Examination of Underpotential Deposition of Copper on Pt(111

Effects of Anions on the Electrodeposition of Cobalt on Pt(111) Electrode. Yenchung Kuo , Weicheng Liao , and ShuehLin Yau. Langmuir 2014 30 (46), 138...
0 downloads 0 Views 680KB Size
Download PDF
Langmuir 2001, 17, 4627-4633

4627

Examination of Underpotential Deposition of Copper on Pt(111) Electrodes in Hydrochloric Acid Solutions with in Situ Scanning Tunneling Microscopy Ze-Lin Wu and Shueh-Lin Yau* Department of Chemistry, National Central University, ChungLi, Taiwan, Republic of China Received October 3, 2000. In Final Form: February 6, 2001 In situ scanning tunneling microscopy (STM) has been used to examine underpotential deposition (UPD) of Cu at Pt(111) electrodes in the solutions of 0.01 M HCl and 1 mM Cu(ClO4)2. Cyclic voltammetry reveals two well-defined features at 0.72 and 0.55 V (vs RHE), where a tailing phenomenon is noted for the former peak. In situ STM imaging reveals a disorder-to-ordered transition of the adlayer, as the electrochemical potential of Pt(111) was stepped from 0.8 to 0.7 V to facilitate the deposition of a sub-monolayer of Cu adatoms. The ordered adlattice can be approximately characterized as (4 × 4), whose irregular intensity modulation of the STM atomic features indicates its incommensuratity. Toward the end of the first UPD feature, deposition of Cu continues, resulting in reconstruction of the (4 × 4) adlattice to (x7×x7)R19.1°. The latter structure is then stable toward further negative potential stepping to the end of the second UPD wave. In situ STM imaging turns fuzzy when the electrochemical potential of Pt(111) is made more negative to the Nernst value. Varying the operation parameters of the STM can result in atomic structures of not only the upper layer of chloride but also the lower metallic adlayer or the Pt(111) substrate.

Introduction Underpotential deposition (UPD) of metal has received much attention in the modern study of interfacial electrochemistry because of its use to tailor the physical and electronic structures of electrodes to enhance their electrocatalytic properties.1-3 UPDs of Cu at Au(111) and Pt(111) are regarded as the model systems, which have been extensively examined by ex situ techniques, such as lowenergy electron diffraction (LEED),4 X-ray photoelectron spectroscopy (XPS),4 Auger electron spectroscopy (AES),5 and reflection high-energy electron diffraction (RHEED),5 and in situ means such as scanning tunneling microscopy (STM),6-8 surface X-ray scattering (SXS),9,10 rotating ringdisk electrode (RRDE),11,12 and chronocoulometry.13 The roles of anions in the UPD processes prove to be one of the important aspects of metal deposition. Depending on the relative interactions among anion, metal adatom, and substrate, anions can act as promoters or competitors for metal deposition. For example, sulfate * To whom correspondence may be addressed. E-mail: sly@ rs250.ncu.edu.tw. TEL: 886-3-4227151-5909. FAX: 886-3-4227664. (1) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 267. (2) Adzic, R. R. In Advances in Electrochemistry and Electrochemical Engineering; Gericher, H., Tobias, C. W., Eds.; Wiley-Interscience: New York, 1984; Vol. 13, pp 159-260. (3) Kolb, D. M. In Advances in Electrochemistry and Electrochemical Engineering; Gericher, H., Tobias, C. W., Eds.; Wiley-Interscience: New York, 1978; Vol. 11. (4) Michaelis, R.; Zei, M. S.; Zhai, R. S.; Kolb, D. M. J. Electroanal. Chem. 1992, 339, 299. (5) Zei. M. S.; Wu, K.; Eiswirth, M.; Ertl, G. Electrochim. Acta 1999, 45, 809. (6) Sashikata, K.; Furuya, N.; Itaya, K. J. Electroanal. Chem. 1991, 316, 361. (7) Matsumoto, H.; Oda, J.; Inukai, J.; Ito, M. J. Electroanal. Chem. 1993, 356, 275. (8) Wu, Z.-L.; Zang, Z.-H.; Yau, S.-L. Langmuir, 2000, 16, 3522. (9) Tidswell, I. M.; Lucas, C. A.; Markovic, N. M.; Ross, P. N. Phys. Rev. B 1995, 51, 10205. (10) Lucas, C. A.; Markovic, N. M.; Ross, P. N. Phys. Rev. B 1997, 56, 3651. (11) Markovic, N. M.; Ross, P. N. Langmuir 1993, 9, 580. (12) Markovic, N. M.; Gasteiger, H. A.; Lucas, C. A.; Tidswell, I. M.; Ross, P. N. Surf. Sci. 1995, 335, 91. (13) Shi, Z.; Lipkowski, J. J. Electroanal. Chem. 1994, 365, 303.

anions act as the templates for the Cu deposit at Au(111) and Pt(111) at the first stage of UPD.6,8,10,13 In contrast, the much stronger interaction between sulfate anions and Rh(111) delays Cu deposition.8 Our previous in situ STM study indicates that UPD of Cu results in a long-ranged ordered (x3×x7) sulfate structure at the onset of bulk deposition of Cu, suggesting that the Cu deposits are likely to be partially charged throughout the UPD processes.8 While the study of the processes of Cu UPD on Pt(111) in sulfate medium finally approaches a status of consenece,8,13 the results obtained in chloride solutions are not all consistent among workers. Rigorous ex situ and in situ studies performed in the past decade have led to different interpretation of the UPD process of Cu onto Pt(111) in the presence of chloride. Zei employed ex situ LEED and RHEED techniques to show the deposition of a full monolayer of Cu at the end of first UPD peak, whereas the second feature is associated with the desorption of chloride, rather than the deposition of Cu.4,5 On the other hand, Markovic et al. used RRDE and SXS to show the continuous deposition of Cu throughout the two UPD features.11,12 One of the possible explanations for these inconsistencies can be associated with the issues of the integrity of emersed electric double layers. The results obtained with in situ means can give more realistic pictures of the electrified interfaces. In addition to studying the adsorption of halide, pseudohalide, sulfate, and organic compounds adsorbed at single-crystal electrodes of Pt, Au, Rh, Pd, and Ir,14 in situ STM was already used to study the structures of Cu UPD at Pt(111) in sulfate- and chloride-containing solutions. The results are however sketchy and insufficient to shed insight into the system.7 The difficulty can arise from the main drawback of STM, the limitation of imaging the uppermost adatoms only. Indeed, in situ STM results in the coadsorbed chloride and sulfate anions atop the Cu deposit at Pt(111), but the Cu adatoms remain hidden to the STM.7,8 On the other hand, it is demonstrated that in some cases in situ STM can selectively image layers at (14) Itaya, K. Prog. Surf. Sci. 1998, 58, 121.

10.1021/la001398f CCC: $20.00 © 2001 American Chemical Society Published on Web 06/22/2001

4628

Langmuir, Vol. 17, No. 15, 2001

Wu and Yau

different depths. For example, the STM can image either the upper organic layers or iodine chemisorbate on Au(111) electrodes.15,16 We show that similar selective imaging can be achieved for the UPD of Cu at Pt(111) in chloride-containing solutions. Experimental Section Pt(111) electrodes were prepared according to a procedure described elsewhere.17,18 The final stage of sample preparation involved annealing the electrodes with a hydrogen flame, followed by rapid quenching in hydrogen-saturated Millipore water. It is important to have a thin water film on the electrodes to prevent contamination in the ambient. Mounting Pt electrodes onto the STM cell could be accomplished within 10 s. All the glassware was cleaned with a hot chromic acid bath, and extreme caution is needed to get high-quality Pt surfaces. Ultrapure hydrochloric acid, sulfuric acid, and copper sulfate were purchased from Merck Inc. (Darmstadt, Germany). The platinum and rhodium wires (purity >99.99%) used for preparing single-crystal electrodes were obtained from Goodfellow Cambridge Limited (Cambridge, U.K.). They were used as received without further purification. Millipore triple-distilled water (resistivity >18.2 MΩ) was used to prepare all the needed solutions. The electrochemical cell consisted of a one-compartment, three-electrode configuration employing a reversible hydrogen electrode (RHE) as the reference electrode and a Pt wire as the counter electrode. The potentiostat was a commercial CHI 700. The STM was a Nanoscope-E (Santa Barbara, CA) and the tip was made of tungsten (0.3 mm diameter) prepared by electrochemical etching in 2 M KOH. After thorough rinsing with water and acetone, a tip was further painted with nail polish for insulation. The leakage current of the tip at the open circuit potential was less than 0.05 nA. More than 80% of the as-prepared tips yielded a good resolution. The scanner was Digital Instruments-made A head. Reversible hydrogen electrodes (RHEs) were used in the electrochemical and STM measurements, and all the potentials in the next section refer to a RHE scale.

Results Cyclic Voltammetry. Figure 1 provides an overview of the cyclic voltammograms (CVs), obtained for wellordered Pt(111) electrodes in three electrolytes, containing 1 mM CuSO4 + 50 mM H2SO4 (dotted line), 1 mM CuSO4 + 0.1 mM KCl + 50 mM H2SO4 (thin line), and 1 mM Cu(ClO4)2 + 0.01 M HCl (thicker line). All the CV profiles contain two features, appearing at different potentials. These peaks are readily attributed to the UPD of Cu. In a 0.05 M sulfuric acid solution UPD of Cu results in two sharp current spikes, not fully separated even at a potential scan rate of 1 mV/s. Adding 0.1 mM potassium chloride to this sulfuric acid solution results in shifting and broadening of these two UPD peaks. In 0.01 M hydrochloric acid, the peak separation nearly doubles. These results are consistent with those reported by others.4,5,11,12 This marked effect of chloride on the deposition of Cu is explained by the formation of a stable CuCl(111) compound layer at the potential region of the first peak.11,12 In contrast, a roughened Pt(111) electrode effected by potential excursion to 1.4 V results in much broader UPD peaks, implying that the degree of ordering of the Pt(111) substrate is important in the formation of the CuCl thin layer. Close inspection of the shape of the first UPD peak reveals a sharp and symmetric peak in 0.05 M sulfuric (15) Kunitake, M.; Batina, N.; Itaya, K. Langmuir 1995, 11, 2337. (16) Sashikata, K.; Sugata, T.; Sugimasa, M.; Itaya, K. Langmuir 1998, 14, 2896. (17) Clavilier, J.; Rodes, A.; El Achi, K.; Zamakhchari, M. A. J. Chim. Phys. 1991, 88, 1291. (18) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Am. Chem. Soc. 1997, 101, 33547.

Figure 1. Cyclic voltammograms at 1 mV/s for Pt(111) electrodes in 0.05 M H2SO4 + 5 mM CuSO4 (dotted line), 0.05 M H2SO4 + 0.1 mM KCl + 5 mM CuSO4 (thin line), and 0.01 M HCl + 1 mM Cu(ClO4)2 (thick line).

acid, but with a marked tailing in 0.01 M HCl. This latter feature was also observed and reported by others,12 so that it is not due to an edge effect or poor quality of the Pt(111) electrodes used in our measurements. It is believed that the different shapes of the UPD peaks seen in Figure 1 have physical significance, suggesting unlike Cu deposition processes in these electrolyte solutions. This is in contrast to the traditional view that two UPD peaks means the formation of two structurally different phases. The complication of the first peak manifests itself in the trace of the positive-going potential scan, which results in a shoulder at 0.7 V to the main peak. Later, we will present in situ STM results to show that the asymmetric shape of the first UPD feature in 0.1 M HCl is associated with the formation of two, rather than one, Cu and Cl mixed adlattices. Another important piece of information from the cyclic voltammograms is the relative amounts of Cu deposited at different potentials. We again focus on the results obtained in 0.01 M HCl because that was the medium used in the STM measurements. We obtain 320 and 80 µC/cm2 charges for the first and second UPD features, after corrected for double-layer charging. Despite the amount of charges in UPD systems that are known to give false coverage of deposited metal, they should however reflect the relative amounts of Cu deposited for each UPD stage. A further complication of coulometric measurements is attributed to the quality of electrodes and cleanliness of electrochemical environment, which partly explains the inconsistency among workers. In Situ Scanning Tunneling Microscopy of Underpotential Deposition of Cu on Pt(111) in 0.01 M HCl + 1 mM Cu(ClO4)2. Figure 2A shows a large STM topography scan of a Pt(111) facet in a solution of 1 mM Cu(ClO4)2 and 0.01 M HCl. This image was acquired at 0.7 V where underpotential deposition of Cu already started. This in situ STM image reveals typical morphology of a (111) facet of Pt, Rh, and Ir.19-21 In addition, this (19) Wan, L. J.; Yau, S.-L.; Itaya, K. J. Phys. Chem. 1995, 99, 9507. (20) Yang, L.-M.; Yau, S.-L. J. Phys. Chem. B 2000, 104, 1769.

Underpotential Deposition of Cu on Pt Electrodes

Langmuir, Vol. 17, No. 15, 2001 4629

Figure 3. A filtered STM atomic image of the (4 × 4) structure (A) and a cross-section plot (B) along the dotted line in the image. The rhombus in A outlines the unit cell.

Figure 2. In situ STM images of Pt(111) at 0.7 V in 0.01 M HCl + 1 mM Cu(ClO4)2. These images reveal the formation of a long-ranged (4 × 4) structure at the first stage of Cu UPD. These images were acquired with -150 mV bias voltage and 5 nA feedback current.

image shows the presence of ordered lattices, which were transformed from the disordered surface prior to the UPD of Cu. In addition, packing imperfections of the adlayer or defects within the Pt(111) substrate appear as randomly distributed pits. Switching to a higher resolution scan (32 × 32 nm) resulted in an STM image shown in Figure 2B, which gives a better view of the atomic structures. In addition to the spatial ordering which spans hundreds of angstroms, a morie pattern is apparent in the image. The cracking-like feature at the right-hand side of the image is attributed to a monatomic height step ledge, running roughly 30° from the close-packed atomic rows of the Pt(111) substrate. Note that the orientation of a Pt(111) substrate can be determined by in situ STM atomic imaging in the hydrogen adsorption region, as previously reported.18,19 A further higher resolution scan (8 × 8 nm) in Figure 2C entails the atomic arrangements within this ordered adlayer, which can be characterized as hexagonal, since any two close-packed atomic rows enclose internal angles of 60 ( 3°. Treating this STM image with a 2D Fourier transform method to remove noise of 0.2 nm or closer results in the STM image shown in Figure 3A. All the protrusions are equally separated from their nearest neighbors by about 0.37 nm. It is important to note that not all protrusions exhibit identical intensity. The atomic corrugation heights differ by 0.023-0.036 nm. At the first glance the modulation of intensity might seem to be (21) Hotlos, J.; Magnussen, O. M.; Behm, R. J. Surf. Sci. 1995, 335, 129.

4630

Langmuir, Vol. 17, No. 15, 2001

Wu and Yau

Figure 4. In situ STM images of Pt(111) at 0.65 V in 0.01 M HCl + 1 mM Cu(ClO4)2. Panel A reveals domain boundaries between (4 × 4) and the new structure, resulting from continuous deposition of Cu. Panels B and C show the long-range ordering and internal atomic arrangements of the (x7×x7)R19.1° structure. Adjusting the imaging conditions from -200 mV and 3 nA to 50 mV and 5 nA reveals the substrate lattice (D).

regular, but close inspection reveals the lack of a longranged periodicity. The cross-section plot in Figure 3B illustrates the relative atomic corrugations along the dotted line shown in Figure 3A. Because the corrugation heights in the STM atomic resolution usually reflect the coordination sites of the adatoms, as found for halide and pseudohalides at noble transition metal surfaces,14 such an irregular modulation pattern of intensity implies complicated registries for adatoms. Thus, it is likely that this structure is incommensurate, despite the fact that this is rare for chemisorption on Pt(111). This view agrees with that of previous SXS measurements.11,12 Despite the lack of a long-range modulation of intensity, one can focus on some local structures and tentatively assign a “(4 × 4)” structure, outlined by the rhombus in Figure 3A. The length of the unit vectors, determined to be 1.1 nm, quadruples that of a Pt lattice (0.278 nm). Also, the unit vectors run parallel to the close-packed directions of the Pt(111) substrate. An ideal (4 × 4) structure would have a coverage of 0.56, and the nearest neighbor spacing (dnn) should be 0.37 nm. Granted the dnn value of 0.363 nm determined by previous SXS measurements, the incommensurate structure is 1.9% compressed from the commensurate one so that the coverage would be slightly larger than 0.56.

Making the potential more negative than 0.65 V brought out a new structure at the expense of (4 × 4). As shown in Figure 4A, a local area is found to contain three domains of ordered atomic arrays with domain boundaries outlined by the dotted lines. The ordered structure in I, readily identified as “(4 × 4)”, is adjacent to two other ordered domains II and III. The latter two domains turn out to be the two rotational domains of a structure to be described next. Measuring the angle enclosed by the close-packed atomic rows of Pt(111) substrate and those of ordered structures is an efficient way to determine the adlattices. In the present case the close-packed directions of Pt(111) are indicated by the atomic rows of (4 × 4) and the atomic rows in domains II and III are rotated (19° from those of (4 × 4). This result is a good indication that the new structure is a (x7×x7)R19.1°, which is know to have two rotational domains. Its population and the degree of ordering prevailed as the potential was set to 0.6 V. As shown by the STM image in Figure 4B, the whole terrace is essentially covered with the (x7×x7)R19.1° structure with countable missing defects. An atomic spacing of about 0.37 nm is determined by the high-resolution STM image in Figure 4C. This result appears to equal that of (4 × 4). It is very difficult for STM to differentiate differences of less than 5%. One cannot determine from this STM result

Underpotential Deposition of Cu on Pt Electrodes

if this is a commensurate (x7×x7)R19.1°. The unit cell of the (x7×x7)R19.1° structure is outlined by the rectangle in Figure 4C. This structure was not found by all the previous ex situ LEED and in situ SXS measurements.4,5,10-12 The RHEED experiments5 indeed identified another structure, namely, (11×x3), at a similar potential. It is not known whether the (x7×x7)R19.1° structure can undergo reconstruction to give (11×x3) upon emersion and transfer to ultrahigh vacuum (UHV). Because the previous in situ SXS measurements11,12 were taken under somewhat different electrolytes, the (x7×x7)R19.1° structure was thus not detected. In addition, we could not obtain good STM imaging of this structure when the concentration of chloride was lower than 0.1 mM. This result suggests that (x7×x7)R19.1° could be a close-pack structure of Cl on a Cu-deposited Pt(111) surface. In addition to probing the uppermost chloride adlayer, it is desirable to examine the structure and coverage of Cu adatoms. Although STM normally images the uppermost adlayer only, it allows imaging of lower atomic planes under certain circumstances. This situation was previously reported for iodine chemisorbed on Au(111) and porphrine adsorbed on iodine-modified Au(111).15,16 Switching the operation conditions of STM allows selective imaging of the adlayers of iodine adsorbate or gold substrate in the former case and imaging of the porphrine or iodine adlayers in the latter case. We adapted this idea and searched for the appropriate operation parameters of STM, which could be used to see the Cu adatoms. We found that a small bias voltage of 50 mV and low feedback current of 1 nA could render an ordered hexagonal array shown in Figure 4D. The close-packed atomic rows are all aligned with those of Pt(111) substrate and the lattice constant of this hexagonal array is only 0.27 nm, significantly smaller than the van der Waals diameter of chlorine atom. Thus, this order pattern has to arise from something other than chloride. Considering the hexagonal structure and its lattice dimension, one can attribute this STM image to the Pt(111) substrate or possibly a pseudomorphic Cu adlayer. The mechanism of STM imaging of adsorbate at a lower plane is not clear. Since it is necessary to wait for a few minutes to retrieve STM atomic imaging of chloride, it is likely that the STM probe at a low bias voltage of 50 mV could remove local copper and chloride structure to allow imaging of the Pt substrate. To further demonstrate the effect of potential on the structural changes, we obtained real-time compound STM images during potential steps. Presented in Figure 5A is an STM image acquired during a downward rastering of the STM tip, as the potential of the Pt(111) electrode was stepped from 0.6 to 0.7 V. The upper half of the STM image is attributed to the (x7×x7)R19.1° structure, whereas the lower half is the (4 × 4) structure. Evidently, the change of structure was instantaneous once the potential was switched. The conversion between these two structures appeared to be highly reversible to the potential. Note that one can measure the enclosed angles between these two ordered phases and determine actually the structures of the adlayers. The fact that the two structures, (4 × 4) and (x7×x7)R19.1°, were imaged before the second UPD peak could explain the tailing shape of the first UPD peak. In other words, the first UPD peak could be a convolution of two poorly resolved features. The conversion between these two was highly reversible, as demonstrated by the compound STM image presented in Figure 5B, obtained as a downward scan, which shows the instantaneous change as the potential was suddenly switched. The reversibility of this structural conversion however

Langmuir, Vol. 17, No. 15, 2001 4631

Figure 5. In situ STM images of Pt(111) in 0.01 M HCl + 1 mM Cu(ClO4)2. These two images were acquired by stepping potential from 0.6 (0.7) to 0.7 (0.6) V for A (B) while the tip scanned to the middle of the frame. The arrows in the figures indicate the scan directions. The tip potential biased -100 mV with respect to the Pt electrode was stepped together.

can be influenced by the concentration of the copper and possibly the chloride anion. The nature of the second UPD peak has been a main issue of this system. Different conclusions were drawn from unlike experimental approaches. We conducted in situ STM imaging experiments by stepping the potential from 0.6 to 0.5 V, the negative end of the second peak. Despite some randomly distributed protrusions, a wellordered structure is evident in Figure 6A and a highresolution STM scan of this structure is shown in Figure 6B. The unit cell of this structure outlined by the rhombus in the figure is determined to be (x7×x7)R19.1°. This arrangement appears to be hexagonal with a dnn value of 0.37 nm. All of the protrusions exhibit the same intensity. Thus, this is essentially the same structure found at 0.6 V in Figure 4C. Thus, the present in situ STM results indicate no chloride desorption in the potential range of the second UPD peak. This result agrees with those of RRDE and SXS measurements.10-12 We again switched the bias voltage from 200 to 50 mV and the feedback current from 5 to 1 nA, trying to image the Cu adlayer underneath. Indeed, in situ STM imaging again revealed

4632

Langmuir, Vol. 17, No. 15, 2001

Wu and Yau

Figure 7. Ball models for the “(4 × 4)” (A) and (x7×x7)R19.1° (B and C) structures consisting of Cu and chloride adatoms on Pt(111). The small and large empty circles represent Pt and Cl atoms, whereas the filled circles are Cu atoms sandwiched between Pt and Cl. The Cu adatoms are assigned to (x7×x7)R19.1° and (1 × 1) lattices for B and C, respectively.

adlayer of Cu atoms of Pt(111) substrate. One can confirm the (x7×x7)R19.1° structure in Figure 6B by referring to the directions of atomic rows defined by Figure 6C. An enclosed angle of 19° is found between the substrate direction and the upper chloride unit vectors. Achieving high-resolution STM imaging of the chloride adlayer at a further negative potential of 0.4 V proved to be difficult, but the copper-like hexagonal array became prominent. It seems the chloride overlayer could become disordered as the potential approached the onset of bulk deposition. Discussion

Figure 6. In situ STM images of Pt(111) at 0.5 V in 0.01 M HCl + 1 mM Cu(ClO4)2. The first two images in A and B reveal the long-ranged order and internal atomic arrangements of the adlayer. Switching imaging conditions from -200 mV and 5 nA to 50 mV and 1 nA led to the image in C. The dotted lines in B represent the close-packed atomic rows determined from the STM image in C.

a hexagonal pattern with a lattice constant of 0.27 nm, as shown by an original STM images in Figure 6C. This result can be associated with the pseudomorphic (1 × 1)

In agreement with the previous results from LEED, RHEED, and SXS,4,5,9-12 in situ STM atomic resolution discloses a highly ordered “(4 × 4)” structure in the early stage of the first UPD peak. Because chloride anions are known to lie on the top of the Cu adlayer, the (4 × 4) structure is likely to associate with the uppermost chloride overlayer. Although copper adatoms are not imaged by the STM, we intend to correlate the STM results to the model derived from SXS and other measurements.12 The model depicted in Figure 7A is essentially the same as that proposed previously by Markovic et al.12 The larger empty circles and smaller spheres respectively represent the upper chloride and the lower copper layers. These layers of Cu and Cl are both hexagonal shown in ideal (4 × 4) symmetry in this model, stacking vertically on Pt(111). It is basically identical to the (111) plane of the CuCl ionic compound with each chloride anion bonded to three Cu adatoms. The corrugation of adatoms as seen in STM atomic resolution usually reflects their registries with respect to the substrate. The situation of the present (4 × 4) structure is somewhat more complicated, because of the influence of the Pt substrate. Although the Pt substrate is located second layer down from the Cl plane, it still can influence the appearance of Cl. As noted in the previous STM imaging of iodine on Pt(111), iodine adatoms chemisorbed at face centered cubic (fcc) and hexagonal close packed (hcp) 3-fold hollow sites result in unlike corrugation heights,22 The model in Figure 7A appears to fit the STM image in Figure 3A in light of the relative corrugation between the two Cl adatoms labeled as 1 and 2 in the model. The former is closer to a 3-fold hollow site, whereas the latter sits nearly on the top of a Pt atom. This (4 × 4) structure is probably not the closest packed lattice, because another structure, (x7×x7)R19.1°, emerges as more Cu adatoms are deposited at more negative potentials. The exact amount of Cu adatoms within this structure is not clear. Granted the previous (22) Schardt, B. C.; Yau, S. L.; Rinaldi, F. Science 1989, 243, 1050.

Underpotential Deposition of Cu on Pt Electrodes

RRDE results and extrapolating the idea of CuCl ionic bilayer, one can draw the ball model in Figure 7B. The model still contains two hexagonal layers of Cu and Cl, but the lattice spacing is now 0.367 nm, which is only 1% less than that of a commensurate (4 × 4) structure. This value is however identical to that found for CuCl bilayer on Au(111), on which a (5 × 5) Cu, Cl mixed adlayer is formed.21 As noted by Hotlos et al., the lattice constants of ionic adlayers are usually smaller than those of bulk crystals.21 Because an in-plane expansion of only 2% can render periodic symmetric coordination of Cu and Cl and thus a commensurate structure of (4 × 4) structure, it is mysterious why the CuCl adlayer forms a likely less stable incommensurate adlayer on Pt(111). Furthermore, we note that the (4 × 4) to (x7×x7)R19.1° reconstruction is concentration dependent. No (x7×x7)R19.1° has been observed when the concentration of Cl is less than 0.1 mM. This finding also explains the differences in CV presented in Figure 1. Granted the preceding description of (x7×x7)R19.1°, which prevails at the end of the first UPD peak, the amount of charges associated with the first UPD feature would be 137 µC/cm2 if one assumes a 1e- reduction scheme from Cu2+ to Cu+ and a coverage of 0.57 within (x7×x7)R19.1°. This value is certainly much lower than all of the coulometric measurements. It seems then the first UPD peak is accompanied by extensive reduction of chlorine species at the electrodes. In terms of the nature of the second UPD feature, although it has been thought to be the desorption of chloride anions or continuous deposition of Cu,9-12 our present STM results, revealing no change of the chloride structure before and after the second UPD peak, lend support to the continuous deposition of Cu. Since the coverage of Cu in the (x7×x7)R19.1° structure is assumed

Langmuir, Vol. 17, No. 15, 2001 4633

to be 0.57, deposition of Cu certainly can proceed to form a pseudomorphoric (1 × 1) Cu adlattice. Figure 7C presents a real-space model of the (x7×x7)R19.1° chloride adlattice, which sits on the top of a pseudomorphic (1 × 1) Cu adlayer on Pt(111). The lack of corrugation among the Cl adatoms can result from their mostly asymmetric adsorption sites, as depicted by the ball model. Conclusion High-quality STM imaging of UPD of Cu at Pt(111) illustrates the important roles of supporting electrolytes in construction of the structures of Cu deposit. With respect to the results found in sulfuric acid, in situ STM in 0.01 M HCl + 1 mM Cu(ClO4)2 reveals that the marked shift of potential for the first UPD peak is indeed associated with precipitation of a CuCl ionic bilayer, arranging in an incommensurate (4 × 4) symmetry. The first stage of Cu UPD is completed with slightly more deposition of Cu and reconstruction to a (x7×x7)R19.1° structure. The second peak at 0.52 V can be associated with the continuous deposition of Cu because in situ STM shows that the (x7×x7)R19.1° Cl structure is stable against a potential step from 0.6 to the end of the second UPD peak at 0.5 V. Desorption of chloride could occur at more negative potential. With suitable imaging parameters, in situ STM imaging was able to discern not only the upper chloride overlayers but also the Cu or Pt substrate atomic lattice. Acknowledgment. The authors thank the National Science Council of the Republic of China for financial support of this research under Contract NSC 892113-M-008-010. LA001398F