Annotation of the results obtained in 2023
It has been established that the photocatalytic properties of the NTAOT array are determined by the physicochemical properties of the nanotube surface. The degradation of photocatalysts is associated with changes in the phase and chemical composition of the surface layer. This is due to the fact that during the process of catalysis, the accumulation of reaction products occurs in a near-surface layer several angstroms thick; detecting changes in the composition of the near-surface composition is extremely difficult. Such changes can be detected only by a limited number of highly sensitive methods, one of which is EPR spectroscopy. It was established that the EPR spectrum of MS NTAOT contains an absorption line corresponding to dangling carbon bonds. The concentration of defects was 1.4·1015 g-1. When illuminated, the amplitude of the EPR signal from these centers increases by approximately 1.8 times. It can be assumed that during the photocatalytic conversion of CO2 into methane and/or methanol, carbon clusters containing carbon atoms with an unpaired electron are formed on the surface of NTAOT. Thus, the concentration of dangling carbon bonds increases. The effect of doping the surface layer with carbon in places of oxygen vacancies is also possible. The appearance of new dangling carbon bonds is possible due to the occupation of oxygen vacancies by carbon. Carbon, as a neutral substitutional impurity, plays the role of a donor. However, compared to an oxygen vacancy, during C• ionization the number of free electrons is smaller. This effect of modification of the surface layer with carbon leads to a decrease in the reducing ability of the NTAOT MS surface and, as a consequence, photocatalytic activity during the conversion of TiO2. The main parameters and processes that have the greatest impact on the efficiency and selectivity of the process of photocatalytic conversion of CO2 are the nature and concentration of paramagnetic centers on the surface of the NTAOT array and their variations during catalysis. The efficiency of the process of photocatalytic reduction of CO2 when using composites based on NTAOT arrays is determined by the ratio of reducing and oxidizing centers and their number. In NTAOT arrays, the following act as reduction and oxidation centers: titanium and oxygen vacancies, and carbon centers. In the case of CuOx-HTAOT composites, the following appear: copper vacancies in copper oxide, oxygen vacancies in copper oxide, as well as copper atoms embedded in the titanium crystal lattice. The processes of passivation of oxygen vacancies in titanium oxide by carbon from CO2 reduce the efficiency of the photocatalytic process of CO2 conversion by reducing the number of electrons available for the reaction. In the case of CuOx-HTAOT composites, a process of passivation of titanium vacancies by carbon atoms may occur, reducing the oxidizing ability of the catalyst surface and, as a consequence, the amount of available H+ and OH- ions. All this may be the reason for the degradation of the photocatalytic properties of NTAOT arrays. In the EPR spectrum of NTAOT/CuOx MSs kept in a CO2 atmosphere in the dark, EPR signals from copper ions Cu2+, a superposition of lines from dangling carbon bonds observed in the original structures, and a weak line from O2- radicals are recorded. The appearance of O2 radicals can be explained by the adsorption of oxygen on oxygen vacancies on the surface of TiO2 and, possibly, on the surface of copper oxide nanoparticles, followed by the capture of electrons from the conduction band. When illuminated, the amplitude of EPR signals from dangling bonds of carbon and O2- radicals increases by a factor of 2 or more (for O2-) and the amplitude of the EPR line from Cu2+ also increases slightly. The increase in the concentration of these radicals is due to the occurrence of redox reactions in a CO2 atmosphere under the influence of illumination, i.e. photocatalysis. An increase in the intensity of the EPR signal from copper bound to the titanium oxide lattice is possible due to the replacement of titanium vacancies. The replacement of titanium vacancies with copper leads to a decrease in the overall oxidizing capacity of the material due to the difference in the number of holes that can enter into the water decomposition reaction. A decrease in the oxidizing capacity of the surface leads to a decrease in the probability of water decomposition and the amount of OH- and H+ available for the reaction. Thus, due to the replacement of some of the initial structural defects by less active centers, degradation of the photocatalyst based on NTAOT arrays occurs. It is also impossible to exclude the formation of amorphous carbon agglomerates on the surface of the material that passes the surface of titanium oxide due to a decrease in the access of CO2 and H2O molecules. The main parameters and processes that have the greatest impact on the efficiency and selectivity of the process of photocatalytic conversion of CO2 are the nature and concentration of paramagnetic centers on the surface of the NTAOT array and their variations during catalysis. The efficiency of the process of photocatalytic reduction of CO2 when using composites based on NTAOT arrays is determined by the ratio of reducing and oxidizing centers and their number. In NTAOT arrays, the following act as reduction and oxidation centers: titanium and oxygen vacancies, and carbon centers. In the case of CuOx-HTAOT composites, the following appear: copper vacancies in copper oxide, oxygen vacancies in copper oxide, as well as copper atoms embedded in the titanium crystal lattice. The processes of passivation of oxygen vacancies in titanium oxide by carbon from CO2 reduce the efficiency of the photocatalytic process of CO2 conversion by reducing the number of electrons available for the reaction. The concentration of defects can be monitored and controlled by varying the parameters of the NTAOT synthesis (introduction of impurities, annealing of samples at different temperatures and in different environments). Illumination of NTAOTs and nanocomposites based on them is also an effective tool. NTAOT/BaTiO3 photocatalysts were obtained and their structural, optical, electrophysical properties and defect characteristics were studied. A new method is proposed for determining the concentration of photoexcited electrons in the conduction band of semiconductors with paramagnetic centers and diagnosing the charge accumulated on defects based on EPR spectroscopy. Photocatalysts based on NTAOT arrays with an increased concentration of defects on the surface have been created, which helps to increase the efficiency of photocatalysts.

 

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Annotation of the results obtained in 2021
The work on the first stage of the project "Development of nanocomposite photocatalytic materials based on titanium dioxide anode nanotubes for energy-efficient reducing of carbon dioxide to energy-capasitive hydrocarbon compounds" was aimed at the formation of arrays of nanotube titanium anode oxide (NTAOT) of various morphologies (two-layer and single-layer structures) and the study of the effect of parameters of copper and copper oxide nanoparticles deposition by different methods with subsequent heat treatment on the chemical and phase composition of the resulting composites NTAOT/CuxO, as well as identifying patterns of influence of the composition and structure of NTAOT/CuxO composites on their optical and electrophysical properties. Electrochemical oxidation of titanium foil in fluorine-containing electrolyte solutions based on ethylene glycol was used to form two-layer (multi-wall) structures of NTAOT with the outer layer representing titanium oxide in anatase modification and the inner layer representing a mesoporous structure either in the rutile modification or in the form of rutile and anatase mixture. For the formation of single-layer (single-wall) NTAOT structures with the anatase crystal structure, an original technique was used to remove the inner layer by chemical treatment of NTAOT in a mixture of sulfuric acid and hydrogen peroxide. The methods of photoinduced deposition and ion layering (SILAR method) were used to introduce copper into the structure of NTAOT in order to form arrays of titanium dioxide nanotubes decorated with copper and copper oxide nanoparticles, the source of copper ions were aqueous solutions of copper salts. To control the copper oxide phase, the samples obtained after modification were subjected to heat treatment in various atmospheres: air, argon, vacuum. The result of this study is to obtain reproducible technological techniques for the formation of NTAOT/CuxO composites. The study showed that during photoinduced deposition in the monochromatic mode (illumination at a wavelength of 375 nm) with a photoexcitation intensity of 0.76 mkW/cm2, copper modification of the NTAOT arrays occurs. Varying the radiation power makes it possible to obtain both micro- and nanoscale copper particles. In the process of ion layering, CuxO nanoparticles are deposited on the surface of the NTAOT arrays. The change in the amount of deposited CuxO copper oxide was carried out by varying the number of ion layering cycles – 10, 30 and 60 monolayers. An increase in the number of copper oxide deposition cycles to 30 by the SILAR method leads to the formation of agglomerates of copper oxide nanoparticles with an average size of about 20 nm. The further increase in the number of deposition cycles to 60 leads to nanoneedles and nanoplates formation with sizes from 40 to 500 nm. Heat treatment in air of NTAOT arrays after deposition of copper oxides by the SILAR method leads to the formation of CuO nanoparticles on the surface of the array. At the same time, annealing in atmospheres with low oxygen content (argon, vacuum) leads to the formation of a mixture of phases CuO, Cu2O on the surface of the NTAOT array. It was found out by the method of energy dispersive X-ray spectroscopy, that the chemical composition of the samples after photoinduced copper deposition and ion layering (deposition of CuxO nanoparticles) includes titanium, oxygen, fluorine, carbon and copper. It was found that radicals (defects) of the carbon dangling bonds type are located on the surface of the inner layer of multi-walled NTAOT (MW-NTAOT), and defects of the type Ti3+/oxygen vacancies are located in the outer layer of nanotubes. Defects of the Ti3+/oxygen vacancies type have been registered in single-walled NTAOT (SW-NTAOT) consisting only of the outer layer. It was found that the obtained arrays of MW-NTAOT most efficiently (compared with SW-NTAOT) convert carbon dioxide (CO2) into methane and methanol. A model of the photocatalytic process of CO2 conversion in NTAOT arrays is proposed, according to which defects (such as carbon dangling bonds studied by electron paramagnetic resonance) act as adsorption centers for CO2 molecules and accumulate an electrons. This leads to acceleration of carbon dioxide conversion to precursors of hydrocarbon fuels – methane and methanol. According to the data on light diffuse reflection and the calculations performed, the value of the band gap for the initial samples of NTAOT and heterostructures of NTAOT/CuxO with a different number of copper oxide deposition cycles (with different CuxO phase content) differs slightly and lies in the range from 3.1 to 3.3 eV. The fluorescence spectra of all samples (NTAOT and NTAOT/CuxO) were also similar. It is established that fluorescence in NTAOT and NTAOT/CuxO structures is caused by interband transitions. Studies have shown that light absorption in the visible region increases in NTAOT/CuxO composites compared to the original NTAOT structures, the greatest effect was observed for 30 cycles of ion layering. At the same time, annealing of NTAOT/CuxO structures in argon enhances the absorption of visible light, and annealing of samples in vacuum leads to a further increase in absorption in the visible region compared with annealing of NTAOT/CuxO in air. It is shown that the conductivity of the initial single-wall and multi-wall structures of NTAOT has similar values. CuxO deposition leads to a significant (by several orders of magnitude) decrease in the conductivity, which may be due to the formation of TiO2/CuxO heterojunctions, which determine the charge transfer processes in structures. The activation energies were determined from the temperature dependences of conductivity, which amounted to 0.26 eV and 0.31 eV for the initial samples of SW-NTAOT and MW-NTAOT, respectively. For both types of composites (single-wall and multi-wall), the activation energy varied slightly and was approximately 0.5 eV. It is shown that annealing of NTAOT/CuxO composites in vacuum leads to a significant increase in conductivity compared to annealing of the samples in air. The observed variations in the optical and electrophysical properties of NTAOT/CuxO composites are probably due to the formation of defects of various types during the formation of NTAOT/CuxO composites, and annealing in different atmospheres leads to variations in the concentrations of defects, the study of which is planned for the next stage of the project. Thus, the results obtained showed the fundamental possibility of forming NTAOT/CuxO composites with reproducible characteristics and controlling their properties by varying the synthesis conditions. Therefore, the experiments, expected results and indicators of publication activity planned for the second year of the project will be achieved. Based on the research results, a scientific paper (Q1 SJR) was published, 2 oral presentations were made at International conferences, one of which was Invited presentation.

 

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Annotation of the results obtained in 2022
In the process of performing the work of the second stage of the project "Development of nanocomposite photocatalytic materials based on titanium dioxide anode nanotubes for energy-efficient reducing of carbon dioxide to energy-capasitive hydrocarbon compounds", arrays of anodic titanium oxide nanotubes (NTAOT) modified by electrochemical deposition with copper nanoparticles were formed and a study of the obtained samples by structural, optoelectronic and electrophysical methods was performed. As follows from the SEM data, the NTAOT morphology does not undergo significant changes upon modification with copper. Defects were identified and their main parameters were determined by the EPR method with the use of numerical simulation. In the initial single-walled NTAOT, only defects of the Ti3+ type/oxygen vacancies localized in the outer layer of the TiO2 nanotubes are detected. In the original multi-walled NTAOT, dangling C⋅ carbon bonds are detected in the inner layer of TiO2 nanotubes and Ti3+/oxygen vacancies are detected in the outer layer. Cu2+ ions embedded in the structure of titanium dioxide and substituting titanium ions in the crystal lattice, as well as copper ions Cu2+ in the CuO phase, were found in NTAOT modified with copper nanoparticles. This indicates the partial oxidation of deposited metallic copper and the formation of copper oxide nanoparticles on the surface of TiO2 nanotubes. Dangling bonds of carbon and Ti3+/oxygen vacancies, which were observed in the original multi-walled samples (without copper), were also detected. In NTAOT/CuxO (both single-walled and multi-walled), copper ions Cu2+ in the CuO phase and O2-radicals and dangling carbon bonds in insignificant concentrations were found. Upon illumination, the Cu2+ concentration irreversibly decreases, which indicates the formation of the Cu2O phase. The concentration of O2 radicals increases upon illumination due to the capture of photoinduced electrons by oxygen molecules adsorbed on the HTAOT surface. The band gap value determined by the method of diffuse reflection of light was the same within errorbars for all obtained samples (NTAOT and composites based on them - NTAOT/CuxO, NTAOT/Cu) and equal to 3.2 ± 0.1 eV. An increase in the coefficient of absorption of visible light by samples containing copper and copper oxide nanoparticles, compared with the original samples, was registered, since the composites contain more defects that create energy levels in the band gap. Using the EPR method, the energy levels of radicals in the band gap were determined and band diagrams were plotted. The concentrations of radicals in the structures under study were calculated, which varied from 10^15 g-1 in the original NTAOT samples to 6 ∙10^16 g^-1 in the NTAOT/CuxO composites. It has been established that the modification of NTAOT with copper by electrochemical deposition leads to a sharp increase in conductivity by 3 orders of magnitude and a decrease in the activation energy of conductivity with an increase in the number of cycles from 0 to 40. The data obtained can be explained by assuming that new donor levels are formed in the band gap of titanium dioxide as a result of the introduction of copper ions into the TiO2 structure. Thermal activation of electrons from these donor levels to the conduction band leads to an increase in the concentration of free charge carriers, and, consequently, to an increase in conductivity. It is shown that at direct current and at low frequencies the conductivity is associated with the motion of electrons along the conduction band, while at high frequencies the hopping mechanism of conduction dominates. The decrease in the conductivity of the HTAOT/CuxO nanocomposites by 3 orders of magnitude compared to the initial HTAOT samples is associated with the formation of p–n heterojunctions on the inner surface of the titanium dioxide nanotubes. Due to the processes of diffusion, as well as recombination with the majority charge carriers, near the heterojunction, regions are formed that are depleted in free charge carriers. The width of these regions is comparable to the thickness of the walls of nanotubes. Therefore, the conductivity of such a structure in the direction perpendicular to the Ti substrate significantly decreases. It has been established that the charge transfer in the samples modified with copper oxide still proceeds along TiO2 nanotubes with n-type conductivity. In this case, charge carriers do not overcome the p-n junction, but move along the walls of the nanotubes. The presence of a p-n junction affects only the concentration of free carriers in nanotubes. Therefore, the current-voltage characteristic of such a structure has a symmetrical, and not a rectifying form typical of a p-n junction. It has been shown that the photocatalytic decomposition of organic molecules under visible light illumination using HTAOT/CuxO nanocomposites is based on redox reactions involving oxygen radical anions. The maximum yield of methanol and the minimum yield of methane are observed for NTAOT samples. The precipitation of copper oxide shifts the selectivity of the conversion process towards the formation of methane due to the migration of photoelectrons from CuO to TiO2. The methane yield reaches a maximum for the HTAOT/CuxO-30 samples. An increase in the number of precipitation cycles from 10 to 60 leads to an increase in the yield of methanol. However, the rate of methanol formation for the samples with the largest amount of deposited copper oxide still remains lower than for the initial NTAOT samples. The free surface of HTAOT decreases with an increase in the number of copper oxide deposition cycles, which leads to a decrease in the rate of CO2 photoconversion to methanol. Note that the decomposition of water on the surface of copper oxide is impossible due to the insufficient positive potential of the top of the valence band. As a consequence, hydrogen ions and hydroxide ions can only form on the TiO2 surface. Thus, the technology for the formation of HTAOT/CuxO composites with reproducible characteristics has been developed, a model of photoelectronic processes and a model describing the transport of charge carriers in HTAOT/CuxO composites have been developed, and features of the mechanism of photocatalytic reduction of CO2 using HTAOT/CuxO composites have been revealed. Based on the results of the research at the second stage, 3 scientific articles were published in the journals Q1, Q2, Q3; 7 reports were made at international conferences, of which 1 was invited, 3 were oral and 3 were posters. Therefore, the work, expected results and indicators of publication activity planned for the third year of the project will be implemented.

 

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