An in-situ STM study of potential-induced changes in the surface topography of Au(100) electrodes

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J. Eleclroanal Chem., 290 (1990) 21-31 Elsevier Sequoia S.A., Lausanne

An in-situ STM study of potential-induced changes in the surface topography of Au(100) electrodes R.J. Nichols, O .M. Magnussen *, J . Hotlos * *, T. Twomey, R .J. Behm * and D.M . Kolb Fritz-Haber-lnstitut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-1000 Berlin 33 (F.R.G.) (Received 23 March 1990)

ABSTRACT

We have used in-situ STM to study the changes in surface topography occurring on flame-annealed Au(100) electrode surfaces, for potential cycles which were limited to the double-layer region and also to potentials of surface oxidation . Even in the former case notable changes in the surface structure occur and it is inferred that anion specific adsorption plays the determining role in such processes . For the first time, STM images are presented of an oxide-covered electrode surface which, for the case of the Au(100) electrode, shows a rough, apparently amorphous form . Images recorded during the initial stages of oxide formation indicate an island-growth mechanism.

INTRODUCTION

In-situ structural characterisation of the metal/ electrolyte interface forms an important basis for a more detailed interpretation of electrochemical

Spectroscopic techniques

processes [1] .

such as in-situ infrared [2] and UV-Vls [3] spectroscopy,

second harmonic generation [4,5] and more recently in-situ X-ray spectroscopies [6-8] have been used to gain knowledge about the structure of adsorbates and the nature of adsorption sites . However, such techniques can provide only indirect information concerning changes in the surface structure . In-situ X-ray diffraction is a promising technique for structural investigations . However, to be observable

[8] by

the X-ray technique, such changes in the surface have to possess long-range order . Scanning tunneling microscopy (STM), on the other hand, can be used to examine

* Institut fur Kristallographie and Mineralogie, Universitat Munchen, Theresienstr. 41, D-8000 Munchen 2, F.R.G . ** Department of Physical Chemistry and Electrochemistry, Jagellonian University, ul . Karasca 3, PL-30-060 Krakow, Poland-

0022-0728/90/$03.50

© 1990 - Elsevier Sequoia S .A .

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changes in the surface topography in real space, i.e., regardless of whether the surfaces are disordered or ordered . Rapid progress has been made since the feasibility of recording STM images of surfaces immersed in electrolytes was demonstrated [9,10] . Applications have included studying the bulk electrodeposition of metals (Pb on Ag [11] and Cu on Cu [12]), the topography of electrofacetted gold electrodes [13] and the restructuring of gold surfaces [14-16] . More recently, the in-situ characterisation of submonolayer adsorbate overlayers, of Cu on Au [17] and iodide on Pt [18] single-crystal substrates, has demonstrated the high degree of resolution which is possible . The stability of gold surfaces and dynamic changes occurring in the surface morphology has been addressed in several STM studies, both in electrolyte [14-16] and in air [16,19,201 . We have demonstrated recently the application of in-situ STM to studying the restructuring of Au(111) surfaces induced by chloride adsorption-desorption cycles [14]. Atomic scale resolution with the ability to distinguish step and terrace sites was shown. Dissolution and redeposition of gold atoms was observed to occur at step sites . Chidsey and co-workers [15] have used in-situ STM to follow the mechanism of roughening, annealing and dissolution of Au(111), in the absence and presence of chloride ions . They have observed the roughening of Au(111) terraces during electrochemical oxidation and reduction and the creation of one-monolayer deep pits. They have provided evidence for mobility of these pits, their growth by fusion and annihilation of pits at step edges. Holland-Moritz et al . [161 have investigated step motion on Au(111) surfaces and have observed the motion of step edges in both air and in 0 .01 M KCl solution near the rest potential . Step edges could be seen to move at typical rates of 50 A/min (1 A = 10 -10 m). They found that this mobility was not spontaneous but was induced by external forces such as the bias between the tip and the surface . This study utilises the in-situ electrochemical STM technique in examining topographical changes of Au(100) electrode surfaces, which are seen to occur with time and upon cycling the electrode potential . For the first time STM images are presented which have been recorded at potentials of electrochemical oxide formation. The structure of the oxide and its effect on the underlying surface topography is assessed, and an island growth mechanism is postulated . EXPERIMENTAL

Lustenberger et al . [21] have described the basic concepts involved in the application of STM to studying surfaces in electrochemical environments . The instrument used in this study was a pocket size STM with a tripod piezoceramic tube assembly as described previously [14] . The tungsten tips were prepared by electrochemical etching and subsequently covered by a thermoplast wax (Apiezon wax), so as to leave only the foremost 50 µm of the tip uncoated . In addition to a conventional three-electrode system (counter, working and reference electrodes), in which the working electrode was at virtual earth, the tip acts as a fourth electrode . The tip potential was chosen so as to minimise any faradaic current flowing to it . It

23 was found that small amounts of Cu e of the tip markedly .

ions in the electrolyte increased the stability

The sample was held in a small macor or Kel-F pot, which constitutes the electrochemical cell, with the counter electrode being placed around the inner rim of this vessel . The reference electrode compartment and the cell were connected by a length of silicone tubing and a Luggin capillary . The now well established flame-annealing method of sample preparation was used [221 . The gold single-crystal discs (8 mm diameter and 2 mm thickness) were held carefully by their edges in a tungsten wire clamp, during the flame-annealing process . Such a treatment involved heating the crystal briefly to ca +800°C, followed by quenching in pure water . Electrolytes were prepared from suprapure chemicals and triply distilled water. All potentials were measured and are quoted with respect to the saturated calomel electrode (SCE). All images shown here were recorded with a tunneling current of 2 .5 nA . This relatively low tunneling current corresponds to a comparatively long tip-to-surface distance (presumably > 1 run, see Fig . 2, ref . 14), hence reducing the possibility of the tip perturbing the surface structure . Typical values of the potential for the tungsten tip ranged from - 100 to + 200 mV versus SCE . RESULTS AND DISCUSSION The gold single-crystal surfaces were prepared as described above . Areas with extended flat terraces spanning over several tens of nm, which were separated by monoatomic high steps, could be located readily on such flame-treated gold crystals . This confirms that high quality single-crystal surfaces may be prepared by such a method . Figure 1 shows a 1000 A x 1000 A area of a Au(111) crystal and is representative of a large number of images which have been obtained . However, for the purposes of studying the stability of gold surfaces in electrolytes it is generally more desirable to select areas containing "distinctive structural features" . These features may be used in evaluating the amount of lateral drift between successive STM images and are useful in ensuring that, within a series of images, the same areas are compared . Figure 2A shows a series of images for Au(100) in 1 M H 2S04 , with several adjacent "U"-shaped steps characterising these images . "Topview" representations,

20A 2Qo A 200A Fig. 1 . An STM image (3-D line scan representation) of a 1000 A X 1000 A area of a Au(111) surface at + 400 mV (SCE), in 0 .05 M H2SO4+5 mM HCI .

A

1 1000 I

Fig . 2 . (A) A series of STM images for Au(100) in 1 M H Z SO4 , showing changes in surface topography which occur within the double-layer charging region . (B) . "Topview" representations of the images in (A) .



N D

25 in which different heights are represented by different shades of grey, show more clearly the step boundaries . In Fig . 2B topview images of the corresponding STM patterns in Fig. 2A are presented. In Fig. 2Ba it can be seen that there are a number of small islands sitting on top of the uppermost terrace . Such monatomic high gold islands have often been observed for flame-treated Au(100) surfaces, which have been maintained at potentials within the "double-layer region". It should also be noted that monoatomic high islands have been seen much less frequently for the Au(111) surface prepared by the method described . An explanation for the origin of such islands may be found when one considers reconstruction of the Au(100) surface . The clean (flame-prepared) Au(100) surface has been shown to possess the reconstructed, hexagonal form, which reverts to the (1x1) structure upon specific adsorption of anions [23]. Under the present experimental conditions the reconstruction was removed upon immersion . Since the reconstructed surface has about 20% more surface atoms than the unreconstructed one, the "hex" -+ (1 x 1) transition causes the extra atoms to be forced upwards and they then coalesce into islands by surface diffusion . Indeed, such a process has been observed by STM upon CO adsorption from the gas phase onto clean Pt(100) [24,25] . Further experiments are being attempted to observe the (5 x 20) -. (1 x 1) reconstruction by in-situ electrochemical STM . However, it has not been possible, using our present experimental methods, to transfer the clean (flame-annealed) Au(100) crystal quickly to the cell, with immersion of the working electrode under potentiostatic control . It may be difficult to obtain the (5 x 20) surface unless we can implement such a method . The gold islands (Fig. 2Ba) are relatively stable at lower electrode potentials . For example, prior to the acquisition of Fig . 2Ba), images were collected over a period of 15 min with the electrode potential being maintained at 400 mV, and these showed only very minor structural changes . However, increasing the potential to + 700 mV (Fig. 2Bb) leads to the dissolution of the smaller islands and changes in the shape of some of the larger ones. In contrast, the step boundaries remain comparatively unchanged in both shape and position. A further increase in the potential to + 1000 mV (Fig. 2Bc) results in a complete dissolution of the islands and a significant roughening of the step edges . Figures 2Bd (+700 mV) and 2Be (+400 mV) complete the cycle . As one can see readily, significant topographical changes have occurred, even for such a cycle, where the potentials are limited to the double-layer region . Figure 3 illustrates changes in surface topography with time for a Au(100) surface in 0.05 M H 2SO4, at a potential of +760 mV . A particularly notable feature of these images is a straightening of the step edges over a period of 10 min . Figure 4 shows STM images as a function of time for Au(100) in 0 .05 M H 2SO4, with the addition of 5 mM HCI . It can be seen that at +500 mV only small changes in topography occur on a timescale of a few minutes, while a rapid dissolution of the islands is seen at +700 mV, with the resulting formation of a large terrace (at the left-hand edge of the STM picture) . The two potentials, 500 and 700 mV vs . SCE,

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Fig . 3 . STM images and topview representations of Au(100) in 0 .05 M H,SO4 at a potential of +760 mV, as a function of time (as marked)_

refer to values before and just after a pronounced spike at 670 mV in the cyclic voltammogram [26] . Although the Cl - coverage is only slightly higher at 700 mV, the influence on the surface topography is increased markedly . It thus appears that the Cl - adsorbate, after it has undergone a structural transition at the spike

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/300A

+500 mV

+700 mV

3004' Fig. 4. STM images of Au(100) in 0 .05 M H2SO4+5 mM HCl as a function of time, at two different electrode potentials. Images were taken consecutively with a time lapse between successive images of ca . 100 s .

potential, interacts more strongly with the substrate, leading to an increased mobility of the gold atoms at steps and kinks . From the examination of a number of STM pattern series for Au(111) it has been noted that changes in the surface topography also appear to occur more rapidly in the presence of Cl - anions [14] . However, no significant differences could be distinguished between electrolytes containing HSO4 and ClO4 anions . Even more pronounced topographical changes in the surface occur when the electrode potential is taken into the oxide formation region . This is illustrated in Fig. 5 for Au(100) + 0 .05 M HC1O4 +1 mM Cu(C104 ) 2 . The addition of Cu" cation to the electrolyte stabilises the tip, particularly at higher tunneling voltages, while not affecting the Au(100) surface at the potentials studied . The dissolution of a gold island and its incorporation into a neighbouring step, which is seen to occur during the first three patterns of this sequence, corresponds with our previous observations at similar potentials (Figs . 2-4) . Extension of the potential into the oxide region results in a dramatic change in the image, which indicates that a

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(i)

600 mV

Fig. 5 . STM images of Au(100) in 0.05 M HCIO4 +1 mM Cu(C104)2, as a function of electrode potential, showing changes induced by oxide formation .

roughened amorphous surface structure has been produced . The image taken in the oxide-dissolution region (Fig. 5g) shows several monoatomic deep pits, on what was previously a flat terrace . This indicates that oxide formation occurs with place exchange between oxygen and gold surface atoms . However, these pits are rapidly filled in to produce a large flat terrace (Figs . 5h and 5i) . As can be seen, from

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Fig . 6, Full width at half height of a histogram peak for a given terrace of Au(100) versus electrode potential . Data were taken from a series of STM images in which the potential was cycled from the double-layer to the oxide region (0) and back again (o). Other details as for Fig. 5 .

comparing the first and final images, a substantial transport of gold atoms has occurred during the formation and stripping of the oxide . Histograms showing the height distribution within an STM image can give an indication of the degree of roughening of terraces upon oxide formation, from a measurement of the full width at half height of the histogram peak for a given terrace . This is plotted against electrode potential in Fig . 6, the data being taken from a series of STM patterns in which the potential was cycled from the double-layer region to the oxide-formation region and back . It is immediately clear from this graph that there is a hysteresis in the terrace roughening . This is as expected, since oxide formation is an irreversible electrochemical process . For the clean surface the width of the histogram is determined by the inherent noise level of the measurement . For the oxidised surface this graph shows that the average height of the observed protrusions is in the range of 2-3 A, with a significant fraction of higher protrusions as reflected by the wings of the histogram peak . Figure 7 shows two successive STM pictures of the same area, for Au(100) in 0 .05 M H 2 SO4 , after stepping the potential from +0.3 to +1 .13 V. The tip scanning directions and the time scales are marked on both images . These show the formation and growth of "hills" (3-10 A high) on the surface with time, and these "hills" are taken to be oxide clusters . Clearly if oxide formation proceeds only through a simple adsorption and two-dimensional growth process, we would expect to see islands of ca . 2 A in height on the surface . The fact that we see "hills" of higher dimensions indicates that these are gold oxide clusters, with the Au atoms being lifted from the surface, and with the oxide formation proceeding through an island growth mechanism . This interpretation is not altered by the fact that due to electronic effects the apparent height of these nuclei does not correspond exactly to their actual geometrical height .

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Fig 7 . Two consecutive images of Au(100) in 0 .05 M H 2SO4 at 1 .13 V, showing the time dependence of oxide-island formation . The direction of the slow scan is indicated by the arrows .

SUMMARY AND CONCLUSIONS Significant changes in surface topography have been observed for Au(100), even for potential cycles which are restricted within the so-called "double-layer region" of the voltammogram. Dissolution and restructuring of monoatomic high islands occur more rapidly than do changes to long, relatively straight step edges, and smaller islands with a higher curvature dissolve more rapidly . The rate of such restructuring processes is far higher at more positive potentials (within the doublelayer region) . From these observations it may be concluded that adsorbed anions play an important role in surface topographical changes by increasing the surface mobility of the gold atoms . We have recorded these images at relatively low tunneling currents (2 .5 nA), where we expect, but cannot be certain, that the tip has a relatively low or negligible contribution to the observed changes in surface topography . However, the observation of changes in the double-layer capacity of metal electrodes with time [27,28] correlates nicely with our data, which depicts mobility in the surface topography . The formation and stripping of an oxide layer can lead to large topographical changes in the gold surface, resulting from the transport of substantial quantities of gold atoms across the surface . Monatomic deep pits and islands can be created on what were previously flat terraces, indicating that a place-exchange process is occurring between the oxygen and gold surface atoms .

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STM images taken at potentials where oxide formation occurs indicate that the oxide surface has a rough, apparently amorphous structure consisting of "hills" sitting upon a surface base, which vary in height between 2 and 10 A. Observations of STM images taken during oxide formation have led to the conclusion that the oxide forms through an island growth process . ACKNOWLEDGEMENTS

We are grateful to the Alexander von Humboldt-Stiftung for a grant to R .J . Nichols and to the Deutsche Forschungsgemeinschaft (via SFB 128 and 338) for financial support of J. Hotlos . REFERENCES 1 D .M . Kolb, Ber. Bunsenges. Phys. Chem ., 92 (1988) 1175 . 2 A . Bewick and S. Pons in R .J .H . Clark and R.E. Hester (Eds.), Advances in Infrared and Raman Spectroscopy, Vol . 12, Wiley-Heyden, Chichester, 1985, p .l . 3 D .M. Kolb, J . Phys. (Paris), 44 (1983) C10-137 . 4 G.L. Richmond, J .M. Robinson and V.L . Shannon, Prog . Surf . Sci., 28 (1988) 1 . 5 A . Friedrich, B . Pettinger, D.M . Kolb, G . Lupke, R . Steinhoff and G. Marowsky, Chem. Phys . Lett ., 163 (1989) 123 . 6 M .G. Samant, M.F . Toney, G.L . Borges, L . Blum and O .R. Melroy, J . Phys . Chem., 92 (1988) 220 . 7 O .R . Melroy, M .G. Samant, G .L. Borges, J .G. Gordon, L. Blum, J .H . White, MJ. Albarelli, M . McMillan and H .D . Abruha, Langmuir, 4 (1988) 728 . . J Kortright, P .N . Ross and L . Blum, J . 8 OR . Melroy, M.F . Toney, G.L . Borges, M .G . Samant,.B Electroanal . Chem., 258 (1989) 403 . 9 R . Sonnenfeld and P .K. Hansma, Science, 232 (1986) 211 . 10 H .Y. Liu, F .-R .F. Fan, C .W . Lin and A.J . Bard, J . Am . Chem. Soc ., 108 (1986) 3838 . 11 R . Christoph, H . Siegenthaler, H . Rohrer and H . Wiese, Electrochim . Acta, 34 (1989) 1011 . 12 X .G. Zhang and U. Summing, Corros . Set., in press . 13 J. G6mez, L . VSsquez, A .M. Bars, C .L . Perdriel and A.J . Arvia, Electrochim . Acta, 34 (1989) 619. 14 J. Wiechers, T. Twomey, D .M. Kolb and R.J. Bchm, J . Electroanal . Chem., 248 (1988) 451 . 15 D .J . Trevor, C .E.D. Chidsey and D .N. Loiacono, Phys. Rev. Lett ., 62 (1989) 929 . 16 E . Holland-Moritz, J. Gordon II, G . Borges and R . Sonnenfeld, Surf. Sci ., submitted . 17 O.M . Magnussen, J . Hotlos, R .J. Nichols, D .M. Kolb and R.J . Behm, Phys . Rev . Lett ., 64 (1990) 2929 . 18 S : L . Yau, C .M. Vitus and B.C. Schardt, J . Am. Chem. Soc ., in press. 19 R. Emch, J . Nogami, M .M . Dovek, C.A. Lang and C.F . Quate, J . Appl . Phys . 65 (1989) 79 . 20 R.C. Jakievic and L . Elie, Phys . Rev . Lett ., 60 (1988) 120 . 21 P. Lustenberger, H . Rohrer, R. Christoph and H . Siegenthaler, J . Electroanal. Chem., 243 (1988) 225 . 22 J . Clavilier, R. Faure, G . Guinet and R. Durand, J . Electroanal . Chem 107 (1980) 205 . 23 D .M . Kolb and I Schneider, Electrochim . Acta, 31 (1986) 929 . 24 W. HOsler, E. Ritter and R.J . Behm, Ber. Bunsenges . Phys . Chem., 90 (1986) 205 . 25 E . Ritter, RJ. Behm, G . Pbtschke and J . Wintterlin, Surf. Sci., 181 (1987) 403 . 26 A. Hamelin and J .P . Seller, C .R . Acad . Sci . (Paris Ser. C, 279 (1974) 371 . 27 M . Fleischmann, J . Robinson and R. Waser, J . Electroanal . Chem ., 117 (1981) 257 . 28 R. Waser and K .G. Weil, J. Electroanal . Chem., 150 (1983) 89 .