Oscar Alejandro Oviedo

Metal Clusters and Nanoalloys

Experimental and simulated electron microscopy in the study of metal nanostructures.- Density-functional theory of free and supported mtal nanoclusters and nanoalloys.- Closed-shell metal clusters.- Optical properties of metal nanoclusters from an atomistic point of view.- Spin-fluctuation theory of cluster magnetism.- Thermodynamics and kinetics using semi-empirical approaches.- Structure and chemical ordering in nanoalloys: Towards nanoalloys phase diagrams.- Modeling of Janus nanoparticles.- Modeling of protected nanoparticles.- Thermodynamic modeling of metallic nanoclusters.

A Straightforward Approach for the Determination of the Maximum Time Step for the Simulation of Nanometric Metallic Systems.

In the present work, we report on a systematic analysis to determine the maximum time step allowed in molecular dynamics simulations applied to study metal systems of current interest in nanoscience. Using the velocity Verlet integration scheme, we have found that it is possible to use a 20 fs time step for the simulation of gold nanosystems. This is roughly an order of magnitude greater than the usually employed integration step (2 to 5 fs). We also propose a general criterion to select this maximum time step for other metallic nanosystems, even in the case of bimetallic nanosystems.

Diffusion mechanisms taking place at the early stages of cobalt deposition on Au(111)

In the present work a detailed atomic-level analysis of some of the main diffusion mechanisms which take place during cobalt adatom deposition are studied within atom dynamics (AD) and the nudged elastic band (NEB) method. Our computer simulations reveal a very fast exchange between Co and Au atoms when the deposit is a single cobalt adatom. However, when the nucleus size increases, a decrease in the exchange probability is observed. Activation energies for different transitions are obtained using AD in combination with the NEB method.

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:

Phenomenology and Thermodynamics of Underpotential Deposition

As can be found from the other chapters of this book, underpotential deposition shows a wide variety of behaviors, which involve the occurrence of several surface phases, formation of submonolayers, monolayers (ML) and eventually the formation of bilayers. Adsorption may be commensurate or incommensurate, where the ML may undergo compression, and metal adatoms may coadsorb with anions to generate new phases. To start the discussion and briefly go into the history of the development of thermodynamics models for upd, we consider a relatively “simple” system, as shown in Fig. 3.1, which corresponds to Ag deposition on Pt(111) [1]. The voltammogram shown there presents three cathodic and three anodic peaks, which correspond to ML formation/desorption(3), bilayer formation/desorption(2) and bulk deposition/oxidation(1) of Ag. The peak potentials of the complementary processes do not coincide, denoting that at the present sweep rate a quasi equilibrium state has still not been reached. For the discussion below, we choose the anodic peaks, that we will denote with E 1, E 2 and E 3 (see Fig. 3.1). At the sight of the features of this voltammogram, and although the abscissa axis gives a measure for the of electrons at the working electrode, it may be appealing to use the position of the peaks found there as a measure for the stability of the different upd ad-phases being formed. In this spirit, Kolb et al. [2–4] introduced in the 1970s the concept of underpotential shift, ΔE upd.

Comment on “Surface thermodynamics reconsidered. Derivation of the Gokhshtein relations from the Gibbs potential; and a new approach to surface stress” by Stephen Fletcher

The present article comments on the article of Stephen Fletcher (Journal of Solid State Electrochemistry Volume 18, Issue 5, pp 1231–1238). The analysis deals with the validity of equation (31 or 40) of the latter for an ideally polarisable interface.