Si 2p binding energy

Figure 6. Comparison of Si 2p binding energies from XPS and chemical shifts from NMR. No Si data is available for the S1-VPI-5A sample.

Table 4.3 Variation of Si 2p binding energy in different compounds (After Castle and West, 1980).

Figure 14.5 Apparent Si 2p binding energy of a muscovite mica sample as a function of water vapor pressure. The incident photon energy is 390 eV, with a flux density of 3x10 mm s . The dotted line indicates the nominal SI 2p BE in muscovite...

In framework silicates, the Al 2p binding energy of Al changes progressively [27, 144, 145] with the replacement of Si by Al in the tetrahedral lattice. Thus, as the Si content of any tetrahedral framework is increased relative to the amount of Al, the combination of the reduced concentration of the interstitial cations and the increased population of Si—O units progressively forces the Al—O bond to be more ionic and as a result the Al 2p binding energy increases (Table VII). This effect has been explained in detail (see [27,144,145,148,150]). 

The lower bound for the Al 2p binding energies for hydrothermally prepared f Al framework aluminosilicates is determined by the Loewenstein rule, which prohibits Al—O—Al bonding. This means that the Si/Al ratio must always be greater than 1.0. Thus, for zeolite Na—A and certain feldspathoids with this 1.0 Si/Al ratio, Al 2p is ca. 73.5 eV (Table VII) [37]. 

An example of surface phase transformation can be found in the work of Jones et al. 85. Pyrrhotite (Fe., S) surfaces in acid solution restructured to a crystalline, defective, tetragonal Fc Si. surface phase due to loss of Fe to solution. The metal-deficient, S-rich product was identified by XRD in combination with XPS. Linear chains of sulfur with a nearest-neighbor distance similar to that of elemental S were observed, but with an S 2p binding energy 0.2-0.6 eV less than that of Ss- As a result of the oxidation process, hydrophilic iron oxides were found at the surface. 

McFeely and co-workers used soft x-ray photoelectron spectroscopy (SXPS) to measure the changes in binding energies of Si(2p) levels after slight exposure to fluorine atoms via dissociative chemisoriDtion of XeF2 [39]. Using synclirotron radiation at 130 eV as the source enabled extreme surface sensitivity. Since this level is split into a... 

As it can be observed in Table 13.1, Ir supported over pure oxides exhibited low acidity, but Ir supported on mixed Nb20s-Si02 displayed an important enhancement in the surface acidity with surface coverage by niobia increases. Binding energies (BE) of core-level electrons and metal surface composition were obtained from XP spectra. The BE values of Si 2p, Ti 2p3/2, Nb 3ds/2 were 103.4, 458.5 and 123 eV respectively, which are exactly the expected values considering the presence of oxides of Si (IV), Ti (IV) and Nb (V). With regard to Ir 4f7/2 core level, a... 

Calibration is done by using the binding energy of a known compound. In SiCVsupported catalysts, for example, one uses the binding energy of the Si 2p... 

The 5950A ESCA spectrometer is interfaced to a desktop computer for data collection and analysis. Six hundred watt monochromatic A1 Ka X-rays are used to excite the photoelectrons and an electron gun set at 2 eV and 0.3 mAmp is used to reduce sample charging. Peak areas are numerically integrated and then divided by the theoretical photoionization cross-sections (11) to obtain relative atomic compositions. For the supported catalyst samples, all binding energies (BE) are referenced to the A1 2p peak at 75.0 eV, the Si 2p peak at 103.0 eV, or the Ti 2p3/2 peak at 458.5 eV. 

The XPS data were obtained with an extensively modified AEI ES-100 photoelectron spectrometer. The samples were analyzed at a pressure typically < 10 g Torr. A magnesium anode (1253.6 eV) was used as the excitation source. The analyzed sample area was of the order of 5 mm2. Survey scans from 0 to 1000 eV were first obtained for each sample to confirm that only the expected elements were present on the fiber surfaces. Subsequently, high resolution spectra were obtained by slowly scanning — 20 eV binding energy windows that included the Si 2p, Al 2p, Ca 2p, B Is, O Is, and C Is photoelectrons, respectively. Integrated peak areas of the photoelectron spectra were determined. The sensitivity factors, which were independently obtained on this spectrometer with oxide standards, were then utilized in the determination of the surface atomic percent compositions of the fibers. 

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