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Understanding the Conformational Dynamics of Organosilanes: γ-APS on Zinc Oxide Surfaces B. Watts,† L. Thomsen,† J. R. Fabien,‡ and P. C. Dastoor*,† Department of Physics, University of Newcastle, Callaghan, New South Wales 2308, Australia, and OneSteel, Industrial Drive, Mayfield, New South Wales 2304, Australia Received July 3, 2001. In Final Form: October 2, 2001 The oscillatory adsorption and orientation of thin films of γ-(aminopropyl)triethoxysilane (γ-APS) on zinc oxide surfaces have been investigated using X-ray photoelectron spectroscopy. Two different values of solution pH were chosen, one above and one below the isoelectric point (IEP) of the substrate. At both values of pH, films of organosilane molecules aligned perpendicular to the surface are produced. Moreover, varying the electrokinetics of the adsorption process radically alters the molecular orientation during oscillatory adsorption. When γ-APS is adsorbed at pH 6.4 (i.e., lower than the IEP of the metal substrate) the molecules are oriented with the silanol groups at the interface, whereas at pH 10.4 the molecules are oriented with the amine groups at the interface. The oscillation in the adsorbed coverage of γ-APS is correlated with either a disordering of the film or a realignment of the γ-APS molecules parallel to the surface.
Introduction Organosilane molecules are employed in a wide range of applications including, for example, biosensors,1 selfassembled monolayers,2 and lubricants.3 They are also increasingly used to promote adhesion between dissimilar materials4 in a wide number of industries, including aerospace, automotive, and construction. Specifically, γ-(aminopropyl)triethoxysilane (γ-APS) has been the coupling agent of choice in many investigations and applications,5-7 with a particular focus on corrosion protection of iron and steel surfaces.9 Although organosilanes are used extensively as adhesion promoters, the exact nature of the processes involved in bonding an organosilane molecule to a metal substrate is not clear.4 In a series of experiments in these laboratories we have shown that the adsorption of a model organosilane (propyltrimethoxysilane (PTMS)) to metal oxide surfaces is complex.10-14 In fact, we have identified an entirely new phenomenon, which we have termed oscillatory adsorp* To whom correspondence may be addressed: tel, 61 49 215426; fax, 61 49 216907; e-mail,
[email protected]. † University of Newcastle. ‡ OneSteel. (1) Andle, J.; Vetelino, K.; Lec, R.; McAllister, D. Proc. IEEE Ultrason. Symp. 1989, 579. (2) Sagiv, J. J. J. Am. Chem. Soc. 1998, 92, 207. (3) de Gennes, P.-G. Rev. Mod. Phys. 1985, 57, 827. (4) Plueddemann, E. P. Silane Coupling Agents; Plenum Press: New York, 1991. (5) Eldridge, B. N.; Buchwalter, L. P.; Chess, C. A.; Goldberg, M. J.; Goldblatt, R. D.; Novak, F. P. In Silanes and Other Coupling Agents; Mittal, K. L., Ed.; VSP: Utrecht, The Netherlands, 1992; pp 305-321. (6) Horner, M. R.; Boerio, F. J.; Clearfield, H. M. In Silanes and Other Coupling Agents; Mittal, K. L., Ed.; VSP: Utrecht, The Netherlands, 1992; pp 241-262. (7) van Ooij, W. J.; Sabata, A. In Silanes and Other Coupling Agents; Mittal, K. L., Ed.; VSP: Utrecht, The Netherlands, 1992; pp 323-343. (8) van Ooij, W. J.; Sabata, A. Surf. Interface Anal. 1993, 20, 475. (9) van Ooij, W. J.; Sabata, A. Scand. J. Metall. 1992, 21, 32. (10) Quinton, J.; Thomsen, L.; Dastoor, P. Surf. Interface Anal. 1997, 25, 931. (11) Quinton, J.; Dastoor, P.; Allison, W. Surf. Sci. 1998, 402-404, 66. (12) Quinton, J. S.; Dastoor, P. C. Surf. Interface Anal. 1999, 28, 12. (13) Quinton, J. S.; Dastoor, P. C. J. Mater. Sci. Lett. 1999, 18, 1833. (14) Quinton, J. S.; Dastoor, P. C. Appl. Surf. Sci. 1999, 152, 131.
tion.11 Our experiments have shown that although these organosilane molecules do attach strongly to metal surfaces, they can subsequently detach (if left for a period of time) and can then reattach again, and so on.12 Studies using secondary ion mass spectrometry indicate that the structure of the organosilane film at short adsorption times is significantly different than that observed at longer adsorption times.13 The situation for hydrolyzed γ-APS, however, is more complicated than that for PTMS, since each molecule possesses two moieties, an amino group and a silanol species, which can interact with an oxide surface. Investigations of the adsorption of γ-APS on iron and steel oxide surfaces have been reported previously, and both moieties are known to interact with the surface.6,7 Recent studies of the adsorption of γ-APS on aluminum alloy surfaces using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) showed that the coverage and structural integrity of the adsorbed γ-APS film were dependent upon the concentration of silane in solution.15 In a previous study of the adsorption of γ-APS on iron oxide surfaces, we demonstrated that this silane also undergoes oscillatory adsorption and that the orientation of the adsorbed molecule is correlated with its adsorption isotherm.16 In this paper we present the results of a systematic study of the adsorption of γ-APS on zinc oxide surfaces at pH 10.4 and pH 6.4. The results show that by varying the electrokinetics of the adsorption system, it is possible to reverse the molecular orientation of the adsorbing system. Furthermore, the oscillation in the adsorbed coverage is also associated with a change in the orientation of the adsorbed film. Experimental Section A polycrystalline zinc sheet (of purity 99.99%) was obtained from Goodfellow Cambridge Ltd. (Cambridge, England). The sheet was cut into 10 × 10 mm squares for dipping and analysis. Silicon nitride, Si3N4, powder (of purity 97% and with a maximum (15) Simpson, T. R. E.; Watts, J. F.; Zhdan, P. A.; Castle, J. E.; Digby, R. P. J. Mater. Chem. 1999, 9, 2935. (16) Quinton, J. S.; Dastoor, P. C. Surf. Interface Anal. 2000, 30, 21.
10.1021/la011058+ CCC: $22.00 © 2002 American Chemical Society Published on Web 01/02/2002
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Figure 1. Variation of normalized γ-APS surface coverage as a function of adsorption time, for zinc samples dipped in a pH 10.4 γ-APS solution, as measured by XPS. The dashed line has been added as a guide to indicate the positions of the maxima and minima in silane coverage discussed in the text. The ratio indicated on the ordinate axis was obtained from the fitted areas under the Si 2p and Zn 2p photoelectron peaks. The error in the measurements has been assuming Gaussian counting statistics. particle size of 10 µm) was also obtained from Goodfellow Cambridge Ltd. γ-(Aminopropyl)triethoxysilane (γ-APS), NH2CH2CH2CH2Si(OCH2CH3)3 (of purity 98%), was supplied by the Aldrich Chemical Co., Inc. (Milwaukee, WI). Solutions of 1% concentration by volume and natural pH (10.4) were prepared by stirring the required quantity of γ-APS into deionized (5 µS MilliQ) water for a period of 5 min, by which time the γ-APS had dissolved completely and was considered to be hydrolyzed. For adsorption at lower pH (6.4), the solutions were acidified with acetic acid. The metal surfaces were all prepared by abrading lightly with 1200 grade abrasive paper and then etched ultrasonically for 1 h in pH 3.0 acetic acid solution. Once etched, the samples were rinsed in deionized (5 µS MilliQ) water and allowed to air-dry prior to dip coating in the desired γ-APS solution. After being dipped, the samples were blown dry with nitrogen and stored under a nitrogen atmosphere to prevent reaction of amine groups with CO2 in air to form ammonium carbamates,17 which may alter the film structure. After being coated, the samples were characterized with XPS using a PHI-550 surface analysis system with a base pressure of better than 2 × 10-10 Torr. The high-resolution XPS spectra were taken with 400-W Al KR unmonochromatized radiation and a double pass cylindrical mirror analyzer with a pass energy of 50 eV. Data analysis involved fitting core level peaks with Lorentzian-Gaussian peak shapes. The calibrated atomic sensitivity factors for the PHI 550 surface analysis system were applied to the measured peak areas to obtain atomic surface concentration.18
Results and Discussion Coverage Dependence Measurements. In accordance with previous studies of adsorption kinetics, the silicon:(silicon + zinc) concentration ratio, as measured by XPS, has been taken as an indication of normalized adsorbate coverage on the zinc surface.10 Figure 1 and Figure 2 show the time dependence of γ-APS adsorption on a native zinc oxide surface for adsorption at pH 10.4 and 6.4, respectively. Clear oscillations in organosilane coverage are observed at both values of pH. The measurements were reproducible with the position of the turning points consistent to within (5 s. In both cases, for the (17) Battjes, K. P.; Barolo, A. M.; Dreyfuss, P. In Silanes and Other Coupling Agents; Mittal, K. L., Ed.; VSP: Utrecht, The Netherlands, 1992; pp 199-213. (18) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; PerkinElmer Corporation, Physical Electronics Division: Eden Praire, MN, 1979.
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Figure 2. Variation of normalized γ-APS surface coverage as a function of adsorption time, for zinc samples dipped in a pH 6.4 γ-APS solution, as measured by XPS. The dashed line has been added as a guide to indicate the positions of the maxima and minima in silane coverage discussed in the text. The ratio indicated on the ordinate axis was obtained from the fitted areas under the Si 2p and Zn 2p photoelectron peaks. The error in the measurements has been assuming Gaussian counting statistics.
Figure 3. Model of possible adsorbate conformations: (a) surface bonding via protonated amine group; (b) surface bonding via protonated amine group and condensed silanol adsorption; (c) surface bonding via condensed silanol-surface interaction.
time period 0-100 s, there appear to be minima at approximately 35-40 and 85-90 s with a maximum at approximately 65-70 s. In addition, the Si:(Si + Zn) ratio is significantly less than unity throughout the adsorption (indicating a strong zinc presence). Indeed, the fact that this ratio does not approach a value of 1, even after relatively long exposure times (>2000 s), suggests that γ-APS multilayers do not form on the surface. Orientation Dependence Measurements. The structure of the γ-APS molecule consists of an amine group and a silane group at either end of the molecule separated by a propyl chain. As shown in Figure 3, the γ-APS molecule is known to bond to the oxide surface via either the silanol and/or the amine group.7 The preferred bonding group, and hence the orientation of the molecule, depends on the isoelectric point of the surface and solution pH.6 The distance between the nitrogen and silicon moieties within the molecule is approximately 6 Å19 and is significant compared to the analysis depth of XPS. In principle, therefore, the variation of the Si:N ratio provides (19) Kaye, G. W. C.; Laby, T. H. Tables of Physical and Chemical Constants, 15th ed.; Longman: London, 1986.
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a measure of the relative numbers of oriented molecules on the surface, since the measured Si:N ratio for molecules with the silicon group aligned toward the surface would be less than the expected value (from the stoichiometry of the molecule) of 1. On the other hand, for molecules with the nitrogen group aligned toward the surface, the Si:N ratio would be greater than 1. Interestingly, a recent XPS study by Lee and Wool also noticed a very weak N 1s signal relative to the Si 2p signal for a monolayer-like film of γ-APS.20 This low signal was attributed to X-ray degradation of the -NH2 layer. However, the explanation that the Si atoms are more stable due to their greater depth seems extremely unlikely given that the attenuation of the X-ray beam by a monolayer of silane will be extremely small. A more likely explanation is that the photoelectron signal from the N 1s atoms is attenuated by the silane overlayer. Indeed, in an earlier XPS and time-of-flight secondary ion mass spectrometry (ToFSIMS) study, it was demonstrated that the variation of the silicon to nitrogen surface concentration ratio as a function of exposure time does provide a measure of the kinetics of the molecular orientation of the adsorbing molecule.16 Thus, plotting the Si:N ratio provides, in principle, conformational information about the adsorbed molecules. However, to quantify the degree of actual molecular orientation from the measured Si:N ratio for the adsorption of γ-APS on zinc oxide, it is necessary to consider the details of the experiment. There are two main aspects of the experiment that need to be considered: (a) the angular acceptance function of the analyzer and (b) the sensitivity factors for the elements of interest (i.e., Si and N). Angular Acceptance of the Instrument. The data were collected using a cylindrical mirror analyzer (CMA), which, as a consequence of its inherent design, actually collects electrons that are emitted through a wide range of angles. The entrance aperture of a CMA is typically arranged so that the analyzer accepts photoelectrons that are ejected over an angular range of approximately 6° about a mean of 42.3° from the axis of the analyzer, at all azimuthal angles.18 It is this large solid angle of acceptance that is one of the advantages of this type of analyzer since it means that the number of collected electrons is relatively large compared to other analyzers. For a sample surface normal that is parallel to the axis of the instrument therefore, the electron takeoff angles accepted by the analyzer would be 42.3 ( 6°. In practice, however, the geometry of the sample to be analyzed is typically arranged so that the sample’s surface normal is tilted at an angle of 40° to the axis of the analyzer.18 Consequently, the analyzer actually accepts electrons that are emitted over a broad range of takeoff angles, as shown in Figure 4. However, the number of electrons collected is a strong function of the angle of electron ejection, Θ, measured relative to the sample’s surface normal. Previous work by Ding and Shimizu to determine the effect of CMA geometry upon escape depth evaluation21 and by Seah and Hunt on optimizing CMA signal-to-noise ratio performance22 have shown how the angle of electron ejection, Θ, is related to the azimuthal angle, φ, about the CMA axis by
cos Θ ) sin R sin θ cos φ + cos R cos θ
(1)
where R is the angle between the sample normal and the CMA axis and θ is the CMA acceptance cone semiangle of 42.3°. (20) Lee, I.; Wool, R. P. Thin Solid Films 2000, 379, 94. (21) Ding, Z.-J.; Shimizu, R. Surf. Interface Anal. 1995, 23, 351. (22) Seah, M. P.; Hunt, C. P. J. Electron Spectrosc. Relat. Phenom. 1994, 67, 151.
Figure 4. Schematic diagram showing the geometry of a cylindrical mirror analyzer.
Figure 5. Relative detection probability of the CMA as a function of electron ejection angle. The ordinate axis has been normalized such that the area under the graph is unity.
However, none of this previous work has derived the collection efficiency of a CMA analyzer as a function of the angle of electron ejection, Θ, since this was not the focus of these earlier studies. The derivation of this angular acceptance function is nontrivial and involves calculating the locus of intersection of the electron ejection cone from the surface with the solid angle of acceptance of the analyzer, the detailed treatment of which will be presented elsewhere.23 Figure 5 shows the collection efficiency, f(Θ), which has been calculated by numerically integrating over all of the electron ejection angles transmitted by a CMA with a finite angular acceptance. Since the function has been normalized so that the area under the graph is equal to unity, f(Θ) shows the relative probability that a photoelectron collected by the analyzer was ejected at an angle, Θ. It is clear from Figure 5 that the collection of photoelectrons that are ejected at grazing angles is enhanced in this analyzer and so attenuation effects are likely to be emphasized. Modifying the Sensitivity Factors. The usual method for relating the area under a given photoelectron peak to the surface concentration of a given elemental species is through its sensitivity factor. The sensitivity factor for a given species, x, is defined by the expression
nx ) Ix/Sx
(2)
where nx is the number density of atoms of species x, Ix is the area under the photoelectron peak associated with the quantitative analysis of species x, and Sx is the so(23) Watts, B.; Thomsen, L.; Dastoor, P. C. Surf. Interface Anal., in press.
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Figure 6. High-resolution photoelectron spectra for Si and N in Si3N4 sample. Table 1. Peak-Fitting Results for Si 2p and N 1s Photoelectron Peaks in Si3N4a element
photoelectron
binding energy (eV)
peak area
λTPP-2M in Si3N4
Si Si N
2p3/2 2p1/2 1s
101.8 102.8 397.0
83.5 167.0 507.6
30.37 25.13
a
The inelastic mean free paths (λTPP-2M) have been calculated using the formula of Tanuma et al.26
called sensitivity factor associated with the relevant photoelectron peak of species x. The Si:N concentration ratio is therefore given by
( )
nSi ISi SN ) nN IN SSi
(3)
Figure 7. Idealized layer structure for silicon nitride crystallites in powder sample. The interlayer spacing is d.
nitrogen atoms will be isotropic. However, the imfp of the silicon 2p photoelectrons is not the same as that of the nitrogen 1s photoelectrons in silicon nitride. Using the predictive formula of Tanuma et al.26 and, assuming that silicon nitride has a band gap of ∼ 5.3 eV,27 the imfp values for silicon and nitrogen in Si3N4 have been calculated and the results are also shown in Table 1. The lower value of the imfp for nitrogen compared to silicon means that, for a homogeneous silicon nitride sample, the measured nitrogen signal would be substoichiometric. Thus, the measured composition, nx, differs from the actual composition, nx0, by a factor R, such that
Clearly, the absolute value of the Si:N surface concentration ratio is therefore critically dependent upon the precise value used for the sensitivity factor for both species in the ratio. More correctly, it is essential that the ratio of the sensitivity factors, for Si and N, be known as precisely as possible. The situation is complicated by the fact that sensitivity factors are a function of a number of experimental parameters, including the transmission function of the apparatus.18 Moreover, the finite sampling depth of the XPS technique means that the defined sensitivity factors can only be applied to homogeneous samples in a reasonably rigorous manner. Thus, for thin film samples, it is important to take into consideration the effect of the inelastic mean free path (imfp) of the ejected photoelectrons. To rigorously quantify the measured Si:N ratio, we have adopted the following approach. First, to ensure that the correct sensitivity factors are used, we have calibrated our system using silicon nitride powder, whose stoichiometry (Si3N4) is well-characterized.24 The areas under the Si 2p and N 1s photoelectron peaks were measured (Figure 6) and the results are shown in Table 1. The binding energies of the Si 2p and N 1s peaks, which have been corrected using adventitious carbon as an energy reference (284.6 eV), agree with those obtained for Si3N4 elsewhere to within (0.3 eV.25 The use of a powder sample ensures that the analyzed material is homogeneous and thus the photoemission from both the silicon and the
To quantify the extent to which the nitrogen signal will be substoichiometric (i.e., the magnitude of R), consider an idealized silicon nitride layered structure shown in Figure 7. The contribution that an elemental species, x, makes to the area under its associated photoelectron peak is reduced by a factor of exp(-nd/λx) for every n layers below the surface from which it originates, where λx is the imfp of species x in silicon nitride and d is the separation between the layers. Thus, the silicon originating from three layers below the surface is attenuated by a factor of exp(-3d/λSi). For an idealized layered structure, therefore,
(24) Jennings, H. M.; Edwards, J. O.; Richman, M. H. Inorg. Chim. Acta 1976, 20, 167. (25) Taylor, J. A.; Lancaster, G. M.; Rabalais, J. W. J. Electron Spectrosc. Relat. Phenom. 1978, 13, 435.
(26) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1994, 21, 165. (27) Iqbal, A.; Jackson, W. B.; Tsai, C. C.; Allen, J. W.; Bates, C. W., Jr. J. Appl. Phys. 1987, 61, 2947.
nx ) Rxnx0
(4)
Multiplying both sides by the sensitivity factor Sx leads to
Ix ) RxIx0
(5)
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the total area under the photoelectron peak (Ix) is made up of the sum of the contributions from all of the analyzed layers, thus
Ix ) Ix0(1 + e-d/λx + e-2d/λx + ...)
(6)
where Ix0 is the contribution originating from the outermost layer. The sum in the parentheses is a simple geometric progression, which sums to
Ix ) Ix0/(1 - e-d/λx)
(7)
The interlayer spacing for Si3N4 is 7.6 Å24 and (from Table 1) for Si3N4, λSi ) 30.37 Å and λN ) 25.13 Å. Thus d/λx < 1 for both Si and N. It is possible to make the assumption that 1 - e-d/λx ∼ d/λx, since for both the Si 2p and the N 1s photoelectron in Si3N4, the difference between 1 - e-d/λx and d/λx is less than 5%. Given that the standard uncertainty in the calculated imfp value is about 19% for inorganic compounds,28 this approximation makes only a marginal difference to the overall uncertainty. Hence, eq 7 can be simplified to
Ix ) Ix0λx/d
(8)
Comparing eqs 5 and 8 leads to
Rx ) λx/d
(9)
The actual Si:N concentration ratio can therefore be written as
nSi0 0
nN
)
( )( )( ) ( )( )( ) ISi SN RN ISi SN λN ) IN SSi RSi IN SSi λSi
(10)
Hence, the sensitivity factor ratio for Si:N is given by
( ) ( )( )( ) SN nSi0 IN λSi ) SSi nN0 ISi λN
(11)
To correct for the effect of attenuation, therefore, the measured peak area ratio needs to be modified by the imfp ratio of Si and N in Si3N4. From the Si 2p and N 1s photoelectron peaks shown in Figure 6, the peak area ratio, IN/ISi, is 2.028, the true concentration ratio, nSi0/nN0, is 0.75 and for Si3N4, λSi ) 30.37 Å and λN ) 25.13 Å. Hence the sensitivity factor ratio is given by
SN 30.37 ) (0.75)(2.028) ) 1.84 SSi 25.13
(
Figure 8. Variation of the silicon:nitrogen surface concentration ratio as a function of adsorption time, for zinc samples dipped in pH 10.4 and pH 6.4 γ-APS solutions, as measured by XPS. The broken lines have been added as a guide to indicate the position of the minimum in the pH 10.4 data (dash-dot line) and the maximum in the pH 6.4 data (dashed line), respectively. The ratio indicated on the ordinate axis was obtained from the fitted areas under the Si 2p and N 1s photoelectron peaks. The error in the measurements has been calculated assuming Gaussian counting statistics.
)
(12)
Using this sensitivity factor ratio, it is possible to plot the absolute Si:N concentration ratio as a function of time for adsorption at pH 10.4 and at pH 6.4, as is shown in Figure 8. The plot shows that, for adsorption at pH 10.4, the Si:N ratio has a value of 1.2-1.3 at short adsorption times and goes through a minimum of about 0.95 at around 40 s adsorption time before returning to a higher value at longer adsorption times. For adsorption at pH 6.4, however, the Si:N ratio has a value of 0.6-0.7 at short adsorption times and goes through a maximum of about 0.95 at around 40 s adsorption time, before returning to a lower value at longer adsorption times. Remarkably, both the minimum (28) Powell, C. J.; Jablonski, A.; Tilinin, I. S.; Tanuma, S.; Penn, D. R. J. Electron Spectrosc. Relat. Phenom. 1999, 98-99, 1.
in the Si:N variation at pH 10.4 and the maximum in the Si:N variation at pH 6.4 occur at the same adsorption time and have the same value of Si:N ratio. To deduce further information from Figure 8, it is necessary to consider the relationship between molecular orientation in the adsorbed film and the absolute value of the Si:N ratio as measured by the CMA instrument. Calculating Idealized Si:N Ratios. It would appear that there are two main observations that can be made from Figure 8. First, there is a significant difference in the equilibrium orientation of the molecule for adsorption conducted at pH 10.4 compared to that at pH 6.4, as indicated by the absolute value of the Si:N ratio. Second, there is a significant change in the orientation of the γ-APS molecule that appears to occur at 40 s for both values of pH. To relate the measured changes in the Si:N ratio to real structural changes of the polymer film, let us now calculate what the expected Si:N ratio would be for a perfectly ordered γ-APS film. Let us first consider the case of an ideal self-assembled monolayer of γ-APS molecules with the N end of the molecule pointing toward the substrate. As discussed earlier, the CMA accepts electrons that are ejected over a range of angles with a collection efficiency, f(Θi), as shown in Figure 5. Therefore, the measured Si:N ratio, which is collected over a range of angles, Θi, will be scaled by both the attenuation factor and the collection efficiency such that
ISi ) IN
∫0π/2 f(Θ)ISi0 dΘ ∫0π/2 f(Θi)IN0 exp(-d/λN cos Θ) dΘ
)
ISi0
∫0π/2 f(Θ) dΘ
IN0
∫0π/2 f(Θi) exp(-d/λN cos Θ) dΘ
)
ISi0 IN0
(13)
1
∫0
π/2
f(Θi) exp(-d/λN cos Θ) dΘ
where f(Θi) has been normalized so that ∫0π/2f(Θ) dΘ ) 1 and λN is the imfp of N 1s photoelectrons in γ-APS.
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Table 2. Values of Inelastic Mean Free Path for Si 2p and N 1s Photoelectrons (ejected by Al X-rays with a photon energy of 1486.6 eV) through a γ-APS Moleculea element
photoelectron
kinetic energy
λTPP-2M in γ-APS
attenuation factor (R′)
Si N
2p 1s
1384.7 1084.7
35.65 29.29
0.64 1.69
a
The inelastic mean free paths (λTPP-2M) have been calculated using the formula of Tanuma et al.26
Multiplying both sides of eq 10 by the appropriate sensitivity factors allows us to write this equation as
( )
nSi nSi0 ) R′ nN nN0
(14)
where R′ is the attenuation factor that incorporates both the effects of the imfp of the ejected photoelectrons and the angular acceptance of the analyzer given by
R′ )
1
∫0
π/2
(15)
f(Θi) exp(-d/λN cos Θ) dΘ
and nSi0 and nN0 are the expected surface concentrations of Si and N in the absence of any attenuation. Since there is only one Si and one N atom per γ-APS molecule, the value of the nSi0/nN0 ratio is unity. Therefore, the ideal value for the Si:N ratio for a layer of γ-APS molecules oriented with the N atom toward the surface, when attenuation is taken into account, is R′. Similarly, it is possible to obtain an expression for the attenuation factor for γ-APS molecules oriented with the Si atom toward the surface, which in this case is given by
R′ )
∫0π/2 f(Θi) exp(-d/λSi cos Θ) dΘ
(16)
Using the NIST imfp database,29 it is possible to calculate the imfp for both Si and N in γ-APS, and the results are shown in Table 2. From the results shown in Table 2, the Si:N ratio expected for an ideal monolayer of γ-APS molecules oriented with the N moiety toward the surface is 1.69 and is 0.64 if the Si moiety is oriented toward the surface. It is also possible to obtain a rough estimate of the error in this theoretical value from the expected error in the imfp, which is expected to dominate the error in the calculation. The data in the NIST database are anticipated to be accurate to within 19% for inorganic materials,28 and recalculating R′ for this variation produces an error in the theoretical Si:N ratio of (0.06. We compared these ideal values with the “plateau” Si:N ratio values (i.e., at adsorption times less than 30 s and greater than 50 s) measured during the experiment. For adsorption at pH 10.4 a plateau Si:N ratio of approximately 1.3 is obtained, which can be compared with the value of 1.65 predicted for an ideally aligned film of γ-APS molecules oriented with the N atom toward the surface. For adsorption at pH 6.4, a plateau Si:N ratio of 0.6 is obtained, which can be compared with the value of 0.65 predicted for an ideally aligned film of γ-APS molecules oriented with the Si atom toward the surface. Horner and co-workers6 studied the adsorption of γ-APS on several metal surfaces with angle-resolved XPS and found that (29) NIST Electron Inelastic-Mean Free-Path Database, Standard Reference Data Program Database 71, Version 1.1; National Institute of Standards and Technology: Gaithersburg, MD, 2000.
Table 3. Approximate IEP and Charge Polarity of the Silanol and Amine Groups of the γ-APS Molecule and Zinc Oxide Surface in γ-APS Solutions at pH 6.4 and pH 10.4 charge polarity charged species
approximate IEP
pH 6.4
pH 10.4
silanol group amine group zinc surface
3-4 10-11 9.0
negative positive positive
negative positive negative
when γ-APS is adsorbed from aqueous solution at pH 10.4, a fraction of the amino groups at the silane oxide interface are irreversibly protonated by interaction with surface hydroxyl groups. Their results showed that γ-APS is adsorbed through the amino group with the extent of amino protonation being strongest on acidic surfaces. Van Ooij et al. showed that the orientation of γ-APS molecules on steel substrates is mixed, with both silanol and amino groups adsorbed at the film-steel interface.7 Thus, it would appear that the equilibrium structure for these molecules adsorbed on the surface at these two different pH values is that of a highly aligned organosilane film, whose orientation can be completely reversed by changing the pH of the adsorbing solution. Comparing and contrasting Figure 1 and Figure 2 with Figure 8 suggests that the graphs are correlated, with the minimum in the Si/(Si + Zn) ratio that occurs at an exposure time of 40-45 s (for both values of pH) coincident with both the corresponding minimum in the Si:N ratio for adsorption at pH 10.4 and the corresponding maximum in the Si:N ratio for adsorption at pH 6.4. Interestingly, both the minimum in the Si:N variation at pH 10.4 and the maximum in the Si:N variation at pH 6.4 have the same value of Si:N ratio of approximately 0.9 ( 0.1. The fact that this value is close to unity suggests that, at this adsorption time, the organosilane molecules either are randomly aligned on the surface or else are aligned parallel to the substrate. Thus, the minimum in surface coverage occurs as the molecules change from being aligned perpendicular to the surface to being either unaligned or aligned parallel to the surface. It is possible to describe the observed behavior for the adsorption of γ-APS on zinc oxide surfaces by considering the changes in surface electrokinetic behavior that occur with changing solution pH. It is the difference between the solution pH and the isoelectric point (IEP) of the material that determines the sign of the surface charge. When the pH is less than the IEP of the surface (or ionisable group), the surface (or ionizable group) is predominantly positively charged as OH- groups are replaced by H+ ions. Vice versa for pH values greater than the IEP, the surface (or ionizable group) possesses a net negative charge. The anticipated charges present on the molecule and the surface for the two different values of pH are shown in Table 3. At pH 10.4, which is the natural pH of the γ-APS solution, the zinc oxide and the silanol moiety are both negatively charged. The amine group is most likely neutral or slightly positively charged if the precise value of the IEP of this group is greater than 10.4. There will be a repulsive interaction between the surface and the silanol group, and thus the anticipated organosilane orientation is for the amine group to point toward the surface. This hypothesis is confirmed by the XPS analysis of the surface film which shows that the equilibrium value for the Si:N ratio is about 1.3, indicating that the γ-APS film is aligned with the amine group at the metal-film interface. At pH 6.4, however, the charge on the zinc oxide surface is
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reversed and, thus, the anticipated organosilane orientation is for the silane group to point toward the surface. Again, the results of the XPS analysis confirm this prediction since the measured equilibrium value of the Si:N ratio is about 0.6 indicating that the γ-APS film is aligned with the silane group at the metal-film interface. The origin of the orientation change that occurs for both values of pH, however, is less clear. Silane films are readily desorbed by OH groups (both in aqueous16 and nonaqueous environments30) resulting in oscillations in silane coverage. Although it is possible to speculate that OH groups may desorb both amine-bonded and silane-bonded γ-APS molecules, it is not clear why this would lead to the observed orientation change. Furthermore, the fact that the observed Si:N ratio is approximately 1 at the minimum coverage means that the precise orientation is ambiguous, since this could result from γ-APS molecules randomly aligned or aligned parallel with the surface. However, the decrease in silane surface coverage is certainly consistent with a lower silane surface density, and thus it is possible to speculate that molecules are aligned parallel to the surface. Further experiments are planned to elucidate this point. (30) Houssiau, L.; Bertrand, P. Appl. Surf. Sci. 2001, 175-176, 399.
Watts et al.
Conclusions The adsorption of γ-APS on zinc oxide surfaces appears to occur via an oscillatory adsorption mechanism. For adsorption times less than about 30 s and greater than about 50 s, the films appear to be highly aligned, with the γ-APS molecules arranged perpendicular to the interface. The orientation of the silane molecules can be controlled by the pH of the adsorbing solution. For adsorption at pH 10.4, the γ-APS molecules adsorb through the amine group, while at pH 6.4 adsorption occurs through the silanol moiety. Furthermore, these observed molecular orientations are entirely consistent with predictions based upon the known isoelectric point of the zinc oxide surface. For adsorption times between 30 and 50 s, the alignment of the adsorbed film changes and it appears that, at this point, the γ-APS molecules are randomly aligned or aligned parallel to the surface. Acknowledgment. Mr. Clifton-Smith and Ms Brown are gratefully acknowledged for assistance with the XPS analysis. The provision of a research scholarship from OneSteel (B.W.) is gratefully acknowledged. LA011058+
