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Fast anodically growth of long, small diameter TiO2 nanotubes by electropolishing of Ti foils in an ethanol-containing solution
Materials Letters 150 (2015) 81–83
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Fast anodically growth of long, small diameter TiO2 nanotubes by electropolishing of Ti foils in an ethanol-containing solution Fatemeh Mohammadpour 1,a, Fahimeh Behzadi a,b,1, Mahmood Moradi a,n a b
Department of Physics, College of Science, Shiraz University, Shiraz 71454, Iran Department of Physics, College of Science, Fasa University, Fasa 73654, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 20 December 2014 Accepted 14 February 2015 Available online 21 February 2015
Anodic growth of TiO2 nanotubes with length of several 10 mm in fluoride containing electrolytes conventionally leads to tube diameter of ca. 90–100 nm with an undesired nanograss appearance on the surface of tubular array. Fabrication of nanotubes with diameters smaller than 50 nm needs mild anodization conditions that lead to tubes shorter than 7 mm. In the present work, we introduce a simple and fast approach for fabrication of highly ordered, long nanotubes, 23 mm, with dramatically small diameter of ca. 29 nm, free of nanograss and top porous layer just by 2 h anodization. This method is based on the creation of small diameter semi-spheres on the Ti surface by electropolishing in an ethanolcontaining electrolyte. Besides pretreatment of Ti foil, we show that controlled stirring speed and temperature of the electrolyte during anodization are crucial factors to achieve this novel morphology. & 2015 Elsevier B.V. All rights reserved.
Keywords: TiO2 nanotube Anodization Small diameter Porous materials Semicondutors
1. Introduction Fabrication of TiO2 nanotubes has received considerable attention over the past decade due to its special semiconducting properties. Photocatalyst production [1], hydrogen generation [2] and construction of dye-sensitized solar cells [3,4] are some of the TiO2 nanotubes applications. The geometry and morphology of the anodic TiO2 nanotubes have significant effects on the performance of these devices. For example, tubes top with no extra oxide layer [5], tube length in the range of 10–20 mm [6] and small tube diameter [7] lead to higher power conversion efficiency in dye-sensitized solar cells. To produce TiO2 nanotubes with small diameter, mild etching conditions are necessary during anodization of Ti foils which are usually provided by anodization in low voltages; whereas its most important problem is producing tubes shorter than 7 mm [7–9]. Recently, Wang et al. [10] have succeeded to fabricate long tubes with small diameter of ca. 35 nm, but in a long anodization time of two days. Here, we investigate the fabrication of the long TiO2 nanotubes with dramatically small diameter of ca. 29 nm by tuning the surface morphology of the Ti foils before anodization. The produced nanotubes have length of ca. 23 mm, just after 2 h anodization. We, also,
n
Corresponding author. Tel.: þ 98 7136137013; fax: þ98 7136460839. E-mail address: [email protected] (M. Moradi). These authors contributed equally to this work and both should be considered as first authors. 1
http://dx.doi.org/10.1016/j.matlet.2015.02.081 0167-577X/& 2015 Elsevier B.V. All rights reserved.
show that controlling temperature and stirring speed of electrolyte during anodization have crucial roles on the final geometry of tubes.
2. Experimental Ti foils (0.25 mm thick, 99.7% purity, Aldrich) were successively cleaned through sonication in acetone, ethanol and deionized water for 10 min during each step and dried under a pure nitrogen stream. One of the samples was used as an as-received sample and the two others were electropolished in two types of electrolyte. One of them was electropolished at 10 1C in an electrolyte solution containing perchloric acid (65%, Merk) and glacial acetic acid (100%, Merk) with volume ratio of 1:9. This sample is named as the non-ethanolic sample, here. The above mentioned electrolyte with additional 15 vol% ethanol was used for electropolishing the other sample at 1 1C which is named as the ethanolic sample. Electropolishing was done in a two-electrode cell where the working and counter electrodes were Ti foil and graphite sheet, respectively. The electropolishing voltage was increased from 0 to 50 V with rate of 1 V/s and held at 50 V for 6 min. The anodization was performed at fixed temperature of 20 ˚C in the two-electrode cell in an ethylene glycol based electrolyte containing 0.3 wt% NH4F and 2 vol% H2O at a constant potential of 60 V and the electrolyte stirring speed of 1100 rpm. To investigate the effect of stirring speed on the nanotubes morphology, different stirring speeds of 0, 1100 and 1800 rpm were chosen for 1 h anodization. Also, the effect of controlling temperature on the fabricated tubes
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F. Mohammadpour et al. / Materials Letters 150 (2015) 81–83
was investigated by anodizing an ethanolic sample at room temperature. Before anodization, roughness factor of the surfaces (RMS) was measured by an atomic force microscopy (Veeco AFM AutoProbe CP-Research). The field emission scanning electron microscopy images were taken by Hitachi FE-SEM S-4160.
3. Results and discussion The as-received, non-ethanolic and ethanolic Ti samples are labeled with A, B and C, respectively. The first column in Fig. 1 with subscribed 1 for labels shows the AFM images of Ti foils with different surface morphologies. The corresponding RMS is given in Table 1. Fig. 1A1 shows a flat surface with abrupt cliffs on the asreceived Ti sample. As can be seen from Fig. 1B1 and C1, electropolishing creates regular and moderate fluctuations on the surface. These semi-sphere undulations are similar to footprints of anodic TiO2 nanotubes on Ti substrate after their removing [11]. In fact, these semi-sphere undulations acts as an initial nucleation layer for growing nanotubes. These footprints on ethanolic sample not only are more regular than the non-ethanolic, but also have smaller diameters. Adding ethanol to the polishing electrolyte results in increase of disintegration of the ions and thereby the increment of the conductivity of the electrolyte. Therefore, the more number of ions per unit area attack on the Ti surface in the ethanolic electrolyte which leads to the higher density of footprints and consequently smaller diameter semispheres on the sample. These footprints act as initial seeding sites for growing tubes where the semispheres diameter correspond the outer diameter of the tubes with size of ca. 92 nm. The inner diameter of the tubes is of ca. 29 nm.
The FE-SEM images in Fig. 1 illustrate the growth stages of TiO2 nanotubes on Ti foils with different surface morphologies. The subscripts 2, 3, 4, and 5 refer to the anodizing time of 1 min, 10 min, 1 h and 2 h, in order. Random and non-uniform pits with large diameter are observed on the as-received sample (Fig. 1A2) due to the uneven initial surface. The pits are distributed more uniformly with smaller diameter on the non-ethanolic Ti surface (Fig. 1B2). This uniformity increases for the ethanolic Ti sample (Fig. 1C2) which can be related to the uniform distribution of the initial seeds on this surface.
Fig. 2. Anodization current density versus time curves for three initial Ti surface conditions: as-received, non-ethanolic and ethanolic samples.
Fig. 1. AFM images of Ti samples at a scale of 1 1 mm2 in different surface conditions: as-received (A1), non-ethanolic (B1), ethanolic (C1). Top-view of FE-SEM images of anodic TiO2 nanotubes on different surface morphologies: as-received (A series), non-ethanolic (B series) and ethanolic (C series) samples at different anodizing time intervals of 1, 10, 60 and 120 min which are subscripted as 2, 3, 4 and 5, respectively. Insets in the last column show cross-section of the corresponding samples after 2 h.
Table 1 Dimensions of grown TiO2 nanotubes on as-received, ethanolic and non-ethanolic samples after 2 h anodization. Sample
RMS (nm)
Inner diameter (nm)
Wall thickness (nm)
Outer diameter (nm)
Length (lm)
Aspect ratio
As-received Non-ethanolic Ethanolic
25.95 11.43 8.14
90 75 95 75 29 75
167 1 127 1 277 1
125 75 120 75 92 75
9.06 23.69 22.70
101 249 783
F. Mohammadpour et al. / Materials Letters 150 (2015) 81–83
83
Fig. 3. FE-SEM images of the anodized ethanolic Ti samples at constant temperature of 20 1C and stirring speed of: (a) 0, (b) 1100, (c) 1800 rpm and (d) corresponding anodization current density curves versus time. (e) FE-SEM image of the anodized ethanolic Ti samples at room temprature and (f) corresponding anodization current density curves versus time.
After 2 h anodization of the samples, a “nanograss” appearance can be seen on the surface of the as-received Ti sample due to chemical etching of tubes top during anodizing (Fig. 1A5); whereas the anodized non-ethanolic Ti sample has a porous layer on the surface with no grass-like appearance, Fig. 1B5. It is noticeable that it can be seen neither “nanograss” appearance nor porous layer on the tubes top of anodized ethanolic Ti sample in contrast to the other samples, Fig. 1C5. It is reported that a rutile layer can be formed at the initial stages of anodization on the polished surfaces [12] that acts as a protective layer against chemical etching of the electrolyte. It seems that the behavior of porous layer on the anodized electropolished surfaces in our work is the same as the rutile layer. The comparison between Fig. 1C4 and C5 shows that the porous layer on tube tops is removed after 2 h anodizing on the ethanolic sample (Fig. 1C5). The inset in Fig. 1C4 shows the grown tubes beneath the porous layer. The nanotube dimensions of the samples after 2 h anodization are summarized in Table 1. Fig. 2 shows the anodization current density versus time curves during the growth of TiO2 nanotubes. As it is known in the anodization process, the current density drops in all cases to reach to its final value [11]. But a minimum in the curve of ethanolic samples is observed which is similar to second anodization processes that results in ordered nanotubes formation. The effect of temperature and stirring speed of the electrolyte on the final geometry of the tubes are other experimental parameters which are investigated in this work. Fig. 3(a–c) shows the FE-SEM images of the grown TiO2 nanotubes on the ethanolic Ti samples after 1 h anodizing at 20 1C and in different stirring conditions such as: (a) 0, (b) 1100 and (c) 1800 rpm. The corresponding current density versus time curves is illustrated in Fig. 3d. As it can be seen from Fig. 3b, the fabricated tubes in electrolyte with stirring speed of 1100 rpm are formed properly and uniformly. A fresh electrolyte reaches to the sample at stirring conditions that result in the increment of the current density, Fig. 3d. As the stirring switched off, the movement of the effective ions decreases and thereby the current density decreases. When the speed of stirring is very fast,
1800 rpm, the more etching of the pore walls is achieved by more effective ions which cause nanopores with bigger diameter, Fig. 3c. Similarly, the current density increases gradually due to the increment of electrolyte temperature at room environment, Fig. 3f. The higher temperature causes faster ion movement inside the tubes and thereby faster etching of the walls is achieved, therefore, the pore diameter increases to the conventional tube diameter of ca. 90 nm, Fig. 3e. 4. Conclusions Long TiO2 nanotubes, 23 mm, with dramatically small diameter of ca. 29 nm were fabricated by anodizing of electropolished Ti foils after 2 h. Electropolishing in an ethanol-containing solution leads to small diameter footprints on the Ti surface which acts as a nucleation layer for growing tubes. We have shown that a fixed temperature of 20 1C and suitable electrolyte stirring speed, 1100 rpm, have significant role in production of this novel structure. References [1] Albu SP, Ghicov A, Macak JM, Hahn R, Schmuki P. Nano Lett 2007;7:1286–9. [2] Whitby M, Quirke N. Nat Nanotechnol 2007;2:87–94. [3] Mohammadpour F, Moradi M, Lee K, Cha G, So S, Kahnt A, Guldi DM, Altomare M, Schmuki P. Chem Commun 2015;51:1631–4. [4] Mohammadpour F, Moradi M, Cha G, So S, Lee K, Altomare M, Schmuki P. J ChemElectroChem 2014. http://dx.doi.org/10.1002/celc.201402368. [5] Yan J, Zhou F. J Mater Chem 2011;21:9406–18. [6] Jennings JJ, Ghicov A, Peter LM, Schmuki P, Walker AB. J Am Chem Soc 2008;130:13364–72. [7] Liu N, Lee K, Schmuki P. Electrochem Commun 2012;15:1–4. [8] Zhu K, Neale NR, Miedaner A, Frank A. Nano Lett 2007;7:69–74. [9] Macak JM, Tsuchiya H, Taveria L, Aldabergerova S, Schmuki P. J Angew Chem Int Ed 2005;44:7463–5. [10] Wang X, Sun L, Zhang S, Wang X. J Appl Mater Interfaces 2014;6:1361–5. [11] Eftekhari A. Nanostructured materials in electrochemistry. Weinheim: WileyVCH; 2008. [12] Kunze J, Seyeux A, Schmuki P. J. Electrochem. Solid-State Lett 2008;11:K11–3.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Fast anodically growth of long, small diameter TiO2 nanotubes by electropolishing of Ti foils in an ethanol-containing solution Fatemeh Mohammadpour 1,a, Fahimeh Behzadi a,b,1, Mahmood Moradi a,n a b
Department of Physics, College of Science, Shiraz University, Shiraz 71454, Iran Department of Physics, College of Science, Fasa University, Fasa 73654, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 20 December 2014 Accepted 14 February 2015 Available online 21 February 2015
Anodic growth of TiO2 nanotubes with length of several 10 mm in fluoride containing electrolytes conventionally leads to tube diameter of ca. 90–100 nm with an undesired nanograss appearance on the surface of tubular array. Fabrication of nanotubes with diameters smaller than 50 nm needs mild anodization conditions that lead to tubes shorter than 7 mm. In the present work, we introduce a simple and fast approach for fabrication of highly ordered, long nanotubes, 23 mm, with dramatically small diameter of ca. 29 nm, free of nanograss and top porous layer just by 2 h anodization. This method is based on the creation of small diameter semi-spheres on the Ti surface by electropolishing in an ethanolcontaining electrolyte. Besides pretreatment of Ti foil, we show that controlled stirring speed and temperature of the electrolyte during anodization are crucial factors to achieve this novel morphology. & 2015 Elsevier B.V. All rights reserved.
Keywords: TiO2 nanotube Anodization Small diameter Porous materials Semicondutors
1. Introduction Fabrication of TiO2 nanotubes has received considerable attention over the past decade due to its special semiconducting properties. Photocatalyst production [1], hydrogen generation [2] and construction of dye-sensitized solar cells [3,4] are some of the TiO2 nanotubes applications. The geometry and morphology of the anodic TiO2 nanotubes have significant effects on the performance of these devices. For example, tubes top with no extra oxide layer [5], tube length in the range of 10–20 mm [6] and small tube diameter [7] lead to higher power conversion efficiency in dye-sensitized solar cells. To produce TiO2 nanotubes with small diameter, mild etching conditions are necessary during anodization of Ti foils which are usually provided by anodization in low voltages; whereas its most important problem is producing tubes shorter than 7 mm [7–9]. Recently, Wang et al. [10] have succeeded to fabricate long tubes with small diameter of ca. 35 nm, but in a long anodization time of two days. Here, we investigate the fabrication of the long TiO2 nanotubes with dramatically small diameter of ca. 29 nm by tuning the surface morphology of the Ti foils before anodization. The produced nanotubes have length of ca. 23 mm, just after 2 h anodization. We, also,
n
Corresponding author. Tel.: þ 98 7136137013; fax: þ98 7136460839. E-mail address: [email protected] (M. Moradi). These authors contributed equally to this work and both should be considered as first authors. 1
http://dx.doi.org/10.1016/j.matlet.2015.02.081 0167-577X/& 2015 Elsevier B.V. All rights reserved.
show that controlling temperature and stirring speed of electrolyte during anodization have crucial roles on the final geometry of tubes.
2. Experimental Ti foils (0.25 mm thick, 99.7% purity, Aldrich) were successively cleaned through sonication in acetone, ethanol and deionized water for 10 min during each step and dried under a pure nitrogen stream. One of the samples was used as an as-received sample and the two others were electropolished in two types of electrolyte. One of them was electropolished at 10 1C in an electrolyte solution containing perchloric acid (65%, Merk) and glacial acetic acid (100%, Merk) with volume ratio of 1:9. This sample is named as the non-ethanolic sample, here. The above mentioned electrolyte with additional 15 vol% ethanol was used for electropolishing the other sample at 1 1C which is named as the ethanolic sample. Electropolishing was done in a two-electrode cell where the working and counter electrodes were Ti foil and graphite sheet, respectively. The electropolishing voltage was increased from 0 to 50 V with rate of 1 V/s and held at 50 V for 6 min. The anodization was performed at fixed temperature of 20 ˚C in the two-electrode cell in an ethylene glycol based electrolyte containing 0.3 wt% NH4F and 2 vol% H2O at a constant potential of 60 V and the electrolyte stirring speed of 1100 rpm. To investigate the effect of stirring speed on the nanotubes morphology, different stirring speeds of 0, 1100 and 1800 rpm were chosen for 1 h anodization. Also, the effect of controlling temperature on the fabricated tubes
82
F. Mohammadpour et al. / Materials Letters 150 (2015) 81–83
was investigated by anodizing an ethanolic sample at room temperature. Before anodization, roughness factor of the surfaces (RMS) was measured by an atomic force microscopy (Veeco AFM AutoProbe CP-Research). The field emission scanning electron microscopy images were taken by Hitachi FE-SEM S-4160.
3. Results and discussion The as-received, non-ethanolic and ethanolic Ti samples are labeled with A, B and C, respectively. The first column in Fig. 1 with subscribed 1 for labels shows the AFM images of Ti foils with different surface morphologies. The corresponding RMS is given in Table 1. Fig. 1A1 shows a flat surface with abrupt cliffs on the asreceived Ti sample. As can be seen from Fig. 1B1 and C1, electropolishing creates regular and moderate fluctuations on the surface. These semi-sphere undulations are similar to footprints of anodic TiO2 nanotubes on Ti substrate after their removing [11]. In fact, these semi-sphere undulations acts as an initial nucleation layer for growing nanotubes. These footprints on ethanolic sample not only are more regular than the non-ethanolic, but also have smaller diameters. Adding ethanol to the polishing electrolyte results in increase of disintegration of the ions and thereby the increment of the conductivity of the electrolyte. Therefore, the more number of ions per unit area attack on the Ti surface in the ethanolic electrolyte which leads to the higher density of footprints and consequently smaller diameter semispheres on the sample. These footprints act as initial seeding sites for growing tubes where the semispheres diameter correspond the outer diameter of the tubes with size of ca. 92 nm. The inner diameter of the tubes is of ca. 29 nm.
The FE-SEM images in Fig. 1 illustrate the growth stages of TiO2 nanotubes on Ti foils with different surface morphologies. The subscripts 2, 3, 4, and 5 refer to the anodizing time of 1 min, 10 min, 1 h and 2 h, in order. Random and non-uniform pits with large diameter are observed on the as-received sample (Fig. 1A2) due to the uneven initial surface. The pits are distributed more uniformly with smaller diameter on the non-ethanolic Ti surface (Fig. 1B2). This uniformity increases for the ethanolic Ti sample (Fig. 1C2) which can be related to the uniform distribution of the initial seeds on this surface.
Fig. 2. Anodization current density versus time curves for three initial Ti surface conditions: as-received, non-ethanolic and ethanolic samples.
Fig. 1. AFM images of Ti samples at a scale of 1 1 mm2 in different surface conditions: as-received (A1), non-ethanolic (B1), ethanolic (C1). Top-view of FE-SEM images of anodic TiO2 nanotubes on different surface morphologies: as-received (A series), non-ethanolic (B series) and ethanolic (C series) samples at different anodizing time intervals of 1, 10, 60 and 120 min which are subscripted as 2, 3, 4 and 5, respectively. Insets in the last column show cross-section of the corresponding samples after 2 h.
Table 1 Dimensions of grown TiO2 nanotubes on as-received, ethanolic and non-ethanolic samples after 2 h anodization. Sample
RMS (nm)
Inner diameter (nm)
Wall thickness (nm)
Outer diameter (nm)
Length (lm)
Aspect ratio
As-received Non-ethanolic Ethanolic
25.95 11.43 8.14
90 75 95 75 29 75
167 1 127 1 277 1
125 75 120 75 92 75
9.06 23.69 22.70
101 249 783
F. Mohammadpour et al. / Materials Letters 150 (2015) 81–83
83
Fig. 3. FE-SEM images of the anodized ethanolic Ti samples at constant temperature of 20 1C and stirring speed of: (a) 0, (b) 1100, (c) 1800 rpm and (d) corresponding anodization current density curves versus time. (e) FE-SEM image of the anodized ethanolic Ti samples at room temprature and (f) corresponding anodization current density curves versus time.
After 2 h anodization of the samples, a “nanograss” appearance can be seen on the surface of the as-received Ti sample due to chemical etching of tubes top during anodizing (Fig. 1A5); whereas the anodized non-ethanolic Ti sample has a porous layer on the surface with no grass-like appearance, Fig. 1B5. It is noticeable that it can be seen neither “nanograss” appearance nor porous layer on the tubes top of anodized ethanolic Ti sample in contrast to the other samples, Fig. 1C5. It is reported that a rutile layer can be formed at the initial stages of anodization on the polished surfaces [12] that acts as a protective layer against chemical etching of the electrolyte. It seems that the behavior of porous layer on the anodized electropolished surfaces in our work is the same as the rutile layer. The comparison between Fig. 1C4 and C5 shows that the porous layer on tube tops is removed after 2 h anodizing on the ethanolic sample (Fig. 1C5). The inset in Fig. 1C4 shows the grown tubes beneath the porous layer. The nanotube dimensions of the samples after 2 h anodization are summarized in Table 1. Fig. 2 shows the anodization current density versus time curves during the growth of TiO2 nanotubes. As it is known in the anodization process, the current density drops in all cases to reach to its final value [11]. But a minimum in the curve of ethanolic samples is observed which is similar to second anodization processes that results in ordered nanotubes formation. The effect of temperature and stirring speed of the electrolyte on the final geometry of the tubes are other experimental parameters which are investigated in this work. Fig. 3(a–c) shows the FE-SEM images of the grown TiO2 nanotubes on the ethanolic Ti samples after 1 h anodizing at 20 1C and in different stirring conditions such as: (a) 0, (b) 1100 and (c) 1800 rpm. The corresponding current density versus time curves is illustrated in Fig. 3d. As it can be seen from Fig. 3b, the fabricated tubes in electrolyte with stirring speed of 1100 rpm are formed properly and uniformly. A fresh electrolyte reaches to the sample at stirring conditions that result in the increment of the current density, Fig. 3d. As the stirring switched off, the movement of the effective ions decreases and thereby the current density decreases. When the speed of stirring is very fast,
1800 rpm, the more etching of the pore walls is achieved by more effective ions which cause nanopores with bigger diameter, Fig. 3c. Similarly, the current density increases gradually due to the increment of electrolyte temperature at room environment, Fig. 3f. The higher temperature causes faster ion movement inside the tubes and thereby faster etching of the walls is achieved, therefore, the pore diameter increases to the conventional tube diameter of ca. 90 nm, Fig. 3e. 4. Conclusions Long TiO2 nanotubes, 23 mm, with dramatically small diameter of ca. 29 nm were fabricated by anodizing of electropolished Ti foils after 2 h. Electropolishing in an ethanol-containing solution leads to small diameter footprints on the Ti surface which acts as a nucleation layer for growing tubes. We have shown that a fixed temperature of 20 1C and suitable electrolyte stirring speed, 1100 rpm, have significant role in production of this novel structure. References [1] Albu SP, Ghicov A, Macak JM, Hahn R, Schmuki P. Nano Lett 2007;7:1286–9. [2] Whitby M, Quirke N. Nat Nanotechnol 2007;2:87–94. [3] Mohammadpour F, Moradi M, Lee K, Cha G, So S, Kahnt A, Guldi DM, Altomare M, Schmuki P. Chem Commun 2015;51:1631–4. [4] Mohammadpour F, Moradi M, Cha G, So S, Lee K, Altomare M, Schmuki P. J ChemElectroChem 2014. http://dx.doi.org/10.1002/celc.201402368. [5] Yan J, Zhou F. J Mater Chem 2011;21:9406–18. [6] Jennings JJ, Ghicov A, Peter LM, Schmuki P, Walker AB. J Am Chem Soc 2008;130:13364–72. [7] Liu N, Lee K, Schmuki P. Electrochem Commun 2012;15:1–4. [8] Zhu K, Neale NR, Miedaner A, Frank A. Nano Lett 2007;7:69–74. [9] Macak JM, Tsuchiya H, Taveria L, Aldabergerova S, Schmuki P. J Angew Chem Int Ed 2005;44:7463–5. [10] Wang X, Sun L, Zhang S, Wang X. J Appl Mater Interfaces 2014;6:1361–5. [11] Eftekhari A. Nanostructured materials in electrochemistry. Weinheim: WileyVCH; 2008. [12] Kunze J, Seyeux A, Schmuki P. J. Electrochem. Solid-State Lett 2008;11:K11–3.