Co-crystallization of zinc oxide (ZnO) and zinc telluride (ZnTe) from a melt at high pressure results in the formation of ZnO-ZnTe composites with interesting physical properties that differ from those of the initial phases. The features of co-crystallization in the ZnO-ZnTe system at high pressure were studied using powder X-ray diffraction, scanning (SEM) and transmission (TEM) electron microscopy, energy dispersive X-ray spectroscopy (EDX), Raman spectroscopy, and electron paramagnetic resonance (EPR) of the quenched samples, and a mechanism of successive (layer-by-layer) crystallization from the melt was proposed. Measurements of the Hall effect and thermoelectric properties revealed that the composites exhibit p-type conductivity. The Seebeck coefficients and temperature dependences of electrical and thermal conductivity were also determined.

Supramolecular system of native barbituric acid and its 1,3-dimethyl derivative with melamine was studied using continuous-wave Q-band EPR spectroscopy. For separation of EPR signals, a special mathematical method was developed and applied. One axial symmetry signal and three singlet signals were detected and interpreted as belonging to superoxide radical and C-centered radicals, correspondingly. Radical activity of the system and predominance of specific radical centers were found to depend on compounds ratio, pH and light irradiation as well as samples storage time and oxygen presence. Quantum chemical calculations were performed to propose the localization of C-centered radicals in both supramolecular systems.

Nanocrystalline TiO2 is a perspective semiconductor gas-sensing material due to its long-term stability of performance, but it is limited in application because of high electrical resistance. In this paper, a gas-sensing nanocomposite material with p-p heterojunction is introduced based on p-conducting Cr-doped TiO2 in combination with p-conducting Cr2O3. Materials were synthesized via a single-step flame spray pyrolysis (FSP) technique and comprehensively studied by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) specific surface area analysis, transition electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and Raman spectroscopy. Gas sensor performance in direct current (DC) mode was studied toward a number of gasses (H2, CO, CH4, NO2, H2S, NH3) as well as volatile organic compounds (VOCs) (acetone, methanol, and formaldehyde) in dry and humid conditions. The long-term stability of the obtained materials' gas sensor performance was evaluated alongside with an ex situ study of structural evolution. High sensitivity toward oxygenated VOCs and a lower detection limit below ppm level with a limited influence of humidity were shown. The long-term gas sensor performance stability of the obtained materials and its connection to the defect structure of doped TiO2 is demonstrated.

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.

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 (NsF = 8.2 × 1015 g−1), newly discovered centers with an unpaired electron localized on an oxygen atom (NsO = 1017 g−1), and centers associated with selenate radicals (NsS = 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.

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.

Carbon nanoparticles (CNPs) are attracting great attention as potential multifunctional agents for biomedical applications because of their bright fluorescence, low toxicity and flexibility of their physico-chemical properties. In the present paper, aqueous solutions of CNPs doped with gadolinium (Gd) (Gd-CNPs) within a widely varying range of Gd concentrations were prepared by hydrothermal synthesis. The influence of Gd doping on the optical properties and magnetic resonance (MR) relaxivity of Gd-CNPs was revealed. The Gd content was determined using X-ray fluorescence and spectrophotometry analysis. The composition of surface functional groups and coordination of Gd ions in Gd-CNPs were established by means of IR absorption spectroscopy and X-ray photoemission spectroscopy (XPS). The optical properties of Gd-CNPs in aqueous solutions were characterized by means of UV-visible-near-IR absorption spectroscopy and photoluminescence measurements with different excitation wavelengths. The local surroundings of Gd ions and paramagnetic centers in Gd-CNPs were probed by using electron paramagnetic resonance (EPR) spectroscopy. MR proton relaxation measurements in aqueous solutions of Gd-CNPs were carried out to determine the effect of Gd concentration on their MR contrasting. The obtained results characterize the coordination of Gd ions in Gd-CNPs and demonstrate new insights for controlling the optical and MR contrast properties of these nanoparticles for biomedical applications.

SnO2/MnOx nanocomposites with manganese content up to [Mn]/([Mn]+[Sn]) = 10 mol.% were prepared using chemical precipitation and impregnation-annealing techniques and examined using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy, mass-spectrometry with inductive coupled plasma. Manganese distribution between the bulk and the surface of SnO2 crystal grains depending on the total manganese concentration was determined and the chemical state of both type (bulk and surface) manganese cations was identified. Strong correlations of sensor signal toward C1 volatile organic compounds (methanol, formaldehyde, formic acid) and C1-C4 carbonic acids (formic, acetic, propionic, butyric) with the total manganese content, manganese distribution and chemical state were revealed.

Photocatalysts based on the anodic single-walled titania nanotubes modified with metal nanoparticles (Au, Pd, Pt) were prepared and investigated. The metal nanoparticles sizes are in a range from 3 to 15 nm. The wall thickness of the nanotubes varied from 14 to 17 nm. It is established that photocatalysts with Au, Pd, Pt produce acetaldehyde during complete ethanol decomposition reaction. Additionally the formation of methane occurs in the TiO2 nanotubes with Pd and Au nanoparticles. We obtained firstly the multifunctional TiO2 nanotubes modified with Pd and Au nanoparticles for use both in air purification and in the production of hydrocarbon fuel precursors.

Nanocomposites based on anodic titanium oxide nanotubes with copper oxide nanoparticles were formed and their structural, optical, and electrophysical properties were studied. Defects in the structure of the samples were identified by electron paramagnetic resonance and it was shown that, as a result of copper oxide deposition, CuO nanoparticles were formed on the surface of nanotubes. It was found that the conductivity of the structure decreases by several orders of magnitude with an increase in the number of deposition cycles. It was shown that this effect could be associated with the formation of TiO2 /CuO heterojunctions on the nanotube surface. It was shown for the first time that an increase in the content of copper oxide in TiO2/CuxO nanocomposites was accompanied by a decrease in conductivity and an increase in the number of defects.

The work addresses the problem of long-term stability of metal oxide materials gas sensor performance and deals with Nb(V) doped nanocrystalline TiO2 which is characterized by reasonable response values for practical application alongside with the absence of response decay during continuous operation. The TiO2 nanomaterials containing up to 4 at% Nb were synthesized using flame spray pyrolysis. Materials are investigated by XRD and EPR methods. The sensor properties were studied by in situ electrical conductivity measurements. Nb(V) reduces the electrical resistance of TiO2 due to the formation of oxygen vacancies and increase in conduction electron concentration, but this effect is limited by the appearance of a significant number of charge traps. Pure TiO2 tended to transform into rutile phase during annealing in the analyte gas mixture, while Nb stabilized the anatase phase. Both pure and doped TiO2 demonstrate stable sensor performance in long-term, however slow growth of baseline resistance and sensor response are observed which is, according to EPR data, connected with slow processes of defect annealing and intergrain contact formation.

The effect of thermobaric treatment (TBT) at & Rcy; = 5.5 GPa T = 1900 degrees C for 4 h on the concentration of single substitutional atoms (P1 centers) in nitrogen-doped synthetic single crystal Ib type diamonds grown by the temperature gradient high-pressure high-temperature (TG-HPHT) method and on their magnetic properties in the temperature range of 2-300 K was investigated. The initial concentration of P1 centers in 2 crystals was 3.0 x 10(19) cm(-3) and 5.6 x 10(19) cm(-3), respectively, which decreased by similar to 96-98 % after TBT. At T < 10 K, an anomalous hysteresis of the magnetic moment is observed in the range of magnetic fields up to H-c = 70 kOe, the amplitude of which is half the value of the paramagnetic saturation moment, corresponding to the number of spins of the P1 centers. The shape of the hysteresis loops after subtracting an intrinsic diamond diamagnetic component and the calculated paramagnetic component indicates a granular (local) superconducting state. The fact that the amplitude of the magnetic moment generated by superconducting currents is equal to half the total number of Bohr magnetons of P1 centers may indicate that elementary currents occur in pairs of adjacent P1 centers. As a result of a decrease in the number of P1 centers in diamonds after TBT, the saturation values of the paramagnetic moment and the amplitude of the diamagnetic moment of the induced superconducting currents decrease proportionally.

Anodic titania nanotubes (TiO2-NT) are very promising for use in photocatalysis and photovoltaics due to their developed surface, symmetrical structure and conductive properties, which, moreover, makes them a convenient matrix for creating various nanocomposites. Herein we propose a new facile way of synthesizing symmetrical TiO2-NT followed by a modification with barium titanate (BaTiO3) nanoparticles, combining the advantages of electrochemical oxidation and hydrothermal synthesis. The electrophysical and optoelectronic properties of the formed nanocomposites have been studied. An asymmetry of the current–voltage characteristics was revealed. It is shown that during the barium titanate deposition, a symmetry-breaking nanoheterojunction TiO2/BaTiO3 is formed. Using EPR spectroscopy, paramagnetic defects (titanium, barium and oxygen vacancies) in the samples were determined. It was observed for the first time that upon illumination of titania nanotubes modified with BaTiO3, the asymmetrical separation of photoexcited charge carriers (electrons and holes) between TiO2-NT and BaTiO3 occurs, followed by the capture of electrons and holes by defects. As a result, the photoinduced charge accumulates on the defects.

The possibility of doping ZnO in its metastable rock salt structure with Li, Na, and K intended to act as acceptor dopants was investigated. For the first time, MgxZn1-xO alloys and pure ZnO with a rock salt structure doped with Li, Na, and K metals was obtained by high-pressure synthesis from pure oxides with the addition of carbonates or acetates of the corresponding metals as dopant sources. Successful stabilization of the metastable rock salt structure and phase purity were confirmed by X-ray diffraction. Transmission electron microscopy was used to study the particle size of nanocrystalline precursors, while the presence of Li, Na, and K metals in rock salt ZnO was detected by electron energy-loss spectroscopy and X-ray photoelectron spectroscopy in MgxZn1-xO alloys. Electron paramagnetic resonance measurements revealed the acceptor behavior of Li, Na, and K dopants based on the influence of the latter on native defects and natural impurities in ZnO-MgO alloys. In addition, diffuse reflectance spectroscopy was used to derive band gaps of quenched rock salt ZnO and its alloys with MgO.

Nanocomposites SnO2/MnOx with various manganese content (up to [Mn]/[Sn] = 10 mol. %) and different manganese distribution were prepared by wet chemical technique and characterized by X-ray diffraction, scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis and mapping, IR and Raman spectroscopy, total reflection X-ray fluorescence, mass-spectrometry with inductive-coupled plasma (ICP-MS), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) spectroscopy. A different distribution of manganese between the volume and the surface of the SnO2 crystallites was revealed depending on the total Mn concentration. Furthermore, the identification of surface MnO2 segregation was performed via Raman spectroscopy. There is a strong dependence of the sensor signal toward CO and, especially, NO) on the presence of MnO2 surface segregation. However, manganese ions intruding the SnO2 crystal structure were shown to not almost effect on sensor properties of the material.

To overcome the low conductance issue of cobalt oxide nanofibers, we have synthesized cobalt oxide-based nanofibers with zinc (Co3O4/Zn) by the method of electrospinning. The structure of materials in this work were characterized extensively using X-ray diffraction, an X-ray fluorescence method, electron microscopy, and infrared and Raman spectroscopy. Nanofibers are shown to have a diameter of approximately 150 nm and consist of crystallites with an average size of about 20 nm. It has been found that the adding of zinc can significantly increase the conductivity of nanofibers: at a zinc concentration of about 15%, the conductivity increases by more than 4 orders of magnitude compared to cobalt oxide nanofibers without zinc and remains of p-type. We have shown that composite consists of ZnxCo3-xO4 and ZnO structures. Additionally, electron paramagnetic resonance (EPR) spectroscopy was conducted to identify the paramagnetic defects in the composites. Findings from the EPR spectroscopy were combined with the aforementioned characterization techniques to illuminate the mechanism of conductivity enhancement. The correlation between the concentration of paramagnetic cobalt atoms in an octahedral environment and the conductivity of Co3O4/ Zn composites indicates that the change in conductivity is associated with a change in the concentration of acceptor levels in the ZnxCo3-xO4 structure. Our results suggest that synthesis of nanofibers with zinc by electrospinning could be a promising method to construct high conductivity cobalt oxide-based nanofibers for environmental applications.

Titanium dioxide (TiO2) nanocrystals are one of the most promising materials for modern photocatalysis applications, having unique properties such as a huge specific surface area and affordability of synthesis. However, the facile fabrication of TiO2-based photocatalysts active in regular daylight remains a major challenge. In this work, aiming to create such a nanocrystalline material by a very simple technology, we provide a detailed analysis of the paramagnetic centers (PCs) and their photoinduced reactions in N- and Nb-codoped TiO2 nanocrystals. The resulting powders show a surface area up to 170 m(2)/g and an intense visible-light photocatalysis of rhodamine 6G with reaction rate k = 0.087 min(-1). Ti3+, N-center dot, and O-2(-) PCs are observed and studied using electron paramagnetic resonance spectroscopy. We also present a new approach to the study of photoinduced processes in nanocrystalline photocatalysts-a simple theoretical model of kinetics of these PCs, which well predicts their behavior and variations of their concentration under illumination. Our results indicate that the obtained Nb-N-TiO2 nanocrystals with a high concentration of PCs on the sample surface can be used in cutting-edge industries, paving the way for the most advanced photocatalytic systems operating in sunlight.

Titania (TiO2) is a widely used semiconductor for the photocatalytic decomposition of organic impurities in air, water and the conversion of CO2 into hydrocarbon fuel precursors. TiO2 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 CO2 to hydrocarbon products and the defect properties of multi-walled and single-walled arrays of TiO2 nanotubes. Methanol and methane have been detected in the CO2 photoreduction process. The photocatalytic evolution rate of multi-walled TiO2 nanotubes is twice as fast for methane as for single-walled TiO2 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 Ti3+/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 CO2 to hydrocarbon compounds and show good prospects for applying TiO2 nanotube arrays.

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/CuxO 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/CuxO 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/CuxO 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.

The thermoelectric properties of Sb2 – xCuxTe3 single crystals (0 ≤ x ≤ 0.10) synthesized by the Bridgman method are studied in the temperature range of 77 K < T < 350 K. It turns out that the hole concentration and electrical conductivity strongly increase, while the Seebeck coefficient slightly decreases when Sb2Te3 crystals are doped with copper. The thermal conductivity of crystals doped with copper is somewhat higher than that of the initial Sb2Te3 crystals. As a result, the thermoelectric figure of merit ZT increases with increasing copper content at T > 300 K. In addition, the quantum mobility of holes μq in Sb2 – xCuxTe3 (0 ≤ x ≤ 0.10), Sb2 – xSnxTe3 (0 ≤ x ≤ 0.01), and Sb2 – xTlxTe3 (0 ≤ x ≤ 0.05) single crystals is measured using data on the Shubnikov–de Haas (SdH) effect. Electron paramagnetic resonance (EPR) measurements show that copper ions in the studied samples are most likely in the spinless Cu+1 state.