Preparation of amorphous precursor for realizing low-temperature NiFe2O4 formation

The present study was conducted to prepare precursors by rapid hydrolysis method for realizing lower temperature formation of NiFe₂O₄ under hydrothermal conditions. The precursor, obtained from a lower concentration of NiCl₂–FeCl₂ mixed solution, was almost amorphous and could be easily converted to NiFe₂O₄ crystal phase at around 130°C by hydrothermal treatment. On the other hand, when a higher concentration of NiCl₂–FeCl₂ solution or trivalent iron salt (FeCl₃) was used as starting reagents, individual crystal phases such as α-Ni(OH)₂, γ-Fe₂O₃ and α-FeOOH were recognized in precursors owing to the difference in hydrolysis rates between Ni²⁺ and Fe²⁺ (or Fe³⁺). These individual crystal phases involved precursors that could not be converted easily to NiFe₂O₄ crystal phase, but needed to treat at higher temperatures for forming NiFe₂O₄ at least 200°C. Thus, the co-existence of individual crystal phases in precursors might prevent the amorphous precursor from forming NiFe₂O₄ crystal phase. The formation of NiFe₂O₄ at lower temperatures is considered to be taken place preferentially from amorphous precursors than individual certain crystal phases.