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A Unique Transformation Route for Synthesis of Rodlike Hollow Mesoporous Silica Particles Xue-Jun Wu, Yuanyuan Jiang, and Dongsheng Xu* Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People's Republic of China ABSTRACT: Rodlike hollow mesoporous silica particles with ordered mesopores parallel to the axis were synthesized by using a surfactant mixture composed of zwitterionic and anionic surfactants as the templates with the assistance of a costructure-directing agent. The products were intact and dispersed hollow particles with a hexagonal cross section and an aspect ratio in the range 1.82.5. The mesostructure was stable even after calcination at 500 °C. Furthermore, the morphology and mesostructure of the products could be tuned through adjusting the surfactant molar ratio, which changed from rods to nearly spheres. After detailed detection of the morphology change of the products at different reaction times, it was found that the products were produced through a specific way that hollows out the chiral mesoporous silica solid nanrods and the addition of triethanolamine is the key step to produce hollow particles. Moreover, removal of surfactants by extraction led to a surface-functionalized sample with positive organic groups on the mesopore surface and with a high specific surface area up to 430 m2 g1, which provided a good candidate for drug delivery and absorption matrix.
’ INTRODUCTION Hollow mesoporous particles can be taken as hierarchically porous materials, which simultaneously possess large pores inside the shells and mesopores at the shells. The large void can be used to load other materials. Meanwhile, the mesopores control the permeability of the shell and provide accessible channels for matter exchange between voids and the outer environment. This feature makes them potential candidates in application fields such as encapsulation, catalysis, and controlled drug delivery.1 Up to now, the widely adopted method for the fabrication of hollow mesoporous particles is based on the soft template method, in which emulsions,2 gas bubbles,3 and vesicles4 are often taken as templates. The produced particles, however, often have less-ordered mesoporous shells and illdefined shape because of low stability and easily distorted properties of the template. It should be noted that hollow mesoporous particles with ordered mesopores can provide accessible channels for molecular diffusion and mass transfer without blocking. These ordered channels also can easily be functionalized with molecules to extend their applications. Moreover, other mesostructured materials templated by these hollow particles also have ordered structures, which will improve their mechanical, chemical, and physical properties. Therefore, it is still desirable to fabricate hollow particles with orderly aligned mesopores. Recently, hollow silica spheres with ordered and radially oriented mesopores have been synthesized by using gas bubbles and emulsions as soft templates, respectively.5,6 In these methods, surfactants or copolymers are also used as additional templates to induce the formation of the mesopores. r 2011 American Chemical Society
To overcome the weakness of the soft template method and produce hollow mesoporous silica with tunable issues, such as the diameters of hollow cores and shell thickness, mesopore ordering, and orientation, a dual-templating method has been advanced, which combined the colloids and surfactants as a template.7 This method requires self-assembly of the surfactants with the inorganic source exclusively on the colloidal surface, which is very sensitive to the reaction conditions. Therefore, the aggregation of the produced particles is often an unavoidable problem. Furthermore, this method is mainly limited to the use of spherical colloids as hard templates, which only produce spherical hollow mesoporous particles. Other hollow mesostructures with different morphologies, such as rods, and especially tubes, also have been widely researched. For example, hollow silica nanotubes with mesoporous walls have been prepared using common surfactants as the templates.8 Among them, there is a unique type of mesostructure, which possesses a helical ordered mesopore arrangement around the tubes.8c Besides these, there are many helical nanotubular structures, such as single- or double-strand helical nantubes and twisted lamellar nanotubes, which can be produced through a solgel transcription process.9 However, most of them lack mesopores in the walls. Especially, Wu et al. have demonstrated that silica nanowires/tubes with kinds of helical mesopororous architectures can be produced inside anodized aluminum oxide nanochannals.10 Received: November 9, 2010 Revised: April 25, 2011 Published: May 23, 2011 11342
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The Journal of Physical Chemistry C Recently, Che’s group developed a method for producing chiral silica nanotubes with helical channels in their walls through the self-assembly of surfactant in the presence of chiral molecules.11 They found that the formation mechanism is based on a crystallization route through hollowing out the chiral mesoporous silica nanorods. In their report, the influences of the different dopants are also noted; only nanotubes with not good morphology or nanowires are produced. Herein, we present a method to synthesize rodlike hollow mesoporous silica particles with ordered mesopores parallel to the axis by using a surfactant mixture composed of zwitterionic and anionic surfactants as the templates with the assistance of a costructure-directing agent (CSDA).11 The formation mechanism of the products was detailed through investigation of the products at different reaction times. It was found that these hollow mesoporous particles were produced through hollowing out the former produced helical silica rods. Furthermore, products with positively charged organic groups functionalized on the surface of nanochannels could be obtained through extraction of the surfactants. The obtained products show a high specific surface area up to 430 m2 g1, and may be potentially used as capsules for controlled release of other materials or as a hard template for replication of other hollow mesoporous particles.
’ EXPERIMENT SECTION Chemicals. N-Dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DDAPS) was purchased from TCI, sodium dodecyl sulfate (SDS) was from ACROS, N-trimethoxysilypropyl-N,N, N-trimethylammonium chloride (TMAPS) was from Fluorochem, and tetraethyl orthosilicate (TEOS) was from Sigma-Aldrich. All chemical agents were used without further purification. Synthesis of Rodlike Hollow Mesoporous Silica Particles. In a typical synthesis of rodlike mesoporous hollow silica particles, 6 mmol of triethanolamine was added into a 20 mL surfactant mixture solution (ctotal = 10 mM) composed of DDAPS and SDS with a 1:1 molar ratio under stirring, and then 0.5 mL of NH3 3 H2O (2.5% NH3) was added. The resulting solution was heated in a water bath at 45 °C, then 112 μL (0.2 mmol) of TMAPS and 315 μL of TEOS were added, and the mixture was stirred for 30 min. The resulting solution was placed in a water bath at 80 °C for 24 h to ensure the complete polymerization of the silica. The product was separated by centrifugation at 12 000 rpm for 3 min, washed by ethanol and water, and dried in ambient atmosphere. Removal of the surfactants was performed through calcination under air at 500 °C for 4 h with a heating ramp of 1.5 °C/min, or through reflux extraction under HCl/ethanol solution for 3 h twice. For studying the effect of the surfactant molar ratio on the mesostructure of the silica shell, DDAPS/SDS molar ratios of 2/3 and 3/2 were also taken to advance the experiment, keeping other conditions the same. Characterization. Transmission electron microscopy (TEM) images were performed on a JEOL 200CX transmission electron microscope under a working voltage of 160 kV. High-resolution TEM (HRTEM) images were performed with a FEI Tecnai F30 FEG-TEM instrument operated at 300 kV. All samples for TEM observations were prepared by dropping the aqueous suspension containing the uniformly dispersed nanoparticles (NPs) onto carbon-coated copper grids. SEM images were obtained with a Hitachi S-4800 instrument. Small-angle X-ray diffraction (SAXRD) patterns were obtained on a Rigaku D/max-2000 diffractometer
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Figure 1. SEM (a, b) and TEM (c) images of the as-prepared sample. (d) HRTEM image of the sample with the incident beam parallel to the axis. The inset in (b) is the SEM image of a broken nanoparticle, and the inset in (d) is the corresponding fast Fourier transform (FFT) pattern.
at the rate of 1°/min. Nitrogen adsorptiondesorption isotherms were measured on a micrometer ASAP 2010 sorption analyzer at 77 K. Specific surface area was calculated by the BET (BrunauerEmmettTeller) method, the pore size distribution was calculated from the adsorption branch using the BJH (BarrettJoynerHalenda) method. The elemental analysis was performed on an Elementar Vario-EL instrument. Solidstate 13C CPMAS NMR spectra were collected on a BRUKER AVANCE III 400 MHz NMR spectrometer.
’ RESULTS AND DISCUSSION Our synthetic method is similar to the anionic surfactant template method for mesoporous silica advanced by Che and Tatsumi,12 in which the positively charged organic groups of CSDA interact electrostatically with the templating anionic surfactant micelles and the alkoxysilane groups of CSDA condense with inorganic precursors. In this approach, CSDA is just like a bridge connecting the organic template and inorganic precursors for the fabrication of mesoporous silica.13 In our synthesis, the mixture composed of zwitterionic and anionic surfactants, substituted only the anionic surfactant, was used as the template to fabricate the silica. A low magnification SEM image displays the overall morphologies of the as-prepared samples (Figure 1a). Most of the samples show apparent rodlike particles, with an aspect ratio of 1.82.5. There are also a few of the byproduct with sphere morphology in the samples. Figure 1b gives a high magnification SEM of the samples, indicating that many samples have a hexagonal rodlike morphology, which can be proven by probing the particles parallel to the axis. The inset in Figure 1b is an SEM image of a broken nanoparticle through grinding, which clearly shows that the product is hollow. The TEM image (Figure 1c) also demonstrates that the samples are indeed hollow particles with a shell thickness in the range 3550 nm. From the walls of the single particles, fringes of mesopores aligned parallel to the axis also can be clearly seen. To confirm this image, an HRTEM image taken with the incident beam parallel to the rodlike particles is also given in Figure 1d, and meanwhile, a fast Fourier transformation (FFT) was conducted over this image to obtain electron diffraction patterns of the mesopores (inset of Figure 1d). Both the HRTEM image and 11343
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Figure 3. SAXRD patterns of the as-prepared samples shown in Figure 2.
Figure 2. TEM images of the as-prepared samples at different aging times at 80 °C. (a) 1, (b) 2, (c) 4, (a) 8, (b) 12, and (c) 24 h.
the FFT pattern reveal that the mesopores arranged into hexagonal style. It should be noted that the rodlike hollow particles may exhibit a little distortion or twist along their axes. This can be partly reflected from Figure 1c, that the fringes of the mesopores cannot be completely detected along the axis of the rod, and can only be seen at near both ends of the rod. This result accords well with the former reports about chiral mesoporous silica.12 However, because the pitch length is much longer than the length of the rodlike particles, the twist is not very clear. In our previous reports, we have presented that CSDA could induce the formation of vesicles in the surfactant mixture consisting of zwitterionic and anionic surfactants under the assistance of the nanoparticles.14 To investigate the formation mechanism of product and study if it is still based on the vesicletemplating method, the structural evolution of the product with time was researched by TEM and small-angle XRD. The product was produced through reaction at 45 °C for 0.5 h, and then kept at 80 °C for another 24 h. Therefore, the samples were taken out at different aging times at 80 °C, and corresponding TEM images are shown in Figure 2. The sample taken after 1 h of aging shows helical mesoporous rod morphology (Figure 2a). The fringes of the mesopores can be clearly detected along the rods. These twodimensional helical mesostructures maintained their morphology after aging for 2 and 4 h (Figure 2b and 2c). However, their surfaces were much rougher than the former ones and some tiny holes could also be detected in the rods after 4 h of aging. The samples were gradually transformed from the solid mesoporous rods to hollow mesoporous rods with prolonged aging time (Figure 2d,e). This transformation was completed through gradually enlarging the diameters of the holes in the rods, and then merging these holes together to form a complete hole along the rods while still keeping the morphology of the rods. After 24 h of aging, all of the solid mesostructure was transformed into
hollow mesoporous rods (Figure 2f). The small-angle XRD patterns of the as-synthesized sample at different reaction times are shown in Figure 3. At the early stage of the aging (1 and 2 h), the XRD patterns of the sample show three peaks, which can be indexed by 10, 11, and 20 reflections of the two-dimensional hexagonal system with d10 spacing of 5.6 nm. The XRD reflection peaks at high angle are very low, which may result from the existence of the surfactants and weaken the reflection contrast. The intensities of the XRD reflection peaks are decreased with prolonged aging time. Finally, only a very broad peak is resolved after 24 h of aging. This result was accorded with the TEM images, which reflects the transformation from the solid mesoporous rods to hollow ones. On the basis of the above results, a hollowing-out process from the solid mesoporous rods to hollow ones while maintaining the helical mesostructure can be observed. This novel route is quite different from the common vesicle-templating method using the surfactants, but it is very similar to the report by Che’s group.11 However, in their report, a chiral molecule was used as the dopant and the product is a high aspect ration nanotube with open ends. This hollowing-out process is also similar to the common Ostwald ripening process to produce a hollow structure.15 To further understand the formation mechanism and the driving force for this process, several blank experiments are also taken. It is found that triethanolamine (TEA) plays a key role in this transformation. Only helical mesoporous silica rods or wires are produced without addition of TEA. It has been reported that TEA can be used as a complex agent for silicate species and as an encapsulator for silica to retard the condensation of the silica source.16 According to these results, the formation mechanism of the hollow mesostructure may be expressed as following: At the early stage, the surfactant mixture and silica source will be selfassembled under the direction of the CSDA, and helical mesoporous silica rods are produced through the solgel process of the silica source. Because of the existence of the TEA, the condensation degree is not very high and the walls are left with many free SiOH or SiOC2H5 groups. Subsequently, the sample is aged at much higher temperature. The outer wall of the rods will be condensed further with the synthesis gel solution. 11344
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The Journal of Physical Chemistry C However, the inner parts of the rods will be gradually dissolved because of the low density of the silica and the less-condensed
Figure 4. (a, b) SEM images and (c, d) TEM images of the as-prepared samples at different molar ratios of DDAPS/SDS: (a, c) 2/3; (b, d) 3/2.
Figure 5. SAXRD patterns of calcinated samples at different surfactant molar ratios.
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wall with a large number of organic groups. In the whole process, the dissolution of the inner part may result from the weak basic property of the TEA or its coordination nature with silicon. It has been reported that hydrothermal treatment can induce a mesostructural change of mesoporous silica.17 The above transformation also provides another way for producing hollow silica mesostructures. To investigate the influence of the surfactant molar ratio on the final morphology of products, DDAPS/SDS molar ratios at 2/3 and 3/2 were also chosen to carry on the synthesis. The corresponding products are displayed in Figure 4. When the DDAPS/SDS molar ratio stays at 2/3, the product becomes very inhomogeneous. There are many spheres and ill-shaped hollow particles (Figure 4a). When the DDAPS/SDS molar ratio turns to 3/2, the product changes to near-sphere hollow particles with multidisperse sizes (Figure 4c). TEM images of the corresponding samples confirmed that the products were all hollow mesostructures (Figure 4b,d). Furthermore, the SAXRD patterns of the samples after calcinations are given in Figure 5. The sample prepared at a DDAPS/SDS molar ratio of 1:1 shows an intense peak at small angle and a weak peak at high angle, which accords with the sample before calcinations. This result also reflects that the mesostructure was still kept after high temperature calcinations. If the surfactant molar ratio were changed to 2/3 or 3/2, the intensities of all peaks decreased or even disappeared. The effects of the surfactant molar ratio on the final morphology and mesostructure of the sample may attribute to the early stage of the self-assembly process of the surfactants and silica source. As we know, there is a synergetic effect between the zwitterionic and anionic surfactants. The morphology of the self-assemblies of the surfactant mixture can be tuned through adjusting some conditions, such as additive, pH, or ratio of surfactants.18 This result also provides another optional way to tune the morphology of the hollow silica mesostructures. Similar to the anionic surfactant template method, the surfactant mixture also can be removed through extraction under HCl/ ethanol solution to keep the quaternary amine groups on the mesoporous walls.5 It should be noted that the mesostructure of
Figure 6. SAXRD pattern (a) and nitrogen absorptiondesorption isotherm (b) of the extracted sample. The inset in (b) is the corresponding pore size distribution. 11345
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Figure 8. TEM images of Au NPs absorbed on (a) extracted sample and (b) calcinated sample.
Figure 7.
13
C CP/MAS NMR spectrum of the extracted sample.
the sample after extraction was completely preserved. The reflection peaks still stayed in the XRD pattern (Figure 6a), which can be attributed to the p6mm system. The nitrogen absorptiondesorption isotherm of the sample after extraction is shown in Figure 6b, which shows a type IV feature with an absorption step at partial pressure between 0.4 and 0.6 due to the capillary condensation of filling nitrogen into the mesopores. Another absorption step in the relative pressure range of 0.60.95 is probably attributed to nitrogen absorption in the cavities of the macropores. The pore size distribution is quite narrow with a peak centered at 3.8 nm, indicating the uniform sizes of the mesopores. The BET specific surface area of the sample is 434 m2 g1 and the pore volume is 0.96 cm3 g1. The 13C CP/MAS NMR spectrum (Figure 7) has confirmed that surfactant molecules were entirely removed by extraction, because there is no resonance signal detected between 20 and 40 ppm, which is attributed to the long alkyl chains of DDAPS and SDS. The resonance signals at 11.08, 18.65, 55.31, and 70.36 ppm should be assigned to C2, C1, C4, and C3 of TMAPS, respectively. The above results demonstrated that DDAPS and SDS surfactant molecules were removed and the quaternary amine groups successfully stayed on the mesopore inner walls. Considering our synthesis, the nitrogen contained in the sample had only originated form TMAPS. Elemental analysis results demonstrate that the loading amount of the organic group (CH2)3Nþ(CH3)3 is 1.5 mmol/g SiO2. To further demonstrate the organic group anchored on the mesopore inner walls, calcinated and extracted mesoporous silica particles were tested for the absorption of small Au nanoparticles (NPs) with diameters of 24 nm.19 After dispersion of the silica particles in water, the Au sol was added into the solution under magnetic stirring for 4 h. Then the products were separated by centrifugation and washed with water several times. As shown in Figure 8a, it can be clearly seen that there are many Au NPs absorbed at both ends, while few NPs were found in the middle of the extracted rodlike silica particles. For calcinated silica particles, however, few Au NPs can be detected on their surface (Figure 8b). Because of the positively charged quaternary amine group on the mesopore inner walls and negatively charged silica skeleton of the extracted sample under neutral environment, the negatively charged Au NPs are prone to absorption at the ends of or into the
mesopores through electrostatic attraction. The calcinated sample does not possess any positively charged organic group after high temperature calcination, so the Au NPs barely absorb on their surface. The remaining organic group on the inner walls may provide an anchored molecular valve for application in controlled drug delivery and absorption.
’ CONCLUSIONS In this paper, a unique hollowing-out process for the preparation of rodlike hollow mesoporous silica particles was presented, in which surfactant mixtures composed of anionic and zwitterionic surfactants were used as the template with the assistance of a CSDA. The products possessed nearly ordered hexagonal mesopore alignment, and the mesopores were parallel to the axis of the sample with helical distortion. It was found that the product was formed through a specific way that hollows out the chiral mesoporous silica solid nanorods while keep the morphology of the product at the early stage. This way is quite different from the common routes to product hollow structures as reported previously. Triethanolamine plays a key role to produce hollow particles, which may result from its weak basic property or coordination nature with silicon. Furthermore, the morphology and mesostructure of the product could be tuned through adjusting the surfactant molar ratio. Because of their nearly ordered mesostructure and the remaining quaternary amine group on the surface of inner mesopore walls after extraction surfactants, these rodlike hollow mesoporous particles may find important applications in the fields of controlled drug delivery and absorption. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work is supported by NSFC (Grant No. 50821061) and MSTC (Grant Nos. NKBRSF 2007CB936201, 2011CB 808702). ’ REFERENCES (1) (a) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453. (b) Marinakos, S. M.; Novak, J. P.; Brousseau, L. C., III; House, A. B.; Edeki, E. M.; Feldhaus, J. C.; Feldheim, D. L. J. Am. Chem. Soc. 1999, 121, 8518. (c) Shchukin, D. G.; Sukhorukov, G. B.; M€ohwald, H. Angew. Chem., Int. Ed. 2003, 42, 4472. (d) Zhu, Y.; Shi, J.; Shen, W.; Dong, X.; Feng, J.; Ruan, M.; Li, Y. Angew. Chem., Int. Ed. 2005, 44, 5083. (e) Du, 11346
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