Shape-controlled Synthesis And Properties Of α-Fe₂O₃

Hematite（α-Fe₂O₃） with the narrow band gap is a stable n-type semiconductor and has the advantages of nontoxicity and low processing cost. At present, the researchers have prepared a series of iron oxides with various morphologies through different synthesis methods and find that the properties of the hematite closely depend on its morphologies and nanostructures. So, the synthesis of the iron oxide with unique morphologies is important for the development of the functional devices. Here,we work on the preparation of hematite nanoparticles and their photocatalytic or gas sensing properties. Based on the preparation of hematite nanoparticles with different morphologies, the mechanisms of the photocatalysis and gas sensing were investigated in depth. By analyzing the hematite nanoparticles’ surfaces, we discovered the associations between the microstructures and the properties of iron oxides. The same analysis methods were applied on the studies Ag/α-Fe₂O₃ microspheres. The primary research contents and results are listed as follows:1. Zn-doped α-Fe₂O₃ microcubes were prepared via a facile hydrothermal method with the existence of Fe（NO3）3 as the ferric source and zinc ions as the structure-directing agents. The morphologies, composition and the nanostructure were illustrated by the data of XRD、EDS、XPS、SEM and TEM. The obtained α-Fe₂O₃ microcubes with the side length ranging from 200 nm to 300 nm contain 5.7 at % of zinc atomic. The influences on the morphologies of the samples were investigated by changing the reaction times, temperatures and the ratio of CFe/CZn. The presence of Zn2+ is the crucial factor that induces the formation of a quasi-cubicstructure, which results from the selective adsorption of [Zn（NH3）4]2 + on the surfaces of the precursor of the hematite particles and their ability to coordinate with ferric ions. In addition,the gas sensing properties of sensors based on as-synthesized Zn-doped α-Fe₂O₃ microcubes to acetone were investigated. The value of S reaches to 44.3 upon exposure of 300 ppm acetone at the 240 °C.2. The novel α-Fe₂O₃ dodecahedron nanocrystals with the diameters of 200 nm were prepared by a facile one-step hydrothermal method using anion（F-）as structure agent.The homogeneous dodecahedron nanoparticles were obtained by optimizing the experimental conditions including reaction temperatures and time. A possible formation mechanism was also proposed according to the morphologies of the products obtained at different stages. To demonstrate the potential applications, the photocatalytic activity of the as-prepared samples（0.5, 1, 2 and 4 h） was evaluated byphoto-degradation of rhodamine B from aqueous solution. Results show that theα-Fe₂O₃ dodecahedron with exposed（012） plane exhibits significantly improved photocatalytic activities.3. Microspheres constructed with a-Fe OOH nanorods were fabricated by SDBS assisted hydrolysis process in an ethanol/H2 O co-solvent system. The evolution formation revealed that SDBS was critical for controlling the assembly of the freshly formed nanocrystallites. The α-Fe OOH microspheres dispersed into the Ag NO3 solutions to obtain the Ag+/α-Fe OOH. Then Ag+/α-Fe OOH transformed to Ag/α-Fe₂O₃ microspheres through the process of calcination. From the TEM of Ag/α-Fe₂O₃ microspheres, we found that Ag nanoparticles with the diameters of 5 nm grew on the surface of nanorods. The formation mechanism of Ag/α-Fe₂O₃ microspheres was proposed by tracking the structure of the products at different growth stages. At240 °C, the Sr of Ag/α-Fe₂O₃ microsphers reaches to 26 in the atmosphere of 300 ppm ethanol gas, which is two times larger than that of the pure nanoparticles. The Ag additive served as an active catalyst creates more active sites that are believed to be crucial for the enhanced sensitivity.