Galvanic Displacement Deposition

Electroless plating should not be confused neither with the electrochemical (galvanic) displacement deposition - process involving the oxidation (dissolution) of the metallic substrate and concomitant reduction of metallic ions in solution - nor with the homogeneous chemical reduction process - indiscriminate deposition over all objects in contact with the solution. 

It is obvious that the galvanic displacement deposition occurs only at the surface of less noble metal (powders or various shape substrates). 

From a theoretical point of view, if a monolayer of the noble metal is deposited onto the surface of less noble metal, then thicker deposits under the conditions of galvanic displacement cannot be deposited. However, as experimentally observed, this is not true. Deposits thicker than one monolayer have frequently been observed using the galvanic displacement deposition. 

Figure 1. Galvanic displacement deposition of a more noble metal (M2) on a less noble metal (Mi) on a flat (a) and powder (b) substrates.

For the electronics and biomedical applications, the galvanic displacement deposition can be successful when very thin films are required and when an appropriate surface pretreatment is carried out to achieve a good adhesion of the deposited metallic film. 

The galvanic displacement deposition proceeds exclusively at the surface of a less noble metal, which acts as the reducing agent. 

By the definition, the galvanic displacement deposition is a heterogeneous process in which the noble metal is deposited at the surface of an active metal [1]. The consequence is that the less noble (or active) metal is oxidized or dissolved in the appropriate solution. As a result, the ions of a more noble metal present in the solution are reduced leading to the deposition of the more noble metal. This situation can be described using the electrochemical half reactions in the following way. 

In the galvanic displacement deposition, the electrons produced due to oxidation or dissolution of the metal Mi, as shown by the reaction (9.1), are further used for the reduction of the metallic ions of the metal M2 (reaction (9.2)). Consequently, a combination of the reactions (9.1) and (9.2) leads to ... 

The reaction (9.3) represents a generic description of the galvanic displacement deposition. This relatively simple process does not require any complicated equipment. Practically speaking, only a beaker (or a reactor) for a solution containing ions of the more noble metal and a less noble metallic substrate immersed into the solution is needed for this process to proceed. Although not very descriptive, other names for the galvanic displacement reaction used in practice include cementation or immersion plating. 

Even not recognized as such, the galvanic displacement deposition of noble metals such as Au or Ag onto Fe, Zn, Cu, or similar substrates is known since the times of early Mediterranean cultures and, possibly, before. In the sixteenth century, the recovery of copper from copper mine waters by contacting dilute process streams with iron scrap was successfully achieved [2]. Since that time, many different galvanic displacement deposition processes have been developed. Examples used on industrial scale include application of aluminum, iron, or zinc powders for the removal of copper, silver, gold, or other noble metals from waste solutions. Similar approaches are used for the solution purification in hydrometallurgical plants, electronics, electrochemical experiments, etc. 

Considering that during the galvanic displacement deposition of copper the aluminum substrate is dissolved, alternatively, the rate of this process can be studied in the following way. Due to dissolution of aluminum, the concentration of Al(III) ions in the solution should increase with time. Eventually, complete aluminum substrate could be dissolved under proper conditions, during the deposition of copper. By applying Faraday s law to Eq. (9.8), the concentration of Al(III)... 

Surface Morphology of Metals Produced via Galvanic Displacement Deposition... 

Galvanic Displacement Deposition onto Semiconductor Substrates... 

Practical applications of the galvanic displacement reaction, when smooth or continuous coatings are required, are, obviously, quite limited. It seems that only very thin coatings limited to several tens of nm with a smooth surface morphology can be achieved by this method. This aspect should be very carefuUy kept in mind for the modem electronics or biomedical applications. Contrarily, when hydromet-allurgical applications are in question, i.e., solution purification, the galvanic displacement deposition is frequently a method of choice. This method is used... 

General Conclusions of the Galvanic Displacement Deposition in Terms of Surface Morphology... 

Metallic powders can also be successfully produced without an external current source from aqueous solutions via electroless deposition. There are two clearly distinguished types of electroless deposition (a) galvanic displacement deposition and (b) autocata-lytic deposition [1]. 

In an ideal thermodynamic case, as soon as the surface of the less noble metal is covered with a film of more noble metal the deposition should stop. However, as experimentally observed, this is not the case. In the galvanic displacement deposition not only thicker coatings but also powders can be obtained. 

Galvanic displacement deposition of metallic powders is achieved as a consequence of the porosity of a more positive metal or due to a simultaneous hydrogen evolution reaction. Many metallic powders can be produced using this simple process. 

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