave emission is presented and discussed in relation to luminescence, the electrochemical experiments and cavitation theory.

Electrochemical reduction of oxygen on mesoporous platinum microelectrodes

The electrochemical reduction of oxygen is studied using electrochemically deposited mesoporous platinum microelectrodes, which exhibit efficient mass transfer of material to the electrode surface and accelerated reduction kinetics when compared to polished microelectrodes.