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IR reflection-absorption study of O2 and H2O interaction with evaporated aluminum films
Surface Science Letters 244 (1991) L l 1 3 - L 1 1 5 North-Holland
Ll13
Surface Science Letters
IR reflection-absorption study of 02 and H20 interaction with evaporated aluminum films E.I. F i r s o v a n d P.A. S h a f r a n o v s k y * Institute for Spectroscopy of the USSR Academy of Science, 142092 Troitzk, Moscow Region, USSR Received 17 July 1990; accepted for publication 19 November 1990
The IR-RA spectra of 02 and H 2 0 chemisorbed on evaporated AI films are presented in the 700-1300 cm - I range. The absorption bands at 845 and 855 cm -1 are assigned to the subsurface A1-O stretching vibration while the band at 940 cm -1 is assigned to the A I - O H stretching vibration of a surface hydroxide layer.
1. Introduction The formation of a transition oxide layer during the oxidation of aluminum surfaces is a process of considerable practical significance as oxide films render the surface passive to most forms of corrosion and therefore makes aluminum a valuable structural material. The barrier type oxide films formed in solutions and natural oxide films formed in air on the surface of evaporated A1 films have usually been studied by IR transmission spectroscopy and IRRA spectroscopy [1,2]. One infrared absorption band observed in the in situ RA spectra has been given different interpretations. This band occurs at approximately 940 c m - k Some authors [1,2] assign this band to the longitudinal mode in A1203 thin films while other authors [3] assign it to a bending mode of hydroxyl groups on the surface of the film (as all the experiments were carried out in air). Other papers consider this band to be the A1-O symmetric stretch vibration mode of barrier type oxide [4]. In this Letter we report and discuss the experiments on oxygen and water interaction with AI film which we suppose will answer some of the
* To whom all correspondence should be addressed.
questions concerning the nature of the transition layer and the origin of the 940 cm -1 absorption band.
2. Experimental details The experiments reported here were made in a stainless steel U H V chamber (USU-4) with a base pressure less than 1 x 10-10 mbar. The chamber is equipped with an evaporation system, monopole mass spectrometer APDM-1 for the residual gas composition control, and a sample holder situated in the middle of the chamber between two KBr IR windows. The procedure of sample preparation was as follows. An amorphous aluminum film ( d = 1000 .~) was evaporated in U H V conditions on a polished glass plate (8 × 2 × 0.4 cm 3). The deposition rate of A1 (99.99% purity) was controlled by measuring the calibrated quartz detector frequency. During A1 deposition, the system pressure increased to 1 x 10 -s mbar. According to mass spectra the pressure increase was attributed to H 2 and N2, which do not influence the processes under investigation at room temperature. An aluminum film was oxidised by 02 gas introduced via a leak valve. 02 gas was obtained by heating of previously degassed K M n O 4. After
0039-6028/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
L 114
E. 1. Firsov, P.A. Shafranovsky / IRRA S of O: and H,O on evaporated A 1films
the oxidation the sample was exposed to H 2 0 (D20) vapor. The I R - R A spectra were recorded with a Bomem DA3-002 F T - I R spectrometer at a grazing incidence angle of about 87 + 2 °. The resolution was 2 cm -1, 1 c m / s scan velocity, 400 scans.
a
0 E 0 .13 k0 .13 0 I
72+°L
O ¢-
3. Results and interpretation
*d
In figs. l b - l d the RA spectra of an oxidised aluminum film obtained at different O 2 exposures (0) are shown. From these spectra one can observe the formation of the absorption band at 845 cm -1 (002 = 24 L, where 1 L = 1 X10 - 6 Torr- s). The band at 855 cm -~ was formed at 0 > 30 L and became dominant at 0o2 = 318 L. It is evident that these two bands may be assigned to a surface E~'po.~l.lre:
T= 500K
eoz/AI L o
b
c
~
.
.
d
~
~
~
24L
~
72L 318L
U tO
_O L
0 _o e
50L
f
O ¢J tO
1400L
£3
L_
i POO
I
900
I
i
f
IO
I 1 0
I
=
h
I
T=3OOK '00
• 960
' 1 ~bo . . . .
frecluencv (cm-1)
13bo"
Fig. 2. I R - R A s p e c t r a of H 2 0 (a) a n d D 2 0 (b) a d s o r b e d o n an oxydised AI film. T h e isotopic shift is a b o u t 15 c m - 1.
alumina layer. It is worth saying that almost the same bands were observed by authors in ref. [5] using diode laser I R - R A spectroscopy of an AI(100) surface interacting with oxygen. They assigned them to a subsurface oxygen. During water interaction with alumina the film that was formed (see figs. l e - l g ) showed an absorption band at 925 cm -1 (at 0H2o > 50 L). In the case of D20 adsorption on an alumivmm film this band shifts to lower frequencies by 15 cm-1 (see fig. 2). The band at 855 cm - t remains constant in its intensity. At 0H2o > 7000 L we noticed the appearance of a small and broad absorption band at 1150 cm -~ which is assigned to the bending mode of hydrogen bonded surface hydroxyl groups [6]. All the facts mentioned above make us assign the band at 925 cm -~ to a symmetric stretch vibration of A 1 , - O H bond in surface hydroxide monolayer, where n = 3 (this value of n may explain small isotopic shift). When we add oxygen to the chamber (fig. le) the small increase in intensity of the 925 cm -1 absorption band is observed.
b
1300
frequency (cm-1) Fig. |. I R - R A spectra of oxygen (b, c, d) and water (e, f, g) a d s o r b e d on e v a p o r a t e d a n AI film o b t a i n e d at different exp o s u r e s ( 0 ( L ) , w h e r e 1 L = 1 0 - 6 T o r r . s ) . T h e resolution is 2 c m -1
4. Conclusions The absorption band at 925 cm-~ was assigned to a symmetric stretch vibration of surface hydroxide monolayer, which has either the form
E.L Firsov, P.A. Shafranovsky / 1 R R A S of O2 and He0 on evaporated Alfilms
surface three-fold bonded hydroxyl or the form of pseudoboehmite. The latter is less probable because we did not observe the two bands at 1090 and 1150 cm -1 which characterise the pseudoboehmite structure [1]. The absorption bands in the 860 cm-1 region were assigned to the alumina film itself. If we examine the IR-RA spectra of a natural alumina film obtained in air [2] we will notice the band at 860 cm-1 as a shoulder of the more intensive band at 940 cm-1. We believe that the results of the experiments with thick oxide films, such as barrier type oxides ( d > 100 ,~, [1,2]) or evaporated sapphire films (d > 1000/k [7]) do not contradict our results. In the case of the increased thickness the aluminum oxide frequency (I, = 855 cm -1) shifts to higher values. Thus for barrier type alumina [1,2] it occurs at 960 cm-1 and overlaps with the hydroxide band and is much stronger in intensity. In the case of very thick films [7] this band is observed at 1025 cm -1 and no hydroxide band may be seen because of the small intensity of the electric field in the vicinity of the hydroxide layer.
Ll15
Acknowledgements The authors would like to express their gratitude to Dr. N. Agladze for the assistance in operation with the Bomem DA3-002. We also thank professor G.N. Zhizhin and Dr. V.A. Yakovlev for fruitful discussions and interest in the work.
References [1] W. Vedder and D. Vermilyea, Trans. Faraday Soc. 113 (1968) 561. [2] A. Maeland, R. Rittenhouse, W. Lahan and P. Romano, Thin Solid Films 21 (1974) 67. [3] W: Bowser and W. Weinberg, Surf. Sci. 64 (1977) 377. [4] G. Dorsey, J. Eleetroehem. Soc. 113 (1966) 169. [5] V. Bermudes, R. Rubinovitz and J. Butler, J. Va¢. Sci. Teehnol. A 6 (1988) 717. [6] J. Lavalley and M. Bensitel, J. Molee. Struct. 175 (1988) 453. [7] A. Vasiliev, N. Gushanskaya, G. Zhizhin and V. Yakovlev, Sov. Zh. Pricl. Spectrosc. 48 (1988) 405.
Ll13
Surface Science Letters
IR reflection-absorption study of 02 and H20 interaction with evaporated aluminum films E.I. F i r s o v a n d P.A. S h a f r a n o v s k y * Institute for Spectroscopy of the USSR Academy of Science, 142092 Troitzk, Moscow Region, USSR Received 17 July 1990; accepted for publication 19 November 1990
The IR-RA spectra of 02 and H 2 0 chemisorbed on evaporated AI films are presented in the 700-1300 cm - I range. The absorption bands at 845 and 855 cm -1 are assigned to the subsurface A1-O stretching vibration while the band at 940 cm -1 is assigned to the A I - O H stretching vibration of a surface hydroxide layer.
1. Introduction The formation of a transition oxide layer during the oxidation of aluminum surfaces is a process of considerable practical significance as oxide films render the surface passive to most forms of corrosion and therefore makes aluminum a valuable structural material. The barrier type oxide films formed in solutions and natural oxide films formed in air on the surface of evaporated A1 films have usually been studied by IR transmission spectroscopy and IRRA spectroscopy [1,2]. One infrared absorption band observed in the in situ RA spectra has been given different interpretations. This band occurs at approximately 940 c m - k Some authors [1,2] assign this band to the longitudinal mode in A1203 thin films while other authors [3] assign it to a bending mode of hydroxyl groups on the surface of the film (as all the experiments were carried out in air). Other papers consider this band to be the A1-O symmetric stretch vibration mode of barrier type oxide [4]. In this Letter we report and discuss the experiments on oxygen and water interaction with AI film which we suppose will answer some of the
* To whom all correspondence should be addressed.
questions concerning the nature of the transition layer and the origin of the 940 cm -1 absorption band.
2. Experimental details The experiments reported here were made in a stainless steel U H V chamber (USU-4) with a base pressure less than 1 x 10-10 mbar. The chamber is equipped with an evaporation system, monopole mass spectrometer APDM-1 for the residual gas composition control, and a sample holder situated in the middle of the chamber between two KBr IR windows. The procedure of sample preparation was as follows. An amorphous aluminum film ( d = 1000 .~) was evaporated in U H V conditions on a polished glass plate (8 × 2 × 0.4 cm 3). The deposition rate of A1 (99.99% purity) was controlled by measuring the calibrated quartz detector frequency. During A1 deposition, the system pressure increased to 1 x 10 -s mbar. According to mass spectra the pressure increase was attributed to H 2 and N2, which do not influence the processes under investigation at room temperature. An aluminum film was oxidised by 02 gas introduced via a leak valve. 02 gas was obtained by heating of previously degassed K M n O 4. After
0039-6028/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
L 114
E. 1. Firsov, P.A. Shafranovsky / IRRA S of O: and H,O on evaporated A 1films
the oxidation the sample was exposed to H 2 0 (D20) vapor. The I R - R A spectra were recorded with a Bomem DA3-002 F T - I R spectrometer at a grazing incidence angle of about 87 + 2 °. The resolution was 2 cm -1, 1 c m / s scan velocity, 400 scans.
a
0 E 0 .13 k0 .13 0 I
72+°L
O ¢-
3. Results and interpretation
*d
In figs. l b - l d the RA spectra of an oxidised aluminum film obtained at different O 2 exposures (0) are shown. From these spectra one can observe the formation of the absorption band at 845 cm -1 (002 = 24 L, where 1 L = 1 X10 - 6 Torr- s). The band at 855 cm -~ was formed at 0 > 30 L and became dominant at 0o2 = 318 L. It is evident that these two bands may be assigned to a surface E~'po.~l.lre:
T= 500K
eoz/AI L o
b
c
~
.
.
d
~
~
~
24L
~
72L 318L
U tO
_O L
0 _o e
50L
f
O ¢J tO
1400L
£3
L_
i POO
I
900
I
i
f
IO
I 1 0
I
=
h
I
T=3OOK '00
• 960
' 1 ~bo . . . .
frecluencv (cm-1)
13bo"
Fig. 2. I R - R A s p e c t r a of H 2 0 (a) a n d D 2 0 (b) a d s o r b e d o n an oxydised AI film. T h e isotopic shift is a b o u t 15 c m - 1.
alumina layer. It is worth saying that almost the same bands were observed by authors in ref. [5] using diode laser I R - R A spectroscopy of an AI(100) surface interacting with oxygen. They assigned them to a subsurface oxygen. During water interaction with alumina the film that was formed (see figs. l e - l g ) showed an absorption band at 925 cm -1 (at 0H2o > 50 L). In the case of D20 adsorption on an alumivmm film this band shifts to lower frequencies by 15 cm-1 (see fig. 2). The band at 855 cm - t remains constant in its intensity. At 0H2o > 7000 L we noticed the appearance of a small and broad absorption band at 1150 cm -~ which is assigned to the bending mode of hydrogen bonded surface hydroxyl groups [6]. All the facts mentioned above make us assign the band at 925 cm -~ to a symmetric stretch vibration of A 1 , - O H bond in surface hydroxide monolayer, where n = 3 (this value of n may explain small isotopic shift). When we add oxygen to the chamber (fig. le) the small increase in intensity of the 925 cm -1 absorption band is observed.
b
1300
frequency (cm-1) Fig. |. I R - R A spectra of oxygen (b, c, d) and water (e, f, g) a d s o r b e d on e v a p o r a t e d a n AI film o b t a i n e d at different exp o s u r e s ( 0 ( L ) , w h e r e 1 L = 1 0 - 6 T o r r . s ) . T h e resolution is 2 c m -1
4. Conclusions The absorption band at 925 cm-~ was assigned to a symmetric stretch vibration of surface hydroxide monolayer, which has either the form
E.L Firsov, P.A. Shafranovsky / 1 R R A S of O2 and He0 on evaporated Alfilms
surface three-fold bonded hydroxyl or the form of pseudoboehmite. The latter is less probable because we did not observe the two bands at 1090 and 1150 cm -1 which characterise the pseudoboehmite structure [1]. The absorption bands in the 860 cm-1 region were assigned to the alumina film itself. If we examine the IR-RA spectra of a natural alumina film obtained in air [2] we will notice the band at 860 cm-1 as a shoulder of the more intensive band at 940 cm-1. We believe that the results of the experiments with thick oxide films, such as barrier type oxides ( d > 100 ,~, [1,2]) or evaporated sapphire films (d > 1000/k [7]) do not contradict our results. In the case of the increased thickness the aluminum oxide frequency (I, = 855 cm -1) shifts to higher values. Thus for barrier type alumina [1,2] it occurs at 960 cm-1 and overlaps with the hydroxide band and is much stronger in intensity. In the case of very thick films [7] this band is observed at 1025 cm -1 and no hydroxide band may be seen because of the small intensity of the electric field in the vicinity of the hydroxide layer.
Ll15
Acknowledgements The authors would like to express their gratitude to Dr. N. Agladze for the assistance in operation with the Bomem DA3-002. We also thank professor G.N. Zhizhin and Dr. V.A. Yakovlev for fruitful discussions and interest in the work.
References [1] W. Vedder and D. Vermilyea, Trans. Faraday Soc. 113 (1968) 561. [2] A. Maeland, R. Rittenhouse, W. Lahan and P. Romano, Thin Solid Films 21 (1974) 67. [3] W: Bowser and W. Weinberg, Surf. Sci. 64 (1977) 377. [4] G. Dorsey, J. Eleetroehem. Soc. 113 (1966) 169. [5] V. Bermudes, R. Rubinovitz and J. Butler, J. Va¢. Sci. Teehnol. A 6 (1988) 717. [6] J. Lavalley and M. Bensitel, J. Molee. Struct. 175 (1988) 453. [7] A. Vasiliev, N. Gushanskaya, G. Zhizhin and V. Yakovlev, Sov. Zh. Pricl. Spectrosc. 48 (1988) 405.