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

TL;DR

This study investigates the nucleation, crystallization, and growth mechanisms of various spinel ferrite nanoparticles using in situ X-ray techniques, revealing how different conditions influence their formation and size control.

Contribution

It provides detailed insights into the nucleation and growth processes of spinel ferrite nanocrystals under hydrothermal conditions, highlighting differences among specific TMs and synthesis parameters.

Findings

Equivalent nucleation processes for all four ferrites under studied conditions

Rapid growth of MnFe2O4 and CoFe2O4 to specific sizes regardless of temperature

Size control feasible for NiFe2O4 and ZnFe2O4 by adjusting reaction time and temperature

Abstract

The nucleation, crystallization, and growth mechanisms of MnFe2O4, CoFe2O4, NiFe2O4, and ZnFe2O4 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% NH4OH 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 MnFe2O4 and CoFe2O4 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 NiFe2O4 (3-30 nm) and ZnFe2O4 (3-12 nm) nanocrystallites.