Erzsébet Szabó-Bárdos

The goal of this work was to prepare modified titanium dioxide catalysts applicable for self-cleaning and disinfecting surfaces, possessing both antibacterial and photocatalytic activity in the visible-light region, via green and affordable synthesis. For this purpose, silverization was chosen due to its antibacterial and electron-capturing effects, and to achieve efficient visible-light excitation, urea was used as a precursor for nitrogen doping. Mechanochemical activation with grinding, as an environmentally friendly process, was applied for the catalyst modification under various conditions, such as the amounts of the modifying substances, the milling time, the ratio of the weights of the material to be ground, and the grinding balls. The photocatalytic activity in the UV and visible range was tested in suspensions with oxalic acid and coumarin as model compounds. The antibacterial effect was measured by the bioluminescence of Vibrio fischeri bacteria. The highest photocatalytic activity in the visible range was observed with the nitrogen-doped titanium dioxide (N-TiO₂) prepared with 10% urea. Silveration of N-TiO₂ (up to 0.2%) decreased photocatalytic activity while improving the antibacterial efficiency. To maximize both effects, mechanical mixtures of the separately modified catalysts (N-TiO₂ and Ag-TiO₂) were also examined in different ratios. The 1:1 mixture provided the optimum combination.

Graphitic carbon nitride (g-C₃N₄) proved to be a promising semiconductor for the photocatalytic degradation of various organic pollutants. However, its efficacy is limited by a fast electron hole recombination, a restricted quantity of active sites, and a modest absorption in the visible range. To overcome these limitations, g-C₃N₄-Bi₂S₃ and g-C₃N₄-ZnS composites were effectively produced utilizing a starch-assisted technique. The findings from FT-IR, XRD, EDX, XPS, BET, SEM, and TEM demonstrated that the enhanced photocatalytic activity of g-C₃N₄-Bi₂S₃ and g-C₃N₄-ZnS composites was primarily due to their improved photocarrier separation and transfer rates. The photocatalyst facilitated the aerobic photocatalytic degradation of colorless contaminants such as coumarin and para-nitrophenol (4-NP). For the decomposition of 4-NP, g-C₃N₄-Bi₂S₃ exhibited a maximum efficiency of 90.86% in UV light and 16.78% in visible light, with rate constants of 0.29 h⁻¹ and 0.016 h⁻¹, respectively. In contrast, g-C₃N₄-ZnS demonstrated a maximum efficiency of 100% in UV light and 15.1% in visible light, with rate constants of 0.57 h⁻¹ and 0.018 h⁻¹, respectively. The bioinspired synthesis combined with the modification with metal sulfides proved to considerably enhance the photocatalytic activity of g-C₃N₄, increasing its potential for practical applicability in environmentally friendly water treatment systems for the efficient removal of recalcitrant organic contaminants.

Penconazole (C₁₂H₁₅Cl₂N₃) is widely used to prevent fungal infection of fruits. Since this toxic fungicide is recalcitrant to biological degradation, it has harmful impacts on aquatic ecosystems. TiO₂-based heterogeneous photocatalysis proved to be an efficient method for its mineralization. To monitor the processes occurring under the influence of illumination, the light absorbance, the pH, and the TOC of the samples were measured. The concentration of the model compound and the degradation products were determined by HPLC and IC. Penconazole did not decompose under UV light (λ_max = 371 nm) without a catalyst. In the presence of TiO₂, mineralization took place. The initial degradation rate in air (7.7 × 10⁻⁴ mM s⁻¹) was 5 times higher than under argon. The formation rate of hydrochloric acid (1.04 × 10⁻³ mM s⁻¹) in the former case significantly contributed to the acidification of the liquid phase. NH₄⁺ also formed, at the rate of 5.9 × 10⁻⁴ mM s⁻¹, and very slightly transformed to NO₃⁻. Due to the intermediates identified by HPLC-MS, hydroxylation, H abstraction, and Cl elimination are involved in the degradation mechanism, in which photogenerated HO^● radicals, conduction-band electrons, and (under air) superoxide radical anions (O₂^●−) play considerable roles. The intermediates proved to be much less toxic than penconazole.

Graphitic carbon nitride (g-C₃N₄) has proved to be a promising heterogeneous photocatalyst in the visible range. It can be used, among others, for the oxidative conversion of environmentally harmful nitrophenols occurring in wastewater. However, its photocatalytic activity needs to be enhanced, which can be achieved by modification with various dopants. In our work, copper-modified g-C₃N₄ was prepared by ultrasonic impregnation of the pristine g-C₃N₄ synthesized from thiourea. The morphology, microstructure, and optical properties of the photocatalysts were characterized by XRD, FT-IR, DRS, SEM, XPS, and TEM. DRS analysis indicated a slight change in both the CB and the VB energies of Cu/g-C₃N₄ compared to those of g-C₃N₄. The efficiency of the photocatalysts prepared was tested by the degradation of nitrophenols. Copper modification caused a sevenfold increase in the rate of 4-nitrophenol degradation in the presence of H₂O₂ at pH = 3. This dramatic enhancement can be attributed to the synergistic effect of copper and H₂O₂ in this photocatalytic system. A minor Fenton reaction role was also detected. The reusability of the Cu/g-C₃N₄ catalyst was demonstrated through five cycles. Copper-modified g-C₃N₄ with H₂O₂ proved to be applicable for efficient visible-light-driven photocatalytic oxidative degradation of nitrophenols.

Titanium-dioxide-based semiconductors proved to be appropriate for photocatalytic application to efficiently degrade emerging organic pollutants such as various herbicides, pesticides, and pharmaceuticals in waters of environmental importance. The characterization of various TiO₂ catalysts, both bare and modified (Ag- and/or N-doped), by mechanochemical treatment was carried out in this work, regarding their structure, morphology, and photocatalytic activity. For the latter investigations, carbamazepine, an antidepressant, proved to be applicable and versatile. The photocatalytic behavior of the catalysts was studied under both UV and visible light. Besides the decomposition efficiency, monitoring the intermediates provided information on the degradation mechanisms. Mechanochemical treatment significantly increased the particle size (from 30 nm to 10 μm), causing a considerable (0.14 eV) decrease in the band gap. Depending on the irradiation wavelength and the catalyst, the activity orders differed, indicating that, in the mineralization processes of carbamazepine, the importance of the different oxidizing radicals considerably deviated, e.g., Ag-TiO₂ < DP25-TiO₂ < ground-DP25-TiO₂ < N-TiO₂ ≈ N-Ag-TiO₂ for O₂^•− and N-TiO₂ ≈ Ag-TiO₂ < N-Ag-TiO₂ < ground-DP25-TiO₂ ≈ DP25-TiO₂ for HO^• generation under UV irradiation. Toxicity studies have shown that the resulting intermediates are more toxic than the starting drug molecule, so full mineralization is required. This could be realized by a synergistic combination of heterogeneous photocatalysis and ozonation.

The synthesis and structural characterization of synthetic zinc oxide and halloysite-based zinc oxide nanocomposites (with 2–28 m/m% ZnO content) are presented. The chemical precipitation of zinc hydroxide precursors and its subsequent drying at 80 °C yielded dominantly zinc oxide (zincite). Thermal treatment at 350 °C completely transformed the remaining precursor to ZnO without causing structural dehydroxylation of the halloysite support. The procedure yielded zinc oxide nanoparticles with 10–22 nm average size having quasi-spherical scale-like morphology. The specific surface area of the synthetic zinc oxide was found to be low (13 m²/g), which was significantly enhanced after nanocomposite preparation (27–47 m²/g). The photocatalytic activity of the prepared nanocomposites was probed by the degradation of a phenolic compound (4-nitrophenol) upon UV irradiation in liquid phase. Compared to their individual constituents, an increased activity of the nanocomposites was observed, while the SSA-normalized photocatalytic activity revealed a synergic effect in nanocomposites above 9 m/m% ZnO content. The nanocomposites were found to be stable at pH = 5.6, with a minor and major mobilization of zinc ions at pH = 12.4 and pH = 1.9, respectively. The toxicity of leachates in different pH environments by Vibrio fischeri bioluminescence indicated low toxicity for ZnO nanoparticles and insignificant toxicity for the nanocomposites. The enhanced photocatalytic activity together with the lower toxicity of the halloysite-ZnO nanocomposites highlight their application potential in water treatment.

Catalysts for visible-light-driven oxidative cleaning processes and antibacterial applications (also in the dark) were developed. In order to extend the photoactivity of titanium dioxide into the visible region, nitrogen-doped TiO₂ catalysts with hollow and non-hollow structures were synthesized by co-precipitation (NT-A) and sol–gel (NT-U) methods, respectively. To increase their photocatalytic and antibacterial efficiencies, various amounts of silver were successfully loaded on the surfaces of these catalysts by using a facile photo-deposition technique. Their physical and chemical properties were evaluated by using scanning electron microscopy (SEM), transmission electron microscopy–energy dispersive X-ray spectroscopy (TEM–EDS), Brunauer–Emmett–Teller (BET) surface area, X-ray diffraction (XRD), and diffuse reflectance spectra (DRS). The photocatalytic performances of the synthesized catalysts were examined in coumarin and 1,4-hydroquinone solutions. The results showed that the hollow structure of NT-A played an important role in obtaining high specific surface area and appreciable photoactivity. In addition, Ag-loading on the surface of non-hollow structured NT-U could double the photocatalytic performance with an optimum Ag concentration of 10⁻⁶ mol g⁻¹, while a slight but monotonous decrease was caused in this respect for the hollow surface of NTA upon increasing Ag concentration. Comparing the catalysts with different structures regarding the photocatalytic performance, silverized non-hollow NT-U proved competitive with the hollow NT-A catalyst without Ag-loading for efficient visible-light-driven photocatalytic oxidative degradations. The former one, due to the silver nanoparticles on the catalyst surface, displayed an appreciable antibacterial activity, which was comparable to that of a reference material practically applied for disinfection in polymer coatings.