X-ray standing waves method

Figure 3.2.2.21 Illustration of the principle of the X-ray standing wave method.

Figure 3.2.2.22 Normal-incidence X-ray standing wave method used for locating the bonding site of the sulfur atom in methylthi-olate adsorbed on Au(lll) (a) top and side views of a cluster model for the adsorbate structure, showing only substrate and sulfur atoms, and indicating also the X-ray standing wave in the bottom drawing, (b) The top...

Mineral-liquid or mineral-gas interfaces under reactive conditions cannot be studied easily using standard UHV surface science methods. To overcome the pressure gap between ex situ UHV measurements and the in situ reactivity of surfaces under atmospheric pressure or in contact with a liquid, new approaches are required, some of which have only been introduced in the last 20 years, including scanning tunneling microscopy [28,29], atomic force microscopy [30,31], non-linear optical methods [32,33], synchrotron-based surface scattering [34—38], synchrotron-based X-ray absorption fine structure spectroscopy [39,40], X-ray standing wave... 

Macroscopic experiments allow determination of the capacitances, potentials, and binding constants by fitting titration data to a particular model of the surface complexation reaction [105,106,110-121] however, this approach does not allow direct microscopic determination of the inter-layer spacing or the dielectric constant in the inter-layer region. While discrimination between inner-sphere and outer-sphere sorption complexes may be presumed from macroscopic experiments [122,123], direct determination of the structure and nature of surface complexes and the structure of the diffuse layer is not possible by these methods alone [40,124]. Nor is it clear that ideas from the chemistry of isolated species in solution (e.g., outer-vs. inner-sphere complexes) are directly transferable to the surface layer or if additional short- to mid-range structural ordering is important. Instead, in situ (in the presence of bulk water) molecular-scale probes such as X-ray absorption fine structure spectroscopy (XAFS) and X-ray standing wave (XSW) methods are needed to provide this information (see Section 3.4). To date, however, there have been very few molecular-scale experimental studies of the EDL at the metal oxide-aqueous solution interface (see, e.g., [125,126]). 

Other X-ray surface probe methods which have not as yet found widespread use but are applicable to electrode/solution interfacial studies are based on X-ray standing waves and glancing angle X-ray diffraction. 

It was nevertheless important to verify this unusual structure by independent experimental methods. This was achieved by a normal-incidence x-ray standing wave (NIXSW) [45, 49] study in 1992, a LEED study [17] in 1994, and in an STM study [50] in 1995. Fig. 7 shows a comparison of experimental LEED spectra with spectra calculated for the Al(l 11)—( 3 x, /3)/ 30°—Na structure with Na atoms adsorbed in substitutional sites. 

Fortunately, the success of surface science, optical and x-ray techniques in the last few decades has provided access for electrochemists to structural information of electrode/electrolyte interfaces. The optical and X-ray spectroscopic techniques have mainly been used in situ, i.e., in the presence of the bulk electrolyte. These techniques include EXAFS (extended x-ray absorption fine structure), SXS (surface x-ray scattering), XSff (x-ray standing wave technique, SERS (surface enhanced Raman scattering), NOM (nonlinear optical methods) IRS (infrared spectroscopy), MS (Mossbauer spectroscopy), RLS (radioactive labelling spectroscopy), STM (scanning tunneling microscopy), and... 

The ability to infer aspects of water structure directly at the mineral/water interface using high-resolution X-ray reflectivity is an important complement to vibrational spectroscopy methods. X-ray reflectivity and other techniques, notably X-ray standing waves, can be used to derive a well-constrained molecular-scale model of the structure and distribution of sorbed ions at or near the mineral/water interface (including physisorbed ions and those in the diffuse portion of the EDL). 

Recently, the method has been combined with an energy-resolved detection of the corresponding photoelectrons, which makes it possible to determine the geometric position of one type of atom in different chemical environments separately. Eor example in the case of PF3 adsorbed on Ni(lll) this so called chemical-shift normal incidence X-ray standing wave (CS-NIXSW) technique makes it possible to determine the position of three different P-species independently [99JAC]. 

Grazing incidence X-ray fluorescence is able to directly determine the surface composition for concentrations ranging from 0.01 M to 1 M under ambient conditions. Coupled with competitive adsorption in mixtnres of salts, this method has the unique ability to distinguish very short-range couplings at the A level. However, it lacks depth sensitivity below abont 5 nm. At solid/solution interfaces, such a sensitivity can be obtained by using the X-ray standing waves technique. 

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