A facile bi-phase synthesis of Fe₃O₄@SiO₂ core–shell nanoparticles with tunable film thicknesses

Abstract

Monodisperse Fe₃O₄@SiO₂ nanoparticles are prepared using hydrazine as a catalyst via a biphase approach without any alcohols or surfactants. Fe₃O₄ seeds can be dispersed well in this system. The sizes of Fe₃O₄@SiO₂ nanoparticles with a single core could be regulated from 20 nm to 50 nm corresponding to SiO₂ shell thickness from 3 nm to 17 nm. Core-free SiO₂ nanoparticles are not observed in this system. The coating process can be implemented at a temperature greater than 90 °C, which results in a short coating duration from 2 h to 8 h for different shell thicknesses. Hydrazine can prevent the Fe₃O₄ core from oxidization during coating at this temperature. Fe₃O₄@SiO₂ nanoparticles have high chemical stability and magnetic saturation. A plausible formation mechanism of these nanoparticles is also presented.

---

Introduction

Magnetic Fe₃O₄ nanoparticles have been extensively applied in various fields,^1–6 such as ferrofluids, magnetic resonance imaging (MRI), drug delivery, labeling and sorting of cells, separation and purification. Most of these applications require the synthesized Fe₃O₄ nanoparticles to be chemically stable and well-dispersed in liquid media, especially in hydrophilic media, which is suitable for biological applications. Several methods have been exploited directly to synthesize water-soluble Fe₃O₄ nanoparticles.^7–11Surface coating modification has been explored to improve dispersibility and chemical stability.^1,3,6 The SiO₂ coating has been commonly used because of its good chemical inertness, hydrophilicity, non-toxicity, and the ease of further surface modification.

The Stöber process and emulsion method are two common approaches to synthesize SiO₂ coated Fe₃O₄ core–shell nanoparticles. The monodisperse Fe₃O₄ core–shell nanoparticles with tunable SiO₂ shell thickness can be prepared with both hydrophilic and oil-dispersed Fe₃O₄ as seeds via emulsion method.^12–15 However, this method is complex such that water, ammonia and TEOS contents, surfactant and helper surfactant, and Fe₃O₄ concentration, directly determine the ultimate structure of Fe₃O₄@SiO₂ core–shell nanoparticles. A large amount of surfactants used to form the emulsion system yields difficulty in separating core–shell nanoparticles from the reaction system.

The Stöber process is introduced to coat SiO₂ on the hydrophilic Fe₃O₄ nanoparticles.^16–21 No surfactants are used in this process. However, alcohol must be used as the reaction medium. The existence of alcohol with a lower polarity relative to the aqueous solution induces significant core aggregation in the Stöber system.^22–24 Yang et al.¹⁶ reported that the surface modification of Fe₃O₄ with citrate groups could improve the ultimate dispersibility of Fe₃O₄@SiO₂ core–shell nanoparticles prepared via the Stöber approach; however Fe₃O₄/SiO₂ composite particle aggregates were attained through a similar process by Hong et al.²⁵ Researchers have pretreated Fe₃O₄ cores with active silica to avoid the formation of aggregation prior to or during the reaction.^26–28 Coating a uniform thin SiO₂ shell Fe₃O₄ to form the Fe₃O₄@SiO₂ core–shell particles with a small size below 50 nm via the Stöber process is difficult. Monodisperse Fe₃O₄@SiO₂ core–shell nanoparticles were obtained until the SiO₂ shell thickness was up to 45 nm.¹⁹ However, a thick SiO₂ shell significantly decreases the magnetization saturation. Yang et al.¹⁶ reported that when the silica shell thickness of magnetic particles was increased to 56 nm, the magnetization saturation dropped dramatically to 3 emu g⁻¹ from 61 emu g⁻¹. SiO₂ magnetite particles with a diameter of 700 nm were also reported to have a magnetization saturation of only ∼0.3 emu g⁻¹.²⁹

Tetraethyl orthosilicate (TEOS) is commonly used to coat silica on Fe₃O₄. The slow hydrolysis of TEOS results in a long duration for the coating of Fe₃O₄ with SiO₂ shell.^12,14,19 Increasing water content accelerates the hydrolysis, but this increase leads to the formation of more aggregates and free-core SiO₂ particles.^13,22 Free-core SiO₂ particles can be reduced by increasing the Fe₃O₄ cores contents.¹² However, a high Fe₃O₄ concentration results in the formation of large magnetite aggregates.^19,26 The formation of free-core SiO₂ particles can be avoided by an equivalently fractionated drop method, but the duration of coating is extended to 128 h.²² Morel et al.³⁰ reported that a sonochemical approach to the synthesis of SiO₂ coating magnetite particles with 1–3.5 nm SiO₂ shell, and the coating process could be accomplished 3 h after ultrasonic treatment.

The silica coating process requires a large number of organic solvents such as alcohol or surfactants, which are adverse to the dispersion of magnetic core and separation of final products. Recently, a bi-phase method has been adopted to synthesize monodisperse SiO₂ nanoparticles with diameters ranging from 12 nm to 45 nm.^31–33 SiO₂ particles with a diameter of ∼200 nm were also synthesized by employing this method.³⁴ Compared with the Stöber approach and the emulsion method, the biphase method requires only water as the reaction medium, with no surfactants used. Water-insoluble TEOS forms an oil phase and separates from water.

In this paper, the biphase method was used to synthesize of Fe₃O₄@SiO₂ core–shell nanoparticles. The alcohol-free aqueous solution is expected to avoid the formation of aggregates, and higher magnetite core concentration is used to eliminate free-core SiO₂ particles. The coating process was implemented at temperatures much higher than 60 °C, which were used in the previous biphase method,^31–33 to accelerate the hydrolysis of TEOS and to exclude alcohol from this process. A short duration of coating is expected. The thin silica shell can be coated on Fe₃O₄ nanoparticles, because the small SiO₂ particles are facilely synthesized via the biphase method.^31–33

Experimental section

Materials

All chemicals were used without any further purification. FeCl₂·4H₂O, FeCl₃·6H₂O (99%), hydrazine (80%), triethanolamiene (TEA, >95%), NaOH (97%) and tetraethyl orthosilicate (TEOS, >99%) was obtained from Aladdin. Absolute ethanol and ammonia (28%) was obtained for Kelong Chemical.

Synthesis of Fe₃O₄ nanoparticles

The Fe₃O₄ nanoparticles were prepared via a modified coprecipitation method by using FeCl₂·4H₂O and FeCl₃·6H₂O as precursors.^18,35 In a typical synthetic procedure, 5 mL of ammonia and 2 mL of hydrazine were diluted with deionized water to 50 mL. 20 mL freshly prepared mixture of 1 g of FeCl₂·4H₂O and 2.7 g of FeCl₃·6H₂O in an aqueous solution was dropwise added into the solution and was stirred at 90 °C for 30 min. 4 g of citric acid dissolved in 10 mL of water was also added into this solution and was stirred at 90 °C for 1.5 h. The products were collected via magnetic separation after cooling to room temperature and were washed alternately with water and acetone thrice. Fe₃O₄ nanoparticles modified with citric groups were attained after vacuum-drying at 30 °C for 12 h. The Fe₃O₄ powders could be redispersed in water to form magnetic fluids.

Synthesis of SiO₂ nanoparticles

The SiO₂ nanoparticles were prepared via a modified biphase method.^31–33 TEOS was directly added into 50 mL of deionized water containing the catalyst and was stirred at 100 °C. Triethanolamine or hydrazine was used as the catalyst. The contents of the catalyst or TEOS were changed to control the product size, and the stirring rates were also varied. The reaction ceased after 20 h. The dispersion was drawn for detection via TEM.

Synthesis of Fe₃O₄@SiO₂ core–shell nanoparticles

The Fe₃O₄@SiO₂ core–shell nanoparticles were synthesized via the biphase method. In a typical synthetic process, 50 mg of Fe₃O₄ was added into 100 mL of deionized water containing 0.5 mL of the hydrazine catalyst. The suspensions were sonicated for 10 min to form Fe₃O₄ magnetic fluids, and 0.4 mL of TEOS was added into the magnetic fluids in a single step. The mixtures were refluxed and stirred at 90 °C for 2 h, and Fe₃O₄@SiO₂ nanoparticles with 6 nm SiO₂ shells were formed. To obtain thicker SiO₂ shells, batch TEOS addition is adopted when the TEOS content exceeds 0.4 mL, that is, after TEOS is added into the reaction system for 2 h, the next batch (0.4 mL per time) of TEOS is added again. And these processes were repeated several times. Other agents also were used as catalysts to attempt the synthesis of Fe₃O₄@SiO₂ instead of hydrazine. An equal amount of triethanolamine (TEA) was used as the catalyst instead of hydrazine. The core–shell nanoparticles were synthesized by directly using ammonia or NaOH as the catalyst. The pH was adjusted to about 9–10, which was consistent with that of hydrazine or TEA. The silica-coating magnetic particles were separated via centrifugation at 13 000 rpm for 30 min at room temperature and were vacuum-dried 50 °C for 12 h. The as-prepared dispersions were drawn and diluted for detection via TEM without further treatment.

Characterization

X-ray diffraction (XRD) pattern was obtained on a Beijing Pgeneral XD-3 X-ray diffractometer with CuKα radiation. Transmission electron microscopy (TEM), High-resolution TEM (HRTEM) and Electron diffraction pattern were performed on a Tecnai G2 F20 system operated at an accelerating voltage of 200 kV. And TEM of the SiO₂ nanoparticles were obtained using Hitachi H7500 system at an accelerating voltage of 80 kV for electrons. The Fourier transform infrared (FTIR) spectroscopy performed by a Nicolet Magna-IR 75° FT-IR spectrometer operating in transmission mode with KBr pellets technique was used to characterize Fe₃O₄ and Fe₃O₄@SiO₂ nanoparticles. The magnetic properties were performed on a superconducting quantum interference device (SQUID) at room temperature. X-Ray photoemission spectroscopy (XPS) was performed using a Thermo ESCALAB 250Xi instrument equipped with a monochromatic Al anode X-ray gun. The Zeta potential and dynamic light Scattering was measured by using Malvern ZEN3690 instrument.

Results and discussions

Fig. 1a shows the XRD pattern of the as-prepared Fe₃O₄ nanoparticles. All diffraction peaks match well with those from the JCPDS card (no. 65-3107) for magnetite, which indicates that the product is magnetite Fe₃O₄. The high-resolution transmission electron micrograph (HRTEM) image (inset of Fig. 1b) illustrates the highly crystalline nature of the nanoparticles with clear lattice fringes. The interplanar spacing is around 0.49 nm, which corresponds to the (111) plane of magnetite Fe₃O₄ and is in agreement with the value of 0.484 nm taken from the JCPDS card (no. 65-3107). The IR result shows an exclusive absorption peak of Fe–O at 567 cm⁻¹, and no higher frequency band at 632 cm⁻¹ is observed, which is the characteristic peak of γ-Fe₂O₃ (ref. 36 and 37) (Fig. 1c). This peak further reveals that the as-resulting samples are pure magnetite Fe₃O₄. The as-prepared Fe₃O₄ dispersion presents a typical brown-black Fe₃O₄ (Fig. 1d), which is different from the clear reddish-brown γ-Fe₂O₃.^11,38 In this reaction system, the N₂H₄ added prior to co-precipitation avoids the oxidation of Fe₃O₄ because of the strong reduction ability of N₂H₄. A pure phase Fe₃O₄ can thus be obtained in this system without the protection of an inert gas.

Fig. 1 XRD pattern (a), TEM (b) images, and IR spectrum (c) of the as-prepared Fe₃O₄ nanoparticles, and the picture of Fe₃O₄ magnetic fluids (d). Inset: HRTEM of Fe₃O₄ nanoparticles.

The average particle size calculated using Scherrer's formula for the strongest peak (311) (Fig. 1a) is about 15 nm. The particle size can be observed directly via TEM (Fig. 1b). The size distribution ranges from 10 nm to 30 nm, with most particles having sizes between 10 and 20 nm. The results are in agreement with those obtained via XRD, but some larger particles about 30 nm can also be observed.

Ultrasonic treatment or surface modification must be implemented to the naked magnetic cores to maintain the dispersibility and stability of magnetic cores in the solvent.^6,39 Fig. 1b indicates that the as-prepared Fe₃O₄ nanoparticles are well-dispersed in aqueous solution, which can be attributed to the introduction of citrate groups in the synthesis of magnetic particles. The functional groups can be identified via FT-IR in Fig. 1c. The bands at 1627, 1558, and 1398 cm⁻¹ correspond to the stretching frequency of the carboxylate groups,^9,35,40 which indicates the existence of citrate anions. These groups endow the magnetic particles more negative charges,^16,35 which enhance the dispersibility and stability of the as-prepared Fe₃O₄ particles in aqueous solution. This observation is supported by the ferrofluidic behavior of Fe₃O₄ dispersion in Fig. 1d.

A biphase method has been developed to prepare the monodisperse SiO₂ nanoparticles by using the basic amino acid as a catalyst.^31–33 We also prepared uniform-sized SiO₂ via the similar method by using hydrazine or TEA as the catalyst in place of the basic amino acid. The TEM image (Fig. S1†) indicates that the diameter of SiO₂ nanoparticles ranges from 10 nm to 50 nm and can be facilely tuned by changing the stirring rates, and the contents of TEOS and the catalyst. Hydrazine or TEA can be used as a catalyst to prepare uniform-sized SiO₂ nanoparticles with tunable diameters via the biphase method.

A modified biphase method was attempted to prepare the Fe₃O₄@SiO₂ core–shell nanoparticles. Fig. 2 shows the TEM images of these particles synthesized at different temperatures with hydrazine as the catalyst. Core–shell particles are hardly observed at 60 °C (Fig. 2a). Non-discrete particles are obtained, and the isolated Fe₃O₄ particles coalesce with the irregular amorphous SiO₂ to form the farraginous Fe₃O₄ and SiO₂ composites. Some Fe₃O₄@SiO₂ core–shell nanoparticles exist when the reaction temperature was increased to 70 °C (Fig. 2b), and isolated SiO₂ particles are also found. No farraginous Fe₃O₄ and SiO₂ composites are observed. Few core-free SiO₂ particles are observed when the reaction temperature was further increased to 80 °C, and a large number of single-core Fe₃O₄@SiO₂ particles are observed (Fig. 2c). The core-free silica particles completely disappear when the reaction temperature exceeded 80 °C, and some well-dispersed particles of Fe₃O₄@SiO₂ core–shell structure are still observed (Fig. 2d and S2†). Thus, the reaction temperature should higher than 80 °C to prepare well-dispersed Fe₃O₄@SiO₂ particles without core-free SiO₂. A temperature of 90 °C is more considered to prepare Fe₃O₄@SiO₂ particles in the reaction system.

Fig. 2 TEM image of Fe₃O₄@SiO₂ core–shell nanoparticles prepared at different temperatures with hydrazine as the catalyst, (a) 60 °C, (b) 70 °C, (c) 80 °C, (d) 90 °C.

The catalyst influences the formation of SiO₂ in the biphase method.^32,33 Different catalysts have been employed to prepare Fe₃O₄@SiO₂ core–shell nanoparticles. The use of TEA as the catalyst can effectively regulate the diameter of the as-prepared SiO₂ nanoparticles. The synthesis of Fe₃O₄@SiO₂ nanoparticles with the TEA catalyst is expected to succeed. Unfortunately, no core–shell Fe₃O₄@SiO₂ particles are observed (Fig. 3a). A large number of core-free SiO₂ particles with smooth surfaces are haphazardly combined with Fe₃O₄ particles. NaOH and ammonia also were also used as catalysts. Fig. 3b and c show that a similar morphology can be observed for these catalysts. The use of NaOH and ammonia as catalysts can both yield nanoparticles with core–shell structures compared when the TEA catalyst is used. However, a large number of core-free SiO₂ particles exist, and the SiO₂ coating layer deposited on the surface of Fe₃O₄ particles is irregular. Some Fe₃O₄@SiO₂ particles with incomplete SiO₂ coating layers are obtained for NaOH and ammonia catalysts compared when the hydrazine catalyst is used. The latter is the most suitable in preparing Fe₃O₄@SiO₂ particles via the biphase method.

Fig. 3 TEM images of Fe₃O₄/SiO₂ composite nanoparticles with different catalysts. (a) TEA, (b) NaOH and (c) ammonia.

The coating of magnetic nanoparticles with SiO₂ by using TEOS is a very slow process. This coating process takes over 10 h or a few days to obtain a homogeneous SiO₂ coating layer on the surface of these particles irrespective of the use of the Stöber process or the microemulsion method.^12,14,19,22 We also analyzed the effect of the duration on coating and the film thickness via the biphase method. The coating process was implemented from 15 min to 20 h. Fig. 4 and S3† show the TEM images of Fe₃O₄@SiO₂ particles, in which 2–3 nm SiO₂ thin shell coated on Fe₃O₄ surface can be observed 15 min after the coating process. However, the thickness of SiO₂ film is inconsistent (Fig. 4a). The thickness of this film increased to around 4 nm when the coating duration was extended to 45 min (Fig. 4b). The thickness remains at about 6 nm when the duration of coating exceeds 1 h. The shells of Fe₃O₄@SiO₂ nanoparticles coated for 1 h (Fig. 4c) are more irregular relative to those coated for a longer time (Fig. 4d and S3†). The uniformity of the SiO₂ shell remains almost consistent when the duration of coating exceeds 2 h, which indicates that the duration of coating to this time is enough to coat SiO₂ on the surface of Fe₃O₄ particles to form uniform and well-dispersed Fe₃O₄@SiO₂ core–shell nanoparticles. Completing the coating process in 2 h is difficult for the Stöber progress or the microemulsion method.^12,16 A rapid sonochemical approach has also been presented to synthesize Fe₃O₄@SiO₂ nanoparticles with 3–3.5 nm SiO₂ shell in 3 h,³⁰ but the ultrasonic treatment results in the surface oxidation of magnetic Fe₃O₄ nanoparticles.^30,39

Fig. 4 TEM image of Fe3O4@SiO2 core–shell nanoparticles prepared at different reaction time. (a)15 min, (b)45 min, (c)1 h, (d)2 h.

The TEOS content adjusts the thickness of SiO₂ shell to obtain silica-capped Fe₃O₄ nanoparticles of tunable thicknesses via the sol–gel process and microemulsion method.^12,21 These contents also are employed to control the thickness of silica in the biphase method. The equivalently fractionated drop method was employed to avoid the formation of core-free SiO₂ particles because an excessive TEOS content results in undesirable core-free SiO₂ particles.¹² Thus, batch TEOS addition is adopted in this method when the TEOS content exceeds 0.4 mL. After TEOS is added into the reaction system for 2 h, 0.4 mL of TEOS is added per time and per batch. Fig. 5 shows the images of the silica-coated Fe₃O₄ particles with varying TEOS contents. Fig. 5a–d show that the silica shell with a thickness ranging from 3 nm to 17 nm can effectively be regulated by changing the TEOS contents. The shape of Fe₃O₄ particles is also maintained during the silica coating process (Fig. 6a). The as-prepared particles retain the polydispersity of original Fe₃O₄ particles with 3 nm SiO₂ shell. Increasing the TEOS content increases the SiO₂ shell thickness, and the as-resulted Fe₃O₄@SiO₂ particles with more regular shell exhibit monodispersity because of the reduction in the relative size distribution.²¹ Core-free SiO₂ particles are not observed even for a TEOS content of 1.6 mL (Fig. 5d). The DLS results also show that the hydrodynamic sizes of samples are slightly larger than those obtained via TEM (Fig. S4†), which indicates that Fe₃O₄@SiO₂ nanoparticles are highly dispersed in aqueous solution.

Fig. 5 TEM image of Fe₃O₄@SiO₂ core–shell nanoparticles with a TEOS content of (a) 0.2 mL, (b) 0.4 mL, (c) 0.8 mL, (d) 1.6 mL.

Fig. 6 XRD pattern (a) and IR spectra (b) of Fe₃O₄@SiO₂ with different contents of TEOS.

The presence of the SiO₂ shell can also be confirmed via XRD measurements. In Fig. 6a, a broad peak near 2θ of about 23° is also observed aside from the peaks belonging to magnetite Fe₃O₄ (JCPDS no. 65-3107), which demonstrates the existence of amorphous SiO₂ shell.^15,28 The broad peak becomes stronger with increasing TEOS content, which indicates that the SiO₂ shell thickness also increases. The IR spectra also confirm the existence of this shell. The bands at 460, 800, and 1108 cm⁻¹ correspond to Si–O bending, Si–O–Si bending, and Si–O–Si stretching, respectively^41,42 (Fig. 6b). This indicates the SiO₂ shell is successfully coated on the surface of Fe₃O₄ particles. The XPS spectra of Si2p also confirm the coating of Fe₃O₄ with this shell (Fig. S5†).

The Fe₃O₄ particles have an isoelectric point of pH ≈ 7 (ref. 39) that do not benefit from the stability of Fe₃O₄ particles in aqueous solution. However, Fig. 7 shows that the as-synthesized Fe₃O₄ modified by citrate groups has an isoelectric point of pH ≈ 3.1, which is lower than that of the unmodified Fe₃O₄. Fe₃O₄ coated with the SiO₂ shell has a lower isoelectric point of pH ≈ 1.9 and is almost consistent with that of SiO₂ particles.⁴³ The Fe₃O₄@SiO₂ particles have a higher negative charge than Fe₃O₄ nanoparticles. The low isoelectric point and high negative charges endow Fe₃O₄@SiO₂ magnetic nanoparticles with better stability and dispersibility, which indicate that the enhancement of these properties for Fe₃O₄ nanoparticles can be facilely achieved by coating with the SiO₂ shell.

Fig. 7 The plots of zeta potential of the Fe₃O₄ particles and Fe₃O₄@SiO₂ with 1.6 mL TEOS.

The magnetic properties of the magnetic particles were also investigated. Fig. 8 shows the magnetization curve of the as-synthesized magnetic Fe₃O₄ particle and the Fe₃O₄@SiO₂ with different TEOS contents. The as-made magnetic Fe₃O₄ particles have a magnetization saturation (M_s) of about 69 emu g⁻¹. However, the M_s of Fe₃O₄@SiO₂ nanoparticles decreases to 29, 15.7, and 8.2 emu g⁻¹ upon increasing the TOES contents to 0.4, 0.8, and 1.6 mL, respectively. Although the M_s of these particles significantly decreases relative to that of uncoated Fe₃O₄, these values indicate a significant enhancement over previously reported studies^29,41. A higher Ms can be obtained for multiple-core Fe₃O₄ or aggregates of Fe₃O₄.¹⁷ However, the final size of the composite particles exceeds 100 nm, which limits their biological applications. The as-prepared Fe₃O₄@SiO₂ with a TEOS content of 0.4 mL has an M_s of 29 emu g⁻¹ and a diameter of 20–30 nm from the TEM image (Fig. 5b). The composite particles exhibit good magnetic responsiveness to the applied magnetic field (inset of Fig. 8). All as-prepared particles in this work also present a very low coercivity (Hc) of about 10 Oe, which is much lower than that of the theoretical value for superparamagnetic particles (Hc ≤ 5 mT = 50 Oe).^30,44 Fe nanoparticles with Hc of 50 Oe are also considered superparamagnetic by Nuetzel et al.,⁴⁵ which indicates that the as-synthesized Fe₃O₄ and Fe₃O₄@SiO₂ particles are superparamagnetic.

Fig. 8 Magnetization curves of the as-synthesized products with different TEOS contents. Inset: the photo of Fe₃O₄@SiO₂ dispersions with 0.4 mL TEOS and its magnetic responsiveness.

In this study, the coating process was fulfilled at a temperature above 90 °C, at which the synthesis of Fe₃O₄ generally must be protected with an inert gas from the oxidation.^18,35 The as-prepared Fe₃O₄@SiO₂ particles with hydrazine as the catalyst can avoid the oxidation of the Fe₃O₄ core without an inert gas. The inset of Fig. 9 shows typical brown-black Fe₃O₄ dispersions retained for the hydrazine catalyst, and typical brown-red γ-Fe₂O₃ is observed for the ammonia catalyst as well as for TEA and NaOH catalysts (results not shown). The difference is also evident from the Fe2p high-resolution XPS spectra (Fig. 9). No satellite peak of Fe2p 3/2 at about 720 eV, the characteristic doublet used in γ-Fe₂O₃,^46,47 is observed for the hydrazine catalyst compared with the XPS spectrum for the ammonia catalyst. A binding energy of about 710.6 for Fe2p 3/2 eV is close to that published in literature,⁴⁶ which reveals that the magnetite Fe₃O₄ in the magnetic silica particles can be well-retained with the hydrazine catalyst. The XPS spectrum for this catalyst is weaker than that for the ammonia catalyst because a more complete and thick shell is coated on the Fe₃O₄ surface for the hydrazine catalyst, which was confirmed via TEM. Morel et al.³⁰ found that the Fe2p 3/2 peak would disappear with the increasing of SiO₂ shell thickness.

Fig. 9 Fe2p high resolution XPS spectra for the coated magnetic particles using NH₃ and N₂H₄ as the catalysts. Inset: their dispersions picture.

We also directly analyzed the chemical stability of Fe₃O₄@SiO₂ particles via comparative acid-corrosion experiment in 1 M HCl. Fig. 10 shows that Fe₃O₄ and its coated dispersions are eroded, but a significant difference can be observed between the two. The Fe₃O₄ dispersion became light after 4 h, and the solution appears completely limpid after 8 h. However, the coated Fe₃O₄ has a much higher chemical stability. The Fe₃O₄ dispersion coated with 0.4 mL of TEOS slightly changes after 72 h and becomes light brown-black after 40 days (Fig. 10b). Increasing the TEOS content increases the SiO₂ shell thickness and significantly enhances the resistance to hydrochloric acid corrosion. Fe₃O₄ coated with 1.6 mL of TEOS hardly changes for 72 h and the dispersion becomes slightly light after 40 days. The SiO₂ shell coated on the Fe₃O₄ particles via the biphase method can protect the Fe₃O₄ core and gives the Fe₃O₄@SiO₂ a high chemical stability.

Fig. 10 Dispersion of magnetic particles in 1 M hydrochloric acid. (a) Fe₃O₄, (b) Fe₃O₄@SiO₂ with 0.4 mL TEOS, (c) Fe₃O₄@SiO₂ with 1.6 mL TEOS.

This method can facilely synthesize monodisperse Fe₃O₄@SiO₂ nanoparticles with a single Fe₃O₄ core without formation of free-core SiO₂ particles. The biphase method can also synthesize Fe₃O₄@SiO₂ nanoparticles with sizes below 50 nm and the reaction duration is short. The solvent only comprised alcohol-free water in this method compared with those via the Stöber and microemulsion methods, thereby ensuring that the Fe₃O₄ cores have good and stable dispersions (Fig. 1b and d). No aggregates of Fe₃O₄ particles were coated by the SiO₂ shell based on the TEM images, although Fe₃O₄ with a high concentration of 500 mg L⁻¹ was used as the seeds in this study, which was much higher than those in literature.^20,26 This indicates that the biphase system is much more propitious to the dispersibility and stability of Fe₃O₄ nanoparticles prior to or during coating.

The water-insoluble TEOS directly forms an oil phase in the biphase system from the absence of alcohol. This phase can be scattered to some oil droplets by stirring (Fig. S6†). Hydrolysis of TEOS therefore occurs only at the interface between the water and the TEOS phase in this system.³² The concentration of SiO₂ species from the hydrolysis of TEOS directly determines the nucleation mode, that is, heterogeneous or homogeneous nucleation. The latter needs a higher supersaturated concentrate¹² and result in the formation of the core-free SiO₂ particles, thereby requiring the maintenance of a low supersaturated concentrate. The hydrolysis of TEOS occurring only at the interface results in a low SiO₂ species concentration. The development of the hydrolysis reduces the size of TEOS droplet, which results in a smaller interface area and retards this reaction. Thus, a low properly supersaturated concentrate can always remain in biphase system. Reaction temperatures above the azeotropic point of ethanol and water mixtures (about 78 °C) eliminate ethanol from the hydrolysis of TEOS, which retains the biphase of TEOS and water. These temperatures accelerate this reaction³⁰ and shorten the reaction duration.

Scheme 1 summarizes a possible formation mechanism of Fe₃O₄@SiO₂ core–shell nanoparticles. Fe₃O₄ magnetic fluids are dispersed better after adding hydrazine because of higher negative charges (Fig. 7). TEOS forms some oil phase droplets while stirring in alcohol-free water. The hydrolysis of TEOS occurs at the interface between the oil phase (TEOS) and water accompanied with the occurrence of condensation in the aqueous solution, which results in the formation of SiO₂ species. These species are deposited on the surface of Fe₃O₄ nanoparticles to form SiO₂ coatings when the concentration is increased to the supersaturated concentrate as determined by the heterogeneous nucleation. The TEOS droplets shrink and eventually disappear when the SiO₂ shell thickness is increased. The increase in the SiO₂ shell thickness is also terminated. Adding more TEOS into the system thickens the SiO₂ shell and facilely produces Fe₃O₄@SiO₂ nanoparticles with the tunable film thicknesses.

Scheme 1 A plausible mechanism of formation of Fe₃O₄@SiO₂ core–shell structure.

Conclusions

We present a biphase approach to prepare Fe₃O₄@SiO₂ core–shell nanoparticles. In this method, an aqueous solution free from alcohols or surfactants is used as the solvent, and oil-phase TEOS droplets are formed in this solution by stirring. Fe₃O₄ seeds have a high dispersibility in this solution. Hydrazine is used as the catalyst and directly protects Fe₃O₄ cores from being oxidized to γ-Fe₂O₃ during coating at temperatures above 90 °C. The SiO₂ shell thickness can facilely be regulated from 3 nm to 17 nm by changing the TEOS content. The formation of core-free SiO₂ particles can be completely avoided, and monodisperse Fe₃O₄@SiO₂ core–shell nanoparticles are obtained via the biphase method. A short duration of coating is observed, and the core–shell nanoparticles have a high chemical stability and magnetization saturation. We explain the plausible formation of Fe₃O₄@SiO₂ core–shell structure via this method.

Acknowledgements

The work described in this paper was supported by a grant from “the China Postdoctoral Science Foundation (2013M531925)” and “Fundamental Research Funds for the Central Universities (XCJK2014C038)”

Notes and references

1. U. Jeong, X. W. Teng, Y. Wang, H. Yang and Y. N. Xia, Adv. Mater., 2007, 19, 33.
2. Y. W. Jun, J. H. Lee and J. Cheon, Angew. Chem., Int. Ed., 2008, 47, 5122.
3. A. H. Lu, E. L. Salabas and F. Schüth, Angew. Chem., Int. Ed., 2007, 46, 1222.
4. R. R. Qiao, C. H. Yang and M. Y. Gao, J. Mater. Chem., 2009, 19, 6274.
5. R. Hao, R. J. Xing, Z. C. Xu, Y. L. Hou, S. Gao and S. H. Sun, Adv. Mater., 2010, 22, 2729.
6. S. Laurent, D. Forge, M. Port, A. Roch, C. Robic, L. V. Elst and R. N. Muller, Chem. Rev., 2008, 108, 2064.
7. J. P. Ge, Y. X. Hu, M. Biasini, C. L. Dong, J. H. Guo, W. P. Beyermann and Y. D. Yin, Chem. – Eur. J., 2007, 13, 7153.
8. C. Hui, C. M. Shen, T. Z. Yang, L. H. Bao, J. F. Tian, H. Ding, C. Li and H. J. Gao, J. Phys. Chem. C, 2008, 112, 11336.
9. J. Liu, Z. K. Sun, Y. K. Deng, Y. Zou, C. Y. Li, X. H. Guo, L. Q. Xiong, Y. Gao, Y. Li and D. Y. Zhao, Angew. Chem., Int. Ed., 2009, 48, 5875.
10. Z. Li, H. Chen, H. B. Bao and M. Y. Gao, Chem. Mater., 2004, 16, 1391.
11. Y. S. Kang, S. Risbud, J. F. Rabolt and P. Stroeve, Chem. Mater., 1996, 8, 2209.
12. H. L. Ding, Y. X. Zhang, S. Wang, J. M. Xu, S. C. Xu and G. H. Li, Chem. Mater., 2012, 24, 4572.
13. M. Stjerndahl, M. Andersson, H. E. Hall, D. M. Pajerowski, M. W. Meisel and R. S. Duran, Langmuir, 2008, 24, 3532.
14. E. S. Jang, J. Korean Chem. Soc., 2012, 56, 478.
15. M. Zhang, B. L. Cushing and C. J. O'Connor, Nanotechnology, 2008, 19, 085601.
16. D. Yang, J. Hu and S. Fu, J. Phys. Chem. C, 2009, 113, 7646.
17. R. Fu, X. Jin, J. Liang, W. Zheng, J. Zhuang and W. Yang, J. Mater. Chem., 2011, 21, 15352.
18. Y. Wang, X. Peng, J. Shi, X. Tang, J. Jiang and W. Liu, Nanoscale Res. Lett., 2012, 7, 86.
19. C. Hui, C. Shen, J. Tian, L. Bao, H. Ding, C. Li, Y. Tian, X. Shia and H. J. Gao, Nanoscale, 2011, 3, 701.
20. Z. Lu, J. Dai, X. Song, G. Wang and W. Yang, Colloids Surf., A, 2008, 317, 450.
21. Y. Lu, Y. Yin, B. T. Mayers and Y. Xia, Nano Lett., 2002, 2, 183.
22. Y. H. Deng, C. C. Wang, J. H. Hu, W. L. Yang and S. K. Fu, Colloids Surf., A, 2005, 262, 87.
23. L. M. Liz-Marzán, M. Giersig and P. Mulvaney, Langmuir, 1996, 12, 4329.
24. C. Graf, D. L. J. Vossen, A. Imhof and A. V. Blaaderen, Langmuir, 2003, 19, 6693.
25. R. Y. Hong, J. H. Li, S. Z. Zhang, H. Z. Li, Y. Zheng, J. M. Ding and D. G. Wei, Appl. Surf. Sci., 2009, 255, 3485.
26. A. P. Philipse, M. P. B. V. Bruggen and C. Pathmamanoharan, Langmuir, 1994, 10, 92.
27. Z. Lu, G. Wang, J. Zhuang and W. Yang, Colloids Surf., A, 2006, 278, 140.
28. D. B. Tada, L. L. R. Vono, E. L. Duarte, R. Itri, P. K. Kiyohara, M. S. Baptista and L. M. Rossi, Langmuir, 2007, 23, 8194.
29. S. H. Im, T. Herricks, Y. T. Lee and Y. Xia, Chem. Phys. Lett., 2005, 401, 19.
30. A. L. Morel, S. I. Nikitenko, K. Gionnet, A. Wattiaux, J. Lai-Kee-Him, C. Labrugere, B. Chevalier, G. Deleris, C. Petibois, A. Brisson and M. Simonoff, ACS Nano, 2008, 2, 847.
31. T. Yokoi, Y. Sakamoto, O. Terasaki, Y. Kubota, T. Okubo and T. Tatsumi, J. Am. Chem. Soc., 2006, 128, 13664.
32. T. Yokoi, J. Wakabayashi, Y. Otsuka, W. Fan, M. Iwama, R. Watanabe, K. Aramaki, A. Shimojima, T. Tatsumi and T. Okubo, Chem. Mater., 2009, 21, 3719.
33. J. Wang, N. A. Sugawara, M. Fukao, T. Yokoi, A. Shimojima and T. Okubo, ACS Appl. Mater. Interfaces, 2011, 3, 1538.
34. K. D. Hartlen, A. P. T. Athanasopoulos and V. Kitaev, Langmuir, 2008, 24, 1714.
35. S. Nigam, K. C. Barick and D. Bahadur, J. Magn. Magn. Mater., 2011, 323, 237.
36. M. Klotz, A. Ayra, C. Guizard, C. Ménager and V. Cabuil, J. Colloid Interface Sci., 1999, 220, 357.
37. A. G. Roca, J. F. Marco, M. d. P. Morales and C. J. Serna, J. Phys. Chem. C, 2007, 111, 18577.
38. C. J. Chen, H. Y. Lai, C. C. Lin, J. S. Wang and R. K. Chiang, Nanoscale Res. Lett., 2009, 4, 1343.
39. C. Yang, G. Wang, Z. Lu, J. Sun, J. Zhuang and W. Yang, J. Mater. Chem., 2005, 15, 4252.
40. X. Liang, X. Wang, J. Zhuang, Y. Chen, D. Wang and Y. Li, Adv. Funct. Mater., 2006, 16, 1805.
41. Y. H. Lien and T. M. Wu, J. Colloid Interface Sci., 2008, 326, 517.
42. J. Chen, Y. G. Wang, Z. Q. Li, C. Wang, J. F. Li and Y. J. Gu, J. Phys.: Conf. Ser., 2009, 152, 012041.
43. Q. Liu, Z. Xu, J. A. Finch and R. Egerton, Chem. Mater., 1998, 10, 3936.
44. D. Heslop, G. McIntosh and M. J. Dekkers, Geophys. J. Int., 2004, 157, 55.
45. J. A. Nuetzel, C. J. Unrau, R. Indeck and R. L. Axelbaum, Proc. Combust. Inst., 2009, 32, 1871.
46. X. Teng and H. Yang, J. Mater. Chem., 2004, 14, 774.
47. T. Fujii, F. M. F. Groot and G. A. Sawatzky, Phys. Rev. B: Condens. Matter Mater. Phys., 1999, 59, 3195.

---

Footnote

† Electronic supplementary information (ESI) available: TEM of SiO₂ nanoparticle prepared by biphase method, TEM of Fe₃O₄@SiO₂ core–shell nanoparticles, DLS size distribution of products, XPS spectra of Fe₃O₄@SiO₂, and picture of TEOS oil droplets in mixture of TEOS and water. See DOI: 10.1039/c3ra47043a

---