Zhou Weiping, Baunach Thorsten, Ivanova Valentina, Kolb Dieter M
Department of Electrochemistry, University of Ulm, 89069 Ulm, Germany.
Langmuir. 2004 May 25;20(11):4590-5. doi: 10.1021/la049903m.
4,4'-Dithiodipyridine (PySSPy) monolayers on Au(111) were investigated by cyclic voltammetry, X-ray photoelectron spectroscopy (XPS) and in situ scanning tunneling microscopy (STM). The studies were performed in solutions of different anions and pHs (0.1 M H2SO4, 0.1 M HClO4, 0.1 and 0.01 M Na2SO4, 0.1 and 0.01 M NaOH). The cyclic current-potential curves in H2SO4 show current peaks at about 0.4 V, which are absent for all other electrolytes at this potential. The XPS data suggest that PySSPy adsorbs via the S endgroup on the gold surface and the S-S bond breaks during adsorption. From the chemical shift of the N(ls) peak, it is concluded that in acidic media the self-assembled monolayer (SAM) is fully protonated, whereas in basic solution it is not. The pKa is estimated to be 5.3. STM studies reveal the existence of highly ordered superstructures for the SAM. In Na2SO4 and H2SO4, a (7 x mean square root of 3) structure is proposed. However, whereas in Na2SO4 solutions the superstructure does not change with potential, in 0.1 M H2SO4 the superstructure is observed only negative of the current peak at +0.4 V. At more positive potentials, the film becomes disordered. The results are compared to those for 4-mercaptopyridine (PyS) SAMs. XPS experiments and current-potential curves indicate that both molecules adsorb in the same manner on Au(111), that is, even in the case of PySSPy the adspecies is PyS. The STM results, however, call for a more subtle interpretation. While in Na2SO4 solutions the observed superstructures are the same for both SAMs, markedly different structures are found for PySSPy and PyS SAMs in 0.1 M H2SO4.
通过循环伏安法、X射线光电子能谱（XPS）和原位扫描隧道显微镜（STM）对金（111）表面的4,4'-二硫代二吡啶（PySSPy）单分子层进行了研究。研究在不同阴离子和pH值的溶液（0.1 M硫酸、0.1 M高氯酸、0.1和0.01 M硫酸钠、0.1和0.01 M氢氧化钠）中进行。硫酸中的循环电流-电位曲线在约0.4 V处显示出电流峰，在该电位下所有其他电解质中均不存在此峰。XPS数据表明，PySSPy通过硫端基吸附在金表面，且在吸附过程中S-S键断裂。从N(ls)峰的化学位移得出，在酸性介质中自组装单分子层（SAM）完全质子化，而在碱性溶液中则不然。估计其pKa为5.3。STM研究揭示了SAM存在高度有序的超结构。在硫酸钠和硫酸中，提出了一种（7×3的平方根）结构。然而，在硫酸钠溶液中，超结构不随电位变化，而在0.1 M硫酸中，仅在+0.4 V处电流峰的负电位下观察到超结构。在更正的电位下，膜变得无序。将结果与4-巯基吡啶（PyS）SAMs的结果进行了比较。XPS实验和电流-电位曲线表明，两种分子以相同方式吸附在金（111）上，即即使对于PySSPy，吸附物种也是PyS。然而，STM结果需要更细致的解释。虽然在硫酸钠溶液中两种SAM观察到的超结构相同，但在0.1 M硫酸中，PySSPy和PyS SAMs的结构明显不同。
