aspects relating to stability of modified passive stratum on tio2 nanostructure

Met. Mater. Int., Vol. 17, No. 2 (2011), pp. 321~327doi: 10.1007/s12540-011-0421-8 Published 26 April 2011

Aspects Relating to Stability of Modified Passive Stratum on TiO2 Nanostructure

Daniela Ionita*, Anca Mazare, Diana Portan, and Ioana Demetrescu

Politehnica University of Bucharest, Faculty of Applied Chemistry and Material Science,Department of General Chemistry, Bucharest, 011061, Romania

(received date: 15 July 2010 / accepted date: 29 November 2010)

Two kinds of nanotube structures differing from the point of view of their dimensions were obtained usinganodizing in two different fluoride electrolytes and these structures were investigated regarding stability. Thenanotubes have diameters of around 100 and 65 nm, respectively, and the testing solutions were simulatedbody fluids (SBF) and NaCl 0.9%. As stability experiments, cyclic voltammetry was performed and ionsrelease was measured. The quantity of released cations in time as a kinetic aspect of passive stratum behaviorwas followed with an inductively coupled plasma mass spectrometer (ICP-MS) and apatite forming in SBFwas found with infrared spectra. This study led to a comparison between the modification and the behaviorof passive stratum on nanotubes as a function of their diameters.

Keywords: biomaterials, anodization, electrochemistry, scanning electron microscopy (SEM), ICP-MS

1. INTRODUCTION

The relationship between biomaterials and nanotechnol-ogy and a definition according to the idea that “nanomaterialis any form of material that is composed of discrete func-tional parts which have dimensions of the order of 100 nm orless” was discussed in the scientific literature [1] in an edito-rial of the journal “Biomaterials”, just two years ago, aftermore than ten years of papers dealing with the nano level. Apart of these articles [2] was devoted to Ti and Ti alloys, takinginto account the fact that materials based on Ti are well knownfor important applications, especially in medicine [3,4].

Titanium stability in various environments, such as bodyfluids, is due to a passive titanium dioxide stratum, which isa mixture of Ti oxides, the predominant one being TiO2 [5].In long term exposure, the behavior and stability of a bioal-loy [6,7] can be different due to oxide hydrolysis or oxygenconsumption [8,9], which may induce pH non-uniformities.When dioxide is present in various types of nanostructures,these structures seem to be more stable and efficient. In thisapproach, titanium dioxide nanotubes are used as nanostruc-tured semiconductor oxides with improved functional photo-catalytic and sensing properties [10]. Considering theseproperties, TiO2 nanotube arrays have attracted wide scien-tific interest in view of their application in self cleaning gassensors, photoelectrolytic material dye sensitized solar cells,

and water photoelectrolysis [11]. Furthermore, the biocom-patible nature of these arrays makes them an excellentchoice for use in biological applications [12,13]. It has beenrecognized that the nano level has the potential to influencevarious properties including biocompatibility phenomena,which are controlled by nanoscale topographical features [13].

Regarding the elaboration of controllable properties ofoxide self organized layers on Ti surface, the anodizing pro-cedure, as a nano-modification technique, has in the lastdecade permitted the creation of nanotubes with mimeticfeatures of natural bone [14], taking into account the geome-try and dimensions of bone components: collagen andhydroxyapatite (HA). In fact, the main organic component ofbone is a triple helix 300 nm in length and 0.5 nm in width.Hydroxyapatite (HA), the second component, has particlesizes around 2040 nm long. Therefore, it is possible toassume that bone cells are accustomed to a nanoscale tubularenvironment rather than to a micron-scale environment [15].

In vitro studies have shown that a nanotubular titania sur-face provides a favorable template for the growth of bonecells. This behavior seems to be strongly dependent on tita-nia nanotubes, according to recent data in the literature [16-18], where the cells cultured on nanotubular surfaces showedhigher adhesion, proliferation, alkaline phosphate activityand bone matrix deposition compared to those grown on flattitanium surfaces. Some of our recent results with the fibro-blast cell indicated a slight preference in terms of cell survivaland adhesion for nanostructure TiO2 with a more hydrophiliccharacter; the electrochemical data revealed that such fea-

*Corresponding author: [email protected]©KIM and Springer

322 Daniela Ionita et al.

tures are connected with better stability in artificial saliva[19].

With these perspectives, nanotubes formation on titaniumalloys by anodizing possesses importance for potentialimplant applications. However, no comprehensive informa-tion is available on the corrosion resistance property of nan-otubular titanium alloys. Corrosion in bioliquids is one of thedecisive properties for the biocompatibility of an implant. Inthis approach, the present paper is an investigation of somestability aspects relating to modification of the passive stra-tum of nanotubes with diameters of around 100 nm and 65 nm.The surface stratum was modified after immersion in simu-lated body fluids (SBF) and the capability of forming apatitein such conditions was discussed as an aspect of biocompat-ibility.

2. EXPERIMENTAL PART

All electrochemical measurements were conducted withtitanium foils (2 mm thickness) that were mechanically pol-ished in order to obtain an appropriate brittle and flat surface.Ti samples were prepared according to details described in aprevious work in which the anodizing system was presentedas well [20]. All the anodic treatments were performed atroom temperature. The anodizing conditions are summarizedin Table 1.

The morphology of the titania nanotubes was investigatedwith an Environmental Scanning Electron Microscope FEI/Phillps XL30 ESEM (SEM). After morphological character-ization, TiO2 nanotubes were tested in simulated body fluids(SBF) and NaCl 0.9 % physiological serum. The SBF com-position is as follows: 142 mmol/l Na+, 5 mmol/l K+, 1 mmol/l Mg2+, 2.5 mmol/l Ca2+, 126 mmol/l Cl−, 10 mmol/l HCO3

− ,mmol/l HPO4

2− mmol/l, 1 mmol/l SO42−.

FTIR spectroscopy was performed with a Shimazu devicein order to determine the chemical structure of the electro-chemical deposition in SBF and to identify the functionalgroups.

Electrochemical potentiodynamic polarization experimentswere carried out in SBF and NaCl 0.9 % solution at 37 ± 1Cemploying a Voltalab 40 potentiostat/galvanostat. The saltconcentration in SBF solution corresponded to that of bodyfluids. A conventional three electrode system with Pt ascounter electrode and saturated calomel electrode (SCE) asreference was used. The scan rate used was 2 mV/s. Tafelextrapolation was followed to determine the corrosionparameters.

The solutions were analyzed with an ICP-MS ELAN DRC-

e from Perkin Elmer SCIEX, U.S., in order to obtain thequantity of released Ti ions from the studied samples andalso to determine the level of evolution of calcium ions asthe measurements took place.

3. RESULTS AND DISCUSSION

In general, in the absence of water in electrolytes, theanodizing process will suffer because of a lack of ions andalso because of the high viscosity of the solutions, viscositywhich can lead to the formation of only a titanium dioxidelayer. The overall reaction for anodic oxidation of titaniumcan be represented as:

H2O → O2 + 4e + 4H+

Ti + O2 → TiO2

In the initial stage of the anodizing process, field-assisted dis-solution dominates chemical dissolution due to the relativelylarge electric field across the thin oxide layer. Small pitsformed due to the localized dissolution of the oxide, repre-sented by the following reaction, act as pore forming centers[21,22]:

TiO2 + 6F- + 4H+ → TiF62- + 2H2O

The pits convert into bigger pores and the pore densityincreases. Subsequently, the pores spread uniformly over thesurface. The pore growth occurs due to the inward movementof the oxide layer at the pore bottom (barrier layer). Thisgrowth supports the concept that tube formation originatesfrom the transition from ordered porous oxide by a “pore-wall-splitting” mechanism, as was described very recently inliterature [23].

Figure 1 presents the S1 surface morphology on whichnanotubes with diameters of about 100 nm were obtained.Figure 2 presents the S2 surface morphology on which nan-otubes with diameters of about 65 nm were obtained.

In the case of S1, the diameter of the nanotubes was 100nm and the wall thickness was 15 nm; for S2, the diameterwas 65 nm and the wall thickness was 17 nm.

Table 1. Electrolyte composition and anodizing conditionsSample Electrolyte composition Voltage Time (min)

S1 HF 0.5 % 20 V 120S2 0.5% HF + 5 g/l Na2HPO4 20 V 120 Fig. 1. TiO2 nanotubes on S1.

Aspects Relating to Stability of Modified Passive Stratum on TiO2 Nanostructure 323

Figures 3 and 4 show the typical current density recordedduring anodizing of Ti at 20 V for samples S1 and S2,respectively.

In both cases the current density in the initial stage dropsdrastically from values of around 4 mA/cm2 to 0.5 mA/cm2

and 1.5 mA/cm2, respectively, after which it starts to growuntil it reaches the steady state value [24].

After anodizing, the chemical structure of the surface wasanalyzed. Figure 5 presents the FTIR analyses of the surfacesamples. It can be seen that the deformation and stretchingvibrations of the OH group at 1618 cm−1 and 3423 cm−1 wereobserved in the IR spectrum of titania nanotubes. This result

confirmed that there is a great deal of water in titania nano-tubes. Due to the strong interaction between the Ti ions andthe OH groups, a shoulder at 3208 cm−1 from the Ti-OHbonds was observed. Such Ti-OH groups lead to a positivecharge according to the reaction:

Ti-OH (basic hydroxide) + H2O [Ti-OH2]+ + OH−,or to a negative charge corresponding to the reaction Ti-OH (acidic hydroxide) + H2O [Ti-O]− + H3O+

The first reaction takes place at acidic pH < 4 and the sec-ond at pH > 9.

At pH between 4 and 9, as is this case, both reactions arepossible, because the acidic and basic hydroxides coexist onthe Ti surface [25].

After the obtaining of nanotubes on these surfaces, thesamples were immersed in SBF solution for different periodsof time: 1 day, 3 days, 5 days and 15 days. The immersion inSBF is not only a bioactivation procedure that is related tothe formation of various phosphates, but was also interpretedas a chemical evaluation of biocompatibility [26,27].

During the immersion in SBF, the pH of the SBF solutionwas monitored for a small period of time (1 day). SignificantpH changes were recorded for sample S2, as can be seenfrom Figure 6, which shows a change in the pH value of thesolution near the coating surface. The pH value of the solu-tion near the coating surface increases from 7.4 to 8 withimmersion time in the initial stage. The growth mechanismof HA is suggested to be as follows:

When the deposited HA coating is immersed in SBF solu-tion, both HA coating dissolution and deposition of HA fromthe solution take place. The dissolution of the HA coatingtends to occur at the edge of needle-like crystals. At the ini-tial stage, the dissolution rate may be even slightly higherthan the deposition rate, resulting in an increase of the pHvalue near the surface regions. With the increase of OH -con-centration, the HA deposition rate should be higher. Also,more OH -ions may be adsorbed on the HA coating surface,which may lead to migration of cations (such as Ca2+) to theHA coating surface. As a result, the solution near the coating

Fig. 2. TiO2 nanotubes on S2.

Fig. 3. Anodization of Ti at 20 V for S1.

Fig. 4. Anodization of Ti at 20 V for S2.

Fig. 5. FTIR spectra of the surface samples with titania nanotube.


surface is supersaturated, and the deposition rate of HA isincreased. Hence, after the initial stage, the HA coating willkeep growing. With a prolonged immersion time, the coatingthickness obviously increases: during the third and fourthdays of immersion and after 5 days of immersion in SBF thesamples were densely covered.

SEM image of sample S2 surface after 5 days of immer-sion in SBF is presented in Fig. 7 together with the EDXspectrum, which permits a determination of the Ca/P ratio as1.58. For sample S1 the SEM image after 5 days remains thesame as that shown in Fig. 1 before immersion in SBF.

Titania nanotube arrays formed in HF solution (sample S1)do not have an apatite forming ability in SBF after 15 daysof immersion, while those formed in HF solution containinga small amount of Na2HPO4 have this ability. Therefore, aunique reason for the apatite forming of the (S2) titania nan-otubes arrays is that some HPO4

2− was incorporated into thenanotubes by electric field and diffusion. After rinsing and

drying of the titania nanotube arrays, HPO42− ions adhere to

the wall of tubes by physical adsorption. After soaking inSBF, HPO4

2− ions adhered on the walls of the tubes canchange to PO4

3− ions in basic solution and move to the nano-tube surface, then Ca2+ ions are adsorbed around PO4

3− byelectrostatic attraction. Once the apatite nuclei are formed,they spontaneously grow by consuming the calcium andphosphate ions from SBF. As a result, apatite nucleates andgrows on the surface of titania nanotube arrays.

Figure 8 presents only the FT-IR spectrum for sample S2after 5 days of immersion in SBF, taking into account thefact that no phosphate formation was observed on S1 afterthe same period of time. This fact sustains the lower bioac-tivity and biocompatibility of the S1 sample.

The spectrum in Fig. 8 shows that the layers are composedof carbonated hydroxyapatite, which idea is supported by thepresence of the bands [28] originating from the stretchingvibration of CO3

2− at 1450 cm−1; a bending mode at 1645 cm−1

originating from H2O could also be observed in the spectrumof the films. A broad phosphate band associated with the P-O asymmetric stretching mode (v3) of the (PO4)3− group wasfound in the region from 1200 to 960 cm−1, indicating a devi-ation of the phosphate ions from their ideal tetrahedral struc-ture. The bending modes of the O-P-O bonds in the phosphates

Fig. 6. pH changes in the SBF solution during the immersion time forsample S2.

Fig. 7. SEM image of the sample S2 surface after 5 days of immersion in SBF.

Fig. 8. FT-IR spectrum of S2 after 15days of immersion in SBF.


were found at 603 and 565 cm−1. The band detected at 875cm could be assigned to the CO3

2− group in carbonated apa-tite as well.

The electrochemical stability test of the two surfaces, S1and S2, was done in SBF and 0.9 % NaCl solution. Figure 9shows the Ti ion release for samples S1 and S2 immersed inSBF.

After 15 days of immersion in SBF, samples S1 and S2were introduced into physiological serum following the evo-lution of Ti ion release for periods of 1, 3, 5, and 15 days ofimmersion.

It can be seen in Fig. 10 that the quantity of Ti ion releasedin the physiological serum is approximately two orders ofmagnitude larger than that released in the SBF solution.

Representative potentiodynamic polarization plots in SBFsolution, obtained for samples S1 and S2 after anodizing, areshown in Fig. 11. Table 2 represents the corresponding cor-rosion parameters according to the potentiodynamic polar-ization analysis results with evaluation of Icorr, Ecorr, Ipass. Icorr,representing the current density, is the main parameter inevaluating electrochemical stability. A higher Icorr value meansless corrosion resistance. Ecorr, the corrosion potential, is abasic indicator of the thermodynamic status. Ipass is one of the

main parameters in describing the passivity process. It is thecurrent corresponding to the start of passivity.

It can be seen that the corrosion current density (Icorr) of thenanotubular S1 sample is significantly higher than that of thenanotubular S2 sample. A high Icorr value suggests less corro-sion resistance property. Both samples exhibited steady pas-sivation regions. The passivation current density (Ipass) of thenanotubular sample S1 was less noble than that of the nano-tubular sample S2.

Figure 12 shows the potentiodynamic curve for nanotubu-lar samples S1 and S2 recorded in NaCl 0.9 % solution. Thecorrosion parameters, including corrosion potential (Ecorr,)density current (Icorr,) and passivity current (Ipass), obtainedfrom the potentiodynamic polarization test are listed in Table

Fig. 9. Ti ion release for samples S1 and S2 immersed in SBF.

Fig. 10. Ti ion release for samples S1 and S2 immersed in physiologi-cal serum.

Fig. 11. Potentiodynamic polarization plot recorded for nanotubularsamples S1(--) and nanotubular samples S2(—) in SBF solution.

Table 2. Corrosion parameters for nanotubular samples in SBF solution

Sample icorr (µAcm−2) Ecorr (mV vs SCE) Ipass (µAcm−2)S1 21.5 −885.1 28.82S2 2.03 −598.6 12.4

Fig. 12. Potentiodynamic polarization plot recorded for nanotubularsamples S1(--) and nanotubular samples S2(—) in NaCl 0.9 % solu-tion.


3. The cyclic polarization curves indicate that there is no filmbreakdown for any of the samples. Both of the samples haveelectrochemical parameters corresponding to very stablebehavior in the stability scale. Regarding the changes ofnumerical values (Icorr, Ecorr, Ipass) according to the pore size ofTiO2, it is well known from literature [29] that despite thefact that a porous structure is supposed to help oseointegra-tion, the pores themselves will introduce a lower electro-chemical stability due to factors such as greater ions release.In the present paper, sample S2, with a lower diameter, is theone with a better stability (smaller Icorr, smaller Ipass, and lession release). The Icorr of the nanotubular sample S2 was lowerthan the Icorr of nanotubular sample S1, which indicates animprovement in corrosion resistance. The Ecorr (vs. SCE) valueof sample S2 is more electropositive than the Ecorr value ofsample S1. The passive region of the nanotubular S2 samplesuggests more stability. Such behavior indicates a highercorrosion resistance for S2 compared to S1 and could possi-bly be correlated with better bioactivity and biocompatibilityfor samples S2 due to smaller ion release from S2 and theability to form phosphate.

4. CONCLUSIONS

1. The electrochemical stability of TiO2 nanotubes in bothtested solutions: SBF and physiological serum, is dependenton nanotube size diameter, the nanotubes with a higherdiameter having a higher corrosion rate.

2. The concentration of Ti ions released in the physiologi-cal solutions is higher for sample S1, on which a depositionof HA was not observed. Such a conclusion sustains the ideathat the HA deposition acts as barrier to the release of Ti ions.

3. The quantity of Ti ions released in the SBF solution isapproximately two orders of magnitude smaller than thatreleased in the physiological serum.

4. Titania nanotube arrays formed in HF solution (S1) donot have an apatite forming ability in SBF after 15 days ofimmersion, while those formed in HF solution containing asmall amount of Na2HPO4 (S2) have this ability. Suchbehavior indicates a better bioactivity and biocompatibilityfor S2 and is correlated with lower Ti ion release, due to asmaller current density.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support

of the Romanian National CNCSIS Grant IDEI No. 1712/2008.

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Table 3. Corrosion parameters for nanotubular samples in NaCl 0.9% solution

Sample icorr (µAcm−2) Ecorr (mV vs SCE) Ipass (µAcm−2)S1 185 −1280 245S2 12 −710 84.5


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aspects relating to stability of modified passive stratum on tio2 nanostructure

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