Synthesis of assembled copper nanoparticles from copper-chelating ...

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Chemical Physics Letters 405 (2005) 49–52 www.elsevier.com/locate/cplett

Synthesis of assembled copper nanoparticles from copper-chelating glycolipid nanotubes Hongwei Zhu a, George John a

b,*

, Bingqing Wei

a,*

Department of Electrical and Computer Engineering and Center for Computation and Technology, Louisiana State University, Baton Rouge, LA 70803, USA b Department of Chemistry, The City College of New York, Cuny, New York, NY 10031, USA Received 3 January 2005; in final form 24 January 2005

Abstract Using glycolipid nanotubes as templates, assembled copper nanoparticles were prepared from copper-chelating amphiphiles by a simple annealing process. The morphology and structure of these nanoparticles were investigated using transmission electron microscopy and scanning electron microscopy. The experimental results showed that the nanoparticles are crystalline and mainly composed of face-centered cubic (fcc) Cu with a narrow size distribution (5 nm). The present method suggests that by using an appropriate template-assisted technique, it is potentially valuable for the fabrication of organized metal nanoparticles arrays. Ó 2005 Elsevier B.V. All rights reserved.

The development of uniform metal nanoparticles with controlled morphology has been an active research field of particular interest because of their applications in the chemical and electronics industries [1,2]. In the last few years, metal nanoparticles have been investigated by various chemical and physical methods. Often, the nanoparticles are required to be non-aggregated and smaller than 50 nm in diameter so that thin dense metal layers can be synthesized. Hence, there is a significant current interest in preparing nanoparticles of small size dispersity and to arrange them in periodically ordered particulate materials [3–5]. One promising synthetic approach is the use of molecular materials that can self-assemble in solution to form unique nanoscale structures as templates which can direct the nucleation and assembly of metal nanoparticles [6–11]. In these studies, lipid nanotubes, peptide nanotubes and fibrils * Corresponding authors. Fax: +1 225 578 5200 (B. Wei), +1 212 650 6107 (G. John). E-mail addresses: [email protected] (G. John), weib@ece. lsu.edu (B. Wei).

0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.01.126

have been used as templates to prepare gold, silver and palladium nanocrystals or nanowires. Here, we describe the fabrication of assembled crystalline copper nanoparticles by a template-assisted technique. In our approach, glycolipid nanotubes were utilized for the generation of nanodispersed metallic–organic chelating amphiphiles. Calcination of these copper complexed nanotubes in argon atmosphere results in assembled copper nanoparticles. First of all, we synthesized the amphiphilic molecules and then self-assembled to form organic nanotubes. In a typical procedure (Fig. 1a), 2,6-diaminopyridine (DAP, 10 mmol, 1.09 g) was dissolved in dry tetrahydrofuran (THF) under argon atmosphere and cooled to 5 °C. The coupling reaction was carried out by the drop-wise addition of oleoyl chloride (10 mmol, 3.30 g) in dry THF in an ice bath using triethylamine (10 mmol, 0.93 g) as the catalyst. The reaction was monitored by thin layer chromatography (TLC), and after the completion of the reaction the excess base was neutralized by the addition of water and worked up to extract the organic layer containing the required product. The

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Fig. 1. (a) Synthetic scheme for glycolipid nanotubes formation. (b) Formation of Cu (II)-chelating amphiphiles. (c) Assembled Cu nanoparticles.

isolated crude product was further purified by column chromatography and the purity was checked by spectroscopic and elemental analysis. The monosubstituted DAP was coupled with glucose via reductive amination under non-degradative conditions. The hydrophilic sugar (glucose, 1 mmol, 0.18 g) was added to the purified DAP in 2:1 pyridine/acetic acid buffer at 80 °C for 1.5 h, and 20-fold excess monoamine was used to achieve maximum coupling. The reaction was monitored by TLC. Reduction occurred by adding an equal amount of borane–dimethylamine complex and heating at 80 °C for an additional hour. The desired compound was purified by column chromatography using ethyl acetate as the eluent. Then, the purified amphiphiles were dispersed in water (5 mg/100 mL) and heated to boiling above its phase transition temperature for 30 min. The fine dispersion were cooled to room temper-

ature at ambient conditions yields a white cotton-like mass consisting of organic nanotubes with an outer diameter of 60–80 nm and an inner diameter of ca. 20 nm [12]. Diaminopyridine (DAP) derivatives are known to function as efficient ligands for the complexation with specific metal ions such as Cu (II), Zn (II) and Co (II) see Ref. [13] and therein, and Ref. [14]. The compound described above possess a metal ion binding subunit in the form of a 2,6-disubstituted DAP moiety. Functional amphiphiles constructed with metal-complexing templates offer additional advantages because they aggregate in aqueous media. Such supramolecular ensembles provide hydrophobic binding sites for the substrates and induce catalytic effects through appropriate functional groups, and further as suitable templates for material synthesis. To test this hypothesis, the glycolipid nanotubes produced thus were dispersed in copper chloride solution for a few hours (Fig. 1b). The organic tubes have functional arms which form chelates with copper in aqueous solution [13]. Washed it many times with excess water, centrifuged and separated the tubes. After drying, the copper complexed tubes were loaded on a clean Si substrate. The substrate was then put in a tube furnace and heated in argon at 500 °C for about 30 min. As-prepared product was characterized by scanning electron microscopy (SEM, JEOL JSM-6330F, equipped with a field emission gun operated at 5 keV) and transmission electron microscopy (TEM, JEOL JEM 2010, at an accelerating voltage of 200 kV). For TEM observation, the particles were scratched from Si substrate and dispersed in acetone, after sonication, a drop of this suspension was drop-cast on a carboncoated copper grid. The samples located at the center of the holes in the copper grid were carefully selected to do selected area electron diffraction (SAED) and EDAX. A background subtraction was needed in order to eliminate the effect of the copper grid. Fig. 2a is a SEM image of the Cu complexed nanotubes located on a Si substrate before annealing, showing the feather-like morphology of alinged nanotube bundles. After annealing, as shown in Fig. 2b, it is found that the host nanotube templates have been removed incompletely with some residual trails left. It has been known that the decomposition of the glycolipid nanotube template starts at 293.6 °C and completely burned out at 657.5 °C [11]. Here, a relative lower temperature (500 °C) was selected in order to investigate how the templates affect on the formation of Cu nanoparticles. Higher resolution SEM images (Fig. 2c–d) reveal unambiguously the dense arrangement of fine Cu nanoparticles on the almost entire surface of the templates which have directed the nucleation and controlled the assembly of Cu nanoparticles. It is worthy of note that, besides those small nanoparticles, 1D arrays of large nanoparticles (50–100 nm) can be occasionally found

H. Zhu et al. / Chemical Physics Letters 405 (2005) 49–52

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Fig. 2. (a) SEM image of as produced copper complexed organic nanotubes. (b) After annealing in argon atmosphere. (c) and (d) Cu nanoparticles formed on nanotube templates. (e) An enlarged view of the square area marked in (d). Scale bar is 500 nm.

(the bold lines indicated by the white arrows) which are supposed to be formed on the nanotube bundles due to the excess supply of local Cu seeds. This size difference suggests that the nanotube template does have an effect on the size distribution of the Cu nanoparticles. In the enlarged view shown in Fig. 2e, one can see Cu nanoparticles show a good alignment along the tube orientation. The size distribution and microstructures of Cu nanoparticles were further investigated by TEM (Fig. 3). A typical TEM image of the Cu nanoparticles is shown in Fig. 3a. They appear to be spherical in shape and fairly uniform in size with a mean diameter of 5 nm. It is notable that the alignment of Cu nanoparticles has been destroyed due to the scratching operation and sonication. While the product of combustion of the host nanotubes can provide further information on the interaction between the nanoparticles and the templates. Fig. 3b is the burnt remains of nanotubes from which nanoparticles have been removed by sonication, showing the presence of many nanopores (marked by dashed circles for guiding eyes) with the same size distribution as the Cu nanoparticles. This porous feature tells that it is the nanotube prevented the as-formed Cu clusters aggregating into large particles and controlled the arrangement of Cu nanoparticles on the templates. The SAED result shown in Fig. 3c indicates that the nanoparticles are crystalline and mainly composed of face-centered cubic (fcc) Cu. From the EDAX result (inset of Fig. 3a), the nanoparticles have been further confirmed as copper with small amount of oxygen and chlorine absorbed on the

surface of nanoparticles or carbon films during the sample preparation. The high resolution TEM images, as shown in Fig. 3d, show the discernible (1 1 1) and (2 0 0) planes of fcc Cu crystal, supporting the above results from the SAED pattern. We believe that the unique 1D nanometer-sized dimension and surface properties of glycolipid nanotubes played important roles in the template-directed formation of assembled Cu nanoparticles. In the copper-nanotubes complexes, Cu (II) ions were stabilized on the surface of the organic nanotubes by forming a series of chelates (Scheme in Fig. 1b). Single-step synthesis of noble metal nanoparticles has been reported in the organic phase of biphasic mixtures of aqueous metal salts and amine-terminated molecular reducing reagents such as hexadecylaniline and aniline [15,16]. In the present case, the amine functional group of the DAP moiety may act as a complexing and reducing agent in presence of sugar moieties and form copper nanoparticles. The copper-chelating nanotubes could break down their chelating bonds upon annealing. Cu nanoparticles then grew through the aggregation of as-decomposed Cu seeds. The presence of argon can prohibit the oxidation of the nascent nanoparticles. The assemblies of Cu nanoparticles were determined by the dimension of the glycolipid nanotube which templated the nanoparticle nucleation and growth. In summary, a novel metal-chelating system was utilized and assembled Cu nanoparticles were successfully prepared by removing the nanotube templates through an annealing process in argon atmosphere. Our studies

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Fig. 3. (a) Low resolution TEM image of Cu nanoparticles and the corresponding EDAX result (inset). (b) Burnt nanotube template after sonication. (c) A typical SAED pattern of Cu nanoparticles. (d) HRTEM images of Cu nanoparticles. Scale bars are 2 nm.

suggest that by using this template-assisted technique, it is possible to fabricate organized metal nanoparticles arrays which can have important applications in the fields of catalysis, superalloys, and thin film coatings in the chemical and electronics industries.

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