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Fabrication and characterization of smooth high aspect ratio zirconia nanotubes
Chemical Physics Letters 410 (2005) 188–191 www.elsevier.com/locate/cplett
Fabrication and characterization of smooth high aspect ratio zirconia nanotubes Hiroaki Tsuchiya, Jan M. Macak, Luciano Taveira, Patrik Schmuki
*
Department of Materials Science, Institute for Surface Science and Corrosion (LKO), University of Erlangen-Nuremberg, Martensstrasse 7, D-91058 Erlangen, Germany Received 13 April 2005; in final form 17 May 2005 Available online 14 June 2005
Abstract In the present work, we report formation of high aspect ratio zirconia nanotubes by electrochemical anodization of zirconium in a 1 M (NH4)2SO4 electrolyte containing 0.5 wt% NH4F. Highly self-organized zirconia nanotubes can be formed with a diameter of 50 nm and a length of 17 lm, i.e. with an aspect ratio of more than 300. The nanotubes show a distinct smooth and straight morphology. XRD investigation reveals that the nanotubes have a cubic crystalline structure directly after anodization, that is, without any further annealing. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction One-dimensional nanoscale oxides such as nanotubes and nanowires have attracted much attention due to their potential applications in nanotechnology. In particular, zirconium oxide, so-called zirconia has been extensively used as a catalyst or as a catalyst support [1,2], as a sensor [3] and as a solid-electrolyte [4–6]. Zirconia has excellent technological properties and characteristics such as chemical and thermal stability, mechanical strength, wear resistance, low electrical conductivity and a high degree of biocompatibility. Recently, much research efforts have focused on the synthesis of zirconia nanotubes by using templating techniques [7–9] and atomic layer deposition [10]. Recently, we reported a simple electrochemical approach to form extremely thick nanoporous zirconia [11] and nanotubular zirconia layers [12,13]. An example is shown in Fig. 1. These porous layers have been formed in H2SO4/NaF electrolytes [12,13]. The treatment led to *
Corresponding author. Fax: +49 9131 852 7582. E-mail address: [email protected] (P. Schmuki).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.05.065
high aspect ratio pore arrays, however the pores that were formed showed a wavy and somewhat irregular wall morphology as apparent in Fig. 1. Anodization done in HF electrolytes also leads to tubular feature that are however comparably irregular and wavy [14]. In this Letter, we show a pronounced improvement of the method, that is, the formation of smooth and straight high aspect ratio zirconia nanotubes.
2. Experimental Samples were zirconium foils (99.8% purity, Goodfellow, England) with 0.1 mm thickness. Prior to the electrochemical treatment, the samples were ultra-sonicated in acetone, isopropanol and methanol successively, followed by rising with deionized (DI) water and finally drying with nitrogen. The electrochemical cell consisted of a conventional three-electrode arrangement with a platinum gauze as a counter electrode and a Huber-Luggin capillary with a Ag/AgCl (1 M KCl) electrode as a reference electrode. The samples were pressed against an o-ring in the electrochemical cell leaving
H. Tsuchiya et al. / Chemical Physics Letters 410 (2005) 188–191
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3. Results and discussion
Fig. 1. SEM images of anodic porous zirconia layer. (a) Top-view and (b) cross-sectional of the porous layer formed on Zr in 1 M H2SO4 + 0.2 wt% NaF electrolyte at 20 V in analogy to [12,13].
1 cm2 of the samples surface exposed to the electrolyte. The electrolyte was 1 M (NH4)2SO4 + 0.5 wt% NH4F and was prepared from analytical grade chemicals and DI water. Electrochemical treatments were carried out using a high-voltage potentiostat Jaissle IMP 88 PC connected to a digital multimeter interfaced to a computer. The electrochemical treatments consisted of a potential sweep from an open-circuit potential (OCP) to 20 V with the sweep rate of 1 V s 1, followed by holding the potential at 20 V for 1 h. After the treatment, the samples were rinsed with DI water and then dried with nitrogen stream. The structure and morphology of the resulting layer were characterized with a Hitachi SEM FE S4800 field-emission scanning electron microscopy (FESEM). Thickness information was obtained by direct SEM cross-sectional observation on samples that were bent until the oxide layer cracked off. Structural information of the layer was obtained using a X-ray diffractometer (Phillips XÕpert-MPD PW3040). The chemical composition of the layer was obtained by X-ray photoelectron spectroscopy (PHI 5600 XPS).
Various electrolytes and electrochemical treatments were tested to improve the morphology of the porous ZrO2 layers from previous works [11–13]. In order to obtain electrochemical information, current–time transients proved to be a valuable tool. Fig. 2 shows such current–time curves recorded during anodization of zirconium in different electrolytes after a potential sweep from OCP to 20 V with a sweep rate of 1 V s 1. The H2SO4/NaF electrolyte was used in previous work and is included as a reference for a case where the fluoride concentration and other parameters were optimized to achieve self-organized pore formation. In the previous work tube formation was carried out in a neutral (NH4)2SO4/NH4F buffer to suppress some disadvantages of the acidic electrolyte [15]. Evidently the (NH4)2SO4 containing 0.5 wt% NH4F shows a very similar characteristics as the H2SO4/NaF electrolyte. Both curves in fluoride-containing electrolytes deviate clearly from the curve in fluoride-free (NH4)2SO4. The higher current density in the fluoride-containing electrolytes indicates an additional dissolution process takes place. This can be attributed to a dissolution of the oxide layer as soluble fluoro-complexes. As pointed out in previous work this enhanced solubility by complex formation is a key factor to achieve pore formation on valve metals. In the fluoride-free electrolyte and as a very well established in literature, only a compact oxide layer was observed. Fig. 3 shows SEM images of the surface layer formed in 1 M (NH4)2SO4 + 0.5 wt% NH4F at 20 V for 1 h after a potential sweep from OCP to 20 V with a scan rate of 1 V s 1 – shown are (a) a top-view, (b) a cross-section and (c) a bottom-view. The cross-section and bottom-
Fig. 2. Current–time curves recorded for different electrolytes during anodization of Zr at 20 V after a potential sweep from OCP to 20 V with a sweep rate of 1 V s 1.
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H. Tsuchiya et al. / Chemical Physics Letters 410 (2005) 188–191
most remarkable difference is that the nanotubes in (NH4)2SO4 + 0.5 wt% NH4F are extremely straight and the irregularities of the pore walls (wavy morphology) does not occur any more. With length of 17 lm, the aspect ratio of the nanotube layer is more than 300. Although the surface morphology is generally very uniform, on some locations, parts of the nanotube layers bended down in a heap, which might be due to the extremely high aspect ratio. Furthermore, by comparing Fig. 1b and Fig. 3b, it becomes clear that the crosssectional feature is relatively different from that in H2SO4 + 0.2 wt% NaF. A key factor to form the straight nanotube layers might be buffering effect of (NH4)2SO4/ NH4F system, thus damping possible alterations in the local pH within the pores – this is analogy with previous work on TiO2 nanotube formation [15] where we showed that the pH-profile within a growing tube can strongly determine the final morphology. Fig. 4 shows an X-ray diffraction pattern of the nanotube layer shown in Fig. 3. It is clear that the layer has a crystalline structure; the peaks corresponding to a cubic zirconia are detected. This is in line with previous work on compact oxide layers formed in a wide variety of electrolytes where the cubic structures were predominant [16]. The observed crystalline structure is particularly noteworthy as porous layers formed on other valve metals such as Ti, W, Nb and Ta typically have an amorphous structure [17]. Therefore – if a crystalline structure is desired – the layer on such valve metals must be transformed to a specific crystalline structure by annealing. However, the fact that the zirconia nanotubes have a cubic crystalline structure directly after anodization without annealing is of a particular advantage in applications that are sensitive to thermal annealing. Additional XPS investigation also showed that the
Fig. 3. SEM images of the zirconia nanotube layer formed on Zr in 1 M (NH4)2SO4 + 0.5 wt% NH4F at 20 V after a potential sweep from OCP to 20 V: (a) top-view; (b) cross-section; (c) bottom-view.
view were obtained from a mechanically cracked sample. It is clear from Fig. 3a that the layer consists of self-organized nanotube arrays of 50 nm in diameter. It is also evident from Fig. 3a and c that the nanotubes are open at the top-end, while they are closed at the bottom. The closed bottom-ends correspond to the barrier oxide layers (essentially the high-field oxide). The features of the top-view and bottom-view of the nanotube layer formed in (NH4)2SO4 + 0.5 wt% NH4F is similar to that in H2SO4 + 0.2 wt% NaF [12,13]. However, the
Fig. 4. XRD patterns for the zirconia nanotube layer formed on Zr in 1 M (NH4)2SO4 + 0.5 wt% NH4F.
H. Tsuchiya et al. / Chemical Physics Letters 410 (2005) 188–191
structure essentially consists of a ZrO2 though only traces of F could be detected in the nanotube layer.
4. Conclusions Smooth high aspect ratio zirconia nanotubes were fabricated by electrochemical anodization of zirconium. The results presented here clearly demonstrate the fabrication of high quality zirconia nanotube layers using a very simple and direct method. The layer produced under the conditions of the present work consists of highly regular arrays of straight nanotubes with a diameter of 50 nm and a length of 17 lm. The most striking improvement from previous work is that the pore walls are completely smooth and straight. The key factor to achieve these straight nanotube layers lies in tailoring the electrochemical conditions by using a buffering electrolyte.
Acknowledgements The authors would like to acknowledge A. Friedrich, H. Hildebrand and U. Marten for SEM, XPS and XRD investigations. The author (L.T.) thanks the CAPES (Brazil) – Bavaria (Germany) program for financial support.
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References [1] F.C. Jentoft, A. Hahn, J. Kro¨hnert, G. Lorenz, R.E. Jentoft, T. Ressler, U. Wild, R. Schlo¨gl, C. Ha¨ßner, K. Ko¨hler, J. Catal. 224 (2004) 124. [2] Q.-H. Zhang, Y. Li, B.-Q. Xu, Catal. Today 98 (2004) 601. [3] B. Pihlar, Z. Cencicˇ, Anal. Chim. Acta 273 (1993) 267. [4] S. Zhuiykov, T. Ono, N. Yamazoe, N. Miura, Solid State Ionics 152–153 (2002) 801. [5] N. Imanaka, M. Kamikawa, S. Tamura, G. Adachi, Sens. Actuators B 77 (2001) 301. [6] G. Lu, N. Miura, N. Yamazoe, Sens. Actuators B 35–36 (1996) 130. [7] J. Bao, D. Xu, Q. Zhou, Z. Xu, Y. Feng, Y. Zhou, Chem. Mater. 14 (2002) 4709. [8] J. Bao, C. Tie, Z. Xu, Q. Ma, J. Hong, H. Sang, D. Sheng, Adv. Mater. 14 (2002) 44. [9] C.N.R. Rao, B.C. Satishkumar, A. Govindaraj, Chem. Commun. 16 (1997) 1581. [10] H. Shin, D.-K. Jeong, J. Lee, M.M. Sung, J. Kim, Adv. Mater. 16 (2004) 1197. [11] H. Tsuchiya, P. Schmuki, Electrochem. Commun. 6 (2004) 1131. [12] H. Tsuchiya, J.M. Macak, I. Sieber, P. Schmuki, Small, in press. [13] H. Tsuchiya, J.M. Macak, I. Sieber, P. Schmuki, Mater. Sci. Forum, accepted. [14] W.-J. Lee, W.H. Smyrl, Electrochem. Solid-State Lett. 8 (2005) B7. [15] J.M. Macak, H. Tsuchiya, P. Schmuki, Angew. Chem. Int. Ed. 44 (2005) 2100. [16] R.A. Ploc, M.A. Miller, J. Nucl. Mater. 64 (1977) 71. [17] H. Tsuchiya, P. Schmuki, unpublished results.
Fabrication and characterization of smooth high aspect ratio zirconia nanotubes Hiroaki Tsuchiya, Jan M. Macak, Luciano Taveira, Patrik Schmuki
*
Department of Materials Science, Institute for Surface Science and Corrosion (LKO), University of Erlangen-Nuremberg, Martensstrasse 7, D-91058 Erlangen, Germany Received 13 April 2005; in final form 17 May 2005 Available online 14 June 2005
Abstract In the present work, we report formation of high aspect ratio zirconia nanotubes by electrochemical anodization of zirconium in a 1 M (NH4)2SO4 electrolyte containing 0.5 wt% NH4F. Highly self-organized zirconia nanotubes can be formed with a diameter of 50 nm and a length of 17 lm, i.e. with an aspect ratio of more than 300. The nanotubes show a distinct smooth and straight morphology. XRD investigation reveals that the nanotubes have a cubic crystalline structure directly after anodization, that is, without any further annealing. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction One-dimensional nanoscale oxides such as nanotubes and nanowires have attracted much attention due to their potential applications in nanotechnology. In particular, zirconium oxide, so-called zirconia has been extensively used as a catalyst or as a catalyst support [1,2], as a sensor [3] and as a solid-electrolyte [4–6]. Zirconia has excellent technological properties and characteristics such as chemical and thermal stability, mechanical strength, wear resistance, low electrical conductivity and a high degree of biocompatibility. Recently, much research efforts have focused on the synthesis of zirconia nanotubes by using templating techniques [7–9] and atomic layer deposition [10]. Recently, we reported a simple electrochemical approach to form extremely thick nanoporous zirconia [11] and nanotubular zirconia layers [12,13]. An example is shown in Fig. 1. These porous layers have been formed in H2SO4/NaF electrolytes [12,13]. The treatment led to *
Corresponding author. Fax: +49 9131 852 7582. E-mail address: [email protected] (P. Schmuki).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.05.065
high aspect ratio pore arrays, however the pores that were formed showed a wavy and somewhat irregular wall morphology as apparent in Fig. 1. Anodization done in HF electrolytes also leads to tubular feature that are however comparably irregular and wavy [14]. In this Letter, we show a pronounced improvement of the method, that is, the formation of smooth and straight high aspect ratio zirconia nanotubes.
2. Experimental Samples were zirconium foils (99.8% purity, Goodfellow, England) with 0.1 mm thickness. Prior to the electrochemical treatment, the samples were ultra-sonicated in acetone, isopropanol and methanol successively, followed by rising with deionized (DI) water and finally drying with nitrogen. The electrochemical cell consisted of a conventional three-electrode arrangement with a platinum gauze as a counter electrode and a Huber-Luggin capillary with a Ag/AgCl (1 M KCl) electrode as a reference electrode. The samples were pressed against an o-ring in the electrochemical cell leaving
H. Tsuchiya et al. / Chemical Physics Letters 410 (2005) 188–191
189
3. Results and discussion
Fig. 1. SEM images of anodic porous zirconia layer. (a) Top-view and (b) cross-sectional of the porous layer formed on Zr in 1 M H2SO4 + 0.2 wt% NaF electrolyte at 20 V in analogy to [12,13].
1 cm2 of the samples surface exposed to the electrolyte. The electrolyte was 1 M (NH4)2SO4 + 0.5 wt% NH4F and was prepared from analytical grade chemicals and DI water. Electrochemical treatments were carried out using a high-voltage potentiostat Jaissle IMP 88 PC connected to a digital multimeter interfaced to a computer. The electrochemical treatments consisted of a potential sweep from an open-circuit potential (OCP) to 20 V with the sweep rate of 1 V s 1, followed by holding the potential at 20 V for 1 h. After the treatment, the samples were rinsed with DI water and then dried with nitrogen stream. The structure and morphology of the resulting layer were characterized with a Hitachi SEM FE S4800 field-emission scanning electron microscopy (FESEM). Thickness information was obtained by direct SEM cross-sectional observation on samples that were bent until the oxide layer cracked off. Structural information of the layer was obtained using a X-ray diffractometer (Phillips XÕpert-MPD PW3040). The chemical composition of the layer was obtained by X-ray photoelectron spectroscopy (PHI 5600 XPS).
Various electrolytes and electrochemical treatments were tested to improve the morphology of the porous ZrO2 layers from previous works [11–13]. In order to obtain electrochemical information, current–time transients proved to be a valuable tool. Fig. 2 shows such current–time curves recorded during anodization of zirconium in different electrolytes after a potential sweep from OCP to 20 V with a sweep rate of 1 V s 1. The H2SO4/NaF electrolyte was used in previous work and is included as a reference for a case where the fluoride concentration and other parameters were optimized to achieve self-organized pore formation. In the previous work tube formation was carried out in a neutral (NH4)2SO4/NH4F buffer to suppress some disadvantages of the acidic electrolyte [15]. Evidently the (NH4)2SO4 containing 0.5 wt% NH4F shows a very similar characteristics as the H2SO4/NaF electrolyte. Both curves in fluoride-containing electrolytes deviate clearly from the curve in fluoride-free (NH4)2SO4. The higher current density in the fluoride-containing electrolytes indicates an additional dissolution process takes place. This can be attributed to a dissolution of the oxide layer as soluble fluoro-complexes. As pointed out in previous work this enhanced solubility by complex formation is a key factor to achieve pore formation on valve metals. In the fluoride-free electrolyte and as a very well established in literature, only a compact oxide layer was observed. Fig. 3 shows SEM images of the surface layer formed in 1 M (NH4)2SO4 + 0.5 wt% NH4F at 20 V for 1 h after a potential sweep from OCP to 20 V with a scan rate of 1 V s 1 – shown are (a) a top-view, (b) a cross-section and (c) a bottom-view. The cross-section and bottom-
Fig. 2. Current–time curves recorded for different electrolytes during anodization of Zr at 20 V after a potential sweep from OCP to 20 V with a sweep rate of 1 V s 1.
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H. Tsuchiya et al. / Chemical Physics Letters 410 (2005) 188–191
most remarkable difference is that the nanotubes in (NH4)2SO4 + 0.5 wt% NH4F are extremely straight and the irregularities of the pore walls (wavy morphology) does not occur any more. With length of 17 lm, the aspect ratio of the nanotube layer is more than 300. Although the surface morphology is generally very uniform, on some locations, parts of the nanotube layers bended down in a heap, which might be due to the extremely high aspect ratio. Furthermore, by comparing Fig. 1b and Fig. 3b, it becomes clear that the crosssectional feature is relatively different from that in H2SO4 + 0.2 wt% NaF. A key factor to form the straight nanotube layers might be buffering effect of (NH4)2SO4/ NH4F system, thus damping possible alterations in the local pH within the pores – this is analogy with previous work on TiO2 nanotube formation [15] where we showed that the pH-profile within a growing tube can strongly determine the final morphology. Fig. 4 shows an X-ray diffraction pattern of the nanotube layer shown in Fig. 3. It is clear that the layer has a crystalline structure; the peaks corresponding to a cubic zirconia are detected. This is in line with previous work on compact oxide layers formed in a wide variety of electrolytes where the cubic structures were predominant [16]. The observed crystalline structure is particularly noteworthy as porous layers formed on other valve metals such as Ti, W, Nb and Ta typically have an amorphous structure [17]. Therefore – if a crystalline structure is desired – the layer on such valve metals must be transformed to a specific crystalline structure by annealing. However, the fact that the zirconia nanotubes have a cubic crystalline structure directly after anodization without annealing is of a particular advantage in applications that are sensitive to thermal annealing. Additional XPS investigation also showed that the
Fig. 3. SEM images of the zirconia nanotube layer formed on Zr in 1 M (NH4)2SO4 + 0.5 wt% NH4F at 20 V after a potential sweep from OCP to 20 V: (a) top-view; (b) cross-section; (c) bottom-view.
view were obtained from a mechanically cracked sample. It is clear from Fig. 3a that the layer consists of self-organized nanotube arrays of 50 nm in diameter. It is also evident from Fig. 3a and c that the nanotubes are open at the top-end, while they are closed at the bottom. The closed bottom-ends correspond to the barrier oxide layers (essentially the high-field oxide). The features of the top-view and bottom-view of the nanotube layer formed in (NH4)2SO4 + 0.5 wt% NH4F is similar to that in H2SO4 + 0.2 wt% NaF [12,13]. However, the
Fig. 4. XRD patterns for the zirconia nanotube layer formed on Zr in 1 M (NH4)2SO4 + 0.5 wt% NH4F.
H. Tsuchiya et al. / Chemical Physics Letters 410 (2005) 188–191
structure essentially consists of a ZrO2 though only traces of F could be detected in the nanotube layer.
4. Conclusions Smooth high aspect ratio zirconia nanotubes were fabricated by electrochemical anodization of zirconium. The results presented here clearly demonstrate the fabrication of high quality zirconia nanotube layers using a very simple and direct method. The layer produced under the conditions of the present work consists of highly regular arrays of straight nanotubes with a diameter of 50 nm and a length of 17 lm. The most striking improvement from previous work is that the pore walls are completely smooth and straight. The key factor to achieve these straight nanotube layers lies in tailoring the electrochemical conditions by using a buffering electrolyte.
Acknowledgements The authors would like to acknowledge A. Friedrich, H. Hildebrand and U. Marten for SEM, XPS and XRD investigations. The author (L.T.) thanks the CAPES (Brazil) – Bavaria (Germany) program for financial support.
191
References [1] F.C. Jentoft, A. Hahn, J. Kro¨hnert, G. Lorenz, R.E. Jentoft, T. Ressler, U. Wild, R. Schlo¨gl, C. Ha¨ßner, K. Ko¨hler, J. Catal. 224 (2004) 124. [2] Q.-H. Zhang, Y. Li, B.-Q. Xu, Catal. Today 98 (2004) 601. [3] B. Pihlar, Z. Cencicˇ, Anal. Chim. Acta 273 (1993) 267. [4] S. Zhuiykov, T. Ono, N. Yamazoe, N. Miura, Solid State Ionics 152–153 (2002) 801. [5] N. Imanaka, M. Kamikawa, S. Tamura, G. Adachi, Sens. Actuators B 77 (2001) 301. [6] G. Lu, N. Miura, N. Yamazoe, Sens. Actuators B 35–36 (1996) 130. [7] J. Bao, D. Xu, Q. Zhou, Z. Xu, Y. Feng, Y. Zhou, Chem. Mater. 14 (2002) 4709. [8] J. Bao, C. Tie, Z. Xu, Q. Ma, J. Hong, H. Sang, D. Sheng, Adv. Mater. 14 (2002) 44. [9] C.N.R. Rao, B.C. Satishkumar, A. Govindaraj, Chem. Commun. 16 (1997) 1581. [10] H. Shin, D.-K. Jeong, J. Lee, M.M. Sung, J. Kim, Adv. Mater. 16 (2004) 1197. [11] H. Tsuchiya, P. Schmuki, Electrochem. Commun. 6 (2004) 1131. [12] H. Tsuchiya, J.M. Macak, I. Sieber, P. Schmuki, Small, in press. [13] H. Tsuchiya, J.M. Macak, I. Sieber, P. Schmuki, Mater. Sci. Forum, accepted. [14] W.-J. Lee, W.H. Smyrl, Electrochem. Solid-State Lett. 8 (2005) B7. [15] J.M. Macak, H. Tsuchiya, P. Schmuki, Angew. Chem. Int. Ed. 44 (2005) 2100. [16] R.A. Ploc, M.A. Miller, J. Nucl. Mater. 64 (1977) 71. [17] H. Tsuchiya, P. Schmuki, unpublished results.