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Apr 7, 2011 - *Shuehlin Yau, e-mail: [email protected], Tel:886-3-4279573, Fax: 886-3-4227664; Wei-Ping Dow, [email protected]...
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In Situ Scanning Tunneling Microscopy Study of 3-Mercaptopropanesulfonate Adsorbed on Pt(111) and Electrodeposition of Copper in 0.1 M KClO4 þ 1 mM HCl (pH 3) PoYu Yen, HsinLing Tu, Hengliang Wu, Sihzih Chen, Walter Vogel,* and ShuehLin Yau* Department of Chemistry, National Central University, Jhongli, Taiwan 320, Republic of China

Wei-Ping Dow* Department of Chemical Engineering, National Chung Hsing University, Taichung 40227, Taiwan, Republic of China

bS Supporting Information ABSTRACT: In situ scanning tunneling microscopy (STM) was used to examine the spatial structure of adsorbed 3-mercaptopropanesulfonate (MPS) molecules on a Pt(111) electrode in 0.1 M KClO4 þ 1 mM HCl þ 107 M MPS (pH 3). Two √ ordered √ MPS structures, Pt(111)(2  2) (θ = 0.25) and ( 3  3)R30 (θ = 0.33) structures were observed at 0.25 V (vs Ag/AgCl). The former (latter) was more important at more negative (positive) potentials. These MPS structures became a disordered adlayer at E > 0.1 V. These restructuring events could result from a progressive of the surface coverage of MPS with potential. Shifting the potential negatively could √ increase √ restore the ordered structures of ( 3  3)R30 and (2  2), but the rather strong Pt-MPS made it difficult for MPS admolecules to desorb from the Pt(111) electrode. By contrast, the MPS adlayer seen in 0.1 M HClO4 was always disordered, regardless of the potential of Pt(111) electrode. (Tu et al., J. Electrochem. Soc. 2010, 157, D206.) It is reasonable to state that potential control, pH, and/or countercations to the sulfonate group of the MPS admolecule could be important in guiding the adsorption of MPS molecules on Pt(111) electrode. Strongly adsorbed MPS molecules on the Pt(111) electrode could impede the rate of Cu2þ reduction, thereby inhibiting rather than accelerating electrodeposition of copper under the present conditions. Real-time STM imaging revealed random nucleation adatoms on Pt(111), followed by lateral growth of Cu nuclei upon further √ of copper √ deposition. Segregated domains of ( 3  3)R30, ascribable to MPS and chloride adspecies, were observed atop a monolayer of Cu deposit prior to the commencement of bulk Cu deposition. With a small overpotential (η < 20 mV), multilayer copper was electroplated on Pt(111) in a layered manner, producing atomically smooth Cu deposit capped by patches of (3  3) MPS. By contrast, the Cu deposit on MPS-modified Pt(111) in 0.1 M HClO4 was decidedly rough, as reported earlier.

1. INTRODUCTION Copper electrodeposition has received great interests recently, because it is recognized as a necessary process to fabricate interconnects in modern semiconductor architecture.114 Aided by chloride anions in the electroplating bath, 3-mercaptopropanesulfonate (MPS) and bis-3-sodiumsulfopropyldi-sulfide (SPS) molecules, can accelerate the reduction of Cu2þ cations and hasten the deposition of Cu. A number of studies have explored the adsorption of MPS or SPS in the presence of chloride and the effects of these chemical species on the electrodeposition of Cu. A number of models are proposed to account for the accelerating effect of SPS or MPS on Cu deposition. It is shown that these species could catalyze the reduction of Cu2þ to Cuþ, the first and the ratedetermining step for the full reduction from Cu2þ to Cu0.15 Meanwhile, some researchers have hypothesized the formation of complexes involving MPSCu2þCl, which facilitates charge transfer between the electrode and Cu2þ cations in the electrolyte.7,16 r 2011 American Chemical Society

A number of in situ and ex situ experimental methods, including ellipsometry, Raman spectroscopy, and scanning tunneling microscopy (STM) have been used to examine the adsorption of MPS and SPS molecules on metallic surfaces such as Cu(111),17 Au(111),18 and Pt(111).19 MPS admolecules are adsorbed in disarray on Cu(111),17 but a few ordered MPS structures were found on Au(111).18 Bae et al. showed that both MPS and SPS molecules are adsorbed so weakly on chloridecovered Cu(100) that they evaded STM imaging.13 However, as noted by Moffat et al., the imaging conditions of STM are detrimental to the outcomes, particularly when the adsorbate is weakly adsorbed.13 On Pt(111), MPS molecules are strained and fail to adapt an ordered structure at all potentials in 0.1 M HClO4 Received: December 5, 2010 Revised: March 21, 2011 Published: April 07, 2011 8110

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The Journal of Physical Chemistry C with or without chloride anions.19 However, √ MPS molecules readily form an ordered Pt(111)(4  2 3)rect structure on Pt(111) preloaded with a monolayer of Cu adatoms, which is transformed into (2  2) as the Cu deposit thickens.19 In studying Cu deposition on patterned chips,20 it is found that a predeposited monolayer of MPS or SPS is sufficient to promote Cu deposition in a plating bath containing no MPS. It is also noted that the extents of alkanethiol adsorbed on gold, silver, and copper electrodes could vary with pH of the dosing solution.21 These studies prompt us to examine whether pH could also influence the adsorption of MPS on ordered Pt(111) electrode, and how pH affected subsequent Cu electrodeposition. It is intriguing to note that raising the pH from 1 to 3 was significant enough to alter the spatial arrangements of MPS admolecules on the Pt(111) electrode and alter the mechanism of Cu deposition. A couple studied are reported to address the effects of pH on the adsorption of organosulfur compounds on Au and Ag substrates.21 The chemical nature of adsorbate seems to be important, as the adsorption of decanethiol is favored by higher pH,21 as opposed to acid-driven adsorption of MPS and SPS.22 As illustrated below, in addition to potential control, pH of the dosing solution of MPS was crucial to the coverage and spatial arrangement of MPS molecules adsorbed on Pt(111) electrode, and the adsorption of MPS could be favored by an acidic medium.

2. EXPERIMENTAL SECTION The Pt(111) electrodes used for STM and voltammetric experiments were homemade from polycrystalline Pt wire. Typically, the pretreatment of a Pt(111) bead electrode involved annealing by a hydrogen torch, followed by quenching in hydrogensaturated ultrapure water. It was then mounted onto the STM cell, equipped with two Pt wires acting as the counter and quasi-reference electrodes.2325 All potentials reported here are converted into a scale of a Ag/AgCl electrode. The potential was scanned from the open-circuit potential (OCP) at ∼0.7 to 0.3 V at 50 mV/s to reduce thermally formed oxide on the Pt(111) electrode. The potential of Pt(111) electrode was held at 0.15 V, followed by adding enough MPS to make a final [MPS] = 107 M in the 0.1 M KClO4 þ 1 mM HCl (pH 3) medium. The STM scanner was a typical A-head (Veeco, Santa Barbara, CA) with a maximal scan size of 500 nm. The tip was a tungsten tip etched by AC in 6 M KOH. Nail polish was applied to coat the whole tip for insulating purposes. The potential of the tip electrode and the feedback current were set normally at 0.4 V and 1 nA, respectively. The electrochemical cell used for voltammetry had a threeelectrode configuration, including a Ag/AgCl reference electrode and a Pt counter electrode. The potentiostat was a CHI 600 (CH Instruments, Austin, TX). Suprapure perchloric acid (HClO4) and hydrochloric acid (HCl) were purchased from Merck (Darmstadt, DFG). Ultrapure water (Millipore, resistivity >18 MΩ cm) was used to prepare the needed electrolytes. Copper perchlorate (Cu(ClO4)2) and MPS were obtained from Aldrich (St. Louis, MO). KClO4 was purchased from Alfa Aesar (Ward Hill, MA). All chemicals were used as received. 3. RESULTS AND DISCUSSION 3.1. Cyclic Voltammetry (CV). The CV profile shown in the blue trace in Figure 1 was recorded at 50 mV/s with Pt(111) electrode immersed in 0.1 M KClO4 with its pH adjusted to 3 by

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Figure 1. CVs obtained at 50 mV/s with Pt(111) in 0.1 M KClO4 with pH adjusted to 3 by adding HClO4 (blue), after adding 103 KCl (red), and after adding 0.1 μM MPS (black).

Figure 2. CVs shown in panel a were obtained at 50 mV/s with Pt(111) in 0.1 M KClO4 þ 1 mM HCl (pH3) þ 1 mM Cu(ClO4)2 without (red) and with (black) 0.1 μM MPS. The D/D0 and E/E0 peaks are due to Cu UPD, whereas F/F0 stems from Cu bulk deposition. The profile spanning between 0.7 and 0.2 (red and black) or 0.03 V (blue) are highlighted in (b). The blue trace in (b) was obtained in the same electrolyte, except [MPS] was 10 μM.

adding enough HClO4. The symmetric peak at 0.42 V (A/A0 ) is due to H2O S OHads þ Hþ þ e, producing/removing OH species on the Pt(111) electrode.26 Underpotential deposition (UPD) for hydrogen was responsible for the current plateau between 0.1 and 0.38 V (B/B0 ), whereas evolution/oxidation of H2 resulted in the diffusion-controlled peak at 0.4 V (C/C0 ). Addition of KCl into the electrolyte (final [KCl] = 1 mM) shifted the A/A0 feature positively to 0.48 V, but hardly affected the B/B0 feature, as revealed by the CV profile shown in the red trace. These CV results show that adsorption of chloride was insignificant at E > 0.4 V, where OH adsorption commenced. Chloride could compete with OH species for surface sites on Pt(111),27,28 leading to the positive shift of the peak potential the of A/A0 feature by about 100 mV. Adding 0.1 μM MPS into a 0.1 M KClO4 þ 1 mM HCl (pH3) electrolyte completely eliminated the A/A0 and B/B0 features, but made little difference to the C/C0 feature (black trace). This CV profile was stable against repetitive potential cycling between 0.7 and 0.8 V, suggesting that MPS admolecules clung firmly to the Pt(111) electrode and they were resistant toward degradation. Despite [MPS] was 4 orders of magnitude less than [Cl] in the electrolyte, it still caused dramatic changes in the CV profile of Pt(111), thereby attesting the strength of Pt-MPS interaction. This view is consistent with the fact that the features of B/B0 (due to hydrogen adsorption) was also eliminated by the adsorption of MPS. Figure 2a shows the jE profiles obtained with Pt(111) electrode in 0.1 M KClO4 þ 1 mM HCl þ 1 mM Cu(ClO4)2 containing no (red trace) or 0.1 μM (black trace) MPS. The 8111

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The Journal of Physical Chemistry C potential sweep began at 0.7 V, going negative first to 0.25 V at a scan rate 50 mV/s. Figure 2b reveals CV scans covering the potential region between 0.7 and 0.2 V to highlight the Cu UPD features. Two pairs of peaks (D/D0 and E/E0 ), attributed to the UPD of Cu, emerged at 0.44 and 0.30 V, and the asymmetric peak of F/F0 at 0.03 V signaled from the overpotential deposition (OPD) of Cu. These results are consistent with those reported earlier,24,29,30 showing that UPD of Cu in chloride-containing electrolyte proceeds in two√steps, √ yielding two well-ordered chloride structures, (4  4) and ( 7  7)R19.1, at the end of each step. The first step of UPD yields about a half monolayer of copper adatoms, followed by continuous deposition to produce a full monolayer of Cu at the end of the second step.24,29 The presence of 0.1 μM MPS in the electrolyte resulted in some differences in the morphology of the profile, as both the UPD and OPD features of Cu decreased in intensity. The UPD peaks still appeared at the same potentials, but broadened noticeably and decreased by ca. 50%. These CV results were hardly changed by repetitive cycles. These results suggest that adsorbed chloride was partly displaced by MPS molecules or MPS and chloride anion were both present on the Pt(111) electrode. Meanwhile, a weak reduction feature emerged at 0.1 V, which is associated with Cu UPD at MPS-occupied domains on Pt(111). Because the UPD features are sensitive to the chemical nature of the adsorbed anions, these CV results indicate that chloride and MPS adspecies segregated into different domains, rather than mixed homogeneously to form a different structure. The relative populations of these two species could vary with their concentrations. To gain more information on the effect of [MPS] on Cu deposition, we conducted voltammetric experiments in the same base electrolyte but with 10 μM MPS, 100 times higher than that used in Figure 2a. The result shown in blue in Figure 2b reveals pronounced changes, as the UPD features seen previously at 0.44 and 0.3 V were no longer present, while a new pair of feature was found at somewhat more negative potentials. This new feature exhibiting quasi-reversible character was also seen for Cu deposition onto Pt(111) in 0.1 M HClO4 containing 10 μM MPS.19 This feature is ascribed to Cu deposition at MPS-occupied Pt(111) sites. Because it emerged at a potential more negative than those found on chloride-covered Pt(111), Cu deposition at MPS-modified Pt(111) was less favorable than at Cl-occupied sites. These results show that MPS and chloride anions could compete for surface sites in the potential region of Cu UPD. MPS could prevail when its concentration was higher than 10 μM in 1 mM chloride medium. Its importance gradually declined upon lowering concentration; the presence of 10 nM MPS in 1 mM chloride made little difference to the CV profile (not shown). Not only the UPD but also the OPD of Cu was inhibited by MPS admolecules, which manifested in the negative shifts in the OPD peaks and decreased in the amount of Cu deposited in the same potential window. For example, the CV scans shown in Figure 2a reveal that the presence of 0.1 μM MPS caused the OPD peak shifted negatively by 30 mV and the accumulated charges due to Cu deposition was lowered to 1.5 mC/cm2, from 1.8 mC/cm2 measured in the absence of MPS. Subsequently, MPS could inhibit, rather than accelerate, Cu deposition on Pt(111), as also noted in 0.1 M HClO4.19 We contend that the strength of MPS adsorption on metal electrode could be one of the critical issues here. If it is adsorbed too strongly on a metal such as Pt, its relocation to the top of the Cu deposit would be difficult. MPS molecules could be buried under the Cu deposit,

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Figure 3. In situ STM images showing the Pt(111) electrified interface at 0.15 V in 0.1 M√ KClO4√ þ 103 M HCl þ 0.1 μM MPS. Panel a reveals two patches of the ( 3  3)R30 structure, highlighted by dotted traces, found amid a more prominent (2  2) structure. Panel b reveals a high-resolution STM scan over an area occupied by the two ordered MPS structures. Molecular-resolution STM images √ shown √ in c and d reveal the internal structures of the (2  2) and ( 3  3)R30 structures (marked by the rhombi).

causing delays in electron transfer involving the reduction of Cu2þ. Slower Cu deposition resulted, as observed here. 3.2. In Situ STM Imaging of MPS Molecules Adsorbed on Pt(111). On the basis of our previous study of alkanethiol adsorbed on Pt(111),31,32 this sort of molecule tends to arrange in ordered structures in the potential region of hydrogen adsorption. Subsequently, in situ STM imaging was first performed at 0.15 V in 0.1 M KClO4 þ 1 mM HCl (pH 3), and an atomic resolution image of the Pt(111) substrate could be obtained (not shown here), as reported previously.33,34 Figure 3a is a constant-current topography STM scan, revealing atomically smooth terraces and monatomic steps (Δz = 0.22 nm). Enough MPS-dosing solution was then added into the STM cell to make a final [MPS] = 0.1 μM. The Pt(111) electrode surface was mostly homogeneous, except two patches marked by dotted traces. These highlighted areas exhibited weaker corrugations. As revealed by the high-resolution STM scans shown in panel b, two different ordered arrays exhibiting distinctively different corrugation heights were present on the electrode surface. These structures were found shortly after the addition of MPS into the STM cell, and they were stable against prolonged STM imaging. These results suggest that these ordered structures were due to adsorbed MPS molecules, rather than remnant sulfur species produced from the degradation of MPS. The internal molecular structures of these ordered MPS structures were visualized by further higher-resolution STM scans, shown in Figure 3c,d. They are both hexagonal, but rotated by 30 from each other. Two nearest neighbors in these arrays are separated by 0.56 and 0.48 nm, respectively. Aided by atomicresolution STM images of the Pt(111) surface, molecular rows in these MPS adlattices are found to align parallel to the Æ110æ and Æ121æ directions of the Pt(111) substrate. This information yields √ √ Pt(111)(2  2) and ( 3  3)R30MPS structures whose coverages are 0.25 and 0.33, respectively. The former (latter) prevailed at more negative (positive) potential, which suggests that the adsorption of MPS on Pt(111) was favored by more 8112

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The Journal of Physical Chemistry C positive potential, despite the MPS adlattice being disordered at E > 0.1 V. The fact that all protrusions seen within an ordered array exhibited the same corrugation heights suggests that MPS admolecules in each structure were adsorbed at the same adsorption sites and in the same molecular configurations. On the other hand, the two structures seen in Figure 3b clearly exhibited different intensity (Δz = 0.03 nm), which implies that MPS admolecules in these two domains could either occupy different types of sites or assumed unlike molecular orientations, for example, in different tilt angles. On the basis of our previous STM results obtained with hexanethiol adsorbed on Pt(111), thiol molecules are thought to favor bonding at 3-fold hollow sites.31,35 But with admolecules √ packed √ more closely with potential, MPS admolecules in the ( 3  3)R30 domain could realign themselves toward the surface normal, as compared with that in the more loosely packed (2  2), which bestowed the dissimilar corrugation heights seen in the STM image.36 The respective ball models for these structures are shown as the insets in Figure 3c,d. Because the intermolecular √ √ spacings of 0.48 and 0.56 nm in the ordered arrays of ( 3  3)R30 and (2  2) are smaller than the physical dimension of MPS (measured 0.65 nm from the sulfur end to the oxygen atoms in the SO3 on the other end of molecule), it is likely that they were oriented upright on the Pt(111) electrode, as illustrated by the model shown in Figure 3b. It is worth noting that thiol molecules with sulfonate endgroups have been examined using Raman spectroscopy,37,38 which indicates that thiol molecules such as MPS could adapt trans and gauche conformations on gold and silver electrodes, depending on the chemical compositions of the media. For example, Kþ, Naþ, and so forth could act as the counter cations to the pending sulfonate groups of MPS admolecules. This mechanism could favor MPS molecules to assume trans molecular conformation with all sulfonate groups facing the solution, which eventually benefits ordering in the MPS adlayer. This line of reasoning could hold for this study, which was conducted in 0.1 M KClO4. In comparison, MPS admolecules were adsorbed in disarray on Pt(111) in 0.1 M HClO4, which could result partly from a lack of this ion-pairing. However, as revealed by our previous study,31 hexanethiol molecules, which cannot form ion pairs with √ cations, √ also produce highly ordered Pt(111)(2  2) and ( 3  3)R30 structures as found with MPS; ion-pairing is not a necessary condition for producing ordered thiol adlayer on Pt(111). Just to shed more insights onto the issue of ion pairing, it would be useful to conduct more STM experiments in electrolyte containing divalent or trivalent cations such as Mg2þ, La3þ, and so forth to see whether this would make any difference to the MPS structure. These ordered MPS structures existed only between 0.3 and 0.1 the potential to E > 0.1 V disrupted the ordered √ V. Shifting √ ( 3  3)R30MPS structure. This order-to-disorder phase transition was, however, reversible, although the reversed process was so slow that it took at least 30 min to restore the ordered MPS structures found at E < 0.1 V. The loss of ordering at E > 0.1 V could result from either an increase of MPS coverage, or simply a disruption of the Kþ SO3 ion-pairing at the interface. Given the rather strong surface bond of PtS, it would be difficult to remove the thiol molecule from the Pt electrode, causing the sluggish restructuring seen by the STM. By contrast, if the ionpairing of KþSO3 was disrupted at E > 0.1 V, reorganization of the electrode double layer would involve shuffling ions at the electrode surface, which should be a facile process.

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In summary, the main driving force causing the change in MPS’s√structure √ at E > 0.1 V was an increase of MPS adsorption. The ( 3  3)R30 structure could be the most compact but still ordered MPS adlattice; forcing more MPS molecules on Pt(111) would occur randomly and yield a disordered structure. Although the adsorption of chloride could become more important with more positive potential,27,30 the Pt(111) electrode was occupied mainly by MPS molecules at E > 0.1 V, as revealed by the poor ordering seen at E > 0.1 V. The highly ordered MPS adlattices seen in this study contrast sharply with the decidedly disordered MPS arrays found on Pt(111) in 0.1 M perchloric acid. Although our previous report shows STM results obtained in 0.1 M HClO4 containing 10 μM MPS (100 times more concentrated than the 0.1 μM MPS used here), we also conducted STM experiments in 0.1 M HClO4 þ 0.1 μM MPS, but results (not shown here) still showed a disordered adlayer. The effect of pH on the adsorption of thiol and disulfide molecules on a gold electrode is outlined as follows:21 The adsorption of decanethiol and dibutyl disulfide molecules on gold is favored in alkaline and acidic media, respectively. The adsorption reactions for decanethiol could be RSH þ Au f RSAuþ þ Hþ þ e. For dibutyl disulfide, RSSR þ Au þ e f RSAuþ þ RS, where acid shifts the equilibrium toward the right because RS is a weak acid. The former reaction scheme could also work for MPS adsorption to Pt(111), if the PtS interaction was strong enough to offset the unfavorable acidic environment for MPS adsorption. (Although the MPS adlayer on Pt(111) was disordered in 0.1 M HClO4, an averaged 0.4 nm spacing between MPS admolecules, as compared to 0.48 nm found in pH 3 media, indicates higher surface coverage of MPS in pH 1 solution.) Depending on the potential, MPS molecules could be adsorbed on Pt(111) in different reactions. For example, if deposition of mps was made at the OCP (∼0.65 V), the Pt electrode could be masked by OH species and the adsorption of MPS could proceed in RSH þ PtOH f RSPt þ H2O. The acidity of the medium could help remove OH species and the subsequent adsorption of MPS. On the other hand, if the Pt electrode was coated with hydrogen atoms at E < 0 V, the adsorption of MPS could proceed in RSH þ PtH f RSPt þ H2. On the other hand, unlike MPS, a number of organosulfur compounds such as hexanethiol √ √could be adsorbed in ordered Pt(111)(2  2) and ( 3  3)R30 in 0.1 M HClO4.31,32 Thus, the sulfonate group in the MPS molecular structure was important in guiding its adsorption on Pt(111), despite the surface bond made via its sulfur headgroup. The most direct contribution of the sulfonate end group to the adsorption of MPS could be pair up with a Kþ cation in the electrolyte. Meanwhile, the structure of the MPS adlayer on Pt(111) in 0.1 M KClO4 (pH7) was also examined. Molecular resolution STM images were also obtained, revealing a (2  2)MPS adlattice as seen in Figure 3c. This result is relegated to the Supporting Information. 3.3. In Situ STM Investigation of UPD of Cu. UPD of Cu on Pt(111) in 0.1 M KClO4 þ 0.1 μM MPS þ 1 mM HCl (pH3) þ 1 mM Cu(ClO4)2 was examined by holding the potential at 0.6 V first, where adsorbed MPS molecules were prominent. Switching potential from 0.6 to 0.2 V then triggered the deposition of a monolayer of Cu. This process was fast and could be finished within seconds. Topography STM image obtained at the completion of this event is shown in Figure 4a, revealing a Pt(111) surface with clear asperities. The close-up STM scans shown in panel b and c reveal structural details at the Pt(111) electrode, 8113

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Figure 4. In situ STM images with successively fine resolution showing the Pt(111) electrified interface as the potential was switched from 0.6 to 0.2 V in 0.1 M KClO4 þ 103 M HCl þ 0.1 μM MPS þ 1√mM Cu(ClO4)2. The flake-like electrode surface comprising ordered ( 3  √ 3)R30 structure of MPS appeared shortly after the shift of potential and stayed unchanged with time. The uneven appearance seen here could arise from segregation of MPS and Cl adsorbates on the Cumodified Pt(111) electrode.

Figure 5. Time-dependent in situ STM images acquired at 0 V in 0.1 M KClO4 þ 103 M HCl þ 0.1 μM MPS þ 1 mM Cu(ClO4)2. These images were consecutively obtained every 30 s after the potential was switched to 0 V from 0.02 V. Cu deposits were imaged as protruded islands, which grew in size and in density to fill nearly the entire Pt(111) surface within 3 min. All images are 300  300 nm.

where patches of ordered arrays are seen. Despite the lack of a long-range ordering, this local hexagonal array with 0.48 nm interparticle spacing and an included angle of 30 with respect √ to the main axis of the Pt(111) substrate, this structure fits ( 3 √ 3)R30 best. Results described above contrast markedly with those found in 0.1 M HClO4, where Cu deposits grow in mushroom-like features on the MPS adlayer.19 It is thought that Cu adatoms could penetrate the MPS adlayer and deposit directly on the Pt surface. The unevenness seen in the STM images in Figure 4b,c could indicate a chemically inhomogeneous interface, which agrees with the CV results shown in Figure 2, suggesting not only MPS but also chloride could be present on the electrode. The average corrugation height of 0.06 nm seen in Figure 4b,c thus could derive from coadsorbed MPS and chloride. √ As found earlier, chloride anions are arranged in Pt(111)( 7  √ 24 This is not what STM 7)R19Cu þ Cl at the end of the UPD. √ √ imaging revealed. Thus, the present ( 3  3)R30 structure is mainly due to MPS admolecules residing on Cu adatoms. Again this contrasts sharply with Cu UPD in pH 1 media, where the Pt(111) surface was saturated with MPS molecules arranged in disarray. 3.4. In Situ STM Investigation of Bulk Cu Deposition. A series of STM images were obtained to unveil the process of Cu deposition onto a Pt(111) electrode preoccupied by a monolayer of MPS (Figure 5). Protruded islands are evident in these STM images, which grew in number with prolonged STM imaging.

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Figure 6. In situ STM images showing the surface morphology of Pt(111) electrode coated with 1.8 monolayer of copper under conditions similar to those of Figure 5. The first image reveals flake-like features, followed by higher-resolution STM scans in b and c revealing ordered adlayer, identified as (3  3)MPS.

These features popped up immediately after the potential was shifted from 0.2 to 0 V, which suggests that they resulted from Cu nucleation. Nearly all seeds seen here were monatomic in height (Δz = 0.22 nm). They grew laterally, along with nucleation of new Cu seeds. Eventually, Cu nuclei coalesced with their neighbors to yield a homogeneous Cu film. These STM results suggest a progressive nucleation-and-growth of Cu deposition. Unlike Cu deposition on bare Pt(111), where surface defects such as steps are usually the preferred sites for nucleation,23 Cu nucleated randomly on MPS-covered Pt(111). This contrast might stem from different mobility of copper adatoms on the substrate. For example, Cu adatoms could be less mobile on the MPS-modified Pt electrode than on bare Pt(111), so that they tended to stick at the sites where they first landed on the electrode. The magnitude of overpotential influenced greatly the mechanism of Cu deposition. A high overpotential (η > 50 mV) favored three-dimensional (3D) Cu deposition and yielded rough Cu film, because Cu adatoms landed on the electrode at a rate too fast to allow atomic diffusion toward most favorable sites.39 These sorts of Cu deposition were seen with or without MPS in the electrolyte, although 3D growth could begin with smaller overpotentials in the presence of MPS.19 Because the second Cu layer nucleated before the first Cu layer was completed, the Cu thin film seemed to grow in a pseudo layer-bylayer mode. It is emphasized that the layered type Cu deposition seen in this study was never observed on MPS-modified Pt(111) electrode in 0.1 M HClO4.19 Higher-resolution STM images were obtained to provide more structural details of the Pt(111) electrode (Figure 6). The corrugated surface morphology reflects how much Cu adatoms were deposited here. If corrugated patches amounting to ca. 80% of the imaged area are assigned as the second layer of Cu, the coverage of Cu would be 1.8 ML (counting the UPD layer as the first layer). A further higher resolution STM scan shown in Figure 6b reveals ordered arrays on corrugated patches. Given the fact that all ordered arrays look the same in the STM image, one can assume that they comprised the same chemical species arranged in the same manner on the Cu-loaded Pt(111). The close-up STM scan shown in Figure 6c reveals that adsorbates are aligned in the close-packed atomic rows of the Pt(111) substrate and the length of the unit vector is 0.77 nm. These STM results lead to a (3  3) structure, which should be attributed to species sitting on top of the Cu deposit. Chloride is unlikely to be solely responsible for because √ this structure, √ chloride is reported to arrange in ( 3  3)R30 on Cu(111).23 Rather, we attribute this (3  3) adlattice to MPS admolecules, which gives rise to a (2  2) structure on Pt(111)supported Cu deposit in 0.1 M HClO4.19 We make it clear that 8114

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The Journal of Physical Chemistry C this (3  3) adlattice was not the only MPS structure observed, as the arrangement of MPS could vary with [MPS] and possibly the thickness of Cu film. These issues will be addressed later. Nevertheless, the present STM results suggest that MPS molecules could also stay afloat during Cu deposition in pH 3 media;an important feature in accelerated Cu deposition, as noted in our previous study conducted in 0.1 M HClO4. In addition, STM results obtained here and previously in 0.1 M HClO4 show that the coverage and spatial arrangement of MPS molecules adsorbed on Cu thin film could also vary with pH of the electrolyte.

4. CONCLUSIONS In situ STM imaging has revealed two highly ordered√MPS arrays characterized as Pt(111)(2  2) (θ = 0.25) and ( 3  √ 3)R30 (θ = 0.33) between 0.3 and 0.1 V in 0.1 M KClO4 þ 103 M HCl þ 0.1 μM MPS. In one single ordered domain, all MPS molecules were adsorbed in the same manner, where their sulfur headgroups were bound to Pt(111) and their alkyl chains oriented upright and pending in the solution. Raising potential could result √ in higher √ surface coverage of MPS, which transformed the ( 3  3)R30 structure into a disarray at E > 0.1 V. By contrast, the MPS adlayer deposited on Pt(111) from 0.1 M HClO4, was always disordered, irrespective of potential. It is possible that the potential of Pt(111) was of prime importance to control the surface coverage and spatial structure of MPS molecules. In pH3 media, the potential could be set low enough to control MPS coverage at 0.33 or below for the production of ordered MPS adlattice without causing severe hydrogen evolution. In contrast, this is not possible in pH1 media, as the potential could not be set low enough to hold back MPS deposition on the Pt(111) electrode. The codeposition of countercations such as Kþ in this study could be another factor influence the organization of MPS admolecules on Pt(111). These results obtained with Pt are useful to guide the study of MPS adsorption on other commonly used metal supports such as Au and Cu. MPS molecule in 0.1 M KClO4 þ 103 M HCl þ 0.1 μM MPS þ 1 mM Cu(ClO4)2 was found to inhibit the electrodeposition of Cu on Pt(111). This effect was more pronounced with higher [MPS] up to 0.1 mM. MPS and Cl adspecies could compete for surface sites and segregated into different domains, √ √ as revealed by STM imaging showing local ( 3  3)R30 structures at the end of Cu UPD (0.2 V). These results imply that within the time domain of STM experiment Cu adatoms were deposited under the MPS adlayer, rather than at some defects of the MPS adlattice. On multilayer Cu deposit STM revealed a (3  3)MPS structure, which indicates that the Cu layers deposited on Pt(111) were atomically flat. Again these results are different from those found in 0.1 M HClO4, where MPS admolecule adsorbed to the Pt(111) electrode so strongly that Cu deposition was possible only at defects in the MPS adlayer. By contrast, the Cu thin film electrodeposited on MPS-modified Pt(111) in 0.1 M HClO4 was decidedly rough. ’ ASSOCIATED CONTENT

bS

Supporting Information. In situ STM image acquired with a Pt(111) electrode at 0.1 V in 0.1 M KClO4 (pH 6.5). An ordered array, identified as Pt(111)(2  2)MPS, as found also in pH 3, 0.1 M KClO4, is seen in this image. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*Shuehlin Yau, e-mail: [email protected], Tel:886-3-4279573, Fax: 886-3-4227664; Wei-Ping Dow, [email protected].

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