Contact potential measurement

For correlating relative Eamo values with values in the UHV scale (0 values), two quantities must be known 0 and A0. Contact potential measurements at metal/solution interfaces can be measured.4 In that case the interfacial structure is exactly that in the electrochemical situation (bulk liquid phase, room temperature). However, 0 to convert E into 0 must be independently known. It may happen that the metal surface state is not exactly the same during the measurements of 0 and A0. 

Contact-potential measurements usually indicate only a small surface dipole associated with chemisorbed hydrogen, and in the case of metals such as tungsten and nickel, the negative end of the dipole appears to be on the outside. While there is some uncertainty about the detailed interpretation of these measurements, they are more readily reconciled with covalent than with ionic bonding. 

Photoelectric measurements at higher temperatures have often shown that chemisorbed hydrogen forms dipoles pointing with their positive ends away from the surface, also in those cases where in later investigations at low temperatures (liquid-air temperatures) contact potential measurements revealed dipoles of reversed direction. It is, therefore, not to be excluded that both types of chemisorption may occur at the same metal surfaces and it is to be expected then that the latter type, which in the case of iron is called 4-type chemisorption ... 

It is, unfortunately, very difficult to compare the experimental data on the decrease of the heats of adsorption with the observed values for the changes in talkali metals on tungsten filaments, where sufficient reliable data are available. Contact potentials, measured when the gases are adsorbed on filaments, are less reliable. Most investigations concerning contact potentials on films have yielded values of surface potentials that are known for nearly complete films, but not for adsorbed layers with low 0 values. There are, moreover, rather serious deviations among the experimental values published by different authors (836). 

Low Energy Reference Electrodes for Investigating Adsorption by Contact Potential Measurements... 

Low field or contact potential measurements on well-defined macroscopic surfaces have an advantage here. The total amount of adsorbed material can be measured separately by flash desorption. Moreover, the contact potential A corresponds to an area average, which is also approached in low field emission measurements. The change in the contact potential in adsorption can therefore be unequivocally related to the dipole moment per adatom through Eq. (32). The difficulty in this approach lies in the preparation of a truly uniform surface of macroscopic size, which has not as yet been accomplished. 

The evidence from changes in conductivity, magnetism, and contact potential measurements is rather conflicting. The most rational explanation is that there are two distinct types of chemisorption of hydrogen on metals. 

A. W. Ritchie (SheU Development Company, Emeryville, California) With regard to Dr. Selwood s comments upon the cleanliness of the surfaces of evaporated metal films, I would hke to call attention to the published work on contact potential measurements. In this work 19 films of copper were evaporated, one on top of another, with the contact potential being measured after each film deposition. The contact potential continued to change until 10 films had been evaporated, after which a constant value for the remaining films was obtained. 

In the hrst section this handbook provides the fundamentals of thermodynamics and kinetics of reference electrodes, and then liquid junction potentials and salt bridges are discussed, as they are involved in almost ah reference systems. The following chapters present the various reference electrodes and systems as they are presently used. A hnal chapter is devoted to the Kelvin probe and discusses this instmment as a reference electrode for contact potential measurements. 

The succeeding material is broadly organized according to the types of experimental quantities measured because much of the literature is so grouped. In the next chapter spread monolayers are discussed, and in later chapters the topics of adsorption from solution and of gas adsorption are considered. Irrespective of the experimental compartmentation, the conclusions as to the nature of mobile adsorbed films, that is, their structure and equations of state, will tend to be of a general validity. Thus, only a limited discussion of Gibbs monolayers has been given here, and none of such related aspects as the contact potentials of solutions or of adsorption at liquid-liquid interfaces, as it is more efficient to treat these topics later. 

If two metals with different work functions are placed m contact there will be a flow of electrons from the metal with the lower work function to that with the higher work fimction. This will continue until the electrochemical potentials of the electrons in the two phases are equal. This change gives rise to a measurable potential difference between the two metals, temied the contact potential or Volta potential difference. Clearly... 

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]. 

Reference electrodes are used in the measurement of potential [see the explanation to Eq. (2-1)]. A reference electrode is usually a metal/metal ion electrode. The electrolyte surrounding it is in electrolytically conducting contact via a diaphragm with the medium in which the object to be measured is situated. In most cases concentrated or saturated salt solutions are present in reference electrodes so that ions diffuse through the diaphragm into the medium. As a consequence, a diffusion potential arises at the diaphragm that is not taken into account in Eq. (2-1) and represents an error in the potential measurement. It is important that diffusion potentials be as small as possible or the same in the comparison of potential values. Table 3-1 provides information on reference electrodes. 

The switching-off method for 7/ -free potential measurement is, according to the data in Fig. 3-5, subject to error with lead-sheathed cables. For a rough survey, measurements of potential can be used to set up and control the cathodic protection. This means that no information can be gathered on the complete corrosion protection, but only on the protection current entry and the elimination of cell activity from contacts with foreign cathodic structures. The reverse switching method in Section 3.3.1 can be used to obtain an accurate potential measurement. Rest and protection potentials for buried cables are listed in Table 13-1 as an appendix to Section 2.4. The protection potential region lies within U[[Pg.326]

Voltmeters and potentiometers The instruments described here are generally referred to as corrosion voltmeters. As mentioned previously, the current flowing through any potential-measurement circuit must be small to avoid errors due to polarisation. Moreover, if the current flow is too large, errors will be introduced owing to the voltage drop caused by the contact resistance between the reference electrode and the electrolyte. It is thus clear that the prime requirement of a potential measurement circuit is high resistance. 

Table 11-2 shows the built-in potential in metal/MEH-PPV/metal structures measured by either electroabsorption [15] or photocurrenl techniques [37] for a variety of contact metals. The uncertainty in both the work function differences and the built-in potential measurements is about 0.1 eV. For all of the structures except the Pt-Ca and Al-Sm devices there is good agreement between the metal work function difference, AW, and the built-in potential, Vhi. This indicates that for a wide range of metal contacts the Schottky energy barrier between the metal and MEH-PPV is well approximated by the ideal Schottky model and that state chaiging, which pins the Schottky energy barrier, is not significant. A built-in potential smaller than the difference between the contact work functions implies that... 

Measurements [113,368] of interfacial (contact) potentials or calculated values of the relative work functions of reactant and of solid decomposition product under conditions expected to apply during pyrolysis have been correlated with rates of reaction by Zakharov et al. [369]. There are reservations about this approach, however, since the magnitudes of work functions of substances have been shown to vary with structure and particle size especially high values have been reported for amorphous compounds [370,371]. Kabanov [351] estimates that the electrical field in the interfacial zone of contact between reactant and decomposition product may be of the order of 104 106 V cm 1. This is sufficient to bring about decomposition. 

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. 

FIG. 28 Changes in contact potential of mica relative to a hydrophobic tip as a function of relative humidity. The tip-sample distance during measurements was maintained at 400 A. At room temperature the potential first decreases by about 400 mV. At -30% RH it reaches a plateau and stays approximately constant until about 80% RH. At higher humidity the potential increases again, eventually becoming more positive than the initial dry mica surface. The changes in surface potential can be explained by the orientation of the water dipoles described in the previous two figures. 

FIG. 32 Top Semilog plot of the time constant t for ionic motion as a function of RH for KF. Bottom Simultaneously measured contact potential. At a critical humidity A, there is a break or a change in slope in these two surface properties. Below A, water solvates preferentially cations at the step edges. Above A, the rates of dissolution (solvation) of anions and cations are similar and water uni-... 

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