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Planar ordered honeycomb-like structures of the MnO2 layered birnessite-type crystalline form have for the first time been synthesized on the surface of Mn(OAc)2 solution drops upon interaction with O3 gas. The obtained network consists of 2D nanocrystals of the thickness up to 10 nm. The prepared material possesses a hierarchical structure and can be used in a fabrication of advanced sensor and electrode materials.

Thin layers formed by the arrays of the vertically oriented 2-dimensional nanocrystals of Birnessite-type manganese oxide CuxMnO2∙nH2O were synthesized for the first time by the reaction of gaseous ozone with the surface of aqueous solutions of manganese (II) acetate and copper (II) acetate salts mixture. The obtained layers were placed onto the surface of indium tin oxide (ITO)–coated glass electrodes and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X-Ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The electrochemical performance of the layers was examined by cyclic voltammetry (CV). It was found that the synthesized structures are formed by the 2D nanocrystals with the thickness of 3–6 nm. The atomic concentration of copper x depends on the ratio of Cu(CH3COO)2 and Mn(CH3COO)2 solution concentrations and can be as much as 0.35. The obtained layers were tested as the electrochemical platform for nonenzymatic hydrogen peroxide sensing. It was shown that the electrode can provide an ultrasensitive H2O2 determination with the limit of detection of 0.4 nМ and linear concentration range that lies in the region of nanomolar concentrations. We assume that the observed effects are due to the morphology of the synthesized layers, which ensures maximum contact of the analyte with the active sites of 2D nanocrystals of CuxMnO2 ∙ nH2O. Comparison of the obtained analytical characteristics with the literature data indicates the good prospects of the synthesized nanomaterial for its use as a working electrode for nonenzymatic H2O2 sensor.

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 HxMnO2·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.