Photocatalytic activity of MoS₂ with water monolayers: Global optimization

Atomically thin MoS₂ has emerged to be promising for photocatalytic water splitting benefiting from its suitable geometrical and electronic structure for light harvesting. A better understanding of how water molecules affect the band edge levels of MoS₂ is critical for promoting the interfacial reactivity. Here, we determine the structures of water monolayers on MoS₂ using global optimizations achieved by molecular dynamics in combination with local minimization. It is shown that cyclic water clusters are formed on a surface through a hydrogen-bonding network. The absolute band edge positions are explored taking into account the derivative discontinuity of the exchange-correlation functional. Shifts in band edges are observed with the increase in H₂O coverage, while bandgaps tend to be slightly decreased. In particular, the band alignment relative to water redox potentials has been investigated in detail. We find that the dimer configuration is likely to suppress the hydrogen evolution reaction (HER), while the polygon clusters lift the conduction band by 0.2-0.7 eV, and thus, they would enhance HER. This effect is explained in terms of the linear dependence of the band edge offset on an interface electric dipole arising from water assemblies.