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Cu2O composite nanowire arrays on Si via AAO template-mediated electrodeposition
Journal of Alloys and Compounds 427 (2007) 213–218
Fabrication of Cu/Cu2O composite nanowire arrays on Si via AAO template-mediated electrodeposition Yueh-Hsun Lee a , Ing-Chi Leu b , Min-Tao Wu a , Jung-Hsien Yen a , Kuan-Zong Fung a,∗ a
Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan b Department of Materials Science and Engineering, National United University, Miao-Li, Taiwan
Received 13 December 2005; received in revised form 23 February 2006; accepted 25 February 2006 Available online 5 April 2006
Abstract Cu/Cu2 O composite nanowire arrays on Si substrate with an Au interlayer have been prepared by anodic aluminum oxide (AAO) templatemediated electrochemical deposition. The pure Cu2 O nanowires are obtained at low current density, 0.05 mA/cm2 , while pure Cu nanowires appear at high current density, 15 mA/cm2 . Between these two current densities, the Cu/Cu2 O composite nanowires are produced and the content of Cu increase as the applied current density is raised. The transmission electron microscopy (TEM) observation indicates that the composite nanowires are composed of isolated Cu and Cu2 O grains, which contradicts to the layered structure observed in the composite films. The spontaneous potential oscillation observed in preparing Cu/Cu2 O composite films is also recorded during the formation of composite nanowires. The periods of spontaneous oscillation are a function of the applied current density. The mechanism of the variation of local pH can account the occurrence of spontaneous potential oscillations in the present system. © 2006 Elsevier B.V. All rights reserved. Keywords: Composite materials; Nanostructure materials
1. Introduction Recently, nanosized materials have received much attention due to their excellent physical and chemical properties. In order to obtain nanostructure, several technologies such as lithographic method, and the bottom up approaches have been developed to promote the progress. The anodic aluminum oxide (AAO) template-mediated technique is currently employed to fabricate the regular and aligned nanoarrays [1,2]. In addition, the electrochemical deposition (ECD) was regarded as an inexpensive and easy method to synthesize material. Combining AAO template and ECD is an efficient method to fabricate nanoarrays of metals and oxides. Our group has reported several results about the anodization of Al foils and films [3], and the combination of electrochemical deposition and AAO template to
∗
Corresponding author. E-mail addresses: [email protected] (Y.-H. Lee), [email protected] (K.-Z. Fung). 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.02.055
prepare oxide nanofibril, nanowires [4,5] and carbon nanotubes [6]. Due to the particular growth characteristics and related optical and electrical properties of Cu/Cu2 O composite [7,8], the AAO template-mediated electrodeposition performed to prepare Cu/Cu2 O nanowires array is discussed in the present study. Electrochemical self-assembly of copper/cuprous oxide layered nanostructures have been reported [9,10]. We will conduct an extended study to characterize the growth process, and to evaluate the possibility of the formation of layered structure in the composite nanowires. In the present study, the AAO template is prepared on Si substrate. And the effect of pH values of NaOH solution served as etching solution for the AAO removal is discussed. The crystal characterization and microstructure features of nanowires are also studied by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The growth behavior of nanowires different from that reported in the literature was found and discussed. The results and information obtained in the present study would be helpful for the develop-
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ment of one-dimensional nanostructure by template-mediated electrodeposition. 2. Experiment High purity Al films with 1.2 m thickness are deposited on to Si wafer with an Au interlayer by E-beam evaporation. The evaporated Au layer is also used as a conductive layer for subsequent electrochemical deposition. The Al-coated Si wafer is then mounted by thermoplastic polymer to control the exposed area in the electrolyte. The anodization of Al is first performed in a bath consisting of 0.3 M oxalic acid solution at 40 V direct current at 13 ◦ C. The samples are then immersed into 0.27 M H3 PO4 solution to remove the barrier layer at the oxide/metal interface. The Cu/Cu2 O nanowires are synthesized from an aqueous solution prepared from 0.6 M Cu(II) and 3 M lactate that contained copper(II) sulfate pentahydrate (Showa) and lactic acid (Riedel-deHa¨en). After adding a certain amount of 5 M NaOH, the solution is then stirred overnight with a magnetic stirplate. The stabilized solution is then adjusted to the desired pH 10. The electrochemical deposition system employed is a conventional three-electrode cell. Si with anodic alumina nanochannels is used as the working electrode. The counter electrode is a Pt foil while an Ag/AgCl in a 3 M NaCl solution is used as the reference electrode. The electrodepositon is performed galvanically with a potentiostat/galvanostat (EG&G, Model 273A) under a constant current density. The current density is set at 0.05 mA/cm2 . The deposition is terminated when the channels of AAO are filled. Then, the deposited specimen is cleaned with deionized water to remove the remaining contaminants. After removing the polymer, deposited samples are immersed in NaOH solution to remove AAO template. The pH of NaOH is varied from 11.0 to 13.5. The duration of immersion is chosen to be 60 min. After immersion, the specimens are rinsed by deionized water to remove remaining NaOH. The porous alumina template and deposited nanowires are then observed by scanning electron microscopy (SEM) and transmission electron microscopy. The structural analysis is conducted using an XRD and TEM.
3. Results and discussion Fig. 1a illustrates a typical top-view SEM image of AAO template. The well-aligned porous alumina channels are fabricated on an Au-coated Si substrate. The diameter of the nanochannels is about 60 nm and the density of pores is approximate 1011 cm−2 . The ratio of open pores to the entirely exposed area is about 33% that was used to determine the applied current density during the electroplating. Fig. 1b shows a SEM side-view image of an AAO template embedding nanowires by electroplating. The Al layer is entirely anodized and transformed to AAO and the straight nanochannel arrays are arranged perpendicularly on the Au/Ti/Si substrate. The uniform-length nanowires are filled at the bottoms of all nanochannels. The Au layer at the bottom of nanochannels is used to enhance the conductivity during the electroplating, and the Ti layer is used as an adhesive buffer layer. After the nanowire arrays are deposited into the AAO on Si, the AAO template is removed by immersing the specimens into the sodium hydroxide solution. In order to obtain the appropriate condition for AAO removal, several NaOH solutions with different pH are used. Fig. 2a shows the surface of deposited sample after immersing in NaOH solution (pH 11). This figure indicates that AAO still remain intact and is not etched by NaOH solution. When the pH of NaOH solution is increased to 11.5, AAO template is partially removed and some nanowires are exposed as shown in Fig. 2b. As pH is increased to 12, all of
Fig. 1. The typical (a) top-view and (b) side-view images of AAO template.
AAO template is removed completely. Thus, well-aligned and free standing nanowires with a diameter of 60 nm can be clearly observe as shown in Fig. 2c. When the pH of NaOH solution is increased to 12.5 as shown in Fig. 2d, the excess dissolution of the deposit is locally observed. While the pH is increased to 13 and 13.5 as shown in Fig. 2e and f, NaOH not only remove the template but also react with the nanowires. The well-aligned nanowires transform to random and tangled fibrous nanostructure. Such a result indicates that the composite nanowires are destroyed after immersing in high pH NaOH solution. The AAO template could not be removed completely at low pH while the crystallinity and morphologies are destroyed at high pH. For a more basic condition, Cu2 O of the composite nanowires were dissolved and only Cu maintained as the tangled fibrous nanostructure. The electrode potential fluctuations with deposition time at different applying current densities during deposition are monitored by recording the time–potential curve. At some chosen current densities, a spontaneous potential oscillation is observed in a wide range between 0.5 and 10 mA/cm2 . As shown in Fig. 3, the electrode potential reaches a stable and more negative potential (∼−0.7 V, as shown in Fig. 3c) for the deposition performed at a high current density (15 mA/cm2 ). In the contrast, the steady state potential for the deposition performed at low current density (0.05 mA/cm2 ) is less negative (∼−0.15 V, as shown in Fig. 3a). At the medium range of current density (0.5–10 mA/cm2 ), a phenomenon of spontaneous potential oscillation occurs. As shown in Fig. 3b, at current density of 5 mA/cm2 , the electrode potential periodically oscillated between about −0.55 and −0.75 V. According to Cu–water sys-
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Fig. 2. The SEM images of obtained Cu/Cu2 O composite nanowires embedding in AAO template immersed at NaOH with different pH for 60 min: (a) 11, (b) 11.5, (c) 12, (d) 12.5, (e) 13 and (f) 13.5.
tem Pourbaix diagram [11], the more negative potential favors the formation of Cu and the less negative one favors the formation of Cu2 O. Besides, the effect of current density and/or pH on the phase composition of the present system has also been well-discussed by Zhou and Switzer [12]. Kenane and Piraux have reported a similar trend during the deposition of Cu/Cu2 O nanowires into track-etched polymer membranes with pore diameter of 230 nm, though the oscillation is observed in a narrower current density range, 1.7–4.7 mA/cm2 , at pH 9.2 [13]. They also show that the oscillating range is 0.25–4.3 mA/cm2 , as the Cu/Cu2 O composite films were deposited at planar substrate. Comparing with the results we obtained, it is found that the oscillating range is dependent on the characteristics of substrate used. For smaller channels, the degree of oscillating is higher and wider. It can be suggested that the oscillating range of current density is significantly affected by the process of mass transport. The XRD patterns of nanowire arrays obtained at different current densities are shown in Fig. 4. The nanowires deposited
at 0.05 mA/cm2 are the pure Cu2 O as shown in Fig. 4a. The reflected peaks are ascribed to the (2 0 0) and (1 1 1) reflections of Cu2 O and those from the substrate (marked with solid stars). When the applied current densities are increased to 3.5 and 5 mA/cm2 , the appearance of Cu reflection can be found, as shown in Fig. 4b and c, respectively. Besides, the intensities of Cu increased with the applied current densities. As the applied current density is increased to 15 mA/cm2 , the reflection of Cu2 O disappears and only that of Cu exists as shown in Fig. 4d. The current density-dependent phase evolution of nanowires is similar to the Cu/Cu2 O composite films. Switzer et al. have shown the self-assembly Cu/Cu2 O layered nanostructure by electrochemical deposition at different substrate [9,10]. For the films deposited at low current density, the pure Cu2 O films are obtained. As the films are deposited at high current density, the pure Cu films are formed. In the medium current density range, the Cu/Cu2 O composite films are obtained with the spontaneous potential oscillation.
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The mechanism based on the variation of local pH has been proposed to account for the spontaneous potential oscillation in copper lactate system as expressed in following [9]: Cu2+ + 2e− → Cu 2+
2Cu
Fig. 3. The electrode potential fluctuations under different current density: (a) 0.05 mA/cm2 , (b) 5 mA/cm2 and (c) 15 mA/cm2 .
Fig. 4. The XRD patterns of nanowires obtained at different current density: (a) 0.05 mA/cm2 , (b) 3.5 mA/cm2 (c) 5 mA/cm2 and (d) 15 mA/cm2 .
−
(1) −
+ 2e + 2OH → Cu2 O + H2 O
(2)
At low pH, the thermodynamically favored reaction is to form metallic copper as expressed in Eq. (1). In contrast to that, the oxidation of metallic copper into cuprous oxide as expressed in Eq. (2) is favored at high pH. During the formation of Cu2 O, the amount of OH− is consumed in the diffusion layer which extends from the OHP (outer Helmholtz plane) into the bulk of the solution, causing a decreased pH on the electrode surface. The decrease of local pH value would favor reaction (1) and form the metallic copper. Then, the OH− is no longer consumed and the reduced concentration of OH− in the diffusion layer is increased back to the high pH value by convection/diffusion from the bulk solution. Namely, the formation of Cu2 O is favored again. As shown in Fig. 4, metallic copper tend to form during the more negative potential period and cuprous trend to form in the less negative potential period. Therefore, the spontaneous potential oscillation during the formation of Cu/Cu2 O nanowires can be ascribed to the variation of surface pH value caused by the different favored electrochemical reaction. The variation of oscillation period as a function of current density is plotted in Fig. 5. It shows that the period of oscillation decreases from 5 to 2.9 s gradually as the current density is increased from 0.5 to 3.5 mA/cm2 . And then the oscillation period maintains at a steady value close to 3 s as the current density is continuously increased till the oscillation disappears. For the correlation of oscillation periods and the applied current density, it may be a direct support to the local pH mechanism. The rate of consumption of OH− increased with
Fig. 5. The correlation of oscillation period and applied current density.
Y.-H. Lee et al. / Journal of Alloys and Compounds 427 (2007) 213–218
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analysis is performed to observe whether the Cu/Cu2 O composite nanowires exhibit the same feature. Fig. 6 shows the TEM dark-field image and diffraction patterns of the nanowires deposited at the current density of 5 mA/cm2 . In Fig. 6a, the bright grains on the nanowires correspond to the Cu2 O grains producing the diffraction ring of Cu2 O(1 1 1). The average grains size are 10–20 nm from the direct image observation. The isolated grains seem to contradict the layered structure that obtained on planar substrate. Fig. 6b illustrates the electron diffraction of the composite nanowires. The identified diffraction rings can be attributed to the reflection planes of Cu and Cu2 O. Herein, there is no evidence for other compounds or crystals except the Cu/Cu2 O composite. The isolated gain structure of the composite nanowires is the most different microstructure feature from the reported layered structure of the composite films. From the discussion above, it is found that the formation of layered structure is dominated by the diffusion of OH− . On the planar substrate, the interface between the electrode and diffusion layer is flat and uniform. Thus, the consumption and subsequent supply of OH− in the interface occurs stably along the solution/solid interface, leading to the formation of a uniform and smooth layer. Liu and Zhou have reported that the diffusion rate of OH− in AAO nanochannels is slower than that on a flat substrate for the deposition of Cu2 O nanowires [15]. Considering the very small path and high aspect ratio in the nanochannels, the diffusion and mass transport of OH− is limited. Therefore, the consumption and supply of OH− near the electrode are very localized, which means that the formation of layered structure is difficult to be found in the present system. The similar results are also observed by Kenane and Piraux in the preparation of Cu/Cu2 O composite nanowires in polycarbonate membranes with a larger pore size than we used and the consequences are the small oscillation period and modulation wavelength of the nanowires [13]. 4. Conclusion
Fig. 6. TEM images of composite nanowires: (a) dark-field image and (b) diffraction pattern.
the applied current density, namely, the oscillation periods are expected to be a decreasing function of the applied current density. And then the oscillating periods maintain at a constant value because of the limitation of OH− mass transport at high current density. Wang et al. have also developed a model for this spontaneous formation of periodic structure [14]. By their experimental observation and modeling, a model based on the coupling of [Cu2+ ] and [H+ ] in the electrodeposition system is proposed to describe the oscillatory phenomena in Cu system. And the oscillating frequency depends on the pH of electrolyte and the applied current/voltage. During the formation of Cu/Cu2 O composite films, the layer by layer nanostructure of Cu and Cu2 O are observed. A TEM
Through the article, we report the fabrication of Cu/Cu2 O composite nanowire arrays on Si by template-mediated electrodeposition. The composition and crystal structure of nanowires can be controlled by simply varied the applied current density. The pure Cu2 O and Cu nanowires are obtained at low applied current density of 0.05 mA/cm2 and high current density of 15 mA/cm2 , respectively. The nanowires deposited at the medium range of current density (0.5–10 mA/cm2 ) are a mixture of Cu and Cu2 O. During the deposition of composite nanowires, a spontaneous potential oscillation occurs. The oscillation periods are a function of the applied current density. Local pH fluctuation nearby the electrode surface can be used to account the phenomenon. The layered composite structure observed in the planar composite films has not been found in the composite nanowires. The microstructure features of composite nanowires are the isolated grains structure. The different mass transport and diffusion behaviors between planar substrate and nanochannels might be responsible for that.
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Acknowledgement The authors gratefully acknowledge the financial support by the National Science Council of Taiwan (Grant No. NSC 942120-M-006-002). References [1] [2] [3] [4]
C.R. Martin, Science 266 (1994). H. Masuda, K. Fukada, Science 268 (1995) 1466. M.T. Wu, I.C. Leu, M.H. Hon, J. Mater. Res. 19 (2004) 888. Y.C. Wang, I.C. Leu, M.H. Hon, J. Mater. Chem. 12 (2002) 2439. [5] C.C. Huang, I.C. Leu, K.Z. Fung, Electrochem. Solid-State Lett. 8 (2005) A204.
[6] J.H. Yen, I.C. Leu, M.T. Wu, C.C. Lin, M.H. Hon, Electrochem. Solid-State Lett. 14 (2005) 841. [7] J.A. Switzer, B.M. Maune, E.R. Raub, E.W. Bohanan, J. Phys. Chem. B 103 (1999) 395. [8] E.D. Mishina, K. Nagai, S. Nakabayashi, Nano Lett. 1 (2001) 401. [9] J.A. Switzer, C.J. Hung, L.Y. Huang, F. Scott Miller, Y. Zhou, E.R. Raub, M.G. Shumsky, E.W. Bohannan, J. Mater. Res. 13 (1998) 909. [10] J.A. Switzer, C.J. Hung, L.Y. Huang, E.R. Switzer, D.R. Kammler, T.D. Golden, E.W. Bohannan, J. Am. Chem. Soc. 120 (1998) 3530. [11] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, National Association of Corrosion Engineers, 1974, p. 386. [12] Y.C. Zhou, J.A. Switzer, Mater. Res. Innovat. 2 (1998) 22. [13] S. Kenane, L. Piraux, J. Mater. Res. 17 (2002) 401. [14] Y. Wang, Y. Cao, M. Wang, S. Zhong, M.Z. Zhang, Y. Feng, R.W. Peng, X.P. Hao, N.B. Ming, Phys. Rev. E 69 (2004) 021607. [15] X.M. Liu, Y.C. Zhou, Appl. Phys. A 81 (2005) 685.
Fabrication of Cu/Cu2O composite nanowire arrays on Si via AAO template-mediated electrodeposition Yueh-Hsun Lee a , Ing-Chi Leu b , Min-Tao Wu a , Jung-Hsien Yen a , Kuan-Zong Fung a,∗ a
Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan b Department of Materials Science and Engineering, National United University, Miao-Li, Taiwan
Received 13 December 2005; received in revised form 23 February 2006; accepted 25 February 2006 Available online 5 April 2006
Abstract Cu/Cu2 O composite nanowire arrays on Si substrate with an Au interlayer have been prepared by anodic aluminum oxide (AAO) templatemediated electrochemical deposition. The pure Cu2 O nanowires are obtained at low current density, 0.05 mA/cm2 , while pure Cu nanowires appear at high current density, 15 mA/cm2 . Between these two current densities, the Cu/Cu2 O composite nanowires are produced and the content of Cu increase as the applied current density is raised. The transmission electron microscopy (TEM) observation indicates that the composite nanowires are composed of isolated Cu and Cu2 O grains, which contradicts to the layered structure observed in the composite films. The spontaneous potential oscillation observed in preparing Cu/Cu2 O composite films is also recorded during the formation of composite nanowires. The periods of spontaneous oscillation are a function of the applied current density. The mechanism of the variation of local pH can account the occurrence of spontaneous potential oscillations in the present system. © 2006 Elsevier B.V. All rights reserved. Keywords: Composite materials; Nanostructure materials
1. Introduction Recently, nanosized materials have received much attention due to their excellent physical and chemical properties. In order to obtain nanostructure, several technologies such as lithographic method, and the bottom up approaches have been developed to promote the progress. The anodic aluminum oxide (AAO) template-mediated technique is currently employed to fabricate the regular and aligned nanoarrays [1,2]. In addition, the electrochemical deposition (ECD) was regarded as an inexpensive and easy method to synthesize material. Combining AAO template and ECD is an efficient method to fabricate nanoarrays of metals and oxides. Our group has reported several results about the anodization of Al foils and films [3], and the combination of electrochemical deposition and AAO template to
∗
Corresponding author. E-mail addresses: [email protected] (Y.-H. Lee), [email protected] (K.-Z. Fung). 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.02.055
prepare oxide nanofibril, nanowires [4,5] and carbon nanotubes [6]. Due to the particular growth characteristics and related optical and electrical properties of Cu/Cu2 O composite [7,8], the AAO template-mediated electrodeposition performed to prepare Cu/Cu2 O nanowires array is discussed in the present study. Electrochemical self-assembly of copper/cuprous oxide layered nanostructures have been reported [9,10]. We will conduct an extended study to characterize the growth process, and to evaluate the possibility of the formation of layered structure in the composite nanowires. In the present study, the AAO template is prepared on Si substrate. And the effect of pH values of NaOH solution served as etching solution for the AAO removal is discussed. The crystal characterization and microstructure features of nanowires are also studied by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The growth behavior of nanowires different from that reported in the literature was found and discussed. The results and information obtained in the present study would be helpful for the develop-
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Y.-H. Lee et al. / Journal of Alloys and Compounds 427 (2007) 213–218
ment of one-dimensional nanostructure by template-mediated electrodeposition. 2. Experiment High purity Al films with 1.2 m thickness are deposited on to Si wafer with an Au interlayer by E-beam evaporation. The evaporated Au layer is also used as a conductive layer for subsequent electrochemical deposition. The Al-coated Si wafer is then mounted by thermoplastic polymer to control the exposed area in the electrolyte. The anodization of Al is first performed in a bath consisting of 0.3 M oxalic acid solution at 40 V direct current at 13 ◦ C. The samples are then immersed into 0.27 M H3 PO4 solution to remove the barrier layer at the oxide/metal interface. The Cu/Cu2 O nanowires are synthesized from an aqueous solution prepared from 0.6 M Cu(II) and 3 M lactate that contained copper(II) sulfate pentahydrate (Showa) and lactic acid (Riedel-deHa¨en). After adding a certain amount of 5 M NaOH, the solution is then stirred overnight with a magnetic stirplate. The stabilized solution is then adjusted to the desired pH 10. The electrochemical deposition system employed is a conventional three-electrode cell. Si with anodic alumina nanochannels is used as the working electrode. The counter electrode is a Pt foil while an Ag/AgCl in a 3 M NaCl solution is used as the reference electrode. The electrodepositon is performed galvanically with a potentiostat/galvanostat (EG&G, Model 273A) under a constant current density. The current density is set at 0.05 mA/cm2 . The deposition is terminated when the channels of AAO are filled. Then, the deposited specimen is cleaned with deionized water to remove the remaining contaminants. After removing the polymer, deposited samples are immersed in NaOH solution to remove AAO template. The pH of NaOH is varied from 11.0 to 13.5. The duration of immersion is chosen to be 60 min. After immersion, the specimens are rinsed by deionized water to remove remaining NaOH. The porous alumina template and deposited nanowires are then observed by scanning electron microscopy (SEM) and transmission electron microscopy. The structural analysis is conducted using an XRD and TEM.
3. Results and discussion Fig. 1a illustrates a typical top-view SEM image of AAO template. The well-aligned porous alumina channels are fabricated on an Au-coated Si substrate. The diameter of the nanochannels is about 60 nm and the density of pores is approximate 1011 cm−2 . The ratio of open pores to the entirely exposed area is about 33% that was used to determine the applied current density during the electroplating. Fig. 1b shows a SEM side-view image of an AAO template embedding nanowires by electroplating. The Al layer is entirely anodized and transformed to AAO and the straight nanochannel arrays are arranged perpendicularly on the Au/Ti/Si substrate. The uniform-length nanowires are filled at the bottoms of all nanochannels. The Au layer at the bottom of nanochannels is used to enhance the conductivity during the electroplating, and the Ti layer is used as an adhesive buffer layer. After the nanowire arrays are deposited into the AAO on Si, the AAO template is removed by immersing the specimens into the sodium hydroxide solution. In order to obtain the appropriate condition for AAO removal, several NaOH solutions with different pH are used. Fig. 2a shows the surface of deposited sample after immersing in NaOH solution (pH 11). This figure indicates that AAO still remain intact and is not etched by NaOH solution. When the pH of NaOH solution is increased to 11.5, AAO template is partially removed and some nanowires are exposed as shown in Fig. 2b. As pH is increased to 12, all of
Fig. 1. The typical (a) top-view and (b) side-view images of AAO template.
AAO template is removed completely. Thus, well-aligned and free standing nanowires with a diameter of 60 nm can be clearly observe as shown in Fig. 2c. When the pH of NaOH solution is increased to 12.5 as shown in Fig. 2d, the excess dissolution of the deposit is locally observed. While the pH is increased to 13 and 13.5 as shown in Fig. 2e and f, NaOH not only remove the template but also react with the nanowires. The well-aligned nanowires transform to random and tangled fibrous nanostructure. Such a result indicates that the composite nanowires are destroyed after immersing in high pH NaOH solution. The AAO template could not be removed completely at low pH while the crystallinity and morphologies are destroyed at high pH. For a more basic condition, Cu2 O of the composite nanowires were dissolved and only Cu maintained as the tangled fibrous nanostructure. The electrode potential fluctuations with deposition time at different applying current densities during deposition are monitored by recording the time–potential curve. At some chosen current densities, a spontaneous potential oscillation is observed in a wide range between 0.5 and 10 mA/cm2 . As shown in Fig. 3, the electrode potential reaches a stable and more negative potential (∼−0.7 V, as shown in Fig. 3c) for the deposition performed at a high current density (15 mA/cm2 ). In the contrast, the steady state potential for the deposition performed at low current density (0.05 mA/cm2 ) is less negative (∼−0.15 V, as shown in Fig. 3a). At the medium range of current density (0.5–10 mA/cm2 ), a phenomenon of spontaneous potential oscillation occurs. As shown in Fig. 3b, at current density of 5 mA/cm2 , the electrode potential periodically oscillated between about −0.55 and −0.75 V. According to Cu–water sys-
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215
Fig. 2. The SEM images of obtained Cu/Cu2 O composite nanowires embedding in AAO template immersed at NaOH with different pH for 60 min: (a) 11, (b) 11.5, (c) 12, (d) 12.5, (e) 13 and (f) 13.5.
tem Pourbaix diagram [11], the more negative potential favors the formation of Cu and the less negative one favors the formation of Cu2 O. Besides, the effect of current density and/or pH on the phase composition of the present system has also been well-discussed by Zhou and Switzer [12]. Kenane and Piraux have reported a similar trend during the deposition of Cu/Cu2 O nanowires into track-etched polymer membranes with pore diameter of 230 nm, though the oscillation is observed in a narrower current density range, 1.7–4.7 mA/cm2 , at pH 9.2 [13]. They also show that the oscillating range is 0.25–4.3 mA/cm2 , as the Cu/Cu2 O composite films were deposited at planar substrate. Comparing with the results we obtained, it is found that the oscillating range is dependent on the characteristics of substrate used. For smaller channels, the degree of oscillating is higher and wider. It can be suggested that the oscillating range of current density is significantly affected by the process of mass transport. The XRD patterns of nanowire arrays obtained at different current densities are shown in Fig. 4. The nanowires deposited
at 0.05 mA/cm2 are the pure Cu2 O as shown in Fig. 4a. The reflected peaks are ascribed to the (2 0 0) and (1 1 1) reflections of Cu2 O and those from the substrate (marked with solid stars). When the applied current densities are increased to 3.5 and 5 mA/cm2 , the appearance of Cu reflection can be found, as shown in Fig. 4b and c, respectively. Besides, the intensities of Cu increased with the applied current densities. As the applied current density is increased to 15 mA/cm2 , the reflection of Cu2 O disappears and only that of Cu exists as shown in Fig. 4d. The current density-dependent phase evolution of nanowires is similar to the Cu/Cu2 O composite films. Switzer et al. have shown the self-assembly Cu/Cu2 O layered nanostructure by electrochemical deposition at different substrate [9,10]. For the films deposited at low current density, the pure Cu2 O films are obtained. As the films are deposited at high current density, the pure Cu films are formed. In the medium current density range, the Cu/Cu2 O composite films are obtained with the spontaneous potential oscillation.
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The mechanism based on the variation of local pH has been proposed to account for the spontaneous potential oscillation in copper lactate system as expressed in following [9]: Cu2+ + 2e− → Cu 2+
2Cu
Fig. 3. The electrode potential fluctuations under different current density: (a) 0.05 mA/cm2 , (b) 5 mA/cm2 and (c) 15 mA/cm2 .
Fig. 4. The XRD patterns of nanowires obtained at different current density: (a) 0.05 mA/cm2 , (b) 3.5 mA/cm2 (c) 5 mA/cm2 and (d) 15 mA/cm2 .
−
(1) −
+ 2e + 2OH → Cu2 O + H2 O
(2)
At low pH, the thermodynamically favored reaction is to form metallic copper as expressed in Eq. (1). In contrast to that, the oxidation of metallic copper into cuprous oxide as expressed in Eq. (2) is favored at high pH. During the formation of Cu2 O, the amount of OH− is consumed in the diffusion layer which extends from the OHP (outer Helmholtz plane) into the bulk of the solution, causing a decreased pH on the electrode surface. The decrease of local pH value would favor reaction (1) and form the metallic copper. Then, the OH− is no longer consumed and the reduced concentration of OH− in the diffusion layer is increased back to the high pH value by convection/diffusion from the bulk solution. Namely, the formation of Cu2 O is favored again. As shown in Fig. 4, metallic copper tend to form during the more negative potential period and cuprous trend to form in the less negative potential period. Therefore, the spontaneous potential oscillation during the formation of Cu/Cu2 O nanowires can be ascribed to the variation of surface pH value caused by the different favored electrochemical reaction. The variation of oscillation period as a function of current density is plotted in Fig. 5. It shows that the period of oscillation decreases from 5 to 2.9 s gradually as the current density is increased from 0.5 to 3.5 mA/cm2 . And then the oscillation period maintains at a steady value close to 3 s as the current density is continuously increased till the oscillation disappears. For the correlation of oscillation periods and the applied current density, it may be a direct support to the local pH mechanism. The rate of consumption of OH− increased with
Fig. 5. The correlation of oscillation period and applied current density.
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analysis is performed to observe whether the Cu/Cu2 O composite nanowires exhibit the same feature. Fig. 6 shows the TEM dark-field image and diffraction patterns of the nanowires deposited at the current density of 5 mA/cm2 . In Fig. 6a, the bright grains on the nanowires correspond to the Cu2 O grains producing the diffraction ring of Cu2 O(1 1 1). The average grains size are 10–20 nm from the direct image observation. The isolated grains seem to contradict the layered structure that obtained on planar substrate. Fig. 6b illustrates the electron diffraction of the composite nanowires. The identified diffraction rings can be attributed to the reflection planes of Cu and Cu2 O. Herein, there is no evidence for other compounds or crystals except the Cu/Cu2 O composite. The isolated gain structure of the composite nanowires is the most different microstructure feature from the reported layered structure of the composite films. From the discussion above, it is found that the formation of layered structure is dominated by the diffusion of OH− . On the planar substrate, the interface between the electrode and diffusion layer is flat and uniform. Thus, the consumption and subsequent supply of OH− in the interface occurs stably along the solution/solid interface, leading to the formation of a uniform and smooth layer. Liu and Zhou have reported that the diffusion rate of OH− in AAO nanochannels is slower than that on a flat substrate for the deposition of Cu2 O nanowires [15]. Considering the very small path and high aspect ratio in the nanochannels, the diffusion and mass transport of OH− is limited. Therefore, the consumption and supply of OH− near the electrode are very localized, which means that the formation of layered structure is difficult to be found in the present system. The similar results are also observed by Kenane and Piraux in the preparation of Cu/Cu2 O composite nanowires in polycarbonate membranes with a larger pore size than we used and the consequences are the small oscillation period and modulation wavelength of the nanowires [13]. 4. Conclusion
Fig. 6. TEM images of composite nanowires: (a) dark-field image and (b) diffraction pattern.
the applied current density, namely, the oscillation periods are expected to be a decreasing function of the applied current density. And then the oscillating periods maintain at a constant value because of the limitation of OH− mass transport at high current density. Wang et al. have also developed a model for this spontaneous formation of periodic structure [14]. By their experimental observation and modeling, a model based on the coupling of [Cu2+ ] and [H+ ] in the electrodeposition system is proposed to describe the oscillatory phenomena in Cu system. And the oscillating frequency depends on the pH of electrolyte and the applied current/voltage. During the formation of Cu/Cu2 O composite films, the layer by layer nanostructure of Cu and Cu2 O are observed. A TEM
Through the article, we report the fabrication of Cu/Cu2 O composite nanowire arrays on Si by template-mediated electrodeposition. The composition and crystal structure of nanowires can be controlled by simply varied the applied current density. The pure Cu2 O and Cu nanowires are obtained at low applied current density of 0.05 mA/cm2 and high current density of 15 mA/cm2 , respectively. The nanowires deposited at the medium range of current density (0.5–10 mA/cm2 ) are a mixture of Cu and Cu2 O. During the deposition of composite nanowires, a spontaneous potential oscillation occurs. The oscillation periods are a function of the applied current density. Local pH fluctuation nearby the electrode surface can be used to account the phenomenon. The layered composite structure observed in the planar composite films has not been found in the composite nanowires. The microstructure features of composite nanowires are the isolated grains structure. The different mass transport and diffusion behaviors between planar substrate and nanochannels might be responsible for that.
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Acknowledgement The authors gratefully acknowledge the financial support by the National Science Council of Taiwan (Grant No. NSC 942120-M-006-002). References [1] [2] [3] [4]
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