Journal of Sol-Gel Science and Technology https://doi.org/10.1007/s10971-019-04949-w ORIGINAL PAPER: FUNCTIONAL COATINGS, THIN FILMS AND MEMBRANES (INCLUDING DEPOSITION TECHNIQUES) Sol–gel–xerogel transformations in the thin layer at the salt solution–gaseous reagent interface and the synthesis of new materials with microtubular morphology Vladislav Gurenko 1 ● Larisa Gulina 1 ● Valeri Tolstoy 1 Received: 11 December 2018 / Accepted: 25 February 2019 © Springer Science+Business Media, LLC, part of Springer Nature 2019 Abstract In this report, we consider the peculiarities of sol–gel–xerogel transformations, which occurs in the thin layer at the FeCl 2 / FeCl 3 water solution and gaseous NH 3 interface. A thin layer consisting of nanoparticles of Fe(OH) 3 and Fe 3 O 4 is formed as a result of interfacial reaction. Depending on the time of treatment with gaseous ammonia, it was possible to obtain either a freestanding layer with a thickness of 1 μm or microtubular structures (scrolls). Synthetic conditions, under which a large number of preferentially oriented microtubes are formed with diameters of about 10 and up to 200-μm long, were determined. The synthesized structures consisted of crystalline Fe 3 O 4 nanoparticles with an average diameter of about 5–10 nm incorporated in an amorphous matrix. Layers and tubes were characterized by X-ray diffraction analysis, scanning and transmission electron microscopy (SEM and TEM), and X-ray photoelectron spectroscopy. The model, concerning sol–gel– xerogel transformations in the thin layer, formed after interaction at the salt solution–gaseous reagent interface, was suggested. The magnetic properties of the products were studied. Graphical Abstract Highlights ● A sol–gel–xerogel transformation in the thin layer at the gas–solution interface has been studied. ● Tubular or planar morphology of the products depended on synthesis conditions. ● The opportunity to obtain an array of oriented microtubes was demonstrated for the first time. ● The layers and tubes based on Fe 3 O 4 demonstrated superparamagnetic properties. Keywords Sol–gel process ● Interface ● Thin films ● Microtubes ● Magnetite * Larisa Gulina l.gulina@spbu.ru 1 Institute of Chemistry, Saint Petersburg State University, 7-9 Universitetskaya Nab., St. Petersburg 199034, Russia 1234567890();,: 1234567890();,: