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Shape-Controlled Catalysis by Cetyltrimethylammonium Bromide Terminated Gold Nanospheres, Nanorods, and Nanoprisms Subrata Kundu,* Sean Lau, and Hong Liang* Materials Science and Mechanical Engineering, Texas A&M UniVersity, College Station, Texas 77843-3123 ReceiVed: December 22, 2008; ReVised Manuscript ReceiVed: February 9, 2009
Shape-controlled Au nanospheres, nanorods, and nanoprisms have been utilized for the first time for selective chemical reduction of different aromatic nitro compounds to the corresponding amino derivatives at room temperature. Careful observation reveals that the reaction was fastest with nanospheres and slowest with nanorods, whereas the rate was intermediate with nanoprisms when keeping the numbers of particles approximately the same. Controlled experiments revealed that our reaction followed the nitroso and hydroxylamine pathway. The yield of the product was very high, and the method should be applicable for very fast catalysis reaction of other nitro compounds. 1. Introduction Synthesis of size- and shape-controlled metal nanoparticles (NPs) has been found to be an active area of research because of the potential applications in chemistry,1 physics,2 biology,3 medicine,4 and electronics.5 NPs, due to their quantum effects and extremely high surface-to-volume ratio, act as excellent catalysts in organic synthesis reactions. The catalytic properties of the NPs depend on the size and shapes of the particles. With the change in size and shapes, the catalytic property also changes. This can be attributed in part to the potential binding sites presented by the atoms situated at the corners and the edges of NPs. It is reported that the rate of a catalyzed reaction increases exponentially with the percentage of these atomic sites.6 It is a major challenge for researchers to understand how to control NP shape in order to maximize the number of coordinatively unsaturated surface sites. The catalytic properties of spherical metal NPs such as Au,7,8 Pt,9 Cu,10 Pd,11 etc. have been studied. Recent reports are Au NPs supported on TiO2,12 polymer-supported Pt carbonyl clusters,13 and carbon nanofiber supported Pt and Pd NPs for catalysis reactions.14 Saha et al.10 reported the chemoselective reduction of aromatic nitro compounds to the corresponding amine derivatives using Cu NPs at 120 °C. Furthermore, other reports conclude that the surface functional groups also influenced catalytic behavior of metal NPs.15,16 The strong interaction between Au NPs with different functional groups such as thiol or pyridyl groups prevent Au NPs from aggregation and provide smaller Au NPs, although these types of strong electronic interaction weaken the catalytic activity of Au NPs. Other reports indicate that the catalytic performance of Au NPs on polymers would be appreciably affected by both the size of the Au NPs and the kinds of polymer structures.17 These Au/polymer beads showed excellent catalytic activity in the decomposition of hydrogen peroxide (H2O2) and the oxidation of glucose with H2O2. In particular, Au NPs highly dispersed on polymers showed higher catalytic activity despite of larger size of Au than those of Au NPs supported on nitrogencontaining polymers such as polyaniline and melamineformaldehyde resin. The reduction of nitro compounds to * To whom correspondence should be addressed. E-mail: skundu@ tamu.edu (S.K.);
[email protected] (H.L.). Phone: 979-862-2578. Fax: 979845-3081.
corresponding amino derivatives with an excess amount of NaBH4 has often been used as a model reaction to examine the catalytic performance of metal NPs. For example, the catalytic activity of Au NPs (8-55 nm) supported on ion-exchange resins has been studied by Pal and co-workers.18 The rate constant increased with a decrease in the size of Au and was proportional to the total surface area of Au NPs. The polymer brush stabilized Au clusters successively improved the catalytic activity compared to that of Pt and Pd NPs.19 In most of the above reports, researchers used only spherical NPs to check their catalytic efficiency, and the reduction took a very long time to complete. In this present report we demonstrated for the first time the shape effects of the Au NPs on the catalysis reaction of aromatic nitro compounds. We synthesized cetyltrimethylammonium bromide (CTAB) stabilized positively charged Au nanospheres, nanorods of different aspect ratios, and nanoprisms for the catalytic reduction of aromatic nitro compounds. Among the Au particles studied, for a particular reaction condition keeping the number of particles the same, the catalytic rate was found to be fastest in the presence of Au nanospheres, medium with nanoprisms, and the slowest with the nanorods. To the best of our knowledge, this is the first example to study the shape effects on catalysis reactions for the reduction of various aromatic nitro compounds. The present process is straightforward, simple, reproducible, and cost-effective. 2. Experimental Section 2.1. Reagents. CTAB (99%), hydrogen tetrachloro aurate, trihydrate (HAuCl4 · 3H2O, 99.9%), and sodium hydroxide (NaOH) were purchased from Sigma-Aldrich and used without further purification. 2,7-Dihydroxy naphthalene (2,7-DHN) was also purchased from Sigma-Aldrich and recrystallized in hot water. Ascorbic acid, sodium borohydride (NaBH4), and trisodium citrate were also purchased from Sigma-Aldrich. Different nitro compounds also purchased from sigma and used as received. Deionized (DI) water was used for all synthesis reaction as well as in the catalysis study. 2.2. Instruments. All of the UV-visible (UV-vis) absorption spectra were recorded in a Hitachi (model U-4100) UV-vis-NIR spectrophotometer equipped with a 1-cm quartz cuvette holder for liquid samples. High-resolution transmission electron microscopy (HR-TEM) (ZEOL ZEM 2010) was used
10.1021/jp811331z CCC: $40.75 2009 American Chemical Society Published on Web 03/11/2009
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TABLE 1: Study of the Rate Constant Values, Percent Yield, and Catalytic Efficiency with Different Au NPs as Catalysts
nano component
substrate 4-NA 4-NA 4-NA 4-NA
Au Au Au Au
approximate available first-order time for full number of particles product and surface area rate constant reduction per unit volume percent (nm2) (k, min-1) (min) (per mL) conversion
nanosphere, (diameter 45 ( 5 nm) nanoprisms, side length 60 nm, width ≈ 4-5 nm nanorod, aspect ratio, 33 ( 0.5 nanorod, aspect ratio, 2.8 ( 0.2
at an accelerating voltage of 200 kV. The NMR spectrum of the reduced products after catalysis was done with an Inova 600 NMR instrument with a frequency of 600 MHz for 1H detection. A domestic microwave (MW) oven (Gold star company, EM-Z200S, 1000 W, 60 Hz) was used for the synthesis of gold nanoprisms. 2.3. Synthesis of Gold Nanospheres, Nanorods, and Nanoprisms. The Au nanospheres and nanorods were synthesized using a slight modification of the seed mediated approach reported by Jana et al.20 For this synthesis we first prepared the gold seed NPs, and then we prepared the larger gold nanospheres and nanorods. The gold nanoprisms were synthesized using our published microwave heating method.21 2.4. Synthesis of Au Seed..20 A 20-mL aqueous solution containing 2.5 × 10-4 M HAuCl4 and 2.5 × 10-4 M trisodium citrate was prepared in a flask. Next, 0.6 mL of ice-cold 0.1 M NaBH4 solution was added to the solution all at once with continuous stirring. The solution turned pink immediately after adding NaBH4, indicating particle formation. The particles in this solution were used as seeds within 2-5 h after preparation. The UV-vis spectrum showed an absorption band maximum at ∼502 nm. The average particle size measured from a transmission electron micrograph was 4 ( 0.7 nm. The citrate serves only as capping agent since it cannot reduce gold salt at room temperature (25 °C). 2.5. Synthesis of Spherical Au NPs..20 Seed-mediated growth techniques were used to prepare three different growth solutions. The first two solutions (1 and 2) contained 0.25 mL of 10 mM HAuCl4, 0.05 mL of 100 mM NaOH, 0.05 mL of 100 mM ascorbic acid, and 9 mL of a 7.5 × 10-2 M CTAB solution. The third solution (3) contained 2.5 mL of 10 mM HAuCl4, 0.50 mL of 100 mM NaOH, 0.50 mL of 100 mM ascorbic acid, and 9 mL of CTAB solution. The nanosphere formation was initiated by adding 1 mL of the preformed seed solution to growth solution 1. After 5 min, one mL of resultant solution 1 was mixed into solution 2 and then again after 5 min all of the resulting growth solution in 2 was added to 3. After the addition, the color of 3 changed from colorless to deep magenta-purple over a period of 30 min. The solution exhibited a plasmon resonance peak at 535-540 nm and had nanospheres with an average diameter of 40-45 nm. This solution was then centrifuged at 8000 rpm for 20 min to remove the excess CTAB. Finally, the precipitated gold nanospheres were redispersed in D.I. water for characterization. 2.6. Synthesis of Smaller (2.8 ( 0.2) Aspect Ratio Au Nanorods..20 Gold nanorods with different aspect ratios were synthesized using slight modification of the seeding protocol as described by Jana et al.20 previously. Briefly, in a clean test tube, 10 mL of growth solution containing 2.5 × 10-4 M HAuCl4 and 0.1 M CTAB was mixed with 0.05 mL of 0.1 M freshly prepared ascorbic acid solution. Next, 0.025 mL of the 4 ( 0.7 nm seed solution was added without further stirring. The solution color changed to reddish brown within 5-10 min. The solution contained 2.8 ( 0.2 aspect ratio rods and a few spheres and plates. The rods were separated by successive
64 86 100 120
6400 3837.7 33900 1470
1.1 × 1020 9.1 × 1019 3.87 × 1018 8.7 × 1019
2.76 × 10-2 2.08 × 10-2 1.82 × 10-2 1.53 × 10-2
p-PDA, p-PDA, p-PDA, p-PDA,
100 100 100 100
centrifugation. The solution was stable for more than three month under ambient conditions. 2.7. Synthesis of Higher (33 ( 0.5) Aspect Ratio Au Nanorod..20 A three-step seeding method was used for this nanorod preparation. Three different test tubes (labeled A, B, and C), each containing 9 mL of a growth solution consisting of 2.5 × 10-4 M HAuCl4 and 0.1 M CTAB, were mixed with 0.05 mL of 0.1 M ascorbic acid. Next, 1.0 mL of the 4 ( 0.7 nm seed solution was mixed with sample A. The color of A turned red within 3 min. After 3 min, 1.0 mL was drawn from solution A and added to solution B, followed by thorough mixing. The color of solution B turned red within 5 min. After 10 min, 1 mL of B was mixed with C. Solution C turned red in color within 15 min. The nanorods with aspect ratio 33 ( 0.5 in solution C were separated by successive centrifugation. The solution was stable for more than three month under ambient conditions. 2.8. Synthesis of Au Nanoprisms..21 For nanoprism synthesis, a solution mixture was prepared by adding 4 mL of (10-1 M) CTAB, 320 µL of (10-2 M) Au (III) solution, 280 µL of (10-2 M) 2,7-DHN, and 10 µL of (1 M) NaOH. The solution was stirred for 30 s and then irradiated by MW for 90 s. For all the above cases, the Au particles formation started after 10-20 s of MW irradiation as observed from color change of the solution mixture and from the UV-visible spectrometry. The solution became pinkish red in color after completion of the reaction indicating the presence of nanoprisms. The resulting solution was centrifuged at 3000 rpm for 10 min for the removal of excess surfactants. The precipitate was redispersed in D.I. water and centrifuged again at 2000 rpm for 5 min. This process was repeated twice. Finally, the precipitate Au NPs were collected and redispersed in water for characterization. The color remained stable for at least three months in a dark ambient environment without change in their optical properties. 2.9. Catalysis of 4-Nitroaniline (4-NA) by Au Nanospheres, Nanorods, and Nanoprisms. For a typical catalysis reaction, 600 µL of 10-3 M stock 4-NA solution was mixed with 4 mL of DI water and stirred for 1-2 min for thorough mixing. After that, 600 µL of a 0.1 M ice-cold solution of sodium borohydride (NaBH4) was added and mixed well. Finally, different volumes of Au NP solution (spheres, rods, or prisms) were added separately to the reaction mixture, keeping the number of Au particles approximately the same in all cases. The approximate number of different particles present per unit volume of solution was given in Table 1. The number of particles or the surface area of the particles in solution was calculated based on the concentration of gold salt used to make the solution. This calculation technique was based on three points into consideration. First, we assume that most of the particles in a particular solution were in same shape as confirmed from TEM images. Second, the density of bulk gold was the same as that of the NPs that had been confirmed by the similar crystal structure and bond lengths. Finally the yield of NPs was assumed to be 100%, i.e., that all the gold salt is converted to gold NPs. After the addition of Au NPs we controlled the pH
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TABLE 2: Study with Other Nitro Compounds Using Au NPs as Catalystsa substrate 4-nitrophenol
2-nitrophenol
4-chloro nitrobenzene
4-iodo nitrobenzene
Meta-dinitro benzene
2-methyl nitrobenzene
4-methyl nitrobenzene
2-methoxy nitrobenzene
nano component
time for full reduction (min)
S P LNR SNR S P LNR SNR S P LNR SNR S P LNR SNR S P LNR SNR S P LNR SNR S P LNR SNR S P LNR SNR
64 87 98 118 66 88 99 117 65 86 97 122 68 89 101 123 69 88 98 118 66 88 99 120 69 90 103 124 67 89 101 122
final product 4-aminophenol
2-aminophenol
4-chloroaniline
4-iodoaniline
meta-nitroanilne
2-methoxyaniline
4-methylaniline
2-methoxyaniline
percent conversion 100 100 100 100 100 100 98 100 93 92 88 90 90 88 86 91 100 100 98 97 88 82 78 89 100 100 92 96 90 86 80 87
a S ) Au nanosphere, P ) Au nanoprism, LNR ) long Au nanorod (aspect ratio) 33 ( 0.5), SNR ) short Au nanorod (aspect ratio ) 2.8 ( 0.2).
of the reaction mixture identical in all cases, and the reaction was monitored using the UV-vis-NIR spectrophotometer over time. The absorption spectra were recorded every 3-7 min until the completion of the reduction. After the completion of the reduction the light yellowish color of 4-NA solution turned colorless due to the formation of the reduced product p-PDA. The completion of the reaction was confirmed from the color of the solution as well as from the UV-vis-NIR spectrum. After the complete reduction, the product was purified and the yield was measured. For the other nitro compound (as shown in Table 2), we changed only the nitro compound instead of 4-NA keeping all other reaction parameters fixed as we did for 4-NA reduction. The reaction was monitored exactly the same way we did for the case of 4-NA. The exact reaction condition was shown at the beginning of this section. 3. Results and Discussion The catalysis reaction with different shaped Au NPs was monitored using a UV-vis-NIR spectrophotometer over time. The CTAB-terminated Au nanorods having aspect ratios of 2.8 ( 0.2 and 33 ( 0.5 and Au nanospheres with average diameter 45 ( 5 nm were synthesized by slight modification of seedmediated approaches.20 The Au nanoprisms with edge length 60 ( 5 nm were synthesized using MW irradiation21 as described in detail in the experimental section. CTAB, which itself might be toxic22,23 was used as a capping agent for the synthesis of the Au NPs. Moreover, CTAB-coated Au NPs are nontoxic.24,25 The toxicity appears due to the unbound CTAB on the Au NPs surface. Thus to obtain “biocompatible” Au
particles, the excess CTAB other than the bilayers25 on the particles surface should be removed. Repeated centrifugation helped to remove the excess surfactants. Figure 1 shows the TEM images of the Au nanospheres (Figure 1A), nanorods of aspect ratio 2.8 ( 0.2 (Figure 1B), nanorods of aspect ratio 33 ( 0.5 (Figure 1C), and nanoprisms of side length 60 ( 5 nm (Figure 1D). The width of the prisms calculated from the AFM image is 3.5 to 4 nm. The inset of all the TEM images shows the corresponding higher magnified images and electron diffraction patterns confirming that the particles are single crystalline. CTAB wrapped on the surface of the particles to control the growth and prevent them from aggregation. The pH of the synthesized Au nanosphere, nanorod, and nanoprism solutions was 4.54, 6.0, and 6.48, respectively. We calculated the ζ potentials of the solutions, which were within +45 to +69 mV. The UV-vis-NIR absorption spectra of the Au NPs are shown in the supporting documents. The nanospheres and nanoprisms show single absorption bands at ∼520 and ∼630 nm, respectively, whereas the Au nanorods show two absorption bands due to transverse and longitudinal oscillation.21 The rods with short aspect ratio (2.8 ( 0.2) show two bands at ∼530 and ∼720 nm, whereas the rods with long aspect ratio (33 ( 0.5) show two bands at ∼520 and ∼780 nm, respectively. The catalytic properties of different shaped gold NPs were examined for the reduction of different aromatic nitro compounds taking 4-nitroaniline (4-NA) as an example in the presence of NaBH4. The reaction was not initiated with Au NPs without NaBH4. The reaction was also very slow in the presence of NaBH4 without Au NPs. Thus, a proper combination of Au
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Figure 1. The TEM images of the Au nanosphere (∼45 ( 5 nm) (A), nanorod of aspect ratio 2.8 ( 0.2 (B), nanorod of aspect ratio 33 ( 0.5 (C), and nanoprisms of side length ∼60 ( 5 nm (D). The width of the prisms calculated from the AFM image is 3.5-4 nm. The inset of each image shows their corresponding higher magnified image and electron diffraction pattern.
NPs and NaBH4 is essential to carry out the reaction. The absorption spectrum of a mixture of 4-NA and NaBH4 shows a band at 380 nm corresponding to intermolecular charge transfer of 4-NA having similarity with others for their study of aromatic nitro compounds.26,27 This band remains almost stable after 2 days (decreasing only 3%), corresponding to only 3-4% of 4-NA converted to the final products as shown in supporting documents. With the addition of Au NPs to the mixture containing 4-NA and NaBH4, the reaction rate was accelerated and finished within 2 h. The successive decrease of the absorption spectra for the reduction of 4-NA using Au nanoprisms is shown in Figure 2A. The band at 380 nm for 4-NA decreased gradually, whereas the new band at 238 nm developed and increased gradually due to the formation of the reaction product p-phenylene diamine (p-PDA) in the solution. Similar types of absorption spectra with other shaped Au NPs were also found and are shown in supporting documents. Keeping all the experimental parameters the same and by varying the shapes of the Au NPs, we observed different catalytic rates. The reaction was fastest in the presence of nanospheres (completed within 64 min), intermediate in the presence of nanoprisms (completed in 86 min), and the slowest in the presence of nanorods (g100 min). The reaction condition maintained firstorder kinetics with respect to 4-NA. The ln(C) vs time (T) plot shows clearly a linear correlation for 4-NA reduction having first-order rate constant (k) value being 2.083 × 10-2 min-1 with respect to nanoprisms shown in Figure 2B. The correlation coefficient and standard deviation for this measurement were
0.997 and 0.045, respectively. The first-order rate equation is, ln[C] ) 0.0208t + ln[C0], where C is the concentration at time t (min) and C0 is the initial concentration. The rate constant values with other shaped Au NPs, the percent yield, the catalytic efficiency, etc., are shown in Table 1. From Table 1, it is shown that in almost all cases the reaction completes within 2 h with products yields g95%. We have studied other nitro compounds, and the results are shown in the Table 2. We have seen that our reactions with the Au NPs as catalysts are 8-10 fold faster than that reported in the literature for the reduction of nitro compounds using metal NPs as catalysts.10,28 The formation of the reduced product p-PDA was also confirmed from the 1H NMR spectra (in CDCl3) of the product (shown in Figure 3) and co-TLC studies. The 1H NMR spectrum consists of three signals at δ 7.27, 6.58, and 3.34 ppm, respectively. The δ 6.58 and δ 3.34 are due to the presence of aromatic protons and amino protons, respectively. The other signal at δ 7.27 is due to the solvent. In our study, we conducted some controlled experiments to know the role of the chemicals used for this catalysis reaction. We already discussed that the reaction was not initiated with Au NPs and 4-NA but without NaBH4. On the other hand, the reaction was very slow in the presence of NaBH4 and 4-NA without Au NPs. Thus, a proper combination of 4-NA, Au NPs, and NaBH4 is very important to carry out the reaction. We also did some experiments using surfactant media (here it is CTAB) instead of water taking different shaped Au NPs. We saw that the reaction is much slower in CTAB medium than that of
5154 J. Phys. Chem. C, Vol. 113, No. 13, 2009
Kundu et al. SCHEME 1: Proposed Reaction Mechanism with Different Shaped Au NPs as Catalysts for the Reduction of 4-NA with NaBH4
Figure 2. (A) The successive decrease of the UV-vis absorption spectra for the reduction of 4-NA using Au nanoprisms as catalysts in the presence of NaBH4. (B) Plot of ln C vs time (T) shows first-order reaction kinetics for 4-NA reduction using Au nanoprisms as catalysts.
aqueous solution. However, the catalytic rate follows exactly the same trend that we observed in aqueous medium. For example, the reaction was fastest in the presence of nanospheres, intermediate in the presence of nanoprisms, and slowest in presence of nanorods. From these control experiments we conclude that the presence of very small amount of surfactant on the NPs surfaces due to capping did not inhibit the catalytic activity of Au NPs, and we easily ruled out the possibility of different reaction rates due to CTAB capping. We also tested the effects of pH on our catalysis reaction in-detail. The pH of 2.5 × 10-3 M Au(III) ions in water was 2.5. The pH of the 4-NA solution (10-3 M), aqueous NaBH4 (0.1 M) solution, and a mixture of 4 mL of 4-NA and 0.6 mL of NaBH4 solution are 6.2, 10.7, and 10.2, respectively. After the addition of different shaped Au NPs to the solution mixture containing 4-NA and NaBH4, we measured the mixed solution pH (that varies slightly) and controlled the pH same in all cases to overcome the observed rate difference due to pH variations. The catalytic efficiency of the NPs depends mainly on two parameters. One is the available active surface area for adsorption and other is the number of NPs present per unit volume. In our reaction we fixed the number of particles (by adding different volumes) to be approximately same (see Experimental section and Table 1) in all cases. Therefore, the variation of the reaction rate or the catalytic efficiency depends mostly on
the available surface area. It has been accepted that with the increase in the surface area of the Au particles, the chances of collision increase as well. As the number of collisions increases, the reaction rate will subsequently increase as observed by others.29 Here in our study, initially the 4-NA molecules are adsorbed on the surface of the Au NPs, which play an important role in the electron transfer process. The electron transfer7,8 occurs from the negatively charged BH4- to the 4-NA via the Au NPs. The 4-NA is then reduced to p-PDA. This electron transfer process depends upon the number of particles present in the solution as well as the available surface area of the catalyst particles. The reaction rate will increase with an increase in the number of particles as well as an increase in surface area of the catalyst particles. The proposed reaction mechanism is shown in Scheme 1. The fastest reaction rate with nanospheres is probably due to the higher surface area than that of the nanoprisms and short aspect ratio nanorods. However, it is unexpected for the rods of a long aspect ratio (having the maximum surface area) to have an intermediate reaction rate. We have calculated that the number of rods present in a unit volume of the solution is less than the spheres and prisms. However, this is not sufficient to explain this phenomenon. One probable reason is that the orientation of the particles in the solution is also important. From the TEM images it is clear that the high aspect ratio rods are closely packed and not oriented separately as single rods. It was reported earlier that special orientation and packing pattern of NPs enhanced the reaction rates.30 This might be the reason for the slow reaction rate of the high aspect ratio rods compare to the spheres and prisms. Jana et al. reported earlier that the orientation of Pd NPs effected the reaction rate of a catalysis reaction.31 At this point, it is not sufficient to explain this observed rate difference of high aspect ratio nanorods. Further research will be conducted to know the details of this catalysis process. On the other hand the higher reaction rate for the prisms over nanorods can be explained by the fact that prisms have more well-defined edges and corners than the rods. Such active regions of the prisms might act as effective catalytic sites for the nitro compound reduction. Those well-defined edges and corners of the prisms are very important as they promote better understanding of the physical properties
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Figure 3. The 1H NMR spectra (in CDCl3) of the product p-PDA.
SCHEME 2: Schematic for the Reaction Pathway for Nitro Compound Reduction
nylhydroxylamine,we obtained 89 and 93% of the final product. From this experiment we conclude that our reaction most likely followed the nitroso and hydroxylamine pathway rather than the azo and hydrazo pathway. 4. Conclusion
during the electron transfer process as predicted by the theory.32-34 We did some preliminary study about the effects of reaction rate with the change in temperature. We observed that the reaction rate increased with the increase in temperature. This is probably due to the increased energy in the Au NPs. At higher temperature, the particles move more quickly leading to an increased probability of favorable collisions between each others. The reduction of the nitro compounds with Au NPs as catalysts can follow two different pathways (Scheme 2). First, the nitro compound forms a nitroso compound, which can condense with hydroxylamine to form an azoxy compound. This azoxy compound then undergoes further reduction to the azo, hydrazo, and finally the amine compound. On the other hand, the nitro compound could reduce to a nitroso compound, then to a hydroxylamine, and finally the amine compound. To find out the exact pathway we carried out our reduction reaction of 4-NA with the intermediate compounds. When we used hydrazobenzene and azo benzene instead of 4-NA and kept the same experimental conditions, we got the amine product only 22 and 26%, respectively, using nanoprisms as catalysts. Similarly, when we used nitroso benzene and phe-
In conclusion, we have demonstrated a very simple and highly efficient pathway for the reduction of different aromatic nitro compounds to the corresponding amino products using different shapes of Au NPs. Our results indicate that the reduction reaction was fastest in the presence of Au nanospheres, intermediate with nanoprisms, and slowest with the nanorods when the numbers of particles were fixed. In all cases the reduction completed within 2 h, which is much shorter than any other earlier reports. The reduction followed first-order kinetics. The controlled experiment reveals that our reaction proceeds via the nitroso and hydroxylamine pathways. In the future, the present method might find wide application for quick organic synthesis as well as other catalysis reactions. Acknowledgment. This research was in part sponsored by the NSF (0506082), the Department of Mechanical Engineering, Texas A&M University; and the Texas Engineering Experiments Station. We wish to thank Dr. Sandip Dey, Department of Chemistry, Texas A&M University, for helping with 1H NMR study. Support for TEM and EDS by Dr. Zhiping Luo at the Microscopy Imaging Center (MIC), Texas A&M University, were greatly appreciated. Supporting Information Available: UV-vis spectrum of different shaped Au NPs and the reduction of other nitro compounds are provided. This material is available free of charge via the Internet at http://pubs.acs.org.
5156 J. Phys. Chem. C, Vol. 113, No. 13, 2009 References and Notes (1) Daniel, M.; Astruc, D. Chem. ReV. 2004, 104, 293. (2) Bae, S.; Lee, S. W.; Takemura, Y. Appl. Phys. Lett. 2006, 89, 252506. (3) Bauer, L. A.; Birenbaum, N. S.; Mayer, G. J. J. Mater. Chem. 2004, 14, 517. (4) Haes, A. J.; Van Duyne, R. P. J. Am. Chem. Soc. 2002, 124, 10596. (5) Kundu, S.; Liang, H. AdV. Mater. 2008, 20, 826. (6) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343. (7) Praharaj, S.; Nath, S.; Ghosh, S. K.; Kundu, S.; Pal, T. Langmuir 2004, 20, 9889. (8) Pradhan, N; Pal, A.; Pal, T. Langmuir 2001, 17, 1800. (9) Prabhuram, J.; Wang, X.; Hui, C. L.; Hsing, I.-M. J. Phys. Chem. B 2003, 107, 11057. (10) Saha, A.; Ranu, B. J. Org. Chem. 2008, 73, 6867. (11) Narayanan, R.; El-Sayed, M. A. J. Catal. 2005, 234, 348. (12) Corma, A.; Serna, P. Science 2006, 313, 332. (13) Maity, P.; Basu, S.; Bhaduri, S.; Lahiri, G. K. AdV. Synth. Catal. 2007, 349, 1955. (14) Takasaki, M.; Motoyama, y.; Higashi, K.; Yoon, S. -H.; Mochida, I.; Nagashima, H. Org. Lett. 2008, 10, 1601. (15) Liu, W.; Yang, X.; Huang, W. J. Colloid Interface Sci. 2006, 304, 160. (16) Liu, W.; Yang, X.; Xie, L. J. Colloid Interface Sci. 2007, 313, 494. (17) Ishida, T.; Kuroda, K.; Kinoshita, N.; Minagawa, W.; Haruta, M. J. Colloid Interface Sci. 2008, 323, 105. (18) Panigrahi, S.; Basu, S.; Praharaj, S.; Pande, S.; Jana, S.; Pal, A.; Ghosh, S. K.; Pal, T. J. Phys. Chem. C 2007, 111, 4596.
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