An investigation on synthesis of Fe₃O₄@nSiO₂@mSiO₂ hybrid particles and peroxidation

Fe₃O₄ particles has attracted lasting interests not only due to its half metallic characteristic of high Curie temperature, large spin polarization, but also from its non-toxic and excellent bio-compatibility.^1–3 Thus the synthesis, assembling and their various properties are extensively exploited for potential applications on biomedicine engineering such as drug delivery, cancer treatment, biosensor, magnetic resonance imaging and so on.^4–9 Most of the applications require nanomaterial to be chemically stable, uniform in size and well dispersed in liquid media. In order to prevent naked nano-particles of Fe₃O₄ to aggregate into large cluster and loose the specific properties, it is quite necessary to combine magnetic particles with the high concentration of surfactant.^10,11 Various inorganic and polymeric materials have been reported as surfactant of magnetic particles.^12–15 Among them, silica coating as an inorganic surfactant is considered to be an ideal supporting material because of its surface activity, reliable chemical stability, biocompatibility, and reactivity with various coupling agents.^16,17 Classical Stöber-method has been demonstrated to be often used for synthesis of the silica shell, however, it yields basically thin thickness on the core surface, which restricts their surface activity and perfectness. One improved method has been developed for preparing mesostructured silica shells with high multi-pore.^18–20 The mesoporous channel system in the silica shell is believed to play an important role in not only the active surface of a core directly accessible, but also the penetration of reactants, products, and solvents through the shell and thus increase of shell thickness. However, in the wrapping process for fabrication of core-shell structured particles, it is easily neglected that a great deal of oxygen in SiO₂ may influence the stoichiometric ratio of Fe₃O₄, and in turn change the magnetic behaviors, which cause the reported magnetism of Fe₃O₄ particles are controversial. Stoichiometric Fe₃O₄ crystal has a cubic inverse spinel structure in which 8 tetrahedral sites (site A) are occupied by Fe³⁺ ions and 16 octahedral sites (site B) are shared by Fe²⁺ ions and Fe³⁺ ions. Its magnetization is determined by the difference of magnetic moment between A and B sites. Peroxidation might cause the lack of Fe ions in B site, thus decrease of the magnetization. In this paper, we prepare the submicron –micron sized Fe₃O₄@nSiO₂@mSiO₂ core-shell structured composite spheres with thicker mesoporous shell by using the improved conventional Stöber poly-condensation method successfully, and examine the magnetic properties and the mechanism behind the magnetism by occupation of Fe ions in Fe₃O₄@nSiO₂@mSiO₂ core-shell particles by Mössbauer spectra.

Fe₃O₄ particles with different size are prepared by the solvothermal methods. Different content of FeCl₃·6H₂O (0.002 M - 0.015 M) is dissolved in 40 ml of ethylene glycol (EG) to form aqueous solution, and 3.6 g of NaAc and 1 g of polyethylene glycol (PEG) are added to the solution subsequently with continuous stirring for 10 min. Then the mixture is placed in a 50 mL Teflon autoclave and reacts at 200 °C for 24 h. After the autoclave is cooled to room temperature naturally, the products are separated by centrifugation with stirring speed of 6000 r/min, washed with ethanol and deionized water several times, and dried for 8 h at 50 °C. The different size of Fe₃O₄ nanoparticles adjusted by changing the concentration of FeCl₃·6H₂O in the solution are marked as A1, A2, A3, A4 from 250 nm to 1 μm.

Fe₃O₄@nSiO₂@mSiO₂ composite particles with layers of mesoporous SiO₂ structure are assembled by improved Stöber method.²¹ Fe₃O₄ particles are firstly treated by dilute sulphuric acid to achieve the rough surface before the shell coating. Two steps are utilized for coverage of shell, firstly a very thin (about 10 nm) layer is covered around the magnetic core with a SiO₂ preferred surface for achieving Fe₃O₄@nSiO₂. During this process, the treated Fe₃O₄ products are dispersed in 80 ml ethanol and 20ml DI water, and then 1.0 ml ammonia (NH₃·H₂O, 28 wt.%) and 0.0001 M of tetraethoxysilane (TEOS) with continuous stirring for 5 h at room temperature to achieve the thin silica-loving shell. The second step is performed by changing content of TEOS during the reaction process for controlling the thickness of SiO₂ shell to obtained Fe₃O₄@nSiO₂@mSiO₂. In this process, first step reactions are repeated and Cetyltrimethyl Ammonium Bromide (CTAB) and NH₃·H₂O are added for mesoporous SiO₂ structure. The particles with different thickness of shell Fe₃O₄@nSiO₂@mSiO₂ are obtained and marked as B1, B2 and B3.

The XRD patterns and SEM images of Fe₃O₄ particles are shown in Fig. 1. From the XRD pattern, all the diffraction peaks are well indexed as inverse spinel structure without impurity peaks, indicating that all samples are actually in single phase of Fe₃O₄. The average lattice constant of 0.838 nm is consistent with that of bulk material. The average grain size can be estimated from the linewidth of main diffraction peaks by using Scherrer formula without considering possible contributions from crystal stress

Here, D is the diameter of the grains, λ is the wavelength of X-ray, 1.542 Å, β is the linewidth of half height of the peak, θ is the diffraction angle, and k is a constant of 0.89. The average grain size increases from 12 nm to 17 nm with increasing the content of FeCl₃·6H₂O, corresponding to particle size, as shown in Table I.

From the SEM images, Fe₃O₄ particles are shown the good shape, uniform size and smoothed surface. The average particle size is increased from 250 nm to 1020 nm with increasing the content of FeCl₃·6H₂O, indicating that the Fe₃O₄ particles consist of small single crystal grains with size of 12 nm to 17 nm, as shown in Table I. From TEM, it is interesting to find that the different orientations of electronic diffraction pattern is exhibited for different size of particles. For a larger size of particles(A3), the electronic diffraction pattern is arranged in order, but for smaller size of particles(A1), the electronic diffraction pattern show a random around a circle, as shown in inset of Fig. 1(a) and (b). Combining the XRD pattern, Fe₃O₄ particles with different size are all polycrystalline and the size of grains in the different size of particle are nearly the same. We speculate that grains may be close more tightly in the smaller particles, and more dispersed in the larger particles, it is possibly arranged in ordered orientations for larger particles as shown in inset of Fig. 1(b).

The hysteresis loops with saturation magnetization, and coercivity measured with VSM at room temperature are also shown in Fig. 1(d). The magnetic parameters of Fe₃O₄ particles exhibit a strong dependence on the particle size as shown in inset of Figs. 1(d). The saturation magnetization M_s increases from 51.0 emu/g to 83.1 emu/g while the coercivity H_c increases from 82.2 Oe to 165.6 Oe with increasing the particle size. Comparing with the bulk value of 92 emu/g,²² M_s is lower, and H_c is large, which may be caused by the small grains in the particles. The size of Fe₃O₄ nanograins in the Fe₃O₄ particles are smaller than the size of single domain and increase with the particle size, thus we guess that could cause the increase of H_c. The dispersed nanograins might lead to a lower magnetization due to the weaker interactions between them, and the M_s increases with increasing particle size due to the increase of nano-grain size, which need to further study.

Fe₃O₄ particles with average particle size of 510 nm (A₂) are selected to prepare the Fe₃O₄@SiO₂ core-shell structure by improved Stöber method. Fig. 2(a–c) shows the TEM images of Fe₃O₄@nSiO₂@mSiO₂ hybrid particles with different mesostructured SiO₂ coating layer thickness from 35 nm, 80 nm to 150 nm, indicating that the thickness of SiO₂ coating can be adjusted successfully. IR spectra of Fe₃O₄ and Fe₃O₄@nSiO₂@mSiO₂ hybrid particles, as shown in Fig. 2(d), exhibits that the products after coating is different from those before coating. The band at 568 cm^-1 is ascribed to Fe-O-Fe stretching vibrations of Fe₃O₄,²³ while the three bands at 467 cm^-1, 1096 cm^-1 and 796 cm^-1 are assigned to the Si-O-Si stretching from the SiO₂,²⁴ which is evidence of the existence of SiO₂.

The XRD spectra of the Fe₃O₄(510 nm)@SiO₂(80 nm) nanoparticles before and after coverage of SiO₂ layer are performed and all diffraction peaks can be corresponding to those of Fe₃O₄ spectra, indicating that the SiO₂ coating layer is amorphous, consistent with the report of Fe₃O₄@SiO₂ particles synthesized by the same method.²⁵ The XRD spectra reveals that the composition and crystal structure of Fe₃O₄ are remain unchanged during the process of covering SiO₂ layer.

Fig. 3 shows that hysteresis loops of the Fe₃O₄@nSiO₂@mSiO₂ hybrid particles with different shell thickness, the hysteresis loop of naked Fe₃O₄ core is also listed for reference. From the Fig. 3, we see that the magnetization of Fe₃O₄@nSiO₂@mSiO₂ particles are 63.2, 49.8, 39.4 emu/g respectively, lower than that of naked Fe₃O₄ particles (64.2 emu/g) due to increasing mass of non-magnetic SiO₂ shell in the samples. The real magnetization of core-shell particles could be estimated by the ratio of the mass between nonmagnetic shell and magnetic core as following expression and would be equal to the saturation magnetization of naked Fe₃O₄ particles, 64.2 emu/g.

Assuming that the saturation magnetization value of the Fe₃O₄ core in various samples with different shell thickness are the same with that of naked core particles, the fraction between mass of SiO₂ layer and Fe₃O₄ core in Fe₃O₄@nSiO₂@mSiO₂ composition material are 0.02, 0.29, 0.63. Based on the spherical mode, the ratio of volume between shell and core are obtained as 0.47, 1.27, 3.01. Taking the density of 5.18×10^-3 kg/m³ for Fe₃O₄, we calculate that the density of SiO₂ are 0.22, 1.18, 1.09 ×10^-3 kg/m³, much lower than the bulk value of 2.6×10^-3 kg/m³, indicating that the SiO₂ shell in Fe₃O₄@SiO₂ composition particles are not so compact and is indeed a mesostructured layer.

The coercivities of Fe₃O₄@nSiO₂@mSiO₂ composition particles with different thickness are 184, 237 and 296 Oe, much larger than that of naked Fe₃O₄ particles of 141 Oe. The increasing coercivity is associated with the packing of particles, and nonmagnetic shell extend the length between magnetic core, described as Hc(p)=Hc(0)(1-p), in which p is a packing factor and is defined as the fraction between magnetic volume V_m to total volume V. If we assume that H_c(p) is the coercivity of composite particles and H_c(0) is the coercivity of the single Fe₃O₄ particle. When the sample is a single particle, the packing factor p equals to 0, H_c(p) = H_c(0), and when the sample is compact completely, p equals to 1, H_c(p) = 0. Thus the thicker the SiO₂ shell is, the smaller the V_m is, and p gets smaller for the increase of shell thickness. For our samples, the pecking fraction p of Fe₃O₄@nSiO₂@mSiO₂ composite particles with different shell thickness are 0.68, 0.44, 0.25, respectively, thus the calculated H_c(p) increases with increasing SiO₂ shell thickness of Fe₃O₄@nSiO₂@mSiO₂.

The Mössbauer measurements and fitting are further preformed to investigate the microstructure of the Fe₃O₄ (510 nm) @SiO₂(80 nm) nanoparticles before and after coverage of SiO₂ layer. As can be seen from Fig. 4, it is found that the coverage layer have a significant effect on the Mössbauer spectra of the composite particles. The ratio of intensity between first split peaks are almost inversed. By using a Normos least-squares fitting program, the fitting curve fits the experimental data well as shown in Fig. 4, and all the parameters for different samples are obtained, as listed in Table II. For particle samples before and after coating, the Mössbauer spectra are well fitted by two split sextet patterns, corresponding to the behavior of Fe ions in A and B ferromagnetic sublattice site of Fe₃O₄.²⁶ 

As it knows, for a perfect bulk magnetite, A site represents tetrahedral site occupied by Fe³⁺ and B site stands for octahedral sites sharing with Fe³⁺ and Fe²⁺. From Table II, we see that the hyperfine field (HMF) in A site and B site show little change before and after covering SiO₂ layer, indicating that the effect of hyperfine interactions could be neglected. However, The isomer shift obviously decreases in A site and increases in B site when particles are covered SiO₂ layer, implying that the environment surrounding Fe ions may be changed in A and B site, which can be seen clearly from the relative absorption intensity RAI ratio I_A/I_B. The ratio of I_A/I_B increase from 35.0% to 36.8% significantly in A site, and decrease from 65.0% to 63.2% in B site, indicating that the transfer of Fe ion from B site to A site occurs with coverage of SiO₂ shell. Such a transfer of Fe ion from B site to A site will causes the decrease of magnetization in principle. For our sample, the transfer of Fe ion from B site to A site might be related to lacking of Fe²⁺ ions, the Fe²⁺ vacancies after covering SiO₂ layer may cause the change of Fe³⁺/Fe²⁺ ratio in octahedral sites.

It is valuable to note that the relative absorption intensity RAI ratio I_A/I_B in B site (65.0%) is nearing 2 times of that in A site (35.0%) for our naked Fe₃O₄ particles, which is close to the theoretical situation. For idea case, there are 24 Fe ions in one unit cell of crystal, eight Fe³⁺ in A site and eight Fe³⁺ plus eight Fe²⁺ in B site. Fe ions in B site are not fixed its valence and electrons transfer between Fe³⁺ and Fe²⁺, which causes the better conductivity of Fe₃O₄. Thus the ratio of Fe ionic number between B and A sites should be 2 for stoichiometric Fe₃O₄. However, for our Fe₃O₄@SiO₂ composition particles, I_A/I_B in B site is decreased to 63.2% and that in A site is increased to 36.8%, which is believed as the origin of the peroxidation. The rich oxygen might come from SiO₂ layer in the interface, which cause the change of the number of Fe ions in A and B site in our Fe₃O₄@SiO₂ composition particles.

In summary, Fe₃O₄ nanoparticles with uniform size, regular shape and good dispersibility are prepared by the solvothermal methods. The size of Fe₃O₄ nanoparticles can be adjusted by changing the concentration of the reactants. The saturation magnetization and the coercivity increases with increasing the particle size. Fe₃O₄@SiO₂ composite particles with layers of mesoporous SiO₂ structure are assembled by improved Stöber method. The different shell thickness of SiO₂ layer can be adjusted from 35∼150nm by the amount of TEOS and multi-coating process. The magnetization of Fe₃O₄@SiO₂ composite particles decrease comparing to its naked core nanomaterial due to its weakened interactions. Mössbauer spectra show that the coverage SiO₂ have a significant effect on the micro-structure. The relative absorption intensity in A site increases as Fe₃O₄ particles are covered by SiO₂ layer, indicating that there are amount of Fe ions from B site to A site, correspondingly, the isomer shift in A site also show an decrease, which is believed that the peroxidation occur during the process of covering SiO₂.

This work was supported by NSFC (No. 11504047, 61427812, 51571062).