Scanning electrochemical microscopy part 13. Evaluation of the tip shapes of nanometer size microelectrodes

The shape of steady-state current-distance (iT-d) curves obtained by scanning electrochemical microscopy (SECM) with a submicrometer conical-type tip is substantially different from that obtained with a microdisk tip. A model is proposed to describe the iT-d curves for tips shaped as cones or spherical segments. In each case a family of theoretical working curves, computed for different values of a single adjustable parameter (the height of a cone or a segment), was used to fit the experimental curves obtained with tips approaching a conductive or insulating substrate. A good fit of the experimental and theoretical curves can be obtained only for a very narrow range of the tip shape parameters, giving confidence in the reliability of the proposed method. The shape parameters for two substantially different microelectrodes are evaluated. A 420 nm radius tip was shown to be a fairly sharp cone with a height-to-radius ratio k of 0.8. An 80 nm radius tip can be represented as either a cone or spherical segment, because of the small k = 0.2. The radius values obtained from iT-d curves agree with those estimated from steady-state current values. The cyclic voltammograms (CVs) obtained at these microtip electrodes have a regular shape with very small capacitive and resistive background. New analytical approximations for iT-d curves are also proposed for SECM with a disk-shaped tip over conductive and insulating substrates.