Fig. 5: Scheme of the proposed mechanism.

Reaction (1) depicts the Massart reaction from Fe²⁺ and Fe³⁺ hydroxide to obtain a number of magnetite seeds N_seeds,1. The precipitation (2) of bare akaganeite with Fe(OH)₂ leads to the formation of magnetite seeds N_seeds,2. The extended LaMer process (3) takes place at c(Fe²⁺) < 10 mmol L⁻¹, where excess akaganeite rods dissolved to Fe(OH)₄^- to ensure a consistent supply with Fe³⁺_. Additionally, the Oswald ripening process (5) can occur, whereby the smaller particles dissolve to promote the growth of the larger particles. During the hydrothermal step, the ion exchange reaction (6) between Fe²⁺ and Co²⁺ results in the synthesis of cobalt-doped particles. If the akaganeite precursor is protected by a silica shell, (2) does not occur, and the dissolution of the nanorods (4) is significantly delayed. As a result, a decreased number of nuclei and a markedly increased particle growth are presented.