Superparamagnetic FexOy@SiO2 CoreâShell Nanostructures

J. Phys. Chem. C 2007, 111, 1999-2007

1999

Superparamagnetic FexOy@SiO2 Core-Shell Nanostructures: Controlled Synthesis and Magnetic Characterization Dongling Ma,†,‡ Teodor Veres,§ Liviu Clime,§ Franç ois Normandin,§ Jingwen Guan,† David Kingston,| and Benoit Simard*,† Steacie Institute for Molecular Sciences and Institute for Chemical Process and EnVironmental Technology, National Research Council of Canada, Ottawa, ON K1A 0R6, Canada and Industrial Materials Institute, National Research Council of Canada, BoucherVille, QC J4B 6Y4, Canada ReceiVed: October 3, 2006; In Final Form: NoVember 14, 2006

We present the results of controlled synthesis of core-shell nanostructures and the influence of silica coating on the magnetic properties of formed hybrid nanoparticles. The core-shell nanoarchitectures composed of magnetic iron oxide cores and amorphous silica shells have been synthesized through a sol-gel approach and characterized by transmission electron microscopy, energy dispersive X-ray analysis, and magnetometry. It is found that many of the hybrid nanoparticles contain a single core. A good control of the silica shell thickness (10-100 nm) has been achieved by adjusting the silane concentration. From temperature-dependent zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements and ZFC model study, it is found that the mean deblocking temperature and effective anisotropy constant remain similar after the iron oxide nanoparticles are coated with about 12 nm thick silica shells. However, the ZFC peak temperature and the ZFC/FC branching point decrease significantly by over 100 K. To understand these interesting phenomena, a specific sample whose surface is modified with a small amount of tetraethoxysilane was prepared. By studying the ZFC/FC and AC behavior of these three kinds of samples, relative contribution from surface anisotropy and magnetic interparticle interactions to the blocking behavior was evaluated. The magnetic core size effect is studied also by comparing two core-shell samples with slightly different core sizes.

1. Introduction Magnetic nanoparticles have attracted considerable attentions because of their broad applications from industrial to biomedical, such as magnetic recording media, drug-targeting delivery, cell labeling and sorting, magnetic resonance imaging, and separation and enrichment of even a trace amount of biospecies.1-3 In many applications, the very fine nanoparticles are preferred because of their enhanced colloidal stability and improved tissue diffusion. Smaller particles can circulate in the blood stream for much longer time than larger particles. Instead of being sequestered and quickly rejected to the liver and spleen, very small particles, less than 40 nm, can cross capillary wall and reach lymph nodes and bone marrow, enabling targeting of these tissues.4 Moreover, with decreasing size especially below 100 nm, the surface-to-volume ratio sharply increases. This in turn provides tremendously increased amount of attaching sites per unit mass for biomolecules. The last but not least important reason for using small magnetic nanoparticles is their remarkable superparamagnetic behavior arising from their quantum size effect. In this case, an assembly of particles behaves like a paramagnet while with extremely large magnetic moments (of the order of 100 000 Bohr magneton for a 10 nm iron nanoparticle) when the temperature is above a certain point, the so-called blocking temperature. This property is very important in applications in which particle agglomeration must * Corresponding author. Email: [email protected]. † Steacie Institute for Molecular Sciences. ‡ Currently, Universite ´ du Que´bec, Institut national de la recherche scientifique, Varennes, QC J3X 1S2, Canada. § Industrial Materials Institute. | Institute for Chemical Process and Environmental Technology.

be avoided upon the removal of magnetic field. Larger magnetic particles, such as ferromagnetic and ferrimagnetic, show remanence and hence remain agglomerated after magnetic confinement. This is clearly not suitable for applications, particularly, in microfluidic devices in which the particles must redisperse immediately after removing the magnetic field, and this is true only for superparamagnetic particles. The direct use of bare magnetic nanoparticles, however, raises new issues. The main concerns are biocompatibility and toxicity, which are of the highest priority for in vivo applications.2 Many highly magnetic materials, such as cobalt or nickel, are toxic.2 Therefore, suitable materials need to be coated onto the particle surface. In addition, the magnetic nanoparticles tend to form large agglomerates and thus fail to achieve the expected functions due to the loss of properties associated with the initial single-domain nanostructures. In addition, the bare magnetic nanoparticles undergo rapid biodegradation when they are exposed to biological environments having various pH values and containing salts at different concentrations.5 Another drawback of using bare magnetic nanoparticles is fluorescence quenching during luminescence detection, which is an extensively utilized and a quite useful technique in biological applications. All these problems can be solved by growing silica shells on the surface of magnetic nanoparticles to isolate them from their surroundings. Another significant advantage of the silica shell is related to the use of well-known conjugation chemistry that allows covalent bonding of various chemical or biological species onto the silica surface. This opens a way for the use of magnetic particles in numerous biomedical applications such as separation devices for molecules or biospecies.

10.1021/jp0665067 CCC: $37.00 Published 2007 by the American Chemical Society Published on Web 01/12/2007

2000 J. Phys. Chem. C, Vol. 111, No. 5, 2007 There has been much research regarding coating nonmagnetic metal nanoparticles, such as gold and silver, with silica to form a core-shell nanoarchitecture.6-10 Single core-shell structures have been developed in controlled manners. For magnetic nanoparticles, difficulty of forming single core-shell nanostructure is largely increased as a result of magnetic interparticle interactions. In many cases, the magnetic cores are clusters of magnetic nanoparticles.11-18 This core structure may deteriorate or even cause the loss of superparamagnetism. Therefore, controlled synthesis is the key aspect for achieving desired properties. In contrast to large efforts toward creating magnetic core-shell nanoparticles with a better quality, much less systematic work has been done to investigate the magnetic properties of the resulting core-shell hybrid structure.11,19 Herein, we show that core-shell nanoparticles can be synthesized with a high yield by using a modified Sto¨ber method.5,20 In addition, their magnetic properties have been fully characterized and analyzed. It is known that silica coating may change the blocking behavior. By preparing the nanoparticles with relatively thin (more accurately, surface modified with silane molecules) and thick shells, we have been able to evaluate the relative contribution of the two phenomena responsible for the change in blocking behavior, namely variations in surface anisotropy and dipolar interparticle interactions. This work provides insight into the magnetic behavior of core-shell systems as well as providing handles for material optimization for practical applications. 2. Experimental Methods 2.1. Materials. Water-based ferrofluid EMG 304 containing magnetic iron oxide (FexOy) nanoparticles was obtained from Ferrotec (USA) Corporation. Tetraethoxysilane (TEOS) was purchased from Gelest, Inc. Ammonium hydroxide (NH4OH, 28-30 wt %) and high purity 2-propanol were both produced by EMD Chemicals, Inc. All chemicals were used as received. High purity water (18 MΩ.cm) was made using a Millipore Q-guard 2 purification system (Millipore Corporation) and used in all the preparations. 2.2. Synthesis of Core-Shell Nanoparticles. Silica coated FexOy nanoparticles (FexOy@SiO2) were prepared via the modified Sto¨ber method.5,20,21 First, a stock colloidal dispersion was prepared by diluting the initial ferrofluid EMG 304 with high purity water in the way that its particle number concentration was around 1014 per mL based on the supplier’s data. For every preparation, the stock dispersion was further diluted with 2.7 times of water by volume and was sonicated with a Branson 5510 sonication bath for 30 min. Following that, a freshly prepared TEOS solution at various TEOS concentrations in 2-propanol was added to the dispersion under vigorous stirring in the volume ratio of 7.1:1. Silica coating was then initiated by adding NH4OH into the mixture. The pH value of the final coating solution was around 11. The resulting dispersion was continuously stirred for at least 5 h at room temperature to allow formation of the silica shells. The synthesized hybrid coreshell particles were repeatedly washed with water and were collected by centrifugation. Depending on the final size of the particles, various centrifugation speeds (4000 to 9000 rpm) or time have been chosen. For a given shell thickness, the particles with different core size distribution were separated by varying the centrifugation time with a given centrifugation speed. The washed particles were dried over acetone for magnetic characterization. The same procedure was employed to prepare the surfacemodified FexOy nanoparticles with a low TEOS concentration

Ma et al. of 1.34 × 10-5 M in 2-propanol. For the preparation of thick shells, which were about 100 nm, TEOS was added in two steps. In the first step, the appropriate amount of TEOS was added to form ∼12 nm silica shells. In the second step, more TEOS was added dropwise. The particles then were washed and treated in the way described above. 2.3. Characterization. Transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analysis were performed using a Philips CM20 FEG microscopy equipped with a Schottky field emission gun at an acceleration voltage of 200 kV. All images were acquired in the bright field mode. Samples were prepared by evaporating several drops of the particle dispersion onto the carbon-coated copper grids. During the EDX analysis, the electron beam could be focused on the core and shell regions, respectively, to collect corresponding chemical information. The X-ray diffraction (XRD) study of the FexOy powder sample was carried out at room temperature with a Scintag X2 diffractometer using a wavelength of 0.154 nm. The spectrum was obtained in step mode with step size of 0.06° and collection time of 20 s. Data were collected within 2θ ranges of 20 to 70°. After background subtraction and performing KR2 stripping, the profile was fitted with the Pearson VII software, which uses mixed functions of Gaussian and Lorentzian. On the basis of this analysis, the average nanocrystallite size D was estimated from the Scherrer equation:22 D ) 0.94λ/β cosθ, where λ is the X-ray wavelength, β is the full-width at half-maximum (in radian) and θ is the diffraction angle. The X-ray photoelectron spectra (XPS) were taken on a Kratos Axis Ultra photoelectron spectrometer equipped with a monochromatic Al KR X-ray source. Analyses were carried out using an accelerating voltage of 14 kV and a current of 10 mA. The pressure in the analysis chamber during analysis was 2.0 × 10-9 Torr. Pass energy for high-resolution scan was 40 eV. DC and AC magnetic properties of the nanoparticles were studied with a Quantum Design PPMS Model 6000 Magnetometer. As-received FexOy or silica coated FexOy nanoparticles, in powder form, were inserted in a gelatin capsule and sealed with Parafilm. For DC measurements, field-dependent magnetization was measured at 300 K for magnetic fields up to 7 T. Temperature-dependent zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements were taken by initially cooling the samples to 2 K in zero and 50 Oe fields, respectively. Then, the magnetization was measured during the heating cycle from 10 to 350 K under a 50 Oe field. AC susceptibility as a function of temperature was measured in zero DC field over the frequency range 10-10 000 Hz with an AC driving magnetic field of 15 Oe. All the magnetization data were normalized with the sample weight. 2.4. Theoretical Model. To extract information about the synthesized hybrid superparamagnetic nanoparticles and evaluate the influence of various preparation methods on their intrinsic physical properties, we implemented a theoretical model for the magnetization obtained in the usual ZFC procedures as described in ref 23. We assume a log-normal distribution of the particle diameters and use the function23

mZFC(T) )

µ0Ms2 HVtotW(Db,∞,3;D) + 3Keff µ0πMs2 V W(0,Db,6;D) (1) 18kBT tot

FexOy@SiO2 Core-Shell Nanostructures

J. Phys. Chem. C, Vol. 111, No. 5, 2007 2001

Figure 2. XRD pattern of the as-received iron oxide nanoparticles.

Figure 1. (A) TEM micrograph and (B) HR-TEM image showing {220} planes of the as-received iron oxide nanoparticles.

for the temperature dependence of the magnetization mZFC in a ZFC measurement, among which

∫DD Dk f(D) dD W(D1,D2,k;D) ) ∞ ∫0 D3 f(D) dD 2

1

(2)

where µ0 is the vacuum absolute permeability, Ms is the saturation magnetization, Keff is the effective anisotropy constant, H is the external applied field, Db is he deblocking diameter, Vtot is the total magnetic volume of the sample, and f(D) is the lognormal probability density distribution of the nanoparticle diameters. Experimental ZFC curves are fitted in the least-square sense with this model and the ZFC peak position Tpeak as well as the deblocking temperature TB for the lognormal mean diameter and the effective anisotropy constant Keff are then identified. 3. Results In Figure 1A, we show a TEM image of the as-received FexOy nanoparticles. They tend to agglomerate during the solvent evaporation process of the TEM sample preparation. Their morphology appears to be irregularly shaped from sphere to oval with the diameter or the longer axis ranging from 5 to 24 nm. The particle size distribution of these particles measured from TEM is considered as log-normal24 with the median diameter d0 ) 9.7 nm, the standard deviation σ ) 0.4 and the mean diameter of 10.5 nm. High-resolution TEM (HR-TEM) image (Figure 1B) shows a lattice fringe of {220} planes of either Fe3O4 or γ-Fe2O3. It is difficult to distinguish these two phases with HR-TEM because both oxides have similar d spacing. The crystalline nature of the bulk material is verified by powder XRD analysis (Figure 2). The diffraction pattern is again characteristics of γ-Fe2O3 and/or Fe3O4, indicating the presence of either or both phases.25 The diffraction peaks are broad because of small dimensions. From the (311) reflection, the volume-weighted thickness in the direction perpendicular to (311) planes of crystallites was

Figure 3. XPS spectrum of Fe 2p of the as-received iron oxide nanoparticles.

calculated to be 10.2 nm using the Scherrer equation. This value is close to the mean diameter of nanoparticles obtained from TEM (10.5 nm), suggesting the highly crystalline nature of these nanoparticles. The chemical composition of Fe2+ and Fe3+ in the as-received FexOy nanoparticles was studied with XPS analysis (Figure 3). The binding energies of about 711 and 724 eV are assigned to the 2p3/2 and 2p1/2 of Fe3+ ions in R-Fe2O3 or γ-Fe2O3, respectively.26 The lack of the evident shoulder around 709 eV, characteristic of the 2p3/2 of Fe2+ ions, suggests that the Fe3O4 phase is in a very low concentration, if there is any. In addition, the observable 2p3/2 satellite of the Fe3+ ions around 719 eV also supports the above conclusion; the presence of the 2p3/2 satellite of Fe2+ ions at 716 eV would lead to a much less resolved 719 eV satellite structure. Therefore, based on XRD and XPS analyses, the as-received FexOy nanoparticles are mainly, if not all, composed of γ-Fe2O3 crystals. In any case, we will use the notation FexOy to represent the magnetic nanoparticles. In Figure 4 we show TEM images of the FexOy nanoparticles after being coated with amorphous silica shells. Clear coreshell architectures are observed because of sufficient intensity contrast between the FexOy cores and the silica shells due to their electron density difference. This conclusion also is supported by the EDX results described below. From the TEM images, it can be seen that most of the hybrid nanoparticles have only single or double cores with a small amount having multiple cores. Bare FexOy and core-free silica nanoparticles were not observed in these cases although some tiny pure silica nanoparticles indeed formed and could be collected by centrifuging at a high speed. The thickness of the silica shells is uniform and the shell surface appears to be quite smooth. The shell thickness is adjusted from 10 nm to around 100 nm (Figure 4) by changing the concentration of TEOS. For the preparation of thinner shells, the required amount of TEOS was added in a single step; for the thicker shells, the TEOS was added in two steps. In this way, the TEOS added in the second step would hydrolyze and react with already formed

2002 J. Phys. Chem. C, Vol. 111, No. 5, 2007

Ma et al.

Figure 5. EDX data of the surface-modified iron oxide nanoparticles.

TABLE 1: Physical and Empirical Parameters as Obtained from ZFC Measurements and Model (eq 1)

Figure 4. (A) TEM micrograph of a group of the core-shell nanoparticles. Higher magnification TEM images showing the coreshell morphology of the particles with the shell thickness of about (B) 12 nm and (C) 40 nm. (D) TEM micrograph showing the large particles with the thick shell thickness ranging from 60 to 100 nm. The size distribution becomes broader. (E) EDX data of the core-shell particles showing chemical information in the core region and the shell region. The TEOS concentrations in isopropanol for (A), (B), (C), and (D) were 1.1 × 10-3, 1 × 10-3, 1.2 × 10-2, and 7 × 10-2 M, respectively.

FexOy@SiO2 seeds to grow thicker shells instead of forming too many core-free silica nanoparticles. The overall particle size is less uniform than that of the thin-shell samples. In addition, as a result of the presence of the thick silica shells, the coreshell morphology could not be resolved using TEM, but could be easily evidenced with EDX. As seen in Figure 4E, iron was detected in the core region, whereas no iron signal was obtained from the shell part. Copper and carbon signals come from the TEM grid. The surface of the FexOy nanoparticles also was modified with TEOS to obtain silane coverage of (at most) a few layers. These cannot be observed directly from the TEM image. However, a trace amount of Si on the surface was identified using EDX (Figure 5) and thus indicating the presence of silane coupling agents on the particle surface. Temperature, frequency, and field dependent behavior of the magnetization was studied for three types of samples: (i) asreceived FexOy nanoparticles, (ii) FexOy@SiO2 core-shell nanoparticles with comparable core and shell sizes (sample A and B), and (iii) surface-modified FexOy nanoparticles through the TEOS. It should be noted that the as-received FexOy nanoparticles are surrounded by about 12 wt % of water

sample

dh (nm)

σ

Tpeak (K)

TB (K)

Keff (× 105J/m3)

as-received FexOy@SiO2 A FexOy@SiO2 B surface-modified

10.5 9.9 9.3 9.9

0.4 0.21 0.22 0.23

≈ 350 152 138 255

52 61 53 91

0.50 0.67 0.67 1.00

dispersible surfactants. The average shell thickness is about 12 nm for both sample A and B. The only difference between them is that sample A has a higher fraction of particles with larger magnetic cores as compared to sample B. This is because sample A was collected before sample B during the same centrifugation process. In these systems, two main differences could be distinguished as the spacing between and chemical environments surrounding the magnetic particles. Three physical quantities are evaluated from the fit with the theoretical model (eq 1): the ZFC peak temperature Tpeak, the deblocking temperature, TB, for the particles with magnetic core diameter as given by the lognormal mean dh, and the effective anisotropy constant, Keff, (see Table 1 and Figure 6). The temperature corresponding to the branching point between ZFC and FC curves (Tbranch) also is directly estimated from the experimental curves. Tbranch determined from the bifurcation, generally dominated by the largest particles, is attributed to the maximum TB, whereas Tpeak determined from the ZFC maximum, mainly dominated by the particles with the average size, may be associated with the mean TB. However, a direct relationship between Tpeak and the mean TB cannot be established without taking into account the mean and the standard deviation of the lognormal distribution.23 For the particles with a wide size distribution, Tbranch would usually be significantly larger than Tpeak. Therefore, to gain a complete understanding it is necessary to take both into account, especially when one works with nanoparticles with broad size distribution. In this context, we identify the Tpeak and Tbranch values for all the samples and compute the TB as well. The experimental and theoretical temperature dependence of DC magnetization of the as-received FexOy, FexOy@SiO2 A and B, and surface-modified FexOy nanoparticles are presented in Figure 6a, b, c, and d, respectively. The magnetization values have been normalized to the magnetic moment at Tpeak value for each sample. Because mainly the shape of the curves and the deblocking temperature are of interest of this study, no weight subtraction of surfactants or silica to get absolute magnetization values has been performed. It could be seen that all ZFC curves show a broad ZFC maximum, characteristic of the magnetic nanoparticles with a wide size distribution,15,27,28 which is consistent with our TEM observations.



Figure 6. Experimental (symbols) and theoretical (line) magnetization curves of (a) the as-received FexOy, (b) the FexOy@SiO2 A, (c) the FexOy@SiO2 B, and (d) the surface-modified FexOy nanoparticles recorded at 50 Oe in ZFC and FC modes.

The Tpeak value of the as-received FexOy is about 350 K (Figure 6a). When the magnetic particles are shelled with 12nm silica, Tpeak sharply decreases by over 150 K. Even for the FexOy@SiO2 samples A and B, which have similar core-shell structure and shell thickness, there is about 14 K difference in their ZFC peak position. Sample A, which has more particles with large magnetic cores, has Tpeak ) 152 K whereas for the sample B, Tpeak ) 138 K. This difference can readily be explained from the particle size distribution. The best fit for the sample A was obtained with d0 ) 9.7 nm and σ ) 0.21 whereas the sample B presents a slightly shifted lognormal distribution toward smaller diameters d0 ) 9.17 nm and larger standard deviations σ ) 0.22. As the nanoparticles in these two samples are similar, the deblocking temperature TB for a given core size will be exactly the same. However, if we choose the lognormal mean diameter dh ) d0 exp{σ2/2} as reference for the comparison between the deblocking temperatures, we will obviously obtain T h B(A) > T h B(B). This is totally because of the distribution shift and not to any variations in intrinsic physical properties of the nanoparticles. This result is consistent with the preparation method of these two samples. For the surface-modified FexOy, Tpeak ) 255 K (i.e., nearly 100 K below that of the as-received FexOy nanoparticles) and 103 and 117 K higher than those of the core-shell samples A and B, respectively. Because the particle distribution does not present an important variation with respect to samples A and B (∆σ ) 0.02 and 0.01, only), we attribute the shift in Tpeak for this sample to some changes in the physical properties of the nanoparticles. Consistently, the T h B for the surface-modified particles (91 K) is significantly higher than those for the samples A and B (61 and 53 K, respectively), highlighting important changes in the superparamagnetic behavior of the nanoparticles in this sample. Its effective anisotropy constant derived from the ZFC model study is the largest.

The ZFC and FC curves of the as-received and the surfacemodified FexOy meet around 300 and 250 K, respectively. In contrast, the bifurcation points Tbranch for the core-shell samples A and B are much lower, being ∼190 and 156 K, respectively. It also is noticed that the FC curves of the as-received and the surface-modified FexOy at temperatures below Tbranch are flatter than those of the core-shell samples, indicating a sizable interaction or aggregation effects in the as-received and surfacemodified FexOy samples.15,29 Another method for investigating the blocking and relaxation behavior of superparamagnetic nanoparticles consists of dynamic susceptibility measurements. These AC susceptibilities have real (i.e., in-phase χ′) and imaginary (i.e., out-of-phase χ′′) components. The maxima in the χ′ (Tpeak,χ′)30 and χ′′ (Tpeak,χ′′)31 curves both have been associated with the deblocking temperature at which the relaxation process is on the same time scale as the measurement process. Because the χ′′ curves represent the magnetic losses and this physical process is very sensitive to the dynamic of relaxation as well as because some of our samples do not reach the maximum in their χ′ curves up to the highest measuring temperature, we mainly utilized the maximum in the χ′′ curves (Tpeak,χ′′) to study the magnetic properties. We performed dynamic susceptibility measurements for the as-received FexOy, FexOy@SiO2 core-shell A and B, and surface-modified FexOy nanoparticles. Because some of the obtained curves are a little noisy, the Gaussian function was used to fit the curves and determine the temperature corresponding to the peak position. The obtained results are shown in Figure 7 for the as-received (a), core-shell FexOy@SiO2 A (b), core-shell FexOy@SiO2 B (c), and surface-modified (d) nanoparticles, respectively. The general trend is that Tpeak,χ′′ shifts toward higher temperatures as the measuring frequency increases. In contrast, the core-shell samples A and B show much smoother susceptibility curves, which enables easy identification


Ma et al.

Figure 8. Temperature dependence of the relaxation time of the magnetization of the as-received FexOy, the core-shell FexOy@SiO2 A and B, and the-surface modified FexOy samples following the Neél theory.

sample, the in-phase susceptibility behavior is similar to that of the as-received FexOy nanoparticles. The peak temperatures are higher than 300 K. The Tpeak,χ′′ values for the surfacemodified sample in the range of 218-285 K, determined from Gaussian fittings, are larger than those for the as-received FexOy nanoparticles in contrast to expectation based on their static ZFC behavior. The difference in the absolute values of blocking temperatures for the same sample determined from the different techniques is in agreement with literature and is due to different sensitivity of techniques, different observation time, and broad size distribution.32 For noninteracting single domain particles, the magnetization relaxation process follows the Neél-Arrhenius law33

( )

τ ) τ0 exp

E kBT

(3)

where τ0 is the characteristic relaxation time and is of the order of 10-9∼10-11 s for ferri- and ferromagnetic particles30,32 and is generally 2 orders of magnitude lower for antiferromagnetic particles.32 E is the anisotropy energy barrier, which may be expressed in a first approximation as the product of effective anisotropy energy constant Keff and the particle volume V. T is the absolute temperature at which point the relaxation time is τ. kB is the Boltzmann’s constant. Because the maximum temperature of the imaginary part of the susceptibility represents a specific temperature Tm at which point the relaxation time matches the time scale of the measurement, the relaxation time can be calculated as 1/f where f is the measuring frequency. Therefore, the relaxation process can be also described as

ln

(1f ) ) ln τ + k ET 0

(4)

B m

Figure 7. Imaginary components of AC susceptibility of (a) the asreceived, (b) the core-shell FexOy@SiO2 A, (c) the core-shell FexOy@SiO2 B, and (d) the surface-modified nanoparticles.

of the deblocking temperatures and their frequency dependence. Both χ′ (not shown here) and χ′′ curves display a single maximum and the maxima shift upward as the frequencies increase. The Tpeak,χ′′ determined from this technique for sample A (138-186 K) is higher than that for sample B (118-161 K) in agreement with our ZFC measurements (see Table 1). The χ′ maxima shift was in the range of 195-235 K and 175-210 K for sample A and B, respectively. For the surface-modified

By plotting ln(1/f) versus 1/Tm, both τ0 and E can be estimated for noninteracting particles. Alternatively, this relationship can be used to determine whether the magnetic particles are interacting or not. It can be seen from Figure 8 that all the samples show linear relationships between ln(1/f) and 1/Tm. The surface-modified sample presents the largest slope indicating the highest effective anisotropy energy barrier. Because this sample shows similar size distribution to the core-shell samples, it can be concluded that the surface-modified sample has a larger effective anisotropy constant than the core-shell samples, as confirmed by the ZFC model (see Table 1). The calculated τ0 values are in the order of 10-13 s for the core-shell samples A and B, and 10-14 s for the as-received and the surface-modified FexOy. All values are smaller than those reported for ferri- and ferromagnetic nano-



Figure 9. Vogel-Fulcher plots for the as-received FexOy, the coreshell FexOy@SiO2 A and B, and the surface-modified FexOy samples using τ0 ) 10-9 s.

particles, indicating magnetic interparticle interactions in all samples. Moreover, smaller calculated τ0 values in the asreceived FexOy and the surface-modified FexOy suggest stronger dipolar interactions. We would like to point out that all these calculated τ0 values do not represent true characteristic relaxation time due to the presence of magnetic interparticle interactions in all cases. In view of magnetic interparticle interactions, the relaxation data were further fitted with the Vogel-Fulcher equation31

τ ) τ0 exp

(

E kB(T - T0)

)

(5)

that considers a cooperative-like (synergetic effect) interparticle interaction and involves a new parameter T0, which is a temperature correction term whose value depends on the strength of the magnetic dipolar interaction. A smaller T0 value suggests a weaker dipolar interaction.34 T0 can be estimated from the linearized Vogel-Fulcher equation

Tm ln

()

τ E τ ) + T0 ln τ0 kB τ0

(6)

where again Tm corresponds to the peak temperature of the imaginary susceptibility and τ equals to 1/f as explained above. τ0 values of 10-9, 10-10, and 10-11 s were used for VogelFulcher plots, and for all samples the best fit was achieved using τ0 ) 10-9 s (Figure 9). The fitted T0 values for the core-shell samples A (58 K) and B (48 K) are close and much smaller than those for the as-received FexOy (107 K) and surfacemodified FexOy (109 K). The results indicate that the as-received FexOy and surface-modified FexOy nanoparticles do interact much more strongly than the core-shell nanoparticles in agreement with the Neél-Arrhenius fittings. Weak magnetic interactions are present in the core-shell samples A and B and are attributed to the presence of large magnetic cores or core clusters. The magnetic interparticle interactions among smallsized particles (e10 nm) are likely to be totally screened by the over 10 nm-thick silica shells.11 The DC field dependent magnetization has been measured for all the samples. The as-received FexOy sample exhibits a saturated magnetization of 53.7 emu/g at 300 K. From thermogravimetric analyses, it is found that the as-received FexOy powder contains around 12 wt % of surfactants. The net saturation magnetization has been calculated by normalizing the observed magnetization of 53.7 emu/g to the net weight of FexOy nanoparticles in the sample. The calculated net saturation magnetization of 61 emu/g is in reasonable agreement with the reported magnetization for the iron oxide35 considering size

Figure 10. Normalized magnetization versus applied magnetic field for the as-received FexOy, the core-shell FexOy@SiO2 A and B, and the surface-modified FexOy samples recorded at 300 K.

effects. All other samples show smaller magnetization values under the same conditions as a result of the presence of nonmagnetic silica materials. All magnetization data were normalized to their corresponding magnetization value at 40 kOe and presented in Figure 10. It can be seen that all samples reveal negligible coercivity and remanence at 300 K, typical of superparamagnetic behavior. The only obvious difference is that all other samples saturate at much lower fields than the surface-modified sample. No clear saturation was observed for this sample even up to 70 kOe, which quite possibly is resulting from large individual anisotropy field. More energy would be required to overcome the higher rotation barrier associated with the larger effective anisotropy constant. By increasing the measuring temperature, the saturation can be expected to occur at lower magnetic fields. 4. Discussion It is known that TB of magnetic nanoparticles are dependent on size, surface states, and mutual interactions of magnetic nanoparticles.36 For the FexOy@SiO2 samples A and B, both of which have silica coating, their surface anisotropy would be similar. In addition, from AC behavior and ZFC study both samples mainly contain similarly weakly interacting particles. However, the average magnetic core sizes are different in these samples. The sample A has more large-sized magnetic cores and accordingly a slightly larger mean core size. Another possibility that cannot be excluded is that some of the large cores in the FexOy@SiO2 A are not single magnetic particles, but magnetic particle clusters. In conclusion, the TB difference between the FexOy@SiO2 A and B is mainly originating from magnetic core size effect. The case is different when comparing the silica shelled and the as-received particles. The main differences between them are surface environments and magnetic interparticle interactions. Quite different blocking behavior in terms of Tpeak, Tbranch, and the shape of ZFC/FC curves was observed. Surprisingly, the calculated TB and Keff only differ slightly (Table 1). To understand the above mentioned interesting phenomena, the contribution from surface anisotropy and magnetic interparticle interactions to the blocking behavior has to be estimated. For this purpose, the surface-modified sample was prepared and was expected to differ from the core-shell samples mainly by the spacing among the magnetic cores (i.e., magnetic dipolar particle interactions) and from the as-received FexOy mainly by surface chemical environments (i.e., surface anisotropy). The first assumption under this expectation is that the surface states of


Ma et al.

Figure 11. The effects of silica coating on the effective anisotropy constant for the as-received FexOy, the core-shell FexOy@SiO2 A and B, and the surface-modified FexOy samples.

the particles coated with silica of different thickness are basically the same. This assumption is fairly reasonable. The second assumption is that the distance among magnetic cores, and thus the magnetic core-core interaction is similar in the surfacemodified sample and the as-received FexOy, proved true experimentally by AC measurements and Vogel-Fulcher fitting. For fine and interacting nanoparticles, contributions from surface anisotropy and particle interactions to the effective anisotropy constant have to be taken into account. Mathematically, this concept can be expressed as37

Keff ) KV + Ks + Kinter

(7)

where Keff, KV, Ks, and Kinter represent the total effective, volume, surface, and interaction anisotropy constants, respectively. Ks is largely influenced by size,38,39 and shape40,41 as well as chemical environments near particle surface.38 In particular, the formation of new covalent bonding at particle surface can result in the modification of surface spin structure. For example, Grasset and coauthors15 found that the cusp in the FC curve of zinc ferrite nanoparticles, possibly as the consequence of a disordered surface spin structure, disappeared after the particle surface was coated with silica. As for magnetic interparticle interactions, according to Philipse’s work,11 magnetic core-core interactions vary with the size of cores and the thickness of isolating layers. Bigger magnetic nanoparticles require thicker layers to screen the magnetic interaction among particles. The model study shows that for a magnetic core of 20 nm in diameter, the distance between cores should be larger than 23 nm to avoid magnetic interactions between them. That is why we are interested in preparing and studying the core-shell samples with an over 10 nm shell; the magnetic particles we used have the largest size of about 20 nm. Further increase of the shell thickness may not yield any evident, further reduction in the magnetic dipolar interaction but will reduce the overall magnetization, which is disadvantageous. For the FexOy@SiO2 and surface-modified particles having comparable log-normal size distributions, we can easily obtain by numerical fit with eq 1 a variation of about 0.3 × 105 J/m3 in the effective anisotropy constant (Table 1), originating mainly from the change of magnetic interparticle interactions. By comparing the effective anisotropy constants of the as-received and surface-modified FexOy, it can be seen the silica coating increases the surface anisotropy, and thus Keff by about 0.5 × 105 J/m3. The increase in Ks due to surface silica coating is counter-balanced by the decrease in Kinter due to the shielding of core-core magnetic interactions by the shells leading to only slightly different Keff in the core-shell samples and the as-received FexOy. Consequently, both kinds of samples without a significant difference in their mean diameter demonstrate similar mean deblocking temperatures TB. Figure 11 has been drawn to demonstrate the effects of silica coating on the effective anisotropy constant for the three types of samples.

The highest Tpeak in the as-received FexOy is attributed to the presence of the highest portion of large-sized particles, or more accurately, nanoparticle clusters, formed due to strong particle interactions during the sample preparation for magnetic measurements. For the surface-modified FexOy, which shows a similar log-normal size distribution as the core-shell samples, the higher Tpeak originates from the increase in effective anisotropy constant due to the increase in surface anisotropy and magnetic interparticle interactions. We would like to point out that although the ZFC model yields a single effective anisotropy constant for each sample, actually there is a distribution of anisotropy constants depending on the size and surface anisotropy of each nanoparticle and its interaction with others. In some cases, one needs to consider the individual particle’s contribution to magnetic behavior such as Tbranch. ZFC/FC curves reflect the blocking behavior with the ZFC/ FC bifurcation point Tbranch dominated by large-sized particles or agglomerates. As magnetic dipolar interactions scale as D6 (D is the diameter of magnetic particles),11 larger particles suffer from larger magnetic interparticle interactions; therefore their blocking behavior tends to be affected more by magnetic interparticle interactions than that of smaller ones in the same sample. As a result, Tbranch is often sensitive to the presence of large particles/agglomerates and magnetic interparticle interactions. This explains why the uncoated FexOy sample shows a significantly higher Tbranch than the core-shell samples in which magnetic interparticle interactions have been largely reduced even for large-sized cores. Similarly, this can be used to explain a much higher Tbranch in the surface-modified sample than those in the core-shell samples although here another factor, surface anisotropy, also contributes. To summarize, the variations of TB for the samples FexOy@SiO2 A and B are readily understood in terms of the variation in the lognormal size distribution whereas the surfacemodified FexOy sample, according to the eq 1, presents an important variation of the effective anisotropy constant compared to other samples. Although the core-shell samples and the as-received sample show similar TB and Keff due to the counteracting effects of Ks and Kinter, the former shows much lower Tpeak and Tbranch, mainly associated with decreased magnetic interparticle interactions. 5. Conclusions We have presented a controlled method for synthesizing silica shells onto iron oxide nanoparticles. Magnetic properties of the as-received and coated nanoparticles were investigated in detail to understand the effect of silica coating on blocking behavior. It is found that after forming over 10-nm silica shells onto the iron oxide nanoparticles, the mean deblocking temperature and effective anisotropy constant remain similar owing to the comparable, counteracting contributions from surface anisotropy and magnetic interparticle interaction. The ZFC peak temperature and the ZFC/FC branching temperature, however, decrease sharply, mainly attributed to the reduction of the magnetic corecore interaction supported by the investigation on AC behavior. The beneficial effect of decreasing magnetic interparticle interactions, by synthesizing about 12-nm silica shell, to the blocking behavior is obvious. However, there must be a critical value of silica shell thickness at which point all magnetic core interactions disappear and the further increase of the shell thickness will then have no effect on the blocking behavior. Modelization also will be carried out on superparamagnetic

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