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Langmuir 1996, 12, 5934-5941
Electromodulated Fourier Transform Infrared Reflectance Spectroscopy at the Gold-Aqueous Tetramethylthiourea Interface Simon N. Port,† Sarah L. Horswell,‡ Rasmita Raval,‡ and David J. Schiffrin*,† Department of Chemistry, University of Liverpool, Liverpool L69 3BX, U.K., and Leverhulme Centre for Innovative Catalysis and IRC in Surface Science, Department of Chemistry, University of Liverpool, Liverpool L69 3BX, U.K. Received April 11, 1996. In Final Form: September 4, 1996X The interaction of tetramethylthiourea with a polycrystalline gold electrode in alkali and acid media has been studied. The behavior of the adsorbate was investigated by cyclic voltammetry, capacitance, and rotating disk measurements, and its orientation at the surface was determined by in-situ subtractively normalized interfacial Fourier transform infrared spectroscopy. In alkaline solutions tetramethylthiourea exhibits reversible potential dependent adsorption. Application of the IR metal-surface selection rule clearly shows that the molecule is adsorbed with the NC(S)N plane parallel to the surface. In acid solutions, tetramethylthiourea (TMTU) is adsorbed at the surface throughout the potential range investigated and at sufficiently positive potentials, dissolution of an Au(I)-TMTU complex into the thin layer between the electrode surface and the cell window was observed.
Introduction The adsorption of organic molecules at the metal/ electrolyte interface has received considerable attention over the past 3 decades due, primarily, to their practical applications as corrosion inhibitors and metal plating bath additives. In particular, thiourea (TU) and tetramethylthiourea (TMTU) are widely used in electroplating and refining.1 Extensive voltammetric studies have been carried out for TU at gold electrodes in acidic solutions.2-5 Additionally, the adsorption of TU at silver and copper electrodes has been studied by surface-enhanced Raman spectroscopy (SERS),6-8 revealing that TU reacts chemically with metallic copper forming copper sulfide.9 In contrast, there is only a limited understanding of TMTU behavior at the metal/electrolyte interface, despite its extensive use as a corrosion inhibitor and accelerator for the electrolytic reduction of metal ions.10 Previous studies of TMTU adsorption on Hg in neutral solution by Ikeda et al.11 indicated a potential dependent orientation of the molecule at the surface. A more recent scanning tunneling microscopy investigation in acid solution has shown that TMTU is adsorbed in a planar conformation at a single crystal Au(111) surface.12,13 The strong
interaction of the sulphur group with metals such as Pd, Zn, Fe and Cu results in the formation of complexes with TMTU, the structures of which have been identified by IR spectroscopy.14-16 Few electrochemical and spectroscopic studies of TMTU adsorbed at surfaces have been undertaken. The present work investigates the adsorption and reactions of TMTU on a polycrystalline gold electrode in alkali and acid media using in-situ infrared reflectance spectroscopy, cyclic voltammetry, capacitance measurements, and rotating disk electrode (RDE) voltammetry. Electromodulated infrared spectroscopy has been shown to be a very powerful surface characterization technique and has been used, for instance, in mechanistic studies of carbon-monoxide electrooxidation on gold,17 adsorption of cyanide on platinum,18,19 adsorption of organic molecules20,21 and many other applications in electrochemical research.22,23 For this reason, subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS)24,25 was extensively used in the present work. TMTU was investigated because the presence of methyl groups is expected to have a stabilizing effect on the molecule with respect to its adsorption at reactive metallic surfaces due to inductive effects, which prevents its oxidation and hence
†
Departrment of Chemistry, University of Liverpool. Leverhulme Centre for Innovative Catalysis and IRC in Surface Science, Department of Chemistry, University of Liverpool. X Abstract published in Advance ACS Abstracts, November 1, 1996. ‡
(1) Szymaszek, A.; Biernat, J.; Pajdowski, Electrochim. Acta 1977, 22, 359. (2) Dhaktode, S. S.; Dhaneshwar, R. G.; Zarapkar, L. R. Proc. Annu. Tech. Meet. Electrochem. Soc. India 1984, 23. (3) Groenewold, T. J. Appl. Electrochem. 1975, 5, 71. (4) Groenewold, T. S. Afr. Inst. Min. Metal. 1977, 1, 217. (5) Zakharrov, V. A.; Bessarabova, I. M.; Songina, O. A.; Timoshkin, M. A. Elektrokhimiya 1971, 7, 1215. (6) Fleischmann, M.; Hill, I. R.; Sundholm, G. J. Electrochem. Soc. 1983, 157, 359. (7) Tian, Z. Q.; Lian, E. M.; Fleischmann, M. Electrochim. Acta 1990, 35, 879. (8) Bukowska, J.; Jackowska, K. J. Electroanal. Chem. 1994, 367, 41. (9) Lahousse, G.; Heerman, L. Bull. Soc. Chim. Belg. 1971, 80, 125. (10) Sykut, K.; Dalmata, G.; Nowicka, B.; Saba, J. J. Electroanal. Chem. 1978, 90, 299. (11) Ikeda, O.; Jimbo, H.; Tamura, H. J. Electroanal. Chem. 1983, 137, 127. (12) Bunge, E.; Nichols, R. J.; Baumga¨rtel, H.; Meyer, H. Ber. Bunsenges. Phys. Chem. 1995, 99, 1243.
S0743-7463(96)00346-0 CCC: $12.00
(13) Bunge, E.; Nichols, R. J.; Roelfs, B.; Meyer, H.; Baumga¨rtel, H. Langmuir 1996, 12, 3060. (14) Lane, T. J.; Yamaguchi, A.; Quagliano, J. V.; Ryan, J. A.; Mizushima, S. J. Am. Chem. Soc. 1959, 81, 3822. (15) Suetaka, W. Bull. Chem. Soc. Jpn. 1967, 40, 2077. (16) Kamata, S.; Hida, A.; Higo, M.; Yazaki, M. Microchem. J. 1994, 49, 194. (17) Edens, G. J.; Hamelin, A.; Weaver, M. J. J. Phys. Chem., 1996, 100, 2322. (18) Ashley, K; Feldhelm, D. L.; Parry, D. B.; Sament, M. G.; Philpott, M. R. J. Electroanal. Chem. 1994, 373, 201. (19) Stuhlmann, C.; Villegas, I.; Weaver, M. J. Chem. Phys. Lett. 1994, 219, 319. (20) Pons, S.; Bewick, A. Langmuir 1985, 1, 141. (21) Port, S. N.; Schiffrin; D. J.; Solomon, T. Langmuir 1995, 11, 4577. (22) Stole, S. M.; Popenoe, D. D.; Porter, M. D. In Electrochemical Interfaces, Modern Techniques for In-situ Interface Characterisation; Abrun˜a, H. D. Ed.; VCH: New York, 1991; p 339. (23) Beden, B.; Lamy, C. In Spectroelectrochemistry, Theory and Practice; Gale, R. J., Ed.; Plenum Press: New York, 1988; p 189. (24) Pons, S.; Davidson, T.; Bewick, A. J. Electroanal. Chem. 1984, 160, 63. (25) Bewick, A.; Pons, S. B. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. N., Hester, R. E., Eds.; London, 1985; Vol. 12, p 1.
© 1996 American Chemical Society
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leads to a chemical reactivity that differs significantly from that of TU. Experimental Section A polycrystalline gold disk electrode (Goodfellows 99.999% purity) of 2 mm thickness and 12 mm diameter was attached with epoxy resin (RS Components) to a glass tube connected to a glass syringe barrel.21 This was then polished to a flat mirror finish with successively finer grades of alumina (Buehler, 1.0, 0.3, and 0.05 µm) and finally cleaned in pure water in an ultrasonic bath. Solutions were prepared using Microselect grade NaOH (Fluka Chemicals), AristaR H2SO4 (BDH), and TMTU (Fluka Chemicals). Quadruply distilled water, twice from alkaline permanganate, was used throughout. A three-electrode cell was employed for all electrochemical and spectroscopic experiments. The counter electrode was a large area platinum gauze and the reference electrode a saturated calomel electrode (SCE). The potential was controlled with a potentiostat and waveform generator (HiTek, England, DT 2101 and PPR1 respectively). The capacitance measurements were performed by applying an ac signal of 10 mV amplitude at 23 Hz and measuring the quadrature and in-phase components of the current with a lock-in amplifier (Stanford, SR830 DSP). The rotating disk experiments were carried out with a CTV101 Radiometer Copenhagen Rotating Disk assembly. The gold disk diameter was 0.5 cm. A dry air purged FTIR spectrometer, BioRad FTS 40, with a liquid nitrogen cooled mercury cadmium telluride (MCT) detector was used for the SNIFTIRS experiments; either p- or s-polarized radiation was employed depending on the type of experiment conducted. The spectroelectrochemical cell was fitted with a CaF2 window which had a spectral cutoff below 1000 cm-1. The potentiostat was switched between two preset potentials, and IR reflectance spectra were collected at each potential. Normalized spectra were obtained by subtracting two spectra (R2 - R1) obtained for different potentials and dividing this difference by R1, the reference spectrum. Thus, the normalized change in reflectance is given by
∆R R2 - R1 ) R1 R1
(1)
The signal-to-noise ratio was improved by collecting 100 interferograms at 4 cm-1 resolution for each potential and calculating the normalized spectra (∆R/R1). This procedure was then repeated 10 times, and the final normalized spectrum was obtained by averaging these data. Individual spectra, R1 and R2, were normally checked for any ghost bands in the normalized spectra which can arise mainly from absorptions due to the window or the electrolyte layer between the electrode and the window.26 Before spectral collection, the solutions were deoxygenated with OFN nitrogen (BOC, Ltd.) and the gold electrode precycled in base electrolyte to the hydrogen and oxygen evolution potentials, thus ensuring a reproducible surface. The electrode was then placed into the spectroelectrochemical cell and positioned against the window ready for spectroscopic measurements. The thin layer cell thus formed usually has a thickness of the order of a few micrometers.27
Results and Discussion TMTU Interaction with Gold in Alkaline Media. Cyclic Voltammetry. Figure 1(i) shows the cyclic voltammogram of Au in contact with 1 mM TMTU in 0.1 M NaOH. A surface wave centered at -0.95 V is clearly observed which shows almost no shift in peak potential with sweep rate. The peak currents are linearly dependent on the sweep rate (Figure 1(ii)), confirming a surface process. These observations indicate a reversible potential dependent adsorption of TMTU. The half width of the voltammetry wave is ∼180 mV, which would correspond (26) Bewick, A.; Kalaji, M.; Larramona, G. J. Electroanal. Chem. 1991, 318, 207. (27) Ashley, K.; Pons, S. Chem. Rev. 1988, 88, 673.
Figure 1. (i) Cyclic voltammogram of gold in 1 mM TMTU and 0.1 M NaOH. Sweep rate was (a) 0.05, (b) 0.10, (c) 0.15, (d) 0.20, (e) 0.25, (f) 0.30, (g) 0.35, (h) 0.40, (i) 0.45, and (j) 0.50 V s-1. (ii) Dependence of current peak height on sweep rate for gold in 1 mM TMTU and 0.1 M NaOH. (These results were corrected for the capacitance charging current contribution by subtraction of the current at the foot of the wave.) (iii) Differential capacity of gold in (a) 1 mM TMTU and 0.1 M NaOH (s) and (b) 0.1 M NaOH alone (- - -).
either to an electroadsorption valency of 0.528 or to the formation of an AuTMTU+ surface complex in a heterogeneous surface due to charge transfer on adsorption. No bulk oxidation of TMTU was observed over the potential range investigated. (28) Bard, A. J.; Faulkner, L. R. In Electrochemical Methods; Wiley: New York, 1980.
5936 Langmuir, Vol. 12, No. 24, 1996
To further investigate this system, the interfacial capacitance was measured over the same potential range. Figure 1(iii) compares the capacitance results for Au in NaOH, with and without the addition of 1 mM TMTU. Data were recorded from negative to positive potentials. The capacitance value at -1.20 V is identical in the absence and presence of TMTU suggesting that at this potential no adsorption of TMTU occurs. However, at more positive potentials, the capacitance in the presence of TMTU shows a sharp increase at -1.05 V, with a maximum at -0.95 V, coincident with the peak observed in the cyclic voltammogram (Figure 1(i)). Finally, at potentials more negative than -0.70 V the capacitance is lower than that for the base electrolyte confirming that TMTU is adsorbed at these potentials. The capacitance behavior on the reverse sweep closely follows the forward trace, confirming that adsorption is reversible. Thus, the above results are a strong indication of potential-dependent TMTU adsorption on the Au surface. Infrared Spectroscopy. Vibrational Assignments for TMTU. The vibrational assignments available for TMTU can be used as a basis for the interpretation of the SNIFTIRS spectra (see later). It should be noted that IR spectroscopic assignments for TMTU have been somewhat ambiguous due to the fact that the normal modes of vibration of TMTU involve extensive coupling between various vibrational motions.29-37 The majority of the vibrational literature on TMTU has relied on an eightbody normal coordinate analysis carried out by Gosavi et al.38 However, a later detailed isotopic study by Anthoni et al.39 with a 20-body normal coordinate calculation provides a more consistent framework for analysis, and the latter has been used for band assignment in the present work. TMTU has a C2 symmetry; an energy-minimized structure of TMTU40 shows that the NC(S)N skeleton is planar (Figure 2). The position of the methyl groups is flexible and is expected to vary depending on local steric constraints. Although TMTU possesses 54 normal modes of vibration, the SNIFTIRS experiment is expected to sense only the strongest IR modes given the low number density of IR active species in the experiment. On this basis, only the bands at 1508, 1470, 1440, 1369, 1360, 1262, 1208, 1131, 1119, and 1096 cm-1 observed by Anthoni et al.39 using a KI disk are expected to give rise to significant absorbances. Table 1 gives a detailed assignment and description of the modes of vibration that give rise to these bands; it is clear from this that many of the vibrational modes are coupled and involve significant amplitude in a number of functional groups. Subtractively Normalized Interfacial FTIR Spectroscopy in Alkaline Solutions. SNIFTIRS spectra of (29) Williams, D. J.; Poor, P. H.; Ramirez, G.; Heyl, B. L. Inorg. Chim. Acta 1988, 147, 21. (30) Wynne, K. J.; Pearson, P. S. Inorg. Chem. 1971, 10, 2735. (31) Wynne, K. J.; Pearson, P. S.; Newton, M. G.; Golen, J. Inorg. Chem. 1978, 11, 1192 (32) Stewart, J. E. J. Phys. Chem. 1957, 26, 248. (33) Gosavi, R. K.; Rao, C. N. R. J. Inorg. Nucl. Chem. 1967, 29, 1937. (34) Rao, C. N. R.; Venkataraghavan, R. Spectrochim. Acta 1962, 18, 541. (35) Randall, H. M.; Fowler, R. G.; Fuson, N.; Dangl, J. R. In Infrared Determination of Organic Structures; Van Nostrand, D., Ed.; New York, 1949. (36) Spinner, E. Spectrochim. Acta 1959, 15, 95. (37) Yamaguchi, A.; Penland, R. B.; Mizushima, S.; Lane, T. J.; Curran, C.; Quagliano, J. V. J. Am. Chem. Soc. 1958, 80, 527. (38) Gosavi, R. K.; Agarwala, U.; Rao, C. N. R. J. Am. Chem. Soc. 1967, 89, 235. (39) Anthoni, U.; Nielsen, P. H.; Borch, G.; Gustavsen, J.; Klaboe, P. Spectrochim. Acta 1977, 33A, 403. (40) Nemesis Version 1.1 (1992), International Molecular Modelling for P.C., Oxford Molecular Ltd.
Port et al.
Figure 2. Energy-minimized structure of TMTU.
Figure 3. SNIFTIRS difference spectra of a gold electrode in 0.1 M NaOH (reference potential -1.10 V).
gold in contact with 0.1 M NaOH and 0.1 M NaOH with TMTU are shown in Figures 3 and 4a, respectively. The reference potential was chosen as -1.10 V on the basis of the voltammetry data since no adsorption of TMTU occurs at this potential. It should be noted that for all applied potentials in the absence of TMTU (Figure 3) only the O-H bending mode of water centered at 1643 cm-1 was observed, corresponding to potential dependent changes of the surface concentration of water at the electrode.51 Normalized IR spectra provide information on the difference of reflectance of the electrode/electrolyte interface at two different potentials, R1 and R2. Both positive and negative bands can appear, the former corresponding to absorbing species present in greater amount or possessing greater IR activity at the first (reference) potential than that at the second potential. The opposite is the case for negative bands. Therefore, the SNIFTIRS data could be expected to show bands arising from (a) depletion of TMTU from the thin layer electrolyte between the electrode and the window resulting from the adsorption process, this would give rise to positive bands at frequencies corresponding to those of TMTU in solution, and (b) adsorption of TMTU on the electrode surface resulting in negative bands if the IR activity of the adsorbed species is allowed by the metal-surface selection rules.41,42 The (41) Greenler, R. G. J. Chem. Phys. 1966, 44, 310.
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Table 1. Fundamentals and Vibrational Band Assignments for TMTU (After Anthoni et al. 39) band wavenumber cm-1
normal mode of vibration
1508 1470
ν7 ν8 ν9 ν34 ν35 ν36 ν37 ν38 ν12 ν39 ν14 ν41 ν15 ν42 ν16 ν43
1440 1369 1360 1262 1208 1131 1119 1096
description νsNCN (19) + δCH3 (27) + δsCH3c-NC (14) + FCH3 (12) + νCS (10) δasCH3 (86) δasCH3 (85) δasCH3 (85) δasCH3 (86) δasCH3 (87) δasCH3 (89) δsCH3 (18) + νasNCN (14) + δasCH3t-NC (17) + νasCH3c-N (19) δsCH3 (104) δsCH3 (97) + νasCH3c-N (13) νsCH3c-N (38) + νsNCN (25) + FCH3t (26) FCH3 (38), νasCH3t-N (31), νasNCN (24) νCS (21) + νsCH3t-N (27) + νsCH3c-N (8) + FCH3c (29) FCH3 (63) FCH3 (84) FCH3 (53) + νasNCN (19)
a
1117 cm-1 at potentials where cyclic voltammetry and capacitance measurements indicate that adsorption occurs. These peaks were not analyzed by the Gaussian model technique43-45 since the spectral resolution and noise level did not warrant further analysis. All the positive IR bands observed in Figure 4a can be assigned to the strongest IR bands associated with TMTU (Table 1). s-Polarized reflectance data (Figure 4b) clearly shows that the depletion bands arise from the TMTU concentration changes in the thin layer due to adsorption. This also shows that the measurement is sensitive enough to detect the number of molecules adsorbed from solution. This behavior clearly indicates that a greater concentration of TMTU exists in the thin solution layer between the electrode and the window at the reference potential -1.10V than at more positive potentials. Depletion of TMTU in the thin layer occurs according to (see later)
TMTU(b) f TMTU(ads)
b
Figure 4. (a) SNIFTIRS difference spectra of a gold electrode in 0.1 M NaOH and 5 mM TMTU (reference potential -1.10 V). (b) SNIFTIRS difference spectra of a gold electrode in 0.1 M NaOH and 5 mM TMTU at -0.60 V for s- and p-polarized radiation (reference potential -1.10 V).
frequency of these possible bands will depend on the perturbation experienced by TMTU upon adsorption. Figure 4a shows that the SNIFTIRS spectra for TMTU exhibits positive bands at 1539, 1389, 1265, 1152, and (42) Pearce, H. A.; Sheppard, N. Surf. Sci. 1976 , 59, 205.
(2)
where b and ads refer to molecules in the bulk thin layer and in the adsorbed state, respectively. The SNIFTIRS results in Figure 4 show that no bands could be attributed to the adsorbed species. The absence of any negative or new bands under conditions where the voltammetry clearly demonstrates that TMTU is adsorbed implies that the adsorbed molecules are not IR active. This can be rationalized in terms of the metal-surface selection rule,41,42 which allows only absorption from those vibrations that give rise to a dipole moment change normal to the surface. The strongest bands of TMTU, outlined in Table 1, involve skeletal stretching and deformations that give rise to large dipole moment changes in the plane of the NC(S)N skeleton (ν7, ν12,ν14, ν15, ν16, ν38 ν39, ν41, ν42, and ν43 in Table 1). Since these vibrational modes are observed only as a depletion from the thin layer, it can be concluded that the TMTU molecules must be adsorbed with their NC(S)N skeletons essentially parallel to the surface so that the metal-surface selection rule makes them IR inactive on adsorption. This is analogous to the geometry adopted by this molecule adsorbed on Au(111) from acid solutions, as demonstrated by recent STM data,12,13 and in ultrahigh vacuum as shown by recent NEXAFS results.46 It should be noted that for this adsorption geometry some of the δ(CH3) deformation (43) Parry, D. B.; Harris, J. M.; Ashley, K. Langmuir 1990, 6, 209. (44) Parry, D. B.; Samant, M. G.; Melroy, O. R. Appl. Spectrosc. 1991, 45, 989. (45) Parry, D. B.; Samant, M. G.; Seki, H.; Philpott, M. R.; Ashley, K. Langmuir 1993, 9, 1878. (46) Roberts, A. J.; Williams, J.; Li, Y.; Shorthouse, L.; Nunney, T.; Woods, G.; Raval, R. To be submitted for publication.
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Figure 6. Current-potential plot at varying rotational speeds for a Au RDE in 0.1 M H2SO4 with 5 mM TMTU (electrode area ) 0.196 cm2, ν ) 2 mV s-1).
Figure 5. (i) Cyclic voltammogram of gold in 1 mM TMTU and 0.1 M H2SO4. Sweep rate was (a) 0.05, (b) 0.10, (c) 0.20, (d) 0.30, (e) 0.40, and (f) 0.50 V s-1. (ii) Dependence of current peak height on sweep rate for gold in 1 mM TMTU and 0.1 M H2SO4. (These results were corrected for the capacitance charging current contribution by subtraction of the current at the foot of the wave.)
vibrations shown in Table 1 at ca. 1470 and 1440 cm-1 are expected to give a dynamic dipole change normal to the surface. It can be concluded that the absence of these bands for the adsorbed species is due to their inherent weakness since they are also not easily observed as depletion bands from the thin layer, in particular since absorption by water vapor in the optical path gives rise to a poor signal to noise performance in this region of the spectrum. Finally, although the SNIFTIRS results infer a planar orientation of the TMTU molecule at the surface, they do not, in this case, give any information on the electronic perturbation experienced by the adsorbate, since the strongest IR vibrations are rendered inactive by the metal-surface selection rule. TMTU Interactions with Gold in Acid Media. Electrochemistry. Figure 5(i) shows the cyclic voltammogram for gold in 0.1 M H2SO4 containing 1 mM TMTU. A surface wave centered at ∼+0.50 V is clearly visible and a plot of peak current versus sweep rate (Figure 5(ii)) is linearly dependent on sweep rate, corresponding to a surface reaction or electroadsorption. The half-width of the voltammetry wave is ∼180 mV as in the case of NaOH solutions. It is proposed that this indicates the formation of a surface AuTMTU+ complex. Comparison with the voltammetry for the base electrolyte showed that in the presence of TMTU, the capacitance was lower than that for the base electrolyte for potentials more negative than
Figure 7. i-1 vs ω-1/2 dependence for the anodic currents shown in Figure 6. The potentials are indicated in the figure.
∼0.30 V. It can be concluded that TMTU is specifically adsorbed on Au in this potential range in agreement with STM observations.12,13 The surface wave observed (Figure 5(i)) can be attributed to electrosorption of TMTU and/or to the initial steps of metal dissolution. It should be noted that the reversible surface wave observed rules out the possibility of oxidation of TMTU since this would lead to a dependence of the peak current (Ip) on sweep rate (ν) of the form Ip ∝ ν1/2 and not to the linear relationship observed. The latter is characteristic of a surface adsorption wave. Simultaneously with the adsorption wave, the anodic dissolution of Au was observed. This was studied with a rotating disk electrode (RDE). Typical polarization curves at different rotation rates are shown in Figure 6, where characteristic anodic dissolution currents can be seen. From the dependence of the current on rotation rate it can be qualitatively ascertained that, surprisingly, the reaction must be sufficiently fast and reversible to give rise to the observed currents. Although this was not the primary objective of the present work, this unusual feature, which is in agreement with the spectroscopic observations, has been further explored. KouteckyLevich plots are illustrated in Figure 7,28 from which the limiting or kinetic currents (jk) corresponding to the anodic dissolution process in the absence of mass transfer effects can be calculated. The corresponding Tafel plot is given in Figure 8; the Tafel slope is 60 mV decade-1. Similar
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Figure 8. Tafel plot for the results shown in Figure 7.
values have been previously found for the dissolution of copper.47 This Tafel slope is unusual as it would correspond to a transfer coefficient R ) 0 for a 1e- reaction. These results can be rationalized considering the dissolution mechanism k1
Au + TMTU(b) y\ z AuTMTU+(ads) + ek
(3)
-1
Figure 9. Potential dependence of log [di-1/dω-1/2] for the results shown in Figure 7. The line (- - -) corresponds to a 60 mV decade-1 slope for comparison.
and from (6)-(8)
j-1 )
j-1 ) k2
k-2 e-n(E-E°′)F/RT k2 1.554Fν-1/6D2/3[TMTU]
θ)
[TMTU](σ) C*
en(E-E°′)(F/RT)
(5)
C* is a scaling bulk concentration corresponding to the solution standard state to which the formal potential of reaction 3 (E0′) refers.50 The current is given by
j ) nFKm[AuTMTU+](σ) ) nFKm{[TMTU](b) [TMTU](σ)} (6) where km is the mass transfer coefficient given by
km ) 1.554ν
2/3
1/2
D ω
-1/67
(7)
ν is the kinematic viscosity, D is the diffusion coefficient, and ω is the rotation rate expressed in hertz. From the mass balance of reaction 4
[AuTMTU+](σ) )
θk2 km + k-2
(8)
(47) De Sanchez, S. R.; Schiffrin, D. J. Corros. Sci. 1982, 22, 585. (48) Discussion on the standard states to be used in this type of reaction can be found in ref 49 and references cited therein. (49) Cheng, Y.; Cunnane, V. J.; Schiffrin, D. J.; Murtoma¨ki, L.; Kontturi, K. J. Chem. Soc., Faraday Trans. 1991, 87, 107. (50) Avaca, L. A.; Kaufmann, S.; Kontturi, K.; Murtoma¨ki, L.; Schiffrin, D. J. Ber. Bunsenges. Phys. Chem. 1993, 97, 70.
1 + 1/2 (σ) ω e-n(E-E°′)F/RT (10) k2F[TMTU+](σ)
-2
AuTMTU+ is the gold(I) complex formed by anodic oxidation; (ads) and (σ) represent the adsorbed state and solution layer at the electrode surface, respectively. From the electrochemical equilibrium (3), and considering Nernstian conditions, i.e., a fast reversible reaction, the surface concentration of the AuTMTU+ complex, θ, is given by:48
(9)
Therefore
followed by
z AuTMTU+(σ) f diffusion (4) AuTMTU+(ads) y\ k
k-2 1 + Fk2kmθ Fk2θ
For the results shown in Figure 6, the measured currents are small compared with the diffusionally controlled current due to mass transfer of TMTU(σ). Therefore, the TMTU(σ) concentration terms in eq 10 can be taken as approximately constant and close to the bulk concentration value. Although an accurate relationship between surface concentration and rotation rate can be derived, the above approach simplifies the analysis. Equation 10 predicts all the observed RDE results. The predicted limiting current (ω f ∞), corresponding to the second term in (10) has a 60 mV decade-1 potential dependence, as observed (Figure 8). The extrapolation predicted from eq 10 is applicable provided the magnitude of the mass transfer coefficient is less than that of the value of the rate constant for reaction 3. The slope of the Koutecky-Levich plots should show a potential dependence given by47
[ ]
d log
dj-1 dω-1/2 nF )dE 2.3RT
(11)
where n is the number of electrons exchanged in reaction 3. The results in Figure 9 confirm that n ) 1. The above analysis gives a strong indication that the oxidation state of gold in the Au-TMTU complex formed on dissolution is +1 and not +3. It should be stressed that in the absence of TMTU, no anodic dissolution is observed for gold in the potential range studied and, hence, the observed anodic currents most likely involve the formation of a soluble Au-TMTU complex. Furthermore, these conclusions are reinforced by STM data on the adsorption of TMTU on Au(111), where surface etching at potentials positive to +0.40 V vs SCE has been observed.12,13
5940 Langmuir, Vol. 12, No. 24, 1996
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Figure 12. SNIFTIRS difference spectra of a gold electrode in 0.1 M H2SO4 and 5 mM TMTU at +0.550 V for s- and p-polarized radiation (reference potential -0.25 V). Note the higher sensitivity for the results with s-polarized radiation. Figure 10. SNIFTIRS difference spectra of a gold electrode in 0.1 M H2SO4 (reference potential -0.25 V).
Figure 11. SNIFTIRS difference spectra of a gold electrode in 0.1 M H2SO4 and 5 mM TMTU (reference potential -0.25 V).
Therefore, the voltammogram in Figure 5(i) can be rationalized in an analogous manner to the TMTU/Au(111) system as follows: between -0.30 V and ∼+0.25 V TMTU is specifically adsorbed, and at potentials positive to this range, anodic dissolution of Au preceded by electrochemical adsorption (reactions 3 and 4) occurs leading to the surface wave observed at ∼+0.50 V and to the simultaneous dissolution reaction. On the reverse sweep the AuTMTU surface complex present in equilibrium is reduced. In order to confirm these processes, insitu FTIR spectroscopy was employed. Subtractively Normalized Interfacial FTIR Spectroscopy in Acid Solutions. SNIFTIRS spectra for gold in contact with 0.1M H2SO4 alone and with added TMTU are shown in Figures 10 and 11, respectively. A reference potential of -0.25 V was chosen for these experiments to avoid hydrogen evolution. In the absence of TMTU only the broad water bending mode at 1643 cm-1 was observed
(Figure 10) corresponding to the potential dependent adsorption of water at the electrode surface.51 The electrochemical data have clearly established that at the reference potential TMTU is already adsorbed at the Au surface. No IR spectral changes are observed between the reference potential and +0.15 V indicating that the adsorbed TMTU layer is stable over this potential range. At potentials positive to +0.15 V bipolar bands are observed. As discussed above, the positive IR bands correspond to depletion of TMTU from the thin layer, whereas the negative bands reflect the formation of a species that absorbs at wavenumbers 20-40 cm-1 shifted with respect to the depletion bands. The positive IR bands could originate from: (i) an increase of TMTU surface coverage leading to a reorientation of the adlayer in which new vibrational modes become allowed, as reported for the H2O/sulfate system45 or (ii) electrochemical dissolution of a TMTU-Au complex into solution. One way to distinguish between these two possibilities is to conduct SNIFTIRS experiments with s-polarized IR radiation. The selection rules for IR reflection from a metal surface indicate that p-polarized radiation will interact with TMTU present both in the thin layer and adsorbed at the surface. In contrast, the interaction of the s-polarized radiation with the adsorbed species is forbidden and therefore this polarization can only excite vibrations of TMTU in the solution thin layer between the electrode and the cell window. Figure 12 shows s- and p-polarized reflectance spectra at +0.55 V. These spectra are very similar showing both positive and negative bands at the same wavenumbers and with similar intensity ratios. However, the s-polarized bands are less intense because the s-field is substantially smaller than the p-field for a distance 2.5-5 µm into solution.25 From this polarization dependent result, it can be concluded that the negative bands observed in the SNIFTIRS experiment are not due to absorbed TMTU but to the formation of a new TMTU species in the thin layer. In addition, the electrochemical RDE evidence strongly suggests that at these potentials dissolution of Au(I) occurs. Furthermore, the vibrational spectra of TMTU complexes with Pd(II), Pt(II), Co(II), Zn(II), Se(II), Te(IV), Cd(II) and Hg(II) show vibrational characteristics30,31,33,38,52,53 which are very similar to those ex(51) Kunimatsu, K.; Bewick, A. Indian J. Technol. 1986, 24, 407. (52) Schafer, M.; Curran, C. Inorg. Chem. 1966, 5, 265.
Interaction at the Gold-Aqueous TMTU Interface
hibited by the observed negative bands, indicating that the anodic dissolution of Au occurs with the formation of a Au(I)-TMTU complex in the thin layer. The observed electrochemical anodic dissolution of Au to yield a complex explains both the depletion of TMTU from the thin layer giving rise to positive IR bands and the appearance of negative IR bands characteristic of complex formation. Finally, vibrational frequency shifts observed for TMTU-metal complexes in solution have been related to the nature of its bonding to the metal. In particular the upshift in frequency of the 1500 cm-1 band has been interpreted in terms of coordination via sulfur.33,38,52 It should be noted that these conclusions have been based on the normal coordinate analysis by Gosavi et al.,38 which attributed this band to largely localized modes δasym(CN) (89%) + δ(NCS) (11%) vibration. On this basis, the increasing frequency of this mode has been rationalized in terms of an increased double bond character of the C-N bond due to increased π-electron delocalization of the N-C-N skeleton when coordination of TMTU occurs through the sulfur atom.30,31,33,38,52,53 However, as can be seen from Table 1, the more detailed calculation by Anthoni et al.39 shows that the normal mode of vibration that gives rise to the 1500 cm-1 band possesses a more complicated character and involves strong coupling of a number of molecular motions, making it more difficult to predict the direct effect of sulfur coordination on the frequency of this mode. Nevertheless, S-metal coordination has been directly established in a number of TMTU complexes by far-IR and X-ray crystallography measurements,30,31,53 and in all cases, the 1500 cm-1 band displays a strong up-shift (53) Marcotrigiano, G.; Battisuzzi, R. J. Inorg. Nucl. Chem. 1974, 36, 3719.
Langmuir, Vol. 12, No. 24, 1996 5941
in frequency. This work, therefore, provides a direct demonstration of the effect of sulfur coordination on the 1500 cm-1 vibrational band. On this basis, it can be concluded that the Au(I)-TMTU complex formed during the dissolution process also involves coordination of TMTU to Au via the sulfur atom. In summary, the reflectance FTIR and RDE electrochemical results provide direct evidence for the formation of a TMTU-Au(I) complex in solution preceded by electrochemical TMTU adsorption, in agreement with STM results.12,13 Conclusions The interaction of TMTU with Au surfaces in alkali and acid solutions has been investigated using in-situ infrared reflectance spectroscopy, capacitance, rotating disk, and cyclic voltammetry measurements. In alkaline solution the electrochemical experiments show a potential dependent adsorption while the IR spectroscopic data give evidence for the adsorption of TMTU involving the carbon, nitrogen, and sulfur skeleton of the compound parallel to the metal surface in a planar configuration. In acid solution, TMTU was found to be adsorbed throughout the potential range. TMTU was not oxidized within the potential range investigated. In addition, at potentials more positive than +0.15 V an anodic dissolution reaction involving the formation of Au(I)-TMTU complexes was observed. It is proposed that coordination in this complex occurs through the sulfur atom. Acknowledgment. The authors thank the EPSRC and the University of Liverpool for equipment support. The authors are indebted to Mr. Bert Chappell for his glassblowing expertise in preparing the SNIFTIRS cells. LA960346G
