Formation of nickelhydroxy sulfide precursor and nickel sulfide in Langmuir and Langmuir–Blodgett films of a nickel complex of octadecylsuccinic acid

Abstract

The influence of parameters on size of NiS particles formed at ambient temperatures by reacting Na 2S with the nickel salt of octadecylsuccinic acid (ODSA) at the air/water interface and in Langmuir–Blodgett films was investigated by changes in surface pressure–molecular area (π– A) and surface potential–molecular area (Δ V– A) isotherms. The contraction in area of ODSA on Ni 2+ subphase is indicative of a strong interaction of the dibasic acid with Ni 2+ leading to a monodentate complex of Ni 2+ with ODSA. The Langmuir monolayers of the nickel complex of ODSA were analysed using Brewster angle microscopy (BAM). In the absence of nickel in the subphase, ODSA exhibited no domains in BAM. The sulfidation reaction on ODSA/Ni 2+ complex at the air/water interface shows two steps as seen in the surface potential vs. time plot corresponding to formation of Ni(OH)S initially which then changes to NiS. The domain size seen with NiS formation increased with time and surface pressure indicating the growth of a two-dimensional solid complex in the monolayer at the air/water interface. The Langmuir–Blodgett (LB) films of ODSA/Ni 2+ on sulfidation transferred onto solid substrates from the two steps in the reaction were characterized using diffuse reflectance UV-visible spectroscopy (DIR-UV-vis) and X-ray photoemission spectroscopy. These experiments confirmed the reduction process involved in the above reaction. Atomic force microscopy experiments on these LB films indicated that there is a variation in the structural organization in the monolayers and in the morphology of the LB film, depending on the initial Ni(OH)S formation which ultimately leads to NiS. The results show that the average particle size which varies between 2 and 3.5 nm depends on the nature of the nickel complexes formed at the air/water interface.