Lee Chong-Yong, Bond Alan M
School of Chemistry, Monash University, Clayton, Victoria 3800, Australia.
Anal Chem. 2009 Jan 15;81(2):584-94. doi: 10.1021/ac801732g.
The level of edge plane defect sites present in highly ordered pyrolytic graphite (HOPG) electrodes has been evaluated via analysis of dc, ac fundamental, and higher-order ac harmonics available from a single large-amplitude Fourier transformed (FT) ac voltammetric experiment. Deliberate introduction of a low level of edge plane defect was achieved by polishing, with a higher level being introduced via electrochemical pretreatment. Kinetics regimes associated with fast electron transfer on the edge plane defect sites and slow electron transfer on the basal plane surface are resolved under ac conditions when using the surface-sensitive Fe(CN)(6) redox probe. However, because of their insensitivity to slow electron transfer, higher-order ac faradaic harmonics almost exclusively detect only the much faster processes that emanate from edge plane defect sites. Thus, detection of fourth- and higher-order ac Faradaic harmonic components that are devoid of background capacitive current is possible at freshly cleaved HOPG in the region near the reversible potential for the Fe(CN)(6) process. Under these circumstances, dc cyclic voltammograms exhibit only reduction and oxidation peaks separated by more than 1 V. The fundamental ac harmonic provides detailed information on the capacitive current, which increases with the level of edge plane defect sites. Apparent charge transfer rate constants also can be derived from peak-to-peak separations obtained from the dc aperiodic component. Estimates of the percentage of edge plane defect sites based on ac higher harmonics, capacitance, and dc aperiodic component that are available from a single experiment have been compared. The edge plane defect levels deduced from capacitance (fundamental harmonic ac component) and higher harmonic Faradaic currents are considered to be more reliable than estimations based on apparent rate constants derived from the dc aperiodic component or conventional dc cyclic voltammogram.
通过对单次大振幅傅里叶变换（FT）交流伏安实验中获得的直流、基波交流和高阶交流谐波进行分析，评估了高度有序热解石墨（HOPG）电极中存在的边缘平面缺陷位点的水平。通过抛光实现了低水平边缘平面缺陷的有意引入，通过电化学预处理引入了更高水平的缺陷。当使用表面敏感的Fe(CN)(6)氧化还原探针时，在交流条件下可以分辨出与边缘平面缺陷位点上的快速电子转移和基面表面上的缓慢电子转移相关的动力学机制。然而，由于高阶交流法拉第谐波对缓慢电子转移不敏感，几乎只检测到来自边缘平面缺陷位点的快得多的过程。因此，在Fe(CN)(6)过程的可逆电位附近区域，对于新劈开的HOPG，可以检测到没有背景电容电流的四阶及更高阶交流法拉第谐波分量。在这些情况下，直流循环伏安图仅显示出分离超过1V的还原峰和氧化峰。基波交流谐波提供了关于电容电流的详细信息，该电流随着边缘平面缺陷位点的水平而增加。表观电荷转移速率常数也可以从直流非周期分量获得的峰峰间距中推导出来。比较了基于单次实验中获得的交流高阶谐波、电容和直流非周期分量对边缘平面缺陷位点百分比的估计。从电容（基波交流分量）和高阶谐波法拉第电流推导出的边缘平面缺陷水平被认为比基于从直流非周期分量或传统直流循环伏安图得出的表观速率常数的估计更可靠。
