Study On The Performance Of Humidity Sensor Based On The Solid Solution Nanofibers

Humidity is an important environmental parameter.The monitoring and control of humidity plays an important role in agriculture,industry,medical care,food preservation and manufacturing industries.In order to meet the growing high requirements for humidity sensors in an intelligent society,there is an urgent need to develop high-performance humidity sensors with high sensitivity,good stability and repeatability,fast response-recovery rate and low cost.Humidity sensitive material is the key factor affecting the performance of the humidity sensor.There are a variety of humidity sensitive materials such as electrolytes,organic polymers and metal oxides etc.,have been developed.Among them,metal oxide nanomaterials attract much attention due to their strong resistance to chemical attack,high thermal stability,small size and large specific surface area.However,single metal oxide cannot meet the requirement of the high-performance humidity sensor,which significantly limits its application in the field of humidity sensing.In view of the above problems,two kinds of hydrophilic metal oxides ZrO₂ and TiO₂ are selected as humidity sensitive materials in this paper.By constructing one-dimensional morphologies combined with doping（substitutional and interstitial）modification,four humidity sensors based on the solid solution nanofibers are developed.Their sensing mechanisms are also investigated systematically.The main contents of this research are as follows:（1）Aiming at the problem of low sensitivity in the ZrO₂-based humidity sensor,a doping modification scheme is proposed,by which the Ti element with the same valence state as zirconium is introduced（substitutional mode）.Ti_xZr_1-xO₂（x=0.0-0.4）nanofibers are prepared by sol-gel method combined with electrospinning technology.The effects of different doping ratios on the microstructural parameters of ZrO₂nanofibers are analyzed through characterization techniques.It is demonstrated that Ti⁴⁺enters ZrO₂ lattice in substitutional mode to form solid solution nanofibers.The performance test results show that the humidity sensor based on Ti_0.1Zr_0.9O₂ solid solution nanofiber has the best humidity sensing performance.The impedance decreases by four orders of magnitude at 11%-95%RH,with a hysteresis of 4%RH and a response/recovery time of 5/45 s.The dielectric loss characteristic of the sensor is analyzed according to the dielectric theory.The sensing mechanism of the humidity sensor based on Ti_0.1Zr_0.9O₂ solid solution nanofiber is analyzed by electrochemical impedance spectroscopy.A large number of oxygen vacancies are formed by the substitution of Zr⁴⁺by Ti⁴⁺,which increases the adsorption of water molecules on the material surface,improves the humidity sensing properties and provides a new idea to enhance the performance of the humidity sensor based on metal oxide.（2）Aiming at the problem of low sensitivity in the TiO₂-based humidity sensor,Zr_xTi_1-xO₂（x=0.0-0.5）nanofibers are prepared by electrospinning technology in combination with the advantages of substitutional modification strategy.The humidity sensing performance of the TiO₂-based humidity sensor is successfully improved by adjusting the doping ratio as well as changing the oxygen vacancy concentration and Ti³⁺content on the surface of the solid solution nanofibers,and the optimal doping ratio（Zr/Ti=1:4）is determined.The humidity sensor based on Zr_0.2Ti_0.8O₂ solid solution nanofiber exhibits good humidity sensing properties with a hysteresis of 3%RH and a response/recovery time of 5/25 s.The sensing process of the humidity sensor based on Zr_0.2Ti_0.8O₂ solid solution nanofiber is systematically investigated through electrochemical impedance spectroscopy.The results show that the key factors affecting the conductivity are different at different relative humidity,which provide guidelines for the subsequent enhancement of the performance of the TiO₂-based humidity sensor.（3）Based on the fact that oxygen vacancy concentration and Ti³⁺content are the key factors to enhance the humidity sensing ability of TiO₂ humidity sensitive materials.The strategy of interstitial doping is proposed in combination the advantages of forming solid solution,which improves the sensing performance of the TiO₂-based humidity sensor.The B_xTi_1-xO₂（x=0.0-0.3）nanofibers are successfully prepared by adjusting the molar ratio.According to the analysis of the characterization results,the B element enters TiO₂ lattice in interstitial mode to form solid solution nanofibers.The oxygen vacancy concentration and Ti³⁺content increase with the increase of doping concentration,so does the sensitivity of the humidity sensor.Taking sensitivity and linearity into account,the humidity sensor based on B_0.2Ti_0.8O₂ solid solution nanofiber presents the best humidity sensing characteristics with low hysteresis（1%RH）,fast response/recovery time（2/13 s）,as well as good stability and reproducibility.The adsorption process of water molecules on the surface of B_0.2Ti_0.8O₂ solid solution nanofiber is analyzed using electrochemical impedance spectroscopy and equivalent circuit model.（4）Based on previous work,LiCl is selected as dopant to improve the performance of the humidity sensor based on boron-titanium solid solution nanofibers.The LiCl/B_0.2Ti_0.8O₂ solid solution nanofibers are successfully prepared by replacing Ti⁴⁺with Li⁺through a substitutional strategy.The hydrophilicity and conductivity of the materials are improved by adjusting the doping ratio and changing defect concentration on the material surface,and the performance of the humidity sensor based on boron-titanium solid solution nanofiber is successfully improved.At 11%-95%RH,the sensitivity of the humidity sensor is significantly improved,the impedance decreases by five orders of magnitude and the response/recovery time is reduced to 1/10 s.In view of the fast response,the humidity sensor is used to monitor the respiratory signal.The practicality of the humidity sensor is demonstrated by identifying the breathing status and assessing the body condition.The reason for the enhanced humidity sensing properties of LiCl/B_0.2Ti_0.8O₂ solid solution nanofibers is analyzed by electrochemical impedance spectroscopy combined with equivalent circuit model.