In this work, microtubes with walls, containing Fe 3 O 4 nanoparticles, obtained by "rolling up" of the interfacial films, were synthesized by the gas-solution interface technique (GSIT), using a mixture of aqueous solutions of FeCl 2 and FeCl 3 and gaseous ammonia. The synthesized microtubes were characterized by Scanning Electronic Microscopy (SEM), Energy-Dispersive X-ray spectroscopy (EDX), X-Ray Diffraction analysis (XRD) and magnetization measurements. It was established that under optimal synthetic conditions the microtube diameter ranged from 5 to 10 µm, the length was up to 120 µm and the thickness of walls was about 0.6 µm, the walls themselves being formed by nanoparticles with a size of about 10 nm. The reversible hysteresis behavior, the low coercive force, the low remanence magnetization and the approaching of Mr/Ms to zero, confirmed the superparamagnetic nature of the synthesized microtubes. A hypothesis on the formation of microtubes by the gas-solution interface technique was proposed.

Anisotropic gradient materials are considered as promising and novel in that they have numerous functional properties and are able to transform into hierarchical microstructures. We report a facile method of gradient inorganic thin film synthesis through diffusion-controlled deposition at the gas–solution interface. To investigate the reaction of interfacial phase boundary controllable hydrolysis by gaseous ammonium, an aqueous solution of FeCl3 and MnCl2 was chosen, as the precipitation pH values for the hydroxides of these metals differ gradually. As a result of synthesis using the gas–solution interface technique (GSIT), a thin film is formed on the surface of the solution that consists of Mn2+(Fe,Mn)2 3+O4 nanoparticles with hausmannite crystal structure. The ratio between iron and manganese in the film can be adjusted over a wide range by varying the synthetic procedure. Specific conditions are determined that allow the formation of a Mn–Fe mixed oxide film with a gradient of composition, morphology, and properties, as well as its further transformation into microscrolls with a diameter of 10–20 μm and a length of up to 300 μm, showing weak superparamagnetic properties. The technique reported provides a new interfacial route for the development of functional gradient materials with tubular morphology.

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