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
HOH
OHM
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
OHH
O
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 spectroscopy
1H 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
(co
unts
)
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