Self-assembly of palladium nanoparticles: synthesis

PAPER

www.rsc.org/materials | Journal of Materials Chemistry

Self-assembly of palladium nanoparticles: synthesis of nanobelts, nanoplates and nanotrees using vitamin B1, and their application in carbon–carbon coupling reactions† Mallikarjuna N. Nadagouda, Vivek Polshettiwar and Rajender S. Varma* Received 30th September 2008, Accepted 26th November 2008 First published as an Advance Article on the web 11th February 2009 DOI: 10.1039/b817112b An environmentally friendly one-step method to synthesize palladium (Pd) nanobelts, nanoplates and nanotrees using vitamin B1 without using any special capping agents at room temperature is described. This greener method, which uses water as a benign solvent and vitamin B1 as a reducing agent, can be extended to prepare other noble nanomaterials such as gold (Au) and platinum (Pt). Depending upon the Pd concentration used for the preparation, Pd crystallized in different shapes and sizes. A lower Pd concentration yielded a plate-like structure where thickness of these plates varied from 100 nm to 250 nm with a length of several microns. An increase in concentration of Pd resulted in the formation of tree-like structures. The Pd plates are grown on a single Pd nanorod backbone mimicking the leaf-like structures. Upon further increase in Pd concentration, Pd nanoplates started becoming thicker by vertically aligning themselves together to form ball-like structures. The synthesized self-assembled Pd nanoparticles were characterized using, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and UV spectroscopy. The Pd nanoparticles showed excellent catalytic activity for several C–C bond forming reactions such as Suzuki, Heck and Sonogashira reactions under microwave (MW) irradiation conditions.

Introduction In nanomaterial chemistry, dendritic hyperbranched structures are formed by hierarchical self-assembly in a non-equilibrium environment.1,2 Study of self-assembled fractal patterns in chemical synthesis has revealed that the distinctive dimensions, that is, size, shape, and chemical functionality of such structures make them potential candidates for the design and invention of new functional nanomaterials.3 Self-assembly of nanoparticles such as nanoclusters, nanowires, nanobelts, and nanotubes, is a technique for construction of elegant electronic and photonic nano-devices.4,5 However, it is exigent to develop easy and sustainable approaches for building hierarchically self-assembled nanoparticles. Single-crystal, one-dimensional (1-D) nanostructures of Pd are attractive as they interconnect to fabricate nanoscale electronic devices. For example, Pd can form reliable and reproducible ohmic contacts with carbon nanotubes (CNTs) because of its relatively high work function and since it can easily wet the carbon surface. This capability allows one to elucidate the intrinsic properties of CNTs and to maximize the performance of CNT-based devices such as field-effect transistors (FETs).6 Another important property of Pd is its exceptional sensitivity toward hydrogen. To this end, polycrystalline, mesoscopic wires Sustainable Technology Division, US Environmental Protection Agency, National Risk Management Research Laboratory, 26 West Martin Luther King Drive, MS 443, Cincinnati, OH, 45268, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available: Fig. S1–S3 (chemical structure of vitamin B1, and TEM images of Pt and Au nanoparticles). See DOI: 10.1039/b817112b

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made of Pd have been utilized for resistance-based detection of hydrogen gas.7 However, polycrystalline wires containing gaps between adjacent grains often shrink after initial exposure to hydrogen and may cause random, irreversible changes to the resistance of a sensing device. It should be possible to overcome this problem by switching to single-crystalline Pd nanowires with better controlled characteristics. One of the simplest ways to generate 1-D nanostructures of metals is to confine their growth within a template. The nanoscale channels in alumina or polycarbonate membranes have been most commonly used for this purpose.8 Other types of templates include mica,9 indium–tin oxide modified with a thin flat film of polypyrrole or polyaniline,10 and pyrolytic graphite,11 as well lithography.12 Engaged in the development of greener and sustainable synthesis of nanomaterials13 and nano-catalysis14 herein we report a greener and simple protocol for self-assembly of Pd nanoparticles with nanobelt, nanoplate and nanotree morphologies using vitamin B1 without using any special capping agents at room temperature. Green chemistry is the design, development, and implementation of chemical products and processes to reduce or eliminate the use and generation of substances hazardous to human health and the environment.13 Strategies to address mounting environmental concerns with current approaches include the use of environmentally benign solvents, biodegradable polymers, and non-toxic chemicals. In the synthesis of metal nanoparticles by reduction of the corresponding metal ion salt solutions, there are three areas of opportunity to engage in green chemistry: (i) choice of solvent, (ii) the reducing agent employed, and (iii) the capping agent (or dispersing agent). In this context, there has also been increasing interest in identifying environmentally friendly materials that are multifunctional. For This journal is ª The Royal Society of Chemistry 2009

example, the vitamin B1 used in this study functions both as a reducing and capping agent for Pd, Au and Pt nanospheres. In addition to its high water solubility, low toxicity, and biodegradability, vitamin B1 is the most widely used vitamin in the world. The rationale behind selecting vitamin B1 is its oxidation and reduction potential (0.4 V), which is very suitable for Pd reduction i.e. basic electrochemical series. This protocol therefore addresses several key requirements from a green chemistry perspective.

Results and discussion Vitamin B1 has an oxidation potential 0.4 V vs. Ag/AgCl electrode.15 The oxidation of vitamin B1 (for structure see ESI Fig. S1†) is well known in alkaline medium and it is a two-electron process, {[C12H17N4OS]+ 4 C12H17N4OS + 3H+ + 2e }. The oxidation potential of vitamin B1 is sufficient to reduce Pd (reduction potential 0.915 V vs. SCE), Ag (0.80 V vs. SCE), Pt (1.20 V vs. SCE) and Au (1.50 V vs. SCE). Formation of nanoplates, nanotrees and nanobelts occurred at room temperature under aqueous conditions. The addition of vitamin B1 to Pd salts resulted in the change in color (slight brown to yellow) indicating the formation Pd nanostructures. Depending upon the Pd concentration used for the preparation, Pd crystallized in different shapes and sizes. For example, lower Pd concentration (Table 1, PD-1 to PD-3) yielded plate-like structure (Fig. 1a–c).

The thickness of these plates varied from 100 to 250 nm with a length of several microns. An increase in concentration of Pd (Table 1, PD-4) resulted in the formation of a tree-like structure (Fig. 1d). The Pd plates are grown on a single Pd nanorod backbone mimicking the leaf-like structures. With slight increase in Pd concentration (Table 1, PD-5 and PD-6), Pd nanoplates started becoming thicker (Fig. 2a–b) by vertically aligning themselves together. Further increases in Pd concentration resulted in interesting Pd ball structures wherein Pd belts aligned vertically upward (Fig. 3 and 4). Although the mechanism for the formation of nanostructures is not clear, we believe the Pd concentration used in the preparation plays a key role in orchestrating the final

Table 1 Synthesis of Pd nanoparticles Entry

Composition

Code

1 2 3 4 5 6 7 8 9 10 11 12

1 mL of PdCl2 (0.01 N) + 5 mL vitamin B1 2 mL of PdCl2 (0.01 N) + 5 mL vitamin B1 3mL of PdCl2 (0.01 N) + 5 mL vitamin B1 4mL of PdCl2 (0.01 N) + 5 mL vitamin B1 5 mL of PdCl2 (0.01 N) + 5 mL vitamin B1 10 mL of PdCl2 (0.01 N) + 5 mL vitamin B1 1 mL of PdCl2 (0.1 N) + 5 mL vitamin B1 2 mL of PdCl2 (0.1 N) + 5 mL vitamin B1 3mL of PdCl2 (0.1 N) + 5 mL vitamin B1 4 mL of PdCl2 (0.1 N) + 5 mL vitamin B1 5 mL of PdCl2 (0.1 N) + 5 mL vitamin B1 10 mL of PdCl2 (0.1 N) + 5 mL vitamin B1

PD-1 PD-2 PD-3 PD-4 PD-5 PD-6 PD-7 PD-8 PD-9 PD-10 PD-11 PD-12

Fig. 1 SEM images of (a) 1 mL, (b) 2 mL, (c) 3 mL and (d) 4 mL PdCl2 (0.01 N) reaction with 5 mL vitamin B1.

This journal is ª The Royal Society of Chemistry 2009

Fig. 2 SEM images of (a) 5 mL, and (b) 10 mL, PdCl2 (0.01 N) reaction with 5 mL vitamin B1.

Fig. 3 SEM images of low and high magnification: (a–b) PD-11 (5 mL PdCl2 (0.1 N) with 5 mL vitamin B1) and (c–d) PD-8 (2 mL, PdCl2 (0.1 N) with 5 mL vitamin B1).

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Fig. 4 SEM images of (a–b) PD-7 (1 mL of PdCl2 (0.1 N) with 5 mL of vitamin B1) and Pd-catalyzed (c) Pd–polypyrrole nanocomposites and (d) Pd–polyaniline nanocomposites.

structure of Pd nanostructures. At higher Pd concentration, more ionic charges floating around help to seed the Pd growth into ball-like structures. On the other hand, at lower Pd concentration, the ionic charges are dilute and ultimately elongated species with belt-like structures are formed. The resulting Pd nanostructures are very reactive and can catalyze aniline and pyrrole to respective polymers such as polyaniline and polypyrrole composites without the addition of any oxidant (Fig. 4). In order to understand the formation of Pd nanostructures, we conducted UV measurements over time. A PD-8 (Table 1) solution was loaded into a UV cuvette and absorption was recorded every 10 minutes. The initial absorption in the visible region (around 420 nm) for PdCl2 salt slowly started decreasing (Fig. 5) and after an hour it disappeared completely. The subsequent continuous absorption in the UV range indicated the characteristic plasmon resonance peak of Pd metal. We conducted GC-MS analysis of the supernatant liquid for the PD-8 (2 mL PdCl2 (0.1 N) with 5 mL vitamin B1) solution (Fig. 6). From GC-MS spectra, we did not observe any major

Fig. 5 The UV spectra of PdCl2 (2 mL, 0.1 N) reaction with vitamin B1 (5 mL) recorded at 10 min intervals (the inset shows the final UV spectra after completion of reaction).

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Fig. 6 GC-MS spectra of filtered supernatant liquid for the 2 mL PdCl2 (0.1 N) with 5 mL vitamin B1.

change in the molecule structure and we believe that the reaction might have happened through an electrochemical reaction wherein only electrons transfer from one entity to the other. To confirm the Pd phase, we subjected the product from the reaction of 2 mL PdCl2 (0.1 N) with 5 mL vitamin B1 (Table 1, PD-8) for XRD analysis. Fig. 7 a–b shows XRD patterns of pure vitamin B1 and PD-8, respectively. It is clear from the pattern that it can be indexed to a cubic system. Other peaks correspond to vitamin B1, which was used to prepare Pd nanostructures. We extended this strategy to make other metals such as Au and Pt but the formation of particle shapes and sizes was different from Pd (ESI Fig. S2 and S3†). The formed Au and Pt nanoparticles were in the size range of 5–20 nm and were spherical in nature. Palladium-catalyzed carbon–carbon cross-coupling reactions are some of the most important processes in synthetic organic chemistry. The Suzuki, Heck and Sonogashira reactions are among the most widely used reactions for the formation of carbon–carbon bonds, and are often catalyzed by soluble Pd complexes with diverse ligands. To investigate the utility of these new and highly structured Pd materials with different morphologies, they were tested as a catalyst for organic transformations involving the aforementioned carbon–carbon (C–C) coupling reactions. Particular attention has been paid to the coupling reaction of aryl halides with aryl boronic acids, alkenes, and alkynes, commonly called Suzuki, Heck and Sonogashira reactions, respectively.16 These reactions are generally catalyzed by soluble Pd complexes with various ligands. However, the efficient separation and subsequent recycling of homogeneous transitionmetal catalysts remains a scientific challenge and an aspect of economical and ecological relevance. The practical use of the ever increasing number of tailor-made transition-metal catalytic This journal is ª The Royal Society of Chemistry 2009

Scheme 1 Pd-Catalyzed Suzuki, Heck and Sonogashira reaction under MW irradiation Table 2 Pd-Catalyzed Suzuki reaction under MW irradiation Aryl boronic Entry Aryl halide acid

Fig. 7 XRD pattern of (a) pure vitamin B1 and (b) PD-2 (2 mL PdCl2 (0.1 N) reacted with 5 mL vitamin B1 product).

species is indeed connected with the problem of separation and reuse of the rather costly catalyst systems. To overcome these problems, it is highly desirable to develop heterogeneous catalysts for industrial applications, as demonstrated by a nanostructured silica-supported Pd catalyst.17 Thus, we decided to explore this Pd-containing material as a catalyst for these reactions (Scheme 1). First, the reaction conditions were optimized using iodobenzene as a substrate. After screening a range of usual inorganic and organic bases and exploring the scope of various solvents, we found that this catalyst is most efficient for the Suzuki reaction in the presence of potassium carbonate as a base and DMF–water (1 : 1) as a solvent at 100 C. Heck and Sonogashira reactions proceed well in the presence of pyridine as a base and acetonitrile as a solvent. Using these optimized reaction conditions, the efficiency of this catalyst was studied for Suzuki, Heck and Sonogashira reaction of various aryl halides and the results are summarized in Tables 2–4. Synthesised Pd nanoparticles showed high catalytic activity for Suzuki reactions (Table 2). Aryl iodide (entries 1–2) and aryl bromide (entries 3–4) efficiently reacted with boronic acid to yield Suzuki products in good to excellent yields. 2-Iodothiophene underwent smooth reaction with various boronic acids (entry 5), providing a useful way for the synthesis This journal is ª The Royal Society of Chemistry 2009

Product

Yield (%)a

1

95

2

89

3

70

4

69

5

75

a

Yields were determined by GC.

of aryl-substituted thiophene heterocycles. It also catalyzed coupling of aryl halides with alkenes (Heck reaction, Table 3) to yield corresponding products in good yield. The utility of the Pd-nanoparticles was then explored for Sonogashira reaction using a variety of substrates (Table 4), and excellent coupling products were obtained. The use of MW-assisted chemistry was due to the efficiency of the interaction of nanoparticles with microwaves, and the reaction mixture can be volumetrically heated under MW irradiation conditions.

Experimental Synthesis of Pd nanoplates and nanotrees About 5 mL of vitamin B1 were placed in a 20 mL glass vial and 1, 2, 3, 4, 5, and 10 mL of PdCl2 (0.01 N) were added at room J. Mater. Chem., 2009, 19, 2026–2031 | 2029

Table 3 Pd-Catalyzed Heck reaction under MW irradiation

Entry

Aryl halide

Alkene

Product

Synthesis of Pd nanobelts Yield (%)a

1

75

2

74

About 5 mL of vitamin B1 were placed in a 20 mL glass vial and 1, 2, 3, 4, 5, and 10 mL of PdCl2 (0.1 N) were added at room temperature in separate experiments, respectively (Table 1). The reaction mixture was hand-shaken for 2 minutes and allowed to react overnight at room temperature. The solution mixture turned to yellow-colored solid particles, indicating the formation of Pd plates. Synthesis of Au and Pt nanoparticles

3

78

4

62

5

65

a


To prepare Au nanoparticles, 1 mL of aqueous 0.01 N HAuCl4 (Aldrich, 99%) and 5 mL of aqueous 0.1 N of vitamin B1 were mixed in a 20 mL glass vial, shaken to ensure thorough mixing, and allowed to settle at room temperature. A similar procedure was repeated for Pt nanoparticles using 0.01 N Na2PtCl6 (Aldrich, 99%). TEM grids were prepared by placing 1 mL of the particle solution on a carbon-coated copper grid and drying at room temperature. The samples for UV spectroscopy measurements were the reaction mixtures dispersed in distilled water. To obtain a better SEM image, the product was drop-casted on carbon tape and allowed to dry and a thin layer of carbon was coated on the surface to make it conducting. Transmission electron microscopy (TEM) was performed with a JEOL-1200 EX II microscope operated at 120 kV. Scanning electron microscopy (SEM) was carried out with JEOL 8400 LV operated at an accelerating voltage of 20 kV.

Table 4 Pd-Catalyzed Sonogashira reaction under MW irradiation

Suzuki reactions Entry Aryl halide Alkyne

Product

Yield (%)a

1

75

2

74

3

73

4

67

Aryl halide (1 mmol), boronic acid (1.2 mmol), K2CO3 (1.5 mmol) and 100 mg of Pd nanoparticles were added to 2 mL DMF–H2O (1 : 1) in a 10 mL crimp-sealed thick-walled glass tube equipped with a pressure sensor and a magnetic stirrer. The reaction tube was then placed inside the cavity of a CEM Discover focused microwave (MW) synthesis system, operated at 100 5 C (temperature monitored by a built-in infrared sensor), power of 200 W (maximum), and pressure of 50 psi (maximum) for 45 minutes. After completion of the reaction, the product was extracted with ethyl acetate and evaporation of solvent yields crude product, which was purified by column chromatography. All products are known in the literature and were identified by comparing their MS spectra with the standard Wiley library. Heck reactions

68

5

a


temperature in separate experiments, respectively (Table 1). The reaction mixture was hand-shaken for 2 minutes and allowed to react overnight at room temperature. The solution mixture turned to yellow-colored solid particles, indicating the formation of Pd nanoparticles. 2030 | J. Mater. Chem., 2009, 19, 2026–2031

Aryl halide (1 mmol), alkene (1.2 mmol), pyridine (1.5 mmol), and 100 mg of Pd nanoparticles were added to 2 mL acetonitrile in a 10 mL crimp-sealed thick-walled glass tube equipped with a pressure sensor and a magnetic stirrer. The reaction tube was then placed inside the cavity of a CEM Discover focused MW synthesis system, operated at 100 5 C (temperature monitored by a built-in infrared sensor), power of 200 W (maximum), and pressure of 80 psi (maximum) for 45 minutes. After completion of the reaction, the product was extracted with ethyl acetate and evaporation of solvent yields crude product, which was purified by column chromatography. All products are known in the literature and were identified by comparing their MS spectra with the standard Wiley library. This journal is ª The Royal Society of Chemistry 2009

Sonogashira reactions

References

Aryl halide (1 mmol), alkyne (1.2 mmol), K2CO3 (1.5 mmol), pyridine (1 mmol) and 100 mg of Pd nanoparticles were added to 2 mL acetonitrile in a 10 mL crimp-sealed thick-walled glass tube equipped with a pressure sensor and a magnetic stirrer. The reaction tube was then placed inside the cavity of a CEM Discover focused MW synthesis system, operated at 100 5 C (temperature monitored by a built-in infrared sensor), power of 200 W (maximum), and pressure of 85 psi (maximum) for 45 minutes. After completion of the reaction, the product was extracted with ethyl acetate and evaporation of solvent yields crude product, which can be purified by column chromatography. All products are known in the literature and were identified by comparing their MS spectra with the standard Wiley library.

1 S. Sun, C. B. Murray, D. Weller, L. Folks and A. Moser, Science, 2000, 287, 1989–1982. 2 C. Didiot, S. Pons, B. Kierren, Y. Fagot-Revurat and D. Malterre, Nat. Nanotechnol., 2007, 2, 617–621. 3 E. V. Shevchenko, D. V. Talapin, N. A. Kotov, S. O’Brien and C. B. Murray, Nature, 2006, 439, 55–59. 4 J. S. Moore and M. L. Kraft, Science, 2008, 320, 620–621. 5 A. M. Kalsin, M. Fialkowski, M. Paszewski, S. K. Smoukov, K. J. M. Bishop and B. A. Grzybowski, Science, 2006, 312, 420–424. 6 D. Mann, A. Javey, J. Kong, Q. Wang and H. Dai, Nano Lett., 2003, 3, 1541–1544. 7 S. Yu, U. Welp, L. Z. Hua, A. Rydh, W. K. Kwok and H. H. Wang, Chem. Mater., 2005, 17, 3445–3450. 8 (a) G. Kartopu, S. Habouti and M. Es-Souni, Mater. Chem. Phys., 2008, 107, 226–230; (b) H. Wang, C. Xu, A. F. Cheng and S. Jiang, Electrochem. Commun., 2007, 9, 1212–1216. 9 Z. Wang, T. Kong, K. Zhang, H. Hu, X. Wang, J. Hou and J. Chen, Mater. Lett., 2007, 61, 251–255. 10 Y. Li, G. Lu, X. Wu and G. Shi, J. Phys. Chem. B, 2006, 110, 24585– 24592. 11 X. Ji, C. E. Banks, W. Xi, S. J. Wilkins and R. G. Compton, J. Phys. Chem. B, 2006, 110, 22306–22309. 12 E. J. Menke, M. A. Thompson, C. Xiang, L. C. Yang and R. M. Penner, Nat. Mater., 2006, 5, 914–919. 13 (a) M. N. Nadagouda and R. S. Varma, Cryst. Growth Des., 2008, 8, 291–295; (b) M. N. Nadagouda and R. S. Varma, Cryst. Growth Des., 2007, 7, 686–690; (c) M. N. Nadagouda and R. S. Varma, Cryst. Growth Des., 2007, 7, 2582–2587; (d) H. Choi, Y. J. Kim, R. S. Varma and D. D. Dionysiou, Chem. Mater., 2006, 18, 5377– 5384; (e) M. N. Nadagouda and R. S. Varma, Green Chem., 2006, 8, 516–518; (f) B. Baruwati, M. N. Nadagouda and R. S. Varma, J. Phys. Chem. C, 2008, 112, 18399–18404; (g) M. N. Nadagouda and R. S. Varma, Green Chem., 2008, 10, 859–862. 14 (a) V. Polshettiwar, M. N. Nadagouda and R. S. Varma, Chem. Commun., 2008, 6318–6320; (b) V. Polshettiwar and R. S. Varma, Chem. Eur. J., 2008, 15(7), 1582–1586; (c) V. Polshettiwar and R. S. Varma, Org. Biol. Chem., 2009, 7, 37–40; (d) V. Polshettiwar, B. Baruwati and R. S. Varma, Green Chem., 2009, 11, 127–131. 15 J. Oni, P. Wesbroek and T. Nyokong, Electroanlysis, 2002, 14, 1165– 1168. 16 L. Yin and J. Liebscher, Chem. Rev., 2007, 107, 133–173. 17 (a) V. Polshettiwar and A. Molnar, Tetrahedron, 2007, 63, 6949–6976; (b) V. Polshettiwar, P. Hesemann and J. J. E. Moreau, Tetrahedron, 2007, 63, 6784–6790; (c) V. Polshettiwar, P. Hesemann and J. J. E. Moreau, Tetrahedron Lett., 2007, 48, 5363–5366.

Conclusions We have developed a convenient synthesis of Pd nanoparticles with nanobelt, nanoplate, and nanotree morphologies. Materials were readily prepared from inexpensive starting materials using vitamin B1 in water without using any capping reagent. This synthesis concept could ultimately enable the fine-tuning of material responses to magnetic, electrical, optical, and mechanical stimuli. These materials also showed excellent catalytic activity for various C–C coupling reactions widely used in organic synthesis.

Acknowledgements MNN and VP were supported by the Postgraduate Research Program at the National Risk Management Research Laboratory administered by the Oak Ridge Institute for Science and Education through an interagency between U.S. Department of Energy and the U.S. Environmental Protection Agency. MNN is thankful to Cristina Bennet-Stamper for use of their microscopic facility.

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