J Porous Mater (2012) 19:745–749 DOI 10.1007/s10934-011-9526-1
Facile synthesis route to monodispersed platelet-like SBA-15 silica Sheng-Zhen Zu • Li-Juan Mao • Abdelhamid Sayari Bao-Hang Han
•
Published online: 18 October 2011 Ó Springer Science+Business Media, LLC 2011
Abstract We here report a facile synthesis route to monodispersed platelet-like SBA-15 mesoporous silica employing kinetically controlled micelles as templates. In previous preparations of SBA-15 silica, it was believed that thermodynamically equilibrated micelles were used as templates. The kinetically controlled micelle has never been used before to generate periodic mesoporous materials with unique morphology. Monodispersed hexagonal platelet-like SBA-15 microparticles were prepared via a very facile synthesis route by tuning the micelle formation process, i.e. the stirring rate and the time of dissolution of the triblock copolymer and formation of the micelles. Shorter micelle formation time and faster stirring are essential to generate platelet-like SBA-15 silica particles. It is expected to apply in the synthesis of a wide variety of mesophase materials. Keywords Mesoporous silica Morphology Micelles Platelet-like SBA-15 Block copolymer
Electronic supplementary material The online version of this article (doi:10.1007/s10934-011-9526-1) contains supplementary material, which is available to authorized users. S.-Z. Zu L.-J. Mao B.-H. Han (&) National Center for Nanoscience and Technology, Beijing 100190, China e-mail:
[email protected] A. Sayari (&) Department of Chemistry, University of Ottawa, Ottawa, ON K1N 6N5, Canada e-mail:
[email protected]
1 Introduction Triggered by the invention of the so-called M41S family of ordered mesoporous silicas (OMS) by the Mobil research group [1], a new field of materials science was developed and has grown dramatically in the past years. Key to this development was not only the synthesis of a wide variety of silica mesophases, but also the discovery of nonsiliceous periodic mesoporous materials such as metal oxides, carbon, polymer, and organosilicates [2]. Further development in the synthesis of periodic mesoporous materials was the use of OMS as a hard template/ mold to produce new materials such as ordered mesoporous carbon and metal oxides, with retention of the original morphology [3–11]. SBA-15 silica [12, 13], possessing the same 2D hexagonal structure as MCM-41, has been among the most extensively investigated mesophases. As for its application potential, SBA-15 shows several advantages over MCM41, due to its larger pores and thicker pore walls, as well as the interconnectivity between adjacent mesoporous channels through micropores or secondary mesopores [14–18]. Much effort has been devoted to the morphological control of mesoporous silicas because of the paramount importance of the morphology and texture of materials with regard to their potential applications [19–31]. The SBA-15 mesoporous silica obtained through the conventional recipe [12, 13] exhibits typical rope-like morphology (bundle of connected micrometer size rods). Employing various silica sources, surfactants, co-surfactants, co-solvents, additives of organic compounds or inorganic salts, as well as varying the relative concentration of these compounds, numerous morphologies were obtained, including hard spheres, fibers, doughnut-, rope-, egg-sausage-, gyroid-, and discoid-like particles, as well as monodispersed rods [27–31].
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We reported earlier a simple, flexible, and high-yield synthesis route to monodispersed SBA-15 silica particles, consisting of hexagonally facetted rods with about 1.5 lm in length and 0.4 lm in diameter under static conditions without any additive [32]. Transmission electron microscopy images revealed that the hexagonally packed mesopores run parallel to the main axis or the c direction of the rods. This indicates that the mesoporous channels are very long (ca. 1.5 lm) compared to the pore size (ca. 8 nm). It is apparent that any defect and/or blockage inside the mesopores will drastically hamper mass transport. Therefore, morphologies with short channels will be more suitable for applications that depend on mass transfer within the pore system such as adsorption, catalysis, and sensing. Using large amounts of decane as additive, which serves as both a swelling agent and confining agent to the silica-polymer composite micelles, Zhang et al. [33] reported the synthesis of an unusual mesoporous SBA-15 silica with parallel channels along the short axis. Furthermore, Zhao and co-workers [30] reported a plate-like morphology employing tetramethyl orthosilicate as silica source. Zin and co-workers [34] prepared monodispersed SBA-15 equilateral hexagonal platelets using triblock copolymer P104 (EO27PO67EO27) and inorganic salt under acidic condition. Very recently, Linton and Alfredsson [35] proposed a mechanism for the aggregate growth of mesoporous SBA-15 particles. Cheng and co-workers [36] prepared SBA-15 silica of platelet shape with short mesochannels using Zr(IV) ions to accelerate the self-assembly of P123 micelles and TEOS. Martens and co-workers [37] also obtained the SBA-15 mesoporous silica with similar morphology at room temperature and Quasi-neutral pH. In previous preparations of SBA-15 silica, it was believed that thermodynamically equilibrated micelles were used as templates. The kinetically controlled micelle has never been used before to generate periodic mesoporous materials with unique morphology. Here, we present a very facile (without any additive) synthesis of monodispersed hexagonal platelet-like SBA-15 microparticles by tuning the micelle formation process, i.e. the stirring rate and the time of dissolution of the triblock copolymer and formation of the micelles.
2 Experimental Triblock copolymer poly (ethylene oxide)-block-poly (propylene oxide)-block-poly(ethylene oxide) (Pluronic P123, EO20PO70EO20, Mw = 5,800) was purchased from Aldrich. Tetraethyl orthosilicate (TEOS, C98%) was commercially available from Fluka. Polypropylene autoclave (Nalgene Straight-Side Wide-Mouth Jars, 125 mL) was purchased from Nalge Nunc International Co.
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The SBA-15 mesoporous silica materials were prepared according to the procedure reported earlier [32, 38] using the same mixture composition but varying the synthesis condition, i.e. the stirring rate and micellization time of triblock copolymer poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) before addition of tetraethyl orthosilicate (TEOS). The preparation procedure was as follows. 2.0 g of P123 was added into the mixture of 15 g of water and 60 g of 2 M HCl aqueous solution in a polypropylene container (125 mL Nalgene Jars), which was stirred magnetically (IKA RCT stirrer) at different rates at 35 °C for 1 h. A clear solution was typically obtained. Then, 4.25 g of TEOS was added to the block copolymer solution under stirring at the same rate. After stirring for 5 min, the homogeneous solution was kept at 35 °C for 20 h under static condition, followed by aging at 90 °C for 2 days under quiescent condition. The solid product was collected by filtration, washed with water, and then dried at 140 °C for 4 h. To remove the surfactant, the as-synthesized white powder was subjected to calcination for 5 h at 550 °C with air flow, the temperature ramp was 5 h. The following methods were applied for analysis: Scanning electron microscopy (SEM) images were acquired on a Hitachi S4800 field-emission scanning electron microscope operating at an accelerating voltage ranging from 2.0 to 15.0 kV. The samples for SEM imaging were prepared by putting a little amount of fine powder on carbon tape which adhibitted on aluminium plate, or dispersing the powder in ethanol and sonicating for about 3 min. A drop of the suspension was placed on the silicon substrate and then dried in air. The samples were sputter-coated with Au for 60 s (resulting in a gold coating of about 10 nm) to reduce charging effects. Transmission electron microscopy (TEM) images were acquired on a JEOL 2010 high-resolution transmission electron microscope at an acceleration voltage of 200 kV. High-resolution scanning transmission electron microscopy (STEM) images were acquired on a Tecnai G2 F20 U-twin field-emission transmission electron microscope at an acceleration voltage of 200 kV. Samples were prepared by dispersing the powder in ethanol and sonicating for about 3 min. One droplet of the suspension was applied to a holey carbon film-coated copper grid and left to dry in air. Nitrogen adsorption–desorption isotherms (T = -196 °C) were obtained with a Micromeritics ASAP 2020M ? C accelerated surface area and porosimetry analyzer. The samples were degassed in the automated degassing system at 150 °C for 10 h. The isotherms were evaluated to give the pore parameters, including specific surface area, average pore size and pore volume. Powder X-ray diffraction (XRD) patterns were taken on an X’Pert PRO MPD multi-purpose X-ray diffraction system using a graphite monochromatized Cu Ka radiation ˚ ). (k = 1.5406 A
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3 Results and discussion Keep the time of dissolution of the triblock copolymer and formation of the micelles constant (1 h), and the stirring rate is large enough (800–1,000 rpm), SBA-15 periodic mesoporous silica with hexagonal platelet-like morphology was obtained. Figure 1 shows SEM images of the calcined material. As seen, the monodispersed platelet-like particles1 are hexagonally facetted and with uniform size of ca. 1 lm in diameter and 400 nm in length. Ryoo and Terasaki et al. [39] observed SBA-15 particles of the same morphology. The platelet-like SBA-15 particles obtained by us are thicker than those reported before [34–36]. Based on SEM observations, the yield of the hexagonal platelet-like morphology approaches 100%. It is worth noting that such a high yield was obtained in all syntheses leading to platelet-like morphology. When the stirring rate is 600 rpm, the same morphology was also obtained in most cases. However, at 500 rpm or lower, the platelet-like morphology was obtained, but not reproducibly. While keeping the stirring rate ([800 rpm) constant, prolonging the stirring/micellization time to 2 or 3 h, the morphology of SBA-15 could not be reproducibly controlled and the products were often composed of platelet-like and rod-like particles. When the dissolution/ micellization time is long (e.g., several hours), the long rod-like particle morphology was obtained regardless of the stirring rate, as we reported previously [32]. It is believed that the platelet-like morphology stems from the shape of micelles of block copolymer. The morphology of SBA-15 silica is sensitive to the feeding recipe and preparation procedure, any delicate variation will result in morphologic change, such as addition of a small amount of Zr(IV) ions in the conventional SBA-15 synthesis solution (which yielded rod-like morphology) produced platelet SBA-15 particles [36]. Since the recipe and the procedures after addition of TEOS we used here were identical to those we used to obtained rod-like morphology [32], the only difference between two syntheses is the time and the stirring rate of dissolution of triblock copolymer P123, the micelles definitely became the key role in the morphology synthesis of SBA-15 silica. The micellization of the triblock copolymer P123 is a kinetic process. However, the kinetically-controlled micelle has never been used before to generate periodic mesoporous materials with unique morphology. In previous preparations of SBA-15 silica by us and others, the dissolution of the polymer and the micellization process were long enough (from several hours to overnight) to afford thermodynamically equilibrated micelles with long rod-like shape. The nanoscale 1
We prefer the name ‘‘platelet’’ (with an aspect ratio of 0.4), rather than rod (with an aspect ratio of ca. 4 in Ref. [32]).
Fig. 1 SEM images (a and b) of platelet-like SBA-15 silica
rod-shaped micelles [40] interact with silicate species to form much larger rod-like polymer-silicate assemblies up to hundreds of nanometers (microrod) as observed by electron microscopy. High-resolution transmission electron microscopy image (Fig. 2) reveals that the mesopores (where the micelles are removed through calcination) are hexagonally packed. The orientation of the mesopores are parallel to the main axis and perpendicular to the hexagonal facet. The STEM image of the SBA-15 particle shown in Figure S1 (Supplementary Information) confirms the above results. This indicated that the length of the mesopores in the plateletlike SBA-15 silica particles is about 400 nm, the aspect ratio is about 50, much smaller than that in our previous rod-like SBA-15 silica ([150) [32]. The XRD pattern (Fig. 3) was also consistent with the occurrence of twodimensional hexagonal symmetry. According to the literature [26, 27], the shape of the surfactant micelles and the resulting silica particles are controlled by several interrelated factors, including the concentration of surfactants, the nature and state of
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Fig. 2 TEM image of platelet-like SBA-15 silica
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Fig. 3 XRD pattern of platelet-like SBA-15 silica
surfactants, the concentration and state of the silicate species, the concentration of the additives (co-solvents, co-surfactants, and inorganic salts), and the stirring rate. It was however, reported that the stirring rate does not strongly influence the particle morphology of MCM-41 silica [19]. The SBA-15 platelet particles in the literature were obtained mainly through variation in surfactants or additives [30, 33–36]. When the dissolution/micellization time is long (e.g., several hours), the rod-like particle morphology was obtained regardless of the stirring rate [32]. When the dissolution/micellization time is short (1 h),2 and the stirring is fast ([800 rpm), the micellization may not reach the thermodynamically stable state (long 2
The time for P123 dissolution is slightly less than 1 h, and decreases with increasing stirring rate.
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rod-like shape), i.e. the micelles are in an intermediate kinetically-controlled state corresponding to a short rod shape. The growth of the micelles of P123 from sphere to rod has been intensively investigated by Denkova et al. [41, 42] and Ganguly et al. [43, 44], respectively, especially in the presence of salt and the time-dependent growth. During the preparation of SBA-15 mesoporous silica, the precipitation occurred after about 30 min during the growth stage (35 °C) under static condition, i.e. the platelet-like morphology of SBA-15 silica was formed at this stage. The fast growth of the silica particles was revealed by monitoring the morphology of silica particles in this stage (35 °C) in control experiments (micellization time: 1 h; stirring at 800 rpm, see Figure S2, Supplementary Information), since the particle shape does not further change significantly after 2 h of growth. Silica particles collected after 4 h showed uniform platelet-like morphology as shown in Figure S2 (Supplementary Information). Furthermore, when the dissolution/micellization time is 2 or 3 h, the micelle might be in the transformation stage from short rod to long rod, and the silica products were mixture of platelet-like and rod-like particles. In general, the formation mechanism of platelet-like SBA-15 could be summarized as follows. Kineticallycontrolled micelles of P123 were used as template to synthesize SBA-15. When the stirring time was short (1 h) and the stirring rate was fast ([800 rpm) in our experiment, the micelles present as a short rod-like shape instead of the thermodynamically stable state (long rod-like shape). These short rod micelles can pack together parallel to each other (shoulder-to-shoulder) to form small assemblies. The short rod micelles and/or the small assemblies would be bound together through the condensation of the silicate sol. The later micelles will attach to the former platelet in a shoulder-to-shoulder way to contribute to the particle growth. It should be pointed out that the growth of micellesilicates is preferably perpendicular to the main axis, i.e. along to the facet of the particle, to give large hexagonal platelet-like silica microparticles. Figure 4 showed a typical nitrogen sorption isotherm for platelet-like SBA-15 mesoporous silica materials, which is characteristic of periodic mesoporous materials. The nitrogen capillary condensation occurs in the relative pressure range of 0.58–0.76. This narrow range indicated that the current mesoporous silicas possess very narrow pore size distribution. The inset in Fig. 4 showing the BJH pore size distribution (PSD) calculated from the adsorption branch confirmed the narrow PSD, where the peak width at half-height is only 1.2 nm. The Brunauer-Emmett-Teller (BET) surface area is 800 m2 g-1, the BJH pore size is 7.8 nm (8.7 nm based on the KJS approach [45]), and the pore volume calculated as the volume of liquid nitrogen adsorbed at the relative pressure of 0.95 was 1.0 cm3 g-1.
Adsorbed Volume, STP/cm g
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Fig. 4 Nitrogen adsorption–desorption isotherm and BJH pore size distribution of platelet-like SBA-15 silica
4 Conclusions In summary, we have prepared monodispersed platelet-like SBA-15 silica through tuning the stirring rate and dissolution/micellization time of the templating block copolymer. Shorter micelle formation time and faster stirring were essential to generate platelet-like particles. The kinetically-based morphological control is expected to apply in the synthesis of a wide variety of mesophase materials. Because of their short pore channels, SBA-15 with platelet-like morphology may prove to be very useful materials for catalysis, adsorption and sensing. Acknowledgment The financial support of the Ministry of Science and Technology of China (grant no. 2011CB932500) and the National Science Foundation of China (Grant no. 91023001) is acknowledged.
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