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Dendrimer-templated AgPd bimetallic nanoparticles
Journal of Colloid and Interface Science 271 (2004) 131–135 www.elsevier.com/locate/jcis
Dendrimer-templated Ag–Pd bimetallic nanoparticles Young-Min Chung and Hyun-Ku Rhee ∗ School of Chemical Engineering and Institute of Chemical Processes, Seoul National University, Kwanak-ku, Seoul 151-742, Republic of Korea Received 17 March 2003; accepted 2 December 2003
Abstract Ultrafine dendrimer-templated Ag–Pd bimetallic nanoparticles with various metal compositions have been prepared successfully using silver(I)–bis(oxalato)palladate(II) complex. The use of an oxalate complex, in which two metal ions exist in one complex, is found to be effective in preventing unfavorable silver halide formation and thus suitable for the formation of Ag–Pd bimetallic nanoparticles. 2003 Elsevier Inc. All rights reserved. Keywords: Dendrimer; Bimetal; Nanoparticles; Template; Nanocomposite; Organic–inorganic hybrid
1. Introduction Dendrimers are highly branched macromolecules and they are generally described as having a structure of spherical shape with a high degree of symmetry [1–8]. With the prospect of dendrimers as a template for the formation of inorganic nanoparticles, various metal nanoparticles have been successfully prepared [9–16]. However, it is worth noting that most previous studies have been confined to monometallic nanoparticles, and bimetallic nanoparticles have not been exploited yet, although there has been an attempt to prepare dendrimer/bimetal nanocomposites using two different metal precursors [17,18]. Recently, we prepared Pt–Pd [19] and Pd–Rh [20] bimetallic nanoparticles in the presence of poly(amidoamine) dendrimers with surface hydroxyl groups (fourth generation, PAMAM-OH) and applied these bimetallic nanoparticles as catalysts for the partial hydrogenation of 1,3-cyclooctadiene. The Pt–Pd system has also been studied by Scott et al. [21]. On the other hand, the preparation of bimetallic nanoparticles containing Ag and noble metals is more difficult than that of our previous Pt–Pd system for the following reasons [22]: First, there are few noble metal complexes soluble in water, except for their halides. Unfortunately, however, Ag+ ion readily reacts with halogen ions to form water-insoluble silver halides. Hence, the choice of a noble metal candidate to be mixed with a silver salt is restricted. In the case of other * Corresponding author.
E-mail address: [email protected] (H.-K. Rhee). 0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2003.12.006
water-soluble noble metal compounds such as cyanides and amine complexes, their strong binding with metal ions may give rise to another serious problem. They are less active than halides for elimination or substitution reactions of ligands, which are prerequisites for the reduction of metal ions [23]. Second, the standard electrode potential of Ag+ /Ag0 is relatively high, so it often occurs that Ag+ is reduced much more rapidly than other metal ions and hence bimetallic particles are hardly formed. In addition, there are some intrinsic problems in the dendrimer templating systems. With PAMAM-OH dendrimer, cumbersome pH controls are required for the stable formation of Ag particles in the dendrimer cavity [24]. Without pH control, Ag nanoparticles can be formed at quite a small metal-to-dendrimer ratio because of the weaker interaction between Ag+ and the internal amine of PAMAM-OH than in the case of Pt or Pd [17,18]. This suggests that only a small amount of Ag+ ion can be spontaneously extracted into dendrimer. With PAMAM dendrimer, precipitation may occur because of noble-metal-induced cross-linking [25]. Only at low metal-to-dendrimer ratios can unfavorable precipitation be prevented. Therefore, it is difficult to prepare Ag–Pd bimetallic nanoparticles with conventional metal precursors such as K2 PdCl4 and AgNO3 in dendrimer templating systems. In this regard, in line with our ongoing effort in the preparation of dendrimer-templated bimetallic nanoparticles, we have aimed here to demonstrate the preparation of Ag–Pd bimetallic nanoparticles using silver(I)– bis(oxalato)palladate(II) complex. The complex seems suit-
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Y.-M. Chung, H.-K. Rhee / Journal of Colloid and Interface Science 271 (2004) 131–135
able for Ag–Pd bimetal formation because two metal ions exist in one complex and thus the unfavorable formation of silver halides can be avoided. Moreover, the oxalate ligand rapidly decomposes by light irradiation or chemical reduction [7]. To the best of our knowledge, this is the first effort to prepare dendrimer-templated Ag–Pd bimetallic nanoparticles.
2. Materials and methods Amine-terminated fourth-generation Starburst poly(amidoamine) (PAMAM) dendrimer having an ethylenediamine core was obtained as 10 wt% methanol solutions (Aldrich). Prior to its use, methanol was removed by rotary evaporation at room temperature. K2 PdCl4 (98%), K2 C2 O4 ·H2 O (99%), and NaBH4 (99%) were supplied from Aldrich and used as received without further purification. All solutions used for preparation of samples for TEM and UV–vis spectra were dialyzed against water for 24 h. Cellulose membranes (Pierce) having a molecular-weight cutoff of 10,000 were soaked in water for 1 h and rinsed thoroughly with deionized water before dialysis. Silver(I)–bis(oxalato)palladate(II) complex was synthesized according to the literature [22,26]. For 2 mmol K2 PdCl4 dissolved in 20 ml water, 4 mmol K2 C2 O4 ·H2 O was added at 60 ◦ C with stirring. Immediately the solution turned orange and K2 [Pd(C2 O4 )2 ]·4H2 O was precipitated. It was recrystallized several times from cold water to remove the byproduct KCl completely. Then, to 20 ml of aqueous solution dissolving 1.5 mmol of K2 [Pd(C2 O4 )2 ], 1.5 mmol of AgNO3 was added at 60 ◦ C with stirring. Upon cooling in an ice water bath, yellow crystals of Ag2 [Pd(C2 O4 )2 ]·3H2 O were precipitated, which were washed with cold water and stored in the dark. The preparation of Ag–Pd bimetallic nanoparticles with dendrimer templates was carried out by a method similar to those for monometallic nanoparticles, except for the use of silver(I)–bis(oxalato)palladate(II) complex. Dilute aqueous solution of PAMAM dendrimer was mixed with aqueous solutions of oxalate complexes at controlled stoichiometries. After the solution was stirred for 1 h, aqueous solution of NaBH4 was slowly added and subsequently the two metal ions were simultaneously reduced to yield zerovalent metal particles. The light yellow dendrimer/(silver(I)– bis(oxalato)palladate(II) complex) solution immediately turned dark brown or black, indicating the formation of colloidal nanoparticles. For the purpose of changing the Ag–Pd composition, AgNO3 was mixed with Ag2 [Pd(C2 O4 )2 ] at a calculated ratio, and the mixed solution were reduced by NaBH4 in the same manner. Monometallic Pd and Ag nanoparticles were prepared by the reduction of K2 PdCl4 and AgNO3 , respectively. All of the resulting nanoparticles were very stable and there was no precipitation for up to three months.
Absorption spectra were recorded on a Perkin–Elmer Lambda 35 UV–vis spectrometer using deionized water as a reference for all measurements. High-resolution transmission electron micrographs (HRTEM) were obtained using a JEOL JEM-3000F transmission electron microscope. The specimens of various bimetallic nanoparticles were prepared by placing a drop of dilute aqueous dendrimer solution on a carbon-coated copper TEM grid and allowing the water to evaporate in air. The HRTEM images were recorded digitally with a chargecoupled-device (CCD) camera (Gattan MSC-794 Model). Average particle sizes and the size distribution of the nanoparticles (approximately 150 particles) were measured from enlarged photographs of TEM images using image analysis software (Scion image).
3. Results and discussion Fig. 1 shows the changes in the absorption spectra of Ag and Pd metals (Ag/Pd ratio = 2) during the course of the complexation with dendrimer and the subsequent reduction. Concerning the metal ions, the mixed absorption peak of two metal ions is observed at 225 nm. After the addition of PAMAM to the solution of the Ag2 [Pd(C2 O4 )2 ], however, an enhanced absorption band appears around 230 nm. This indicates that two metal ions are complexed with the dendrimer. Although it is difficult to explain the UV–vis spectra of complexation, the change in absorption spectra may result from the interaction between metal precursor and dendrimer. The absorption band of ‘b’ is different from the sum of ‘a’ (metal precursor) and ‘d’ (dendrimer). After reduction, the band observed in the former case completely disappears and a new broad absorption band appears over a wide wavelength range. Fig. 2 presents a series of UV–vis spectra of Ag–Pd bimetallic nanoparticles with various Ag/Pd ratios. As the Ag/Pd ratio increases, the λmax shifts to a longer wavelength. Moreover, it should be noted that the spectra of the resulting nanoparticles are different not only from those of the monometallic Ag or Pd nanoparticles but also from those of their physical mixtures. Therefore, the characteristic absorption spectra strongly suggest the formation of Ag–Pd bimetallic nanoparticles. The change in the absorption spectra of the bimetallic nanoparticles from those of monometallic ones can be primarily attributed to the change in dielectric function caused by mixing the two different metal atoms [27–29]. The broadening of the plasmon peak results from an overlap between surface plasmon band and interband transitions. According to Creighton et al. [28], silver displays approximately freeelectron behavior in the visible range, which gives rise to a sharp absorption band, while other metals including Pd are less free-electron metals, resulting in a broad absorption band. An intermediate bandwidth between Ag and Pd shown in Fig. 2 is consistent with the theory and may be considered
Y.-M. Chung, H.-K. Rhee / Journal of Colloid and Interface Science 271 (2004) 131–135
Fig. 1. Variations in the UV–vis spectra of Ag and Pd metals during the course of the complexation and the subsequent reduction: (a) 4 mM Ag2 [Pd(C2 O4 )2 ]; (b) complexation of metal ions with 40 mM dendrimer; (c) reduction with 40 mM NaBH4 ; and (d) PAMAM dendrimer (generation 4). The concentration of dendrimer in (b) refers to the surface functional groups. The concentrations of both total metal ion and dendrimer are kept constant irrespective of the Ag/Pd ratio.
Fig. 2. UV–vis spectra of Ag–Pd bimetallic nanoparticles with various Ag/Pd ratios.
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as additional evidence for the formation of Ag–Pd bimetallic nanoparticles. Concerning the mechanism, one may refer to the mechanism suggested by Torigoe et al. [22,29], i.e., (1) reduction of Ag+ by NaBH4 , (2) an electron acceptor is adsorbed onto Ag0 surface which leads to a decrease in the electron density of Ag0 and the Fermi potential goes down, and (3) Ag0 induced reduction of Pd2+ with the oxidative elimination of C2 O2− 4 leads to an elevation of the Fermi potential. The representative HRTEM image of G4-NH2 (Ag2 0 / Pd1 0 ) is shown in Fig. 3. The microscopy demonstrates that the particle size is uniform and the shape is nearly spherical. Bimetallic nanoparticles with a diameter of ∼2.4 ± 0.2 nm are observed. The formation of quite monodisperse nanoparticles suggests the effectiveness of dendrimers acting as stabilizers for the prevention of aggregation. Although the particle size distribution of Ag–Pd bimetallic nanoparticles is somewhat broad compared to the cases of monometallic and bimetallic noble metal nanoparticles [19,20], the size distribution of the resulting nanoparticles is rather uniform compared to that of the Ag nanoparticles shown in Fig. 4. It is to be noted, however, that one may expect some errors when the particle size distribution is determined on the basis of the TEM image because of the aggregation on the grid and the expansion of the flattened nanoparticles in comparison with the solution method. To confirm whether the nanoparticles are a mixture of Ag and Pd nanoparticles or are bimetallic, EDS analysis was carried out and both Ag and Pd were detected. However, it was rather difficult to acquire the elements of one particle because the spatial resolution (ca. 5 nm) was not high enough to detect the X-rays generated from a single particle. In the present study, therefore, only the average compositions are discussed. The variations of particle size with the change of Ag/Pd ratio are presented in Fig. 5. It is observed that the average particle size increases with increased Ag content. Compared with Ag nanoparticles, which show larger and
Fig. 3. HRTEM image and particle size distribution of Ag–Pd bimetallic nanoparticles with on Ag/Pd ratio of 2.
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Fig. 4. TEM image and particle size distribution of Ag nanoparticles.
Acknowledgment Financial aid from the Brain Korea 21 Program supported by the Ministry of Education is gratefully acknowledged.
References
Fig. 5. Variations of particle size with the change of Ag/Pd ratio.
broader size distributions, the particle size distributions of Ag–Pd bimetallic nanoparticles remain in the range of 2– 5 nm regardless of Ag/Pd ratios. The narrow particle size distributions may be further evidence of bimetal formation.
4. Conclusion In summary, ultrafine dendrimer-templated Ag–Pd bimetallic nanoparticles with various metal compositions have been successfully prepared. The characteristic absorption spectra can be described as a result of an overlap between surface plasmon band and interband transitions, indicating the formation of Ag–Pd bimetallic nanoparticles. Moreover, the particle size distribution is dependent upon the Ag/Pd ratio but remains in a narrow region. The resulting Ag–Pd bimetallic nanoparticles can be useful in examining optical spectra, SERS (surface-enhanced Raman scattering) activity, or photoelectrochemical reactions [22]. Other types of dendrimer-templated bimetallic nanoparticles will be reported in due course.
[1] G.R. Newkome, C.N. Moorefield, F. Vögtle, Dendritic Molecules: Concepts, Synthesis, Perspectives, VCH, Weinheim, 1996. [2] Y.-M. Chung, H.-K. Rhee, Chem. Commun. 238 (2002). [3] Y.-M. Chung, H.-K. Rhee, Catal. Lett. 82 (3–4) (2002) 249. [4] D.A. Tomalia, H.D. Durst, Top. Curr. Chem. 165 (1993) 193. [5] M. Fischer, F. Vögtle, Angew. Chem. Int. Ed. 38 (7) (1999) 884. [6] L. Balogh, R. Valluzzi, G.L. Hagnauer, K.S. Laverdure, S.P. Gido, D.A. Tomalia, J. Nanoparticle Res. 1 (3) (1999) 353. [7] J.-A. He, R. Valluzzi, K. Yang, T. Dolukhanyan, C. Sung, J. Kumar, S.K. Tripathy, L. Samuelson, L. Balogh, D.A. Tomalia, Chem. Mater. 11 (11) (1999) 3268. [8] M.F. Ottaviani, R. Valluzzi, L. Balogh, Macromolecules 35 (13) (2002) 5105. [9] M. Zhao, L. Sun, R.M. Crooks, J. Am. Chem. Soc. 120 (1998) 4877. [10] M. Zhao, R.M. Crooks, Adv. Mater. 11 (3) (1999) 217. [11] K. Esumi, A. Suzuki, N. Aihara, K. Usui, K. Torigoe, Langmuir 14 (1998) 3157. [12] K. Esumi, A. Suzuki, A. Yamahira, K. Torigoe, Langmuir 16 (2000) 2604. [13] L. Balogh, D.A. Tomalia, J. Am. Chem. Soc. 120 (1998) 7355. [14] F. Gröhn, B.J. Bauer, Y.A. Akpalu, C.L. Jackson, E.J. Amis, Macromolecules 33 (2000) 6042. [15] P.N. Floriano, C.O. Noble IV, J.M. Schoonmaker, E.D. Poliakoff, R.L. McCarley, J. Am. Chem. Soc. 123 (2001) 10,545. [16] K. Esumi, K. Miyamoto, T. Yoshimura, J. Colloid Interface Sci. 254 (2002) 402. [17] R.M. Crooks, B. Lemon, L.K. Yeung, M. Zhao, Top. Curr. Chem. 212 (2000) 81. [18] R.M. Crooks, M. Zhao, L. Sun, V. Chechik, L.K. Yeung, Acc. Chem. Res. 34 (3) (2001) 181. [19] Y.-M. Chung, H.-K. Rhee, Catal. Lett. 85 (3–4) (2003) 159. [20] Y.-M. Chung, H.-K. Rhee, J. Mol. Cat. A 206 (1–2) (2003) 291. [21] R.W.J. Scott, A.K. Datye, R.M. Crooks, J. Am. Chem. Soc. 125 (2003) 3708.
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[22] K. Torigoe, K. Esumi, Langmuir 9 (1993) 1664. [23] A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, 5th ed., Wiley, New York, 1988. [24] J. Zheng, M.S. Stevenson, R.S. Hikida, P.G.V. Patten, J. Phys. Chem. B 106 (2002) 1252. [25] M. Zhao, R.M. Crooks, Angew. Chem. Int. Ed. 11 (3) (1999) 217.
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[26] Dictionary of Inorganic Compounds, 1st ed., Chapman & Hall, London, 1992, and references therein. [27] M.P. Andrews, G.A. Ozin, J. Phys. Chem. 90 (1986) 2929. [28] J.A. Creighton, D.G. Eadon, J. Chem. Soc., Faraday Trans. 87 (24) (1991) 3881. [29] K. Torigoe, Y. Nakajima, K. Esumi, J. Phys. Chem. 97 (1993) 8304.
Dendrimer-templated Ag–Pd bimetallic nanoparticles Young-Min Chung and Hyun-Ku Rhee ∗ School of Chemical Engineering and Institute of Chemical Processes, Seoul National University, Kwanak-ku, Seoul 151-742, Republic of Korea Received 17 March 2003; accepted 2 December 2003
Abstract Ultrafine dendrimer-templated Ag–Pd bimetallic nanoparticles with various metal compositions have been prepared successfully using silver(I)–bis(oxalato)palladate(II) complex. The use of an oxalate complex, in which two metal ions exist in one complex, is found to be effective in preventing unfavorable silver halide formation and thus suitable for the formation of Ag–Pd bimetallic nanoparticles. 2003 Elsevier Inc. All rights reserved. Keywords: Dendrimer; Bimetal; Nanoparticles; Template; Nanocomposite; Organic–inorganic hybrid
1. Introduction Dendrimers are highly branched macromolecules and they are generally described as having a structure of spherical shape with a high degree of symmetry [1–8]. With the prospect of dendrimers as a template for the formation of inorganic nanoparticles, various metal nanoparticles have been successfully prepared [9–16]. However, it is worth noting that most previous studies have been confined to monometallic nanoparticles, and bimetallic nanoparticles have not been exploited yet, although there has been an attempt to prepare dendrimer/bimetal nanocomposites using two different metal precursors [17,18]. Recently, we prepared Pt–Pd [19] and Pd–Rh [20] bimetallic nanoparticles in the presence of poly(amidoamine) dendrimers with surface hydroxyl groups (fourth generation, PAMAM-OH) and applied these bimetallic nanoparticles as catalysts for the partial hydrogenation of 1,3-cyclooctadiene. The Pt–Pd system has also been studied by Scott et al. [21]. On the other hand, the preparation of bimetallic nanoparticles containing Ag and noble metals is more difficult than that of our previous Pt–Pd system for the following reasons [22]: First, there are few noble metal complexes soluble in water, except for their halides. Unfortunately, however, Ag+ ion readily reacts with halogen ions to form water-insoluble silver halides. Hence, the choice of a noble metal candidate to be mixed with a silver salt is restricted. In the case of other * Corresponding author.
E-mail address: [email protected] (H.-K. Rhee). 0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2003.12.006
water-soluble noble metal compounds such as cyanides and amine complexes, their strong binding with metal ions may give rise to another serious problem. They are less active than halides for elimination or substitution reactions of ligands, which are prerequisites for the reduction of metal ions [23]. Second, the standard electrode potential of Ag+ /Ag0 is relatively high, so it often occurs that Ag+ is reduced much more rapidly than other metal ions and hence bimetallic particles are hardly formed. In addition, there are some intrinsic problems in the dendrimer templating systems. With PAMAM-OH dendrimer, cumbersome pH controls are required for the stable formation of Ag particles in the dendrimer cavity [24]. Without pH control, Ag nanoparticles can be formed at quite a small metal-to-dendrimer ratio because of the weaker interaction between Ag+ and the internal amine of PAMAM-OH than in the case of Pt or Pd [17,18]. This suggests that only a small amount of Ag+ ion can be spontaneously extracted into dendrimer. With PAMAM dendrimer, precipitation may occur because of noble-metal-induced cross-linking [25]. Only at low metal-to-dendrimer ratios can unfavorable precipitation be prevented. Therefore, it is difficult to prepare Ag–Pd bimetallic nanoparticles with conventional metal precursors such as K2 PdCl4 and AgNO3 in dendrimer templating systems. In this regard, in line with our ongoing effort in the preparation of dendrimer-templated bimetallic nanoparticles, we have aimed here to demonstrate the preparation of Ag–Pd bimetallic nanoparticles using silver(I)– bis(oxalato)palladate(II) complex. The complex seems suit-
132
Y.-M. Chung, H.-K. Rhee / Journal of Colloid and Interface Science 271 (2004) 131–135
able for Ag–Pd bimetal formation because two metal ions exist in one complex and thus the unfavorable formation of silver halides can be avoided. Moreover, the oxalate ligand rapidly decomposes by light irradiation or chemical reduction [7]. To the best of our knowledge, this is the first effort to prepare dendrimer-templated Ag–Pd bimetallic nanoparticles.
2. Materials and methods Amine-terminated fourth-generation Starburst poly(amidoamine) (PAMAM) dendrimer having an ethylenediamine core was obtained as 10 wt% methanol solutions (Aldrich). Prior to its use, methanol was removed by rotary evaporation at room temperature. K2 PdCl4 (98%), K2 C2 O4 ·H2 O (99%), and NaBH4 (99%) were supplied from Aldrich and used as received without further purification. All solutions used for preparation of samples for TEM and UV–vis spectra were dialyzed against water for 24 h. Cellulose membranes (Pierce) having a molecular-weight cutoff of 10,000 were soaked in water for 1 h and rinsed thoroughly with deionized water before dialysis. Silver(I)–bis(oxalato)palladate(II) complex was synthesized according to the literature [22,26]. For 2 mmol K2 PdCl4 dissolved in 20 ml water, 4 mmol K2 C2 O4 ·H2 O was added at 60 ◦ C with stirring. Immediately the solution turned orange and K2 [Pd(C2 O4 )2 ]·4H2 O was precipitated. It was recrystallized several times from cold water to remove the byproduct KCl completely. Then, to 20 ml of aqueous solution dissolving 1.5 mmol of K2 [Pd(C2 O4 )2 ], 1.5 mmol of AgNO3 was added at 60 ◦ C with stirring. Upon cooling in an ice water bath, yellow crystals of Ag2 [Pd(C2 O4 )2 ]·3H2 O were precipitated, which were washed with cold water and stored in the dark. The preparation of Ag–Pd bimetallic nanoparticles with dendrimer templates was carried out by a method similar to those for monometallic nanoparticles, except for the use of silver(I)–bis(oxalato)palladate(II) complex. Dilute aqueous solution of PAMAM dendrimer was mixed with aqueous solutions of oxalate complexes at controlled stoichiometries. After the solution was stirred for 1 h, aqueous solution of NaBH4 was slowly added and subsequently the two metal ions were simultaneously reduced to yield zerovalent metal particles. The light yellow dendrimer/(silver(I)– bis(oxalato)palladate(II) complex) solution immediately turned dark brown or black, indicating the formation of colloidal nanoparticles. For the purpose of changing the Ag–Pd composition, AgNO3 was mixed with Ag2 [Pd(C2 O4 )2 ] at a calculated ratio, and the mixed solution were reduced by NaBH4 in the same manner. Monometallic Pd and Ag nanoparticles were prepared by the reduction of K2 PdCl4 and AgNO3 , respectively. All of the resulting nanoparticles were very stable and there was no precipitation for up to three months.
Absorption spectra were recorded on a Perkin–Elmer Lambda 35 UV–vis spectrometer using deionized water as a reference for all measurements. High-resolution transmission electron micrographs (HRTEM) were obtained using a JEOL JEM-3000F transmission electron microscope. The specimens of various bimetallic nanoparticles were prepared by placing a drop of dilute aqueous dendrimer solution on a carbon-coated copper TEM grid and allowing the water to evaporate in air. The HRTEM images were recorded digitally with a chargecoupled-device (CCD) camera (Gattan MSC-794 Model). Average particle sizes and the size distribution of the nanoparticles (approximately 150 particles) were measured from enlarged photographs of TEM images using image analysis software (Scion image).
3. Results and discussion Fig. 1 shows the changes in the absorption spectra of Ag and Pd metals (Ag/Pd ratio = 2) during the course of the complexation with dendrimer and the subsequent reduction. Concerning the metal ions, the mixed absorption peak of two metal ions is observed at 225 nm. After the addition of PAMAM to the solution of the Ag2 [Pd(C2 O4 )2 ], however, an enhanced absorption band appears around 230 nm. This indicates that two metal ions are complexed with the dendrimer. Although it is difficult to explain the UV–vis spectra of complexation, the change in absorption spectra may result from the interaction between metal precursor and dendrimer. The absorption band of ‘b’ is different from the sum of ‘a’ (metal precursor) and ‘d’ (dendrimer). After reduction, the band observed in the former case completely disappears and a new broad absorption band appears over a wide wavelength range. Fig. 2 presents a series of UV–vis spectra of Ag–Pd bimetallic nanoparticles with various Ag/Pd ratios. As the Ag/Pd ratio increases, the λmax shifts to a longer wavelength. Moreover, it should be noted that the spectra of the resulting nanoparticles are different not only from those of the monometallic Ag or Pd nanoparticles but also from those of their physical mixtures. Therefore, the characteristic absorption spectra strongly suggest the formation of Ag–Pd bimetallic nanoparticles. The change in the absorption spectra of the bimetallic nanoparticles from those of monometallic ones can be primarily attributed to the change in dielectric function caused by mixing the two different metal atoms [27–29]. The broadening of the plasmon peak results from an overlap between surface plasmon band and interband transitions. According to Creighton et al. [28], silver displays approximately freeelectron behavior in the visible range, which gives rise to a sharp absorption band, while other metals including Pd are less free-electron metals, resulting in a broad absorption band. An intermediate bandwidth between Ag and Pd shown in Fig. 2 is consistent with the theory and may be considered
Y.-M. Chung, H.-K. Rhee / Journal of Colloid and Interface Science 271 (2004) 131–135
Fig. 1. Variations in the UV–vis spectra of Ag and Pd metals during the course of the complexation and the subsequent reduction: (a) 4 mM Ag2 [Pd(C2 O4 )2 ]; (b) complexation of metal ions with 40 mM dendrimer; (c) reduction with 40 mM NaBH4 ; and (d) PAMAM dendrimer (generation 4). The concentration of dendrimer in (b) refers to the surface functional groups. The concentrations of both total metal ion and dendrimer are kept constant irrespective of the Ag/Pd ratio.
Fig. 2. UV–vis spectra of Ag–Pd bimetallic nanoparticles with various Ag/Pd ratios.
133
as additional evidence for the formation of Ag–Pd bimetallic nanoparticles. Concerning the mechanism, one may refer to the mechanism suggested by Torigoe et al. [22,29], i.e., (1) reduction of Ag+ by NaBH4 , (2) an electron acceptor is adsorbed onto Ag0 surface which leads to a decrease in the electron density of Ag0 and the Fermi potential goes down, and (3) Ag0 induced reduction of Pd2+ with the oxidative elimination of C2 O2− 4 leads to an elevation of the Fermi potential. The representative HRTEM image of G4-NH2 (Ag2 0 / Pd1 0 ) is shown in Fig. 3. The microscopy demonstrates that the particle size is uniform and the shape is nearly spherical. Bimetallic nanoparticles with a diameter of ∼2.4 ± 0.2 nm are observed. The formation of quite monodisperse nanoparticles suggests the effectiveness of dendrimers acting as stabilizers for the prevention of aggregation. Although the particle size distribution of Ag–Pd bimetallic nanoparticles is somewhat broad compared to the cases of monometallic and bimetallic noble metal nanoparticles [19,20], the size distribution of the resulting nanoparticles is rather uniform compared to that of the Ag nanoparticles shown in Fig. 4. It is to be noted, however, that one may expect some errors when the particle size distribution is determined on the basis of the TEM image because of the aggregation on the grid and the expansion of the flattened nanoparticles in comparison with the solution method. To confirm whether the nanoparticles are a mixture of Ag and Pd nanoparticles or are bimetallic, EDS analysis was carried out and both Ag and Pd were detected. However, it was rather difficult to acquire the elements of one particle because the spatial resolution (ca. 5 nm) was not high enough to detect the X-rays generated from a single particle. In the present study, therefore, only the average compositions are discussed. The variations of particle size with the change of Ag/Pd ratio are presented in Fig. 5. It is observed that the average particle size increases with increased Ag content. Compared with Ag nanoparticles, which show larger and
Fig. 3. HRTEM image and particle size distribution of Ag–Pd bimetallic nanoparticles with on Ag/Pd ratio of 2.
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Fig. 4. TEM image and particle size distribution of Ag nanoparticles.
Acknowledgment Financial aid from the Brain Korea 21 Program supported by the Ministry of Education is gratefully acknowledged.
References
Fig. 5. Variations of particle size with the change of Ag/Pd ratio.
broader size distributions, the particle size distributions of Ag–Pd bimetallic nanoparticles remain in the range of 2– 5 nm regardless of Ag/Pd ratios. The narrow particle size distributions may be further evidence of bimetal formation.
4. Conclusion In summary, ultrafine dendrimer-templated Ag–Pd bimetallic nanoparticles with various metal compositions have been successfully prepared. The characteristic absorption spectra can be described as a result of an overlap between surface plasmon band and interband transitions, indicating the formation of Ag–Pd bimetallic nanoparticles. Moreover, the particle size distribution is dependent upon the Ag/Pd ratio but remains in a narrow region. The resulting Ag–Pd bimetallic nanoparticles can be useful in examining optical spectra, SERS (surface-enhanced Raman scattering) activity, or photoelectrochemical reactions [22]. Other types of dendrimer-templated bimetallic nanoparticles will be reported in due course.
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