Electrodeposition principles

Preparation of basis metals for plating Preliminary cleaning of various basis metals follows the broad principles used for electrodeposited nickel. 

Thin films of ternary cadmium sulfoselenide CdS Sei-x have been electrodeposited from a non-aqueous organic bath by Baranski et al. [67], as well as by Loutfy and Ng [121], who employed similar principles (deposition from ethylene glycol solution... 

On account of the fact that the electrode potential of molybdenum is more negative than the discharge potential of hydrogen, principle difficulties arise to cathodically electrodeposit molybdenum chalcogenide films from aqueous solutions. Theoretically, the deposition of pure molybdenum by electrolytic reduction of molybdates in acidic aqueous solutions is possible according to the reaction... 

Hodes G (1995) Electrodeposition of 11-Vl semiconductors, in Rubinstein I (ed.) Physical Electrochemistry Principles, Methods, and Applications. Marcel Dekker, New York... 

Brenner A (1963) Electrodeposition of alloys - Principles and practice. Academic Press, New York... 

In the case of molten salts, the functional electrolytes are generally oxides or halides. As examples of the use of oxides, mention may be made of the electrowinning processes for aluminum, tantalum, molybdenum, tungsten, and some of the rare earth metals. The appropriate oxides, dissolved in halide melts, act as the sources of the respective metals intended to be deposited cathodically. Halides are used as functional electrolytes for almost all other metals. In principle, all halides can be used, but in practice only fluorides and chlorides are used. Bromides and iodides are thermally unstable and are relatively expensive. Fluorides are ideally suited because of their stability and low volatility, their drawbacks pertain to the difficulty in obtaining them in forms free from oxygenated ions, and to their poor solubility in water. It is a truism that aqueous solubility makes the post-electrolysis separation of the electrodeposit from the electrolyte easy because the electrolyte can be leached away. The drawback associated with fluorides due to their poor solubility can, to a large extent, be overcome by using double fluorides instead of simple fluorides. Chlorides are widely used in electrodeposition because they are readily available in a pure form and... 

Electroless deposition as we know it today has had many applications, e.g., in corrosion prevention [5-8], and electronics [9]. Although it yields a limited number of metals and alloys compared to electrodeposition, materials with unique properties, such as Ni-P (corrosion resistance) and Co-P (magnetic properties), are readily obtained by electroless deposition. It is in principle easier to obtain coatings of uniform thickness and composition using the electroless process, since one does not have the current density uniformity problem of electrodeposition. However, as we shall see, the practitioner of electroless deposition needs to be aware of the actions of solution additives and dissolved O2 gas on deposition kinetics, which affect deposit thickness and composition uniformity. Nevertheless, electroless deposition is experiencing increased interest in microelectronics, in part due to the need to replace expensive vacuum metallization methods with less expensive and selective deposition methods. The need to find creative deposition methods in the emerging field of nanofabrication is generating much interest in electroless deposition, at the present time more so as a useful process however, than as a subject of serious research. 

Consequently, ions such as BF4 and PFf), which might be expected to complex or solvate an electroactive metal species, are not expected to be reduced and to influence alloy composition, unlike AICI4 and A ECU. In principle, this should lead to better control of alloy composition since the concentrations of the electroactive species may be controlled independently. For example, one can avoid an electroactive species such as [Ti(AlCl4)3] which is likely responsible for the limited composition range found in Ti-Al alloys electrodeposited from chloroaluminates. 

Chronopotentiometry. Paunovic and Oechslin (8) measured the adsorption of peptone on lead-tin alloy electrodes using chronopotentiometric and double-layer measurements. This case is different from the adsorption of HCOOH because peptone is not an electroactive species in the conditions smdied but only blocks the surface used for the electrodeposition of lead-tin alloys from solutions containing Sn and Pb ions. Chronopotentiometric analysis is based on the following principles (7). In the absence of adsorption, the relationship between the transition time r (for reduction of Sn and Pb in this case), the bulk concentration c° of the substance reacting at the electrode, and the current I is given by the equation... 

Alloy deposition is almost as old an art and/or science as is the electrodeposition of individual metals. (Brass deposition, for instance, was invented circa 1840 ) In the last analysis, as can well be expected, alloy deposition is subject to the same scientific principles as individual metal plating. Indeed, progress in either of the two has almost always depended on similar advances in electrodeposition science and/or technology. 

Electrodeposition presents, in principle, several advantages for the investigation and production of layered alloys. Among these are the tendency of electrodeposited materials to grow epitaxial and thus to form materials with a texture influenced by the substrate. Electrodeposition can be used in systems that do not lend themselves to vacuum deposition. The electrodeposition process is inexpensive and can be upscaled with relative ease for use on large parts further, it is a room-temperature technology. This last point may be important for systems in which undesirable interdiffusion between the adjacent layers may readily occur. 

In 1986 Yahalom and Zadok (4) pointed to methods to produce composition-modulated alloys by electrodeposition, initially for the copper-nickel couple. They obtained modulation to thicknesses down to 8 A. The principle of the method is as follows. 

Eutectic mixtures of imidazolium chloride with GaCb and InCl3 have also been reported [99, 100], These metals will be of limited interest for electrodeposition although some studies have been made on the deposition of semiconductors [115, 116]. Other metal halides that have been used include AuCb, NiCh and C0CI2 [106, 107]. These tend to have higher melting points than other metal salts for the reasons explained above. They have been used for synthetic applications and while, in principle, they could be used for electrodeposition there are better alternatives that would be more suitable. 

The same principle has been used for the deposition of InSb alloys [115]. Anionic liquid based on InCh is made and SbCh is added. InSb alloy was electrodeposited but there was also some elemental In and Sb in the deposits. The In Sb ratio could be varied by altering the deposition potential. 

Comparing this approach with previous work - except the studies on solid electrolytes - ionic liquids have two distinct advantages over aqueous or organic solvents (i) Due to their extremely low vapor pressure ionic liquids can be used without any problem in standard plasma vacuum chambers, and the pressure and composition in the gas phase can be adjusted by mass flow controllers and vacuum pumps. As the typical DC or RF plasma requires gas pressures of the order of 1 to 100 Pa, this cannot be achieved with most of the conventional liquid solvents. If the solvent has a higher vapor pressure, the plasma will be a localised corona discharge rather than the desired extended plasma cloud, (ii) The wide electrochemical windows of ionic liquids allow, in principle, the electrodeposition of elements that cannot be obtained in aqueous solutions, such as Ge, Si, Se, A1 and many others. Often this electrodeposition leads to nanoscale products, as shown e.g. by Endres and coworkers [60]. 

It was John Wilkes who realized that room-temperature molten salts would only experience a widespread interest and uptake if they were stable under environmental conditions. Wilkes group published details of the first such liquid in 1992 using the BF]j" and the PFj anions, the latter showing a miscibility gap with water. Thus these liquids could, in principle, be made water free. (Today we know that ionic liquids containing BFJ and PF are subject to decomposition in the presence of water.) Electrochemical studies showed that even these early ionic liquids had wide electrochemical windows of about 4 V with cathodic limits of-2 to -2.5 V. vs. NHE. This cathodic limit should, from the thermodynamic point of view, be wide enough to electrodeposit many reactive elements. 

In Chapter 1 we explain the motivation and basic concepts of electrodeposition from ionic liquids. In Chapter 2 an introduction to the principles of ionic liquids synthesis is provided as background for those who may be using these materials for the first time. While most of the ionic liquids discussed in this book are available from commercial sources it is important that the reader is aware of the synthetic methods so that impurity issues are clearly understood. Nonetheless, since a comprehensive summary is beyond the scope of this book the reader is referred for more details to the second edition of Ionic Liquids in Synthesis, edited by Peter Wasserscheid and Tom Welton. Chapter 3 summarizes the physical properties of ionic liquids, and in Chapter 4 selected electrodeposition results are presented. Chapter 4 also highlights some of the troublesome aspects of ionic liquid use. One might expect that with a decomposition potential down to -3 V vs. NHE all available elements could be deposited unfortunately, the situation is not as simple as that and the deposition of tantalum is discussed as an example of the issues. In Chapters 5 to 7 the electrodeposition of alloys is reviewed, together with the deposition of semiconductors and conducting polymers. The deposition of conducting polymers... 

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