- Email: [email protected]
Size-controlled preparation of silver nanoparticles by a modified polyol method
Colloids and Surfaces A: Physicochem. Eng. Aspects 366 (2010) 197–202
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Size-controlled preparation of silver nanoparticles by a modified polyol method Tao Zhao a , Rong Sun a,∗ , Shuhui Yu a , Zhijun Zhang b , Limin Zhou c , Haitao Huang d,∗ , Ruxu Du e a
Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, 1068 Xueyuan Avenue, Shenzhen University Town, Shenzhen 518055, PR China Key Lab for Special Functional Materials, Ministry of Education, Henan University, Kaifeng 475001, PR China c Department of Mechanical Engineering, the Hong Kong Polytechnic University, Hon Hum, Kow Loon, Hong Kong, PR China d Department of Applied Physics, the Hong Kong Polytechnic University, Hon Hum, Kow Loon, Hong Kong, PR China e Institute of Precision Engineering, the Chinese University of Hong Kong, Shatin, NT, Hong Kong, PR China b
a r t i c l e
i n f o
Article history: Received 3 February 2010 Received in revised form 17 May 2010 Accepted 2 June 2010 Available online 11 June 2010 Keywords: Size-controlled preparation Ag nanoparticles Polyol method
a b s t r a c t In this work, size-controlled silver nanoparticles were prepared with liquid phase chemical method in the ethylene glycol/polyvinylpyrrolidone media. Crystal structure, size, thermal properties and surface chemistry state of the silver nanoparticles were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR), thermal analysis (TA) and X-ray photoelectron spectroscopy (XPS). The particle size can be conveniently adjusted to 80 nm, 50 nm, 30 nm and 10 nm by controlling the experimental parameters such as the ratio of PVP to AgNO3 , and the amount of ammonia added as complexing agent. The as-obtained silver nanoparticles can be dispersed in water, ethanol and other polar solvents, which has attractive applications in electrical and biological fields. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Silver nanoparticles have a wide range of applications in many fields, such as surface-enhanced Raman spectroscopy [1], catalysts [2,3], anti-bacterial materials [4,5] and lubricating materials [6]. Recently, it was reported that Ag nanoparticles exhibit promising cytoprotective activities towards HIV-infected T-cells [7,8] and HBV [9]. Ag/polymer composite [10,11] is approach towards high-k materials for embedded capacitor application. The dielectric behavior of such composites is strongly dependent on the size, shape and distribution of the filling particles [12,13]. Nanoparticles with controlled particle size are of key importance because the optical, and magnetic properties of these nanoparticles depend strongly on their size [14,15]. Therefore, the synthesis of size-controlled nanoparticles is of great importance for both fundamental research and technical applications. Especially, nanoparticles that can be dispersed in a specific media, such as the polar or non-polar solvents, are most desirable in order to meet different application requirements. Up to now, various methods have been employed to prepare Ag nanoparticles with different size and shape, such as UV irradiation [16], electron irradiation [17], photochemical method [18], sonoelectrochemical method [19], microwave irradiation [20], chemical reduction [21–25], atom beam sputtering [26]. However, most of
the reported methods involve more than one steps and sometimes produce unsafe chemicals. Besides, the derived Ag nanoparticles with the above methods are hard to be dissolved in polar solvents, which limits their applications. For example, use Ag nanoparticles as fillers in dielectric composites with polar polymers as the matrix, or as the anti-bacterial and antiviral agents in aqueous media. The purpose of this study is to synthesize size-controlled Ag nanoparticles that can be dispersed in polar solvents using a method that is simple and environmental friendly. As one of the liquid phase chemical methods, polyol method is simple and environmentally friendly, without need for inert gas protection. It has been proven that the polyol method is a convenient and versatile method for the preparation of Ag [27], Pd [28], Te [29] BiIn [30], FePt [31], metal oxides [32,33] and chalcogenide [34] nanoparticles. The polyol involved in the reaction can act as a solvent, stabilizer, and reducing agent which limits particles growth and prevent agglomeration. In previous studies, our research group [30,33] has prepared ZnO nanotubes and Bi–In alloy nanoparticles in polyols. Preparations of Ag nanoparticles in polyols have been reported before. However, almost all the studies focused on the morphology control [27,35], while the control of particle size using the polyol process is rarely reported. In this paper, a simple and efficient polyol process was explored to synthesize the size-controlled Ag nanoparticles that can be dispersed in polar solvents. The formation mechanism of Ag nanoparticles has been investigated in details. 2. Experimental
∗ Corresponding authors. E-mail addresses: [email protected] (R. Sun), [email protected] (H. Huang).
Silver nitrate (AgNO3 ) (impurity >99%, Kermel Chemical Co., Tianjin, China), polyvinylpyrrolidone (PVP-K30, MW 40,000)
0927-7757/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.06.005
198
T. Zhao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 366 (2010) 197–202
Fig. 1. XRD pattern of PVP capped Ag nanoparticles.
(Kermel Chemical Co., Tianjin, China), ethylene glycol (impurity >98%, Kermel Chemical Co., Tianjin, China) and ammonia (28%, Kermel Chemical Co., Tianjin, China) were used as raw materials. In a typical procedure, an appropriate amount of PVP (0.17–1.7 g) was dissolved in 10 ml ethylene glycol and heated up to 160 ◦ C in an oil bath, until it turned from colorless to light yellow. Then, 0.170 g of AgNO3 in another 10 ml ethylene glycol was added dropwise into the above solution. and the reaction allowed to proceed for 4 h at this temperature., A brown colloidal dispersion was formed which indicated the formation of silver nanoparticles. The colloidal dispersion was cooled down to room temperature and mixed with a certain amount of acetone to allow for the generation of brown precipitate The precipitate was collected after centrifugation (8000 rpm for 30 min) and washed with acetone for three times, followed by drying at 40 ◦ C for 5 h in a vacuum dryer. X-ray diffraction (XRD) patterns were recorded with Philips X’Pert Pro X-ray powder diffraction instrument (the Netherlands,
Fig. 2. TEM images of PVP capped Ag nanoparticles prepared with different mass ratios of PVP to AgNO3 , (a) 8:1, (b) 10:1, (c) 15:1, (d) 20:1, (e) 8:1 (with ammonia) and (f) 10:1 (with ammonia).
T. Zhao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 366 (2010) 197–202
199
Philips Company). The operation voltage and current were 40 kV and 40 mA, respectively, with a scanning rate of 10◦ /min. The morphology of the as-synthesized Ag nanoparticles was examined on a JEOL JEM 100CX-II transmission electron microscope (TEM) at an acceleration voltage of 100 kV. The prepared particles were dispersed in ethanol with ultrasonic agitation for 2 min, and then deposited on a copper grid covered with a perforated carbon film. They were then subject to TEM observation. Nicolet Avatar 360-type Fourier transform infrared spectroscopy (FT-IR, the United States Nicolet Corporation) was used to characterize the surface structure of Ag nanoparticles in the wavelength range of 4000–400 cm−1 . The prepared Ag nanoparticles were mixed with KBr powder and pressed into a pellet for measurement. Background correction was made on the basis of the spectrum from a reference pure KBr pellet. Thermogravimetry analysis (TGA) and differential thermal analysis (DTA) were conducted in nitrogen on a Seiko EXSTAR 6000 thermal analysis system (TG & DTA, Japan’s Seiko Corporation) at a scanning rate of 10 ◦ C/min. The surface chemical state of the Ag nanoparticles was also investigated by X-ray photoelectron spectroscopy (XPS, AXIS Ultra Kratos Analytical Ltd.), using monochromatised Al K˛ radiation with an anode voltage of 15 kV and emission current of 3 mA. All the XPS spectra were calibrated using the main component of the C1s signal, attributed to aromatic carbons, at 284.8 eV. 3. Results and discussion 3.1. Crystal structure analysis The XRD spectrum of the as-synthesized PVP capped Ag nanoparticles of a typical size (30 nm) is shown in Fig. 1. Five main characteristic diffraction peaks for silver were observed at 2 = 38.2◦ , 44.5◦ , 64.7◦ , 77.5◦ , and 81.8◦ , which correspond to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) crystallographic planes of face-centered cubic (fcc) Ag crystals, respectively (JCPDS No. 870720). The result confirms that the samples were metallic silver nanoparticles with cubic crystal structure. No diffraction peaks of Ag2 O were detected in Fig. 1, which suggests that the PVP capped layer on Ag nanoparticles prevents Ag nanocores from oxidation. 3.2. Size-controlled Ag nanoparticles With the aim of preparing monodispersed silver nanoparticles with different sizes, several AgNO3 to PVP ratios were tried in order to determine the optimum conditions. The AgNO3 concentration was fixed, while the amount of PVP added was varied in the range of 0.17–1.7 g. Fig. 2a and b shows TEM images of PVP capped Ag nanoparticles prepared with various PVP to AgNO3 ratios. When the mass ratio of PVP to AgNO3 were slightly increased from 8:1 to 10:1, the size of Ag nanoparticles decrease drastically from 80 to 50 nm. However, if we further increase the mass ratio of PVP to AgNO3 to 15:1 or 20:1, it found that the size of the nanoparticles changed not obviously, as shown in Fig. 2c and d. The reason may be as: when the mass ratio of PVP to AgNO3 was above 10:1, it was found that the viscosity of the reaction system increased dramatically. The increased viscosity makes it difficult for the Ag atoms to diffuse in the system and results in an almost constant particle size that is independent of the PVP concentration. Therefore, it is difficult to prepare nanoparticles with size less than 50 nm by merely increasing PVP concentration. To overcome the problem, an appropriate amount of ammonia was added to the AgNO3 solutions during the reaction process. The pH of the AgNO3 solutions was adjusted to be 10 using ammonia solution. As a result, well dispersed Ag nanoparticles with smaller sizes of 30 and 10 nm were obtained, as shown in Fig. 2e and f,
Fig. 3. FT-IR spectra of (a) PVP and (b) PVP capped Ag nanoparticles.
respectively. In this reaction, NH3 and Ag+ form Ag (NH3 )2 + , which is more difficult to be reduced than Ag+ does. From the potential point of view, Ag+ /Ag0 electrical potential is 0.7991 V, while [Ag (NH3 )2 ]+ /Ag0 electric potential is 0.3719 V at standard temperature and pressure, so the reduction of Ag+ to silver metal is easier than [Ag (NH3 )2 ]+ . Therefore, when the redox reaction took place, the silver grain growth was suppressed and the grains were coated with PVP, leading to smaller sizes of silver nanoparticles. A typical reduction reaction of a silver cations. First, the ethylene glycol lost water and changed into acetaldehyde. Then, Ag+ or [Ag (NH3 )2 ]+ were reduced by acetaldehyde to Ag0 . The reaction could be expressed as HO–(CH2 )2 –OH → CH3 CHO + H2 O +
Ag + CH3 CHO → Ag
0
[Ag (NH3 )2 ]+ + CH3 CHO → Ag0
(1) (2) (3)
3.3. FT-IR analysis Fig. 3 shows the IR spectra of PVP and PVP coated Ag nanoparticles. For PVP spectrum shown in Fig. 3a, the bands at 2920
200
T. Zhao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 366 (2010) 197–202
and 2885 cm−1 are attributed to the asymmetric and symmetric stretching vibrations of –CH2 –, respectively. The bands at 722 cm−1 are attributed to the bending vibrations in plane of the long chain –(CH2 )n –(n > 4). 1095 cm−1 and 1075 cm−1 are the C–N asymmetric and symmetric stretching vibrations, respectively. The bands between 1400 and 1500 cm−1 are three groups bond absorption peaks of –N–C–. The bands at 1661 cm−1 is attributed to the stretching vibration of the –C O. All the absorption peaks are related to the characteristic absorption of PVP. The bands at 3430 cm−1 are attributed to the stretching vibration absorption of –OH of adsorbed water, as PVP has a strong hygroscopicity. The IR spectrum of PVP capped Ag nanoparticles with an average diameter of 30 nm is shown in Fig. 3b. The –CH2 – vibration bands are detected, but the asymmetric and symmetric stretching vibration peaks of C–N shift from 1095 and 1075 cm−1 to 1118 and 1080 cm−1 , respectively. The stretching vibration peak of –C O also moves from 1661 to 1668 cm−1 . These shifted peaks indicate that the N or O atoms of the PVP molecules interact with the surface of Ag nanoparticles by chemical absorption. 3.4. Thermal behavior Fig. 4 shows the TGA and DTA curves of PVP capped Ag nanoparticles. The mass loss before 100 ◦ C in the TGA curve is due to the evaporation of adsorbed water. The weight loss from 100 to 200 ◦ C is attributed to the decomposition of a small amount of organic molecules in the sample. An obvious mass loss is observed from 404 to 457 ◦ C, which corresponds to the decomposition of the capping layer of the sample. Above 457 ◦ C, the TGA curve tends to be smooth and flat, and the sample reaches a constant weight. The total mass loss of PVP coated Ag nanoparticles is about 86%. There is an endothermic peak in the vicinity of 146 ◦ C in the DTA curve which corresponds to the melting of the organic molecules in the sample. The endothermic peak near 450 ◦ C is attributed to the
Fig. 4. TG–DTA curves of PVP capped Ag nanoparticles.
decomposition of PVP. The thermal analysis results confirm that the Ag nanoparticles are coated with PVP. 3.5. XPS analysis Fig. 5 shows the XPS spectra of PVP. It can be seen that the occurrence of carbon, oxygen, and nitrogen signals confirms the presence of the typical PVP. Through curve fitting it can be found that the C1s spectrum is composed of four peaks with binding energies of 284.8, 285.6, 286.1, and 287.6 eV, which are attributed to the C1 (and C4 ), C3 (and C5 ), C2 , and C6 of the PVP molecule, respectively. O1s electronic binding energy at 531.3 eV is attributed to the carboxyl (C O) oxygen atom, and that at 533.4 eV corresponds to the binding energy of the adsorbed water. N1s electronic binding energy is at 399.6 eV.
Fig. 5. Molecular formula of PVP repeated unit, with carbon atom numbering and XPS spectra of PVP.
T. Zhao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 366 (2010) 197–202
201
Fig. 6. XPS spectra of PVP capped Ag nanoparticles.
Fig. 6 shows the XPS spectra of the PVP capped Ag nanoparticles, where the C1s electronic binding energies at 284.8, 285.6, 286.1, and 287.6 eV are the same as those of PVP. Curve fitting in the O1s region leads to two peaks at 531.2 and 532.8 eV, originated from carboxyl (C O) oxygen atoms of PVP and hydroxyl (C–OH) oxygen atoms of ethylene glycol, respectively. Compared with PVP, the N1s electron binding energy increases to 400.3 eV, which implies that the lone pair electrons of N atoms are transferred to the Ag nanoparticles, leading to the decrease of the electron cloud densities around the N atoms. Ag3d electronic binding energy at 368.1 eV of the Ag nanoparticles is the same as that of the metallic Ag. The oxygen signals from Ag2 O (O1s electron binding energy at 529.2 eV) and AgO (O1s electron binding energy at 528.5 eV) are not found. Due to the interaction between the N atoms of PVP and the Ag nanoparticles, the N → Ag coordination bond could form on the surface of the Ag nanoparticles, which corresponds to the blue shift of the C–N stretching vibration, as indicated in Fig. 3b. Therefore, both the XRD and XPS analysis indicate that PVP prevents Ag nanoparticles from being oxidized.
4. Conclusion In summary, an optimal condition for the synthesis of size controlled, well dispersed Ag nanoparticles in ethylene glycol/PVP system has been developed by controlling the mass ratios of PVP to AgNO3 , and by adding a certain amount of ammonia as the complexing agent. The average particle size can be varied from 10 to 80 nm with a narrow size distribution by controlling the amount of PVP and ammonia added. The particles size decreases with increasing PVP concentration in a certain range and the addition of ammonia to the precursor solution can further decrease the Ag particle size. FT-IR and XPS analysis results show that the N
atoms in PVP coordinates to the Ag core surface and form a coordination bond between a N atom and a silver at the surface of the core. The size-controlled, well dispersed Ag nanoparticles prepared are capable of being dispersed in water, ethanol, and other polar solvents which have potential applications in dielectric and biological fields. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 20971089, No. 50807038) and the research funding from National S&T Major Project with the contact No. 2009ZX02038 is greatly acknowledged. References [1] (a) P. Setua, A. Chakraborty, D. Seth, M.U. Bhatta, P.V. Satyam, N. Sarkar, Synthesis, optical properties, and surface enhanced Raman scattering of silver nanoparticles in nonaqueous methanol reverse micelles, J. Phys. Chem. C 111 (2007) 3901–3907; (b) S. Panigrahi, S. Praharaj, S. Basu, S.K. Ghosh, S. Jana, S. Pande, T. Jiang, H. Vo-Dinh, T. Pal, Self-assembly of silver nanoparticles: synthesis, stabilization, optical properties, and application in surface-enhanced Raman scattering, J. Phys. Chem. B 110 (2006) 13436–13444. [2] R.J. Chimentão, I. Kirm, F. Medina, X. Rodríguez, Y. Cesteros, P. Salagre, J.E. Sueirasa, Different morphologies of silver nanoparticles as catalysts for the selective oxidation of styrene in the gas phase, Chem. Commun. (2004) 846–847. [3] Q.y. Wei, B. Li, C. Li, J.q. Wang, W. Wang, X.j. Yang, PVP-capped silver nanoparticles as catalysts for polymerization of alkyls lanes to siloxane composite microspheres, J. Mater. Chem. 16 (2006) 3606–3608. [4] H.Y. Kong, Y.S. Jang, Antibacterial properties of novel poly(methyl methacrylate) nanofiber containing silver nanoparticles, Langmuir 24 (2008) 2051–2056. [5] Y.W. Zhang, H.S. Peng, W. Huang, Y.F. Zhou, X.H. Zhang, D.Y. Yan, Hyperbranched poly(amidoamine) as the stabilizer and reductant to prepare colloid silver nanoparticles in situ and their antibacterial activity, J. Phys. Chem. C 112 (2008) 2330–2336.
202
T. Zhao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 366 (2010) 197–202
[6] L. Sun, Z.J. Zhang, Z.S. Wu, H.X. Dang, Synthesis and characterization of DDP coated Ag nanoparticles, Mater. Sci. Eng. A 379 (2004) 378–383. [7] J.L. Elechiguerra1, J.L. Burt, J.R. Morones1, A.C. Bragado, X.X. Gao, H.H. Lara, M.J. Yacaman, Interaction of silver nanoparticles with HIV-1, J. Nanobiotechnol. 3 (2005) 6. [8] W.Y. Sun, R. Chen, N.P.Y. Chung, C.M. Ho, C.L.S. Lin, C.M. Che, Silver nanoparticles fabricated in Hepes buffer exhibit cytoprotective activities toward HIV-1 infected cells, Chem. Commun. (2005) 5059–5061. [9] L. Lu, R.W. Sun, R. Chen, C.K. Hui, C.M. Ho, J.M. Luk, G.K. Lau, C.M. Che, Silver nanoparticles inhibit hepatitis B virus replication, Antivir. Ther. 13 (2008) 253–262. [10] L. Qi, B.I. Lee, S.H. Chen, W.D. Samuels, G.J. Exarhos, High-dielectric-constant silver-epoxy composites as embedded dielectrics, Adv. Mater. 17 (2005) 1777–1781. [11] Y. Shen, Y.H. Lin, C.W. Nan, Interfacial effect on dielectric properties of polymer nanocomposites filled with core/shell-structured particles, Adv. Funct. Mater. 17 (2007) 2405–2410. [12] D.M. Grannan, J.C. Garland, D.B. Tanner, Critical behavior of the dielectric constant of a random composite near the percolation threshold, Phys. Rev. Lett. 46 (1981) 375–378. [13] Y. Shen, Y. Lin, M. Li, C.W. Nan, High dielectric performance of polymer composite films induced by a percolating interparticle barrier layer, Adv. Mater. 19 (2007) 1418–1422. [14] C.S. Seney, B.M. Gutzman, R.H. Goddard, Correlation of size and surfaceenhanced Raman scattering activity of optical and spectroscopic properties for silver nanoparticles, J. Phys. Chem. C 113 (2009) 74–80. [15] W.S. Seo, H.H. Jo, K. Lee, B. Kim, S.J. Oh, J.T. Park, Size-dependent magnetic properties of colloidal Mn3 O4 and MnO nanoparticles, Angew. Chem., Int. Ed. 43 (2004) 1115–1117. [16] H.T. Huang, Y. Yang, Preparation of silver nanoparticles in inorganic clay suspensions, Compos. Sci. Technol. 68 (2008) 2948–2953. [17] K.A. Bogle, S.D. Dhole, V.N. Bhoraskar, Silver nanoparticles: synthesis and size control by electron irradiation, Nanotechnology 17 (2006) 3204– 3208. [18] B. Pietrobon, V. Kitaev, Photochemical synthesis of monodisperse sizecontrolled silver decahedral nanoparticles and their remarkable optical properties, Chem. Mater. 20 (2008) 5186–5190. [19] J.J. Zhu, S.W. Liu, O. Palchik, Y. Koltypin, A. Gedanken, Shape-controlled synthesis of silver nanoparticles by pulse sonoelectrochemical methods, Langmuir 16 (2000) 6396–6399.
[20] S. Kundu, K. Wang, H. Liang, Size-controlled synthesis and self-assembly of silver nanoparticles within a minute using microwave irradiation, J. Phys. Chem. C 113 (2009) 134–141. [21] D. David, J.R. Evanoff, G. Chumanov, Size-controlled synthesis of nanoparticles. 1. “Silver-Only” aqueous suspensions via hydrogen reduction, J. Phys. Chem. B 108 (2004) 13948–13956. [22] I.P. Santos, L.M.L. Marzn, Formation and stabilization of silver nanoparticles through reduction by N,N-dimethylformamide, Langmuir 15 (1999) 948–951. [23] T. Yonezawa, S. Onoue, N. Kimizuka, Preparation of highly positively charged silver nanoballs and their stability, Langmuir 16 (2000) 5218–5220. [24] A. Taleb, C. Petit, M.P. Pileni, Synthesis of highly monodisperse silver nanoparticles from AOT reverse micelles: a way to 2D and 3D self-organization, Chem. Mater. 9 (1997) 950–959. [25] S.A. Harfenist, Z.L. Wang, M.M. Alvarez, I. Vezmar, R.L. Whetten, Highly oriented molecular Ag nanocrystal arrays, J. Phys. Chem. 100 (1996) 13904–13910. [26] Y.K. Mishra, S. Mohapatra, D. Kabiraj, B. Mohanta, N.P. Lalla, J.C. Pivin, D.K. Avasthi, Synthesis and characterization of Ag nanoparticles in silica matrix by atom beam sputtering, Scr. Mater. 56 (2007) 629–632. [27] Y.G. Sun, B.M. Gates, Y.N. Xia, Crystalline silver nanowires by soft solution processing, Nano Letters 2 (2002) 165–168. [28] Y.J. Xing, J.Y. Chen, B. Wiley, Y.N. Xia, Understanding the role of oxidative etching in the polyol synthesis of Pd nanoparticles with uniform shape and size, J. Am. Chem. Soc. 127 (2005) 7332–7333. [29] Y.J. Zhu, X.L. Hu, Tellurium nanorods and nanowires prepared by the microwave-polyol method, Chem. Lett. 33 (2004) 760–761. [30] G.F. Xiao, Y.B. Zhao, X.L. Meng, Z.S. Wu, Z.J. Zhang, Shape-controlled synthesis of BiIn alloy nanostructures, J. Alloys Compd. 437 (2007) 329–331. [31] C. Liu, X.W. Wu, T. Klemmer, N. Shukla, X.M. Yang, D. Weller, A.G. Roy, M. Tanase, D. Laughlin, Polyol process synthesis of monodispersed FePt nanoparticles, J. Phys. Chem. B 108 (2005) 6121–6123. [32] P.l. Zhu, J.W. Zhang, Z.S. Wu, Z.J. Zhang, Microwave-assisted synthesis of various ZnO hierarchical nanostructures: effects of heating parameters of microwave oven, Crystal Growth Des. 8 (2008) 3148–3153. [33] C. Feldmann, H.O. Jungk, Polyol-mediated preparation of nanoscale oxide particles, Angew. Chem., Int. Ed. 40 (2001) 359–362. [34] Y.W. Zhao, Y. Zhang, H. Zhu, G.C. Hadjipanayis, J.Q. Xiao, Low-temperature synthesis of hexagonal (wurtzite) ZnS nanocrystals, J. Am. Chem. Soc. 124 (2004) 6874–6875. [35] Y.G. Sun, Y.N. Xia, Large-scale synthesis of uniform silver nanowires through a soft, self-seeding, polyol process, Adv. Mater. 14 (2002) 833–837.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Size-controlled preparation of silver nanoparticles by a modified polyol method Tao Zhao a , Rong Sun a,∗ , Shuhui Yu a , Zhijun Zhang b , Limin Zhou c , Haitao Huang d,∗ , Ruxu Du e a
Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, 1068 Xueyuan Avenue, Shenzhen University Town, Shenzhen 518055, PR China Key Lab for Special Functional Materials, Ministry of Education, Henan University, Kaifeng 475001, PR China c Department of Mechanical Engineering, the Hong Kong Polytechnic University, Hon Hum, Kow Loon, Hong Kong, PR China d Department of Applied Physics, the Hong Kong Polytechnic University, Hon Hum, Kow Loon, Hong Kong, PR China e Institute of Precision Engineering, the Chinese University of Hong Kong, Shatin, NT, Hong Kong, PR China b
a r t i c l e
i n f o
Article history: Received 3 February 2010 Received in revised form 17 May 2010 Accepted 2 June 2010 Available online 11 June 2010 Keywords: Size-controlled preparation Ag nanoparticles Polyol method
a b s t r a c t In this work, size-controlled silver nanoparticles were prepared with liquid phase chemical method in the ethylene glycol/polyvinylpyrrolidone media. Crystal structure, size, thermal properties and surface chemistry state of the silver nanoparticles were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR), thermal analysis (TA) and X-ray photoelectron spectroscopy (XPS). The particle size can be conveniently adjusted to 80 nm, 50 nm, 30 nm and 10 nm by controlling the experimental parameters such as the ratio of PVP to AgNO3 , and the amount of ammonia added as complexing agent. The as-obtained silver nanoparticles can be dispersed in water, ethanol and other polar solvents, which has attractive applications in electrical and biological fields. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Silver nanoparticles have a wide range of applications in many fields, such as surface-enhanced Raman spectroscopy [1], catalysts [2,3], anti-bacterial materials [4,5] and lubricating materials [6]. Recently, it was reported that Ag nanoparticles exhibit promising cytoprotective activities towards HIV-infected T-cells [7,8] and HBV [9]. Ag/polymer composite [10,11] is approach towards high-k materials for embedded capacitor application. The dielectric behavior of such composites is strongly dependent on the size, shape and distribution of the filling particles [12,13]. Nanoparticles with controlled particle size are of key importance because the optical, and magnetic properties of these nanoparticles depend strongly on their size [14,15]. Therefore, the synthesis of size-controlled nanoparticles is of great importance for both fundamental research and technical applications. Especially, nanoparticles that can be dispersed in a specific media, such as the polar or non-polar solvents, are most desirable in order to meet different application requirements. Up to now, various methods have been employed to prepare Ag nanoparticles with different size and shape, such as UV irradiation [16], electron irradiation [17], photochemical method [18], sonoelectrochemical method [19], microwave irradiation [20], chemical reduction [21–25], atom beam sputtering [26]. However, most of
the reported methods involve more than one steps and sometimes produce unsafe chemicals. Besides, the derived Ag nanoparticles with the above methods are hard to be dissolved in polar solvents, which limits their applications. For example, use Ag nanoparticles as fillers in dielectric composites with polar polymers as the matrix, or as the anti-bacterial and antiviral agents in aqueous media. The purpose of this study is to synthesize size-controlled Ag nanoparticles that can be dispersed in polar solvents using a method that is simple and environmental friendly. As one of the liquid phase chemical methods, polyol method is simple and environmentally friendly, without need for inert gas protection. It has been proven that the polyol method is a convenient and versatile method for the preparation of Ag [27], Pd [28], Te [29] BiIn [30], FePt [31], metal oxides [32,33] and chalcogenide [34] nanoparticles. The polyol involved in the reaction can act as a solvent, stabilizer, and reducing agent which limits particles growth and prevent agglomeration. In previous studies, our research group [30,33] has prepared ZnO nanotubes and Bi–In alloy nanoparticles in polyols. Preparations of Ag nanoparticles in polyols have been reported before. However, almost all the studies focused on the morphology control [27,35], while the control of particle size using the polyol process is rarely reported. In this paper, a simple and efficient polyol process was explored to synthesize the size-controlled Ag nanoparticles that can be dispersed in polar solvents. The formation mechanism of Ag nanoparticles has been investigated in details. 2. Experimental
∗ Corresponding authors. E-mail addresses: [email protected] (R. Sun), [email protected] (H. Huang).
Silver nitrate (AgNO3 ) (impurity >99%, Kermel Chemical Co., Tianjin, China), polyvinylpyrrolidone (PVP-K30, MW 40,000)
0927-7757/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.06.005
198
T. Zhao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 366 (2010) 197–202
Fig. 1. XRD pattern of PVP capped Ag nanoparticles.
(Kermel Chemical Co., Tianjin, China), ethylene glycol (impurity >98%, Kermel Chemical Co., Tianjin, China) and ammonia (28%, Kermel Chemical Co., Tianjin, China) were used as raw materials. In a typical procedure, an appropriate amount of PVP (0.17–1.7 g) was dissolved in 10 ml ethylene glycol and heated up to 160 ◦ C in an oil bath, until it turned from colorless to light yellow. Then, 0.170 g of AgNO3 in another 10 ml ethylene glycol was added dropwise into the above solution. and the reaction allowed to proceed for 4 h at this temperature., A brown colloidal dispersion was formed which indicated the formation of silver nanoparticles. The colloidal dispersion was cooled down to room temperature and mixed with a certain amount of acetone to allow for the generation of brown precipitate The precipitate was collected after centrifugation (8000 rpm for 30 min) and washed with acetone for three times, followed by drying at 40 ◦ C for 5 h in a vacuum dryer. X-ray diffraction (XRD) patterns were recorded with Philips X’Pert Pro X-ray powder diffraction instrument (the Netherlands,
Fig. 2. TEM images of PVP capped Ag nanoparticles prepared with different mass ratios of PVP to AgNO3 , (a) 8:1, (b) 10:1, (c) 15:1, (d) 20:1, (e) 8:1 (with ammonia) and (f) 10:1 (with ammonia).
T. Zhao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 366 (2010) 197–202
199
Philips Company). The operation voltage and current were 40 kV and 40 mA, respectively, with a scanning rate of 10◦ /min. The morphology of the as-synthesized Ag nanoparticles was examined on a JEOL JEM 100CX-II transmission electron microscope (TEM) at an acceleration voltage of 100 kV. The prepared particles were dispersed in ethanol with ultrasonic agitation for 2 min, and then deposited on a copper grid covered with a perforated carbon film. They were then subject to TEM observation. Nicolet Avatar 360-type Fourier transform infrared spectroscopy (FT-IR, the United States Nicolet Corporation) was used to characterize the surface structure of Ag nanoparticles in the wavelength range of 4000–400 cm−1 . The prepared Ag nanoparticles were mixed with KBr powder and pressed into a pellet for measurement. Background correction was made on the basis of the spectrum from a reference pure KBr pellet. Thermogravimetry analysis (TGA) and differential thermal analysis (DTA) were conducted in nitrogen on a Seiko EXSTAR 6000 thermal analysis system (TG & DTA, Japan’s Seiko Corporation) at a scanning rate of 10 ◦ C/min. The surface chemical state of the Ag nanoparticles was also investigated by X-ray photoelectron spectroscopy (XPS, AXIS Ultra Kratos Analytical Ltd.), using monochromatised Al K˛ radiation with an anode voltage of 15 kV and emission current of 3 mA. All the XPS spectra were calibrated using the main component of the C1s signal, attributed to aromatic carbons, at 284.8 eV. 3. Results and discussion 3.1. Crystal structure analysis The XRD spectrum of the as-synthesized PVP capped Ag nanoparticles of a typical size (30 nm) is shown in Fig. 1. Five main characteristic diffraction peaks for silver were observed at 2 = 38.2◦ , 44.5◦ , 64.7◦ , 77.5◦ , and 81.8◦ , which correspond to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) crystallographic planes of face-centered cubic (fcc) Ag crystals, respectively (JCPDS No. 870720). The result confirms that the samples were metallic silver nanoparticles with cubic crystal structure. No diffraction peaks of Ag2 O were detected in Fig. 1, which suggests that the PVP capped layer on Ag nanoparticles prevents Ag nanocores from oxidation. 3.2. Size-controlled Ag nanoparticles With the aim of preparing monodispersed silver nanoparticles with different sizes, several AgNO3 to PVP ratios were tried in order to determine the optimum conditions. The AgNO3 concentration was fixed, while the amount of PVP added was varied in the range of 0.17–1.7 g. Fig. 2a and b shows TEM images of PVP capped Ag nanoparticles prepared with various PVP to AgNO3 ratios. When the mass ratio of PVP to AgNO3 were slightly increased from 8:1 to 10:1, the size of Ag nanoparticles decrease drastically from 80 to 50 nm. However, if we further increase the mass ratio of PVP to AgNO3 to 15:1 or 20:1, it found that the size of the nanoparticles changed not obviously, as shown in Fig. 2c and d. The reason may be as: when the mass ratio of PVP to AgNO3 was above 10:1, it was found that the viscosity of the reaction system increased dramatically. The increased viscosity makes it difficult for the Ag atoms to diffuse in the system and results in an almost constant particle size that is independent of the PVP concentration. Therefore, it is difficult to prepare nanoparticles with size less than 50 nm by merely increasing PVP concentration. To overcome the problem, an appropriate amount of ammonia was added to the AgNO3 solutions during the reaction process. The pH of the AgNO3 solutions was adjusted to be 10 using ammonia solution. As a result, well dispersed Ag nanoparticles with smaller sizes of 30 and 10 nm were obtained, as shown in Fig. 2e and f,
Fig. 3. FT-IR spectra of (a) PVP and (b) PVP capped Ag nanoparticles.
respectively. In this reaction, NH3 and Ag+ form Ag (NH3 )2 + , which is more difficult to be reduced than Ag+ does. From the potential point of view, Ag+ /Ag0 electrical potential is 0.7991 V, while [Ag (NH3 )2 ]+ /Ag0 electric potential is 0.3719 V at standard temperature and pressure, so the reduction of Ag+ to silver metal is easier than [Ag (NH3 )2 ]+ . Therefore, when the redox reaction took place, the silver grain growth was suppressed and the grains were coated with PVP, leading to smaller sizes of silver nanoparticles. A typical reduction reaction of a silver cations. First, the ethylene glycol lost water and changed into acetaldehyde. Then, Ag+ or [Ag (NH3 )2 ]+ were reduced by acetaldehyde to Ag0 . The reaction could be expressed as HO–(CH2 )2 –OH → CH3 CHO + H2 O +
Ag + CH3 CHO → Ag
0
[Ag (NH3 )2 ]+ + CH3 CHO → Ag0
(1) (2) (3)
3.3. FT-IR analysis Fig. 3 shows the IR spectra of PVP and PVP coated Ag nanoparticles. For PVP spectrum shown in Fig. 3a, the bands at 2920
200
T. Zhao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 366 (2010) 197–202
and 2885 cm−1 are attributed to the asymmetric and symmetric stretching vibrations of –CH2 –, respectively. The bands at 722 cm−1 are attributed to the bending vibrations in plane of the long chain –(CH2 )n –(n > 4). 1095 cm−1 and 1075 cm−1 are the C–N asymmetric and symmetric stretching vibrations, respectively. The bands between 1400 and 1500 cm−1 are three groups bond absorption peaks of –N–C–. The bands at 1661 cm−1 is attributed to the stretching vibration of the –C O. All the absorption peaks are related to the characteristic absorption of PVP. The bands at 3430 cm−1 are attributed to the stretching vibration absorption of –OH of adsorbed water, as PVP has a strong hygroscopicity. The IR spectrum of PVP capped Ag nanoparticles with an average diameter of 30 nm is shown in Fig. 3b. The –CH2 – vibration bands are detected, but the asymmetric and symmetric stretching vibration peaks of C–N shift from 1095 and 1075 cm−1 to 1118 and 1080 cm−1 , respectively. The stretching vibration peak of –C O also moves from 1661 to 1668 cm−1 . These shifted peaks indicate that the N or O atoms of the PVP molecules interact with the surface of Ag nanoparticles by chemical absorption. 3.4. Thermal behavior Fig. 4 shows the TGA and DTA curves of PVP capped Ag nanoparticles. The mass loss before 100 ◦ C in the TGA curve is due to the evaporation of adsorbed water. The weight loss from 100 to 200 ◦ C is attributed to the decomposition of a small amount of organic molecules in the sample. An obvious mass loss is observed from 404 to 457 ◦ C, which corresponds to the decomposition of the capping layer of the sample. Above 457 ◦ C, the TGA curve tends to be smooth and flat, and the sample reaches a constant weight. The total mass loss of PVP coated Ag nanoparticles is about 86%. There is an endothermic peak in the vicinity of 146 ◦ C in the DTA curve which corresponds to the melting of the organic molecules in the sample. The endothermic peak near 450 ◦ C is attributed to the
Fig. 4. TG–DTA curves of PVP capped Ag nanoparticles.
decomposition of PVP. The thermal analysis results confirm that the Ag nanoparticles are coated with PVP. 3.5. XPS analysis Fig. 5 shows the XPS spectra of PVP. It can be seen that the occurrence of carbon, oxygen, and nitrogen signals confirms the presence of the typical PVP. Through curve fitting it can be found that the C1s spectrum is composed of four peaks with binding energies of 284.8, 285.6, 286.1, and 287.6 eV, which are attributed to the C1 (and C4 ), C3 (and C5 ), C2 , and C6 of the PVP molecule, respectively. O1s electronic binding energy at 531.3 eV is attributed to the carboxyl (C O) oxygen atom, and that at 533.4 eV corresponds to the binding energy of the adsorbed water. N1s electronic binding energy is at 399.6 eV.
Fig. 5. Molecular formula of PVP repeated unit, with carbon atom numbering and XPS spectra of PVP.
T. Zhao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 366 (2010) 197–202
201
Fig. 6. XPS spectra of PVP capped Ag nanoparticles.
Fig. 6 shows the XPS spectra of the PVP capped Ag nanoparticles, where the C1s electronic binding energies at 284.8, 285.6, 286.1, and 287.6 eV are the same as those of PVP. Curve fitting in the O1s region leads to two peaks at 531.2 and 532.8 eV, originated from carboxyl (C O) oxygen atoms of PVP and hydroxyl (C–OH) oxygen atoms of ethylene glycol, respectively. Compared with PVP, the N1s electron binding energy increases to 400.3 eV, which implies that the lone pair electrons of N atoms are transferred to the Ag nanoparticles, leading to the decrease of the electron cloud densities around the N atoms. Ag3d electronic binding energy at 368.1 eV of the Ag nanoparticles is the same as that of the metallic Ag. The oxygen signals from Ag2 O (O1s electron binding energy at 529.2 eV) and AgO (O1s electron binding energy at 528.5 eV) are not found. Due to the interaction between the N atoms of PVP and the Ag nanoparticles, the N → Ag coordination bond could form on the surface of the Ag nanoparticles, which corresponds to the blue shift of the C–N stretching vibration, as indicated in Fig. 3b. Therefore, both the XRD and XPS analysis indicate that PVP prevents Ag nanoparticles from being oxidized.
4. Conclusion In summary, an optimal condition for the synthesis of size controlled, well dispersed Ag nanoparticles in ethylene glycol/PVP system has been developed by controlling the mass ratios of PVP to AgNO3 , and by adding a certain amount of ammonia as the complexing agent. The average particle size can be varied from 10 to 80 nm with a narrow size distribution by controlling the amount of PVP and ammonia added. The particles size decreases with increasing PVP concentration in a certain range and the addition of ammonia to the precursor solution can further decrease the Ag particle size. FT-IR and XPS analysis results show that the N
atoms in PVP coordinates to the Ag core surface and form a coordination bond between a N atom and a silver at the surface of the core. The size-controlled, well dispersed Ag nanoparticles prepared are capable of being dispersed in water, ethanol, and other polar solvents which have potential applications in dielectric and biological fields. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 20971089, No. 50807038) and the research funding from National S&T Major Project with the contact No. 2009ZX02038 is greatly acknowledged. References [1] (a) P. Setua, A. Chakraborty, D. Seth, M.U. Bhatta, P.V. Satyam, N. Sarkar, Synthesis, optical properties, and surface enhanced Raman scattering of silver nanoparticles in nonaqueous methanol reverse micelles, J. Phys. Chem. C 111 (2007) 3901–3907; (b) S. Panigrahi, S. Praharaj, S. Basu, S.K. Ghosh, S. Jana, S. Pande, T. Jiang, H. Vo-Dinh, T. Pal, Self-assembly of silver nanoparticles: synthesis, stabilization, optical properties, and application in surface-enhanced Raman scattering, J. Phys. Chem. B 110 (2006) 13436–13444. [2] R.J. Chimentão, I. Kirm, F. Medina, X. Rodríguez, Y. Cesteros, P. Salagre, J.E. Sueirasa, Different morphologies of silver nanoparticles as catalysts for the selective oxidation of styrene in the gas phase, Chem. Commun. (2004) 846–847. [3] Q.y. Wei, B. Li, C. Li, J.q. Wang, W. Wang, X.j. Yang, PVP-capped silver nanoparticles as catalysts for polymerization of alkyls lanes to siloxane composite microspheres, J. Mater. Chem. 16 (2006) 3606–3608. [4] H.Y. Kong, Y.S. Jang, Antibacterial properties of novel poly(methyl methacrylate) nanofiber containing silver nanoparticles, Langmuir 24 (2008) 2051–2056. [5] Y.W. Zhang, H.S. Peng, W. Huang, Y.F. Zhou, X.H. Zhang, D.Y. Yan, Hyperbranched poly(amidoamine) as the stabilizer and reductant to prepare colloid silver nanoparticles in situ and their antibacterial activity, J. Phys. Chem. C 112 (2008) 2330–2336.
202
T. Zhao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 366 (2010) 197–202
[6] L. Sun, Z.J. Zhang, Z.S. Wu, H.X. Dang, Synthesis and characterization of DDP coated Ag nanoparticles, Mater. Sci. Eng. A 379 (2004) 378–383. [7] J.L. Elechiguerra1, J.L. Burt, J.R. Morones1, A.C. Bragado, X.X. Gao, H.H. Lara, M.J. Yacaman, Interaction of silver nanoparticles with HIV-1, J. Nanobiotechnol. 3 (2005) 6. [8] W.Y. Sun, R. Chen, N.P.Y. Chung, C.M. Ho, C.L.S. Lin, C.M. Che, Silver nanoparticles fabricated in Hepes buffer exhibit cytoprotective activities toward HIV-1 infected cells, Chem. Commun. (2005) 5059–5061. [9] L. Lu, R.W. Sun, R. Chen, C.K. Hui, C.M. Ho, J.M. Luk, G.K. Lau, C.M. Che, Silver nanoparticles inhibit hepatitis B virus replication, Antivir. Ther. 13 (2008) 253–262. [10] L. Qi, B.I. Lee, S.H. Chen, W.D. Samuels, G.J. Exarhos, High-dielectric-constant silver-epoxy composites as embedded dielectrics, Adv. Mater. 17 (2005) 1777–1781. [11] Y. Shen, Y.H. Lin, C.W. Nan, Interfacial effect on dielectric properties of polymer nanocomposites filled with core/shell-structured particles, Adv. Funct. Mater. 17 (2007) 2405–2410. [12] D.M. Grannan, J.C. Garland, D.B. Tanner, Critical behavior of the dielectric constant of a random composite near the percolation threshold, Phys. Rev. Lett. 46 (1981) 375–378. [13] Y. Shen, Y. Lin, M. Li, C.W. Nan, High dielectric performance of polymer composite films induced by a percolating interparticle barrier layer, Adv. Mater. 19 (2007) 1418–1422. [14] C.S. Seney, B.M. Gutzman, R.H. Goddard, Correlation of size and surfaceenhanced Raman scattering activity of optical and spectroscopic properties for silver nanoparticles, J. Phys. Chem. C 113 (2009) 74–80. [15] W.S. Seo, H.H. Jo, K. Lee, B. Kim, S.J. Oh, J.T. Park, Size-dependent magnetic properties of colloidal Mn3 O4 and MnO nanoparticles, Angew. Chem., Int. Ed. 43 (2004) 1115–1117. [16] H.T. Huang, Y. Yang, Preparation of silver nanoparticles in inorganic clay suspensions, Compos. Sci. Technol. 68 (2008) 2948–2953. [17] K.A. Bogle, S.D. Dhole, V.N. Bhoraskar, Silver nanoparticles: synthesis and size control by electron irradiation, Nanotechnology 17 (2006) 3204– 3208. [18] B. Pietrobon, V. Kitaev, Photochemical synthesis of monodisperse sizecontrolled silver decahedral nanoparticles and their remarkable optical properties, Chem. Mater. 20 (2008) 5186–5190. [19] J.J. Zhu, S.W. Liu, O. Palchik, Y. Koltypin, A. Gedanken, Shape-controlled synthesis of silver nanoparticles by pulse sonoelectrochemical methods, Langmuir 16 (2000) 6396–6399.
[20] S. Kundu, K. Wang, H. Liang, Size-controlled synthesis and self-assembly of silver nanoparticles within a minute using microwave irradiation, J. Phys. Chem. C 113 (2009) 134–141. [21] D. David, J.R. Evanoff, G. Chumanov, Size-controlled synthesis of nanoparticles. 1. “Silver-Only” aqueous suspensions via hydrogen reduction, J. Phys. Chem. B 108 (2004) 13948–13956. [22] I.P. Santos, L.M.L. Marzn, Formation and stabilization of silver nanoparticles through reduction by N,N-dimethylformamide, Langmuir 15 (1999) 948–951. [23] T. Yonezawa, S. Onoue, N. Kimizuka, Preparation of highly positively charged silver nanoballs and their stability, Langmuir 16 (2000) 5218–5220. [24] A. Taleb, C. Petit, M.P. Pileni, Synthesis of highly monodisperse silver nanoparticles from AOT reverse micelles: a way to 2D and 3D self-organization, Chem. Mater. 9 (1997) 950–959. [25] S.A. Harfenist, Z.L. Wang, M.M. Alvarez, I. Vezmar, R.L. Whetten, Highly oriented molecular Ag nanocrystal arrays, J. Phys. Chem. 100 (1996) 13904–13910. [26] Y.K. Mishra, S. Mohapatra, D. Kabiraj, B. Mohanta, N.P. Lalla, J.C. Pivin, D.K. Avasthi, Synthesis and characterization of Ag nanoparticles in silica matrix by atom beam sputtering, Scr. Mater. 56 (2007) 629–632. [27] Y.G. Sun, B.M. Gates, Y.N. Xia, Crystalline silver nanowires by soft solution processing, Nano Letters 2 (2002) 165–168. [28] Y.J. Xing, J.Y. Chen, B. Wiley, Y.N. Xia, Understanding the role of oxidative etching in the polyol synthesis of Pd nanoparticles with uniform shape and size, J. Am. Chem. Soc. 127 (2005) 7332–7333. [29] Y.J. Zhu, X.L. Hu, Tellurium nanorods and nanowires prepared by the microwave-polyol method, Chem. Lett. 33 (2004) 760–761. [30] G.F. Xiao, Y.B. Zhao, X.L. Meng, Z.S. Wu, Z.J. Zhang, Shape-controlled synthesis of BiIn alloy nanostructures, J. Alloys Compd. 437 (2007) 329–331. [31] C. Liu, X.W. Wu, T. Klemmer, N. Shukla, X.M. Yang, D. Weller, A.G. Roy, M. Tanase, D. Laughlin, Polyol process synthesis of monodispersed FePt nanoparticles, J. Phys. Chem. B 108 (2005) 6121–6123. [32] P.l. Zhu, J.W. Zhang, Z.S. Wu, Z.J. Zhang, Microwave-assisted synthesis of various ZnO hierarchical nanostructures: effects of heating parameters of microwave oven, Crystal Growth Des. 8 (2008) 3148–3153. [33] C. Feldmann, H.O. Jungk, Polyol-mediated preparation of nanoscale oxide particles, Angew. Chem., Int. Ed. 40 (2001) 359–362. [34] Y.W. Zhao, Y. Zhang, H. Zhu, G.C. Hadjipanayis, J.Q. Xiao, Low-temperature synthesis of hexagonal (wurtzite) ZnS nanocrystals, J. Am. Chem. Soc. 124 (2004) 6874–6875. [35] Y.G. Sun, Y.N. Xia, Large-scale synthesis of uniform silver nanowires through a soft, self-seeding, polyol process, Adv. Mater. 14 (2002) 833–837.