Electrochimica Acta 83 (2012) Contents lists available at SciVerse ScienceDirect. Electrochimica Acta

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1 Electrochimica Acta 83 (212) 2 29 Contents lists available at SciVerse ScienceDirect Electrochimica Acta jou rn al h om epa ge: Growth mechanism and morphology control of double-layer and bamboo-type TiO 2 nanotube arrays by anodic oxidation Dongsheng Guan, Paul J. Hymel, Ying Wang Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA 783, USA a r t i c l e i n f o Article history: Received 28 May 212 Received in revised form 7 August 212 Accepted 8 August 212 Available online xxx Keywords: TiO 2 nanotubes Anodic oxidation Current transient Double-layer Ion diffusion a b s t r a c t We have synthesized multilayer and bamboo-type TiO 2 nanotube arrays via alternating-voltage anodization steps in hydrous ethylene glycol (EG) containing NH F and investigated their growth mechanisms using experimental and theoretical approaches. Current transients are recorded to study real-time morphological evolution of anodic TiO 2 films during anodization at high and low voltages (V high, V low ). Current changes after each voltage ramp to V high are observed along with sequential origination of pits, pores and tubes in a compact barrier layer at the base of oxide film. Two anodization steps at V high separated by one step at V low with equal holding time yield double-layer smooth-walled TiO 2 nanotubes. However, repetition of this sequence does not produce nanotubes of more layers, but makes lower-layer nanotubes longer and induces ridges on their walls to form bamboo-type tubes. Formation mechanisms of doublelayer TiO 2 consisting of smooth-walled or bamboo-type nanotubes are explored. A proper holding time of low-voltage anodization is required for ridge formation, but ridge spacing is determined by high-voltage anodization time. The ridge spacing increases linearly with the high-voltage anodization time, and can be theoretically calculated for lower-layer bamboo-type nanotubes formed in EG electrolytes with 5 vol% H 2 O. Less water (2 vol%) in electrolyte results in larger ridge spacing, while more water ( 15 vol%) not only reduces the ridge spacing, but also causes instability during growth of multilayer TiO 2 nanotube arrays and eventually leads to formation of a disordered porous TiO 2 structure. 212 Elsevier Ltd. All rights reserved. 1. Introduction Self-assembled TiO 2 nanotube arrays formed by anodic oxidation of Ti were firstly reported in 21, and soon after they have drawn tremendous attention due to their well-ordered structure, large surface area and broad applications [1]. In the past decade, anodic TiO 2 nanotubes have been widely used in dye-sensitized solar cells (DSSCs) [2,3], photo catalysis [,5], gas sensing [6 8], water splitting [9,], biomedical materials [11,12], and lithiumion batteries (LIBs) [13,1]. One advantage of these TiO 2 nanotubes is their easy fabrication via anodic oxidation. The length, diameter and wall features of tubes can be adjusted by tuning synthesis conditions, such as anodization voltage or current (constant [15 17] or alternating [18,19]), electrolyte composition and anodization time. More recently, there have been some research efforts to further increase the surface area of anodic TiO 2 nanotubes, such as fabrication of multilayer TiO 2 nanotube arrays with extra porous interlayers [2,21] or bamboo-type TiO 2 nanotubes with ridges on outer tube walls by anodizing Ti under alternating-voltage (AV) Corresponding author. Tel.: ; fax: address: ywang@lsu.edu (Y. Wang). conditions [18,19]. These new TiO 2 nanostructures provide larger surface area due to extra interfaces or rough walls, while retaining the vertically ordered one-dimensional nanostructure. In addition, they enable more light scattering when used in DSSCs, and provide more flexibility for dimension tuning and morphology engineering. For example, our group synthesized and explored growth mechanism of double-layer and sandwich-structured smooth-walled TiO 2 nanotube arrays via modified AV anodization conditions [22]. Double-layer TiO 2 nanotube arrays can also be prepared by using stepping-voltage anodization [23], or by anodizing Ti in alternating aqueous and organic electrolytes [2,25]. Regarding bamboo-type TiO 2 nanotube arrays, Zhang et al. [26] achieved ordered bambootype TiO 2 nanotubes in electrolytes with different viscosity and water content. Li et al. [27,28] employed AV anodization to synthesize ordered TiO 2 nanotubes with bamboo-shaped upper section and smooth-walled lower section. Schmuki and co-workers [29] integrated bamboo-type TiO 2 nanotube arrays into DSSCs; the new DSSCs yielded a photo conversion efficiency 55% higher than that of DSSCs based on smooth-walled nanotubes with identical film thickness of 8 m, due to larger surface area of bamboo-type nanotubes for more dye loading and enhanced light scattering for more photon absorption. In addition, Xie et al. [3] synthesized bamboo-type TiO 2 nanotubes with various ridge densities on tube walls, which /$ see front matter 212 Elsevier Ltd. All rights reserved.

2 D. Guan et al. / Electrochimica Acta 83 (212) found applications in DSSCs for up to 9% higher photo conversion efficiencies than smooth-walled nanotubes. Consequently, much attention is focused on the bamboo-type TiO 2 nanotube due to its unique advantages and potential applications in photo catalysts, LIBs and DSSCs [31]. To date, there are only a few papers about bamboo-type TiO 2 nanotube arrays as summarized above. Syntheses in these reports involve electrolytes containing hazardous HF, high voltage pairs (12 V/ V), and slow TiO 2 nanotube growth rate (e.g., 12 nm/min and 37 nm/min) [18,29]. Though formation process of bambootype TiO 2 nanotubes is briefly outlined [18], there is no report about fundamental explorations of growth mechanism underlying the synthesis and no clear understanding of factors that control morphological features such as ridge spacing on bamboo-type nanotubes. Ridge spacing provides a simple way for measuring electrochemical growth rate of anodic TiO 2 nanotubes. It should be noted that the growth rate cannot be calculated directly from the final tube length divided by total anodic time, since the length of nanotubes is decided by both the growth process of their roots towards substrate and the dissolution process of their tops into electrolyte. However, ridge spacing on bamboo-type TiO 2 nanotubes is the intact outcome from the electrochemical growth length within a given time period, and thus it can be used to calculate the electrochemical growth rate. On the other hand, the ridge spacing can be calculated from the growth rate of TiO 2 nanotubes or electrochemical etching rate of Ti, which has been briefly mentioned in some early reports [32,33]. In the present work, we report facile synthesis of doublelayer and bamboo-type TiO 2 nanotube arrays using non-toxic electrolytes (ethylene glycol containing NH F and H 2 O) and relatively lower voltage pairs (6/ V) with faster tube growth rate (6 7 nm/min). We synthesize and explore growth mechanism of double-layer TiO 2 nanostructures composed of smooth-walled nanotubes in the upper layer and bamboo-type nanotubes in the lower layer. For comparison purposes, single-layer smooth-walled TiO 2 nanotubes and double-layer smooth-walled TiO 2 nanotubes are fabricated as well. Current measurements are used to study the real-time growth process and morphological evolution of these anodic TiO 2 nanostructures. Formation mechanism of double-layer TiO 2 nanotubes with or without a bamboo-type layer is proposed based on ion diffusion-controlled process inside tubes and intertube cavities. Fundamental factors that affect the morphology of bamboo-type TiO 2 nanotubes are studied with both experimental approaches and theoretical calculations and by manipulating water content in electrolyte and adjusting high or low voltage anodization time. To the best of our knowledge, so far there is no report about the effect of water content and low-voltage anodization duration on ridge formation. 2. Experimental Ti foils (99.5 wt%) ( mm mm.25 mm) in this study were purchased from Alfa Aesar. Prior to any electrochemical treatment, Ti foils were degreased and rinsed by sonicating in ethanol and deionized water. A two-electrode cell with a Pt mesh as the counter electrode was assembled for electrochemical anodization. Electrolytes were anhydrous ethylene glycol (EG) with.3 wt% NH F and 2 15 vol% H 2 O. All the solutions were prepared from reagent grade chemicals and deionized water. The voltage was supplied by a DC power supply with digital display (Model 1623A, PK Precision). A Data Acquisition/Data Logger Switch Unit (Aglient 397A) was employed to record real-time anodic current. To prepare multilayer TiO 2 nanotubes, the anodization process consists of several alternating high and low-voltage anodization steps, as shown in Fig. 1. The voltage is first increased from zero to V high with Applied Potential/V 6 t 1 t 3 t 2 t 5 t 7 t 9 t t 6 t 8 1V s -1 Fig. 1. Anodization sequence for the formation of double-layer and bamboo-type TiO 2 nanotube arrays. a ramp rate of 1 V s 1 and remains for a time t 1, then drops to V low and is kept at V low for a time t 2, followed by increasing to V high with a rate of 1 V s 1 and being kept at V high for a time t 3. Such voltage alteration is repeated for different TiO 2 nanotube structures. For comparison purpose, single-layer TiO 2 nanotubes were prepared by anodizing Ti sheets at 6 V for 2 min. All the experiments were carried out at room temperature. After anodization was completed, the samples were immediately rinsed in deionized water and dried in air. A FEI Quanta 3D FEG scanning electron microscope (SEM) was used to characterize the morphology of TiO 2 nanotube arrays formed on the front side of Ti foils. 3. Results and discussion 3.1. Morphology evolution of TiO 2 nanotube arrays Table 1 summarizes various anodization conditions and morphological features of resultant TiO 2 nanotube arrays. It can be seen that the TiO 2 tubular structure evolves from single layer to double-layer structure of smooth nanotubes, and further to doublelayer structure composed of smooth tubes above and bamboo-type tubes below, under constant voltage (CV) anodization and different alternating high and low-voltage anodization conditions. Clearly, number of anodizing steps and their holding time affects the quantity and spacing of bamboo ridges. The formation mechanism of these nanostructures and factors affecting their morphological features are discussed with both SEM observations and electrochemical analyses in the following sections. Fig. 2 shows SEM images of TiO 2 nanotube arrays synthesized under CV or AV conditions in EG electrolytes containing.3 wt% NH F and 5 vol% H 2 O. The direct anodization at 6 V leads to formation of single-layer TiO 2 nanotube arrays (CV-NT, Fig. 2a). The tubular structure is composed of hollow nanotubes (Fig. S1a) with a hemispherical closed bottom. These tubes have smooth walls with an average length of 3. m and their outer diameter is slightly increased from the top (12 nm) to bottom (152 nm). Our previous results demonstrate that the 1:1 ratio of highvoltage anodization time (t 1 ) to low-voltage anodization time (t 2 ) is a critical parameter for synthesis of double-layer TiO 2 nanotube arrays by using three high/low-voltage anodization steps in EG electrolytes containing.3 wt% NH F and 5 vol% H 2 O at room temperature [22]. This in-situ preparation of double-layer TiO 2 nanotubes is much more convenient than the two-step anodization method using alternating electrolytes with different compositions reported in literature [2,25]. Fig. 2b shows AV-NT1 composed of smooth-walled nanotubes in both the upper layer and lower layer. The upper layer reaches a thickness of 1.6 m where the nanotubes have an outer diameter of 152 nm, and the lower layer is 1.7 m thick and the outer tube diameter becomes a little larger ( 172 nm). SEM image in Fig. S1b shows these upper-layer tubes

3 22 D. Guan et al. / Electrochimica Acta 83 (212) 2 29 Table 1 Single-layer and double-layer TiO 2 nanotubes synthesized in EG electrolytes containing.3 wt% NH F and 5 vol% H 2O under different voltage conditions. Applied potential (V) Layer structure of films Number of ridges Ridge spacing Time (min) t 1 t 3 t 5 t 7 t 9 t 2 t t 6 t 8 CV-NT 2 S AV-NT1 S/S AV-NT2 2/3 2/3 2/3 2/3 2/3 S/B 1 5 AV-NT S/B 2 8 AV-NT S/B 3 65 AV-NT5 S/B 1 16 S: single-layer smooth nanotubes; S/S: double-layer structure composed of smooth nanotubes in both layers; S/B: double-layer structure composed of smooth nanotubes in the upper layer and bamboo-type nanotubes in the lower layer. Fig. 2. Cross-sectional SEM images of different TiO 2 nanotube arrays synthesized in EG electrolytes containing.3 wt% NH F and 5 vol% H 2O: (a) CV-NT, (b) AV-NT1, (c) AV-NT2, (d) AV-NT3, (e) AV-NT and (f) AV-NT5. (Insets) The whole single-layer or double-layer structure of TiO 2 nanotubes.

4 D. Guan et al. / Electrochimica Acta 83 (212) with round open entrances. In addition, it is noted that Ti is fully oxidized during formation of either single-layer or double-layer structures by anodization, yielding TiO 2 nanotubes [22]. The high/low-voltage anodization steps are then repeated for more times to explore morphological evolution of TiO 2 nanotubes. Anodic oxidation of Ti with three anodization steps at 6 V for 2/3 min separated by two steps at V for 2/3 min yields a new type of double-layer nanotube arrays (AV-NT2) consisting of closed-bottom smooth-walled nanotubes in the upper layer and bamboo-type nanotubes (one ridge on each tube) in the lower layer (Fig. 2c). This double-layer structure is 3. m thick and the spacing between the bamboo ridge and tube base is about 5 nm. Likewise, AV-NT3 is composed of smooth-walled nanotubes in the upper layer and bamboo-type nanotubes (two ridges on each tube) in the lower layer (Fig. 2d), with a total thickness of 2.9 m; and the spacing between the two neighboring ridges on the lower-layer nanotubes is 8 nm. AV-NT consists of smooth-walled nanotubes in the upper layer and bamboo-type nanotubes with three ridges on each tube in the lower layer (Fig. 2e). In this structure, the top layer is composed of small closed-bottom nanotubes with a length of.5 m and an outer diameter of 9 nm, which covers a layer of bamboo-type nanotubes that are larger ( 16 nm in outer diameter) and much longer ( 2.6 m). The ridge spacing (or the length of tube sections) is 65 nm, a little longer than the length of top-layer nanotubes, since the top layer suffers from dissolution by electrolyte during the anodization process. Open entrances can be also observed in top-layer TiO 2 nanotubes, as shown in its top-view SEM image (Fig. S1c). It is observed that these five different nanotube arrays in Fig. 2a e have similar total thicknesses ( m), since the total high-anodization voltage time for growing these nanotubes is the same (2 min). Furthermore, if we increase both high-voltage and low-voltage anodization time (e.g. from to min) while keeping t 1 /t 2 as 1 in the multiple-step anodization sequence, double-layer TiO 2 nanotube arrays are still resulted. For instance, in AV-NT5 the upper-layer nanotubes are smooth and closed-bottom with a length of 1.3 m and the lower-layer bamboo-type nanotubes is 5.7 m long (Fig. 2f). Open entrances are also observed in the top-view SEM image of upper-layer nanotubes (Fig. S1d). Interestingly, some lower-layer nanotubes only have one bamboo ridge on their outer walls. The spacing from this single ridge to tube top (or the length of 1st section) is 1.6 m, almost one fourth of the total length of lower-layer nanotubes, and thus it is assumed that two ridges disappear in subsequent alteration of high/low-voltage anodization steps. Such morphological discrepancies suggest changes in the local anodization conditions for tube growth beneath a thicker upper layer and a longer tube section. It is ensured that the closely-packed long tubes hinder inward diffusion of ionic species (e.g. F ions and H 2 O) from reaching the tube base, which creates a different electrolyte environment there, with respect to anodization for AV-NT that produces shorter uppertubes and tube sections Anodic current versus time transients It is well known that current time transients during anodic oxidation of Ti under CV or AV conditions reveal details about different stages of the film growth process and corresponding morphologies [3 37]. Fig. 3a and b shows current time transients for CV-NT grown in agitated electrolyte. When the applied voltage goes up to 6 V at the beginning, the initial anodic current is increased as well and reaches to its maximum. In this process, water is decomposed fast at anode to produce O 2 ions and Ti is oxidized quickly to form a compact oxide film. The compact film hinders ion transport and eventually causes an abrupt current drop. The formation of an initial compact film is commonly regarded as the stage I of TiO 2 nanotube growth, as noted in the current plot in Fig. 3b. Later, some species, especially F ions, aggregate in regions with high surface energy (e.g. micro cracks) and selectively dissolve oxides there to originate tiny pits and pores in the compact film [38]. The development of pits and pores converts the film into a porous structure and attenuates the film thickness, which facilitates ion transport through this film to slow down the current drop (or usually to increase the anodic current slightly during anodization of Ti in static electrolytes) [36,37]. Formation of the porous film is the growth stage II of TiO 2 nanotube arrays. As the pores grow deeper, the inter-pore sites become high-surface-energy regions attracting F ions for faster dissolution of oxides there [38,39]. As a result, cavities emerge between the pores, and turn the porous film into a tubular structure. An array of parallel nanotubes appears with a thin compact barrier layer underneath, and their primary development in depth continues to attenuate the oxide film and retard the current drop, but later their steady-state growth yields relatively stable current as shown in Fig. 3b. Formation of the tubular film is the growth stage III of anodic TiO 2 nanotubes. Hence, it can be concluded that the current transient measurement provides insight for exploring the morphological evolution of anodic oxide film grown on Ti substrate under constant voltage steps. Fig. 3c and d displays the current time plots recorded during growth of AV-NT1. Similarly, a current peak appears after the initial voltage ramp to 6 V, due to the formation of a compact oxide film (stage I), and then the current drops at a reducing rate due to the origination of pores or tubes in the oxide film (stage II and III). Within the tube growth, the voltage is quickly altered to V at 66 s, yielding an instant current drop by 5.8 times. However, the current quickly recovers a little and then drops (inset of Fig. 3d), suggesting that the oxide film becomes a little thinner first and then thicker. The temporarily thinning oxide film is due to weaker oxidation of Ti and slower movement of barrier layer at V, while H + and F ion profiles achieved at 6 V continue to dissolve oxides of barrier layer at a fast speed. After a short while, these species are mostly consumed and their profiles are finally adjusted according to anodization conditions at V. The barrier layer grows to be thicker, yielding a small current drop, but the subsequent pit or pore formation in this layer yields an increased current. If the anodization step at V is performed for a sufficient time (e.g. 3 min), small tubes can be formed under the barrier layer, but the holding time of min here only allows origination of tiny pores, as described in our earlier work [22]. When the voltage is tuned to be 6 V again, the current increase/decrease/increase sequence vividly indicates the three growth stages of a new layer of nanotubes under the first one [3,35]. Surprisingly, this current variation sequence represents typical current time behaviors of anodizing Ti in static hydrous electrolytes [35,37], indicating that ion diffusion process dominates the mass transfer inside the nanotube layer, rather than the convection process in stirred bulk electrolyte [22]. The final current plateau tells a steady growth state of the new-layer nanotubes at 6 V, and its height is lower than the small current peak before it. Fig. 3e and f presents the current time plots recorded during growth of AV-NT. The first two high-voltage anodization steps produce similar current changes as in Fig. 3d, suggesting formation of double-layer TiO 2 nanotube arrays. However, the subsequent three anodization steps at 6 V yield different current time transients featured by their current plateaus higher than the peak before it. For example, the current starts to rise at 18 s and reaches a peak, as the voltage goes back to 6 V, but later drops a little due to thickened barrier layer under the lower-layer nanotubes. Then the pit or pore origination attenuates the barrier layer, which could only cause the current to increase until reaching a plateau the same as the previous one. However, the current keeps increasing to form a plateau higher than the peak right before it.

5 2 D. Guan et al. / Electrochimica Acta 83 (212) 2 29 Applied Potential/V a 5 35 Stage II b 6 2 I/mA Stage III 1V s -1 5 Stage I t/s 8 12 Applied Potential/V c 5 d 6 I/mA I II III 1V s I II III t/s Applied Potential/V e 5 f 35 6 I/mA V s t/s Applied Potential/V g 5 h I/mA V s t/s 5 6 Fig. 3. Anodization sequences and current time curves recorded during anodic growth of different TiO 2 nanotube arrays in stirred EG electrolytes containing.3 wt% NH F and 5 vol% H 2O: (a and b) CV-NT, (c and d) AV-NT1, (e and f) AV-NT, (g and h) AV-NT5.

6 D. Guan et al. / Electrochimica Acta 83 (212) It is assumed that growth of bigger pores in the barrier layer involves after the voltage returns to 6 V, and further attenuates the barrier layer. In the presence of formed tube section above, these tubes and inter-tube cavities are most likely to be the bigger pores, which are much larger than nascent pits originated at 6 V. Besides, identical current time plots appear in the final two high-voltage steps as well, suggesting that the same morphological features for the tubes emerge in the three anodization steps. At this point, it is understood that there is a competitive relation between growth of former tube sections and nascent pits along the vertical direction to the Ti substrate. Since ion diffusion to the bottom is slowed down by the closely arrayed nanotube layer and a compact thin barrier layer above, pit development is difficult inside the base barrier layer, which is incomparable to the re-started growth of formed tubes and cavities. Therefore, the newly-born pits are consumed or eaten, which greatly reduces the probability of developing a new nanotube layer from these pits. Moreover, the temporarily thickened barrier layer leaves some debris on tube walls to generate ridges, yielding bamboo-type tubes [22]. Fig. 3g and h exhibits current time transient recorded during growth of longer nanotubes AV-NT5. It can be seen that this current time plot is much like that shown in Fig. 3f, suggesting some similarities in the growth of double-layer TiO 2 nanotubes during multi-voltage alterations. However, one distinct difference between the two plots is the current changes in the final two voltage switches from V to 6 V. The current increases after the voltage is back to 6 V, then drops slightly, and soon after rises to a current plateau higher than the current peak right before it. The reduced current drop indicates that the barrier layer changes very little in thickness at the voltage switch, thanks to much fewer ions reaching the base barrier layer through further thicker tube layer and tube sections above. Consequently, pit formation tends to be slowed down or even suppressed in barrier layer, which is easier to be replaced by re-started growth of former tubes and inter-tube cavities, yielding smooth tube sections Growth mechanism of double-layer and bamboo-type TiO 2 nanotubes Our previous work reports in detail that the growth of single-layer TiO 2 nanotubes is highly dependent on stable ion concentration profiles (e.g. ph gradient) established inside nanotubes and cavities under constant-voltage anodizing conditions (Fig. a), and efficient variations of the concentration profiles on the switch of high and low anodic voltages will induce their evolution to a double-layer structure with smooth tubes in both the upper layer and lower layer (Fig. b and c) [22]. It should be noted that tiny pits and pores emerge and grow in the barrier layer when the lowvoltage anodization proceeds, as shown in Fig. b. Here we further discuss the formation of ridges on the lower-layer nanotubes when more alternating-voltage steps are applied, by still taking H + ion concentration (or ph gradient) profile inside tubes as an example to simplify our model. In the second high-voltage anodization process, water decomposition at the tube bottom yields H + ions and a stable ph gradient profile is established to allow the steady growth of lower-layer nanotubes (Fig. d). If the voltage is switched to a lower value ( V) again, fewer H + ions are produced and the ph gradient profile in lower-layer nanotubes becomes less steep (Fig. e). After a short while, tiny pits and pores are originated in the barrier layer under the low voltage, but their growth is relatively slow since the closedbottom upper-layer tubes with a barrier layer below hinder ions outside (e.g. F ions) from diffusion to the bottom of lower-layer tubes, and ions inside (e.g. H + ions) from diffusion to the entrance of upper-layer tubes. Hence, within the holding time of low voltage that is equal to time of previous high voltage step, the ph gradient profile in the lower layer fails to have sufficient variation for initiation of a third tube formation at the subsequent high-voltage step. A subsequent step back to a high voltage (6 V) would thicken the barrier layer near the substrate, which then is attenuated by growth of existing pits and pores. However, the restarted growth of former tubes and inter-tube cavities is faster than them, and thus they are consumed. This helps to attenuate the barrier layer and facilitate ion transport through it, and further increase the anodic current as shown in current time plots in Fig. 3f. In particular, the concave cavity deepens relatively fast towards the convex triangular pillar of metal among three neighboring tube bases, leaving some residues of thickened barrier layer on walls of two neighboring tubes to form ridges, yielding bamboo-type features (Fig. 2e). Thereby, the first tube section of bamboo-type nanotubes is generated in the lower layer. More ridges are formed in the same way during the following high/low-voltage anodization steps, since the formed upper-layer tubes and ridge layers retard ion diffusion through the whole tubular layer. Hence, the anodic current transients for the final three voltage switches from V to 6 V are the same, as shown in Fig. 3f. So far it has been known that ion diffusion inside nanotubes and cavities is very significant not only for initiation of new nanotube layer below, but also for formation of bamboo ridges on the lowerlayer tubes. In some cases, high-voltage anodization proceeds for a long period, resulting in long upper-layer tubes and long tube sections with large spacing between ridges shown in Fig. 2f. These nanotubes are closely packed, and their gaps become narrow when near to the bottom, which makes ion diffusion much more difficult. As the voltage is altered back to a high value, thickness of the barrier layer changes very little in which tiny pits possibly emerge later. However, these pits grow so slowly that the re-grown tubes and cavities consume them immediately. As a result, formation of other bamboo ridges after the first one is suppressed, yielding long smooth tube walls with fewer ridges. At this point, it is assumed that the smooth tube section is easier to be produced in organic solvent-based electrolytes with less water and slower ion diffusion. 3.. Role of low-voltage anodization According to the growth model above, a sufficiently long low-voltage anodization step is critical for growth of doublelayer nanotubes, and even for origination of an extra layer of small nanotubes between the upper layer and lower layer [22]. Fig. 5b shows SEM image of double-layer nanotubes formed by four anodization steps at 6 V for min separated by three steps at V for min, min and 2 min, respectively (Fig. 5a). The resulted structure is also composed of short closed-bottom nanotubes in the upper layer and bamboo-type nanotubes with two ridges on each in the lower-layer. The spacing between bamboo ridges is 6 nm, which is close to the ridge spacing in AV-NT, indicating that different lowvoltage anodization time has no effect on the ridge spacing. Moreover, it is noted earlier that the total thicknesses of five different TiO 2 nanotube arrays in Fig. 2a e are quite close ( m) due to the same total time (2 min) of high-voltage anodization steps, though their total low-voltage anodization time ranges from min to 16 min. These results indicate that the ridge spacing and nanotube length are dependent on highvoltage anodization time, rather than low-voltage anodization. This can also be understood from the current time plots in Fig. 3f and h showing much smaller anodic current at lowvoltage steps than at high-voltage steps. Theoretically, within the same time period, much larger quantity of electric charge is transferred at a high-voltage step than at a low-voltage step, and Ti is mostly oxidized to form oxides during high-voltage

7 26 D. Guan et al. / Electrochimica Acta 83 (212) 2 29 Fig.. Schematic showing growth of double-layer structure containing smooth-walled TiO 2 nanotubes or bamboo-type TiO 2 nanotubes via anodic oxidation: (a) ph gradient profile in nanotubes during their steady growth at a high voltage, (b) much less steep ph gradient and tube formation when a low voltage is kept for a sufficiently long period (t 1/t 2 1), (c) formation of double-layer smooth TiO 2 nanotubes, (d) ph gradient profile in double-layer nanotubes during their steady growth at a high voltage, (e) less steep ph gradient in the lower-layer nanotubes and pit or pore formation in the barrier layer after the voltage is altered to a low value, (f) formation of double-layer structure with long bamboo-type nanotubes in the lower layer. anodization to achieve nanotube growth, according to Faraday s law []. On the other hand, a proper low-voltage anodization step is indispensable for ridge formation on the lower-layer bamboo-type nanotubes. Fig. 5d shows SEM image of double-layer TiO 2 nanotubes formed by four anodization steps at 6 V for min separated by three steps at V for min, 2 min and 1 min, respectively (Fig. 5c). The first low-voltage anodization step ( V for min) helps initiation of a new tubular layer under short upper-layer nanotubes. The second low-voltage step (2 min) facilitates formation of a ridge on each nanotube in the lower layer. However, no more ridges are observed in the SEM image in Fig. 5d, indicating that the third low-voltage step (1 min) is too short to induce ridge formation. This can be explained by the ion-diffusion-controlled in-situ growth mode of double-layer and bamboo-type TiO 2 nanotubes above. Inside the lower-layer tubes, ion diffusion is so slow that the ion concentration profile established at previous highvoltage anodization (6 V for min) can change little within too short time (1 min), and thus pit formation cannot be induced here. Once the high voltage is applied again, only growth of tubes and inter-cavities is re-started, yielding smooth tube sections. Hence, a proper ratio of high/low-voltage anodization time is critical for wall features of lower-layer nanotubes in double-layer structures. In summary, low-voltage anodization time needs to be long enough to induce ridge formation on bamboo-type nanotubes, but overly long low-voltage anodization time has no effect on the ridge spacing.

8 D. Guan et al. / Electrochimica Acta 83 (212) Fig. 5. Anodization sequences and cross-sectional SEM images of TiO 2 nanotubes formed in EG electrolytes containing.3 wt% NH F and 5 vol% H 2O under different conditions: (a and b) four anodization steps at 6 V for min separated by three steps at V for min, min and 2 min; (c and d) four anodization steps at 6 V for min separated by three steps at V for min, 2 min and 1 min, respectively Effects of high-voltage anodization time and water content on ridge spacing It has been reported in literature that bamboo-type TiO 2 nanotubes with dense ridges have larger surface area and thus show enhanced DSSC efficiency as photo-anodes compared to smoothwalled TiO 2 nanotubes [29,3]. Our study above has demonstrated that the ridge spacing is dependent on high-voltage anodization steps. Therefore, we can manipulate and optimize morphological features of bamboo-type TiO 2 nanotubes for device applications. Fig. 6b shows SEM image of double-layer TiO 2 nanotubes formed by five anodization steps at 6 V for 2 min separated by four steps at V for 2 min (Fig. 6a). This structure contains one layer of short closed-bottom nanotubes ( 185 nm long) covering a layer of longer bamboo-type nanotubes with three ridges on each. The ridge spacing is 33 nm. When the anodization time is further reduced to 1 min (Fig. 6c), the ridges become much denser with a spacing of 165 nm (Fig. 6d), while the total thickness of the nanotube array in Fig. 6b (1.3 m) remains similar to that in Fig. 6d (1.8 m) due to the same total time of high-voltage anodization steps ( min). As more high/low-voltage anodization steps are applied, dissolution of upper-layer nanotubes becomes more severe, yielding a layer of directional nanowires on the top of bamboo-type nanotubes as shown in Fig. 6d. Fig. 7 summarizes the correlation between ridge spacing of various nanotubes observed in SEM images presented earlier and high-voltage anodization time and water content used for their syntheses. It can be clearly seen that the ridge spacing increases linearly from 165 nm to 16 nm, with the high-voltage anodization time from 1 min to min. The theoretical ridge spacing can be derived from nanotube length (L) grown within a given time as follows: L = QM (Fnı) where Q is the circulated charge (C cm 2 ), M is molecule weight of TiO 2 (79.9 g mol 1 ), F is the Faraday constant (96,5 C equiv 1 ), n is the number of electrons involved in the reaction and ı is density of TiO 2 (3.8.1 g cm 3 ) [1]. The circulated charge is calculated by integrating the corresponding current time plot. For example, the current time plot corresponding to the fourth high-voltage anodization ( s in Fig. 3f) gives a total circulated charge of 2.16 C cm 2, and leads to a theoretical oxide growth length of 112 nm. However, the oxide film appears on both the front side and back side of Ti substrate, and simultaneous growth of nanotubes at the two sides is lopsided: tube length is longer at the front side than at the back side [2]. For anodization in EG electrolyte with.3 wt% NH F and 2 vol% H 2 O at 6 V, the tube length is 29% bigger at the front side than at the back side [2] and thus we estimate the theoretical ridge spacing of our sample to be 65 nm, very close to the actual spacing of 65 nm between ridges on AV-NT (Fig. 2e). Correspondingly, the theoretical electrochemical growth rate of these nanotubes is 162 nm per minute. Furthermore, it can be seen from Fig. 3f and h that the partial circulated charge mostly depends on width of current plateaus of high-voltage anodization steps. Since the plateaus are parallel to the time axis, the linear relationship between ridge spacing and high-voltage anodization time can be anticipated. (1)

9 28 D. Guan et al. / Electrochimica Acta 83 (212) 2 29 Fig. 6. Anodization sequences and cross-sectional SEM images of double-layer TiO 2 nanotube arrays synthesized in EG electrolytes containing.3 wt% NH F and 5 vol% H 2O under different conditions: (a) five anodization steps at 6 V for 2 min separated by four steps at V for 2 min, (b) bamboo-type nanotubes in the lower layer (inset: the whole double-layer nanotube array); (c) ten anodization steps at 6 V for 1 min separated by nine steps at V for 1 min, (d) bamboo-type nanotubes in the lower layer (inset: the whole nanotube array). In addition, water content also affects the ridge spacing in bamboo-type nanotubes. For example, the ridge spacing is 65 nm for double-layer TiO 2 nanotubes grown in EG electrolyte with 5 vol% H 2 O, and 1 nm for those formed in EG electrolyte with 2 vol% H 2 O (Fig. S2), obtained via the same anodization sequence shown in Fig. 3e. The discrepancy can be explained as follows: less water in electrolytes yields thinner barrier layers [3] at the Ti Spacing between ridges/nm vol% H 2 O 5 vol% H 2 O vol% H 2 O Fig. 7. Relationship between ridge spacing and time for high-voltage anodization step used for synthesis of double-layer structures with lower-layer bamboo-type TiO 2 nanotubes in EG electrolytes containing.3 wt% NH F and 2 vol% H 2O. 9 substrate because the donation of oxygen becomes more difficult and thus it is harder to form oxides []. Ionic transport across the barrier layer is enhanced, which thereby accelerates the motion of this layer toward substrate, resulting in faster growth of nanotubes. On the other hand, a thicker barrier layer is formed to hinder ionic transport and thus reduce the growth rate of nanotubes when water content is increased. As a result, the ridge spacing becomes shorter, as evidenced by 65 6 nm for bamboo-type nanotubes formed by the same anodization sequences in EG electrolytes with increasing water content from 5 vol% to vol% (Figure S3). We also found that a further increase of water content in EG electrolyte to 15 vol% under the same anodization sequence yields disordered porous or spongeous layers, due to growth instability induced by excessive water for fierce anodic reactions, which is in agreement with our previous findings [22].. Conclusions We have synthesized double-layer smooth-walled TiO 2 nanotubes and double-layer TiO 2 nanotubes with bamboo-type nanotubes in the lower layer via alternating-voltage anodization of Ti in NH F-containing ethylene glycol electrolytes. Anodic current is recorded during growth of various TiO 2 nanotube arrays, in order to evaluate their morphological evolutions at different stages. With voltages applied, the increase/decrease/increase sequences of current represent formation of oxide barrier layer, pores and nanotubes, respectively. A plateau of current appears upon stable growth of TiO 2 nanotubes. Based on these findings, growth mechanisms of double-layer structures containing smooth-walled

10 D. Guan et al. / Electrochimica Acta 83 (212) or bamboo-type nanotubes are proposed and ion diffusion in nanotubes is considered to be a dominant factor. A sufficient variation of ion concentration profiles in upper-layer nanotubes during high/lower-voltage alteration can induce formation of a new tube layer underneath. The morphology of lower-layer nanotubes can be tuned by repeating the high-voltage and low-voltage anodization steps, and thus bamboo-type nanotubes are obtained. Sufficient time of low-voltage anodization step is required for formation of ridges, but the spacing between neighboring ridges relies on the time of high-voltage anodization step. In particular, the ridge spacing is found to be increased linearly from 165 nm to 16 nm, with the time of high-voltage anodization step from 1 min to min, for lower-layer bamboo-type nanotubes formed in EG electrolytes with.3 wt% NH F and 5 vol% H 2 O. In addition, decreasing water content in electrolytes results in larger ridge spacing on lower-layer bamboo-type nanotubes, owning to origination of a thin barrier layer, while higher water content causes smaller ridge spacing and even instability for growth of double-layer nanotubes. Double-layer and bamboo-type TiO 2 nanotubes have larger surface area than single-layer nanotubes with the same length, due to extra layers or interfaces present between two neighboring nanotubes. Their surface area can be further enlarged by producing denser ridges on longer bamboo-type tubes, simply by repeating high-voltage and low-voltage anodization steps. Controllable synthesis of double-layer and bamboo-type TiO 2 nanotubes and fundamental understanding of their growth characteristics in this report provide direct paths to optimize dimensions and morphology of various TiO 2 nanotube arrays for maximizing TiO 2 -based device performances such as DSSCs. 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