The role of surfactants in induced electrodeposition of Zn–Mo layer from citrate solutions

Abstract

The influence of various additives on the process of electrodeposition of Zn–Mo coatings from citrate solutions was examined. Potentiostatic deposition, Wavelength dispersive X-ray fluorescence (WDXRF), X-ray diffraction and micro-Raman spectroscopy were carried out to analyze the role surfactants of electrodeposition of Zn–Mo layers. Atomic force microscopy was applied to investigate the surface morphologies. The mechanism describing the action of surfactant during the co-deposition process of Zn–Mo has been proposed. It was found that the additive can adsorb specifically on the Inner Helmholtz Plane (IHPs) on the cathode and consequently limiting the approaching of other molecules (complexes) to the surface. Reduction processes are associated mainly with the formation and adsorption of zinc citrate complexes and mixed Zn(I)–Mo(VI)-citrate complexes those further reduce to metallic zinc and metallic zinc-molybdenum alloy (two hexagonal metallic phases). What is remarkable by hindering the reduction of the Zn(I)-citrate complex, the surfactant inhibits the reduction of zinc to metallic zinc and at the same time facilitates (catalyzes) the formation of the Zn(I)–Mo(VI)-citrate complex. The presence of only PEG 20000 in the bath causes deposition of one (Zn,Mo) hcp phase. While the presence in the bath of CTAB, Triton X-100 or D-sorbitol makes that two hcp phases can create: (Zn) and (Zn,Mo) or one hcp phase (Zn) and one oxide phase (MoO2).However, absence of surfactants or not optimal concentration of surfactants in the electrolyte and low ratio of concentrations of Zn(II) to Mo(VI) cause the reduction of molybdenum complexes only to molybdenum oxides which blocked the cathode surface, when in the case of greater ratio of concentrations of Zn(II) to Mo(VI) (increase the reduction rate of zinc citrate complexes in comparison to mixed Zn(I)–Mo(VI)-citrate complexes and molybdate citrate complexes ([(MoO4)H3cit]3-)) the metallic zinc, zinc oxides, and molybdenum oxides can co-deposit on the electrode surface (composite coatings of oxide nanoparticles dispersed within a zinc matrix).