Revealing the H-bonding nature of water bilayers on Au(111)

•We have revealed the bonding nature of two contrasting H-bonding types existing in water bilayers on Au (111), with clear evidences for covalent and electrostatic components.•Regardless of the H-up or H-down bilayer, we found that the symmetric and asymmetric H-bonds are 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 are resulted 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.