Highly Sensitive Humidity Sensors For Low-Humidity Detection

Accurate detection of low humidity is very difficult,and sensors used for lowhumidity detection have problems such as poor sensitivity,slow response,large drift,and nonlinearity.In recent years,various low-humidity detection methods have been continuously improved,expanding the applicability of sensors.Impedance lowhumidity sensors have advantages such as portability,flexibility,and low cost,which have attracted much attention.Aiming at the development needs of highly sensitive impedance low-humidity sensors,a series of mesoporous silica composite humidity sensing materials were designed based on the high specific surface area,high porosity,high stability,adjustable pore structure,abundant active sites,and easy modification of mesoporous silica.The low-humidity sensing performance of the prepared humidity sensors was researched,and the detection ability of the sensors in practical application scenarios was explored.This dissertation mainly focuses on following research:1.In view of the problem of poor sensitivity in impedance low-humidity sensors,the feasibility of using electrolyte materials based on mesoporous silica for lowhumidity sensors was explored.Firstly,mesoporous silica was treated with concentrated hydrochloric acid to introduce hydroxyl functional groups;Then,the hydrophilicity and ionic conductivity of mesoporous silica were improved by introducing imidazole bromide based poly(ionic liquid)s by hydrogen bonding.The humidity sensing film was prepared by in-situ thermal polymerization,and the lowhumidity sensor with good sensing performance was obtained by adjusting the content of poly(ionic liquid)s.The preferred sensor has a response of 806% in 7–33% RH range,a response time of 8 s,a recovery time of 13 s,a humidity hysteresis of 1.0% RH,and good long-term stability.The research results indicate that the pore structure of mesoporous silica plays an important role in low-humidity sensing processes,enhancing the adsorption of water molecules and promoting ion transport through capillary force.2.To solve the problem that weak hydrogen bonding forces could not ensure the uniform distribution of poly(ionic liquid)s on mesoporous silica,poly(ionic liquid)s were grafted onto specific positions(the channels,outer surface,both the channels and outer surface)in mesoporous silica through thiol-ene click reaction to endow the mesoporous silica specific active sites for adsorption of water molecules.Lowhumidity sensors with different contents of poly(ionic liquid)s grafted in the channels were prepared in situ,and the optimal content of grafted poly(ionic liquid)s was determined.The sensing performance of mesoporous silica low-humidity sensors grafted with poly(ionic liquid)s in the channels,on the outer surface,both in the channels and on the outer surface in the range of 7–33% RH was investigated.The sensing performance of the mesoporous silica low-humidity sensor grafted with poly(ionic liquid)s in the channels(response of 265%,response time of 25 s,recovery time of 9 s,and humidity hysteresis of 1.3% RH)is better than that of the mesoporous silica low-humidity sensor grafted with poly(ionic liquid)s on the outer surface(response of 186%,response time of 22 s,recovery time of 16 s,and humidity hysteresis of 1.5% RH).Compared with the single grafting method,the sensitivity of the mesoporous silica low-humidity sensor grafted with poly(ionic liquid)s both in the channels and on the outer surface was greatly improved.The response is up to 4112%,the response time is 3 s,and the humidity hysteresis is 0.6% RH.Finally,a humidity sensing mechanism based on constructing dual-channel water molecule transport to achieve high sensitivity of low-humidity sensing was proposed,that is,the channels and outer surfaces with the same hydrophilicity simultaneously adsorb water molecules,and the strong synergistic effect generated by the hybrid interface significantly increases the number of dissociated ions,leading to large changes in impedance signals.3.In order to expand the practical application potential of low-humidity sensors,a strategy of constructing dual-function sensors based on humidity activated ammonia sensors was proposed.Low-humidity sensors based on a composite system of mesoporous silica,Au nanoparticles and gelatin were prepared,and the sensing performance was studied.The response of the preferred sensor in the range of 0.4–30%RH is 423%,and the response time is 28 s.At 30% RH,the initial impedance value of the sensor is as low as ~6.6 MΩ,and the response to 50 ppm NH3 is 78%.The sensing mechanism was analyzed via the sensing performance test results and complex impedance spectra,and the contribution of each component in the sensing film to different sensing processes was clarified.Finally,the potential application of the developed low-humidity sensor was verified by water loss monitoring experiments in eggs.Below 35% RH,the developed low-humidity sensor has better linearity in humidity output values than commercial hygrometer,and the humidity value difference between the two is less than 3.8% RH.4.A strategy for developing self-powered humidity sensors by combining humidity generators with low-humidity sensors was proposed to address the issue of lowhumidity sensors requiring external power supply.The characteristics of mesoporous silica were improved by selecting a polymerizable deep eutectic solvent with strong hydrophilicity and high ionic conductivity.A low-humidity sensor and a biomimetic humidity generator based on mesoporous silica/polymerizable deep eutectic solvent ionogels were prepared.The low-humidity sensing performance,self-power supply performance and mechanism were researched.The self-power supply capability was verified by charging capacitors and lighting LED lamps.The response of the prepared low-humidity sensor in the range of 5–30% RH is up to 1600%,with a humidity hysteresis of 0.8% RH and a response time of 21 s.At 33% RH,the device can produce an open-circuit voltage of ~600 m V.