Follow:

XPS Principle

https://doi.org/10.3390/ma15072540

XPS Principle and Experimental Details

XPS is a well-established and widely used surface-sensitive spectroscopic technique for studying the elemental composition of the surface of SOFC cathode layers within the first few nanometers [2]. This photoelectron spectroscopic technique is used to quantitatively analyze the surface electronic structure of crystalline solid through the photoelectron spectra. The basic principle of the photoelectron effect was explained by Albert Einstein in 1905. Kai Siegbahn constructed an XPS instrument that can analyze photoelectron emissions and allow the speciation analysis of the sample surface. In XPS, the sample is irradiated by a soft X-ray (typically 1–3 keV) source (AlKα or MgKα) with an energy of  to ionize electrons from the surface, as shown in Figure 3. The photoelectron penetrates the samples between 1 nm and 10 nm, where the photon is absorbed by an atom in the solid, leading to ionization and emission of the core electron in different directions. The atom then releases energy by the emission of an Auger electron, in which the L electron falls to fill the core-level vacancy. The KLL Auger electron is then emitted to conserve energy released during initial emission. The photoelectric effect generates free electrons with certain kinetic energy (Ekin����) and is measured by a detector.
Figure 3. X-ray photoelectron spectroscopy experimental setup.
The number of photoelectrons emitted as a function of their kinetic energy (Ekin����) can be measured using an electron energy analyzer, and the corresponding photoelectron spectrum can be recorded. The XPS instrument measures the kinetic energy of all collected electrons. The photoelectron spectrum includes the photoelectron and Auger electron lines. Each element produces a characteristic set of peaks in the photoelectron spectrum at particular binding energy (EB) values that directly identify the element on the surface of the sample analyzed. The XPS records the core-level values of the elements in electronvolt (eV). The XPS spectral lines are identified by the shell (e.g., 1s, 2s, and 2p), from which the electrons are emitted.

Typical XPS spectra represent intensity versus EB, where the intensity area reveals the concentration and binding energy reveals the speciation. The spectrum reveals the electron energy distribution in the material. The position and height (intensity) of each peak in the photoelectron spectrum provide the desired information on the chemical state, elemental composition, empirical formula determination, electronic state, and oxidation state of the sample surface. These data are used to determine the binding energy of the ejected electron to obtain information about the electronic structure using the equation:

EB=hvEkinΦ��=ℎ�−����−Φ

where h is the Planck constant (6.63 × 10−34 J s), ν is the frequency (Hz) of radiation, EB is the electron binding energy, Ekin is the kinetic energy of freed electrons, and ΦΦ is the work function. Binding energy is an important parameter in the XPS analysis to obtain information about the electronic structure. The work function of the material is the difference between the Fermi level (EF��) and the vacuum level (Evac����). For an electron to be ejected or emitted from the solid, the energy of the photoelectron must be sufficient to overcome its attraction to the material or ΦΦ of the material. Table 1 represents the binding energy levels from the ejected electron and their corresponding orbital from which the electrons are ejected for selected elements.

Table 1. Binding energy of electrons for selected elements in their elemental form.
Elements Binding Energy (eV)
1s 2s 2p1/2 2p3/2 3s 3p1/2 3p3/2 3d3/2 3d5/2
La 38,925 6266 5891 5483 1362 1209 1128 853 836
Sr 16,105 2216 2007 1904 358.7 280.3 270 136 134.2
Ba 37,441 5989 5627 5247 1293 1137 1063 795.7 780.5
Sm 46,834 7737 7312 6716 1723 1541 1420 1110.9 1083.4
Gd 50,239 8376 7930 7243 1881 1688 1544 1221.9 1189.6
Y 17,038 2373 2156 2080 392 310.6 298.8 157.7 155.8
Zr 17,998 2532 2307 2223 430.3 343.5 329.8 181.1 178.8
Nb 18,986 2698 2465 2371 466.6 376.1 360.6 205 202.3
Tb 51,996 8708 8252 7514 1968 1768 1611 1276.9 1241.1
Pd 24,350 3604 3330 3173 671.6 559.9 532.3 340.5 335.2
Co 7709 925.1 793.2 778.1 101 58.9 58.9
Fe 7112 844.6 719.9 706.8 91.3 52.7 52.7
Ni 8333 1008.6 870 852.7 110.8 68 67.2
Cr 5989 696 583.8 574.1 74.1 42.2 42.2
Ca 4038.5 438.4 349.7 346.2 44.3 25.4 25.4
Mn 6539 769.1 649.4 638.7 82.3 47.2 47.2
O 531 22

Leave a Comment