Hydrogen Evolution at the In Situ MoO₃/MoS₂ Heterojunctions Created by Nonthermal O₂ Plasma Treatment

Nonprecious two-dimensional materials, especially molybdenum sulfide (MoS₂), have gained considerable attention due to their unique catalytic activity and stability for the hydrogen evolution reaction (HER). However, the constraint of MoS₂ lies on the inactive basal planes, and the catalytic activity is limited to the active edge sites only. In this work, we are incorporating different defects into the multilayered MoS₂ nanoflakes by the nonthermal oxygen (O₂) plasma technique to improve its catalytic activity. A mild O₂ plasma technique has been used to introduce different concentrations of dopants to improve the HER of 2D-MoS₂ and to enhance the electron mobility of MoS₂ nanosheets for fast electron transfer. Oxygen has been incorporated in MoS₂ nanosheets which formed the nanocomposite material MoS_2–xO_x with sulfur-vacancies by controlled oxygen (O) plasma exposure. The best performance is obtained after irradiation of 30 s of O₂ plasma with an overpotential of 250 mV at 10 mA cm^–2. By employing density functional theory (DFT), it has also been shown that optimized insertion of oxygen in MoS₂ can influence the electron density of states, leading to the faster charge transfer and consequent increase in catalytic activity. Our experimental (XRD, XPS, TEM) and theoretical (DFT) calculations show that O-doped 2D-MoS₂ with S-edge termination exhibits superior activity and stability for the HER activity with low overpotential due to the fast electron transfer. The synergism between the optimal defect concentration along with the enhanced electron mobility increases the electrochemical activities in an acidic medium.