The semiconductor properties and tin segregation mechanism in the passive film formed on the electrodeposited Ni-Sn coatings

Abstract

The nanocrystalline Ni-Sn coatings (average grain size 15.78 nm) formed of relatively ordered circular particles covering the entire surface characterized with nodule-like endings were successfully electrodeposited using pulse electrodeposition technique. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) and X-ray diffraction (XRD) were used to analyse the film microstructure. The corrosion resistance and semiconducting properties of Ni-Sn coatings were investigated in borate buffer solution. The EIS measurements showed that Ni-Sn alloys developed, in the passive zone, a good corrosion resistance as demonstrated by a thin film thickness, the low capacitance value, high polarization resistance, and the high value of electric field strength. Mott-Schottky analysis showed that the passive film formed on Ni-Sn coatings presents an p-n heterojunction characteristic indicating that the charge carrier densities are composed of cation (NA) and anion (ND) vacancies. The high density of point defects (NA + ND ∼ 1021cm−3) induces a high electronic conductivity in the Ni-Sn coatings passive film. The XPS analysis showed that the passive film formed on Ni-Sn alloys is composed of NiO, Ni(OH)2, NiOOH, SnO, and SnO2 species and an enrichment of Sn in the passive film. The mechanisms of passive film growth and Sn segregation in the Ni-Sn passive film are suggested in conjunction with the Point Defect Model (PDM). The good corrosion resistance and high electronic conductivity achieved in this work suggest that Ni-Sn Coating is a good candidate for water electrolysis applications.