Correlation of electrochemistry at carbon electrodes with surface microstructural changes induced by laser irradiation.

摘要

Electrochemistry is found in many analytical and industrial applications. Glassy carbon, (GC), is an attractive electrode material but its surface is poorly understood. The low heterogeneous electron transfer rate constant, (k{dollar}spcirc{dollar}), typical of conventionally prepared GC may be increased by intense pulsed laser irradiation. The purpose of this work is to better understand the enhancement of k{dollar}spcirc{dollar} at carbon electrodes using laser irradiation.; For carbon electrodes, k{dollar}spcirc{dollar} has been proposed as depending on surface roughness, cleanliness, oxygen functionalities and fractional coverage of graphitic edge plane, (F{dollar}sb{lcub}rm e{rcub}{dollar}). Enhancement of k{dollar}spcirc{dollar} for {dollar}rm Fe(CN)sb6sp{lcub}-3/-4{rcub}{dollar} at the oxide free and atomically smooth basal plane of highly oriented pyrolytic graphite, (HOPG), was investigated initially. Increases in the differential double layer capacitance, (C{dollar}spcirc{dollar}), and k{dollar}spcirc{dollar} showed well defined thresholds and similar trends with increasing power density. Quantitative expressions are proposed which independently relate C{dollar}spcirc{dollar} and k{dollar}spcirc{dollar} to F{dollar}sb{lcub}rm e{rcub}{dollar}. Calculations of F{dollar}sb{lcub}rm e{rcub}{dollar} using observed C{dollar}spcirc{dollar} and k{dollar}spcirc{dollar} yield similar values. Enhancement of k{dollar}spcirc{dollar} for {dollar}rm Fe(CN)sb6sp{lcub}-3/-4{rcub}{dollar} at basal plane HOPG by laser irradiation is due primarily to increases in F{dollar}sb{lcub}rm e{rcub}{dollar}. Increasing F{dollar}sb{lcub}rm e{rcub}{dollar} by only 3% is associated with an increase in k{dollar}spcirc{dollar} by 10{dollar}sp5{dollar}, indicating that only minor surface modification is required.; Enhancement of k{dollar}spcirc{dollar} for {dollar}rm Fe(CN)sb6sp{lcub}-3/-4{rcub}{dollar} at polished GC electrodes by multiple pulse low power density laser irradiation was also investigated. A k{dollar}spcirc{dollar} of above 0.3 cm/sec was observed for the first time. High rates were verified by adherence to Nicholson theory, semiintegration and by comparison of voltammograms to that calculated by finite difference. Although surface roughness and F{dollar}sb{lcub}rm e{rcub}{dollar} were not changed as k{dollar}spcirc{dollar} increased, C{dollar}spcirc{dollar} increased monotonically with the number of laser pulses. A clean fractured GC surface exhibited a k{dollar}spcirc{dollar} of 0.5 cm/sec for {dollar}rm Fe(CN)sb6sp{lcub}-3/-4{rcub}{dollar} and was also very active for ascorbic acid and dopamine oxidation, indicating that the base microstructure of the GC has inherently high k{dollar}spcirc{dollar}. These observations are consistent with a surface cleaning mechanism for laser activation of a polished GC electrode. A quantitative expression based on surface cleaning is proposed which may account for the observed relationship between C{dollar}spcirc{dollar} and k{dollar}spcirc{dollar} at polished GC upon low power density multiple pulse laser irradiation.; Current transients induced by pulsed laser irradiation at platinum, basal plane HOPG and GC electrodes were also studied. Transients at platinum and HOPG electrodes are primarily due to double layer charging. Transients at GC are composed of a short term capacitive charging component and longer term second order Faradaic current component.