Topochemical reduction of FeCo-oxide to FeCo-alloy nanosystems into a SiO2 matrix†

This study focuses on the synthesis of metallic magnetic nanosystems embedded in mesoporous silica (SiO₂), and the impact of matrix porosity, controlled by temperature treatment, on the efficiency of H₂ reduction process. The reduction of FeCo oxides to the corresponding alloy nanosystems was first optimized, identifying FeCo with 50 at% Fe as the optimal composition due to its high saturation magnetization (∼242 A m² kg⁻¹) and oxidation onset temperature (∼440 °C). Then, the FeCo-oxide nanocomposites were synthesized into SiO₂via sol–gel self-combustion under thermal treatments, to properly tune the surface area of the silica matrix. By controlling the annealing temperature, the specific surface area (SA) of the matrix decreases from ∼512(1) m² g⁻¹ to ∼345(1) m² g⁻¹ when annealed to 900 °C in air. Following topochemical reduction in H₂, the structural properties of the obtained FeCo–SiO₂ nanocomposites have been analyzed using X-ray powder diffraction and magnetic properties were evaluated to establish a correlation between matrix SA and reduction capability. The decrease of SA leads to incomplete reduction at higher temperatures, with the formation of Fe_YO_X/Co_XO_Y intermediates. This work underscores the critical role of matrix porosity in achieving a delicate balance to ensure both the efficient conversion of nanostructured oxide to their metallic state and the preservation of their magnetic and structural integrity.