The formation of Liesegang patterns of cobalt hexacyanoferaate Prussian blue analogue in polyacrylamide gels

The formation of patterns in nature has always fascinated scientists who try to understand the mechanism behind this complex phenomenon. Liesegang patterns (LPs) are one category of periodic precipitations patterns formed by the reaction- diffusion (RD) system, studied and modeled by chemists and mathematicians for over a century. The majority of the focus has been on understanding the formation of these patterns, modeling them, and studying the effect of different gel types, gel concentrations, and inner and outer concentrations on various systems. There are only a few examples in the literature showing the effect of temperature, electric or magnetic field, and mechanical force on LPs and controlling the pattern formation by these external stimuli, and more importantly, showing a possible application of these systems. In this work, we show a novel approach to form LPs in a hydrogel medium, track the band formation with temperature, characterize each band, and last but not least, suggest a useful application for these LPs. This study mainly concentrates on the pattern formation of cobalt Prussian blue, but LPs of some other Prussian Blue Analogues (PBAs) are also shown. First, we propose a new method to form PBAs LPs formation. Then, we track the band formation by changing the system's temperature. To understand the effect of temperature on the microscale, we characterize each band using SEM, EDX, and XPS to analyze particles size and Co/Fe ratio. Finally, we use each band as electrocatalysts for water oxidation reactions in water splitting.