Formation of Fe(III)–oxides on the magnetite surfaces in the low–temperature hydrothermal reaction

抄録

Low–temperature hydrothermal experiments were conducted to investigate mineral formation on the magnetite surfaces. The synthesized magnetite single crystals grown up to approximately 100 µm in size had a truncated–octahedral shape enclosed by {111}, {100}, and {110} planes, but the {100} plane was composed of many micro/nano pyramid arrays. The as grown magnetite was hydrothermally treated in deoxidized water at 100 °C for 30 days. The molecular hydrogen generation was detected by methylene blue colorimetric measurement. SEM observations showed that after the hydrothermal reaction the magnetite (100) surfaces were selectively dissolved and enormous number of hematite (α–Fe₂O₃) nanoparticles were epitaxially grown on the magnetite (111) surfaces. At the shallow depth of the magnetite (100) surfaces within 200 nm, magnetite was transformed to maghemite (γ–Fe₂O₃). This is the first report that maghemite is formed as a Fe(III)–oxide by the low–temperature hydrothermal alteration of magnetite. Both the selective dissolution and maghemite formation on the magnetite (100) surfaces would be responsible for the anisotropic diffusion property of Fe²⁺ cations in magnetite.