Room-temperature Mössbauer spectra of composite materials obtained by precipitation of β-FeOOH (0.10 M, 0.37 M, and 0.55 M concentrations of FeCl₃·6H₂O) at the presence of MWCNT

The F-series samples were synthesized using a procedure based on the hydrolysis of trivalent iron (with FeCl₃·6H₂O as the iron precursor) followed by precipitation, with the main objective being to evaluate how the molar concentration of the precursor affects the structural and morphological characteristics of the resulting material. In a typical synthesis, a 1 M aqueous NaOH solution was added dropwise to an aqueous FeCl₃·6H₂O solution under continuous stirring using an IKA C-MAG HS 7 magnetic stirrer, while maintaining the reaction temperature in the range of 55–65 °C. The resulting colloidal sols were then kept at 65 °C to initiate gelation and to allow gel aging for 24 h. After aging, the precipitate was separated by centrifugation and washed several times with distilled water until the pH of the supernatant reached near-neutral values (pH 5.5–6.0). Finally, the products were dried at 100 °C overnight. Three systems (F1, F2, and F3) were prepared using different initial molar concentrations of FeCl₃·6H₂O in the precursor solution, namely 0.10 M, 0.37 M, and 0.55 M, respectively. Composite materials based on the β-FeOOH samples of the F-series and multi-walled carbon nanotubes (MWCNTs) were prepared using the same approach as previously described for the S-series composites. The β-FeOOH/MWCNT composites were obtained by mechanical mixing of the components, with a β-FeOOH-to-carbon mass ratio of 4:1. Homogenization of the mixture was carried out by ultrasonication for 2 h in an acetone medium. The resulting composite samples were denoted as F1+MWCN, F2+MWCN, and F3+MWCN.