Work function of metals

Kiena A and Wo]ciechowski 1981 Work function of metals relation between theory and experiment Prog. Surf. Sci. 11 293-338... 

On the other hand, surface physicists often measure 0 which represents the work function of metals as modified by adsorption of polar (water) molecules.35-39 What they are measuring (although they may not realize it) is precisely the potential of zero charge of the given metal in the UHV scale. The value of 0 is exactly known in that case, but the relevance of the value of A0 is in doubt.32,33 In fact, only a few layers of a solvent... 

J. Holzl, and F.K. Schulte, Work Function of Metals, in Solid Surface Physics, Springer-Verlag, Berlin (1979), pp. 1-150. 

J.O.M. Bockris, and S.D. Argade, Work function of metals and the potential at which they have zero charge in contact with solutions,/. Chem. Phys. 49(11), 5133 (1968). 

Filaments are usually refractory metals such as tungsten or iridium, which can sustain high temperatures for a long time (T > 3000 K). The lifetime of filaments for electron sources can be prolonged substantially if an adsorbate can be introduced that lowers the work function on the surface so that it may be operated at lower temperature. Thorium fulfills this function by being partly ionized, donating electrons to the filament, which results in a dipole layer that reduces the work function of the tungsten. In catalysis, alkali metals are used to modify the effect of the work function of metals, as we will see later. 

Several methods exist for measuring the electron work functions of metals. In all these methods one determines the level of an external stimulus (light, heat, etc.) required to extract electrons from the metal. 

Electron work functions of metals in solution can be determined by measurements of the current of electron photoemission into the solution. In an electrochemical system involving a given electrode, the photoemission current ( depends not only on the light s frequency v (or quantum energy hv) but also on the potential E. According to the quantum-mechanical theory of photoemission, this dependence is given by... 

Smoluchowski R. 1941. Anisotropy of the electronic work function of metals. Phys Rev 60 661-674. 

Ekardt W. 1984. Work function of metal particles Self consistent spherical jelUum-background model. Phys Rev B 29 1558-1564. 

The work functions of metals correlate with their surface energies Fig. 4.11 shows this for sp metals. Hence the surface energy of a metal with a high work function is lowered when it is covered with a monolayer of a metal with a lower work function and lower surface energy. 

Figure 2-11 compares the observed work function, 4>, with that calculated based on the jeUium model as a function of the electron density, n.,in metals here, n, is represented in terms of the Wigner-Seitz radius which is inversely proportional to the cube root of n.. The chemical potential term (p. = —1.5 to-2.5 eV) predominates in the work function of metals of low valence electron density, while on the contrary the surface term (- e x = -0-1 -5.0 eV) predominates for... 

It should be noted that the correlations being discussed here are far from perfect and exceptions can be found in nearly each of the reaction series. (For the ethylene-hydrogen and deuterium-ammonia reactions, the correlation between catalytic activity and per cent d-character is nearly quantitative.) This is to be expected in view of the experimental difficulties involved in preparing clean and reproducible metal surfaces, particularly where different metals are being compared. In any attempt to correlate catalytic properties with work functions, it should also lie recognized that the work function is affected by adsorption, and therefore that the work functions of metals under catalytic conditions, or even their relative order, may be somewhat different than those of the clean metals. 

J. O M. Bockris and S. D. Aigade, Work Function of Metals and the Potential at which They Have Zero Charge in Contact with Solutions, J. Chem, Phys. 49 5133 (1968). 

As early as 1928, Roginskil and Schul tz (4) stressed the importance of electronic considerations, and Rideal and Wansbrough-Jones (5) related the work function of metals to the activation energy for their oxidation. Brewer, 1928 (6), Schmidt, 1933 (7), and Nyrop, 1935 (8) proposed that the surface must be capable of effecting ionization of the adsorbed species in some catalytic processes. Lennard-Jones (9) in his... 

An important consideration for the electronics of semiconductor/metal supported catalysts is that the work function of metals as a rule is smaller than that of semiconductors. As a consequence, before contact the Fermi level in the metal is higher than that in the semiconductor. After contact electrons pass from the metal to the semiconductor, and the semiconductor s bands are bent downward in a thin boundary layer, the space charge region. In this region the conduction band approaches the Fermi level this situation tends to favor acceptor reactions and slow down donor reactions. This concept can be tested by two methods. One is the variation of the thickness of a catalyst layer. Since the bands are bent only within a boundary layer of perhaps 10-5 to 10 6 cm in width, a variation of the catalyst layer thickness or particle size should result in variations of the activation energy and the rate of the catalyzed reaction. A second test consists in a variation of the work function of the metallic support, which is easily possible by preparing homogeneous alloys with additive metals that are either electron-rich or electron-poor relative to the main support metal. 

Much of our knowledge about the electronic structure of gas-phase clusters comes from ionization potential measurements as a function of cluster size. This is analogous to the measurement of the work function of metals. Kappes et al. recently reviewed ionization potential data for a large number of systems. Recently results were reported for Nb and V clusters as well. Some electron affinity measurements have also been reported. Most recently... 

The energy required by an electron to escape from the surface of a crystalline solid is called the work function (9) of the material. It is a characteristic parameter for its electron emission behavior. The work function of metals is in the range of 2 to 6eV and correlates mainly with the electronegativity of the element (Table 1.23). 

Thus, in the approximation of a uniform dipole layer, the resulting electrostatic potential has the form of a sharp step of hight AV = Aixpn. This effect is well-known and is used in the surface double layer model for the work function of metals (see (45), (46)). In our case the height of the step AV = Airpn 0.1 V at the interface. Thus, the electric field in the region of a step with an effective thickness a is equal Ez = Airpn/a, opposite to the direction of the CTE dipole... 

Fig. 52a. Electronic potential for He in a high electric field. Dotted curve at right repeats potential in free space. —work function of metal I—ionization potential xc— distance from image plane at which electron level in atom lines up with Fermi level.

It has been recognized for several years from ultra-high-vacuum investigations that alkali metals can lower the work function of metal surfaces [for review, see Ref. 75]. In its simplest form, the work function is defined as the minimum energy required to extract one electron from a metal. Although this definition is correct, it does not lend itself to description of how an adsorbate can alter the work function of a metal. 

R. Smoluchowsky. Anisotropy of the Electronic Work Function of Metals. Phys. Rev. 60 661 (1941). 

FIGURE 4. A. Semiconductor band edges and work function of metals... 

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