Preparation of well-ordered hexagonal particles of synthetic cobalt smectites containing electrochemically active cobalt(II) sites

Abstract

The preparation of ordered cobalt smectites consisting of regular hexagonal platelets, up to 300 nm in diameter, is described. They were obtained by hydrothermal treatment at 250 °C of reaction mixtures prepared by adding silicic acid to cobalt hydroxide suspensions produced by the addition of NaOH to cobalt(II) chloride solutions. Selected area electron diffraction gave well-resolved spots in hexagonal arrangements showing that the platelets consisted of ordered coherent stacks of several clay layers, rather than the turbostratic stacking of layers usually found for natural smectites. Simply inverting the order of addition of NaOH and silicic acid during the preparation of the reaction mixture gave a smectite composed of much smaller particles and containing no regular hexagonal platelets. Lowering the temperature used in the hydrothermal treatment produced less regular clay particles. Increasing the pressure during the hydrothermal treatment by adding argon to the autoclave did not affect the morphology of the clay particles. These differences in the morphology of the clay particles were found to affect the electrochemical behavior of thin films of the clays. The voltammetric waves obtained for [Ru(bpy) 3] 2+ cation adsorbed in films of the clays containing the large hexagonal particles was much larger than in films of clays containing no such particles. The fraction of the adsorbed cations that were electrochemically accessible increased from 8.5% in clays containing no hexagonal particles to more than 50% in clays containing the hexagonal particles.