Study On The Moisture-induced Discoloration Behavior Of NiBr₂ Humidity Sensor

Humidity occupies an increasingly important position with the development of the times.Surface adsorption mechanism is the humidity sensitive mechanism of most traditional humidity sensors.However,it is remains challenging to detect trace-level moisture changes in the low-humidity environment for conventional humidity sensor because the surface conductive layer is almost unaffected by changes in water molecules concentration.The bulk adsorption behavior of hydrochromic materials for water molecules can change the physical properties of the materials,such as crystal structure,which has brought about the widespread attention of researchers in the field of humidity sensor.In this work,the discoloration behavior,humidity sensing properties,humidity sensing mechanism and improvement of humidity sensing properties of hydrochromic material Nickel(II)bromide(NiBr₂)are studied in detail.The main contents are divided into three parts:1.NiBr₂ material is prepared by a simple wet chemical method.The color of NiBr₂film shows yellow below 24%RH and green above that.Various characterization experiments of NiBr₂ are carried out.The results show that the discoloration mechanism of NiBr₂is the material transition,which makes NiBr₂ absorb water and transform into NiBr₂·6H₂O.This indicates that the bulk adsorption behavior of NiBr₂ to water molecules will lead to the change of the crystal structure of NiBr₂ itself.It is expected that the material can be used in the detection of low humidity environment.2.A ceramic substrate-based humidity sensor based on NiBr₂ is prepared.NiBr₂humidity sensor delivers a tremendous impedance change nearly 5 orders of magnitude in the low relative humidity(RH)ranging from 7%to 24%and the sensitivity is about 40M?/%RH,the response time is only about 2 s in low humidity environment.The impedance of NiBr₂ humidity sensor on ceramic substrate is only 3 orders of magnitude in the medium-high humidity environment of 26%-95%RH.Meanwhile,the sensitivity of the humidity sensor(500?/%RH)in the medium-high humidity environment is only1/10⁵ of the sensitivity of that in low humidity environment,which indicates that the sensitivity of the sensor to water molecules caused by the bulk absorption behavior is better than that dominated by the surface adsorption behavior.The results demonstrate that the bulk absorption behavior-mediated reversible material transition provides efficient guidance to realize low-humidity detection.Interestingly,the reversible bulk adsorption behavior will be affected by the penetrability of the humidity sensor substrate.The corresponding detection range of humidity sensor dominated by the bulk adsorption behavior is broadened to 75%RH by using loose porous fiber filter paper-based humidity sensor.The results demonstrate that the bulk adsorption behavior-mediated reversible material transition opens a new dimension for the development of high-performance humidity sensors.3.NiBr₂-PVA material is prepared by using PVA as the loading material,and the humidity sensing performance of NiBr₂-PVA humidity sensor are tested.The recovery time of NiBr₂-PVA humidity sensor is 82 s,which is 48%higher than that of NiBr₂humidity sensor.In the long-term test,the impedance response of NiBr₂-PVA humidity sensor decreased by less than 0.5 orders of magnitude,while the impedance response of NiBr₂ humidity sensor decreased by nearly 4 orders of magnitude.In addition,compared with NiBr₂ humidity sensor,NiBr₂-PVA humidity sensor also has excellent repeatability and anti-gas interference ability.The improvement of humidity sensing performance of NiBr₂-PVA humidity sensor is due to the characteristics of PVA film-forming and easy adsorption on the substrate,which can effectively improve the problem of easy deliquescence of NiBr₂ material.Therefore,the above analysis shows that using other materials to load NiBr₂ material is expected to make NiBr₂ material get practical application in low humidity detection.