Abstract
Owing to the unique order mesopores, mesoporous SBA-15 could be used as the carrier of the magnetic nanoparticles. The magnetic nanoparticles in the frame and the mesopores lead to the exchange-coupling interaction or other interactions, which could improve the magnetic properties of SBA-15 nanocomposites. Mesoporous Fe/SBA-15 had been prepared via in situ anchoring Fe2O3 into the frame and the micropores of SBA-15 using the sol-gel and hydrothermal processes. Co3O4 nanoparticles had been impregnated into the mesopores of Fe/SBA-15 to form mesoporous Fe/SBA-15-Co3O4 nanocomposites. XRD, HRTEM, VSM, and N2 physisorption isotherms were used to characterize the mesostructure and magnetic properties of the SBA-15 nanocomposites, and all results indicated that the Fe2O3 nanoparticles presented into the frame and micropores, while the Co3O4 nanoparticles existed inside the mesopores of Fe/SBA-15. Furthermore, the magnetic properties of SBA-15 could be conveniently adjusted by the Fe2O3 and Co3O4 magnetic nanoparticles. Fe/SBA-15 exhibited ferromagnetic properties, while the impregnation of Co3O4 nanoparticles greatly improved the coercivity with a value of 1424.6 Oe, which was much higher than that of Fe/SBA-15.
1. Introduction
SBA-15, a type of mesoporous zeolite, features large uniform mesopores arranged into a two-dimensional (2D) hexagonal structure with the micropores (1~2 nm) in the frame [1, 2]. Due to its unique and regular pore structures, tunable pore sizes (5–30 nm), and high surface areas, SBA-15 has found many potential applications in catalysis, separation, drug targeting, and so forth [3–11]. However, the relatively inert chemical reactivity observed in SBA-15 had significantly limited its wide practical applications. Meanwhile, chemical modification to SBA-15 was one of the approaches for reaching its full potential. After a variety of surface modifications, such as the introduction of guest molecules through coating or incorporations and surface or mesopore grafting of metal atoms, the obtained new materials were suitable for many new applications, such as catalysis, separation, and drug delivery.
A number of studies have been reported on the modification of SBA-15 with implantation of various metal atoms such as Al, Ti, Co, and Fe [11–20]. Park et al. [9] utilized polyethylene oxide (PEO) as an encapsulating agent to disperse NiO onto SBA-15. Yang et al. [18] incorporated Pd nanoparticles in the micropores of SBA-15, where selective surface functionalization of the mesopores and micropores of SBA-15 were developed. Yiu et al. [20] synthesized novel magnetic Fe metal-silica (Fe/SBA-15) and magnetite-silica (Fe3O4/SBA-15) nanocomposites with high Fe contents through temperature programmed reduction, which exhibited superparamagnetic properties with a total magnetization value of 17 emu·g−1.
Magnetic mesoporous silica materials were much in demand as promising carriers for drug delivery [10, 21–23]. Moreover, magnetic mesoporous materials were attractive supports for protein or enzymes immobilization [24, 25], with which drug effects and separation could be operated easily with a magnet. To search for mesoporous materials with the better magnetic response, a great deal of work had been done in the area. For example, Zhu et al. [10] prepared a novel magnetic and temperature-responsive drug delivery system based on poly(N-isopropylacrylamide) (PNIPAM) modified SBA-15 containing magnetic γ-Fe2O3 nanoparticles. Magnetically separable Fe/SBA-15 nanocomposites were synthesized by Lin et al. through selective deposition of Fe2O3 nanoparticles into the micropores of SBA-15, and Fe2O3 nanoparticles embedded in the micropores exhibited superparamagnetic properties [22].
In our previous work, Fe/SBA-15 [26, 27] and CoFe2O4-doped Fe/SBA-15 [28] were reported, where Fe2O3 nanoparticles were anchored into the frame or micropores and CoFe2O4 nanoparticles were confined inside the mesopores of Fe/SBA-15. Magnetic properties, including saturation magnetization intensity (Ms) and coercivity (Hc), were able to be controlled to a certain extent. In order to further adjustment of magnetic performance of SBA-15, especially for Hc, we attempted to implant Co3O4 nanoparticles into the mesopores of Fe/SBA-15 in this paper. Herein, Fe/SBA-15 was firstly prepared, and then Co3O4 nanoparticles were implanted into its mesopores (denoted Fe/SBA-15-Co3O4 below). The microstructure and magnetic properties of the obtained SBA-15 samples were analyzed by XRD, HRTEM, VSM, and N2 physisorption isotherms, and the results were discussed in detail.
2. Experimental
2.1. Synthesis
In a typical synthesis procedure, triblock copolymer P123 (EO20PO70EO20, 2.00 g) and ferric nitrate (Fe(NO3)3·9H2O) were dissolved in HCl (2.00 M, 60 mL) before tetraethoxysilane (TEOS, 4.50 mL) was added. The different amounts of ferric nitrate were added into the mixture with the Fe to Si molar ratios of 0, 0.04, 0.08, 0.12, and 0.16 in each separated synthesis. After stirring for 24 h at 313 K, the mixture was transferred to an autoclave and hydrothermally treated for another 24 h at 373 K. The obtained solid by filtration was dried at 353 K and calcined at 823 K for 6 h to remove the triblock copolymer and form Fe2O3 in the final products. The final products were denoted Fe/SBA-15, or specifically %Fe/SBA-15, where represented the molar ratio of Fe to Si.
Fe/SBA-15-Co3O4 nanocomposites were synthesized through ethanol impregnation. Owing to the mesostructure revealed below, 8%Fe/SBA-15 was adopted as a template to produce Fe/SBA-15-Co3O4 nanocomposites. After cobalt nitrates (Co(NO3)2·6H2O: 0.001, 0.005, 0.01, and 0.02 mol each) were dissolved in ethanol (25 mL), 8%Fe/SBA-15 (1.00 g) were added. All mixtures were kept stirring at room temperature until ethanol was completely volatilized. After dried at 318 K for 24 h, all samples were calcined at 823 K for 6 h with a ramp of 2°C·min−1. The Co3O4 doped samples were denoted Fe/SBA-15-Co-, where represented the mole of Co(NO3)2·6H2O in 25 mL of ethanol.
2.2. Characterizations
X-Ray powder diffraction (XRD: XD-5A, wavelength 0.154 nm) was recorded on an X-ray diffractometer equipped with a Cu target in a 2 range between 15° and 80° with a scanning rate of 0.02°/step. High resolution transmission electron microscopy (HRTEM: JEM-1200EX) was used for analyzing the microstructure of all samples. The specimens for HRTEM were well dispersed in dehydrated ethanol by ultrasound and dropped on Cu grids. Nitrogen physisorption experiments were carried out at 77 K on a Micromeritics ASAP 2020 surface area and porosity analyzer. All samples were outgassed at 200°C in the port of the adsorption analyzer for 4 h before nitrogen physisorption. Specific surface areas were calculated via the Brunauer-Emmett-Teller (BET) model, and pore size distribution curves were obtained from the adsorption branch using the Barrett-Joyner-Halenda (BJH) method. The magnetic properties of the as-prepared SBA-15 nanocomposites were measured up to 2 T at room temperature on a Vibrating Sample Magnetometer (VSM: 7407 Model) from Lake Shore Cryotronics, Inc., USA. From the obtained hysteresis loops, Ms and Hc were determined.
3. Results and Discussion
Figure 1 presented the XRD patterns of a series of all Fe/SBA-15 samples with all peaks normalized according the strongest peak in each curve. A very wide diffraction peak was found at 20–30° in all patterns that was attributed to amorphous silica. When , only a broad peak of silica was observed and the peaks of α-Fe2O3 were undetected at a high scanning angle, indicating that α-Fe2O3 nanoparticles were well dispersed in SBA-15. As , α-Fe2O3 nanoparticles with phase structure R-3c(167) were detected. Based on the previous work with small-angle XRD [26] and N2 adsorption-desorption isotherms below, α-Fe2O3 nanoparticles should exist in the frame of SBA-15. The intensity ratio of the hematite α-Fe2O3 peak to the silica peak increased with x. Such change was attributed to the better crystallization of α-Fe2O3 for the larger grain size, which could be easily detected by XRD.
The N2 adsorption-desorption isotherms (Figure 2(a)) and pore size distribution curves (Figure 2(b)) of all as-prepared Fe/SBA-15 samples were given in Figure 2, and Table 1 summarizes the data of the surface area, pore volume, and pore size of pure SBA-15 and Fe/SBA-15. Clearly, the N2 physisorption isotherms of all Fe/SBA-15 belonged to type IV classification with a type H1 hysteresis loop at high relative pressure, indicating that as-prepared Fe/SBA-15 still possessed a well-defined hexagonal pore structure same as the pure SBA-15. Furthermore, the pronounced capillary condensation step observed for all Fe/SBA-15 samples showed no significant change compared to pure SBA-15. Coupled with the pore volume results in Table 1, α-Fe2O3 doping had a little effect on the pore volume of SBA-15, and all samples showed a narrow pore size distribution. The most probable pore size of all Fe/SBA-15 samples in Figure 2(b) was always larger than the average pore size, which indicated that a considerable portion of micropores existed in the frame of SBA-15. When Fe atoms in situ replacing Si were rooted in the SBA-15 frame and form α-Fe2O3, part of α-Fe2O3 entered the micropores. As a result, the micropores were filled and the average pore size of Fe/SBA-15 increased. Particularly, the most probable pore size of Fe/SBA-15 was larger than that of SBA-15, suggesting the partial addition of α-Fe2O3 came into the micropores. Given the results in Figure 2(b) and Table 1, it could be concluded that no α-Fe2O3 presented in the mesopores of SBA-15 when , which accorded with previous studies [26]. With an increasing , the average pore size of Fe/SBA-15 increased to a maximum of 7 nm at . When , partial α-Fe2O3 nanoparticles came into the mesopores of Fe/SBA-15 and thus the average pore size decreased. As it could be seen from Table 1 that the surface area decreased, it was likely because the mesopores of Fe/SBA-15 were partially blocked.
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The above results indicated that all as-prepared Fe/SBA-15 still possessed a well-defined 2D hexagonal pore structure, which was confirmed by HRTEM image of 8%Fe/SBA-15 in Figure 3. Ordered channels in the structure were clearly observed along the [110] zone axis (perpendicular to the mesopores of SBA-15), suggesting that α-Fe2O3 nanoparticles caused no damage to the mesostructure of SBA-15. The mesopore diameter in Figure 3 was measured at about 8 nm, similar to the pore size determined by the N2 physisorption isotherms. Just like the above analysis, α-Fe2O3 nanoparticles were present in the frame or micropores of Fe/SBA-15 when .
Furthermore, the magnetic properties of as-prepared Fe/SBA-15 were characterized by VSM with results shown in Figure 4. Apparently, the addition of α-Fe2O3 nanoparticles improved the magnetic properties and all Fe/SBA-15 samples exhibited ferromagnetic properties. Hc of Fe/SBA-15 was approximately 244.4 Oe, which was much larger than that of pure α-Fe2O3 (1.0 Oe). Normally for magnetic nanoparticles, Hc was related to the diameter of the magnetic single-domain size and supermagnetism critical size. When the diameter was larger than the supermagnetism critical size while smaller than the single-domain size, Hc increased with an increasing size of nanoparticles. Such rule also governed the α-Fe2O3 nanoparticles in Fe/SBA-15. Hence, Hc of the Fe/SBA-15 samples increased with higher loading of α-Fe2O3 nanoparticles. Since α-Fe2O3 were spin antiferromagnetic material, Ms of Fe/SBA-15 was very low, Ms strengthened from 0.6 emu·g−1 to 0.85 emu·g−1 with an increasing from 4 to 16.
As shown in Table 1, the average pore size of 8%Fe/SBA-15 was the largest at about 7.0 nm, so 8%Fe/SBA-15 was chosen as a hard template to synthesize Fe/SBA-15-Co3O4 nanocomposites. As seen from normalized XRD patterns in Figure 5, Co3O4 nanoparticles were successfully implanted into 8%Fe/SBA-15, and the diffraction peak intensity of spine phase Co3O4 became larger with an increasing . The widened peak of amorphous silica observed at 20–30° becomes weak and almost disappears when , which indicates that Co3O4 nanoparticles became more crystalline with an increasing y.
Figure 6 presented the N2 adsorption-desorption isotherms (Figure 6(a)) and pore size distribution curves (Figure 6(b)) of 8%Fe/SBA-15-Co3O4, while Table 2 summarized their structural parameters. Typical type H1 hysteresis loops were observed for all isotherms in Figure 6(a), indicating a well-defined mesostructure in the hard template 8%Fe/SBA-15. However, the loop became shorter with an increasing . The relative pressure in the hysteresis loop for each sample was also enhanced with an increasing , likely due to the increase in the pore dimensions. This conclusion was confirmed below by the average pore size summarized in Table 2. It was understood that small pores were easily blocked by Co3O4 nanoparticles, which led to an increase in the average pore diameter. The pore volume of 8%Fe/SBA-15-Co3O4 decreased greatly with an increasing down to 0.19 cm3 g−1 for 8%Fe/SBA-15-Co-0.02, which was only about one fifth of 0.86 cm3 g−1 for Fe/SBA-15 and one third of 0.56 cm3 g−1 for Fe/SBA-15-Co-0.001. The most probable pore size in Figure 6(b) was similar to that of the template of 8%Fe/SBA-15, revealing only partial mesopores were blocked by the Co3O4 nanoparticles. In summary, Co3O4 nanoparticles partially occupied the mesopores of Fe/SBA-15, which resulted in the decrease of surface area and pore volume [29–31].
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An HRTEM image of 8%Fe/SBA-15-Co-0.02 was shown in Figure 7, where ordered mesopores were observed clearly along the [110] zone axis. Consistent with the N2 adsorption-desorption isotherms in Figure 6(a), Fe/SBA-15-Co-0.02 still retained the same ordered 2D hexagonal p6mm structure as pure SBA-15. The mesochannel structure of the hard template was also observed after the introduction of Co3O4. It could also be seen that 8%Fe/SBA-15-Co-0.02 actually had the ordered mesopores with a diameter of about 8 nm. Moreover, it was clear in Figure 7(a) that a part of mesopores were free of Co3O4 nanoparticles, agreeing well with the volume change in Table 2. On the other hand, Co3O4 nanowires were observed in Figure 7(b) after SBA-15 was removed with a hot NaOH solution (2.0 M). Thus, the Co3O4 nanoparticles are successfully implanted into the mesopores of Fe/SBA-15.
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Finally, the magnetic properties of 8%Fe/SBA-15-Co3O4 were measured and discussed by VSM with results summarized in Figure 8. As shown, both Hc and Ms of Fe/SBA-15-Co3O4 were greatly affected by the introduction of Co3O4 nanoparticles. The Co3O4 nanoparticles belonged to the spine phase structure consisting of . Since had a large magnetic anisotropy, it gave a higher Hc to the Co3O4 materials. Hc increased with an increasing before reaching 0.02 with the maximum value of 1424.6 Oe, which was much higher than that of Fe/SBA-15 (about 240 Oe). Meanwhile, Co3O4 was antiferromagnetic with zero net magnetic moment. As a result, Ms decreased for higher density 8%Fe/SBA-15-Co3O4 due to the addition of Co3O4 nanoparticles. Thus, Ms decreased from 0.7 emu·g−1 for 8%Fe/SBA-15 to 0.3 emu·g−1 for 8%Fe/SBA-15-Co-0.02. Compared with our previous work [28], through introducing the different magnetic nanoparticles to SBA-15, the magnetic parameters of SBA-15 nanocomposites could be controlled to a certain extent for potential applications in magnetic drug targeting.
4. Conclusion
Mesoporous Fe/SBA-15 materials were prepared by sol-gel and hydrothermal processes, and Co3O4 nanoparticles were implanted into the mesopores of Fe/SBA-15 through impregnation. XRD, HRTEM, and N2 physisorption isotherms results indicated that the synthesis of Fe2O3 loaded SBA-15 caused no damage to the microstructure of SBA-15, while the Co3O4 nanoparticles exist in the mesopores of Fe/SBA-15. Because α-Fe2O3 is spin antiferromagnetic, Fe/SBA-15 exhibits ferromagnetism with low Ms from 0.6 emu·g−1 for 4%Fe/SBA-15 to 0.85 emu·g−1 for 16%Fe/SBA-15. With the introduction of antiferromagnetic Co3O4 nanoparticles, Hc of Fe/SBA-15-Co3O4 was affected greatly and increases from 200 Oe for Fe/SBA-15 to 1424.6 Oe for Fe/SBA-15-Co-0.02, while Ms decreased slightly due to the higher density for the additional Co3O4. Such impregnation method enabled us to easily improve Ms and Hc and enhance the magnetic response, which promised great potentials in application such as magnetic drug targeting.
Acknowledgments
The research was funded by the National Natural Science Foundation of China (nos. 21001098, 51172220, and 51172219), Zhejiang Province Science Foundation (no. Z4090462), and Innovation Team Foundation of Science and Technology Department of Zhejiang Province (no. 2010R50016).