The Characteristic Anodic Currents in HF

If the electrolyte is composed of HF and an organic solvent a dependence of n on the kind of solvent and on the residual water content is observed, while the substrate orientation showed no effect on ru, [Chl5], 

The characteristic shape of the anodic voltammogram of a Si electrode in aqueous fluoride media, as shown for example in Fig. 3. Id, is surprisingly stable against changes in fluoride concentration (cF) or pH. When the potential of a p-type Si electrode is swept anodic of OCP a steep current rise near 0 V is observed, followed by a sharp peak (Jj) and a narrow plateau (J2). Then a second broad maximum (Ji) is found around a positive bias of 1.5-2.5 V, followed by a broad plateau (J4) extending over several volts, as shown in Fig. 4.7. When electrode rotation is used, these curves are pen-reproducible for a given solution. The hysteresis of the curves approaches zero for slow sweeps [Ch3]. 

In a detailed rotating-disk electrode study of the characteristic currents were found to be under mixed control, showing kinetic as well as diffusional limitations [Ha3]. While for low HF concentrations ( 1 M) kinetic limitations dominate, the regime of high HF concentrations ( 1 M) the currents become mainly diffusion controlled. However, none of the relevant currents (J1 to J4) obeys the Levich equation for any values of cF and pH studied [Etl, Ha3]. According to the Levich equation the electrochemical current at a rotating disk electrode is proportional to the square root of the rotation speed [Le6], Only for HF concentrations below 1 mol 1 1 and a fixed anodic potential of 2.2 V versus SCE the traditional Levich behavior has been reported [Cal 3]. 

The characteristic currents J4 to J4 are a unique property of aqueous HF, they are not observed in electrolytes composed of anhydrous HF and an organic solvent or in electrolytes free of HF [Ril]. All currents J4 to ]4 show a similar dependence on the 

The currents J1 to J4 and thereby the dissolution rate in fluoride media shows a dependence on the nature and concentration of the cation present in the electrolyte. Ji to J4 increase by more than one order of magnitude if a small cation, like Li+, is replaced by a large one, like Rb+ or Cs+. A catalytic effect of the cation on breaking of Si-O bonds is assumed [Hall, Ha 12]. 

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