- Email: [email protected]
Nanoporous Ag prepared from the melt-spun Cu-Ag alloys
Solid State Sciences 13 (2011) 1379e1384
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Nanoporous Ag prepared from the melt-spun Cu-Ag alloys Guijing Li, Xiaoping Song, Zhanbo Sun*, Shengchun Yang, Bingjun Ding, Sen Yang, Zhimao Yang, Fei Wang MOE Key Laboratory for Non-equilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 February 2011 Received in revised form 1 April 2011 Accepted 12 April 2011 Available online 22 April 2011
Nanoporous Ag ribbons with different morphology and porosity were achieved by the electrochemical corrosion of the melt-spun Cu-Ag alloys. The Cu-rich phase in the alloys was removed, resulting in the formation of the nanopores distributed across the whole ribbon. It is found that the structures, morphology and porosity of the nanoporous Ag ribbons were dependent on the microstructures of the parent alloys. The most of ligaments presented a rod-like shape due to the formation of pseudoeutectic microstructure in the melt-spun Cu55Ag45 and Cu70Ag30 alloys. For nanoporous Ag prepared from Cu85Ag15 alloys, the ligaments were camber-like because of the appearance of the divorced microstructures. Especially, a novel bamboo-grove-like structure could be observed at the cross-section of the nanoporous Ag ribbons. The experiment reveals that nanoporous Ag ribbons exhibited excellent enhancement of surface-enhanced Raman scattering (SERS) effect, but a slight difference existed due to the discrepancy of their morphology. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Cu-Ag alloys Melt spinning Electrochemical corrosion Nanoporous silver SERS effect
1. Introduction Because of their large specific surface area, lower density and excellent chemical stability, nanoporous noble metals have extensive potential applications in the fields of catalysts [1e3], optics [4], sensors [5] and medical treatment [6]. A number of nanoporous metals and alloys, such as nanoporous Au [7e11], Au-Pt [12], Pt-Ru [13], Pd [14] and Ag-Pt [15], have been extensively studied. Recently, nanostructured Ag has also attracted much interest due to its excellent SERS effect and catalytic performance [16e19]. Hence, Ag-Al [20e22] and Ag-Zn [23,24] alloys have been utilized as the parent systems to investigate nanoporous Ag by dealloying. Dealloying is widely used to obtain nanoporous noble metals [25e27]. The process is that one or more elements are removed selectively from the parent alloys [28e30], which involves the decomposition of solid solution or intermetallic compounds and the recomposition of residual components. The decomposition process of intermetallic compounds, such as Ag2Al [20], however, is restricted by the alloy compositions, which can have an evident influence on the uniform of nanoporous structures. But as for preparation of some nanoporous noble metals, it is only necessary to remove one or more phases from the parent alloys during dealloying. However, the process and the relationship between the nanoporous structures and the parent microstructures have not been studied in detail.
* Corresponding author. Tel./fax: þ86 29 82665995. E-mail address: [email protected] (Z. Sun). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.04.007
Due to the low solid solubility, the Cu-Ag alloys only consist of separated Ag-rich and Cu-rich phases. Therefore, they can serve as an ideal parent alloy system to prepare nanoporous Ag by dealloying. Morrish et al. [31] have prepared nanoporous Ag by selective oxidation of Cu to Cu2þ from cast eutectic Ag-Cu alloys. Nevertheless, few studies have reported on investigating nanoporous Ag with high porosity and different morphology by chemical or electrochemical dealloying from Ag-Cu alloys. In the present investigation, the nanoporous Ag ribbons with different morphology and porosity were prepared successfully by the electrochemical corrosion of the melt-spun Cu-Ag alloys. The relationship between nanoporous Ag and the microstructures of parent alloys and the formation mechanism of nanoporous structures were discussed. The SERS spectra of Rhodamine 6G adsorbed on the nanoporous Ag ribbons were measured. 2. Experimental Melt-spun Cu55Ag45, Cu70Ag30 and Cu85Ag15 (at.%) alloys were used to prepare nanoporous Ag. The alloys fabricated by pure Ag (99.99%) and pure Cu (99.9%) were melted in a vacuum arc furnace and prepared into thin ribbons by single roller melt spinning. The width and thickness of ribbons were approximately 2 mm and 30 mm, respectively. The surface and cross section of the ribbons were polished, and the microstructures were characterized by a JEOL JSM-7000 F microscope (SEM) equipped with a backscattered electron detector. The Cu-Ag ribbons were corroded by the electrochemical method on a CHI 1130 A potentiostat at room
1380
G. Li et al. / Solid State Sciences 13 (2011) 1379e1384
Fig. 1. Backscattered electron images of the melt-spun Cu-Ag alloys. (a), (b) and (c) Plan-view of Cu55Ag45, Cu70Ag30 and Cu85Ag15, respectively; (d) section-view of Cu85Ag15.
temperature, and the setting potential value was 0.05 V. The 0.5 M CuSO4 aqueous solution was used as the electrolyte. The Cu-Ag ribbons served as the anode while a silver rod the cathode. The corroded Cu-Ag ribbons were cleaned by diluted hydrochloric acid and absolute ethyl alcohol.
The morphology and the phase structures of the corroded ribbons were characterized by the SEM, a JEM-2100 high-resolution transmission electron microscope (HTEM), and a Bruker D8 advanced X-ray diffraction (XRD), respectively. The corroded ribbons were broken off and the morphology of the cross sections
Fig. 2. Plane-view SEM images of the nanoporous Ag prepared by the electrochemical corrosion of the melt-spun (a) Cu55Ag45, (b) Cu70Ag30, (c) Cu85Ag15 alloys for 24 h, and (d) XRD patterns of the corroded Cu85Ag15 and Cu70Ag30 alloy.
G. Li et al. / Solid State Sciences 13 (2011) 1379e1384 Table 1 The EDX results of the corroded melt-spun Cu-Ag alloys by electrochemical method for 24 h. Samples
Cu (at.) %
Ag (at.) %
Ag45Cu55 Ag30Cu70 Ag15Cu85
4.17 1.50 0.00
95.83 98.50 100
were observed. A Laboratory Ram JY-HR800 spectrometer was employed to characterize the SERS effect. The spectra obtained by immersing the nanoporous Ag ribbons in the 0.1 mM Rhodamine 6G solution for 20 min were measured at the operating wavelength of 632.8 nm, the output laser power 17 mw/100 (D2 filter) and the exposure time 1 s. 3. Results and discussion Fig. 1 shows the microstructures of the melt-spun Cu-Ag alloys. The dark phase was Cu-rich, and the bright was Ag-rich. The microstructures were very fine due to the melt spinning. It can be seen from Fig. 1a and b that the Cu55Ag45 and Cu70Ag30 alloys were
1381
composed of pseudoeutectic and divorced eutectic microstructures. Where, a portion of the Ag-rich phase had a long rod-like or granular shape, and some rods were parallel to the ribbon plane while some were perpendicular. The microstructure of Cu70Ag30 alloy was similar to that of Cu55Ag45, but more short rod-like shapes can be observed. As shown in Fig. 1c, a typical divorced eutectic microstructure appeared in the melt-spun Cu85Ag15 alloy. The Ag-rich phase with a camber-like or short rod-like shape was distributed along the Cu-rich grain boundaries. The cross-section of the Cu85Ag15 alloy, as shown in Fig. 1d, mainly presented a typical column structure, which caused the Ag-rich phase in the form of a rod-like shape perpendicular to the specimen plane. The cross-section size of the column Cu-rich grains was less than 1 mm, and the Ag-rich phase was very fine. It is noticed that some Ag-rich rods, as marked by the arrow, had some branches. The above results indicate that the Cu-Ag alloys with different microstructures have been obtained. Fig. 2 shows the surface morphology of the nanoporous Ag ribbons by the electrochemical corrosion of the melt-spun Cu-Ag alloys and their XRD patterns. As can be seen from Fig. 2a, a large number of pores with an average size about w80 nm can be observed from the corroded Cu55Ag45 alloy. The ligaments in the
Fig. 3. Section-view SEM images showing the corroded Cu-Ag alloys. (a) Cu55Ag45, (b) Cu70Ag30, (c) Cu85Ag15; (d), (e) and (f) are the magnifying images of (a), (b) and (c), respectively.
1382
G. Li et al. / Solid State Sciences 13 (2011) 1379e1384
porous structure were composed of two kinds of morphology. Some ligaments were with a long rod-like or plate-like shape parallel to each other, while some presented a short rod-like shape. Besides, many linear channels with a width of about 60 nm were observed clearly. It is suggests that they arose from the corroded grain boundaries in the parent alloy. As the Cu content was increased to 70%, the corroded Cu-Ag alloy exhibited a porous structure with a linked pore channel and a higher porosity. Most of ligaments had a short rod-like or particle-like shape and the corroded grain boundaries became fewer (Fig. 2b). From Fig. 2c, it can be observed that the morphology of the corroded Cu85Ag15 alloy was characterized by an open nanoporous structure. All the ligaments exhibited a camber-like or short rod-like shape. The parallel ligaments with a long-rod shape and the corroded grain boundaries, as shown in Fig. 2a, cannot be observed. Fig. 2d indicates the phase structures of the corroded Cu-Ag alloys. It is clear that only fcc-Ag diffraction peaks appeared. Table 1 shows the EDX results of the corroded Cu-Ag alloys. The ribbon was close to pure Ag after the Ag15Cu85 was corroded. However, a few Cu atoms were residual with an increase of the Ag content in the parent alloys under the same corrosion procedure, which indicates that the Cu-Ag alloys could be corroded more easily as the increase of Cu content. From these results, it can be concluded that nanoporous Ag ribbons with different morphology have been achieved successfully by the electrochemical corrosion of the melt-spun Cu-Ag alloys. Fig. 3 illustrates the cross-sectional view of the nanoporous Ag ribbons. As shown in Fig. 3aec, the nanopores and ligaments were distributed across the entire ribbons. The ligaments of nanoporous Ag prepared from the Cu55Ag45 and Cu70Ag30 alloy, as shown in Fig. 3d and e, were composed of a rod-like and sheet-like shape, which was similar to their surface morphology. But the corroded
Fig. 4. HARTEM images of the corroded Cu70Ag30 alloy.
grain boundaries which appeared in surface (Fig. 2a) could not be observed. Fig. 3f shows the morphology of nanoporous Ag prepared from the Cu85Ag15 ribbons. The pore size was slightly larger than that from Cu70Ag30. It is noticed that an interesting morphology presented in the cross-section of the nanoporous Ag ribbons from the Cu85Ag15 ribbons. Where, the straight Ag stocks were interconnected by branches, forming an oriented nanoporous structure. This morphology was described as bamboo-grove-like Ag nanoporous structure in the present work. It also suggests that nanoporous Ag prepared from the Cu85Ag15 ribbons had higher porosity than from Cu55Ag45 and Cu70Ag30. Fig. 4 shows the TEM and HRTEM images of nanoporous Ag prepared from Cu70Ag30 ribbons. As shown in Fig. 4a, the pores in the nanoporous Ag ribbons were inter-connected. The rod-like ligaments were intercommunicated and mutually overlapped. The Ag ligaments had a ladder-like character on the rough surface in the atomic scale, as shown in Fig. 4b. Fig. 5 shows the SERS spectra of R6G deposited in the corroded Cu55Ag45, Cu70Ag30 and Cu85Ag15 alloys. The spectrum of the corroded Cu40Ag60 alloy was also measured for the sake of contrastive analysis. The Raman intension was enhanced at the
Fig. 5. SERS spectra of the corroded Cu-Ag alloys (a) and the intensity of the first peaks (b). a, b, c, and d indicate the effect of nonporous Ag prepared from the melt-spun Cu85Ag15, Cu70Ag30, Cu55Ag45 and Cu40Ag60, respectively.
G. Li et al. / Solid State Sciences 13 (2011) 1379e1384
bands of 613, 773, 1186, 1364, 1515 and 1651 cm1, and the SERS signals increase with the increase of the Cu contents in the parent alloys. It suggests that the nanoporous Ag prepared from Cu-Ag alloys had excellent SERS effect. The nanoporous Ag ribbons with different morphology and porosity have been achieved from the melt-spun Cu-Ag alloys. According to different standard potentials between Cu phase (Cu/ Cu2þ ¼ 0.34 V) and Ag phase (Ag/Agþ ¼ 0.80 V), the Cu-rich phase can be removed from the Cu-Ag alloy by electrochemical corrosion, and the residual Ag will form a nanoporous structure subsequently. Generally, the formation of noble nanoporous structure involves the decomposition of intermetallic compounds or solid solutions from the parent alloys [32,33]. However, in the present work, the Ag-rich phase evaluated ligaments and formed the nanoporous Ag after the Cu-rich phase was removed by electrochemical corrosion. A supersaturated solid solution of Ag in Cu appeared due to the melt spinning. The Ag released from the Cu-based solid solution would cause a slight growth of the primary Ag-rich phase. This means that a dealloying is included in the formation of the nanoporous Ag. However, Compared Figs. 2 and 3 with Fig. 1, it is found that the morphology of nanoporous structures could be observed in the parent alloys, which means that the characters of the Ag-rich phase remained after the Cu-rich phase was removed. Although the Ag phase might dissolve more Cu atoms compared to the equilibrium state, the effect on the morphology of the nanoporous structures was less. Therefore, a name of selective phase corrosion is preferred to describe the formation of nanoporous Ag in the present work. Because of the selective phase corrosion, the dependence of nanoporous Ag on the parent microstructures is presented. As the parent alloys were composed of pseudoeutectic and divorced eutectic microstructures, the ligaments in nanoporous Ag showed the inherited morphology (Fig. 2a and b) from the Ag-rich phase with the long rod-like or granular shape (Fig. 1a and b). The morphology of the Ag-rich phase in pseudoeutectic microstructure was slightly different due to the variation of alloy compositions, resulting in the ligaments with different shapes, such as sheet, long rod and particle sometimes. During the formation of the nanoporous structure, the Ag phase connected each other and built up the ligaments with a step-like character in the atomic scale (Fig. 4). In addition, due to the melt spinning, the fine microstructures of the Cu-Ag alloys could provide more grain boundaries, which contributed to the formation of nanopores being distributed across the entire ribbon (Fig. 3aec). Nanoporous Ag prepared from the Cu70Ag30 alloy had a higher porosity than that from Cu55Ag45 as more Cu was removed from the parent alloys. As the Cu content was increased to 85%, more short rod-like Ag ligaments appeared in the surface of the nanoporous Ag ribbons due to the formation of the divorced eutectic microstructure. It is noticed that the rod-like Ag-rich phase spread along the Cu-rich grains in the parent microstructure (Fig. 1c and d). After the Cu-rich phase was removed, the Ag phase evolved into the skeleton of the nanoporous structure accompanying volume shrinkage (Figs. 1b and 2c). The furcated Ag phase spread along the column Cu-rich grains in the cross section (Fig. 1d) and became the stocks and branches of the ligaments, which resulted in the formation of the continuous bamboo grove morphology with higher porosity (Fig. 3c and f). In addition, the corroded grain boundaries, as shown in Fig. 2a, could not be observed because the Cu grain boundaries were almost occupied by the Ag-rich phase. These results suggest that the morphology and porosity of nanoporous Ag could be controlled by a reasonable design and adoption of preparation for different applications. The rough surface and morphology of nanoporous metals play an important role in the SERS signal enhancement [34,35]. The
1383
nanoporous Ag ribbons prepared from the melt-spun Cu-Ag alloys had a strong SERS effect due to the adsorption of a bicontinuous pore structure, rough surface and step structure (Fig. 4). With the increase of Cu in the parent alloys, the formed nanoporous Ag had a higher porosity (Fig. 3), which could provide more short-distance hot-spots that contributed to the enhancement of surfaceenhanced Raman scattering [36]. Therefore, the SERS signals from the corroded Cu-Ag alloys with high Cu contents (Fig. 5) could get a much higher enhancement. The corroded Cu40Ag60 alloy did not form an interlinking porous structure with high porosity due to the lower Cu content, which resulted in the weak SERS effect. 4. Conclusions Nanoporous Ag ribbons with different morphology and porosity were prepared successfully by the selective phase corrosion of the melt-spun Cu-Ag alloys. Both the pore channels and ligaments had the size of less than 100 nm. The morphology and porosity of nanoporous Ag were strongly dependent on the microstructures of the parent alloys. The cross-section of nanoporous Ag showed the novel bamboo-grove-like morphology due to the columnar structure of the melt-spun Cu85Ag15 alloy. The nanoporous Ag ribbons exhibited excellent SERS effect. These results indicate that the electrochemical corrosion of the melt-spun Cu-Ag alloys is an effective and controllable method to prepare nanoporous Ag. Acknowledgements This work is sponsored by National Basic Research Program of China (2010CB635101), the National Science Foundation of China (51071116, 50901056, 50871081, 50871080, 51071117, 51002117), and the National 863 Program Projects of China (2009AA03Z320). References [1] S. Polarz, B.J. Smarsly, Nanosci. Nanotechno. 6 (2002) 581e612. [2] C.X. Xu, J.X. Su, X.H. Xu, P.P. Liu, H.J. Zhao, F. Tian, Y. Ding, J. Am. Chem. Soc. 129 (2007) 42e43. [3] L.F. Liu, E. Pippel, R. Scholz, U. Gösele, Nano Lett 9 (2009) 4352e4358. [4] L.H. Qian, W. Shen, B. Das, B. Shen, G.W. Qin, Chem. Phys. Lett. 479 (2009) 259e263. [5] A.K.M. Kafi, A. Ahmadalinezhad, J.P. Wang, D.F. Thomas, A. Chen, Biosens. Bioelectron. 25 (2010) 2458e2463. [6] A. Pujia, D.F. Angelis, D. Scumaci, M. Gaspari, C. Liberale, P. Candeloro, G. Cuda, E.D. Fabrizio, Mater. Sci. Eng. B. 169 (2010) 111e113. [7] M. Kim, W.J. Ha, J.W. Anh, H.S. Kim, S.W. Park, D.Y. Lee, J. Alloys Compound 484 (2009) 28e32. [8] J. Biener, A.M. Hodge, J.R. Hayes, C.A. Volkert, L.A. Zepeda-Ruiz, A.V. Hamza, F. Farid, Nano Lett. 6 (2006) 2379e2382. [9] M. Hakamada, M. Mabuchi, Mater. Lett. 62 (2008) 483e486. [10] Z.N. Liu, L.H. Huang, L.L. Zhang, H.Y. Ma, Y. Ding, Electrochim. Acta 54 (2009) 7286e7293. [11] A.M. Hodge, J.R. Hayes, J.A. Caro, J. Biener, A.V. Hamza, Adv. Eng. Mater. 8 (2006) 853e856. [12] C.X. Xu, R.Y. Wang, M.W. Chen, Y. Zhang, Y. Ding, Phys. Chem. Chem. Phys. 12 (2010) 239e246. [13] C.X. Xu, L. Wang, X.L. Mu, Y. Ding, Langmuir 26 (2010) 7437e7443. [14] M. Hakamada, M. Mabuchi, J. Alloys Compound 479 (2009) 326e329. [15] J.N. Gao, X.L. Ren, F.Q. Tang, Photograph. Sci. Photochem. 24 (2006) 360e366. [16] H.H. Zhou, D.Y. Wu, J.Q. Hu, T.Q. Lian, Zh. Q. Tian, J. Chin, Light Scatt. 16 (2004) 221e224. [17] X.Zh. Sun, L.H. Lin, Zh.C. Li, Zh.J. Zhang, J.Y. Feng, Mater. Lett. 63 (2009) 2306e2308. [18] C. Shi, M.J. Cheng, Z.P. Qu, X.H. Bao, Appl. Catal. B, Environmental 51 (2004) 171e181. [19] K. Brandt, M.E. Chiu, D.J. Watson, M.S. Tikhov, R.M. Lambert, J. Am. Chem. Soc. 131 (2009) 17286e17290. [20] X.G. Wang, Zh. Qi, C.C. Zhao, W.M. Wang, Z.H. Zhang, J. Phys. Chem. C 113 (2009) 13139e13150. [21] C.X. Xu, Y.Y. Li, F. Tian, Y. Ding, ChemPhysChem 11 (2010) 3320e3328. [22] Z.H. Zhang, Y. Wang, Z. Qi, W.H. Zhang, J.Y. Qin, J. Frenzel, J. Phys. Chem. C 113 (2009) 12629e12636. [23] Z.Q. Li, B.Q. Li, Z.X. Qin, X. Lu, J. Mater. Sci. 45 (2010) 6494e6497. [24] F.H. Yeh, C.C. Tai, J.F. Huang, I.W. Sun, J. Phys. Chem. B. 110 (2006) 5215e5222.
1384
G. Li et al. / Solid State Sciences 13 (2011) 1379e1384
[25] J. Erlebacher, M.J. Aziz, A. Karma, N. Dimitrov, K. Sieradzki, Nature 410 (2001) 450e453. [26] J.C. Thorp, K. Sieradzki, L. Tang, P.A. Crozier, M. Nastasi, D. Mitlin, S.T. Picraux, Appl. Phys. Lett. 88 (2006) 557. [27] X.L. Tan, Y.J. Tang, Y. Liu, J.S. Luo, K. Li, X.B. Liu, Mate. Rev. 23 (2009) 68e71. [28] X. Lu, E. Bischoff, R. Spolenak, T.J. Balk, Scripta Mater. 56 (2007) 557e560. [29] Zh.H. Zhang, Y. Wang, Y.Z. Wang, X.G. Wang, Zh. Qi, H. Ji, Ch.C. Zhao, Scripta Mater. 62 (2010) 137e140. [30] S. Parida, D. Kramer, C.A. Volkert, H. Rösner, J. Erlebacher, J. Weissmüller, PRL 97 (2006) 1e4.
[31] R. Morrish, A.J. Muscat, Chem. Mater. 21 (2009) 3865e3870. [32] Q. Zhang, X.G. Wang, Zh. Qi, Y. Wang, Zh.H. Zhang, Electrochim. Acta 54 (2009) 6190e6198. [33] H.B. Lu, Y. Li, F.H. Wang, Corros. Sci. 48 (2006) 2106e2119. [34] L.W. Guo, X.T. Zhang, Z.L. Du, Y.B. Huang, Y.J. Mo, Spectrosc. Spectr. Anal. 21 (2001) 16e18. [35] V.S. Tiwari, T. Oleg, G.K. Darbha, W. Hardy, J.P. Singh, P.C. Ray, Chem. Phys. Lett. 446 (2007) 77e82. [36] S.Y. Lee, L. Hung, G.S. Lang, J.E. Cornett, I.D. Mayergoyz, O. Rabin, ACS Nano 4 (2010) 5763e5772.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Nanoporous Ag prepared from the melt-spun Cu-Ag alloys Guijing Li, Xiaoping Song, Zhanbo Sun*, Shengchun Yang, Bingjun Ding, Sen Yang, Zhimao Yang, Fei Wang MOE Key Laboratory for Non-equilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 February 2011 Received in revised form 1 April 2011 Accepted 12 April 2011 Available online 22 April 2011
Nanoporous Ag ribbons with different morphology and porosity were achieved by the electrochemical corrosion of the melt-spun Cu-Ag alloys. The Cu-rich phase in the alloys was removed, resulting in the formation of the nanopores distributed across the whole ribbon. It is found that the structures, morphology and porosity of the nanoporous Ag ribbons were dependent on the microstructures of the parent alloys. The most of ligaments presented a rod-like shape due to the formation of pseudoeutectic microstructure in the melt-spun Cu55Ag45 and Cu70Ag30 alloys. For nanoporous Ag prepared from Cu85Ag15 alloys, the ligaments were camber-like because of the appearance of the divorced microstructures. Especially, a novel bamboo-grove-like structure could be observed at the cross-section of the nanoporous Ag ribbons. The experiment reveals that nanoporous Ag ribbons exhibited excellent enhancement of surface-enhanced Raman scattering (SERS) effect, but a slight difference existed due to the discrepancy of their morphology. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Cu-Ag alloys Melt spinning Electrochemical corrosion Nanoporous silver SERS effect
1. Introduction Because of their large specific surface area, lower density and excellent chemical stability, nanoporous noble metals have extensive potential applications in the fields of catalysts [1e3], optics [4], sensors [5] and medical treatment [6]. A number of nanoporous metals and alloys, such as nanoporous Au [7e11], Au-Pt [12], Pt-Ru [13], Pd [14] and Ag-Pt [15], have been extensively studied. Recently, nanostructured Ag has also attracted much interest due to its excellent SERS effect and catalytic performance [16e19]. Hence, Ag-Al [20e22] and Ag-Zn [23,24] alloys have been utilized as the parent systems to investigate nanoporous Ag by dealloying. Dealloying is widely used to obtain nanoporous noble metals [25e27]. The process is that one or more elements are removed selectively from the parent alloys [28e30], which involves the decomposition of solid solution or intermetallic compounds and the recomposition of residual components. The decomposition process of intermetallic compounds, such as Ag2Al [20], however, is restricted by the alloy compositions, which can have an evident influence on the uniform of nanoporous structures. But as for preparation of some nanoporous noble metals, it is only necessary to remove one or more phases from the parent alloys during dealloying. However, the process and the relationship between the nanoporous structures and the parent microstructures have not been studied in detail.
* Corresponding author. Tel./fax: þ86 29 82665995. E-mail address: [email protected] (Z. Sun). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.04.007
Due to the low solid solubility, the Cu-Ag alloys only consist of separated Ag-rich and Cu-rich phases. Therefore, they can serve as an ideal parent alloy system to prepare nanoporous Ag by dealloying. Morrish et al. [31] have prepared nanoporous Ag by selective oxidation of Cu to Cu2þ from cast eutectic Ag-Cu alloys. Nevertheless, few studies have reported on investigating nanoporous Ag with high porosity and different morphology by chemical or electrochemical dealloying from Ag-Cu alloys. In the present investigation, the nanoporous Ag ribbons with different morphology and porosity were prepared successfully by the electrochemical corrosion of the melt-spun Cu-Ag alloys. The relationship between nanoporous Ag and the microstructures of parent alloys and the formation mechanism of nanoporous structures were discussed. The SERS spectra of Rhodamine 6G adsorbed on the nanoporous Ag ribbons were measured. 2. Experimental Melt-spun Cu55Ag45, Cu70Ag30 and Cu85Ag15 (at.%) alloys were used to prepare nanoporous Ag. The alloys fabricated by pure Ag (99.99%) and pure Cu (99.9%) were melted in a vacuum arc furnace and prepared into thin ribbons by single roller melt spinning. The width and thickness of ribbons were approximately 2 mm and 30 mm, respectively. The surface and cross section of the ribbons were polished, and the microstructures were characterized by a JEOL JSM-7000 F microscope (SEM) equipped with a backscattered electron detector. The Cu-Ag ribbons were corroded by the electrochemical method on a CHI 1130 A potentiostat at room
1380
G. Li et al. / Solid State Sciences 13 (2011) 1379e1384
Fig. 1. Backscattered electron images of the melt-spun Cu-Ag alloys. (a), (b) and (c) Plan-view of Cu55Ag45, Cu70Ag30 and Cu85Ag15, respectively; (d) section-view of Cu85Ag15.
temperature, and the setting potential value was 0.05 V. The 0.5 M CuSO4 aqueous solution was used as the electrolyte. The Cu-Ag ribbons served as the anode while a silver rod the cathode. The corroded Cu-Ag ribbons were cleaned by diluted hydrochloric acid and absolute ethyl alcohol.
The morphology and the phase structures of the corroded ribbons were characterized by the SEM, a JEM-2100 high-resolution transmission electron microscope (HTEM), and a Bruker D8 advanced X-ray diffraction (XRD), respectively. The corroded ribbons were broken off and the morphology of the cross sections
Fig. 2. Plane-view SEM images of the nanoporous Ag prepared by the electrochemical corrosion of the melt-spun (a) Cu55Ag45, (b) Cu70Ag30, (c) Cu85Ag15 alloys for 24 h, and (d) XRD patterns of the corroded Cu85Ag15 and Cu70Ag30 alloy.
G. Li et al. / Solid State Sciences 13 (2011) 1379e1384 Table 1 The EDX results of the corroded melt-spun Cu-Ag alloys by electrochemical method for 24 h. Samples
Cu (at.) %
Ag (at.) %
Ag45Cu55 Ag30Cu70 Ag15Cu85
4.17 1.50 0.00
95.83 98.50 100
were observed. A Laboratory Ram JY-HR800 spectrometer was employed to characterize the SERS effect. The spectra obtained by immersing the nanoporous Ag ribbons in the 0.1 mM Rhodamine 6G solution for 20 min were measured at the operating wavelength of 632.8 nm, the output laser power 17 mw/100 (D2 filter) and the exposure time 1 s. 3. Results and discussion Fig. 1 shows the microstructures of the melt-spun Cu-Ag alloys. The dark phase was Cu-rich, and the bright was Ag-rich. The microstructures were very fine due to the melt spinning. It can be seen from Fig. 1a and b that the Cu55Ag45 and Cu70Ag30 alloys were
1381
composed of pseudoeutectic and divorced eutectic microstructures. Where, a portion of the Ag-rich phase had a long rod-like or granular shape, and some rods were parallel to the ribbon plane while some were perpendicular. The microstructure of Cu70Ag30 alloy was similar to that of Cu55Ag45, but more short rod-like shapes can be observed. As shown in Fig. 1c, a typical divorced eutectic microstructure appeared in the melt-spun Cu85Ag15 alloy. The Ag-rich phase with a camber-like or short rod-like shape was distributed along the Cu-rich grain boundaries. The cross-section of the Cu85Ag15 alloy, as shown in Fig. 1d, mainly presented a typical column structure, which caused the Ag-rich phase in the form of a rod-like shape perpendicular to the specimen plane. The cross-section size of the column Cu-rich grains was less than 1 mm, and the Ag-rich phase was very fine. It is noticed that some Ag-rich rods, as marked by the arrow, had some branches. The above results indicate that the Cu-Ag alloys with different microstructures have been obtained. Fig. 2 shows the surface morphology of the nanoporous Ag ribbons by the electrochemical corrosion of the melt-spun Cu-Ag alloys and their XRD patterns. As can be seen from Fig. 2a, a large number of pores with an average size about w80 nm can be observed from the corroded Cu55Ag45 alloy. The ligaments in the
Fig. 3. Section-view SEM images showing the corroded Cu-Ag alloys. (a) Cu55Ag45, (b) Cu70Ag30, (c) Cu85Ag15; (d), (e) and (f) are the magnifying images of (a), (b) and (c), respectively.
1382
G. Li et al. / Solid State Sciences 13 (2011) 1379e1384
porous structure were composed of two kinds of morphology. Some ligaments were with a long rod-like or plate-like shape parallel to each other, while some presented a short rod-like shape. Besides, many linear channels with a width of about 60 nm were observed clearly. It is suggests that they arose from the corroded grain boundaries in the parent alloy. As the Cu content was increased to 70%, the corroded Cu-Ag alloy exhibited a porous structure with a linked pore channel and a higher porosity. Most of ligaments had a short rod-like or particle-like shape and the corroded grain boundaries became fewer (Fig. 2b). From Fig. 2c, it can be observed that the morphology of the corroded Cu85Ag15 alloy was characterized by an open nanoporous structure. All the ligaments exhibited a camber-like or short rod-like shape. The parallel ligaments with a long-rod shape and the corroded grain boundaries, as shown in Fig. 2a, cannot be observed. Fig. 2d indicates the phase structures of the corroded Cu-Ag alloys. It is clear that only fcc-Ag diffraction peaks appeared. Table 1 shows the EDX results of the corroded Cu-Ag alloys. The ribbon was close to pure Ag after the Ag15Cu85 was corroded. However, a few Cu atoms were residual with an increase of the Ag content in the parent alloys under the same corrosion procedure, which indicates that the Cu-Ag alloys could be corroded more easily as the increase of Cu content. From these results, it can be concluded that nanoporous Ag ribbons with different morphology have been achieved successfully by the electrochemical corrosion of the melt-spun Cu-Ag alloys. Fig. 3 illustrates the cross-sectional view of the nanoporous Ag ribbons. As shown in Fig. 3aec, the nanopores and ligaments were distributed across the entire ribbons. The ligaments of nanoporous Ag prepared from the Cu55Ag45 and Cu70Ag30 alloy, as shown in Fig. 3d and e, were composed of a rod-like and sheet-like shape, which was similar to their surface morphology. But the corroded
Fig. 4. HARTEM images of the corroded Cu70Ag30 alloy.
grain boundaries which appeared in surface (Fig. 2a) could not be observed. Fig. 3f shows the morphology of nanoporous Ag prepared from the Cu85Ag15 ribbons. The pore size was slightly larger than that from Cu70Ag30. It is noticed that an interesting morphology presented in the cross-section of the nanoporous Ag ribbons from the Cu85Ag15 ribbons. Where, the straight Ag stocks were interconnected by branches, forming an oriented nanoporous structure. This morphology was described as bamboo-grove-like Ag nanoporous structure in the present work. It also suggests that nanoporous Ag prepared from the Cu85Ag15 ribbons had higher porosity than from Cu55Ag45 and Cu70Ag30. Fig. 4 shows the TEM and HRTEM images of nanoporous Ag prepared from Cu70Ag30 ribbons. As shown in Fig. 4a, the pores in the nanoporous Ag ribbons were inter-connected. The rod-like ligaments were intercommunicated and mutually overlapped. The Ag ligaments had a ladder-like character on the rough surface in the atomic scale, as shown in Fig. 4b. Fig. 5 shows the SERS spectra of R6G deposited in the corroded Cu55Ag45, Cu70Ag30 and Cu85Ag15 alloys. The spectrum of the corroded Cu40Ag60 alloy was also measured for the sake of contrastive analysis. The Raman intension was enhanced at the
Fig. 5. SERS spectra of the corroded Cu-Ag alloys (a) and the intensity of the first peaks (b). a, b, c, and d indicate the effect of nonporous Ag prepared from the melt-spun Cu85Ag15, Cu70Ag30, Cu55Ag45 and Cu40Ag60, respectively.
G. Li et al. / Solid State Sciences 13 (2011) 1379e1384
bands of 613, 773, 1186, 1364, 1515 and 1651 cm1, and the SERS signals increase with the increase of the Cu contents in the parent alloys. It suggests that the nanoporous Ag prepared from Cu-Ag alloys had excellent SERS effect. The nanoporous Ag ribbons with different morphology and porosity have been achieved from the melt-spun Cu-Ag alloys. According to different standard potentials between Cu phase (Cu/ Cu2þ ¼ 0.34 V) and Ag phase (Ag/Agþ ¼ 0.80 V), the Cu-rich phase can be removed from the Cu-Ag alloy by electrochemical corrosion, and the residual Ag will form a nanoporous structure subsequently. Generally, the formation of noble nanoporous structure involves the decomposition of intermetallic compounds or solid solutions from the parent alloys [32,33]. However, in the present work, the Ag-rich phase evaluated ligaments and formed the nanoporous Ag after the Cu-rich phase was removed by electrochemical corrosion. A supersaturated solid solution of Ag in Cu appeared due to the melt spinning. The Ag released from the Cu-based solid solution would cause a slight growth of the primary Ag-rich phase. This means that a dealloying is included in the formation of the nanoporous Ag. However, Compared Figs. 2 and 3 with Fig. 1, it is found that the morphology of nanoporous structures could be observed in the parent alloys, which means that the characters of the Ag-rich phase remained after the Cu-rich phase was removed. Although the Ag phase might dissolve more Cu atoms compared to the equilibrium state, the effect on the morphology of the nanoporous structures was less. Therefore, a name of selective phase corrosion is preferred to describe the formation of nanoporous Ag in the present work. Because of the selective phase corrosion, the dependence of nanoporous Ag on the parent microstructures is presented. As the parent alloys were composed of pseudoeutectic and divorced eutectic microstructures, the ligaments in nanoporous Ag showed the inherited morphology (Fig. 2a and b) from the Ag-rich phase with the long rod-like or granular shape (Fig. 1a and b). The morphology of the Ag-rich phase in pseudoeutectic microstructure was slightly different due to the variation of alloy compositions, resulting in the ligaments with different shapes, such as sheet, long rod and particle sometimes. During the formation of the nanoporous structure, the Ag phase connected each other and built up the ligaments with a step-like character in the atomic scale (Fig. 4). In addition, due to the melt spinning, the fine microstructures of the Cu-Ag alloys could provide more grain boundaries, which contributed to the formation of nanopores being distributed across the entire ribbon (Fig. 3aec). Nanoporous Ag prepared from the Cu70Ag30 alloy had a higher porosity than that from Cu55Ag45 as more Cu was removed from the parent alloys. As the Cu content was increased to 85%, more short rod-like Ag ligaments appeared in the surface of the nanoporous Ag ribbons due to the formation of the divorced eutectic microstructure. It is noticed that the rod-like Ag-rich phase spread along the Cu-rich grains in the parent microstructure (Fig. 1c and d). After the Cu-rich phase was removed, the Ag phase evolved into the skeleton of the nanoporous structure accompanying volume shrinkage (Figs. 1b and 2c). The furcated Ag phase spread along the column Cu-rich grains in the cross section (Fig. 1d) and became the stocks and branches of the ligaments, which resulted in the formation of the continuous bamboo grove morphology with higher porosity (Fig. 3c and f). In addition, the corroded grain boundaries, as shown in Fig. 2a, could not be observed because the Cu grain boundaries were almost occupied by the Ag-rich phase. These results suggest that the morphology and porosity of nanoporous Ag could be controlled by a reasonable design and adoption of preparation for different applications. The rough surface and morphology of nanoporous metals play an important role in the SERS signal enhancement [34,35]. The
1383
nanoporous Ag ribbons prepared from the melt-spun Cu-Ag alloys had a strong SERS effect due to the adsorption of a bicontinuous pore structure, rough surface and step structure (Fig. 4). With the increase of Cu in the parent alloys, the formed nanoporous Ag had a higher porosity (Fig. 3), which could provide more short-distance hot-spots that contributed to the enhancement of surfaceenhanced Raman scattering [36]. Therefore, the SERS signals from the corroded Cu-Ag alloys with high Cu contents (Fig. 5) could get a much higher enhancement. The corroded Cu40Ag60 alloy did not form an interlinking porous structure with high porosity due to the lower Cu content, which resulted in the weak SERS effect. 4. Conclusions Nanoporous Ag ribbons with different morphology and porosity were prepared successfully by the selective phase corrosion of the melt-spun Cu-Ag alloys. Both the pore channels and ligaments had the size of less than 100 nm. The morphology and porosity of nanoporous Ag were strongly dependent on the microstructures of the parent alloys. The cross-section of nanoporous Ag showed the novel bamboo-grove-like morphology due to the columnar structure of the melt-spun Cu85Ag15 alloy. The nanoporous Ag ribbons exhibited excellent SERS effect. These results indicate that the electrochemical corrosion of the melt-spun Cu-Ag alloys is an effective and controllable method to prepare nanoporous Ag. Acknowledgements This work is sponsored by National Basic Research Program of China (2010CB635101), the National Science Foundation of China (51071116, 50901056, 50871081, 50871080, 51071117, 51002117), and the National 863 Program Projects of China (2009AA03Z320). References [1] S. Polarz, B.J. Smarsly, Nanosci. Nanotechno. 6 (2002) 581e612. [2] C.X. Xu, J.X. Su, X.H. Xu, P.P. Liu, H.J. Zhao, F. Tian, Y. Ding, J. Am. Chem. Soc. 129 (2007) 42e43. [3] L.F. Liu, E. Pippel, R. Scholz, U. Gösele, Nano Lett 9 (2009) 4352e4358. [4] L.H. Qian, W. Shen, B. Das, B. Shen, G.W. Qin, Chem. Phys. Lett. 479 (2009) 259e263. [5] A.K.M. Kafi, A. Ahmadalinezhad, J.P. Wang, D.F. Thomas, A. Chen, Biosens. Bioelectron. 25 (2010) 2458e2463. [6] A. Pujia, D.F. Angelis, D. Scumaci, M. Gaspari, C. Liberale, P. Candeloro, G. Cuda, E.D. Fabrizio, Mater. Sci. Eng. B. 169 (2010) 111e113. [7] M. Kim, W.J. Ha, J.W. Anh, H.S. Kim, S.W. Park, D.Y. Lee, J. Alloys Compound 484 (2009) 28e32. [8] J. Biener, A.M. Hodge, J.R. Hayes, C.A. Volkert, L.A. Zepeda-Ruiz, A.V. Hamza, F. Farid, Nano Lett. 6 (2006) 2379e2382. [9] M. Hakamada, M. Mabuchi, Mater. Lett. 62 (2008) 483e486. [10] Z.N. Liu, L.H. Huang, L.L. Zhang, H.Y. Ma, Y. Ding, Electrochim. Acta 54 (2009) 7286e7293. [11] A.M. Hodge, J.R. Hayes, J.A. Caro, J. Biener, A.V. Hamza, Adv. Eng. Mater. 8 (2006) 853e856. [12] C.X. Xu, R.Y. Wang, M.W. Chen, Y. Zhang, Y. Ding, Phys. Chem. Chem. Phys. 12 (2010) 239e246. [13] C.X. Xu, L. Wang, X.L. Mu, Y. Ding, Langmuir 26 (2010) 7437e7443. [14] M. Hakamada, M. Mabuchi, J. Alloys Compound 479 (2009) 326e329. [15] J.N. Gao, X.L. Ren, F.Q. Tang, Photograph. Sci. Photochem. 24 (2006) 360e366. [16] H.H. Zhou, D.Y. Wu, J.Q. Hu, T.Q. Lian, Zh. Q. Tian, J. Chin, Light Scatt. 16 (2004) 221e224. [17] X.Zh. Sun, L.H. Lin, Zh.C. Li, Zh.J. Zhang, J.Y. Feng, Mater. Lett. 63 (2009) 2306e2308. [18] C. Shi, M.J. Cheng, Z.P. Qu, X.H. Bao, Appl. Catal. B, Environmental 51 (2004) 171e181. [19] K. Brandt, M.E. Chiu, D.J. Watson, M.S. Tikhov, R.M. Lambert, J. Am. Chem. Soc. 131 (2009) 17286e17290. [20] X.G. Wang, Zh. Qi, C.C. Zhao, W.M. Wang, Z.H. Zhang, J. Phys. Chem. C 113 (2009) 13139e13150. [21] C.X. Xu, Y.Y. Li, F. Tian, Y. Ding, ChemPhysChem 11 (2010) 3320e3328. [22] Z.H. Zhang, Y. Wang, Z. Qi, W.H. Zhang, J.Y. Qin, J. Frenzel, J. Phys. Chem. C 113 (2009) 12629e12636. [23] Z.Q. Li, B.Q. Li, Z.X. Qin, X. Lu, J. Mater. Sci. 45 (2010) 6494e6497. [24] F.H. Yeh, C.C. Tai, J.F. Huang, I.W. Sun, J. Phys. Chem. B. 110 (2006) 5215e5222.
1384
G. Li et al. / Solid State Sciences 13 (2011) 1379e1384
[25] J. Erlebacher, M.J. Aziz, A. Karma, N. Dimitrov, K. Sieradzki, Nature 410 (2001) 450e453. [26] J.C. Thorp, K. Sieradzki, L. Tang, P.A. Crozier, M. Nastasi, D. Mitlin, S.T. Picraux, Appl. Phys. Lett. 88 (2006) 557. [27] X.L. Tan, Y.J. Tang, Y. Liu, J.S. Luo, K. Li, X.B. Liu, Mate. Rev. 23 (2009) 68e71. [28] X. Lu, E. Bischoff, R. Spolenak, T.J. Balk, Scripta Mater. 56 (2007) 557e560. [29] Zh.H. Zhang, Y. Wang, Y.Z. Wang, X.G. Wang, Zh. Qi, H. Ji, Ch.C. Zhao, Scripta Mater. 62 (2010) 137e140. [30] S. Parida, D. Kramer, C.A. Volkert, H. Rösner, J. Erlebacher, J. Weissmüller, PRL 97 (2006) 1e4.
[31] R. Morrish, A.J. Muscat, Chem. Mater. 21 (2009) 3865e3870. [32] Q. Zhang, X.G. Wang, Zh. Qi, Y. Wang, Zh.H. Zhang, Electrochim. Acta 54 (2009) 6190e6198. [33] H.B. Lu, Y. Li, F.H. Wang, Corros. Sci. 48 (2006) 2106e2119. [34] L.W. Guo, X.T. Zhang, Z.L. Du, Y.B. Huang, Y.J. Mo, Spectrosc. Spectr. Anal. 21 (2001) 16e18. [35] V.S. Tiwari, T. Oleg, G.K. Darbha, W. Hardy, J.P. Singh, P.C. Ray, Chem. Phys. Lett. 446 (2007) 77e82. [36] S.Y. Lee, L. Hung, G.S. Lang, J.E. Cornett, I.D. Mayergoyz, O. Rabin, ACS Nano 4 (2010) 5763e5772.