The Journal of
Physical Chemistry
0 Copyright, 1989, by the American Chemical Society
VOLUME 93, NUMBER 6
MARCH 23, 1989
LETTERS I n Situ Scanning Tunneling Microscopy Studies of the Underpotential Deposition of Lead on Au(ll1) M. P. Green,*'+ K. J. Hanson,t D. A. Scherson,§ X. Xing,§ M. Richter,+ P. N. R O S S ,R. ~ Carr," and I. Lindad Department of Applied Physics, Stanford University, Stanford, California 94305; AT& T Bell Laboratories, 600 Mountain Ave., Murray Hill, New Jersey 07974: Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106; Materials and Chemical Sciences Laboratory, Lawrence Berkeley Laboratory, Berkeley, California 94720; and Stanford Synchrotron Radiation Laboratory, Stanford, California 94305 (Received: December 12, 1988)
A modified scanning tunneling microscope (STM) was used in situ to examine the changes in the surface topography induced by the underpotential deposition of a monolayer of lead on Au( 11 1). Comparison of the STM images before and after deposition shows that an essentially conformal layer of lead covers the gold substrate. However, pit sites and terrace boundaries display topographic changes due to additional lead-on-lead bonding at step edges.
Introduction Considerable progress has been made over the past few years toward the development of scanning tunneling microscopy (STM) as an in situ probe of electrochemical interfaces.' A particular area in which this technique can provide much needed structural information is metal underpotential deposition (UPD). This phenomenon refers to the ability of certain metal ions to undergo deposition on appropriate electrode materials at potentials positive of the reversible potential for bulk deposition of the metal. Underpotential deposition involves one or in some cases up to two monolayers and reflects a higher affinity of the adsorbate atoms
'
Department of Applied Physics, Stanford University, Stanford, CA 94305. 'AT4T Bell Laboratories, 600 Mountain Ave., Murray Hill, NJ 07974. f Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106. Materials and Chemical Sciences Laboratory, Lawrence Berkeley Laboratory, Berkeley, CA 94720. 11 Stanford Synchrotron Radiation Laboratory, Stanford, CA 94305.
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for the foreign substrate than for a surface of the same element. Among the most interesticg aspects of UPD is the possibility of changing in a controlled and reversible fashion the coverage of the adsorbate, making it in principle possible to study detailed aspects of two-dimensional nucleation and growth. This Letter will present in situ S T M results for the UPD of lead on Au( 111). The use of single crystals as electrode substrates is particularly convenient as complicating effects associated with site heterogeneity can to a large extent be avoided. The UPD of lead on Au( 11 1) has been extensively studied by a variety of ex situ and in situ spectroscopic techniques2 and thus affords an ideal model system to be examined.
Ex@menta' Section Au(ll1) films of about 1000-8, thickness were grown epitaxially on freshly cleaved mica surfaces, as described in detail elsewhere.3 (1) Sonnenfeld, R. In Modern Aspects of Electrochemistry; Bockris, J. O M . , Conway, B., Eds., to be published. ( 2 ) Hamelin, A. Mod. Aspects Electrochem. 1986, 16, 1.
0 1989 American Chemical Society
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2182 The Journal oJPhysica1 Chemislry, Vol. 93, No. 6, 1989
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POTENTIOSTAT
Letters
I-----------T , , WORKING ELECTRODE
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I
I
COUNTER TUNNELING
I
I
z w
LL
LL
I
2
-20 0
iI I L
IL_-------
I I I
I
I
0.5
10
1.5
2.0
APPLIED POTENTIAL,(V V I Pb/Pb+21
I
Figure 2. Cyclic voltammagram of lead underpotential deposition on Solution: 5 mM Pb(NO1)*in 0.05 M HCIO,. Sweep rate:
Au(l1 I).
15 mV/s.
- - - - - - -ELECTROCHEMICAL CELL
Figure 1. Schematic representation of electrochemical cell and elm tronics configuration. The tunneling voltage between the tip and gold electrode, V,,is held constant. This induces a tunneling current, I,, to flow across the gap.
The films were transferred to the S T M instrument through air without taking special precautions to exclude oxygen. STM images of these surfaces in air were found to exhibit terraced planes, often pitted with monolayerdeep depressions. In situ STM experiments were performed with a conventional three-electrode electrochemical cell using a platinum counter electrode and a PbjPh” reference electrode. Solutions of 5 mM Pb(NO,), in 0.05 M HCIO, were prepared with water that had been filtered, deionized, and purged with N,. A drop of solution was placed onto the Au( I I I ) film with a syringe and held in place by surface tension. In this arrangement the surface of the solution is exposed to the atmosphere, and therefore effects due to the presence of dissolved gasses, particularly oxygen, could not be avoided (vide infra). Cyclic voltammetry measurements were performed with a Pine Instruments RDE 3 potentiostat. STM images were obtained with a PZT tube scanner driven by a computer-generated waveform capable of producing a 256 X 256 point image every 50 s (5-Hz line scan rate). The data were recorded in the constant-current mode, wherein the images map a surface of constant tunneling resistance. It is especially important with liquid tunneling media, such as electrolytic solutions, to take care that thermal drift rates are within reasonable levels. The cylindrical symmetry of the stainless steel microscope body helped to reduce the drift to typical values of IO 8,jmin. In situ images were obtained by connecting the electrochemical cell and potentiostat to the STM, as shown in Figure I . A constant potential difference between the tip and sample was maintained: hence, the tunneling voltage remained invariant as the working electrode potential was swept. In this mode the tip underwent a parallel potential sweep offset by the applied tunneling voltage. A positive tunneling voltage was always applied so that the potential difference between the tip and the reference electrode was never negative enough to allow deposition on the tip. This configuration is advantageous because a constant tunneling voltage is maintained and spectroscopic effects are not present in the topographic image. However, it has the drawback that as the tip potential changes relative to the reference so does the background electrochemical current measured by the STM. Although this current can he as low as IO pA near the open circuit potential, it can increase lo several nanoamperes at potentials in the UPD region. In order to overcome this difficulty, it was found necessary to tunnel at currents as high 40 nA. Although the background current may be large, it is relatively steady a t fixed working electrode potential. Since the potential was swept slowly with respect to the image scan rate, the images produced were found to be the same as those observed in the absence of an externally applied potential. Tunneling tips capable of affording ex situ
0.05 M HCIO, at 60 mV V, = 200 mV, I, = 2.0 “A. scan size = 400 X 400 A, atomic step height between terraces = 2.4 0.3 A. Figure 3. Au( I I I ) electrode imaged through YS Pb/Pb’+.
*
atomic resolution on Au(l11) were produced by simply manually cutting 20 mil Ptjlr wire. The ionic background current was reduced by coating the tips with fingernail polish, leaving only the end bare, to minimize the tip surface area in contact with the electrolytic solution.
Results and Discussion The cyclic voltammetry curve for lead UPD on Au( 11 I ) is given in Figure 2. Typical features attributed to the deposition and stripping of the lead overlayer (150 mV vs Pb/Pb2+) are superimposed on a negative current background. The latter can be ascribed to the reduction of dioxygen from the atmosphere, an effect that is not expected to modify the nature of the UPD deposits. As the potential was swept in the positive direction, a negative-going peak (arrow) was observed. This phenomenon is specific to the PbjAu(l1 I ) system and has been associated with the reduction of nitrate on the partially lead covered gold surface.’ Integration of the current, after background subtraction, yields atomsjcm’, a value that a deposition coverage of about 1 X compares favorably with that reported by Melroy et al? and earlier by Schultz and Dickertmand for a hexagonal close-packed (hcp) layer. Figure 3 shows a STM image of a Au( 11 I ) electrode obtained in the lead-free HCIO. electrolyte a t 60 mV (vs Pb/Pb2+). In the lower half of this image two plateaus can be seen separated by a 12&8,-wide “valley” located one atomic step below the surface of the plateaus. A monolayer-deep depression or pit (about 20 8, in diameter), adjacent to a monolayer-high “island” (a small raised terrace of gold on the substrate), is visible on the left side of the image. The height of each of these features was measured (4) Domcncch. I. G.; Climent. M.A.; Aldaz, A,: Vazquez, I. L.; Clavilicr. I. 3. Eleelroonol. Chem. 1983, 159, 223. ( 5 ) Mclroy, 0.; Kanamawa. K.; Gordon, J. G.; Buttry.
D. Langmub 1986.
9 697
(3) Pashley. D. W. Ado. Phys. 1965. 14, 327.
(6) Schultz. J. W.; Dickcrtmann.
D. SurJ Sei. 1976, 54. 489
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 2183
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Figure 5. Correspondence between the position of the islands befoorc and after the dcposition of lead can be seen by a superposition of a line drawing showing the position of the gold islands on the substrate (Figure 4 A ) over the image of the lead-covered surface (Figure 4C).
af& (C) underpotential deposition of a lead monolayer. V, = 3 1 mV. I, = 40 nA. scan sizc = 400 X 400 step height = 2.4 + 0.3 A.
A,
to be 2.4 f 0.3 8, after calibration by measuring single and multiple atomic steps of Au(l11) in air. This value agrees with the expected figure of 2.35 A for a single atomic step on Au(lI1). Images of gold electrodes taken under the electrolytic solution reveal the same topographic features seen in air.' It has been found, however, that noise levels increase with more positive electrode potentials, possibly because oxides are present on the surface. (7) Chidsey, C. E.;Loiaeono. D.N.;Sleator, T.;Nakahara, S. .Sur5 Sci. 1988, zoo. 4s.
Figure 4A-C shows images of a 400 X 400 8,electrode region taken during a potential sweep from 600 to 60 mV. Figure 4A shows the stable structure of the gold surface before deposition. This area exhibits a monolayer-high step edge which runs diagonally across the image delineating two terraces. Both the upper and lower terraces contain several small (about 15-,& diameter) islands (marked "I") raised one monolayer and several monolayer-deep pits ("P"). Figure 4 8 shows the S T M image of the same area as the potential of the working electrode is swept from 600 to 60 mV. This potential range brackets the potential at which the UPD peak for lead deposition occurs (150 mV). Since a full image takes 50 s to produce, the potential of the working electrode can change by 750 mV from the top to the bottom of the picture for a potential sweep rate of I5 mV/s. When the potential passes 150 mV (arrow). a dramatic change . . . . shown in - occurs in the topography the S T M image. Figure 4C shows the subsequent STM frame, with the potential heldat 60 mV. The effect of the lead coverage can now be seen over the entire region. The step edge is still visible but is sharper, the terrace surface now appears to be much smoother, the pits have been filled in, and the diameter of the islands has increased to about 60 8,. Again, at this potential the image is stable and, except for a slight thermal drift, is identical with that shown in the lower half of Figure 4B. The images of Figure 4 provide clear evidence that the deposition of a monolayer of lead sharply changes the STM image of the gold surface. Assuming that the observed differences before and after deposition are due solely to topographical changes of a lead adlayer on the gold surface, images can be interpreted in the following way: Lead will adsorb in such a way as to minimize the surface free energy. It does this by filling the pits and by lowering the density of kinks at terrace edges, even though this implies some lead-on-lead bonding and the deposition of somewhat more than a single monolayer. It will also deposit in such a way as to minimize the lattice strain with the gold substrate, with which there is a 21% mismatch. Near a step edge there will be excess lattice strain which could be eased by additional coverage. The growth from the small gold islands into larger lead-covered islands is supported by the fact that there is a correspondence between the location of the islands on the initial surface and on the lead-covered surface, as shown in Figure 5 . The deposition of more than one monolayer in the UPD region is possible (e.& the case of lead on silver8) but has not been observed on this system.
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Other workers report charge transfer corresponding to a single hcp layersg Unfortunately, the uncertainty in the coulometry in the present experiment was too large to substantiate this point. The STM image is not necessarily a representation of only the metal atoms on the surface, as the presence of adsorbed anions could also contribute to the image. The changes shown in Figure 4 were not observed when the potential of the working electrode was swept in the same range in a solution free of lead ions. However, since the potential of zero charge (PZC) shifts about 400 to 500 mV in the negative direction upon the deposition of lead, the adsorption of anions would be enhanced on the leadcovered surface vs the lead-free surface. The use of tunneling microscopy to study monolayer deposition on an electrode surface is very new, and much work is needed before the STM images can be explained definitively. Nevertheless, it is clear that this technique will be able to make a unique (8) Juttner, K.; Siegenthaler, H. Electrochim. Acta 1978, 23, 971 (9) Bewick, A,; Thomas, B. J . Elecrroanal. Chem. 1975, 65, 91 I .
contribution to the study of electrode surfaces.
Summary It has been demonstrated that scanning tunneling microscopy can be an effective in situ technique to study the very early stages of electrodeposition. In situ scanning tunneling microscopy results have indicated that the underpotential deposition of lead on Au( 11 I ) is accompanied by an overall smoothing of the surface features and an increase in the size of the flat terraces. The first of these changes has been attributed to a shift in the potential of zero charge in the negative direction after underpotential deposition which would enhance the specific adsorption of nitrate on the surface. The increase in the size of the terraces results evidently from the adsorption of lead atoms on a lead adlayer at the periphery of the terraces. Acknowledgment. We thank Rex Wright and Lloyd Lacomb for help with the image processing. This research was partially supported by N S F Grant DMR-84-14569, DOE Grant AC0382-ER-13000, and DARPA Contract NOOO14-85-K-0388.
Solute-Dependent Solvent Force Constants for Ion Pairs and Neutral Pairs in a Polar Solvent Emily A. Cartert and James T. Hynes* Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-021 5 (Received: January 23, 1989)
The solvent force constants k characterizing the fluctuations of a polar solvent in the presence of a solute ion pair A'Band a neutral pair AB are determined by molecular dynamics simulation. The origin of the observed difference in the k values and the consequences for electron-transfer rate-reaction free energy gap behavior are discussed.
Introduction In the presence of an immersed solute, a polar solvent continuously fluctuates due to the translational and reorientational motions of its constituent molecules. The concept of a (harmonic) free energy curve governing these fluctuations as a function of a solvent coordinate plays a central role in many aspects of solvation dynamics; examples include electron-transfer reactions,' time-dependent fluorescence,* and heavy particle charge transfers such as SN2 reaction^.^ With a few exception^,^" it is usually assumed that the force constant k characterizing the curvature of the well in this free energy curve is independent of the charge distribution of the solute; the standard dielectric continuum Marcus theory' for electron transfers provides a well-known illustration. In a recent series of papers, Kakitani and Mataga (KM) and co-workerse have argued that k should instead strongly increase with increasing solute charge, and they have predicted significant consequences of this for rate-energy gap behavior for various classes of photochemical electron-transfer reactions. Here we present the first molecular dynamics (MD) computer simulations addressed to this question. Solvation free energy curves for an ion pair (IP) A'B- and a corresponding neutral pair (NP) AB in a model polar solvent are found, and the associated solvent force constants are defined and determined to differ for the IP and the NP. The origin of the difference and some consequences for electron-transfer rates are briefly described. Formulation A convenient microscopic level choice of a solvent coordinate is'.'.* AE, the difference of the potential energy of a given set of 'Permanent address: Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90024-1569.
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solvent molecule configurations in the presence of the IP and NP:
AE =
vsolv,lP
- vsolv.NP
(1)
In the limit where the short-range nonelectrostatic solute-solvent interactions are identical for the IP and NP, the collective coordinate AE is determined exclusively by the electrostatic Coulomb interactions and reduces to the Coulomb interaction energy of the IP solute with the polar solvent molecules in their given configurations. In the point dipole approximation for the solvent molecules, AE would reduce to
A E = - 1 d r P(r)-[EoA(r- rA) + EoB(r - rB)]
(2)
where P(r) is the solvent orientational polarization at r and Eoi(r (1) Marcus, R. A. Annu. Reu. Phys. Chem. 1964, 15, 155. (b) Newton, M. D.; Sutin, N. Annu. Reo. Phys. Chem. 1984, 35, 437. (c) Hynes, J . T. J . Phys. Chem. 1986, 90, 3701. (2) Hynes, J. T.; van der Zwan, G. J . Phys. Chem. 1985, 89, 4181. (3) (a) Gertner, B. J.; Bergsma, J . P.; Wilson, K. R.; Lee, S.; Hynes, J. T. J . Chem. Phys. 1987,86, 1377. (b) Lee, S.; Hynes, J. T. J . Chem. Phys. 1988, 88, 6853. (4) (a) Kakitani, T.; Mataga, N. Chem. Phys. 1985, 93, 381. (b) Kakitani, T.; Mataga, N. J . Phys. Chem. 1985, 89, 4752. (c) Kakitani, T.; Mataga, N. J . Phys. Chem. 1985,89, 8. (5) (a) Kakitani, T.; Mataga, N. J . Phys. Chem. 1986, 90, 993. (b) Kakitani. T.;Mataga, N. J . Phys. Chem. 1987, 91, 6277. (6) (a) Kakitani, T.; Mataga, N. Chem. Phys. Lett. 1986, 124, 437. (b) Hatano, Y . ;Saito, M.; Kakitani, T.; Mataga, N. J . Phys. Chem. 1988, 92, 1008,
( 7 ) Warshel, A. J . Phys. Chem. 1982, 86, 2218. (8) (a) Hwang, J. K.; Warshel, A. J . Am. Chem. SOC.1987, 109,715. (b) Kuharski, R. A,; Bader, J. S.;Chandler, D.; Sprik, M.; Klein, M. L.; Impey, R. W . J . Chem. Phys. 1988, 89, 3248.
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