Strategy for Patterning Titania Dendrites by Gas-Solution Interaction at Droplet Surfaces.

Interaction of the solution droplet surface with gaseous components of the environment can lead to the formation of highly ordered patterns, such as dendrites. Here, we show that these structures can be spontaneously created during the open-air interaction of aqueous solution drop of titanium(III) salt with gaseous NH3 at the contact boundary thereof. The conditions have been identified under which radially ordered dendritic patterns can form on the surface of the TiCl3 solution droplet. The formation of these self-organized dendrite patterns can be attributed to the surface instability manifesting in Marangoni thermal flows in a droplet occurring during open-air fabrication. The composition of as-synthesized structures corresponds to coprecipitated crystalline NH4Cl and amorphous TiO2nH2O. After thermal treatment at 450 °C, TiO2 with the anatase crystal lattice is formed; meanwhile, the ordered dendrite patterns are preserved.

Oxygen Generation Using Catalytic Nano/Micromotors.

Gaseous oxygen plays a vital role in driving the metabolism of living organisms and has multiple agricultural, medical, and technological applications. Different methods have been discovered to produce oxygen, including plants, oxygen concentrators and catalytic reactions. However, many such approaches are relatively expensive, involve challenges, complexities in post-production processes or generate undesired reaction products. Catalytic oxygen generation using hydrogen peroxide is one of the simplest and cleanest methods to produce oxygen in the required quantities. Chemically powered micro/nanomotors, capable of self-propulsion in liquid media, offer convenient and economic platforms for on-the-fly generation of gaseous oxygen on demand. Micromotors have opened up opportunities for controlled oxygen generation and transport under complex conditions, critical medical diagnostics and therapy. Mobile oxygen micro-carriers help better understand the energy transduction efficiencies of micro/nanoscopic active matter by careful selection of catalytic materials, fuel compositions and concentrations, catalyst surface curvatures and catalytic particle size, which opens avenues for controllable oxygen release on the level of a single catalytic microreactor. This review discusses various micro/nanomotor systems capable of functioning as mobile oxygen generators while highlighting their features, efficiencies and application potentials in different fields.

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.

Interface-Assisted Synthesis of the Mn3-xFexO4 Gradient Film with Multifunctional Properties.

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)23+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.

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.

Interface-Assisted Synthesis of Single-Crystalline ScF3 Microtubes.

Scandium fluoride (ScF3) microtubes with nanoscale wall thickness were for the first time successfully synthesized by an interface-assisted technique at the surface of a scandium nitrate aqueous solution without the addition of any surfactant as a result of interaction with hydrofluoric acid from the gaseous phase in only 30 min. X-ray diffraction analysis, scanning electron microscopy, helium ionic microscopy, transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) were used to examine the morphology and crystal structure of ScF3 microtubes. The results show that the ScF3 microtube is single-crystalline and has a hexagonal structure. A hypothetical model of thin-walled microtube formation is proposed.

Synthesis of birnessite structure layers at the solution-air interface and the formation of microtubules from them.

H(x)MnO2·nH2O layers have been successfully produced through a facile low-temperature process at the solution-air interface without using any templates. The crystalline structures of layers can be tuned by the compositions and the pH of the growth solutions. The analysis of birnessite-like layers indicates that they are formed by nanosheets approximately 4-6 nm thick that are oriented for the most part normally to the interface. Our results demonstrated that 1-3-µm-thick layers can roll up into microtubules 20 to 100 µm in diameter and up to 10 mm long. The hypothesis explaining the formation of the microtubular structures is assumed.