Silica Nanotubes through a

Aug 30, 2010 - Dongming Liu , Qiong Wu , Richard L. Andersson , Mikael S. Hedenqvist , Stefano Farris , Richard T. Olsson. Journal of Materials Chemis...
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Synthesis of Well-Defined Silica and Pd/Silica Nanotubes through a Surface Sol-Gel Process on a Self-Assembled Chelate Block Copolymer Minchao Zhang, Wangqing Zhang,* and Shengnan Wang Key Laboratory of Functional Polymer Materials of Ministry of Education, Institute of Polymer Chemistry, Nankai UniVersity, Tianjin 300071, China ReceiVed: July 19, 2010; ReVised Manuscript ReceiVed: August 14, 2010

The synthesis of well-defined silica nanotubes and Pd nanoparticle-decorated silica nanotubes is reported. The synthesis of silica nanotubes involves (1) formation of a 1-D template of the core-corona threadlike micelles through self-assembly of poly(ethylene glycol)-block-poly(4-vinylpyridine) in water, (2) a directed surface sol-gel process of tetraethylorthosilicate (TEOS) on the template of the threadlike micelles, and (3) calcination to remove the template. Because of the inherently pendent catalyst sites of the poly(4-vinylpyridine) block on the threadlike micelles, the surface sol-gel process is directed onto the template, and therefore, formation of irregular silica aggregates is avoided. Following the proposed method, well-defined silica nanotubes with thicknesses ranging from 3 to 17 nm are produced by changing the weight ratio of TEOS/micelles. Also benefiting from the chelate poly(4-vinylpyridine) block, Pd nanoparticles are introduced into the cavum of silica nanotubes initially through coordination between the poly(4-vinylpyridine) block with the Pd precursor, followed by reduction with NaBH4 aqueous solution. 1. Introduction Silica nanotubes have attracted much attention due to their potential applications in both academia and industry.1,2 Among the various methodologies to synthesize silica nanotubes,3,4 the sol-gel process on 1-D templates, including nanowires,5,6 nanorods,7 and carbon nanotubes and nanofibers,8,9 is considered to be the most valid. Generally, there are two mechanisms for transcription of the template into shape-controlled silica materials. One is the solution mechanism that involves hydrolysis and polycondensation of a silica precursor, such as tetraethylorthosilicate (TEOS), in the presence of a free catalyst.10 Following this solution mechanism, the silica precursor and a suitable template, such as peptide amphiphile nanofibers,11 carbon nanotubes/nanofibers,12 porous alumina membranes,13,14 and laurylamine hydrochloride,15 are first mixed, then a soluble catalyst, including NH3, NaOH, or HCl aqueous solution, is added, and then the sol-gel process is initiated and silica nanotubes with different morphologies are synthesized. The other is the surface mechanism proposed by Shinkai et al,16 following which the component to catalyze the formation and growth of silica is appended on the template surface, and therefore, the sol-gel process takes place directly and exclusively on the template. This surface directed sol-gel process leads to a unitary product, which is very important when clean and well-defined materials are needed.17 In recent years, metal-decorated silica nanotubes have attracted much interest, and this silica/metal hybrid is expected to have many advantages and possible applications in medicine, physics, chemistry, and catalysis.18-23 Metal-decorated silica nanotubes are usually achieved initially by modifying silica nanotubes with thiol or amine, followed anchoring metal nanoparticles.24-26 However, silica modification is usually tedious and sometimes may degrade the silica,24-27 and therefore, a valid synthesis methodology is desired. * To whom correspondence should be addressed. E-mail: wqzhang@ nankai.edu.cn. Tel: 86-22-23509794. Fax: 86-22-23503510.

Self-assembly of a block copolymer in a block-selective solvent is demonstrated to be a powerful methodology to produce various shape-controlled supramolecular materials, including spherical core-corona micelles, vesicles, nanorods, and nanowires.28,29 The variety of these supramolecular materials in both shape and composition provides a great convenience to producing inorganic or polymer/inorganic materials when they are used as a template.30-35 For example, Manners and coworkers have prepared silica nanowires employing the cylindrical micelles of poly(ferrocenyldimethylsilane)-block-poly(2vinylpyridine) as a template.31 We have prepared single Au nanoparticles and Au nanoparticle clusters on self-assembled micelles of the thermoresponsive and pH-responsive coordination triblock copolymer.36 Herein, we propose a valid synthesis of well-defined silica nanotubes and palladium-decorated silica nanotubes (Pd/Si nanotubes) through the self-assembled core-corona threadlike micelles of the coordination block copolymer of poly(ethylene glycol)-block-poly(4-vinylpyridine) (PEG-b-P4VP). Because the PEG-b-P4VP core-corona threadlike micelles contain the pendent catalyst of the Lewis alkaline P4VP block, the surface sol-gel process of TEOS is directed exclusively onto the template of the threadlike micelles, and therefore, well-defined silica nanotubes are produced. Besides, Pd nanoparticles can be introduced into the cavum of the silica nanotubes through coordination between the pyridine ligand in the chelate P4VP block and the Pd precursor, and therefore, Pd/Si nanotubes are fabricated. 2. Experimental Section 2.1. Materials. TEOS (98%, Alfa Aesar), NaBH4 (>98.9%, Tianjin Chemical Company), and PdCl2 (>99%, Alfa Aesar) were used as received. The block copolymer of PEG114-bP4VP28 (MW ) 9.7 × 103 g/mol; polydispersity index, PDI ) 1.22; and the subscripts represent the polymerization degrees) was synthesized as discussed elsewhere.37 Deionized water was used.

10.1021/jp106690q  2010 American Chemical Society Published on Web 08/30/2010

Synthesis of Silica and Pd/Silica Nanotubes

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SCHEME 1: Schematic Synthesis of the Silica Nanotubes and Pd/Silica Nanotubes

2.2. Preparation of the Template of the Threadlike Micelles. The self-assembled threadlike micelles were prepared by dispersing 0.030 g of PEG114-b-P4VP28 in 100.0 mL of water. The mixture was kept at room temperature overnight with magnetic stirring, and then a pearl blue dispersion was observed. 2.3. Preparation of Silica Nanotubes. Into a flask, a given amount of TEOS (0.060-0.30 g) was initially dispersed in 50 mL of water by means of an ultrasonic bath set at 20 °C (KQ200KDE, 40 kHz, 200 W, Zhoushan, China). A 50 mL portion of an aqueous dispersion of the threadlike micelles was then added. The surface sol-gel process was performed initially at room temperature with gentle stirring for 4 days and then aged at 50 °C for 8 h. The resultant rodlike silica/micelles hybrid was collected by centrifugation, washed with ethanol (3 × 5 mL), dried at 40 °C under vacuum, and lastly calcined at 550 °C for 3 h in air to produce silica nanotubes. 2.4. Preparation of Pd Nanoparticle-Decorated Silica Nanotubes of Pd/Silica. Two methods were employed to prepare Pd/silica nanotubes. Following method 1, 3.5 mL of PdCl2 aqueous solution (2.5 × 10-3 mmol/mL) was initially added into 50.0 mL of an aqueous dispersion of the PEG114-bP4VP28 threadlike micelles. After being kept at room temperature with magnetic stirring for about 4 h, 5.0 mL of NaBH4 aqueous solution (2.0 × 10-2 mol/L) was added. The resultant Pd/micelles composite was collected by centrifugation and further dispersed in 50.0 mL of water. The surface sol-gel process on the Pd/micelles composite was started by adding the aqueous dispersion of the Pd/micelles composite into the aqueous dispersion of TEOS (50.0 mL, 6.0 mg/mL). After the sol-gel process at room temperature for 4 days, aging at 50 °C for 8 h, and calcination at 550 °C for 3 h in air, the Pd/Si nanotubes were obtained. Following method 2, 3.5 mL of PdCl2 aqueous solution (2.5 × 10-3 mmol/mL) and 50.0 mL of an aqueous dispersion of TEOS (6.0 mg/mL) were added into 50.0 mL of an aqueous dispersion of the PEG114-b-P4VP28 threadlike micelles. After being kept at room temperature for 4 days with magnetic stirring, 5.0 mL of NaBH4 aqueous solution (2.0 × 10-2 mol/L) was added. After aging at 50 °C for 8 h, the resultant was collected by centrifugation, washed with ethanol (3 × 5 mL), dried at 40 °C under vacuum, and lastly calcined at 550 °C for 3 h in air to produce Pd/Si nanotubes.

2.5. Characterization. Products were characterized by 1H NMR, TEM, XRD, XPS, and nitrogen gas adsorption-desorption analysis. The 1H NMR spectra were recorded on a UNITY PLUS-400 spectrometer using CDCl3 or D2O as a solvent. The TEM measurement was conducted by using a Philips T20ST transmission electron microscope at an acceleration voltage of 200 kV, whereby a small drop of the sample was deposited onto a carbon-coated copper grid and dried at room temperature under atmospheric pressure. XPS analysis was performed with a Kratos Axis Ultra DLD spectrometer employing a monochromated Al KR X-ray source (1486.6 eV) and a delay line detector (DLD). The XRD measurement was performed on a Rigaku D/max 2500 X-ray diffractometer. N2 adsorption-desorption isotherms at 77 K were conducted on a Micromeritics TriStar 3000 apparatus, and the BET surface areas were calculated using the BET equation. 3. Results and Discussion The synthesis of silica nanotubes as shown in Scheme 1 involves (1) formation of a 1-D template of the core-corona threadlike micelles through self-assembly of PEG114-b-P4VP28 in water, (2) a surface sol-gel process of TEOS on the template of the threadlike micelles to form a rodlike silica/micelles hybrid, and (3) calcination to remove the template to fabricate silica nanotubes. Pd nanoparticles are introduced into the cavum of the silica nanotubes initially through coordination between the chelate P4VP block and the Pd precursor, followed by reduction with NaBH4 aqueous solution before or during the surface sol-gel process, as shown in Scheme 1. Selection of the chelate block copolymer of PEG114-b-P4VP28 is due to two concerns. First, the self-assembled core-corona threadlike micelles, which have a hydrophilic PEG corona and a P4VP core, contain the pendent catalyst of the Lewis alkaline P4VP block to initiate the surface sol-gel process of TEOS.38 Second, the chelate P4VP block in which the pyridine ligand can coordinate with the Pd precursor provides a convenience to introducing Pd nanoparticles into silica nanotubes.28 3.1. Preparation and Characterization of the Template of the Threadlike Micelles. The template of the core-corona threadlike micelles is prepared by dispersing PEG114-b-P4VP28 in water at room temperature. After PEG114-b-P4VP28 is added into water and kept at room temperature overnight with magnetic

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Zhang et al.

Figure 1. TEM images of the threadlike micelles (A), rodlike silica/micelles hybrids (B), and silica nanotubes with wall thicknesses of 17 (C, D), 3 (E), and 8.6 nm (F).

stirring, a pearl blue dispersion is formed, indicating formation of micelles. TEM observation confirms that the block copolymer self-assembles into threadlike micelles (Figure 1A). The width of the threadlike micelles is about 40 nm, and the length is estimated to be ∼1 um. Proton NMR analysis suggests that the threadlike micelles in water have a hydrophilic PEG corona and a P4VP core (Figure S1 in the Supporting Information). 3.2. Preparation and Characterization of Silica Nanotubes. After adding TEOS into the micelle dispersion (weight ratio of TEOS/micelles ) 20), the surface sol-gel process on the template of the threadlike micelles is carried out at room temperature without additional free catalyst. TEM observation indicates that the sole product of the rodlike silica/micelles hybrid with a width of ∼60 nm is produced (Figure 1B), confirming the directed sol-gel process of TEOS on the threadlike micelles due to the inherently pendent catalyst sites of the P4VP block. Upon calcination, well-defined silica nanotubes with a wall thickness of 17 nm (Figure 1C,D) are produced. Moreover, the wall thickness of silica nanotubes can be tuned by the weight ratio of TEOS/micelles. For example, when the weight ratio of TEOS/micelles equals 4 and 15, silica nanotubes with wall thicknesses of ∼3 nm (Figure 1E) and 8.6 nm (Figure 1F) are produced. As far as we know, the silica nanotubes with a wall thickness of ∼3 nm, as shown in Figure 1E, may be one of the most thin ones so that the outline of the silica nanotubes is just observed by TEM. Compared with the general sol-gel process of TEOS catalyzed by NH3 aqueous solution,10 the present surface sol-gel process at room temperature is very mild and a relatively long time of 4 days is needed, possibly because the P4VP block is a weaker Lewis alkaline catalyst than NH3 aqueous solution. An attempt to accelerate the sol-gel process is made by addition of NH3 aqueous solution (pH ) 10) to the aqueous dispersion of the threadlike micelles. As expected, the sol-gel process runs much faster, whereas no silica nanotubes, but irregular aggregates (Figure S2 in the Supporting Information), are synthesized. We think the formation of the irregular silica aggregates is ascribed to the sol-gel translation in aqueous solution catalyzed by the free NH3 catalyst. Therefore, the surface sol-gel process on the core-corona threadlike PEG114-

Figure 2. Nitrogen adsorption/desorption isotherm of the silica nanotubes with a 17 nm wall (•, adsorption; O, desorption).

b-P4VP28 micelles without free catalyst, as shown in Scheme 1, provides the advantage of synthesis of well-defined silica nanotubes without unwanted irregular silica aggregates. It is found that the wall thickness of the silica nanotubes is larger than half of the width difference between the rodlike silica/micelles hybrid and the threadlike micelles. For example, on the basis of the width of the threadlike micelles (40 nm, Figure 1A) and the rodlike silica/micelles hybrid synthesized at a weight ratio of TEOS/micelles ) 20 (60 nm, Figure 1B), the theoretical wall thickness of the silica nanotubes is 10 nm, which is thinner than those of 17 nm, as shown in Figure 1C,D. This indicates that the TEOS molecules penetrate the PEG corona to approach the P4VP core on which sol-gel process is initiated, as shown in Scheme 1. It is expected, during the sol-gel process, that the polymer chains of the penetrated PEG block are enveloped within the resultant silica nanotubes, which will produce meso- and/or micropores just as the synthesis of mesoporous silica materials employing the typical structuredirecting agent of Pluronic.39,40 This hypothesis is confirmed by the N2 adsorption-desorption analysis of the resultant silica nanotubes. Figure 2 shows the nitrogen adsorption/desorption isotherm of the silica nanotubes with a wall thickness of 17 nm. Clearly, a typical hysteresis loop is observed, suggesting mesopores in the silica nanotubes.41-43 Furthermore, a relatively

Synthesis of Silica and Pd/Silica Nanotubes

Figure 3. XRD pattern of the silica nanotubes (A), the silica-coated Pd@micelles composite (B), and Pd/silica nanotubes synthesized following method 1 (C).

high adsorption is observed at a relative pressure near zero, suggesting micropores in the wall of the silica nanotubes due to the enveloped PEG block in the wall of the silica nanotubes.44,45 The BET surface area of the silica nanotubes, 556 m2/g, is also calculated based on the Brunauer-EmmettTeller (BET) equation. The typical silica nanotubes with a 17 nm wall are also characterized by XRD analysis. As shown in Figure 3, pattern A, a broad diffraction peak centered around 23° in the XRD pattern is observed, indicating an amorphous structure of the silica nanotubes.46,47 3.3. Preparation and Characterization of Pd/Silica Nanotubes. Pd nanoparticles are introduced into the cavum of the silica nanotubes with a 17 nm wall by two methods. Following the first method (method 1), PdCl2 aqueous solution is initially added into the aqueous dispersion of the threadlike micelles and then the Pd precursor of PdCl2 is immobilized through coordination with the P4VP block because P4VP is a typical chelate polymer.28 When NaBH4 aqueous solution is added, Pd nanoparticles are in situ synthesized on the threadlike micelles and then the threadlike composite of Pd@micelles are produced. After the sol-gel process on the composite of Pd@micelles and then calcination at 550 °C in air, Pd/Si nanotubes are fabricated (Figure 4A). From the TEM image shown in Figure 4A, it is easily discerned that 4.4-8.3 nm Pd nanoparticles are encapsulated within the cavum of the silica nanotubes. The present Pd/Si nanotubes are much different from those of the surface-doped ones,48,49 in which the metal nanoparticles are deposited on the surface of the silica nanotubes. Following the second method (method 2), immobilization of the Pd precursor and the sol-gel process are combined into a single step; that is, the Pd precursor and TEOS are added into the

J. Phys. Chem. C, Vol. 114, No. 37, 2010 15643 aqueous dispersion of the threadlike micelles at one time. TEM observation (Figure 4B) indicates that the size of Pd nanoparticles encapsulated within the silica nanotubes, 8.7-12.2 nm, is larger than those synthesized following method 1, which is also confirmed by XRD analysis discussed subsequently. Clearly, both of the two methods to produce Pd/Si nanotubes do not distort or change the silica nanotubes themselves, which provides the methodology with great potential. Furthermore, we believe, other metal nanoparticles, such as Au, Ag, Pt, Cu, and CdS, can be encapsulated within the silica nantubes following a similar strategy as discussed above, and therefore, various metal-decorated silica nanotubes can be produced. The synthesis of the Pd/Si nanotubes is also tracked by XRD analysis. Figure 3, patterns B and C, shows the XRD patterns of the silica-coated Pd@micelles composite and the Pd/Si nanotubes following method 1. Similar to the XRD pattern of the silica nanotubes, a broad diffraction peak centered around 23° is also observed in Figure 3, patterns B and C.46,47 In Figure 3, pattern B, a peak centered around 39.5° corresponding to the diffraction of the (111) lattice plane of Pd nanoparticles is also observed, indicating immobilization of Pd nanoparticles in the silica-coated Pd@micelles composite. Herein, it is pointed out that Pd content in the silica-coated Pd@micelles composite is as low as 2.7 wt %, which makes the peak of the (111) lattice plane of Pd nanoparticles quite feeble, as shown in Figure 3, pattern B. After calcination at 550 °C in air, the polymer template is removed, and the Pd/Si nanotubes nanotubes are fabricated. It is found that part of the Pd nanoparticles are oxidized, as indicated by the diffractions of (101), (110), (110), (200), and (211) lattice planes of PdO nanoparticles and the (111) lattice plane of Pd nanoparticles, as shown in Figure 3, pattern C, and Figure S3 (in the Supporting Information). Thus, the encapsulated metal nanoparticles within the silica nanotubes are the mixture of Pd and PdO nanoparticles. We believe that the oxidation of Pd nanoparticles can be avoided when the calcination is performed under inert gas atmospheres. According to the Scherrer equation,50 the average size of the encapsulated PdO nanoparticles, 8.4 nm, is calculated utilizing the diffraction peak of the (101) lattice plane,51 which is generally consistent with those by the TEM observation, as shown in Figure 4A. The Pd/Si nanotubes synthesized from method 2 are also characterized by XRD (Figure S4 in the Supporting Information), and the calculated size of the encapsulated PdO nanoparticles is 12.6 nm. X-ray photoelectron spectroscopy (XPS) analysis of the Pd/ silica nanotubes synthesized following method 1 is further made, and the XPS spectrum is shown in Figure 5. The peaks corresponding to the Si 2p (101.7 eV), Pd 3d (335.4 eV), and

Figure 4. TEM images of the Pd/silica nanotubes synthesized following method 1 (A) and method 2 (B).

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Figure 5. XPS spectrum of the Pd/silica nanotubes synthesized following method 1.

O 1s (531.1 eV) are clearly observed. On the basis of their relative intensity of Pd, Si and O, 1.2 wt % Pd in the Pd/Si nanotubes (Figure 5) is obtained, which is much lower than the value of 4.6 wt % measured by atomic absorption spectroscopy (AAS) analysis. The much lower Pd content by XPS analysis further confirms that the Pd and/or PdO nanoparticles are encapsulated within the cavum of the silica nanotubes. 4. Conclusion In summary, well-defined silica nanotubes are synthesized through a surface sol-gel process of TEOS on the template of self-assembled core-corona threadlike micelles of PEG114-bP4VP28. With the benefit from the inherent catalytic sites of the Lewis alkaline P4VP block on the threadlike micelles, the surface sol-gel process of TEOS is directed onto the template, and therefore, formation of irregular aggregates is avoided. By changing the weight ratio of TEOS/micelles, well-defined silica nanotubes with wall thicknesses ranging from 3 to 17 nm are produced. Also ascribed to the chelate P4VP block on the template of the threadlike micelles, Pd nanoparticles can be introduced into the cavum of silica nanotubes through coordination between the P4VP block and the Pd precursor, and therefore, Pd-decorated silica nanotubes of Pd/silica nanotubes are fabricated. The proposed synthesis is anticipated to be a general methodology to synthesize silica nanotubes and metal nanoparticle-decorated silica nanotubes. Acknowledgment. The financial support by the National Science Foundation of China (No. 20974051) and the Tianjin Natural Science Foundation (No. 09JCYBJC02800) is gratefully acknowledged. Supporting Information Available: Text as well as Figures S1-S4 showing the 1H NMR spectra, TEM image, and XRD pattern. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Zollfrank, C.; Scheel, H.; Greil, P. AdV. Mater. 2007, 19, 984– 987. (2) Ruscher, C. H.; Bannat, I.; Feldhoff, A.; Ren, L. R.; Wark, M. Microporous Mesoporous Mater. 2007, 99, 30–36. (3) Bae, C.; Yoo, H.; Kim, S.; Lee, K.; Kim, J.; Sung, M. A.; Shin, H. Chem. Mater. 2008, 20, 756–767. (4) Fan, R.; Wu, Y.; Li, D.; Yue, M.; Majumdar, A.; Yang, P. J. Am. Chem. Soc. 2003, 125, 5254–5255. (5) Zhu, J.; Peng, H.; Connor, S. T.; Cui, Y. Small 2009, 5, 437–439. (6) Yin, Y.; Lu, Y.; Sun, Y.; Xia, Y. Nano Lett. 2002, 2, 427–430. (7) Obare, S. O.; Jana, N. R.; Murphy, C. J. Nano Lett. 2001, 1, 601– 603.

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