How Does Water Wet a Surface?

Abstract

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.

A multi-technique study of CO2 adsorption on Fe3O4 magnetite

“…The alternative explanation is that the twolayer structure is unstable against the growth of multilayer islands. The growth of 3D clusters on a wetting layer has been observed for water on Ru(0001) and was linked to the formation of non-ice-like wetting layer structures due to a mixture of hydrogen bonding and a strong interaction with the substrate, 61 similar to Stranski-Krastanov growth. The CO 2 /Fe 3 O 4 (001) system differs in that the second layer forms completely, and island growth would have to remove CO 2 from this structure in a process similar to detwetting.…”

The promotional role of water in heterogeneous catalysis: mechanism insights from computational modeling

“…Water can exist in all three physical states of matter, but in chemistry, on the surface of catalysts, the states of water are diverse and dynamic, depending on the surface types and the thermodynamic conditions. On metal surfaces, both molecular water or water clusters and dissociated products of water (H and OH) were identified using scanning tunneling microscopy (STM) . At low temperatures, the intact water clusters ranging from monomer up to monolayer were detected on Ru(0001) surfaces, while at temperatures above 150 K, partially dissociated layers were formed, as shown in Figure , reflecting a temperature dependence of water states on metal surface.…”

“…On metal surfaces, both molecular water or water clusters and dissociated products of water (H and OH) were identified using scanning tunneling microscopy (STM) . At low temperatures, the intact water clusters ranging from monomer up to monolayer were detected on Ru(0001) surfaces, while at temperatures above 150 K, partially dissociated layers were formed, as shown in Figure , reflecting a temperature dependence of water states on metal surface. When water was absorbed on Ru(0001) with a small amount of carbon impurities, various surface species, including molecular H 2 O, H 2 O–C complexes, H, OH, O and CH species, were identified, indicating that the water adsorption, dissociation, and related reactions occurred on metal surfaces, while on relatively inert metal surfaces, such as Au surface, water mainly exits in its molecular state .…”

Probing equilibrium of molecular and deprotonated water on TiO 2 (110)

“…adsorbate dynamics | water | dissociative adsorption | titanium dioxide | kinetic barriers W ater is ubiquitous in the environment and, as such, the nature of its interactions with interfaces can determine the outcome of a broad range of processes that include wetting, dissolution, precipitation, phase transformation, corrosion, and catalytic and environmental reactions (1)(2)(3)(4)(5)(6)(7). In this regard, the relative stability of molecularly and dissociatively bound species can be of critical importance with the preferred configuration being controlled by many factors including surface structure, acid/base properties, defects, impurities, water coverage, and temperature (8)(9)(10)(11)(12)(13).…”