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

Vladimir Kolesnichenko

Department of Chemistry, Xavier University of Louisiana

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.

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

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

Ternary iron oxides with Cubic Inverse Spinel structureMIIFe2O4 (MII = Mg, Mn, Fe, Co, Ni, Cu, Zn)

ferrimagnets

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

Solvents / chelating agents

HOOH HO

OO

OH

OH

OH diethylene glycol: = 32; b.p. 245oC

HOO

OH

Reagents: MCl2 + 2 FeCl3 + 8 NaOH

a) Formation of metal chelate alkoxide complexes in parent alcohol solutions

] 2-O

O O

MCl

Cl

H

H OHOH

M

O

O O

2 H2O

- 2 Cl-

O

O O

MCl

Cl ClCl

M

O

O O

H

H

] 2-2 OH-

-2H2O

H

HO

O

O

MCl2(H2O)x + -X H2O

b) Nucleation and growth of the nanoparticles

O

O O

FeOH

OHHO

O O

MOH

OHH

H

+ 2 MFe2O4 + 2 H2O + 3

H

HO

O

O

Methods of Characterization

Transmission electron microscopy (TEM) combined with EDX analysis

X-ray diffraction

Elemental analysis

FT-IR spectroscopy1H NMR spectrometry

Dynamic Light Scattering

Zeta-potential measurements

Magnetic measurements using SQUID magnetometer

TEM Image For FeFe2O4

Wide-area TEM image for FeFe2O4

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

ZFC and FC curves for 4 nm particles of Fe2O3

0 50 100 150 200 250 300

0.5

1.0

1.5

2.0

2.5

3.0

3.5M

, em

u/g

Temperature, K

Fe2O3 at 50 Oe

Hysteresis Plot for FeFe2O4 (4 nm from TEM)

-60000 -40000 -20000 0 20000 40000 60000-100

-80

-60

-40

-20

0

20

40

60

80

100

M, e

mu/

g

Field, Oe

Fe3O4 at 300K

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

30 40 50 60 700

200

400

600

800

1000

Inte

nsity

(cou

nts)

2 theta (deg)

Synthesis of Nanocrystalline Ferrites by Decomposition ofMetal Chelates in Non-aqueous Solutions

MCl

Cl ClCl

M

O

Z O

H

H

2-2 OH-

H

H

MCl2 + -X H2O - 2 H2O

2-

MCl

Cl

H

H OHOH

M2 H2O

- 2 Cl-

FeOH

OHH

MOH

OHH

H

+ 2n (MFe2O4)n + 2n H2O + 3n

H

H

n

O

O

Z

O

O

Z

O

O

Z O

O

Z

O

O

Z O

O

Z

O

O

Z

Z = O or NCH3

Inorg. Chem., 2002, 41, 6137Chem. Mater, 2004, 16, 5527

Powder X-ray Diffractograms for Fe3O4

a) Synthesized inb) Synthesized inc) Synthesized in

+

HOO

OH

HOO

OH

HON

OH

CH3

HON

OH

CH3

Nanocrystals of Fe3O4 Synthesized In DifferentComplexing Media

Solvent/Chelating Agent:

HOO

OH

Solvent/Chelating Agent: 1:1 mixture

HOO

OH

HON

OH

CH3

Solvent/Chelating Agent:

HON

OH

CH3

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

Thermogravimetric curve for Fe3O4

2 °/min, air

1H NMR spectrum of the DEG recovered from the nanocrystals’ surface

DMSO was used as a standard for integration

TEM image of nanocrystals recovered from aqueous colloid

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.

+ n L →

LLLLLL

LLL

LLL

LLLLLLL

Modification of the Nanocrystal’s Surface

Me3N+ CO2HCl-

OH

O

OH

HO

OO

OH

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.).

OH OOH

O

HO

OH

OH OOH

O

HO

The DLS spectra of magnetite citrate colloids.Red – pH 7.5

Green – pH 4.8Blue – pH 4.5

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)

Citrate Tartrate Malate

The reference peak* intensity turned > 90% (pH↑)

7.4 7.8 8.8

The reference peak* intensity is still > 90% (pH↓)

4.9 7.2 decomposes

The reference peak* intensity turned 0% (pH↓)

4.5 6.9 decomposes

Isoelectric point 3.6 4.4 4.3

* the reference peak 7-9 nm in the DLS spectra pH↑ - titration with base pH↓ - titration with acid

The proposed binding modes of citric and tartaric acids

O FeFe O

OO O

OHO

O FeFe O

OOO O

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.

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)

Physical measurements performed by:

• magnetic measurements - Leonard Spinu and Cosmin Radu (UNO)• TEM – Jibao He (Tulane)

We gratefully acknowledge the support of this work by

Xavier University, Center for Undergraduate Research,Advanced Materials Research Institute (UNO),

DOD/DARPAand

National Institutes of Health