Electrodes enhancement factors

A qualitatively similar behaviour was obtained during C3H6 epoxidation on Ag 43 Enhancement factor A values of the order of 150 were measured.43 Both the rates of epoxidation and oxidation to C02 increase with I>0 and decrease with I<0. The intrinsic selectivity to propylene oxide was very low, typically 0.03 and could be increased only up to 0.04 by using positive currents. This was again an exploratory study, as no reference electrode was used, thus T and UWr could not be measured 43... 

The expression d In a/d In c is known as the thermodynamic factor and is a special case of the Wagner factor (or thermodynamic enhancement factor) which plays an important role for the kinetic properties of electrodes. This term indicates the deviation from ideality of the mobile component. For ideal systems this quantity becomes 1 and comparison with Pick s first law yields... 

Duyne and co-workers estimated enhancement factors on the order of 105 to 106 for pyridine on rough silver electrodes. The value was obtained from a comparison between surface-enhanced and normal bulk Raman signals from pyridine by taking into account the different number of molecules on the electrode and in solution. The size of the enhancement was found to correlate with the electrode roughness, indicating that enhancement occurs via a strong electromagnetic field. On the other hand, the dependence of the... 

A number of new ideas have been put forward recently. A X7 enhancement of the generated second harmonic power was obtained by ultraviolet patterning to produce a chirped periodic nonlinear susceptibility on a 60 pm wide channel waveguide [41 ]. A complete analysis of theory and experiment is given, illustrating the potential of this new technique, very competitive with the contact electrode method developed earlier [42], and first introduced without the chirping and with an enhancement factor of about 1.5 to 2 by the same group [43,44]. 

Table 1. Enhancement factors and FWHMs of COad in different electrodes.

The enhancement factor of the field near a long defect (made of n Cq capacitors) perpendicular to the electrodes is given by... 

The variation of the enhancement factor Air versus the thickness dm of the Ru him is plotted in Figure 12(b) [47], which shows an asymmetrical volcano curve. As stated previously, similar results of variation of Ajr versus dm were observed on nm-Pt/GC and nm-Pd/GC electrodes. The maximum values of Air measured on the three kind of electrodes can be sorted in a descending order of nm-Pd/GC > nm-Ru/ GC > nm-Pt/GC. 

CO adsorption on an nm-Pd/GC electrode in alkaline solution (Section 21.4.3) we know that the nm-Ru/GC exhibited also AIREs in alkaline solution. The enhancement factor Air of IR absorption of COad is 33, which is larger than the Air measured in acid solutions. We can see that both the COl and COr bands are shifted linearly to higher wavenumbers when the s is increased positively. The Stark effect of the COr band is 34 cm V It may be noteworthy that in an early study [50] of CO adsorption on a massive Ru electrode in alkaline solutions, only one COad band near 1970cm was observed and assigned incorrectly to IR absorption of bridge-bonded CO species. 

There are four main problems which arise when enhancement factors are evaluated. The first problem is that we generally do not know the amount of material on the surface, especially in electrochemical, colloid, and thin-film studies conducted in the ambient or in solution. There are only a few studies which have used radiochemical techniques for electrodes " or colloids. " A few studies attempted to measure surface coverages from the... 

Akins " reported an enhancement of about 2000 for cyanine chloride when adsorbed on a silver sol. However, the dye absorption is centered around 530 nm, while for the Raman studies a 564-nm excitation was used, i.e., a slightly off-resonance situation. An absorption feature, not present for the dye by itself or the sol alone, was seen at about 575 nm. Akins interprets it as due to a dye aggregate. However, as will be seen in Section II.5, similar bands are commonplace in SERS systems. In a separate study, Li et alP tried to evaluate the enhancement factor for cyanine chloride at 488 nm (in the absorption band) on a silver electrode and on a sol. Their estimate is a factor of about 50 for the colloid and 10" on the electrode. One should recall the difficulties inherent in such determinations (see Section II.I). 

A small number of ORCs is usually sufficient to achieve the maximum enhancement factors for silver electrodes. Indeed, repeated cycling beyond four or five times has been observed to have a detrimental effect on the SERS enhancement. A rather different situation is found for gold, however. It has... 

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