Work function of a metal

As discussed in Section I.3(i), AX indicates the variation in the work function of a metal as an interface is created by bringing a solid and a liquid in contact. In principle, it should be possible to compare AX values with A values measured directly in gas phase experiments. This is the aim of UHV synthesis of the electrochemical double layer868 in which the electrode interface is created molecule by molecule, starting with the bare metal surface. It is thus possible to obtain evidence of ion-water interactions that can be envisaged from electrochemical measurements but that are not directly demonstrable. Wagner55 has given a recent comprehensive review of electrochemical UHV experiments. 

The potential of zero charge measures, on a relative scale, the electron work function of a metal in an electrochemical configuration, i.e., immersed in a solution rather than in a vacuum. Converted to an absolute value (UHV scale) and compared with the classic electron work function of the given metal, the difference between the two quantities tells us what occurs from the local structural point of view as the metal comes in contact with the solution. 

The work function, , of a metal surface can be measured relatively easily and when using the Kelvin probe technique, in situ, i.e., during catalyst operation.54,55 Three techniques are the most commonly used54-58 ... 

Consider an atom approaching the surface in Fig. 6.23. If the upper level of the atom originally contained an electron, then upon adsorption it will transfer part of this electron density to the metal and become positively charged. This is the case with alkali atoms. The atom forms a dipole with the positive end towards the outside, which counteracts the double layer that constitutes the surface contribution to the work function of the metal (Fig. 6.13). Thus alkali atoms reduce the work function of a metal surface simply because they all have a high-lying s electron state that tends to donate charge to the metal surface. 

Following the ideas of Section 2.3, devise a suitable cycle to derive the work function of a metal deposition reaction this will involve the energy of sublimation of the metal. 

An interesting correlation exists between the work function of a metal and its pzc in a particular solvent. Consider a metal M at the pzc in contact with a solution of an inert, nonadsorbing electrolyte containing a standard platinum/hydrogen reference electrode. We connect a platinum wire (label I) to the metal, and label the platinum reference electrode with II. This setup is very similar to that considered in Section 2.4, but this time the metal-solution interface is not in electronic equilibrium. The derivation is simplified if we assume that the two platinum wires have the same work function, so that their surface potentials are equal. The electrode potential is then ... 

A close relation between the work function of a metal and its ys has been suspected by several investigators. Thus32, the ys of fee metals is supposed to be... 

Several methods have been employed to measure the energy of this state in nonpolar liquids. The methods fall into three categories the change in work function of a metal when immersed in the liquid, photoionization, and field ionization. Of these, the latter, in which field ionization of high-lying Rydberg states is utilized to locate Fq, has in recent years provided what are considered to be the most accurate values of Fq in fluid Ar, Kr, and Xe. 

One of the earliest determinations of the change of photoelectric work function of a metal surface due to adsorption was made by Ives and Olphin... 

Figure 2 shows a photoelectric cell suitable for the direct determination of the photoelectric work function of a metal under the conditions of chemisorption. The electrically conducting catalyst, used as a cathode, either is inserted as a metal foil B or is evaporated from E to B. A metal layer coating the inside of the photoelectric cell serves as the anode, with a lead wire C. B can be heated electrically the leads fc are sealed into quartz and connected to an instrument (electrometer or amplifier) for... 

In the literatme, the work function of a metal, p (in eV), is often used to estimate the degree of charge transfer at semiconductor/metal junctions. The work function of a metal is defined as the minimum potential experienced by an electron as it is removed from the metal into a vacuum. The work function ip is often nsed in lieu of the electrochemical potential of a metal, because the electrochemical potential of a metal is difficult to determine experimentally, whereas tp is readily accessible from vacuum photoemission data. Additionally, the original model of semiconductor/metal contacts, advanced by Schottky, utilized differences in work functions, as opposed to differences in electrochemical potentials, to describe the electrical properties of semiconductor/metal interfaces. A more positive work function for a metal (or more rigorously, a more positive Fermi level for a metal) would therefore be expected to produce a greater amount of charge transfer for an n-type semiconductor/metal contact. Therefore, use of metals with a range of tp (or fip.m) values should, in principle, allow control over the electrical properties of semiconductor/metal contacts. 

Using the law of conservation of energy, relate the work function of a metal to the wavelength of light used to eject electrons in the photoelectric effect and the kinetic energy of those electrons (Section 4.4, Problems 25-28). 

A simple and accurate method of comparing the scales of energies and electrode potentials follows from Reference 22 and lies in the calculation of the work function of a metal, which is in the electrolyte solution at the reference-electrode potential. This is illustrated in Fig. 2, which shows a metal in equilibrium with an ox/red couple in the solution (so that their electrochemical potential levels coincide Fredox = Fmet)- The change in the free energy of reaction (16) is the sum of the energies of stages c and d of the cycle shown in Fig. 1, that is the quantity Fredox (which coincides, as was already noted, with Fq for = Cred)-... 

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. 

The work function of a metal surface is reduced when the surface area is covered with a fraction of adsorbed... 

Another application of the EA method was to determine the shift in the work function of a metal (and the consequent Schottky barrier shift when used as a contact in diodes) that can be induced by self-assembled monolayers (SAMs) [69,70,146-148]. The metal work function can be controlled by attaching a monolayer of polar molecules to the surface of the metal. Because of the inherent ordering of the monolayers (especially for thiol-based adsorbates on copper gold or silver electrodes), the molecular dipoles are oriented relative to... 

Coverage Effect on the Work Function of a Metal Films Covered... 

COVERAGE EFFECT ON THE WORK FUNCTION OF A METAL FILMS COVERED WITH ORGANIZED ORGANIC SPACER... 

The photoelectric work function of a metal is the minimum energy needed to eject an electron by irradiating the metal with light. For calcium, this work function equals 4.34 X 10 J. What is the minimum frequency of light for the photoelectric effect in calcium ... 

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