Anodic Titania Nanotubes Grown on Titanium Tubular Electrodes

Feb 24, 2014 - School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798,. Singapore...
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Anodic Titania Nanotubes Grown on Titanium Tubular Electrodes Lidong Sun,†,‡ Xiaoyan Wang,§ Meilin Li,† Sam Zhang,*,§ and Qing Wang*,† †

Department of Materials Science and Engineering, Faculty of Engineering, NUSNNI-NanoCore, National University of Singapore, Singapore 117576, Singapore ‡ School of Materials Science and Engineering, Chongqing University, Chongqing 400044, People’s Republic of China § School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore S Supporting Information *

ABSTRACT: In the past decade, research into growth and application of anodic titania nanotubes has been focused on planar titanium electrodes. Although patterned, curved, or cylindrical substrates were also employed in a number of applications, the study of nanotubes grown on a titanium tubular electrode is rather inadequate, despite their expected uses in thermal fluids. In this study, growth of titania nanotubes on tubular electrodes was investigated. It was found that nanotubes are formed at both outer and inner surfaces of the electrode. The nanotube length (or growth rate in the first 30 min) at the outer surface decreases gradually from the side facing the cathode to that at the other side, while the length at the inner surface smears out this trend. This is due to the effect of the electric field emanating from the potential drop in the organic electrolyte. The variation of nanotube diameter just echoes such a tendency of potential drop. The influence of electrode orientation during anodization on the resulting features of nanotubes was also examined and discussed. The nanotube geometry is thus tailorable for particular applications.

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to a normal tube (0.617 and 0.077−0.102 W m−1 K−1 for amorphous titania nanotubes in the axial and radial directions, respectively,30 and 22 W m−1 K−1 for pure titanium metal). This cooling process could be further suppressed if the outer surface of the pipe is coated with nanotubes as well, thus conserving the energy efficiently. Another potential application is to enhance the volumetric flow rate of a liquid in a nanotubecoated tube (or pipe) using superhydrophobic or superoleophobic nanotubes.31−33 In addition, other alternative materials of interest with similar nanostructured coatings make the pertinent choice abundant, as summarized previously.34 However, up until now, the study of the growth of nanotubes on titanium tubular electrodes has been inadequate. Yada et al. fabricated sodium titanate nanotubes on both inner and outer surfaces of a titanium microtube by hydrothermal treatment,35 which was not an anodization method nor were titania nanotubes produced, as is the case here. Yu and co-workers prepared a free-standing tube comprising TiO2 nanotube arrays by etching away the metal core of an as-anodized titanium wire in a mixture of bromine and methanol.36 It is however impossible to fabricate nanotubes at the inner surface of the electrode using such an etching technique, not to mention the environmentally unfriendly process. Paulose and co-workers once mentioned the cylindrical titania membrane made by anodizing a titanium pipe while

INTRODUCTION Anodic growth of titania nanotube arrays has been studied extensively since first reported.1,2 To date, tube geometry (e.g., tube length,3−7 pore diameter,8−10 wall thickness,11 intertube spacing,12 wall roughness,13 etc.) can be well-tailored by controlling anodization conditions. This enables the nanotubes to be applied in various applications, such as dye-sensitized solar cells,14−17 water splitting,11 gas sensors,18,19 drug delivery,20 etc. Most of these initial studies are based on planar titanium electrodes. When all of the surfaces of a planar metal electrode are exposed to an anodizing electrolyte, a conformal coating of anodic nanotubes forms on the electrode,21,22 as illustrated in Figure 1a. Detailed investigation reveals that the tube length at the side facing the cathode is longer than that at the opposite side.23 Accordingly, similar conformal coatings can also be produced on patterned,24 curved,22 and thus cylindrical electrodes,25 as shown in panels b and c of Figure 1. A typical example is the use of titanium wire, which has found interesting applications in flexible dye-sensitized solar cells.26−29 One intuitive question is what will happen when the cylindrical electrode becomes tubular. It is anticipated that nanotubes grow at both inner and outer surfaces of a titanium tube with the in situ anodization process, hence forming conformal coatings, as depicted in Figure 1d. Such a tube (or a long pipe) may exhibit interesting behaviors when used in thermal fluids with well-tailored configuration. For instance, when a hot gas or liquid passes through a nanotube-coated pipe, it will be cooled slowly in view of the drastically reduced thermal conductivity in the presence of nanotubes, as compared © 2014 American Chemical Society

Received: September 10, 2013 Revised: February 22, 2014 Published: February 24, 2014 2835

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EXPERIMENTAL SECTION

Detailed experimental conditions are described in our previous report.37 In brief, all of the experiments were carried out in ethylene glycol (anhydrous, 99.8%, Sigma-Aldrich) consisting of 0.3 wt % ammonium fluoride (98+%, ACS reagent, Sigma-Aldrich) and 2 vol % deionized water at 60 V and room temperature (∼20 °C) for 30 min. The choice of 30 min was to avoid the formation of bundled nanotubes (or nanowires) at the nanotube tops under prolonged anodization,34 which will affect the evaluation of nanotube length. Titanium foils (0.127 mm, 99.7% purity, Sigma-Aldrich) were fashioned into tubular shape, with the joints being sealed with HiBond (bottom layer to seal the joints) and Kapton (top layer to protect the Hi-Bond tape) tapes very carefully to prevent the electrolyte from leaking along the joints. The titanium tubes were employed as the anodes, while a planar platinum mesh (25 × 50 mm) was used as the cathode. Three different setups were used in this study: (1) The tubular anodes (9 mm in diameter) were placed vertically (i.e., axial direction perpendicular to the liquid surface) and dipped into the electrolyte solution partially for about 17 mm, as illustrated in Figure 2a. The distance between the center of the anode

Figure 1. Illustrations of anodic titania nanotubes grown on (a) planar, (b) curved, (c) cylindrical, and (d) tubular titanium electrodes. Arrows denote the nanotubes on the electrodes and point from tube bottoms to mouths. Note that the nanotubes are produced with a two-electrode configuration, with a planar cathode parallel to the respective anode (axial direction). Left column, geometric shapes of titanium anodes; right column, corresponding horizontal cross-sections of the anodes.

Figure 2. Schematic diagrams of different electrode orientations during anodization with the tubular electrode (a) perpendicular to the liquid surface and parallel to the cathode, (b) immersing in the solution and parallel to the cathode, and (c) immersing in the solution and perpendicular to the cathode. In all cases, the cathode was placed perpendicular to the plane of the paper. Photographs of titanium tubular substrates (d and e) before and (f) after electrochemical anodization at 60 V for 30 min. The outer and inner surfaces cut from an as-anodized titanium tube are shown in panels g and h, respectively.

highlighting the importance of critical point drying.7 No detailed methods or results were presented in their paper. Other accessible literature seldom covers the relevant topics. As such, it is still unclear how titania nanotubes grow when anodizing a titanium tube. Additionally, the tubular electrode provides a straightforward way to reveal the role of the electric field in nanotube growth, because the anodizing environment is quite different inside and outside the tubular electrode depending upon anode orientation. Our previous work indicates that the field resulting from the potential drop in an organic electrolyte is essential in affecting the growth of nanotubes.23 Longer nanotubes are thus developed at the side facing the cathode. This was proved indirectly by tailoring the conductivity of the electrolyte to alter the potential drop and, hence, the electric field. With the tubular electrode, the field in the electrolyte solution will continue to influence the tube growth at the outer surface, which might be mostly screened inside the tubular electrode. In this study, experiments were judiciously designed to obtain insight into nanotube growth on the tubular electrode, consolidating the ground for future applications.

and the cathode was approximately 22.5 mm. (2) The tubular anodes (9 mm in diameter and 30 mm in length) were immersed totally in the electrolyte solution and placed parallel to the liquid surface and the cathode, as shown in Figure 2b. The distance between the center of the anode and the cathode was set to 22.5 mm. (3) The tubular anodes of different sizes (9 and 5 mm in diameter and 30 mm in length) were immersed totally in the electrolyte solution and placed parallel to the liquid surface while perpendicular to the cathode, as sketched in Figure 2c. The distance between one end of the anode and the cathode was about 14 mm. Panels d−h of Figure 2 show a few photographs of the tubular anodes before and after anodization with the experimental setup in Figure 2a. The experimental conditions using conventional planar titanium foils as the anodes were the same as above, except for the electrode dimension, which will be described in detail when referenced. Crosssectional views of the nanotubes were observed from mechanically fractured samples by field emission scanning electron microscopy (FESEM, Zeiss, SUPRA 40), which were used to determine the length of the nanotubes. 2836

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Figure 3. FESEM cross-sectional views of anodic nanotubes at the (a, b, and c) outer and (d, e, and f) inner surfaces of a tubular electrode at the (a and d) front, (b and e) side, and (c and f) back regions with respect to the cathode, as sketched in Figure 2a. Illustration displays the positions where top (∼1/6), middle (∼3/6), and bottom (∼5/6) rows were taken from the part immersing in the electrolyte solution.

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RESULTS AND DISCUSSION Panels g and h of Figure 2 display the respective outer and inner surfaces of a tubular electrode after anodization. Obviously, new coatings were developed on both surfaces upon anodization. The microscopic morphology of the coatings is shown in Figure 3. To look into the nanotube features, 18 different positions were selected and characterized on each tubular electrode, 9 of which were from the outer surface (left panel in Figure 3), while the others were from the corresponding inner surface (right panel in Figure 3). Each surface was divided into three typical regions: the region facing the cathode (referred to as “front”; panels a and d of Figure 3), the region opposite the cathode (referred to as “back”; panels c and f of Figure 3), and the region in between (referred to as “side”; panels b and e of Figure 3). Each region was further split into three parts: the part close to the liquid/gas interface (referred to as “top”; top row in Figure 3), the part near the electrode bottom (referred to as “bottom”; bottom row in Figure 3), and the part in between (referred to as “middle”; middle row in Figure 3). It reveals that nanotube arrays are formed at both outer and inner electrode surfaces, regardless of the position examined. Nevertheless, the nanotube length varies with the position, implying different growth rates at each area. To elucidate the ways that nanotubes grow on the tubular electrode, nanotube lengths are compiled (partially) on the basis of the FESEM images in Figure 3, as summarized in Figure 4. It shows that nanotubes at the outer surface (top three curves) are generally longer than that at the inner counterpart (bottom three curves). This is related to the supply of F− ions, which are critical for nanotube growth.34,37 The outer surface is in direct contact with the bulk electrolyte, where F− ions can diffuse to the surface easily and continuously. In contrast, to reach the inner surface, the ions have to enter

Figure 4. Length of anodic nanotubes at different positions on a titanium tubular electrode. All of the points were from one single electrode for reasonable comparison, while each point was based on at least 10 FESEM images. FO, front at the outer surface; SO, side at the outer surface; BO, back at the outer surface; FI, front at the inner surface; SI, side at the inner surface; and BI, back at the inner surface, as indicated in Figure 2a.

from the electrode bottom and diffuse along the axial direction. This results in the relatively shorter nanotubes inside the electrode. The diffusion process also affects the length (or growth rate) at different parts at the inner surface. That is, the nanotubes become shorter and shorter as the ions diffuse from the bottom to the middle and eventually to the top. Accordingly, to eliminate or reduce the length difference inside the electrode, it will be effective to submerge the whole tube into the electrolyte solution (e.g., the axial direction parallel to the liquid surface and cathode), as shown in Figure 2b. In this 2837

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Figure 5. Top view illustrations of the distribution of electric lines of force for (a) planar and (b) tubular anodes. The electric lines of force inside the tubular electrode are not shown for clarity. The drawing is partially based on the two-dimensional electric potential distribution in an electrochemical cell (see ref 38).

Figure 6. Nanotubes grown in the presence of two planar anodes: large and small foils. (a) Length of the nanotubes at the front (LF and SF) and back (LB and SB) of the large (LF and LB) and small (SF and SB) foils. The inset illustrates the arrangement of the two anodes and one cathode (CE) during anodization. (b−e) FESEM cross-sectional views of the nanotubes at the relative parts of the anodes.

case, the F− ions can diffuse into the tube from either side of the tubular electrode, which will be discussed later. We now turn to the nanotubes at the outer surface. Apparently, nanotube length exhibits the following sequence: front (FO) > side (SO) > back (BO). This is due to the potential drop in the organic electrolyte, which, in turn, affects mass transport in the electrolyte solution and the driving force for nanotube growth. For mass transport, considering a conventional planar anode as in Figure 5a, the ionic flux at the front of the anode consists of two components: ion diffusion under a concentration gradient and ion drift under an electric field. Because most of the current flows directly through the interpolar region between the electrodes,38 the ionic flux at the back of the anode mostly originates from the ion diffusion, with little influences from the potential drop at the front. Therefore, the larger ionic flux contributes to a larger tube growth rate at the front. In principle, the driving force for tube growth is the potential drop over the barrier layer,39 which is weakened at the back side, owing to the additional potential drop from the bending electric lines of force, as shown in Figure 5a. These two factors account together for the larger growth rate of nanotubes at the front of a planar anode. In the case of a tubular electrode, a similar process governs the tube

growth rate at the outer electrode surface. That is, the potential drop in the organic electrolyte solution gives rise to a gradually decreased ionic flux and driving force from the front to the back region, as depicted in Figure 5b. Consequently, the tube growth rate decreases progressively along the anode surface from the front to the back, as shown in Figure 4. The magnitude of the potential drop is roughly estimated to be about 0.71 V/mm (see the Supporting Information) in the organic electrolyte solution, suggesting its substantial impact on tube growth rate. Another effect of the potential drop is to cause variation in nanotube diameter, as will be discussed later. Interestingly, the nanotubes at the inner surface show no obvious difference between regions at the front (FI), side (SI), and back (BI). This can be ascribed to the screening of the electric field inside the tubular electrode; that is, the nanotube growth inside the anode is nearly unaffected by the potential drop in the interpolar region, because most of the electric lines of force are closed at the outer electrode surface. In this case, the ionic flux emanates almost from ion diffusion only, thereby obliterating the varying trend exhibited at the outer surface. To demonstrate the screening effect, control experiments with planar titanium electrodes were further carried out. In these experiments, two anodes were used: one large foil (28 × 20 2838

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Figure 7. (a−f) FESEM images showing anodic nanotubes grown at different parts on the side position of a tubular electrode, as in Figure 2a. (g) Corresponding nanotube outer diameter and wall thickness that were fabricated at the outer and inner surfaces of the tubular electrode. The wall thickness at the top and middle parts on the inner surface is not shown, because of the difficulty in evaluating the very thin nanotube walls.

Figure 8. Length of anodic nanotubes grown at different positions on a titanium tubular electrode, which was placed (a) parallel to the cathode as in Figure 2b and (b) perpendicular to the cathode as in Figure 2c. FO, front at the outer surface; SO, side at the outer surface; BO, back at the outer surface; FI, front at the inner surface; SI, side at the inner surface; and BI, back at the inner surface. Typical FESEM images are given in Figures S1 and S2 of the Supporting Information.

mm, indicating the width × depth dipping into the solution, so do the dimensions mentioned below) directly facing the platinum cathode (25 × 15 mm) and the other small foil (12 × 15 mm) located behind the large foil with an inter space of 7 mm, as illustrated in the inset of Figure 6a. In this scenario, the electric field (from the potential drop in the electrolyte solution) imposed on the small foil as well as the back side of the large foil is largely screened by the large titanium foil. As such, the nanotube length at the front of the large anode (LF) should be longer than that at its corresponding back side (LB) and that at the front (SF) and back (SB) sides of the small counterpart, while the latter three should show comparable length without the influence of the electric field at the

interpolar region (i.e., between the front of the large foil and the cathode). Figure 6a presents the length pertinent to the above locations, which just echoes the above discussion. In our previous study,23 double-sided tube growth on a titanium foil produces longer nanotubes at the front, as compared to the back. Herein, the nanotubes grown on the large foil are consistent with the conclusion; however, that on the small foil exhibits an opposite propensity. This can be attributed to the shared electrolyte between the back of the large foil and the front of the small foil, giving rise to a limited ion source in contrast to the bulk electrolyte at the back of the small foil. Therefore, the growth of nanotubes on the small foil behind the large foil mostly depends upon the diffusion process in view of 2839

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the field screening effect. This accounts for the comparable nanotubes at each same part inside the tubular electrode, as summarized in Figure 4. Panels b−e of Figure 6 display nanotubes grown at different parts of the two planar anodes. Obviously, the outer diameter of the nanotubes at the front of the large foil is much larger than that of the other three, which, however, display similar diameters. This further confirms the effect of the potential drop in the organic electrolyte, which induces the differences in the diameter. However, even though the potential drop (at the barrier layer) is supposed to be smaller on the small foil in consideration of the enlarged distance to the cathode, comparable diameters were obtained. This suggests that the ionic flux also plays an essential role in nanotube growth. The screening of the field produces a similar ionic flux and, thereby, a similar diameter. In parallel, the nanotube diameter at the side position (see Figure 2a) of the tubular anode was also examined, as shown in panels a−f of Figure 7 and summarized in Figure 7g. It reveals that the nanotube outer diameter and wall thickness are comparable at the outer anode surface. In contrast, the outer diameter at the inner surface decreases from the bottom to the middle and to the top of the anode, in line with the changes in nanotube length. This can be ascribed to a weakened potential drop at the barrier layer and reduced ionic flux from the bottom to the top inside the tubular anode, consistent with the results given in panels b−e of Figure 6. To investigate the effect of electrode orientation on the nanotube growth rate, experiments were also carried out with the setups illustrated in panels b and c of Figure 2. As mentioned above, when immersing the whole electrode into the electrolyte solution (Figure 2b), the differences in nanotube length at the inner surface can be reduced, as a consequence of double estuaries at both ends of the tubular electrode, while the variation of nanotubes at the outer surface follows the same trend presented in Figure 4, as exhibited in Figure 8a. This rule holds provided that the electrode axial direction is parallel to the cathode. When the axial direction is normal to the surface of the cathode when immersing in the electrolyte (Figure 2c), the distance to the cathode affects the nanotube growth rate substantially, according to our previous work;37 that is, the growth rate decreases gradually at both surfaces from one end close to the cathode toward the other end, while it may increase again when approaching the other estuary, as shown in Figure 8b. It also indicates that the length variation at the inner surface from the end to the center is reduced with the increase in the electrode diameter, as disclosed in Figure 8b. It is anticipated that the variation will be enhanced with the increase in the electrode length. In addition, if only one surface is required to coat the nanotubes, the other surface can be protected intentionally during anodization. Accordingly, the nanotubes grown on a titanium (or other possible metals) tubular electrode can be well-tailored by designing the anodization process judiciously for particular applications.

electrode. However, the growth rate decreases progressively from the inner bottom to the top, because of the diffusion process, which is available from only one end of the electrode, and also the weakened driving force at the barrier layer. The nanotube diameter at the outer surface is comparable in a particular region, while it decreases at the inner surface from the bottom to the top of the anode, which further confirms the role of the potential drop in the organic electrolyte. The nanotube features can be well-tailored by adjusting the electrode orientation during anodization. The present study provides a basis for future application of the nanotube-coated tubes or pipes in thermal fluids and other relevant areas.

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ASSOCIATED CONTENT

S Supporting Information *

Estimation of the potential drop in the organic electrolyte solution and FESEM images of anodic nanotubes produced with experimental setups, as in panels b and c of Figure 2. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: +65-67904400. Fax: +65-67924062. E-mail: [email protected]. *Telephone: +65-65167118. Fax: +65-67763604. E-mail: qing. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Qing Wang is grateful to the financial support of the National Research Foundation Singapore under its Competitive Research Program (CRP Award NRF-CRP8-2011-04) and National University of Singapore−Faculty of Engineering(NUS−FOE) Energy Research for Sustainability Initiatives (R284000089112). Lidong Sun acknowledges the startup grant under the “100 Young Talents Program” of Chongqing University. The authors thank Dr. James Robert Jennings at the Department of Materials Science and Engineering, National University of Singapore for his help with improving the language in the manuscript.

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REFERENCES

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CONCLUSION A conformal coating of anodic nanotubes can be formed on a titanium tubular electrode that is perpendicular to the liquid surface. The nanotube growth rate at the outer anode surface decreases gradually from the side facing the cathode to that at the opposite side, because of the potential drop in the organic electrolyte, which affects mass transport and the driving force for nanotube growth. This trend is absent at the inner surface, as a result of the field screening effect inside the tubular 2840

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dx.doi.org/10.1021/la500050q | Langmuir 2014, 30, 2835−2841