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tubular layers on Mg alloy (WE43)
Electrochemistry Communications 12 (2010) 796–799
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Anodic growth of self-ordered magnesium oxy-fluoride nanoporous/tubular layers on Mg alloy (WE43) Metehan C. Turhan, Robert P. Lynch 1, Himendra Jha, Patrik Schmuki, Sannakaisa Virtanen ⁎ Department of Materials Science, WW4-LKO, University of Erlangen-Nuremberg, Martensstraße 7, 91058 Erlangen, Germany
a r t i c l e
i n f o
Article history: Received 3 March 2010 Received in revised form 27 March 2010 Accepted 29 March 2010 Available online 2 April 2010
a b s t r a c t In the present work we show that self-ordered nanotubular and nanoporous magnesium oxy-fluoride structures can be grown on a magnesium-based alloy when anodized in a non-aqueous (ethylene glycol) HF electrolyte. The morphology of the surface structures varies with applied potential and anodisation time. Tubular and porous structures with different sizes and orientation can be grown. © 2010 Elsevier B.V. All rights reserved.
Keywords: Anodisation Mg surface Nanoporous surface Nanotubular surface Porous magnesium Non-aqueous electrolytes
1. Introduction Over the past decades, a variety of self ordering electrochemical processes have been found to produce oxide nanostructures from aqueous or non-aqueous fluoride-containing solutions [1]. Under optimized conditions resulting surface features are regular tubular (nanotubular) or porous structures. There are many works reported in the literature on the formation of ordered porous oxides of metals including valve metals, such as Al [2,3], Ta [4], Zr [5,6], Nb [7,8], Ti [9–11], and Hf [12], but only a few attempts can be found based on Mg and Mg alloys. Since magnesium hardly forms defined oxide films in aqueous solutions, it may be a promising approach to use nonaqueous solutions for anodisation, particularly with the aim of reducing chemical dissolution during anodisation. Brunner et al. [13] showed the formation of a black, porous oxide-layer on Mg from water free methanol or ethanol electrolytes containing nitrate ions. Ono and Asoh [14] studied anodisation of pure Mg in non-aqueous solutions where they showed the formation of a barrier layer in a solution consisting of a mixture of amine, ethylene glycol and water. They also investigated the formation of anodic oxide films on magnesium surfaces in alkaline-fluoride solutions [15] and for the first time, using alkaline aluminate solutions, they reported formation of cylindrical cellular structures [16].
⁎ Corresponding author. E-mail address: [email protected] (S. Virtanen). 1 Present address: Instytut Chemii Fizycznej (IChF), Polskiej Akademii Nauk (PAN), ul. Kasprzaka 44/52, 01-224 Warszawa, Poland. 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.03.036
Such surface modifications might be especially interesting for utilizing of Mg alloys in biomedical applications, as a biodegradable metallic implant material: a subject of steadily growing interest [17]. In order to optimize the biological performance, the corrosion behaviour of the material as well as its interactions with cells need to be tailored. In view of biomedical applications, it is interesting to mention that on TiO 2 nanotubes adhesion, proliferation and differentiation of cells have been demonstrated to be controlled by the morphology (tube diameter) of nanotubular layers [18]. In addition, in the case of TiO2, formation of hydroxyapatite has also been shown to be strongly enhanced by a nanotubular surface [19,20]. For Mg alloys, simple chemical surface treatments such as chemical passivation or biofunctionalization by soaking in simulated body fluids were recently shown to drastically influence adhesion and survival of cells on corroding Mg surfaces [21], but specific surface nanostructuring of Mg alloy surfaces has not been studied in this context. The aim of the present work was to explore possibilities to achieve electrochemical self-organized nanostructuring of Mg alloy surfaces. In this paper we report the formation of nanopores and nanotubes on a magnesium alloy surface and show the dependence of porosification on anodisation time and potential. The alloy selected (WE43) is a rare-earth metal containing Mg alloy and it is one of the likely candidates for biodegradable implants [22,23]. 2. Experimental Since HF acts as a weak acid and does not dissociate in non-aqueous solvents such as EG, electrolyte was aged (see below). During the aging
M.C. Turhan et al. / Electrochemistry Communications 12 (2010) 796–799
of the solution, titanium was dissolved and therefore the conductivity was increased by formation of (TiF6)2− ions [10]. Electrolytes of ethylene glycol (EG) were prepared with addition of hydrofluoric acid (HF): of 500 ml volume and 0.2 M HF concentration. From this solution 50 ml was ‘aged’ by anodising of Ti overnight for 12 h at a constant potential of 120 V in a two electrode cell with the solution as the electrolyte. The Ti working electrode was sealed to the cell by an o-ring, to expose 1 cm2 of the electrode surface to the electrolyte, by pressing a Cu plate, which acted as electrical contact, to the back of the electrode. A platinum foil was used as the counter electrode. After aging, the aged solution (50 ml) was stirred together with the rest of the solution (450 ml, non-aged) for 10 min. This mixture was found to have the optimum conductivity to obtain porous surface layers. WE43 Mg alloy samples — cut from a rod with a diameter of 25 mm — were anodised in the resulting electrolyte using the same electrochemical cell and counter electrode as used for electrolyte aging. Prior to anodising, the samples were ground with 1200 grit emery paper. The chemical composition of WE43 alloy is consisting of 3.7–4.3% Y with 2.2–4.4% rare-earth elements (Nd) and 0.4% Zr with balance Mg. To investigate the effect of applied potential on the resulting surface morphology, potential ramp experiments were performed. The potential ramps were from the 0 V to a maximum potential, Vmax, (of 20, 40, 70, 120, 160, 200, 240 and 300 V) at a sweep rate of 1 V s− 1 after which the potential was held constant at Vmax for 1 additional hour. In addition, effect of the anodisation duration was investigated for potential ramp experiments with a Vmax of 70 V by applying the same initial sweep to Vmax (70 V) but by varying the duration the experiment was held at Vmax. A DC-Voltage source (Heinzinger 350-2) interfaced to a personal computer via a National Instruments GPIB controller for USB, attached to a Keithley 188 model digital multimeter, was used for anodisation and curves were monitored by in-house software programmed in LabView. A field-emission scanning-electron-microscope (Hitachi FE-SEM S-4800) equipped with an energy dispersive X-ray analyser (EDAX) was used to investigate the morphology and composition of the surface layers. 3. Results and discussion Anodisation of the Mg WE43 samples at different potentials (as described in the Experimental section) indicates a formation of a compact layer of magnesium fluorides or magnesium oxy-fluorides (SEM and EDX results, not shown) with some incomplete passivation for up to Vmax = 40 V, which is followed by a drop in current containing a shoulder (a typical example of which is shown in Fig. 1 for 40 V). On such samples randomly distributed surface features such as macro pitting was observed, but no nano porous or tubular surface morphology could be detected. At higher potentials, typical passivating I–t curves are obtained as shown in Fig. 1. After the initial potential sweep to the desired potential, a drop in current density occurs — this can be attributed to surface coverage by the growing layers. After this initial drop of current density, the curve has a shoulder shape followed by a further, more rapid current decrease; probably concurrent dissolution and passivation reactions are occurring on the surface (as would be in line with expectations for self-organizing processes [1]). The slower decrease of current for 120 V than 70 V suggests a higher overall dissolution rate at the higher potential in the early stages of anodisation. The surface morphology for 70 V and 120 V anodisation, however, drastically differs from anodisation at lower potentials. At 70 V elongated nanoporous structures (nanopores/nanotubes) on the Mg alloy surface are observed (Fig. 2). At this potential, some large recessed areas are formed on the surface; focusing in these regions reveals nanostructures of both nanopores and nanotubes on the surface (Fig. 2a–f). For example, Fig. 2a shows highly packed nanotubular structures with an average diameter of 100 nm. Porous structures near these tubes with an average pore diameter of 70 nm are shown in
797
Fig. 1. Current transient as a function of time recorded during electrochemical anodisation of WE43 Mg alloy by potential ramps to three different maximum potentials (40, 70 and 120 V) in 0.2 M HF ethylene glycol solution at a sweep rate of 1 V s− 1.
Fig. 2b. Moreover, well defined and separated nanotubes are also found in these regions having an average diameter of 70 nm (Fig. 2c). Lastly, different types of long columns are also detected (Fig. 2d). Among all these different types of surface structures, tubular forms are the most frequently occurring structures. EDX measurements indicate that in this case passivation is possible not only by oxide formation but also by formation of Mg fluorides or oxy-fluorides (Fig. 3).The observation of different surface morphologies at different surface areas could be due to the fact that the alloy studied is a multiphase alloy, and different morphologies could be formed on different phases. However, until now a correlation of the surface morphology with the substrate microstructure was not possible. Further work is required to elucidate the origin of the different types of surface morphologies. When the applied potential is increased to 120 V, the resulting surface morphology displays a lesser presence of elongated porous structures as compared to surface structures formed at 70 V but still displays tubular (Fig. 2e) and pore like (Fig. 2f) features. This may be an indication of excess field supported chemical etching at higher anodisation potentials, in accordance with the slower decrease of current with time. This effect becomes even more significant at even higher potentials. The morphology in general is similar to TiO2 nanotubes; however, it is not entirely clear yet, if indeed a barrier layer is present at the bottom of the tubes. Although under the present conditions structures as ideal as grown for Al or Ti could not be attained; the experiments at 70 V represent an optimum potential for obtaining of well defined tubular structures on Mg. In order to study the initiation phase of nanotube formation, a potential ramp to Vmax = 70 V followed by a short anodisation time of 3 min at Vmax was carried out. Fig. 4 and the details in its insets show four different structural layers: the first layer (I) is a compact layer (top), the second layer (II) is porous and the third layer (III) is the propagation layer, i.e., the initiation of these structures is in agreement with a sequence frequently observed for self-organizing oxide growth [1]: I, compact layer formation; II, penetration and pore formation underneath the compact layer; and III, ordered pore growth. The growing pores finally lead to a dimpled scalloped surface (IV). With increasing anodisation time, the surface shows well-ordered tubular structures with diameters of between 50 and 80 nm (as shown for 1 h anodisation in Fig. 2). Overall, anodisation time increases tube order and abundance but for durations greater than 2 h an increasing
798
M.C. Turhan et al. / Electrochemistry Communications 12 (2010) 796–799
Fig. 2. SEM images of a WE43 magnesium alloy surface regions anodised in 0.2 M HF ethylene glycol solution at 70 V (a–d) and 120 V (e, f) during 1 h: a) nanotubular structures; b) nanoporous structures; c) separated tubular structures; d) long hollow column structures; e) nanotubular structures; and f) nanoporous structures.
loss of the porous structures is observed. This can be attributed to dissolution processes that begin to dominate the anodisation reaction. The mixed type of surface morphology (tubular and porous) and the loss of the tubular layers after extended anodisation suggest that oxide formation and chemical etching play an important interacting role in the formation of self-ordered oxide features. This suggests that the anodisation potential, the type of chemical solvent used, or the
addition of water to solution may influence the surface morphology in a similar way to what has been observed in the case of aluminum and titanium anodisation, and the effect of these parameters on Mg anodisation will require further exploration so as to achieve greater degree and range of ordering. 4. Conclusions Potentiostatic anodisation of a biorelevant magnesium alloy (WE43) in HF containing non-aqueous electrolyte, after an initial potential
Fig. 3. EDX spectra of WE43 alloy surface after anodisation by ramping the potential from 0 to 20 V and then holding it at this potential for 1 h in Ti aged 0.2 M HF ethylene glycol solution. Inset table: weight and atomic percentage ratio of the elements on the surface.
Fig. 4. SEM image of a WE43 magnesium alloy surface anodised in 0.2 M HF ethylene glycol solution at 70 V for 3 min along with a) an inset figure of surface region II; b) an inset figure of surface region III; and c) an inset figure of surface region IV.
M.C. Turhan et al. / Electrochemistry Communications 12 (2010) 796–799
sweep, can lead to growth of ordered oxy-fluoride nanostructures (nanopores and nanotubes). Optimum conditions for the formation of porous/tubular structures were found to be potential- and timedependent. In particular, it should be noted that an optimum parameter window exists. When the anodisation time or potential is too high, the resulting surface morphology is destroyed by excess chemical etching. In view of practical applications, these nanostructured layers on Mg alloys have a considerable potential in functionalising the surface, for instance in biomedical applications of Mg and its alloys. Acknowledgement The authors would like to thank the German Research Foundation (DFG) for financial support. References [1] [2] [3] [4] [5]
A. Ghicov, P. Schmuki, Chem. Comm. 20 (2009) 2791. J.W. Diggle, T.C. Downie, C.W. Goulding, Chem. Rev. 69 (1969) 365. H. Masuda, K. Fukuda, Science 268 (1995) 1466. I. Sieber, B. Kannan, P. Schmuki, Electrochem. Solid State Lett. 8 (2005) J10. H. Tsuchiya, J.M. Macak, A. Ghicov, L. Taveira, P. Schmuki, Corros. Sci. 47 (2005) 3324.
799
[6] H. Tsuchiya, J.M. Macak, I. Sieber, P. Schmuki, Small 1 (2005) 722. [7] S. Ono, T. Nagasaka, H. Shimazaki, H. Asoh, 206th Meeting of the Electrochemical Society, Honolulu, HI, 3–8 Oct, 2004, p. 788, (meeting abstracts). [8] I. Sieber, H. Hildebrand, A. Friedrich, P. Schmuki, Electrochem. Commun. 7 (2005) 97. [9] V. Zwilling, E. Darque-Ceretti, A. Boutry-Forveille, D. David, M.Y. Perrin, M. Aucouturier, Surf. Sci. Anal 27 (1999) 629. [10] J.M. Macak, H. Tsuchiya, P. Schmuki, Angew. Chem. Int. Ed. 44 (2005) 2100. [11] J.M. Macak, H. Tsuchiya, L. Taveira, S. Aldabergerova, P. Schmuki, Angew. Chem. Int. Ed. 44 (2005) 7463. [12] H. Tsuchiya, P. Schmuki, Electrochem. Commun. 7 (2005) 49. [13] J.G. Brunner, R. Hahn, J. Kunze, S. Virtanen, J. Electrochem. Soc. 156 (2009) C62. [14] S. Ono, H. Asoh, Mater. Sci. Forum 957 (2009) 419. [15] S. Ono, H. Kijima, N. Masuko, J. Jap. Ins. Light Met. 52 (2002) 115. [16] S. Ono, M. Miyake, H. Asoh, J. Jap. Ins. Light Met. 54 (2004) 544. [17] F. Witte, N. Hort, C. Vogt, S. Cohen, K.U. Kainer, R. Willumeit, F. Feyeraberd, Curr. Opin. Solid State Mater. Sci. 12 (2008) 68. [18] J. Park, S. Bauer, K. von der Mark, P. Schmuki, Nano Lett. 7 (2007) 1686. [19] H. Tsuchiya, J.M. Macak, L. Müller, J. Kunze, F. Müller, P. Greil, S. Virtanen, P. Schmuki, J. Biomed. Mat. Res. 85A (2008) 167. [20] A. Kodama, S. Bauer, A. Komatsu, H. Asoh, S. Ono, P. Schmuki, Acta Biomater. 5 (2009) 2322. [21] C. Lorenz, J.G. Brunner, P. Kollmannberger, L. Jaafar, B. Fabry, S. Virtanen, Biomaterialia 5 (2009) 2783. [22] R. Rettig, S. Virtanen, J. Biomed. Mat. Res. 77A (2006) 534. [23] R. Rettig, S. Virtanen, J. Biomed. Mat. Res. 88A (2009) 359.
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Anodic growth of self-ordered magnesium oxy-fluoride nanoporous/tubular layers on Mg alloy (WE43) Metehan C. Turhan, Robert P. Lynch 1, Himendra Jha, Patrik Schmuki, Sannakaisa Virtanen ⁎ Department of Materials Science, WW4-LKO, University of Erlangen-Nuremberg, Martensstraße 7, 91058 Erlangen, Germany
a r t i c l e
i n f o
Article history: Received 3 March 2010 Received in revised form 27 March 2010 Accepted 29 March 2010 Available online 2 April 2010
a b s t r a c t In the present work we show that self-ordered nanotubular and nanoporous magnesium oxy-fluoride structures can be grown on a magnesium-based alloy when anodized in a non-aqueous (ethylene glycol) HF electrolyte. The morphology of the surface structures varies with applied potential and anodisation time. Tubular and porous structures with different sizes and orientation can be grown. © 2010 Elsevier B.V. All rights reserved.
Keywords: Anodisation Mg surface Nanoporous surface Nanotubular surface Porous magnesium Non-aqueous electrolytes
1. Introduction Over the past decades, a variety of self ordering electrochemical processes have been found to produce oxide nanostructures from aqueous or non-aqueous fluoride-containing solutions [1]. Under optimized conditions resulting surface features are regular tubular (nanotubular) or porous structures. There are many works reported in the literature on the formation of ordered porous oxides of metals including valve metals, such as Al [2,3], Ta [4], Zr [5,6], Nb [7,8], Ti [9–11], and Hf [12], but only a few attempts can be found based on Mg and Mg alloys. Since magnesium hardly forms defined oxide films in aqueous solutions, it may be a promising approach to use nonaqueous solutions for anodisation, particularly with the aim of reducing chemical dissolution during anodisation. Brunner et al. [13] showed the formation of a black, porous oxide-layer on Mg from water free methanol or ethanol electrolytes containing nitrate ions. Ono and Asoh [14] studied anodisation of pure Mg in non-aqueous solutions where they showed the formation of a barrier layer in a solution consisting of a mixture of amine, ethylene glycol and water. They also investigated the formation of anodic oxide films on magnesium surfaces in alkaline-fluoride solutions [15] and for the first time, using alkaline aluminate solutions, they reported formation of cylindrical cellular structures [16].
⁎ Corresponding author. E-mail address: [email protected] (S. Virtanen). 1 Present address: Instytut Chemii Fizycznej (IChF), Polskiej Akademii Nauk (PAN), ul. Kasprzaka 44/52, 01-224 Warszawa, Poland. 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.03.036
Such surface modifications might be especially interesting for utilizing of Mg alloys in biomedical applications, as a biodegradable metallic implant material: a subject of steadily growing interest [17]. In order to optimize the biological performance, the corrosion behaviour of the material as well as its interactions with cells need to be tailored. In view of biomedical applications, it is interesting to mention that on TiO 2 nanotubes adhesion, proliferation and differentiation of cells have been demonstrated to be controlled by the morphology (tube diameter) of nanotubular layers [18]. In addition, in the case of TiO2, formation of hydroxyapatite has also been shown to be strongly enhanced by a nanotubular surface [19,20]. For Mg alloys, simple chemical surface treatments such as chemical passivation or biofunctionalization by soaking in simulated body fluids were recently shown to drastically influence adhesion and survival of cells on corroding Mg surfaces [21], but specific surface nanostructuring of Mg alloy surfaces has not been studied in this context. The aim of the present work was to explore possibilities to achieve electrochemical self-organized nanostructuring of Mg alloy surfaces. In this paper we report the formation of nanopores and nanotubes on a magnesium alloy surface and show the dependence of porosification on anodisation time and potential. The alloy selected (WE43) is a rare-earth metal containing Mg alloy and it is one of the likely candidates for biodegradable implants [22,23]. 2. Experimental Since HF acts as a weak acid and does not dissociate in non-aqueous solvents such as EG, electrolyte was aged (see below). During the aging
M.C. Turhan et al. / Electrochemistry Communications 12 (2010) 796–799
of the solution, titanium was dissolved and therefore the conductivity was increased by formation of (TiF6)2− ions [10]. Electrolytes of ethylene glycol (EG) were prepared with addition of hydrofluoric acid (HF): of 500 ml volume and 0.2 M HF concentration. From this solution 50 ml was ‘aged’ by anodising of Ti overnight for 12 h at a constant potential of 120 V in a two electrode cell with the solution as the electrolyte. The Ti working electrode was sealed to the cell by an o-ring, to expose 1 cm2 of the electrode surface to the electrolyte, by pressing a Cu plate, which acted as electrical contact, to the back of the electrode. A platinum foil was used as the counter electrode. After aging, the aged solution (50 ml) was stirred together with the rest of the solution (450 ml, non-aged) for 10 min. This mixture was found to have the optimum conductivity to obtain porous surface layers. WE43 Mg alloy samples — cut from a rod with a diameter of 25 mm — were anodised in the resulting electrolyte using the same electrochemical cell and counter electrode as used for electrolyte aging. Prior to anodising, the samples were ground with 1200 grit emery paper. The chemical composition of WE43 alloy is consisting of 3.7–4.3% Y with 2.2–4.4% rare-earth elements (Nd) and 0.4% Zr with balance Mg. To investigate the effect of applied potential on the resulting surface morphology, potential ramp experiments were performed. The potential ramps were from the 0 V to a maximum potential, Vmax, (of 20, 40, 70, 120, 160, 200, 240 and 300 V) at a sweep rate of 1 V s− 1 after which the potential was held constant at Vmax for 1 additional hour. In addition, effect of the anodisation duration was investigated for potential ramp experiments with a Vmax of 70 V by applying the same initial sweep to Vmax (70 V) but by varying the duration the experiment was held at Vmax. A DC-Voltage source (Heinzinger 350-2) interfaced to a personal computer via a National Instruments GPIB controller for USB, attached to a Keithley 188 model digital multimeter, was used for anodisation and curves were monitored by in-house software programmed in LabView. A field-emission scanning-electron-microscope (Hitachi FE-SEM S-4800) equipped with an energy dispersive X-ray analyser (EDAX) was used to investigate the morphology and composition of the surface layers. 3. Results and discussion Anodisation of the Mg WE43 samples at different potentials (as described in the Experimental section) indicates a formation of a compact layer of magnesium fluorides or magnesium oxy-fluorides (SEM and EDX results, not shown) with some incomplete passivation for up to Vmax = 40 V, which is followed by a drop in current containing a shoulder (a typical example of which is shown in Fig. 1 for 40 V). On such samples randomly distributed surface features such as macro pitting was observed, but no nano porous or tubular surface morphology could be detected. At higher potentials, typical passivating I–t curves are obtained as shown in Fig. 1. After the initial potential sweep to the desired potential, a drop in current density occurs — this can be attributed to surface coverage by the growing layers. After this initial drop of current density, the curve has a shoulder shape followed by a further, more rapid current decrease; probably concurrent dissolution and passivation reactions are occurring on the surface (as would be in line with expectations for self-organizing processes [1]). The slower decrease of current for 120 V than 70 V suggests a higher overall dissolution rate at the higher potential in the early stages of anodisation. The surface morphology for 70 V and 120 V anodisation, however, drastically differs from anodisation at lower potentials. At 70 V elongated nanoporous structures (nanopores/nanotubes) on the Mg alloy surface are observed (Fig. 2). At this potential, some large recessed areas are formed on the surface; focusing in these regions reveals nanostructures of both nanopores and nanotubes on the surface (Fig. 2a–f). For example, Fig. 2a shows highly packed nanotubular structures with an average diameter of 100 nm. Porous structures near these tubes with an average pore diameter of 70 nm are shown in
797
Fig. 1. Current transient as a function of time recorded during electrochemical anodisation of WE43 Mg alloy by potential ramps to three different maximum potentials (40, 70 and 120 V) in 0.2 M HF ethylene glycol solution at a sweep rate of 1 V s− 1.
Fig. 2b. Moreover, well defined and separated nanotubes are also found in these regions having an average diameter of 70 nm (Fig. 2c). Lastly, different types of long columns are also detected (Fig. 2d). Among all these different types of surface structures, tubular forms are the most frequently occurring structures. EDX measurements indicate that in this case passivation is possible not only by oxide formation but also by formation of Mg fluorides or oxy-fluorides (Fig. 3).The observation of different surface morphologies at different surface areas could be due to the fact that the alloy studied is a multiphase alloy, and different morphologies could be formed on different phases. However, until now a correlation of the surface morphology with the substrate microstructure was not possible. Further work is required to elucidate the origin of the different types of surface morphologies. When the applied potential is increased to 120 V, the resulting surface morphology displays a lesser presence of elongated porous structures as compared to surface structures formed at 70 V but still displays tubular (Fig. 2e) and pore like (Fig. 2f) features. This may be an indication of excess field supported chemical etching at higher anodisation potentials, in accordance with the slower decrease of current with time. This effect becomes even more significant at even higher potentials. The morphology in general is similar to TiO2 nanotubes; however, it is not entirely clear yet, if indeed a barrier layer is present at the bottom of the tubes. Although under the present conditions structures as ideal as grown for Al or Ti could not be attained; the experiments at 70 V represent an optimum potential for obtaining of well defined tubular structures on Mg. In order to study the initiation phase of nanotube formation, a potential ramp to Vmax = 70 V followed by a short anodisation time of 3 min at Vmax was carried out. Fig. 4 and the details in its insets show four different structural layers: the first layer (I) is a compact layer (top), the second layer (II) is porous and the third layer (III) is the propagation layer, i.e., the initiation of these structures is in agreement with a sequence frequently observed for self-organizing oxide growth [1]: I, compact layer formation; II, penetration and pore formation underneath the compact layer; and III, ordered pore growth. The growing pores finally lead to a dimpled scalloped surface (IV). With increasing anodisation time, the surface shows well-ordered tubular structures with diameters of between 50 and 80 nm (as shown for 1 h anodisation in Fig. 2). Overall, anodisation time increases tube order and abundance but for durations greater than 2 h an increasing
798
M.C. Turhan et al. / Electrochemistry Communications 12 (2010) 796–799
Fig. 2. SEM images of a WE43 magnesium alloy surface regions anodised in 0.2 M HF ethylene glycol solution at 70 V (a–d) and 120 V (e, f) during 1 h: a) nanotubular structures; b) nanoporous structures; c) separated tubular structures; d) long hollow column structures; e) nanotubular structures; and f) nanoporous structures.
loss of the porous structures is observed. This can be attributed to dissolution processes that begin to dominate the anodisation reaction. The mixed type of surface morphology (tubular and porous) and the loss of the tubular layers after extended anodisation suggest that oxide formation and chemical etching play an important interacting role in the formation of self-ordered oxide features. This suggests that the anodisation potential, the type of chemical solvent used, or the
addition of water to solution may influence the surface morphology in a similar way to what has been observed in the case of aluminum and titanium anodisation, and the effect of these parameters on Mg anodisation will require further exploration so as to achieve greater degree and range of ordering. 4. Conclusions Potentiostatic anodisation of a biorelevant magnesium alloy (WE43) in HF containing non-aqueous electrolyte, after an initial potential
Fig. 3. EDX spectra of WE43 alloy surface after anodisation by ramping the potential from 0 to 20 V and then holding it at this potential for 1 h in Ti aged 0.2 M HF ethylene glycol solution. Inset table: weight and atomic percentage ratio of the elements on the surface.
Fig. 4. SEM image of a WE43 magnesium alloy surface anodised in 0.2 M HF ethylene glycol solution at 70 V for 3 min along with a) an inset figure of surface region II; b) an inset figure of surface region III; and c) an inset figure of surface region IV.
M.C. Turhan et al. / Electrochemistry Communications 12 (2010) 796–799
sweep, can lead to growth of ordered oxy-fluoride nanostructures (nanopores and nanotubes). Optimum conditions for the formation of porous/tubular structures were found to be potential- and timedependent. In particular, it should be noted that an optimum parameter window exists. When the anodisation time or potential is too high, the resulting surface morphology is destroyed by excess chemical etching. In view of practical applications, these nanostructured layers on Mg alloys have a considerable potential in functionalising the surface, for instance in biomedical applications of Mg and its alloys. Acknowledgement The authors would like to thank the German Research Foundation (DFG) for financial support. References [1] [2] [3] [4] [5]
A. Ghicov, P. Schmuki, Chem. Comm. 20 (2009) 2791. J.W. Diggle, T.C. Downie, C.W. Goulding, Chem. Rev. 69 (1969) 365. H. Masuda, K. Fukuda, Science 268 (1995) 1466. I. Sieber, B. Kannan, P. Schmuki, Electrochem. Solid State Lett. 8 (2005) J10. H. Tsuchiya, J.M. Macak, A. Ghicov, L. Taveira, P. Schmuki, Corros. Sci. 47 (2005) 3324.
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