Multi-functional biomimetic graphene induced transformation of Fe3O4 to ε-Fe2O3 at room temperature

Abstract

Epsilon-iron oxide (ε-Fe₂O₃) has been synthesized in large yields (≈73.7%) in a colloidal form at ambient conditions. Being embedded in biomimetic graphene, the synthesized thermodynamically unstable monoclinic phase is prevented from transforming to other phases. We have used the same protein–polymer mixture both for exfoliating natural graphite and as templating agents for iron oxide nanoparticles. X-ray diffraction of the composites confirms the formation of the ε-Fe₂O₃ phase with minor quantities (≈26.3%) of cubic magnetite (Fe₃O₄). The particle size and distribution was studied using high resolution transmission electron microscopy which clearly shows self-assembled dense nanoparticles on graphene sheets. This exercises strain on graphene; evident from the highly broadened D and G bands of Raman measurements and the blue shifting of the G band. X-ray photoelectron spectra shows signatures of iron oxide, graphene and protein in the sample; deconvoluted C1s, O1s and N1s core level peaks confirm both the attachment of the nanoparticles with the substrate and Fe2p core level peaks reveal the high spin oxidation state of Fe³⁺ ions. Magnetic measurements confirm the superparamagnetic nature of the composites; the lack of coercivity unexpected of this polymorph may be explained by the low magnetocrystalline anisotropy of the randomly oriented graphene sheets. We suspect that graphene attracts the maximum ferric (Fe³⁺) ions of the mixed ferrous/ferric ions in the system resulting in ferrous (Fe²⁺) cation substitution which also results in the reduction of coercivity. Exchange bias was also observed at low temperature in this antiferro–ferrimagnetic hybrid film.