Surface potential work function

Atomic force microscopy (AFM) and electrochemical atomic force microscopy (ECAFM) have proven usefiil for the study of nucleation and growth of electrodeposited CP films on A1 alloy [59]. AFM was used to study adhesion between polypyrrole and mild steel [60], whereas electric force microscopy (EFM) has been used to study local variations in the surface potential (work function) of CP films [61]. AFM with a conductive tip permits a nanoscale AC impedance measurement of polymer and electrolyte interfaces, permitting differentiation between highly conductive amorphous regions and less-conductive crystalline regions of the CP film [62]. 

Fig. 7. Photoelectric work function for Au, Pt and Ag electrodes emersed from 0.1 M HC104 at different potentials. Work function was measured by means of UPS (He 1). Arrows indicate work functions of clean polycrystalline surfaces. After [20].

Is there any relevance of this new potential, work function, to electrochemistry The main idea is that because of its nature, the work function can be considered fingerprints of individual metals. If the electrode studied is a metal, then the work function is expected to be a relevant physical property in electrochemistry. It is involved in all electrochemical processes and accounts for effects observed on metals with different surface orientations. An example of these effects is given in Fig. 6.46. Obviously, different metals would have different chemical potentials, and that would account for the different values of d> in Fig. 6.46. But what about the differences observed, for example, for two of the crystalline faces of silver (Ag) For both crystals He is clearly the same thus the work function difference arises from different dipole layers at surfaces with different surface geometry. Another important involvement of in electrochemistry is in the determination of the absolute electrode potential, as will be explained in the next section. 

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. 

In electrochemical conditions, in the case of no adsorption, when the density of charge is zero at the metal surface the work function is linearly related to the potential, and the pzc varies with the CO in parallel with the y plot (see Section V.l). 

The electron work function of a crystal is the difference between bulk chemical potential (Fermi level),, and the total electrostatic potential barrier at the surface. Adsorption of atoms or molecules on the surface can not change the bulk chemical potential but certainly the surface barrier if the adsorbed species develops a dipole moment in the process of adsorption or has a permanent dipole moment which becomes oriented in the electrostatic field at the surface. The work function change, A< ), is therefore eqrral to the electrostatic surface potential barrier cormected with a dipole momerrt via the classic Helrrrholtz eqrration ... 

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. 

The scanning Kelvin probe (SKP) provides a measure of the Volta potential (work function) that is related to the corrosion potential of the metal, withont touching the corroding surface [24]. The technique can give a corrosion potential distribution, with a spatial resolution of 50 to 100 pm, below highly isolating polymer films. The SKP is an excellent research tool to study the initiation of corrosion at the metal/poly-mer interface. 

Solid electrolyte cells are work-function probes for the gas-ex-posed, catalytically active catalyst-electrode surfaces that is, the change A(e0) in catalyst surface average work function e0 is equal to eAVwR-The catalyst potential Vwr with respect to a reference electrode can be varied both by changing the gaseous composition and/or by polarizing the catalyst/solid electrolyte interface. 

B. Volta Potentials, Surface Potential Differences, and the Thermionic Work Function... 

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... 

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]. 

In order for the primary photoelectron, which is bound to the surface atom with binding energy to be detected ia xps, the electron must have sufficient kinetic energy to overcome, ia addition to E the overall attractive potential of the spectrometer described by its work function, Thus, the measured kinetic energy of this photoelectron, Ej is given by... 

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]). 

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... 

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.,... 

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... 

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. 

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...

---