Research On Gas Sensing Mechanism And Response Characteristics Of Molybdenum Selenide-based Sensors For Detecting Harmful Gases In Greenhouse

With the rapid improvement of China’s agricultural modernization level and the increasing material needs of people,the planting level and output in the greenhouse are put forward higher requirements,the growth and development of crops in the greenhouse and the physical condition of workers are closely related to the air quality in the greenhouse.Because the greenhouse is in a closed environment for a long time,if the ventilation is not timely,it is easy to cause the accumulation of harmful gases.These harmful gases will not only harm the health of workers in the greenhouse,but also reduce the vitality of plants with strong growth and affect the quality.It is an important measure to ensure productivity and production efficiency by effectively detecting the composition and concentration of harmful gases in greenhouse and then controlling them within an appropriate concentration range.Molybdenum selenide（Mo Se₂）has been widely used in the field of gas sensing because of its excellent physical and chemical properties and multi-dimensional hierarchical structure.Pure Mo Se₂ nano materials have low response to harmful gases in greenhouse and narrow working range.Metal doping can effectively improve their adsorption characteristics and gas sensing response.At present,the metal doping modification and gas sensing mechanism of Mo Se₂ based sensitive materials are not perfect,so they can not achieve good application results.Therefore,it is of great significance to study the micro adsorption mechanism and actual response characteristics of Mo Se₂ based gas sensor for detecting harmful gases in greenhouse.The main harmful gases NH₃,SO₂ and NO₂ in the greenhouse were selected as the research object in this paper.Based on the density functional theory,the intrinsic and the structural parameters and electronic properties of noble metal palladium（Pd）and platinum（Pt）doped Mo Se₂ crystal gas adsorption system are discussed in detail.The modification mechanism and gas sensing mechanism of Pd and Pt doping on Mo Se₂sensitive materials are systematically studied;Based on the hydrothermal method,the intrinsic and metal Pd and Pt doped Mo Se₂ based sensitive materials with multi-dimensional hierarchical structure were prepared,and their chemical element composition and crystal morphology and structure were studied by means of microscopic characterization;The prepared Mo Se₂ based sensitive material is used to make a side heated planar gas sensor.The response characteristics of the sensor to the main harmful gases NH₃,SO₂ and NO₂ in three kinds of greenhouse are tested through the built gas sensing test platform to verify the feasibility of the theoretical mechanism.The main research work and conclusions of this paper are as follows:（1）Based on the density functional theory,the Intrinsic,Pd and Pt doped Mo Se₂crystal plane model are established to explore the doping modification mechanism of Pd and Pt on Mo Se₂ materials from two aspects of structural parameters and electronic properties.After Pd doping,a new density of states peak is formed at the top of the valence band,which reduces the energy band gap,reduces the potential barrier required for electron transition,and is conducive to improving the probability of charge transition.Pt doping is mainly due to the strong hybridization between the d-electron orbital of Pt atom and Mo Se₂ system,the emergence of a new characteristic peak of density of states between the top of valence band and the bottom of conduction band,the formation of an obvious intermediate energy level,the significant reduction of the energy required for electronic transition and the expansion of several rates of electronic transition.（2）The Intrinsic,Pd and Pt doped Mo Se₂ based models and their adsorption models for NH₃,SO₂ and NO₂ were established.The adsorption properties of Mo Se₂ based gas sensing materials for harmful gases NH₃,SO₂ and NO₂ in greenhouse were analyzed by calculating the parameters of each model,such as atomic structure change,doping binding energy,gas adsorption energy,charge transfer amount,electronic density of states,deformation charge density and so on.The results show that the intrinsic Mo Se₂ system has weak adsorption capacity for NH₃,SO₂ and NO₂.After adsorbing gas,the absolute value of adsorption energy of Mo Se₂ system is less than 0.6e V.The charge transfer basically occurs in the gas molecule itself or the material itself,which belongs to weak physical adsorption.The adsorption energy is negative after Pd or Pt doping,and the adsorption process can proceed spontaneously.At the same time,the absolute value of the adsorption energy increases to more than 0.6e V,indicating that the adsorption changes from weak physical adsorption to chemical adsorption.For the adsorption of NH₃,SO₂and NO₂,Pd-Mo Se₂ system shows better adsorption capacity for NH₃,with the maximum charge transfer capacity of 0.238e,while Pt-Mo Se₂ system has the best adsorption capacity for NO₂,with the maximum charge transfer capacity of 0.254e.（3）The Pure,Pd and Pt doped Mo Se₂ flower like sensitive materials were prepared by hydrothermal method.The successful synthesis of flower like Mo Se₂ based materials and the beneficial effects of the introduction of metal Pd and Pt on the flower like structure of Mo Se₂ were verified by XRD,SEM,EDS,XPS and other morphology and structure characterization methods.The three prepared Mo Se₂-based samples all present multidimensional hierarchical nanoflower-like structures.After Pd doping,the surface of Mo Se₂ flower-like materials is loaded with fine metal particles.Such rough surface morphology can provide more active sites for the adsorption of gas molecules.The flower-like nanomaterial of Pt-doped Mo Se₂ is composed of two-dimensional nanosheets crossed and stacked irregularly.Such a complex layered form provides a larger specific surface area for the adsorption of gas molecules.（4）Based on the built gas sensing test platform,the concentration response characteristics,response recovery characteristics and repetition stability of the Mo Se₂based gas sensor to NH₃,SO₂ and NO₂ were tested at room temperature.The gas sensing test shows that Pd or Pt doping can effectively improve the detection performance of Mo Se₂ sensor for NH₃,SO₂ and NO₂.The response of Pd-Mo Se₂ sensor to 20μL/L NH₃,SO₂ and NO₂ at room temperature is 4.12,3.53 and 3.95,which are 2.67,2.04 and 2.39times of that of Pure Mo Se₂ sensor,respectively.Moreover,the detection limit of NH₃can reach 5μL/L,and the detection of low concentration of NH₃ can be realized.The response of Pt-Mo Se₂ sensor to 20μL/L NH₃,SO₂ and NO₂ at room temperature is as high as 4.72,5.92 and 6.88,which are 3.07,3.42 and 4.17 times of Pure Mo Se₂ sensor,respectively.The response of Pt-Mo Se₂ sensor to 2μL/L NO₂ can reach 3.88.The results showed that it could detect NO₂ with low concentration and high sensitivity.