Cyclic Voltammetric Responses for Inlaid Microdisks with Shields of Thickness Comparable to the Electrode Radius: A Simulation of Reversible Electrode Kinetics

Abstract

Given common methods of preparing inlaid microdisk electrodes, shield thicknesses are often comparable to disk radii. Equations appropriate to characterizing the steady state, cyclic voltammetric response for microdisks embedded in in6nite insulating planes poorly approximate the steady state response at these electrodes because, on the time scale to achieve steady state, diffusion around the shield from behind the plane of the electrode contributions to the flux. A finite difference simulation with a nonuniform, expanding spatial grid is presented for reversible electrode kinetics over a range of scan rates s a c i e n t to follow the transition from linear to radial diffusion, including flux around the edge of the shield. The voltammetric response is influenced critically by shield thickness and electrode radius; a method is proposed and veritied experimentally to determine radius and shield thickness. The equations for infinitely thick insulators underestimate the current at a thinly shielded microdisk by 549%. Generation of a nonuniform, expanding grid is presented.Microelectrodes have become a major tool for studying nonstandard electrochemical systems. Solutions without added frozen solutions,6 and gas phase environments7-12 ' Present address: American Cyanamid,Dibble, T.; Bandyopadhyay, S.; Ghoroghchian, J.; Smith, J. J.; Sarfarazi, F.; (2) Cassidy, J.; Khoo, S. B.; Pons, S.; Fleischmann, M. J. Phys. Chem. 1985, (3) Bond, A M.; Lay, P. A.

Oxidation of hydroxide ions from dilute unbuffered solutions of ammonia at platinum microdiscs surrounded by thick and thin insulating sheaths

“…For a microdisc embedded on an infinite insulating plane, as the IS electrode can be regarded (insulating thickness much larger than the electrode radius), k ¼ 4 [24]. For the FS electrode, k depends on the relative dimensions of the thickness of the insulating sheath (b) and the electrode radius [17][18][19][20][21][22]. Approximate analytical equations relating k, b and a were derived by fitting data obtained from numerical procedures [17][18][19]21,22].…”

“…However, in the CV recorded at the FS electrode, the wave during the backward scan almost exactly retraces that during the forward scan, whereas the CV recorded at the IS displays a larger hysteresis. These results indicate that at the FS electrode the steady state is achieved faster [18,27]. Table 1 summarises relevant voltammetric parameters, includes the thickness of the insulating electrophoretic paint of the FS electrode, which was measured from SEM images, k values and the radii of the two types of electrode, which were calculated from the experimental steady-state limiting current, the other parameters in Eq.…”

“…In fact, over the potential region where the main wave occurs, surface oxides are formed concurrently [3,29]. This phenomenon gives rise to a transient current which, at a given scan rate, is probably greater at the IS electrode, because the achievement of the steady state takes a relatively longer time [18]. Differences in the surface state of the two types of electrodes, which could also explain the different types of voltammetric behaviours described above, may reasonably be excluded by the fact that both types of electrodes underwent the same electrochemical pre-treatment to refresh the electrode surface before each measurement.…”

“…The Pt microdiscs employed are surrounded by either a thick or a very thin insulating sheath. Microdiscs with shields of thickness comparable to the electrode radius are attractive, as on the time scale of standard voltammetric measurements, the diffusion field undergoes a transition from linear to radial symmetry, and is established radially behind the plane of the electrode and shield [17][18][19][20][21][22] (see Fig. 1a).…”

Analysis of positive feedback currents at the scanning electrochemical microscope

“…(30). However, at RG < 10 the difference becomes significant due to mediator diffusion from the back of the tip [15,25]. The correction for the finite insulator thickness [24] can be introduced by replacing the factor ''4'' in Eq.…”