Christopher J. Slevin

FABRICATION AND CHARACTERISATION OF NANOMETRE-SIZED PLATINUM ELECTRODES FOR VOLTAMMETRIC ANALYSIS AND IMAGING

Abstract A simple procedure is described for the fabrication of micrometre to nanometre scale Pt electrodes. These electrodes are prepared by etching a fine Pt wire, which is subsequently coated with an electrophoretic paint, deposited anodically or cathodically. The electrodes are characterised by scanning electron microscopy and steady-state linear sweep voltammetry. Voltammetric measurements of the oxidation of aqueous ferrocyanide at electrodes with effective radii varying from 1 μm to 10 nm show the expected increase in irreversibility with increasing mass transport rate. The electrodes are shown to be particularly promising as imaging probes for scanning electrochemical microscopy.

Scanning electrochemical microscopy: beyond the solid/liquid interface

Recent progress has seen scanning electrochemical microscopy (SECM) emerge as a powerful technique for probing physicochemical interfacial processes, beyond the solid/liquid and electrode/electrolyte interfaces that were originally of primary interest. This review (with 92 references) assesses recent developments in SECM as a methodology for investigating liquid/liquid and liquid/gas interfaces, along with processes of biochemical and biophysical significance.

Micro ring–disk electrode probes for scanning electrochemical microscopy

Abstract The construction and characterisation of ring–disk (RD) microelectrodes suitable for use in scanning electrochemical microscopy (SECM) is reported. Such RD electrodes are proposed as probes for novel generator–collector SECM experiments. In this case, the interaction of both the reactants and products with the substrate under investigation can be followed simultaneously from a single approach curve to the substrate. Examples of such approach curves to conducting and insulating substrates are given to demonstrate the potential of this new mode of SECM operation.

Probing the oxidative etching kinetics of metals with the feedback mode of the scanning electrochemical microscope

The conditions under which the feedback mode of the scanning electrochemical microscope (SECM) can be used to probe the oxidative etching kinetics of metals most effectively are considered. In this application, the oxidant of interest is electrogenerated at the tip ultramicroelectrode (UME) of an SECM from a solution containing only the reduced form of the redox couple. In this study, tris (2,2′-bipyridyl)ruthenium(III)[Ru(bipy)33+] is generated in aqueous potassium nitrate solutions through the oxidation of Ru(bipy)32+, and bromine is formed through the oxidation of bromide in aqueous sulfuric acid solutions. The oxidant diffuses to an unbiased metal substrate, where it may induce oxidative etching, resulting in the anodic dissolution of the metal. In the process, the mediator is converted back to its reduced form and fed back to the tip by diffusion, thereby enhancing the current. When the substrate is effectively infinite in size compared to the tip UME, its potential is fixed by the bulk solution conditions at a value that also promotes diffusion-controlled positive feedback of the mediator. Thus, feedback current measurements under these conditions may not simply reflect the kinetics of etching. SECM imaging experiments with Pt substrate electrodes, which are inert with respect to anodic dissolution with the two oxidants of interest, demonstrate that this complication can be circumvented by decreasing the size of the substrate electrode to the dimensions of the tip UME. Under these conditions, substrate potential-driven feedback at an unbiased metal becomes negligible, allowing etching kinetics to be studied in isolation. Investigations of Cu etching with both Ru(bipy)33+ and Br2, under these conditions reveal, unambiguously, that the metal dissolution process is diffusion-controlled under all of the conditions examined, suggesting that the heterogeneous rate constant controlling the etching process is large. In the case of the reaction with Ru(bipy)33+, a minimum rate constant of 0.4 cm s–1 is estimated for the interfacial reaction, assuming a first-order process. On a general level, the studies provide new information on the imaging capabilities of SECM in the feedback mode with unbiased substrates.

Proton transport in radiation-grafted membranes for fuel cells as detected by SECM

Scanning electrochemical microscopy (SECM) has been applied to investigate counter ion transport through four different proton conducting membranes with poly(styrene sulfonic acid) side chains. These membranes, intended for the polymer electrolyte fuel cell, are based on PVDF and PVDF-co-HFP matrix materials and have been prepared by an irradiation grafting method. SECM is found to be suitable for mapping variations in proton diffusion coefficient and concentration in these inhomogeneous membranes. It was found that the variations in these parameters are most considerable in a membrane with a high degree of grafting. Ionic conductivities measured with impedance spectroscopy were in agreement with calculated values obtained on the basis of the SECM measurements.

A new approach to the measurement of transfer rates across immiscible liquid/liquid interfaces

The use of ultramicroelectrodes (UMEs) to induce and quantitatively measure the transport of chemical species across immiscible liquid organic/water (o/w) interfaces is considered. In this application, a disc-shaped UME located in the aqueous phase, at micrometre distances from the interface of interest, is employed to drive the transfer process, intially at equilibrium, in the direction of the aqueous phase. This is achieved by applying a potential to the UME, sufficient to electrolyse the target species at a diffusion-controlled rate. The resulting depletion in the local aqueous concentration provides the thermodynamic force for the transfer process and species crossing the interface contribute to the UME current flow, the magnitude of which depends on the transfer kinetics. The technique is illustrated with model studies on the extraction/stripping of Cu2+ from aqueous solutions by oxime ligands in heptane and 1,2-dichloroethane (DCE).

Resolution of coupled electron transfer–ion transfer processes at liquid/liquid interfaces by visualisation of interfacial concentration profiles

A powerful approach for investigating heterogeneous electron transfer (ET) reactions at liquid/liquid (oil/water) interfaces is described and illustrated with studies of the reactions between IrCl62– or Fe(CN)63– in an aqueous phase and decamethylferrocene (DMFc) or ferrocene (Fc) in 1,2-dichloroethane (DCE).