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J. Phys. Chem. B 2006, 110, 383-387

383

Synthesis of Palladium Nanoparticles by Sonochemical Reduction of Palladium(II) Nitrate in Aqueous Solution Abderrafik Nemamcha,† Jean-Luc Rehspringer,*,‡ and Djameledine Khatmi† Laboratoire d’Analyse Industrielle et Ge´ nie des Mate´ riaux, De´ partement de Chimie Industrielle, Faculte´ des Sciences et de l’Inge´ nierie, UniVersite´ 08 Mai 1945 de Guelma, B.P. 401, Guelma 24000, Alge´ rie, and Groupe des Mate´ riaux Inorganiques, Institut de Physique et de Chimie des Mate´ riaux de Strasbourg, UMR 7504 CNRS/ULP 23, rue de Loess, 67034 Strasbourg Cedex 2, France ReceiVed: June 30, 2005; In Final Form: October 28, 2005

The sonochemical synthesis of stable palladium nanoparticles has been achieved by ultrasonic irradiation of palladium(II) nitrate solution. The starting solutions were prepared by the addition of different concentrations of palladium(II) nitrate in ethylene glycol and poly(vinylpyrrolidone) (PVP). The resulting mixtures were irradiated with ultrasonic 50 kHz waves in a glass vessel for 180 min. The UV-visible absorption spectroscopy and pH measurements revealed that the reduction of Pd(II) to metallic Pd has been successfully achieved and that the obtained suspensions have a long shelf life. The protective effect of PVP was studied using Fourier transform infrared (FT-IR) spectroscopy. It has been found that, in the presence of ethylene glycol, the stabilization of the nanoparticles results from the adsorption of the PVP chain on the palladium particle surface via the coordination of the PVP carbonyl group to the palladium atoms. The effect of the initial Pd(II) concentration on the Pd nanoparticle morphology has been investigated by transmission electron microscopy. It has been shown that the increase of the Pd(II)/PVP molar ratio from 0.13 × 10-3 to 0.53 × 10-3 decreases the number of palladium nanoparticles with a slight increase in particle size. For the highest Pd(II)/PVP value, 0.53 × 10-3, the reduction reaction leads to the unexpected smallest nanoparticles in the form of aggregates.

1. Introduction Nanoscale materials have received considerable attention because the particles in the nanometric size range are thought of as a bridge between molecules and bulk materials. These nanomaterials often exhibit very interesting chemical, optical, electronic, and magnetic properties that are unachievable in bulk materials.1,2 Moreover, ultrafine particles of noble metals have attracted particular interest because their increased number of edges, corners, and faces gives them a high surface/volume ratio and therefore they are useful in various fields of chemistry.3,4 The development of the preparation of uniform palladium nanoparticles becomes a very important issue in their application to heterogeneous catalysis and to the water denitrification processes.5 For this purpose, the catalytic particles have to be as small as possible with a high accessible surface. Aggregates have to be vanished. Several synthetic approaches and different metal precursors have been applied to generate palladium nanoparticles having different shapes and sizes: chemical reduction of PdCl2 by NaBH46 and by arc-discharge,7 magnetic stirring at 80 °C of Pd(OAc)2,8 reduction in supercritical carbon dioxide of Pd(OAc)2,9 thermally induced reduction of Pd(Fod)2,10 and sonochemical reduction of PdCl2.11,12 To prevent the formation of undesired agglomerates of palladium nanoparticles, the processes have been performed in the presence of various surfactant molecules.2,3,13-16 The ultrasonic reduction method of palladium salts (chlorides, acetates) in various media (aqueous, polyols) has been used to * Corresponding author. E-mail: [email protected] Phone: (33) 388 107 190. Fax: (33) 388 107 247. † Universite ´ 08 Mai 1945 de Guelma. ‡ Institut de Physique et de Chimie des Mate ´ riaux de Strasbourg.

generate novel palladium nanoparticles with a much smaller size, higher surface area, and narrower size distribution than those prepared by other methods.11,12 However, few studies have been performed on the preparation and characterization of palladium nanoparticles produced by reduction of palladium(II) nitrate and on the effect of protective agents.17,18 In this work, we present a study of the characteristics of the palladium nanoparticles obtained by sonochemical reduction of palladium(II) nitrate in aqueous solution with ethylene glycol and poly(vinylpyrrolidone) as reducing and stabilizer agents, respectively. The formation, size, and shape of palladium nanoparticles will be discussed as a function of metal precursor concentration and of the effect of the surfactant molecules. 2. Experimental Procedure 2.1. Materials. The metal precursor palladium(II) nitrate (Pd(NO3)2) was purchased from Aldrich with a purity of 99%, ethylene glycol (EG) from Prolabo was used as a reducing agent and solvent,13 and poly(vinylpyrrolidone) (PVP) (K-30, average molecular weight 40 000) from Aldrich was used as a protective agent. All reagents were used as received without further purification. Double deionized water was also employed during this procedure. 2.2. Preparation of the Palladium Nanoparticle Suspensions. The palladium nitrate solution was prepared by dissolving 1.25 mg of Pd(NO3)2 in 30 mL of deionized water. A mixture of 40 mL of ethylene glycol and 5 × 10-6 mol (0.2 g) of PVP was mixed under magnetic stirring in a glass vessel for 15 min. After that, different amounts of Pd(NO3)2 solutions0.5, 1, 1.5, and 2 mLswere added to the mixture, corresponding to 0.66

10.1021/jp0535801 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/08/2005

384 J. Phys. Chem. B, Vol. 110, No. 1, 2006

Figure 1. Schematic of the ultrasonic irradiation experiment used in this study.

× 10-3 mol of Pd(NO3)2 in sample A, 1.33 × 10-3 mol of Pd(NO3)2 in sample B, 2 × 10-3 mol of Pd(NO3)2 in sample C, and 2.66 × 10-3 mol of Pd(NO3)2 in sample D, respectively. The covered vessel was fixed and irradiated with ultrasonic waves, necessary for the nitrate reduction,19 for 180 min using a multiwave ultrasonic generator (50 kHz). The experimental setup is schematically shown in Figure 1. 2.3. Characterization of Samples. The UV-visible absorption spectra of the prepared suspensions were obtained by an UV-visible spectrophotometer from Hitachi with quartz cuvettes over wavelengths from 250 to 750 nm. The acidity of the irradiated solution was measured by a pH meter (Crison pH 25). Equivalent amounts of suspension were diluted in a constant volume of distilled water and then analyzed at room temperature. The protective action of stabilizer agent was investigated using Fourier transform infrared (FT-IR) spectroscopy. For the measurements, a drop of the colloidal suspensions was deposited on a KBr pellet and analyzed using an ATI Matson Genesis series FT-IR instrument. The shape and size of the palladium particles were investigated using transmission electron microscopy (TEM, Topcon 002B operating at 200 kV). Samples for TEM were prepared by dissolving a palladium nanoparticle suspension in ethanol and placing a drop of the obtained mixture onto a carbon-coated copper grid, followed by the natural evaporation of the solvent at room temperature. 3. Results and Discussion After ultrasonic irradiation of the palladium(II) nitrate mixtures, the color of the solutions turned from the initial pale yellow to a dark brown. This observation indicates the reduction of Pd(II) ions by organic radicals (eqs 1-3) according to the mechanism proposed by Okitsu et al.11 and adopted by Chen et al.12 It has been well established that the chemical effects of ultrasound irradiation are, primarily, attributed to its acoustic cavitation: formation, growth, and implosive collapse of bubbles in the irradiated liquid. During the collapse, there is a high concentration of energy from the conversion of the kinetic energy of the liquid’s motion into heating of the contents of the bubble.11,12,18,19 In addition, Rae et al.20 reported that the presence of shear forces of polymers in aqueous solution may, also, initiate the sonochemical reaction in the fluid. In this study,

Nemamcha et al.

Figure 2. pH changes of the sample solutions obtained by different palladium(II) nitrate contents.

the reduction of palladium(II) ions could occur according to the following reactions:

H2O 9 8 •OH + •H ultrasonic irradiation

(1)

HOCH2CH2OH + •OH (•H) f HOCH2C• HOH + H2O (H2) (2) nPd(II) + 2nHOCH2C• HOH f nPd(0) + 2nHOCH2CHO + 2nH+ (3) The pH measurements on the obtained suspensions have been carried out to get information about the reduction of Pd(II) ions to palladium atoms Pd(0).21 The pH changes of the irradiated solutions in the presence of EG and PVP are presented in Figure 2. The pH values of the samples of palladium solution decrease remarkably with the metal in precursor concentration. This variation is presumably attributed to the amount of produced palladium atoms which increases the H+ ion concentration in the irradiated solutions, according to eq 3. The pH curve shape indicates clearly that the acidity of the solution decreases progressively from low concentration in Pd(II) (sample A) to high concentration (sample D). This result confirms the dependence of Pd(II) reduction on the precursor concentration in the starting solution in the presence of organic radicals. The palladium nanoparticle formation has been investigated using UV-visible spectroscopy in the 250-750 nm range. Figure 3 shows the absorption spectra of palladium colloidal suspensions after 180 min of ultrasonic irradiation and the absorption of Pd(NO3)2 solution, used as a reference sample, for comparison. The absorption bands presented in the reference sample spectrum are attributed to the absorption of palladium(II) species in the starting solution.2,8,11,22 The absence of the absorption peaks above 300 nm in all of the samples shows the full reduction of the initial Pd(II) ions.8,11 Yonezawa et al.23 have ascribed the absence of absorption bands to the total reduction of Pd(II). The same assignment was made by Ho et al.10 during thermally reduced-induced reduction of Pd(fod)2. In addition, the spectra of the irradiated samples present broad continuous absorptions in the UV-visible range. These absorptions are typical of those of palladium nanoparticles.3,8,11,12,23,24 Thus, it can be assumed that we have obtained palladium colloid having a particle size of less than 10 nm.1

Sonochemical Synthesis of Palladium Nanoparticles

Figure 3. UV-visible spectra of palladium nanoparticle suspensions prepared with different concentrations of palladium(II) nitrate solution after 180 min of ultrasonic irradiation: (A) 0.66 × 10-3 mol; (B) 1.33 × 10-3 mol; (C) 2.0 × 10-3 mol; (D) 2.66 × 10-3 mol and Pd(NO3)2 solution.

Figure 4. Photographs of palladium nanoparticle suspensions: (A) 0.66 × 10-3 mol; (B) 1.33 × 10-3 mol; (C) 2.0 × 10-3 mol; (D) 2.66 × 10-3 mol.

The stability of nanoparticle suspensions was studied through the direct observation method reported by Ca´rdinas-Trivin˜o et al.24 As shown in Figure 4, it was clearly found that the obtained palladium nanoparticle suspensions absorb visible light strongly and thus have a black color. Furthermore, no segregation between the solvent and the black palladium particles was observed for more than 15 months. The stabilization of nanoparticles has been investigated using FT-IR spectroscopy. The obtained spectra of palladium nanoparticle solutions are shown in Figure 5. In all spectra, the absorption bands appearing in the 1665-1640 cm-1 range are assigned to the stretching vibration mode of the poly(vinylpyrrolidone) carbonyl in the nanoparticle suspensions. Sze´raz et al.25 have studied the solvent effect on the adsorption of PVP molecules on γ-alumina. The shift of the carbonyl group position toward low frequencies has been assigned first to the interaction between the EG and the PVP and second to the adsorption of PVP molecules on the γ-alumina surface. Moreover, during the preparation of a PVP/sodium montmorillonite nanocomposite, Koo et al.26 attributed the shift of the CdO (PVP) absorption toward low frequencies to the interaction between CdO (PVP) and the silicate surface. In our case, the shift of the PVP carbonyl position in the EG-PVP-Pd suspension, compared to the Cd O (PVP) adsorption position at 1679 cm-1 and to the Cd0 (PVP-EG) position at 1669 cm-1, is essentially due to the interaction of the CdO (PVP) with the palladium nanoparticle surface. This result is in good agreement with those reported in earlier studies,22,24,25 where it has been concluded that the PVP

J. Phys. Chem. B, Vol. 110, No. 1, 2006 385

Figure 5. FT-IR spectra restricted to the PVP carbonyl absorption region for the sample solutions obtained by different palladium(II) nitrate concentrations after 180 min of ultrasonic irradiation: (A) 0.66 × 10-3 mol; (B) 1.33 × 10-3 mol; (C) 2.0 × 10-3 mol; (D) 2.66 × 10-3 mol.

can protect the metal nanoparticles via the carbonyl group. According to these results, one can suggest that the stabilization of the formed palladium nanoparticles may result (i) mainly from the adsorption of the PVP chain on the palladium particle surface via the coordination of the PVP carbonyl group to the palladium atoms and (ii) additionally from the steric effect of the PVP polymer long chain. In Figure 5, one can observe a slight shift of the PVP carbonyl peak position toward higher frequencies from 1645 to 1651 and 1658 cm-1, with the increase in Pd(II) ion concentration from 0.66 × 10-3 mol (A) to 1.33 × 10-3 mol (B) and to 2 × 10-3 mol (C), respectively. This behavior can be explained by the fact that the number of adsorbed PVP molecules on the particle surface is restricted by the long carbon chain of the polymer. Therefore, the absorption position of the PVP carbonyl depends strongly on the particle number in the suspension. In fact, at low Pd(II) concentration, the interaction between Pd nanoparticles and the PVP molecules is strong. All of the particles are protected with an optimal number of PVP molecules. However, when the Pd(II) concentration increases, the total signal of the CdO (PVP) increases and reaches a high value for sample C. This means that the total interaction between CdO (PVP) and Pd nanoparticle surfaces is low and suggests that the average Pd particle covered surface decreases with an increase of Pd(II) content. Unexpectedly, for the highest Pd content, sample D (2.66 × 10-3 mol), the CdO (PVP) absorption shifts slightly back to a lower frequency (1656 cm-1), as can be expected for an increase of the interaction between PVP molecules and Pd nanoparticles. The palladium nanoparticle shape and size in the colloidal solutions and their distribution have been investigated using transmission electron microscopy. Figure 6 illustrates the morphology of the prepared palladium nanoparticles. First, for all samples, the sonochemical reduction of Pd(II) ions leads to a sharp distribution of nanoparticles which are clearly separated and have, in general, a rounded shape. Besides this, the corresponding electron diffraction pattern is shown in Figure 6E. The values of interplanar distance dhkl calculated from the diffraction rings are in good agreement with the standard ASTM values for bulk palladium. This indicates the existence of a facecentered cubic structure of palladium in the nanoparticles.

386 J. Phys. Chem. B, Vol. 110, No. 1, 2006

Nemamcha et al.

Figure 6. Transmission electron micrographs of the samples prepared from different concentrations of palladium(II) nitrate solution after 180 min of ultrasonic irradiation: (A) 0.66 × 10-3 mol; (B) 1.33 × 10-3 mol; (C) 2.0 × 10-3 mol; (D) 2.66 × 10-3 mol. (E) Electron diffraction pattern of the obtained palladium nanoparticles.

The TEM micrographs show that the size of our palladium nanoparticles is affected by the changes in initial Pd(II) content. For sample A (0.66 × 10-3 mol), the resulting Pd nanoparticles are highly dispersed and have a diameter of about 3-4 nm (Figure 6A). A further increase in initial Pd(II) concentration leads to the formation of nanoparticles having sharp shapes and particle diameters of about 4-5 and 5-6 nm for samples B (1.33 × 10-3 mol) and C (2 × 10-3 mol), respectively (Figure 6B,C). For the highest Pd content, sample D (2.66 × 10-3 mol), the nanoparticles are always packed in larger light aggregates with an average diameter close to 2-3 nm but the grains seem not to be stacked together. At this point, we have to stress that mean particle size is not only related to the initial Pd(II) concentration but also seems to be in relation with the relative amount of PVP to the available metallic surface.

For a better interpretation of the obtained results, the mean particle number and the global particle surface have been roughly estimated using the following formulas:

Nparticles )

6M[Pd(II)]

Sparticles )

6M[Pd(II)] Fd

πFd3

where [Pd(II)] is the initial Pd content, d is the mean particle diameter observed by TEM, M is the Pd molecular weight, and F is the Pd bulk density. The number of PVP molecules available for each Pd nanoparticle (PVPn/Nparticles) and the ratio between the number

Sonochemical Synthesis of Palladium Nanoparticles

J. Phys. Chem. B, Vol. 110, No. 1, 2006 387

TABLE 1: Characteristics of the Palladium Nanoparticle Suspensions

A (0.66 × 10-3 mol) B (1.33 × 10-3 mol) C (2.0 × 10-3 mol) D (2.66 × 10-3 mol)

mean measured diameter (nm)

calculated number of particles

global particle surface (nm2)

PVPn/Nparticles

PVPn/Sparticles (molecules/nm2)

3.5 4.5 5.5 2.5

0.26 × 1018 0.24 × 1018 0.19 × 1018 2.84 × 1018

9.87 × 1018 15.46 × 1018 17.74 × 1018 55.67 × 1018

11.74 12.38 16.12 1.06

0.31 0.19 0.17 0.05

of PVP molecules and the global metallic surface (PVPn/Sparticles) have been calculated for each sample. The obtained values for the nanoparticle suspensions are given in Table 1. It appears clearly that the increase of initial [Pd(II)] surprisingly decreases the number of Pd particles for samples A-C. For sample D, a strong increase is observed instead; the total nanoparticle surface increases slightly for samples A-C, but for sample D, it increases dramatically. According to our results, it is found that, at the lowest Pd(II)/ PVP molar ratio (sample A), the colloid is formed by a high number of stabilized nanoparticles with small diameters. For higher Pd(II)/PVP molar ratios (samples B and C), the number of stabilized nanoparticles decreases and the mean particle diameter increases slightly but with an increase in global particle surface. Unexpectedly, for high initial Pd(II)/PVP (sample D), the obtained suspension is formed by aggregation of smaller nanoparticles with a strong increase in total particle surface. The FT-IR analysis shows that the interaction between CdO (PVP) and the Pd nanoparticles is lower and close to that of the PVP-EG solution and the calculated PVPn/Nparticles value of about 1. All of these results suggest that an over PVP concentration is enough to protect the palladium particle until a definite number of PVP for each particle surface to be covered. In other words, if the number of PVP molecules is in sufficient quantity to overcoat all of the particle surfaces with a PVPn/ Nparticles value in the range 11-16 molecules by particle, each particle can be stabilized and can grow when new Pd(0) is produced. However, if the number of palladium nuclei generated is so high that the PVPn/Nparticles value is less than 11, or when PVPn/Sparticle is less than 0.16 molecules/nm2, aggregation occurs. However, the adsorption of ethylene glycol on the particle surface prevents them from sharing a contact surface and avoids grain sintering and abnormal grain growth, as reported by Okitsu et al.11 PVP is therefore expelled on the aggregate surface. 4. Conclusions In this study, it has been demonstrated that the synthesis of palladium nanoparticles with mean sizes of about 3-6 nm can be successfully prepared by sonochemical reduction of palladium(II) nitrate in aqueous solution with EG in the presence of PVP as a stabilizer agent. UV-visible spectroscopy, pH measurements, FT-IR analysis, and TEM observations have been performed, and the analysis of the results revealed the following: (1) The obtained colloids have a black color and have a long shelf life. (2) The PVP protects the palladium nanoparticles by the adsorption of its molecules on the particle surface via the coordination of the CdO group with the palladium atoms.

(3) The initial palladium ion concentration affects the number, the dispersion, and the size of palladium nanoparticles. It has been shown that the increase of the Pd(II)/PVP molar ratio decreases the number of palladium nanoparticles and increases their size from 3 to 6 nm for low Pd(II) content. They are coated by a shell of protective PVP and are unaggregated. In contrast, for the highest value of Pd(II)/PVP, the formation of a high number of particles with a PVPn/Nparticles value of ≈1 occurs and PVP is unable to protect each particle. This leads to the formation of aggregates of the smallest nanoparticles (mean size

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