Scanning electrochemical microscopy. 57. SECM tip voltammetry at different substrate potentials under quasi-steady-state and steady-state conditions

We discuss SECM tip voltammetry, where a UME tip is held above a conductive substrate within about a tip radius and a tip voltammogram is recorded as its potential is slowly scanned while the substrate is held at a fixed potential. When the potential of the substrate is changed, the series of steady-state tip voltammograms provide information about the reactants and products. When the potential of the substrate, ES, is set so that the reaction at the substrate is opposite to that at the tip (the usual SECM conditions), a total positive feedback (tpf) tip voltammogram is recorded. When the substrate potential is set to values where the reaction at the substrate is the same as that occurring on the tip, the tip is shielded from the species in the bulk solution. Depending upon the substrate potential, this can cause total shielding (ts) or a voltammogram that is the result of partial feedback/partial shielding (pf-ps). The result is a series of tip voltammograms that are characterized by tpf, pf-ps, or ts, depending upon ES. Experimental tip voltammograms resulting from the reversible reduction of TCNQ and oxidation of ferrocene in MeCN are reported. These are compared with those from simulations and approximate equations developed to describe the features of the tip voltammograms generated under tpf, ts, or pf-ps conditions. The effect of the diffusion coefficient ratio on the ability of the UME tip to reach a true steady state is also addressed and possible applications, e.g., obtaining information about the reversibility of an electrochemical reaction, the product of an electrochemical reaction, the stability of that product, or the diffusion coefficients of the electroactive species, are discussed.