Polyoxometalates Composite Nanotubes with

May 22, 2008 - A nanotubular catalytic reactor composed of an parallel array of Pt-loaded polyoxometalates/polyelectrolyte nanotubes is presented. Pt/...
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J. Phys. Chem. C 2008, 112, 8875–8880

8875

Parallel Array of Pt/Polyoxometalates Composite Nanotubes with Stepwise Inside Diameter Control and Its Application in Catalysis Zhuo Ma, Qiang Liu, Zhi-Min Cui, Shao-Wei Bian, and Wei-Guo Song* Beijing National Laboratory of Molecular Sciences (BNLMS), Institute of Chemistry, the Chinese Academy of Sciences Beijing 100190, People’s Republic of China ReceiVed: January 24, 2008; ReVised Manuscript ReceiVed: March 12, 2008

A nanotubular catalytic reactor composed of an parallel array of Pt-loaded polyoxometalates/polyelectrolyte nanotubes is presented. Pt/POM composite nanotubes with precisely controlled wall thickness as well as tube inside diameters are prepared by a layer by layer method. Negative-charged polyoxometalates (POMs) and positive-charged polyelectrolytes were alternatively coated onto the inside walls of the porous polycarbonate template. The wall thicknesses as well as the inside diameters of the tubes are precisely controlled by the number of coating bilayers, with a stepwise tailoring at 2.2 nm per step and up to at least 40 steps. Scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, UV, fluorescence spectroscopy, and solid-state NMR characterization of the POM/polyelectrolyte tubes were carried out. The Pt/POM nanotubular reactor with smaller inside diameters shows higher catalytic activities for cyclohexene hydrogenation, indicating promising potential in designing reactors with specific sizes for specific reactions. 1. Introduction Nanoheterogeneous catalysis is emerging as a promising frontier in catalysis field.1,2 Milder reaction conditions, superior selectivities, and even unprecedented catalytic activities (for example Nanogold3,4) are found from nanocatalysts. In nanocatalysis, the sizes of the catalysts as well as the reactor are in the nanorange, which is in the same order of the sizes of participating molecules. Understanding of catalysis at the molecular level within a nanoreactor will enable the researchers to design better catalysis system. One example of nanocatalytic reactor is the catalytic reaction inside a zeolite nanocage. If we envision a zeolite cage as an isolated nanoreactor, within the cage the active site, reactant molecules, intermediate species, and product molecules are all individual molecules. However, we are not able to assess the performance of one individual “nanoreactor” in a zeolite, because in a real catalytic test unit many zeolite particles are used, so we can not track the reaction on any specific zeolite particles, let along a specific nanoreactor within a zeolite particle. Tubular materials composed of parallel array of organic/ inorganic composite tubes have shown promising potentials in applications such as storage and controlled release of chemicals,5 gas sensors,6 conductors,7 and catalysis.8–10 For tubular systems, precisely controlled wall thickness as well as the tube diameter are essential for better performance in applications.11 For example, transport selectivity observed in bioseparations is influenced by the nanotube inside diameter.12,13 In terms of shape selectivity, tubular micronanostructured materials with precisely controlled wall thickness and internal diameters are very useful. In catalysis, the ability to precisely tailor the inside diameter of a nanotubular system will enable us to choose “correct” diameters for different catalysis reactions. The so-called confinement effect, which is mostly limited to zeolite type materials and small molecules, can be exploited for larger molecules using tubular structures.14 For example, in bioinspired catalysis, a * To whom correspondence should be addressed. Phone and fax: (86)1062557908. E-mail: [email protected].

tabular reactor loaded with enzyme with an inside diameter that fits the sizes of the reactant molecules may promote the reaction.15–17 Polyoxometalates (POMs) are one group of inorganic metal oxide clusters with special chemical, structural, and electronic properties and have attracted great interest for their applications in several fields, such as catalysis, medicine release, optical devices, electronics, and magnetic materials.18–21 POMs as building blocks for functional composite materials have shown promising prospects in catalysis, including redox reactions and acid/base catalyzed reactions.22–24 Most POMs are anions (balanced by cations such as Na+) and are soluble in water. They form various framework structures, which enable researchers to develop the “smart” catalysis system that can be regenerated by itself.25,26 POMs-based nanotubular materials will have functional properties of POM as well as advantages of tubular systems. However, synthesis of high quality POM-based nanotubular structures is seldom reported.27,28 In this report, a straightforward and highly versatile template assisted layer-by-layer (LBL) method is developed to produce POM/polyelectrolyte nanotubes. The LBL method is a very effective method to coat the substrates with uniform layer and controllable thickness. The method is depicted in Scheme 1. Three POMs were used as a negative-charged layer to be combined with a positive-charged polyelectrolyte layer. Four high quality tubular materials were prepared with precise wall thickness control under mild conditions. The diameter of the tubes is readily controlled by the number of opposite-charged layers, and the tube diameter can be controlled from 250 to 40 nm stepwise at 2.2 nm per step. The POMs tubes are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), UV, fluorescence spectroscopy, and solid-state NMR techniques. The POM/polyelectrolyte hybrid tubes are then used as the support for well-dispersed Pt nanoparticles. The Pt/POM nanotubular reactors with different inside diameters

10.1021/jp800703w CCC: $40.75  2008 American Chemical Society Published on Web 05/22/2008

8876 J. Phys. Chem. C, Vol. 112, No. 24, 2008 SCHEME 1: Schematic Diagram Illustrating the Fabrication Process of POM/Polyelectrolyte Nanotubes through LBL Coating and Removal of the PC Template

show different activities for cyclohexene hydrogenation, indicating promising potential in designing specific reactor for specific reactions. 2. Experimental Section 2.1. Chemicals and Materials. Polyelectrolytes, include poly(ethyleneimine) (PEI; MW 50 000), poly(stryenesulfonate) (PSS; MW 70 000), poly(allylamine hydrochloride) (PAH; MW 70 000), and poly(diallyldimethylammonium chloride) (PDDA; MW 200 000-350 000) were purchased from Aldrich and used as received. The polyelectrolyte powders were then dissolved in water. The following solutions were used to prepare POM/ polyelectrolyte nanotubes: aqueous PEI solution (10-2 M), aqueous PSS solution (10-2 M with 1 M NaCl), aqueous PAH solution (10-2 M with 1 M NaCl), aqueous PDDA solution (10-2 M), and aqueous POM (1 × 10-3 M, NaAc-HAc buffer solution pH 4.0-4.2) solution. POMs with the compositions of R-K6P2W18O62 (P2W18), Na12P2W15O56 · 18H2O (P2W15), and Na9EuW10O36 · 32H2O (EuW10) were prepared according to the references (see Supporting Information).29,30 Polycarbonate (PC) membranes with an average pore diameter of 200 nm and average pore length of about 8 µm were obtained from Whatman Corp (see Supporting Information for a SEM image of the pores). All aqueous solutions were prepared using purified water from a Millipore Milli-Q system. 2.2. POM/Polyelectrolyte Nanotubes Synthesis and Characterization. To prepare POM/polyelectrolyte nanotubes, PC substrate was first pretreated with PEI, then PSS, and finally PAH solution in sequence. In each step, the substrate was immersed into PEI, PSS, or PAH solutions for 20 min. After pretreatment, the PC membrane was rinsed with deionized water and then dried in nitrogen atmosphere. Then the PC template was immersed into the POM solution followed by polyelectrolyte solutions for 20 min each. After each coating step, the substrate was cleaned by deionized water and dried in nitrogen. This process was repeated until the desired number (n) of bilayers of POM/polyelectrolyte was obtained. The whole process was carried out at room temperature. The PC template can be dissolved by dichloromethane to obtain pure POM/polyelectrolyte composite tubes. In some cases, the PC template was retained as support for the tubes. The multilayer tubular systems thus obtained were labeled as (POM/polyelectrolyte)n, where n denotes the number of the bilayers. In the present study, (P2W15/PAH)n, (P2W18/PAH)n, (EuW10/PAH)n, and (P2W15/PDDA)n were prepared through this method.

Ma et al. SCHEME 2: Experimental Setup for Catalytic Testing

The POM/polyelectrolyte samples were characterized by SEM (Hitachi S-4300). TEM and EDX were performed on a JEOL J1020 instrument with 200 kV accelerating voltage. UV-vis spectra were recorded on a JASCO V-530 spectrometer. Luminescence spectra were measured on an F-4500 fluorescence spectrophotometer using a xenon lamp (260 nm) as the excitation source. Solid-state NMR experiments were performed with a 7.5 mm double-resonance probe on a triple-resonance 5 mm probe on a Varian Infinityplus-400 system. 2.3. Catalytic Testing. In the catalyst experiment, the PC template was retained as support for the tubes. In a typical experiment, three pieces of PC membrane with (P2W15/PAH)10, (P2W15/PAH)20, and (P2W15/PAH)30 were respectively immersed in PPh3-modified Pt/toluene solution that was prepared according to a literature method31 for 10 h to obtain the Pt/(P2W15/PAH)n catalyst. The color of Pt colloidal solutions changed from dark to almost colorless, and the initially white PC membranes became dark brown. X-ray photoelectron spectroscopy (XPS, ESCALab220I-XL with Al/Mg cathode radiation) and TEM (Philips Tecnai F30) were used to characterize the nature of the platinum nanoparticles. A flow reactor system was built for the catalytic testing as illustrated in Scheme 2. The experiment was carried out at room temperature under 10 SCCM (standard cubic centimeter per minute) flow rate of H2 that bubbles through cyclohexene. The cyclohexene hydrogenation reaction was monitored using an Agilent 5890 GC equipped with a FID detector. 3. Results and Discussion The concept of LBL method in this study, as illustrated in Scheme 1, is to alternatively coat the inside walls of the pores on the template with positively charged polyelectrolyte and negatively charged POM. Electrostatic attraction between opposite-charged layers enables strong bindings between each layer. PC has a strong affinity toward PEI, so pretreatment with PEI and then PSS generates a layer of PEI and PSS bilayer as the starting bilayer with negatively charged PSS layer available to start the coating of PAH/POM bilayers. To investigate the properties of the resulted POM/polyelectrolyte tubes, the PC template can be dissolved by a organic solvent dichloromethane. For catalysis applications, the template is retained as it helps to preserve the space isolation and orientation of the tubes. Figure 1 illustrates SEM images of typical architectures of the as-prepared four (POM/ polyelectrolyte)15 samples after the PC template is dissolved. The tubes are stuck together to a bunch with clear-cut ends on both sides. The length between both ends is about 7.5 µm, which is close to the thickness of the PC template. It also can be seen that many tubes are not parallel to each other, and some of them actually intersect with other tubes and form branches. Such branched structures are most likely due to the nature PC templates. The pores on the PC template are produced by laser etching of PC membrane. Apparently the pores in the PC template are not all parallel; some of them are intersected. Such intersected tubes may help to form a framework structure, which enhances the mechanistic strength of the material.

Parallel Array of Pt/Polyoxometalates Composite Nanotubes

Figure 1. SEM images of (POM/polyelectrolyte)n nanotubes: panels a-d are (P2W15/PAH)15, (P2W18/PAH)15, (EuW10/PAH)15, and (P2W15/ PDDA)15, respectively.

Figure 2. The TEM images of the (POM/polyelectrolyte)15 tubes (after ultrasonicating for 20 min): panels a-d are (P2W15/PAH)15, (P2W18/ PAH)15, (EuW10/PAH)15, and (P2W15/PDDA)15, respectively.

Three samples using PAH as polyelectrolytes (Figure 1a-c) are high quality tubular materials, as well-formed tubes with similar structures can be seen from their SEM images, suggesting that the preparation method is effective and reliable. However, the sample prepared using PDDA (Figure 1d) is only partly developed into tubes. From their structures, three POMs used in this study have 6, 9, and 12 negative charges on each molecule, respectively, but the positive charges on PAH or PDDA are hard to assess. Because (P2W15/PAH)15 tubes are well-developed, poor tubular structure on (P2W15/PDDA)15 is likely due to weaker electrostatic attractions as PDDA may not have enough positive charge density. Figure 2 shows typical TEM images of four (POM/polyelectrolyte)15 samples. Tubular structures of all samples are visible from their TEM images, which show void cores and dark walls. The outside diameters are between 200 and 250 nm, which are controlled by the pore sizes of staring PC membrane. The wall thickness is roughly 40 nm. To test the physical strength of the tubes, TEM images in Figure 2 were acquired from nanotubes after PC membrane was removed and were sonicated for 20 min. After sonication, we still observed a large number of POM tubes, indicating that many POM tubes can tolerate sonication, and indicating that the tubes structure are quite stable.

J. Phys. Chem. C, Vol. 112, No. 24, 2008 8877

Figure 3. The EDX spectrum of the (P2W15/PAH)15 tubes.

Figure 4. The TEM images of (P2W15/PAH)n composite nanotubes: (a) n ) 25 with a thickness of 60 nm, (b) n ) 15 with a thickness of 40 nm, and (c) n ) 7 with a thickness of 20 nm. The scale bars are 200 nm. (d) Deduced linear relationship between wall thickness (nm) and the number of bilayers.

Figure 3 shows the energy-dispersive X-ray (EDX) spectra of (P2W15/PAH)15 nanotubes. Besides the main elements from substrate and the sample, such as carbon, oxygen and copper, phosphorus and tungsten were detected. The observed P-W atomic ratio of 0.19:1.42 is very close to the elemental composition of the starting POM material (P2W15). Other three nanotubes show similar EDX results (see Supporting Information), indicating that the POM retains its chemical structure in the tubes. To control the wall thickness and consequently the inside diameter of the tubes, three (P2W15/PAH)n nanotubes where n ) 25, 15, and 7, respectively, are prepared, and their TEM images are shown in Figure 4. The wall thickness are about 60, 40, and 20 nm for n ) 25, 15, and 7, respectively. The wall thickness shows a near linear relationship with the number of POM/PAH bilayers. Figure 4 also show linear decrease of the inside pore diameters, implying that both wall thickness and inside pore diameter can be readily and precisely controlled by the choosing desired number of POM/PAH layers.

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Ma et al.

Figure 6. The TEM images of (P2W15/PAH)n composite nanotubes: (a) n ) 40 and (b) n ) 50.

Figure 5. UV-vis spectra of (P2W15/PAH)n with n ) 0-15. The inset shows the absorbance at 198 nm as a function of n.

To further explore the growth process of the multilayer and the relationship between physical properties and the number of POM/PAH layers, UV-vis spectra of (P2W15/PAH)15 are acquired after each cycle of POM/PAH coating on a quartz slide.32 As shown in Figure 5, the absorbance at characteristic wavelength of 198 nm (corresponding to P2W15) increases as the number of POM/PAH layers increase. The whole spectrum for each cycle has the same absorbance peaks, indicating that the incorporation of POM anions into the composite does not cause any structural alteration. The inset in Figure 5 displays the plots of the absorbance values at 198 nm as a function of n. Absorbance intensity increases nearly linearly with increasing n, indicating quantitative and reproducible control of the POM/ PAH film thickness on silicon disk.33 Though this result is from a flat surface and may not be directly extended to a cylinder type substrate, we expect that the same linear relationship exist for P2W15/PAH tubes. However, PC membrane is not suitable for UV spectroscopy. Similar linear increases of absorbance with the increase of n are found from (P2W18/PAH)n, (EuW10/PAH)n, and (P2W15/PDDA)15 films on quartz slide (shown in Supporting Information). From the linear relation in Figure 5 and TEM measurement results in figure 4, a linear relationship between the wall thickness T (nm) and the number of the bilayer is deduced and shown in Figure 4d, as T (nm) ) 2.2n + 5.3, where the value 5.3 is likely due to the thickness of the pretreatment PEI/PSS/ PAH layers, and coefficient value 2.2 is like the thickness of one POM/PAH bilayer. Such relationship establishes that the LBL method developed in this study can control the tube diameter stepwise by 2.2 nm per step; consequently, the tube inside diameter is also controlled by the same step length. For POM/PAH membranes on a flat surface, the number of layers seems to be unlimited. However, POM/PAH tubes are restricted by two factors. One is the size of the template pore; the other is the migration of the macromolecules inside the charged polyelectrolyte cylinders and their adsorption on oppositely charged surfaces,. We attempted to obtain (P2W15/ PAH)n composite nanotubes with n ) 40 and 50, respectively. Figure 6 depicts the TEM images of these two samples. For n ) 40 sample, the tubular structure is still visible with an average inside diameter of about 40 nm and wall thickness of about 90 nm, which agrees very well with the trend in Figure 4. However, for n ) 50 sample, the void interior of the tube is hardly seen. Figure 6b shows a nearly clogged structure. Apparently, about 40 bilayers are close to the maximum amount of coatings that can produce stable tubular structures.

Figure 7. 31P solid state NMR CP-MAS spectra of (P2W15/PAH)n, where n ) 7 (a), 15 (b), and 25 (c), respectively, which were acquired from (P2W15/PAH)n with PC template and with identical NMR conditions.

For (EuW10/PAH)15 tubes, because Eu is a photoluminescence active species, we studied the photoluminescence behavior of the sample. The photoluminescence spectrum of Na9EuW10O36 · 32H2O powder and (EuW10/PAH)15 tubes dispersed in dichloromethane exhibit similar photoluminescence (shown in Supporting Information). This shows that EuW10 in as-prepared (EuW10/PAH)15 tubes retains its intrinsic photoluminescence properties. These results further establish that POM species did not change their structures and chemical properties in the tubes. Because 31P is an abundant and NMR active isotope, solid state NMR of the (P2W15/PAH)n samples were acquired to further investigate the effect of wall thickness. Because the presence of PC did not interfere with 31P, PC membranes were not removed in solid-state NMR studies. The 31P CP/MAS solid state NMR spectra of the (P2W15/PAH)n with PC template are shown in Figure 7. Each sample shows a two-line spectrum with signals at δ ) -7.4 and δ ) -14.3 ppm, which were tentatively assigned to two 31P atoms in different chemical environments in a P2W15 anion. Though CP/MAS is not a quantitative NMR method, the peak intensities from n ) 25 tubes are roughly two times higher than that from n ) 7 sample, which is consistent with thicker walls on n ) 25 sample. The shapes of NMR spectra did not change from n ) 7 to n ) 25, suggesting that P2W15 anions in different layers did not interact with each other. The synthesis scheme in this study is a bottom-up method from the wall thickness point of view; it is a bottom-up method as it is increased stepwise from blank. These materials provide an ideal opportunity for building a nanotubular catalytic reactor array. A tubular reactor, packed with catalysts, is a typical design

Parallel Array of Pt/Polyoxometalates Composite Nanotubes

J. Phys. Chem. C, Vol. 112, No. 24, 2008 8879 TABLE 1: Cyclohexene Conversion on Pt/(P2W15/PAH)n Nanotubular Catalytic Reactor

Figure 8. (a) TEM image of Pt/POM nanotubes. (b) The HRTEM image of Pt particle shown by an arrow in the left micrograph.

for continuous flow reactions. However, due to the mechanical difficulties, even in a so-called microflow reactor, the tubular reactor is at least a few micron meters in terms of inside diameter. A nanotubular reactor with nanocatalysts coated on the internal walls of the POM nano tubes is a true nanocatalytic reactor, with several features that are appealing. First, because the catalysts are nanocatalysts and are coated on the inside wall of the nanotube, the center of the tube is void, minimizing the impediment of flow and both gas and liquid can flow through. Along the traveling path, reactants have full access to the catalysts on the wall because the distance between the reactants and the wall is only several nanometers. This results in facile mass flow as well as high turnover frequency. Second, it is a bona fide nanotubular flow reactor with definite flow of gases or liquids stream flowing from one end to the other end. Third, the inside diameter of the tube can be precisely controlled from several hundred nanometers to a few nanometers. This allows large range of molecules to go through the tube and the inside diameter of the nanotube can be tuned to fit different catalytic reaction, that is, a 100 nm tube for large molecules and a 10 nm tube for small molecules. As a preliminary example, we tested the effect of the inside diameter to the cyclohexene hydrogenation reaction. The Pt nanoparticles were loaded into the inside walls of POM tubes with 10, 20, and 30 bilayers. When preparing Pt/POM composites, PC membranes were not removed, so the POM tubes’ external surfaces are not exposed, and Pt nanoparticles can only be loaded at the interior of the pores. The presence of PC membrane also ensures that the flow gas only goes through the Pt/POM nanotubes. The Pt nanoparticles are prepared in a separate flask before being loaded into the pores of the POM tubes. All three Pt/POMs samples were prepared from the same batch of Pt nanoparticle colloidal solutions. The XPS spectrum of the Pt/nanotubes composite shows the presence and nature of platinum. The Pt4f peaks (shown in Supporting Information) consist of two pairs of doublets. The more intense doublets (71.4 and 74.8 eV) are due to metallic platinum, and the other doublet is due to the Pt(II). From XPS spectrum, we found approximately 87.4% Pt(0) and 12.6% Pt(II). Figure 8a is a typical TEM image of Pt/POM nanotubes (PC membrane was removed). HRTEM image shown in Figure 8b shows dispersed Pt nanoparticles at the pore mouth area of the composite tube. A lattice spacing of 0.2285 nm was obtained from the images, corresponding to the interplanar distance of the Pt (111) face. The flow reaction was carried out at room temperature as illustrated in Scheme 2. In this reaction design, the whole piece of template (with Pt/POM tubes in it) was placed perpendicular to the gas flow direction, so that the tubes are parallel to the gas flow direction. This design can direct gas flow to go through all tubes, so that the Pt nanoparticles have the maximum

n

10

20

30

inside diameter nm conversion/% TOF of each tubular reactor (108/s)

150 5 1.1

100 11 2.4

60 26 5.6

opportunity to catalyze the reaction. The conversions of cyclohexene using different tubes as Pt supports are listed in Table 1. Apparently the conversion is controlled by the internal diameters of the nanotubular reactors. Pt catalysts with smaller inside diameter (more bilayers) show higher conversion. Because the total flow rate is the same for all three samples, linear flow rate should be faster for Pt/POM nanotubes with smaller inside diameters, and the average resident time for cyclohexene is shorter, which may lead to lower catalytic activity. However, for smaller inside diameters the diffusion courses to the Pt nanoparticles is shorter, which may have resulted in higher catalytic activity. We believe that shorter diffusion course is the dominate factor in this system. In a control experiment, bare POMs tubes showed no catalytic activity. Because the Pt/POM nanotubes are parallel and evenly isolated by the PC template, all nanotubes can be considered identical, so the performance of each tubular reactor can be accurately assessed from their overall performances. There are about 4 × 108 pores on each PC template. From the hydrogen flow rate and cyclohexene vapor pressure, approximately 9 × 1017 cyclohexene molecules per second flowed through these tubes. So the turn over frequencies (TOF) are 1.1 × 108, 2.4 × 108, and 5.6 × 108 per second for one single nanotubular reactor with 150, 100, and 60 nm internal diameters, respectively. We are trying to accurately measure the loading as well as the size distributions of the Pt nanoparticles on the Pt/POM composites; this will enable us to measure the TOF on each Pt nanoparticles. However, because the internal surfaces of the nanotubes decrease as the internal diameters decrease, we believe there will be less Pt nanoparticles on the 60 nm nanotubes than the other two larger tubes, so the TOF of each Pt nanoparticles in the 60 nm nanotubes will be significantly higher. In future work, we will exploit the ability to control the internal diameter of the nanotubes for catalysis, such as using nanotubular reactor with desired internal diameters to study the catalytic reactions or selective adsorptions that involve larger molecules including peptide and fused aromatics. 4. Conclusions On the basis of a LBL method and a porous PC template, Pt/POM composite nanotubes were prepared. Various characterization methods, including SEM, TEM, UV, fluorescence spectroscopy, and solid-state NMR are employed to study the structure and composition of the tubes. Precise control of the tube wall thickness and inside tube diameter at 2.2 nm per step is achieved by controlling the number of POM/polyelectrolyte bilayer. Pt/POM composite nanotubes are used as nanotubular reactors, showing promising potentials in nanocatalysis. The internal diameter of the nanoreactor can be designed to fit the sizes of participating molecules. Acknowledgment. Financial support from the National Natural Science Foundation of China (NSFC 20673125 and 50725207), Ministry of Science and Technology (MOST 2007CB936403), and the Chinese Academy of Sciences are gratefully acknowledged. The authors thank Professors ChunLi Bai and Li-Jun Wan for valuable advice. The authors are

8880 J. Phys. Chem. C, Vol. 112, No. 24, 2008 grateful to Professor Feng Deng for help in NMR measurements and Mr. Yun-Feng Qiu for POM preparations. Supporting Information Available: All structures of the referring compounds. SEM images of PC membranes. The UV-vis and IR spectrum of the as-prepared POM compositions. The EDX spectrum of the (P2W18/PAH)15, (EuW10/PAH)15, and (P2W15/PDDA)15. The UV-vis spectra of (P2W18/PAH)n, (EuW10/PAH)n, and (P2W15/PDDA)n with n ) 0-15. The photoluminescence spectra of EuW10 powder and (EuW10/ PAH)n nanotubes dispersed in dichloromethane. XPS spectrum of the Pt/POM tubes. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ibanez, F. J.; Zamborini, F. P. J. Am. Chem. Soc. 2008, 130, 622. (2) Qian, Y.; Wen, W.; Adcock, P. A.; Jiang, Z.; Hakim, N.; Saha, M. S.; Mukerjee, S. J. Phys. Chem. C 2008, 112 (4), 1146–1157. (3) Corma, A.; Serna, P. Science 2006, 313, 332. (4) Abad, A.; Almela, C.; Corma, A.; Garcia, H. Chem. Commun. 2006, 3178. (5) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. AdV. Mater. 2003, 15, 353. (6) Vayssieres, L.; Keis, K.; Hagfeldt, A.; Lindquist, S. E. Chem. Mater. 2001, 13, 4395. (7) Wan, M.; Huang, J.; Shen, Y. Synth. Met. 1999, 101, 708. (8) Wang, C. C.; Kei, C. C.; Yu, Y. W.; Perng, T. P. Nano Lett. 2007, 7, 1566. (9) Funk, S.; Hokkanen, B.; Burghaus, U.; Ghicov, A.; Schmuki, P. Nano Lett. 2007, 7, 1091. (10) Mitchell, D. T.; Lee, S. B.; Trofin, L.; Li, N.; Nevanen, T. K.; Soderlund, H.; Martin, C. R. J. Am. Chem. Soc. 2002, 124, 11864. (11) Hou, S.; Harrell, C. C.; Trofin, L.; Kohli, P.; Martin, C. R. J. Am. Chem. Soc. 2004, 126, 5674. (12) Jirage, K. B.; Hulteen, J. C.; Martin, C. R. Science 1997, 278, 655.

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