Department of Physics, University of Erlangen-Nürnberg , 91058 Erlangen, Germany.
Materials Sciences Division, Lawrence Berkeley National Laboratory, University of California , Berkeley, California 94720, United States.
Acc Chem Res. 2015 Oct 20;48(10):2783-90. doi: 10.1021/acs.accounts.5b00214. Epub 2015 Sep 29.
The adsorption and reactions of water on surfaces has attracted great interest, as water is involved in many physical and chemical processes at interfaces. On metal surfaces, the adsorption energy of water is comparable to the hydrogen bond strength in water. Therefore, the delicate balance between the water-water and the water-metal interaction strength determines the stability of water structures. In such systems, kinetic effects play an important role and many metastable states can form with long lifetimes, such that the most stable state may not reached. This has led to difficulties in the theoretical prediction of water structures as well as to some controversial results. The direct imaging using scanning tunneling microscopy (STM) in ultrahigh vacuum at low temperatures offers a reliable means of understanding the local structure and reaction of water molecules, in particular when interpreted in conjunction with density functional theory calculations. In this Account, a selection of recent STM results on the water adsorption and dissociation on close-packed metal surfaces is reviewed, with a particular focus on Ru(0001). The Ru(0001) surface is one where water adsorbs intact in a metastable state at low temperatures and where partially dissociated layers are formed at temperatures above ∼150 K. First, we will describe the structure of intact water clusters starting with the monomer up to the monolayer. We show that icelike wetting layers do not occur on close-packed metal surfaces but instead hydrogen bonded layers in the form of a mixture of pentagonal, hexagonal, and heptagonal molecular rings are observed. Second, we will discuss the dissociation mechanism of water on Ru(0001). We demonstrate that water adsorption changes from dissociative to molecular as a function of the oxygen preadsorbed on Ru. Finally, we briefly review recent STM experiments on bulk ice (Ih and Ic) and water adsorption on insulating thin films. We conclude with an outlook illustrating the manipulation capabilities of STM in respect to probe the proton and hydrogen dynamics in water clusters.
表面上水的吸附和反应引起了极大的兴趣，因为水参与了界面处的许多物理和化学过程。在金属表面上，水的吸附能与水中氢键的强度相当。因此，水-水和水-金属相互作用强度之间的微妙平衡决定了水结构的稳定性。在这样的体系中，动力学效应起着重要的作用，许多亚稳态可以形成长寿命，使得最稳定的状态可能无法达到。这导致了对水结构的理论预测的困难，并导致了一些有争议的结果。在低温超高真空下使用扫描隧道显微镜（STM）进行直接成像提供了一种可靠的手段来理解水分子的局部结构和反应，特别是当与密度泛函理论计算结合使用时。在本综述中，选择了最近关于在密排金属表面上水吸附和离解的 STM 结果进行了回顾，特别关注 Ru(0001)。Ru(0001)表面是一种在低温下以亚稳态完整吸附水的表面，在高于约 150 K 的温度下形成部分离解层。首先，我们将描述完整水团簇的结构，从单体开始到单层。我们表明，冰状润湿层不会在密排金属表面上发生，而是观察到以五边形、六边形和七边形分子环的混合物形式存在氢键层。其次，我们将讨论水在 Ru(0001)上的离解机制。我们证明，随着 Ru 上预吸附氧的增加，水吸附从离解转变为分子。最后，我们简要回顾了最近关于体冰（Ih 和 Ic）和绝缘薄膜上水吸附的 STM 实验。我们以展望结束，说明 STM 在探测水团簇中质子和氢动力学方面的操纵能力。
