Study On The Hydrochromic Behavior Of NiI₂-based Hybrid Materials

With the development of manufacturing technology and society,humidity monitoring has received extensive attention due to its important role in agricultural production,environmental monitoring,electronic applications,food storage and many other fields.Different from the electrical response-based humidity sensors,colorimetric humidity sensors based on moisture-induced color-change materials directly convert the change of environmental humidity parameter into visual color-changes without the need of complex circuit and display devices.However,hydrochromic materials still suffer from many challenges that make them difficult to achieve practical applications,such as slow color change,irreversible hydrochromism,and so on.By mean of the organic hybrid process,certain functional groups(for example,hydrophobic and hydrophilic groups)are introduced into the moisture-induced color-change materials,which is expected to improve the color-change behavior of the moisture-induced color-change materials.In this work,NiI₂/CF₃(CF₂)₇I and NiI₂/TBAI were prepared by hybridizing NiI₂ with CF₃(CF₂)₇I and TBAI respectively,which realized the rapid color-change of NiI₂.The main research results are as follows:1.NiI₂/CF₃(CF₂)₇I prepared by organic hybridization,is found to be of reversible and fast color-change behavior from black to transparent.Its phase,functional group,chemical component,hydrophilicity and absorbance were analyzed by high-temperature in-situ Xray diffraction(XRD),Fourier transform infrared spectroscopy(FT-IR),angle-resolved Xray photoelectron spectroscopy(ARXPS),water contact angle,UV-vis absorption spectra,and so on.NiI₂/CF₃(CF₂)₇I film and humidity sensor were prepared by spin-coating and drip-coating,respectively,and color-change behaviors and humidity sensing properties of NiI₂/CF₃(CF₂)₇I were tested.The results show that,compared with pure NiI₂ film,the speed of color-change from transparent to black for NiI₂/CF₃(CF₂)₇I film is obviously faster than that of NiI₂ film in a dry environment,and the recovery time NiI₂/CF₃(CF₂)₇I humidity sensor is about 1/8 of that of NiI₂ humidity sensor at the humidity range of 11%-95% RH.According to ARXPS results,the fast desorption behavior can be attributed to the introduction of hydrophobic groups(CF₃(CF₂)₇·)inside NiI₂ film,which makes the interior of NiI₂/CF₃(CF₂)₇I film more hydrophobic than the pure NiI₂ film to accelerate the desorption of water molecules.By means of the fast hydrochromic behavior,a NiI₂/CF₃(CF₂)₇I humidity switch based on solar cell is developed without power supply to realize humidity threshold detecting.In addition,the applications of NiI₂/CF₃(CF₂)₇I hybrid material in fingertip humidity detection and anti-counterfeiting are also explored.2.NiI₂/TBAI hybrid material was prepared by organic hybridization,and its phase,functional group,reflectance and hydrophilicity were studied by high-temperature in-situ XRD,FT-IR,UV/vis reflectance spectrum and water contact angle.The film and humidity sensor based on NiI₂/TBAI were prepared to study color-change behaviors and humidity sensing properties of NiI₂/TBAI.It was found that it has different colors under different humidity,such as red,yellow and black,and the conversion between these colors is reversible.The color-change behavior can be attributed to the coordination between NiI₂,water molecules and TBAI.By introducing amphiphilic TBAI materials,the color-change behavior and humidity sensing properties of NiI₂ were improved,especially the desorption property of NiI₂/TBAI humidity sensor for water molecules was greatly improved,and the recovery time of NiI₂/TBAI humidity sensor has been reduced to 1/9 of that of NiI₂ humidity sensor.In addition,the NiI₂/TBAI humidity sensor has good sensitivity and stability in the range of 11% ? 95% RH,as well as small hysteresis.Based on the unique moisture-induced color-change characteristics and good humidity sensing properties,the applications of NiI₂/TBAI hybrid material in watercolor pen and breath detection are explored.