Metal ferrites are materials with applications in catalysis, magnetic hyperthermia, and energy storage. Metal ferrites, composed of cations based on Fe, Ni, Co, Mn and Cu elements, which can change their valence (i.e., oxidation state) under applied potential or in presence of reducing or oxidizing agents, enabling them to participate in heterogeneous catalysis through redox reactions, or as active material in the electrodes of supercapacitors. Unfortunately, metal ferrites have tendency to be agglomerated, which is detrimental for applications in which high surface area is required. This inconvenience can be partially solved through the fabrication of composites with 2D materials, such as reduced graphene oxide. In this work, we present the hydrothermal synthesis of NiFe2O4 and CoFe2O4 nanoparticles on graphene oxide (GO) and their application for chromium removal from water. The GO was previously synthesized by the Tour method. The effect of pH on the reaction yield in the hydrothermal synthesis of the composites is presented. The role of ammonium hydroxide in the reaction mixture is proposed. Results of X-ray diffraction, X-ray photoelectron spectroscopy (XPS), Raman dispersion, and surface area are presented. Transmission electron microscopy (TEM) measurements were performed to assess the dispersion of the nanoparticles on the graphene oxide, which is relevant for applications such as water purification. Spherical nanoparticles with an average size of approximately 14 nm were obtained. This size decreased up to 7 nm when they were grown over GO. BET surface areas between 70 and 90 m2g-1 were obtained for the nanocomposites synthesized at pH 12. Magnetic moments for the CoFe2O4/rGO and NiFe2O4/rGO nanocomposite at 300 K was 46.6 and 29.2 emu/g, respectively, which allows them to be used as magnetically separable catalysts. The best performance of the nanocomposites in the removal of Cr(III) ions from water was achieved by the NiFe2O4/rGO nanocomposite.

Performance of NiFe2O4 and CoFe2O4 nanoparticles grown over graphene oxide as adsorbent materials for Cr(III) removal from water

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