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Silicon dioxide hollow microspheres with porous composite structure: Synthesis and characterization
Journal of Colloid and Interface Science 362 (2011) 253–260
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Silicon dioxide hollow microspheres with porous composite structure: Synthesis and characterization Xiuli Yan, Zhongli Lei ⇑ Key Laboratory of Applied Surface and Colloid Chemistry, Shaanxi Normal University, Ministry of Education, School of Chemistry and Materials Science, Xi’an 710062, PR China
a r t i c l e
i n f o
Article history: Received 16 December 2010 Accepted 25 June 2011 Available online 7 July 2011 Keywords: Hollow porous SiO2 microsphere Micro-gel template Surface morphology
a b s t r a c t In this paper, a strategy for hollow porous silica microspheres with ideally flower structure is presented. SiO2/PAM hybrid composite microspheres with porous were synthesized by the reaction that the porous polyacrylamide (PAM) micro-gels immersed in tetraethoxysilane (TEOS) anhydrous alcohol solution and water in a moist atmosphere, with ammonium hydroxide as a catalyst. The SiO2 hollow microspheres with porous were obtained after calcination of the composite microspheres at 550 °C for 4 h. The morphology, composition, and crystalline structure of the microspheres were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermo-gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FI-IR), and X-ray diffraction (XRD), N2 absorption analysis, respectively. The results indicated that the obtained hollow porous SiO2 microspheres were a perfect flower structure. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Recently, hollow porous materials with size-tunable have been widely studied because of its special properties and potential applications in many fields, such as drug delivery [1,2], catalysis applications [3], gas sensor [4], absorption [5], high-capacity reservoirs to store chemicals, biological agents, or other nanoparticles [6], and so forth. For example, Li et al. fabricated hollow porous silica via a sol–gel route with two different structure-directing templates, and the produced porous hollow silica nanoparticles (PHSNs) were applied as controlled pesticide release carriers to study the effects of the shell thickness on the loading efficiency for avermectin, the UV-shielding property for the loaded avermectin and the controlled release of the loaded avermectin from the carriers. Li et al. found that at least 18% of the loaded avermectin still remained un-decomposed inside the PHSNs carriers after UV irradiation for 720 min [7]. Hollow porous silica materials, as one of the hollow porous materials, have also received considerable attention because of its good biocompatibility, and very high specific surface area. Various morphologies such as silica ellipsoids, asymmetrical silica dumbbells, mesoporous silica nanospheres, and hollow microspheres have been reported using different strategies [8–12]. Recently, Zhang et al. synthesized hollow silica spheres with hexagonally arrayed cylindrical nanochannels along the radial direction using a one-pot sol–gel/emulsion approach ⇑ Corresponding author. Address: College of Chemistry and Materials Science, Shaanxi Normal University, Xi’an 710062, PR China. Fax: +86 29 85307774. E-mail address: [email protected] (Z. Lei). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.06.062
[13]. The result shows the spheres have very high specific surface area up to 700 m2/g, the spheres can be potentially used as capsules for controlled release of drug and dyes. Most recently, Wang et al. fabricated mesoporous silica hollow spheres with ordered surface morphologies using a single-step, emulsion template method [14]. They used PEG–PPG–PEG as a template, n-octane as core materials, and benzyl alcohol as emulsion generator to fabricate hierarchical mesoporous silica hollow spheres with monodisperse microcapsule and uniform shells. Although a great deal of scientists concentrate on the preparation of hollow porous silica microspheres, few scientists report about the preparation of well-defined flower hollow porous silica morphologies with the micrometer scale. We use template replication method to prepare the hollow porous silica spheres. The template replication method is the most extensive, effective and easier approach for the preparation of hollow porous materials. Inorganic particles [15,16], surfactant [17] and polymeric micro-gel particles [18] are as the template material. However, methods of fabrication of hollow porous materials have mainly focused on using the polymer as a template to prepare the hollow microspheres. Thereby, we use the porous PAM microspheres as a template in the paper. Moreover, the polymeric hydro-gel has three-dimensional network structure [19]. The hollow porous silica spheres can be synthesized through hydrolysis and condensation reactions of TEOS in the emulsion system containing TEOS, water, anhydrous alcohol, and ammonium hydroxide. Here, we present a method for preparation of SiO2 hollow microspheres with porous composed of a uniquely tunable shaped outer morphology. It contains that PAM micro-gels were prepared
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according to the reverse suspended aggregation, PAM microspheres with porous structures were prepared by the freeze-drying treatment, silica coats, and the polymer components were removed via heat treatment. After thermal decomposition of the polymer components in the silica shell, SiO2 hollow microspheres with porous will be obtained.
2. Materials and methods 2.1. Materials Acrylamide (AM) was purified by re-crystallization in acetone. N,N0 -methylene bisacryamide (BA), cyclohexane, acetone, tetraethoxysilane (TEOS, 98%), ammonium per-sulfate (APS), span-60, ammonium hydroxide (NH3H2O, 25%), and anhydrous alcohol were of analytical grade. All these chemicals were used without further purification. Water used in the experiments was doubledistilled.
2.2. Synthesis 2.2.1. Synthesis of PAM micro-gels At 35 °C, 244.8 rpm, 90 ml of cyclohexane and 0.60 g of Span-60 were added to a 250 ml three-neck flask equipped with a mechanical stirrer, a nitrogen inlet, and a Hirsch funnel. The mixture was stirred vigorously under nitrogen purging until the surfactant was uniformly dispersed. At the same time, 2.00 g of AM, 0.10 g of BA, and 0.12 g of APS dissolved into 10 ml of double-distilled water were added slowly into the flask via the Hirsch funnel. After the solution being dripped off, the temperature rose to 68 °C. The new mixture was stirred continuously under a nitrogen atmosphere at 68 °C for 4 h. The PAM micro-gels were collected and washed alternatively with double-distilled water and anhydrous alcohol, and the white product was dried overnight under ambient conditions. The SEM images of the resulting PAM micro-gels are shown in Fig. 1a.
2.2.2. Synthesis of PAM porous microspheres PAM microspheres with porous structures were prepared by the freeze-drying treatment [20]. A typical preparation is as follows: 0.1 g of PAM micro-gel was swollen by adding 8 ml of doubledistilled water and then the sample, after being quenched in liquid nitrogen, was freeze-dried by an ALPHA1-2 freeze-drying instrument at 55 °C for 24 h. The PAM micro-gels with porous structures were finally obtained. The typical SEM images of the freeze-dried PAM micro-gels are shown in Fig. 1b.
2.2.3. Synthesis of PAM/SiO2 composite microspheres The composite microspheres of PAM/SiO2 were prepared by the following steps. At room temperature, 0.1 g of the freeze dried PAM micro-gels were immersed in 1.5 ml of Tetraethoxysilane (TEOS, 98%), 47.5 ml of anhydrous alcohol, 5 ml of double-distilled water and 1 ml of ammonium hydroxide (NH3H2O, 25%) in a small beaker. Then, the small beaker was placed in a closed large beaker containing water for 12 h. After being washed with anhydrous alcohol, the white PAM/SiO2 composite microspheres were obtained. Finally, the hollow porous silica microspheres were obtained by calcination at 550 °C for 4 h. The synthesis process is shown in Scheme 1. 2.3. Characterization The morphologies of the PAM micro-gels, the freeze-dried PAM micro-gels, the PAM/SiO2 composite microspheres, and the SiO2 hollow microspheres with porous were examined by Philips scanning electron micros-copy (SEM), using an accelerating voltage of 20 kV (the samples were coated with a thin layer of gold before measurement). The elements in the samples were probed by the EDX analysis accessory to the scanning electron microscopy (SEM). To investigate the ‘‘hollow’’, the composite microspheres were dispersed in alcohol and observed under transmission electron microscopy (TEM, JEM-2100), using an accelerating voltage of 200 kV. The IR spectra were recorded on an AVTAR360 Nicolet Fourier transform infrared (FT-IR) spectrometer using a KBr pellet. Thermo gravimetric analyses (TGA) were performed using a SDT Q600 V8.0 Build 95 instrument. The composite powders were heated to 650 °C in oxygen at a scan rate of 5 °C/min, and the observed mass loss was attributed to the quantitative pyrolysis of the polymeric component. The X-ray diffraction (XRD) spectra were taken from 2h angles from 10° to 70° at a scan rate of 8°/ min. The accelerating voltage and electric current were 35 kV and 40 mA. The BET (Brunauer–Emmett–Teller) specific surface area (ABET) was calculated from the adsorption data in the relative pressure ranging from 0.04 to 0.1. The pore size (Dp) distribution was calculated from the adsorption branch of isotherms using the BJH formula, respectively. 3. Results and discussion 3.1. Morphology of PAM micro-gels Fig. 1 shows typical SEM images of the PAM micro-gels and the water-swollen PAM after freeze-drying. It can be seen in Fig. 1 that the PAM micro-gels are perfectly spherical with a diameter of about 40 lm (see Fig. 1a). And the slight irregularities in the smooth surfaces, this is probably the result of grinding and crushing. Whereas,
Fig. 1. SEM images of: (a) PAM microgels with alcohol, (b) the water-swollen PAM after freeze-drying.
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by water swollen
PAM micrgels
treated by freeze-drying
added into Tetraethoxysilane anhydrous alcohol solution
placed in a closed beaker
washed by anhydrous samples
added into ammonium hydroxide
alcohol
water
at 550°C calcination 4h
SiO2 /PAM
SiO2 hollow microspheres with porous
Scheme 1. Procedure for preparation of SiO2 hollow microspheres with porous.
see the Fig. 1b, the water-swollen PAM micro-gels, after being treated by freeze-drying, exhibit porous structures, and the diameter of the freeze-dried PAM micro-gels are about 60 lm. 3.2. Characterizations of SiO2 hollow microspheres with porous Fig. 2 shows optimum SEM, TEM images of SiO2 hollow microspheres with porous. Fig. 2c shows typical SEM images of PAM/SiO2 microspheres. In comparison with Fig. 2c, the diameter of SiO2 hollow microspheres with porous is slightly decreased, and the spherical surface morphology forms a sharp change. This is likely to calcine process of silicon the gelatin in the result of shrinkage [21,22]. Fig. 2a exhibits a perfectly whole microsphere with a diameter of about 50 lm and the spherical surface morphology is flower-like structure. Fig. 2b clearly shows that the product is
hollow with its body porous and flower-like structure. TEM images more clearly reveal that the resultant product is a hollow structure, and the shell has a porous appearance. That may be the outermost layer of the end of the shadow caused by flower-like structure. The diameter is about 20 lm (Fig. 2d), which is slight smaller than the particle size in SEM images. This may be because the particle size is uneven. In conclusion, the surface of the product is attributed to an ideal flower wrinkly surface, not smooth porous structure. Such a unique surface structure in the controlled spherical morphology is unprecedented. Moreover, the micro-pore size (Dp) distribution estimated by the BJH method reveals the micro-pores is 1–3 nm in diameter (Figs. 8 and 10). And that there are also pores whose size is distributed between 50 and 100 nm (Fig. 8a, b and 10d). Fig. 3 shows the FT-IR spectra of the PAM micro-gel (a), the PAM/SiO2 composite microspheres (b), PAM/SiO2 composite
Fig. 2. SEM images (a, b) of SiO2 hollow microspheres with porous and (c) of PAM/SiO2 microspheres: (a) a whole microsphere, and (b) a broken microsphere, (c) PAM/SiO2 microspheres. (d) TEM images d of the whole SiO2 hollow microspheres with porous.
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microspheres after calcination at 550 °C (c) and pure SiO2 microspheres (d). Similarities and differences are clear by comparing these spectra. The characteristic absorption bands for PAM are clearly observed as shown in Fig. 5a, such as the carbonyl group of PAM at 1639.98 cm1 and the amide group of PAM at 3439.25 cm1, respectively. These characteristic absorption bands basically disappear after calcination at 550 °C (cf. Fig. 5a–c) as a result of decomposition of organic components. It is obviously found that the peaks at about 808.63 and 1104.18 cm1 appear after calcination. The results are attributed to the Si–O–Si asymmetric stretching and symmetric stretching bands of SiO2 [23]. Compared with pure SiO2 microspheres, the peaks at around 950 and 560 cm1 belong to silanol disappear after calcination at 550 °C because of the condensation leading silanol disappear [24]. Furthermore, the peak at 3100 cm1 disappeared after calcination at 550 °C, which is attributed to decrease in silanol. Fig. 4 shows the EDX spectrum of PAM/SiO2 composite microspheres before (a) and after calcination at 550 °C (b). The obvious characteristic peaks for C, N, O, and Si elements, originating from composite microspheres, are found in Fig. 6a, indicating that SiO2 is, indeed, incorporated in the PAM microspheres. At the same time, the characteristic peaks for N elements, originating from microspheres after calcination at 550 °C, vanish in Fig. 6b, indicating that SiO2 hollow microspheres have been obtained. Thermo gravimetric analysis of PAM/SiO2 and after calcination at 550 °C is shown in Fig. 5. The result shows that a loss of 9.32% in weight, below 246.41 °C, is assigned to releasing water physic absorbed in the composite microspheres [25]. From 246.41 to 440.20 °C, a loss of 62.02 wt.% could be ascribed to the decomposition of PAM and loss of water from crystal transformation. The XRD patterns of PAM/SiO2 after calcination at 550 for 4 h are shown in Fig. 6. It can be seen only a single broad peak at 2h 20° in the spectra for PAM/SiO2 after calcination at 550 °C. This indicates that SiO2 hollow microspheres with porous is in the amorphous state [14]. Based on the aforementioned observations, we propose a mechanism to explain the formation of such morphology microspheres via a sol–gel deposited method. When the freeze dried PAM micro-gels were completely immersed in the sol mixture [ethanol + water + TEOS + ammonium hydroxide], the freeze dried PAM micro-gels acted as a micro-reactor, Si(OR)4 where R is a – C2H5, three kinds of reactions occur depending on the involved functional groups, hydrolysis of Si–OR to Si–OH (Eq. (1)), alcoholproducing condensation between Si–OR and Si–OH (Eq. (2)), and water-producing condensation between two Si–OH (Eq. (3)).
ðROÞ3 Si—OR þ H2 O
hydrolysis
esterification
RO3 Si—OH þ ROH
ð1Þ
Fig. 3. FT-IR absorption spectra of the PAM (a); PAM/SiO2 before (b) after calcination at 550 °C (c); and pure SiO2 microspheres (d).
ðROÞ3 Si—OH þ RO—SiðROÞ3
alcohol condensation
alcoholysis
RO3 Si—O—SiðORÞ3 þ ROH
ðROÞ3 Si—OH þ HO—SiðORÞ3
water condensation
hydrolysis
ð2Þ
RO3 Si—O—SiðORÞ3 þ H2 O
ð3Þ
TEOS was used as the precursor for the silica shell in the reaction. The first step is hydrolysis of TEOS. It is followed by slower poly-condensation reactions, and the reaction occurs on the – NH2 surface of the freeze dried PAM micro-gels that serve as a framework for deposition of a silica gel network. Generally speaking, condensation cannot start until hydrolysis has proceeded to some extent and then the two steps occur simultaneously [26]. In practice, the relative rate of hydrolysis and condensation varies with the reaction conditions. Therefore, the reaction pathway and the structure of silica gel prepared from TEOS vary widely depending on the preparation conditions. Among various parameters in the reaction conditions, pH has been known to have the largest effect on the preferential site for each reaction, as well as on the structure of the silica gel obtained [27]. At low pH, acid catalysis promotes hydrolysis but hinders both condensation and dissolution reactions [28]. Consequently, for the synthesis of silica particles using acid catalysis, small growth is observed and smaller, homogeneous particles are formed [29].
Fig. 4. EDX spectrum of PAM/SiO2 composite microspheres before (a) and after calcination at 550 °C (b).
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Fig. 6. XRD patterns of PAM/SiO2 after calcination at 550. Fig. 5. Thermo-gravimetric analysis of PAM/SiO2 (a) and after calcination at 550 °C (b).
Namely, the silicon particles evenly wrapped in the PAM, it cannot be the structure of aforementioned observations. Base catalysis of the hydrolysis and condensation reactions, in contrast, promotes fast condensation and dissolution. This leads to the production of an inhomogeneous system due to rapid condensation of all the hydrolyzed precursor monomers and to the formation of dense silica particles by the ripening of aggregates formed during the collision of droplets. As a result, the micro-particles are dense; namely, their scattering length is that of amorphous silica (this is a good agreement with that of XRD), and aggregate of
the silicon particles present the structure of aforementioned observations (Fig. 2a) [30,31]. 3.3. Effects of amount of the reactants on the morphology and Porosity of SiO2 hollow microspheres with porous
3.3.1. TEOS The SiO2 hollow microspheres with porous were obtained after calcination of the PAM/SiO2 composite microspheres at 550 °C for 4 h, and the PAM/SiO2 composite microspheres were synthesized at room temperature with the volume composition of TEOS varied;
Fig. 7. SEM images of the products synthesized with the composition of TEOS varied. The volume composition of the reactants was 1:x:47.5:5NH3H2O:TEOS:C2H5OH:H2O, where x = (a) 1.5, (b) 6, (c) 12, and (d) 25, and inset shows SEM image of the broken microsphere.
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X. Yan, Z. Lei / Journal of Colloid and Interface Science 362 (2011) 253–260 Table 1 Micropore parameters determined by BET. Specific surface area (m2/g) 8.85
Porous volume (cm3/g) 0.02
Mean pore diameter (nm) 10.51
NH3H2O Hollow porous SiO2 microspheresr Hollow porous SiO2 microspheress Hollow porous SiO2 microspherest Hollow porous SiO2 microspheresu
248.08
0.5537
1.84
123.08
0.3014
2.08
101.18
0.2773
2.18
89.82
0.2489
2.30
TEOS
381.96
0.8349
1.96
182.82
0.4508
2.18
135.13
0.2663
2.28
52.48
0.2394
2.45
Sample
PAM porous microspheres
Fig. 8. The pore size distribution by the BJH method using adsorption branch (DP: pore diameter; VP: pore volume; the volume of TEOS (a) 1.5, (b) 6, (c) 12, and (d) 25).
the volume composition of the reactants was 1:{:47.5:5NH3H2O:TEOS:C2H5OH:H2O, where x was the volume composition of TEOS. The representative SEM images of the products with the composition of TEOS varied are shown in Fig. 7. When the other conditions kept changeless, x was increased from 1.5 to 25; the morphology of the resultant products formed a great change (see Fig. 3a–d) from perfectly flower microsphere to smooth structure. In addition, the pore diameter of the resultant products was gradually increased as the increase of TEOS volume (see Fig. 8a–d). This was because the addition of TEOS can induce the formation of large amounts of the textural pores [32]. At the same time, it is shown that the micropores parameters determined by BET in Table 1. It can be seen in Table 1 that the BET surface area and total pore volume were gradually decreased with increasing x; the ABET at x = 1.5 and 25 were 381.96 and 52.48 m2/g, respectively; the VP at x = 1.5 and 25 were ca. 0.8349 and 0.2394 cm3/g, respectively. These results imply that the pore structure is changed from porous to smooth structure.
Hollow porous SiO2 microspheresv Hollow porous SiO2 microspheresw Hollow porous SiO2 microspheresx Hollow porous SiO2 microspheresy
The condition for preparation of the hollow porous SiO2 microspheres r s t u v w x y are shown in Figs. 9a–d and 7a–d.
And the mean micro-pore diameter was slightly increased with increasing x. 3.3.2. Ammonium hydroxide According to the proposed method for preparation of PAM/SiO2 composite microspheres, the interfacial reaction between TEOS located at the immersed PAM microspheres and water in a moist atmosphere with ammonium hydroxide as a catalyst is employed to incorporate SiO2 into the template microspheres. Because, by
Fig. 9. SEM images of the products synthesized with different concentration of ammonium hydroxide. (a) 10%, (b) 15%, (c) 20%, and (d) 25%, and inset shows SEM image of the broken microsphere.
X. Yan, Z. Lei / Journal of Colloid and Interface Science 362 (2011) 253–260
Fig. 10. The pore size distribution by the BJH method using adsorption branch (DP: pore diameter; VP: pore volume; the concentration of ammonium hydroxide (a) 10%, (b) 15%, (c) 20%, and (d) 25%).
changing the pH of the solution, wide control of the size and shape of the silica polymers evolving is achieved. The rate of hydrolysis of TEOS (Eq. (1)) exhibits a minimum at pH = 7 and increases exponentially at either lower or higher pH. In contrast, the rate of condensation (Eqs. (2) and (3)) exhibits a minimum at pH = 2 and a maximum around pH = 7, where SiO2 solubility and dissolution rates are maximized [33]. So, it is necessary to investigate the effect of concentration of the ammonium hydroxide on the morphology of the composite microspheres. No silica product was obtained when concentration of the ammonium hydroxide was 0, likely because of the lack of catalysis for the hydrolysis of TEOS followed by condensation of the resultant products. When the other conditions kept changeless, concentration of the ammonium hydroxide was increased from 10% to 25%, the morphology of the resultant products had a great diversification range from porous crinkly to smooth surface morphology (see Fig. 9a–d). This likely is because the increase in ammonium hydroxide concentration as a base led to the slight increase in the pH in the solution. When concentration of the ammonium hydroxide was 10% and 20%, the
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pH was found to be approximately 7.0 and 8.0, respectively. We know that the rate of hydrolysis of TEOS, condensation of silicate species and the solubility of silicate species are drastically changed at the boundary between pH = 8 and 9 [34,35]. When concentration of the ammonium hydroxide was 25%, the ABET and VP were 89.82 m2/g and 0.2489 cm3/g, respectively. However, When concentration of the ammonium hydroxide was 10%, the ABET and VP were 248.08 m2/g and 0.5537 cm3/g, respectively. It suggests that the pore structure is changed range from porous crinkly to smooth surface morphology (see Fig. 10a–d). This can because the hydrolysis, condensation and dissolution reactions promote fast when concentration of the ammonium hydroxide was 25%. This leads to the production of an inhomogeneous system due to rapid condensation of all the hydrolyzed precursor monomers and to the formation of dense silica particles by the ripening of aggregates formed during the collision of droplets. As a result, the micro particles are dense; namely, their scattering length is amorphous silica (this is a good agreement with that of XRD). And the amorphous silica is basically no porosity [28]. In other words, the micro particle synthesis results in the formation of relatively large matrix particles of low and even negligible porosity. Consequently, the morphology of the resultant products might be transformed from porous crinkly to smooth surface, leading to the change in pores structure.z
4. Conclusions We have demonstrated that the hollow porous silica composite microspheres can be successfully synthesized through the following procedure: the porous polyacrylamide (PAM) micro-gels saturated with tetraethoxysilane (TEOS) anhydrous alcohol solution and water in a moist atmosphere, with ammonium hydroxide as a catalyst. The hollow porous silica spheres with controllable surface morphologies have been synthesized through calcination of the PAM/SiO2 composite microspheres at 550 °C for 4 h. The resultant products have three typical surface morphologies: (1) an ideal flower surface; (2) a porous wrinkly morphology; (3) a wrinkly
Fig. 11. TEM images of PAM/SiO2 microspheres with deposition times of SiO2 for 8 h (a), 12 h (b), and 16 h (c) after calcination at 550 °C.
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surface. In addition, we investigate the effects of the volume of TEOS and the concentration of ammonium hydroxide on the morphologies of hollow porous silica spheres. The results indicate that the surface morphologies, the pore size, as well as the thickness of silica shell can be effectively adjusted by changing the volume of TEOS and the concentration of ammonium hydroxide. The pore size of hollow porous silica spheres increases as the amount of TEOS and the concentration of ammonium hydroxide increases.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Silicon dioxide hollow microspheres with porous composite structure: Synthesis and characterization Xiuli Yan, Zhongli Lei ⇑ Key Laboratory of Applied Surface and Colloid Chemistry, Shaanxi Normal University, Ministry of Education, School of Chemistry and Materials Science, Xi’an 710062, PR China
a r t i c l e
i n f o
Article history: Received 16 December 2010 Accepted 25 June 2011 Available online 7 July 2011 Keywords: Hollow porous SiO2 microsphere Micro-gel template Surface morphology
a b s t r a c t In this paper, a strategy for hollow porous silica microspheres with ideally flower structure is presented. SiO2/PAM hybrid composite microspheres with porous were synthesized by the reaction that the porous polyacrylamide (PAM) micro-gels immersed in tetraethoxysilane (TEOS) anhydrous alcohol solution and water in a moist atmosphere, with ammonium hydroxide as a catalyst. The SiO2 hollow microspheres with porous were obtained after calcination of the composite microspheres at 550 °C for 4 h. The morphology, composition, and crystalline structure of the microspheres were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermo-gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FI-IR), and X-ray diffraction (XRD), N2 absorption analysis, respectively. The results indicated that the obtained hollow porous SiO2 microspheres were a perfect flower structure. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Recently, hollow porous materials with size-tunable have been widely studied because of its special properties and potential applications in many fields, such as drug delivery [1,2], catalysis applications [3], gas sensor [4], absorption [5], high-capacity reservoirs to store chemicals, biological agents, or other nanoparticles [6], and so forth. For example, Li et al. fabricated hollow porous silica via a sol–gel route with two different structure-directing templates, and the produced porous hollow silica nanoparticles (PHSNs) were applied as controlled pesticide release carriers to study the effects of the shell thickness on the loading efficiency for avermectin, the UV-shielding property for the loaded avermectin and the controlled release of the loaded avermectin from the carriers. Li et al. found that at least 18% of the loaded avermectin still remained un-decomposed inside the PHSNs carriers after UV irradiation for 720 min [7]. Hollow porous silica materials, as one of the hollow porous materials, have also received considerable attention because of its good biocompatibility, and very high specific surface area. Various morphologies such as silica ellipsoids, asymmetrical silica dumbbells, mesoporous silica nanospheres, and hollow microspheres have been reported using different strategies [8–12]. Recently, Zhang et al. synthesized hollow silica spheres with hexagonally arrayed cylindrical nanochannels along the radial direction using a one-pot sol–gel/emulsion approach ⇑ Corresponding author. Address: College of Chemistry and Materials Science, Shaanxi Normal University, Xi’an 710062, PR China. Fax: +86 29 85307774. E-mail address: [email protected] (Z. Lei). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.06.062
[13]. The result shows the spheres have very high specific surface area up to 700 m2/g, the spheres can be potentially used as capsules for controlled release of drug and dyes. Most recently, Wang et al. fabricated mesoporous silica hollow spheres with ordered surface morphologies using a single-step, emulsion template method [14]. They used PEG–PPG–PEG as a template, n-octane as core materials, and benzyl alcohol as emulsion generator to fabricate hierarchical mesoporous silica hollow spheres with monodisperse microcapsule and uniform shells. Although a great deal of scientists concentrate on the preparation of hollow porous silica microspheres, few scientists report about the preparation of well-defined flower hollow porous silica morphologies with the micrometer scale. We use template replication method to prepare the hollow porous silica spheres. The template replication method is the most extensive, effective and easier approach for the preparation of hollow porous materials. Inorganic particles [15,16], surfactant [17] and polymeric micro-gel particles [18] are as the template material. However, methods of fabrication of hollow porous materials have mainly focused on using the polymer as a template to prepare the hollow microspheres. Thereby, we use the porous PAM microspheres as a template in the paper. Moreover, the polymeric hydro-gel has three-dimensional network structure [19]. The hollow porous silica spheres can be synthesized through hydrolysis and condensation reactions of TEOS in the emulsion system containing TEOS, water, anhydrous alcohol, and ammonium hydroxide. Here, we present a method for preparation of SiO2 hollow microspheres with porous composed of a uniquely tunable shaped outer morphology. It contains that PAM micro-gels were prepared
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according to the reverse suspended aggregation, PAM microspheres with porous structures were prepared by the freeze-drying treatment, silica coats, and the polymer components were removed via heat treatment. After thermal decomposition of the polymer components in the silica shell, SiO2 hollow microspheres with porous will be obtained.
2. Materials and methods 2.1. Materials Acrylamide (AM) was purified by re-crystallization in acetone. N,N0 -methylene bisacryamide (BA), cyclohexane, acetone, tetraethoxysilane (TEOS, 98%), ammonium per-sulfate (APS), span-60, ammonium hydroxide (NH3H2O, 25%), and anhydrous alcohol were of analytical grade. All these chemicals were used without further purification. Water used in the experiments was doubledistilled.
2.2. Synthesis 2.2.1. Synthesis of PAM micro-gels At 35 °C, 244.8 rpm, 90 ml of cyclohexane and 0.60 g of Span-60 were added to a 250 ml three-neck flask equipped with a mechanical stirrer, a nitrogen inlet, and a Hirsch funnel. The mixture was stirred vigorously under nitrogen purging until the surfactant was uniformly dispersed. At the same time, 2.00 g of AM, 0.10 g of BA, and 0.12 g of APS dissolved into 10 ml of double-distilled water were added slowly into the flask via the Hirsch funnel. After the solution being dripped off, the temperature rose to 68 °C. The new mixture was stirred continuously under a nitrogen atmosphere at 68 °C for 4 h. The PAM micro-gels were collected and washed alternatively with double-distilled water and anhydrous alcohol, and the white product was dried overnight under ambient conditions. The SEM images of the resulting PAM micro-gels are shown in Fig. 1a.
2.2.2. Synthesis of PAM porous microspheres PAM microspheres with porous structures were prepared by the freeze-drying treatment [20]. A typical preparation is as follows: 0.1 g of PAM micro-gel was swollen by adding 8 ml of doubledistilled water and then the sample, after being quenched in liquid nitrogen, was freeze-dried by an ALPHA1-2 freeze-drying instrument at 55 °C for 24 h. The PAM micro-gels with porous structures were finally obtained. The typical SEM images of the freeze-dried PAM micro-gels are shown in Fig. 1b.
2.2.3. Synthesis of PAM/SiO2 composite microspheres The composite microspheres of PAM/SiO2 were prepared by the following steps. At room temperature, 0.1 g of the freeze dried PAM micro-gels were immersed in 1.5 ml of Tetraethoxysilane (TEOS, 98%), 47.5 ml of anhydrous alcohol, 5 ml of double-distilled water and 1 ml of ammonium hydroxide (NH3H2O, 25%) in a small beaker. Then, the small beaker was placed in a closed large beaker containing water for 12 h. After being washed with anhydrous alcohol, the white PAM/SiO2 composite microspheres were obtained. Finally, the hollow porous silica microspheres were obtained by calcination at 550 °C for 4 h. The synthesis process is shown in Scheme 1. 2.3. Characterization The morphologies of the PAM micro-gels, the freeze-dried PAM micro-gels, the PAM/SiO2 composite microspheres, and the SiO2 hollow microspheres with porous were examined by Philips scanning electron micros-copy (SEM), using an accelerating voltage of 20 kV (the samples were coated with a thin layer of gold before measurement). The elements in the samples were probed by the EDX analysis accessory to the scanning electron microscopy (SEM). To investigate the ‘‘hollow’’, the composite microspheres were dispersed in alcohol and observed under transmission electron microscopy (TEM, JEM-2100), using an accelerating voltage of 200 kV. The IR spectra were recorded on an AVTAR360 Nicolet Fourier transform infrared (FT-IR) spectrometer using a KBr pellet. Thermo gravimetric analyses (TGA) were performed using a SDT Q600 V8.0 Build 95 instrument. The composite powders were heated to 650 °C in oxygen at a scan rate of 5 °C/min, and the observed mass loss was attributed to the quantitative pyrolysis of the polymeric component. The X-ray diffraction (XRD) spectra were taken from 2h angles from 10° to 70° at a scan rate of 8°/ min. The accelerating voltage and electric current were 35 kV and 40 mA. The BET (Brunauer–Emmett–Teller) specific surface area (ABET) was calculated from the adsorption data in the relative pressure ranging from 0.04 to 0.1. The pore size (Dp) distribution was calculated from the adsorption branch of isotherms using the BJH formula, respectively. 3. Results and discussion 3.1. Morphology of PAM micro-gels Fig. 1 shows typical SEM images of the PAM micro-gels and the water-swollen PAM after freeze-drying. It can be seen in Fig. 1 that the PAM micro-gels are perfectly spherical with a diameter of about 40 lm (see Fig. 1a). And the slight irregularities in the smooth surfaces, this is probably the result of grinding and crushing. Whereas,
Fig. 1. SEM images of: (a) PAM microgels with alcohol, (b) the water-swollen PAM after freeze-drying.
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by water swollen
PAM micrgels
treated by freeze-drying
added into Tetraethoxysilane anhydrous alcohol solution
placed in a closed beaker
washed by anhydrous samples
added into ammonium hydroxide
alcohol
water
at 550°C calcination 4h
SiO2 /PAM
SiO2 hollow microspheres with porous
Scheme 1. Procedure for preparation of SiO2 hollow microspheres with porous.
see the Fig. 1b, the water-swollen PAM micro-gels, after being treated by freeze-drying, exhibit porous structures, and the diameter of the freeze-dried PAM micro-gels are about 60 lm. 3.2. Characterizations of SiO2 hollow microspheres with porous Fig. 2 shows optimum SEM, TEM images of SiO2 hollow microspheres with porous. Fig. 2c shows typical SEM images of PAM/SiO2 microspheres. In comparison with Fig. 2c, the diameter of SiO2 hollow microspheres with porous is slightly decreased, and the spherical surface morphology forms a sharp change. This is likely to calcine process of silicon the gelatin in the result of shrinkage [21,22]. Fig. 2a exhibits a perfectly whole microsphere with a diameter of about 50 lm and the spherical surface morphology is flower-like structure. Fig. 2b clearly shows that the product is
hollow with its body porous and flower-like structure. TEM images more clearly reveal that the resultant product is a hollow structure, and the shell has a porous appearance. That may be the outermost layer of the end of the shadow caused by flower-like structure. The diameter is about 20 lm (Fig. 2d), which is slight smaller than the particle size in SEM images. This may be because the particle size is uneven. In conclusion, the surface of the product is attributed to an ideal flower wrinkly surface, not smooth porous structure. Such a unique surface structure in the controlled spherical morphology is unprecedented. Moreover, the micro-pore size (Dp) distribution estimated by the BJH method reveals the micro-pores is 1–3 nm in diameter (Figs. 8 and 10). And that there are also pores whose size is distributed between 50 and 100 nm (Fig. 8a, b and 10d). Fig. 3 shows the FT-IR spectra of the PAM micro-gel (a), the PAM/SiO2 composite microspheres (b), PAM/SiO2 composite
Fig. 2. SEM images (a, b) of SiO2 hollow microspheres with porous and (c) of PAM/SiO2 microspheres: (a) a whole microsphere, and (b) a broken microsphere, (c) PAM/SiO2 microspheres. (d) TEM images d of the whole SiO2 hollow microspheres with porous.
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microspheres after calcination at 550 °C (c) and pure SiO2 microspheres (d). Similarities and differences are clear by comparing these spectra. The characteristic absorption bands for PAM are clearly observed as shown in Fig. 5a, such as the carbonyl group of PAM at 1639.98 cm1 and the amide group of PAM at 3439.25 cm1, respectively. These characteristic absorption bands basically disappear after calcination at 550 °C (cf. Fig. 5a–c) as a result of decomposition of organic components. It is obviously found that the peaks at about 808.63 and 1104.18 cm1 appear after calcination. The results are attributed to the Si–O–Si asymmetric stretching and symmetric stretching bands of SiO2 [23]. Compared with pure SiO2 microspheres, the peaks at around 950 and 560 cm1 belong to silanol disappear after calcination at 550 °C because of the condensation leading silanol disappear [24]. Furthermore, the peak at 3100 cm1 disappeared after calcination at 550 °C, which is attributed to decrease in silanol. Fig. 4 shows the EDX spectrum of PAM/SiO2 composite microspheres before (a) and after calcination at 550 °C (b). The obvious characteristic peaks for C, N, O, and Si elements, originating from composite microspheres, are found in Fig. 6a, indicating that SiO2 is, indeed, incorporated in the PAM microspheres. At the same time, the characteristic peaks for N elements, originating from microspheres after calcination at 550 °C, vanish in Fig. 6b, indicating that SiO2 hollow microspheres have been obtained. Thermo gravimetric analysis of PAM/SiO2 and after calcination at 550 °C is shown in Fig. 5. The result shows that a loss of 9.32% in weight, below 246.41 °C, is assigned to releasing water physic absorbed in the composite microspheres [25]. From 246.41 to 440.20 °C, a loss of 62.02 wt.% could be ascribed to the decomposition of PAM and loss of water from crystal transformation. The XRD patterns of PAM/SiO2 after calcination at 550 for 4 h are shown in Fig. 6. It can be seen only a single broad peak at 2h 20° in the spectra for PAM/SiO2 after calcination at 550 °C. This indicates that SiO2 hollow microspheres with porous is in the amorphous state [14]. Based on the aforementioned observations, we propose a mechanism to explain the formation of such morphology microspheres via a sol–gel deposited method. When the freeze dried PAM micro-gels were completely immersed in the sol mixture [ethanol + water + TEOS + ammonium hydroxide], the freeze dried PAM micro-gels acted as a micro-reactor, Si(OR)4 where R is a – C2H5, three kinds of reactions occur depending on the involved functional groups, hydrolysis of Si–OR to Si–OH (Eq. (1)), alcoholproducing condensation between Si–OR and Si–OH (Eq. (2)), and water-producing condensation between two Si–OH (Eq. (3)).
ðROÞ3 Si—OR þ H2 O
hydrolysis
esterification
RO3 Si—OH þ ROH
ð1Þ
Fig. 3. FT-IR absorption spectra of the PAM (a); PAM/SiO2 before (b) after calcination at 550 °C (c); and pure SiO2 microspheres (d).
ðROÞ3 Si—OH þ RO—SiðROÞ3
alcohol condensation
alcoholysis
RO3 Si—O—SiðORÞ3 þ ROH
ðROÞ3 Si—OH þ HO—SiðORÞ3
water condensation
hydrolysis
ð2Þ
RO3 Si—O—SiðORÞ3 þ H2 O
ð3Þ
TEOS was used as the precursor for the silica shell in the reaction. The first step is hydrolysis of TEOS. It is followed by slower poly-condensation reactions, and the reaction occurs on the – NH2 surface of the freeze dried PAM micro-gels that serve as a framework for deposition of a silica gel network. Generally speaking, condensation cannot start until hydrolysis has proceeded to some extent and then the two steps occur simultaneously [26]. In practice, the relative rate of hydrolysis and condensation varies with the reaction conditions. Therefore, the reaction pathway and the structure of silica gel prepared from TEOS vary widely depending on the preparation conditions. Among various parameters in the reaction conditions, pH has been known to have the largest effect on the preferential site for each reaction, as well as on the structure of the silica gel obtained [27]. At low pH, acid catalysis promotes hydrolysis but hinders both condensation and dissolution reactions [28]. Consequently, for the synthesis of silica particles using acid catalysis, small growth is observed and smaller, homogeneous particles are formed [29].
Fig. 4. EDX spectrum of PAM/SiO2 composite microspheres before (a) and after calcination at 550 °C (b).
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Fig. 6. XRD patterns of PAM/SiO2 after calcination at 550. Fig. 5. Thermo-gravimetric analysis of PAM/SiO2 (a) and after calcination at 550 °C (b).
Namely, the silicon particles evenly wrapped in the PAM, it cannot be the structure of aforementioned observations. Base catalysis of the hydrolysis and condensation reactions, in contrast, promotes fast condensation and dissolution. This leads to the production of an inhomogeneous system due to rapid condensation of all the hydrolyzed precursor monomers and to the formation of dense silica particles by the ripening of aggregates formed during the collision of droplets. As a result, the micro-particles are dense; namely, their scattering length is that of amorphous silica (this is a good agreement with that of XRD), and aggregate of
the silicon particles present the structure of aforementioned observations (Fig. 2a) [30,31]. 3.3. Effects of amount of the reactants on the morphology and Porosity of SiO2 hollow microspheres with porous
3.3.1. TEOS The SiO2 hollow microspheres with porous were obtained after calcination of the PAM/SiO2 composite microspheres at 550 °C for 4 h, and the PAM/SiO2 composite microspheres were synthesized at room temperature with the volume composition of TEOS varied;
Fig. 7. SEM images of the products synthesized with the composition of TEOS varied. The volume composition of the reactants was 1:x:47.5:5NH3H2O:TEOS:C2H5OH:H2O, where x = (a) 1.5, (b) 6, (c) 12, and (d) 25, and inset shows SEM image of the broken microsphere.
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X. Yan, Z. Lei / Journal of Colloid and Interface Science 362 (2011) 253–260 Table 1 Micropore parameters determined by BET. Specific surface area (m2/g) 8.85
Porous volume (cm3/g) 0.02
Mean pore diameter (nm) 10.51
NH3H2O Hollow porous SiO2 microspheresr Hollow porous SiO2 microspheress Hollow porous SiO2 microspherest Hollow porous SiO2 microspheresu
248.08
0.5537
1.84
123.08
0.3014
2.08
101.18
0.2773
2.18
89.82
0.2489
2.30
TEOS
381.96
0.8349
1.96
182.82
0.4508
2.18
135.13
0.2663
2.28
52.48
0.2394
2.45
Sample
PAM porous microspheres
Fig. 8. The pore size distribution by the BJH method using adsorption branch (DP: pore diameter; VP: pore volume; the volume of TEOS (a) 1.5, (b) 6, (c) 12, and (d) 25).
the volume composition of the reactants was 1:{:47.5:5NH3H2O:TEOS:C2H5OH:H2O, where x was the volume composition of TEOS. The representative SEM images of the products with the composition of TEOS varied are shown in Fig. 7. When the other conditions kept changeless, x was increased from 1.5 to 25; the morphology of the resultant products formed a great change (see Fig. 3a–d) from perfectly flower microsphere to smooth structure. In addition, the pore diameter of the resultant products was gradually increased as the increase of TEOS volume (see Fig. 8a–d). This was because the addition of TEOS can induce the formation of large amounts of the textural pores [32]. At the same time, it is shown that the micropores parameters determined by BET in Table 1. It can be seen in Table 1 that the BET surface area and total pore volume were gradually decreased with increasing x; the ABET at x = 1.5 and 25 were 381.96 and 52.48 m2/g, respectively; the VP at x = 1.5 and 25 were ca. 0.8349 and 0.2394 cm3/g, respectively. These results imply that the pore structure is changed from porous to smooth structure.
Hollow porous SiO2 microspheresv Hollow porous SiO2 microspheresw Hollow porous SiO2 microspheresx Hollow porous SiO2 microspheresy
The condition for preparation of the hollow porous SiO2 microspheres r s t u v w x y are shown in Figs. 9a–d and 7a–d.
And the mean micro-pore diameter was slightly increased with increasing x. 3.3.2. Ammonium hydroxide According to the proposed method for preparation of PAM/SiO2 composite microspheres, the interfacial reaction between TEOS located at the immersed PAM microspheres and water in a moist atmosphere with ammonium hydroxide as a catalyst is employed to incorporate SiO2 into the template microspheres. Because, by
Fig. 9. SEM images of the products synthesized with different concentration of ammonium hydroxide. (a) 10%, (b) 15%, (c) 20%, and (d) 25%, and inset shows SEM image of the broken microsphere.
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Fig. 10. The pore size distribution by the BJH method using adsorption branch (DP: pore diameter; VP: pore volume; the concentration of ammonium hydroxide (a) 10%, (b) 15%, (c) 20%, and (d) 25%).
changing the pH of the solution, wide control of the size and shape of the silica polymers evolving is achieved. The rate of hydrolysis of TEOS (Eq. (1)) exhibits a minimum at pH = 7 and increases exponentially at either lower or higher pH. In contrast, the rate of condensation (Eqs. (2) and (3)) exhibits a minimum at pH = 2 and a maximum around pH = 7, where SiO2 solubility and dissolution rates are maximized [33]. So, it is necessary to investigate the effect of concentration of the ammonium hydroxide on the morphology of the composite microspheres. No silica product was obtained when concentration of the ammonium hydroxide was 0, likely because of the lack of catalysis for the hydrolysis of TEOS followed by condensation of the resultant products. When the other conditions kept changeless, concentration of the ammonium hydroxide was increased from 10% to 25%, the morphology of the resultant products had a great diversification range from porous crinkly to smooth surface morphology (see Fig. 9a–d). This likely is because the increase in ammonium hydroxide concentration as a base led to the slight increase in the pH in the solution. When concentration of the ammonium hydroxide was 10% and 20%, the
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pH was found to be approximately 7.0 and 8.0, respectively. We know that the rate of hydrolysis of TEOS, condensation of silicate species and the solubility of silicate species are drastically changed at the boundary between pH = 8 and 9 [34,35]. When concentration of the ammonium hydroxide was 25%, the ABET and VP were 89.82 m2/g and 0.2489 cm3/g, respectively. However, When concentration of the ammonium hydroxide was 10%, the ABET and VP were 248.08 m2/g and 0.5537 cm3/g, respectively. It suggests that the pore structure is changed range from porous crinkly to smooth surface morphology (see Fig. 10a–d). This can because the hydrolysis, condensation and dissolution reactions promote fast when concentration of the ammonium hydroxide was 25%. This leads to the production of an inhomogeneous system due to rapid condensation of all the hydrolyzed precursor monomers and to the formation of dense silica particles by the ripening of aggregates formed during the collision of droplets. As a result, the micro particles are dense; namely, their scattering length is amorphous silica (this is a good agreement with that of XRD). And the amorphous silica is basically no porosity [28]. In other words, the micro particle synthesis results in the formation of relatively large matrix particles of low and even negligible porosity. Consequently, the morphology of the resultant products might be transformed from porous crinkly to smooth surface, leading to the change in pores structure.z
4. Conclusions We have demonstrated that the hollow porous silica composite microspheres can be successfully synthesized through the following procedure: the porous polyacrylamide (PAM) micro-gels saturated with tetraethoxysilane (TEOS) anhydrous alcohol solution and water in a moist atmosphere, with ammonium hydroxide as a catalyst. The hollow porous silica spheres with controllable surface morphologies have been synthesized through calcination of the PAM/SiO2 composite microspheres at 550 °C for 4 h. The resultant products have three typical surface morphologies: (1) an ideal flower surface; (2) a porous wrinkly morphology; (3) a wrinkly
Fig. 11. TEM images of PAM/SiO2 microspheres with deposition times of SiO2 for 8 h (a), 12 h (b), and 16 h (c) after calcination at 550 °C.
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surface. In addition, we investigate the effects of the volume of TEOS and the concentration of ammonium hydroxide on the morphologies of hollow porous silica spheres. The results indicate that the surface morphologies, the pore size, as well as the thickness of silica shell can be effectively adjusted by changing the volume of TEOS and the concentration of ammonium hydroxide. The pore size of hollow porous silica spheres increases as the amount of TEOS and the concentration of ammonium hydroxide increases.
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