Crystallization of Prussian Blue analogues at the air-water interface using an octadecylamine monolayer as a template

Abstract

We demonstrate the use of a Langmuir monolayer as a templating agent for growth of oriented crystals of metal(II) -hexacyanoferrate(III) (where metal can be Ni, Co, or Cu), popularly known as the Prussian Blue analogues, crystals of which are difficult to grow by conventional means. Crystalline films of metal(II) -hexacyanoferrate(III) are deposited at the air-water interface, under an octadecylamine (ODA) monolayer in a Langmuir trough, upon addition of K₃Fe(CN)₆ solution into the subphase containing aqueous metal chloride solution. Template action to promote crystallization at the air-water interface appears to be provided by the protonated ODA molecules. In the absence of the template (without ODA), no crystals are deposited and instead a precipitation reaction occurs resulting in the formation of a nonoriented cubic M₃[Fe(CN)₆]₂·nH₂O (M = Ni, Co, or Cu) precipitate. Crystalline Ni-hexacyanoferrate films formed through the template action were characterized using X-ray diffraction (XRD), transmission electron microscopy, X-ray photoelectron spectroscopy, and so forth. We also report initial results on the formation of crystalline Co(II)- and Cu(II)-hexacyanoferrate(III) films by XRD results.