Studies On The Preparation And Performance Of New Two-dimensional Nanocomposite Membrane Formaldehyde Gas Sensor

Air pollution prevention is the key for winning the blue sky defense battle and creating a good production and living environment for the people.Gas sensor can realize the rapid identification and detection of toxic and harmful gases that converts gas volume signals into electrical signals,which has become a research hotspot.Semiconductor gas sensors have been widely used in various fields in our life,but during the long-term research and application,there are still some key problems that need to be solved urgently for this type of semiconductor sensors including:（1）different types of sensitive materials have different corresponding sensing mechanisms;（2）The characteristics of semiconductor materials limit the application of gas sensors with harsh environments such as high temperature and drying;（3）many gas sensors have response to almost all gases with poor selectivity,and poor anti-interference ability.2D nanomaterials are a favorable choice for a new generation of low-cost,miniaturized,and low-power gas sensors due to the unique electrical properties.For two-dimensional semiconductor materials such as graphene and molybdenum disulfide,the two-dimensional structure exposes a large number of atoms on the surface with high specific surface area,which is very conducive for the adsorption and diffusion of the target gas to achieve high sensitivity;In addition,two-dimensional nano-semiconductor materials have the characteristics of a complete lattice and high carrier mobility.It is easy to increase the sensing active site of the gas by making defects on the surface or connecting functional groups,which will also greatly improve the sensitivity and selectivity.In the present work,the preparation and performance of a new two-dimensional composite membrane formaldehyde sensor were studied,focusing on the key problems of unclear sensing mechanism,low sensitivity and poor selectivity of traditional formaldehyde gas sensors.Based on reduced graphene oxide（RGO）or molybdenum dichloride（MoS₂）gas-sensitive materials,a novel formaldehyde gas sensor was consisted by hydroxylamine hydrochloride（H₃NO·HCl）,polyvinylidene fluoride（PVDF）film,RGO and interdigitated electrodes.Moreover,the gas sensitivity performance of the sensor including sensitivity,selectivity,response-recovery time,stability,working temperature,humidity and detection limit was studied by a homemade gas sensitivity test system.Gas chromatography mass spectrometry（GC-MS）was used to study the mechanism of gas-sensitive response.The main content of the paper is as following:1.Studies on the Preparation and Performance of RGO Composite Film Formaldehyde Sensor:Based on the sensitivity of RGO to electron-deficient substances,a new formaldehyde gas-sensitive sensor was prepared by using the formaldehyde specific oximation reaction.The chemical reduction method was used to prepare RGO nanosheets at room temperature,and the structural morphology and reduction degree of the products were characterized.The sensor uses RGO as a single gas-sensitive material and is composed of a layered structure of H₃NO·HCl,PVDF film,RGO sensitive film,and aluminum oxide interdigital electrode.The gas-sensing performance of RGO composite thin film sensors including sensitivity,selectivity,response and recovery time,stability,operating temperature and humidity,detection limit,etc.were systematically studied at room temperature.The experimental results show that under normal temperature and humidity conditions,the sensitivity of the sensor to formaldehyde gas of 21.2 mg·m^-3 HCHO is up to 75%.The selectivity of the RGO sensor for HCHO is 10³ times of the other high concentration VOCs such as methanol,ethanol,toluene,methyl chloride and water.In a 15-day continuous test,the relative standard deviation（RSD）of the sensor sensitivity is as low as 5.83%;The response value of the sensor increases linearly with the increase of HCHO concentration,and the detection limit of formaldehyde gas is as low as 0.0304mg·m^-3.2.Studies on the Preparation and Performance of MoS₂ Composite Film Formaldehyde Sensor:Based on the sensitivity of RGO to electron-deficient substances,a novel gas-sensitive conduction mechanism was designed using formaldehyde-specific oxime reaction,and a new formaldehyde gas-sensitive sensor was prepared.MoS₂ nanosheets were prepared by using mechanical ultrasonic peeling method at room temperature,and the structure and morphology of the obtained products were also characterized.The sensor uses MoS₂ as a single gas-sensitive material and is composed of a layered structure of H₃NO·HCl,PVDF film,RGO sensitive film,and aluminum oxide interdigital electrode.The gas-sensing performance of MoS₂ composite thin film sensors was studied at room temperature.The experimental results show that the sensitivity to 8 mg·m^-3 is up to 295%,which is more than 6 times that of RGO HCHO gas sensor under normal temperature and humidity conditions.3.Studies of Sensing Mechanism:The response sensing mechanism of the sensor was studied by gas chromatography-mass spectrometry（GC-MS）.In the process of sensor gas response,GC-MS was used to detect the oxime reaction product formaldehyde oxime.HCl vapor was further injected into the gas sensor cavity to confirm the sensor response mechanism.When the RGO/HA-HCl sensor was exposed to the HCHO,the oximation reaction would release the HCl vapor immediately.Subsequently,the released HCl vapor diffuses to the RGO surface to capture electrons from the RGO sheets and leave an equal number of carrier holes on the RGO sheets which decreases the resistance.By purging with air,the adsorbed HCl vapor would desorb from the RGO,which could induce the recombination of holes and electrons leading to the recovery the resistance of the RGO sheets.