X-ray spectroscopic analysis of stochastic, periodic precipitation in Co−Fe-Based PBAs

What is it about?

Inorganic precipitates sometimes spontaneously create complex, interesting banding structures in gels, which are potentially applicable in material 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), designated 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 that 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.

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Why is it important?

In recent decades, interest in Liesegang banding has experienced a resurgence, partially because of its relevance in materials engineering; Liesegang banding is potentially applicable for assembling functional devices on the micro- and nanoscale with a spatial distribution by self-organization processes. Such technological applications require a deeper understanding of the Liesegang banding of various functional materials. Unfortunately, however, most of the inorganic compounds that can create Liesegang bands are not very valuable as functional materials that are available as magnets and/or electrodes. Prussian blue analogues are cyano-bridged coordination polymers that have a similar structure to that of Prussian blue. PBAs have attracted significant attention as functional materials because of their diverse magnetic and electrochemical properties. Recently, we found that the Mn−Fe-based PBAs (Mn−Fe PBAs; M = Mn and M′ = Fe) precipitated in water-glass gels containing Mn2+ and [Fe(CN)6]3− ions and formed the Liesegang bands using a combination of time-resolved X-ray fluorescence (XRF) spectroscopy and position-dependent X-ray absorption near-edge structure (XANES) spectroscopy. In this study, we have investigated the precipitation patterns of the Co−Fe PBAs in water-glass gels. By examining the concentration dependencies of the coprecipitated ions on the bandings in detail, we observed (stochastic) Liesegang bands of Co−Fe PBAs for the first time. The obtained results 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 phenomena in gels.

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