Dissolution kinetics surface morphology

Higgins S.R., Boram L.H., Eggleston C.M., Coles B.A., Compton R.G., and Knauss K.G. (2002a) Dissolution kinetics, step and surface morphology of magnesite (104) surfaces in acidic aqueous solution at 60°C by atomic force microscopy under defined hydro-dynamic conditions. /. Phys. Chem. B 106, 6696-6705. 

The electroless deposition of metals on a silicon surface in solutions is a corrosion process with a simultaneous metal deposition and oxidation/dissolution of silicon. The rate of deposition is determined by the reduction kinetics of the metals and by the anodic dissolution kinetics of silicon. The deposition process is complicated not only by the coupled anodic and cathodic reactions but also by the fact that as deposition proceeds, the effective surface areas for the anodic and cathodic reactions change. This is due to the gradual coverage of the metal deposits on the surface and may also be due to the formation of a silicon oxide film which passivates the surface. In addition, the metal deposits can act as either a catalyst or an inhibitor for hydrogen evolution. Furthermore, the dissolution of silicon may significantly change the surface morphology. 

Information about the actual dissolution mechanism based on surface adsorption layers may be obtained from analysis of kinetic data on the undersaturation dependence of dissolution rate and observation of surface morphology of etched surfaces (36). 

The fundamental reason for the uneven distribution of reactions is that the rate of electrochemical reactions on a semiconductor is sensitive to the radius of curvature of the surface. This sensitivity can either be associated with the thickness of the space charge layer or the resistance of the substrate. Thus, when the rate of the dissolution reactions depends on the thickness of the space charge layer, formation of pores can in principle occur on a semiconductor electrode. The specific porous structures are determined by the spatial and temporal distributions of reactions and their rates which are affected by the geometric elements in the system. Because of the intricate relations among the kinetic factors and geometric elements, the detail features of PS morphology and the mechanisms for their formation are complex and greatly vary with experimental conditions. 

The morphology of weathered feldspar surfaces, and the nature of the clay products, contradicts the protective-surface-layer hypothesis. The presence of etch pits implies a surface-controlled reaction, rather than a diffusion (transport) controlled reaction. Furthermore, the clay coating could not be "protective" in the sense of limiting diffusion. Finally, Holdren and Berner (11) demonstrated that so-called "parabolic kinetics" of feldspar dissolution were largely due to enhanced dissolution of fine particles. None of these findings, however, addressed the question of the apparent non-stoichiometric release of alkalis, alkaline earths, silica, and aluminum. This question has been approached both directly (e.g., XPS) and indirectly (e.g., material balance from solution data). 

Thus, whether a metal can be deposited by electroless deposition onto a silicon surface depends on the redox potential and its relative position to the band edges and on whether the silicon can be dissolved under those conditions. On the other hand, whether the deposition can be sustained to cover the entire surface area depends, on nucleation and growth kinetics of the deposits, the catalytic effect of the deposits on silicon dissolution and hydrogen evolution and the evolution of the morphology of the surface. The formation of a continuous and uniform metal film by electroless deposition is intrinsically difficult because a certain amount of bare silicon surface area is required for silicon dissolution in order to sustain the deposition. 

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. 

An additional merit of the channel electrode set-up is that the exposed calcite surface can be viewed in situ, by light microscopy, allowing the crystallographic nature of the dissolution process to be linked directly to kinetic measurements. Furthermore, it should be noted that ex-situ studies on the morphology of crystal surfaces subjected to dissolution in the channel set-up are likely to provide greater information, by nature of the non-uniform accessibility of the crystal surface to protons, than might be anticipated from the same measurements on a crystal surface exposed to a uniform flux of electrolyte (vide infra). 

As soon as the catalyst particles are pushed out, the freed surface is exposed again to the carbon source, and growth proceeds further via carbon dissolution and diffusion throngh or on the surface of the particle. It has been proposed that the surface structure of the metallic particles directly affects the kinetics of carbon decomposition and the morphology of the carbon deposit at this stage of the growth mechanism. 

---