Preparation of silver sulfide-coated nickel sulfide monodispersed fine particles for use in solid-state nickel ion selective electrodes

A nickel ion-selective electrode (Ni(II)-ISE) was fabricated using monodispersed fine silver sulfide (Ag₂S)-coated nickel sulfide (NiS) particles 5 μm in diameter. The NiS particles were prepared by a method which involved the homogeneous precipitation from a mixed solution of thioacetamide with and without urea and nickel nitrate at 70°C or 90°C over a 3 h period. The surface coating with the NiS paiticles with an Ag₂S layer was carried out by an ion-exchange method, in which the NiS particles were dispersed into ethanol containing silver nitrate at 0°C for 0.5 h or 2.5 h. The amount of Ag₂S coating the surface of the NiS particles increased with an increase in the time used for the coating reaction. The Ag₂S content in the Ag₂S-coated NiS particles obtained at 2.5 h of the coating reaction was 7.4 wt%, and the thickness of the Ag₂S layer was estimated to be 50 nm. The resulting Ag₂S-coated NiS particles were pressed into a pellet, and the pellet was then sintered at 500°C under an atmosphere of N₂ for 3 h. The Ni(II)-ISE was fabricated using the pellet as the sensing membrane. An Ni(II)-ISE based on a mixture of Ag₂S and NiS particles was also fabricated in a manner similar to that based on the Ag₂S-coated NiS particles for comparison. The Ni(II)-ISE fabricated using the Ag₂S-coated NiS particles (coating time: 2.5 h) showed a near-Nernstitian response to Ni(II) in the concentration range from 10^-3 to 10^-1 M. This electrode showed a much better response than the electrode fabricated using the mixture of Ag₂S and NiS particles.