| Formaldehyde,the main gas of indoor volatile organic compounds,has become the primary source of pollution of working and living environment,which seriously threatens people’s health,life,and property safety.Thus,real-time monitoring of formaldehyde in environmental gases is of great significance.In the field of gas sensor technology,Sn O2,as a typical n-type semiconductor gas-sensing material,has been a hot spot of gas sensor research due to its good thermal stability and chemical stability,high sensitivity to certain gases and low cost.Nevertheless,Sn O2-based gas sensor is difficult to meet the needs of practical application,which attributes to the low response and the high power consumption toward formaldehyde.Therefore,it is of great value to improve the performance of gas sensor by adjusting Sn O2microstructure and constructing composite materials.In this thesis,the first-principles simulations based on the density functional theory were performed to calculate the crystal structure,electronic structure and gas adsorption properties on surfaces for purpose of investigating the synergistic enhancement mechanism of Sn O2/graphene composite material and providing theoretical basis for subsequent experimental study.Sn O2/r GO composite nanostructures with six morphologies in different dimensions were synthesized via hydrothermal approach,including Sn O2 nanoparticles/r GO nanocomposites,Sn O2raspberry-like hierarchical structure/r GO nanocomposites,Sn O2nanorods arrays/r GO nanocomposites,Sn O2 nanosheets arrays/r GO nanocomposites,Sn O2 needle-like sphere 3D hierarchical structure/r GO nanocomposites,and Sn O2 cube-like 3D hierarchical structure/r GO nanocomposites.The crystal structure,morphology characteristics,surface properties and formaldehyde sensing performances of different dimensions Sn O2/r GO composite nanostructures were studied.The main research contents and results are as follows:（1）Theoretical calculation of gas adsorption on the surface of Sn O2/graphene composite materialThe first-principles simulations based on the density functional theory were performed to calculate the adsorption properties of O2 and HCHO on graphene@Sn O2 surface.The calculation results indicate that O2 tends to adsorb on the graphene@Sn O2（101）surface when O2 and HCHO molecules are adsorbed on graphene@Sn O2（110）surface and graphene@Sn O2（101）surface,respectively.Furthermore,more electrons transfer from the surface to O2,which is conducive to the formation of more adsorbed oxygen ions,leading to the high-resistance state of graphene@Sn O2 composite material.The adsorption energies of HCHO on graphene@Sn O2（101）surface are also lower.It is beneficial for formaldehyde to react fully with adsorbed oxygen ions,thus causing more significant resistance changes during the process of gas-sensing reaction.（2）Construction of zero-dimensional Sn O2/graphene composite nanostructures with formaldehyde sensing propertiesThe two zero-dimensional Sn O2/graphene nanostructures were constructed by hydrothermal approach,namely Sn O2 nanoparticles/r GO nanocomposites and Sn O2 raspberry-like hierarchical structure/r GO nanocomposites.The response of Sn O2nanoparticles/r GO composite nanomaterials to 50 ppm HCHO is 99 at operating temperature of 250℃,while the response of Sn O2 raspberry-like hierarchical structure/r GO composite nanomaterials to 50 ppm HCHO reaches as high as 2420 at operating temperature of 100℃.The notable discrepancies on formaldehyde sensing performances of Sn O2 nanoparticles/r GO composite nanomaterials and Sn O2 raspberry-like hierarchical structure/r GO composite nanomaterials are mostly related to the differences of the form of composite structure,the grain size and the crystallization degree.（3）Construction of one-dimensional Sn O2/graphene composite nanostructures with formaldehyde sensing propertiesSn O2 nanorods arrays/r GO composite nanomaterials were fabricated by a nanocrystal-seeds-directing hydrothermal approach.Through the study of the crystal growth process of Sn O2 nanorods arrays/r GO composite nanomaterials,the mechanism of crystal growth is proposed.Sn O2 nanorods arrays/r GO composite nanomaterial exhibits a special 1D array structure/2D multi-level and multi-dimensional composite structure,and possesses the largest specific surface area and the highest concentration of adsorbed oxygen ions.Therefore,Sn O2nanorods arrays/r GO composite nanomaterials demonstrate the best formaldehyde sensing performances,whose response to 50 ppm HCHO is as high as 5594 at operating temperature of50℃.（4）Construction of two-dimensional Sn O2/graphene composite nanostructures with formaldehyde sensing propertiesOrdered Sn O2 nanosheets arrays grown on r GO as substrate were fabricated via a one-pot hydrothermal approach.The response of Sn O2 nanosheets arrays/r GO composite nanomaterials to 50 ppm HCHO reaches 324 at operating temperature of 150℃.Sn O2 nanosheets arrays/r GO composite nanomaterials exhibit mediocre formaldehyde sensing performances,which may be mainly attributed to the significantly lower concentration of adsorbed oxygen ions among the six different Sn O2/r GO composite nanostructures.（5）Construction of three-dimensional Sn O2/graphene composite nanostructures with formaldehyde sensing propertiesThe two three-dimensional Sn O2/graphene nanostructures were constructed by hydrothermal approach,namely Sn O2 needle-like sphere 3D hierarchical structure/r GO nanocomposites and Sn O2 cube-like 3D hierarchical structure/r GO nanocomposites.Through the study of formaldehyde sensing performances,the response of Sn O2 needle-like sphere 3D hierarchical structure/r GO composite nanomaterials to 50 ppm HCHO reaches 128 at operating temperature of 200℃,while the response of Sn O2 cube-like 3D hierarchical structure/r GO composite nanomaterials to 50 ppm HCHO reaches 359 at operating temperature of 200℃.The notable discrepancies on formaldehyde sensing performances of Sn O2 needle-like sphere3D hierarchical structure/r GO nanocomposites and Sn O2 cube-like 3D hierarchical structure/r GO nanocomposites are mainly attributed to that the concentration of adsorbed oxygen ions and ID/IG ratio of Sn O2 cube-like 3D hierarchical structure/r GO nanocomposites are higher than that of Sn O2 needle-like sphere 3D hierarchical structure/r GO nanocomposites. |
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