Facet-Dependent Catalytic Activity of Gold Nanocubes, Octahedra

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Facet-Dependent Catalytic Activity of Gold Nanocubes, Octahedra, and Rhombic Dodecahedra toward 4‑Nitroaniline Reduction Chun-Ya Chiu, Pei-Ju Chung, Ka-Un Lao, Ching-Wen Liao, and Michael H. Huang* Department of Chemistry and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua University, Hsinchu 30013, Taiwan S Supporting Information *

ABSTRACT: In this study, cubic, octahedral, and rhombic dodecahedral gold nanocrystals synthesized by a seed-mediated growth method were employed as catalysts for the examination of facet-dependent catalytic activity toward NaBH4 reduction of p-nitroaniline to p-phenylenediamine at different temperatures. Different amounts of the nanocrystal solutions were used so that all samples contain particles with the same total surface area. UV−vis absorption spectra monitored the reaction progress. Rhombic dodecahedra showed the best catalytic efficiency at all the temperatures examined. Nanocubes have higher reaction rates than those of octahedra from 25 to 36 °C, so the catalytic activity for the reduction reaction follows the order of {110} > {100} > {111}. However, the reaction rates for octahedra increase rapidly with rising temperature; their reaction rate surpasses that for the nanocubes at 40 °C. Rate constants and activation energies were determined, again showing that the activation energy is lowest for rhombic dodecahedra. Density functional theory (DFT) calculations indicate highest binding energy between p-nitroaniline and the Au(110) plane. The results reveal rhombic dodecahedral gold nanocrystals as highly efficient catalysts.

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INTRODUCTION An important nanomaterials research direction involves the preparation of nanocrystals with excellent shape and size control. Successful syntheses of metal and semiconductor nanocrystals with a high degree of particle shape uniformity such as the formation of cubes and octahedra offer opportunities to examine their facet-dependent catalytic, electrical, and molecular adsorption properties with greater accuracy.1−12 For example, dramatic differences in the photodegradation of methyl orange have been found for Cu2O nanocubes, octahedra, and rhombic dodecahedra.5,7,8 A single Cu2O cube is barely electrically conductive, while an octahedron is remarkably conductive.9 If the polyhedral nanocrystals are monodispersed in size, they can be used as building blocks to form long-range self-assembled superlattices.13−16 Previously, we have reported the successful syntheses of gold nanocubes, octahedra, and rhombic dodecahedra with excellent size and shape control by a seedmediated growth approach.17,18 Their comparative surfaceenhanced Raman scattering (SERS) sensitivity for the detection of thiophenol at very low concentrations has been studied, showing rhombic dodecahedra as the best SERS substrates.19 Another interesting research topic for these gold nanocrystals is their facet-dependent catalytic activity. A typical gold-catalyzed reaction is borohydride reduction of aromatic nitro compounds to the corresponding amino compounds. Gold mediates the electron transfer from borohydride to the aromatic nitro © 2012 American Chemical Society

compounds. Various gold nanostructures have been examined as catalysts in this type of reaction.20−28 Liang et al. have compared the reaction rates at room temperature for a variety of nitro compounds using cetyltrimethylammonium bromide (CTAB) terminated gold nanospheres, long nanorods, and nanoprisms as catalysts.20 The number of gold nanoparticles were kept approximately the same. Because of large surface area differences, the comparison of catalytic efficiency of different gold nanostructures is less useful and insightful. In another study comparing the reduction rates using approximately the same number of poly(N-vinyl-2-pyrrolidone) (PVP)-stabilized gold nanoparticles with three different sizes, the same surface area problem exists.21 Use of PVP with different molecular weights to synthesize these gold nanoparticles complicates the analysis. In all these studies, the facet-dependent catalytic efficiency of gold nanocrystals cannot be addressed. It is thus most interesting to use gold nanocubes, octahedra, and rhombic dodecahedra bounded exclusively by respective {100}, {111}, and {110} faces to evaluate how different facets affect their catalytic activity. Such well-controlled study is not possible until the recent development of their synthetic methods. Received: August 6, 2012 Revised: October 18, 2012 Published: October 29, 2012 23757

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Figure 1. SEM images of (a) gold octahedra, (b) nanocubes, and (c) rhombic dodecahedra. Scale bars are equal to 100 nm. Edge lengths a are indicated in the drawings and used to determine the particle sizes and surface areas.

Then, 10 μL of 0.01 M NaBr was introduced for the growth of nanocubes and rhombic dodecahedra. To make octahedra, 5 μL of 0.01 M KI was added instead of NaBr. Finally, 90, 220, and 150 μL of 0.04 M ascorbic acid was introduced for the synthesis of nanocubes, octahedra, and rhombic dodecahedra, respectively. Total solution volume in each vial is 10 mL. The solution color turned colorless after the addition of ascorbic acid, indicating the reduction of Au3+ to Au+ species. Next, 25 μL (for nanocubes and rhombic dodecahedra) or 50 μL (for octahedra) of the seed solution was added to the growth solution in vial A with shaking until the solution color turned light pink (∼5 s). Then, 25 μL (for nanocubes and rhombic dodecahedra) or 50 μL (for octahedra) of the solution in vial A was transferred to vial B with thorough mixing for ∼10 s. The solution in vial B was left undisturbed for 15 min for particle growth, and centrifuged three times at 2500 rpm for 10 min (Hermle Z323 centrifuge). The water above the particles containing surfactant was carefully removed, and 1 mL of deionized water was subsequently added. After centrifugation, the gold nanocrystals were collected in vials. Determination of Particle Amounts Needed for the Catalysis Experiments. About 400 μL of the gold nanocrystal solution was dropped into a clean and weighted vial cap. Nanoparticles were allowed to dry completly in about two days. The vial cap was weighted again to obtain the mass of the gold nanocrystals. Using the density of gold, total particle volume can be calculated. From the total particle volume and the average edge length of a particular particle sample, the number and total surface areas of nanocrystals in 400 μL of its solution can be obtained. Different volumes of the nanocrystal solutions having the same total surface area were used for their comparative catalytic activity experiments. Catalysis Experiments. For a typical catalysis reaction, 500 μL of 1.0 × 10−3 M 4-NA solution was added to 4 mL of deionized water and stirred thoroughly. After that, different volumes of the Au nanocrystal solution were added to the reaction mixture (i.e., 102 μL for nanocubes, 95 μL for octahedra, and 347 μL for rhombic dodecahedra), keeping the total surface area of Au particles approximately the same in all cases at 4.50 cm2 (see Table S1, Supporting Information). The reaction mixture was maintained at 25, 28, 32, 36, and 40 °C using a water bath. Finally, 1.2 mL of 0.1 M ice-cold NaBH4 solution was added and mixed well to start the reaction. Next, 800 μL of the solution was transferred to a narrow-slit cuvette for spectral measurements without dilution. The solution was discarded after taking a spectrum, and fresh solution from the same heated mixture was used for next run. If many spectra were needed for a particular sample, the volume of the stock

In this work, we have used gold nanocubes, octahedra, and rhombic dodecahedra as catalysts to follow the rates of NaBH4 reduction of p-nitroaniline to p-phenylenediamine at different temperatures for the first time. The nanocrystals were synthesized by almost the same reaction conditions, thus eliminating any possible surfactant effect on the reaction rate. Nanocrystals with approximately the same total surface area were employed, so that the results can be attributed to the different gold surfaces exposed. Rate constants and activation energies were obtained. Interesting relative temperaturedependent reaction rates have been observed for cubes and octahedra. Density functional theory (DFT) calculations of pnitroaniline on the three surface planes of gold have been performed to provide additional insights and assist in the explanation of the experimental results.

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EXPERIMENTAL SECTION Chemicals. Hydrogen tetrachloroaurate (III) trihydrate (HAuCl4·3H2O, Aldrich, 99.9%), p-nitroaniline (4-NA, Alfa Aesar, 98%), cetyltrimethylammonium chloride (CTAC, TCI, 95%), sodium borohydride (NaBH4, Sigma-Alderich, 98%), ascorbic acid (AA, Riedel-de Haën, 99.7%), sodium bromide (NaBr, UCW), and potassium iodide (KI, J. T. Baker) were used without further purification. Ultrapure distilled and deionized water (18.3 MΩ) was used for all solution preparations. Synthesis of Gold Nanocubes, Octahedra, and Rhombic Dodecahedra. The gold nanocrystals were synthesized following our previously reported procedures.17,18 The synthetic procedure begins with the preparation of gold seed solution. A volume of 10 mL of aqueous solution containing 2.5 × 10−4 M HAuCl4 and 0.10 M CTAC was prepared. Concurrently, 10 mL of 0.02 M ice-cold NaBH4 solution was made. To the HAuCl4 solution was added 0.45 mL of the NaBH4 solution with stirring. The resulting solution turned brown immediately, indicating the formation of gold particles. The seed solution was aged for 1 h at 30 °C to decompose excess borohydride. To make the gold nanocrystals with different morphologies, two vials were labeled A and B. A growth solution was prepared in each of the two vials. First, 0.32 g of CTAC surfactant was added. Depending on the morphology of gold nanocrystals to be synthesized, slightly different volumes of deionized water were added to each vial (i.e., 9.625 mL for nanocubes, 9.475 mL for octahedra, and 9.565 mL for rhombic dodecahedra). The concentration of CTAC in the growth solution is equal to 0.10 M. The vials were then kept in a water bath set at 25 °C. To both vials were added 250 μL of 0.01 M HAuCl4 solution. 23758

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Figure 2. Time-dependent UV−vis absorption spectra for the borohydride reduction of 4-NA at (a−c) 25 and (d−f) 36 °C using (a,d) octahedral, (b,e) cubic, and (c,f) rhombic dodecahedral gold nanocrystal catalysts.

solution prepared was doubled. UV−vis absorption spectra were recorded at different time periods depending on the reaction rate. After the completion of the reduction reaction, the light yellowish p-nitroaniline solution turned colorless due to the formation of p-phenylenediamine (p-PDA) product. Instrumentation. Scanning electron microscopy (SEM) images of the nanocrystals were obtained using a JEOL JSM7000F electron microscope. UV−vis abosrption spectra were taken with the use of a JASCO V-570 spectrophotometer. X-ray

photoelectron spectroscopy (XPS) analysis was performed using an ULVAC-PHI Quantera SXM spectrometer. Data were recorded using a monochromatized Al anode as the excitation source and the C1s peak was chosen as the reference line.

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RESULTS AND DISCUSSION

The gold nanocrystals were synthesized by a seed-mediated growth approach in the presence of CTAC surfactant. Trace amounts of NaBr was added in the production of nanocubes 23759

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Figure 3. ln[4-NA] versus time plots using gold nanocubes, octahedra, and rhombic dodecahedra as catalysts for the reduction of 4-NA carried out at (a) 25, (b) 28, (c) 32, (d) 36, and 40 °C.

crystalline gold octahedra, nanocubes, and rhombic dododecahedra. Particles with monodispersed shapes and sizes were obtained, so they can easily form ordered packing structures. Their X-ray diffraction patterns and transmission electron microscopy characterization, including their selected-area

and rhombic dodecahedra, while a trace amount of KI was introduced in the formation of octahedra. Because they were prepared by almost the same synthetic conditions, they are ideal for comparison of their catalytic activity with greater accuracy. Figure 1 shows SEM images of the synthesized single23760

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product formation is still far from complete even after 240 min of reaction. Even though the major product band at 240 nm was not reached by octahedra and nanocubes at 25 °C (235 nm for octahedra at 240 min and 238 nm for nanocubes at 70 min) and at 28 °C for octahedra (235 nm at 70 min), the product band eventually reaches 240 nm by extending the reaction time. The reaction proceeds to form the final product at higher temperatures. As expected, the reaction rates increase at higher temperatures for all the samples. For example, it takes around 50 min for rhombic dodecahedra to complete the conversion of 4-NA at 25 °C, yet the conversion is finished in just 12.5 min for rhombic dodecahedra at 36 °C. It is also interesting to note that, at higher temperatures (e.g., at 36 °C), reduction rate increases dramatically for the octahedra relative to that for the nanocubes. To further evaluate the relative reaction rates of these nanocrystals, plots of ln[4-NA] versus time for the reduction of 4-NA carried out at the five temperatures using gold nanocubes, octahedra, and rhombic dodecahedra as catalysts are prepared and shown in Figure 3. Absorbance values are converted to concentrations in the preparation of these plots. The linear fits for these data points indicate that the reduction reaction follows first-order kinetics. The excess amount of NaBH4 used is also consistent with the reaction order. From the slope of the best fit line, the rate constant k can be determined. Rate constants are provided in Figure 3 and Table S1 (Supporting Information). The plots reveal that rhombic dodecahedra always maintain the largest k values among the different particle morphologies at all temperatures, so these nanocrystals bounded by the {110} facets are indeed the best catalysts for the reduction reaction. Nanocubes outperform octahedra at 25−32 °C, but their reaction rates are still much slower than those of the rhombic dodecahedra at these temperatures. From 32−36 °C, the reduction rate for octahedra increases dramatically and the difference in the reaction rate between octahedra and nanocubes narrows. Remarkably, the reaction rate for octahedra surpasses that for nanocubes at 40 °C and approaches that for rhombic dodecahedra. This strong temperature-dependent catalytic activity is an important observation not reported in previous studies. The reason for such behavior is unclear at present. From the k values, a plot of ln k versus 1/T for the reduction of 4-NA at 25, 28, 32, 36, and 40 °C using gold nanocubes, octahedra, and rhombic dodecahedra as catalysts is prepared and presented in Figure 4. From the best fit lines, one can see that the ascending trend with increasing temperatures for the rhombic dodecahedra and nanocubes are similar, while that for the octahedra goes more rapidly. The slope of the best fit line is equal to −Ea/R, where Ea is the activation energy and R is the universal gas constant. From the line slopes, activation energies of the nanocrystals were claculated. Activation energies for the gold rhombic dodecahedra, nanocubes, and octahedra are 62.2, 90.5, and 189.3 kJ/mol, respectively. The results show that the gold rhombic dodecahedra are the best catalysts because the activation energy required for the reaction to occur is the lowest, while the highest activation energy is needed to carry out this same reaction on the gold octahedra. Since the reaction takes place on the particle surfaces, the catalytic activity differences are ascribed to the different facets exposed (i.e., {110} > {100} > {111}). To assist in the explanation of the experimental observations and gain more insights into the molecular interactions with the various gold surfaces, we have performed state-of-the-art

electron diffraction patterns, have been reported previously.15,17,18 The gold nanocubes, octahedra, and rhombic dodecahedra have average edge lengths of 61, 45, and 44 nm, respectively. The corresponding sizes for octahedra (opposite corner-to-corner distance) and rhombic dodecahedra (opposite face-to-face distance) are 63.6 and 71.8 nm, respectively. For accurate catalytic activity comparison, total surface area of the particles should be kept the same. The total particle weight in 400 μL of the nanocrystal solution was determined for each sample (see Supporting Information Table S1 for the particle weights and calculations). From the particle edge length, the volume and total surface area of a single nanocrystal can be calculated, and the mass of a single nanocrystal is obtained. Approximate number of nanocrystals and thier total surface area in this volume of particle solution were determined. With this information, different volumes of the nanocrystal solutions were used for the catalysis experiments such that the total particle surface area is the same for the three samples. After adding the nanocrystal solution to the 4-NA solution, ice-cold NaBH4 solution was introduced to start the reduction reaction. The reaction progress was monitored by taking the UV−vis absorption spectra of the reaction solution as a function of time at different temperatures. To confirm that it is metallic gold acting as the catalysts, XPS spectra of the three nanocrystal samples were taken before the catalysis experiments (see Supporting Information Figures S1 and S2). Gold nanocubes, octahedra, and rhombic dodecahedra all show Au 4f7/2 and 4f5/2 peaks at binding energies of ∼83.4 and 87.1 eV, respectively (or peak-to-peak distance of 3.7 eV). These peak positions are consitent with the Au0 oxidation state.29 If the Au+ species was present, a peak is expected to appear at 84.9 eV.29 The mechanism of reduction of aryl nitro compounds to the aryl amine compounds has been reported to follow either a direct route or a condensation route.20,30,31 In the more probable direct route, BH4− and the aryl nitro compund are both adsorbed on the gold surface first.30 BH4− serves as the hydrogen source and gold should act as the electron donor. The aryl nitro compound is first reduced to the nitroso compound and then quickly to the corresponding hydroxylamine compound. The hydroxylamine compound is finally reduced to the aryl amine product. However, the different gold surfaces influencing the rate of this reduction reaction has not been elucidated. This study addresses this important issue to identify the best gold surface for the reaction. Figure 2 gives time-dependent UV−vis absorption spectra for the borohydride reduction of 4-NA at 25 and 36 °C using octahedral, cubic, and rhombic dodecahedral gold nanocrystal catalysts. The corresponding time-dependent UV−vis absorption spectra for the reaction carried out at 28, 32, and 40 °C are available in Figure S3, Supporting Information. 4-NA displays a strong absorption band at 380 nm and another band at 227 nm. In the absence of Au nanoparticles, no reduction reaction occurred for a mixture of 4-NA and NaBH4 even after a week (see Figure S4, Supporting Information). The reduction product p-phenylenediamine (or benezene-1,4-diamine) shows a strong absorption band at 240 nm and another band at 305 nm (see Figure S3, Supporting Information). Figures 2 and S3 (Supporting Information) show continuous decrease of the 380 nm band of 4-NA and gradual increase of the 240 nm band of p-phenylenediamine as the reaction proceeds at reaction temperatures of 32−40 °C. At 25 °C, rhombic dodecahedra exhibited the highest reduction rate, followed by that of the nanocubes. Octahedra displayed the slowest reduction rate; 23761

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the most stable structure as shown in Figure 5. The BE between gold cluster and 4-NA was obtained by subtracting the 4-NAcluster energy from the total energy of the bare gold cluster and the bare 4-NA. BE = E4‐nitroaniline + Egold cluster − E4‐nitroaniline−gold cluster

Positive value of BE indicates a stabilization of the 4-NA-gold cluster system with respect to the bare 4-NA and bare gold cluster. All BEs are corrected for the basis set superposition error (BSSE) by the counterpoise scheme of Boys.36 All calculations were performed by the Gaussian09 software package.37 The BE between 4-NA and Au(111), Au(110), and Au(100) surfaces are 11.702, 14.947, and 11.945 kcal/mol, respectively, as shown in Table 1. The BE between 4-NA and Au(110) surface is much larger than those for the other two surfaces. The distances between the nitrogen atom (−NO2) of 4-NA and gold atom on Au(111), Au(110), and Au(100) surfaces are 3.311, 3.003, and 3.337 Å, respectively. The distances between the oxygen atom of 4-NA and gold atom on Au(111), Au(110), and Au(100) surfaces are 2.957, 2.397, and 2.920 Å, respectively. The N−Au and O−Au distances between 4-NA and Au(110) surface are shorter than on the other two surfaces, which indicate that the N−Au and O−Au bonds between 4-NA and Au(110) surface are stronger than those for the other two surfaces. This result is also reflected by the highest BE between 4-NA and Au(110) surface among the three surfaces. Therefore, Au(110) surface is much easier than the other two surfaces to bind 4-NA and easily reduces 4-NA to pphenylenediamine. Furthermore, the distances of nitrogen and oxygen atoms of 4-NA to the gold atoms on the Au(110) surface are much shorter than those for the other two surfaces. This close interaction also facilitates hydrogen and electron transfer. The calculations also reveal strong benzene ring interaction with the Au surfaces. The calculations qualitatively support the experimental results that the relative catalytic activity for the 4-NA reduction reaction follows the order of {110} > {100} > {111} surfaces, although the similar BEs for the Au(100) and Au(111) surfaces are best matched to data recorded at 36 °C.

Figure 4. Plot of ln k versus 1/T for the reduction of 4-NA at 25, 28, 32, 36, and 40 °C using gold nanocubes, octahedra, and rhombic dodecahedra as catalysts. The slope of each line multiplied by 8.314 gives the value of activiation energy in J/mol.

density functional theory (DFT) calculations to determine the binding energies (BEs) of 4-NA on the Au {100}, {111}, and {110} surfaces and offer the most stable molecular binding geometries on these surfaces. The gold surfaces for adsorption of 4-NA were modeled using gold clusters with 25 gold atoms distributed in two layers. The model of Au(111) and Au(110) was set up by arranging 16 and 9 gold atoms in the first and the second layer, respectively. The Au(100) surface was mimicked by 13 gold atoms in the first layer and 12 gold atoms in the second layer. All subsequent DFT calculations for binding energies between 4-NA and gold surfaces were performed using Becke 88 exchange32 combined with Perdew 86 correction33 functionals (BP86) with long-range correction34 (LC-BP86) allied with LANL2DZ double-ζ quality basis set with the corresponding ECP35 for gold atoms and 6-311G(2df) basis set for carbon, hydrogen, nitrogen, and oxygen atoms. Clusters of Au(111), Au(110), and Au(100) were first optimized under LC-BP86/LANL2DZ level of theory. Afterward, Au(111), Au(110), and Au(100) clusters were fixed at their optimized structures, and 4-NA was placed in different initial positions on the three surfaces to perform the geometry optimizations. The geometric structure with the lowest total energy was chosen as

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CONCLUSION Gold nanocubes, octahedra, and rhombic dodecahedra synthesized by a seed-mediated growth approach have been employed to examine their comparative catalytic activity toward

Figure 5. (a) Top views and (b) side views of the most stable structure of 4-nitroaniline on Au(111) (left), Au(110) (middle), and Au(100) (right) surfaces. 23762

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Table 1. DFT Calculation Results of the Distances of Nitrogen and Oxygen Atoms to the Closest Gold Atoms on the Au(111), Au(110), and Au(100) Surfaces and the Binding Energies of 4-Nitroaniline on These Gold Surfaces surfaces

distances of the nitrogen atom to the closest gold atom (Å)

distances of the oxygen atom to the closest gold atom (Å)

BEs for 4-nitroaniline and corresponding gold surfaces (kcal/mol)

BEs for 4-nitroaniline and corresponding gold surfaces (J/mol)

Au(111) Au(110) Au(100)

3.311 3.003 3.337

2.957 2.397 2.920

11.702 14.947 11.945

48961.17 62538.25 49977.88

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NaBH4 reduction of p-nitroaniline to p-phenylenediamine at different temperatures. Different amounts of these nanocrystals were used for the reaction so that the total particle surface area is the same for all the samples. Rhombic dodecahedra were found to exhibit the highest reduction rate, followed by those of nanocubes and octahedra in the temperature range of 25−36 °C. However, the reaction rate for octahedra increases significantly at higher temperatures and becomes faster than that for nanocubes at 40 °C. The rate constants and activation energies for the three gold nanostructures were determined. Rhombic dodecahedra have the lowest activation energy among the three particle shapes. DFT calculations show that the binding energy between p-nitroaniline and the Au(110) plane is highest among the three surfaces of gold nanocrystals. The DFT results support the experimental observations that the relative catalytic activity for the reduction reaction follows the order of {110} > {100} > {111}. Thus, rhombic dodecahedral gold nanocrystals are very valuable because they are catalytically highly efficient.

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ASSOCIATED CONTENT

S Supporting Information *

XPS spectra, additional UV−vis absorption spectra, and tables of particle weights, total surface areas, rate constants, and activation energies. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Council of Taiwan for the support of this research (NSC 98-2113-M-007-005-MY3). We also thank Professor Chin-Hui Yu for helpful discussion.

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dx.doi.org/10.1021/jp307768h | J. Phys. Chem. C 2012, 116, 23757−23763