XANES surface probe

XPS has typically been regarded primarily as a surface characterization technique. Indeed, angle-resolved XPS studies can be very informative in revealing the surface structure of solids, as demonstrated for the oxidation of Hf(Sio.sAso.5)As. However, with proper sample preparation, the electronic structure of the bulk solid can be obtained. A useful adjunct to XPS is X-ray absorption spectroscopy, which probes the bulk of the solid. If trends in the XPS BEs parallel those in absorption energies, then we can be reasonably confident that they represent the intrinsic properties of the solid. Features in XANES spectra such as pre-edge and absorption edge intensities can also provide qualitative information about the occupation of electronic states. 

XANES spectra measured the surface chemistry and the TEY probe indicated that sulfur for all concentrations was in the sulfide form. 

XANES contains information of the stereochemical details (coordination geometry and bond angles) and EXAFS gives information about local structures in terms of atomic radial distribution (distances) around the central atom. Some techniques are very deep probes while others are strictly surface oriented e.g., with the EPMA and SIMS, the probe depth is 1 nm with XPS, 2 to 7 nm with EXAFS, 0.5 nm, with XANES, 5 to 50 nm, with AES, 1 to 3 nm, and with ISS, it is only 0.3 nm. 

For EXAFS and particularly for XANES, data analysis is complex. The oscillation frequency/bond distance dependence means that extensive use is made of Fourier transform analysis. Most applications to date have been in the EXAFS region. In order to acquire sufficiently strong signals in a reasonable time, use has to be made of high-intensity photon fluxes, which are available at synchrotron facilities. These provide a broad-band tuneable source of high-intensity radiation, but the reduced number of facilities limits widespread dissemination of the technique. Reflection (fluorescent detection) mode is usually preferred to transmission. Experiments can be conducted in any phase, and the probing of electrode surfaces in situ is an important application. 

In many materials problems, for example at surfaces or interfaces, the chemical composition and nuclear coordinates are not fully known. Indeed, any information which can be obtained by theory on these basic structural properties will be useful, in conjunction with experiment. Spatially Resolved Electron Energy Loss Spectroscopy (SREELS), X-ray near-edge absorption (XANES) and emission, Mossbauer spectra, etc. provide site-specific probes which can be combined with theory to help resolve structures. 

The experimental detection and quantification of surface species on in situ soil particles and other natural colloids is a difficult area of research because of the sample heterogeneity, low surface concentrations, and the necessity to investigate the solid adsorbents in the presence of water. Unambiguous information can be obtamed only with in situ surface spectroscopy, such as x-ray photoelectron (XPS), extended x-ray absorption hne structure (EXFAS), x-ray absorption near-edge structure (XANES), melastic electron tunneling (lETS), and electron energy loss (EELS) spectroscopies. Recent advances in the development of nonevasive, in situ spectroscopic scarmed-probe and microscopic techniques have been applied successfully to study mineral particles in aqueous suspensions (Hawthorne, 1988 Hochella and White, 1990). 

Electrons ejected after the core ionization can be measured either selectively by their energy as Auger electrons or unselected as the so-called total electron yield. Due to the small free path that electrons have in condensed matter, these electrons stem from a thin layer of the surface of the sample. Under these conditions, XAS becomes a surface-sensitive probe [41] known as SEXAFS (Surface EXAFS) and NEXAFS (Near Edge X-ray Absorption Fine Structure with the same meaning as XANES,but applied exclusively to near-edge spectra detected using surface-sensitive measurements). These methods have become very important... 

In order to get answers to these questions, the ability to better characterize catalysts and electrocatalysts in situ under actual reactor or cell operating conditions (i.e., operando conditions) with element specificity and surface sensitivity is crucial. However, there are very few techniques that lend themselves to the rigorous requirements in electrochemical and in particular fuel cell studies (Fig. 1). With respect to structure, in-situ X-ray diffraction (XRD) could be the method of choice, but it has severe limitations for very small particles. Fourier transform infra red (FTTR), " and optical sum frequency generation (SFG) directly reveal the adsorption sites of such probe molecules as CO," but cannot provide much information on the adsorption of 0 and OH. To follow both structure and adsorbates at once (i.e., with extended X-ray absorption fine stmcture (EXAFS) and X-ray absorption near edge stmc-ture (XANES), respectively), only X-ray absorption spectroscopy (XAS) has proven to be an appropriate technique. This statement is supported by the comparatively large number of in situ XAS studies that have been published during the last decade. 16,17,18,19,20,21,22,23,24,25 highly Versatile, since in situ measme-... 

N K-edge X-ray absorption near edge stracture (XANES) spectra of the samples were measured at the BL-8B1 station of UVSOR-n at the Institute for Molecular Science, Okazaki, Japan. Data were recorded at room temperature in total electron yield mode, and the X-ray energy dependence of the N Auger election yield was monitored. Considering the escape depth of the Auger electrons, the spectra probe the sample fiom the surface up to a few nanometers in depth. 

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