X-ray and Neutron Techniques for Nanomaterials Characterization by Challa S. S. R. Kumar

Author:Challa S. S. R. Kumar
Language: eng
Format: epub
Publisher: Springer Berlin Heidelberg, Berlin, Heidelberg

4.2 XAFS

4.2.1 Trends of In Situ XAFS

Since in situ XAFS does not require long-range order, in contrast to in situ SXS, it is applicable not only to single-crystal electrodes but also to various materials such as nanoparticles, metal complexes, and biomolecules adsorbed on polycrystal electrode surfaces and, of course, polycrystal electrodes themselves. Thus, taking advantage of the XAFS method, there have been many structural studies on polycrystalline types of electrodes, especially metal and alloy nanoparticles, in recognition of their technological applications to electrocatalysts.

Since XAFS is not inherently surface sensitive because of the relatively long penetration length of x-rays, various techniques were combined to selectively extract the information from the surfaces of interest. At the very early stage, most of the XAFS studies on surface analysis were performed using electron detection techniques which are surface sensitive but are not suitable for interfaces buried under liquid layers. Thus, various tailor-made in situ electrochemical cells suitable for each application have been developed as summarized in Sects. 3.2.2 and 3.2.3.

Pioneering structural studies at electrode/electrolyte interfaces using in situ XAFS were reported in the late 1980s by several groups [220] in relation to surface passivation films on metal surfaces [49, 50, 221], metal complexes [27], chemisorbed species [47, 222], and UPD metal monolayers [223–226] on single-crystal substrates, metal oxides [26, 227, 228], and materials for batteries and fuel cells [25].

Since one of the most attractive points of in situ measurements is detection and identification of intermediate species adsorbed on electrode surfaces, surface-adsorbed layers became a primary object of study. Abruña et al. successfully detected the Pt–I bond at an iodine monolayer adsorbed on a Pt(111) single-crystal surface in contact with an electrolyte solution [222]. Their initial experiment was performed with an incident angle of 45°, resulting in obscure signals due to Compton and elastic scatterings from the substrate. Subsequently, a configuration with a grazing angle was adopted, where total external reflection occurs, leading to significant suppression of Compton and elastic scatterings and an improvement in the signal-to-noise ratio. They also applied this technique for a structural determination of the Cu UPD layer formed on the Au(111) surface [223]. Fitting of the EXAFS oscillation at the Cu K edge showed that Cu atoms sit in the threefold hollow site of Au(111) with an oxygen species on top of the deposited Cu.

In research on corrosion/inhibition, McBreen et al. measured potential cycle dependence of the structures of the surface Ni-OH species of the Ni oxide polycrystal electrode using the transmission mode, which was explained in Sect. 3.2.2. In addition, time-resolved DXAFS was readily adopted for this system after its development. They also utilized the DXAFS technique to monitor the continuous change of oxidation states and local coordination structures of the Ni oxide [229, 230]. Davenport et al. measured the potential-dependent oxidation number of Cr in a polycrystal Al–Cr alloy using the fluorescence mode, which was explained in Sect. 3.2.3.

Since these pioneering studies were reported, many structural researches at the electrode/electrolyte interface involving species such as metal complexes [231–235],

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