Modified Hard-Template Synthesis of Superparamagnetic Hollow Spinel Ferrite Spheres and Their Characterization

Herein, we present a simple modified hard-template approach for the fabrication of uniform superparamagnetic hollow maghemite (γ-Fe
₂
O
₃
) spheres. Hollow spheres were prepared by the attachment of ferric and ferrous ions onto the surface of the spherical silica (SiO
₂
) particles synthesized by the Stöber process. The synthesis consisted of four major steps. In the first step, monodispersed spherical silica particles with a narrow size distribution of around (497.5 ± 4.5) nm were prepared as hard templates. Second, Fe
²⁺
/Fe
³⁺
ions were adsorbed onto the surface of the SiO
₂
particles in the aqueous solution. In the third step, Fe
²⁺
/Fe
³⁺
ions at the SiO
₂
surface slowly nucleated and crystallized in the form of spinel γ-Fe
₂
O
₃
nanoparticles. SiO
₂
templates were dissolved simultaneously when [NaOH/NaHCO3] (pH 11) and [Na
₂
CO
₃
/NaHCO
₃
] (pH 9) buffer solutions were added successively until the reaction medium reached pH of 3.0 ± 0.1. In the fourth step, the addition of an alkali solution accelerated the crystallization and formation of γ-Fe
₂
O
₃
nanocrystallites via the precipitation mechanism. The formation of the hollow spherical structures and their morphology were investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The crystallinity and the phase analysis of the samples were characterized by means of X-ray powder diffractometry [X-ray diffraction (XRD)] and Fourier-transform infrared (FTIR) spectroscopy. The specific surface area and the specific magnetization of the prepared samples were also defined. It is important to note that the third step presents a novel approach [Patent GB 2526659] for the preparation of superparamagnetic hollow spheres, avoiding high temperatures of calcination or environmentally questionable etching, which usually employ strong solvents and acids. Consequently, our synthesis is faster, more economical and environmentally friendly when compared with other relevant procedures described in the literature.