Work function of surface

Actual electron transfer does occur in oxidation/reduction, or "redox", reactions. In this type of reaction, there is a change in the oxidation state of the adsorbate. A simple example is the chemisorption of an alkali atom, in which it becomes a 1+ ion, transferring its outer electron to empty electron orbitals of the substrate. It is the large electric dipole moment created by this charge transfer process that lowers the work function of surfaces on which alkali atoms are adsorbed (e.g., "cesiation") by up to several eV. This type of bonding is generally strong, and it can also be either molecular or dissociative. 

Work Function of Surfaces, Electropositive and Electronegative Promoters... 

Work function of surface under discharge not known. 

One of the techniques for measuring the work function of surfaces uses the thermal emission of electrons from the heated surface under study [4]. According to the Richardson-Schottky equation, the electron current density, I, depends on the surface temperature, T, and the work function, / ... 

Why do you think the Cassie equation Eq. X-27 might work better than Eq. X-28 for predicting the contact angle as a function of surface polarity ... 

Wlien an electrical coimection is made between two metal surfaces, a contact potential difference arises from the transfer of electrons from the metal of lower work function to the second metal until their Femii levels line up. The difference in contact potential between the two metals is just equal to the difference in their respective work fiinctions. In the absence of an applied emf, there is electric field between two parallel metal plates arranged as a capacitor. If a potential is applied, the field can be eliminated and at this point tire potential equals the contact potential difference of tlie two metal plates. If one plate of known work fiinction is used as a reference electrode, the work function of the second plate can be detennined by measuring tliis applied potential between the plates [ ]. One can detemiine the zero-electric-field condition between the two parallel plates by measuring directly the tendency for charge to flow through the external circuit. This is called the static capacitor method [59]. 

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.

Surface ionization. Takes place when an atom or molecule is ionized when it interacts with a solid surface. Ionization occurs only when the work function of the surface, the temperature of the surface, and the ionization energy of the atom or molecule have an appropriate relationship. 

Using Equation (1.7) calculate the velocity of photoelectrons ejected from a sodium metal surface, with a work function of 2.46 eV, by ultraviolet light of wavelength 250 nm. 

Work functions of alkali mefal surfaces are only a few electronvolfs so fhaf fhe energy of near ultraviolef radiation is sufticienf to produce ionization. 

Mdissociates as a positive ion. Conversely, the enhanced ion yields of the cesium ion beam can be explained using a work function model, which postulates that because the work function of a cesiated surface is drastically reduced, there are more secondary electrons excited over the surface potential barrier to result in enhanced formation of negative ions. The use of an argon primary beam does not enhance the ion yields of either positive or negative ions, and is therefore, much less frequently used in SIMS analyses. 

Electron-tunneling Model. Several models based on quantum mechanics have been introduced. One describes how an electron of the conducting band tunnels to the leaving atom, or vice versa. The probability of tunneling depends on the ionization potential of the sputtered element, the velocity of the atom (time available for the tunneling process) and on the work function of the metal (adiabatic surface ionization, Schroeer model [3.46]). 

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. 

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... 

It seems hard to support the above hypothesis on the basis of work function measurements for Hg in the presence of residual gases. Adsorption of water indeed reduces the work function and this is also the case with inert gases. There remains the possibility of surface oxidation by residual oxygen, but the values of Ayr measured with the Hg stream have been shown42,43 to be stable even in the presence of 02 impurities provided the gas flows rapidly, as was the case during the experiments. The same conclusion has been reached recently by measuring the work function of Hg in ambient gas.46... 

The contact potential difference between Hg and water (actually a dilute aqueous solution of a surface-inactive electrolyte) has been measured42,43 to be -0.25 V. The negative sign means that the work function of Hg decreases upon contact with water. Since 4.50( 0.02) cV is the currently accepted5 value for 0 of Hg, the value of 0 for the uncharged metal (at the potential of zero charge) is 4.25 eV. 

The discrepancy would be resolved if about 4.8 eV were the actual work function of clean Hg. In this case, however, it would be difficult to understand why 4.50 eV has been consistently measured it is hard to imagine what kind of contamination is responsible for such a highly reproducible situation. On the other hand, if 4.80 eV were the value of for clean Hg, then most of the other metals would show a decrease in work function upon water adsorption less negative than Hg, which is at variance with the expected chemistry of metal surfaces (see later discussion). 

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... 

Figure 16. Plot of the potential of zero charge, Eamo, vs. the electron work function of several low-index and stepped surfaces of Au. E a0 and measured on the same...

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. 

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... 

It should be emphasized that is the actual, promoter modified, work function of the catalyst surface and not that of a clean metal surface for which we reserve the symbol o- It should also be clarified that the kinetic constant kR is also expected to vary with . Since, however, we have no rules on how it varies with we will attempt here to rationalize some classical promotional kinetics treating it as a constant. What is amazing is that this procedure works, which indicates that the promoter action effect on kD andkA, together with the 1-0P term, is dominant. 

Indirect ( through the metal ) interaction due to the redistribution of electrons in the metal. In this case an electropositive promoter decreases the work function of the surface and this in turn weakens the chemisorptive bond of electropositive (electron donor) adsorbates and strengthens the chemisorptive bond of electronegative (electron acceptor) adsorbates. 

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