Green Room-Temperature Synthesis and Spatiotemporal Dynamics of Iron Oxide Polymorphs and Transition Metal-Substituted Ferrites Using the Reaction-Diffusion Framework

Abstract

Reaction-diffusion systems offer a green, efficient and scalable framework for studying chemical processes where reactants are transported by diffusion and undergo transformations, resulting in spatiotemporal patterns and dynamic behaviors. These systems enable room-temperature synthesis, providing an environmentally friendly alternative for the production of complex materials. Iron oxides, composed of iron and oxygen, are widely recognized for their natural abundance and pivotal roles in geological and biomedical applications. This work presents a novel method for synthesizing iron oxide polymorphs using the reaction-diffusion framework (RDF), where substances are transformed by chemical reaction and transported into a gel matrix by diffusion. Specifically, the diffusion of a strong alkaline solution into an agar gel impregnated with Fe(II)/Fe(III) ions initiates a precipitation reaction, resulting in continuous strata of colored precipitates corresponding to distinct iron oxide polymorphs. These polymorphs include the yellow α-polymorph, goethite, which is kinetically favored and transforms gradually into the thermodynamically stable black polymorph, magnetite. By analyzing the spatiotemporal dynamics of these strata, information about the kinetics of solid-state transformations between polymorphs can be inferred. The dynamics of these transformations provide insights into the reaction mechanisms and growth processes within the gel medium. The resulting solids were characterized using scanning electron microscopy (SEM), powder X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), Fourier-transform infrared spectroscopy (FT-IR), and diffuse reflectance spectroscopy. Additionally, the effects of various parameters, including the concentrations of the inner and outer matrices, gel thickness, and different alkaline solutions, were systematically studied. Transition metal-substituted iron oxides, MFe2O4 (M = Ni, Zn, Co), were successfully synthesized using the same green, room-temperature reaction-diffusion framework, exhibiting continuous precipitation dynamics similar to their iron oxide counterparts. Furthermore, the influence of a static magnetic field on the spatiotemporal dynamics of these systems will be explored, potentially revealing new insights into the interplay between magnetic forces and reaction-diffusion phenomena in these materials.