- Email: [email protected]
Synthesis of Au–Pd bimetallic nanoparticles under energetic irradiation fields
ARTICLE IN PRESS Radiation Physics and Chemistry 78 (2009) 1049–1053
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Synthesis of Au–Pd bimetallic nanoparticles under energetic irradiation fields N. Taguchi a, A. Iwase a, N. Maeda a, T. Kojima b, R. Taniguchi b, S. Okuda b, T. Akita c, T. Abe d, T. Kambara d, H. Ryuto d, F. Hori a, a
Department of Materials Science, Osaka Prefecture University, Gakuen-cho, Sakai, Osaka 599-8531, Japan Radiation Research Center, Osaka Prefecture University, Gakuen-cho, Sakai, Osaka 599-8570, Japan c National Institute of Advanced Industrial Science and Technology (AIST), Midorigaoka-8-31, Ikeda, Osaka 563-8577, Japan d The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198, Japan b
a r t i c l e in fo
Keywords: Au–Pd nanoparticle synthesis g-rays Pulsed electrons GeV heavy ions Core–shell structure Dose rate dependence
abstract Bimetallic Au–Pd nanoparticles were synthesized under high-energy irradiation fields (1.17 and 1.33 MeV g-rays, 9 MeV electrons, and 1.6 GeV C ions) from solutions containing Au3+ and Pd2+ and cationic surfactant (sodium dodecyl sulfate). Particles synthesized by the irradiation were observed using conventional transmission electron microscope (TEM) and annular dark-field scanning transmission electron microscopy (ADF-STEM). The particles synthesized by g-rays and C ion irradiation exhibit core–shell structure with a Au-core and a Pd-shell. The dependence of the size distribution of nanoparticles on the dose rate is discussed. & 2009 Elsevier Ltd. All rights reserved.
1. Introduction Nanoparticles are of great interest from the viewpoint of using them as new functional materials for applications such as catalysts, because nanoparticles show peculiar properties which are not observed in bulk materials. Especially, bimetallic nanoparticles show new properties associated with the variations of compositions and structures (Edwards et al., 2008). It has been found that sonochemically synthesized Au–Pd nanoparticles using sodium dodecyl sulfate (SDS) as a surfactant have a core–shell structure (Akita et al., 2008). The electronic structures of Au–Pd core–shell nanoparticles are different from Au or Pd monometallic nanoparticles (Hori et al., 2007; Taguchi et al., 2008; Tanaka et al., 2008), and show higher activity for hydrogenation of 4 pentenoic acid (Mizukoshi et al., 2000; Takatani et al., 2003). As peculiar properties of nanoparticles strongly depend on their shapes and structures, it is very important to study various synthesis processes to control them. In our previous work, we obtained Au monometallic nanoparticles using various kinds of high-energy irradiation fields in aqueous solutions (Maeda et al., 2006). In the present study, the same method is used for synthesizing bimetallic Au–Pd nanoparticles. Through the transmission electron microscope (TEM) observation and ultraviolet–visible (UV–vis) measurements, Remita et al. (2003, 2005) have already reported the synthesis of bimetallic nanoparticles such as Au–Pt, Au–Pd, and so
Corresponding author. Tel./fax: +8172 254 9812.
E-mail address: [email protected] (F. Hori). 0969-806X/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2009.05.007
on in various kinds of irradiation fields. They have concluded that the dose rate is an important factor to control the particle size and structures (Remita et al., 2003, 2005; Redjala et al., 2006). To study the dose rate effect on the size and structure of irradiationsynthesized nanoparticles more clearly, we made annular darkfield scanning transmission electron microscopy (ADF-STEM) observations with energy-dispersive X-ray spectroscopy (EDS) measurements for Au–Pd nanoparticles synthesized for various kinds of irradiation as well as conventional transmission electron microscope observations and ultraviolet–visible measurements. In this report, the dependence of size distribution and the local structure of irradiation-synthesized nanoparticles on the kind of irradiation fields is discussed.
2. Experimental Aqueous solutions were prepared in given concentrations (0.5 mM Au3+ ions, 0.5 mM Pd2+ ions) of noble metal complexes (NaAuCl4 2H2O, PdCl2 2NaCl 3H2O) with an cationic surfactant of 8.0 mM SDS. Aqueous solution was poured into polystyrene cells. Each irradiation was performed in air. As the dose rate was too small to increase the temperature of solutions, temperature of each solution was kept at room temperature during the irradiation. The g-rays and electron irradiation were performed at Radiation Research Center of Osaka Prefecture University. For the g-ray irradiation, 60Co source was used. The energies of g-rays from 60Co isotope were 1.17 and 1.33 MeV. The mean dose rate was 40 Gy/min and the maximum dose was 1.0 104 Gy.
ARTICLE IN PRESS 1050
N. Taguchi et al. / Radiation Physics and Chemistry 78 (2009) 1049–1053
3 50
0Gy 1000Gy
40
7500Gy
2
Distribution (%)
Absorbance (arb. u.)
3000Gy
10000Gy
1
30
20
10
0 200
300
400 500 Wavelength (nm)
600
0
700
0
8
16
24 32 40 48 56 Particle Diameter (nm)
64
72
0
8
16
24 32 40 48 56 Particle Diameter (nm)
64
72
0
8
16
24 32 40 48 56 Particle Diameter (nm)
64
72
3 50
0Gy 1500Gy
40
10000Gy
Distribution (%)
Absorbance (arb. u.)
5000Gy
2
1
30
20
10
0 200
300
400 500 Wavelength (nm)
600
0
700
100
3
2
80
12300Gy
Distribution (%)
Absorbance (arb. u.)
0Gy 1500Gy 3000Gy 7500Gy
1
60
40
20
0 200
300
400
500 Wavelength (nm)
600
700
0
Fig. 1. UV–vis absorption spectra for solutions irradiated with (a) g-rays, (b) C ions, and (c) electrons.
Fig. 2. Size histograms of Au–Pd nanoparticles synthesized by (a) g-rays, (b) C ions, and (c) electrons.
The electron beam irradiation was performed using an electron linear accelerator. The energy of electrons was 9 MeV. Electron beam was pulsed, and the pulse frequency was 30 Hz.
Maximum dose was 1.2 104 Gy. The mean dose rate was 4.1 103 Gy/min. The 1.6 GeV C ion irradiation was performed at E5B beam line of RIKEN ring cyclotron accelerator. The dose rate
ARTICLE IN PRESS N. Taguchi et al. / Radiation Physics and Chemistry 78 (2009) 1049–1053
1051
was about 3.0 102 Gy/min, and the maximum dose was 1.0 104 Gy. Chemical reactions in aqueous solutions during the irradiation were traced by UV–vis spectroscopy (Shimadzu UV-2550) and the shapes and the structures of colloidal products were observed by conventional TEM (JEOL JEM-200CX) and STEM (JEOL JEM-3000F equipped with EDS and ADF-STEM systems). Samples for TEM and STEM observations were made by putting a drop of colloidal solutions on TEM meshes and drying them in a vacuum.
Au plasmon peak shifts to the lower wave length side with increasing the ion fluence. This trend is hardly observed in case of g-ray irradiation. The reason why the Au plasmon peak shifts by the ion irradiation, however, remains uncertain. For the electron beam irradiation, the absorption spectra indicate the gradual reduction of metal ions. The intensity of absorption peak for Au particles (around 520 nm) cannot be seen after the irradiation. For all kinds of irradiation processes, the reduction of the metallic ions is completed when the dose reaches about 104 Gy.
3. Results and discussion
3.2. TEM and STEM observations
3.1. UV–vis measurements
3.2.1. Mean particle size Fig. 2 shows the particle size distribution for each irradiation (g-rays, electrons or C ions). The mean diameters of nanoparticles for the g-ray, C ion, and electron irradiation are 25.5, 12.2, 3.2 nm, respectively. For the g-rays and the C ion irradiation, the particle size is widely distributed. On the other hand, nanoparticles synthesized by the electron beam irradiation show a narrow size distribution. Smaller particles with a narrow distribution in size are obtained through the high dose rate irradiation such as electron irradiation (4.1 103 Gy/min), and larger particles with a wide distribution in size are obtained through the low dose rate
Fig. 1(a)–(c) shows UV–vis spectra of each solution for g-ray, C ion, and electron irradiation, respectively. For the g-rays and the C ion irradiation, UV–vis spectra show that an absorption peak is located around 520 nm, which is due to Au particles (called the plasmon peak), and that the absorption peak decreases with increasing the irradiation dose. This result indicates that Au particles are gradually covered with Pd atoms during the irradiation. The Au plasmon peaks, however, still remain even after g-rays or C ion irradiation. Fig. 1(b) shows that the position of
Fig. 3. Conventional TEM and STEM images of synthesized Au–Pd nanoparticles: (a) g-rays, (b) C ions, and (c) electrons. The scale bar in conventional TEM images is 100 nm.
ARTICLE IN PRESS 1052
N. Taguchi et al. / Radiation Physics and Chemistry 78 (2009) 1049–1053
irradiation such as g-ray irradiation (40 Gy/min). This trend is quite similar to the results for the irradiation-synthesized Au–Pt, Au–Pd systems, which have been reported by (Remita et al., 2003, 2005; Redjala et al., 2006). In case of low dose rate irradiation, as
the reduction rate is very small, the growth of nanoparticles tends to occur easily. In contrast, the high dose rate irradiation causes much nucleation that dominates the particle growth, leading to the synthesis of small-size particles.
100
100 C
80
80
60
60
40
40 Pd
Cu
20
Counts (arb. u.)
Cu Cu
Cu
0 C
80
60
Pd
20
Cu
Cu
0
80
60 Pd
40
Cu
40
Cu
C
Au
20
0
Au
Pd Cu
Au
Cu
Au
20
Au
Cu
Cu
Au
0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Energy (keV)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Energy (keV) 100 80 C
60 40
Counts (arb. u.)
Counts (arb. u.)
C
Cu
Au
20 Cu
Pd
Cu
Au
Au
0 80 C
60 40 20
Cu
Au Cu
Pd
Cu
Au
Au
0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Energy (keV) Fig. 4. EDS spectra in local area for Au–Pd nanoparticles synthesized by (a) g-rays, (b) C ions, and (c) electrons.
ARTICLE IN PRESS N. Taguchi et al. / Radiation Physics and Chemistry 78 (2009) 1049–1053
3.2.2. The structure of Au–Pd nanoparticles TEM and ADF-STEM micrographs of Au–Pd nanoparticles synthesized by the g-ray, C ion, and electron irradiation are shown in Fig. 3(a)–(c), respectively. The results of EDS analysis are also shown in Fig. 4. In STEM micrographs, bright regions in each particle strongly reveal Au-cores, and they are surrounded by weakly contrasted regions which correspond to Pd shells (Fig. 3(a) and (b)). The STEM analyses have confirmed that the morphology of many nanoparticles synthesized by the g-rays and C ions is clearly-separated core–shell structure with a Au-core and a Pd-shell. The structure of these core–shell particles is similar to that synthesized by the sonochemical method (Akita et al., 2008). The EDS spectra in the local area of core–shell nanoparticles synthesized by the g-rays and C ions strongly support the results of STEM micrographs (Fig. 4(a) and (b)). At low dose rate process such as the g-rays and the C ion irradiation, Au3+ and Pd2+ ions reduction process is successive, Au3+ ions in aqueous solution are reduced first, and the reduction of Pd2+ ions follows the reduction of Au3+ ions. Then, core–shell-structured nanoparticles can be synthesized. The STEM observation also reveals that Pd-shell thickness of the nanoparticles in g-ray irradiation is not uniform as well as the shapes. In fact, there are some nanoparticles where the Pd-shell does not entirely cover the Au-core. This result well explains the experimental fact that the plasmon peaks of Au also remain in UV–vis spectra even after the irradiation. On the other hand, for the nanoparticles synthesized by the electron irradiation, it is very difficult to see a clear contrast of Au and Pd atoms in the STEM image, because of very small nanoparticle sizes shown in Fig. 3(c). Some nanoparticles seem to have core–shell structures, but others seem to show mixed Au–Pd structures. EDS spectra for the electron irradiation are shown in Fig. 4(c). Due to the spatial resolution of the electron microscope, however, the local structure of the nanoparticles can hardly be discussed. In the present study, plasmon peak of Au cannot be seen. The change in plasmon peak for the particle size has been reported so far for Au nanoparticles. Au plasmon peak is quite weak and broad for the mean particle size of 3.4 nm (Shimizu et al., 2003). The mean particle size of the present Au–Pd nanoparticles synthesized by electron irradiation is about 3.2 nm. Thus, it is difficult to discuss the particle structures from the view of the Au plasmon peak. Remita et al. and Redjala et al. have reported the trend of the synthesis of Au–Pd nanoparticles using g-rays and electron beam irradiation. At high dose rate such as electron beam irradiation, the reduction agents such as radicals are generated in a very dense state in aqueous solutions, and alloyed Au–Pd nanoparticles are formed (Remita et al., 2003; Redjala et al., 2006). However, in the present study, the STEM micrograph for some nanoparticles does not show a uniform contrast. The presence of nanoparticles which do not have a homogeneous composition (for example, core-shelllike structure) is also not completely deniable. Finally, we note here the dose rate for pulsed electron irradiation. In the discussion of this report, we have adopted the mean dose rate (4.1 103 Gy/min) and not the dose rate in each pulse (3.4 107 Gy/min). Considering the time interval of the nucleation and growth of the nanoparticles during irradiation, it is more reasonable to adopt the mean dose rate for the discussion of dose rate effect. The very high dose rate in a short pulse may, however, have some effects on the primary reduction process. To clarify this pulse effects, the comparison of the present result for
1053
pulsed electron irradiation with that for DC electron irradiation is needed.
4. Summary Aqueous solutions with Au3+ and Pd2+ ions were irradiated with 1.17, 1.33 MeV g-rays, 9 MeV pulsed electrons, or 1.6 GeV C ions. In all kinds of irradiation processes, nano-sized Au–Pd particles were synthesized. Not only the size distribution but also the structure of nanoparticles was studied by means of STEM observation with EDS analysis as well as a conventional TEM and UV–vis observations. For low dose rate, the structure of Au–Pd nanoparticles tends to be a core–shell structure with a large diameter. The structure of the Au–Pd core–shell nanoparticles is composed of well-separated Au-core and Pd-shell. For high dose rate, smaller-sized Au–Pd nanoparticles are synthesized. Thus, dose rate is one of important parameters to control the size of nanoparticles synthesized by energetic irradiation fields.
Acknowledgement The authors would like to thank Prof. R. Oshima, Dr. S. Tanaka, and Dr. K. Okazaki for the useful discussions on the synthesis of the nanoparticles and their catalytic activity. This work was supported by the Japan Society for the Promotion of Science (JSPS) Research (Grant-in-Aid for Scientific Research, B, no. 17360314). References Akita, T., Hiroki, T., Tanaka, S., Kojima, T., Kohyama, M., Iwase, A., Hori, F., 2008. Analytical TEM observation of Au–Pd nanoparticles prepared by sonochemical method. Catal. Today 131, 90–97. Edwards, J.K., Thomas, A., Carley, A.F., Herzing, A.A., Kiely, C.J., Hutchings, G.J., 2008. Au–Pd supported nanocrystals as catalysts for the direct synthesis of hydrogen peroxide from H2 and O2. Green Chem. 10, 388–394. Hori, F., Kojima, ., Tanaka, S., Akita, T., Iwai, T., Onitsuka, T., Taguchi, N., Iwase, A., 2007. Characterization of sonochemically synthesized Au–Pd nanoparticles by using slow positron beam. Phys. Status Solidi (C) 4 (10), 3895–3898. Maeda, N., Hiroki, T., Hori, F., Okuda, S., Taniguchi, R., Kojima, T., Kambara, T., Abe, T., Iwase, A., 2006. Synthesis of Au nano-particles under energetic irradiation fields. Mater. Res. Soc. Symp. Proc. 90 E, 0900-O06-16. 1–6. Mizukoshi, Y., Fujimoto, T., Nagata, Y., Oshima, R., Maeda, Y., 2000. Characterization and catalytic activity of core–shell structured gold/palladium bimetallic nanoparticles synthesized by the sonochemical method. J. Phys. Chem. B 104, 6028–6032. Redjala, T., Remita, H., Apostolescu, G., Mostafave, M., Thomazeau, C., Uzio, D., 2006. Bimetallic Au–Pd and Ag–Pd clusters synthesised by g or electron beam radiolysis and study of the reactivity/structure relationships in the selective hydrogenation of Buta-1,3-Diene. Oil Gas Sci. Technol. Rev. IFOP 61 (6), 789–797. Remita, H., Etcheberry, A., Belloni, J., 2003. Dose rate effect on bimetallic gold–palladium cluster structre. J. Phys. Chem. B 107, 31–36. Remita, H., Lamre, I., Mostafavi, M., Balanzat, E., Bouffard, S., 2005. Comparative study of metal clusters induced in aqueous solutions by g-rays, electron or C6+ ion beam irradiation. Radiat. Phys. Chem. 72, 575–586. Shimizu, T., Teranishi, T., Hasegawa, S., Miyako, M., 2003. Size evolution of alkanethiol-protected gold nanoparticles by heat treatment in the solid state. J. Phys. Chem. B 107, 2719–2724. Taguchi, N., Hori, F., Iwai, T., Iwase, A., Akita, T., Tanaka, S., 2008. Study of Au–Pd core–shell nanoparticles by using slow positron beam. Appl. Surf. Sci. 255, 164–166. Takatani, H., Kago, H., Nakanishi, M., Kobayashi, Y., Hori, F., Oshima, R., 2003. Characterization on noble metal alloy nanoparticles prepared by ultrasound irradiation. Rev. Adv. Mater. Sci. 5, 232–238. Tanaka, S., Taguchi, N., Akita, T., Hori, F., Kohyama, M., 2008. First-principles calculations of Pd/Au(1 0 0) interfaces with adsorbates. Solid State Phenom. 139, 47–52.
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Synthesis of Au–Pd bimetallic nanoparticles under energetic irradiation fields N. Taguchi a, A. Iwase a, N. Maeda a, T. Kojima b, R. Taniguchi b, S. Okuda b, T. Akita c, T. Abe d, T. Kambara d, H. Ryuto d, F. Hori a, a
Department of Materials Science, Osaka Prefecture University, Gakuen-cho, Sakai, Osaka 599-8531, Japan Radiation Research Center, Osaka Prefecture University, Gakuen-cho, Sakai, Osaka 599-8570, Japan c National Institute of Advanced Industrial Science and Technology (AIST), Midorigaoka-8-31, Ikeda, Osaka 563-8577, Japan d The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198, Japan b
a r t i c l e in fo
Keywords: Au–Pd nanoparticle synthesis g-rays Pulsed electrons GeV heavy ions Core–shell structure Dose rate dependence
abstract Bimetallic Au–Pd nanoparticles were synthesized under high-energy irradiation fields (1.17 and 1.33 MeV g-rays, 9 MeV electrons, and 1.6 GeV C ions) from solutions containing Au3+ and Pd2+ and cationic surfactant (sodium dodecyl sulfate). Particles synthesized by the irradiation were observed using conventional transmission electron microscope (TEM) and annular dark-field scanning transmission electron microscopy (ADF-STEM). The particles synthesized by g-rays and C ion irradiation exhibit core–shell structure with a Au-core and a Pd-shell. The dependence of the size distribution of nanoparticles on the dose rate is discussed. & 2009 Elsevier Ltd. All rights reserved.
1. Introduction Nanoparticles are of great interest from the viewpoint of using them as new functional materials for applications such as catalysts, because nanoparticles show peculiar properties which are not observed in bulk materials. Especially, bimetallic nanoparticles show new properties associated with the variations of compositions and structures (Edwards et al., 2008). It has been found that sonochemically synthesized Au–Pd nanoparticles using sodium dodecyl sulfate (SDS) as a surfactant have a core–shell structure (Akita et al., 2008). The electronic structures of Au–Pd core–shell nanoparticles are different from Au or Pd monometallic nanoparticles (Hori et al., 2007; Taguchi et al., 2008; Tanaka et al., 2008), and show higher activity for hydrogenation of 4 pentenoic acid (Mizukoshi et al., 2000; Takatani et al., 2003). As peculiar properties of nanoparticles strongly depend on their shapes and structures, it is very important to study various synthesis processes to control them. In our previous work, we obtained Au monometallic nanoparticles using various kinds of high-energy irradiation fields in aqueous solutions (Maeda et al., 2006). In the present study, the same method is used for synthesizing bimetallic Au–Pd nanoparticles. Through the transmission electron microscope (TEM) observation and ultraviolet–visible (UV–vis) measurements, Remita et al. (2003, 2005) have already reported the synthesis of bimetallic nanoparticles such as Au–Pt, Au–Pd, and so
Corresponding author. Tel./fax: +8172 254 9812.
E-mail address: [email protected] (F. Hori). 0969-806X/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2009.05.007
on in various kinds of irradiation fields. They have concluded that the dose rate is an important factor to control the particle size and structures (Remita et al., 2003, 2005; Redjala et al., 2006). To study the dose rate effect on the size and structure of irradiationsynthesized nanoparticles more clearly, we made annular darkfield scanning transmission electron microscopy (ADF-STEM) observations with energy-dispersive X-ray spectroscopy (EDS) measurements for Au–Pd nanoparticles synthesized for various kinds of irradiation as well as conventional transmission electron microscope observations and ultraviolet–visible measurements. In this report, the dependence of size distribution and the local structure of irradiation-synthesized nanoparticles on the kind of irradiation fields is discussed.
2. Experimental Aqueous solutions were prepared in given concentrations (0.5 mM Au3+ ions, 0.5 mM Pd2+ ions) of noble metal complexes (NaAuCl4 2H2O, PdCl2 2NaCl 3H2O) with an cationic surfactant of 8.0 mM SDS. Aqueous solution was poured into polystyrene cells. Each irradiation was performed in air. As the dose rate was too small to increase the temperature of solutions, temperature of each solution was kept at room temperature during the irradiation. The g-rays and electron irradiation were performed at Radiation Research Center of Osaka Prefecture University. For the g-ray irradiation, 60Co source was used. The energies of g-rays from 60Co isotope were 1.17 and 1.33 MeV. The mean dose rate was 40 Gy/min and the maximum dose was 1.0 104 Gy.
ARTICLE IN PRESS 1050
N. Taguchi et al. / Radiation Physics and Chemistry 78 (2009) 1049–1053
3 50
0Gy 1000Gy
40
7500Gy
2
Distribution (%)
Absorbance (arb. u.)
3000Gy
10000Gy
1
30
20
10
0 200
300
400 500 Wavelength (nm)
600
0
700
0
8
16
24 32 40 48 56 Particle Diameter (nm)
64
72
0
8
16
24 32 40 48 56 Particle Diameter (nm)
64
72
0
8
16
24 32 40 48 56 Particle Diameter (nm)
64
72
3 50
0Gy 1500Gy
40
10000Gy
Distribution (%)
Absorbance (arb. u.)
5000Gy
2
1
30
20
10
0 200
300
400 500 Wavelength (nm)
600
0
700
100
3
2
80
12300Gy
Distribution (%)
Absorbance (arb. u.)
0Gy 1500Gy 3000Gy 7500Gy
1
60
40
20
0 200
300
400
500 Wavelength (nm)
600
700
0
Fig. 1. UV–vis absorption spectra for solutions irradiated with (a) g-rays, (b) C ions, and (c) electrons.
Fig. 2. Size histograms of Au–Pd nanoparticles synthesized by (a) g-rays, (b) C ions, and (c) electrons.
The electron beam irradiation was performed using an electron linear accelerator. The energy of electrons was 9 MeV. Electron beam was pulsed, and the pulse frequency was 30 Hz.
Maximum dose was 1.2 104 Gy. The mean dose rate was 4.1 103 Gy/min. The 1.6 GeV C ion irradiation was performed at E5B beam line of RIKEN ring cyclotron accelerator. The dose rate
ARTICLE IN PRESS N. Taguchi et al. / Radiation Physics and Chemistry 78 (2009) 1049–1053
1051
was about 3.0 102 Gy/min, and the maximum dose was 1.0 104 Gy. Chemical reactions in aqueous solutions during the irradiation were traced by UV–vis spectroscopy (Shimadzu UV-2550) and the shapes and the structures of colloidal products were observed by conventional TEM (JEOL JEM-200CX) and STEM (JEOL JEM-3000F equipped with EDS and ADF-STEM systems). Samples for TEM and STEM observations were made by putting a drop of colloidal solutions on TEM meshes and drying them in a vacuum.
Au plasmon peak shifts to the lower wave length side with increasing the ion fluence. This trend is hardly observed in case of g-ray irradiation. The reason why the Au plasmon peak shifts by the ion irradiation, however, remains uncertain. For the electron beam irradiation, the absorption spectra indicate the gradual reduction of metal ions. The intensity of absorption peak for Au particles (around 520 nm) cannot be seen after the irradiation. For all kinds of irradiation processes, the reduction of the metallic ions is completed when the dose reaches about 104 Gy.
3. Results and discussion
3.2. TEM and STEM observations
3.1. UV–vis measurements
3.2.1. Mean particle size Fig. 2 shows the particle size distribution for each irradiation (g-rays, electrons or C ions). The mean diameters of nanoparticles for the g-ray, C ion, and electron irradiation are 25.5, 12.2, 3.2 nm, respectively. For the g-rays and the C ion irradiation, the particle size is widely distributed. On the other hand, nanoparticles synthesized by the electron beam irradiation show a narrow size distribution. Smaller particles with a narrow distribution in size are obtained through the high dose rate irradiation such as electron irradiation (4.1 103 Gy/min), and larger particles with a wide distribution in size are obtained through the low dose rate
Fig. 1(a)–(c) shows UV–vis spectra of each solution for g-ray, C ion, and electron irradiation, respectively. For the g-rays and the C ion irradiation, UV–vis spectra show that an absorption peak is located around 520 nm, which is due to Au particles (called the plasmon peak), and that the absorption peak decreases with increasing the irradiation dose. This result indicates that Au particles are gradually covered with Pd atoms during the irradiation. The Au plasmon peaks, however, still remain even after g-rays or C ion irradiation. Fig. 1(b) shows that the position of
Fig. 3. Conventional TEM and STEM images of synthesized Au–Pd nanoparticles: (a) g-rays, (b) C ions, and (c) electrons. The scale bar in conventional TEM images is 100 nm.
ARTICLE IN PRESS 1052
N. Taguchi et al. / Radiation Physics and Chemistry 78 (2009) 1049–1053
irradiation such as g-ray irradiation (40 Gy/min). This trend is quite similar to the results for the irradiation-synthesized Au–Pt, Au–Pd systems, which have been reported by (Remita et al., 2003, 2005; Redjala et al., 2006). In case of low dose rate irradiation, as
the reduction rate is very small, the growth of nanoparticles tends to occur easily. In contrast, the high dose rate irradiation causes much nucleation that dominates the particle growth, leading to the synthesis of small-size particles.
100
100 C
80
80
60
60
40
40 Pd
Cu
20
Counts (arb. u.)
Cu Cu
Cu
0 C
80
60
Pd
20
Cu
Cu
0
80
60 Pd
40
Cu
40
Cu
C
Au
20
0
Au
Pd Cu
Au
Cu
Au
20
Au
Cu
Cu
Au
0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Energy (keV)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Energy (keV) 100 80 C
60 40
Counts (arb. u.)
Counts (arb. u.)
C
Cu
Au
20 Cu
Pd
Cu
Au
Au
0 80 C
60 40 20
Cu
Au Cu
Pd
Cu
Au
Au
0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Energy (keV) Fig. 4. EDS spectra in local area for Au–Pd nanoparticles synthesized by (a) g-rays, (b) C ions, and (c) electrons.
ARTICLE IN PRESS N. Taguchi et al. / Radiation Physics and Chemistry 78 (2009) 1049–1053
3.2.2. The structure of Au–Pd nanoparticles TEM and ADF-STEM micrographs of Au–Pd nanoparticles synthesized by the g-ray, C ion, and electron irradiation are shown in Fig. 3(a)–(c), respectively. The results of EDS analysis are also shown in Fig. 4. In STEM micrographs, bright regions in each particle strongly reveal Au-cores, and they are surrounded by weakly contrasted regions which correspond to Pd shells (Fig. 3(a) and (b)). The STEM analyses have confirmed that the morphology of many nanoparticles synthesized by the g-rays and C ions is clearly-separated core–shell structure with a Au-core and a Pd-shell. The structure of these core–shell particles is similar to that synthesized by the sonochemical method (Akita et al., 2008). The EDS spectra in the local area of core–shell nanoparticles synthesized by the g-rays and C ions strongly support the results of STEM micrographs (Fig. 4(a) and (b)). At low dose rate process such as the g-rays and the C ion irradiation, Au3+ and Pd2+ ions reduction process is successive, Au3+ ions in aqueous solution are reduced first, and the reduction of Pd2+ ions follows the reduction of Au3+ ions. Then, core–shell-structured nanoparticles can be synthesized. The STEM observation also reveals that Pd-shell thickness of the nanoparticles in g-ray irradiation is not uniform as well as the shapes. In fact, there are some nanoparticles where the Pd-shell does not entirely cover the Au-core. This result well explains the experimental fact that the plasmon peaks of Au also remain in UV–vis spectra even after the irradiation. On the other hand, for the nanoparticles synthesized by the electron irradiation, it is very difficult to see a clear contrast of Au and Pd atoms in the STEM image, because of very small nanoparticle sizes shown in Fig. 3(c). Some nanoparticles seem to have core–shell structures, but others seem to show mixed Au–Pd structures. EDS spectra for the electron irradiation are shown in Fig. 4(c). Due to the spatial resolution of the electron microscope, however, the local structure of the nanoparticles can hardly be discussed. In the present study, plasmon peak of Au cannot be seen. The change in plasmon peak for the particle size has been reported so far for Au nanoparticles. Au plasmon peak is quite weak and broad for the mean particle size of 3.4 nm (Shimizu et al., 2003). The mean particle size of the present Au–Pd nanoparticles synthesized by electron irradiation is about 3.2 nm. Thus, it is difficult to discuss the particle structures from the view of the Au plasmon peak. Remita et al. and Redjala et al. have reported the trend of the synthesis of Au–Pd nanoparticles using g-rays and electron beam irradiation. At high dose rate such as electron beam irradiation, the reduction agents such as radicals are generated in a very dense state in aqueous solutions, and alloyed Au–Pd nanoparticles are formed (Remita et al., 2003; Redjala et al., 2006). However, in the present study, the STEM micrograph for some nanoparticles does not show a uniform contrast. The presence of nanoparticles which do not have a homogeneous composition (for example, core-shelllike structure) is also not completely deniable. Finally, we note here the dose rate for pulsed electron irradiation. In the discussion of this report, we have adopted the mean dose rate (4.1 103 Gy/min) and not the dose rate in each pulse (3.4 107 Gy/min). Considering the time interval of the nucleation and growth of the nanoparticles during irradiation, it is more reasonable to adopt the mean dose rate for the discussion of dose rate effect. The very high dose rate in a short pulse may, however, have some effects on the primary reduction process. To clarify this pulse effects, the comparison of the present result for
1053
pulsed electron irradiation with that for DC electron irradiation is needed.
4. Summary Aqueous solutions with Au3+ and Pd2+ ions were irradiated with 1.17, 1.33 MeV g-rays, 9 MeV pulsed electrons, or 1.6 GeV C ions. In all kinds of irradiation processes, nano-sized Au–Pd particles were synthesized. Not only the size distribution but also the structure of nanoparticles was studied by means of STEM observation with EDS analysis as well as a conventional TEM and UV–vis observations. For low dose rate, the structure of Au–Pd nanoparticles tends to be a core–shell structure with a large diameter. The structure of the Au–Pd core–shell nanoparticles is composed of well-separated Au-core and Pd-shell. For high dose rate, smaller-sized Au–Pd nanoparticles are synthesized. Thus, dose rate is one of important parameters to control the size of nanoparticles synthesized by energetic irradiation fields.
Acknowledgement The authors would like to thank Prof. R. Oshima, Dr. S. Tanaka, and Dr. K. Okazaki for the useful discussions on the synthesis of the nanoparticles and their catalytic activity. This work was supported by the Japan Society for the Promotion of Science (JSPS) Research (Grant-in-Aid for Scientific Research, B, no. 17360314). References Akita, T., Hiroki, T., Tanaka, S., Kojima, T., Kohyama, M., Iwase, A., Hori, F., 2008. Analytical TEM observation of Au–Pd nanoparticles prepared by sonochemical method. Catal. Today 131, 90–97. Edwards, J.K., Thomas, A., Carley, A.F., Herzing, A.A., Kiely, C.J., Hutchings, G.J., 2008. Au–Pd supported nanocrystals as catalysts for the direct synthesis of hydrogen peroxide from H2 and O2. Green Chem. 10, 388–394. Hori, F., Kojima, ., Tanaka, S., Akita, T., Iwai, T., Onitsuka, T., Taguchi, N., Iwase, A., 2007. Characterization of sonochemically synthesized Au–Pd nanoparticles by using slow positron beam. Phys. Status Solidi (C) 4 (10), 3895–3898. Maeda, N., Hiroki, T., Hori, F., Okuda, S., Taniguchi, R., Kojima, T., Kambara, T., Abe, T., Iwase, A., 2006. Synthesis of Au nano-particles under energetic irradiation fields. Mater. Res. Soc. Symp. Proc. 90 E, 0900-O06-16. 1–6. Mizukoshi, Y., Fujimoto, T., Nagata, Y., Oshima, R., Maeda, Y., 2000. Characterization and catalytic activity of core–shell structured gold/palladium bimetallic nanoparticles synthesized by the sonochemical method. J. Phys. Chem. B 104, 6028–6032. Redjala, T., Remita, H., Apostolescu, G., Mostafave, M., Thomazeau, C., Uzio, D., 2006. Bimetallic Au–Pd and Ag–Pd clusters synthesised by g or electron beam radiolysis and study of the reactivity/structure relationships in the selective hydrogenation of Buta-1,3-Diene. Oil Gas Sci. Technol. Rev. IFOP 61 (6), 789–797. Remita, H., Etcheberry, A., Belloni, J., 2003. Dose rate effect on bimetallic gold–palladium cluster structre. J. Phys. Chem. B 107, 31–36. Remita, H., Lamre, I., Mostafavi, M., Balanzat, E., Bouffard, S., 2005. Comparative study of metal clusters induced in aqueous solutions by g-rays, electron or C6+ ion beam irradiation. Radiat. Phys. Chem. 72, 575–586. Shimizu, T., Teranishi, T., Hasegawa, S., Miyako, M., 2003. Size evolution of alkanethiol-protected gold nanoparticles by heat treatment in the solid state. J. Phys. Chem. B 107, 2719–2724. Taguchi, N., Hori, F., Iwai, T., Iwase, A., Akita, T., Tanaka, S., 2008. Study of Au–Pd core–shell nanoparticles by using slow positron beam. Appl. Surf. Sci. 255, 164–166. Takatani, H., Kago, H., Nakanishi, M., Kobayashi, Y., Hori, F., Oshima, R., 2003. Characterization on noble metal alloy nanoparticles prepared by ultrasound irradiation. Rev. Adv. Mater. Sci. 5, 232–238. Tanaka, S., Taguchi, N., Akita, T., Hori, F., Kohyama, M., 2008. First-principles calculations of Pd/Au(1 0 0) interfaces with adsorbates. Solid State Phenom. 139, 47–52.