Crystal growth electrocrystallization

Walsh, F.C. and M.E. Herron, Electrocrystallization and electrochemical control of crystal growth fundamental considerations and electrodeposition of metals. Journal of Physics D Applied Physics, 1991. 24(2) p. 217. 

He started to work at the Chemical Faculty of Sofia University where he became a professor and the head of the Department of Physical Chemistry, in 1947. Kaishev founded the Institute of Physical Chemistry of the Bulgarian Academy of Sciences in 1958, and helped to establish the Central Laboratory of Electrochemical Power Sources [i]. Kaishev started to collaborate with - Stran-ski in Berlin in 1931 [iii] and became his assistant in Sofia in 1933. They laid the fundamentals of the crystal growth theory. They proposed the first kinetic theory of the two-dimensional nucleation and growth. The spiral type growth during electrocrystallization was first observed by Kaishev on silver [iii]. On the history of the creation of the molecular-kinetic theory of crystal growth see [iv]. 

Refs. [i] Toschev S, Milchev A, Stoyanov S (1972) J Crystal Growth 13/14 123 [ii] Gunawardena, GA, Hills G), Scharifker BR (1981) J Elec-troanal Cheml30 99 [iii] Milchev A, Tsakova V (1985) Electrochim Acta 30 133 [iv] Milchev A (2002) Electrocrystallization fundamentals of nucleation and growth. Kluwer, Boston... 

All of the general precautions that hold for chemical crystal growth methods (Section III.A.l) must be observed in electrocrystallization experiments also (purity of starting materials and solvent light- and vibration-free environment). If a radical-cation salt is to be prepared, 5 mL of donor solution (1 to 5 mM) is placed in the half-cell that contains the anode. 

Before electrocrystallization is initiated the solvent and anionic derivative are placed in the cathode compartment of the cell while the solvent, anionic derivative, and organic donor are loaded in the anode (oxidizing) compartment. Platinum electrodes are then inserted in both compartments and oxidation, with concomitant crystal growth at the anode, is accomplished using either constant voltage or constant current techniques. In the case of (TMTSF)2X, crystals grow on the anode according to the reaction ... 

Electrociystallization denotes nucleation and crystal growth in electrochemical sterns under the influence of an electric field [1.1-1.21]. Electrocrystallization of metals takes place at an electronic conducting substrate / ionic conducting electrolyte interface including, in general, three stages ... 

The sequence i)-iv) corresponds to increasing inhibition of the electrocrystallization process accompanied by increasing cathodic overvoltage [6.27, 6.28, 6.37]. Examples are shown in Fig. 6.1 [6.8]. A special texture type is produced by the so-called rhythmic-lamellar crystal growth, representing an oscillation reaction (Fig. 6.2) [6.38]. 

In the case of DA compounds, both donor and acceptor molecules may undergo oxidation towards fractional oxidation states. In general, their half wave potentials are similar and formation of the DA compound may compete with crystal growth of FOSC phases. This feature was illustrated in the case of TTF[Ni(dmit)2]2. Single crystals of this phase were electrocrystallized using TTF and TBA[Ni(dmit)2]. The cyclic voltammogram of each compound shows that [Ni(dmit)2] is oxidized at a potential 120 mV lower than that of TTF. However, addition of TTF to a solution of (Bun)4[Ni(dmit)2] inhibits the formation of the (Bun)4 o.29[Nl(dmit)2] FOSC phase in favor of the TTF[Ni(dmit)2]2 one.76... 

Black, shiny single crystals with distorted-hexagon-shape (3 x 2 x 0.05 mm ) of k-(BEDT-TTF)2Cu(NCS)2 were prepared by the electrochemical oxidation of BEDT-TTF (prepared from CS2 by conventional methods) in 1,1,2-trichloroethane (TCE), benzonitrile or THF in the presence of (1) CuSCN, KSCN and 18-crown-6 ether, (2) K(18-crown-6 ether) Cu(NCS)2 or (3) CuSCN and TBA-SCN. For electrolytes (1) or (3), undissolved materials remained on the bottom of the cell during the course of electrocrystallization, but the precipitation did not affect the crystal growth. Crystals were grown in H cells (total volume ca. 20 mL) or modified H cells, where one cell compartment is an Erlenmeyer flask (total volume ca. 100 mL). The anode and cathode were separated by a medium-porosity frit. 

A solution containing 105.0 g (31 mmol) of tetrabutylammonium hydrogen sulfate (recrystallized once from acetone) in 150 mL of triply distilled water is placed in a plastic beaker (500 mL) and placed in an ice th to cool. Then a solution of 95% HFSO3 (18.8 mL, 31 mmol) (Aldrich) is added dropwise with stirring. A white precipitate immediately forms. This precipitate is filtered and washed with copious amounts of ice-cold triply distilled water until the wash solution shows pH near 7.0. The precipitate is then dried and twice recrystallized from ethyl acetate to yield 9 g (8% of theoretical yield) of colorless, platelike crystals of very pure [CH3(CH2)3]4NFS03 (mp = 179-180°C). A neutral salt of very high purity is essential for any electrocrystallization portions of this synthesis, otherwise spurious reactions occur and crystal growth fails. 

Let us look at the growth sites more closely and consider factors which affect their concentration on the metal electrode the composition of the solution and the potential. Like growth sites of any other crystal, those on electrodes are found at the grain boundaries and at crystal dislocations and imperfections. These depend on the history of the electrode, whether it was stretched, hammered or annealed and thus will also depend on the history of the electrode. If the solution contains species that adsorb on the electrode at the crystal growth sites, will be smaller than if the solution was free from such species. This alone demands the exercise of great care in handling experiments and experimental data on electrocrystallization. 

Figure 1. Schematic representation of a conventional electrocrystallization cell. In this example, donor molecules are oxidized at the anode in the presence of A", resulting in crystal growth of the electrode.

The above discussion indicates a relatively poor understanding of the mechanistic aspects of electrocrystallization, clearly suggesting opportunities in both experimental and theoretical (modeling) areas. This will require careful studies of the role of electrochemical parameters and solvent composition in crystal growth, as well as methods that can probe the influence of these factors, preferably in a dynamic fashion. 

Under these conditions, in the presence of an overpotential, metal ions may discharge on the surface of the crystal of bulk M. This type of electrodeposition is called electrochemical crystal growth or electrocrystallization on a like substrate and will be considered in Chapter 4. Polarization to a potential E < E may be also imposed on a foreign substrate. If this is the case then it is possible to form metal nuclei on its surface. A nucleus is a cluster of metal atoms that carries the physical properties of the new crystalline phase and it is the nucleus formation what we consider as a precursor of the overall electrociystaUization phenomenon. 

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... 

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). 

Deposition potential — is the required value to observe the appearance of a new phase in the course of a -> electrocrystallization process. See, - equilibrium forms of crystals and droplets, - nucleation and growth kinetics, -> nucleation overpotential. 

The fifth part deals with growth mechanisms of single crystal faces. The growth by 2D nucleation of quasi-perfect faces as well as the spiral growth mechanism of real crystal faces are discussed. Experimental verification is presented for the case of silver electrocrystallization. 

There is no fundamental theory for electrocrystallization, in part because of the complexity of the process of lattice formation in the presence of solvent, surfactants, and ionic solutes. For example, the growth of zinc dendrites is little understood, although it represents a significant limitation to the performance of zinc-containing battery systems. Investigations at the atomic level in parallel with studies on nonelectrochemical crystallization would be rewarding. 

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