Nanosized titanium dioxide (TiO₂) is currently being actively studied by the global scientific community, since it has a number of properties that are important from a practical point of view. One of these properties is a large specific surface, which makes this material promising for use in photocatalysts, sensors, solar cells, etc. In this work, we prepared photocatalysts based on TiO₂ nanotubes for converting carbon dioxide (CO₂) into energy-intensive hydrocarbon compounds. Efficient gas-phase CO₂ conversion in the prepared single-walled TiO₂ nanotube-Cu_xO composites was investigated. Parameters of defects (radicals) in composites were studied. Methanol and methane were detected during the CO₂ photoreduction process. In single-walled TiO₂ nanotubes, only Ti³⁺/oxygen vacancy defects were detected. The Cu²⁺ centers and O₂⁻ radicals were found in TiO₂ nanotube-Cu_xO composites using the EPR technique. It has been established that copper oxide nanoparticles are present in the TiO₂ nanotube-Cu_xO composites in the form of the CuO phase. A phase transformation of CuO to Cu₂O takes place during illumination, as has been shown by EPR spectroscopy. It is shown that defects accumulate photoinduced charge carriers. The mechanism of methane and methanol formation is discussed. The results obtained are completely original and show high promise for the use of TiO₂-Cu_xO nanotube composites as photocatalysts for CO₂ conversion into hydrocarbon fuel precursors.

We report a comprehensive study on the effect of H₂SeO₄ electrolyte temperature on the composition, defect, morphological, and luminescent properties of porous anodic aluminum oxide (AAO). An increase in the synthesis temperature led to a decrease in the AAO cell diameter from 85–115 nm to 38–58 nm (depending on the electrolyte concentration) and enhanced the etching of the AAO walls, which even resulted in the disintegration of the AAO into individual fibers at 40 °C. The selenium concentration in the samples formed in 0.5–1.5 M H₂SeO₄ in the temperature range of 5–40 °C did not exceed 2 at.% and fell below the detection limit at 40 °C. The formation of a nanocrystalline Al₂O₃ phase was observed in the H₂SeO₄ electrolyte at 40 °C. The samples exhibited weak photoluminescence. We identified three types of paramagnetic centers in AAO formed in H₂SeO₄: F⁺ centers (N_sF = 8.2 × 10¹⁵ g⁻¹), newly discovered centers with an unpaired electron localized on an oxygen atom (N_sO = 10¹⁷ g⁻¹), and centers associated with selenate radicals (N_sS = 6 × 10¹⁸ g⁻¹). By comparing the photoluminescence spectra and defect concentrations, we conclude that the luminescence of AAO formed in selenic acid is exclusively due to F⁺ centers, while other paramagnetic centers do not contribute.

For the efficient operation of various TiO₂-based devices, it is important to understand the patterns of electric charge transport. In the present paper TiO₂-C-Cu nanocomposites were synthesized by the electrochemical method. The band gap energy Eg of all systems was found to be approximately the same, 3.2 eV. Both copper ions replacing titanium ions and copper ions within the CuO phase were detected. The modification of TiO₂-C nanotubes by copper led to a significant increase in conductivity and photocurrent, which may be associated with the formation of new donor states (Ti³⁺ centers) creating levels in the band gap of TiO₂-C-Cu. The characteristics of charge carrier transport (including photocurrent) in TiO₂-C-Cu materials were revealed for the first time. The conductivity at DC and at low frequencies of AC is due to the movement of electrons along the conduction zone, whereas at high frequencies there is a hopping mechanism of conduction. The acquired original results testify to the potential usage of TiO₂-C-Cu nanocomposites in the field of catalysis and photoelectrochemistry.

The complex study of the influence of the H2SeO4 electrolyte temperature on the porous anodic aluminum oxide (AAO) composition and defects, morphological, luminescent properties is performed. The synthesis temperature increasing leads to decrease of AAO cell diameter from 85-115 nm to 38-58 nm (depending on the electrolyte concentration) and increases AAO walls etching that can even lead to AAO etching to individual fibers at 40°C. Selenium concentration in the samples formed in 0.5-1.5 M H₂SeO₄ at 5-40°C does not exceed 2 at.% and becomes undetectable at 40°C. It is established that the formation of the nanocrystalline phase Al₂O₃ in H₂SeO₄ electrolyte is observed at 40°C. The samples exhibit very weak photoluminescence. It was shown that in AAO formed in H2SeO4 there are 3 types of paramagnetic centers: F⁺ centers (NₛF=8.2·1015 g^-1), newly discovered centers in which the unpaired electron belongs to an oxygen atom (NₛO=1017 g^-1), and paramagnetic centers associated with selenate radicals (NₛS=6·1018 g^-1). By comparison of the photoluminescence spectra and defect concentrations, it is assumed that the luminescent properties of AAO obtained in selenic acid are exclusively determined by F⁺ centers. The centers associated with other reaction products do not contribute to the AAO luminescent properties.

A correlation was established between the photoluminescence intensity and the concentration of oxygen vacancies (F⁺-centers) in nanoporous aluminum oxide (por-Al₂O₃). For the first time, in amorphous por-Al₂O₃ synthesized in selenic acid, unusual spin centers were discovered, which are oxygen radicals with orthorhombic symmetry and a concentration of 10¹⁷ g^–1.

Anodic aluminum oxide has been successfully prepared using different acid concentrations in the electrolyte, anodization voltages, and current densities. F⁺ spin centers were detected in all samples. The photoluminescence (PL) band at λ₁ = 465 nm represents PL from F⁺. The PL band at λ₂ = 635 nm is due to radiative recombination of photoexcited electrons in nonparamagnetic centers. The obtained samples are promising for the creation of optical sensors for biomarkers.