The interaction of water with solid surfaces: fundamental aspects revisited

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Abstract

Water is perhaps the most important and most pervasive chemical on our planet. The influence of water permeates virtually all areas of biochemical, chemical and physical importance, and is especially evident in phenomena occurring at the interfaces of solid surfaces. Since 1987, when Thiel and Madey (TM) published their review titled ‘The interaction of water with solid surfaces: fundamental aspects’ in Surface Science Reports, there has been considerable progress made in further understanding the fundamental interactions of water with solid surfaces. In the decade and a half, the increased capability of surface scientists to probe at the molecular-level has resulted in more detailed information of the properties of water on progressively more complicated materials and under more stringent conditions. This progress in understanding the properties of water on solid surfaces is evident both in areas for which surface science methodology has traditionally been strong (catalysis and electronic materials) and also in new areas not traditionally studied by surface scientists such as electrochemistry, photoconversion, mineralogy, adhesion, sensors, atmospheric chemistry and tribology. Researchers in all these fields grapple with very basic questions regarding the interactions of water with solid surfaces such as how is water adsorbed, what are the chemical and electrostatic forces that constitute the adsorbed layer, how is water thermally or non-thermally activated and how do coadsorbates influence these properties of water. The attention paid to these and other fundamental questions in the past decade and a half has been immense. In this review, experimental studies published since the TM review are assimilated with those covered by TM to provide a current picture of the fundamental interactions of water with solid surfaces.

Introduction

The richness of the molecular properties of water and its appearance in so many interfacial systems make it one of the most interesting and important adsorbates. Depending on the system, adsorbed water may be present as a molecule or it may dissociate. It may become a cationic species by accepting a proton or it may become an anion by donating a proton. As an adsorbed molecule, it may bind to a surface through a variety of means such as electrostatics, charge transfer or hydrogen bonding. It has been detected on surfaces as monomers, dimers, larger multimers, 2D bilayers, 3D clusters and a variety of ice structures. Its molecular dipole readily responds to its environment such that orientations from hydrogen-end ‘up’ to hydrogen-end ‘down’ have been observed through manipulations of coverage, surface structure, coadsorbate or applied potential. The water molecule can be very inert on some substrates, while on others it readily oxidizes the surface and near-surface regions. In interacting with coadsorbates, it can behave in its most popular role as a solvent by hydrating/solvating its neighbors, it can react with its neighbors, or it can compete aggressively for adsorption sites by displacing its neighboring coadsorbates from the surface. The water molecule is also an excellent probe for studying substrate properties, such as catalytic reactivity, redox processes, site distributions, defect influence, adsorption capacity and electronic corrugation, among others. Cooperative effects, such as surface dissolution, etching and bulk-phase oxidation, result from the combined solvent and chemical properties of water at high pressures and as a liquid. These and many other interesting properties of water manifest themselves in a variety of important applied fields such as catalysis, electrochemistry, material science, electronic devices, photocatalysis and photoconversion, corrosion, geochemistry, adhesion, sensors, tribology, astrophysics and astrochemistry, and even membrane science.

The objective of this review is to highlight the many advancements in understanding the fundamental properties of adsorbed water that have taken place since the 1987 review by Thiel and Madey [1] (hereafter referred to as TM). Many of the unique properties of water as a molecule and as a solid/liquid/gaseous substance can be found in TM, and therefore are not reiterated in this review. Instead, this review will focus on the large body of literature published since 1987 on the interactions of water with well-ordered surfaces, drawing also from TM and papers reviewed therein. The restriction to well-ordered surfaces, which in most cases involves single-crystals, has two purposes. The first is that by starting with a basic understanding the structure of the solid surface one can better understand the properties of water as an adsorbate. The second reason, which is perhaps more practical, is that a water review that included all solid surfaces (single-crystals as well as polycrystallines, powders, amorphous materials, etc.) would be far too extensive based on the large number of water–single-crystal studies published since the TM review (see Appendix A). A final restriction to this review is that only experimental studies will be the focus. Although theoretical studies of the water–solid surface interaction provide invaluable insights that are often beyond the grasp of many experimental approaches, inclusion of these studies would also render a review that is unmanageably large given the number of theoretical studies published since the TM review. Nevertheless, discussion has been included at points in this review on selected theoretical studies that address particular experimental issues.

The structure of the review is based on four general categories. The first three categories, molecular water, dissociated water and coadsorbed water, involve studies under ultrahigh vacuum (UHV) conditions in which the integrity of the water–surface interaction is critically controlled and a large set of experimental methods are available to the researcher. The fourth category involves the interactions of water with surfaces under non-UHV conditions. Improvements in in situ characterization since the TM review have enabled researchers to go beyond the restrictions of UHV and probe molecular-level processes of water at high pressures (relative to UHV) and under aqueous conditions.

Section snippets

Molecularly adsorbed water

Literature on the molecular properties of water on solid surfaces is extremely vast, encompassing many disciplines and a large number of material systems. For the purposes of this review, the topic of molecularly adsorbed water has been broken into five broad categories. The first four topics relate, more or less, to substrate-independent physical properties of molecularly adsorbed water, namely its electronic structure, its vibrational properties, its tendency to form local and/or long-range

Dissociation of water on surfaces

A major objective in studying water–surface interactions is determining whether water is molecularly versus dissociatively adsorbed because the presence or absence of dissociation has significant implications for many chemical processes (see TM and other reviews [67], [370], [371], [372], [373]). For example, water dissociation is important in the surface and bulk oxidation many materials. The chemical reactivities of the water dissociation products (OH, H and O) are very different from that of

Coadsorption of water

In virtually all cases in which water–solid surface interactions are important, the encounter between molecule and surface does not occur without the presence of many spectators in the form of coadsorbed species. These coadsorbates can be active or passive participants in the chemistry of the adlayer, and water may influence their surface properties with the same likelihood that they influence water. Coadsorption effects are often manifested in terms of site blocking or displacement, enhance

Liquid or high-pressure water in contact with surfaces

One of the major advancements since the TM review in the study of the interaction of water with solid surfaces has been the ability of researchers to study surfaces under non-UHV conditions using techniques such as scanning probe microscopy and IRAS. Such efforts allow for examination of a wide variety of phenomena (such as bulk hydroxylation, surface dissolution, electrochemical charge transfer or ion solvation) that can at best only be modeled under UHV conditions. In this section, non-UHV

Conclusions and future directions

The importance of the fundamental interactions of molecular water with solid surfaces to many fields is evident from the quantity and diversity of work published on the subject since the TM review. This volume of work also reflects the success with which researchers have had in characterizing the molecular-level properties of adsorbed water. Traditional surface science techniques (such as XPS, UPS, LEED, HREELS, IRAS and TPD) continue to be used to reveal new insights into the water–surface

Acknowledgements

The author wishes to thank the many authors who responded to requests for reprints and preprints of their works, as well as those authors who provided Figures and other information for this review. The author wishes to personally thank Bruce Bunker, Scott Chambers, Jim Cowin, John Daschbach, Zdenek Dohnalek, Bill Epling, Greg Exarhos, Kim Ferris, Greg Herman, Steve Joyce, Bruce Kay, John LaFemina, Thom Orlando, Chuck Peden, Craig Perkins, Kevin Rosso, Jim Rustad and Li-Qiong Wang for fruitful

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