Surface spectroscopy LEIS

Remember 7.4 Local Electrochemical Impedance Spectroscopy (LEIS) is a relatively new and underutilized technique that is useful for exploring the influence cf surface heterogeneities on the impedance response. 

In the study of the surface phases of the Pt-Sn system, as well as of other binary systems, a variety of experimental methods are available. Surface spectroscopies based on ion or electron interaction with the surface provide composition information with a depth resolution that can go from a few atomic layers (X-ray photoelectron spectroscopy, XPS and Auger electron spectroscopy, AES) to single atomic layer resolution. The latter can be obtained by low energy ion scattering (LEIS) a method which has been extensively used for the study ot the Pt-Sn system. Since surface spectroscopic methods are rather well known we will not review them in detail here. 

Recently, two new electrochemical mapping techniques have become available the scanning vibrating electrode technique (SVET) and the localized electrochemical impedance spectroscopy (LEIS) technique. These techniques provide the capability to identify and monitor electrochemical behavior down to the micron level. These represent significant advances over traditional electrochemical methods (cyclic voltammetry, EIS, and even EQCM), which provide data that reflect only an average over the entire sample surface. Although such data are very useful, a major drawback is that no local or spatial information is obtained. 

Lillard and coworkers developed a method called Local Electrochemical Impedance Spectroscopy LEIS [39). It relies on the fact that ac current densities in the solution very near to the working electrode are proportional to the local impedance properties of the electrode. In order to determine the current densities normal to the surface, the ac potential drop was measured between planes parallel to the electrode surface employing a two-electrode microprobe. A schematic of a commercially available experimental setup for LEIS is... 

The local electrochemical impedance spectroscopy (LEIS) " is another scanning probe technique that can map the ac impedance distribution over an electrode surface. In LEIS a sinusoidal voltage perturbation between the working and reference electrode is maintained by driving an ac current between the working... 

In situ characterization. Catalysts should preferably be investigated under the conditions under which they are active in the reaction. Various reasons exist why this may not be possible, however. For example, lattice vibrations often impede the use of EXAFS, XRD and Mossbauer spectroscopy at reaction temperatures the mean free path of electrons and ions dictates that XPS, SIMS and LEIS are carried out in vacuum, etc. Nevertheless, one should strive to choose the conditions as close as possible to those of the catalytic reaction. This means that the catalyst is kept under reaction gases or inert atmosphere at low temperature to be studied by EXAFS and Mossbauer spectroscopy or that it is transferred to the vacuum spectrometers under conditions preserving the chemical state of the surface. 

Techniques based on the interaction of ions with solids, such as secondary ion mass spectrometry (SIMS) and low-energy ion scattering (LEIS) have undoubtedly been accepted in catalyst characterization, but are by no means as widely applied as for example X-ray photoelectron spectroscopy (XPS) or X-ray diffraction (XRD). Nevertheless, SIMS, with its unsurpassed sensitivity for many elements, may yield unique information on whether or not elements on a surface are in contact with each other. LEIS is a surface technique with true outer layer sensitivity, and is highly useful for determining to what extent a support is covered by the catalytic material. Rutherford backscattering (RBS) is less suitable for studying catalysts, but is indispensable for determining concentrations in model systems, where the catalytically active material is present in monolayer (ML)-like quantities on the surface of a flat model support. 

In this article, the study of alloy electrodes by a combination of EC with LEISS is discussed. LEIS spectroscopy is unparalleled in its ability to interrogate only the outermost layer this is because of the repulsive nature of ion-atom interactions that serves to mask interactions between sub-surface species with the probe ions. LEISS is an established surface physics technique but its adoption in surface electrochemical investigations has not been pervasive. For the work reviewed here, an instrument that incorporates EC with LEISS, LEED, XPS and TPD was employed. 

Fig. 3.2 Characterization of Ptj Ni(hkr) surfaces with (a) Auger Electron Spectroscopy (AES) (b) LEIS (revealed complete segregation of Pt for aU three orientations and the formation of the Pt-skin structure (c) UPS (confirmed that the position of d-band center is structure sensitive (d-f) LEED the Pt3Ni(l 11) surface exhibits a (1 X 1) pattern, Pt3Ni(100) has a (1 x 5) reconstruct pattern, and Pt3Ni(110) exhibits (1 x 2) periodicity. Reprinted with permission from [23], copyright 2007 by American Association for the Advancement of Science...

Ion scattering spectroscopy (ISS) was introduced by Smith [25] using noble gas ions and has become a powerful tool for surface analysis. In low-energy ion scattering (LEIS), a monoenergetic beam of low-energy ions in the 0.2-5-keV range is directed... 

---