Methylene blue (MB) is widely used as a test material in photodynamic therapy and photocatalysis. These applications require an accurate determination of the MB concentration as well as the factors affecting the temporal evolution of the MB concentration. Optical absorbance is the most common method used to estimate MB concentration. This paper presents a detailed study of the dependence of the optical absorbance of aqueous methylene blue (MB) solutions in a concentration range of 0.5 to 10 mg·L⁻¹. The nonlinear behavior of optical absorbance as a function of MB concentration is described for the first time. A sharp change in optical absorption is observed in the range of MB concentrations from 3.33 to 4.00 mg·L⁻¹. Based on the analysis of the absorption spectra, it is concluded that this is due to the formation of MB dimers and trimers in the specific concentration range. For the first time, a strong, thermally induced discoloration effect of the MB solution under the influence of visible and sunlight was revealed: the simultaneous illumination and heating of MB solutions from 20 to 80 °C leads to a twofold decrease in the MB concentration in the solution. Exposure to sunlight for 120 min at a temperature of 80 °C led to the discoloration of the MB solution by more than 80%. The thermally induced discoloration of MB solutions should be considered in photocatalytic experiments when tested solutions are not thermally stabilized and heated due to irradiation. We discuss whether MB is a suitable test material for photocatalytic experiments and consider this using the example of a new photocatalytic material—boron oxynitride (BNO_x) nanoparticles—with 4.2 and 6.5 at.% of oxygen. It is shown that discoloration is a complex process and includes the following mechanisms: thermally induced MB photodegradation, MB absorption on BNO_x NPs, self-sensitizing MB photooxidation, and photocatalytic MB degradation. Careful consideration of all these processes makes it possible to determine the photocatalytic contribution to the discoloration process when using MB as a test material. The photocatalytic activity of BNO_x NPs containing 4.2 and 6.5 at.% of oxygen, estimated at ~440 μmol·g⁻¹·h⁻¹. The obtained results are discussed based on the results of DFT calculations considering the effect of MB sorption on its self-sensitizing photooxidation activity. A DFT analysis of the MB sorption capacity with BNO_x NPs shows that surface oxygen defects prevent the sorption of MB molecules due to their planar orientation over the BNO_x surface. To enhance the sorption capacity, surface oxygen defects should be eliminated.

Molybdenum sulfide is a very promising catalyst for the photodegradation of organic pollutants in water. Its photocatalytic activity arises from unsaturated sulfur bonds, and it increases with the introduction of structural defects and/or oxygen substitutions. Amorphous molybdenum sulfide (a-MoS_xO_y) with oxygen substitutions has many active sites, which create favorable conditions for enhanced catalytic activity. Here we present a new approach to the synthesis of a-MoS_xO_y and demonstrate its high activity in the photodegradation of the dye methylene blue (MB). The MoS_xO_y was deposited on hexagonal boron oxynitride (h-BNO) nanoflakes by reacting h-BNO, MoCl₅, and H₂S in dimethylformamide (DMF) at 250 °C. Both X-ray diffraction analysis and high-resolution TEM show the absence of crystalline order in a-MoS_xO_y. Based on the results of Raman and X-ray photoelectron spectroscopy, as well as analysis by the density functional theory (DFT) method, a chain structure of a-MoS_xO_y was proposed, consisting of MoS₃ clusters with partial substitution of sulfur by oxygen. When a third of the sulfur atoms are replaced with oxygen, the band gap of a-MoS_xO_y is approximately 1.36 eV, and the valence and conduction bands are 0.74 eV and −0.62 eV, respectively (relative to a standard hydrogen electrode), which satisfies the conditions of photoinduced splitting of water. When illuminated with a mercury lamp, a-MoS_xO_y/h-BN_xO_y nanohybrids have a specific mass activity in MB photodegradation of approximately 5.51 mmol g⁻¹ h⁻¹, which is at least four times higher than so far reported values for nonmetal catalysts. The photocatalyst has been shown to be very stable and can be reused.

The development of high-effective photodetectors operating in a wide spectral range is an important technological task. In this work we have demonstrated that the detection bandwidth ofh-BN photodetectors in the UV range can be extended due to the surface plasmon resonance (SPR) effect. Theoretical calculations showed that, among Al, Au, Ag, and Cu, Al is the most suitable metal for theh-BN UV sensible detectors due to the SPR effect in the middle UV range. Based on the theoretical predictions, a simple and highly efficient method for obtainingh-BN/Al nanocomposites for localized SPR-based UV detectors was developed. It was demonstrated that theh-BN/Al material is sensitive to UV radiation with a wavelength of 266 nm that is far away of the detection limit of 240 nm inherent for pureh-BN.