*3.1.1. Electroless deposition*

This type of process is done without the application of any external power source. The process involves autocatalytic chemical reduction reactions to deposit metal layers. The electroless plating deposition principle is shown in Fig.4, where the reduction of metal cations occurs by electrons produced from a reducing agent. The actual electrochemical nature of the process written for the z-valent metal in the form of two half reactions is also shown in Fig. 4 [39]. The final product of the reactions comes in the form of metal deposition on the substrates. The most important feature of the electroless deposition process is that it is capable of metalizing semiconducting or even non-conducting surfaces, such as plastics, ceramics and glass.

**Figure 4.** Electroless plating deposition principle for: the oxidation of the reductant and metal layer formation from the reduction of metal ions [39].

The Ni electroless plating bath is composed of the following bath compositions [40]:

**•** A source of nickel from nickel sulphate (NiSO4[H2O]6) or nickel chloride, (NiCl2 6H2O).


Additionally, a small amount of ammonium hydroxide (NH4OH) is also added to elevate the pH of the solution. The pH of the plating bath should be maintained at around 8 ~ 10 for bath stability and uniform deposition rates [10, 16, 41]. The chemical reactions based on catalytic oxidation-reduction between hypophosphite ions and Ni can be described as the sum of two simultaneous steps [27]:

$$\text{Step 1: }\text{ H}\_2\text{PO}\_2^- + \text{H}\_2\text{O} \rightarrow \text{HPO}\_3^{2-} + 2\text{H}^+ + \text{H}^+ \tag{1}$$

$$\text{Step 2: }\ 2\text{H}^{\cdot} + \text{Ni}^{2+} \rightarrow \text{Ni} + \text{H}\_{2} \tag{2}$$

$$\text{Sum: } 2\text{H}\_2\text{PO}\_2^- + 2\text{H}\_2\text{O} + \text{Ni}^{2+} \rightarrow \text{Ni} + \text{H}\_2 + 4\text{H}^+ + 2\text{HPO}\_3^{2-} \tag{3}$$
