Preparation Of Co₃O₄-based P-n Heterojunction Composites And Gas Sensitivity To Low-concentration Formaldehyde

Formaldehyde as a class 1 carcinogen seriously endangers people’s health.Formaldehyde sensors have become an essential tool for indoor air quality monitoring and industrial gas leak detection.It is of great practical significance to develop a highly sensitive formaldehyde sensor with fast response/recovery,low cost,low energy consumption,long service life and low detection limit.Metal-oxide-semiconductor（MOS）gas sensors have become one of the most popular gas sensing materials due to their portability,simple operation,wide variety,excellent performance and low cost.Cobalt tetroxide（Co₃O₄）,as a representative p-type MOS,has received extensive attention in the field of gas sensing.However,a single MOS material has disadvantages such as low sensitivity,high operating temperature,poor selectivity and high detection limit.How to strengthen the gas-sensing characteristics of MOS and improve the comprehensive performance of MOS gas-sensing sensors has become a key problem for researchers to solve urgently.The construction of p-n heterojunction can improve the amount of adsorbed oxygen and electrical properties on the surface of the material,so it becomes one of the important ways to strengthen MOS sensors.In this paper,Co₃O₄ with catalytic properties is used as the host material,and Co₃O₄-based p-n heterojunction composites are designed and synthesized by a two-step hydrothermal method and subsequent calcination method.Three different n-type metal oxides of Zn O,Fe₂O₃ and In₂O₃ are used.The semiconductors were compound modified p-type semiconductor Co₃O₄ to explore its gas-sensing properties and gas-sensing strengthening mechanism.The specific research work is as follows:（1）A series of Zn O-Co₃O₄ composites with different Zn/Co ratios were prepared by a two-step method,and their sensing properties to formaldehyde were systematically studied.The effects of reaction temperature,reaction time,calcination temperature and different molar ratios on the gas sensitivity of the material were investigated.The research results show that the Zn O-Co₃O₄composites prepared at a Zn/Co molar ratio of 1:2,a reaction temperature of 90℃,a reaction time of 10 h,and a calcination temperature of 600℃can resist 50 ppm formaldehyde gas at a working temperature of 170℃.The sensitivity can reach96,the response time is 80 s,the recovery time is 128 s,the sensitivity is 2 times that of pure Co₃O₄ material,and 48 times that of pure Zn O material,and the response value of Zn O-Co₃O₄ composite material to 0.1 ppm formaldehyde gas is2.4,and the response value is 2.4.The time is 3 s.Through a series of characterizations,it is found that the enhancement of gas sensing performance is attributed to the formation of p-n heterojunction after recombination and the increase of specific surface area,more surface defects and lower built-in resistance to improve the charge transfer ability,thereby effectively improving the gas sensing performance.（2）Co₃O₄ was modified by Fe₂O₃ with catalytic activity,and a series of Fe₂O₃-Co₃O₄ composites were synthesized by a two-step method.The effects of reaction time,calcination temperature and different molar ratios on the gas sensitivity of the material were investigated.The research results show that Fe₂O₃-Co₃O₄ composites obtained at a Fe/Co molar ratio of 1:5,a reaction temperature of 90℃,a reaction time of 12 h,and a calcination temperature of 400℃can resist 50 ppm formaldehyde gas at a working temperature of 170℃.The sensitivity can reach 110,the response time is 27 s,the recovery time is 134 s,the sensitivity is 20 times that of pure Co₃O₄ material,27.5 times that of pure Fe₂O₃material,and the response value of Fe₂O₃-Co₃O₄ composite material to 0.1 ppm formaldehyde gas is 2.8,The response time is 2.5 s.Through a series of characterizations,it is found that the composite material has a larger specific surface area,more surface defects and a lower forbidden band width,and the heterojunction effect can effectively improve the gas sensing performance of the material.（3）Using n-type semiconductor In₂O₃ and p-type semiconductor Co₃O₄ for compounding,a series of In₂O₃-Co₃O₄ compound materials are prepared by a two-step method.The effects of calcination temperature and different molar ratios on the gas sensitivity of the material were investigated.The research results show that the In₂O₃-Co₃O₄ composites synthesized at a molar ratio of In/Co of 1:2,a reaction temperature of 90℃,a reaction time of 12 h,and a calcination temperature of 500℃can resist 50 ppm formaldehyde gas at a working temperature of 130℃.The sensitivity can reach 194,the response time is 30 s,the sensitivity is 15.6 times that of pure Co₃O₄ material,and 45.8 times that of pure In₂O₃ material,and the response value of In₂O₃-Co₃O₄ composite material to0.05 ppm formaldehyde gas is 12,and the response time is 20 s.Through a series of characterizations,it was found that the addition of In₂O₃ significantly increased the specific surface area of the material to provide more active sites and facilitate the adsorption of gas molecules.At the same time,the construction of the p-n heterostructure expanded the interfacial effect,thereby achieving a substantial improvement in gas sensing performance.