Sumi Takayoshi, Uosaki Kohei
Physical Chemistry Laboratory, Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan.
J Phys Chem B. 2004 May 20;108(20):6422-8. doi: 10.1021/jp049558+.
Electrochemical oxidative adsorption and reductive desorption of a self-assembled monolayer (SAM) of decanethiol on a Au(111) single crystal electrode were examined in 0.1 M KOH ethanol solution containing various concentrations of decanethiol ranging from 1 muM to 1 mM. Anodic and cathodic current peaks corresponding to the adsorption and desorption of decanethiol, respectively, were observed in cyclic voltammograms of a Au(111) single crystal electrode obtained in 0.1 M KOH ethanol solution containing more than 10 muM of decanethiol. Positions of both peaks depended on the concentration of decanethiol, and they shifted negatively by ca. 0.057 V/decade with increase in decanethiol concentration. This result confirms that the adsorption and desorption of decanethiol is a one-electron process. The reductive charge, which consists of desorption charge and capacitive charge, increased when the sweep rate was decreased and the decanethiol concentration was increased and reached the saturated value of 103 (+/-5%) muC cm-2, which corresponds to the reductive charge of thiol SAM of full coverage with a ( radical3 x radical3)R30 degrees structure. Potentiostatic SAM formation was also investigated by holding the potential at +0.1 V. The reductive charge, i.e., the coverage of the SAM, increased with time and reached the saturated value of 103 (+/-5%) muC cm-2, corresponding to full coverage, after holding the potential at +0.1 V for a certain period of time. The time when the amount of adsorbed thiolate reached full coverage depended on the concentration of decanethiol. The higher the concentration was, the faster full coverage was reached. The desorption peak shifted negatively as the holding time at +0.1 V was increased even after the adsorbed amount had reached full coverage. These results suggest that the ordering of decanethiol SAMs requires a much longer time than the time required for full coverage adsorption. The position of the reductive desorption peak was independent of the thiol concentration if the electrode was kept at +0.1 V for long enough so that a highly ordered SAM was formed. The cathodic peak shifted negatively as the sweep rate was increased, showing that reductive desorption of the SAM was rather slow. The rate constant for the reductive desorption was determined from the potential dependent peak shift to be 0.24 s-1, which is in good agreement with the value obtained for a SAM prepared without potential control, indicating that the quality of the electrochemically prepared SAM is as good as that of the SAM prepared nonelectrochemically.
在含有浓度范围从1 μM到1 mM的各种癸硫醇的0.1 M KOH乙醇溶液中，研究了癸硫醇自组装单分子层（SAM）在Au(111)单晶电极上的电化学氧化吸附和还原解吸过程。在含有超过10 μM癸硫醇的0.1 M KOH乙醇溶液中获得的Au(111)单晶电极的循环伏安图中，分别观察到对应于癸硫醇吸附和解吸的阳极和阴极电流峰。两个峰的位置取决于癸硫醇的浓度，并且随着癸硫醇浓度的增加，它们负向移动约0.057 V/十倍浓度变化。该结果证实癸硫醇的吸附和解吸是一个单电子过程。由解吸电荷和电容性电荷组成的还原电荷，在扫描速率降低且癸硫醇浓度增加时增大，并达到饱和值103（±5%）μC cm-2，这对应于具有(√3×√3)R30°结构的完全覆盖的硫醇SAM的还原电荷。还通过将电位保持在 +0.1 V来研究恒电位SAM的形成。在将电位保持在 +0.1 V一段时间后，还原电荷即SAM的覆盖度随时间增加并达到饱和值103（±5%）μC cm-2，对应于完全覆盖。吸附的硫醇盐达到完全覆盖的时间取决于癸硫醇的浓度。浓度越高，达到完全覆盖的速度越快。即使在吸附量达到完全覆盖后，随着在 +0.1 V的保持时间增加，解吸峰也负向移动。这些结果表明，癸硫醇SAM的有序化所需的时间比完全覆盖吸附所需的时间长得多。如果电极在 +0.1 V保持足够长的时间以形成高度有序的SAM，则还原解吸峰的位置与硫醇浓度无关。随着扫描速率的增加，阴极峰负向移动，表明SAM的还原解吸相当缓慢。由电位依赖性峰位移确定的还原解吸速率常数为0.24 s-1，这与在无电位控制下制备的SAM所获得的值非常一致，表明电化学制备的SAM的质量与非电化学制备的SAM的质量一样好。
