In-Situ Generation of Nitrogen-Doped MoS₂ Quantum Dots Using Laser Ablation in Cryogenic Medium for Hydrogen Evolution Reaction

Here, nitrogen doped molybdenum disulfide quantum dots (N-MoS₂ QDs) are fabricated by making use of the pulsed laser ablation (PLA) process in liquid nitrogen (LN₂) as a dopant agent. In fact, LN₂ contributes the rapid condensation of the plasma plume to form MoS₂ QDs, optimizing the conditions for the synthesis of N-doped MoS₂ with p-type property. The structural/optical features of the synthesized products are studied using transmission electron microscopy (TEM), absorption spectroscopy, photoluminescence (PL) spectroscopy techniques, and X-ray photoelectron spectroscopy (XPS). The TEM image shows the creation of MoS₂ QDs with 5.5 nm average size. UV-vis and PL spectroscopy confirm the formation of N-MoS₂ QDs according to the dominant peaks. The Tuck plot gives a direct band-gap of 4.34 eV for MoS₂ QDs. Furthermore, XPS spectroscopy reveals Mo-N bonding, indicating nitrogen doping as evidence of p-type MoS₂ QDs. Thus, PLA provides a single-stage way to the clean and green synthesis of the MoS₂ QDs suspension without a need for high vacuum devices and additional chemical components. Regarding the pristine MoS₂, the N-MoS₂ QDs benefit from a low overpotential of −0.35 V at −10 mA/cm² per µg alongside a low Tafel slope of 300 mV/dec. Subsequently, the lower R_ct value of N-MoS₂ QDs verifies the enhancement of the charge transfer kinetics mainly due to the elevated electronic conductivity. Furthermore, the quasi-rectangular cyclic voltammetry (CV) as well as the larger current window demonstrate a notable electrocatalytic activity. The former is based on the enhanced active sites in favor of N-MoS₂ QDs against other samples of interest. Thereby, it is discovered that the N-doped MoS₂ QD acts as an effective catalyst to notably improve the performance of the hydrogen evolution reaction (HER).