Anodic oxides dissolution

Figure 9-16 illustrates the polarization curves for the anodic oxidative and the cathodic reductive dissolution of ionic compound semiconductors. The anodic oxidative dissolution proceeds readily at p-type semiconductor electrodes in which the mqjority charge carriers are holes whereas, the cathodic reductive dissolution proceeds readily at n-type semiconductor electrodes in which the majority charge carriers are electrons. 

Fig. 9-16. Polarization curves of anodic oxidative dissolution and cathodic reductive dissolution of semiconductor electrodes of an ionic compound MX iiixcp) (iMxh )== anodic oxidative (cathodic reductive) dissolution current solid curve = band edge level pinning at the electrode interface, dashed curve = Fermi level pinning.

In addition to water rinse, certain other procedures also produce flatter surfaces. It has been reported that a BHF-treated surface is flatter than one treated with HE. A photocurrent-assisted etching followed by a dark current transient has been found to reduce microscopic roughness. A multistep process, involving formation of an anodic oxide, dissolution in 0.2 M NH4F at pH 4 and then at pH 4.9, produced a completely monohydride-terminated (111) surface. ... 

Compound semiconductors may be subject to both anodic oxidative dissolution and cathodic reductive dissolution. Let us now consider a metal oxide, MO, in aqueous solution. The anodic dissolution will occur with the oxidation of oxide ions at the metal oxide surface producing gaseous oxygen. Concurrently, the metal ions transfer from the metal oxide across the oxide-solution interface forming hydrated metal ions in the aqueous solution ... 

One of the anodic oxidative dissolution of metal oxides frequently encountered in the corrosion of stainless steels is the transpassive dissolution of metallic chromium ... 

This is essentially a corrosion reaction involving anodic metal dissolution where the conjugate reaction is the hydrogen (qv) evolution process. Hence, the rate depends on temperature, concentration of acid, inhibiting agents, nature of the surface oxide film, etc. Unless the metal chloride is insoluble in aqueous solution eg, Ag or Hg ", the reaction products are removed from the metal or alloy surface by dissolution. The extent of removal is controUed by the local hydrodynamic conditions. 

Formation of the first layer (a monolayer) of passivating oxide film on a denuded metal surface occurs very simply by the loss of protons from the adsorbed intermediate oxidation products, such intermediates being common to both dissolution and passivation processes . Thus for example, the first oxidative step in the anodic oxidation of nickel is the formation of the unstable adsorbed intermediate NiOH by... 

Similar considerations apply to oxidation. An anion which is considerably more stable than water will be unaffected in the neighbourhood of the anode. With a soluble anode, in principle, an anion only needs be more stable than the dissolution potential of the anode metal, but with an insoluble anode it must be stable at the potential for water oxidation (equation 12.4 or 12.5) plus any margin of polarisation. The metal salts, other than those of the metal being deposited, used for electroplating are chosen to combine solubility, cheapness and stability to anode oxidation and cathode reduction. The anions most widely used are SOj", Cl", F and complex fluorides BF4, SiFj , Br , CN and complex cyanides. The nitrate ion is usually avoided because it is too easily reduced at the cathode. Sulphite,... 

CV of solutions of lithium bis[ salicy-lato(2-)]borate in PC shows mainly the same oxidation behavior as with lithium bis[2,2 biphenyldiolato(2-)-0,0 ] borate, i.e., electrode (stainless steel or Au) passivation. The anodic oxidation limit is the highest of all borates investigated by us so far, namely 4.5 V versus Li. However, in contrast to lithium bis[2,2 -biphenyl-diolato(2-)-0,0 Jborate based solutions, lithium deposition and dissolution without previous protective film formation by oxidation of the anion is not possible, as the anion itself is probably reduced at potentials of 620-670 mV versus Li, where a... 

Spent anode residues from electrorefining (which contain approximately 20-30 percent of the plutonium fed to the process) are either recycled back to electrorefining, or, if high enough in impurities, are oxidized and sent to oxide dissolution. The spent salt is sent to aqueous dissolution (see Figure 1). 

Hence, in the absence of a redox system in solution the anodic reaction of FeS2 yields iron oxide/hydroxide and water-soluble sulfate ions. The compound does not undergo non-oxidative dissolution. 

Each of these reactions occurs in its own typical potential range. Several reactions may occur in parallel. The oxidation of solution components and the evolution of oxygen and chlorine are discussed in Chapter 15, the formation of surface layers in Section 16.3. In the present section we discuss anodic metal dissolution. 

In the late 1960s it was discovered (Entina, 1968 Binder et al., 1972) that a strong synergy effect exists in the platinum-ruthenium system. Alloys of these two metals are two to three orders of magnitude more active catalytically for the anodic oxidation of methanol than pure platinum, whereas pure ruthenium is altogether inactive for this reaction. Prolonged exploitation of such anodes is attended by a gradual decrease in catalytic activity of the alloys because of slow anodic dissolution of ruthenium from the surface layer. A similar simation is seen for platinum-tin alloys, which... 

Using electrons for the electrolytic reduction of metal salts, Reetz and coworkers have introduced a further variation to the tetraalkylammoniumhalide-stabilization mode [192-198]. The overall electrochemical process can be divided into the following steps (i) oxidative dissolution of the sacrificial Metbuik anode, (ii) migration of Met ions to the cathode, (iii) reductive formation of... 

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... 

Satisfactory agreement of experiments with kinetic laws, described by Eqs. (44) and (45), are observed only for tantalum and niobium, when the current efficiency approaches 100%. Even for these metals, certain deviations occur which could be attributed to space charge effects,82 electronic leakage currents,83 or other factors. In the case of aluminum, these deviations are relatively large, as, even in barrier-forming electrolytes, some oxide dissolution takes place from the very beginning of voltage supply to an anodized sample.32... 

Analysis of experimental data shows that the dependence of the geometrical parameters of oxides on the temperature and concentration of electrolyte is different for galvanostatic and potentio-static conditions (Fig. 35).221 It appears that potentiostatic anodization is limited mainly by processes in the bulk of the oxide and thus is not influenced by temperature (Fig. 35b), whereas the galvanostatic anodization regime involves oxide dissolution processes at the O/S interface depending both on Tel and Cel. 

Fig. 3. Current-potential curves for anodic oxidation of H2CO on different metals. Dotted lines current attributable to the anodic dissolution of Cu and Co electrodes. Solution composition 0.1 mol dm-3, 0.175 mol dm 3 EDTA, pH = 12.5, T — 298 °K. Adapted from ref. 38.

For the case of Si02 etching, HF, (HF)2 and HF2- are assumed to be the active species [Vel, Jul]. If HC1 is added to the solution the concentration of the HF2-ion becomes negligible, which leaves HF and its polymers to be the active species [Ve3]. Because for high current densities the electrochemical dissolution of silicon occurs via a thin anodic oxide layer it can be concluded that, at least for this regime, the same species are active. This is supported by the observation that F- is... 

In contrast to acidic electrolytes, chemical dissolution of a silicon electrode proceeds already at OCP in alkaline electrolytes. For cathodic potentials chemical dissolution competes with cathodic reactions, this commonly leads to a reduced dissolution rate and the formation of a slush layer under certain conditions [Pa2]. For potentials slightly anodic of OCP, electrochemical dissolution accompanies the chemical one and the dissolution rate is thereby enhanced [Pa6]. For anodic potentials above the passivation potential (PP), the formation of an anodic oxide, as in the case of acidic electrolytes, is observed. Such oxides show a much lower dissolution rate in alkaline solutions than the silicon substrate. As a result the electrode surface becomes passivated and the current density decreases to small values that correspond to the oxide etch rate. That the current density peaks at PP in Fig. 3.4 are in fact connected with the growth of a passivating oxide is proved using in situ ellipsometry [Pa2]. Passivation is independent of the type of cation. Organic compounds like hydrazin [Sul], for example, show a behavior similar to inorganic ones, like KOH [Pa8]. Because of the presence of a passivating oxide the current peak at PP is not observed for a reverse potential scan. 

If the anodic potential is increased, the current density becomes larger than JPS and dissolution occurs via an intermediate anodic oxide film. Hence the reaction can be separated into electrochemical oxide formation according to reaction (4.2) and chemical dissolution of the oxide due to HF, (HF)2 or HF2 [So2] ... 

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