Surface effects work function

The catalytic significance of Fig. 9.12 is that it represents the differences in the effective work functions that a molecule experiences upon adsorption at different positions on the surface. As explained in the Appendix, a low work function of the substrate enhances the capability of the substrate to donate electrons into empty chemisorption orbitals of the adsorbate. If such an orbital is antibonding with respect to an intramolecular bond of the adsorbed molecule, the latter is weakened due to a higher electron occupation. 

The same picture holds for physical adsorption on metal surfaces. The polarization of the adsorbed molecules causes dipoles pointing with their positive ends away from the metal surface. The work function of the metal will be lowered by this effect, and it seems as if the increase of the normal nonselective photoelectric emission of metals by the adsorption of water molecules (122) or molecules of organic substances such as pyridine, propionic acid, and benzene (123) or alcohol, diethyl ether, and acetone (124) is caused by this effect. The explanation, which, many years ago, was given by the author (125), viz., polarization by positive hydrogen ions which should still be present, may seem to be unnecessary and obsolete. 

Table 7.1 Effective work functions and surface-state densities of polymers.

Many investigations with surfaces have been carried out in this and other laboratories using the ion-bombardment method of cleaning. These include (1) structure investigations of the surface plane on clean surfaces, (2) work-function determinations, (3) adsorption measurements, (4) catalysis, (5) surface recombination velocity, (6) surface conductivity, and (7) field effect. One of the significant finds indicates that the relative positions of the atoms in the clean 100 surface planes of germanium and silicon are not the same as those of similar planes in the bulk crystals, but that these relative positions are the same when a monolayer of oxygen is adsorbed on these surfaces (9). 

The work function is an important parameter that influences the catalytic behavior of the metal surface. The work function effect has been clearly demonstrated in the case of the NEMCA (non-Faradaic electrochemical modification of catalytic activity) effect, where it has been shown experimentally that catalytic reaction rates depend exponentially on the work function change of the metal surface ([126-130] and references therein). Work function changes in the case of NEMCA are caused by backspillover of anions on the metal surface, and they reflect changes in the surface potential. 

The effect of adsorbed molecules or atoms on the surface s work function (wf) raised interest already in the beginning of the century. Langmuir established that molecules or atoms vdiich have an ionization potential lower than the surface wf, tend to reduce the effective wf as they are adsorbed. Later Gomer and co-workers made quantitative measurements both on metal surfaces covered with alkalies and for adsorption of rare gases3 4. It was found that while the first monolayer cause a decrease, the second either has no effect, or may cause an increase in the wf. 

Negative ion yield is proportional to the electron affinity of the element. Sputter yield depends on the difference between electron affinity of the desired atom and the effective work function. Work function varies upon the environment of the surface of the sample. Physical conditions of the sample affect the properties of atoms on the surface. The probability of negative ion formation is enhanced by the presence of Cs layer at the surface of the sample and electron cloud near the sample surface. Samples are mixed with metallic powder (e.g., Ag or Nb) to improve the thermal and electrical conductivity. Ion-atom collision kinematics reduces the sputter yield for heavy elements. Production of negative ions is at the maximum for normal incidence of the sputtering beam, but the total sputter rate, which means positive, negative, and neutral emission, increases when the angle of incidence is away from the normal. Atomic ion current is very low or zero for some elements. In that case, selection of one molecular ion out of many possible molecular ions (like oxides, hydrides, or carbides) becomes important (Tuniz et al. 1998). 

Another modification scheme of note is the selective addition of tin-phenoxides on an ITO surface, a process developed by Schwartz and coworkers for the modification of the effective work function of ITO surfaces [13, 80]. Because of the low concentration of tin sites on such surfaces, this modification scheme, which does not modify exposed indium sites, places the functional groups at some distance from each other, a type of modification not afforded by other modification schemes based on covalent bond formation, or chemisorption. 

The work functions of metal surfaces show a marked dependence on the crystal planes that constitute the surface. For polycrystal metal surfaces, the experimentally determined value of the work function is an average of the work functions of all the crystal planes that are present on the surface. It is made up of the contributions from the individual crystal planes. If /a, / and tp are defined as the mean effective work functions for thermionic electron emission, positive surface ionization, and negative surface ionization, respectively, it is generally evident that /+ > /, and / =... 

Briefly, Schottky showed that a potential barrier to electron emission from a metal surface into a vacuum is lowered by an applied field. Consequently, the effective work function of an electrode is lowered by an applied field. The current enhancement can be described as... 

Comparisons of polymer OLEDs generally show an order of magnitude improvement in light emission efficiency for PPV-based devices when the surface of the ITO anode is treated with oxygen plasmas or acid cleaning methods (which modify the effective work function of the TTO surface). However, a thousand fold improvement results from the use of hole injeetion layers such as PEDOT PSS. 

While field ion microscopy has provided an effective means to visualize surface atoms and adsorbates, field emission is the preferred technique for measurement of the energetic properties of the surface. The effect of an applied field on the rate of electron emission was described by Fowler and Nordheim [65] and is shown schematically in Fig. Vlll 5. In the absence of a field, a barrier corresponding to the thermionic work function, prevents electrons from escaping from the Fermi level. An applied field, reduces this barrier to 4> - F, where the potential V decreases linearly with distance according to V = xF. Quantum-mechanical tunneling is now possible through this finite barrier, and the solufion for an electron in a finite potential box gives... 

The final technique addressed in this chapter is the measurement of the surface work function, the energy required to remove an electron from a solid. This is one of the oldest surface characterization methods, and certainly the oldest carried out in vacuo since it was first measured by Millikan using the photoelectric effect [4]. The observation of this effect led to the proposal of the Einstein equation ... 

Therefore, the ratio of the number of ions to the number of neutrals desorbing from a heated filament depends not only on the absolute temperature but also on the actual surface coverage of ions and neutrals on the filament (C, C ) and crucially on the difference between the ionization energy and work function terms, I and (j). This effect is explored in greater detail in the following illustrations. 

Schematic diagram showing how placing a thin layer of highly dispersed carbon onto the surface of a metal filament leads to an induced dipolar field having positive and negative image charges. The positive side is always on the metal, which is much less electronegative than carbon. This positive charge makes it much more difficult to remove electrons from the metal surface. The higher the value of a work function, the more difficult it is to remove an electron. Effectively, the layer of carbon increases the work function of the filament metal. Very finely divided silicon dioxide can be used in place of carbon.

Photoelectron spectroscopy involves the ejection of electrons from atoms or molecules following bombardment by monochromatic photons. The ejected electrons are called photoelectrons and were mentioned, in the context of the photoelectric effect, in Section 1.2. The effect was observed originally on surfaces of easily ionizable metals, such as the alkali metals. Bombardment of the surface with photons of tunable frequency does not produce any photoelectrons until the threshold frequency is reached (see Figure 1.2). At this frequency, v, the photon energy is just sufficient to overcome the work function

[Pg.289]

Phofoelectron spectroscopy is a simple extension of the photoelectric effect involving the use of higher-energy incident photons and applied to the study not only of solid surfaces but also of samples in the gas phase. Equations (8.1) and (8.2) still apply buf, for gas-phase measuremenfs in particular, fhe work function is usually replaced by fhe ionization energy l so fhaf Equation (8.2) becomes... 

Thermionic Emission - Because of. the nonzero temperature of the cathode, free electrons are continuously bouncing inside. Some of these have sufficient energy to overcome the work function of the material and can be found in the vicinity of the surface. The cathode may be heated to increase this emission. Also to enhance this effect, cathodes are usually made of, or coated with, a low work-function material such as thorium. 

For a metal/solution interface, the pcz is as informative as the electron work function is for a metal/vacuum interface.6,15 It is a property of the nature of the metal and of its surface structure (see later discussion) it is sensitive to the presence of impurities. Its value can be used to check the cleanliness and perfection of a metal surface. Its position determines the potential ranges of ionic and nonionic adsorption, and the region where double-layer effects are possible in electrode kinetics.8,10,16... 

By 19884 it became obvious that the NEMCA effect, this large apparent violation of Faraday s law, is a general phenomenon not limited to a few oxidation reactions on Ag. Of key importance in understanding NEMCA came the observation that NEMCA is accompanied by potential-controlled variation in the catalyst work function.6 Its importance was soon recognized by leading electrochemists, surface scientists and catalysis researchers. Today the NEMCA effect has been studied already for more than 60 catalytic systems and does not seem to be limited to any specific type of catalytic reaction, metal catalyst or solid electrolyte, particularly in view of... 

Thus, as will be shown in this book, the effect of electrochemical promotion (EP), or NEMCA, or in situ controlled promotion (ICP), is due to an electrochemically induced and controlled migration (backspillover) of ions from the solid electrolyte onto the gas-exposed, that is, catalytically active, surface of metal electrodes. It is these ions which, accompanied by their compensating (screening) charge in the metal, form an effective electrochemical double layer on the gas-exposed catalyst surface (Fig. 1.5), change its work function and affect the catalytic phenomena taking place there in a very pronounced, reversible, and controlled manner. 

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