Anodic dissolution fundamentals

The fundamental and applied electrochemistry of the silicon/electrolyte interface is presented in an authoritative review by Dr. Gregory Zhang, with emphasis in the preparation of porous silicon, a material of significant technological interest, via anodic dissolution of monocrystalline Si. The chapter shows eloquently how fundamental electrokinetic principles can be utilized to obtain the desired product morphology. 

One of the central fundamental topics in the electrochemistry of corrosion is the atomic-scale mechanism of the sequence of steps by which Fe (the most important metal in construction engineering) dissolves anodically. There are several mechanisms, and the one most investigated has a rate-determining step FeOH — FeOH+ + Cq. The involvement of OH implies that the anodic dissolution rate is pH dependent. 

A better method, from the point of view of fundamentals, is to plot the log of the current densities of the anodic dissolution current and that of the cathodic partner reaction as a function of potential, but at a given pH, respectively. The common log i at which they intersect determines the corrosion rate. These Evans-Hoar diagrams are fundamentally correct and tell whether the corrosion will be significant. However, the relevant data, which would have to take into account the presence of oxide films, etc., is at present sparse, so that Evans-Hoar diagrams are largely of value for teaching principles and seldom for giving industrially useful information on demand. 

In this chapter, the conditions for the formation of PS, the relation between the formation conditions and PS morphology, and the mechanisms for the formation of PS and morphology are discussed. The various aspects of surface condition, nature of reactions, and reaction kinetics that are fundamentally involved in the anodic dissolution of silicon are discussed in Chapters 2-5. 

A quantitative description of the diverse morphological features of PS requires the integration of the aspects discussed above as well as the fundamental reaction processes involved in silicon/electrolyte interface structure, anodic dissolution, and anodic oxide formation and dissolution as detailed in Chapters 2-5. Any mathematical formulation for the mechanisms of PS formation without such a global integration would be limited in the scope of its validity and in the power to explain details. In addition, a globally and microscopically accurate model would also require the full characterization of all of the morphological features of PS in relation to all of the... 

Since metals have very high conductivities, metal corrosion is usually electrochemical in nature. The term electrochemical is meant to imply the presence of an electrode process, i.e. a reaction in which free electrons participate. For metals, electrochemical corrosion can occur by loss of metal atoms through anodic dissolution, one of the fundamental corrosion reactions. As an example, consider a piece of zinc, hereafter referred to as an electrode, immersed in water. Zinc tends to dissolve in water, setting up a concentration of Zn ions very near the electrode... 

Fundamentals. The composition of liquids with respect to both identity and concentration of dissolved species can be determined with inductively coupled plasma atomic emission spectrometry (ICP-AES) [972]. The employed spectrometer can be coupled directly with an electrochemical cell wherein processes like corrosion or anodic dissolution occur. Continuous aspiration of very small liquid volumes transferred into the spectrometer allows determination of rates of dissolution as a function of various experimental parameters like electrode potential [973]. 

There is no doubt that specific adsorption of anions and organic species occurs in a wide potential range as the experiments were carried out in 0.1 mol dm HCIO4 supporting electrolyte at low concentrations (10 -10 mol dm ) of the species studied. It was found that the adsorption of anions exerts some influence on the anodic dissolution rate however, this effect is very far from that observed in the case of thiourea. In the latter case, the rate of anodic dissolution was significantly influenced by the presence of thiourea, even at its very low concentration. This behavior was ascribed to the strong interaction of thiourea with the metal ions and the formation of complexes both on the metal surface and in the solution phase. The phenomena presented were considered as a result of the adsorption competition of the species studied with C104 ions. Three fundamental mechanisms were formulated for the active dissolution ... 

The problem of introducing solutes into melts may not always be inconsiderable. A number of entry-port systems have been described/ but the most elegant solution to the problem for fundamental studies may be anodic dissolution. Otherwise, the availability or ease of preparation of adequately anhydrous material can present serious difficulties. ... 

The compact, nonporous anodic alumina film is the most suitable for fundamental investigations. It is grown by anodization, mostly under constant-current (galvanostatic) conditions, in neutral solutions of borates, tartrates, citrates, and phosphates, all of which possess significant buffering capacity and hence do not allow significant dissolution of the oxide. 

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