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A novel method of supporting gold nanoparticles on MWCNTs: Synchrotron X-ray reduction
China Particuology 5 (2007) 237–241
A novel method of supporting gold nanoparticles on MWCNTs: Synchrotron X-ray reduction Kuan-Nan Lin a , Tsung-Yeh Yang b , Hong-Ming Lin a,∗ , Yeu-Kuang Hwu b , She-Huang Wu a , Chung-Kwei Lin c a Department of Materials Engineering, Tatung University, Taipei 104, Taiwan, China Institute of Physics, Academia Sinica, 128 Academia Road, Nankang, Taipei 115, Taiwan, China c Department of Materials Science and Engineering, Feng Chia University, 407 Taichung, Taiwan, China b
Received 15 September 2006; accepted 16 March 2007
Abstract Gold nanoparticles decorating the surface of multiwalled carbon nanotubes (MWCNTs) are prepared by photochemical reduction. The gold clusters form different interesting geometrical faceted shapes in accordance to time duration of synchrotron X-ray irradiation. The shape of nanogold could be spherical, rod-like, or triangular. Carbon nanotubes serve as optimal templates for the heterogeneous nucleation of gold nanocrystals. These nanocrystal structures are characterized by transmission electron microscope (TEM) and element analysis by energy dispersive spectroscopy (EDS). © 2007 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. Keywords: Gold; MWCNT; Heterogeneous nucleation; Synchrotron X-ray irradiation
1. Introduction Since Iijima (1991) first observed by HRTEM the multiwalled carbon nanotubes (MWCNTs), extensive work has been carried out worldwide. Carbon nanotubes have shown intriguing physical and chemical properties in catalytic reactions. This is followed by current research focussed on size and shape control as well as the synthesis of functional hybrid noble metal/CNTs catalytic materials, such as Pt/CNTs, Pd/CNTs, Au/CNTs, Ni/CNTs, and Ag/CNTs (Bonet et al., 1999; Bonet, Grugeon, Herrera Urbina, Tekaia-Elhsissen, & Tarascon, 2002; Chen, Lee, & Liu, 2004; Huang, Chiang, Huang, & Sheen, 1998). Gold nanoparticles have been found to be capable of transforming CO to CO2 . Synthesizing gold nanoparticles on modified multiwalled carbon nanotubes (MWCNTs) by means of wet chemical routes has been demonstrated, including effective ways of attaching gold nanoparticles either inside or outside of MWCNTs through the use of surfactants or heat treatments in NH3 (Jiang & Gao, 2003). The possibility of using synchrotron X-ray for the direct reduction of gold precursor solutions was first explored by Ma, ∗
Corresponding author. Tel.: +886 2 2586 6030; fax: +886 2 2593 6897. E-mail address: [email protected] (H.-M. Lin).
Moldovan, Mancini, and Rosenberg (2000), showing the high Au nanocrystals growth rate of 40 nm/min and the uniform Au grain size of around 200 nm. We have succeeded in preparing pure spherical Au nanoparticles via radiation induced reduction. Compared to other photochemical methods, including UV light and Xenon lamp irradiation, the gold nanoparticles are distinguished for their face-selective growth (Kim, Song, & Yang, 2002; Miranda & Ahmadi, 2005; Song, Kim, Kim, & Yang, 2005). It was noted that a small amount of silver nitrate (AgNO3 ) was critical for the formation of Au nanorods (AuNRs). Because of their preferred crystal overgrowth, the gold nanostructures showed morphological anisotropy (Wang, 2000). Synchrotron X-ray produced by third generation synchrotron radiation facilities was used to produce hybrid Au/CNTs catalytic materials in shorter time as compared to other synthesis methods. The different shapes of gold nanoparticles attached on MWCNTs for different exposure times were directly observed by TEM and identified by EDS. 2. Experimental Multi-wall carbon nanotubes (MWCNTs) used in this study were provided by CNT Co. Ltd. from Korea. The as received MWCNTs, 5 g, were first boiled in 250 mL concentrated nitric
1672-2515/$ – see front matter © 2007 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.cpart.2007.03.007
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K.-N. Lin et al. / China Particuology 5 (2007) 237–241
acid (Wako, 141-01361) at about 150 ◦ C for 3 or 6 h to remove impurities, then filtered and washed with distilled water to acidfree, and finally dried at 80 ◦ C overnight to obtain purified MWCNTs. A 0.01 g purified MWCNTs are then well dispersed in 50 mL distilled water by ultrasound. Then, 0.4 mL of 0.1 M NaOH (UNION Chemical Works Ltd.) and 1 mL of 0.05 M HAuCl4 ·4H2 O (Wako, 99%) are injected into the MWCNTs solution, respectively. During X-ray irradiation, the 3 or 6 h acid treatment MWCNTs based reactive solution was sampled at 1, 10, and 30 min exposure time individually. Each experiment was performed at 01A beamline of NSRRC (Nation Synchrotron Radiation Research Center, Taiwan), operating at 1.5 GeV. The unmonochromatized (“white”) beam with no optical elements except beryllium and silicon windows was used in the experiments. The beam size was controlled at 0.6 cm × 1.3 cm by tungsten slits. Fig. 1 shows the diagram of 01A beamline energy range and the calculated flux of synchrotron X-ray. The morphology and structure of the synthesized products were examined by a transmission electron microscope (HITACHI 800 STEM) operated at 175 kV and a field emission electron microscope (FE-TEM, JEOL, JEM-2100F) operated at 200 kV. TEM observation was performed to compare the
Fig. 1. Diagram of energy range and calculated flux of X-ray.
microstructure of the samples. Element analysis was carried out by means of energy dispersive spectroscopy (EDS). 3. Results and discussion When synchrotron X-ray irradiates the solution, solvated electrons and other hydroxides are produced, including H3 O+ , H2 , H2 O2 , OH− , and H+ (Marignier, Belloni, Delcourt, &
Fig. 2. TEM images of the reacting colloids with 3 h acid treatment MWCNTs exposed in X-ray for 1, 10, and 30 min as shown in (a), (b), and (c), respectively.
K.-N. Lin et al. / China Particuology 5 (2007) 237–241
Chevalier, 1985), according to the following reaction: H2 O + irradiation → e− (solvated) + H3 O+ + H2 + H2 O2 + OH− + H+ . (1) Solvated electrons react with gold ions to yield gold nanoparticles as follows: e− (solvated) + Au3+ → Au2+ , −
e (solvated) + Au
2+
+
→ Au ,
e− (solvated) + Au+ → Au clusters.
(2) (3) (4)
Simultaneously, heterogeneous nucleation of gold nanoparticles occurred on the MWCNTs. Fig. 2(a–c) shows the TEM images of the reacting colloids with 3-h acid treated MWCNTs exposed to X-ray for 1, 10 and 30 min, respectively, showing that the number of gold nanoparticles attached onto the multiwalled carbon nanotubes is fewer for 1 and 10 min. The nanoparticles are mostly spherical in shape and around 25 nm in size, but at 30 min gold nanorods and gold nanotriangles are evident and the average size of Au has risen to around 85 nm. Fig. 3(a–c) shows respectively the corresponding results for the 6 h purified MWCNTs exposed to X-ray for 1, 10 and
239
30 min. Fig. 3(d) shows the element analysis of 30 min irradiation sample as determined by energy dispersive spectroscopy (EDS). After 1 and 10 min X-ray exposures, the gold nanoparticles are mainly spherical, nanorods are rare, unless when irradiation was prolonged to 30 min, gold nanorods, nanotriangles, and nanopentagons became evident. After 30 min X-ray exposures, the average size of gold nanoparticles was around 35 nm. Upon detailed examination, adjacent gold nanoparticles were found to grow on MWCNTs presenting two or three different shapes, as shown in Fig. 4, that is, different crystal planes of gold nanoparticles can contact one another on a carbon nanotube and form similar surface nanostructures. This phenomenon is worth further studying. Surface energies differ for different crystallographic planes, generally in the sequence of γ {1 1 1} < γ {1 0 0} < γ {1 1 0} . For a spherical single crystal, its surface contains high-index crystallographic planes, resulting in higher surface energy. Facets tend to form on particle surfaces to increase the portion of lowerindex planes. Therefore, for particles smaller than 10–20 nm, the surface is that of a polyhedron. In general, almost all gold nanorods are single crystals and contain no twins or dislocations (Wang, 2000).
Fig. 3. TEM images of the reacting colloid for 6 h acid treatment MWCNTs exposed to X-ray for 1, 10, and 30 min, as shown in (a), (b), and (c), respectively. The element characterization by EDS is shown in (d).
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Fig. 4. FE-TEM images showing different shapes of Au nanoparticles on purified MWCNTs after 6 h acid treatment. The inset in (b) represents schematically the crystalline planes of gold nanotriangle.
Fig. 5. FE-TEM images showing twinned Au nanotriangles and nanorods bond to one another for the 3 and 6 h acid purified MWCNTs, as shown in (a) and (b), respectively. The inset in (b) represents schematically the crystalline planes of gold nanorods.
In this study, twinned Au nanotriangles and nanorods bond to one another for the 3 and 6 h acid purified MWCNTs, as shown in Fig. 5(a) and (b). These twinned Au nanotriangles and nanorods might have been induced by synchrotron irradiations. Usually, the face-centered cubic structure of metallic nanocrystals has {1 1 1} twins. The multiple twins shown here represent structural configuration resulting possibly from low surface energy. The MWCNTs after acid treatment possess lower surface energy defects to result in the gold nanoparticles with corresponding suitable surface energy crystallographic planes wetting the outside MWCNT walls. For instance, the (1 1 0) surface of gold nanorods is more often seen to wet the MWCNTs, leading to stronger interaction between Au and MWCNTs. Also, the surface atoms of gold nanorods could occupy new equilibrium positions, implying that the specific surface nanostructures arise from the crystalline defects of purified MWCNTs.
ing synchrotron X-ray irradiation, Au nanocrystals nucleate heterogeneously and grow via a gold–ion reduction process, e− (solvated) + Au+ → Au clusters, where photoelectrons reduce the gold ions to metal. Because of different defects distribution on the MWCNT wall after acid treatment, gold shows various kinds of crystalline shapes with their appropriate faces attached to the MWCNT surface. The study demonstrates a novel method for supporting gold nanoparticles on MWCNTs as a new catalytic material. Acknowledgements The authors would like to thank the National Science Council and National Synchrotron Radiation Research Center (NSRRC) for financially supporting this research. References
4. Conclusions In this study, synchrotron X-ray irradiations have been used successfully to synthesize gold nanoparticles coated on acid-treated multiwall carbon nanotubes (MWCNTs). Dur-
Bonet, F., Delmas, V., Grugeon, S., Herrera Urbina, R., Silvert, P.-Y., & TekaiaElhsissen, K. (1999). Synthesis of monodisperse Au, Pt, Pd, Ru and Ir nanoparticles in ethylene glycol. Nanostructured Materials, 11, 1277–1284. Bonet, F., Grugeon, S., Herrera Urbina, R., Tekaia-Elhsissen, K., & Tarascon, J.-M. (2002). In situ deposition of silver and palladium nanoparticles
K.-N. Lin et al. / China Particuology 5 (2007) 237–241 prepared by the polyol process, and their performance as catalytic converters of automobile exhaust gases. Solid State Science, 4, 665–670. Chen, W., Lee, J., & Liu, Z. (2004). Preparation of Pt and PtRu nanoparticles supported on carbon nanotubes by microwave-assisted heating polyol process. Materials Letter, 58, 3166–3169. Huang, C.-Y., Chiang, H.-J., Huang, J.-C., & Sheen, S.-R. (1998). Synthesis of nanocrystalline Ag–Pd alloys by chemical reduction method. Nanostructured Materials, 10, 1393–1400. Iijima, S. (1991). Helical microtubules of graphitic carbon. Nature, 354, 56–58. Jiang, L. Q., & Gao, L. (2003). Modified carbon nanotubes: An effective way to selective attachment of gold nanoparticles. Carbon, 41, 2923– 2929. Kim, F., Song, J. H., & Yang, P. (2002). Photochemical synthesis of gold nanorods. Journal of the American Chemical Society, 124, 14316– 14317.
241
Ma, Q., Moldovan, N., Mancini, D. C., & Rosenberg, R. A. (2000). Synchrotronradiation-induced, selective-area deposition of gold on polyimide from solution. Applied Physics Letters, 76, 2014–2016. Marignier, J. L., Belloni, J., Delcourt, M. O., & Chevalier, J. P. (1985). Microaggregates of non-noble metals and bimetallic alloys prepared by radiation-induced reduction. Nature, 317, 344–345. Miranda, O. R., & Ahmadi, T. S. (2005). Effects of intensity and energy of CW UV light on the growth of gold nanorods. Journal of Physical Chemistry B, 109, 15724–15734. Song, J. H., Kim, F., Kim, D., & Yang, P. (2005). Crystal overgrowth on gold nanorods: Tuning the shape, facet, aspect ratio, and composition of the nanorods. Chemistry- A European Journal, 11, 910–916. Wang, Z. L. (2000). Transmission electron microscopy of shape-controlled nanocrystals and their assemblies. Journal of Physical Chemistry B, 104, 1153–1175.
A novel method of supporting gold nanoparticles on MWCNTs: Synchrotron X-ray reduction Kuan-Nan Lin a , Tsung-Yeh Yang b , Hong-Ming Lin a,∗ , Yeu-Kuang Hwu b , She-Huang Wu a , Chung-Kwei Lin c a Department of Materials Engineering, Tatung University, Taipei 104, Taiwan, China Institute of Physics, Academia Sinica, 128 Academia Road, Nankang, Taipei 115, Taiwan, China c Department of Materials Science and Engineering, Feng Chia University, 407 Taichung, Taiwan, China b
Received 15 September 2006; accepted 16 March 2007
Abstract Gold nanoparticles decorating the surface of multiwalled carbon nanotubes (MWCNTs) are prepared by photochemical reduction. The gold clusters form different interesting geometrical faceted shapes in accordance to time duration of synchrotron X-ray irradiation. The shape of nanogold could be spherical, rod-like, or triangular. Carbon nanotubes serve as optimal templates for the heterogeneous nucleation of gold nanocrystals. These nanocrystal structures are characterized by transmission electron microscope (TEM) and element analysis by energy dispersive spectroscopy (EDS). © 2007 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. Keywords: Gold; MWCNT; Heterogeneous nucleation; Synchrotron X-ray irradiation
1. Introduction Since Iijima (1991) first observed by HRTEM the multiwalled carbon nanotubes (MWCNTs), extensive work has been carried out worldwide. Carbon nanotubes have shown intriguing physical and chemical properties in catalytic reactions. This is followed by current research focussed on size and shape control as well as the synthesis of functional hybrid noble metal/CNTs catalytic materials, such as Pt/CNTs, Pd/CNTs, Au/CNTs, Ni/CNTs, and Ag/CNTs (Bonet et al., 1999; Bonet, Grugeon, Herrera Urbina, Tekaia-Elhsissen, & Tarascon, 2002; Chen, Lee, & Liu, 2004; Huang, Chiang, Huang, & Sheen, 1998). Gold nanoparticles have been found to be capable of transforming CO to CO2 . Synthesizing gold nanoparticles on modified multiwalled carbon nanotubes (MWCNTs) by means of wet chemical routes has been demonstrated, including effective ways of attaching gold nanoparticles either inside or outside of MWCNTs through the use of surfactants or heat treatments in NH3 (Jiang & Gao, 2003). The possibility of using synchrotron X-ray for the direct reduction of gold precursor solutions was first explored by Ma, ∗
Corresponding author. Tel.: +886 2 2586 6030; fax: +886 2 2593 6897. E-mail address: [email protected] (H.-M. Lin).
Moldovan, Mancini, and Rosenberg (2000), showing the high Au nanocrystals growth rate of 40 nm/min and the uniform Au grain size of around 200 nm. We have succeeded in preparing pure spherical Au nanoparticles via radiation induced reduction. Compared to other photochemical methods, including UV light and Xenon lamp irradiation, the gold nanoparticles are distinguished for their face-selective growth (Kim, Song, & Yang, 2002; Miranda & Ahmadi, 2005; Song, Kim, Kim, & Yang, 2005). It was noted that a small amount of silver nitrate (AgNO3 ) was critical for the formation of Au nanorods (AuNRs). Because of their preferred crystal overgrowth, the gold nanostructures showed morphological anisotropy (Wang, 2000). Synchrotron X-ray produced by third generation synchrotron radiation facilities was used to produce hybrid Au/CNTs catalytic materials in shorter time as compared to other synthesis methods. The different shapes of gold nanoparticles attached on MWCNTs for different exposure times were directly observed by TEM and identified by EDS. 2. Experimental Multi-wall carbon nanotubes (MWCNTs) used in this study were provided by CNT Co. Ltd. from Korea. The as received MWCNTs, 5 g, were first boiled in 250 mL concentrated nitric
1672-2515/$ – see front matter © 2007 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.cpart.2007.03.007
238
K.-N. Lin et al. / China Particuology 5 (2007) 237–241
acid (Wako, 141-01361) at about 150 ◦ C for 3 or 6 h to remove impurities, then filtered and washed with distilled water to acidfree, and finally dried at 80 ◦ C overnight to obtain purified MWCNTs. A 0.01 g purified MWCNTs are then well dispersed in 50 mL distilled water by ultrasound. Then, 0.4 mL of 0.1 M NaOH (UNION Chemical Works Ltd.) and 1 mL of 0.05 M HAuCl4 ·4H2 O (Wako, 99%) are injected into the MWCNTs solution, respectively. During X-ray irradiation, the 3 or 6 h acid treatment MWCNTs based reactive solution was sampled at 1, 10, and 30 min exposure time individually. Each experiment was performed at 01A beamline of NSRRC (Nation Synchrotron Radiation Research Center, Taiwan), operating at 1.5 GeV. The unmonochromatized (“white”) beam with no optical elements except beryllium and silicon windows was used in the experiments. The beam size was controlled at 0.6 cm × 1.3 cm by tungsten slits. Fig. 1 shows the diagram of 01A beamline energy range and the calculated flux of synchrotron X-ray. The morphology and structure of the synthesized products were examined by a transmission electron microscope (HITACHI 800 STEM) operated at 175 kV and a field emission electron microscope (FE-TEM, JEOL, JEM-2100F) operated at 200 kV. TEM observation was performed to compare the
Fig. 1. Diagram of energy range and calculated flux of X-ray.
microstructure of the samples. Element analysis was carried out by means of energy dispersive spectroscopy (EDS). 3. Results and discussion When synchrotron X-ray irradiates the solution, solvated electrons and other hydroxides are produced, including H3 O+ , H2 , H2 O2 , OH− , and H+ (Marignier, Belloni, Delcourt, &
Fig. 2. TEM images of the reacting colloids with 3 h acid treatment MWCNTs exposed in X-ray for 1, 10, and 30 min as shown in (a), (b), and (c), respectively.
K.-N. Lin et al. / China Particuology 5 (2007) 237–241
Chevalier, 1985), according to the following reaction: H2 O + irradiation → e− (solvated) + H3 O+ + H2 + H2 O2 + OH− + H+ . (1) Solvated electrons react with gold ions to yield gold nanoparticles as follows: e− (solvated) + Au3+ → Au2+ , −
e (solvated) + Au
2+
+
→ Au ,
e− (solvated) + Au+ → Au clusters.
(2) (3) (4)
Simultaneously, heterogeneous nucleation of gold nanoparticles occurred on the MWCNTs. Fig. 2(a–c) shows the TEM images of the reacting colloids with 3-h acid treated MWCNTs exposed to X-ray for 1, 10 and 30 min, respectively, showing that the number of gold nanoparticles attached onto the multiwalled carbon nanotubes is fewer for 1 and 10 min. The nanoparticles are mostly spherical in shape and around 25 nm in size, but at 30 min gold nanorods and gold nanotriangles are evident and the average size of Au has risen to around 85 nm. Fig. 3(a–c) shows respectively the corresponding results for the 6 h purified MWCNTs exposed to X-ray for 1, 10 and
239
30 min. Fig. 3(d) shows the element analysis of 30 min irradiation sample as determined by energy dispersive spectroscopy (EDS). After 1 and 10 min X-ray exposures, the gold nanoparticles are mainly spherical, nanorods are rare, unless when irradiation was prolonged to 30 min, gold nanorods, nanotriangles, and nanopentagons became evident. After 30 min X-ray exposures, the average size of gold nanoparticles was around 35 nm. Upon detailed examination, adjacent gold nanoparticles were found to grow on MWCNTs presenting two or three different shapes, as shown in Fig. 4, that is, different crystal planes of gold nanoparticles can contact one another on a carbon nanotube and form similar surface nanostructures. This phenomenon is worth further studying. Surface energies differ for different crystallographic planes, generally in the sequence of γ {1 1 1} < γ {1 0 0} < γ {1 1 0} . For a spherical single crystal, its surface contains high-index crystallographic planes, resulting in higher surface energy. Facets tend to form on particle surfaces to increase the portion of lowerindex planes. Therefore, for particles smaller than 10–20 nm, the surface is that of a polyhedron. In general, almost all gold nanorods are single crystals and contain no twins or dislocations (Wang, 2000).
Fig. 3. TEM images of the reacting colloid for 6 h acid treatment MWCNTs exposed to X-ray for 1, 10, and 30 min, as shown in (a), (b), and (c), respectively. The element characterization by EDS is shown in (d).
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K.-N. Lin et al. / China Particuology 5 (2007) 237–241
Fig. 4. FE-TEM images showing different shapes of Au nanoparticles on purified MWCNTs after 6 h acid treatment. The inset in (b) represents schematically the crystalline planes of gold nanotriangle.
Fig. 5. FE-TEM images showing twinned Au nanotriangles and nanorods bond to one another for the 3 and 6 h acid purified MWCNTs, as shown in (a) and (b), respectively. The inset in (b) represents schematically the crystalline planes of gold nanorods.
In this study, twinned Au nanotriangles and nanorods bond to one another for the 3 and 6 h acid purified MWCNTs, as shown in Fig. 5(a) and (b). These twinned Au nanotriangles and nanorods might have been induced by synchrotron irradiations. Usually, the face-centered cubic structure of metallic nanocrystals has {1 1 1} twins. The multiple twins shown here represent structural configuration resulting possibly from low surface energy. The MWCNTs after acid treatment possess lower surface energy defects to result in the gold nanoparticles with corresponding suitable surface energy crystallographic planes wetting the outside MWCNT walls. For instance, the (1 1 0) surface of gold nanorods is more often seen to wet the MWCNTs, leading to stronger interaction between Au and MWCNTs. Also, the surface atoms of gold nanorods could occupy new equilibrium positions, implying that the specific surface nanostructures arise from the crystalline defects of purified MWCNTs.
ing synchrotron X-ray irradiation, Au nanocrystals nucleate heterogeneously and grow via a gold–ion reduction process, e− (solvated) + Au+ → Au clusters, where photoelectrons reduce the gold ions to metal. Because of different defects distribution on the MWCNT wall after acid treatment, gold shows various kinds of crystalline shapes with their appropriate faces attached to the MWCNT surface. The study demonstrates a novel method for supporting gold nanoparticles on MWCNTs as a new catalytic material. Acknowledgements The authors would like to thank the National Science Council and National Synchrotron Radiation Research Center (NSRRC) for financially supporting this research. References
4. Conclusions In this study, synchrotron X-ray irradiations have been used successfully to synthesize gold nanoparticles coated on acid-treated multiwall carbon nanotubes (MWCNTs). Dur-
Bonet, F., Delmas, V., Grugeon, S., Herrera Urbina, R., Silvert, P.-Y., & TekaiaElhsissen, K. (1999). Synthesis of monodisperse Au, Pt, Pd, Ru and Ir nanoparticles in ethylene glycol. Nanostructured Materials, 11, 1277–1284. Bonet, F., Grugeon, S., Herrera Urbina, R., Tekaia-Elhsissen, K., & Tarascon, J.-M. (2002). In situ deposition of silver and palladium nanoparticles
K.-N. Lin et al. / China Particuology 5 (2007) 237–241 prepared by the polyol process, and their performance as catalytic converters of automobile exhaust gases. Solid State Science, 4, 665–670. Chen, W., Lee, J., & Liu, Z. (2004). Preparation of Pt and PtRu nanoparticles supported on carbon nanotubes by microwave-assisted heating polyol process. Materials Letter, 58, 3166–3169. Huang, C.-Y., Chiang, H.-J., Huang, J.-C., & Sheen, S.-R. (1998). Synthesis of nanocrystalline Ag–Pd alloys by chemical reduction method. Nanostructured Materials, 10, 1393–1400. Iijima, S. (1991). Helical microtubules of graphitic carbon. Nature, 354, 56–58. Jiang, L. Q., & Gao, L. (2003). Modified carbon nanotubes: An effective way to selective attachment of gold nanoparticles. Carbon, 41, 2923– 2929. Kim, F., Song, J. H., & Yang, P. (2002). Photochemical synthesis of gold nanorods. Journal of the American Chemical Society, 124, 14316– 14317.
241
Ma, Q., Moldovan, N., Mancini, D. C., & Rosenberg, R. A. (2000). Synchrotronradiation-induced, selective-area deposition of gold on polyimide from solution. Applied Physics Letters, 76, 2014–2016. Marignier, J. L., Belloni, J., Delcourt, M. O., & Chevalier, J. P. (1985). Microaggregates of non-noble metals and bimetallic alloys prepared by radiation-induced reduction. Nature, 317, 344–345. Miranda, O. R., & Ahmadi, T. S. (2005). Effects of intensity and energy of CW UV light on the growth of gold nanorods. Journal of Physical Chemistry B, 109, 15724–15734. Song, J. H., Kim, F., Kim, D., & Yang, P. (2005). Crystal overgrowth on gold nanorods: Tuning the shape, facet, aspect ratio, and composition of the nanorods. Chemistry- A European Journal, 11, 910–916. Wang, Z. L. (2000). Transmission electron microscopy of shape-controlled nanocrystals and their assemblies. Journal of Physical Chemistry B, 104, 1153–1175.