Origin of Asymmetry of Paired Nanogap Voltammograms Based on Scanning Electrochemical Microscopy: Contamination Not Adsorption

Abstract

Formation of a nanometer-wide gap between tip and substrate electrodes by scanning electrochemical microscopy (SECM) enables voltammetric measurement of ultrafast electron-transfer kinetics. Herein, we demonstrate the advantage of SECM-based nanogap voltammetry to assess the cleanness of the substrate surface in solution by confirming that airborne contamination of highly oriented pyrolytic graphite (HOPG) causes the nonideal asymmetry of paired nanogap voltammograms of (ferrocenylmethyl)trimethylammonium (Fc(+)). We hypothesize that the amperometric response of a 1 μm-diameter Pt tip is less enhanced in the feedback mode, where more hydrophilic Fc(2+) is generated from Fc(+) at the tip and reduced voltammetrically at the HOPG surface covered with airborne hydrophobic contaminants. The tip current is more enhanced in the substrate generation/tip collection mode, where less charged Fc(+) is oxidized at the contaminated HOPG surface. In fact, symmetric pairs of nanogap voltammograms are obtained with the cleaner HOPG surface that is exfoliated in humidified air and covered with a nanometer-thick water adlayer to suppress airborne contamination. This result disproves a misconception that the asymmetry of paired nanogap voltammograms is due to electron exchange mediated by Fc(2+) adsorbed on the glass sheath of the tip. Moreover, weak Fc(+) adsorption on the HOPG surface causes only the small hysteresis of each voltammogram upon forward and reverse sweeps of the HOPG potential. Significantly, no Fc(2+) adsorption on the HOPG surface ensures that the simple outer-sphere pathway mediates ultrafast electron transfer of the Fc(2+/+) couple with standard rate constants of ≥12 cm/s as estimated from symmetric pairs of reversible nanogap voltammograms.

Adsorption and Electron‐Transfer Mechanisms of Ferrocene Carboxylates and Sulfonates at Highly Oriented Pyrolytic Graphite

“…The low reverse response indicates that the product of the forward scan, Fc + CH 2 CO 2 − , was not adsorbed on the HOPG surface. Overall, the ET of the Fc + CH 2 CO 2 − /FcCH 2 CO 2 − couple is coupled with the adsorption of only FcCH 2 CO 2 − and, subsequently, mediated through the outer‐sphere pathway (Figure B).…”

“…A Milli‐Q Advantage A10 water purification system (EMD Millipore) was used to obtain UV‐treated ultrapure water with a TOC value of 2–3 ppb as measured by using an internal online TOC monitor. Purified water was collected in a class 100 vertical laminar flow hood (AC632TLFC, AirClean Systems, Raleigh, NC) equipped with a bonded carbon filter (ACF100, AirClean Systems) to minimize airborne contamination . The Milli‐Q system was fed with the water (15.0 MΩ cm) purified from tap water by using Elix 3 Advantage (EMD Millipore).…”

“…CHI 660B (CH Instruments, Austin, TX) was used to measure CV with compensation of ohmic potential drop, which was measurable owing to the high current based on the desorption of ferrocene derivatives at fast scan rates of up to 10 V/s despite small resistances of ∼15 Ω. A water‐protected HOPG surface was obtained by peeling HOPG under humidified conditions surrounded by dry ice in air filtered through activated carbon and was immediately immersed in the electrolyte solution filled in the sealed cell . Pt and Ag/AgCl wires were used as counter and reference electrodes, respectively.…”

“…We argued that these voltammograms prove outer‐sphere ET because inner‐sphere ET requires the adsorption of both forms of a redox couple . Moreover, self‐exchange ET between the adsorbed reductant and the non‐adsorbed oxidant is hindered because the production of the adsorbed oxidant is highly unfavorable . Furthermore, the concerted oxidation of the adsorbed reductant to the non‐adsorbed oxidant is unlikely owing to the Frank‐Condon principle…”

“…By contrast, only reduced forms of the ferrocene derivatives are adsorbed on the carbon surfaces (Figure B) to yield characteristic cyclic voltammograms (CVs) based on the weak or strong adsorption of the reactants . We argued that these voltammograms prove outer‐sphere ET because inner‐sphere ET requires the adsorption of both forms of a redox couple . Moreover, self‐exchange ET between the adsorbed reductant and the non‐adsorbed oxidant is hindered because the production of the adsorbed oxidant is highly unfavorable .…”