Liquids work function

As a metal comes in contact with a liquid polar phase (a solvent), the situation can be depicted as in Fig. 2. The electron work function will be modified by A4 so that... 

According to Fig. 2, as M comes in contact with S,3 4 the electron distribution at the metal surface (giving the surface potential XM) will be perturbed X ) The same is the case for the surface orientation of solvent molecules (Xs + SXS). In addition, a potential drop has to be taken into account at the free surface of the liquid layer toward the air (xs). On the whole, the variation of the electron work function (if no charge separation takes place as assumed at the pzc of a polarizable electrode) will measure the extent of perturbation at the surfaces of the two phases, i.e.,... 

It has been consistently found that small amounts of Pb in Sn + Pb alloys cause an appreciable decrease in the electron work function of Sn, which is in good agreement with data for liquid Sn + Pb alloys.816-818 The surface activity of Pb has been found to increase as the temperature decreases.817,818... 

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. 

One basic reason which made the absolute electron potential problem so complicated to solve in aqueous electrochemistry is the experimental difficulty of measuring work functions on metal surfaces covered with liquid films or in contact with liquids and their vapours. 

Figure 7.1. Definition of absolute electron potential in aqueous electrochemistry according to Trasatti16 in a classical (a) and liquid covered (b) electrode geometry. Point C corresponds to the zero energy level. O0 is the work function of the bare electrode surface and AC>(=eA P) is the work function modification induced by the presence of the electrolyte layer (b). Reprinted with permission from Elsevier Science.

It is also worth noting that the one-to-one correspondence between change in (ohmic drop-free) catalyst potential and work function in solid-state electrochemistry,7,8 may also be applicable to the work function of liquid-free gas-exposed electrode surfaces in aqueous electrochemistry.8 Such surfaces, created when gases are consumed or produced on an electrode surface, may also play a role in the observed NEMCA behaviour. The one-to-one correspondence between eAUwR and AO is strongly reminiscent of the similar one-to-one relationship established with emersed electrodes previously polarized in aqueous solutions,9,10 as already discussed in Chapter 7. 

Surface Ionization Sources. In this system, a low ionization potential atom (e.g. caesium) is adsorbed on a high work function metal (e.g. iridium). The temperature is raised so that the rate of desorption exceeds the rate of arrival of the atoms at the surface, and the Cs is then desorped as ions with very small energy spread (< 1 eY). The spot size - current characteristics of these sources lie between liquid metal and plasma discharge sources. 

Calculations using oscillatory ion profiles have also been performed61 and give better surface profiles and work functions for liquid metals. Such oscillatory profiles are expected on the following... 

The decrease of the work function of Rb and Cs upon oxidation is also seen in quite different experiments. The wetting properties of liquid He on the metal surfaces is critically dependent on the mutual interaction, which is determined by the work function [63, 64]. Discrete He droplets are stable to higher temperature on Cs than on Rb, the latter in this property coming dose to Cs upon oxidation. 

With the potential large matrix of both anions and cations, it becomes clear that it will be impossible to screen any particular reaction in all possible ionic liquids. Work is clearly needed to determine how the properties of ionic liquids vary as functions of anion/cation and establish which, if any, properties change in a systematic way. 

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. 

In the photoelectric method, Fq is obtained as the change in work function, (f>, of a metal when immersed in the liquid. Thus Fq is given by the dilference in work functions ... 

The experiments were performed in stainless steel UHV chambers which were equipped with the instrumentation necessary to perform Auger Electron Spectroscopy (AES), X-ray Photoelectron Spectroscopy (XPS), UV Photoelectron Spectroscopy (UPS), Low Energy Electron Diffraction (LEED), work function measurements (A( )), High Resolution Electron Energy Loss Spectroscopy (HREELS), and Temperature Programmed Desorption (TPD). The Au(lll) crystal was heated resist vely and cooled by direct contact of the crystal mounting block with a liquid nitrogen reservoir. The temperature of the Au(lll) crystal was monitored directly by means of a... 

The results shown in Fig. 24 indicate that this assumption is correct. The photoelectric emission of a platinum surface at X = 265.5 mg diminishes after the N2O capsule, still cooled with liquid air, has been smashed (F in Fig. 2). The emission current becomes entirely unmeasure-able after removal of the cooling medium. This corresponds to a considerable increase of the work function in the presence of N2O. 

The electronic interaction of the relatively large molecules of phthalocyanine shows (Fig. 30) a considerable temperature effect (77a). In an experiment demonstrating this effect, the platinum foil (B in Fig. 2) was covered by the dye molecules until the work function was lowered to 4.32 volts at room temperature. If B was cooled by pouring liquid air into the upper tube of the photocell (a in Fig. 30), the photoelectric sensitivity increased and remained constant as long as liquid air was added. If the liquid air evaporated (6 in Fig. 30), the photoemission dropped to the original value at room temperature. This effect was arbitrarily reproducible. The calculation of the work function 4> and the constant M by the curves of Fowler [see Equation (5) in section III,la] in Fig. 31 gives = 4,32 volts, log M = —12.17 at room temperature (curve I), and = 4.15 volts, log M = —12.17 at low temperature (curve II). While... 

The papers in the second section deal primarily with the liquid phase itself rather than with its equilibrium vapor. They cover effects of electrolytes on mixed solvents with respect to solubilities, solvation and liquid structure, distribution coefficients, chemical potentials, activity coefficients, work functions, heat capacities, heats of solution, volumes of transfer, free energies of transfer, electrical potentials, conductances, ionization constants, electrostatic theory, osmotic coefficients, acidity functions, viscosities, and related properties and behavior. 

Debye and Huckel (J) have derived an expression for the work function of an ion in an ion atmosphere in solution. They and others (J, S, 4) have applied this function to various phenomena in liquid media. The authors (2) have previously deduced, in a similar way, the field around a dipole and have combined it with Onsagers (5) theory of polar liquids to obtain an equation that explains the electrostatic effects on the rates of reaction between ions and dipolar molecules (2). The equation has been applied (2,6,7,8) to the rates of several ion-dipolar molecular reactions. 

An electron in the conductivity band is quasi-free, since for it to escape from the solid or the liquid we must supply the electron with the so-called work function, which equals the energy an electron has at the bottom of the conductivity band taken with an opposite sign. (This energy is measured from the energy of an electron in vacuum.) Denoting it as V0, we can write the relation between the ionization potential 7C and the external emission threshold as... 

Implicit in this equation is the assumption of constancy of the selectivity coefficient K, and the assumption of constancy of the term Vq. Those assumptions put this otherwise useful equation in the category of empirical relationships, the same as the Nikolskij-Eisenman equation. An example of such a response is shown in Fig. 6.36 in which WF CHEMFET with doped polyaniline/camphorsulfonic/ionic liquid gate was exposed to stepwise changes of ammonia concentration. In this case ammonia acts as an electron donor, thus lowering the work function of the selective layer. The value of 5g determined from the slope of (6.101) was found to be 0.6. 

Davis, J. H., Jr. Working salts syntheses and uses of ionic liquids containing functionalized ions. In Ionic Liquids Industrial Applications for Green Chemistry, eds. 

The liquid metal Ion source could help dramatically In the area of+negatlve secondary Ion mass spectrometry where bombarding with Cs ions has proven very beneficial (19). This is due to the large Increase In negative ion emission from a cesiated, low work function surface. Unfortunately, the sources used to produce cesium ions are of the surface ionization type (20) which has a low brightness. If the liquid metal field emission Ion source can be developed to operate reliably with liquid cesium, then small, high-current density ion beams could be formed. Currently, however, the reactive nature of the metal makes the liquid metal field emission cesium Ion source more of a research project than a routine analytical technique. 

As already stated above, a cesium-metal surface, when exposed to oxygen at liquid-air temperatures, is spontaneously covered with a chemisorbed layer of oxygen. As the fully oxidized product which is formed by oxidation at higher temperatures proves to be cesium superoxide (Cs02), we may assume that the chemisorbed layer formed at — 180°C. also consists of chemisorbed 02 ions. As the work function of cesium has a low value, the difference in energy between levels A and D in Fig. 18 is rather small and, consequently, minimum E of the chemisorption curve will be appreciably lower than level A. 

In general, the diffusion equation depends on all the microscopic parameters. The microscopic parameters of van Kampen s model are the local values of the effective trap density cr, which is density times cross-section and work function . The traditional diffusion relation of Eq. (6.305) is valid only for isotropic diffusion and under the restrictive conditions that cr cx exp( homogeneous system with nontrivial geometry. Equation (6.306) is valid when the effective trap concentration is constant, which is more realistic for liquids. 

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