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In Situ Time-Resolved XAFS Study on the Formation Mechanism of Cu Nanoparticles Using Poly(N-vinyl-2-pyrrolidone) as a Capping Agent Shun Nishimura, Atsushi Takagaki, Shinya Maenosono,* and Kohki Ebitani* School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan Received September 17, 2009. Revised Manuscript Received December 8, 2009 The formation mechanism of copper nanoparticles (NPs) using poly(N-vinyl-2-pyrrolidone) (PVP) as a capping agent was investigated by measurements of X-ray diffraction (XRD), transmission electron microscopy (TEM), in situ timeresolved X-ray adsorption fine structure (XAFS) analysis, in situ UV-vis spectroscopy, and an indicator method. XAFS analyses, in combination with TEM observations and the indicator method, revealed that the stable intermediates such as Cu(OH)2 and Cuþ-PVP intermediate were formed during an induction period of nucleation of Cu NPs, which play a critical role in the Cu NP formation. Our results suggest that the PVP capping agent is important not only to protect NPs from overgrowth and aggregation but also to control the reaction kinetics of NP formation.
Introduction Chemically synthesized nanoparticles (NPs) have been attracting much attention for their size-dependent properties1-3 and their use in a wide range of applications such as paints and inks,4,5 biolabeling and drug delivery,6-9 MRI contrast agents,10-12 photovoltaic devices,13,14 and catalysts.15-17 Various synthetic methods of NPs to control their size and shape have been reported.18-20 In view of previous studies, the physicochemical properties of NPs were strongly associated with reaction conditions such as pH, temperature, molar ratio of precursors, kinds of capping agent, reducing agent and metal sources, etc. Usually, *Corresponding authors. E-mail:
[email protected] (S.M.), ebitani@ jaist.ac.jp (K.E.). (1) Jin, R.; Cao, Y. C.; Hao, E.; Metraux, G. S.; Schatz, G. C.; Mirkin, C. A. Nature 2003, 425, 487. (2) Park, T. J.; Papaefthymiou, G. C.; Viescas, A. J.; Moodenbaugh, A. R.; Wong, S. S. Nano Lett. 2007, 7, 766. (3) Zheng, J.; Zhang, C.; Dickson, R. M. Phys. Rev. Lett. 2004, 93, 077402. (4) Gotoh, A.; Uchida, H.; Ishizaki, M.; Satoh, T.; Kaga, T.; Okamoto, S.; Ohta, S.; Sakamoto, M.; Kawamoto, T.; Tanaka, T.; Tokumoto, H.; Hara, M.; Shiozaki, S.; Yamada, M.; Miyake, M.; Kurihara, M. Nanotechnology 2007, 18, 345609. (5) Parka, B. K.; Kima, D.; Jeonga, S.; Moona, J.; Kim, J. S. Thin Solid Films 2007, 515, 7706. (6) Rieter, W. J.; Pott, K. M.; Taylor, K. M. L.; Lin, W. J. Am. Chem. Soc. 2008, 130, 11584. (7) Jin, R.; Wu, G.; Li, Z.; Mirkin, C. A.; Schatz, G. C. J. Am. Chem. Soc. 2003, 125, 1643. (8) Gao, X.; Yu, K. M. K.; Tam, K. Y.; Tsang, S. C. Chem. Commun. 2003, 2998. (9) Chen, Y.; Chi, Y.; Wen, H.; Lu, Z. Anal. Chem. 2007, 79, 960. (10) Zhao, M.; Beauregard, D. A.; Loizou, L.; Davletov, B.; Brindle, K. M. Nat. Med. 2001, 7, 1241. (11) Maenosono, S.; Suzuki, T.; Saita, S. J. Magn. Magn. Mater. 2008, 320, L79. (12) Jun, Y. W.; Huh, Y. M.; Choi, J. S.; Lee, J. H.; Song, H. T.; Kim, K.; Yoon, S.; Kim, K. S.; Shin, J. S.; Suh, J. S.; Cheon, J. J. Am. Chem. Soc. 2005, 127, 5732. (13) Schaller, R.; Klimov, V. Phys. Rev. Lett. 2004, 92, 186601. (14) Kikuchi, E.; Kitada, S.; Ohno, A.; Aramaki, S.; Maenosono, S. Appl. Phys. Lett. 2008, 92, 173307. (15) Xie, X.; Li, Y.; Liu, Z. Q.; Haruta, M.; Shen, W. Nature 2009, 458, 746. (16) Mori, K.; Sugihara, K.; Kondo, Y.; Takeuchi, T.; Morimoto, S.; Yamashita, H. J. Phys. Chem. C 2008, 112, 16478. (17) Gill, C. S.; Price, B. A.; Jones, C. W. J. Catal. 2007, 251, 145. (18) Xia, Y.; Xiong, Y.; Lim, B.; Skrablak, S. E. Angew. Chem., Int. Ed. 2009, 48, 60. (19) Chen, J.; Lima, B.; Leea, E. P.; Xia, Y. Nano Today 2009, 4, 81. (20) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025.
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however, the formation of NPs is dominated by a rapid nucleation process, and thus, it is difficult to clarify the role of intermediates on the formation kinetics. Some researchers have tried to ascertain the NP formation mechanisms.21-24 For instance, Saita and Maenosono performed a detailed study on the formation mechanism of FePt NPs by analyzing the relationship between the morphology of the NPs formed and the synthetic conditions. They determined that the FePt NPs were formed via the nucleation of Pt followed by a slow growth process of Fe and Pt on the Pt nuclei.25 Zou and coworkers investigated the formation process of CdSe NPs using in situ UV-vis adsorption spectroscopy.26 They concluded that the CdSe NPs were formed by a fast growth process, in which H2Se gas might play a crucial role in the formation process. Kumar et al. studied the formation process of Cu NPs stabilized with sulfobetaine using a microfluidic synthesis with CuCl2 source.27 They proposed that Cuþ intermediate species could be formed as linear and tetracoordinated Cuþ-sulfobetaine complexes during the reducing process on the basis of the UV-vis spectra and X-ray adsorption fine structure (XAFS) data of asformed Cu NPs under different conditions. These approaches to understand the NP formation mechanism are imperative for controlling NP morphology. However, the role of capping agents in the reaction kinetics has received little attention because the in situ observation of intermediates during the reaction is generally extremely difficult. In situ XAFS analysis is useful for dynamic monitoring of the reduction process of metals. Many researchers have attempted to understand the formation dynamics of active metal species on supports during preparation, activation, and reaction process (21) Besson, C.; Finney, E. E.; Finke, R. G. J. Am. Chem. Soc. 2005, 127, 8179. (22) Pong, B. K.; Elim, H. I.; Chong, J. X.; Ji, W.; Trout, B. L.; Lee, J. Y. J. Phys. Chem. C 2007, 111, 6281. (23) Pongpeerapat, A.; Wanawongthai, C.; Tozuka, Y.; Moribe, K.; Yamamoto, K. Int. J. Pharm. 2008, 352, 309. (24) Ethayaraja, M.; Dutta, K.; Bandyopadhyaya, R. J. Phys. Chem. B 2006, 110, 16471. (25) Saita, S.; Maenosono, S. Chem. Mater. 2005, 17, 6624. (26) Deng, Z.; Cao, L.; Tang, F.; Zou, B. J. Phys. Chem. B 2005, 109, 16671. (27) Song, Y.; Doomes, E. E.; Prindle, J.; Tittsworth, R.; Hormes, J.; Kumar, C. S. S. R. J. Phys. Chem. B 2005, 109, 9330.
Published on Web 12/29/2009
DOI: 10.1021/la904248z
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using in situ XAFS analysis.28-36 However, the above-mentioned studies were carried out only for the inhomogeneous nucleation and growth processes of NPs on the surface of the support material without a capping agent. Therefore, the system is relatively simpler than the case of the homogeneous nucleation of isolated NPs in the presence of capping agents. Recently, Harada and Inada have investigated the formation processes of isolated Rh and Pd NPs synthesized via the photoreduction of RhCl3 and PdCl2, respectively, in the presence of poly(N-vinyl-2-pyrrolidone) (PVP) using in situ energy-dispersive XAFS (DXAFS) analysis and UV-vis spectroscopy.37 In the case of Rh NPs, they found that there was an induction period before the nucleation of NPs, and the reduction of RhCl3 took place during this period. On the contrary, in the case of Pd NPs, there was no induction period. They concluded that the reduction of RhCl3 occurred in a stepwise fashion through three other Rh/ Cl species, while the reduction of PdCl2 occurred directly. Unfortunately, however, the role of PVP in the reaction kinetics has remained an open question. In this study, we comprehensively investigated the formation mechanisms of Cu NPs synthesized using PVP as a capping agent, which has been widely prepared,38-42 by in situ quick XAFS (QXAFS) in combination with other analytical techniques. This combination of analytical techniques is valuable to investigate the formation mechanisms of Cu NPs not only for the basic understanding of the role of the PVP capping agent but also for the development of mass production techniques.
Experimental Section Cu NPs were synthesized by a wet-chemical reduction method according to a literature technique43 with some modifications. Briefly, Cu(II) acetate [Cu(OAc)2] (15 mM) and poly(N-vinyl-2pyrrolidone) (PVP) were simultaneously dissolved into pure water in a flask, and then an aqueous solution of sodium borohydride (NaBH4) and sodium hydroxide (NaOH) was injected into the Cu(OAc)2/PVP aqueous solution under an N2 atmosphere at room temperature. The molar ratio of mixed colloidal solution was Cu(OAc)2/PVP/NaBH4/NaOH/H2O = 1.0/0.033/1.0/ 1.0/3.7. In order to investigate the NP formation dynamics, transmission electron microscopy (TEM) and in situ QXAFS analyses were conducted. TEM and high-resolution TEM (HR-TEM) (28) Shido, T.; Yamaguchi, A.; Inada, Y.; Asakura, K.; Nomura, M.; Iwasawa, Y. Top. Catal. 2002, 18, 53. (29) Yamamoto, T.; Suzuki, A.; Nagai, Y.; Tanabe, T.; Dong, F.; Inada, Y.; Nomura, M.; Tada, M.; Iwasawa, Y. Angew. Chem., Int. Ed. 2007, 46, 9253. (30) Bus, E.; Miller, J. T.; Kropf, A. J.; Prins, R.; Bokhoven, J. A. Phys. Chem. Chem. Phys. 2006, 8, 3248. (31) Suzuki, A.; Yamaguchi, A.; Chihara, T.; Inada, Y.; Yuasa, M.; Abe, M.; Nomura, M.; Iwasawa, Y. J. Phys. Chem. B 2004, 108, 5609. (32) Fernandez, A.; Caballero, A.; Gonzalez-Elipe, A. R.; Herrmann, J. M.; Dexpert, H.; Villain, F. J. Phys. Chem. 1995, 99, 3303. (33) Nagai, Y.; Dohmae, K.; Teramura, K.; Tanaka, T.; Guilera, G.; Kato, K.; Nomura, M.; Shinjoh, H.; Matsumoto, S. Catal. Today 2009, 145, 279. (34) Teramura, K.; Okuoka, S.; Yamazoe, S.; Kato, K.; Shishido, T.; Tanaka, T. J. Phys. Chem. C 2008, 112, 8495. (35) Okumura, K.; Honma, T.; Hirayama, S.; Sanada, T.; Niwa, M. J. Phys. Chem. C 2008, 112, 16740. (36) Okumura, K.; Matsui, H.; Sanada, T.; Arao, M.; Honma, T.; Hirayama, S.; Niwa, M. J. Catal. 2009, 265, 89. (37) Harada, M.; Inada, Y. Langmuir 2009, 25, 6049. (38) Park, B. K.; Jeong, S.; Kim, D.; Moon, J.; Lim, S.; Kim, J. S. J. Colloid Interface Sci. 2007, 311, 417. (39) Zhang, H.; Ren, X.; Cui, Z. J. Cryst. Growth 2007, 304, 206. (40) Zhang, H.; Cui, Z. Mater. Res. Bull. 2008, 43, 1583. (41) Liu, C. M.; Guo, H. B.; Xu, Z. Y.; Wu, Z. Y.; Weber, J. Microelectron. Eng. 2003, 66, 107. (42) Zhu, J.; Wang, Y.; Wang, X.; Yang, X.; Lu, L. Powder Technol. 2008, 181, 249. (43) Hirai, H.; Wakabayashi, H.; Komiyama, M. Bull. Chem. Soc. Jpn. 1986, 59, 367.
4474 DOI: 10.1021/la904248z
images were taken by sampling a small amount of the reaction solution every few minutes and casting onto a carbon-coated copper grid and then drying in vacuum. The samples were analyzed using a Hitachi H-7100 transmission electron microscope operated at 100 kV and a Hitachi H-9000NAR transmission electron microscope operated at 300 kV. Cu-K edge (8980.3 eV) XAFS spectra were recorded every 68 s at the range of the Cu-K edge X-ray absorption near-edge structure (XANES) region during the reaction by the fluorescence method using a Si(111) monochromator at the BL01B1 station in the SPring-8 synchrotron radiation facility, Japan. For the in situ QXAFS measurements, Cu(OAc)2/PVP solution was loaded into a plastic cell (PMMA, 1 1 4.5 cm3) under an N2 atmosphere. Subsequently, NaBH4/NaOH solution was injected into the cell using a microsyringe under stirring. Note that the composition of the reaction solution was the same as the case of the reaction in the flask. The injection of NaBH4/NaOH solution to the Cu(OAc)2/ PVP aqueous solution was counted as the beginning of the reaction. The spot size of X-ray was 5 1 mm2, and the volume irradiated by X-rays was only 2% of the reaction solution. All operations were conducted under an N2 atmosphere at room temperature. The measurement was started immediately after the injection of the reducing agent into the Cu(II) solution. The X-ray energy was calibrated using Cu foil as a reference. To observe the color variation, both absorption spectra and photographs were taken periodically. The in situ UV-vis spectra were taken every 87 s at the range of 500-1000 nm using a PerkinElmer Lambda 35 UV-vis spectrometer. The reaction conditions were the same as those in the case of in situ XAFS measurements. Powder X-ray diffraction (XRD) patterns of Cu NPs were obtained in reflection geometry using a Rigaku RINT2000 X-ray diffractometer at room temperature with Cu KR radiation (wavelength 1.542 A˚). The sample was kept in vacuum overnight before measurement. The average crystallite size of Cu metal was estimated from the Cu(111) diffraction peak by the Scherrer formula.
Results and Discussion Figure 1a shows the TEM image of the sample obtained immediately after injection of the NaBH4/NaOH solution in the presence of PVP. The large reticulated structure, which corresponds to gel-like Cu(OH)2, was detected at the beginning of the reaction. No Cu NPs were observed in the TEM image. Note that Cu(OH)2 was formed when NaOH was dripped into the Cu(OAc)2 solution as shown in Figures 1e,f, and the large reticulated structure, which is quite similar to Figure 1a, was observed. Figures 1b-d show the TEM images of the samples obtained 5, 15, and 60 min after the beginning of the reaction, respectively. In Figure 1b, spherical NPs with a wide size distribution were formed on the framework of gel-like Cu(OH)2. After 15 min reaction, Cu NPs of 4 nm mean diameter with narrow size distribution were observed as shown in Figures 1c,d. The size distribution of Cu NPs showed little change when the reaction time was increased more than 15 min. Figure 2 shows the HR-TEM image of as-synthesized Cu NPs. The lattice spacing d was measured to be 0.21 and 0.25 nm from the HR-TEM image corresponding to the Cu(111) and Cu2O(111) plane, respectively. The as-synthesized Cu NPs with a polycrystalline body were obtained. On the other hand, large Cu agglomerates were formed on the framework of gel-like Cu(OH)2 immediately after the injection of the reducing agent in the absence of PVP as shown in Figure 3. Figure 4 shows the XRD patterns of Cu NPs synthesized in the presence and the absence of PVP. In the case of Cu NPs synthesized in the presence of PVP, only the Cu metal phase was observed (Figure 4a). The mean crystallite sizes of Cu NPs were estimated to be 10.7, 11.1, and 12.1 A˚ for the samples Langmuir 2010, 26(6), 4473–4479
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Figure 1. TEM images of the colloidal Cu NP dispersion: (a) 0, (b) 5, (c) 15, and (d) 60 min after injection of the NaBH4/NaOH solution. Parts e and f show the XRD pattern and TEM image of Cu(OH)2 formed by adding NaOH into the Cu(OAc)2 solution.
Figure 3. TEM images of the Cu NPs synthesized in the absence of PVP: (a) 0, (b) 5, and (c) 15 min after injection of the NaBH4/ NaOH solution. Figure 2. HR-TEM images of as-synthesized single Cu NPs. Only the Cu(111) plane (d = 0.21 nm) is observed for a Cu NP in the right panel, while both Cu(111) and Cu2O(111) (d = 0.25 nm) planes are observed for a NP in the left panel.
obtained 15, 30, and 60 min after injection of the NaBH4/NaOH solution, respectively. Several sharp peaks observed in lower angle regions (
