PHYSICS, CHEMISTRY AND APPLICATION OF NANOSTRUCTURES, 2011
INFLUENCE OF SYNTHESIS CONDITIONS ON THE SIZE,
MORPHOLOGY AND STRUCTURE OF IRON OXIDE
PARTICLES
A. FILIPOVICH, M. IVANOVSKAYA, D. KOTSIKAU, V. PANKOV
Research Institute for Physical Chemical Problems, Belarusian State University
14 Leningradskaya, Minsk, 220030, Belarus. E-mail: ivanovskaya@bsu.by
The possibility to control the structure, size and morphology of iron oxide nanoparticles
prepared by the sol-gel approach has been studied. Two procedures were used to obtain γFe2O3 and Fe3O4 samples: i) spray pyrolysis of FeII + FeIII salt solutions or Fe3O4·nH2O
colloidal solutions; ii) chemical precipitation of FeII or FeII + FeIII hydroxides with
subsequent peptization of the sediment and drying. SiO2 sol, chloride ions and surfaceactive agents (surfactants) were added into starting solutions to modify the surface of the
resulting iron oxide nanoparticles and to control their size. Structural features of the
samples were examined by XRD, TEM and SEM, IR spectroscopy and Mössbauer
spectroscopy.
1. Introduction
Nanoparticles of γ-Fe2O3 are among the most frequently studied systems due to
their considerable value for biomedical applications [1−4]. In this regard, there
are certain requirements for particle size, surface and magnetic parameters of
γ-Fe2O3. The magnetic characteristics can be managed within some limits by
changing the size, shape and structure of γ-Fe2O3 nanoparticles.
The aim of the work is a comparative evaluation of the size, morphology
and structure of γ-Fe2O3 particles obtained by different variants of the sol-gel
synthesis.
2. Experimental
The γ-Fe2O3 powders were obtained by: i) combined precipitation of FeII/FeIII
hydroxides with ammonia solution and transformation of the precipitate into a
sol form by the peptization with HNO3 and ultrasonic treatment; ii) spray
pyrolysis of an aerosol of aqueous Fe(NO3)3 + FeSO4 solution; iii) spray
pyrolysis of the Fe3O4∙nH2O sol, obtained by the method i. The effect of
additives, such as silica sol and chloride ions, on the formation of γ-Fe2O3
particles was studied.
The samples were characterized by means of X-ray diffraction (XRD)
analysis, Fourier-transform infrared spectroscopy (FTIR), transmission and
scanning electron microscopy (TEM and SEM), and Mössbauer spectroscopy.
XRD analysis was carried out on a HZG−4A diffractometer by using Ni-filtered
Co K radiation. IR-spectra were recorded on an AVATAR FTIR-330
1
2
spectrometer. TEM/ED examinations were performed with a LEO 906E and a
JEOL 4000 EX transmission electron microscopes. The resonance spectra were
recorded in air at 298 K and processed by using a SM2201 Mössbauer
spectrometer equipped with a 15 mCi 57Co (Rh) source.
3. Results and Discussion
3.1. Sol-gel synthesis of γ-Fe2O3
EM data evidence that the sol, prepared by chemical precipitation of FeII or FeII
+ FeIII hydroxides with subsequent peptization of the sediment, contains
spherically shaped particles of Fe3O4 with a size of 6−7 nm (Figure 1). After
annealing at 300°C, Fe3O4 completely transforms into γ-Fe2O3. The formation of
γ-Fe2O3 and the absence of Fe2+ in the sample were confirmed by Mössbauer
spectroscopy (Table 1), which indicates the presence of a cubic phase of iron
oxide (δ = 0.34 mm/s, Δ = −0.03 mm/s, B = 49.1 T). The γ-Fe2O3 → α-Fe2O3
transition starts after annealing at 400°C.
Table 1. Phase composition and particle size of γ-Fe2O3 as a function of annealing temperature
according to XRD data
50°C
300°C
Phase
d, nm
Fe3O4
6-7
Phase
γ-Fe2O3
400°C
500°C
800°C
d, nm
Phase
d, nm
Phase
d, nm
Phase
d, nm
6-7
γ-Fe2O3
α-Fe2O3
7-8
−
α-Fe2O3
γ-Fe2O3
30-40
10
α-Fe2O3
70-80
The growth of iron oxide particles and the transformation of γ-phase into αphase takes place when the annealing temperature is increased.
Figure 1. TEM image of the γ-Fe2O3 sample,
annealed at 300°C.
Figure 2. SEM image of the products
obtained by aerosol pyrolysis of Fe(NO3)3 +
FeSO4 solution with the addition of SiO2.
3
3.2. Spray pyrolysis of solutions of FeII and FeIII salts
The products of the pyrolysis of FeII and FeIII salts solution at 350−400°C are Xray amorphous phase and represent spherically shaped particles with the
predominant size of 0.5−1.0 mm (Figure 2).
The adition of silica sol to the salts solution does not prevent the growth of
the particles. The IR-spectroscopy data indicate the presence of γ-Fe2O3
(absorption bands at 480 and 660 cm−1), α-Fe2O3 (530 cm-1) and sulfate ions
(595 cm-1) in the pyrolysis products.
3.3. Spray pyrolysis of Fe3O4∙nH2O sol
According to the XRD data, the products of the pyrolysis of Fe3O4∙nH2O sol
carried out at 350°C appeared to be crystalline and contained the particles of
ferrimagnetic γ-Fe2O3 phase. The primary particle size, according to TEM, is
7−8 nm (Figure 3). These particles are combined to form spherical globules. The
IR spectroscopy confirms the formation of γ-Fe2O3 phase structure
(560−580 cm−1, 635 cm−1, 690 cm−1) and the absence of α-Fe2O3 phase in the
pyrolysis products at 350°C.
Silica sol addition to the initial Fe3O4 sol does not prevent the aggregation
of γ-Fe2O3 particles. Only the addition of the chloride ions (as KCl) to the
Fe3O4∙nH2O + SiO2∙nH2O sol mixture inhibits the aggregation of iron oxide
particles.
According to the TEM data, synthesis of γ-Fe2O3 by pyrolysis of
Fe3O4∙nH2O sol in the presence of chloride ions results in particles of 20−80 nm
with the predominant size of 30−45 nm (Figure 4). Particles of this size are
preferred for biomedical applications [4, 5].
Figure 3. TEM image of the products
obtained by spray pyrolysis of Fe3O4∙nН2О
sol.
Figure 4. TEM image of the products obtained
by spray pyrolysis of Fe3O4∙nН2О sol with the
addition of SiO2 and KCl.
4
4. Conclusion
1. Sol-gel method, which includes the combined hydrolysis of Fe II and FeIII salts
with the formation of Fe3O4, transformation of the sediment in colloidal state by
peptization, and heating at 300°C, leads to the formation of γ-Fe2O3 particles
with d = 6−7 nm.
2. Pyrolysis of the Fe(NO3)3 + FeSO4 solution does not lead to a singlephase product; thus, along with the ferrimagnetic γ-Fe2O3 phase it contains αFe2O3 and FeSO4.
3. Spray pyrolysis of Fe3O4∙H2O sol leads to the formation of aggregated
spherical particles of γ-Fe2O3 with d = 0.5−1.0 mkm. Primary particle size is
7−8 nm. The γ-Fe2O3−SiO2 nanocomposites were formed by adding silica sol to
the initial Fe3O4 sol. The silica sol does not prevent γ-Fe2O3 particles from
aggregation.
4. Chloride ions, introduced into the Fe3O4 sol, prevent effectively the
aggregation of γ-Fe2O3 particles, formed by spray pyrolysis.
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