The use of scanning electrochemical microscopy (SECM) to measure charge transport diffusion constants (D CT ) in metallopolymers of the type [Os(bpy) 2 (PVP) n Cl]Cl, bpy ) 2,2′-bipyridyl and PVP ) poly(4vinylpyridine), is described. In this approach, a triple potential step technique is employed in which the ultramicroelectrode (UME) tip of the SECM is used to electrogenerate a solution phase oxidant, Ru(CN) 6 3-, in an initial potential step, via the oxidation of Ru(CN) 6 4-. This moiety diffuses from the tip to the underlying polymer film where electron transfer occurs, causing the local oxidation of the polymer-bound complex of Os II to Os III . The form of the current-time characteristic in this step provides information on the kinetics of the ET process between the solution species and the polymer-bound moiety, as well as the concentration of redox-active species in the polymer film. This process creates lateral concentration gradients of Os II and Os III along the film. After the first potential step, a waiting period is introduced in which Ru(CN) 6 3is converted back to Ru(CN) 6 4at the UME and Os II can recover in concentration by electron self-exchange between Os III and Os II moieties. After a defined time, the potential of the UME is switched again to cause the generation of the solution-phase oxidant, Ru(CN) 6 3-. The current-time behavior associated with this step is influenced significantly by the extent of lateral electron hopping in the waiting period. It is shown that SECM is capable of measuring D CT values as low as 10 -10 cm 2 s -1 with good precision. We report experimental measurements on spin-coated films of [Os(bpy) 2 (PVP) n Cl]Cl, where n ) 5 or 10, which indicate that D CT is affected significantly by redox site loading and film structure (as determined by atomic force microscopy).

Mass transport to micrometer-sized electrodes in a microjet (wall-tube) electrode configuration is examined experimentally and through finite element modeling. Electrochemical imaging experiments reveal that local mass transport is highly sensitive to the lateral position of the nozzle with respect to the electrode. When these two components are arranged coaxially, there is a pronounced minimum in the mass transfer rate to the electrode, as determined from transport-limited current measurements. Small lateral displacements of the nozzle from the coaxial position lead first to an increase in mass transport, with the current reaching a maximum at a displacement of around one nozzle radius (50 μm). For larger lateral displacements of the nozzle from the coaxial position, the limiting current gradually decreases with increasing distance. The implications of these observations for practical applications of the microjet electrode are considered. Voltammetric measurements on the oxidation of IrCl6 3- in aqueous solution, with the electrode and nozzle coaxial are shown to be in good agreement with simulation of mass transport. Increasing the solution viscosity dramatically decreases mass transport to the electrode, with the reduction in the diffusion coefficient of the redox species as the major factor.

The surface processes accompanying dissolution from the (010) surface of potassium ferrocyanide trihydrate in aqueous solution have been determined using an integrated electrochemical-atomic force microscope (IE-AFM). This instrument employs a Pt-coated AFM tip that functions as an electrode, as well as a conventional topographical imaging device. The dissolution process was induced by oxidizing ferrocyanide at the tip, from an initially saturated solution, thereby creating a local undersaturation at the crystal-solution interface. The subsequent dissolution behavior was imaged in real time as a function of the diffusional driving force. At low driving force, the dissolution process proceeds mainly by the retreat of preexisiting steps aligned in the dominant [102] direction. With increasing driving force, dissolution occurs via the unwinding of steps from screw dislocation sites, leading to the formation of macroscopic etch pits, with an outline morphology determined by the [102] and [201] directions. At the highest driving forces the density of etch pits corresponds closely with the observed density of growth hillocks on the crystal surface, prior to dissolution. The surface dynamics observed under diffusion-controlled conditions are consistent with earlier scanning electrochemical microscopy kinetic studies (Macpherson,