Crystal growth electrochemical methods

Dynamics of Crystal Growth hi the preceding section we illustrated the use of a lattice Monte Carlo method related to the study of equilibrium properties. The KMC and DMC method discussed above was applied to the study of dynamic electrochemical nucleation and growth phenomena, where two types of processes were considered adsorption of an adatom on the surface and its diffusion in different environments. 

The subhalides of tellurium are an especially important class of solid state compounds, and they have been the subject of intensive studies, so that a rather complete picture of their chemistry and their properties has been obtained in recent years. Because of their high tellurium content they contain fragments of the homonuclear tellurium chains their modified tellurium structures are of great current interest with respect to possibly significant physical properties. Consequently, the results of various investigations on the synthesis of the compounds, on phase analysis by thermal methods, on crystal growth, on the structures, on spectroscopic, thermodynamic, optical, photoelectric, electrochemical properties have been reported in the last two decades. In a comprehensive review (237) all significant results are reported and discussed in detail so that the present chapter will be restricted to some selected and chemically important features. 

Some work has been reported on deposition of hydroxyapatite under hydrothermal conditions, that is much above 100 °C. This includes a study by Liu, Savino and Yates (2011) who coated hydroxyapatite on titanium, stainless steel, aluminium and copper substrates by a seeded hydrothermal deposition method. The deposition strategy included an electrochemical reaction to form quickly a thin layer of HAp seed crystals. Subsequent hydrothermal crystal growth from the seed layer resulted in dense and durable HAp films. In a typical hydrothermal synthesis, a solution of Na2EDTA (0.20 M) and Ca(NOs)2 (0.20 M) was prepared in 15 ml water and a solution of (NH4)2HP04 (0.12 M) in 15 ml water was prepared in a second container. The two source solutions were mixed together after the pH of each solution was raised to 10.0 with ammonium hydroxide. The resulting combined solution was stirred at room temperature for about 20 min and then transferred to a Teflon-lined stainless steel pressure vessel of 40 ml internal volume. 

Electrochemical crystal growth in the liquid phase is a very convenient method to get high-quality single crystals, as demonstrated by Bechgaard [6]. 

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. 

Undoubtedly, the most important contribution of Budevski et al. [58,70-72] to the field of electrocrystaUization consists in the refinement of the capillary method developed in 1951 by Kaischev, Bliznakov, and Scheludko [49] for studying the crystal growth phenomena. What Evgeni Budevski and his co-workers did was to isolate a single, defect-free crystallographic face of a silver crystal in a glassy capiUaty and to record the current due to the formation and spread of single and multiple two-dimensional (2D) clusters on its surface at a constant electrochemical overpotential / [71, 72]. 

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. 

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