X-ray spectroscopic analysis of stochastic, periodic precipitation in Co–Fe-Based Prussian blue analogues

Abstract

Inorganic precipitates sometimes spontaneously create complex, interesting banding structures in gels, which are potentially applicable in materials engineering. A new member of such self-organizing bandings, i.e., stochastic, periodic banding of the precipitates of Co–Fe-based Prussian blue analogues (Co–Fe PBAs), denoted as “pattern-A”, was observed in water-glass gels. The pattern-A periodic bands did not obey empirical scaling laws that many Liesegang bands follow, illustrating the importance of ripening effects. Time-resolved X-ray fluorescence (XRF) and position-dependent X-ray absorption near-edge structure (XANES) measurements were conducted for a pattern-A sample, which was prepared from 0.30 M CoCl₂ and 0.05 M K₃[Fe(CN)₆] solutions. XRF results demonstrated that the Co–Fe PBA distributions fluctuated highly over the sample tube, including the turbid zone, and suggested that significant Ostwald ripening of the Co–Fe PBAs takes place in the periodic bands. XANES results indicated that the local structures and symmetries of the Co–Fe PBAs in the periodic bands were highly uniform. Furthermore these results are consistent with a model where the periodic bands contain a relatively high concentration of Co–Fe PBAs, with the most dominant local structure being Co(OC)₄(NCFe)₂, co-existing with [Co(H₂O)₆]²⁺ ions. These findings not only provide an interesting piece of information for future self-organization processes of Co–Fe PBAs, but also confirm the potential of the combined X-ray spectroscopic techniques for in-depth analysis of the periodic precipitation phenomenon in gels.