HPW-Anchored UiO-66 Metal-Organic Framework: A Promising Photocatalyst Effective toward Tetracycline Hydrochloride Degradation and H2 Evolution via Z-Scheme Charge Dynamics

Insight into Fe-O-Bi electron migration channel in MIL-53(Fe)/Bi4O5I2 Z-scheme heterojunction for efficient photocatalytic decontamination

Building a heterojunction is a fascinating option to guarantee sufficient carrier separation and transfer efficiency, but the mechanism of charge migration at the heterojunction interface has not been thoroughly studied. Herein, MIL-53(Fe)/Bi4O5I2 photocatalyst with a Z-scheme heterojunction structure is constructed, which achieves efficient photocatalytic decontamination under solar light. Driven by the newly-built internal electric field (IEF), the formation of Fe-O-Bi electron migration channel allows for rapid separation and transfer of charge carriers at the heterojunction interface, confirmed by the material characterization and density functional theory (DFT) calculation. The narrower band gap and improved visible light response also contribute to the enhanced photocatalytic activity of composite materials. With levofloxacin as the target pollutant, the optimal MIL-53(Fe)/Bi4O5I2 achieves complete removal of pollutant within 150 min, the photocatalysis rate of which is ca. 4.4 and 26.0 times that of pure Bi4O5I2 and MIL-53(Fe), respectively. Simultaneously, the optimal composite material exhibits satisfactory photodegradation of seven fluoroquinolones, and the photocatalysis rates are as follows: lomefloxacin > ciprofloxacin > enrofloxacin > norfloxacin > pefloxacin > levofloxacin > marbofloxacin. DFT calculations reveal a positive relationship between degradation rate and Fukui index (ƒ0) of main carbon atoms in seven fluoroquinolones. This study sheds light on the existence of electron migration channels at Z-scheme heterojunction interface to ensure sufficient photoinduced carrier transfer, and reveals the influence of pollutant structure on photolysis rate.

In situ synthesis of porous metal-organic frameworks NH2-UiO-66 on tea stem biochar and application in odours adsorption

Multiple odour nuisance in livestock farming is a notorious problem that has a significant impact on the living environment of surrounding communities. Adsorbents based on metal-organic framework (MOF) materials show great promise for controlling odour pollution, as they offer a high specific surface area, a controllable structure and an abundance of active sites. However, the MOF formation process is prone to problems such as pore clogging or collapse and reduced porosity, which limits its further application. In this study, a series of odour adsorbents were prepared by in situ growth of NH2-UiO-66 on tea stem biochar (TSBC) using a hydrothermal method and named UiO (Zr)-TSBCx. The physical and chemical properties and composition of UiO (Zr)-TSBCx have been systematically characterized using SEM, TEM, XRD, FT-IR, N2 adsorption-desorption and XPS. The release of odours from the pig farm effluent was monitored using in-situ continuous Proton-Transfer-Reaction Mass Spectrometry (PTR-MS), and the obtained primary compositions were tested for further adsorption. In dynamic adsorption experiments focused on butyric acid, UiO (Zr)-TSBC2 showed a high adsorption capacity of 3.99 × 105 μg/g and exceptional structural stability. UiO (Zr)-TSBC2 showed variable adsorption efficiencies for different odorous gases, with the best performance for the removal of ammonia, toluene and butyric acid. It also demonstrated the ability to rapidly mitigate instantaneous high concentrations of hydrogen sulfide (H2S), methanethiol and toluene resulting from agitation. Additionally, based on the relationship between the adsorption amount and the structural characteristics of the adsorbent as well as the nature of the odours, a possible adsorption mechanism of UiO (Zr)-TSBC2 for a variety of odours released from pig farm effluent was proposed. This work demonstrates a novel approach to promote deodorization applications in livestock and poultry farming environments by the in-situ growth of NH2-UiO-66 on biochar prepared from tea stem.

A review on development and modification strategies of MOFs Z-scheme heterojunction for photocatalytic wastewater treatment, water splitting, and DFT calculations

Increasing water pollution and decreasing energy reserves have emerged as growing concerns for the environment. These pollution are due to the dangerous effects of numerous pollutants on humans and aquatic organisms, such as hydrocarbons, biphenyls, pesticides, dyes, pharmaceuticals, and metal ions. On the other hand, the need for a clean environment, finding alternatives to fossil and renewable fuels is very important. Hydrogen (H2) is regarded as a viable and promising substitute for fossil fuels, and a range of methodologies have been devised to generate this particular source of energy. Metal-organic frameworks (MOFs) are a new generation of nanoporous coordination polymers whose crystal structure is composed of the juxtaposition of organic and inorganic constituent units. Due to their flexible nature, regular structure, and high surface area, these materials have attracted much attention for removing various pollutants from water and wastewater, and water splitting. MOFs Z-scheme heterojunctions have been identified as an economical and eco-friendly method for eliminating pollutants from wastewater systems, and producing H2. Their low-cost synthesis and unique properties increase their application in various energy and environment fields. The heterojunctions possess diverse properties, such as exceptional surface area, making them ideal for degradation and separation. The development and formulation of Z-scheme heterojunctions photocatalytic systems using MOFs, which possess stable and potent redox capability, have emerged as a successful approach for addressing environmental pollution and energy shortages in recent times. Through the utilization of the benefits offered by MOFs Z-scheme heterojunctions photocatalysts, such as efficient separation and migration of charge carriers, extensive spectrum of light absorption, among other advantages, notable enhancements can be attained. This review encompasses the synthesis techniques, structure, and properties of MOFs Z-scheme heterojunctions, and their extensive use in treating various wastewaters, including dyes, pharmaceuticals, and heavy metals, and water splitting. Also, it provides an overview of the mechanisms, pathways, and various theoretical and practical aspects for MOFs Z-scheme heterojunctions. Finally, it thoroughly assesses existing challenges and suggests further research on the promising applications of MOFs Z-scheme in industrial-scale wastewater treatment.

CO2 Enrichment Boosts Highly Selective Infrared-Light-Driven CO2 Conversion to CH4 by UiO-66/Co9S8 Photocatalyst

Photocatalytic CO2 reduction to high-value chemicals is an attractive approach to mitigate climate change, but it remains a great challenge to produce a specific product selectively by IR light. Hence, UiO-66/Co9S8 composite is designed to couple the advantages of metallic photocatalysts and porous CO2 adsorbers for IR-light-driven CO2-to-CH4 conversion. The metallic nature of Co9S8 endows UiO-66/Co9S8 with exceptional IR light absorption, while UiO-66 dramatically enhances its local CO2 concentration, revealed by finite-element method simulations. As a result, Co9S8 or UiO-66 alone does not show observable IR-light photocatalytic activity, whereas UiO-66/Co9S8 exhibits exceptional activity. The CH4 evolution rate over UiO-66/Co9S8 reaches 25.7 µmol g-1 h-1 with ca.100% selectivity under IR light irradiation, outperforming most reported catalysts under similar reaction conditions. The X-ray absorption fine structure spectroscopy spectra verify the presence of two distinct Co sites and confirm the existence of metallic Co─Co bond in Co9S8. Energy diagrams analysis and transient absorption spectra manifest that CO2 reduction mainly occurs on Co9S8 for UiO-66/Co9S8, while density functional theory calculations demonstrate that high-electron-density Co1 sites are the key active sites, possessing lower energy barriers for further protonation of *CO, leading to the ultra-high selectivity toward CH4.

A Visible Light-Driven α-MnO2/UiO-66-NH2 S-Scheme Photocatalyst toward Ameliorated Oxy-TCH Degradation and H2 Evolution

Photocatalytic hydrogen production and pollutant degradation using a heterogeneous photocatalyst remains an alternative route for mitigating the impending pollution and energy crisis. Hence, the development of cost-effective and environmentally friendly semiconducting materials with high solar light captivation nature is imperative. To overcome this challenge, α-MnO2 nanorod (NR)-modified MOF UiO-66-NH2 (UNH) was prepared via a facile solvothermal method, which is efficient toward H2 evolution and oxy-tetracycline hydrochloride (O-TCH) degradation. The field-emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HR-TEM) results of the α-MnO2@UNH (MnU) hybrid reveals its nanorod embedded in MOF matrix, and the X-ray photoelectron spectroscopy (XPS) result confirms the interaction of UNH moiety with α-MnO2 NRs. Additionally, the outstanding separation of photogenerated excitons and the charge-transfer efficacy are further validated by photoluminescence (PL), time-resolved photoluminescence (TRPL), electrochemical impedance spectroscopy (EIS), and transient photocurrent analysis, which are the key causes for photoactivity augmentation in the MnU composites. The MnU-2 composite shows a superior O-TCH degradation efficiency of 93.23% and an excellent H2 production rate of about 410.6 μmol h-1 upon light irradiation. This study provides significant evidence in favor of the suggested mediator-free S-scheme-adapted charge migration path, and it effectively explains the enhanced exciton separation leading to extraordinary catalytic efficiency of the proposed composite.

Ti3C2@UiO-TCPP Schottky junction photoelectrochemical sensor for detecting alkaline phosphatase through the steric hindrance effect of phosphopeptide

Alkaline phosphatase (ALP) is a major biomarker for clinical diagnosis, but detection methods of ALP are limited in sensitivity and selectivity. In this paper, a novel method for ALP determination is proposed. A photoelectrochemical (PEC) sensor was prepared by growing UiO-tetratopic tetrakis (4-carbox-yphenyl) porphyrin (TCPP) in situ between layered Ti3C2 through a one-pot hydrothermal method. The obtained Schottky heterojunction photoelectric material Ti3C2@UiO-TCPP not only has a large light absorption range but also greatly improves the efficiency of photogenerated electron hole separation and thereby enhances sensitivity for PEC detection. The phosphate group on the phosphorylated polypeptide was utilized to form a Zr-O-P bond with the zirconium ion on UiO-66, and then photocurrent decreases due to the steric hindrance effect of phosphorylated polypeptides, that is, the hindrance of electron transfer between the photoelectric material and a solution. The specific interaction between ALP and phosphorylated polypeptides shears the bond between phosphate and zirconium ion on UiO-66 in the peptides then weakens the hindrance effect and increases the photocurrent, thus realizing ALP detection. The linear range of ALP is 0.03-10,000 U·L-1, and the detection limit is 0.012 U·L-1. The method is highly sensitive and selective, and has been applied in detection of ALP in serum samples.

Fabrication of UiO-66/GCN, a Hybrid Photocatalyst, for Effective Degradation of Ciprofloxacin, Toxicity Estimation, and Its Antibacterial Activity

Fabrication of a metal-organic framework-based photocatalyst has been gaining much interest due to its higher surface area and reasonable band gap, enhancing its photocatalytic activity. This study attempted a facile synthesis of the hybrid photocatalyst UiO-66 doped with graphitic carbon nitride (GCN) by a simple solvothermal method. This composite minimized the drawback related to photogenerated charge transfer and recombination and helped the absorption of visible light. The material was investigated by using various instrumental techniques. In this work, ciprofloxacin (CIP), a fluoroquinolone drug, was chosen as a target micropollutant, and a photodegradation experiment was carried out by using UiO-66, GCN, and UiO-66/GCN under a visible light source, which exhibited 81.85, 69.48, and 93.60% of degradation, respectively. Finally, liquid chromatography mass spectrometry analysis and theoretical computation were carried out to identify the CIP degradation mechanism, and T.E.S.T. software was used to investigate the toxicity of the intermediate products. Apart from photocatalytic activity, the prepared material was also tested for its antibacterial properties against Staphylococcus aureus and Escherichia coli.

Synthesis of cellulose cotton-based UiO-66 MOFs for the removal of rhodamine B and Pb(II) metal ions from contaminated wastewater

The presence of pollutants in drinking water has become a significant concern recently. Various substances, including activated carbon, membranes, biochar, etc., are used to remove these pollutants. In the present study, a new composite comprising cotton fabric and a mixture of Metal-Organic Frameworks (MOFs) was synthesized and used as an adsorbent for eliminating pollutants from wastewater. At first, the UiO-66 MOFs were prepared by a simple method of reacting Zirconium (IV) chloride (ZrCl4) and p-Phthalic acid (PTA) after successful preparation of UiO-66 then modified its surface with amino functional groups by reacting with APTES to obtain UiO-66-NH2. Moreover, the cellulose cotton fabric (CF) surface was modified with Polydopamine (PDA) and obtained CF@PDA. Further, with the help of EDC-HCl and NHS, the UiO-66-NH2 grafted on the surface of the CF@PDA and finally obtained CF@PDA/UiO-66-NH2. In addition, the adsorption study was performed toward RhB dye and Pb(II) metal ion pollutants. The maximum adsorption toward RhB dye was 68.5 mg/g, while toward Pb(II) metal ions was 65 mg/g. In addition, the kinetic study was also conducted and the result favoured the Pseudo-second order kinetic study. The adsorption isotherm was also studied and the Langmuir model was more fitted as compared with the Freundlich model. Moreover, the material has excellent regeneration and recycling ability after ten cycles. The significant adsorption ability, the novel combination of cotton and MOFs, and the recycling feature make our material CF@PDA/UiO-66-NH2 a promising potential absorbent material for wastewater treatment and even in other important areas of water research.

Sustainable production of Fe-doped MnO2 nanoparticles for accelerated tetracycline antibiotic detoxification

Manganese dioxide (MnO2) has been recognized as one of the natural systems' most active mineral oxidants. However, when it comes to catalytic oxidation of antibiotic applications, pure MnO2 falls short in delivering satisfactory performance. Hence, a set of Fe3+-doped porous MnO2 (0.02Fe-MnO2, 0.1Fe-MnO2, and 0.14Fe-MnO2) nanoparticles were synthesized here via a convenient and energy-efficient one-step reaction method. A series of experiments revealed that Fe-doping strategy enhances the properties of MnO2 host by suppressing the crystalline structure, increasing the amount of surface oxygen defects, and modifying the Mn3+/Mn4+ ratio. Specifically, the tetracycline (TC) removal efficiency of 0.14Fe-MnO2 reaches 92% without the need for any additional co-oxidant, representing a 20% improvement over pristine MnO2 nanoparticles. Moreover, this process shows a fast dynamic (achieving 70% of TC removal in just 5 min) and demonstrates pH-resistance, maintaining high TC removal efficiency (≥90%) over a wide pH range of 3.0-9.0. Mechanical studies reveal that the degradation of TC can be attributed to the oxidation by reactive oxygen radicals and Mn3+, with 1O2 being the primary radical involved in the reaction, accounting for 55% of TC removal. Importantly, cytotoxicity testing indicates that the biotoxicity of TC toward organisms can be effectively mitigated using 0.14Fe-MnO2 nanomaterial. This study presents a readily applicable candidate for economically and conveniently eliminating of environmental TC pollution, thereby reducing the threat posed by TC pollution to the ecosystem.

Synthesis of Dual p-n Heterojunction of Ni/Mn-MOF-74/CdS@Co3O4 Photocatalyst as a Photoassisted Fenton-like Catalyst for Removal of Tetracycline Hydrochloride

In this study, a novel composite material, Ni/Mn-MOF-74/CdS@Co3O4 was synthesized. This material consisted of a dual p-n heterojunction, which enabled efficient separation and transfer of charge carriers. Compared to a single p-n heterojunction, the presence of this dual heterojunction significantly enhanced the overall efficiency. The improved efficiency could be attributed to the unique properties of the constituent semiconductors. Co3O4 exhibited p-type semiconductor properties, while Ni/Mn-MOF-74 and CdS exhibited n-type semiconductor properties. By a combination of these materials to form a composite photocatalyst, a Z-type heterojunction was created at the interface of the p-n junction. This design established an internal electric field at both ends, effectively separating the photogenerated electrons and holes in each individual photocatalyst. As a result, the respective photocatalytic activities of the materials were maximized. To demonstrate the practical application of this composite material, it was utilized for the activation of peroxymonosulfate under visible light irradiation, with the aim of enhancing the photocatalytic degradation efficiency of tetracycline hydrochloride. The photocatalytic mechanism of Ni/Mn-MOF-74/CdS@Co3O4 in activating peroxymonosulfate and degrading tetracycline hydrochloride was investigated in detail. Furthermore, the toxicity of tetracycline hydrochloride and its intermediates was evaluated by using toxicity evaluation software.

Bisthiourea immobilized UiO-66-NH2 supported Fe2O3 nanoparticles to accelerate dual centers Fenton-like reaction

In this paper, an efficient catalyst UiO-66-BTU/Fe₂O₃ was synthesized by using bisthiourea modified zirconium-based metal organic framework (Zr-MOF). The UiO-66-BTU/Fe₂O₃ system features outstanding Fenton-like activity that is 22.84 times and 12.91 times larger than Fe₂O₃ and conventional UiO-66-NH₂/Fe₂O₃ system. It also exhibits good stability, broad pH range and recycle ability. Through comprehensive mechanistic investigations, we have ascribed the excellent catalytic performance of the UiO-66-BTU/Fe₂O₃ system to ¹O₂ and HO as the reactive intermediates, cause Zr centers can make complexation with Fe to form dual centers. Meanwhile, the CS on the bisthiourea can form Fe-S-C bonds with Fe₂O₃, reducing the redox potential of Fe(III)/Fe(II) and influencing the decomposing of H₂O₂, which indirectly regulate the interaction between Fe and Zr to accelerate electron transfer during the reaction. This work exhibits the design and understanding of the iron oxides incorporated in modified MOFs with excellent Fenton-like catalytic performance to remove phenoxy acid herbicides.

Metal-organic frameworks in photocatalytic Z-scheme heterojunctions: an emerging technology

There is an urgent need for cleaner production processes for chemicals. An efficient and promising alternative for such reactions is heterogeneous photocatalysis, which works on the principle of converting (visible) light, including solar energy, into chemical energy. To that end, properly designed semiconductor based photocatalysts are necessary to trigger the photocatalytic reactions. Many commonly used photocatalysts have too large bandgaps (3-3.4 eV) to use visible light and a too low surface area for efficient production. Metal-organic frameworks (MOFs) have emerged as an encouraging class of materials for photocatalytic applications due to their (i) large surface area and porosity that facilitate adsorption towards chemicals, (ii) tunable crystallinity and optical and electronic properties for efficient light absorption in the visible region, (iii) tunable composition and functionality that make them versatile photocatalysts for a wide range of reactions, and (iv) facile development of composites with other semiconductors to produce Z-scheme heterojunctions that can effectively suppress the recombination of photogenerated charges. Ongoing research has started focusing on the judicious construction of Z-scheme heterojunctions in MOFs, to mimic natural photosynthesis, such that the MOF photocatalysts have higher light harvesting capacity, spatially separated reductive and oxidative active sites, and well-preserved redox ability. This review provides a concise compilation of the recent progress in the development and applications of MOF-based Z-scheme photocatalysts, their advanced characterization, and future perspectives for further advancements.

Nanoscale Multimodal Analysis of Sensitive Nanomaterials by Monochromated STEM-EELS in Low-Dose and Cryogenic Conditions

Scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS) provides spatially resolved chemical information down to the atomic scale. However, studying radiation-sensitive specimens such as organic-inorganic composites remains extremely challenging. Here, we analyzed metal-organic framework nanoparticles (nanoMOFs) at low-dose (10 e^-/Ų) and liquid nitrogen temperatures, similar to cryo-TEM conditions usually employed for high-resolution imaging of biological specimens. Our results demonstrate that monochromated STEM-EELS enables damage-free analysis of nanoMOFs, providing in a single experiment, signatures of intact functional groups comparable with infrared, ultraviolet, and X-ray data, with an energy resolution down to 7 meV. The signals have been mapped at the nanoscale (<10 nm) for each of these energy spectral ranges, including the chemical features observed for high energy losses (X-ray range). By controlling beam irradiation and monitoring spectral changes, our work provides insights into the possible pathways of chemical reactions occurring under electron exposure. These results demonstrate the possibilities for characterizing at the nanoscale the chemistry of sensitive systems such as organic and biological materials.

Substituted Ti(IV) in Ce-UiO-66-NH2 Metal-Organic Frameworks Increases H2 and O2 Evolution under Visible Light

Metal-organic frameworks (MOFs) as photocatalysts have received increasing attention. In this work, a dual metal-substituted UiO-66-NH₂ (Ti/Ce-MOF) containing different Ti/Ce mole ratios (x = 0-2.46) has been prepared via post-synthetic exchange between Ce-UiO-66 and TiCl₄, followed by amination. The solid had a high surface area (828-937 m²/g) and a large pore volume (0.451-0.507 m³/g). Under visible light, Ti/Ce-MOF showed x-dependent activity for H₂O reduction and oxidation on a film electrode, respectively. However, such a change for H₂ evolution in a Na₂S/Na₂SO₃ aqueous solution was observed only after CdS loading. In combination with the photoluminescence and band parameters, we propose that the photoactivity of Ti/Ce-MOF for redox reaction is determined by its ability for electron transfer. Furthermore, there is an interfacial electron transfer from Ti/Ce-MOF to CdS and a hole transfer from CdS to Ti/Ce-MOF, respectively, significantly improving the efficiency of charge separation for redox reactions. This work offers a new insight that Ti substitution benefits the performance of Ce-based MOF.

Recognition and quantitative analysis for six phthalate esters (PAEs) through functionalized ZIF-67@Ag nanowires as surface-enhanced Raman scattering substrate

A novel surface enhanced Raman scattering (SERS) method was developed based on Ag nanowires embedded into functionalized metal-organic framework ZIF-67 (ZIF-67@Ag NWs) composite as substrate, which was applied for rapid recognition and sensitive detection of six PAEs. The Raman signals for PAEs detection were gained at ZIF-67@Ag NWs substrate mainly due to the "sharp tip effect" of rough Ag nanowires and excellent absorptive capacity of ZIF-67 to capture targeted molecules into the electromagnetic field. Different structural PAEs, including carbon chain lengths, isomers, and substituents, were evaluated for SERS performance and characteristic peaks under the optimal conditions. The SERS spectra proved that different PAEs exhibited some typically characteristic peaks in favor of recognizing and distinguishing them. The ZIF-67@Ag NWs as SERS substrate was successfully applied to detect six PAEs and exhibited wide linear ranges, low detection limit (LOD), excellent repeatability and stability (such as dibutyl phthalate DBP: linear range of 10^-2 ∼ 10^-12 mol/L, LOD 3 × 10^-13 mol/L). The ZIF-67@Ag NWs substrate by SERS was implemented to determine trace DBP in plastics with satisfactory recoveries of 82.5 % ∼ 108.3 %. The proposed ZIF-67@Ag NWs substrate may provide an effective and promising SERS platform for recognition and quantitative determination of different structural PAEs in environment.

Hydrolytically stable mixed ditopic linker based zirconium metal organic framework as a robust photocatalyst towards Tetracycline Hydrochloride degradation and hydrogen evolution

In the existing eco-crisis, designing and engineering an efficient as well as water stable photocatalyst for energy conversion and pollutant abatement remains crucial. In this regard, a mixed linker type zirconium metal organic framework (Zr-MOF) with terepthalic acid based ditopic linkers were utilized to design a single component photocatalyst through single step solvothermal method to utilize photons from visible light illumination towards hydrogen energy (H2) production and Tetracycline Hydrochloride (TCH) degradation. The one pot synthesized mixed linker based Zr-MOF displays visible light absorption through band gap tuning, superior exciton segregation and oxygen vacancy that cumulatively supports the enhancement in the photocatalytic output with respect to their pristine counterparts. Additionally, the X-ray photoelectron spectroscopy, optical and electrochemical studies strongly reinforces the above claims. The prepared mixed linker Zr-MOF showed superior photocatalytic H2 evolution performance of 247.88 µmol h-1 (apparent conversion efficiency; ACE = 1.9%) that is twice than its pristine Zr-MOFs. Moreover, in TCH degradation, the mixed linker MOF displays an enhanced efficacy of 91.8 % and adopts pseudo-first order type kinetics with a rate constant value of 0.032. Typically, the active species participating for the TCH photo-degradation follows the order of hydroxyl (OH.) < superoxide (O2.-) radicals. Consequently, the mixed linker Zr-MOF could be effectively used as a robust photocatalyst exhibiting boosted TCH degradation and H2 production.

Synthesis of Z-schemes 0D–3D heterojunction bi-functional photocatalyst with ZnInCuS alloyed QDs supported BiOI MF for H2O2 production and N2 fixation

Photocatalytic H2O2 and NH3 production on Zn–Cu–In–S QDs coupled with BiOI MFs via a Z-scheme charge transfer dynamic.

Metal-Organic Frameworks as Photocatalysts for Solar-Driven Overall Water Splitting

Metal-organic frameworks (MOFs) have been frequently used as photocatalysts for the hydrogen evolution reaction (HER) using sacrificial agents with UV-vis or visible light irradiation. The aim of the present review is to summarize the use of MOFs as solar-driven photocatalysts targeting to overcome the current efficiency limitations in overall water splitting (OWS). Initially, the fundamentals of the photocatalytic OWS under solar irradiation are presented. Then, the different strategies that can be implemented on MOFs to adapt them for solar photocatalysis for OWS are discussed in detail. Later, the most active MOFs reported until now for the solar-driven HER and/or oxygen evolution reaction (OER) are critically commented. These studies are taken as precedents for the discussion of the existing studies on the use of MOFs as photocatalysts for the OWS under visible or sunlight irradiation. The requirements to be met to use MOFs at large scale for the solar-driven OWS are also discussed. The last section of this review provides a summary of the current state of the field and comments on future prospects that could bring MOFs closer to commercial application.

A comprehensive review on water remediation using UiO-66 MOFs and their derivatives

Metal-organic frameworks (MOFs) are a versatile class of porous materials offering unprecedented scope for chemical and structural tunability. On account of their synthetic versatility, tunable and exceptional host-guest chemistry they are widely utilized in many prominent water remediation techniques. However, some of the MOFs present low structural stabilities specifically in aqueous and harsh chemical conditions which impedes their potential application in the field. Among the currently explored MOFs, UiO-66 exhibits structural robustness and has gained immense scientific popularity. Built with a zirconium-terephthalate framework, the strong Zr-O bond coordination contributes to its stability in aqueous, chemical, and thermal conditions. Moreover, other exceptional features such as high surface area and uniform pore size add to the grand arena of porous nanomaterials. As a result of its stable nature, UiO-66 offers relaxed admittance towards various functionalization, including synthetic and post-synthetic modifications. Consequently, the adsorptive properties of these highly stable frameworks have been modulated by the addition of various functionalities. Moreover, due to the presence of catalytically active sites, the use of UiO-66 has also been extended towards the degradation of pollutants. Furthermore, to solve the practical handling issues of the crystalline powdered forms, UiO-66 has been incorporated into various membrane supports. The incorporation of UiO-66 in various matrices has enhanced the rejection, permeate flux, and anti-fouling properties of membranes. The combination of such exceptional characteristics of UiO-66 MOF has expanded its scope in targeted purification techniques. Subsequently, this review highlights the role of UiO-66 in major water purification techniques such as adsorption, photocatalytic degradation, and membrane separation. This comprehensive review is expected to shed light on the existing developments and guide the inexhaustible futuristic scope of UiO-66 MOF.

Preparation and photocatalytic performance of UIO-66/La-MOF composite

To improve the photocatalytic degradation efficiency of photocatalytic materials UIO-66 and La-MOFs under visible-light irradiation, a series of photocatalytic materials with La and Zr as metal centers and terephthalic acid (H₂BDC) and 2-amino terephthalic acid (H₂ATA) as organic ligands were prepared by solvothermal method. The photocatalytic materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-visible (UV-vis) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, and Mott-Schottky test. The photocatalytic degradation performance to Rhodamine B of the catalysts was fully investigated. Results show that the H₂ATA series had stronger visible-light absorption capacity and better photocatalytic performance. The 0.35 La/Zr-H₂ATA composite showed the best photocatalytic degradation. The quenching experiments confirmed that the active species in the photocatalytic degradation were the holes and superoxide radicals. The possible mechanisms of the carrier migration paths in the energy level matching for La/Zr-H₂BDC and La/Zr-H₂ATA were also discussed in detail.

Visible-Light-Responsive UiO-66(Zr) with Defects Efficiently Promoting Photocatalytic CO2 Reduction

It is of great importance to understand the relationship between the structure and properties at the atomic level, which provides a solid platform for the design of efficient heterogeneous catalysts. However, it remains a challenge to elucidate the roles of the structure of reaction sites in the catalytic activity of active sites due to the lack of understanding of the structure of specific active site species. Herein, taking the metal-organic framework (MOF) UiO-66(Zr) as a prototype, MOF catalysts with all-solid-state frustrated Lewis pairs (FLPs) Zr³⁺-OH were synthesized in situ by adding acetic acid (HAc) as a modulator. By introducing missing linkers, UiO-66(Zr) first becomes a visible-light-responsive photocatalyst for CO₂ reduction. The in situ Fourier transform infrared (FTIR) spectrum reveals that b-CO₃^2- is the key intermediate for the activation of CO₂ molecules through FLPs Zr³⁺-OH. Moreover, defective UiO-66(Zr) could "self-breath" by surface hydroxyls. This finding not only provides a new avenue for utilizing UV-responsive MOFs by defect engineering but also sheds light on its catalytic activity at the atomic level.

Preparation of formate-free PMA@MOF-808 catalysts for deep oxidative desulfurization of model fuels

Due to the increasingly serious environmental problems caused by the combustion of sulfides in fuel, deep desulfurization of fuel became particularly urgent. Herein, the catalyst (PMA@MOF-808) of the Zr-based metal-organic framework (MOF-808) encapsulating phosphomolybdic acid (PMA) was prepared via a one-pot hydrothermal method. Besides, the formate ions of PMA@MOF-808 were removed by posttreatment with methanol, resulting in formate-free PMA@MOF-808-H catalysts with unsaturated open metal sites. The as-synthesized catalysts were systematically characterized by XRD, FT-IR, SEM, BET, TGA, ¹H NMR and XPS. The catalysts were also applied in catalytic oxidation desulfurization of fuel. The results indicated that the introduction of PMA and the removal of formate ions can improve the desulfurization performance of catalysts. Formate-free 0.2-PMA@MOF-808-H catalyst can reach 100% desulfurization rate for DBT. Besides, the kinetic properties were studied, and the apparent activation energy was 29.34 kJ/mol.

Chemical Vapor Deposition Growth of MoS2 on g-C3N4 Nanosheets for Efficient Removal of Tetracycline Hydrochloride

MoS₂ was vertically grown on g-C₃N₄ nanosheets by chemical vapor deposition to prepare nanocomposites named MS-CN samples. Because of a large-surface area of 545.2 m²·g^-1 and a total pore volume of 1.7 cm³·g^-1, the sample MS-CN revealed fast and large adsorption capacity for tetracycline hydrochloride (TCH). The adsorption kinetics model proved that TCH could be rapidly adsorbed within 5 min, and chemical adsorption was dominant. For single-component adsorption of TCH, the maximum adsorption capacity was ∼154 mg/g. The monolayer adsorption was carried out on the surface of MS-CN. Both of the film and intra-particle diffusion were considered as significant processes to facilitate adsorption. Thermodynamic parameters indicate that the adsorption of TCH is a spontaneous endothermic process. The adsorption of TCH was highly pH-dependent. The maximum adsorption capacity of TCH was obtained in the case of pH ∼ 7. After four adsorption and desorption cycles, MS-CN still maintained well-adsorption performance. Multiple adsorption mechanism, pore filling, electrostatic force, π-π conjugation, and hydrogen bonding interactions were studied. Because of fast adsorption, large adsorption capacity, and high stability, it is a promising adsorbent for antibiotics.

Bi@BiOx(OH)y modified oxidized g-C3N4 photocatalytic removal of tetracycline hydrochloride with highly effective oxygen activation

A novel Bi@BiOx(OH)y-modified oxidized g-C₃N₄ photocatalyst was successfully prepared via wet chemical reduction under alkaline conditions for the tetracycline hydrochloride removal. The prepared materials were characterized comprehensively and fully. Sufficient structural representation analyses confirmed the successful loading of Bi in the form of Bi@BiOx(OH)y complex beads. Based on basic photocatalytic studies, 10% (mass percentage) was found to be the best metal Bi loading. DRS, PL, transient photocurrent and EIS have explored the improvement of the photochemical properties of materials by loading Bi@BiOx(OH)y groups, particularly the improvement of photocatalytic properties by the SPR effect and electron traps. 10%Bi-OxCN exhibited the most suitable particle size of nonagglomerated Bi-metal groups, the largest specific surface area (43.53 m² g^-1), the most adsorption sites and the most significant photocurrent (8.694 × 10^-2 mA cm^-2) (7.78 times that of OxCN). This indicated that 10%Bi-OxCN had good adsorption capacity and excellent light response capability. In addition, 10%Bi-OxCN showed the best tetracycline hydrochloride removal efficiency (96.0%), with ∙O₂^- as the main active substance and ¹O₂ as the second most important substance made of ∙O₂^- and h⁺. The excellent photocatalytic effect and good reusability were fundamentally dependent on the modification of OxCN by Bi@BiOx(OH)y groups to produce a large number of active substances (including the separation efficiency of electron-hole pairs and the generation efficiency of ∙O₂^- and ¹O₂). These advantages are all related to the high specific surface area, a large number of active sites, narrow bandgap width, Bi-SPR effect, and BiOx(OH)y electron trap caused by successful loading of Bi groups.

Metal organic framework-based Janus nanomaterials: rational design, strategic fabrication and emerging applications

Janus nanoparticles (JNPs) with dual segments comprising chemically distinct compositions have garnered the attention of researchers in the past few years. The combination of different materials with diversified morphology, topology, and distinct physico-chemical characteristics into the single Janus nanocrystal has yielded multifarious capabilities for a myriad of emerging applications involving catalysis, gas separation, electro-catalysis, adsorption and energy storage. However, the traditional Janus entities significantly lack the need for populous active sites and high surface area. To overcome the textural hurdles and improve the functionalities of JNPs, porous MOFs were eventually introduced into Janus particles. MOFs are well endowed with varied pore apertures, structures, large surface areas and tailored characteristics, making them potentially invaluable for Janus fabrication. Depending upon the usage, MOFs can be explored to design Metal@MOF, polymetalic@MOF, MOF@MOF and MOF-derived JNPs. In this regard, we have represented a holistic summarization of the design, synthesis and emerging applications of a rising class of multi-functionalized MOF-based Janus nanomaterials. Moreover, this article will significantly aid researchers with a vision of creating dual-composition porous nanomaterials as the MOF-based Janus nanoparticles is at infancy.

Constructing Schottky junctions via Pd nanosheets on DUT-67 surfaces to accelerate charge transfer

The separation, transfer and recombination of charge often affect the rate of photocatalytic reduction of CO₂. Schottky junctions can promote the rapid separation of space charge. Therefore, in this paper, Pd nanosheets were grown on the surface of DUT-67 by a hydrothermal method, and a Schottky junction was constructed between DUT-67 and Pd. Under the action of the Schottky junction, the CO yield of 0.3-Pd/DUT-67 reached 12.15 μmol/g/h, which was 17 times higher than that of DUT-67. Efficient charge transfer was demonstrated in photochemical experiments. The large specific surface area and the increased light utilization rate also contributed to the increase in the CO₂ reduction efficiency. In addition, the mechanism of Pd/DUT-67 photocatalytic reduction of CO₂ was proposed.

Mixed-Valence Bimetallic Ce/Zr MOF-Based Nanoarchitecture: A Visible-Light-Active Photocatalyst for Ciprofloxacin Degradation and Hydrogen Evolution

A mixed-valency bimetallic Ce/Zr MOF with Ce³⁺/Ce⁴⁺ ions incorporated and an oxygen vacancy-rich single-component photocatalyst have been designed through the one-step solvothermal route to harness photons from the visible-light spectrum for green energy (H₂) generation and ciprofloxacin (CIP) degradation. The one-pot-engineered bimetallic Ce/Zr MOF shows visible-light-active characteristics accompanied by a narrower band gap, along with enhanced exciton separation and superior ligand-to-metal charge transfer (LMCT), due to the presence of an interconvertible Ce³⁺/Ce⁴⁺ ions pair in comparison to its pristine MOF counterpart. The Ce ion insertion led to increase in electron density around the Zr⁴⁺ ion, along with generation of some oxygen vacancies (OV), which cumulatively led to the rise in the photo-reaction output. The synthesized UNH (Ce/Zr 1:1) MOF displayed a boosted photocatalytic H₂ production rate of 468.30 μmol h^-1 (ACE = 3.51%), which is around fourfolds higher than that of pristine MOFs. Moreover, for CIP photodegradation, the UNH (Ce/Zr 1:1) shows an enhanced efficiency of 90.8% and follows pseudo-first-order kinetics with a rate constant of 0.0363. Typically, the active species involved in the photo-redox reaction of the CIP photodegradation follows the order hydroxyl radical (OH^•) < superoxide radical (O₂^•-), as confirmed by the TA and NBT tests. Consequently, the bimetallic Ce/Zr MOF can be readily employed as a robust photocatalyst with enhanced tendencies towards CIP degradation and H₂ evolution.

Robust direct Z-scheme exciton transfer dynamics by architecting 3D BiOI MF-supported non-stoichiometric Cu0.75In0.25S NC nanocomposite for co-catalyst-free photocatalytic hydrogen evolution

Designing promising photocatalytic systems with wide photon absorption and better exciton separation ability is a cutting-edge technology for enhanced solar-light-driven hydrogen production. In this context, non-stoichiometric Cu0.75In0.25S nanocrystals (CIS NCs) coupled with three-dimensional (3D) BiOI micro-flowers (BOI MFs) were synthesized through an ultra-sonication strategy forming a CIS-BOI heterojunction, which was well supported by XRD, photocurrent, XPS and Mott-Schottky analyses. Further, the co-catalyst-free CIS-BOI binary hybrid shows improved hydrogen evolution, i.e., 588.72 μmol h-1, which is 3.2 times greater than the pristine CIS NC (183.97 μmol h-1). Additionally, the binary composite confers an apparent conversion efficiency (ACE) of 9.44% (8.90 × 1016 number of H2 molecule per sec), which is extensively attributed to the robust charge carrier separation and transfer efficiency via the direct Z-scheme mechanism (proved through superoxide and H2 evolution activity). Moreover, the broad photon absorption range and productive exciton separation over the CIS-BOI composite are substantially justified by UV-Vis DRS, PL, EIS and photocurrent measurements.

Incorporating nitrogen vacancies in exfoliated B-doped g-C3N4 towards improved photocatalytic ciprofloxacin degradation and hydrogen evolution

The e-BCN fabricated from the double calcination process exhibited significant photocatalytic activity towards photocatalytic ciprofloxacin degradation and hydrogen generation.

Cerium versus zirconium UiO66 metal–organic frameworks coupled with CdS for H2 evolution under visible light

Metal–organic frameworks (MOFs) as highly porous photocatalysts have received increasing attention.

Facile synthesis of magnetic framework composite MgFe2O4@UiO-66(Zr) and its applications in the adsorption-photocatalytic degradation of tetracycline

Recently, metal-organic framework (MOF)-based hybrid composites have attracted significant attention in photocatalytic applications. In this work, MgFe₂O₄@UiO-66(Zr) (MFeO@UiO) composites with varying compositions were successfully synthesized via facile in situ assemblies. Depositing the UiO-66(Zr) framework onto ferrite nanoparticles yielded a composite structure having a lower bandgap energy (2.28-2.60 eV) than that of the parent UiO-66(Zr) (~3.8 eV). Moreover, the MFeO@UiO composite exhibited magnetic separation property and improved porosity. The removal experiment for tetracycline (TC) in aqueous media revealed that the MFeO@UiO composite exhibited a high total TC removal efficiency of ca. ~94% within 45-min preadsorption and 120-min visible-light illumination, which is higher than that of pristine ferrite and UiO-66(Zr). The enhanced photodegradation efficiency of MFeO@UiO is attributed to efficient interfacial charge transfer at the heterojunction and the synergistic effect between the semiconductors. Radical scavenging experiments revealed that photogenerated holes (h⁺) and hydroxyl radicals (·OH) were the major reactive species involved in TC photodegradation. Moreover, the prepared MFeO@UiO nanocomposite showed good recyclability and renewability, making it a potential material for wastewater treatments.

Black titania an emerging photocatalyst: review highlighting the synthesis techniques and photocatalytic activity for hydrogen generation

The TiO₂ semiconductor photocatalyst is in the limelight of sustainable energy research in recent years because of its beneficial properties. However, its wide band-gap and rapid exciton recombination rate makes it a lame horse, and reduces its photocatalytic efficiency. Recently, researchers have developed facile methods for lowering the band-gap, so that it captures a wide range of solar spectrum, but the efficiency is still way behind the target value. After the discovery of black titania (B-TiO₂), the associated drawbacks of white TiO₂ and its modified forms were addressed to a large extent because it not only absorbs photons in a broad spectral range (UV to IR region), but also modifies the structural and morphological features, along with the electronic properties of the material, significantly boosting the catalytic performance. Hence, B-TiO₂ effectively converts solar energy into renewable chemical energy i.e. green fuel H₂ that can ultimately satisfy the energy crisis and environmental pollution. However, the synthesis techniques involved are quite tedious and challenging. Hence, this review summarizes various preparation methods of B-TiO₂ and the involved characterization techniques. It also discusses the different modification strategies adopted to improve the H₂ evolution activity, and hopes that this review acts as a guiding tool for researchers working in this field.

1D/2D constructed Bi2S3/Bi2O2CO3 direct Z-Scheme heterojunction: A versatile photocatalytic material for boosted photodegradation, photoreduction and photoelectrochemical detection of water-based contaminants

In this work, two-dimensional Bi₂O₂CO₃ disk is synthesized, followed by the growth of Bi₂S₃ over Bi₂O₂CO₃ via topotactic transformation by controlling the amount of thiourea under hydrothermal conditions. The synthesized composite catalyst is investigated for photocatalytic oxidation and reduction of tetracycline hydrochloride and hexavalent chromium under visible light irradiation. High interfacial contact between the Bi₂O₂CO₃ disk0 and Bi₂S₃ fiber is confirmed via high-resolution microscopic imaging. Enhanced light absorption and increased charge carrier separation is observed after the formation of the Bi₂S₃/Bi₂O₂CO₃ composite. The Bi₂S₃/Bi₂O₂CO₃ composite grown using 1 mmol of thiourea shows approximately 98% degradation of tetracycline hydrochloride after 120 min and 99% Cr(VI) reduction after 90 min of photochemical reaction under visible light irradiation. The charge separation is due to the formed internal electric field at the interface, which upon light irradiation follows a z-scheme charge transfer hindering the recombination at the Bi₂S₃ and Bi₂O₂CO₃ interface, thereby contributing efficiently to the photochemical process. In addition, the mechanism of the photochemical reaction for the degradation of pollutants is supported using quencher and probe experiments. Furthermore, photoelectrochemical detection of antibiotic in aqueous solution is conducted to understand the sensing feasibility of the synthesized system.

Impact of Iron in Three-Dimensional Co-MOF for Electrocatalytic Water Oxidation

The integration of iron (Fe) into a cobalt metal-organic framework (Co-MOF) tunes the electronic structure of the parent MOF as well as enhances their electrocatalytic characteristics. By using pyrazine and hydrofluoric acid, we have synthesized three-dimensional Co-MOF [CoFC₄H₄N₂(SO₄)_0.5], (1), and Fe-MOF [FeFC₄H₄N₂(SO₄)_0.5], (2), through a single-step solvothermal method. Further, a series of bimetallic (having both Co and Fe metal centers) MOFs [Co_1-xFe_xFC₄H₄N₂(SO₄)_0.5] were synthesized with variable concentrations of Fe, and their electrocatalytic performances were analyzed. The optimized amount of Fe significantly impacted the electrocatalytic behavior of the bimetallic MOF toward water oxidation. Particularly, the Co_0.75Fe_0.25-MOF needs only 239 and 257 mV of overpotential to deliver 10 and 50 mA/cm² current density, respectively, in alkaline electrolytic conditions. The Co_0.75Fe_0.25-MOF shows a lower Tafel slope (42 mV/dec.) among other bimetallic MOFs and even the commercial RuO₂, and it has excellent durability (with ∼8 mV increases in overpotential after 18 h of electrolysis) and 97.05% Faradaic efficiency, which further evident its catalytic excellency. These findings explore the intrinsic properties of MOF-based electrocatalysts and prospect the suitability for future water electrolysis.

Carboxymethyl cellulose/tetracycline@UiO-66 nanocomposite hydrogel films as a potential antibacterial wound dressing

Designing an antibacterial agent with a suitable water vapor permeability, good mechanical properties, and controlled antibiotic release is a promising method for stopping bacterial infection in wound tissue. In this respect, this work aims to prepare novel flexible polymeric hydrogel films via integrating UiO-66 into the polymeric carboxymethyl cellulose (CMC) hydrogel for improving the mechanical and antibiotic release performances. First, we performed a green hydrothermal synthetic method to synthesis UiO-66 and followed by encapsulating Tetracycline (TC) through immersion in its aqueous solution. Also, the casting technique was utilized to integrate different concentrations of the TC-encapsulated UiO-66 (TC@UiO-66, 5% to 15%) in the polymeric CMC matrix (CMC/TC@UiO-66) cross-linked by citric acid and plasticized by glycerol. The release performance showed a low initial burst release with a controlled release over 72 h in the artificial sweat and simulated wound exudate (PBS, pH 7.4) media. The in vitro cytotoxicity and antibacterial activity results revealed a good cytocompatibility toward Human skin fibroblast (HFF-1) cells and a significant activity against both E. coli and S. aureus with 1.3 and 1.7 cm inhibition zone, respectively. The obtained results recommend CMC/TC@UiO-66 films as a potential antibacterial wound dressing.

Hydrolytically stable citrate capped Fe3O4@UiO-66-NH2 MOF: A hetero-structure composite with enhanced activity towards Cr (VI) adsorption and photocatalytic H2 evolution

Design and facile fabrication of a magnetically separable hetero-structure photocatalyst as well as an adsorbent having dual green benefits towards energy conversion and pollutant remediation are quite indispensable in the current scenario. In this regard, a composite of citrate capped Fe₃O₄ and UiO-66-NH₂ has been designed to remediate Cr (VI) by adsorption and harvest photons from visible light for clean energy (H₂) conversion. The material was prepared by the union of citrate capped Fe₃O₄ (CCM) and versatile aqueous stable Zr-based MOF (UiO-66-NH₂) through in-situ solvothermal method. The composite of CCM with MOF (MU-2) was studied through sophisticated analysis techniques; PXRD, FT-IR, BET, UV-Visible DRS, PL, TG, HRTEM and XPS etc. to reveal the inherent characteristics of the material. BET surface analysis revealed high specific surface area (572.13 m² g^-1) of MU-2 in comparison to its pristine MOF. Furthermore, the dual function composite MU-2's VSM studies showed that its magnetic saturation is 3.07 emu g^-1 that is suitable for magnetic separation after desired reaction from aqueous media. The Cr (VI) sorption studies revealed that the composite adsorbent (MU-2) showed maximum monolayer adsorption capacity (Q_m) of 743 mg g^-1 which followed pseudo second order kinetics. Moreover, the sorption thermodynamics revealed that the process was spontaneous and endothermic in nature. In addition to it, the synthesized composite material displayed enhanced activity towards photocatalytic H₂ evolution with a maximum evolution rate of 417 µmole h^-1 with an apparent conversion efficiency (ACE) of 3.12 %. Typically, MU-2 displays high adsorptions of Cr (VI) as well as some extent of Cr (VI) reduction owning to its populous active sites and free carboxylate groups respectively. Moreover, the synergistic effect of CCM and UNH in the composite resulted in Z scheme mediated charge transfer mechanism that showed enhanced H₂ photo-evolution rates. Hence, MU-2 can be readily utilized as magnetically retrievable dual function composite for Cr (VI) adsorption and photocatalytic H₂ evolution.