Structural stability study of cobalt ferrite-based

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 2357–2358
Structural stability study of cobalt ferrite-based nanoparticle
using micro Raman spectroscopy
M.A.G. Solera,*, T.F.O. Meloa, S.W. da Silvaa, E.C.D. Limab, A.C.M. Pimentab,
V.K. Garga, A.C. Oliveiraa, P.C. Moraisa
a
!
Instituto de F!ısica, Universidade de Bras!ılia, Nucleo
de F!ısica Aplicada, 70919-970 Bras!ılia-DF, Brazil
b
! 74001-970 Goiania-GO,
#
Instituto de Qu!ımica, Universidade Federal de Goias,
Brazil
Abstract
Micro Raman scattering was used to study the structural stability of cobalt ferrite-based (CoFe2O4) nanoparticles,
under illumination with the 514 nm line, at 7 mW laser power. Different samples were investigated after performing the
steps of the magnetic fluid (MF) preparation. Raman spectra of samples peptized at 0.25 mol/l perchloric acid showed
features similar to bulk maghemite. However, samples peptized at 0.75 mol/l perchloric acid showed features similar to
the Fe3O4 phase.
r 2004 Elsevier B.V. All rights reserved.
PACS: 75.50.Mm; 74.62.Bf; 63.22.+m
Keywords: Magnetic fluid; Raman scattering; Cobalt ferrite; Structural stability
The chemical synthesis of spinel ferrite-based nanoparticles, passivated and peptized as stable ionic
magnetic fluids (MFs), represents a very important step
towards the engineering of specific magnetic carriers.
The structural stability of the MF nanoparticles is
essential in all technical and biological applications [1].
Compared to magnetite (Fe3O4) nanoparticles, recent
results observed in the visible range showed the higher
structural stability of cobalt ferrite (CoFe2O4) nanoparticles upon laser illumination [2]. This observation
indicates that cobalt ferrite nanoparticles are more
reliable as magnetic drug carriers in biological applications, as for instance in the photodynamic therapy [3].
Also, it is well known that in initially stable ionic MFs
the suspended nanoparticles could progressively be
dissolved in low pH values, harming the long-term MF
stability [4]. In this study, micro Raman spectroscopy
was used to investigate the influence of the peptization
condition on the structural stability of cobalt ferrite
nanoparticles suspended as ionic MFs.
*Corresponding author. Tel.: +55-61-3072900; fax: +55-613072363.
E-mail address: soler@unb.br (M.A.G. Soler).
Stable CoFe2O4-based ionic MF samples were obtained following the three-step procedure described in
the literature [5]. In the first step, cobalt ferrite
nanoparticles (sample S1) were synthesized by coprecipitating Co(II) and Fe(III) ions in alkaline medium. In
the second step, sample S1 was submitted to the
passivation process using concentrated ferric nitrate
solution under boiling condition (sample S2). Finally, in
the third step, peptization of non-passivated (sample S1)
and passivated (sample S2) nanoparticles were performed using different perchloric acid concentration
(0.25 and 0.75 mol/l). Twenty-four hours after peptization the four MF samples were precipitated using
acetone. The precipitates were dried in air at room
temperature to produce four samples (S3, S4, PS1, and
PS2). Samples S3 and S4 refer to sample S1 peptized at
0.25 and 0.75 mol/l, respectively. Samples PS1 and PS2
refer to sample S2 peptized at 0.25 and 0.75 mol/l,
respectively. The average CoFe2O4 nanoparticle diameter was estimated by X-ray diffraction measurements
as 8.6 nm. The Raman system used to record the roomtemperature spectra was a commercial triple spectrometer equipped with a CCD detector. The 514 nm line of
a CW Argon ion laser was used for excitation of the
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2003.12.582
ARTICLE IN PRESS
M.A.G. Soler et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 2357–2358
2358
samples, and the incident power was kept at 7 mW,
elevating the temperature up to 200 C at the focus.
It was observed that samples S1, S2, PS1, and S3
present similar Raman spectra (see Fig. 1(a)). The best
curve fitting of the spectra, using lorentzian bandshaped lines, showed the presence of five structures at
220, 310, 467, 624, and 690 cm1. This finding is in very
good agreement with the five optical active Raman
modes (A1g+Eg+3F2g), characteristic of the cubic
inverse-spinel structure O7h ðF d 3% mÞ space group [6].
Further, except for the downshift observed in all Raman
features, the spectrum showed in Fig. 1(a) is similar to
the spectrum of bulk maghemite (g-Fe2O3) presented in
the literature [7]. The observed downshift is due to the
largest Co-atom mass compared to the Fe-atom mass.
The similarities between the Raman features of the
nominal CoFe2O4 samples and the Raman features of
bulk maghemite suggests that samples S1, S2, PS1, and
S3 present the crystal structure of the cobalt-modified gFe2O3 reported in the literature [8]. This is supported by
the room-temperature values of the hyperfine fields of
463 and 414 kOe, attributed to Fe3+ at A and B sites [9],
.
as obtained by Mossbauer
Spectroscopy. Finally,
excitation of samples S1, S2, PS1, and S3 in a wide
CoFe2O4 - passivated
range of optical intensity (0.7–70 mW), do not induce
any significant change in the Raman features showed in
Fig. 1(a).
The Raman spectra of samples PS2 and S4 are similar
to one another and quite different from the Raman
spectra of samples S1, S2, PS1, and S3. The Raman
spectrum of sample PS2 (see Fig. 1(b)) presents seven
lines at 190, 300, 340, 475, 516, 610, and 680 cm1.
Except for two extra Raman modes at 190 and
475 cm1, the five observed lines are typical of the cubic
inverse-spinel structure, characteristic of the nominal
Fe3O4 structure [7]. The extra Raman peak at 475 cm1
is related to the O-site mode that reflects the local lattice
effect in the octahedral sublattice of CoFe2O4. Note that
the Raman spectra recorded from samples PS2 and S4,
at 0.7 mW laser power, are similar to the Raman spectra
quoted in Fig. 1(a). However, differences between the
spectra of samples PS1 and PS2, at 7 mW laser power,
are not clear yet. Atomic absorption measurements
indicated that samples peptized at 0.75 mol/l release
more Co-atom to the aqueous medium than samples
peptized at 0.25 mol/l. Further, there are evidences in the
literature that Co- and Fe-atoms may self-rearrange in
the crystalline structure when heated above room
temperature [10]. Finally, the differences between the
samples PS1 and PS2 could be attributed to the
annealing process induced by the laser excitation.
The authors acknowledge the financial support of the
Brazilian agencies FINEP/CTPETRO, CAPES, FINATEC, and CNPq.
514 nm
7 mW
Raman Intensity
References
(a) 0.25 mol/L
(b) 0.75 mol/L
200
300
400
500
Wavenumber
600
700
800
(cm-1)
Fig. 1. Raman spectra of samples PS1 (peptized at 0.25 mol/l)
and PS2 (peptized at 0.75 mol/l).
.
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