Metal surfaces, work function

Theory of metal surfaces work function N. D. Lang and W. Kohn, Phys. Rev. B 3, 1215 (1970). Coupled partial ion-transfer steps in the anodic dissolution of metals. D. Vanmaekelbergh and B. H. Erne, J. Electrochem. Soc. 146, 2488 (1999). 

Electronegative adatoms cause significant changes in the metal surface electronic stmcture, manifest as changes in the surface work function. In general electronegative additives increase the work function of the metal substrate. Typical examples are shown in Figures 2.9 and 2.10 for the adsorption of Cl and coadsorption of Cl and O on the work function of... 

Since Bis via Gauss s Law of electrodynamics proportional to the local excess free charge it follows that the term fjeV VGj is proportional to the net charge stored in the metal in region G. This net charge, however, was shown above to be zero, due to the electroneutrality of the backspillover-formed effective double layer at the metal/gas interface and thus Dfje w.Gj must also vanish. Consequently Eq. (5.47) takes the same form with Eq. (5.19) where, now, O stands for the average surface work function. The same holds for Eq. (5.18). 

These measurements have verified that the work function of an electrode, emersed with the double layer intact, depends only on the electrode potential and not on the electrode material or the state of the electrode (oxidized or covered with submonolayer amounts of a metal) [20]. Work function measurements on emersed electrodes do not serve the same purpose as in surface science investigations of the solid vacuum interface. At the electrochemical interface, any change of the work function by adsorption is compensated by a rearrangement of the electrochemical double layer in order to keep the applied potential i.e. overall work function, constant. Work function measurements, however, could well be used as a probe for the quality of the emersion process. Provided the accuracy of the measurement is good enough, a combination of electrochemical and UPS measurements may lead to a determination of the components of equation (4). 

For electrons in a metal the work function is defined as the minimum work required to take an electron from inside the metal to a place just outside (c.f. the preceding definition of the outer potential). In taking the electron across the metal surface, work is done against the surface dipole potential x So the work function contains a surface term, and it may hence be different for different surfaces of a single crystal. The work function is the negative of the Fermi level, provided the reference point for the latter is chosen just outside the metal surface. If the reference point for the Fermi level is taken to be the vacuum level instead, then Ep = —, since an extra work —eoV> is required to take the electron from the vacuum level to the surface of the metal. The relations of the electrochemical potential to the work function and the Fermi level are important because one may want to relate electrochemical and solid-state properties. 

Electrical properties of upd metal layers also differ from the corresponding bulk metals. The work function of upd layers changes with coverage, reaching the value for the bulk metal far beyond the monolayer level [117]. It is expected that lateral interactions among adatoms will differ from interactions between the substrate and the adatoms in the direction perpendicular to the electrode surface and that both types of interaction will also change with coverage [115]. 

It is possible to be more definite about the influence of face on the work function, which is a measure of the electron affinity of the metal. The work function has been shown to depend on crystal face (19). The question of the relationship of the work function and the specific surface energy of the surface to its chemical activity has been discussed by Suhrmann (20). 

Figure 3. Calculated and experimental variation of electron work function of various refractory metals with equilibrium adsorbed Cs on surfaces. Work function is plotted vs. ratio of surface temperature to temperature of the Cs reservoir, which...

An alternative interpretation of the phenomenon of metal-support interactions induced by doping of semiconductive carriers with aliovalent cations is based on the theory of electrochemical promotion or the NEMCA effect. According to this interpretation, the charge carriers transported from the carrier to the metal particles are oxygen ions, which diffuse to the surface of the metal particles, thus altering the surface work function and, subsequently, chemisorptive and catalytic parameters. Work is currently in progress to elucidate the mechanism of induction of metal-support interactions by carrier doping. 

Provided that there is no additional surface charge, fj, is a pure bulk term which is independent of any electrostatic potential. The term is the contribution of surface dipoles [1, 2] (Fig. 2.1). Such a dipole can be caused by an unsymmetrical distribution of charges at the surface because there is a certain probability for the electrons to be located outside the surface. In the case of compound semiconductors, dipoles based on the surface structure caused by a particular ionic charge distribution occur. These effects depend on the crystal plane and on the reconstruction of the surface atoms [3, 4]. These dipole effects also influence the electron affinity and ionization energy. In the case of metals, the work function is a directly measurable quantity, and for semiconductors it is calculable from ionization measurements. However, the relative contributions of fi and ex are not accessible experimentally and data given in the literature are based on theoretical calculations (see e.g. ref. [1]). 

The adsorbate substrate complex excitation mechanism predicts an action spectrum analagous to the absorption spectra of the complex. The direct mechanism predicts a photochemical action spectrum similar to that of the gas phase molecule. The final mechanism, dissociative electron attachment (DEA), suggests that the action spectrum should be referenced to the absorbance of the substrate, modified by the surface work function and electron attachment cross section of the adsorbate. The DEA mechanism appears to be of importance for many metal and semiconductor substrates, especially for the case of photochemistry induced by anomalously low energy radiation. 

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