Valence band XPS

The change of the initial state energy of the silver core levels during size reduction clearly suggests that there should be size dependent changes in the valence band DOS of the Ag nanoparticles, which can be visualized by valence band XPS and UPS measurements. 

In Figure 8 [146] we present the valence band XPS and UPS spectra of the silver nanoparticles at different stages of the size reduction process. The contribution of the substrate was subtracted. The parameter at each spectrum is the measured Ag/Si ratio. 

Figure 8. Valence band XPS (a) and UPS (b) spectra of silver islands on native oxide covered Si(l 0 0) during bombardment with 1 keV Ar" ions. Substrate related contributions are removed. Numbers at each spectra stand for the Ag/Si ratio determined from the appropriate XPS core level spectra. The uppermost curve is the spectrum of polycrystalline bulk Ag. (Reprinted from Ref [146], 1998, with permission from Elsevier.)...

An attractive feature of applying XPS to study these skutterudites is that the valence states of all atoms can be accessed during the same experiment. As in the study of the MnP-type compounds, these types of investigations also provide insight into bonding character and its relation to electronegativity differences. This information is obtained by analysing both core-line and valence band XPS spectra. 

In principle, valence band XPS spectra reveal all the electronic states involved in bonding, and are one of the few ways of extracting an experimental band structure. In practice, however, their analysis has been limited to a qualitative comparison with the calculated density of states. When appropriate correction factors are applied, it is possible to fit these valence band spectra to component peaks that represent the atomic orbital contributions, in analogy to the projected density of states. This type of fitting procedure requires an appreciation of the restraints that must be applied to limit the number of component peaks, their breadth and splitting, and their line-shapes. 

UPS studies of supported catalysts are rare. Griinert and coworkers [45] recently explored the feasibility of characterizing polycrystalline oxides by He-II UPS. A nice touch of their work is that they employed the difference in mean free path of photoelectrons in UPS, V 2p XPS and valence band XPS (below 1 nm, around 1.5 nm, and above 2 nm, respectively) to obtain depth profiles of the different states of vanadium ions in reduced V205 particles [45]. However, the vast majority of UPS studies concern single crystals, for probing the band structure and investigating the molecular orbitals of chemisorbed gases. We discuss examples of each of these applications. 

Figure 6. Valence band XPS of fresh and spent catalysts. A significant decrease in the energy gap between the 3d levels of Cu and Co and a decrease in the overall bandwidth are observed on spent catalysts. Fe 3d level decreases on spent catalysts at 0.25 < x > 0.75. Reprinted from Journal of Catalysis, 210, Mathew T., et al., 2002, 405-417 with permission from Elsevier.

The results of a preliminary study of a sample of berkelium oxide (Bk02, Bk203, or a mixture of the two) via X-ray photoelectron spectroscopy (XPS) included measured core- and valence-electron binding energies (162). The valence-band XPS spectrum, which was limited in resolution by photon broadening, was dominated by 5f-electron emission. 

Sn forms a silicide but not a carbide. Niles et al [106] have used core-level and valence-band XPS to study Sn on ion-bombarded P-SiC. The first monolayer grows as a uniform layer of a-Sn, while subsequent layers form clusters of P-Sn. Annealing in the 400- 1000°C range leads to out-diffusion of Si into the Sn layer with formation of Sn silicide. The reacted surface layer withstands anneals at as high as 1000°C without desorption of Sn. 

In Fig. 21. la-d, valence photoelectron spectra refiect the differences in the chemical structures between four polymers (PE, PS, PMMA, PVC). For the valence band XPS spectra in Fig. 21.la-d, the calculated spectra correspond well to the experimental ones. It can be predicted from the present MO results that valence XPS spectra of the polymers reflect the electronic state at the ground state of each polymer due to the good accordance of simulated spectra with the experimental results. 

Although for wide band metallic materials the valence band XPS spectra probably provide a fairly direct picture of the occupied band structure, where the valence electrons are localized the correlation with the final hole state must be considered. This is apparent for 4/compounds and also quite nicely for FeFa . The situation for other transition metal compounds with possibly wider d bands and greater covalent mixing is less clearcut, but a similar effect is observed in the exchange splitting of core levels in magnetic ions . 

Valence band XPS is more sensitive to surface functionalized sjjecies, although the surface appears to be identical to the bulk... 

Fig. 3.60. The valence band XPS spectra of Gd, Dy, and Er (Baer and Busch, 1974). The solid curves on the right are the density-of-states curves calculated by Keeton and Loucks (1968).

Fig. 21, Valence-band XPS spectra for Am metal and AmH showing trivalence for both systems. Positive chemical shifts indicate electron transfer from metal to hydrogen. After Cox et al. (1992).

Figure 15.3. Valence band XPS spectra of WC, Pt, and W [61]. (From Beimett LH, Cuthill JR, McAlister AJ, Erickson NE. Electronic structure and catalytic behavior of tungsten carbide. Science 1974 184 56365. Reprinted with permission from AAAS.)...

Finally, the UPS results are compared with the valence band XPS data presented in Fig. 2.38. These spectra are very weak and require accumulation times of ca 10 hours, which may be compared to the UPS data which are available within 5 minutes. The spectra were obtained after 72 hours of reduction. The intensity at the zero-energy point implies the presence of different amounts of elemental iron in the two samples. This assignment is, however, not wholly unambiguous. Ferrous ions exhibit a characteristic structure at a binding energy of 1 eV (arrow in Fig. 

Valence-band XPS spectra require more sophisticated methods of calculation. The author has found that scattered-wave Xa and ab initio molecular orbital calculations arc good approaches, which he has developed so that they may be used to generate a predicted valence-band. spectrum simultaneously with the experimental observations (25). Agreement with experiment has been found to be excellent. In the valence band, the separation of spectral features, and their relative intensities, which can be compared with calculations and model compounds, provide the chemical information. As in the core region... 

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