Plate, Wire, Mesh, Microsphere, and Microtube Composed of Sodium

Feb 2, 2007 - Titanate Nanotubes on a Titanium Metal Template ... 1 Honjyo, Saga 840-8502, Japan, CREST, Japan Science and Technology agency, 4-1-8 Ho...
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Langmuir 2007, 23, 2815-2823

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Plate, Wire, Mesh, Microsphere, and Microtube Composed of Sodium Titanate Nanotubes on a Titanium Metal Template Mitsunori Yada,*,† Yuko Inoue,† Masafumi Uota,‡ Toshio Torikai,† Takanori Watari,† Iwao Noda,§ and Takao Hotokebuchi| Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga UniVersity, 1 Honjyo, Saga 840-8502, Japan, CREST, Japan Science and Technology agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan, Japan Medical Materials Corporation, Uemura Nissei Bldg. 9F 3-3-31 Miyahara, Yodogawa-ku, Osaka 532-0003, Japan, and Department of Orthopaedic Surgery, Faculty of Medicine, Saga UniVersity, 5 Nabeshima, Saga 849-8501, Japan ReceiVed September 11, 2006. In Final Form: December 9, 2006 Sodium titanate nanotube/titanium metal composites were synthesized by hydrothermal treatment of titanium metals with various morphologies such as plate, wire, mesh, microsphere, and microtube at 160 °C in aqueous NaOH solution and by the subsequent fixation treatment by calcination at 300 °C. The surface of the composite was covered with sodium titanate nanotubes with a diameter of approximately 7 nm, and the core part of the composite was titanium metal phase. The raw titanium metal acts as a template or a morphology-directing agent of micrometer size or more to arrange the nanotubes as well as a titanium source for the formation of nanotubes. The concentration of titanium species increases in the reaction solution as the dissolution of titanium metal is accelerated by the reaction between titanium and OH-. Furthermore, with an increase in concentration of titanium species in the reaction solution, the titanium species are reprecipitated as sodium titanate nanotubes onto the titanium metal. Titanium metal with a large surface area and volume can form sodium titanate nanotubes on the surface of the titanium metal, though titanium metal with a small volume and surface area tends to dissolve with the hydrothermal treatment. Even in the synthesis using titanium metal with a small volume and surface area, sodium titanate nanotubes are formed and cover the surface of the titanium metal by adding another titanium metal as a source of titanium species in the reaction solution.

Introduction Recently, the synthesis of titanium dioxide and titanate nanotubes (hereafter referred to as TNT) with an outer diameter of approximately 6-10 nm by the hydrothermal treatment of titanium dioxide powder in alkaline solution has been reported1-4 and has been attracting much attention. Because TNT has a large specific surface area, uniform one-dimensional nanopores, and cation exchangeability between titanate layers, various applications have been investigated such as cation exchange properties,5 precursors of perovskite nanotubes,6 electrochromism,7 bone regeneration,8 proton conduction,9 and so forth as well as photoinduced hydrophilicity,10 photocatalysts,11 and dyesensitizing solar batteries,12 which are all applications of conventional titanium dioxide. To maximize the characteristics * To whom correspondence should be addressed. Phone: +81-952-288682. Fax: +81-952-28-8682. E-mail: [email protected]. † Department of Chemistry and Applied Chemistry, Saga University. ‡ CREST, Japan Science and Technology agency. § Japan Medical Materials Corporation. | Department of Orthopaedic Surgery, Saga University. (1) (a) Kasuga, T.; Hiramatsu, M.; Hoson, A. Langmuir 1998, 14, 3160. (b) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. AdV. Mater. 1999, 11, 1307. (2) Chen, Q.; Zhou, W.; Du, G.; Peng, L.-M. AdV. Mater. 2002, 14, 1208. (3) Tasi, C.-C.; Teng, H. Chem. Mater. 2006, 18, 367-373. (4) Armstrong, G.; Armstrong, A. R.; Canales, J.; Bruce, P. G. Chem. Commun. 2005, 2454-2456. (5) Sun, X.; Li, Y. Chem. Eur. J. 2003, 9, 2229. (6) Mao, Y.; Banerjee, S.; Wong, S. S. Chem. Commun. 2003, 408. (7) Tokudome, H.; Miyauchi, M. Angew. Chem., Int. Ed. 2005, 44, 1974. (8) Kubota, S.; Johkura, K.; Asanuma, K.; Okouchi, Y.; Ogiwara, N.; Sasaki, K.; Kasuga, T. J. Mater. Sci.: Mater. Med. 2004, 15, 1031. (9) Thorne, A.; Kruth, A.; Tunstall, D.; Irvine, J. T. S.; Zhou, W. J. Phys. Chem. B 2005, 109, 5439. (10) Tokudome, H.; Miyauchi, M. Chem. Commun. 2004, 958. (11) Tokudome, H.; Miyauchi, M. Chem. Lett. 2004, 33, 1108. (12) Uchida, S.; Chiba, R.; Tomiha, M.; Masaki, N.; Sirai, M. Electrochemistry 2002, 70, 418.

of the nanotube and to use them efficiently, preventing their excessive aggregation and arrangement at larger than micrometer or centimeter size is considered important. As an example of TNT’s arrangement, Tian et al.13 reported that TNTs were observed on a titanium plate treated in aqueous NaOH solution after coating the titanium plate with titanium dioxide powder. The reported TNT thin film formed on the titanium plate seems not always to be stable; however, the TNT thin film was reported to be exfoliated when its thickness increased with increasing reaction time. Formation of TNT nanotube thin film by alternate layer deposition method with successful control of its thickness is reported.7,10 Although several reports of TNT thin film formation exist, none of them deal with controlling TNT’s arrangement and organization in various complicated shapes of micrometer or centimeter size. In this paper, we report the synthesis and organization of TNT of size larger than a micrometer, using titanium metals. The titanium metal acts as a template for the organization as well as a titanium source. Although Nakahira and co-workers14 have already reported the synthesis of TNT powders using titanium metal powders and sponges as a source, the titanium metals were only used as a source and not as a template for organization of TNT into micrometer-sized morphologies as in our study. Therefore, the originality of our study is to use titanium metal as a morphology-directing material. Using titanium metals as a titanium source is useful for the organization of TNT of more than a micrometer size, since various shapes of titanium metals such as plate, wire, mesh, microsphere, and microtube are already commercially available. First, the procedure of formation and (13) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Xu, H. J. Am. Chem. Soc. 2003, 125, 12384. (14) (a) Isu, N.; Nakahira, A.; Yamazaki, T. Jpn. Kokai Tokkyo Koho 2006. (b) Nakahira, A.; Kubo, T. Mater. Integr. 2004, 18, 15.

10.1021/la062654c CCC: $37.00 © 2007 American Chemical Society Published on Web 02/02/2007

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Figure 1. Photograph (a, g), TEM (b, j, m), and SEM (c-f, h, i, k, l) images for the as-grown (a-f) and the 300 °C calcined (g-m) products obtained after the 20-h (a-j) and the 3-h (k-m) reactions. (a) Overall image of the plate, (b) nanotubes observed in the thin film, (c) cross section of the thin film, (d) enlarged image of the TNT phase, (e) enlarged image of the dense sodium titanate phase without TNT, (f) back side of the thin film, (g) overall image of the plate, (h) surface of the plate, (i) enlarged image of the plate, (j) nanotubes observed in the thin film, (k) surface of the plate, (l) enlarged image of the plate, and (m) nanotubes observed in the thin film.

fixation of a TNT thin film on titanium plate was investigated. Next, this procedure was applied to investigate the synthesis of a TNT/Ti composite in which various titanium metal materials with controlled shapes of a micrometer size were used as templates and titanium metal sources. These included titanium wire with a diameter of a micrometer, mesh woven from the titanium wire, titanium microspheres, and titanium microtube. Experimental Section Various titanium metal materials were immersed in 20 mL of 10 mol L-1 aqueous NaOH solution in a Tefron container, and reactions

were carried out at 160 °C for 3 and 20 h. Titanium plate (20 mm × 20 mm × 2 mm, Japan Medical Materials Corporation), titanium wire (lengths: 5 cm, 24 cm and diameters: 53.4 µm, 104.4 µm, 203.7 µm, The Nilaco Corporation), titanium mesh (woven from the titanium wire with a diameter of 104.4 µm, 20 mm × 20 mm, The Nilaco Corporation), titanium tube (inner diameter: 800 µm, outer diameter: 1 mm, length: 1 cm, The Nilaco Corporation), and titanium spheres (diameter: 850-1180 µm, weight: 0.21-0.24 g, Japan Medical Materials Corporation) were used as metal titanium sources. Transmission electron microscopy (TEM) was carried out using a Hitachi H-800MU instrument. Scanning electron microscopy (SEM)

Sodium Titanate Nanotubes on a Titanium Template

Figure 2. XRD patterns of the plates obtained after the 20-h (a) and the 3-h (b) reactions and of the original titanium plate (c). Peak assignment: b titanium metal, O sodium titanate nanotube. was performed using a Hitachi S-3000N. X-ray microanalysis (EDX) was conducted with an EDAX Genesis 2000 instrument. Powder X-ray diffraction (XRD) measurements were made on a Shimadzu XRD-6100 instrument with Cu KR radiation.

Results and Discussion Synthesis and Fixation of Sodium Titanate Nanotubes on Titanium Plate. First, the growth and fixation of TNT on titanium plate were investigated. After hydrothermal treatment for 20 h,

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the surface of the plate changed to pale, indicating the formation of a thin film on the titanium plate. Moreover, since the reaction solution turned turbid white, the formation of particles in the solution was also confirmed. To wash out NaOH and the particles that adhered to the surface of the thin film, the plate was washed with water after the reaction. The thin film immediately exfoliated as shown in Figure 1a, and then a surface with a metallic luster similar to that of titanium metal appeared on the surface of the plate. The greater part of the film was posited to consist of nanotubes with an outer diameter of approximately 7 nm, since only nanotubes were observed by TEM after ultrasonic dispersion of the film as shown in Figure 1b. Through EDX analysis, the mol fraction of Na/Ti/O for the obtained film was determined to be 1:1.947:4.943. The film was thus assumed to be Na2Ti4O9‚ H2O, though some titanate structures, such as A2Ti2O5‚H2O,15 A2Ti3O72, H2Ti4O9‚H2O,16 and lepidocrocite titanates,17 have been assigned as nanotube constituents (A ) Na or H) as summarized by Tasi and Teng.3 The formation of sodium titanate nanotubes or TNT was thus confirmed; this is similar to the experimental result, in which titanium dioxide was used as the source of titanium. Moreover, by detailed SEM observation of the cross section of this film, the film thickness was determined to be approximately 20.2 µm, as shown in Figure 1c. The thickness of the TNT phase was determined to be approximately 19.2 µm, by the existence of fibrous morphologies because of TNT as shown in Figure 1d, and the thickness of the dense sodium titanate phase without TNT was determined to be approximately 1.0 µm as shown in Figure 1e. Although fibrous morphologies were observed on the surface of the film, the back of the film was flat with no visible fibers as shown in Figure 1f. Flat smooth image, which differed from the original titanium metal image, was also observed on the surface of the plate after the film exfoliation. Furthermore, since very small amounts of Na were detected on the surface of the plate by EDX analysis, and the plate displayed a metallic luster after film exfoliation, a small amount of the sodium titanate phase appeared to remain on the surface of the

Figure 3. SEM images of the surfaces of the wires synthesized using the wires of diameters 53.4 µm (a), 104.4 µm (b), and 203.7 µm (c) after the 3-h reaction.

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Figure 4. SEM (a, b, d, e) and TEM (c, f) images of the wires synthesized using the 203.7-µm (a-c) diameter wire and the 104.4-µm (d-f) diameter wire obtained after the 20-h reaction. (a) Obtained wire, (b) enlarged image of the wire’s surface, (c) nanotubes observed in the wire, (d) obtained wire, (e) enlarged image of the wire’s surface, and (f) nanotubes observed in the wire.

plate. Therefore, the film exfoliation was considered to occur at the interface between the titanium metal phase and the sodium titanate phase without TNT. On the basis of the above results, the formation of the TNT thin film can be explained as follows: (1) the concentration of titanium species18 such as TiO32-, TiO2(OH)22-, and possibly polytitanates TinO2n+m2m- in the reaction solution increases as the dissolution of titanium is accelerated by the reaction between titanium and OH-; (2) titanium species are reprecipitated as sodium titanate with an increase in the concentration of titanium species in the reaction solution; and (3) since the concentration of the titanium species in the reaction solution is expected to increase with time, the sodium titanate phase without TNT is formed when the concentration of titanium species is low and the TNT phase is formed after the concentration becomes sufficiently high. The TNT-free sodium titanate phase formed at low concentrations of titanium species may be amorphous sodium titanate, because (15) Yang, J.; Jin, Z.; Wang, X.; Li, W.; Zhang, J.; Zhang, S.; Guo, X.; Zhang, Z. J. Chem. Soc., Dalton Trans. 2003, 3898. (16) Nakahira, A.; Kato, W.; Tamai, M.; Isshiki, T.; Nishio, K.; Aritani, H. J. Mater. Sci. 2004, 39, 4239. (17) Ma, R.; Bando, Y.; Sasaki, T. Chem. Phys. Lett. 2003, 380, 577. (18) Wu, D.; Liu, J.; Zhao, X.; Li, A.; Chen, Y.; Ming, N. Chem. Mater. 2006, 18, 547.

Kim et al.19 have already reported the synthesis of sodium titanate with an amorphous structure formed on the surface of a titanium plate by reaction of the plate in 5 mol L-1 NaOH solution at 60 °C. In a hydrothermal reaction in concentrated alkaline solution, as in this experiment, TNT is considered to be synthesized on a titanium plate because of the simultaneous dissolution of titanium species from the titanium plate and redeposition of sodium titanate nanotubes on the plate. Since the concentration of titanium species in the reaction solution is considered to affect the type of sodium titanate cluster and the formation rate of the sodium titanate phase, the concentration of titanium species, together with temperature and other concentrations, is also considered to be a factor in determining the type of phase formed. Moreover, to prevent exfoliation of the thin TNT film, the as-synthesized plate was slowly dried at room temperature after the hydrothermal treatment without washing it with water. Although NaOH crystals were observed on the plate, the thin film still adhered to the plate. When the plate was washed with water after heat treatment at 300 °C for 1 h in air, although the NaOH crystals dissolved, the thin TNT film still adhered to the plate firmly and no exfoliation was observed. Slow drying of the (19) Kim, H.-M.; Miyaji, F.; Kokubo, T.; Nakamura, T. J. Biomed. Mater. Res. 1996, 32, 409.

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Figure 5. SEM (a-d) and TEM (e, f) images of the wires synthesized using the 53.4-µm diameter wire wound onto the titanium plate after the 3-h (a, c, e) and the 20-h (b, d, f) reactions. (a, b) Low-magnification images of the wires, (c, d) enlarged images of the wire’s surfaces, and (e, f) nanotubes observed in the wires.

plate at room temperature and subsequent heat treatment produced a titanium plate coated with a thin TNT film, as shown in Figure 1g. Figure 1h shows the smooth surface of the obtained plate. It has already been reported that the crystal structure and morphology of TNT are retained at 300 °C.5 TNT formation was confirmed by the fibrous morphologies observed on the surface of the thin film in an enlarged SEM image (Figure 1i) and nanotubes observed in a TEM image of the thin film (Figure 1j). Moreover, in an XRD pattern of the thin film, only diffraction peaks characteristic of TNT2 were observed along with peaks assigned to titanium metal; peaks assigned to NaOH were not observed as shown in Figure 2a. The reason for this stable coating is probably because polycondensation of hydroxyl groups in the interface area between the titanium plate and the TNT-free sodium titanate phase occurred by the heat treatment at 300 °C, and TNT was firmly fixed on the plate. The slow drying process is also considered to be important for the fixation of TNT onto the titanium plate, since the thin film exfoliated from the titanium plate by drying at 60 °C. The formation and fixation of the TNT thin film were also observed in the reaction after 3 h. A smooth surface (Figure 1k) and fibrous morphologies (Figure 1l) similar to those of the 20-h product were observed in the SEM images. Nanotubular structures similar to those of the 20-h product were also observed in Figure 1m. The thickness of 13.1 µm for the

film obtained after the 3-h reaction was smaller than that of 20.2 µm for the film obtained after the 20-h reaction. The thicknesses of the films indicate that the thickness of the film is controllable by the reaction time. In an XRD pattern of the 3-h product shown in Figure 2b, peaks assigned to TNT were present similar to that of the 20-h product. Moreover, the XRD patterns indicated that the intensity of the peaks, corresponding to TNT, increased as the reaction time increased; this was considered to be due to the increase in the amount of TNT formation. Furthermore, since the relative intensity of the diffraction peak at 2θ ) 40° corresponding to the 101 reflection of the titanium plate decreased as the reaction time increased as shown in Figure 2a-c, it is suggested that the dissolution rate of the 101 plane was greater than that of other crystal planes. Through EDX analysis, the mol fraction of Na/Ti/O for the film was determined to be 1:1.986: 5.361. The obtained film was also assumed to be Na2Ti4O9‚H2O similar to that of the 20-h product. Major differences between the as-synthesized 3-h product and the 20-h product are that exfoliation of the thin TNT film for the 3-h product hardly occurred when the plate was directly washed with water after the synthesis. The TNT thin film obtained after a reaction time of 3 h seemed to adhere strongly when compared to that obtained after 20 h. This strong adhesion observed in the short reaction time is similar to the results reported by Tian et al.13 Possibly,

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Figure 6. Photograph (a, b), SEM (c-h), and TEM (i, j) images of the meshes obtained after the 3-h (a, c, d, g, i) and the 20-h (b, e, f, h, j) reactions. (a, b) Overall images of the meshes, (c-f) enlarged images of the meshes, (g, h) more enlarged images of the surfaces of the meshes, and (i, j) nanotubes observed in the wires.

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Figure 7. SEM (a, b) and TEM (c) images of the microsphere obtained after the 3-h reaction. (a) Overall image of the microsphere, (b) enlarged surface of the microsphere, and (c) nanotubes observed in the surface of the microsphere.

Figure 8. Photograph (a), SEM (b, c), and TEM (d) images of the microtube obtained after the 3-h reaction. (a) Overall image of the microtube, (b) enlarged surface of the microtube, (c) enlarged inner surface of the microtube, (d) nanotubes observed in the inner surface of the microtube.

it was not exfoliated because of the small surface tension and the thin thickness. Since the procedure of synthesis and fixation of the TNT film onto a titanate plate was established as mentioned above, the thickness of the film can be controlled more effectively than before. Wire Composed of Sodium Titanate Nanotube/Titanium Composite. By applying this procedure, the synthesis and fixation of TNT were investigated using titanium metal wires of diameters 53.4, 104.4, and 203.7 µm and length 5 cm as titanium sources. As a result, after the hydrothermal treatment for 3 h, sodium titanate with an irregular morphology was formed on the surface

of the titanium wires as shown in Figure 3a-c, and only a small amount of nanotubes was observed in the products synthesized using titanium wires of diameters 104.4 and 203.7 µm. The diameters decreased from 53.4 and 104.4 µm for the original wires to 36.3 and 93.8 µm for the wires after a reaction time of 3 h, respectively. Moreover, after the hydrothermal treatment for 20 h, both the 53.4- and 104.4-µm titanium wires completely dissolved. The reason for the complete dissolution of the original 53.4- and 104.4-µm wires is because the dissolution rates of titanium species from the wires were faster than the redeposition rate of sodium titanate on the surface of the wires. On the other

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hand, the diameter of the 203.7-µm wire increased to 211.3 µm after 3 h. This is due to the formation of bulky sodium titanate, since the thickness of the sodium titanate phase was 4.7 µm and the thickness of the titanium metal phase was 202.0 µm. Moreover, in the 20-h reaction using the 203.7-µm diameter wire, the wire shape was maintained without complete dissolution and nanotubes were partially observed on the surface of the wire. The diameter of the wire increased from 203.7 µm for the original wire to 206.1 µm for the 20-h product, and the thickness of the sodium titanate phase including TNT increased from 4.7 µm for the 3-h product to 11.3 µm for the 20-h product, although the thickness of the titanium metal phase decreased from 202.0 µm for the 3-h product to 183.5 µm for the 20-h product. These differences depending on the diameters of the original wires are explained as follows. Surface area and surface texture strongly affect the concentration of titanium species in the reaction solution. The amount of titanium species in the reaction solution increases with an increase in the diameter of the wire, since the surface area of the wire increases with an increase in the diameter. Additionally, the difference in the surface texture of the wires also affects the concentration of titanium species near their surfaces. Detailed SEM observations of the original titanium wires confirmed that the surfaces of the wires of diameter 203.7 µm (Supporting Information 1b) and 104.4 µm (Supporting Information 1c) were porous, but the surface of the wire of diameter 53.4 µm (Supporting Information 1d) was relatively smooth. The concentration of dissolved titanium species would be higher near the wire and lower as the distance from the wire increases. In particular, the concentration of titanium species near the porous surface would be higher than that near the smooth surface. Therefore, in the experiments using the 104.4- and 203.7µm diameter wires, the amount of titanium species dissolved per unit of time and the concentration of the titanium species would be large because of their larger diameters and porous surfaces, and consequently, the concentration of titanium species would be sufficiently high for the formation of TNT as the dissolution of titanium proceeded. On the other hand, since the surface area of the 53.4-µm diameter wire was predicted to be smaller than the 104.4- and 203.7-µm diameter wire because of its diameter and smooth surface, the concentration of titanium species dissolved per unit of time would also be small. Therefore, the concentration of titanium species would be too low for the formation of TNT, and consequently, sodium titanate with irregular morphology was formed without TNT by the synthesis. When the amount of titanium was increased further to increase the surface area and amount of titanium, namely, when similar synthesis was carried out using a wire of length 24 cm and diameter 203.7 µm, the amount of TNT formed at a reaction time of 3 h increased and most of the surface was covered with TNT, and sodium titanate with an irregular morphology was occasionally observed. After 20 h of reaction, only the formation of TNT was confirmed on the surface of the wire as shown in Figure 4a-c. The smooth surface shown in Figure 4a is different from the surface of the original 203.7-µm wire shown in Supporting Information 1b. Uniform fibrous morphologies were observed as shown in Figure 4b and the fibers were confirmed to be nanotubes similar to those on the titanium plate as shown in Figure 4c. In the cross section of the obtained wire, the interface between the TNT and the titanium phase was clearly observed: on the outside was the sodium titanate phase mainly composed of TNT and on the core side was the titanium metal phase. The diameter of the wire was 214.2 µm, the thickness of the TNT phase was 11.1 µm, and the diameter of the titanium phase was 192.0 µm. This result also indicates that the reaction proceeded

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with the dissolution of titanium species from the surface and its redeposition as TNT. Moreover, even in the experiment using a wire of length 24 cm and diameter 104.4 µm, the amount of TNT formed at 3-h reaction time increased, and the formation of TNT confirmed on the surface of the wire that the 20-h reaction was different from the experiment using a wire of length 5 cm and diameter 104.4 µm. Figure 4d-f shows SEM and TEM images for the product obtained from the 104.4-µm wire at a reaction time of 20 h. The diameter of the wire was 116.5 µm, the thickness of the TNT phase was 16.0 µm, and the diameter of the titanium phase was 84.5 µm. The reason for the complete coverage of TNT on the wire without dissolution is that the redeposition rate of sodium titanate nanotubes on the wire’s surface became faster than the dissolution rate of titanium species from the wire with an increase in its length. The surface of the 104.4-µm wire after 20 h of reaction was rough as shown in Figure 4d in comparison with the flat and smooth surface of the wire obtained from the 203.7-µm diameter wire after 20 h of reaction as shown in Figure 4a. This difference in the surface texture is due to the porosity of the original wire. The fast formation rate of titanium species because of the porous texture observed in the 104.4-µm diameter wire would result in the rough surface composed of TNT for the obtained wire, since the surface texture of the original 104.4-µm diameter wire is porous in comparison with that of the original 203.7-µm diameter wire as shown in Supporting Information 1b and 1c. On the other hand, in the experiment using a wire of diameter 53.4 µm and length 24 cm, no nanotubes were observed on the surface of the obtained wire at a reaction time of 3 h and the wire completely dissolved at a reaction time of 20 h. It is considered that since the amount of titanium species dissolved from the wire would be small even in the experiment using the 53.4-µm-diameter wire of length 24 cm, the concentration of titanium species would be too low for the formation of TNT, and consequently, sodium titanate with an irregular morphology was formed without TNT by the synthesis using the 53.4-µm wire. Taking into consideration the above discussion, a similar hydrothermal and fixing treatment was performed using a wire of diameter 53.4 µm and length 24 cm wound onto the abovementioned titanium plate, which could act as a source of titanium species. Wired morphologies remained for 3- and 20-h reactions as shown in Figure 5a and 5b, respectively, which is different from the complete dissolution for the direct hydrothermal treatments of the titanium wire of diameter 53.4 µm and length 24 cm at a 20-h reaction time. The formation of TNT was observed on the surfaces of both wires, and fibrous morphologies were observed for the 3- and 20-h products as shown in the SEM images of Figure 5c and 5d, respectively. TEM images of the products also indicate a uniform nanotubular structure similar to that of the titanium plate as shown in Figure 5e and 5f, respectively. The diameter of the wire increased from 53.4 µm for the original wire to 94.0 µm for the 20-h product and was 57.0 µm for the 3-h product. Although the thicknesses of the titanium metal phase at the reaction times of 3 and 20 h were 46.8 and 45.1 µm, the thickness of the sodium titanate layers significantly increased from 5.1 µm for the 3-h reaction product to 24.4 µm for the 20-h reaction product, measured on the cross section of the wire. Since the amount of titanium species reprecipitated on the wire, supplied by the dissolution from the titanium plate, was larger than the amount of titanium species dissolved from the wire, the surface of the wire was covered with TNT and the amount of TNT increased with time. These results also indicate that dense concentration of titanium species near the titanium surface is required for the formation of TNT on the

Sodium Titanate Nanotubes on a Titanium Template

titanium wire. Consequently, the wire with TNT/Ti composite was considered to be formed. Similar experiments using 104.4and 203.7-µm wires wound onto the titanium plate also obtained complete coating of TNT on both wires at reaction times of 3 and 20 h. This technique would thus enable the formation of TNT on titanium wires of different diameters. Mesh Composed of Sodium Titanate Nanotube/Titanium Composite Microwires. A similar experiment was conducted on a mesh woven with the 104.4-µm wire. Figure 6a and 6b shows that although the 104.4-µm wire of length 5 cm dissolved after the 20-h reaction, the shape of mesh was maintained after the 3- and 20-h reactions. After the synthesis, the lower right parts of the mesh in Figure 6a and 6b were cut for SEM observation. Although the surface of the product obtained after the 3-h reaction was flat and smooth as shown in Figure 6c and 6d, the surface of the sample obtained after the 20-h reaction was rough as shown in Figure 6e and 6f. Uniform fibers were observed on the surfaces of both products as shown in Figure 6g and 6h. The formation of TNT was also confirmed in both products, as shown in TEM images of Figure 6i and 6j. Therefore, it is clarified that TNT can be synthesized using various shapes of titanium wire fabrics larger than a micrometer. After the 20-h reaction, the diameter of the wire increased to 125.5 µm for the 20-h product from its original 104.4 µm and was 114.1 µm for the 3-h product. The thickness of the sodium titanate phase including TNT increased from 5.2 µm for the 3-h reaction to 15.5 µm for the 20-h reaction, and the titanium metal phase’s diameter of 103.7 µm for the 3-h product decreased to that of 94.6 µm for the 20-h product. Since the amount of titanium using mesh in the experiment was approximately 85 times as much as that used in the experiment with wire (with a length of 5 cm and diameter of 104.4 µm), the amount of titanium dissolved in the reaction solution was larger in the experiment using the mesh than in the experiment using the wire. Moreover, since the distance between wires constituting the mesh was small, the concentration of titanium species near the wire also rapidly increased to a level sufficient for formation of TNT. It was consequently suggested that since the reprecipitation of TNT was more rapid than the dissolution of titanium species, the shape of the mesh could be maintained after 20 h of reaction, and the mesh coated with TNT was obtained. On the basis of the above results, TNT applications can be largely extended by the hydrothermal treatment of a cloth woven with titanium wires and by weaving a cloth with TNT/Ti wires. Microspheres Composed of Sodium Titanate Nanotube/ Titanium Composite. Similar experiments with reaction times of 3 and 20 h were conducted for titanium microspheres. After the reactions, the surfaces of the microspheres obtained were covered with thin TNT films. SEM and TEM images for the 3-h product are shown in Figure 7. Figure 7a is an image of the (20) (a) Wang, H.; Li, X.; Nakamura, H.; Miyazaki, M.; Maeda, H. AdV. Mater. 2002, 14, 1662. (b) Miyazaki, M.; Kaneno, J.; Uehara, M.; Fujii, M.; Shimiz, H.; Maeda, H. Chem. Commun. 2003, 648-649.

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obtained sphere and shows the smooth surface of the sphere, which is different from the surface of the original titanium microsphere shown in Supporting Information 1e. The surface was covered with uniform fibers as shown in Figure 7b and was confirmed to be nanotubes with an average diameter of 6 nm as shown in Figure 7c. The thickness of the sodium titanate phase including TNT increased from 4.3 µm for the 3-h product to 27.1 µm for the 20-h product. Microtube Composed of Sodium Titanate Nanotube/ Titanium Composite. A similar experiment with a reaction time of 3 h was performed on a titanium microtube. The microtube shown in Figure 8a was cut in half, and the inner and outer surfaces of a section were observed as shown in Figure 8b-d. Figure 8b shows smooth inner and outer surfaces of the microtube, which is different from the surface of the titanium microtube shown in Supporting Information 1f. Although the microtube in Figure 8b appears to be slightly deformed and damaged, the shape was generated only when the microtube was fixed with an appliance to cut the tube. Both outer and inner surfaces of the microtube were covered with uniform fibers as shown in Figure 8c and were confirmed to be nanotubes with an average diameter of 7 nm as shown in Figure 8d. The thickness of the sodium titanate phase including TNT was approximately 5.9 µm.

Conclusion In this study, we report the procedure for synthesis and fixation of TNT on titanium metals for various morphologies such as plate, wire, mesh, microtube, and microsphere. Since the TNT/ Ti composite wires have a softness, flexibility, and mechanical strength similar to metal titanium because of the existence of titanium metal in its core part, they can be fabricated into various shapes of cloth, fiber, and so forth of centimeter or meter size using conventional spinning techniques. Therefore, TNT’s applications,5-12 as listed in the introduction, would be remarkably expanded. Furthermore, tubes of micrometer size, the inside of which is modified with TNT, can contribute much to the improvement in microreactor technology, since the function of a microreactor is known to be improved by modification of its inside with various particles.20 Moreover, titanium metal has been used for implants such as prosthetic hip implants or dental implants, and titanium metal covered with TNT as obtained in this study would be promising as a novel implant with strong osteoconductive activity because of the excellent bone regeneration properties8 of TNT. Acknowledgment. This research was partially supported by the Ministry of Education, Science, Sports, and Culture, Grantin-Aid for Young Scientists (A), 16685021. Supporting Information Available: SEM images of the enlarged surfaces for the original titanium metals. (a) Plate, (b) wire of diameter 203.7 µm, (c) wire of diameter 104.4 µm, (d) wire of diameter 53.4 µm, (e) microsphere of diameter 850-1180 µm, (f) microtube of inner diameter 800 µm and outer diameter 1000 µm. This material is available free of charge via the Internet at http://pubs.acs.org. LA062654C