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Characterization of aluminum hydroxide thin film on metallic aluminum powder
Materials Letters 57 (2003) 2907 – 2913 www.elsevier.com/locate/matlet
Characterization of aluminum hydroxide thin film on metallic aluminum powder Nsoki Phambu * Department of Chemistry, Johnson C. Smith University, Charlotte, NC 28262, USA Received 23 September 2002; accepted 28 September 2002
Abstract This article reports on the thickness and nature of the passivation film of industrial aluminum powders, which are of a great technological interest. These powders are used in the iron and steel industry as agents of reduction and pigment and as solid fuel ˚, of engines and rockets. Grain size was 9 Am. The XPS results enabled the determination of the thickness of the thin film, 35 A and to confirm the presence of aluminum hydroxide in the film, without indicating the type of aluminum hydroxide. The Raman spectrum revealed that the aluminum hydroxide of the passivation film was a bayerite. Only the infrared spectra allowed the characterization of both the nature and the presence of reactive sites on the powder: the passivation film was made up of + bayerite containing surface groups of AlOH1/2 type. I emphasize, herein, the spectroscopic performance carried out for 2 obtaining the infrared and Raman spectra of the thin film of a thickness less than 5 nm. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Aluminum substrate; Bayerite; Characterization methods; Diffuse reflectance infrared Fourier transform spectra; Nanostructure; Raman spectra; Thin film; XPS spectra
1. Introduction Metallic aluminum and its products of passivation have only been extensively studied during the last 40 years. The first studies by Hart [1], in 1957, and later further by Vedder and Vermilyea [2], in 1969, investigated the nature of oxides and hydroxides which could grow on a previously electro-polished aluminum foil, immersed in water. The layers observed ˚ of thickness. They were able to exceeded the 100 A establish the different natures of hydroxides that formed as a function of the temperature of the bath.
* Tel.: +1-704-378-1298; fax: +1-704-378-1213. E-mail address: [email protected] (N. Phambu).
Below 60 jC, oxidation film grows in three major steps: (1) growth of an amorphous layer of oxyhydroxide, (2) growth and transformation of the amorphous film into boehmite, and (3) change at the end of growth to give an external film of bayerite which will inhibit its own growth. To solve the problem of oxidation in a given atmosphere, investigations were made on aluminum films deposited under ultra-high vacuum and oriented substrates [3– 6]. The (100), (110) and (111) faces constituted the main part of the studied interfaces. Monitoring and analysis of the reactions were generally made by the AES and XPS techniques or by infrared spectroscopy with grazing angle. The infrared spectroscopes on samples provided some information by using techniques of reflection
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01395-2
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N. Phambu / Materials Letters 57 (2003) 2907–2913
with grazing angle or of multiple reflection [7 – 9]. Very few works have been directly interested in the range of O – H stretching modes. The range 800 –1500 cm 1, where Al – OH and Al – OAl vibrations are expected, has been more fully investigated. The controversy for the assignment of a fluctuating band between 940 and 970 cm 1 [2,7,9 –11], is still under investigation. From diverse studies, we have concluded that this band is a band of Al –OH deformation mode. For Firsov and Shafranovsky [9], this band was a Al – OH stretching mode. The study of the deposited films by the High Resolution Electron Energy Loss Spectroscopy (HREELS) has been done [6,12,13]. However, this technique does not easily cover the area of O – H stretching modes due to a bad signal-to-noise ratio in this area; consequently, studies of this type were done specially made within the 0– 2000 cm 1 range. In the literature studies on the metallic aluminum and (oxy)hydroxide interface, studies that treat metallic powders using vibration spectroscopes or the vibration spectroscopes used on industrial aluminum are not found. Infrared spectroscopy has not been used for characterizing a layer of passivation ranging ˚ , but only for characterizing between 20 and 60 A the first states of oxidation or films of more than ˚. 300 A The objective of this study was therefore to show how infrared and Raman spectroscopes are able to characterize a film of oxidation of less than ˚ on a metallic aluminum powder. For the first 50 A time, the range of OH stretching modes was also investigated. These aluminum powders are of significant technological interest because they can be used as solid fuel in engines. Moreover, mixtures of aluminum powder with oxygen have a low temperature of combustion, the reaction being highly exothermic and able to reach very high pressures of explosion.
3. Characterization methods 3.1. Infrared spectrometry The infrared spectra were obtained with a Fourier Transform infrared spectrometer device, Perkin-Elmer System 2000. An easy and rapid characterization was obtained by collecting the diffuse reflectance spectrum of the sample melted with KBr at 10% in weight, with the optical device Harrick HVCDRP. The spectral resolution was 4 cm 1. The detector used was a broad band DTGS. 3.2. Raman spectrometry Raman spectra were recorded with a triple-subtractive-monochromator Jobin Yvon T64000 spectrometer equipped with a confocal microscope. The detector was a charged– coupled device cooled by liquid nitrogen. The Raman spectra were excited by a laser beam at 514.53 nm emitted by an argon laser (Stabilite 2017, Spectra Physics), focused on the samples with a diameter of about 1.5 Am and a power of about 20 mW. The Raman backscattering was collected through the microscope objective ( 50) and dispersed by an 1800-groove/mm grating to obtain 2.7-cm 1 spectral resolution. The precision of the wave number in a vacuum was better than 0.8 cm 1. 3.3. XPS XPS spectra of the Al 2p and O 1s photoelectron lines were recorded with a PHI 5400 ESCA set at a constant analyzer pass energy of 35.75 eV with a step size of 0.1 eV, using unmonochromatised incident Mg X-ray radiation.
4. Results 4.1. Infrared spectra
2. Materials The metallic aluminum powder was provided by Pechiney (France) and was used without further purification. My sample presents a specific surface area of 0.25 m2/g measured by an adsorption isotherm of nitrogen.
At first, no signal could be detected in order to obtain the spectrum of a pellet of a mixture of aluminum powder and potassium bromide. When the powder was mixed 10% w/w with KBr, a diffuse reflectance infrared Fourier Transform (DRIFT) spectrum with a satisfactory signal-to-noise ratio was obtained (Fig. 1). On
N. Phambu / Materials Letters 57 (2003) 2907–2913
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Fig. 1. DRIFT spectrum of the passivation layer formed on aluminum powder diluted in KBr (specific area = 0.25 m2/g and grain size = 9 Am).
this spectrum, the spectral components of the bayerite at 3656 and 3549 cm 1 as well as at 980 cm 1 were found, with also a very strong component at 3464 and 1031 cm 1. The very intense component at 3464 cm 1 was accompanied by that at 3440 cm 1 on enlargement (Fig. 2). I thus found the spectral specificities of samples with large specific surface area, with a very intense component at 3464 cm 1. It was demonstrated
in a earlier work that this component at 3464 cm 1 is due to surface groups of AlOH21/2 + type [14,15]. These surface groups are the most reactive [16,17]. 4.2. Raman spectra Obtaining the Raman spectrum was difficult. After many unsuccessful attempts using traditional geometry
Fig. 2. DRIFT spectrum of the passivation layer formed on aluminum powder diluted in KBr (specific area = 0.25 m2/g and grain size = 9 Am) in the region of OH stretching modes.
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Fig. 3. Raman spectrum of the passivation layer formed on aluminum powder.
in macro, I tried to obtain the retro diffusion spectrum of the powder under microscope. Fig. 3 shows the spectra obtained with different objectives. An extremely weak and uncertain signal was obtained when an objective ( 50) with weak numerical aperture (N.A. f 0.3) was used and the excitation laser focused only on one grain. On the other hand, the objective ( 100) (N.A. 0.95) provided a spectrum of good quality. That is the traditional Raman spectrum of the bayerite, with wave numbers at 3651, 3543 and 3427 cm 1 (with the shoulder at 3440 cm 1). In addition, a very weak signal was detected at approximately 3470 cm 1.
4.3. XPS spectra: evaluation of thickness It has been reported that it is possible to obtain, from the binding energies of the band O(1s), the difference between oxide and hydroxide on common materials [5,18]. However, it is difficult with an aluminum powder to know if a hydroxide or an oxide has been obtained. Indeed, the charge effects can be more significant on a metallic powder than on metallic sheets. In addition, since the investigated surface of the powder that is made up of ‘‘metallic balls’’ is rough, different angles are thus measured compared to the film of a
Fig. 4. Al 2p XPS spectra of a pure bayerite sample, an aluminum sheet and a aluminum powder are given for comparison.
N. Phambu / Materials Letters 57 (2003) 2907–2913
sheet sample. The XPS spectrum that was obtained on our powder is seen in Fig. 4. However, the O/AlIII ratio from the XPS spectrum is a very quantitative information. The ratio of our sample was 2.7, confirming that we have a form close to a Al(OH)3 hydroxide (since boehmite, diaspore AlOOH would give a ratio of 2, and that alumina would give 1.5). In the range corresponding to the transitions from the 2p orbitals of Al (around 75 eV), the spectrum also displays a band corresponding to aluminum atoms not oxidized. This fact demonstrates that the passivation film was very thin. If we had a sheet sample covered with a perfectly homogeneous film of Al(OH)3, the thickness estimated from the AlB/ ˚ . This number gives the AlIII ratio would be about 35 A order of magnitude of what we can imagine, but with the assumptions made, it does not make absolute sense for the powder sample used in our study.
5. Discussion 5.1. Obtaining the Raman and infrared spectra Obtaining the Raman spectrum of the passivation film, which has a relatively low thickness, is practi-
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cally impossible in macro- or micro-sampling using an objective of weak numerical aperture because the excitation of the laser arrives at almost perpendicularly angle to the surface of the metallic aluminum and is thus reflected. Because of the Maxwell equations on the metal surfaces, the electric field is almost nonexistent at the interface. On the other hand, with a very large numerical aperture, a part of the interface is irradiated in a grazing pattern. Thus, the path of the beam in the hydroxide layer is longer and, in addition, the electric field perpendicular to the hydroxide/metal interface exists and is even amplified by a ‘‘mirror effect’’. The simple observations indicate that it is possible to obtain the Raman spectrum of layers as ˚ but only by using objectives with large thin as 35 A numerical aperture. In addition, the advantage of having a metallic support allows amplifying the existing field and bringing back all the Raman diffusion into a single hemisphere that is almost entirely viewed by the microscope objective. Although the infrared spectrum was obtained by using the diffuse reflectance accessory, I may consider that it is a double transmission spectrum through the passivation film, with a specular reflection with grazing angle on the metallic core of the powder grains
Fig. 5. Schematic representation of the multiple reflection – absorption phenomenon. The balls represent the aluminum particles. The sample is a pure aluminum powder.
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(Fig. 5). As previously stated, the grazing angles relative to the surface do not enable the cancellation of the electric field in the film. Fig. 5 enables visualizing of the phenomenon of multiple reflection – absorption. If this effect is real, I must state that beyond a certain concentration of aluminum in KBr, the spectrum should not change any more in intensity and remain identical to that observed for a non-diluted aluminum powder. This is what is observed in my experiments. For this limiting concentration, I estimate that the incident beam is entirely reflected by aluminum at least once, and that the projection on the horizontal plan of the balls of aluminum covers all the irradiated section (Fig. 5). 5.2. Decomposition of the infrared spectrum and statement on the division of the bayerite of the passivation A decomposition method was used to integrate the intensity of each infrared component. Fig. 6 gives the results of the decomposition of the spectrum with the decomposition parameters used in an earlier work [15], only the widths at mid-height and the intensities are initially free. I find the traditional spectrum of the bayerite with the four characteristic vibrations at 3656, 3560, 3548, and 3420 cm 1, in relative inten-
sities very close to those found in the absorption spectra, with only the band at 3420 cm 1 appearing weaker. In addition to this spectrum, I see the very intense band at 3465 cm 1 with that at 3438 cm 1. If I suppose that the DRIFT spectrum obtained here is a real absorption spectrum, I can, from the infrared spectrum information and particularly from the relative intensity of the band at 3465 cm 1 compared to that at 3656 cm 1, semi-quantitatively evaluate the lateral specific surface area developed by the bayerite of passivation. While referring to Fig. 12 of Ref. [15], I find approximately a lateral specific surface area of 3 m2/g of bayerite. If I consider that my film is of a uniform layer, the thickness of 3 to 4 nm evaluated by XPS gives a specific surface area of bayerite of approximately: 4p ð4:5 AmÞ2 4p ð4:5 AmÞ2 4 nm 2:53 g=cm3 c 130 to 100 m2 =g of bayerite These two results indicate that bayerite on the metal is covering by exposing outside the basal hydroxides, and thus has only a few lateral hydroxides [15]. A diagrammatic vision would be to consider ‘‘flat’’ plans of bayerite on the surface of the metal, with openings for lateral hydroxides.
Fig. 6. Decomposition results for a pure bayerite sample. The spectrum was obtained in transmission without dilution.
N. Phambu / Materials Letters 57 (2003) 2907–2913
6. Conclusions The infrared diffuse reflectance technique is a powerful tool for obtaining the equivalent of a multiple absorption –reflection spectrum with grazing angle of a film of few nanometers on a metallic powder support. In Raman spectroscopy, the use of objective with very large numerical aperture is required for obtaining incidence angles large enough to have a sufficiently significant component of the electric field perpendicular to the surface. I also was able to determine the nature of the hydroxide of surface, which is impossible by other techniques (XPS, X-rays or other).
Acknowledgements The author is grateful to Dr. Dan Lucero for the reviews and useful comments.
References [1] R.K. Hart, Trans. Faraday Soc. 53 (1957) 1020 – 1027. [2] W. Vedder, D. Vermilyea, Trans. Faraday Soc. 65 (1969) 561.
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[3] A. Misra, H.D. Bist, M.S. Navati, R.K. Thareja, J. Narayan, Mater. Sci. Eng., B 79 (1) (2001) 49 – 54. [4] M. Legros, G. Dehm, R.M. Keller-Flaug, E. Arzt, K.J. Hemker, S. Suresh, Mater. Sci. Eng., A 309 – 310 (2001) 463 – 467. [5] L.P.H. Jeurgens, W.G. Sloof, F.D. Tichelaar, C.G. Borsboom, E.J. Mittemeijer, Appl. Surf. Sci. 144 – 145 (1999) 11 – 15. [6] J.E. Crowel, J.G. Chen, J.T. Yates Jr., J. Electron Spectrosc. Relat. Phenom. 39 (1986) 97. [7] P. Mertens, Surf. Sci. 71 (1978) 161 – 173. [8] G.W. Poling, J. Electrochem. Soc. 116 (1969) 958. [9] E.I. Firsov, P.A. Shafranovsky, Surf. Sci. 244 (1991) L113 – L115. [10] R.W. Hannah, Appl. Spectrosc. 17 (1963) 23. [11] G.A. Dorsey Jr., J. Electrochem. Soc. 113 (1966) 169 – 172. [12] J.G. Chen, J.E. Crowel, J.T. Yates Jr., Surf. Sci. 172 (1986) 97. [13] J.G. Chen, P. Basu, L. Ng, J.T. Yates Jr., Surf. Sci. 194 (1988) 397 – 418. [14] N. Phambu, Preparation d’hydroxides d’aluminium. Caracterisation structurale, morphologique et superficielle. Application a l’etude d’une couche de passivation d’aluminium. PhD, thesis, Univ. Nancy, Nancy, France, 1996. [15] N. Phambu, B. Humbert, A. Burneau, Langmuir 16 (15) (2000) 6200 – 6207. [16] E. Molisa, O. Barresa, H. Marchanda, E. Sauzeata, B. Humbert, F. Thomas, Colloids Surf., A Physicochem. Eng. Asp. 163 (2 – 3) (2000) 283 – 292. [17] N. Phambu, Appl. Spectrosc. 56 (6) (2002) 756 – 761. [18] M. Textor, Amstutz, Anal. Chim. Acta 297 (1994) 15 – 26.
Characterization of aluminum hydroxide thin film on metallic aluminum powder Nsoki Phambu * Department of Chemistry, Johnson C. Smith University, Charlotte, NC 28262, USA Received 23 September 2002; accepted 28 September 2002
Abstract This article reports on the thickness and nature of the passivation film of industrial aluminum powders, which are of a great technological interest. These powders are used in the iron and steel industry as agents of reduction and pigment and as solid fuel ˚, of engines and rockets. Grain size was 9 Am. The XPS results enabled the determination of the thickness of the thin film, 35 A and to confirm the presence of aluminum hydroxide in the film, without indicating the type of aluminum hydroxide. The Raman spectrum revealed that the aluminum hydroxide of the passivation film was a bayerite. Only the infrared spectra allowed the characterization of both the nature and the presence of reactive sites on the powder: the passivation film was made up of + bayerite containing surface groups of AlOH1/2 type. I emphasize, herein, the spectroscopic performance carried out for 2 obtaining the infrared and Raman spectra of the thin film of a thickness less than 5 nm. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Aluminum substrate; Bayerite; Characterization methods; Diffuse reflectance infrared Fourier transform spectra; Nanostructure; Raman spectra; Thin film; XPS spectra
1. Introduction Metallic aluminum and its products of passivation have only been extensively studied during the last 40 years. The first studies by Hart [1], in 1957, and later further by Vedder and Vermilyea [2], in 1969, investigated the nature of oxides and hydroxides which could grow on a previously electro-polished aluminum foil, immersed in water. The layers observed ˚ of thickness. They were able to exceeded the 100 A establish the different natures of hydroxides that formed as a function of the temperature of the bath.
* Tel.: +1-704-378-1298; fax: +1-704-378-1213. E-mail address: [email protected] (N. Phambu).
Below 60 jC, oxidation film grows in three major steps: (1) growth of an amorphous layer of oxyhydroxide, (2) growth and transformation of the amorphous film into boehmite, and (3) change at the end of growth to give an external film of bayerite which will inhibit its own growth. To solve the problem of oxidation in a given atmosphere, investigations were made on aluminum films deposited under ultra-high vacuum and oriented substrates [3– 6]. The (100), (110) and (111) faces constituted the main part of the studied interfaces. Monitoring and analysis of the reactions were generally made by the AES and XPS techniques or by infrared spectroscopy with grazing angle. The infrared spectroscopes on samples provided some information by using techniques of reflection
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01395-2
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N. Phambu / Materials Letters 57 (2003) 2907–2913
with grazing angle or of multiple reflection [7 – 9]. Very few works have been directly interested in the range of O – H stretching modes. The range 800 –1500 cm 1, where Al – OH and Al – OAl vibrations are expected, has been more fully investigated. The controversy for the assignment of a fluctuating band between 940 and 970 cm 1 [2,7,9 –11], is still under investigation. From diverse studies, we have concluded that this band is a band of Al –OH deformation mode. For Firsov and Shafranovsky [9], this band was a Al – OH stretching mode. The study of the deposited films by the High Resolution Electron Energy Loss Spectroscopy (HREELS) has been done [6,12,13]. However, this technique does not easily cover the area of O – H stretching modes due to a bad signal-to-noise ratio in this area; consequently, studies of this type were done specially made within the 0– 2000 cm 1 range. In the literature studies on the metallic aluminum and (oxy)hydroxide interface, studies that treat metallic powders using vibration spectroscopes or the vibration spectroscopes used on industrial aluminum are not found. Infrared spectroscopy has not been used for characterizing a layer of passivation ranging ˚ , but only for characterizing between 20 and 60 A the first states of oxidation or films of more than ˚. 300 A The objective of this study was therefore to show how infrared and Raman spectroscopes are able to characterize a film of oxidation of less than ˚ on a metallic aluminum powder. For the first 50 A time, the range of OH stretching modes was also investigated. These aluminum powders are of significant technological interest because they can be used as solid fuel in engines. Moreover, mixtures of aluminum powder with oxygen have a low temperature of combustion, the reaction being highly exothermic and able to reach very high pressures of explosion.
3. Characterization methods 3.1. Infrared spectrometry The infrared spectra were obtained with a Fourier Transform infrared spectrometer device, Perkin-Elmer System 2000. An easy and rapid characterization was obtained by collecting the diffuse reflectance spectrum of the sample melted with KBr at 10% in weight, with the optical device Harrick HVCDRP. The spectral resolution was 4 cm 1. The detector used was a broad band DTGS. 3.2. Raman spectrometry Raman spectra were recorded with a triple-subtractive-monochromator Jobin Yvon T64000 spectrometer equipped with a confocal microscope. The detector was a charged– coupled device cooled by liquid nitrogen. The Raman spectra were excited by a laser beam at 514.53 nm emitted by an argon laser (Stabilite 2017, Spectra Physics), focused on the samples with a diameter of about 1.5 Am and a power of about 20 mW. The Raman backscattering was collected through the microscope objective ( 50) and dispersed by an 1800-groove/mm grating to obtain 2.7-cm 1 spectral resolution. The precision of the wave number in a vacuum was better than 0.8 cm 1. 3.3. XPS XPS spectra of the Al 2p and O 1s photoelectron lines were recorded with a PHI 5400 ESCA set at a constant analyzer pass energy of 35.75 eV with a step size of 0.1 eV, using unmonochromatised incident Mg X-ray radiation.
4. Results 4.1. Infrared spectra
2. Materials The metallic aluminum powder was provided by Pechiney (France) and was used without further purification. My sample presents a specific surface area of 0.25 m2/g measured by an adsorption isotherm of nitrogen.
At first, no signal could be detected in order to obtain the spectrum of a pellet of a mixture of aluminum powder and potassium bromide. When the powder was mixed 10% w/w with KBr, a diffuse reflectance infrared Fourier Transform (DRIFT) spectrum with a satisfactory signal-to-noise ratio was obtained (Fig. 1). On
N. Phambu / Materials Letters 57 (2003) 2907–2913
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Fig. 1. DRIFT spectrum of the passivation layer formed on aluminum powder diluted in KBr (specific area = 0.25 m2/g and grain size = 9 Am).
this spectrum, the spectral components of the bayerite at 3656 and 3549 cm 1 as well as at 980 cm 1 were found, with also a very strong component at 3464 and 1031 cm 1. The very intense component at 3464 cm 1 was accompanied by that at 3440 cm 1 on enlargement (Fig. 2). I thus found the spectral specificities of samples with large specific surface area, with a very intense component at 3464 cm 1. It was demonstrated
in a earlier work that this component at 3464 cm 1 is due to surface groups of AlOH21/2 + type [14,15]. These surface groups are the most reactive [16,17]. 4.2. Raman spectra Obtaining the Raman spectrum was difficult. After many unsuccessful attempts using traditional geometry
Fig. 2. DRIFT spectrum of the passivation layer formed on aluminum powder diluted in KBr (specific area = 0.25 m2/g and grain size = 9 Am) in the region of OH stretching modes.
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Fig. 3. Raman spectrum of the passivation layer formed on aluminum powder.
in macro, I tried to obtain the retro diffusion spectrum of the powder under microscope. Fig. 3 shows the spectra obtained with different objectives. An extremely weak and uncertain signal was obtained when an objective ( 50) with weak numerical aperture (N.A. f 0.3) was used and the excitation laser focused only on one grain. On the other hand, the objective ( 100) (N.A. 0.95) provided a spectrum of good quality. That is the traditional Raman spectrum of the bayerite, with wave numbers at 3651, 3543 and 3427 cm 1 (with the shoulder at 3440 cm 1). In addition, a very weak signal was detected at approximately 3470 cm 1.
4.3. XPS spectra: evaluation of thickness It has been reported that it is possible to obtain, from the binding energies of the band O(1s), the difference between oxide and hydroxide on common materials [5,18]. However, it is difficult with an aluminum powder to know if a hydroxide or an oxide has been obtained. Indeed, the charge effects can be more significant on a metallic powder than on metallic sheets. In addition, since the investigated surface of the powder that is made up of ‘‘metallic balls’’ is rough, different angles are thus measured compared to the film of a
Fig. 4. Al 2p XPS spectra of a pure bayerite sample, an aluminum sheet and a aluminum powder are given for comparison.
N. Phambu / Materials Letters 57 (2003) 2907–2913
sheet sample. The XPS spectrum that was obtained on our powder is seen in Fig. 4. However, the O/AlIII ratio from the XPS spectrum is a very quantitative information. The ratio of our sample was 2.7, confirming that we have a form close to a Al(OH)3 hydroxide (since boehmite, diaspore AlOOH would give a ratio of 2, and that alumina would give 1.5). In the range corresponding to the transitions from the 2p orbitals of Al (around 75 eV), the spectrum also displays a band corresponding to aluminum atoms not oxidized. This fact demonstrates that the passivation film was very thin. If we had a sheet sample covered with a perfectly homogeneous film of Al(OH)3, the thickness estimated from the AlB/ ˚ . This number gives the AlIII ratio would be about 35 A order of magnitude of what we can imagine, but with the assumptions made, it does not make absolute sense for the powder sample used in our study.
5. Discussion 5.1. Obtaining the Raman and infrared spectra Obtaining the Raman spectrum of the passivation film, which has a relatively low thickness, is practi-
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cally impossible in macro- or micro-sampling using an objective of weak numerical aperture because the excitation of the laser arrives at almost perpendicularly angle to the surface of the metallic aluminum and is thus reflected. Because of the Maxwell equations on the metal surfaces, the electric field is almost nonexistent at the interface. On the other hand, with a very large numerical aperture, a part of the interface is irradiated in a grazing pattern. Thus, the path of the beam in the hydroxide layer is longer and, in addition, the electric field perpendicular to the hydroxide/metal interface exists and is even amplified by a ‘‘mirror effect’’. The simple observations indicate that it is possible to obtain the Raman spectrum of layers as ˚ but only by using objectives with large thin as 35 A numerical aperture. In addition, the advantage of having a metallic support allows amplifying the existing field and bringing back all the Raman diffusion into a single hemisphere that is almost entirely viewed by the microscope objective. Although the infrared spectrum was obtained by using the diffuse reflectance accessory, I may consider that it is a double transmission spectrum through the passivation film, with a specular reflection with grazing angle on the metallic core of the powder grains
Fig. 5. Schematic representation of the multiple reflection – absorption phenomenon. The balls represent the aluminum particles. The sample is a pure aluminum powder.
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N. Phambu / Materials Letters 57 (2003) 2907–2913
(Fig. 5). As previously stated, the grazing angles relative to the surface do not enable the cancellation of the electric field in the film. Fig. 5 enables visualizing of the phenomenon of multiple reflection – absorption. If this effect is real, I must state that beyond a certain concentration of aluminum in KBr, the spectrum should not change any more in intensity and remain identical to that observed for a non-diluted aluminum powder. This is what is observed in my experiments. For this limiting concentration, I estimate that the incident beam is entirely reflected by aluminum at least once, and that the projection on the horizontal plan of the balls of aluminum covers all the irradiated section (Fig. 5). 5.2. Decomposition of the infrared spectrum and statement on the division of the bayerite of the passivation A decomposition method was used to integrate the intensity of each infrared component. Fig. 6 gives the results of the decomposition of the spectrum with the decomposition parameters used in an earlier work [15], only the widths at mid-height and the intensities are initially free. I find the traditional spectrum of the bayerite with the four characteristic vibrations at 3656, 3560, 3548, and 3420 cm 1, in relative inten-
sities very close to those found in the absorption spectra, with only the band at 3420 cm 1 appearing weaker. In addition to this spectrum, I see the very intense band at 3465 cm 1 with that at 3438 cm 1. If I suppose that the DRIFT spectrum obtained here is a real absorption spectrum, I can, from the infrared spectrum information and particularly from the relative intensity of the band at 3465 cm 1 compared to that at 3656 cm 1, semi-quantitatively evaluate the lateral specific surface area developed by the bayerite of passivation. While referring to Fig. 12 of Ref. [15], I find approximately a lateral specific surface area of 3 m2/g of bayerite. If I consider that my film is of a uniform layer, the thickness of 3 to 4 nm evaluated by XPS gives a specific surface area of bayerite of approximately: 4p ð4:5 AmÞ2 4p ð4:5 AmÞ2 4 nm 2:53 g=cm3 c 130 to 100 m2 =g of bayerite These two results indicate that bayerite on the metal is covering by exposing outside the basal hydroxides, and thus has only a few lateral hydroxides [15]. A diagrammatic vision would be to consider ‘‘flat’’ plans of bayerite on the surface of the metal, with openings for lateral hydroxides.
Fig. 6. Decomposition results for a pure bayerite sample. The spectrum was obtained in transmission without dilution.
N. Phambu / Materials Letters 57 (2003) 2907–2913
6. Conclusions The infrared diffuse reflectance technique is a powerful tool for obtaining the equivalent of a multiple absorption –reflection spectrum with grazing angle of a film of few nanometers on a metallic powder support. In Raman spectroscopy, the use of objective with very large numerical aperture is required for obtaining incidence angles large enough to have a sufficiently significant component of the electric field perpendicular to the surface. I also was able to determine the nature of the hydroxide of surface, which is impossible by other techniques (XPS, X-rays or other).
Acknowledgements The author is grateful to Dr. Dan Lucero for the reviews and useful comments.
References [1] R.K. Hart, Trans. Faraday Soc. 53 (1957) 1020 – 1027. [2] W. Vedder, D. Vermilyea, Trans. Faraday Soc. 65 (1969) 561.
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[3] A. Misra, H.D. Bist, M.S. Navati, R.K. Thareja, J. Narayan, Mater. Sci. Eng., B 79 (1) (2001) 49 – 54. [4] M. Legros, G. Dehm, R.M. Keller-Flaug, E. Arzt, K.J. Hemker, S. Suresh, Mater. Sci. Eng., A 309 – 310 (2001) 463 – 467. [5] L.P.H. Jeurgens, W.G. Sloof, F.D. Tichelaar, C.G. Borsboom, E.J. Mittemeijer, Appl. Surf. Sci. 144 – 145 (1999) 11 – 15. [6] J.E. Crowel, J.G. Chen, J.T. Yates Jr., J. Electron Spectrosc. Relat. Phenom. 39 (1986) 97. [7] P. Mertens, Surf. Sci. 71 (1978) 161 – 173. [8] G.W. Poling, J. Electrochem. Soc. 116 (1969) 958. [9] E.I. Firsov, P.A. Shafranovsky, Surf. Sci. 244 (1991) L113 – L115. [10] R.W. Hannah, Appl. Spectrosc. 17 (1963) 23. [11] G.A. Dorsey Jr., J. Electrochem. Soc. 113 (1966) 169 – 172. [12] J.G. Chen, J.E. Crowel, J.T. Yates Jr., Surf. Sci. 172 (1986) 97. [13] J.G. Chen, P. Basu, L. Ng, J.T. Yates Jr., Surf. Sci. 194 (1988) 397 – 418. [14] N. Phambu, Preparation d’hydroxides d’aluminium. Caracterisation structurale, morphologique et superficielle. Application a l’etude d’une couche de passivation d’aluminium. PhD, thesis, Univ. Nancy, Nancy, France, 1996. [15] N. Phambu, B. Humbert, A. Burneau, Langmuir 16 (15) (2000) 6200 – 6207. [16] E. Molisa, O. Barresa, H. Marchanda, E. Sauzeata, B. Humbert, F. Thomas, Colloids Surf., A Physicochem. Eng. Asp. 163 (2 – 3) (2000) 283 – 292. [17] N. Phambu, Appl. Spectrosc. 56 (6) (2002) 756 – 761. [18] M. Textor, Amstutz, Anal. Chim. Acta 297 (1994) 15 – 26.