Oxidized Regions

Equation (12) works until coalescence starts between adjacent nuclei (Fig. 38). Assuming the symmetry of the growing process, our three-dimensional system is reduced to a problem of two dimensions, so Avrami s treatment can be applied  

Here/r(f) is the electrical current flowing after coalescence, 4xt is the extended value of intensity given by Eq. (19), and ext is the extended oxidation area without considering the existence of coalescence, referenced to the total film area (A). The value of S . can be easily deduced from Eq. (13)  

---

Oxidized regions are uniform in composition and consequently in charge density at every polarization time. Regions of neutral polymer have, as well, a uniform composition. Both oxidized and neutral regions have an amorphous structure. 

The surface concentration of polymeric segments relaxed by conformational movements on the borders of the oxidized regions a) can be expressed as follows ... 

Taking into account that the amount of charge consumed during relaxation at a given overpotential under pure conformational relaxation processes is proportional to the relaxation area of the oxidized regions ... 

Figure 12.4 A series of SFG spectra in the CO stretch region of chemisorbed CO on polycrystalline Pt in a CO-free 0.1 M H2SO4 electrolyte. The atop spectra were fit to (12.5) (see text) to extract the amplitude, frequency, and width [Lu et al., 2005 Lagutchev et al, 2006] (each displayed data point is the average of three or five spectra). The electrode potential was swept at a rate of 5 mV/s, and SFG spectra were obtained every 200 ms. Spectra were obtained at 1 mV intervals, but, to avoid congestion in the plot, averaged spectra are displayed at 10 mV intervals in the pre-oxidation region (V < 0.43 V) and at 3.3 mV intervals in the oxidation region (V > 0.43 V) [Lu et al., 2005].

Figure 12.5 CO stripping voltammogram with a CO- tee 0.1 M H2SO4 electrolyte. Compare the data in Fig. 12.4 the CO oxidation region begins at V = 0.43 V. After CO stripping, hydrogen adsorption/desorption peaks and the beginning of the Pt oxidation range are shown.

When the oxide is formed by anodizing in acid solutions and the sample is then left to rest at the OCP, some dissolution can occur. This process has been studied by a numbers of authors,70-75 especially in relation to porous oxides [cf. Section 111(4)]. It was found that pore walls are attacked, so that they are widened and tapered to a trumpet-like shape.70 71 Finally, the pore skeleton collapses and dissolves, at the outer oxide region. The outer regions of the oxide body dissolve at higher rates than the inner ones.9,19 The same is true for dissolution of other anodic oxides of valve metals.76 This thickness dependence is interpreted in terms of a depth-dependent vacancy concentration in the oxide75 or by acid permeation through cell walls by intercrystalline diffusion, disaggregating the microcrystallites of y-alumina.4... 

In the latter case, electron uptake occurs after decarboxylation of the ketoacid via Fe3+, which is able to take up electrons in strongly oxidized regions of the black-band iron sediments. Fe-ions act catalytically on the process of thioester formation, which can then occur without the help of enzymes. Thus, it was solar UV irradiation which carried the prebiotic thioesters across the energy threshold. 

Table 3.1 Peak potential, charge passed and corresponding oxide coverage for the various oxide regions in the anodic sweep of the Pt cyclic voltammogram...

In the oxidized region, holes and cation vacancies are preferred, so the electroneutrality equation is approximated by... 

Figure 18. Cyclic voltammograms on Au(III) in sulfuric acid solution. (A) In O.SmAf H2SO4 at lOmAf s", (B) in 50mM H2SO4 + O.lmAf Ag at 2mA/ s, and (C) in 0.5mA/ H2SO4 + 0.1 itiA/ Ag at 2 mV s. The dotted curve of (C) represents a cyclic voltam-mogram in the gold oxidation region. (From Ref. 19.)...

Pore initiation in anodic films produced at constant current density occurs by the merging of locally thickening oxide regions, which appear related to the substrate substructure, and the subsequent concentration of current into the residual thin regions. 

Figure 18. Schematic model of the structure of Pt particles on an voltammogram in relation to the electrode potential. The hydrogen, double layer, and oxide regions are based on cyclic voltammetry. The lattice disorder decreases in the order D>A>C>B." (Reproduced with permission from ref 40. Copyright 1993 ElsevierSequoia S.A., Lausanne.)...

The thin films are electrochemically active in both the oxidation and reduction regions, due to their having the electrochemically active zinc porphyrin units. The electrochemical processes are accompanied by color changes of the film, between black and yellowish-green in the oxidation region, and between red and yellowish-green in the reduction region. 

In the oxide region of gold electrodes, residual lead can be present, resulting from irreversible adsorption, and presence of hydroxide species has been postulated ]265]. Pb UPD on Au(lll) has also been studied in selected organic solvents, mainly propylene carbonate [284]. Results similar to those in aqueous solutions have been obtained. Deposition of Pb on Au electrodes coated by silver has also been studied [285]. Depending on the silver layer thickness, results typical for Pb deposited on Au or Ag have been obtained. 

Q mall amounts of aliphatic amines profoundly affect the reactions be- tween simple organic molecules and oxygen in the gas phase. Inhibition occurs in the slow oxidation region and, at higher temperatures, ignition is either delayed or prevented. 

---