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Magnetic properties of novel superparamagnetic iron oxide nanoclusters and their peculiarity under annealing treatment
Accepted Manuscript Title: Magnetic properties of novel superparamagnetic iron oxide nanoclusters and their peculiarity under annealing treatment Author: Marin Tadic Slavko Kralj Marko Jagodic Darko Hanzel Darko Makovec PII: DOI: Reference:
S0169-4332(14)02181-3 http://dx.doi.org/doi:10.1016/j.apsusc.2014.09.181 APSUSC 28837
To appear in:
APSUSC
Received date: Revised date: Accepted date:
14-8-2014 25-9-2014 27-9-2014
Please cite this article as: M. Tadic, S. Kralj, M. Jagodic, D. Hanzel, D. Makovec, Magnetic properties of novel superparamagnetic iron oxide nanoclusters and their peculiarity under annealing treatment, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.09.181 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Magnetic properties of novel superparamagnetic iron oxide nanoclusters and their peculiarity under annealing treatment
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Marin Tadica,*, Slavko Kraljb,c, Marko Jagodicd, Darko Hanzele, Darko Makovecb Condensed Matter Physics Laboratory, Vinca Institute of Nuclear Sciences, University of
Department for Materials Synthesis, Jožef Stefan Institute, Ljubljana SI-1000, Slovenia
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b
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Belgrade, POB 522, 11001 Belgrade, Serbia
c
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Nanos Scientificae d.o.o. (Nanos Sci.), Teslova 30, Ljubljana, Slovenia
Institute of Mathematics, Physics and Mechanics, 1000 Ljubljana, Slovenia e
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Abstract
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Jozef Stefan Institute, Jamova 39, Ljubljana, Slovenia
The aim of this work is to present the magnetic properties of novel superparamagnetic
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iNANOvativeTM silicaTM nanoclusters. A TEM analysis showed that these nanoclusters, approximately 80 nm in size, contained an assembly of maghemite nanoparticles in the core and an amorphous silica shell. The maghemite nanoparticles in the core were approximately 10 nm in size, whereas the uniform silica shell was approximately 15 nm thick. The number of magnetic nanoparticles that were densely packed in the core of the single nanocluster was estimated to be approximately 67, resulting in a high magnetic moment for the single nanocluster of mnc~1.2·106 µB. This magnetic property of the nanocluster is advantageous for its easy manipulation using an external magnetic field, for example, in biomedical applications, such as drug delivery, or for magnetic separation in biotechnology. The magnetic properties of the iNANOvativeTM silicaTM 1 Page 1 of 40
nanoclusters were systematically studied, with a special focus on the influence of the magnetic interactions between the nanoparticles in the core. For comparison, the nanoclusters were annealed for 3 h at 300°C in air. The annealing had no influence on the nanoparticles’ size and
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phase; however, it had a unique effect on the magnetic properties, i.e., a decrease of the blocking temperature and a weakening of the inter-particle interactions. We believe that this surprising
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observation is related to the thermal decomposition of the organic surfactant on the surfaces of
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the nanoparticles’ at the high annealing temperatures, which resulted in the formation of amorphous carbon inside the nanocluster.
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Keywords: iron oxide, maghemite, surface effects, magnetic properties, superparamagnetism
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(SPION), self-assembly nanoparticles, clusters, AC susceptibility, inter-particle interactions.
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1. Introduction
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Research on nanosized materials with their low-dimensional physical and chemical properties has focused considerable attention on 3d oxides in recent years [1-23]. Iron oxides show
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interesting properties, both for fundamental investigations and practical applications [24-60]. The magnetic properties of these materials were found to depend on their size, shape and microstructure [61-65]. In particular, iron oxides are an attractive and heterogeneous class of materials covering a wide range of magnetic properties, from antiferromagnetic, spin-glass, commensurate, canted and weak ferromagnetic to ferromagnetic [2,29,64-67]. As the size of the particles decreases, the magnetic properties of the iron oxides exhibit interesting properties, which differ from those of conventional bulk materials, because of nanoscale confinement and surface effects. If the particles are small enough, the direction of the magnetic moment in a single-domain nanoparticle fluctuates due to thermal agitation, leading to superparamagnetic 2 Page 2 of 40
behavior above the blocking temperature TB, and to the spatial freezing of these moments below TB. Moreover, for closely spaced iron oxide particles, the magnetostatic interactions influence the magnetic behavior and slow down the relaxation time τ of the magnetic moments of neighboring
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magnetic nanoparticles. Accordingly, the agglomerates of the nanoparticles strongly affect the magnetic properties, producing an increase in the blocking temperature TB and so reduce the
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intrinsic properties of the superparamagnetic nanoparticle, as has been observed, both
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experimentally and theoretically [68-70]. Therefore, the magnetic properties of closely spaced magnetic nanoparticles are influenced by inter-particle interactions and usually differ
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significantly from those of isolated nanocrystals and their bulk counterparts, thus opening up the opportunity for novel properties and practical applications. Although there have been a large
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number of studies on iron oxides in their nanoparticle form, less attention has been paid to the
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synthesis and studies of the magnetic properties of nanoparticle cluster structures. In particular,
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the preparation of different cluster nanostructures has recently been reported, leading to novel and improved magnetic properties for these materials [28-37]. Moreover, these cluster
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nanoparticles show enhanced magnetic moments, compared to a single magnetic nanoparticle. This leads to easier manipulation with an external magnet, which is a desirable feature for various practical applications. Therefore, it is challenge for various applications, especially biomedical ones, to develop magnetic nanostructure systems with large magnetic moments for the particles and, at the same time, superparamagnetic properties. In addition, the collective properties of nanoparticle clusters are not completely understood yet, and are currently the subject of intensive research [71-73]. So, it is a challenge to develop an efficient method for the synthesis of nanoparticles in a cluster structure with high magnetic moments and superparamagnetic properties. 3 Page 3 of 40
Among the iron oxides, maghemite (γ-Fe2O3) is one of the most important and widely utilized magnetic materials in various forms, such as bare and coated nanoparticles, nanowires, nanorods, nanotubes, and nanoparticles embedded in a matrix [62,74-76]. This has been under
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extensive investigation in order to understand the influence of size, shape, anisotropy and surface effects, as well as inter-particle interactions, on its magnetic properties [2,64,65,77,78].
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Maghemite is a ferrimagnetic material that is widely used for various applications due to its
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biocompatibility, non-toxicity and room-temperature superparamagnetism when in the form of nanoparticles with a size below approximately 20 nm [1,2]. Nanocrystalline maghemite has been using
different
methods,
including
co-precipitation,
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prepared
sol–gel,
hydrothermal,
mechanochemical, thermal decomposition and other preparation techniques, for different
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applications and properties [1,2]. In many cases the nanoparticles are aggregated, creating a
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serious disadvantage with respect to their application. To overcome this problem, nanoparticles
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are coated using various coatings: silica, carbon, polymers and different organic molecules. Silica has been widely used in the coating of magnetic nanoparticles, due to its nontoxic nature, optical
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transparency, chemical inertness, thermal stability, high biocompatibility, adjustable pore diameter, very high specific surface area with abundant Si-OH bonds on the surface and nonmagnetic properties. Currently, the research on maghemite has been directed towards improving its properties for different practical applications. Accordingly, the increased demands from different modern applications have led to novel nanocrystal designs and the development of advanced, ferromagnetic iron-oxide-based materials [30-58]. Tan et al. reported on PEOlated Fe3O4@SiO2 nanoparticles with a size of about 30 nm and magnetization MS=15.21 emu/g [36]. They showed, using a Langevin fit, that the magnetic moment of the particles depends on the particle size and increases with an increasing particle size. Benelmekki et al. obtained spheres 4 Page 4 of 40
with a diameter of 400 nm where the maghemite nanoparticles are distributed in a silica matrix [38]. The magnetic saturation of these spheres is about 10 emu/g. Zhao et al. fabricated magnetic core/mesoporous silica shell nanospheres with a particle diameter of about 270 nm [41]. They
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obtained a saturation magnetization value of MS=27.3 emu/g and a large coercivity at room temperature.
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The aim of this work was to present the magnetic properties of novel iNANOvativeTM
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silicaTM nanoclusters with a high magnetic moment and superparamagnetic properties, which are desirable for biomedical applications. The magnetic properties of the iNANOvativeTM silicaTM
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and annealed samples were systematically studied using AC and DC magnetic measurements. An analysis of the results shows a high magnetic moment for the superparamagnetic nanocluster,
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which is estimated to be mp=1.2·106 μB. We also found that the annealing treatment used for the
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nanoclusters produces interesting changes in the magnetic properties: lowering the TB, weakening
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2. Experimental
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the inter-particle interactions and providing an increase in the saturation magnetization MS.
The superparamagnetic iNANOvative™ silica nanoclusters (sample S0) were kindly provided by a Nanos Scientificae d.o.o. (Nanos Sci). These nanoclusters were synthesized by the selfassembly of primary maghemite nanoparticles followed by coating of the nanoclusters with a layer of silica. First, single maghemite (γ-Fe2O3) nanoparticles were synthesized using precipitation from an aqueous solution. In brief, a solution of Fe2+ (0.027 mol L-1) and Fe3+ (0.023 mol L-1) ions was precipitated with concentrated ammonia (25 %) in two steps. In the first step, the pH value of the solution was raised to pH=3 and maintained at a constant value for 30 5 Page 5 of 40
min to precipitate the iron hydroxides. In the second step, the pH value was further increased to pH=11.6 to oxidize the iron (II) hydroxide with oxygen from the air, forming a spinel product. After an ageing time of 30 min the synthesized nanoparticles were thoroughly washed with a
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diluted ammonia solution at pH = 10.5. The procedure was described in our previous publication [40]. Nanos Sci. roughly controls the size of the nanoclusters by the use of a proper combination
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of the modified polyacrylic acid (PAA) and polyvinylpyrrolidone (PVP). PAA is used as a
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capping agent/surfactant for the maghemite nanoparticles, whereas the PVP is used as a promoter for further silica-shell syntheses. The precise control of the nanocluster size is finally achieved
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using a high-gradient magnetic separator (HGMS). The synthesis procedures were developed exclusively by Nanos Sci. and they are suitably protected. To study the influence of nanoparticle
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coating on the magnetic properties of the nanoclusters S0, the as-received sample was annealed in
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air at 300 ºC for 3 h (sample S300).
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The crystalline structure and morphology were characterized by X-ray diffraction (XRD, Bruker D8 Advance, Cu Kα radiation, λ=1.5406 Ǻ) and transmission electron microscopy (TEM,
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JEM-2100F). Afterwards, the samples were studied using Mössbauer spectroscopy. The spectra were collected at room temperature with a constant-acceleration spectrometer using a 57Co source in rhodium. The velocity scale was calibrated with a 25-mm-thick metallic iron foil. The magnetic measurements were performed on a commercial Quantum Design MPMS-XL-5 SQUID-based magnetometer over a wide range of temperatures (2–300 K) and applied magnetic fields (up to 5 T). The same instrument was used for AC magnetization measurements carried out in the 1 Hz≤ν≤1000 Hz frequency range in a temperature region encompassing the blocking temperature values. 3. Results and Discussion 6 Page 6 of 40
The formation of the magnetic nanoclusters is schematically illustrated in Fig.1. The nanoclusters were synthesized by the self-assembly of the superparamagnetic iron oxide nanoparticles. Afterwards, the nanoclusters were coated with a silica shell. The structure and the
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phase composition of the samples were revealed by XRPD (Fig. 2). Fig. 2(a) and (b) show the XRPD diffraction patterns of the as-received nanoclusters (S0) and the annealed (S300)
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nanoclusters, respectively. The XRPD patterns in Fig. 2 reveal the nanocrystal nature of the
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particles in both samples. It is clear that the position and relative intensity of all the diffraction peaks (S0 and S300 samples) correspond to the iron oxide spinel structure, and that no other
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reflections were detected, except for a broad peak at approximately 23º, originating from the amorphous material in the samples. The crystal sizes of the nanoparticles were calculated from
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the XRPD diffraction-peak broadening using the Scherrer formula. The determined average
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crystallite sizes were d(S0)=10.3 nm and d(S300)=10.6 nm for the samples S0 and S300,
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respectively. These values show that the nanoparticle crystal size was not affected to any significant extent by the annealing treatment at 300 ºC. The samples were also studied using
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Mössbauer spectroscopy. The Mössbauer spectra were recorded at room temperature with a constant-acceleration spectrometer, with and without an applied external magnetic field (Fig. 3). The velocity and isomer shifts were calibrated against a metallic iron foil. The spectra were fitted using the Recoil program. The spectra recorded without any applied magnetic field are typical of superparamagnetic behavior for S0 and S300 and show a very broad line, with no discrete lines that would indicate magnetic ordering. The spectra recorded in the external magnetic field of 0.27 T applied perpendicular to the gamma rays show magnetic ordering. The relative intensities of the resonance lines of 3:4:1:1:4:3 confirm this ordering. The determined fitting parameters of the hyperfine magnetic fields, Hhf=467(26) kOe (sample S0) and 473.3(51) kOe (sample S300), the 7 Page 7 of 40
isomer shifts δ=0.338(18) mm/s (sample S0) and 0.341(32) mm/s (sample S300), and the almostzero quadrupole splitting are unambiguously compatible with nanosized maghemite [79]. Moreover, the Mössbauer hyperfine parameters of the S0 and S300 samples are almost the same,
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indicating nearly negligible differences between the local environments of the Fe atoms.
The sizes and morphologies of the samples were investigated using a TEM analysis (Fig. 4).
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It can be seen that in both, the as-received and the annealed nanoclusters, there is a spherical
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shape and a core-shell nanostructure consisting of the nanoparticle cluster core and the amorphous silica shell. A TEM observation further revealed that the diameters of the S0 (Fig.
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4(a) and (b)) and S300 (Fig. 4(d) and (e)) nanoclusters are approximately 80 nm and the thickness of the amorphous silica shell is approximately 15 nm. In TEM images taken at a higher
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magnification (Fig. 4(c) and (f)) it is clear that the nanocluster’ core is composed of small,
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maghemite nanoparticles with diameters of about 10 nm, which is in good agreement with the
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crystal size obtained from the XRPD. The TEM observations also revealed that after the annealing of the S0 nanoclusters in air at 300 ºC the structure and morphology of the S300
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nanoclusters were very similar to the S0, i.e., the sizes of the nanoclusters and the magnetic nanoparticles, as well as that of the silica shell thickness, were not changed. We can expect that the superparamagnetic nanoparticles in the as-received nanocluster core are not in close contact, because of the surfactants at their surfaces. In the annealed nanoclusters S300 we suggest that the thermal decomposition of the surfactants during the annealing treatment and in the absence of oxygen inside the nanoclusters led to the formation of an amorphous carbon ”coat” around the maghemite nanoparticles. The resulting amorphous carbon can act as a barrier to suppress the aggregation and crystal growth of the magnetic nanoparticles, and can also act as a shield that
8 Page 8 of 40
prevents any strong inter-particle interactions. The presence of the carbon in the S300 sample was observed by energy-dispersive X-ray spectroscopy (EDS) analysis in the TEM. The magnetic properties of the samples were characterized by measurements of the
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temperature dependence of the AC and DC susceptibility and the magnetic field dependence of the magnetization. These properties of the S0 and S300 samples were investigated using a
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SQUID magnetometer. In order to investigate the dynamic properties and the inter-particle
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interactions between the magnetic nanoparticles, AC susceptibility measurements were made at four different frequencies, i.e., 1, 10, 100 and 1000 Hz (Figs. 5(a) sample S0, and 5(b) sample
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S300). First, we checked for the existence of inter-particle interactions in the samples using the Néel-Brown theory of superparamagnetism [80,81]. The magnetic moments of non-interacting,
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single-domain magnetic nanoparticles with a uniaxial anisotropy fluctuate between the two
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directions of the easy axis of magnetization with a relaxation time τ that obeys the Arrhenius law: (1)
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τ=τ0exp(Ea/kBTB) ,
where Ea is the energy barrier to magnetization reversal in a single particles, kB is Boltzman’s
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constant, and τ0 is the attempt frequency. The attempt frequency τ0 should be within the 10-9 to 10-13 range, as reported in the literature [4,17,22,69]. The values of τ0<10-24 s determined from the fits of the Arrhenius law to our experimental data are much lower than the above-quoted limit. This reveals the existence of inter-particle interactions in the investigated samples. Second, we determined the presence and strength of the inter-particle interactions in the samples from the frequency dependence of TB by using the empirical parameter C1=ΔTB/(TBΔlogν), where TB denotes the average value of the blocking temperature in the range of applied frequencies ν, whereas ΔTB denotes the difference between the maximum and the minimum value of TB (Fig. 5(a) and (b)). In this way we obtained C1(S0)=0.055 and C1(S300)=0.069 for the samples S0 and 9 Page 9 of 40
S300, respectively. The value of the C1 parameter for a non-interacting magnetic nanoparticle system lies in the range ~0.1–0.13 and decreases with the increasing strength of the inter-particle interactions [82,83]. These results show the existence of inter-particle interactions in both
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samples, and that the annealed sample S300 has weaker inter-particle interactions. The obtained values for the attempt frequency τ0 in the Arrhenius law and the empirical parameter C1 imply
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dependence of TB should be fitted to the Vogel-Fulcher law [84]:
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that the interactions among the nanoparticles are not negligible and that the frequency
(2)
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τ=τ0exp[Ea/kB(TB-T0)] ,
where τ=1/ν (ν=1, 10, 100 and 1000 Hz) and the parameter T0 is a measure of the inter-particle
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interaction strength. The fits of Eq. (2) to the experimental data are given in Fig. 5(c) for sample S0 and 5(d) for sample S300 (solid lines). The obtained values for the fit parameters are:
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τ0(S0)=4.48·10-11 s, Ea(S0)/kB=1264 K, T0(S0)=92.6 K for sample S0 and τ0(S300)=3.08·10-10 s,
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Ea(S300)/kB=1149 K, T0(S300)=70.66 K for sample S300. The obtained magnetic anisotropy
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parameters Ea/kB were used to estimate the effective anisotropy constant Keff, using the relation KeffV=Ea, where V denotes the volume of the particle. For particles with a diameter d(S0)=10.4 nm and d(S300)=10.6 nm, this relation gives Keff(S0)=2.96·105 erg/cm3 and Keff(S300)=2.54·105 erg/cm3, whereas the value for the magnetocrystalline anisotropy constant for the bulk maghemite is Kv=4.7·104 erg/cm3. The substantially higher values of Keff in the investigated samples suggest that in the case of our nanoparticle systems, additional contributions to the anisotropy constant exist, e.g., the particle surface anisotropy, magnetoelastic, and shape anisotropy as well as that the inter-particle interactions play an important role in these systems [17,85]. Finally, we calculated the value of another empirical parameter, C2=(TB-T0)/TB, using the T0 values obtained 10 Page 10 of 40
from the Vogel-Fulcher fit. The obtained values are C2(S0)=0.43 and C2(S300)=0.52, whereas the expected value of the C2 parameter for non-interacting systems is about one [82,83]. It was also experimentally determined that the value of C2 decreases with increasing interactions among the
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particles. These calculated values confirm the existence of interactions in both samples, and of weaker inter-particle interactions in sample S300.
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The temperature dependences of the magnetization M(T) for both samples were recorded in
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the zero-field-cooled (ZFC) and field-cooled (FC) regimes (Figs. 6(a) for sample S0 and 6(b) for sample S300). The magnetizations were measured in a weak applied magnetic field (H=100 Oe)
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as a function of increasing temperature after the sample was cooled, with and without an applied magnetic field. The ZFC curves exhibit maxima at around TB(S0)=110 K and TB(S300)=88 K,
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corresponding to the blocking temperatures TB of the samples S0 and S300, respectively. The
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ZFC and FC curves split significantly below TB: the ZFC magnetization decreases sharply while
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the FC magnetization rises steadily (Figs. 6(a) and (b)). On one hand, the continuous rise of the FC curves shows that strong inter-particle interactions do not exist in the samples due to the
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coating of the maghemite nanoparticles with the surfactant (S0) and the amorphous carbon (S300). On the other hand, a plateau and/or peak in the FC curves was observed in the case of the systems with strong inter-particle interactions [86]. The higher value of Tmax in the ZFC curve of the sample S0 can be explained as being a consequence of the stronger dipolar magnetic interactions among the maghemite nanoparticles, as shown by the AC susceptibility studies. Such interactions slow the magnetic relaxation and increase Tmax, as reported in the literature [87]. The magnetic field dependences of the magnetization M(H), were measured at temperatures below TB at T=5 K for both samples (S0 and S300) and are shown in Fig. 7(a) and 7(b). In Fig. 7, it is clear that the magnetizations are almost saturated in high magnetic fields for both samples, as 11 Page 11 of 40
also observed for well-crystallized ferrimagnetic iron oxide nanomaterials. The obtained hysteresis loops are symmetric around the origin (Fig. 7(a) and (b), insets), with coercivity, remanence and saturation magnetization HC(S0)=180 Oe, Mr(S0)=6.4 emu/g, and MS(S0)=29.4 emu/g
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for the sample S0 (Fig. 7(a)) and HC(S300)=138 Oe, Mr(300)=6.3 emu/g, and MS(300)=33.4 emu/g for the sample S300 (Fig. 7(b)). The results of the field-dependent magnetization measurements
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show that the S300 has a higher magnetization (Fig. 7). After the annealing treatment, the
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saturation magnetization is increased by about ≈13.6 % of the value of the sample S0 (Fig. 7). This increase in the magnetization of the annealed sample can be attributed to the degradation
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and decomposition of the surfactants and their release of gas products during the annealing treatment from the sample, i.e. to the higher content of the magnetic material in the sample S300.
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The M(H) dependencies were also recorded at T=300 K for both samples S0 (Fig. 8(a)) and S300
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( Fig. 8(b)). The absence of both coercivity and remanence suggests superparamagnetic behavior
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at this temperature. The values of the saturation magnetization of the samples MS0=24.75 emu/g and MS300=28.85 emu/g were determined as the measured values of the magnetization obtained at
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H= 3 T. The observed saturation magnetizations are related to the silica-coated nanoclusters. The organic surfactant or amorphous carbon as well as the silica shell decrease the saturation magnetization of the produced maghemite nanoclusters. The saturation magnetization of the bare maghemite nanoparticles is around MS=66 emu/g. The magnetization vs. magnetic field dependence was fitted using the modified Langevin function (Fig. 9(a)): M=MS[coth(mpH/kBT)-(kBT/mpH)]+χH ,
(3)
where χ is the high-field susceptibility. The mass of the sample (S0) was normalized to the mass of the magnetic material (Fig. 9(a)). The mass of iron oxide in the formed silica-coated nanoclusters was determined from the magnetic properties of the bare maghemite nanoparticles 12 Page 12 of 40
(MS=66 emu/g). The determined magnetic moment of the maghemite nanoparticles using the Langevin fit was mp=18005 µB. An estimation of the mean particle size can be made by using the expression mp=πd3MS/6 for the magnetic moment of a particle, which assumes that the
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nanoparticle has a spherical shape, and where d denotes the diameter of that sphere. From the fitting parameters and the above relation for the magnetic moment, the mean particle diameter
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was determined to be d(S0)=10.2 nm. In addition, this value is in good agreement with the value
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obtained during the XRPD and TEM studies. Assuming the magnetic nanoclusters of diameter DS=80 nm, a of shell thickness 15 nm and maghemite nanoparticles of 10 nm size, we estimated
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the number of nanoparticles per magnetic nanocluster to be N=67. From the above-listed results we obtained the following magnetic moment for the single magnetic nanocluster mnc=1.2·106 µB.
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The unchanged magnetic structure of the magnetic nanoparticles after the annealing treatment is
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also evidenced in the data obtained from the magnetic measurements. The overlap of the M/MS
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vs. H curves at constant temperature T=300 K (Fig. 9(b)) for both samples indicates that the magnetization behavior is preserved, independently of the coating and the annealing treatment.
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We can conclude that both samples are composed of almost identical maghemite nanoparticles (good agreement with XRPD and TEM studies) and that those inter-particle interactions affect the magnetic properties of the samples. This is also supported by the results of the AC magnetic susceptibility measurements, where the temperature in which χ' peaks at different frequencies following the Vogel-Fulcher model, a feature commonly found in systems with magnetic dipolar interactions. Finally, we can conclude that direct contact between the magnetic nanoparticles in the samples was avoided by coating, where a layer of surfactant (sample S0) or amorphous carbon (sample S300) acts so as to change the magnetic properties by altering the strength of the magnetic dipolar interaction. The decrease of the interactions in the annealed sample S300 13 Page 13 of 40
indicates that the maghemite nanoparticles are better isolated and shield each other with the
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amorphous carbon.
4. Conclusions
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Our novel core-shell nanoclusters were superparamagnetic and had high magnetic moments at the same time, which is desirable for practical applications. The direct contacts between the
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maghemite nanoparticles inside the nanoclusters were made impossible by surfactants at their
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surfaces (sample S0) or by amorphous carbon (sample S300), so as to suppress the strong interparticle interactions, while maintaining the superparamagnetic behavior at room temperature. It
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was also shown that for the annealed sample (S300) the average crystallite and particle sizes of the maghemite nanoparticles estimated from the XRPD and TEM do not show an appreciable
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change in comparison to the as-received nanoclusters. Moreover, the TEM analysis confirmed the
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core-shell nanostructure of the nanoclusters with a diameter of 80 nm, composed of a cluster of
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maghemite nanoparticles in the core and an amorphous silica shell with a thickness of 15 nm. The nanoparticles in the core had a size of 10 nm, for both samples. The number of maghemite nanoparticles in the nanocluster’ core was estimated to be N=67. This internal microstructure produced the high magnetic moment of the superparamagnetic nanocluster, which is calculated to be mnc=1.2·106 µB. It is expected that on the basis of these values the magnetic nanoclusters would be an excellent material, for biomedical applications to magnetic separation and drug delivery. Both the DC magnetization and AC susceptibility measurements indicate the nanoscale magnetic properties of the as-received S0 and annealed S300 samples with blocking temperatures of TB(S0)=110 K and TB(S300)=88 K, respectively. The AC susceptibility study shows that the Vogel-Fulcher model for single-domain interacting magnetic nanoparticles is applicable, and it 14 Page 14 of 40
was used to determine the empirical parameters C2(S0)=0.43 and C2(S300)=0.52, which show the presence of inter-particle interactions between the magnetic nanoparticles and somewhat weaker interactions in the S300. The significantly different and unexpected magnetic properties of the
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annealed sample S300 (lower TB and weaker inter-particle interactions) in comparison with S0 can be explained by the thermal decomposition of the SPION’s surfactants in an atmosphere
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without oxygen inside the nanoclusters and the production of amorphous carbon, which coated
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the maghemite nanoparticles. The analysis of the magnetization results indicates that amorphous carbon shields the magnetic nanoparticles better. The above results suggest that the observed
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changes in the magnetic properties of the annealed nanoclusters are due to the different strengths of the inter-particle interactions in the samples, and not due to the intrinsic properties of the
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magnetic nanoparticles. Therefore, our results show that the inter-particle interactions between
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the magnetic nanoparticles may decrease when annealing the nanoclusters. These findings are
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important for the design of magnetic materials, and such well-defined nanoclusters may be used for the subsequent engineering of their surfaces and functionalization for biomedical applications.
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This study may also be useful as a motivation for synthesizing other metal oxide cluster structures with novel and useful properties for both fundamental research and various practical applications.
Acknowledgements
The support by the Ministry of Higher Education, Science and Technology of the Republic of Slovenia within the National Research Program is acknowledged. The authors acknowledge the use of equipment in the Center of Excellence on Nanoscience and Nanotechnology (Nanocenter). The studied materials were provided by Nanos Scientificae d.o.o. (www.nanos-sci.com), Teslova ulica 30, 1000 Ljubljana, Slovenia. M.T. acknowledges Serbian Ministry of Science for financial 15 Page 15 of 40
support (Grant no. III 45015) and Professor Veljko Dmitrasinovic (Institute for Physics,
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d
M
an
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cr
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Belgrade) for his comments.
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Figure captions
Figure 1. Formation mechanism of core-shell nanocluster: as-synthesized maghemite
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nanoparticles (a and d), nanoparticle cluster (b and e) and coated by silica nanocluster (c and f). Figure 2. The XRPD patterns of the samples S0 (a) and S300 (b). The Miller indices (hkl) of the
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diffraction peaks are shown in the diffraction patterns.
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Figure 3. Mössbauer spectra of samples S0 and S300 recorded at room temperature. Figure 4. The TEM and HRTEM images of the maghemite (γ-Fe2O3) nanoparticle clusters
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coated with a silica shell for S0 (a, b, c) and S300 (d, e, f) samples.
Figure 5. Temperature dependence of the in phase χ' measured ac-susceptibility at different
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frequencies from 1 Hz to 1000 Hz corresponding to the S0 (a) and S300 (b) samples. Relaxation
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fit with the Vogel–Fulcher law.
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time τ as a function of TB-1 for S0 (c) and S300 (d) samples. The solid lines correspond to the best
Figure 6. The ZFC and FC curves performed at 100 Oe for samples S0 (a) and S300 (b).
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Figure 7. Magnetization vs. magnetic field measured at 5 K for samples S0 (a) and S300 (b). The insets show the zoom view of M-H loops. Figure 8. Magnetization vs. applied magnetic field measured at 300 K for samples S0 (a) and S300 (b). Inset: field-dependent magnetization curve of bare maghemite nanoparticles. Figure 9. (a) Field-dependent magnetization curve of S0 at 300 K. The solid line is fit to a Langevin function. (b) Plot of M/MS vs. T using the data at T=300 K for samples S0 and S300.
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Research Highlights >Magnetic properties of γ‐Fe2O3 nanoclusters and their thermal decomposition. >SPION clusters show superparamagnetism and high magnetic moments mnc~1.2·106 µB. >The TEM shows maghemite
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S0169-4332(14)02181-3 http://dx.doi.org/doi:10.1016/j.apsusc.2014.09.181 APSUSC 28837
To appear in:
APSUSC
Received date: Revised date: Accepted date:
14-8-2014 25-9-2014 27-9-2014
Please cite this article as: M. Tadic, S. Kralj, M. Jagodic, D. Hanzel, D. Makovec, Magnetic properties of novel superparamagnetic iron oxide nanoclusters and their peculiarity under annealing treatment, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.09.181 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Magnetic properties of novel superparamagnetic iron oxide nanoclusters and their peculiarity under annealing treatment
a
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Marin Tadica,*, Slavko Kraljb,c, Marko Jagodicd, Darko Hanzele, Darko Makovecb Condensed Matter Physics Laboratory, Vinca Institute of Nuclear Sciences, University of
Department for Materials Synthesis, Jožef Stefan Institute, Ljubljana SI-1000, Slovenia
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b
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Belgrade, POB 522, 11001 Belgrade, Serbia
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Nanos Scientificae d.o.o. (Nanos Sci.), Teslova 30, Ljubljana, Slovenia
Institute of Mathematics, Physics and Mechanics, 1000 Ljubljana, Slovenia e
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Abstract
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Jozef Stefan Institute, Jamova 39, Ljubljana, Slovenia
The aim of this work is to present the magnetic properties of novel superparamagnetic
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iNANOvativeTM silicaTM nanoclusters. A TEM analysis showed that these nanoclusters, approximately 80 nm in size, contained an assembly of maghemite nanoparticles in the core and an amorphous silica shell. The maghemite nanoparticles in the core were approximately 10 nm in size, whereas the uniform silica shell was approximately 15 nm thick. The number of magnetic nanoparticles that were densely packed in the core of the single nanocluster was estimated to be approximately 67, resulting in a high magnetic moment for the single nanocluster of mnc~1.2·106 µB. This magnetic property of the nanocluster is advantageous for its easy manipulation using an external magnetic field, for example, in biomedical applications, such as drug delivery, or for magnetic separation in biotechnology. The magnetic properties of the iNANOvativeTM silicaTM 1 Page 1 of 40
nanoclusters were systematically studied, with a special focus on the influence of the magnetic interactions between the nanoparticles in the core. For comparison, the nanoclusters were annealed for 3 h at 300°C in air. The annealing had no influence on the nanoparticles’ size and
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phase; however, it had a unique effect on the magnetic properties, i.e., a decrease of the blocking temperature and a weakening of the inter-particle interactions. We believe that this surprising
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observation is related to the thermal decomposition of the organic surfactant on the surfaces of
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the nanoparticles’ at the high annealing temperatures, which resulted in the formation of amorphous carbon inside the nanocluster.
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Keywords: iron oxide, maghemite, surface effects, magnetic properties, superparamagnetism
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(SPION), self-assembly nanoparticles, clusters, AC susceptibility, inter-particle interactions.
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1. Introduction
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Research on nanosized materials with their low-dimensional physical and chemical properties has focused considerable attention on 3d oxides in recent years [1-23]. Iron oxides show
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interesting properties, both for fundamental investigations and practical applications [24-60]. The magnetic properties of these materials were found to depend on their size, shape and microstructure [61-65]. In particular, iron oxides are an attractive and heterogeneous class of materials covering a wide range of magnetic properties, from antiferromagnetic, spin-glass, commensurate, canted and weak ferromagnetic to ferromagnetic [2,29,64-67]. As the size of the particles decreases, the magnetic properties of the iron oxides exhibit interesting properties, which differ from those of conventional bulk materials, because of nanoscale confinement and surface effects. If the particles are small enough, the direction of the magnetic moment in a single-domain nanoparticle fluctuates due to thermal agitation, leading to superparamagnetic 2 Page 2 of 40
behavior above the blocking temperature TB, and to the spatial freezing of these moments below TB. Moreover, for closely spaced iron oxide particles, the magnetostatic interactions influence the magnetic behavior and slow down the relaxation time τ of the magnetic moments of neighboring
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magnetic nanoparticles. Accordingly, the agglomerates of the nanoparticles strongly affect the magnetic properties, producing an increase in the blocking temperature TB and so reduce the
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intrinsic properties of the superparamagnetic nanoparticle, as has been observed, both
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experimentally and theoretically [68-70]. Therefore, the magnetic properties of closely spaced magnetic nanoparticles are influenced by inter-particle interactions and usually differ
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significantly from those of isolated nanocrystals and their bulk counterparts, thus opening up the opportunity for novel properties and practical applications. Although there have been a large
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number of studies on iron oxides in their nanoparticle form, less attention has been paid to the
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synthesis and studies of the magnetic properties of nanoparticle cluster structures. In particular,
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the preparation of different cluster nanostructures has recently been reported, leading to novel and improved magnetic properties for these materials [28-37]. Moreover, these cluster
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nanoparticles show enhanced magnetic moments, compared to a single magnetic nanoparticle. This leads to easier manipulation with an external magnet, which is a desirable feature for various practical applications. Therefore, it is challenge for various applications, especially biomedical ones, to develop magnetic nanostructure systems with large magnetic moments for the particles and, at the same time, superparamagnetic properties. In addition, the collective properties of nanoparticle clusters are not completely understood yet, and are currently the subject of intensive research [71-73]. So, it is a challenge to develop an efficient method for the synthesis of nanoparticles in a cluster structure with high magnetic moments and superparamagnetic properties. 3 Page 3 of 40
Among the iron oxides, maghemite (γ-Fe2O3) is one of the most important and widely utilized magnetic materials in various forms, such as bare and coated nanoparticles, nanowires, nanorods, nanotubes, and nanoparticles embedded in a matrix [62,74-76]. This has been under
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extensive investigation in order to understand the influence of size, shape, anisotropy and surface effects, as well as inter-particle interactions, on its magnetic properties [2,64,65,77,78].
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Maghemite is a ferrimagnetic material that is widely used for various applications due to its
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biocompatibility, non-toxicity and room-temperature superparamagnetism when in the form of nanoparticles with a size below approximately 20 nm [1,2]. Nanocrystalline maghemite has been using
different
methods,
including
co-precipitation,
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prepared
sol–gel,
hydrothermal,
mechanochemical, thermal decomposition and other preparation techniques, for different
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applications and properties [1,2]. In many cases the nanoparticles are aggregated, creating a
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serious disadvantage with respect to their application. To overcome this problem, nanoparticles
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are coated using various coatings: silica, carbon, polymers and different organic molecules. Silica has been widely used in the coating of magnetic nanoparticles, due to its nontoxic nature, optical
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transparency, chemical inertness, thermal stability, high biocompatibility, adjustable pore diameter, very high specific surface area with abundant Si-OH bonds on the surface and nonmagnetic properties. Currently, the research on maghemite has been directed towards improving its properties for different practical applications. Accordingly, the increased demands from different modern applications have led to novel nanocrystal designs and the development of advanced, ferromagnetic iron-oxide-based materials [30-58]. Tan et al. reported on PEOlated Fe3O4@SiO2 nanoparticles with a size of about 30 nm and magnetization MS=15.21 emu/g [36]. They showed, using a Langevin fit, that the magnetic moment of the particles depends on the particle size and increases with an increasing particle size. Benelmekki et al. obtained spheres 4 Page 4 of 40
with a diameter of 400 nm where the maghemite nanoparticles are distributed in a silica matrix [38]. The magnetic saturation of these spheres is about 10 emu/g. Zhao et al. fabricated magnetic core/mesoporous silica shell nanospheres with a particle diameter of about 270 nm [41]. They
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obtained a saturation magnetization value of MS=27.3 emu/g and a large coercivity at room temperature.
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The aim of this work was to present the magnetic properties of novel iNANOvativeTM
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silicaTM nanoclusters with a high magnetic moment and superparamagnetic properties, which are desirable for biomedical applications. The magnetic properties of the iNANOvativeTM silicaTM
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and annealed samples were systematically studied using AC and DC magnetic measurements. An analysis of the results shows a high magnetic moment for the superparamagnetic nanocluster,
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which is estimated to be mp=1.2·106 μB. We also found that the annealing treatment used for the
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nanoclusters produces interesting changes in the magnetic properties: lowering the TB, weakening
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2. Experimental
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the inter-particle interactions and providing an increase in the saturation magnetization MS.
The superparamagnetic iNANOvative™ silica nanoclusters (sample S0) were kindly provided by a Nanos Scientificae d.o.o. (Nanos Sci). These nanoclusters were synthesized by the selfassembly of primary maghemite nanoparticles followed by coating of the nanoclusters with a layer of silica. First, single maghemite (γ-Fe2O3) nanoparticles were synthesized using precipitation from an aqueous solution. In brief, a solution of Fe2+ (0.027 mol L-1) and Fe3+ (0.023 mol L-1) ions was precipitated with concentrated ammonia (25 %) in two steps. In the first step, the pH value of the solution was raised to pH=3 and maintained at a constant value for 30 5 Page 5 of 40
min to precipitate the iron hydroxides. In the second step, the pH value was further increased to pH=11.6 to oxidize the iron (II) hydroxide with oxygen from the air, forming a spinel product. After an ageing time of 30 min the synthesized nanoparticles were thoroughly washed with a
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diluted ammonia solution at pH = 10.5. The procedure was described in our previous publication [40]. Nanos Sci. roughly controls the size of the nanoclusters by the use of a proper combination
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of the modified polyacrylic acid (PAA) and polyvinylpyrrolidone (PVP). PAA is used as a
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capping agent/surfactant for the maghemite nanoparticles, whereas the PVP is used as a promoter for further silica-shell syntheses. The precise control of the nanocluster size is finally achieved
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using a high-gradient magnetic separator (HGMS). The synthesis procedures were developed exclusively by Nanos Sci. and they are suitably protected. To study the influence of nanoparticle
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coating on the magnetic properties of the nanoclusters S0, the as-received sample was annealed in
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air at 300 ºC for 3 h (sample S300).
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The crystalline structure and morphology were characterized by X-ray diffraction (XRD, Bruker D8 Advance, Cu Kα radiation, λ=1.5406 Ǻ) and transmission electron microscopy (TEM,
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JEM-2100F). Afterwards, the samples were studied using Mössbauer spectroscopy. The spectra were collected at room temperature with a constant-acceleration spectrometer using a 57Co source in rhodium. The velocity scale was calibrated with a 25-mm-thick metallic iron foil. The magnetic measurements were performed on a commercial Quantum Design MPMS-XL-5 SQUID-based magnetometer over a wide range of temperatures (2–300 K) and applied magnetic fields (up to 5 T). The same instrument was used for AC magnetization measurements carried out in the 1 Hz≤ν≤1000 Hz frequency range in a temperature region encompassing the blocking temperature values. 3. Results and Discussion 6 Page 6 of 40
The formation of the magnetic nanoclusters is schematically illustrated in Fig.1. The nanoclusters were synthesized by the self-assembly of the superparamagnetic iron oxide nanoparticles. Afterwards, the nanoclusters were coated with a silica shell. The structure and the
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phase composition of the samples were revealed by XRPD (Fig. 2). Fig. 2(a) and (b) show the XRPD diffraction patterns of the as-received nanoclusters (S0) and the annealed (S300)
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nanoclusters, respectively. The XRPD patterns in Fig. 2 reveal the nanocrystal nature of the
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particles in both samples. It is clear that the position and relative intensity of all the diffraction peaks (S0 and S300 samples) correspond to the iron oxide spinel structure, and that no other
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reflections were detected, except for a broad peak at approximately 23º, originating from the amorphous material in the samples. The crystal sizes of the nanoparticles were calculated from
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the XRPD diffraction-peak broadening using the Scherrer formula. The determined average
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crystallite sizes were d(S0)=10.3 nm and d(S300)=10.6 nm for the samples S0 and S300,
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respectively. These values show that the nanoparticle crystal size was not affected to any significant extent by the annealing treatment at 300 ºC. The samples were also studied using
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Mössbauer spectroscopy. The Mössbauer spectra were recorded at room temperature with a constant-acceleration spectrometer, with and without an applied external magnetic field (Fig. 3). The velocity and isomer shifts were calibrated against a metallic iron foil. The spectra were fitted using the Recoil program. The spectra recorded without any applied magnetic field are typical of superparamagnetic behavior for S0 and S300 and show a very broad line, with no discrete lines that would indicate magnetic ordering. The spectra recorded in the external magnetic field of 0.27 T applied perpendicular to the gamma rays show magnetic ordering. The relative intensities of the resonance lines of 3:4:1:1:4:3 confirm this ordering. The determined fitting parameters of the hyperfine magnetic fields, Hhf=467(26) kOe (sample S0) and 473.3(51) kOe (sample S300), the 7 Page 7 of 40
isomer shifts δ=0.338(18) mm/s (sample S0) and 0.341(32) mm/s (sample S300), and the almostzero quadrupole splitting are unambiguously compatible with nanosized maghemite [79]. Moreover, the Mössbauer hyperfine parameters of the S0 and S300 samples are almost the same,
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indicating nearly negligible differences between the local environments of the Fe atoms.
The sizes and morphologies of the samples were investigated using a TEM analysis (Fig. 4).
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It can be seen that in both, the as-received and the annealed nanoclusters, there is a spherical
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shape and a core-shell nanostructure consisting of the nanoparticle cluster core and the amorphous silica shell. A TEM observation further revealed that the diameters of the S0 (Fig.
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4(a) and (b)) and S300 (Fig. 4(d) and (e)) nanoclusters are approximately 80 nm and the thickness of the amorphous silica shell is approximately 15 nm. In TEM images taken at a higher
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magnification (Fig. 4(c) and (f)) it is clear that the nanocluster’ core is composed of small,
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maghemite nanoparticles with diameters of about 10 nm, which is in good agreement with the
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crystal size obtained from the XRPD. The TEM observations also revealed that after the annealing of the S0 nanoclusters in air at 300 ºC the structure and morphology of the S300
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nanoclusters were very similar to the S0, i.e., the sizes of the nanoclusters and the magnetic nanoparticles, as well as that of the silica shell thickness, were not changed. We can expect that the superparamagnetic nanoparticles in the as-received nanocluster core are not in close contact, because of the surfactants at their surfaces. In the annealed nanoclusters S300 we suggest that the thermal decomposition of the surfactants during the annealing treatment and in the absence of oxygen inside the nanoclusters led to the formation of an amorphous carbon ”coat” around the maghemite nanoparticles. The resulting amorphous carbon can act as a barrier to suppress the aggregation and crystal growth of the magnetic nanoparticles, and can also act as a shield that
8 Page 8 of 40
prevents any strong inter-particle interactions. The presence of the carbon in the S300 sample was observed by energy-dispersive X-ray spectroscopy (EDS) analysis in the TEM. The magnetic properties of the samples were characterized by measurements of the
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temperature dependence of the AC and DC susceptibility and the magnetic field dependence of the magnetization. These properties of the S0 and S300 samples were investigated using a
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SQUID magnetometer. In order to investigate the dynamic properties and the inter-particle
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interactions between the magnetic nanoparticles, AC susceptibility measurements were made at four different frequencies, i.e., 1, 10, 100 and 1000 Hz (Figs. 5(a) sample S0, and 5(b) sample
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S300). First, we checked for the existence of inter-particle interactions in the samples using the Néel-Brown theory of superparamagnetism [80,81]. The magnetic moments of non-interacting,
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single-domain magnetic nanoparticles with a uniaxial anisotropy fluctuate between the two
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directions of the easy axis of magnetization with a relaxation time τ that obeys the Arrhenius law: (1)
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τ=τ0exp(Ea/kBTB) ,
where Ea is the energy barrier to magnetization reversal in a single particles, kB is Boltzman’s
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constant, and τ0 is the attempt frequency. The attempt frequency τ0 should be within the 10-9 to 10-13 range, as reported in the literature [4,17,22,69]. The values of τ0<10-24 s determined from the fits of the Arrhenius law to our experimental data are much lower than the above-quoted limit. This reveals the existence of inter-particle interactions in the investigated samples. Second, we determined the presence and strength of the inter-particle interactions in the samples from the frequency dependence of TB by using the empirical parameter C1=ΔTB/(TBΔlogν), where TB denotes the average value of the blocking temperature in the range of applied frequencies ν, whereas ΔTB denotes the difference between the maximum and the minimum value of TB (Fig. 5(a) and (b)). In this way we obtained C1(S0)=0.055 and C1(S300)=0.069 for the samples S0 and 9 Page 9 of 40
S300, respectively. The value of the C1 parameter for a non-interacting magnetic nanoparticle system lies in the range ~0.1–0.13 and decreases with the increasing strength of the inter-particle interactions [82,83]. These results show the existence of inter-particle interactions in both
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samples, and that the annealed sample S300 has weaker inter-particle interactions. The obtained values for the attempt frequency τ0 in the Arrhenius law and the empirical parameter C1 imply
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dependence of TB should be fitted to the Vogel-Fulcher law [84]:
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that the interactions among the nanoparticles are not negligible and that the frequency
(2)
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τ=τ0exp[Ea/kB(TB-T0)] ,
where τ=1/ν (ν=1, 10, 100 and 1000 Hz) and the parameter T0 is a measure of the inter-particle
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interaction strength. The fits of Eq. (2) to the experimental data are given in Fig. 5(c) for sample S0 and 5(d) for sample S300 (solid lines). The obtained values for the fit parameters are:
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τ0(S0)=4.48·10-11 s, Ea(S0)/kB=1264 K, T0(S0)=92.6 K for sample S0 and τ0(S300)=3.08·10-10 s,
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Ea(S300)/kB=1149 K, T0(S300)=70.66 K for sample S300. The obtained magnetic anisotropy
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parameters Ea/kB were used to estimate the effective anisotropy constant Keff, using the relation KeffV=Ea, where V denotes the volume of the particle. For particles with a diameter d(S0)=10.4 nm and d(S300)=10.6 nm, this relation gives Keff(S0)=2.96·105 erg/cm3 and Keff(S300)=2.54·105 erg/cm3, whereas the value for the magnetocrystalline anisotropy constant for the bulk maghemite is Kv=4.7·104 erg/cm3. The substantially higher values of Keff in the investigated samples suggest that in the case of our nanoparticle systems, additional contributions to the anisotropy constant exist, e.g., the particle surface anisotropy, magnetoelastic, and shape anisotropy as well as that the inter-particle interactions play an important role in these systems [17,85]. Finally, we calculated the value of another empirical parameter, C2=(TB-T0)/TB, using the T0 values obtained 10 Page 10 of 40
from the Vogel-Fulcher fit. The obtained values are C2(S0)=0.43 and C2(S300)=0.52, whereas the expected value of the C2 parameter for non-interacting systems is about one [82,83]. It was also experimentally determined that the value of C2 decreases with increasing interactions among the
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particles. These calculated values confirm the existence of interactions in both samples, and of weaker inter-particle interactions in sample S300.
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The temperature dependences of the magnetization M(T) for both samples were recorded in
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the zero-field-cooled (ZFC) and field-cooled (FC) regimes (Figs. 6(a) for sample S0 and 6(b) for sample S300). The magnetizations were measured in a weak applied magnetic field (H=100 Oe)
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as a function of increasing temperature after the sample was cooled, with and without an applied magnetic field. The ZFC curves exhibit maxima at around TB(S0)=110 K and TB(S300)=88 K,
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corresponding to the blocking temperatures TB of the samples S0 and S300, respectively. The
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ZFC and FC curves split significantly below TB: the ZFC magnetization decreases sharply while
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the FC magnetization rises steadily (Figs. 6(a) and (b)). On one hand, the continuous rise of the FC curves shows that strong inter-particle interactions do not exist in the samples due to the
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coating of the maghemite nanoparticles with the surfactant (S0) and the amorphous carbon (S300). On the other hand, a plateau and/or peak in the FC curves was observed in the case of the systems with strong inter-particle interactions [86]. The higher value of Tmax in the ZFC curve of the sample S0 can be explained as being a consequence of the stronger dipolar magnetic interactions among the maghemite nanoparticles, as shown by the AC susceptibility studies. Such interactions slow the magnetic relaxation and increase Tmax, as reported in the literature [87]. The magnetic field dependences of the magnetization M(H), were measured at temperatures below TB at T=5 K for both samples (S0 and S300) and are shown in Fig. 7(a) and 7(b). In Fig. 7, it is clear that the magnetizations are almost saturated in high magnetic fields for both samples, as 11 Page 11 of 40
also observed for well-crystallized ferrimagnetic iron oxide nanomaterials. The obtained hysteresis loops are symmetric around the origin (Fig. 7(a) and (b), insets), with coercivity, remanence and saturation magnetization HC(S0)=180 Oe, Mr(S0)=6.4 emu/g, and MS(S0)=29.4 emu/g
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for the sample S0 (Fig. 7(a)) and HC(S300)=138 Oe, Mr(300)=6.3 emu/g, and MS(300)=33.4 emu/g for the sample S300 (Fig. 7(b)). The results of the field-dependent magnetization measurements
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show that the S300 has a higher magnetization (Fig. 7). After the annealing treatment, the
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saturation magnetization is increased by about ≈13.6 % of the value of the sample S0 (Fig. 7). This increase in the magnetization of the annealed sample can be attributed to the degradation
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and decomposition of the surfactants and their release of gas products during the annealing treatment from the sample, i.e. to the higher content of the magnetic material in the sample S300.
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The M(H) dependencies were also recorded at T=300 K for both samples S0 (Fig. 8(a)) and S300
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( Fig. 8(b)). The absence of both coercivity and remanence suggests superparamagnetic behavior
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at this temperature. The values of the saturation magnetization of the samples MS0=24.75 emu/g and MS300=28.85 emu/g were determined as the measured values of the magnetization obtained at
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H= 3 T. The observed saturation magnetizations are related to the silica-coated nanoclusters. The organic surfactant or amorphous carbon as well as the silica shell decrease the saturation magnetization of the produced maghemite nanoclusters. The saturation magnetization of the bare maghemite nanoparticles is around MS=66 emu/g. The magnetization vs. magnetic field dependence was fitted using the modified Langevin function (Fig. 9(a)): M=MS[coth(mpH/kBT)-(kBT/mpH)]+χH ,
(3)
where χ is the high-field susceptibility. The mass of the sample (S0) was normalized to the mass of the magnetic material (Fig. 9(a)). The mass of iron oxide in the formed silica-coated nanoclusters was determined from the magnetic properties of the bare maghemite nanoparticles 12 Page 12 of 40
(MS=66 emu/g). The determined magnetic moment of the maghemite nanoparticles using the Langevin fit was mp=18005 µB. An estimation of the mean particle size can be made by using the expression mp=πd3MS/6 for the magnetic moment of a particle, which assumes that the
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nanoparticle has a spherical shape, and where d denotes the diameter of that sphere. From the fitting parameters and the above relation for the magnetic moment, the mean particle diameter
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was determined to be d(S0)=10.2 nm. In addition, this value is in good agreement with the value
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obtained during the XRPD and TEM studies. Assuming the magnetic nanoclusters of diameter DS=80 nm, a of shell thickness 15 nm and maghemite nanoparticles of 10 nm size, we estimated
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the number of nanoparticles per magnetic nanocluster to be N=67. From the above-listed results we obtained the following magnetic moment for the single magnetic nanocluster mnc=1.2·106 µB.
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The unchanged magnetic structure of the magnetic nanoparticles after the annealing treatment is
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also evidenced in the data obtained from the magnetic measurements. The overlap of the M/MS
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vs. H curves at constant temperature T=300 K (Fig. 9(b)) for both samples indicates that the magnetization behavior is preserved, independently of the coating and the annealing treatment.
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We can conclude that both samples are composed of almost identical maghemite nanoparticles (good agreement with XRPD and TEM studies) and that those inter-particle interactions affect the magnetic properties of the samples. This is also supported by the results of the AC magnetic susceptibility measurements, where the temperature in which χ' peaks at different frequencies following the Vogel-Fulcher model, a feature commonly found in systems with magnetic dipolar interactions. Finally, we can conclude that direct contact between the magnetic nanoparticles in the samples was avoided by coating, where a layer of surfactant (sample S0) or amorphous carbon (sample S300) acts so as to change the magnetic properties by altering the strength of the magnetic dipolar interaction. The decrease of the interactions in the annealed sample S300 13 Page 13 of 40
indicates that the maghemite nanoparticles are better isolated and shield each other with the
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amorphous carbon.
4. Conclusions
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Our novel core-shell nanoclusters were superparamagnetic and had high magnetic moments at the same time, which is desirable for practical applications. The direct contacts between the
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maghemite nanoparticles inside the nanoclusters were made impossible by surfactants at their
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surfaces (sample S0) or by amorphous carbon (sample S300), so as to suppress the strong interparticle interactions, while maintaining the superparamagnetic behavior at room temperature. It
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was also shown that for the annealed sample (S300) the average crystallite and particle sizes of the maghemite nanoparticles estimated from the XRPD and TEM do not show an appreciable
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change in comparison to the as-received nanoclusters. Moreover, the TEM analysis confirmed the
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core-shell nanostructure of the nanoclusters with a diameter of 80 nm, composed of a cluster of
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maghemite nanoparticles in the core and an amorphous silica shell with a thickness of 15 nm. The nanoparticles in the core had a size of 10 nm, for both samples. The number of maghemite nanoparticles in the nanocluster’ core was estimated to be N=67. This internal microstructure produced the high magnetic moment of the superparamagnetic nanocluster, which is calculated to be mnc=1.2·106 µB. It is expected that on the basis of these values the magnetic nanoclusters would be an excellent material, for biomedical applications to magnetic separation and drug delivery. Both the DC magnetization and AC susceptibility measurements indicate the nanoscale magnetic properties of the as-received S0 and annealed S300 samples with blocking temperatures of TB(S0)=110 K and TB(S300)=88 K, respectively. The AC susceptibility study shows that the Vogel-Fulcher model for single-domain interacting magnetic nanoparticles is applicable, and it 14 Page 14 of 40
was used to determine the empirical parameters C2(S0)=0.43 and C2(S300)=0.52, which show the presence of inter-particle interactions between the magnetic nanoparticles and somewhat weaker interactions in the S300. The significantly different and unexpected magnetic properties of the
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annealed sample S300 (lower TB and weaker inter-particle interactions) in comparison with S0 can be explained by the thermal decomposition of the SPION’s surfactants in an atmosphere
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without oxygen inside the nanoclusters and the production of amorphous carbon, which coated
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the maghemite nanoparticles. The analysis of the magnetization results indicates that amorphous carbon shields the magnetic nanoparticles better. The above results suggest that the observed
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changes in the magnetic properties of the annealed nanoclusters are due to the different strengths of the inter-particle interactions in the samples, and not due to the intrinsic properties of the
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magnetic nanoparticles. Therefore, our results show that the inter-particle interactions between
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the magnetic nanoparticles may decrease when annealing the nanoclusters. These findings are
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important for the design of magnetic materials, and such well-defined nanoclusters may be used for the subsequent engineering of their surfaces and functionalization for biomedical applications.
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This study may also be useful as a motivation for synthesizing other metal oxide cluster structures with novel and useful properties for both fundamental research and various practical applications.
Acknowledgements
The support by the Ministry of Higher Education, Science and Technology of the Republic of Slovenia within the National Research Program is acknowledged. The authors acknowledge the use of equipment in the Center of Excellence on Nanoscience and Nanotechnology (Nanocenter). The studied materials were provided by Nanos Scientificae d.o.o. (www.nanos-sci.com), Teslova ulica 30, 1000 Ljubljana, Slovenia. M.T. acknowledges Serbian Ministry of Science for financial 15 Page 15 of 40
support (Grant no. III 45015) and Professor Veljko Dmitrasinovic (Institute for Physics,
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M
an
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Belgrade) for his comments.
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Figure captions
Figure 1. Formation mechanism of core-shell nanocluster: as-synthesized maghemite
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nanoparticles (a and d), nanoparticle cluster (b and e) and coated by silica nanocluster (c and f). Figure 2. The XRPD patterns of the samples S0 (a) and S300 (b). The Miller indices (hkl) of the
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diffraction peaks are shown in the diffraction patterns.
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Figure 3. Mössbauer spectra of samples S0 and S300 recorded at room temperature. Figure 4. The TEM and HRTEM images of the maghemite (γ-Fe2O3) nanoparticle clusters
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coated with a silica shell for S0 (a, b, c) and S300 (d, e, f) samples.
Figure 5. Temperature dependence of the in phase χ' measured ac-susceptibility at different
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frequencies from 1 Hz to 1000 Hz corresponding to the S0 (a) and S300 (b) samples. Relaxation
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fit with the Vogel–Fulcher law.
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time τ as a function of TB-1 for S0 (c) and S300 (d) samples. The solid lines correspond to the best
Figure 6. The ZFC and FC curves performed at 100 Oe for samples S0 (a) and S300 (b).
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Figure 7. Magnetization vs. magnetic field measured at 5 K for samples S0 (a) and S300 (b). The insets show the zoom view of M-H loops. Figure 8. Magnetization vs. applied magnetic field measured at 300 K for samples S0 (a) and S300 (b). Inset: field-dependent magnetization curve of bare maghemite nanoparticles. Figure 9. (a) Field-dependent magnetization curve of S0 at 300 K. The solid line is fit to a Langevin function. (b) Plot of M/MS vs. T using the data at T=300 K for samples S0 and S300.
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Research Highlights >Magnetic properties of γ‐Fe2O3 nanoclusters and their thermal decomposition. >SPION clusters show superparamagnetism and high magnetic moments mnc~1.2·106 µB. >The TEM shows maghemite
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