Research On Key Technologies Of Gas Sensor Based On Black Phosphorus

With the improvement of people’s quality of life,new challenges are presented to the detection technology and methods of environmental pollution gases.Therefore,there is an urgent need for highly sensitive gas sensors that can detect concentrations as low as ppb.Black phosphorus(BP)is a kind of P-type semiconductor,similar to graphene,and has unique properties when its number of layers is small to several,which is called few-layer black phosphorus nanosheets.Few-layer black phosphorus nanosheets have the characteristics of unique anisotropy,tunable direct bandgap structure,high specific surface area and carrier mobility,and are ideal and highly sensitive gas-sensing materials.Poor stability caused by easy degradation is the biggest challenge in the application of black phosphorus gas sensors.Since there are five electrons in the outermost layer of black phosphorus,each phosphorus atom only forms four chemical bonds with three adjacent phosphorus atoms,and a large number of residual electrons tend to accumulate on the surface of black phosphorus when the number of black phosphorus layers is reduced to several layers,forming adsorption site.These adsorption sites not only participate in gas adsorption,but also are the main factors for the high sensitivity of black phosphorus gas sensing;they also react with oxygen and water molecules,which is the cause of its easy degradation.In this thesis,a resistive gas sensor using black phosphorus as a sensitive material is prepared,and two different n-doping methods of Benzyl viologen(BV)and Sn O quantum dots are designed to improve its humidity resistance and improve its gas sensitivity.Systematic analysis and research,combined with various characterization analysis methods,carried out an in-depth discussion on its gas-sensitivity mechanism.The main content includes the following aspects:(1)In the thesis,the degradation mechanism of black phosphorus has been fully investigated and researched,and a method of preparing gas sensor by nitrogen protection liquid phase exfoliation and transfer process and vacuum heating is proposed for the first time.The AFM results show that the prepared black phosphorus is a few-layer black phosphorus nanosheets with a thickness of less than 10 nm.The characterization confirms that the oxygen element in the prepared black phosphorus nanosheets is equivalent to that of the bulk black phosphorus,which can effectively avoid the oxidation of black phosphorus during the preparation process.This method is generally applicable to easily degradable 2D materials such as MXene.The prepared black phosphorus gas sensor has a response of-33% to 50 ppb nitrogen dioxide at room temperature,and its response value and detection limit are the best values of the existing research on a single black phosphorus material gas sensor.This part of the research content is based on the degradation mechanism of black phosphorus,and the preparation method from few-layer black phosphorus nanosheets to gas sensors is studied.Problems such as poor attenuation need to be solved urgently.(2)Aiming at the problems of weak recovery and poor moisture resistance of black phosphorus gas sensors,BV was used to do surface doping of few-layer black phosphorus nanosheets,and this surface doping would preferentially occupy the high-energy adsorption sites on the surface of black phosphorus.Since these high-energy adsorption sites are tightly bound to the gas,it is difficult for nitrogen dioxide molecules to desorb,resulting in poor sensor recovery.The reduction of high-energy adsorption sites improves the recovery of black phosphorus on the one hand,and the moisture resistance on the other hand.At the same time,BV provides additional electrons during nitrogen dioxide adsorption to compensate for the reduced response due to the reduction of adsorption sites.The gas sensing test results show that the BP-BV composite gas sensor has high sensitivity,good recovery,repeatability,long-term stability and excellent moisture resistance at room temperature,in comparison with other black phosphorus-based nitrogen dioxide sensors Shows a clear advantage,with a minimum detection of 25 ppb nitrogen dioxide,while the theoretical detection limit is as low as 3.3 ppb.In this chapter,the working mechanism of BP-BV gas sensing is described in detail,and the experimental design scheme based on BV surface doping to reduce the electron adsorption sites on the surface of black phosphorus and improve the recovery and moisture resistance of black phosphorus is verified.The adsorption site is the main factor for its nitrogen dioxide sensitivity and easy degradation.(3)Existing researches on black phosphorus gas sensors mainly focus on detecting nitrogen dioxide and humidity,which have not fully exploited the potential of black phosphorus in gas sensing.In this paper,based on few-layer black phosphorus nanosheets,Sn O quantum dots were grown in situ on them by a one-step hydrothermal method,and the specific adsorption sites for ammonia gas were constructed on their surfaces to improve the response of black phosphorus to ammonia gas.Although BP and Sn O are both P-type semiconductors,the n-doping of Sn O on black phosphorus is realized by using the difference in work function of the two.Ultraviolet photoelectron spectroscopy shows that the Fermi level of black phosphorus nanosheets is lower than that of Sn O quantum dots.At the Fermi level,in order to reach the equilibrium of the Fermi level,electrons will be transferred from Sn O to black phosphorus.This electron transfer effectively improves the moisture resistance and thus improves its stability.The BP-Sn O composite sensor achieves excellent selectivity to ammonia by improving the response to ammonia and reducing the response to other gases.The results of the gas sensing test show that the BP-Sn O composite sensor has ultra-high sensitivity,good repeatability,stability,selectivity and moisture resistance to ammonia,and can detect ammonia as low as 0.5 ppm,and the theoretical detection is as low as 0.8 ppt.Compared with the gas sensors related to black phosphorus and Sn O,the BP-Sn O composite sensor has the best response to ammonia gas at room temperature.