Study On Synthesis And Gas-sensitive Properties Of ZIF-8/ZIF-71@?-MoO₃ Materials

As a typical N-type semiconductor material with wide band gap,the research and application of?-MoO₃ material in gas sensor field is still in the research stage,compared with ZnO and SnO₂ sensors.Existing studies have shown that?-MoO₃ gas sensors still have shortcomings such as general sensitivity,high operating temperature,long recovery time,and so on.In order to improve its performance,people have carried out a lot of and in-depth exploration,such as adjusting the structure and morphology of materials,doping heterogeneous elements,and constructing semiconductor heterojunctions.These methods have greatly improved the sensitivity of the sensor,but the improvement in selectivity is far from enough.By constructing the ZIF-8/71@?-MoO₃ core-shell structure,the selectivity of the?-MoO₃ gas sensor can be effectively improved by the molecular sieving effect and pre-concentration effect of the ZIF-8/71 pore size.This work takes?-MoO₃ as the research object.In order to improve the performance of the?-Mo O3 sensor,the ZIF-8/71 film coated on the surface of?-MoO₃ nanosheets was explored as a molecular sieve barrier.The main research contents are as follows:1.With the help of TiO₂ seed layer assisted growth method,?-MoO₃ nanosheets are directly grown on ceramic tubes,and different types of surfactants are added to adjust the morphology of?-MoO₃ nanosheets.The gas-sensing performance data show that the presence of surfactants will affect the sensor performance in different degrees.CTAB promotes most of the?-MoO₃ to be converted to h-Mo O₃,resulting in the?-MoO₃-CTAB sensor being unresponsive to the test gas.While the?-MoO₃-PEG4000 and?-MoO₃-DBS sensor are responsive to the test gas,their performance is far inferior to that of the?-MoO₃ sensor without surfactant.The performance of the?-MoO₃ sensor without any surfactant is relatively excellent.When the working temperature is 220^oC,its response to 50 ppm TEA and NH₃ is about 250 and17 respectively.However,the outstanding response of the?-MoO₃ sensor to TEA severely limits its use for monitoring other gases.2.The strategy of coating ZIF-8 on?-MoO₃ nanosheets surface as a gas screening barrier was proposed and proved.The optimal synthesis conditions of ZIF-8@?-Mo O3 composite materials were determined by adjusting the conditions of hydrothermal growth temperature,growth time,growth solution concentration and so on.On this basis,different solvents were used as template agents to regulate the pore size of ZIF-8.The results show that the ZIF-8@?-MoO₃(H₂O)sensor has a good response(35)to 50 ppm NH₃ at 280?,which is about 7 times that of the?-MoO₃ sensor.At the same time,when the interfering gas TEA exists,the selectivity of ZIF-8@?-MoO₃(H₂O)sensor for NH₃ is increased from S_AB=0.15(?-MoO₃ sensor)to S_AB=4.3,which is nearly improved 28 times.Therefore,the ZIF-8@?-MoO₃(H₂O)sensor is capable of eliminating false positive or interference signals to detect NH₃.3.ZIF-71 with larger pore size and more prominent hydrophobicity was chosen to coat?-MoO₃ nanosheets.After synthesizing ZIF-71@?-MoO₃ composite material,the influence of different solvents on its structure and properties was studied.Compared with other solvents,the ideal ZIF-71 film can be obtained using methanol as the solvent.The results show that the?-MoO₃ sensor has little selectivity(0.22)to NH₃ in the presence of TEA gas at 280?.The selectivity(3.89)and sensitivity(36)of the ZIF-71@?-MoO₃(Methanol)sensor to 50 ppm NH₃are better than that of the?-MoO₃ sensor,and the influence of environmental humidity on it is also reduced.The improvement of gas sensitivity is attributed to the molecular sieving effect and pre-concentration effect of the ZIF-71 film.