Binding energy of elements

Peak identification in spectra is the primary task in qualitative analysis. For example, data in Figure 7.13 can be used to identify peaks in AES spectra. Table 7.1 lists the binding energies of elements in core levels. Such energies are the primary source for peak identification of XPS spectra. Peak identification is a non-trivial task in electron spectroscopy such as XPS. In additional to the spectrum features discussed in the previous section, other factors such as chemical shift and surface charge can complicate peak identification. The following section briefly reviews the important factors that, particularly for XPS, affect peak identification. 

Table 2 Binding energy of elements in chemical functions of biochemical compounds, Ref. 134 unless otherwise specified...

X-ray photoelectron spectroscopy (XPS), also called electron spectroscopy for chemical analysis (ESCA), is described in section Bl.25,2.1. The most connnonly employed x-rays are the Mg Ka (1253.6 eV) and the A1 Ka (1486.6 eV) lines, which are produced from a standard x-ray tube. Peaks are seen in XPS spectra that correspond to the bound core-level electrons in the material. The intensity of each peak is proportional to the abundance of the emitting atoms in the near-surface region, while the precise binding energy of each peak depends on the chemical oxidation state and local enviromnent of the emitting atoms. The Perkin-Elmer XPS handbook contains sample spectra of each element and bindmg energies for certain compounds [58]. 

An XPS spectrum consists of a plot of N(E)/E, the number of photoelectrons in a fixed small interval of binding energies, versus E. Peaks appear in the spectra at the binding energies of photoelectrons that are ejected from atoms in the solid. Since each photoemission process has a different probability, the peaks characteristic of a particular element can have significantly different intensities. 

TEM observation and elemental analysis of the catalysts were performed by means of a transmission electron microscope (JEOL, JEM-201 OF) with energy dispersion spectrometer (EDS). The surface property of catalysts was analyzed by an X-ray photoelectron spectrometer (JEOL, JPS-90SX) using an A1 Ka radiation (1486.6 eV, 120 W). Carbon Is peak at binding energy of 284.6 eV due to adventitious carbon was used as an internal reference. Temperature programmed oxidation (TPO) with 5 vol.% 02/He was also performed on the catalyst after reaction, and the consumption of O2 was detected by thermal conductivity detector. The temperature was ramped at 10 K min to 1273 K. 

No new peaks were observed in the mechanical mixtures. The binding energies of all elements were the same in the pure phases and in the mixtures [14]. Figure 1 shows the apparent atomic percentages of molybdenum and cobalt, as given by XPS, on the surface of the sulfided pure phases and mechanical mixtures. In both cases, the experimental results are close to the theoretical values calculated according to Equation 2. 

Figure 2.5 illustrates two spectra recorded from a sample of iron using (a) Al Ka radiation, and (b) Mg Ko, radiation. The binding energy of the peaks are characteristic of each element. There is a difference in hv between these sources of 233 eV, so, as expected from equation (2.1), the XPS peaks on spectrum (a) are displaced 233 eV relative to those in spectrum (b). The spectrum was taken over a wide energy range to detect all possible peaks of elements present in the surface. The 2p and 3p peaks from iron are identified, as well as the Is peak from carbon which was present as a contaminant. 

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