Research on Modification of Fe₃O₄ Magnetic Nanoparticles with Two Silane Coupling Agents

Abstract

As a novel functional nanomaterial, Fe₃O₄ magnetic nanoparticles (MNPs) modified by different surfactants have attracted and are attracting worldwide interest. In this research, we introduced two different silane coupling agents to modify Fe₃O₄ MNPs instead of a single surfactant to achieve complete coating and functionalization. The modification mechanism was also explained. Techniques such as TEM, XRD, FT-IR, TG-DSC, and VSM were applied to characterize the obtained modified Fe₃O₄ sample. From these techniques, the following information is obtained: The characteristic bands of TEOS and KH-792 were present in the FT-IR spectra and in the XPS plots of modified Fe₃O₄ MNPs, demonstrating that the silane coupling agents were present in the sample obtained after the modification. The TG analysis of the modified sample showed complete decomposition at 228 °C. The mass ratio of the sample obtained before and after the modification was close to 29:65. The XRD patterns show that the modified Fe₃O₄ MNPs possessed an identical reverse spinel crystal structure as an unmodified Fe₃O₄ sample. The modification decreased the saturation magnetization of Fe₃O₄ MNPs from 70.04 emu/g to 57.41 emu/g and the coating did not change the superparamagnetism of Fe₃O₄ MNPs.

1. Introduction

The Fe₃O₄ MNPs possess many special characteristics totally different from bulk substances, which have attracted interest both for its scientific values and technological applications. The functional materials have been exploited in many fields such as ferrofluids [1,2], magnetic record [3], biomedicine [4,5], catalyst [6], electronic technique [7], etc., owing to their unique strong superparamagnetism, high saturation magnetization, and good biological compatibility. Various applications require the introduction of special functional groups to realize new features. Moreover, Fe₃O₄ is prone to be oxidized to Fe₂O₃ and totally lose magnetism due to its high surface activity [8]. The functionalization of Fe₃O₄ MNPs by surfactants can meet the two requirements. Oleic acid [9], lauric acid [10], SDBS [11], PEG [12], etc., are common surfactants used to modify Fe₃O₄ MNPs. Additionally, all these surfactants are hydrocarbons and their chemical properties are active and easy to be dissolved by solvents, thus stripping from the Fe₃O₄ surface and losing the modification effect. Furthermore, Fe₃O₄ modified by these conventional surfactants lacks low temperature adaptability and biocompatibility, so its applications are limited. Recently, some scientists have applied silane coupling agent as a surfactant to modifying Fe₃O₄ MNPs.

In Chen’s study, silane coupling agent KH550 was used to modify NdFe₁₂N_x magnetic powders and prevented the NdFe₁₂N_x powders from corrosion and oxidation to ensure excellent durability and stability [13]. In Zhu’s experiment, four types of silane coupling agents were used to modify HNTs/Fe₃O₄ composite respectively, and it was found that the anilino-methyl-triethoxysilane (KH-42)-modified HNTs/Fe₃O₄ composite exhibited the highest capacity for adsorption [14]. To enhance the dispersive capacity in organic solvents, KH-570 was used to modify nano-Sb₂O₃ particles by a covalent bond (Sb-O-Si) [15]. Fe₃O₄ was coated by silica via hydrolysis and the condensation of TEOS and the mean size was 216.9 nm. The mean size was 275 ± 16.1 nm and the saturation magnetization was 2.9 emu/g when using sodium citrate prior to coating with silica in stabilization. Subsequently, the surface of particles was functionalized by amine groups using N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (KH-792). The results obtained by zeta potential revealed that the highest value of the isoelectric point change, indicating a more efficient surface functionalization, occurred when the KH-792 concentration of 90 mM was used [16]. In Abedi’s study, a citric acid-modified isocyanate silane coupling agent (MMSN-NCO-CA) was used and the magnetic nanoparticles they synthesized with MMSN-NCO-CA were smaller in pore size by only 3.52 nm; this magnetic particle could be used in the medical field [17].

All these experiments mentioned above used a single silane coupling agent as a surfactant, since the application of mixed silane coupling agents has not been developed. When modified by a silane coupling agent, SiO₂ usually deposits on the surface of Fe₃O₄ MNPs and forms the core–shell structure, but it is difficult to continue functionalization on the SiO₂ surface. The chance of a further reaction to prepare other composites is restrained. At the same time, the excessive deposition of SiO₂ leads to a size increase in Fe₃O₄ MNPs, which is not conducive to the preparation of stable and suspended ferrofluid based on Fe₃O₄ MNPs. In this research, we introduced two silane coupling agents to modify Fe₃O₄ MNPs to solve these problems. The modification mechanism of silane coupling agents was explained and the properties of modified Fe₃O₄ were tested and analyzed.

2. Materials and Experiments

2.1. Materials

Ferrous sulfate (FeSO₄·7H₂O) and ferric trichloride (FeCl₃·6H₂O) were purchased from Xilong Scientific Co., Ltd. (Shantou, China). Absolute ethyl alcohol and ammonium hydroxide (25%) were procured from Beijing Chemical Industry Group Co., Ltd. (Beijing, China). The silane coupling agents TEOS (Tetraethyl orthosilicate) and KH-792 (N-(2-aminoethyl)-3-aminopropyltrimethoxysilane) were obtained from Beijing Chemical Industry Group Co., Ltd. (Beijing, China). and Yangzhou Wanhe Chemical Co., Ltd. (Yangzhou, China), respectively. The purity grades of the chemicals analytical reagent (AR) and used as received without further purification. The water used in all the experimental processes was deionized water (the resistivity is 18.2 ΜΩ.cm) filtered using an ultrapure water machine (PSDK2-20-C) in the lab.

2.2. Experiments

Co-precipitation [18,19] is a simpler, more tractable and efficient method to control the size and composition of Fe₃O₄ MNPs. Bare Fe₃O₄ MNPs synthesized using co-precipitation follow the chemical equation: Fe²⁺ + 2Fe³⁺ + 8OH⁻ = Fe₃O₄ + 4H₂O. Under certain reaction conditions, the chemical reaction continues, resulting in a size increase in bare Fe₃O₄ MNPs. Both TEOS and KH-792 are used as surfactants to coat the surface of bare Fe₃O₄ MNPs to terminate the co-precipitation chemical reaction of Fe₃O₄ MNPs and control the particle size. Considering Fe²⁺ will be partially oxidized into Fe³⁺ in the non-protective atmosphere, the mole ratio of Fe²⁺:Fe³⁺ was adjusted to 1:1.75. The color of iron salts dissolved with 25% ammonia added as a precipitant at less than 60 °C and by stirring immediately changed from reddish-brown to black. Under stirring for 20 min, bare Fe₃O₄ MNPs were generated at this stage. The Fe₃O₄ MNPs were washed with deionized water until pH = 9 and then dispersed in anhydrous ethanol, heating to 65 °C and adding TEOS and KH-792 into the flask for 2 h with continuous stirring. The modified Fe₃O₄ MNPs were collected by magnet and obtained after washing, filtering, and drying.

2.3. Characterization

The morphology and composition of synthesized Fe₃O₄ nanoparticles were characterized by X-ray diffraction (XRD, Brucker D8, Berlin, Germany), Fourier transform infrared spectroscopy (FT-IR, Bruker VERTEX 70V, Woodlands, TX, America), and transmission electron microscopy (TEM, JEOL-2100, Japan Electronics Corporation, Tokyo, Japan), respectively. The size distribution was tested by Malvern Zatersizer Nano-ZS. The surface tension of silane coupling agents was tested using a JZ-200 surface tension meter. The contact angles were tested using a photographic process in the angle measurement method (HKCA-15 HARKE, Beijing, China, droplet 3 μL). The Fe₃O₄ synthesized were diluted and ultrasonically dispersed for 20 min and tested using the Zatasizer Nano ZS90 laser particle size analysis from Malvern, UK. The magnetic property was tested using a vibrating sample magnetometer (VSM, Lakeshore 8604,Lexington, MI, America), the thermal behavior was tested using TG-DSC (Netzsch STA449F3, Selb, Germany), and the elementary composition was tested using X-ray photo-electron spectroscopy (XPS, PHI Quantera SXM, ULVAC-PHI, Chigasaki, Japan).

3. Results and Discussion

3.1. Research on Modification Mechanism of Two Silane Coupling Agents

The reaction model of silane coupling agent modification on the Fe₃O₄ surface can be divided into four steps, as shown in Figure 1.

(1) Hydrolysis: Silicon nucleus is negatively charged due to nucleophilic reaction of OH^-, leading to the shift of electron cloud to OC₂H₅ or OCH₃, which weakens and breaks the Si-O bond. Si-OH is formed during the hydrolysis reaction. (2) Condensation reaction: -Si(OH)₃ as a weak acid turns into an alkali after dehydrogenation in alkaline conditions. Dehydration polymerization occurs when nucleophilic attack is launched on silicon atoms. Polysiloxane (Si-O in main chain) with a low molecular weight is formed by dehydration condensation between the adjacent Si-OH of silanol. (3) Form of hydrogen bond: In aqueous solution, Fe₃O₄ shows Lewis acidity owing to the interaction between uncompleted 3d orbitals in Fe²⁺ and Fe³⁺ and lone-pair electrons in 2p orbitals in oxygen atom of H₂O [20]. The interaction reduces dissociation energy and weakens O-H bond in H₂O molecules, resulting in chemisorption of -OH on Fe₃O₄ surface [21]. Furthermore, hydrogen bond is formed between Si-OH in siloxane and -OH on Fe₃O₄ surface. (4) Dehydration and form of covalent bond: by continuous heating during the modification process, a covalent bond is formed between siloxane and -OH on Fe₃O₄ surface by dehydration reaction and finally stable coating is formed. It is generally believed that only one of the three -Si-OH is bonded with -OH on Fe₃O₄ surface and the other two have condensation reaction with -Si-OH in other silane or present a free state.

3.2. Thermal Stability Analysis

The experiment temperature of modified Fe₃O₄ was 25–350 °C and that of the unmodified Fe₃O₄ was 25–800 °C, as shown in Figure 2 and Figure 3. The decomposition temperature of the silane coupling agents, oxidation of Fe₃O₄, mass ratio, modification amount of silane coupling agent, and the thermal oxygen stability of Fe₃O₄ were obtained by studying the mass change, heat release, and absorption of Fe₃O₄ before and after modification.

From the two thermogravimetric loss curves, it can be seen that 6% and 7.2% weight losses were generated in the range of 25–100 °C, mainly from the water of hydration due to nanoscale surface active adsorption. Because of the hydrophobic effect of silane coupling agents, the adsorption water of modified Fe₃O₄ was slightly less than that of exposed Fe₃O₄. The Fe₃O₄ modified by TEOS and KH-792 gradually produced about 29% weight loss with the temperature increasing from 160 °C to 228 °C and simultaneously a large strong exothermic peak was formed in the DSC curve. The weight loss mainly resulted from heat a decomposition reaction of silane coupling agent under air atmosphere with volatile products generated such as NH₃ and CO₂. The mass ratio of silane coupling agent to Fe₃O₄ core was 29:65. It is inferred that the applied temperature range of modified Fe₃O₄ was not higher than 228 °C.

However, there was no obvious mass loss at 150 °C and 590 °C in unmodified Fe₃O₄ as the temperature increased, but two strong exothermic peaks were caused by a phase transition of Fe₃O₄ [22]. Fe₃O₄ and γ-Fe₂O₃ are cubic systems and their lattice constants are very close. The oxidation of Fe₃O₄ to γ-Fe₂O₃ only increases the valence, that is, the Fe²⁺ changes to Fe³⁺ and there is no change in the position of the nucleus. However, α-Fe₂O₃ is an orthorhombic system and its lattice constant is far from that of γ-Fe₂O₃ (α-Fe₂O₃ lattice constant a = 0.5034 nm, while γ-Fe₂O₃ lattice constant a = 0.8347). When γ-Fe₂O₃ is transformed into α-Fe₂O₃, the vacancies of Fe³⁺ and Fe²⁺ in the crystal cells both need to move greatly, the adjustment range is large, and more energy is released. In addition, the oxidation of Fe₃O₄ to γ-Fe₂O₃ and α-Fe₂O₃ is a process of crystallization and the maximum exothermic peak appears at 590 °C. Our research showed that the superparamagnetism of Fe₃O₄ disappeared at 260 °C and only γ-Fe₂O₃ remained. When the temperature is higher than 650 °C, all the γ-Fe₂O₃ would turn into non-magnetic single α-Fe₂O₃ [23]. According to the analysis of the TG–DSC curves, the silane coupling agent has been successfully modified on the Fe₃O₄ surface and its thermal-oxidative stability has been improved.

3.3. Component Analysis

(1)

FT-IR spectroscopy analysis

The FT-IR spectroscopy of Fe₃O₄ MNPs modified by TEOS and KH-792 silane coupling agents is shown in Figure 4.

In the infrared spectrum, 564 cm⁻¹ corresponds to the strong stretching vibration absorption peak of Fe-O in Fe₃O₄ [24]. Firstly, there is a large and strong absorption peak at 3430 cm⁻¹, which can be indexed to Si-OH. Si easily adsorbs -OH in an alkali solution after the breakage of Si-OC₂H₅ and Si-OCH₃. Secondly, the characteristic peak partly belongs to the -OH’ stretching vibration absorption peak caused by the absorbed water on the Fe₃O₄ surface. Thirdly, the strong vibration absorption peak of N-H in KH-792 also strengthens the peak. The stretching vibration absorption peak of Si-O-Si is usually between 1029–1101 cm⁻¹, for example 1042 cm⁻¹ in this figure, which indicates that dehydration polymerization occurred between the two silane coupling agents; 1630 cm⁻¹ is the in-plane bending vibration absorption peak of N-H in KH-792. The weak absorption peaks at 2975 cm⁻¹ and 1405 cm⁻¹ belong to the stretching vibration absorption peak and in-plane bending vibration absorption peak of C-H in -OCH₃ or -OCH₂CH₃ inside the chains of the silane coupling agent. The existence of C-H indicates that the -OCH₃ or -OCH₂CH₃ together with Si is only partially replaced by -OH. The characteristic peaks of Fe₃O₄ and silane functional groups appear simultaneously in the infrared spectrum of the sample, which indicates that the silane coupling agent has been successfully modified on the surface of Fe₃O₄ MNPs. The characteristic peaks of Fe₃O₄ and the silane coupling agent simultaneously exist in the infrared spectrum, which indicates that the two silane coupling agents have been successfully modified on the Fe₃O₄ surface.

(2)

X-ray photoelectron spectroscopy analysis

The XPS photoelectron spectrum of Fe₃O₄ modified by TEOS and KH-792 silane coupling agents is shown in Figure 5.

The XPS analysis shows that the samples modified by TEOS and KH-792 mainly contained Fe, O, Si, C, N, and H. This is consistent with the elements in Fe₃O₄ MNPs and the silane coupling agent molecules. It proves that silane coupling agent has been successfully coated on the Fe₃O₄ surface. The main binding energies of the Fe2p spectrum are at 723.53 eV and 709.94 eV, which belong to the spectral band of Fe2p_1/2 and Fe2p_3/2 [12]. The O1s spectrum with a binding energy of 529.20 eV is mainly composed of the lattice oxygen (O²⁻) in the Fe₃O₄ and the -O- of the ether bond in the silane coupling agent. The energy spectra of Si2p_1/2 and Si2p_3/2 in Si2p appear at 101.21 eV and 92.98 eV; 284.31 eV represents C1s binding energy of –CH₃ or –CH₂CH₃ in the silane coupling agent; 391.88 eV is the binding energy of N1s in KH-792. The element contents obtained by XPS photoelectron spectroscopy are shown in

Table 1. Element contents of modified Fe₃O₄ after TEOS and KH-792.

Element	Fe2p	O1s	Si2p	C1s	N1s
Content	17.34	51.49	9.14	17.61	4.42

.

3.4. Crystal Structure

The XRD patterns of modified Fe₃O₄ MNPs are shown in Figure 6. The XRD pattern shows that there were obvious diffraction peaks at 2θ = 30.0°, 35.4°, 43.1°, 53.6°, 56.9°, and 62.4° corresponding to the crystal planes of Fe₃O₄ on (220), (311), (400), (422), (511), and (440), respectively. The position and intensity of the main diffraction peaks were basically consistent with the characteristic peaks of the PDF card (JCPDS 19-0629) [25]. The silane coupling agent was noncrystal and without diffraction peaks. The modification caused no change of the Fe₃O₄ crystal phase of the inverse spinel structure. The average grain size of the Fe₃O₄ nanoparticles modified by a silane coupling agent was approximately 9.7 nm as calculated by the Scherrer formula. ${\mathrm{D}}_{\mathrm{hkl}} = {\mathrm{Nd}}_{\mathrm{hkl}} = \frac{0.89\mathit{\lambda }}{{\mathit{\beta }}_{\mathrm{hkl}}\cos \mathit{\theta }}$, λ is the X-ray wavelength of incident light; θ is the Bragg Angle of diffracted hkl; and β_hkl is the half-peak width of the diffracted hkl in radians. K = 0.89 and Cu-Kα, λ = 0.15406 nm. The silane coupling agent modified on the Fe₃O₄ surface formed an organic layer and prevented Fe³⁺, Fe²⁺, and OH^- from crystal growing, thereby controlling the particle size of Fe₃O₄.

3.5. Micromorphology Analysis

The TEM patterns of Fe₃O₄ MNPs before and after modification with silane coupling agents are shown in Figure 5. It can be seen from the figure that the exposed Fe₃O₄ nanoparticles agglomerated seriously before modification and the boundary was not clear, as shown in Figure 7a. Most of the particles adhered to each other to form agglomerations, which could not be dispersed by ultrasound. However, the Fe₃O₄ nanoparticles modified by the silane coupling agent were spherical and well-dispersed with a clear boundary, without adhesion and agglomeration, and the particle size decreased also. Before the addition of silane coupling agent, the precursor Fe³⁺ and Fe²⁺ were in an oversaturated state. The numerous crystal nuclei formed instantly when precipitant was added. The heat released by Fe₃O₄ crystal promoted the rapid growth of the crystal further. Moreover, the Fe₃O₄ particles agglomerated spontaneously to reduce the surface energy owing to the surface effect of the nanoparticles. Meanwhile, mutual attraction between magnetic dipole moments also led to continuous agglomeration. The addition of the silane coupling agent formed chemisorption on the Fe₃O₄ surface to terminate the co-precipitation reaction in time to prevent the continuous co-precipitation and the uncontrollable crystallization [26]. At the same time, the steric effect functioned on the Fe₃O₄ surface by the swing of the silane coupling agent hydrophobic chain, which inhibited the agglomeration of Fe₃O₄ and reduced Fe₃O₄ size, thereby achieving the purpose of controlling the particle size and distribution, as it shown in Figure 7b.

3.6. Particle Size and Size Distribution

Figure 8 shows the size distribution of Fe₃O₄ MNPs before and after modification of silane coupling agents. The size distribution of Fe₃O₄ without modification was mainly in the range of 20–40 nm and the average diameter was 30 nm. After modification, the size distribution was reduced to 7–15 nm significantly and the average diameter was 10 nm, consistent with the results calculated by XRD. The synthesis of Fe₃O₄ by co-precipitation was carried out in water. NH₃·H₂O was used as a precipitant to process the crystallization reaction. Without a silane coupling agent, the Fe₃O₄ MNPs nucleated explosively and grew quickly, while when silane coupling agents were added, chemisorption occurred easily between nanoparticles and silane coupling agents. Then, the further chemical reaction of Fe₃O₄ was blocked by modification, thereby inhibiting the growth of particles.

3.7. Low Surface Tension

The surface tension values of several surfactants are listed in Figure 9 [3]. Compared with hydrocarbon surfactants, the surface tension of KH-792 and TEOS was 24 mN/m and 25 mN/m, respectively, only slightly larger than the fluorocarbon surfactant. The siloxane side chain was perpendicular to the plane formed by the Si-O and could rotate or vibrate about the Si-O bond axis. Because the Si-O bond was longer, the hydrogen atoms on the alkyl group could be spread out similar to umbrellas. The rotation of the alkyl groups occupied a large space and increased the distance between neighboring molecules. Meanwhile, due to the shielding effect of the alkyl group, the Si-O interaction was slight, molar volume was large, and surface tension was small [27]. This is the reason why the siloxane coupling agent was easy to spread on the Fe₃O₄ interface.

3.8. Contact Angle

The photographic process was used in angle measurement, as shown in Figure 10, the contact angles of TEOS and KH-792 were 4.4° and 42.13°, respectively; they were less than 90° and could form a wetted surface with good hydrophilicity. This is the premise of the stable coating on Fe₃O₄. The smaller the contact angle is, the faster it is to form wetting and spreading on the surface of nanoparticles and the easier it is to form a solid coating. Alkoxy in both KH-792 and TEOS’s molecular chain can form a hydrogen bond with Fe₃O₄ nanoparticles, which can strengthen the force between the silane coupling agent and Fe₃O₄ nanoparticles.

3.9. Magnetic Analysis

The hysteresis curves before and after modification with the silane coupling agent are shown in Figure 11.

It can be seen from the hysteresis curves that the coercivity and remanence of Fe₃O₄ approximate zero and the magnetization curves are similar to “S” either with or without modification, because the modification of the silane coupling agent does not change the superparamagnetism of Fe₃O₄. The saturation magnetization of the bulk Fe₃O₄ was 92 emu/g [28], while that of the synthesized Fe₃O₄ MNPs without modification was 70.04 emu/g, which is mainly due to the fact that the nanometer size effect leads to the increase in the disordered atomic structure and decrease in the magnetic moment. The silane coupling agents slightly reduced the saturation magnetization to 57.41 emu/g after modification. There are three reasons for this: First, the silane coupling agents are non-magnetic material and the per unit mass of the magnetic material can be reduced [23]. Moreover, the interaction between Fe₃O₄ MNPs was weakened by modification. With the increase in nanoparticles in specific surface areas, the surface defects increased, meaning it was harder for magnetic moments to align in the direction of the magnetic field. This effect has been proved by other surfactants [29]. Finally, the magnetic performance is proportional to the particle size [30]. The growth of Fe₃O₄ was limited and particle size decreases due to the coating of the silane coupling agents, so the magnetization intensity was also decreased.

4. Conclusions

The two silane coupling agents, TEOS and KH-792, were modified on Fe₃O₄ MNPs surface using hydrolysis, a condensation reaction, and dehydration. The silane coupling agents did not change the inverse spinel structure of Fe₃O₄ MNPs but could refine the grains. The average particle size decreased from 30 nm to 10 nm0 and the Fe₃O₄ MNPs were in homogeneous dispersion after modification. The characteristic peaks of TEOS and KH-792 were present in the FT-IR and XPS of the modified Fe₃O₄ MNPs, proving that silane coupling agents have been modified on the surface of Fe₃O₄ MNPs. The modification decreased the saturation magnetization of Fe₃O₄ MNPs from 70.04 emu/g to 57.41 emu/g and the superparamagnetism still remained. Additionally, the coating was completely decomposed at 228 °C. The mass ratio of coating to bare Fe₃O₄ was close to 29:65. Fe₃O₄ was partially transformed into γ-Fe₂O₃ near 150 °C and γ-Fe₂O₃ into α-Fe₂O₃ near 600 °C. Over 680 °C, the Fe₃O₄ was finally oxidized to α-Fe₂O₃ and lost its magnetic properties. The modified nanoparticles can be combined with polyethylene glycol, water-based cellulose, or other small molecules of silica gel to achieve potential applications in areas such as biomedicine, 3D printing, or soft robot development.