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Self-Assembly of Aminosilane Films on Silicidized Iron M. Rebhan,*,† A. Plagge, M. Rohwerder,‡ and M. Stratmann‡ Lehrstuhl fu¨ r Korrosion und Oberfla¨ chentechnik, Institut fu¨ r Werkstoffwissenschaften 4, Universita¨ t Erlangen-Nu¨ rnberg, Martensstr. 7, 91058 Erlangen, Germany Received January 29, 2001. In Final Form: December 12, 2001 It is shown that aminomethoxysilane (AEAPSi) forms almost a monolayer thick film during a hydrolysis on silicidized iron. The molecules chemisorb from a solution in water within a few minutes in a selfassembly (SA) process. IR and X-ray photoelectron spectroscopy measurements provide evidence for a chemical reaction of AEAPSi with the surface. The surface is covered by more than a half and less than one monolayer after 10 min duration of the self-assembly process (0.5 < θ < 1). The resulting SA film is ordered with the silicon-containing head of the AEAPSi molecules at the interface and the amino group at the outer surface. The AEAPSi molecules have a small tilt against the normal axis.
1. Introduction Covering and protecting iron and steel by polymeric coatings is a challenging task not only from the point of basic research but also from the point of industrial application. This process is necessary for many applications such as coating steel with colorful and corrosionprotecting paint for cars, washing machines, facades of houses, and so forth. But organic molecules adhere on the iron surface only if the surface is oxide free.1-3 Then, for example, mercaptane molecules form stable chemical bonds with the iron surface.4 Unfortunately, this can only be realized in high-vacuum systems because even a small amount of oxygen leads to a reoxidation of the iron surface. On the other side, huge coils of steel with widths of several meters run through the lines (speed of the coils ≈ 1 m/min) for the mass production of, for example, car steel. But unfortunately, for technical reasons it is not possible to integrate high-vacuum chambers into these production lines. Therefore, mercaptane molecules cannot be applied for the mass production of steel. Some kinds of silicon organic molecules adhere well on glass and silicon oxide surfaces.5,6 They form stable SiO-Si bonds with the substrate after hydrolysis in water:
nRSi(OCH3)3 + mH2O f HO(-SiR(OH)O-)nH (1) Then the OH group at the organic molecule RSi reacts with hydroxide bound to the surface Si(OH), * Corresponding author. E-mail: matthias.rebhan@ mchp.siemens.de. Tel: ++49-89-636-48507. Fax: ++49-89-63648555. † Present address: Siemens AG, CT D2P, Otto-Hahn-Ring 6, 81730 Munich, Germany. ‡ Present address: Max Planck Institute for Iron Resarch, MaxPlanck-Str. 1, 40237 Du¨sseldorf, Germany. (1) Reynders, B.; Stratmann, M. Fresenius’ J. Anal. Chem. 1991, 341, 406-407. (2) Reinartz, C.; Fu¨rbeth, W.; Stratmann, M. Fresenius’ J. Anal. Chem. 1995, 353, 657-660. (3) Stratmann, M.; Rohwerder, M.; Reinartz, C.; Bram, Ch.; Jung, Ch. ECASIA ’97: 7th European Conference on Applications of Surface and Interface Analysis; Olefjord, I., Nyberg, L., Briggs, D., Eds.; John Wiley & Sons: New York, 1997. (4) Vago, E. R.; de Weldige, K.; Rohwerder, M.; Stratmann, M. Fresenius’ J. Anal. Chem. 1995, 353, 316-319. (5) Plueddemann, E. P. Silane coupling agents; Plenum Press: New York, 1982. (6) Arkles, B.; Steinmetz, J. R.; Zazyczny, J.; Mehta, P. In Silanes and other Coupling agents; Mittal, K. L., Ed.; VSP: Utrecht, 1992; p 91.
RSi(OH) + Si(OH) f RSi-O-Si + H2O
(2)
while the tail R of the molecule points away from the surface. Equation 1 shows that the silicon organic molecules are connected with each other by Si-O-Si bonds. They form a two-dimensional network via the SiO-Si bonds. Therefore, a stable binding of polymeric coatings to the iron via the Si-O-Si bonds should be possible if the iron is covered by silicon oxide. On top of it, a thin layer of an adhesion promoter connects to the coating. The hydrolysis and the following reaction with the surface are summarized by eqs 1 and 2. The detailed reaction mechanism is described in ref 6. The overall aim of this project is to improve the adhesion of polymeric coatings on iron and steel. Hence, a silicon oxide rich surface is created on iron by a chemical vapor deposition (CVD) process as the first step. The reaction mechanism and the resulting surface structure are described elsewhere.7-10 In the second step, silicon organic molecules react during a self-assembly (SA) process with this surface. The reaction conditions and the properties of the resulting layer are described in this paper. This SA film acts as an adhesion promoter to polymeric coatings. The polymers were put on top of the system afterward, and then the polymer was cured so that the SA film and the coating reacted with and connected to each other. Finally, the electrochemical properties and the corrosion behavior of this system are shown in another paper.10,11 A schematic drawing of the whole multilayer system is given in Figure 1. It consists of the Fe substrate covered by an amorphous layer of a-SiO2:Fe. On top of it, the selfassembly film acts as an adhesion promoter to polymeric coatings. 2. Experimental Procedure Pure iron samples (Fe content > 99.995 at. %) with low carbon and sulfur content (C < 0.001 at. %, S < 0.0001 at. %) were used. (7) Rebhan, M.; Rohwerder, M.; Stratmann, M. Appl. Surf. Sci. 1999, 140, 99. (8) Rebhan, M.; Stratmann, M. In Werkstoffwoche ’98; Dimigen, H., Paatsch, W., Eds.; Wiley-VCH: 1999; Vol. IX, p 160. (9) Rebhan, M.; Meier, M.; Plagge, M.; Rohwerder, M.; Stratmann, M. Appl. Surf. Sci. 2001, 178. (10) Rebhan, M. Herstellung und Charakterisierung eines mehrkomponentigen Schichtsystems auf Siliziumbasis als neuartiger Korrosionsschutz von Eisenoberfla¨ chen und Haftvermittler zu Lacken; ShakerVerlag: Aachen, 2000. (11) Rebhan, M.; Rohwerder, M.; Stratmann, M. Mater. Corros. 2001, 52, 936.
10.1021/la010151v CCC: $22.00 © 2002 American Chemical Society Published on Web 03/07/2002
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Figure 1. Schematic drawing of the different surface layers: On top of the iron substrate an about 10 nm thick layer of amorphous SiO2 with Fe inclusions is deposited (a-SiO2:Fe). The Fe content decreases when reaching the outer surface. A monolayer of AEAPSi adsorbs on top of it and acts further on as an adhesion promoter to polymeric coatings. A layer of SiO2 about 10 nm thick was deposited in a CVD process of SiH4/H2O at 770 °C.9,10 It follows from the analysis of X-ray photoelectron spectroscopy (XPS) and glancing incidence X-ray diffraction (XRD) measurements that the surface consists of amorphous SiO2 with iron inclusions (a-SiO2:Fe). The iron concentration is 3 at. % at the outer surface according to XPS analysis. The thickness of the a-SiO2:Fe layer is about 10 nm which was measured by XPS and AES sputtering.9 A solution of 1 vol % n-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEAPSi, H2N(CH2)2NH(CH2)3Si(OCH3)3) in water at pH ) 9 was taken for the SA experiments. The samples were immersed in the liquid for 0.5-60 min. A 20 vol % solution of 1-iodotridecafluorohexane (ITDFH, I(CF2)5CF3) in acetone was used for the nucleophile substitution12 which was done at 45 °C for 30 min.
R′NH + I(CF2)5CF3 f R′N(CF2)5CF3 + IH
(3)
Then the samples were put in an ultrasonic bath with water or acetone, respectively, to dissolve the physisorbed molecules while the chemisorbed ones stay on the surface. Afterward, they were rinsed in the solvent and dried in nitrogen gas. Then they were brought immediately to the measuring equipment in a dry, clean box. ITDFH exchanges the hydrogen atoms which are bound to the nitrogen of the AEAPSi by an IR sensitive substance. This is necessary since the infrared spectrum of AEAPSi cannot be distinguished from slight pollution caused by the transport of the sample through air to the IR. During the transport and the alignment, the sample was exposed to air which can lead to the adsorption of organic molecules containing CH2. These vibrations appear also in the IR spectrum. A detailed description follows below. A Phi 5600 ESCA was used for angle-resolved XPS (ARXPS). The electron takeoff angle ϑ is defined as the angle between the sample surface normal and the analyzer axis. Before the ARXPS measurements were performed, the distance of the sample from the X-ray source was adjusted by hand. Its final position was reached at the maximum count rate of the detector. The distance of the sample from the detector was kept the same for all experiments. So the absolute intensity of the signal varied from measurement to measurement even if the samples were prepared in the same way. Therefore, only relative values of intensities are used in the analysis. In this case, the results are independent of the distance of the X-ray source from the sample. Other measuring conditions such as the type of the (12) Organikum, 17th ed.; Becker, H. G. O., Domschke, G., et al., Eds.; VEB Deutscher Verlag der Wissenschaften: Berlin, 1988.
Figure 2. IRRAS signal of a self-assembly film of AEAPSi before (a) and after a nucleophile substitution with ITDFH (b). The broad CF2 and CF3 vibrations can be seen around 1150 cm-1. XPS source (Al KR, pω ) 1486.6 eV), their intensity, and the measurement duration and angles were the same for all samples. An infrared reflection-absorption spectrometer (IRRAS) by Nicolet (model 740) was utilized to detect CH2, N-H, and C-F vibration modes. The silicidized iron sample without the SA was taken as a reference.
3. Results and Discussion 3.1. Chemisorption of AEAPSi. Methoxysilanes such as the used AEAPSi react during the hydrolysis in the way given by eq 1.5,6 In the present case, R is H2N(CH2)2NH(CH2)3. The Si(OH) group reacts with the silicon hydroxide on the surface to form Si-O-Si bindings (eq 2). The surface is conducting due to the iron content in the a-SiO2:Fe layer.9 Hence, IRRAS is utilized to illustrate the adsorption of AEAPSi on the surface. CH2 vibration modes at 2920 cm-1 appear in the IRRA spectrum of the AEAPSi films (Figure 2a) with an absorbance of the order of 3.5 × 10-3. But N-H vibrations cannot be detected. This is caused by the following reasons: R has only got two nitrogen atoms. The interaction of N-H vibrations and infrared photons is weak.13 If the electric field of IR photons is not oriented along the N-H dipoles, then the absorption and therefore the IR signal is weak. On the other hand, the measured CH2 vibrations cannot solely be attributed to the methylene in the R since the samples were carried through air before the IRRAS measurements. This might cause a slight pollution of the (13) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasseli, J. G. The handbook of infrared and Raman characteristic frequencies of organic molecules; Academic Press: Boston, 1991.
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Figure 3. (a) Schematic drawing of the AEAPSi molecule; (b) shows an ordered layer of AEAPSi on the silicidized surface. The N atoms are indicated which are necessary for the reaction with the ITDFH.
surface which cannot be distinguished from the CH2 signal of the SA film. A nucleophile substitution reaction with ITDFH is used to exchange the hydrogen atoms (eq 3) which are bound to the nitrogen. Since ITDFH contains C-F bonds, the resulting molecule is more sensitive to IR photons than the N-H bonds of the sole AEAPSi. The resulting IRRA spectrum is shown in Figure 2b. The broad signal at 1150 cm-1 is attributed to CF2 and CF3 vibrations.13 Since a chemical reaction of ITDFH with the silicon oxide surface can be excluded, only the reaction of eq 3 can explain the CF2/3 modes in Figure 2b. Therefore, it must be concluded that the AEAPSi reacted with the surface during the self-assembly as proposed in eq 1. As shown in the schematic drawing of Figure 3a, the ITDFH molecules can react with the AEAPSi at the three N-H sites of the molecule. But even if the AEAPSi is well ordered on the surface as in Figure 3b, the ITDFH molecules are not necessarily ordered after the reaction with the AEAPSi. Especially the ITDFH molecules, which react with the N-H molecules at the tail of the AEAPSi, are not necessarily orientated along the axis of the AEAPSi. They can have a wide range of angles in the direction of the surroundings. Therefore, a broad CF2/3 signal results from the IR measurements. The coverage of the self-assembly film can be estimated from the spectrum of Figure 2b: The AEAPSi has got three N-H bonds which act as possible reaction partners for the nucleophile substitution. Since the substitution took place at elevated temperature close to the boiling point of ITDFH, it can be assumed that every accessible N-H molecule reacted according to eq 3. Assuming that the AEAPSi is well ordered (similar to Figure 3b), the two N-H bonds which form the outer surface are easily accessible. But the inner N-H bond is more difficult to reach by the ITDFH. When the AEAPSi is not well ordered, for example, they lie parallel to the surface, then each N-H bond is accessible. At least two substitution reactions take place per molecule of AEAPSi. And the average value will be higher than two but less than three of course. So there are 10-15 CF2 groups per molecule of AEAPSi on the surface which leads to an absorbance value of 2.6 × 10-4 cm-1 per CF2. The absorbance of a CH2 vibration of orientated films is about 1.2-1.8 × 10-4 cm-1.14-16 Also, the absorbance cross section of CF2 and of CH2 is about the same.15 Taking this into account and considering the variation of the absorbance coefficient given above, then it must be concluded that the surface coverage of AEAPSi θ is more than half a monolayer and less than one monolayer (0.5 < θ < 1). The results of the IR measurement are summarized below: (14) Jung, Ch. Ph.D. Thesis, Universita¨t Erlangen-Nu¨rnberg, Erlangen, 1998. (15) Jung, D. R.; Czanderna, A. W. Crit. Rev. Solid State Mater. Sci. 1994, 19 (1), 1. (16) Yang, D. B.; Wakamatsu, T. Surf. Interface Anal. 1996, 24, 803.
Figure 4. Dependence of the Fe 2p peak intensity on the angle ϑ. The ratio of the uncovered and AEAPSi-covered XPS Fe/Fe0 intensity is shown. The duration of the self-assembly is 20 min.
AEAPSi cannot be detected directly by IR measurements due to the weak interaction of N-H with the IR photons and due to possible pollution caused by the preparation conditions. This signal superposes with the CH2/3 of the AEAPSi. The nucleophile substitution reaction with ITDFH shows that the AEAPSi chemisorbs on the surface. These IR measurements show that ITDFH is not well orientated. But conclusions cannot be drawn from this about the orientation of the AEAPSi. The coverage of AEAPSi is 0.5 < θ < 1. 3.2. Homogeneous Distribution. As shown above, the IR measurement cannot be used to find out if the AEAPSi molecules are orientated or not. Also, it is not clear by now if the surface is covered homogeneously by AEAPSi molecules or not. To solve these questions, ARXPS measurements are performed. The reduction of the XPS intensity of the substrate atoms due to the absorption in the self-assembly film can be utilized for this purpose. Figure 4 presents the ratio of the Fe 2p signal before and after the SA of AEAPSi as a function of the takeoff angle ϑ. It shows a good logarithmic dependence of IFe/ IFe,0 over 1/cos ϑ. IFe represents the intensity of the Fe 2p photoelectron attenuated by the AEAPSi film (thickness dSA), λFe is the attenuation length of the film (λFe ) 2.5 nm18), and IFe,0 is the iron intensity of the clean surface. The exponential decay of IFe/IFe,0 can only be explained if the coverage of the surface is homogeneous.17 This can be made clear by a simple gedanken experiment: Only the part of the Fe XPS signals suffers from absorption by the AEAPSi molecules which pass through them. In the case of very low surface coverage (θ , 1), almost no Fe signal is absorbed and hence the same angle dependence appears as in the case of the uncovered surface. Therefore, a very low coverage leads to a constant function of IFe/IFe,0. On the other hand, the absorption function decays exponential with the thickness of the AEAPSi layer dSA. If the surface is covered by almost a monolayer of AEAPSi (θ ≈ 1), an exponential function of IFe/IFe,0 is expected. The X-rays which pass the layer at normal incidence are attenuated by the absorption within a dSA thick layer. But if the angle ϑ is very high, then the photons see a much longer path dSA/cos ϑ and hence the absorption (17) Yang, D.-Q.; Sun, Y.; Da, D.-A. Appl. Surf. Sci. 1999, 144, 451. (18) Tunama, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1993, 21, 165.
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increases dramatically. Therefore, the intensity ratio IFe/ IFe,0 falls with increasing ϑ. The equation below follows from this gedanken experiment:
(
dSA IFe ) (1 - θ) + θ exp IFe,0 λFe cos ϑ
)
(4)
In the case of high coverage (θ ≈ 1), this equation simplifies to
ln
( )
dSA IFe ∝IFe,0 λFe cos ϑ
(5)
Comparing this gedanken experiment and eqs 4 and 5 with the measured results given in Figure 4, it follows that the surface coverage ϑ is much larger than 0 and the coverage is homogeneous. On the other hand, the data do not fit exactly with the straight line. This gives a hint that not a complete monolayer of AEAPSi is chemisorbed on the surface (0 , ϑ < 1). Taking the attenuation length λFe as given above, the thickness of the SA film can be calculated from the slope of the straight line. The resulting thickness of the selfassembly film dSA is 1.02 nm. The length of the AEAPSi molecule can be calculated with the knowledge of the length of the C-C, C-N, Si-C, and Si-O bonds19 and the assumption that the angles of the chains R are tetraeder angles of 110°. Then the AEAPSi molecule has a length of dAEAPSi ) 1.16 nm. Therefore, the average orientation of the molecules relative to the surface normal can be estimated as θSA ) 28°. It is assumed in this calculation that the molecules form ordered regions and that they are also orientated. It will be shown in the next section that these assumptions are justified. It was shown in this section that the distribution of AEAPSi over the surface is homogeneous. This was concluded from the angle-dependent absorption of the X-rays of the Fe atoms in the surface. The average orientation of the molecules relative to the surface normal is 28°. The XPS data and the IR measurements are consistent in the way that both show that the surface is covered by AEAPSi. It must also be concluded from the XPS and IR data that the coverage θ is less than a monolayer. 3.3. Orientation of the AEAPSi Molecules. ARXP spectra can be used to receive information about the orientation of the AEAPSi. Figure 5 shows that the ratio of the IN/IFe intensity increases with the takeoff angle ϑ of the electrons. The iron signal is related to the surface of the silicidized iron sample, and the nitrogen signal, to the amino groups in the AEAPSi. According to the structure of the AEAPSi, only the methoxy group can react with the SiO2 surface of the substrate. Therefore, the bond between the surface and the AEAPSi is formed at the silicon-containing head of the molecule. Since the samples were put into an ultrasonic bath and rinsed afterward, it must be concluded that the AEAPSi chemisorbed on the surface which was shown in section 3.1. Taking this information together with the results of the ARXPS, it follows that the tail of the AEAPSi does not lie on the surface but stands almost normal to it with the amine groups into the direction of the surrounding air. The distance of the nitrogen in the NH and NH2 from the outer surface can be calculated (dNH2 ) 0.06 nm, dNH (19) Handbook of chemistry and physics; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1990.
Figure 5. The measured N/Fe XPS intensity increases with the angle ϑ (dots). The steady function describes the N/Fe ratio of a monolayer of an AEAPSi film with a 28° tilting angle (eq 6) (SA duration of the film is 20 min).
Figure 6. The XPS measurements cannot distinguish if the molecules are tilted to the right or to the left in this twodimensional drawing.
) 0.42 nm).19 A schematic drawing of the AEAPSi molecule is given in Figure 3a. Figure 3b illustrates the position of the two N atoms in the AEAPSi molecule in an ordered region. In the case of high coverage (θ ≈ 1), the ratio IN/IFe is given by the function
(
) { (
) )}
dNH2 cos θSA IN dSA IN,0 1 ) exp × exp + IFe IFe,0 λFe cos ϑ 2 λN cos ϑ
(
exp -
dNH cos θSA λN cos ϑ
(6)
λN ) 3.3 nm is the attenuation length of nitrogen,18 and θSA is the angle of the AEAPSi molecule relative to the surface normal. The function and the experimental data are plotted in Figure 5. They adapt well if θSA ) 28° as is demonstrated in the graph. The XPS measurement cannot prove whether all molecule axes are parallel to each other and hence orientated in the same way. It is also possible that there are regions with the same orientation neighbored to other regions with a different orientation (as sketched in Figure 6). But the XPS measurement showed that the average tilt of the molecules relative to the surface normal is 28° in each region. Summarizing this, the XPS measurement proved that the AEAPSi molecules are orientated. Their silane head is at the interface to the silicidized surface and the amine groups at the outer surface. 3.4. Time Dependence of the Self-Assembly. The ratio of the IN/IFe signal increases with the duration of the self-assembly (Figure 7). It reaches a saturation value at about 10 min. Combining this with the IR and ARXPS results, the self-assembly takes about 10 min. A source of error in this figure is caused by the absolute value of the intensity I. In the case of low surface coverage, the
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1.02 nm. Together with the estimated length of the molecules, the average tilting angle of the AEAPSi is calculated as θ ) 28°. Angle-dependent XPS signals of the N and the Fe signal confirm this too. Finally, the XPS IN/IFe signal reaches a saturation value after about 10 min of self-assembly. It must be concluded from this that the silicidized surface is covered by more than half but less than one monolayer of the AEAPSi film after 10 min of the selfassembly.
Figure 7. Normalized N/Fe XPS intensity as function of selfassembly duration (dots). It reaches a saturation value at about 10 min.
XPS nitrogen intensity IN is small and therefore the error xI/I is relatively strong. 4. Summary The nucleophile substitution reaction by ITDFH proved the chemisorption of the AEAPSi self-assembly film on the silicidized iron. This is confirmed by XPS measurement both by detecting nitrogen which is attributed to amine groups of the AEAPSi and by measuring the intensity ratio IFe/IFe,0. The thickness of the SA film is calculated as
5. Outlook This system of Fe/SiO2/AEAPSi is very interesting for the car and steel industries for many applications such as corrosion protection and improvement of the stability of polymeric coatings on steel. The great advantage of the AEAPSi self-assembly film lies in its simple and easy handling. Also, the reaction of the AEAPSi with the surface needs only a few minutes. Finally, it seems to be an easy task to apply this system not only on iron but also on different types of steels. The described system improves the corrosion resistance and the delamination of polymeric coatings. These results, which are shown in another paper,10,11 are useful for many purposes. Acknowledgment. We thank the Deutsche Forschungsgemeinschaft which financed this research within the Sonderforschungsbereich 292 “Mehrkomponentige Schichtsysteme” as Project D6. LA010151V
