Electrocrystallization

The possibility of electrochemically producing CgQ anions in a defined oxidation state by applying a proper potential can be used to synthesize fulleride salts by electrocrystallization [39, 75-80]. An obvious requirement for this purpose is the insolubility of the salt in the solvent to be used for the electrocrystallization process. This can be achieved by choosing the proper solvent, the oxidation state of Cjq and the counter cation, which usually comes from the supporting electrolyte. 

Fulleride anions are often more soluble, especially in more polar solvents, than the parent fullerenes. For example, in bulk electrolysis experiments with tetra-n-butylammonium perchlorate (TBACIO4) as supporting electrolyte, carried out in acetonitrile where Cjq is completely insoluble, fairly concentrated, dark red-brown solutions of 50 can be obtained [81]. Upon reoxidation, a quantitative deposition of a neutral Cjq film on the surface of a gold/quartz crystal working electrode takes place. This Cjq film can be stepwise reductively doped with TBA, leading to (Cjo ) 

Whereas Cjq is insoluble and inert in liquid ammonia without any cosolvent, the fulleride anions Cjq n = 1 ), generated electrochemically with KI as supporting electrolyte, dissolve completely in this polar medium [15]. Further reductions lead to the ammonia-insoluble potassium salts of the penta- and hexaanions. 

The formation of crystalline fulleride salts at the electrode occurs when less polar solvents and bulky cations are used for the electrosynthesis. The first fulleride salt was synthesized by Wudl by bulk electrolysis of in o-dichlorobenzene with tetraphenylphosphonium chloride as supporting electrolyte [39, 80]. This black microcrystalline material with the composition (Ph4P )3(Cgg )(Cr)2 exhibits an ESR line with a g-value of 1.9991 and a line width of 45 G at room temperature. Single crystals of the slightly different salts (Ph4P )2(Cgg )(Cr) and (Ph4P )2(C50 )(Br ) could be obtained by electrocrystallization and their crystal structure was determined [82, 83]. Magnetic measurements showed the presence of unpaired spins. 

Electrocrystallization in the presence of [bis(triphenylphosphoranylidene)]ammo-nium chloride (PNPCl) leads to the single crystalline salt (PNP )(Cgo )(C5H5Cl), which shows no electrical conductivity [77]. Cjq is surrounded by bulky PPN-cation and crystal solvent and has no short contact between neighboring Cgg anion radicals 

The formation or dissolution of a new phase during an electrode reaction such as metal deposition, anodic oxide formation, precipitation of an insoluble salt, etc. involves surface processes other than charge transfer. For example, the incorporation of a deposited metal atom (adatom [146]) into a stable surface lattice site introduces extra hindrance to the flow of electric charge at the electrode—solution interface and therefore the kinetics of these electrocrystallization processes are important in the overall electrode kinetics. For a detailed discussion of this subject, refs. 147—150 are recommended. 

It is necessary to make a distinction between the initial stages of phase formation at electrodes and further growth or thickening of the deposition where local potential distribution and mass transport conditions determine the morphology. 

If the electrocrystallization is controlled by formation of two- or three-dimensional isolated nuclei, the current—overpotential relationship has a stronger dependence on 17 than predicted by the Butler—Volmer equation for charge transfer control [151] 

Growth of isolated nuclei at an electrode surface is eventually limited when they start to coalesce due to their number and size and the size of the electrode area. Analysis of the overlap problem can be performed by use of the Avrami theorem [152] and leads to maxima in the current—time curves at constant potential. Potentiostatic conditions are convenient for the study of these phenomena because electrochemical rate coefficients and surface concentration conditions are well controlled. 

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Wu S, Lipkowski J, TylizczakT and Hitchcock H P 1995 Effect of anion adsorption on early stages of copper electrocrystallization at Au(111) surface Prog. Surf. Sc/. 50 227-36... 

A different problem of spacing, which is closely related to directional solidification but has some additional degrees of freedom, is electrocrystallization. We would like to mention at least one example of greater significance, namely the production of porous silicon [131], which is still not very well understood today. 

The importance of phase transformations in anodic electrocrystallization processes has been demonstrated for the mercury/sulflde system, which exhibits a... 

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]. 

By electrodeposition of CuInSe2 thin films on glassy carbon disk substrates in acidic (pH 2) baths of cupric ions and sodium citrate, under potentiostatic conditions [176], it was established that the formation of tetragonal chalcopyrite CIS is entirely prevalent in the deposition potential interval -0.7 to -0.9 V vs. SCE. Through analysis of potentiostatic current transients, it was concluded that electrocrystallization of the compound proceeds according to a 3D progressive nucleation-growth model with diffusion control. 

Elwell D (1981) Electrocrystallization of semiconducting materials from molten salt and organic solutions. J Cryst Growth 52 741-752... 

Peter LM (1978) The electrocrystallization of cadmium sulphide films on cadmium. Electrochim Acta 23 165-174. 

Obretenov W, Bostanov V, Popov V (1982) Stochastic character of two-dimensional nucleation in the case of electrocrystallization of silver. J Electroanal Chem 132 273-276... 

Cation-radical salts with pentafluorophenyl gold(III) anions such as (TTFPh2)2.s[Au(C6F5)2Cl2] and (TTFPh2)[Au(C6F5)2l2], where TTFPh2 is the donor molecule 4,4 -diphenyltetrathiafulvalene, can be performed by electrocrystallization techniques [83]. 

Cathodic deposition (electrocrystallization) of metals is the basic process in electrometallurgy and electroplating. 

The structure of metallic deposits is determined primarily by the size, shape (faceting), type of arrangement, and mutual orientation of the crystallites. Two factors may influence the orientation and spatial alignment of the microcrystals in electrocrystallization the field direction (or direction of the electric current) and the nature of the substrate. The deposits are said to have texture when the crystallites are highly oriented in certain directions. Epitaxy implies that the lattice is altered under the influence of the substrate. 

Until the advent of modem physical methods for surface studies and computer control of experiments, our knowledge of electrode processes was derived mostly from electrochemical measurements (Chapter 12). By clever use of these measurements, together with electrocapillary studies, it was possible to derive considerable information on processes in the inner Helmholtz plane. Other important tools were the use of radioactive isotopes to study adsorption processes and the derivation of mechanisms for hydrogen evolution from isotope separation factors. Early on, extensive use was made of optical microscopy and X-ray diffraction (XRD) in the study of electrocrystallization of metals. In the past 30 years enormous progress has been made in the development and application of new physical methods for study of electrode processes at the molecular and atomic level. 

Cl. Decroly, A. Mukhtar and R. Winand, Comparative Study of Electrocrystallization of Tantalum and Niobium from Molten Fluoride Mixtures, J. Electrochem Soc. Electrochemical Sciences, p. 905, Sept. 1968. 

The involvement of halogen bonding in conducting molecular materials is essentially based on the use of halogenated TTFs in electrocrystallization experiments with counter ions of Lewis base character prone to act as halogen bond acceptors. This concept was first successfully introduced by Imakubo... 

Since the discovery of the first organic conductors based on TTF, [TTF]C1 in 1972 [38] and TTF - TCNQ in 1973 [39], TTF has been the elementary building block of hundreds of conducting salts [40] (1) charge-transfer salts if an electron acceptor such as TCNQ is used, and (2) cation radical salts when an innocent anion is introduced by electrocrystallization [41]. In both cases, a mixed-valence state of the TTF is required to allow for a metallic conductivity (Scheme 5), as the fully oxidized salts of TTF+ cation radicals most often either behave as Mott insulators (weakly interacting spins) or associate into... 

Furthermore, electrocrystallization of the diodo derivatives DIPS and DIPSe afforded original salts with threefold symmetry and varying stoichiometries, such as (DIPS)3(PF6)(PhCl)i.i5 or (DIPSe)3(PF6)i.33 (CH202)1.2 [91]. hi these salts, the halogen bond is further enhanced as... 

Following Dehnicke s report in 1996 on the co-crystallization of diiodoacety-lene (DIA) with various halide anions to form two-dimensional networks through C-I X interactions [93], Kato et al. investigated these supramo-lecular anions in the electrocrystallization of classical TTFs such as BEDT-TTF. 

Scheme 13 Polymeric anionic networks formed upon electrocrystallization of BEDT-TTF with X (Cl , Br ) in the presence of iodoalkynes or iodoperfluoroarenes...

Fig. 5.18 Potentiostatic methods (A) single-pulse method, (B), (C) double-pulse methods (B for an electrocrystallization study and C for the study of products of electrolysis during the first pulse), (D) potential-sweep voltammetry, (E) triangular pulse voltammetry, (F) a series of pulses for electrode preparation, (G) cyclic voltammetry (the last pulse is recorded), (H) d.c. polarography (the electrode potential during the drop-time is considered constant this fact is expressed by the step function of time—actually the potential increases continuously), (I) a.c. polarography and (J) pulse polarography...

Sections 5.6.2 and 5.6.3 dealt with the deposition of metals from complexes these processes follow the simple laws dealt with in Sections 5.2 and 5.3, particularly if they take place at mercury electrodes. The deposition of metals at solid electrodes (electrocrystallization) and their oxidation is connected with the kinetics of transformation of the solid phase, which has a specific character. A total of five different cases can be distinguished in these processes ... 

The basic properties of electrocrystallization can best be illustrated by the example of the deposition of a metal on an electrode of a different material (case 1). 

The electrocrystallization on an identical metal substrate is the slowest process of this type. Faster processes which are also much more frequent, are connected with ubiquitous defects in the crystal lattice, in particular with the screw dislocations (Fig. 5.25). As a result of the helical structure of the defect, a monoatomic step originates from the point where the new dislocation line intersects the surface of the crystal face. It can be seen in Fig. 5.48 that the wedge-shaped step gradually fills up during electrocrystallization after completion it slowly moves across the crystal face and winds up into a spiral. The resultant progressive spiral cannot disappear from the crystal surface and thus provides a sufficient number of growth... 

Fig. 5.49 A microphotograph of low pyramid formation during electrocrystallization of Ag on a single crystal Ag surface. (By courtesy of E. Budevski)...

The kinetics of electrocrystallization conforms to the above description only under precisely defined conditions. The deposition of metals on polycrystalline materials again yields products with polycrystalline structure, consisting of crystallites. These are microscopic formations with the structure of a single crystal. 

Macroscopic growth during electrocrystallization occurs through fast movements of steps, 10-4-10 5cm high, across the crystal face. Under certain conditions, spirals also appear, formed of steps with a height of a thousand or more atomic layers, so that they can be studied optically (Fig. 5.50). 

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