Potentiostatic oxide formation

Meincke H, Ebling DG, Heinze J, Tacke M, Bbttner H (1998) Potentiostatic oxide formation on lead selenide single crystals in alkaline solutions. J Electrochem Soc 145 2806-2812... 

Typical anodization curves of silicon electrodes in aqueous electrolytes are shown in Fig. 5.1 [Pa9]. The oxidation can be performed under potential control or under current control. For the potentiostatic case the current density in the first few seconds of anodization is only limited by the electrolyte conductivity [Ba2]. In this respect the oxide formation in this time interval is not truly under potentiostatic control, which may cause irreproducible results [Ba7]. In aqueous electrolytes of low resistivity the potentiostatic characteristic shows a sharp current peak when the potential is switched to a positive value at t=0. After this first current peak a second broader one is observed for potentials of 16 V and higher, as shown in Fig. 5.1a. The first sharp peak due to anodic oxidation is also observed in low concentrated HF, as shown in Fig. 4.14. In order to avoid the initial current peak, the oxidation can be performed under potentiodynamic conditions (V/f =const), as shown in Fig. 5.1b. In this case the current increases slowly near t=0, but shows a pronounced first maximum at a constant bias of about 19 V, independently of scan rate. The charge consumed between t=0 and this first maximum is in the order of 0.2 mAs cnT2. After this first maximum several other maxima at different bias are observed. 

The surface of the electrode must remain constant as the potential is changed in a series of potentiostat, steady-state measurements. Apart from difficulties connected with impurity adsorption, which are reduced if the experiments are carried out sufficiently quickly (Cliapter 8), it may be that thermodynamically the most stable state of the surface changes with potential most commonly by means of oxide formation. If this is suspected, it is helpful to keep observing the electrode surface during the potential measurements. The methods used must be in situ spectroscopic ones (see Section 7.5.15) FTIR or ellipsometiy are the most readily applied (see Sections 7.5.15.2 and 7.5.16, respectively). 

A good insight into the anodic oxide formation is gained from potentiostatic pulse measurements. Figure 19 shows current transients i t) of anodic oxide formation on aluminum at pH = 6.0. Various potential steps from 0 V (hess) were chosen to an oxide formation potential between 3.3 and 5.9 V [77]. This corresponds to an increase in field strength from 6.6 to 10.1 MVcm . The initial film thickness of 7.4 nm is given by a prepolarization to 3V (hess). Each experiment must be performed on a different sample with respect to the irreversible... 

Fig. 20 Oxide formation on Cu and corrosion during a potentiostatic transient in dependence of log t. U = 1.5 V, pH 9. The total anodic charge q+ is obtained by integration of i over t. Coulometric measurements of oxide reduction after the formation at tf yield the charges

Oxide Growth Kinetics and Mechanism. Formation of oxide films by potentiostatic polarization and their characterization by CV enables distinction of various oxide states as a function of the polarization conditions, here Ep, tp and T. This method allows precise determination of the thickness of oxide films with accuracy comparable to the most sensitive surface science techniques 4-7J1-20), CV may be considered the electrochemical analog of temperature programmed desorption, TPD, and one may refer to it as potential programmed desorption, PPD. Theoretical treatment of such determined oxide reduction charge densities by fitting of the data into oxide formation theories leads to derivation of important kinetic parameters of the process as a function of the polarization conditions. The kinetics of electro-oxidation of Rh at the ambient temperature were studied and some representative results are reported in ref 24. The present results are an extension of the previous experiments and they involve temperature dependence studies. 

FIGURE 3.19 Species coverage in the oxide formation and reduction model in the limit of zero scan rate (potentiostatic) compared with EC-XPS data from Wakisaka et al. (2010). (Reprinted from Electrocatalysis, Mechanistic principles of platinum oxide formation and reduction, 2014, 1-11, Rinaldo et al. Copyright (2014) Springer. With permission.)... 

On the basis of experimental findings Heinze et al. propose the formation of a particularly stable, previously unknown tertiary structure between the charged chain segments and the solvated counterions in the polymer during galvanostatic or potentiostatic polymerization. During the discharging scan this structure is irreversibly altered. The absence of typical capacitive currents for the oxidized polymer film leads them to surmise that the postulated double layer effects are considerably smaller than previously assumed and that the broad current plateau is caused at least in part by faradaic redox processes. 

In a recent publication, Schafer and coworkers point out the utility of the electrode as a reagent which is effective in promoting bond formation between functional groups of the same reactivity or polarity [1]. They accurately note that reduction at a cathode, or oxidation at an anode, renders electron-poor sites rich, and electron-rich sites poor. For example, reduction of an a, -un-saturated ketone leads to a radical anion where the )g-carbon possesses nucleophilic rather than electrophilic character. Similarly, oxidation of an enol ether affords a radical cation wherein the jS-carbon displays electrophilic, rather than its usual nucleophilic behavior [2]. This reactivity-profile reversal clearly provides many opportunities for the formation of new bonds between sites formally possessing the same polarity, provided only one of the two groups is reduced or oxidized. Electrochemistry provides an ideal solution to the issue of selectivity, given that a controlled potential reduction or oxidation is readily achieved using an inexpensive potentiostat. 

Investigations into the effect of ultrasound upon these polymerisation processes began in the mid 1980 s when Akbulut and Toppare [81] examined the potentiostatic control of a number of copolymerisations. In such copolymerisations initiation takes place once a potential in excess of the oxidation potential of either monomer has been applied. However, often potentials even higher than these are required due to the formation at the electrode of a polymer film. These films create a resistance to the passage of current in the bulk medium with consequent reductions in the possible electrochemical reactions and therefore reductions in the rate and the yield. The use of ultrasound has been rationalised in terms of its removal of this layer in a... 

Often the first step in the electropolymerization process is the electrooxida-tive formation of a radical cation from the starting monomer. This step is commonly followed by a dimerization process, followed by further oxidation and coupling reactions. Well-adhered films can thus be formed on the surface in galvanostatic, potentiostatic, or multiscan experiments. The behavior of elec-... 

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