Synthesis of Amphiphilic Ionic Liquids Terminated Gold Nanorods and

Ionic Liquids Terminated Gold Nanorods and Their Superior Catalytic Activity ... Colloid and Interface Chemistry, Shandong University, Ministry of...
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Synthesis of Amphiphilic Ionic Liquids Terminated Gold Nanorods and Their Superior Catalytic Activity for the Reduction of Nitro Compounds Xiangtao Bai, Yanan Gao, Hong-guo Liu, and Liqiang Zheng* Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education, Jinan, 250100, China ReceiVed: July 06, 2009; ReVised Manuscript ReceiVed: August 04, 2009

Uniform gold nanorods were prepared via a three-step seed-mediated growth method using a long-chain ionic liquid (IL), 1-dodecyl-3-methylimidazolium bromide (C12mimBr), as a capping agent. Both AgNO3 and HNO3 were used in the synthesis process. The aspect ratio, R, of the nanorods was increased when AgNO3 was replaced by HNO3. HRTEM revealed that these well-crystallized nanorods are all enclosed by five {100} facets and their cross section is pentagon. The interaction energies between the individual surfactants and different gold crystalline planes were calculated using a molecular dynamics simulation. The results showed that the interaction energies between the C12mimBr and different gold crystalline planes were smaller than those of CTAB based system. The catalytic experiments showed that the short gold nanorods had excellent catalytic efficiency for the reduction of nitro compounds. Introduction Size- and shape-controlled metal nanoparticles have received much attention during the past several decades as they play important roles in various areas of materials science. These metal nanoparticles, especially gold nanorods, have been widely exploited for use in catalysis,1,2 biosensing,3 and optics4 due to their interesting physicochemical and optoelectronic properties. Because their longitudinal plasmon band is highly sensitive to their aspect ratios, gold nanorods have many potential applications.5,6 They may be used as scattering7,8 and twophoton-absorption9 chromophores for biological imaging of blood vessels and cancer cells. They also have some potential uses in biological sensors,10 carriers for drug delivery,11 and agents for photothermal cancer therapy.8,12 Many methods have been developed for preparing Au nanorods, such as electrochemical deposition,13 photochemistry,14 and seed-mediated synthesis.15,16 The Ag(I)-mediated seeded-growth procedure was first performed by Murphy.15 Afterward, the method had been widely used in the preparation of gold nanorods. The introduction of Ag+ in growth solutions significantly improves the yield of Au nanorods.17-19 synthetic additives were also used instead of AgNO3, such as nitric acid20 and halide,21 and uniform and monodispersed gold nanorods were synthesized in large quantity. In the past few years, the synthesis of inorganic materials using ionic liquids (ILs) has attracted increasing interest.22-26 Various shaped gold nanomaterials have been obtained in the presence of ILs, such as dendrite nanostructures,22 microscale nanoplates,24 and caplike nanosheets.27 Many reports are focused on the use of dodecyltrimethylammonium bromide (CTAB) as a capping agent. Recently, amphiphilic long-chain ILs have received considerable attention because of their advantages over traditional surfactants in the preparation of ordered selforganized structures. For example, long-chain ILs have been used as synthetic templates to prepare mesoporous silica.28-30 Our group has reported the properties of the Langmuir mono* Towhomcorrespondenceshouldbeaddressed.E-mail:[email protected]. Phone: +86-531-88366062. Fax: +86-531-88564750.

layer formed by a long-chain IL, 1-hexadecyl-3-methylimidazolium bromide (C16mimBr) and used it as templates to synthesize single crystalline Au nanoplates.31 Using gold nanoparticles for various catalysis reactions has been widely reported.32-34 Because of high surface-to-volume ratio and high surface energy, their surface atoms are very active and can act as excellent catalysts in organic synthesis reactions. The nanoparticles used as catalysts are usually supported on a substrate or stabilized by surfactant molecules.33 In addition to the size and shapes of gold nanostructures, the support substrates and the surfactants are also vital parameters for catalysis. Furthermore, other reports conclude that the surface functional groups also influenced catalytic behavior of metal nanoparticles.35,36 The strong interaction between Au nanoparticles and different functional groups prevents Au nanoparticles from aggregation, but the interaction weakens the catalytic activity of gold nanoparticles. The reduction of nitro compounds with an excess amount of NaBH4 has often been used as a model reaction to examine the catalytic performance of metal nanoparticles.1,2 For example, Liang et al.1 studied the catalysis of nitro compounds with different shaped nanoparticles. They found that the reaction rate was fastest in the presence of Au nanospheres and slowest for Au nanorods, whereas the reaction rate was intermediate when Au nanoprisms were used instead. Mandal et al.2 reported that the polygonal gold nanoparticles have higher catalytic activity by a factor of 300-1000 in nitrophenol reduction reaction compared to spherical nanoparticles. Herein, we report the growth of gold nanorods with uniform size Via a three-step seed-mediated growth method as reported by Murphy,15 but a long-chain IL, 1-dodecyl-3-methylimidazolium bromide (C12mimBr), was used instead of CTAB. HRTEM reveals that these well crystallized nanorods are all enclosed by five {100} facets and their cross section is pentagon. The interaction energies between the individual surfactants and different gold crystalline planes were calculated using a molecular dynamics simulation. We also tested the catalytic activity of these nanorods and the result shows that the nanorods have excellent catalytic efficiency for the reduction of nitro compounds.

10.1021/jp906378d CCC: $40.75  2009 American Chemical Society Published on Web 09/02/2009

Amphiphilic Ionic Liquids Terminated Gold Nanorods Experimental Section Chemicals. Long-chain IL, C12mimBr, was synthesized according to the literature.37-39 All chemicals for the synthesis of IL were purchased from ACROS and used as received. In a typical synthesis, an excess of 1-dodecanebromine (31.90 g, 0.128 mol) was mixed with 1-methylimidazole (10.26 g, 0.125 mol), an appropriate amount of dichloromethane was added as solvent. The mixture was put into a 250 mL flask, refluxed at 75-80 °C for 48 h, and then cooled to room temperature. Dichloromethane and the unreacted reactants were removed under reduced pressure, leaving a white waxy solid. The product was further purified by recrystallization from ethyl acetate at least four times and dried under vacuum for 24 h. The purity of the product was ascertained by a 1HNMR spectrum in CDCl3. Tetrachloroaurate trihydrate (HAuCl4 · 3H2O, 99.99%), potassium borohydride (KBH4, >98%), and L-(+)-ascorbic acid (>99%) were purchased from Shanghai Chemical Reagent Co. Ltd. and used as received. All other chemicals were analytical grade and used without further purification. Preparation of Initial Gold Seed Particles. Gold nanorods were synthesized using a seed-mediated method, as described by Murphy et al.15,40 Spherical Au nanoparticle seeds were first prepared as the following: 0.25 mL of 0.01 M HAuCl4 aqueous solution and 7.5 mL of 0.05 M C12mimBr aqueous solution were added into a 20 mL glass vial. The solution was mixed by mild stirring. A portion (0.6 mL) of 0.01 M ice-cold NaBH4 aqueous solution was then poured into the above solution, followed by rapid mixing. The seed solution was kept at room temperature and used within at least 4 h after preparation. Three-Step Seeding Synthesis of Gold Nanorods. Growth solution was prepared by mixing 24 mL of 0.1 M C12mimBr and 24 mL of 0.001 M of HAuCl4 aqueous solutions. This growth solution (total 48 mL) was divided into three parts and labeled A (4 mL), B (4 mL), and C (40 mL). To the growth solution A and B, 50 µL of 0.1 M ascorbic acid was added. However, 500 µL of 0.1 M ascorbic acid and an appropriate concentrated AgNO3 or HNO3 were added to the solution C. After that, 200 µL of seed solution was added into the growth solution A, and stirred for 10 s. Then, 200 µL of solution A was transferred to B and stirred for 10 s. Finally, an appropriate amount of growth solution B (typically 1 mL) was transferred to C and mixed for 5 s. The solution was kept at room temperature for 12 h. Short gold nanorods were obtained in the presence of AgNO3. These short gold nanorods were concentrated and separated from the small nanospheres by centrifugation. Then they were redispersed in water for further characterizations. Relatively long gold nanorods were prepared when using HNO3 in the synthesis process, which were precipitated at the bottom of the tube after reaction. The supernatant was poured out and the precipitation was redispersed in water. (Scheme shown in Figure S1, Supporting Information) Characterization. The gold nanorods were characterized by transmission electron microscopy (TEM) (JEM-100CX II (JEOL)), high resolution TEM (HRTEM) (JEM-2100), scanning electron microscopy (SEM) (JEOL JSM-7600F), UV-vis spectroscopy (HITACHI U-4100), and X-ray powder diffraction (XRD; D8 Advance X-ray Diffractometer, Cu KR radiation source (40 kV)). Numerical Simulation. Molecular dynamics (MD) simulation in canonical (constant atom number, volume, and temperature, NVT) ensemble was employed to calculate the interaction energies between the surfactants and different gold crystalline planes. The simulation method and structural analysis are similar to Zeng’s work.41 The interaction energies between the indi-

J. Phys. Chem. C, Vol. 113, No. 41, 2009 17731 vidual surfactants (i.e., C12mimBr and CTAB) and different gold crystalline planes (i.e., (100), (110), and (111)) were calculated. In order to obtain the interaction energy for each gold-surfactant system, three models were established: (1) the three-dimensional model consists of one gold crystal plane and a certain amount of surfactant molecules on the gold surface; (2) gold crystal plane only, and (3) surfactants only. All simulations were performed by using the Discover Module of Materials Studio 4.4. The COMPASS force field was used to calculate the interaction potential energy. All the systems were subjected to energy minimization for the structural optimization before MD simulation. The NVT simulations lasted for 50 ps with a time step of 1 fs. Simulation data were collected in the last 20 ps for statistical and structural analysis. Catalysis of p-Nitroaniline and p-Nitrophenol by Short Au Nanorods. In a typical catalysis reaction, 300 µL of 10-3 M p-nitroaniline or p-nitrophenol stock solution was mixed with 2.4 mL of deoinized (DI) water and stirred for 1-2 min for thorough mixing. After that, 300 µL of the obtained Au nanoparticles solution was added and mixed well. Finally, 300 µL of 0.1 M ice-cold solution of KBH4 was added to the reaction mixture. The reaction was monitored with a UV-vis-NIR spectrophotometer. The absorption spectra were recorded every 4-10 min. Results and Discussion Formation of Au Nanorods. The existence of AgNO3 is essential for the preparation of Au nanorods in high yield, although it is not clear how it works.40,42 Here we also used this Ag(I)-mediated seeded-growth procedure to prepare the Au nanorods, but C12mimBr was used as a replacement of CTAB. Figure 1 shows the typical TEM images of the gold nanorods obtained under different conditions, together with their corresponding size distribution analyses. A large quantity of small gold nanorods can be observed. When the concentration of AgNO3 and C12mimBr in the growth solution was 0.75 mM and 0.05 M, respectively, very short Au nanorods were obtained in high yield, accompanied by a small amount of Au spherical or polyhedral nanoparticles, as shown in Figure 1a. When the concentration of AgNO3 was increased to 1.25 mM but the concentration of C12mimBr was left constant, a large quantity of nanorods was also obtained, as shown in Figure 1c. The nanorods obtained when the concentration of C12mimBr was increased to 0.1 M and the concentration of AgNO3 was kept constant are shown in Figure 1e. The particle size distributions were evaluated from the image analysis; the average lengths of the Au nanorods were determined to be 23.7 ( 2.4, 22.4 ( 2.6, and 24.9 ( 3.6 nm, as shown in Figure 1b,d,f, respectively, and the diameters were determined to be 9.6 ( 0.9, 8.0 ( 1.4 and 6.8 ( 1.6 nm, respectively. So the aspect ratios R (R ) L/D) are about 2.5, 2.8 and 3.7, respectively. All the data indicated their uniformity in size. In addition, we need to point out that the variation of the size is not caused by the different three-dimensional orientation of these nanorods. It can be seen from Figure 1a,c that the concentration of Ag+ slightly affects the size of the nanorods. However, no nanorods were found without AgNO3 in the reaction system. It can be seen from Figure 1a,e that the concentration of C12minBr has a greater influence on the size of the nanorods, which is different from the CTAB-based system, in which the existence of Ag+ is not necessary for the formation of gold nanorods, but necessary for the high yield of nanorods.42 It has also been proved that at least 0.1 M CTAB is necessary.15,40,42 To the best of our knowledge, there is no report on how the concentration of CTAB

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influences the diameter of Au nanorods. Most studies are focused on how to get a longer nanorod, resulting in a large aspect ratio, R. In our experiments, there is no obvious difference in the length of the nanorods, the difference of the aspect ratio, R, is mainly caused by the diameter of nanorods. What causes the difference between CTAB and C12mimBr? We think that it may be because the different interaction energies (U) of surfactants on the different crystal planes of Au (such as (111) and (110)). The adsorption of surfactant molecules on a gold surface can be investigated by the MD simulation. Theoretically, molecular adsorption can be reflected from the interaction strength between the molecules and crystal surface. In order to verify this deduction, the interaction energies (U) were calculated for different surfactant-gold plane systems, given by

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U ) Etotal - Esurface - Esaa

(1)

where Etotal is the total energy of the crystal surface and surfactant, Esurface is the surface energy after removal of the surfactant, Esaa is the energy after removal of the surface. The calculated potential energy can reveal the interaction feature between the surfactant and gold surface; a negative value indicates an attractive interaction while a positive value means a repulsive interaction. The results of the MD calculation are listed in Table 1. We can see that all the interaction energies between the C12mimBr and different crystal planes of Au are smaller than those of CTAB, suggesting that the interaction between C12mimBr and Au crystal planes is weaker than that of CTAB. On the other hand, the hydrophilic group of C12mimBr is also much bigger than that of CTAB. Therefore, we can infer

Figure 1. TEM micrographs of as-prepared gold nanorods synthesized under the different conditions (a) 0.75 mM AgNO3, 0.05 M C12mimBr; (c) 1.25 mM AgNO3, 0.05 M C12mimBr; (e) 0.75 mM AgNO3, 0.1 M C12mimBr and their corresponding size histograms (b, d, f).

Amphiphilic Ionic Liquids Terminated Gold Nanorods TABLE 1: The Calculated Interaction Energies (kcal mol-1) for Various Surfactant-Gold Plane Systems Au(111) Au(110) Au(100)

CTAB

C12mimBr

-216.5 -134.1 -131.9

-174.1 -148.4 -65.3

that the bilayers formed by C12mimBr on different Au crystal planes may be much looser than those formed by CTAB. In other words, the restriction of C12mimBr bilayer for crystal growth is smaller than CTAB. Therefore, the nanorods are dominated by both the concentration of Ag+ and C12mimBr. Because of the weaker interaction of C12mimBr with Au, only short Au nanorods can be obtained. According to the literatures, most of the Au nanorods have a polyhedral structure and the cross section is octagonal or pentagon shaped.43,44 Figure 2a shows a typical HRTEM image of the Au nanorods. The lattice spacing measured from Figure 2a is about 0.207 nm, corresponding to the d200 of Au. Figure 2b shows the electron diffraction of the Au nanorod. The diffraction rings correspond to {111}, {200}, {220}, {311}, and {331} from the center to the outside, respectively. Compared with Mann’s44 report, we believe that each Au nanorod is enclosed by five {100} facets and its cross section is a pentagon. The MD simulation has revealed that the interaction energy between the C12mimBr and Au {100} planes is much lower compared to CTAB, so a high concentration of C12mimBr is favorable to direct the gold nanorods to grow along a certain orientation. Therefore, at an increased C12mimBr concentration, a larger R, can be obtained (from 2.5 to 3.7, as shown in Figure 1a,e). The face-centered cubic (fcc) structure of Au nanorods was confirmed by the X-ray diffraction (XRD) pattern in Figure 2c. The XRD peaks of the nanorods at 38.21, 44.41, and 64.61° could be assigned as (111), (200), and (220) reflection lines, which were in agreement with the diffraction standard of Au (JCPDS 04-0784). Diffraction peaks from the {111} (nanorods ends) and {200} (nanorods body) faces are identical to those of other Au nanorods with different length and width dimensions. Wu et al.20 reported that the addition of nitric acid is more conducive to generate the gold nanorods with large R in high yield. Recently, Huang and co-workers16 have reported the synthesis of gold nanowires with tunable diameters from 16 to 66 nm and lengths up to 10 µm using a room temperature acidic solution route. In our case, when nitric acid was used instead of AgNO3, Au nanorods with large R can also be obtained, as shown in Figure 3a. These nanorods are straight and very uniform in dimensions with an average length of about 200 nm, and the average R of about 15. Both the length and R are smaller than Wu’s report, consistent with the above analysis of short Au nanorods. Some nanoplates were also observed as byproducts in the products. This is a typical problem in the growth process of long nanorods.16,20,43 The optical properties of these gold nanorods were characterized by UV-vis absorption spectroscopy. Figure 3b shows the UV-vis absorption spectrum of the two gold nanorods obtained in the presence of AgNO3 and HNO3, respectively. The short gold nanorods show two distinct plasmon absorption bands around 525 and 780 nm (Figure 3b, curve 1). As is known, gold nanorods typically exhibit a transverse surface plasmon resonance (SPR) absorption band at about 510-520 nm and a longitudinal SPR band that is red shifted to the near-IR region when the Au nanorods have high R values.45 Spherical Au particles also show an SPR band at approximately 520 nm and the band is usually red shifted to a longer wavelength with increasing the particle size.46 As there

J. Phys. Chem. C, Vol. 113, No. 41, 2009 17733 are some Au spherical byproducts, we cannot distinguish which led to the absorption at 525 nm. The absorption band around 780 nm is due to the longitudinal SPR absorption. This is in agreement with a recent report that used the discrete dipole approximation method for the determination of the position of longitudinal SPR band for gold nanorods

λmax ) 96R + 418

(2)

where R refers to the aspect ratios of Au nanorods.6 With an average R of 3.7, the predicted position of the longitudinal SPR band is at 773 nm, which is in agreement with our experimental data. The longer nanorods obtained in the presence of HNO3 show one absorption band at 590 nm (Figure 3b, curve 2), which is also due to either the transverse SPR of gold nanorods or the spherical byproducts. The longitudinal SPR absorption of the nanorods is observable in the near-IR region. From the inset of Figure 3b, it can be seen clearly that there is an absorption peak starting from about 1500 nm. Catalytic Activities of Short Gold Nanorods in the Borohydride Reduction of Nitro Compounds. P-aminophenol is an important intermediate in organic synthesis, medicine, and dyes. It was widely used in the production of medicines, such as paracetamol and clofibrate. It also can be used for dyes preparation, such as azo dyes and sulfur dyes. P-aminophenol is mainly prepared by catalytic hydrogenation of p-nitrophenol. It is known that anisotropic gold nanoparticles exhibit better catalytic activities over their spherical counterparts.2 Thus, we have examined the performance of these short gold nanorods as catalysts for the borohydride reduction of p-nitrophenol into the corresponding amino derivative. It has been reported that in the absence of gold nanoparticles the mixture of p-nitrophenol and potassium borohydride (KBH4) showed an absorption band at λmax ) 400 nm corresponding to the p-nitrophenolate. This peak remained unaltered with time, suggesting that the reduction did not take place in the absence of a catalyst.2 However, the addition of a small amount of purified gold nanoparticles to the reaction mixture caused fading and ultimate bleaching of the yellow reaction mixture in quick succession. The UV-vis absorbance at 400 nm of p-nitrophenolate was also observed in our investigated system. Time-dependent absorption spectrum of this reaction mixture showed a gradual disappearance of the peak when catalyzed by the as-prepared short Au nanorods, that is, sample in Figure 1a (Figure 4a). The result indicates that the gold nanorods can successfully catalyze the reduction reaction. In this reduction process, as the concentration of KBH4 in the reaction mixture far exceeds the concentration of p-nitrophenol, the rate is assumed to follow first order kinetics. The logarithm of the absorbance of p-nitrophenolate at 400 nm (ln A) will then decrease linearly with reaction time. The apparent rate constant (ka) of the catalytic reaction can be calculated from a linear regression of the slope of ln A versus time. Figure 4b shows the plot of ln A versus time for Figure 4a. The plot is a straight line, indicating that the reduction reaction follows first order kinetics. The first-order rate constant, ka, was calculated to be 7.4 × 10-2 min-1 for sample in Figure 1a. The same procedure (see Supporting InformationFigure S2) was also carried out to obtain the ka for samples shown in Figure 1c,e and ka is about 6.8 × 10-2 min-1 and 5.8 × 10-2 min-1, respectively. This is comparable with Mandal’s report,2 but we need to note that very little amount of catalysts are used in our experiments, which will be discussed in the following. The catalytic properties of the gold nanorods shown in Figure 1a were also examined for the reduction of another aromatic

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Figure 2. (a) A typical HRTEM image of final gold nanorods. The measured distance is the distance of {200}. (b) The corresponding electron diffraction pattern of the Au nanorods (1 to 5 corresponding to {111} {200} {220} {311} and {331}). (c) X-ray diffraction of Au nanorods with diffraction peaks indexed to the crystalline Au0 nanorods. The (111) face corresponds to the end of the nanowires..

Figure 3. TEM image of nanorods synthesized in the presence of HNO3 (a) and UV-vis-NIR spectra of the nanorods obtained in the presence of AgNO3 and HNO3, respectively (b); the inset shows NIR range of curve 2.

nitro compound, p-nitroaniline. Its reduction product, p-phenylenediamine, is a widely used intermediate. It can be used in the preparation of azo dyes, fur dyes, polymers, rubber antioxidant, and photo developer. Using the same procedure (Figure 4c,d), the first-order rate constant, ka, was calculated to be 4.4 × 10-2 min-1. These results reveal that the reduction reaction rate is comparable with those using CTAB capping Au nanorods as catalyst.1,2 The number of nanoparticles in solution was calculated according to Liang’s1 three assumptions. First, most of the gold particles in the solution were in same shape and mostly had the same size. Second, nanorods and bulk gold has the same density. Finally, the yield of nanoparticles was assumed to be 100%. On the basis of these assumptions, the approximate number of the nanorods in our system is about 1011 mL-1, that

is, 10-9 mmol/mL. It is worthy noting that the number of nanorods in our catalytic reaction system is much less than that reported by Liang’s1 work in which 8.7 × 1019 mL-1 gold nanorods catalysts were used to reach ka of 1.53 × 10-2 min-1, a similar value with our work. The aspect ratio, R, of the nanorods they used is about 2.8, very close to our nanorods. The length of nanorods (about 45 ( 5 nm) is even larger than ours. Why do our nanorods have such a high catalytic efficiency? It is well known that there are two parameters that affect the interface catalytic reaction, the available active surface area for adsorption and the number of nanoparticles presented per unit volume. Both the number of the particles and the surface area are smaller in our investigated system! As we know, there are many catalyst active sites on the surface of metal surface. It is

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Figure 4. (a) Successive UV-vis absorption spectra of the borohydride reduction of p-nitrophenol catalyzed by the as-prepared gold nanorods. (b) Plots of ln A (A ) absorbance of p-nitrophenolate at 400 nm) versus time for the reduction of pnitrophenol catalyzed by gold nanorods. (c,d) Successive UV-vis absorption spectra and Plots of ln A versus time for the p-nitroaniline based system.

the existence of these active sites that causes the catalytic activity. The reactants must be combined with the active sites so that the reaction will proceed. However, there is the competitive adsorption between the reactants and the capping reagents. As we mentioned above, the interaction between the C12mimBr and Au crystal planes is weaker than that of CTAB. So the desorption of C12mimBr from Au crystal planes should be much easier than that of CTAB. Thus the gold nanorods capped with C12mimBr may provide more active sites in the same area. Moreover, there may be some synergistic effect between the C12mimBr and gold nanorods. So the nanorods have a high catalytic activity. This is just our initial speculation and the mechanism has to be further researched in the future work. To confirm our conclusion, we also prepared polygonal gold nanoparticles with C12mimBr as capping agent. The morphology and size of these polygonal nanoparticles are shown in Supporting Information, Figure S3. Under the same conditions, the ka is about 1.7 × 10-1 min-1, which is also faster than Liang’s1 report. This suggests that the catalysis activity of our Au nanospheres is also higher, further confirming that the gold nanoparticles capped with C12mimBr can provide more active sites than those capped with CTAB. Conclusions In summary, the uniform-sized gold nanorods have been prepared via a three-step seed-mediated growth method using a long-chain ionic liquid (IL), C12mimBr, as a capping agent. The aspect ratio, R, of the nanorods was increased when HNO3 was used to replace AgNO3 in the synthesis process. HRTEM revealed that these well-crystallized nanorods are all enclosed

by five {100} facets and their cross section is pentagon. Molecular dynamics simulation showed that the interaction energies between the C12mimBr and different gold crystalline planes were smaller than those of CTAB based system. Because of this reason, the Au nanorods prepared with C12mimBr have more catalyst active sites and thus exhibited excellent catalytic efficiency for the reduction of nitro compounds. Acknowledgment. The authors are grateful to the National Natural Science Foundation of China (No. 20773081 and Z2007B06) and National Basic Research Program (2007CB808004 and 2009CB930101). Supporting Information Available: A flow diagram of seed-growth method, UV-vis spectra of different catalytic experiments of Au nanoparticles are provided. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kundu, S.; Lau, S.; Liang, H. J. Phys. Chem. C 2009, 113, 5150– 5156. (2) Rashid, M. H.; Mandal, T. K. AdV. Funct. Mater. 2008, 18, 2261– 2271. (3) Parab, H. J.; Chen, H. M.; Lai, T.-C.; Huang, J. H.; Chen, P. H.; Liu, R.-S.; Hsiao, M.; Chen, C.-H.; Tsai, D.-P.; Hwu, Y.-K. J. Phys. Chem. C 2009, 113, 7574–7578. (4) Zheng, Y. B.; Jensen, L.; Yan, W.; Walker, T. R.; Juluri, B. K.; Jensen, L.; Huang, T. J. J. Phys. Chem. C 2009, 113, 7019–7024. (5) Pe’rez-Juste, J.; Pastoriza-Santos, I.; Liz-Marza’n, L. M.; Mulvaney, P. Coord. Chem. ReV. 2005, 249, 1870–1901. (6) Brioude, A.; Jiang, X. C.; Pileni, M. P. J. Phys. Chem. B 2005, 109, 13138–13142.

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