Microstructure andMagnetic Properties of Highly Ordered SBA-15 Nanocomposites Modified with Fe 2 O 3 and Co 3 O 4 Nanoparticles

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.


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][4][5][6][7][8][9][10][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.
Magnetic mesoporous silica materials were much in demand as promising carriers for drug delivery [10,[21][22][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-Fe 2 O 3 nanoparticles.Magnetically separable Fe/SBA-15 nanocomposites were synthesized by Lin et al. through selective deposition of Fe 2 O 3 nanoparticles into the micropores of SBA-15, and Fe 2 O 3 nanoparticles embedded in the micropores exhibited superparamagnetic properties [22].
In our previous work, Fe/SBA-15 [26,27] and CoFe 2 O 4doped Fe/SBA-15 [28] were reported, where Fe 2 O 3 nanoparticles were anchored into the frame or micropores and CoFe 2 O 4 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 Co 3 O 4 nanoparticles into the mesopores of Fe/SBA-15 in this paper.Herein, Fe/SBA-15 was firstly prepared, and then Co 3 O 4 nanoparticles were implanted into its mesopores (denoted Fe/SBA-15-Co 3 O 4 below).The microstructure and magnetic properties of the obtained SBA-15 samples were analyzed by XRD, HRTEM, VSM, and N 2 physisorption isotherms, and the results were discussed in detail.

Experimental
2.1.Synthesis.In a typical synthesis procedure, triblock copolymer P123 (EO 20 PO 70 EO 20 , 2.00 g) and ferric nitrate (Fe(NO 3 ) 3 •9H 2 O) 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 Fe 2 O 3 in the final products.The final products were denoted Fe/SBA-15, or specifically x%Fe/SBA-15, where x represented the molar ratio of Fe to Si.

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 2θ 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.

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 x ≤ 4, only a broad peak of silica was observed and the peaks of α-Fe 2 O 3 were undetected at a high scanning angle, indicating that α-Fe 2 O 3 nanoparticles were well dispersed in SBA-15.As x ≥ 8, α-Fe 2 O 3 nanoparticles with phase structure R-3c(167) were detected.Based on the previous work with small-angle XRD [26] and N 2 adsorption-desorption isotherms below, α-Fe 2 O 3 nanoparticles should exist in the frame of SBA-15.The intensity ratio of the hematite α-Fe 2 O 3 peak to the silica peak increased with x.Such change was attributed to the better crystallization of α-Fe 2 O 3 for the larger grain size, which could be easily detected by XRD.
The N 2 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 N 2 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-Fe 2 O 3 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    1, it could be concluded that no α-Fe 2 O 3 presented in the mesopores of SBA-15 when x < 8, which accorded with previous studies [26].With an increasing x, the average pore size of Fe/SBA-15 increased to a maximum of 7 nm at x = 8.When x > 8, partial α-Fe 2 O 3 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.
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-Fe 2 O 3 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 N 2 physisorption isotherms.Just like the above analysis, α-Fe 2 O 3 nanoparticles were present in the frame or micropores of Fe/SBA-15 when x < 8. 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-Fe 2 O 3 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-Fe 2 O 3 (1.0Oe).Normally for magnetic nanoparticles, Hc was related to the diameter of the magnetic singledomain 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-Fe 2 O 3 nanoparticles in Fe/SBA-15.Hence, Hc of the Fe/SBA-15 samples increased with higher loading of α-Fe 2 O 3 nanoparticles.Since α-Fe 2 O 3 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 x 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-Co 3 O 4 nanocomposites.As seen from normalized XRD patterns in Figure 5, Co 3 O 4 nanoparticles were successfully implanted into 8%Fe/SBA-15, and the diffraction peak intensity of spine phase Co 3 O 4 became larger with an increasing y.The widened peak of amorphous silica observed at 20-30 • becomes weak and almost disappears when y ≥ 0.01, which indicates that Co 3 O 4 nanoparticles became more crystalline with an increasing y.
Figure 6 presented the N 2 adsorption-desorption isotherms (Figure 6    8%Fe/SBA-15.However, the loop became shorter with an increasing y.The relative pressure in the hysteresis loop for each sample was also enhanced with an increasing y, 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 Co  Co 3 O 4 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.

Conclusion
Mesoporous Fe/SBA-15 materials were prepared by sol-gel and hydrothermal processes, and Co
(a)) and pore size distribution curves (Figure 6(b)) of 8%Fe/SBA-15-Co 3 O 4 , while Table2summarized their structural parameters.Typical type H1 hysteresis loops were observed for all isotherms in Figure6(a), indicating a well-defined mesostructure in the hard template
3 O 4 nanoparticles, which led to an increase in the average pore diameter.The pore volume of 8%Fe/SBA-15-Co 3 O 4 decreased greatly with an increasing y down to 0.19 cm 3 g −1 for 8%Fe/SBA-15-Co-0.02,which was only about one fifth of 0.86 cm 3 g −1 for Fe/SBA-15 and one third of 0.56 cm 3 g −1 for Fe/SBA-15-Co-0.001.The most probable pore size in Figure6(b) was similar to that of the template of 8%Fe/SBA-15, revealing only partial mesopores were blocked by the Co 3 O 4 nanoparticles.In summary, Co 3 O 4 nanoparticles partially occupied the mesopores of Fe/SBA-15, which resulted in the decrease of surface area and pore volume[29][30][31].An HRTEM image of 8%Fe/SBA-15-Co-0.02 was shown in Figure7, where ordered mesopores were observed clearly along the [110] zone axis.Consistent with the N 2 adsorptiondesorption isotherms in Figure6(a), Fe/SBA-15-Co-0.02still 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 Co 3 O 4 .It could also be seen that 8%Fe/SBA-15-Co-0.02actually had the ordered mesopores with a diameter of about 8 nm.Moreover, it was clear in Figure7(a) that a part of mesopores were free of Co 3 O 4 nanoparticles, agreeing well with the volume change in Table 2. On the other hand, Co 3 O 4 nanowires were observed in Figure 7(b) after SBA-15 was with a hot NaOH solution (2.0 M).Thus, the Co 3 O 4 nanoparticles are successfully implanted into the mesopores of Fe/SBA-15.Finally, the magnetic properties of 8%Fe/SBA-15-Co 3 O 4 were measured and discussed by VSM with results summarized in Figure 8.As shown, both Hc and Ms of Fe/SBA-15-Co 3 O 4 were greatly affected by the introduction of Co 3 O 4 nanoparticles.The Co 3 O 4 nanoparticles belonged to the spine phase structure consisting of Co 2+ [Co 3+ ] 2 O 4 .Since Co 2+ had a large magnetic anisotropy, it gave a higher Hc to the Co 3 O 4 materials.Hc increased with an increasing y 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, Co 3 O 4 was antiferromagnetic with zero net magnetic moment.As a result, Ms decreased for higher density 8%Fe/SBA-15-Co 3 O 4 due to the addition of