Anodization Fabrication of Highly Ordered TiO2 Nanotubes - The

Jun 9, 2009 - This Article focuses on the fabrication of highly ordered nanotubes and some novel nanostructures of titania (TiO2) with a two-step anod...
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J. Phys. Chem. C 2009, 113, 12759–12765

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Anodization Fabrication of Highly Ordered TiO2 Nanotubes Shiqi Li, Gengmin Zhang,* Dengzhu Guo, Ligang Yu, and Wei Zhang Key Laboratory for the Physics and Chemistry of NanodeVices and Department of Electronics, Peking UniVersity, Beijing 100871, China ReceiVed: April 2, 2009; ReVised Manuscript ReceiVed: May 7, 2009

This Article focuses on the fabrication of highly ordered nanotubes and some novel nanostructures of titania (TiO2) with a two-step anodization method. The first-step anodization was actually a pretreatment of the Ti foil surface and provided well-ordered imprints that served as a template for the further growth of nanotubes. As a result, the TiO2 nanotubes growing in the second-step anodization appreciably outperformed those fabricated with the conventional one-step Ti anodization in terms of size uniformity and arrangement orderliness. The parameters of the anodization were then modulated to obtain more complex structures. When the voltage in the second-step anodization was lower than that in the first-step anodization, a lotus root-shaped TiO2 nanostructure, in which each imprint contained several smaller nanopores, was achieved. When the second anodization was further divided into two stages, double-layered nanotube arrays were synthesized. They contained two distinctly separated parts, i.e., the bamboo-shaped upper one and the smooth-walled lower one. These results have demonstrated the effectiveness and controllability of the two-step anodization method in producing high-quality TiO2 nanotubes, which are believed to have potential applications in such fields as solar cells, photonic crystals, and hydrogen storage. 1. Introduction Titania (TiO2) has been widely investigated due to its application prospects in such areas as gas sensors,1-5 photocatalysis,6 and photovoltaic cells.7-9 It is well-known that properties and performance of TiO2 are dependent partly on its crystallinity and morphology. Especially, TiO2 nanotubes usually have advantages over TiO2 films because the former can provide large surface-to-volume ratio and unidirectional electrical channel.10-18 In this context, the anodization route has been developed for the preparation of the TiO2 nanotubes. In their pioneering works, Gong et al. obtained TiO2 nanotubes by the anodization of a pure Ti sheet in an aqueous solution of hydrofluoric (HF) acid.19 From then on, significant progress has been further made in this field.20,21 In many applications of the TiO2 nanotube arrays, e.g., dyesensitized solar cells (DSSCs) and hydrogen storage, smooth topography and orderly arrangement are desired.22-24 More interestingly, the possibility of photon manipulating with the TiO2 photonic crystal has also been suggested due to the high refractive index (2.5-2.9) and minimal absorption in the visible spectrum of TiO2.25-27 The demand for a strict periodicity of the photonic crystal entails very good uniformity of the TiO2 nanotubes in an array. In this sense, the morphology of the TiO2 nanotube arrays, e.g., the smoothness of the layer top and the orderliness of the nanotubes, still remains to be further improved. Though highly ordered Al2O3 nanopores are routinely obtainable by the anodization of Al,28 the anodization of Ti usually gives rise to TiO2 nanotube arrays with rough top surfaces and poor alignment. So far, several effective approaches to fabricating highly ordered TiO2 nanostructures have been developed, mainly including ion track lithography,29,30 atomic layer deposition * Corresponding author. Phone: 86-10-62751773. Fax: 86-10-62762999. E-mail: [email protected].

(ALD),31 and self-organization.32,33 Among them, the two-step anodization of a Ti foil is the most convenient and economical method.32,34 The work described in this paper is largely devoted to further improving the morphology of the TiO2 nanotube arrays on the basis of the two-step anodization method. Herein, a well-textured Ti surface was obtained after the removal of the nanotube layer generated in the first-step anodization. Then, the Ti foil that had experienced this pretreatment was anodized again for the eventual growth of highly ordered TiO2 nanotubes. It is worth emphasizing that in this work the detachment of the first nanotube layer from the Ti foil was achieved by an ultrasonic treatment instead of using an adhesion tape.34,35 This modification has helped avoid possible mechanical damage to the Ti surface and also greatly improved the uniformity and alignment of the TiO2 nanotubes. As a result, high-quality arrays of TiO2 nanotubes have been achieved. Moreover, some novel structures of TiO2 based on its highly ordered nanotube arrays were also achieved. On the one hand, the bamboo-type nanotube-based DSSCs show a significantly higher efficiency than those based on smooth-walled tubes due to substantial increase in dye loading achieved by the bamboo rings;36 on the other hand, however, reducing the dimensionality of transport and recombination in DSSCs can also increase the efficiency.22 Herein, in an effort to integrate both the above two advantages to the samples, double-layered TiO2 nanotube arrays with two distinctly separated parts were synthesized. The upper parts of the samples assumed a bamboolike shape and the lower parts had smooth walls. 2. Experimental Section The principal component of the electrolyte used in the Ti anodization was ethylene glycol (C2H6O2). Importantly, the solution also contained 0.25% (in mass) NH4F and 1% (in volume) H2O. The electrolyte was aged under a 60 V voltage for 60 h before being formally used in anodization. A Ti foil,

10.1021/jp903037f CCC: $40.75  2009 American Chemical Society Published on Web 06/09/2009

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Figure 1. Results of the first-step anodization: (a) the SEM image of the Ti surface outside the anodization region (inset is the EDX); (b) the Ti surface after the removal of the nanotube layer (inset is the EDX); (c) the SEM image of the nanotube layer generated in the first-step anodization (inset was obtained when the sample was titled by 45° from the incident electron beam); (d) the bottom side of the nanotube layer that shed from the Ti foil; (e) the Ti foil surface exposed after the ultrasonic removal of a nanotube layer fabricated with an insufficiently aged electrolyte.

0.1 mm in thickness and 99.7% in purity, was cleaned ultrasonically in turn in acetone, deionized water, and ethanol. Then, the foil was bound to the electrolytic cell with an O-ring and used as the anode. The cathode was a piece of graphite, whose area was about four times that of the Ti foil. The anode and the cathode were separated by approximately 3 cm. The reaction was driven by a dc power source. All the experiments were done at room temperature. The fabrication of the TiO2 nanotubes presented here featured an anodization that included two steps. In the first step, which was actually a pretreatment, a Ti foil was anodized at 60 V for 24 h, and a layer of nanotubes grew on the foil surface. Then, the nanotube layer was removed ultrasonically in deionized water, and the glossy underlying Ti was exposed. As will be elaborated in the next section, a pattern was left on the foil surface after the removal of the nanotube layer, and it would play a key role in the further growth of well-aligned nanotubes. In the second step of the anodization, the pretreated Ti foil was used as the anode again, and the voltage applied to it was either 60 or 30 V in different experiments. As will be illustrated in the next section, nanotubes resulted from the 60 V voltage and lotus-root-like nanostructures from the 30 V voltage. After the conclusion of the two-step anodization, the sample was cleaned ultrasonically in ethanol and finally rinsed in turn in deionized water and ethanol. Such means as scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX) were used to characterize the samples. The FEI XL-30 scanning electron microscope (SEM), which was also attached with an EDX spectroscope, and the H-9000NAR transmission electron microscope (TEM) worked under the 15 and 300 kV accelerating voltages, respectively. 3. Results and Discussion 3.1. Preparatory Anodization. In most of the experiments in this field, nanotubes were directly grown on Ti foils.37,38 Since the surface of a Ti foil is corrugated, the nanotubes on it are usually in a disordered array. The result of the first-step anodization in this work is given in Figure 1, which confirms that indeed a one-step anodization is insufficient for obtaining well-aligned and uniform nanotubes. Several Ti foils were submitted to the anodization, and some of the resulting nanotube layers were ultrasonically peeled off. Figure 1a, which is an

SEM image of the Ti surface outside the anodization region, shows that the Ti surface was very rough and irregular. In contrast, Figure 1b shows the regularly ordered Ti surface inside the anodization region exposed after the removal of the nanotube layer. This surface with regular hexagon-shaped imprints conduced to further growth of nanotubes. As shown in the inset of Figure 1a, the EDX of a Ti foil before any pretreatment contains the O peak. This is in agreement with the common knowledge that a Ti foil is generally covered with an oxide layer before receiving any treatment.39 As shown in the inset of Figure 1b, no major O peak can be found in the EDX of the imprints; thus, their component was mainly Ti instead of its oxides. That is, the oxygen on the Ti surface was largely removed in the first-step anodization. Figure 1c gives the SEM images of the nanotube arrays that grew on the rough Ti surface in the first-step anodization. The nanotubes had a remarkable disparity in length, and the array surface was considerably undulated. Figure 1d is the SEM image of the bottom side of the nanotube layer that was peeled off from the Ti foil. As seen from the direction of the incident electron beam, the bottom side of the nanotube layer shown in Figure 1d has an approximate 6-fold symmetry. First, the bottoms of the nanotubes all assume a rough hexagonal shape. Second, each nanotube is surrounded by 6 closest neighboring ones. The outer diameters of these nanotubes are on average similar to those of the imprints shown in Figure 1b. That is, the two patterns in Figure 1b,d are in commensuration. It is almost common knowledge that the electrolyte needs an aging process before it can be used for an anodization fabrication of TiO2 nanotubes,40,41 though the exact mechanism of this aging process is still poorly understood. As introduced in section 2, this practice was also followed in this work. For confirmation of its role in the anodization, an electrolyte that had only experienced a 10-h aging process, as against the aging process as long as 60 h for other samples in this work, was intentionally used in the anodization, and the result is given in Figure 1e, which shows the Ti foil surface exposed after the ultrasonic removal of a nanotube layer. Compared with the result shown in Figure 1b, the whole pattern in Figure 1e is quite irregular. The boundaries between the hexagonal imprints in Figure 1e are not well developed, either. Thus, it has been made clear that a thorough aging of the electrolyte is also indispensable to obtaining a regular imprint pattern.

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Ti + 2H2O - 4e- f TiO2 + 4H+

(1)

Then, chemical dissolution of the oxide as soluble fluoride complexes began to compete with the anodic oxidation, and a direct complexation of the high-field transported Ti cations also occurred at the oxide/electrolyte interface:46,47

Figure 2. Nanotube array generated in the second-step anodization under a 60 V voltage: (a) top view; (b) observed from a 45° angle; (c) the bottom side of the nanotube layer after its removal from the Ti foil; (d) the exposed Ti surface after the removal of the nanotube layer. (e) The I-t curve during the nanotube formation.

3.2. Second-Step Anodization. After the first-step anodization and the removal of the nanotube layer, the Ti foil was assembled back to the electrolytic cell again as the anode for the second-step anodization. Figure 2 shows the porous layer generated under a 60 V anodizing voltage in the second-step anodization. The nanotube array shown in Figure 2a,b is much more uniform in alignment and length than that shown in Figure 1c, confirming the necessity of the first-step anodization as a pretreatment. Well-ordered porous layers of anodized aluminum oxide (AAO) are readily available.42,43 Nonetheless, within the authors’ knowledge, reports on arrays of TiO2 nanotubes with such good alignment and length uniformity are still rare. Just like the sample after the first-step anodization, the nanotube layer shown in Figure 2a,b was also peeled off from the Ti foil ultrasonically. The bottom side of the nanotube layer and the imprint pattern left on the Ti surface are shown in Figure 2c,d, respectively. As indicated in the inset of Figure 2a, the nanotubes have a circular internal transverse section. Nonetheless, as each nanotube is surrounded by 6 nearest neighbors, its outer section and the pattern it leaves on the Ti foil surface both assume a hexagonal shape, as shown in Figure 2d and the inset of Figure 2c. In Figure 2b, each nanotube has 6 protrusions at the fringe of its top end. The sizes of the hexagons formed by these protrusions are in good agreement with those of the imprints in Figure 1b, suggesting that the nanotubes in the second-step anodization directly developed from the imprint pattern left on the Ti surface. It is also worth pointing out that the imprint patterns left on the Ti foil surface after the first-step and the second-step anodizations, respectively shown in Figures 1b and 2d, do not show obvious disparity in uniformity and orderliness. Therefore, as is argued in the preceding paragraph, aging the electrolyte was more effective in improving the quality of the imprint pattern than performing more anodization. It is generally accepted that the formation mechanism of the TiO2 nanotubes resembles that of the AAO nanopores.44,45 As introduced in section 2, the electrolyte contained a trace amount

TiO2 + 6F- + 4H+ f [TiF6]2- + 2H2O

(2)

Ti4+ + 6F- f [TiF6]2-

(3)

The current-time (I-t) curve acquired during the growth of the nanotubes, shown in Figure 2e, is consistent with the above reactions. The rapid drop in the initial stage of the curve resulted from the generation of the barrier layer described by reaction 1. Then, the current increased slightly when reaction 2 dominated momentarily, because the ion transport became possible again as the oxide layer was thinned. While both field-assisted dissolution and chemical dissolution made a contribution to the dissolution of the oxide layer, it is the field-assisted dissolution that dominated this stage due to the relatively large electric field across the thin oxide layer.45,48 Finally, when a balance was reached among reactions 1-3, the current stabilized at a certain value.47 Actually, in the first-step anodization, the nanotube formation, which depended remarkably on local electric field and solution diffusion rate, did not begin uniformly across the Ti foil surface. First, further growth of the oxide layer generated by the electrochemical oxidation in reaction 1 was controlled by fieldassisted transport of Ti4+ cations and O2- anions through the layer;47 second, local acidification of the solution, which was indispensable to the chemical dissolution of TiO2 in reaction 2, was largely determined by the establishment of a pH gradient.49 Therefore, the nanotube growth took place preferentially at some locations that can simultaneously provide sufficiently high local electric field and a narrow channel-shaped morphology. The former was necessary for the oxidation of Ti and the generation of H+ cations as well, and the latter was favorable to the accumulation of the generated H+ cations. As shown in Figure 1a, the morphology of the Ti surface before the pretreatment was irregular; thus, the nanotube growth on it commenced with a rather random spatial distribution. In the anodization of the Ti substrate into TiO2, the formation of the relatively ordered array was the result of a competition between the initially existent nanotubes. During an autocatalysis sequence, only the nanotubes with optimal starting conditions, such as a large depth and a good orientation at the beginning, were allowed to grow continuously and finally evolve into members in the array.50 Though this self-organization mechanism was effective in modulating the nanotube growth, the eventual result was still not satisfactory, as shown in Figure 1c. The random initial growth still had a negative influence on the orderliness and uniformity of the array. Moreover, the bending of some pores during the autocatalysis was also a concern.50 In contrast, when the pretreated Ti foil, shown in Figure 1b, was used as the anode, both the electric field and the morphology had a regular distribution across the surface at the very beginning. Actually, the ordered imprints played the role of template for the growth of the nanotubes, and the initial

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Figure 3. Nanopores fabricated on a mechanically polished Ti foil.

randomness was effectively avoided. As a result, the uniformity and orderliness of the nanotube arrays were greatly improved in the second-step anodization. For a better understanding of the role of the imprints left after the first-step anodization, it is worth pointing out that a very flat surface of a Ti foil is not necessarily conducive to orderly growth of a nanotube layer. A specially processed Ti foil was purchased and used as the anode in a conventional one-step anodization. It had been mechanically polished so that its surface was very flat and smooth.23,51 Figure 3 shows the nanopores that grew on the flat surface of this Ti foil. Their diameter and depth are not so uniform compared with those of the nanotubes shown in Figure 2. Some pores are actually very shallow, indicating the lack of strong field necessary for keeping the anodization progressing at some locations. This phenomenon is easily explainable. Since the surface was very flat, the electric field across it was nearly equal everywhere, and the anodization could start only at some random locations. Thus, the nanopores with poor uniformity resulted. While on a surface with regular imprints, e.g., the one shown in Figure 1b, the strongest electric field occurred at the bottoms of the scallop-shaped hemispherical imprints and further growth of the nanotubes naturally started there.44,49 Moreover, after the Ti foil was cut using scissors, the side profile of the TiO2 layer generated in the second step anodization was exposed, and the results of SEM and TEM observation are given in Figure 4. Unlike AAO, the TiO2 layer in Figure 4a is not a continuous oxide layer with nanopores embedded in it. Instead, the layer is actually an assembly of nanotubes that pack closely to each other. Moreover, as can be clearly seen in the inset of Figure 4a, small gaps exist between the abutting nanotubes.52 The generation of these gaps is ascribed mainly to the mechanical stress in the oxide tubes.50 Also, the nanotube layer in Figure 4a is covered with a porous film; thus, it appears as a nanopore array in the top view. In some laboratories, TiO2 nanotube arrays covered with porous films were fabricated by anodizing mechanically polished Ti substrates. The films can effectively prevent tube tops from being attacked by chemical dissolution.23,51 The length of a nanotube does not increase when the rate of oxidation at the Ti/oxide interface at the bottom equals the rate of dissolution at the oxide/electrolyte interface at the top. Hence, preparation of long nanotubes is possible only when the tube tops are well protected. The TiO2 nanotube arrays prepared by the two-step anodization in this work were also covered with porous film and thus had the potential of being further prolonged in future work. Figure 4b shows that the nanotubes are closed at the oxide/ Ti interface. It is argued that there exists a thin dense interface between the porous AAO and the underlying Al substrate.53 Similarly, Figure 4b indicates that a barrier layer also occurred between the TiO2 nanotubes and the metallic Ti. This observation has confirmed Albu et al.’s argument that the growth of the nanotubes is preceded by the growth of a compact anodic oxide.41 The compact oxide layer moves further into the metal with thickness unchanged when the rate of oxide growth at the

Li et al. metal-oxide interface equals that of oxide dissolution at the oxide-electrolyte interface.45 Figure 4a,b respectively show that the nanotubes are open at the top and closed at the bottom. From now on, the open end of a nanotube is referred to as the “mouth”. The two figures also disclose that the nanotubes have smaller cavities near the bottom than the top. The inner diameter at the mouth shown in the inset of Figure 4a is around 100 nm while that at the bottom shown in Figure 4b is around 50 nm. During the formation of the hollow nanotubes, as the dissolution proceeded from the top to the bottom, the upper parts of the nanotubes were exposed to the solution for a longer time than the lower parts.54 Hence the walls of the nanotubes at the mouths were apparently thinner than those at the bottom. As shown in Figures 2a and 4b, the wall thickness of the nanotubes is 17 and 50 nm at the mouth and the bottom, respectively. It is also noticed that the side walls of the nanotubes have obvious thickness variation, often referred to as ripples;55 one example is given in Figure 4c. This is a common phenomenon in the fabrication of nanotubes by anodization.52 So far, the ripples are ascribed to the periodic oscillations of the current in anodization,47 and it has been reported that the bamboo-shaped nanotube, which is actually a nanostructure with more drastic ripples along the side walls, is available when the dc voltage is replaced by an ac voltage.41 The registered voltage in an anodization under a galvanostatic mode has a tendency to oscillate, which is attributed to sequential growth and lift-off of TiO2 nanotube layers.56 The steady-state current density of the anodization is diffusion limited.57 Thus, growth of nanotubes is steadier in electrolytes with large viscidities than in an aqueous electrolyte.37 Herein, the current density-time curve under the potentiostatic mode was also recorded. As shown in Figure 4d, the current oscillation is small. Accordingly, the ripples shown in Figure 4c are also small. The rate of the nanotube growth is largely determined by that of the chemical dissolution of the TiO2 at the nanotube bottom, which, as is clear in eq 2, further depends on the local pH value there. According to eq 1, the local acidification of the solution in the vicinity of the nanotube bottom results from the anodization of Ti driven by the external current. Therefore, when the current of the anodization dropped to a relatively low value during the oscillation, H+ ions became less available and the dissolution rate of the TiO2 was accordingly lowered. Thus, the growth of the nanotubes slowed down considerably, and the ripples resulted along the sidewalls of the nanotubes.41 At the mouth, the diffusion of the H+ ions was much easier than that inside the nanotubes. Hence, the pH gradient was not well established at the beginning, and the initial dissolution of the TiO2 should be relatively slow but quite uniform. Consequently, as shown in Figure 4a, no ripple is found near the nanotube mouths. For the purpose of probing the mechanism behind the ripple formation, here the constant applied voltage was replaced by a periodic square-waved voltage in the second-step anodization, whose value was alternately 30 and 60 V. The structure shown in Figure 4e was obtained when both the 30 and 60 V voltages lasted 10 s in one period. In this case, the application of a squarewaved voltage did not make obvious difference from the constant voltage that yielded Figure 4c. When the duration of the 30 V voltage was prolonged to 90 s, a bamboo-shaped structure, shown in Figure 4f, was attained. It is believed that the formation of the nanotube, i.e., the dissolution of the TiO2, almost stopped under a voltage as low as 30 V. The “bamboo joints” in Figure 4f were developed shortly after the transition

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Figure 4. Side view of the TiO2 nanotubes generated in the second-step anodization: (a) the side view of the nanotubes; (b) the bottom of the nanotubes and the barrier layer; (c) the ripples; (d) the current-time curve under a potentiostatic mode; (e) the bamboo-shaped tubes generated under a square-waved anodizing voltage (both the 30 V and the 60 V voltages lasted 10 s in one period); (f) the bamboo-shaped tubes generated under another square-waved anodizing voltage (the 30 V voltage lasted 90 s and the 60 V voltage still lasted 10 s).

Figure 5. Lotus-root-shaped nanostructure obtained under a 30 V anodizing voltage in the second-step anodization: (a) top view; (b) side view.

to the 60 V voltage. Therefore, the results presented in Figure 4e,f have confirmed that indeed the periodic oscillation of the anodizing current was responsible for the ripples. It is worth pointing out that bamboo-shaped TiO2 nanotubes are also readily attainable in aqueous electrolyte.58 Since the aqueous electrolyte has small viscidity, the bamboo-shaped nanotubes can result from the oscillation of current even under constant anodizing voltage. In contrast, as in this work, when an organic electrolyte with a relatively large viscidity is employed, a constant voltage usually gives rise to ripples along the nanotube walls, and bamboo-shaped structures are obtainable only when the voltage is subjected to a dramatic periodic change. 3.3. Lotus-Root-Shaped Nanostructures and DoubleLayered Nanostructures. In other experiments, the anodizing voltage was varied, and different nanostructures were obtained. For example, Figure 5 gives the result of the second-step anodization under a 30 V anodizing voltage. The first-step anodization was similar to that employed in the above work. That is, it was performed under a 60 V voltage, and the nanotube layer generated in it was ultrasonically removed. The nanostructure in it has two levels and resembles a lotus root in shape. The first level consists of cells less than 0.2 µm in size, and one of them is highlighted by a hexagon in the inset of Figure

5a. The pores with smaller diameters, one of them highlighted by a circle in the inset of Figure 5a, inside the cells constitute the second-level structure. It is noticed that no such nanopores extend across any neighboring cells. That is, the nanopores all evolved in the interior of the cells. On average, the size of the cells is similar to that of the imprints shown in Figure 1b. Therefore, it is reasonable to believe that the cells in Figure 5 correspond to the imprints in Figure 1b. It is known from Figure 5b that the nanostructure is also an assembly of nanotubes, and the cells of the first-level structure indeed correspond to the imprints shown in Figure 1b. It merits pointing out that this lotus-root-shaped nanostructure was available only when the anodizing voltage in the second-step anodization was low enough. As shown in Figure 2a, it was not obtained when the anodizing voltage was 60 V in the second-step anodization. Since the anodizing voltage in the second-step anodization, 30 V, was lower than that in the first-step anodization, 60 V, the nanotubes generated in the second-step anodization were thinner than those generated in the first-step anodization.20,44 Therefore, several nanotubes simultaneously developed inside one imprint, and the lotus-root-shaped nanostructure resulted. Under appropriate conditions, TiO2 nanotube arrays with different structures along the longitudinal direction of the tubes have been attained by previous researchers. For example, Yang et al. and Macak et al., respectively, synthesized double-layered TiO2 nanotube arrays by a two-step anodization, in which the first-step anodization was performed in an aqueous electrolyte and the second-step anodization in a nonaqueous electrolyte. In their nanotube arrays, rough and large-diameter nanotubes constituted the upper parts while smooth and small-diameter ones constituted the lower parts.57,59 For another example, Yasuda and Schmuki obtained multilayer zirconium titanate nanotube arrays by alternately applying and cutting off the anodizing voltage. Interestingly, they claimed that the formation

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Figure 6. Double-layered TiO2 nanotube arrays.

of new nanotubes in the lower part of the array started in the gaps between the existing nanotubes in the upper part.60 In this work, as shown in Figure 6, double-layered nanotube arrays with morphology different from those in the above examples were achieved. Here the two-step anodization route was still employed, and the first-step anodization was similar to that used in obtaining the nanotube arrays shown in Figure 2. It is the second-step anodization that was modulated. After the pretreatment, regularly distributing hexagonal imprints were left on the Ti foil surface. The sequential second-step anodization consisted of two stages. First, the anodizing voltage jumped between 60 and 30 V for 24 h, with the 60 V voltage lasting 10 s and the 30 V voltage 90 s in each period. The bambooshaped upper part of the layer in Figure 6 was generated in this stage. Then, without being cutting off, the voltage was switched to and continuously kept at 60 V for another 3 h, and the lower part of the array, which was an assembly of smooth nanotubes, resulted. Different from those in the above-mentioned works, the nanotubes in the upper part of Figure 6 do not pause at the boundary between the upper and the lower parts. Instead, they continuously evolve from the bamboo-shaped part to the smooth-walled part. Moreover, the nanotubes in the two parts have a similar diameter. Therefore, it is reasonable to believe that the nanotubes in the lower part started growing directly from the bottom of those in the upper part. There were two key points in fabricating the double-layered nanotube array shown in Figure 6. First, in order to avoid possible disruption between the nanotubes of the upper part and the lower part, the anodizing voltage should not be cut off when being switched from the first stage to the second one. Second, in order to avoid possible disparity in diameter between the nanotubes in the upper part and the lower part, the anodizing voltage in the second stage should be equal to the larger one of the two voltages in the first stage. Furthermore, as indicated by the arrow, the interface between the upper and the lower parts is quite distinct and flat. This phenomenon is attributable to the pretreatment of the Ti foil, which ensured that the growth of the bamboo-shaped nanotubes started at a relatively flat surface and thus stopped almost in the same plane. 4. Conclusion In summary, by a two-step anodization method, which featured a preparatory anodization as a pretreatment of the Ti foil surface, highly ordered TiO2 nanotube arrays have been achieved. Two key issues were found to be crucial in guaranteeing the orderliness and alignment of the nanotube arrays. First, after the first-step anodization, the nanotube layer should be removed ultrasonically instead of with an adhesion tape, so that possible damage to the imprints left on the Ti foil could be avoided. Second, the electrolyte should be subjected to a sufficiently long aging process before being used in the anodization. In the oxide layer, the nanotubes packed closely to each other and were covered by a layer of porous film at the top; thus, the layer appeared to be a nanopore array in the top

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