Metal-organic framework as a photocatalyst: Progress in modulation strategies and environmental/energy applications

Progress in the design, synthesis, and modification of metal-organic frameworks (MOFs) has immensely helped expand their applications in a wide variety of research fields. Such developments offered great opportunities for upgrading their efficiencies in diverse photocatalytic applications (e.g., N₂/CO₂ reduction, H₂ generation, organic synthesis, and environmental remediation) through enhanced conversion/storage of solar energy. The MOF-based photocatalytic platforms are, nonetheless, subject to many practical problems (e.g., inapplicability for industrial upscaling and thermodynamic instability under environmental conditions). In this review, the effects of synthesis/modification strategies on MOF photocatalysis have been discussed with respect to the type of inorganic nodes, the modulation of organic ligands, and the pre-/post-synthesis modification in MOF networks (i.e., MOF-based composite). Particular emphasis was placed on the technical advances achieved in the photoelectronic/catalytic performances of MOFs in multiple energy/environmental (redox) reactions based on both experimental and theoritical analyses. Further, the technical merits/disadvantages of MOF photocatalysts (in terms of structural defects, light absorption, active sites, and kinetic/thermodynamic stability) have been evaluated in relation to quantum efficiency and charge transfer mechanisms in various photo-redox reactions. The pursuit of strategies for enhanced kinetic stability of MOFs have also been highlighted based on the nature/strength of coordination modes, the inertness of metal centers, and the functionality of ligand types. Lastly, the current limitations of MOF-based photocatalysts are addressed with respect to their practical applications at industrial scales along with a discussion on their future use.     

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.     

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.     

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.     

Photocatalytic H2 evolution of porous silicon derived from magnesiothermic reduction of mesoporous SiO2

In this work, porous silicon (PSi) was synthesized by magnesiothermic reduction of mesoporous SiO₂ (MCM-41) and its photocatalytic hydrogen evolution performance was investigated. The unique mesoporous structure of PSi expands the band gap of silicon and shifts its conduction band to a more negative position. As a result, excellent photocatalytic water splitting efficiency of 604.7 μmol h⁻¹ g⁻¹ under visible-light radiation is recorded for the synthesized PSi photocatalysts without loading noble metal cocatalysts. This study presented a promising visible light response photocatalysts for the generation green renewable hydrogen energy basing on PSi material deriving from simple magnesiothermic reduction of mesoporous SiO₂.     

Oxygen vacancy induced Pt-decorated MOF photocatalyst for hydrogen production

The photocatalytic performance has remained challenging due to the rapid recombination of photoexcited electron-hole (e-h) pairs. To overcome this problem, creating oxygen vacancies on the surface of semiconductors has been an effective strategy. Herein, we report the effects of oxygen vacancies (Ov) on photocatalytic HER performance of Pt nanoparticles (NPs) anchored on UiO-66-NH₂. In contrast, under the same amount of Pt NPs, UiO-66-NH₂ with high oxygen vacancies (denoted as Pt/UN-Ovh) exhibit superior photocatalytic H₂ generation than the catalyst with low oxygen vacancies (denoted as Pt/UN-Ovl) under visible-light irradiation. Based on the experimental characterization and theoretical calculations, the high oxygen vacancies not only stabilize the Pt NPs on the substrate (UiO-66-NH₂), but also develop the strong interaction between Pt NPs and support thereby Pt NPs traps more electrons from substrate and provides protons for H₂ production inhibiting the electron-hole recombination. This work provides novel strategy for enhancing the photocatalysts performance of MOF based materials.     

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.     

A review on bismuth-based composite oxides for photocatalytic hydrogen generation

Bismuth-based composite oxides are always considered the best visible-light photocatalysts for oxygen production. However, they are failed to photocatalytic reduce the hydrogen from water, due to their lower conduction band made up by Bi 6p and O 2p. Thus, it is significant to modulate their levels of the conduction and valence bands satisfying the redox potential for both H⁺/H₂ and O₂/H₂O, which will directly lead to discovering new visible-light materials for photocatalytic hydrogen generation. Recent years, some modified bismuth-based composite oxides have been reported to achieve photocatalytic hydrogen production. In this paper, a review of photocatalytic hydrogen generation by bismuth-based composite oxides is presented, mainly including energy band engineering, Z-scheme overall water splitting, and strategies for photocatalytic activity improvement.     

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.     

Enhanced photocatalytic hydrogen evolution on TiO2 employing vanadium carbide as an efficient and stable cocatalyst

In an attempt to construct efficient and robust photocatalysts/systems for solar H₂ evolution from water splitting, the development of highly active and stable H₂ evolution cocatalysts is crucial yet remains a great challenge. Herein, we present that vanadium carbide (VC) can serve as an efficient cocatalyst when integrated with TiO₂ for photocatalytic H₂ evolution. With 15 wt% VC, the obtained TiO₂/VC (15 wt%) composite photocatalyst (denoted as TV15) shows the highest photocatalytic H₂ evolution rate of 521.4 μmol h⁻¹ g⁻¹, while the pristine TiO₂ hardly shows H₂ evolution activity. The apparent quantum efficiency (AQE) of H₂ evolution reaches up to 2.3% under light irradiation of 365 nm. Notably, the TV15 exhibits excellent photocatalytic stability for H₂ evolution over four cycles of continuous light irradiation of 20 h. The enhanced activity of TV15 can be attributed to the cocatalyst effects of VC, which can not only effectively capture the photogenerated electrons of TiO₂ to greatly enhance the charge separation efficiency but also significantly reduce the overpotential of H₂ evolution reaction, thus enhancing the photocatalytic activity of TiO₂/VC towards H₂ evolution. This work provides a new insight to rationally design and develop efficient photocatalysts using active and stable transition metal carbides as cocatalysts.     

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.     

Current progress on 3D graphene-based photocatalysts: From synthesis to photocatalytic hydrogen production

Hydrogen (H₂) is considered an alternative energy carrier for future clean energy systems in many applications. The three-dimensional (3D) graphene is one of the promising candidates for various applications especially in photocatalytic H₂ production due to its high electron conductivity, mechanical stability, fast electron transfer, and large surface area. Exploring the changes in the physical properties from different dimensionalities can be interesting because a 3D structure may improve the photocatalytic efficiency in terms of enhancing the light adsorption, increasing the accessible active surface, and improving the charge transport. Graphene can act as an electron acceptor and cocatalyst, and combining the graphene with metal oxides, transition metal dichalcogenides, or other semiconducting materials can enhance the photocatalytic activity of composites. Therefore, the synthesis, characterization, mechanism, and performance of the 3D graphene-based photocatalyst in the photocatalytic H₂ production are comprehensively discussed. The current progress and future challenges in the H₂ generation is also discussed in this review.     

Hydrogen storage in Ca-decorated,B-substituted metal organic framework

The feasibility to store hydrogen in calcium-decorated metal organic frameworks (MOFs) is explored by using first-principles electronic structure calculations. We show that substitution of boron atoms into the benzene ring of the MOF linker substantially enhances the Ca binding energy to the linker as well as the H₂ binding energy to Ca. The Kubas interaction between H₂ molecules and Ca added in the MOF gives rise to a large number of bound H₂'s (8H₂'s per linker) with the binding energy of 20 kJ/mol, which makes the system suitable for reversible hydrogen storage under ambient conditions.     

Recent advances in covalent organic framework (COF) nanotextures with band engineering for stimulating solar hydrogen production: A comprehensive review

Due to the continuous consumption of fossil fuels, natural reserves are depleting and it has been earnest need for developing new sources of energy. Among the several solar energy conversion techniques, photocatalytic hydrogen (H₂) generation is regarded as one of the most promising routes. Till date, several metal-based semiconductor materials have been investigated, however, H₂ generation is not substantial with the notion of sustainable development. Current research trends show the growing interest in advanced and metal free photocatalyst materials such as covalent organic frameworks (COFs) due to their several benefits such as crystalline porous polymers with pre-designed architectures, large surface area, exceptional stability, and ease of molecular functionalization. By combining COFs with other functional materials, composites may be created that display unique characteristics that exceed those of the separate components. This work provides a comprehensive development on COFs as a photocatalysts and their composites/hybrids for photocatalytic hydrogen generation with a focus on visible-light irradiation. To reduce the dependency on novel metals and overcome the drawbacks of individual material, the creation of composite materials based on covalent-organic frameworks (COFs) are systematically discussed. In addition, advantages in terms of performance, stability, durability of composites/hybrids COFs for photocatalytic hydrogen production in reference to traditional catalysts are investigated. Different composites such as metals loading, morphological development, band engineering, and heterojunctions are systematically discussed. Finally, challenges and opportunities associated with constructing COF-based catalysts as future research prospective for chemistry and materials science are highlighted.     

A critical review in strategies to improve photocatalytic water splitting towards hydrogen production

Water splitting for hydrogen production under light irradiation is an ideal system to provide renewable energy sources and to reduce global warming effects. Even though significant efforts have been devoted to fabricate advanced nanocomposite materials, the main challenge persists, which is lower efficiency and selectivity towards H₂ evolution under solar energy. In this review, recent developments in photo-catalysts, fabrication of novel heterojunction constructions and factors influencing the photocatalytic process for dynamic H₂ production have been discussed. In the mainstream, recent developments in TiO₂ and g-C₃N₄ based photo-catalysts and their potential for H₂ production are extensively studied. The improvements have been classified as strategies to improve different factors of photocatalytic water splitting such as Z-scheme systems and influence of operating parameters such as band gap, morphology, temperature, light intensity, oxygen vacancies, pH, and sacrificial reagents. Moreover, thermodynamics for selective photocatalytic H₂ production are critically discussed. The advances in photo-reactors and their role to provide more light distribution and surface area contact between catalyst and light were systematically described. By applying the optimum operating parameters and new engineering approach on photoreactor, the efficiency of semiconductor photocatalysts for H₂ production can be enhanced. The future research and perspectives for photocatalytic water splitting were also suggested.     

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.     

Free-standing CuS–ZnS decorated carbon nanotube films as immobilized photocatalysts for hydrogen production

Free-standing carbon nanotube films (CNTF) with entangled carbon nanotubes (CNT) were used as conductive supports for the preparation of CuS–ZnS/CNTF composite as immobilized photocatalysts for H₂ production. The surface morphology, crystalline property, surface chemistry, and optical properties of the CuS–ZnS/CNTF photocatalysts were investigated. The effects of forming CuS–ZnS heterojunction and conductive CNTF on the separation of photogenerated charges and photocatalytic hydrogen production activity of CuS–ZnS/CNTF photocatalysts were evaluated by the photocatalytic hydrogen production tests. Conductive CNT films can prevent the recombination of photogenerated electron–hole pairs. The deposition of CuS nanoparticles on the ZnS/CNTF leads to higher photocatalytic activity which can be attributed to the effective electron–hole separation. Introducing ZnS and CuS makes the photocatalyst surface more hydrophilic. The porous structure contributed to the effective contact between the sacrificing agents and the photocatalysts, leading to enhanced H₂ production activity.     

Estimation of system-level hydrogen storage for metal-organic frameworks with high volumetric storage density

Metal organic framework (MOF) materials have emerged as the adsorbent materials with the highest H₂ storage densities on both a volumetric and gravimetric basis. While measurements of hydrogen storage at the material level (primarily at 77 K) have been published for hundreds of MOFs, estimates of the system-level hydrogen storage capacity are not readily available. In this study, hydrogen storage capacities are estimated at the system-level for MOFs with the highest demonstrated volumetric and gravimetric H₂ storage densities. System estimates are based on a single tank cryo-adsorbent system that utilizes a type-1 tank, multi-layer vacuum insulation, liquid N₂ cooling channels, in-tank heat exchanger, and a packed MOF powder inside the tank. It is found that with this powder-based system configuration, MOFs with ultra-high gravimetric surface areas and hydrogen adsorption amounts do not necessarily provide correspondingly high volumetric or gravimetric storage capacities at the system-level. Meanwhile, attributes such as powder packing efficiency and system cool-down temperature are shown to have a large impact on the system capacity. These results should shed light on the material properties that must to be optimized, as well as highlight the important design challenges for cryo-adsorbent hydrogen storage systems.     

Photocatalytic generation of useful hydrocarbons and hydrogen from acetic acid in the presence of lanthanide modified TiO2

Pure and Ln³⁺ modified TiO₂ photocatalysts (Ln³⁺ = Eu³⁺ or Sm³⁺ ions) synthesized by a sol-gel method as well as commercially available P25 were applied for photocatalytic generation of useful hydrocarbons and hydrogen from acetic acid. Structure and surface properties of the photocatalysts were characterized by XRD, UV-vis/DR, FT-IR and N₂ adsorption-desorption measurements. The main gaseous products of CH₃COOH decomposition were CH₄ and CO₂. Trace amounts of C₂H₆ and H₂ were also detected in the reaction mixture. Moreover, it was observed that the quantities of all identified gases increased with elongation of irradiation time. The most active photocatalysts towards CH₄ and H₂ generation were Ln³⁺/TiO₂ containing 0.05 mol % of Sm and Eu, respectively. The results revealed that the Ln³⁺ modification can improve the effectiveness of the photocatalysts compared to pure and commercially available TiO₂ provided that a proper amount of modifying ions is used.     

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.