ZIF-8 Derived Synthesis Of α-Fe₂O₃ Based Micro-nano Heterostructures And Their Gas Sensing Properties

In this paper,a series ofα-Fe₂O₃ micro/nanostructures with different morphology were synthesized by solvothermal method,including one-dimensional nanorods,two-dimensional nanosheets and three-dimensional rod-like self-assembled structures.By adjusting the reaction conditions,the growth mechanism and gas sensing properties ofα-Fe₂O₃ micro/nanostructures were studied.The loading of heterogeneous phases onα-Fe₂O₃ surface and the functionalization processes were realized by hydrothermal method,ultrasonic method and room temperature stirring method.The gas sensing properties ofα-Fe₂O₃-based composites were significantly enhanced,and the gas-sensitizing enhancement mechanism for different systems was investigated.The specific research contents of this paper is as follows:1.Regulation of the structure ofα-Fe₂O₃/SnO₂/ZnO composite nanorods and the study of their gas sensing properties.The precursor of Fe₂（OH）₂CO₃ was prepared by hydrothermal method in the glycerol and ethanol system with NaHCO₃ as the base source.And the one dimensionalα-Fe₂O₃ nanorods were prepared by calcining at 450 ^oC.Then,α-Fe₂O₃/SnO₂nanorods were obtained by stirring at room temperature with SnCl₂·2H₂O as the tin source in the HCl system of 0.1 M.The experimental results show thatα-Fe₂O₃/SnO₂ nanorods exhibit different distribution states when the concentrations of SnCl₂·2H₂O are 1.3,2.2,and 3.1 mmol,respectively.When the concentration of SnCl₂·2H₂O is 2.2 mmol,the nanorods can be composed of SnO₂ andα-Fe₂O₃ nanoparticles uniformly distributed and exhibit the optimal performances of sensitivity and selectivity,which is due to the combination of uniform heterojunctions and the effect of large surface area.The effect of ZIF-8 content on the morphology and composition ofα-Fe₂O₃/SnO₂/ZnO nanorods was investigated by adjusting the concentrations of 2-methimidazole and Zn（NO₃）₂·6H₂O.When the mole ratio of2-methimidazole and Zn（NO₃）₂·6H₂O is 1:1.5,the resulting composites show the highest gas sensing properties,which can be mainly ascribed to the formation of multiphase heterojunctions for increasing the electron transmission rate.2.A series of hexagonal-likeα-Fe₂O₃/ZnO/Au nanoplate heterostructures with tunable morphologies and superior ethanol gas-sensing performance were successfully synthesized via the facile multi-step reaction processes.Hexagonal-likeα-Fe₂O₃ nanoplates with uniform size around 150 nm were employed as new sensor substrates for loading the well-distributed ZnO and Au nanoparticles with adjustable size distribution on the different surfaces.The Brunauer-EmmeQ-Teller（BET）surface areas ofα-Fe₂O₃ andα-Fe₂O₃/ZnO samples were evaluated to be 37.94 and 61.27 m²/g,respectively,whileα-Fe₂O₃/ZnO/Au composites possessed the highest value of 79.08 m²/g.It was exciting that theseα-Fe₂O₃ based functional materials can exhibit outstanding sensing properties to ethanol.When the ethanol concentration was 100 ppm,the response value ofα-Fe₂O₃/ZnO/Au composites can reach up to 170,which was 14.6 and 80.3 times higher than that ofα-Fe₂O₃/ZnO and pureα-Fe₂O₃,respectively.The recycling stability and long time effectiveness can be availably maintained within 30 days,as well as the response and recovery times were shortened to 2 and 5 s,respectively.Significantly,the response value ofα-Fe₂O₃/ZnO/Au composite was still up to 63 at an operating temperature of 280 ^oC even though the ethanol concentration decreased to 10 ppm.The enhanced gas sensing mechanism would be focused on the synergistic effects of phase compositions,surface heterogeneous structures,large specific surface area,and the selective depositions of Au nanoparticles inα-Fe₂O₃/ZnO/Au sensors.Specially,the synergistic effect of different surface heterostructures referring toα-Fe₂O₃/ZnO/Au and their novel electron transport processes on the surfaces are first investigated and discussed in details.It is expected that hexagonal-likeα-Fe₂O₃/ZnO/Au nanoplate heterostructures with excellent sensing performance can be the promising highly-sensitive materials in the actual application for monitoring and detecting ethanol.3.Synthesis of the self-assembledα-Fe₂O₃/ZnFe₂O₄ composites induced by ZIF-8 and study of their gas sensing properties.With FeCl₂·4H₂O as the iron source,a three-dimensional self-assembled FeOOH micro/nanostructures was prepared by hydrothermal method in aqueous solution.A series ofα-Fe₂O₃ structures from the nanorods to self-assembled structures can be obtained at 450 ^oC by adjusting the concentration of FeCl₂·4H₂O in the system.The test results show that the tubable morphologies and particle sizes ofα-Fe₂O₃ products play an important role for the gas sensitive properties to TEA.The effects of ZIF-8 contents on the morphology and composition ofα-Fe₂O₃/ZnFe₂O₄ composites were investigated by controlling the concentrations of 2-methimidazole and Zn（NO₃）₂·6H₂O.It can be found that the core/shell microstructural heterojunctions will predominantly affect the performance ofα-Fe₂O₃/ZnFe₂O₄composites.