Effect of pH on the Langmuir Monolayers of Nickel Stearate and the Synthesised Nickel Oxide Thin Films

Abstract

NiO is an attractive transition metal oxide in regard of its optical, electrical, structural, mechanical and magnetic properties. Nickel oxide thin films find important applications in varied fields such as photovoltaic devices, smart windows, as counter electrode, electro chromic coating etc. So their formation and its control is an important topic of research. Langmuir Blodgett (LB) thin film deposition method is known for the precise control of thin film parameters. In this work the thin uniform films of nickel oxide were synthesized from the Ni-stearate LB films. Stable and compact Langmuir monolayers are the first step in LB film deposition. Their characteristics decide the quality of LB films and the morphology of the final phase formation. Ni-stearate Langmuir monolayers were deposited and characterized on the surface of the Langmuir trough. It was observed that as the concentration and pH of subphase increased the compaction of the film and its static elasticity in the solid phase increased indicating an increase in Ni2+ ion incorporation. An important result to emerge from these studies was that for very high pH (8.3) the structure of the films opens up. Analysis of π-A isotherms indicated that pH 7.9 had good parameters for the LB deposition. The oscillating barrier characterizations of Ni stearate monolayers at different frequencies and pH values were recorded. It also confirmed the stability of the monolayers at pH 7.9. Transparent Nickel oxide thin films on glass substrate were synthesized by deposition of 10, 15, 19 layers of Ni-stearate at 7.9 pH followed by drying, removal of stearate chain and calcination. The presence of Ni in thin films was checked by EDS. The synthesized nickel oxide thin film has the band gap of 4.04 eV. The thin films formed by deposition of 15 layers of nickel stearate deposited at pH 7.9 were continuous and uniform. The effect of pH on the final film morphology was determined by depositing 15 layers of Ni stearate for 6.9 and 8.3 pH values. The results showed that these films are non-continuous and have large grain size distribution.