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Soft solution approach to prepare crystalline titania films
Scripta Materialia 46 (2002) 705–709 www.actamat-journals.com
Soft solution approach to prepare crystalline titania films Jin-Ming Wu a
a,b,*
, Satoshi Hayakawa a, Kanji Tsuru a, Akiyoshi Osaka
a
Faculty of Engineering, Biomaterials Laboratory, Okayama University, Tsushima Naka, Okayama-shi 700-8530, Japan b Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Received 13 November 2001; accepted 16 January 2002
Abstract Titania films with crystal structures of anatase or a mixture of anatase and rutile were prepared through a soft solution approach. The crystalline titania film resulted from crystallization of the previously deposited amorphous gel in an acidic solution. A low pH value of the solution favored the formation of rutile. Ó 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Titanium; Oxides; Thin films; Chemical synthesis
1. Introduction Crystalline titania (TiO2 , anatase or rutile) films are of great interest in many scientific applications such as photoelectronic optical devices, gas sensors and photo-catalysts [1–12]. They also attract attention of researchers in biomaterials: an anatase surface layer promotes significantly the bioactivity of titanium and its alloys, which are of practical importance for them to be used as orthopaedic and dental implants [13–15]. Increasing the surface area of the film is a promising way to improve the desired functions because almost all of these applications utilize chemical reactions on the surface [1]. Unfortunately, most of the reported routes to fabricate crystalline titania films, such as sol–gel
* Corresponding author. Address: Faculty of Engineering, Biomaterials Laboratory, Okayama University, Tsushima Naka, Okayama-shi 700-8530, Japan. Tel.: +81-86-251-8214; fax: +81-86-251-8263. E-mail address: [email protected] (J.-M. Wu).
processing [1–5], cathodic electrodeposition [6–8] and direct oxidation of metallic titanium foils [9], involve heating to high temperatures, which causes grain coarsening and hence decrease the surface area of the film. Therefore, low temperature synthesis of anatase, rutile, or a mixture of both phases has been focuses of many studies [10–12]. In this paper, we reported a much simpler soft solution approach to prepare crystalline titania films on titanium substrates.
2. Experimental procedures Reagent grade TaCl5 (Nacalai Tesque, Inc. Kyoto, Japan) was dissolved in a 30 wt.% hydrogen peroxide solution to give a concentration of 3.0 mM (designated as H2 O2 /Ta solution hereafter). Titanium pieces with a size of 10 10 1 mm3 were cut from a commercially available pure titanium (cpTi) plate supplied by Kobe steel Ltd., Osaka, Japan, using a spark-cutting machine. They were then pickled at 60 °C for 2 min in a
1359-6462/02/$ - see front matter Ó 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 6 2 ( 0 2 ) 0 0 0 5 6 - 8
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J.-M. Wu et al. / Scripta Materialia 46 (2002) 705–709
1:1.5:6 (in volume) mixture of a 55 mass% HF aqueous solution, a 60 mass% HNO3 aqueous solution and distilled water (DW), followed by ultrasonically rinsing three times in DW for 15 min in total. Each piece of the cpTi samples was soaked in 10 ml of the H2 O2 /Ta solution in a 50 ml polyethylene bottle (25 mm in diameter) with a tight screw cap, and kept in an oven at 80 °C for 8 h and 3 d. Some cpTi samples after soaking in the H2 O2 / Ta solution at 80 °C for 8 h were subjected to ageing at 80 °C for 3 d in DW, 0.25 M HCl and 1 M NaOH solution (10 ml for each piece), respectively. After soaking, they were washed ultrasonically with DW for 5 min to detach loosely attached particles and dried at 60 °C overnight. Thin film X-ray diffraction (TF-XRD) tests were performed using a RAD IIA powder diffractrometer. The samples were scanned at a ð1=3Þ° min1 scanning rate using Cu Ka radiation, k ¼ 0:15405 nm, at 40 KV and 25 mA. The morphology of the titania films was examined using scanning electron microscope (SEM, JEOL JSM6300, Japan). Before the SEM examination, samples were coated with a layer of gold to improve the electrical conductivity.
3. Results Fig. 1a and b show the TF-XRD patterns of cpTi soaked in the H2 O2 /Ta solution at 80 °C for 8 h and 3 d, respectively. Broad bands around 25.3° and 48.1° in 2h, which correspond to the two main anatase peaks of (1 0 1) and (2 0 0) respectively, appeared after up to 8h of soaking. This suggests that the titania film formed at early soaking stage was almost amorphous or consisted of anatase with very low degree of crystallinity. To be precise, we simply call such titania layer amorphous gel. After 3 d of soaking, peaks corresponding to anatase and rutile were observed. These peaks are broad and weak in intensity, suggesting that the grain size of the crystalline titania may fall into the range of nanometers. Fig. 1c–e illustrate the TF-XRD patterns of cpTi soaked in the H2 O2 /Ta solution at 80 °C for 8 h followed by ageing at 80 °C for 3 d in 0.25 M HCl, 1 M NaOH and DW, respectively. Ageing in
Fig. 1. TF-XRD patterns of cpTi after soaking in the H2 O2 /Ta solution at 80 °C for (a) 8 h, (b) 3 d; in the H2 O2 /Ta solution at 80 °C for 8 h followed by ageing at 80 °C for 3 d in (c) 0.25 M HCl, (d) 1 M NaOH and (e) DW.
the 0.25M HCl solution resulted in a mixture of anatase and rutile (Fig. 1c). Fig. 1d shows that, except the appearance of sodium chloride, which came from the combination of sodium ions in the ageing solution with chloride ions incorporated in the titania gel, ageing in a basic solution induced no significant change compared to Fig. 1a. Acute peaks corresponding to anatase can be seen on samples aged in DW (Fig. 1e). After soaking cpTi at 80 °C for 3 d, the pH values of the remained DW and H2 O2 /Ta solution were measured to be 6.30 and 1.80 respectively. Therefore, Fig. 1 suggests that crystalline titania films can be obtained by ageing the previously deposited amorphous film in an acidic solution. A low pH value of the solution favored the formation of rutile.
J.-M. Wu et al. / Scripta Materialia 46 (2002) 705–709
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Fig. 2. Surface morphology of pickled cpTi subjected to soaking in the H2 O2 /Ta solution at 80 °C for 3 d: (a) low magnification and (b) high magnification.
Fig. 2 shows the morphology of cpTi soaked in the H2 O2 /Ta solution at 80 °C for 3 d. The low magnification image shown in Fig. 2a was similar to that obtained after pickling in the mixed acid solution. This suggests that the crystalline titania thin film covered homogeneously on the cpTi substrate. Fig. 2b indicates that the crystalline film consisted of minor titania particles less than 100 nm. The titania films adhered strongly to the titanium substrates, as they sustained the subsequent ultrasonic cleaning.
4. Discussion Recently, Shimizu et al. [10] deposited crystalline titania (anatase) thin films on glass and organic substrates at 40–70 °C using titanium tetrafluoride (TiF4 ) aqueous solutions. Baskaran et al. [11] prepared titania thin films on organic interfaces through a biomimetic processing at temperatures lower than 100 °C. Petkov et al. [12] demonstrated that titania gel with mixed amorphous and anatase structure can be obtained after drying at 110 °C the film dip-coated from a solution of tetra-iso-propoxytitanate (TPOT) in ethanol, which was left to mature for nine weeks. These studies suggest that crystalline titania films can be obtained at low temperature through soft solution approach. To date, only amorphous titania gels with forms of TiO2 nH2 O, Ti(OH)x and some other
complexes of superoxide radicals coordinated to Ti(IV) have been yielded through interactions between titanium and hydrogen peroxide under low temperatures (<80 °C) [14–18]. Wang et al. [15] indicated that heat treatment at temperatures above 300 °C transformed the amorphous gels to crystalline films consisting mainly of anatase. In the present experiment, we obtained crystalline titania thin films on cpTi substrates using the H2 O2 /Ta solution at a quite low temperature of 80 °C. The crystalline titania thin films are believed to result from crystallization of the previously formed amorphous gels in an acidic environment. A low pH value favored the formation of rutile (Fig. 1). More recently, Grader et al. [19] prepared an alumina xerogels using AlCl3 , Pri2 O and CH2 Cl2 under argon through a sol–gel process. They found that, wetting the 650 °C-calcined xerogels with water at room temperature results in continuous crystallization to a-Al2 O3 upon subsequent heating starting at a temperature of 450 °C. Wetting with water at increased temperatures of 50 and 70 °C accelerates the low-temperature crystallization process. They proposed that the crystallization proceeds in a dissolution precipitation way in water. Seo et al. [20,21] also reported recently the synthesis of anatase nanocrystalline powders by ageing in boiling water for 24 h the amorphous titanium hydroxide, which was obtained from the reaction between TiOCl2 and NH4 OH solutions. However, they gave no explanations. Similar to
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J.-M. Wu et al. / Scripta Materialia 46 (2002) 705–709
Grader et al. [19], the present crystalline titania may also result from the dissolution of the previously deposited amorphous titania gel and the concurrently precipitation of the titania crystals on the titanium substrates. This can find support from the observation that a white pulverous layer formed after soaking the amorphous titania gel in acidic solutions (including DW) at 80 °C for 3 d, which was readily removed by ultrasonic cleaning. Another possible route to obtain the crystalline titania under the present soft solution condition is the in situ crystallization of the amorphous titania gel. The Hþ in the acidic solution attacked the O atoms between the BTi–O–TiB bonds and finally broke these bonds, thus favored relocation of the related Ti, O atoms to form an ordered structure, which was more stable in thermodynamics. In addition, the aqueous environment improved the diffusivity of atoms remarkably, which also helped the nucleation and growth of crystalline phases, either in the dissolution and precipitation mechanism, or through the in situ crystallization route. The result that high Hþ concentration favors the formation of rutile agrees well with Seo et al. [20]. They found that, ageing the amorphous titanium hydroxide in DW, 0.1 and 0.5 M HCl solutions results in anatase powders; while rutile powders are obtained by ageing in the 2 M HCl solution [20]. It is noted that, in the present experiment, rutile also appeared when ageing the amorphous titania gel in 0.25 M HCl solution (Fig. 1c). This difference may be ascribed to the different chemical reactions leading to the amorphous titania gel. Yang et al. [22] prepared nanocrystalline titania particles by hydrothermally treating titanium alkoxide species at 200 and 240 °C for 2 h. They found that a pretreatment of peptizing the titanium alkoxide with acid solution (HNO3 ) favored the formation of rutile during the hydrothermal treatment. However, details concerning how an acidic environment promotes the rutile formation remain unclear. The present method to synthesize crystalline titania films is simple and economical. Compared to the generally used sol–gel methods, it is a quite environmental friendly approach, as it involves neither extremely low or high temperatures, nor toxic reagents.
5. Conclusions Soaking titanium in a 30 wt.% hydrogen peroxide solution containing 3 mM tantalum chloride at 80 °C for up to 8 h deposited an amorphous titania gel on the metal surface. In an acidic solution, this amorphous gel crystallized to anatase or a mixture of anatase and rutile, under a low temperature of 80 °C. A low pH value of the solution favored the formation of rutile. Acknowledgements Financial supports by the QOL project of the Society of Non-Traditional Technology, NEDO 00Z45005x and Grant-in-Aid for Scientific Research, the Ministry of Education, Japan (#12558109) are gratefully acknowledged. JinMing 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. References [1] Kajihara K, Yao T. J Sol–Gel Sci Techn 2000;17:239. [2] Negishi N, Takeuchi K, Ibusuki T. Appl Surface Sci 1997;121/122:417. [3] Keddie JL, Braun PV, Giannelis EP. J Am Ceram Soc 1994;77:1592. [4] Imai H, Hirashima H, Awazu K. Thin solid films 1999;351:91. [5] Olofinjana AO, Bell JM, Jamting AK. Wear 2000;241:174. [6] Natarajan C, Nogami G. J Electrochem Soc 1996;143: 1547. [7] Karuppuchamy S, Amalnerkar DP, Yamaguchi K, Yoshida T, Sugiura T, Minoura H. Chem Lett 2001:78. [8] Zhitomirsky I. J Eur Ceram Soc 1999;19:2581. [9] Gouma PI, Mills MJ, Sandhage KH. J Am Ceram Soc 2000;83:1007. [10] Shimizu K, Imai H, Hirashima H, Tsukuma K. Thin solid films 1999;351:220. [11] Baskaran S, Song L, Liu J, Chen YL, Graff GL. J Am Ceram Soc 1998;81:401. [12] Petkov V, Holzhuter G, Troge U, Gerber T, Himmel B. J Non-Cryst Solids 1998;231:17. [13] Kaneko S, Tsuru K, Hayakawa S, Takemoto S, Ohtsuki C, Ozaki T, et al. Biomaterials 2001;22:875. [14] Ohtsuki C, Iida H, Hayakawa S, Osaka A. J Biomed Mater Res 1997;35:39.
J.-M. Wu et al. / Scripta Materialia 46 (2002) 705–709 [15] Wang XX, Hayakawa S, Tsuru K, Osaka A. J Biomed Mater Res 2000;52:171. [16] Tengvall P, Elwing H, Lundstrom I. J Colloid Interface Sci 1988;130:405. [17] Tengvall P, Lundstrom I. Clinical Mater 1992;9:115. [18] Komarneni S, Roy R, Breval E. J Am Ceram Soc 1985;68: C-41.
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[19] Grader GS, Shter GE, Avnir D, Frenkel H, Sclar D, Delev A. J Sol–Gel Sci Techn 2001;21:157. [20] Seo DS, Lee JK, Kim H. J Crystal Growth 2001;233: 298. [21] Seo DS, Lee JK, Lee EG, Kim H. Mater Lett 2001;51:115. [22] Yang J, Mei S, Ferreira JMF. J Am Ceram Soc 2000;83: 1361.
Soft solution approach to prepare crystalline titania films Jin-Ming Wu a
a,b,*
, Satoshi Hayakawa a, Kanji Tsuru a, Akiyoshi Osaka
a
Faculty of Engineering, Biomaterials Laboratory, Okayama University, Tsushima Naka, Okayama-shi 700-8530, Japan b Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Received 13 November 2001; accepted 16 January 2002
Abstract Titania films with crystal structures of anatase or a mixture of anatase and rutile were prepared through a soft solution approach. The crystalline titania film resulted from crystallization of the previously deposited amorphous gel in an acidic solution. A low pH value of the solution favored the formation of rutile. Ó 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Titanium; Oxides; Thin films; Chemical synthesis
1. Introduction Crystalline titania (TiO2 , anatase or rutile) films are of great interest in many scientific applications such as photoelectronic optical devices, gas sensors and photo-catalysts [1–12]. They also attract attention of researchers in biomaterials: an anatase surface layer promotes significantly the bioactivity of titanium and its alloys, which are of practical importance for them to be used as orthopaedic and dental implants [13–15]. Increasing the surface area of the film is a promising way to improve the desired functions because almost all of these applications utilize chemical reactions on the surface [1]. Unfortunately, most of the reported routes to fabricate crystalline titania films, such as sol–gel
* Corresponding author. Address: Faculty of Engineering, Biomaterials Laboratory, Okayama University, Tsushima Naka, Okayama-shi 700-8530, Japan. Tel.: +81-86-251-8214; fax: +81-86-251-8263. E-mail address: [email protected] (J.-M. Wu).
processing [1–5], cathodic electrodeposition [6–8] and direct oxidation of metallic titanium foils [9], involve heating to high temperatures, which causes grain coarsening and hence decrease the surface area of the film. Therefore, low temperature synthesis of anatase, rutile, or a mixture of both phases has been focuses of many studies [10–12]. In this paper, we reported a much simpler soft solution approach to prepare crystalline titania films on titanium substrates.
2. Experimental procedures Reagent grade TaCl5 (Nacalai Tesque, Inc. Kyoto, Japan) was dissolved in a 30 wt.% hydrogen peroxide solution to give a concentration of 3.0 mM (designated as H2 O2 /Ta solution hereafter). Titanium pieces with a size of 10 10 1 mm3 were cut from a commercially available pure titanium (cpTi) plate supplied by Kobe steel Ltd., Osaka, Japan, using a spark-cutting machine. They were then pickled at 60 °C for 2 min in a
1359-6462/02/$ - see front matter Ó 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 6 2 ( 0 2 ) 0 0 0 5 6 - 8
706
J.-M. Wu et al. / Scripta Materialia 46 (2002) 705–709
1:1.5:6 (in volume) mixture of a 55 mass% HF aqueous solution, a 60 mass% HNO3 aqueous solution and distilled water (DW), followed by ultrasonically rinsing three times in DW for 15 min in total. Each piece of the cpTi samples was soaked in 10 ml of the H2 O2 /Ta solution in a 50 ml polyethylene bottle (25 mm in diameter) with a tight screw cap, and kept in an oven at 80 °C for 8 h and 3 d. Some cpTi samples after soaking in the H2 O2 / Ta solution at 80 °C for 8 h were subjected to ageing at 80 °C for 3 d in DW, 0.25 M HCl and 1 M NaOH solution (10 ml for each piece), respectively. After soaking, they were washed ultrasonically with DW for 5 min to detach loosely attached particles and dried at 60 °C overnight. Thin film X-ray diffraction (TF-XRD) tests were performed using a RAD IIA powder diffractrometer. The samples were scanned at a ð1=3Þ° min1 scanning rate using Cu Ka radiation, k ¼ 0:15405 nm, at 40 KV and 25 mA. The morphology of the titania films was examined using scanning electron microscope (SEM, JEOL JSM6300, Japan). Before the SEM examination, samples were coated with a layer of gold to improve the electrical conductivity.
3. Results Fig. 1a and b show the TF-XRD patterns of cpTi soaked in the H2 O2 /Ta solution at 80 °C for 8 h and 3 d, respectively. Broad bands around 25.3° and 48.1° in 2h, which correspond to the two main anatase peaks of (1 0 1) and (2 0 0) respectively, appeared after up to 8h of soaking. This suggests that the titania film formed at early soaking stage was almost amorphous or consisted of anatase with very low degree of crystallinity. To be precise, we simply call such titania layer amorphous gel. After 3 d of soaking, peaks corresponding to anatase and rutile were observed. These peaks are broad and weak in intensity, suggesting that the grain size of the crystalline titania may fall into the range of nanometers. Fig. 1c–e illustrate the TF-XRD patterns of cpTi soaked in the H2 O2 /Ta solution at 80 °C for 8 h followed by ageing at 80 °C for 3 d in 0.25 M HCl, 1 M NaOH and DW, respectively. Ageing in
Fig. 1. TF-XRD patterns of cpTi after soaking in the H2 O2 /Ta solution at 80 °C for (a) 8 h, (b) 3 d; in the H2 O2 /Ta solution at 80 °C for 8 h followed by ageing at 80 °C for 3 d in (c) 0.25 M HCl, (d) 1 M NaOH and (e) DW.
the 0.25M HCl solution resulted in a mixture of anatase and rutile (Fig. 1c). Fig. 1d shows that, except the appearance of sodium chloride, which came from the combination of sodium ions in the ageing solution with chloride ions incorporated in the titania gel, ageing in a basic solution induced no significant change compared to Fig. 1a. Acute peaks corresponding to anatase can be seen on samples aged in DW (Fig. 1e). After soaking cpTi at 80 °C for 3 d, the pH values of the remained DW and H2 O2 /Ta solution were measured to be 6.30 and 1.80 respectively. Therefore, Fig. 1 suggests that crystalline titania films can be obtained by ageing the previously deposited amorphous film in an acidic solution. A low pH value of the solution favored the formation of rutile.
J.-M. Wu et al. / Scripta Materialia 46 (2002) 705–709
707
Fig. 2. Surface morphology of pickled cpTi subjected to soaking in the H2 O2 /Ta solution at 80 °C for 3 d: (a) low magnification and (b) high magnification.
Fig. 2 shows the morphology of cpTi soaked in the H2 O2 /Ta solution at 80 °C for 3 d. The low magnification image shown in Fig. 2a was similar to that obtained after pickling in the mixed acid solution. This suggests that the crystalline titania thin film covered homogeneously on the cpTi substrate. Fig. 2b indicates that the crystalline film consisted of minor titania particles less than 100 nm. The titania films adhered strongly to the titanium substrates, as they sustained the subsequent ultrasonic cleaning.
4. Discussion Recently, Shimizu et al. [10] deposited crystalline titania (anatase) thin films on glass and organic substrates at 40–70 °C using titanium tetrafluoride (TiF4 ) aqueous solutions. Baskaran et al. [11] prepared titania thin films on organic interfaces through a biomimetic processing at temperatures lower than 100 °C. Petkov et al. [12] demonstrated that titania gel with mixed amorphous and anatase structure can be obtained after drying at 110 °C the film dip-coated from a solution of tetra-iso-propoxytitanate (TPOT) in ethanol, which was left to mature for nine weeks. These studies suggest that crystalline titania films can be obtained at low temperature through soft solution approach. To date, only amorphous titania gels with forms of TiO2 nH2 O, Ti(OH)x and some other
complexes of superoxide radicals coordinated to Ti(IV) have been yielded through interactions between titanium and hydrogen peroxide under low temperatures (<80 °C) [14–18]. Wang et al. [15] indicated that heat treatment at temperatures above 300 °C transformed the amorphous gels to crystalline films consisting mainly of anatase. In the present experiment, we obtained crystalline titania thin films on cpTi substrates using the H2 O2 /Ta solution at a quite low temperature of 80 °C. The crystalline titania thin films are believed to result from crystallization of the previously formed amorphous gels in an acidic environment. A low pH value favored the formation of rutile (Fig. 1). More recently, Grader et al. [19] prepared an alumina xerogels using AlCl3 , Pri2 O and CH2 Cl2 under argon through a sol–gel process. They found that, wetting the 650 °C-calcined xerogels with water at room temperature results in continuous crystallization to a-Al2 O3 upon subsequent heating starting at a temperature of 450 °C. Wetting with water at increased temperatures of 50 and 70 °C accelerates the low-temperature crystallization process. They proposed that the crystallization proceeds in a dissolution precipitation way in water. Seo et al. [20,21] also reported recently the synthesis of anatase nanocrystalline powders by ageing in boiling water for 24 h the amorphous titanium hydroxide, which was obtained from the reaction between TiOCl2 and NH4 OH solutions. However, they gave no explanations. Similar to
708
J.-M. Wu et al. / Scripta Materialia 46 (2002) 705–709
Grader et al. [19], the present crystalline titania may also result from the dissolution of the previously deposited amorphous titania gel and the concurrently precipitation of the titania crystals on the titanium substrates. This can find support from the observation that a white pulverous layer formed after soaking the amorphous titania gel in acidic solutions (including DW) at 80 °C for 3 d, which was readily removed by ultrasonic cleaning. Another possible route to obtain the crystalline titania under the present soft solution condition is the in situ crystallization of the amorphous titania gel. The Hþ in the acidic solution attacked the O atoms between the BTi–O–TiB bonds and finally broke these bonds, thus favored relocation of the related Ti, O atoms to form an ordered structure, which was more stable in thermodynamics. In addition, the aqueous environment improved the diffusivity of atoms remarkably, which also helped the nucleation and growth of crystalline phases, either in the dissolution and precipitation mechanism, or through the in situ crystallization route. The result that high Hþ concentration favors the formation of rutile agrees well with Seo et al. [20]. They found that, ageing the amorphous titanium hydroxide in DW, 0.1 and 0.5 M HCl solutions results in anatase powders; while rutile powders are obtained by ageing in the 2 M HCl solution [20]. It is noted that, in the present experiment, rutile also appeared when ageing the amorphous titania gel in 0.25 M HCl solution (Fig. 1c). This difference may be ascribed to the different chemical reactions leading to the amorphous titania gel. Yang et al. [22] prepared nanocrystalline titania particles by hydrothermally treating titanium alkoxide species at 200 and 240 °C for 2 h. They found that a pretreatment of peptizing the titanium alkoxide with acid solution (HNO3 ) favored the formation of rutile during the hydrothermal treatment. However, details concerning how an acidic environment promotes the rutile formation remain unclear. The present method to synthesize crystalline titania films is simple and economical. Compared to the generally used sol–gel methods, it is a quite environmental friendly approach, as it involves neither extremely low or high temperatures, nor toxic reagents.
5. Conclusions Soaking titanium in a 30 wt.% hydrogen peroxide solution containing 3 mM tantalum chloride at 80 °C for up to 8 h deposited an amorphous titania gel on the metal surface. In an acidic solution, this amorphous gel crystallized to anatase or a mixture of anatase and rutile, under a low temperature of 80 °C. A low pH value of the solution favored the formation of rutile. Acknowledgements Financial supports by the QOL project of the Society of Non-Traditional Technology, NEDO 00Z45005x and Grant-in-Aid for Scientific Research, the Ministry of Education, Japan (#12558109) are gratefully acknowledged. JinMing 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. References [1] Kajihara K, Yao T. J Sol–Gel Sci Techn 2000;17:239. [2] Negishi N, Takeuchi K, Ibusuki T. Appl Surface Sci 1997;121/122:417. [3] Keddie JL, Braun PV, Giannelis EP. J Am Ceram Soc 1994;77:1592. [4] Imai H, Hirashima H, Awazu K. Thin solid films 1999;351:91. [5] Olofinjana AO, Bell JM, Jamting AK. Wear 2000;241:174. [6] Natarajan C, Nogami G. J Electrochem Soc 1996;143: 1547. [7] Karuppuchamy S, Amalnerkar DP, Yamaguchi K, Yoshida T, Sugiura T, Minoura H. Chem Lett 2001:78. [8] Zhitomirsky I. J Eur Ceram Soc 1999;19:2581. [9] Gouma PI, Mills MJ, Sandhage KH. J Am Ceram Soc 2000;83:1007. [10] Shimizu K, Imai H, Hirashima H, Tsukuma K. Thin solid films 1999;351:220. [11] Baskaran S, Song L, Liu J, Chen YL, Graff GL. J Am Ceram Soc 1998;81:401. [12] Petkov V, Holzhuter G, Troge U, Gerber T, Himmel B. J Non-Cryst Solids 1998;231:17. [13] Kaneko S, Tsuru K, Hayakawa S, Takemoto S, Ohtsuki C, Ozaki T, et al. Biomaterials 2001;22:875. [14] Ohtsuki C, Iida H, Hayakawa S, Osaka A. J Biomed Mater Res 1997;35:39.
J.-M. Wu et al. / Scripta Materialia 46 (2002) 705–709 [15] Wang XX, Hayakawa S, Tsuru K, Osaka A. J Biomed Mater Res 2000;52:171. [16] Tengvall P, Elwing H, Lundstrom I. J Colloid Interface Sci 1988;130:405. [17] Tengvall P, Lundstrom I. Clinical Mater 1992;9:115. [18] Komarneni S, Roy R, Breval E. J Am Ceram Soc 1985;68: C-41.
709
[19] Grader GS, Shter GE, Avnir D, Frenkel H, Sclar D, Delev A. J Sol–Gel Sci Techn 2001;21:157. [20] Seo DS, Lee JK, Kim H. J Crystal Growth 2001;233: 298. [21] Seo DS, Lee JK, Lee EG, Kim H. Mater Lett 2001;51:115. [22] Yang J, Mei S, Ferreira JMF. J Am Ceram Soc 2000;83: 1361.