? 2023Understanding H-bonded water on metal surfaces is crucial for electrocatalysis. Using density functional theory calculations, herein, we have revealed the bonding nature of two contrasting H-bonding types existing in water bilayers on Au (111), with clear evidence for covalent and electrostatic components. Regardless of the H-up or H-down bilayer, we found that the symmetric and asymmetric H-bonds respectively resulted from the shallow (3a1) and deep (1b2) orbital splitting in hybridizations with 1b2 and 3a1 orbitals themselves. We have revealed that all H-bonding types originate from complete σ resonances, with both the bonding and antibonding H-bonding states populated at several energy levels. The highest principle states are generally found to be dominated by lone-pair electrons of the proton acceptors for both the symmetric and asymmetric H-bonds. For all H-bonding types, their electrostatic interactions result from alternating positive and negative regions in their charge redistribution. In comparison to the H-up bilayer, the enhanced adsorbate-metal interactions in the H-down bilayer make its H-bonds more electrostatic with more significant electron pairing. The knowledge of H-bonding types in water bilayers would have advanced our current understanding of H-bonding interactions beyond the ordered water?hydroxyl phase on metal surfaces (Materials Today Advances, 2021, 12, 100,172) and pointed to the importance of manipulating H-bonds at electronic levels.
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