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Ellipsometric and Raman Spectroscopic Study of Thermally Formed Films on Titanium E. Hristova University "Cyril and Methodius," Skopje, Macedonia

Lj. Arsov University "Cyril and Methodius," Skopje, Macedonia

Branko N. Popov University of South Carolina - Columbia, [email protected]

Ralph E. White University of South Carolina - Columbia, [email protected]

Follow this and additional works at: http://scholarcommons.sc.edu/eche_facpub Part of the Chemical Engineering Commons Publication Info Journal of the Electrochemical Society, 1997, pages 2318-2323. © The Electrochemical Society, Inc. 1997. All rights reserved. Except as provided under U.S. copyright law, this work may not be reproduced, resold, distributed, or modified without the express permission of The Electrochemical Society (ECS). The archival version of this work was published in the Journal of the Electrochemical Society. http://www.electrochem.org/ Publisher's link: http://dx.doi.org/10.1149/1.1837811 DOI: 10.1149/1.1837811

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Raman Spectroscopic Spectroscopic Study Ellipsometric and Ellipsometric and Raman Study of of Thermally Thermally Formed Films Formed Films on Titanium Hristova and and Lj. Lj. Arsov Arsov E. Hristova Methodius," Skopje Skopje 91000, 91000, Macedonia and Methodius," of Technology Technology and Faculty Faculty of and Metallurgy, University "Cyril and Macedonia

B. N. Popov* R.E. E.White* White* B. Popov* and R. 29208, USA USA South Carolina Carolina29208, of South South Carolina, Carolina,Columbia, Columbia, South Universityof of Chemical ChemicalEngineering, Engineering,University Departmentof Department ABSTRACT ABSTRACT atmospheric pressure. pressure. The Thermal formed by by heating beating titanium titanium samples samples in in air at atmospheric The were formed films on titanium surfaces surfaces were Thermal films optical constants, temperatures and and times of heating heating were were invesinvesfilms at various various temperatures of the the formed formed films thickness, and structure of optical constants, thickness, The complex complexindex index of ofrefraction refraction and and the the thickness thickness of of generated generated films films and Raman Raman spectroscopy. spectroscopy. The tigated by by ellipsometry ellipsometry and were determined by comparing experimental loci A and and 'IY 'I' obtained obtained by by ellipsometric ellipsometric measurements measurementswith withtheoretical theoretical comparing the experimental were computed A A vs. vs. I4' curves. curves. ItIt was and porosity porosity of of formed formed films films increase increasewith with inhomogeneity and the thickness thickness inhomogeneity was found found that the computed increasing temperature and the duration of the thermal treatment. treatment. Beyond Beyond aa certain certain critical critical temperature, temperature,the the appearance appearance increasing transformed from amorphous of some some Raman Raman bands bands and and changes changes in in their their intensities intensities indicated indicated that the film transformed amorphous to to microcrysmicrocryscrystalline structure. talline and crystalline

Introduction titanium and its intensively have been been intensively its oxides oxides have Metallic Metallic titanium studied the last as aa result result of of their their two decades decades as last two during the studied during This of industry. industry. This wide branches of in various various branches applications in wide applications spontaresistance due metal has aa high due to to the spontahigh corrosion corrosion resistance neous formation of of aa thin thin natural mm film on on its surface.t surface.' This This room temformed in atmospheric atmospheric pressure film can pressure at room temcan be formed perature. of the the films artificially be artificially films can be The thickness thickness of perature. The increased by either anodically oxidizing the the substrate substrate at at anodically oxidizing increased by potentials2 steady-state potentials2 more positive positive than the steady-state potentials more temperatures.3 Ion or at high high temperatures.3 Ion oxidation at or by by thermal thermal oxidation implantation films causes causesstructural structural amoramoroxide films implantation in these oxide influence phization and defects which influence lattice defects high energy energy lattice and high the electrode capacity, the the rate rate of transfer reacreacof electron electron transfer electrode capacity, 4 film. tions, and of the the film.4 properties of optical properties and the optical of anodically anodically and The optical properties properties and structure and structure of thermally formed oxide films are are of importance importance in in underunderphotoelectrochemiand photoelectrochemistability and corrosion stability standing standing the corrosion cal activity and their their applications applications in in oxides and of titanium oxides activity of solar energy and electrodes electrodes for for electrosyntheelectrosyntheenergy conversion conversion and properties sis.5 In recent recent years, years, the the structure structure and optical optical properties sis.5 In of the anodic studied extensively studied have been been extensively oxide films films have anodic oxide by Raman Onthe the other other and ellipsometry.°'7 ellipsometry.7 On spectroscopy and Raman spectroscopy the thermally thermally exist on on the fragmented data only fragmented hand, hand, only data exist and spectroscopy and Raman spectroscopy formed utilizing Raman oxide films films utilizing formed oxide ellipsometry."' ellipsometry.3'9 employ xMost of the numerous studies employ experimental studies numerous experimental Most of investigate the ray and electron techniques to investigate the diffraction techniques electron diffraction crystalline et al.3 al. Douglass et films. Douglass of the the thermal thermal films. crystalline structure of have the existence of titanium titanium dioxide with rutile rutile dioxide with existence of have shown shown the structure, havedetermined determinedthe thestrucstrucal." have while David et al.1° structure, while ture to These disdismodification. These and rutile rutile modification. to be both anatase and crepancies arise from from the the surface and the the irregularities and surface irregularities crepancies arise nonuniformity of the the film, which is is more in prominent in more prominent film, which nonuniformity of one thermally formed temperatures. This This is is one films at high temperatures. formed films studies ellipsometric studies more ellipsometric reasons why why more of the primary reasons the primary have formed oxides oxides than anodically formed performed on anodically have been been performed thermally formed ones. ones. rutile structure structure only rutile Kozlowski et al.t1 showedthat that only al." showed Kozlowski et exists for thin oxide vs.aa saturated saturated films formed formed at 2.5 2.5 V V vs. oxide films calomel Since they they have have etched etched chemicalchemical(SCE). Since calomel electrode (SCE). the the electrochemically electrochemically polished ly the polished surface surface prior prior to to the anodic film growth, growth, they they assumed assumedthat that the the surface surface etching etching anodic film electropolishing formed during during electropolishing removes the oxide oxide film film formed removes the which the structure the structure of the influences the strongly influences further strongly which further which anodic during electropolishing electropolishing which formed during oxide film film formed anodic oxide further influences the the structure of the anodic anodic structure of strongly influences further strongly oxide film. Their Their conclusion that the the substrate surface substrate surface conclusion that oxide film. transform can completely completely transform preparation oxidation can before oxidation preparation before * Electrochemical Member. Active Member Society Active Electrochemical Society *

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oxide the structure of the anodically formed formed oxide the anodically the crystalline crystalline structure using chemical because by film is questionable questionable because by using chemical etching etching itit was not possible possibletoto obtain obtainaa substrate substratewithout withoutaanaturally naturally was not formed oxide film. formed film. The objectives of this work were were by by using using Raman specRaman specobjectives of troscopy and ellipsometry to study various structural structural study various ellipsometry to troscopy and surfaces transitions during of titanium surfaces during thermal oxidation of crysproperties and and to optical properties and the the crystheir optical to determine determine their talline structure. structure.

Experimental Experimental (99.7% pure) Ti samples samples (99.7% Sample preparation.-The preparation—The Ti pure) were were machined flat in in the diameter of with aa diameter form of of disks disks with the form machined flat 20 mm and a height of mm. One One base base of of each each disk diskwas was 10 mm. of 10 abraded with 600 carbide paper paper lubricated lubricated silicon carbide 600 grade grade silicon with distilled washed disks were were washed use, the disks water. Prior to use, distilled water. ultrasonically in alcohol alcohol and and electropolished electropolishedtotoaa mirror mirror ultrasonically in brightness 12. Ref. 12. suggested in Ref. procedure suggested to the the procedure according to brightness according oxide electropolishing, an procedure.-After electropolishing, Heating procedure—After an oxide surface. film begins begins to to form form spontaneously spontaneously on on the the titanium titanium surface. Due was not not reasonreasoncontamination, itit was high rate of contamination, Due to the high able On the the below 100°C. 100°C. On of titanium titanium below oxidation of able to study the oxidation because other hand, 600°C was chosen chosen as as an an upper limit limit because 600°C was when the samples samples are are oxidized oxidizedatatthis thistemperature, temperature,the thedifdiffusion of oxygen oxygen into into the the bulk bulk of of the the substrate substrate becomes becomes the the fusion of rate limiting process. Due to to the the mass mass transfer transfer limitations limitations process. Due bemorphology bethe surface surface morphology present this temperature, temperature, the present at this comes very inhomogeneous. Thisnonuniform nonuniformsurface surface crecreinhomogeneous. This comes very ates problems bemeasurements beellipsometric measurements the ellipsometric with the problems with from cause the diffracted incident light light from of the incident reflectance of diffracted reflectance the Ti more pronounced pronouncedininrelation relation with with surface becomes becomes more Ti surface the specular measurethe ellipsometric ellipsometric measureThis causes causes the one. This specular one. and accuracy. accuracy. in sensitivity sensitivity and ments to decrease decrease in begin to ments to begin measurements ellipsometric measurements Before oxidation, ellipsometric thermal oxidation, Before thermal were each sample sample in in order order to to determine determine the the were carried out for each treatment. Then, quality surface treatment. Then, the the preliminary surface of the the preliminary quality of sample was placed placed inside inside aa tubular tubular oven. The temperature, temperature, oven. The sample was measured thermocouple, was was stabilized stabilized automatically automatically measured by a thermocouple, samples were were heated heated for The samples within an accuracy of + 1°C. The accuracy of various times ranging from 30 30 s to 8 h. specimen was completed, the specimen After the oxidation was was completed, oxidation was removed from the the oven oven and and cooled cooled to to room room temperature temperature in in removed from spectroscopic a desiccator. Next, ellipsometric ellipsometric and Raman spectroscopic desiccator. Next, Measurements for the samples. samples. Measurements tests for were performed performed on the tests were various sameach temperature, using various time and temperature, oxidation time each oxidation ples, were repeated repeated at at least least three three times times and the the values for values for ples, were further computations were values of of the the single single average values were the average measurements. After the the measurements, the oxide film oxide film measurements, the measurements. After was removed by mechanical mechanical means means and and repolished repolished in in order order removed by to reuse the sample the next oxidation. oxidation. sample for the

Inc. Society,Inc. ElectrochemicalSociety, © The 1997The No. 7, 7, July July1997 144, No. Soc., Vol. Vol. 144, J. Electrochem. Soc., Electrochemical J.

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1997The © The ElectrochemicalSociety, Society,Inc. Inc. J. Electrochemical J. Electrochem. Soc., Soc., Vol. Vol. 144, 144, No.7, No. 7, July July1997 Apparatus.-The Apparatus.—The ellipsometric ellipsometric analyses analyses were were executed executed with aa Rudolph Rudolph Research Research type type 43603-200 43603-200 thin film ellipsometer at aa wavelength of 546.1 nm and and an an incident incident angle angle someter 546.1 nm of 70°. 700. The Raman spectra were R-800T were recorded recorded by by an an ASCO ASCO R-800T (triple-type Thelaser laser light light for for Raman Raman (triple-type monochromator). monochromator). The excitation was 514.5 nmradiation radiation emitted emitted by by an an argon argon ion ion 514.5 nm laser with an The samples samples an incident incident power power of of 10 10 to to 15 15 mW. mW. The were were illuminated illuminated through through the objective objective of of aa microscope microscope which also served served to to collect collect the the Raman light. Since Since the the which also Raman light. laser beam only increased the the temperature temperature of of the the point point of of only increased illumination less thermal degradation degradation of the film film of the less than 5°C, 5°C, thermal was not a problem. problem.

Results and Discussion Discussion Ellipsometry.-Ellipsometry one of of the the most most conveconveEllipsometry.—Ellipsometry is one

nient and of the the and accurate techniques techniques for the measurement measurement of thickness and refractive refractive index index of of thin thin films films on on solid solid sursurthickness and faces. In ellipsometric measurements, the the oxide-free metal ellipsometric measurements, oxide-free metal surface and the refractive index of of surface the estimation estimation of of complex complex refractive the bare most importance. importance. An Anincorincorbare metal metal surface surface is of the most

rect determination of the the optical constants of of the the bare bare determination of optical constants

metal surface of error error in ellipsometric measone source source of ellipsometric meassurface is one urements. In this work, we have have taken taken the the refractive refractive index index for the oxide-free Ti surface from from our our previous previous ellipsometellipsomet-

nc of electropolished electropolished and and cathodically cathodically ric measurements measurements of polarized polarized Ti Ti surfaces surfaces at at—0.6 -0.6 V vs. vs. SCE SCE in 0.5 0.5 M M H,S04. H2SO4. 66

At this potential reduction of ofnatural natural potential the electrochemical electrochemical reduction oxide film uniformly across across the the electrode surelectrode suroxide film occurred occurred uniformly face. The computed computed values values of of the the complex complex refractive refractive index index valof free metal metal surface surfacewerehs wereh = 2.94 2.94 (1 (1 — - 1.217i). 1.217i). These valof free ues are are very very close close to tothe the literature literature data data for for Ti prepared by by ues Ti prepared evaporation in aa vacuum evaporation vacuum and measured measured in high vacuum.'3 vacuum. 3 By reducing reducing the the oxide oxide cathodically, cathodically,one onecannot cannot be be certain certain By

that natural oxide oxide film film isiscompletely completely reduced. reduced. that the the natural According to literature, literature, the the cathodic reduction isis incaincacathodic reduction According to oxide-free Ti Ti surface pable of producing producing an oxide-free surface because because of of the formation of of aa surface surface hydride hydride layer.2 layer. 2 The The preliminary preliminary studies studies carried carried out out to to determine determine the the influence of gas gas pressure pressure on on titanium titanium oxidation reaction influence of oxidation reaction rate in in the the temperature temperature range range between between 100 100 and and 600°C 600°C rate showed that showed that pressure pressure has has very very little little effect effect on on the the oxide oxide film indicating that the reaction film growth, growth, indicating reaction between between oxygen oxygen and Ti and Ti is not not occurring occurring on the the base metal. metal. Instead, Instead, the oxygen rate-determining step is is probably probably the the diffusion diffusion of of oxygen rate-determining atoms through aa thin atoms thin oxide oxide film. film. Below Below 400°C, 400°C, the the titanititanireaction rate (logarithmic (logarithmic growth growth rate) rate) um oxidation um oxidation reaction shows noticeable variation variation with with oxygen oxygen pressure pressure shows slightly slightly noticeable range of of 11to to 10 10 mm mm Hg. Hg. At At 500°C 500°C and up to 600°C, 600°C, in the range the titanium titanium oxidation oxidation reaction reaction follows follows the parabolic parabolic the growth rate and is growth is independent independent of of pressure in the range of 11 to to 760 760 mm mm Hg. Hg. These Theseresults resultsare arein in agreement agreement with with those those obtained by and Hurlen.' Hurlen.'66 by Kofstadt Kofstadtetetal.2413 al. 4' 5 and shows the variations variations of ellipsometric ellipsometric parameFigure 11 shows parameP during ters A and and 'P during the the thermal thermal oxidation oxidation of of Ti Ticarried carriedout out at lowest investigated temperature of of 100°C. 100°C. By By fitting fitting the the experimentally obtained relative phase phase experimentally obtainedpoints points ofof the the relative 'P prechange A A and change and the the relative relative amplitude amplitude attenuation attenuation 'P sented in Fig. Fig. 11 with the the theoretically theoretically calculated calculated A vs. t sented vs. 'P curves, it was curves, was possible possible to to determine determine accurately accurately the the thickthick— ness d and ness and complex complex refractive refractive index index [n2 [2i, = = n n(l 2 (l - ik2 )], where a2 where n2 and and k2 k2 are are the the relative relative index index (real (real part) part) and and the of the absorption index (imaginary (imaginary part) part) of the thermally thermally formed formed films, respectively. curves were were films, respectively.The The theoretical theoretical A A7vs. vs. P'P curves computed utilizing computed utilizing Fresnel's Fresnel's equations equations" tg'P tgP .. exp A) = = exp (i A)

k

. d(ñ. d(K2~ — - i

p,'

sin' sin2 p,)1/2

29.5 29.5

9o n 29.0 111 111

112

113 113

[2]-'

114 114

116 116

115

A (cieg) (deg) Fig. 1. Dependence of ellipsometric during Fig. 1. Dependence of ellipsometric parameters parameters A A and and T 'P during the denote experimentally oxidation ofofTiTi at at 100°C. 100°C. Cycles Cycles denote experimentally the thermal oxidation measured A A and 'P' as aa function function of of time. time. The The solid solidline linedenotes denotes the the measured best theoretical fit fit to the experimental experimental data computed computed for aa growgrowbest theoretical ing film. of 00 nm nm corresponds corresponds to to the the value value of film. AAFilm film thickness thickness of of A A and 'P for aa bare 'P for bare metal metal substrate. substrate.

for fixed of the refractive fixed values values of refractive index index of surrounding surrounding while the the surrounding media Ail ñ, = 1, For the surrounding media media f1. For media il.2. 1, while value refractive index index of of metal metal substrate substrate value of the complex complex refractive previous work. work. - 1.217 1.217 .·i) i, == 2.94 2.94 (1 (1 — i) was taken from our previous ii, amplitude attenuation, Eq. 1, represents relative relative amplitude In Eq. 1, tg1 tgt represents attenuation, A A Fresnel relative phase phase change, change, rr6 and is relative and r, represent represent the the Fresnel reflections coefficients reflections coefficients for for light light polarized polarizedparallel parallel and and perperrespectively, while pendicular to the the plane plane of ofincidence, incidence, respectively, pendicular while the subscripts subscripts 1, 1, 2, 2, and 33 correspond correspond to to medium, medium, film, film, and and metal substrate, substrate, respectively. respectively. The The complex complex refractive refractive metal = a, = a, indexes ii,l = n1 (1 — - k, .· i), ni n2 (1 — - k, k .· i), and113 n3 = n3 indexes i), il2 1), and a3 (1 (1 — - k3i) correspond to medium, medium, film, film, and andmetal metal substrate, substrate, k,i) correspond respectively, n,a, and k2 and respectively, andk,k, n2, a,, and k,, and n3 a, and andk3k, are are relative relative indexes and extinction indexes of medium, medium, film, film, and and metal metal substrate. of phase of the substrate. In Eq. Eq. 2, 2, S represents represents change change of phase of beam the film, film, dd isis the the film film thickness, thickness, kk is is wavewavebeam crossing crossing the length, while p, p, is the angle of incidence. incidence. Figure the variations shows the variations of of the the ellipsometrically ellipsometrically Figure 2 shows r measured parameters A A and and I'P during the the thermal thermal oxidaoxidameasured parameters during tion of Ti Ti samples samples for for the the highest highest investigated investigated temperature temperature of The refractive refractive index, 112a, andfilm filmthickness, thickness,d, d, of 600°C. 600C. The 4in, and of the formed formed films films could could be be determined determined in in the the same same manmanner as byfitting fitting the the experimentally experimentally determined determined as for for 100°C 100°C by values of of A and and ·'P with with theoretical theoretical computed computed curves. curves. The The fitting procedure procedure was performed by specially prepared fitting was performed by aa specially prepared

I

60 60

I

I

40 40 a' ", I 4,

0

oH 141 run

20 20

00

I

0,-ni,,

85mm, 8nmin,

180 180

,

270 270 A

exp (-2ib) (—2i3)

1 2i5) 1+ + r,,, rl,2, · r2, exp (-2i8) r,3 (— 3,,exp [11 [1] r,, ++ r2,3,,· rl,2,, exp(-2i) (—2i5) ?-,,,- exp

== 2

-u -o

90 90

rl,2,p + + r23(—2iS) 2,3,p(-2i8) r,,

1+ + ;,, 12r,,,, 2,,,

30.0 a,

a) ,

360/0 360/0

90 90

180

(deg) (ae9)

Fig. 2. 2. Dependence Dependence of ellipsometric parameters 'P during during Fig. of ellipsometric parameters A and and 'P oxidation of of TiTi at at 600°C. 600°C. Cycles Cycles denote the the thermal thermal oxidation denote experimentally experimentally measured A A and 'P' as as aa function function of of time. time. The The solid solid line linedenotes denotes the the measured best theoretical theoretical fit the experimental experimental data computed computed for for aagrowgrowbest fit to the ing ing him film (thickness (thickness increments increments are are indicated indicated in in the the curve). curve). A film film thickness of of 00 nm correspondsto to the the value value of of A and and 'P thickness nm corresponds P for for aa bare bare metal subsfrate. substrate. metal


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J. Soc., Vol. Vol. 144, 144, No. No. 7, 7, July July1997 1997The © The ElectrochemicalSociety, Society, Inc. Inc. J. Electrochem. Soc., Electrochemical

was determined program in in which wl rich n2 i2 was determined by given given computer program values of the values the film film thickness thick less in in an anincreasing increasing direction.17 direction." During the computation conly one theoretical theoretical function, function, TT == )nly one from family of of theoretical theoretical funcfuncom the family f (AT, PT) was searched searched fro 'P,.) was

ftions (, which had had minimal minim, al distance tions which distance to to the the experimental experimental function, E = ff(AE, function, E). W then the experimentally experimentally measured measured 'YE). When (, T and for minimal distance by by minimal distance AT and'YE E were were superimposed superin posed for computed poin tsofoftheoretical theoretical curve, curve, the computed AE andand 'PTAPT points the fitting fitting

procedure was over. Duri procedure was over. During the fitting fitting procedure, the corcorng the procedure, the rect values searched pa parameters were possible possible to to obtain obtain values of searched irameters were only if the only if the number number of of theoretical t]heoretical points points M, M, were were many many the experimentally expeerimentally measured times larger than the In measured values values N. N. In this the number theoretical points points were were at at least least this study, study, the number of off theoretical thos;e measured ten times larger than those measured experimentally. experimentally. Kinetic analyses Kinetic analyseswere were also also carried carried out out to to verify verify the the influence of influence of the temperat temperature film ure on the kinetic law law of the film thickness show the film film thickness thickness thickness growth. Figures Figures s 3 and 4 show growth determined determined ellipo )smetrically during growth elliposmetrically during thermal thermal oxioxidation at temperatures temperatures ranging ranging from from 100 100 to to 300°C 300°C and and dation from 400 respectively. Thefinal final time time of of the the specspec400 to 600°C, 600°C, respec tively. The imen thermal imen thermal heating heating at at a given given temperature temperature was was deterdetermined by the the noticeable variations of the position position of of ellipellipnoticeable v variations of sometric parameters parameters A and 'I'P between between two two successive successive sometric measurements. experimentally measmeasmeasurements. Next, Next, by by fitting the experimentally ured zAvs. each tempertempervs. 'P P curves curves with theoretical theoretical ones ones for each ature, of film film thickness thickness growth growth was was ature, the kinetic kinetic rate rate law la w of it the experimental experimental kinetic kinetic data, determined. In determined. In order order to to ffit a computer of computer program program was was prepared prepared which which consists consists of 6 laws. presently known presently known kinetic laws.'6

the first At was yellow. yellow. At first observable observable interference interference color color was

longer oxidation times times and and at medium temperatures, the the longer oxidation medium temperatures, oxidation anisotropy occurs occurs which which causes causes the the grains grains to oxidation anisotropy turn orange, orange, dark dark brown, brown, and and finally finally purple. purple. At At higher higher turn temperatures growth temperatures there there is a permanent permanent film thickness thickness growth which time consists of grains grains with with for longer longer oxidation oxidation time consists of which for different color morphology thickness. morphology and thickness. It was the film film growth growth can can be be was found found that that below below 375°C, 375°C, the described with the the logarithmic rate law. At intermediate described with logarithmic rate law. At intermediate temperatures the film film thickness thickness does doesnot notfollow followany any temperatures (375°C) (375°C) the of the curve curve shape shape of the known known kinetic kinetic equations. equations. At 400°C, 400°C, the of the film thickness growth growth corresponds corresponds to to the the cubic cubic rate. rate. film thickness The data obtained were straight straight The experimental experimental data obtained at 425°C 425°C were lines plot, and it was was difficult difficult to to determine if the lines on a cubic plot, oxidation oxidation was was cubic cubic or parabolic. parabolic. For For temperatures temperatures from from 450 to 600°C, 600°C,the theoxidation oxidationrate rate can can be be best best described described as as a 450 to parabolic increase parabolic increase in in thickness. thickness. Theoretically, Theoretically, aa parabolic rate rate law law occurs occurs after aa logarithmic logarithmic except in in the case case when when the diffusion of of the the ions ions controlled by diffusion the oxide oxide film growth is controlled through deviation from fromthe theparabolic parabolic rate rate law law through the the film. film. The The deviation was by the appearance was accompanied accompanied by appearance of of white white blisters. blisters. Curve (on d vs. Curve 3 in Fig. 4, 4, obtained obtained at at 600°C, 600°C, showed showed (on vs. t plot) plot) that after after the the initial initial parabolic parabolic rate, rate, the film growth growth rate changes changes to almost almost a linear rate. According According to Kofstad Kofstad et al.,15 theactivation activationenergy energyfor for diffusion diffusionofoftitanium titanium ions ions al.,1 5 the (in rutile), from from the the metal metal surface surface toward toward the the film, film, isis highhigh(in rutile), er than through the the oxide than the the diffusion diffusion of the oxygen oxygen ions through oxide

tures. tures. A A mechanism mechanism based based on on the the diffusion diffusion of of oxygen oxygen explains the explains the linear oxidation. oxidation. The The same same transformation transformation 9 '20 was was also also observed observed for for other othermetals.19"° metals.

10 10

C

Fig. off. For For aa film film thickness thickness of 15 to 20 20 nm, nm, 15 to Fig. 3 tend to level off.

metal surface. surface. Accordingly, AccordinlIv the nrefilm to the metal the latter latter will will predominate at lower temperatures, while diffusion of of titanititanidominate lower temperatures, while diffusion um ions will control control the the oxide oxide growth growth at at higher higher temperatemperaum ions will

12 12

EE

As shown As 100 shown in in Fig. Fig. 3,3, initially initially at temperature temperature range of 100

to 300°C rate of was 300°C aa high high rate of the the film film thickness thickness growth growth was observed. With With increasing increasingexposure exposuretime, time,the the curves curves inin observed.

8

'1

6

44 0.5 0.5

1.5

1 1

2.5

2

(mn) log log t (in)

th determined determined ellipsometrically ellipsometrically during during growth Fig. 3. 3. Film Film thickness thickness grow Fig. and (3) (3)300°C. 3000C. Ti heating at: (1) (1)100, 100, (2) (2)200, 20(0,and Ti heating

150 1 50

________________________________________ I

3/ 3

1 00 00

/

2~~~2

?EC c0

According to our microscopic studies, the the oxide According to microscopic studies, oxide film film formed shows epitaxial epitaxial growth. growth. The The film film is is comcomformed at 100°C 100°C shows pact, uniform, uniform, and it it replicates replicates the substrate substrate grains. grains. At temperatures and 600°C, the growth growth of of the the temperatures between between 500 500 and 600°C, the oxide grains is is rapid; rapid; the the grains grains are are large and and the film film conconoxide grains tinues to show show epitaxial epitaxial growth. growth. Oxidation Oxidation leads leads to to aa gengeneral of the the oxide eral thickening thickening of oxide film film and and whiskers whiskers begin begin to form on the the top of this this film. At these these temperatures, temperatures, oxide oxide form on film. At twinning and deformation in the the substrate twinning and deformation in substrate are are likely likely to occur. The computed refractive refractive indexes indexes and and the the corresponding corresponding The computed kinetic laws of of thermally formed titanium titanium oxide oxide kinetic growth growth laws thermally formed presented in Table Table I for for various various temperatures. temperatures. films are presented According to literature, literature, the the oxide growth up up to to 250°C According to oxide film film growth 250°C follows follows logarithmic logarithmic increase increase in in thickness. thickness. Above Above 250°C, 250°C, the oxide the oxide film film growth growth is is parabolic. parabolic. However, However, at at higher higher temperatures, the rate rate21may deviate from the initial temperatures, the may deviate from the initial parabola, Thereisisa adiscrepancy discrepancyininthe the parabola, becoming becominglinear.21'22 linear. ' 22 There literature about the critical literature critical temperature temperature at at which which the therate rate low occurs which which probably probably results results from from different different low change change occurs techniques used to study this phenomenon as vacuum vacuum phenomenon such as microbalance,1s interference interference color color method,23 method 23 pressure microbalance," pressure 25 method, 24 microgravimetry, and change method,24 microgravimetry,25 anddetermination determination of of absorbed Thereare are aa few few absorbed oxygen oxygen on metallic metallic surface.21 surface.2" There attempts in the literature to investigate the investigate the kinetics kinetics of the oxide film growth growth by by using using ellipsometry. ellipsometry. Boulben Boulben et et al.8 al.8

50 50 Table Table I.. Computed Computed refractive refractive indexes indexesand and corresponding corresponding kinetic kinetic law law of thermally formed formed titanium titanium oxide oxidefilms filmsatatvarious varioustemperatures. temperatures.

0 00

I

55

Temperature (°C) (C) Temperature

I

10 10

15 15

20

(nin2)300

(mm2) (min)

Fig. determined ellipsomefrically ellipsometrically during during Fig. 4. Film thickness growth determined Ti heating heating at: (1) (1)400, 400, (2) (2)500, 500, and and(3) (3)600°C. 600°C. Ti

Kinetic law Kinetic law

723 == n2 (1 A ( — - ik) ik2)

100 100 200 200 300 400 400

nh = = 2.75 2.75 11 — - 0.001 0.001 0.006 - 0.006 n2 = = 2.73 2.73 11 — n, == 2.71 2.71 (11 — - 0.007 0.007 fl3 - 0.056 0.056 n = = 2.46 2.46 (11 —

i

soo 500 600 600

h2 = = 2.40 2.40 (1 (1- 0.065 0.065 n2 = 2.15 2.15 (1 — 0.111 0.111

i) i)

i il

Logarithmic Logarithmic Logarithmic Logarithmic Logarithmic Logarithmic Transition Transition to to parabolic parabolic Parabolic Parabolic Parabolic Parabolic


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JJ.Electrochem. Electrochemical Society, Vol.144, 144,No. No.7,7,July July1997 1997The © The Electrochemical Society,Inc. Inc. Electrochem.Soc., Soc.,Vol.

carried measurements inin the the temperacarried out out ellipsometric ellipsometric measurements temperature range Theyfound found that that only only 420°C. They between 300 300 and and 420°C. range between kinetic data. data. parabolic rate law fits their experimental experimental kinetic fits their parabolic rate reported by ellipsometric kinetic Contrarily, the Contrarily, the ellipsometric kinetic data data reported by Kucirek" while for for to 480°C, 480°C, while logarithmic law law up up to Kucirek 2 6 fit fit the the logarithmic temperatures the observed observed oxide oxide film film 700°C the from 600 600 up up to 700°C temperatures from growth kinetic data data fit fit the law which which isis in in the parabolic parabolic law growth kinetic agreement with the results obtained agreement with obtained in this study. There are a few few studies studies in in literature literature regarding the varivariregarding the There are ation of of the the oxide film function of oxide film indexes as a function of refractive refractive indexes of refractive refractive growth. the variation variation of Kucirek26 studied studied the growth. Kucirek" indexes as a function function of the the film film thickness thickness growth growth for fortherther27 mally formed oxide oxide films, films, while while Arsov Arsovetetal. at." studied the the studied mally formed variation of anodically formed formed oxide oxide indexes of anodically of refractive refractive indexes films. In this this work, work, aa detailed detailed study study was was carried carried out out to to films. In determine the effect effect of of temperature on the the values of values of temperature on determine the refractive indexes. each temperature temperature in in Table Table I,I, itit was was indexes. For For each assumed that the the refractive refractive indexes indexes n2 it, and and kk, are indepenindepenassumed that 2 are the results results dent Our assumption assumption and and the of film film thickness. thickness. Our dent of obtained for it, with the the value value n2 are are in in good good agreement agreement with obtained for 26 reported studied only only the the variation variation of of n2 it, who studied reported by Kucirek" Kucirek who and k, function of the the film film thickness thickness growth growth at at aa fixed fixed k2 as a function temperature of 600°C. Accordingtotothis this author author the the n2 it, 600°C. According temperature of slightly decreases from from 2.18 2.18toto 2.1 2.1 for for aa wide wide interval interval of of slightly decreases (from 20 film 20 to to 250 250 nm), nm),while whilekk, has constant constant film thickness thickness (from 2 has values In this this study, for the the same same temperatemperastudy, for 0.05. In values of about 0.05. respecture, were 2.15 2.15 and and 0.111, 0.111, respecof it, n2 and k, k2 were ture, the values values of value tively. The observed differencebetween betweenthe the kk, observed difference tively. The 2 value 26 obtained study with with the the value value reported reported by byKucirek Kucirek" obtained in this study from different probably results results from probably different techniques techniques used used to to prepare prepare the thermal heating. to our our According to before thermal heating. According the Ti Ti surfaces surfaces before 7 previous study,' the previous study,'7 the electropolished electropolished Ti Ti surface surface has has higher higher when values of the the metal metal substrate substrate when of refractive refractive indexes indexes of values of chemically polished compared with compared with chemically polished surfaces. surfaces. ItIt was was also also indexes have found refractive indexes have larger larger influinflufound that measured measured refractive decrease of ence on k, on it,. n2. The The observed observed decrease of n2 n, in in ence on k2 than than on Table with an an increase in the the temperature temperature index index of of air airisis increase in Table II with have a equal to one, one, the porous film approximately film should should have approximately equal compact oxide smaller refractive smaller refractive index, index, n2, it,, than the compact oxide film. film. High values values observed observed for for the the absorption absorption coefficient, coefficient, kk,, at 2, at of these also indicate high temperatures high temperatures also indicate aa porous porous nature nature of these films. films. For a thin oxide formed at at aa lower lowertemperature, temperature, the the oxide film, film, formed film. close to the nonabsorbing nonabsorbing film. k must must be be very very close values values of k, (i.e., kk,2 = 0). higher temperatures, temperatures, the the oxygen oxygenion iontranstrans(i.e., 0). At higher increases leading port number increases leading to to higher higher internal internal comprescompressive stress the film film which sive stress in in the which causes causes the the film film to to lose lose its its becomes microfilm becomes homogeneity. The homogeneity. Thestructure structure of of the the film micronormally absorb absorb more porous defects which which normally more porous with surface defects break. Higher film before before the break. than the the amorphous amorphous film Higher values values absorption index, index, observed of the of the absorption observed at at higher higher temperatures temperatures caused surface roughness roughness caused increase of the surface also indicate also indicate an increase by the porous structure of the film. film. This This assumption assumption can canbe be figure, for oxide film verified As shown shown in in this figure, for oxide film verified in Fig. 2. As clearly 130 nm, experimental points clearly thickness above above 130 thickness nm, the the experimental deviate from the the fitted fitted curve curve due due to to the the augmentation augmentation of of deviate from remained oxide surface surface remained 130 nm, porosity. Up film porosity. Up to 130 nm, the the oxide bright. With the oxide oxide thickness, thickness, the With further further increase increase of the experiment was surface surface became became opaque. opaque. At At this this point point the the experiment was observed complex because the observed stopped because complex index index of of refraction refraction 2.15 change significantly significantly from began to change began fromaa value valueofof nñ, = 2.15 (1 — comtheoretical comwhich corresponds - 0.111 0.111 . i) which corresponds to to the the theoretical curve. puted curve. Ramait Raman spectroscopy.—The spectroscopy.-The Raman Ramaneffect effectisisaasatisfactory satisfactory

electron x-ray and electron procedure to alternative alternative procedure to the the standard standard x-ray

for the detection of diffraction of crystal crystal structure. structure. diffraction techniques for Raman spectroscopy offers Raman spectroscopy offers the the advantages advantages of of more more simple simple of measurements. measurements. rapidity of sample preparation sample preparation and rapidity

film of of an electropolished electropolished Ti The natural oxide oxide film Ti surface surface The measurements measurements using x-ray diffraction. diffraction. The were studied using were by short short angle angle cameras cameras have haveshown shownthat thatthe thenatural natural oxide oxide to dd lines which correspond peaks for was TiO, TiO, with film was with peaks for lines which correspond to existence structure. 28 Hugot Hugot Le-Goff" Le-Goff" reported rutile structure.'8 reported the existence oxide of active bands for for very very thin thin TiO TiO, natural oxide active Raman Raman bands 2 natural anatase. ReRefilms different than those and anatase. different than those of bulk rutile and

0 cently, Pankuch obtained the active Raman cently, Pankuch et et al. at." obtained the active Raman bands bands thin oxide oxide film film on on Ti Ti surface for a thin surface which which indicated indicated a high high disorder of of the amorphous amorphous phase consisting of disorder phase consisting of TiO2 TiO, and and Ti2 O3 . Besides Besides these still the chemical chemical composicomposiTi,O,. these studies, studies, still tion, structure, structure, and and crystal crystal modification modification of tion, of natural natural very very thin titanium oxide oxide films films is not not known. known. The Raman The Raman spectra spectra of of the the sample sample heated heated at at aa temperatemperature of of 100°C 100°C did did not not show show any any active active Raman Raman bands bands even even after 66 h of thermal heating. after heating. Between and 12 12 hh of of heatheatBetween 66 and ing the film film thickness thickness growth growth was was negligible negligible (0.2 (0.2 nm). nm). ing the Raman spectra of of Ti Ti samples samples heated at at 500°C 500°C for for 30, Raman spectra 30, 180, 180, and 360 min shown in Fig. 5. 5. The The first visible and 360 mm are are shown visible Raman Raman

bands appear appear at 500°C and an oxidation oxidation time bands at 500°C and for for an time of of 30 min. 30 mm. At At this this temperature, temperature, the the spectrum develops develops slowly slowly and there there was was not not aa big big change changeininthe theRaman Ramanbands' bands'intenintensities with time, even after sities time, even after 6 hh of of heating. heating. Only Only two two bands bands appear, located appear, located at at 447 447 and and 612 612 cm-'. cm'. These These bands bands correcorrethe E5 Eg and A15 A,, modes spond to the modesofofthe theTiO TiO, rutilephase. phase. The The 2 rutile bands forming forming the bands the rutile rutile structure structure which which should should appear appear at at 143 and are missing. missing. 143 and 236 236 cm cm'- ' are As As shown shown in in Fig. Fig. 6,6,the thebands' bands' intensities intensities increased increased if the the Ti samples temperature of of 600°C. Ti samples were were heated heated at at a temperature 600°C. At At this this temperature complete Raman temperature aa complete Raman spectrum spectrum ofof TiO, TiO, with with rutile structure structure develops develops within 30 mm. min. Three Three strong strong rutile within 30 bands, including including one one broad one shoulder, shoulder, can bands, broad band and one can be be distinguished in spectrum. The distinguished in the the spectrum. The spectrum spectrum isisstable stablefor for several days days at air. With several at room room temperature temperature in air. With the the appearappearance of the Raman bands bands at 500°C 500°C transformation of ance the first Raman transformation of the oxide oxide film from an begins. film from an amorphous amorphous to to rutile rutile phase begins. The Raman sample heated to to 10 100°C The Raman spectrum of aa Ti sample 0°Cwhich which previously heated at 600°C 600°C showed was previously showed only only the the bands bands due due

447 612 612 447

0

0

>

3

Oni

~~~~~~~~~~~~2

c c_ a' 0) C

250 250

750 750

250 1250

Wovenumber Wovenumber(cm-') (cnf') Fig. Roman spectra spectra of Ti heated for various varioustimes times at at500°C. 500°C.(1) (1) Fig. 5. Roman heated for 30, (2) (2) 180, 180, and and (3) (3) 360 360 mm. min. 30,

3

a >U,

C C 4' C C

250

750 750

1250

Wavenumber (crrV') Woverumber (crri') Fig. Roman spectra spectra of 360 mm. min. Fig. 6. Roman of Ti Ti heated heated at 600°C 600°C for 360


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to rutile rutile structure structure indicating indicating that that the to the amorphous amorphous film film

irreversibly to transforms irreversibly to rutile structure. structure. The observed in Fig. 6 became The bands observed became more more intense intense as asthe the exposure time time increases increases due due to to increased increased film exposure film thickness. thickness. However, the However, the film film thickness thickness isis not not the the only only factor factor responresponsible for the intensity of of Raman Raman bands. bands. As sible As shown shown in in Fig. Fig. 5, titanium oxide film film with calculated calculated thickness titanium oxide thickness of of 42 42 nm nm

develops Raman develops Raman spectrum spectrumwithin within 30 30 min mm exposure exposure atat 500°C. On 500°C. Onthe the other other side, side, for for the the same same oxide oxide film film thickness thickness at lower lower temperature temperature (400°C) (400°C) even but at even after after 7 h of expoexposure, Raman spectra were not not observed observed indicating indicating the sure, Raman spectra were the importance of substrate surface surface morphology morphology and importance of the substrate and the the breakdown process. breakdown process.When Whenthe the breakdown breakdown ofof the the film film begins, crystallization of the oxide begins, the the crystallization of the oxide occurs. the occurs. With With the progress of progress of the the breakdown breakdown the the crystallite crystallite state state formation formation also progresses progresses causing causing the oxide film to also to become become inhomoinhomogeneous. Thus, geneous. Thus, two two coupled coupled processes, processes,surface surface film filmformaformation and its its destruction, destruction, contribute contribute to tion to its its inhomogeneity inhomogeneity resulting number of structural defects. defects. These resulting in in a number defects These defects can be observed observed microscopically microscopically as can as aa variety variety of of different different interference colors interference colors which which exist exist between between the the grains grains of of the the oxide. It seems that breakdown process oxide. It seems that the breakdown process starts starts earlier earlier the appearance appearance of than the of the the first first Raman Raman bands. bands. ItIt isis still still not clear clear if the breakdown breakdown is is provoked provoked by the oxide oxide cryscrystallization, or the film film crystallization crystallization causes tallization, causes the the creation creation are manifested manifested as as breakdown. of defects defects which are breakdown. Evidently, Evidently, the conventional Raman Raman spectroscopy the conventional spectroscopy isis not not sensitive sensitive enough to detect the the earliest earliest stages enough to detect stages of of the the breakdown breakdown when the local local formation formation of of very when very short short range range structural structural order begins. begins. The The laser order laser beam beam falls falls on on aa large large surface surface area area 2 (about 11 mm2), mm ), and of the sample (about and in in the the Raman Raman scattered scattered light average values, light average values, in in which which amorphous amorphous structures structures prepreregistered. dominate, is registered. is still not clear why why the It is the appearance appearance of of anatase anatase phase phase is observed observed in anodic oxidation oxidation instead is in anodic instead of of rutile rutile which which appears in thermal oxidation oxidation of titanium. It was found appears found by by Arsov al.31 that Arsov et al.3' that during during the the anodic anodic oxidation, oxidation, the the rutile rutile structure appears appears after after the the anatase structure anatase by by local local heating heating of of electrode surface spark effect. electrode surface caused caused by by the the spark result effect. The The result presented presented in this study that the rutile rutile is is the only only detected detected are not in in agreement agreement with structure are with the results results obtained obtained by by 0 David al.,' who who claim claim that that at David et et al.,'° at temperature temperature ranges ranges between 400 400 and 500°C 500C two between two crystalline of crystalline modifications modifications of TiO2, namely, Ti02, namely,both both rutile rutile and and anatase, anatase, exist. exist. The The electron electron diffraction technique technique was used diffraction used in their study to detect the oxide crystalline crystalline modifications. modifications. The oxide The presence presence of of subsuboxides Ti05, TiO, Ti oxides Ti203, andTiO TiOwere were also also not not excluded. excluded. ItIt is 2,O,, and not possible possible for amount of anatase not for a small amount anatase to to exist exist in the before detecting film before detecting the rutile structure. The frequency frequency of of most intense intense band, band, Eg Eg mode the most mode in in anatase anatase is is centered at centered at 144 cm nearly equal equal to to that thatof 144 cm' 'which isis nearly ofthe theBig Big mode mode in centered at at 143 143 cm1. cm-'. Taking Taking into account rutile centered account the the enorenormous intensity intensity of of the anatase anatase band7 mous band which which is is approxiapproxi30 times more intense mately 30 times more intense than than that one one which which belongs belongs to rutile rutile structure, structure, it is is expected expected that the to the existence existence of of anatase in thermally formed formed oxide film film to to provoke provoke the thefirst first - ' appearance of appearance ofthe theactive activeband bandtotobebeatat144 144cmcm, , which which was not observed observed in our our studies. studies. Also was Also the the strongest strongest Ti203 Ti 02 3 which appear appear at 279 bands which 279 and and 350 350cm cm''were were not not detectdetected in our study. study. The results The results obtained obtained in this study study are are in in agreement with agreement with those reported reported by by Douglas Douglas and Landuyt.3 Landuyt.3 Using those Using electron electron these authors have shown diffraction technique technique these shown that on on Ti Ti samples oxidized samples oxidized at various various times times in in air air under under atmospheratmospheric pressure pressure and and temperatures temperatures ranging ic 450 and and ranging between between 450 700C an 700°C an oxide oxide isis formed formedwith with aa rutile rutile modification. modification. Raman spectroscopy spectroscopy measurements measurements performed The Raman performed so so have shown shown three far have three well-known well-known structural structural modifications modifications of Ti02: TiO: rutile, rutile, anatase, anatase,and andbrookite,729'3° brookite 7,29,30 which are of which are

tetragonal, orthorhombic, orthorhombic, and tetragonal, respectively. respectively. For tetragonal, For 3 3 , n2 n2 = = 2.622; 2.622; for anatase pp = = 3.9 g/cm rutile pp = 4.22 4.22 g/cm , g/cm3, g/cm3, 3 n2 = 2.544, 4.23 g/cm3, 2.745. 2.544, and and for for brookite brookite p = 4.23 g/cm , n n2 2 = 2.745. These values in agreement agreement with with the thePauling These values are in Paulingtheory,32 theory, 32 our previous previous measurements measurements carried out on with our on anatase,6 anatase, 6 obtained for rutile in this study. and with the results obtained The study. The above data the real real part of above data indicate indicate that that the of the the refractive refractive

indexes decrease decrease with decreasing decreasing density. density. Accordingly, Accordingly, the indexes the observed n2 in observed decrease of n2 in Table TableII isis caused caused by by the the decrease decrease of of the the film film density density which which results results from from augmentation augmentation of of the the porosity at higher higher temperatures. temperatures. On porosity On the the other other hand, the hand, the presence of of oxygen presence oxygen vacancies vacancieswas was indicated indicated by by aa noticenoticeable absorption in in the the visible visible range.33 range. 33 In able absorption In both, both, rutile rutile and and anatase, there there are are two two elongated elongated titanium-oxygen titanium-oxygen bonds anatase, which occur occur at 0.1946 and which at 0.1946 and 0.1984 0.1984nm nmfor forrutile rutile and and at at

0.1937 0.193 7and and0.1966 0.1966nm nmfor foranatase anataseinin each eachoctahedron. octahedron. The The lengthening of lengthening of these these bonds bonds isis greater greater in in the the case case of of rutile, rutile, which causes causes a relative relative weakening weakening of which of the the bonds bonds and, and, conconsequently, energy requirements sequently, aa lowering lowering of of the the energy requirements for for the excitation. This phenomenon can excitation. This phenomenon can be be correlated correlated with with the the optical optical properties properties of of these these materials materials by by measuring measuring their their complex indexes. The complex refractive refractive indexes. The vacancies vacancies are are less less numernumerous in anatase structure than in in rutile, causing of causing the value value of k k2 for rutile rutile to to be be bigger bigger than than the for anatase. anatase. For For aa the k2 k2 for 2 for thicker film with structure, the thicker film with a rutile structure, the oxygen oxygen vacancies vacancies are the presence are associated associated with with the presence of of Ti Ti ions ions which which cause cause aa larger porosity porosity than than in anatase anatase and and consequently consequently larger larger larger values values for k2 than in in anatase. anatase. k2 than

Conclusions Thermal films on titanium surfaces surfaces were formed Thermal films formed by by heatheating titanium substrates in in air at at atmospheric atmospheric pressure. ing pressure. The The optical constants, optical constants, thickness, thickness, and and structure structure of of the the formed formed films at various various temperatures temperatures and films and time time of of heating heating were were investigated using using ellipsometry ellipsometry and Raman Raman spectroscopy. spectroscopy. investigated The natural oxide oxide film film which which spontaneously The spontaneously forms forms on on electropolished electropolished Ti Ti surfaces surfaceshas has an an amorphous amorphous structure. structure. At At temperatures below below 375°C, 375C, the temperatures the film film thickness thickness growth growth can can be described logarithmic rate be described with with the the logarithmic rate law. At intermediintermedilaw. At ate temperatures temperatures (375°C) (375C) the the film film thickness thickness growth growth does does not follow follow any the kinetic kinetic equations. equations. The not any of of the best agreeThe best agreement with ment with the the experimental experimental data data was wasobtained obtained with withfourth fourth degree reaction At 400°C, 400C, the degree reaction rate. At the curve curve shape shape of of the the film film thickness growth corresponds to the thickness growth corresponds the cubic cubic rate. rate. At At 425°C, 425C, the reaction rate can be attributed as the reaction rate can be attributed as transition transition between between cubic and parabolic rate law. law. For cubic For temperatures temperatures from from 450 450 to to 600C, the 600°C, theoxidation oxidationrate rate can canbe bebest bestdescribed describedasasa aparaparabolic increase bolic increase in in thickness. thickness. Up Up to to 500°C 500C the the structure structure of of is amorphous amorphous TiO2 TiO, while the film is while above above this this temperature temperature amorphous film film transforms only to the amorphous transforms only to rutile rutile structure. Manuscript submitted submitted Aug. Aug. 8, 1996; Manuscript 1996; revised revisedmanuscript manuscript received April 9, 9, 1997. 1997. received The University of The ofSouth South Carolina Carolinaassisted assisted in inmeeting meetingthe the publication costs of publication of this article. REFERENCES REFERENCES 1. Lj. Lj. Arsov, Arsov, J. J. Eng. Eng. Phys., Phys., XX, XX, 55 (1979). (1979). 1. 2. Lj. Arsov, Arsov, Electrochim. Electrochim. Acta, 6, 6, 663 663 (1992). (1992). 2. Lj. D. Douglass Douglass and and J. J. van van Landuyt, Landuyt, Acta 3. D. Acta Metall., Metall., 14, 14, 491 491

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Spectroelectrochemical Study Spectroelectrochemical Study of of the the Role Role Played Played by by Carbon Carbon Functionality Functionality in inFuel Fuel Cell Cell Electrodes Electrodes */ a'b A. W. Harding,,b S. C. C.Roy, Roy," Harding,0I A. and K. K. M. Thomas°" A. E. E.RusseII,*} Russell,I * b and Thomas "ob

"Northern Carbon 'Northern Research Laboratories (NCRL) and Department of Carbon Research Laboratories (NCRL) and 1Department of Chemistry, Chemistry, University University of ofNewcastle, Newcastle, Newcastle-upon-Tyne NE1 NEI 7R 7RU, Newcastle-upon-Tyne U, England England ABSTRACT ABSTRACT absorption spectroscopy spectroscopy was X-ray absorption was used used to to identify identify specific specifictypes typesofofnitrogen nitrogenand andsulfur-based sulfur-basedcarbon carbonfunctionality functionality the carbon carbon black black supports supports of of fuel fuel cell cell anodes present in the of these these functional functional groups groups on on the the elecelecanodes and cathodes. cathodes. The effects effects of trocatalytic performance of carbon, during oxygen trocatalytic performance of small small platinum platinum particles, particles, dispersed dispersed on on the the carbon, during methanol methanol oxidation oxidation and and oxygen reduction were were assessed. assessed. Electrodes Electrodes functionalized functionalizedwith withnitrogen nitrogenhad had enhanced enhanced catalytic catalyticactivities activities toward toward oxygen oxygenreducreduction and methanol Although electrodes electrodeswith withsulfur sulfurfunctionality functionalityhad hadhigher higher methanol oxidation oxidation relative relative to untreated electrodes. electrodes. Although oxygen reduction carbons, the oxygen reduction activities activities than than untreated carbons, the activity activity of these these electrodes electrodes toward toward methanol methanoloxidation oxidationwas wasfound found to be lower lower than than electrodes electrodes manufactured manufactured from from untreated untreated carbon. carbon. ItIt was was found found that that carbon carbon supports supports functionalized functionalized with with initiated the the formation formation of both nitrogen nitrogen and sulfur initiated of Pt Pt particles smaller smaller in size on untreated untreated carbon carbon size than those observed observed on supports. supports.

Introduction The The generation of electricity electricity by by low low temperature temperature fuel fuel generation of cells, such cells, such as the the direct direct methanol methanol fuel fuel cell cell (DMFC), (DMFC), is increasingly perceived increasingly perceivedas as an an advantageous advantageous alternative alternative to to existing methods of power generation generation by, by, for example, existing methods of power for example, combustion engines in cars. Relatively internal combustion Relatively clean clean techtechnology such as as this this will will aid aid reduction reduction of of atmospheric atmospheric polpolnology such lution by generating generating power power without without the the accompanying accompanying lution release of compounds associated associated with fossil fossil release of harmful harmful NO, NO compounds combustion. A fuel combustion. A recent recent overview overview of of underlying underlying principrinciDMFC can be found in Ref. 1. The ples behind the DMFC The electroelectrochemical reactions cell are are oxygen oxygen chemical reactionstaking taking place place inin the the cell reduction at the reduction the cathode cathode and methanol methanol oxidation oxidation at at the the anode. Both of anode. of these these reactions reactions occur occur on on small small particles particles of of typically 1 to 10 10 nm platinum, typically nm in in diameter, diameter dispersed dispersed on a carbon black support. carbon support. surface of the carbon black support is composed The surface of composed of graphene layers. layers. Functional Functional groups, predomigraphene groups, which which are are predomioxygen, nitrogen, nantly oxygen, nitrogen, or sulfur sulfur based, based, are are located located at at the the edges of graphene planes, present as as heteroatom heteroatom edges of the graphene planes, 2or or present functionality planes. The The design functionality within the planes.2 design of of supported supported catalysts, such as Pt/C, Pt/C, is is being being steadily steadily advanced advanced by by catalysts, such as information concerning concerning metal-support metal-support interactions interactions (MSIs) (MSIs) information between surface between surface carbon carbon functional functional groups groupsand andthe the particparticulate metal. metal. Recently, Recently, an an extended extended x-ray x-ray absorption absorption fine fine structure (EXAFS) (EXAFS) study catalysts, composed composed of structure study of of Pt/C Pt/C catalysts, of carbons impregnated impregnatedwith with [Pt(NH,3)]Cl 2 and and H2PtC16,3 H2 PtCl 6 ,3 has carbons (Pt(NH3)4JC12 has allowed distinctions distinctions to be drawn between allowed between different different metal metal precursor-support interactions. precursor-support interactions. A A considerable considerable amount amount of of literature centered literature centered on on the MSI MSI between between carbon carbon functionalfunctionalparticulate Pt, and the influence ity and particulate on particle particle influence it has on size and and dispersion dispersion can found in Ref. Ref. 4-8. 4-8. Work Work by size can be found by Antonucci et al., for Antonucci for example, example, has has shown shownthat that an an increase increase the concentration concentration of of oxygenated oxygenated carbon in the carbon functional functional groups (e.g.,-COOH, (e.g.,-COOH, -OH), groups -OH),asasdetermined determinedby byx-ray x-rayphotophoto**Electrochemical Electrochemical Society Society Active Active Member Member.

spectroscopy (XPS), electron spectroscopy (XPS),decreases decreasesPt Ptsurface surfacearea area for for aa given given loading, loading, by increasing increasing Pt crystallite crystallite size.4 size.4 Pt particle size is an Pt an important parameter which which affects affects the reactivity of of platinized platinized fuel fuelcell cellelectrodes.9 electrodes.9 Recently, Recently,

notable enhancements enhancements of of short-term short-term electrocatalytic electrocatalytic notable oxygen reduction'° reduction" have been activity toward oxygen been observed observed on on Pt/C catalysts catalysts which which contain contain relatively relatively small Pt/C small platinum platinum crystallites. This crystallites. This reduction reduction of of average average Pt Pt particle particle size size isis brought about reacting a a Pt Pt precursor precursor solution solution brought about by by reacting [Na 6 Pt(SO3)4] with [Na5Pt(SOj4] withcarbon carbon supports supports containing containing nitrogen nitrogen and sulfur functionalities. It is is therefore therefore important important to to be be functionalities. It able to isolate able isolate the various various types types of carbon carbon functionality functionality present on these supports, supports, in order to establish present on these in order establish links links between specific between specific types types of of surface surface functionality functionality and and elecelectrocatalytic activity. activity. trocatalytic Experiments designed Experiments designed to to examine examine the the nature nature of of carbon carbon functionality in carbonaceous carbonaceous materials materials often often employ functionality employ '2 XPS. 4""1 Recently, XPS has XPS.4'11"2 Recently, XPS hasbeen beenused usedtotodetermine determine types of sulfur sulfur functionalities functionalities present of present inin the the carbon carbon black black (Vulcan XC-72) fuel cell cell electrodes.13 electrodes. 3 X-ray (Vulcan XC-72) supports supports of fuel X-ray absorption near near edge edge spectroscopy spectroscopy (XANES) (XANES) has has also also been been effectively used effectively usedtoto study study nitrogen nitrogen and and sulfur sulfur functionalifunctionali4- 7 ties ties in in fossil fossil fuels.14-17 Thismethod methodhas hasseveral severaladvantages advantages fuels.' " This over XPS. The The most most notable notable of of these these is improved resoluover XPS. improved resolution of spectra, with less overlap of bands. bands. overlap of The work presented presented in paper employs employs XANES XANES to The in this paper examine carbon examine the the types types of of carbon carbon functionality functionality on on the the carbon assesses electrocatalytic support and assesses electrocatalytic performance performance of of particulate Pt Pt dispersed reducdispersed on on the support toward oxygen oxygen reducand methanol oxidation. tion and oxidation. Three different types of of carcardifferent types black were bon black were used: used: untreated, untreated, nitrogen-functionalized, nitrogen-functionalized, and sulfur functionalized. functionalized.

Experimental Experimental

Synthesis Synthesis

characterization of of new and characterization new carbon carbon

blacks.-Introduction carbon black—The black.-The blacks —mt roductionofofsulfur sulfur to to carbon Vulcan Vulcan XC-72 XC-72 was was functionalized functionalizedby byaddition addition of of sulfur sulfur containing groups as as described described previously.'0 previously. Briefly, Briefly, a 1:2 1:2
