Electrically active complex

Besides the electrically active complexes discussed above, there is indirect evidence for the existence of neutral complexes. In close analogy to the observations in silicon and several III-V materials it appears that hydrogen passivates deep and shallow acceptors. Because of the small concentrations of these neutral centers, all attempts to detect them directly with local vibrational mode (LVM) spectroscopy or electron paramagnetic resonance (EPR) have been unsuccessful. 

It is responsible to suppose that the regularities established for very simple model could serve as a guide to more complex cases. The results obtained for model system show that at least two parameters, namely the rate constant, k i, and the complexation degree, O (see Eq. (1.12)), are responsible for deviations from the equiUbrium distribution. This effect is most pronounced at low concentrations of electrically active complexes that are generated by the preceding reaction. Of course, any preliminary premise on system lability should be validated or disproved by the respective simulations. 

The peculiarities of electrode processes considered next have no analogs among the so-called noncomplex systems. In their case, the same species (M" solvate-complexes) participate in both diffusion mass transport and the charge transfer step. On the contrary, in the solutions containing complexes, mass transport is determined by the flows of all particles including M", whereas the electrode potential depends only on some components within the system electrically active complexes (EACs) and products of their reduction (ligands). 

Systems of metal complexes are noted for a greater variety of mechanisms of electrochemical processes than solutions of solvate complexes. One of the main reasons determining this feature is that by their composition alone, these complex solutions are more comphcated. The material presented at the begiiming of this book shows that such solutions can contain a large number of complex and hgand species. In studying the mechanism of electrochemical processes, first and foremost, one must determine the composition of the complex, which directly participates in the charge transfer step, or, in other words, its chemical formula. Hereinafter, this species shall be termed as an electrically active complex (EAC). 

We saw that formal kinetic equations apart from kinetic parameters also contain surface concentrations Cj of electrically active species. It follows from the material presented in previous chapters that differs from the corresponding bulk values because a diffusion layer with certain concentration profiles forms at the electrode surface. Moreover, another reason due to which surface concentrations change is adsorption phenomena, which form a certain structure called a double electrode layer (DEL) at the boundary metal solution. It is clear that in kinetic equations, it is necessary to use local concentrations of reactants and products, that is, concentrations in that region of DEL where electrically active particles are located. The second effect produced by DEL is related to the fact that a potential in the localization of the electrically active complex (EAC) differs from the electrode potential. Therefore, activation energy of the electrochemical process does not depend on the entire jump of the potential at the boundary but on its part only, which characterizes the change in the potential in the reaction zone. In this connection, the so-called Frumkin correction appears in the electrochemical kinetic equations, which is related to the evaluation of the local potential i// [1]. 

Surface concentrations were used in the analysis of experimental voltammo-grams obtained for Cu electrode whose surface was mechanically renewed in the course of measurements [4]. To determine the number of ligand particles in the electrically active complex CuCNp", the following equation was suggested ... 

At higher polarizations, a direct discharge of AuCN diffusing from the bulk of solution to the electrode is possible. In Ref. [57], the assumption is made that the electrically active complex is AuCN, which at low ti, is reduced insofar as it is present in the surface layer. It follows from Tafel slopes reported in Ref. [57] that = 0.5. More complete kinetic data have been obtained at 60 C for AuCN reduction [56] = 0.7, ig = 0.82mAcm , = 3.9 x 10 cms . ... 

To determine the composition of the electrically active complex in the Cu(II)-glycolic acid solutions, the EIS and IPS methods (see Section 6.2) were used. The composition of IPS series (Table 8.3) was calculated using material balance equations with [Cu ] = 5.3mM, pH 5.3, and different [L]. Copper electrodes in these solutions acquire actually the same equilibrium potential equal to 0.239 0.001 V. Experimental Nyquist plots (the interrelation between real, Z, and imaginary, Z, components of the impedance) and the EC applied... 

Figure 9.5 Tafel plots normalized to the ratio of Cj/Cg, where c is the respective concentration of the electrically active complex given at the plots. Transformation of RDE voltammet-...

To reveal the reasons for such specific voltammetric behavior, it is necessary to analyze the influence of cathodic polarization on CU2O stability. Kinetic relations (5.19) and (5.20) derived for stepwise charge transfer mechanism can be used for this purpose, taking into account that monoligand complexes of Cu(II) and Cu(I) (CuL+ and CuL, respectively) are electrically active. With this provision, also using the expressions for stability constants for the electrically active complex (EAC), it... 

According to the concepts of thermodynamics, equilibrium characteristics of the reversible electrochemical process depend on the initial and final states and do not depend on the mechanism of equilibration. Therefore, any version of Nernst equation applied to the charge transfer process involving any metal complex should yield the same equihbrium potential. This means that there is no thermodynamic hmitation on the composition of the electrically active complex, and any metal-containing species may be treated as a possible EAC. 

Furthermore, interactions between native (nonstoichiometry) defects like vacancies and impurities/dopants are observed in GaAs, resulting in electrically active complexes that influence compensation in SC GaAs and, therefore, have to be taken into account. Post-growth heat treatment of the crystals can by appHed to influence the defect equilibria. 

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