Bioactive Titanium Oxide-Based Nanostructures Prepared by One

Article pubs.acs.org/JPCC

Bioactive Titanium Oxide-Based Nanostructures Prepared by OneStep Hydrothermal Anodization Chun-Yi Chen,*,†,‡ Kazunari Ozasa,†,‡ Ken-ichi Katsumata,§ Mizuo Maeda,‡ Kiyoshi Okada,§ and Nobuhiro Matsushita§ †

Department of Electrochemistry, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 2268503, Japan RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 3510198, Japan § Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 2268503, Japan ‡

S Supporting Information *

ABSTRACT: A method to rapidly fabricate highly crystalline titania and titanate nanomaterials is required to facilitate their use in biomedical applications such as bone implants. In this study, a new one-step “hydrothermal anodization” (HTA) method to modify titanium surfaces for biomedical application is presented. In HTA, rapid formation of highly crystalline nanostructures is realized by combining the advantages of hydrothermal treatment and anodization. Multiple reactions including recrystallization/ dissolution and electric field-enhanced oxidation/dissolution result in HTA providing wide working windows for both anodic oxidation and dissolution of titanium. The effects of reaction temperature, applied current, and electrolyte concentration are studied. Crystalline sodium titanate nanowires with an average length of 1.0 μm are synthesized at a growth rate of 500 nm h−1, and a titanium surface with a roughness of ca. 1.5 nm is obtained by HTA. Immersion of the nanowires in simulated body fluid to examine their bioactivity shows that porous nanoflakes of crystalline hydroxyapatite were deposited after 1 week. This highlights the potential of this new fabrication process to produce titanium surfaces suitable for biomedical applications.

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12 nm and length of 100−600 nm.11 Although the HT method can easily produce highly crystalline nanostructures, it requires long reaction times to prepare sufficiently large quantities of ST nanostructures because the growth rate of the nanostructures is relatively low due to the dissolution−recrystallization mechanism.12 Alternatively, anodization in an electrolyte containing fluoride ions can be used to fabricate self-organized titania nanostructures with higher growth rate than the HT method. Zwilling et al. fabricated self-organized 500 nm long amorphous titania nanotubes in an electrolyte containing HF.13 Albu et al. succeeded in obtaining longer amorphous nanotubes (180 nm in diameter and 145 μm in length) at a growth rate of 14.7 μm h−1 in an NH4F electrolyte containing ethylene glycerol.14 However, titania nanotubes prepared by anodization are amorphous; high-temperature annealing is required to increase their crystallinity.15 Allam et al. fabricated crystalline titania nanotubes without high-temperature annealing by pretreatment with an oxidant. In this case, the length of the nanotubes was limited to the thickness of the preprepared dense oxide films.16 Alkali electrolytes can be used to synthesize nanostructures of other materials by anodization at room temperature.

INTRODUCTION In recent years, titania and titanate nanomaterials including nanowires (NWs)1 and nanotubes2,3 have drawn much attention because of their widespread application in, for example, dye-sensitized solar cells,4 photocatalysts,5 hydrogen evolution,6 drug delivery,7 and bone implants.8 Pertaining to bone implants, the formation of nanostructured titania and titanate layers on titanium are promising to improve the bioactivity and biocompatibility of titanium. Many synthetic methods9 to fabricate titania and titanate nanostructures have been investigated such as sol−gel and hydrothermal (HT) methods, chemical vapor deposition, electrodeposition, and anodization. HT and anodization possess advantages over other methods because they are simple and cost-effective and can be used for mass production of Ti-based nanostructures. The HT method is widely used to fabricate sodium titanate (ST) nanostructures in alkali solution, which is an important precursor for titania and other titanate compounds. Highly crystalline nanostructures can be prepared by HT because the dissolution−recrystallization mechanism of HT treatment is effective to grow nanocrystals. Miyauchi prepared ST nanorods by the HT method and synthesized single crystalline strontium titanate nanorod arrays by subsequent ion exchange and annealing.10 Yu et al. treated titania powders by the HT method at 110 °C in 10 M NaOH solution for 90 h and obtained polycrystalline titania nanotubes with an outer diameter of 9− © 2012 American Chemical Society

Received: November 9, 2011 Revised: March 1, 2012 Published: March 6, 2012 8054

dx.doi.org/10.1021/jp210783w | J. Phys. Chem. C 2012, 116, 8054−8062

The Journal of Physical Chemistry C

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Sreekantan et al. synthesized ZnO nanoneedles on a Zn foil in 4 M NaOH electrolyte with a current density of 8.33 mA cm−2 for 1 h.17 Hossain et al. fabricated mesoporous SnO spheres in an electrolyte containing 0.1 M NaOH, 0.05 M NH4F, and ethylene glycerol at 4 V for 10 h.18 Xie et al. grew thin layers of nanonetworks of titania at 20 V for 45 min in 10 M NaOH,19 while Chiang et al. fabricated titania nanonetworks with a thickness of ca. 350 nm by applying a high current of 0.2 A in 5 M NaOH electrolyte for 30 min.20 The growth rate of nanostructures formed by anodization is higher than that compared with the HT method; however, the crystallinity of titania nanostructures prepared by anodization at room temperature is low.19,20 We have developed a “one-step hydrothermal anodization method” (HTA) to rapidly prepare bioactive titanium oxidebased nanostructures with high crystallinity. HTA involves dissolution−recrystallization by HT treatment and electric field-enhanced oxidation/dissolution by anodization. Therefore, nanostructures form rapidly through anodization and crystallinity is improved by simultaneous HT treatment. We describe the characteristics of one-step HTA and discuss the mechanism of nanostructure formation in terms of current- and electrolyteconcentration effects. To show that the nanostructures prepared by one-step HTA are suitable for use as bone implants, we evaluate their ability to promote apatite formation upon immersion in simulated body fluid (SBF).

microscope (AFM, MFP-3D Asylum Research, Santa Barbara, CA). The crystallographic structures of the samples were investigated with an X-ray diffractometer (XRD, RINT-2000, Rigaku, Japan) and transmission electron microscope (TEM, JEOL JEM-2100, Japan, operated at 200 keV). The composition of the samples was determined with an X-ray photoelectron spectroscope (XPS, VG Scientific, ESCALAB 250, Sussex, UK). XPS measurements were performed with Al Kα (hν = 1486.6 eV) as the X-ray source and a pass energy of 100 eV (300 eV) for single elemental (survey) spectra. The C1s peak at 284.6 eV was used as a reference binding energy for calibration. Raman spectra of the samples were obtained using a Raman spectrometer (Jobin Yvon T64000, France) with an excitation laser (λ = 514.5 nm), which was operated with an output power of 50 mW. To evaluate the bioactivity, 1 cm2 NW samples were immersed in SBF solution (30 mL)22 containing ionic concentrations of 142.0 mmol L−1 Na+, 5.0 mmol L−1 K+, 1.5 mmol L−1 Mg2+, 2.5 mmol L−1 Ca2+, 148.8 mmol L−1 Cl−, 4.2 mmol L−1 HCO3−, 1.0 mmol L−1 HPO42−, and 0.5 mmol L−1 SO42− and buffered at pH 7.4. The samples immersed in SBF in individual polypropylene vials were stored in an incubator at 37 °C for 30 min and 1, 2, 7, and 12 days. After immersion, the samples were rinsed with distilled water, dried in air, and then prepared for SEM, XPS, XRD, and Raman measurements.

EXPERIMENTAL SECTION Titanium foil (99.5%, 0.127 mm thick, Nilaco Corp., Japan) with a size of 1 cm × 4 cm was ultrasonically degreased in acetone and deionized water prior to one-step HTA. HTA was carried out in an autoclave21 with an electrochemical cell. Titanium foil was used as the working electrode, a platinum plate as the counter electrode, and Ag/AgCl as the reference electrode, as shown in Figure 1. The distance between the

RESULTS HTA Conditions versus Surface Morphology. Figure 2a shows a typical as-prepared surface obtained by HTA under a current density of 5.0 mA cm−2. The surface was homogeneous and covered with dense NWs. No nanostructure formed without anodic current, as shown in Figure 2b. Figure 3 shows the dependence of the surface morphology on the NaOH concentration used for HTA. The surface morphology obtained with 0.5 M NaOH (Figure 3a) possessed a thin layer of flakes with lateral sizes of 200−300 nm. Upon increasing the NaOH concentration to 5.0 M, the surface morphology changed from nanoflakes to NWs, as shown in Figure 3b. In contrast, the surface obtained with 10 M NaOH had no features (Figure 3c). We also monitored the voltage response during HTA using different concentrations of NaOH. The concentration of the electrolyte significantly affected the voltage response during HTA, as shown in Figure 4. Figure 4a shows that the higher the concentration of NaOH, the lower the voltage during HTA. The steady state voltages for 0.5, 5, and 10 M NaOH are ca. 5.78, 1.65, and 0.82 V, respectively. The voltage response during the initial stage of HTA shows different behaviors for the different concentrations of NaOH, as shown in Figure 4b. In 10 M NaOH, the voltage gradually increased to 0.82 V within 240 s. In 5 M NaOH, the voltage first increased and then reached a plateau at 0.82 V from 15 to 50 s. The voltage increased further to a steady state of 1.65 V after 60 s of HTA. In contrast, the voltage of the reaction in 0.5 M NaOH increased significantly at the start of HTA, and then continued to increase slowly to ca. 5.78 V for the remainder of the reaction. AFM observation indicated that the sample obtained with 10 M NaOH (Figure 5b) possessed a smoother surface than the as-purchased foil (Figure 5a). The root-mean-square (rms) surface roughness was 33.5 nm for the as-purchased samples, whereas it was only 1.5 nm for those prepared by HTA with 10 M NaOH.

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Figure 1. Schematic diagram of the experimental equipment used for one-step HTA.

working and counter electrodes was 4 cm. The area of the working electrode immersed in electrolyte was 6 cm2. HTA was conducted at 150 °C in 0.5−10 M NaOH electrolyte under an electric current of 5.0 mA cm−2 for 0−120 min. The morphology of the as-prepared samples was observed with a field-emission scanning electron microscope (FESEM, S4500 and S-5200, Hitachi, Japan) and an atomic force 8055



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Figure 2. SEM images of titanium surfaces treated at 150 °C for 2 h in 5 M NaOH (a) with and (b) without an applied current of 5.0 mA cm−2.

Figure 3. SEM images of titanium surfaces after HTA at 150 °C for 2 h in (a) 0.5 M, (b) 5 M, and (c) 10 M NaOH with an applied current of 5.0 mA cm−2.

NWs Prepared by HTA. Figure 6 shows TEM images of the NWs prepared by HTA obtained in 5 M NaOH electrolyte under a current density of 5.0 mA cm−2 for 120 min. The diameter of the NWs was 8−12 nm. The process of NW formation was traced by observing cross-sectional SEM images of the samples after different reaction times of 0−120 min under a current density of 5.0 mA cm−2 (Figure 7). After heating the sample to 150 °C, the surface was covered with a dense layer of particles with a depth of ca. 400 nm (Figure 7a). After HTA for 10 min, the thickness of the layer increased and the surface became porous (Figure 7b). After HTA for 20 min, randomly oriented nanorod-like structures with a thickness of ca. 600 nm formed, and the thickness of the dense particulate layer increased to 600 nm (Figure 7c). Between 30 and 60 min, the dense layer grew thicker and the nanorods were further elongated. After 120 min, the diameter of the nanorods decreased, and quasi-aligned NWs with a length of ca. 1 μm thickness on a dense layer of particles with a thickness of ca. 750 nm. To compare the growth rates of different techniques, we examined anodization, HT, and HTA using the same electrolyte (5 M NaOH), current (5.0 mA cm−2, anodization and HTA only), and temperature (150 °C, HT and HTA only). The thickness of the oxide layer obtained after 120 min of anodization, HT, and HTA was approximately 60 nm, 1.2 μm, and 1.75 μm, respectively (Figure S1 in the Supporting Information). Nanostructures were produced on a dense oxide layer by HTA, whereas only a dense particulate film was

obtained by HT. These results clearly show that HTA produces thicker nanostructures than HT or anodization in the same period. In addition, anodization formed amorphous nanostructures, while those formed by HTA were highly crystalline (Figure S2, Supporting Information). The crystallinity of the NWs was evaluated by highresolution (HR) TEM, XRD, and Raman spectroscopy. A HRTEM image of a NW prepared by HTA in Figure 6b shows clear lattice fringes inside the NW, indicating it is crystalline, not amorphous. Fringe spacings were 0.36 and 0.76 nm, which are close to the (110) and (002) phases of Na2Ti9O19 (ST), respectively. XRD patterns of NWs prepared by HTA (and the accompanying dense layer) are given in Figure 8b, together with that of an untreated substrate for comparison in Figure 8a. By comparing the peak positions in Figure 8b with those of previous reports23−25 and the XRD database (JCPDS No. 331293), the broad peaks at 24.7 and 48.1° were attributed to the (110) and (020) planes of Na2Ti9O19, although they were also close to the (101) and (200) phases of anatase (JCPDS No. 211272). A Raman spectrum of the NWs and dense layer after HTA for 2 h is shown in Figure 9a. The peaks at 144, 396, 517, and 639 cm−1 are consistent with anatase TiO2,26,27 while the peak located at 276 cm−1 is attributed to Na−Ti−O.28,29 This suggests that the NWs and accompanying dense layer prepared by HTA are a mixture of anatase TiO2 and ST, although the structures of ST synthesized by HT and HTA are still unclear. 8056



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Figure 6. (a) TEM image and (b) HRTEM image of NWs prepared by HTA at 150 °C in 5 M NaOH for 2 h.

survey spectrum (Figure 10a) revealed major peaks for Na1s, Ti2p, and O1s and a minor C1s peak from surface contamination. The positions of the peaks all correspond to ST, not TiO2.34 Evaluation of Bioactivity by Immersion in SBF. To evaluate the bioactivity of NWs prepared by HTA, the formation of hydroxyapatite (HAp) on NWs by immersing them in SBF for up to 12 days was examined. After immersion for 30 min, terrace-like structures partially covered the surface of the NWs prepared by HTA, as shown in Figure 11b. The most probable compound in the terraces is calcium titanate, which is by XPS spectra showing Ca2p peaks at binding energies of 346.7 eV (Ca2p1/2) and 350.2 eV (Ca2p3/2) (Figure 12d).35,36 No P2p or S2p3/2 peaks were detected in the XPS spectrum. The Ti2p peaks at binding energies of 458.1 eV (Ti2p1/2) and 463.9 eV (Ti2p3/2) in Figure 12a were attributed to Ti−O bonds.34,36 The Na1s peak (Figure 12b) was caused by uncovered NWs on the surface.34,36 The main peak at 529.8 eV in Figure 12c was also attributed to Ti−O bonds. In general, calcium titanate grows prior to HAp on titania- and titanate-based materials.34,37 Calcium titanate formed on the NWs prepared by HTA is significantly faster than that reported previously.38 Terrace structures covered almost the entire surface of the NWs after immersion in SBF for 1 day, as shown in Figure 11c; it was estimated that at least 70% of the surface was covered with calcium titanate after immersion for 1 day. The positions of the Ti2p, Na2p, O1s, and Ca2p peaks remain the same. A new P2p peak appears in the XPS spectrum, as shown in Figure 12e. This suggests that small amounts of HAp nuclei formed,34 although no specific structure is observed on the surface by SEM (Figure 11c). The formation of HAp nuclei within 1 day is considerably faster than that previously reported.34

Figure 4. (a) Voltage−time response during HTA of titanium at a constant current density of 0.5 mA cm−2 and temperature of 150 °C for 2 h in different concentrations of NaOH. The initial change in voltage is magnified in part b.

The XRD and Raman data indicate that the NWs were mostly crystalline, so further annealing, which is frequently needed for implant materials,30−33 is not required for NWs prepared by HTA. Both XRD and Raman data indicate the presence of anatase TiO2 and ST, although the HRTEM image of a NW only reveals the crystal phase of Na2Ti9O19. To determine the chemical compositions of NWs grown on the titanium surface, they were also examined by XPS, as shown in Figure 10. A

Figure 5. AFM micrographs (5 μm2) of (a) a bare titanium surface with a rms of ca. 33.5 nm and (b) an electropolished titanium surface with a rms of ca. 1.5 nm. [Insets: (a) photographs of bare titanium and (b) electropolished titanium foil]. 8057



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Figure 7. Cross-sectional SEM images of ST layers on titanium foil following HTA at 150 °C in 5 M NaOH for different periods. The bottom edge of the above images (indicated by arrows) is the interface between the titanium substrate and dense oxide layer.

Figure 8. X-ray diffraction patterns of titanium foil (a) before and (b) after HTA, and (c) HAp layer grown on the ST layer after SBF immersion for 12 days.

Figure 9. Raman spectra of HTA-treated samples (a) before and (b) after SBF immersion for 12 days.

After immersion for 2 days, the terraces grew and coalesced into a flat surface, as shown in Figure 11d. The intensities of the Ti2p, Na1s, Ca2p, and P2p peaks were 0.81, 0.71, 1.18, and 1.25 times higher, respectively, than those after immersion for 1 day. The decrease in Ti2p and Na1s indicates that the area of uncovered NWs decreased as the terrace structures grew, whereas the increases in Ca2p and P2p reveal the growth of calcium titanate and HAp nuclei. HAp was not observed in the SEM image (Figure 11d) because the nuclei were still small and partially embedded or covered by calcium titanate. A large number of spherical structures with a diameter of 1− 3 μm grew on the surface of the NWs after immersion for 7 days, as shown in Figure 11e. A large shift of the O1s peak by 1.6 eV to a higher binding energy of 531.4 eV, which corresponds to the P−O bond of Hap,34,39 was observed. This change indicates that the spherical structures present after 1

week were HAp. The Ca2p1/2 and Ca2p3/2 peaks shifted to higher binding energies of 347.5 and 351.1 eV, respectively, and the intensity of the P2p peak was 1.83 times higher than that after immersion for 2 days. These changes indicate that a large amount of HAp formed on the NW surface.34 The intensities of the Ti2p and Na1s peaks decreased appreciably because HAp covered much of the surface. After immersion for 12 days, spherical HAp covered the entire surface (Figure 11f), and the Ti2p and Na1s peaks completely disappeared. The positions of the O1s, Ca2p, and P2p peaks remained the same as those after immersion for 7 days, while the peak intensities were 1.17−1.43 times higher. HAp formation after immersion for 12 days is also considerably faster than that reported previously on a titanium-based surface.8 A high-resolution SEM image of the HAp formed on the NWs prepared by HTA is shown in Figure 13. Porous nanoflakes 8058



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Figure 10. (a) Wide-range survey scan and narrow scans of XPS spectra for (b) Na1s, (c) Ti2p, and (d) O1s peaks detected from NWs prepared by HTA at 150 °C for 2 h in 5 M NaOH.

Figure 11. SEM images of HAp growth on the surface of NWs prepared by HTA for different immersion times. The cracks in parts d and e were formed during SEM sample preparation.

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Figure 12. XPS spectra of (a) Ti2p, (b) Na1s, (c) O1s, (d) Ca2p, and (e) P2p peaks detected on the surface of NWs prepared by HTA after different SBF immersion times, and (f) schematic illustration of HAp growth.

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DISCUSSION HTA Conditions versus Surface Morphology. The conditions used during HTA such as applied current, electrolyte concentration, and temperature were studied. HTA and HT treatment were conducted at 150 °C for 2 h, but NWs only formed by HTA (Figure 2). Although both processes involve high temperature, the temperature-driven rate of oxidation was low. As Shao et al. reported previously, long reaction times are required to form nanostructures by HT treatment.42 Therefore, the electric current used during HTA provides an additional driving force that promotes oxidation and chemical dissolution reactions. The electrolyte concentration is also an important parameter in HTA. The difference in surface morphology between Figure 3a and b shows that the formation of NWs was accelerated when the concentration of NaOH was increased from 0.5 to 5 M. Consequently, we proposed that 5 M NaOH is the optimum concentration for NW fabrication by HTA. When 10 M NaOH was used, a smooth titanium surface was obtained instead of NWs (Figures 3c and 5b). Such electropolishing, i.e., anodic dissolution rather than anodic oxidation, was predominant at higher NaOH concentrations (≥10 M). Smoothing of titanium is a desirable outcome for some applications, and it is interesting that it can be achieved with HTA in alkali electrolytes, when NWs/nanotubes form using conventional HT methods with similar alkali concentrations.42−44 It should also be noted that our one-step HTA requires only a single

Figure 13. SEM image of HAp formed on ST layers.

with a thickness of ca. 20 nm were observed. An XRD pattern (Figure 8c) of the as-formed HAp layer possessed intense peaks at 25.9 and 31.9° that were consistent with the (002) and (211) planes of HAp (JCPDS No. 09-432), respectively. These peaks revealed that the HAp formed on the NWs was crystalline. A Raman spectrum of the HAp (Figure 9b) exhibited peaks at 429, 446, 580, 590, 607, and 962 cm−1 that correspond to the vibrational modes of the phosphate ions in HAp, confirming that a dense HAp layer formed on the NWs prepared by HTA.39−41 8060



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mainly TiO2, as shown in Figure 7f. The possible formation mechanisms are as follows:

inorganic alkali electrolyte to realize effective electropolishing. This makes it advantageous to other electropolishing methods that require multicomponent organic solvents containing hydrochloric acid.45−47 The voltage responses for different NaOH concentrations used in HTA also support the occurrence of different reactions such as anodic dissolution and anodic oxidation. The concentration of NaOH had a significant effect on the response voltage during HTA, with a higher concentration resulting in a lower voltage. For 10 M NaOH, the voltage gradually increased to a constant 0.82 V within 240 s (Figure 4b), and a smooth electropolished surface was obtained (Figure 3c). The increase in voltage may be caused by dissolution of the dense particulate layer (Figure 6a) that formed during heating to 150 °C. After this dense layer dissolved, anodic dissolution proceeded at 0.82 V; the low voltage implies that this surface showed low resistance. In the case of 5 M NaOH, the voltage first increased rapidly and reached a plateau at 0.82 V. This increase in voltage is also attributed to the presence of a dense particulate layer. It should be noted that the voltage reached a plateau at 0.82 V, which is similar to the voltage observed for anodic dissolution in 10 M NaOH electrolyte. This plateau from 15 to 50 s suggests anodic dissolution dominated anodic oxidation during this period. The voltage then increased further, reaching a constant voltage of 1.65 V after 60 s. This suggests that anodic dissolution and oxidation compete with each other and reach a balance after 60 s. The cross-sectional SEM image in Figure 6b also suggests that two reactions occurred during HTA; the increased layer thickness was attributed to anodic oxidation, and the porous surface was formed through dissolution. In contrast, the voltage response in 0.5 M NaOH is different from those in 5 and 10 M NaOH. The voltage in 0.5 M NaOH increased significantly to >4 V at the beginning of HTA, and then continued to increase slowly to 5.78 V. However, only a thin layer of nanoflakes was obtained using 0.5 M NaOH. There are several possible reasons for the high response voltage when the concentration of NaOH is low. For example, a low OH− concentration can retard anodic oxidation in reaction 1 shown below. Low NaOH concentration also reduces the conductivity of the electrolyte, so a high voltage is required for this to be overcome. Moreover, the shift to high voltage may also cause the reaction to change from oxidation to side reactions such as oxygen evolution. Investigation of the conditions for HTA revealed that the structures produced can be readily tuned from NWs to smooth surfaces by careful control of reaction conditions. NWs Prepared by HTA. NWs were prepared on a dense particulate layer by one-step HTA under specific conditions. The morphology, chemical composition, and crystallographic structure of the NWs and dense layer were examined. The diameter of the NWs was much smaller than those prepared by conventional anodization20 and HT methods.42−44 It is possible that chemical dissolution driven by HT processes and electric field-induced chemical etching both enhance the splitting of nanostructures to form such narrow NWs. In addition, the dense layer provides evidence that oxidation of the titanium foil proceeds simultaneously with etching of the oxide surface into NWs during HTA. Both the XRD and Raman data indicated the NWs and dense layer were a mixture of anatase TiO2 and ST. However, the HRTEM image in Figure 6b and XPS spectra in Figure 10 show that the NWs are ST. Therefore, we conclude that NWs are ST, and the dense particulate layer is

Ti + 3OH−(aq) − 3e− Anodization

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ TiO2 ·H2O(s) +

1 H2(g) 2

(1)

9TiO2 · H2O(s) + 2Na+ + 2e− Hydrothermal

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Na2Ti 9O19(s) + H2(g) + 8H2O

(2)

Reactions 1 and 2 represent anodic oxidation and HT reaction, respectively, and proceed simultaneously in HTA. The titanium substrate is anodically oxidized to form a TiO2 layer (1), and sodium ions from the NaOH electrolyte readily react with the outer part of the TiO2 layer to form Na2Ti9O19 NWs (2). These two parallel reactions result in the formation of ST NWs on a dense layer of TiO2. It should also be noted that the NWs prepared by HTA are crystalline. In general, nanostructures prepared by anodization in alkali electrolyte are mostly amorphous.19,20 An XRD pattern of the sample prepared by anodization at room temperature did not show any crystalline peaks from TiO2 or ST (data not shown here). This clearly indicates that nanostructures with higher crystallinity can be produced by HTA than by anodization at room temperature. This difference was caused by the higher reaction temperature used for HTA facilitating crystallization of NWs. Evaluation of Bioactivity by Immersion in SBF. The NWs prepared by HTA are highly bioactive because calcium titanate and HAp can form on them in a short time, as shown in Figures 11 and 12. The bioactivity of the NWs is attributed to their surface area, chemical composition, and crystallinity. For instance, nucleation of calcium titanate readily occurs on the NWs because of their large effective surface area. Meanwhile, the release of Na+ ions from the surface of ST also accelerates the growth of calcium titanate because Na+ ions exchange with H3O+ ions to form Ti−OH, which induces further reaction with Ca2+ ions in SBF to form calcium titanate.40 It is also proposed that the crystallinity of the NWs promotes HAp formation because specific crystal structures such as anatase or rutile TiO2 are preferred for apatite formation.48−51 The crystalline ST NWs may possess a suitable atomic arrangement to allow epitaxial growth of HAp nuclei as well as crystalline TiO2. HAp nanoflakes that were slightly smaller than that reported by Oh et al. were obtained.30 These porous nanoflakes could be described as “nano-inspired nanostructures”;30 the NWs induce Hap flake formation on a similar scale to their own.

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CONCLUSIONS We have developed a novel one-step technique called HTA to produce nanostructures. By altering the conditions of HTA, the morphology can be readily tuned from nanoflakes with a lateral size of 200−300 nm to NWs with a diameter of 8−12 nm to a smoothened titanium surface. Importantly, the as-grown NWs are crystalline, and their length growth rate is as high as 500 nm h−1. Titanium surfaces were electropolished into smoother surfaces with an rms roughness of 1.5 nm at high concentrations of NaOH (above 10 M). The mechanisms responsible for both the formation of crystalline NWs and smoothing of titanium surfaces were primarily electric fieldenhanced oxidation and dissolution under HTA conditions; i.e., 8061



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oxidation contributes to the formation of crystalline NWs, whereas dissolution enhances the surface smoothing of titanium and splitting of NWs. The as-grown NWs were immersed in SBF to test their bioactivity. Calcium titanate formed rapidly (within 30 min), HAp nuclei formed after immersion for 1 day, and after immersion for 7 days, a large amount of HAp nanoflakes had formed. The rapid growth of calcium titanate and HAp was attributed to the crystallinity and high density of NWs prepared by HTA. This study indicates that one-step HTA is a versatile method that can be used not only to rapidly prepare nanostructures of titanate with high crystallinity but also to produce smooth titanium surfaces. Both NWs and electropolished titanium surfaces prepared by HTA are potentially useful in biomedical and dental applications, particularly bone implants and artificial hip joints.

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ASSOCIATED CONTENT

S Supporting Information *

Cross-sectional SEM images and X-ray diffraction patterns of samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-45-924-5369. Fax: +81-45-924-5358. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Professor M. Sone and Mr. T.F. Mark Chang for voltage−time measurement and Ms. H. Ito for XPS measurement. C.-Y.C. is grateful for financial support from RIKEN IPA program.

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REFERENCES

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