Anal. Chem. 2004, 76, 604-609
Infrared-Visible Sum Frequency Generation Investigation of Cu Corrosion Inhibition with Benzotriazole Zachary D. Schultz, Mary Ellen Biggin, Jeffrey O. White, and Andrew A. Gewirth*
Department of Chemistry and Frederick Seitz Materials Research Laboratory, University of Illinois at UrbanasChampaign, Urbana, Illinois 61801
Infrared-visible sum frequency generation spectroscopy is used to investigate the corrosion inhibitor benzotriazole (BTAH) adsorbed on Cu(100) and Cu(111) in acidic solution. Potential-dependent in situ spectra indicate that the adsorbed molecule is the benzotriazole anion (BTA-) at all potentials investigated. The Cu(100) surface is shown to form an ordered adlayer at all potentials probed, while the Cu(111) face is shown to be disordered at negative potentials, but to order with applied positive potential. The ordered adlayer is shown to consist of the BTA- in two configurations, one coordinated to the surface and Cu+ ions in solution and the other coordinated only to the surface. The BTA- coordinated to Cu+ is shown to be more stable with respect to Cl- addition than BTAcoordinated to only the surface. This study demonstrates the viability of using sum frequency generation to study corrosion inhibition in situ. Organic and inorganic molecules are commonly used to inhibit corrosion processes on metal electrodes.1 These molecules adhere to the metal surface and form a film that retards metal dissolution. An important question concerns the way in which these molecules associate with the electrode surface and their mode of action. One way to examine these surfaces is through the use of vibrational spectroscopy. The recent development of infrared-visible sum frequency generation (SFG) spectroscopy provides the considerable advantages of true surface sensitivity, ability to interrogate well-defined surfaces, such as single crystals, and being an absolute measurement. Infrared-visible SFG spectroscopy is a surface-sensitive vibrational probe used to elucidate interfacial phenomena.2 SFG is an effective probe of the electrochemical interface3 and can distinguish surface from bulk properties in polymeric systems.4 The recent development of robust, tabletop tunable infrared lasers makes SFG more attractive to examine a variety of interfacial problems.2 Despite these advantages, there have been no studies of corrosion inhibitors using SFG. * To whom correspondence should be addressed. E-mail: agewirth@ uiuc.edu. Tel.: 217-333-8329. Fax: 217-333-2685. (1) Newman, R. C.; Sieradzki, K. Science 1994, 263, 1708-1709. (2) Richmond, G. L. Chem. Rev. 2002, 102, 2693-2724. (3) Tadjeddine, A.; Le Rille, A.; Pluchery, O.; Vidal, F.; Zheng, W. Q.; Peremans, A. Phys. Status Solidi A 1999, 175, 89-107.
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The interfacial sensitivity attendant SFG arises from the nonlinear effect that gives rise to the SFG signal. SFG is a secondorder nonlinear process in which a visible laser and a tunable infrared laser combine to produce a signal at the sum frequency. Under the electric-dipole approximation, SFG is forbidden in any material with inversion symmetry. At an interface, the inversion symmetry of bulk systems is broken, allowing SFG to probe the behavior in this region without contribution from surrounding bulk species. The SFG signal is composed of two components as shown in eq 1. In eq 1, the first component (Aeiφ) is a nonresonant portion
|
ISFG ∝ Aeiφ +
∑ (ω n
IR
βn - ωn) + iΓn
|
2
IVisIIR
(1)
from the mixing of the visible and infrared beams at the interface. This component varies, depending on the properties of the interface, but is generally regarded as constant for a given system. The second component is a Lorentz-type function that represents the change in SFG intensity arising from resonance with a vibration at the interface. β is related to the product of the infrared and Raman transition amplitudes. This means that only a mode that is both IR and Raman active will be observed by SFG.5 Benzotriazole (C6H3N3H, BTAH) is a molecule used to control the corrosion and dissolution of Cu. Through the loss of the amine hydrogen, BTA- forms a polymeric film with Cu ions in solution creating a barrier layer,6 which protects the surface at both anodic and cathodic potentials.7 Voltammetry of the Cu-BTA system in 0.1 M H2SO4 is featureless between the anodic dissolution wave (+0.25 V vs Ag/AgCl) and hydrogen evolution (-0.7 V vs Ag/ AgCl).8 While the barrier film is insensitive to the crystal orientation,8 there is empirical evidence that corrosion resistance of Cu exposed to BTAH is surface structure dependent, with (100) > (110) > (111).9 Scanning tunneling microscopy (STM) investigations showed that an adlayer forms in the presence of BTAH at negative (4) Chen, Z.; Shen, Y. R.; Somorjai, G. A. Annu. Rev. Phys. Chem. 2002, 53, 437-465. (5) Shen, Y. R. The Principles of Nonlinear Optics; John Wiley & Sons: New York, 1984. (6) Cotton, J. B.; Scholes, I. R. Br. Corros. J. 1967, 2, 1-5. (7) Poling, G. W. Corros. Sci. 1970, 10, 359-370. (8) Biggin, M. E.; Gewirth, A. A. J. Electrochem. Soc. 2001, 148, C339-C347. (9) Mayanna, S. M.; Setty, T. H. V. Corrosion Sci. 1975, 15, 627-637. 10.1021/ac035169k CCC: $27.50
© 2004 American Chemical Society Published on Web 12/23/2003
potentials.10 At positive potentials, however, the formation of the Cu-BTA film over the surface made imaging difficult. The STM images at negative potentials indicate that the BTAH adlayer is dependent upon surface orientation and anions present in solution, particularly Cl-.11 Unfortunately, STM does not provide information as to the chemical identity of the moieties being imaged. Surface-enhanced Raman scattering (SERS) studies indicate that the identity of the molecule adsorbed to the surface is potential dependent. , BTAH is found at potentials negative of -400 mV versus SCE. However, at cathodic potentials, the surface species resembles the polymeric Cu-BTA film.12,13 Unfortunately, SERS requires the use of a roughened substrate and is not strictly surface sensitive.14 We used polarization modulated-infrared reflection adsorption spectroscopy (PM-IRRAS) to show that the CuBTA film forms reversibly in Cl- containing electrolyte but irreversibly in sulfate solution.8 PM-IRRAS is sensitive to material within 2-3 µm normal to the electrode surface making it predominantly sensitive to the barrier film, rather than the electrode surface specifically. The presence of the Cu-BTA film in this region thus obscures the signal from the metal surface. To obtain information about the nature of the species on the Cu surface, and their dependence on potential, crystal orientation, and solution composition, we report here the results of studies on BTAH adsorption on Cu in acidic solution utilizing SFG spectroscopy. These studies provide insight into the mechanism of adhesion of the polymeric BTA-Cu film with the Cu surface and the way in which this important molecule inhibits Cu corrosion. EXPERIMENTAL SECTION BTAH (99%; Aldrich) was recrystallized prior to use. Highpurity sulfuric and hydrochloric acids (J. T. Baker Ultrex II) were used. All solutions were prepared using ultrapure water (Milli-Q UV plus, Millipore Inc., 18.2 MΩ cm). All experiments were performed using 75 mM BTAH in 0.1 M H2SO4. The Cu(100) and Cu(111) single crystals were obtained from Monocrystals Co. The Cu single crystals were mechanically polished with alumina paste down to 0.3 µm and were then electropolished for 2 min in 66% orthophosphoric acid aqueous solution containing ∼0.1 M Cu ions. The surface orientation of the Cu crystals was confirmed by using Laue´ X-ray backscattering. The infrared and visible beams required for the SFG experiment were provided by a Euroscan SFG spectrometer that has been described in detail previously.15 Briefly, the system consists of an all-solid-state Nd:YAG laser operating at 25 Hz, producing ∼100 pulses of 12-ps duration per laser burst. The output of the Nd:YAG laser is split by a 33% beam splitter, with one-third being doubled to provide the visible beam at 532 nm. The rest of the beam pumps an optical parametric oscillator (OPO) with an AgGaS2 crystal to provide tunable infrared from 4 to 10 µm. In other configurations, this instrument is tunable from 2.5 to 20 µm with a resolution of better than 2 cm-1. The resolution of this (10) Magnussen, O. M.; Behm, R. J. MRS Bull. 1999, 16-23. (11) Polewska, W.; Vogt, M. R.; Magnussen, O. M.; Behm, R. J. J. Phys. Chem. B 1999, 103, 10440-10451. (12) Youda, R.; Nishihara, H.; Aramaki, K. Corros. Sci. 1988, 28, 87-96. (13) Chan, H. Y. H.; Weaver, M. J. Langmuir 1999, 15, 3348-3355. (14) Campion, A.; Kambhampati, P. Chem. Soc. Rev. 1998, 27, 241-250. (15) Mani, A. A.; Dreesen, L.; Hollander, P.; Humbert, C.; Caudano, Y.; Thiry, P. A.; Peremans, A. Appl. Phys. Lett. 2001, 79, 1945-1947.
experiment was limited by bandwidth of the infrared beam and is estimated to be ∼2 cm-1, based on the width of water lines observed in the infrared absorption spectrum. The OPO produced 1-2 mW of infrared power, measured at the sample, in the examined region. The visible and infrared beams are co-propagating and incident upon the sample at 55° and 65°, respectively. The SFG spectra are normalized against a simultaneously obtained spectrum from a ZnS crystal by means of a 10% CaF2 beam splitter located just before the sample. The reference allows for corrections due to laser fluctuation and other changes in IR power. The visible, infrared, and sum frequency beams are all p-polarized in these experiments. SFG measurements were performed using a Kel-F/glass spectroelectrochemical cell described previously8 based on designs in the literature.16 The co-propagating beams pass through a 60° trapezoidal CaF2 prism and are reflected off the single crystal surface and back through the prism. The amount of solution between the prism and the crystal surface is minimized (on the order of a few micrometers) by pressing the crystal against the prism via rubber bands. A Au wire was used as the counter electrode with a Ag/AgCl reference. All potentials presented in this paper are given relative to Ag/AgCl. The cell was partly filled with solution before the crystal was introduced to ensure the presence of solution between the crystal and the prism. All solutions were bubbled with N2 for ∼20 min prior to the measurements, and the cell was maintained under a N2 atmosphere during the experiment. In potential step experiments, the system was held at a specified potential for ∼3 min before the SFG spectrum was obtained in order to ensure that the system was at equilibrium. The effects of Cl- were measured by adding picomoles of Cl- via syringe into the cell. The correct potentialdependent behavior of the SFG bands was verified by potential excursions between -0.70 and +0.20 V prior to Cl- addition. The potential during Cl- addition was maintained at 0.0 V. PM-IRRAS measurements were made as reported previously.8 Transmission infrared measurements on NaBTA in KBr were made using a Nicolet FT-IR spectrometer. NaBTA was prepared by neutralization of BTAH with NaOH. Vibrational mode analysis was performed using Gaussian 9817 under the B3LYP method and the 6-31G* basis set. Results were scaled by 0.97 to correct for electron correlation effects. RESULTS AND DISCUSSION SFG from BTA on Cu(100). Shown in Figure 1 is a SFG spectrum acquired from the Cu(100) surface in 0.1 M H2SO4 and 75 mM BTAH at 0.10 V versus Ag/AgCl. The spectrum shows four major peaks, at 1563, 1492, 1445, and 1399 cm-1, labeled 1-4, (16) Bethune, D. S.; Luntz, A. C.; Sass, J. K.; Roe, D. K. Surf. Sci. 1988, 197, 44-66. (17) Frisch, M. J.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Challacombe, A. N. M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian Revision A.9 ed.; Gaussian, Inc.: Pittsburgh, PA, 1998.
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Table 1. Observed and Calculated Vibrational Frequencies for the Modes of BTA- Examined in This Study
Figure 1. SFG spectrum of Cu(100) in 0.1 M H2SO4 and 75 mM BTAH at 0.10 V. O represent experimental data points. The line is the result of fit to eq 1. Peak numbers are shown.
respectively. Band 4 at 1399 cm-1 exhibits a shoulder, labeled band 5, with a maximum at 1389 cm-1. We used a nonlinear leastsquares fit of the experimental spectrum to eq 1, with the result shown as the solid line in Figure 1. In this fit, the vibrational transitions appear as peaks. In their work on polycrystalline Cu, Klenerman and co-workers showed that subtle changes in Cu surface chemistry could lead to substantial changes in the phase of the SFG response, which yields SFG transitions evident as dips.18 Indeed, SFG of CN on Cu(111) evinced a derivative line shape.19 The agreement between the fit and the experimental points shows that phase considerationssproblematic in other situations19sare not important here. SFG spectroscopy in this region of the infrared is problematic due to the difficulty in producing sufficient infrared radiation and the interference from gas-phase H2O. We are aware of only one other system that has been studied in this region20 without the use of a free electron laser. The rotational-vibrational levels of H2O act to deplete the tunable IR prior to reaching the cell. Our setup uses a reference to account for this depletion, but minor deviations in beam overlap upon entering the in situ cell can occur, leading to higher noise in the spectrum, especially in the highenergy region. BTA- and BTAH both exhibit vibrational bands in the energy region shown in Figure 1. Upon the loss of the amine H, there is a shift in the position of the benzyl stretching modes from 1595, 1515, and 1462 cm-1 to 1575, 1490, and 1445 cm-1.21 These latter three energies are very close to what is observed in Figure 1, which strongly suggests that the molecule adsorbed to the Cu(100) surface is BTA-, not BTAH. We note that position of band 1 at the 1563-cm-1 band is ∼10 cm-1 too low in energy for this assignment. However, the low signal level and higher noise in this region caused by overlap with the ro-vibration lines of water contribute to a poorer fit at this energy and higher indeterminacy in the band position. All three of the high-energy bands seen in Figure 1 behave similarly, and this coupled with their similar origin leads us to focus only on band 3 in subsequent analysis of the benzyl ring stretches. Band 5 at 1389 cm-1 has been assigned to a combined benzyltriaz ring stretching mode.8,21 Band 4 at 1399 cm-1 has not been previously observed, and its assignment will be presented below. (18) Lin, S.; Oldfield, A.; Klenerman, D. Surf. Sci. 2000, 464, 1-7. (19) Matranga, C.; Wehrenberg, B. L.; Guyot-Sionnest, P. J. Phys. Chem. B 2002, 106, 8172-8175. (20) Peremans, A.; Caudano, Y.; Thiry, P. A.; Dumas, P.; Zhang, W. Q.; Le Rille, A.; Tadjeddine, A. Phys. Rev. Lett. 1997, 78, 2999-3002. (21) Rubim, J.; Gutz, I. G. R.; Sala, C.; Orville-Thomas, J. J. Mol. Struct. 1983, 100, 571-583.
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We performed calculations on the free BTA- anion to help explain the origin of the vibrational modes followed in this study. Table 1 lists the peaks observed in SFG, the calculated vibrational frequency, and a representation of the mode involved. Table 1 shows that the locus of motion for peaks 1-3 is located in the benzyl ring, in agreement with previous results.21 Peak 4 shows a much stronger displacement vector in the triazole portion of the molecule than any of the other observed modes. Since the molecule is known to adhere to the surface via the triazole nitrogens,22 it is reasonable to expect this mode to be significantly affected upon adsorption to the Cu surface. This gas-phase analysis of the vibrational spectroscopy of BTA- of course does not fully account for the behavior of the molecule when it is complexed as an oligimer or when it is adsorbed on the surface. These latter calculations are considerably more advanced than that presented here and will be the focus of future work. Figure 2 provides a comparison between transmission IR of solid NaBTA in KBr (Figure 2A), the Cu-BTA film obtained using in situ PM-IRRAS at 0.10 V in a solution containing 75 mM BTAH (22) Chan, H. Y. H.; Weaver, M. J. Langmuir 1999, 15, 3348-3355.
Figure 2. (A) IR absorption spectrum of NaBTA in a KBr pellet. (B) In situ PM-IRRAS spectrum of Cu-BTA film at 0.10 V. (C) SFG spectrum of Cu(100)-BTA interface at 0.10 V. Peak numbers are shown. Figure 4. Potential dependence of intensity for (A) SFG of Cu(100) peaks 3 (9), 4 (b), and 5 (2) (offset for clarity), (B) PM-IRRAS of peak 5, and (C) SFG of Cu(111) peak 3. Solid symbols indicate anodic scan, and open symbols are the cathodic scan. Overlapped symbols appear open.
Figure 3. Potential-dependent SFG spectrum obtained from the Cu(100)-BTA interface at the indicated potentials in the anodic scan (A) and cathodic scan (B). Arrows indicate scan direction. The spectra are offset for clarity.
and 0.1 M H2SO4 (Figure 2B), and the in situ SFG spectrum at 0.1 V (Figure 2C). Figure 2 shows that peaks 3 and 5 are common to all three spectroscopies. Band 5 from NaBTA evinces some splitting as a consequence of a Fermi resonance,22 but the splitting is to lower energy and is relatively small. Figure 2 shows clearly that band 4 at 1399 cm-1 is a new feature unique to SFG. The emergence of a new peak in SFG spectroscopy is unusual and is indicative of events at the Cu(100)-BTA interface. Parenthetically, the agreement in energy for bands 3 and 5 between all three techniques provides strong support for the accuracy of our SFG fit. The effects of applied potential to the Cu surface were investigated to elucidate the behavior of the surface adlayer. Figure 3 shows the potential dependence of the SFG signal between 1375 and 1475 cm-1 in both the initial anodic and subsequent cathodic direction. The potential range was chosen based on previously reported voltammetry8 showing the onset of hydrogen evolution (-0.65 V) and copper dissolution (0.15 V). One interesting feature observed is that the peak energies are constant throughout the potential range examined. This implies that the identity of the adsorbed monolayer is constant with respect to potential, in contrast to the results of previous SERS studies.12,13 Vibrational Stark effects have been observed at electrode surfaces for strongly coupled adsorbates such as CO, NO, and SCN.23 However, this effect is not seen here, possibly due to a Stark tuning coefficient too small to be resolved by our instrument.
A second interesting observation is the change in intensities of the peaks 3-5 with respect to potential. In Figure 3A, as the potential is made positive, the intensity of peak 3 increases and the intensity ratio of peaks 4 and 5 changes. Upon making the potential negative, peak 3 loses intensity and peak 4 gains intensity. These intensity changes are plotted in Figure 4A. Figure 4A shows that the intensity of peak 5 begins to increase at potentials positive of ∼-0.2 V and then remains constant upon the subsequent cathodic scan. This irreversibility is in contrast to that observed for peaks 3 and 4. The behavior of peak 4 is similar to that of peak 3, except that peak 4 decreases in intensity at positive potentials while peak 3 increases in intensity at the same potentials. The behavior of peaks 3 and 4 could occur as a result of (a) a change in polarizability of the molecule due to surface charge, (b) a reorientation of the molecule at the interface, or (c) a change in packing density of the molecule on the surface. Figure 4B shows the potential dependence of the intensity of peak 5 as measured by in situ PM-IRRAS. Intriguingly, the irreversibility of the intensity of this peak with potential is similar to that seen by the same peak in SFG. Since the IRRAS measurement probes the Cu-BTA film above the surface, this similarity in behavior suggests that peak 5 in the SFG corresponds to BTA molecules acting as a tether at the surface for the film. This assignment implies that peak 4 must be associated with BTA molecules at the surface that are not coordinated to the barrier film. Peaks 4 and 5 likely split because they correspond to a mode involving the triazole ring, where coordination is known to occur.13 Peak 3 (and peaks 1 and 2) corresponds to a benzyl stretching mode that is insensitive to coordination. SFG from BTA on Cu(111). Association of BTAH with Cu(111) was examined using SFG spectroscopy in order to further probe the coordination of this molecule with Cu surfaces. Figure (23) Wasileski, S. A.; Koper, M. T. M.; Weaver, M. J. J. Am. Chem. Soc. 2002, 124, 2796-2805.
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Figure 5. Potential-dependent SFG spectrum obtained from the Cu(111)-BTA interface at the indicated potentials in the anodic scan (A) and cathodic scan (B). Arrows indicate scan direction. The spectra are offset for clarity.
Figure 6. SFG intensity from Cu(100) peaks 3 (9), 4 (b), and 5 (2) (offset for clarity) as a function of Cl- concentration in the cell.
5 shows in situ potential-dependent SFG obtained from BTA on Cu(111) in 0.1 M H2SO4. At positive potentials, the figure shows three peaks at 1391 (peak 5), 1399 (peak 4), and 1445 cm-1 (peak 3); these match well with the corresponding features on the Cu(100) surface. This result again suggests that BTA- is the species at the interface. Figure 5 also shows that the intensity of peak 3 increases with positive potential and decreases with negative potential. The intensity of peak 3 is plotted as a function of potential in Figure 4C. As with the Cu(100) case, the intensity in this band is reversible with potential. Figure 4 shows that the onset of the intensity change in peak 3 for the Cu(111) and Cu(100) differs by ∼400 mV. This value tracks the difference in work function for the two surfaces.24 It is difficult to determine whether there are other new features around peak 3. In fact, it would be difficult to assign peak 3 above the noise in this region if it did not show marked intensity increase with potential. The interference from water vapor could easily be responsible for peaklike features around 1440 and 1455 cm-1. The region around 1395 cm-1 for the Cu(111) case shows markedly different behavior relative to Cu(100). At negative potentials, there are at least three peaks at 1386 (peak 7), 1395 (peak 6), and 1399 cm-1 (peak 4). As the potential becomes positive, peak 7 disappears and peak 6 decreases in intensity. At the most positive potentials, peak 5 reappears. At 0.20 V, peak 7 is completely gone and peak 6 is obscured by the emergence of peak 5. As the potential is made negative again, we see the intensities and identities of the peaks return to a condition similar to that initially observed. The intensity of peak 3 for Cu(111) shown in Figure 4C as a function of potential exhibits significant hysteresis. Peak 3 does not show a large change in intensity until the potential is made positive of 0.05 V. Upon making the potential negative, the intensity of peak 3 persists until a potential of -0.25 V is reached. This hysteresis in peak intensity correlates exactly with the emergence and disappearance of peak 5 on the Cu(111) surface. At a potential positive of 0.05 V, peak 5 is clearly visible, and peak 5 persists on the cathodic scan until a potential of -0.25 V is obtained. Understanding of the differences between the Cu(111) and Cu(100) systems makes recourse to the STM studies. While SFG alone cannot determine surface order unambiguously, we note
the correlation between SFG features and surface order previously determined by in situ STM measurements. The Cu(111)-BTA surface exhibits a disordered STM image at negative potentials,11 while the Cu(100) surface is ordered in the same potential region.25 Additionally, it is understood that SFG intensity can increase with increasing surface order.26 The Cu(111) surface exhibits BTApeaks 3-5 at energies nearly identical to those found on Cu(100). STM revealed the presence of an ordered BTA- adlayer on Cu(100). The spectral features from the Cu(100) surface show no variance in frequency as a function of potential, supporting constancy in the adlayer structure. On Cu(111) in the negative potential region, STM shows that the surface is disordered.11 This disorder is reflected in the SFG spectra for Cu(111) by a loss in intensity for peak 3 and the emergence of other peaks in the triazole stretch region, corresponding to heterogeneity in BTACu surface association. The ability of Cu(100) and BTA- to maintain a more ordered adlayer may explain why the (100) surface is more resistant to corrosion than (111). SERS results suggested that the adsorbed species on Cu(poly) was BTAH in the negative potential region and BTA- in the positive potential region. This result is somewhat different from the SFG results reported here, which show that BTA- is present throughout the potential region studied. We make two comments regarding this discrepancy. First, we note that the increasing number of features observed on the Cu(111) surface at negative potentials could complicate clear identification of surface species in the SERS measurement, which may sample a number of different interfacial structures. Second, SERS enhancement extends 1-2 nm from the surface and could easily incorporate species in the bulk that are excluded by SFG, including uncomplexed BTAH molecules trapped in the oligomeric film. SFG arises only from the interface. Titration with Cl-. To further understand the role of the surface adlayer in the corrosion process, we performed a titration by adding Cl- to a Cu(100)-BTA surface held at a fixed potential of 0.0 V. It has been shown in solution that the Cu-BTA film is unstable relative to CuCl at pH
