Synthesis And Gas-sensitive Mechanism Of Highly Active CoV₂O₆/MoS_2-xSe_x Heterojunction

Nitrogen dioxide（NO₂）is a toxic and harmful gas that can cause harm to the environment and life health.Its sources include various high temperature combustion and related industrial processes.In view of the great harm and wide distribution of NO₂,it is urgent to develop low-cost and portable NO₂ gas sensors,and resistive sensors have been widely studied and concerned.Among various gas sensitive materials,metal oxides have high sensitivity,but their operation usually requires high temperature conditions.Two-dimensional transition metal chalcogenides（TMDs 2D）materials have great room temperature sensing potential,but there is a huge gap in the response compared to metal oxides.In order to make up for the defects of each material,this paper combines multivalent bimetallic oxide CoV₂O₆ material with TMDs(Mo S₂ and alloying Mo S_2-xSe)nanomaterials to prepare CoV₂O₆/Mo S₂ and CoV₂O₆/Mo S_2-xSe_x heterojunction structures,and realizes high-performance NO₂ sensing at room temperature.In addition,in order to further improve the gas sensitivity of heterojunction,the selenization conversion synthesis and gas sensing properties of Mo S_2-xSe_x nanomaterials were also studied.The prepared Mo S_2-xSe_xnanomaterials exhibited higher gas sensing properties at room temperature.The specific research contents are as follows:（1）CoV₂O₆ nanomaterials with different morphologies have been prepared by a simple wet chemical method.In this part of the work,N,N-dimethylformamide（DMF）and methanol solvents were mixed with ultrapure water in a certain proportion to form different reaction solvents,and CoV₂O₆ nanomaterials with different morphologies were prepared.For enhance the performance of CoV₂O₆,the influences of growth time on the morphology and gas sensing properties of CoV₂O₆ were also studied.By adjusting the morphology,the optimal working temperature of the sensors can be reduced from 260℃to 100℃,which even has gas response at near room temperature（40℃）.The response of the best CoV₂O₆ nanomaterial to 50 ppm NO₂reached 459.6%.（2）In order to realize the gas sensing response at room temperature,CoV₂O₆/Mo S₂ and CoV₂O₆/Mo S_2-xSe_x heterostructures were prepared by hydrothermal method by combining CoV₂O₆ with Mo S₂ and Mo S_2-xSe_x nanomaterials,respectively.Both heterojunction structures achieve gas-sensing response at room temperature.Compared with CoV₂O₆/Mo S₂heterostructure,CoV₂O₆/Mo S_2-xSe_x heterostructure exhibits better gas sensing performance at room temperature.The response of the optimal CoV₂O₆/Mo S_2-xSe_x heterostructure to 50 ppm NO₂ reached 97.1%at room temperature.（3）2D materials usually generate strong quantum confinement effect and weak dielectric shielding effect,thereby reducing the lifetime of carriers and the performance of devices.The above defects can be effectively suppressed by introducing the built-in electric fields.In this work,a ternary alloyed Mo S_2-xSe_x nanocomposites has been prepared by converting few-layered Mo S₂nanomaterials through a selenization process.Owing to the local built-in electric field,the response,response/recovery rate,selectivity,repeatability and detection limit of Mo S_2-xSe_x at room temperature have been greatly improved.The best Mo S_2-xSe_x sensor has a response of 127.5%to 1 ppm NO₂,and its response to each concentration of NO₂ is 2-3 times that of the Mo S₂ sensor.The actual detection limit reaches 50 ppb,and the theoretical detection limit achieves 4.0 ppb.In summary,multivalent bimetallic oxide CoV₂O₆ nanomaterials can achieve gas sensing at near room temperature,and the combination of them with TMDs nanomaterials to prepare heterostructures has great room temperature sensing potential.Ternary alloyed TMDs have better development prospects than their binary counterparts in the field of gas sensing,which provides a new idea for the development of advanced gas sensing materials.