The deposition of electroless Ni–P film is a complex auto-catalytic process. \({\text{H}}_{2} {\text{PO}}_{2}^{ - }\) ions act as the reducing agent that participate in oxidation and reduction reactions. \({\text{H}}_{2} {\text{PO}}_{2}^{ - }\) was absorbed on the catalytic surface and then reacted with H2O to form \({\text{H}}_{ 2} {\text{PO}}_{3}^{ - }\), free electrons and hydrogen ions (H+), according to the following reactions (1) and (2) (Hsu et al. 2009):
$${\text{H}}_{2} {\text{PO}}_{2}^{ - } + {\text{H}}_{ 2} {\text{O}} \to {\text{H}}_{2} {\text{PO}}_{3}^{ - } + 2e^{ - } + 2H^{ + }$$
(1)
$${\text{Ni}}_{{}}^{2 + } + 2{\text{e}}^{ - } \to {\text{Ni}}^{0}$$
(2)
According to the reaction Eqs. (1) and (2), the deposition of electroless Ni could be described by the following reaction (3), which could occur in either alkaline or acid environment.
$${\text{Ni}}^{{ 2 { + }}} + {\text{H}}_{2} {\text{PO}}_{2}^{ - } + {\text{ H}}_{ 2} {\text{O }} \to {\text{H}}_{2} {\text{PO}}_{3}^{ - } + 2{\text{H}}^{ + } + {\text{Ni}}^{0}$$
(3)
For the alkaline solution, the main reactions for conventional deposition of electroless Ni–P film will be expressed by the following equations according to the hydride transfer mechanism (Zhang et al. 2011):
$${\text{H}}_{2} {\text{PO}}_{2}^{ - } + 2 {\text{OH}}_{{}}^{ - } \to {\text{H}}^{ - } + {\text{H}}_{2} {\text{PO}}_{3}^{ - } + {\text{H}}_{ 2} {\text{O}}$$
(4)
$$N{\text{i}}_{{}}^{2 + } + 2H^{ - } \to {\text{Ni}}^{0} + {\text{ H}}_{ 2}\uparrow$$
(5)
$$H_{2} PO_{2}^{ - } + H_{{}}^{ - } \to 2OH_{{}}^{ - } + P + 1/ 2 {\text{H}}_{ 2}\uparrow$$
(6)
$${\text{H}}_{ 2} {\text{O }} + {\text{H}}_{{}}^{ - } \to {\text{OH}}_{{}}^{ - } + {\text{H}}_{ 2}\uparrow$$
(7)
According to the Eqs. (4)–(7), the hydride transfer mechanism explains the occurrence of hydrogen evolution in a bath solution containing \({\text{H}}_{ 2} {\text{PO}}_{2}^{ - }\). Abdel Hamid et al. (Abdel Hamid 2003) pointed out that the increase in pH value for the alkaline solution resulted in the decrease of the deposition rate due to the consumption of \(OH_{{}}^{ - }\) ions according to the following Eq. (8):
$${\text{H}}_{2} {\text{PO}}_{2}^{ - } + {\text{OH}}^{ - } \to {\text{ H}}_{\text{ads}} {\text{ + H}}_{2} P{\text{O}}_{3}^{ - } + {\text{e}}^{\text{ - }}$$
(8)
The combination of two H atoms will result in the hydrogen evolution (Eq. 9) and reduction of \(N{\text{i}}_{{}}^{2 + }\) ions (Eq. 10)
$$2 {\text{H}}_{\text{ads}} \to {\text{ H}}_{ 2}\uparrow$$
(9)
$$2 {\text{H}}_{\text{ads}} + {\text{Ni}}_{{}}^{2 + } \to {\text{ Ni}}^{0} + {\text{ 2H}}_{{}}^{ + }$$
(10)
The reduced Ni mainly resulted from the reaction among the Ni ions and the reductant radicals (Abrantes and Correia 1994; Iwasa et al. 1968). However, the Si substrate could be oxidized in aqueous alkaline solution as following reaction Eq. (11):
$${\text{Si }} + {\text{ 2OH}}^{\text{ - }} \to {\text{ SiO}}_{ 2} + {\text{ H}}_{ 2} + {\text{ 2e}}^{ - }$$
(11)
On the other hand, as the Si substrate was immersed in the aqueous alkaline solution, the galvanic displacement reaction occurred, which is a spontaneous reaction. The spontaneous reaction for Ni deposition can be expressed as following Eq. (12) (Hsu et al. 2009):
$$2 {\text{Ni}}^{ 2+ } + {\text{ Si }} \to {\text{ 2Ni}}^{0} + {\text{ Si}}^{ 4+ }$$
(12)
During this spontaneous reaction, the Si substrate surface was oxidized and became a catalytic surface inducing further co-deposition of Ni–P film. Therefore, the oxidation and reduction reactions of H2PO2
− ions began to occur, followed by co-deposition of Ni–P film. In this work, Si substrate was not done by sensitizing and activation processes, the Ni nucleation on the surface of Si substrate was limited due to self-activation. The evidence could be observed under dense layer with a sparse particle distribution (Fig. 3b). The EDS pattern in Fig. 4 also proved that the Ni nucleation was Ni nuclei particles were formed preferentially in initial nucleation stage. The above discussion has been ascertained by further experiments, as shown in Fig. 9. Figure 9 shows the SEM micrographs and chemical composition distribution of fracture surface of the film. The P-content in the deposit was approximately 24 ± 0.2 wt%. The thickness of the film was uniform and dense. However, the interface between the film and the substrate was not homogeneous due to the effect of the substrate surface. There are many big particles like some nodules infiltrated into Si substrate. The EDS line profile showed that Ni concentration has a sharp peak with a sudden increase at the interface between the deposit and the substrate, which is assertive evidence to prove that Ni nuclei particles were formed preferentially in initial nucleation stage.
The reducing agent was added and the nucleated Ni particles were produced, and then Ni–P film was grown rapidly. Coincidentally with Ni reduction, \({\text{H}}_{2} {\text{PO}}_{2}^{ - }\) ions are reduced to elemental P and oxidized to \({\text{H}}_{ 2} {\text{PO}}_{3}^{ - }\) as the following reaction (13):
$$2 {\text{H}}_{ 2} {\text{PO}}_{2}^{ - } \to {\text{H}}_{ 2} {\text{PO}}_{3}^{ - } + {\text{P}} + {\text{OH}}_{{}}^{ - } + 1/ 2 {\text{H}}_{2}$$
(13)
Island growth was evident and remarkable sparse sphere particle nucleation was present. Then, islands grew and then met one another and drove to form a continuous film. Adhesion of the Ni–P film with Si substrate is naturally poor because of the sparsely distributed Ni nuclei particles in initial nucleation stage (see Fig. 3b, Fig. 4b and Fig. 9). Nuclei are firstly bonded to the substrate surface. In initial nucleation stage, nucleation rate of Ni were insufficient, which could cause a weak adhesion between the Ni–P layer and Si substrate (Wei et al. 2015). Lack of attachment areas and sites on the Si wafer caused the Ni–P layer to peel off easily.