Study On Humidity Sensor Based On Carbon Nano Film And Its Composite

In recent years,with the continuous development of the Internet of Things industry,many emerging application fields have put forward higher requirements for humidity monitoring.Currently,commercial humidity sensors generally have problems such as low sensitivity and long response time,which seriously hinder its application in these emerging fields.Therefore,it is imperative to develop humidity sensors with novel and high sensing performance.Carbon nanomaterials have the characteristics of large specific surface area,high carrier mobility,good mechanical properties,stable physical and chemical properties,and room temperature conductivity,which largely compensate for the higher power consumption of traditional oxide semiconductor sensors and the poor stability of polymer sensors,thus providing a new direction and idea for the realization of high-performance humidity sensors.In this paper,based on the structure and properties of carbon nano-film materials,several nano-diamond-based carbon nano-films and their composite film humidity-sensitive materials are proposed.Combined with different types of transducer devices,humidity sensors with different characteristics were constructed.On this basis,the humidity sensitivity characteristics and humidity sensitivity mechanism of the prepared humidity sensors were studied and analyzed in detail,and try to apply some of these prepared sensors to these emerging fields.The main content summarized as follows:（1）A fast and flexible wearable humidity sensor based on nano-diamond（ND）is proposed.The ND-modified flexible humidity sensor was prepared by deposting the ND dispersion on the polyimide flexible interdigital electrode substrate with spin-coating method.The sensitivity,repeat stability,response and recovery time of the sensor were studied,respectively.The results show that the response/recovery time of the sensor has a decrease with the reduce of the bending radius,but it has a sub-second response/recovery time under each different bending conditions.In addition,the sensor not only shows good repeatability under different bending conditions,but also keeps the same performance after multiple bending.In view of the good flexibility and fast response time of the humidity sensor,it was applied to human breathing detection.The results show that the sensor can respond to the breathing state under different sports conditions.This research provides a potential application for humidity sensors in monitoring human health activities such as sports and fitness.（2）A highly sensitive humidity sensor based on reduced graphene oxide/nanodiamond（r GO/ND）composite film was prepared by thermal reduction method.The frequency-capacitance response,hysteresis characteristics,and long-term stability of the prepared r GO/ND humidity sensor were investigated.The results show that the r GO/ND humidity sensor has an ultra-high sensitivity（13086 p F/%RH）and a good low moisture（<33%）sensitivity（539 p F/%RH）at the frequency of 100 Hz,which is 30 times more sensitive than pure r GO and pure ND film humidity sensors and exceeds most reported capacitive humidity sensors in literatures.At the same time,the sensor shows a low hysteresis characteristic（3.5%RH）.A detailed analysis of the moisture-sensitive response mechanism of the r GO/ND modified humidity sensor was carried out using complex impedance spectroscopy（CIS）,and a possible sensing mechanism model was developed to account for the low hysteresis characteristics of the sensor.（3）A highly sensitive and fast humidity sensor based on nanodiamond-loaded flower-like molybdenum disulfide（Mo S₂/ND）composite films was fabricated via a hydrothermal synthesis method.The structure and morphology of the prepared moisture-sensitive material were characterized by scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,and X-ray photoelectron spectroscopy（XPS）.The response sensitivity,frequency characteristics,response/recovery characteristics,humidity hysteresis characteristics,and temperature characteristics of the Mo S₂/ND composite film sensor were investigated.The results show that the Mo S₂/ND composite film humidity sensor has higher sensitivity than that of the pure Mo S₂ humidity sensor.The sensitivity of the sensor is 5 times higher than that of the pure Mo S₂ humidity sensor,and the sensor also shows fast response and recovery time（less than 1 s）.Based on the complex impedance Cole-Cole and Bode plots,the humidity-sensitive mechanism of the sensor was analyzed and discussed in detail.Given the good sensitivity as well as the fast response and recovery time of the sensor,the sensor is applied to monitor the humidity of human finger skin,and the results confirm that the sensor can respond well to the approach and departure of fingers.On this basis,the non-contact control of the LED brightness by fingers is realized,which provides a potential direction for future non-contact applications.（4）A high stability quartz crystal microbalance（QCM）humidity sensor based on nano-diamond/multi-walled carbon nanotubes（ND/MWCNTs）composite film was proposed.Benefiting from the high mechanical modulus of ND,it was dispersed into carbon nanotubes to form the higher mechanical modulus composite film of ND/MWCNTs.The humidity sensitivity of the QCM humidity sensor modified by ND/MWCNTs composite films was studied by impedance analysis and oscillating circuit.The results show that,compared with pure ND sensitive film modified QCM humidity sensor,the quality factor（Q）of the QCM humidity sensor modified by ND/MWCNTs composite film is about four times of that of the pure ND film modified QCM humidity sensor.And the higher Q value can be maintained for several months,the reason may be related to the network chain structure formed by good dispersion of ND/MWCNT composite films,which exhibits high mechanical modulus,thus reducing the internal stress caused by viscoelasticity of water absorption,so the loss of acoustic energy was reduced.The dynamic oscillating circuit analysis displays that the QCM humidity sensor modified by ND/MWCNTs composite film also has fast response/recovery time,good repeatability,low hysteresis,and good long-term stability.This research provides a good idea and method for constructing high-stability QCM humidity sensors.（5）A quartz crystal microbalance（QCM）humidity sensor based on pencil-drawn method is proposed.The QCM humidity sensor was fabricated with by painting directly pencil traces on the surface of QCM electrode as sensitive film.Scanning electron microscope（SEM）,atomic force microscopy（AFM）and Raman spectroscopy were used to characterize the morphology and composition of the films,and impedance analysis and oscillating circuit was applied to analyze the humidity sensing performance.The results show that the QCM humidity sensor has a very high Q value.In the entire humidity range,the Q value only drops from47,923 to 43,849,which is better than most QCM humidity sensors reported previously.In addition,the sensor shows good long-term stability in long-term test,the maximum frequency deviation is less than 5%,and the sensor has fast response recovery time.This study shows that pencil drwaing is a simple,green and cost-effective method for constructing high-performance QCM humidity sensors.