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Aust. J. Chem. 2008, 61, 833–836

Formation and Stabilization of Anisotropic Palladium and Platinum Nanoparticles in Aqueous Polymer Solution Using Microwave Irradiation Angshuman Pal,A Sunil Shah,A Debjani Chakraborty,A and Surekha DeviA,B A Department

of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 390002, Gujarat, India. B Corresponding author. Email: [email protected]

Anisotropic palladium and platinum nanoparticles were synthesized by reduction of the corresponding metal ions with hydrazine using polyacrylamide as a stabilizing agent in aqueous medium under microwave irradiation. The formation of particles was confirmed by UV-visible spectroscopy. The size and shape of the particles were determined using transmission electron microscopy. Rapid microwave heating resulted in ‘star-shaped’ palladium nanoparticles, but platinum nanoparticles were observed to be spherical with a distinctly visible 3–4 nm coating of polyacrylamide on their surface, which was not observed for the palladium particles. The Pt nanoparticles were used as a catalyst in the redox reaction. Manuscript received: 10 October 2007. Final version: 23 September 2008.

The past couple of decades have witnessed worldwide exponential growth of activities in the field of nanoscience, driven both by the excitement of understanding new science and by the potential hope for newer applications and economic impacts. The largest activity in this field at present is in the synthesis of nanoparticles of different sizes and shapes, e.g., by bottom-up or top-down techniques. Although many future applications such as sensors, medical diagnosis, and homogeneous catalysis will make use of the specific properties of the individual nanoparticles, they can also be used in nanoelectronics, optoelectronics, photonics, and heterogeneous catalysis. Interest in colloidal noble metal nanoparticles protected by polymers is increasing, as these materials offer options to combine properties that originate from both the nanoparticles and the polymers.[1–3] Polymer-protected noble metal colloids are usually prepared from suitable metal precursors by various in-situ reactions in the presence of the protective polymer. Through suitable choice of the precursors, reduction conditions, and protective polymer, it is possible to control not only the particle size but also the particle shape and morphology, which can be an additional tool to engineer the optical or catalytic properties.[1] Tristany et al.[4] have reported the synthesis of Pt nanoparticles organised in rod or wire-shaped superstructures that result from the combination of an organometallic route, which leads to size-controlled nanoparticles, with the use of a fluorinated aniline as a stabilizing agent. Among all the metals, Pd and Pt nanoparticles have their own place because of their unique catalytic activity and their use in other applications. Palladium nanoparticles serve as the primary catalyst for the low temperature reduction of pollutants emitted from automobiles[5,6] and in organic reactions, such as Suzuki, Heck, and Stille couplings.[7–11] By tailoring the size and/or shape, one can, in principle, enhance their catalytic performance in a range of applications.[12,13] Pd nanoparticles of various morphologies have also been prepared in the presence of surfactant,[14,15] with

the mediation of RNAs,[16] through the thermal decomposition of a Pd–surfactant complex,[17] and via use of a coordinating ligand.[18] Platinum is a catalyst for several significant reactions, including evolution of hydrogen, reduction of oxygen, oxidation of hydrogen, oxidation of methanol, and hydrogenation.[19–22] Recent developments in the synthesis of Pt nanoparticles with surface capping agents[23,11] permitted control over the nanoparticles’ size, shape, and crystallinity and, therefore, opened the possibility for systematic studies of nanometer-scale catalysis. El-Sayed and co-workers have synthesized polyacrylate-capped Pt nanocrystallites in cubic, tetrahedral, octahedral, and various other shapes.[24] Here in this communication we report ‘star’-shaped Pd nanoparticles and spherical Pt nanoparticles synthesized by microwave irradiation. To our knowledge, this is the first report for the anisotropic synthesis of Pd and Pt nanoparticles using polyacrylamide as a stabilizing agent and hydrazine as a reducing agent under microwave irradiation. Chlorides of palladium and platinum were taken as metal precursors and hydrazine hydrate (0.25 M) was used for the reduction of the metal ions. An aqueous polyacrylamide (0.1%, v/v) solution was used as a stabilizing agent and a 0.01 M concentration was used for both the metal ions. A typical reaction volume that contained 10 mL of 0.1% (v/v) polyacrylamide solution, with 3 × 10−4 M metal and 2.5 × 10−3 M reducing agent was irradiated with microwaves in an LG make MG 605AP microwave oven for 5 min at grill conditions. Over the conventional synthesis methods, microwavemediated syntheses have the advantage of improved kinetics of the reaction generally by one or two orders of magnitude, because of rapid initial heating and the generation of localized high-temperature zones at reaction sites. This may be because all the species involved in the nanoparticle formation reaction are simultaneously thermally excited within a very short time.

© CSIRO 2008

10.1071/CH07353

0004-9425/08/110833

834

A. Pal et al.

After the microwave irradiation, the colour of the solution changed from yellow to dark brown, as a result of the disappearance of palladium ions, which indicates the reduction of Pd2+ and formation of Pd nanoparticles. However, in the case of Pt nanoparticles a black solution was obtained. Characterization of the Pd and Pt nanoparticles was achieved by 1.0

Absorbance

Pd2⫹ 0.5

Absorbance

1.2

Pt nanoparticle

0.6

Pt4⫹ Pd nanoparticles 0.0 300

400

500

600

700

800

Wavelength [nm]

0.0 300

400 Wavelength [nm]

500

600

Fig. 1. UV-visible spectra of Pd and Pt ions. The insets show spectra of the Pd and Pt nanoparticles.

UV-visible spectrometry. Although there are no characteristic absorption bands for Pd and Pt nanoparticles, absorption spectra that result from the scattering of palladium nanoparticles could be used as an index for the formation of palladium nanoparticles (Fig. 1). The absorption peaks at 245 and 327 nm are ascribed to the ligand-to-metal charge-transfer transition in [PdCl4 ]2− ions by Xiong et al.[25] Wang et al. have reported a ligand-tometal charge transfer transition in [PtCl4 ]2− at 215 nm.[26] In the present study, no absorption band was observed in the UV-visible region for the aqueous PtCl4 solution, except for a small hump at 247 nm (Fig. 1). However, in the case of the PdCl2 solution, absorption bands were observed at 422, 309, and 236 nm. Fig. 2 illustrates the morphology of palladium nanoparticles observed by transmission electron microscopy (TEM). Anisotropic (star) growth of the Pd nanoparticles is clearly seen in the photographs where the nucleation starts from one point and then grows into a star shape. In the case of platinum, formation of spherical particles was observed. TEM images of spherical Pt nanoparticles with a distinct coating of polymer on the metal surface are shown in Fig. 3. The thickness of the coating was calculated from the two-dimensional images and was observed to be 3–4 nm (Fig. 3b). The coating of the particles may be responsible for the observed difference in growth pattern from the palladium nanoparticles. The particle size distribution of the Pd and Pt nanoparticles, obtained from dynamic light scattering analysis, is given in Fig. 4. The observed effective diameters for palladium and platinum nanoparticles are 180 and 30 nm, respectively. A catalytic property of Pt nanoparticles for the electrontransfer reaction between potassium ferricyanide and sodium (a)

1 µm

100 nm

(b)

1 µm

Fig. 2.

TEM photographs of the Pd nanoparticles.

200 nm

Fig. 3. (a) TEM images of the Pt nanoparticles. (b) Expanded section from (a).

Pd and Pt Nanoparticles in Aqueous Polymer Solution

835

100

thiosulfate was monitored by UV-visible spectroscopy. In a typical reaction, a 2 mL solution that contained the synthesized Pt nanoparticles, 0.2 mL of 0.001 M potassium ferricyanide, and 0.2 mL of 0.1 M sodium thiosulfate was placed in a 1 cm3 quartz cell. The reaction was carried out at 25 ± 0.1◦ C. Yang and his group have reported that the electron transfer reaction between potassium ferricyanide and sodium thiosulfate is catalyzed by noble metals like Pt.[27] The redox reaction between Fe(CN)3− 6 and S2 O2− 3 proceeds by electron transfer through the surface of the Pt particles, where the Pt particles act as highly dispersed electrodes. The decrease in the intensity of the Fe(CN)3− 6 bands with and without polyacrylamide-protected Pt nanoparticles as a function of time was monitored by measuring the absorption at 420 nm at 25 ± 0.1◦ C. The absorption was measured at 5 min time intervals. Fig. 5 shows a continuous decrease in absorbance at 420 nm with respect to time in the presence of polyacrylamidecoated Pt nanoparticles. However, results presented in the inset of

Pd

Intensity

80

60

40

20

0 0

100

200 300 Diameter [nm]

400

500

100 Pt

0.35 80

0.3 ⫺In A420

40

0.2 y ⫽ 0.002x ⫹ 0.229 R 2 ⫽ 0.930

0.15 0.1 0.05

20

0 0

10

20 Time [min]

0 0

50

Particle size distributions of Pd and Pt nanoparticles.

0.14

5 min

0.8

0.12

40 min

0.10 0.08 0.06 0.04 0.02

0.6

0.00 300

400

500

600

Wavelength [nm]

0.4

0.2 350

40

Fig. 6. Pseudo-first-order plot of −ln A420 against time for the determination of the rate of the reaction in Fig. 5 with polyacrylamide protected Pt nanoparticles.

Absorbance

Fig. 4.

30

100

Diameter [nm]

Absorbance

Intensity

0.25 60

400

450

500

550

Wavelength [nm] 3− Fig. 5. UV-Visible spectra that indicate the change of Fe(CN)3− 6 concentration during the reaction between Fe(CN)6 ◦ C in the presence of Pt nanoparticles, and the inset shows an uncatalyzed reaction in the absence of Pt and S2 O2− at 25 3 nanoparticles.

836

Fig. 5 show little variation in absorbance at 420 nm with respect to time in the absence of Pt nanoparticles. Hence, it is proposed that the reaction between potassium ferricyanide and thiosulfate is catalyzed by Pt nanoparticles. The pseudo first-order plot of −ln A370 against time showed a linear relationship with a correlation coefficient of 0.980 (Fig. 6). The rate constant value for the reaction, in the presence of the polyacrylamide-protected Pt nanoparticles, obtained from the slope of the straight line, was observed to be 0.002 min−1 . Our results are consistent with Yang’s observation. Microwave-mediated synthesis provides an effective environment for the synthesis of stable anisotropic palladium and platinum nanoparticles, which can be achieved by reducing the corresponding metal ions using a water-soluble polymer, polyacrylamide as a stabilizing agent, and hydrazine as a reducing agent. A microwave rapid heating process can be a good alternative approach for the preparation of metallic nanoparticles. The method resulted in the formation of star-shaped palladium and spherical platinum nanoparticles. The synthesized Pt nanoparticles were used as a catalyst in the redox reaction between potassium ferricyanide and thiosulfate. Acknowledgement The authors thank GUJCOST (Gandhinagar, Gujarat) for financial support.

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