Covalent Dispersion of Surfactant-Encapsulated Polyoxometalates

Nov 18, 2009 - In this paper, we have exhibited a novel strategy for the construction of polyoxometalate (POM) and metal nanoparticle (MNP) integrated...
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Covalent Dispersion of Surfactant-Encapsulated Polyoxometalates and In Situ Incorporation of Metal Nanoparticles in Silica Spheres Yuanyuan Zhao, Wei Qi, Wen Li, and Lixin Wu* State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, P. R. China Received September 16, 2009. Revised Manuscript Received November 1, 2009 In this paper, we have exhibited a novel strategy for the construction of polyoxometalate (POM) and metal nanoparticle (MNP) integrated silica spheres. To introduce MNPs into the POM embedded silica spheres, we employed the controllable in situ synthesis of MNPs through the photoreduction property of POMs that were pre-incorporated into silica spheres. Through electrostatic encapsulation with hydroxyl-terminated cationic surfactants, the POM polyanions with photoinduced redox property formed surfactant encapsulated clusters first. The complex was then grafted into a silica matrix by means of a co-condensation with hydrolyzed tetraethoxylsilicane covalently, and stable silica spheres containing surfactant-encapsulated POMs were obtained. The dispersion and the concentration of POM complex in the silica spheres can be tuned in a quite large extent, where the structure and property of POMs were maintained. In addition, the POMs can be photochromically changed to the reduced state through the irradiation with UV light. The well-dispersed POMs in a hydrophobic microenvironment within the hybrid spheres can be used as reductants for the in situ synthesis of MNPs. More significantly, the size and content of MNPs were tuned by controlling the experiment condition and the possible locations of both POMs and MNPs were characterized by IR, XPS, TEM and Raman spectra. The hybrid silica spheres combining both POMs and MNPs may provide potential applications in catalysis and antibacterial materials.

Introduction Polyoxometalates (POMs) with sizes of one to several nanometers and structural variety, as a rich family of metal-oxygen clusters, exhibit diverse catalytic, electrochemical, photophysical and redox properties.1 To apply these properties in applicable materials systems, well developed methods concerning the surface modification provide convenient routes to change the microenvironment of POMs and perform their features in specially designed matrices.2-6 For example, some POMs have been introduced into confined domains (Langmuir-Blodgett film, layer-by-layer assembly, zeolite and so on), realizing photo- and *To whom correspondence should be addressed. E-mail: [email protected]. (1) (a) Special issue on “Polyoxometalates” . Chem. Rev. 1998, 98, 1-390. (b) Pope, M. T.; M€uller, A. Polyoxometalate Chemistry: From Topology via Self-Assembly to Application; Kluwer: Dordrecht, The Netherlands, 2001. (2) (a) Kurth, D. G.; Lehmann, P.; Volkmer, D.; C€olfen, H.; Koop, M. J.; M€uller, A.; Chesne, A. D. Chem.;Eur. J. 2000, 6, 385–393. (b) Zhang, H.; Lin, X. K.; Yan, Y.; Wu, L. X. Chem. Commun. 2006, 4575–4577. (c) Li, H. L.; Qi, W.; Li, W.; Sun, H.; Bu, W. F.; Wu, L. X. Adv. Mater. 2005, 17, 2688–2692. (d) Li, W.; Bu, W. F.; Li, H. L.; Wu, L. X.; Li, M. Chem. Commun. 2005, 3785–3787. (e) Zhang, T. R.; Spitz, C.; Antonietti, M.; Faul, C. F. J. Chem.;Eur. J. 2005, 11, 1001–1009. (f) Zhang, J.; Song, Y.-F.; Cronin, L.; Liu, T. B. J. Am. Chem. Soc. 2008, 130, 14408–14409. (3) (a) Liu, S. Q.; Volkmer, D.; Kurth, D. G. J. Cluster Sci. 2003, 14, 405–419. (b) Zhang, T. R.; Liu, S. Q.; Kurth, D. G.; Faul, C. F. J. Adv. Funct. Mater. 2009, 19, 642–652. (c) Gao, G. G.; Xu, L.; Wang, W. J.; An, W. J.; Qiu, Y. F. J. Mater. Chem. 2004, 14, 2024–2029. (4) (a) Zhang, L.; Shen, Y. H.; Xie, A. J.; Li, S. K.; Wang, C. Mater. Chem. 2008, 18, 1196–1203. (b) Kishore, P. S.; Viswanathan, B.; Varadarajan, T. K. Nanoscale Res. Lett. 2008, 3, 14–20. (c) Kida, T. Langmuir 2008, 24, 7648–7650. (5) (a) Qi, W.; Li, H. L.; Wu, L. X. Phys. Chem. B 2008, 112, 8257–8263. (b) Xu, M.; Liu, C. L.; Xu, Y.; Li, W.; Wu, L. X. Colloids Surf. A 2009, 333, 46–52. (6) Qi, W.; Li, H. L.; Wu, L. X. Adv. Mater. 2007, 19, 1983–1987. (7) (a) Crooks, R. M.; Zhao, M. Q.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181–190. (b) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001, 412, 169–172. (c) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729–7744. (8) (a) Jain, K. K. Expert. Rev. Mol. Diagn. 2003, 3, 153–161. (b) Haes, A. J.; Chang, L.; Klein, W. L.; Van Duyne, R. P. J. Am. Chem. Soc. 2005, 127, 2264–2271. (c) Liu, G.; Garrett, M. R.; Men, P.; Zhu, X. W.; Perry, G.; Smith, M. A. Biochim. Biophys. Acta 2005, 1741, 246–252.

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electro-chromism as well as the redox capability.3-5 On the other hand, metal nanoparticles (MNPs) attracted considerable interests due to their potential applications in catalysis,7 medicine,8 and optical devices.9 Though the preparation methods of metal nanoparticles (MNPs) have been extensively investigated,10 POMs exhibit a specific role for this purpose, because the inorganic clusters can serve as both the reductants and the stabilizers,11-13 and the process may provide an effective approach for the synergic catalysis of the oxidation and hydrogenation.14 Thus, developing a simple method to combine POMs and MNPs in one confined domain in a well dispersed state is of interests in both fundamental research and in potential applications. Currently, most of the reported examples regarding POM reduction limit in aqueous solutions,11-13 films,4a,b,5b liquid-liquid interface,4c and bulk silica.5a Through modification with ammonium surfactant bearing hydrophobic alkyl chains, surfactant encapsulated POMs (SEPs) with hydroxyl groups outside could be prepared and incorporated into sol-gel bulk and films. And, the property of POMs was confirmed well kept in the solid materials.5,6 (9) (a) Link, S.; Ei-Sayed, M. A. Annu. Rev. Phys. Chem. 2003, 54, 331–366. (b) Thomas, K. G.; Kamat, P. V. Acc. Chem. Res. 2003, 36, 888–898. (c) Maier, S. A.; Kik, P. G.; Atwater, H. A.; Meltzer, S.; Harel, E.; Koel, B. E.; Requicha, A. A. G. Nat. Mater. 2003, 2, 229–232. (10) Feldheim, D. L.; Foss, C. A., Jr. Metal Nanoparticles: Synthesis, Characterization, and Applications; Marcel Dekker Inc.: New York and Basel, 2002. (11) (a) Troupis, A.; Hiskia, A.; Papaconstantinou, E. Angew. Chem., Int. Ed. 2002, 41, 1911–1914. (b) Sanyal, A.; Mandal, S.; Sastry, M. Adv. Funct. Mater. 2005, 15, 273–280. (12) (a) Zhang, G. J.; Keita, B.; Dolbecq, A.; Mialance, P.; Secheresse, F.; Miserque, F.; Nadjo, L. Chem. Mater. 2007, 19, 5821–5823. (13) (a) Mandal, S.; Selvakannan, P. R.; Pasricha, R.; Sastry, M. J. Am. Chem. Soc. 2003, 125, 8440–8441. (b) Yang, L. B.; Shen, Y. H.; Xie, A. J.; Liang, J. J.; Zhu, J. M.; Chen, L. Eur. J. Inorg. Chem. 2007, 1128–1134. (14) (a) Maayan, G.; Neumann, R. Chem. Commun. 2005, 4595–4597. (b) Hetterley, R. D.; Kozhevnikova, E. F.; Kozhevnikov, I. V. Chem. Commun. 2006, 782–784. (c) De bruyn, M.; Neumann, R. Adv. Synth. Catal. 2007, 349, 1624–1628. (d) Boujday, S.; Blanchard, J.; Villanneau, R.; Krafft, J.-M.; Geantet, C.; Louis, C.; Breysse, M.; Proust, A. ChemPhysChem 2007, 8, 2636–2642.

Published on Web 11/18/2009

DOI: 10.1021/la903501h

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Scheme 1. Schematic Drawings of Overall Procedures for the Preparation of SEP-1-Based SiO2 Hybrid Spheres, Photochromic Process, and Synthetic Route of MNPs in the Hybrid Sphere Matrix

Figure 1. (a) IR spectra of POM-1, SEP-1 and HUDAH, and (b) amplified IR spectra of SEP-1 (blue), SEP-1/SiO2-2 (black), and pure SiO2 spheres (red). water (18.2 MΩ) from a Milli-Q Water System was used throughout the experiments.

To increase the surface area and stability of the POMs, especially when they are to be used as catalysts, one of the best choices is to incorporate them into silica nanospheres. However, the known methods through directly mixing POMs with the condensation systems or sucking POMs into silica spheres are difficult to avoid the aggregation, leakage and instability from the matrices.15 To overcome these disadvantages, in this paper, we reported a new approach to fabricate POM-doped SiO2 hybrid spheres through supramolecular modification and St€ober method, and to apply the hybrid objects as the microreactors performing the in situ preparation of MNPs, as schematically presented in Scheme 1. Through the encapsulation of [EuP5W30O110]12- (POM-1) with hydroxyl terminated undecyldimethylammonium hydrobromide, we prepared surfactant encapsulated POM-1 (SEP-1), an organic-inorganic supramolecular complex. By co-condensation with the alkylorthosilicate, the SEP-1 complexes were covalently doped into SiO2 spheres. The hybrid silica spheres display obvious advantages: the complexes are well dispersed in the silica spheres uniformly and well fixed in inner spheres chemically; the POMs are located at the microenvironment formed by the hydrophobic surfactants; the structure and stability of POMs are well retained; and the POMs in the hybrid spheres can communicate with outside stimulus, such as photoreduction, which can be employed for the in situ preparation of MNPs. More significantly, the hybrid nanospheres integrated multicomponents, serving as the catalyst carrier, should be potentially useful for the oxidation and hydrolysis of some organic molecules through the synergetic catalysis of POMs and MNPs as well as the hydrophobic microenvironment.14,16

Experimental Section Materials. Dimethylamine, silver nitrate, chloroauric acid, chloroplatinic acid, tetraethyl orthosilicate (TEOS), and ammonia were purchased from Beijing Chemical Reagents Company. 11-Bromoundecanol with a purity of 97% was the product of Sigma-Aldrich. Except that the TEOS was redistilled under vacuum just before use, all the chemicals were used without any further purification. Ethanol is in excellent purity, and ultra pure (15) (a) Green, M.; Harries, J.; Wakefield, G.; Taylor, R. J. Am. Chem. Soc. 2005, 127, 12812–12813. (b) Kishore Kumar, G. D.; Baskaran, S. J. Org. Chem. 2005, 70, 4520–4523. (c) Zhang, F.-M.; Wang, J.; Yuan, C.-S.; Ren, X.-Q. Catal. Lett. 2005, 102, 171–174. (d) Said, A. A.; Abd El-Wahab, M. M. M.; Alian, A. M. J. Chem. Technol. Biotechnol. 2007, 82, 513–523. (e) Haber, J.; Pamin, K.; Matachowski, L.; Mucha, D. Appl. Catal. A- Gen. 2003, 256, 141–152. (16) (a) Inumaru, K.; Ishihara, T.; Kamiya, Y.; Okuhara, T.; Yamanaka, S. Angew. Chem., Int. Ed. 2007, 46, 7625–76328. (b) Jiang, C. J.; Lesbani, A.; Kawamoto, R.; Uchida, S.; Mizuno, N. J. Am. Chem. Soc. 2006, 128, 14240–14241. (c) Mandal, S.; Das, A.; Srivastava, R.; Sastry, M. Langmuir 2005, 21, 2408–2413.

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Preparation of SEP-1 and SEP-1 Doped Hybrid Silica Spheres. POM-1 with potassium counterions was synthesized

according to a published procedure.17 The synthetic procedure of the surfactant 11-hydroxylundecyldimethylamine hydrobromide (HUDAH 3 Br) was adopted as reported.5a Simply, 1.89 g (42 mmol) of (CH3)2NH and 1.5 g (6 mmol) of Br(CH2)11OH with an initial molar ratio of 7:1 were dissolved in 80 mL of ethanol. The reaction mixture was refluxed with stirring for 24 h, and the solvent and excess dimethylamine were removed under the reduced pressure. A white crude product was obtained and purified over silica gel chromatography with 20:1 (v/v) of chloroform and methanol as eluent. Pure HUDAH 3 Br was obtained as a white powder (yield: 52%) and was characterized by 1H NMR (500 MHz, DMSO-d6, δ): 1.25 (m, 14 H), 1.40 (m, 2 H), 1.59 (m, 2 H), 2.75 (s, 6 H), 3.01 (t, 2 H), 3.37 (m, 2 H), 4.32 (t, 1 H), 9.29 (s, 1 H). ESI-MS: 216.2 and 13C NMR for HUDAH 3 Br match the expected chemical formula (Figures S1 and S2). The SEP-1 complex was prepared according to the reported method.5a,6 Typically, 5 mL of POM-1 aqueous solution (0.008 mmol/mL) was added dropwise into a 20 mL aqueous solution of HUDAH 3 Br (0.024 mmol/mL) with stirring, and the initial molar ratio of HUDAH 3 Br to POM-1 was controlled at 12:1. After stirring for 1.5 h, the yielded precipitate was filtered and washed with deionized water several times, then dried in vacuum until the weight remained constant, giving the complex SEP-1. The IR spectrum exhibits the characteristic absorptions of HUDAH at v = vas (O-H) 3403 cm-1, vas (CH2) 2924 cm-1, vs (CH2) 2851 cm-1, v (NH) 2746 cm-1, δ (CH2) 1468 cm-1, vs (C-O) 1054 cm-1, and POM-1 at v = v (P-Oa) 1158, 1062 cm-1, vas (W-Od) 980 cm-1, vas (W-Ob-W) 914 cm-1, vas (W-Oc-W) 785 cm-1 (Figure 1a). The elemental analysis (EA) result (Anal. Calcd: C, 17.52; H, 3.41; N, 1.57. Found: C, 18.00; H, 3.26; N, 1.60) suggests a consistent formula of (HUDAH)11HEuP5W30O110 3 3H2O. As an example, the procedure for the preparation of SEP-1doped silica spheres was performed as follows: 16.0 mg of SEP-1 was dissolved in a mixed solvent of 50 mL of ethanol and 5 mL of water. Then, 1.0 mL of TEOS was injected into this mixed solution with mechanical agitation, and 1.0 mL of ammonia (28 wt %) was added in 1 min subsequently. After 5 h of reaction, additional 0.5 mL of TEOS was added into the reaction system. The stirring was stopped after another 5 h, and the mixed solution was centrifuged. The precipitate was redispersed in water under sonication and suffered the centrifugation and washing several times to remove the residual ethanol and ammonia, as well as the unreacted residues. The yielded blue-white precipitate was dispersed in 5 mL of water for the measurements. To examine the residues left in the reaction mixture, all the supernatants were collected, concentrated to ca. 2 mL for UV-vis spectral and ICP analysis. In Situ Reduction of Metal Ions. The SEP-1 incorporated silica spheres in aqueous solution were irradiated with a 300-W (17) Creaser, I.; Heckel, M. C.; Neitz, R. J.; Pope, M. T. Inorg. Chem. 1993, 32, 1573–1578.

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high-pressure mercury lamp at a 15-cm distance at room temperature kept by a small fan. In 15 min, the solution was poured into deaerated solutions of metal ions (10 μmol/mL for AgNO3, 13 μmol/mL for HAuCl4, 6.5 μmol/mL for H2PtCl6, respectively). The mixed solutions were sealed and placed in dark, and centrifuged at a speed of 12000 rpm when the color of solutions changed completely. The color of the precipitate was purple black from the AgNO3 solution and mauve from HAuCl4 solution and became whiter than the original from H2PtCl6 solution. The precipitate from AgNO3 solution was washed with dilute HNO3 solution (pH = 1.9) and then water to exclude the residual silver ions. The precipitates from HAuCl4 and H2PtCl6 solutions were just washed three times with deionized water. All the precipitates were finally redispersed in water through sonication. Measurements. 1H NMR and 13C NMR spectra were recorded on a Bruker UltraShieldTM spectrometer. The EA result was obtained on a Flash EA1112 from ThermoQuest Italia SPA The thermalgravimetric analysis (TGA) was carried out on a Perkin-Elmer 7 series thermal analysis system. The ESI-MS experiments were performed on an IonSpec HiRes Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR-MS) (Lake Forest, CA), equipped with a Micromass Z-spray electrospray source and a 7-T shielded superconducting magnet. Fourier transform infrared (FT-IR) spectra were performed on a Bruker IFS66 V FT-IR spectrometer equipped with a DGTS detector (32 scans) and recorded with a resolution of 4 cm-1. The fluorescence spectrophotometer used in the measurements was HITACHI F-4500. The inductively coupled plasma-optical emission spectrometer (ICP-OES) used to determine the concentration of a wide range of elements in solutions was Thermo Scientific iCAP ICP-OES 6000 Series. X-ray photoelectron spectroscopic (XPS) measurements were performed on an ES-CALAB Mark (VG Company, U.K.) photoelectron spectrometer using a monochromatic Al KR X-ray source. Transmission electron microscopic (TEM) measurements were finished on a JEOL-2010 electron microscope operating at 200 KV. Scanning electron microscopic (SEM) images were acquired on a JEOL FESE-M 6700F electron microscope. Raman spectra were recorded on a laser confocal Raman microscope (Renishaw 1000 model) equipped with a CCD detector and a holographic notch filter. Radiations of 488, 514.5, and 785 nm from an air-cooled argon ion laser (Spectra-Physics model 163-C4260) were used as the excitation light sources. Laser power at the sample position was no more than a few milliwatts. The microscope attachment was a Leica DMLM system, and a 50  objective was used to focus the laser beam onto a spot of approximate 1 μm in diameter. The ζ potential was recorded on a Nano ZS (Red badge) ZEN3600 from Malvern.

Results and Discussion Structure Characterization of SEP-1. Because of the electrostatic interaction serving as the driving force, the cationic surfactants HUDAH can easily cover the anionic surface of POM-1s, forming SEP-1 complexes, as presented in Scheme 1.5a,6 Because of the hydrophobicity of HUDAH and charge neutralization, SEP-1s become water-insoluble and precipitate from pure water. Thus, the SEP-1 possesses an inorganic core of POM-1 and an organic shell of HUDAH with a hydroxyl group at the terminal toward outside. The EA result proved the chemical formula of SEP-1, (HUDAH)11HEuP5W30O110 3 3H2O, as predicted and presented in literature identically.5a TGA shows 0.22 wt % loss before 110 °C, which is derived from losing crystal water, matching well the value of 0.18 wt % calculated from the structural formula. The exact residue of 74.26% in total at 900 °C (Figure S3) is also in agreement with the calculated value of 72.74%, based on the assumption that the organic component has decomposed completely and all the inorganic residuals are SiO2, Eu2O3, and WO3. Meanwhile, potassium counterions of POM-1 were not found in the XPS spectrum of the complex, indicating Langmuir 2010, 26(6), 4437–4442

Figure 2. UV-vis spectra of (a) SEP-1 in the mixed solvent of ethanol and water (red), pure POM-1 in water (blue), pure SiO2 spheres in water (black) and SEP-1-doped SiO2 spheres in water (purple), and (b) POM-doped SiO2 spheres before (black) and after (red) UV irradiation.

the full replacement of potassium ions. These data confirm that the 11 negative charges in one POM-1 cluster are neutralized with corresponding number of HUDAH cations. In comparing with free pure state in 1H NMR spectrum, the chemical shifts of N-CH3 and N-CH2 of HUDAH in SEP-1 move to low field quite much and become broadened, indicating that it is the quaternary ammonium head interacts with POM-1 electrostatically. In addition, as shown in the IR spectra (Figure 1a), the bands of W-Oc-W vibrations shift from 777 and 740 cm-1 in free POM-1 state to 785 and 737 cm-1 in SEP-1,2e,5a and the vibration of v (NHþ) changes from a sharp peak at 2696 cm-1 in free HUDAH 3 Br to a weak broad band around 2740 cm-1 in SEP-1, indicating the tight combination of inorganic cluster with organic component due to the electrostatic interaction. Raman spectra (Figure S4a) further confirm the complexation of POM-1 and HUDAH in SEP-1. The vibrations at 968 and 659 cm-1 assigned to W-O and W-O-W modes of POM-1 appear in the spectrum of SEP-1, indicating the surface modification and the structural stability of POM-1. The vibrations at 2970, 2901, 2852, and 1443 cm-1, which are attributed to the stretching and distortion modes of HUDAH alkyl chains, respectively, are also found at almost the same frequencies in the spectrum of SEP-1. The intensity of the vibration appeared at 2696 cm-1 decreases greatly, indicative of the electrostatically binding of ammonium head of HUDAH with POM-1 in the complex. It should be noted that the strong bands at ca. 2500 and 3200 cm-1 are not derived from Raman vibrations but from the fluorescence of europium substituted POM-1s as these bands change with the alternation of excitation wavelength from 514.5 to 488 nm (Figure S4b). From the UV-vis spectra (Figure 2a), it can be seen that the band at 274 nm for SEP-1 in the mixed solvent of ethanol and water shows the characteristic O f W ligand to metal charge transfer (O f W LMCT) absorption,3,4 suggesting the well kept structure of POM-1 in SEP-1. The small band shift in comparison to that of pure POM-1 reveals the change of its surface microenvironment. Thus, the hybrid we prepared should be the expected complex. As the purpose enwrapping the POMs with the as-prepared surfactant molecules is to incorporate them into silica matrix in a well dispersed state, the modification of the hydrophobic terminal of alkyl-chains with hydroxyl groups exhibits at least three main merits: making SEP-1s dissolve in the mixed solvent of ethanol and water easily; attaching and cross-linking SEP-1s to the silica backbones chemically; fabricating a hydrophobic microenvironment and enhancing the stability of POM-1 as a nanoreactor. In this case, the chemical structure of SEP-1 confirms that the same amount of free hydroxyl groups as that of surfactant molecules toward outside of the complex should be benefit for the reaction with TEOS, realizing the functionalized silica spheres. DOI: 10.1021/la903501h

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Figure 3. (a) SEM micrograph, (b) TEM micrograph of SEP-1/ SiO2-1 spheres, and magnified images of single spheres (c) from part b and (d) from pure SiO2 spheres.

Fabrication and Structure of SEP-1-Doped SiO2 Spheres. The SEP-1-doped silica spheres were prepared through the hydrolysis and gelation of TEOS in the SEP-1 dispersed solution. The SEM image (Figure 3a) shows that the objects we obtained are spherical structure with a size around 150 nm. To ascertain the percentage of SEP-1 in silica spheres, the concentrated supernatant of the sol-gel reaction solution collected after the centrifugation was first characterized through UV-vis spectrum and ICP measurement. As it was expected that all silica spheres were separated out through centrifugation, the band at 274 nm should source from the residual SEP-1. By employing the absorption coefficient of the mixture solution fitting the absorption of pure SEP-1 under the known concentration and Lambert-Beer law, we estimated that more than 99.2% of SEP-1s incorporated into silica spheres. ICP data indicate the contents of 18.7 μg/mL for W and 50.0 μg/mL for Si, which means 65.0 μg/ mL (ca. 0.8 wt %) of SEP-1, and 214.0 μg/mL of SiO2 or 742 μg/ mL of TEOS (ca. 0.16 wt %) left in the residue solution. According to this route and examination, we prepared two SEP1-doped SiO2 samples with SEP-1 content of 3.9% and 13.6%, which are named as SEP-1/SiO2-1 and SEP-1/SiO2-2, respectively. The SEP-1-doped SiO2 spheres from the precipitate were characterized in detail. In the IR spectrum, accompanied by the band around 943 cm-1 derived from the absorption of SiO2, three weak bands appear at 918, 936, and 955 cm-1 in association with the absorptions of POM-1, revealing the existence of SEP-1 in the silica nanospheres and the structural stability during the doping process (see Figure 1b). Because of the low incorporation content of SEP-1 and binding percentage with silica matrix, it is difficult to discern the C-O-Si vibration band convincingly. The UV-vis spectrum (Figure 2a) of SEP-1/SiO2-2 spheres shows a shoulder band around 274 nm, attributing to the absorption of SEP-1, in addition to the absorption of SiO2 spheres. Considering the possible measurement errors, the molar ratio of Eu:P:W (1:4.1:32.1) found in SiO2 spheres from ICP approximately corresponds to the composition of Eu:P:W (1:5:30) in POM-1. In addition, from the similar values of P (2p) energy level of pure POM-1 and SEP-1 in silica sphere, we also know that the cluster structure of the POM-1 is maintained (Figure S5). The XPS 4440 DOI: 10.1021/la903501h

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spectrum of the precipitate without etching only shows elements of Si (2p, 103.20 eV) and O (1s, 532.50 eV) with a little residual C (1s, 284.65 eV) (Figure 4, parts a and b). The residual C is probably derived from the trace incompletely hydrolyzed TEOS because the peak position and intensity does not change under Ar ion etching. This case can also be observed in the XPS data of pure SiO2 spheres. In comparison to this, the XPS result of the hybrid spheres encountered etching in 1 wt % HF solution for 1 min exhibit the elements of SEP-1, supporting the successful introduction of SEP-1 complex into silica spheres. The evident existence of elements C (1s: 284.55, 286.12, 287.76, and 288.85 eV), N (2p: 401.81 eV), P (2p: 133.80 eV), W (4f 7/2: 35.58 eV) and O (1s: 530.9 eV) (Figure 4a) further confirm the incorporation of SEP-1 in the spheres. The binding energy peaks ascribed to C element can be assigned to four chemical environments, C-C, C-N, C-O-H, and C-O-Si bonds in turn (Figure 4c).5 The existence of C-O-Si bond confirms the condensation of the Si-OH group hydrolyzed from TEOS and the terminal hydroxyl group from SEP-1. The relative peak area ratio of C-N and C-O estimated from XPS spectrum is ca. 3.1:1, very close to the corresponding C atom ratio (3:1), calculated from the molecular structure of SEP-1. This result strongly supports the assignment for the condensation between Si-OH and C-OH. In addition, we found that ca. 55.3% of -OH groups on SEP-1 were covalently linked to the silica matrix by comparing the relative peak areas of C atoms belonging to C-O-H and C-O-Si. That means, averagely, more than six -OH groups in one complex are covalently linked to silica backbones. In consistence with the XPS results, the zeta potential of the hybrid spheres appears at -43.2 mV, in agreement with zeta potential of pure SiO2 spheres, -45.2 mV. These results mean that SEP-1 has been introduced into the SiO2 spheres successfully, and almost all of SEP-1 complexes locate at the inner of the spheres. The dispersion of SEP-1 in silica spheres could be verified by TEM (Figure 3b). The amplified TEM image (Figure 3c) confirms that the hybrid spheres exhibit a SEP-1 dispersed core/shell structure. The inside part clearly corresponds to the SEP-1-doped SiO2 domain because inside the sphere the dark dots within the size of 2-4 nm, which are dispersed uniformly, can be definitely assigned to SEP-1 complexes composed of heavy metal ions. In contrast to the SEP-1-doped SiO2, the pure SiO2 spheres display a smooth and homogeneous inner part (Figure 3d). As the second portion of TEOS was added, the thin darkish shell corresponds to the pure SiO2 and no SEP-1s was observed clearly. We have proved that SEPs normally form aggregations in organic solutions, even in water.18 Obviously, the good dispersion of SEP-1s in the silica matrix should be derived from the immobilization of SEP-1 by covalently binding to silica matrix, which separates the complexes from the aggregations effectively. In Situ Reduction of Metal Ions in SEP-1-Doped SiO2 Spheres. Similar to the reports occurred in solution systems and the color change of the complex embedded in silica bulk, the SEP1-doped SiO2 spheres displayed evident photoreduction property under the radiation, as well.4,5,11,13 After 15 min of irradiation with UV light, the IR spectrum maintains almost the same as that before photoirradiation. The characteristic O f W LMCT band of POM-1 around 274 nm also can be clearly observed in the UV-vis spectrum after the irradiation, suggesting that the frame structure of POM-1 is well retained.5,6 However, in contrast to the strong absorption of LMCT band, a new broad absorption band (18) (a) Li, H. L.; Sun, H.; Qi, W.; Xu, M.; Wu, L. X. Angew. Chem., Int. Ed. 2007, 46, 1300–1303. (b) Yan, Y.; Li, B.; Li, W.; Li, H.; Wu, L. X. Soft Mater. 2009, DOI: 10.1039/B912011D.

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Figure 4. XPS spectra of (a) SEP-1/SiO2-1 spheres before (black) and after (red) etching by 1 wt % of HF for 1 min, and the amplifications of the C element for the sample surface (b) before and (c) after etching by HF solution (black) as well as the corresponding simulation curves (red and blue).

Figure 5. XPS spectra of Ag (3d level) generated in SEP-1-doped SiO2 spheres on: (a) surface without washing; (b) surface washed several times with water; (c) surface washed with dilute HNO3 (pH = 1.9); (d) inner part of spheres etched by 1 wt % HF for 1 min.

appears in the visible region around 730 nm after the irradiation, and can be definitely assigned to the W5þ f W6þ intervalence charge transfer (IVCT).4,5,11,13 This characteristic absorption band of heteroblue indicates the formation of reduced POM-1 (Figure 2b).4,5,11,13 As reported, POM-1s at the reduced state can be used as the reductant for the preparation of MNPs in aqueous solution.13,14 Considering the fact that the pH of isoelectric point (iep) is ca. 2.0 for SiO2 colloids, to avoid the metal ions being adsorbed onto the surface of SiO2 directly and be sure of them going through the porous channel in the spheres and reaching the reaction locations, the pH of gold and platinum ion solutions was adjusted to around 1.9 through the formation of HAuCl4 and H2PtCl6. When the photoirradiated hybrid spheres was poured into deoxygenized solution of metal ions, the mixed solution changed its color from colorless to purple for AgNO3, from golden to golden red for HAuCl4, while little color change occurs for H2PtCl6 system. The color changes distinctly demonstrate the reduction of metal ions to MNPs through the reaction with POM1s, and the increased plasma resonance absorption supports the formation of MNPs (Figure S6). After these precipitates have been etched by 1.0 wt % of HF to peel off the surface layer, the XPS data of the hybrid sphere cores reveal the existence of Ag0 (3d5/2 367.49 eV) (Figure 5), Au0 (4f7/2 83.50 eV), and Pt0 (4f7/2 71.98 eV) (Figure S7), though there are some changes in contrast to the reported ones.19 The TEM images further illustrate that the (19) (a) Fuggle, J. C.; Kallne, E.; Watson, L. M.; Fabian, D. J. Phys. Rev. B 1977, 16, 750–761. (b) He, S. T.; Yao, J. N.; Xie, S. S.; Jiang, P.; Pang, S. J.; Gao, H. J. Chem. Phys. Lett. 2001, 343, 28–32. (c) Bird, R. J.; Swift, P. J. Electron Spectrosc. Relat. Phenom. 1980, 21, 227–240. (d) Drawdy, J. E.; Hoflund, G. B.; Gardner, S. D.; Yngvadottir, E.; Schryer, D. R. Surf. Interface Anal. 1990, 16, 369–374.

Langmuir 2010, 26(6), 4437–4442

Figure 6. TEM images of (a) Ag nanoparticles grown in SEP-1/ SiO2-1 and (b) SEP-1/SiO2-2, (c) Au nanoparticles grown in SEP1/SiO2-1 and (d) SEP-1/SiO2-2, and (e) Pt nanoparticles grown in SEP-1/SiO2-1 and (f) SEP-1/SiO2-2.

MNPs disperse in the SiO2 matrix with low SEP-1 content uniformly, and no obvious dark dots with larger size are found (Figure 6a), indicating the enrichment of MNPs in SEP-1-doped silica spheres. This is quite different from those results that the silica spheres are grown from one, oligo- or multimetal nanoparticle cores.20 A possible reason that the adsorbed metal atoms in the SEP-1-doped silica spheres do not aggregate into larger nanoparticles may be derived from the confined volume around the POM-1 reaction site in the hybrid spheres. As a comparison, for the SEP-1-doped silica spheres first encountering same soakage in the Ag ion solution and then the UV irradiation, the generated Ag nanoparticles on silica spheres become scanty but larger in size, and spread both inside and outside (Figure S8). Similar results for Au and Pt nanoparticles reduced in lower SEP-1 content silica spheres can also be found in Figure 6, parts c and e, indicative of the common efficiency of the present method. (20) (a) Aslan, K.; Wu, M.; Lakowicz, J. R.; Geddes, C. D. J. Am. Chem. Soc. 2007, 129, 1524–1525. (b) Shi, Y.-L.; Asefa, T. Langmuir 2007, 23, 9455–9462.

DOI: 10.1021/la903501h

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Article

Figure 7. Raman spectra of (a) SEP-1/SiO2-1 with (red) and without (black) Ag nanoparticles and (b) SEP-1/SiO2-2 with (red) and without (black) Ag nanoparticles, excited at 514.5 nm.

Compared with SEP-1/SiO2-1, SEP-1/SiO2-2 displays a stronger capability for the reduction of metal ions. As shown in Figure 6b, obvious aggregation of Ag nanoparticles was found both in inner and on the surface of SEP-1/SiO2-2 spheres with a size of several nanometers. However, the phenomenon was not obvious in the reduction of Au and Pt metal ions through SEP-1/SiO2-2 (Figure 6, parts d and f), although the size of the MNPs seems to increase in some extent because the observed structure becomes rough. As the two kinds of SEP-1/SiO2 spheres are confirmed to possess similar structure, the observed difference should source from the amount of SEP-1s in the silica spheres. The higher content of SEP-1s can supply more reactive sites of POM-1s and the hydrophobic microenvironment derived from alkyl chains of surfactant molecules, leading to the accommodation of more MNPs. On the other hand, it is possible that Ag atoms have a faster diffusion rate than Au atoms in the hydrophobic environment as reported in the literature,21 which could accelerate the growth of particles and the aggregation. XPS analysis for these metal clusters grown in SEP-1/SiO2-2 spheres, which experienced a wash of dilute HNO3 (pH = 1.9), shows the existence of Ag atoms on the surface of SEP-1/SiO2-2 spheres (Figure 5), but shows no Au and Pt atoms on the surface (Figure S7). A possible reason for the difference may be derived from the faster diffusion rate of Ag ions due to its smaller volume and positive charge comparing to the larger and negative charged AuCl4- and PtCl62-, which speed up the reduction of Agþ ions in nanoreactors and induce the particle aggregation. In addition, the rich reactive sites of POM-1s can reduce more Agþ ions and result in access Ag atoms overbrimming from the limit space of the hybrid spheres. It is noted that only one electron is needed for the reduction of each Agþ, while three electrons are needed for each AuCl4-, and four electrons are needed for each PtCl62-. Therefore, for the same SEP-1/SiO2 hybrid nanoreactor, more Ag atoms can be produced than Au and Pt atoms, as verified from the ICP-OES analysis through Ag:W30 = 16.2:1 for Ag/POM/ SiO2-1, Au:W30 = 3.6:1 for Au/POM/SiO2-1 and Pt:W30 = 2.9:1 for Pt/POM/SiO2-1. On the basis of this result, each photoreduced POM-1 can reduce about 16 Ag, 4 Au and 3 Pt ions, respectively. We examined the location of the yielded MNPs in inner silica spheres and their association with the reaction site of SEP-1 through surface enhanced Raman scattering (SERS). For low content SEP-1 doping silica spheres, Raman enhancement is not obvious due to the very low amount of MNPs and their small size, as shown in Figure 7a. For the high content SEP-1-doped silica spheres, the increased amount of MNPs make Raman scattering strength enhance quite much, due to the SERS effect. Because the SERS effect requires a specified plasma resonance frequency for (21) Roos, C.; Schmidt, M.; Ebenhoch, J.; Baumann, F.; Deubzer, B.; Weis, J. Adv. Mater. 1999, 11, 761–766.

4442 DOI: 10.1021/la903501h

Zhao et al.

the fixed laser wavelength, the suitable size of MNPs is needed. As shown in Figure 7b, the vibrations of POM-1 and cationic headgroup of surfactants are greatly enhanced at 818 (νs WOc-W), 959 (ν W-Od) and 1300 cm-1 (δ C-N), indicating that Ag nanoparticles are nearby the interface between POM-1 and HUDAH of SEP-1, as the enhancement normally occurs at a close distance between the metal surface and the molecules to be enhanced based on the SERS mechanism.22 This result is in good consistent with reported results occurred in thin films.6 Of course, from the intensity enhancement of Si-O-Si vibration at 1045 cm-1, we also know that some of generated Ag nanoparticles are attached on the silica matrix. Au nanoparticles through in situ reduction in high SEP-1-doped silica spheres also display similar SERS effect based on the enhanced intensities of POM-1, further supporting the assignment concerning the location of MNPs in silica spheres (Figure S9).

Conclusions In conclusion, we have successfully prepared POM-1 incorporated silica spheres through the supramolecular encapsulation of POM-1 with single chain quaternary ammonium molecules bearing a hydroxyl terminal group. Because of the chemical bonding to the silica matrix by co-hydrolyzation and condensation with TEOS, the surfactant enwrapped POM can be well dispersed in the formed silica spheres uniformly and stably. The approach exhibits obvious merits: the content of SEPs can be well controlled and almost all POMs are incorporated inside of the silica spheres fixedly and located at an embedded hydrophobic microenvironment. Most importantly, the properties of POMs are well maintained in the silica spheres. The expected photoreduction of POMs through UVirradiation can occur inside the silica spheres. By employing the reduction property of the heteropolyblue, we can prepare MNPs inside of the hybrid spheres through soaking in metal ion solutions. The formed MNPs are proved in very small size and well dispersed inside the spheres close to the SEPs, which provide an effective path for the potential application, as a new hybrid nanomaterial for the synergetic catalysis of POM and MNPs. We believe that the present method also exhibits a general path for the preparation of silica spheres incorporated other inorganic clusters or particles. Acknowledgment. This work was financially supported by the National Basic Research Program of China (2007CB808003), National Natural Science Foundation of China (20703019, 20731160002, 20973082, 50973042), 111 Project (B06009) for supporting the visit of Prof. I. Kim at Pusan National University and fruitful discussion with him, and open project of State Key Laboratory of Polymer Physics and Chemistry of CAS. Supporting Information Available: Figures showing 13C NMR and ESI-MS spectra of HUDAH 3 Br; TGA curve of SEP-1; Raman spectra of POM-1, SEP-1, HUDAH 3 Br, SEP1/SiO2-2, and Au/SEP-1/SiO2-2; XPS spectra of P (2p) energy level of pure POM-1 and SEP-1/SiO2-1, Au (4f) energy level on the surface and in inner of Au/SEP-1/SiO2-2 spheres, Pt (4f) energy level on the surface and in inner of Pt/SEP-1/SiO2-2 spheres; UV-vis spectra of SEP-1/SiO2-1, SEP-1/SiO2-2, Ag/ SEP-1/SiO2-1, Au/SEP-1/SiO2-1, and Ag/SEP-1/SiO2-2 solutions; and TEM image of SEP-1/SiO2-1, which undergoes primarily soaking in AgNO3 solution and subsequent irradiation of UV light. This material is available free of charge via the Internet at http://pubs.acs.org. (22) (a) Chang, R. K.; Furtak, T. E. Surface Enhanced Raman Scattering; Plenum Press: New York, 1982. (b) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783. (c) Morton, S. M.; Jensen, L. J. Am. Chem. Soc. 2009, 131, 4090–4098.

Langmuir 2010, 26(6), 4437–4442