- Email: [email protected]
Pt bimetallic nanoparticles
Chemical Physics Letters 420 (2006) 484–488 www.elsevier.com/locate/cplett
Local structural characterization of Au/Pt bimetallic nanoparticles H.M. Chen a, H.-C. Peng a, R.S. Liu a
a,*
, S.F. Hu b, L.-Y. Jang
c
Department of Chemistry, National Taiwan University, #1, Section 4, Roosevelt Road, Taipei 106, Taiwan, ROC b National Nano Device Laboratories, Hsinchu 300, Taiwan, ROC c National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan, ROC Received 28 October 2005; in final form 21 December 2005 Available online 26 January 2006
Abstract In this study, we examined the amount-dependent change in morphology for a series of Au/Pt bimetallic nanoparticles synthesized using chemical reduction. The Au/Pt molar ratio was varied from 1/1 to 1/4 to synthesize Pt shell layers with different thicknesses. We have obtained that these bimetallic nanoparticles can form flower-like nanoparticles. Moreover, an extended X-ray absorption fine structure (EXAFS) analysis was used to demonstrate the structure of Au/Pt bimetallic nanoparticles. The EXAFS results confirmed the formation of a core-shell structure and inter-diffusion between Au and Pt atoms. The composition of the shell layer was found to be Pt-enriched Au/Pt alloy. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction Nanoparticles of noble metals are interesting because of the transition between quantum and bulk properties. In addition, their unusual chemical and physical properties may lead to many potential applications in semiconductors, catalysts, photonic crystals, magnetic materials, and so on [1–3]. A common example is the depression of the melting temperature (Tm) in metallic systems at a nanoscale [4–8]. The size effect may appear in bimetallic systems during alloy formation. Mori and co-workers reported the spontaneous alloying of Cu [9], Zn [10], and Pb [11] with gold. In addition, Tomohiro et al. [12] also reported the spontaneous alloying of Ag with gold at ambient temperature. Bimetallic catalysts have been considered to be valuable for investigating the relationship between the catalytic activity and structure of catalysts [13–15]. Metal catalysts composed of two different metallic elements are of interest from both scientific and technological points of view, bimetallic nanoparticles also show a different surface plasma band compared to pure metals [16–19]. It is *
Corresponding author. Fax: +886 2 23636359. E-mail address: [email protected] (R.S. Liu).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.12.086
quite challenging to prepare bimetallic nanoparticles with a controlled composition distribution. Moreover, the rigorous structural characterization of bimetallic nanoparticles is important. Although transmission electron microscopy (TEM) has been shown to be a powerful tool for the analysis of nanoparticles, it is limited to study the chemical bonds and structural characteristics of multicomponent materials. In contrast, X-ray absorption spectroscopy (XAS) can provide information about the nanostructure (e.g., coordination number, interatomic distance, and oxidation state of absorption atoms). This structural information is averaged over all of the X-ray-excited atoms in the entire sample, but since a large percentage of the atoms in nanoparticles are at the interface, the information obtained is sensitive to the bimetallic interfacial structure. In this study, we demonstrate systematic variation in the shape and spectrum of bimetallic nanoparticles, using tannic acid and trisodium citrate dehydrate as reductants in hot (95 °C) aqueous solution. Furthermore, analysis by extended X-ray absorption fine structure (EXAFS) was useful for identifying the random or preferred occupation of sites around specific atoms. Using EXAFS, the interatomic distances, the variations in distances, and the identity
H.M. Chen et al. / Chemical Physics Letters 420 (2006) 484–488
and the number of nearest neighboring atoms (coordination numbers labeled below CN) within the first few coordination shells of the X-ray-excited atoms can be determined. Using EXAFS, we demonstrated the structure of Au/Pt bimetallic nanoparticles suspended in aqueous solution. 2. Experiments 2.1. Preparation of metallic nanoparticles Hydrogen tetrachloraurate (III) hydrate, tannic acid (95%), trisodium citrate dehydrate (99%), and hydrogen hexachloroplatinate (IV) hydrate were obtained from Acros Organics. The water used throughout this work was reagent-grade water produced by a Milli-Q SP ultrapure-water purification system from Nihon Millipore Ltd., Tokyo. A solution consisting of 0.2 mM AuCl 4 and 0–0.8 mM PtCl2 was prepared by dissolving the corre6 sponding crystals in water. The reductant consisted of solutions of 4 mL of tannic acid (2%) and 1 mL of trisodium citrate (1%). Colloidal metallic nanoparticles were prepared by adding reductant to solutions containing Au and Pt metal salts at different ratios. The reaction mixture was stirred for 15 min at 100 °C, and then stirred for an additional 10 min without heating. 2.2. Characterization of Au/Pt bimetallic clusters The surface morphology of the samples was examined by a transmission electron microscope (TEM; JEM2000EX, 200 kV). Specimens were obtained by placing several drops of the colloidal solution on a Formvar-covered copper grid and evaporating it in air at room temperature. For the XAS measurement, the Au/Pt bimetallic nanoparticle solution was placed in aluminum cells and sealed in a plastic bag. Au LIII-edge and Pt LIII-edge X-ray absorption spectra were recorded on the BL17C1 beam line of NSRRC, which has been designed for such experiments. The data were collected in transmission mode with nitrogen and argon mixed gas-filled ionization chambers as detectors. Energy calibrations were carried out with Au and Pt metal foils, and the first inflection points were assigned to 11.918 and 11.564 eV, respectively. To remove an energy shift problem, X-ray absorption spectra for Au and Pt metal foils were measured simultaneously in every measurement when the metal foils were positioned before the window of the third ion chamber. 2.3. EXAFS data analysis EXAFS analyses were carried out using an analysis package, ‘REX’ or ‘REX2000’, coded by Rigaku [20]. EXAFS oscillations [v(k)] were elucidated from the data by a spline smoothing method and normalized. The normalized k3-weighted EXAFS spectra, k3v(k), were Fou˚ 1 to rier-transformed within k range from 2.0 to 13 A
485
show the contribution of each bond pair in the Fourier transform (FT) peak. The experimental Fourier-filtered spectra were obtained from the inverse Fourier transformation with a Hanning window function in the r space range ˚ to determine the structural paramebetween 1.0 and 3.3 A ters for each bond pair. The structural parameters, such as the coordination number, the interatomic distance and the Debye–Waller factor, were used as adjustable parameters in the fitting process for the EXAFS spectra. The data were then analyzed by curve-fitting methods in k space using the following equation: X S i N i F i ðk i Þe2r2i k2i sinð2k i ri þ ui ðk i ÞÞ vðkÞ ¼ k i r2i i qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k i ¼ k 2 2me DEi =h2 where Si, Ni, ri, and ri, are an amplitude reduction factor, a coordination number, a Debye–Waller factor, and a bond distance for the ith shell, respectively. Fi(k) and ui(k) are backscattering amplitude and a phase shift function, respectively. The phase shift and amplitudes were calculated using FEFF8 [21]. DEi is the difference between the experimentally determined photoelectron kinetic energy (Ek) and absorption edge (E0), which is used for theoretical calculations for phase shift and amplitude functions of the ith shell. Si was fixed at 0.8, which was obtained from the corresponding metal foil. For EXAFS analysis of bimetallic nanoparticles, the extraction result of two kinds metal from corresponding spectra must be compare strictly. However, the absorption edge of these two metal are too close to analyze the structure information from EXAFS. The spectrum range of Pt is so short (350 eV), merely the spectrum of Au can be extracted in this study. 3. Results and discussion TEM photographs of nanoparticles are shown in Fig. 1. The bimetallic nanoparticles formed irregular particles as the ratio of Au/Pt decreased. This unusual result seems to be because of the strong collisions among the nucleus, which might make the nucleation of Pt atoms. When a much Pt ions was employed, a high concentration of Pt was present. A faster nucleation rate with plenty of metal ions available resulted in crystal growth much less selective and hence produced irregular nanocrystals. The TEM image in Fig. 2 shows the formation of small metallic clusters on the surface. The HRTEM image shows that these nanoclusters are crystalline, as indicated clearly by atomic lattice fringes. In fact, we have obtained nanoclusters as small as 2 nm when the ratio of Au/Pt was 1/4, noting that these bimetallic nanoparticles can form flower-like nanoparticles. When high concentration of Pt ions was employed, much small clusters deposited on the metallic core instead of increase the size of Au/Pt nanoparticles. As is well known, in addition to the collision energy and the sticking coefficient, the rates of nucleation and growth are determined mainly by the probabilities of collisions
486
H.M. Chen et al. / Chemical Physics Letters 420 (2006) 484–488
Fig. 1. TEM micrographs of nanoparticles: (A) Au; (B) Au/Pt (1/1); (C) Au/Pt (1/2) and (D)Au/Pt (1/3).
between several atoms, between one atom and a nucleus, and between two or more nuclei [22]. This causes the probability of an effective collision between several atoms to become much higher than the other two collisions, resulting in the formation of Pt small clusters on the metal core. The resulting products are in the shape of a flower-like structure nanoparticles. The UV/Vis absorption spectra of Au nanoparticles, Pt nanoparticles, and Au/Pt (1/1) bimetallic nanoparticles are shown in Fig. 3. The absorption band at 530 nm is surface plasmon absorption corresponding to Au nanoparticles. However, no significant surface plasmon absorption was observed at 530 nm in Au/Pt (1/1) bimetallic nanoparticles. The spectrum of bimetallic nanoparticles is easy to distinguish, indicating that Au/Pt (1/1) nanoparticles are not physical mixtures of individual metallic nanoparticles. By observing the change in color during the reaction, it is possible to elucidate the process of formation of Au/Pt bimetallic nanoparticles. In this study, the color turned red immediately after the addition of
reductant due to the formation of Au nanoparticles. In the preparation of bimetallic nanoparticles, the reaction solution turned red immediately after adding the reductant, which suggested that the color change was due to the formation of gold nanoparticles. After several minutes, Pt ions were reduced to deposit on the surface of Au core particles. The solution changed continuously from red to brown at this stage which the surface plasma absorption around 410 nm, indicating that a new phase was formed to make the color of the solution different than those with mono-component nanoparticles (as shown in Fig. 3). To obtain better evidence of the structural parameters of Au/Pt bimetallic nanoparticles, EXAFS was performed. Fourier-transformed EXAFS spectra at the Au LIII-edge of colloidal dispersions of Au nanoclusters, Pt nanoparticles, and Au/Pt bimetallic nanoclusters with Au/Pt ratios of 1/1, 1/2, 1/3, and 1/4 are shown in Fig. 4. The main peak (at a larger radial distance) can be assigned to the Au/Au and Au/Pt bonds, which is
H.M. Chen et al. / Chemical Physics Letters 420 (2006) 484–488
Fig. 2. TEM micrographs of Au/Pt (1/4) nanoparticles.
487
Fig. 4. Fourier-transformed EXAFS spectra at the Au LIII-edge of colloidal dispersions of Au/Pt bimetallic nanoclusters at Au/Pt ratios of 1/ 0, 1/1, 1/2, 1/3, and 1/4.
the bulk. To compare the coordination numbers with those in a random alloy model, the coordination numbers calculated from the random model are shown in Table 1. The simple relation can be expressed as [13] N AuPt ¼ ðX Pt =X Au ÞN AuAu
Fig. 3. UV/Vis absorption spectra of Au, Pt, and Au/Pt (1/1) bimetallic nanoparticles.
˚ by curve-fitting analysis (Table determined to be 2.8 A 1). The appearance of a peak at a smaller radial distance ˚ ) was due to interference from Pt LIII-edge oscilla(1.9 A tion. Although the oscillation amplitude decays with energy, there is too much Pt to avoid this interference. However, the peak-appearance phenomenon supports the existence of Pt atoms, and the peak gradually increases with a decrease in the Au/Pt ratio. The structure parameters of colloidal dispersions of Au/Pt bimetallic nanoclusters were determined on the basis of two-shell fitting as shown in Table 1. The shorter bond distance for the Au/Pt bond indicates that Pt atoms have a smaller radius than Au, which was consistent with the value of
NAuPt and NAuAu represent the coordination numbers of Pt atoms around Au atoms and Au atoms around Au atoms, respectively. XPt and XAu are the atomic fractions of Pt and Au, respectively, in bimetallic nanoclusters. As shown in Table 1, the coordination numbers are quite different from those calculated for the random model. Since the Au/Pt ratio is 1/1, the CN (coordination number) values for Au and Pt atoms around Au atoms are determined to be 7.5 ± 0.4 and 2.9 ± 0.4, respectively. This result is significantly different from the value in the random model, which indicates that the structure of Au/Pt at this stage is not a random alloy. This observation is consistent with the above results in this study, suggesting that the structure of Au/Pt (1/1) is core–shell-type rather than a random alloy. In the case of Au/Pt (1/2), the CN values of Au and Pt atoms around Au atoms are determined to be 5.8 ± 0.9 and 5.3 ± 0.9, respectively. The CN values of Pt atoms around Au atoms significantly increased due to an increase in Pt atoms, which confirms the deposition of Pt atoms on the surface of bimetallic nanoclusters. On the other hand, the Pt atoms deposited on the surface migrated from the surface to the core of the particles, which caused the CN values of Au atoms around Au atoms to decrease. Pt has a smaller atomic radius than Au, which enhances the diffusion of Pt atoms. As can be seen in Table 1, a further increase in Pt hardly affects the CN values of Au and Pt atoms around Au atoms. This finding indicates that excess
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Table 1 Curve-fitting analyses of EXAFS data for Au and Au/Pt bimetallic nanoparticles Au/Pt (molar ratio)
Absorbing metal
Scattering metal
CN
˚) Radial distance (A
Alloy
1/0 1/1
Au Au Au Au Au Au Au Au Au
Au Au Pt Au Pt Au Pt Au Pt
9.0 ± 0.6 7.5 ± 0.4 2.9 ± 0.4 5.8 ± 0.9 5.3 ± 0.9 5.7 ± 0.3 5.4 ± 0.4 5.8 ± 0.3 5.9 ± 0.5
2.84 ± 0.02 2.88 ± 0.03 2.79 ± 0.04 2.87 ± 0.03 2.81 ± 0.06 2.87 ± 0.05 2.80 ± 0.07 2.89 ± 0.06 2.80 ± 0.06
– 5.2 5.2 3.5 6.9 2.6 7.8 2.1 8.3
1/2 1/3 1/4
Pt atoms would not diffuse into the core of bimetallic nanoclusters, and instead form small clusters and deposited on the surfaces of the particles. This proposal is consistent with the above results drawn from TEM, which suggested the formation of a flower-like structure of Pt clusters on the surfaces of Au/Pt bimetallic nanoparticles. This could be explained by the following process. The reduction of 2 AuCl 4 ions is complete before the reduction of PtCl6 ions takes place. The AuCl4 ions are first reduced and form a Au core, and then the PtCl2 ions in the solution are 6 reduced onto the surface of the Au core particles. At the same time, the interdiffusion of Au and Pt atoms leads to the formation of an alloy. Since there are more residual PtCl2 ions than AuCl 6 4 ions in the solution, the Au/Pt alloy is rich in Pt. As a result, the difference in the rates of reduction of the Au and Pt ions leads to the formation of a core-shell structure. The alloying of Au and Pt at the interface may be caused by a relatively high density of defects or vacancies at the interface of the two metal species. Diffusion in metals readily proceeds via the migration of atoms in vacancy defects. A similar explanation has been reported for Au/Ag alloying [9,12]. 4. Conclusion Core–shell-structure Au/Pt nanoparticles were synthesized. The structural parameters drawn from EXAFS also confirm the formation of a core-shell-type structure and an alloying process between Au/Pt atoms. In the case of a Au/ Pt (1/4) sample, Pt clusters formed a string-like structure on the surface. This investigation may provide general synthetic concepts for the synthesis of bimetallic nanoparticles. This special structure of Au/Pt bimetallic nanoparticles may exhibit unique catalytic properties. We are currently working along this direction.
Acknowledgment We thank the National Science Council of Taiwan, for financially supporting this research under Contract No. NSC 94-2113-M-002-006. References [1] L.L. Beecroft, C.K. Ober, Chem. Mater. 9 (1997) 1302. [2] P.V. Kamat, Chem. Rev. 93 (1993) 267. [3] A.J. Hoffman, G. Mils, H. Yee, M.R. Hoffman, J. Phys. Chem. 99 (1995) 4414. [4] A.N. Goldstein, C.M. Echer, A.P. Alivisatos, Science 256 (1992) 1425. [5] P.Z. Pawlow, Phys. Chem. 65 (1909) 545. [6] M. Haase, A.P. Alivisatos, J. Phys. Chem. 96 (1992) 6756. [7] C.-C. Chen, A.B. Herhold, C.S. Johnson, A.P. Alivisatos, Science 276 (1997) 398. [8] S. Tolbert, A.B. Herhold, L.E. Brus, A.P. Alivisatos, Phys. Rev. Lett. 76 (1996) 4384. [9] H. Mori, H. Yasuda, T. Kamino, Philos. Mag. Lett. 69 (1994) 279. [10] H. Yasuda, H. Mori, Phys. Rev. Lett. 69 (1992) 3747. [11] H. Yasuda, H. Mori, Philos. Mag. A 73 (1996) 567. [12] S. Tomohiro, A.B. Bruce, Z. Zhenyuan, M. Dan, F.V. Charles, J.G. Daniel, J. Am. Chem. Soc. 124 (2002) 11989. [13] N. Toshima, M. Harada, T. Yonezawa, T. Kushihashi, K. Asakura, J. Phys. Chem. 95 (1991) 7448. [14] N. Toshima, M. Harada, Y. Yamazaki, K. Asakura, J. Phys. Chem. 96 (1992) 9927. [15] N. Toshima, T. Yonezawa, New J. Chem. (1998) 1179. [16] W.H. Sang, K. Yunsoo, K. Kwan, J. Colloid Interf. Sci. 208 (1998) 272. [17] S. Link, Z.L. Wang, M.A. EI-Sayed, J. Phys. Chem. B 103 (1999) 3529. [18] M.P. Mallin, C.J. Murphy, Nano Lett. 2 (2002) 1235. [19] K. Mallik, M. Mandal, N. Pradhan, T. Pal, Nano Lett. 1 (2001) 319. [20] K. Asakura, in: Y. Iwasawa (Ed.), X-ray Absorption Fine Structure for Catalysts and Surfaces, World Scientific, Singapore, 1996, p. 33. [21] J.J. Rehr, R.C. Albers, Rev. Mod. Phys. 72 (2001) 621. [22] M.-L. Wu, D.-H. Chen, T.-C. Huang, Chem. Mater. 13 (2001) 599.
Local structural characterization of Au/Pt bimetallic nanoparticles H.M. Chen a, H.-C. Peng a, R.S. Liu a
a,*
, S.F. Hu b, L.-Y. Jang
c
Department of Chemistry, National Taiwan University, #1, Section 4, Roosevelt Road, Taipei 106, Taiwan, ROC b National Nano Device Laboratories, Hsinchu 300, Taiwan, ROC c National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan, ROC Received 28 October 2005; in final form 21 December 2005 Available online 26 January 2006
Abstract In this study, we examined the amount-dependent change in morphology for a series of Au/Pt bimetallic nanoparticles synthesized using chemical reduction. The Au/Pt molar ratio was varied from 1/1 to 1/4 to synthesize Pt shell layers with different thicknesses. We have obtained that these bimetallic nanoparticles can form flower-like nanoparticles. Moreover, an extended X-ray absorption fine structure (EXAFS) analysis was used to demonstrate the structure of Au/Pt bimetallic nanoparticles. The EXAFS results confirmed the formation of a core-shell structure and inter-diffusion between Au and Pt atoms. The composition of the shell layer was found to be Pt-enriched Au/Pt alloy. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction Nanoparticles of noble metals are interesting because of the transition between quantum and bulk properties. In addition, their unusual chemical and physical properties may lead to many potential applications in semiconductors, catalysts, photonic crystals, magnetic materials, and so on [1–3]. A common example is the depression of the melting temperature (Tm) in metallic systems at a nanoscale [4–8]. The size effect may appear in bimetallic systems during alloy formation. Mori and co-workers reported the spontaneous alloying of Cu [9], Zn [10], and Pb [11] with gold. In addition, Tomohiro et al. [12] also reported the spontaneous alloying of Ag with gold at ambient temperature. Bimetallic catalysts have been considered to be valuable for investigating the relationship between the catalytic activity and structure of catalysts [13–15]. Metal catalysts composed of two different metallic elements are of interest from both scientific and technological points of view, bimetallic nanoparticles also show a different surface plasma band compared to pure metals [16–19]. It is *
Corresponding author. Fax: +886 2 23636359. E-mail address: [email protected] (R.S. Liu).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.12.086
quite challenging to prepare bimetallic nanoparticles with a controlled composition distribution. Moreover, the rigorous structural characterization of bimetallic nanoparticles is important. Although transmission electron microscopy (TEM) has been shown to be a powerful tool for the analysis of nanoparticles, it is limited to study the chemical bonds and structural characteristics of multicomponent materials. In contrast, X-ray absorption spectroscopy (XAS) can provide information about the nanostructure (e.g., coordination number, interatomic distance, and oxidation state of absorption atoms). This structural information is averaged over all of the X-ray-excited atoms in the entire sample, but since a large percentage of the atoms in nanoparticles are at the interface, the information obtained is sensitive to the bimetallic interfacial structure. In this study, we demonstrate systematic variation in the shape and spectrum of bimetallic nanoparticles, using tannic acid and trisodium citrate dehydrate as reductants in hot (95 °C) aqueous solution. Furthermore, analysis by extended X-ray absorption fine structure (EXAFS) was useful for identifying the random or preferred occupation of sites around specific atoms. Using EXAFS, the interatomic distances, the variations in distances, and the identity
H.M. Chen et al. / Chemical Physics Letters 420 (2006) 484–488
and the number of nearest neighboring atoms (coordination numbers labeled below CN) within the first few coordination shells of the X-ray-excited atoms can be determined. Using EXAFS, we demonstrated the structure of Au/Pt bimetallic nanoparticles suspended in aqueous solution. 2. Experiments 2.1. Preparation of metallic nanoparticles Hydrogen tetrachloraurate (III) hydrate, tannic acid (95%), trisodium citrate dehydrate (99%), and hydrogen hexachloroplatinate (IV) hydrate were obtained from Acros Organics. The water used throughout this work was reagent-grade water produced by a Milli-Q SP ultrapure-water purification system from Nihon Millipore Ltd., Tokyo. A solution consisting of 0.2 mM AuCl 4 and 0–0.8 mM PtCl2 was prepared by dissolving the corre6 sponding crystals in water. The reductant consisted of solutions of 4 mL of tannic acid (2%) and 1 mL of trisodium citrate (1%). Colloidal metallic nanoparticles were prepared by adding reductant to solutions containing Au and Pt metal salts at different ratios. The reaction mixture was stirred for 15 min at 100 °C, and then stirred for an additional 10 min without heating. 2.2. Characterization of Au/Pt bimetallic clusters The surface morphology of the samples was examined by a transmission electron microscope (TEM; JEM2000EX, 200 kV). Specimens were obtained by placing several drops of the colloidal solution on a Formvar-covered copper grid and evaporating it in air at room temperature. For the XAS measurement, the Au/Pt bimetallic nanoparticle solution was placed in aluminum cells and sealed in a plastic bag. Au LIII-edge and Pt LIII-edge X-ray absorption spectra were recorded on the BL17C1 beam line of NSRRC, which has been designed for such experiments. The data were collected in transmission mode with nitrogen and argon mixed gas-filled ionization chambers as detectors. Energy calibrations were carried out with Au and Pt metal foils, and the first inflection points were assigned to 11.918 and 11.564 eV, respectively. To remove an energy shift problem, X-ray absorption spectra for Au and Pt metal foils were measured simultaneously in every measurement when the metal foils were positioned before the window of the third ion chamber. 2.3. EXAFS data analysis EXAFS analyses were carried out using an analysis package, ‘REX’ or ‘REX2000’, coded by Rigaku [20]. EXAFS oscillations [v(k)] were elucidated from the data by a spline smoothing method and normalized. The normalized k3-weighted EXAFS spectra, k3v(k), were Fou˚ 1 to rier-transformed within k range from 2.0 to 13 A
485
show the contribution of each bond pair in the Fourier transform (FT) peak. The experimental Fourier-filtered spectra were obtained from the inverse Fourier transformation with a Hanning window function in the r space range ˚ to determine the structural paramebetween 1.0 and 3.3 A ters for each bond pair. The structural parameters, such as the coordination number, the interatomic distance and the Debye–Waller factor, were used as adjustable parameters in the fitting process for the EXAFS spectra. The data were then analyzed by curve-fitting methods in k space using the following equation: X S i N i F i ðk i Þe2r2i k2i sinð2k i ri þ ui ðk i ÞÞ vðkÞ ¼ k i r2i i qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k i ¼ k 2 2me DEi =h2 where Si, Ni, ri, and ri, are an amplitude reduction factor, a coordination number, a Debye–Waller factor, and a bond distance for the ith shell, respectively. Fi(k) and ui(k) are backscattering amplitude and a phase shift function, respectively. The phase shift and amplitudes were calculated using FEFF8 [21]. DEi is the difference between the experimentally determined photoelectron kinetic energy (Ek) and absorption edge (E0), which is used for theoretical calculations for phase shift and amplitude functions of the ith shell. Si was fixed at 0.8, which was obtained from the corresponding metal foil. For EXAFS analysis of bimetallic nanoparticles, the extraction result of two kinds metal from corresponding spectra must be compare strictly. However, the absorption edge of these two metal are too close to analyze the structure information from EXAFS. The spectrum range of Pt is so short (350 eV), merely the spectrum of Au can be extracted in this study. 3. Results and discussion TEM photographs of nanoparticles are shown in Fig. 1. The bimetallic nanoparticles formed irregular particles as the ratio of Au/Pt decreased. This unusual result seems to be because of the strong collisions among the nucleus, which might make the nucleation of Pt atoms. When a much Pt ions was employed, a high concentration of Pt was present. A faster nucleation rate with plenty of metal ions available resulted in crystal growth much less selective and hence produced irregular nanocrystals. The TEM image in Fig. 2 shows the formation of small metallic clusters on the surface. The HRTEM image shows that these nanoclusters are crystalline, as indicated clearly by atomic lattice fringes. In fact, we have obtained nanoclusters as small as 2 nm when the ratio of Au/Pt was 1/4, noting that these bimetallic nanoparticles can form flower-like nanoparticles. When high concentration of Pt ions was employed, much small clusters deposited on the metallic core instead of increase the size of Au/Pt nanoparticles. As is well known, in addition to the collision energy and the sticking coefficient, the rates of nucleation and growth are determined mainly by the probabilities of collisions
486
H.M. Chen et al. / Chemical Physics Letters 420 (2006) 484–488
Fig. 1. TEM micrographs of nanoparticles: (A) Au; (B) Au/Pt (1/1); (C) Au/Pt (1/2) and (D)Au/Pt (1/3).
between several atoms, between one atom and a nucleus, and between two or more nuclei [22]. This causes the probability of an effective collision between several atoms to become much higher than the other two collisions, resulting in the formation of Pt small clusters on the metal core. The resulting products are in the shape of a flower-like structure nanoparticles. The UV/Vis absorption spectra of Au nanoparticles, Pt nanoparticles, and Au/Pt (1/1) bimetallic nanoparticles are shown in Fig. 3. The absorption band at 530 nm is surface plasmon absorption corresponding to Au nanoparticles. However, no significant surface plasmon absorption was observed at 530 nm in Au/Pt (1/1) bimetallic nanoparticles. The spectrum of bimetallic nanoparticles is easy to distinguish, indicating that Au/Pt (1/1) nanoparticles are not physical mixtures of individual metallic nanoparticles. By observing the change in color during the reaction, it is possible to elucidate the process of formation of Au/Pt bimetallic nanoparticles. In this study, the color turned red immediately after the addition of
reductant due to the formation of Au nanoparticles. In the preparation of bimetallic nanoparticles, the reaction solution turned red immediately after adding the reductant, which suggested that the color change was due to the formation of gold nanoparticles. After several minutes, Pt ions were reduced to deposit on the surface of Au core particles. The solution changed continuously from red to brown at this stage which the surface plasma absorption around 410 nm, indicating that a new phase was formed to make the color of the solution different than those with mono-component nanoparticles (as shown in Fig. 3). To obtain better evidence of the structural parameters of Au/Pt bimetallic nanoparticles, EXAFS was performed. Fourier-transformed EXAFS spectra at the Au LIII-edge of colloidal dispersions of Au nanoclusters, Pt nanoparticles, and Au/Pt bimetallic nanoclusters with Au/Pt ratios of 1/1, 1/2, 1/3, and 1/4 are shown in Fig. 4. The main peak (at a larger radial distance) can be assigned to the Au/Au and Au/Pt bonds, which is
H.M. Chen et al. / Chemical Physics Letters 420 (2006) 484–488
Fig. 2. TEM micrographs of Au/Pt (1/4) nanoparticles.
487
Fig. 4. Fourier-transformed EXAFS spectra at the Au LIII-edge of colloidal dispersions of Au/Pt bimetallic nanoclusters at Au/Pt ratios of 1/ 0, 1/1, 1/2, 1/3, and 1/4.
the bulk. To compare the coordination numbers with those in a random alloy model, the coordination numbers calculated from the random model are shown in Table 1. The simple relation can be expressed as [13] N AuPt ¼ ðX Pt =X Au ÞN AuAu
Fig. 3. UV/Vis absorption spectra of Au, Pt, and Au/Pt (1/1) bimetallic nanoparticles.
˚ by curve-fitting analysis (Table determined to be 2.8 A 1). The appearance of a peak at a smaller radial distance ˚ ) was due to interference from Pt LIII-edge oscilla(1.9 A tion. Although the oscillation amplitude decays with energy, there is too much Pt to avoid this interference. However, the peak-appearance phenomenon supports the existence of Pt atoms, and the peak gradually increases with a decrease in the Au/Pt ratio. The structure parameters of colloidal dispersions of Au/Pt bimetallic nanoclusters were determined on the basis of two-shell fitting as shown in Table 1. The shorter bond distance for the Au/Pt bond indicates that Pt atoms have a smaller radius than Au, which was consistent with the value of
NAuPt and NAuAu represent the coordination numbers of Pt atoms around Au atoms and Au atoms around Au atoms, respectively. XPt and XAu are the atomic fractions of Pt and Au, respectively, in bimetallic nanoclusters. As shown in Table 1, the coordination numbers are quite different from those calculated for the random model. Since the Au/Pt ratio is 1/1, the CN (coordination number) values for Au and Pt atoms around Au atoms are determined to be 7.5 ± 0.4 and 2.9 ± 0.4, respectively. This result is significantly different from the value in the random model, which indicates that the structure of Au/Pt at this stage is not a random alloy. This observation is consistent with the above results in this study, suggesting that the structure of Au/Pt (1/1) is core–shell-type rather than a random alloy. In the case of Au/Pt (1/2), the CN values of Au and Pt atoms around Au atoms are determined to be 5.8 ± 0.9 and 5.3 ± 0.9, respectively. The CN values of Pt atoms around Au atoms significantly increased due to an increase in Pt atoms, which confirms the deposition of Pt atoms on the surface of bimetallic nanoclusters. On the other hand, the Pt atoms deposited on the surface migrated from the surface to the core of the particles, which caused the CN values of Au atoms around Au atoms to decrease. Pt has a smaller atomic radius than Au, which enhances the diffusion of Pt atoms. As can be seen in Table 1, a further increase in Pt hardly affects the CN values of Au and Pt atoms around Au atoms. This finding indicates that excess
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Table 1 Curve-fitting analyses of EXAFS data for Au and Au/Pt bimetallic nanoparticles Au/Pt (molar ratio)
Absorbing metal
Scattering metal
CN
˚) Radial distance (A
Alloy
1/0 1/1
Au Au Au Au Au Au Au Au Au
Au Au Pt Au Pt Au Pt Au Pt
9.0 ± 0.6 7.5 ± 0.4 2.9 ± 0.4 5.8 ± 0.9 5.3 ± 0.9 5.7 ± 0.3 5.4 ± 0.4 5.8 ± 0.3 5.9 ± 0.5
2.84 ± 0.02 2.88 ± 0.03 2.79 ± 0.04 2.87 ± 0.03 2.81 ± 0.06 2.87 ± 0.05 2.80 ± 0.07 2.89 ± 0.06 2.80 ± 0.06
– 5.2 5.2 3.5 6.9 2.6 7.8 2.1 8.3
1/2 1/3 1/4
Pt atoms would not diffuse into the core of bimetallic nanoclusters, and instead form small clusters and deposited on the surfaces of the particles. This proposal is consistent with the above results drawn from TEM, which suggested the formation of a flower-like structure of Pt clusters on the surfaces of Au/Pt bimetallic nanoparticles. This could be explained by the following process. The reduction of 2 AuCl 4 ions is complete before the reduction of PtCl6 ions takes place. The AuCl4 ions are first reduced and form a Au core, and then the PtCl2 ions in the solution are 6 reduced onto the surface of the Au core particles. At the same time, the interdiffusion of Au and Pt atoms leads to the formation of an alloy. Since there are more residual PtCl2 ions than AuCl 6 4 ions in the solution, the Au/Pt alloy is rich in Pt. As a result, the difference in the rates of reduction of the Au and Pt ions leads to the formation of a core-shell structure. The alloying of Au and Pt at the interface may be caused by a relatively high density of defects or vacancies at the interface of the two metal species. Diffusion in metals readily proceeds via the migration of atoms in vacancy defects. A similar explanation has been reported for Au/Ag alloying [9,12]. 4. Conclusion Core–shell-structure Au/Pt nanoparticles were synthesized. The structural parameters drawn from EXAFS also confirm the formation of a core-shell-type structure and an alloying process between Au/Pt atoms. In the case of a Au/ Pt (1/4) sample, Pt clusters formed a string-like structure on the surface. This investigation may provide general synthetic concepts for the synthesis of bimetallic nanoparticles. This special structure of Au/Pt bimetallic nanoparticles may exhibit unique catalytic properties. We are currently working along this direction.
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