Spectroscopic Investigation of the Adsorption and Oxidation of

Langmuir 2004, 20, 8773-8780

8773

Spectroscopic Investigation of the Adsorption and Oxidation of Thiourea on Polycrystalline Au and Au(111) in Acidic Media Gonzalo Garcıá,† Jose´ L. Rodrı´guez,† Gabriela I. Lacconi,‡ and Elena Pastor*,† Departamento de Quı´mica Fı´sica, Universidad de La Laguna, 38071 La Laguna, Spain, and INFIQC, Departamento de Fisicoquı´mica, Facultad de Ciencias Quı´micas, Universidad Nacional de Co´ rdoba, 5000 Co´ rdoba, Argentina Received April 22, 2004. In Final Form: July 8, 2004 In the present paper, a systematic electrochemical investigation on thiourea (TU) electrooxidation was developed on polycrystalline and (111) single-crystal gold electrodes in 0.1 M perchloric acid. The combination of cyclic voltammetry with in situ Fourier transform infrared spectroscopy (FTIRS) and differential electrochemical mass spectrometry techniques have allowed the nature of the species formed during the electroadsorption and electrooxidation of TU to be established. FTIRS experiments were performed in D2O to clean up the region of the H2O bending around 1600 cm-1. It was concluded that TU adsorbs tilted on the surface in the 0.05-0.40 VRHE potential range. A dual-path reaction mechanism was evidenced in the oxidation process. The first pathway takes place from adsorbed TU at E > 0.40 VRHE and implies the formation of [Au(I)-(TU)2]+, which is oxidized to NH2CN and S0 at E > 0.80 VRHE. In a following oxidation step at E > 1.20 V, N2, CO2, and HSO4-/SO42- were produced. The second parallel reaction occurs from TU in solution at E > 0.50 VRHE to form (TU)22+. All these species were characterized from the spectroscopic experiments. Similar results were obtained for both surfaces.

1. Introduction Organic reagents, such as thiourea (TU), are often employed in metal electrodeposition with the purpose to modify the properties of the deposit (for example, smoothness, brightness, or grain refinement). Specifically, TU is used to enhance the morphology of copper and silver electrodeposits,1-6 refining the deposit grain size by adsorption on active nucleation sites.4 TU in solution is able to reduce transition metals, like copper, silver, and gold, with formation of complex species,3,7-12 with the remarkable production of linear bidentate gold complexes when the metal acts with an oxidation state of +1.13 The crystal and molecular structure of different TU-metal complexes has been determined by X-ray.8,9 Over the past few years, in the metallurgical industry, TU has received considerable interest as an alternative complexing agent to cyanide for gold leaching. The formation of a cationic complex ([Au(I)(TU)2]+) was * To whom correspondence should be addressed. Telephone: +34922318028. Fax: +34922318002. E-mail: [email protected]. † Universidad de La Laguna. ‡ Universidad Nacional de Co ´ rdoba. (1) Suarez, D. F.; Olsen, F. A. J. Appl. Electrochem. 1992, 22, 1002. (2) O’Keefe, T. J.; Hurst, L. R. J. Appl. Electrochem. 1978, 8, 109. (3) Fleischmann, M.; Hill, I. R.; Sundholm, G. J. Electroanal. Chem. 1983, 157, 359. (4) Lacconi, G. I.; Macagno, V. A. Electrochim. Acta 1994, 39, 2605. (5) Alodan, M.; Smyrl, W. Electrochim. Acta 1998, 44, 299. (6) Tarallo, A.; Heerman, L. J. Appl. Electrochem. 1999, 29, 585. (7) Szymaszek, A.; Biernat, J.; Pajdowski, L. Electrochim. Acta 1977, 22, 359. (8) Piro, O. E.; Piatti, R. C. V.; Bolzań, A. E.; Salvarezza, R. C.; Arvia, A. J. Acta Crystallogr., Sect. B 2000, 56, 993. (9) Piro, O. E.; Castellano, E. E.; Piatti, R. C. V.; Bolzań, A. E.; Arvia, A. J. Acta Crystallogr., Sect. C 2000, 58, m252. (10) Bolzań, A. E.; Wakenge, I. B.; Piatti, R. C. V.; Salvarezza, R. C.; Arvia, A. J. J. Electroanal. Chem. 2001, 501, 241. (11) Joy, V. T.; Srinivasan, T. K. K. Spectrochim. Acta, Part A 1999, 55, 2899. (12) El Hajbi, A.; Chartier, P. J. Electroanal. Chem. 1987, 227, 159. (13) Elschenbroich, C.; Salzer, A. In Organometallics: a concise introduction; VCH: Weinheim, Germany, 1989; Vol. 11, p 178.

established in acidic conditions. This reaction presents advantages over the conventional cyanidation: lower environmental impact, lower toxicity, greater selectivity toward gold, and faster kinetics of gold dissolution.14-17 Hence, knowledge of the interaction of TU with the electrode surface will lead to a greater understanding of the role and activity of the additive during electrodeposition processes. The electrochemical adsorption and oxidation of TU in different media has been studied in the past by cyclic voltammetry and coulometry.10,18-24 It was concluded that TU adsorbs on diverse metals, poisoning the electrode with sulfur species after oxidation of the molecule. In situ vibrational spectroscopies were also used to investigate these reactions.3,11,12,22,25-33 Surfaceenhanced Raman spectroscopy has shown that TU adsorbs (14) Li, J.; Miller, J. D. Hydrometallurgy 2002, 63, 215. (15) Go¨nen, N. Hydrometallurgy 2003, 69, 169. (16) Ubaldini, S.; Fornari, P.; Massidda, R.; Abbruzzese, C. Hydrometallurgy 1998, 48, 113. (17) Juarez, C. M.; Dutra, A. J. B. Miner. Eng. 2000, 13, 1083. (18) Kirchnerova, J.; Purdy, W. C. Anal. Chim. Acta 1981, 123, 83. (19) Bolzań, A. E.; Wakenge, I. B.; Salvarezza, R. C.; Arvia, A. J. J. Electroanal. Chem. 1999, 475, 181. (20) Reddy, S. J. J.; Krishnan, V. N. J. Electroanal. Chem. 1970, 27, 473. (21) Garcıá, G.; Macagno, V. A.; Lacconi, G. I. Electrochim. Acta 2003, 48, 1273. (22) Vandeberg, P. J.; Johnson, D. C. J. Electroanal. Chem. 1993, 362, 129. (23) Khan, S. H.; Rizvi, S. M. H.; Khan, A. A.; Rhaman, S. M. F. J. Electroanal. Chem. 1970, 27, 473. (24) Bolzań, A. E.; Piatti, R. C. V.; Arvia, A. J. J. Electroanal. Chem. 2003, 552, 19. (25) Loo, B. H. Chem. Phys. Lett. 1982, 89, 346. (26) Tian, Z. Q.; Lian, Y. Z.; Fleischmann, M. Electrochim. Acta 1990, 35, 879. (27) Kim, H.; Kim, J. J. J. Raman Spectrosc. 1993, 24, 77. (28) Reents, B.; Plieth, W.; Macagno, V. A.; Lacconi, G. I. J. Electroanal. Chem. 1998, 453, 121. (29) Brown, G. M.; Hope, G. A.; Schweinsberg, D. P.; Fredericks, P. M. J. Electroanal. Chem. 1995, 380, 161. (30) Holze, R.; Schomaker, S. Electrochim. Acta 1990, 35, 613. (31) Yan, M.; Liu, K.; Jiang, Z. J. Electroanal. Chem. 1996, 408, 225.

10.1021/la048978n CCC: $27.50 © 2004 American Chemical Society Published on Web 08/26/2004

8774

Langmuir, Vol. 20, No. 20, 2004

strongly on the metal surface via the sulfur atom in a wide potential range and desorbs at sufficiently negative potentials.3,11,12,25-30 On the other hand, the combination of Fourier transform infrared spectroscopy (FTIRS) with electrochemical techniques enables the acquisition of molecular information concerning the direct interaction of the adsorbate at the metal/electrolyte interface.34,35 Consequently, in situ FTIRS appears also to be a useful technique to study the interaction of organic plating additives. The electrochemical and spectroscopic behavior of TU in acidic media on polycrystalline gold [Au(pc)] has been studied by Bolzań et al. applying cyclic voltammetry,24,33 rotating disk, and ring-disk techniques,24 in a 0.1-50 mM concentration range, as well as FTIRS for a 50 mM solution.33 They concluded that the electrooxidation of TU and electrodissolution of gold are concomitant processes at E < 0.90 VSHE with the formation of formamidine disulfide (FDS) and [Au(I)(TU)2]+, respectively. Then, a second oxidation reaction takes place with the production of carbon dioxide, sulfate ions, and CNcontaining products. No adsorbates apart from sulfate at E > 1.20 VSHE were reported from FTIRS experiments,33 whereas from the electrochemical ones they proposed the presence of adsorbed TU, FDS, S, and CN.24 The same authors concluded a perpendicular adsorption of TU, and probably FDS, from scanning tunneling microscopy (STM) images obtained on Au(111) in a 50 mM solution in neutral media.36 A question that is still open relates to the identification of all intermediates and products in the charge-transfer processes involving TU adsorbed on different metals. For an in-depth knowledge of the influence of the surface structure on TU behavior, as well as for the elucidation of some contradictory data obtained by different authors, identification of the oxidation products of TU becomes crucial. In this context, the interaction between this molecule with polycrystalline and single-crystal electrodes could be relevant. Accordingly, we have undertaken a systematic investigation on the electrochemical behavior of TU using conventional electrochemical methods combined with a flow cell, as well as spectroscopic in situ techniques such as FTIRS and differential electrochemical mass spectrometry (DEMS). Using an appropriate experimental setup, the latter technique can be used to characterize submonolayers of adsorbates by means of the anodic or cathodic stripping of the layer.37,38 DEMS allows the detection of volatile and gaseous reaction products and intermediates with excellent sensitivity. To our knowledge, no DEMS studies of TU have been previously reported. Thus, the present work refers to the electroadsorption, oxidation, and reduction processes of TU in HClO4 solutions at (pc) and (111) single-crystal gold surfaces. The nature of the adsorbed species and the oxidation products is discussed on the basis of the spectral data (32) Papapanayiotou, D.; Nuzzo, R. N.; Alkire, R. C. J. Electrochem. Soc. 1998, 145, 3366. (33) Bolzań, A. E.; Iwasita, T.; Arvia, A. J. J. Electroanal. Chem. 2003, 554-555, 49. (34) Iwasita, T.; Nart, F. C. In Advances in Electrochemical Science and Engineering; Gerischer, H., Tobias, C. W., Eds.; VCH: New York, 1995; Vol. 4, p 123. (35) Iwasita, T.; Nart, V. F. C. Prog. Surf. Sci. 1997, 55, 271. (36) Azzaroni, O.; Andreasen, G.; Blum, B.; Salvarezza, R. C.; Arvia, A. J. J. Phys. Chem. B 2000, 104, 1395. (37) Wolter, O.; Heitbaum, J. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 2. (38) Baltruschat, H. In Interfacial Electrochemistry: Theory, Experiment and Applications; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; p 577.

Garcıá et al.

obtained at different potentials and compared with the electrochemical results. 2. Experimental Section Solutions were prepared from Millipore Milli-Q water and analytical grade reagents. The working solutions were 1-20 mM TU (Riedel-de Hae¨n) in 0.1 M perchloric acid (70% Merck). Deuterium oxide (99.9% Cambridge Isotope Laboratories, p.a.) was employed in some infrared experiments to avoid water interferences. Freshly prepared solutions were purged with argon (99.998%, Air Liquide). All experiments were carried out in electrochemical flow cells using a three-electrode assembly at room temperature. The working electrode was carefully flame-annealed and cooled close to the ultrapure water surface in voltammetric studies and FTIRS experiments. Then it was transferred to the electrochemical cell protected with a drop of water. A gold sheet was used as the counter electrode and a reversible hydrogen electrode (RHE) in the supporting electrolyte was employed as the reference electrode. All potentials in the text are referenced to this electrode. 2.1. Cyclic Voltammetric Studies. TU voltammetric studies were performed using an Au(pc) foil (1.158 cm2 real area) and an Au(111) disk (0.027 cm2 real area) as working electrodes. A hanging meniscus configuration was employed during the measurements with the single-crystal electrode. To characterize the quality of the surface electrode, cyclic voltammograms (CVs) were recorded in the supporting electrolyte solution between 0.05 and 1.65 V at a scan rate of 0.05 V s-1. Corresponding CVs for TU solutions were obtained in the same experimental conditions. It should be noted that the electrode contacts the solution at 0.05 V. 2.2. FTIRS Experiments. FTIRS experiments were performed with a Bruker Vector 22 spectrometer equipped with a mercury cadmium telluride detector. Parallel (p) and perpendicular (s) polarized IR lights were employed. A small glass flow cell with a 60° CaF2 prism at its bottom was used. The geometric area of (pc) and single-crystal disk electrodes was 0.785 cm2. The cell and experimental arrangements have been described in detail elsewhere.34,35 FTIR spectra were acquired from the average of 100 scans, obtained with 8 cm-1 resolution at selected potentials, by applying 0.05 V single potential steps from a reference potential, in the positive- and negative-going directions. The reflectance ratio R/R0 was calculated, where R and R0 are the reflectances measured at the sample and the reference potential, respectively. In this way, positive and negative bands represent the loss and gain of species at the sampling potential, respectively. s- and p-polarized light were used to distinguish between adsorbed and solution species. The p-polarized radiation carries information about the species in the bulk solution and also adsorbed molecules on the electrode surface. The s-polarized light only is active with solution species.34,35 2.3. DEMS Experiments. DEMS experiments were carried out in a 2 cm3 plexiglass flow cell directly attached to the vacuum chamber of the mass spectrometer (Balzers QMG112) with a Faraday cup detector. The working electrode was a porous gold layer sputtered onto a poly(tetrafluoroethylene) membrane (Scimat, Ltd., 200/40/60). This membrane forms the interface between the electrochemical cell and the ionization chamber of the mass spectrometer. The experimental setup allows the simultaneous acquisition of mass spectrometric cyclic voltammogram (MSCVs) for selected masses and conventional CVs at a scan rate of 0.01 V s-1. Details of this method have been given elsewhere.37,38 The working electrode was activated in the supporting electrolyte solution by potential cycling between the onset for hydrogen and oxygen evolution. After electrode activation, the TU-containing solution was introduced in the cell under potential control and the CVs and MSCVs for pre-set mass-to-charge ratios (m/z) were recorded.

3. Results and Discussion 3.1. Cyclic Voltammetry. Parts A and B of Figure 1 exhibit the first CV after contacting the 10 mM TU solution at 0.05 V (solid lines) for Au(pc) and Au(111), respectively. Also, the voltammetric profiles corresponding to the

Electrochemical Behavior of TU on Au Electrodes

Figure 1. (A) CVs for Au(pc) in 0.1 M HClO4 (dashed line) and 10 mM TU + 0.1 M HClO4 (solid line). (B) CVs for Au(111) in 0.1 M HClO4 (dashed line) and 10 mM TU + 0.1 M HClO4 (solid line). v ) 0.05 V s-1.

supporting electrolyte are given (dashed lines). When a recently flame-annealed and cooled Au(pc) electrode is introduced in the TU solution and the potential scan is started, the onset potential for the oxidation is established at 0.30 V (Figure 1A). The inset in this figure shows the development of a small reversible peak at 0.46 V when the upper potential is limited to 0.65 V. At more positive potentials, two anodic contributions are observed: the first one at 0.96 V and the largest one at 1.40 V. In the case of Au(111) (Figure 1B), the CV presents a similar behavior: a small wave centered at 0.50 V and two voltammetric peaks at 1.06 and 1.38 V. For both Au surfaces, the anodic current decreases during the reverse potential scan and presents an anodic current loop with respect to the positive-going potential scan in the ranges of 0.83-0.66 V and 0.93-0.67 V for Au(pc) and Au(111), respectively. This loop resembles those usually observed for metal electrodissolution in aggressive environments.33 However, in our case, this is not the process, as will be demonstrated later. At more negative potentials (E < 0.65 V), three features are apparent. The first one is observed as a hump at 0.48 V, very close to a well-defined cathodic peak around 0.35 V, and finally, a third small contribution is located at 0.12 V. In the corresponding second CVs (not sown for the sake of clarity), the peaks at 0.96 and 1.06 V for Au(pc) and Au(111), respectively, appear at more negative potentials, around 0.71 V, for both surfaces. As a result of the complicated nature of the CVs, the assignment of the voltammetric features will be discussed later in section 3.4 together with the results from in situ FTIRS and DEMS. In that section, the mechanism and range of the electroadsorption, oxidation, and reduction processes will be established. 3.2. FTIRS. A series of s-polarized spectra for Au(pc) and Au(111) in a 10 mM TU + 0.1 M HClO4/H2O solution can be seen in Figure 2A,B, respectively. From these spectra, similar TU electrochemical behavior is concluded

Langmuir, Vol. 20, No. 20, 2004 8775

Figure 2. s-polarized-light FTIR spectra (100 scans, 8 cm-1 resolution) for (A) Au(pc) and (B) Au(111) in 10 mM TU + 0.1 M HClO4/H2O at different potentials. Reference potential ) 0.05 V.

on both gold surfaces considering that the same bands develop with the same potential dependence. Thus, three positive-going contributions are apparent at 1620-1623, 1486, and 1405 cm-1, whereas five negative features are located at 2343, 2240, 1519-1529, 1435, and about 1200 cm-1. We consider now the assignment of all these bands. The broad positive one around 1620 cm-1, present in all spectra, is due to the O-H bending mode of water,39 which disturbs the spectral region between 1700 and 1400 cm-1. This fact implies an uncertainty in the position of the bands observed in this range related to TU adsorption and oxidation processes. Moreover, the presence of some features could be obscured. Although the contribution from the solvent cannot be avoided, the signals in this region can be studied using deuterium oxide, because the bending mode appears in this case around 1200 cm-1 (see later). For E > 0.40 V, two positive bands develop at 1405 and 1486 cm-1, corresponding to the symmetric and asymmetric N-C-N stretching vibration modes, respectively, associated with the depletion of TU in the thin layer.31-33 It has to be mentioned that the CS stretching as well as the NH2 rocking modes also contribute to these band intensities.31-33 The consumption of TU has to be related to the adsorption and oxidation reactions occurring in this potential region. Accordingly, the spectra show, at the same potentials, a negative contribution near 1435 cm-1 that reflects the formation of a new soluble species. As previously reported,33 this band can be assigned to an Au(I) complex with two TU molecules bonded to the metal through the sulfur atoms ([Au(I)-(TU)2]+). The contribution located at 1519-1529 cm-1 is strongly affected by the water band, and, therefore, its assignment in H2O is quite difficult and will be reconsidered later in D2O. The appearance of all these bands is almost coincident with the onset potential for the oxidation of TU during the positive-going potential scan in the CVs in Figure 1A,B (0.30 V, the small shift is easily explained assuming that the experimental procedure for FTIRS is potentiostatic whereas for CV is potentiodynamic). (39) Nakamoto, K. In Infrared and Raman Spectra of Inorganic and Coordination Compounds; John Wiley & Sons: New York, 1997.

8776


Garcıá et al.

Table 1. Principal Infrared Bands of Undeuterated and Deuterated Species wavenumber, cm-1

assignment

references

1405 (TU) 1486 (TU) 2240 (NH2CN) 1435 ([Au(I)-(TU)2]+) 2343 (CO2) 1206 (HSO4-) 1385 (TU-d4) 1516 (TU-d4) 1397 (TU22+-d8) 1634 (TU22+-d8) 1397 ([Au(I)-(TU-d4)2]+) 1561 ([Au(I)-(TU-d4)2]+)

νs CN2 + ν CS + F NH2 νas CN2 + σ NH2 ν CN νs CN2 + ν CS + F NH2 νas CO2 νas SO νs CN2 + ν CS νas CN2 νs CN2 + ν CS νas CN2 νs CN2 + ν CS νas CN2

11, 31-33 11, 31-33 40 33 39 39 31-33 31 31, 33 31, 33 33 33

On the other hand, for E g 0.80 V another broad negative feature is observed at 2240 cm-1, which can be assigned to the stretching mode of a CtN group,39,40 revealing the formation of cyanamide.31-33 The production of NH2CN from TU has been previously established from hydrometallurgical studies.14,15 Finally, spectra recorded at E > 1.20 V show two negative bands around 1200 and 2343 cm-1 due to the O-S-O stretching mode of bisulfate ions and the O-C-O asymmetric stretching of carbon dioxide, respectively.39 Band assignment for the features in Figure 2A,B are compiled in Table 1. It has to be mentioned that no differences were observed between the spectra acquired with p- and s-polarized light. This fact could suggest that no adsorption species from TU are formed, or at least cannot be detected. However, the strong water band obscures an important wavenumber region, and no conclusions can be obtained until the experiments were performed in deuterated water. For this reason the corresponding in situ FTIR spectra using p- and s-polarized radiation were recorded in 10 mM TU + 0.1 M HClO4/D2O. Again, the same bands are apparent with Au(pc) and Au(111), and, therefore, only the series obtained with the single crystal are given in Figure 3A,B. First, for the interpretation of the spectra it is necessary to consider that, in D2O, the amine hydrogens in TU readily exchange with deuterium from the solvent and, therefore, the deuterated form (TU-d4) is expected in the solution.31-33 Table 1 also summarizes the band assignments for deuterated species. A close comparison of the two spectral series in Figure 3A,B reveals some differences for E e 0.40 V. The p series (Figure 3A) exhibits two bipolar bands at 1585-1517 and 1387-1371 cm-1 whereas with s-polarized radiation only positive bands at 1518 and 1385 cm-1 are present (Figure 3B). This result evidences the existence of adsorbed species from TU-d4. Considering that s-polarized light is only sensitive to solution species, positive contributions at 1518 and 1385 cm-1 in Figure 3B can be assigned to the N-C-N asymmetric and symmetric stretching of TU-d4 in solution32,33 due to the consumption of this compound. These features correspond to those at 1486 and 1405 cm-1 in Figure 2A,B with a shift justified by the presence of deuterium attached to the N atoms instead of hydrogen. On the other hand, the negative bands at 1585 and 1387 cm-1, which are only visible with p-polarized light, have to be assigned to adsorbed species. These signals seem to correspond to the same vibrations of TU-d4 in solution previously mentioned, with a displacement in their position due to the bonding with the metal.32,33 The upshift can be explained in terms of an increase in the double

No differences between s- and p-polarized light spectra at E > 0.40 V are observed in Figure 3A,B. So, all features have to be associated with solution species. The negative bands at 1562 and 1395 cm-1 point out the formation of the complex species ([Au(I)-(TU)2]+), as both signals can be seen in the transmission spectrum of the complex.33 However, in this reference, the band at 1395 cm-1 in the spectra for TU oxidation has been interpreted as a contribution of FDS. Obviously, this compound has a vibrational mode at this wavenumber (see Table 1), but its intensity is very small, as it can be seen in ref 31. In fact, in accordance with its potential dependence (parallel to that at 1562 cm-1), the contribution of the gold complex prevails. Moreover, the same bands have been detected during dissolution of gold in TMTU solutions producing

(40) SDBS web. http://www.aist.go.jp/RIODB/SDBS/ (accesed Oct 2002).

(41) Port, S. N.; Horswell, S. L.; Raval, R.; Schiffrin, D. J. Langmuir 1996, 12, 1934.

Figure 3. FTIR spectra (100 scans, 8 cm-1 resolution) for Au(111) in 10 mM TU + 0.1 M HClO4/D2O at different potentials. (A) p-polarized and (B) s-polarized light. Reference potential ) 0.05 V.

bond character of the C-N bond originated from an increment of π-electron delocalization in the N-C-N skeleton. Following this reasoning, the coordination of TU with the metal must be through the sulfur atom.3,11,12,32,33,36 In relation to adsorbed TU, the presence of both the N-C-N symmetric and asymmetric stretching modes in the spectra can only be explained, according with the surface selection rule,34,35 when a tilt orientation of TU is considered. A perpendicular molecular adsorption on the surface implies that the symmetric stretching vibration should be IR active whereas the N-C-N asymmetric mode should be inactive. On the other hand, a parallel molecular adsorption leads to the absence of both the symmetric and asymmetric vibrations. This result is in accordance with the STM images for tetramethylthiourea (TMTU) in acid media, which shows that this molecule adsorbs slightly tilted on the Au(111) surface.41 On this basis, the first step of the TU interaction with gold is the adsorption of this molecule through the S atom with a certain angle with respect to the surface of the electrode:

Au + TU(aq) a AuTU(ad)

(1)


Langmuir, Vol. 20, No. 20, 2004 8777

Figure 4. Potential dependence of the integrated band intensities for (A) TU, (B) [Au(I)-(TU2)]+, (C) (TU2)2+, (D) NH2CN, (E) HSO4-, and (F) CO2 in 0.1 M HClO4 at Au(pc).

Figure 5. Potential dependence of the integrated band intensities for (A) TU, (B) [Au(I)-(TU2)]+, (C) [TU2]2+, (D) NH2CN, (E) HSO4-, and (F) CO2 in 0.1 M HClO4 at Au(111).

[Au(I)TMTU]+,42 and in this case, no FDS can be formed. This assignment is important for the establishment of the reactions that takes place in the different potential ranges, avoiding erroneous interpretations. As before, TU isotopic exchange explains the frequency shift when compared with the band position in the spectra in Figure 2A,B. The corresponding reaction can be written as

will be described together (only the differences will be mentioned). TU Consumption (Figures 4A and 5A). In the 0.050.40 V potential range, the intensity of the TU band (1516 cm-1) is almost constant suggesting that this molecule already adsorbs at the reference potential (0.05 V) and the consumption in this region is very small. Only reaction 1 takes place. The strong consumption of TU in the thin layer occurs at E > 0.4 V during the positive-going potential steps, mainly in the 0.40-0.80 potential range. The band intensity remains almost constant between 0.80 and 1.65 V, indicating that most of the products formed in this region proceed not from TU(aq) but from intermediate species formed at more negative potentials. For E < 0.80 V during the negative-going steps, the amount of TU increases again in the thin layer, but the final value is lower than the initial one. Then, it can be concluded that most of the TU consumed is recovered from reversible processes (the production of [Au(I)-(TU)2]+ and (TU)22+ in reactions 2 and 3), but the other part is irreversibly oxidized during the potential excursion up to 1.65 V (oxidation to cyanamide and sulfur, which finally produces CO2, N2, and sulfate species in reactions 4-7, see later). Gold Dissolution (Figures 4B and 5B). Parallel to the strong decrease in the intensity of the spectral feature at 1516 cm-1 between 0.40 and 0.80 V (Figures 4A and 5A), a sharp increase in the intensity of the band at 1561 cm-1, related to the gold complex, is observed (Figures 4B and 5B). This behavior reveals that dissolution of Au in reaction 2 with the formation of [Au(I)-(TU)2]+ is the main pathway at these potentials. The intensity of the band diminishes for E > 0.80 V until it disappears at 1.20 V. A small signal is recorded in the negative-going potential step direction, and, therefore, dissolution of gold is not responsible for the anodic loop in the CVs in Figure 1A,B. These results confirm that the formation of the gold complex requires the previous adsorption of TU, which cannot occur on the gold oxide.

AuTU(ad) + TU(aq) a [Au(I)-(TU)2]+(aq) + e- (2) The development of a third negative band at 1634 cm-1 for E > 0.50 V in Figure 3A,B, which increases in intensity as the potential is set to more positive values, is remarkable. This feature was hidden by the water band in the experiments performed in 10 mM TU + 0.1 M HClO4/H2O and can be assigned to the production of protonated FDS (TU)22+ in the solution.31 This vibration corresponds to the sCdNs imino groups. The following reaction describes this oxidation process:

2TU(aq) + [Au] a (TU)22+(aq) + 2e- + [Au] (3) An overview of all the processes occurring in the whole potential region (0.05-1.65 V) is obtained from the plot of the integrated band intensities for the compounds detected in the FTIR spectra, as a function of the applied potential. For each species, one characteristic vibrational mode has been selected: TU (1516 cm-1), [Au(I)-(TU)2]+ (1561 cm-1), and (TU)22+ (1634 cm-1) from the spectra recorded in D2O and NH2CN (2240 cm-1), HSO4- (1206 cm-1), and CO2 (2343 cm-1) from the undeuterated experiments. Figures 4 and 5 show the results on Au(pc) and Au(111), respectively. As expected, similar behavior is observed for these surfaces, and accordingly, both figures (42) Bunge, E.; Nichols, R. J.; Roelfs, B.; Meyer, H.; Baumga¨rtel, H. Langmuir 1996, 12, 3060.

8778


FDS Production (Figures 4C and 5C). The second process at E > 0.40 V is the yield of protonated FDS (TU)22+ according to reaction 3, which can be followed through the signal at 1634 cm-1 (Figures 4C and 5C). The corresponding plot shows the onset at 0.50 V and a progressive increase until 1.20 V, where a plateau is achieved, in contrast with the peak described by the signal related to gold dissolution. The behavior of this band also differs from that at 1516 cm-1 during the negative-going potential direction: the production of (TU)22+ is still relevant, with an increment from 0.90 to 0.40 V, where the intensity falls, attaining 0 at 0.20 V. During the increase in the 0.90-0.40 V potential range, no further depletion of TU is apparent. However, the consumption of the gold complex in this region which produces free TU molecules could explain this fact. The formation of FDS explains the anodic current observed in the corresponding CVs in this potential region during the reverse potential scan with both gold surfaces (Figure 1A,B). It must be considered that the reduction of the gold oxide is completed at 0.90 V in the reverse scan for both surfaces, producing a very clean and reactive surface, which is free of adsorbed TU. Then, bulk TU can react on the electrode to form (TU)22+ through reaction 3. Comparing Figure 4C with Figure 5C, the higher production of (TU)22+ on Au(111) than on Au(pc) is evident. Cyanamide Formation (Figures 4D and 5D). The main production of NH2CN (2240 cm-1) begins at 0.80 V just with the sharp decrease in the production of the gold complex, attaining the maximum at 1.30 V. Therefore, the formation of NH2CN could be related with the diminution of the band intensity related to [Au(I)-(TU)2]+, according with the following reaction:

[Au(I)-(TU)2]+(aq) f 2NH2CN(aq) + 4H+(aq) + 2S0 + 3e- + Au (4) It should be mentioned that small amounts of cyanamide are detected on Au(111) in the 0.40-0.80 V potential range but not in the case of Au(pc). The intensity of this signal during the reverse potential step direction can be explained in terms of the diffusion of the species previously formed and retained in the thin layer. No adsorbed cyanamide can be established from our spectra. However, preliminary results on the oxidation of isolated TU adsorbed on gold, applying a flow cell procedure, suggest that adsorbed cyanamide is produced as an intermediate at E < 1.20 V.43 Production of Sulfate Species (Figures 4E and 5E). At E g 1.20 V a new band appears at 1206 cm-1 (Figure 2A,B) in the spectra acquired in H2O (in D2O this spectral region is obscured by the bending vibrational mode of the solvent), related to bisulfate ions. A fast increase in the intensity is observed, attaining the maximum production at 1.65 V in the positive-going step direction. In the negative run, no formation is shown (only the diffusion out of the thin layer seems to occur). The yield of sulfate species is explained assuming that residual S0 formed on the gold surface in reaction 4 at more negative potentials (0.80 < E < 1.20 V) is oxidized at E g 1.20 V, producing bisulfate ions: (43) Garcıá, G.; Lacconi, G.; Rodrı´guez, J. L.; Pastor, E. Manuscript in preparation.

Garcıá et al.

S0 + 4H2O f HSO4-(aq) + 7H+(aq) + 6e-

(5)

Then the acid/base equilibrium leads to the formation of sulfate:

HSO4-(aq) a SO42-(aq) + H+(aq)

(6)

Sulfate ions in solution exhibit a characteristic band near 1100 cm-1, which overlaps with the signal from perchlorate in the solution and, therefore, cannot be considered for this analysis. However, a behavior parallel to that for the 1200 cm-1 signal should be expected. Yield of CO2 (Figures 4F and 5F). The band at 2343 cm-1 is associated with the formation of CO2. The onset potential for CO2 (1.20 V) is coincident with the onset potential for the decrease in the intensity of the band related to NH2CN. Accordingly, it can be considered that the former is a product from the oxidation of NH2CN:

NH2CN(aq) + 2H2O f N2(g) + CO2(g) + 6H+(aq) + 6e- (7) The maximum in the CO2 production is observed at 1.65 V, and the shape of the band intensity versus potential plot for the negative-going steps displays the slow diffusion of the CO2 trapped in the thin layer. Comparing Figures 4F and 5F, the lower intensity values for Au(111) are remarkable, and, therefore, it can be concluded that reaction 7 is favored on Au(pc). 3.3. DEMS. The production of N2 in reaction 7 cannot be detected with FTIRS but can be detected with DEMS. Also other gaseous products, which are not IR-active, can be followed with the mass spectrometric technique. For this reason, TU reactions have also been studied with DEMS. Figure 6 shows the results for porous Au(pc) in a 20 mM TU + 0.1 M HClO4/H2O solution. Figure 6A exhibits the CV obtained between -0.15 and +1.65 V, whereas the corresponding MSCVs for the mass signals with a potential-dependent response are given in Figure 6B. No potential-dependent mass signals were observed for the ratios m/z ) 17 ([NH3]•+) and 16 ([NH2]+) associated with the formation of ammoniac or for m/z ) 30 ([NO]+), which implies that nitrogen oxides are not produced in this potential range. Assuming the absence of the latter compounds, the mass signal for m/z ) 44 in Figure 6B is only due to CO2. In agreement with FTIRS results, the onset for its production is established at 1.20 V attaining the maximum at the upper potential limit. The mass signal for m/z ) 28 describes a parallel potential dependence to that of m/z ) 44. The former ratio can be due to nitrogen ([N2]•+) formation during the oxidation process (reaction ) or to the contribution of carbon monoxide ([CO]+) formed in the DEMS chamber from the fragmentation of CO2. It is possible to distinguish between both possibilities considering that, for the first case, the formation of N2 would imply the same potential dependence for the mass signal for m/z ) 14 ([N]+) due to the fragmentation of the molecule, and for the second case, the ratio between the intensities of the mass signals for m/z ) 44 and m/z ) 28 must be on the order of the magnitude of that from the expected fragmentation for CO2.44 Figure 6B displays the variation with the potential for m/z ) 14, which indeed behaves parallel to the signal for m/z ) 28. Moreover, the intensity ratio for m/z ) 44 (44) Stenhagen, E., Abrahamsson, S., McLafferty, F. W., Eds. Atlas of Mass Spectral Data; Interscience: New York, 1969.


Langmuir, Vol. 20, No. 20, 2004 8779

Figure 7. (A) CVs for 20 mM TU in 0.1 M HClO4 at porous gold (solid line) and for the supporting electrolyte (dotted line). (B) MSCVs recorded for the mass signal for m/z ) 2 ([H2]•+) for the TU-containing solution (solid line) and for the supporting electrolyte (dotted line). v ) 0.01 V s-1. Figure 6. (A) CV for 20 mM TU in 0.1 M HClO4 at porous gold and (B) corresponding MSCVs recorded for the mass signals for m/z ) 44 ([CO2]•+), 28 ([N2]•+), 14 ([N]+), and 34 ([SH2]•+). v ) 0.01 V s-1.

Scheme 1a

to m/z ) 28 is lower than that expected from the CO2 fragmentation. Then, it can be concluded that N2 is produced during oxidation of TU. This fact fortifies the postulated reaction 7 as the two gaseous products, N2 and CO2, have been actually detected. Figure 6B also exhibits the potential dependence of the mass signal for m/z ) 34. The onset potential is observed at 0.25 V with the maximum production at 0.05 V during the negative-going potential scan. This maximum is coincident with the contribution recorded in the CV (Figure 6A). If the upper potential limit is established at 0.65 V, no potential dependence was observed for m/z ) 34 (not shown). Then, the signal in the +0.25 to -0.1 V potential range in Figure 6B is due to the reduction of species formed in the anodic wave at E > 0.80 V during the positive run in the CV (Figure 6A). Assuming that reaction 4 occurs in this potential range, the mass signal for m/z ) 34 can be assigned to H2S formed during the reverse scan from the reduction of S0.

S0 + 2H(ad) f H2S(g)

(8)

From these data, it is obvious that the oxidation of S atoms to sulfate ions (reactions 5 and 6) is not complete during the potential scan up to 1.65 V at the sweep rate used in DEMS experiments (0.01 V s-1). Finally, Figure 7 corresponds to the CVs in the hydrogen evolution region and the MSCVs for its production (m/z ) 2, [H2]•+) with (solid line) and without (doted line) TU in the solution. It should be noted that the anodic potential limit is established at 0.30 V avoiding the gold dissolution and TU oxidation regions. A shift in the H2 evolution of about 0.20 V to more negative potentials is observed in the presence of TU. This result is an indirect demonstration of TU adsorption at E < 0.30 V. 3.4. Summary. A global overview of the electrochemical behavior of TU in an acidic medium on Au(pc) and Au(111) electrodes can be obtained combining cyclic voltam-

a The numbers in scheme correspond to the number for the corresponding reaction in the text. It should be noted that all soluble and gaseous species in this scheme have been detected by FTIRS [TU, (TU)22+, [Au(I)(TU)2]+, NH2CN, CO2, and HSO4-] and/or DEMS (CO2, N2, and H2S).

metry with in situ spectroscopic techniques (FTIRS and DEMS). Similar results were observed for both surface structures. Species formed at the different potential ranges are summarized in Scheme 1. Four potential regions can be distinguished according with the processes involved: (1) 0.05 < E < 0.40 V. From the analysis of the FTIR spectra, it is evident that TU molecules adsorb with a tilt orientation on clean gold surfaces according to reaction 1. DEMS results also support the presence of TU(ad), which shifts the hydrogen evolution to more negative potentials (Figure 7). (2) 0.40 < E < 0.80 V. The IR spectra prove the existence of a dual-path oxidation process. At low potentials, the

8780


gold dissolution with the reversible formation of a complex (detected by FTIRS) from adsorbed TU as described in reaction 2 prevails. This reaction is related with the first anodic wave in the 0.40-0.60 V potential range in the CVs during the positive sweep, whereas the corresponding cathodic process of gold deposition occurs in the same region during the negative-going potential scan (inset in Figure 1A,B). Also in the same potential range, the formation of (TU)22+ was observed in the spectra, but its production increases very slowly in the potential region 2 and becomes more important in region 3. Reaction 3 describes this pathway, which is responsible for the pair of redox peaks in the CVs at about 1.00 and 0.35 V during the positive and negative scans, respectively.14 This experimental behavior can be explained considering that the strong metal-TU bond probably arises from the σ donation from the ligand to empty metal orbitals, along with back-donation from filled metal d orbitals to antibonding ligand orbitals.3 Thus, it is assumed that TU adsorbs through the S atom (potential region 1).36 As the potential is set to more positive values, the strength of the metal-sulfur σ bond increases, and, hence, the strength of the CS bond decreases. In the vicinity of the potential of zero charge, a discharge of one electron from gold is possible, and in the presence of TU, the complex [Au(I)(TU)2]+ is formed. Then, TU molecules in the solution that find free sites at the electrode surface can react to form (TU)22+. (3) 0.80 < E < 1.20 V. According to FTIRS, the concentration of the gold complex in the thin layer decreases because it suffers oxidation with the yield of cyanamide through reaction 4. In fact, the latter reaction, together with the formation of (TU)22+, are the processes in the potential region of the peak around 1.00 V in the CVs in Figure 1. The presence of S0 on the gold surfaces was indirectly established from DEMS experiments, as H2S was detected at E < 0.25 V (Figure 6B, reaction 8).

Garcıá et al.

(4) 1.20 < E < 1.65 V. Oxidation of cyanamide was established because the presence of its oxidation products (CO2 and N2) was confirmed by FTIRS (CO2) and DEMS (CO2 and N2). The appearance of bands related with sulfate ions (reactions 5 and 6) also confirms indirectly the previous formation of S0. These processes are related with the third contribution in the CV (the peak centered around 1.40 V in Figure 1A,B) in the positive run. 4. Conclusions The electrochemical behavior of TU on Au(pc) and Au(111) in acidic media was investigated using CV, FTIRS, and DEMS techniques. The electroadsorption and electrooxidation reactions of TU were established, and it was concluded that they are similar on both gold surfaces. The adsorption process occurs at potentials more negative than 0.40 V, and a tilt orientation of TU was proposed to justify the bands in the FTIR spectra. The electrooxidation reactions take place at more positive potentials in two parallel pathways. Adsorbed TU is implied in the production of a gold complex [Au(I)(TU)2]+ in a first step. Afterward, for E > 0.80 V electrooxidation of this gold complex produces NH2CN and S0, which are oxidized in a third step at E > 1.20 V to produce N2, CO2, and HSO4-/SO42-. The other parallel reaction involves bulk TU molecules to form (TU)22+. It must be noted that the maximum in the formation of this dimmer is produced in the negativegoing potential direction, once the gold surface is cleaned from the gold oxide. Acknowledgment. The financial support from CONICET, SECyT(UNC), ANPCyT, Agencia Co´rdoba Ciencia S.E. and Gobierno Autońomo de Canarias (PI2003/ 070), and DGES (PB2002-01685) is gratefully acknowledged. G.G. thanks the MCYT for the FPI grant. LA048978N

Spectroscopic Investigation of the Adsorption and Oxidation of

Recommend Documents