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Studies of preparation of palladium nanoparticles protected by dendrons

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INSTITUTE OF PHYSICS PUBLISHING

NANOTECHNOLOGY

Nanotechnology 15 (2004) 1716–1719

PII: S0957-4484(04)80821-3

Studies of preparation of palladium nanoparticles protected by dendrons Guohua Jiang, Li Wang1 , Tao Chen, Haojie Yu and Jianjun Wang The State Key Lab of Polymer Reaction Engineering, College of Materials Science and Chemical Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China E-mail: opl [email protected] (Li Wang)

Received 18 May 2004, in final form 14 October 2004 Published 3 November 2004 Online at stacks.iop.org/Nano/15/1716 doi:10.1088/0957-4484/15/12/003

Abstract Instead of the usual surfactants, palladium (Pd) nanoparticles have been prepared in the presence of the dendrons G1-CH3 and 12G1-CH3 , and investigated by TEM and absorption. It is found that the Pd particles’ size and shape can be controlled by the molar ratio of metal ions to the dendrons, the metal ion concentration and the surface-groups of the dendrons. The possible formation process of the Pd particles is discussed. (Some figures in this article are in colour only in the electronic version)

1. Introduction The preparation of nanoparticles is one of the rapidly emerging research fields because of their potential applications to sensor devices, catalysis, nanoelectronics and many other areas [1]. They are being heralded as the next generation of building blocks for designing modern materials [2]. Therefore, the development of preparation technology of metal nanoparticles with different sizes and shapes is important. Several methods involving micelle, Langmuir–Blodgett film (LB), zeolite, twophase liquid–liquid and organometallic techniques have been used in recent years for preparing metal nanoparticles [3]. However, the chemical reduction of metal salts in their liquid phase is the most convenient method for large scale preparation of nanoparticles, whereby metal ions are reduced by a reducing agent such as sodium borohydride, sodium citrate, carbon monoxide, hydrogen or alcohol [4]. Unfortunately, usually resultant nanoparticles tend to aggregate in solution because of their small size [5]. One of the effective strategies to avoid this is to protect nanoparticles with protective agents including protein cages [6], polymer matrices [7], surfactant vesicles [8], etc. Dendrons are a class of highly branched, monodisperse and globular synthetic molecules. They have also been used to control nanoparticle formation and isolated nanoparticles of different sizes were obtained [9]. There are a few reports in which some metal particles were prepared in the presence of dendrons by photoreduction [10] or by the addition of 1 Author to whom any correspondence should be addressed.

0957-4484/04/121716+04$30.00 © 2004 IOP Publishing Ltd

chemical reductants [11]. For example, Taubert et al [12] reported a synthesis process of gold nanoparticles in the presence of a stiff polyphenylene dendron template with 16 thiomethyl groups on the outside. Esumi et al [13] have reported the preparation of metal dendrimer nanocomposites in the presence of poly(amidoamine) dendrons (PAMAM) by a wet chemical reduction method. They found that the particles’ growth is accelerated with decreasing PAMAM dendron concentration as well as decreasing generation. Generally, dendrimers of lower generation tend to exist in relatively open forms, while higher generation dendrimers take a spherical three-dimensional structure, which is very different from linear polymers adopting random-coil structures. Further, dendrimers might provide reaction sites including their interior or periphery and it is suggest that dendrimers act as a very effective protective colloid for preparing metal nanoparticles [14]. It is well known that Pd0 is an excellent catalyst for organic reactions in aqueous, organic and fluorocarbon solutions. Moreover, Pd0 nanoparticles have more excellent catalytic properties for their small size. However, the preparation of metal nanoparticles with special size and shape in the solution is still difficult. The work described herein introduces a method for the preparation of nano-palladiumcored dendrimers (NPDs), using the dendrons methyl 3,4,5tri(benzyloxy) benzoate (G1-CH3 ) and methyl 3,4,5-tri(ndodecyl-1-oxy) benzoate (12G1-CH3 ) as protective agents; stable Pd nanoparticles were formed by reduction of H2 PdCl4 with N2 H4 . The influential factors for the forming and growth of the metal nanoparticles were studied.

Printed in the UK

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Studies of preparation of palladium nanoparticles protected by dendrons

O O

O CO2CH3

O

G1-CH3

O

CO2CH3 O

12G1-CH3

2. Experimental details 0.4 C

The precursor metal compound was palladium chloride (PdCl2 , 99%, Guangdong Dahao Refined Chemical Co.). H2 PdCl4 was obtained by dissolving PdCl2 in 0.2 M HCl solution. Hydrazine monohydrate (N2 H4 ·H2 O, 99%) was obtained from Across. Tetrabutyl ammonium bromide ((n-C4 H9 )4 NBr, 99%) was purchased from the Chinese Medical Co. Other chemicals included dichloromethane (CH2 Cl2 , 99%) and acetone ((CH3 )2 CO, 99%), which were purchased from Hangzhou Chemical Reagent Co. and used as received. Dendrons G1-CH3 and 12G1-CH3 were synthesized according to the literature [15]. 2.2. Preparation of palladium nanoparticles Palladium nanoparticles were prepared with a phasetransferred process by chemical reduction method. Briefly, to a solution of H2 PdCl4 in deionized water was added (n-C4 H9 )4 NBr in acetone/CH2 Cl2 . H2 PdCl4 was phasetransferred into the organic phase using (n-C4 H9 )4 NBr. The mixture was stirred for 1 h. Then the dendron solution was added to the above mixture. Subsequent dropwise addition of a freshly prepared aqueous solution of N2 H4 caused an instant colour change. The resulting mixture was stirred under dry Ar for an additional 24 h at room temperature. An orange–yellow organic phase containing NPDs and a colourless aqueous phase were obtained. 2.3. Characterization The morphologies and sizes of the sample were investigated by transmission electron microscopy (TEM) with a JEOL LEM-200CX, using an accelerating voltage of 160 kV. The specimens were drop cast onto a carbon-coated copper grid sample holder followed by natural evaporation at room temperature. UV–visible (UV–vis) spectra were acquired with an HP8453 spectrophotometer.

3. Results and discussion A dendron, for example G1-CH3 or 12G1-CH3 [15], is a segment of dendrimer that possesses a focal point onto which the branching units of a dendritic architecture are attached. If the focal point is capable of metal complexation, the specific metal–dendron interactions can be utilized to control reactions at this site, and these interactions controlled in a confined and localized area may be used for the controlled growth and stabilization of metal nanoparticles [11b]. Such a situation is validated here during the production of NPDs using dendrons G1-CH3 and 12G1-CH3 in which the carboxyl is in the

Frequency

2.1. Materials

0.3 0.2 0.1 0.0

10

20 30 40 50 Diameter/nm

60

Figure 1. TEM images (A, B, D) of Pd nanoparticles that obtained at various molar ratios of G1-CH3 :Pd2+ and histograms of size distribution (C) of the sample from B. (Molar ratio of G1-CH3 :Pd2+ = 22.2:1 (A); 8.9:1 (B) and 4.5:1 (D). [H2 PdCl4 ] = 4.5 × 10−3 mol l−1 , [TBAB] = 4.7 × 10−3 mol l−1 , [G1-CH3 ] = 2 × 10−2 mol l−1 , [N2 H4 ] = 1.0 mol l−1 .)

focal point as a protective agent. The dendron focal point appears to be only reasonable site where the particles may nucleate and grow, because the dendron outside lacks metalstabilizing groups, while the loose dendron inside may not be able to sustain the confinement of the particles within the branching units. The oxygen atoms in the carboxyl moiety and even the conjugated six-membered ring can potentially bind metals [11b]. The morphologies and sizes of the sample were investigated by transmission electron microscopy (TEM). First, we found that the present palladium nanoparticle size can be affected by the molar ratio of G1-CH3 to metal ions. H2 PdCl4 was phase-transferred into the well stirred solution in the presence of G1-CH3 using (n-C4 H9 )4 NBr. Subsequent dropwise addition of a freshly prepared aqueous solution of N2 H4 caused an instant colour change, which indicated the formation of Pd0 particles. Figure 1 shows TEM images and the size distribution of palladium nanoparticles prepared at various molar ratios of G1-CH3 /Pd2+ with fixed H2 PdCl4 at 4.5 × 10−3 mol l−1 at room temperature. When the molar ratio of G1-CH3 :Pd2+ = 22.2:1, the resultant Pd nanoparticles have an irregular shape and the diameter of the particles ranges from 5 to 20 nm (figure 1(A)). When decreasing the molar ratio of G1-CH3 /Pd2+ (G1-CH3 :Pd2+ = 8.9:1), the shape of the nanoparticles changes from irregular to spherical or near-spherical (figure 1(B)). The size distribution of resultant palladium nanoparticles is 15–45 nm in the diameter and no particles larger than 50 nm are observed as shown in figure 1(C). Further decreasing the molar ratio of G1-CH3 :Pd2+ = 4.5:1, the presence of particles have spherical shape and 30–100 nm in diameter (figure 1(D)). The particles’ size changes with the molar ratio of G1-CH3 to metal ions may be due to the stabilizing molecule, G1-CH3 , which is absorbed on the particles surfaces preventing their aggregation. The nanoparticles’ size increases with the decreasing molar ratio of G1-CH3 to metal ions. 1717

G Jiang et al

Absorbance

0 .4 0 .3 0 .2 0 .1 0 .0 200

300

400

500

600

700

800

W a v e len g th /n m Figure 2. UV–vis spectrum of NPDs in the organic phase of the sample from figure 1(A).

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Figure 3. The TEM images of the as-prepared samples with different metal concentrations. (A, [H2 PdCl4 ] = 7.5 × 10−4 mol l−1 ; B, [H2 PdCl4 ] = 4.5 × 10−3 mol l−1 , C, [H2 PdCl4 ] = 4.5 × 10−2 mol l−1 , [TBAB] = 4.7 × 10−3 mol l−1 ; [G1-CH3 ] = 2 × 10−2 mol l−1 .) 0.20 Frequency

This phenomenon can be explained with the mechanism of the nucleation and growth of metal nanoparticles [16]. Formation of metal nanoparticles from reduction of metal ions involves two processes, that is, the nucleation and growth of nuclei. The metal ions are reduced into the metal atoms by reducing agents. Several metal atoms collide together to form a metal nucleus, which can grow further into nanoparticles or cluster by attachment of the atoms produced subsequently. A minimum number of the atoms are required to form a stable nucleus. There is competition between the nucleation and growth. In the general field of colloid chemistry, it is known that fast nucleation relative to growth results in small particle size [17]. When the molar ratio of G1-CH3 to the metal ions is relatively high, the nucleation rate of the Pd particles may be higher relative to the growth rate. This means that the nucleation process would be superior to the growth once the Pd atoms were formed. Thus, the diameter of the resultant Pd nanoparticles is relatively small. When the molar ratio of G1-CH3 to the metal ions is relatively low, the disintegration of the smaller particles or nuclei below a critical size and the particles are thermodynamically unstable because of the large surface energy. The growth rate of the Pd particles may be higher relative to the nucleation rate. Thus, the atoms formed at the latter period of reduction were mainly used in collisions with the nuclei already formed, instead of in the formation of new nuclei. The atoms from the reduction reaction and the disintegration then attach themselves to form larger particles that exceed the critical size, and this is in accordance with the crystal growth rule that larger particles grow at the expense of the smaller ones [18]. Figure 2 shows the absorption spectrum of NPDs in the organic phase. There is no obvious surface plasmon (SP) band and the spectrum consists of a smoothly increasing absorption at increasing energy. The spectrum obtained was similar to those of the palladium protected Fr´echet-type dendritic polyaryl ether disulfide reported by Gopidas and coworkers [19]. It was found that the size and the shape of the Pd particles change with the metal concentration. As shown in figure 3, when the PdCl42− concentration is 7.5 × 10−4 mol l−1 , on addition of 5 ml H2 PdCl4 to the well stirred solution in the presence of 5 ml of G1-CH3 ([G1-CH3 ] = 2 × 10−2 mol l−1 ), the mean diameter of the particles is 40–60 nm, and the shapes of some particles are spherical and a few particles are triangular (figure 3(A)). When the PdCl42− concentration increases to 4.5 × 10−3 mol l−1 , the mean particle diameter is 70–90 nm, and the shapes of most particles are irregular

B

0.15 0.10 0.05 0.00

5

10 15 20 Diameter/nm

25

Figure 4. TEM images of the as-prepared samples in the presence of 12G1-CH3 ([H2 PdCl4 ] = 4.5 × 10−3 mol l−1 ; [TBAB] = 4.7 × 10−3 mol l−1 ; [12G1-CH3 ] = 2 × 10−2 mol l−1 ).

(figure 3(B)). Further increasing the PdCl42− concentration to 4.5× 10−2 mol l−1 , the mean particle diameter is 250–350 nm, and only the spherical particles are obtained (figure 3(C)). It can be interpreted that the particle size increases with increasing concentration of PdCl42− . This may be due to an increase in the crystal growth rate as the PdCl42− concentration is increased, resulting in the change of the particles’ size and shape. When the concentration of metal ions is relatively low, the relative growth rate of different faces of the particles is also distinct for the relatively low growth rate. The final shapes of the particles is the relative growth rate on [100] and [111]. When the concentration of metal ions is relatively high, the relative growth rate of different faces of the particles is not distinct for the relatively high growth rate, and therefore leads to larger particles with spherical shape. In order to investigate the effect of the out-groups of the dendron on the size and shape of Pd particles, we also perform the experiment when 12G1-CH3 replaces G1-CH3 . From figure 4, we can see that the size and shape of Pd particles are different when G1-CH3 is replaced by 12G1-CH3 . When H2 PdCl4 solution is added to the mixture in the presence of 12G1-CH3 , with molar ratio of 12G1-CH3 :Pd2+ = 8.9:1, the Pd particles are all fine and nearly monodispersed. Figure 4 shows a typical TEM photograph and the size distribution. The mean diameter of the particles is 10–20 nm. An amazing thing was also observed from the TEM image: some Pd nanoparticles, which have almost the same diameter,

Studies of preparation of palladium nanoparticles protected by dendrons

O

O

O

O

O O

O O

O

O

O O CH3 H3 C O O C O C O OO CH 3 H3 C C

G 1-CH3 or 12G1-C H3

O

O C CH3 H3 C C OO OO C

O

O

O

O

O O

2 (n-C4H9)4N

O

O

PdC l 42

O

O

H 3 C C C H3 C O O OO C O CH3 H3C O O O C C O O O CH 3 H 3C O C OO OO C H 3 C C CH3

O

or O

O

O

O O O O

O

O O

O O O O O O O

O O

Figure 5. Schematic presentation of Pd-G1-CH3 and Pd-12G1-CH3 nanocrystalline growth controlled by the dendrons.

connected and formed into a linear shape. In fact, nanoparticles tend to aggregate because of their small size and high surface energy. Another important fact is that the alkyl groups in 12G1-CH3 are more flexible than the benzyl groups in G1-CH3 . The steric requirement of 12G1-CH3 is less than that of G1-CH3 . Given the weak force between the NPDs, particle aggregation conceivably occurs and could be interpreted by a diffusion aggregation mechanism [20]. We also can find that the mean size of the particles obtained here is smaller than that in the presence of G1-CH3 . This might be a result of the effect of the stabilized molecules and the result is similar to the other stabilizing molecules [21]. The dendrons with the benzyl groups on the surface have the greater steric requirement than those with alkyl groups on the surface because the benzyl groups are more rigid than the alkyl groups. In other words, the more hindered periphery leads to less compaction because of the increasing open space between the branching units. Hence, there is more room for Pd atoms to agglomerate in G1-CH3 than 12G1-CH3 (figure 5). When the PdCl42− anions were reduced by the reducing agents, the Pd atoms aggregated at room temperature. It is thus possible to control the particles’ size and shape by controlling the surfacegroups of the dendrimer.

4. Conclusion In summary, Pd nanoparticles were obtained in the presence of dendrimers G1-CH3 and 12G1-CH3 . The influencing factors that affect the size and shape of the particles also have been studied. From the results that we obtained, we found that the molar ratio of the mental ion to the dendrons, the metal concentration and the reducing agents affect the size and shape of the particles. The use of dendrons with different surface-groups helps to control the shapes and sizes of the product. This work also demonstrates that the dendroncontrolled method provides an effective and simple way of preparing metal materials with well defined morphologies that provide advantages in certain applications. Furthermore, this work should be easily expanded to the preparation of other dendron-stabilized metal nanoparticles.

Acknowledgments Financial support by the Science and Technology Commission of Zhejiang Province (2004C34005) and Ningbo Science and Technology Program are gratefully acknowledged.

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