Anodic Electrocrystallization

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

The approaches to HTSC electrosynthesis known to data are classified below according to the nature of the corresponding electrochemical processes. It should be emphasized that so far only process (anodic electrocrystallization) allow one to obtain HTSCs directly. The other cases deal with the fabrication of HTSC precursors or their electrochemical processing. 

The kinetics of anodic electrocrystallization has been studied in detail only for a limited group of systems. Oxides, hydroxides, and simple and complex salts can be the products of this process. The electrochemical synthesis of crystalline oxides as the... 

Fig. 5. Characteristics of thallium cuprate TlCuO(OH) electrosynthesized by anodic electrocrystallization [348] (a) structure calculated on the basis of X-ray crystal analysis (b) resistivity vs. temperature of the deposit on a copper substrate measured by the two-probe technique.

The anodic electrocrystallization [348] is carried out at a lower temperature than any other known method for the electrosynthesis (and synthesis in general) of any HTSC to date, and it is undoubtly of interest to develop it further and to extend it to other materials. In this regard, the possibility of formation of mixed-valence oxide [332,353] merits notice because in common thallium HTSCs the effective valence of thallium is less than three [361,362] (the corresponding T1(I) content is about 10%, i.e., close to the composition of the mixed-valence oxide). 

One more interesting prospect for anodic electrocrystallization for HTSC synthesis is the possibility of electrosynthesizing TI2O3-TIOF compositions [363]. 

Anodic Electrocrystallization in Low-Temperature Alkaline Melts Norton Method... 

Electrochemical studies in alkaline melts are complicated by the absence ol detailed information on the chemistry of corresponding systems, and in particular or solvation processes. Moist melts generally represent the limiting case of strong base solutions. It would undoubtedly useful to study the changes in the kinetics and the composition of the products of anodic electrocrystallization in a model system upor the gradual transition from common alkaline solutions to concentrated bases, anc then to alkaline melts with different water contents. 

The more traditional approach has already been used in anodic electrocrystallization processes to produce nanocompositions and superlattices of mixed Ti-Pb oxides [341-347]. With HTSC materials, initial steps have been made in this direction in studies on the electrochemical deposition of conductive polymers on the surface of microband YBCO electrodes [28,50,433]. In the resulting composition, the reversible transition from the HTSC/metal junction (at the high doping degree of the polymer) to the HTSC/semiconductor junction has been achieved. The properties of these compositions allow one to control the shift over a wide interval. 

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. 

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 ... 

For oxide electrocrystallization, the last condition is the most strenuous, sine many oxides are insulating non-stoichiometric compounds, however, are sufficient conductive. When two (or more) substances are codeposited, certain specific feature of the crystallization can be expressed for both cathodic and anodic processes on th basis of the thermodynamics of binary (or more complex) systems. If stabl multicomponent phases exist, then it is their deposition (not the deposition of mixture of simpler products) that preferentially proceeds in a certain potential regioi In such cases, intermetallic compounds are deposited in cathodic processes, and th deposition of mixed oxides takes place in anodic processes. These products ca represent both chemical compounds and solid solutions. 

Where Na is number of moles of A transformed. The counter electrode reaction must be chosen carefully in undivided cells to prevent reaction with the target product. The use of a sacrificial counter electrode may be satisfactory. The ideal solution is a paired electrosynthesis, i.e., when both cathodic and anodic processes are of interest. In most electrolyses, oxygen, which is electroactive, is a poison and must be removed by bubbling an inert gas through the solution or by vacuum techniques. When the electrolysis is complete, the product must be recovered. Obviously, there is no problem when the product precipitates or electrocrystallizes. The work-up of the solution may be facilitated by an appropriate choice of the experimental conditions. In... 

There is an extensive literature on the electrosynthesis of coordination compounds, which has been periodically reviewed.34"38 In the last 10 years, significant progress has been made in electrocrystallization and in the use of sacrificial anodes. 

Phase formation controlled by a charge transfer reaction Me"+ + ne M across the interface, such as 2D electrocrystallization [114, 115], UPD[2, 22, 116], and 2D anodic passivation [117,118] gives the following expression for AGj... 

How should the adsorption of additives be measured The most straightforward way is the determination of surface concentration. Some methods were already discussed in Chapter 4. These methods are complemented by a very effective method, anodic stripping. The application of this method to study the influence of additives in electrocrystallization was first described by Ogden and Tench. ° The most important application was achieved in the process control of copper deposition. This is explained in Figure 7.25. 

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. 

---