Underpotential Deposition Of Pb Research Articles

New Electrochemical Approach for Synthesis of Nanoporous Silver

Cu-Ag alloy films were electrodeposited on Au substrates to serve as precursor alloys for synthesizing finely-structured nanoporous Ag (NPS) structures. Two innovative approaches, surface limited redox replacement (SLRR) and defect mediated growth (DMG) along with overpotential deposition (OPD), were comparatively utilized to fabricate Cu-Ag alloy films. The electrolyte for these novel approaches contained Pb2+ ions to serve either as a sacrificial metal to be replaced by the co-depositing Cu and Ag (in SLRR) or as mediating metal to facilitate the 2D growth of both alloy constituents (in DMG). The resulting alloy films from both approaches displayed superior uniformity and miscibility compared to the OPD alloy, as evidenced by electrochemical scanning electron microscopy (SEM) and X-ray diffraction characterization routines. In a subsequent step, NPS structures were generated through the de-alloying of as-deposited Cu-Ag alloys, as illustrated by SEM imaging that revealed ligament and pore sizes with a thickness in the ballpark of 40 nm. Also, surface area measurements done by a Pb underpotential deposition assay suggested a surface enhancement ratio nearly five times higher than that of flat Ag. Furthermore, various de-alloying potentials were assessed to determine the optimal de-alloying potential for the best outcome of the de-alloying process.

The Deposition of Lead on a Pt(100) Electrode and the Effects on the Oxidation of Formic Acid

The electrodeposition of foreign metals on a platinum (Pt) electrode can be used to alter the electrochemical properties of the modified electrode. The current study employed in situ scanning tunneling microscopy (STM) to characterize the spatial structures of a monolayer and multilayer lead (Pb) electrodeposited on an ordered Pt(100) electrode in 0.1 M perchloric acid (HClO4) with and without formic acid under potential control. The Pt(100) bead crystal used in this study was pretreated by a modified annealing and quenching method, producing a long-range ordered, unreconstructed (1 × 1) surface. The underpotential deposition (UPD) of Pb on the Pt(100) electrode resulted in a pair of sharp and reversible peak at 0.5 V (vs. Ag/AgCl), prior to the bulk deposition at E < -0.45 V in 0.1 M HClO4 + 1 mM Pb(ClO4)2. Cyclic potential sweeping in the UPD regimes resulted in a stable voltammogram, but bulk Pb deposition led to varying morphologies of the profiles. Atomic resolution STM images were obtained to reveal a well-ordered Pt(100)-(√2 × √2)R45°-Pb structure and elongated rows of Pb aggregates and local (√2 × 2√2) R45° in the initial and final stages of Pb UPD, respectively. Bulk Pb deposition at E < -0.5 V led to a crystalline Pb film, but a roughened Pt(100) surface was imaged after the Pb deposit was anodically stripped. The activity of the Pt(100) electrode toward formic acid oxidation was nearly tripled after being modified with a sub-monolayer of Pb, and increased by another 20 % when it was loaded with multilayer Pb.

Impact of Surface Composition Changes on the CO2-Reduction Performance of Au-Cu Aerogels.

Over the past decades, the electrochemical CO2-reduction reaction (CO2RR) has emerged as a promising option for facilitating intermittent energy storage while generating industrial raw materials of economic relevance such as CO. Recent studies have reported that Au-Cu bimetallic nanocatalysts feature a superior CO2-to-CO conversion as compared with the monometallic components, thus improving the noble metal utilization. Under this premise and with the added advantage of a suppressed H2-evolution reaction due to absence of a carbon support, herein, we employ bimetallic Au3Cu and AuCu aerogels (with a web thickness ≈7 nm) as CO2-reduction electrocatalysts in 0.5 M KHCO3 and compare their performance with that of a monometallic Au aerogel. We supplement this by investigating how the CO2RR-performance of these materials is affected by their surface composition, which we modified by systematically dissolving a part of their Cu-content using cyclic voltammetry (CV). To this end, the effect of this CV-driven composition change on the electrochemical surface area is quantified via Pb underpotential deposition, and the local structural and compositional changes are visually assessed by employing identical-location transmission electron microscopy and energy-dispersive X-ray analyses. When compared to the pristine aerogels, the CV-treated samples displayed superior CO Faradaic efficiencies (≈68 vs ≈92% for Au3Cu and ≈34 vs ≈87% for AuCu) and CO partial currents, with the AuCu aerogel outperforming the Au3Cu and Au counterparts in terms of Au-mass normalized CO currents among the CV-treated samples.

Enhancing Electrocatalytic Synthesis of Glycine with CuPb1ML Electrode Synthesized via Pb UPD

AbstractIn the field of reductive organic electrosynthesis, the hydrogen evolution reaction (HER) is considered a parasitic reaction that lowers the Faradaic efficiency of the synthesis. Metals with a high overpotential for HER are often used to prevent this. However, this limits the catalytic materials that can be used in these reactions. To expand upon the scope of available electrocatalysts, we prepared a CuPb electrode via underpotential deposition (UPD). We thereby created an electrocatalyst with a single monolayer of Pb, CuPb1ML, in which Pb weight loading is only 415 ng cm−2, yet its properties could still effectively inhibit HER. The CuPb1ML electrode was used in the electrosynthesis of glycine from oxalic acid and hydroxylamine. This reaction served as a model for a C−N bond forming reaction in acidic aqueous media. The CuPb1ML electrode was compared against a pure Pb and Cu metal electrode. The CuPb1ML electrode showed a Faradaic efficiency for glycine production of 57 %, which was 9‐fold higher than Cu and rivaled the Pb electrode. The catalytic activity of CuPb1ML was 211 μmol h−1 cm−2, which is higher than both Cu and Pb. The mechanism of the electroreduction was then studied via in situ Fourier Transform Infra‐Red (FTIR) spectroscopy. These results hinted to an evolution of the electrocatalyst during the electrolysis reaction, which was then studied via Scanning Electron Microscopy (SEM) and X‐Ray Photoelectron spectroscopy (XPS). We found that the Pb monolayer restructured during catalysis, forming microparticles that were active in the reaction based on the listed experiments. Pb alloying into the lattice, which can occur during UPD, also lowered the HER, further facilitating glycine synthesis. Thus, our research also shows how Pb UPD impacts the catalytic properties of a metal both through monolayer deposition as well as surface alloying.

The Effect of Halide Adsorption on Pb Underpotential Deposition on Au

In many systems with a large lattice mismatch, the formation of surface alloys during deposition has been reported and theoretically predicted.1 Pb monolayer deposition on Au is one such system that has been studied extensively in the past in the UHV and electrochemical environment as the underpotential deposition.2-4 Pb UPD on Au(111) in perchlorate solution has been a model UPD system studied by many surface techniques.2-6 In particular, the in-situ surface stress and the in-situ STM studies5,6 have demonstrated changes associated with the surface alloying at the partial Pb coverage. Our recent electrochemical study showed the effect of extended polarisation and potential cycling on the surface alloying in this system.7 This talk will present further work on this system by examining the effect of Cl- and Br- adsorption on Pb UPD and surface alloying.The formation of the UPD monolayer in the presence of halides is often accompanied by competitive adsorption, changes in the kinetics and energetics of deposition, and the co-adsorption that could stabilise and create new stages/phases of growth. In contrast to many other model UPD systems, the effect of halide adsorption in this system has not been explored much. To our knowledge, there are only a few studies reported so far. The in situ X-ray studies of Pb UPD on Au in the presence of Cl- and Br- by Wang et al. 8 suggested the formation of a series of ordered structures referred to as metal-halide hybrid structures. However, their model analysis did not include surface-alloying as a possible origin of the observed ordered structures.The effect of Cl- and Br- adsorption on Pb UPD was studied by gradually increasing the concentrations of ions (from 5μM-10mM) in the perchlorate solution. The results as illustrated in the Figure below show that both Cl- and Br - compete with Pb UPD causing some of the characteristic peaks to shift towards more negative potentials. The cyclic voltammetry in Cl- reveals the similar stages of the Pb UPD formation as those observed in perchlorate. The most significant differences have been observed in Br- solutions. For bromide concentration above 50 μM pronounced surface reconstruction-type peaks were observed suggesting ordered structures that could include both Br and Au. The difference in the electrochemical behaviour between the perchlorate and halide containing solutions are also visible in the chronoamlerometric current transients in the UPD region. Besides the fundamental interest in understanding halide effects on this UPD system, the interest was in the surface alloy (Pt-Au) formation during polarisation experiments. The comparison with the results of surface alloying in the perchlorate solutions will be presented and discussed. References F. Besenbacher, L. P. Nielsen, and P. T. Sprunger, The Chemical Physics of Solid Surfaces, D. A. King, D. P. Woodruff, Eds. Elsevier 8, 207-257 (1997).E. Herrero, L. J. Buller, and H. D. Abruña, Chemical Reviews, 101 (7), 1897-1930 (2001).A. Hamelin, J. Electroanal. Chem,165(1-2), 167-180, (1984).O. A. Oviedo, L. Reinaudi, S. G. Garcia, and E. P. M. Leiva, Underpotential Deposition. From Fundamentals and Theory to Applications at the Nanoscale, Springer (2016).J. Nutariya, J. Velleuer, W. Schwarzacher, and N. Vasiljevic, ECS Transactions, 28 (25), 15-25 (2010).J. W. Shin, U. Bertocci, and G. R. Stafford, J. Phys. Chem. C, 114 (17), 7926-7932 (2010).A.Szczepanska and N. Vasiljevic J. Electrochem. Soc .169, 112509 (2022).J.X. Wang, I.K. Robinson, J.E. DeVilbiss, R.R. Adzic, J. Phys. Chem. B, 104 (33), 7951-7959, (2000). Figure 1

Roughness Factors of Electrodeposited Nanostructured Copper Foams

Copper-based electrocatalytic materials play a critical role in various electrocatalytic processes, including the electroreduction of carbon dioxide and nitrate. Three-dimensional nanostructured electrodes are particularly advantageous for electrocatalytic applications due to their large surface area, which facilitates charge transfer and mass transport. However, the real surface area (RSA) of electrocatalysts is a crucial parameter that is often overlooked in experimental studies of high-surface-area copper electrodes. In this study, we investigate the roughness factors of electrodeposited copper foams with varying thicknesses and morphologies, obtained using the hydrogen bubble dynamic template technique. Underpotential deposition (UPD) of metal adatoms is one of the most reliable methods for estimating the RSA of highly dispersed catalysts. We aim to illustrate the applicability of UPD of lead for the determination of the RSA of copper deposits with hierarchical porosity. To find the appropriate experimental conditions that allow for efficient minimization of the limitations related to the slow diffusion of lead ions in the pores of the material and background currents of the reduction of traces of oxygen, we explore the effect of lead ion concentration, stirring rate, scan rate, monolayer deposition time and solution pH on the accuracy of RSA estimates. Under the optimized measurement conditions, Pb UPD allowed to estimate roughness factors as high as 400 for 100 µm thick foams, which translates into a specific surface area of ~6 m2·g−1. The proposed measurement protocol may be further applied to estimate the RSA of copper deposits with similar or higher roughness.

Understanding reactivity of self-assembled monolayer-coated electrodes: SAM-induced surface reconstruction

Thiolate self-assembled monolayers (SAMs) are often used to modify surface properties, including catalytic activity. These SAMs can also induce reconstruction of some metallic surfaces. Here we show, through formation and subsequent removal of thiolate SAMs from Au polycrystalline electrocatalysts, that irreversible changes to the underlying metal surface can lead to significant changes in catalytic properties, irrespective of specific interactions that might occur between thiolate molecules and various reactants. Using underpotential deposition of Pb as a surface probe, we find that across a range of different thiolates, SAMs tend to increase the proportion of (111)-facets on Au, but they simultaneously increase the defect density upon these and other facets. These changes generally lead to delayed onset but higher maximum activity toward formic acid oxidation, which is hypothesized to relate to changes in both the density of appropriate active site ensembles and interactions of intermediates and site-blocking hydroxyl species with newly generated defects. The impacts of reconstruction are further illustrated through measured shifts in selectivity for electroreduction of crotonaldehyde, with reconstructed catalysts changing the favored product from butanal to crotyl alcohol. Thus, complex surface reorganization may play a significant role in the catalytic behaviors of SAM-modified surfaces.

Surface Alloying During Pb Underpotential Deposition on Au(111)

The surface alloying during Pb underpotential deposition (UPD) on Au(111) films was studied using electrochemical techniques. This UPD system has been known for its unusual stress behavior associated with surface alloying during epitaxial monolayer (ML) formation. The characteristic cyclic voltammetry of Pb UPD on Au(111) exhibits an anodic peak at the most positive potentials that does not have a symmetric cathodic counterpart. The peak can be associated with the surface structural changes due to the Pb dealloying from the top substrate layer. Two electrochemical approaches were used to study the surface transformations: i) extended polarization (up to 60 min) at high Pb coverage of 0.85 ML, and ii) repeated cycling 1150 times between the potentials corresponding to 0.25 ML and 1 ML Pb coverages. In both approaches, it was observed that with the increased time of polarization or number of potential cycles, the prominent UPD peaks gradually reduced in magnitude, became broader and lost their original double-peaks structure. At the same time, the dealloying (the most anodic) peak shifted positive about 0.1 V and increased in magnitude. Quantitative analysis of the changes estimated the coverage of Pb alloying with a surface of 0.28–0.30 ML.

In Situ Analysis of the Facets of Cu-Based Electrocatalysts in Alkaline Media Using Pb Underpotential Deposition.

Establishing relationships between the surface atomic structure and activity of Cu-based electrocatalysts for CO2 and CO reduction is hindered by probable surface restructuring under working conditions. Insights into these structural evolutions are scarce as techniques for monitoring the surface facets in conventional experimental designs are lacking. To directly correlate surface reconstructions to changes in selectivity or activity, the development of surface-sensitive, electrochemical probes is highly desirable. Here, we report the underpotential deposition of lead over three low index Cu single crystals in alkaline media, the preferred electrolyte for CO reduction studies. We find that underpotential deposition of Pb onto these facets occurs at distinct potentials, and we use these benchmarks to probe the predominant facet of polycrystalline Cu electrodes in situ. Finally, we demonstrate that Cu and Pb form an irreversible surface alloy during underpotential deposition, which limits this method to investigating the surface atomic structure after reaction.

Au(001) Thin Films: Impact of Structure and Mosaicity on the Oxygen Reduction Reaction in Alkaline Medium

Au(001) thin films with thickness varying from ca. 25 to 83 nm were deposited on MgO(001) substrates using pulsed laser deposition. Epitaxial growth of (001) Au on the (001) MgO single-crystal substrate was observed with a Au(001)[010]//[010](001)MgO cube-on-cube epitaxial relationship for substrate temperature ≤200 °C. Detailed X-ray reciprocal space mapping (RSM) analysis and atomic force microscopy (AFM) imaging of films with different thicknesses were conducted. There is a nonmonotonic dependence of both the in-plane and out-of-plane lattice parameters with the films’ thickness, while the lateral coherence length increases with the thickness. The maximum tensile strain measured was 0.3% for 65 nm thick Au(001) film. Cyclic voltammetry (CV) in acidic and alkaline media and Pb underpotential deposition (Pb UPD) measurements were used as fingerprints to assess the structure of the surface. All films exhibited the characteristic response of Au(001) single-crystal surfaces, although with some differences in the potential region between 0.7 and 1.2 V vs reversible hydrogen electrode (RHE), which corresponds to the adsorption and partial discharge of OH– anions at the electrode surface. Since epitaxial Au(001) thin films do not lend themselves to steady-state voltammetry with rotating disks or microelectrodes due to their square shape and the insulating nature of MgO substrates, the electrocatalytic activity of the films for the oxygen reduction reaction (ORR) in the alkaline electrolyte was assessed by sampled current voltammetry. It was shown that both the onset potential (Eonset) and the half-wave potential (E1/2) for the ORR do not show any dependence on strain but rather become less negative (more active) with an increase of the lateral coherence length. These data are rationalized in the context of recent density functional theory (DFT) calculations and the mechanisms governing the ORR in alkaline electrolytes.

Restructuring of 4H phase Au nanowires and its catalytic behavior toward CO electro-oxidation

Au nanowires in 4H crystalline phase (4H Au NWs) are synthesized by colloid solution methods. The crystalline phase and surface structure as well as its performance toward electrochemical oxidation of CO before and after removing adsorbed oleylamine molecules (OAs) introduced from its synthesis are evaluated by high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), underpotential deposition of Pb (Pb-upd) and cyclic voltammetry. Different methods, i.e. acetic acid cleaning, electrochemical oxidation cleaning, and diethylamine replacement, have been tried to remove the adsorbed OAs. For all methods, upon the removal of the adsorbed OAs, the morphology of 4H gold nanoparticles is found to gradually change from nanowires to large dumbbell-shaped nanoparticles, accompanying with a transition from the 4H phase to the face-centered cubic phase. On the other hand, the Pb-upd results show that the sample surfaces have almost the same facet composition before and after removal of the adsorbed OAs. After electrochemical cleaning with continuous potential scans up to 1.3 V, CO electro-oxidation activity of the 4H Au sample is significantly improved. The CO electro-oxidation activity is compared with results on the three basel Au single crystalline surfaces reported in the literature, possible origins for its enhancement are discussed.

Study on electrodeposited CuFe film on Cu electrode as an efficient catalyst for electrochemical reduction of nitrate

In this research, a novel CuFe catalyst was fabricated using a simple, single-step electrodeposition method on a traditional Cu electrode. CuFe films from various electrodeposition baths with different Cu and Fe ratios were prepared by chronopotentiometry and characterised by X-ray diffraction (XRD), Scanning electron microscope (SEM), Energy dispersive X-ray analysis (EDX), and Pb underpotential deposition (Pb-UPD). The Cu electrode coated with CuFe8020 (from electrodeposition bath containing 80% Cu and 20% Fe) showed the highest reduction current among the film produced. The Cu electrode coated with CuFe8020 exhibited three times higher nitrate reduction efficiency and less undesired products like NO2− và NH4+ than the traditional Cu.

Electrochemical Filling of Nanoporous Gold with Silver via Surface-Limited Redox Replacement

Nanoporous (np) metals feature a unique network of interconnected pores and ligaments structured at a length scale of a few to hundreds of nanometers. As a result, np metals have an inherently higher surface area-to-volume ratio resulting in lower melting temperature compared to their bulk metal counterparts. Most applications of np metals have been associated with electrocatalysis. However, in recent years, a growing interest in the use of np metals in microelectronics have been pursued. Among these applications is the development of np-Cu-based chip-to-substrate interconnections that is achieved by Cu-on-Cu bonding. np-Cu is an ideal candidate for solder alternatives because of its relatively lower melting and sintering temperatures. Furthermore, the incorporation of a thin film of Sn in the pores and/or the surface of the np-Cu ligaments could further lower its sintering temperature while maintaining low resistivity. In order to fabricate these composites, there must be strict control over the uniform and flat deposition of the Sn thin film on np-Cu.Here we present a proof-of-concept study on the development of an all-electrochemical metal-filling of np structures. Conventional methods like electron beam induced deposition and overpotential deposition (OPD) have been developed for the filling of porous structure. However, these approaches showed only partial filling that are also accompanied by the growth of three-dimensional protrusions forming outside of the pores. In order to overcome these complications, we developed an approach based on electrochemical atomic layer deposition by surface-limited redox replacement (SLRR). This method allows for a layer-by-layer deposition of metals that has been shown to form relatively flat and uniform deposits. In this study, the model porous structure is np-Au, which is prepared by the selective de-alloying of pre-deposited Au-Cu alloys. The np-Au structure was then filled with Ag via SLRR (in a one-cell configuration) that is realized by successive cycles of the adsorption of a Pb monolayer, serving as a sacrificial metal, that is immediately replaced by Ag via electroless galvanic displacement.A comparative study suggests that the SLRR approach results in a more uniform Ag-filling compared to a stepwise potentiostatic OPD approach that was also done independently. Morphological characterization via surface and cross-sectional scanning electron microscopy (SEM) reveals progressive filling of np-Au with Ag. Additionally, electrochemical BET characterization via cyclic voltammetry of Pb-underpotential deposition (Pb-UPD) was also used to monitor the filling process by measuring the electrochemical surface area of the Ag-filled np-Au composite. The findings from this study will be discussed in detail. Reference:Castillo, Y. Xie, and N. Dimitrov “Filling in nanoporous gold with silver via bulk deposition and surface-limited redox replacement approaches” Electrochimica Acta, 2021, 380, 138196.

Preparation of high surface area Cu‐Au bimetallic nanostructured materials by co‐electrodeposition in a deep eutectic solvent

Our sustainable future requires finding new, affordable and green routes to prepare nanostructured materials used in renewable energy conversion. In this work we present an electrodeposition method in a deep eutectic solvent (DES) to prepare bimetallic high surface area nanostructures of Cu and Au with tunable structure and composition. The metal electrodeposition performed in choline chloride within a urea deep eutectic solvent allows us to tailor the size, morphology and elemental composition of the deposits. We combine electrochemical methods with scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDS) to characterize the electrodeposited nanostructured materials. We assess the increase of the electroactive surface area through the analysis of the lead underpotential deposition (UPD) on the prepared films. Integrated Pb UPD charge values of ca. 1600–4000 μC/cm2 for the prepared Cu-Au films have been calculated, suggesting a 5–14 fold increase of the active surface area compared to flat surfaces of polycrystalline Cu or Au. Our work reports a versatile and environmentally friendly route for the electrodeposition of Cu-Au bimetallic nanostructures in a DES. The combination of a tailored morphology and composition with the high active surface area of the nanostructured materials show that electrodeposition in DES is promising for the development of multimetallic electrocatalysts.

Surface characterization of copper electrocatalysts by lead underpotential deposition

Cu electrodes are model catalysts for a wide variety of interesting energy conversion reactions. Structure sensitivity typically plays an important role in the mechanism of different electrocatalytic reactions. However, quantifying the presence of different domains on Cu electrocatalysts remains a challenge. Herein, we evaluate the lead (Pb) underpotential deposition (UPD) on well-defined Cu electrode surfaces in presence of chloride, as a tool to analyse the surface state and determine the presence and distribution of domains on Cu catalysts. A polycrystalline Cu electrode was specifically modified by applying different electrochemical pre-treatments. Then, its surface ordering was tested by Pb UPD. Results were also compared with well-defined Cu single crystalline electrodes, to estimate the ratio between 〈1 0 0〉, 〈1 1 1〉 facets and defects on the pre-treated Cu electrode. The voltammetric profile of Pb UPD is strongly sensitive to the changes in the structure of the Cu electrode, allowing to monitor the appearance of new defects and domains. We highlight that Pb UPD can be used to provide the electrochemical fingerprint of Cu catalysts and decouple the contribution of different facets and defects in the surface structure.

Interaction of CO with Gold in an Electrochemical Environment

We present a joint theoretical–experimental study of CO coverage and facet selectivity on Au under electrochemical conditions. With in situ attenuated total reflection surface-enhanced IR spectroscopy, we investigate the CO binding in an electrochemical environment. At 0.2 V versus SHE, we detect a CO band that disappears upon facet-selective partial Pb underpotential deposition (UPD), suggesting that Pb blocks certain CO adsorption sites. With Pb UPD on single crystals and theoretical surface Pourbaix analysis, we eliminate (111) terraces as a possible adsorption site of CO. Ab initio molecular dynamics simulations of explicit water on the Au surface show the adsorption of CO on (211) steps to be significantly weakened relative to the (100) terrace due to competitive water adsorption. This result suggests that CO is more likely to bind to the (100) terrace than (211) steps in an electrochemical environment, even though Au steps under gas-phase conditions bind CO* more strongly. The competition between water and CO adsorption can result in different binding sites for CO* on Au in the gas phase and electrochemical environments.

Effect of the residual silver and adsorbed lead anions towards the electrocatalytic methanol oxidation on nanoporous gold in alkaline media

Nanoporous gold (NPG) obtained by potentiostatic dealloying of an AuAg master alloy was pulverized, filled into a cavity microelectrode and its surface electrochemistry was investigated in NaOH and KOH solutions. This method yielded highly resolved undistorted voltammograms of this polycrystalline material with clear evidences for surface segregation of Ag during storage in air. One cycle in 0.1 M H2SO4 can remove most of the residual Ag from the surface as evidenced by voltammetry after back transfer to alkaline solution and by X-ray photoelectron spectroscopy (XPS). After removal of residual Ag from the surface, the surface undergoes rephasing forming wider {111} and {110} terraces as evidenced by Pb underpotential deposition (UPD). By adjustment of the bulk concentration of PbII species, a selective partial coverage of the high internal surface area of NPG could be intentionally adjusted. Lead species remain attached to the NPG surface at potentials positive of the UPD regions either as plumbates or PbIV species as evidenced by XPS. Those tools enable to disentangle effects on the electrocatalysis of the methanol oxidation reaction (MOR) in different potential regions for 1 M methanol in different concentrations of NaOH and KOH. The MOR commences at lower potentials in the presence of residual Ag. At very high potential, the presence of Ag species promotes the oxidation to CO2/CO32-. Tafel analysis after selective blocking of specific facets shows that the {110} terraces promote especially the first step of MOR while the {111} terraces enhance the rate of further steps that are rate-limiting at higher potentials. While high coverages by Pb UPD layers or adsorbed plumbate anions inhibit electrooxidation, the behavior of NPG electrodes with low coverages of Pb species is reminiscent to NPG electrodes with residual Ag. This could be caused by promoting the methoxide and/or OH- adsorption at low overpotentials and catalyzing complete oxidation by a surface bound PbIV species at higher potentials. The catalytic currents increase with base concentration and are higher for KOH compared to NaOH. Comparable effects of base concentration on the parameters of the Tafel lines suggest that this effect is a result of slightly stronger deprotonation of methanol in bulk KOH solutions.

Filling in nanoporous gold with silver via bulk deposition and surface-limited redox replacement approaches

This work reports on the development of an all-electrochemical metal-filling of nanoporous structures via conformal deposition. The study focuses on the Ag-filling of nanoporous Au (np-Au) using both overpotential (bulk) deposition (OPD) and electrochemical atomic layer deposition by surface-limited redox replacement (SLRR). The SLRR protocol shows advantageous generation of uniform Ag filling by layer-by-layer deposition throughout the nanometer-sized porous substrate as opposed to OPD that features drawbacks including incomplete filling and clustered deposition outside of the pores. The np-Au structure to be filled with Ag is obtained by de-alloying of Cu from an electrodeposited Au-Cu precursor alloy. In both approaches, the Ag-filled np-Au composites feature an electrochemically active surface area that is comparable to that of the original precursor alloy, as monitored by cyclic voltammetry of Pb underpotential deposition (Pb UPD). Scanning electron microscopy (SEM) is also employed to characterize both the composites’ surface and cross-section morphology and the SEM findings generally corroborate the Pb UPD results. Finally, a secondary Ag de-alloying was performed to ascertain that uniformity of the Ag filling via OPD and SLRR.

In-situ high-resolution scanning electron microscopy observation of electrodeposition and stripping of lead in an electrochemical cell

A compact electrochemical cell capped with a silicon nitride (SiN) film of 50 nm thick as an electron window was developed for a side-entry holder of a high-resolution scanning electron microscope (SEM). Electrodeposition and stripping of lead (Pb) were examined on a gold electrode formed on the SiN film, which was faced inside the cell filled with an aqueous solution of lead nitrite. The SEM images of growth and dissociation of Pb were recorded simultaneously with cyclic voltammograms, controlled with a two-terminal potentiostat. Particulate growth of Pb at the edge of a gold (Au) electrode was observed at the underpotential deposition of Pb, followed by dendrite growth of Pb at higher deposition potentials. The growth mode changed depending on the edge morphology of the Au electrode. This indicated that in-situ microscopic observation was invaluable for understanding the phenomena of electrodeposition in electrochemistry.

Surface-Alloying during Pb Underpotential Deposition on Au

Surface-alloys can form spontaneously during the deposition of one metal onto another, especially in systems with a large lattice mismatch that do not have bulk counterparts (so-called immiscible systems). The spontaneous surface alloying has been widely studied in Ultra High Vacuum (UHV) [1], but these materials have not been exploited much in practical applications. Surface-alloying in an electrochemical environment is an ideal route to the design of new functional materials. Electrochemically controlled surface-alloying would allow room-temperature conditions and better scalability making it more viable and versatile for practical applications.The focus of our study was the surface-alloying during underpotential deposition (UPD) of Pb on Au(111) [2,3]. As shown in Fig 1.a, the cyclic voltammetry (CV) of Pb UPD on Au (111) has asymmetric anodic and cathodic peaks (labelled in the order of their associated appearance). The most positive anodic peak A2 is often associated with the dealloying of Pb incorporated in the Au surface. The electrochemical measurements and the in-situ Scanning Tunnelling Microscopy (STM) characterisation during Pb UPD on Au(111) have shown that the Pb ML stripping is followed by a significant Au surface roughening [2]. During Pb ML formation and dissolution, the top-surface Au atoms get displaced, creating highly mobile Au ad-islands and vacancy islands [2]. The in-situ surface stress measurements have shown a tensile stress relaxation caused by the alloying/dealloying of Pb-Au during Pb ML formation and dissolution [2,4-5].In this study, we used two different electrochemical protocols to examine the surface alloying process: 1) repeated potential cycling within the UPD range and 2) polarisation at a fixed potential in the UPD region. By following the changes in electrochemical behaviour, we established the range of potentials and conditions at which the surface-alloys form. In addition to that, we investigated the structural and compositional changes by surface sensitive techniques such as X-ray Photoelectron Spectroscopy (XPS), Ultraviolet Photoelectron Spectroscopy (UPS), Energy-Filtered Photoemission Electron Microscopy (EF-PEEM) and Work Function (WF) mapping.To study the 'dynamic' alloying, the potential was cycled repeatedly between 0.0 V and 0.5 V (vs Pb/Pb2+), as marked in Fig 1.a, over the potential range negative from the A2 anodic peak. The potential was cycled up to 400 times. After each 100th cycle, the potential was extended to the whole Pb UPD region to observe the changes in the cathodic and anodic peaks. For illustration, Fig 1.b and Fig 1.c show the changes of peaks A2 and A3 with the number of cycles. While both peaks show changes of the shape and total charge, we observe a significant shift of the peak A2 suggesting dramatic changes associated with the morphology of the surface and the amount of alloyed Pb in the top surface layer. The Pb-Au surface alloy composition, based on the charge under the A2 peak, increase from 17% up to 25% of Pb (after 400 cycles) as shown in Fig 1.d. Similar changes of the CV peaks have been observed after the polarisation at the fixed potential of 0.2 V (vs Pb/Pb2+) in the Pb UPD region for different times (15 min - 60 min).Furthermore, the Pb UPD on Au(111) has been studied in the presence of chloride. These results show gradual changes in positions and shapes of all characteristic peaks of the Pb UPD leading to complete disappearance of the A2 dealloying peak, which have not been reported before.