Displacement deposition

In Sect. 3.3, we have shown that in the case of electroless deposition the reducing agent Red in the solution is the electron source, the electron-donating species that give electrons to the catalytic surface and the metal ions Mz+ at the interface. In this section, we show that the substrate itself can also be the electron-donating species. 

we show that, in general, Ox/Red (Mz+/M) couples with high standard electrode potentials are reduced by Ox/Red (Mz+/M) couples with low standard electrode potentials. In other words, low reduces high. 

Consider a strip of Zn placed in a solution of CUSO4 (Fig. 24). Consider the 

a layer of metallic Cu is deposited on the zinc, while Zn dissolves into solution. Metallic Zn under these conditions reduces the Cu2+ ions. This reaction is called a displacement deposition of Cu on Zn. 

The thickness of the deposited metal in the displacement deposition is self-limiting, since the displacement deposition process needs exposed (free) substrate surface in order to proceed. 

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Activation by Displacement Deposition. Silicon can be made catalytic for electroless deposition of Ni by replacing the surface Si atoms with Ni atoms (58,62) ... 

This reaction is called displacement deposition, because the nickel ions in solution simply displace the silicon at the surface. The substrate. Si, acts here as a reducing agent, as discussed in Chapter 9. Copper may be deposited on Si from HF acid solutions (69). In the presence of HF, Si is oxidized into [SiF5] . 

Activation by Thermal Decomposition of Metallic Oxides. The surface of alumina, AI2O3, may be activated by employing laser or ultraviolet irradiation to decompose AI2O3 (68). Decomposition of AI2O3 results in the generation of aluminum particles that are catalytic for electroless deposition of Cu (the first reaction probably is displacement deposition). 

The overall displacement deposition reaction in general Ox/Red (M /M) terms is given by... 

We described one example of this type of electrochemical deposition in Section 5.7 when we considered processes on a strip of Zn placed in a solution of CUSO4 (Fig. 5.11). In Chapter 5 we stated that there are two partial reactions in that system, as in an electroless system. In displacement deposition of Cu on Zn, electrons are supplied in the oxidation reaction of Zn ... 

Figure 9.1. Relationship between partial reactions in displacement deposition.

This is obtained via combination of the two partial electrode reactions, oxidation and reduction, reaction (9.5) and (9.6), respectively. Thus, in the displacement deposition of Cu on a Zn substrate, a layer of metallic Cu is deposited on the zinc while Zn dissolves into solution (Fig. 5.11). We stated that this reaction is possible since the Zn/Zn system has an electrode potential lower than that of the Cu/Cu system (Table 5.1 and Fig. 5.10). The overall displacement deposition reaction according to Eq. (9.7) can be considered as the reaction of the electrochemical cell... 

When Er is larger than Ei, reduction occurs at the right-hand electrode [Eqs. (9.4) and (9.6)]. Thus, when S > 0, the overall displacement deposition reaction [Eqs. (9.1) and (9.7)] will occur from left to right. The reaction is spontaneous (feasible) in a direction from left to right since AG is negative for pKJsitive values of [Eqs. (9.8) and (9.9)]. This is in agreement with earlier discussions in Chapter 5 and Eigure 5.10. 

The same value of f is obtained experimentally. Since f is pKJsitive, +1.10 V, and AG is negative [Eq. (9.9)], the overall displacement deposition reaction (9.7) proceeds spontaneously from left to right. 

Let us determine whether we can use the displacement deposition technique to deposit Sn on a Cu substrate. The simplest way to determine this is to use the principle presented in Figures 5.10 and 9.1. According to this principle, Sn cannot be deposited by displacement on a Cu substrate since the standard electrode potential of a Cu /Cu couple is more positive than that of an Sn +/Sn couple ... 

The kinetics and mechanisms of the displacement deposition of Cu on a Zn substrate in alkaline media was studied by Massee and Piron (5). They determined that at the beginning of the deposition process, the rate is controlled by activation. The activation control mechanism changes to diffusion control when the copper covers enough of the Zn surface to facilitate further deposition of copper. This double mechanism can explain the kinetic behavior of the deposition process. 

The mechanisms of the crystal-building process of Cu on Fe and A1 substrates were studied employing transmission and scanning electron microscopy (1). These studies showed that a nucleation-coalescence growth mechanism (Section 7.10) holds for the Cu/Fe system and that a displacement deposition of Cu on Fe results in a continuous deposit. A different nucleation-growth model was observed for the Cu/Al system. Displacement deposition of Cu on A1 substrate starts with formation of isolated nuclei and clusters of Cu. This mechanism results in the development of dendritic structures. 

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