Synthesis and characterization of a mixture of CoFe2O4 and MgFe2O4 from layered double hydroxides: Band gap energy and magnetic responses

• A mixture of nanocrystals of CoFe2O4 and MgFe2O4 was obtained.
• MgO, CoFe2O4 and MgFe2O4 were detected in the calcined samples.
• A diminution the crystal size of spinel ferrites, generates a low Eg values.
• The superparamagnetic behaviour is attributed to the nanocrystals of ferrite.

A mixture of nanocrystals of cobalt ferrite and magnesium ferrite was obtained from Layered Double Hydroxides (LDH) through a co-precitation method with a theoretical molar ratio M2+:Fe3+=3:1, where M2+represents Mg2+ and/or Co2+. The molar ratios between Co2+:Fe3+ were 0.0 (0Co), 0.2 (5Co), and 0.4 (10Co). In order to assess the effect on the properties of the LDH and their oxides, the molar percentages were 0, 5 and 10%. Two different synthesis methods were evaluated; (i) ageing at room temperature (rt), and (ii) hydrothermal ageing at 200 °C in autoclave (ht), both methods needed 15 h of ageing. Then, these LDH were calcined in air atmosphere at 550 °C for 10 h. The calcined materials were characterized by X-ray diffraction (XRD), thermogravymetric analysis (TGA), temperature-programmed reduction (TPR), infrared spectroscopy with Fourier transform (FTIR), Diffuse Reflectance UV–visible spectroscopy (UV–vis-DRS), Mössbauer spectroscopy and inductively coupled plasma optical emission spectroscopy (ICP–OES). The magnetic response was analyzed using a vibrating sample magnetometer (VSM). The band gap energy of the iron oxides was determined through the UV–vis-DRS analysis. Through these studies it was possible to identify the presence of a mixture of cobalt ferrite and magnesium ferrite. Samples did not show hematite and cobalt oxides, but the presence of MgO in the periclase phase was determined. This magnesium oxide promoted a good dispersion of the ferrites. Moreover, when a single ferrite phase of Co or Mg was formed, a diminution of the crystal size with consequent enlarged values of band gap energy was observed. Thus, materials synthesized by room temperature ageing promoted the superparamagnetic behaviour of samples, attributed to the content of the cobalt ferrite structure in nanocrystals. In regard to the estimated band gap energy, all samples exhibited low levels. These results indicate that these solids would be suitable for photocatalysts use in all visible light range, and that they could be easily removed from the reaction medium by a simple magnetic separation procedure.