Contact potential difference measurements

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

Kondratenko, E.V., Buyevskaya, O., and Baems, M. Mechanistic insights in the activation of oxygen on oxide catalysts for the oxidative dehydrogenation of ethane from pulse experiments and contact potential difference measurements. J. Mol Catal A Chem. 2000,158, 199. 

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]. 

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 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 contact potential difference between Hg and H20, are a subject of continued dispute. Efforts have been made in this chapter to try to highlight the elements of the problem. However, a specialized 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. 

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-...

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. 

A distinction must be made between Ihe coniacl potentials in air and the so-called "intrinsic contact potentials in a vacuum with all adsorbed gases removed According to Millikan, the intrinsic potential difference between two metals A and B is expressed by VAfl = (i(v4 - vg /e. in which It is Planck s constant. t, i and ug are the critical frequencies of phutoeleclric emission for the two metals (see Photoelectric Effect), and e is the electronic charge. In any ease, if (he electronic work funclions of the metals are )>,t and ps- the contact potential difference is V. sa = [p./, - pn)/e. The work funclions. and hence V, . are in general dependent upon the medium surrounding the metals. Accurate measurements of these potentials are. unlorlunnlely, very difficult. 

The work function was determined from contact potential differences (CPD) measured using a gold reference electrode. Ten-mm diameter... 

The Kelvin probe (a vibrating capacitor) is used to measure the difference in work function of two electronic materials. It is often incorrectly called a Contact Potential Difference (CPD) measurement. Explain why this term is incorrect. 

This is a measurable quantity which is usually called the contact potential difference [4, 6]. The equivalent potential difference for the inner potentials is known as the Galvani potential difference1,... 

Comprehensive experimental and theoretical investigations of self-oscillations in oxidation reactions were begun in the 1970s by Slinko and coworkers (hydrogen oxidation on Ni and Pt). Their experimental methods are the optimal combination of electrothermography (ETM) with continuous measurements of the contact potential difference (CPD) between the catalyst and a reference electrode. It permitted them to observe simultaneously... 

Fig. 5 (a) The contact potential difference (CPD) measured between the gold-coated monolayer of polyalanine and a gold substrate as a function of temperature, (b-d) The photoelectron spectra that were measured at 297 K, 264 K, and 250 K, respectively. The signal intensity is plotted vs the photoejected electrons kinetic energy. The photon energy used is 5 eV. Separate spectra are shown for a clockwise circularly polarized (+ red curve) photon beam and a counterclockwise circularly polarized (— blue curve) photon beam, (c) The photoelectron spectrum at 264 K, where the CPD is zero (see a). Here the spectrum does not depend on laser polarization and does not exhibit a broad resonance. PHYSICAL REVIEW B 68, 115418 (2003). Copy right permission granted. 

Fig. 7.13 Diagram of a modified Kelvin method for measurement of the contact potential difference between two different metals, 1 and 2.

Methods of work function measurement are of two types, electron emission methods and condenser methods. In the former method heating, irradiation with light of a suitable wavelength or application of sufficiently strong electrical fields is used to cause electrons to tunnel through the surface potential barrier. The latter method consists of measuring the contact potential difference between the surface under study and a reference electrode. 

Fig. 14. Cell for the measurement of contact potential difference by the static condenser method.

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