Role of assisting reagents on the synthesis of α-Fe 2 O 3 by microwave-assisted hydrothermal reaction

Currently, the controlled synthesis of well-defined metal oxide nanoparticles has received intense scientific attention to modulate the properties of materials through the size and shape. This paper focuses on providing a better understanding about the growth, morphology, size, and crystal structure of α-Fe2O3 nanoparticles synthesized by a microwave-assisted hydrothermal route, considering a detailed analysis of the influence of the reaction temperature, time, and ratio of assisting reagents (sodium citrate and urea) as well as the effects of calcination temperature. According to X-Ray Diffraction analysis, the α-Fe2O3 crystalline phase was directly prepared at 200 °C for 60 min using only urea, while materials with low crystallinity were obtained using only sodium citrate as well as sodium citrate:urea (in both ratio 1:1 and 2:2.5). Upon calcination at 600 °C, the crystallization of α-Fe2O3 started in the material prepared with a sodium citrate:urea ratio of 2:2.5. Scanning and Transmission Electron Microscopies results revealed that the materials synthesized using urea, sodium citrate, and sodium citrate:urea ratio of 1:1 are formed by nanoparticles less than 100 nm without a defined morphology, whereas the materials prepared using sodium citrate:urea ratio of 2:2.5 showed well-defined nanospheres with average sizes between 150 and 250 nm constituted by self-assembled crystals smaller than 10 nm. The shape and size of the nanospheres did not undergo significant changes even at high thermal treatments, such as 800 °C. Based on equilibrium diagrams, the role of each chemical agent was disclosed, indicating that the modulated precipitation through soluble complexes is a very important factor in controlling the hierarchical organization of the particles to form the nanospheres. This study demonstrates the versatility of sodium citrate and urea as assisting reagents to prepare a variety of α-Fe2O3 nanoparticles. Besides, these results could be useful in extending new ideas for the synthesis of other nanomaterials.