Effects of NH4+/citrate complexing agent ratio on Ni–Mo and Ni–Mo–O electrodeposits from ammonium citrate baths

Abstract

To understand the effect of complexing agents (i.e., ammonium and citrate) in nickel–molybdenum electrodeposition, calculation of the concentration of various Ni and Mo species as a function of pH and initial concentration of metal ions and complexing agents was performed. In addition, linear sweep voltammetry and Hull cell experiments were systematically investigated to understand the effect of current density and ammonium-to-citrate ratio to film compositions, morphology, and crystallinity. The results indicated that Ni(NH3)32+ played a critical role in induced co-deposition mechanism of Ni–Mo alloys, which involved the reduced Ni and absorbed H atoms. Microstructure analysis of deposits indicated that the transition from smooth laminarly grown amorphous Ni–Mo–O composites to columnar and nanocrystalline metallic Ni–Mo alloys with a globular structure as the ammonium-to-citrate molar ratio increases. The highest Mo content of alloys was as high as 19 at%, and up to 70 at% O was present in the composites.

Effect of solution pH, precursor ratio, agitation and temperature on Ni-Mo and Ni-Mo-O electrodeposits from ammonium citrate baths

“…More citrate ions in electrolyte leads to greater Ni-citrate complexes compared to Ni-ammonia complexes. The effect of citrate concentration further confirms that Ni-ammonia complexes are presumably the chemically active species for Ni reduction, particularly Ni(NH 3 ) 3 2+ (To et al, 2022). Since Ni and Mo content in metallic Ni-Mo alloys are inversely proportional, the higher Mo content might be owing to lower relative Ni content.…”

“…In previous work, the effects of complexing agents (i.e., citrate ions and ammonia) were systematically investigated and found the key role of complexing agents in the phase transition between metallic Ni-Mo alloys and Ni-Mo-O composites as well as Ni-ammonia complexes for Ni reduction ( To et al, 2022 ). In this work, the citrate concentration varied from 0.05 M to 0.25 M while fixing the concentration of the other components (Solutions #1 and #2) to study the effect of citrate ions on the Mo content of metallic Ni-Mo alloys.…”

“…Therefore, a series of work which correlates the initial electrolyte composition with concentration of chemically active complexes and deposit composition to deconvolute the mechanism as well as to study the effects of synthesis parameters was conducted. In the first part of this series, the effects of ammonium-to-citrate molar ratio on the electrodeposition of Ni-Mo and Ni-Mo-O was investigated experimentally as well as correlated with the simulated fraction of species ( To et al, 2022 ). It was suggested that ammonium ions play an important role in the fraction of chemically active nickel species and thereby the formation of metallic Ni-Mo.…”