Size Control Using Electrochemical Methods

Electrochemical methods are very useful in structural studies but are barely applicable for preparative aims. The cause is the limited stability of cation radicals. It is difficult to do low-temperature preparative electrolysis, and the main problem is to dispose of the large amount of heat generated during the electrode work. That is, not much current can be passed through an ordinary-sized electrode without generating too much heat. When potential and temperature control are necessary, only small quantities of a material can be obtained in a reasonable period of time. When potential and temperature control are not necessary, as in Kolbe electrolysis, anodic oxidation is indeed useful as a preparative method. 

SERS active structures can be prepared by a variety of chemical physical and electrochemical methods described in Sect. 4.1. The chemical preparation of colloidal nanoparticles is frequently used (Sect. 4.1.1). An interesting electrochemical preparation procedure is the so-called double-pulse technique. This method is an electrochemical tool for controlling the metal deposition with respect to particle size and particle density (Sect. 4.1.2). 

The electrochemical synthesis developed by Reetz and co-workers offers at present the most rational method for control of particle size. Researchers have obtained at will almost monodisperse samples of colloidal Pd and Ni between 1 and 6nm using variable-current densities and suitable adjustment of further essential parameters [12]. For thermal decomposition methods the resulting particle size has been found to depend on the heat source [44f]. Size control has also been reported for the sonochemical decomposition method [45e] and y-radiolysis [48]. 

In the standard chemical preparation methods, the properties, especially the size and size distribution of the nanoparticles, are defined by the choice of the reaction conditions, reactant concentrations, etc. The use of electrochemical techniques to generate nuclei has the advantage that the supersaturation is determined by the applied potential or current density. Thus, the size of the particles can be controlled by electrochemical instrumentation rather than by changing the experimental conditions. Reetz and Helbig [115] demonstrated how electrochemical methods can be used to produce metal colloids of nanometer size and more importantly how particle size can be controlled in a simple manner by adjusting the current density [159]. First, a sacrificial anode was used as the source of the metal ions, which were then reduced at the cathode. Later, a more general approach was introduced, where metal salts were used as the starting material [160]. The particles were stabilized by alkylammonium or betaine salts. With a suitable choice of surfactants, the electrochemical method can be applied in the preparation of different shapes of particles, e.g., nanorods [161]. 

In spite of the exquisite control of reaction rate and duration afforded by electrochemical methods, electrodeposition has hardly been used for preparing nanomaterials. An exception to this generalization is the synthesis of nanoparticles and nanorods using the template synthesis method pioneered by Martin (1-6), Moskovits and co-workers (7-9), and Searson and co-workers (10-16). Template synthesis (Scheme 16.1.1) involves the electrodeposition of materials into the pores of ultrafiltration membranes (e.g., Nuclepore and Anopore ) that have uniform, cylindrical, or prismatic pores of a particular size. 

Porous anodic alumina (PAA) is a closely packed structure, consisting of many hexagonal cells, in the center of which is a vertical pore. PAA attractiveness is in its low range of pore sizes in diameter and this size is well controlled in the course of its formation. Deposition of In, Cd and Zn in these pores is implemented by electrochemical method using pulsed deposition, which ensures a void-free PAA pore filling by metal. 

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