Controlled Synthesis And Properties Of Hierarchical α-Fe₂O₃ Micro Structures

Hierarchical structure usually can be regarded as a higher dimension of micro- or nanostructure composed of low dimensional, nano-building blocks. Hierarchical structure with less agglomerated configurations exhibit high specific surface area and well-aligned nano-size pores, which is beneficial for the species diffusion, and hence the reaction rate can be increased. Hematite（α-Fe₂0₃）, as an environmentally friendly n-type semiconductor（Eg = 2.3eV） and the most stable iron oxide with low fabrication cost, has applications in a variety of fields.Flower- like α-Fe₂0₃ microspheres with porous shell have been synthesized by a facile solution method reacted at 95℃ using sodium citrate as surfactant. The growth mechanism of hierarachical structure was discussed from the morphology evolution with the reaction time and the amount of sodium citrate added. Sodium citrate may induce the polar growth of the α-Fe₂0₃ into nanosheets, and then nanosheets self-assembled into flower- like microspheres. The acetone gas-sensing properties were measured at the working temperature of 340℃. The response to 100 ppm acetone is 15.9, with the response and recovery time of 13 s, 39 s, respectively. The response to low acetone concentration as 1 ppm can reach the value of 6.3. The flower- like α-Fe₂0₃ exhibits a significantly improved photocatalytic activity in the photodegradation of Congo red, the degradation rate reaches 95%. Hierarchical structure could not only benefit the transfer of the charge but also allows the rapid diffusion.In order to achieve the regulation of the microstructure, effect of the calcined temperature and hydrothermal treatment of the precursor on microstructure was studied. As the calcined temperature increases, the sample becomes loosely and crystallinity increased. Meanwhile, the nanosheets become rough and a large number of pores appear. The samples’ s response to 100 ppm acetone at 340℃ was 16.9, with the response and recovery time of 15 s, 20 s, respectively. After hydrothermal treatment, the structure becomes more loosely, more porous shell appear. The response to 100 ppm acetone at 340℃ reaches 30.3, with the response and recovery time of 10 s, 21 s, respectively.In this dissertation, α-Fe₂0₃ hollow microspheres were also synthesized through a hydrothermal method. The growth mechanism can be regarded as combining a quasimicroemulsion through Ostwald ripening with a self-assembly process in constructing the hollow microspheres. And the hierarchical structure about 1-2 μm is assembled from nanoballs with diameter 200-300 nm. The response to 100 ppm acetone at 340℃ was 10.2, with the response and recovery time of 1.5 s, 0.2 s, respectively. The degradation rate of Congo red is 64%.