Further Comments on “Engineering Shape Anisotropy of Fe₃O₄-γ-Fe₂O₃ Hollow Nanoparticles for Magnetic Hyperthermia”

In their earlier paper, Niraula et al. (ACS Appl. Nano Mater. 2021, 4, 3148–3158) described the morphological, compositional, and magnetic properties of three different magnetite/maghemite or Fe3O4/γ-Fe2O3, hollow nanoparticles, referred to herein as nanorings, short-nanotubes, and long-nanotubes. Scanning electron microscopy indicates that these nanoparticles have lengths of 275 ± 51, 411 ± 92, and 515 ± 98 nm and outer diameters of 201 ± 55, 251 ± 46, and 229 ± 42 nm, respectively, dimensions that are all rather similar in view of their distributions, as is shown in a figure herein. Further, the lengths indicate that these nanoparticles are far larger than what are normally considered nanoparticles. Rietveld refinement of the powder X-ray diffraction patterns presumably reveals the presence of Fe3O4, γ-Fe2O3, and small amounts of α-Fe2O3 in some of the nanoparticles; unfortunately, the lack of refinement details make the validity of these compositions at least problematic. The published iron-57 Mössbauer spectral analysis is marginal. An alternative analysis of both the reported X-ray lattice parameters and the Mössbauer spectral results for the three nanoparticles in terms of solid solutions of magnetite and maghemite, AFe3+[BFe1–3δ2+Fe1+2δ3+□δ]O4, where □ represents a vacancy, δ = 0 corresponds to magnetite, Fe3O4, and δ = 0.333 corresponds to maghemite, γ-Fe2O3, is proposed herein. In the presence of the expected magnetite Verwey transition, the Mössbauer spectral analysis is formulated with the stoichiometry AFe3+[BFe2(1–3δ)2.5+Fe5δ3+□δ]O4, and as far as we can tell, this model is consistent with the Rietveld X-ray diffraction analysis. The values of δ = 0.28(2) and 0.30(1) obtained from the X-ray diffraction and Mössbauer spectral analyses, respectively, indicate that the composition of the nanoparticles is very close to γ-Fe2O3, in contrast to the earlier conclusion. During the course of this reformulation, numerous errors in the mathematical expressions, and in some cases their subsequent misuse, have been discovered and corrected herein whenever possible.