PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Reversible post-synthesis tuning of the superparamagnetic blocking
temperature of c-Fe2O3 nanoparticles by adsorption and desorption of
Co(II) ions{
German Salazar-Alvarez,*a Jordi Sort,b Abdusalam Uheida,c Mamoun Muhammed,c Santiago Suriñach,a
Maria Dolors Baróa and Josep Noguésb
Received 8th September 2006, Accepted 19th October 2006
First published as an Advance Article on the web 2nd November 2006
DOI: 10.1039/b613026g
The influence of the post-synthesis adsorption of Co(II) ions on the structural and magnetic
properties of maghemite (c-Fe2O3) nanoparticles with a mean particle size of about 10 nm has
been investigated. It is shown that the step-wise adsorption of Co(II) can controllably increase the
blocking temperature, TB, of the system up to 60 K with respect to that of untreated particles,
while neither the particle size nor the particle size distribution are significantly modified. This is
accompanied by a four-fold increase in the coercivity, HC, at low temperatures. Using a selective
leaching of the previously adsorbed Co(II) ions the TB and HC values of the pristine c-Fe2O3
nanoparticles are recovered. Hence, a reversible and controllable tailoring of the magnetic
properties (e.g., TB and HC) of the c-Fe2O3 nanoparticles can be achieved by a simple adsorption
and desorption process of Co(II) ions after their synthesis.
Introduction
Magnetic nanoparticles, and in particular iron oxides, are used
in widespread technological applications,1–3 ranging from
biomedicine and biodiagnostics4–7 to engineering and industrial applications.8–11 Some of them require the nanoparticles
to be in the superparamagnetic state, for example, to avoid
interparticle aggregation. However for applications like
recording media the nanoparticles must remain ferromagnetic.
Moreover, the use of magnetic nanoparticles in sensors calls
for the particles to be magnetically soft without being superparamagnetic. Thus, the magnetic properties of nanoparticles
often need to be adjusted to match specific purposes.12 In this
context, the improvement of the coercivity, HC, of acicular,
sub-micron sized, c-Fe2O3 particles for their application
in recording media has been extensively studied since the
1960s.13–15 This enhancement has been achieved by (i)
adsorbing Co(II) ions on the surface of the particles, (ii)
growing an epitaxial layer of CoFe2O4, or (iii) body-doping
c-Fe2O3 particles.16 Despite the number of publications
dealing with such improvement of the magnetic properties of
acicular submicron particles, very little work has been carried
out on smaller, i.e., few nanometers, sized particles.17–19
Chakrabarti et al.17 reported the synthesis of Co-body-doped
c-Fe2O3 nanoparticles with an average diameter of about
6 nm, where, by varying the Co doping, the blocking
a
Dept. de Fı́sica, Universitat Autònoma de Barcelona, E-08193,
Bellaterra, Spain. E-mail: [email protected]; Fax: +34-93-5812155
b
Institució Catalana de Recerca i Estudis Avançats (ICREA) and Dept.
de Fı́sica, Universitat Autònoma de Barcelona, E-08193, Bellaterra,
Spain
c
Materials Chemistry Division, Royal Institute of Technology,
SE-100 44, Stockholm, Sweden
{ Electronic supplementary information (ESI) available: information
on the surface composition and structure analyses by X-ray photoelectron spectroscopy. See DOI: 10.1039/b613026g
322 | J. Mater. Chem., 2007, 17, 322–328
temperature, TB, of the system increases from 306 K (0 mol%
Co (II) doping) to 337 K (5 mol% Co (II) doping). Conversely,
Frankamp et al.18 tuned the TB of an assembly of dendrimerencapsulated iron oxide nanoparticles by changing the
interparticle distance within a polymeric matrix with the
consequent variation of the dipolar interactions while
Frandsen et al.19 investigated the increase of TB occurring
when physically contacting c-Fe2O3 with CoO.
In this work we demonstrate that, by using a simple postsynthesis route, the blocking temperature and the coercivity of
y10 nm c-Fe2O3 nanoparticles can be easily enhanced by the
adsorption of Co(II) ions without modifying their particle size
or size distribution. Furthermore, the effect of Co(II) surfacedoping of the particles is shown to be reversible, i.e. removing
the adsorbed Co(II) ions, by leaching, reverts to the original
values of untreated c-Fe2O3 nanoparticles. To our knowledge
this is the first report of a reversible, post-synthesis, tuning of
TB and HC of nanoparticles by a simple surface doping–
leaching process.
Experimental
Chemicals
Cobalt(II) nitrate, iron(II) and iron(III) chlorides of analytical
grade (Aldrich), and all other chemicals were used as received
without further purification. High purity water (resistivity of
18 MV cm, Milli-Q Millipore) was employed throughout all
the experiments.
Preparation of nanoparticles
The synthesis of the maghemite (c-Fe2O3) nanoparticles is
described in detail elsewhere.20 Briefly, 35 cm3 of an aqueous
solution of 1 mol dm23 Fe3+ and 0.5 mol dm23 Fe2+ were
added under stirring in argon atmosphere to 450 cm3 of a
This journal is ß The Royal Society of Chemistry 2007
0.7 mol dm23 NH4OH aqueous solution at 70 uC. The reaction
was allowed to proceed for 45 min after which the black gel
formed by the particles (Fe3O4) was decanted with the help of
an external magnet. The gel was rinsed several times with
water and 300 cm3 of a 0.01 mol dm23 nitric acid solution
(used as mild oxidising agent), followed by thorough rinsing
with water. The resulting dark brownish particles (Fe32xO4)
were further dialyzed against a 0.01 mol dm23 HNO3 aqueous
solution for 24 h, which yielded particles with a red–brown
colour (c-Fe2O3).
The adsorption of Co(II) ions was undertaken by mixing for
10 min 50 mg of nanoparticles with 10 cm3 of an aqueous
solution containing a given concentration of Co2+ with a
constant ionic strength of 0.01 mol dm23 NaNO3. The
suspension was shaken for about 10 min at 22 ¡ 1 uC and
the particles were retrieved by centrifugation. The concentration of Co in the aqueous solutions was determined by atomic
absorption spectroscopy (AAS-200, Varian Inc.). The amount
of adsorbed Co was calculated from the concentration
difference between the initial (t = 0 min) and final (t =
10 min) Co concentrations. Powder samples were prepared by
drying under air at 90 uC in a conventional oven. Leaching of
Co was carried out by soaking a given amount of particles in
an aqueous solution of nitric acid, with a varying concentration, for 24 h. The particles were then washed with water,
collected with a magnet and dried under vacuum.
The equilibrium concentration, C (in mgCo (gc-Fe2O3)21), of
adsorbed Co can be expressed as:
C~ðCi {Cf Þ
V
m
(1)
where Ci and Cf are the initial and at equilibrium Co
concentrations in the aqueous phase, respectively (aqueous
concentrations are given in mg dm23), V is the total aqueous
volume (= 0.01 dm3), and m is the mass of iron oxide used for
the experiments (= 0.05 g).
Characterization techniques
The particles were imaged with a high resolution transmission
electron microscope (HRTEM, JEM-2011 and JEM-2010F,
Jeol) operating at an accelerating voltage of 200 kV. Specimens
were prepared by dispersing a suspension of the particles
in ethanol under sonication and then depositing a drop
onto carbon-coated copper grids. The excess of solvent was
removed by gentle heating with an infrared lamp. Particle size,
DP, and particle size distribution, sP, were obtained by
measuring the equivalent diameter of .250 particles from
TEM micrographs and fitting the results with a log-normal
distribution function. Selected-area electron diffraction
(SAED) in conjunction with HRTEM analyses were used to
determine the local crystallographic phases. The microstructure of the particles was characterized by X-ray diffraction
(XRD) using a Philips 3050 diffractometer with Cu Ka
radiation (l = 0.154184 nm). Analysis of the XRD data was
undertaken with a full pattern fitting procedure based on the
Rietveld method using the program MAUD.21 The magnetic
properties of the samples were measured with a SQUID
magnetometer (MPMS 7XL, Quantum Design) using a
This journal is ß The Royal Society of Chemistry 2007
maximum applied field of 50 kOe. The specific surface area
of the powders was determined using the multipoint Brunauer–
Emmet–Teller method (BET, ASAP2000, Micromeritics
Corporation) using nitrogen as the adsorbate gas. X-Ray
photoelectron spectroscopy, XPS, experiments were performed
in a PHI 5500 Multitechnique System (Physical Electronics
Division, Perkin Elmer Inc.) with a monochromatic
X-ray source (Al Ka line of 1486.6 eV and 350 W), placed
perpendicular to the analyzer axis and calibrated using the Ag
3d5/2 line with a full width at half maximum (FWHM) of
0.8 eV. The charging effect on the spectra was corrected by
setting the signal of adventitious carbon to 285 eV. Thermal
characterization of the samples was carried out with a
differential scanning calorimeter, DSC (DSC-7, Perkin Elmer
Inc.), using a heating rate of 40 K min21 and by thermogravimetric analysis, TGA (TGA-7, Perkin Elmer Inc.), using a
heating rate of 10 K min21.
Results
Table 1 shows a list of the samples studied with the corresponding amount of adsorbed Co as well as some other physical
characteristics.
A particle size of about 10 nm and particle size distribution
of about 2.7 nm remained practically the same, within the
experimental error, for all of the samples (Table 1). Therefore,
for simplicity, only the TEM images and particle size
distributions corresponding to the samples with no Co(II)
adsorbed (CF0) and the one with C = 200 mgCo (gc-Fe2O3)21
(CF200) are shown in Fig. 1. From this figure it can be seen
that the particles, which are very similar for both samples, are
spheroidal and somewhat aggregated.
High resolution TEM (HRTEM) micrographs are shown in
Fig. 2. The figure shows crystalline particles with lattice fringes
corresponding to the (311) (d = 0.25 nm), (220) (d = 0.29 nm)
and (111) (d = 0.48 nm) reflections of inverse spinel iron oxide
(JCPDS card no. 39-1346). The crystallographic phase of the
samples was further characterized with selected-area electron
diffraction. As can be seen in Fig. 2, all the reflections
correspond to the inverse spinel structure.
The crystallographic composition of the powders was also
followed by X-ray powder diffraction (XRD). Fig. 3 shows the
corresponding X-ray diffractograms for the sample with no Co
(CF0) and, for comparison, that of a sample prepared with
200 mgCo (gc-Fe2O3)21 of adsorbed Co(II) (CF200). All of the
measured samples with and without adsorbed Co(II) showed
the presence of only the inverse spinel structure with a lattice
parameter of about 0.836 nm, i.e., similar to that reported for
disordered maghemite, c-Fe2O3.22 The presence of maghemite
Table 1 List of samples with their cobalt content, particle size, DP,
particle size distribution, sP, and the lattice cell parameter of the cubic
spinel structure, a
Sample
C/mgCo (gc-Fe2O3)21
DP ¡ sP/nm
a/nm
CF500
CF200
CF35
CF2
CF0
457.6 ¡ 0.9
200.0 ¡ 0.4
34.00 ¡ 0.10
2.000 ¡ 0.004
0.000 ¡ 0.004
9.5
10.2
10.1
9.4
10.2
0.836(1)
0.836(0)
0.836(4)
0.836(0)
0.836(1)
¡
¡
¡
¡
¡
2.8
2.7
3.0
2.2
2.9
J. Mater. Chem., 2007, 17, 322–328 | 323
Fig. 3 XRD patterns of (a) the pristine c-Fe2O3 particles (CF0) and
(b) the Co(II) adsorbed particles (CF200). The spectra below the
diffractograms correspond to the difference between the experimental
pattern (circles) and the calculated one (line). The bars indicate the
positions of the inverse spinel reflections, JCPDS card no. 39-1346.
Fig. 1 TEM micrographs of (a) pristine c-Fe2O3 (CF0) and (c)
Co(II)-adsorbed c-Fe2O3 particles (CF200), with their corresponding
particle size distributions and the fitting to a log-normal distribution
(b and d).
Fig. 2 HRTEM micrographs of (a) pristine c-Fe2O3 (CF0) and (b)
Co-adsorbed c-Fe2O3 particles (CF200). The lattice fringes correspond
to the crystallographic planes (311) (d = 0.25 nm), (220) (d = 0.29 nm),
and (111) (d = 0.48 nm). The corresponding SAED patterns are also
shown (c and d) with the diffraction rings indexed for inverse spinel
iron oxide (JCPDS card no. 39-1346).
was further confirmed by XPS (see ESI{). Besides, from the
width of the XRD peaks, an average crystallite size of about
10 nm was obtained for all samples, in agreement with the
TEM results, with negligible microstrain.
As can be seen from the hysteresis loops in Fig. 4(a), HC
increases dramatically after Co(II) adsorption. Actually,
HC rapidly increases after adsorbing more than 2 mgCo
(gc-Fe2O3)21, leveling off towards 200 mgCo (gc-Fe2O3)21. The
maximum increment in coercivity (measured at 10 K) is of
about 550 Oe, i.e., almost a four-fold increase with respect to
pristine c-Fe2O3 nanoparticles. This increment is obtained at a
concentration near 200 mgCo (gc-Fe2O3)21, which is about the
amount of cobalt necessary to form a continuous monolayer of
CoFe2O4. Moreover, after doubling this concentration of Co
(II) (as in sample CF500) the coercivity does not increase
324 | J. Mater. Chem., 2007, 17, 322–328
further significantly, indicating that the effect saturates at
about one monolayer. Experiments carried out on the lower
end of the adsorbate concentration indicate that there is a
Co(II) concentration threshold for the HC increase. Indeed, the
adsorption of 0.02 mg Co per gram of iron oxide (not shown)
does not increase significantly the coercivity, varying it only
from 188 Oe (pristine particles) to 189 Oe. From Fig. 4(b) it
can also be observed that the saturation magnetization, MS, of
the samples decreases with the concentration of adsorbed
Co(II). This is consistent with the progressive formation of the
CoFe2O4 surface layer (as will be discussed later). The steeper
decrease of MS for the CF500 sample indicates the formation
of non-magnetic material. Actually, the possibility of forming
a fraction of Co(OH)2 can be inferred from the chemical
conditions for the preparation of the sample CF500.23 Such a
compound is amorphous (thus not detectable by XRD) and
also antiferromagnetic and would, therefore, contribute to a
reduction of the overall saturation magnetization.
Fig. 5 shows the zero-field cooled (ZFC) and field-cooled
(FC) measurements carried out on the samples with pristine
particles (CF0) and with adsorbed cobalt (CF200). From the
figure it can be clearly observed that the blocking temperature
(CF0) y
(given by the maximum of MZFC(T)) of CF0, TZFC
B
260 K, is significantly lower than the one for CF200, TZFC
B
(CF200) y 350 K. However, the rather broad and flat peak in
MZFC(T) (due to the particle size distribution and the interrather unreliable.
actions) makes the determination of TZFC
B
Hence, to obtain TBHC of the different samples in a systematic
way, the temperature dependence of the coercivity in the range
T ¡ TB has been fitted to:
"
k #
T
(2)
HC ~HC ð0Þ 1{
TB
where HC(0) is a constant that depends on MS and the anisotropy
constant, TB is the superparamagnetic effective blocking temperature and the exponent k relates to different experimental
parameters, e.g., anisotropy type, random nanoparticle arrangement and interactions.24 Note that this relation extends the
classical one with k = K obtained for non-interacting and
randomly oriented nanoparticles with uniaxial anisotropy,25 to
This journal is ß The Royal Society of Chemistry 2007
Fig. 4 (a) Hysteresis loops recorded at 10 K for samples (%) CF0, (#) CF2, and (n) CF200. Shown in the inset is the enlargement of the central
part of the loops. (b) Variation of the coercivity (open symbols) and saturation magnetization (closed symbols), measured at 10 K, H10K
and M10K
C
S ,
as a function of the concentration of adsorbed Co(II) ions. The curves are plotted in semi-log form to highlight the differences at low
concentrations. An adsorbed concentration of 1022 mgCo (gc-Fe2O3)21 has been arbitrarily assigned to sample CF0. The lines are a guide to the eye.
Fig. 5 Temperature dependence of the zero field-cooled (ZFC; open
symbols) and field-cooled (FC; filled symbols) magnetization for the
pristine c-Fe2O3 (CF0, squares) and Co(II)-adsorbed c-Fe2O3 particles
(CF200, circles).
take into account the less ideal conditions often found experimentally. From the temperature dependence of HC (Fig. 6(a)) it can be
seen that for the sample with no Co(II) adsorbed (CF0) HC
vanishes at about 150 K, whereas HC for CF200 remains finite up
to about 220 K. Similarly, the squareness ratio, MR/MS, for
sample CF200 is significantly larger and disappears at higher
temperatures than that for CF0 (Fig. 6(b)). Since the packing
fraction of all the samples is kept constant, these results indicate
that the superparamagnetic transition temperature, TBHC , is much
higher for the Co(II)-adsorbed samples. As can be seen from the
solid lines in Fig. 6(a), eqn (2) fits reasonably well to the
experimental data. The TBHC values obtained, shown in Fig. 6(b),
evidence a drastic increase of the effective blocking temperature
by the post-synthesis Co(II) adsorption process. The maximum
increase in TBHC is about 60 K, i.e. almost a 40% increase of TB
with respect to the pure c-Fe2O3 nanoparticles. It is noteworthy
that TBHC vTBZFC , as is commonly observed in nanoparticles when
analysed using these approaches.26
To determine the reversibility of the TB and HC enhancement and hence the feasibility of fully tuning the magnetic
properties of the particles in cycles, i.e., by adsorbing and
desorbing Co(II) ions, sample CF200 was subject to a
progressive leaching process to selectively and controllably
remove the previously adsorbed Co(II) ions. In this way,
approximately 10% of the adsorbed Co was removed using a
1024 mol dm23 nitric acid solution whereas about 17% of the
adsorbed cobalt was removed by soaking the particles in a
1022 mol dm23 nitric acid solution. In both cases the particle
size and distribution were not modified. As can be seen in
Fig. 7, both TB and HC quickly decrease as the Co(II) content
is reduced, i.e., a mere reduction of about 34 mgCo (gc-Fe2O3)21
brings both TB and HC close to the values exhibited by the
pure c-Fe2O3 (CF0). Moreover, the saturation magnetization
Fig. 6 Temperature variation of (a) the coercivity, HC, and (b) the squareness ratio, MR/MS, of samples (%) CF0, (#) CF2, and (n) CF200,
where the continuous lines in (a) are fittings to the data based on eqn (2); (c) dependence of the evaluated effective blocking temperatures, TBHC , of
the samples, as a function of the concentration of adsorbed Co(II) ions. The lines in (b) and (c) are guides for the eye.
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 322–328 | 325
Fig. 7 Variation of coercivity measured at 10 K (#), H10K
C , and the
effective blocking temperature ($),TBHC , as a function of the cobalt
concentration, C, for the samples treated with acid to leach the Co(II)
ions. The broken lines indicate HC and TBHC for the pristine c-Fe2O3
particles (CF0). The continuous lines are guides for the eye.
of the samples tends to increase for lower Co(II) concentrations to reach the value corresponding to the sample CF0 (not
shown). It is worth mentioning that previous reports have also
demonstrated that, in Co(II)-adsorbed acicular, sub-micronsized particles, leaching of Co(II) ions with aqueous solutions
of hydrochloric acid could be used to lower the coercivity of
the samples,27 however, the leaching was not selective and
iron ions were also released. Conversely, Jolivet and Tronc
also studied the selective leaching of Co2+ from aqueous
suspensions of CoFe2O4 by using a solution of HClO4.28
Similarly, the leaching of cobalt ions with solutions of nitric
acid used in this work is selective due to its lower complexation
strength towards iron as compared with HCl. Hence, the
loss of iron ions could be kept negligible, within the
instrumental error.
Discussion
Magnetite, Fe3O4 (or FeIIFeIII2O4), possesses the inverse spinel
structure which is based on the framework of a close-packed
array of oxygen ions with the iron ions occupying some of the
octahedral and tetrahedral positions. In the ideal inverse spinel
unit cell structure (with 8 formula units per unit cell) the
totality of the Fe2+ ions occupy 8 octahedral positions while
the Fe3+ ions are equally distributed between 8 octahedral and
8 tetrahedral positions, hence the formula per unit cell can be
written as (FeIII8)[FeIII8FeII8]O32. Alternatively, maghemite
(c-Fe2O3) can be understood as a Fe2+-deficient structure
with a unit cell composition (FeIII8)[FeIII40/3%8/3]O32 where %
represents vacancies. In the absence of vacancy ordering the
structure remains cubic with a slightly smaller lattice
parameter than magnetite.22 From the magnetic standpoint,
the structure of inverse spinels can be conceived ideally as a
two-sublattice system where the 8 Fe3+ tetrahedral ions
arrange antiparallel to 8 Fe3+ octahedral ions due to the
superexchange interactions through oxygen and thus the
magnetization is dictated by the remaining 8 divalent
octahedral ions. In the case of Co(II), with 3 unpaired valence
electrons, this would result in a unit cell moment of 24 mB
whereas in the case of maghemite (although the structure
is still not well understood), 8 Fe3+ tetrahedral ions are
antiparallel to the 40/3 Fe3+ octahedral ions, hence a net
moment of 80/3 mB per unit cell is expected since Fe3+ possesses
326 | J. Mater. Chem., 2007, 17, 322–328
5 unpaired electrons. Experimentally, the saturation
magnetization of c-Fe2O3 has indeed been found to be larger
than that of CoFe2O4 (at 10 K, MS = 82.2 and 80.3 emu g21,
respectively).22,29 Nevertheless, the distribution of cations (and
vacancies) strongly depends on the thermal history and
thus, fine particles with the inverse spinel structure obtained
by chemical methods often show an incomplete degree of
inversion which results in a ratio of filled octahedral to filled
tetrahedral positions different from that present in the bulk
form.30 Such a ratio can be determined experimentally since it
is known that, in the case of the spinel structure, the intensity
of the (220) reflection is due only to the concentration of ions
in tetrahedral positions (denoted as (A)) whereas the intensity
of the (222) reflection is related only to the concentration of
ions in octahedral positions (denoted as [B]). Hence, the
intensity ratio I[222]/I[220] provides us with information on
the different occupancy ratios, [B]/(A), of the metal ions in the
spinel structure.31 Fig. 8 shows that the occupancy ratio
increases with the concentration of adsorbed cobalt. In other
words, the relative concentration of ions in octahedral sites
increases with the concentration of adsorbed ions, which
shows that these ions are located preferentially in such
octahedral positions, as suggested by Tronc et al.32 It has also
been shown23,32 that the Co(II) sorption capacity of spinel iron
oxide nanoparticles is much higher than the number of
available surface sites for the ion exchange provided by those
hydroxylated surfaces as determined for metal oxides in
contact with water.33 Moreover, adsorption of Co(II) in spinel
iron oxides (Fe3O4) has shown a sorption capacity of about
one order of magnitude higher than that observed in a
corundum-like iron oxide such as hematite (a-Fe2O3).34
The large sorption capacity has been explained in terms of
surface complexation, i.e., formation of MFe–O–Co+ and
(MFe–O)2–Co species,23,34 and surface ion-exchange, i.e.,
FeIIFeIII2O4(surf) + xCo2+ = CoIIxFeII(12x)FeIII2O4(surf) +
xFe2+.23 However, in the case of maghemite, the adsorption
has been explained in terms of structural adsorption through
the diffusion of the ions into the first monolayers with the most
probable exchange with octahedral vacancies in the maghemite
structure.23 The XPS spectra of the core lines of Fe and Co
show that the energy of the iron line increases with the
adsorption of cobalt, and that cobalt exists as Co(II) (see
Fig. 8 Variation of the occupancy ratio, [B]/(A), as a function of the
concentration of adsorbed Co(II) ions, C. The curves are plotted in
semi-log form to highlight the differences at low concentrations. An
adsorbed concentration of 1022 mgCo (gc-Fe2O3)21 has been arbitrarily
assigned to sample CF0. The line is a guide for the eye.
This journal is ß The Royal Society of Chemistry 2007
ESI{). Both results are consistent with the formation of
CoFe2O4.35,36 The increase of the [B]/(A) ratio, the CoFe2O4
features observed in XPS, the absence of crystallographic
phases other than spinel ferrite together with the absence of a
significant weight loss in the temperature range 473–673 K37
(samples CF0 through CF200) can then only be explained by
the formation of a surface layer with a composition near that
of CoFe2O4 and thus rule out the precipitation of a layer of
cobalt hydroxides.38,39 Furthermore the enhancement of HC
is also consistent with the diffusion of cobalt into the inverse
spinel lattice to form a surface layer compound with a
composition close to that of cobalt ferrite, CoFe2O4, as has
been reported for larger particles.16 In the case of surface
adsorption of Co(II) on a-Fe2O3 there is no formation of a
highly anisotropic surface layer and thus the effect of Co(II)
adsorption on TB is negligible.40 Actually, the formation of
CoO nanoparticles or a CoO shell on the c-Fe2O3 nanoparticles could also explain the increase in TB and HC.19,41
However, the lack of CoO in our structural analyses (XRD,
TEM and XPS) leads us to discard this possibility.
Three main mechanisms (which are not necessarily mutually
exclusive) have been put forward to explain the increase of
the effective anisotropy, K, of cobalt-adsorbed c-Fe2O3 nanoparticles: (i) increase of the surface anisotropy due to the
presence of the Co(II) ions,11 (ii) exchange coupling between
the surface cobalt ferrite layer and the maghemite core42
(either through a spring magnet43 or exchange bias44 type of
coupling) and (iii) reduction of the surface roughness leading
to a modification of the nucleation for reversal.39 Note that the
change of HC and TB can be understood in terms of an increase
of K since HC 3 K/MS and TB 3 K.
Leaching of the adsorbed Co(II) ions yielded samples that
showed a dramatic reduction in the coercivity, i.e., a lowering
in the Co concentration by about 17% (relative to that of
sample CF200), brought about a reduction in the coercivity of
about 300 Oe (see Fig. 6), which is much more than what one
would expect from the dependence of HC on the initial Co(II)
adsorption (see from Fig. 4(b) that HC should still be around
700 Oe for a Co(II) concentration of 170 mgCo (gc-Fe2O3)21).
The strong hysteresis in the change of the magnetic properties
with adsorption–desorption of Co(II) ions probably indicates
that during the adsorption and desorption of the Co(II) ions
different mechanisms for the control of the magnetic
properties may be dominant. For example, leaching has been
observed, in some systems, to increase the porosity and surface
roughness,45 which could perhaps enhance such a mechanism
for HC and TB reduction. Nitrogen adsorption isotherms
did not indicate the formation of micropores after the
leaching, although the specific surface area increased from
about 90 m2 g21 (for both CF0 and CF200) to about
110 m2 g21 after leaching, which can be attributed to an
increase of the surface roughness of the particles, which could
control the leaching process and thus the magnetic properties.
Finally, note that due to the drying process at 90 uC some of
the cobalt may have diffused into the core of the particle and
hence would be difficult to leach.
These results open the door to the possibility to synthesize a
single, generic, type of particle, which can later be modified to
adapt to the needs of a specific application.
This journal is ß The Royal Society of Chemistry 2007
Conclusions
In summary, c-Fe2O3 nanoparticles were surface-doped with
Co(II) ions using a simple post-synthesis route. The gradual
adsorption of Co(II) brought about a steady increase of the
coercivity and blocking temperature of the particles. These
effects can be ascribed to the formation of a surface layer
with a composition near that of cobalt ferrite, which increases
the effective anisotropy of the nanoparticles. Removing the
adsorbed Co(II) ions by leaching in nitric acid results in a loss
of the enhanced blocking temperature and coercivity roughly
back to the untreated c-Fe2O3 values. Hence, we demonstrate
the possibility to reversibly tailor the blocking temperature and
coercivity of c-Fe2O3 nanoparticles by a straightforward, postsynthesis, surface adsorption and desorption of Co(II) ions.
Acknowledgements
This work is supported by the Spanish CICYT (MAT200401679), the Catalan DGR (2005-SGR-00401) and the
European Union research network NEXBIAS (HPRN-CT2002-00296). G.S.A. acknowledges the financial support of
the research network NEXBIAS. The authors thank Julia
Garcı́a-Montaño and the Servei de Microscòpia for their
technical assistance.
References
1 S. P. Gubin, Y. A. Koksharov, G. B. Khomutov and G. Y. Yurkov,
Russ. Chem. Rev., 2005, 74, 489.
2 M. A. Willard, L. K. Kurihara, E. E. Carpenter, S. Calvin and
V. G. Harris, Int. Mater. Rev., 2004, 49, 125.
3 X. Batlle and A. Labarta, J. Phys. D: Appl. Phys., 2002, 35, R15.
4 P. Tartaj, M. P. Morales, S. Veintemillas-Verdager, T. GonzálezCarreño and C. J. Serna, J. Phys. D: Appl. Phys., 2003, 36, R182.
5 U. Häfeli, W. Schütt, J. Teller and M. Zborowski, Scientific and
clinical applications of magnetic carriers, Kluwer Academic Press/
Plenum Publishers, Amsterdam, 1997.
6 G. M. Whitesides, R. J. Kazlauskas and L. Josephson, Trends
Biotechnol., 1983, 1, 144.
7 M. Shinkai, J. Biosci. Bioeng., 2002, 94, 606.
8 M. Pardavi-Horvath, J. Magn. Magn. Mater., 2000, 215–216, 171.
9 J. L. Simonds, Phys. Today, 1995, 48, 26.
10 D. E. Speliotis, J. Appl. Phys., 1967, 38, 1207.
11 K. Raj and R. Moskowitz, J. Magn. Magn. Mater., 1990, 85, 233.
12 S. B. Darling and S. D. Bader, J. Mater. Chem., 2006, 15, 4189.
13 A. E. Berkowitz, W. J. Schuele and P. J. Flanders, J. Appl. Phys.,
1968, 39, 1261.
14 Y. Imaoka, S. Umeki, Y. Kubota and Y. Tokuoka, IEEE Trans.
Magn., 1978, 14, 649.
15 J. E. Knowles, IEEE Trans. Magn., 1981, 17, 3008.
16 A. Eiling, IEEE Trans. Magn., 1987, 23, 16.
17 S. Chakrabarti, S. K. Mandal and S. Chaudhuri, Nanotechnology,
2005, 16, 506.
18 B. L. Frankamp, A. K. Boal, M. T. Tuominen and V. M. Rotello,
J. Am. Chem. Soc., 2005, 127, 9731.
19 C. Frandsen, C. W. Ostenfeld, M. Xu, C. S. Jacobsen, L. Keller,
K. Lefmann and S. Mørup, Phys. Rev. B, 2004, 70, 134416.
20 G. Salazar-Alvarez, Synthesis, Characterisation and Applications
of Iron Oxide Nanoparticles, PhD Thesis, Royal Institute of
Technology, Stockholm Sweden, 2004.
21 L. Lutterotti and S. Gialanella, Acta Mater., 1997, 46, 101.
22 R. M. Cornell and U. Schwertmann, The Iron Oxides, 2nd edn,
Wiley-VCH,Weinheim, 2003.
23 A. Uheida, G. Salazar-Alvarez, E. Björkman, Y. Zhang and
M. Muhammed, J. Colloid Interface Sci., 2006, 298, 501.
24 C. de Julián Fernández, Phys. Rev. B, 2005, 72, 054438.
25 C. P. Bean and J. D. Livingston, J. Appl. Phys., 1959, 30, 120S.
J. Mater. Chem., 2007, 17, 322–328 | 327
26 (a) E. Lima, A. L. Brandl, A. D. Arelaro and G. F. Goya, J. Appl.
Phys., 2006, 99, 083908; (b) J. Vejpravová, V. Sechovský, J. Plocek,
D. Nižňanský, A. Hutlová and J. L. Rehspringer, J. Appl. Phys.,
2005, 97, 124308.
27 M. P. Sharrock, P. J. Picone and A. H. Morrish, IEEE Trans.
Magn., 1983, 19, 1466.
28 J. P. Jolivet and E. Tronc, J. Colloid Interface Sci., 1988, 125, 688.
29 P. I. Slick, in Ferromagnetic Materials, ed. E. P. Wohlfarth, Noth
Holland, Amsterdam, 1980, vol. 2.
30 G. A. Sawatzky, F. van der Woude and A. H. Morrish, Phys. Rev.,
1969, 187, 447.
31 V. Šepelák, D. Baabe, D. Mienert, D. Schultze, F. Krumeich, F. J.
Litterst and K. D. Becker, J. Magn. Magn. Mater., 2003, 257, 377.
32 E. Tronc, J. P. Jolivet, J. Lefevbre and R. Massart, J. Chem. Soc.,
Faraday Trans. 1, 1984, 80, 2619.
33 H. Tamura, A. Tanaka, K. Mita and R. Furuichi, J. Colloid
Interface Sci., 1999, 209, 225.
34 H. Tamura, N. Katayama and R. Furuichi, J. Colloid Interface
Sci., 1997, 195, 192.
35 T. Fujii, F. M. F. de Groot, G. A. Sawatzky, F. C. Voogt,
T. Hibma and K. Okada, Phys. Rev. B, 1999, 59, 3195.
328 | J. Mater. Chem., 2007, 17, 322–328
36 S. A. Chambers, R. F. C. Farrow, S. Maat, M. F. Toney, L. Folks,
J. G. Catalano, T. P. Trainor and G. E. Brown, J. Magn. Magn.
Mater., 2002, 246, 124.
37 In this temperature range the dehydration of cobaltous hydroxide
species to render cobalt oxide takes place according to the reaction
3 Co(OH)2 + KO2 = Co3O4 + 3H2O. See Z. P. Xu and H. C. Zeng,
J. Mater. Chem., 1998, 8, 2499.
38 M. Kishimoto, S. Kitaoka, H. Andoh, M. Amemiya and
F. Hayama, IEEE Trans. Magn., 1981, 17, 3029.
39 A. E. Berkowitz, F. E. Parker, E. L. Hall and G. Podolsky, IEEE
Trans. Magn., 1988, 24, 2871.
40 C. Frandsen and S. Mørup, J. Magn. Magn. Mater., 2003, 266, 36.
41 J. Nogués, V. Skumryev, J. Sort, S. Stoyanov and D. Givord, Phys.
Rev. Lett., 2006, 97, 157203.
42 A. Aharoni, J. Appl. Phys., 1988, 63, 4605.
43 E. F. Kneller and R. Hawig, IEEE Trans. Magn., 1991, 27,
3588.
44 J. Nogués, J. Sort, V. Langlais, V. Skumryev, S. Suriñach,
J. S. Muñoz and M. D. Baró, Phys. Rep., 2005, 422, 65.
45 F. Watari, P. Delavignette, J. Van Landuty and S. Amelinckx,
J. Solid State Chem., 1983, 48, 49.
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