Research On In₂O₃-based Highly Selective Gas Sensor

Gas sensors are widely used in industrial detection,medical warning,environmental monitoring,and medical diagnosis because of their low price,simple fabrication process,high sensitivity,and good stability.Currently commercial metal oxide semiconductor gas sensors are often used in complex environments where multiple gases are present,and the selectivity of the gases restricts the practical application of metal oxide semiconductor gas sensors.In order to solve the shortcomings of low selectivity of metal-semiconductor gas sensors,previous researchers usually advocate to increase the specific surface by changing the morphology of gas-sensitive materials,to enhance the sensitivity by constructing heterojunctions or by adding catalysts,but neglect to investigate whether the electronic structure and crystal phase structure of the gas-sensitive materials themselves affect the gas-sensitive performance.In this paper,we systematically investigate how the electronic structure and crystalline structure of the gas-sensitive material In₂O₃affect the gas-sensitive performance based on a typical semiconductor metal oxide In₂O₃,and provide new ideas for the design of highly selective metal oxide semiconductor gas sensors.The main research of this paper is as follows.（1）A series of In₂O₃ graded spheres（HS）samples with different band gaps were prepared by varying the amount of urea in the reaction conditions,showing different sensing properties for formaldehyde and ethanol.The experiments showed that In₂O₃-0.05 HS exhibited excellent selectivity for formaldehyde at low temperature（180°C）.With the increase of the reaction solvent-urea,the selectivity of the sample for formaldehyde decreased,but the sensitivity to ethanol increased,indicating a clear connection between the band gap and the sensing performance.The UV-Vis absorption spectroscopy tests and the results of the reaction mechanism on different samples showed that the change in the amount of urea in the reaction conditions changed the energy gap,leading to a difference in the selectivity of the sensor for acetone and formaldehyde.（2）By preparing nanofibers with different morphologies（In₂O₃NF）and flower-like nanofibers（In₂O₃FNF）,we gained insight into how the different phase structures change the gas sensing performance.The experiments show that In₂O₃NF has good selectivity for 100 ppm acetone gas at 180°C,excellent sensing performance,as well as high sensitivity（72）and ultra-fast response time（1 s）.In contrast,the In₂O₃FNF showed significantly enhanced selectivity and gas sensing performance for formaldehyde gas at 180°C,with 14.3 times higher sensitivity than the In₂O₃NF.This gas sensing performance can be attributed to the difference in oxidation capacity of the chemisorbed oxygen species caused by the change in phase structure,which affects the difference in gas selectivity.（3）Constructing p-n heterojunctions is usually considered as an effective method to improve the gas sensing performance of nanomaterials.By preparing In₂O₃/Co₃O₄core/shell layered heterostructures（In₂O₃/Co₃O₄HHS）,we understand how the construction of p-n heterojunctions can improve the gas sensing performance by changing the Fermi energy level.It is shown that In₂O₃/Co₃O₄HHS has good selectivity,excellent gas-sensing performance and high response to formaldehyde gas at 180℃.In general,the low selectivity of gas sensors limits the development of gas sensors,and we are eager to find the important factors affecting the selectivity of gases and investigate the mechanism to eventually create highly selective materials.In this thesis,we demonstrate through three experiments that the variation of band gap,the phase structure of the sample and the Fermi energy level can indirectly lead to differences in the selectivity of the gas sensor for the gas,which provides new ideas for designing highly selective gas sensors.