Understanding the role of vacancy-vacancy interaction and hydrogen-hydrogen coupling in hydrogen bonding on MoS₂ surfaces

MoS₂ is a promising nonprecious electrochemical catalyst for catalyzing hydrogen production from water. It has been found that (i) the catalytic reaction activity depends on the concentration of sulfur vacancies, and (ii) the free energy of hydrogen adsorption varies with the coverage of hydrogen adsorbates. However, microscopically, the physical factors responsible for such finding remain unclear. In this talk, we will present a microscopic model to understand the effects of vacancy-vacancy interaction and hydrogen-hydrogen coupling. We find that the effect of vacancy-vacancy interaction on hydrogen-surface bonding becomes considerable only when they are separated by less than 1.1 nm. When one hydrogen atom is adsorbed at a sulfur vacancy, the interaction between this sulfur vacancy and others nearby will be largely disabled. Such disabling decreases the adsorption energy and hence strengthens the binding. Similarly, the hydrogen-hydrogen interaction also becomes important only when they are close enough. However, instead of being disabled, such interaction is introduced upon hydrogen adsorption, which increases the adsorption energy and weakens the binding. The generation of those results will also be briefly discussed.