11014
J. Phys. Chem. 1996, 100, 11014-11018
Mechanism of Organosilane Self-Assembled Monolayer Formation on Silica Studied by Second-Harmonic Generation Xiaolin Zhao and Raoul Kopelman* Department of Chemistry, UniVersity of Michigan, Ann Arbor, Michigan 48109 ReceiVed: September 12, 1995; In Final Form: April 8, 1996X
Organosilane self-assembled monolayer (SAM) formation from solution on fused silica surfaces has been studied through the surface silanol group chemical reaction. The method is based on second-harmonic generation (SHG) efficiency as a function of pH. Direct experimental support is provided for the following organosilane SAM structure at the silica surface: (1) the organosilane SAM formation does not have any preference toward different surface sites; (2) during organosilane SAM formation process at the silica surface, most of the silica surface functional groups remain intact and therefore maintain their surface chemistry; (3) the silica surface is much rougher than the organosilane SAM on top of it and therefore provides more surface functional groups per unit area. This detailed understanding of the organosilane SAM formation may provide guidance for the proper application of SAM to various surface modification techniques, e.g., in microelectronic engineering or near-field optical chemical sensors.
I. Introduction Self-assembled monolayer (SAM) formation can be used to modify surface properties and has therefore become more and more important in surface science, molecular recognition, electrochemistry, microelectronic engineering, nanotechnological structures, bioactive surfaces, and many other fields.1 In practice, one of the most commonly used SAMs is the organosilane monolayer on hydroxy surfaces such as silica, sapphire, and oxidized silicon. While techniques such as contact angle measurements,2 ellipsometry,3 UV-vis spectroscopy,4 FTIR spectroscopy,5 ESCA at variable angles,6 X-ray,7 AFM,8,9 and STM10 have been applied to characterize the organosilane SAM surface, ironically little is known about the detailed formation process. Due to the covalent nature of SAM formation, it is easy to assume that SAM formation is a 1:1 reaction between the monolayer and surface. AFM studies suggested that the SAM/silica is always smoother in topography than the bare silica surface. Although AFM provides valuable information about the SAM surface topography, it still does not provide a detailed mechanism of SAM formation. Organosilane SAM formation on silica/silicon is believed to be accomplished through surface adsorption/hydration/silanization.12 In the process, the silanol (SiOH) groups on the glass are believed to react with either R-SiCl3 or R-Si(O(CH2)nCH3)3 through a dehydration process and thus form strong chemical bonds. This process is not necessarily limited to the surface, and under some conditions the SAM may develop in three dimensions because the dehydration may happen between monomer and SAM instead of between monomer and surface functional group. Valuable information such as the detailed bonding structure between the SAM and the surface is still missing. The technique chosen for the study of SAM formation from a chemical reaction point of view was second-harmonic generation (SHG). This provides reliable information on the surface charge density of the silica surface. The pKa of the surface functional group silanol, -SiOH, is somewhere between 4 and 9; therefore, the surface can have severely different charge densities under different bulk pH conditions, depending on the ionization of -SiOH. SAM formation must consume -SiOH * Author to whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, June 1, 1996.
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and therefore should modify the charge density at the surface. By studying the SHG signal dependence on bulk pH with and without a SAM, it is possible to retrieve details about the structure of the SAM surface. For convenience, let us consider two special chemical equilibria
HA+ + H2O h A + H3O+
(type I)
HA + H2O h A- + H3O+
(type II)
of the functional groups studied; the amino and pyridine groups belong to type I, and the carboxylic and silanol groups belong to type II. The questions that are addressed in this paper are the following: (a) How much can SAM formation modify the surface charge density? (b) Is SAM formation selective to different surface sites? (c) Can one use the pH response of the surface charge density as a pH sensor mechanism? II. SHG Method and Silica Surface Charge Density Measurement The basic idea of the method used here is to measure the relative populations of the acid and base at the interface using SHG. At a charged surface where the third-order process cannot be neglected, the SHG response at a distance z from the surface is
E(2ω, z) ∼ χ(2)(2ω, ω,ω)E(ω)E(ω)δ(z) + χ(3)(2ω, 0, ω,ω)E(0, z)E(ω)E(ω) (1) where E is the electric field at the specific frequency and χ is the susceptibility of a certain order. χ(2) is the surface specific term under the electric dipole approximation, and χ(3) is the thirdorder susceptibility of the bulk water in our studies. The total SHG response must be integrated over all distances z and thus we have
E(2ω) ∼ χ(2)(2ω,ω,ω)E(ω)E(ω) + χ(3)(2ω, 0, ω,ω)Ψ(0)E(ω)E(ω) ) A + BΨ(0) (2) where Ψ(0) is the electric potential at the charge plane, A is related only to the second-order susceptibility, and B is related © 1996 American Chemical Society
Organosilane Self-Assembled Monolayer Formation
J. Phys. Chem., Vol. 100, No. 26, 1996 11015
only to the third-order susceptibility of the material within the electrostatic field. Ψ(0) can be related to the surface charge density for a given electric double-layer model such as the Gouy-Chapman model.14 This latter model has been shown to be a good model for charged air/water interfaces, up to high ionic strengths.14 Recalling that the surface charge is due to the ionization of the surface functional groups, consider the acid-base equilibrium at the surface for an amine or pyridine functional group (type I). When the activity coefficients are neglected, the equilibrium constant for this process can be expressed as
Ksa )
[A]S[H3O+]S [HA+]S
)
[H3O+]SNA NHA+
(3)
where S denotes the properties at the surface and NA and NHA+ are the number densities (molecules/cm2) of the amine and ammonium groups at the surface, respectively. The type II acid-base equilibrium at the surface, with silanol or carboxylic acid functional groups, is
Ksa )
[A-]S[H3O+]S [HA]S
)
[H3O+]SNANHA
(4)
where NA- and NHA are the number densities(molecules/cm2) of silanolate (-SiO-) and silanol (-SiOH) at the surface, respectively. The value of pHs at a charged surface is related to the bulk value, pHb, as follows:15
pHs ) pHb + (eΨ/2.3RT)
(5)
The electric potential Ψ is due to the presence of the surface charge density σ, which in this case arises from the charged form of the surface functional groups, e.g., for amine, σ ) eNHA+. Under the simple Gouy-Chapman model of electrical double layers, the surface potential is given by15
Ψ ) (2kT/e) sinh-1[σ(π/2CkT)1/2]
(6)
where C is the total number of ions/cm3 in bulk, the dielectric constant of the medium, T the temperature in Kelvin, k the Boltzmann constant, and e the electronic charge. III. Experimental Section One face of a UV grade fused silica 60° prism (ESCO Products) serves as the silica surface for these studies. It was cleaned in three steps: (1) heating with chromerge at high temperature (>100 °C) for over 3 h; (2) soaking in saturated KOH/methanol for 0.5 h; (3) soaking in 1:1 HCl/methanol for 0.5 h. The cleaned prism was put in a dry N2 box to dry out just before monolayer deposition. A sample cell to hold the various pH solutions was constructed by gluing together glass slides with a face of the prism serving as a side wall. The 532 nm light output from a 10 Hz Quanta-Ray Nd:YAG laser, with 10 ns pulse width, is used as the fundamental excitation light. The light is focused into the prism through one of its side faces in a total internal reflection geometry using a 1 m focal length lens. The reflected signal exits the other side face and passes through filters (a CoCl2 solution and a UG5 filter) and a monochromator, and the second-harmonic intensity at 266 nm is measured by a photomultiplier tube (PMT). The signal is averaged for 100 laser pulses using a digital oscilloscope. The oscilloscope’s internal 1 MΩ termination results in sufficient signal voltage and an extended decay time so that many decay points can be digitized per laser pulse. The
uncertainty of the measurement is less than 5% of the SHG intensity and is mainly due to laser power fluctuations. Prism coating with trimethoxysilyl or triethoxysilyl monolayer is performed in a N2 atmosphere by heating the sample in a 1% anhydrous toluene solution with 1 mM acetic acid at temperatures ramping up from room temperature to 60-70 °C over the period of 1 h. The sample is removed from solution and rinsed with anhydrous toluene several times and finally baked at 150 °C for 20 min. Coating of the trichlorosilyl monolayer is also performed in a dry N2 atmosphere by dipping the sample into a 1% anhydrous toluene solution for 5 min, followed by washing with anhydrous toluene several times, and then baking the sample at 150 °C for 20 min. All SAM coatings were tested qualitatively by monitoring the wetting properties to check the surface homogeneity and to ensure that the coating was successful. Anhydrous toluene (>99%) and anhydrous acetic acid (>99%) were received from Aldrich. The toluene was dried by passing it through silica gel prior to use. The solutions of various pH were made by mixing 0.1 N HCl, KOH, and KCl at constant ionic strength. A Corning pH meter was used to measure the solution’s pH immediately after each SHG measurement. IV. Results and Discussion There can be many contributions to the second-harmonic signal. It should be noted that there is a significant difference between the SHG in the presence of a strong electric field and the conventional surface SHG, which is due to the broken symmetry at the interface. In our case, the main signal is from the bulk molecular dipoles oriented in an electric field near the charged interface. Therefore, it can be much larger than the conventional surface SHG because there are many more responding bulk molecules in the electric field induced SHG than in the surface SHG. In fact, previous studies on various charged interfaces using SHG have shown that the SHG signal from the χ(3) bulk contribution is equal to or larger than the χ(2) surface contribution when the surface is fully charged.13 From previous studies,13 the SHG response of the silica/water interface to changes in the bulk pH was shown to be mainly due to the electrostatic field of the interface region and the χ(3) contribution from water molecules. The χ(2) contribution is quite small and is regarded to be relatively constant with changing pH.13 For silica/water interfaces at very high pH, the SHG signal is dominated by the χ(3) contribution from water molecules.13 The relative χ(3) and χ(2) contributions to the total SHG signal strength can be obtained by comparing signal strength at pH 2 with that at any other pH, because the silica surface has an isoelectronic point around pH 2, where the total positive surface charge and the total negative surface charge cancel and thus give rise to a zero χ(3) contribution to the total SHG signal. This has led to the conclusion that, for the silica/ water interface, the χ(2) contribution can be neglected with respect to the χ(3) contribution.13 This result is corroborated by our own measurements, where the SHG intensity from silica/ water interfaces at bulk pH 2 is at least 20 times less than that at pH 13 (see Figure 1). This indicates that χ(3), not χ(2), is the dominating term for the present systems. A number of studies, including NMR,17-19 IR,20 and potentiometry,21-23 have suggested that there are at least two types of silanol groups at the aqueous solution/silica gel surface. The SHG studies of the silica/water interface14 indeed indicated two kinds of surface sites with quite different pKa values, one around 4 and the other around 9 (see Figure 1). This manifests itself in the SHG signal vs pH curve for the uncoated silica surface as two separate titration curves. Since the pKa of normal silanol is around 9,
11016 J. Phys. Chem., Vol. 100, No. 26, 1996
Figure 1. Second-harmonic field from a bare silica surface at 0.1 N ionic strength solution vs bulk pH.
this suggests that some of the surface functional groups at the silica interface take the normal silanol form. For the materials studied here, the χ(2) contribution of the octadecyl and phenyl self-assembled monolayers has no pH dependence since no chemical structure change is involved with the bulk pH changes. Therefore, the pH dependence must come from the χ(3) of some bulk material, such as water molecules in the electric double layer. The χ(2) of silanol and silanolate at the silica surface has been shown13 to be negligible compared with the χ(3) of water at high pH and can be regarded as pH independent. Therefore, the SHG signal changes as a function of pH for uncoated silica, octadecyl, and phenyl SAMs can be analyzed. For the pyridine self-assembled monolayer, the bulk pH change will result in a SAM structure change because the pKa of pyridine is between 9 and 10. This introduces some difficulties into the quantitative analysis of the pyridine SAM result. The SHG can also be generated from the two dry surfaces of the coupling prism because our SHG signal is collected in a total internal reflection geometry configuration. Nevertheless, this part of the signal is pH independent and can be regarded simply as background. To answer the question of whether the SAM formation is selective with respect to different surface sites, phenyl- and octadecyl-terminated SAMs were used to coat the surface. These coatings terminated the silanol surface functional group with an inert hydrocarbon group and should therefore reduce the surface charge density at all pH values, provided that the SAM is linked to the silica surface functional group directly. While the large polarizability of phenyl is expected to magnify both the second- and third-order susceptibility contributions, the SHG signal enhancements for the octadecyl should not be as large, due to its small polarizability. This is indeed what was observed (see Figure 2, for example, where the difference between the octadecyl-coated surface and that of the bare silica surface is small). However, if all of the silanol surface functional groups on a silica surface had been replaced by the phenyl or octadecyl groups, there should be no observed pH dependence arising from the third-order process. To the contrary, the pH response curve in Figure 2, for 3-phenylpropyland octadecyl-coated silica surface, has the same trend as that of the bare silica surface. It is thus obvious from Figure 2 that the silanol has not been completely replaced with hydrocarbon groups. To be able to quantitatively analyze the surface charge density at the silica/water interface, one must separate the χ(2) contribution from the total signal. This is a complicated matter unless all materials are far away from resonance or the resonance part of the susceptibility is not dominating. Octadecyl and water
Zhao and Kopelman
Figure 2. Second-harmonic field from nonreactive functional group coated silica surfaces at 0.1 N ionic strength solution vs bulk pH.
Figure 3. Second-harmonic field from pyridine coated silica surfaces at 0.1 N ionic strength solution vs bulk pH.
TABLE 1
material
intensity (pH ∼2) I2
intensity (pH ∼8) I8
intensity (pH ∼13) I13
[(I13)1/2 - (I2)1/2]/ [(I8)1/2 - (I2)1/2]
bare silica octadecylsilane 3-PhPr-silane
0.094 0.22 1.0
1.2 1.5 3.5
2.4 2.7 5.5
1.57 1.56 1.54
are far from resonance at both 532 and 266 nm. while the phenyl and pyridine are in resonance at 266 nm. This resonance introduces only limited signal enhancement, if any. For example, see Figures 2 and 3, where the phenyl- and pyridinecoated SAM curves are only 25% higher than the bare silica surface curve at pH 13. Considering that these two samples have a strong χ(2) background due to the side prism surfaces, the signal enhancement is minimal; therefore, the nonresonant part of the nonlinear susceptibility should still be dominating. To the first degree of approximation, we will assume that the phases for all materials are either 0° or 180°, regardless of the resonant effect. Under this assumption, recalling that the signal at pH 2 is a pure χ(2) contribution, one need only subtract the electric field value at pH 2 from that at other pH values for the same chemical surface to obtain the χ(3) value. As was seen in Figure 2, the SHG response vs bulk pH is actually a titration curve with two distinct pKa values. For a titration process with more than one pKa, one can estimate the relative concentration of each species by looking at the relative plateau height for each pKa. The results are presented in Table 1. The most notable feature of the phenyl and octadecyl coating data is that the relative height of the first plateau vs that at the
Organosilane Self-Assembled Monolayer Formation
Figure 4. Illustration of SAM formation mechanisms: (a) a simple direct chemical reaction that links SAM and surface functional groups directly; (b) SAM formation mechanism supported by our experiment, which involves at least three stagessphysical adsorption to previouly formed water film at the solid surface, hydration, and polymerization.
highest pH (Figure 2) is roughly the same as for the bare surface, provided that the signal strength at pH 2 (χ(2)) is subtracted (see Table 1). If SAM formation prefers only the surface sites with a pKa of 8-9, one should expect the modification of the SHG response only in the high pH range because the direct SAM linkage will preferentially consume the groups with pKa of 8-9. If SAM formation prefers only the groups with a pKa of 4-5, then the plateau around pH 7-9 should be greatly reduced relative to the rising response at high pH due to the preferentially consumed surface group with a pKa of 4-5. Therefore, one immediate conclusion is that SAM formation at the silica surface is not selective to different surface sites. Note that the -SiOSiR (R ) 4-phenylpropyl or octadecyl) coating can only terminate the pH sensitive surface groups instead of replacing them with different responsive groups like amines or pyridines; the isoelectronic point should remain unchanged and, therefore, at the isoelectronic point pH 2, the χ(2) contributes to the whole SHG exclusively. The large baseline difference between the phenyl-coated and uncoated surfaces is attributed to the χ(2) process in addition to the χ(2) background from the other two surfaces of the prism. The fact that the absolute and relative heights of the plateau around pH 7-9 to that of pH 13 are roughly the same, after the χ(2) contribution is eliminated from the total SHG signal, strongly suggests that the SAM has no selectivity toward different surface silanol sites. Another conclusion drawn from the results of the 4-phenylbutyl and octadecyl coatings is that most of the silanol groups at the silica surface remain sensitive to the bulk pH after the coating. In fact, the SHG vs pH curve has the same shape if the χ(2) is subtracted. Nevertheless, from the phenyl coating it is difficult to determine how much the surface functional groups have been modified by SAM formation because the strong χ(2) contribution from the phenyl groups may be the sole reason for the higher signal magnitude (even though the phenyl group has a smaller χ(3) than that of water24). The interpretation of the octadecyl coating results are much easier because most of the simple hydrocarbons such as cyclohexane have a χ(3) which is close to that of water.24 There could be two explanations for the higher signal strength compared to water for the octadecyl coating: (a) the silyl linkage to the surface and between the SAM molecules (see Figure 4) can hydrolyze to produce silanol and
J. Phys. Chem., Vol. 100, No. 26, 1996 11017 therefore provide extra charges, in addition to the charges at the silica surface; or (b) the χ(3) of hydrocarbons is slightly higher than that of water and therefore results in the slightly higher signal response. If the first explanation is correct, the signal strength at pH >9 must be greatly enhanced because the hydration of Si-O-Si should produce the normal -SiOH with a pKa around 8. These results negate the first option. If SAM formation does not modify significantly the surface chemistry of the silanol surface groups, i.e., the charge density dependence of the bulk pH retains almost the same shape for these two SAM formations, the only logical conclusion is that most of the surface silanol groups remain intact during the SAM formation process and the SAM is linked to the silica through only a few SiO-Si bridges. This provides direct experimental support for the SAM formation mechanism illustrated in Figure 4b. The formation of covalently bound silane monolayers on a silica surface was postulated to involve three steps (see Figure 4a): (1) hydrolysis of SiCl3 or Si(OMe)3 to Si(OH)3; (2) adsorption of Si(OH)3 to the hydroxy surface; (3) dehydration and polymerization to form Si-O-Si networks between monolayer silyl groups and surface silanol groups. However, this simple scheme has never been supported by direct experimental evidence. Under the most strictly anhydrous conditions, SiCl3 is capable of forming SiO-Si with surface SiOH through a gas phase reaction at elevated temperature.25 Most experimental conditions, however, will still permit some trace water on the silica surface. Another commonly accepted mechanism12 is (1) quick adsorption of a water film, (2) physical adsorption of a monomer monolayer, and (3) chemical reaction that links the silanes together and anchors the net to the silica after dehydration. This latter mechanism, although lacking strong direct experimental support, does explain the fact that the roughness of a silanated silica surface is lower than that of the original surface. The acidbase reaction of the surface with and without coating demonstrates that the surface silanol chemistry is the same with and without the SAM and therefore supports the second SAM formation mechanism mentioned above. With this in mind, one can easily explain the nonselectivity toward the different surface sites of SAM formation because there is not much linking between the SAM and the surface except occasional anchoring. This conclusion is consistent with other recent studies3,26 which suggest that the organosilane SAMs prepared on vastly different surfaces, such as hydrolyzed gold and oxidized silicon, have similar IR spectra, and thus the SAM phase structure is the same whether or not there are any silyloxy links between the substrate surface and the SAM film. These studies suggest that the SAM structure is similar to that of a Langmuir-type insoluble monolayer film on a solid support. To further test our model, one SAM was prepared using 3-(βpyridinyl)propylsilane. Its SHG vs pH response is presented in Figure 3. One can immediately see the large difference in the pH response, especially the large dip around pH 4-5. Pyridine, when ionized, is positively charged with a pKa of around 9.5. This would cancel the negative charge due to the surface silanol group. Note that at pH
