In Situ Scanning Tunneling Microscopy of Underpotential Deposition

J. Phys. Chem. B 2001, 105, 5489-5496

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In Situ Scanning Tunneling Microscopy of Underpotential Deposition of Copper at Pt(100) Electrodes Coated with an Iodine Monolayer Chia-Haw Shue and Shueh-Lin Yau* Department of Chemistry, National Central UniVersity, ChungLi, Taiwan, R.O.C. ReceiVed: January 17, 2001; In Final Form: March 31, 2001

Reported here are in situ scanning tunneling microscopy (STM) results of underpotential deposition (UPD) of copper at an iodine-modified Pt(100) electrode. The cyclic voltammograms reveal that an iodine adlayer strongly adsorbed on Pt(100) resulted in a 350 mV delay of Cu deposition with respect to that of a bare Pt(100) electrode. High-quality in situ STM imaging reveals the atomic structures of the iodine adlayers before and after the deposition of Cu adatoms. Depending on the dosage of iodine vapor to an annealed Pt(100) electrode, two ordered structures of the iodine adatoms are formed and are characterized as (x2 × 5x2)R45° and (x2 × 9x2)R45° with coverage of 0.6 and 0.55, respectively. The predominant interadsorbate interaction results in adsorption of iodine at either symmetric 4-fold or asymmetric sites. Since the Cu deposit displaces the iodine adatoms on Pt(100), the iodine adatoms with weaker surface bonding are replaced first, leading to anisotropic deposition. Regardless of the initial iodine structures on Pt(100), deposition of Cu results in a c(2 × 2) structure. The deposition and dissolution processes are reversible with respect to the Cu adatoms but the iodine overlayer undergoes irreversible reconstruction from (x2 × 5x2)R45° to c(2 × 2). In situ STM atomic imaging at the onset of bulk Cu deposition also reveals protruding islands with ordered c(2 × 2) structure of iodine adatoms, suggesting a pseudomorphic structure of the 2nd Cu adlayer.

Introduction In the past decade the phenomenon of underpotential deposition (UPD) at well-defined electrodes have been a main focus in the study of electrified interfaces.1,2 UPD of Cu at low millerindex platinum electrodes has been one of the model systems and the advance of many modern techniques renders atomic details of these electrochemical events.3-8 For example, it is possible to determine precisely the amount and structures of the metal adatoms and the coadsorbed anions deposited as a function of electrochemical potential. The anions of halide exert marked effects on the reduction of Cu2+ cations.6-8 Driven by the need to minimize the surface energy of surfaces, the Cu deposit goes underneath the halides, adsorbing directly at Pt electrodes.6,7 As the structural effect has been an important aspect of singlecrystal electrochemistry, we intend to use in situ STM to gain insight into the UPD process at Pt(100) and compare to the results observed at Pt(111).9 Meanwhile, Markovic et al. have employed ex situ techniques of low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and in situ probes of rotating ring-disk electrodes (RRDE), and surface X-ray scattering (SXS) techniques to examine UPD of Cu at Pt(100) in the presence of sulfate, chloride, and bromide anions.10,11 It is found that regardless of the chemical nature of the anions, Cu UPD at Pt(100) always proceeds through a pseudomorphic (1 × 1) structure till the completion of a full monolayer of Cu deposit.10,11 A submonolayer of bromide anions is found to sit atop the Cu plane and arrange in an ordered structure of c(2 × 2). This conclusion agrees with a previous LEED study.12 Whereas a variety of the in situ and ex situ means have been used to study UPD of Cu at Pt(100), highly surface sensitive in * Corresponding author. Tel: 886-3-4227151-5909. Fax: 886-3-4227664. E-mail: [email protected].

situ scanning tunneling microscopy (STM) has not been used to examine this system thus far. As demonstrated in the past decade,13 in situ STM with its sub-nanometer resolution has been one of the most important tools in the study of electrified interfaces. The present study is intended to elucidate the deposition processes of Cu at Pt(100) modified with a full monolayer of strongly adsorbed iodide adatoms. In contrast to the previous STM studies of iodine at Pt(100),14-17 additional iodine overlayers are identified by the present high-resolution STM imaging. Deposition of Cu, as known to involve placeexchanges with the iodine adatoms, occurred first near iodine sites of weaker chemical bonding force before a pseudomorphic layer was formed. Thus, anisotropic deposition of Cu at an iodine-modified Pt(100) electrode is orientation-dependent, at least at the initial stage of Cu deposition. Experimental Section Pt(100) electrodes were prepared according to the Clavilier’s procedure.18 The platinum wires (purity >99.99%) used for preparing single-crystal electrodes were obtained from Goodfellow Cambridge Limited (Cambridge, UK). The quality of the crystals were checked by conducting cyclic voltammetry in 0.1 M HClO4 and H2SO4, where the pretreatment of electrode involved annealing by a hydrogen torch and quenching in hydrogen-saturated Millipore water. A thin water film on the electrodes appears to be effective in protecting the electrodes during their transfer from the quenching tube to the electrochemical cell. It is now realized that the procedure of electrode pretreatment controls the surface structures of Pt(100) electrodes. The adsorption of oxide in the course of cooling always gives rise to a rough surface.19 Ultrapure perchloric acid, sulfuric acid, and copper perchlorate were purchased from Merck Inc.(Darmstadt, Germany). They

10.1021/jp010172y CCC: $20.00 © 2001 American Chemical Society Published on Web 05/17/2001

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Figure 1. Cyclic voltammograms of bare (broken line) and iodinemodified (solid and dotted lines) Pt(100) electrodes in 0.1 M HClO4 and 5 mM Cu(ClO4)2. The solid line represents the first cathodic potential scan from 0.85 to -0.15 V. The scan rates were 5 mV/s.

were used as received without further purification. Millipore triple distilled water (resistivity >18.2 MΩ) was used to prepare all the needed solutions. The STM was a Nanoscope-E (Santa Barbara, CA) and the tip was made of tungsten (diameter 0.3 mm) 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. Reversible hydrogen electrodes (RHE) were used in the electrochemical and STM measurements but all of the potentials in the followings refer to a saturated calomel reference (SCE) scale. Results and Discussion I. Cyclic Voltammograms of Pt(100) Electrodes. We first obtained cyclic voltammograms of the as-prepared Pt(100) electrodes in HClO4 to ensure the quality of the electrodes as well as the cleanliness of the electrochemical environment. The resultant CVs (not shown here) are consistent with those reported by others.18-20 Dosing Pt(100) electrodes with iodine vapor was done after annealing by a hydrogen torch for 20 min. The iodinecoated Pt(100) electrodes resulted in nearly featureless current vs potential curves. The processes, associated with hydrogen adsorption/desorption and the oxidation of electrode, were entirely inhibited. The iodine adlayer exerts a marked effect on the deposition of copper, as illustrated by the results in Figure 1. The broken and solid traces respectively represent the CVs for deposition of Cu onto Pt(100) from 5 mM Cu(ClO4)2 and 0.1 M HClO4 without and with the adsorption of iodine overlayer. The dotted line was acquired as the steady-state potential scans for iodinemodified Pt(100). It is known that deposition of Cu at Pt electrodes is a slow process, the UPD peak at 0.6 V would broaden substantially at scan rates of 50 mV/s or higher. Only when the scan rate is lower than 5 mV/s did the UPD features become well-defined peaks. These results agree with the reported results.11-12 The reversible UPD features near 0.6 V shift negatively by 350 mV and broaden significantly when the Pt(100) electrode is coated with a monolayer of iodine. In fact, the effect of iodine adsorbate on Cu UPD was previously noted in the case of Pt(111), although it is less pronounced. In comparison, a bromide adlayer on Pt(100) gives rise to a nearly 400 mV difference

Shue and Yau between the deposition and dissolution of Cu at a scan rate of 10 mV/s.11 It is also noted that the first CV scan (solid trace) differs from the steady-state CV (dotted trace) in Figure 1. This hysteresis can be explained by the STM results, showing the irreversible restructuring of the iodine adlayer due to the deposition of Cu. Because the UPD features overlap with that of bulk deposition, it is difficult to get an accurate measurement of the amount of charge associated with UPD of Cu. A rough estimate suggests a 395 µC/cm2 charge, which is about 5% less than the theoretical value of 414 µC/cm2 for a full monolayer of Cu adatoms reduced from Cu2+ cations. Our results agree with those determined by RRDE.12 These results indicate that the structures of an adsorbate can be influential to the process of Cu UPD if the adsorbate-substrate interaction is strong enough to compete with the incoming Cu adatoms. On the other hand, this view does not apply to the case of Cu UPD at iodinecoated Pt(111) electrodes, as the structures of iodine adatoms has no effect on Cu deposition.9 II. In Situ STM Imaging. 1. The Structures of Iodine OVerlayer. We obtain in situ STM images of the structures of iodine adatoms on Pt(100) at 0.7 V in 5 mM Cu(ClO4)2 and 0.1 M HClO4. The Pt(100) electrode was cooled in an iodine stream for 5 min to give an iodine-saturated Pt(100) surface. Figure 2 displays some of the typical STM images obtained at a terrace site. The first image in 2A reveals a nearly perfect iodine adlayer on Pt(100) in an area of 32 × 32 nm, except two irregularities appearing at the upper left-hand corner of the image. It seems that this iodine structure contains parallel lines with alternating intensity running diagonally across the image. Figure 2B reveals the internal atomic arrangements of this structure. All of the parallel lines apparently are made of equally separated atomic protrusions with interatomic spacing of 0.4 nm. The spacing between two parallel lines are however not uniform; they appear to form pairwise pattern. These results are reminiscence of the previous STM studies14-17 and this iodine superlattice can be readily determined to be (x2 × 5x2)R45°. The rectangle in the image outlines the unit cell of this structure and a corresponding real-space model is shown in Figure 2C. This model readily explains the appearance of the (x2 × 5x2)R45° structure; the pairwise protrusions are attributed to the iodine adatoms residing at asymmetric types of sites; whereas the trenches between the protrusions are associated with the iodine atoms adsorbed at 4-fold hollow sites. Sashikata et al. already configured the same ball model.22 This view is certainly in line with the previous findings of iodine at Pt(111).23 The closest spacing between two iodine atoms, 0.382 nm, corresponds to a 8.2% compression from the value determined by the Pt(111)-(3 × 3) structure. The coverage of 0.6 for this Pt(100) - (x2 × 5x2)R45° structure is 36% higher than that of Pt(111)-(3 × 3).23 Thus, 0.6 is likely to be the saturated coverage of iodine at Pt(100). As reported by others,14-17 this (x2 × 5x2)R45° adlattice represents the most important iodine structure at Pt(100). However, we note that varying the dosing condition of the Pt(100) electrode can lead to different iodine structures. The following presents the STM results of a Pt(100) electrode treated with iodine vapor for 15 s after a hydrogen torch annealed it. Figure 3A displays an STM atomic image of another well-ordered iodine structure, tentatively assigned as (x2 × 9x2)R45°, whereas Figure 3B shows the coexistence of this structure (domain I) with the aforementioned (x2 × 5x2)R45° (domain II) on the same terrace. The spacing between two lines is 1.8 and 1.0 nm, respectively. The

Underpotential Deposition of Cu at Pt(100) Electrodes

Figure 2. In situ STM images of iodine overlayers on Pt(100) and the corresponding atomic models. The iodine adlayer was deposited by exposing a red-hot Pt(100) crystal to iodine vapor for 5 min under a nitrogen stream. These images were acquired at 0.7 V in 0.1 M HClO4 and 5 mM Cu(ClO4)2.

reason that the directions of the atomic protrusions in Figure 3A and B appear to be rotated by 90° is that they were obtained

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Figure 3. Same as Figure 2, except the iodine dosing time is decreased from 5 min to 15 s under a nitrogen stream.

at different spots on the sample. The rectangle in Figure 3A represents the unit cell of this structure, which can be reconciled by the real-space model in Figure 3C. It consists of pairwise protruding rows, separated by 1.6 nm. Between these features are three dimmer protruding atomic rows. The relative corrugation height seen in the STM image is associated with iodine

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Figure 4. Time-dependent in situ STM images showing the morphological changes after the deposition of a monolayer of Cu adatoms. While 4A was obtained before the potential was stepped from 0.7 to 0.3 V, 4B, C, and D were obtained consecutively 20, 40, and 60 s after the step. The scan areas for all the images are 100 × 100 nm.

adatoms adsorbed respectively at near-top, atop sites, and symmetric 4-fold hollow sites. These magnitudes of corrugation differences are measured to be 0.01 and 0.04 nm when compared to the brightest features. According to the model in Figure 3C, the coverage of this iodine adlayer is determined to be 0.55, slightly lower than that of Pt(100) - (x2 × 5x2)R45°. Again, we emphasize that the population of these structures hinges on the dosing time of the Pt(100) electrode with iodine vapor. 2. Underpotential Deposition of Cu at Iodine-Coated Pt(100). We first present the time-dependent in situ STM images, obtained by stepping the potential of an iodine-coated Pt(100) electrode from 0.7 to 0.3 V in HClO4 solution containing 5 mM Cu2+. High-resolution STM imaging reveals that the iodine adatoms form a well-ordered (x2 × 5x2)R45° structures at 0.7 V. Figure 4 presents a series of time-dependent STM images were acquired after the potential step. The scan areas of all the images are 100 × 100 nm. The first image (Figure 4A), obtained prior to the potential step is followed by three images acquired at 20 (4B), 40 (4C), and 60 s (4D) recorded in the course of Cu deposition. The variations of these STM images indicate an upward drift during the imaging process. However, this does

not obscure the importance of these results, which highlight Pt(100) electrode morphological changes in the course of Cu deposition. In addition to revealing well-defined terrace and step features, internal atomic packing of iodine atoms was imaged as parallel lines at the terraces. While the well-ordered (x2 × 5x2)R45° structure predominates, packing defects within the iodine adlayer seems to be inevitable, frequently found near steps or pits on the terraces. The potential step from 0.7 to 0.3 V immediately renders changes of the surface structures, as seen in the STM image of Figure 4B. Deposition of Cu results in randomly distributed islands, seemingly elongated in parallel to the directions of the atomic rows. This result reflects the nucleation-and-growth mechanism of Cu deposition, where packing defects near steps and pits serve as the nucleation sites. The deposition fronts then moved into the terraces until the whole surface is covered with Cu. As the deposition of Cu proceeds, some protruding islands randomly emerge on the terraces. The density of the islands gradually increases (Figure 4C) but finally reaches a steady state, as revealed by Figure 4D. Nearly a monolayer of Cu adatoms was deposited in a period of 60 s. These islandlike features,

Underpotential Deposition of Cu at Pt(100) Electrodes

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Figure 5. High-resolution STM images of an iodine-modified Pt(100) electrode after the deposition of a partial (A) full (B and C) Cu UPD layer. The former was collected at 0.4 V and the latter were obtained at 0.2 V. Figure 5D shows the real-space model of the c(2 × 2)-Cu,I mixed adlattice.

accompanying the deposition of Cu, tentatively arise from the reconstruction of the iodine adatoms. A decrease of the coverage results in exclusion of iodine adatoms in the process of Cu deposition. This issue becomes clear after revealing the structure of Cu and I by the STM. These extra iodine species are likely to be atomic at this potential, whereas they could be reduced at further more negative potential. High-resolution STM images in Figure 5 reveal the internal atomic structures of the iodine adlayers in the course of Cu deposition. Figure 5A is a portion of Figure 4B, showing the surface state of Pt(100) with its 60% area covered with Cu atoms at 0.3 V. The elongating islands in the directions of the atomic lines indicate the anisotropic nature of the UPD process. Figure 5B reveals the surface morphology along with a well-resolved atomic pattern seen at the end of Cu UPD (0.2 V). In addition to the well-ordered atomic arrays, nanometer-scaled protruding islands with step height of 0.23 nm emerge. These features are tentatively associated with iodine adatoms squeezed out of the original (x2 × 5x2)R45° structure or possibly the deposition

of second layer of Cu. Figure 5C reveals the well-ordered surface structures, presumably attributed to iodine adatoms. This structure can be assigned as c(2 × 2), where the close-packed spacing for the iodine atoms is 0.393 nm. Because all of the protrusions exhibit similar intensity, it is reasonable to propose that all of them reside at similar types of sites. A ball model is depicted in Figure 5D to reconcile this structure. The Cu adatoms are assigned to 4-fold hollow sites, rendering the formation of a pseudomorphic (1 × 1) structure on Pt(100). Similarly, all of the iodine adatoms occupy 4-fold hollow sites on the Cu plane. Stickney et al. previously used AES to establish this iodine-copper-platinum sandwich structure.24 The insertion of Cu adatoms between iodine and Pt inevitably lessens the strong I-Pt surface bonding and thus restructures the iodine structure from a compact (x2 × 5x2)R45° to a more open c(2 × 2) structure. This atomic rearrangement and reduction of coverage then result in exclusion of roughly 17% of the iodine atoms to the uppermost layer. This explains the changes observed in Figure 4.

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Figure 6. Time-dependent in situ STM images showing the UPD of Cu at a domain covered with the Pt(100)-(x2 × 9x2)R45° structure. The first image (6A) is obtained at 0.7 V, followed by a potential step to 0.45 V to give STM images in 6B and C. Figure 6D was acquired at 0.4 V, followed by stepping potential back to 0.45 V to show the reversibility of the process. Figure 6F was obtained at 0.35 V where more Cu adatoms were deposited, leading to the formation of local c(2 × 2) structures. The scan areas for 6A-E are 50 × 50 nm and 6F is 7 × 7 nm.

By the same token, we then used in situ STM to investigate the deposition of Cu at domains coated with Pt(100)(x2 × 9x2)R45° iodine structure. The details of this structure are already shown in Figure 3. The in situ STM imaging experiments were conducted by stepping the potential from 0.7 to 0.4 V and time-dependent STM images (Figure 6) were

acquired simultaneously. Note that Cu UPD is only partial at 0.4 V. The well-resolved surface features, including the domain wall in the middle of the image, indicate that thermal drift, frequently encountered in STM imaging, is minimal throughout the imaging period of 8 min. The winding protrusion cutting across the terrace is likely to be an anti-phase domain boundary.

Underpotential Deposition of Cu at Pt(100) Electrodes The parallel lines are associated with the aforementioned (x2 × 9x2)R45° iodine adlattice. As the potential is stepped from 0.7 to 0.4 V, a few short (