Unlike metal or semiconductor electrodes, the surface charge resulting from the protonation or deprotonation of insulating mineral oxides is highly localized and heterogeneous in nature. In this work the Stark active C≡N stretch of potassium thiocyanate is used as a molecular probe of the heterogeneity of the interfacial electrostatic potential at the α-Al2O3(0001)/H2O interface. Vibrational sum frequency generation (vSFG) measurements performed in the OH stretching region suggest that thiocyanate species organize interfacial water similarly to halide ions. Changes in the electrostatic potential are then tracked via Stark shifts of the vibrational frequency of the thiocyanate stretch. Our vSFG measurements show that we can simultaneously measure the vSFG response of SCN- ions experiencing charged and neutral surface sites. We assign local potentials of +308 and -154 mV to positively and negatively charged aluminol groups that are present at pH = 4 and pH = 10, respectively. Thiocyanate anions at positively charged surface sites and negatively charged surface sites and those participating in contact ion pairing adopt similar orientations and are oppositely oriented relative to thiocyanate ions near neutral surface sites. All four species followed Langmuir adsorption isotherms. Density functional theory-molecular dynamics (DFT-MD) simulations of SCN- near the neutral α-Al2O3(0001)/H2O interface show that the vSFG response in the C≡N stretch region originates from a SCN-H-O-Al complex, suggesting the surface site specificity of these experiments. To our knowledge this is the first spectroscopic measurement of local potentials associated with a heterogeneously charged surface. The ability to probe the evolution of local charges in situ could provide vital insight into many industrial, electrochemical, and geochemically relevant interfaces.

中文翻译：

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探索 α-Al2O3(0001)/H2O 界面的异质电荷分布

与金属或半导体电极不同，绝缘矿物氧化物质子化或去质子化所产生的表面电荷本质上是高度局部化和异质的。在这项工作中，硫氰酸钾的 Stark 活性 C≡N 段用作 α-Al2O3(0001)/H2O 界面处界面静电势异质性的分子探针。在 OH 拉伸区域中进行的振动和频率生成 (vSFG) 测量表明，硫氰酸盐物质组织界面水类似于卤离子。然后通过硫氰酸盐拉伸振动频率的斯塔克位移跟踪静电势的变化。我们的 vSFG 测量表明，我们可以同时测量经历带电和中性表面位点的 SCN 离子的 vSFG 响应。我们将 +308 和 -154 mV 的局部电位分配给分别在 pH = 4 和 pH = 10 下存在的带正电和带负电的铝醇基团。带正电表面位点和带负电表面位点的硫氰酸根阴离子以及那些参与接触离子配对的阴离子采用相似的取向，并且相对于中性表面位点附近的硫氰酸根离子的取向相反。所有四种物质都遵循朗缪尔吸附等温线。中性 α-Al2O3(0001)/H2O 界面附近 SCN-的密度泛函理论-分子动力学 (DFT-MD) 模拟表明，C≡N 拉伸区域中的 vSFG 响应源自 SCN-HO-Al 复合物，表明这些实验的表面位点特异性。据我们所知，这是与异质带电表面相关的局部电位的第一次光谱测量。原位探测局部电荷演变的能力可以提供对许多工业、电化学和地球化学相关界面的重要洞察。