电化学阻抗研究形核机理

发表时间:2018-1-14 14:48:52 文章来源:广德教育网 手机版

导读:广德教育网友为您分享以下“电化学阻抗研究形核机理”资讯,希望对您有所帮助,感谢您对codmst.的支持!

Electrochemical impedance spectroscopy study of the nucleation and growth of apatite on chemically treated pure titanium

C.X. Wang *, M. Wang

School of Mechanical and Production Engineering, Nanyang Technological University, 50Nanyang Avenue, Singapore 639798, Singapore

Received 3July 2001; accepted 24July 2001

Abstract

Bone-like apatite formed on the surface of pure titanium pretreated with NaOH solution after having been immersed in simulated body fluid (SBF).In the present study, electrochemical impedance spectroscopy (EIS)measurement was used to investigate the nucleation and growth of apatite on chemically treated pure titanium immersed in the SBF solution. An appropriate equivalent circuit model was constructed to describe the nucleation and growth of apatite. It was found that EIS is a useful method for investigating the nucleation and growth of bone-like apatite on pure titanium pretreated with NaOH solution. It may quantify apatite growth on pretreated titanium substrates. D 2002Elsevier Science B.V . All rights reserved.

Keywords:Electrochemical impedance spectroscopy; Alkaline treatment; Pure titanium; Bone-like apatite; Nucleation and growth

1. Introduction

Calcium phosphate ceramics, such as hydroxy-apatite (HA)-coatedtitanium and its alloys, are among the most promising implant materials for orthopedic and dental applications. However, prob-lems such as low bond strength between the coating and the substrate and nonuniformity across the thick-ness of the coating are often encountered with these coatings [1].

Recently, it has been reported that chemically treated titanium can induce bone-like apatite forma-tion in vitro and in vivo [2–4],which means that

titanium and its alloys have potential bioactivity. Treatment with a NaOH solution produces a sodium titanate gel layer on titanium surface. The gel layer has the ability to induce formation of bone-like apatite during immersing in simulated body fluid (SBF)and, thus, is considered bioactive. The gel layer can initiate apatite nucleation on itself. Once apatite nucleation occurs, it spontaneously grows by taking calcium and phosphate ions from the sur-rounding environment.

The qualitative observation of nucleation and growth of apatite on pretreated pure titanium could be done using conventional methods such as X-ray diffraction (XRD),scanning electron microscope (SEM),etc. The main objective of this study was focused on a quantitative investigation of the nuclea-tion and growth of apatite by using electrochemical impedance spectroscopy (EIS)measurements.

0167-577X/02/$-see front matter D 2002Elsevier Science B.V . All rights reserved. PII:S 0167-577X (01) 00532-8

*

Corresponding author. Tel.:+65-790-5332;fax:+65-791-

1859.

E-mail address:mcxwang@ntu.edu.sg(C.X.Wang).

/locate/matlet

May 2002

Materials Letters 54(2002)30

–36

2. Materials and methods

2.1. Surface treatment of titanium substrates A mercially available pure titanium substrate (discsof dimensions F 15?3mm) was mechan-ically polished and ultrasonically cleaned with ace-tone and alcohol. These discs were soaked in 5.0M NaOH solution at 60j C for 24h, then gently washed with distilled water and finally dried at 37j C for 24h.

2.2. Immersing of pretreated substrates in simulated body fluid (SBF)

An acellular simulated body fluid (SBF)with pH 7.4and ion concentrations (inmM:Na +142.0, K +5.0, Mg 2+1.5, Ca 2+2.5, Cl à147.8, HCO 3à4.2, HPO 42à1.0, SO 42à0.5) nearly equal to those of human blood plasma was previously proposed by Kokubo et al. [5],and has been extensively con-firmed to reproduce the in vivo apatite formations on bioactive materials [5,6].The SBF was prepared by dissolving reagent grade chemicals of NaCl, NaHCO 3, KCl, K 2HPO 4á3H 2O, MgCl 2á6H 2O, CaCl 2and Na 2SO 4into distilled water and buffering at 7.40with tris -hydrochlomethyl-amminomethane ((CH2OH) 3-CNH 3) and hydrochloric acid at 36.5j C.

After the alkaline treatment, the treated titanium substrates were immersed in SBF. At regular inter-vals, the specimens were removed from SBF, washed with distilled water and acetone and dried at room temperature. Some of the specimens were used for microstructural characterization and the others were used for electrochemical impedance spectroscopy measurements.

2.3. Microstructural characterizations

X-ray diffraction (XRD)was employed to analyze the structure of titanium substrate, gel layer and bone-like apatite. A thin-film X-ray diffractometer (XRD,Rigaku X-ray diffractometer) was used. The morphologies of the specimens were examined under scanning electron microscopy (SEM,JEOL JSM 5600LV).

2.4. Electrochemical impedance spectroscopy (EIS)measurements

The electrochemical impedance spectroscopy (EIS)measurements were made using a lock-in am-plifier (Model5210, EG&GInstrument) coupled to a Potentiostat –Galvanostat System (Model273A, EG&GParc.), which was connected to a three-elec-trode electrochemical cell. A platinum foil was

used

Fig. 1. SEM micrographs of untreated titanium and titanium treated with 5.0M NaOH solution at 60j C for 24h:(a)untreated titanium substrate, (b)NaOH-treated titanium substrates.

C.X. Wang, M. Wang /Materials Letters 54(2002)30–3631

as counter electrode and a saturated calomel electrode

(SCE)was used as a reference electrode. The treated

titanium specimens were used as the working electro-

des. EIS spectra were obtained at open-circuit poten-

tial of the specimens in SBF, with an amplitude of 10

mV . The frequency span was from 100KHz down to

1mHz. Data registration and analysis were performed

on an interfaced puter. The spectra were then

interpreted using the nonlinear least square fitting

procedure developed by Boukamp [7].The quality

of fitting to the equivalent circuit was judged firstly by

the chi-square value, and secondly by the error dis-tribution vs. frequency paring experimental with simulated data [7].3. Results 3.1. Scanning electron microscopy Fig. 1shows scanning electron microscopy (SEM)micrographs of the surfaces of titanium substrates that were soaked in 5.0M NaOH solution at 60j C for 24h in parison to the untreated

titanium

Fig. 2. SEM micrographs of the surfaces of pretreated titanium substrates immersed in the SBF solution at 36.5j C at regular intervals:(a)1week, (b)3

weeks.

Fig. 3. TF-XRD pattern of titanium substrate treated with 5.0M NaOH at 60j C for 24h. C.X. Wang, M. Wang /Materials Letters 54(2002)30–36

32

substrates. It can be seen that a porous network structure was formed on the surface of titanium with the NaOH treatment.

Fig. 2shows SEM micrographs of the surfaces of NaOH pretreated titanium substrates were immersed in simulated body fluid (SBF)at 36.5j C for regular intervals. As can be seen, after 1week’simmersion in SBF solution, apatite nuclei formed on the surface of the pretreated titanium substrates. Then, the ap-atite nuclei grew and the amount of apatite increased with the extension of immersion time. In the stage of nuclei formation (Fig.2a), there were some spherical apatite islands on the network structure. With the increase in the immersion time, islands of apatite grew, and the network structure was gradually cov-ered by apatite, and with the further increase in the immersion time, the growing apatite islands coa-lesced, and the network structure was pletely

covered with apatite (Fig.2b). This indicated that this network structure formed on the titanium surface by alkaline treatment could induce the nucleation and enhance the formation of apatite, which made titanium to be bioactive. 3.2. XRD results

Fig. 3shows the TF-XRD pattern of the surface of titanium treated with 5.0M NaOH solution at 60j C for 24h. A broad bump and small peaks at around 24, 28and 48j were observed in the XRD pattern, in-dicating that the surface porous network layer, which was formed by the NaOH treatment, is an amorphous sodium titanate phase [2,4].

Fig. 4shows the TF-XRD patterns of alkaline-treated titanium immersed in the SBF solution at 36.5j C at regular intervals. In parison to

the

Fig. 5. Typical bode plot for alkaline-treated titanium immersed in the SBF solution (8

weeks).

Fig. 4. TF-XRD patterns of alkaline-treated titanium immersed in the SBF solution at 36.5j C at regular intervals:(a)1, (b)2, (c)3, (d)4, (e)6and (f) 8weeks.

C.X. Wang, M. Wang /Materials Letters 54(2002)30–3633

pattern in Fig. 3, all the new peaks appeared in the patterns in Fig. 4are ascribed to crystalline bone-like apatite, indicating that the network structure formed on the surface of titanium could induce the nucleation and growth of bone-like apatite on titanium. In the stage of nucleation (Fig.4a), the counts of the peaks for apatite were very low, and peaks for titanium substrate were also observed. With an increase in immersion time in simulated body fluid, the counts of the peaks for apatite were getting higher, and no peaks for the titanium substrate were observed (Fig.4b–f),indicating the growth of apatite and the surface was fully covered with apatite.

3.3. Electrochemical impedance spectroscopy A typical EIS spectrum is shown in Fig. 5. When alkaline-treated titanium is exposed to simulated body

fluid, the spectra appears very different and varies significantly with exposure time, especially for the spectra at higher frequencies, which indicates the nucleation and growth of apatite. It is interesting to note that the remarkable change in the spectrum coincided with the nucleation and growth of apatite on the pretreated titanium. 3.4. Analysis of EIS spectra

For analysis of the impedance data, a software program, ‘EquivalentCircuit,’was used. The program used a variety of electrical circuits to numerically fit the measured impedance data. A constant phase ele-ment (CPE),Q , are used for the equivalent circuit in this study.

Fig. 6shows the equivalent circuit based on a three-layer model (inner,hydrogel and apatite layers),

which

Fig. 6. Equivalent circuits used for alkaline-treated titanium immersed in the SBF solution at 36.5j C at regular intervals, and schematic representation of the apatite, hydrogel and inner layers of oxide film on treated titanium. Notations:R e is the solution resistance; C b , R b are the inner layer capacitance and resistance; C p , R p are the hydrogel layer capacitance and resistance; C a , R a are the apatite layer capacitance and resistance.

Table 1

EIS spectra fitting results for alkaline-treated titanium immersed in the SBF solution for various periods of time Time 1h 1week 2weeks 3weeks 4weeks 6weeks 8weeks R a (V ) 78

167

199

211

346

356

418

C a (F)6.3?10à63.6?10à42.3?10à41.1?10à42.9?10à42.9?10à41.7?10à4R p (V ) 2.5?1032.0?1031.2?1035.6?1043.8?1031.1?1036.8?102C p (F)2.9?10à41.7?10à41.9?10à41.6?10à45.7?10à45.3?10à54.7?10à5R b (V ) 1.3?1079.3?1072.7?1081.9?1071.3?1075.4?1072.8?108C b (F)

3.1?10à5

2.2?10à5

2.8?10à5

2.7?10à5

3.8?10à5

3.4?10à5

2.7?10à5

C.X. Wang, M. Wang /Materials Letters 54(2002)30–36

34

can be satisfactorily used for fitting the spectra obtained from alkaline-treated titanium immersed in simulated body fluid at different periods of time. The fitting results are listed in Table 1. As can be seen that R a (apatitelayer resistance) continuously increases with the exposure time. At the apatite nucleation stage (from1h to 1week), even though apatite could not be clearly seen under scanning electron microscopy (SEM)observation, the increase in R a indicated apatite nucleation. And the growth of apatite corresponded to the continuous increase of R a . 4. Discussion

A broad bump in the TF-XRD pattern of titanium treated with NaOH solution at 60j C for 24h suggests that the network structure on the surface is an amor-phous sodium titanate hydrogel. When the treated titanium discs immersed in simulated boy fluid sol-ution at 36.5j C, this hydrogel layer could induce the nucleation and growth of apatite on the surface. After about 1-week nucleation time, islands of apatite were seen on the surface of pretreated titanium under SEM observation, and weak peaks for apatite with peaks for titanium substrate appeared in the TF-XRD pattern. With an increase in immersion time in simulated body fluid, islands of apatite were seen to grow and coa-lesce on pretreated titanium under SEM observation, and strong peaks of apatite with no peaks for titanium substrates appeared in the TF-XRD patterns.

Electrochemical impedance microscopy analysis has been shown to be a useful method for investigat-ing the nucleation and growth of bone-like apatite on pretreated titanium. Based on the EIS spectra, a three-layer (inner,hydrogel and apatite layers) model was used to interpret the obtained spectra. The results coincided with those obtained from SEM and TF-XRD very well. At the apatite nucleation stage, even though no apatite nuclei formed on the surface (apatitecould not be clearly seen under SEM observation), the increase of resistance in the outmost surface of pre-treated titanium discs, which resulting from the changes on the surface due to releases of Na +ions, formation of Ti–OHgroups, calcium titanate and apatite nuclei [3],indicated apatite nucleation. With an increase in immersion time in simulated body fluid, islands of apatite (apatitenuclei) were seen grow and

coalesce on pretreated titanium under SEM. The continuous increase of the resistance of apatite layer indicated the growth of apatite on the pretreated titanium. In parison to the results obtained from other methods such as SEM, TF-XRD, XPS, etc., the growth of apatite are quantitative. Therefore, EIS can be used to quantify apatite growth on pretreated titanium.

5. Conclusions

Bone-like apatite formed on the surface of titanium pretreated with NaOH solution after the pretreated titanium discs had been immersed in the simulated body fluid solution. Electrochemical impedance spec-troscopy (EIS)has been shown to be a useful method for investigating the nucleation and growth of bone-like apatite on pretreated titanium. At the apatite nuc-leation stage, even though apatite could not be clearly seen under scanning electron microscopy (SEM),the increase in electrical resistance in the outmost surface of pretreated titanium discs indicated apatite nuc-leation. With an increase in immersion time in simu-lated body fluid, islands of apatite were seen to grow and coalesce on pretreated titanium under SEM. The growth of apatite corresponded to the increase in electrical resistance of the surface layer. EIS can be used to quantify apatite growth on pretreated titanium.

Acknowledgements

The authors would like to thank Nanyang Techno-logical University (NTU)for funding the research. Wang Changxiang thanks Nanyang Technological University for providing a research fellowship. Assis-tance provided by technical staff in the School of MPE, NTU, is gratefully acknowledged.

References

[1]W.R. Lacefield, Hydroxyapatite coatings, in:P. Ducheyne, J.E.

Lemons (Eds.).Bioceramics:Material Characteristics Versus In Vivo Behavior. Annals of The New York Academy of Science, New York, 1988, pp. 72–80.

[2]H.M. Kim, F. Miyaji, T. Kokubo, S. Nishiguchi, T. Nakamura,

Graded surface structure of bioactive titanium prepared by

C.X. Wang, M. Wang /Materials Letters 54(2002)30–3635

chemical treatment, Journal of Biomedical Materials Research 45(1999)100–107. [3]H. Takadama, H.M. Kim, T. Kokubo, T. Nakamura, An x-ray photoelectron spectroscopy study of the process of apatite for-mation on bioactive titanium metal. Journal of Biomedical Ma-terials Research 55(2001)185–193. [4]H.M. Kim, F. Miyaji, T. Kokubo, T. Nakamura, Preparation of bioactive Ti and its alloys via simple chemical surface treat-

ment. Journal of Biomedical Materials Research 32(1996)

409–417. [5]T. Kokubo, H. Kushitani, S. Sakka, T. Kitsugi, T. Yamamuro,

Solution able to reproduce in vivo surface-structure changes in

bioactive glass –ceramic A-W. Journal of Biomedical Materials

Research 24(1990)721–734.

[6]M.R. Filguerias, G. LaTorre, L.L. Hench, Solution effects on

the surface reaction of a bioactive glass. Journal of Biomedical

Materials Research 27(1993)445–453.

[7]B.A. Boukamp, A nonlinear least squares fit procedure for analysis of immittance data of electrochemical systems. Solid State Ionics 20(1986)31–44. C.X. Wang, M. Wang /Materials Letters 54(2002)30–36

36


百度搜索“广德教育”,专业资料,生活学习,尽在广德教育网codmst.,您的在线图书馆
电化学阻抗谱图分析  电化学阻抗   电化学阻抗  
以上关于电化学阻抗研究形核机理的相关信息是广德教育网收集并且整理,仅为查考。
关于我们  服务范围  版权声明  文章投稿  网站地图 | 口号:与你分享知识的乐趣
版权所有:广德教育网(www.codmst.com)教育信息门户 @ 为每个爱好学习的同学提供最好的教育