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Selective synthesis of copper nanoplates and nanowires via a surfactant-assisted hydrothermal process
Materials Chemistry and Physics 120 (2010) 1–5
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Selective synthesis of copper nanoplates and nanowires via a surfactant-assisted hydrothermal process Shuling Xu a , Xun Sun b , Hong Ye a , Ting You a , Xinyu Song a,∗ , Sixiu Sun a,∗ a Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, 27 Shanda South Road, Jinan, Shandong 250100, PR China b State Key Laboratory of Crystal Materials, Shandong University, 27 Shanda South Road, Jinan, Shandong 250100, PR China
a r t i c l e
i n f o
Article history: Received 12 October 2008 Received in revised form 27 August 2009 Accepted 31 October 2009 Keywords: Nanostructures Crystal growth Chemical synthesis Transmission electron microscopy
a b s t r a c t A facile solution-phase process has been demonstrated for the selective preparation of single-crystalline Cu nanoplates and nanowires by reducing Cu+ with ascorbic acid (VC) in the presence of cetyltrimethylammonium bromide (CTAB) or cetyltrimethylammonium chloride (CTAC). To study the formation process of nanoplates and nanowires, samples obtained at various stages of the growth process were studied by TEM and XRD. The possible mechanism was discussed to elucidate the formation of different morphologies of Cu nanostructures. UV–vis spectra of the Cu nanoplates and nanowires were recorded to investigate their optical properties, which indicated that the as-prepared Cu nanostructures exhibited morphology-dependent optical property. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the past few years, much effort has been devoted to the controlled synthesis of metal nanostructures because of their unique chemical and physical properties that are different from those of the bulk materials [1]. Among all metals, Cu is the most commonly used in electrical conductivity owing to its low price and stability at high frequencies. Therefore, the synthesis of Cu micro/nanostructures has been extensively investigated during the past several years. Many synthetic methods, including polyol process, templated synthesis, reverse micelles, microemulsion, electrochemical deposition, chemical solution process, chemical vapor deposition and irradiation method, were applied to synthesis Cu nano/micro structures with different shapes [2–10]. Most recently, 1D and 2D Cu nanostructures with regular shapes, in particular, have generated intense interest due to its potential application as an essential component in the further generation of nanodevices [11–14]. However, in contrast to a large amount of reports on wire-like and plate-like nanostructures of some facecentered cubic metal (such as Ag and Au) [15–24], the synthesis of Cu nanoplates and nanowires is mainly limited. Thus, successful synthetic strategies are still a challenge for the preparation of low-dimensional Cu micro/nanostructures, and their formation becomes favorable only in a slow reduction process. In this paper, we describe a facile hydrothermal process to selective syn-
∗ Corresponding authors. Tel.: +86 531 8836 4879; fax: +86 531 8856 446. E-mail addresses: [email protected] (X. Song), [email protected] (S. Sun). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.10.049
thesis of anisotropic Cu nanocrystal. Two different morphologies, nanoplates and nanowires were obtained by utilizing different surfactants. Plate-like Cu crystals were synthesized when CTAB was used as surfactant, while wire-like structures were obtained if CTAB was substituted by CTAC. 2. Experimental All of the chemical reagents in this experiment were analytical grade, and used without further purification. In a typical procedure of Cu nanoplates, 0.15 mmol CuCl was added into 25 mL of distilled water with magnetic stirring, and then 0.5 mmol VC was dissolved into the solution. Finally, 0.075 mmol CTAB and 0.5 mL of NaOH (1 M) were added into the above mixture sequentially. After being vigorously stirred for 10 min, the mixture was put into a Teflon-lined autoclave of 30-mL capacity and maintained at 120 ◦ C for 1 h, then cooled to room temperature naturally. The products were collected, and washed thoroughly with deionized water and absolute ethanol. Finally they were dried in a vacuum oven before further characterization. When 0.075 mmol CTAB was substituted by 0.075 mmol CTAC, whereas other reaction conditions were kept the same, Cu nanowires were obtained. The samples were characterized by XRD on a German Bruker D8 X-ray diffractometer with Ni filtered Cu K␣ radiation ( = 1.5418 Å). TEM and SAED patters were characterized by a JEM-100CXII at an accelerating voltage of 100 kV. HRTEM image was carried out on a JEOL 2100 transmission electron microscope. UV–vis absorption spectra were taken using a Hitachi U-4100 spectrophotometer.
3. Results and discussion The phase and purity of the nanoplates were confirmed by the XRD pattern shown in Fig. 1(a). All the diffraction peaks can be indexed orderly to Cu (1 1 1), (2 0 0), (2 2 0) planes of the face-centered cubic structure of Cu (JCPDS file NO. 65-9026). No characteristic peak from impurity is detected. TEM image
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S. Xu et al. / Materials Chemistry and Physics 120 (2010) 1–5
Fig. 1. XRD patterns of the obtained Cu of (a) nanoplates; (b) nanowires.
shows that the plate-like structures with a diameter of about 800–1200 nm (Fig. 2(a)). Furthermore, Fig. 2(b) shows the HRTEM image of the nanoplate. The HRTEM image reveals parallel fringes with a space of 0.21 nm, which corresponds to the (1 1 1) lat-
tice planes of the face-centered cubic Cu. The SAED patter shown in the inset of Fig. 2(b) confirms that the nanoplate is a single crystal. In order to understand the formation process of the Cu nanoplates, samples obtained at various stages of the growth process were studied by TEM and XRD. Fig. 3(a) gives a TEM image of the samples collected after 20 min of the reaction. Here some platelike structures were formed, but the majority of the products were irregular nanostructures. After 40 min of the reaction, it was found that most of the products were well-defined plates (Fig. 3(b)), but a few irregular nanoparticles still existed. Fig. 3(c) and (d) shows XRD patterns of the products obtained at 120 ◦ C for 20 and 40 min, respectively. From the pattern in Fig. 3(c), it could be seen that CuBr, CuCl and Cu coexisted in the products, after the reaction proceeded for 20 min. The presence of CuBr could attribute to the reaction between CuCl and Br− anion (from CTAB). With a longer reaction time of 40 min, the peak of CuCl disappeared and the intensity of CuBr peaks weakened (Fig. 3(d)). It was found that CTAB played an important role in the formation of the nanoplates. To clearly show the effect of CTAB on the morphology of the Cu nanoparticles, the experiments were carried out at different CTAB amounts and kept the other conditions changeless. In the absence of CTAB (Fig. 4(a)), irregular Cu particles were obtained. As shown in Fig. 4(b), nanoparticles, nanowires and nanoplates were obtained when the amount of CTAB was 0.025 mmol. When the amount of CTAB was increased
Fig. 2. TEM images of (a) Cu nanoplates; (b) HRTEM image of a nanoplate. The inset shows the SAED pattern of an individual nanoplate.
Fig. 3. TEM images of plate-like copper particles obtained at different reaction time: (a) 20 min; (b) 40 min. XRD patterns of the products at reaction time of (c) 20 min and (d) 40 min.
S. Xu et al. / Materials Chemistry and Physics 120 (2010) 1–5
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Fig. 4. TEM images of Cu particles prepared under different amounts of CTAB: (a) 0 mmol; (b) 0.025 mmol; (c) 0.050 mmol; (d) 0.125 mmol.
to 0.050 mmol, little Cu nanowire was obtained in addition to the nanoplates (Fig. 4(c)). If the CTAB amount was further increased to 0.125 mmol, the products were thick nanoplates with a diameter of about 300–600 nm (Fig. 4(d)). The results indicated that the morphology of final products could be controlled by the amount of CTAB. On the basis of above results, it can be concluded that CTAB serves as both Br source and surfactant direct the growth of Cu nanoplates. To further clarify the role of the Br− anion in CTAB, we have conducted the control experiments by substituting CTAB with CTAC under the standard conditions. As shown in Fig. 5(a) nanowires were generated in the presence of CTAC, implying that CTA+ alone is not sufficient for the shape control of nanoplates. Fig. 1(b) shows XRD pattern of the products obtained when CTAC was used as surfactant. All the diffraction peaks in the patterns are indexed to the structure of face-centered cubic Cu (JCPDS file NO. 65-9026). The HRTEM image of an individual nanowire (Fig. 5(b)) reveals parallel fringes with a space of 0.21 nm, which is consistent with the space of (1 1 1) lattice planes, confirming that each nanowire is a [0 1 1]-oriented growth direction. The SAED pattern (inset in Fig. 5(b)) indicates that the copper nanowire is a single crystal.
The time-dependent morphology evolutions of the nanowires were studied by TEM. Fig. 6(a) and (b) shows TEM images of the products obtained at 120 ◦ C for 20 and 40 min, respectively. As shown in Fig. 6(a), a few wire-like particles were obtained after the reaction mixture was heated for 20 min. When the reaction time was prolonged to 40 min, the major products were nanowires with lengths of several micrometers (Fig. 6(b)). Fig. 6(c) is a XRD pattern of the products collected after 20 min of the reaction. Here Cu and CuCl coexisted in the products, indicating CuCl was partially reduced by VC under current conditions. After the reaction proceeded for 40 min, the intensity of CuCl peaks weakened, whereas that of the corresponding ones of Cu intensified since CuCl gradually transformed into Cu (Fig. 6(d)). The above experimental results show that Cu nanoplates and nanowires can be selective synthesized by utilizing different surfactants. For a face-centered cubic metal, the thermodynamically favored shapes are truncated nanocubes and multiple twinned particles. To obtain the shapes other than the thermodynamic ones, the kinetics of nucleation and growth must be carefully controlled [17]. Nanoplates and nanowires are intrinsically higher in energy than the thermodynamically favored shapes, and their formation becomes favorable only in a slow reduction process. It is well known
Fig. 5. TEM images of (a) Cu nanowires and (b) HRTEM image of typical nanowires. The inset shows the SAED pattern of an individual nanowire.
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S. Xu et al. / Materials Chemistry and Physics 120 (2010) 1–5
Fig. 6. TEM images of wire-like copper particles obtained at different reaction time: (a) 20 min and (b) 40 min. XRD patterns of the products at reaction time of (c) 20 min and (d) 40 min.
that cuprous halide has low solubility in aqueous solution and it could not be completely dissolved under current conditions. The free Cu+ was maintained at a low level in the solution and the generation of Cu atom came through a slow reduction process. When the reduction becomes slow enough, kinetic control will take over in both nucleation and growth of Cu nanoparticles. In our system, when CTAB was used as surfactant, the release rate of the free Cu+ in the system was controlled by the newly form CuBr. Due to the less solubility of CuBr (PKsp = 8.28) than CuCl (PKsp = 5.92) in the aqueous solution, the free Cu+ concentration in the solution was decreased. The lower Cu+ concentration may provide favorable condition for the growth of anisotropic plate-like structures in the presence of CTA+ . When CTAB was substituted by CTAC, release rate of the Cu+ from CuCl was beneficial to the formation of the nanowires. If the concentrations of CTAB were low, CuCl cannot transform into CuBr completely. Thus, the release rate of the Cu+ was controlled by both CuCl and CuBr, nanowires and nanoplates coexisted in the products. It is reasonable to explain the presence of nanowires in addition to nanoplates as shown in Fig. 4(b) and
(c). Consequently, the release rate of the free Cu+ in the system and the selective adsorption of CTA+ play critical roles in controlling the morphology of the products. UV–vis spectra of as-synthesized Cu particles were recorded to investigate theirs optical property. As seen from Fig. 7(a), Cu nanoplates have absorption at 599 nm. For the nanowires, the absorption shows a slight blue shift to 588 nm (Fig. 7(b)), which is in good agreement with the reported values for Cu nanowires [8]. Though the reason of blue shift is unclear, the results indicate that the as-prepared Cu nanostructures exhibited morphologydependent optical property. 4. Conclusions In summary, Cu nanoplates and nanowires have been selectively synthesized via surfactant-assisted hydrothermal processes. A series of experiments confirm that the release rate of the free Cu+ in the system and the selective adsorption of CTA+ play critical roles in controlling the morphology of the products. The release rate of the free Cu+ was controlled by CuBr when CTAB was used as surfactant, and Cu nanoplates were obtained. If CTAB was substituted by CTAC, the release rate of the Cu+ from CuCl provided favorable conditions for the growth of nanowires. This synthetic route may be applied to morphology control synthesis of other anisotropic metal nanoparticles. Acknowledgement We greatly acknowledge the financial support of the 973 project of China (No. 2005CB623601). References
Fig. 7. UV–vis spectra of as-synthesized (a) Cu nanowires and (b) Cu nanoplates.
[1] D. Yu, V.W. Yam, J. Am. Chem. Soc. 126 (2004) 13200. [2] L. Qi, J. Ma, J. Shen, J. Colloid Interface Sci. 186 (1997) 498. [3] I. Lisiecki, M. Biorling, L. Motte, B. Ninham, M.P. Pileni, Langmuir 11 (1995) 2385. [4] Y. Zhao, J.J. Zhu, J.M. Hong, N. Bian, H.Y. Chen, Eur. J. Inorg. Chem. 20 (2004) 4072. [5] H.A. Becerril, B.M. Stoltenberg, C.F. Monson, A.T. Woolley, J. Mater. Chem. 14 (2004) 611. [6] H.C. Shin, J. Dong, M. Liu, Adv. Mater. 15 (2003) 1610. [7] M. Zhang, S. Lenhert, M. Wang, L. Chi, N. Lu, H. Fuchs, N. Ming, Adv. Mater. 16 (2004) 409.
S. Xu et al. / Materials Chemistry and Physics 120 (2010) 1–5 [8] Z. Liu, Y. Yang, J. Liang, Z. Hu, S. Li, S. Peng, Y. Qian, J. Phys. Chem. B 107 (2003) 12658. [9] S. Kapoor, D.K. Palit, T. Mukherjee, Chem. Phys. Lett. 355 (2002) 383. [10] H. Chol, S.H. Park, J. Am. Chem. Soc. 126 (2004) 6248. [11] J.E. Millstone, G.S. M’etraux, C.A. Mirkin, AdV. Funct. Mater. 16 (2006) 1209. [12] N. Fan, L. Xu, J. Li, X. Ma, Y. Qian, J. Cryst. Growth 299 (2007) 212. [13] W. Wang, G. Li, Z. Zhang, J. Cryst. Growth 299 (2007) 158. [14] T.K. Huang, T.H. Cheng, M.Y. Yen, W.H. Hsiao, L.S. Wang, F.R. Chen, J.J. Kai, C.Y. Lee, H.T. Chiu, Langmuir 23 (2007) 5722. [15] X. Sun, S. Dong, E. Wang, Chem. Lett. 34 (2005) 968.
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[16] B. Liu, J. Xie, J.Y. Lee, Y.P. Ting, J.P. Chen, J. Phys. Chem. B 109 (2005) 15256. [17] Y. Xiong, I. Washio, J. Chen, H. Cai, Z.Y. Li, Y. Xia, Langmuir 22 (2006) 8563. [18] H.C. Chu, C.H. Kuo, M.H. Huang, Inorg. Chem. 45 (2006) 808. [19] Y. Sun, Y. Xia, Adv. Mater. 15 (2003) 695. [20] Y. Sun, B. Mayers, Y. Xia, Nano Lett. 3 (2003) 675. [21] L. Jiang, S. Xu, J. Zhu, J. Zhang, J. Zhu, H. Chen, Inorg. Chem. 43 (2004) 5877. [22] W.L. Huang, C.H. Chen, M.H. Huang, J. Phys. Chem. C 111 (2007) 2533. [23] N.R. Jana, L. Gearheart, C.J. Murphy, J. Phys. Chem. B 105 (2001) 4065. [24] J. Bai, Y. Qin, C. Jiang, L. Qi, Chem. Mater. 19 (2007) 3367.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Selective synthesis of copper nanoplates and nanowires via a surfactant-assisted hydrothermal process Shuling Xu a , Xun Sun b , Hong Ye a , Ting You a , Xinyu Song a,∗ , Sixiu Sun a,∗ a Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, 27 Shanda South Road, Jinan, Shandong 250100, PR China b State Key Laboratory of Crystal Materials, Shandong University, 27 Shanda South Road, Jinan, Shandong 250100, PR China
a r t i c l e
i n f o
Article history: Received 12 October 2008 Received in revised form 27 August 2009 Accepted 31 October 2009 Keywords: Nanostructures Crystal growth Chemical synthesis Transmission electron microscopy
a b s t r a c t A facile solution-phase process has been demonstrated for the selective preparation of single-crystalline Cu nanoplates and nanowires by reducing Cu+ with ascorbic acid (VC) in the presence of cetyltrimethylammonium bromide (CTAB) or cetyltrimethylammonium chloride (CTAC). To study the formation process of nanoplates and nanowires, samples obtained at various stages of the growth process were studied by TEM and XRD. The possible mechanism was discussed to elucidate the formation of different morphologies of Cu nanostructures. UV–vis spectra of the Cu nanoplates and nanowires were recorded to investigate their optical properties, which indicated that the as-prepared Cu nanostructures exhibited morphology-dependent optical property. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the past few years, much effort has been devoted to the controlled synthesis of metal nanostructures because of their unique chemical and physical properties that are different from those of the bulk materials [1]. Among all metals, Cu is the most commonly used in electrical conductivity owing to its low price and stability at high frequencies. Therefore, the synthesis of Cu micro/nanostructures has been extensively investigated during the past several years. Many synthetic methods, including polyol process, templated synthesis, reverse micelles, microemulsion, electrochemical deposition, chemical solution process, chemical vapor deposition and irradiation method, were applied to synthesis Cu nano/micro structures with different shapes [2–10]. Most recently, 1D and 2D Cu nanostructures with regular shapes, in particular, have generated intense interest due to its potential application as an essential component in the further generation of nanodevices [11–14]. However, in contrast to a large amount of reports on wire-like and plate-like nanostructures of some facecentered cubic metal (such as Ag and Au) [15–24], the synthesis of Cu nanoplates and nanowires is mainly limited. Thus, successful synthetic strategies are still a challenge for the preparation of low-dimensional Cu micro/nanostructures, and their formation becomes favorable only in a slow reduction process. In this paper, we describe a facile hydrothermal process to selective syn-
∗ Corresponding authors. Tel.: +86 531 8836 4879; fax: +86 531 8856 446. E-mail addresses: [email protected] (X. Song), [email protected] (S. Sun). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.10.049
thesis of anisotropic Cu nanocrystal. Two different morphologies, nanoplates and nanowires were obtained by utilizing different surfactants. Plate-like Cu crystals were synthesized when CTAB was used as surfactant, while wire-like structures were obtained if CTAB was substituted by CTAC. 2. Experimental All of the chemical reagents in this experiment were analytical grade, and used without further purification. In a typical procedure of Cu nanoplates, 0.15 mmol CuCl was added into 25 mL of distilled water with magnetic stirring, and then 0.5 mmol VC was dissolved into the solution. Finally, 0.075 mmol CTAB and 0.5 mL of NaOH (1 M) were added into the above mixture sequentially. After being vigorously stirred for 10 min, the mixture was put into a Teflon-lined autoclave of 30-mL capacity and maintained at 120 ◦ C for 1 h, then cooled to room temperature naturally. The products were collected, and washed thoroughly with deionized water and absolute ethanol. Finally they were dried in a vacuum oven before further characterization. When 0.075 mmol CTAB was substituted by 0.075 mmol CTAC, whereas other reaction conditions were kept the same, Cu nanowires were obtained. The samples were characterized by XRD on a German Bruker D8 X-ray diffractometer with Ni filtered Cu K␣ radiation ( = 1.5418 Å). TEM and SAED patters were characterized by a JEM-100CXII at an accelerating voltage of 100 kV. HRTEM image was carried out on a JEOL 2100 transmission electron microscope. UV–vis absorption spectra were taken using a Hitachi U-4100 spectrophotometer.
3. Results and discussion The phase and purity of the nanoplates were confirmed by the XRD pattern shown in Fig. 1(a). All the diffraction peaks can be indexed orderly to Cu (1 1 1), (2 0 0), (2 2 0) planes of the face-centered cubic structure of Cu (JCPDS file NO. 65-9026). No characteristic peak from impurity is detected. TEM image
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S. Xu et al. / Materials Chemistry and Physics 120 (2010) 1–5
Fig. 1. XRD patterns of the obtained Cu of (a) nanoplates; (b) nanowires.
shows that the plate-like structures with a diameter of about 800–1200 nm (Fig. 2(a)). Furthermore, Fig. 2(b) shows the HRTEM image of the nanoplate. The HRTEM image reveals parallel fringes with a space of 0.21 nm, which corresponds to the (1 1 1) lat-
tice planes of the face-centered cubic Cu. The SAED patter shown in the inset of Fig. 2(b) confirms that the nanoplate is a single crystal. In order to understand the formation process of the Cu nanoplates, samples obtained at various stages of the growth process were studied by TEM and XRD. Fig. 3(a) gives a TEM image of the samples collected after 20 min of the reaction. Here some platelike structures were formed, but the majority of the products were irregular nanostructures. After 40 min of the reaction, it was found that most of the products were well-defined plates (Fig. 3(b)), but a few irregular nanoparticles still existed. Fig. 3(c) and (d) shows XRD patterns of the products obtained at 120 ◦ C for 20 and 40 min, respectively. From the pattern in Fig. 3(c), it could be seen that CuBr, CuCl and Cu coexisted in the products, after the reaction proceeded for 20 min. The presence of CuBr could attribute to the reaction between CuCl and Br− anion (from CTAB). With a longer reaction time of 40 min, the peak of CuCl disappeared and the intensity of CuBr peaks weakened (Fig. 3(d)). It was found that CTAB played an important role in the formation of the nanoplates. To clearly show the effect of CTAB on the morphology of the Cu nanoparticles, the experiments were carried out at different CTAB amounts and kept the other conditions changeless. In the absence of CTAB (Fig. 4(a)), irregular Cu particles were obtained. As shown in Fig. 4(b), nanoparticles, nanowires and nanoplates were obtained when the amount of CTAB was 0.025 mmol. When the amount of CTAB was increased
Fig. 2. TEM images of (a) Cu nanoplates; (b) HRTEM image of a nanoplate. The inset shows the SAED pattern of an individual nanoplate.
Fig. 3. TEM images of plate-like copper particles obtained at different reaction time: (a) 20 min; (b) 40 min. XRD patterns of the products at reaction time of (c) 20 min and (d) 40 min.
S. Xu et al. / Materials Chemistry and Physics 120 (2010) 1–5
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Fig. 4. TEM images of Cu particles prepared under different amounts of CTAB: (a) 0 mmol; (b) 0.025 mmol; (c) 0.050 mmol; (d) 0.125 mmol.
to 0.050 mmol, little Cu nanowire was obtained in addition to the nanoplates (Fig. 4(c)). If the CTAB amount was further increased to 0.125 mmol, the products were thick nanoplates with a diameter of about 300–600 nm (Fig. 4(d)). The results indicated that the morphology of final products could be controlled by the amount of CTAB. On the basis of above results, it can be concluded that CTAB serves as both Br source and surfactant direct the growth of Cu nanoplates. To further clarify the role of the Br− anion in CTAB, we have conducted the control experiments by substituting CTAB with CTAC under the standard conditions. As shown in Fig. 5(a) nanowires were generated in the presence of CTAC, implying that CTA+ alone is not sufficient for the shape control of nanoplates. Fig. 1(b) shows XRD pattern of the products obtained when CTAC was used as surfactant. All the diffraction peaks in the patterns are indexed to the structure of face-centered cubic Cu (JCPDS file NO. 65-9026). The HRTEM image of an individual nanowire (Fig. 5(b)) reveals parallel fringes with a space of 0.21 nm, which is consistent with the space of (1 1 1) lattice planes, confirming that each nanowire is a [0 1 1]-oriented growth direction. The SAED pattern (inset in Fig. 5(b)) indicates that the copper nanowire is a single crystal.
The time-dependent morphology evolutions of the nanowires were studied by TEM. Fig. 6(a) and (b) shows TEM images of the products obtained at 120 ◦ C for 20 and 40 min, respectively. As shown in Fig. 6(a), a few wire-like particles were obtained after the reaction mixture was heated for 20 min. When the reaction time was prolonged to 40 min, the major products were nanowires with lengths of several micrometers (Fig. 6(b)). Fig. 6(c) is a XRD pattern of the products collected after 20 min of the reaction. Here Cu and CuCl coexisted in the products, indicating CuCl was partially reduced by VC under current conditions. After the reaction proceeded for 40 min, the intensity of CuCl peaks weakened, whereas that of the corresponding ones of Cu intensified since CuCl gradually transformed into Cu (Fig. 6(d)). The above experimental results show that Cu nanoplates and nanowires can be selective synthesized by utilizing different surfactants. For a face-centered cubic metal, the thermodynamically favored shapes are truncated nanocubes and multiple twinned particles. To obtain the shapes other than the thermodynamic ones, the kinetics of nucleation and growth must be carefully controlled [17]. Nanoplates and nanowires are intrinsically higher in energy than the thermodynamically favored shapes, and their formation becomes favorable only in a slow reduction process. It is well known
Fig. 5. TEM images of (a) Cu nanowires and (b) HRTEM image of typical nanowires. The inset shows the SAED pattern of an individual nanowire.
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S. Xu et al. / Materials Chemistry and Physics 120 (2010) 1–5
Fig. 6. TEM images of wire-like copper particles obtained at different reaction time: (a) 20 min and (b) 40 min. XRD patterns of the products at reaction time of (c) 20 min and (d) 40 min.
that cuprous halide has low solubility in aqueous solution and it could not be completely dissolved under current conditions. The free Cu+ was maintained at a low level in the solution and the generation of Cu atom came through a slow reduction process. When the reduction becomes slow enough, kinetic control will take over in both nucleation and growth of Cu nanoparticles. In our system, when CTAB was used as surfactant, the release rate of the free Cu+ in the system was controlled by the newly form CuBr. Due to the less solubility of CuBr (PKsp = 8.28) than CuCl (PKsp = 5.92) in the aqueous solution, the free Cu+ concentration in the solution was decreased. The lower Cu+ concentration may provide favorable condition for the growth of anisotropic plate-like structures in the presence of CTA+ . When CTAB was substituted by CTAC, release rate of the Cu+ from CuCl was beneficial to the formation of the nanowires. If the concentrations of CTAB were low, CuCl cannot transform into CuBr completely. Thus, the release rate of the Cu+ was controlled by both CuCl and CuBr, nanowires and nanoplates coexisted in the products. It is reasonable to explain the presence of nanowires in addition to nanoplates as shown in Fig. 4(b) and
(c). Consequently, the release rate of the free Cu+ in the system and the selective adsorption of CTA+ play critical roles in controlling the morphology of the products. UV–vis spectra of as-synthesized Cu particles were recorded to investigate theirs optical property. As seen from Fig. 7(a), Cu nanoplates have absorption at 599 nm. For the nanowires, the absorption shows a slight blue shift to 588 nm (Fig. 7(b)), which is in good agreement with the reported values for Cu nanowires [8]. Though the reason of blue shift is unclear, the results indicate that the as-prepared Cu nanostructures exhibited morphologydependent optical property. 4. Conclusions In summary, Cu nanoplates and nanowires have been selectively synthesized via surfactant-assisted hydrothermal processes. A series of experiments confirm that the release rate of the free Cu+ in the system and the selective adsorption of CTA+ play critical roles in controlling the morphology of the products. The release rate of the free Cu+ was controlled by CuBr when CTAB was used as surfactant, and Cu nanoplates were obtained. If CTAB was substituted by CTAC, the release rate of the Cu+ from CuCl provided favorable conditions for the growth of nanowires. This synthetic route may be applied to morphology control synthesis of other anisotropic metal nanoparticles. Acknowledgement We greatly acknowledge the financial support of the 973 project of China (No. 2005CB623601). References
Fig. 7. UV–vis spectra of as-synthesized (a) Cu nanowires and (b) Cu nanoplates.
[1] D. Yu, V.W. Yam, J. Am. Chem. Soc. 126 (2004) 13200. [2] L. Qi, J. Ma, J. Shen, J. Colloid Interface Sci. 186 (1997) 498. [3] I. Lisiecki, M. Biorling, L. Motte, B. Ninham, M.P. Pileni, Langmuir 11 (1995) 2385. [4] Y. Zhao, J.J. Zhu, J.M. Hong, N. Bian, H.Y. Chen, Eur. J. Inorg. Chem. 20 (2004) 4072. [5] H.A. Becerril, B.M. Stoltenberg, C.F. Monson, A.T. Woolley, J. Mater. Chem. 14 (2004) 611. [6] H.C. Shin, J. Dong, M. Liu, Adv. Mater. 15 (2003) 1610. [7] M. Zhang, S. Lenhert, M. Wang, L. Chi, N. Lu, H. Fuchs, N. Ming, Adv. Mater. 16 (2004) 409.
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