QuasiparticleEnergy,Electron–Hole Excitation,and Optical Property in Oxygen-Substituted MoS₂ Monolayers

Atomically thin transition metal dichalcogenides (TMDs) have attracted widespread attention due to their unique optical, electronic, and catalytic properties. During their synthesis, unavoidable oxygen incorporation is believed to be related to enhanced hydrogen evolution photocatalysis and reduced hydrogen adsorption free energy. It is noted that in two-dimensional (2D) materials, reduced dielectric screening will significantly strengthen electron–electron and electron–hole interactions. This makes predictions and understanding of quasiparticle and excitonic properties inaccessible in oxygen-substituted MoS₂. In this work, we employ a first-principles density-functional theory and GW-Bethe-Salpeter equation (GW-BSE) approach to investigate the quasiparticle electronic structure, excitonic effects, and optical properties of oxygen-substituted MoS₂ monolayers. We find a direct-to-indirect band gap transition induced by oxygen substitution. The global quasiparticle band gap stays at 2.66 eV up to 5.56% doping, drops sharply to 2.00 eV at 50%, and then stabilizes. The optical gap remains 2.03 eV at low doping, rises to 2.19 eV at 50%, and then fluctuates moderately. On the other hand, band edges approach redox potentials with increasing oxygen concentrations (below 12.5%) compared with the pristine MoS₂ monolayer. Such evolution can suppress carrier recombination and could be related to enhanced hydrogen/oxygen evolution photocatalysis. Our findings offer insights into the role of oxygen in the excited-state properties of 2D TMDs and guide the design of efficient photocatalytic water-splitting systems.