Titania (TiO ₂ ) is a widely used semiconductor for the photocatalytic decomposition of organic impurities in air, water and the conversion of CO ₂ into hydrocarbon fuel precursors. TiO ₂ in the form of nanotubes arrays is the most attractive for practical use because of the morphological advantages providing more favorable diffusion of photocatalytic reaction products and a low recombination rate of photogenerated electrons and holes. We have carried out a comparative study of the photocatalytic activity of gas-phase conversion of CO ₂ to hydrocarbon products and the defect properties of multi-walled and single-walled arrays of TiO ₂ nanotubes. Methanol and methane have been detected in the CO ₂ photoreduction process. The photocatalytic evolution rate of multi-walled TiO ₂ nanotubes is twice as fast for methane as for single-walled TiO ₂ nanotubes after four hours of irradiation and four times faster for methanol. The type and features of the structural defects have been investigated by EPR spectroscopy. For the first time, it has been shown that Ti ³⁺ /oxygen vacancy centers are mainly located inside the outer layer of nanotubes, while carbon dangling bonds have been observed directly on the surface of the inner layer. Carbon defects have been found to be the centers of adsorption and accumulation of photoinduced charge carriers. The results are entirely new; they clarify the role of different types of defects in the photocatalytic conversion of CO ₂ to hydrocarbon compounds and show good prospects for applying TiO ₂ nanotube arrays.

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.

This study demonstrates the ability to control the properties of TiO₂–CuO_x composite layers for photocatalytic applications by using a simple electrophoretic deposition method from isopropanol-based suspension. To obtain uniform layers with a controlled composition, the surfactant sodium lauryl sulfate was used, which influenced the electrophoretic mobility of the particles and the morphology of the deposited layers. The TiO₂–CuO_x composite layers with different CuO_x contents (1.5, 5.5, and 11 wt.%) were obtained. It is shown that the optical band gap measured by UV–VIS–NIR diffuse reflectance spectra. When CuO_x is added to TiO₂, two absorption edges corresponding to TiO₂ and CuO_x are observed, indicating a broadening of the photosensitivity range of the material relative to pure TiO₂. An open-circuit potential study shows that by changing the amount of CuOx in the composite material, one can control the ratio of free charge carriers (n and p) and, therefore, the catalytic properties of the material. As a result, the TiO₂–CuO_x composite layers have enhanced photocatalytic activity compared to the pure TiO₂ layer: methanol yield grows with increasing CuO_x content during CO₂ photoreduction.

Titania is very famous photocatalyst for decomposition of organic pollutants. Its photocatalytic properties significantly depend on the morphology and chemical composition of the samples. Herein, the TiO₂ nanotubes/Cu_xO nanoheterostructures have been synthesized and the effect of heat treatment performed in molecular atmospheres of air and argon on their photoelectrochemical and photocatalytic properties has been studied. The prepared samples have a higher reaction rate constant compared to TiO₂ nanotubes in the decomposition reaction of methylene blue molecules. It is established that in argon treated nanoheterostructures, the copper oxide is present in two phases, CuO and Cu₂O, while in air treated ones there is only CuO. In the TiO₂ nanotubes/Cu_xO samples, Cu²⁺ ions and molecular O₂⁻ radicals were detected while in TiO₂ nanotubes only carbon dangling bond defects are present. The dynamics of O₂⁻ radicals under illumination are discussed. It was shown that the TiO₂ nanotubes do not exhibit photocatalytic activity under visible light. The mechanism of the photocatalytic reaction on the surface of the TiO₂ nanotubes/Cu_xO samples was proposed. It is assumed that a photocatalytic decomposition of organic molecules under visible light at the surface of the nanoheterostructures under investigation is realized mainly by the reaction of these molecules with photogenerated O₂⁻ radicals. The results obtained are completely original and indicate the high promise of the prepared photocatalysts.

Symmetric titania nanotube arrays (TiO₂ NTs) are a well-known photocatalyst with a large surface area and band edge potentials suitable for redox reactions. Thermal treatment of symmetrical arrays of TiO₂ nanotubes in argon was used to change the carbon content of the samples. The influence of the carbon content in the structure of symmetrical TiO₂ NTs on their photoelectrochemical properties and photocatalytic activity in the conversion of CO₂ into organic fuel precursors has been studied. The structure, chemical, and phase composition of obtained samples were studied by X-ray analysis, Raman spectroscopy, and SEM with energy dispersive analysis. It is established that carbon-related defects in the samples accumulate electrons on the surface required for the CO₂ conversion reaction. It has been shown for the first time that varying the carbon content in symmetric TiO₂ NTs arrays by annealing at different temperatures in argon makes it possible to control the yield of methane and methanol in CO₂ conversion. It is revealed that too high a concentration of carbon dangling bonds promotes the growth of CO₂ conversion efficiency but causes instability in this process. The obtained results show a high promise of symmetric carbon-doped TiO₂ NTs arrays for the photocatalytic conversion of CO₂.