Controlled microwave-assisted and pH-affected growth of ZnO structures and their photocatalytic performance

Unraveling the impact of microwave-assisted techniques in the fabrication of yttrium-doped TiO2 photocatalyst

In this study, we investigate the role of microwave technology in the fabrication of yttrium-doped TiO2 through a comparative analysis of hydrothermal techniques. Microwave-assisted hydrothermal synthesis offers advantages, but a comprehensive comparison between microwave-assisted and conventional methods is lacking. Therefore, in our investigation, we systematically evaluate and compare the morphological, structural, and optical properties of yttrium-doped TiO2 samples synthesized using both techniques. The X-ray diffraction (XRD) patterns confirm the anatase tetragonal structure of the synthesized TiO2-Y systems, while the larger ion radius of yttrium (Y3+) compared to titanium (Ti4+) presents challenges for yttrium to incorporate into the TiO2 lattice. The X-ray Photoelectron Spectroscopy (XPS) revealed a significant difference in the atomic content of yttrium between the TiO2-Y systems synthesized using microwave-assisted and conventional methods. This finding suggests that the rapid microwave method is more effective in successfully doping TiO2 with rare earth metals such as yttrium. The photo-oxidation of carbamazepine (CBZ) using TiO2-Y systems demonstrated high efficiency under UV-LED light. Microwave-synthesized TiO2-Y demonstrates improved photo-oxidation efficiency of CBZ, attributed to enhanced absorption, charge transfer, surface area, and crystallite size. Overall, the microwave-synthesized TiO2-Y systems showed promising performance for the photo-oxidation of CBZ, with improved efficiency compared to conventional synthesis methods.

Insight into (Electro)magnetic Interactions within Facet-Engineered BaFe12O19/TiO2 Magnetic Photocatalysts

A series of facet-engineered TiO2/BaFe12O19 composites were synthesized through hydrothermal growth of both phases and subsequent deposition of the different, faceted TiO2 nanoparticles onto BaFe12O19 microplates. The well-defined geometry of the composite and uniaxial magnetic anisotropy of the ferrite allowed alternate interfaces between both phases and fixed the orientation between the TiO2 crystal structure and the remanent magnetic field within BaFe12O19. The morphology and crystal structure of the composites were confirmed by a combination of scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses together with the detailed study of BaFe12O19 electronic and magnetic properties. The photocatalytic activity and magnetic field effect were studied in the reaction of phenol degradation for TiO2/BaFe12O19 and composites of BaFe12O19 covered with a SiO2 protective layer and TiO2. The observed differences in phenol degradation are associated with electron transfer and the contribution of the magnetic field. All obtained magnetic composite materials can be easily separated in an external magnetic field, with efficiencies exceeding 95%, and recycled without significant loss of photocatalytic activity. The highest activity was observed for the composite of BaFe12O19 with TiO2 exposing {1 0 1} facets. However, to prevent electron transfer within the composite structure, this photocatalyst material was additionally coated with a protective SiO2 layer. Furthermore, TiO2 exposing {1 0 0} facets exhibited significant synergy with the BaFe12O19 magnetic field, leading to 2 times higher photocatalytic activity when ferrite was magnetized before the process. The photoluminescence emission study suggests that for this particular combination, the built-in magnetic field of the ferrite suppressed the recombination of the photogenerated charge carriers. Ultimately, possible effects of complex electro/magnetic interactions within the magnetic photocatalyst are shown and discussed for the first time, including the anisotropic properties of both phases.

Comparative study of TiO2-Fe3O4 photocatalysts synthesized by conventional and microwave methods for metronidazole removal

This study focused on a direct comparison of conventional hydrothermal and microwave treatment during the synthesis of TiO₂-Fe₃O₄ photocatalyst, which is an effective catalyst for decomposing metronidazole. The photocatalyst underwent various characterization analyses, including X-ray diffraction, Raman spectroscopy, transmission electron microscopy, energy dispersive X-ray, and diffuse reflectance spectroscopy. The Raman spectroscopy analysis revealed that the materials obtained through the conventional hydrothermal treatment consisted of separate phases of anatase and magnetite. On the other hand, the materials synthesized using the microwave process showed a noticeable shift in the E_g band (143 cm^-1) and its half-width towards higher wavenumbers. This shift is likely due to the introduction of Fe ions into the TiO₂ lattice. Additionally, both conventional hydrothermal and microwave synthesis routes produced TiO₂-Fe₃O₄ systems with superparamagnetic properties, as demonstrated by SQUID magnetic measurements. The TEM analysis revealed that the materials synthesized using the microwave process exhibited higher homogeneity, with no noticeable large aggregates observed. Finally, this work proposed a convenient LED photoreactor that effectively utilized the photo-oxidative properties of TiO₂-Fe₃O₄ photocatalysts to remove metronidazole. Combining photoactive TiO₂-Fe₃O₄ catalysts with an energy-efficient LED reactor resulted in a low electrical energy per order (E_EO).

Hydrothermal Synthesis of ZnO Superstructures with Controlled Morphology via Temperature and pH Optimization

Zinc oxide, as a widely used material in optics, electronics, and medicine, requires a complete overview of different conditions for facile and easily reproducible syntheses. Two types of optimization of ZnO hydrothermal preparation from zinc acetate and sodium hydroxide solution are presented, which allowed for obtaining miscellaneous morphologies of materials. The first was a temperature-controlled synthesis from 100 to 200 °C, using citric acid as a capping agent. The formation of hexagonal rods at the lowest temperature was evidenced, which agglomerated to flower-like structures at 110 and 120 °C. It was followed by transformation to flake-like roses at 160 °C, up to disordered structures composed of nanosized plates (>180 °C). The transformations were generated through a temperature change, which had an impact on the diffusion effect of hydroxide and citrate complexes. The second optimization was the hydrothermal synthesis free of organic additives and it included only a pH variation from 7.5 to 13.5. It was found that by utilizing a slow-dropping process and varying amounts of NaOH solutions, it is possible to obtain well-formed hexagonal pellets at pH 8.0-8.5. Strongly basic conditions of pH 11.0 and 13.5 impeded superstructure formations, giving small elongated particles of ZnO. All samples were characterized by high phase purity and crystallinity, with a specific surface area of 18-37 m²/g, whereas particle size distribution indicated a predominance of small particles (<1 μm).

Synergistic effect of modified pore and heterojunction of MOF-derived α-Fe2O3/ZnO for superior photocatalytic degradation of methylene blue

Mesoporous heterojunction MOF-derived α-Fe₂O₃/ZnO composites were prepared by a simple calcination of α-Fe₂O₃/ZIF-8 as a sacrificial template. The optical properties confirm that coupling of both the modified pore and the n-n heterojunction effectively reduces the possibility of photoinduced charge carrier recombination under irradiation. The mesoporous Fe(25)ZnO with 25% loading of α-Fe₂O₃ exhibited the best performance in MB degradation, up to ∼100% after 150 minutes irradiation, higher than that of pristine ZnO and α-Fe₂O₃. Furthermore, after three cycles reusability, mesoporous Fe(25)ZnO still showed an excellent stability performance of up to 95.42% for degradation of MB. The proposed photocatalytic mechanism of mesoporous Fe(25)ZnO for the degradation of MB corresponds to the n-n heterojunction system. This study provides a valuable reference for preparing mesoporous MOF-derived metal oxides with an n-n heterojunction system to enhance MB photodegradation.

Effect of Nitrogen Doping in GO as Support in ZnO/GO-N Compounds and Their Photocatalytic Assessment to Degrade the Lignin Molecule

Control of the recombination process and improvement of transport charge carriers could be achieved in photocatalysts by modifying the catalytic support. In the present study, our goal was to study the effect of nitrogen doping on graphene oxide sheets using doping sources such as urea, thiourea, or ethylenediamine to produce GO-N catalytic supports which were used to form ZnO/GO-N systems. The synthesis of ZnO and GO-N was carried out through a hydrothermal process under microwave heating. The ZnO/GO-N compounds were tested to study the degradation of the lignin molecule under UV irradiation. A set of characterization techniques were used to study the ZnO/GO-N compounds, including XPS analyses which confirmed the N-doping in the samples. The ZnO compound reached 40% of lignin degradation in 70 min, while the ZnO/GO-N compound produced 79% of lignin degradation, also in 70 min evidencing the positive effect of the GO-N support. The best results of degradation were obtained when thiourea was used as the N-doping media.

Insight into the LED-assisted deposition of platinum nanoparticles on the titania surface: understanding the effect of LEDs

This paper proposes a novel LED-assisted deposition of platinum nanoparticles on the titania surface. For the first time, this process was supported by a UV-LED solution. We used two light sources with different wavelengths (λ_max = 365 and 395 nm), and power (P = 1, 5, and 10 W) because the photodeposition process based on LEDs has not been defined. The TiO₂-Pt material was discovered to be nano-crystalline anatase particles with nano-platinum particles deposited on the surface of titanium dioxide. Furthermore, the luminescence intensity decreased when Pt was added to TiO₂, indicating that charge carrier recombination was reduced. The spectra matching of the photocatalyst and LED reactor was performed for the first time in this work. We proposed a convenient LED reactor that focused light in the range of 350-450 nm, allowing us to effectively use photo-oxidative properties of TiO₂-Pt materials in the process of removing 4-chlorophenol. In the presented work, the LED light source plays a dual role. They first induce the platinum photodeposition process, before becoming an important component of tailored photoreactors, which is an important innovative aspect of this research.

A Study on Doping and Compound of Zinc Oxide Photocatalysts

As an excellent semiconductor photocatalyst, zinc oxide is widely used in the field of photocatalysis and is regarded as one of the most reliable materials to solve environmental problems. However, because its band gap energy limits the absorption of visible light and reduces the efficiency of catalytic degradation, it needs to be doped with other substances or compounded with other substances and precious metal. This paper summarizes the research on this aspect at home and abroad in recent years, introduces the doping of transition metal ions by zinc oxide, the compounding of zinc oxide with precious metals or other semiconductors, and the prospect of further improving the catalytic efficiency of zno photocatalyst is also put forward.

The In Situ Hydrothermal and Microwave Syntheses of Zinc Oxides for Functional Cement Composites

This study presents the results of research on cement mortars amended with two zinc oxides obtained by two different methods: hydrothermal ZnO-H and microwave ZnO-M. Our work indicates that, in contrast to spherical ZnO-H, ZnO-M was characterized by a columnar particle habit with a BET surface area of 8 m²/g, which was four times higher than that obtained for hydrothermally obtained zinc oxide. In addition, ZnO-M induced much better antimicrobial resistance, which was also reported in cement mortar with this oxide. Both zinc oxides showed very good photocatalytic properties, as demonstrated by the 4-chlorophenol degradation test. The reaction efficiency was high, reaching the level of 90%. However, zinc oxides significantly delayed the cement binder setting: ZnO-H by 430 min and ZnO-M by 380 min. This in turn affected the increments in compressive strength of the produced mortars. No significant change in compressive strength was observed on the first day of setting, while significant changes in the strengths of mortars with both zinc oxides were observed later after 7 and 28 days of hardening. As of these times, the compressive strengths were about 13-15.5% and 12-13% higher than the corresponding values for the reference mortar, respectively, for ZnO-H and ZnO-M. There were no significant changes in plasticity and flexural strength of mortars amended with both zinc oxides.

Design and Microwave-Assisted Synthesis of TiO2-Lanthanides Systems and Evaluation of Photocatalytic Activity under UV-LED Light Irradiation

The TiO2-Eu and TiO2-La systems were successfully synthesized using the microwave method. Based on the results of X-ray diffraction analysis, it was found that regardless of the analyzed systems, two crystal structures were noted for the obtained samples: anatase and rutile. The analysis, such as XPS and EDS, proved that the doped lanthanum and europium nano-particles are present only on the TiO2 surface without disturbing the crystal lattice. In the synthesized systems, there were no significant changes in the bandgap energy. Moreover, all the obtained systems were characterized by high thermal stability. One of the key objectives of the work, and a scientific novelty, was the introduction of UV-LED lamps into the metronidazole photo-oxidation pathway. The results of the photo-oxidation study showed that the obtained TiO2 systems doped with selected lanthanides (Eu or La) show high efficiency in the removal of metronidazole, and at the same consuming nearly 10 times less electricity compared to conventional UV lamps (high-pressure mercury lamp). Liquid-chromatography mass-spectrometry (LC-MS) analysis of an intermediate solution showed the presence of fragments of the degraded molecule by m/z 114, 83, and 60, prompting the formulation of a plausible photodegradation pathway for metronidazole.

Application of Spinel and Hexagonal Ferrites in Heterogeneous Photocatalysis

Semiconducting materials display unique features that enable their use in a variety of applications, including self-cleaning surfaces, water purification systems, hydrogen generation, solar energy conversion, etc. However, one of the major issues is separation of the used materials from the process suspension. Therefore, chemical compounds with magnetic properties have been proposed as crucial components of photocatalytic composites, facilitating separation and recovery of photocatalysts under magnetic field conditions. This review paper presents the current state of knowledge on the application of spinel and hexagonal ferrites in heterogeneous photocatalysis. The first part focuses on the characterization of magnetic (nano)particles. The next section presents the literature findings on the single-phase magnetic photocatalyst. Finally, the current state of scientific knowledge on the wide variety of magnetic-photocatalytic composites is presented. A key aim of this review is to indicate that spinel and hexagonal ferrites are considered as an important element of heterogeneous photocatalytic systems and are responsible for the effective recycling of the photocatalytic materials.

The TiO2-ZnO Systems with Multifunctional Applications in Photoactive Processes-Efficient Photocatalyst under UV-LED Light and Electrode Materials in DSSCs

The main goal of the study was the hydrothermal-assisted synthesis of TiO₂-ZnO systems and their subsequent use in photoactive processes. Additionally, an important objective was to propose a method for synthesizing TiO₂-ZnO systems enabling the control of crystallinity and morphology through epitaxial growth of ZnO nanowires. Based on the results of X-ray diffraction analysis, in the case of materials containing a small addition of ZnO (≥5 wt.%), no crystalline phase of wurtzite was observed, proving that a high amount of modified titanium dioxide can inhibit the crystallization of ZnO. The transmission electron microscopy (TEM) results confirmed the formation of ZnO nanowires for systems containing ≥ 5% ZnO. Moreover, for the synthesized systems, there were no significant changes in the band gap energy. One of the primary purposes of this study was to test the TiO₂-ZnO system in the photodegradation process of 4-chlorophenol using low-power UV-LED lamps. The results of photo-oxidation studies showed that the obtained binary systems exhibit good photodegradation and mineralization efficiency. Additionally, it was also pointed out that the dye-sensitized solar cells can be a second application for the synthesized TiO₂-ZnO binary systems.