Enhanced Performances of PbS Quantum-Dots-Modified MoS<sub>2</sub> Composite for NO<sub>2</sub> Detection at Room Temperature

The modification of the material surface by the second-phase particles enables the electron interaction on the Fermi level or the energy band between different materials, which can achieve the improvement of gas-sensing properties. Herein, a novel composite of PbS quantum-dots-modified MoS<sub>2</sub> (MoS<sub>2</sub>/PbS) is synthesized by combination of hydrothermal method with chemical precipitation and fabricated into the gas sensor to investigate its enhanced gas-sensing properties caused by the modification of PbS quantum dots at room temperature. It is found that the responsivity of MoS<sub>2</sub>/PbS is obviously higher than that of pure MoS<sub>2</sub> gas sensor throughout the whole test range, and MoS<sub>2</sub>/PbS gas sensor has better selectivity compared with pure MoS<sub>2</sub> gas sensor at room temperature. The response of MoS<sub>2</sub>/PbS gas sensor is about 50 times higher than that of MoS<sub>2</sub> gas sensor at 100 ppm NO<sub>2</sub> concentration. The recovery behavior is greatly improved, and the resistance of MoS<sub>2</sub>/PbS gas sensor can return completely with almost no drift (the recovery ratio is more than 99%). The enhanced gas-sensing properties of MoS<sub>2</sub>/PbS, which are superior to those of pure MoS<sub>2</sub>, are ascribed to the large surface area of MoS<sub>2</sub> combined with the high responsivity of PbS quantum dots for NO<sub>2</sub>. The formation of heterojunctions leads to the competitive adsorption of the target gases, which can prevent MoS<sub>2</sub> from being oxidized, further improving the stability of gas sensor. Furthermore, to profoundly discuss the enhanced performances and the sensing mechanism, the molecular models of adsorption systems are constructed to calculate the adsorption energies and the diffusion characters of NO<sub>2</sub> via density functional theory. We expect that our work can offer a useful guideline for enhancing the gas-sensing properties at room temperature.