ZnO-MoS₂ nano-composites with excellent light-activated NO₂ gas sensitivity and MB photocatalytic degradation efficiency

Effective monitoring of toxic and harmful gases and rapid degradation of organic pollutants are essential to reduce the hazards of air and water pollution. In this study, the MoS₂ nanosheets prepared by hydrothermal method were coupled into the ZnO nanoparticles prepared by sol-gel method to form ZnO-MoS₂ nano-composites via a facile ultrasonic chemical route. The structure, morphology and surface chemical component of synthesized materials were characterized by XRD, SEM, TEM and XPS. The characterizations show that multilayer MoS₂ nanosheets are well dispersed among ZnO nanoparticles, and ZnO-MoS₂ composites have good crystallinity and abundant surface defects. The photoelectric properties were explored by UV-vis diffuse reflectance spectrum, photoluminescence spectra (PL) and surface photovoltage spectra (SPV). The results reveal that the formation of ZnO-MoS₂ heterostructure improves the utilization of light and promotes the effective separation of photo-carriers. The UV light-activated gas sensitivity test using NO₂ as the target gas preformed at room temperature saw that the prepared ZnO-MoS₂ gas sensor exhibited excellent gas sensing properties with good sensitivity, recoverability, stability and selectivity, which could effectively respond to low concentration NO₂. The response of the optimized ZnO-MoS₂ sensor with 5wt%MoS₂ to 0.47 mg/m³ NO₂ reached 19.6%. Meanwhile, the gas sensing performance was found to be greatly influenced by the adsorption of O₂ molecule on the surface of the materials, and ZnO-MoS₂ gas sensor possessed much higher gas sensitivity to NO₂ under oxygen free conditions. In addition, the photocatalytic degradation of methylene blue (MB) under simulated sunlight reveal that the ZnO-MoS₂ composites can rapidly remove the high concentration of MB (15 mg/L) in aqueous solution within 40 min by combined action of adsorption and photocatalysis, thereinto, the ZnO-MoS₂ sample with 10wt%MoS₂ shows a reaction rate constant as high as 0.032 min⁻¹. Mechanism analysis shows that the improvement of gas sensing and photocatalytic performance of ZnO-MoS₂ composites mainly attribute to the better absorbability of MoS₂ and the promotion of photocarrier separation rate caused by combination.