Hexagonal Mesostructure and Its Disassembly into Nanofibers of a

Hexagonally mesostructured polyoxometalate-based hybrid was prepared by a mild-solution method through the self-assembly of an amphiphilic poly(ethyle...
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Hexagonal Mesostructure and Its Disassembly into Nanofibers of a Diblock Molecule/Polyoxometalate Hybrid Xiankun Lin, Yinglin Wang, and Lixin Wu* State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, People’s Republic of China Received January 2, 2009. Revised Manuscript Received February 7, 2009 Hexagonally mesostructured polyoxometalate-based hybrid was prepared by a mild-solution method through the self-assembly of an amphiphilic poly(ethylene oxide) octadecyldimethylammonium (C18NEO12) and a polyanionic silicotungstic acid (HSiW). The composition of the hybrid was characterized through 1H NMR, infrared spectroscopy, mass spectroscopy, elemental analysis, and thermogravimetric analysis, indicating an approximate formula (C18NEO12)1.6H2.4(SiW12O40). The structure of the hybrid was investigated by transmission electron microscopy (TEM), scanning electron microscopy, X-ray powder diffraction, and contact angle measurement. The hybrid is a rodlike grain with the hexagonal mesostructure, which consists of one-dimensional column micelles in which alkyl chains of C18NEO12 locate at the center and poly(ethylene oxide) (PEO) part at the outside, with HSiWs anchoring at the interface, but more close to the PEO part. Furthermore, the observation of the formation process of hybrids by TEM shows that ethanol plays an important role in the self-assembly. More importantly, the hexagonal mesostructure can transform into nanofibers comprising nanofibrils in water through the disassembly.

Introduction The organization of various functional nano-objects such as diverse nanoparticles, nanorods, nanotubes, and nanospheres in both bulk and solution is of great interest in chemistry, materials, and biology due to the demand of novel properties and applications of them.1-5 Many nano-objects of metals, semiconductors, fullerene,6 etc., have been explored extensively, including organized structures and the structural transformations. However, organizing multiple-charged inorganic clusters such as polyoxometalates (POMs) is still limited, although they have been proved promising functional objects.7-9 POM-based hexagonal or monoclinic10-16 hybrid *To whom correspondence should be addressed. E-mail: wulx@jlu. edu.cn. (1) Ofir, Y.; Samanta, B.; Rotello, V. M. Chem. Soc. Rev. 2008, 37, 1814–1823. (2) Crookes-Goodson, W. J.; Slocik, J. M.; Naik, R. R. Chem. Soc. Rev. 2008, 37, 2403–2412. (3) Arumugam, P.; Xu, H.; Srivastava, S.; Rotello, V. M. Polym. Int. 2007, 56, 461–466. (4) Haryono, A.; Binder, W. H. Small 2006, 2, 600–611. (5) Kotov, N. A. Nanoparticle Assemblies and Superstructures; CRC Press: Boca Raton, FL, 2006. (6) Nurmawati, M. H.; Ajikumar, P. K.; Renu, R.; Sow, C. H.; Valiyaveettil, S. ACS Nano 2008, 2, 1429–1436. (7) Special issue on polyoxometalates: Chem. Rev. 1998, 98, 1. :: (8) Pope, M. T.; Muller, A. Angew. Chem., Int. Ed. Engl. 1991, 30, 34–48. (9) Long, D.-L.; Burkholder, E.; Cronin, L. Chem. Soc. Rev. 2007, 36, 105–121. (10) Stein, A.; Fendorf, M.; Jarvie, T. P.; Mueller, K. T.; Benesi, A. J.; Mallouk, T. E. Chem. Mater. 1995, 7, 304–313. (11) Janauer, G. G.; Dobley, A.; Guo, J. D.; Zavalij, P.; Whittingham, M. S. Chem. Mater. 1996, 8, 2096–2101. (12) Taguchi, A.; Abe, T.; Iwamoto, M. Adv. Mater. 1998, 10, 667–669. (13) Taguchi, A.; Abe, T.; Iwamoto, M. Microporous Mesoporous Mater. 1998, 21, 387–393. (14) Ciesla, U.; Demuth, D.; Leon, R.; Petroff, P.; Stucky, G.; Unger, K.; :: Schuth, F. J. Chem. Soc., Chem. Commun. 1994, 1387-1388. (15) Yun, H.-s.; Kuwabara, M.; Zhou, H. S.; Honma, I. Thin Solid Films 2007, 515, 2842–2846. (16) Polarz, S.; Smarsly, B.; Antonietti, M. ChemPhysChem 2001, 457–461.

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mesostructures as well as one-dimensional (1D) fibrous assemblies17-19 have advantages of morphologies for the promising applications such as conduction in nanodevices,20 ceramic products with well-defined mesostructure, and catalysis.21 Specifically, linear amphiphile/POM aggregates with well-proportioned structure in water have not been observed due to the lacking of a suitable approach, while amphiphiles are powerful matrixes for POM-based assemblies and can optimize the properties of the hybrid assemblies.22-26 To fabricate new aggregated structures of amphiphile/ POM hybrids, a new strategy should be considered. Recently, the disassembly of mesostructures has been applied successfully to synthesize well-defined structural building units from the block copolymer/nanoparticle mesostructures27 and mesoporous silica nanocubes from the three-dimensionally ordered macroporous structures.28 Such a top-down methodology can provide new nanostructures which are difficult to (17) Alam, M. A.; Kim, Y.-S.; Ogawa, S.; Tsuda, A.; Ishii, N.; Aida, T. Angew. Chem., Int. Ed. 2008, 47, 2070–2073. :: (18) Yelamanchili, R. S.; Walther, A.; Muller, A. H. E.; Breu, J. Chem. Commun. 2008, 489–491. (19) Carraro, M.; Sartorel, A.; Scorrano, G.; Maccato, C.; Dickman, M. H.; Kortz, U.; Bonchio, M. Angew. Chem., Int. Ed. 2008, 47, 7275–7279. (20) Cho, B.-K.; Jain, A.; Gruner, S. M.; Wiesner, U. Science 2004, 305, 1598-1601. (21) Boettcher, S. W.; Fan, J.; Tsung, C.-K.; Shi, Q. H.; Stucky, G. D. Acc. Chem. Res. 2007, 40, 784–792. :: (22) Kurth, D. G.; Lehmann, P.; Volkmer, D.; Colfen, H.; Koop, M. J.; :: Muller, A.; Chesne, A. D. Chem.;Eur. J. 2000, 6, 385–393. (23) Bu, W. F.; Li, H. L.; Sun, H.; Yin, S. Y.; Wu, L. X. J. Am. Chem. Soc. 2005, 127, 8016–8017. (24) Li, H. L.; Sun, H.; Qi, W.; Xu, M.; Wu, L. X. Angew. Chem., Int. Ed. 2007, 46, 1300–1303. (25) Zhang, H.; Lin, X. K.; Yan, Y.; Wu, L. X. Chem. Commun. 2006, 4575–4577. (26) Clemente-Le on, M.; Ito, T.; Yashiro, H.; Yamase, T.; Coronado, E. Langmuir 2007, 23, 4042–4047. (27) Warren, S. C.; Disalvo, F. J.; Wiesner, U. Nat. Mater. 2007, 6, 156–161. (28) Li, F.; Wang, Z. Y.; Stein, A. Angew. Chem., Int. Ed. 2007, 46, 1885–1888.

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obtain otherwise and may be feasible in many fields.29 However, the implementation of this methodology is rare for the amphiphile/inorganic cluster mesostructures. To perform such disassembly, obtaining assembled mesostructures in the first step may be crucial, which may require suitable design and choice of the building blocks, that is, the effective combination of self-assembly and disassembly. Herein, we focus on whether the assembly and disassembly can be employed to prepare new POM-based ordered structures and how to obtain the required mesostructures. We report a novel amphiphile/POM hybrid with hexagonal mesostructure prepared through self-assembly of silicotungstic acid (H4SiW12O40, denoted as HSiW) and an amphiphilic diblock molecule (C18NEO12 3 Ts) (Chart 1). Through properly disintegrating such mesostructure, 1D amphiphile/POM hybrid aggregates in water are observed for the first time.

Experimental Section Materials. Unless otherwise noted, the chemicals were all obtained from commercial suppliers and used without further purification. Silicotungstic acid (H4SiW12O40, denoted as HSiW), phosphotungstic acid (H3PW12O40, denoted as HPW), and p-toluenesulfonyl chloride (TsCl) were purchased from Sinopharm Chemical Reagent Co. Ltd. CH3(OCH2CH2)nOH (average Mn = 550, Mw/Mn = 1.03) was purchased from Fluka, and N,N-dimethyloctadecylamine with purity of 89% was the product of Acros Organics. CH3(CH2)17(OCH2CH2)nOH (average Mn = 711, denoted as C18EO10) was a product from Aldrich. Preparation of C18NEO12 3 Ts. The amphiphilic diblock molecule C18NEO12 3 Ts was synthesized according to such a route: beginning with the tosylation of poly(ethylene oxide) monomethyl ether, and then the obtained product reacting with N,N-dimethyloctadecylamine to give the target product. p-Toluenesulfonyl Poly(ethylene oxide) Monomethyl Ether.30 To the mixture of CH3(OCH2CH2)nOH (Mn = 550, Mw/Mn = 1.03) (8.72 g, 15.9 mmol) and 16% sodium hydroxide aqueous solution (5.1 mL) in an ice salt bath was added dropwise the THF (5 mL) solution of TsCl (4.02 g, 21.1 mmol) with continuous stirring for 4 h. The resulting mixture was further stirred at room temperature for 25 h and then poured into ice water. The aqueous solution was extracted with three portions of chloroform. The combined organic extract was dried over anhydrous MgSO4, filtered, and concentrated under the reduced pressure to give product (11.30 g) in 99% yield as a colorless oil. 1H NMR (500 MHz, CDCl3, δ): 2.45 (s, 3H), 3.37 (s, 3H), 3.54-3.73 (m, 46H), 4.16 (t, J = 5 Hz, 2H), 7.35 (d, J = 8 Hz, 2H), 7.80 (d, J = 8 Hz, 2H).

Oligo(oxyethylene)octadecyldimethylammonium 4-Methylbenzenesulfonate (C18NEO12 3 Ts). p-Toluenesulfonyl poly

(ethylene oxide) monomethyl ether (3.46 g, 4.85 mmol) and N, N-dimethyloctadecylamine (1.78 g, 6.00 mmol) were dissolved in anhydrous acetonitrile. The mixture was heated at reflux for 21 h. Solvent was removed using a rotary evaporator, and the crude product was purified by silica gel column chromatography with chloroform/methanol (20:1 in v/v) as eluent to give the compound C18NEO12 3 Ts (1.22 g, 24.8%) as a white solid. 1 H NMR (500 MHz, DMSO-d6, δ): 7.48 (d, J = 8 Hz, 2H), 7.12 (d, J = 8 Hz, 2H), 3.82 (t, 2H), 3.41-3.58 (m, 46H), 3.28 (m, 2H), 3.24 (s, 3H), 3.04 (s, 6H), 2.29 (s, 3H), 1.65 (m, 2H), 1.24 (m, 30H), 0.86 (t, J = 6.5 Hz, 3H); MALDI-TOF MS (m/z): 662.8 [M+], 706.8 [M+], 751.5 [M+], 795.8 [M+], 837.9 [M+], 882.8 [M+], 927.1 [M+], 968.8 [M+]. (29) Pradeep, C. P.; Long, D.-L.; Streb, C.; Cronin, L. J. Am. Chem. Soc. 2008, 130, 14946–14947. (30) Ouchi, M.; Inoue, Y.; Liu, Y.; Nagamune, S.; Nakamura, S.; Wada, K.; Hakushi, T. Bull. Chem. Soc. Jpn. 1990, 63, 1260–1262.

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Chart 1. Chemical Structures of C18NEO12 3 Ts, C18EO10, and Polyhedron Structure of HSiW

Preparation of C18NEO12/HSiW Hybrid. C18NEO12 3 Ts (0.414 mmol) in water (14 mL) was stirred at room temperature, and then HSiW (0.103 mmol) dissolved in water (4 mL) was added dropwise. After stirring for 2 h, a clear solution was obtained, and ethanol (50 mL) was added dropwise with stirring. A white precipitate formed and was collected by filtration, washed with a bit of ethanol, and dried in vacuum overnight at 50 °C, giving the hybrid in a yield of 50.2%. 1H NMR (500 MHz, DMSO-d6, δ): 3.83 (t, 2H), 3.41-3.58 (m, 46H), 3.28 (m, 2H), 3.24 (s, 3H), 3.04 (s, 6H), 1.66 (m, 2H), 1.24 (m, 30H), 0.86 (t, 3H). MALDI-TOF MS (m/z): 707.1 [M+], 752.0 [M+], 797.5 [M+], 840.8 [M+], 884.4 [M+], 927.1 [M+]. IR (KBr): ν = 3436, 2957, 2923, 2853, 1635, 1467, 1397, 1353, 1298, 1250, 1097, 970, 920, 883, 797 cm-1. Elemental analysis: Anal. Calcd for (C18NEO12)1.6H2.4(SiW12O40): C 20.04; H 3.57; N 0.52. Found: C 20.27; H 3.25; N 0.47. Examination of C18NEO12/HSiW Hybrid through Transmission Electron Microscopy (TEM). One milliliter of C18NEO12 3 Ts aqueous solution with a concentration of 34.5 mg/mL and 0.29 mL of HSiW aqueous solution with a concentration of 85.3 mg/mL were mixed under stirring at room temperature. Here, the mole ratio of C18NEO12 3 Ts and HSiW was kept at 4:1. After stirring for 3 h, ethanol was added dropwise, and the mixed solution was sampled for TEM measurements. Several drops of the solution or dispersion were placed on a carbon-coated copper grid and dried at room temperature through suction by filter papers. Preparation of C18EO10/HSiW Hybrid. As the solubility of C18EO10 in water is less than 1 mg mL-1 at room temperature, the preparation of the control C18EO10/HSiW hybrid is different from that of the C18NEO12/HSiW hybrids and was carried out through the coprecipitation from ethanol. The nonionic surfactant C18EO10 (1.04 g, 1.46 mmol) was dissolved in 4.5 mL of ethanol, and then silicotungstic acid (1.05 g, 0.37 mmol) in 2.0 mL of ethanol was added dropwise into the C18EO10 solution with stirring at room temperature. The obtained white precipitate was filtered and washed with ethanol and then dried overnight at 30 °C in vacuum to give the control hybrid with a yield of 206.6 mg. Characterization. 1H NMR spectra were recorded on a Bruker Avance 500 instrument using d6-DMSO as solvent and TMS as internal reference. Elemental analysis (C, H, N) was performed on a Vario EL Elemental Analyzer from Elementar Analysensysteme GmbH. FT-IR spectra were carried out on a Bruker IFS 66v FT-IR spectrometer equipped with a DTGS detector (32 scans) with a resolution of 4 cm-1 from pressed KBr pellets. Thermogravimetric analysis (TGA) was conducted with a PerkinElmer Diamond TG/DTA instrument with a heating rate of 10 °C/min under flowing air. The matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra were recorded on a LDI-1700 mass spectrometer (Linear Scientific Inc.) with DMSO as solvent. Atomic force microscopy (AFM) images were carried out with a commercial instrument (Digital Instrument, Nanoscope III, and Dimension 3000) at Langmuir 2009, 25(11), 6081–6087

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room temperature in air. Scanning electron microscopy (SEM) images were collected on a JEOL JSM-6700F field emission scanning electron microscope. Transmission electron microscopy (TEM) images were obtained with a Hitachi H8100 electron microscope with accelerating voltage of 200 KV without staining. Several drops of sample solution or dispersion were placed on a carbon-coated copper grid, following sucking by filter papers and drying at room temperature. X-ray diffraction (XRD) experiments were carried out on a Philips PW1710 BASED X-ray diffractometer. Contact angles of water drops :: were measured with sessile drop method using a KRUSS DSA10-MK2 contact angle measuring system at ambient temperature. For the contact angle measures, the hybrid powder was impressed as a flat and smooth slide using a tablet compression machine.

Results and Discussion Hexagonal Mesostructure of C18NEO12/HSiW Hybrid. The designed C18NEO12 3 Ts is a substitute of octadecyltrimethylammonium p-toluenesulfonate, in which one of the methyl groups has been replaced by a poly(ethylene oxide) (PEO) chain (Chart 1). The HSiW, as one of the most common Keggin-type POMs, is a strong acid and possesses four negative charges.7 Under the given pH, HSiW maintains its chemical structure well and accommodates more binding sites in comparison to the case of HPW with three negative charges, which is favorable to enhance the interaction between organic and inorganic components. As mentioned in the following discussion, the acidic POMs are necessary for the formation of hybrids because the neutral POMs cannot protonize the PEO chains effectively. The preparation of the hybrid was carried out by mixing C18NEO12 3 Ts and HSiW aqueous solutions with a molar ratio of 4:1, giving a white precipitate on the occasion of addition of ethanol. Here, the organic component, as a diblock molecule connected through a quaternary ammonium group, should tend to form both hydrophobic and hydrophilic domains through microphase separation. The cationic quaternary ammonium group can firmly anchor the polyanion through electrostatic interaction, which is similar to the route that has been employed to modify the surface of POMs with cationic surfactants, forming surfactantencapsulated polyoxometalates (SEPs).22-25 However, an additional interaction should be considered in the present case. As reported by Neumann and Assael,31 PEO chains can strongly complex with heteropolyacid through electrostatic attraction due to the formation of oxonium ions. As a result, hydrophilic polyacid should disperse in hydrophilic PEO domains rather than in hydrophobic alkyl chain areas, and the rigidity of inorganic polyanion clusters should enhance the microphase segregation to form highly ordered mesostructures. The obtained hybrid is no longer soluble in water and common organic solvents except for DMSO and DMF. 1 H NMR spectra in deuterated DMSO indicate that the chemical shifts and the integral area ratios are coincident with those of C18NEO12 3 Ts, except that the signals corresponding to the p-toluenesulfonic anions disappear completely. This result suggests that the cationic C18NEO12 maintains its chemical structure and interacts with HSiW electrostatically through the ion replacement. The mass spectrum of the hybrid figures a molecular weight distribution (31) Neumann, R.; Assael, I. J. Chem. Soc., Chem. Commun. 1989, 547–548.

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Figure 1. IR spectra of (a) the C18NEO12/HSiW hybrid and (b) the C18EO10/HSiW hybrid and HSiW.

similar to that of C18NEO12 3 Ts, suggesting that the precipitation does not change the molecular weight distribution of organic component. In the IR spectrum of the hybrid (Figure 1a), the bands at 797, 883, 920, and 970 cm-1 are attributed to νas(W-Oc-W), νas(W-Ob-W), νas(Si-Oa), and νas(W-Od), respectively,32 clearly revealing that HSiWs keep their Keggin structures well after the composition. The CH2 wagging mode appearing at 1353 cm-1 shows that PEO chains are amorphous,33 which may derive from disordered stacking or the effect of small molecular weight. The characteristic asymmetric bending vibration of oxonium ions at about 1720 cm-1 was not observed because of the low content comparing with the large molecular weight of the hybrid (ca. 4200 g mol-1, as follows) or the resistance of bending vibrations in solid state.34 Combining these results and elemental analysis, an approximate formula for the chemical composition of the hybrid complex is proposed to be (C18NEO12)1.6H2.4(SiW12O40). The thermogravimetric analysis obtained in air also confirms this formula. The 30.9% (in w/w) of weight loss between 150 and 700 °C is coincident with the calculated value 32.0% according to the given formula, assuming the complete decomposition of the organic components and transformation of HSiW into corresponding oxides. To demonstrate the interaction between PEO chains and HSiW, we prepared a composite consisting of HSiW and a nonionic surfactant CH3(CH2)17(OCH2CH2)nOH (Mn = 711, C18EO10). C18EO10 possesses a similar structure to C18NEO12 but without the bridging group of quaternary (32) Rocchiccioli-Deltcheff, C.; Fournier, M.; Franck, R.; Thouvenot, R. Inorg. Chem. 1983, 22, 207–216. (33) Dissanayake, M. A. K. L.; Frech, R. Macromolecules 1995, 28, 5312–5319. (34) Yang, G. C.; Guo, H. W.; Wang, M. K.; Huang, M. H.; Chen, H.; Liu, B. F.; Dong, S. J. J. Electroanal. Chem. 2007, 600, 318–324.

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Figure 2. (a) SEM image of the hybrid nanorods viewed in a large area and amplified TEM images of the hybrids viewed (b) along the axis and (c) cross section. All images were captured from sample chloroform suspensions. (d) Powder X-ray diffraction pattern of the hybrid solid. ammonium (Chart 1). In the IR spectrum of the C18EO10/ HSiW hybrid, the bands attributed to νas(W-Oc-W), νas(W-Ob-W), νas(Si-Oa), and νas(W-Od) have obvious shifts compared with the spectrum of HSiW, indicating the interaction between HSiW and PEO chains of C18EO10 in the control hybrid (Figure 1b).15,35 Furthermore, these bands are almost the same as the ones in the IR spectrum of the C18NEO12/HSiW hybrid. The interaction between HSiW and PEO chains should also exist in the C18NEO12/HSiW hybrid. In addition, the formula of the C18NEO12/HSiW hybrid (C18NEO12)1.6H2.4(SiW12O40) also shows the hydronium ions exist in the hybrid. The hydronium ions can provide the bridge for the interaction between HSiW and PEO chains. Scanning electron microscopy (SEM) micrographs show the morphology of the C18NEO12/HSiW hybrid suspended in chloroform (Figure 2a), rodlike grains with a dimension of up to ca. 1 μm in length and an average ca. 150 nm in width. Interestingly, as shown in the transmission electron microscopic (TEM) image (Figure 2b), the rodlike grains are constituted of parallel stripes with regular spacings. Normally, the dark regions with ca. 2 nm of thickness should correspond to the domains of HSiW due to the heavy atoms contained in POMs. Owing to the strong electrostatic interaction with HSiW, the PEO part of the organic component should also exist in the area where POMs locate. In contrast to this, the light regions with the same size scale should be attributed to the hydrophobic alkyl chains because this part is incompatible with both PEO and HSiW parts. Although these parallel arrays may imply different packing morphologies, the X-ray diffraction (XRD) pattern of the hybrid reveals an obvious hexagonal structure rather than a layered  (35) Stangar, U. L.; Groselj, N.; Orel, B.; Colomban, P. Chem. Mater. 2000, 12, 3745–3753.

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structure. The diffraction peaks can be well indexed as (100), (110), (200), (210), (300), (220), (310), and (400) planes of √ the hexagonal phase with a = b = 4.3 nm (2d100/ 3) and an unknown c (Figure 2d). The intensity of peak (300) is the strongest one in all diffractions, suggesting the preferred orientation of the hybrid nanorods. The preferred orientation is a usual phenomenon for some nonspherical particles, especially for rod- or flakelike crystalline grains. The additional diffraction at 2θ = 5.8° (d = 1.5 nm) cannot be assigned definitely. One of the possibilities is that there is an ordered array of HSiW in PEO domains. Owing to the fact that each HSiW is electrostatically surrounded by cations, the combination of HSiW diameter (0.9 nm) and the average lateral size (ca. 0.6 nm) of C18NEO12 as well as hydrogen atoms is in agreement with the d value of the diffraction. Normally, the full observation of the cross section of the C18NEO12/HSiW hybrid nanorods is relatively difficult because the stripes are arrayed along the long axis of the nanorods. Fortunately, a lateral view to some short nanorods occasionally standing on end definitely proves the onedimensional structure because the hexagonal arrayed dotted rather than layered domains are found very distinctly (Figure 2c). The measured lattice constant of ca. 4.5 nm is coincident with the result from XRD measurement. In addition, the contact angle of water drops on the flat compressed slide of the hybrid is 33.8 ( 1.4°, indicating that the surface of the nanorods is hydrophilic. Therefore, we suppose a possible packing model: in the hexagonal packing, the 1D structures should be the column micelles composed of hydrophobic alkyl chains surrounding by the PEO chain matrix, in which HSiWs locate at the interface composed of quaternary ammonium groups, but more close to the PEO part (Scheme 1). This stacking structure is geometrically and energetically favorable due to the larger volume of Langmuir 2009, 25(11), 6081–6087

Lin et al. Scheme 1. Illustration of the Hexagonally Mesostructured Nanorods and Its Disintegration into Nanofibersa

a The alkyl and PEO chains and HSiW anions are represented as black and red lines and blue circles, respectively. Marked dashed line circle shows the cross section of a nanofibril.

the PEO chain than the alkyl chain in one C18NEO12 cation. The suggested structure also matches the result of the contact angle measurement and the TEM observation that the gray side views of the linear structure locate in black surroundings due to the strong contrast caused by heavy atoms of HSiW. Because of the strong complexation between C18NEO12 and HSiW, especially the cross-linking interaction of PEO chains, the present packing model is also in agreement with the fact that the hybrid is insoluble in common solvents. Obviously, such POM-based nanorods possess larger surface area than common bulk POMs, displaying the potential in catalysis.18 Meanwhile, the hexagonally mesostructured nanorods may also be applied as precursors to prepare mesoporous materials through appropriate treatments such as heating to a high temperature. The detailed observation of preparation process is helpful for understanding the formation mechanism of the hybrids. We investigated the evolution of aggregated structures in C18NEO12 3 Ts/HSiW mixture solution versus addition of ethanol by TEM. When no ethanol was added, the solution was clear and colorless, and no stable aggregates were found to generate. Adding 3 mL of ethanol into the sample solution led to a light blue opalescence immediately, while no stable aggregates were observed initially. After 5 min of standing, the solution became opaque with oily streaks and straight beltlike aggregates comprising nanofibrils appeared (Figure 3a). Meanwhile, the small fragments of beltlike aggregates that may serve as the nuclei grew into long and wide belts, and the process speeded up when prolonging the standing time and increasing the added ethanol (Figure 3b). The solution became turbid, and the white precipitates emerged finally, implying that rodlike grains with black and white strips have grown up (Figure 3c,d). Obviously, ethanol plays an important role in the formation of hybrid assemblies. For common polar solvents, only alcohols such as methanol, ethanol, and isopropyl alcohol can cause the precipitation of the hybrid in ethanol-aqueous solution. It is well-known that the mixture of CH3(OCH2CH2)nOH and HSiW is soluble in water, but they can precipitate in ethanol. Therefore, a possible reason is that the addition of adequate ethanol enhances the interaction between C18NEO12 and HSiW and reduces the polarity of the mixed solvent remarkably, leading to the formation of the aggregates and growing up despite the hydrophilic surface of the nanorods. Meanwhile, the microphase separation of incompatible parts leads to the highly ordered structure. Hence, we suppose that the lateral electrostatic interaction-induced cross-linking Langmuir 2009, 25(11), 6081–6087

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between protonated PEO chains and HSiWs directs the formation of linear elementary column micelles as well as long belts and finally nanorods of hybrids composed of the column micelles. Different from the previous reports,10-15 the present hexagonal mesostructure shows the morphology of nanorods, which may derive from such a self-assembly process. In addition, we checked the effect of the mole ratio of C18NEO12 3 Ts to HSiW with the same preparation process. When the molar ratio keeps at 8:1, 4:1, or 1:1, the chemical compositions of the obtained hybrids are almost the same (Table S3 in the Supporting Information), implying a optimal ratio of organic to inorganic components in the precipitate. For the ratio at 8:1, nanorods with parallel stripes are also obtained (Figure S8). But for the ratio at 1:1, nanorod morphologies are rare, and only some stripes are found (Figure S9). In this case, the excess protons from HSiWs maybe influence the assembly and growth of the column micelles so that nanorods cannot be observed readily. In the present approach, POMs should be able to provide protons and stable in the acidic condition for the protonation of PEO chains. When mixing C18NEO12 3 Ts and POM salts (e.g., Na9EuW10O36) and then adding ethanol, no hybrid precipitate was observed. We also tried to prepare hybrids containing another common Keggin-type POM, HPW, with the mole ratio of C18NEO12 3 Ts to HPW at 3:1. However, the composition of the obtained hybrid powder seems heterogeneous according to the elemental analysis due to the large viscosity of the mixed solution. Nanofibers of C18NEO12/HSiW Hybrid in Water. Ordered structures and the cooperation of multiple supramolecular interactions in the hybrids may imply the possibility for obtaining new structures by the controlled decomposition of the mesostructures. In light of this idea, we investigated the aggregated structure of the C18NEO12/HSiW hybrids in the water. After 30 min of sonication at 70 °C, the hybrid solid can dissolve in water. The aqueous solution becomes transparent and maintains stable in half a year, and the concentration can reach as high as 0.75 mg mL-1. The appeared Tyndall phenomenon implies that tiny aggregates exist in the aqueous solution. TEM images exhibit long nanofibers with a width of 30-50 nm (Figure 4a). From the amplified image, these nanofibers are proved to be composed of fine nanofibrils (Figure 4b). The width of black and white regions of a nanofibril is estimated ca. 6.5 nm, larger than the one observed in the nanorods. From the consistent IR spectrum of nanofibers with that of bulk hybrids, we believe that the Keggin structure of HSiWs keeps quite well after the disassembling treatment (Figure S10). The nanofibers can be also observed by AFM through casting the sample solution on mica and then sucking the excessive solvent with a filter paper, indicating that the nanofibers have developed in solution rather than the volatilization process of solvent on the substrate (Figure S11). It is obvious that the sonication and heating of the aqueous solution partially weakens the interaction between PEO chains and polyanions bridged by oxonium ions. As a result, several adjacent columns disintegrate from the hexagonal mesostructure into nanofibers that are composed of nanofibrils in aqueous solution (Scheme 1). We believe that the stretching and relaxing of PEO chains from the binding with HSiWs extend the distance between the white regions of the adjacent nanofibrils up to ca. 6.5 nm. DOI: 10.1021/la900014j

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Figure 3. TEM images of self-assembled structures in the mixed aqueous solution of C18NEO12 3 Ts and HSiW by gradually increasing the volume of added ethanol by (a) 3 mL (sampling after 5 min), (b) 4 mL, to (c) 20 mL, where (d) is an image in a larger area.

Figure 4. TEM images of (a) the nanofibers obtained through dissolution of the hybrid in water by sonication and heating and (b) the higher magnification of the nanofibers. To understand the effect of quaternary ammonium group on the construction of nanofibers, the nonionic surfactant CH3(CH2)17(OCH2CH2)nOH (Mn = 711, C18EO10), possessing a similar structure to C18NEO12 but without the bridging group of quaternary ammonium, was chosen as a comparison to mix with HSiW (Chart 1). For the C18EO10/ HSiW hybrids, the nanorods with regular stripes along the long axis can also be observed by TEM (Figure S12), which is analogous to the structure of the C18NEO12/HSiW hybrid. However, the C18EO10/HSiW hybrid dissolves totally in water by heating, and no apparent aggregates can be observed by TEM. Evidently, the reason is that the binding force between C18EO10 and HSiW is much weaker than that occurring in the C18NEO12/HSiW complex because of lacking additional strong charge interaction between quaternary ammonium group and HSiW. This case directs that 6086

DOI: 10.1021/la900014j

the C18EO10/HSiW hybrid could not make the nanofibers stable enough in the solution because the nanofibers need much stronger interaction against the dissolution of water than bulk nanorods needed under the same conditions. Different from the disassembly of the block copolymer/inorganic particle mesostructures,27 the additional electrostatic interaction should be a necessary factor in the present case by making the column structures strong enough to keep the aggregation during the disassembly process.

Conclusions In conclusion, we constructed a POM-based hexagonal mesostructure with nanorod morphology. The nanofibers comprising nanofibrils in water are obtained through the Langmuir 2009, 25(11), 6081–6087

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partial disassembly of the C18NEO12/HSiW mesostructure. The mesostructure consists of 1D column micelles in which alkyl chains of the amphiphiles locate at the center and PEO part locates at the outside, with HSiWs anchoring at the interface, but more close to the PEO part. In the self-assembly process, ethanol promotes the formation of nanorod morphology of the hybrids, and in the disassembly, quaternary ammonium group plays a key role to stabilize the nanofibers. The present results may imply that the proper application of disassembly of amphiphile/cluster assemblies can be employed to fabricate new nanostructures based on POMs or other inorganic functional clusters. Since both HSiWs and PEO are candidates for ionic conductor36 and POMs can undergo redox reactions without changing their structures,37 the obtained hexagonal packing and 1D structures may (36) Zhao, X.; Xiong, H.-M.; Xu, W.; Chen, J.-S. Mater. Chem. Phys. 2003, 80, 537–540. (37) Troupis, A.; Hiskia, A.; Papaconstantinou, E. Angew. Chem., Int. Ed. 2002, 41, 1911-1914.

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facilitate the application of POMs in protonic conduction and catalysis.38 Acknowledgment. This work was financially supported by the National Basic Research Program (2007CB808003), National Natural Science Foundation of China (20574030, 20703019, 20731160002), PCSIRT of Ministry of Education of China (IRT0422), and Open Project of State Key Laboratory of Polymer Physics and Chemistry, CAS. The authors thank the 111 project for the visit and helpful discussions with Dr. T. Liu at Lehigh University. Supporting Information Available: Characterization of C18NEO12 3 Ts and the C18NEO12/HSiW hybrid, TEM images and elemental analysis results of hybrids at the different mole ratio of C18NEO12 3 Ts to HSiW, IR spectrum and AFM image of the nanofibers, and TEM images and XRD pattern of the control C18EO10/HSiW hybrid. This material is available free of charge via the Internet at http://pubs.acs.org. (38) Sanyal, A.; Mandal, S.; Sastry, M. Adv. Funct. Mater. 2005, 15, 273–280.

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