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Efficient suppression of nanograss during porous anodic TiO2 nanotubes growth
Applied Surface Science 314 (2014) 505–509
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Efficient suppression of nanograss during porous anodic TiO2 nanotubes growth Qunfang Gui a,b , Dongliang Yu a,b , Dongdong Li b,∗ , Ye Song a , Xufei Zhu a,∗ , Liu Cao a , Shaoyu Zhang a , Weihua Ma a , Shiyu You a a b
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China
a r t i c l e
i n f o
Article history: Received 26 May 2014 Received in revised form 6 July 2014 Accepted 9 July 2014 Available online 15 July 2014 Keywords: Anodic titanium oxide Nanotubes Nanograss Sacrificed layer
a b s t r a c t When Ti foil was anodized in fluoride-containing electrolyte for a long time, undesired etching-induced “nanograss” would inevitably generate on the top of porous anodic TiO2 nanotubes (PATNTs). The nanograss will hinder the ions transport and in turn yield depressed (photo) electrochemical performance. In order to obtain nanograss-free nanotubes, a modified three-step anodization and two-layer nanostructure of PATNTs were designed to avoid the nanograss. The first layer (L1 ) nanotubes were obtained by the conventional two-step anodization. After washing and drying processes, the third-step anodization was carried out with the presence of L1 nanotubes. The L1 nanotubes, serving as a sacrificed layer, was etched and transformed into nanograss, while the ultralong nanotubes (L2 ) were maintained underneath the L1 . The bi-layer nanostructure of the nanograss/nanotubes (L1 /L2 ) was then ultrasonically rinsed in deionized water to remove the nanograss (L1 layer). Then much longer nanotubes (L2 layer) with intact nanotube mouths could be obtained. Using this novel approach, the ultralong nanotubes without nanograss can be rationally controlled by adjusting the anodizing times of two layers. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Porous anodic TiO2 nanotubes (PATNTs) have attracted considerable scientific interests because of their interesting formation mechanisms [1–3] and extensive applications [4–6]. Longer nanotubes with higher aspect ratio count for more active area, thus, in order to obtain well-ordered and long PATNTs, many successful approaches have been employed for assembling the hierarchically structured TiO2 nanotubes [7–9]. However, particularly long nanotubes have to be grown by extended anodizing time, often a disintegration of the top end of nanotubes due to chemical dissolution is observed, and then the ordered PATNTs transform into TiO2 nanowires or nanograss [10,11]. The fabrication of much longer (higher aspect ratio) nanotubes without nanograss is still a great challenge. For a given rate of pore formation, the chemical dissolution of the oxide at the pore mouth by F− ions determines the nanotube length [11]. The TiO2 nanotubes with nanograss have been reported in many literatures [11–14]. The top etching or bundling of the nanotubes not only affects the exciton transport
∗ Corresponding authors. Tel.: +86 25 84315949; fax: +86 25 84303029. E-mail addresses: [email protected] (D. Li), [email protected] (X. Zhu). http://dx.doi.org/10.1016/j.apsusc.2014.07.046 0169-4332/© 2014 Elsevier B.V. All rights reserved.
and recombination dynamics in PATNTs-based solar cells [15], but also has an impact on the infiltration of interacted materials [16]. Therefore, mainly four techniques have been put forward to avoid or remove the disordered bundled nanograss: (1) formation of a less soluble top layer, (2) supercritical drying, (3) sonication directly and (4) multilayer architecture [10,15,17,18]. Schmuki and coworkers [10,19] have employed an optimized photoresist and rutile layer on the Ti substrate before the anodic tube growth, which improved the morphology in some extend. However, the nanograss issue is far from being solved [14–17]. Therefore, it is of great significance to eliminate the bundled nanograss on the top end of nanotubes. In this work, the formation mechanism of the bundled nanograss on the top end of nanotubes was elaborated. The evolution of top etching and appearance of nanograss seem inevitably for a long time anodizing process. Herein, using a modified three-step anodization, the obtained PATNTs can be free from bundling compared with those obtained by typical two-step anodization for the same duration. The first nanotube layer (L1 ) anodized for a shorter time (T1 ), serves as a protective layer for the second nanotube layer (L2 ) anodized for a longer time (T2 ). While the second nanotube layer formed, the L1 layer was etched and transformed into nanograss, yielding a nanograss/nanotubes (L1 /L2 ) bi-layer structure. The nanograss top layer can be easily removed by comparison
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with other techniques because of the existence of the separated interface between L1 and L2 , after mild ultrasonic treatment, the top surface morphology of the PATNTs is quite neat. As a result, much longer (higher aspect ratio) nanotubes with intact nanotube mouths, i.e., the L2 nanotubes without nanograss, can be obtained ultimately. This technique is particularly significant for applications that require open and long nanotubes.
2. Experimental The commercial Ti foils (purity 99.7%, 0.2 mm thickness) were ultrasonically cleaned in acetone, ethanol and deionized water successively for 10 min, respectively. In order to obtain the prepatterned Ti substrates, the first anodization was performed. The electrolyte was ethylene glycol solution containing 0.3 wt.% NH4 F and 2 vol% H2 O. All Ti substrates were anodized for the first time at 60 V for 1 h in a conventional two-electrode cell with a carbon rod as cathode. Then the as-anodized samples were ultrasonicated in deionized water at 320 W for 10 min to remove the preformed TiO2 nanotubes gaining the pre-patterned Ti substrates. After washed and dried in the air, the pre-patterned Ti substrates were anodized for the second time. All the PATNTs were prepared at approximately 20 ◦ C under an applied voltage of 60 V. In order to compare the formation process of the bundled nanograss, the pre-patterned Ti substrates were anodized at 60 V for different anodizing times of 1 h, 2 h, 3 h and 4 h, respectively. In order to obtain nanograss-free nanotubes, bi-layer nanostructure of PATNTs was designed to avoid the formation of nanograss. The first layer (L1 ) of the nanotubes was obtained in the secondstep anodization for a shorter time (T1 = 20 min). The sample with the first layer nanotubes was then rinsed in deionized water for
5 min, dried in N2 gas flow for 3 min at room temperature. After washing and drying, the sample with the first layer nanotubes was put into the electrolyte for the third-step anodization with a longer duration (T2 = 2 h, 4 h and 6 h). The L1 top layer was etched gradually and transformed into nanograss during the formation of the second layer (L2 ) underneath, leading to a bi-layer structure of the nanograss/nanotubes (L1 /L2 ). The obtained bi-layer samples were ultrasonicated in deionized water at 160 W for 8 min to remove the L1 nanograss layer, and then much longer L2 nanotubes without nanograss could be obtained. The morphologies of PATNTs were characterized by a field-emission scanning electron microscope (FESEM, FEI Quanta 600).
3. Results and discussion In order to understand the formation mechanism of the bundled nanograss, the evolution of top etching or the collapse of nanotubes need to be discussed. Fig. 1 shows the FESEM images of the top surface and cross-section morphologies of PATNTs anodized at 60 V for different durations of 1 h, 2 h, 3 h and 4 h, respectively. With the extension of time, a series of changes of the top surface and cross-section morphologies can be observed in Fig. 1. Fig. 1a and b show that when the anodization time increased from 1 h to 2 h, the wall thickness was reduced due to the electrochemical etching near the top of the nanotubes. And the tubes also started disintegrating as seen from the cross section view (Fig. 1a and b). When the anodization time further increased to 3 h, as illustrated in Fig. 1c, the nanotubes near the top surface were broken up, even without the nanotubular shape. When the anodization time extended to 4 h,
Fig. 1. Surface (insets) and cross section morphologies of the PATNTs anodized at 60 V for different times: (a) 1 h, (b) 2 h, (c) 3 h, and (d) 4 h.
Q. Gui et al. / Applied Surface Science 314 (2014) 505–509
as shown in Fig. 1d, bundled nanograss could be found on the entire surface, covering the top of the nanotubes. Based on above results, the evolution of nanograss may be explained by the following three steps: (1) thinning the tube wall thickness of nanotubes near the top surface, (2) breaking up the nanotubes in the upper section, (3) splitting nanotubes into nanograss. With prolonged anodizing time, the wall thickness in the upper section became thinner due to the chemical dissolution of TiO2 in fluoride-containing electrolyte (TiO2 + 4H+ + 6F− → [TiF6 ]2− + 2H2 O). Thinning of nanotube walls or perforating of nanotube walls in the upper parts may be associated with the distribution of fluorides preferentially at boundary triple points [20]. Unceasingly anodization will results in the collapse of the thin nanotubes or bundling nanograss (Fig. 1c and d) owing to a lack of mechanical support [20]. From the above, the top etching seems inevitable when anodization for a longer time without any protection. So we adopted a novel approach to obtain self-organized nanotube layers with significantly improved tube morphology. That is, introduce a protective nanotube layer L1 before gaining a desired controllable nanotube layer L2 . In comparison with the conventional anodizing process (without protective layer), Fig. 2 shows the current–time characteristics during the growth of nanotubes without and with a protective layer at 60 V for 20 min. For the conventional unprotected sample, the current decreased sharply and then increased [20–22], lastly reached a stable current after approximately 8 min. These three stages correspond to three processes, i.e., the formation of the barrier oxide film, the pore generation on the barrier film surface, and the self-ordering process of the pore development [20–25]. For the sample with the protective layer, the value of the stable current is smaller than that of the unprotected sample. The lower current density is ascribed to the lower ionic transport velocity due to the existence of protective layer.
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Fig. 2. Current–time curves during anodization process with and without protective layer. The inset is a closer look at current oscillations in the range of 0–30 min.
Fig. 3 shows the FESEM images of the top and cross-section morphologies of bi-layer PATNTs prepared beneath a protective layer. The second layer was fabricated by the third-step anodization for 2 h, 4 h, and 6 h, respectively. During the formation of the underneath nanotubes, the protective L1 layer was gradually etched and transformed into nanograss, and then the bi-layer structure of the nanograss/nanotubes (L1 /L2 ) was fabricated. Obviously, the protective layer has collapsed seriously with the extension of the third-step anodization time. Meanwhile, the lengths of L2 nanotubes increase with the extension of third-step anodization time. Interestingly, the L2 layer reaches 54.1 m with markedly intact feature even after 6 h anodization because of the protection of L1 layer. The morphology of the L2 nanotubes is dramatically improved compared with that of the unprotected sample after 3 h anodization (Fig. 1c).
Fig. 3. Cross-sectional FESEM images of TiO2 nanotube arrays formed under protective layer. (a) T1 = 20 min, T2 = 2 h; (b) T1 = 20 min, T2 = 4 h; (c) and (d) T1 = 20 min, T2 = 6 h. The red arrows show the boundaries of the two layers. The insets show the nanotube lengths and the boundaries of the L1 /L2 interfaces (red arrows). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. FESEM images of the nanotubes before (a) and after (b) mild ultrasonic treatment to remove the top nanograss. The inset of (b) shows the top surface of the L2 layer nanotubes after mild ultrasonic treatment. (c) Cross-sectional FESEM images of bi-layer TiO2 nanotube arrays, the first nanotube layer (upper part) anodized for 1 h, the second nanotube layer (lower part) anodized for 2 h. The inset shows the arc base of the first layer nanotubes.
Here, Fig. 3a and d show obviously the existence of the separated interface (boundary) between L1 and L2 . Thus, the binding capacity between L1 and L2 is weak because of the boundary. The nanograss L1 layer can be easily removed by ultrasonication. Fig. 4a also clearly shows the separated interface between the L1 and L2 layer. And after the bi-layer structure ultrasonically cleaned in deionized water, the L2 nanotubes without nanograss could be obtained ultimately as shown in Fig. 4b. Fig. 4a and b illustrate the FESEM images of the bi-layer nanograss/nanotube structure as well as the neat nanotubes after removal of nanograss layer. We consider that the separated interface between the L1 and L2 layer (as shown in Fig. 4a) results from the treatment process of washing and drying after the second anodization. The washing and drying steps are important for the formation of separated interface (L1 /L2 ) in the third-step anodization. The third-step anodization in the fluoride-containing electrolyte may begin from the interface between the Ti substrate and the L1 layer (protective layer), i.e., the L2 layer nanotubes grow again from the Ti substrate. The growth of new barrier layers in the third-step anodization can be proved by the current–time curve in Fig. 2. Fig. 2 shows the whole process of PATNT growth (three stages), and the first stage (current-decrease stage) corresponds to the new barrier layer formation of L2 nanotubes [23,24]. This consequence was proved by the cross-sectional images in Fig. 4c. We can find that the arc base of the first layer (L1 ) nanotubes are sealed, that is to say, the L2 nanotubes in the lower layer start growing at the separated interface between the existing nanotubes (L1 layer) and the Ti substrate. 4. Conclusions In order to obtain well-ordered, neat and rather long TiO2 nanotube arrays, a successful approach is desired for removal of the top etching layer. In this work, a modified three-step anodization
and nanograss/nanotube bi-layer nanostructure were developed. The first layer is obtained by the second step anodization, which serves as a sacrifice to weaken the dissolution effect on the second layer nanotubes. Moreover, the washing and drying processes before the third-step anodization facilitates the formation of a separated interface between L1 and L2 layers. The weak binding capacity due to the natural separated interface is beneficial for the selective removal of top nanograss layer. Using this novel approach, nanotubes with desired length and intact tubular morphology are produced by adjusting the anodization time. The high aspect ratio and nanograss free structure would promote the PATNTs electrode performances in a variety of electrochemical energy conversion and storage devices.
Acknowledgement This work was supported financially by the National Natural Science Foundation of China (Grant Nos. 61171043, 51377085, 51102271, 21276127).
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Efficient suppression of nanograss during porous anodic TiO2 nanotubes growth Qunfang Gui a,b , Dongliang Yu a,b , Dongdong Li b,∗ , Ye Song a , Xufei Zhu a,∗ , Liu Cao a , Shaoyu Zhang a , Weihua Ma a , Shiyu You a a b
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China
a r t i c l e
i n f o
Article history: Received 26 May 2014 Received in revised form 6 July 2014 Accepted 9 July 2014 Available online 15 July 2014 Keywords: Anodic titanium oxide Nanotubes Nanograss Sacrificed layer
a b s t r a c t When Ti foil was anodized in fluoride-containing electrolyte for a long time, undesired etching-induced “nanograss” would inevitably generate on the top of porous anodic TiO2 nanotubes (PATNTs). The nanograss will hinder the ions transport and in turn yield depressed (photo) electrochemical performance. In order to obtain nanograss-free nanotubes, a modified three-step anodization and two-layer nanostructure of PATNTs were designed to avoid the nanograss. The first layer (L1 ) nanotubes were obtained by the conventional two-step anodization. After washing and drying processes, the third-step anodization was carried out with the presence of L1 nanotubes. The L1 nanotubes, serving as a sacrificed layer, was etched and transformed into nanograss, while the ultralong nanotubes (L2 ) were maintained underneath the L1 . The bi-layer nanostructure of the nanograss/nanotubes (L1 /L2 ) was then ultrasonically rinsed in deionized water to remove the nanograss (L1 layer). Then much longer nanotubes (L2 layer) with intact nanotube mouths could be obtained. Using this novel approach, the ultralong nanotubes without nanograss can be rationally controlled by adjusting the anodizing times of two layers. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Porous anodic TiO2 nanotubes (PATNTs) have attracted considerable scientific interests because of their interesting formation mechanisms [1–3] and extensive applications [4–6]. Longer nanotubes with higher aspect ratio count for more active area, thus, in order to obtain well-ordered and long PATNTs, many successful approaches have been employed for assembling the hierarchically structured TiO2 nanotubes [7–9]. However, particularly long nanotubes have to be grown by extended anodizing time, often a disintegration of the top end of nanotubes due to chemical dissolution is observed, and then the ordered PATNTs transform into TiO2 nanowires or nanograss [10,11]. The fabrication of much longer (higher aspect ratio) nanotubes without nanograss is still a great challenge. For a given rate of pore formation, the chemical dissolution of the oxide at the pore mouth by F− ions determines the nanotube length [11]. The TiO2 nanotubes with nanograss have been reported in many literatures [11–14]. The top etching or bundling of the nanotubes not only affects the exciton transport
∗ Corresponding authors. Tel.: +86 25 84315949; fax: +86 25 84303029. E-mail addresses: [email protected] (D. Li), [email protected] (X. Zhu). http://dx.doi.org/10.1016/j.apsusc.2014.07.046 0169-4332/© 2014 Elsevier B.V. All rights reserved.
and recombination dynamics in PATNTs-based solar cells [15], but also has an impact on the infiltration of interacted materials [16]. Therefore, mainly four techniques have been put forward to avoid or remove the disordered bundled nanograss: (1) formation of a less soluble top layer, (2) supercritical drying, (3) sonication directly and (4) multilayer architecture [10,15,17,18]. Schmuki and coworkers [10,19] have employed an optimized photoresist and rutile layer on the Ti substrate before the anodic tube growth, which improved the morphology in some extend. However, the nanograss issue is far from being solved [14–17]. Therefore, it is of great significance to eliminate the bundled nanograss on the top end of nanotubes. In this work, the formation mechanism of the bundled nanograss on the top end of nanotubes was elaborated. The evolution of top etching and appearance of nanograss seem inevitably for a long time anodizing process. Herein, using a modified three-step anodization, the obtained PATNTs can be free from bundling compared with those obtained by typical two-step anodization for the same duration. The first nanotube layer (L1 ) anodized for a shorter time (T1 ), serves as a protective layer for the second nanotube layer (L2 ) anodized for a longer time (T2 ). While the second nanotube layer formed, the L1 layer was etched and transformed into nanograss, yielding a nanograss/nanotubes (L1 /L2 ) bi-layer structure. The nanograss top layer can be easily removed by comparison
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Q. Gui et al. / Applied Surface Science 314 (2014) 505–509
with other techniques because of the existence of the separated interface between L1 and L2 , after mild ultrasonic treatment, the top surface morphology of the PATNTs is quite neat. As a result, much longer (higher aspect ratio) nanotubes with intact nanotube mouths, i.e., the L2 nanotubes without nanograss, can be obtained ultimately. This technique is particularly significant for applications that require open and long nanotubes.
2. Experimental The commercial Ti foils (purity 99.7%, 0.2 mm thickness) were ultrasonically cleaned in acetone, ethanol and deionized water successively for 10 min, respectively. In order to obtain the prepatterned Ti substrates, the first anodization was performed. The electrolyte was ethylene glycol solution containing 0.3 wt.% NH4 F and 2 vol% H2 O. All Ti substrates were anodized for the first time at 60 V for 1 h in a conventional two-electrode cell with a carbon rod as cathode. Then the as-anodized samples were ultrasonicated in deionized water at 320 W for 10 min to remove the preformed TiO2 nanotubes gaining the pre-patterned Ti substrates. After washed and dried in the air, the pre-patterned Ti substrates were anodized for the second time. All the PATNTs were prepared at approximately 20 ◦ C under an applied voltage of 60 V. In order to compare the formation process of the bundled nanograss, the pre-patterned Ti substrates were anodized at 60 V for different anodizing times of 1 h, 2 h, 3 h and 4 h, respectively. In order to obtain nanograss-free nanotubes, bi-layer nanostructure of PATNTs was designed to avoid the formation of nanograss. The first layer (L1 ) of the nanotubes was obtained in the secondstep anodization for a shorter time (T1 = 20 min). The sample with the first layer nanotubes was then rinsed in deionized water for
5 min, dried in N2 gas flow for 3 min at room temperature. After washing and drying, the sample with the first layer nanotubes was put into the electrolyte for the third-step anodization with a longer duration (T2 = 2 h, 4 h and 6 h). The L1 top layer was etched gradually and transformed into nanograss during the formation of the second layer (L2 ) underneath, leading to a bi-layer structure of the nanograss/nanotubes (L1 /L2 ). The obtained bi-layer samples were ultrasonicated in deionized water at 160 W for 8 min to remove the L1 nanograss layer, and then much longer L2 nanotubes without nanograss could be obtained. The morphologies of PATNTs were characterized by a field-emission scanning electron microscope (FESEM, FEI Quanta 600).
3. Results and discussion In order to understand the formation mechanism of the bundled nanograss, the evolution of top etching or the collapse of nanotubes need to be discussed. Fig. 1 shows the FESEM images of the top surface and cross-section morphologies of PATNTs anodized at 60 V for different durations of 1 h, 2 h, 3 h and 4 h, respectively. With the extension of time, a series of changes of the top surface and cross-section morphologies can be observed in Fig. 1. Fig. 1a and b show that when the anodization time increased from 1 h to 2 h, the wall thickness was reduced due to the electrochemical etching near the top of the nanotubes. And the tubes also started disintegrating as seen from the cross section view (Fig. 1a and b). When the anodization time further increased to 3 h, as illustrated in Fig. 1c, the nanotubes near the top surface were broken up, even without the nanotubular shape. When the anodization time extended to 4 h,
Fig. 1. Surface (insets) and cross section morphologies of the PATNTs anodized at 60 V for different times: (a) 1 h, (b) 2 h, (c) 3 h, and (d) 4 h.
Q. Gui et al. / Applied Surface Science 314 (2014) 505–509
as shown in Fig. 1d, bundled nanograss could be found on the entire surface, covering the top of the nanotubes. Based on above results, the evolution of nanograss may be explained by the following three steps: (1) thinning the tube wall thickness of nanotubes near the top surface, (2) breaking up the nanotubes in the upper section, (3) splitting nanotubes into nanograss. With prolonged anodizing time, the wall thickness in the upper section became thinner due to the chemical dissolution of TiO2 in fluoride-containing electrolyte (TiO2 + 4H+ + 6F− → [TiF6 ]2− + 2H2 O). Thinning of nanotube walls or perforating of nanotube walls in the upper parts may be associated with the distribution of fluorides preferentially at boundary triple points [20]. Unceasingly anodization will results in the collapse of the thin nanotubes or bundling nanograss (Fig. 1c and d) owing to a lack of mechanical support [20]. From the above, the top etching seems inevitable when anodization for a longer time without any protection. So we adopted a novel approach to obtain self-organized nanotube layers with significantly improved tube morphology. That is, introduce a protective nanotube layer L1 before gaining a desired controllable nanotube layer L2 . In comparison with the conventional anodizing process (without protective layer), Fig. 2 shows the current–time characteristics during the growth of nanotubes without and with a protective layer at 60 V for 20 min. For the conventional unprotected sample, the current decreased sharply and then increased [20–22], lastly reached a stable current after approximately 8 min. These three stages correspond to three processes, i.e., the formation of the barrier oxide film, the pore generation on the barrier film surface, and the self-ordering process of the pore development [20–25]. For the sample with the protective layer, the value of the stable current is smaller than that of the unprotected sample. The lower current density is ascribed to the lower ionic transport velocity due to the existence of protective layer.
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Fig. 2. Current–time curves during anodization process with and without protective layer. The inset is a closer look at current oscillations in the range of 0–30 min.
Fig. 3 shows the FESEM images of the top and cross-section morphologies of bi-layer PATNTs prepared beneath a protective layer. The second layer was fabricated by the third-step anodization for 2 h, 4 h, and 6 h, respectively. During the formation of the underneath nanotubes, the protective L1 layer was gradually etched and transformed into nanograss, and then the bi-layer structure of the nanograss/nanotubes (L1 /L2 ) was fabricated. Obviously, the protective layer has collapsed seriously with the extension of the third-step anodization time. Meanwhile, the lengths of L2 nanotubes increase with the extension of third-step anodization time. Interestingly, the L2 layer reaches 54.1 m with markedly intact feature even after 6 h anodization because of the protection of L1 layer. The morphology of the L2 nanotubes is dramatically improved compared with that of the unprotected sample after 3 h anodization (Fig. 1c).
Fig. 3. Cross-sectional FESEM images of TiO2 nanotube arrays formed under protective layer. (a) T1 = 20 min, T2 = 2 h; (b) T1 = 20 min, T2 = 4 h; (c) and (d) T1 = 20 min, T2 = 6 h. The red arrows show the boundaries of the two layers. The insets show the nanotube lengths and the boundaries of the L1 /L2 interfaces (red arrows). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. FESEM images of the nanotubes before (a) and after (b) mild ultrasonic treatment to remove the top nanograss. The inset of (b) shows the top surface of the L2 layer nanotubes after mild ultrasonic treatment. (c) Cross-sectional FESEM images of bi-layer TiO2 nanotube arrays, the first nanotube layer (upper part) anodized for 1 h, the second nanotube layer (lower part) anodized for 2 h. The inset shows the arc base of the first layer nanotubes.
Here, Fig. 3a and d show obviously the existence of the separated interface (boundary) between L1 and L2 . Thus, the binding capacity between L1 and L2 is weak because of the boundary. The nanograss L1 layer can be easily removed by ultrasonication. Fig. 4a also clearly shows the separated interface between the L1 and L2 layer. And after the bi-layer structure ultrasonically cleaned in deionized water, the L2 nanotubes without nanograss could be obtained ultimately as shown in Fig. 4b. Fig. 4a and b illustrate the FESEM images of the bi-layer nanograss/nanotube structure as well as the neat nanotubes after removal of nanograss layer. We consider that the separated interface between the L1 and L2 layer (as shown in Fig. 4a) results from the treatment process of washing and drying after the second anodization. The washing and drying steps are important for the formation of separated interface (L1 /L2 ) in the third-step anodization. The third-step anodization in the fluoride-containing electrolyte may begin from the interface between the Ti substrate and the L1 layer (protective layer), i.e., the L2 layer nanotubes grow again from the Ti substrate. The growth of new barrier layers in the third-step anodization can be proved by the current–time curve in Fig. 2. Fig. 2 shows the whole process of PATNT growth (three stages), and the first stage (current-decrease stage) corresponds to the new barrier layer formation of L2 nanotubes [23,24]. This consequence was proved by the cross-sectional images in Fig. 4c. We can find that the arc base of the first layer (L1 ) nanotubes are sealed, that is to say, the L2 nanotubes in the lower layer start growing at the separated interface between the existing nanotubes (L1 layer) and the Ti substrate. 4. Conclusions In order to obtain well-ordered, neat and rather long TiO2 nanotube arrays, a successful approach is desired for removal of the top etching layer. In this work, a modified three-step anodization
and nanograss/nanotube bi-layer nanostructure were developed. The first layer is obtained by the second step anodization, which serves as a sacrifice to weaken the dissolution effect on the second layer nanotubes. Moreover, the washing and drying processes before the third-step anodization facilitates the formation of a separated interface between L1 and L2 layers. The weak binding capacity due to the natural separated interface is beneficial for the selective removal of top nanograss layer. Using this novel approach, nanotubes with desired length and intact tubular morphology are produced by adjusting the anodization time. The high aspect ratio and nanograss free structure would promote the PATNTs electrode performances in a variety of electrochemical energy conversion and storage devices.
Acknowledgement This work was supported financially by the National Natural Science Foundation of China (Grant Nos. 61171043, 51377085, 51102271, 21276127).
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