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Surface characteristics of porous anodic TiO2 layer for biomedical applications
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Materials Chemistry and Physics 109 (2008) 10–14
Materials science communication
Surface characteristics of porous anodic TiO2 layer for biomedical applications Han-Jun Oh a,∗ , Jong-Ho Lee b , Young-Jig Kim c , Su-Jeong Suh c , Jun-Hee Lee d , Choong-Soo Chi e a
Department of Materials Science, Hanseo University, Haemie-Myun, Chung-Nam, Seosan 352-820, Republic of Korea b Department of Chemistry, Hanseo University, Seosan 352-820, Republic of Korea c Department of Metallurgical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea d Department of Materials Science and Engineering, Donga University, Busan 604-714, Republic of Korea e School of Advanced Materials Engineering, Kookmin University, Seoul 136-702, Republic of Korea Received 17 August 2007; received in revised form 6 November 2007; accepted 8 November 2007
Abstract To improve the preferential nucleation of calcium phosphate by incorporated ions during anodization, titanium oxide (TiO2 ) films for implant applications were synthesized by electrochemical process in an electrolyte with sodium silicate solution as an additive. The surface morphologies of the anodic TiO2 films were dependent on the electrolyte. The anodic oxide films formed in the electrolyte with high additive content and for long anodic applied time demonstrated the greater precipitation capability of the bioactive Ca–P compounds. © 2007 Elsevier B.V. All rights reserved. Keywords: Anodic TiO2 layer; Biomaterials; Electrochemical techniques; Energy dispersive analysis of X-rays (EDS); X-ray photo-emission spectroscopy (XPS)
1. Introduction For application as a high performance biomedical material with good biocompatibility, an anodic titanium oxide (TiO2 ) layer was fabricated on Ti substrate by electrochemical method. Compared with conventional methods like plasma spray method [1,2] or sol–gel technique [3,4], the electrochemical method introduces major advantages and the oxide films fabricated by anodization not only show various surface morphologies [5], but also contain valuable chemical species incorporated from the electrolyte, which can be more effective for bioactive calcium phosphate formation after being implanted in the human body. In anodizing process, surface morphologies and characteristics of the anodic titania films depend sensitively on applied anodic potentials. At anodic potential range lower than 40 V, the self-organized nanotubular layer [6–9] can be achieved, which has attracted considerable interest because of its various functional properties [10,11] and potential technological applications such as gas sensor [12,13], and photocatalysis [14]. Meanwhile, anodic process at a potential range higher than 150 V, ∗
Corresponding author. Tel.: +82 41 660 1442; fax: +82 41 660 1119. E-mail address: [email protected] (H.-J. Oh).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.11.022
often also referred to as plasma electrolytic oxidation [15] or spark discharge anodizing, not only improves the surface properties [16,17], including adhesion to substrate metal, corrosion and friction, but also changes the chemical compositions of the anodic oxide layer depending on the electrolytes. The present work investigated the surface characteristics of anodic titania formed by anodizing at higher applied voltage and the effects of incorporated chemical species in the anodic TiO2 layer on the formation of bioactive apatite in biological fluid. To improve the preferential nucleation of bioactive calcium phosphate by the incorporation of chemical species during anodization, the anodic TiO2 layer was prepared by electrochemical process in a H2 SO4 /H3 PO4 mixture electrolyte containing sodium silicate solution as an additive for the incorporation of chemical species into the anodic surface during anodic oxidation in the electrolyte. Biological evaluations were carried out in a simulated body fluid (SBF) [18–20]. 2. Experimental Commercial grade titanium specimens (99.6 wt.%) were used for anodic oxidation. The titanium specimens were mechanically polished and degreased in n-hexane for 6 min, and then washed and dried. After the pretreatment, the titanium oxide films for application of biomaterials were prepared by anodizing
H.-J. Oh et al. / Materials Chemistry and Physics 109 (2008) 10–14 at a constant voltage of 180 V for 30 min in 0.9 M H2 SO4 /0.1 M H3 PO4 mixture, with 0.9 M H2 SO4 /0.1 M H3 PO4 /0.1 M additive, and 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive mixture solution. And for analysis of the effect of anodic time on chemical properties of the anodic oxide surface the anodizing was accomplished for 60 min in 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive solution. Sodium silicate solution was used as an additive. Anodizing was performed using a two-electrode system controlled with dc power supply in a glass cell. The exposed titanium area as working electrode in the electrolyte was 3 cm × 4 cm. A wide titanium steel sheet was used as the cathode. The microstructures of the anodic TiO2 films were observed using a scanning electron microscope (JSM5410/EDS). The phase of the films was identified by X-ray diffraction (XRD) (Philips, PW1710) and the chemical states were analyzed by X-ray photoelectron spectrometry (XPS, PHI 5700) at an accelerating voltage of 15 kV and current of 30 mA with magnesium K␣ radiation. After the anodizing process, the biological test was performed by soaking the anodic films (8 mm × 10 mm × 0.5 mm) in 50 mL of simulated body fluid (SBF) with an ion concentration (Na+ : 142, K+ : 5.0, Mg2+ : 1.5, Ca2+ : 1.5, Cl− : 147.8, HCO3− : 4.2, HPO4 2− : 1.0, and SO4 2− : 0.5 mM) nearly equal to that of human blood plasma, as developed by Kokubo and co-workers [18,19]. The SBF was prepared by dissolving reagent grade chemicals of NaCl, KCl, NaHCO3 , K2 HPO4 , MgCl2 , CaCl2 and Na2 SO4 into distilled water and buffering at pH 7.40 with tris (hydroxymethyl) aminomethane and 1 M HCl at 37 ◦ C. After immersion of the films in an SBF solution at 37 ◦ C in the incubator for 3 days, the surface morphology was observed by SEM.
3. Results and discussion 3.1. Morphology of the anodic TiO2 layer The anodizing process is shown in Fig. 1. For fabrication of the TiO2 layer, an anodic constant current of 35 mA cm−2 was applied on the titanium specimen, where the anodic potential was increased gradually with time. As the anodizing potential rea-
11
Fig. 1. Schematic diagram of the anodizing process.
ched a value of 180 V, the process was transferred to a constant potential mode, and the potential was maintained at 180 V. The variations of the microstructure and morphology of the anodic TiO2 layer are shown in Fig. 2, which clearly shows the dependence of the layer’s surface morphology on the electrolyte composition, and also the gradual decrease in cell structure and pore size with the addition of the additives in the electrolyte. The morphology of the anodic oxide layer did not show any distinct change with anodic time, as can be seen Fig. 2c and d. Nevertheless, the number of ions incorporated from the electrolyte to anodic titania were affected by the anodic time.
Fig. 2. SEM images of the surface morphologies of the anodic TiO2 films at 180 V in 0.9 M H2 SO4 /0.1 M H3 PO4 for 30 min (a), 0.9 M H2 SO4 /0.1 M H3 PO4 /0.1 M additive for 30 min (b), 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 30 min (c), and 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 60 min (d).
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Fig. 3. X-ray diffraction patterns of the anodic titania films formed at 180 V in 0.9 M H2 SO4 /0.1 M H3 PO4 for 30 min (a), 0.9 M H2 SO4 /0.1 M H3 PO4 /0.1 M additive for 30 min (b), 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 30 min (c), and 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 60 min (d).
3.2. Characteristics of the anodic TiO2 layer The X-ray diffraction results of anodic titania films showed peaks related to the anatase phases [21] and titanium substrate (Fig. 3). The XRD results of the anodic films formed in the H2 SO4 /H3 PO4 mixtures show the predominance of the X-ray peak at 2θ = 25.25 (1 0 1), which was identified as the crystal of anatase (Fig. 3a). However, the main growth plane of anatase gradually decreased with the addition of the electrolyte additive. These ions incorporated from the additive were assumed to act as an interruption for (1 0 1) predominant growth of anatase during anodization, thereby allowing anatase to be grown with various crystallographic planes. The peaks of the new crystalline compounds due to the ions incorporated from the electrolyte are not shown clearly in Fig. 3. The evidence of the ions incorporated on the anodic titania layer can be explained from the XPS results. Fig. 4 shows the XPS wide scan spectrum of an anodic TiO2 layer formed at 180 V. The XPS spectrum indicates the differences in chemical properties of the anodic oxide surface. Regarding the spectrum peaks about the chemical composition of the anodic TiO2 surface, it was reported that the O1s originating from the TiO2 layer has a typical binding energy of 530.3 eV, while the binding energies of organic oxygen-containing species and the oxides of several other metals are generally in the range of 532–534 eV [22]. The major O1s binding energy of the surface that is shown in Fig. 4a and b at 530.3 eV originated from TiO2 , while the binding energy of the surface shown in Fig. 4c
and d indicates the presence of O-containing functional groups and other oxides incorporated from the electrolyte with sodium silicate solution as an additive. However, the major O1s binding energy of 530.3 eV is not shifted with small additive amounts (Fig. 4b). The Na2p3 and Si2p peaks also indicate the migration during anodizing of the sodium and silicon ions in the additive across to the anodic TiO2 layer.
Fig. 4. XPS spectra of the anodic titania film formed at 180 V in 0.9 M H2 SO4 /0.1 M H3 PO4 for 30 min (a), 0.9 M H2 SO4 /0.1 M H3 PO4 /0.1 M additive for 30 min (b), 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 30 min (c), and 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 60 min (d).
H.-J. Oh et al. / Materials Chemistry and Physics 109 (2008) 10–14
13
Fig. 5. SEM photographs of the specimens soaked in SBF for 3 days. The anodic TiO2 film formed at 180 V in 0.9 M H2 SO4 /0.1 M H3 PO4 for 30 min (a), 0.9 M H2 SO4 /0.1 M H3 PO4 /0.1 M additive for 30 min (b), 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 30 min (c), and 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 60 min (d).
3.3. Immersion test in SBF solution The bioactivity of these anodic TiO2 films was evaluated in SBF. After immersion in an SBF solution at 37 ◦ C for 3 days, the surface of the anodic films was observed by SEM with energy dispersive spectroscopy (EDS). Fig. 5 shows the morphologies of the anodic TiO2 surface after soaked in SBF for 3 days. The SEM images show the hydroxyapatite-like precipitates on the surface of all the specimens. The precipitates existed in all of the anodic oxide films, but for the layer formed in the 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 60 min, more apparently obvious Ca–P compound particles appeared (Fig. 5d). Therefore, the anodic oxide film
formed in high additive concentration over a long anodic time demonstrates the higher precipitation capability of bioactive Ca–P compounds. To investigate the active site and reaction mechanism for the heterogeneous nucleation of the earliest calcium phosphate phase, Sahai et al. [23] reported that silanol groups (4SiOH) providing a stereochemical match for O atoms bonded to Ca2+ on the (0 0 1) face of hydroxyapatite. This result indicated that silicate trimer (silanol, 4SiOH) is the active site for the nucleation of the calcium phosphate compound. The result of Fig. 5 indicated that the ions incorporated during anodizing from the electrolyte with additive, such as Si and O groups in the surface, are promoted to form silanols, which act as preferential sites for the nucleation of the Ca–P compound in biological fluid.
Fig. 6. SEM micrographs and EDS analysis for the precipitated particles on the surface of the anodic layer formed in 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 30 min after immersion test in SBF for 3 days.
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EDS analysis was performed to examine the chemical elements of the particles precipitated on the TiO2 layer after immersion test in SBF. Fig. 6 shows the SEM micrographs and EDS analysis results for the precipitated particles on the surface of the anodic layer formed in 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 30 min after immersion test in SBF for 3 days. The homogeneous, precipitated particles were revealed to be Ca–P–O rich compounds by the EDS analysis. 4. Conclusions In order to facilitate the preferential nucleation of bioactive calcium phosphate compounds on TiO2 layer, anodic titania layers for biomedical applications were fabricated by electrochemical method in electrolytes with sodium silicate solution. The bioactive properties of the fabricated anodic TiO2 films and their surface characteristics were investigated. The surface morphology of the anodic TiO2 films was dependent on the amount of additive during anodizing, and the cell structure of the films gradually disappeared with increasing additive concentration in the electrolyte. The ions incorporated on the anodic titania surface acted as preferential nucleation sites for calcium phosphate by interaction with Ca2+ ion in the biological fluid. The anodic oxide films formed at high additive content and long anodic applied time demonstrated the greater precipitation capability of the bioactive Ca–P compounds. References [1] K.A. Grossm, C.S. Chai, G.S.K. Kannangara, B.B. Nissam, J. Mater. Sci. Mater. Med. 9 (1998) 839.
[2] M.O. Oji, J.V. Wood, S. Downes, J. Mater. Sci. Mater. Med. 10 (1999) 869. [3] W. Weng, S. Zhang, K. Cheng, H. Qu, P. Du, G. Shen, J. Yuan, G. Han, Surf. Coat. Technol. 167 (2003) 292. [4] W. Weng, J.L. Baptista, Biomaterials 19 (1998) 125. [5] H.-J. Oh, J.-H. Lee, Y. Jeong, Y.-J. Kim, C.-S. Chi, Surf. Coat. Technol. 198 (2005) 247. [6] J.M. Macak, H. Tsuchiya, P. Schmuki, Angew. Chem. Int. Edit. 44 (2005) 2100. [7] J.M. Macak, H. Tsuchiya, L. Taveira, S. Aldabergerova, P. Schmuki, Angew. Chem. Int. Edit. 44 (2005) 7463. [8] L.V. Taveira, J.M. Macak, H. Tsuchiya, L.F.P. Dick, P. Schmuki, J. Electrochem. Soc. 152 (2005) B405. [9] K. Shankar, G.K. Mor, A. Fitzgerald, C.A. Grimes, J. Phys. Chem. C 111 (2007) 21. [10] S. Sirivisoot, C. Yao, X. Xiao, B.W. Sheldon, T.J. Webster, Nanotechnology 18 (2007) 365102. [11] C.A. Grimes, J. Mater. Chem. 17 (2007) 1451. [12] K. Zakrzewska, M. Radecka, M. Rekas, Thin Solid Films 310 (1997) 161. [13] A. Rothschild, F. Edelman, Y. Komem, F. Cosandey, Sens. Actuators B 67 (2000) 282. [14] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Nano Lett. 5 (2005) 191. [15] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73. [16] Y.-J. Park, K.-H. Shin, H.-J. Song, V 253, Appl. Surf. Sci. (2007) 6013. [17] H.-J. Song, M.-K. Kim, G.-C. Jung, M.-S. Vang, Y.-J. Park, Surf. Coat. Technol. 201 (2007) 8738. [18] K. Hata, T. Kokubo, T. Nakamura, T. Yamamuro, J. Am. Ceram. Soc. 78 (1995) 1049. [19] Y. Abe, T. Kokubo, T. Yamamuro, J. Mater. Sci. Mater. Med. 1 (1990) 233. [20] J.-H. Lee, S.-E. Kim, Y.-J. Kim, C.-S. Chi, H.-J. Oh, Mater. Chem. Phys. 98 (2006) 39. [21] M.A. Fox, M.T. Dulay, Chem. Rev. 93 (1993) 54. [22] C. Massaro, P. Rotolo, F. DeRiccardis, E. Milella, A. Napoli, M. Textor, N.D. Spencer, D.M. Brunette, J. Mater. Sci. Mater. Med. 12 (2001) 225. [23] N. Sahai, M. Anseau, Biomaterials 26 (2005) 5763.
Materials Chemistry and Physics 109 (2008) 10–14
Materials science communication
Surface characteristics of porous anodic TiO2 layer for biomedical applications Han-Jun Oh a,∗ , Jong-Ho Lee b , Young-Jig Kim c , Su-Jeong Suh c , Jun-Hee Lee d , Choong-Soo Chi e a
Department of Materials Science, Hanseo University, Haemie-Myun, Chung-Nam, Seosan 352-820, Republic of Korea b Department of Chemistry, Hanseo University, Seosan 352-820, Republic of Korea c Department of Metallurgical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea d Department of Materials Science and Engineering, Donga University, Busan 604-714, Republic of Korea e School of Advanced Materials Engineering, Kookmin University, Seoul 136-702, Republic of Korea Received 17 August 2007; received in revised form 6 November 2007; accepted 8 November 2007
Abstract To improve the preferential nucleation of calcium phosphate by incorporated ions during anodization, titanium oxide (TiO2 ) films for implant applications were synthesized by electrochemical process in an electrolyte with sodium silicate solution as an additive. The surface morphologies of the anodic TiO2 films were dependent on the electrolyte. The anodic oxide films formed in the electrolyte with high additive content and for long anodic applied time demonstrated the greater precipitation capability of the bioactive Ca–P compounds. © 2007 Elsevier B.V. All rights reserved. Keywords: Anodic TiO2 layer; Biomaterials; Electrochemical techniques; Energy dispersive analysis of X-rays (EDS); X-ray photo-emission spectroscopy (XPS)
1. Introduction For application as a high performance biomedical material with good biocompatibility, an anodic titanium oxide (TiO2 ) layer was fabricated on Ti substrate by electrochemical method. Compared with conventional methods like plasma spray method [1,2] or sol–gel technique [3,4], the electrochemical method introduces major advantages and the oxide films fabricated by anodization not only show various surface morphologies [5], but also contain valuable chemical species incorporated from the electrolyte, which can be more effective for bioactive calcium phosphate formation after being implanted in the human body. In anodizing process, surface morphologies and characteristics of the anodic titania films depend sensitively on applied anodic potentials. At anodic potential range lower than 40 V, the self-organized nanotubular layer [6–9] can be achieved, which has attracted considerable interest because of its various functional properties [10,11] and potential technological applications such as gas sensor [12,13], and photocatalysis [14]. Meanwhile, anodic process at a potential range higher than 150 V, ∗
Corresponding author. Tel.: +82 41 660 1442; fax: +82 41 660 1119. E-mail address: [email protected] (H.-J. Oh).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.11.022
often also referred to as plasma electrolytic oxidation [15] or spark discharge anodizing, not only improves the surface properties [16,17], including adhesion to substrate metal, corrosion and friction, but also changes the chemical compositions of the anodic oxide layer depending on the electrolytes. The present work investigated the surface characteristics of anodic titania formed by anodizing at higher applied voltage and the effects of incorporated chemical species in the anodic TiO2 layer on the formation of bioactive apatite in biological fluid. To improve the preferential nucleation of bioactive calcium phosphate by the incorporation of chemical species during anodization, the anodic TiO2 layer was prepared by electrochemical process in a H2 SO4 /H3 PO4 mixture electrolyte containing sodium silicate solution as an additive for the incorporation of chemical species into the anodic surface during anodic oxidation in the electrolyte. Biological evaluations were carried out in a simulated body fluid (SBF) [18–20]. 2. Experimental Commercial grade titanium specimens (99.6 wt.%) were used for anodic oxidation. The titanium specimens were mechanically polished and degreased in n-hexane for 6 min, and then washed and dried. After the pretreatment, the titanium oxide films for application of biomaterials were prepared by anodizing
H.-J. Oh et al. / Materials Chemistry and Physics 109 (2008) 10–14 at a constant voltage of 180 V for 30 min in 0.9 M H2 SO4 /0.1 M H3 PO4 mixture, with 0.9 M H2 SO4 /0.1 M H3 PO4 /0.1 M additive, and 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive mixture solution. And for analysis of the effect of anodic time on chemical properties of the anodic oxide surface the anodizing was accomplished for 60 min in 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive solution. Sodium silicate solution was used as an additive. Anodizing was performed using a two-electrode system controlled with dc power supply in a glass cell. The exposed titanium area as working electrode in the electrolyte was 3 cm × 4 cm. A wide titanium steel sheet was used as the cathode. The microstructures of the anodic TiO2 films were observed using a scanning electron microscope (JSM5410/EDS). The phase of the films was identified by X-ray diffraction (XRD) (Philips, PW1710) and the chemical states were analyzed by X-ray photoelectron spectrometry (XPS, PHI 5700) at an accelerating voltage of 15 kV and current of 30 mA with magnesium K␣ radiation. After the anodizing process, the biological test was performed by soaking the anodic films (8 mm × 10 mm × 0.5 mm) in 50 mL of simulated body fluid (SBF) with an ion concentration (Na+ : 142, K+ : 5.0, Mg2+ : 1.5, Ca2+ : 1.5, Cl− : 147.8, HCO3− : 4.2, HPO4 2− : 1.0, and SO4 2− : 0.5 mM) nearly equal to that of human blood plasma, as developed by Kokubo and co-workers [18,19]. The SBF was prepared by dissolving reagent grade chemicals of NaCl, KCl, NaHCO3 , K2 HPO4 , MgCl2 , CaCl2 and Na2 SO4 into distilled water and buffering at pH 7.40 with tris (hydroxymethyl) aminomethane and 1 M HCl at 37 ◦ C. After immersion of the films in an SBF solution at 37 ◦ C in the incubator for 3 days, the surface morphology was observed by SEM.
3. Results and discussion 3.1. Morphology of the anodic TiO2 layer The anodizing process is shown in Fig. 1. For fabrication of the TiO2 layer, an anodic constant current of 35 mA cm−2 was applied on the titanium specimen, where the anodic potential was increased gradually with time. As the anodizing potential rea-
11
Fig. 1. Schematic diagram of the anodizing process.
ched a value of 180 V, the process was transferred to a constant potential mode, and the potential was maintained at 180 V. The variations of the microstructure and morphology of the anodic TiO2 layer are shown in Fig. 2, which clearly shows the dependence of the layer’s surface morphology on the electrolyte composition, and also the gradual decrease in cell structure and pore size with the addition of the additives in the electrolyte. The morphology of the anodic oxide layer did not show any distinct change with anodic time, as can be seen Fig. 2c and d. Nevertheless, the number of ions incorporated from the electrolyte to anodic titania were affected by the anodic time.
Fig. 2. SEM images of the surface morphologies of the anodic TiO2 films at 180 V in 0.9 M H2 SO4 /0.1 M H3 PO4 for 30 min (a), 0.9 M H2 SO4 /0.1 M H3 PO4 /0.1 M additive for 30 min (b), 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 30 min (c), and 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 60 min (d).
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H.-J. Oh et al. / Materials Chemistry and Physics 109 (2008) 10–14
Fig. 3. X-ray diffraction patterns of the anodic titania films formed at 180 V in 0.9 M H2 SO4 /0.1 M H3 PO4 for 30 min (a), 0.9 M H2 SO4 /0.1 M H3 PO4 /0.1 M additive for 30 min (b), 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 30 min (c), and 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 60 min (d).
3.2. Characteristics of the anodic TiO2 layer The X-ray diffraction results of anodic titania films showed peaks related to the anatase phases [21] and titanium substrate (Fig. 3). The XRD results of the anodic films formed in the H2 SO4 /H3 PO4 mixtures show the predominance of the X-ray peak at 2θ = 25.25 (1 0 1), which was identified as the crystal of anatase (Fig. 3a). However, the main growth plane of anatase gradually decreased with the addition of the electrolyte additive. These ions incorporated from the additive were assumed to act as an interruption for (1 0 1) predominant growth of anatase during anodization, thereby allowing anatase to be grown with various crystallographic planes. The peaks of the new crystalline compounds due to the ions incorporated from the electrolyte are not shown clearly in Fig. 3. The evidence of the ions incorporated on the anodic titania layer can be explained from the XPS results. Fig. 4 shows the XPS wide scan spectrum of an anodic TiO2 layer formed at 180 V. The XPS spectrum indicates the differences in chemical properties of the anodic oxide surface. Regarding the spectrum peaks about the chemical composition of the anodic TiO2 surface, it was reported that the O1s originating from the TiO2 layer has a typical binding energy of 530.3 eV, while the binding energies of organic oxygen-containing species and the oxides of several other metals are generally in the range of 532–534 eV [22]. The major O1s binding energy of the surface that is shown in Fig. 4a and b at 530.3 eV originated from TiO2 , while the binding energy of the surface shown in Fig. 4c
and d indicates the presence of O-containing functional groups and other oxides incorporated from the electrolyte with sodium silicate solution as an additive. However, the major O1s binding energy of 530.3 eV is not shifted with small additive amounts (Fig. 4b). The Na2p3 and Si2p peaks also indicate the migration during anodizing of the sodium and silicon ions in the additive across to the anodic TiO2 layer.
Fig. 4. XPS spectra of the anodic titania film formed at 180 V in 0.9 M H2 SO4 /0.1 M H3 PO4 for 30 min (a), 0.9 M H2 SO4 /0.1 M H3 PO4 /0.1 M additive for 30 min (b), 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 30 min (c), and 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 60 min (d).
H.-J. Oh et al. / Materials Chemistry and Physics 109 (2008) 10–14
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Fig. 5. SEM photographs of the specimens soaked in SBF for 3 days. The anodic TiO2 film formed at 180 V in 0.9 M H2 SO4 /0.1 M H3 PO4 for 30 min (a), 0.9 M H2 SO4 /0.1 M H3 PO4 /0.1 M additive for 30 min (b), 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 30 min (c), and 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 60 min (d).
3.3. Immersion test in SBF solution The bioactivity of these anodic TiO2 films was evaluated in SBF. After immersion in an SBF solution at 37 ◦ C for 3 days, the surface of the anodic films was observed by SEM with energy dispersive spectroscopy (EDS). Fig. 5 shows the morphologies of the anodic TiO2 surface after soaked in SBF for 3 days. The SEM images show the hydroxyapatite-like precipitates on the surface of all the specimens. The precipitates existed in all of the anodic oxide films, but for the layer formed in the 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 60 min, more apparently obvious Ca–P compound particles appeared (Fig. 5d). Therefore, the anodic oxide film
formed in high additive concentration over a long anodic time demonstrates the higher precipitation capability of bioactive Ca–P compounds. To investigate the active site and reaction mechanism for the heterogeneous nucleation of the earliest calcium phosphate phase, Sahai et al. [23] reported that silanol groups (4SiOH) providing a stereochemical match for O atoms bonded to Ca2+ on the (0 0 1) face of hydroxyapatite. This result indicated that silicate trimer (silanol, 4SiOH) is the active site for the nucleation of the calcium phosphate compound. The result of Fig. 5 indicated that the ions incorporated during anodizing from the electrolyte with additive, such as Si and O groups in the surface, are promoted to form silanols, which act as preferential sites for the nucleation of the Ca–P compound in biological fluid.
Fig. 6. SEM micrographs and EDS analysis for the precipitated particles on the surface of the anodic layer formed in 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 30 min after immersion test in SBF for 3 days.
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EDS analysis was performed to examine the chemical elements of the particles precipitated on the TiO2 layer after immersion test in SBF. Fig. 6 shows the SEM micrographs and EDS analysis results for the precipitated particles on the surface of the anodic layer formed in 0.9 M H2 SO4 /0.1 M H3 PO4 /0.4 M additive for 30 min after immersion test in SBF for 3 days. The homogeneous, precipitated particles were revealed to be Ca–P–O rich compounds by the EDS analysis. 4. Conclusions In order to facilitate the preferential nucleation of bioactive calcium phosphate compounds on TiO2 layer, anodic titania layers for biomedical applications were fabricated by electrochemical method in electrolytes with sodium silicate solution. The bioactive properties of the fabricated anodic TiO2 films and their surface characteristics were investigated. The surface morphology of the anodic TiO2 films was dependent on the amount of additive during anodizing, and the cell structure of the films gradually disappeared with increasing additive concentration in the electrolyte. The ions incorporated on the anodic titania surface acted as preferential nucleation sites for calcium phosphate by interaction with Ca2+ ion in the biological fluid. The anodic oxide films formed at high additive content and long anodic applied time demonstrated the greater precipitation capability of the bioactive Ca–P compounds. References [1] K.A. Grossm, C.S. Chai, G.S.K. Kannangara, B.B. Nissam, J. Mater. Sci. Mater. Med. 9 (1998) 839.
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