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Exploring Electrosorption at Iron Electrode with in Situ Surface-Enhanced Infrared Absorption Spectroscopy Sheng-Juan Huo,† Jin-Yi Wang,† Jian-Lin Yao,*,‡ and Wen-Bin Cai*,† Shanghai Key Laboratory for Molecular Catalysis and Innovative Materials and Department of Chemistry, Fudan University, Shanghai 200433, China, and Department of Chemistry, Suzhou University, Suzhou 215123, China Surface-enhanced infrared absorption spectroscopy (SEIRAS) in attenuated total reflection (ATR) configuration has been extended to the Fe electrode/electrolyte interface in neutral and weakly acidic solutions for the first time. The SEIRA-active Fe film electrode was obtained through a potentiostatic electrodeposition of a virtually pinhole-free 40 nm-thick Fe overfilm onto a 60 nm-thick Au underfilm chemically predeposited on the reflecting plane of an ATR Si prism. The infrared absorption for CO adlayer at the Fe film electrode measured with ATR-SEIRAS was enhanced by a factor of larger than 34, as compared to that at a Fe bulk electrode with external infrared absorption spectroscopy in the literature. More importantly, the unipolar band shape enabled the reliable determination of the Stark tuning rates of CO adlayer at Fe electrode. In situ ATR-SEIRAS was also applied to study the electrosorption of a typical corrosion inhibitor benzotriazole (BTAH) on Fe electrode as a function of potential, providing additional spectral information at positive potentials in support of the formation of a polymer-like surface complex FeIIm(BTA)n as the corrosion-resistant layer. Iron is perhaps the most popular metal in the world with unique and technologically important chemical properties. On one hand, Fe is essential in many (electro)catalytic reactions; for example, Fe is the main catalyst in the synthesis of ammonia, and Fe-alloyed materials are developed as efficient CO-tolerant anode and cathode catalysts for proton-exchanged membrane fuel cells.1,2 On the other hand, Fe is prone to corrosion due to its active property; intensive interests have been directed to the corrosion-inhibition on Fe.3 Therefore, extension and application of high-sensitivity in situ molecular spectroscopies4,5 are of great * To whom correspondence should be addressed. W.-B.C.: e-mail, wbcai@ fudan.edu.cn; phone, +86-21-55664050; fax, +86-21-65641740. J.-L.Y.: e-mail,
[email protected]; phone, +86-512-65880359. † Fudan University. ‡ Suzhou University. (1) Watanabe, M.; Zhu, Y. M.; Uchida, H. J. Phys. Chem. B 2000, 104 (8), 1762–1768. (2) Lefevre, M.; Proietti, E.; Jaouen, F.; Dodelet, J. P. Science 2009, 324 (5923), 71–74. (3) Melendres, C. A. In Electrochemical and Optical Techniques for the Study and Monitoring of Metallic Corrosion; Ferreira, M. G. S.; Melendres, C. A., Eds.; Kluwer: Dordrecht, 1991; p 355. (4) Lipkowski, J., Ross, P. N., Eds. Adsorption of Molecules at Metal Electrodes; VCH: New York, 1992. 10.1021/ac1002323 2010 American Chemical Society Published on Web 05/18/2010
interest for probing the adsorption and reaction involved in the (electro)catalysis and corrosion-inhibition at the Fe electrode. Along this line, surface-enhanced Raman spectroscopy (SERS) has been reported to be the study of relevant topics by Tian’s group;6-8 it was found that the surface enhancement for a roughened Fe electrode was about 3 to 4 orders of magnitude as low as that for a typical SERS substrate of Ag, Au, or Cu. In contrast, the theory of surface-enhanced IR absorption spectroscopy (SEIRAS) predicts a comparable surface enhancement factor for nearly all appropriately structured metal nanoparticle films. As well as being demonstrated in previous reports,9-23 electrochemical SEIRAS in attenuated total reflection (ATR) configuration is a powerful analytical tool for the characterization of the adsorption and reaction at metal surfaces. (5) Sun, S. G., Christensen, P. A., Wieckowski, A., Eds. In-Situ Spectroscopic Studies of Adsorption at the Electrode and Electrocatalysis; Elsevier: Amsterdam, 2007. (6) Cao, P. G.; Gu, R. N.; Tian, Z. Q. Langmuir 2002, 18 (20), 7609–7615. (7) Cao, P. G.; Yao, J. L.; Ren, B.; Mao, B. W.; Gu, R. A.; Tian, Z. Q. Chem. Phys. Lett. 2000, 316 (1-2), 1–5. (8) Yao, J. L.; Ren, B.; Huang, Z. F.; Cao, P. G.; Gu, R. A.; Tian, Z. Q. Electrochim. Acta 2003, 48 (9), 1263–1271. (9) Ataka, K.; Yotsuyanagi, T.; Osawa, M. J. Phys. Chem. 1996, 100 (25), 10664– 10672. (10) Osawa, M. In Handbook of Vibrational Spectroscopy; Chalmers, J. M.; Griffiths, P. R., Eds.; John Wiley & Sons: Chichester, 2002; Vol. 1. (11) Bjerke, A. E.; Griffiths, P. R.; Theiss, W. Anal. Chem. 1999, 71 (10), 1967– 1974. (12) Aroca, R. F.; Ross, D. J.; Domingo, C. Appl. Spectrosc. 2004, 58 (11), 324A– 338A. (13) Delgado, J. M.; Orts, J. M.; Rodes, A. Electrochim. Acta 2007, 52 (14), 4605–4613. (14) Yajima, T.; Uchida, H.; Watanabe, M. J. Phys. Chem. B 2004, 108 (8), 2654– 2659. (15) Shao, M. H.; Adzic, R. R. J. Phys. Chem. B 2005, 109 (35), 16563–16566. (16) Futamata, M.; Luo, L. Q.; Nishihara, C. Surf. Sci. 2005, 590 (2-3), 196– 211. (17) Shiroishi, H.; Ayato, Y.; Kunimatsu, K.; Okada, T. J. Electroanal. Chem. 2005, 581 (1), 132–138. (18) Yan, Y. G.; Li, Q. X.; Huo, S. J.; Ma, M.; Cai, W. B.; Osawa, M. J. Phys. Chem. B 2005, 109 (16), 7900–7906. (19) Huo, S. J.; Xue, X. K.; Yan, Y. G.; Li, Q. X.; Ma, M.; Cai, W. B.; Xu, Q. J.; Osawa, M. J. Phys. Chem. B 2006, 110 (9), 4162–4169. (20) Huo, S. J.; Wang, J. Y.; Sun, D. L.; Cai, W. B. Appl. Spectrosc. 2009, 63 (10), 1162–1167. (21) Xue, X. K.; Wang, J. Y.; Li, Q. X.; Yan, Y. G.; Liu, J. H.; Cai, W. B. Anal. Chem. 2008, 80 (1), 166–171. (22) Chen, Y. X.; Heinen, M.; Jusys, Z.; Behm, R. B. Angew. Chem., Int. Ed. 2006, 45 (6), 981–985. (23) Han, B.; Li, Z. H.; Wandlowski, T. Anal. Bioanal. Chem. 2007, 388 (1), 121–129.
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To the best of our best knowledge, extension of SEIRAS to Fe electrodes has not been attempted, although traditional external IR reflection absorption spectroscopy (IRRAS) of CO on UHVsputtered Fe films has detected enhanced but badly distorted IR absorption in the solid-gas interface.24,25 Also, notably, surface infrared spectroscopic study on the Fe electrode remains largely unexplored. Cuesta et al. pioneered in the investigation of CO adsorption on a bulk Fe electrode using the external IRRAS with the so-called “thin-layer structure”.26 Nevertheless, the reported signals of adsorbed CO were very weak and even severely distorted by the bulk water absorption, complicating the data interpretation. To address these issues, one priority of the present work is to demonstrate that electrochemical ATR-SEIRAS with much higher surface sensitivity and undistorted spectral response can be extended to a Fe electrode prepared through the two-step wet process strategy as developed in our group using CO as the surface probe molecule.18-20 This strategy combines the initial electroless deposition of a conductive Au underfilm on Si with a subsequent electrodeposition of a virtually pinhole-free desired metal overfilm. Furthermore, we will demonstrate that this ATR-SEIRAS can be applied to the study of the electrosorption of a typical corrosion inhibitor, i.e., benzotriazole (BTAH) on this electrode, yielding useful vibrational information complementary to SERS results. SERS has an advantage of providing lower frequency bands that are inaccessible to SEIRAS. However, the complexity of the enhancement mechanisms (and, thus, the surface selection rule) of SERS complicates the spectral analysis.27,28 In fact, due to the special chemical enhancement mechanism, SERS appears to yield the largest signals for many metals at more negative potentials, where the actual coverage of an adsorbate is rather low. In contrast, more realistic spectral information can be obtained with ATR-SEIRAS at the more positive potentials, which is important for understanding the corrosion-inhibition mechanism. Unfortunately, no previous infrared spectroscopic study on the potential dependent electrosorption of a typical corrosion inhibitor at the Fe electrode has been found in the literature, and therefore, it is the other priority of the present work to extend in situ ATRSEIRAS to this area. EXPERIMENTAL SECTION Preparation and Characterization of Fe Overfilm. Initially a 60 nm-thick Au film was chemically deposited on the basal plane of a hemicylindrical undoped Si prism (PASTEC, Japan, Osaka), according to the procedures described in refs 18 and 19. The Aucoated Si prism was rinsed with Milli-Q water and assembled into the custom-made spectroelectrochemical cell (vide infra). The surface of the Au film was cleaned in 0.1 M HClO4 by cycling the electrode potential between 0.0 and 1.45 V at 50 mV s-1 for about five cycles to remove possible surface contaminants to attain a stable and reproducible cyclic voltammogram. After the cell was thoroughly rinsed with a copious amount of Milli-Q (24) Krauth, O.; Fahsold, G.; Magg, N.; Pucci, A. J. Chem. Phys. 2000, 113 (15), 6330–6333. (25) Krauth, O.; Fahsold, G.; Pucci, A. J. Chem. Phys. 1999, 110 (6), 3113– 3117. (26) Cuesta, A.; Gutierrez, C. J. Phys. Chem. 1996, 100 (30), 12600–12608. (27) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. J. Phys.: Condens. Matter 1992, 4 (5), 1143–1212. (28) Moskovits, M. Rev. Mod. Phys. 1985, 57 (3), 783–826.
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water, a Fe overfilm was formed on the Au underfilm electrode using a potentiostatic deposition in the bath of 0.01 M FeSO4 + 10-4 M H2SO4 + 0.01 M H3BO3 at -1.0 V for 600 s. Then, the spectroelectrochemical cell was thoroughly rinsed with a copious amount of Milli-Q water and quickly injected with the deaerated phosphate buffer solution (PBS, pH 6.9) followed by immediate potential control at -1.0 V in order to reduce the native oxides on Fe surfaces. Alternatively, the plating solution in the spectroelectrochemical cell was replaced with a 0.1 M KClO4 (pH 5.8) electrolyte through a homemade flow system under the potential control at -1.0 V. The geometrical surface area of the Fe working electrode is 1.33 cm-2. A Pt gauze and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. A CHI 660B electrochemistry workstation (CHI instruments, Shanghai) was employed for potential control. Inductively coupled plasma atomic emission spectroscopy (ICPAES, IRIS Intrepid, Thermo Elemental) was used to determine the concentration of dissolved Fe and Au ions from the metal films on Si in hot aqua-regia of a known volume. With a known amount of Au and Fe elemental compositions, the mass equivalent thicknesses of Au underfilm and Fe overfilm were estimated to be around 60 and 40 nm, respectively, by assuming the same densities as the corresponding bulk metals and using the geometry area for calculation. Atomic force microscopy (AFM) images of the Au film chemically deposited on Si before and after electrodeposition of the Fe overfilm were acquired with a tapping mode under ambient conditions with a Pico-SPM (Molecular Imaging, Tempe, AZ). Si cantilevers having spring constants between 1.2 and 5.5 N m-1 were used at resonance frequencies between 60 and 90 kHz. The XPS spectra of a Fe-coated Au/Si sample were recorded on a Perkin-Elmer PHI-5000C ESCA system equipped with a dual X-ray source, of which the Al KR radiation (hν ) 1486.6 eV) anode and a hemispherical energy analyzer were used. The background pressure during data acquisition was maintained at 10-9 Pa. Measurements were performed at a pass energy of 93.90 eV. The survey XPS spectrum (0-1200 eV) and the narrow spectra of all the elements with much high resolution were both recorded. Data were analyzed using the PHI-MATLAB software provided by PHI Corporation. All the chemicals used were of A.R. grade and from Sinopharm Chemical Reagent Coorporation. Suprapure Milli-Q water (>18 MΩ cm) was used to prepare desired solutions. Ar (99.999% pure) and CO (99.91% pure) gases were purchased from Shanghai Yunguang Industrial Gas Company and Shanghai Reici Chuangyi Instruments Company, respectively. In Situ ATR-SEIRA Spectroscopy. Our homemade flow spectroelectrochemical cell (vide supra) was similar to that reported in ref 9, except that a new inlet and outlet for replacing solution were added in designing the flow cell with the inlet connected to a sealed reservoir containing the deareated electrolyte via a polyethylene tubing. The electrolyte was driven out of the reservoir to the cell by applying an Ar gas pressure to the former. As for an in situ ATR-SEIRAS measurement, briefly, an unpolarized infrared radiation from a ceramic source was focused at the interface by being passed through the hemicylindrical Si prism at an incident angle of 70°, and the totally reflected radiation
Figure 1. AFM images for (a) Au underfilm chemically deposited on a Si wafer and (b) Fe overfilm electrodeposited on the Au underfilm.
was detected. Infrared spectra were recorded on a Magna-IR ESP System 760 Fourier transform infrared spectrometer (Nicolet) equipped with a liquid-nitrogen-cooled MCT detector. The sample compartment of the FT-IR spectrometer was purged with 20 L min-1 of CO2- and H2O-free nitrogen. The spectrometer was operated with a resolution of 4 cm-1, and 128 interferograms were coadded to each single-beam spectrum. All spectra except Figure 4 are shown in the absorbance units defined as A) -log (R/R0), where R and R0 represent the intensities of the infrared radiation reflected from the electrode at the sample and the reference potentials, respectively. RESULTS AND DISCUSSION AFM and XPS Charaterizations of Films. The Au film formed on Si looked golden shiny and turned silvery shiny after the electrodeposition of the 40 nm-thick Fe overfilm. Figure 1 shows the AFM images for the Au underfilm chemically deposited on a Si wafer without and with the Fe overfilm. The Au underfilm was composed of regular and uniform nanoparticles with a mean size of ca. 60-80 nm in diameter, while the Fe overfilm exhibited closely compact irregular nanoislands with significantly increased particle sizes. The z-axis heights for the Au underfilm and Fe overfilm were similar to the mean thicknesses evaluated by ICPAES. Both visual inspection and AFM characterization indicated totally different surfaces before and after electrodeposition. The elemental composition of top surface atomic layers of the Fe overfilm can be identified from a survey XPS spectrum as shown in Figure 2. With the exception of the C(1s) peak, all the other peaks could be assigned to photoemission and Auger transitions of Fe and O as indicated. The Fe 2p peaks at binding energies (BEs) of 712 and 725 eV in the right inset indicate that two types of Fe chemical species, one reduced and one oxidized, were detected.29,30 The peak at lower binding energy can be attributed to the nominally metallic state, and that at higher energy can be attributed to the Fex+ state. The peaks in the survey spectrum located at 23.1, 55.3, 93.1, 711.8, 724.6, and 847 eV (29) Fujii, T.; de Groot, F. M. F.; Sawatzky, G. A.; Voogt, F. C.; Hibma, T.; Okada, K. Phys. Rev. B 1999, 59 (4), 3195–3202. (30) Fujii, T.; Alders, D.; Voogt, F. C.; Hibma, T.; Thole, B. T.; Sawatzky, G. A. Surf. Sci. 1996, 366 (3), 579–586.
Figure 2. Survey XPS spectrum of an electrodeposited Fe overfilm on a Au underfilm chemically deposited on a Si substrate and the narrow spectra of Au 4f and Fe 2p (inset).
correspond to O2s, Fe3p, Fe3s, Fe2p3/2, Fe2p1/2, Fe2s, respectively. The peak at 660.2 eV is due to FeLMM, and the one at 766.1 eV is due to OKLL. However, no peaks assignable to Au could be observed, as shown in the left inset. Because XPS detects the elemental composition of surface layers with a skin depth of around 3-5 nm, the presence of Au-Fe alloys in the surface layers of our electrodeposited Fe overfilm can be excluded, suggesting that the electrodeposited Fe overfilm is sufficiently pinhole-free to be representative of a polycrystalline Fe film electrode. Electrochemical Characterization. The cyclic voltammogram of the Fe film electrode in 0.1 M phosphate buffer solution (PBS, pH 6.9, dotted trace in Figure 3a) shows an anodic peak at -0.56 V with a charge of 4.2 mC cm-2 leading to surface passivation. After a cyclic potential excursion, the electrode was set at -1.0 V while CO was bubbled in for 30 min to attain a saturated adsorption of CO. The anodic peak shifted positively to -0.29 V with a charge of 4.0 mC cm-2 (solid trace in Figure 3a), attributable to partial CO and Fe oxidations. The inhibited electro-oxidation of Fe together with the suppressed hydrogen evolution at negative potentials can be attributed to the poisoning of Fe surfaces by the chemically adsorbed CO. In the second cycle, Analytical Chemistry, Vol. 82, No. 12, June 15, 2010
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Figure 3. (a) Cyclic voltammograms of a Fe film electrode in 0.1 M phosphate buffer solution (pH 6.9) in a high-purity Ar atmosphere (dotted curve) and in CO-saturated solution (solid and dashed curves for 1st and 2nd cycles, respectively). (b) Cyclic voltammograms of the CO-predosed Fe film electrode in a phosphate buffer solution after purging CO with Ar, 1st cycle (solid curve) and 2nd cycle (dashed curve). Scan rate: 50 mV s-1.
the anodic peak was located at -0.35 V with the charge being ca. 4.1 mC cm-2, suggestive of CO readsorption onto Fe at negative potentials (dashed trace in Figure 3a). Figure 3b shows the voltammetric responses of a Fe film electrode in PBS after being predosed with CO by bubbling in gaseous CO followed by Ar sparging at -1.0 V. (Note that the same CO predosing procedure for the Fe electrodes was used in the subsequent SEIRAS measurements.) The anodic voltammetric response of the CO-predosed Fe electrode (solid trace in Figure 3b) was similar to that of a Fe electrode in the CO-saturated solution (solid trace in Figure 3a), suggestive of strong chemical adsorption of CO on Fe surfaces. Upon CO-stripping, the voltammogram of the CO-predosed Fe electrode in the second cycle (dashed trace in Figure 3b) resembles that of an as-prepared Fe electrode in the Ar-saturated PBS (dotted trace in Figure 3a), suggestive of the reoxidation of the freshly reduced Fe sites after the first oxidation and reduction cycle. It should be pointed out that the voltammetric features were close to those obtained with the bulk counterparts,26 rendering the following SEIRAS measurement meaningful. SEIRAS of CO at Fe Electrodes. For the whole potential range under investigation, we were unable to find a potential at which CO could be totally oxidized at Fe electrodes. In order to compare the spectrum obtained by ATR-SEIRAS with that obtained by IRRAS in the literature,26 in Figure 4, we show the differential spectrum calculated according to ∆R/R ) (R/R0) - 1. The reference and sample single-beam spectra for CO adsorption on Fe were collected at -0.9 and -0.7 V (vs SCE), respectively, very close to the conditions reported by Cuesta et al.26 With this convention, a positive band indicates that a species present at the reference potential is consumed at the sample potential. Conversely, a negative band indicates that a new species, not present at the reference potential, is produced at the sample potential. A bipolar band indicates a Stark shift of a chemisorbed species present at both the reference and sample potentials. As shown in Figure 4, the S/N ratio for the differential spectrum is substantially enhanced, as compared to that obtained by IRRAS. The positive-going and negative-going peaks at 1954 and 1985 cm-1 correspond to CO absorptions at -0.9 and -0.7 5120
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Figure 4. Differential SEIRA spectrum for a Fe film electrode in COsaturated PBS solution; the reference and sample potentials were -0.9 and -0.7 V, respectively.
V, respectively, with a peak to peak intensity height of 0.051 ∆R/R, which is 34-fold as strong as that reported for a bulk Fe electrode with IRRAS (i.e., 0.0015 ∆R/R in Figure 6a of ref 26). For a more reasonable evaluation of surface enhancement factor, one should calibrate the contributions from surface roughness factor, adsorbate coverage, incident angle, and polarization state of an IR irradiation.19 The surface coverages of CO used in the two measurements are the same, namely, the saturated coverage; the incidence angles at the electrode surface are quite close, i.e., around 65-70°. On the other hand, the surface roughness factor of the Fe film electrode is 1.5 to 2 times as large as that of the polished Fe bulk electrode used in IRRAS, as estimated by comparing the anodic stripping charges of the two CO-predosed Fe electrodes,26 which raises the band intensity by a factor of less than 2 in the ATR-SEIRAS measurement. However, since p-polarized and unpolarized IR irradiations were, respectively, used in the IRRAS and ATR-SEIRAS measurements, more than 2-fold enhancement of the band intensity went to the IRRAS measurement, depending on the optical constants of the metal film and the incidence angle. Therefore, the SEIRA enhancement factor should be higher than 34 after taking into account all the above effects. In addition, the peaks around 3600 cm-1 are due to the interfacial free water interacting strongly with the underlying CO adlayer at Fe electrode which could only be identified in the ATR-SEIRAS measurement. Anyway, it should be mentioned that the bipolar band shape makes the interpretation of the spectra difficult. Specifically, it is impossible to determine the Stark tuning rate for CO adlayer at the Fe electrode. In order to obtain “absolute” spectra of adsorbed CO species and the Stark tuning rate, the single-beam spectra collected at stepwisely increased potentials for a CO-predosed Fe electrode were taken as the sample spectra and were referenced to the ones collected for a Fe electrode in the blank PBS solution at the same corresponding potentials prior to bubbling CO (for the sample spectra collected up to -0.8 V) or referenced to the one collected at -0.8 V in the blank PBS solution (for the sample spectra collected at potentials higher than -0.7 V). The latter measurement was taken to avoid the anodic dissolution and/or passivation of the Fe film in a blank PBS at higher potentials; see Figure 3. With these tactics, the potential dependent SEIRA spectra for CO
Figure 5. Potential dependent multistep SEIRA spectra for the COpredosed Fe film electrode in 0.1 M PBS. For the potential range of -1.1 to -0.8 V, the reference single-beam spectra were taken at the corresponding potentials in neat PBS before introducing CO. All the spectra at higher potentials are referenced to the one single-beam spectrum acquired at -0.8 V in neat PBS.
adsorbed at the Fe electrode are shown in Figure 5. Unlike the previous report, normally directed and unipolar CO absorption bands were obtained with significantly increased intensities. Only one CO absorption band at 1940-2000 cm-1 showed up, in agreement with previous IRRAS results at solid/gas31,32 and solid/electrolyte26 interfaces, which can be assigned to the linearly bonded CO (COL) on Fe surface.26 An increased baseline shift from high-to-low wavenumber following the CO feature may be ascribed to the adsorption of CO at different facets of polydispersed Fe nanoislands25,33,34 and/or to the so-called “Fano-like” effect for CO adsorbed on these Fe surfaces.25 Compared to the CO adsorptions on Ni19 and Co20 electrodes in the same PBS, the ratio of COL versus COM (CO bounded to multiple metal atom sites) increased in the order of Fe (COL only) > Co (COL more than COM) > Ni (COM more than COL). Figure 6 clearly shows the different spectral features for CO adsorption on these three electrodes under otherwise the same conditions. This difference may be explained by an increasingly stronger back-donation of 3d electrons from a transition metal to CO 2π* orbitals with increasing number of d-electrons, given that the valence electron structures of Fe, Co, and Ni are 4s23d6, 4s23d7, and 4s23d8, respectively. Anderson et al35,36 applied the atom superposition and electron delocalization molecular orbital theory and two-layer thick cluster models to predict the binding site preferences of CO on Pd and Pt surfaces. It was concluded that different CO binding site preferences are a result of metal valence band positions, i.e., a suitable match of the valence band and the 2π* orbitals of CO. Metal orbital size and surface atom spacing differences are insufficient to alter the dominance of the band positions on the binding preference.35 Stronger 2π* mixing (31) Wadayama, T.; Kubo, K.; Yamashita, T.; Tanabe, T.; Hatta, A. J. Phys. Chem. B 2003, 107 (16), 3768–3773. (32) Tanabe, T.; Buckmaster, R.; Ishibashi, T.; Wadayama, T.; Hatta, A. Surf. Sci. 2001, 472 (1-2), 1–8. (33) Park, S.; Wieckowski, A.; Weaver, M. J. J. Am. Chem. Soc. 2003, 125 (8), 2282–2290. (34) Park, S.; Wasileski, S. A.; Weaver, M. J. J. Phys. Chem. B 2001, 105 (40), 9719–9725. (35) Anderson, A. B.; Awad, M. K. J. Am. Chem. Soc. 1985, 107 (26), 7854– 7857. (36) Mehandru, S. P.; Anderson, A. B. J. Phys. Chem. 1989, 93 (5), 2044–2047.
Figure 6. Comparison of SEIRA spectra for CO adsorbed on Fe, Co, and Ni film electrodes in CO-saturated 0.1 M PBS (pH 6.9) at -0.8 V vs SCE. Reference spectrum was measured in blank PBS at -0.8 V before introducing CO. For the preparation of Co and Ni overfilm electrodes, see refs 19 and 20 for details.
Figure 7. Plots of the peak intensity (upper panel) and frequency (lower panel) of the υCO band as a function of applied potential; data are adapted from Figure 5.
with enhanced surface back-donation to the empty CO 2π* orbitals favors high coordinate sites, as in the case of CO on Pd surfaces. For CO adsorption on the Pt(111) electrode, a negative potential shifts up the valence band of Pt(111), closer to the 2π* orbitals, leading to the increased multicoordinated CO sites. As can be seen from the valence electron structure of the iron triad, the number of d-electrons increases in the sequence of Fe (4s23d6), Co (4s23d7), and Ni (4s23d8). On the other hand, the density function theory calculation indicates the d-band center decreases in the order of Fe (-0.92 eV), Co (-1.17 eV), and Ni (-1.29 eV).37 In comparison to the d-band center of Pd (-1.83 eV) and Pt (-2.25 eV),37 the d-band center of Ni is closest to that of Pd and that of Fe is furthest apart, suggesting that among the iron triad the valence band of Ni is most effective for mixing with the 2π* orbitals of CO leading to the strongest back-donation. Thus, bridge-bonded CO was observed predominantly on Ni. At the other extreme, CO on Fe preferred a linear bonding. The integrated intensity for the COL band kept nearly constant in the potential range from -1.1 to -0.5 V (Figure 7, upper panel) and started to decrease at ca. -0.5 V, corresponding to (37) Ruban, A.; Hammer, B.; Stoltze, P.; Skriver, H. L.; Norskov, J. K. J. Mol. Catal. A 1997, 115 (3), 421–429.
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Figure 9. Structures of the cationic, neutral, and anionic forms of benzotriazole.
Figure 8. Cyclic voltammograms at a scan rate of 50 mV s-1 for a Fe film electrode in 0.1 M KClO4 in the absence (solid line) and the presence of 10 mM BTAH (dotted line).
the onset potential for the anodic oxidation of Fe. The band could survive even at 0.1 V with its integrated intensity being about 60% of that at -1.1 V. We tentatively attributed the bands at potentials higher than -0.5 V to the CO adsorption on the passivated Fe surface. Owing to the largely unipolar COL feature with a much higher S/N ratio, for the first time, we were able to obtain the Stark tuning rate for CO at Fe electrode. As shown in the lower panel of Figure 7, two potential regions with different values of dν/dE were clearly seen. The Stark tuning rate for COL was 67 cm-1 V-1 in the potential range from -1.1 to -0.5 V, whereas that was 26.5 cm-1 V-1 from -0.4 to 0.1 V. The different Stark shifts reflected that the Fe surface was changed around -0.5 V, in agreement with the cyclic voltammetric measurement shown in Figure 3. The passivation of Fe surface in the higher potential region could induce a larger ohmic drop, retarding the actual potential effect on the CO adsorption and oxidation on Fe sites. It should be mentioned that these valuable spectral data were not able to be obtained in the previous IRRAS measurement,26 due to the lower S/N ratio and bipolar band shape. Nevertheless, the potential reversibility of the Stark shifts was not further explored, in consideration of the fact that the passivated Fe surface at higher potentials could not be reversibly recovered in the negative-going potential movement. In fact, the spectral band feature was significantly distorted in the latter process, preventing a reliable determination of the Stark shifts. In the following, we will further take the advantage of the SEIRA effect of the as-prepared electrode to study the potential dependent electrosorption behavior of BTAH at Fe electrode. In Situ SEIRAS of Benzotriazole (BTAH) at Fe Electrode. Figure 8 shows the cyclic voltammograms obtained on an as-deposited Fe electrode in 0.1 M KClO4 in the absence and the presence of 10 mM BTAH, respectively. In the blank KClO4 solution, the anodic dissolution current was observed at potentials positive of -0.6 V. In the backward scan, hydrogen evolution commenced at -0.80 V. Unlike the cyclic voltammograms in the PBS, the anodic oxidation did not lead to the surface passivation of Fe electrode in 0.1 M KClO4 in the potential range of interest, possibly due to the decreased local pH as the anodic reaction proceeded in the absence of a buffer 5122
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solution26 as well as the anion effect. Both the anodic metal dissolution and the cathodic hydrogen evolution reaction were greatly suppressed after the addition of BTAH. It is well-known that the neutral form of BTAH (pKa ≈ 8.37) is present in a weakly acidic or neutral solution. BTAH may be protonated by H+ to form BTAH2+ in a strongly acidic solution and deprotonated to form BTA- in a strongly basic solution,8 as depicted in Figure 9. Since the protonation or deprotonation occurs at the triazole ring, it is, thus, possible to differentiate the form of BTAH on a metal surface by analyzing the changes in the spectral features of the triazole ring-related vibrational modes. Our preliminary measurement was conducted in the weakly acidic solution (0.1 M KClO4 + 10 mM BTAH, pH 5.5) for the reasons as follows. First, BTAH is mostly used as an efficient corrosion inhibitor in solutions near neutrality rather than in strongly acidic ones, since the surface protective layer can be destructed by the protonation of BTA- in the latter.8 Second, the as-prepared 40 nm-thick Fe film electrode tends to dissolve in acidic electrolytes, inducing a significant change in the spectral background baseline and making the spectroscopic analysis difficult. The normal infrared spectrum of solid BTAH and the ATRSEIRA spectra obtained at the interface of Fe electrode and BTAHcontaining solution are shown in Figure 10a,b, respectively. At potentials negative of -0.6 V, no characteristic features in the spectra were observed. At potentials positive of -0.4 V, several bands showed up in the spectral region between 1500 and 1000 cm-1. The up-going bands at 1487, 1447, 1390, 1288, and 1187 cm-1 increased with increasing potential, indicative of the interfacial structural change at more positive potentials. The deprotonation of BTAH to form BTA- on the Fe surface can be distinguished by comparing the characteristic bands as listed in columns 1 and 2 of Table 1. The in-plane N-H bending at 1092 cm-1 and the asymmetric triazole ring stretching at 1418 cm-1 corresponding to BTAH (Figure 10a) were no longer observed in the ATR-SEIRAS measurement of BTAH adsorption on the Fe surface (Figure 10b). Moreover, the band at 1209 cm-1 for the triazole ring breathing mode was red-shifted to 1187 cm-1 accompanied with a decreased intensity, suggestive of the deprontonation of BTAH upon its adsorption on Fe electrode, in agreement with a previous SERS report,38 in which the ν(N-N-N) breathing mode in triazole ring was red-shifted from 1210 to 1165 cm-1. In Figure 10a, the bands at 1456, 1514, and 1594 cm-1, characteristic of the C-C ring stretching modes of BTAH, were red-shifted to 1447, 1487, and 1575 cm-1 in Figure 10b, respectively, in further support of that BTAH adsorbed onto Fe surface in the deprotonated form BTA-.38 (38) Rubim, J. C.; Gutz, I. G. R.; Sala, O.; Orvillethomas, W. J. J. Mol. Struct. 1983, 100, 571–583.
Figure 10. (a) IR spectrum of BTAH powder in transmission mode. (b) Potential dependent SEIRA spectra for the adsorption of BTA on a Fe film electrode in 0.1 M KClO4 + 10 mM BTAH; the potential was indicated as the spectra, and the reference spectrum was collected at -1.0 V.
Table 1. Vibrational Frequencies of BTAH and BTA and their Assignments BTAH solid IR
Fe-BTA SEIRAS
980 1005 1092 1121 1146 1209 1267 1282 1384 1418 1456 1498 1514 1594
972 1002
a
1132 1164 1187 1262 1288 1390
Fe-BTA SERS8
BTAH solution Raman6
Fen(BTA)m complex Raman8
1122 1150 1190
1126 1148 1219
1133 1150 1191
1282 1389
1278 1385
1286 1387
1463
1443 1489
1447 1487 1509 1575
1509 1596
1579
assignment Tz ring bending Bz ipba+CH ipb NH ipb combination combination Tzb ring breathing combination skeletal + CH bending Tz ring stretching Tz ring stretching (as.) Bzc ring stretching Bz ring stretching Bz ring stretching (CdC) Bz ring stretching
ipb ) in-plane bending. Tz ) triazole c. Bz ) benzene. b
c
BTAH can form a polymer-like layer on a metal surface at positive potentials in roughly neutral solutions, resulting in an enhanced anticorrosion capability of BTAH. In this type of polymer-like layer, a BTA- ligand (the deprotonated form of BTAH) provides two N sites to central metal ions, and each metal ion may interact with more than one BTA- species, depending on the nature of the metal. In fact, on the basis of their SERS results, Rubim et al. suggested that the surface protective layer on Cu in a BATH-containing solution was a polymer-like [CuIBTA]n in which each surface Cu(I) was coordinated with two nitrogen atoms of different BTAH triazole rings to form a compact corrosion-resistant layer (the schematic diagram can be seen in diagram (III) of ref 38); later, more vibrational spectroscopic measurements further supported the proposed [CuIBTA]n structure.39-41 In the SERS study on the BTAH adsorption on the Fe electrode, Tian et al6,8 proposed a similar polymer-like surface complex FeIIm(BTA)n as the corrosion-resistant layer, in analogy to the proposed [CuIBTA]n structure but with a more general (39) Musiani, M. M.; Mengoli, G.; Fleischmann, M.; Lowry, R. B. J. Electroanal. Chem. 1987, 217 (1), 187–202. (40) Chant, H. Y. H.; Weaver, M. J. Langmuir 1999, 15 (9), 3348–3355. (41) Biggin, M. E.; Gewirth, A. A. J. Electrochem. Soc. 2001, 148 (5), C339– C347.
formulation, on the basis of the observation of the 1133 and 1191 cm-1 bands characteristic of triazole ring-breathing vibrations both in the SERS spectra and the normal Raman spectrum of a synthesized Fe(II)-BTA complex (column 5, Table 1). Our ATR-SEIRAS spectral evidence further indicated the formation of the suggested surface complex at higher potentials where BTA- ligands were coordinated to the oxidized state of Fe, namely, Fe(II) species. The IR features at 1132 and 1187 cm-1 for the BTA- adsorption at Fe electrode (column 1, Table 1) were in accordance with the suggested FeIIm(BTA)n protective layer. At positive potentials, BTA- anions were mainly attached to the Fe surface with their triazole rings to form the surface complex, and due to the steric hindrance, some BTA- anions could also approach to the Fe surface with the benzene rings; both forms of BTA- contributed to the observed signals associated to the vibrational modes of the benzene ring, i.e., the peaks at 1447, 1509, and 1575 cm-1. It should be pointed out that at initially negative potentials (
