Tetravalent dissolution

For the p-type substrate a significant number of electrons are collected at the backside, as shown in the top part of Fig. 3.2. This is true not only for the illuminated p-type electrode but also if the electrode is kept in the dark, which indicates that electrons are injected during the tetravalent dissolution reaction. In the regime of oscillations the electron injection current is found to oscillate, too [CalO]. 

Fig. 4.4 Reaction scheme proposed for the anodic, tetravalent dissolution of silicon electrodes in aqueous HF. The reaction can be separated into two parts first an oxide is...

FIGURE 5.61. Mechanism of the anodic dissolution of silicon in low HF concentrations at higher anodic potentials (tetravalent dissolution). After Memming and Schwandt. " ... 

Figure 2.62 Chronoamperometric conditioning of n-Si(l 11) in 0.1 M NH4F, pH 4 potentials A and C correspond to the respective light intensities in Figure 2.60 and indicate divalent and tetravalent dissolution,...

The Electron Transfer Step. Inner-sphere and outer-sphere mechanisms of reductive dissolution are, in practice, difficult to distinguish. Rates of ligand substitution at tervalent and tetravalent metal oxide surface sites, which could be used to estimate upward limits on rates of inner-sphere reaction, are not known to any level of certainty. 

At higher anodic potentials an anodic oxide is formed on silicon electrode surfaces. This leads to a tetravalent electrochemical dissolution scheme in HF and to passivation in alkaline electrolytes. The hydroxyl ion is assumed to be the active species in the oxidation reaction [Drl]. The applied potential enables OH to diffuse through the oxide film to the interface and to establish an Si-O-Si bridge under consumption of two holes, according to Fig. 4.4, steps 1 and 2. Details of anodic oxide formation processes are discussed in Chapter 5. This oxide film passivates the Si electrode in aqueous solutions that are free of HF. 

For the electrochemical dissolution of Si in electrolytes composed of anhydrous HF and an organic solvent a reaction is proposed that is similar to the divalent dissolution in aqueous HF. However, molecular hydrogen is not observed and four charge carriers are consumed per dissolved silicon atom, as in the tetravalent case [Pr7, Ril]. 

In experiments at the UNILAC accelerator at GSI, Darmstadt, the tantalum isotopes 168 170Ta were transported by the He(KCl) gas-jet and deposited on a polyethylene frit in ARCA II. The dissolution of the collected tantalum activity from the frit was investigated as a function of the a-HiB concentration. Because of the smaller column size in ARCA II (1.6x8 mm) which might cause an earlier breakthrough of the tetravalent and trivalent metal ions it was desirable to decrease the a-HiB concentration. Even with 34 pL of 0.025 M a-HiB, dissolution of >75% of the tantalum activity was achieved in 2 s. The time required for the complete elution of Ta from the column was about 4 s. Similar experiments performed at the Mainz TRIGA reactor with "mNb confirm this result as shown in Figure 14. 

The electrodeposition of tin, Sn, has been reported in both basic and acidic EMICI-AICI3 ionic liquid [26]. A divalent tin species, Sn(II), can be introduced by the anodic dissolution of metallic tin. The introduction of a tetravalent tin species, Sn(lV), is also possible by dissolving tin tetrachloride, SnCL,. However, the evaporation of SnCl4 occurs in the case of an acidic ionic liquid. The irreversible reduction of Sn(IV) to Sn(II) occurs at around 0.91 and —0.9 V in the acidic and basic ionic liquids, respectively. The electrodeposition of metallic Sn is possible by the reduction of Sn(II) ... 

Hydroxides. Pure and mixed metal actinide hydroxides have been studied for their potential utility in nuclear fuel processing. At the other end of the nuclear cycle, the hydroxides are important in spent fuel aging and dissolution, and environmental contamination. Tetravalent actinides hydrolyze readily, with Th more resistant and Pu more likely to undergo hydrolysis than and Np. All of these ions hydrolyze in a stepwise marmer to yield monomeric products of formula An(OH) with = 1,2,3 and 4, in addition to a number of polymeric species. The most prevalent and well characterized are the mono- and tetra-hydroxides, An(OH) and An(OH)4. Characterization of isolated bis and tri-hydroxides is frustrated by the propensity of hydroxide to bridge actinide centers to yield polymers. For example, for thorium, other hydroxides include the dimers. 

Perchlorates and iodates. Thorium perchlorate forms upon dissolution of thorium hydroxide in perchloric acid and crystallizes as Th(C104)4 4H20. The precipitation of tetravalent actinides as iodates has long been used to separate these elements from lanthanides at low pH. One of the earliest forms that Pu was isolated in was that of Pu(I03)4. The structure and most properties of Pu(103)4 are currently unknown, but a remarkable feature is that it is insoluble in 6M HNO3. 

Dissolution of the calcium fluoride in aluminum nitrate-nitric acid oxidizes the plutonium to the tetravalent hexanitrate complex (3), while the transplutonium nuclides remain in the trivalent state. The only actinides retained by a nitrate-form anion-exchange column are thorium, neptunium, and plutonium. The uranium distribution coeflBcient under these conditions is about ten, but uranium should not be present at this point since hexavalent uranium does not carry on calcium fluoride (4). 

In terms of decontamination, the ceric leaching appears adequate for HEPA filters. The experimental studies (5>15 >l6) have achieved 99 99% dissolution of the most refractory of the tetravalent actinides. However, the leaching of the incinerator ashes (5l>16 ) has been less successful, since as much as 5 of the alpha activity initially in this waste does not dissolve with ceric treatment alone. However, this last 5 may be recovered by dissolving the ash residuum entirely with fluoride. (This 5 loss would represent about 0.05 of the actinide feed to the main plant.)... 

The divalent dissolution of Si was then described by a two-dimensional reaction scheme at a kink site on a silicon surface (not shown) assuming the surface to be covered by fluorine instead of hydroxyl groups [8]. It was further postulated that the unstable silicon difluoride changes into a stable tetravalent form by a disproportionation reaction [9] ... 

The anodic behavior of p-type Si electrodes is quite different for lower HF concentrations. The current increases, but not really exponentially, with rising anodic polarization, it passes a maximum and increases again slowly at higher anodic potentials [8] (Fig. 8.6). The current increases with the rotation speed to of the electrode. Since the current does not follow a tu /z dependence (Levich relation [11]) the relationship cannot be determined entirely by diffusion. At electrode potentials below the peak, silicon is dissolved again in the divalent state, as already reported above in the case of high HF concentrations. Here also H2 formation was observed. At electrode potentials beyond the current peak, as shown in Fig. 8.6, the dissolution was found to occur via the tetravalent state of Si and the H2 evolution disappeared at p-type electrodes [8]. These results were confirmed 25 years later [12]. Experiments performed using the thin slice arrangement (see Chapter 4) have shown that the anodic reactions occur only via the valence band at all electrode potentials [8]. 

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