Electrical potential barrier

A similar experiment to that noted above can be performed, but now let the interface be populated by a molecular layer at constant n and known interface electrical potential. A molecule adsorbing at such an interface must do work against the electrical potential barrier, as well as against the interfacial pressure. We get... 

Often a rather slow adsorption at the air-water interface has been observed. Whether this is due to electrical potential barriers, or whether a particular orientation is required of the arriving molecule before it can enter the monolayer has not yet been clearly demonstrated. For small ions taking part in reactions at interfaces, such as hydroxyl and permanganate, the latter effect has never been observed, although Alexander (29) claims that ion exchange below monolayers of amines is a slow process. The present author considers that this may be explained in terms of a slow desorption of one ionic species rather than as a slow approach of the other. A gradual change in the structure of the amine film is also a possibility. 

The method of Trumit avoids problems that may arise with a spreading solvent. However, a factor that was not studied was the effect of the electric charge carried by the protein and the accompanying electrical potential barrier produced at the interface, as discussed in Section III,C. For this reason, it is preferable, when using the Trurnit method, to have the protein at a pH close to its isoelectric point to ensure complete spreading. 

Figure 9.4 Recombination pathways of photogenerated charge carriers in an n-type semiconductor-based photoelectrochemical cell. The electron-hole pairs can recombine through a current density in the bulk of the semiconductor, the depletion region, or through defects (trap states) at the semiconductor/liquid interface, iss- Charges can also tunnel through the electric potential barrier near the surface, 4 or can transfer across the interface, The bold arrows indicate the favourable current processes in the operation of a photoelectrochemical cell. The hollow arrows indicate the processes that oppose the excess of charge carriers generated by light absorption.

We will offer some familiar examples. Studies undertaken in 1986 by C. Pijolat demonstrate the influence of the electrode s nature on the response of a sintered Sn02 sensor to benzene effect. These results (Figure 8.1) show the difference between gold and platinum as the metallic elements used to facilitate the electric contacts by inlaid wires. The effects of the metal are explained by a significant difference in temperature on the response curve. This difference in the sensitivity of the sensors can be attributed to phenomena activated by the temperature, among which electric potential barriers created at the oxide/metal interface. 

FIGURE 4.44. The effect of normalized electric potential barrier, eztp/kT, on the diffusive transport kinetics in the... 

The discussion in the previous paragraph mentions causes which could affect the exchange current across the interface. The electrical potential barrier, though, depends on the difference in the Fermi level of the two sohds, and the difference in the unperturbed chemical potential of the mobile ions for the two sohds before contact is estabhshed. The contact introduces changes in the distribntion of the point defects in the space charge and the corresponding internal electrical field. This wiU be discussed in more detail in Section III on grain boimdaries. 

C.Q. Sun, C.L. Bai, Modelling of non-uniform electrical potential barriers for metal surfaces with chemisorbed oxygen. J. Phys. Condens. Matter 9(27), 5823-5836 (1997)... 

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