Metal electrodeposition on single crystal metal surfaces mechanisms, structure and applications

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Abstract

This paper is a short review about the principle, mechanisms and applications of epitaxial electrochemical growth of ultrathin metal films on single crystal metal electrode surfaces. Starting from an atomic scale view of electrodeposition the parallel is made with the parent technique molecular beam epitaxy (MBE) in ultra high vacuum (UHV). A first section is devoted to homoepitaxy studies. Subsequent sections deal with selected studies of heteroepitaxy of magnetic materials and electrocatalysts.

Introduction

The physicochemical properties (magnetic, catalytic, optical, etc.) of metallic nanostructures deposited on a metal surface critically depend on their size, morphology and crystal structure. The chemical interactions with the substrate and the environment are also a crucial issue. In vacuum conditions, several decades of advanced surface science have contributed to give a reliable atomistic description of nucleation and growth for a precise control of metal on metal homoepitaxy and heteroepitaxy [1], [2]. Electrodeposition remains comparatively more empirical even though an atomic view of the process has now emerged [3], [4] thanks to the use of in situ scanning tunnelling microscopy (STM) [5], [**6] and in situ synchrotron based X-ray techniques on well defined single crystal electrodes [7], [8]. A full understanding of the interface structural characterizations may be completed by analytical surface characterizations using in situ vibrational techniques [4], [3].

In electrodeposition the metal species are dissolved in solution in the form of solvated cations or complexes. Metal phase formation requires reducing them by transfer of electrons from the substrate according to the reaction MZ+ + ze  M which standard redox potential is E0 = E(MZ+/M). For convenience, one often defines the deposition overpotential η = E0  U: electrodeposition requires therefore applying η > 0 or an electrode potential U smaller than E0. In some instances under potential deposition (UPD) is observed, i.e. deposition occurs for η < 0 [9]. The UPD process resembles an adsorption stage with the formation of a uniform monolayer composed of metal adatoms and anions [10]. This phenomenon is the archetype of a surface-limited reaction. It is sensitive to the nature of the anions and in some instance a specific anion may induce UPD [11]. UPD will not be addressed in this paper. The reader may consult a recent review about the application of surface-limited reactions to deposit layers and superlattices of II–VI semiconductors [12]. Electrodeposition is also suitable to deposit various epitaxial oxide layers [13], [14] ZnO [15], [16] and II–VI semiconductors [17] with excellent electronic and optical properties. These data will not be reviewed here for lack of space.

The scope of this paper is to review overpotential metal electrodeposition on well defined single crystal metal electrodes and to discuss the principles and the mechanisms of deposition on the atomic scale and to compare them with those involved in the parent technique molecular beam epitaxy (MBE) in ultra high vacuum (UHV). Homoepitaxy is considered in a first section as a test ground of theory. The following sections deal with selected studies of heteroepitaxy in the field of magnetism and electrocatalysis. The self-ordered growth of nanostructures, which requires controlled growth on pre structured surfaces, was discussed recently [*18].

Section snippets

Homoepitaxy

In overpotential deposition the flux of adatoms F is proportional to the faradic current corresponding to the reduction of MZ+ species. For small η the Butler–Volmer equation applies and F  [exp{αzη/kT}  1], with α the transfer coefficient (0 < α < 1) and all other symbols have their usual meaning [19]. F becomes however independent of η above a critical η value because the rate of deposition becomes limited by the transport of metal species to the electrode surface: In these conditions F  D0C0/δ with

Heteroepitaxy of ultrathin magnetic films on Au(1 1 1)

Magnetism is one field where the results of fundamental research quickly find interest in industrial applications. For instance studies of materials growth and the physics of magnetic multilayers found applications in the design of recording media and GMR read heads (Giant magnetoresistance) [25], [26]. Electrodeposition could be considered as an alternative method for fabricating read heads since electroplated magnetic multilayers exhibit a GMR [27].

More recent fundamental studies have

Heteroepitaxy of ultrathin films for electrocatalysis studies

Numerous surface science studies have been aimed at investigating the interplay between the catalytic activity of a material and its surface structure and composition. The so-called d-band theory has rationalized many observations by relating effects to changes in the energetic position of the d-band centre as a function of the atom spacing and the chemical environment [**43]. Two approaches are described below, in which the control of electrodeposition was mandatory to put in evidence and

Concluding remarks

We hope that the examples described in this brief review demonstrate that electrodeposition is a powerful technique to grow epitaxial layers on well defined electrode surfaces. Electrodeposited metal layers may have a different structure and/or a much smoother morphology compared with the corresponding MBE layers, which is of further interest in fundamental studies of the physicochemical properties of a given system. The difficulty with electrodeposition is however predicting film morphology.

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