Preparation By Pyrolyzation Of Metallic Organic Frameworks And Gas-sensing Properties Of MoO₃-based Composites

With the rapid development of modern science and technology,the problem of air pollution is becoming more and more serious.Among them,volatile organic compounds（VOCs）are common indoor and outdoor pollutants,which cause harm to the environment and human physiological system.Therefore,the development of gas sensors with high sensitivity and selectivity is of great significance to environmental protection and human health.In this paper,Mo O₃materials were prepared by pyrolysis metal organic framework method.The effects of calcination temperature on the morphology and properties of the materials were investigated,and its formation mechanism was explored.On this basis,Co₃O₄-Mo O₃ and Ni O-Mo O₃ composites were synthesized by constructing p-n heterostructures.The results show that Mo O₃ and its composites prepared in this paper have good gas sensing properties to triethylamine.The specific research work is as follows:1.Mo O₃ materials with different calcination temperatures were successfully prepared by pyrolytic metal organic skeleton method with ammonium molybdate[（NH₄）₉Mo₇O₂₄·6H₂O]as raw material and 2-methylimidazole（C₄H₆N₂）as template.The effects of calcination temperature on its morphology and gas sensing properties and its formation mechanism were explored.The results show that the calcination temperature has a great influence on the morphology and gas sensing properties of the material.When the calcination temperature is300～500℃,the synthesized Mo O₃ is a nano sheet with a size of about 300 nm and a thickness of 60 nm.When the calcination temperature is 600～700℃,the synthesized Mo O₃ has a lamellar structure with a size of about 10～100 um.When the calcination temperature is 400℃and the working temperature is 350℃,the material has short response time to 100 ppm triethylamine,strong stability and high sensitivity of 409.5.2.In order to improve the sensitivity of Mo O₃ to low concentration triethylamine gas,p-type Ni O and n-type Mo O₃ were compounded to prepare Ni O-Mo O₃ composites with p-n heterojunction.The effects of calcination temperature and molar ratio of Mo and Ni on the gas sensing properties of the materials were studied.The results show that when the working temperature is300℃,the calcination temperature is 400℃,and the molar ratio of Mo and Ni is 9:1,the sensitivity of Ni O-Mo O₃ composite to 100 ppm triethylamine is 1226,which is 20.4 times that of pure Mo O₃ at 300℃,and its sensitivity to 1 ppm triethylamine is 13.06,which significantly improves its gas sensing response to low concentration triethylamine gas.The enhancement of gas sensing performance is due to the formation of a p-n heterojunction at the interface between Ni O and Mo O₃.3.In order to further improve the sensitivity of Mo O₃ to low concentration triethylamine gas and reduce its working temperature,Co³⁺with catalytic effect was introduced to prepare Co₃O₄-Mo O₃ composite.The effects of molar ratio of Mo and Co and calcination temperature on the structure and gas sensing characteristics of the material were investigated.It is found that when the molar ratio of Mo and Co is 28:1 at the calcination temperature of 400℃,the Co₃O₄-Mo O₃ composite shows superior gas sensing performance at the working temperature of 250℃.Its sensitivity to 100 ppm triethylamine is 1063,which is26 times that of pure Mo O₃ at 250℃（40.9）,and the sensitivity to 1 ppm triethylamine is 9.64.The gas sensing performance of low concentration triethylamine is significantly enhanced.Compared with Mo O₃,the optimum working temperature of Co₃O₄-Mo O₃ composite is reduced by 100℃.It provides a way to detect low concentration triethylamine gas.This is because p-n heterojunction is formed at the interface after Mo O₃ and Co₃O₄ are combined,and Co³⁺has excellent catalytic properties,which can well convert oxygen molecules into adsorbed oxygen,and then have redox reaction with triethylamine.