Role of Akaganeite (β-FeOOH) in the Growth of Hematite (α-Fe₂O₃) in an Inorganic Silica Hydrogel

The use of an inorganic hydrogel as a means to modulate the hierarchical architectures of oxide compounds requires an understanding of the effect of the matrix on intermediate phases. In this work, we report on the crystallization of akaganeite (β-FeOOH), both within a silica hydrogel and from aqueous solution, with a focus on understanding the chemical effects of pH, [Fe 3+ ], and [Cl – ], in concert with the physical effects of the silica hydrogel, on the ultimate formation of hematite (α-Fe 2 O 3 ). A distinct physical consequence of the hydrogel crystallization microenvironment is the stabilization of akaganeite as three-dimensional assemblies, in contrast to the discrete rods that form in solution. Chemically, we find that [Fe 3+ ] affects the size of akaganeite crystals, while [H + ] determines the aspect ratio. We also identify that crystal splitting is correlated to high [Cl – ]. In addition, we demonstrate that planar, branched aggregates of akaganeite rods are favored at high [H + ] and are associated with a pathway to hematite that proceeds through the goethite polymorph (α-FeOOH). With these results, we highlight the physical and chemical variables of the crystallization microenvironment that dictate the structural features of akaganeite crystals and their corresponding hematite forms.