Room Temperature Gas Sensing Properties Of Tin Based Dichalcogenide And Its Heterostructures

Gas sensors have become extremely necessary for effective detection and quantification of flammable,explosive,and toxic gases in a variety of applications,such as pollution abatement,hazard warning,and public security.With the advent of Internet of Things,it is urgent to develop low power consumption and high-performance gas sensors.Chemoresistive-type gas sensors have been widely employed due to their low cost,ease of fabrication,and well-integrated circuit compatibility.Unfortunately,traditional gas sensors based on metal oxide semiconductors usually need high operating temperature（200-600°C）,which revokes their advantages in intended applications in low-power integrated devices.Two-dimensional nanomaterials have promoted the development of gas sensing field.Layered metal dichalcogenides are particularly competitive by virtue of their large specific surface area,superb electrical properties,and high surface reactivity.Among LMDs,Sn-based dichalcogenides（SnS₂ and SnSe₂）have attracted considerable attention for low temperature and room temperature gas sensing due to their low cost,earth-abundant,and environmentally-friendly elements.However,the room temperature sensing for SnS₂ and SnSe₂ are still confronted with great challenges.For example,it is hard to achieve both high response and rapid response/recovery speed,and the sensors are usually designed to be effective for a single gas.Besides,it is usually lack of in-depth understanding of sensing mechanism.Therefore,to solve the above problems,this dissertation used structure design,novel heterostructure construction,and light illumination to rationally modify SnS₂ and SnSe₂ materials,so as to realize the optimization of room temperature gas sensing performance.Besides,the sensing mechanism was also analyzed to provide theoretical and experimental guidance for the design of other high-performance sensing materials.The main research works are as follows:The densely restacked structures of layered materials would obscure active sites and reduce the diffusion of detected gases,thereby weakening the intrinsic sensing performance.To solve this issue,hierarchical SnSe₂ was designed and prepared through a facile solvothermal method to realize high-performance NO₂ sensing.The flower-like hierarchical structure assembled from thin nanosheets has a large internal space,which is conducive for the adsorption and diffusion of gas molecules.Morphology analysis exhibited that with the reaction time increasing,SnSe₂ gradually grew from stacked sheets to flower-like hierarchical structure.BET and EPR results showed that the hierarchical SnSe₂ had a larger specific surface area and more Se vacancy defects,which could promote the adsorption of target gas molecules.The sensor based on hierarchical SnSe₂ demonstrated a high response value of 1200%（triple that of SnSe₂ nanosheets）toward 10 ppm NO₂.On the basis of preserving the hierarchical structure,the Sn O₂/SnSe₂heterostructure was constructed by in-situ oxidation calcination method with SnSe as the precursor,and the high-sensitivity H₂S sensing was realized at room temperature.Cosharing with the Sn atom enabled the formation strong Se-Sn-O chemical bonds at the heterointerface.DFT calculations demonstrated that there is a significant decrease in the bandgap in the heterointerface betweenSn O₂ and SnSe₂,indicating the faster electron transport kinetics.The gas sensor based onSn O₂/SnSe₂ heterostructure exhibited ultrahigh response toward H₂S at room temperature（resistance ratio=32to 10 ppm）,roughly 4.5 and 16 times higher than that of pure Sn O₂ and SnSe₂,respectively.The main reasons for the performance improvement of the heterostructure are as follows:on the one hand,due to the formation of n-n heterojunction,the electron depletion layer at the interface could provide additional gas adsorption sites to promote the adsorption of H₂S gas molecules,which is conducive to improving the sensitivity;On the other hand,the interface coupling of coshared Sn atoms effectively improves the charge transfer efficiency and accelerates the sensing response/recovery speed.From the perspective of heterointerface optimization,the rational designed Ag₂S/SnS₂ heterostructure was constructed by in-situ cation exchange method based on the easy substitution reaction of Sn atoms inSnS₂,thus to realize efficient detection of NO₂ at room temperature.The morphology characterization results showed that Ag₂S nanocrystals grew in situ on the surface of hierarchical SnS₂through the bridging S atoms.DFT calculations showed that the obvious charge rearrangement and accumulation occur at the heterogeneous interface between Ag₂S and SnS₂,which leads to strong electron coupling,thus effectively accelerating charge transport and improving the room temperature conductivity of the heterostructure.The sensor based on Ag₂S/SnS₂ heterostructure could simultaneously maintain high sensitivity and fast response/recovery characteristics to NO₂ at room temperature.The optimal Ag₂S/SnS₂ sensor demonstrated extremely high response values（286%）and short response/recovery time（17 s/38 s）to 1 ppm NO₂.To develop bifunctional gas sensors for advanced sensing platforms,we demonstrated a visible-light-modulated strategy to realize the bifunctional gases detection by using Bi₂S₃/SnS₂ heterostructure as the sensing materials.Compared to pristine SnS₂,the Bi₂S₃/SnS₂ heterostructure exhibited enhanced electronic conductivity and strong photo-absorption over the visible range.The sensor based on Bi₂S₃/SnS₂ demonstrated superior sensitivity toward NO₂ under light irradiation（14.0towards 500 ppb NO₂）,and high selectivity to H₂S in the dark at room temperature（12.3 towards 500 ppb H₂S）.Such distinctive light-dependent bifunctional sensing behavior of the Bi₂S₃/SnS₂ heterostructure is mainly attributed to that light irradiation could trigger the desorption of pre-adsorbed oxygen on the surface of sensing materials.Light illumination could regulate the number of NO₂ and H₂S adsorption sites.Thus,the sensor based on Bi₂S₃/SnS₂ successfully achieved bifunctional sensing capability:it was ultrasensitive to NO₂ under light irradiation but highly selective to H₂S in the dark.