Stephan Kozhukharov

The influence of suspension depth and pollutant concentration on the rate and efficiency of photocatalytic degradation was investigated in aqueous TiO₂ suspensions using methyl orange (MO) as a model pollutant. Both the reaction rate and the efficiency increased by more than one order of magnitude upon a relatively small decrease in suspension layer thickness. The reaction rate exhibited a complex N-shaped dependence on dye concentration, deviating from the monotonic behavior predicted by the Eley–Rideal and Langmuir–Hinshelwood mechanisms, as well as from the relationship derived in our previous study. To elucidate the origin of this behavior, nanoparticle aggregation was examined by sedimentation kinetics, low-acceleration centrifugation, and scanning electron microscopy (SEM). The results suggest that the interplay between enhanced dye adsorption and the reduction in the available photocatalyst surface area due to aggregation leads to the appearance of a maximum in the concentration dependence of the reaction rate. The relationship between reaction rate and photocatalytic efficiency was also analyzed. Although both parameters are correlated, efficiency values strongly depend on the selected reaction time interval, which complicates direct comparison between studies employing different experimental protocols. Consequently, the reaction rate appears to be a more reliable parameter for describing photocatalytic kinetics.

Environmental and health concerns drive the search for environmentally benign anodizing electrolytes. This study presents a comparative assessment of anodic films formed on AA2024-T3 alloy in 0.5 M solutions of seven biocompatible organic acids. Anodization was performed at 15 mA cm-2 for 30 min at 20 °C following identical preliminary treatments. The influence of electrolyte composition on film barrier properties, surface morphology, wettability, and chemical composition was evaluated using Electrochemical Impedance Spectroscopy, Potentiodynamic Polarization, Optical Metallographic Microscopy, Scanning Electron Microscopy, sessile drop tests, and X-ray Photoelectron Spectroscopy. The obtained barrier abilities followed the order: malic > malonic > tartaric > citric > oxalic > maleic acid, while glycine exhibited a detrimental effect on film properties. Additional experiments were conducted on films produced in malic acid after thermal treatment in boiling distilled water for 30 min. Although the treatment slightly reduced barrier performance, it significantly improved repeatability by converting the surfaces from completely hydrophobic to fully hydrophilic. The relationships between electrolyte composition, surface characteristics, and corrosion-protective properties are systematically examined and discussed.

In this work, Ce-doped ZnO thin films at various contents of cerium were deposited on glass substrates by thermal vacuum evaporation to study the influence of Ce concentration on their optical, structural, morphological, and photocatalytic behavior. Pure ZnO and Ce-doped ZnO films doped with 2% and 5% Ce were characterized by SEM, XRD, AFM, UV–VIS spectroscopy, and ellipsometry. The XRD analysis confirmed that all the films retained the hexagonal wurtzite structure, while Ce incorporation induced lattice strain and reduced crystallite size, particularly at higher doping levels. SEM and AFM studies showed that films with 2% Ce exhibited smaller grain size and lower roughness, whereas 5% Ce-doped films showed grain growth and increased roughness. Pure ZnO films displayed high transparency (>90%), whereas Ce incorporation caused a red shift in the absorption edge and narrowing of the optical band gap due to defect-related states and lattice distortion. Photocatalytic experiments revealed that Ce doping improved charge carrier separation and increased the number of oxygen vacancies. Among all samples, the 2% Ce-doped ZnO film demonstrated the highest photocatalytic efficiency. These findings highlight the importance of controlled Ce doping in tuning the microstructure, optical properties, and photocatalytic performance of ZnO thin films, making them suitable for environmental remediation and optoelectronic applications.

The recent endeavor to establish a sustainable society, with respect to environmental protection and occupational health prevention, imposes the need for the development of environmentally friendly anodization electrolytes. In addition, these electrolytes should be composed of biocompatible organic acids derived from renewable sources. In response to these challenges, a need arises to seek environmentally conscious alternatives to the widely used sulfuric acid anodization electrolytes. Accordingly, a comparative study was performed on the anodic polarization of AA2024-T3 aircraft alloy samples for 30 min at 0 or 20 °C. The respective electrolytes were composed of 0.5 M solutions of oxalic, citric, tartaric acids, or glycine. The comparative analysis included optical metallographic microscopy (OMM), scanning electron microscopy (SEM), determination of color characteristics and wettability, chemical composition analysis by X-ray photoelectron spectroscopy (XPS), and assessment of the corrosion protective properties of the obtained layers. The latter were defined by the application of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization scanning (PDS) after 24 h of exposure to a 0.5% NaCl solution. Among the most important conclusions is that the barrier properties of the layers obtained in citric and tartaric acid electrolytes remarkably exceed those of the film obtained in oxalic acid. The use of glycine does not result in film formation at all. The process temperature did not have as strong an effect as the electrolyte composition.

Humidity sensing elements based on sodium-doped titanium dioxide (Na-doped TiO₂) were prepared using a sol–gel method in the presence of cerium ions and sintered at 400 °C and 800 °C. Titanium (IV) n-butoxide and a saturated solution of diammonium hexanitratocerate in isobutanol served as starting materials. Sodium hydroxide and sodium tert-butoxide were used as inorganic and organometallic sodium sources, respectively. The influence of sodium additives on the properties of the humidity sensing elements was systematically investigated. The surface morphologies of the obtained layers were examined by scanning electron microscopy (SEM). Elemental mapping was conducted by energy-dispersive X-ray (EDX) spectroscopy, and structural characterization was performed using X-ray diffractometry (XRD). Electrical properties were studied for samples sintered at different temperatures over a relative humidity range of 15% to 95% at 20 Hz and 25 °C. Experimental results indicate that sodium doping enhances humidity sensitivity compared to undoped reference samples. Incorporation of sodium additives increases the resistance variation range of the sensing elements, reaching over five orders of magnitude for samples sintered at 400 °C and four orders of magnitude for those sintered at 800 °C.

The anodic behavior of zinc electrodes is important for energy storage, corrosion protection, electrochemical processing, and other practical applications. This study investigates the anodic galvanostatic polarization of zinc foil and bulk electrodes in aqueous oxalic acid solutions, revealing significant differences in their electrochemical behavior, particularly in induction period durations. The induction period’s duration depended on electrolyte concentration, current density, and temperature. Notably, the temperature dependence of the kinetics exhibited contrasting trends: the induction period for foil electrodes increased with temperature, while that of bulk electrodes decreased. Chemical analysis and polishing treatment comparisons showed no significant differences between the foil and bulk electrodes. However, Scanning Electron Microscopy (SEM) observations of samples anodized at different temperatures, combined with Inductively Coupled Plasma–Optical Emission Spectroscopy (ICP-OES) analysis of dissolved electrode material, provided insights into the distinct anodic behaviors. X-ray Diffraction (XRD) studies further confirmed these findings, revealing a crystallographic orientation dependence of the anodic behavior. These results provide detailed information about the electrochemical properties of zinc electrodes, with implications for optimizing their performance in various applications.

The dependence of the reaction rate on the solution layer thickness discovered in a previous work could be a powerful tool for investigating photocatalytic reactions. A reduction of the apparent rate constant with the growth of the dye concentration was found using this dependence. This decrement follows a hyperbolic law. This dependence is explained based on the observed increment of the solution conductivity. In addition, it is confirmed experimentally that the reaction rate decreases with the solution depth growth. The possibility of independent determination of the reaction rate constant and the adsorption equilibrium constant has been discussed. Additionally, it is demonstrated that the vessel’s reflective bottom could increase the chemical reaction rate. The reason why other authors have not yet reported this effect is also discussed.

The present research is dedicated to the systematic characterization of the surface characteristics of AA2024-T3 specimens, after the formation of Anodic Aluminium Oxide (AAO) layer, deposition of a Ceruim Conversion Coating (CeCC) and combination of these methods. The latter procedure was performed at two temperatures (20°C and 50°C). The investigations comprised combined AAO/CeCC coating primers, as well. All the obtained protective layers were subjected to systematic evaluations for determination of their colour characteristics, wettability and topology. The latter, including the layer thickness, was observed by both Optical Metallographic (OMM) and Scanning Electron (SEM) Microscopy. The outcome analysis of this study provides an additional contribution to the contemporary discussion on the formation of combined AAO/CeCC coating primers.