The Synthesis Of Hierarchical SnO₂-based Nanomaterials And Their Gas Sensing Properties For Vocs

In recent years,with the rapid development of industry,air pollution is entering more and more serious,and air pollution has become an"invisible killer"which seriously endangers human health.Gas sensors have great application prospects in the detection,monitoring and alarming of toxic and harmful gases.How to enhance the gas sensing response and the selectivity is an urgent problem for researchers.In the past few years,it has become the focus that researchers modified the present sensor materials to improve their gas sensing properties.In this paper,SnO₂ was used as the research object.Various SnO₂-based hierarchical nanostructures were synthesized via a facial hydrothermal method,and were applied in VOCs gases detection.Furthermore,the structure,surface chemical states,electrochemical properties and the band structure of the as-prepared nanomaterials were systematically analyzed,which deeply investigated the mechanism of the enhancement in gas sensing performance.The main works are summarized as follows:1.SnO₂ nanotubes were produced by using EDTA as reaction template via a hydrothermal process;sphere-liked SnO₂nanostructures consisting of nanoparticles were synthesized by adding SDS into hydrothermal reaction;olive-like SnO₂nanostructures and flower-like SnO₂nanostructures were fabricated by via a hydrothermal process with different reaction times.The HCHO sensing measurements of these four different SnO₂nanostructures were carried out.Sphere-liked SnO₂nanostructures consisting of nanoparticles exhibit outstanding performances including low working temperature(175?),high response(125 towards 100 ppm HCHO),fast response and recovery times(12 s/45 s)and obvious selectivity.We suggested that these extraordinary performances of sphere-liked SnO₂nanostructures were attributed to unique nanostructure that nanospheres composed of nanoparticles.2.A series of Zn-SnO₂ samples with different Zn²⁺/Sn⁴⁺molar ratios were characterized to confirm the crystal structures.By regulating the reaction time,reaction temperature,and the concentration of urea and Na OH,we confirm the best synthesis conditions and Zn dopants were also confirmed to facilitate the orientation growth of SnO₂ crystal in alkaline environment.The growth mechanism of Zn-doping SnO₂ nanostructures was proposed.And the exposed crystal facets of each sample were analyzed by high-resolution transmission electron microscopy.More importantly,it was proved that the difference in the exposed facets was induced by low Zn-doping energy of(101)facets compared with that of the(110)facets.Moreover,the(101)facets of SnO₂ crystal were well engineered by mediating the concentration of Zn²⁺.Experimental and density functional theory(DFT)calculation results confirmed that the Zn doping in the(101)surface of SnO₂ nanostructure could facilitate the generation of ionosorbed oxygen species and optimize the electronic structure.When applied to detecting triethylamine(TEA),the SnO₂ nanostructure with exposed well-engineered(101)surface exhibited an excellent sensitivity towards TEA with the detection limit of 50 ppb at low working temperature(70?).And the sensing mechanism behind the corresponding redox process was first discussed in this work.3.The exploration of gas selectivity is an important part to understand surface gas-solid chemical reaction and improve gas sensing performance.In this chapter,the selectivity measurements of four different SnO₂ nanomaterials synthesized in chapter 3 were implemented,and the mechanism of selectivity was explored by DFT calculations.The in situ Fourier transform infrared(FTIR)spectrometer was used to investigate the completed reaction process between TEA and three Zn-SnO₂ nanomaterials exposed with different(101)facets,which identified the intermediate products and final products in these processes.The results revealed that the well-engineered(101)facets of SnO₂ crystal exhibited the strongest adsorption towards TEA,and the final products including H₂O,CO₂,and NO₂easily were disported from the well-engineered(101)facets,resulting in the high response and the fast response-recovery time.