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Ribbon-like and rod-like hydroxyapatite crystals deposited on titanium surface with electrochemical method
Materials Letters 61 (2007) 4062 – 4065 www.elsevier.com/locate/matlet
Ribbon-like and rod-like hydroxyapatite crystals deposited on titanium surface with electrochemical method Wei Ye, Xiao-Xiang Wang ⁎ Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Received 11 December 2006; accepted 9 January 2007 Available online 23 January 2007
Abstract Homogeneous coatings were attained by electrochemical method in electrolytes containing Ca2+ and PO3− 4 ions with Ca/P ratio being 1.67. SEM observation showed that the hydroxyapatite (HAp,Ca10(PO4)6(OH)2) crystals prepared with higher concentration electrolyte (4 × 10− 2 M Ca2+) are ribbon-like with thickness of nanometer size, a morphology seldom reported previously. In an electrolyte of lower concentration (6 × 10− 4 M Ca2+), the HAp crystals formed are rod-like with a hexagonal cross section and diameter of about 70–80 nm. XRD patterns and IR spectra confirmed that the coatings consist of HAp crystals. TEM micrographs and SAD indicated that the longitude direction for both ribbon-like and rod-like crystal is [002], and the flat surface of the ribbon is (110). HRTEM showed that the ribbon-like crystal is a mixture of HAp and octacalcium phosphate (OCP, Ca8H2 (PO4)6.5H2O). © 2007 Elsevier B.V. All rights reserved. Keywords: Hydroxyapatite; Electrochemical; Ribbon-like; Rod-like; Nanomaterials; Ceramics
1. Introduction The chemically or physically synthesized hydroxyapatite (HAp, Ca10(PO4)6(OH)2) has the same crystal structure and similar chemical composition as the bone mineral [1], and as such is able to bond with bone firmly. Therefore, the HAp is considered to be the most suitable implant material in terms of bioactivity [2]. However, a shortcoming of this ceramics lies in its brittleness and insufficient strength. So, hydroxyapatite has been used as coatings on the load-bearing metallic implants such as titanium and its alloys (e.g. Ti6Al4V). To overcome drawbacks of the commercially provided plasma-sprayed HAp coating, various mild coating techniques have been studied in recent years, such as the electrochemical, biomimetic and biological coating techniques [3,4]. Among them the electrochemical technique is the most efficient deposition method. Shirkhanzadeh has shown that in the electrolytes of lower calcium concentration and higher pH, the HAp may be directly precipitated and thus does not contains HPO4− 2 group; while in electrolytes of higher calcium concentration, the HAp may be ⁎ Corresponding author. Tel.: +86 057187952255; fax: +86 057187952255. E-mail address: [email protected] (X.-X. Wang). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.01.040
precipitated via precursor phase and thus HPO4− 2 group still remains in the crystal [5]. In his work, however, the morphologies of those HAp crystals have not been clarified. In the present work, we conducted the HAp precipitation on the cathodical titanium surface using the similar electrolytes as Shirkhanzadeh used and put the emphasis on the SEM and HRTEM morphology characterization of the HAp crystals. 2. Materials and methods Commercially pure titanium (CP Ti) plates of 10 × 10 × 1 mm dimensions pre-etched in an acidic mixture (HF:HNO3: H2O = 1:3:10) were used as the working electrode (cathode) for the deposition of calcium phosphates, while a platinum (Pt) plate was used as the counter electrode. The electrolytes were prepared by dissolving reagent-grade Ca (NO 3 ) 2 and NH4H2PO4 (or NaH2PO4) with Ca/P ratio being 1.67 in deionized water and their pH values were adjusted by ammonia. To improve the conductivity of the electrolytes, 0.1 M NaNO3 was added. Two electrolytes with Ca2+ concentration of 4 × 10− 2 M and 6 × 10− 4 M were used, corresponding to the pH values of 5 and 6 respectively. The deposition process was conducted with a DC power source operated at 3.0 V at 85 °C.
W. Ye, X.-X. Wang / Materials Letters 61 (2007) 4062–4065
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The crystal structure of the coatings was examined using an X-ray diffractometer (XRD, Philips XD-98) with Cu Kα radiation. The morphology of the coatings was examined using a scanning electron microscope (FSEM, SIRION-100). Spectroscopic analysis of the grown HAp microcrystals was carried out by FT-IR (AVATAR) using KBr pellet technique. The TEM samples were separated in an ethanol solution using ultrasonic vibration and then picked up with the TEM copper grids coated with amorphous carbon film for TEM (JEM-2010HR) examinations. The examinations were conducted in HRTEM systems with a maximum acceleration voltage of 200 kV. 3. Results and discussion Homogeneous coatings on the titanium substrates were precipitated in the electrolytes with Ca2+ concentration of 6 × 10− 4 and 4 × 10− 2 M after 2 h of electrochemical processing. XRD patterns of the powders scratched from the cathode in Fig. 1 show that the two coatings exhibit typical apatite peaks at 2θ of 25.9° and 31−33°, which correspond to the HAp (002) diffraction and a combination of the poorly resolved (211), (112) and (300) diffractions. The peaks from both coatings match well with the standard HAp patterns. FT-IR studies also confirmed that the coatings consist of HAp crystals (Fig. 2). The stretching mode of the OH− group appears at 1651 and 3572 cm− 1. The bands that correspond −1 to the internal modes of PO3− 4 group occur at v1 (600, 570 cm ) and v2 −1 −1 (1085, 1033 cm ). The OH stretching band at 3572 cm is considered as the indication of HAp structure [6]. SEM observation shows that the HAp crystals prepared with lower concentration electrolyte (6 × 10− 4 M Ca2+) are rod-like with a hexagonal cross section and diameters of about 70–80 nm (Fig. 3a), a morphology frequently reported previously [7]. Fig. 3(b) and (c) show the TEM image and the selected area diffraction pattern of the rod-like crystals, respectively. The diffraction pattern can be indexed as the HAp crystal with the longitude direction of [002], and the six surface of the rod are (100). The HAp crystals with hexagonal prism morphology have been synthesized with hydrothermal and electrochemical techniques [8]. Particularly, Ban [9] obtained electrochemically the rod-like HAp crystals with defined hexagonal cross section and a diameter of 150 nm. High resolution transmission electron
Fig. 1. XRD patterns of the HAp powder scratched from the cathode: (a) the ribbon-like HAp and (b) the rod-like HAp.
Fig. 2. FT-IR spectra of the two types of deposition: (a) the nanoribbon-like HAp and (b) the nanorod-like HAp.
Fig. 3. SEM (a) and TEM (b) micrograph of the nanorod-like HAp coating; (c) the selected area diffraction pattern from the marked HAp crystal in (b).
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microscopy indicated that the longitudinal direction of the rod is parallel to [001]. In the present work, the electrolyte with much dilute Ca2+ concentration was employed and the rod-like HAp crystal has a much smaller diameter. The smaller size of the HAp crystal may be beneficial to the bioactivity of the coating. In the electrolyte of higher Ca2+ concentration, the ribbon-like HAp crystal was obtained with thickness of tens of nanometer and rectangular cross section, the morphology seldom reported previously. Fig. 4(a) is the SEM image of the ribbon-like HAp. Different morphology of crystal may imply the different mechanism of formation. Several authors have suggested that the electrolytes of higher Ca2+ concentration usually accompanied with lower pH value and the octacalcium phosphate (OCP, Ca8H2(PO4)6·5H2O) could be a precursor phase for HAp formation. Brown and Smith suggested that OCP is the original precipitate on which biological apatite nucleates [10]. In the present work, the electrolyte associated with ribbon-like HAp has a pH value of 5.0, more acidic than the electrolyte depositing rod-like HAp (pH = 6.0). Therefore, it might be possible that in the electrolyte of higher Ca2+ concentration the OCP was formed during the deposition and took the morphology of ribbon. To prove it, we employed
Fig. 5. Lattice image of the ribbon-like crystal: (a) the HRTEM fringes of the crystal; (b) enlarged image of the framed area in (a), showing the crystal containing both OCP and HAp structures whose boundaries are indicated with the dash lines.
transmission electron microscopy (TEM) to detect the evidence of the transformation of OCP to HAp in the formation of the ribbon-like crystal. Fig. 4(b) and (c) show the TEM image and the selected area diffraction pattern of the ribbon-like crystals. The diffraction pattern can be indexed as HAp crystal with the electron beam direction of [110], suggesting that the longitude direction of the ribbon is [002] and the flat surface is (110). High-resolution TEM (HRTEM) images enabled us to obtain detailed crystallographic information of the OCP to HAp transformation. Fig. 5 shows the lattice image. Contrasting variation with interplanar spacings of approximately 0.34 nm was observed in the area of the well-defined lattice image, indicating the (002) plane of the hydroxyapatite. An enlarged area containing both OCP and HAp structures is showed in Fig. 5(b), revealing that the ribbon-like crystal is really a mixture of two calcium phosphate phases (OCP and HAp).
4. Conclusions
Fig. 4. SEM (a) and TEM (b) micrograph of the nanoribbon-like HAp coating; (c) the selected area diffraction pattern of the HAp crystal in (b).
(1) The morphology of hydroxyapatite crystal can be effectively controlled by the concentration of the electrolytes prepared for the deposition. When Ca2+ concentration is 6 × 10− 4 M, nanorod-like HAp crystal can be deposited on the titanium surface. (2) In the electrolyte with Ca2+ concentration of 4 × 10− 2 M, the nanoribbon-like HAp can be prepared. (3) TEM micrographs and SAD indicated that the longitude direction for both ribbon-like and rod-like crystal is [002], and the flat surface of the ribbon is (110). HRTEM showed that the ribbon-like HAp crystals were transformed from OCP.
W. Ye, X.-X. Wang / Materials Letters 61 (2007) 4062–4065
Acknowledgement This study was financially supported by the National Natural Science Foundation of China (NSFC) under the grant number 50571088 and by Science Program of Zhejiang Province under the grant number 2005C23006. References [1] L. Addadi, S. Raz, S. Weiner, Adv. Mater. 15 (2003) 959. [2] J.C. Merry, I.R. Gibson, et al., J. Mater. Sci., Mater. Med. 9 (1998) 779.
[3] [4] [5] [6] [7] [8] [9] [10]
4065
X.-X. Wang, L. Xie, R. Wang, Biomaterials 26 (2005) 6229. X.-X. Wang, S. Hayakawa, K. Tsuru, et al., Biomaterials 23 (2002) 1353. M. Shirkhanzadeh, J. Mater. Sci., Mater. Med. 9 (1998) 67. M. Ashok, N. Meenakshi Sundaram, et al., Mater. Lett. 57 (2003) 2066. X. Lu, Z.F. Zhao, Y. Leng, J. Cryst. Growth 284 (2005) 506. Y. Fujishiro, T. Sato, A. Okumura, J. Mater. Sci., Mater. Med. (1995) 172. S.J. Ban, S. Maruno, J. Biomed. Mater. Res. 42 (1998) 387. W.E. Brown, J.P. Smith, Nature 1962 (1962) 1050.
Ribbon-like and rod-like hydroxyapatite crystals deposited on titanium surface with electrochemical method Wei Ye, Xiao-Xiang Wang ⁎ Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Received 11 December 2006; accepted 9 January 2007 Available online 23 January 2007
Abstract Homogeneous coatings were attained by electrochemical method in electrolytes containing Ca2+ and PO3− 4 ions with Ca/P ratio being 1.67. SEM observation showed that the hydroxyapatite (HAp,Ca10(PO4)6(OH)2) crystals prepared with higher concentration electrolyte (4 × 10− 2 M Ca2+) are ribbon-like with thickness of nanometer size, a morphology seldom reported previously. In an electrolyte of lower concentration (6 × 10− 4 M Ca2+), the HAp crystals formed are rod-like with a hexagonal cross section and diameter of about 70–80 nm. XRD patterns and IR spectra confirmed that the coatings consist of HAp crystals. TEM micrographs and SAD indicated that the longitude direction for both ribbon-like and rod-like crystal is [002], and the flat surface of the ribbon is (110). HRTEM showed that the ribbon-like crystal is a mixture of HAp and octacalcium phosphate (OCP, Ca8H2 (PO4)6.5H2O). © 2007 Elsevier B.V. All rights reserved. Keywords: Hydroxyapatite; Electrochemical; Ribbon-like; Rod-like; Nanomaterials; Ceramics
1. Introduction The chemically or physically synthesized hydroxyapatite (HAp, Ca10(PO4)6(OH)2) has the same crystal structure and similar chemical composition as the bone mineral [1], and as such is able to bond with bone firmly. Therefore, the HAp is considered to be the most suitable implant material in terms of bioactivity [2]. However, a shortcoming of this ceramics lies in its brittleness and insufficient strength. So, hydroxyapatite has been used as coatings on the load-bearing metallic implants such as titanium and its alloys (e.g. Ti6Al4V). To overcome drawbacks of the commercially provided plasma-sprayed HAp coating, various mild coating techniques have been studied in recent years, such as the electrochemical, biomimetic and biological coating techniques [3,4]. Among them the electrochemical technique is the most efficient deposition method. Shirkhanzadeh has shown that in the electrolytes of lower calcium concentration and higher pH, the HAp may be directly precipitated and thus does not contains HPO4− 2 group; while in electrolytes of higher calcium concentration, the HAp may be ⁎ Corresponding author. Tel.: +86 057187952255; fax: +86 057187952255. E-mail address: [email protected] (X.-X. Wang). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.01.040
precipitated via precursor phase and thus HPO4− 2 group still remains in the crystal [5]. In his work, however, the morphologies of those HAp crystals have not been clarified. In the present work, we conducted the HAp precipitation on the cathodical titanium surface using the similar electrolytes as Shirkhanzadeh used and put the emphasis on the SEM and HRTEM morphology characterization of the HAp crystals. 2. Materials and methods Commercially pure titanium (CP Ti) plates of 10 × 10 × 1 mm dimensions pre-etched in an acidic mixture (HF:HNO3: H2O = 1:3:10) were used as the working electrode (cathode) for the deposition of calcium phosphates, while a platinum (Pt) plate was used as the counter electrode. The electrolytes were prepared by dissolving reagent-grade Ca (NO 3 ) 2 and NH4H2PO4 (or NaH2PO4) with Ca/P ratio being 1.67 in deionized water and their pH values were adjusted by ammonia. To improve the conductivity of the electrolytes, 0.1 M NaNO3 was added. Two electrolytes with Ca2+ concentration of 4 × 10− 2 M and 6 × 10− 4 M were used, corresponding to the pH values of 5 and 6 respectively. The deposition process was conducted with a DC power source operated at 3.0 V at 85 °C.
W. Ye, X.-X. Wang / Materials Letters 61 (2007) 4062–4065
4063
The crystal structure of the coatings was examined using an X-ray diffractometer (XRD, Philips XD-98) with Cu Kα radiation. The morphology of the coatings was examined using a scanning electron microscope (FSEM, SIRION-100). Spectroscopic analysis of the grown HAp microcrystals was carried out by FT-IR (AVATAR) using KBr pellet technique. The TEM samples were separated in an ethanol solution using ultrasonic vibration and then picked up with the TEM copper grids coated with amorphous carbon film for TEM (JEM-2010HR) examinations. The examinations were conducted in HRTEM systems with a maximum acceleration voltage of 200 kV. 3. Results and discussion Homogeneous coatings on the titanium substrates were precipitated in the electrolytes with Ca2+ concentration of 6 × 10− 4 and 4 × 10− 2 M after 2 h of electrochemical processing. XRD patterns of the powders scratched from the cathode in Fig. 1 show that the two coatings exhibit typical apatite peaks at 2θ of 25.9° and 31−33°, which correspond to the HAp (002) diffraction and a combination of the poorly resolved (211), (112) and (300) diffractions. The peaks from both coatings match well with the standard HAp patterns. FT-IR studies also confirmed that the coatings consist of HAp crystals (Fig. 2). The stretching mode of the OH− group appears at 1651 and 3572 cm− 1. The bands that correspond −1 to the internal modes of PO3− 4 group occur at v1 (600, 570 cm ) and v2 −1 −1 (1085, 1033 cm ). The OH stretching band at 3572 cm is considered as the indication of HAp structure [6]. SEM observation shows that the HAp crystals prepared with lower concentration electrolyte (6 × 10− 4 M Ca2+) are rod-like with a hexagonal cross section and diameters of about 70–80 nm (Fig. 3a), a morphology frequently reported previously [7]. Fig. 3(b) and (c) show the TEM image and the selected area diffraction pattern of the rod-like crystals, respectively. The diffraction pattern can be indexed as the HAp crystal with the longitude direction of [002], and the six surface of the rod are (100). The HAp crystals with hexagonal prism morphology have been synthesized with hydrothermal and electrochemical techniques [8]. Particularly, Ban [9] obtained electrochemically the rod-like HAp crystals with defined hexagonal cross section and a diameter of 150 nm. High resolution transmission electron
Fig. 1. XRD patterns of the HAp powder scratched from the cathode: (a) the ribbon-like HAp and (b) the rod-like HAp.
Fig. 2. FT-IR spectra of the two types of deposition: (a) the nanoribbon-like HAp and (b) the nanorod-like HAp.
Fig. 3. SEM (a) and TEM (b) micrograph of the nanorod-like HAp coating; (c) the selected area diffraction pattern from the marked HAp crystal in (b).
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W. Ye, X.-X. Wang / Materials Letters 61 (2007) 4062–4065
microscopy indicated that the longitudinal direction of the rod is parallel to [001]. In the present work, the electrolyte with much dilute Ca2+ concentration was employed and the rod-like HAp crystal has a much smaller diameter. The smaller size of the HAp crystal may be beneficial to the bioactivity of the coating. In the electrolyte of higher Ca2+ concentration, the ribbon-like HAp crystal was obtained with thickness of tens of nanometer and rectangular cross section, the morphology seldom reported previously. Fig. 4(a) is the SEM image of the ribbon-like HAp. Different morphology of crystal may imply the different mechanism of formation. Several authors have suggested that the electrolytes of higher Ca2+ concentration usually accompanied with lower pH value and the octacalcium phosphate (OCP, Ca8H2(PO4)6·5H2O) could be a precursor phase for HAp formation. Brown and Smith suggested that OCP is the original precipitate on which biological apatite nucleates [10]. In the present work, the electrolyte associated with ribbon-like HAp has a pH value of 5.0, more acidic than the electrolyte depositing rod-like HAp (pH = 6.0). Therefore, it might be possible that in the electrolyte of higher Ca2+ concentration the OCP was formed during the deposition and took the morphology of ribbon. To prove it, we employed
Fig. 5. Lattice image of the ribbon-like crystal: (a) the HRTEM fringes of the crystal; (b) enlarged image of the framed area in (a), showing the crystal containing both OCP and HAp structures whose boundaries are indicated with the dash lines.
transmission electron microscopy (TEM) to detect the evidence of the transformation of OCP to HAp in the formation of the ribbon-like crystal. Fig. 4(b) and (c) show the TEM image and the selected area diffraction pattern of the ribbon-like crystals. The diffraction pattern can be indexed as HAp crystal with the electron beam direction of [110], suggesting that the longitude direction of the ribbon is [002] and the flat surface is (110). High-resolution TEM (HRTEM) images enabled us to obtain detailed crystallographic information of the OCP to HAp transformation. Fig. 5 shows the lattice image. Contrasting variation with interplanar spacings of approximately 0.34 nm was observed in the area of the well-defined lattice image, indicating the (002) plane of the hydroxyapatite. An enlarged area containing both OCP and HAp structures is showed in Fig. 5(b), revealing that the ribbon-like crystal is really a mixture of two calcium phosphate phases (OCP and HAp).
4. Conclusions
Fig. 4. SEM (a) and TEM (b) micrograph of the nanoribbon-like HAp coating; (c) the selected area diffraction pattern of the HAp crystal in (b).
(1) The morphology of hydroxyapatite crystal can be effectively controlled by the concentration of the electrolytes prepared for the deposition. When Ca2+ concentration is 6 × 10− 4 M, nanorod-like HAp crystal can be deposited on the titanium surface. (2) In the electrolyte with Ca2+ concentration of 4 × 10− 2 M, the nanoribbon-like HAp can be prepared. (3) TEM micrographs and SAD indicated that the longitude direction for both ribbon-like and rod-like crystal is [002], and the flat surface of the ribbon is (110). HRTEM showed that the ribbon-like HAp crystals were transformed from OCP.
W. Ye, X.-X. Wang / Materials Letters 61 (2007) 4062–4065
Acknowledgement This study was financially supported by the National Natural Science Foundation of China (NSFC) under the grant number 50571088 and by Science Program of Zhejiang Province under the grant number 2005C23006. References [1] L. Addadi, S. Raz, S. Weiner, Adv. Mater. 15 (2003) 959. [2] J.C. Merry, I.R. Gibson, et al., J. Mater. Sci., Mater. Med. 9 (1998) 779.
[3] [4] [5] [6] [7] [8] [9] [10]
4065
X.-X. Wang, L. Xie, R. Wang, Biomaterials 26 (2005) 6229. X.-X. Wang, S. Hayakawa, K. Tsuru, et al., Biomaterials 23 (2002) 1353. M. Shirkhanzadeh, J. Mater. Sci., Mater. Med. 9 (1998) 67. M. Ashok, N. Meenakshi Sundaram, et al., Mater. Lett. 57 (2003) 2066. X. Lu, Z.F. Zhao, Y. Leng, J. Cryst. Growth 284 (2005) 506. Y. Fujishiro, T. Sato, A. Okumura, J. Mater. Sci., Mater. Med. (1995) 172. S.J. Ban, S. Maruno, J. Biomed. Mater. Res. 42 (1998) 387. W.E. Brown, J.P. Smith, Nature 1962 (1962) 1050.