Electrochemical scanning tunneling microscopy of silver adlayers on

Scanning Tunneling Microscopy Investigation of Silver Deposition upon ... Processes of Porphyrin Adlayers on Iodine-Modified Au(111) in Solution: In S...
0 downloads 0 Views 3MB Size
Download PDF
8780

J. Phys. Chem. 1993,97, 8780-8785

Electrochemical Scanning Tunneling Microscopy of Silver Adlayers on Iodine-Coated Au( 11 1) in Perchloric Acid Solution Satoru Sugita, Takayuki Abe, and Kingo Itaya' Department of Engineering Science. Faculty of Engineering, Tohoku University, Sendai 980, Japan Received: February 23, 1993; In Final Form: June 3, I993

The underpotential deposition (UPD) of Ag on an iodinecoated Au( 11 1) surfacehas been examined in perchloric acid solution by electrochemical scanning tunneling microscopy (STM).A ( 5 x 4 3 ) structure for the iodine adlayer was observed over wide terraces before initiating the UPD of Ag. Two different structures of ( 3 x 3 ) were found for the first UPD layer of Ag. It was expected by STM measurements that the iodine exchanges and exists above the layer of bulk deposited Ag. The structure of iodine on bulk deposited Ag is a simple ( d 3 X d 3 ) R 3 O o with no regularly arrayed phase boundaries.

Introduction The underpotential deposition (UPD) of hydrogen and metal adlayers on well-ordered substrates at potentials more positive than the reversible Nernst potential has long been investigated because it is well known that UPD processes areextremelysensitive to the atomic structure of the substrate surface.'-3 Voltammetric features of the adsorption-desorption processes of hydrogen and variousmetals strongly depend on the crystallographicorientation of the substrate as well as the supporting electrolyte. In order to more fully understand the role of the substrate surface structure on UPD processes, electrochemicalscanningtunnelingmicroscopy (STM) and related techniques such as atomic force microscopy (AFM) have been recognized as the first direct methods for the structural characterization of electrodes in electrolyte solutions under potential contr01.~Atomicresolutionof individualadatoms in the UPD layers have been achieved for Cu,M Ag,9Jo Pb," Hg,I2and Bi13on Au( 111)and Cu14andAg150nPt( 111). Atomic images of strongly adsorbed iodine,l6*3bromine,u and carbon monoxide (CO)2s9uhave also been observed on Pt, Au, and Rh single-crystal electrodes. We have previously reported on the UPD of Ag on Au( 111) in sulfuric acid solution,10and it was noted that the UPD occurs in at least three steps on a Au( 111) surface prepared by the flame-annealingquenching technique. A ( 4 3Xd3)R3Oo structure was observed after the formation of the first UPD layer. On the other hand, a totally different structure has been reported for exactly the same system using AFMS9 For the UPD of Cu on Au( 11l), it has been concluded that the ( d 3 X d 3 ) R 3 O oinitial structure in sulfuric acid solution reconstructs to the (5x5) structure in the presence of chloride The above results indicate that the structure of UPD layers is extremely sensitive to coadsorbed anions such as chloride, bromide, and iodide. Hubbard and co-workershave carried out detailed studies of the UPD of Ag, as well as other metals, on iodine-pretreated Pt( 111) using low-energy electron diffraction (LEED)27-30 and recently developed angular distribution Auger microscopy (ADAM).31*32 They reported ( 3 x 3 ) structures of the first Ag adlayer on an iodine-pretreated Pt( 111). In this paper, we describe the electrochemical STM investigation of the UPD of Ag on an iodine-coated Au( 111) surface in perchloric acid solution. Atomic images of the iodine adlayer on Au(ll1) before and after the UPD of Ag are presented. An atomically resolved iodine structure is also revealed on the bulk deposited Ag layer on the Au( 111) surface.

Experimental Section Au single-crystal beads were prepared at the end of Au wire (99.99%)by a method as described in a previous paper.33 It has 0022-365419312097-8780$04.00/0

been shown that atomically flat terrace-step structures are consistently observed on the (1 11) facet formed on the singlecrystal bead in an octahedral config~ration.~JOJ3One of the (1 11) facets was directly used for STM experiments. Mechanically exposed (1 11) surfaces were carefully prepared with an accuracy in the angle of

II

2.0 -

I

-

A

-

>-

B

2.0 -

I

-

"

0 --

[1io]

6.0 -

r

I

0

I

I

0

I

2.0

I

I

4.0

I

1

I 2.0

I

1

I 4.0

I

1

1

6.0

X/nm

6.0

X/nm

b

I

I

Figure 4. An STM top view of 7.0 X 7.0 nm2 area obtained at 1.05 V. The [ 1 IO] directionof the (1 1 1) surface was indicated by an arrow. The

E 0.05

potential of the tip was 1 .O V. The tunneling current was 10 nA. Scan

f5 0.00

speed was 20 ms/line.

0.0 nm) observed on the I-Au( 111) surface is quite large compared with that of the Au atoms (ca. 0.02 nm) on a Au( 111)-(1X 1) surface as described in our previous paper? However, it is obvious that STM measurementsmust be performedwith minimalthermal drift in x-y directions in order to discriminate between the (d3X43)R3O0 and ( 5 x 4 3 ) iodine adlayer structures because both are very similar in terms of the direction of atomic rows as reported previously.22J3J8 Figure 4 shows an example of a high-resolution STM image of the I-Au(ll1) surface obtained at 1.05 V. The image has been treated using a band-pass filter to reduce the signal noise. It can be observed that there is significant distortion of atomic rows of iodine in the directions, labeled I, 11,and 111. The image cannot be interpreted as the (d3Xd3)R30° structure, which predicts a distanceof 0.50 nm between ordered maxima with 30' rotation relativeto the underlyingAu lattice. This adlayer, rather, has a ( 5 x 4 3 ) structure as proposed previously based on the significant distortion in the atomic rows of I and I11 as indicated in Figure 4. In this structure, the atomic row in the direction labeled I1 is rotated by 30° (*lo) with respect to the Au(ll1) lattice, while the rows of labeled I and I11 are rotated by 34.7O (f1O ) . It can also be seen that the row in the direction labeled I is at an angle of 55' (&lo)relative to that of 11. An angle of 60' is expected for the (43Xd3)R30° structure. In addition to this rotational distortion, the average distance between two nearest-neighbor iodineatoms is found to be 0.49 nm (f0.02 nm) in the row labeled I1 and 0.43 nm (h0.02 nm) in the rows labeled I and 111. Thisobservationis consistentwith the (5x43) structure proposed pre~iously.2~~~3~3~ Note that no appreciable change in imagewas observed when theelectrodepotential wasvaried within the double-layer region. The ( 5 x 4 3 ) structure has consistently been observed in a potential region between 0.2 and 1.2 V. Therefore, we can conclude that the iodine adlayer on Au( 111) has the (5x43) structure in pure HClO4 solution in the absence of iodide. Exactlythe samestructure has been observedby Weaver and co-workers at a limited potential region in a HC104 solution in the presenceof iodide.2u3 In addition to the ( 5 x 4 3 ) structure, they also observed a high-coverage iodine layer with a (7x7)R21.8O at potentials more positive than 0.3 V vs SCE in a solution containing 0.5 mM NaI.22923However, we found in this study that a similar structure of (7X7)R21.8' with a long-range hexagonal pattern appeared at potentials more positive than ca. 1.3 V,near the onset of the oxidation of the iodine layer as shown in Figure 1. More interestingly, the formation of pits as well as

C E 0.05 f5

0.00

t

0.1

0.2 X/nm

I

I

0.1

0.2

0.3

0.4

I

0.4 Wnm Figure 5. STM top views (a) of 7.0 X 7.0 nm2 area obtained at 0.9 V in a 0.1 M HC104 + 1 mM AgC104 solution. The corrugations(c and d) along the atomic rows A and B, respectively. The potential of the tip was 1.2 V. The tunneling current and scan speed were the same as in Figure 4.

0.0

0.3

single atomic defects was observed at ca. 1.4 V. This observation is believed to be due to a partial oxidation of the iodine layer exposing the underlying substrate. Details of the oxidation of the adsorbed iodine are of our special interest. Atomic Structureof UPD Layers. The structure of the iodine adlayer was first examined before starting the UPD of Ag in the solution of 0.1 M HC104containing 1 mM AgC104. When the electrode potential was held at 1.2 V,the ( 5 x 4 3 ) structure was consistently observed over the wide terraces indicating that no structural changeoccurs in the iodineadlayer even in the presence of Ag+. Moreimportantly, it was found that the (5x43) structure persists even when the electrode potential was very slowly scanned in the negative direction to 1.15 V. As discussed above, a large amount of charge (ca. 65 pC/cm2) is already consumed between 1.3 and 1.15 V, prior to the first Ag UPD peak. This suggests that the shoulder that appears before the first peak (I) is not due to a simple UPD of Ag. Some interaction between the iodine and Ag+ in the solution might be occurring which gives rise to additional double-layer charging current. Completely different atomic images appeared soon after the electrode potential was passed through the first peak (I). Figure 5a shows an example of STM image obtained at 0.9 V. The orientation of the single crystal of Au( 111) with respect to the scan direction of the piezoelectric tube was not identical to that for the determination of the structure of I-Au( 111) as shown in Figure 4. We found in Figure Sa that the atomic rows are almost completely parallel to those of Au( 111)within experimental error (f 1O ) . The observed atomic distance is 0.43 nm (f0.02 nm). Figure Sb,c presents the corrugations along the atomic rows (A and B) indicated by the arrows in Figure Sa. An ordered

STM of Ag Adlayers on Iodine-Coated Au( 111)

The Journal of Physical Chemistry, Vol. 97, No. 34, 1993 8783 [1 TO]

I

U-'

I

Figure 6. An illustrative depiction of Au(l11)(3X3)-1, Ag (dl = 8, = 4/9h

corrugation with an almost uniform height of ca. 0.06 nm can be seen along the atomic row labeled A in Figure Sa. On the other hand, two different kinds of alternatingatoms appear alternatively along the row labeled B. It is clear that the two different alternatingrows exist alternativelyin all three directionsuniformly after the first UPD process. Hubbard and co-workers have previously proposed two (3x3) structures for the first UPD layer of Ag on an I-Pt( 111) surface based on LEED a n a l y s i ~ .Figure ~ ~ ~ ~6 shows a depiction of a (3x3) structure. In this model, four Ag atoms underneath the iodinelayer are positioned directly on top of the Au atoms making up the four corners of the unit cell. The other four atoms in the unit cell are located at bridge sites. Iodine atoms are simply sitting at the 3-fold hollow sites in the Ag layer. Therefore, the three iodine atoms located near each corner of the unit cell appear as image features higher than the iodine atoms sitting in the center of the unit cell. The STM data shown in Figure 5 are in agreement with this model. The surface coverage of Ag of the (3x3) structureis equal to 4/9. The number of atoms in an ideal Au(ll1)-(1x1) surface is 1.39 X 1015/cm2,which corresponds to 223 pC/cm2, assuming the formation of an epitaxial layer of Ag with a 1:l ratio of Ag to Au atoms involving one-electron transfer to Ag+. Consequently, the expected charge consumed for the first UPD of Ag to form the (3x3) structure should be equal to 99.1 pC/cm*. The experimental value of 70 pC/cm2 is in rough agreement with the theoretical value. It must be noted that a differently oriented (3x3) structure was sometimesobserved on the same surface,although the (3x3) structure shown in Figure 5 seemed to be the predominant one after the formation of the first Ag layer. Figure 7 shows an example of STM images for an alternate (3x3) structure. The image has been digitally filtered to remove substantial noise. It was not easy to find domains showing the new (3x3) structure. However, it can be seen in the image shown in Figure 7 that the four corners in the (3x3) adlattice appeared as bright spots with an equal corrugation height. Weak spots could be found in between the two bright spots. It is also clear that the atomic rows are parallel to those found in Figure 5. Theseobservationssuggest that the second type of the (3x3) structure may correspond to a previously proposed (3x3) structure with a surface coverage of OAg = 5 / 9 and 81 = 4/9 on Pt(ll1) (Figure 2d in ref 28). Nevertheless, we feel that more experiments will be necessary to clearly resolve the image of this second (3x3) structure, because only few our trials presented this atomic image. It was also observed that pits could be seen in the large-area STM images after the formation of the first Ag layer. Figure 8 shows an STM image acquired in area of 25 X 25 nm2 where the (3x3) structure shown in Figure 5 can be still seen on the

I

I

0

2.0

I

1

I

1

6.0

4.0

X/nm Figure 7. An STM top view of 7.0 X 7.0 nm2 area obtained at 0.9 V. The tunneling current and scan speed were the same as indicated in

Figure 5.

2ol :]

I

>- 10

O-J I

0

I

I

10

I

I

I

20

X/nm Figure 8. An STM top view of 25 X 25 nm2 area obtained at 0.9 V.The tunneling current and scan speed were the same as indicated in Figure 5.

brighter terraces. The pits, appearing as darker areas, have basically triangular or hexagonal shapes where the step edgesare parallel to the atomicrowsof iodineof the (3x3) structure. These pits completely disappeared, leaving very flat terraces when the first Ag layer was stripped away by changing the electrode potential from 0.9 to 1.3 V. The ( 5 x 4 3 ) structure of the I-Au adlayer shown in Figure 4 can be restored at the potential of 1.3 V. The appearance of the pits during the first UPD most likely results from a change in the surface coverageof iodine (81) during the UPD of Ag. The ( 5 x 4 3 ) structure of iodine has a 81 value of 0.40. On the other hand, the surface coverages of Ag and iodine of the (3x3) structure shown in Figure 6 are equal to 4/9. It is apparent that 81 of the (3x3) structure is larger than that of the (5x43). Therefore, we can expect that the iodine layer on Au( 111) must rearrange to the (3x3) structure to accommodate the Ag atoms during the first UPD, leaving areas with lower coverage of iodine. Atomic structures inside the pit areas could not clearly be resolved, although some ordered structures can be seen near the step edge and below letter A shown in Figure 8. A variable height in the pit's step edge was observed. A height of ca. 0.1 nm is found for pit (A) which appears brighter than (B)as shown in Figure 8. In a darker pit (B), the step height is about 0.2 nm. These observations suggest that iodine atoms with lower coverages also exist in the pits. It is obvious that the surface coverage of iodine in the pit is not uniform. Such iodine atoms are thought to diffuse in the pits. Note that the UPD of

8704 The Journal of Physical Chemistry, Vol. 97, No. 34, 1993 [1io]

6.0 -

4.0

f

-

E

5 2.0 -

0-

I 0

I

I

2.0

I

I 4.0

I

1

1

6.0

X/nm

Figure 9. An STM top view of 7.0 X 7.0 nm2 area of a bulk deposited Ag surface obtained at 0.5 V. The other conditions were the same as in Figure 5.

Ag should occur even in the pits because the observed step heights are lower than that expected for an iodine adlayer atomic step. When the electrode potential was further scanned in the negative direction and held at 0.8 V, it was importantly found that the (3x3) structure as shown in Figure 5 was consistently observed. No evidencewas found in the atomic resolutionimages for further deposition of Ag on the surface with the (3x3) structure. In the case of the UPD of Ag on an I-Pt(l1 l), a changein theLEEDpattem from (3x3) to( 18x18)has previously been reported for the second UPD.Z7-30 Several model structures for (18X 18) were proposed by adding Ag atoms on the top of the iodine layer with a (18X 18)periodi~ity.2~~299~~ In lower-resolution images acquired over large area, we observed islands after the second UPD. Islands with heights of ca. 0.1 nm appeared on the atomically flat terrace. The diameter of these islands was in the range 2-3 nm. This observation suggests that further UPD of Ag might be occurring on the areas having lower iodine coverage (observed after the first UPD peak, Figure 7), although it was not clearly resolved in this study that such islands were mainly formed in the holes shown in Figure 8. More detailed investigations are needed for further discussion. Bulk Deposition of Ag. Because the third UPD peak is very close to the onset of the bulk deposited of Ag, it was not possible in this work to determine the atomic structure of the third UPD (111) layer. A progressive growth of the bulk deposited Ag layer on Au( 111) has already been observed in lower-resolution images when the electrode potential was held at near 0.69 V. In addition, an unstable step motion was observed at the equilibrium potential which is thought to bedue to a relatively fast equilibrium reaction (Ag+ e- Ag) at the step site similar to that reported in our previous work. 10 To eliminate the complicationdue to the equilibrium reaction and further deposition of Ag during the STM observation, the surface with bulk deposited Ag was investigated in a fairly dilute solution of Ag+, less than 0.01 mM. The bulk deposition of Ag was carried out at 0.5 V while the tip was retracted from the surface to suppressthe screening effect of the tip on the diffusion of Ag+. After the deposition of ca. 25 equivalent layers, STM images were acquired at the same potential. Monatomic step and terrace structures were consistently observed over an area of 150 X 150nm2in low-resolution images, suggestingthat the bulk deposition of Ag occurs epitaxially as previously observed for the Ag deposition on a clean Pt(ll1) surface.ls However, a very clear atomic structure could be seen in high-resolution images acquired on atomically flat terraces. Figure 9 shows a top view of an area of 7.0 X 7.0 nm2. The corrugation amplitude is ca. 0.06 nm. In comparison with the STM images showed in Figure

+

Sugita et al. 5, the image in Figure 9 can be interpreted as the ( 4 3 x 4 3 ) R30°, because the atomic rows are rotated nearly 30° with respect to those of Au( 111) and the atomic distance is ca. 0.48 nm. No significantdistortion in the atomic rows can be found on the bulk deposited Ag surface. The above observation makes it possible to draw a conclusion that the iodine layer with the ( 4 3 x 4 3 ) R30° structure exists as the outermost layer on the Ag surface. This conclusion is consistent with LEED and Auger analysis of the UPD of Ag on Pt( 111) ~ystem.~~-~O However, it must be pointed out that a nearly perfect (d3Xd3)R3Oo structure, as shown in Figure 9, is extended over a quite large area. Hubbard and co-workers reported LEED patterns for a bulk deposited Ag surface on iodine-adsorbedPt( 111) surface as well as an iodine layer on a bulk single crystal of Ag( 11l).39 The LEED pattern was interpreted as the (2/3Xd3)R30°, which is consistent with our present observation, but showed a distinctive 1/ 17 index triangular ~ p l i t t i n g . ~ ”According ~ ~ ~ ~ ~ to a LEED simulation calculation, they proposed a model structure of a (d3X43)R3O0 array with antiphase boundaries.39 On the basis of more recent ADAM measurements, Hubbard and co-workers have also proposed a new structure for the Pt( 11l)/Ag/I system which does not contain antiphase domains but still produces a LEED pattern with split 4 3 spots.32 As described above, we did not resolvesuch antiphase boundaries in our STM images. It has been shown in our previous worklothat the first close-packed Ag layer is epitaxially formed on the Au(ll1) in a sulfuric acid solution, because the lattice constants of Ag and Au agree to within 0.3%. An almost completecommensuracycan be expected for the Ag layers on Au(ll1). Finally, it is noteworthy that a complete correspondencein the STM images has been observed during the anodic stripping of the Ag layers. After achieving the atomic resolution of (d3Xd3)R3Oo on the bulk deposited Ag layer, the (3x3) and ( 5 x 4 3 ) structuresreappeared after the I11and I strippingpeaks, respectively. The observed ( 5 x 4 3 ) structure after the potential cycle was almost identical to that shown in Figure 4. No clear defects in the ( 5 x 4 3 ) structure were observed. The above result strongly indicates that a stable iodine layer exists on the top layers of Ag and Au during the deposition and dissolution of Ag. Conclusions

It has been reported in our previous paper that the UPD of Ag on a clean Au( 111) occurs in three different steps in sulfuric acid.10 It has been shown in this study that the UPD of Ag occurs similarly in three steps on an I-Au( 111) in perchloric acid. However, the atomic structures reported in this paper are totally different from those observed on a clean Au( 111) surface. The structure of iodineadsorbed on Au( 111) is predominantly ( 5 Xd3) rather than (d3Xd3)R3O0. This structure is found to exist at all potentials in the double-layer region in 0.1 M HClOd in the absence of Ag+ and iodide in the solution. In the presence of Ag+, the same iodine structure is observed before the initial UPD peak. This structureistransformed to the (3x3) structuresduring the first UPD peak. The (3x3) structuresis consistentlyobserved even after the second UPD peak, suggesting that the second UPD occurs at defect areas with lower densities of adsorbed iodine. The structure of the third UPD layer could not be resolved in the present study. However, a clear atomic image appears after the bulkdeposition of Ag, indicating that the iodineatomsare located on the outermost layer of Ag with a (d3Xd3)R3O0 structure. The STM image showing the (d3Xd3)R3Oo structure shows no evidence of the existence for antiphase boundaries. Exactly the same STM images are observed during the anodic stripping of deposited Ag, indicating that the iodine adlayer is stable on the electrode surface during the deposition and dissolution cycle of Ag. Acknowledgment. We thank Dr. G. M.Swain for his help in writing the manuscript. The authors also express their thanks

STM of Ag Adlayers on Iodine-Coated Au( 11 1) to Drs. A. T. Hubbard, D. G. Frank, and C. A. Doyle (University of Cincinnati) for sending us their helpful comments on this manuscript and their paper32prior to its publication. The work was supported by the Ministry of Education, Science and Culture, Grant-in Aid for Research No. 04241103.

References and Notes (1) Kolb, D. M. In Advances in Electrochemistry and Electrochemical Engineering, Gerisher, H., Tobias, C. W., Eds.; Wiley: New York, 1978; Vol. 11, p 125. (2) Conway, B. E. Prog. Surf.Sci. 1984, 16, 1. (3) Clavilier, J.; Rodes, A.; Achi, K. E.I.; Zamakhchari, M. A. J . Chim. Phys. 1991,88, 1291. (4) (a) Cataldi, T. R. I.; Blackham, I. G.; Briggs, G. A. D.; Pethica, J. B.; Hill, H. A. 0. J. Electroanal. Chem. 1990,90, 1. (b) Christensen, P. A. Chem. Soc. Rev. 1992, 197. (5) Magnussen, 0. M.; Hotlos, J.; Nichols, R. J.; Kolb, D. M.; Behm, R. J. Phys. Rev. Lett. 1990, 64, 2929. (6) Magnussen, 0. M.; Hotlos, J.; Beitel, G.; Kolb, D. M.; Behm, R. J. J. Vac. Sci. Technol. 1991, 89, 969. (7) Hachiya, T.; Honbo, H.; Itaya, K. J . Electroanal. Chem. 1991,315, 275.

( 8 ) Manne, S.; Hansma, P. K.; Massie, J.; Elings, V. B.; Gewirth, A. A. Science 1991, 251, 183. (9) Chen, C.-H.; Vesecky, S. M.; Gewirth, A. A. J. Am. Chem. SOC. 1992, 114, 451. (10) Hachiya, T.; Itaya, K. Ultramicroscopy 1992, 42-44, 445. (11) Tao, N. J.; Pan, J.; Li, Y.; Oden, P. I.; DeRose, J. A.; Lindsay, S. M. Sur/. Sci. Lett. 1992, 271, L338. (12) Chen, C.-H.; Gewirth, A. A. Phys. Rev. Lett. 1992, 68, 1571. (13) Chen, C.-H.; Gewirth, A. A. J . Am. Chem. SOC.1992, 114, 5439. (14) Sashikata, K.; Furuya, N.; Itaya, K. J . Electroanal. Chem. 1991, 316, 361. (15) Kimizuka, N.; Itaya, K. Faraday Discuss., in press. (16) Schardt, B. C.; Yau, S.-L.;Rinaldi, F. Science 1989, 243, 1050. (17) Yau, S.-L.; Vitus, C. M.; Schardt, B. C. J . Am. Chem. SOC.1990, 112, 3677.

The Journal of Physical Chemistry, Vol. 97, No. 34, 1993 8785 (18) Chang, S.-C.; Yau, S.-L.;Schardt, B. C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 4787. (19) Vogel, R.; Bartruschat, H. Surf.Sci. Lett. 1991, 259, L739. (20) Vogel, R.; Kamphausen, I.; Barkuschat, H. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 525. (21) McCarley, R. L.; Bard, A. J. J . Phys. Chem. 1991, 95,9618. (22) Haiss, W.; Sass, J. K.; Gao, X.;Weaver, M. J. Surf. Sci. Lett. 1992, 274, L593. (23) Gao, X.;Weaver, M. J. J. Am. Chem. SOC.1992, 114, 8544. (24) Tao, N. J.; Lindsay, S.M. J. Phys. Chem. 1992,96, 5213. (25) Vitus, C. M.; Chang, S.-C.; Schardt, B. C.; Weaver, M. J. J . Phys. Chem. 1991,95,7559. (26) Yau, S.-L.; Gao, X.;Chang, S.-C.; Schardt, B. C.; Weaver, M. J. J. Am. Chem. SOC.1991, 113,6049. (27) Hubbard, A. T. Chem. Rev. 1988, 88, 633. (28) Stickney, J. L.;Rosasco, S.D.; Song, D.; Schardt, B. C.; Hubbard, A. T. Surf. Sci. 1983, 130, 326. (29) Hubbard,A.T.;Stickney, J. L.;Rosasco,S.D.;Soriaga,M.P.;Song, D. J. Electroanal. Chem. 1983, 150, 165. (30) Wieckowski, A,; Schardt, B. C.; Rosasco, S. D.; Stickney, J. L.; Hubbard, A. T. Surf.Sci. 1984,146, 115. (31) Batina, N.; Chyan, 0. M. R.; Frank, D. G.; Golden, T.; Hubbard, A. T. Natunvissenschaften 1990, 77, 557. (32) Frank, D.G.;Chyan,O. M. R.;Golden,T.;Hubbard,A. T. Submitted

for publication. (33) Honbo, H.; Suaawara, S.;Itaya, K. Anal. Chem. 1990. 62, 2424. (34) (a) Angerstein-Kozlowska, H.i Conway, B. E.; Hamelin, A.; Viciu, L. S. Electrochim. Acta 1986, 31, 1051. (b) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Viciu, L. S.J. Elecrroanal. Chem. 1987, 228, 429. (35) Rodriguez, J. F.; Mebrahtu, T.; Soriaga, M. P. J. Electroanal. Chem. 1987, 233, 283. (36) Rodriguez, J. F.; Soriaga, M. P. J.Electrochem. Soc. 1988,135,616. (37) Paffett, M. T.; Campbell, C. T.; Taylor, T. N. Langmuir 1987, I, 741. (38) Bfavo, B. G.; Michelhaugh, S.L.; Soriaga, M. P.; Villegas, I.; Suggs, D. W.: Sticknev. J. L. J. Phvs. Chem. 1991. 95. 5245. (39) Salaita, G. N.; Lu, F:; Davidson,L.L:; Hubbard, A. T. J. Elec?roanal. Chem. 1987, 229, 1.