Synthesis of Fe(OH) 3 Microtubes at the Gas–Solution Interface and Their Use for the Fabrication of Fe 2 O 3 and Fe Microtubes

Synthesis of the FeOOH Microtubes with Inner Surface Modified by Ag Nanoparticles

Lepidocrocite (γ-FeOOH) microtubes with scroll morphology prepared by gas-solution interface technique (GSIT) have been modified by silver nanoparticles (Ag NPs). The successive ionic layer deposition (SILD) was first used for the synthesis of the Ag NPs on the lower surface of a solid film freely lying on the surface of a solution. The sizes of Ag NPs are about 15 nm after one synthesis cycle, and their diameters reach 35 nm after three SILD cycles. As a result of vacuum-drying, the modified film is transformed into microtubes with a diameter of about 10 μm and a length of 150 μm in such a way that the inner surface of the microtube is modified by Ag NPs. The catalytic properties of the microtubes have been observed by the decomposition of H2O2 in aqueous solution. The Ag/FeOOH microtubes move in hydrogen peroxide solutions with an average speed of 117 μm/s. This result is based on the synergetic effect between lepidocrocite nanosheets and Ag NPs, which results in the modified microtubes having enhanced mobility.

Formation of Ordered Honeycomb-like Structures of Manganese Oxide 2D Nanocrystals with the Birnessite-like Structure and Their Electrocatalytic Properties during Oxygen Evolution Reaction upon Water Splitting in an Alkaline Medium

In this work, a chemical reaction between gaseous ozone and aqueous solution of Mn(CH₃COO)₂ in drops has been researched. It has been shown that the formation of H _x MnO₂·nH₂O nanocrystals with a morphology of nanosheets and a birnessite-like crystal structure with a thickness of 5-8 nm is observed on the surface of drops. These nanocrystals are oriented spontaneously to the solution-gas interface and constitute peculiar ribbons with a width of 1-2 μm, some of which form ordered honeycomb structures (OHS) with a 5-20 μm cell size. To explain the observed effect, the scheme of chemical reactions that take place at the interface between the surface of a drop and ozone has been modeled, and it can be described using a diffusion pattern model taking into account the action of "force fields" on the surface of a drop, which arise due to its curvature. After the drop is dried, these structures practically retain their morphology and form a fractal structure with a geometric area equal to the area of the drop base on the surface of the substrate. The study of the electrocatalytic properties of these structures revealed that they are active electrocatalysts in the oxygen evolution reaction (OER) during water electrolysis in alkaline medium. The most efficient of the obtained electrocatalysts are characterized by an overpotential value of 284 mV at a current of 10 mA/cm² and the Tafel coefficient of 37.7 mV/dec and are currently one of the best among pure manganese oxides. Finally, it has also been assumed that this effect is explained by the morphological features of the structures obtained, which contribute to the removal of oxygen bubbles from the electrode surface during electrolysis.