Atomic structure at the interface

The atomic structure at the interface depends upon the shape of the metallic clusters in contact with the substrate and upon the phase coherence between the deposit and the substrate lattice. As far as clusters are concerned, before understanding how the substrate modifies their structure, it is worthwhile recalling some of their atomic characteristics in the free state. 

Geometric structure of free clusters Most of the properties of small clusters in the free state are determined by the outer atoms which have an incomplete coordination shell. Defining an elementary atomic volume 4nRl/3, and assuming that an n-atom cluster is like a liquid drop of volume 4nR /3) = n4nR /3 , one may estimate the number of surface atoms ns, and the fraction of under-coordinated atoms in the cluster / = Us/n. Here, Us is equal to 4nR /nR and typical values of / are 0.86 and 0.4 for clusters of 100 and 1000 atoms. Up to a few hundred atoms, the cluster properties are thus mainly determined by their surface. However, the size criterion is only qualitatively valid, since the actual structure and shape of the particles are also important. Much work has been performed during the last ten years to understand the peculiarities of these small clusters (Joyes, 1990 Jortner, 1992)  

At a more microscopic level, the arguments developed in Chapter 2 for surface relaxation, which are based on the competition between covalent energy and short-range atom-atom repulsion, also lead to contractions of bond-lengths in the neighbourhood of under-coordinated atoms. 

Shape of supported clusters When clusters grow or are deposited on surfaces some of the peculiarities of isolated clusters persist, at least qualitatively, and they have to be considered in the interpretation of experiments. However, their shape is no longer uniquely determined by their surface energy r, but also by the substrate surface energy ts and the interfacial energy Ti. Winterbottom (1967) has written down the equilibrium condition for the shape of a particle on a substrate, at given volume, temperature and chemical potential  

The generalization of the Wulf construction to solid-solid interfaces leads to the discrimination of four cases (Fig. 5.4, left column), which correspond, respectively, to  

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All of these techniques, although powerful, do not reveal the structure and geometric arrangement of atomic species at the interface. Thus, in spite of its importance, our knowledge of the structure of the electrode/solution interface at the atomic level is still very rudimentary. 

Browning, N. D. and Pennycook, S. J. (1996). Direct experimental determination of the atomic structure at internal interfaces. J. Phys. D 29, 1779-88. 

In this contribution, we will first provide an overview of the nature of the systems and phenomena and the modeling and computational challenge which they represent. In the following two sections we describe calculations of the electronic and atomic structure of the interface and of electron transfer at the interface. In each case we present some details of our own results involving copper-water interfaces and electron transfer from a copper ion in... 

To investigate further the change in electron distribution within the metal cluster we used CO as a probe molecule to monitor the modifications induced in the Ni6 clusters by the interaction with the support. We found that when CO is adsorbed on the one-layer Ni6 cluster deposited on alumina the CO vibrational frequency is substantially blue shifted compared with free Ni6. This indicates that charge transfer from the Ni overlayer to the oxide occurs, with a consequent reduction of back-donation into the Ni-CO bond. When, on the other hand, CO is adsorbed on the two-layer Nig cluster deposited on alumina we found little change in the CO adsorption properties compared with the situation without substrate. The metal layer in direct contact with the substrate is partially oxidized whereas the second Ni layer is almost unperturbed by the substrate. The change in electronic structure at the interface is rapidly screened for the upper metal layers thus, Ni atoms of the second layer where CO is adsorbed behave similarly in supported and unsupported clusters. 

Surface X-ray diffraction is now a well-estabhshed technique for probing the atomic structure at the electrochemical interface, and, since the first in-situ synchrotron X-ray study in 1988 [1], several groups have used the technique to probe a variety of electrochemical systems [1, 2, 9]. It is beyond the scope of this article to provide a comprehensive description of basic X-ray diffraction from surfaces. Readers are referred to the excellent reviews by Feidenhans l [10], Fuoss and Brennan [11], and Robinson and Tweet [12] for exphcit details. It should be noted that, throughout this review, the acronym SXS is used to describe the X-ray measurements, although all of the results are obtained by X-ray diffraction. 

XAS (EXAFS)-investigations of copper catalysts under real catal rtic conditions indicated the presence of a metallic copper bulk and reversible small changes of the Cu-Cu nearest neighbour distances and coordination numbers correlated with the methanol-conversion and with the oxygen content in the gas phase. These changes were interpreted as the formation of a nanocrystalline copper bulk structure by reversible intercalation of atomic oxygen at the interface of the nanociystallites and not in the regular Cu lattice [5]. 

Lucas, C.A. (1999) Atomic structure at the electrochemical interface. Journal of Physics D Applied Physics, 32, A198-A201. 

From an experimental standpoint, information on the dye binding modes at the semiconductor/dye interface, are conventionally accessed by vibrational spectroscopy [Fourier Transform InfraRed (FT-IR) spectroscopy and Surface-Enhanced Raman Spectroscopy (SERS)] [228-237]. These techniques can provide structural details about the adsorption modes as well as information on the relative orientation of the molecules anchored onto the oxide surface. Photoelectron Spectroscopy (PES) has also been successfully employed to characterize the dye/oxide interface for a series of organic dyes [238-242]. The analysis of the PES spectra yields information on the molecular and electronic structures at the interface, along with basic indications of the dye coverage and of the distance of selected atoms from the... 

Compared to the results of photoelectron spectroscopy, which are very sensitive to changes in charge distribution and electronic structure, we believe that the examination of the vibrational properties of such complexes offers a more direct probe to the actual chemical structure at the interface. In recent works [118, 120], we have described the evolution of the vibrational spectrum calculated for a polyene molecule, octatetraene, upon bonding of two A1 atoms, in order to model the Al/polyacetylene interface formation. These theoretical results indicate that important changes can be expected in the experimental infrared spectrum as a consequence of (i) the formation of Al-C covalent bonds and (ii) strong modifications in the bond pattern along the chain. 

Electron diffraction investigations showed that epitaxy did indeed exist when one metal was electrodeposited on another, but that it persisted for only tens or hundreds of atomic layers beyond the interface. Thereafter the atomic structure (or lattice) of the deposit altered to one characteristic of the plating conditions. Epitaxy ceased before an electrodeposit is thick enough to see with an optic microscope, and at thicknesses well below those at which pseudomorphism is observed. 

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