Palladium Electrodeposition onto Pt(100): Two-Layer Underpotential

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Palladium electrodeposition onto Pt(100): two layers underpotential deposition Bruno A. F. Previdello, Eric Sibert, Mireille Maret, and Yvonne Soldo-Olivier Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03968 • Publication Date (Web): 13 Feb 2017 Downloaded from http://pubs.acs.org on February 14, 2017

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Palladium electrodeposition onto Pt(100): two layers underpotential deposition

Bruno A.F. Previdelloa,b,+, Eric Siberta,b*, Mireille Maretc,d, Yvonne Soldo-Oliviere,a,b a University Grenoble Alpes, LEPMI, F-38000 Grenoble, France. b CNRS, LEPMI, F-38000 Grenoble, France c University Grenoble Alpes, SiMAP, F-38000 Grenoble, France. d CNRS, SiMAP, F-38000 Grenoble, France e CNRS, Institut Néel, F-38042 Grenoble, France + present address: Institute of Chemistry of São Carlos, University of São Paulo, P.O. Box 780, 13560-970 São Carlos, São Paulo, Brazil * corresponding author: [email protected] Keywords: electrodeposition, underpotential deposition, platinum single crystal, atomic force microscopy, palladium

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Abstract

Electrodeposition of first Pd layers onto Pt(100) was investigated using cyclic voltammetry at low scan rate (0.1 mV.s-1). Ultra-thin films were characterized by cyclic voltammetry in 0.1M H2SO4 solution and with ex situ AFM (Atomic Force Microscopy). For the first time, we evidenced the underpotential character of the two first Pd layers deposition, characterized by a two steps mechanism, each step corresponding to the deposition of a complete Pd atomic layer. For thicker deposits, especially above 10 monolayers as equivalent thickness, the electrochemical characterization displays a strong irreversibility and a broadening of the adsorption/desorption peaks, associated with a reduction of long range ordered flat areas. Ex situ AFM images are in agreement with this description. They show rough thick deposits and the growth of (100)-oriented rectangular shaped islands with their sides aligned with the two [011] and [0-11] perpendicular directions of the (100) Pt surface.

Introduction Metals deposition is of primary interest in several applications, like micro-electronic interconnect manufacturing, catalysts modification or protective film coating. Morphology and structure of the deposit may have an important role, as they may influence the properties of the material [1]. Several possible growth modes are observed, depending on substrate, deposited material and deposition methods. In Frank–van der Merwe [2], each atomic plane is completed before the start of the next one (layer-by-layer mechanism). In opposite, the Volmer-Weber mode [3] corresponds to the growth of 3D islands without layer completion. Finally, in the intermediate Stranski–Krastanov mode [4] the deposit starts with a layer-bylayer mechanism up to a critical thickness and is followed by a 3D island formation.


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Among the numerous physical and chemical methods available for metallic deposition, electrochemistry is a valuable process allowing operating in a quite simple and reproducible way at room temperature and atmospheric pressure. Several parameters can be adjusted in order to optimize the film properties, mainly deposition potential and the use of inorganic and organic additives. Pd is of primary interest not only for its electrocatalytic properties [5-7], but also for gas purification [8] or hydrogen storage [9-12]. Electrochemical deposition of Pd onto Pt(111) has been extensively studied [12-16]. Independently from the anions in the solution, the first Pd layer can be obtained in underpotential deposition (UPD) condition i.e. at potential higher than the Nernst one. This layer is pseudomorphic. We have recently shown that thicker films are also pseudomorphic and quite smooth up to about 10 atomic planes when deposition is performed in presence of additional chloride in the electrolytic solution [17]. Less work has been done on Pd deposition onto Pt(100) [6, 16, 18-22]. Llorca et al. [18] used spontaneous and forced deposition methods to prepare Pd/Pt(100) layers. For Pd films thinner than one monolayer (ML), they put a clean Pt(100) surface in contact with a droplet of sulfuric acid solution containing Pd2+ ions and waited for Pd spontaneous deposition. In the case of thicker deposits, Pd deposition was forced by exposing the droplet to an Ar+H2 stream. The characterization of the samples by cyclic voltammetry in sulfuric acid solution showed the presence of several adsorption peaks as a function of the film thickness. For thin deposits, the signature of the Pt(100) substrate progressively decreases while a first pair of peaks at +0.17 VRHE appears, associated with adsorption/desorption processes on the first Pd atomic plane. This description suggests that the first Pd layer is characterized by the growth of two-dimensional islands. Also, adsorption charge under the peak corresponding to adsorption/desorption on Pt(100) symmetrically decreases with the increase of that corresponding to the first Pd ML, signature that each Pd atom is adsorbed on one Pt site. The 3 ACS Paragon Plus Environment

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authors point out that such description is compatible with a pseudomorphic growth of the first Pd layer. For thicker deposits, a second pair of peaks at +0.27 VRHE grows, attributed to adsorption/desorption on the second and following Pd MLs. Such peaks become broader with thickness. Voltammetry reveals that the second Pd ML starts depositing before the completion of the first one, which remains incompletely covered at least up to an equivalent thickness of 10 ML. These findings point out that Pd/Pt(100) films obtained with spontaneous or forced deposition are rough and that only the first Pd atomic plane is complete at least up to an equivalent thickness of about 10 ML. Gómez et al. [19] obtained Pd films about 10 ML thick by forced deposition in a H2SO4 solution containing 10-3 M PdSO4. Characterization in H2SO4 solution shows a single pair of very sharp peaks at +0.28 VRHE again associated with adsorption/desorption on the second and following Pd layers. By using CO displacement experiments, authors show that the cathodic peak is a combination of hydrogen and sulfate adsorption/desorption. Voltammetric profiles in different electrolytic environment and infrared spectra of adsorbed CO indicate that the Pd/Pt(100) electrode has the same electrochemical behavior as bulk Pd(100), except for the hydrogen absorption process. In particular, Cu UPD on the Pd films shows the same details as for Cu UPD on bulk Pd(100) electrode, confirming that the growth is epitaxial with a Pd(100)-(1x1) surface structure. Inukai et al. [16] performed Pd deposition while cycling at 50 mV.s-1 in a sulfuric acid solution containing 10-4M PdCl2. Electrochemical surface characterization during deposition revealed the same pair of adsorption peaks associated with adsorption/desorption processes on the first Pd layer, as shown by Llorca et al. [18]. In situ infrared spectroscopy of CO adsorbed on Pd/Pt(100) confirms the formation of two-dimensional islands for the first Pd ML and suggests its pseudomorphic character.


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Attard et al. [22] obtained Pd/Pt(100) deposits using metal evaporation in ultra-high vacuum conditions. Combination of low-energy electron diffraction (LEED), Auger spectroscopy and cyclic voltammetry shows that palladium grows as islands with pseudomorphic character up to about 2.3 ML. Authors observed vestiges of the palladium first layer peak and clean Pt(100)-(1 × 1) surface could still be discerned. Characterization in sulfuric acid solution exhibits adsorption peaks at the same potentials as seen by Llorca et al. [18]. The comparison between Pd and Pt Auger peak-to-peak heights allows authors to prove that the pair of electrochemical peaks at low potential is univocally associated to adsorption/desorption on the first Pd layer. They also noted that adsorption peaks at higher potential exhibits a maximum for 2 Pd ML, followed by a broadening and a small shift towards higher potentials for thicker deposits. Baldauf et al. [6] made electrochemical deposition at constant potential from sulfuric acid solution containing 0.1 mM H2PdCl4 salt. They directly monitored the quantity of deposited Pd by coulometry. As signatures in H2SO4 solution are very similar to previous works, they conclude that deposits morphology is similar to that of the films obtained with the other deposition techniques. Ball et al. [20] prepared Pd deposits by fast cycling (20 mV.s-1) as in [16], but with 10-5 M PdO salt. With in situ surface X-ray scattering in sulfuric acid solution, they characterized two PdnML/Pt(100) samples, with n≈1 and n>2. The pseudomorphic growth of the Pd films was pointed out, but formation of three dimensional islands begins to take place prior to the completion of the first monolayer. For the thin deposit, authors observed the growth of the second atomic plane (coverage 0.15) before the completion of the first one (coverage 0.8), in agreement with their electrochemical measurements. For the thicker film, the first Pd atomic plane is fully deposited and 3D islands up to 7 layers high are present.


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In order to improve the smoothness of existing deposits, Alvarez et al. [21] proposed a method using NO. The initial Pd deposit with an equivalent coverage slightly higher than one ML is firstly obtained by fast cycling (50 mV.s-1) in sulfuric acid solution containing 10-5 M PdSO4 salt. Secondly, once NO is adsorbed on the electrode put in contact with NO saturated solution, it is reductively stripped off in sulfuric acid solution. After this procedure, electrochemical characterization only exhibits the signature of the first Pd ML. As far as we know, this is the only experiment where a two-dimensional (one single Pd layer) Pd1ML/Pt(100) film was obtained. The smoothing effect of NO stripping was not investigated on thicker deposits. The deposition mechanisms of the very first atomic plane(s) may be different from that of the following ones. Their electrochemical deposition may occur not only at a potential higher than that of the next planes, but also at a potential higher than the Nernst one. This is the so called underpotential deposition. A first theoretical approach on UPD was proposed by Kolb et al. [23], suggesting that UPD is present when a less noble metal is deposited on a more noble one. Due to his surface limited character, it may help producing full layer deposition, pointing to Frank–van der Merwe or Stranski–Krastanov growth modes. UPD has been widely investigated on different single crystal surfaces, allowing a better defined deposition process thanks to homogeneously distributed and well defined active sites [24]. Depending on substrate nature and orientation, and on deposited metal, UPD may occur in one or several steps, involving deposition up to two complete layers, with reversibility ranging from good to poor between deposition and dissolution. Several examples illustrate these purposes. Completion of the first copper UPD layer onto Pt(111) [25-31] and onto Au(111) [32-36] splits into two steps after the addition of chloride ions in the acidic solution. The first third of ML is deposited at a higher potential value than the following two thirds. Cu UPD on Pt is also very sensitive to crystal orientation, as shown by the very different shapes 6 ACS Paragon Plus Environment

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of the voltamperommetries on Pt(111), Pt(110) and Pt(100) [30]. Silver deposition is also strongly affected by the nature of the single crystal used as substrate. Ag deposition onto Au(100) in HClO4 solution [37, 38] shows an UPD character for the first layer [37]. Differently, Ag can be under potentially deposited in acidic media up to two atomic layers on the three Pt basal orientations, although deposition potential of each layer is strongly affected by crystal orientation [39, 40]. Bi UPD on Pt(111) is highly irreversible, where no Bi dissolution can be obtained after UPD in sulfuric acid solution [41]. Conversely, copper UPD on Pt(111) in presence of chloride exhibits one of the best reversibility with only 25 mV between deposition and dissolution peaks [27]. A detailed review of UPD on single crystals is given by Hereo et al. [24]. From the theoretical point of view, calculations suggest that the strength of UPD, as described by the potential difference between UPD and bulk deposition, not only depends on surface orientation but is also stronger on less compact surfaces [42]. This has been observed for Ag first layer UPD on Pt basal planes, where UPD strength is the highest on Pt(110), middle one on Pt(100) and lowest one on Pt(111) [39, 40]. Pd UPD has been already experimentally reported for Pt(111) [15] and for the less noble metal Au(100) [43]. The existence of Pd UPD on Pt(100) is theoretically predicted [44] at a potential value 70 mV higher than on Pt(111), due to its less compact surface. Moreover, as Pt is more noble than Au, UPD effect should be stronger on Pt(100) than on Au(100) [23]. Nevertheless, as far as we know, and despite the use of different deposition methods, the presence of Pd UPD on Pt(100) has never been reported. As discussed in the previous paragraphs, Pd/Pt(100) films studied in the literature are rather rough, with the second Pd atomic plane growth starting before the completion of the first one and with the completion of the second atomic plane only for equivalent thicknesses larger than about 10 layers.


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In this context, we want to investigate an alternative deposition method for the Pd/Pt(100) system, scanning the potential at very low rate (0.1 mV.s-1) from a sufficiently high potential value, where no deposition occurs, down to the bulk deposition region. This method already allowed the deposit of quite smooth Pd/Pt(111) thin films [12, 15], but was never used for Pd/Pt(100). We especially want to address the existence of Pd UPD onto Pt(100).

Experimental Pt(100) single crystal (5 mm diameter, 4 mm thick) was purchased at Mateck. The procedure used to regenerate the extreme surface of the platinum crystal is described elsewhere [27]. Briefly, it was flame annealed (hydrogen/air), cooled down to room temperature under reducing atmosphere (Ar + 10% H2) and finally quenched in ultra-pure water saturated with the same gas mixture. In order to avoid air contamination, the crystal surface was protected by a droplet of ultra-pure water during transfers. All electrolytes were prepared from H2SO4 (Merck, suprapur), HCl (Merck, suprapur), PdCl2 (Alfa Aesar, 99.99%), MilliQ grade ultra-pure water (18.2 MΩcm, 3ppb TOC). The solutions were de-aerated using argon (Air Liquide N60). All experiments were carried out at room temperature (25°C). Two glass cells were used for the electrochemical experiments. One was employed for the electrochemical deposition, the other one was dedicated to the characterization of the crystal surface and of the Pd film in 0.1 M H2SO4. Details on the cells are described elsewhere [12]. A Reversible Hydrogen Electrode (RHE) was use as a reference electrode. Measurements were done with a PAR 273A potentiostat controlled by CorrWare. Before each deposition experiment, a cyclic voltammogram was recorded in 0.1 M H2SO4 (red curve in fig. 5) to check that the Pt(100) surface was well-ordered and free from any contamination [45]. It also eliminates thermal oxygen possibly produced by the flame annealing. 8 ACS Paragon Plus Environment

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Pd depositions were carried out in a 0.1 M H2SO4 + 10-4 M PdCl2 solution. Extra chlorides (3·10-3 to 9·10-3 M HCl) have been added because they seem to have a smoothing role for Pd deposition onto Pt, as we have shown for Pd/Pt(111) using surface X-ray diffraction [46]. At the beginning of each deposition experiment, Pt(100) electrode was introduced at a controlled potential value of +0.95 VRHE, ensuring that surface oxide formation and associated damages do not occur and that Pd does not deposit. Potential was scanned down very slowly at 0.1 mV.s-1, in order to allow a better separation between UPD and bulk deposition [12, 47]. Potential lower limit (+0.7 VRHE, in the Pd bulk deposition region) was maintained until the desired amount of deposited palladium was obtained. This was monitored through the coulometric charge q passed in the deposition time interval. One palladium ML corresponds to a charge qML equal to 418 µC cm-2, assuming a two-electron transfer process and one deposited Pd atom per surface Pt atom. As palladium deposition may not follow a layer-bylayer growth mode, palladium coverage is expressed as an equivalent number n of complete ML corresponding to the measured charge q=n·qML. In the following, Pd deposits will be referred to as PdnML/Pt(100). After deposit, PdnML/Pt(100) surface was thoroughly rinsed with ultra pure water and then characterized in a 0.1 M H2SO4 solution, free of any trace of palladium ions. At the end of each experiment, Pd film was removed from Pt(100) substrate by applying a potential of +2 V in 1 M HClO4: the formed surface oxide was then dissolved in 10 % HCl aqueous solution [48]. Ex situ Atomic Force Microscopy (AFM) images were acquired using a Veeco system in the tapping mode. Images were treated with WSxM version 2.0 Beta 6.0 software. Topographic images were obtained on three deposits: n=2, 4 and 14. AFM images were processed using Gwyddion software.

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Cyclic voltammetry in deposition solution Figure 1 shows one and a half voltammetric cycle of a freshly prepared Pt(100) electrode in the deposition solution. During the positive-going sweep, current instantaneously toggles from Pd deposition to Pd dissolution at +0.797 VRHE. This highlights the reversible character of the Pd(II)/Pd(0) system in our experimental conditions. Therefore, this value can be considered as the Nernst potential of Pd(II)/Pd(0) (vertical line in figure 1), although the real value may be few mV higher due to Pd(II) concentration diminution in front of the electrode. Our experimental evaluation of the Nernst potential is in accordance with theoretical calculation using PdCl42-/Pd(0) standard potential [49]. Starting at high potential, a first deposition peak A is observed at +0.868 VRHE, about 70 mV higher than the Nernst value. This is a clear signature of an UPD process associated with peak A. Integrating this peak in the 0.95-0.855 VRHE range, we find a charge equal to 395 µC cm-2, very close to that expected for the deposition of one complete Pd ML (1 Pd atom for each surface Pt atom, 418 µC cm-2, neglecting Cl- desorption). This result suggests that peak A may correspond to a deposition equivalent to one Pd atomic layer. The attribution of this charge will be investigated later using electrochemical characterization. Before discussing the other features of the voltammetric cycle shown in figure 1, we further investigate deposition peak A. Figure 2 presents Pd deposition on Pt(111) and on Pt(100) in the same experimental conditions (deposition solution, scan rate, etc.). In both cases, an UPD peak A is present, but UPD is easier on the more open surface Pt(100), with the peak position at higher potentials (+0.87 VRHE) compared to Pt(111) (+0.84 VRHE) [15]. Our experimental results confirm that, as predicted by theoretical calculations, UPD effect is stronger (i.e. at higher potential compared to Nernst value) on less compact surfaces [42], even if we find a smaller value of that (70 mV) suggested by calculations [44]. 10 ACS Paragon Plus Environment

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Contrarily to Pd/Pt(111), a second deposition peak B (fig. 1 and 2) is present for Pd/Pt(100), with a current maximum at +0.787 VRHE. This potential and most of the charge under peak B are below the Nernst potential. Nevertheless, deposition current starts 20 mV above equilibrium potential, suggesting the presence of an UPD process. The fact that part of the deposition occurs below the Nernst potential may be ascribed to a kinetic issue despite the slow scan rate. Although not common, UPD process for more than one layer has already been observed for the deposition of Ag on Pt(111), (110) [39] and Pt(100) [40]. Calculation of the charge under peak B is made difficult by the fact that it is not well separated from bulk deposition (peak C). Even so, the charge calculated by current integration from 0.825 VRHE down to 0.776 VRHE (current minimum between peaks B and C) is equal to 415 µC cm-2, suggesting that peak B corresponds to a deposit equivalent to one atomic layer. The complete assignment of the A and s will be discussed in detail in paragraph 3.3, thanks to the electrochemical characterization of the film’s surface. Peak C (fig. 1) is more likely a plateau, corresponding to the diffusion limit of Pd2+ species coming from the solution bulk. In this region, we notice that the current intensity decreases with cycling, even if we cannot find an explanation for this behavior. Only one dissolution peak D (+0.87 VRHE) is observed during the positive scan (fig. 1). The overall large charge and the uncertainty associated with it do not allow unambiguously correlating it to specific deposition peaks. Inset in figure 1 shows a cyclic voltammetry with potential interval limited to the deposition peak. The absence of any significant oxidation current in the positive back scan suggests that deposited Pd is not dissolved at least up to +0.95 VRHE. This is confirmed by the absence of Pd UPD (peak A) during the second negative scan (dashed line in figure 1), whatever performing a full potential cycle (main figure) or just a cycle limited to first deposition peak A (inset). Potential excursion to higher potential could possibly dissolve the Pd deposit on Pt(100), but oxide formation on the freshly uncovered Pt 11 ACS Paragon Plus Environment

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surface would be present at the same time and contribute to the measured charges. Correspondingly, in the same experimental conditions Lebouin et al. [12] observed only a partial dissolution of the first Pd/Pt(111) layer at a potential 150 mV higher than that of its deposition. Hence, both systems Pt(111) and Pt(100) present a high irreversibility between deposition and dissolution of the first UPD peak, at least at the intermediate extra-chloride concentration considered in our experiment. In opposite, deposition associated with peak B (figure 1) seems to completely dissolve after one cycle, as shown by the fact that the peak does not significantly evolve with cycling, neither in its position nor concerning the associated charge. All these observations show that peak D corresponds to dissolution of both peak B and bulk Pd deposits. Finally, deposition voltammetries clearly show the presence of a two-step mechanism for the first layers Pd electrodeposition. The UPD character of the first deposition peak A in the Pd deposit is the first experimental proof of its theoretically predicted existence. More unexpectedly, measurements suggest the presence of Pd UPD on Pt(100) also for the second step, even if possible kinetic hindering doesn’t allow to definitely state about that at this stage of the analysis. Charges under peaks A and B seems to correspond each to one complete monolayer deposition, but this point will be discussed in the following section dedicated electrochemical characterization of the film surface.

Effect of Chloride concentration The presence of several complexes involving Pd2+ and Cl- and their concentration are influenced by chloride addition to the deposition solution, which shifts the equilibrium potential of the Pd(II)/Pd(0) system[50]. The deposition behavior may also be changed, especially in the UPD domain, as already seen for Pd/Pt(111) [12]. With the aim of investigating this aspect, we studied the deposition voltammetries obtained with three 12 ACS Paragon Plus Environment

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different chloride concentrations. Also, we looked for best suited chloride concentration, allowing a better separation between UPD and bulk deposition. Figure 3 shows the Pd deposition curves obtained in 10-4 M PdCl2 + 0.1 M H2SO4 + x HCl, with different chloride concentrations (x = 3·10-3 M, 6·10-3 M, 9·10-3 M). Changing the concentration of HCl does not modify the general deposition profile as described in paragraph 3.1: the whole curve is simply shifted towards lower potential values with [HCl] increasing. The potential of peak A is shifted toward lower values by the same value (about 48 mV from 3·10-3 M up to 9·10-3 M) as for the Nernst potential (vertical lines, figure 3). Peak B is characterized by a smaller shift (32 mV), evidencing the presence of different deposition mechanisms for peak A and peak B. We also note that with chloride concentration increase, peak B position shifts towards higher potential values relative to Nernst potential: for 9 10-3 it is entirely at potential higher than Nernst value, although it is still not completely separated from bulk deposition (region C). For the first time, our experiment gives the indication that a two steps UPD process, each having a charge equivalent to one Pd atomic layer, is present for Pd electrodeposition onto Pt(100). Another point which can be addressed looking at the Pd deposition curves as a function of the chloride concentration concerns the separation at high potential between oxide formation on Pt surface and Pd deposition. As mentioned in the experimental section, the electrode is put in contact with the solution at a potential (+0.95 VRHE) where neither oxide forms or Pd deposits, allowing a good control of the film deposition process. Indeed, figure 3 shows that the separation gap between these two phenomena increases with extra chloride addition, making this highest concentration (9·10−3 M HCl) the most suitable one in the purpose of controlling Pd deposition. For this reason, experiments discussed in the following are made using this highest chloride concentration value. 13 ACS Paragon Plus Environment

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In order to go deeper into the comprehension of the first layers Pd deposition onto Pt(100) and to possibly confirm the fact that the two-step UPD process corresponds to the layer-bylayer deposition of the two first layers, we considered the electrochemical characterization of the films.

Electrochemical characterization Pd nanofilms electro-deposited in 10-4 M PdCl2 + 0.1 M H2SO4 + 9·10-3 M HCl solution were characterized by cyclic voltammetry in 0.1 M H2SO4 solution. Figure 4 shows the CV of a freshly prepared Pd1ML/Pt(100) deposit. During the first about 10 voltammetric cycles, profile changes before reaching a stationary state. A broad reduction peak around +0.12 VRHE decreases while a sharper pair of peaks at +0.175 VRHE increases while cycling. Similar changes were observed for thicker deposits, although steady state was more quickly reached. Such behavior was already observed for this system [18]. It was attributed to surface annealing i.e. a smoothing of the surface by displacement of some Pd atoms from upper layers to vacancies in lower ones. In the following, characterization curves correspond to stationary voltammetric profiles. Figure 5 displays the voltammetric profiles obtained for three different films: Pd1ML, Pd1.5ML and Pd2ML. Free surface of Pt(100) is also shown for comparison. Depending on the thickness, two pairs of sharp peaks are observed, similarly to previous works [18, 21, 22]. We used the attributions initially proposed by Llorca et al. [18] and confirmed by Markovic et al. [20] using in situ surface X-ray diffraction. Peaks 1;1’, measured at +0.175 VRHE, are associated with adsorption/desorption of hydrogen/(hydrogeno)sulfate on the surface of the first Pd atomic plane in contact with the electrolyte. Peaks 2;2’, at +0.267/+0.278 VRHE, are attributed to adsorption/desorption on the surface of second and following Pd atomic planes in contact with the electrolyte. In opposite to previous works, in the case of Pd1ML/Pt(100) we only 14 ACS Paragon Plus Environment

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observe peaks 1;1’ corresponding to adsorption on the free surface of the first Pd atomic plane. Voltammograms do not reveal any signature of the free Pt(100) surface, as shown by the comparison with the curve corresponding to clean Pt(100), nor of the second and following layers (peaks 2;2’). Such measurements demonstrate that Pd1ML/Pt(100) film is described by one complete Pd atomic plane, where second atomic plane deposition does not begin before first atomic plane completion. This is the first time that such a structure has been observed for Pd electrodeposition onto Pt(100). This finding supports the fact that deposition peak A (figures 1 to 3) truly corresponds to the deposition of the first atomic Pd layer and that chloride contribution is negligible. We can conclude that peak A corresponds to the UPD of the first Pd atomic plane onto Pt(100). As the film thickness increases up to 1.5 ML, the intensity of peaks 1;1’ decreases, signature of the fact that the 1st Pd atomic plane is progressively covered. As expected, at the same time peaks 2;2’ are growing, confirming the presence of higher layers. For Pd2ML/Pt(100) peaks 1;1’ have completely disappeared and only peaks 2;2’ are present and their position has not changed between 1.5 ML to 2 ML. This is the clear signature that the first Pd atomic plane has been completely covered. Several authors previously showed that the first Pd/Pt(100) layer is pseudomorphic both in absence [19, 20] or in presence [16] of chloride. Should the second layer deposition be pseudomorphic, as Ball et al. [20] found for Pd/Pt(100) in absence of chloride, or relaxed (lattice parameter 0.7 % smaller), the only way to have agreement between the total charge of the Pd2ML deposit (corresponding to near 2 atomic layers) and the electrochemical characterization (pointing out the complete coating of the first layer) is the deposition of two complete atomic layers. Hence, the growth of the second Pd layer is completed before further layers formation. This observation clearly indicates that we are in presence of a Frank-van der Merwe growth mechanism up to two atomic planes. This layer-by-layer growth is in agreement with an UPD process not only for 15 ACS Paragon Plus Environment

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the first, but also for the second Pd layer, confirming the discussion we have made in the previous paragraphs on the position of deposition peak B (figure 3). As far as we know, this is the first time that a layer-by-layer Pd growth on Pt(100) up to two atomic planes was obtained. Negative scan at low scan rate (0.1 mV s-1) in conjunction with extra chloride addition reveals to be a very powerful deposition method, allowing UPD deposition of the two first Pd layers onto Pt(100). The presence of a 2D second layer growth allows interpreting the fact that peaks 1;1’, characteristic of the free first Pd layer, become less reversible once the second layer begins to grow. Indeed, Pd first layer free surface is now fragmented by the second Pd atomic plane domains, and they have a less extended long-range order compared to Pd1ML. Hence, our results suggest that 1;1’ peaks irreversibility could be related to the width of the free first Pd layer terraces.

While increasing Pd equivalent thickness from 2 up to 16 ML (figure 6), only peaks 2;2’ are present, but their amplitude and position are significantly affected. Irreversibility between positive and negative scan strongly increases: potential difference is equal to 9 mV for Pd2ML/Pt(100), to 23 mV for 7 ML and reaches around 50 mV for both 10 ML and 16 ML. The broadening and a small shift towards higher potentials of the peaks were already observed by Attard et al. [22] for deposits thicker than about 2ML. If irreversibility is linked to the size of long range ordered areas, as we discussed in the previous paragraph, this means that this size decreases with thickness, but seems to reach a limit at about 10 ML as equivalent thickness. Peak broadening is also observed, particularly between the 2ML deposit and the three thicker ones. Finally, one should mention the continuous peaks charge increase with thickness. These three findings have also been reported


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in the literature for Pd/Pt(111) [11-13, 15] and Pd/Au(111) [10] and has been associated to an increase of the surface roughness with the film thickness due to 3D growth. This behavior is much more pronounced for Pd/Pt(100) than for Pd/Pt(111): irreversibility rapidly grows in the first case between 2 and 10 ML, while it progressively takes place and is clearly seen only beyond about 20 ML on Pt(111) [12]. Surface X-ray diffraction on Pd/Pt(111) showed that deposits up to 20 ML have a limited roughening, with about 10 complete pseudomorphic atomic planes [17]. Hence, the present electrochemical characterization gives a clear indication that Pd/Pt(100) films morphology is completely different compared to Pd/Pt(111), suggesting a more pronounced 3D growth beyond the two first UPD layers.

Atomic Force Microscopy In order to evaluate the roughness of deposits and it evolution with thickness, we performed ex situ AFM measurements for Pd deposits with three different Pd equivalent thicknesses (2ML, 4ML and 14ML). The AFM images shown in Figure 7 are topographic top views: darker (brighter) colors refer to the lower (higher) parts of the surface. Pd2ML/Pt(100) surface (fig. 7A) is quite flat: the estimated roughness (RMS) is equal to about 0.3 nm, corresponding to 1.4 atomic planes. Only a few Pd islands have been observed, as the one seen in the middle of the picture, with a height of 2 nm. The Pd4ML deposit (fig. 7B) is characterized by a higher RMS equal to 0.9 nm (i.e. 4.5 atomic planes) and more islands are present. This is in agreement with a tridimensional growth suggested by our electrochemical surface characterization, during which over-potential deposition (OPD) conditions were used for deposits thicker than 2ML. Accordingly, AFM images show that between Pd2ML to Pd4ML deposits, the roughness increases more rapidly than the nominal thickness, in good agreement with a change of the growth mode between UPD (up to 2 ML) and OPD for thicker films. 17 ACS Paragon Plus Environment

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For the Pd14ML/Pt(100) surface (fig. 7C), the film presents a different surface morphology compared to the thinner ones. RMS is equal to about 1.3 nm (about 6.8 atomic planes). The roughness increase is in agreement with the broadening and large irreversibility of the 2,2’ electrochemical peaks for thicker Pd/Pt(100) deposits (10 and 14 ML), as discussed in electrochemical characterization section (see fig. 6). But compared to the images (A) and (B), well defined rectangular shaped islands are now present, with preferential orientation (fig. 7 C) and typical height between 3 and 4 nm (fig. 7 D). The edges are oriented along two perpendicular directions that should correspond to the compact [011] and [01-1] orientations of the Pt substrate, suggesting the formation of (111) side-wall facets, energetically more favorable [51]. However the slopes of island facets extracted from the AFM images are between 10° and 15°, i.e. much smaller than the expected angle between the (100) bottom facet and (111) side-wall facets, equal to 54.7°. Such deviation cannot be attributed to a convolution effect due to the AFM tip radius (close to 10nm). Therefore (111) side-wall facets separated from narrow (100) terraces could explain the observed slopes. The morphology of the Pd14ML deposit is very different from that of thinner films but also from that of Pd film grown on Pt(111) surface with comparable thickness. Ex situ AFM images for Pd16ML/Pt(111) reveal a quite low roughness, equivalent to 2 atomic planes, and the absence of well-defined islands. [12].

Conclusion Palladium deposition onto Pt(100) was investigated using negative potential scan at low scan rate in presence of extra chloride for deposition and cyclic voltammetry in 0.1M H2SO4 solution coupled with ex situ AFM for surface roughness characterization. For the first time, we could evidence the presence of two UPD processes. The first UPD corresponds to the first 18 ACS Paragon Plus Environment

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complete Pd atomic plane deposition. It was predicted by theoretical calculations, but never experimentally observed. Its potential is higher than the one of UPD for Pd/Pt(111), confirming that UPD process is stronger on more open surfaces. Pt(100) presents a high irreversibility between deposition and dissolution of the first UPD Pd layer. For the first time, we could also clearly identify the presence of another Pd UPD process, corresponding to the deposition of the second complete atomic plane. Pd/Pt(100) layer-bylayer deposition mechanisms for the two first layers is definitely different compared to Pd/Pt(111), where only the first Pd layer is underpotentially deposited. For thicker deposits, characteristic peaks of adsorption/desorption processes on the Pd surface rapidly broaden and become less reversible. Their behavior is in complete agreement with the presence of two different growth mechanisms as a function of the thickness: layerby-layer up to 2 ML and 3D for thicker films. Ex situ AFM pictures are in agreement with this morphology and they also point out the presence of “pyramidal” structures for Pd14ML, accompanied by a reduced surface extension of (100) terraces. Acknowledgment B. Previdello would like to acknowledge the French Ministry of Higher Education and Research for his granting. [1] J. Nordlund, A. Roessler, G. Lindbergh, The influence of electrode morphology on the performance of a DMFC anode, Journal of Applied Electrochemistry, 32 (2002) 259-265. [2] F.C. Frank, J.H. van der Merwe, One-Dimensional Dislocations. II. Misfitting Monolayers and Oriented Overgrowth, Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, 198 (1949) 216-225. [3] M. Volmer, A. Weber, Zeitschrift für Physikalische Chemie, 119 (1926) 277. [4] I.N. Stranski, L. Krastanov, Zur Theorie der orientierten Ausscheidung von Ionenkristallen aufeinander, Sitzungsber. Akad. Wiss. Wien, Math.-naturwiss. Kl. IIb, 146 (1938) 797–810. [5] M.J. Llorca, J.M. Feliu, A. Aldaz, J. Clavilier, Formic acid oxidation on Pdad + Pt(100) and Pdad + Pt(111) electrodes, Journal of Electroanalytical Chemistry, 376 (1994) 151. [6] M. Baldauf, D.M. Kolb, Formic Acid Oxidation on Ultrathin Pd Films on Au(hkl) and Pt(hkl) Electrodes, Journal of Physical Chemistry, 100 (1996) 11375. 19 ACS Paragon Plus Environment

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[7] L.A. Kibler, A.M. El-Aziz, R. Hoyer, D.M. Kolb, Tuning Reaction Rates by Lateral Strain in a Palladium Monolayer, Angewandte Chemie International Edition, 44 (2005) 2080. [8] G.J. Grashoff, C.E. Pilkington, C.W. Corti, The Purification of Hydrogen, Platinum Metals Review, 27 (1983) 157-169. [9] M. Baldauf, D.M. Kolb, A hydrogen adsorption and absorption study with ultrathin Pd overlayer on Au(111) and Au(100), Electrochimica Acta, 38 (1993) 2145. [10] H. Duncan, A. Lasia, Mechanism of hydrogen adsorption/absorption at thin Pd layers on Au(1 1 1), Electrochimica Acta, 52 (2007) 6195. [11] H. Duncan, A. Lasia, Hydrogen Adsorption / Absorption on Pd/Pt(111) multilayers, Journal of Electroanalytical Chemistry, 621 (2008) 62. [12] C. Lebouin, Y. Soldo-Olivier, E. Sibert, P. Millet, M. Maret, R. Faure, Electrochemically elaborated palladium nanofilms on Pt(1 1 1): Characterization and hydrogen insertion study, Journal of Electroanalytical Chemistry, 626 (2009) 59-65. [13] M.J. Ball, C.A. Lucas, N.M. Marković, V.R. Stamenković, P.N. Ross, Jr., From submonolayer to multilayer––an in situ X-ray diffraction study of the growth of Pd films on Pt(111), Surface Science, 518 (2002) 201. [14] G.A. Attard, A. Bannister, The electrochemical behaviour of irreversibly adsorbed palladium on Pt(111) in acid media, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 300 (1991) 467-485. [15] R. Hoyer, L.A. Kibler, D.M. Kolb, The initial stages of palladium deposition onto Pt(111), Electrochimica Acta, 49 (2003) 63-72. [16] J. Inukai, M. Ito, Electrodeposition processes of palladium and rhodium monolayers on Pt(111) and Pt(100) electrodes studied by IR reflection absorption spectroscopy, Journal of Electroanalytical Chemistry, 358 (1993) 307. [17] Y. Soldo-Olivier, M.C. Lafouresse, M. De Santis, C. Lebouin, M. De Boissieu, É. Sibert, Hydrogen Electro-Insertion into Pd/Pt(111) Nanofilms: An in Situ Surface X-ray Diffraction Study, Journal of Physical Chemistry C, 115 (2011) 12041-12047. [18] M.J. Llorca, J.M. Feliu, A. Aldaz, J. Clavilier, Electrochemical structure-sensitive behaviour of irreversibly adsorbed palladium on Pt(100), Pt(111) and Pt(110) in acidic medium, Journal of Electroanalytical Chemistry, 351 (1993) 299. [19] R. Gómez, A. Rodes, J.M. Pérez, J.M. Feliu, A. Aldaz, Electrochemical and in situ FTIRS studies of the CO adsorption at palladium and rhodium multilayers deposited on platinum single crystal surfaces II. Pt(100) substrate, Surface Science, 344 (1995) 85. [20] M.J. Ball, C.A. Lucas, N.M. Marković, V.R. Stamenković, P.N. Ross, Jr., Surface X-ray scattering studies of the growth of Pd thin films on the Pt(0 0 1) electrode surface and the effects of the adsorption of CO, Surface Science, 540 (2003) 295. [21] B. Álvarez, A. Berná, A. Rodes, J.M. Feliu, Electrochemical properties of palladium adlayers on Pt(100) substrates, Surface Science, 573 (2004) 32. [22] G.A. Attard, R. Price, Electrochemical investigation of a structure sensitive growth mode: palladium deposition on Pt(100)-hex-R0.7° and Pt(100)-(1 × 1), Surface Science, 335 (1995) 63-74. [23] D.M. Kolb, M. Przasnyski, H. Gerischer, Underpotential deposition of metals and work function differences, Journal of Electroanalytical Chemistry, 54 (1974) 25-38. [24] E. Herrero, L.J. Buller, H.D. Abruña, Underpotential Deposition at Single Crystal Surfaces of Au, Pt, Ag and Other Materials, Chemical Reviews, 101 (2001) 1897. [25] K. Sashikata, N. Furuya, K. Itaya, In situ scanning tunneling microscopy of underpotential deposition of copper on platinum(111) in sulfuric acid solutions, Journal of Electroanalytical Chemistry, 316 (1991) 361. 20 ACS Paragon Plus Environment

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[26] I.M. Tidswell, C.A. Lucas, N.M. Marković, P.N. Ross, Jr., Surface-structure determination using anomalous x-ray scattering : Underpotential deposition of copper on Pt(111), Physical Review B, 51 (1995) 10205. [27] Y. Soldo, E. Sibert, G. Tourillon, J.-L. Hazemann, J.-P. Lévy, D. Aberdam, R. Faure, R. Durand, In situ X-ray absorption study of copper under potential deposition on Pt(111): role of the anions on the Cu structural arrangement, Electrochimica Acta, 47 (2002) 3081. [28] J.H. White, H.D. Abruña, An In situ surfaces EXAFS study of copper underpontential deposition on Pt(111)Part I. The observation of strong in-plane scattering at submonolayer coverage, Journal of Electroanalytical Chemistry, 274 (1989) 185. [29] N.M. Marković, H.A. Gasteiger, P.N. Ross, Jr., Copper Electrodeposition on Pt(111) in the presence of chloride and (Bi)sulfate : rotating Ring-Pt(111) disk electrode studies, Langmuir, 11 (1995) 4098. [30] P.C. Andricacos, P.N. Ross, Jr., The underpotential deposition of Cu on Pt single crystals prepared in ultra-high vacuum system, Journal of Electroanalytical Chemistry, 167 (1984) 301. [31] N.M. Marković, P.N. Ross, Jr., Effect of Anions on The Underpotential Deposition of Cu on Pt(111) and Pt(100) surfaces, Langmuir, 9 (1993) 580. [32] O.M. Magnussen, J. Hotlos, R.J. Nichols, D.M. Kolb, R.J. Behm, Atomic structure of Cu adlayers on Au(100) and Au(111) electrodes observed by in situ scanning tunneling microscopy, Physical Review Letters, 64 (1990) 2929. [33] A. Tadjeddine, D. Guay, M. Ladouceur, G. Tourillon, Electronic and Structural Characterization of Underpotentially Deposited SubMonolayers and Monolayers of Copper on Gold(111) Studied by In Situ X-Ray-Absorption Spectroscopy, Physical Review Letters, 66 (1991) 2235. [34] M.F. Toney, J.N. Howard, J. Richer, G.L. Borges, J.G. Gordon, O.R. Melroy, D. Yee, L.B. Sorensen, Electrochemical Deposition of Copper on a Gold Electrode in Sulfuric Acid : Resolution of the Interfacial Structure, Physical Review Letters, 75 (1995) 4472. [35] S. Wu, Z. Shi, J. Lipkowski, A.P. Hitchcock, T. Tyliszczak, Early Stages of Copper Electrocrystallization: Electrochemical and in Situ X-ray Absorption Fine Structure Studies of Coadsorption of Copper and Chloride at the Au(111) Electrode Surface, Journal of Physical Chemistry B, 101 (1997) 10310-10322. [36] X.H. Xia, L. Nagle, R. Schuster, O.M. Magnussen, R.J. Behm, The kinetics of phase transitions in underpotentially deposited Cu adlayers on Au(111), Physical Chemistry Chemical Physics, 2 (2000) 4387-4392. [37] S.G. Garcia, D. Salinas, C. Mayer, J.R. Vilche, H.J. Pauling, S. Vinzelberg, G. Staikov, W.J. Lorenz, Silver electrodeposition on Au(100): structural aspects and mechanism, Surface Science, 316 (1994) 143-156. [38] K. Ogaki, K. Itaya, In situ scanning tunneling microscopy of underpotential and bulk deposition of silver on gold (111), Electrochimica Acta, 40 (1995) 1249-1257. [39] F. El Omar, R. Durand, R. Faure, Underpotential deposition of Ag on Pt(111), Pt(100) and Pt(110) electrodes, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 160 (1984) 385-392. [40] A.M. Bittner, The growth of the second underpotentially deposited silver layer on Pt(100), Journal of Electroanalytical Chemistry, 431 (1997) 51. [41] J. Clavilier, J.M. Feliu, A. Aldaz, An irreversible structure sensitive adsorption step in bismuth underpotential deposition at platinum electrodes, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 243 (1988) 419-433. [42] E.P.M. Leiva, Recent developments in the theory of metal upd, Electrochimica Acta, 41 (1996) 2185-2206. 21 ACS Paragon Plus Environment

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[43] L.A. Kibler, M. Kleinert, D.M. Kolb, Initial stages of Pd deposition on Au(hkl). Part II:Pd on Au(100), Surface Science, 461 (2000) 155. [44] M.I. Rojas, M.G. Del Pópolo, E.P.M. Leiva, Simulation Study of Pd Submonolayer Films on Au(hkl) and Pt(hkl) and Their Relationship to Underpotential Deposition, Langmuir, 16 (2000) 9539-9546. [45] J. Clavilier, R. Faure, G. Guinet, R. Durand, Electrochemical adsorption behaviour of Pt(100) in sulphuric acid solution, Journal of Electroanalytical Chemistry, 127 (1981) 281. [46] C. Lebouin, Y. Soldo-Olivier, E. Sibert, M. De Santis, F. Maillard, R. Faure, Evidence of the Substrate Effect in Hydrogen Electroinsertion into Palladium Atomic Layers by Means of in Situ Surface X-ray Diffraction, Langmuir, 25 (2009) 4251-4255. [47] E. Sibert, L. Wang, M. De Santis, Y. Soldo-Olivier, Mechanisms of the initial steps in the Pd electro-deposition onto Au(111), Electrochimica Acta, 135 (2014) 594-603. [48] E. Sibert, F. Ozanam, F. Maroun, R.J. Behm, O.M. Magnussen, Diffusion-limited electrodeposition of ultrathin Au films on Pt(111), Surface Science, 572 (2004) 115. [49] A.J. Bard, R. Parsons, J. Jordan, Standard Potentials in Aqueous Solution, (1985). [50] L.A. Kibler, M. Kleinert, R. Randler, D.M. Kolb, Initial stages of Pd deposition on Au(hkl). Part I:Pd on Au(111), Surface Science, 443 (1999) 19. [51] M. Maret, F. Liscio, D. Makarov, B. Doisneau-Cottignies, F. Ganss, J.-M. Missiaen, M. Albrecht, Growth temperature effect on the structure of CoPt islands on NaCl(001) studied by grazing-incidence small-angle X-ray scattering, Journal of Applied Crystallography, 47 (2014) 102-109.


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E / VRHE Figure 1. Pt(100) in 10-4 M PdCl2 + 3•10-3 M HCl + 0.1 M H2SO4 at 0.1 mV s-1. Continuous line: first cycle. Dashed line: first half of the second cycle. Vertical line: equilibrium potential of Pd(II)/Pd(0) in solution experimentally fixed at the j=0 value. Inset: cycle limited down to +0.825 VRHE.

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E / VRHE Figure 2. Pt(100) (dashed line) and Pt(111) (continuous line) in 10-4 M PdCl2 + 3•10-3 M HCl + 0.1 M H2SO4 at 0.1 mV s-1. Vertical line: equilibrium potential of Pd(II)/Pd(0) experimentally determined as discussed in the paper.


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E / VRHE Figure 3. Pt(100) in 10-4 M PdCl2 + 0.1 M H2SO4 + x HCl (x = 3·10-3 M continuous line; x = 6·10-3 M dashed line; x = 9·10-3 M dotted line) at 0.1 mV s-1. Vertical lines: theoretical equilibrium potential of Pd2+/Pd in the corresponding solutions [49].


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E / VRHE Figure 4. Cyclic voltammetry of Pd1ML/Pt(100) in 0.1 M H2SO4, 10 mV s-1; first ten cycles. Arrows indicate evolution through cycling.


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Figure 5. CV of PdxML/Pt(100) in 0.1 M H2SO4, 10mV s-1. x=1: thick continuous line, x=1.5: dotdashed line, x=2: dashed line, Pt(100): red line.


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Figure 7 Ex situ AFM images of PdxML/Pt(100); 500x500 nm²; (A) x = 2 ML, (B) x = 4 ML; (C) x = 14 ML; (D) profile along the green segment in (C)


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