Structural, morphological and photocatalytic properties of Ni-Mn ferrites: Influence of the Ni:Mn ratio

Mixed Ni-Mn ferrite spinel nanoparticles embedded in SiO2 matrix were prepared by sol-gel method, followed by annealing at 300 and 1000 °C. The thermal analysis and Fourier transform infrared spectroscopy confirmed the formation of Ni-, Mn- and Co- malonate precursors and their decomposition to ferrites. The X-ray diffraction data showed the presence of poorly crystallized Ni-Mn ferrite at 300 °C and of Ni-Mn ferrite accompanied by α-Fe2O3, cristobalite, quartz and Fe2SiO4. at 1000 °C. The transmission electron microscopy revealed spherical shape ferrite particles of 19–46 nm diameter, that increase with the increase of Ni content and annealing temperature. For the samples with low Ni content, the pores are larger, their size reaching 100 Å, while for the samples with high Ni content the pores are smaller, their size being lower than 50 Å. The specific surface area increases with the increase of Ni content up to 0.7/0.3 Ni/Mn ratio and after remains constant. The isotherms were used to calculate the rate constants for the decomposition of malonate precursors to ferrites and the activation energy for each ferrite. The samples show a broad response in both UV and visible range, with the maximum in visible domain. Ni0.5Mn0.5Fe2O4 sample shows the best sonophotocatalytic activity against Rhodamine B solution under visible light irradiation. The prepared Ni-Mn ferrites may have potential application for water and wastewater decontamination.