Synthesis of Nanoscale γ-Fe 2 O 3 Powders with Hydrates via Microwave-Assisted Heat Treatment

Herein, the effect of microwave irradiation on the formation of γ-Fe 2 O 3 phase was investigated. The structural and chemical features of the γ-Fe 2 O 3 synthesized via microwave-assisted heat treatment (MWH) were examined in terms of precursors. Three different types of precursors, i.e., FeC 2 O 4 ∙2H 2 O (FH), FeOOH, and Fe 3 O 4 , were used, and each precursor was heated in a temperature range of 100–300 °C via MWH. Nanocrystalline γ-Fe 2 O 3 powders with a crystallite size of ~ 19 nm were synthesized when the source FH was heat-treated at 120 °C for 20 min by MWH. The activation energy required for the formation of γ-Fe 2 O 3 powders was 21.4 kJ/mol, which is approximately one-third or one-quarter of the activation energy (68.5 and 87.9 kJ/mol) reported for the synthesis of γ-Fe 2 O 3 via conventional heating. The lattice parameter of the synthesized γ-Fe 2 O 3 phase expanded to 8.360 Å at 120 °C, whereas it contracted to 8.351 Å at 200 °C. This variation can be attributed to the content of the surface-absorbed OH group. The values of the saturated magnetization and coercive force of the powders increased by approximately threefold with increasing crystallinity. Graphic Abstract