Zhang Jingdong, Bilic Ante, Reimers Jeffrey R, Hush Noel S, Ulstrup Jens
Department of Chemistry, Nano.DTU, Technical University of Denmark, DK-2800 Lyngby, Denmark.
J Phys Chem B. 2005 Aug 18;109(32):15355-67. doi: 10.1021/jp050797m.
The structural organization, catalytic function, and electronic properties of cysteamine monolayers on Au(111) have been addressed comprehensively by voltammetry, in situ scanning tunneling microscopy (STM) in anaerobic environment, and a priori molecular dynamics (MD) simulation and STM image simulation. Two sets of voltammetric signals are observed. One peak at -(0.65-0.70) V (SCE) is caused by reductive desorption of cysteamine. The other signal, at -(0.25-0.40) V consists of a peak doublet. The pH dependence of the latter suggests that the origin is catalytic dihydrogen evolution. The doublet feature is indicative of two distinct cysteamine configurations. Cysteamine monolayer formation from initial nucleation to a highly ordered phase has been successfully observed in real time using oxygen-free in situ STM. Random cellular patterns, disordered adlayer formation accompanied by high step edge mobility, and ultimately a highly ordered (square root 3 x 4) R30 degrees lattice are observed sequentially. Pits are formed due to enclosure of the mobile edges during the adsorption process. In the highly ordered cysteamine layer, each unit has two spots with apparent 0.6 A height difference in STM images. The coverage 5.7 +/- 0.1 x 10(-10) mol cm(-2) determined by voltammetry supports that the spots represent two individual cysteamine molecules. A priori MD and density functional simulations hold other clues to the image interpretation and indicate that the NH(3)(+) groups dominate the tunneling contrast. A wide range of interface structures, showing variations in the sulfur binding site and orientation, gauche and trans conformers, and especially hydrogen-bonding interactions, are examined, from which it is concluded that the adsorbate structure is controlled by interactions with the solvent rather than with the substrate.
通过伏安法、厌氧环境下的原位扫描隧道显微镜（STM）以及先验分子动力学（MD）模拟和STM图像模拟，全面研究了金（111）表面半胱胺单层的结构组织、催化功能和电子性质。观察到两组伏安信号。在-(0.65 - 0.70) V（SCE）处的一个峰是由半胱胺的还原脱附引起的。另一个信号在-(0.25 - 0.40) V，由一个双峰组成。后者的pH依赖性表明其起源是催化析氢。双峰特征表明存在两种不同的半胱胺构型。使用无氧原位STM实时成功观察到了从初始成核到高度有序相的半胱胺单层形成过程。依次观察到随机的蜂窝状图案、伴随着高台阶边缘迁移率的无序吸附层形成，以及最终高度有序的（√3×4）R30°晶格。在吸附过程中，由于移动边缘的包围形成了凹坑。在高度有序的半胱胺层中，每个单元在STM图像中有两个明显高度差为0.6 Å的斑点。通过伏安法测定的覆盖度为5.7±0.1×10⁻¹⁰ mol cm⁻²，这支持了这些斑点代表两个单独的半胱胺分子。先验MD和密度泛函模拟为图像解释提供了其他线索，并表明NH₃⁺基团主导隧道对比度。研究了广泛的界面结构，这些结构在硫结合位点和取向、gauche和反式构象以及特别是氢键相互作用方面存在变化，由此得出结论，吸附质结构是由与溶剂而非与底物的相互作用控制的。
