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Ultrasonic-assisted ultra-rapid synthesis of monodisperse meso-SiO2@Fe3O4 microspheres with enhanced mesoporous structure
Ultrasonics Sonochemistry xxx (2013) xxx–xxx
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Ultrasonic-assisted ultra-rapid synthesis of monodisperse meso-SiO2@Fe3O4 microspheres with enhanced mesoporous structure Hongfei Liu, Shengfu Ji ⇑, Hao Yang, Huan Zhang, Mi Tang State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China
a r t i c l e
i n f o
Article history: Received 15 June 2013 Received in revised form 19 August 2013 Accepted 19 August 2013 Available online xxxx Keywords: Ultrasonic Ultra-rapid Core–shell structure meso-SiO2@Fe3O4 microspheres Magnetic Mesoporous silica
a b s t r a c t A core–shell-type of meso-SiO2@Fe3O4 microsphere was synthesized via an ultrasonic-assisted surfactant-templating process using solvothermal synthesized Fe3O4 as core, tetraethoxysilane (TEOS) as silica source, and cetyltrimethyl ammonium bromide (CTAB) as templates. The samples were characterized by FT-IR, XRD, TEM, N2 adsorption–desorption technology, and vibrating sample magnetometer (VSM). The results show that as-prepared meso-SiO2@Fe3O4(E) and meso-SiO2@Fe3O4(C) microspheres, treated by acetone extraction and high temperature calcination, respectively, still maintain uniform core–shell structure with desirable mesoporous silica shell. Therein, the meso-SiO2@Fe3O4(E) microspheres possess a distinct pore size distribution in 1.8–3.0 nm with large specific surface area (468.6 m2/g) and pore volume (0.35 cm3/g). Noteworthily, the coating period of this ultrasonic-assisted method (40 min) is much shorter than that of the conventional method (12–24 h). The morphology of microspheres and the mesoporous structure of silica shell are significantly influenced by initial concentration of CTAB (CCTAB), ultrasonic irradiation power (P) and ultrasonic irradiation time (t). The acceleration roles of ultrasonic irradiation take effect during the whole coating process of mesoporous silica shell, including hydrolysis-condensation process of TEOS, co-assembly of hydrolyzed precursors and CTAB, and deposition of silica oligomers. In addition, the use of ultrasonic irradiation is favorable for improving the homogeneity of silica shell and the monodispersity of meso-SiO2@Fe3O4 microspheres. Ó 2013 Published by Elsevier B.V.
1. Introduction Recently, various types of magnetic mesoporous silica nanocomposites (MMSNs) with magnetic responsibility and mesoporous structure have attracted much attention due to their potential applications in catalysis, drug/DNA/gene delivery, water treatment, multimodal imagine, and so on [1,2]. Among them, core–shell-type MMSN microspheres with large Fe3O4 spherical cores (100–300 nm) and MCM-41 family ordered mesoporous silica shells have been the subject of extensive researches because to their unique features, including high thermal and chemical stabilities, favorable biocompatibility and superparamagnetism, simple functionality, high available surface area, and large pore volume [3–5]. The most representative core–shell-type MMSN microspheres (Fe3O4@nSiO2@mSiO2) with perpendicularly aligned mesoporous shells were firstly synthesized by Zhao and coworkers through a two-step coating process [4]. Subsequently, some modified core–shell-type MMSN microspheres with different specific function were obtained upon the similar two-step coating process [5–7]. During this two-step coating process, uniform Fe3O4
⇑ Corresponding author. Tel./fax: +86 10 64419619. E-mail address: [email protected] (S. Ji).
microspheres are firstly precoated with a thin silica layer through a modified stöber process, and this layer is indispensable for the deposition of uniform mesoporous silica shell through a surfactant-templating approach with cetyltrimethyl ammonium bromide (CTAB) as the templates. Therein, the thin silica layer and uniform mesoporous silica shell are derived from slow hydrolysis of tetraethyl orthosilicate (TEOS) (at least 12–24 h of mechanical stirring at room temperature). As a result, relatively complex preparation routes and long-time coating processes limit their applications in some way. Additionally, these above mentioned core–shell-type MMSN microspheres exhibit poor monodispersity which is bad for their applications in bio-fields and liquid–solid heterogeneous catalysis. Therefore, exploration of new strategies to rapidly and facilely synthesize core–shell-type MMSN microspheres with desirable monodispersity is in high demand. Over the past decades, ultrasonic method has been widely used in the preparation of powdery nanomaterials owing to its special cavitation process composed of formation, growth, and collapse of microbubbles in liquid solution [8–10]. During this process, extremely high peak temperature (>5000 K), pressure (>20 MPa), and cooling rates (over 1011 K/s) are attained inside the cavities due to the transient collapsing of the bubbles [11], meanwhile, microjet stream and shock wave are generated inside the liquids solution [12]. Considered these unique properties in the ultrasonic
1350-4177/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.ultsonch.2013.08.010
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irradiated solution, the ultrasonic technique was used in the synthesis of zeolites and mesoporous silicas, such as MCM-22 [13], SBA-15 [14], and MCM-41 [15], and it was proven that the ultrasonic treatment was effective to shorten synthesis period and improve pore structure. Very recently, ultrasonic approach has been successfully applied for the fabrication of various composites with distinct core–shell structure [16–18]. Among them, a rapid sonochemical synthesis of monodisperse nonaggregated Fe3O4@SiO2 magnetic nanoparticles with nonporous silica shell was reported [12], and it was shown that the coating process was accelerated many-fold in the presence of a 20 kHz ultrasonic field. Therefore, it is believed that the ultrasonic holds many prospects for resolving aforementioned problems, long-time preparation time and poor monodispersity, in the synthesis of core–shell-type MMSN microspheres. In this work, we report, for the first time, an ultrasonic-assisted ultra-rapid synthesis of core–shell meso-SiO2@Fe3O4 microspheres with Fe3O4 core and homogeneous mesoporous silica shell by a CTAB-templating approach. The structure and physical properties of as-prepared meso-SiO2@Fe3O4(E) and meso-SiO2@Fe3O4(C) microspheres, which are treated by acetone extraction and high temperature calcination, respectively, were studied by XRD, TEM, N2 adsorption–desorption technique and VSM. The influences of initial concentration of CTAB, ultrasonic irradiation power and ultrasonic irradiation time to the structural properties of mesoSiO2@Fe3O4(E) microspheres were investigated systematically. Finally, the formation mechanism of meso-SiO2@Fe3O4 microspheres prepared by ultrasonic-assisted method was discussed. 2. Experimental 2.1. Synthesis of Fe3O4 Briefly, 4.32 g of FeCl36H2O and 12.0 g of sodium acetate were dissolved in 160 mL of glycol under stirring. The obtained homogeneous yellow solution was transferred to a 200 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed and heated at 200 °C. After heated for 12 h, the autoclave was naturally cooled to room temperature. The obtained black magnetite particles were separated with a permanent magnet, washed with ethanol for six times, and dried in vacuum at 60 °C for 24 h. 2.2. Synthesis of meso-SiO2@Fe3O4 0.1 g of as-prepared Fe3O4 microspheres was dispersed into the mixture solution of 80 mL of ethanol, 10 mL of deioned water, 4 mL of concentrated ammonia aqueous solution (28 wt%), and a certain amount of CTAB. After this, the mixture solution was homogenized by ultrasonication (frequency = 40 kHz, power = 150 W) for 10 min to form a homogeneous dispersion. Subsequently, TEOS (0.25 mL) in ethanol (24 mL) was injected into the synthesis system under ultrasonic irradiation. After the reaction was performed for a certain time, the product was separated with a permanent magnet, washed with deionized water for three times, and dried in vacuum at 50 °C for 12 h to obtain the meso-SiO2@ Fe3O4 microspheres. The template removal was studied by two different methods: (i) Solvent extraction: as-prepared meso-SiO2@Fe3O4 microspheres were dispersed in acetone and kept at 80 °C under N2 protection for 24 h. This procedure was repeated twice. The product was defined as meso-SiO2@Fe3O4(E). (ii) High temperature Calcination: as-prepared meso-SiO2@Fe3O4 microspheres were calcined for 3 h at 450 °C under N2 protection. The heating rate was set to 1 °C/min. The product was defined as meso-SiO2@Fe3O4(C).
To study the influence of different preparation conditions on the structure and properties of meso-SiO2@Fe3O4(E) particles, various conditions including initial concentration of CTAB (CCTAB = 0– 10.29 mmol/L), ultrasonic irradiation power (P = 60–150 W) and ultrasonic irradiation time (t = 5–40 min) were chosen and their effects were investigated during the preparation process. 2.3. Characterization Power X-ray diffraction (XRD) patterns were collected on a D/Max 2500 VB2+/PC diffractometer with Ka irradiation of Cu (k = 1.5418 Å, 200 kV, 50 mA). The N2 adsorption–desorption analysis was tested on an ASAP 2020 M automatic specific surface area and aperture analyzer, and the specific surface area determination and pore volume analysis were performed by Brunauer– Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively. Transmission electron microscopy (TEM) and highresolution (HR) TEM images were taken with a JEOL (JEM-2100) transmission electron microscope operated at 200 kV accelerating voltage. Magnetic properties of the samples were measured on a vibration sample magnetometer (VSM; Lake Shore Model 7400) under magnetic fields up to 20 kOe. 3. Results and discussion 3.1. Characteristics of as-prepared meso-SiO2@Fe3O4 microspheres 3.1.1. XRD of samples Fig. 1 shows the typical XRD patterns of Fe3O4, meso-SiO2@Fe3O4(E) and meso-SiO2@Fe3O4(C), where meso-SiO2@Fe3O4 particles were synthesized under following condition: CCTAB = 6.86 mmol/L, P = 150 W and t = 30 min. From Fig. 1a, the wide-angle XRD patterns of all samples show obvious diffraction peaks, which can be readily indexed to face center cubic magnetite phase (Fe3O4, JCPDS No. 19-0629). For meso-SiO2@Fe3O4(E) and meso-SiO2@Fe3O4(C), the broad peak at 2h = 20–30° can be assigned to the amorphous silica (SiO2, JCPDS No. 29-0085) [6]. From Fig. 1b, the low-angle XRD patterns of meso-SiO2@Fe3O4(E) and meso-SiO2@Fe3O4(C) present obvious (1 0 0) peak at around 2.5° and invisible (1 1 0) and (2 0 0) peaks, which suggest the ordered mesopore symmetry, revealing the existence of short-range ordered mesoporous structure [5,6]. The decrease of intensity of (1 0 0) peak for meso-SiO2@Fe3O4(C) sample may be due to the high temperature treatment, which results in the decrease of ordering of the mesoporous structure. 3.1.2. TEM of samples The TEM images of meso-SiO2@Fe3O4 samples are shown in Fig. 2. For meso-SiO2@Fe3O4(E), monodisperse microspheres with uniform core–shell structure are clearly exhibited in Fig. 3a, from which it is obvious that the Fe3O4 core is well coated by a silica shell with thickness of about 50 nm. From HRTEM image (Fig. 3c), we can observe abundant of wormlike mesopores in the silica shell, which is different with perpendicularly aligned mesopores presented in Fe3O4@nSiO2@mSiO2 microspheres prepared by traditional two-step coating process [4–6]. Noteworthily, mesoSiO2@Fe3O4(C) microspheres still exhibit uniform core–shell structure with about 50 nm silica shell (Fig. 3b), and the silica shell keep the similarly wormlike mesopores (Fig. 3d), reflecting the favorable thermostability of as-prepared meso-SiO2@Fe3O4 microspheres. 3.1.3. N2. adsorption–desorption analysis of samples In order to directly characterize the textural properties of asprepared meso-SiO2@Fe3O4 microspheres, meso-SiO2@Fe3O4(E) and meso-SiO2@Fe3O4(C) microspheres were analyzed by N2
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Fig. 3. (a) N2 adsorption–desorption isotherms and (b) pore size distribution curves of samples. Fig. 1. (a) Wide-angle XRD patterns and (b) low-angle XRD patterns of samples.
Fig. 2. TEM images of the samples: (a) and (c) meso-SiO2@Fe3O4(E); (b) and (d) meso-SiO2@Fe3O4(C).
adsorption–desorption technology. Fig. 3 shows the N2 adsorption–desorption isotherms and the pore size distribution curves
of Fe3O4 and meso-SiO2@Fe3O4 microspheres. It can be seen that the Fe3O4 particles exhibit III-type isotherm and corresponding pore size distribution curve presents non-porous structure. For meso-SiO2@Fe3O4(E), we can observed a typical IV-type curve with a steep increase in the P/P0 range of 0.04–0.25 (Fig. 3a), and corresponding pore size distribution curve presents a sharp peak centered at the mean value of 2.3 nm (Fig. 3b). The products obtained via traditional two-step coating process have the same N2 adsorption–desorption characteristic [4–6]. Unusually, there is an obvious H1 hysteresis loop consisting one broad capillary condensation step at P/P0 of 0.45–0.95 indicated by black arrow in Fig. 3a, which indicates the presence of textual mesopores [5]. The same phenomenon was observed in the ultrasonic-assisted synthesis of ordered mesoporous silica SBA-15 [14]. The BET surface area and BJH pore volume of meso-SiO2@Fe3O4(E) microspheres are calculated to be 468.6 m2/g and 0.35 cm3/g, respectively, which are higher than that of samples obtained by traditional two-step coating process [4–6]. For meso-SiO2@Fe3O4 (C), the N2 adsorption–desorption isotherm exhibits a similar IVtype isotherm with an unobvious H1 hysteresis loop, but the pore size distribution curve presents a relatively broad peak at value of 1.8–3.0 nm, confirming the decrease of ordering of the mesopore structure after high temperature calcination. The BET surface area and BJH pore volume of meso-SiO2@Fe3O4(E) microspheres decrease to 356.3 m2/g and 0.29 cm3/g, respectively. 3.1.4. VSM of samples Fig. 4 shows the magnetization curves of Fe3O4 and mesoSiO2@Fe3O4 microspheres. All curves appear nonlinear and reversible characteristic with no hysteresis (zero coercivity and no remanence), exhibiting superparamagnetic behavior. The
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saturation magnetization (Ms) values of meso-SiO2@Fe3O4(C) and meso-SiO2@Fe3O4(E) are 44.72 and 40.38 emu/g, respectively, which can completely meet the requirement of the magnetic separation. The slight increase of Ms. values for meso-SiO2@Fe3O4(C) microspheres may be due to the Ostwald ripening during high temperature calcination [19]. From above results, it can be found that, in the ultrasonic-assisted synthesis of meso-SiO2@Fe3O4 microspheres, the use of ultrasonic can effectively shorten the coating period of mesoporous silica from 12–24 h to 40 min, and obtained meso-SiO2@Fe3O4 microspheres still exhibit uniform core–shell structure with desirable mesoporous silica shell after acetone extraction or high temperature calcination. 3.2. Effect of synthesis conditions on structure and properties of asprepared meso-SiO2@Fe3O4(E) microspheres In consideration of the extraordinary mesoporous shell of mesoSiO2@Fe3O4(E) microspheres. It is necessary to study several representative processing parameters, such as initial concentration of CTAB, ultrasonic irradiation power and ultrasonic irradiation time, which may influence the structure and/or properties of product in the ultrasonic-assisted synthesis process. Therefore, three series of meso-SiO2@Fe3O4(E) samples were prepared when any of the above variables was changed independently of the others. 3.2.1. Initial concentration of CTAB Fig. 5 shows the low-angle XRD patterns of meso-SiO2@Fe3O4(E) microspheres which were synthesized with different CCTAB, whereas the other variables were held constant (P = 150 W and t = 30 min). As shown in Fig. 5, no diffraction peak is observed in the low-angle XRD pattern (a) of sample prepared without CTAB. The patterns (b–f) of samples obtained with CTAB present obvious (1 0 0) peak at around 2.5° and invisible (1 1 0) and (2 0 0) peaks, revealing the existence of short-range ordered mesoporous structure [5,6]. And the ordering of this mesoporous structure is improved with the CCTAB increased from 1.76 to 6.86 mmol/L according to the gradually enhanced intensity of (1 0 0) peak. Moreover, nearly unchanged intensity of (1 0 0) peak at 10.29 mmol/L of CCTAB suggests that excess CTAB is unnecessary for ultrasonic-assisted synthesis of meso-SiO2@Fe3O4 microspheres. Fig. 6 shows the N2 adsorption–desorption isotherms and pore size distribution curves of meso-SiO2@Fe3O4(E) microspheres
Fig. 4. Magnetization curves of samples: (a) pure Fe3O4; (b) meso-SiO2@Fe3O4(E); (c) meso-SiO2@Fe3O4(C).
Fig. 5. Low-angle patterns of meso-SiO2@Fe3O4(E) microspheres obtained using different CCTAB: (a) 0; (b) 1.76; (c) 3.43; (d) 5.19; (e) 6.86; and (f) 10.29 mmol/L. (P = 150 W and t = 30 min).
obtained with different CCTAB. The sample obtained without CTAB presents non-porous characteristics. In the presence of CTAB, the N2 adsorption–desorption isotherms of all samples exhibit distinct IV-type isotherm with H1 hysteresis loop, and corresponding pore size distribution curves present a distinct peak at value of 1.8– 3.0 nm, indicating the presence of short-range ordered mesopores [4]. Furthermore, the intensity of the pore size distribution gradually enhances with the increase of CCTAB from 1.76 to 6.86 mmol/L, confirming the improvement of the mesoporous structure, but it remains nearly unchanged at CCTAB of 10.29 mmol/L. Meanwhile, it can be seen from Table 1 that the BET surface area and BJH pore volume of samples increase first and then decrease with the increase of CCTAB. Fig. 7 shows the TEM images of meso-SiO2@Fe3O4(E) microspheres obtained using different CCTAB. It can be observed from Fig. 7a–c and e–g that as-prepared meso-SiO2@Fe3O4(E) microspheres keep monodisperse core–shell structure and their mesoporous structure of silica shell becomes increasingly clear in the CCTAB range of 0–6.86 mmol/L. However, the microspheres obtained at 10.29 mmol/L of CCTAB show serious aggregation which may account for their slight decrease of BET surface area and BJH volume. From above results it can be inferred that the monodispersion and mesoporous structure of as-prepared meso-SiO2@Fe3O4(E) microspheres are significantly influenced by CCTAB. Low CCTAB has adverse effects on mesoporous structure of silica shell and high CCTAB goes against the monodispersion of microspheres. The optimum CCTAB for ultrasonic-assisted synthesis of meso-SiO2@Fe3O4(E) microspheres is about 6.86 mmol/L. 3.2.2. Ultrasonic irradiation power Fig. 8 shows the TEM images of meso-SiO2@Fe3O4(E) microspheres which were synthesized under different P of 60, 90, 120, and 150 W, respectively, whereas the other variables were held constant (CCTAB = 6.86 mmol/L and t = 30 min). From Fig. 8a, it can be seen that Fe3O4 microspheres exhibit serious aggragation because of the non-uniform mesoporous SiO2 coating under 60 W of P. With the P increased from 90 to 150 W, the uniformity of the mesoporous silica shell and the monodispersity of meso-SiO2@Fe3O4(E) microspheres are improved obviously (Fig. 8b–d). Therefore, it can be concluded that the P have important effects on the architectural feature of obtained microspheres [12,17].
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Same results were shown in ultrasonic synthesis of mesoporous molecular sieve (MCM-41) [15]. Therefore, it can be believed that using relatively high P is beneficial to synthesize monodisperse meso-SiO2@Fe3O4(E) microspheres with favorable mesoporous silica shell.
Fig. 6. (a) N2 adsorption–desorption isotherms and (b) pore size distribution curves of the meso-SiO2@Fe3O4(E) microspheres obtained using different CCTAB. (P = 150 W and t = 30 min).
Fig. 9 shows the N2 adsorption–desorption isotherms and pore size distribution curves of meso-SiO2@Fe3O4(E) microspheres obtained using different P. As a whole, all samples exhibit distinct IV-type isotherms with an obvious H1 hysteresis loop (Fig. 9a), and corresponding pore size distribution curves emerge a distinct peak at 1.8–3.0 (Fig. 9b). With the increase of P, the intensity of the pore size distribution enhances apparently, and the BET surface area and BJH pore volume keep continuous increase (Table 1), indicating the enhancement of mesoporous structure of the silica shell.
3.2.3. Ultrasonic irradiation time To gain insight into the formation process of mesoporous silica shell of the meso-SiO2@Fe3O4(E) microspheres under ultrasonic irradiation, time-dependent experiments were carried out. Fig. 10 shows the TEM images of meso-SiO2@Fe3O4(E) microspheres under different t of 5, 10, 20, 30, and 40 min, respectively, whereas the other variables were held constant (CCTAB = 6.86 mmol/L and P = 150 W). In general, it can be observed from Fig. 10 that all samples present monodisperse core–shell microspheres (Fig. 10a–e), and the thickness of silica shell increases from 5 to 50 nm (Fig. 10f–j) with the extension of t, revealing the gradual formation of that silica shell. When t is only 5 min, only a thin SiO2 (average thickness of 5 nm) layer without obvious mesoporous structure can be observed (Fig. 10a and f). Subsequently, as t prolonged to 10 min, it can be noticed from the image contrast that there exist obvious wormlike mesopores in the silica shell (Fig. 10b and g), and that wormlike mesoporous structure is increasingly clear when t is 20 min (Fig. 10c and h). With further extended t to 30 min and more, the morphology structure and thickness (about 50 nm) of mesoporous silica shell keep unchanged (Fig. 10d,e,i and j), revealing the fully formed of the mesoporous silica shell during the 30 min of synthesis time. Fig. 11 shows the N2 adsorption–desorption isotherms and pore size distribution curves of meso-SiO2@Fe3O4(E) microspheres synthesized under different t. As shown in Fig. 11, when t is only 5 min, the N2 adsorption–desorption isotherm and pore size distribution curve confirm that there is no mesoporous structure in the thin silica shell. As t prolonged to 10 min, the sample exhibits distinct IV-type isotherm and corresponding pore size distribution curve emerge a broad peak in 1.8–3.0 nm. With t further extended to 20 min and more, the intensity of the pore size distribution gradually enhances. In addition, Table 1 shows that the BET surface area and BJH pore volume of samples increase gradually and then remain nearly unchanged with the extension of t. Therefore, we can conclude that there is close relationship between mesoporous structure of silica shell and t. The mesoporous silica shell of as-prepared meso-SiO2@Fe3O4(E) microspheres is composed by two parts: inner nonporous layer, and outer mesoporous layer with a pore size distribution in 1.8–3.0 nm.
Table 1 Textural and structural characteristics of samples prepared in different preparation conditions.
a b c d e f
Sample
CCTABa (mmol/L)
Pb (W)
tc (min)
SBET (m2 g
1 2 3 4 5 6 7 8 9 10 11 12 13
0 1.76 3.43 5.19 6.86 10.29 6.86 6.86 6.86 6.86 6.86 6.86 6.86
150 150 150 150 150 150 60 90 120 150 150 150 150
30 30 30 30 30 30 30 30 30 5 10 20 40
32.3 142.6 209.0 306.4 468.6 398.1 153.9 315.5 414.3 23.7 103.4 245.2 441.4
1 d
)
VBJHe (cm3 g 0.07 0.11 0.16 0.24 0.35 0.32 0.13 0.26 0.31 0.04 0.17 0.22 0.34
1
)
Dvf (nm) 8.0 3.3 3.1 2.8 2.3 2.6 3.3 2.9 2.4 7.2 5.9 3.2 2.3
Initial concentration of CTAB. Ultrasonic irradiation power. Ultrasonic irradiation time. Multipoint BET surface area. BJH method cumulative desorption pore volume. Average pore diameter.
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Fig. 7. TEM images of the meso-SiO2@Fe3O4(E) microspheres obtained using different CCTAB: (a) and (e) 0; (b) and (f) 3.43; (c) and (g) 6.86; (d) and (h) 10.29 mmol/L (P = 150 W and t = 30 min).
Fig. 8. TEM images of the meso-SiO2@Fe3O4(E) microspheres obtained under different P: (a) 60; (b) 90; (c) 120; (d) 150 W. (CCTAB = 6.86 mmol/L and t = 30 min).
3.3. Formation mechanism of ultrasonic-assisted synthesized mesoSiO2@Fe3O4 microspheres Based on the above results and discussion, the ultrasonic-assisted synthesis process of meso-SiO2@Fe3O4 microspheres can be summarized in schematic illustration as shown in Scheme 1. Compared with the classical two-step coating process [4–6], the ultrasonic-assisted synthesis route has only one CTAB-templating approach, but as-prepared meso-SiO2@Fe3O4 microspheres present similar core–shell structure with an inner nonporous silica layer. Actually, the presence of this inner nonporous silica layer can not only serve as nucleation seeds for growth of mesoporous silica layer [2] but also protect magnetic cores to improve the thermal and chemical stability of meso-SiO2@Fe3O4 microspheres [20].
Fig. 9. (a) N2 adsorption–desorption isotherms and (b) pore size distribution curves of the meso-SiO2@Fe3O4(E) microspheres obtained under different P. (CCTAB = 6.86 mmol/L and t = 30 min).
In addition to simplify the coating procedure, the applied ultrasonic environment plays some significant roles in formation of meso-SiO2@Fe3O4 microspheres. In conventional synthesis of core–shell-type MMSN microspheres, the formation process of
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Fig. 10. TEM images of the meso-SiO2@Fe3O4(E) microspheres obtained in different t: (a) and (f) 5 min; (b) and (g) 10 min; (c) and (h) 20 min; (d) and (i) 30 min; (e) and (j) 40 min. (CCTAB = 6.86 mmol/L and P = 150 W).
oligomers. Finally, these precipitated nanosized oligomers deposited on the magnetic particles, shaping an ordered mesostructured silica layer. In our ultrasonic-assisted synthesis of monodisperse meso-SiO2@Fe3O4 microspheres, the ultrasound gave full play to the role of process intensification. Therein, the most obvious performance was ultra-rapid synthesis time. The acceleration role of the ultrasound was mainly manifested in following aspects. On the one hand, because the hydrolysis-condensation process of TEOS was known to be temperature sensitive [21], transient high temperature produced by cavitation process would lead to strong local acceleration of this hydrolysis-condensation process. On the other hand, microjet stream and shock wave generated inside the liquids solution by cavitation process could effectively enhance the mass transfer [22–24], thereby improving the co-assembly of hydrolyzed precursors and templates (CTAB). Furthermore, the ultrasonic could effectively intensify the secondary nucleation on the solid surface [12,23,24], thereby accelerate the deposition of silica oligomers on the magnetic particles. Therefore, the synthetic process of meso-SiO2@Fe3O4 microspheres was comprehensively accelerated by ultrasound. Besides the acceleration role, ultrasonically forced oscillations of the cavitation bubble could also improve the homogeneity of silica shell and the monodispersity of mesoSiO2@Fe3O4 microspheres [12,17], hence the meso-SiO2@Fe3O4 microspheres synthesized in ultra-rapid coating period could still maintain favorable monodispersity with uniform mesoporous silica shell.
Fig. 11. (a) N2 adsorption–desorption isotherms and (b) pore size distribution curves of the meso-SiO2@Fe3O4(E) microspheres obtained in different t. (CCTAB = 6.86 mmol/L and P = 150 W).
mesoporous silica shell based on CTAB-templating stöber method has been summarized as follow [2]: firstly, TEOS molecules were alkaline hydrolyzed, and then hydrolyzed precursors and templates (CTAB) co-assembled into composite micelles through electrostatic interaction. Subsequently, the composite micelles aggregated, precipitated from the solution because of the condensation of hydrolyzed precursors, and formed the nanosized
Scheme 1. Formation microspheres.
of
ultrasonic-assisted
synthesized
meso-SiO2@Fe3O4
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4. Conclusions In summary, ultrasonic technique has been successfully applied for the CTAB-templating synthesis of monodisperse core–shelltype meso-SiO2@Fe3O4 microspheres with favorable mesoporous silica shell. Compared with the conventional synthetic method, the use of ultrasonic irradiation can effectively simplify the coating procedure from two steps to only one step and greatly shorten coating period of mesoporous silica shell from 12–24 h to 40 min. Even so, as-prepared meso-SiO2@Fe3O4 microspheres maintain uniform core–shell structure with desirable mesoporous silica shell after acetone extraction or high temperature calcination. Therein, the mesoporous silica shell of meso-SiO2@Fe3O4(E) microspheres presents unique two-layer structure: inner nonporous layer, and outer mesoporous layer with a pore size distributions in 1.8–3.0 nm. In addition, the monodispersion and architectural features of as-prepared meso-SiO2@Fe3O4(E) microspheres are significantly influenced by initial concentration of CTAB, ultrasonic irradiation power and time. Especially, strong ultrasonic irradiation power is beneficial to obtain monodisperse meso-SiO2@Fe3O4(E) microspheres with favorable mesoporous silica shell. Finally, we also conclude that the ultrasound play the acceleration role during the whole coating process of mesoporous silica shell, including hydrolysis-condensation of TEOS, co-assembly of hydrolyzed precursors and CTAB, and deposition of silica oligomers. And ultrasonically forced oscillations of the cavitation bubble is favorable for improving the homogeneity of silica shell and the monodispersity of meso-SiO2@Fe3O4 microspheres. Acknowledgements Financial funds from the National Natural Science Foundation of China (Grant No. 21173018 and 21136001) are gratefully acknowledged. References [1] J. Liu, S.Z. Qial, Q.H. Hu, G.Q. Lu, Magnetic nanocomposites with mesoporous structures: synthesis and applications, Small 7 (2011) 425–443. [2] Y.H. Deng, Y. Cai, Z.K. Sun, D.Y. Zhao, Magnetically responsive ordered mesoporous materials: a burgeoning family of functional composite nanomaterials, Chem. Phys. Lett. 510 (2011) 1–13. [3] W.R. Zhao, J.L. Gu, L.X. Zhang, H.R. Chen, J.L. Shi, Fabrication of uniform magnetic nanocomposite spheres with a magnetic core/mesoporous silica shell structure, J. Am. Chem. Soc. 127 (2005) 8916–8917. [4] Y.H. Deng, D.W. Qi, C.H. Deng, X.M. Zhang, D.Y. Zhao, Superparamagnetic highmagnetization microspheres with an Fe3O4@SiO2 core and perpendicularly aligned mesoporous SiO2 shell for removal of microcystins, J. Am. Chem. Soc. 130 (2008) 28–29.
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Ultrasonic-assisted ultra-rapid synthesis of monodisperse meso-SiO2@Fe3O4 microspheres with enhanced mesoporous structure Hongfei Liu, Shengfu Ji ⇑, Hao Yang, Huan Zhang, Mi Tang State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China
a r t i c l e
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Article history: Received 15 June 2013 Received in revised form 19 August 2013 Accepted 19 August 2013 Available online xxxx Keywords: Ultrasonic Ultra-rapid Core–shell structure meso-SiO2@Fe3O4 microspheres Magnetic Mesoporous silica
a b s t r a c t A core–shell-type of meso-SiO2@Fe3O4 microsphere was synthesized via an ultrasonic-assisted surfactant-templating process using solvothermal synthesized Fe3O4 as core, tetraethoxysilane (TEOS) as silica source, and cetyltrimethyl ammonium bromide (CTAB) as templates. The samples were characterized by FT-IR, XRD, TEM, N2 adsorption–desorption technology, and vibrating sample magnetometer (VSM). The results show that as-prepared meso-SiO2@Fe3O4(E) and meso-SiO2@Fe3O4(C) microspheres, treated by acetone extraction and high temperature calcination, respectively, still maintain uniform core–shell structure with desirable mesoporous silica shell. Therein, the meso-SiO2@Fe3O4(E) microspheres possess a distinct pore size distribution in 1.8–3.0 nm with large specific surface area (468.6 m2/g) and pore volume (0.35 cm3/g). Noteworthily, the coating period of this ultrasonic-assisted method (40 min) is much shorter than that of the conventional method (12–24 h). The morphology of microspheres and the mesoporous structure of silica shell are significantly influenced by initial concentration of CTAB (CCTAB), ultrasonic irradiation power (P) and ultrasonic irradiation time (t). The acceleration roles of ultrasonic irradiation take effect during the whole coating process of mesoporous silica shell, including hydrolysis-condensation process of TEOS, co-assembly of hydrolyzed precursors and CTAB, and deposition of silica oligomers. In addition, the use of ultrasonic irradiation is favorable for improving the homogeneity of silica shell and the monodispersity of meso-SiO2@Fe3O4 microspheres. Ó 2013 Published by Elsevier B.V.
1. Introduction Recently, various types of magnetic mesoporous silica nanocomposites (MMSNs) with magnetic responsibility and mesoporous structure have attracted much attention due to their potential applications in catalysis, drug/DNA/gene delivery, water treatment, multimodal imagine, and so on [1,2]. Among them, core–shell-type MMSN microspheres with large Fe3O4 spherical cores (100–300 nm) and MCM-41 family ordered mesoporous silica shells have been the subject of extensive researches because to their unique features, including high thermal and chemical stabilities, favorable biocompatibility and superparamagnetism, simple functionality, high available surface area, and large pore volume [3–5]. The most representative core–shell-type MMSN microspheres (Fe3O4@nSiO2@mSiO2) with perpendicularly aligned mesoporous shells were firstly synthesized by Zhao and coworkers through a two-step coating process [4]. Subsequently, some modified core–shell-type MMSN microspheres with different specific function were obtained upon the similar two-step coating process [5–7]. During this two-step coating process, uniform Fe3O4
⇑ Corresponding author. Tel./fax: +86 10 64419619. E-mail address: [email protected] (S. Ji).
microspheres are firstly precoated with a thin silica layer through a modified stöber process, and this layer is indispensable for the deposition of uniform mesoporous silica shell through a surfactant-templating approach with cetyltrimethyl ammonium bromide (CTAB) as the templates. Therein, the thin silica layer and uniform mesoporous silica shell are derived from slow hydrolysis of tetraethyl orthosilicate (TEOS) (at least 12–24 h of mechanical stirring at room temperature). As a result, relatively complex preparation routes and long-time coating processes limit their applications in some way. Additionally, these above mentioned core–shell-type MMSN microspheres exhibit poor monodispersity which is bad for their applications in bio-fields and liquid–solid heterogeneous catalysis. Therefore, exploration of new strategies to rapidly and facilely synthesize core–shell-type MMSN microspheres with desirable monodispersity is in high demand. Over the past decades, ultrasonic method has been widely used in the preparation of powdery nanomaterials owing to its special cavitation process composed of formation, growth, and collapse of microbubbles in liquid solution [8–10]. During this process, extremely high peak temperature (>5000 K), pressure (>20 MPa), and cooling rates (over 1011 K/s) are attained inside the cavities due to the transient collapsing of the bubbles [11], meanwhile, microjet stream and shock wave are generated inside the liquids solution [12]. Considered these unique properties in the ultrasonic
1350-4177/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.ultsonch.2013.08.010
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irradiated solution, the ultrasonic technique was used in the synthesis of zeolites and mesoporous silicas, such as MCM-22 [13], SBA-15 [14], and MCM-41 [15], and it was proven that the ultrasonic treatment was effective to shorten synthesis period and improve pore structure. Very recently, ultrasonic approach has been successfully applied for the fabrication of various composites with distinct core–shell structure [16–18]. Among them, a rapid sonochemical synthesis of monodisperse nonaggregated Fe3O4@SiO2 magnetic nanoparticles with nonporous silica shell was reported [12], and it was shown that the coating process was accelerated many-fold in the presence of a 20 kHz ultrasonic field. Therefore, it is believed that the ultrasonic holds many prospects for resolving aforementioned problems, long-time preparation time and poor monodispersity, in the synthesis of core–shell-type MMSN microspheres. In this work, we report, for the first time, an ultrasonic-assisted ultra-rapid synthesis of core–shell meso-SiO2@Fe3O4 microspheres with Fe3O4 core and homogeneous mesoporous silica shell by a CTAB-templating approach. The structure and physical properties of as-prepared meso-SiO2@Fe3O4(E) and meso-SiO2@Fe3O4(C) microspheres, which are treated by acetone extraction and high temperature calcination, respectively, were studied by XRD, TEM, N2 adsorption–desorption technique and VSM. The influences of initial concentration of CTAB, ultrasonic irradiation power and ultrasonic irradiation time to the structural properties of mesoSiO2@Fe3O4(E) microspheres were investigated systematically. Finally, the formation mechanism of meso-SiO2@Fe3O4 microspheres prepared by ultrasonic-assisted method was discussed. 2. Experimental 2.1. Synthesis of Fe3O4 Briefly, 4.32 g of FeCl36H2O and 12.0 g of sodium acetate were dissolved in 160 mL of glycol under stirring. The obtained homogeneous yellow solution was transferred to a 200 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed and heated at 200 °C. After heated for 12 h, the autoclave was naturally cooled to room temperature. The obtained black magnetite particles were separated with a permanent magnet, washed with ethanol for six times, and dried in vacuum at 60 °C for 24 h. 2.2. Synthesis of meso-SiO2@Fe3O4 0.1 g of as-prepared Fe3O4 microspheres was dispersed into the mixture solution of 80 mL of ethanol, 10 mL of deioned water, 4 mL of concentrated ammonia aqueous solution (28 wt%), and a certain amount of CTAB. After this, the mixture solution was homogenized by ultrasonication (frequency = 40 kHz, power = 150 W) for 10 min to form a homogeneous dispersion. Subsequently, TEOS (0.25 mL) in ethanol (24 mL) was injected into the synthesis system under ultrasonic irradiation. After the reaction was performed for a certain time, the product was separated with a permanent magnet, washed with deionized water for three times, and dried in vacuum at 50 °C for 12 h to obtain the meso-SiO2@ Fe3O4 microspheres. The template removal was studied by two different methods: (i) Solvent extraction: as-prepared meso-SiO2@Fe3O4 microspheres were dispersed in acetone and kept at 80 °C under N2 protection for 24 h. This procedure was repeated twice. The product was defined as meso-SiO2@Fe3O4(E). (ii) High temperature Calcination: as-prepared meso-SiO2@Fe3O4 microspheres were calcined for 3 h at 450 °C under N2 protection. The heating rate was set to 1 °C/min. The product was defined as meso-SiO2@Fe3O4(C).
To study the influence of different preparation conditions on the structure and properties of meso-SiO2@Fe3O4(E) particles, various conditions including initial concentration of CTAB (CCTAB = 0– 10.29 mmol/L), ultrasonic irradiation power (P = 60–150 W) and ultrasonic irradiation time (t = 5–40 min) were chosen and their effects were investigated during the preparation process. 2.3. Characterization Power X-ray diffraction (XRD) patterns were collected on a D/Max 2500 VB2+/PC diffractometer with Ka irradiation of Cu (k = 1.5418 Å, 200 kV, 50 mA). The N2 adsorption–desorption analysis was tested on an ASAP 2020 M automatic specific surface area and aperture analyzer, and the specific surface area determination and pore volume analysis were performed by Brunauer– Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively. Transmission electron microscopy (TEM) and highresolution (HR) TEM images were taken with a JEOL (JEM-2100) transmission electron microscope operated at 200 kV accelerating voltage. Magnetic properties of the samples were measured on a vibration sample magnetometer (VSM; Lake Shore Model 7400) under magnetic fields up to 20 kOe. 3. Results and discussion 3.1. Characteristics of as-prepared meso-SiO2@Fe3O4 microspheres 3.1.1. XRD of samples Fig. 1 shows the typical XRD patterns of Fe3O4, meso-SiO2@Fe3O4(E) and meso-SiO2@Fe3O4(C), where meso-SiO2@Fe3O4 particles were synthesized under following condition: CCTAB = 6.86 mmol/L, P = 150 W and t = 30 min. From Fig. 1a, the wide-angle XRD patterns of all samples show obvious diffraction peaks, which can be readily indexed to face center cubic magnetite phase (Fe3O4, JCPDS No. 19-0629). For meso-SiO2@Fe3O4(E) and meso-SiO2@Fe3O4(C), the broad peak at 2h = 20–30° can be assigned to the amorphous silica (SiO2, JCPDS No. 29-0085) [6]. From Fig. 1b, the low-angle XRD patterns of meso-SiO2@Fe3O4(E) and meso-SiO2@Fe3O4(C) present obvious (1 0 0) peak at around 2.5° and invisible (1 1 0) and (2 0 0) peaks, which suggest the ordered mesopore symmetry, revealing the existence of short-range ordered mesoporous structure [5,6]. The decrease of intensity of (1 0 0) peak for meso-SiO2@Fe3O4(C) sample may be due to the high temperature treatment, which results in the decrease of ordering of the mesoporous structure. 3.1.2. TEM of samples The TEM images of meso-SiO2@Fe3O4 samples are shown in Fig. 2. For meso-SiO2@Fe3O4(E), monodisperse microspheres with uniform core–shell structure are clearly exhibited in Fig. 3a, from which it is obvious that the Fe3O4 core is well coated by a silica shell with thickness of about 50 nm. From HRTEM image (Fig. 3c), we can observe abundant of wormlike mesopores in the silica shell, which is different with perpendicularly aligned mesopores presented in Fe3O4@nSiO2@mSiO2 microspheres prepared by traditional two-step coating process [4–6]. Noteworthily, mesoSiO2@Fe3O4(C) microspheres still exhibit uniform core–shell structure with about 50 nm silica shell (Fig. 3b), and the silica shell keep the similarly wormlike mesopores (Fig. 3d), reflecting the favorable thermostability of as-prepared meso-SiO2@Fe3O4 microspheres. 3.1.3. N2. adsorption–desorption analysis of samples In order to directly characterize the textural properties of asprepared meso-SiO2@Fe3O4 microspheres, meso-SiO2@Fe3O4(E) and meso-SiO2@Fe3O4(C) microspheres were analyzed by N2
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Fig. 3. (a) N2 adsorption–desorption isotherms and (b) pore size distribution curves of samples. Fig. 1. (a) Wide-angle XRD patterns and (b) low-angle XRD patterns of samples.
Fig. 2. TEM images of the samples: (a) and (c) meso-SiO2@Fe3O4(E); (b) and (d) meso-SiO2@Fe3O4(C).
adsorption–desorption technology. Fig. 3 shows the N2 adsorption–desorption isotherms and the pore size distribution curves
of Fe3O4 and meso-SiO2@Fe3O4 microspheres. It can be seen that the Fe3O4 particles exhibit III-type isotherm and corresponding pore size distribution curve presents non-porous structure. For meso-SiO2@Fe3O4(E), we can observed a typical IV-type curve with a steep increase in the P/P0 range of 0.04–0.25 (Fig. 3a), and corresponding pore size distribution curve presents a sharp peak centered at the mean value of 2.3 nm (Fig. 3b). The products obtained via traditional two-step coating process have the same N2 adsorption–desorption characteristic [4–6]. Unusually, there is an obvious H1 hysteresis loop consisting one broad capillary condensation step at P/P0 of 0.45–0.95 indicated by black arrow in Fig. 3a, which indicates the presence of textual mesopores [5]. The same phenomenon was observed in the ultrasonic-assisted synthesis of ordered mesoporous silica SBA-15 [14]. The BET surface area and BJH pore volume of meso-SiO2@Fe3O4(E) microspheres are calculated to be 468.6 m2/g and 0.35 cm3/g, respectively, which are higher than that of samples obtained by traditional two-step coating process [4–6]. For meso-SiO2@Fe3O4 (C), the N2 adsorption–desorption isotherm exhibits a similar IVtype isotherm with an unobvious H1 hysteresis loop, but the pore size distribution curve presents a relatively broad peak at value of 1.8–3.0 nm, confirming the decrease of ordering of the mesopore structure after high temperature calcination. The BET surface area and BJH pore volume of meso-SiO2@Fe3O4(E) microspheres decrease to 356.3 m2/g and 0.29 cm3/g, respectively. 3.1.4. VSM of samples Fig. 4 shows the magnetization curves of Fe3O4 and mesoSiO2@Fe3O4 microspheres. All curves appear nonlinear and reversible characteristic with no hysteresis (zero coercivity and no remanence), exhibiting superparamagnetic behavior. The
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saturation magnetization (Ms) values of meso-SiO2@Fe3O4(C) and meso-SiO2@Fe3O4(E) are 44.72 and 40.38 emu/g, respectively, which can completely meet the requirement of the magnetic separation. The slight increase of Ms. values for meso-SiO2@Fe3O4(C) microspheres may be due to the Ostwald ripening during high temperature calcination [19]. From above results, it can be found that, in the ultrasonic-assisted synthesis of meso-SiO2@Fe3O4 microspheres, the use of ultrasonic can effectively shorten the coating period of mesoporous silica from 12–24 h to 40 min, and obtained meso-SiO2@Fe3O4 microspheres still exhibit uniform core–shell structure with desirable mesoporous silica shell after acetone extraction or high temperature calcination. 3.2. Effect of synthesis conditions on structure and properties of asprepared meso-SiO2@Fe3O4(E) microspheres In consideration of the extraordinary mesoporous shell of mesoSiO2@Fe3O4(E) microspheres. It is necessary to study several representative processing parameters, such as initial concentration of CTAB, ultrasonic irradiation power and ultrasonic irradiation time, which may influence the structure and/or properties of product in the ultrasonic-assisted synthesis process. Therefore, three series of meso-SiO2@Fe3O4(E) samples were prepared when any of the above variables was changed independently of the others. 3.2.1. Initial concentration of CTAB Fig. 5 shows the low-angle XRD patterns of meso-SiO2@Fe3O4(E) microspheres which were synthesized with different CCTAB, whereas the other variables were held constant (P = 150 W and t = 30 min). As shown in Fig. 5, no diffraction peak is observed in the low-angle XRD pattern (a) of sample prepared without CTAB. The patterns (b–f) of samples obtained with CTAB present obvious (1 0 0) peak at around 2.5° and invisible (1 1 0) and (2 0 0) peaks, revealing the existence of short-range ordered mesoporous structure [5,6]. And the ordering of this mesoporous structure is improved with the CCTAB increased from 1.76 to 6.86 mmol/L according to the gradually enhanced intensity of (1 0 0) peak. Moreover, nearly unchanged intensity of (1 0 0) peak at 10.29 mmol/L of CCTAB suggests that excess CTAB is unnecessary for ultrasonic-assisted synthesis of meso-SiO2@Fe3O4 microspheres. Fig. 6 shows the N2 adsorption–desorption isotherms and pore size distribution curves of meso-SiO2@Fe3O4(E) microspheres
Fig. 4. Magnetization curves of samples: (a) pure Fe3O4; (b) meso-SiO2@Fe3O4(E); (c) meso-SiO2@Fe3O4(C).
Fig. 5. Low-angle patterns of meso-SiO2@Fe3O4(E) microspheres obtained using different CCTAB: (a) 0; (b) 1.76; (c) 3.43; (d) 5.19; (e) 6.86; and (f) 10.29 mmol/L. (P = 150 W and t = 30 min).
obtained with different CCTAB. The sample obtained without CTAB presents non-porous characteristics. In the presence of CTAB, the N2 adsorption–desorption isotherms of all samples exhibit distinct IV-type isotherm with H1 hysteresis loop, and corresponding pore size distribution curves present a distinct peak at value of 1.8– 3.0 nm, indicating the presence of short-range ordered mesopores [4]. Furthermore, the intensity of the pore size distribution gradually enhances with the increase of CCTAB from 1.76 to 6.86 mmol/L, confirming the improvement of the mesoporous structure, but it remains nearly unchanged at CCTAB of 10.29 mmol/L. Meanwhile, it can be seen from Table 1 that the BET surface area and BJH pore volume of samples increase first and then decrease with the increase of CCTAB. Fig. 7 shows the TEM images of meso-SiO2@Fe3O4(E) microspheres obtained using different CCTAB. It can be observed from Fig. 7a–c and e–g that as-prepared meso-SiO2@Fe3O4(E) microspheres keep monodisperse core–shell structure and their mesoporous structure of silica shell becomes increasingly clear in the CCTAB range of 0–6.86 mmol/L. However, the microspheres obtained at 10.29 mmol/L of CCTAB show serious aggregation which may account for their slight decrease of BET surface area and BJH volume. From above results it can be inferred that the monodispersion and mesoporous structure of as-prepared meso-SiO2@Fe3O4(E) microspheres are significantly influenced by CCTAB. Low CCTAB has adverse effects on mesoporous structure of silica shell and high CCTAB goes against the monodispersion of microspheres. The optimum CCTAB for ultrasonic-assisted synthesis of meso-SiO2@Fe3O4(E) microspheres is about 6.86 mmol/L. 3.2.2. Ultrasonic irradiation power Fig. 8 shows the TEM images of meso-SiO2@Fe3O4(E) microspheres which were synthesized under different P of 60, 90, 120, and 150 W, respectively, whereas the other variables were held constant (CCTAB = 6.86 mmol/L and t = 30 min). From Fig. 8a, it can be seen that Fe3O4 microspheres exhibit serious aggragation because of the non-uniform mesoporous SiO2 coating under 60 W of P. With the P increased from 90 to 150 W, the uniformity of the mesoporous silica shell and the monodispersity of meso-SiO2@Fe3O4(E) microspheres are improved obviously (Fig. 8b–d). Therefore, it can be concluded that the P have important effects on the architectural feature of obtained microspheres [12,17].
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Same results were shown in ultrasonic synthesis of mesoporous molecular sieve (MCM-41) [15]. Therefore, it can be believed that using relatively high P is beneficial to synthesize monodisperse meso-SiO2@Fe3O4(E) microspheres with favorable mesoporous silica shell.
Fig. 6. (a) N2 adsorption–desorption isotherms and (b) pore size distribution curves of the meso-SiO2@Fe3O4(E) microspheres obtained using different CCTAB. (P = 150 W and t = 30 min).
Fig. 9 shows the N2 adsorption–desorption isotherms and pore size distribution curves of meso-SiO2@Fe3O4(E) microspheres obtained using different P. As a whole, all samples exhibit distinct IV-type isotherms with an obvious H1 hysteresis loop (Fig. 9a), and corresponding pore size distribution curves emerge a distinct peak at 1.8–3.0 (Fig. 9b). With the increase of P, the intensity of the pore size distribution enhances apparently, and the BET surface area and BJH pore volume keep continuous increase (Table 1), indicating the enhancement of mesoporous structure of the silica shell.
3.2.3. Ultrasonic irradiation time To gain insight into the formation process of mesoporous silica shell of the meso-SiO2@Fe3O4(E) microspheres under ultrasonic irradiation, time-dependent experiments were carried out. Fig. 10 shows the TEM images of meso-SiO2@Fe3O4(E) microspheres under different t of 5, 10, 20, 30, and 40 min, respectively, whereas the other variables were held constant (CCTAB = 6.86 mmol/L and P = 150 W). In general, it can be observed from Fig. 10 that all samples present monodisperse core–shell microspheres (Fig. 10a–e), and the thickness of silica shell increases from 5 to 50 nm (Fig. 10f–j) with the extension of t, revealing the gradual formation of that silica shell. When t is only 5 min, only a thin SiO2 (average thickness of 5 nm) layer without obvious mesoporous structure can be observed (Fig. 10a and f). Subsequently, as t prolonged to 10 min, it can be noticed from the image contrast that there exist obvious wormlike mesopores in the silica shell (Fig. 10b and g), and that wormlike mesoporous structure is increasingly clear when t is 20 min (Fig. 10c and h). With further extended t to 30 min and more, the morphology structure and thickness (about 50 nm) of mesoporous silica shell keep unchanged (Fig. 10d,e,i and j), revealing the fully formed of the mesoporous silica shell during the 30 min of synthesis time. Fig. 11 shows the N2 adsorption–desorption isotherms and pore size distribution curves of meso-SiO2@Fe3O4(E) microspheres synthesized under different t. As shown in Fig. 11, when t is only 5 min, the N2 adsorption–desorption isotherm and pore size distribution curve confirm that there is no mesoporous structure in the thin silica shell. As t prolonged to 10 min, the sample exhibits distinct IV-type isotherm and corresponding pore size distribution curve emerge a broad peak in 1.8–3.0 nm. With t further extended to 20 min and more, the intensity of the pore size distribution gradually enhances. In addition, Table 1 shows that the BET surface area and BJH pore volume of samples increase gradually and then remain nearly unchanged with the extension of t. Therefore, we can conclude that there is close relationship between mesoporous structure of silica shell and t. The mesoporous silica shell of as-prepared meso-SiO2@Fe3O4(E) microspheres is composed by two parts: inner nonporous layer, and outer mesoporous layer with a pore size distribution in 1.8–3.0 nm.
Table 1 Textural and structural characteristics of samples prepared in different preparation conditions.
a b c d e f
Sample
CCTABa (mmol/L)
Pb (W)
tc (min)
SBET (m2 g
1 2 3 4 5 6 7 8 9 10 11 12 13
0 1.76 3.43 5.19 6.86 10.29 6.86 6.86 6.86 6.86 6.86 6.86 6.86
150 150 150 150 150 150 60 90 120 150 150 150 150
30 30 30 30 30 30 30 30 30 5 10 20 40
32.3 142.6 209.0 306.4 468.6 398.1 153.9 315.5 414.3 23.7 103.4 245.2 441.4
1 d
)
VBJHe (cm3 g 0.07 0.11 0.16 0.24 0.35 0.32 0.13 0.26 0.31 0.04 0.17 0.22 0.34
1
)
Dvf (nm) 8.0 3.3 3.1 2.8 2.3 2.6 3.3 2.9 2.4 7.2 5.9 3.2 2.3
Initial concentration of CTAB. Ultrasonic irradiation power. Ultrasonic irradiation time. Multipoint BET surface area. BJH method cumulative desorption pore volume. Average pore diameter.
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Fig. 7. TEM images of the meso-SiO2@Fe3O4(E) microspheres obtained using different CCTAB: (a) and (e) 0; (b) and (f) 3.43; (c) and (g) 6.86; (d) and (h) 10.29 mmol/L (P = 150 W and t = 30 min).
Fig. 8. TEM images of the meso-SiO2@Fe3O4(E) microspheres obtained under different P: (a) 60; (b) 90; (c) 120; (d) 150 W. (CCTAB = 6.86 mmol/L and t = 30 min).
3.3. Formation mechanism of ultrasonic-assisted synthesized mesoSiO2@Fe3O4 microspheres Based on the above results and discussion, the ultrasonic-assisted synthesis process of meso-SiO2@Fe3O4 microspheres can be summarized in schematic illustration as shown in Scheme 1. Compared with the classical two-step coating process [4–6], the ultrasonic-assisted synthesis route has only one CTAB-templating approach, but as-prepared meso-SiO2@Fe3O4 microspheres present similar core–shell structure with an inner nonporous silica layer. Actually, the presence of this inner nonporous silica layer can not only serve as nucleation seeds for growth of mesoporous silica layer [2] but also protect magnetic cores to improve the thermal and chemical stability of meso-SiO2@Fe3O4 microspheres [20].
Fig. 9. (a) N2 adsorption–desorption isotherms and (b) pore size distribution curves of the meso-SiO2@Fe3O4(E) microspheres obtained under different P. (CCTAB = 6.86 mmol/L and t = 30 min).
In addition to simplify the coating procedure, the applied ultrasonic environment plays some significant roles in formation of meso-SiO2@Fe3O4 microspheres. In conventional synthesis of core–shell-type MMSN microspheres, the formation process of
Please cite this article in press as: H. Liu et al., Ultrasonic-assisted ultra-rapid synthesis of monodisperse meso-SiO2@Fe3O4 microspheres with enhanced mesoporous structure, Ultrason. Sonochem. (2013), http://dx.doi.org/10.1016/j.ultsonch.2013.08.010
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Fig. 10. TEM images of the meso-SiO2@Fe3O4(E) microspheres obtained in different t: (a) and (f) 5 min; (b) and (g) 10 min; (c) and (h) 20 min; (d) and (i) 30 min; (e) and (j) 40 min. (CCTAB = 6.86 mmol/L and P = 150 W).
oligomers. Finally, these precipitated nanosized oligomers deposited on the magnetic particles, shaping an ordered mesostructured silica layer. In our ultrasonic-assisted synthesis of monodisperse meso-SiO2@Fe3O4 microspheres, the ultrasound gave full play to the role of process intensification. Therein, the most obvious performance was ultra-rapid synthesis time. The acceleration role of the ultrasound was mainly manifested in following aspects. On the one hand, because the hydrolysis-condensation process of TEOS was known to be temperature sensitive [21], transient high temperature produced by cavitation process would lead to strong local acceleration of this hydrolysis-condensation process. On the other hand, microjet stream and shock wave generated inside the liquids solution by cavitation process could effectively enhance the mass transfer [22–24], thereby improving the co-assembly of hydrolyzed precursors and templates (CTAB). Furthermore, the ultrasonic could effectively intensify the secondary nucleation on the solid surface [12,23,24], thereby accelerate the deposition of silica oligomers on the magnetic particles. Therefore, the synthetic process of meso-SiO2@Fe3O4 microspheres was comprehensively accelerated by ultrasound. Besides the acceleration role, ultrasonically forced oscillations of the cavitation bubble could also improve the homogeneity of silica shell and the monodispersity of mesoSiO2@Fe3O4 microspheres [12,17], hence the meso-SiO2@Fe3O4 microspheres synthesized in ultra-rapid coating period could still maintain favorable monodispersity with uniform mesoporous silica shell.
Fig. 11. (a) N2 adsorption–desorption isotherms and (b) pore size distribution curves of the meso-SiO2@Fe3O4(E) microspheres obtained in different t. (CCTAB = 6.86 mmol/L and P = 150 W).
mesoporous silica shell based on CTAB-templating stöber method has been summarized as follow [2]: firstly, TEOS molecules were alkaline hydrolyzed, and then hydrolyzed precursors and templates (CTAB) co-assembled into composite micelles through electrostatic interaction. Subsequently, the composite micelles aggregated, precipitated from the solution because of the condensation of hydrolyzed precursors, and formed the nanosized
Scheme 1. Formation microspheres.
of
ultrasonic-assisted
synthesized
meso-SiO2@Fe3O4
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4. Conclusions In summary, ultrasonic technique has been successfully applied for the CTAB-templating synthesis of monodisperse core–shelltype meso-SiO2@Fe3O4 microspheres with favorable mesoporous silica shell. Compared with the conventional synthetic method, the use of ultrasonic irradiation can effectively simplify the coating procedure from two steps to only one step and greatly shorten coating period of mesoporous silica shell from 12–24 h to 40 min. Even so, as-prepared meso-SiO2@Fe3O4 microspheres maintain uniform core–shell structure with desirable mesoporous silica shell after acetone extraction or high temperature calcination. Therein, the mesoporous silica shell of meso-SiO2@Fe3O4(E) microspheres presents unique two-layer structure: inner nonporous layer, and outer mesoporous layer with a pore size distributions in 1.8–3.0 nm. In addition, the monodispersion and architectural features of as-prepared meso-SiO2@Fe3O4(E) microspheres are significantly influenced by initial concentration of CTAB, ultrasonic irradiation power and time. Especially, strong ultrasonic irradiation power is beneficial to obtain monodisperse meso-SiO2@Fe3O4(E) microspheres with favorable mesoporous silica shell. Finally, we also conclude that the ultrasound play the acceleration role during the whole coating process of mesoporous silica shell, including hydrolysis-condensation of TEOS, co-assembly of hydrolyzed precursors and CTAB, and deposition of silica oligomers. And ultrasonically forced oscillations of the cavitation bubble is favorable for improving the homogeneity of silica shell and the monodispersity of meso-SiO2@Fe3O4 microspheres. Acknowledgements Financial funds from the National Natural Science Foundation of China (Grant No. 21173018 and 21136001) are gratefully acknowledged. References [1] J. Liu, S.Z. Qial, Q.H. Hu, G.Q. Lu, Magnetic nanocomposites with mesoporous structures: synthesis and applications, Small 7 (2011) 425–443. [2] Y.H. Deng, Y. Cai, Z.K. Sun, D.Y. Zhao, Magnetically responsive ordered mesoporous materials: a burgeoning family of functional composite nanomaterials, Chem. Phys. Lett. 510 (2011) 1–13. [3] W.R. Zhao, J.L. Gu, L.X. Zhang, H.R. Chen, J.L. Shi, Fabrication of uniform magnetic nanocomposite spheres with a magnetic core/mesoporous silica shell structure, J. Am. Chem. Soc. 127 (2005) 8916–8917. [4] Y.H. Deng, D.W. Qi, C.H. Deng, X.M. Zhang, D.Y. Zhao, Superparamagnetic highmagnetization microspheres with an Fe3O4@SiO2 core and perpendicularly aligned mesoporous SiO2 shell for removal of microcystins, J. Am. Chem. Soc. 130 (2008) 28–29.
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Please cite this article in press as: H. Liu et al., Ultrasonic-assisted ultra-rapid synthesis of monodisperse meso-SiO2@Fe3O4 microspheres with enhanced mesoporous structure, Ultrason. Sonochem. (2013), http://dx.doi.org/10.1016/j.ultsonch.2013.08.010