Aqueous Phase Synthesis of Palladium Tripod Nanostructures for

Jul 18, 2012 - ABSTRACT: In this work, palladium tripod nanocrystals have been synthesized by mixing an aqueous solution of cetyltrimethylammonium ...
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Aqueous Phase Synthesis of Palladium Tripod Nanostructures for Sonogashira Coupling Reactions Yi-Ting Chu, Kaushik Chanda, Po-Heng Lin, 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 work, palladium tripod nanocrystals have been synthesized by mixing an aqueous solution of cetyltrimethylammonium bromide (CTAB) surfactant, Na2PdCl4, copper acetate, and ascorbic acid at 30 °C for 3 h. Addition of a small amount of copper ion source is critical to the formation of these tripods with a pod length reaching 100 nm. The incorporation of Cu atoms into the Pd tripods has been verified. The entire Pd tripod is single-crystalline with their branches growing along the [111] and [200] directions. Formation of side branches can be observed in some tripods. Triangular nanoplates are initially formed and evolved into the tripod structure in 20−30 min of reaction. Further growth leads to elongation of the pods. The large Pd tripods can serve as active and recyclable catalysts for a broad range of Sonogashira coupling reactions in water using a variety of aromatic halides containing electrondonating and -withdrawing substituents.

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INTRODUCTION Excellent control of nanocrystal morphologies is important not only because nanomaterials with enhanced chemical and physical properties can be obtained but also because novel particle structures may be synthesized.1,2 For palladium, various nanocrystal morphologies such as nanocubes, octahedra, icosahedra, nanoplates, nanobars, and nanorods have been synthesized.3−6 However, Y-shaped Pd tripods cannot be easily prepared.7 Simple branched metal nanocrystals such as bipods, tripods, and tetrapods are a major class of nanostructures receiving significant attention due to their tunable optical responses and possible enhanced catalytic activity.8−14 Tilley et al. first reported the synthesis of branched Pd nanocrystals in toluene containing less than 40% tripods in a pressure reaction vessel.7 The Pd precursor was decomposed under 3 bar of hydrogen gas in the vessel. Short-branched particles with pod lengths less than 20 nm were formed using a surfactant mixture of 1:1 oleylamine and oleic acid. This procedure is complicated and not readily accessible to others. Extensively branched and leaflike Pd nanostructures have also been synthesized using a high-pressure system, NaCl-assisted growth, kinetically controlled growth, or through the addition of polyglycol/ethylene glycol; however, simple Pd multipods were not obtained.15−19 Development of a simple method to make Pd tripods without the use of a high-pressure system is highly desirable. In this work, we describe a simple and direct method for the synthesis of large Pd tripod nanocrystals in aqueous solution at a reaction temperature of 30 °C. The key to the production of tripods is the addition of a small amount of copper ion source. Previously, we have reported a seed-mediated and copper ionassisted approach to the growth of long Pd nanorods and extensively branched nanocrystals.5 The crystal structures of © 2012 American Chemical Society

these Pd tripods and their growth process have been carefully examined. Cu incorporation into the Pd tripods was also demonstrated. Pd nanostructures have been employed to catalyze Suzuki, Heck, and Sonogashira coupling reactions.5,20−24 Various organic solvents have been used for many of the reactions. The synthesized Pd tripods were tested as catalysts for Sonogashira coupling reactions in water using a variety of aromatic halides containing electron-donating and -withdrawing substituents. They were found to serve as highly active and recyclable catalysts despite their large sizes.

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EXPERIMENTAL SECTION Synthesis of Pd Tripods. To synthesize Pd tripod nanocrystals, a 2.0 mM Na2PdCl4 solution was first prepared by dissolving 0.0355 g of deep-brown PdCl2 powder and 0.876 g of NaCl in 100 mL of deionized water. In a 20 mL vial, 0.0546 g of CTAB surfactant was dissolved in 9.475 mL of deionized water. Next, 1.25 mL of 2.0 mM Na2PdCl4 solution and 25 μL of 4.0 mM copper(II) acetate, or Cu(OAc)2, were added to the vial. After shaking the vial for 5 s, the vial was placed in a water bath set at 30 °C. Then 250 μL of 0.1 M L(+)-ascorbic acid was introduced, and the solution was stirred. The total solution volume is 10 mL. Final concentrations of Na2PdCl4, CTAB, Cu(OAc)2, and ascorbic acid are 2.5 × 10−4 M, 1.5 × 10−2 M, 1.0 × 10−5 M, and 2.5 × 10−3 M, respectively. The mixture was left undisturbed for 3 h in the water bath for particle growth. The solution gradually turned charcoal gray. The Pd products were collected by centrifugation at 6000 rpm Received: June 6, 2012 Revised: July 5, 2012 Published: July 18, 2012 11258

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Figure 1. Representative (a) SEM and (b) TEM images of Pd tripods. Inset shows an enlarged SEM image of a tripod revealing its nonplanar structure. Scale bar of the inset is equal to 100 nm. (c) XRD pattern of the Pd tripods. Standard XRD pattern of Pd is also provided. (d) UV−vis spectrum of the Pd tripods.

for 10 min. Then the products were centrifuged twice in deionized water to remove the surfactant. Pd Tripod-Catalyzed Sonogashira Coupling Reactions. A mixture of CuI (0.0022 g, 2.5 mol %), PPh3 (0.0064 g, 5 mol %), and KOH (0.055 g, 0.98 mmol, 2.0 equiv) is placed in a 25 mL round-bottomed flask containing degassed water (2 mL). After stirring the solution for 5 min, iodobenzene 1a (0.10 g, 0.49 mmol, 1.0 equiv) and phenyl acetylene 2a (0.075 g, 0.74 mmol, 1.5 equiv) were added into the solution, immediately followed by the addition of the colloidal solution of Pd tripod nanocrystals (1.0 mg, 2 mol %). Subsequently, the reaction mixture was heated to 100 °C with stirring for 6 h. The progress of the reaction was monitored by TLC. After completion of the reaction, the solvent was removed under reduced pressure. The residue left was redissolved in ethyl acetate (5 mL) and washed with water. The combined organic layer was dried over anhydrous Na2SO4. The combined filtrate was subjected to evaporation to obtain the crude compound 3a which was purified over a silica gel column (60−120 mesh) using hexane as eluent to provide the coupling product 3a. The same reaction was repeated with different substituents on aryl halides containing electrondonating and -withdrawing groups which show no remarkable changes on the catalytic activity. Catalyst Recyclability. The recyclability of Pd tripods as catalysts was also surveyed. After completion of the reaction, the reaction mixture was diluted with ethyl acetate (10 mL) and washed with water to separate the coupling product from the Pd nanocrystals. To the aqueous solution containing the Pd tripod nanocrystals, freshly measured quantities of cuprous iodide, triphenylphosphine, and potassium hydroxide were added. After stirring the solution for 5 min, iodobenzene and phenyl acetylene were added into the solution. Subsequently, the reaction mixture was heated to 100 °C with stirring. The progress of the reaction was monitored by TLC. After

completion of the reaction, the same process was repeated for two consecutive trials without loss of their catalytic activity. Characterization. Transmission electron microscopy (TEM) characterization was performed on a JEOL JEM-2100 microscope with an operating voltage of 200 kV. Ni grid was used as the sample holder for TEM analysis. Scanning electron microscopy (SEM) images of the samples were obtained using a JEOL JSM-7000F electron microscope. X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD-6000 diffractometer with Cu Kα radiation. One drop of the Pd tripod solution was added to an optical microscope cover slide and dried naturally. The slide was mounted onto a sample holder for the XRD measurements. UV−vis absorption spectra were taken using a JASCO V-570 spectrophotometer. X-ray photoelectron spectroscopy (XPS) characterization was carried out on a ULVAC-PHI Quantera SXM high-resolution XPS spectrometer. 1H NMR (300 MHz, or 400 MHz) and 13C NMR (75 MHz, or 100 MHz) were recorded with a Varian 400, or Bruker DRX300 spectrometer. Chemical shifts are reported in ppm relative to the internal solvent peak (δ = 7.26 and 77.0 ppm, respectively, for CDCl3). Coupling constants, J, are given in Hz. Multiplicities of peaks are given as d (doublet), m (multiplet), s (singlet), t (triplet). TLC plates were Merck silica gel 60 F254 on aluminum. Flash column chromatography was performed with silica gel (60−100 mesh). All reagents required for the Sonogashira coupling reactions are available commercially.

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RESULTS AND DISCUSSION Previously we have reported that the addition of a tiny amount of a copper ion source in the synthesis of Pd nanorods by a seed-mediated growth approach can elongate the Pd nanorods to 200−300 nm in length.5 Further increase in the amount of copper ion source introduced leads to the formation of Pd multipods. In the present study, it was found that the addition 11259

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Figure 2. (a, b) TEM image and its corresponding SAED pattern of a single Pd tripod nanocrystal synthesized after 3 h of reaction. (c) TEM images and SAED pattern of another Pd tripod. (d−l) Magnified TEM images of the three branches of the Pd tripod shown in panel a, corresponding SAED patterns of each pod, and high-resolution TEM images of the square region of each pod.

of a sufficiently large amount of Cu(OAc)2 resulted in the growth of a large number of Pd tripods. Figure 1 gives the SEM and TEM images of the synthesized Pd tripod nanocrystals. Many Y-shaped Pd tripods have been formed. The branches can reach lengths of over 100 nm, so the entire Pd tripods have sizes of more than 200 nm and are considered large branched metallic nanostructures. Figure S1 in the Supporting Information (SI) gives a size distribution plot of the Pd tripods in terms of pod length. Most tripods have pod lengths greater than 100 nm. The tripod yield is estimated to be around 50% with the rest composed of mostly nanoparticles and more extensively branched structures. Figure S2, SI provides a largearea SEM image showing a large number of Pd tripod nanostructures can be prepared by this simple method. Inset of Figure 1a shows an enlarged view of a slightly tilted Pd tripod revealing its nonflat structure. Sheetlike feature can be seen growing from the main pods. Some tripods also develop side branches from their main stems (see Figure S3, SI). The side branches run parallel to the three directions of the main stems. The present synthetic conditions have significantly

reduced the formation of such side growth to yield many clean tripods. XRD pattern of the tripod nanostructures matches exactly with the standard diffraction pattern for Pd (see Figure 1c). The (111), (200), (220), and (311) reflection peaks are at 40.26, 46.78, 68.30, and 82.22° 2θ, respectively. No indication of the formation of Cu−Pd alloy has been found from the XRD pattern and TEM lattice fringe images shown later. UV−vis absorption spectrum of the Pd tripods and byproducts show two broad bands centered at 400 nm and ∼1250 nm, as well as a shoulder band at 260 nm. On the basis of the elongated branch structure of the tripods like that of long Pd nanorods, the 260 nm peak and the near-infrared band are attributed to light absorption by the tripods and possibly the extensively branched nanocrystals.5 The 400 nm band should mainly come from light absorption by the irregularly shaped nanoparticles. Further structural analysis of individual Pd tripods was performed by TEM characterization. Figure 2 shows TEM images and selected-area electron diffraction (SAED) patterns of two different Pd tripods. Results of a detailed analysis on all three branches of the tripod shown in Figure 2a are also 11260

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provided. The tripod surface is not smooth; its contour displays many irregularities resulting from slightly different extents of side growth. The SAED patterns taken over individual tripods indicate that the entire Pd tripod is single-crystalline. SAED patterns of the three pods all give the same diffraction pattern with the same spot orientation. High-resolution TEM images reveal (111) and (200) lattice fringes of Pd with respective dspacing of 1.93 and 2.22 Å. Combining the SAED patterns and high-resolution TEM images, the three pods were found to grow along the [111] and [200] directions. Figure 3 presents TEM and high-resolution TEM image of another Pd tripod nanocrystal. High-resolution TEM images of the tip and stem regions of the tripod show the same (111) lattice planes with

the same lattice orientation running across the entire tripod, further verifying the single-crystalline nature of Pd tripods. The (111) lattice planes are aligned at different angles to the surface facets. Surface irregularities and protrusions (see Figure 3e) suggest that the tripods are bounded by various surface facets, including possibly various high-index facets due to the presence of surface atomic steps. Energy-dispersive X-ray spectroscopy (EDS) elemental mapping analysis of Pd tripods has been performed (see Figure S4, SI). Copper signals have been detected throughout a single tripod. A copper atomic percentage of 6.2% has been obtained over a large scanned area of more than 1 μm2. The results show that a small amount of copper is persistently incorporated into the growing Pd tripods. Additional EDS analysis was carried out to compare copper contents in Pd tripods, multipods, and more spherically shaped particles observed in the solution after tripod growth. Figure 4 gives

Figure 4. TEM images and single-spot EDS analysis of (a) a Pd tripod, (b) a multipod, and (c) a spherical nanoparticle found in the same sample.

their TEM images and the single-spot EDS results. The central regions of Pd tripods and multipods show Cu contents of around 5−6%, whereas a lower Cu content was recorded for the tripod stem region (4.77%). Cu content is only 0.45% for the spherical Pd nanoparticle shown in Figure 4, although a Cu content of 1.6% has been measured for another spherical particle. The results support the importance of persistent copper incorporation in the formation of Pd tripods and multipods. Because of a reduction potential of Cu lower than but close to that of Pd (0.34 V vs 0.60−0.623 V), we previously thought that a small amount of Cu(II) ions added can be directly reduced to Cu and deposited on the initially formed Pd particles.5 However, ascorbic acid, a weak reducing agent, cannot directly reduce Cu(II) ions to Cu atoms (dehydroascorbic acid +2H+ + 2e− ⇄ ascorbic acid + H2O with a standard reduction potential of 0.39 V). Instead, Cu(II) ions may be reduced to Cu(I) ions first by ascorbic acid in the presence of bromide and chloride ions forming CuBr and CuCl (Cu2+ + Br− + e− ⇄ CuBr with E° = 0.640 V and Cu2+ + Cl− + e− ↔ CuCl with E° = 0.538 V). Here bromide and chloride sources come from CTAB surfactant and Na2PdCl4. With their fairly large solubility product constant values (Ksp of CuBr = 6.27 × 10−9 and Ksp of CuCl = 1.72 × 10−7), some amount of Cu(I) ions should be present in the solution and be reduced to copper atoms by ascorbic acid (Cu+ + e− ⇄ Cu with E° = 0.518 V). The formation of CuBr and CuCl may explain the low copper content in the Pd tripods. To further confirm the presence of zero-valence Cu atoms in the Pd tripods, XPS spectrum of the

Figure 3. (a) TEM image of another Pd tripod nanostructure. (b−g) High-resolution TEM images of the tip and stem portions of the tripod. The imaged areas are labeled in panel a. 11261

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for Pd−Cu bimetallic materials.25,26 The Cu 2p3/2 peak is centered at 931.4 eV, whereas the Cu 2p1/2 peak is located at 950.8 eV. The Cu 2p3/2 peak position also matches well with literature reports for metallic copper.25,26 XPS results confirm the reduction of Cu(II) ions by ascorbic acid and the existence of copper atoms in the Pd tripods. Copper ions are involved in continuous reduction and oxidation cycles in the presence of palladium ions, and this small quantity of copper atom deposition facilitates the directional growth of long branches into large tripods. Such a growth strategy is similar to that typically used to make long gold nanorods by adding a small amount of AgNO3. Previously we have shown in the synthesis of Pd nanorods and multipods that the same structures could not be produced if Cu(OAc)2 was replaced by metal ions with higher or much lower reduction potentials than that of palladium such as AgNO3, Ni(OAc)2, and Zn(OAc)2.5 Here we have also replaced the Cu(OAc)2 solution with the same volume and concentration of Zn(OAc)2 solution to grow Pd tripods. Star-shaped and broad leaflike structures, rather than Pd tripods, were generated (see Figure S5, SI). Thus, introduction of a copper ion source is indispensible to the growth of Pd tripods. To understand how these Pd tripods are formed, the growth process has been followed by examining intermediate products produced during tripod synthesis (see Figure 6). After 10 min

synthesized tripod sample was obtained and is provided in Figure. 5. Copper gives very weak signals, suggesting that copper atoms are located in the interior of the Pd tripods and multipods. Alternatively, EDS overestimates the copper content in Pd tripods. The Pd 3d5/2 peak has a binding energy (BE) of 334.8 eV, while BE for the Pd 3d3/2 peak is 340.1 eV. These peak positions are consistent with values previously reported

Figure 6. TEM images of the intermediate structures observed after (a, b) 10 and (c−f) 20−30 min of reaction.

of reaction, triangular nanoplates with sizes of 20−30 nm were observed along with some multiply twinned particles. The unique triangular platelike structure is most relevant to the tripod shape. The multiply twinned nanoparticles should remain twinned when they grow to larger sizes and other shapes and should produce a SAED pattern comprising two sets of diffraction spots.27 Since they are not single-crystalline, the twinned particles are unlikely to evolve into Pd tripods. In the period of 20−30 min of reaction, some of these triangular nanoplates have developed short branches by incorporating surrounding tiny Pd nanoparticles. The tripods also become

Figure 5. (a) Full XPS spectrum of the Pd tripod sample. (b, c) XPS spectra showing the Pd 3d and Cu 2p peaks. 11262

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nonplanar in shape. Some small but structurally recognizable Pd tripods have also been formed in this time period. After 1 h of reaction, single branches have grown to 60−90 nm in length (see Figure S6, SI). A detailed structural analysis shows the examined tripod is single-crystalline, and its branches also grow along the [111] and [200] directions with the same surface irregularities. The reaction conditions have been varied to see how the amount and choice of copper ions introduced affect the product morphology (see Figures S7 and S8, SI). Without adding any copper ion source, large triangular prismatic structures and starshaped nanocrystals with sizes of 100−300 nm were synthesized. These structures are similar to those observed by replacing Cu(OAc)2 solution with Zn(OAc)2 solution in the growth of Pd tripods. Addition of 40 μL of 4.0 mM Cu(OAc)2 solution to the reaction mixture yielded extensively branched structures. Thus, it is necessary to control the amount of copper ion source to obtain a high yield of Pd tripod nanostructures. When Cu(OAc)2 was replaced with the same amount of CuCl2 as the copper ion source, Pd tripods were also synthesized but at a lower yield. Use of the same volume of Cu(NO3)2 solution also formed Pd tripods with a yield similar to that obtained using Cu(OAc)2 solution. All these reagents can serve as suitable copper ion sources for Pd tripod generation. The Pd tripods were employed as catalysts for Sonogashira coupling reactions in water using a variety of aromatic halides containing electron-donating and -withdrawing substituents. Table 1 summarizes the reaction conditions and lists the various reactions tested and the synthesized products. The detailed product characterization is available in the SI. Because of the surface irregularities, these Pd tripods should possess various high-index surface planes in addition to normal lowindex facets.20,28 This structural feature may enhance the catalytic activity of the Pd tripods. In the first example, their use as recyclable catalysts was investigated in the reaction between iodobenzene and phenyl acetylene to form diphenyl acetylene. Product yield was 93% for the first run after 6 h of reaction at 100 °C, and 91 and 90% for the second and third cycles, respectively. The tripod structure remains unchanged after the reaction (see Figure S9, SI). The results demonstrate that the Pd tripods can be easily recovered and reused without loss of catalytic activity. The turnover frequency (TOF) was calculated to be 16.4 h−1 for the first run of this reaction (TOF = moles of product/(moles of catalyst × reaction time)). Here moles of catalyst refer to moles of Pd atoms, not moles of tripod particles. Since the interior Pd atoms are not involved in catalysis, the actual catalytic activity for the surface Pd atoms of the tripods should be much higher than the TOF number indicates. TOF values of 14.4 and 12.5 have been calculated respectively for long Pd nanorods and branched nanocrystals catalyzing a Suzuki coupling reaction, but these TOF values cannot be directly compared.5 No other TOF values for Pd nanostructures catalyzing Sonogashira coupling reactions are available for comparison. The Pd tripods were subsequently used as catalysts for a wide range of Sonogashira reactions with different aromatic halides and alkynes as listed in Table 1. Excellent product yields of 85−93% have been achieved for these reactions. By comparison, micrometer-sized platelike Pd structures have been reported to catalyze a range of Sonogashira reactions by a microwave heating approach with product yields of 67−75%.29 Pyridine base and acetonitrile solvent were used. The reaction temperature was 100 °C, and the microwave heating time was 45 min. The versatility of the

Table 1. Sonogashira Coupling Reactions between Different Aryl Halides and Alkynes Using Pd Tripods As the Catalysts

a

Determined on the basis of the weight of purified samples (%). Compounds 3a−b, 3d−e, and 3i were characterized by comparison of 1H and 13C NMR spectra with literature data. b

Pd tripod nanocrystals for a broad scope of Sonogashira reactions and their easy separation from the products implies that they may be employed for catalyzing many important bioactive molecules.

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CONCLUSION In summary, large Pd tripod nanostructures have been synthesized for the first time by simply making an aqueous mixture of CTAB surfactant, Na2PdCl4, Cu(OAc)2, and ascorbic acid at 30 °C for 3 h. The tripods are single-crystalline with their branches growing along the [111] and [200] directions. The incorporation of copper atoms into the Pd tripods has been verified. The tripods appear to evolve from triangular nanoplate particles. Once the tripod shape is established, the branches continue to grow longer, reaching 100 nm in length. They were found to serve as effective and recyclable catalysts for a broad spectrum of Sonogashira coupling reactions. Their use in many other Pd-catalyzed reactions can be expected. 11263

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ASSOCIATED CONTENT

S Supporting Information *

Large-area SEM image of Pd tripods, EDS spectrum, and elemental mapping, additional SEM and TEM characterization of Pd tripods, and spectroscopic characterization of the products obtained from the Sonogashira reactions. 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

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REFERENCES

We thank the National Science Council of Taiwan for the support of this research (NSC 98-2113-M-007-005-MY3 and NSC 100-2811-M-007-032).

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