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Morphology control of silver nano-crystals through a polyol synthesis
Solid State Sciences 13 (2011) 1719e1723
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Morphology control of silver nano-crystals through a polyol synthesis Xiaolin Luo a, *, Yashao Chen b, Desuo Yang a, Zongxiao Li a, Yinfeng Han a a b
Shaanxi Key Laboratory for Phytochemistry, Chemistry & Chemical Engineering Department, Baoji University of Art and Science, Baoji 721013, China Key Laboratory of Applied Surface and Colloid Chemistry, Shaanxi Normal University, Ministry of Education, School of Chemistry and Materials Science, Xi’an 710062, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 March 2011 Received in revised form 25 May 2011 Accepted 25 June 2011 Available online 18 July 2011
Silver nano-crystals in the forms of nanowires and triangular nanoplates were successfully synthesized through a polyol synthesis. The morphologies changed from nanowires to triangular nanoplates by adjusting the HNO3 concentration. The samples were systematically characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscropy, thermogravimetric analysis and UVevisible Spectra. Examination of nanoparticles at different stages of the reaction revealed that the circular nanoparticles, twinned seeds grew to become triangular nanoplates and nanowires respectively. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Silver Crystal growth Morphology Etching
1. Introduction Metal nanostructures have generated significantly scientific interest in the past several decades due to their unusual optical properties as well as their novel chemical and catalytic properties [1,2]. Regarding their optical properties, the most important optical excitations in metal nanostructures arise from surface plasmon resonances (SPR), which are collective oscillations of the conductive electrons of individual nanoparticles [3e5]. When the wavelength of light couples with the oscillation frequency of the conduction electrons, the surface plasmon resonance arises. In addition, the optical properties of a metal nanostructure such as silver nanoparticles are strongly related to their size, shape, composition and crystallinity, all of which may improve the practical use of silver nanoparticles for sensing and spectroscopic devices [6]. Much effort has been devoted to the synthesis of silver nanoparticles with different shapes such as rods [7], wires [8,9], triangular plates [4,5,10,11], cubes [12], bipyramids [13]. Two kinds of solution-phase approaches for the synthesis of silver triangular nanoplates are normally used. One is a photo-induced method of selecting appropriate radiation wavelengths to control the parameters of the nanoplates. Another method is thermal synthesis. For this method, small molecules or polymers are necessary because these structure-direct agents selectively adsorb on certain crystal
planes, thus inducing anisotropic growth on different planes [14]. For instance, Métraux and Mirkin developed an effective thermal route to synthesize silver triangular nanoplates with additives such as NaBH4, H2O2, poly(vinyl pyrrolidone) (PVP) and citrate [11]. Recently, Zhang et al. applied a novel method based on a unique water/PVP/n-pentanol interface to synthesize triangular silver nanoplates with controlled edge lengths [15]. The polyol reducing approach is an effective and simple method of synthesizing silver nanoparticles. Xia et al. [2,4,16,17] applied this method to synthesize silver nanowires successfully, which evolve into nanocubes, nanobars, nanorices, and nanobelts through the addition of chemicals such as bromide and sodium citrate. Im et al. [18] reported the synthesis of silver nanocubes by polyol reducing approach and discussed the role of HCl in promoting cube perfection and monodispersity. Others also discussed the function of structure directing agents such as PVP in the polyol synthesis of silver nanomaterials [19,20]. To the researchers’ knowledge, no reports on the use of HNO3 as an etchant to control the morphology transformation of silver crystals are currently on record. This paper describes polyol synthesis which enables the control of the morphologies of silver nano-crystals. 2. Experimental section 2.1. Synthesis
* Corresponding author. Tel./fax: 86 0917 3566589. E-mail address: [email protected] (X. Luo). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.06.024
In a typical procedure, 5 mL AgNO3 solution(2.0 103 mol L1, in water) and nitric acid(HNO3, 70 wt%) are dispersed in 20 mL
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X. Luo et al. / Solid State Sciences 13 (2011) 1719e1723
are characterized by UVevisible extinction spectra (Lambd 950, PerkineElmer Corporation, USA).
Table 1 Reaction conditions of the samples. Sample name
Reaction time (h)
nPVP: nAgNO3
Usage of HNO3(mL)
A B C D E F G H
2 2 2 2 0.5 4 0.5 4
0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55
0 1 1.5 3 1 1 0 0
ethylene glycol(EG, 99.8 wt%, AR) and stirred for 10 min at room temperature. After that, PVP (Mw z 30 000) is then added, and the mixture is vigorously stirred for another 20 min. After mixing, the suspension is transferred into a dark, covered Teflon autoclave to avoid the influence of light. The autoclave is then sealed and placed into a preheated electric oven at 120 C for a fixed time without agitation. The products are diluted with ethanol or demonized water and centrifuged at 4000 rpm for 10 min. Centrifugation is repeated five times until the supernatant become colorless. Finally, the powder obtained is dried in an oven at 80 C for 5 h. 2.2. Characterization The crystal phase is analyzed by powder X-ray diffraction (XRD, D/Max-3c, Rigalcu, Japan) using Cu Ka radiation, with the diffraction angle (2q) at a range of 30e80 . The morphology of the crystals is examined by scanning electron microscope (SEM Quanta 200, Philips-FEI, Holland) after coating with Au and transmission electron microscope (TEM, JEM-3010, JEOL, Japan) operated at 300 kV. The thermogravimetric analysis (TG) was performed using PyrisⅡthermobalance (TA, USA). The optical characters of the products
3. Result and discussion 3.1. Transformation of silver nano-crystals by adjusting the usage of HNO3 The synthesizing conditions are summarized in Table 1. A study focused on the influence of HNO3 concentration on the crystal morphology provides insight into their roles in this process. In this work, when HNO3 was omitted from the reactants (Table 1: sample A), the mixture of silver nanowires with the average length more than 100 mm and a litter of nanoparticles were observed (Fig. 1A). The average diameter of the nanowires is about 60 nm as shown in TEM image. When the usage of HNO3 was fixed at 1 mL, the morphologies of the crystals transformed into triangular nanoplates with edge lengths of 3 mm; some spherical nanoparticles were also present (Fig. 1B). Notably, the ratio between the edge length and the thickness of a triangular nanoplate should be relatively high because most of the nanoplates display curly planes. Increasing the dosage of HNO3 to 1.5 mL, the flat surfaces of the triangular nanoplates enlarged, forming little silver fragments (Fig. 1C). And then, the obtained sample D was consisted of 85% spherical nanoparticles about 30 nm in size, 10% nanorods about 100 nm in length and 5% triangular nanoplates with symmetric size distribution in the case of using 3 mL HNO3 (Fig. 1D). The samples were also characterized by XRD. As shown in Fig. 2, all diffraction peaks of the products can be perfectly indexed to the face-centered cubic (FCC) structure of silver (JCPDS File 04-0783). No other phase appeared, demonstrating that this sample is phasepure. Note that the diffraction intensity ratio between peaks (111) and (200) of sample A was about 2.4, which consisted of the theoretical value (2.5). However, the diffraction intensity ratio
Fig. 1. SEM and TEM images of samples AeD in Table 1.
X. Luo et al. / Solid State Sciences 13 (2011) 1719e1723
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3.2. Crystallization process of the silver nanowires and nanoplates
Fig. 2. XRD patterns of samples A and B in Table 1.
between these two peaks exhibited a very high value of about 22.5. This indicates the enrichment of {111} crystalline planes compared with {100} planes in the sturcture of the triangular nanoplate (Fig. 1B). As a result, HNO3 is critical for the transformation of the crystal morphologies because the etching function of HNO3 for silver crystals. Wiley et.al [21] used iron ions (FeⅡ) to eliminate the etching effect of oxygen dissolved in reactants, resulting in transformation from nanowires to nanocubes. Through HNO3, the dissolution of silver crystals is substantially accelerated, and the period of Ostwald ripening alters synchronously in this system. Therefore, the rate of crystallization is limited to ensure a kinetically controlled process; the silver crystal seeds tend to form nanoplates rather than other thermodynamic structures.
Fig. 3AeE shows the SEM and TEM images of samples EeH in Table 1, respectively. As shown in Fig. 3A, circular nanoparticles with an average diameter of 150 nm were obtained at synthesis time t ¼ 0.5 h. By increasing the reaction time to 2 h, the triangular nanoplates grew several microns in edge length, whereas the spherical particles dissolved to about 200 nm in size (Fig. 1B). At t ¼ 4 h, silver particles grown continuously. And triangular nanoplates with asymmetrical edge lengths and more favorable thicknesses were also obtained (Fig. 3B). In fact, when the dispersion of the silver particles was continuously heated at a high temperature, the small crystals dissolved for the benefit of larger ones via a process called Ostwald ripening. In this phenomenon, the ripening process became intense after the addition of HNO3. However, when the synthesizing conditions were the same as those in sample E except for the addition of HNO3, some silver nanoparticles were obtained as shown in Fig. 3C. The stark contrast across each nanoparticle implies that each silver nanoparticle contained several single-crystal subunits. The inset gives the TEM image of a several cross section (at a slightly higher magnification), basically disclosing the different orientations of lattice fringes resulting from the differences in crystal orientation induced by the twin defects. In the following process, these twinned particles grew into nanowires (Fig. 1A). Comparing with the single-crystal nanoparticles of sample E, HNO3 preferentially etches twinned particles because of the higher density of defects on their surfaces. This selective etching is responsible for the formation of single-crystal triangular nanoplates. Prolonging the synthesis time to 4 h, the silver nanowires were also obtained, furthermore, the diameter of the nanowires grew from 60 nm of the sample A to about 100 nm (Fig. 3D). Form the analysis above, it is concluded that in the early period of crystal formation, singlecrystal nanoplates dominantly consume the newly formed silver atoms to form triangular nanoplates with the etching effect of
Fig. 3. SEM and TEM images of samples EeH, (A): sample E; (B): sample F; (C): sample G; (D): sample H.
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Fig. 4. TG curve of samples A and B in Table 1.
HNO3. Without HNO3, the twinned nanoparticles form dominantly then subsequently grow into a thermodynamically favorable morphology such as nanowires. 3.3. Characterization of the silver nanowires and nanoplates Fig. 4 shows the TG curve of the sample A and B with the several washing and centrifuging times. For these two samples, a four-step weight decline pattern was observed from w70e680 C. The first weight loss at 80e135 C was attributable to water evaporation; the second corresponds to ethylene glycol desorption. The last two lapses at 416e670 C correspond to the decomposition of PVP on the surface of silver nanoplates, which is consisted with the previous work [3]. It is worth noting that the weight loss of PVP on nanowires is about 8% that is much more than 2% on nanoplates. The UVevisible spectroscopic method was also used to track the optical properties of silver nanowires and nanoplates because silver nanomaterials display size-dependent SPR properties. Fig. 5A shows the UVevisible spectra of the samples G, A and H. For the sample G, the UV-Visble curve displays adsorption peaks concentrating from 350 to 380 nm. The peak at 355 nm could be attributed to the longitudinal mode of nanoparticles, while the last two peaks correspond to the transverse mode of silver nanoparticles [3]. As the nanoparticles grew into nanowires, the longitudinal and transverse plasmon mode (at w370 nm) was greatly increased in intensity. Fig. 5B shows the UVevisible spectra of the samples E, B and F. The extinction curves of these three samples display a similar
Fig. 5. UV-Visble spectra of the samples, (A. a): sample G; (A. b): sample A; (A. c): sample H; (B. a): sample E; (B. b): sample B; (B. c): sample F.
trend. The shoulder at about 340 nm should be attributed to out-ofplane quadrupole resonances, whereas the adsorption peak at 450 nm corresponds to out-of-plane dipole resonances. When the synthesis time is fixed at 2 h, the blue shift of in-plane dipole resonances to 665 nm shows that the obtained triangular nanoplates are truncated. Furthermore, the samples B and F exhibit a broad adsorption in the wavelength region of visible light, indicating the existence of a broad distribution in size and shape for the silver particles. 4. Conclusion In summary, silver nano-crystals with different morphologies were successfully synthesized by reducing AgNO3 in the presence of PVP. By adjusting the concentration of HNO3, the morphologies of the silver crystals changed from nanowires to triangular nanoplates and then to nanoparticles, which were consistent with the variation in XRD and UVeVisible results. Comparing these two crystallization process of silver nanowires and nanoplates, it is confirmed that the selective etching of HNO3 for the twined crystals occurred at the early stage of crystal formation, which induced transformation from the thermodynamically favorable nanowires to triangular nanoplates. This work provides simple but effective methods for potentially controlling the morphology of silver nano-crystals.
X. Luo et al. / Solid State Sciences 13 (2011) 1719e1723
Acknowledgment This work has been supported in part by the Research Foundation of the Baoji University of Arts and Sciences (No. ZK08117). And the authors also thank the support of Research Foundation of Shaanxi Key Laboratory (No. 09 JS068). References [1] N. Tian, Z.Y. Zhou, S.G. Sun, Y. Ding, Z.L. Wang, Science 316 (2007) 732e735. [2] Y.G. Sun, Y.N. Xia, Science 298 (2002) 2176e2179. [3] Y.G. Sun, Y.D. Yin, B.T. Mayers, T. Herricks, Y.N. Xia, Chem. Mater. 14 (2002) 4736e4745. [4] Y.G. Sun, Y.N. Xia, Adv. Mater. 15 (2003) 695e699. [5] M. Maillard, S. Giorgio, M.P. Pileni, Adv. Mater. 14 (2002) 1084e1086. [6] S. Nie, S.R. Emory, Science 275 (1997) 1102e1106. [7] M. Tsuji, K. Matsumoto, N. Miyamae, T. Tsuji, X. Zhang, Cryst. Growth Des. 7 (2007) 311e320.
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[8] X.M. Sun, Y.D. Li, Adv. Mater. 17 (2005) 2626e2630. [9] K.K. Caswell, C.M. Bender, C.J. Murphy, Nano Lett. 3 (2003) 667e669. [10] S. Lv, H. Suo, H. Wang, C. Wang, J. Wang, Y. Xu, C. Zhao, Solid State Sci. 59 (2008) 380e384. [11] G.S. Métraux, C.A. Mirkin, Adv. Mater. 17 (2005) 412e415. [12] M. Rycenga, J.M. Mclellan, Y.N. Xia, Adv. Mater. 20 (2008) 2416e2420. [13] B.J. Wiley, Y.J. Xiong, Z.Y. Li, Y.D. Yin, Y.N. Xia, Nano Lett. 6 (2006) 765e768. [14] X.H. Liu, F. Zhang, R. Huang, C.F. Pan, J. Zhu, Cryst. Growth Des. 8 (2008) 1916e1923. [15] J.H. Zhang, H.Y. Liu, P. Zhan, L. Wang, N.B. Ming, Adv. Funct. Mater. 17 (2007) 1558e1566. [16] B. Wiley, T. Herricks, Y.G. Sun, Y.N. Xia, Nano Lett. 4 (2004) 1733e1739. [17] B.J. Wiley, Y. Chen, J.M. McLellan, Y.J. Xiong, Z.Y. Li, D. Ginger, Y.N. Xia, Nano Lett. 7 (2007) 1032e1036. [18] S.H. Im, Y.T. Lee, B. Wiley, Y. Xia, Angew. Chem. Int. Ed. 44 (2005) 2154e2157. [19] Y. Gao, P. Jiang, D.F. Liu, H.J. Yuan, X.Q. Yan, Z.P. Zhou, J.X. Wang, L. Song, L.F. Liu, W.Y. Zhou, G. Wang, C.Y. Wang, S.S. Xie, J. Phys. Chem. B. 108 (2004) 12877e12881. [20] Y.G. Sun, B. Mayers, T. Herricks, Y.N. Xia, Nano Lett. 3 (2003) 955e960. [21] B. Wiley, Y.G. Sun, Y.N. Xia, Langmuir 21 (2005) 8077e8080.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Morphology control of silver nano-crystals through a polyol synthesis Xiaolin Luo a, *, Yashao Chen b, Desuo Yang a, Zongxiao Li a, Yinfeng Han a a b
Shaanxi Key Laboratory for Phytochemistry, Chemistry & Chemical Engineering Department, Baoji University of Art and Science, Baoji 721013, China Key Laboratory of Applied Surface and Colloid Chemistry, Shaanxi Normal University, Ministry of Education, School of Chemistry and Materials Science, Xi’an 710062, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 March 2011 Received in revised form 25 May 2011 Accepted 25 June 2011 Available online 18 July 2011
Silver nano-crystals in the forms of nanowires and triangular nanoplates were successfully synthesized through a polyol synthesis. The morphologies changed from nanowires to triangular nanoplates by adjusting the HNO3 concentration. The samples were systematically characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscropy, thermogravimetric analysis and UVevisible Spectra. Examination of nanoparticles at different stages of the reaction revealed that the circular nanoparticles, twinned seeds grew to become triangular nanoplates and nanowires respectively. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Silver Crystal growth Morphology Etching
1. Introduction Metal nanostructures have generated significantly scientific interest in the past several decades due to their unusual optical properties as well as their novel chemical and catalytic properties [1,2]. Regarding their optical properties, the most important optical excitations in metal nanostructures arise from surface plasmon resonances (SPR), which are collective oscillations of the conductive electrons of individual nanoparticles [3e5]. When the wavelength of light couples with the oscillation frequency of the conduction electrons, the surface plasmon resonance arises. In addition, the optical properties of a metal nanostructure such as silver nanoparticles are strongly related to their size, shape, composition and crystallinity, all of which may improve the practical use of silver nanoparticles for sensing and spectroscopic devices [6]. Much effort has been devoted to the synthesis of silver nanoparticles with different shapes such as rods [7], wires [8,9], triangular plates [4,5,10,11], cubes [12], bipyramids [13]. Two kinds of solution-phase approaches for the synthesis of silver triangular nanoplates are normally used. One is a photo-induced method of selecting appropriate radiation wavelengths to control the parameters of the nanoplates. Another method is thermal synthesis. For this method, small molecules or polymers are necessary because these structure-direct agents selectively adsorb on certain crystal
planes, thus inducing anisotropic growth on different planes [14]. For instance, Métraux and Mirkin developed an effective thermal route to synthesize silver triangular nanoplates with additives such as NaBH4, H2O2, poly(vinyl pyrrolidone) (PVP) and citrate [11]. Recently, Zhang et al. applied a novel method based on a unique water/PVP/n-pentanol interface to synthesize triangular silver nanoplates with controlled edge lengths [15]. The polyol reducing approach is an effective and simple method of synthesizing silver nanoparticles. Xia et al. [2,4,16,17] applied this method to synthesize silver nanowires successfully, which evolve into nanocubes, nanobars, nanorices, and nanobelts through the addition of chemicals such as bromide and sodium citrate. Im et al. [18] reported the synthesis of silver nanocubes by polyol reducing approach and discussed the role of HCl in promoting cube perfection and monodispersity. Others also discussed the function of structure directing agents such as PVP in the polyol synthesis of silver nanomaterials [19,20]. To the researchers’ knowledge, no reports on the use of HNO3 as an etchant to control the morphology transformation of silver crystals are currently on record. This paper describes polyol synthesis which enables the control of the morphologies of silver nano-crystals. 2. Experimental section 2.1. Synthesis
* Corresponding author. Tel./fax: 86 0917 3566589. E-mail address: [email protected] (X. Luo). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.06.024
In a typical procedure, 5 mL AgNO3 solution(2.0 103 mol L1, in water) and nitric acid(HNO3, 70 wt%) are dispersed in 20 mL
1720
X. Luo et al. / Solid State Sciences 13 (2011) 1719e1723
are characterized by UVevisible extinction spectra (Lambd 950, PerkineElmer Corporation, USA).
Table 1 Reaction conditions of the samples. Sample name
Reaction time (h)
nPVP: nAgNO3
Usage of HNO3(mL)
A B C D E F G H
2 2 2 2 0.5 4 0.5 4
0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55
0 1 1.5 3 1 1 0 0
ethylene glycol(EG, 99.8 wt%, AR) and stirred for 10 min at room temperature. After that, PVP (Mw z 30 000) is then added, and the mixture is vigorously stirred for another 20 min. After mixing, the suspension is transferred into a dark, covered Teflon autoclave to avoid the influence of light. The autoclave is then sealed and placed into a preheated electric oven at 120 C for a fixed time without agitation. The products are diluted with ethanol or demonized water and centrifuged at 4000 rpm for 10 min. Centrifugation is repeated five times until the supernatant become colorless. Finally, the powder obtained is dried in an oven at 80 C for 5 h. 2.2. Characterization The crystal phase is analyzed by powder X-ray diffraction (XRD, D/Max-3c, Rigalcu, Japan) using Cu Ka radiation, with the diffraction angle (2q) at a range of 30e80 . The morphology of the crystals is examined by scanning electron microscope (SEM Quanta 200, Philips-FEI, Holland) after coating with Au and transmission electron microscope (TEM, JEM-3010, JEOL, Japan) operated at 300 kV. The thermogravimetric analysis (TG) was performed using PyrisⅡthermobalance (TA, USA). The optical characters of the products
3. Result and discussion 3.1. Transformation of silver nano-crystals by adjusting the usage of HNO3 The synthesizing conditions are summarized in Table 1. A study focused on the influence of HNO3 concentration on the crystal morphology provides insight into their roles in this process. In this work, when HNO3 was omitted from the reactants (Table 1: sample A), the mixture of silver nanowires with the average length more than 100 mm and a litter of nanoparticles were observed (Fig. 1A). The average diameter of the nanowires is about 60 nm as shown in TEM image. When the usage of HNO3 was fixed at 1 mL, the morphologies of the crystals transformed into triangular nanoplates with edge lengths of 3 mm; some spherical nanoparticles were also present (Fig. 1B). Notably, the ratio between the edge length and the thickness of a triangular nanoplate should be relatively high because most of the nanoplates display curly planes. Increasing the dosage of HNO3 to 1.5 mL, the flat surfaces of the triangular nanoplates enlarged, forming little silver fragments (Fig. 1C). And then, the obtained sample D was consisted of 85% spherical nanoparticles about 30 nm in size, 10% nanorods about 100 nm in length and 5% triangular nanoplates with symmetric size distribution in the case of using 3 mL HNO3 (Fig. 1D). The samples were also characterized by XRD. As shown in Fig. 2, all diffraction peaks of the products can be perfectly indexed to the face-centered cubic (FCC) structure of silver (JCPDS File 04-0783). No other phase appeared, demonstrating that this sample is phasepure. Note that the diffraction intensity ratio between peaks (111) and (200) of sample A was about 2.4, which consisted of the theoretical value (2.5). However, the diffraction intensity ratio
Fig. 1. SEM and TEM images of samples AeD in Table 1.
X. Luo et al. / Solid State Sciences 13 (2011) 1719e1723
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3.2. Crystallization process of the silver nanowires and nanoplates
Fig. 2. XRD patterns of samples A and B in Table 1.
between these two peaks exhibited a very high value of about 22.5. This indicates the enrichment of {111} crystalline planes compared with {100} planes in the sturcture of the triangular nanoplate (Fig. 1B). As a result, HNO3 is critical for the transformation of the crystal morphologies because the etching function of HNO3 for silver crystals. Wiley et.al [21] used iron ions (FeⅡ) to eliminate the etching effect of oxygen dissolved in reactants, resulting in transformation from nanowires to nanocubes. Through HNO3, the dissolution of silver crystals is substantially accelerated, and the period of Ostwald ripening alters synchronously in this system. Therefore, the rate of crystallization is limited to ensure a kinetically controlled process; the silver crystal seeds tend to form nanoplates rather than other thermodynamic structures.
Fig. 3AeE shows the SEM and TEM images of samples EeH in Table 1, respectively. As shown in Fig. 3A, circular nanoparticles with an average diameter of 150 nm were obtained at synthesis time t ¼ 0.5 h. By increasing the reaction time to 2 h, the triangular nanoplates grew several microns in edge length, whereas the spherical particles dissolved to about 200 nm in size (Fig. 1B). At t ¼ 4 h, silver particles grown continuously. And triangular nanoplates with asymmetrical edge lengths and more favorable thicknesses were also obtained (Fig. 3B). In fact, when the dispersion of the silver particles was continuously heated at a high temperature, the small crystals dissolved for the benefit of larger ones via a process called Ostwald ripening. In this phenomenon, the ripening process became intense after the addition of HNO3. However, when the synthesizing conditions were the same as those in sample E except for the addition of HNO3, some silver nanoparticles were obtained as shown in Fig. 3C. The stark contrast across each nanoparticle implies that each silver nanoparticle contained several single-crystal subunits. The inset gives the TEM image of a several cross section (at a slightly higher magnification), basically disclosing the different orientations of lattice fringes resulting from the differences in crystal orientation induced by the twin defects. In the following process, these twinned particles grew into nanowires (Fig. 1A). Comparing with the single-crystal nanoparticles of sample E, HNO3 preferentially etches twinned particles because of the higher density of defects on their surfaces. This selective etching is responsible for the formation of single-crystal triangular nanoplates. Prolonging the synthesis time to 4 h, the silver nanowires were also obtained, furthermore, the diameter of the nanowires grew from 60 nm of the sample A to about 100 nm (Fig. 3D). Form the analysis above, it is concluded that in the early period of crystal formation, singlecrystal nanoplates dominantly consume the newly formed silver atoms to form triangular nanoplates with the etching effect of
Fig. 3. SEM and TEM images of samples EeH, (A): sample E; (B): sample F; (C): sample G; (D): sample H.
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Fig. 4. TG curve of samples A and B in Table 1.
HNO3. Without HNO3, the twinned nanoparticles form dominantly then subsequently grow into a thermodynamically favorable morphology such as nanowires. 3.3. Characterization of the silver nanowires and nanoplates Fig. 4 shows the TG curve of the sample A and B with the several washing and centrifuging times. For these two samples, a four-step weight decline pattern was observed from w70e680 C. The first weight loss at 80e135 C was attributable to water evaporation; the second corresponds to ethylene glycol desorption. The last two lapses at 416e670 C correspond to the decomposition of PVP on the surface of silver nanoplates, which is consisted with the previous work [3]. It is worth noting that the weight loss of PVP on nanowires is about 8% that is much more than 2% on nanoplates. The UVevisible spectroscopic method was also used to track the optical properties of silver nanowires and nanoplates because silver nanomaterials display size-dependent SPR properties. Fig. 5A shows the UVevisible spectra of the samples G, A and H. For the sample G, the UV-Visble curve displays adsorption peaks concentrating from 350 to 380 nm. The peak at 355 nm could be attributed to the longitudinal mode of nanoparticles, while the last two peaks correspond to the transverse mode of silver nanoparticles [3]. As the nanoparticles grew into nanowires, the longitudinal and transverse plasmon mode (at w370 nm) was greatly increased in intensity. Fig. 5B shows the UVevisible spectra of the samples E, B and F. The extinction curves of these three samples display a similar
Fig. 5. UV-Visble spectra of the samples, (A. a): sample G; (A. b): sample A; (A. c): sample H; (B. a): sample E; (B. b): sample B; (B. c): sample F.
trend. The shoulder at about 340 nm should be attributed to out-ofplane quadrupole resonances, whereas the adsorption peak at 450 nm corresponds to out-of-plane dipole resonances. When the synthesis time is fixed at 2 h, the blue shift of in-plane dipole resonances to 665 nm shows that the obtained triangular nanoplates are truncated. Furthermore, the samples B and F exhibit a broad adsorption in the wavelength region of visible light, indicating the existence of a broad distribution in size and shape for the silver particles. 4. Conclusion In summary, silver nano-crystals with different morphologies were successfully synthesized by reducing AgNO3 in the presence of PVP. By adjusting the concentration of HNO3, the morphologies of the silver crystals changed from nanowires to triangular nanoplates and then to nanoparticles, which were consistent with the variation in XRD and UVeVisible results. Comparing these two crystallization process of silver nanowires and nanoplates, it is confirmed that the selective etching of HNO3 for the twined crystals occurred at the early stage of crystal formation, which induced transformation from the thermodynamically favorable nanowires to triangular nanoplates. This work provides simple but effective methods for potentially controlling the morphology of silver nano-crystals.
X. Luo et al. / Solid State Sciences 13 (2011) 1719e1723
Acknowledgment This work has been supported in part by the Research Foundation of the Baoji University of Arts and Sciences (No. ZK08117). And the authors also thank the support of Research Foundation of Shaanxi Key Laboratory (No. 09 JS068). References [1] N. Tian, Z.Y. Zhou, S.G. Sun, Y. Ding, Z.L. Wang, Science 316 (2007) 732e735. [2] Y.G. Sun, Y.N. Xia, Science 298 (2002) 2176e2179. [3] Y.G. Sun, Y.D. Yin, B.T. Mayers, T. Herricks, Y.N. Xia, Chem. Mater. 14 (2002) 4736e4745. [4] Y.G. Sun, Y.N. Xia, Adv. Mater. 15 (2003) 695e699. [5] M. Maillard, S. Giorgio, M.P. Pileni, Adv. Mater. 14 (2002) 1084e1086. [6] S. Nie, S.R. Emory, Science 275 (1997) 1102e1106. [7] M. Tsuji, K. Matsumoto, N. Miyamae, T. Tsuji, X. Zhang, Cryst. Growth Des. 7 (2007) 311e320.
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