Evaluation of the Humidification Effect of Street Trees Based on All-Inorganic Lead-Free K₂CuBr₃ Humidity Sensors

A suitable humidity environment is of great significance to the healthy and comfortable life of urban residents, and the humidification effect of street trees is crucial for effectively regulating microclimates in big cities. Therefore, evaluating the humidification capacity among species is a rewarding research aiming at providing bases for selecting appropriate street tree species. The leaves of trees generate water vapor through transpiration to slowly increase the environmental humidity, and humidity sensors with giant response, excellent linearity, and reliable repeatability are more conducive to analyzing and evaluating the subtle process in detail. Here, the antisolvent method was adopted to simplify the synthesis process of lead-free K2CuBr3, and the fabricated impedance-type humidity sensor based on K2CuBr3 microrods satisfied these requirements. The all-inorganic lead-free halide K2CuBr3 overcomes two drawbacks, including the toxicity of Pb and the moisture instability of most reported halide perovskites used in the field of humidity sensing. The impedance variation exceeds 106 Ω (162 Ω to 182 MΩ), with the relative humidity varying from 12 to 95%. In addition, the analysis of the corresponding complex impedance spectroscopy revealed the sensing mechanism. The intrinsic properties of K2CuBr3 are reflected at low humidity, and the proton transfer gradually dominates the sensing paths with increasing humidity. Furthermore, the K2CuBr3 humidity-sensitive layer is employed to evaluate the transpiration of street trees for the first time, and the successful evaluation of the humidification effect indicates more applications of lead-free halides.