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AN IN-SITU REFLECTION-SPECTROSCOPIC STUDY APPLIED TO ANODIC OXIDE FILMS ON
IRON, NICKEL, AND TITANIUM
T. Ohtsuka, K. Azumi, N. Sato
To cite this version:
T. Ohtsuka, K. Azumi, N. Sato. AN IN-SITU REFLECTION-SPECTROSCOPIC STUDY APPLIED
TO ANODIC OXIDE FILMS ON IRON, NICKEL, AND TITANIUM. Journal de Physique Colloques,
1983, 44 (C10), pp.C10-191-C10-194. �10.1051/jphyscol:19831039�. �jpa-00223496�
JOURNAL DE PHYSIQUE
Colloque CIO, supplément au n°12, Tome M, décembre 1983 page C10-191
AN I N - S I T U REFLECTION-SPECTROSCOPIC STUDY APPLIED TO ANODIC OXIDE FILMS ON IRON/ N I C K E L , AND TITANIUM
T. Ohtsuka, K. Azumi and N. Sato
Electrochemistry Laboratory, Faculty of Engineering > Hokkaido University, Sapporo, 060 Japan
Résumé - La réflectométrie à 3 paramètres est utilisée pour caractériser les films minces d'oxyde sur fer, nickel et titane. Le film de passivation sur le fer pré- sente un spectre d'absorption, et donc des niveaux de transition des électrons, analogues à ceux du Fe?0- massique. Le spectre optique du film de passivation du nickel ne présente pas de structure caractéristique mais le coefficient d'extinc- tion dépend beaucoup du potentiel anodique. Le film anodique d'oxyde de titane présente une bande interdite d'environ 3,2 eV, qui est du même ordre de grandeur que celle du TiCL massique (3,05 e V ) .
A b s t r a c t - 3-Parameter r e f l e c t o m e t r y i s a p p l i e d t o c h a r a c t e r i z e the anodic t h i n oxide f i l m s on i r o n , n i c k e l , and t i t a n i u m . The p a s s i v a t i o n f i l m on i r o n e x h i b i t s the absorption spectrum and hence the t r a n s i t i o n l e v e l s o f e l e c t r o n analogous t o t h a t o f the bulk F e ? ^ . The p a s s i v a t i o n f i l m on n i c k e l have no c h a r a c t e r i s t i c f e a t u r e i n the o p t i c a l spectrum, but i t s e x t i n c t i o n index i s much dependent on the anodic p o t e n t i a l . The anodic oxide f i l m on t i t a n i u m have a band gap about 3.2 eV, which i s comparable t o the band gap 3.05 eV o f bulk TiOg.
INTRODUCTION
The t h i n anodic oxide f i l m formed on metals o f t e n plays an important r o l e t o p r o t e c t the s u b s t r a t e metals against c o r r o s i o n i n aqueous s o l u t i o n . Since the composition o f the t h i n p r o t e c t i v e f i l m i s s e n s i t i v e t o the environment w i t h which the f i l m i s i n c o n t a c t , i n - s i t u measurements o f the f i l m s are r e q u i r e d . The method using p o l a r i z e d - l i g h t r e f l e c t i o n on t h e m e t a l l i c e l e c t r o d e can be used t o
c h a r a c t e r i z e the surface f i l m s i n - s i t u i n aqueous s o l u t i o n i n terms o f the complex r e f r a c t i v e index ( rip = ng - i k 2 ) and thickness ( d ) . I n t h i s paper an i n - s i t u 3-parameter (3-P) r e f l e c t o m e t r y i s a p p l i e d t o the study on the spectroscopic p r o p e r t i e s o f the anodic oxide f i l m s on i r o n , n i c k e l , and t i t a n i u m . The 3-P technique i s indispensable t o q u a n t i t a t i v e d e s c r i p t i o n o f the surface t h i n f i l m , because there are three unknown physical q u a n t i t i e s ( n g , k2 and d ) of the surface f i l m t o be e s t i m a t e d / 1 , 2 / . I n t h i s 3-P r e f l e c t o m e t r y , three r e f l e c t i v i t y changes a t three d i f f e r e n t s t a t e s o f l i g h t p o l a r i z a t i o n have been measured. One o f the r e f l e c t i v i t y changes i s a f u n c t i o n o f the r e l a t i v e phase d i f f e r e n c e between p- and s - p o l a r i z e d l i g h t . The other two are the r e f l e c t i v i t y changes o f p- and
s - p o l a r i z e d l i g h t s / 1 / . The spectra o f complex r e f r a c t i v e index and the
thickness o f the anodic oxide f i l m are measured a t a wavelength range from X = 640 t o 340 nm .
EXPERIMENTAL
The apparatus used f o r 3P r e f l e c t o m e t r y consisted o f a l i g h t source, a spectrometer, an p o l a r i z e r , a sample e l e c t r o d e , an a n a l y s e r , and a p h o t o m u l t i - p l i e r . For s t a b i l i z i n g the apparatus, the l i g h t source was made of a 24W-power tungsten lamp connected t o a l i g h t - f e e d b a c k system, and the d e t e c t i o n of i n t e n s i t y o f r e f l e c t e d l i g h t was conducted w i t h AC a m p l i f i c a t i o n using a l i g h t chopper and a l o c k - i n a m p l i f i e r . Three r e f l e c t i v i t y changes d u r i n g the f i l m growth and
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831039
CIO-192 JOURNAL DE PHYSIQUE
r e d u c t i o n were measured a t t h r e e azimuth s e t t i n g s o f t h e a n a l y s e r (A=OO, 45"and 9 0 ' ) a t a f i x e d azimuth o f t h e p o l a r i z e r ( P = 40"). The i n c i d e n c e a n g l e o f l i g h t was
@ = 55.06". Numerical e s t i m a t i o n o f t h e t h r e e unknowns o f n2,
,
and d from t h e t h r e e r e f l e c t i v i t y changes was made w i t h t h e F r e s n e l ' s and Drude s e q u a t i o n s f o rK3
p o l a r i z e d - l i g h t r e f l e c t i o n b y t h e Newton-Raphson method o f successive approximations/3/.
RESULT
An example o f t h e r e f l e c t i v i t y - c h a n g e s p e c t r a , measured f o r t h e t h r e e d i f f e r e n t s t a t e s o f p o l a r i z e d l i g h t , i s shown i n Fig.1, where t h e r e l a t i v e r e f l e c t i v i t y change f o r t h e a n o d i c p a s s i v e f i l m on i r o n i s g i v e n by
I n ~ q . ( I ) , R(d=O) and R(d) a r e r e s p e c t i v e l y t h e r e f l e c t i v i t i e s o f t h e s u r f a c e reduced a t c o n s t a n t c u r r e n t o f 5 y ~ c m - 2 f o r 1500 s i n b o r a t e s o l u t i o n a t pH 8.4 and o f t h e s u r f a c e subsequently o x i d i z e d a t c o n s t a n t p o t e n t i a l o f E = 1.43V vs HESS ( hydrogen e l e c t r o d e a t t h e same s o l u t i o n ) f o r 1 h i n phosphate s o l u t i o n a t pH 3.1
.
S u b s c r i p t s p, s, and a i n F i g . 1 i n d i c a t e t h e a n a l y z e r s e t t i n g s a t A = 0°, 90°, and 45", r e s p e c t i v e l y . ( AR/R ) p and ( AR/R ) s correspond t o t h e r e l a t i v e r e f l e c t i v i t y o f p - p o l a r i z e d and s - p o l a r i z e d l i g h t , r e s p e c t i v e l y . ( AR/R)ai s a l s o t h e r e l a t i v e r e f l e c t i v i t y change determined m a i n l y by t h e phase d i f f e r e n c e between p-and s - p o l a r i z e d l i g h t s / l / .
From t h e t h r e e v a l u e s o f ( AR/R ) t h e complex r e f r a c t i v e i n d e x and t h i c k n e s s o f t h e f i l m a r e e s t i m a t e d a t each wavelength as shown i n Fig.2. The complex r e f r a c t i v e i n d e x o f i r o n s u b s t r a t e has been measured by u s i n g a r o t a t i n g - a n a l y s e r e l l i p s o m e t e r and t h e r e f r a c t i v e i n d e x o f s o l u t i o n by u s i n g P u l f r i c h r e f r a c t o m e t e r . The c a l c u l a t i o n o f t h e complex r e f r a c t i v e i n d e x and t h i c k n e s s o f t h e f i l m was made i n two steps. I n t h e f i r s t s t e p o f c a l c u l a t i o n t h e Newton-Raphson method o f
L - , 1 8 , I
Wave Length, A / l o 2 n m
F i g . 1 I n - s i t u s p e c t r a o f r e l a t i v e r e f l e c t i v i t y change (AR/R) o f t h e p a s s i v e f i l m formed on i r o n a t E = 1.43V i n phosphate s o l u t i o n a t pH 3.1.
Wave Length, A / Idnrn .-
F i g . 2 S p e c t r a o f complex r e f r a c t i v e i n d e x and t h i c k n e s s o f t h e p a s s i v e f i l m s on i r o n c a l c u l a t e d f r o m t h e r e l a t i v e r e f l e c t i v i t y changes (AR/R) ( s e e t e x t )
successive approximation was a p p l i e d t o estimate a s e t o f n2, k2 and d a t each wavelength. Since t h e value o f thickness thus c a l c u l a t e d i s s c a t t e r e d w i t h i n '10%
e r r o r as shown i n Fig.2, t h e second c a l c u l a t i o n was made on n2 and k2 by t h e l e a s t square e s t i m a t i o n w i t h t h e thickness, d, f i x e d a t an average value. Fig.2 i n d i c a t e s t h a t t h e f i l m i s t h i c k e r i n n e u t r a l s o l u t i o n than i n a c i d i c s o l u t i o n a t t h e same anodic p o t e n t i a l and t h a t t h e r e f r a c t i v e index o f t h e f i l m i s l a r g e r i n a c i d i c s o l u t i o n than i n n e u t r a l s o l u t i o n . No s i g n i f i c a n t d i f f e r e n c e i s found i n t h e e x t i n c t i o n index o f t h e f i l m s formed i n a c i d i c and n e u t r a l s o l u t i o n s .
Frpm t h e e x t i n c t i o n index the a b s o r p t i o n spectrum o f t h e passive f i l m on i r o n can be estimated and t h e r e s I t s a r e shown i n Fig.3, where t h e absorption
c o e f f i c i e n t , 2a = 4 n k 2 ~ - 1 ( m-
Y
), i s p l o t t e d a g a i n s t t h e wave l e n g t h . I n Fig.3 t h e r e s u l t a t another p o t e n t i a l o f E = 0.52V i n borate s o l u t i o n i s a l s o shown, t h e p o t e n t i a l which i s l e s s noble than t h e Flade p o t e n t i a l o f i r o n p a s s i v a t i o n . A l l the a b s o r p t i o n s p e c t r a e x h i b i t t h e same tendency and t h e two absorption edges are found a t 2.0eV and 2.6eV. These absorption spectra resemble t h a t o f a-Fez0 f i l m (44nm t h i c k ) on alumina r e p o r t e d by Gardner et.a1/4/. According t o t h e c a l c u f a t i o n and review o f Debnath e t a1/5/ t h e two a b s o r p t i o n edges s t a r t i n g from 2.0eV and from 2.6eV correspond t o t h e e l e c t r o n i c t r a n s i t i o n o f Fe 3d+Fe 3d and t h a t o f 0 2p+Fe 3d, r e s p e c t i v e l y . The above r e s u l t o f t h e a b s o r p t i o n spectrum i n d i c a t e s t h a t t h e e l e c t r o n i c s t r u c t u r e o f t h e passive f i l m does n o t e s s e n t i a l l y d i f f e r from t h a t o f t h e Fe 03.~ 6 e s i m i l a r measurements were a1 so conducted w i t h t h e n i c k e l p a s s i v a t i o n f i l m i n borate s o l u t i o n . The r e s u l t s a r e g i v e n i n Fig.4. No c h a r a t e r i s t i c f e a t u r e can be found i n t h e spectrum o f complex r e f r a c t i v e index o f t h e f i l m which i s about 1 nm t h i c k . The e x t i n c t i o n index, however, i s seen t o depend on t h e anodic p o t e n t i a l , i n c r e a s i n g from k2 = 0.1 a t E = 0.4V near t h e p a s s i v a t i o n p o t e n t i a l t o k = 1.0 a t E = 1.64V i n t h e secondary passive p o t e n t i a l region. The p o t e n t i a l dependent k2 i n d i c a t e s t h a t t h e f i l m composition changes g r a d u a l l y w i t h p o t e n t i a l .
The t i t a n i u m p a s s i v a t i o n f i l m a t t a i n s much l a r g e r thickness than t h e passive f i l m s on i r o n and n i c k e l . Fig.5 shows t h e complex r e f r a c t i v e i n d i c e s o f t h e f i l m s 33nm t h i c k and 25nm t h i c k . The r e f r a c t i v e index g r a d u a l l y increases w i t h
Wavelength
,
A / 102nrn -'E Photon Energy
,
hu /eV 4.0 3.5 3.0 2.6 2.4 2.2 2.02 5 1 I ! I I I I I
Photon Energy, h u /eV 4 3 . 5 3 215
1.64V ( dav,= 1 . 3 2 n y )
-.A
*-.-.-.-.- -.-.
2s I. 14 V ( da$0.92nm)
\
Wavelength
,
X/102nm Fe Passive FilmFig.3 Absorption c o e f f i c i e n t specta o f Fig.4 Spectra o f complex r e f r a c t i v e t h e passive f i l m s on i r o n c a l c u l a t e d index and thickness o f t h e passive from t h e e x t i n c t i o n index k2. f i l m s formed on n i c k e l i n borate
s o l u t i o n a t pH 8.4.
JOURNAL DE PHYSIQUE
Photon Eneray -. , h v / eV
4 3.5 3 2.5 2
I I I I I
TI / pH 8.4 borate soln.
4 - -
':
Wavelength, A / l o 2 nrn
Fig.5 Spectra o f complex r e f r a c t i v e index o f the anodic oxide f i l m s formed on t i t a n i u m i n b o r a t e s o l u t i o n a t pH 8.4.
decreasing wave1 ength, and t h e e x t i n c t i o n index a l s o increases f o r wavelength smal- l e r than 390nm showing t h e absorption edge a t 3.2 eV. This absorption edge i s s l i g h t l y l a r g e r than t h e band gap (3.05V) o f the b u l k Ti02. This discrepancy may probably be a t t r i b u t e d t o t h e h y d r a t i o n o f t h e anodic oxide f i l m .
REFERENCES
1) T.Ohtsuka and K.E. Heusler, J. E l e c t r o a n a l
.
Chem.,2,
(1979) 319 : K.E, Heusler and T. Ohtsuka, S u r f . Sci.,101,
(1980) 1942) C.T. Che and B.D. Cahan, J. Electrochem. Soc.,
2,
(1982) 171 3) W.-K. Paik and J. O'M. Bockris, S u r f . Sci.,8,
(1971) 614) R.F.G. Gardner, F. Sweett and D.W. Tanner, J. Phys. Chem. S o l i d s ,
