Work function adsorption induced

Figure 2,11. Work function changes induced by S adsorption on Ni(100) and Pt(lll) surface.6,38 Reprinted from ref. 6 with permission from Elsevier Science.

The last point is confirmed by measuring the work function changes upon CO chemisorption on clean and alkali-promoted metal surfaces. Figures 2.16 and 2.17 show the work function changes induced by CO adsorption on a K/Pt(lll) and on a Na/Ru(1010) surface respectively, for various alkali... 

Fig. 4 Work function changes induced by water adsorption on Pt(l 11) for various predosed Cl coverages.64...

Figure 16 Work function changes induced by the adsorption of CO on Rh(lll)...

Table 11.1 Comparison of work function changes induced by adsorption of molecular monolayer on either Cu or Au.

Juarez, M. F. and Santos, E. 2013. Electronic anisotropy at vicinal Ag(lln) surfaces Work function changes induced by steps and hydrogen adsorption. J. Phys. Chem. C 117 4606-4618. 

With respect to the work-function changes induced by H2O adsorption there is always a decrease of 0.6 to 1.3 eV observed indicating that the lower half of the bilayer -pointing to the metal substrate -interacts with the oxygen part. For higher exposures multilayer ice is condensed which does not contribute very much to a further work-function decrease as the H2O dipoles are apparently randomly oriented. 

Fig. 21. Calibration of the work-function change induced by CO adsorption of Ru(OOOl) vs. coverage at 200 K relative coverage taken from integration of TPD traces absolute calibration at 0 = 0.33 by comparison with LEED. From (83P2].

Fig. 37. Work-function changes induced by Cu adsorption on the W(112) face as a function of the substrate temperature. Open circles Acpm x full circles Acps t crosses Acp ji . From [77M3].

Figure 2.5. Potassium (a) and sodium (b) induced work function changes for adsorption at 100 K (open circles) and after annealing to 350 K or upon alkali adsorption at 350 K (open triangles) on Al single crystals.23 Reprinted with permission from the American Vacuum Society.

Figure 2.31. CO induced work function changes during adsorption on Ni(l 11) modified with increasing amounts of oxygen.88 Reprinted with permission from Elsevier Science.

The same effect exists for adsorption on a metal surface from the gas phase. In this case the adsorbate-induced dipole potential changes the work function by an amount A. If nad is the number of adsorbed molecules per unit area, the component mx of the dipole moment of single adsorbed molecule can be inferred from the relation ... 

Figure 9.11 Promoter-induced binding energy shifts of Ar, Kr and Xe photoemission peaks with respect to adsorption on the clean metal as a function of the distance of the adsorption site to the nearest potassium atom on a potassium-promoted Rh( 111) surface. These curves reflect the variation of the surface potential (or local work function) around an adsorbed potassium atom. Note the strong and distance-dependent local work function at short distances and the constant local work function, which is lower than that of clean Rh( 111) at larger distances from potassium. The lowering at larger distances depends on the potassium coverage. The averaged distances between the potassium atoms are 1.61, 1.32 and 1.20 nm for coverages of 2.7, 4.1 and 5.0% respectively, vertical lines mark the half-way distances. Lines are drawn as a guide to the eye (adapted from Janssens et al. [38]).

It is now believed, however, that this intrinsic or a priori surface heterogeneity is not the most general cause of the effect under consideration and that much of the observed decrease in the heat of adsorption with increasing surface -coverage arises from effects associated with the adsorption process itself (Boudart, 85). Among these are repulsions between the adsorbed molecules and changes in the work function of the metal. The latter effect, which is due to the dipoles associated with the adsorbed species, is particularly important and has been termed induced heterogeneity (Boudart, 85) or the work function effect (de Boer, 69, US). 

There is very little experimental evidence pointing to enrichment in solid solutions. Bouwman et al. 15) studied the surface composition of Ag-Pd alloys by measuring the changes in work function of the alloys upon adsorption of CO. The result they obtained after brief exposition to CO at room temperature is shown in Fig. 5. Since the dependence on alloy concentration is nonlinear in the silver concentration, enrichment in silver is concluded, this enrichment being a function of the concentration of the alloys. After 16 hours exposure to a CO atmosphere Bouwman et al. found an additional increase in work function, which they ascribed to chemisorption-induced enrichment... 

Fig. 9.n Promoter-induced binding energy shifts of Ar, Kr and Xe photoemission peaks with respect to adsorption on the clean metal as a function of the distance of the adsorption site to the nearest potassium atom on a potassium-promoted Rh(lll) surface. These curves reflect the variation of the surface potential (or local work function) around an adsorbed potassium atom. Note the strong and distance-dependent local work function... 

In a similar fashion, adsorption of atoms or molecules onto a surface can raise or lower the value of the work function. Figures 16 and 17 illustrate the affect of CO and Na adsorption on the work function of Rh(l 11). For the case of carbon monoxide, the presence of the double layer at the surface leads to the polarization of CO with the negative end of the induced dipole oriented away from the surface. The presence of this dipole reinforces the dipole potential and conseqnently increases the work function. For the case of sodinm, charge transfer occurs upon adsorption leaving an electropositive element at the surface. This in turn decreases the effective magnitude of the dipole potential and lowers the work fhnction. The data of Figure 17 also illustrate the role of adsorbate-adsorbate interactions as depolarization of the sodinm occurs at near-monolayer coverage and reduces the effect. The adsorption of gas molecules on insulator... 

In fact, as can be seen in Figure 11, upon crossplotting Ed versus the work function change. A , induced by the alkali ad-atoms, a straight line with a positive slope is obtained. For nonactivated adsorption, as is the case here, this implies a... 

In Table 11.1 we show a compilation of the theoretical and experimental results obtained for benzene and cyclohexane, a saturated hydrocarbon, deposited on Au and Cu substrates. The change of the work function induced by benzene adsorption has been calculated using the Helmholtz equation by as-... 

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