Hierarchical Composite Structure Of Metal Oxide Semiconductor And Its Sensing Enhancement Mechanism

In recent years,modern industry and technology have greatly improved the living standard of human society.However,modern technology has also brought side effects to our daily life,which has seriously affected our daily life and health.In addition,the use of specific biomarkers in exhaled breath for disease diagnosis has attracted widespread attention because of its key advantages in noninvasive,real-time and potentially inexpensive diagnosis.Therefore,a variety of sensors have been designed and developed to monitor the ambient air quality and human health,and prevent the occurrence of some dangerous situations.Among many sensors,Metal Oxide Semiconductor(MOS)gas sensor is widely studied and applied in practical production and life because of its portability,durability,easy maintenance and high precision.However,MOS gas sensors still have some disadvantages in real life,such as high energy consumption,poor selectivity,and harsh environment and so on.In order to improve the sensitivity of the sensor,the hierarchical structure material is often used;in order to improve the gas selectivity of the sensor,the composite material is often used.Based on this,this paper focuses on the design and control of valence state,hierarchical topology and morphology of oxides,and systematically studies the synthesis,growth mechanism and sensing enhancement mechanism of hierarchical composite MOS.The research results of this paper provide a series of technical routes and methods for the synthesis and preparation of high-performance hierarchical composite MOS sensing materials with simple and short process.At the same time,based on the valence state design to improve the sensing performance of materials,it also provides a new idea for the development of a new generation of high-performance hierarchical composite variable valence compound sensing materials.The specific contents of this paper are as follows:(1)Monovalent Zn₄CO₃(OH)₆·H₂O nanosheets were prepared by hydrothermal method as precursors.Hierarchical ZnO with different morphologies was prepared by controlling the calcination temperature(350?,400?,500?600?and 700?),and the gas sensing properties also were changed.The experimental results show that the best process condition for obtaining the hierarchical ZnO nanonets is the calcination temperature of 500?.When the working temperature is 280?,the ZnO nanonets sensor shows the highest sensitivity in 100 ppm C₂H₅OH environment,which is 398;the corresponding response and recovery time are 4 s and 63 s,respectively;the minimum detection limit concentration for C₂H₅OH gas is 5 ppm,and the sensitivity is3.In addition,the ZnO nanonets sensor also shows good reproducibility,selectivity and humidity resistance.Combined with the experimental characterization analysis,the model of grain boundary sensing enhancement effect with univalent hierarchical structure was established.A large number of oxygen vacancies at the grain boundaries lead to the increase of the thickness of the depletion layer,and then lead to a greater change of the grain barrier in air and ethanol environment.(2)The hierarchical structure of multivalence Sn_xO_y was prepared by hydrothermal and calcination method.First,the hierarchical structure Sn₃O₄nanoflower with Sn ion as+2.67 valence was prepared by hydrothermal synthesis,and then the SnO₂ nanoflower of Sn+4 with valence structure was prepared by adjusting calcining temperature.With the increase of calcination temperature,the surface of Sn₃O₄ is gradually oxidized to SnO₂,and different surface states of Sn based oxides are obtained.The experimental results show that the Sn₃O₄ sensor does not show gas sensitivity to reducing gas,while the SnO₂ sensor shows good gas sensitivity to reducing gas.Especially for H₂S gas,when the operating temperature is 180?,SnO₂sensor has a high sensitivity in 2 ppm H₂S environment,which is 2.6;the corresponding response and recovery time of 127 s and 273 s,respectively.In addition,the results show that SnO₂ sensor has good reproducibility,selectivity and humidity resistance.It is found that the formation of gas sensing properties is due to the surface bonding state caused by the fact that the surface of the material does not occupy the electron orbit.A multi valence hierarchical model of surface state sensing mechanism is established.The influence of surface state constructed by different valence states on gas sensitivity is related to the electronegativity of adsorbed surface atoms and the electronic interaction between gas molecules and adsorbed surface.(3)The self-assembled composite valence hierarchical structure W_xO_y was prepared by hydrothermal method in one step.By adjusting the ratio of enthanol to H₂O in the solvent to control the hydrolysis rate of tungsten ions,hierarchical structure W₁₈O₄₉ nanowires,self-assembled hierarchical composite WO₃ nanosheets/W₁₈O₄₉nanowires(HHW)and hierarchical WO₃ nanoflowers were prepared.The results show that the self-assembly growth of complex valence hierarchical structure is mainly controlled by the competition and balance among surfactant concentration,precipitation coefficient and hydrolysis rate of tungsten ion.The optimum process condition for obtaining the hierarchical structure of complex valence is 0.175 ml H₂O in ethanol solution.When the operating temperature is 160?,HHW sensor has a high sensitivity in 10 ppm NO₂ environment,which is 1687;the corresponding response and recovery time of 36 s and 13 s,respectively;the minimum detection limit concentration of NO₂ is 100 ppb and sensitivity is 3.In addition,the HHW sensor also showed good reproducibility,selectivity and humidity resistance.Combined with the experimental characterization analysis,a gas sensing enhancement model with complex valence hierarchical structure was established,which is the synergistic enhancement effect of nanowires and nanosheets.In the complex valence structure,W₁₈O₄₉ nanowires provide fast electron separation and transport,and WO₃ nanosheets provide a large number of adsorption sites.(4)In₂S₃ nanoflower with hierarchical structure was prepared by rapid and mild microwave assisted hydrothermal synthesis.Subsequently,the In₂S₃/In₂O₃heterojunction nanomaterials with different interfaces were prepared by adjusting calcination time.The gas sensing properties of the materials change accordingly.The experimental results show that the best process conditions for obtaining In₂S₃/In₂O₃ are calcination time of 1 hour(ISO-1),and the prepared materials have the best gas sensitivity.When the operating temperature is 160?,ISO-1 sensor has good sensitivity in 10 ppm NO₂ environment,which is 251;the corresponding response and recovery time of 95 s and 31 s,respectively.The minimum detection limit concentration of NO₂is 100 ppb and the sensitivity is 2.In addition,the results show that ISO-1 sensor has good reproducibility,selectivity and humidity resistance.Through further experimental analysis and theoretical calculation,it is shown that the highest adsorption energy is obtained when the interface heterojunction is constructed.The interface enhancement effect model of hierarchical structure is established.When the interface is constructed,the dielectric constant of the material increases and the hole concentration decreases,which leads to the Debye length in the material and increases the sensing performance.