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 structure
MIIFe2O4 (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]
- 4 H2O
MFe2O4
Solvents / chelating agents
HO
OH
HO
O
O
OH
HO
OH
OH
O
OH
diethylene glycol:  = 32; b.p. 245oC
Reagents: MCl2 + 2 FeCl3 + 8 NaOH
a) Formation of metal chelate alkoxide complexes
in parent alcohol solutions
H
O
H
O
MCl2(H2O)x +
O
Cl
2 OH-
Cl
M
-X H2O
O
H
O
-2H2O
OH
] 2-
Cl
O
Cl
M
O
O
b) Nucleation and growth of the nanoparticles
O
Cl
M
O
H
O
M
O
+ 2
OH
2 H2O
OH
OH
Fe
O
OH
OH
M
- 2 Cl-
O
H
O
OH
OH
H
O
] 2-
Cl
O

OH
H
O
MFe2O4 + 2 H2O + 3
O
O
H
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
5.3 nm
16 %
6.6 nm
11 %
CoFe2O4 NiFe2O4
4.2 nm
18 %
5.1 nm
15 %
ZnFe2O4
5.6 nm
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
3.5
Fe2O3 at 50 Oe
3.0
M, emu/g
2.5
2.0
1.5
1.0
0.5
0
50
100
150
200
Temperature, K
250
300
Hysteresis Plot for FeFe2O4 (4 nm from TEM)
100
80
60
40
M, emu/g
20
0
-20
Fe3O4 at 300K
-40
-60
-80
-100
-60000
-40000
-20000
0
Field, Oe
20000
40000
60000
X-ray diffractogram for FeFe2O4 nanoparticles:
4 nm from TEM; 5.3 nm from XRD
1000
Intensity (counts)
800
600
400
200
0
30
40
50
2 theta (deg)
60
70
Synthesis of Nanocrystalline Ferrites by Decomposition of
Metal Chelates in Non-aqueous Solutions
H
O
H
O
Z
MCl2 +
Z
O
H
Cl
M
Z
O
H
O
n
2 H2O
Z
H
O
+ 2n
OH
OH
OH
2-
Cl
O
Cl
- 2 H2O
Z
O
Z = O or NCH3
OH
OH
OH
Fe
Z
2 OH
-
M
OH
M
- 2 Cl-
OH
OH
M
Z
H
O
2-
Cl
Cl
M
-X H2O
O
Cl
H
O

(MFe2O4)n + 2n H2O + 3n
O
O
H
Inorg. Chem., 2002, 41, 6137
Chem. Mater, 2004, 16, 5527
Z
Powder X-ray Diffractograms for Fe3O4
a) Synthesized in
b) Synthesized in
c) Synthesized in
HO
HO
HO
O
O
N
CH3
OH
OH
OH
+
HO
N
CH3
OH
Nanocrystals of Fe3O4 Synthesized In Different
Complexing Media
Solvent/Chelating Agent:
HO
O
OH
Solvent/Chelating Agent:
1:1 mixture
HO
OH
O
HO
OH
N
CH3
Solvent/Chelating Agent:
HO
N
CH3
OH
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.
L LL
L
L
+nL →
L
L
L
LL
L
L
L
L
L
L
L
LL
Modification of the Nanocrystal’s Surface
Reactions of Aqueous Colloids of Fe3O4 With Carboxylic Acids
Me3N+
Cl-
O
CO2H
OH
HO
OH
O
OH
HO
O
O
OH
OH
O
OH
O
HO
OH
O
OH
FT-IR spectra of the isolated solids evidenced no binding of
monocarboxylic and binding of dicarboxylic acids
and hydroxy-carboxylic acids (citric, tartaric, etc.).
The DLS spectra of magnetite citrate colloids.
Red – pH 7.5
Green – pH 4.8
Blue – pH 4.5
The pH values representing substantial aggregation and deaggregation 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
OH
O
O
Fe
O
Fe
O
O
O
O
O
Fe
O
O
Fe
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/DARPA
and
National Institutes of Health