Low Humidity Sensors Based On Metal Organic Frameworks

Low humidity sensors have important applications in goods storage,semiconductor manufacturing,national defense and security.Among them,impedance type low humidity sensors have attracted much attention because of their small size,easy integration and low cost.In recent years,researchers have reported low humidity sensors based on different types of sensitive materials,but they still have some shortcomings such as low sensitivity and large humidity hysteresis.In this thesis,aiming at the development demand of high-performance low humidity sensors,metal-organic framework humidity-sensitive materials with different structural characteristics are developed by using the characteristics of easy modification,high porosity and good stability of metal-organic framework,and the effects of metal-organic framework structure,hydrophilic group structure and content on the humidity-sensitive performance of devices are studied.This paper provides novel ideas for the development of low humidity sensors with high sensitivity,low hysteresis and high stability.This thesis mainly focuses on following research contents:1.Aiming at the problem of poor film forming ability of metal organic framework,in-situ click polymerization on the interdigital electrode was proposed to prepare metal organic framework based polyelectrolyte film humidity sensors.Firstly,hydrophilic groups with different contents（sodium sulfonate and ionic liquid（IL））were introduced into the metal organic framework by a one-step synthesis method.Then,polyelectrolyte sensing films were prepared in batch by the in-situ crosslinking polymerization on the interdigital electrode,the humidity sensors based on Ui O-66-SO₃Na and IL-Ui O-66 were obtained accordingly.The optimized Ui O-66-SO₃Na sensor exhibits a two orders of magnitude response in the humidity range of 33–95%RH with a hysteresis of 1.2%RH,and a response time of 3.1 s.The optimized IL-Ui O-66 sensor exhibits low humidity sensing properties（humidity range of 5–30%RH）with a good linearity（R²=0.994）,a hysteresis of 0.2%RH and a response time of 0.4 s.2.Aiming at the low response value of metal organic framework based low humidity sensor,large content of hydrophilic ILs（EMZC and IL-COOH）was introduced into the metal organic frameworks,and the hydrophilicity of the sensitive materials was controlled by the content of ILs,low humidity sensors based on IL@Ui O-66-NH₂ and IL-MOF-801 were prepared accordingly.The optimized IL@Ui O-66-NH₂ sensor shows a linear response（R²=0.992）and a hysteresis of 0.5%RH in the low humidity range of 5–30%RH.The optimized IL-MOF-801 sensor exhibits a high sensitivity（20.3）with a good linearity（R²=0.997）,a low humidity hysteresis（～1%RH）,a fast response（0.4 s）,a high humidity resolution（1%RH）,and high stability（over 100 days）in the low humidity range of 0–35%RH.3.On the basis of realizing that the enhanced conductivity of sensing film can improve the response of low humidity sensor,Au nanoparticles（NPs）was introduced into the metal organic framework.By the ability of decomposing water molecules of Au NPs,low humidity sensors based on Au-Ui O-66-SO₃Na were developed.The response of the obtained sensors is negatively correlated with the particle size of Au NPs.The sensing range of the optimized sensor is 5–30%RH,and the response is9.04 with a humidity hysteresis of 0.48%RH,a response time of 9.2 s and high stability（over 30 days）.The sensing mechanism of the sensor was analyzed by temperature characteristics,quartz electronic microbalance（QCM）and complex impedance spectroscopy（CIS）,and the application of the sensor in food storage was explored.4.The high impedance of the sensor under low humidity is one of the reasons that limit the stability of the output signal.In order to reduce the impedance value of the sensor under dry environment,low humidity sensors based on gelatin/glycerin/IL-MOF hydrogel were developed by the simulation of neuron system.The strong hydrophilicity,high boiling point of glycerin and the hydrogen bond networks between glycerin,gelatin and IL-MOF enhanced the hydrophilicity of the sensitive film and reduced the initial impedance value of the sensor.The optimized gelatin/glycerin/IL-MOF hydrogel shows dual-mode（voltage,impedance）humidity sensing characteristics.The humidity range of the self-powered humidity sensor is 0–75%RH,the output voltage of the sensor is 95 m V at 0%RH,the full range voltage response is larger than 0.2 V,and can charge the capacitor.The humidity range of the self-powered low humidity sensor is 0–23%RH with a response of 1.5 and a linearity of 0.998.The impedance humidity sensing range of the optimized low humidity sensor is 0–23%RH,and the sensor exhibits a high sensitivity（40.2）with a small humidity hysteresis（～0.1%RH）and excellent reusability.Finally,the application of the sensor in low humidity visual detection was explored.