Surface X-Ray Diffraction Study

Figwe 5. Proposed shiictural models for Ru(OOOl) oxidation, where the O, S, and Ru atoms are represented by die open, lightly-shaded, and heavily- shaded circles, respectively. The layer spacings are given in nm, and coverage is given in mono-layers. Reprinted widi permission from Copyright (2001) American Chemical Society. 

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The structure of alumina on NiAl(l 1 0) was the subject of a surface X-ray diffraction study by Stierle et al. [46]. The model derived by Stierle et al. from the analysis of the X-ray diffraction data was based on a strongly distorted double layer of hexagonal oxygen ions, where the Al ions are hosted with equal probability on octahedral- and tetrahedral-coordinated sites the resulting film structure was closely related to bulk k-A1203. An attractive feature of Stierle s model was that it provided a natural explanation of the domain structure of the alumina overlayer, which is induced by a periodic row matching between film and substrate lattices. However, as pointed out recently by Kresse et al. [47], this structure model has two bonds with... 

The presence of a hydrated film (FeOOH) was found to depend on the concentration of iron cations at the passive layer-solution interface [64]. In situ surface X-ray diffraction studies indicated that the film consists of a spinel crystal structure [65]. 

Wark, Whitlock, and co-workers [72]-[75] extend these ideas in shock compression of < 111 >-oriented silicon single crystals. The method of producing the shock wave differs from previous X-ray diffraction studies, but the basic concepts are the same. Higher X-ray fluences result in a time resolution of 0.05-0.1 ns. This permits a sequence of exposures at various irradiances and delay times, thus mapping the interatomic spacing of the shock-compressed surface as a function of time. 

A number of surface diffraction techniques can be employed in the structural study of electrochemical interfaces, depending on the details of the system under study. For bulk materials or thick films (such that the X-ray beam only samples that layer) conventional diffraction experiments can be performed and, in fact, a number of in situ X-ray diffraction studies of this type have been reported.126 129 In the case of thin films or monolayers, two different techniques can be employed and these are the reflection-diffraction technique introduced by Marra and Eisenberger,3 ), 32 and the technique based on surface truncation rods.131 In the first case, the incident X-ray beam impinges on the sample at an angle below... 

It is difficult to reconcile these very different views of the interaction of water and clay surfaces. Sposito (8.) has attempted this. He points out that the thermodynamic properties have an essentially infinite time scale, whereas the spectroscopic measurements look at some variant of the vibrational or a predecessor of the diffusional structure of water. It is possible that the thermodynamic properties reflect a number of cooperative interactions which can be seen only on a very long time scale. Still, the X-ray diffraction studies seemingly also operate on as long a time scale as the thermodynamic properties. There is still not a clear choice between the short-range and long-range interaction models. 

For the purposes of this chapter, which focuses on comparisons of isocyanide binding in transition metal complexes and isocyanide adsorption on metal surfaces, we first summarize known modes of isocyanide binding to one, two and three metals in their complexes. In such complexes, detailed structural features of isocyanide attachment to the metals have been established by single-crystal X-ray diffraction studies. On the other hand, modes of isocyanide attachment to metal atoms on metal surfaces are proposed on the basis of comparisons of spectroscopic data for adsorbed isocyanides with comparable data for isocyanides in metal complexes with known modes of isocyanide attachment. 

Adsorption and oxidation of NO adlayers on Pt(lll) was studied by using in situ surface X-ray diffraction and ex situ X-ray photoemission spectroscopy [86]. Molecular adsorption of NO was observed independently of the preparation of the adlayer. 

Ocko etal. [57, 58] have studied adsorption of bromide on Au(lOO) using in situ surface X-ray diffraction (SXD) in combination with electrochemical measurements. Low surface excess of bromide ions at Au(100)-(hex) caused a lifting of the... 

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