Silvana Graciela García

Ag UPD on Au(100) and Au(111)

The underpotential deposition (UPD) of Ag in the systems Au(hkl)/Ag + , ClO - 4 and Au(hkl)/ Ag + , SO 2- 4 with (hkl) = (100), (111) was studied by in situ STM and different electrochemical techniques including charge-coverage measurements with the twin-electrode thin-layer (TTL) technique. Ag UPD was found to occur stepwise and similar in both systems. At relatively high underpotentials expanded and commensurate Ag adlayers Au(100)-c(√2x5√2)R45°Ag and Au(111)-(4x4)Ag are formed. At low underpotentials, condensed commensurate overlayers with Au(hkl)-(1 x 1)Ag structures are formed via a first order phase transition. On Au(111), the condensed Ag phase is preferentially created on monatomic steps, whereas on Au(100) a simultaneous 2D nucleation of Ag on flat terraces is observed. Electrosorption valency measurements in the system Au(hkl)/Ag + , SO 2- 4 show γ = z = 1, indicating that coadsorption or competitive adsorption of anions can be excluded. Electrochemical and in situ STM results indicate similar charge-coverage behaviour in the system Au(hkl)/Ag + , ClO - 4 . Ag UPD on Au(hkl) occurs at positive potentials with respect to the potentials of zero charge of Au(hkl) and Ag(hkl). Therefore, a nearly constant anion coverage in the entire Ag UPD range cannot be excluded.

STM tip-induced local electrochemical dissolution of silver

Local dissolution/deposition processes under in situ scanning tunneling microscopy (STM) imaging conditions are studied in the systems Ag(111)/Ag+, ClO4− and Ag(111)/Ag+, SO42−. The results show that in both systems the local kinetics of these processes strongly depend on the polarization conditions. At STM-tip potentials more positive than the Ag/Ag+ equilibrium potential, a local dissolution of the Ag(111) substrate is observed even at cathodic substrate overpotentials at which the overall substrate current density is cathodic. This tip-induced Ag dissolution is in agreement with results obtained recently in the system Cu(111)/Cu2+. The enhanced local Ag dissolution is explained by a reduced Ag+ concentration underneath the STM tip promoted by both an electrostatic repulsion of Ag+ and a reduction of the mass transport due to the shielding effect of the tip. The possibility for a preparation of negative Ag nanostructures by STM tip-induced electrochemical dissolution is demonstrated.

Nanocrystallization process of the Fe69.5Cu1Nb3B9Si13.5Cr4 FINEMET-type alloy : an AFM study

The different devitrification stages of Fe 69.5 Cu 1 Nb 3 B 9 Si 13.5 Cr 4 (F-Cr4) amorphous alloy were studied by atomic force microscopy (AFM) on samples annealed at 400, 600 and 670°C for different treatment times. The AFM images showed no topographic changes in the alloys treated at 400°C, indicating that only structural relaxations took place at this annealing temperature. Samples thermally treated at 600°C showed hemispherical features uniformly distributed on the surface, suggesting initiation of the nanocrystallization process. The nanocrystals formed had a size distribution of 15-40 nm, which is slightly affected by the annealing time. At 670°C the crystallization phenomena occurred with the formation of large crystals and an increase in roughness. The corrosion behaviour of the nanocrystalline alloys in 2 M HCl solution was correlated with the AFM studies. The potentiodynamic experiments for these alloys showed a significant decrease of the current density peak at the active-passive transition in relation to the F-Cr4 amorphous alloy. The results indicated that the corrosion resistance and passivating ability of the alloys are improved by the thermal treatment.

In situ STM study of electrocrystallization of Ag on Ag(111)

The electrocrystallization process was studied in the system Ag(111)/Ag+, SO4= by in situ scanning tunneling microscopy (STM). The results show that Ag deposition occurs preferentially at step edges following a layer-by-layer growth mechanism, but polarization and imaging conditions greatly affect the local kinetics of this process. At STM-tip potentials more positive than the Ag/Ag+ equilibrium potential, a local dissolution of the substrate underneath the tip is observed even at low negative substrate overpotentials, at which the overall substrate current density is cathodic. An in situ STM imaging of Ag deposition was possible at sufficiently high negative substrate overpotentials. An estimation of the local deposition current density, however, indicates that the deposition rate underneath the STM-tip is reduced. These results are explained by the presence of an electric field between the STM-tip and the substrate, which affects the potential distribution directly underneath the tip, producing a large shielding of the diffusive flux of Ag+ ions.

Underpotential deposition and involved alloy formation of cadmium on silver particles modified HOPG substrates

Cadmium underpotential deposition (UPD) on Ag particles modified highly ordered pyrolytic graphite (HOPG) surfaces, and the involved alloy formation were studied by conventional electrochemical techniques. Voltammetric results indicated that the Cd UPD followed an adsorption behavior different from that observed for massive Ag electrodes and Ag particles supported on vitreous carbon. Nanometer-sized bimetallic Cd–Ag particles were characterized by ex situ atomic force microscopy (AFM). Initially, AFM images show Ag deposits of similar size distributed preferably on HOPG step edges. No remarkable morphological changes are observed on the surface after the subsequent Cd deposition, suggesting that the Cd particles are deposited selectively over the Ag crystals. From the analysis of desorption spectra, employing different polarization times, and density functional theory (DFT) calculations, the formation of a Cd–Ag surface alloy could be inferred.

Applications of Underpotential Deposition on Bulk Electrodes as a Model System for Electrocatalysis

The underpotential deposition (upd) of metals may modify the catalytic activity of substrates in several ways. For the sake of simplicity, we divide the types of impacts that upd metals can produce in four types, although they all can in principle be acting on a given reaction at the same time.

Experimental Techniques and Structure of the Underpotential Deposition Phase

The electrochemical deposition of metals on foreign substrates is a complex process which includes a number of phase formation phenomena. The very initial electrodeposition stages of a metal, M, on a foreign substrate, S, involve adsorption reactions as well as two- and/or three-dimensional nucleation and growth processes. The most important factors determining the mechanism of electrochemical M phase formation on S are the binding energy between the metal adatoms (Mads) and S, as well as the crystallographic misfit between the 3D M bulk lattice parameters and S. As we have shown in Fig. 1.3, when the binding energy between the depositing M-adatoms and the substrate atoms exceeds that between the atoms of the deposited metal, low dimensional iD metal phases (i = 0, 1 and 2) are formed onto the foreign metal substrate. This phenomenon, introduced in Chap. 1 as underpotential deposition (upd) [1–4], has been known for a long time and it has been intensively subject of study in the past decades since 1970s. This has been demonstrated by many studies of the upd process of different metals on mono- and polycrystalline substrates as well as reviews on the subject. The understanding of the nature of this phenomenon as conceived in the middle 1990s can be found in the book of Lorenz et al. [1]. Reviews available in the literature include the works of Kolb et al., Abruna et al., Sudha and Sangaranarayanan, Aramata [5–8], and the work of Szabo [3] concerning the theoretical aspects of upd, updated by Leiva [9], and also the works of Adžic [10] and Kokkinidis [11], concerning mainly the catalytic effects of upd adatoms.

Underpotential Deposition and Related Phenomena at the Nanoscale: Theory and Applications

Macroscopic materials composed by transition metals such as Ag, Au, Cu, Pd, and Pt are ductile, malleable, display excellent electrical and heat conductivity and high optical reflectivity. These properties have allowed these materials to be widely used in several areas, such as electrical contacts and conductors and the catalysis of chemical reactions. When the size of these materials decreases to the nanometric scale, these particles show unique properties, which cannot be observed in macroscopic-sized materials. The number of synthesis methods of these nanomaterials and their new and possible technological applications has increasingly grown during the last decade. This progress has inevitably led to the commercialization of several nanomaterials. For example, Ag nanoparticles (NPs) have been used as a type of antimicrobial reactive of broad spectrum in Medicine, in mass consumer products, in antiseptic aerosols of domestic use, in antimicrobial water filters coatings, etc. [1]. Apart from these commercial applications, nanomaterials are largely used in the field of research, e.g. Plasmonics, Medicine, reactivity, combustion cells, etc. [2, 3].

What Is Coming Next

In the following sections we will discuss on some tendencies and prospects concerning underpotential deposition (upd) research, related to both theoretical and experimental works.

Modelling of Underpotential Deposition on Bulk Electrodes

As discussed in Chaps. 2 and 3, a wide variety of experimental techniques have allowed to obtain a wealth of information of upd systems. This information concerns: