ZIF-8 derived hollow CuO/ZnO material for study of enhanced photocatalytic performance

Micro-nanobubble-assisted As(III) removal from water by Ni-doped MOF materials

Micro-nanobubbles (MNBs) can form reactive oxygen species (ROS) with high oxidizing potential. In this study, nickel-doped metal-organic framework materials (MOFs) capable of activating molecular oxygen were synthesized using trivalent arsenic (As(III)) as a target pollutant and combined with peroxymonosulfate (PMS) to construct a MOF/MNB/PMS system. The results included the rapid oxidation of As(III), the successful absorption of oxidized As(V), and finally the efficient removal of As. The effects of pH, amount of PMS used, and preparation time of MNBs on the As removal performance of the MOF/MNB/PMS system were investigated experimentally. The changes in the properties of the materials before and after the reaction were analyzed by XPS, and it was found that the main active sites on the surface of the MOFs were the metal elements and the pyridine nitrogen near the carbon atom. The regular morphology and elemental composition of the MOFs were determined by TEM scanning and EDS test, which indicated the presence of nickel. XRD tests before and after the reaction showed that the MOFs were structurally stable. The results of the free radical burst experiments show that the single linear oxygen (1O2) is the main active substance in the system, and that the MNBs are key factors by which the system achieves efficient oxidation performance. In addition to providing a sustainable supply of molecular oxygen to the MOFs during the reaction process, coupling the MNBs with PMS was found to improve the oxidation capacity of the system. The results of this study thus provide a new concept for As removal and advanced oxidation in water bodies.

Visible-light photooxidation of ciprofloxacin utilizing metal oxide incorporated sol-gel processed La-doped NaTaO3 nanoparticles: A comparative study

The supper dissemination of antibiotic waste in water resources has exponentially progressed the vital water and soil pollution that affect human health and the environment. Consequently, there have been several types of research anticipated for the green mineralization of such pollutants. Herein, we intended a surfactant-aided sol-gel formation of lanthanum-doped sodium tantalate (LNTO) nanocrystals. The synthesized 13 nm averaged-size perovskite LNTO nanocrystals were responsive to visible-light irradiation by incorporation of 4.4-5.2 nm oxide nanoparticles, namely Bi₂O₃, CdO, Fe₂O₃, and CuO at 4.0 wt% through coprecipitation. The formed nanomaterials unveiled mesostructured surface textures with specific surface areas of 199-229 m² g^-1. The obtained nanoceramics were employed for the mineralization of 10 ppm of ciprofloxacin antibiotic (CPF) as an emerging antibiotic waste in water under visible light irradiation. The CuO-incorporated LNTO exhibited the best photocatalytic oxidation of CPF after 120 min compared with other oxides with an excellent photoreaction rate of 0.0343 min^-1 which is 49 times higher than the pure LNTO. The 2.0 gL^-1 CuO/LNTO-dose achieved the full photooxidation of CPF at an oxidation speed of 0.0738 min ^-1 within just 1.0 h of visible light irradiation and magnificent regeneration ability. This enhanced activity of CuO/LNTO is regarded as significant light absorption and a bandgap energy reduction to 2.12 eV. Besides that, the heterojunction between CuO and LNTO amended the photogenerated carrier mobility and separation as concluded from the photoluminescence and photocurrent exploration. This comparative work suggests the proper design of low bandgap oxide decoration of solution-based perovskite oxide photocatalysts for promoting the visible-light mineralization of antibiotics in water.

Green Synthesis of a CuO-ZnO Nanocomposite for Efficient Photodegradation of Methylene Blue and Reduction of 4-Nitrophenol

CuO-ZnO nanocomposites (NCs) were synthesized using an aqueous extract of Verbascum sinaiticum Benth. (GH) plant. X-ray diffraction (XRD), spectroscopic, and microscopic methods were used to explore the crystallinity, optical properties, morphology, and other features of the CuO-ZnO samples. Furthermore, catalytic performances were investigated for methylene blue (MB) degradation and 4-nitrophenol (4-NP) reduction. According to the results, CuO-ZnO NCs with 20 wt % CuO showed enhanced photocatalytic activity against MB dye with a 0.017 min^-1 rate constant compared to 0.0027 min^-1 for ZnO nanoparticles (NPs). Similarly, a ratio constant of 5.925 min^-1 g^-1 4-NP reductions was achieved with CuO-ZnO NCs. The results signified enhanced performance of CuO-ZnO NCs relative to ZnO NPs. The enhancement could be due to the synergy between ZnO and CuO, resulting in improved absorption of visible light and reduced electron-hole (e^-/h⁺) recombination rate. In addition, variations in the CuO content affected the performance of the CuO-ZnO NCs. Thus, the CuO-ZnO NCs prepared using V. sinaiticum Benth. extract could make the material a desirable catalyst for the elimination of organic pollutants.

Transformation of metal-organic frameworks with retained networks

Metal-organic frameworks (MOFs) are a class of crystalline porous coordination materials with systematically designable network structures and tunable properties, demonstrating great potential for applications in diverse fields. However, the generally poor stability of dynamic coordination bonds in MOFs hinders their practical applications in harsh environments. Although MOFs have been used as precursors and templates for the production of various derivatives with enhanced stability via thermal treatment, the extreme thermolytic conditions often destroy the network structures, consequently resulting in obvious decreases in porosity and surface areas with undesired characteristics. This feature article discusses the generally used pathways for the transformation of MOFs and the advanced fabrication methods for the production of various MOF-derived materials. We particularly emphasize the recent progress in the designed strategies for customization and derivation tailoring of MOFs, which could produce MOF-derived functional materials with remaining framework skeletons and inherited characteristics (surface area, porosity and properties) of the parent MOFs, exhibiting great promise for practical applications.

Application of natural minerals in photocatalytic degradation of organic pollutants: A review

Photocatalysis is an effective, inexpensive and environmentally friendly technology for the decomposition of various aqueous organic pollutants and plays an increasingly critical role in the degradation of pollutants. Natural minerals are abundant natural resources on Earth and can be obtained directly from nature. Natural minerals are excellent photocatalyst carriers that are environmentally friendly, low in price, and will not cause secondary pollution to the environment. Natural minerals have the characteristics of a large specific surface area, providing more active centres, and adsorbing pollutants to concentrate catalysis. Natural minerals are also excellent photocatalysts, such as haematite and magnetite, which play a very good role in the degradation of water pollutants. Studies that make full use of natural minerals are of great significance. This review covers the latest research on natural minerals as photocatalytic composite materials to degrade organic pollutants in water, including three parts: the classification of natural minerals, the structural description of natural mineral composites, and the photocatalytic degradation of organic pollutants by natural mineral composites. In addition, the current limitations and opinions of natural mineral composites are discussed to achieve better results in applying natural minerals.

Current advancements on the fabrication, modification, and industrial application of zinc oxide as photocatalyst in the removal of organic and inorganic contaminants in aquatic systems

Industrial wastewaters contain hazardous contaminants that pollute the environment and cause socioeconomic problems, thus demanding the employment of effective remediation procedures such as photocatalysis. Zinc oxide (ZnO) nanomaterials have emerged to be a promising photocatalyst for the removal of pollutants in wastewater owing to their excellent and attractive characteristics. The dynamic tunable features of ZnO allow a wide range of functionalization for enhanced photocatalytic efficiency. The current review summarizes the recent advances in the fabrication, modification, and industrial application of ZnO photocatalyst based on the analysis of the latest studies, including the following aspects: (1) overview on the properties, structures, and features of ZnO, (2) employment of dopants, heterojunction, and immobilization techniques for improved photodegradation performance, (3) applicability of suspended and immobilized photocatalytic systems, (4) application of ZnO hybrids for the removal of various types of hazardous pollutants from different wastewater sources in industries, and (5) potential of bio-inspired ZnO hybrid nanomaterials for photocatalytic applications using renewable and biodegradable resources for greener photocatalytic technologies. In addition, the knowledge gap in this field of work is also highlighted.

Polylactic acid/polyvinyl alcohol-quaternary ammonium chitosan double-layer films doped with novel antimicrobial agent CuO@ZIF-8 NPs for fruit preservation

ZIF-8, a subclass of metal organic frameworks (MOFs), was employed as the CuO carriers because of its high surface areas and good dispersibility. A novel antibacterial agent CuO@ZIF-8 was synthesized by environmentally-friendly direct calcination strategy, and introduced into the composite double-layer films for packing materials. The double-layer films were prepared via solution casting method with polylactic acid (PLA) and polyvinyl alcohol (PVA)-quaternary ammonium chitosan as the matrix of outer layer and inner layer, respectively; and CuO@ZIF-8 nanoparticles were introduced into the PVA-quaternary ammonium chitosan layer. The double-layer films exhibited superior antibacterial activity resulted from the uniform dispersion of CuO by ZIF-8 carriers. The elongation at break was enhanced and up to 17.13%, about 2.4-fold that of PLA films. Meanwhile, the films provided low water vapor permeability and strong UV-barrier ability which were attributed to the lay-by-layer casting, CuO@ZIF-8 doping and TiO₂ addition. Cherry tomato preservation experiment revealed that the composite films retarded the growth of harmful microorganisms on the fruit surface. MTT assay confirmed the cytocompatibility of the films. The easily fabricated double-layer films presented potential possibility in the field of biodegradable food packaging.

Enhanced photocatalytic activities of a hierarchical ZnO/V2C MXene hybrid with a close coupling heterojunction for the degradation of methyl orange, phenol and methylene blue dye

A hierarchical ZnO/V2C MXene hybrid exhibited enhanced photocatalytic performance due to its close coupling heterojunction facilitating photo-generated carrier transfer.

Metal Exchange of ZIF-8 and ZIF-67 Nanoparticles with Fe(II) for Enhanced Photocatalytic Performance

Zeolitic imidazolate frameworks (ZIFs), such as ZIF-8 and ZIF-67, were found to be efficient catalysts. However, ZIFs are not used much in photocatalysis due to their low photocatalytic activity for most reactions. The photocatalytic activity can be improved by modifying the framework by exchanging the Zn(II) ions (ZIF-8) and Co(II) ions (ZIF-67) with a more photocatalytically active metal(II) ion to form an efficient bimetallic ZIF photocatalyst. Redox-active iron (Fe)-based materials are known to be highly potent photocatalysts. Thus, incorporating iron into ZIFs could significantly enhance their photocatalytic performance. In this study, we modified nanosized ZIF-8(Zn) and ZIF-67(Co) via metal (Fe²⁺) exchange to produce bimetallic frameworks that are photocatalytically more active than their parent ZIFs. Nanosized ZIF-8 and ZIF-67 were synthesized isothermally in either water or methanol under ambient conditions. From these, Fe-containing bimetallic ZIF-8 and ZIF-67 nanoparticles were synthesized via the metal exchange, and their performance on the photocatalytic degradation of dye was evaluated. The morphology and crystal structures of the pristine ZIF-8 and ZIF-67 nanoparticles were retained to a large extent during the iron exchange. Their Brunauer-Emmett-Teller (BET) surface areas decreased by less than 15% for nZIF-8 and less than 12% for nZIF-67. The binding energy values on X-ray photoelectron spectroscopy (XPS) confirmed the preservation of the oxidation state of Fe(II) during the exchange process. A remarkably higher catalytic activity was observed for the photocatalytic degradation of dye by the Fe-exchanged nZIF-8 and nZIF-67 compared to their parent ZIFs. This proved that the incorporation of Fe(II) centers into the ZIF framework enhanced the photocatalytic activity of the framework dramatically. In addition, these catalysts can be regenerated and reused without an appreciable loss in activity.

Emerging Hybrid Nanocomposite Photocatalysts for the Degradation of Antibiotics: Insights into Their Designs and Mechanisms

The raising occurrence of antibiotics in the global water bodies has received the emerging concern due to their potential threats of generating the antibiotic-resistive and genotoxic effects into humans and aquatic species. In this direction, the solar energy assisted photocatalytic technique offers a promising solution to address such emerging concern and paves ways for the complete degradation of antibiotics with the generation of less or non-toxic by-products. Particularly, the designing of hybrid photocatalyticcomposite materials has been found to show higher antibiotics degradation efficiencies. As the hybrid photocatalysts are found as the systems with ideal characteristic properties such as superior structural, surface and interfacial properties, they offer enhanced photoabsorbance, charge-separation, -transfer, redox properties, photostability and easy recovery. In this context, this review study presents an overview on the recent developments in the designing of various hybrid photocatalytic systems and their efficiency towards the degradation of various emerging antibiotic pharmaceutical contaminants in water environments.

Facile synthesis of Ag2O/ZnO/rGO heterojunction with enhanced photocatalytic activity under simulated solar light: Kinetics and mechanism

Ag₂O/ZnO/rGO heterojunction photocatalysts were synthesized via a rapid microwave hydrothermal method for photocatalytic degradation of bisphenol A (BPA) under simulated solar light. Ag doping efficiently decreased the bandgap of ZnO, and loading on rGO inhibited the recombination of photoinduced electron-hole pairs. The highest BPA removal rate (80%) was achieved with an Ag doping ratio of 5% and a GO loading ratio of 3 wt%. The enhanced photocatalytic performance was attributed to the narrower bandgap and the improved separation efficiency of electron-hole pairs. Moreover, the recycling experiments proved that Ag₂O/ZnO/rGO possessed excellent photostability. Hole (h⁺) and •OH played crucial roles in the photocatalytic system. The degradation pathway of BPA including hydroxylation and the cleavage of covalent bonds was proposed. The toxicity assessment of intermediates elucidated that most of intermediates were less toxic than BPA. The as-prepared Ag₂O/ZnO/rGO exhibited outstanding photostability and pH adaptability, having great potential to be applied to the degradation of emerging organic pollutants in wastewater.

ZIF-8 calcination derived Cu2O–ZnO* material for enhanced visible-light photocatalytic performance

The mechanism of TC degradation over Cu₂O–ZnO* rich in oxygen vacancies.

ZnO encapsulants: Design and new view

ZnO encapsulants with capsular configurations (e.g. a large inner cavity, sizeable pore, low density and high specific surface area) have attracted considerable attention as effective and promising candidates in various fields owing to the merits of ZnO (e.g. UV protection, photoelectric catalysis, gas sensitivity, antibacterial effect). However, the research on ZnO encapsulants has not yet reached the eruptive stage. This probably due to their high morphological flexibility and relatively low structural strength that is not easy to control during the preparation process. In this review, the principles of cavity-generating and pore-forming are firstly discussed in depth after going through the synthesis of hollow ZnO in the past ten years. Moreover, the regulation of cavity diameter and pore size of different synthetic strategies is investigated. Then, the research progress of ZnO encapsulants is debated in detail from the loading and release of functional materials and the corresponding characterization. Finally, some potential designs and new views on the future research and development of ZnO encapsulants are concluded.