XPS and UPS

1 XPS and UPS X-ray Photoelectron Spectroscopy (XPS) is a technique capable of providing the elemental composition of the outer l-5 nm from any solid, although insulators are difficult, to detection limits down to 0.1% (detection limits are element dependent) with some spedation information also available. All elements from Li to U are detectable. The spatial resolution can be down to 2 pm. This technique does so by directing au x-ray beam (Al-ka is most often used) at the solid of interest. This induces core electron emission (valence electrons are also produced). Elemental identification is made possible as the core electron energies are element/level specific. No prior sample preparation is needed, but analysis must be carried out under UHV conditions. 

The primary difference between UPS and XPS lies in the energy of the photons used to induce the emissions, i.e. UPS can only sample valence electrons, whereas XPS samples both valence and core electrons. As the photon energy used in UPS is more closely matched to the valence electron binding energies, this technique is far more sensitive to the valence region than XPS. No prior sample preparation is needed, but UHV is required. 

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Although the division into XPS and UPS is conceptually artificial if is often a practically useful one because of fhe dififerenf experimenfal fechniques used. 

PracticalSu face Analysts, edited by D. Briggs and M. P. Seah, published by J. Wiley Handbook of XPS and UPS, edited by D. Briggs. Both contain extensive discussion on use of XPS for surface and material analysis. 

C. R. Brundle. In Molecular Spectroscopy. K. R. West, Ed.) Heyden, London, 1976. This review discusses both the use of XPS and UPS in studying adsorption and reactions at surfaces. 

Palladium clusters deposited on amorphous carbon have been studied by XPS and UPS [28] and both techniques show broadening of the d-band peak as cluster size increases. The d-threshold shifts towards Ep as cluster size increases. In UPS studies the d-emission of the single atom has its peak at 3.0 eV below Ep, whereas the d-threshold is 2eV below Ep. Palladium clusters evaporated onto Si02 have been studied by UPS [38]. At large coverages of the Pd metal evaporated (> 10 atoms/cm ), a high emission intensity at Ep excited with photons of 21.2 eV (He(I)) or 40.8 eV (He(II)) as excitation source, is observed. This feature is characteristic in the spectra from bulk Pd samples. At the lowest metal coverage (3 x 10 atoms/cm ),... 

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. 

Oxygen activation of molecules at metal surfaces was first established in the 1970s by surface spectroscopies (XPS and UPS) over a wide temperature range (80-400 K). Furthermore, the distinction was made between the reactivity of partially covered surfaces and the relative inactivity of the oxide monolayer. 

What can be learnt from XPS about electrochemical processes will be demonstrated and discussed in the main part of this chapter by means of specific examples. Thereby a survey of new XPS and UPS results on relevant electrode materials will be given. Those electrode materials, which have some potential for a technical application, are understood as practical and will be discussed with respect to the relevant electrochemical process. The choice of electrode materials discussed is of course limited. Emphasis will be put on those materials which are relevant for technical solid polymer electrolyte cells being developed in the author s laboratory. 

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. 

The interpretation of XPS data is not always straightforward as is exemplified by different conclusions drawn by different investigators for the same electrode reaction. These discrepancies can be overcome if certain standards for electrode preparation, emersion and transfer processes are developed. The effects of the relative complexity of the emersed electrochemical interface on XPS and UPS data analysis in terms of (electro)chemical shifts and work function changes have to be considered. 

The nature of the final state depends upon the energy, hv, of the exciting photons. In X-ray photoelectron spectroscopy (XPS) the exciting photons are provided by sources such as A1 Ka (1,486 eV) or Mg Ka (1,253 eV) and excitation of the core electrons of the molecules is observed. In UV photoelectron spectroscopy (UPS), Hel (21.2eV) or Hell (40.8 eV) radiation is used and excitation from the valence region of the neutral molecule is observed. XPS and UPS are surface-sensitive techniques, which are capable of providing extremely useful information on the chemical nature of a surface or interface and, in the case of the XPS, the conformational state of the molecules at the surface [64]. 

Perkins, C. L. Hasoon, F. S. Al-Thani, H. A. Asher, S. E. Sheldon, P. 2005. XPS and UPS investigation of NH4OH-exposed Cu(In,Ga)Se2 thin films. Proceedings of the 31st IEEE Photovoltaics Specialists Conference (Lake Buena Vista, FL Ian. 3-7). IEEE, New York. pp. 255-258. 

XPS and UPS are based on the photoelectric effect, in which an atom absorbs a photon of energy hv, next a core or valence electron with binding energy Eh is ejected with kinetic energy (Fig. 3.2) ... 

The combined UPS and XPS facilities available in the Vacuum Generators ESCA-3 Spectrometer (the name by which the prototype became known) provided a powerful means (5, 3a) of exploring solids, their surfaces, and in turn the reasons for their inherent chemical reactivities. We shall see that XPS and UPS enable both qualitative and quantitative characterization of the solid surface at the atomic level, the chemical environment of a particular... 

Interest in studying formic acid adsorption on metals by XPS and UPS was stimulated largely by its use as a probe molecule for investigating the role of the electronic factor in heterogeneous catalysis as in the work of Schwab (70), Dowden and Reynolds (71), Eley and Leutic (72), and Fahren-fort et al. (73). The advantages of XPS and UPS are fourfold. 

Current views on the surface enrichment of one component over another in alloy systems are, surprisingly, more a consequence of gas titration and Auger electron spectroscopy than XPS and UPS. There is little doubt, however, that looking to the future XPS will provide important clues regarding the mechanism of bimetallic catalysts, the significance of promoters. 

The significance of the development of photoelectron spectroscopy over the last decade for a better understanding of solid surfaces, adsorption, surface reactivity, and heterogeneous catalysis has been discussed. The review is illustrative rather than exhaustive, but nevertheless it is clear that during this period XPS and UPS have matured into well-accepted experimental methods capable of providing chemical information at the molecular level down to 10% or less of a monolayer. The information in its most rudimentary state provides a qualitative model of the surface at a more sophisticated level quantitative estimates are possible of the concentration of surface species by making use of escape depth and photoionization cross-section data obtained either empirically or by calculation. 

Thanks to the extensive literature on Aujj and the related smaller gold cluster compounds, plus some new results and reanalysis of older results to be presented here, it is now possible to paint a fairly consistent physical picture of the AU55 cluster system. To this end, the results of several microscopic techniques, such as Extended X-ray Absorption Fine Structure (EXAFS) [39,40,41], Mossbauer Effect Spectroscopy (MES) [24, 25, 42,43,44,45,46], Secondary Ion Mass Spectrometry (SIMS) [35, 36], Photoemission Spectroscopy (XPS and UPS) [47,48,49], nuclear magnetic resonance (NMR) [29, 50, 51], and electron spin resonance (ESR) [17, 52, 53, 54] will be combined with the results of several macroscopic techniques, such as Specific Heat (Cv) [25, 54, 55, 56,49], Differential Scanning Calorimetry (DSC) [57], Thermo-gravimetric Analysis (TGA) [58], UV-visible absorption spectroscopy [40, 57,17, 59, 60], AC and DC Electrical Conductivity [29,61,62, 63,30] and Magnetic Susceptibility [64, 53]. This is the first metal cluster system that has been subjected to such a comprehensive examination. 

In Fig. 2, the two methods (XPS and UPS) are illustrated. Very often, a combination of the two methods is employed (XPSAJPS) the advantages of this combination will be illustrated later when the properties of photoemission cross-sections will be discussed. 

In the photoemission spectroscopic method, it is customary to distinguish between a core level region, which is probed in XPS (see Fig. 2), and which contains the response coming from the bound or core levels, and a valence band region, which is explored by both XPS and UPS, and which contains the response coming from the outer electrons in a solid, those of the ground state energy bands. 

Several practical aspects of the photoelectron technique will be discussed here. First, we shall concentrate upon the surface specificity of XPS and UPS. Then the sample preparation procedures will be reviewed. Thereafter, the charging effect, the energy calibration and the problems of handhng radioactive materials will be discussed. Lastly, a short review of similar topics applied to BIS will be given. 

Spurious effects due to incompletely removed oxides layers are very likely to be recorded and misinterpreted in photoemission experiments from the very oxidizable U-metal surface. However, considering only high resolution XPS and UPS data for clean surfaces as well as the measurements using synchrotron radiation , it can be... 

O Jg). A mechanism of the oxidation of Ag(lll) submonolayer has been proposed on the basis of combined cyclic voltammetric, in situ SERS, ex situ XPS and UPS studies. Danckwerts et al. have published the results of electrochemical and SHG... 

In conclusion, although both the O2 lattice and the OH" on the surface are well characterized by XPS, the evidence for O" from XPS and UPS must be treated with considerable caution since it has proved very difficult to correlate the details of the Of 1 s) binding energy unambiguously with the charge and environment of the adsorbed oxygen and to distinguish clearly when OH- may be present as a contaminant. 

To illustrate the appearance of AES, XPS, and UPS spectra, we have reproduced in Figure 2 (18) spectra of nickel and nonstoichiometric nickel oxide that were taken with the three techniques. It also illustrates different and complementary information available from a multi-technique analysis on one sample. 

Direct chemical information is obtainable only from XPS and UPS, with XPS by far the easier to interpret. Similar information can be determined indirectly by an experienced investigator using AES by noting small peak shifts... 

Solid metal hydrides specifically have been reviewed here, but XPS and UPS can serve as tools to study vapors or volatile liquids. Much of the original work with these two methods involved organic molecules only later were solid surfaces studied. Therefore, they should always be considered as helpful analytical instruments for examining the bonding chemistry of organometallic compounds. This symposium covered mainly organometallic hydrides, and they are prime candidates for photoelectron spectroscopy study. 

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