Suppressing γ- to α-Fe2O3 transformation using a reduced graphene oxide hydrogel

Abstract

Maghemite (γ-Fe₂O₃) is an important magnetic material for wide utilization; however, it tends to transform into hematite (α-Fe₂O₃) under high temperature and humidity (HT and H) conditions. Here, we report that this phase transition can be completely suppressed by a reduced graphene oxide (rGO) hydrogel. Experiments demonstrate that γ-Fe₂O₃ first dissolves in water and then recrystallizes into α-Fe₂O₃ in a hydrothermal autoclave at 180 °C (an HT and H environment). In contrast, after adding γ-Fe₂O₃ to graphene oxide (GO) solution, its transformation into α-Fe₂O₃ is completely prevented during hydrothermal treatment up to 240 °C. The possible mechanism for this phase transition suppression is that the rGO hydrogel formed through hydrothermal reduction of GO fully encapsulates γ-Fe₂O₃ to prevent its contact with water and subsequent dissolution. This work highlights the stabilizing role of the rGO hydrogel in improving the stability of metastable metal oxides under HT and H conditions.