XPS signals

Madey and co-workers followed the reduction of titanium with XPS during the deposition of metal overlayers on TiOi [87]. This shows the reduction of surface TiOj molecules on adsorption of reactive metals. Film growth is readily monitored by the disappearance of the XPS signal from the underlying surface [88, 89]. This approach can be applied to polymer surfaces [90] and to determine the thickness of polymer layers on metals [91]. Because it is often used for chemical analysis, the method is sometimes referred to as electron spectroscopy for chemical analysis (ESCA). Since x-rays are very penetrating, a grazing incidence angle is often used to emphasize the contribution from the surface atoms. 

Figure 5.37. Transient effect of constant current application and interruption on the Pt/YSZ catalyst potential UWr and on the XPS signal at Eb = 528.8 eV (location of 8-0 Is peak) and at Eb = 181.7 eV (electrochemically shifted position of the Zr 3d5/2 peak).6 Reprinted with permission from the American Chemical Society.

One can go a step further and use the /P//s ratio for a quantitative estimate of the dispersion. Through the years, several methods have been proposed to predict XPS intensity ratios for supported catalysts. Angevine et al. [29] modeled their catalyst with crystallites on top of a semi-infinite support, as sketched in Fig. 3.9a. However, as the inelastic mean free path of, for example, Si02 is 3.7 nm, photoelectrons coming from particles inside pores as deep as 10 nm below the surface still contribute to the XPS signal and the assumption of a semi-infinite support is probably too simple. Indeed, the model predicts /P//s ratios that may be a factor of 3 too high [30],... 

Figure 3.16 Experimental and theoretical azimuthal dependence of Ag 3d XPS signal from a Ag (110) surface at three different take-off angles (note that the latter are defined with respect to the surface, in contrast to all other figures in this book adapted from Takahashi et al. [41]).

As an example, XPS has been used to analyze modifications induced by 2 to 20 eV electrons incident on a hydrogen-passivated and sputtered Si(lll) surface, onto which had been physisorbed thin films of H2O [293,294] and CF4 [295]. In both cases, following the electron-induced dissociation of the molecular adsorbate, a new XPS signal associated with the chemisorption of either O or F onto the Si surface was observed and an effective cross section for chemisorption was then calculated. This cross section for electron-induced chemisorption of oxygen from an H2O bilayer onto a hydrogen-passivated Si(l 11) surface is shown in Fig. 24 as a function of Ei [293,294]. The low energetic threshold for the chemisorption process (i.e., 5.2 eV) has been interpreted as due to the formation of OH via the DEA process... 

Figure 10.3 XPS Signal and N/x electrons of Sm2Fe17 after bombardment in N2.23...

Figure 10.7 XPS signals of Sm2Fe17 ingot plates (A) CIs, (B) NAv, and (C) OIs. Samples (a) as-obtained, (b) treated by the glow discharge in N2-H2 (molar ratio = 1 2), and (c) treated by the glow discharge in CH4-N2-H2 (molar ratio = 1 1 4). Plasma treatment conditions 723 K and 2 h. Sputtered depth dashed line, 0 nm solid line, 25 im.24...

The depth of analysis (d) in XPS is approximately given by 3/ sin 6 [21] where l is the mean escape depth and 6 is the take-off angle of the photoelectron with respect to the sample surface plane. Thicknesses of surface coverage layers on different samples were estimated from the attenuation of the XPS signal from the substrate by the overlayer using the relation [22] In [/ // +1] = d/l sin 6, where d is the overlayer thickness, R is the ratio of photoelectron signal intensities from the overlayer to substrate of any particular element, and is the photoelectron intensity from the same element of infinite thickness. 

This may also provide an alternative method for calibrating the XPS signal. 

The ionisation potential of a core electron depends, to a small extent, on the chemical environment of the atom in question, and chemical shifts of up to about 10 eV can be observed. For example, the C(ls) XPS signal for molecularly adsorbed carbon monoxide on polycrystalline iron at 290 K shows a peak at 285.5 eV, which is... 

Using X-ray Photoelectron Spectroscopy (XPS) a distinction is made between free amine groups and amines in interaction with the surface. The N Is XPS signal of APTS modified silica shows two contributing bands, at binding energies of 400 and 402 eV (figure 9.25). 

In this respect, Kuipers made an important point (as illustrated in Fig. 3.10c), namely that layers of thickness x which cover the support to a fraction 6, have the same dispersion as hemispheres of radius 2 x, or spheres with a diameter 3x. Even more interesting is the fact that these three particle shapes with the same surface-to-volume ratio give virtually the same fp/fs intensity ratio in XPS when they are randomly oriented in a supported catalyst The authors tentatively generalized the mathematically proven result to the following statement that we quote literally For truly random samples the XPS signal of a supported phase which is present as equally sized but arbitrarily shaped convex particles is determined by the surface/volume ratio. Thus, in Kuipers model the XPS intensity ratio fp/fs is a direct measure of the dispersion, independent of the particle shape. As the mathematics of the model is beyond the scope of this book, the interested reader... 

The quantitative evaluation of the XPS signals requires a background correction according to Shirley [47], and a further calculation on the basis of the spectra of standards. For this purpose the standard spectra were described with a set of Gauss-... 

Lorentz curves with a tail function for the asymmetry of the XPS signals for these transition elements. These sets were kept constant in position, shape and size relative to each other. The XPS spectrum of an actual specimen was described with a least-square fit by variation of the size of the appropriate standard sets [36]. Various standards were prepared and measured for a subsequent data analysis of actual specimens. Only some few examples are mentioned here. Pure metal standards are Ar-sputter cleaned specimens. Fe(III) oxide corresponds to a thick passive layer formed at the positive end of the passive range. For Fe(II), a passive layer formed on Fe5Cr is reduced in 1 M NaOH at ca. = -1.0 V (SHE) [12]. For NiO, oxide grown at 1000 °C on pure Ni in air was used as a standard. For Ni(III)oxyhydroxide, NiOOH was deposited by oxidation of Ni2+ from weakly alkaline solution or formed... 

Film formation in 1 M NaOH at potentials E > 0.72 V or the oxidation of anodic Ag20 at these potentials yields a thick AgO film. These films have been investigated qualitatively and quantitatively in detail with XPS. The width of the Ag 3d5/2 XPS signal suggests the presence of two Ag ions, Ag+ and Ag3+, which has... 

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