Fe₂O₃ :Al³⁺ obtained by the modified Pechini method

Full text: Hematite (α-Fe₂O₃ ), the most stable iron oxide, with n-type semiconducting properties under ambient conditions, is of scientific and technological importance because of its application as catalyst, pigment, magnetic material, gas sensor, and for lithium-ion batteries [1]. The present work aims at synthesizing and characterizing Fe₂O₃ , Fe_1.9Al_0.1O₃ , Fe_1.8Al_0.2O₃, Fe_1.7Al_0.3O₃ powders, displaying the hematite structure. The modified Pechini method was used for this synthesis, with control of the carbon elimination to decrease organic matter and avoid aggregates. The polymeric resins underwent a primary calcination at 350 deg C for 2 h, leading to the partial elimination of carbon and forming the powder precursors, which were milled in a spex mill and calcined in O₂ atmosphere at 300 deg C for 12 h. Another calcination was carried out between 600 and 1000 deg C for 4 h. The characterization was done by X-ray diffraction (XRD), UV-visible and infrared spectroscopy were used for the characterization. The XRD patterns indicated the presence of the hematite phase in the samples calcined between 700 and 1000 deg C. For the samples Fe_1.8Al_0.2O₃ and Fe_1.7Al_0.3O₃ , magnetite was formed as intermediate phase at 600 deg C, indicating that a reducing atmosphere is formed during synthesis. The dominant features of the hematite infrared spectrum are absorption bands in the wavenumber range from about 700 to 200 cm^-1 which are caused by crystal lattice vibrations [2]. In this work, the bands were observed at 443 cm^-1 and 534 c^-1. According to the literature [3], the alpha-hematite gap is approximately 2.1 eV, while the gap found in this work is close to this value, being equal to 1.9 eV. References: [1] C. Wu et al. J. Phys. Chem. B. 110, 17806 (2006) [2] S. Krehula et al. J. Alloys. Compd. 545, 200 (2012) [3] C. Xia et al. Phys. Lett. A. 377, 1943 (2013). (author)