Abstract
Composite additives have an important influence on the process of metal electrodeposition and the quality of a metal deposited layer. In this work, the additive thiourea (TU) was compounded with cetyltrimethyl ammonium chloride (CTAC), sodium dodecyl sulfate (SDS) and polyethylene glycol 20,000 (PEG20000), and their effect on the cyclic voltammetric behavior, electrochemical nucleation mechanism, crystallographic orientations and surface morphology of the nickel electrodeposition in ammoniacal solution were experimentally investigated. The results show that the introduction of composite additives resulted in a stronger cathodic polarization and increased the nucleation overpotential (NOP) values significantly, which had an important impact on forming compact and smooth nickel deposits. The chronoamperometry analysis indicated that the reduction in nickel followed the 3D progressive nucleation mechanism in the presence of composite additives at the step potential of −1.16 V and −1.18 V. Smoother and finer nickel films were found using scanning electron microscopy (SEM) images as the composite additives were used. X-ray diffraction revealed that all nickel deposit samples had the face-centered cubic structure, and five main crystal planes were displayed by the presence of composite additives in the electrolyte. Furthermore, the diffraction peaks of (111) and (200) crystal planes were slightly shifted toward lower 2θ values when thiourea was used in combination with additive CTAC or PEG20000. These results were beneficial for understanding the mechanisms and facilitating the rational design of additives for metal nickel electrodeposition.
Highlights
Electrodeposited nickel films have been used widely as a matrix of composite electrochemical coatings and many other applications, such as surface engineering or materials applications, due to their excellent mechanical, anticorrosion, electrocatalytic, thermal, decorative, wear-resistance and magnetic properties [1,2,3,4,5,6,7]
A typical current hysteresis loop appeared in the negative scan direction at the cathodic parts of the curves, which indicate that a nucleation/growth process was involved and an overpotential was required in the nickel electrodeposition process in the presence of additives
The potential difference between the nucleation potential (Enu ) and the crossover potential (Eco ) is defined as nucleation overpotential (NOP), which is regarded as a measure of cathode polarization, and higher NOP value means a stronger polarization [28]
Summary
Electrodeposited nickel films have been used widely as a matrix of composite electrochemical coatings and many other applications, such as surface engineering or materials applications, due to their excellent mechanical, anticorrosion, electrocatalytic, thermal, decorative, wear-resistance and magnetic properties [1,2,3,4,5,6,7]. Hydrogen evolution is inevitable, which influences current efficiency, quality of the nickel deposits and harsh corrosiveness to equipment in these conventional acidic nickel-plating baths, even though various measures, such as adding additives, changing composition of solutions, pH, temperature and substrate types, have been taken to interfere in the cathode interface process. Compared with single additive thiourea, the composite additives increased the nucleation overpotentials more significantly, especially when thiourea was compounded with CTAC This may be related to the co-adsorption and strong synergetic effect of the two additives, which are much more likely to transfer to the cathode interface under the application of electric field and further adsorb on the active sites of the electrode. Anodic peak with positive potential was mainly attributed to the oxidation of thiourea and the other one was the dissolution of nickel deposits or the formation of nickel hydroxide
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