First‐Principles Insights into Asymmetric Surface Reactivity of Janus <scp>WSTe</scp> Monolayers

ABSTRACT The intrinsic asymmetric surface reactivity of Janus WSTe monolayers is investigated using first‐principles calculations. The broken inversion symmetry generates a macroscopic polarization that manifests as an effective built‐in electric field of approximately 0.08 V/Å, driving spatial separation of frontier molecular orbitals and anisotropic orbital hybridization. The Te‐terminated surface exhibits a higher electrophilic Fukui function of 0.18 e/Å 3 , while the S‐terminated surface shows a higher nucleophilic Fukui function of 0.15 e/Å 3 . ELF analysis reveals diffuse delocalized electron basins at the Te‐terminated surface and compact localized basins at the S‐terminated surface. Hydrogen chemisorbs strongly at the S‐terminated surface with an adsorption energy of −2.33 eV, whereas carbon monoxide exhibits only weak physisorption. The preferential H adsorption at the S‐terminated surface correlates with its higher nucleophilic Fukui function, though the detailed orbital matching mechanism involves both the electron‐accepting character of the S site and the radical reactivity of the H atom. These results establish a quantum mechanical correlation among structural asymmetry, electronic structure, and surface chemical reactivity.