Article pubs.acs.org/JPCC
Dendrimer-Encapsulated Pt Nanoparticles: An Artificial Enzyme for Hydrogen Production Tianjun Yu,† Wen Wang,† Jinping Chen,*,† Yi Zeng,*,† Yingying Li,† Guoqiang Yang,*,‡ and Yi Li*,† †
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People's Republic of China ‡ Beijing National Laboratory for Molecular Sciences (BNLMS), Key laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People's Republic of China S Supporting Information *
ABSTRACT: Two series of dendrimer encapsulated Pt nanoparticles (DENPt) were created by using sixth generation poly(amidoamine) (PAMAM) dendrimers terminated with different numbers of hydroxyl groups (s-G6-OH and t-G6OH) to mimic hydrogenases. Pt nanoparticles act as the active site to generate H2 by reducing H+, and dendrimers provide cavities to maintain the integrity of small Pt nanoparticles and prevent agglomeration. The artificial hydrogenases (t-G6-OH/ Ptx and s-G6-OH/Ptx) were successfully applied to a lightinduced hydrogen production system with Pt-tppa+, ethyl viologen, and TEOA as photosensitizer, electron relay, and sacrificial reagent, respectively, exhibiting excellent stability and efficient catalytic activity. No passivation effect is caused by the periphery hydroxyl groups of dendrimers. The optimal size of the Pt clusters consists of 200 Pt atoms, and the most adapted pH value is 9 to gain the highest catalytic efficiency in the applied hydrogen production system. This study provides a new strategy for developing artificial hydrogenases by using dendritic architectures.
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INTRODUCTION The requirement to develop inexpensive renewable energy sources has stimulated new approaches to mimic natural photosynthesis in the conversion and storage of solar energy.1 Hydrogen (H2), with high specific enthalpy of combustion and a benign combustion product (water), is considered to be a potential alternative to fossil fuels. Given that hydrogen does not exist in minable quantities on Earth, it must be produced by the reverse reaction of combustion with enough energy input.2 A near-term hydrogen source is reforming of fossil fuels, resulting in little to reduce dependence on fossil fuels and emission of carbon dioxide. Therefore, finding efficient ways for hydrogen production by solar water splitting remains a great challenge.3−6 In nature, hydrogen photoproduction proceeds in hydrogenase, a catalytic center bound to a series of conserved protein motifs.7 The active site of [Fe−Fe] hydrogenase consists of a dimetal center with CO and CN ligands, and a cubane [4Fe4S] cluster as electron relay, which is located in an organized protein architecture precisely.2,8,9 [Fe−Fe] hydrogenases have high turnover rates of up to 9000 per enzyme per second; however, they are very sensitive to oxygen, and most of them are irreversibly inactivated after exposure to O2. Therefore, the task of creating robust artificial hydrogenases is of paramount importance.5,10−13 © 2012 American Chemical Society
Platinum (Pt) has a low overpotential for hydrogen evolution.14 Because it is costly and resource-limited, a number of approaches on the basis of the Pt nanoparticles have been done to maximize the catalytic efficiency of Pt on a per atom basis for developing economically feasible catalysts.15−18 Increasing the surface area and overcoming the passivation effect of stabilizers are considered useful ways benefiting the catalytic efficiency. Recently, Douglas et al. developed an artificial hydrogenase by using Pt clusters deposited on a protein cage architecture and gained a high initial hydrogen production rate.19 Dendrimers, like natural enzymes, possess well-defined structures with suitable cavities, which are particularly well-suited for hosting metal nanoparticles with controlled size, stability, and solubility in diameters of several nanometers and are easily prepared on a large scale. More importantly, dendrimers are highly permeable, and there is little passivation effect on the surface of encapsulated nanoparticles, resulting in high catalytic activity. Crooks, Tomalia, and Esumi are pioneers in the preparation of dendrimer encapsulated metal nanoparticles with various periphery modified poly(amidoamine) (PAMAM) dendrimers.20−22 Different kinds of metal nanoparticles, such as monometallic, bimetallic alloy, and Received: March 6, 2012 Revised: April 25, 2012 Published: April 26, 2012 10516
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Scheme 1. Visual Expression for the DENPt Preparation Process
Preparation of DENPt. A freshly prepared aqueous solution of H2PtCl6 was added into the t-G6-OH or s-G6OH aqueous solution with fixed concentration, and the molar ratios of Pt to dendrimer were kept at 100:1, 200:1, 300:1, 400:1, etc., respectively. Afterward, 1.5 time volume of methanol (vs water) was added, and the mixture was stirred and refluxed for 4 h to allow the Pt ions to be fully reduced by methanol. Upon reduction, the solution color changed from colorless to golden-brown. After methanol was evaporated, the DENPt aqueous solution was obtained and diluted with water to a certain volume ready for use. Preparation of the DENPt Modified Electrode. A glassy carbon electrode (GCE) was polished with 1.0 and 0.3 μm alumina powder, followed by sonication in water for 10 min, and then dried under flowing N2 gas after rinsing with water and acetone. The freshly polished GCE was placed in a DENPt aqueous solution ([Pt] = 1 × 10−4 M) containing 0.1 M LiClO4, and the potential of the electrode was scanned three times between 0 and 1.0 V by using a platinum wire as the counter electrode and a Ag/AgCl electrode as the reference electrode. Hydrogen Production. The hydrogen production experiments were carried out in a Pyrex reactor with a magnetic stir. Dilute acetic acid or NaOH was used to control the pH value. The sample solutions were deaerated by three freeze−pump− thaw cycles at a pressure of 5 × 10−5 Torr and warmed to ambient temperature prior to irradiation. A 500 W Xe lamp was used as the visible light source, and a filter was used to cut off the light below 400 nm and the infrared light. The generated photoproduct of H2 was characterized by GC analysis using a 5 Å molecular sieve column, a TCD detector, and N2 carrier gas with methane as an internal standard.
core−shell dendrimer-encapsulated metal nanoparticles, have been prepared successfully by chemical reduction of metal ions within PAMAM dendrimers.23−28 Herein, we attempt to take advantage of dendritic architectures and prepare small and well-defined Pt clusters as the catalytic center by using dendrimers as templates and stabilizers to create artificial hydrogenases. For this purpose, we prepared Pt nanoparticles (DENPt) encapsulated by sixthgeneration poly(amidoamine) dendrimers terminated with different numbers of hydroxyl groups (s-G6-OH and t-G6OH). The artificial hydrogenases DENPt were successfully applied to a hydrogen production system by excitation of a photosensitizer with visible light.
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EXPERIMENTAL SECTION Materials. Reagents were purchased from Acros, Alfa Aesar, or Beijing Chemicals and were used without further purification unless otherwise noted. Milli-Q deionized water (Millipore) was used in the DENPt preparation and the light-driven hydrogen generation experiments. Methanol (MeOH) and toluene (PhCH3) were dried with sodium and distilled under N2 atmosphere. Instrumentation. 1H NMR (400 MHz) spectra were obtained from a Bruker Avance Π-400 spectrometer with tetramethylsilane as an internal standard. ESI mass spectra were recorded on a Waters GCT Premier apparatus. Absorption and emission spectra were run on a Shimadzu UV-1601PC spectrometer and a Hitachi F-4500 spectrometer, respectively. The analysis of hydrogen production was carried out on a Beifen GC-3420 with a TCD detector. The electrochemical experiments were determined by using glassy carbon electrodes on CHI600C. High-resolution transmission electron microscopy (HRTEM) images were obtained by using a JEM-2100F transmission electron microscope with a point-to-point resolution of 0.19 nm. The quantum yields for hydrogen production were measured by irradiation with a 440 nm laser (PicoQuant LDH-D-C-440 laser head with PDL 828 Sepia II driver system, CW mode). Synthesis of Sensitizer and PAMAM Dendrimers. The sensitizer Pt(II) terpyridyl phenylacetylide perchlorate ((Pttppa)ClO4) was synthesized by the reaction of [Pt(terpy)Cl]Cl (terpy = 4′-(4-tolylphenyl)-2,2′,6′,2″-terpyridine) with phenylacetylene in the presence of CuI as a catalyst according to an early reported method.29 The PAMAM dendrimers terminated with triple-hydroxyl groups (t-G6-OH) or single-hydroxyl groups (s-G6-OH) were synthesized by using G5.5 PAMAM to react with tris(hydroxymethyl)aminomethane or 2-aminoethanol, respectively, according to the method reported by Tomalia.30−32 The detailed synthesis and the characterization data are supplied in the Supporting Information.
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RESULTS AND DISCUSSION Preparation and Characterization of DENPt. The sixthgeneration PAMAM dendrimers terminated with triplehydroxyl groups (t-G6-OH) and single-hydroxyl group (s-G6OH) were chosen to prepare DENPt. The high-generation dendrimers possess cavities large enough to encapsulate Pt nanoparticles with size above the threshold for hydrogen producing catalysis,19 and the different numbers of peripheral hydroxyl group are used to examine the encapsulation and passivation effects of dendrimers. The visual expression of the preparation of DENPt is illustrated in Scheme 1. The Pt ions provided by H2PtCl6 were first coordinated with tertiary amines of t-G6-OH or s-G6-OH and then reduced by reductant to form DENPt. Usually, the preparation of dendrimer encapsulated metal nanoparticles is prepared by using PtCl42− as the starting material and BH4− as the reductant, which requires several days to reach full complexation of Pt ion with tertiary amines to avoid precipitation of zerovalent Pt prior to 10517
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reduction.33 To achieve the complete reduction, a much more excessive amount of reduction agent of BH4− is needed, resulting in a tedious purification process to get rid of the excess BH4− and B4O72−. In the present work, for the first time, we used methanol as the reductant to prepare of DENPt by borrowing the idea of preparation of Pt nanoparticles by alcohol reduction in the presence of polymer stabilizers.34 The excessive amount of methanol was added after the aqueous solution of H2PtCl6 and s-G6-OH (or t-G6-OH) was stirred only for 10 min, and the disappearance of the PtCl62− absorption band at 265 nm was monitored after 2 h (Supporting Information Figure S1). To ensure accomplishment of reduction, the reaction was prolonged for another 2 h, and DENPt was obtained by easily evaporating the excessive methanol. No observable precipitation of zerovalent Pt was observed in the preparation of Ptx (x ≤ 400), and the details are discussed in the next paragraph. The use of methanol reductant shortened the preparation time of dendrimer encapsulated Pt nanoparticles from ∼4 days to ∼4 h and simplified the purification process. To optimize the catalytic efficiency of DENPt, a series of DENPt with various sizes (t-G6-OH/Ptx and s-G6-OH/Ptx) were prepared. The t-G6-OH/Ptx (x = 100, 200, 300, 400, 500, 600) were obtained by starting with different mole ratios of H2PtCl6 to t-G6-OH (100, 200, 300, 400, 500, and 600) reacting under the same experiment conditions, as illustrated in Figure 1. The DENPt aqueous solutions show a dark brown
and Pt600 can be ascribed to the limited cavity size of t-G6-OH and s-G6-OH, which cannot fully encapsulate the Pt nanoparticles with 500 or more Pt atoms, and the capacity of sixthgeneration PAMAM dendrimer is limited by Pt400. High-resolution TEM images were used to characterize the prepared DENPt. Figure 2 and Supporting Information Figure
Figure 1. Photograph of t-G6-OH/Ptx (x = 100, 200, 300, 400, 500, 600) aqueous solutions prepared by using CH3OH as the reducing agent.
color, and macroscopic black precipitates were observed in the cases of t-G6-OH/Pt500 and t-G6-OH/Pt600. Similarly, the aqueous solutions of s-G6-OH/Ptx (x = 100, 200, 300, 400) were prepared with no observable precipitate (Supporting InformationFigure S2). Although there are only 254 interior tertiary amine groups for t-G6-OH and s-G6-OH, the formation of nanoparticles containing more than 254 Pt atoms is achieved by using methanol reductant, which can be described by the mechanism proposed by Teranishi et al.34 Initially, the PtCl62− ions coordinate with tertiary amines of dendrimers, accompanied by the elimination of some Cl− ions from PtCl62−, and then the Pt4+ ions are reduced to Pt0 atoms to form the Pt nuclei form. The reduction of Pt4+ and the growth of the Pt nuclei proceed at the same time to form the Pt nanoparticles with the complete elimination of Cl−. The reduction of Pt4+ releases some unoccupied tertiary amines, which further coordinate with the free PtCl62− ions in solution, allowing the formation of nanoparticles with more than 254 Pt atoms. The formation of precipitates in the preparation of Pt500
Figure 2. HRTEM images and particle-size distribution histograms for t-G6-OH/Pt100, t-G6-OH/Pt200, t-G6-OH/Pt300, and t-G6-OH/Pt400 by using CH3OH as the reducing agent.
S3 provide the HRTEM micrographs, together with the corresponding size-distribution histograms for t-G6-OH/Ptx and s-G6-OH/Ptx (x = 100, 200, 300, 400), illustrating a high degree of monodispersity of the Pt clusters. Analysis of ∼100 randomly selected nanoparticles for each sample reveals the average diameter of t-G6-OH/Ptx and s-G6-OH/Ptx (x = 100, 200, 300, 400), which increases with an increase in the ratio of H2PtCl6 to dendrimer; the data are summarized in Table 1. The theoretical values for intradendrimer Pt nanoparticles are calculated by assuming all the added H2PtCl6 are reduced and encompassed within the smallest sphere possessing a facecentered cubic (fcc) crystal. The results from TEM experiments are consistent with the calculated ones, indicative of a complete 10518
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electrochemical cycles or even after sonication under acidic conditions. The catalytic efficiency of DENPt for light-induced hydrogen production was examined by using Pt-tppa+ and ethyl viologen (EV2+) as the photosensitizer and the electron relay, respectively, and TEOA as the sacrificial electron donor. The electron transfer process from the photosensitizer Pt-tppa+ to the electron relay EV2+ was confirmed by steady state spectroscopy. The luminescence of Pt-tppa+ (5.0 × 10−5 M) was quenched by EV2+ efficiently with a quenching rate constant of (3.8 ± 0.2) × 1010 M−1 s−1 (Supporting Information Figure S5). Moreover, the formation of blue EV•+ was observed upon irradiation of Pt-tppa+−EV2+ MeCN/ H2O solution with visible light in the presence of sacrificial regent TEOA, which cut off the back electron transfer pathway. The absorption intensity of EV•+ at maxima of 400 and 605 nm increased substantially with irradiation time (Supporting Information Figure S6). The light-driven hydrogen production was conducted by illuminating a solution (10 mL, MeCN/H2O = 2/3) containing DENPt ([Pt] = 5 × 10−5 M), Pt-tppa+ (5 × 10−5 M), EV2+ (1 × 10−4 M), and TEOA (200 mM) at certain pH values. The production of hydrogen was monitored and analyzed by gas chromatography. The schematic diagram of the H2 production catalyzed by DENPt is shown in Scheme 2. EV•+ was used to drive the reaction, which was generated via oxidation of TEOA by irradiation of Pt-tppa+ with visible light.
Table 1. Average Particle Sizes and Calculated Values Based on 100, 200, 300, and 400 Pt Atoms sample t-G6-OH/ Pt100 t-G6-OH/ Pt200 t-G6-OH/ Pt300 t-G6-OH/ Pt400
average diameter (nm)
sample
average diameter (nm)
calcd valuea (nm)
1.5 ± 0.2
s-G6-OH/Pt100
1.6 ± 0.2
1.4
1.9 ± 0.2
s-G6-OH/Pt200
1.9 ± 0.2
1.8
2.0 ± 0.2
s-G6-OH/Pt300
2.0 ± 0.2
2.1
2.2 ± 0.3
s-G6-OH/Pt400
2.2 ± 0.3
2.3
a
Calculated using the equation R = (3nVg/4π)1/3, where R is radius of the Pt nanoparticle, n is the number of moles of Pt, and Vg is the molar volume of Pt (9.10 cm3/mol).35
conversion of H2PtCl6 to Pt and each Pt particle being wrapped by one dendrimer molecule. Application of DENPt to Hydrogen Production. First, the catalytic activity of DENPt was evaluated by measuring cathodic polarization curves of the t-G6-OH/Ptx (x = 100, 200, 300, 400)-modified working electrodes. According to previous reports that alcohols and glycols could be linked to carbon surfaces at positive potentials,36,37 DENPt was grafted to a glassy carbon electrode via the terminated hydroxyl groups by electrochemical process. The t-G6-OH/Ptx (x = 100, 200, 300, 400)-modified electrodes show a clearly positive potential shift compared with GCE and the t-G6-OH grafted one, as shown in Figure 3, indicative of their catalytic ability for the H+ reduction
Scheme 2. Schematic Diagram of Light Driven Hydrogen Production Catalyzed by DENPt
Figure 4 illustrates the H2 production results upon 6 h irradiation at a pH value of 9. The catalytic efficiency evidently varies with the size of the Pt nanoparticles of DENPt. t-G6OH/Ptx and s-G6-OH/Ptx show a similar trend. This is
Figure 3. Cathodic polarization curves of GCE, t-G6-OH and t-G6OH/Ptx (x = 100, 200, 300, 400)-modified GCE electrodes in 0.1 M KCl aqueous solution (scan rate, 5 mV/s; electrode area, 0.071 cm2; reference electrode, Ag/AgCl).
process.38−40 t-G6-OH/Pt200 exhibits the highest catalytic activity; the catalytic activities of t-G6-OH/Pt300 and t-G6OH/Pt400 are similar and take second place, which can be ascribed to the less surface area. For t-G6-OH/Pt100, the sluggish catalytic behavior may be attributed to the small size of the Pt nanoparticle, which is below some threshold limit required for full catalytic activity, and the weak contact of Pt nanoparticles with the glassy carbon electrode surface caused by the sizable surrounding t-G6-OH molecule. A similar trend of catalytic activity for s-G6-OH/Ptx (x = 100, 200, 300, 400) was observed (Supporting Information Figure S4). Furthermore, it is worth noting that both t-G6-OH/Ptx and s-G6-OH/Ptx (x = 100, 200, 300, 400)-modified electrodes are stable after 50
Figure 4. DENPt size dependence of hydrogen production catalyzed by t-G6-OH/Ptx and s-G6-OH/Ptx (x = 100, 200, 300, 400) in MeCN/H2O (2/3, v/v) solution at pH = 9. ([Pt] = [Pt-tppa+] = 5 × 10−5 M, [EV2+] = 1 × 10−4 M, [TEOA] = 200 mM). 10519
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consistent with a previous report of the size-dependent activity of Pt41 and our electrocatalytic hydrogen production results. According to a previous report, the minimum number of Pt atoms required for H2-producing activity is about 50, and a large number is needed for full catalytic activity. Therefore, the less efficient catalytic efficiency for t-G6-OH/Pt100 and s-G6OH/Pt100 may be a consequence of their smaller size. The maxima of turnover numbers were observed in the t-G6-OH/ Pt200 and s-G6-OH/Pt200 catalytic systems, indicating that the optimal number of Pt atoms for DENPt is 200. The lower catalytic efficiency for t-G6-OH/Ptx and s-G6-OH/Ptx (x = 300, 400) can be ascribed to the smaller surface area of the Pt clusters, which decreases the virtual amount of catalyst due to the inactive interior Pt atoms. Similar catalytic activity for t-G6OH/Ptx and s-G6-OH/Ptx indicates no passivation effect caused by the periphery hydroxyl groups. To optimize the experiment conditions, light-induced hydrogen production was conducted by using t-G6-OH/Pt200 and s-G6-OH/Pt200 as the catalyst at different pH values upon irradiation for 6 h. The pH value of the catalytic system was adjusted by addition of an aqueous solution of acetic acid (0.1 M) or sodium hydroxide (0.1 M) prior to irradiation. As shown in Figure 5, the pH value affects the hydrogen production
Figure 6. Time dependence of hydrogen production catalyzed by tG6-OH/Pt200 and s-G6-OH/Pt200 in MeCN/H2O (2/3, v/v) solution at pH = 9 ([Pt-tppa+] = 5 × 10−5 M, [EV2+] = 1 × 10−4 M, [TEOA] = 200 mM).
irradiation of the solutions (dendrimer/Pt200 = 5 × 10−5 M, Pttppa+ = 5 × 10−5 M, EV2+ = 1 × 10−4 M, TEOA = 200 mM in 10 mL MeCN/H2O = 2/3) with 440 nm laser at pH = 9 for 1 h, giving initial catalytic efficiencies of 0.07 ± 0.02 f for both tG6-OH/Pt200 and s-G6-OH/Pt200. The cyclability of the catalysts t-G6-OH/Pt200 and s-G6-OH/ Pt200 was also checked. After the hydrogen production ceased, readdition of t-G6-OH/Pt200 or s-G6-OH/Pt200 to the system resulted in no more hydrogen evolution, indicating that the passivation of the system is not caused by the deactivation of the catalysts. Replenishment of the sensitizer (Pt-tppa+, 5 × 10−5 M) to the passive system initiated the hydrogen production again with ∼80% efficiency of the first cycle, demonstrating that the ceasing of hydrogen production was due to the consumption of sensitizer and the catalytic activities of tG6-OH/Pt200 and s-G6-OH/Pt200 can last for several cycles.
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CONCLUSIONS In summary, we have synthesized robust artificial hydrogenases, DENPt, by using a well-defined dendritic architecture encapsulating a Pt cluster in a spatially selective manner, which has been successfully applied to the hydrogen production system. The Pt clusters act as the active site to reduce H+ to the form H2. The dendrimers, t-G6-OH and s-G6-OH, provide cavities to maintain the integrity of small Pt nanoparticles and prevent agglomeration. The preparation of DENPt has been improved efficiently by using methanol as the reduction regent. The present study provides a new strategy for developing artificial hydrogenases by using dendritic architectures.
Figure 5. pH value dependence of hydrogen generation catalyzed by tG6-OH/Pt200 and s-G6-OH/Pt200 in MeCN/H2O (2/3, v/v) solution after 6 h of irradiation ([Pt] = [Pt-tppa+] = 5 × 10−5 M, [EV2+] = 1 × 10−4 M, [TEOA] = 200 mM).
dramatically. The maximum turnover numbers corresponding to the amount of hydrogen production were achieved at pH = 9 for both t-G6-OH/Pt200 and s-G6-OH/Pt200 catalytic systems, manifesting the optimal pH value to be 9. When the pH value of the catalytic solution is below 7, the hydrogen production can be neglected because the protonation of TEOA renders TEOA an inefficient sacrificial electron donor to the oxidized photosensitizer. At pH values above 10, the hydrogen production involving the proton reduction is reduced due to the low concentration of H+. At the optimal pH value of 9, the maximum turnover numbers were achieved (300 ± 30 and 270 ± 22 catalyzed by tG6-OH/Pt200 and s-G6-OH/Pt200, respectively) upon 25 h of irradiation (Figure 6). The initial hydrogen production rates are almost the same, and the turnover number catalyzed by t-G6OH/Pt200 is slightly higher than that by s-G6-OH/Pt200 after a long-time irradiation, which may be caused by the different stability of DENPt. The t-G6-OH with more hydroxyl groups at the periphery can encapsulate a Pt nanoparticle more compactly and result in higher stability of the DENPt. The quantum yields of hydrogen production were also measured by
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ASSOCIATED CONTENT
S Supporting Information *
Experimental procedures for the synthesis of all compounds and characterization data; monitoring spectra during the t-G6OH/Ptx (x = 100, 200, 300, 400) formation process; photograph of s-G6-OH/Ptx (x = 100, 200, 300, 400) solution; HRTEM images and particle size distribution histograms for sG6-OH/Ptx (x = 100, 200, 300, 400); cathodic polarization curves of s-G6-OH/Ptx (x = 100, 200, 300, 400) modified GCE electrode; emission quenching and Stern−Volmer plot of Pt-tppa+ by EV2+; UV−vis spectra of the mixed solution upon irradiation by visible light. This material is available free of charge via the Internet at http://pubs.acs.org. 10520
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(29) Yang, Q. Z.; Wu, L. Z.; Wu, Z. X.; Zhang, L. P.; Tung, C. H. Inorg. Chem. 2002, 41, 5653−5655. (30) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Macromolecules 1986, 19, 2466−2468. (31) Tomalia, D. A.; Hall, M.; Hedstrand, D. M. J. Am. Chem. Soc. 1987, 109, 1601−1603. (32) Tomalia, D. A.; Durst, H. D. Top. Curr. Chem. 1993, 165, 193− 313. (33) Knecht, M. R.; Weir, M. G.; Myers, V. S.; Pyrz, W. D.; Ye, H. C.; Petkov, V.; Buttrey, D. J.; Frenkel, A. I.; Crooks, R. M. Chem. Mater. 2008, 20, 5218−5228. (34) Miyake, M.; Teranishi, T.; Hosoe, M.; Tanaka, T. J. Phys. Chem. B 1999, 103, 3818−3827. (35) Ye, H.; Crooks, J. A.; Crooks, R. M. Langmuir 2007, 23, 11901− 11906. (36) Maeda, H.; Yamauchi, Y.; Hosoe, M.; Li, T. X.; Yamaguchi, E.; Kasamatsu, M.; Ohmori, H. Chem. Pharm. Bull. 1994, 42, 1870−1873. (37) Maeda, H.; Katayama, K.; Matsui, R.; Yamauchi, Y.; Ohmori, H. Anal. Sci. 2000, 16, 293−298. (38) Koca, A.; Ozcesmeci, M.; Hamuryudan, E. Electroanalysis 2010, 22, 1623−1633. (39) Nyokong, T.; Mbambisa, G.; Tau, P.; Antunes, E. Polyhedron 2007, 26, 5355−5364. (40) Kaneko, M.; Zhao, F.; Zhang, J.; Abe, T.; Wohrle, D. J. Mol. Catal., A: Chem. 1999, 145, 245−256. (41) Greenbaum, E. J. Phys. Chem. 1988, 92, 4571−4574.
AUTHOR INFORMATION
Corresponding Author
*E-mails:
[email protected];
[email protected]; zengyi@ mail.ipc.ac.cn;
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grant Nos. 21173245, 21073215, 21004072, 21002109), the Chinese Academy of Sciences, and the National Basic Research Program (Grant No. 2010CB934500) is gratefully acknowledged. J. Chen thanks the Beijing Nova Program for financial support.
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