Sonoelectrochemistry

Sonoelectrochemistry can be considered as the interaction of sound (hence SONO) with electrochemistry which is itself the interconversion of electrical and chemical energies. Whilst this chapter will concentrate on the application of ultrasound to important industrial processes such electrodeposition (or electroplating) and electo-or-ganic synthesis, it is important to first introduce the concept of electrochemistry, for those who are unfamiliar, so that we will have a better understanding as to what precisely happens in an electrochemical or electroplating process and how the application of ultrasound will be of benefit. 

In general, any ionic solid can be pictured as a unit cell in which the cations are surrounded by a number of anions and the anions are surrounded by a number of cations. A typical example is that of sodium chloride (Fig. 6.1). 

On addition to water, the lattice is destroyed and the solid dissolves to create a conducting solution of solvated sodium and chloride ions in water. Since some short range order still exists between the two ions we may naively picture the solvated solution as follows (Fig. 6.2). So what happens when we insert a piece of metal into solution. 

By analogy, when we immerse a metal in a solution of ions, something happens to set up an electric potential difference between the metal and the solution. The metal and the ions do not exist as independent entities. Let us take the example of immersing a piece of copper metal into aqueous copper sulphate. 

When we insert the copper metal into the copper sulphate solution, we find that some of the copper ions deposit on the copper metal. They do this by accepting electrons from the metal conduction band thereby leaving the metal with a small positive charge (Eq. 6.1). 

The effect of ultrasound in electrochemistry, i.e. that the application of ultrasonic energy can increase the rate of electrolytic water cleavage, was discovered as early 

Ultrasound and electrochemistry provide a powerful combination for several reasons. Ultrasound is well known for its capacity to promote heterogeneous reactions, mainly through increased mass-transport, interfacial cleaning, and thermal effects. Effects of ultrasound in electrochemistry may be divided into several important branches (1) Ultrasound greatly enhances mass transport, thereby altering the rate, and sometimes the mechanism, of the electrochemical reactions. 

However, it is only recently that the potential benefits of combining sonochemistry with electrochemistry have increasingly been studied. It should be noted that electrochemical methods, mainly electrodeposition, are well established for the preparation of metals and semiconductor nanomaterials (for a review see Mastai et al. [146]). 

There are several aspects of ultrasound which recommend its use in conjunction with electrochemical processes (Table 10.6) and it is therefore somewhat surprising that there is a dearth of information on ultra-sonically enhanced electrochemistry until quite recently [28]. 

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The cleaning or depassivation eflect is of great importance in sonoelectrochemistry, as it can be employed to wash off surface-adsorbed species and reduce blocking of the electrode by adsorption of reaction products. This eflect has been reported, for example, for the depassivation of iron electrodes and for the removal of deposits and in the presence of polymer films on the electrode surface. However, damage of the electrode surface, especially for materials of low hardness such as lead or copper, can also occur under harsh experimental conditions and applied intensities [70, Tf, 80]. 

Sonoelectrochemistry has been employed in a number of fields such as in electroplating for the achievement of deposits and films of higher density and superior quality, in the deposition of conducting polymers, in the generation of highly active metal particles and in electroanalysis. Furtlienuore, the sonolysis of water to produce hydroxyl radicals can be exploited to initiate radical reactions in aqueous solutions coupled to electrode reactions. 

Apphcations of ultrasound to electrochemistry have also seen substantial recent progress. Beneficial effects of ultrasound on electroplating and on organic synthetic apphcations of organic electrochemistry (71) have been known for quite some time. More recent studies have focused on the underlying physical theory of enhanced mass transport near electrode surfaces (72,73). Another important appHcation for sonoelectrochemistry has been developed by J. Reisse and co-workers for the electroreductive synthesis of submicrometer powders of transition metals (74). 

An example of enhancement in mass transfer by acoustic cavitation is the increase in the limiting current density in electrolysis [79], The electrochemistry with ultrasound is called sonoelectrochemistry. Another example is ultrasonic cleaning [80], Soluble contaminants on a solid surface dissolve into the liquid faster with acoustic cavitation. Insoluble contaminants are also removed from a solid surface with ultrasound. This is also induced by acoustic cavitation in many cases, but in some other cases it is by acoustic streaming [81-85],... 

Abstract In the last decade, the sonoelectrochemical synthesis of inorganic materials has experienced an important development motivated by the emerging interest in the nanostructures production. However, other traditional sonoelectrochemical synthesis such as gas production, metal deposits and metallic oxide films have also been improved with the simultaneous application of both electric and ultrasound fields. In this chapter, a summary of the fundamental basis, experimental set-up and different applications found in literature are reported, giving the reader a general approach to this branch of Applied Sonoelectrochemistry. 

Sonoelectrochemistry has also been used for the efficient employment of porous electrodes, such as carbon nanofiber-ceramic composites electrodes in the reduction of colloidal hydrous iron oxide [59], In this kind of systems, the electrode reactions proceed with slow rate or require several collisions between reactant and electrode surface. Mass transport to and into the porous electrode is enhanced and extremely fast at only modest ultrasound intensity. This same approach was checked in the hydrogen peroxide sonoelectrosynthesis using RVC three-dimensional electrodes [58]. 

Deposits of materials other than metals can be obtained by sonoelectrochemistry and, among them, metal oxides are of particular interest specially in electrochemistry. 

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Synthesis of metallic magnesium by sonoelectrochemistry Hass I, Gedanken A (2008) Chem Commun 1795-1797... 

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A novel method of generating finely divided zinc metal is by the use of pulsed sonoelectrochemistry using an ultrasonic horn as the cathode [85], Normal electrolysis of ZnCl2 in aqueous NH4CI affords a zinc deposit on the cathode. When the electrolysis is pulsed at 300 ms on/off and the cathode is pulsed ultrasonically at a 100 ms 200 ms on off ratio the zinc is produced as a fine powder. This powder is considerably more active than commercial zinc powder e. g. in the addition of allyl bromide to benzaldehyde (Eq. 3.9). 

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