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.

Nanostructured hexagonal rare-earth orthoferrites (h-RfeO₃, R = Sc, Y, Tb-Lu) are well known as a highly effective base for visible-light-driven heterojunction photocatalysts. However, their application is limited by metastability, leading to difficulties in synthesis due to the irreversible transformation to a stable orthorhombic structure. In this work, we report on a simple route to the stabilization of h-YbFeO₃ nanocrystals by the synthesis of multiphase nanocomposites with titania additives. The new I-type heterojunction nanocomposites of o-YbFeO₃/h-YbFeO₃/r-TiO₂ were obtained by the glycine–nitrate solution combustion method with subsequent heat treatment of the products. An increase in the mole fraction of the h-YbFeO₃ phase in nanocomposites was found with the titanium addition, indicating its stabilizing effect via limiting mass transfer over heat treatment. The complex physicochemical analysis shows multiple contacts of individual nanocrystals of o-YbFeO₃ (44.4–50.6 nm), h-YbFeO₃ (7.5–17.6 nm), and rutile r-TiO₂ (~5 nm), confirming the presence of the heterojunction structure in the obtained nanocomposite. The photocatalytic activity of h-YbFeO₃/o-YbFeO₃/r-TiO₂ nanocomposites was evaluated by the photo-Fenton degradation of the methyl violet under visible light (λ ≥ 400 nm). It was demonstrated that the addition of 5 mol.% of TiO₂ stabilizes h-YbFeO₃, which allowed us to achieve a 41.5 mol% fraction, followed by a three-time increase in the photodecomposition rate constant up to 0.0160 min⁻¹.

The volume of dye production in the chemical industry is growing rapidly every year. Given the global importance of clean water resources, new wastewater treatment solutions are required. Utilizing photocatalysis by harvesting solar energy represents a facile and promising solution for removing dangerous pollutants. This study reports the possibility of increasing the photocatalytic activity of g-C₃N₄ by creating nanocomposites with ZnO. Exfoliated g-C₃N₄/ZnO nanocomposites were synthesized by heat treatment of urea and subsequent ultrasonic exfoliation of the colloidal solution by introducing zinc acetate. The uniformity of the distribution of ZnO nanoparticles is confirmed by the method of elemental mapping. The obtained X-ray diffractograms of the obtained nanocomposites show typical X-ray reflections for g-C₃N₄ and ZnO. It was found that the introduction of oxide into g-C₃N₄ leads to an increase in the specific surface area values due to the developed ZnO surface. The maximum value of the specific surface area was obtained for a sample containing 7.5% ZnO and was 75.2 m²/g. The g-C₃N₄/7.5% ZnO sample also demonstrated increased photocatalytic activity during the decomposition of methylene blue under the influence of visible light, which led to a twofold increase in the reaction rate compared to initial g-C₃N₄.

Photo-Fenton-like oxidation of organic substances is one of the key advanced oxidation processes based on the reversible Fe2+↔Fe3+ transition and the generation of a strong oxidant ·OH in the presence of H2O2 and is currently considered as a promising method for the purification of polluted aqueous media. However, the absence of effective and stable photocatalysts of this process, operating under the action of visible light, necessitates the exploratory studies, mainly among iron oxides and ferrites of various compositions and structures. In this work, using the method of solution combustion followed by heat treatment in air the heterojunction nanocomposites based on ytterbium orthoferrite and cerium dioxide of the composition o-YbFeO3/h-YbFeO3/CeO2 (0–20 mol.%) with high absorption in the visible region and advanced photo-Fenton-like activity were obtained. The nanocomposites were studied by EDS, SEM, XRD, BET, and DRS methods. The photo-Fenton-like activity of the nanocomposites was investigated during the degradation of methyl violet under the action of visible (λmax = 410 nm) radiation. As a result, the formation of I-type heterojunction based on stable rhombic (55.4–79.0 nm) and metastable hexagonal (19.5–24.0 nm) modifications of ytterbium orthoferrite (o-YbFeO3 and h-YbFeO3, respectively) and cubic cerium dioxide CeO2 (13.2–19.2 nm) nanocrystals was established. It was shown that the obtained nanocomposites had foamy morphology and were characterized by a specific surface in the range of 9.1–25.0 m2/g, depending on the CeO2 content. It was found that nanocrystalline components were chemically and phase-pure, uniformly spatially distributed over the nanocomposite, and had multiple contacts with each other. Based on this fact and the established electronic structure of the nanocomposite components, the formation of I-type heterojunction with the participation of o-YbFeO3 (Eg = 2.15 eV), h-YbFeO3 (Eg = 2.08 eV), and CeO2 (Eg = 2.38 eV) was shown, the presence of which increased photocatalytic activity of the resulting nanocomposite. The optimal content of CeO2 in the nanocomposite was 5%, and the o-YbFeO3/h-YbFeO3/CeO2–5% sample was characterized by the highest rate constant of photo-Fenton-like degradation of methyl violet under the action of visible light equal to k = 0.138 min–1, which was 2.5 to 5 times higher than for nanocomposites based on ytterbium orthoferrite. The obtained results obtained indicate that the developed nanocomposites can be considered as a promising basis for the advanced oxidation processes for the purification of aqueous media from organic pollutants.