Fig. 4: Comparison of the shapes, sizes, and cobalt contents of the cobalt-doped ferrites by changing the thickness of the silica shell of the precursors.

The akaganeite (hollow, orange), aka@0.12PAA (half-filled, grey), or aka@0.12PAA@SiO₂ (filled, blue) precursor with a shell thickness varying between 2.2 nm and 6.8 nm was converted in a hydrothermal step with a filling volume of 55% at a temperature of 190 °C. a TEM images indicate a change in the shape from cubic to truncated cubic particles by increasing the silica shell of the precursors. b The morphological and crystallite diameter, as well as c the cobalt-to-iron ratio of the resulting cobalt-doped particles, increase with the silica shell thickness. EDX mapping and corresponding intensity line scan profiles of the cobalt-doped particles synthesized with d akaganeite, e aka@0.12PAA, or f aka@0.12PAA@5.6SiO₂ confirm the presence of Co (green), Fe (yellow), O (red), and Si (turquoise). However, the distribution includes only the intensities of Si, Co, Fe, and O, which were normalized to 100 atom%. Therefore, the absolute values should be lower when considering all elements, as other components are not included in this normalization. g The formation of MNPs from bare akaganeite indicates the presence of a magnetite-like phase (brown particle). Smaller MNPs with a high cobalt content, indicated by a green color, can be synthesized by adding a polymer additive. The diameter and cobalt content per unit volume can be adjusted by converting a precursor system with a silica shell. As the cobalt content in the suspension increases, along with the thickness of the silica shell and the diameter of the particles, the mechanism of formation of the magnetite core changes. h Fig. 4c is depicted in dependence on the size and cobalt content per volume, summarizing the data from F-AAS, XRD, and TEM.