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Wetting by water is ubiquitous on solid surfaces in nature and technology. The degree of wetting, wettability, depends on the physical and chemical nature of the surface, whether it is hydrophilic or hydrophobic. The wettability of a metal surface has a strong influence on interfacial chemistry in electrocatalysis and atmospheric corrosion in humid environments. The wettability of a surface has traditionally been characterized by the contact-angle of water droplets at ambient conditions, which gives little microscopic insight into the interaction of water with surfaces. In contrast, surface science studies in ultra-high vacuum (UHV) and at low temperatures have provided detailed information on the interaction of water with surfaces at a molecular level. Yet most processes of interest in real systems take place at ambient or higher pressures and elevated temperatures. Hence it is very important to carry out molecular-level studies of the wetting water layer on metals under realistic ambient conditions. Here we have investigated the wetting of water on two different Cu surfaces of (110) and (111) orientations at near ambient conditions (p(H2O)= 1 Torr, T= 295 K), using an in-situ x-ray photoemission spectroscopy.
We report strikingly different wetting properties of Cu(110) and Cu(111) at near ambient conditions of water pressure (1 Torr) and temperature (295 K); the Cu(110) surface is covered to saturation with a mixed OH and H2O layer (wetting), while the Cu(111) surface remains clean and adsorbate-free (non-wetting). We show that wetting is controlled by the presence of OH groups on the surface that stabilize water molecules via a strong H-bonding interaction. The different wettability on two Cu surfaces is thus attributed to a lower activation barrier for water dissociation on Cu(110) compared to Cu(111), which facilitates the hydroxylation of the Cu(110) surface. We also show that wetting properties can be substantially modified by controlling the formation of OH groups using preadsorbed oxygen. Our results clearly indicate that water chemistry, through the formation of OH groups, plays a decisive role in the wetting of metal surfaces at near ambient conditions.
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