Specific features of the formation of orthorhombic and hexagonal holmium orthoferrite during heat treatment of the products of glycine-nitrate combustion have been studied. Photocatalytic activity of...

In this work, a simple solution combustion synthesis followed by heat treatment was proposed to produce nanocrystalline h-YbFeO3/o-YbFeO3 heterojunction and phase-pure o-YbFeO3. A detailed investigation on the formation process of h-YbFeO3 and o-YbFeO3 nanocrystals was carried out using a wide variety of methods, including EDS, SEM, PXRD, 57Fe Mössbauer spectroscopy, DRS. The most prospective samples of the h-YbFeO3/o-YbFeO3 nanocomposite and o-YbFeO3 nanopowder were examined towards the photo-Fenton decolorization of methyl violet (MV) under visible light. It was shown that the proposed approach allows for obtaining h-YbFeO3 and o-YbFeO3 nanocrystals with an average size of 15.7–21.1 nm and 41.1–65.5 nm, respectively, and with a specific surface area of up to 16 m2 g−1. It was found that the h-YbFeO3/o-YbFeO3 nanocomposite has a mesoporous structure and foam-like morphology, while the o-YbFeO3 nanopowder is non-porous and represented by agglomerates of individual nanoparticles with isometric morphology. Based on the DRS results, it was found that the band gap value was 2.05 eV and 2.16 eV for the h-YbFeO3/o-YbFeO3 and o-YbFeO3 samples, respectively, providing strong visible-light absorption of the samples. It was shown that the electron structure of the h-YbFeO3/o-YbFeO3 nanocomposite refers to I type heterojunction, leading to effective electron and hole transfer from o-YbFeO3 to h-YbFeO3 bands. This feature determines the higher photocatalytic activity of the h-YbFeO3/o-YbFeO3 nanocomposite as compared to pure o-YbFeO3 in the process of the Fenton-like decolorization of methyl violet under visible light with the pseudo-first-order reaction rate of 0.004848 min−1 and 0.003123 min−1, respectively. In conclusion, the mechanism of the photocatalytic decolorization of MV over the heterojunction nanocomposite was proposed to explain its high activity in the Fenton-like process.

Nanocrystals of gadolinium orthoferrite (GdFeO₃) with morphology close to isometric and superparamagnetic behavior were successfully synthesized using direct, reverse and microreactor co-precipitation of gadolinium and iron(III) hydroxides with their subsequent heat treatment in the air. The obtained samples were investigated by PXRD, FTIR, low-temperature nitrogen adsorption-desorption measurements, HRTEM, SAED, DRS and vibration magnetometry. According to the X-ray diffraction patterns, the GdFeO₃ nanocrystals obtained using direct co-precipitation have the smallest average size, while the GdFeO₃ nanocrystals obtained using reverse and microreactor co-precipitation have approximately the same average size. It was shown that the characteristic particle size values are much larger than the corresponding values of the average crystallite size, which indicates the aggregation of the obtained GdFeO₃ nanocrystals. The GdFeO₃ nanocrystals obtained using direct co-precipitation aggregate more than the GdFeO₃ nanocrystals obtained using reverse co-precipitation, which, in turn, tend to aggregate more strongly than the GdFeO₃ nanocrystals obtained using microreactor co-precipitation. The bandgap of the obtained GdFeO₃ nanocrystals decreases with decreasing crystallite size, which is apparently due to their aggregation. The colloidal solutions of the obtained GdFeO₃ nanocrystals with different concentrations were investigated by ¹H NMR to measure the T₁ and T₂ relaxation times. Based on the obtained r₂/r₁ ratios, the GdFeO₃ nanocrystals obtained using microreactor, direct and reverse co-precipitation may be classified as T₁, T₂ and T₁–T₂ dual-modal MRI contrast agents, respectively.

The concentration of microplastic particles (with size from 100 µm to 5 mm) and their chemistry were studied in water, subsoils and sediments in the coastal area of the Neva Bay, the Gulf of Finland. The characteristics determined in the study were the dominating shapes of the particles, their size range, polymer chemistry and concentrations in different media, i.e., in water, bottom sediments, and coastal subsoils. Microplastic concentrations in bottom sediments and coastal subsoils are far in excess of their concentration in water mass. The spatial distribution of microplastic particles was studied in water, coastal subsoils, and bottom sediments of the Neva Bay, the Gulf of Finland. Microplastic concentration at the water edge was found to decrease with the distance from St. Petersburg City and Neva mouth, along either the northern or the southern coast. After reaching the Gulf of Finland, the Neva water, polluted by microplastic particles, is diluted and continues moving and distributing in accordance with the dominating westward hydrodynamic transport.