J. Phys. Chem. 1996, 100, 8617-8620
8617
Observation of Uniaxial Structures of Underpotentially Deposited Cadmium on Au(111) with in Situ Scanning Tunneling Microscopy Joseph C. Bondos, Andrew A. Gewirth,* and Ralph G. Nuzzo* Department of Chemistry and Frederick Seitz Materials Research Laboratory, UniVersity of Illinois, Urbana, Illinois 61801 ReceiVed: February 23, 1996X
The atomic structures of several distinct adlayers of Cd underpotentially deposited onto a Au(111) surface in sulfuric acid electrolyte were revealed by in situ scanning tunneling microscopy (STM). Three ordered adlattices were observed, all of which have a long-range linear morphology rotated by 30° from the substrate’s lattice directions. Complete geometric descriptions and proposed structures are presented for each. A purely electrostatic model is insufficient to explain the banded morphology which is observed; therefore, other structure-forming interactions, including lattice strain, are discussed as possible contributors.
We report results of direct in situ STM imaging studies of Cd underpotential deposition (upd)1-3 on a Au(111) electrode in H2SO4 electrolyte. Three distinct overlayer structures are observed, all of which are defined by a banded morphology. These overlayers display a general transition to more compact overlayer structures with increasingly negative potentials. The striped morphology of the Cd overlayer on the Au(111) substrate is an unexpected and puzzling result and provides a point of contrast to other upd adlayers. Upd of Cd has been investigated primarily with classical electrochemical methods.4-6 Scanning probe microscope images of Cd upd are restricted to work examining Cd on Cu(111) 7 and onto upd adlayers of S, Se, and Te.8-10 In these studies the Cd adlayers are found to reflect the symmetry of the substrate. Cd adlayers on Pt or Pd are found to enhance the latter’s activity toward the oxidation of formic acid and several small alcohols.11-14 Finally, Cd adlayers on Au(111) in acid solution act as a catalyst for the electroreduction of nitrates to nitrites.15,16 There is no structural insight available into the origin of this catalytic activity. The Au(111) substrates used in this work were flame-annealed Au films17 deposited on borosilicate glass (Dirk Schro¨er Inc., Berlin). Aqueous solutions were made of 1 mM CdSO4 (99% from Aldrich, used as received)18 and 0.1 M ultrapure H2SO4 (J. T. Baker, Ultrex, also used as received) in Millipore-Q purified water. Constant current STM measurements were performed with a Nanoscope-E electrochemical scanning tunneling microscope (ECSTM, Digital Instruments, Santa Barbara, CA) according to established procedures.19 The counter and reference electrodes were Pt and Au wire,20 respectively. All of the potentials in this paper are reported versus the bulk Cd deposition potential which is ca. -491 mV versus NHE in 0.1 M H2SO4. This means that Cd upd on Au(111) occurs at the most negative potential of any upd system thus far studied. Figure 1 shows a cyclic voltammogram obtained from a Au(111) surface in an aqueous solution comprised of 0.1 M H2SO4 and 1 mM CdSO4 at a scan rate of 50 mV/s. This CV exhibits two sets of peaks. The peak potentials of the deposition waves occur at 450 and 181 mV versus Cd/Cd2+, while the corresponding stripping peaks are seen at potentials of 465 and 270 mV. In the region positive of the upd peaks, between 700 and 1200 mV, ECSTM images reveal a hexagonal lattice with an X
Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(96)00569-2 CCC: $12.00
Figure 1. Cyclic voltammogram of Cd upd on Au(111) in a solution of 0.1 M H2SO4 and 1.0 mM CdSO4. The scan rate is 50 mV/s.
interatomic spacing of 0.29 ( 0.02 nm. The spacing and symmetry evinced in this image are identical with that anticipated for the Au(111) lattice. This structure was also observed in solutions not containing Cd. In the upd region, three distinct overlayers (I, II, and III) are observed. Overlayer I is visible in the region 200-350 mV, overlayer II is visible in the region 100-300 mV, and overlayer III occurs between 0 and 200 mV. All three overlayer structures exist over large regions of the surface, typically persisting for more than 100 nm. Also, all three overlayer structures exist in the potential range where catalytic activity for nitrate reduction occurs.15,16 Figure 2a shows a 7.5 nm by 7.5 nm STM image of overlayer I obtained at a potential of 250 mV. This image shows a linear morphology with bands, composed of four parallel rows of spots, repeated across the figure. Each type of spot (row) has been labeled as a, b, c, or d. The separation between common spots along the row is 0.45 ( 0.02 nm and the distance between sites measured perpendicular to the row direction on adjacent bands is 1.33 ( 0.02 nm. The closest distance seen in the image, occurring between spots a and d, is 0.26 ( 0.02 nm, while the spacing between spots a and b and spots b and c is 0.43 ( 0.02 nm. Spots a, b, and c are collinear within a band. By comparing the image shown in Figure 2a with those of the bare Au(111) surface, obtained earlier in the same experiment, it is © 1996 American Chemical Society
8618 J. Phys. Chem., Vol. 100, No. 21, 1996
Letters
Figure 2. (a) 7.5 nm × 7.5 nm representative ECSTM image of overlayer I obtained at 250 mV versus Cd/Cd2+. (b) Proposed structure for overlayer I. a spots are denoted by the lightest gray circles, b spots are identified by black circles, c spots are denoted by charcoal gray circles, and the circles for d spots are somewhat darker gray than those for a spots. Spots are shown schematically for clarity of presentation. (c) 7.5 nm × 7.5 nm representative ECSTM image of overlayer II obtained at 225 mV versus Cd/Cd2+. (d) Proposed structure for overlayer II. f spots are identified by black circles, and g spots are denoted by white circles filled with black dots. Spots are again shown schematically.
found that the rows propagate along a direction rotated 30° from the underlying Au(111) lattice directions. An underlying lattice is visible below the a, b, c, and d spots (this is also true of the images of overlayers II and III presented below). This lattice has the spacing, direction, and symmetry of the Au(111) substrate. The dark regions visible in the underlying lattice have the correct dimensions, symmetry, and placement to be straightforwardly assigned to the 3-fold hollows of the underlying Au(111) surface. On the basis of this assumption and the geometry of the ordered adlayer relative to the Au(111) surface, the simple structure shown in Figure 2b is proposed for overlayer I. It should be noted that all of the spots in this structure do not fall on surface sites of the same symmetry and further that all of the spots in Figure 2a do not appear equivalent (the a spots are larger than either the b or c spots, which are about the same size, and are, in turn, much larger than d spots). The apparent coverage of Cd is dependent on which spots are assigned to this species. Inclusion of a spots only gives an apparent Cd coverage of 1/9 relative to Au(111). Addition of type b spots adds another 1/9 of a monolayer of Cd to the apparent coverage. Inclusion of all spots gives an apparent coverage of 4/9. The likely identities of the spots are discussed below. Figure 2c shows a representative 7.50 nm by 7.50 nm image of overlayer II taken at 225 mV. This overlayer also has a linear
morphology with bands of three parallel rows of spots being repeated across the image. As before, the ordering of the overlayer is long-ranged and propagates along a direction rotated 30° from the underlying Au(111) lattice directions. The distance between spots, labeled f or g, along the rows is 0.45 ( 0.02 nm. The distance between rows of f spots, irrespective of whether or not they are separated by a row of g spots, is 0.60 ( 0.02 nm and the closest distance between f and g spots is 0.38 ( 0.02 nm. An angle of 90° is formed between the row direction and a line containing the f spots. Figure 2d shows a structure consistent with the geometric arrangement of the spots seen in the STM image. The f spots are centered on bridging sites while the g spots are atop of Au lattice atoms. The apparent coverage with only f spots being counted is 1/4 and, if the g spots are also included, increases to 3/ of a monolayer. Finally, the f spots appear visually to be 8 larger than the g spots. Figure 3a shows a representative 7.50 nm by 7.50 nm image of overlayer III taken at 200 mV. Overlayer III also shows a striking linear morphology but differs from overlayers I and II in that the bands are not simple groups of parallel rows but rather are bands of a hexagonal overlayer. The closest distance between spots is 0.28 ( 0.02 nm and the bands propagate along a 30° rotated direction. The bands are of varying width and have long-range order. The hexagonal adlayer in the bands appears to be locally (1 × 1) with the adsorbed spots in atop
Letters
Figure 3. (a) 7.5 nm × 7.5 nm representative ECSTM image of overlayer III obtained at 200 mV versus Cd/Cd2+. A hexagon has been superimposed to identify the hexagonal adlattice. (b) Proposed structure for a random area of overlayer III. Overlayer spots (gray circles) are shown schematically for clarity of presentation.
positions. Also, all spots in the adlayer appear to be identical. The varying band width makes the calculation of an apparent coverage impossible. The Cd-Cd nearest-neighbor distance is 0.298 nm,21 which is slightly greater than the 0.288 nm21 Au-Au spacing. This means that if these bands appear to be locally (1 × 1), one of four things must be true. The first is that the overlayer is actually rotated by a very small angle in a manner reminiscent of the structures of Pb or Tl upd overlayers on Ag or Au substrates.22 This does not appear to be the case. A small rotation would be difficult or impossible to detect perpendicular to the row direction due to the narrow nature of the bands. However, one should be able to detect it at the edge of a band by looking along the band’s direction and comparing the placing of the adlayer spots to the spots of the underlying Au(111) lattice. If the image is examined in this way, no rotation is observed. The second possibility is that there is a corrugation normal to the surface. This corrugation would result in a variation in the z dimension that is well within the error of the measurement and thus cannot be confirmed or ruled out on the basis of current data. The third possibility is simply that the overlayer is compressed. This would result in the overlayer having inherent compressive strain. This also cannot be ruled out at this time. Finally, it is possible although unlikely that the overlayer is not composed of Cd. The images reported above raise two related issues. First, this is the first report of an overlayer formed through upd on a
J. Phys. Chem., Vol. 100, No. 21, 1996 8619 (111) surface that has a rowlike structure. Second, the identity of species on the surface in these images is unclear. The open structures observed in other upd systems are often explained to be a result of repulsive, electrostatic forces due to anions coadsorbed with the metal atoms. However, it is impossible for these interactions to be the lone driving forces for the three overlayers seen in this system. These forces are isotropic and cannot give rise to a one-dimensional overlayer on a substrate of this symmetry. There are few adlayers of atomic adsorbates on metals described in the literature that result in a linear/banded morphology, and most of those that are reported are of the missing row type. These systems fit into four main groups. They are the absorption of alkali metals,23-25 hydrogen,26-28 and nonmetal atoms (i.e., O and S) on metals29 and fcc metal reconstructions.30-32 Fcc(111) faces reconstruct into large bands of hexagonal structure typified by the (23 × x3), “herringbone”, reconstruction of Au(111).33,34 Most of the linear overlayers involve some type of substrate metal reconstruction, and very few of these are found on faces of hexagonal symmetry. Missing row(s) structures, such as those referred to above, are not observed in the Cd upd on Au(111) system. Consequently, the literature on these structures is unlikely to be useful in elucidating the reason for the observed upd adlayers assuming a linear morphology. It is known that reconstructions of this type occur in such a manner as to alleviate strain in the surface layer; the atomic density is increased upon the formation of the new surface structure, thereby bringing it closer to the bulk atomic density for the metal.34,35 These considerations may be of importance in the creation of overlayer III. Furthermore, the Au reconstructions have been found to be stable in acidic solution at the potentials of the Cd upd region.30-32 This negative potential range (more negative than in any other upd system studied) may act as an additional driving force for strain alleviation and result in a banded motif similar as in the abovementioned surface systems. Another possibility for the banded morphology in structure III is that the adlayer may be templated by the less dense overlayers (I and II). It is likely that there are spots in overlayers I and II not ascribable to Cd. There are four possibilities for the identity of the non-Cd adlayer spots since only three chemical species in addition to Cd are present at large concentrations or have a literature precedent for their adsorption in a upd system. These species are SO42-/HSO4-, OH-, and Au. The sharp nature of the more positive set of voltammetric peaks indicates the possibility of coabsorption of electrolyte anions along with the Cd at this potential (about 500 mV).36 Therefore, it is possible that some of the spots, especially those seen in overlayer I, are actually SO42- or HSO4- ions. Hydroxide has been found to coadsorb in upd systems,37 and Cd is known to form stable hydroxides in solution of the type CdOH+, Cd(OH)2, and Cd2(OH)3+.38,39 According to the Pourbaix diagram (potential versus pH) for Cd,38 however, these species should occur at conditions far from those utilized in this study and have been discounted in the Cd/Cu(111) system.7 It is unlikely, therefore, that hydroxide accounts for some of the observed spots seen in overlayers I and II. Finally, the suggestion that the upd of an electrodeposited metal results in the reconstruction of the substrate metal is to the best of our knowledge unprecedented. The possibility cannot be totally ignored, though, especially due to the potential range. For a complete accounting of anions and their presence in various potential regions, one would need to perform coulometry with different anion and adatom concentrations at varying values of pH. Such studies are currently in progress in our laboratory.
8620 J. Phys. Chem., Vol. 100, No. 21, 1996 Using a scanning tunneling microscope, we have observed three banded upd Cd adlattices on Au(111). The identities of the observed spots at this point are unknown but appear to be limited to Cd, SO42-/HSO4-, and Au. The origin of these banded structures is unknown but comparison of this system with literature precedent indicates that strain relief along with surface atomic density increases motivated by the rather negative potential range in this system may be a contributing factor. Pure electrostatics due to anions, the main structure determining factor in most upd adlayers, is insufficient to explain this morphology. Acknowledgment. J.C.B. acknowledges an NSF Graduate Student Fellowship (GER-92-53864) and partial support from the National Science Foundation (CHE 9300995). The support of this work by the Department of Energy through the Seitz Materials Research Laboratory (DEFG02-91ER45349) is gratefully acknowledged. References and Notes (1) Magnussen, O. M.; Hotlos, J.; Nichols, R. J.; Kolb, D. M.; Behm, R. J. Phys. ReV. Lett. 1990, 64, 2929. (2) Adzic, R. R. Electrocatalytic Properties of the Surfaces Modified by Foreign Metal Adatoms; John Wiley and Sons: New York, 1984; Vol. 13, p 159. (3) Magnussen, O. M.; Hotlos, J.; Bettel, G.; Kolb, D. M.; Behm, R. J. J. Vac. Sci. Technol. B 1991, 9, 969. (4) Jovicevic, J. N.; Despic, A. R.; Drazic, D. M. Electrochim. Acta 1976, 22, 577. (5) Martins, M. E.; Hernandez-Creus, A.; Salvarezza, R. C.; Arvia, A. J. J. Electroanal. Chem. 1994, 375, 141. (6) Romeo, F. M.; Tucceri, R. I.; Posadas, D. Surf. Sci. 1988, 203, 186. (7) Ge, M.; Gewirth, A. A. Surf. Sci. 1994, 324, 140. (8) Colleti, L. P.; Teklay, D.; Stickney, J. L. J. Electroanal. Chem. 1994, 369, 145. (9) Demir, U.; Shannon, C. Langmuir 1994, 10, 2794. (10) Suggs, D. W.; Stickney, J. L. Surf. Sci. 1993, 290, 375. (11) El-Shafei, A. A.; Abd El-Maksoud, S. A.; Moussa, M. N. H. Z. Phys. Chem. (Munich) 1992, 177, 211. (12) El-Shafei, A. A.; Shabanah, H. M.; Moussa, M. N. H. J. Power Sources 1993, 46, 17. (13) Ritzoulis, G.; Georgolios, N. J. Electroanal. Chem. 1994, 370, 219. (14) Adzic, R. R.; Simic, D. N.; Drazic, D. M.; Despic, A. R. J. Electroanal. Chem. Interfacial Electrochem. 1975, 61, 117.
Letters (15) Huang, H.; Zhao, M.; Xing, X.; Bae, I. T.; Scherson, D. J. Electroanal. Chem. 1990, 293, 279. (16) Xing, X.; Scherson, D. A.; Mak, C. J. Electrochem. Soc. 1990, 137, 2166. (17) Will, T.; Dietterle, M.; Kolb, D. M. The Initial Stages of Electrolytic Copper Deposition: An Atomistic View. In Nanoscale Probes of the Solid/ Liquid Interface; Gewirth, A. A., Siegenthaler, H., Eds.; Kluwer Academic Publishers: Boston, 1995; Vol. 288; pp 137-162. (18) The principal impurity in this compound is water. The Clconcentration of the sample used was
