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 CoFe-based Prussian blue analogues (CoFe 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 CoFe PBA distributions fluctuated highly over the sample tube, including the turbid zone, and suggested that significant Ostwald ripening of the CoFe PBAs takes place in the periodic bands. XANES results indicated that the local structures and symmetries of the CoFe 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 CoFe 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 CoFe PBAs, but also confirm the potential of the combined X-ray spectroscopic techniques for in-depth analysis of the periodic precipitation phenomenon in gels.
Journal of Analytical Atomic Spectrometry – Royal Society of Chemistry
Published: May 3, 2018
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