Crystallization partial reactions

Processes at metal/metal-ion electrodes include crystallization partial reactions. These are processes by which atoms are either incorporated into or removed from the crystal lattice. Hindrance of these processes results in crystallization overpotential The slowest partial reaction is rate determining for the total overall reaction. However, several partial reactions can have low reaction rates and can be rate determining. 

The overall reaction, Eq. (1), may take place in a number of steps or partial reactions. There are four possible partial reactions charge transfer, mass transport, chemical reaction, and crystallization. Charge-transfer reactions involve the transfer of charge carriers (ions or electrons) across the double layer. This is the basic deposition reaction. The charge-transfer reaction is the only partial reaction directly affected by the electrode potential. In mass transport processes, the substances consumed or formed during the electrode reaction are transported from the bulk solution to the interphase (double layer) and from the interphase to the bulk solution. This mass transport takes place by diffusion. Chemical reactions involved in the overall deposition process can be homogeneous reactions in the solution and heterogeneous reactions at the surface. The rate constants of chemical reactions are independent of the potential. In crystallization partial reactions, atoms are either incorporated into or removed from the crystal lattice. 

In this chapter we discuss the electrochemical model of electroless deposition (Sections 8.2 and 8.3), kinetics and mechanism of partial reactions (Sections 8.4 and 8.5), activation of noncatalytic surfaces (Section 8.6), kinetics of electroless deposition (Section 8.7), the mechanism of electroless crystallization (Section 8.8), and unique properties of some deposits (Section 8.9). 

A comparison of the results using this method and the rate of electroless copper deposition determined gravimetrically shows that the best results are obtained with the Le Roy equation applied to the polarization data in the anodic range. It is interesting to note that here, in the metal deposition as in the corrosion (9), the partial reaction, which does not involve destruction or building of a crystal lattice of metal substrate, gives better results (this is hardly surprising, of course). 

The overpotential rj is required to overcome the hindrance of the overall electrode reaction, which is usually composed of the sequence of partial reactions. There are four possible partial reactions, as described in Sect. 3.1.1 and, thus, four types of rate control charge transfer, diffusion, chemical reaction, and crystallization. Four different kinds of overpotential are distinguished and the total overpotential rj can be considered to be composed of four components... 

Similarly, partial reaction currents in electroless copper-plating solution can be extracted using electrochemical quartz crystal microgravimetry (EQCM) to in situ monitor the rate of copper deposition under open-circuit conditions and as a function of the electrode potential... 

The synthesis of pure rutile is difficult, as the crystallization normally yields mixtures of two, or even all three, polymorphs. Rutile is usually prepared via a hydrothermal synthesis from chlorides and oxychlorides of titanium seeded with rutile nanocrystals at temperatures below 250 °C. The addition of hydrochloric acid and aqueous alcohol solutions facilitates the preparation of rutile at temperatures between 40 and 90 °C [148]. Despite the risk of contamination, mineralizers (e.g., Sn02, NH4CI or N aCl) are often used in order to reduce the size of rutile crystals. The reaction times of the hydrothermal synthesis of rutile can be significantly reduced by microwave irradiation [149]. A single-phase rutile with nanosized, well-dispersed particles prepared by a 2 h treatment of partially hydrolyzed 0.5 M TiCU solution at 160 °C is shown in Figure 1.9. 

The fluoride [Me AlF] is tetrameric in the crystal. It is isoelectronic with [Me SiO] (p. 113) and has a very similar structure. The eight-membered ring is pucWed and the bond angles / AlFAl large (146°). Four coordinate aluminium is also present in alkoxides and amino- derivatives which result from partial reaction of R3AI with alcohols or amines (p. 82 ). Cubane type tetramers e.g. (PhAlNPh) are formed when triphenylaluminium is heated with an arylamine. 

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