Effect of the mediator in feedback mode-based SECM interrogation of indium tin-oxide and boron-doped diamond electrodes

Abstract

Indium tin-oxide (ITO) and polycrystalline boron-doped diamond (BDD) have been examined in detail using the scanning electrochemical microscopy technique in feedback mode. For the interrogation of electrodes made from these materials, the choice of mediator has been varied. Using${\text{Ru}}{\left( {{\text{CN}}} \right)}^{{4 - }}_{{{\text{6 }}{\left( {{\text{aq}}} \right)}}} ,$ ferrocene methanol (FcMeOH),${\text{Fe}}{\left( {{\text{CN}}} \right)}^{{{\text{3}} - }}_{{{\text{6 }}{\left( {{\text{aq}}} \right)}}}$ and${\text{Ru}}{\left( {{\text{NH}}_{{\text{3}}} } \right)}^{{3 + }}_{{{\text{6 }}{\left( {{\text{aq}}} \right)}}} ,$ approach curve experiments have been performed, and for purposes of comparison, calculations of the apparent heterogeneous electron transfer rates (k _app) have been made using these data. In general, it would appear that values of k _app are affected mainly by the position of the mediator reversible potential relative to the relevant semiconductor band edge (associated with majority carriers). For both the ITO (n type) and BDD (p type) electrodes, charge transfer is impeded and values are very low when using FcMeOH and${\text{Fe}}{\left( {{\text{CN}}} \right)}^{{{\text{3}} - }}_{{{\text{6 }}{\left( {{\text{aq}}} \right)}}}$ as mediators, and the use of ${\text{Ru}}{\left( {{\text{NH}}_{{\text{3}}} } \right)}^{{3 + }}_{{{\text{6}}{\left( {{\text{aq}}} \right)}}}$ results in the largest value of k _app. With ITO, the surface is chemically homogeneous and no variation is observed for any given mediator. Data is also presented where the potential of the ITO electrode is fixed using a ratio of the mediators ${\text{Fe}}{\left( {{\text{CN}}} \right)}^{{{\text{3}} - }}_{{{\text{6}}{\left( {{\text{aq}}} \right)}}}$ and ${\text{Fe}}{\left( {{\text{CN}}} \right)}^{{{\text{4}} - }}_{{{\text{6}}{\left( {{\text{aq}}} \right)}}} .$ In stark contrast, the BDD electrode is quite the opposite and a range of k _app values are observed for all mediators depending on the position on the surface. Both electrode surfaces are very flat and very smooth, and hence, for BDD, variations in feedback current imply a variation in the electrochemical activity. A comparison of the feedback current where the substrate is biased and unbiased shows a surprising degree of proportionality.