Physical Sciences Division, Physical and Computational Sciences Directorate , Pacific Northwest National Laboratory , Richland , Washington 99352 , United States.
School of Chemical and Biological Engineering , Washington State University , Pullman , Washington 99364 , United States.
J Am Chem Soc. 2019 Feb 6;141(5):2135-2142. doi: 10.1021/jacs.8b12483. Epub 2019 Jan 24.
Muscovite mica (001) is a widely used model surface for controlling molecular assembly and a common substrate for environmental adsorption processes. The mica (001) surface displays near-trigonal symmetry, but many molecular adsorbates-including water-exhibit unequal probabilities of alignment along its three nominally equivalent lattice directions. Buried hydroxyl groups within the muscovite structure are speculated to be responsible, but direct evidence is lacking. Here, we utilize vibrational sum frequency generation spectroscopy (vSFG) to characterize the orientation and hydrogen-bonding environment of near-surface hydroxyls inside mica. Multiple distinct peaks are detected in the O-H stretch region, which we attribute to Si/Al substitution in the SiO tetrahedron and K ion adsorption above the hydroxyls based on density functional theory simulations. Our findings demonstrate that vSFG can identify the absolute orientation of -OH groups and, hence, the surface termination at a mica surface, providing a means to investigate how -OH groups influence molecular adsorption and better understand mica stacking-sequences and physical behavior.
白云母（001）是一种广泛用于控制分子组装的模型表面，也是环境吸附过程的常见基底。云母（001）表面呈现近三角对称，但许多分子吸附物——包括水——在其三个名义上相等的晶格方向上的排列概率并不相等。白云母结构中的埋置羟基基团被认为是造成这种情况的原因，但缺乏直接证据。在这里，我们利用振动和频产生光谱（vSFG）来表征云母内部近表面羟基的取向和氢键环境。在 O-H 伸缩区域中检测到多个不同的峰，我们根据密度泛函理论模拟将其归因于 SiO 四面体中的 Si/Al 取代和羟基上方的 K 离子吸附。我们的发现表明，vSFG 可以确定 -OH 基团的绝对取向，从而确定云母表面的表面终止，为研究 -OH 基团如何影响分子吸附以及更好地理解云母堆积序列和物理行为提供了一种手段。
