Application of Infrared Spectrum in Synthesis of NiFe2O4 Nanoparticles

A sol-gel method was used to synthesizeNiFe2O4nanoparticles. The sol solution was prepared through nitrates dissolved in melted citric acid for 30 min by means of an ultrasonic cleaner, maintaining the molar proportion of iron and nickel equal to 2∶1 and joining a little PEG(polyethylene glycol) 200 as the dispersing agent. The sol solution was evaporated at 70℃ for 12 h to formwetgel, then dried at 110℃ in a drying oven for 24 h. The dried gel was annealed at different temperatures. Their composition and structure were measured in the synthesizing process using the infrared absorption-transmittance spectrum. The infrared diffuse reflection spectra and X-ray diffraction (XRD) spectra were measured for the specimens annealed at different temperatures. The average grain sizes were calculated from XRD spectra using Scheer′s equation. It was discussed for the effect of the NiFe2O4 nanoparticle size on infrared diffuse reflection spectra. The results show that the infrared absorption-transmittance spectra can reflect the changes of composition and structure in various stages from sol to heat-treatment and provide basis for the optimum annealed temperature in the synthesis of NiFe2O4with the sol-gel method. The grain sizes are different for the samples annealed at different temperatures and the annealed temperature must be more than 400℃for eliminating organic compounds completely. The higher the temperature, the bigger the grain size and the smaller the Kubelka-Mnnk function (K-M) values. In a certain range of grain sizes, the relationship of grain size and the logarithm of the K-M value is linear. When the grain size is big enough, the K-M value almost doesn′t change and the effect of grain size disappears.