- Email: [email protected]
Solvent-induced shape evolution of PVP protected spherical silver nanoparticles into triangular nanoplates and nanorods
Journal of Colloid and Interface Science 289 (2005) 402–409 www.elsevier.com/locate/jcis
Solvent-induced shape evolution of PVP protected spherical silver nanoparticles into triangular nanoplates and nanorods T.C. Deivaraj a , Neeta L. Lala b , Jim Yang Lee a,b,∗ a Singapore–MIT Alliance, National University of Singapore, Singapore 117576 b Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260
Received 29 November 2004; accepted 29 March 2005 Available online 17 May 2005
Abstract The reaction between silver nitrate and poly(N -vinyl-2-pyrrolidone) (PVP) in pyridine at ambient conditions could lead to the formation of spherical nanoparticles or quadrilateral and triangular silver nanoplates, depending on the silver-to-PVP ratio used. It is proposed that the spherical Ag nanoparticles, which were formed early in the reaction, were transformed into nanoplates through an Ostwald ripening process driven by the bridging flocculation of small spherical Ag nanoparticles. This unique and hitherto unreported shape evolution process was carefully followed by a combination of techniques, viz., UV–visible spectroscopy, TEM, and powder X-ray diffraction. 2005 Elsevier Inc. All rights reserved. Keywords: Ag nanoparticles; Poly(N -vinyl-2-pyrrolidone) (PVP); Nanoplates; Nanoprisms; Ostwald ripening
1. Introduction Lights of appropriate wavelengths can cause the collective oscillation of conduction electrons in crystalline nanoparticles of silver and gold, a phenomenon known as surface plasmon resonance (SPR). The wavelength of the SPR band generally red-shifts with increasing dielectric constant of the environment, increasing particle size, or increasing extent of particle aggregation [1]. Thus, the color of the colloidal metal varies depending on the method of preparation, which affects the size [2], shape [3,4], state of aggregation [5–7], and degree of hollowness in the particles [8], as well as the dielectric properties of the surrounding medium [9,10]. The upsurge in research activities on the synthesis of nanogold and nanosilver with controlled geometries [5,6,11] is motivated by the size and shape tunable optical properties of these metallic nanoparticles, which have many projected applications in nanoelectronics, optical filters, photon energy * Corresponding author. Fax: +65 6779 1936.
E-mail address: [email protected] (J.Y. Lee). 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.03.076
transport, and surface-enhanced Raman or fluorescence scattering, just to name a few [12–21]. Several preparatory routes to silver nanoprisms and plates have recently been reported [22–30]. The room temperature transformation of spherical particles into nanoprisms and triangular nanoplates can be carried out using UV light [22] or by a thermal process under the ordinary lighting of a chemical laboratory [23,29,30]. Of late, Callegari et al. described a photochemical conversion process in which spherical Ag nanoparticles were converted into triangular plates of various sizes after irradiation with lights of appropriate wavelength(s) [30]. Other methods of preparation of Ag nanoplates include seed-mediated growth in the presence of cetyltrimethylammonium bromide (CTAB) soft templates [26,27], and by boiling a dimethyl formamide (DMF) solution of AgNO3 in the presence of poly(N -vinyl2-pyrrolidone) (PVP) [28]. This paper presents an even simpler alternative in which triangular nanoplates and nanorods of silver are formed evolutionarily by an Ostwald ripening process starting from spherical Ag nanoparticle precursors, without any field (UV or visible light irradiation)
T.C. Deivaraj et al. / Journal of Colloid and Interface Science 289 (2005) 402–409
403
Table 1 Composition of reactants used Sample
Conc. of AgNO3 (mM)
Conc. of PVP (mM)
Ratio of Ag+ to PVP monomer units
Morphology of Ag nanoparticles (after centrifuging)
Ag-1 Ag-2 Ag-3 Ag-4 Ag-5 Ag-6
7.8 7.8 15.5 31.1 77.7 155.4
20.0 40.0 40.0 40.0 40.0 40.0
232 464 232 116 46 23
Spherical (∼90%), polyhedral Spherical (∼60%), polyhedral Quadrilateral (∼30%) and triangular plates (∼70%) Truncated and nontruncated triangular plates (∼90%) Nanorods (∼30%) Circular disks and spherical
or thermally induced activation. The observation appears to be unprecedented and is deserving of further investigation as a method of preparation of anisotropic nanoparticles.
2. Experimental 2.1. Materials and characterization techniques AgNO3 , poly(N -vinyl-2-pyrrolidone) (PVP) (MW = 10,000), and pyridine were obtained from Aldrich and used without further purification. Deionized water was distilled by a Milli-Q water purification system. All glassware were treated with aqua regia, followed by copious washing with distilled water before drying in an oven. UV–visible spectra of the samples were recorded by a Shimadzu 2450 spectrophotometer. A JEOL JEM2010 microscope was used to obtain all TEM images of the nanoparticles. For TEM measurements a drop of the metal nanoparticle solution was dispensed onto a 3-mm copper grid coated with a continuous carbon film. Excess solution was removed by an absorbent paper. X-ray diffraction (XRD) patterns of the nanoparticles were recorded by a Rigaku D/Max-3B diffractometer using CuKα radiation. All samples were vacuum-dried before the measurements. 2.2. Preparation of Ag nanoparticles In a typical experiment, measured amounts of PVP and AgNO3 were separately dissolved in 5 ml each of pyridine and then mixed well to form a homogeneous mixture, which was left unstirred in the dark for several days. Details on the quantities of the reactants used are given in Table 1. The solution turned yellow when the PVP and AgNO3 solutions were mixed, indicating the formation of Ag nanoparticles. The reaction mixture was monitored by UV–visible spectroscopy to follow the nanoparticle growth process. Nanotriangles and nanorods could be separated from the spherical nanoparticles by centrifuging the mixed particle system at 6000 rpm for 10 min.
Fig. 1. X-Ray diffraction of Ag nanoparticles obtained from Ag-1.
3. Results and discussion 3.1. Formation of Ag nanoparticles Pastoriza-Santoz and Liz-Marzán have recently reported the formation of nanoprisms of silver using DMF as the solvent. It appears that both the nature of the organic solvent (polarity and donor characteristics) and the PVP-toAgNO3 ratio are important to the formation of silver nanoprisms [28]. Pyridine is used in lieu of DMF as the solvent for this study for the following reasons: (a) pyridine is a polar and strongly coordinating solvent, and (b) pyridine can dissolve PVP and AgNO3 well at room temperature. Acetonitrile could be another solvent to use, if it were not for its limited solubility for PVP, which constrains the range of concentrations that can be used in the experiments. Ag nanoparticles were spontaneously formed when the pyridine solutions of AgNO3 and PVP were mixed, demonstrating the function of PVP as a reducing agent cum particle stabilizing agent. The inference is congruent with current findings in the literature [31–33]. The reduction of AgNO3 by PVP could also take place in aqueous solutions, although the reaction was not immediately apparent at low PVP concentrations (<0.5 wt%), and changes in the UV–vis spectra could only be detected after several days. A typical XRD spectrum of the silver nanoparticles formed is shown in Fig. 1. It shows peaks at 37.8◦ , 43.8◦ , 64.2◦ , and 77.2◦ , which agree well with the (111), (200),
404
T.C. Deivaraj et al. / Journal of Colloid and Interface Science 289 (2005) 402–409
Fig. 2. UV–visible spectra of Ag-1–Ag-6 after centrifuging (6000 rpm, 10 min).
(220), and (311) diffractions of face centered cubic (fcc) silver (JCPDS File 04-0783). 3.2. UV–visible spectroscopic studies and TEM examinations A pale yellow color appeared immediately when a pyridine solution of AgNO3 was mixed with a pyridine solution of PVP. The UV–visible spectra of reaction mixtures with different AgNO3 -to-PVP ratios stored up to a day showed a peak only at ∼420 nm, indicating the lone presence of spherical Ag nanoparticles and TEM imaging confirmed the same. However, the color of the reaction mixture would deepen after a few days, from the initial pale yellow to brown, or to dark green or purple, depending on the (increasing) silver content in the mixture. Mie’s theory predicts only a single surface plasmon resonance (SPR) band in the absorption spectra of spherical nanoparticles. Anisotropic particles, on the other hand, could give rise to two or more SPR bands depending on the shape of the particles [34]. It has been observed that the number of SPR bands increases with decreasing symmetry of the nanoparticles [6]. Spherical nanoparticles, circular disks, and triangular nanoplates of silver show one, two, and three peaks, respectively. After several days, Ag-1 and Ag-2 still displayed UV– visible spectral characteristics of spherical Ag nanoparticles. The spectra of Ag-3 to Ag-6, on the other hand, had an additional absorption peak at long wavelength, more commonly found in anisotropic particles. However, the intensity of the long-wavelength absorption was quite weak, suggesting that the anisotropic particles were present among a majority of spherical particles. Centrifugation at 6000 rpm for 10 min was adequate to separate most of the anisotropic particles in the (Ag-3–Ag-6) samples from the reaction mixture. The UV–visible spectra of all centrifuged samples are shown in Fig. 2. After centrifugation, samples Ag-1 and Ag-2 showed absorption maxima at 413 and 419 nm, respectively. The absorption peaks are not symmetrical and a weak broad hump could be identified at longer wavelengths, suggesting
Fig. 3. TEM image of Ag-1.
Fig. 4. TEM images of Ag-2.
the possible presence of a small population of anisotropic Ag particles. TEM images of Ag-1 and Ag-2 in Figs. 3 and 4, respectively, lend support to this inference. While Ag-1 consisted mostly of spherical nanoparticles of different sizes (mean diameter of 73.8 nm and standard deviation of 20.1 nm), anisotropic Ag particles (rods, hexagonal plates and prisms) were more easily identified in the Ag-2 sample. Nevertheless the spherical particles in Ag-2 have statistics comparable to those of Ag-1 (mean diameter = 72.8 nm, standard deviation = 16.7 nm).
T.C. Deivaraj et al. / Journal of Colloid and Interface Science 289 (2005) 402–409
Fig. 5. TEM images of nanoplates of silver obtained from Ag-3.
Fig. 6. TEM images of triangular nanoplates of silver obtained from Ag-4.
The UV–visible spectra of Ag-3 and Ag-4 showed two absorption peaks in the low wavelength region, at 341 and 416 nm and at 331 and 410 nm, respectively. The absorption in the near infrared (NIR) region was rather strong, obscuring the detection of discrete absorption peaks in the visible region of the UV–vis spectrum. Two possible reasons for the strong absorption in the NIR region may be proposed: (a) absorption due to the in-plane dipole resonance mode associated with nanoplates of edge length greater than 100 nm [29], and (b) absorption due to the strong coupling between the nanoparticles arising from aggregation of silver nanoparticles [35–37]. In addition, it has also been suggested that for plates of a given thickness, absorption peaks at longer wavelengths are associated with larger lateral size of the
405
Fig. 7. TEM images of nanorods of silver obtained from Ag-5.
plates [30]. The presence of nanoplates with an average edge length above 100 nm was confirmed by TEM (Figs. 5 and 6, showing mean edge lengths of 149.2 nm for Ag-3 and 155.0 nm for Ag-4). The TEM images also showed that the nanoplates in Ag-3 varied greatly in morphology (triangular plates with truncated and sharp edges, rods and quadrilateral plates). The nanoplates in Ag-4, in contrast, were relatively more uniform and were predominantly triangular in shape. Nevertheless both triangular and truncated triangular plates could be found. A few of the triangular plates were oriented perpendicularly on the TEM grids, allowing their thickness to be measured. The measurements showed an average plate thickness of ∼16 nm for these nanotriangles. The UV–visible spectrum of Ag-5 showed absorption maxima at 328 and 411 nm and a very broad peak at 670 nm (FWHM = 225 nm). Similarly, the spectrum of Ag-6 showed absorption at 337, 409, and 666 nm (FWHM = 147 nm), except that it was no longer dominated by the last peak. TEM images revealed more significant differences between these samples in terms of particle morphology. TEM examination of Ag-5 showed the existence of nanorods about 100 nm in length (Fig. 7). On the other hand, Ag-6 showed a near-linear arrangement of aggregated spherical silver nanoparticles, nanodisks, and nanorods (Fig. 8). It has been reported that silver nanoparticles that assemble themselves in a largely linear fashion have spectral properties similar to those of 1D structures such as nanorods and nanowires [38]. Sample Ag-6 appears to be an example of this. 3.3. Evolution of shapes Shape evolution was detected in Ag-3, Ag-4, and Ag-5. The data from Ag-5 are used here as an example to illustrate the different stages in the evolution process. For
406
T.C. Deivaraj et al. / Journal of Colloid and Interface Science 289 (2005) 402–409
Fig. 8. TEM images of nanodisks and spherical nanoparticles for silver (Ag-6).
Ag-5, mixing the reactant solutions (AgNO3 and PVP) in pyridine produced a yellow solution initially, which slowly darkened to a brown solution. At this stage the TEM image showed only spherical Ag nanoparticles. Two weeks later the color of the reaction mixture changed to green and TEM examination showed spherical particles coexisting with some triangular plates. The reaction mixture was deep purple in about a month and the TEM image showed predominantly silver nanorods. There were no further changes in the solu-
(a)
tion color and particle morphology after this stage. Fig. 9 shows the UV–vis and TEM snapshots taken during the various stages of the morphological evolution. Several reports have attributed the transformation of small nanoparticles into other particle geometries to the Ostwald ripening process [39]. Control experiments in which poly(ethylene oxide) (PEO) was used instead of PVP did not result in the formation of anisotropic silver particles. This indicates that the PVP presence is essential to the shape evolution process, which also corroborates the observations of others [40]. However, the exact role of PVP remains a puzzle. It has been speculated that the kinetics of adsorption and desorption of PVP on different crystallographic planes of the nanoparticles are different, thus leading to the eventual development of a nonspherical geometry [41]. Ostwald ripening is most likely the driving force for the shape evolution. This hypothesis is substantiated by the following two observations: (a) A progressive change occurs in the UV–visible spectra of all samples from the initially single SPR absorption at ∼410 nm into more complex spectral features. Red-shifting of the ∼410 nm (indicating particle agglomeration) occurred invariably before the development of anisotropy (e.g., see Fig. 9 for the changes in the UV–visible spectrum of Ag-5 with time). (b) TEM shows a progressive growth in particle size concomitantly with the shape evolution (Fig. 10), which agrees well with the known characteristics of Ostwald ripening of forming large particles by “dissolving” smaller particles. In the same figure a triangular and a quadrilateral particle showing an unusual corrugated topography could also be identified, which are probably snapshots of an intermediate stage in the Ostwald ripening growth process. Ostwald ripening was also cited by
(b)
Fig. 9. (a–c) TEM images showing the morphological evolution as seen in Ag-5; (d) UV–vis spectra of the reaction mixture.
T.C. Deivaraj et al. / Journal of Colloid and Interface Science 289 (2005) 402–409
(c)
407
(d) Fig. 9. Continued.
(a)
(b)
Fig. 10. (a, b) TEM snapshots showing particles with corrugated morphology.
Shankar et al. as the mechanism of growth for the triangular nanoplates of gold formed in the reduction of chloroauric acid by lemon grass extract [42]. 3.4. Effects of reactant composition, solvent, and other synthesis details Current literature suggests that the morphology of the silver nanoparticles formed is strongly dependent on the mole
ratio of PVP (in monomer units) to AgNO3 [43]. Table 1 shows the ratio as the number of monomer units of PVP per Ag+ ion as well as the number of polymer units per Ag+ ion. It can be seen that Ag-1 and Ag-3 had the same PVPto-Ag ratio, and yet the silver nanoparticles formed from these two reaction mixtures had remarkably different particle morphologies. These experimental results indicate that the PVP-to-AgNO3 ratio is a necessary but not sufficient
408
T.C. Deivaraj et al. / Journal of Colloid and Interface Science 289 (2005) 402–409
condition to determine the development of particular particle morphology in a given system. Control experiments were carried out using the same amounts of AgNO3 and PVP but with pyridine replaced by ethanol. Only 7 nm spherical Ag particles were formed and shape evolution was not detected within the same time span (TEM image in Supplementary Material). This observation, together with Pastoriza-Santoz and Liz-Marzán’s synthesis [28] of silver nanoprisms only in DMF underlines the importance of solvent in directing the formation of anisotropic silver nanoparticles. The concentrations of silver nitrate used in this work that resulted in anisotropic Ag nanoparticle formation were comparable to those used by Pastoriza-Santoz and Liz-Marzán [28]. However, Pastoriza-Santoz and LizMarzán believed that higher concentrations of PVP would lead to a more isotropic growth of the nanoparticles, and used only low PVP concentrations in their synthesis. The PVP concentration was relatively high in this study (0.02 M), but it had no apparent adverse effect on the formation of anisotropic Ag nanoparticles. In yet another study, He and co-workers obtained Ag nanoparticles by microwave irradiation of pyridine solutions of silver salts using PVP as the stabilizing agent [44]. The PVP concentrations adopted in that study were lower than the levels used here and yet the particles obtained were almost exclusively spherical in shape. As it has been reported earlier that light may play a role in the shape evolution process involving Ag [22,23,29,30], the experiments were performed in light as well as in the dark; however, the outcome did not vary greatly, suggesting that lighting was not a determining factor in the formation of nanoplates and nanorods. All these observations indicate that the actual concentration of silver ions and PVP, and the choice of solvent are important factors controlling the formation of anisotropic Ag nanoparticles.
4. Conclusions In summary, we report here an easy method for preparing spherical, triangular, and quadrilateral silver nanoparticles by judiciously varying the reactant ratios. Under the right conditions, it was even possible for the nanoplates to evolve into nanorods. Interestingly, no field effect (thermal, microwave, or light radiation) was needed to fuel the shape evolution process. Contrary to current belief, anisotropic Ag nanoparticles could still be formed despite a high PVP monomer: Ag+ ion ratio provided that the solvent and the silver ion concentrations were appropriately chosen. It is the collective effect of the concentrations of Ag+ and PVP and the choice of solvent that controls the destiny of the shape evolution process. This study therefore underlines the complexity of the anisotropic growth of Ag nanoparticles. A satisfactorily rationalization of all aspects of the experimental observations would require further and more detailed investigations.
Acknowledgments This research was funded by the Molecular Engineering of Chemical and Biological Systems (MEBCS) program of the Singapore MIT Alliance, National University of Singapore. The authors would like to thank the Institute of Materials Research and Engineering for access to their TEM facility.
Supplementary material The online version of this article contains additional supplementary material. Please visit DOI: 10.1016/j.jcis.2005.03.076.
References [1] M. Zangeneh, R. Terrill, in: J.A. Schwarz, C.I. Contescu, K. Putyera (Eds.), Dekker Encyclopedia of Nanoscience and Nanotechnology, vol. 5, Dekker, New York, 2000, p. 3819. [2] U. Kreibig, L. Genzel, Surf. Sci. 156 (1985) 678. [3] D. Sarkar, N.J. Halas, Phys. Rev. E 56 (1997) 1102. [4] Y.Y. Yu, S.S. Chang, C.L. Lee, C.R.C. Wang, J. Phys. Chem. B 101 (1997) 6661. [5] S. De, A. Pal, N.R. Jana, T. Pal, J. Photochem. Photobiol. A 131 (2000) 11. [6] Y. Sun, Y. Xia, Analyst 128 (2003) 686. [7] J.P. Novak, D.L. Feldheim, J. Am. Chem. Soc. 122 (2000) 3979. [8] Y. Sun, Y. Xia, Anal. Chem. 74 (2002) 5297. [9] C.L. Haynes, R.P. Van Duyne, J. Phys. Chem. B 105 (2001) 5599. [10] S. Underwood, P. Mulvaney, Langmuir 10 (1994) 3427. [11] Y. Wu, P. Yang, J. Am. Chem. Soc. 123 (2001) 3165. [12] A.C. Templeton, W.P. Wuelfing, R.W. Murray, Acc. Chem. Res. 33 (2000) 27. [13] L.R. Garrell, Anal. Chem. A 61 (1989) 401. [14] T.W. Ebbesen, H.J. Lezec, H.F. Ghaemi, T. Thio, P.A. Wolff, Nature 391 (1998) 667. [15] J. Pendry, Science 285 (1999) 1687. [16] K.D. Hermanson, S.O. Lumsdon, J.P. Williams, E.W. Kaler, O.D. Velev, Science 294 (2001) 1082. [17] B. Knoll, F. Keilmann, Nature 399 (1999) 134. [18] P.M. Tessier, O.D. Velev, A.T. Kalambur, J.F. Rabolt, A.M. Lenhoff, E.W. Kaler, J. Am. Chem. Soc. 122 (2000) 9554. [19] P. Das, H. Metiu, J. Phys. Chem. 89 (1985) 4680. [20] J.J. Storhoff, R. Elghanian, R.C. Mucic, C.A. Mirkin, R.L. Letsinger, J. Am. Chem. Soc. 120 (1998) 1959. [21] L.A. Dick, A.D. McFarland, C.L. Haynes, R.P. Van Duyne, J. Phys. Chem. B 106 (2002) 853. [22] R. Jin, Y.C. Cao, C.A. Mirkin, K.L. Kelly, G.C. Schatz, J.G. Zheng, Science 294 (2001) 1901. [23] R. Jin, Y.C. Cao, E. Hao, G.S. Metraux, G.C. Schatz, C.A. Mirkin, Nature 425 (2003) 487. [24] D.O. Yener, J. Sindel, C.A. Randall, J.H. Adair, Langmuir 18 (2002) 8692. [25] S. Chen, D.L. Carroll, Nano Lett. 2 (2002) 1003. [26] S. Chen, Z. Fan, D.L. Carroll, J. Phys. Chem. B 106 (2002) 10777. [27] M. Maillard, S. Giorgio, M.P. Pileni, Adv. Mater. 14 (2002) 1084. [28] I. Pastoriza-Santos, L.M. Liz-Marzán, Nano Lett. 2 (2002) 903. [29] Y. Sun, B. Mayers, Y. Xia, Nano Lett. 3 (2003) 675. [30] A. Callegari, D. Tonti, M. Chergui, Nano Lett. 3 (2003) 1565. [31] B. Jose, J.H. Ryu, Y.J. Kim, H. Kim, Y.S. Kang, S.D. Lee, H.S. Kim, Chem. Mater. 14 (2002) 2134.
T.C. Deivaraj et al. / Journal of Colloid and Interface Science 289 (2005) 402–409
[32] H.H. Huang, X.P. Ni, G.L. Loy, C.H. Chew, K.L. Tan, F.C. Loh, J.F. Deng, G.Q. Xu, Langmuir 12 (1996) 909. [33] P. Jiang, S.-Y. Li, S.-S. Xie, Y. Gao, L. Song, Chem. Eur. J. 10 (2004) 4817. [34] G. Mie, Ann. Phys. 25 (1908) 377. [35] P.V. Kamat, M. Flumiani, G.V. Hartland, J. Phys. Chem. B 102 (1998) 3123. [36] J.P. Novak, D.L. Feldheim, J. Am. Chem. Soc. 122 (2000) 3979. [37] J. Hranisavljevic, N.M. Dimitrijevic, G.A. Wurtz, G.P. Wiederrecht, J. Am. Chem. Soc. 124 (2002) 4536.
409
[38] H. Ma, Y. Jiao, B. Yin, S. Wang, S. Zhao, S. Huang, W. Pan, S. Chen, F. Meng, Chem. Phys. Chem. 5 (2004) 713. [39] A.R. Roosen, W.C. Carter, Physica A 261 (1998) 232. [40] Y. Sun, B. Gates, B. Mayers, Y. Xia, Nano Lett. 2 (2002) 165. [41] F. Bonet, K. Tekaia-Elhsissen, K.V. Sarathy, Bull. Mater. Sci. 23 (2000) 165. [42] S.S. Shankar, A. Rai, B. Ankamwar, A. Singh, A. Ahmad, M. Sastry, Nat. Mater. 3 (2004) 482. [43] Y. Sun, Y. Yin, B.T. Mayers, T. Herricks, Y. Xia, Chem. Mater. 14 (2002) 4736. [44] R. He, X. Qian, J. Yin, Z. Zhu, J. Mater. Chem. 12 (2002) 3783.
Solvent-induced shape evolution of PVP protected spherical silver nanoparticles into triangular nanoplates and nanorods T.C. Deivaraj a , Neeta L. Lala b , Jim Yang Lee a,b,∗ a Singapore–MIT Alliance, National University of Singapore, Singapore 117576 b Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260
Received 29 November 2004; accepted 29 March 2005 Available online 17 May 2005
Abstract The reaction between silver nitrate and poly(N -vinyl-2-pyrrolidone) (PVP) in pyridine at ambient conditions could lead to the formation of spherical nanoparticles or quadrilateral and triangular silver nanoplates, depending on the silver-to-PVP ratio used. It is proposed that the spherical Ag nanoparticles, which were formed early in the reaction, were transformed into nanoplates through an Ostwald ripening process driven by the bridging flocculation of small spherical Ag nanoparticles. This unique and hitherto unreported shape evolution process was carefully followed by a combination of techniques, viz., UV–visible spectroscopy, TEM, and powder X-ray diffraction. 2005 Elsevier Inc. All rights reserved. Keywords: Ag nanoparticles; Poly(N -vinyl-2-pyrrolidone) (PVP); Nanoplates; Nanoprisms; Ostwald ripening
1. Introduction Lights of appropriate wavelengths can cause the collective oscillation of conduction electrons in crystalline nanoparticles of silver and gold, a phenomenon known as surface plasmon resonance (SPR). The wavelength of the SPR band generally red-shifts with increasing dielectric constant of the environment, increasing particle size, or increasing extent of particle aggregation [1]. Thus, the color of the colloidal metal varies depending on the method of preparation, which affects the size [2], shape [3,4], state of aggregation [5–7], and degree of hollowness in the particles [8], as well as the dielectric properties of the surrounding medium [9,10]. The upsurge in research activities on the synthesis of nanogold and nanosilver with controlled geometries [5,6,11] is motivated by the size and shape tunable optical properties of these metallic nanoparticles, which have many projected applications in nanoelectronics, optical filters, photon energy * Corresponding author. Fax: +65 6779 1936.
E-mail address: [email protected] (J.Y. Lee). 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.03.076
transport, and surface-enhanced Raman or fluorescence scattering, just to name a few [12–21]. Several preparatory routes to silver nanoprisms and plates have recently been reported [22–30]. The room temperature transformation of spherical particles into nanoprisms and triangular nanoplates can be carried out using UV light [22] or by a thermal process under the ordinary lighting of a chemical laboratory [23,29,30]. Of late, Callegari et al. described a photochemical conversion process in which spherical Ag nanoparticles were converted into triangular plates of various sizes after irradiation with lights of appropriate wavelength(s) [30]. Other methods of preparation of Ag nanoplates include seed-mediated growth in the presence of cetyltrimethylammonium bromide (CTAB) soft templates [26,27], and by boiling a dimethyl formamide (DMF) solution of AgNO3 in the presence of poly(N -vinyl2-pyrrolidone) (PVP) [28]. This paper presents an even simpler alternative in which triangular nanoplates and nanorods of silver are formed evolutionarily by an Ostwald ripening process starting from spherical Ag nanoparticle precursors, without any field (UV or visible light irradiation)
T.C. Deivaraj et al. / Journal of Colloid and Interface Science 289 (2005) 402–409
403
Table 1 Composition of reactants used Sample
Conc. of AgNO3 (mM)
Conc. of PVP (mM)
Ratio of Ag+ to PVP monomer units
Morphology of Ag nanoparticles (after centrifuging)
Ag-1 Ag-2 Ag-3 Ag-4 Ag-5 Ag-6
7.8 7.8 15.5 31.1 77.7 155.4
20.0 40.0 40.0 40.0 40.0 40.0
232 464 232 116 46 23
Spherical (∼90%), polyhedral Spherical (∼60%), polyhedral Quadrilateral (∼30%) and triangular plates (∼70%) Truncated and nontruncated triangular plates (∼90%) Nanorods (∼30%) Circular disks and spherical
or thermally induced activation. The observation appears to be unprecedented and is deserving of further investigation as a method of preparation of anisotropic nanoparticles.
2. Experimental 2.1. Materials and characterization techniques AgNO3 , poly(N -vinyl-2-pyrrolidone) (PVP) (MW = 10,000), and pyridine were obtained from Aldrich and used without further purification. Deionized water was distilled by a Milli-Q water purification system. All glassware were treated with aqua regia, followed by copious washing with distilled water before drying in an oven. UV–visible spectra of the samples were recorded by a Shimadzu 2450 spectrophotometer. A JEOL JEM2010 microscope was used to obtain all TEM images of the nanoparticles. For TEM measurements a drop of the metal nanoparticle solution was dispensed onto a 3-mm copper grid coated with a continuous carbon film. Excess solution was removed by an absorbent paper. X-ray diffraction (XRD) patterns of the nanoparticles were recorded by a Rigaku D/Max-3B diffractometer using CuKα radiation. All samples were vacuum-dried before the measurements. 2.2. Preparation of Ag nanoparticles In a typical experiment, measured amounts of PVP and AgNO3 were separately dissolved in 5 ml each of pyridine and then mixed well to form a homogeneous mixture, which was left unstirred in the dark for several days. Details on the quantities of the reactants used are given in Table 1. The solution turned yellow when the PVP and AgNO3 solutions were mixed, indicating the formation of Ag nanoparticles. The reaction mixture was monitored by UV–visible spectroscopy to follow the nanoparticle growth process. Nanotriangles and nanorods could be separated from the spherical nanoparticles by centrifuging the mixed particle system at 6000 rpm for 10 min.
Fig. 1. X-Ray diffraction of Ag nanoparticles obtained from Ag-1.
3. Results and discussion 3.1. Formation of Ag nanoparticles Pastoriza-Santoz and Liz-Marzán have recently reported the formation of nanoprisms of silver using DMF as the solvent. It appears that both the nature of the organic solvent (polarity and donor characteristics) and the PVP-toAgNO3 ratio are important to the formation of silver nanoprisms [28]. Pyridine is used in lieu of DMF as the solvent for this study for the following reasons: (a) pyridine is a polar and strongly coordinating solvent, and (b) pyridine can dissolve PVP and AgNO3 well at room temperature. Acetonitrile could be another solvent to use, if it were not for its limited solubility for PVP, which constrains the range of concentrations that can be used in the experiments. Ag nanoparticles were spontaneously formed when the pyridine solutions of AgNO3 and PVP were mixed, demonstrating the function of PVP as a reducing agent cum particle stabilizing agent. The inference is congruent with current findings in the literature [31–33]. The reduction of AgNO3 by PVP could also take place in aqueous solutions, although the reaction was not immediately apparent at low PVP concentrations (<0.5 wt%), and changes in the UV–vis spectra could only be detected after several days. A typical XRD spectrum of the silver nanoparticles formed is shown in Fig. 1. It shows peaks at 37.8◦ , 43.8◦ , 64.2◦ , and 77.2◦ , which agree well with the (111), (200),
404
T.C. Deivaraj et al. / Journal of Colloid and Interface Science 289 (2005) 402–409
Fig. 2. UV–visible spectra of Ag-1–Ag-6 after centrifuging (6000 rpm, 10 min).
(220), and (311) diffractions of face centered cubic (fcc) silver (JCPDS File 04-0783). 3.2. UV–visible spectroscopic studies and TEM examinations A pale yellow color appeared immediately when a pyridine solution of AgNO3 was mixed with a pyridine solution of PVP. The UV–visible spectra of reaction mixtures with different AgNO3 -to-PVP ratios stored up to a day showed a peak only at ∼420 nm, indicating the lone presence of spherical Ag nanoparticles and TEM imaging confirmed the same. However, the color of the reaction mixture would deepen after a few days, from the initial pale yellow to brown, or to dark green or purple, depending on the (increasing) silver content in the mixture. Mie’s theory predicts only a single surface plasmon resonance (SPR) band in the absorption spectra of spherical nanoparticles. Anisotropic particles, on the other hand, could give rise to two or more SPR bands depending on the shape of the particles [34]. It has been observed that the number of SPR bands increases with decreasing symmetry of the nanoparticles [6]. Spherical nanoparticles, circular disks, and triangular nanoplates of silver show one, two, and three peaks, respectively. After several days, Ag-1 and Ag-2 still displayed UV– visible spectral characteristics of spherical Ag nanoparticles. The spectra of Ag-3 to Ag-6, on the other hand, had an additional absorption peak at long wavelength, more commonly found in anisotropic particles. However, the intensity of the long-wavelength absorption was quite weak, suggesting that the anisotropic particles were present among a majority of spherical particles. Centrifugation at 6000 rpm for 10 min was adequate to separate most of the anisotropic particles in the (Ag-3–Ag-6) samples from the reaction mixture. The UV–visible spectra of all centrifuged samples are shown in Fig. 2. After centrifugation, samples Ag-1 and Ag-2 showed absorption maxima at 413 and 419 nm, respectively. The absorption peaks are not symmetrical and a weak broad hump could be identified at longer wavelengths, suggesting
Fig. 3. TEM image of Ag-1.
Fig. 4. TEM images of Ag-2.
the possible presence of a small population of anisotropic Ag particles. TEM images of Ag-1 and Ag-2 in Figs. 3 and 4, respectively, lend support to this inference. While Ag-1 consisted mostly of spherical nanoparticles of different sizes (mean diameter of 73.8 nm and standard deviation of 20.1 nm), anisotropic Ag particles (rods, hexagonal plates and prisms) were more easily identified in the Ag-2 sample. Nevertheless the spherical particles in Ag-2 have statistics comparable to those of Ag-1 (mean diameter = 72.8 nm, standard deviation = 16.7 nm).
T.C. Deivaraj et al. / Journal of Colloid and Interface Science 289 (2005) 402–409
Fig. 5. TEM images of nanoplates of silver obtained from Ag-3.
Fig. 6. TEM images of triangular nanoplates of silver obtained from Ag-4.
The UV–visible spectra of Ag-3 and Ag-4 showed two absorption peaks in the low wavelength region, at 341 and 416 nm and at 331 and 410 nm, respectively. The absorption in the near infrared (NIR) region was rather strong, obscuring the detection of discrete absorption peaks in the visible region of the UV–vis spectrum. Two possible reasons for the strong absorption in the NIR region may be proposed: (a) absorption due to the in-plane dipole resonance mode associated with nanoplates of edge length greater than 100 nm [29], and (b) absorption due to the strong coupling between the nanoparticles arising from aggregation of silver nanoparticles [35–37]. In addition, it has also been suggested that for plates of a given thickness, absorption peaks at longer wavelengths are associated with larger lateral size of the
405
Fig. 7. TEM images of nanorods of silver obtained from Ag-5.
plates [30]. The presence of nanoplates with an average edge length above 100 nm was confirmed by TEM (Figs. 5 and 6, showing mean edge lengths of 149.2 nm for Ag-3 and 155.0 nm for Ag-4). The TEM images also showed that the nanoplates in Ag-3 varied greatly in morphology (triangular plates with truncated and sharp edges, rods and quadrilateral plates). The nanoplates in Ag-4, in contrast, were relatively more uniform and were predominantly triangular in shape. Nevertheless both triangular and truncated triangular plates could be found. A few of the triangular plates were oriented perpendicularly on the TEM grids, allowing their thickness to be measured. The measurements showed an average plate thickness of ∼16 nm for these nanotriangles. The UV–visible spectrum of Ag-5 showed absorption maxima at 328 and 411 nm and a very broad peak at 670 nm (FWHM = 225 nm). Similarly, the spectrum of Ag-6 showed absorption at 337, 409, and 666 nm (FWHM = 147 nm), except that it was no longer dominated by the last peak. TEM images revealed more significant differences between these samples in terms of particle morphology. TEM examination of Ag-5 showed the existence of nanorods about 100 nm in length (Fig. 7). On the other hand, Ag-6 showed a near-linear arrangement of aggregated spherical silver nanoparticles, nanodisks, and nanorods (Fig. 8). It has been reported that silver nanoparticles that assemble themselves in a largely linear fashion have spectral properties similar to those of 1D structures such as nanorods and nanowires [38]. Sample Ag-6 appears to be an example of this. 3.3. Evolution of shapes Shape evolution was detected in Ag-3, Ag-4, and Ag-5. The data from Ag-5 are used here as an example to illustrate the different stages in the evolution process. For
406
T.C. Deivaraj et al. / Journal of Colloid and Interface Science 289 (2005) 402–409
Fig. 8. TEM images of nanodisks and spherical nanoparticles for silver (Ag-6).
Ag-5, mixing the reactant solutions (AgNO3 and PVP) in pyridine produced a yellow solution initially, which slowly darkened to a brown solution. At this stage the TEM image showed only spherical Ag nanoparticles. Two weeks later the color of the reaction mixture changed to green and TEM examination showed spherical particles coexisting with some triangular plates. The reaction mixture was deep purple in about a month and the TEM image showed predominantly silver nanorods. There were no further changes in the solu-
(a)
tion color and particle morphology after this stage. Fig. 9 shows the UV–vis and TEM snapshots taken during the various stages of the morphological evolution. Several reports have attributed the transformation of small nanoparticles into other particle geometries to the Ostwald ripening process [39]. Control experiments in which poly(ethylene oxide) (PEO) was used instead of PVP did not result in the formation of anisotropic silver particles. This indicates that the PVP presence is essential to the shape evolution process, which also corroborates the observations of others [40]. However, the exact role of PVP remains a puzzle. It has been speculated that the kinetics of adsorption and desorption of PVP on different crystallographic planes of the nanoparticles are different, thus leading to the eventual development of a nonspherical geometry [41]. Ostwald ripening is most likely the driving force for the shape evolution. This hypothesis is substantiated by the following two observations: (a) A progressive change occurs in the UV–visible spectra of all samples from the initially single SPR absorption at ∼410 nm into more complex spectral features. Red-shifting of the ∼410 nm (indicating particle agglomeration) occurred invariably before the development of anisotropy (e.g., see Fig. 9 for the changes in the UV–visible spectrum of Ag-5 with time). (b) TEM shows a progressive growth in particle size concomitantly with the shape evolution (Fig. 10), which agrees well with the known characteristics of Ostwald ripening of forming large particles by “dissolving” smaller particles. In the same figure a triangular and a quadrilateral particle showing an unusual corrugated topography could also be identified, which are probably snapshots of an intermediate stage in the Ostwald ripening growth process. Ostwald ripening was also cited by
(b)
Fig. 9. (a–c) TEM images showing the morphological evolution as seen in Ag-5; (d) UV–vis spectra of the reaction mixture.
T.C. Deivaraj et al. / Journal of Colloid and Interface Science 289 (2005) 402–409
(c)
407
(d) Fig. 9. Continued.
(a)
(b)
Fig. 10. (a, b) TEM snapshots showing particles with corrugated morphology.
Shankar et al. as the mechanism of growth for the triangular nanoplates of gold formed in the reduction of chloroauric acid by lemon grass extract [42]. 3.4. Effects of reactant composition, solvent, and other synthesis details Current literature suggests that the morphology of the silver nanoparticles formed is strongly dependent on the mole
ratio of PVP (in monomer units) to AgNO3 [43]. Table 1 shows the ratio as the number of monomer units of PVP per Ag+ ion as well as the number of polymer units per Ag+ ion. It can be seen that Ag-1 and Ag-3 had the same PVPto-Ag ratio, and yet the silver nanoparticles formed from these two reaction mixtures had remarkably different particle morphologies. These experimental results indicate that the PVP-to-AgNO3 ratio is a necessary but not sufficient
408
T.C. Deivaraj et al. / Journal of Colloid and Interface Science 289 (2005) 402–409
condition to determine the development of particular particle morphology in a given system. Control experiments were carried out using the same amounts of AgNO3 and PVP but with pyridine replaced by ethanol. Only 7 nm spherical Ag particles were formed and shape evolution was not detected within the same time span (TEM image in Supplementary Material). This observation, together with Pastoriza-Santoz and Liz-Marzán’s synthesis [28] of silver nanoprisms only in DMF underlines the importance of solvent in directing the formation of anisotropic silver nanoparticles. The concentrations of silver nitrate used in this work that resulted in anisotropic Ag nanoparticle formation were comparable to those used by Pastoriza-Santoz and Liz-Marzán [28]. However, Pastoriza-Santoz and LizMarzán believed that higher concentrations of PVP would lead to a more isotropic growth of the nanoparticles, and used only low PVP concentrations in their synthesis. The PVP concentration was relatively high in this study (0.02 M), but it had no apparent adverse effect on the formation of anisotropic Ag nanoparticles. In yet another study, He and co-workers obtained Ag nanoparticles by microwave irradiation of pyridine solutions of silver salts using PVP as the stabilizing agent [44]. The PVP concentrations adopted in that study were lower than the levels used here and yet the particles obtained were almost exclusively spherical in shape. As it has been reported earlier that light may play a role in the shape evolution process involving Ag [22,23,29,30], the experiments were performed in light as well as in the dark; however, the outcome did not vary greatly, suggesting that lighting was not a determining factor in the formation of nanoplates and nanorods. All these observations indicate that the actual concentration of silver ions and PVP, and the choice of solvent are important factors controlling the formation of anisotropic Ag nanoparticles.
4. Conclusions In summary, we report here an easy method for preparing spherical, triangular, and quadrilateral silver nanoparticles by judiciously varying the reactant ratios. Under the right conditions, it was even possible for the nanoplates to evolve into nanorods. Interestingly, no field effect (thermal, microwave, or light radiation) was needed to fuel the shape evolution process. Contrary to current belief, anisotropic Ag nanoparticles could still be formed despite a high PVP monomer: Ag+ ion ratio provided that the solvent and the silver ion concentrations were appropriately chosen. It is the collective effect of the concentrations of Ag+ and PVP and the choice of solvent that controls the destiny of the shape evolution process. This study therefore underlines the complexity of the anisotropic growth of Ag nanoparticles. A satisfactorily rationalization of all aspects of the experimental observations would require further and more detailed investigations.
Acknowledgments This research was funded by the Molecular Engineering of Chemical and Biological Systems (MEBCS) program of the Singapore MIT Alliance, National University of Singapore. The authors would like to thank the Institute of Materials Research and Engineering for access to their TEM facility.
Supplementary material The online version of this article contains additional supplementary material. Please visit DOI: 10.1016/j.jcis.2005.03.076.
References [1] M. Zangeneh, R. Terrill, in: J.A. Schwarz, C.I. Contescu, K. Putyera (Eds.), Dekker Encyclopedia of Nanoscience and Nanotechnology, vol. 5, Dekker, New York, 2000, p. 3819. [2] U. Kreibig, L. Genzel, Surf. Sci. 156 (1985) 678. [3] D. Sarkar, N.J. Halas, Phys. Rev. E 56 (1997) 1102. [4] Y.Y. Yu, S.S. Chang, C.L. Lee, C.R.C. Wang, J. Phys. Chem. B 101 (1997) 6661. [5] S. De, A. Pal, N.R. Jana, T. Pal, J. Photochem. Photobiol. A 131 (2000) 11. [6] Y. Sun, Y. Xia, Analyst 128 (2003) 686. [7] J.P. Novak, D.L. Feldheim, J. Am. Chem. Soc. 122 (2000) 3979. [8] Y. Sun, Y. Xia, Anal. Chem. 74 (2002) 5297. [9] C.L. Haynes, R.P. Van Duyne, J. Phys. Chem. B 105 (2001) 5599. [10] S. Underwood, P. Mulvaney, Langmuir 10 (1994) 3427. [11] Y. Wu, P. Yang, J. Am. Chem. Soc. 123 (2001) 3165. [12] A.C. Templeton, W.P. Wuelfing, R.W. Murray, Acc. Chem. Res. 33 (2000) 27. [13] L.R. Garrell, Anal. Chem. A 61 (1989) 401. [14] T.W. Ebbesen, H.J. Lezec, H.F. Ghaemi, T. Thio, P.A. Wolff, Nature 391 (1998) 667. [15] J. Pendry, Science 285 (1999) 1687. [16] K.D. Hermanson, S.O. Lumsdon, J.P. Williams, E.W. Kaler, O.D. Velev, Science 294 (2001) 1082. [17] B. Knoll, F. Keilmann, Nature 399 (1999) 134. [18] P.M. Tessier, O.D. Velev, A.T. Kalambur, J.F. Rabolt, A.M. Lenhoff, E.W. Kaler, J. Am. Chem. Soc. 122 (2000) 9554. [19] P. Das, H. Metiu, J. Phys. Chem. 89 (1985) 4680. [20] J.J. Storhoff, R. Elghanian, R.C. Mucic, C.A. Mirkin, R.L. Letsinger, J. Am. Chem. Soc. 120 (1998) 1959. [21] L.A. Dick, A.D. McFarland, C.L. Haynes, R.P. Van Duyne, J. Phys. Chem. B 106 (2002) 853. [22] R. Jin, Y.C. Cao, C.A. Mirkin, K.L. Kelly, G.C. Schatz, J.G. Zheng, Science 294 (2001) 1901. [23] R. Jin, Y.C. Cao, E. Hao, G.S. Metraux, G.C. Schatz, C.A. Mirkin, Nature 425 (2003) 487. [24] D.O. Yener, J. Sindel, C.A. Randall, J.H. Adair, Langmuir 18 (2002) 8692. [25] S. Chen, D.L. Carroll, Nano Lett. 2 (2002) 1003. [26] S. Chen, Z. Fan, D.L. Carroll, J. Phys. Chem. B 106 (2002) 10777. [27] M. Maillard, S. Giorgio, M.P. Pileni, Adv. Mater. 14 (2002) 1084. [28] I. Pastoriza-Santos, L.M. Liz-Marzán, Nano Lett. 2 (2002) 903. [29] Y. Sun, B. Mayers, Y. Xia, Nano Lett. 3 (2003) 675. [30] A. Callegari, D. Tonti, M. Chergui, Nano Lett. 3 (2003) 1565. [31] B. Jose, J.H. Ryu, Y.J. Kim, H. Kim, Y.S. Kang, S.D. Lee, H.S. Kim, Chem. Mater. 14 (2002) 2134.
T.C. Deivaraj et al. / Journal of Colloid and Interface Science 289 (2005) 402–409
[32] H.H. Huang, X.P. Ni, G.L. Loy, C.H. Chew, K.L. Tan, F.C. Loh, J.F. Deng, G.Q. Xu, Langmuir 12 (1996) 909. [33] P. Jiang, S.-Y. Li, S.-S. Xie, Y. Gao, L. Song, Chem. Eur. J. 10 (2004) 4817. [34] G. Mie, Ann. Phys. 25 (1908) 377. [35] P.V. Kamat, M. Flumiani, G.V. Hartland, J. Phys. Chem. B 102 (1998) 3123. [36] J.P. Novak, D.L. Feldheim, J. Am. Chem. Soc. 122 (2000) 3979. [37] J. Hranisavljevic, N.M. Dimitrijevic, G.A. Wurtz, G.P. Wiederrecht, J. Am. Chem. Soc. 124 (2002) 4536.
409
[38] H. Ma, Y. Jiao, B. Yin, S. Wang, S. Zhao, S. Huang, W. Pan, S. Chen, F. Meng, Chem. Phys. Chem. 5 (2004) 713. [39] A.R. Roosen, W.C. Carter, Physica A 261 (1998) 232. [40] Y. Sun, B. Gates, B. Mayers, Y. Xia, Nano Lett. 2 (2002) 165. [41] F. Bonet, K. Tekaia-Elhsissen, K.V. Sarathy, Bull. Mater. Sci. 23 (2000) 165. [42] S.S. Shankar, A. Rai, B. Ankamwar, A. Singh, A. Ahmad, M. Sastry, Nat. Mater. 3 (2004) 482. [43] Y. Sun, Y. Yin, B.T. Mayers, T. Herricks, Y. Xia, Chem. Mater. 14 (2002) 4736. [44] R. He, X. Qian, J. Yin, Z. Zhu, J. Mater. Chem. 12 (2002) 3783.