Effects of Reaction Temperature and Time on Structural, Morphological and Optical Properties of Magnetite Quantum Dots Prepared by Solvothermal Method

Magnetite quantum dots were synthesized by solvothermal method in a conventional reflux setup, in an inert environment, at different reaction temperatures and times. Powder X-ray diffraction patterns of the as-prepared iron oxide confirmed magnetite crystalline phases irrespective of the reaction conditions. TEM micrographs revealed that the iron oxides obtained at 200, 180 and 165 °C are spherical in shapes with mean particle sizes of 3.25, 3.3 and 3.90 nm respectively, showing an increase in particle size with decreasing temperature. TEM micrographs revealed that particles obtained at 6 and 3 h have a mean particle size of 3.25 nm while particles obtained at 4.5-h has a bigger mean particle size of 7.5 nm. The results show that irrespective of the synthetic parameters, the average particle sizes of the magnetite are in the range 3–8 nm and could be described as quantum dots. Optical absorption spectra of the as-prepared magnetite quantum dots exhibited maximum absorption in the range of 200–210 nm. Fe3O4-1, Fe3O4-2 and Fe3O4-3 synthesized at 200, 180 and 165 °C have energy band gaps of 5.49, 5.13 and 5.01 eV respectively, showing a decrease in band gap with decreasing temperature. Fe3O4-4 and Fe3O4-6 synthesized at 6 and 3 h have optical band gap of 5.44 eV while Fe3O4-5 synthesized at 4.5 h has a band gap of 3.75 eV. FTIR spectra confirmed the passivation of the surface of the as-prepared magnetite quantum dots by oleate.