© Copyright 1996 American Chemical Society
SEPTEMBER 4, 1996 VOLUME 12, NUMBER 18
Letters Quantitative Surface Stress Measurements on Au(111) Electrodes by Scanning Tunneling Microscopy Wolfgang Haiss Chemistry Department, University of Liverpool, Liverpool L69 3BX, England
Ju¨rgen-Kurt Sass* Fritz-Haber-Institut der MPG, Faradayweg 4-6, D-14195 Berlin, Germany Received March 11, 1996. In Final Form: June 10, 1996X Using macroscopic cantilevers of evaporated Au(111) films on glass substrates in conjunction with scanning tunneling microscopy, we performed quantitative surface stress measurements of copper underpotential deposition in bromide and chloride acid solutions. Upon differentiation of the stress data with respect to potential, definite signatures of the corresponding voltammograms were obtained, in contrast to previous contentions of the applicability of the Lippmann equation to solid electrodes which predicts such signatures only for the second derivative of the stress.
We have recently shown1,2 that relative surface stress data, with excellent signal-to-noise ratio, can be obtained in an electrochemical environment by scanning tunneling microscopy (STM) on thin, circularly supported Au(111)on-mica films. In gas-phase studies3 the preferred sample geometry is that of a cantilever, for which the measured deflection, usually obtained by capacitive techniques, can be directly related to the surface stress by the Stoney formula.4 Early electrochemical work actually also made use of this type of electrode, although a very large sample had to be used and the sensitivity achieved with laser diffraction patterns was fairly low.5 In a recent report6 a miniature AFM cantilever was employed for stress measurements in solution. However, the crystallinity and topography of an evaporated metal film or of a bulk singlecrystal can be much better controlled and assessed when the sample dimensions are in the millimeter range. The X
Abstract published in Advance ACS Abstracts, July 15, 1996.
(1) Haiss, W.; Sass, J. K. J. Electroanal. Chem. 1995, 386, 267. (2) Haiss, W.; Sass, J. K. J. Electroanal. Chem., in press. (3) Ibach, H. J. Vac. Sci. Technol. 1994, A12, 2240. (4) Mu¨ller, P.; Kern, R. Surf. Sci. 1994, 301, 386. (5) Fredlein, R. A.; Damjanovic, A.; Bockris, J. O’M. Surf. Sci. 1971, 25, 261. (6) Raiteri, R.; Butt, H.-J. J. Phys. Chem. 1995, 99, 15728.
S0743-7463(96)00222-3 CCC: $12.00
Figure 1. Schematic illustration of the electrochemical cell employed for STM surface stress measurements with a cantilever electrode.
use of such larger cantilevers is reported in this communication. Due to their low noise our quantitative stress data can be differentiated and can thus be compared to thermodynamic considerations.2,7 A schematic illustration of our electrochemical cantilever cell is shown in Figure 1. The shape of the Kel-F © 1996 American Chemical Society
4312 Langmuir, Vol. 12, No. 18, 1996
Figure 2. Voltammogram and surface stress data (full circles, cathodic; open circles, anodic scan), recorded simultaneously (dE/dt ) 20 mV/s), in acid bromide solution (0.1 M H2SO4 + 1 mM CuSO4 + 1 mM CsBr): (a) voltammogram; (b) voltstressogram; (c) first derivative of (b).
cell is an inverted T, with the tip entering through the vertical arm. The two halves of a plug support the cantilever sample on one side of the horizontal arm, while on the other side reference and counter electrodes are introduced. The samples with dimensions 9 × 2 × 0.55 mm3 consist of glass substrates (AF 45, E ) 66 kN/mm2, V ) 0.235, Berliner Glas KG) covered on one side by evaporated chromium (2 nm) and gold (200 nm). They are flame-annealed in the manner described previously.8 All other experimental procedures have also been outlined in previous work.1,2,8-11 (7) See, for example: Lang, G.; Heusler, K. E. J. Electroanal. Chem. 1995, 377, 1. (8) Haiss, W.; Lackey, D.; Besocke, K. H.; Sass, J. K. J. Chem. Phys. 1991, 95, 2193. (9) Haiss, W.; Sass, J. K.; Gao, X.; Weaver, M. J. Surf. Sci. 1992, 274, L593. (10) Haiss, W.; et al. In Atomic Force Microscopy/Scanning Tunneling Microscopy; Cohen, S. H., et al., Eds.; Plenum: New York, 1994; p 423. (11) Haiss, W. PhD Thesis, Technical University Berlin, 1994.
Letters
Figure 3. Voltammogram and surface stress data (full circles, cathodic; open circles, anodic scan), recorded simultaneously (dE/dt ) 20 mV/s), in acid chloride solution (0.1 M H2SO4 + 1 mM CuSO4 + 1 mM CsCl): (a) voltammogram; (b) voltstressogram; (c) first derivative of (b).
The adsorption process that we have studied by our novel technique is underpotential deposition of copper in both bromide- and chloride-containing acid solutions. These systems seem to be quite well understood now,2 not the least because of the coverage data provided by recent chronocoulometry studies.12-15 Briefly stated, in the case of bromide the fairly high coverage at more positive potentials decreases only slightly toward negative potentials, whereas for chloride the variation is somewhat larger. Concurrent with the onset of copper deposition, distinct long-range ordered structures of bromide and chloride are formed. These phases, i.e., the anions, are (12) Shi, Z.; Lipkowski, J. J. Electroanal. Chem. 1994, 369, 283. (13) Shi, Z.; Lipkowski, J. J. Phys. Chem. 1995, 99, 4170, and references therein. (14) Shi, Z.; Wu, S.; Lipkowski, J. J. Electroanal. Chem. 1995, 384, 171. (15) Wu, S.; Shi, Z.; Lipkowski, J.; Hitchcock, A. P.; Tyliszczak, T. Submitted for publication in J. Phys. Chem.
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imaged by STM and seen to persist up to copper monolayer completion.2,16 The voltammogram and the quantitative stress data that we have obtained are shown in Figure 2. Flameannealed gold films on glass substrates have been shown8 to consist of large (111) terraces (with diameters up to several hundred nanometers), which explains the close resemblance of Figure 2a to single-crystal results.2,12,13 Two major stages of copper deposition may be distinguished. The first two peaks in the negative direction result in a fractional coverage, whereas the third peak brings about monolayer completion. The stress curve in Figure 2b can be qualitatively understood along the lines recently proposed for the single crystal films on mica.1,2 By charge depletion of the metal surface, adsorbed anions produce a compressive stress, which upon copper deposition in the submonolayer range is counteracted by charge donation. This electropositive action of the copper is essentially lost, however, when the closed metallic monolayer is formed. The fact that the magnitude of the adsorbate-induced stress changes is somewhat smaller than in the gas phase3 can most likely be attributed to hydration interactions which compete with the chemisorption bond. It is perhaps also instructive to relate the measured values of surface stress changes to the calculated surface stress17 of clean Au(111), which is 2.77 N/m. This qualitative type of surface science interpretation3 of our electrochemical data is not sufficient though, since there are thermodynamic aspects to be taken into account for a solid-electrolyte interface.2,7 Specifically, we have to consider the relevance of the well-known Lippmann equation
dγ/dE ) -σm
(1)
which relates the derivative of the surface stress γ with respect to the electrode potential E to the charge on the metal σm. This equation is rigorously valid for liquid metals in equilibrium with the solution and has been claimed to be adequate also for solid electrodes.18 However, for solid electrodes a second term on the right-hand side of eq 1 may be dominant, which in surface science terms is related to the change of the surface energy upon stretching the interface.17 (16) Toney, M. F.; et al. Phys. Rev. Lett. 1995, 75, 4472. (17) Needs, R. J.; Godfrey, M. J.; Mansfield, M. Surf. Sci. 1991, 242, 215. (18) Mohilner, D. M.; Beck, T. R. J. Phys. Chem. 1979, 83, 1160.
Further differentiation of eq 1 with respect to E provides a convenient test of this aspect
d2 γ/(dE)2 ) -dσm/dE ) I/(dE/dt)
(2)
since the current I is measured in the voltammogram. Upon differentiating our surface stress data the surprising result is that already the first derivative of γ (cf. Figure 2c), which should resemble the charge, has all the signatures of the current. Only the reversal of the sign of the signal at about 0.1 V is reminiscent of the charge. Particularly on the negative scan, however, the ratio of first derivative and current is essentially constant in the range of the two submonolayer peaks, which would not be expected for the charge. In Figure 3 we show that this intriguing result is not unique to the bromide anion. For chloride the behavior of the surface stress is again characterized by a close similarity of the voltammogram to the first derivative of γ.2 As mentioned above, chloride differs from bromide in that a larger variation of the coverage takes place in the region of positive potentials.14 The copper coverage deduced from chronocoulometry data15 shows an increase toward cathodic potentials which is not unlike the bromide case, such that the interpretation of the stress data could follow the same lines here. In conclusion, we have shown that the use of macroscopic cantilevers in conjunction with STM provides low-noise, quantitative surface stress data, which can be analyzed with respect to the thermodynamic equations frequently applied to metal-electrolyte interfaces. Definite evidence has been obtained that these equations do not reproduce the behavior observed for copper underpotential deposition on Au(111) surfaces and that presumably the Lippmann equation has to be augmented for the case of solid, singlecrystal electrodes. The reason why the chronocoulometry procedures,12-15 which are based on the simple Lippman equation, seem to give fairly accurate coverage data is currently under investigation.19 Acknowledgment. Valuable discussions with J. K. Gimzewski and R. Parsons are gratefully acknowledged. We thank R. Nichols for a critical reading of the manuscript. Preparation of the gold films by K. Grabitz was much appreciated. W.H. would like to thank the Deutsche Forschungsgemeinschaft for financial support. LA9602224 (19) Parsons, R. Submitted for publication in Solid State Ionics.
