Contact potential differences

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]. 

Conditions of equilibrium, 92 Conduction of heat, 48, 84, 454 Configuration, 22, 107 Connodal curve, 243 Conservation of energy, 35 Contact potential differences, 470 Continuity of states, 174 Corresponding states, 228, 237 Creighton. See Southern. 

If the two Cu cables are short circuited while the cell is broken into two parts by splitting the liquid phase, it can easily be proved that the same AE as for cell (12a) is measured as a contact potential difference (cpd) between the two solutions. In fact... 

A third experimental configuration was proposed by Kolb and Hansen40 emersed electrodes. If an electrode is emersed from a solution while the control of the potential is maintained, the solvent layer dragged off with the metal (Fig. 3) would reproduce UHV conditions, but with potential control and at room temperature, as in the actual electrode situation. This appears to be the most convenient configuration for measuring 0. However, there are doubts that the solvent layer retains the properties of a bulk phase. It has in fact been demonstrated41 that a contact potential difference exists between an electrode in the emersed state and the same electrode regularly immersed in solution. 

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 two perturbation terms are specific to the given interface and are experimentally inseparable. They measure the contact potential difference at the M/S contact. However, since no cpd is measured in this case <5/M + S%s are grouped into a single quantity denoted by X, called the interfacial... 

Conversion of Earno into an absolute (UHV) scale rests on the values of ff-0 and for Hg used as areference surface. While the accuracy of is indisputable, the experimental value of experimental approach to the measurement of 0 (and A0 upon water adsorption) of Hg would definitely remove any further ambiguity as well as any reasons not to accept certain conclusions. 

Thus if the working and counter electrodes are made of different metals, then Eq. (5.35) gives the cpd (contact potential difference) of the two metals ... 

Why in a contact potential difference (CPD) experiment the potentials are nonzero (and the O are unaffected) while in a NEMCA setup P vanishes and O is affected by Uwr ... 

In contrast to the case of the 8CB liquid crystal, no contact potential differences between first and second layers were observed with these lubricants. This indicates that there is no special orientation of the dipole active end groups, or perhaps that the end groups form hydrogen-bonded pairs with neighboring molecules so as to give no net dipole moment. 

FIG. 30 Top Square voltage waveform applied to the tip. Bottom. Corresponding changes in tip deflection (converted to force after multiplying by the lever spring constant). There is a net attractive force for both the positive and the negative cycles, but it takes time to reach the final force value. Note that the square-wave voltage is not symmetrical around zero. An offset is applied to compensate for the contact potential difference between the tip and the surface. This offset is dependent on humidity and is equal to the potential difference between the tip and the sample. (From Ref. 78.)... 

Consider two conductors, a and (3, in mutual contact in a vacnnm (Fig. 9.2). Each of them has a certain surface potential these potentials are and respectively. Between the conductors the Galvani potential is established. The potential difference between points a and b located in the vacnnm jnst ontside condnctors a and P, respectively, is called the Volta potential or the outer or contact potential difference, of this pair of conductors. Taking into acconnt that the potential difference between two points is independent of the path taken between these points, we have... 

Cutlip and Kenney (44) have observed isothermal limit cycles in the oxidation of CO over 0.5% Pt/Al203 in a gradientless reactor only in the presence of added 1-butene. Without butene there were no oscillations although regions of multiple steady states exist. Dwyer (22) has followed the surface CO infrared adsorption band and found that it was in phase with the gas-phase concentration. Kurtanjek et al. (45) have studied hydrogen oxidation over Ni and have also taken the logical step of following the surface concentration. Contact potential difference was used to follow the oxidation state of the nickel surface. Under some conditions, oscillations were observed on the surface when none were detected in the gas phase. Recently, Sheintuch (46) has made additional studies of CO oxidation over Pt foil. 

Contact potential difference measurements, os-cillatoiy reactions, 39 85 Contact synergy model, 40 183 Contaminant distribution... 

Trasatti ° assumed that the value of at ct = 0 is constant (-0.31 V) and independent of the nature of the solvent. Therefore, if the contact potential difference at cr = 0 is known, the values of Sx for a given metal can be calculated. It should be noted that the idea that the potential shift due to the interaction of metal electrons with solvent is independent of the nature of the solvent is open to criticism. For example, the local solvent field can interfere with electron distribution in the metal in the vicinity of the interface. The data obtained for a mercury electrode and different solvents show that the contact potential difference is mainly determined by the orientation of solvent dipoles at the interface. The positive values of gjUdip)o are due to orientation of the solvent dipoles with their negative ends directed toward the mercury surface. 

The metal-solvent interaction is expected to depend on the donicity of the solvent the higher the donor number of the solvent the stronger the solvent-metal interaction should be. Hence, a correlation between the contact potential difference A>// (a = 0) and the donor number of the solvent should be observed. However, this correlation for the Hg electrode is rather poor, with the most deviant point having been found for water, that is, for the case of the strongest dipole-dipole interaction in the bulk. The correlation is better when acceptor numbers of solvents are taken into account. ... 

Fig. 4-13. Energy levels of electrons in an electrode partially immersed in an electrolyte solution m/s = outer potential (contact potential difference) between the electrode and the electrolyte solution.

Figure 5.3. Contact potential difference between two dissimilar conductors Ef, Fermi level.

The term A (Pt,M) appears in all measurements and thus does not influence the order of the measured electrode potentials. It is the potential difference that appears when two dissimilar conductors come into contact. Since the Fermi energies of two different metals are in general different, a flow of electrons occurs that tends to equalize the Fermi energies (i.e., their chemical potential). The Fermi level is either (1) the uppermost (the top) filled energy level in a partially occupied valence band of electrons in a solid, or (2) the boundary between the filled and the empty states in a band of electrons in a solid (Chapter 3). This electron flow charges up one conductor relative to the other and the contact potential difference results (Fig. 5.3). 

An experimental measurement of the band lineup between the CdS and solar-grade polycrystalline CIGS has been made using contact potential difference (Kelvin probe) measurements in air [10]. This lineup is shown in Figure 9.3. In particular, it shows that no spike was found in the conduction band. The presence of such a spike (believed to occur from previous studies either on single crystals... 

The difference in electrostatic potential which exists between the inside and the outside of the metal is termed the surface potential. The related properties—the work function and the contact potential difference—respectively measure free energy changes when electrons are moved from one conductor to a vacuum and from one conductor to another. The thermodynamic basis of these properties has been reviewed by Herring and Nichols (6), and Chalmers (7) has considered the theory of contact potentials. 

Fig. 6.29. If electrode M, and the connecting wires M2 are dissimilar metals, a contact potential difference PC /M, at the metal Mumetal M2 interface is generated in the measurement process in addition to the extra metal-solution potential difference PDm2/S-...

Further, the contact potential difference between two metals depends on the composition of the two metals and is unaffected by potential difference across the cell, and hence,... 

In this representation, Pt (and not Pt) has been written in at the right to show that a contact potential difference will arise where the platinum wire from the high-input impedance voltmeter (Fig. 7.14) contacts the copper electrode. The symbol //is used to indicate that the potential due to the junction between the solutions containing the H+ and Cu2+ has been minimized. 

The concept of the Galvani potential should be distinguished from that of the contact potential difference, which is widely used in physics to describe contacts of two electronic conductors. In an electrode-electrolyte system the contact potential difference Ai/l which is frequently called the Volta potential, represents the difference of electrostatic potentials between two points located in the same (vapor) phase near free surfaces of contacting electrode and electrolyte solution (see Fig. 1). Let us note that the Volta potential can be measured directly, but the Galvani potential cannot, since it represents the potential difference between points in different phases. 

In Tables V and VI data for the work functions for different organic solid compounds are summarized. It is peculiar that for anthracene no reliable value could be found and the margin of

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