Reductive electrocrystallization

It is interesting to note that in a further confirmation of the redox-active character of the polypyridine ligands, reductive electrocrystallization of [Mn(bipy)3]2+ and [Mn(terpy)2]2+ complexes (M = Fe, Ru, Os) afforded the corresponding neutral species [M(bipy)3]° and [M(terpy)2]°. Based on the relevant bond lengths, it seems likely that the two added electrons enter the polypyridine ligands according to the formulation [MII(bipy-)2(bipy)]° and [Mn(terpy )2]°.111,112... 

The species that results from the reductive electrocrystallization of the sodium complex of tris-bipy cryptate, the schematic structure of which is shown in figure 2, has been called a "cryptatium." The name was chosen to express the dual nature of its procedence, since it is part cryptate and part sodium metal. It must be stressed that this cryptatium material is electroneutral, thus forming an expanded atom structure. Figure 2 also shows the schematic structure of an electride and that of a simple sodium atom, to illustrate two additional and extreme situations. In one, the electride, the complexation of the metal ion by the cryptand is so strong that the electron is essentially expelled from its interaction with the cationic center. In the other, the simple sodium atom, the outermost electron resides in an s orbital of the metallic center. Cryptatium thus represents an in-between situation, where the electron is not totally expelled from its interaction with the cation but it does not reside on the cation either, but rather on the ligand. The result is an expanded-metal type structure, where the electron is localized in the ligand structure. 

Cathodic electrodeposition of microcrystalline cadmium-zinc selenide (Cdi i Zn i Se CZS) films has been reported from selenite and selenosulfate baths [125, 126]. When applied for CZS, the typical electrocrystallization process from acidic solutions involves the underpotential reduction of at least one of the metal ion species (the less noble zinc). However, the direct formation of the alloy in this manner is problematic, basically due to a large difference between the redox potentials of and Cd " couples [127]. In solutions containing both zinc and cadmium ions, Cd will deposit preferentially because of its more positive potential, thus leading to free CdSe phase. This is true even if the cations are complexed since the stability constants of cadmium and zinc with various complexants are similar. Notwithstanding, films electrodeposited from typical solutions have been used to study the molar fraction dependence of the CZS band gap energy in the light of photoelectrochemical measurements, along with considerations within the virtual crystal approximation [128]. 

Sometimes, semiconductivity depends on the type of a structural phase that arises from synthesis. Thus, in the case of (TCNQ) Cu the semiconducting phase is thermodynamically disfavored. To prepare this semiconductor, Harris et al. (2005) proposed to perform the reduction of TCNQ in acetonitrile at glass-carbon, gold, or platinum electrode in the presence of Cu. This allows the electrocrystallization of sparingly soluble TCNQCu semiconducting phase to occur by a nucleation... 

The structure was found to be (Ph4P )2(C5o )Clj l (x = 0,15), whereas the chlorine is formed by reduction of methylene chloride. This salt is isostmctural and isomorphous with (Ph4P )2(Cgo )(Cr) [82]. Electrocrystallization with tetra-phenylarsonium halides gives analogous results [80, 83]. 

The kinetics of the electrodeposition and electrocrystallization of titanium were studied in alkali chloride melts by Haarberg et al. [140], The cathodic reduction Ti(III)/Ti(II) was very irreversible, and the Ti(II)/Ti reduction was found to be quasi-reversible, as shown in the voltammogram in Figure 14. In LiCl-KCl... 

The simultaneous application of ultrasonic irradiation to an electrochemical reaction which has been termed sonoelectrochemistry has been shown to produce a variety of benefits in almost any electrochemical process. These include enhanced chemical yield in electrosynthesis and the control of product distribution improved electrochemical efficiency in terms of power consumption, improved mixing, and diffusion in the cell minimization of electrode fouling accelerated degassing and often a reduction in the amount of process-enhancing additives required. In a major chapter devoted to this topic, Suki Phull and Dave Walton have attempted to cover the majority of applications of ultrasound in electrochemistry including electrochemical synthesis, electroanalytical chemistry, battery technology, electrocrystallization, electroinitiated polymerization, and electroplating. 

Stable electrocatalyst for long-term performances that are typical for metal deposition and electrocrystallization and reduction reactions with the adsorbed intermediates such as cuprous ions. For example, most noble metals such as platinum and rhodium [19,20],... 

Summary. An STM study has been initiated to investigate the various processes associated with electrodeposition of Cu-Ni multilayers on Cu(100). The substrates were prepared by electropolishing in phosphoric acid followed by immersion in 10 mmol/1 HCl. A (V2 x V2)R45° adlattice of oxidatively adsorbed chlorine is formed under these conditions. The adlayer stabilizes the surface steps in the <100> direction which corresponds to the close packed direction of the chloride adlattice. In dilute (millimolar) solutions of cuprous ion, reduction occurs under mass transport control with the electrocrystallization reaction proceeding by step flow in the <100> direction. At more negative potentials chloride is partially desorbed. Coincidentally, the highly kinked metal steps become Mzzy and move towards adopting the close-packed <110> orientation of the metal lattice. Preliminary experiments on heteroepitaxial nickel deposition reveal regions where electrocrystallization on Cu(100) occurs via step flow in the <110> direction. 

Galvanostatic reduction is another alternative for metal electrocrystallization in CPs. The metal nucleation and growth occurs at a continuously varying overpotential and therefore it is not suitable for gaining insight into the kinetics of the metal electrodeposition. Nevertheless, this approach provides a helpful opportunity to assess the involvement of CP reduction in the overall process, and to explore fine differences in the reductive behavior of CP materials synthesized under various electrochemical conditions [180-183,185,189]. 

Peter et al. [18] emphasized the role of the effect of uncompensated ohmic drop, and analyzed the current transients within the framework of the two-dimensional electrocrystallization model, taking into account instantaneous and progressive nu-cleations. Three-dimensional expansion of growth centers was also considered. It was found that the reduction is only rapid as long as the film remains in its conducting state. (A more detailed analysis of this problem is provided in Sect. 6.6.) It was also suggested that the electroneutrality is maintained by fast proton transport at short times. 

Electrocrystallization is carried out under galvanostatic control as potentials are poorly defined in this system. High current densities are used to drive the reduction at competitive rates relative to back-reactions such as reoxidation or dissolution. An initial current density of 50 mA/cm is usual. As the experiment proceeds, crystals grow on the cathode causing the current density to drop. Therefore, the current is stepped up over the course of approximately 2 h to a maximum of 200 mA/cm. The reaction is allowed to proceed for about an hour more. To end the experiment, the current is turned off and the electrode immediately raised above the surface of the melt. The electrode with attached crystals is carefully removed from the furnace. 

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