Structural evolution of iron forming iron oxide in a deep eutectic-solvothermal reaction

Area of Science:

• Materials Science
• Nanotechnology
• Green Chemistry

Background:

• Deep eutectic solvents (DES) offer sustainable alternatives for nanomaterial synthesis.
• Understanding solvent-solute interactions is crucial for controlling nanoparticle formation.
• Iron oxide nanoparticles have diverse applications in catalysis, medicine, and data storage.

Purpose of the Study:

• To investigate the structural and dynamic processes of iron oxide nanoparticle formation in a choline chloride:urea DES.
• To elucidate the role of water in the solvothermal synthesis of iron oxide nanoparticles.
• To characterize the influence of DES structure on nanoparticle morphology.

Main Methods:

• Extended X-ray absorption fine structure (EXAFS) for initial Fe3+ speciation and local order.
• Neutron and X-ray diffraction with empirical potential structure refinement (EPSR) for atomistic structure.
• In situ small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) for mesoscale changes and nucleation/growth.
• Monitoring reaction kinetics and nanoparticle morphology.

Main Results:

• Iron salts form octahedral [Fe(L)3(Cl)3] complexes in DES, inducing structural rearrangements.
• Fe3+ complexation with urea hydrolysis products leads to oligomer formation.
• Hydrated DES promote rapid reaction, nucleation, and growth.
• Pure DES exhibit a delayed nucleation phase after 5000 s.
• Hydrated DES favor 1D nanoparticle morphology due to selective choline capping, while pure DES show restricted morphology control.

Conclusions:

• Water significantly accelerates iron oxide nanoparticle formation and influences morphology in DES.
• The DES structure and choline capping play critical roles in directing nanoparticle growth.
• This work provides fundamental insights into solvothermal nanomaterial synthesis using DES for greener chemical processes.