Ultrasmall iron oxide nanoparticles: synthesis, surface chemistry and magnetic properties

1. Ultrasmall iron oxide nanoparticles: synthesis, surface chemistry and magnetic properties Vladimir Kolesnichenko Department of Chemistry, Xavier University of Louisiana

2. The Purpose Nanocrystals of the magnetic metals and metal oxides are used as: - recording media - components of miniature electronic devices - sensors - ferrofluids - labeling agents and carriers in biology - diagnostic and therapeutic tools in medicine.

3. The Idea • To develop new methods of synthesis of the various nanocrystalline metals and metal oxides featuring: • - Scalability (non-hazardous simple technique + high yield) • - Improved quality of the products: • high purity, variable crystal size with narrow size distribution, • high crystal ordering • - Nanocrystals are non-aggregated with the surface available for chemical modification • - Advanced properties of the products: colloid and surface chemistry, magnetic properties

4. The Approach • Homogeneous solution synthesis • Kinetically-controlled crystals’ nucleation and growth • Not using surfactants or strong capping ligands • Using polar coordinating solvents with high boiling points

5. Ternary iron oxides with Cubic Inverse Spinel structure MIIFe2O4 (MII = Mg, Mn, Fe, Co, Ni, Cu, Zn) ferrimagnets

6. Metal precursors tested Metal chlorides – hydrated or anhydrous: Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+ Fe3+ The reference reaction: co-precipitation in aqueous medium M2+ + 2 Fe3+ + 8 OH- [M(OH)2+2Fe(OH)3] MFe2O4 - 4 H2O

7. Solvents / chelating agents diethylene glycol:  = 32; b.p. 245oC Reagents: MCl2 + 2 FeCl3 + 8 NaOH

8. a) Formation of metal chelate alkoxide complexes in parent alcohol solutions b) Nucleation and growth of the nanoparticles

9. Methods of Characterization Transmission electron microscopy (TEM) combined with EDX analysis X-ray diffraction Elemental analysis FT-IR spectroscopy 1H NMR spectrometry Dynamic Light Scattering Zeta-potential measurements Magnetic measurements using SQUID magnetometer

10. TEM Image For FeFe2O4

11. Wide-area TEM image for FeFe2O4

12. Synthesized nanocrystalline ferrites • MnFe2O4 FeFe2O4 CoFe2O4 NiFe2O4 ZnFe2O4 • 5.3 nm 6.6 nm 4.2 nm 5.1 nm 5.6 nm • 16 % 11 % 18 % 15 % 12 % • All products are: • - highly crystalline: • obtained with yield of 75-90% • non-aggregated although contain no surfactants

13. ZFC and FC curves for 4 nm particles of Fe2O3

14. Hysteresis Plot for FeFe2O4 (4 nm from TEM)

15. X-ray diffractogram for FeFe2O4 nanoparticles: 4 nm from TEM; 5.3 nm from XRD

16. Synthesis of Nanocrystalline Ferrites by Decomposition of Metal Chelates in Non-aqueous Solutions Inorg. Chem., 2002, 41, 6137 Chem. Mater, 2004, 16, 5527

17. Powder X-ray Diffractograms for Fe3O4 • Synthesized in • Synthesized in • Synthesized in +

18. Nanocrystals of Fe3O4 Synthesized In Different Complexing Media

19. Characterization of the Nanocrystals’ Surface TGA – in air, agron or vacuum, 2 °/min. The results: weight loss 7.4% for 5 nm and 3.4% for 12 nm particles @ 175-325 °C EDX – the experiment combined with TEM study The results: 0 - 2.4 wt.% of Cl and 0 % of Na FT-IR spectrometry. The results: characteristic vibrations for DEG and NMDEA molecules 1H NMR spectrometry – performed after the samples were decomposed and the organic component was isolated. Integration was used for semiquantitative analysis. The results: ~ 3 wt.% of DEG

20. Thermogravimetric curve for Fe3O4 2 °/min, air

21. 1H NMR spectrum of the DEG recovered from the nanocrystals’ surface DMSO was used as a standard for integration

22. TEM image of nanocrystals recovered from aqueous colloid

23. Nanocrystals’ Surface Derivatization The surface of the precipitated nano-powders remains passivated against agglomeration but active in metal-ligand reactions. This offers the opportunity to perform post-synthesis reactions targeting the advanced core/shell nanocomposites and the organic shell-modified nanoparticles for various applications. L L L L L L L L L + n L → L L L L L L L L L L

24. Modification of the Nanocrystal’s Surface Reactions of Aqueous Colloids of Fe3O4 With Carboxylic Acids FT-IR spectra of the isolated solids evidenced no binding of monocarboxylic and binding of dicarboxylic acids and hydroxy-carboxylic acids (citric, tartaric, etc.).

25. The DLS spectra of magnetite citrate colloids.Red – pH 7.5Green – pH 4.8Blue – pH 4.5

26. The pH values representing substantial aggregation and de-aggregation events during titration of aqueous colloids with 0.01M HCl and 0.01M NaOH (monitored by DLS method) * the reference peak 7-9 nm in the DLS spectra pH↑ - titration with base pH↓ - titration with acid

27. The proposed binding modes ofcitric and tartaric acids

28. Conclusions • - Controlling the rate of crystallization of metal oxides in • solutions can be achieved by changing the mechanism of • reaction of their formation from ionic metathesis to molecular • nucleophilic substitution reactions. • Hydrolysis of metal alkoxide complexes in non-aqueous • solutions at the elevated temperature yields colloidal • metal oxide nanocrystals. • Surface of the precipitated nanopowders is passivated • against agglomeration by the adsorbed DEG, but is active in • metal-ligand reactions. • Bridging α-hydroxy-carboxylic acids demonstrate strong • attachment to the nanocrystals surface in aqueous colloids.

29. Participating Researchers Galina Goloverda (Xavier, professor) Yann Remond (AMRI, undergrad. student) Daniela Caruntu (AMRI, grad. student) Charles O’Connor (AMRI, director) Vincent Vu (Xavier, undergrad. student) Gabriel Caruntu (AMRI, postdoctoral fellow)

30. Physical measurements performed by: • magnetic measurements - Leonard Spinu and Cosmin Radu (UNO) • TEM – Jibao He (Tulane)

31. We gratefully acknowledge the support of this work by Xavier University, Center for Undergraduate Research, Advanced Materials Research Institute (UNO), DOD/DARPA and National Institutes of Health