Rapid and sustainable synthesis of iron(III) oxide nanoparticles with photocatalytic properties using microwave heating

This work aimed to obtain iron oxide nanoparticles for use in heterogeneous photocatalysis through a fast, sustainable, and relatively unexplored route in photocatalyst production. A microwave-assisted heating synthesis method was employed to produce iron oxides on a nanometric scale given its shorter reaction time and lower reagent consumption compared to other methods. The iron(III) nitrate salt was heated in a microwave muffle furnace at temperatures 200–500 °C for 10 min. Iron(III) oxides (Fe₂O₃) were obtained, amorphous at 200 and 300 °C (particle size 95 and 71 nm, respectively) and crystalline with rhombohedral structure at 400 and 500 °C (particle size 47 and 42 nm, respectively) with band gap values between 1.87 and 2.01 eV, showing strong absorption in the ultraviolet and visible spectral regions. The most promising samples were obtained at 200 (IOM200) and 300 °C (IOM300), with larger specific areas (76 and 126 m² g⁻¹), where the percentage of photocatalytic discoloration was 97 % and 89 % for methyl orange (MO) and 98 and 97 % for methylene blue (MB), respectively. For samples IOM200 and IOM300 the discoloration of the MO was due to the adsorption process combined with heterogeneous photocatalysis. The second-order kinetic model fitted well to experimental data for the MO, whereas for the MB the pseudo-first-order kinetic model best fitted the experimental data. In addition, the iron(III) oxides nanoparticles were also reusable in at least three cycling runs, showing good recycling stability, meeting the principles of green chemistry and sustainable environmental objectives.