Micro Structural Characterization of Bio Medical Titanium

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Microstructural characterization of biomedical titanium

oxide film fabricated by electrochemical method

Han-Jun Oha, Jong-Ho Leeb, Yongsoo Jeongc, Young-Jig Kimd, Choong-Soo Chie,*

aDept. of Materials Science, Hanseo University, Seosan 352-820, South KoreabDept. of Chemistry, Hanseo University, Seosan 352-820, South Korea

cKorea Institute of Machinery and Materials, Changwon 641-010, South KoreadDept. of Metallurgical Engineering, Sungkyunkwan University, Suwon 440-746, South KoreaeSchool of Advanced Materials Engineering, Kookmin University, Seoul 136-702, South Korea

Available online 18 November 2004

Abstract

For an application as biomedical materials of high performance with a good biocompatibility, the anodic TiO2 layer on Ti substrate has

been fabricated by electrochemical method, and the characteristics of anodic titania film have been investigated. X-ray diffraction (XRD)

results indicate that the titania film formed in acidic electrolyte with additives is mainly composed of anatase structure containing rutile. From

the analysis of chemical states of the anodic film using X-ray photoelectron spectroscopy (XPS), phosphorus and sulfur were observed in the

anodic film, which were penetrated from the electrolyte into the oxide layer during anodic process. From the result of biological evaluation in

simulated body fluid (SBF), the anodic TiO2 film was effective for bioactive property.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Anodizing; Titanium oxide; X-ray diffraction; Photoelectron spectroscopy

1. Introduction

In biomedical implants and dental fields, titanium has

been widely utilized for excellent corrosion-resistance and

biocompatibility. However, Ti and its alloys are non-

bioactive after being implanted in bone. Thus, for further

improvement in biocompatibility the various implant sur-

face modifications have been investigated. These surface

modifications have included sandblasting [1], acid etching

[2], combination of sandblasting and acid etching [3], sol

gel technique [46], deposition of Ti coatings using plasmaspraying, and deposition of calcium phosphate or hydrox-

yapatite (HA) coatings [711]. In this study, for the purpose

of improvement in biocompatibility the anodic TiO2 layer

on Ti substrate was fabricated by electrochemical method in

acidic solution, and the characteristics of anodic titania film

has been examined. And the biological evaluation of the

anodic TiO2 films formed in acidic solution was performed

in a simulated body fluid.

2. Experimental

A commercial grade pure titanium (99.6 wt.%) was used

for anodization. The titanium specimens with dimensions of

8100.5 mm were mechanically polished and degreasedin n-hexane for 6 min, and then washed and dried. After the

pretreatment, the titanium oxide films for application ofbiomaterials were prepared by anodizing at a constant

voltage of 180 V in 1.5 M H2SO4, 0.3 M H3PO4 and 0.3 M

H2O2 mixture solution. For the comparison of bioactive test,

the etched titanium without anodization was prepared by

pickling in 30 vol.% nitric acid and 3 vol.% of hydrofluoric

acid. The microstructures of anodic TiO2 film and thickness

were observed using a scanning electron microscope

(JSM5410/EDS). Identification of the phase of anodic

TiO2 films was carried out using an XRD (Philips,

PW1710). The analysis of chemical states by X-ray photo-

0257-8972/$ - see front matterD 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.surfcoat.2004.10.029

* Corresponding author. Tel.: +82 2 910 4666; fax: +82 2 910 4320.

E-mail address: [email protected] (C.-S. Chi).

Surface & Coatings Technology 198 (2005) 247 252

www.elsevier.com/locate/surfcoat


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electron spectroscopy (XPS, PHI 5700) was performed at an

accelerating voltage of 15 kV and current of 30 mA with amagnesium Ka radiation.

After the anodic oxidation, the TiO2 films were soaked in

50 mL of simulated body fluid (SBF) with ion concentration

(Na+: 142, K+: 5.0, Mg2+: 1.5, Ca2+: 1.5, Cl: 147.8,

HCO3: 4.2, HPO42: 1.0, and SO4

2: 0.5 mM) nearly equal

to human blood plasma, which was developed by Kokubo et

al. [12]. The SBF was prepared by dissolving reagent grade

chemicals of NaCl, KCl, NaHCO3, K2HPO4, MgCl2, CaCl2and Na2SO4 into distilled water, and buffered at pH 7.40

with tris (hydroxymethyl) aminomethane and 1 M HCl at 37

8C. After the immersion in a SBF solution at 37 8C in the

normal incubator for 5 days, surface of sample was observed

by SEM.

3. Results and discussion

3.1. Characteristics of anodic TiO2 film

To obtain the anodic TiO2 layer an anodic constant

current of 35 mA/cm2 was applied on titanium surface, and

then the anodic potential was increased slowly. When the

anodizing potential reached a value of 180 V, anodizingprocess was carried out at a constant potential of 180 V.

During anodizing process the variations of surface mor-

phology of titanium is exhibited in Fig. 1. At the early stage

of anodization the anodic TiO2 layer was uniform and less

porous than that of anodic film formed at higher potentials.

At higher potential than 90 V the anodizing process was led

to increased gas evolution and also frequent sparking

phenomena. At the same time, anodic pore cell structure

began to nucleate at the surface, and the irregular arrays of

cell structure have gradually changed to regular morphol-

ogy. Under constant potential of 180 V the pore cell size

increased with the anodic time The anodic titanium oxide

film formed by electrochemical method at 180 V for 30 min

in 1.5 M H2SO4, 0.3 M H3PO4 and 0.3 M H2O2 mixture

solution is shown in Fig. 2a. The analyses of pore

distribution were performed using Image analyzer (Image

Proplus, Media Cybernetics), and thickness was observed

using SEM, and the results are shown in Fig. 2b.

Fig. 2a shows the variation of pore diameters of TiO2 cell

structure with anodic time. At an early stage of anodization

the diameters of the pores are smaller and distributions are

Fig. 1. Surface morphologies of anodic TiO2 films with anodizing time at 180 V in 1.5 M H2SO4, 0.3 M H3PO4 and 0.3 M H2O2 mixture solution.

Fig. 2. Electron micrograph of cross sectioned TiO2 film (a) and the relationship between average pore diameter and anodic film thickness with anodizing time

at 180 V in 1.5 M H2SO4, 0.3 M H3PO4 and 0.3 M H2O2 mixture solution (b).

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more scattered. As can be seen in Fig. 2a, the growth of pore

diameter of cell structure with anodic time increases rapidly

in the beginning stage of anodizing, which indicates average

pore diameter of 0.32 Am for 5 min, 0.56 Am for 30 min,

and then growth of pore diameter increases linearly.

Therefore the cell structure on anodic oxide film revealed

a normal frequency distribution by gradual growth of earlygenerated micropores, and the thickness of TiO2 film

increased linearly with anodic time. The pore diameter

and layer thickness of anodic TiO2 increase with anodic

time, and the anodic film thickness is dependent on anodic

time with a rate of 3.15102 Am/min at 180 V.Fig. 3 shows the X-ray diffraction (XRD) patterns of the

anodic titania film with anodic time. The anatase phase with

rutile revealed in XRD results in early stage of anodization.

And titanium peak observed in pattern is due to the titanium

substrate. As the anodic reaction increased, the X-ray peak

intensities of anatase gradually increased, and the crystalline

phase of anodic films was predominantly anatase. It is wellknown that the TiO2 has three crystal structures such as

anatase, rutile and brookite, and the anatase TiO2 is more

reactive than the rutile [13]. Fig. 4 shows XPS wide scan

spectrum of an anodic TiO2 layer formed at 180 V for 30

min. From the analysis of narrow scan of XPS spectrum, the

chemical states of O-1s, P-2p, S-2p, and Ti-2p were

identified in the forms of P2O5, SO42, TiO2 (anatase,

rutile), Ti2O3. Therefore, during anodization in the electro-

lyte of H2SO4, H3PO4 and H2O2 mixture solution, it is

found that P and S were incorporated into the anodic oxide

layer. And also it can be described that P migrates inward

across the TiO2 layer in the initial stage of anodization, and

S in the later stage of it. Fig. 5 shows depth profile of anodic

titania obtained from the XPS analysis. In Fig. 5, the

phosphorus is infiltrated throughout the oxide films, while

sulfur is detected in oxide films close to the electrolyte/

oxide interface. This suggests that during anodization the

inward diffusion of phosphorus or phosphate ion in

electrolyte to the oxide/electrolyte interface is dominant

rather than that of sulfur.

3.2. Effect of electrolytes

The anodic oxide films on titanium are formed by

anodization in electrolyte, for which anodic reaction in

combination with electrical-field driven metal and oxygen

ion diffusion lead to the formation of an oxide film at the

anode surface, which can be written as:

Ti4ox 4H2OaqfTiOH4ox 2H2

fTiO2xOH2x 2 xH2O

In these reactions the structure and morphology of anodicoxides can be varied by controlling the process parameters,

such as electrolyte composition. To observe the effects of

electrolytes on the microstructure of anodic TiO2 film, the

anodization was performed at 180 V for 30 min in various

electrolyte solutions. The electrolytes were 1.5 M H2SO4solution, 1.5 M H2SO4/0.3 M H2O2 mixture, 1.5 M H2SO4/

0.3 M H3PO4 mixture, and 1.5 M H2SO4/0.3 M H3PO4/0.3

M H2O2 mixture solution.

Fig. 3. X-ray diffraction pattern of the TiO2 film with anodizing time.

Fig. 4. XPS spectrum of anodic TiO2 formed at 180 V for 30 min in 1.5 M

H2SO4, 0.3 M H3PO4 and 0.3 M H2O2 mixture solution.

Fig. 5. Depth profile of anodic TiO2 film formed at 180 V for 30 min.

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Fig. 6 shows microstructures of anodic titania prepared

at 180 V for 30 min in various solution. The anodic TiO2films formed in 1.5 M H2SO4 solution and 1.5 M H2SO4/

0.3 M H2O2 mixture represent smaller cell structures than

those formed in 1.5 M H2SO4/0.3 M H3PO4 mixture and

1.5 M H2SO4/0.3 M H3PO4/0.3 M H2O2 mixture.

However, considerable changes in the surface morphologyare not seen only with addition of hydrogen peroxide as

additive.

From the results of thickness measurement the average

thickness of anodic titania film formed in 1.5 M H2SO4was 2.5 Am, 2.8 Am for H2SO4/H2O2 mixture, 3.2 Am for

H2SO4/H3PO4, and 3.6 Am for H2SO4/H3PO4/H2O2mixture. These results indicate that due to the addition of

hydrogen peroxide, the film thickness increases slightly by

about 12%, but for addition of phosphoric acid, the

thickness increased significantly. In general, during anod-

ization for valve metal the oxidation takes place at metal/

oxide interface; on the other hand, oxide surface/electrolyteinterface dissolves in acid electrolyte. Therefore surface

morphology and layer thickness can be strongly associated

with electrolyte. In aluminum anodization for fabrication

of nanopore arrays in anodic alumina, Li et al. [1416]

showed that anodic alumina surface with larger cell

structure was observed by using phosphoric acid solution

rather than sulfuric acid as electrolyte. However, for an

addition of hydrogen peroxide, oxide layer can be more

grown without substantial change in morphology, due to

migration of oxygen containing ions from H2O2 in

electrolyte. The chemical reaction of the titanium surface

with peroxides has been proposed by Tengvall et al. [17].

From the results of Fig. 6 and thickness measurements we

therefore assume that differences in the morphology and

thickness of anodic films are concerned mostly with the

addition of phosphoric acid, and for the addition of

hydrogen peroxide the anodic film thickness was affected

slightly.

Fig. 6. SEM images showing surface oxide morphology formed at 180 V for 30 min in 1.5 M H2SO4 solution (a), 1.5 M H2SO4/0.3 M H2O2 mixture (b), 1.5 M

H2SO4/0.3 M H3PO4 mixture (c), and 1.5 M H2SO4/0.3 M H3PO4/0.3 M H2O2 mixture solution (d).

Fig. 7. XRD patterns of anodic TiO2 film thickness formed at 180 V for 30

min in 1.5 M H2SO4 solution (a), 1.5 M H2SO4/0.3 M H2O2 mixture (b),

1.5 M H2SO4/0.3 M H3PO4 mixture (c), and 1.5 M H2SO4/0.3 M H3PO4/

0.3 M H2O2 mixture solution (d).



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Fig. 7 shows the X-ray diffraction patterns of the anodic

titania film formed at 180 V for 30 min in various solutions.

The phase of the titania films was basically anatase, and the

rutile was also observed together with titanium peaks from

the substrate. And it is exhibited that crystalline phase of the

anodic oxide was dependent on the electrolytic compositions,

especially for the film in Fig. 7d, indicating that anatase

structure was predominant. The biological evaluation of these

anodic TiO2 films formed in 1.5 M H2SO4/0.3 M H3PO4/0.3

M H2O2 mixture solution was performed in a simulated body

fluid. After the immersion in a SBF solution at 37 8C in 5

days, the surface of anodic film was observed by SEM with

energy dispersive spectroscopy analysis (EDS).

For the comparison of biocompatibility, etched titanium

specimen prepared in HNO3/HF mixture was performed in

SBF. Fig. 8 shows the morphologies of etched titanium

and anodic TiO2 surface after immersion in SBF for 5

days. These results suggest that the particles were

precipitated on the surface of anodic film in SBF, but

no apparent changes appeared on the surface of the etched

titanium, as can be seen in Fig. 8b and d, respectively.

From the result of EDS analysis, as shown in Fig. 9, the

precipitated particles on the surface of anodic film is

revealed as Ca-rich compounds, which will be steadily

converted to hydroxyapatite crystalline phase that is

similar in composition and structure to bone apatite. The

anodic TiO2 film formed by electrochemical method has a

porous and relatively rough morphology, and composes of

anatase and rutile structure. From the results in Figs. 8

and 9, it is clearly suggested that these microporous

Fig. 8. SEM photographs of specimens soaked in SBF for 5 days. Morphologies of pure titanium surface etched in HF/HNO 3 (a) and (b), anodized in 1.5 M

H2SO4/0.3 M H3PO4/0.3 M H2O2 mixture solution (c) and (d). Original magnification, 1000 (a), 10,000 (b), 1000 (c), 10,000 (d).

Fig. 9. SEM micrograph and line profiles for precipitation particles on surface of anodic layer soaked in SBF for 5 days. The precipitated particles indicates Ca-

rich compound by EDS analysis.

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surface features and titanium oxide structure are all

effective for biocompatibility.

4. Conclusions

The anodic titanium oxide film for biomedical appli-cations was synthesized by anodic oxidation in acid

solution, and the surface characteristics of anodic TiO2layer has been evaluated. The major structure of anodic

TiO2 film was revealed to anatase, and surface morphol-

ogy exhibited a porous cell structure. For the effects of

electrolytes on the microstructure of anodic TiO2 film, the

differences in the morphology and thickness of anodic

films concerned mostly with the addition of phosphoric

acid. And for the addition of hydrogen peroxide the

anodic film thickness was affected slightly. The compo-

nent elements of the electrolyte, P and S, were found in

the oxide layer, which were incorporated from theelectrolyte into the oxide. From the result of biological

evaluation in simulated body fluid, the TiO2 anodic film

was effective for bioactive property.

Acknowledgments

This work has been supported by the Kookmin University.

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Micro Structural Characterization of Bio Medical Titanium

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