Research On Humidity Sensitivity Of Inorganic Halide Perovskite And Its Humidity Sensing Application

Humidity is one of the environmental factors that cannot be ignored.The measurement of humidity is of great significance in various fields of human daily production and life.Humidity-sensitive materials are the core elements of humidity sensing devices,and their humidity-sensitive properties have an important impact on the sensing performance of the sensor,such as the detection range,humidity responsiveness,and response speed.Traditional moisture-sensitive ceramics and moisture-sensitive polymers are limited due to the need for catalyst assistance to improve the humidity response and the difficulty of desorption of water molecules due to the inclusion of hydrophilic functional groups,respectively.Halide perovskites are favorable for obtaining large humidity responses due to their exceptionally sensitive surface properties to water molecules and are easily desorbed by water molecules because they do not contain hydrophilic functional groups.At the same time,halide perovskites are low-cost and solution-processable,which is beneficial to the miniaturization and integration of sensors.Therefore,halide perovskites are a promising new type of moisture-sensing material.In this paper,a new type of humidity sensor based on inorganic halide perovskite is taken as the research object,and related research work is carried out for the purpose of improving the humidity responsiveness and expanding the sensing application.The results show that the preparation cost and inherent toxicity of inorganic halide perovskites can be reduced by adjusting the preparation process and constituent elements.Its humidity-sensing properties can be enhanced by tuning the crystal size and surface ligands,and the sensing performance can be optimized by tuning the microscopic morphology and sensing mechanism.In addition,the practicality and effectiveness of inorganic halide perovskites in the field of humidity sensing are demonstrated through practical experiments.The specific research in this paper is as follows:（1）The effect of different crystal sizes on the humidity sensing performance of halide perovskites was investigated.The humidity response properties of nanocrystalline and microcrystalline all-inorganic halide perovskite crystals with different microscopic sizes and the effect of crystal microscopic size on their humidity response and recovery behavior were investigated.The results show that small-sized nanocrystals can generate impedance responses at low humidity due to multiple response mechanisms of electron tunneling and surface defect passivation.However,the impedance of large-sized microcrystals does not change significantly at lower humidity due to the lack of electron tunneling effect and poor film connectivity.The sensing mechanism of the two sizes of crystalline thin films under high humidity is the proton transport effect after forming a continuous water film,but the small-sized nanocrystalline film requires less water molecular weight to form a continuous water film and has a small humidity response.This is attributed to its excessive grain boundary and surface defects hindering carrier transport,which also makes the response time longer.The large-sized microcrystalline thin films are stacked due to the large crystal size so that the pores of the thin film are larger,and more water molecules can be accommodated when a continuous water film is formed.Therefore,the microcrystals have a greater impedance response under high humidity,and their better crystallinity is beneficial to the improvement of the response speed.（2）Influence of the chain-length structure of surface ligands on the humidity-sensitive properties of halide perovskite crystallites.Lead-free Cs₃Cu₂Br₅ crystallites were passivated by two surface ligands with different chain lengths,short-chain octylamine（OLA）and long-chain oleylamine（OAm）,and the corresponding humidity sensors were prepared.The sensing performance test results show that the impedance changes of the fabricated humidity sensor from 12%to95%relative humidity are 10⁵Ω（OLA）and 10⁴Ω（OAm）,respectively.The response/recovery speeds are 24.9/8.8 s（OLA）and 18.5/7.8 s（OAm）,respectively.The analysis of the moisture-sensing properties of ligands with different chain lengths showed that the short-chain OLA was beneficial to the formation of porous films with stronger water absorption,and its stronger hydrophilicity was helpful for improving the humidity response.Long-chain OAm helps to increase the density of the film and improve the detection limit of the sensor,and its strong hydrophobicity is beneficial to shorten the response and recovery time.（3）The moisture-sensing properties of the novel one-dimensional halide perovskite crystallites and their practical effects for the detection of plant transpiration were studied.A lead-free one-dimensional K₂CuBr₃ humidity sensor was prepared,and the humidity response characteristics of halide perovskite crystallites with one-dimensional morphology were investigated.The results show that the porous one-dimensional structure of K-based metal halides is more favorable for water molecule adsorption and electron transport.The fabricated sensor exhibits a large impedance response of over 10⁶Ωfrom 12%to 95%relative humidity changes,while maintaining excellent linearity and maintaining good humidity cycling repeatability.In addition,the accurate detection of different plant transpiration intensities and patterns by the prepared K₂CuBr₃ impedance humidity sensor proves the practicality of one-dimensional lead-free halide perovskites in humidity sensing applications.（4）The optimization effect of quartz crystal microbalance（QCM）structure on the humidity sensing performance of one-dimensional halide perovskite and its practical expansion effect in high frequency humidity change scenarios were studied.To improve the defects of slow response speed and small humidity test range of impedance type humidity sensor,a QCM type humidity sensor based on one-dimensional K₂CuBr₃ was developed.The prepared QCM-type humidity sensor exhibited an excellent log-linear response（R²=0.98626）in the humidity range of 11-95%RH.The humidity detection device has a faster response/recovery speed（10.63/4.31 s）than the impedance type device.The results show that combining the high humidity sensitivity of K₂CuBr₃with the high-quality change sensitivity of QCM plays a comprehensive role in optimizing the performance of the humidity sensor.Furthermore,the frequency-shift sensing mechanism of the mass loading effect was revealed by Fourier transform infrared spectroscopy（FTIR）and the Langmuir adsorption model.The application test results of accurate monitoring of the human body’s real-time breathing state and breathing mode prove the practicability of the QCM-type humidity sensor in high-performance demand scenarios with high-frequency changes in humidity.