Valence band chemical state information

More than a decade ago, Hamond and Winograd used XPS for the study of UPD Ag and Cu on polycrystalline platinum electrodes [11,12]. This study revealed a clear correlation between the amount of UPD metal on the electrode surface after emersion and in the electrolyte under controlled potential before emersion. Thereby, it was demonstrated that ex situ measurements on electrode surfaces provide relevant information about the electrochemical interface, (see Section 2.7). In view of the importance of UPD for electrocatalysis and metal deposition [132,133], knowledge of the oxidation state of the adatom in terms of chemical shifts, of the influence of the adatom on local work functions and knowledge of the distribution of electronic states in the valence band is highly desirable. The results of XPS and UPS studies on UPD metal layers will be discussed in the following chapter. Finally the poisoning effect of UPD on the H2 evolution reaction will be briefly mentioned. 

To determine the BEs (Eq. 1) of different electrons in the atom by XPS, one measures the KE of the ejected electrons, knowing the excitation energy, hv, and the work function, electronic structure of the solid, consisting of both localized core states (core line spectra) and delocalized valence states (valence band spectra) can be mapped. The information is element-specific, quantitative, and chemically sensitive. Core line spectra consist of discrete peaks representing orbital BE values, which depend on the chemical environment of a particular element, and whose intensity depends on the concentration of the element. Valence band spectra consist of electronic states associated with bonding interactions between the... 

Ultra-violet photoemission spectroscopy (UPS) probes the density of states, and ion neutralization spectroscopy (INS) and surface Penning ionization (SPI) provide similar information with probes of ions and metastable atoms, respectively. Angle-resolved UPS can determine the valence band structure. X-ray Photoelectron Spectroscopy (XPS) provides information on chemical shifts of the atomic core levels, and this can also help in understanding chemical bonding at the surface. 

As stated in Sec. 3.1, valuable information on the mechanism of chemical etching processes can similarly be obtained by studying the electrochemical behavior of the interface. In the particular case of GaP, the conclusion that open-circuit etching of GaP single crystals in acidic Br2 solutions proceeds via a chemical mechanism arises from two experimental observations. Firstly, current-potential measurements at p-GaP show that Br2 cannot inject holes into the valence band of GaP, so that elec-... 

The recent electron spectrometer provides highly resolved spectrum for valence state XPS, which supplies us with very useful information for discussion on chemical bonding, when combined with an appropriate theoretical analysis as mentioned above. Therefore, an accurate calculation of electronic state is required for such a purpose. The DOS calculated by DV-Xa method has been demonstrated to reproduce well the valence state XPS for some oxyanions compared with other theoretical calculations. The calculation was made using a simple model cluster XO4 " with T symmetry, thus the theoretical analysis was insufficient for the valence structure in details. This has significantly been improved by a careful analysis with more realistic model clusters which are determined from crystallographic data for those oxyanions. The comparison of the theoretical and experimental spectra for PO4 ions is shown in Fig.8. The agreement is very good even for the fine structure in the valence band. 

Several techniques that provide information about composition and structure on the molecular level were discussed. For instance, secondary ion mass spectroscopy (SIMS), XPS which provide information about surface composition and the chemical environment and bonding of surface species, and ultraviolet photoelectron spectroscopy (UPS), which probes the density of electronic states in the valence band of materials. Also, the low energy electron diffraction (LEED) and high resolution energy electron loss spectroscopy (HREELS) are electronscattering techniques that are uniquely suited to yield the structure of the surface... 

However, the density-of-states (DOS) in the valence band can be quite complex, as was shown to be the case in Sect. 3.10, which means that extracting information regarding the chemical/electronic environment of a material will not be as straightforward as with XPS. 

Ultraviolet photoelectron spectroscopy is capable of providing chemical state and electronic structure information from materials. However, due to the complex nature of the density-of-states (DOS) in the valence band, it is more difficult to extract this information, as compared to XPS, usually requiring band-structure calculations and other spectroscopies. By observation of the onset of photoelectron emission, work function measurements may be made using UPS. Like XPS, UPS is non-destructive. However, UPS cannot typically provide quantitative information. 

UV photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED) are most commonly applied in this context. In the first method, UPS, electrons are excited by UV light (sources He I = 21.22 eV He ii = 40.82 eV) and information on the electronic structure of the valence band region is obtained. The second method, XPS, provides information about the elemental composition and the valence states of the elements. Here, X-ray excitation is used (possible radiation sources MgK = 1253.6eV or A K = 1486.6 eV). In both methods, the emitted electrons are analyzed as current densities in dependence of their kinetic energy. Since the XPS signals depend not only on elemental composition but are also sensitive to the chemical environment of specific atoms, valuable information on a molecular structure can be obtained (see Chapter 8). LEED is used for the analysis of the geometric structure of the surface. Details of these and other methods applicable in combined electrochemical/UHV systems are very well discussed in a review article byjagermann [23]. 

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