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Adsorbate-Induced Etching of Au(111) Surfaces: A Combined in-Situ Infrared Spectroscopy and Scanning Tunneling Microscopy Study E. Bunge,†,‡ S. N. Port,§ B. Roelfs,†,‡ H. Meyer,† H. Baumga¨rtel,*,‡ D. J. Schiffrin,§ and R. J. Nichols§ Atotech Deutschland GmbH, Erasmusstrasse 20-24, 10553 Berlin, Germany, Department of Chemistry, The University of Liverpool, Liverpool L69 3BX, U.K., Freie Universita¨ t Berlin, Takustrasse 3, 14195 Berlin, Germany Received June 5, 1996. In Final Form: August 21, 1996X In this paper the initial stages of the etching of Au(111) electrode surfaces in the presence of adsorbed tetramethylthiourea (TMTU) are investigated using the complementary methods of in-situ IR spectroscopy and STM. Scanning tunneling microscopy (STM) has been used in-situ to examine changes in surface topography associated with the surface etching. STM images show that the etching process proceeds from step edges and eventually leads to the exposure of monatomic steps which are orientated at 60°/120° with respect to each other. This indicates that the more lowly co-ordinated kink atoms are more readily etched than “straight” steps, leading to an appearance of anisotropic etching of steps. In-situ IR spectroscopy confirms that etching leads to the formation of gold-TMTU complexes, which are soluble in the electrolyte solution. It is also apparent from these spectroscopic data that no significant TMTU decomposition occurs during the anodic surface etching. The potential dependence of the IR band intensity of the Au-TMTU complex indicates the important role which adsorbed TMTU plays in the etching process. Upon subsequently lowering the electrode potential a slow growth of the step edges is seen by in-situ STM. Deductions concerning the growth could be made by following the development of the surface topography with in-situ STM. Growth preceded two-dimensionally from substrate step edges and gradually gave rise to semicircular step edges which advanced slowly across the surface. This can be interpreted as the nucleation and growth of gold islands by electrodeposition of the TMTU-complexed gold.
1 Introduction Organic adsorbates have wide ranging applications in electrochemical technology.1,2 Examples include their use as corrosion inhibitors or as additives in metal-plating baths, where they are used to control the growth morphology of the metallic deposit.3,4 Thiourea and thiourea derivatives have been effectively used in such applications.5 The importance of such additives has provided a strong motivation for scientific investigations of their electrochemical behavior. In a recent paper in this journal, a detailed structural model for the adsorption of tetramethylthiourea (TMTU) was derived from high-resolution scanning tunneling microscopy (STM) images with submolecular resolution.6 It was deduced that TMTU is adsorbed on the electrode surface with its molecular NC(S)N “skeleton” orientated parallel to the electrode surface. In addition, well-ordered superstructures were identified. Another important aspect is the stability of the surface in the presence of organic adsorbates. In this respect, it is well recognized that adsorbates can exert a large †
Atotech Deutschland GmbH. Freie Universita¨t Berlin. § The University of Liverpool. X Abstract published in Advance ACS Abstracts, November 15, 1996. ‡
(1) Pletcher, D.; Walsh, F. Industrial Electrochemistry, 2nd ed.; Chapman and Hall Ltd.: London, 1990. (2) Gabe, D. Principles of Metal Surface Treatment and Protection; Pergamon Press: New York, 1978. (3) Nichols, R. J.; Beckmann, W.; Meyer, H.; Batina, N.; Kolb, D. M. J. Electroanal. Chem. 1992, 330, 381. (4) Nichols, R. J.; Bach, C. E.; Meyer, H. Ber. Bunsenges. Phys. Chem. 1993, 97 (8), 1012. (5) Szymaszek, A.; Biernat, J.; Pajdowski, Electrochim. Acta 1977, 22, 359. (6) Bunge, E.; Nichols, R. J.; Roelfs, B.; Meyer, H.; Baumga¨rtel, H. Langmuir 1996, 12, 3060.
S0743-7463(96)00548-3 CCC: $14.00
influence on surface structure and dynamics.7-16 Reconstruction behavior and surface mobility have been seen to differ in the presence of adsorbates.7,14,15 STM is a particularly powerful technique for observing such morphological changes of electrode surfaces.7,14-16 Several groups have used in-situ STM to follow the mechanism of roughening, annealing, and dissolution of Au(111) surfaces, in the presence of strong adsorbates such as chloride7,14-16. Similarly, a relatively high mobility of gold surfaces in aqueous electrolytes has been observed in the presence of organic adsorbates such as thiols.8-13,17 More recently it was noted that Au(111) electrode surfaces show signs of etching at moderate anodic potentials in the presence of tetramethylthiourea.6 In this paper we investigate this anodic dissolution of Au(111) and the subsequent deposition of gold-TMTU complex from solution, using the complementary methodologies of in-situ STM and infrared spectroscopy. Microscopic aspects of the anodic dissolution have been (7) Nichols, R. J.; Magnussen, O. M.; Hotlos, J.; Twomey, T.; Behm, R. J.; Kolb, D. M. J. Electroanal. Chem. 1990, 290, 21. (8) Sondag Huethorst, J. A. M.; Scho¨nenberger, C.; Fokking, L. G. J. J. Phys. Chem. 1994, 98, 6826. (9) McCarley, R. L.; Dunaway, D. J.; Willicut, R. J. Langmuir 1993, 9, 2775. (10) Scho¨nenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokking, L. G. J. Langmuir 1994, 10, 611. (11) Delamarche, E.; Michel, B.; Kang, H.; Gerber, C. Langmuir 1994, 10, 4103. (12) Bucher, J.-P.; Santesson, L.; Kern, J. Langmuir 1994, 10, 979. (13) McDermott, C. A.; McDermott, M. T.; Green, J.-B.; Porter, M. D. Submitted for publication in J. Phys. Chem. (14) (a) Trevor, D. J.; Chidsey, C. E. D.; Loiacono, D. N. Phys. Rev. Lett. 1989, 62, 929. (b) Trevor, D. J.; Chidsey, C. E. D. J. Vac. Sci. Technol. 1991, B9, 964. (15) Holland-Moritz, E.; Gordon, J.; Borges, G.; Sonnenfeld, R. Langmuir 1991, 7, 301. (16) Holland-Moritz, E.; Gordon, J.; Kanazawa, K.; Sonnenfeld, R. Langmuir 1991, 7, 1981. (17) Rohwerder, M.; Deweldige, K.; Vago, E.; Viefhaus, H.; Stratmann, M. Thin Solid Films 1995, 264, 240.
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examined with in-situ STM, while in-situ infrared spectroscopy has been used to characterize the formation of Au-TMTU complexes. This combination of infrared spectroscopy and STM is particularly powerful, since IR spectroscopy can be used to directly identify molecular species which are formed, while STM can be used to study changes in the electrode surface at the nanoscale. 2 Experimental Section 2.1. Electrodes and Solution. Au(111) working electrodes were used for all experiments. In the case of the in-situ IR spectroscopy measurements, a disk shaped Au(111) single crystal electrode (Goodfellow), 2 mm thick and 12 mm diameter connected to a 10 cm gold wire, was used. This gold electrode was carefully flame annealed at yellow heat prior to the spectroscopic measurements and transferred to the IR cell after quenching with ultrapure water. Working electrodes for the STM measurements consisted of a 200 nm thick gold film with a 2 nm thick chromium underlayer vacuum evaporated onto “Tempax” glass (AF45, Berliner Glas K.G.). These samples were flame annealed in a similar manner to the disk gold single crystals. Atomic resolution STM images of such gold film on glass samples show that these samples possess large terraces with a (111) orientation.18,19 A platinum wire counter electrode was used for the STM measurements, while a large area platinum gauze counter electrode was used for in-situ IR. A saturated calomel reference electrode was chosen for the IR measurements, while a Ag-wire reference electrode was more suitable for the STM measurements. This Ag-wire reference was tested and found to be stable in the electrolytes used. In all cases the electrode potentials quoted here are versus the saturated calomel electrode (SCE). Solutions were prepared with H2SO4 (Merck Suprapur) and tetramethylthiourea (Aldrich), purified by recrystallization. 2.2. STM. The in-situ STM imaging was conducted with a Topometrix TMX 2000 with a bipotentiostat. The STM tips were fabricated either from cut Pt-Ir (80:20) or etched tungsten, both coated with nail polish. 2.3. Infrared Spectroscopy. Infrared spectra were obtained using the subtractively normalized interfacial Fourier transform infrared spectroscopy technique (SNIFTIRS).20,21 A threeelectrode cell with a CaF2 IR window (spectral cutoff below 1000 cm-1) was employed for all spectroscopic experiments. After flame annealing, the solid gold working electrode was placed in the SNIFTIRS cell and aligned against the IR window with a glass plunger. The thin layer cell thus formed usually has a thickness in the order of a few micrometers.22 The potential was controlled with a potentiostat and waveform generator (HiTek; England, DT2101 and PPR1), the potentiostat was switched between two preset potentials, and IR reflectance spectra were collected at each potential. Before spectral collection the solutions were purged with nitrogen (oxygen free, BOC Ltd). A dry air purged FTIR spectrometer (BioRad FTS40) with a liquid nitrogen cooled mercury cadmium telluride (MCT) detector was used for all the SNIFTIRS experimens.23 Either p- or s-polarized radiation was employed depending on the experiment conducted. 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/R ) (R2 - R1)/R1
(1)
Consider that a new molecular species is formed at potential 2 (E2), which is not present at E1, and that this species gives rise (18) Haiss, W.; Lackey, D.; Sass, J. K.; Besocke, K. H. J. Chem. Phys. 1991, 95, 2193. (19) Haiss, W.; Lackey, D.; Sass, J. K.; Meyer, H.; Nichols, R. J. Chem. Phys. Lett. 1992, 200, 343. (20) Pons, S.; Davidson, T.; Bewick, A. J. Electroanal. Chem. 1984, 160, 63. (21) Bewick, A.; Pons, S. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley Heyden: New York, 1985; Vol. 12. (22) Ashley, K.; Pons, S. Chem. Rev. 1988, 88, 673. (23) Port, S. N.; Schiffrin, D. J.; Solomum, T. Langmuir 1995, 11, 4577.
to new IR absorption peaks. These peaks will point downward in the ∆R/R (SNIFTIRS) spectrum, since increased IR absorption at E2 is equivalent to decreased reflectivity at E2, i.e. (R2 - R1)/R1 < 1. In other words, in simple situations negative going bands are related to species which are in excess at E2, while positive going ones correspond to species in excess at the reference potential (E1). The signal-to-noise ratio of the IR spectrum was improved by collecting a number of interferograms. For these experiments, 100 interferograms were collected at each potential and the normalised spectra (∆R/R1) calculated. This procedure was then repeated 10 times, and the final normalized spectrum was obtained by averaging these data.
3 Results Figure 1 shows a 105 × 105 nm area of the Au(111) surface. A highly curved monoatomic step edge (0.24 ( 0.1 nm height) and a screw dislocation are distinctive features in this area of the surface. Image A was taken following a potential step from +60 to +400 mV, with images B-D then being taken at time intervals of 1.25, 2.5, and 3.75 min, respectively. These images are representative of the anodic etching of Au(111) in the presence of TMTU and are similar to images which have been previously presented.6 This publication identified that TMTU-induced etching of the Au(111) surface proceeds from step edges. In this study here we further analyze the gradual nonisotropic evolution of step morphology, which is apparent in sequences such as shown in Figure 1. In these images the characteristic screw dislocation, located midway up on the right-hand side, acts as a “landmark”, ensuring that the same area of the surface is compared in a series of images. During this sequence there is a substantial etching of the curved, and hence highly kinked, step line which runs through the center of the image, while the screw dislocation hardly changes. The highly curved step develops into straight step edges meeting at 60°/120° (note that the major substrate directions of the Au(111) surface are orientated at 60° with respect to one another). We conclude from such images that highly kinked step edges are most rapidly etched, to eventually reveal straight step edges which are orientated along the major substrate directions. We can hence deduce that removal of the coordinatively more saturated gold atoms from straight step edges is significantly slower than removal of less coordinated kink atoms. Etching of screw dislocations is also very slow for similar reasons. From analysis of sequences such as those shown in Figure 1, we can rank the rate of TMTU-induced etching of local surface sites: kinks > straight steps, screw dislocation edges . in-plane terrace atoms. Further evidence that the retreat of step edges is associated with surface etching and gold dissolution into the electrolyte has been provided by analysis of electrolyte solutions, which had been in contact with Au(111) polarized at +400 mV. Data for the analysis of electrolyte solutions for gold ions have been previously presented.6 Significant concentrations (ca. 10-6 M) of gold ions were detected in electrolytes (0.1 M H2SO4 + 2 mM TMTU) left in contact with Au(111) held at a potential of +400 mV for 18 h. This formation of gold-TMTU complexes by etching of the Au(111) surface has been further investigated by in-situ infrared spectroscopy. Infrared spectroscopy provides a useful complementary method for monitoring the surface etching by directly following the formation of Au-TMTU complexes. SNIFTIRS spectra of Au(111) in contact with 0.1 M H2SO4 and 5 mM TMTU are shown in Figure 2. Since these SNIFTIRS difference spectra are likely to contain bands arising from uncomplexed TMTU in solution, it is first
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Figure 1. STM images of Au(111) in 0.1 M H2SO4 + 1 × 10-5 M TMTU at +400 mV. Images B-C were taken 1.25, 2.5, and 3.75 min after image A. Tunneling current parameters: constant current mode, It ) 3.8 nA, Vbias ) +100 mV.
on an eight body determination carried out by Gosavi et al.33 A more recent analysis by Anthoni et al.34 using a 20-body analysis provides a more reliable interpretation and has been used for band assignment in this paper. TMTU may be thought of as a rather simple molecule; however under C2 symmetry it still has 54 normal modes of vibration, most of which are strongly coupled. Nevertheless, only the strongest bands are expected to be apparent in the SNIFTIRS spectra; these are at 1508, 1470, 1369, 1360, 1262, 1208, 1131, 1119, and 1096 cm-1. These bands are listed in Table 1. It should be noted that all the bands are strongly coupled with the NC(S)N skeleton stretching and deformations (ν7, ν12, ν14, ν15, ν16, ν38, ν39, ν41, and ν42). In order to assign spectral features to Au-TMTU complex formation, it is necessary to consider in detail spectral differences between TMTU and TMTU-metal Figure 2. SNIFTIRS spectra of Au(111) in 0.1 M H2SO4 + 5 × 10-3 M TMTU (-300 mV, reference potential).
necessary to consider IR spectral assignments for TMTU. In the literature there are a number of detailed studies for TMTU in both solution and the solid phass.24-32 However, ambiguities exist with many analyses relying (24) Williams, D. J.; Poor, P. H.; Ramirez, G.; Heyl, B. L. Inorg. Chim. Acta 1988, 147, 21. (25) Wynne, K. J.; Pearson, P. S. Inorg. Chem. 1971, 10, 2735.
(26) Wynne, K. J.; Pearson, P. S.; Newton, M. G.; Golen, J. Inorg. Chem. 1978, 11, 1192. (27) Stewart, J. E. J. Phys. Chem. 1957, 26, 248. (28) Gosavi, R. K.; Rao, C. N. R. J. Inorg. Nucl. Chem. 1967, 29, 1937. (29) Rao, C. N. R.; Venkataraghavan, R. Spectrochim. Acta 1962, 18, 541. (30) Randall, H. M.; Fowler, R. G.; Fuson, N.; Dangl, J. R. In Infrared Determination of Organic Structures D. Van Nostrand: New York, 1949. (31) Spinner, E. Spectrochim. Acta 1959, 15, 95. (32) Yamaguchi, A.; Penland, R. B.; Mizushima, S.; Lane, T. J.; Curran, C.; Quagliano, J. V. J. Am. Chem. Soc. 1958, 80, 527. (33) Gosavi, R. K.; Agarwala, U.; Rao, C. N. R. J. Am. Chem. Soc. 1967, 89, 235. (34) Anthoni, U.; Nielsen, P. H.; Borch, G.; Gustavsen, J.; Klaboe, P. Spectrochim. Acta 1977, 33A, 403.
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Table 1. IR Band Assignments after Anthoni et al.34 band wavenumber (cm-1)
normal mode of vibration
1508
ν7
1470
ν8 ν9 ν34 ν35 ν36 ν37 ν38
1440 1369 1360 1262 1208 1131
ν12 ν39 ν14 ν41 ν15
1119 1096
ν42 ν16 ν43
description νsNCN + δCH3 + δsCH3c -NC + FCH3 + νCS δasCH3 δasCH3 δasCH3 δasCH3 δasCH3 δasCH3 δsCH3 + νasNCN + δasCH3t - NC + νasCH3c -N δsCH3 δsCH3 + νasCH3c -N νsCH3c -N + νsNCN + FCH3t FCH3, νasCH3t -N, νasNCN νCS + νsCH3t -N + νsCH3c -N + FCH3c FCH3 FCH3 FCH3 + νasNCN
Figure 3. s- and p-polarized SNIFTIRS spectra of Au(111) in 0.1 M H2SO4 + 5 × 10-3 M TMTU (-300 mV, reference potential).
Table 2. Comparison of IR Shifts for TMTU-Metal Complexes band wavenumber (cm-1) species
NCN
CdS
TMTU (crystalline) TMTU solution (SNIFTIRS) Cd-TMTU (crystalline) Co-TMTU (crystalline) Pd-TMTU (crystalline) Pt-TMTU (crystalline) Zn-TMTU (crystalline) Au-TMTU solution (SNIFTIRS)
1504 1537 1550 1542 1549 1600 1544 1571
1126 1126 1108 1098 1101 1102 1104 1111
complexes. Although, no IR characterization, specifically for Au-TMTU complexes, could be found in the literature, it is known that TMTU forms complexes with numerous transition metals, for example, Pd(II), Cd(II), Hg(II), Cr(III), Fe(III), Zn(II), Pt(II), and Co(II), for which IR data are available.28,33,35 Upon complexation TMTU shows marked vibrational frequency shifts for all of these metals, and this has been related to the nature of the bonding to the metal. Table 2 shows a comparison of two of the strongest IR bands for a range of TMTU complexes. From this table it is clear that all complexes, when compared to uncomplexed crystalline TMTU, exhibit shifts to higher frequencies for the 1504 cm-1 band (NCN antisymmetric stretch) and to lower frequencies for the 1126 cm-1 band (CdS stretch).28,33,35 From previous capacitance data it has been inferred that TMTU is adsorbed over a wide potential range, bounded by hydrogen evolution at the cathodic limit and TMTU desorption at the anodic limit.6,36 The reference potential for the SNIFTIRS spectra shown in Figure 2 was -300 mV, chosen to avoid hydrogen evolution. At this potential TMTU is strongly adsorbed.6,36 For potentials up to +100 mV, no spectral changes were observed, confirming that the TMTU layer is stable within this potential range. At +300 mV and above, bipolar bands are clearly observed, the origin of which will be discussed below. The SNIFTIRS spectra consist of both positive- and negative-going bands. The positive bands at 1537, 1371, 1280, and 1153 cm-1 correspond to TMTU in solution, implying that there is a higher concentration of TMTU in (35) Schafer, M.; Curran, C. Inorg. Chem. 1966, 5, 265. (36) Bunge, E.; Nichols, R. J.; Baumga¨rtel, H.; Meyer, H. Ber. Bunsenges. Phys. Chem. 1995, 99, 1243.
Figure 4. Plot of band height versus electrode potential for various IR bands arising from Au-TMTU complexes formed by etching of the Au(111) surface.
the thin layer at the lower potential limit (the reference potential, -300 mV). These positive bands could be due to either (i) further adsorption of TMTU at the metal interface at the higher potential or (ii) consumption of solution-free TMTU to form new chemical species at the higher potential. Analysis of the negative-going bands in these spectra enables us to identify which of these processes occurs. Negative-going bands correspond to the formation of new species, since they are significantly shifted from those of TMTU in solution. In Table 2 bands seen in the SNIFTIRS spectra are compared with those of TMTUmetal complexes. This comparison leads us to conclude that these SNIFTIRS bands arise from TMTU-Au complexes. For instance, the band at 1571 cm-1 is shifted by 38 cm-1 with respect to that of uncomplexed TMTU. This is directly comparable to the 30-40 cm-1 shifts typically seen for this vibrational mode on going from TMTU to TMTU-metal complexes. A sharp band at 1111 cm-1 is also seen in the SNIFTIRS spectra and is also comparable to a band seen for metal-TMTU complexes. For metal-TMTU complexes this band is considerably sharper than the corresponding vibrational band for uncomplexed TMTU.27 Similarly, a broad positive shoulder is observed at 1126 cm-1, in the SNIFTIRS spectra. Hence, the broad shoulder can be assigned to solutionfree TMTU and the sharp negative band at 1111 cm-1 to the Au-TMTU complex. Therefore, the SNIFTIRS results and their good correlation with literature spectra for metal-TMTU complexes, provide direct evidence for the formation of a Au-TMTU complex during etching.
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Figure 5. A series of in situ STM images of the surface in 0.1 M H2SO4 + 2.5 × 10-5 M TMTU at +20 mV (vs SCE) after polarization to +400 mV. Images B-D were taken 2.5, 5.5, and 8 min after image A. Tunneling current parameters: constant current mode, It ) 6.3 nA, Vbias ) +110 mV.
To confirm whether the Au-TMTU complex formed at anodic potentials are surface or solution species, SNIFTIRS experiments using s-polarized IR radiation were performed at the Au(111)/electrolyte interface. The comparison between s- and p-polarized spectra is noteworthy, in that only p-polarized light has an appreciable field strength at the electrode surface and hence it alone carries information on vibrations of adsorbed species. By contrast, the field strength of the s-polarized radiation vanishes at the electrode surface and only gradually increases in magnitude up to distances of about 2.5 µm (at 2000 cm-1) away from the surface. Therefore, adsorbed species cannot give rise to bands in the s-polarized spectrum. However, species produced or consumed from the thin layer will give spectral features in both s- and p-polarized spectra, although band intensities will be typically significantly lower in the s-spectra. As can be seen in Figure 3 all the IR bands seen for TMTU with p-polarized radiation are also clearly observed with the s-radiation. It should be noted that the intensities are reduced, as expected, but more importantly the relative intensities of the positive and negative bands are similar, confirming their interrelated character. Since all the IR bands are observed with the s-radiation it is concluded that the electrochemically generated Au-TMTU complex is formed in the thin layer and not adsorbed at the Au(111) surface.
Despite the wealth of IR data for metal-TMTU complexes it is difficult to unequivocally determine the TMTU ligand co-ordination, although several groups have assumed that the bonding to the metal is through the sulfur.12,28 This difficulty in assignment arises from the strong coupling of vibrational modes. However, the overall conclusion is that a solution-free Au-TMTU complex is formed by anodic etching of the surface with TMTU decomposition not being apparent. The evolution of the bands for the Au-TMTU complex can give an indication of the potential dependence of the complex formation. This is shown in Figure 4. There is good correlation between the electrode potential at which surface etching is seen by STM and the evolution of the IR bands for the complex. It is also interesting that the IR band intensity reaches a maximum as the electrode potential is increased and then falls at more anodic potentials. At these more anodic potentials direct desorption of TMTU has been previously observed by insitu STM.6 It was proposed that TMTU is displaced by sulfate at these more anodic potentials. This would explain the reduction in the rate of metal complex formation by etching and highlights the important role of the adsorbed TMTU in the etching process. The in-situ infrared data and the STM clearly illustrate that the gold surface is slowly etched at sufficiently anodic potentials, with Au-TMTU complex being formed in
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Figure 6. A schematic representation of the anisotropic surface etching and the isotropic deposition of Au-TMTU complexes at step edges of the Au(111) surface in TMTU-containing electrolytes.
solution. This complex can be subsequently electrodeposited if the potential is lowered. Figure 5 shows a series of images taken at +20 mV. Prior to acquisition of this series, the Au(111) electrode was polarized at +400 mV, resulting in the formation of complexed gold in the electrolyte solution. In this STM image series a gradual advance of step edges is seen. The step edges have a distinctive semicircular morphology which is in distinct contrast to the step edge morphology generated by etching the surface at the more anodic potentials. The development of the surface morphology allows us to make certain conclusions about the mechanism of the electrodeposition of the gold-TMTU complex. The growth and propagation of distinctive semicircular step edges point to nucleation of the metallic deposit at step edges followed by a twodimensional growth, which is isotropic with respect to the substrate surface. The observations for the etching and deposition are schematically depicted in Figure 6. 4 Conclusion In-situ scanning tunneling microscopy is a powerful method for following morphological changes of surfaces during etching. In the presence of adsorbed TMTU, curved
Bunge et al.
and kinked step edges of the Au(111) substrate are preferentially etched at moderate anodic potentials. The development of straight step edges meeting at 60°/120° is consistent with co-ordinatively less highly saturated kink atoms in steps being etched much more rapidly than the more highly co-ordinated gold atoms in straight step edges. Screw dislocation edges are also more resilient to TMTUinduced surface etching. In-situ infrared spectroscopy has been used to confirm that the etching process yields TMTU-Au complexes, with there being no sign of TMTU decomposition. The formation of gold-TMTU complexes has been further characterized by in-situ infrared spectroscopy. Spectral features corresponding to vibrational modes of Au-TMTU complexes could be assigned by analysis of the spectra and comparison with infrared spectra of metal-TMTU complexes. Although spectra of Au-TMTU complexes are not available in the literature, the spectral shifts for vibrational modes of TMTU upon Au-TMTU complex formation are similar to those of other metal-TMTU complexes which have been synthesized and characterized. Analysis of band intensities for the vibrational modes of TMTU-Au complexes indicates that the rate of surface etching increases with increasing anodic potential to a maximum and then falls. The maximum corresponds closely with the electrode potential where TMTU desorption begins. We have also investigated the subsequent deposition of gold from Au-TMTU complexes. The deposit propagates two-dimensionally from growth centers at step edges of the substrate to yield semicircular steps which form the growth front. The 2D growth is isotropic in the surface plane. Acknowledgment. Funding for part of this work was provided by the University of Liverpool, The Royal Society, Deutsche Forschungsgemeinschaft, and the Nuffield Foundation. The SNIFTIRS cell, specially designed for use with the single crystal, was constructed by Bert Chappell, to whom we are grateful. LA960548C
