Underpotential Deposition Region Research Articles

Scalable Production of Monolayer Shell(Pt)@Core(Pd) Nanoparticles by Electroless Cu UPD for Oxygen Reduction Reaction

In this article, we discuss an electroless under potential deposition (UPD) method to synthesize core@shell monolayer nanoparticles in bulk quantities. In this electroless UPD method, potential is controlled and maintained in the UPD region by a redox couple. This electroless path enables the production of core@shell monolayer catalysts on any type of support, unlike the conventional UPD, where potential is controlled by an external power source which allows for the deposition only on conductive supports such as carbon. This was demonstrated by synthesizing Pt(shell)@Pd(core) nanopartricles on both conductive (Vulcan carbon) and non-conductive (alumina) supports by Cu UPD-galvanic replacement method. Core-shell structure of the Pd-Pt nanoparticles was confirmed by STEM characterization. The thickness of Pt shell in Pt@Pd nanoparticles was examined by analytical and experimental observations. Furthermore, catalytic activity of Pt@Pd/C nanoparticles, synthesized by the electroless Cu UPD-galvanic replacement method, was examined for oxygen reduction reaction.

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.

Effect of thiourea on electrocrystallization of Cu–Sn alloys from sulphate electrolytes

In this work, the effect of thiourea on the initial stages of the Cu–Sn electrodeposition in the potential range of the underpotential deposition (UPD) of tin from the sulphate electrolyte was studied by linear sweep voltammetry. The mechanism of Cu–Sn alloy nucleation and growth was proposed based on the results of chronoamperometry, chronopotentiometry, and scanning electron microscopy. Thiourea in the concentrations of 0.001–0.010 g/L showed a significant inhibitory effect on the reduction of copper ions. Analysis of the current-time dependencies revealed that Cu–Sn deposition in the UPD region of tin from thiourea-containing solutions proceeds through 3D diffusion-controlled growth of copper clusters. The nucleation type and the main nucleation parameters were calculated based on the Scharifker and Hills theoretical model. Galvanostatic experiments proved that the UPD of tin occurs on the surface of initially formed copper clusters. Surface characterization of Cu–Sn coatings deposited in the presence of thiourea showed a good correlation with electrochemical experiments.

Electrodeposition of Ag Overlayers onto Pt(111): Structural, Electrochemical and Electrocatalytic Properties

Epitaxially grown Ag overlayers have been fabricated by electrochemical deposition onto Pt(111). The electrochemical behaviour of these Ag overlayers has been studied by cyclic voltammetry, and their adsorption properties are significantly influenced by the underlying Pt(111) substrate and markedly different from those of Ag(111). A characteristic voltammetric peak for OH adsorption on pseudomorphic Ag islands has been observed for alkaline solution. A deposition–dissolution hysteresis in the underpotential deposition region for the Ag bilayer suggests exchange processes between subsurface Ag and Pt atoms. Theoretical DFT calculations confirm the stability of a pseudomorphic Ag monolayer. However, it is shown for two and three Ag layers that the formation of sandwich structures is theoretically favoured, i.e. Ag layers tend to be separated by single Pt layers. While Ag displaces hydrogen adsorbed in the underpotential region, the activity of Ag monolayers for the hydrogen evolution reaction (HER) is very close to that of Pt(111). Also, Tafel slopes for HER on the first pseudomorphic Ag monolayer on Pt(111) and for blank Pt(111) are almost identical, whereas thicker overlayers are more Ag-like. It is shown by theoretical calculations for the case of an Ag monolayer on Pt(111) that hydrogen can be adsorbed on the Pt subsurface layer.

Determining Kinetic Barriers to Alkaline Hydrogen Oxidation and Evolution Via Oxide Supports

It has long been recognized that the reaction rates of the hydrogen oxidation and hydrogen evolution reactions (HOR and HER) are slower in basic than acidic electrolytes, even though the surface intermediate of adsorbed hydrogen is independent of solution pH. Understanding the root of this observation is critical to designing catalysts for a multitude of electrochemical reactions with relevance to energy conversion and storage. In this work, we undertake a fundamental investigation relating interfacial water structure to the reaction barrier for hydrogen underpotential adsorption and to the HER/HOR kinetics. The pH-dependence of HOR/HER kinetics has been attributed to a variety of reasons. The research groups of Gasteiger and Yan have both suggested that in base, hydroxide ions stabilize the Pt-H bond for stronger binding and slower catalysis1,2. Central to this hypothesis is an experimentally measured shift with pH in the peak potential for hydrogen underpotential deposition on the (100) and (110) steps on the Pt surface. Although it is unclear why pH would stabilize adsorbed hydrogen, stronger binding would push platinum further to the right of the volcano peak and decrease the overall activity. Markovic et al proposed instead that in base, the water recombination/dissociation step of HOR/HER requires specific adsorption of hydroxide ions and therefore optimal binding to two adsorbates3. One observation that has been less discussed than either hydrogen or hydroxide adsorption is the effect of pH not only on the apparent hydrogen adsorption energy, but the kinetic barrier to adsorption. Koper et al attributed this observation to a pH-dependent water orientation at the surface, and hypothesized that “H-down” in acid vs “H-up” in base resulted in slower transfer of hydrogen to and from the surface4. The same pH-dependence of water orientation was recently supported computationally by Rossmeisl's group5. In this work, we examine specifically the hypothesis that water orientation governs the rate of hydrogen adsorption and thus the overall HER/HOR kinetics. The native oxide formed on chromium metal is known from colloidal literature to have a potential of zero charge (Epzc) of 1.22V vs. RHE6. We hypothesize that if water orientation truly governs the alkaline HOR/HER rate, the negative oxide surface charge on chromium oxide will induce an H-down water orientation adjacent to platinum nanoparticles supported on the surface. This will result in faster hydrogen adsorption kinetics and thus, higher activity for alkaline HOR/HER than extended platinum surfaces. To test this hypothesis, platinum nanoparticles were synthesized via pulsed electrodeposition on chromium oxide. The nanoparticle loading was controlled by the duration of deposition pulse, with longer times corresponding to higher coverage as well as larger particles. The hydrogen adsorption kinetics were measured by cyclic voltammetry in the hydrogen underpotential deposition region. Varying the sweep rate results in greater peak separation that can be used to extract rate constants for the adsorption reaction via traditional electroanalysis7. A typical voltammogram of Pt/CrOx in 0.1M KOH is shown in Fig. 1a. As the scan rate increases, kinetic barriers cause greater peak separation (dEp) for hydrogen adsorption on the 100 and 110 facets of platinum. For lower loadings with shorter pulse periods, both smaller currents and smaller peak separations are observed due to the lower Pt surface area. The effects of surface area can be accounted for by normalizing current to Pt surface area as measured by Hupd coulometry. The resulting surface-adsorption Tafel plot is shown in Fig 1b. The peak potential separation dEp is much greater for the larger nanoparticles with higher loading. This difference suggests that smaller particles have qualitatively more rapid adsorption kinetics due to the advantageous water orientation induced by negatively charged CrOx. The results of this study contribute to resolving a long-standing paradox in electrocatalysis and surface science by highlighting the importance of kinetic barriers, as well as adsorption energies, and suggest the ability to modulate alkaline hydrogen activity through metal-support interactions. Future work will discuss the relation of hydrogen underpotential deposition kinetics to HER/HOR activity and the effects of different oxide supports. (1) Durst et al. Energy Environ. Sci. 2014, 2255. (2) Sheng et al. Nat. Commun. 2015, 5848. (3) Strmcnik et al, Nat. Chem. 2013, 1. (4) Van Der Niet et al. Catal. Today 2013, 105. (5) Rossmeis et al. Phys. Chem. Chem. Phys. 2013, 10321. (6) McCafferty, E. Electrochim. Acta 2010, 1630. (7) Angerstein-Kozlowska et al. J. Electroanal. Chem. Interfacial Electrochem. 1979, 1. Figure 1

H3PO3 electrochemical behaviour on a bulk Pt electrode: adsorption and oxidation kinetics

Polybenzimidazole-type polymer doped with H3PO4 is commonly used as the proton-conductive phase in high-temperature proton-exchange membrane fuel cells. However, H3PO4 is not stable during fuel cell operation and undergoes reduction by hydrogen on a Pt surface to phosphorus compounds in a lower oxidation state, such as H3PO3. In this work the kinetics of H3PO3 oxidation on Pt electrode was studied, including an investigation of H4P2O6 as a possible oxidation intermediate. H3PO3 adsorption in hydrogen underpotential deposition region was described by a triple Langmuir isotherm corresponding to adsorption on specific Pt crystalline planes. Co-adsorption of hydrogen as well as SO42−, HSO4− ions decreased the total amount of adsorbed H3PO3. The determined apparent charge transfer coefficients of H3PO3 anodic oxidation on a metallic Pt surface were found to be concentration and temperature-dependent, indicating that the nature of the anodic process is complex. From chronopotentiometric measurements of H3PO3 and H4P2O6 oxidation on a preoxidised Pt surface it was concluded that, while H3PO3 is oxidised by means of a chemical reaction with PtOx, H4P2O6 undegoes anodic oxidation on the PtOx surface. According to voltammetry and bulk electrolysis experiments H4P2O6 is not formed as an intermediate product during electrochemical oxidation of H3PO3 on a metallic Pt surface.

Carbon supported Pt-Y2O3 and Pt-Gd2O3 nanoparticles prepared via carbonyl chemical route towards oxygen reduction reaction: Kinetics and stability

The Pt-M2O3/C (M = Y and Gd) catalysts were synthesized via carbonyl chemical route, and heat-treated at 300 °C. Both catalysts are nanoparticulated with a face-centered cubic (fcc) structure with no secondary phase present as revealed by X-ray diffraction (XRD) patterns similar to Pt/C, generated in the same way. Data analyses of Pt-M2O3/C XRD patterns via the Williamson–Hall method were performed. The stacking fault, and micro-strain values increased with respect to Pt/C catalyst. The transmission electron microscopy (TEM) further revealed that nano-particles are homogeneously dispersed in both Pt/C and Pt-M2O3/C (M = Y and Gd). The mean particle size is close, ca. 3.4 nm for Pt/C, ca. 4.1 nm for Pt-Y2O3/C and ca. 3.6 nm for Pt-Gd2O3/C sample. Although the surface electrochemical studies (cyclic and CO-stripping voltammograms) showed similar Pt surface behavior, the kinetics of oxygen reduction reaction (ORR) on Pt-M2O3/C (M = Y and Gd) catalysts was higher than the homemade Pt/C and commercial Pt/C catalysts (20 wt. %, Johnson Matthey). After 6000 potential cycles (0.6–1.0 V vs. RHE) of accelerated stability test (AST), the remaining Pt active surface (based on hydrogen underpotential deposition region) and kinetic current density (at 0.9 V vs. RHE) in Pt-M2O3/C (M = Y and Gd) catalysts were higher than the reference Pt/C, and commercial Pt/C catalysts. These findings assess the positive effect of M2O3 (M = Y and Gd) in improving the activity and stability of Pt NPs towards ORR.

(Invited) In Situ Stress and Nanogravimetric Measurements during Underpotential Deposition

The underpotential deposition (UPD) of metals onto foreign metal substrates involves monolayer or submonolayer formation at potentials that are positive of the reversible electrodeposition of the bulk metal. UPD is particularly important to the understanding of electrodeposition where processes in the UPD region can be expected to influence the growth and subsequent properties of bulk thin films. In electrocatalysis, sub-monolayers of metals such as Bi, Pb, and Tl on noble metal surfaces have shown enhanced catalytic activity for a variety of electroreduction processes. Modified catalytic activity has also been observed for Pd and Pt overlayers consisting of a few monolayers. For this reason UPD processes are being examined by a variety of in situ techniques, including wafer curvature, in order to better understand the relationship between the structure of the UPD adlayers and the electrocatalytic activity. Changes in surface stress can be measured by monitoring the curvature of a wafer or cantilever while in solution and under potential control. Surface stresses on the order of 0.01 N/m can be resolved, sufficient to study the adsorption of molecular monolayers onto the electrode surface. Equation (1) quantifies the change in surface stress associated with epitaxial overlayer growth1, ΔF = Δfo + Δfpzc + Δh + Σcohtf (1) where Δfo is the intrinsic surface stress difference between the overlayer and the substrate, Δfpzc is an electrocapillary term that reflects the change in capacitance and potential of zero charge (pzc) associated with changing the surface from the substrate metal to the overlayer metal, Δh is the interface stress, and ΔΣcohtf is the product of the coherency stress and the film thickness. For sub-monolayers, Eqn. (1) may also contain an additional term that captures the contribution of the compressive strain due to island size effects2. Although the misfit term in Eqn. (1) is often assumed to dictate the overall change in surface stress, this is not always the case. The upd of Al and Cu onto Au are clear examples where the stress change is compressive even though the lattice misfit is positive. In some cases, pseudomorphic film growth of several monolayers allows the coherency term to be distinguished from the interfacial terms, which are independent of film thickness. Measurements of this type have been made for the electrodeposition of Pd onto (111)-textured Au3, and have resulted in estimates of Δh for the Pd-Au interface to be about -1.2 N/m. This is similar in both sign and magnitude to values reported for Ag-Cu4. This talk will examine the surface stress changes associated with underpotential deposition, paying particular attention to the interfacial contributions, in addition to that associated with lattice misfit.

Fundamental Aspects of ad-Metal Dissolution and Contamination in Low and Medium Temperature Fuel Cell Electrocatalysis: A Cu Based Case Study Using In Situ Electrochemical X-ray Absorption Spectroscopy

Electrochemical methods (cyclic voltammetry and rotating ring disk electrode) and in situ X-ray absorption spectroscopy (XAS) were used in conjunction to study copper adsorption onto carbon-supported platinum nanoparticles over the operating potential range of proton exchange membrane fuel cell (PEMFC) cathodes and anodes (∼0.0–1.0 V vs the reference hydrogen electrode, RHE). Our purpose was to better understand the detrimental effects of Cu leaching from high-activity dealloyed PtCux electrocatalysts. These studies were conducted in CuSO4-doped 0.1 N solutions of HClO4 and H2SO4 under both inert and oxygenated conditions. Over the anode potential range (∼0.0–0.3 V vs RHE), concentrations of Cu2+ as low as 10 μM were found to coat the active Pt surfaces, thereby drastically inhibiting the hydrogen oxidation reaction. Over the Cu underpotential deposition region (0.35–0.70 V vs RHE), Cu2+ concentrations ≥0.05 mM resulted in Cu deposition onto Pt. This was found to lower the oxygen reduction reaction activi...

Observations in the H-Pd{111} System via Stress and EQCM Measurements

The under-potential deposition (UPD) & absorption of hydrogen in thin film palladium (45–75 nm) was investigated electrochemically utilizing both EQCM and thin film stress measurement techniques. The total frequency change during H absorption (desorption) as measured by EQCM is attributed to both an increase (decrease) in mass, Δfm, as well as the associated thin film stress evolution, Δfs. In order to deconvolute these two contributions, the changes in the Pd{111} thin-film stresses were measured directly by a high-resolution cantilever curvature-based stress monitoring technique. Mass incorporation and stress generation due to hydrogen absorption were recorded during cyclic voltammetric scans of increasingly cathodic potentials as well as during chronoamperometric holds in the hydrogen UPD region. The combination of stress measurements and mass uptake of hydrogen in the purely elastic regime provided a measure of a biaxially constrained partial molar volume of hydrogen in the Pd thin films, = 0.44 cm3/mol. Additionally, the occurrence of hysteresis and dramatic changes in slope of stress vs. H concentration during loading and unloading of thin films is explained in the context of elastic-plastic transitions that occur in the film.

Study of CO Oxidation on Polycrystalline Pt Electrodes in Acidic Solution by ATR-SEIRAS

Adsorption and electro-oxidation of CO on a polycrystalline Pt electrode in acidic solutions were systematically revisited by in situ attenuated-total-reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) in conjunction with related Gram–Schmidt response analysis. CO was either adsorbed in the double-layer region, i.e., 0.45 V (RHE) (denoted as CO@DL) or in the hydrogen underpotential deposition region, i.e., 0.1 V (RHE) (denoted as CO@UPD). The results indicate that the CO@UPD and H2Ofree coexisted structure (or simply costructure) forms only at a sufficiently high global CO coverage (H2Ofree denotes hydrogen-bonding-broken water); In contrast, the CO@DL and H2Ofree costructure forms in an earlier adsorption phase, less dependent on the global CO coverage. The partial oxidation of CO from solution and weakly adsorbed COL at the active sites is suggested to yield a prepeak that occurs with the relaxation of the COad-H2Ofree costructure and the disorganization of the outer water net layers. In the main oxidation process, the oxidation of CO@UPD tends to proceed via the “mean-field approximation” kinetics due to the high COad mobility resulting from the oxidation prepeak. The oxidation process of CO@DL is, however, likely via the “nucleation and growth” kinetics due to the good stability of the local CO@DL and H2Ofree costructured islands. The H2Ofree can be better assigned to the “probe” of the local COad coverage rather than the main oxygenated species for COad oxidation according to the spectral results for both CO@UPD and CO@DL.

Detection of lead ions in picomolar concentration range using underpotential deposition on silver nanoparticles-deposited glassy carbon electrodes

The efficacy of silver-deposited glassy carbon electrode for the determination of lead ions at the sub-nanomolar concentration ranges is investigated. The silver nanoparticles are electrodeposited on glassy carbon electrode using chronoamperometry and the electrode surface is characterized using SEM. Lead ions are detected in the region of underpotential deposition. The analysis is performed in square wave mode in the stripping voltammetry without the removal of oxygen. The detection limit of 10pM has been obtained with a constant potential of −0.7V during the electrodeposition step for a period of 50s. The interference of surfactants in the detection of lead ions is also studied.

Nanoscale Electrodeposition of Ga on Au(111) from Ionic Liquids

Electrocrystallization of Ga on Au(111) in two different ionic liquid electrolytes has been investigated by in situ electrochemical STM imaging with partly nanometer resolution. In chlorogallate-containing melts underpotential deposition (UPD) leads to surface alloying whereby a Ga−Au surface layer is formed on top of Au(111). In AlCl3−[C4mim]Cl + 5 mM GaCl3 melts, 2D-electrochemical phase formation is driven by conventional nucleation and growth of 2D Ga islands. Prior to the UPD processes anion adsorption of GaCl4− and AlCl4−, respectively, occurs which is characterized by well-ordered anion layer structures with comparable lattice constants. At potentials negative of the UPD region, 3D Ga clusters are formed. On the basis of their shape and the coexistence of small and large clusters in contact, it is concluded that the clusters in the size range 5−30 nm are solid. Taking into account that the temperatures during electrodeposition ranged up to 313 K—above the melting point of α-Ga of 303 K—this observation is remarkable.

Thallium underpotential deposition on silver nanoparticles: size-dependent adsorption behaviour

We report the size-dependent adsorption of thallium on silver nanoparticles (AgNPs) in the region of underpotential deposition; the phenomenon is observed for large nanoparticles, and bulk silver, but absent for nanoparticles below 50 nm diameter.

Electrochemical Measurement of the Surface Alloying Kinetics of Underpotentially Deposited Ag on Au(111)

The cyclic voltammetry characterizing underpotential deposition (UPD) of Ag onto Au(111) varies in the literature with respect to the characteristic UPD peaks in both position and number. Rooryck et al. (1) confirmed that the discrepancy in terms of peak position, specifically the initial UPD to which a third of a monolayer of deposition is attributed, is due to a variation in the quality of the surface. Clean, smooth Au(111) surfaces yield a peak position of 0.53 V vs Ag0/Ag+, while rough disordered surfaces yield a peak position of 0.61 V vs Ag0/Ag+. Repetitive potential cycling in the UPD region resulted in a gradual shift in peak position, with time as the deposited Ag alloyed with, and was stripped from the surface leaving vacancies. We provide a methodology for tracking the rate at which UPD Ag alloys with the Au(111) surface without the use of continuous potential cycling. A simple kinetic model is developed for the surface alloying of Ag on Au(111), from which we extract an activation barrier and attempt frequency for this process. Notably, we introduce a novel technique for the inexpensive parallel fabrication of Au(111) single crystals that allowed us to build statistics and ensured reproducibility of our data.

Mechanism of formation and electronic structure of semiconducting ZnSb nanoclusters electrodeposited from an ionic liquid

Electrocrystallization of Sb and the compound semiconductor ZnSb has been investigated by in situ SPM methods at the electrified ionic liquid/Au(1 1 1) interface at an elevated temperature of 50 °C for the first time employing the ionic liquid ZnCl 2–[C 4mim] +Cl − (45:55). Prior to the underpotential deposition (UPD) process of Sb, ZnCl 3 − anions adsorb on the gold surface at the open-circuit potential (OCP). An ordered region – showing the characteristic of a Moiré-like pattern – coexists with a disordered region indicative of an interfacial phase transition. When the potential is reduced to −0.40 V versus Pt/Pt(II), 2D electrocrystallization of Sb starts showing a typical 3 × 3 structure of the first monolayer. Further decreasing the potential to −0.5 V a second layer of Sb islands occurs. Stepping the potential from the UPD region to −0.60 V, the OPD of Sb sets in showing randomly dispersed clusters of homogeneous size. Near the ZnSb deposition potential, at ∼−0.95 V, a nearly homogeneous distribution of clusters of spherical shape with diameters up to 15 nm is found. Their corresponding STS curves exhibit an obvious semiconducting behaviour with a gap-energy of ∼0.6 ± 0.2 eV. Experiments at deposition conditions on the Sb-rich or Zn-rich side relative to the ZnSb deposition potential show an obvious doping effect – in the case of Zn excess – which is revealed by the corresponding normalized conductance (NC) spectra.

Electrochemical silver deposition on adsorbate modified Au(111) electrode

The silver underpotential deposition (UPD) on thymine modified and unmodified electrodes was investigated using classical electrochemical methods combined with photoemission experiments. On unmodified electrodes positive of the Nernst potential two silver monolayers were formed. The first formed monolayer differs in its work function as well as in its 3d-band binding energy from the second monolayer/bulk material. On thymine modified electrodes only one silver monolayer is formed whereas the formation of the second monolayer is completely blocked in the UPD region. On both, the deposited silver monolayer and the bulk silver, thymine is adsorbed topmost, interacting via its N(3) atom with the surface. XPS spectra reveal an increased electron density at the N(3) in the adsorbed state compared to the signal arising from vacuum deposited thymine multilayers. The metal deposition in electrochemical environment includes two steps, firstly on the electrode adsorbed thymine has to be removed by silver ions; secondly thymine has to be readsorbed on the topmost metal layer. Therefore the influence of adsorbed thymine on metal deposition processes is strongly correlated to the energy balance between the adsorption energies of metal adatoms, thymine on the electrode and thymine on the final topmost layer. Adjusting these energies leads to the different influence of chemisorbed thymine on the formation of the first and second silver monolayer.

In situ STM investigation of spinodal decomposition and surface alloying during underpotential deposition of Cd on Au(111) from an ionic liquid

The electrodeposition and anodic dissolution of Cd on Au(111) in an acidic chloroaluminate ionic liquid (MBIC-AlCl(3), 42 : 58 mol%) have been investigated by cyclic voltammetry and in situ STM. In the Cd underpotential deposition region, various nanostructures can be distinguished. At a potential of 0.95 V vs. Al/Al(iii), a transformation from a well ordered AlCl(4)(-) adlayer to a ( radical3 x radical19) superstructure, presumably due to Cd-AlCl(4)(-) coadsorption, is observed. Reducing the potential to 0.45 V, surface alloying of Cd and Au occurs, which is evidenced for the first time by typical spinodal structures occurring both during deposition and dissolution of the surface alloy layer having a hexagonal structure. At still lower potentials below 0.21 V, a layer-by-layer growth of bulk Cd sets in.

Codeposition of copper and tin from acid sulphate solutions containing polyether sintanol DS-10 and micromolar amounts of halides

Cu(II) and Sn(II) reduction in acid sulphate solutions containing polyether laprol DS-10 was investigated using voltammetric XPS and XRD techniques. Bright yellow bronze coatings can be deposited at potentials ( E) that are positive than equilibrium potential ( E eq Sn ) of Sn|Sn 2+ electrode. Here, Sn(II) reduction might be treated as underpotential deposition (UPD) of tin on foreign (copper) substrate. Further incorporation of tin into integral Cu–Sn crystallic lattice yields the mixture of pure copper, α-CuSn phase and intermediate hexagonal hcp phase. The formation of free tin phase occurs at E < E eq Sn . This gives rise for strong inhibitive adsorption of sintanol that manifests itself in the development of deep voltammetric minimum. Addition of halides results in the shift of codeposition potential to more negative values and in the increase of copper content in the coatings deposited in the UPD region. The action of halides intensifies in the sequence Cl − < Br − < I −. If iodide concentration exceeds 2–3 μM, deposition of yellow bronze becomes impossible.

Photoinduced Thermal Copper Reduction onto Gold Nanocrystals under Potentiostatic Control

We study the photoreduction of adsorbed copper ions onto Au nanoparticles, on an indium tin oxide (ITO) electrode in an aqueous electrochemical cell, as a function of applied voltage and laser intensity. The photocurrent is a nonlinear function of laser intensity and increases sharply with cathodic voltage in the underpotential deposition region. The photoreduction is attributed to laser heating of the Au nanoparticles rather than "hot electron" processes. Numerical simulation of the Butler-Volmer kinetic equation using experimental parameters predicts a several orders of magnitude increase in current for a temperature rise of a few Kelvin.