During preparation and even after the preparation of magnetite nanoparticles, the color of the prepared NPs is black. But thereafter it gradually changes to brownish even when sample is subjected to ultrasonification for significant time imidiately after preparation. I wanted to know the chemical changes occured during this and how differ the brown sample from original black sample in terms of its composition of ions.
For Fe oxides and hydroxides, color is an important factor that indicates possible phase changes. Interpretation based only on color but may not be reliable. Some analysis is needed. Best powder X-ray diffraction, or some magnetic measurement. Reliable separation of magnetite from maghemite by powder diffraction is difficult (approximately according to unit cell value), but the method excludes other oxides and hydroxides of Fe.
However, it is possible to hypothesize that the oxidation converted magnetite to maghemite during sonication. Maghemite is dark brown, strongly ferrimagnetic, but nanoparticle magnetism is a complex matter.
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