Application Of NiI₂-based Humidity Driven Discoloration Materials

With the development of science and society,humidity detection plays an important role in life,production and other fields.The traditional humidity detection mostly uses the electrical mode to detect,which requires professional testing instruments.The development of humidity-driven discoloration materials provides a new method for humidity detection,which can be detected by naked eye observation.In this paper,NiI₂ and NiI₂/?CH₃?₄NI materials are used for research objects,and the discoloration behavior and humidity sensing properties of NiI₂ and NiI₂/?CH₃?₄NI materials were tested,respectively.And the discoloration mechanism and humidity sensitive mechanism of the materials were explored.The main contents are divided into two parts:1. NiI₂ material was obtained by wet chemical method.The phase composition,crystal structure and absorption spectrum of NiI₂ in dry and wet state were studied by a series of in-situ characterization.The mechanism of the discoloration behavior of NiI₂ film is analyzed,because NiI₂ will change to NiI₂·6H₂O after absorbing water.The discoloration behaviors of NiI₂ film were tested.It was found that the NiI₂ film exhibits reversible black-to-transparent discoloration behavior with the change of humidity,and the discoloration behaviors can be controlled by the substrate of film.The theoretical calculation results show that the NiI₂ material has larger crystal surface energy after wet chemical treatment.In addition,the electrical humidity sensing properties of NiI₂ in ultra-low humidity environment are measured.The results show that the impedance change of NiI₂ humidity sensor is about 2 orders of magnitude in the range of 0.4%RH?11%RH,and the linearity is 0.992,showing excellent sensitivity and linearity.The humidity sensing mechanism of ultra-low humidity detection is also analyzed.It is considered that the excellent humidity sensing properties of NiI₂ are due to bulk adsorption of humidity.Furthermore,the humidity sensing properties of the sensor in wide range of RH are also tested.The response time of NiI₂ sensor is only 3 s,and the sensor shows excellent repeatability.Due to the humidity sensitive characteristics of NiI₂ in optical and electrical aspects,the NiI₂ optical/electric dual-mode flexible humidity detection wristband was fabricated.The wristband can detect the humidity by two modes,observing the color change with naked eyes and electrical test,and the maximum error of electrical test is only 3.9%RH.This dual-mode humidity detection method provides a new research method for humidity detection.2. NiI₂/?CH₃?₄NI hybrid material was prepared by organic hybridization,and its crystal structure,surface composition and absorption spectrum were studied by X-Ray Diffraction ?XRD?,angle resolved X-ray photoelectron spectroscopy?ARXPS?and water contact angle etc.The discoloration behaviors of NiI₂/?CH₃?₄NI hybrid material were studied,which exhibits faster and more reliable discoloration behaviors than that of NiI₂.The enhanced discoloration behaviors are attributed to the introduction of?CH₃?₄NI,which improved the hydrophilicity and hydrophobicity of the surface and interior of NiI₂ film,respectively.Using NiI₂/?CH₃?₄NI hybrid material as the sensing material,the humidity sensor was fabricated,and its humidity sensing properties were measured.It was found that recovery characteristics of the sensor were greatly improved,and the recovery time was only one third of NiI₂ sensor.Finally,combining with its unique optical color changing characteristics and electrical humidity sensitive characteristics,the applications of NiI₂/?CH₃?₄NI hybrid material in optical/electrical dual-mode flexible humidity detection wristband,paper-based sensor and smart window are explored.