ИСТИНА

Nanostructured titanium dioxide (TiO2) is currently being actively studied by the world scientific community, since it has a number of properties that are important from a practical point of view [1]. Very important property is the presence of a large specific surface area accessible to environmental molecules, which makes this material promising for use as photocatalysts or sensors [2]. The actual task of today is the development of efficient photocatalysts based on TiO2 nanotubes for converting carbon dioxide into more energy-intensive hydrocarbon compounds and the study of their electronic properties, in particular, the nature and main characteristics of defects. Therefore, the aim of this study was to identify the spin centers (defects) in the TiO2 nanotubes (TiO2 NTs) with different morphology and to study their properties. Since the most of defects in the TiO2 is paramagnetic, we used EPR spectroscopy. The synthesis of multi-walled and single-walled TiO2 nanotubes (MW-TiO2 NTs and SW-TiO2 NTs) is described in [3]. Note that the multi-wall NTs means that ones have an outer layer of pure titania and an inner layer of carbon doped titania; single-wall NTs have only an outer layer. Copper oxide CuxO was deposited onto the surface of the MW-TiO2 NTs arrays using the Successive Ionic Layer Adsorption and Reaction (SILAR) method. The source of copper ions was an aqueous solution of CuCl2•2H2O, the pH of which was adjusted to 10 with a solution of 25% ammonia (NH4OH). A solution of ethyl alcohol heated to 70°C with deionized water in a ratio of 1:3 was used as an anion source. The SILAR method consists of three stages. At the first stage, the sample is immersed in an aqueous solution of copper chloride for 30 seconds, which contains [Cu(NH3)4]+2 ions. In the second step, the sample is immersed in a solution of ethyl alcohol with deionized water for 7 seconds. At the third stage, the sample is washed in deionized water for 30 seconds. The amount of copper oxide CuxO deposited on MW-TiO2 NTs was varied by varying the number of ion deposition cycles: 10, 30, and 60 monolayers (samples MW-TiO2NTs/CuxO-10, MW-TiO2NTs/CuxO-30, MW-TiO2NTs/CuxO-60 and SW-TiO2NTs/CuxO-10, SW-TiO2NTs/CuxO-30, SW-TiO2NTs/CuxO-60, respectively). After ion layering, the resulting structures were subjected to heat treatment in an air atmosphere at a temperature of 300°C for 60 minutes at a heating rate of 30°C/min. The EPR spectrum of the initial MW-TiO2NTs samples is dominated by the signal from dangling carbon bonds (С∙), the Landé g factor g = 2.0027, while the EPR spectrum of SW-TiO2NTs is due to the Ti3+/oxygen vacancies centers, g1=1.9961, g2=.9697 [4]. The presence of carbon in the samples is probably due to the presence of ethylene glycol in the electrolyte and is also confirmed by elemental analysis data. The EPR spectra of the TiO2NTs/CuxO nanocomposites are a superposition of several EPR signals. First, a strong EPR signal from copper ions Cu2+ (g=2.1612) [4] is recorded, which indicates the presence of the CuO phase. We also observed a superposition of lines from defects of the C∙ type (detected in the initial structures), and from O2- radicals (g1=2.029, g2=2.009, g3=2.003). The appearance of O2- radicals can be explained by the adsorption of oxygen molecules on oxygen vacancies on the surface of TiO2 and, probably, on the surface of copper oxide nanoparticles, followed by the capture of electrons from the conduction band. This can lead to a limitation of the electron transport and, accordingly, to a decrease in the conductivity in the TiO2NTs/CuxO nanocomposites compared to the initial TiO2 NTs, what we observe in the experiment. With an increase in the number of copper oxide deposition cycles, the intensity of the line from Cu2+ ions increases. In addition, the intensity of the EPR signal from O2- radicals also increases. The results obtained open up new possibilities for the development of photocatalysts based on TiO2NTs/CuxO composites. The study was supported by a grant from Russian Science Foundation № 21-19-00494.