High‐Performance Humidity Sensor Based on CsPdBr3 Nanocrystals for Noncontact Sensing of Hydromechanical Characteristics of Unsaturated Soil

The humidity sensors based on metal halide perovskite have achieved outstanding improvement. However, there are few reports of perovskite nanocrystals for humidity sensors. Herein, CsPdBr 3 nanocrystals with high surface‐to‐volume ratio are synthesized by the in situ spin‐coating method. Subsequently, the humidity sensor based on CsPdBr 3 nanocrystals is prepared and the response mechanism is demonstrated to originate from the formation of the Pd––O bond on the surface by performing density functional theory calculation. The humidity sensor shows a fast response/recovery time of 0.68/2.43 s in a relative humidity range from 88.9%RH to 4.7%RH; the fast response/recovery properties are used to monitor human respiratory and noncontact switch. Moreover, the hydromechanical characteristics of unsaturated soil are obtained by noncontact testing with the humidity sensor for the first time in geological engineering, and the strong correlation between relative humidity and hydromechanical characteristics is revealed by mathematical derivation, which lays the theoretical foundation for the noncontact real‐time sensing of hydromechanical characteristics of unsaturated soil. These findings not only open the door to the development of perovskite nanocrystals materials for humidity measurement, but also offer a new method for nondestructive and real‐time test of the mechanical properties of soil in geological engineering.