| Porous metal oxides are an important class of sensitive materials,and their gas sensing properties have a great influence on their morphologies,structure and composition etc.MOFs are a novel class of highly porous materials that constructed by metal ions or metal ion clusters and poly-functional organic ligands,and have the advantages of simply tunable sizes and morphologies.Based on the structural characteristics,MOFs can be as the self-sacrificial templates for the fabrication of porous metal oxides and nanocomposites.The objective of this work is to synthesizeα-Fe2O3 and nanocomposites with porous structures using Fe-based MOFs as templates,the differences of gas sensing behavior related to the structures were investigated in detail,and the gas sensing mechanism of as-prepared samples was also proposed.The main researches are shown as follows:1.α-Fe2O3 porous nanorods(PNRs)with controlled morphologies were simply synthesized by thermolysis of Fe-based metal-organic framework(MIL-88A)at 500℃.The as-resulted α-Fe2O3 nanostructures were well preserved according to the original MIL-88A morphology.The results indicated that the amount of fumaric acid(FMA)was the key to adjust the morphologies of Fe-MOF precursors.With the amount of fumaric acid decreasing,the lengths of α-Fe2O3 nanorods increased from 800 nm to 6 μm,and the width decreased from 200 nm to 100 nm.Further experiments were conducted to explain the different gas sensing properties of the as-prepared α-Fe2O3 PNRs.The sensitivities of sample S1 towards 1000 ppm of acetone gas reached 19.3,and the gas detection range of 20-1000 ppm.The high sensitivity of sample S1 was mainly attributed to the relative higher specific surface area(27.48 m2 g-1)and higher percentages of oxygen vacancies(35.7%)compared to the other samples.In addition,according to the experimental results the gas sensing mechanism with oxygen vacancy(Ov)as the main medium was proposed.2.A novel porous heterostructure of TiO2/α-Fe2O3 nanorods were synthesized by the hydrolysis and deposition process using MIL-88A as sacrificial templates.The shell thickness of TiO2 can be controlled through adjusting the amounts of added TBOT.The gas sensing tests indicate that the TiO2α-Fe2O3 heterostructure exhibits significantly improved gas sensitivity and selectivity properties in comparison with the pristine α-Fe2O3 nanorods sensor.Thereinto,the TiO2/α-Fe2O3-2 sensor exhibits relatively good sensing performance,the response to 1000 ppm acetone is up to 27.02,as high as 4 times compared to the pure α-Fe2O3(7.14).The detection limit for acetone could be lower than 100 ppb.When α-Fe2O3 nanorods was decorated by TiO2,the electrons in TiO2 transfer to α-Fe2O3 nanorods,form an electron accumulation layer in α-Fe2O3.The electron accumulation layer contributes to the adsorption of more oxygen molecules,which can enhance the gas-sensing properties of TiO2/α-Fe2O3 heterostructure.3.Spindle-like porous Zn-doped α-Fe2O3 nanostructure was fabricated by the thermal decomposition of the Zn-doped MIL-88B precursors.The results show that the dosage of Zn2+ had a remarkable effect on the morphology of product.With increasing Zn2+ concentration,the morphology of α-Fe2O3 evolved from typical octahedron to regular dodecahedron.Gas-sensing measurements revealed that the porous Zn-doped α-Fe2O3 nanostructure has been changed from the n-type semiconductor to p-type semiconductor,and present the highest sensitivity to acetone and a low detection limit(at least 100 ppb).According to the XPS results,the percentages of oxygen vacancies(39.58%)of α-Fe2O3 increased after Zn doping,and its resistance reduced accordingly.Therefore,the sensitivity of Zn-α-Fe2O3 is improved. |
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