Synthesis of ordered mesoporous CoFe2O4-containing silica by self-assembly process

Journal of Magnetism and Magnetic Materials 331 (2013) 198–203

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Synthesis of ordered mesoporous CoFe2O4-containing silica by self-assembly process Xiaoyan Yuan, Laifei Cheng n, Wen Liu, Litong Zhang Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an Shaanxi 710072, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 April 2012 Received in revised form 16 November 2012 Available online 29 November 2012

Magnetic CoFe2O4-containing silica with an ordered mesoporous structure was prepared by the selfassembly associated with triblock copolymer, tetraethyl orthosilicate, ferric nitrate and cobalt nitrate. Ammonia water was added to adjust the pH value for collecting the mixture, and then the products were obtained at various temperatures in air. The final products were investigated by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, nitrogen adsorption–desorption isotherm, and vibrating sample magnetometer measurements. As a result, the as-prepared magnetically hybrid porous silica possesses ordered 2-D hexagonal (p6mm) mesoporosity with uniform poresize distribution and high surface areas (up to 283 m2/g at 1000 1C). A pure CoFe2O4 with a high degree of crystallization was formed in the amorphous silica matrix at 1000 1C. In addition, this self-assembly method can be applied to prepare other composites with highly ordered mesostructures. Such nanocomposites with hydrophilic and magnetic framework showed a good dispersibility in water and an easy separation procedure. Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved.

Keywords: CoFe2O4 nanoparticle Mesoporous silica Hydrothermal Magnetic property

1. Introduction It is well known that mesoporous materials are promising candidates for various applications due to their high efficiency, including catalysis, sorption, separation, and so are used in gas sensors, photonic and electronic devices [1–5], which attracted a lot of researchers’ attention in this area. Up to now, various forms of ordered mesoporous materials, such as silica [1,6,7], carbon [2,8], metal oxides [9–11], metal nitrides [12–14], polymers [15] and non-oxide ceramics [16,17], have been fabricated via ‘‘templating’’ (cooperative assembly of surfactants or nanocasting) pathways. Mesoporous silica is a shining star in this material family, due to its highly ordered mesoporous structure, good pore-connectivity and thermal stability. Up to now, mesoporous silica is widely used as hard templates for preparing new mesoporous materials and tunable nanoscale channels for the formation of nanoparticles or nanorods. Magnetic nanocomposites with well-defined mesoporous structures, shapes, and tailored properties are of great scientific and technological interest [18,19]. The nanocomposites can offset and ameliorate magnetic nanoparticles in several unavoidable problems, such as easy oxidization in air, intrinsic instability over long periods, and easy aggregation accompanied with significantly

n

Corresponding author. Tel./fax: þ 86 029 8848 6068. E-mail address: [email protected] (L. Cheng).

decrease of the interfacial areas, which maybe cause the reduction of magnetism and dispersibility [20,21]. Thus many researchers devote a great deal of money and manpower in developing the magnetic nanocomposites that can prevent aggregation to enlarge the application fields. Zhao and colleagues have prepared Fe species within the channels of mesoporous SBA-15 using Fenton’s reagent (Fe2 þ –H2O2) as an iron precursor at low temperature [22]. The host materials with the iron oxide nanoclusters show significant catalytic properties. Wang et al. prepared CoFe2O4/Fe2O3SBA-15 mesoporous nanocomposites via the hydrothermal treatment and impregnation method [23]. The Fe2O3 and CoFe2O4 nanoparticles were confined in the frame and mesopores of SBA15. Yan et al. synthesized the magnetic CoFe2O4 nanoparticles within the channels of as-prepared SBA-15 via the two-step nanocasting and subsequent calcination [24]. Du et al. also prepared mesoporous materials CoFe2O4-SBA-15 with heatresistant magnetism from the impregnation of cobalt salt, iron salt, and citric acid with as-synthesized SBA-15 [25]. In our paper, we demonstrate a ‘‘one-pot’’ route for the synthesis of magnetically separable ordered mesoporous silica via block-copolymer self-assembly process followed by a direct calcined process. In the present study, we described a facile route for the synthesis of magnetic and separable ordered mesoporous CoFe2O4-containing silica via block-copolymer self-assembly process followed by a direct calcined process. Tetraethyl orthosilicate, iron nitrate and cobalt nitrate were used as the precursors, and triblock copolymer P123 as a soft template. This method can

0304-8853/$ - see front matter Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.11.050

X. Yuan et al. / Journal of Magnetism and Magnetic Materials 331 (2013) 198–203

be applied to impart some properties in other porous composites. The magnetic properties of the samples were intensively investigated. Moreover, their dispersibility in the aqueous solution, and recycling and reuse were studied as well.

2. Experiment 2.1. Chemicals Triblock poly(ethylene oxide)-b-poly(propylene oxide)-bpoly(ethylene oxide) copolymer Pluronic P123 (Mw¼5800 g/ mol, EO20PO70EO20) was purchased from Aldrich Chemical Inc. Tetraethyl orthosilicate (TEOS, analytical reagent, AR) was purchased from Tianjin Kermel Chemical Reagent Company. Fe(NO3)3  9H2O (AR), Co(NO3)2  6H2O (AR) and aqueous ammonia (25 wt%) (AR) were purchased from Tianjin Chemical Reagent Company. Other chemicals were purchased from the Shanghai Chemical Company. All chemicals were used in the received state without any further purification. Deionized water was used in all experiments. 2.2. Synthesis of the magnetic mesoporous CoFe2O4-containing silica materials. The method for the preparation of CoFe2O4-containing silica was similar to the previously described process, which is employed for the synthesis of Fe/SBA-15 by Wang et al. [26]. In this process, P123 was used as a structure directing agent, TEOS as a silica precursor, and ferric nitrate and cobalt nitrate as magnetic precursors. In a typical process, 2.0 g of triblock copolymer P123 was dissolved in a mixture solution of 60.0 g of 2 M HCl and 15.0 g of water under stirring at 35 1C for 6 h, then 0.194 g of Co(NO3)2  6 H2O and 0.538 g of Fe(NO3)3  9 H2O were dissolved in the above solution, and 4.25 g of tetraethyl orthosilicate (TEOS) was added while stirring rigorously. The content of CoFe2O4 is about 12.8 wt% by theoretical calculation. After being kept at 35 1C for 20 h under vigorous stirring, the mixture was heated to 130 1C for 24 h in a PTFE autoclave without stirring. After the pH value was adjusted to about 7 using ammonia water, which was slowly added dropwise (2 drops/min, controlled by transfer pipette) with stirring rigorously by mechanical agitator, the milkmixture gradually changed into orange red, recovered by filtration, and was dried in oven. Finally, the products were calcined in air at 400 1C, 600 1C, 800 1C, and 1000 1C for 4 h with a heating rate of 1 1C/min. The samples sintered at different temperatures were correspondingly denoted as CFO-400, CFO-600, CFO-800 and CFO-1000. 2.3. Characterization Transmission electron microscopy (TEM) measurements were conducted on a FEI-T20 microscope operated at 200 kV, to reveal the ordered structure of the samples. Powdery samples were firstly dispersed in ethanol with the aid of sonication and then collected using carbon-film-covered copper grids for TEM analyses. Powder small-angle X-ray diffraction (SA-XRD) and wideangle X-ray diffraction (WA-XRD) patterns were achieved by using a Philipps X’Pert PRO X-ray diffraction system (Cu Ka radiation, 0.15406 nm), operating at 40 mA and 40 kV. The step sizes of SA-XRD and WA-XRD are 0.0051/s and 0.021/s, respectively. Nitrogen absorption–desorption isotherm measurements were performed on a Micrometitics ASAP 2020 volumetric absorption analyzer at  196 1C. Before adsorption–desorption isotherm measurements, the samples were outgassed at 150 1C for 4 h in the degas port of the analyzer. The Brunauer–Emmett–

199

Teller (BET) method was utilized to calculate the specific surface area of each sample, and the pore-size distribution was derived from the absorption branch of the corresponding isotherm using the Barrett–Joyner–Halenda (BJH) method. The total pore volume was estimated from the amount adsorbed at a relative pressure of P/P0 ¼ 0.99. The magnetic properties of the samples were tested by vibrating sample magnetometer (VSM: LakeShore-7407, 2T,) at room temperature. The morphology of the final products was observed on a field emission scanning electron microscope (FESEM, JSM 6701F).

3. Results and discussion In order to verify the ordered mesoporous structures, SA-XRD measurements were employed. Fig. 1 shows the corresponding SAXRD patterns of the CFO-400, CFO-600, CFO-800, and CFO-1000. It is obviously observed that three distinct diffraction peaks appeared in each pattern, which is similar to the pattern of mesoporous silica SBA-15 [1], indexed as the (100), (110) and (200) reflections attributed to the two-dimensional hexagonal p6mm symmetry. It is worth to point out the diffraction peak (100) displaces to the larger angle with the increase of the calcination temperature. The d100-spacing values and the cell parameters are listed in Table 1. The d100 values of CFO-400 and CFO-600 were not significant, because the sintering temperature was too low to cause obvious contraction of pore-value. When the sintering temperature increased, the d100 values of CFO-800 and CFO-1000 had a notable reduction, which is mainly due to the densification of the composites’ pore walls in high temperature [24]. Fig. 2 shows the typical WA-XRD patterns for magnetic composites at different calcination temperatures. In the case of CFO-400, no obvious peak appeared, revealing the amorphous state of the composites. One small peak at 35.61 in the pattern of the CFO-600 displays the characteristic diffraction peaks of CoFe2O4 crystallites nature [24], which indicate that the CoFe2O4 has formed and grown at 600 1C. When the calcination temperature is raised up to 800 1C, the pattern displays the main five peaks at 30.11, 35.61, 43.11, 57.21 and 62.71 assigned to the 220, 311, 400, 511 and 440 planes, repectively, providing evidence that the sample’s dominant phase is CoFe2O4 (JPDS Card number 221086) with the expected inverse spinel structure [27]. With the increasing calcination temperature, diffraction peaks of CoFe2O4 become stronger; all the diffraction peaks of CFO-1000 match well with those of the CoFe2O4 phase. There is no other diffraction peaks observed, suggesting a pure CoFe2O4 phase was formed. The strongest diffraction peak (311) of the CoFe2O4 phase becomes sharper with the elevated temperature, indicating the increase of the particle size. Using Scherrer’s formula, the grain size of magnetic crystallites calcinated at 600, 800 and 1000 1C can be estimated to be 6, 9 and 21 nm, respectively. Moreover, it should be mentioned that the broad band at 21–231 existed in each pattern is related with silica, suggesting its amorphous nature. The ordered mesoporous structure of magnetic CoFe2O4loaded silica was further evidenced by TEM images. As shown in Fig. 3, two samples exhibited characteristic arrangement of cylindrical channels in large domains, indicating the samples have good ordered 2-D hexagonal mesostructure. This is in accordance with the SA-XRD testing. The dark spots doped in the frameworks of matrix material, proving the magnetic nanoparticles have a good dispersity. In the low magnification image (shown in Fig. 3a), the magnetic particles are mostly embedded in the amorphous mesopore silica on a large scale. The high magnification image of CFO-1000 (Fig. 3b) clearly shows that the magnetic particles are randomly located in the silica matrix. With the increasing temperature, the magnetic particles further grow and

X. Yuan et al. / Journal of Magnetism and Magnetic Materials 331 (2013) 198–203

Intensity (a.u.)

40000 (100)

20000

CFO-400

Intensity (a.u.)

200

30000 20000 (110) (200)

10000

CFO-600

10000 (110) (200) 0

0 2 2 Theta (degrees)

4

2 2 Theta (degrees)

4

5000

4000

Intensity (a.u.)

5000 Intensity (a.u.)

(100)

CFO-800

3000 2000 (100) 1000

(110) (200)

4000

2000 1000

0

CFO-1000

3000

(100)

(110) (200)

0 2 2 Theta (degrees)

2 2 Theta (degrees)

4

4

Fig. 1. SA-XRD patterns of CoFe2O4-incorporated mesoporous silica nanocomposites treated at different temperatures.

Table 1 Textural data of the nanocomposites as-prepared samples. Sample

d100 Spacing (nm)

Cell parameter a (nm)

SBET (m2/g)

Pore volume (cm3/g)

Pore size (nm)

Ms (emu/g)

Mr (emu/g)

Hc (Oe)

CFO-400 CFO-600 CFO-800 CFO-1000

10.7 10.6 8.7 7.8

12.4 12.3 10.0 8.8

645 589 499 283

1.3 1.2 1.0 0.6

8.0 7.8 7.5 6.6

0.04 0.29 2.60 5.12

0.003 0.023 1.26 2.74

98 126 246 2839

3000 CFO-400 CFO-600 CFO-800 CFO-1000

(311)

Intensity (a.u.)

2500 (220)

2000

(400) (222)

1500

(440) (511) (422)

(533)

1000 500 0 20

30

50 40 60 2 Theta (degrees)

70

80

Fig. 2. WA-XRD patterns of ordered mesoporous nanocomposites.

penetrate the pore walls and the mesoporous orderliness of the samples is decreased. The corresponding selected-area electron diffraction (SAED) pattern (the inset of Fig. 3b) exhibits electron diffraction spots and rings and all of them are indexed to the diffraction planes of CoFe2O4 nanoparticles. The morphology of the as-prepared mesoporous SBA-15 and that of the nanocomposites of CoFe2O4-incorporated meso-silica were studied by SEM. As shown in Fig. 4a, the SBA-15 exhibits the morphology with the length of the long axis around 20–50 mm.

As shown in Fig. 4b and c, the morphology of CFO-400 and CFO1000 is quite similar to the pristine SBA-15. From the magnified image of CFO-1000 shown in Fig. 4d, it can be obviously observed that the rodlike length is piled up by many smaller rodlike with sizes about 2  0.7 mm2. This similarity in the morphology indicates that the magnetic-particles incorporated do not destroy the microstructure of mesoporous silica. Meanwhile, this also suggests that the mesoporous CoFe2O4-containing silica composites have high thermal stability. Nitrogen isotherms adsorption–desorption and the corresponding pore-size distribution curves of CoFe2O4-containing silica calcinated at different temperatures are shown in Fig.5. As seen from Fig. 5a, all the isotherms show representative type-IV curves with H1 hysteretic loops characteristic for the relatively high uniform distribution of the pore size [28,29]. In high sintering temperature, the steep hysteretic loops can also be clearly observed, which suggests the retention of the high uniformity of the mesoporous structure. Compared with the CFO-400 sample, the capillary condensation steps of CFO-600, CFO-800 and CFO-1000 are shifted to lower relative pressures, implying the smaller mesopore size, which is consistent with the pore-size distribution calculated from the adsorption branch of the isotherms by BJH algorithm (shown in Fig. 5b). The values of BET surface areas, average pore diameters and the total pore volumes are listed in Table 1. It can be seen that, the BET surface area, the average pore diameter and total pore volume of the samples drastically change with the increasing sintering temperature increases. As temperature increases from 400 to 1000 1C, the BET surface area drops from 645 to 283 m2/g, the pore size decreases from 8.0 to 6.6 nm, and the pore volume from 1.3 to

X. Yuan et al. / Journal of Magnetism and Magnetic Materials 331 (2013) 198–203

(222)

(400)

201

(311)

(440)

Fig. 3. TEM images of the CFO-400 (a) and CFO-1000 (b) with the selected-area electron diffraction (SAED) pattern (inset).

Fig. 4. SEM images of SBA-15 (a), CFO-400 (b) and CFO-1000 (c, d).

1200 1000

Fractional Pore Volume (cm3g-1nm-1)

Quantity Adesorbed (cm3/gSTP)

1400 CFO-400 CFO-600 CFO-800 CFO-1000

800 600 400 200 0 0.0

0.4 0.6 0.8 0.2 Relative Pressure (P/P0)

1.0

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

CFO-400 CFO-600 CFO-800 CFO-1000

10

20 30 40 Pore Diameter (nm)

50

Fig. 5. (a) Nitrogen isotherms adsorption–desorption isotherms of CFO-400, CFO-600, CFO-800 and CFO-1000. The Y-axis of the isotherms of CFO-400, CFO-600 and CFO1000 is moved up to 500, 200 and  50, respectively. (b) The corresponding pore-size distribution of the samples calculated from the adsorption branch of the isotherms using the BJH algorithm.

0.6 cm3/g. Furthermore, it should be mentioned that, during the high temperature calcination, the framework of porous composites contracts and perhaps is partially destroyed, which would result in the decrease in the BET surface area of the final

composite products [30,31]. It is reported that the values for mesoporous SBA-15 calcinated at 1000 1C drop to 259 m2/g, 6.9 nm and 0.5 cm3/g [24]. The decrease of samples’ textural data with the increase of temperature is mainly due to the framework

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X. Yuan et al. / Journal of Magnetism and Magnetic Materials 331 (2013) 198–203

6 CFO-400 CFO-600 CFO-800 CFO-1000

M (eum/g)

4 2 0

3 min

-2 -4 -6 -10000

-5000

0 5000 Field (G)

10000

Fig. 6. (a) Magnetization curves of the ordered mesoporous CFO-400 and CFO-1000 nanocomposites at room temperature. (b) The optical photos of the CFO-1000 sample dispersed in aqueous solution and treated by a magnetic field.

densification of mesoporous silica combined with the growth of the CoFe2O4 crystallites. To investigate the magnetic properties of CoFe2O4-incorporated silica nanocomposites, the room temperature hysteresis loops were tested by VSM, as given in Fig. 6a. And the corresponding magnetic parameters including saturation magnetization (Ms), remnant magnetization (Mr) and coercivity (Hc) were summarized in Table 1. It was found that the influence of the calcination temperature on the magnetic properties is very obvious. The M–H curves of CFO-400 and CFO-600 take on an incomplete reversibility and the corresponding Mr is very small. As the calcination temperature increases to 800 and 1000 1C, the typical saturated hysteresis loops appear in the corresponding M– H curves, which exhibit the characteristic ferromagnetic behavior. According to the model built by Berkowitz and his coworkers, the magnetic particle growth and the average particle size of a magnetic material affect its magnetic behavior [32], and as the WA-XRD data described before. Therefore, the increase of the magnetic data is mainly attributed to the growth of the CoFe2O4 nanoparticles with the elevated temperature. The sample of CFO1000 has the coercivity (Hc) of 2.839 kOe, which is higher than that of the CoFe2O4/SiO2 nanocomposites reported by Yan et al. (Hc¼1.302 kOe) [24] and Xiao et al. (Hc¼1.241 kOe) [33], and the CoFe2O4 nanoparticles embedded in silica matrix reported by Cerda et al. (Hc ¼1.1 kOe) [34]. Subtracted the weight of the nonmagnetic silica, the Ms of CFO-1000 is 40.5 emu/g. This value is lower than that of the reported bulk CoFe2O4 (80.8 emu/g) [27], due to many complex factors: the microstructure, residual strain, nanosize of CoFe2O4 particles and the interaction of silica [24,33]. From the Fig. 6b, we can see that the nanocomposite CFO-1000 sample was dispersed in water with the aid of sonication, exhibiting good dispersibility in the aqueous solution. Just after 3 min the aqueous solution has become clear by a magnetic field, indicating that the mixture can be easily separated, easily recycled and reused. We believe that the composites have potential use as an adsorbent in industrial waste water.

4. Conclusion In summary, we demonstrate a process of simple route for the synthesis of magnetically ordered mesoporous silica via the block-copolymer self-assembly process followed by a direct calcined process. This synthesis route is related to the interaction between cobalt and iron ions and the TEOS matrix. The XRD, SEM, TEM and nitrogen sorption measurements reveal that the magnetic silica nanocomposites have high ordered meso-structure. At the temperature of 1000 1C, the composite has high crystallization

degree and exhibits ordered mesopores structure with high specific surface areas. Meanwhile, the composites exhibit good dispersibility in the aqueous solution and can be easily separated by a magnetic field. This method can be applied to prepare other composites with high ordered mesostructures. We believe that the composite materials containing different metals or metal oxides would have potential applications, such as in catalysis, adsorption, optics, separation and as supporters.

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