The Formation of a Nanoscale Hard Magnetic Ferrite from Prussian Blue Nanocrystals

Abstract

Epsilon iron oxide (ε-Fe2O3) is a nanoscale iron oxide polymorph with a large coercive field value (> 20 kOe at room temperature). High yield ε-Fe2O3 syntheses involve the use of a SiO2 matrix to prevent agglomeration during high temperature sintering (> 900°C).1-3 Herein, we present a new synthesis method of ε-Fe2O3 at low temperature and ambient pressure which uses no protective matrix, via a precursor of Prussian blue with a small amount of embedded ε-Fe2O3 nanoparticles.Core ε-Fe2O3 nanoparticles, prepared using a standard method,4 were sonicated in K4[Fe(CN)6](aq) solution before 2M HCl(aq) was added. The obtained sample (denoted core-shell) was sintered in air at 300°C, 400°C and 500°C for 2 hours.Fig. 1a shows TEM images of the core-shell, 300°C, 400°C and 500°C sintered samples. ε-Fe2O3 nanoparticles are shown to have spherical morphology, dTEM = 23 ± 7 nm, whereas Prussian blue nanocrystals are larger cubic or spherical particles. The sintered samples at 300°C, 400°C and 500°C show aggregations of spherical and cubic nanoparticles which become more distinct as the temperature increases.Fig. 1b shows the XRD spectra with Rietveld analysis. The core-shell sample shows peaks assigned to ε-Fe2O3 (24%, orthorhombic, Pna21) and Prussian blue (76%, cubic, Fm3m). In the sintered samples, Prussian blue peaks are not present, though the peaks corresponding to the core ε-Fe2O3 remain with the particle size calculated via the first principles method consistent in all samples, ca. 25 nm. At 300°C, a second phase of ε-Fe2O3 and γ-Fe2O3 with particle sizes ca. 3 nm are present, alongside a small amount of α-Fe2O3. At 400°C, the proportion of small size Fe2O3 nanoparticles decreases, and the proportion of ca. 25 nm ε-Fe2O3 and α-Fe2O3 increases. At 500°C, the only phases present are ca. 25 nm ε-Fe2O3 (92%) with a small amount of macroscale α-Fe2O3 (8%).Prussian blue was shown to convert to ε-Fe2O3 at high yield when sintered at 500°C in the presence of ε-Fe2O3 core nanoparticles. This is the lowest temperature ε-Fe2O3 has been reportedly prepared at in ambient pressures and may pave the way to a lower energy synthesis method for this hard magnetic ferrite.  Fig. 1 a) TEM images and b) XRD spectra of respective samples