The Chemistry of Spinel Ferrite Nanoparticle Nucleation, Crystallization, and Growth

Henrik L. Andersen*, Cecilia Granados-Miralles, Kirsten M. Ø. Jensen, Matilde Saura-Múzquiz, Mogens Christensen

*Corresponding author for this work

Research output: Contribution to journal/Conference contribution in journal/Contribution to newspaperJournal articleResearchpeer-review

Abstract

The nucleation, crystallization, and growth mechanisms of MnFe 2O 4, CoFe 2O 4, NiFe 2O 4, and ZnFe 2O 4 nanocrystallites prepared from coprecipitated transition metal (TM) hydroxide precursors treated at sub-, near-, and supercritical hydrothermal conditions have been studied by in situ X-ray total scattering (TS) with pair distribution function (PDF) analysis, and in situ synchrotron powder X-ray diffraction (PXRD) with Rietveld analysis. The in situ TS experiments were carried out on 0.6 M TM hydroxide precursors prepared from aqueous metal chloride solutions using 24.5% NH 4OH as the precipitating base. The PDF analysis reveals equivalent nucleation processes for the four spinel ferrite compounds under the studied hydrothermal conditions, where the TMs form edge-sharing octahedrally coordinated hydroxide units (monomers/dimers and in some cases trimers) in the aqueous precursor, which upon hydrothermal treatment nucleate through linking by tetrahedrally coordinated TMs. The in situ PXRD experiments were carried out on 1.2 M TM hydroxide precursors prepared from aqueous metal nitrate solutions using 16 M NaOH as the precipitating base. The crystallization and growth of the nanocrystallites were found to progress via different processes depending on the specific TMs and synthesis temperatures. The PXRD data show that MnFe 2O 4 and CoFe 2O 4 nanocrystallites rapidly grow (typically <1 min) to equilibrium sizes of 20-25 nm and 10-12 nm, respectively, regardless of applied temperature in the 170-420 °C range, indicating limited possibility of targeted size control. However, varying the reaction time (0-30 min) and temperature (150-400 °C) allows different sizes to be obtained for NiFe 2O 4 (3-30 nm) and ZnFe 2O 4 (3-12 nm) nanocrystallites. The mechanisms controlling the crystallization and growth (nucleation, growth by diffusion, Ostwald ripening, etc.) were examined by qualitative analysis of the evolution in refined scale factor (proportional to extent of crystallization) and mean crystallite volume (proportional to extent of growth). Interestingly, lower kinetic barriers are observed for the formation of the mixed spinels (MnFe 2O 4 and CoFe 2O 4) compared to the inverse (NiFe 2O 4) and normal (ZnFe 2O 4) spinel structured compounds, suggesting that the energy barrier for formation may be lowered when the TMs have no site preference.

Original languageEnglish
JournalACS Nano
Volume18
Issue14
Pages (from-to)9852-9870
Number of pages19
ISSN1936-0851
DOIs
Publication statusPublished - Apr 2024

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