SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 2006; 38: 182–185Published online in Wiley InterScience(www.interscience.wiley.com). DOI: 10.1002/sia.2261

Film formation and characterization of anodic oxideson titanium for biomedical applications

Christian Jaeggi,1 Philippe Kern,1∗ Johann Michler,1 Joerg Patscheider,2 Joy Tharian2 andFrans Munnik3

1 EMPA Materials Science and Technology, Feuerwerkerstrasse 39, 3602 Thun, Switzerland2 EMPA Materials Science and Technology, Ueberlandstrasse 129, 8600 Duebendorf, Switzerland3 Ion Beam Analysis Center, 8a Rue Jambe-Ducommun, 2400 Le Locle, Switzerland

Received 1 July 2005; Revised 24 November 2005; Accepted 29 November 2005

For a better understanding of the oxide growth and final film properties upon anodization of titaniumin sulfuric and phosphoric acid containing electrolytes, the electrochemical behavior as studied by a.c.impedance was correlated to microstructural analysis (TEM, Raman). Chemical depth profiling of thefilms was performed with glow discharge optical emission spectroscopy (GDOES), XPS and RBS. Thefitted capacitances and resistances from a.c. impedance measurements were greatly influenced by theongoing crystallization as well as by film porosity as a function of increasing anodization potential.GDOES revealed a small sulfur contamination (with its maximum before the oxide–metal interface) uponanodization in 1 M H2SO4 and a significant phosphorus content throughout the oxides for films grownin 1 M H3PO4, showing an accumulation at the surface. Small impurities of carbon in all films as well asan accumulation of hydrogen at/after the interface oxide–metal were also observed. Quantification of thehydrogen content by elastic recoil detection analysis (ERDA) indicated a peak concentration of 20–30 at%.Copyright 2006 John Wiley & Sons, Ltd.

KEYWORDS: anodic oxides; titanium oxide; crystallization; film morphology; electrochemical impedance spectroscopy;chemical depth profiling

INTRODUCTION

Upon anodic polarization titanium has the ability to form rel-atively thick oxide layers. Anodization is frequently used inbiomedical industry for color coding and corrosion resistanceimprovement. Depending on oxidation conditions (e.g. elec-trolyte type and concentration and applied potentials) thefilms can grow dense or porous, amorphous or crystalline.1 – 3

It has also been found by Sul4 that the bone response tooxidized implants was clearly influenced by the choice ofanodization electrolytes.

In an earlier work, we investigated the growth of anodicoxides in 1 M sulfuric acid and their suitability as maskmaterial for electrochemical micromachining of biomedicaldevices.5 In this work, surfaces anodized in sulfuric andphosphoric acid containing electrolytes, both often used inanodic oxidation of Ti-like materials, were compared withrespect to film growth and chemical composition (electrolyteimpurities) of the resulting thin oxides.

EXPERIMENTAL

Sample preparation and anodization – Commercially puretitanium discs (grade 2) with a diameter of 15 mm and a

ŁCorrespondence to: Philippe Kern, EMPA Materials Science andTechnology, Feuerwerkerstrasse 39, 3602 Thun, Switzerland.E-mail: philippe.kern@empa.ch

thickness of approximately 1 mm were used. Their prepa-ration and the anodic oxidation setup has been describedelsewhere.5 The electrolytes were prepared from 95–97%H2SO4 and 85% H3PO4 (p.a. Merck), respectively. Anodicoxidation was performed applying a sweep rate of 5 V/supto different end potentials.

Electrochemical characterization – Electrochemical impe-dance spectroscopy (EIS) measurements were performedas described earlier in Ref. 5 using an Autolab poten-tiostat/galvanostat (PGSTAT30) with frequency responseanalysis (FRA) module. The Autolab software was used fordata fitting and simulation. When using equivalent circuitsfor simulation, the complex impedance Z of a constant phaseelement (CPE) is defined as Z D 1/�CÐiÐω�n, with ω being theangular frequency, i D p

��1� and n being the ‘exponent’representing a measure of nonideality of the capacitance. Forsimplicity reasons, in this work, the values obtained for Care referred to as the capacitance.

Raman – Raman spectra were measured in backscatteringgeometry on a Renishaw Ramascope 2000 using a HeNe laser(� D 633 nm).

TEM – TEM was performed on Philips CM-300 and EM-430 microscopes at 300 keV.

Chemical depth profiling – Glow discharge optical emissionspectroscopy (GDOES) (Jobin-Yvon 5000 RF) was used forthe estimation of film thickness and chemical depth profiling(for more details see Ref. 5). Rutherford backscattering

Copyright 2006 John Wiley & Sons, Ltd.

Formation and characterization of anodic oxides on titanium 183

spectroscopy (RBS) and elastic recoil detection analysis(ERDA) spectra were obtained by irradiation with a 2 MeVHe-ion beam on a Van de Graaff accelerator.

RESULTS AND DISCUSSION

Anodization and oxide thicknessLinear sweep anodic oxidation in 1 M H3PO4 was similarlywell reproducible in terms of current–voltage behavior andresulting interference color as in 1 M H2SO4. In phosphoricacid, however, sparking at the surface due to dielectricbreakdown of the film was not observed until 150 V, asopposed to ¾90–100 V in sulfuric acid.

The thickness of a series of sulfuric acid oxides wasmeasured by ellipsometry and was found to be in goodagreement with the thickness estimation obtained fromGDOES (¾2 nm/V) up to 80 V as reported in Ref. 5. Fora given potential, phosphoric acid oxides were generallyslightly thinner than oxides grown in sulfuric acid. Asteeper increase in growth rate was observed after ¾150 V,corresponding to the sparking potential in 1 M H3PO4.

MicrostructureRaman investigations of both sulfuric and phosphoric acidoxides revealed that already at 10 V, a small amount ofcrystallinity is present, evidenced by the appearance of themost prominent peak of the anatase phase at ¾144 cm�1.

For a given anodization potential, sulfuric acid oxidesshow a clearer crystalline signal than phosphoric acid oxides.For example, an oxide grown to 100 V in H3PO4 showsalmost the same Raman signal as an oxide grown to 50 Vin H2SO4.

TEM observations revealed that despite a very uniforminterference color both sulfuric and phosphoric acid oxidesexhibit porosity already at 20 V. Figure 1 demonstrates amultilayered nature of these TiO2 thin films for an oxidegrown in 1 M H2SO4, exhibiting an inner dense layer, acentral porous part and an outer dense layer. Also wellvisible is the presence of nanocrystalline regions around theporous zone, supporting the Raman observations. Habazakiet al. reported that the growth of pores was associated withthe formation of nanocrystallites.6

Nanocrystalline regions have also been observed byannular dark-field TEM images in both sulfuric and phospho-ric 20 V acid oxides (not shown). Interestingly, the crystalliza-tion was found to start in different regions. While crystallineregions in the 20 V sulfuric acid oxide were observed aroundthe porous central part and in the outermost oxide layer, the20 V phosphoric acid oxide showed some nanocrystallinityexclusively near the interface oxide–metal. The amount ofnanocrystals observed in the thin films clearly supportsthe idea of a slower crystallization rate in phosphoric acidelectrolyte.

AC impedanceBode plots of a series of phosphoric acid oxides are shown inFig. 2. The total film resistance is decreasing with increasinganodization potentials from 10 up to 100 V. A 10 V oxide,like native titanium oxide, showed one time constant as a

Figure 1. TEM bright-field image of a 20 V sulfuric acid oxide.The insert shows an overview of its layered structure,exhibiting a porous middle region surrounded by dense parts.Crystallization is mainly observed in the porous part and theouter dense part; the inner dense region appears mainlyamorphous.

Figure 2. Bode representation of the EIS spectra of a series ofanodic oxides grown in 1 M H3PO4: 10 V (�), 20 V (�), 100 V(°) and 150 V (ð). Impedance values are connected by adashed line and phase values by a solid line. Data points at thelowest frequencies are excluded from the fitting.

very broad phase representing the typical ‘constant phaseelement’ behavior. At 20 V, a second time constant appears,corresponding to a second capacitance, as seen by a secondpeak in the recorded phase. This second time constant isshifted to higher frequencies with increased anodizationpotential up to the sparking potential (¾150 V).

The appearance of a second capacitance is explainedby the formation of pores, and its growing dominance isinterpreted as an increased presence of porosity within theoxide. The EIS behavior above the sparking potential is

Copyright 2006 John Wiley & Sons, Ltd. Surf. Interface Anal. 2006; 38: 182–185DOI: 10.1002/sia

184 C. Jaeggi et al.

mainly influenced by the appearance of open porosity and isnot studied in more detail in the present paper.

The development of EIS spectra of 1 M H2SO4 oxideshas been discussed in Ref. 5. The general behavior is similarin both electrolytes up to ¾80 V. However, the two timeconstants are more clearly separated in phosphoric acidoxides, starting from 20 V.

On the basis of TEM imaging and impedance data, aserial (two) layer model5 consisting of two parallel (R-CPE)elements combined in series was chosen for data fitting,clearly simplifying the highly complex nature of theseoxides. In the equivalent circuit RE-�CPEP-RP�-�CPED-RD�used, RE, RP and RD are the resistances of the electrolyte(E), the porous part (P) and the dense part (D) of the oxide,respectively. CPEP and CPED are the constant phase elementsof the porous and the dense part, respectively. Figure 3shows the evolution of the fitted values for RD and CP in bothinvestigated electrolytes (CP values are normalized with thecapacitance value of the respective 20 V oxide �2.53 µF forH3PO4 oxides and 0.804 µF for H2SO4 oxides and RD valuesare normalized with the resistance found for the native oxidein the respective electrolyte �7.7 M� for H3PO4 oxides and496 k� for H2SO4 oxides).

The resistance RD of the dense part shows a similarbehavior after 10 V for both electrolytes. It rapidly decreasesup to ¾20–30 V, followed by a much slower decreasewith increasing potential for H3PO4 and stays almostconstant for H2SO4. The strong decrease of RD despitethe increasing film thickness can be explained by ongoingcrystallization, which is reported to produce a path of higherelectrical conductivity.2 The initial increase of RD at 10 V inH3PO4, indicating the dominance of the film growth overcrystallization, again points toward a later crystallizationcompared to anodization in H2SO4. The explanation for thecapacitance (CD) behavior is similar. The initial capacitancedrop observed is due to the dominating increase in filmthickness following the parallel plate capacitor equation

C D εÐε0ÐA/d �1�

Figure 3. (a) Capacitance (CP) values of the porous part andresistance (RD) values of the dense part of a series of anodicoxides grown in sulfuric acid (HS, circles) and those grown inphosphoric acid (HP, squares) obtained from fitting to theequivalent circuit described in the text. The error bars shownwere obtained as uncertainty from data fitting.

where C is the capacitance, ε is the dielectric constant of thematerial, ε0 is the permittivity of vacuum, A is the surfacearea and d is the layer thickness. The clear change in slopeat 10 V is due to the influence of a change in the dielectricconstant of the film as crystallization leads to an increase inε (εamorphous ¾D 4–40, εanatase ¾D 48).7,8

Also supporting the slower crystallization in phosphoricacid is the capacitance behavior CP of the porous layer partFig. 3. Its initial, strong decrease (due to increasing filmthickness) is stopped earlier in 1 M H3PO4 oxides than in 1 M

H2SO4 oxides with increasing anodization potential.Similar to the results shown in Ref. 5 for oxides grown in

1 M H2SO4, the resistance RP of the porous oxide part of oxidesgrown in 1 M H3PO4 corresponds well to measured pittingpotentials, generally increasing with higher anodizationpotential.

Chemical depth profilingXPS revealed a constant Ti : O ratio of 1 : 2 through the oxides,underlining the TiO2 stoichiometry.

Besides showing a constant Ti : O ratio, GDOES detects abroad sulfur contamination peak in the sulfuric acid oxides5

located before the oxide–metal interface and decreasingtoward the surface. A maximum concentration of ¾2.5 at%for an oxide anodized to 80 V was found. The oxidesgrown in the H3PO4 electrolyte all exhibited a phospho-rus peak at the outer surface, coinciding with an increaseof O-signal, pointing toward adsorbed phosphate groups.From the surface, P rapidly decreased to a plateau valueof ¾5 at%, as shown in Fig. 4. The surface P-peaks becamemore pronounced with increasing oxide thicknesses, pos-sibly due to increased surface roughness. Also a carboncontamination was found, decreasing after a surface concen-tration of ¾5 at% to a constant content of ¾1–3 at% in alloxides.

Ion beam analysis (IBA) yields a profile of concentrationsagainst the areal density (in at/cm3). The sample densityis needed to convert the areal density to a thickness innanometers. The thickness of 105 nm obtained for an 80 Voxide grown in 1 M H2SO4, Fig. 5, using the density of

Figure 4. GDOES depth profile of an oxide grown in 1 M H3PO4

to 80 V. The phosphorus (P) and carbon (C) concentrationswere multiplied by 3 for better visibility. The qualitativehydrogen signal (dashed line) is included.

Copyright 2006 John Wiley & Sons, Ltd. Surf. Interface Anal. 2006; 38: 182–185DOI: 10.1002/sia

Formation and characterization of anodic oxides on titanium 185

Figure 5. RBS/ERDA depth profile of an oxide grown in 1 M

H2SO4 to 80 V.

anatase (3.9 g/cm3) is smaller compared to the thicknessdetermined by ellipsometry (¾160–170 nm). This is due tothe porous nature of the oxide, changing the average density.The amount of porosity can be quantified by comparing thetwo thicknesses, yielding a maximum value of 34–38%. Thisvalue seems overestimated as a result of the film being notentirely crystalline yet (as found by TEM analyses) and thedensity of amorphous TiO2 not being known.

The hydrogen peak seen qualitatively by GDOES wasverified by IBA. Quantification yielded a maximum con-centration of ¾20–30 at%. The observed accumulation ofhydrogen at the interface metal–oxide needs further investi-gation with respect to its origin, chemical state and its effecton the hydrogen embrittlement sensitive Ti materials usedin biomedical applications.9

CONCLUSIONS

Crystallization of anodic oxides on Ti in both sulfuricand phosphoric acid containing electrolytes was found tostart at potentials as low as 10 V (observed with Raman),strongly influencing the fitted capacitance and resistancevalues obtained from a.c. impedance data. The presence ofa porous layer was observed at potentials as low as 20 V byTEM and EIS. Further crystallization was found to progressslower in H3PO4 than in H2SO4, in agreement with a shift ofthe sparking potential toward higher anodization potentials.

Chemical depth profiling by XPS, GDOES and RBS/ERDA revealed stoichiometric titania containing small S- andP-contaminations that clearly originate from the anodizationelectrolytes. The presence of hydrogen accumulated in thesubstrate close to the interface metal–oxide was indicated byGDOES as well as ERDA and is currently under investigation.

AcknowledgementsSpecial thanks go to M. Aeberhard (EMPA) for helping with GDOESmeasurements and M. Parlinska from CIME/EPFL, Lausanne, forTEM imaging.

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Copyright 2006 John Wiley & Sons, Ltd. Surf. Interface Anal. 2006; 38: 182–185DOI: 10.1002/sia