Key role of antibonding electron transfer in bonding on solid surfaces

The description of the chemical bond between a solid surface and an atom or a molecule is the fundamental basis for understanding a broad range of scientific problems in heterogeneous catalysis, semiconductor device fabrication, and fuel cells. Widespread understandings are based on the molecular orbital theory and focused on the degree of filling of antibonding surface-adsorbate states that weaken bonding on surfaces. The unoccupied antibonding surface-adsorbate states are often tacitly assumed to be irrelevant. Here, we show that the unoccupancy of those antibonding surface-adsorbate states is related to antibonding electron transfer: Electrons that would occupy these antibonding states are transferred to the lower-energy Fermi level. This antibonding electron transfer leads to an energy gain, which plays a critically important role in controlling the trends of bond formation on surfaces that are often inhomogeneous and defective. This finding is illustrated from the first-principles study of hydrogen adsorption on ${\mathrm{MoS}}_{2}$ surfaces. A clear linear relationship between the energies of antibonding electron transfer and hydrogen adsorption is identified. Active sites of hydrogen evolution reaction on ${\mathrm{MoS}}_{2}$ are found to originate from the in-gap states induced by sulfur vacancies or edges. The emerging picture is general and suited for both metal and semiconductor surfaces. It also offers a physically different explanation for the well-known $d$-band model for hydrogen adsorption on transition-metal surfaces.