Autocatalytic deposition

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

In the autocatalytic deposition there are various reducing agents (e.g., formaldehyde, hydrazine, hypophosphite, ascorbic acid, polyhydroxy alcohols, and hydrogen) that have been reported in the literature. The nature of the reducing agent can significantly influence the kinetics of electroless deposition as well as surface morphology and physicochemical properties of deposits. Once when initiated, 

In consideration of the so-called autocatalytic deposition processes, the following facts must be taken into consideration. 

This type of metallic ion reduction can take place in the bulk solution or only at the catalytically active surfaces. 

When the deposition is carried out at the solid surfaces, they (surfaces) must properly be activated to initiate the electroless deposition. 

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This can be accomplished by means of two different processes (1) an electrodeposition process in which z electrons (e) are provided by an external power supply, and (2) an electroless (autocatalytic) deposition process in which a reducing agent in the solution is the electron source (no external power supply is involved). These two processes, electrodeposition and electroless deposition, constitute the electrochemical deposition. In this book we treat both of these processes. In either case our interest is in a metal electrode in contact with an aqueous ionic solution. Deposition reaction presented by Eq. (1.1) is a reaction of charged particles at the interface between a solid metal electrode and a liquid solution. The two types of charged particles, a metal ion and an electron, can cross the interface. 

The electro-copper plating from an aqueous solution of a copper ion has an electrochemical mechanism involving the electron transfer with an external source of electric current. Copper is autocatalytically deposited on the carbon surface as a function of reaction time [10]. 

O autocatalytic deposition, A codeposition with Ni or Co, galvanic displacement. 

Electroless (autocatalytic) deposition takes place on catalytic surfaces (see Sect. 3.3... 

The autocatalytic deposition of Ni and Co was intensively studied, although the most realistic mechanism has not yet been proposed. In the explanation of the mechanisms of autocatalytic deposition, the authors use the principles of the electrodeposition, although there are significant differences among the two processes. 

When compared with electrodeposition, in the example of iron group of metals or their alloys, the proposed mechanism explaining the autocatalytic deposition must take into consideration the following aspects. 

In the autocatalytic deposition of Ni, Co, and Fe, different kinetic behavior is observed. For example, Ni is the most easily deposited by autocatalytic deposition with the common reducing agents in the acidic and alkaline conditions. Cobalt is usually deposited in alkaline solutions and with great difficulty in acidic solutions. Fe is deposited with a great difficulty, frequently with the help of small currents, resulting from displacement reactions of more electronegative metals. 

There is an obvious difference in the kinetic behavior between the electrodeposition and autocatalytic deposition of metals such as Pb, Cd, and Zn. These metals are easily electrode-posited at low overvoltages and high exchange currents. However, they act as inhibitors or stabilizers when added in very small amounts to solutions from which an autocatalytic deposition of iron group of metals or alloys is carried out. 

In the electrodeposition of alloys from the iron group of metals, anomalous deposition is observed, i.e., the more electronegative metal tends to be deposited preferentially. In the autocatalytic deposition this behavior is not observed. 

Autocatalytic deposition can easily be utilized to completely coat a nonconductive surface starting from few catalytic nuclei usually consisting of palladium or similar. It can also start on a glass surface by simple adsorption of hydrolyzed nickel ions on the surface. 

The different morphology of the Ni-P deposits produced during the autocatalytic deposition (homogenous with pores) and electrodeposition (lamellar), when process is carried out at low temperature and containing phosphite or hypophosphite.11... 

Autocatalytic deposition of nickel and other metals was investigated by means of the electrochemical techniques.1 3 For this purpose, both steady-state and transient methods were applied. The results of these studies give important information about the reactions occurring during the autocatalytic deposition and about the surface where the intermediate species of reaction are stabilized by adsorption. 

In many other cases, however, there is a direct influence of the reducing agent on the metal deposition reaction. This is observed for NiP autocatalytic deposition, de Minjer22 showed that it is possible to deposit nickel cathodically when the electrode is connected through a liquid junction to another nickel electrode, at which the reducing agent oxidation proceeds independently. However, the rate of deposition under these conditions, based on de Minjer s observations, is much lower when the two reactions are separated than when they occur simultaneously at the same surface. 

H2 evolution. The phosphorous content in the NiP deposit was about 16% by mass, which is a result of the phosphite reduction. At the glass frit, NiP deposition also started, showing that there is no need of a metallic surface to initiate the autocatalytic deposition process. This observation can be attributed to the nickel hydroxide species adsorbed at the glass surface, which is obviously a suitable catalyst for the initiation of the deposition process. 

Codeposition of P or B as seen in the autocatalytic deposition of Ni or Co with hypophosphite or boron-containing reducing... 

A careful consideration of all the above points suggests that the mechanism based on intermediate hydrolyzed species is clearly more heuristic.1,5,6,15 This approach explains the importance of the adsorption of colloids at the surface at which the autocatalytic deposition takes place. 

Using the mechanisms of intermediate hydrolyzed species is very beneficial in understanding and the explanation of autocatalytic deposition of other metals, e.g., Cu, Ag, Au, or similar, as shown in Sect. II.2. 

Palladium electroless deposition was used for coating of Nafion 117 for the application as a membrane in direct methanol fuel cell.59 After the activation of the polymer membrane (Nafion 117) with Pd(II) complexes, reduction was carried out with sodium boro-hydride, NaBH4. Further, autocatalytic deposition of palladium was performed using a commercially available solution. Compared to bare Nafion, the Nafion/Pd composites considerably reduced methanol crossover. This resulted in enhanced cell performance, which was attributed to the existence of the Pd layer at the surface of polymer. 

Better results can be obtained for the electroless deposition (ELD) in solutions. In this case the protective ligands can probably be dissolved in the liquid phase and thus diffuse away from the seeds. Of the several deposition baths considered, each consisting of a soluble gold salt and reducing reagent, only the one based on the sulfite complex of Au and hydroxylamine resulted in dense and relatively thick (up to 800 nm) gold layers. The reason for this behaviour remains unclear but there are hints that complex reactions are taking place in the autocatalytic deposition process. 

Autocatalytic deposition is the most widely used type of plating of metals from aqueous solutions, without an application of the external electrical current or potential. This process is frequently called electroless or chemical deposition, although, these terms do not precisely describe the autocatalytic deposition. 

The reducing agent and a proper catalyst for the initiation of the autocatalytic deposition play very important roles in order to achieve the desired properties of the final product. In general terms, observations for commonly used reducing agents and respective catalysts, based on the published literature [1, 16, 17], are summarized as follows ... 

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