Humidity-Sensing Performance Of CeO₂/Ionic Liquid Hybrid Materials

With the rapid development of science and technology,humidity sensor has been widely used in many fields,such as meteorology,industrial production,environmental monitoring and so on.The fast response humidity sensor is of great significance in real-time measurement.For example,the measurement of respiratory frequency in medical,normal adults breathe 16-20 times per minute,so the sensor response time and recovery time must be less than 3 s,to detect the respiratory frequency with respiratory disorder,needs higher requirements;the national standard material research center believes that "For the relative humidity measuring instrument,the accuracy is still very difficult to achieve 2%RH",high sensitivity can provide higher precision,is conducive to control humidity.The response time of ordinary humidity sensor is 8s-30s,and the measurement precision is 5%RH,which cannot meet the above requirements.Therefore,the development of low-cost high-performance humidity sensor needs is becoming more and more urgent.The metal oxide semiconductor humidity sensor has many advantages such as high sensitivity,low cost and long service life,which can further improve the humidity sensing performance.As one of the most important rare earth oxides,CeO₂（Cerium Oxide）has been widely used in many fields,such as hydrogen storage materials,fuel cells,gas sensors and so on.In the literature,it has the disadvantages of low sensitivity and long response time.This paper chooses CeO₂ as the base material,by decreasing the particle size and ionic liquid composite two aspects to enhance the humidity sensitive properties of CeO₂,using complex impedance analysis method,combined with the equivalent circuit model,using Zview the simulation software,to explain its humidity sensitive mechanism.Pure CeO₂ nanoparticles were synthesized by hydrothermal method.Under the optimum operating frequency of 1kHz,when the humidity changes from 11RH%to 97%RH,the sensitivity reaches 4 orders of magnitude,the humidity hysteresis is about 2%,and response time and recovery time are 10s and 3s,respectively.TheCeO₂ materials with small particles have a larger specific surface area,which is benefit for the adsorption of water molecules,so it has the characteristics of high sensitivity.The change of the complex impedance curve from low humidity to high humidity has experienced a process from a semicircle to a half circle connecting a line segment and then gradually disappearing into a half circle.The mechanism of the humidity sensing is analyzed.At low humidity,the ionization and polarization of CeO₂ particles play an important role in the conductance,while the ionization and polarization of the adsorbed water molecules play an important role in the conductance at high humidity.Synthesis of CeO₂ with high hydrophilic ionic liquid（[Bmim]Br:1-butyl-3-methyl imidazole bromide）composite material by ion thermal method.Under the optimum operating frequency of 100Hz,when the humidity changes from 11RH%to 97%RH,the sensitivity reaches 3 orders of magnitude,the humidity hysteresis is about 1%,response time and recovery time are 4s and Is,respectively.Compared with the pure Ce02 nanoparticles,the response time and recovery time are significantly improved,and remained stable in 2 weeks.The main reason is that[Bmim]Br has a high ionic conductivity,hydrophilic and structural orientation,it promotes the ion diffusion,so that the sensor has the characteristics of rapid response.Compared with the pure CeO₂ nanoparticles,the[Bmim]Br/CeO₂ composite nanoparticles can form a continuous and uniform water layer in a low humidity environment.Synthesis of CeO₂ with low hydrophilic ionic liquid([C₁₆mim]Br:1-sixteen alkyl-3-methyl imidazole bromide)composite material by ion thermal method.Under the optimum operating frequency of 100Hz,when the humidity changes from 11RH%to 97%RH,the sensitivity reaches 3 orders of magnitude,the humidity hysteresis is about 2%,response and recovery time are 8s and Is,respectively.The performance was better than that of CeO₂ nanoparticles with the same size,but slightly less than that of[Bmim]Br/CeO₂ composite nanoparticles.The main reason is that the hydrophilic phase of[Ci6mim]Br/CeO₂ is weaker than that of[Bmim]Br/CeO₂ composite nanoparticles,so it is difficult to improve the humidity sensitive properties of the material.Similar changes of the whole complex impedance characteristic curve of[C₁₆mim]Br/CeO₂ composite trend and[Bmim]Br/CeO₂ composite;humidity sensitive mechanism it is with the[Bmim]Br/CeO₂ composite materials are roughly the same,the electron ion conductive type.