Nanocrystalline Titania Made from Interactions of Ti with Hydrogen Peroxide Solutions Containing Tantalum Chloride Jin-Ming Wu,*,†,‡ Satoshi Hayakawa,† Kanji Tsuru,† and Akiyoshi Osaka†
CRYSTAL GROWTH & DESIGN 2002 VOL. 2, NO. 2 147-149
Biomaterials Laboratory, Faculty of Engineering, Okayama University, Tsushima Naka, Okayama shi 700-8530, Japan, and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China Received June 20, 2001;
Revised Manuscript Received December 11, 2001
ABSTRACT: Crystalline titania film and powders with crystallite size in the range of nanometers are of great interest in the fields of optical devices, gas sensors, catalysts, and biomaterials. This paper reports a soft solution approach to prepare crystalline titania. After Ti was soaked in hydrogen peroxide solutions containing 3.0 mM tantalum chloride at 80 °C for 3 days, crystalline titania films consisting of anatase and rutile were deposited on Ti substrates. While Ti was soaked in tantalum-free hydrogen peroxide solution followed by aging in hydrogen peroxide solution containing tantalum ions, titania films with well-crystallized anatase were obtained. The crystalline titania resulted from crystallization of the previously deposited amorphous gel. Addition of tantalum ions in hydrogen peroxide solutions helps to obtain the crystalline titania and favors especially the formation of rutile phase. Nanosized crystalline titania (TiO2) film and powders are of great interest in applications such as optical devices, gas sensors, and catalysts.1,2 In biomaterial regions, the film is used to provide bioinert materials with bioactivity,3,4 while the powders are used as ceramic adsorbents in blood purification therapies.5 Until now, many methods have been explored to prepare the crystalline titania, such as sol-gel,5-7 hydrothermal synthesis,1,2 cathodic electrodeposition,3,8 direct oxidation of metallic Ti foils,9 and biomimetic process.10 Unfortunately, titania (film or powder) obtained by most of these methods is amorphous.1,2,4-7 Subsequent heat treatment at temperatures over 300 °C is needed to induce crystallization of the amorphous titania, and thus decreases the specific surface area and in turn deteriorates desired properties. Therefore, low-temperature synthesis of anatase, rutile, or a mixture of both phases attracts many researchers’ attention.1 Recently, Shimizu et al. deposited crystalline titania (anatase) thin films on glass and organic substrates at 40-70 °C using aqueous solutions of titanium tetrafluoride (TiF4).11 Petkov et al. showed that, after being dried at 110 °C, titania gel with mixed amorphous and anatase structure can be obtained by dip-coating from a solution of tetraiso-propoxytitanate (TPOT) in ethanol, which was left previously to mature for nine weeks.12 Preparations of nanocrystalline titania particles under low temperatures through the sol-gel method have also been reported.1,2,13-15 This paper reports a relatively simple route to prepare titania films with nanosized crystallites on Ti substrates under mild conditions. Reagent grade TaCl5 (Nacalai Tesque, Inc., Kyoto, Japan) and hydrogen peroxide solution (30 wt % in water, Santoku Chemcal Industries Co., Ltd. Tokyo, Japan) were used to prepare solutions with nominal concentrations of xH2O2/yTaCl5 (x ) 3-30 wt %; y ) 0, * Corresponding author. Tel. +86-571-87951532, Fax. +86-57187951708, E-mail:
[email protected]. † Okayama University. ‡ Zhejiang University.
3 mM). Pieces of commercially available pure Ti (10 × 10 × 1 mm3) were pickled at 60 °C for 2 min in a 1:1.5:6 (in volume) mixture of 55% HF, 60% HNO3, and distilled water, and then ultrasonically rinsed three times in distilled water for a total of 15 min. One piece of Ti was soaked in 10 mL of the xH2O2/yTaCl5 solutions in a polyethylene bottle (25 mm in diameter) with tight screw cap and kept in an oven at 80 °C for 8 h and 3 days. After the samples were soaked, they were washed ultrasonically with distilled water for 5 min to dispose of the loosely attached particles and dried at 60 °C overnight. Thin film X-ray diffraction (TF-XRD) was performed using a RAD IIA powder diffractrometer. The samples were scanned at a scanning rate of (1/3)° min-1 using Cu KR radiation, λ ) 0.15405 nm, at 40 KV and 25 mA. Figure 1b,c shows TF-XRD patterns of Ti soaked in the 30 wt % H2O2/3 mM TaCl5 solution (designated as the H2O2/Ta solution hereafter) at 80 °C for 8 h and 3 days, respectively. As compared to as-pickled Ti (Figure 1a), broad haloes around 25.3° and 48.1° in 2θ are evident in the XRD pattern of Ti soaked for 8 h (Figure 1b). This suggests that an amorphous titania gel imbedded with poorly crystallized anatase deposited on Ti surfaces. With prolonged soaking of the sample for 3 days, peaks corresponding to crystalline titania with both anatase and rutile can be found (Figure 1c). According to the Scherrer formula, dhkl ) kλ/(Bcos(2θ)), where λ is the wavelength of the Cu KR radiation (λ ) 0.15405 nm), θ is the Bragg’s diffraction angle, B is the full width at half-maximum (FWHM) intensity of the peak, and k is a constant (usually ∼0.94);2 crystallite sizes for anatase (d101) and rutile (d110) were estimated to be 11.6 and 11.4 nm, respectively. This may be an overestimation; however, it is safe to say that the crystallite size of the presently low-temperature synthesized titania fell into the range of nanometers. A layer of white powders appeared on the Ti surface upon soaking in the H2O2/Ta solution at 80 °C for 3 days. These powders were collected and accumulated
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Figure 1. TF-XRD patterns of Ti before (a) and after soaking in the H2O2/Ta solution at 80 °C for 8 h (b) and 3 days (c). XRD pattern for powders formed on Ti surfaces is shown in (d).
Figure 2. TF-XRD patterns of Ti after soaking in xH2O2/3 mM TaCl5 solutions with different hydrogen peroxide concentrations (x: wt %) at 80 °C for 3 days.
for the XRD test. Figure 1d indicates that they possessed similar phases to the film. Therefore, the present method is applicable to prepare crystalline titania powders. The titania film with a mixture of anatase and rutile was also obtained by soaking Ti in solutions containing 9-30 wt % hydrogen peroxide and 3 mM TaCl5 (Figure 2). The FWHM of both peaks did not change remarkably with the H2O2 concentration. This suggests that changing the H2O2 concentration has minor effects, if any, on the crystallite size of the crystalline titania film. In most cases, the intensity of the XRD peak corresponding to rutile exceeded that of anatase. However, no clear
Wu et al.
Figure 3. TF-XRD patterns of Ti soaked in 30 wt % H2O2 solution at 80 °C for 8 h (a) and 3 days (b). The XRD pattern of Ti soaked in 30 wt % H2O2 solution at 80 °C for 8 h followed by aging in the H2O2/Ta solution at 80 °C for 3 days is shown in (c).
relationship between the rutile/anatase intensity ratio and the H2O2 concentration can be generalized, probably due to the complexity of the crystallization process. Figure 3 indicates that only amorphous titania imbedded by poorly crystallized anatase was obtained by soaking Ti in hydrogen peroxide solutions without addition of TaCl5 at 80 °C for 8 h (Figure 3a) and 3 days (Figure 3b). However, the amorphous titania gel shown in Figure 3a transformed to well-crystallized anatase when followed by aging in the H2O2/Ta solution at 80 °C for 3 days (Figure 3c). Interactions between metallic Ti and hydrogen peroxide resulted in an amorphous titania gel in forms of TiO2‚nH2O, Ti(OH)x and some other complexes of superoxide radicals coordinated to Ti(IV).16 This amorphous gel crystallized to anatase when heating at a temperature higher than 300 °C.4 It is clear from Figure 1c and Figure 3b that crystalline titania can be obtained with prolonged soaking for 3 days only in the TaCl5contained H2O2 solution, not in the 30 wt % H2O2 solution. Therefore, the addition of Ta(V) ions in hydrogen peroxide solutions helps to obtain the crystalline titania. In addition, amorphous titania resulted from soaking Ti in Ta-free hydrogen peroxide solution crystallized mainly to anatase upon subsequent aging in the H2O2/Ta solution (Figure 3c), whereas continuously soaking in the H2O2/Ta solution for 3 days resulted in both anatase and rutile (Figure 1c). It may be discerned that Ta(V) ions incorporated in the amorphous titania favors the formation of rutile phase. However, interactions between Ti and hydrogen peroxide are quite complicated and many works remain to clarify mechanisms concerning the low-temperature crystallization process. These include the structural evolution of the titania, the roles hydrogen peroxide and Ta(V) ions play on the crystallization process, effects of the aging solution nature, etc. Some results will be reported later. Conclusions Crystalline titania consisting of anatase and rutile was prepared through interactions between Ti and H2O2
Nanocrystalline Titania from Interactions of Ti and H2O2
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solutions containing 3.0 mM TaCl5 at 80 °C for 3 days. Well-crystallized anatase film was also obtained by soaking Ti in 30 wt % H2O2 solution at 80 °C for 8 h followed by subsequent aging in the TaCl5-contained H2O2 solution at 80 °C for 3 days. The crystalline titania resulted from crystallization of the previously deposited amorphous gel. Addition of tantalum ions in H2O2 solutions helps to obtain the crystalline titania and favors especially the formation of rutile phase.
(3) Osaka, A.; Wang, X. X.; Hayakawa, S.; Tsuru, K. Bioceramics 2001, 13, 263-266. (4) Wang, X. X.; Hayakawa, S.; Tsuru, K.; Osaka, A. J. Biomed. Mater. Res. 2000, 52, 171-176. (5) Takashima, S.; Takemoto, S.; Tsuru, K.; Hayakawa, S.; Osaka, A. Bioceramics 2001, 13, 889-892. (6) Keddie, J. L.; Braun, P. V.; Giannelis E. P. J. Am. Ceram. Soc. 1994, 77, 1592-1596. (7) Imai, H.; Hirashima, H.; Awazu, K. Thin Solid Films 1999, 351, 91-94. (8) Zhitomirsky, I. J. Eur. Ceram. Soc. 1999, 19, 2581-2587. (9) Gouma, P. I.; Mills, M. J.; Sandhage, K. H. J. Am. Ceram. Soc. 2000, 83, 1007-1009. (10) Baskaran, S.; Song, L.; Liu, J.; Chen, Y. L.; Graff, G. L. J. Am. Ceram. Soc. 1998, 81, 401-408. (11) Shimizu, K.; Imai, H.; Hirashima, H.; Tsukuma, K. Thin Solid Films 1999, 351, 220-224. (12) Petkov, V.; Holzhuter, G.; Troge, U.; Gerber, Th.; Himmel, B. J. Non-Cryst. Solids 1998, 231, 17-30. (13) Wei, Y.; Wu, R. T.; Zhang, Y. F. Mater. Lett. 1999, 41, 101103. (14) Liu, X. H.; Yang, J.; Wang, L.; Yang, X. J.; Lu, L. D.; Wang, X. Mater. Sci. Eng. A 2000, 289, 241-245. (15) Seo, D. S.; Lee, J. K.; Kim H. J. Cryst. Growth 2001, 233, 298-302. (16) Tengvall, P.; Elwing, H.; Lundstrom, I. J. Coll. Inter. Sci. 1988, 130, 405-413.
Acknowledgment. One of the authors, Jin-Ming Wu, gratefully appreciates the financial support of the Venture Business Laboratories, the Graduate School of Natural Sciences, Okayama University. This work was performed when he was on leave from Zhejiang University, P. R. China. Partial financial support by the Society of Non-Traditional Technology and Wesco Science Promotion Foundation are also acknowledged. References (1) Yang, J.; Mei, S.; Ferreira, J. M. F. J. Am. Ceram. Soc. 2000, 83, 1361-1368. (2) Yang, J.; Mei, S.; Ferreira, J. M. F. J. Am. Ceram. Soc. 2001, 84, 1696-1702.
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