Delivered by Ingenta to: Sung Kyun Kwan University IP : 115.145.200.148 Thu, 06 May 2010 12:02:47 RESEARCH ARTICLE Copyright © 2010 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 10, 4259–4265, 2010 Fabrication of Free Standing Anodic Titanium Oxide Membranes with Clean Surface Using Recycling Process Xianhui Meng 1 , Tae-Young Lee 1 , Huiyu Chen 1 , Dong-Wook Shin 2 , Kee-Won Kwon 3 , Sang Jik Kwon 4 , and Ji-Beom Yoo 1 2 ∗ 1 School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon, 440-476, Korea 2 SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon, 440-476, Korea 3 School of Information and Communication Engineering, Sungkyunkwan University, Suwon, 440-476, Korea 4 Electronics Engineering, Kyungwon University, Senongnam, 461-701, Korea Large area of self-organized, free standing anodic titanium oxide (ATO) nanotube membranes with clean surfaces were facilely prepared to desired lengths via electrochemical anodization of highly pure Ti sheets in an ethylene glycol electrolyte, with a small amount of NH 4 F and H 2 Oat50V, followed by self-detachment of the ATO membrane from the Ti substrate using recycling processes. In the first anodization step, the nanowire oxide layer existed over the well-arranged ATO nanotube. After sufficiently rinsing with water, the whole ATO layer was removed from the Ti sheet by high pressure N 2 gas, and a well-patterned dimple layer with a thickness of about 30 nm existed on the Ti substrate. By using these naturally formed nano-scale pits as templates, in the second and third anodization process, highly ordered, ver tically aligned, and free standing ATO membranes with the anodic aluminum oxide (AAO)-like clean surface were obtained. The inter-pore distance and diameter was 154 ± 2 nm and 91 ± 2 nm, the tube arrays lengths for 25 and 46 hours were 44 and 70 m, respectively. The present study demonstrates a simple approach to producing high quality, length controllable, large area TiO 2 membrane. Keywords: Titanium Oxide, Nanotube Membrane, Recycling Process, Dimple Template. 1. INTRODUCTION Nanomaterials have attracted increasing attention over the past few decades because of their excellent properties and wide applications as compared to their bulk coun- terparts. One-dimensional nanostructures such as nano- tubes, nanorods, nanofibers, nanoribbons and nanowires, are expected to play important roles in the fabrication of functional nanodevices. 1 Recently, special interest has been focused on TiO 2 nanostructures, which have many potential applications in gas sensors 2 and biocompatible, 3 photovoltaic, 4 and photocatalytic 5 6 materials, to name a few. The ability to control the structure and properties of the nanostructural TiO 2 materials will be very useful for these applications. Generally, there are three strategies used in the fabri- cation of TiO 2 nanotubes: template synthesis, 7 hydrother- mal methods, 8 and electrochemical synthesis (anodization ∗ Author to whom correspondence should be addressed. of a Ti sheet). Typically, the template-directed synthesis method involves the fabrication of a nanoporous template, backfilling with the TiO 2 precursors, then removing the template to yield the resulting nanostructures. But these processes are complicated. The complete infiltration of the precursors, the uniform formation of the TiO 2 nanotube array, and a good transfer of products are always hard to obtain. The hydrothermal method is a simple way to prepare individual nanotubes, but only randomly aligned tubes can be obtained. In addition, this approach only pro- duces protonated titanate nanotubes (H 2m TiO 2n+m  rather than TiO 2 . These disadvantages limit the further electrical applications in nanodevices. In 1999 it was reported that porous TiO 2 nanostructures could be fabricated by elec- trochemically anodizing a Ti sheet in an acid electrolyte containing a small amount of hydrofluoric acid (HF). 9 Since then, many research groups have paid considerable attention to this field, because anodization opens up ways to easily produce closely packed tube arrays with a self- organized vertical alignment. J. Nanosci. Nanotechnol. 2010, Vol. 10, No. 7 1533-4880/2010/10/4259/007 doi:10.1166/jnn.2010.2209 4259 Delivered by Ingenta to: Sung Kyun Kwan University IP : 115.145.200.148 Thu, 06 May 2010 12:02:47 RESEARCH ARTICLE Fabrication of Free Standing Anodic Titanium Oxide Membranes with Clean Surface Using Recycling Process Meng et al. So far, there have been three generations of anodic tita- nium oxide (ATO) nanotubes. In the first generation, TiO 2 nanotubes were fabricated in HF-based aqueous solutions. Some other inorganic acids and fluoride compounds, such as H 3 PO 4 /NaF, 10 NH 4 F/(NH 4  2 SO 4 , 11 and Na 2 SO 4 /NaF 12 were used as a substitute for poisonous HF. Due to the rapid chemical dissolution rate, the length of the obtained nanotube was limited to 500 nm. These low-aspect-ratio tubes could not satisfy the requirements for applications in dye sensitized solar cells (DSSCs) or filter membranes. Through adjusting the pH of the electrolyte, the second generation of ATO nanotubes was fabricated and tube length was increased to a few micrometers. 13 In the third generation, by using an almost water-free polar solution, especially in viscous glycerol 14 or ethylene glycol 15 elec- trolytes, tens, even hundreds, of micron-length ATO nano- tubes were easily prepared. However, the above-mentioned TiO 2 nanotubes were attached to the Ti substrate, which may limit their wider application. Recently, many groups have attempted to make self-organized, free standing TiO 2 nanotube arrays. Firstly, Jan M. Macak and co-workers directly fabricated a flow-through TiO 2 nanotubular membrane by selective dissolution of Ti substrate and successfully used as a photocatalytic microreactor. 16 This method was not only a laborious process, which took more than 10 hours, but also used a toxic bromine-containing methanol solution. Then, Wang et al. optimized the solution by using pure methanol evaporation to delaminate the layer between the membrane and the Ti foil. 17 Paulose et al. described a process includ- ing a critical point drying method by which nanotube array films were able to be transformed into free-standing mem- branes and applied it to control the diffusion of phenol red. 18 More recently, Park et al. reported the preparation of a membrane by immersing the Ti sheet with ATO in aque- ous HCl solution for 1 h, and made a glass-based DSSC. 19 However, the above membrane surfaces were always cov- ered by clumps of TiO 2 nanotubes or nanowires. To use the free standing TiO 2 nanotubes for a variety of applications, e.g., DSSCs, this top layer should be removed to infiltrate dye molecules and redox electrolytes into nanopores. Post treatments, such as sonication or Al 2 O 3 milling, sometimes needed several repetitions to ensure complete removal of the top layer of nanowires. These processes can cause the membrane to crack. So, our aim is to end up with as-received membranes with a clean surface at desired tube lengths. Herein, we report a simple and safe method to achieve large scale, self-organized, freestanding, and length con- trollable ATO nanotube membranes by using the dimpled substrate in the recycling process. 2. EXPERIMENTAL DETAILS Pure titanium foil (250 m in thickness and 99.7% purity) was purchased from Sigma Aldrich and cut into the desired size (4.0 × 5.0 cm). A direct current power supply (Agilent E3612A) was used as the voltage source for the anodiza- tion. The anodization process was carried out in a two- electrode configuration with titanium foil as the working electrode and graphite (5.0 × 5.0 cm) as the counter elec- trode under constant potential at 20  C. The titanium sheets were ultrasonicated in acetone, isopropanol and methanol each for 5 minutes, followed by rinsing with de-ionized (DI) water and drying in N 2 gas. The first electrochem- ical treatment was carried out at 50 V in ethylene gly- col with 0.3 wt% NH 4 Fand2v%H 2 O for 24 hours. The as-prepared sample was rinsed immediately with DI water for several minutes and dried with high pressure N 2 gas to remove the first ATO nanotube layer. Using the as-prepared patterned substrate without any treatment, the second anodic process was carried out at 50 V for 25 hours. Then the sample was rinsed with DI water for 3 minutes and placed on a filter paper to evaporate the water at room temperature. The free standing ATO mem- brane spontaneously separated from the substrate without bending. Using the same approach, we anodized the sub- strate obtained from the second anodization for 46 hours in the third process. The morphology of the ATO was examined by JEOL JSM-7000 field emission scanning electron microscopy (FESEM). The cross-section photographs were obtained by observing a mechanically fractured ATO layer trans- ferred onto an adhesive carbon tape coated with 3 nm Pt. The specimen for transmission electron microscopy (TEM, JEOL-2100F) at 200 kV was prepared by soni- cating a small part of the ATO membrane in ethanol for 30 minutes, and observing on a carbon-coated copper grid. The dimple substrate after first anodization was charac- terized by Atomic Force Microscopy (AFM). In order to examine the phase transition, the as-prepared sample was annealed at 450  C in air for 3 hours at a heating rate of 1  C/min. TEM images were also obtained using the above-mentioned method. X-ray diffraction (XRD) was performed (Rigaku, Rotafles) using Cu K (0.15416 nm) radiation. 3. RESULTS AND DISCUSSION After anodization for 24 hours, the ATO layer with the Ti substrate was washed with DI water for 3 min. A sud- den color change was observed, which indicated that water had completely penetrated into the pores and reached the substrate. When this happened, the ATO layer was eas- ily removed by subsequent blowing with high pressure N 2 gas and broken into several pieces. The easy detachment of the ATO layer discovered here is a notable feature, particularly given that long-time immersion of the ATO layer in methanol or HCl solution was not required. The ATO membrane and the Ti substrate were characterized as follows. 4260 J. Nanosci. Nanotechnol. 10, 4259–4265, 2010 Delivered by Ingenta to: Sung Kyun Kwan University IP : 115.145.200.148 Thu, 06 May 2010 12:02:47 RESEARCH ARTICLE Meng et al. Fabrication of Free Standing Anodic Titanium Oxide Membranes with Clean Surface Using Recycling Process (a) (c) (d) (b) Fig. 1. FESEM images of the as-prepared sample from (a) top view, (b) bottom view, (c) and (d) cross-sectional views. In Figure 1(c), a TEM image was inserted to show the tubular structure. The sample was anodized for 24 h. Figure 1(a) shows the typical morphology of the top- surface TiO 2 membrane. The image clearly reveals that the entire surface of the ATO layer is covered with a layer of nanowires, which is attributed to the long time split of the tube induced chemical etching in a “bamboo-splitting” mode. 20 21 The nanowires were randomly arranged and intersected. As shown in Figure 1(b), the bottoms of the TiO 2 nanotubes were closed. From the different magnified cross-section views (Figs. 1(c and d)), it can be seen that the height of the first anodized ATO nanotubes membrane with the nanowires is about 57 m, and there is no tran- sition area between the wire oxide layer and ordered bulk ATO nanotubes. The TEM image in Figure 1(c) shows the tubular structure of the titanium oxide nanotubes fabricated in first process. From the AFM image of the patterned substrate after removing the first ATO layer in Figure 2, it is evident that well-patterned dimples existed in the entire Ti sheet. The height of the dimples was approximately 30 nm and the average inter-distance is approximately 155 nm. The dim- ple size is dependent on the applied potential with bigger potentials resulting in larger dimples as shown in Figure 3. This corrugated substrate was used as a patterned template in the second anodization process. After 25 hours’ anodization in the same way as men- tioned above, the as-prepared sample, after sufficient wash- ing with DI water, was put on the filter paper for drying at room temperature without blowing with N 2 gas. Figure 4 shows a digital image of a brownish free standing TiO 2 nanotube membrane obtained directly after self-peeling off from the substrate. Because a large graphite plate electrode was used instead of the usual Pt wire electrode, a more 0 59 nm 44.25 29.5 14.75 0 191 382 573 764 955 nm Fig. 2. AFM image of the corrugated substrate produced after removing the TiO 2 nanotubes array. J. Nanosci. Nanotechnol. 10, 4259–4265, 2010 4261 Delivered by Ingenta to: Sung Kyun Kwan University IP : 115.145.200.148 Thu, 06 May 2010 12:02:47 RESEARCH ARTICLE Fabrication of Free Standing Anodic Titanium Oxide Membranes with Clean Surface Using Recycling Process Meng et al. (a) (b) (d)(c) Fig. 3. FESEM images of the as-prepared sample at different voltages, from a bottom view: (a) 30 V, (b) 40 V, (c) 50 V, and (d) 60 V. The size of dimple pits increased as the anodization voltage increasing. stable and larger equivalent electrical field was generated, and a large scale free standing sample was obtained. The evaporation of water caused the light brownish membrane to gradually separate from the Ti substrate as a result of surface-tension-driven delamination, owing to the presence of water and F − ions in the defect areas between the ATO layer and Ti substrate. Also, slowly drying, instead of fast drying by heating or N 2 gas blowing, gave a high area membrane without cracks. Fig. 4. A photograph of a free standing TiO 2 nanotube membrane. As shown in Figures 5(a and b), it can be seen that although the anodizing time was almost the same as the first step, the surface of the second ATO was quite clean and regular after having used the patterned substrate. The surface looks similar to the anodic aluminum oxide sur- face, in which the tubes are closely packed with each other and ordered on a large scale, instead of yielding indi- vidual tubes. In some places, around the cracks of the AAO-like surface, buried TiO 2 nanotubes were observed. The morphology of the patterned pits provided preferen- tial dissolution sites for anodization, which worked as pore nucleation centers for growth of homogeneous pores. This effect can be understood when taking into account the preexisting balance between the rates of oxide formation and oxide dissolution. As a result, when the same voltage was applied, the growth of the nanotube was restricted to the vertical direction against the substrate, and formed an AAO-like surface. The patterned surface oxide layer that was over the ATO layer protected the outside channel of the nanotube from the etching fluoride electrolyte, so the tube did not split along the sidewall of the tube. There were no nanowires formed or thinned-out tube walls, when the anodization was carried out for an extended time. After 25 hours’ anodization, the membrane thickness reached 44 m and there were no clumps of nanotubes from the side view (Fig. 5(c)). The morphology of these open tube vertical arrays offers an excellent pathway for the 4262 J. Nanosci. Nanotechnol. 10, 4259–4265, 2010 Delivered by Ingenta to: Sung Kyun Kwan University IP : 115.145.200.148 Thu, 06 May 2010 12:02:47 RESEARCH ARTICLE Meng et al. Fabrication of Free Standing Anodic Titanium Oxide Membranes with Clean Surface Using Recycling Process (a) (c) (b) (d) Fig. 5. FESEM and TEM images of the as-prepared sample from (a), (b) top views, (c) cross-sectional view, and (d) scale bar = 100 nm. The sample was anodized for 25 hours in the second step. (a) (c) (d) (b) Fig. 6. FESEM images of the as-prepared sample from (a), (b) top views, and (c, d) cross-sectional views. The sample was anodized for 46 hours in the third step. J. Nanosci. Nanotechnol. 10, 4259–4265, 2010 4263 Delivered by Ingenta to: Sung Kyun Kwan University IP : 115.145.200.148 Thu, 06 May 2010 12:02:47 RESEARCH ARTICLE Fabrication of Free Standing Anodic Titanium Oxide Membranes with Clean Surface Using Recycling Process Meng et al. (a) (b) (c) Fig. 7. Some SEM images of samples with different anodization times: (a) 10 min, (b) 1 hour, and (c) 11 hours. The chart shows the correlation between the thickness of free standing ATO membrane and the anodization time. (a) (b) (c) Fig. 8. TEM images and related SAED patterns shown in the inset of ATO nanotube membranes (a) before annealing treatment and (b) after annealing treatment. Scale bar is 10 nm. (c) XRD patterns of the ATO nanotubes membranes. Line “a” is before annealing treatment and line “b” is after annealing treatment (JCPDS. 21-1272). 4264 J. Nanosci. Nanotechnol. 10, 4259–4265, 2010 Delivered by Ingenta to: Sung Kyun Kwan University IP : 115.145.200.148 Thu, 06 May 2010 12:02:47 RESEARCH ARTICLE Meng et al. Fabrication of Free Standing Anodic Titanium Oxide Membranes with Clean Surface Using Recycling Process percolating and transferring of both electrons and small molecules (i.e., dye). Figure 5(d) shows the transmission electron microscopy image of nanotubes. This TEM image confirms that after 30 minutes of sonication, the nanotubes separated from each other. The walls of the nanotubes are very smooth and straight over their entire length. The thickness of the walls is around 30 nm, and the diameter of the walls is about 90 nm. In order to examine whether long durations of anodiza- tion etched the patterned suface layer or not, the third anodization process was carried out for 46 hours. The results were also shown in Figure 6. The ATO with AAO- like clean surface, (as the same as ATO in the second step), suggested that chemical dissolution did not etch the surface regardless of anodized time. It was found that the inter-pore distance, diameter, and thickness is 154 ± 2 nm, (which agrees well with the AFM data (in Fig. 2)), 91± 2 nm, and 70 m respectively (Figs. 6(a, c)). From both bottom and top views, comparing to 25 hours’ sample, more and better ordered hexagons were obtained (Figs. 6(b, d)). The correlation between the thickness of the free stand- ing ATO membrane and the anodization time is shown in Figure 7. One could use this chart to choose the anodiza- tion time according to the desired tube length, which would be very helpful for further applications. The as-prepared TiO 2 nanotube membrane was amor- phous as revealed by TEM imaging (Fig. 8(a)) and the XRD pattern (Fig. 8(c) line a), in which broad spectra ranging from 20 to 80 degree is clearly evident. In order to use the nanotubular structures of ATO membrane in DSSCs, the initially amorphous phase should be converted into photoactive anatase or rutile phase. Thus, the as- prepared sample was annealed at 450  C in air for 3 h at a heating rate of 1  C/min to induce crystallinity. In XRD pattern (Fig. 8(c) line b), all the observed peaks were indexed to anatase (JCPDS. 21-1272). No other peaks related to other phases or impurities were detected. From the TEM image and the SEAD pattern in Figure 8(b), evi- dence of polycrystalline rings also showed that after the annealing treatment, the amorphous titanium oxide nano- tubes converted into anatase phase. 4. CONCLUSIONS Several anodization processes were carried out in a 0.3 wt% NH 4 F/2 v% H 2 O/ C 2 H 6 O 2 solution at 50 V. Large scale membranes of ATO nanotubes were obtained using a plate graphite electrode instead of a Pt wire electrode under a constant voltage. Using the 30 nm thickness dim- ple layer as a patterned titanium substrate obtained after removing the ATO layer, gave the TiO 2 membranes a clean surface. In this simple recycling method, utilizing the patterned nano-scale pits as templates could get desired length, as a function of the anodization time, of titanium oxide nanotube membranes with clean surfaces. Acknowledgments: Financial support from BK21 Project through the School of Advanced Materials Science and Engineering is gratefully acknowledged. References and Notes 1. S. Ju, A. Facchetti, Y. Xuan, J. Liu, F. Ishikawa, P. Ye, C. Zhou, T. J. Marks, and D. B. Janes, Nat. Nanotechnol. 2, 378 (2007). 2. S. Yoriya, H. E. Prakasam, O. K. Varghese, K. Shankar, M. Paulose, G. K. Mor, T. A. Latempa, and C. A. Grimes, Sensor Lett. 4, 334 (2006). 3. J. Park, S. Bauer, K. V. D. Mark, and P. Schmuki, Nano Lett. 7, 1686 (2007). 4. G. K. Mor, K. Shankar, M. Paulose, O. K. Varghese, and C. A. Grimes, Nano Lett. 6, 215 (2006). 5. G. K. 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