Metal-organic frameworks for CO2 photoreduction

Metal-organic frameworks (MOFs) have attracted much attention because of their large surface areas, tunable structures, and potential applications in many areas. In recent years, MOFs have shown much promise in CO₂ photoreduction. This review summarized recent research progresses in MOF-based photocatalysts for photocatalytic reduction of CO₂. Besides, it discussed strategies in rational design of MOF-based photocatalysts (functionalized pristine MOFs, MOF-photosensitizer, MOF-semiconductor, MOF-metal, and MOF-carbon materials composites) with enhanced performance on CO₂ reduction. Moreover, it explored challenges and outlook on using MOF-based photocatalysts for CO₂ reduction.     

Current trends in structural development and modification strategies for metal-organic frameworks (MOFs) towards photocatalytic H2 production: A review

Photocatalytic H₂ generation using semiconductor photocatalysts is considered as a cost-effective and eco-friendly technology for solar to energy conversion; however, the present photocatalysts have been recognized to depict low efficiency. Currently, porous coordination polymers known as metal-organic frameworks (MOFs) constituting flexible and modifiable porous structure and having excess active sites are considered to be appropriate for photocatalytic H₂ production. This review highlights current progress in structural development of MOF materials along with modification strategies for enhanced photoactivity. Initially, the review discusses the photocatalytic H₂ production mechanism with the concepts of thermodynamics and mass transfer with particular focus on MOFs. Elaboration of the structural categories of MOFs into Type I, Type II, Type III and classification of MOFs for H₂ generation into transition metal based, post-transition metal based, noble-metal based and hetero-metal based has been systematically discussed. The review also critically deliberate various modification approaches of band engineering, improvement of charge separation, efficient irradiation utilization and overall efficiency of MOFs including metal modification, heterojunction formation, Z-scheme formation, by introducing electron mediator, and dye based composites. Also, the MOF synthesized derivatives for photocatalytic H₂ generation are elaborated. Finally, future perspectives of MOFs for H₂ generation and approaches for efficiency improvement have been suggested.     

A review on graphitic carbon nitride (g-C3N4) – metal organic framework (MOF) heterostructured photocatalyst materials for photo(electro)chemical hydrogen evolution

Graphitic carbon nitride (g-C₃N₄)-based heterostructured photocatalysts have recently attracted significant attention for solar water splitting and photocatalytic hydrogen (H₂) evolution, because of their alterable physicochemical, optical and electrical properties, such as tunable band structure, ultrahigh specific surface area and controllable pore size, defect formation and active sites. On the other hand, metal-organic frameworks (MOFs) possess a favorable surface area, permanent porosity and adjustable structures that allow them to be suitable candidates for diverse applications. In this review, we therefore comprehensively discuss the structural properties of heterogeneous g-C₃N₄/MOF-based photocatalysts with a special emphasis on their photocatalytic performance regarding the mechanism of heterogeneous photocatalysis, including advantages, challenges and design considerations.     

Novel strategies to tailor the photocatalytic activity of metal–organic frameworks for hydrogen generation: a mini-review

This review provides a recompilation of the most important and recent strategies employed to increase the efficiency of metal–organic framework (MOF)-based systems toward the photocatalytic hydrogen evolution (PHE) reaction through specific strategies: tailoring the photocatalytic activity of bare MOFs and guest@MOF composites, formation of heterojunctions based on MOFs and various photocatalysts, and inorganic photocatalysts derived from MOFs. According to the data reported in this mini-review, the most effective strategy to improve the PHE of MOFs relies on modifying the linkers with new secondary building units (SBUs). Although several reviews have investigated the photocatalytic activity of MOFs from a general point of view, many of these studies relate this activity to the physicochemical and catalytic properties of MOFs. However, they did not consider the interactions between the components of the photocatalytic material. This study highlights the effects of strength of the supramolecular interactions on the photocatalytic performance of bare and MOF-based materials during PHE. A thorough review and comparison of the results established that metal–nanoparticle@MOF composites have weak van der Waals forces between components, whereas heterostructures only interact with MOFs at the surface of bare materials. Regarding material derivatives from MOFs, we found that pyrolysis destroyed some beneficial properties of MOFs for PHE. Thus, we conclude that adding SBUs to organic linkers is the most efficient strategy to perform the PHE because the SBUs added to the MOFs promote synergy between the two materials through strong coordination bonds.     

Metal-organic frameworks (MOFs)-based efficient heterogeneous photocatalysts: Synthesis,properties and its applications in photocatalytic hydrogen generation,CO2 reduction and photodegradation of organic dyes

Metal-organic frameworks (MOFs) are a new class of functional materials having porous structures that show extraordinary specific surface areas, and tunable surface chemistry; hence, they hold great potential as photocatalysts. This review describes the fundamentals of MOFs and possible new research directions in the area of heterogeneous MOFs that can provide enhanced photocatalytic performance, especially for hydrogen production, degradation of emerging organic pollutants, and CO₂ reduction. The role of MOFs as multifunctional photocatalysts for light-stimulated organic reactions through an effective combination of metal/ligand/guest-based photocatalysts is discussed. Recent literature is discussed critically on the design and selection of materials, with possible directions to improve their catalytic properties. Furthermore, this comprehensive review systematically discusses the current developments of various MOFs-based hybrid nanostructures as multifunctional photocatalysts from different points, including several synthetic methodologies, key features, photocatalytic mechanism, and various influencing parameters to enhance catalytic efficiency. The recent achievements are critically discussed in the designing and selection of MOFs-based functional materials, with directions to effectively improve their catalytic properties for various photocatalytic applications. The article also summarizes with challenges and future prospects for the cost-effective and large scale photocatalytic applications of MOFs-based heterostructured catalysts.     

2D CoP supported 0D WO3 constructed S-scheme for efficient photocatalytic hydrogen evolution

For heterojunction composite photocatalyst, intimate contact interface is the key to the carrier transfer separation conditions. Due to the interface contact, the electron transfer rate between catalysts can be increased during photocatalytic hydrogen production, therefore, we design the close contact of 0D/2D heterojunction, which greatly enhanced the photocatalytic hydrogen production activity of the composite catalyst. The composite catalyst WO₃/CoP was obtained by simple high temperature in situ synthesis. Moreover, it was proved by photoelectric chemistry and fluorescence tests that appropriate conduction band and valence band locations of WO₃ and CoP provided a favorable way for thermodynamic electron transfer. In addition, fluorescence results showed that WO₃ load effectively promoted photoelectron-hole transfer and increased electron lifetime. The formation of S-scheme heterojunctions can make more efficient use of useful photogenerated electrons and prevent the photogenerated electron-hole recombination of CoP itself, further promote the liveness of photocatalytic H₂ evolution. Meanwhile, the study of Metal-organic frameworks (MOFs) materials further promoted the application of MOFs derivatives in the field of photocatalytic hydrogen evolution, and provided a reference for the rational design of composite catalysts for transition metal phosphide photocatalysts.     

Fabrication of metal-doped BiOI/MOF composite photocatalysts with enhanced photocatalytic performance

An octahedron-like metal-organic framework (MOF) was successfully synthesized by the ultrasound irradiation synthesis method. Meanwhile, a novel visible-light-driven Cu-doped BiOI/MOF composite photocatalyst was also synthesized by the microwave irradiation. MOFs, a new class of porous crystalline materials, have attracted tremendous attention considering their broad applications because MOF possesses the repeated crystalline structures and thereby improving the harvest of solar energy and transportation of charge carriers. In this article, the result has revealed that the appropriate modification of BIOI by incorporating MOF and copper could reach the maximum hydrogen yield under visible light irradiation. The hydrogen evolution for 10% v/v lactic acid and 0.30 g L^?1 of 3In/BiOI/4MOF composite photocatalyst had the maximum of 269.1 μmol h^?1 g^?1 for 6 h. However, as the overall time in a course of irradiation is prolongated until 48 h, 3Cu/BiOI/4MOF has the higher hydrogen evolution efficiency (7685.2 μmol) than that of 3In/BiOI/4MOF. Therefore, this study has substantially demonstrated to enhance the photocatalytic efficiency of BiOI photocatalyst using MOF and copper modified BiOI.     

In situ modification of Co-MOF with graphene oxide for enhanced photocatalytic hydrogen production

Metal-organic frameworks (MOFs) are one of the most promising precursors for the fabrication of advanced photocatalysts. In this report, we present a stable in water MOF based on earth-abundant cobalt (Co-BDC) as a highly active catalyst for visible-light-driven H₂ generation. The rate of H₂ production sensitized by eosin Y (EY) over Co-BDC reached 14.5 mmol g^?1 h^?1. By in situ addition of graphene oxide suspension to the photocatalytic system, the hydrogen production efficiency was further enhanced by a factor of 6.6.     

Fabrication of CuO/MoO3 p-n heterojunction for enhanced dyes degradation and hydrogen production from water splitting

The development of excellent photocatalytic material is highly required for energy and environmental applications. In this study, visible light responsive p-n heterojunction photocatalysts based on CuO/MoO₃ with varying ratios of CuO were prepared by the facile hydrothermal method. The crystalline structure, surface morphology, chemical compositions and optical properties of the synthesized photocatalysts were studied using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), photoluminescence (PL) techniques and UV–Vi's absorption spectroscopy. The results showed that the 5%CuO/MoO₃ nanocomposite displayed enhanced photocatalytic performance for the production of hydrogen (98.5 μmol h^?1g^?1) and degradation of dyes rhodamine B (RhB) and alizarine yellow (AY) than all other samples. Furthermore, 5% CuO/MoO₃ composite exhibited excellent stability after five consecutive cycles for both RhB and AY dyes. Overall, the improved photocatalytic performance of 5%CuO/MoO₃ composite was due to increased adsorption of visible light, good surface morphology, enhanced charge separation/transfer which inhibited recombination of electrons and holes. This study could encourage the synthesis of novel and effective p-n heterojunction photocatalysts for practical applications.     

Green synthesis of well dispersed TiO2/Pt nanoparticles photocatalysts and enhanced photocatalytic activity towards hydrogen production

Deposition of Pt NPs with preferred dispersion and morphologies on TiO₂ have been the focus of studies in photocatalytic and photoelectrochemical hydrogen production. Green synthesis of TiO₂/Pt NPs nanocomposites with narrow size distribution of Pt NPs still remain a challenge. Herein, we report that sucrose is highly efficient for the preparation of well-dispersed TiO₂/Pt NPs photocatalysts. Moreover, the sucrose could act as an electron donor, showing higher hydrogen production activity under simulated sunlight than pure water. The as-synthesized photocatalysts have been characterized by techniques of transmission electron microscopy (TEM), energy dispersive X-ray spectrometer (EDX), and diffuse reflectance spectroscopy (DRS). Compared with TiO₂/Pt NPs photocatalysts prepared through conventional photodeposition, the photocatalysts as prepared showed higher photocatalytic efficiency. Moreover, the catalyst could be reused easily without apparent degradation of their original photocatalytic activities. This approach presents a promising and low-cost strategy to improve the photocatalytic performance of TiO₂ from biomass.     

Nanoporous metal organic framework materials for smart applications

Abstract

This review is concerned with the recent advances in metal organic framework (MOF) materials. We highlight the unique combination of physicochemical and thermomechanical characteristics associated with MOF-type materials and illustrate emergent applications in three challenging technological sectors: energy, environmental remediation and biomedicine. MOFs represent an exciting new class of nanoporous crystalline solids constituting metal ions/clusters and multifunctional organic linkages, which self-assemble at molecular level to generate a plethora of ordered 3D framework materials. The most intriguing feature of a MOF lies in its exceptionally large surface area, far surpassing those of the best activated carbons and zeolites. Next generation multifunctional materials encompassing MOF based thin films, coatings, membranes and nanocomposites have potential for exploitation in an immense array of unconventional applications and smart devices. We pinpoint the key technological challenges and basic scientific questions to be addressed, so as to fulfil the translational potential for bringing MOFs from the laboratory into commercial applications.     

Hetero-nanostructured metal oxide-based hybrid photocatalysts for enhanced photoelectrochemical water splitting – A review

This review is mainly focused on nanostructured metal oxide-based efficient photocatalysts for photoelectrochemical (PEC) water splitting applications. Owing to their distinctive physical and chemical properties, metal-oxide nanostructures have attracted a wide research interest for solar power-stimulated water splitting applications. Hydrogen generation by solar energy-assisted water splitting is a clean and eco-friendly route that can solve the energy crisis and play a significant role in efforts to save the environment. In this review, synthesis strategies, control of morphology, band-gap properties, and photocatalytic application of solar water splitting using hierarchical hetero-nanostructured metal oxide-based photocatalysts, such as titanium dioxide (TiO₂), zinc oxide (ZnO), and tungsten/wolfram trioxide (WO₃), are discussed.     

Recent development in band engineering of binary semiconductor materials for solar driven photocatalytic hydrogen production

Photocatalytic hydrogen production from water splitting is a promising approach to develop sustainable renewable energy resources and limits the global warming simultaneously. Despite the significant efforts have been dedicated for the synthesis of semiconductor materials, key challenge persists is lower quantum efficiency of a photocatalyst due to charge carrier recombination and inability of utilizing full spectrum of solar light irradiation. In this review, recent developments in binary semiconductor materials and their application for photocatalytic water splitting toward hydrogen production are systematically discoursed. In the main stream, fundamentals and thermodynamic for photocatalytic water splitting and selection of photo-catalysts has been presented. Developments in the binary photocatalysts and their efficiency enhancements though surface sensitization, surface plasmon resonance (SPR) effect, Schoktty barrier and electrons mediation toward enhanced hydrogen production has been deliberated. Different modification approaches including band engineering, coupling of semiconductor catalysts, construction of heterojunction, Z-scheme formation and step-type photocatalytic systems are also discussed. The binary semiconductor materials such as TiO₂, g-C₃N₄, ZnO, ZnS, Fe₂O₃, CdS, WO₃, rGO, V₂O₅ and AgX (Cl, Br and I) are systematically disclosed. In addition, role of sacrificial reagents for efficient photocatalysis through reforming and hole-scavenger are elaborated. Finally, future perspectives for photocatalytic water splitting towards renewable hydrogen production have been suggested.     

Metal-organic frameworks as highly efficient electrodes for long cycling stability supercapacitors

Among a large variety of energy storage technologies, supercapacitors possess special advantages such as rapid charge/discharge, high power density, safety, and environmental friendliness to meet the requirement of specific applications. The common electrode materials of supercapacitors, including porous carbon, conductive polymers, and metal oxides/hydroxides, have their own benefits and drawbacks in energy density and stability. Owing to the big surface area and controllable porosity, the metal-organic frameworks (MOFs) have been explored as important candidates for supercapacitor applications. This mini-review focuses on the recent advances of MOF-based materials including pristine MOFs, MOFs composite materials, and MOF-derived materials in the development of long cycling life supercapacitors. The devices discussed here mean those with capacitive retention rates of more than 90% after 10,000 cycles and high energy density. In addition, we also describe the fundamental knowledge of supercapacitors, highlight the stabilization mechanism of MOFs, and propose the strategies to enhance the stability of MOF-based supercapacitor electrodes.     

Recent progress in metal-doped TiO2, non-metal doped/codoped TiO2 and TiO2 nanostructured hybrids for enhanced photocatalysis

Titanium dioxide remains a benchmark photocatalyst with high stability, low cost, and less toxicity, but it is active only under UV light; thus, in practical applications using visible light, its catalytic reactions are stalled. To enhance its catalytic activity under visible light, non-metal/codoped TiO₂ structures are being studied. These structures improve the photocatalytic activity of TiO₂ in visible light by reducing its energy bandgap. This might be useful in wastewater treatment for the photocatalytic degradation of organic contaminants under visible and UV light irradiation. In this intensive review, we describe recent developments in TiO₂ nanostructured materials for visible-light driven photocatalysis, such as (i) mechanistic studies on photo-induced charge separation to understand the photocatalytic activity and (ii) synthesis of non-metal doped/codoped TiO₂ and TiO₂ nanostructured hybrid photocatalysts. Furthermore, the effects of various parameters on their photocatalytic efficiency, photodegradation of various organic contaminants present in wastewater, and photocatalytic disinfection are delineated.     

Recent advancements in MOFs synthesis and their green applications

Metal organic frameworks (MOFs) are crystalline, hybrid, materials containing metal ions and organic molecules; components that together form three-dimensional structures. At the present, the large-scale production necessary for the commercialization of MOFs based industrial processes seems to be the greatest challenge, mainly due to the fact that MOFs require harsh operating conditions and large amounts of toxic solvents during their synthesis which result in added production costs and raise safety and environmental related concerns. This review provides an overview on the recent developments in the synthesis of MOFs and their green applications. The currently used solvothermal processes for the synthesis of MOFs necessitate the use of hazardous and non-renewable solvents such as such as dimethylformamide (DMF), dimethylformamide (DEF), methanol, ethanol, and acetonitrile. Green and economical routes for MOFs synthesis that are non-solvent based have been rigorously under study to facilitate the commercialization of synthesis processes that are safe and environmentally benign. Such processes include water-based MOF synthesis, mechanochemical MOFs synthesis and the use of super-critical fluids and ionic liquids as solvents to replace the conventionally used ones. The paper also discusses the recent developments in the applications of MOFs in innovative and vital applications such as hydrogen production, fuel cells, and water desalination. Challenges and corrective actions attributed to the use of MOFs in these applications are also presented and discussed.     

Recent advances in non-metals-doped TiO2 nanostructured photocatalysts for visible-light driven hydrogen production,CO2 reduction and air purification

The generation of hydrogen and oxygen from the photocatalytic water splitting reaction under visible light is a promisingly renewable and clean source for H₂ fuel. The transition metal oxide semiconductors (e.g. TiO₂, WO_3, ZnO, and ZrO₂) are have been widely used as photocatalysts for the hydrogen generation. Because of safety, low cost, chemical inertness, photostability and other characteristics (bandgap, corrosion resistance, thermal and environmental stability), TiO₂ is considered as a most potential catalyst of the semiconductors being investigated and developed. However, the extensive applications of TiO₂ are hampered by its inability to exploit the solar energy of visible region. Other demerits are lesser absorbance under visible light, and recombination of photogenerated electron-hole pairs. In this review, we focus on the all the possible reactions taking place at the catalyst during photo-induced H₂ from water splitting reaction, which is green and promising technology. Various parameter affecting the photocatalytic water splitting reactions are also studied. Predominantly, this review is focussed on bandgap engineering of TiO₂ such as the upward shift of valence band and downward shift of conduction bands by doping process to extend its light absorption property into the visible region. Furthermore, the recent advances in this direction including various new strategies of synthesis, multiple doping, hetero-junction, functionalization, perspective and future opportunities of non-metals-doped TiO₂-based nanostructured photocatalysts for various photocatalytic applications such as efficient hydrogen production, air purification and CO₂ reduction to valuable chemicals have been discussed.     

A critical review on core/shell-based nanostructured photocatalysts for improved hydrogen generation

Photocatalytic water splitting into gaseous hydrogen and oxygen in the presence of semiconductor photocatalysts under a visible spectrum of solar irradiation is one of the most promising processes for plummeting energy demands and environmental pollution. Among the successful photocatalytic materials, the core/shell nanostructures show promising results owing to their fascinating morphology that protects the surface features of the core besides the effective separation of photo-excitons resulting in an enhanced rate of hydrogen production up to 162 mmol h⁻¹g⁻¹_cat, which is a notable highest value reported in the literature. In this review, we have focused on the basic characteristics of the core-shell structure-based semiconductor photocatalytic systems and their efficient water-splitting reactions under light irradiation. Comprehensive detail on various synthesis methods of core-shell nanostructures, shell thickness-dependent properties, charge-transfer reaction mechanisms, and photocatalytic stability are highlighted in this review. Core-shell nanostructured materials have been extensively used as a photocatalyst, co-catalyst, and by coupling with supporting materials to improve the apparent quantum efficiency up to 45.6%. Besides, important photocatalytic properties that influence the redox reactions i.e., effective exciton separation, the effect of different light sources/wavelengths, surface charge modeling, photocatalytic active sites, and turnover frequency (TOF) have also been focused on and extensively described. Finally, the present and future prospects of the core-shell nanostructured photocatalysts for solar energy conversion into green hydrogen production have been expounded.     

Metal-organic-framework based catalyst for hydrogen production: Progress and perspectives

Fossil fuel shortage and global warming have inspired scientists to search for alternative energy sources which are green, renewable, and sustainable. Hydrogen formed from water splitting has been considered as one of the most promising candidates to replace traditional fuels due to its low production cost and zero-emission. Metal-organic frameworks (MOFs) have been considered as potential catalysts for hydrogen production from water splitting account for their flexible structure, ultra-large surface area, and chemical component diversification. This paper reviews different kinds of MOF-related electrocatalysts, involving metals, metal oxides, single atoms, metal phosphides, metal nitrides, and metal dichalcogenides for hydrogen production. Also, MOF-based photocatalysts consisting of pristine MOFs, MOFs as supporters, and MOF-derived heterojunction architectures are reviewed. The finding of MOF-based catalysts for hydrogen generation is summarized. The pros and cons of different MOF-based materials as catalysts for water splitting are discussed. Finally, current challenges and the potential developments of these unique materials as catalysts are also provided.     

Recent strategies to improve the catalytic activity of pristine MOFs and their derived catalysts in electrochemical water splitting

In recent years, MOF-based materials, including pristine metal-organic frameworks (MOFs) and their derivatives, are one kind of the most popular materials, and have been widely used in biomedicine, sensing, energy storage, and catalysis due to their unique pore structures, abundantly accessible metal nodes, large specific surface areas, tailorable function, and morphology. Most of them have shown satisfactory performances in specific fields. However, there are still some urgent problems need to be solved in the application of MOF-based materials in electrocatalytic water splitting, especially the facile and controllable synthesis, and limited activity and stability. Herein, first this article mainly reviewed the applications and challenges of MOF-based materials in electrocatalytic water splitting. Then, typical development strategies (e.g., construction of multi-metallic site of MOF-based materials, structural morphology designing and controlling, engineering of doping, defect, vacancy, coupling with conductive carrier or constructing heterostructures) were highlighted in this review. Finally, perspectives on the development of MOF-based materials were provided for fabricating more efficient water splitting electrocatalysts.