General Considerations of Electroless Deposition

Studies and practices of plating of different types of coatings from aqueous solutions without an external current source were described in the literature as electroless deposition. The research was predominantly related to the deposition of metals and/or alloys. However, it seems that the deposition of compounds without an external current source was in a way neglected. It is to be noted that electroless deposition of oxides, salts, polymers, etc. is also possible. 

Metal electrodeposition, where the reduction of metal ions takes place, is always a cathodic reaction. In most of the cases electrodeposition of oxides is the anodic process resulting from the oxidation of metals. 

In the analogy, for the electroless deposition of metals, the presence of reducing agents is needed, while for the electroless deposition of compounds, oxidizing agents are usually required. 

In terms of kinetics and mechanisms, electroless deposition processes have many similarities. In an attempt to analyze the electroless deposition, several mechanisms such as atomic hydrogen, hydride ion, metal hydroxide, electrochemical, and universal have been proposed.1-3 It is important to note that these mechanisms were developed for cases of nickel and copper electroless deposition, which were the most widely studied metals in this respect. Based on the proposed mechanisms, most of the features of electroless deposition can be explained. However, there are some characteristics of electroless deposition, which cannot be explained using these mechanisms. The major problems arise when attempting to generalize the proposed models explaining the mechanistic aspects. 

Electroless deposition can be defined as a deposition of solid phase of continuous coatings (films) or powders of metals, alloys, or compounds from aqueous or nonaqueous solutions or melts without an external current source. Based on this definition, electroless deposition is a purely chemical process, which involves oxido-reduction 

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