In Situ Fabrication of Porous MOF/COF Hybrid Photocatalysts for Visible-Light-Driven Hydrogen Evolution

Two-Dimensional Organic-Inorganic van der Waals Hybrids

Two-dimensional organic-inorganic (2DOI) van der Waals hybrids (vdWhs) have emerged as a groundbreaking subclass of layer-stacked (opto-)electronic materials. The development of 2DOI-vdWhs via systematically integrating inorganic 2D layers with organic 2D crystals at the molecular/atomic scale extends the capabilities of traditional 2D inorganic vdWhs, thanks to their high synthetic flexibility and structural tunability. Constructing an organic-inorganic hybrid interface with atomic precision will unlock new opportunities for generating unique interfacial (opto-)electronic transport properties by combining the strengths of organic and inorganic layers, thus allowing us to satisfy the growing demand for multifunctional applications. Here, this review provides a comprehensive overview of the latest advancements in the chemical synthesis, structural characterization, and numerous applications of 2DOI-vdWhs. Firstly, we introduce the chemistry and the physical properties of the recently rising organic 2D crystals (O2DCs), which feature crystalline 2D nanostructures comprising carbon-rich repeated units linked by covalent/noncovalent bonds and exhibit strong in-plane extended π-conjugation and weak interlayer vdWs interaction. Simultaneously, representative inorganic 2D crystals (I2DCs) are briefly summarized. After that, the synthetic strategies will be systematically summarized, including synthesizing single-component O2DCs with dimensional control and their vdWhs with I2DCs. With these synthetic approaches, the control in the dimension, the stacking modes, and the composition of the 2DOI-vdWhs will be highlighted. Subsequently, a special focus will be given on the discussion of the optical and electronic properties of the single-component 2D materials and their vdWhs, which will be closely relevant to their structures, so that we can establish a general structure-property relationship of 2DOI-vdWhs. In addition to these physical properties, the (opto-)electronic devices such as transistors, photodetectors, sensors, spintronics, and neuromorphic devices as well as energy devices will be discussed. Finally, we provide an outlook to discuss the key challenges for the 2DOI-vdWhs and their future development. This review aims to provide a foundational understanding and inspire further innovation in the development of next-generation 2DOI-vdWhs with transformative technological potential.

II-Scheme Heterojunction Frameworks Based on Covalent Organic Frameworks and HKUST-1 for Boosting Photocatalytic Hydrogen Evolution

Covalent organic frameworks (COFs) are one type of promising polymer semiconductors in solar-driven hydrogen production, but majority of COFs-based photocatalytic systems show low photocatalytic efficiency owing to lack of metal active sites. Herein, we reported II-Scheme heterojunction frameworks based on COF (TpPa-1) and metal-organic framework (HKUST-1) for highly efficient hydrogen production. The coordination bonding directed self-assembly of HKUST-1 on the surface of TpPa-1 endows the heterojunction frameworks (HKUST-1/TpPa-1) with strong interface interaction, optimized electronic structures and abundant redox active sites, thus remarkably boosting photocatalytic hydrogen evolution. The hydrogen evolution rate for optimal HKUST-1/TpPa-1 is as high as 10.50 mmol g-1 h-1, which is significantly enhanced when compared with that of their physical mixture (4.13 mmol g-1 h-1), TpPa-1 (0.013 mmol g-1 h-1) and Pt-based counterpart (6.70 mmol g-1 h-1). This work offers a facile approach to the construction of noble-metal-free II-Scheme heterojunctions based on framework materials for efficient solar energy conversion.

Impact of Postsynthetic Modification on the Covalent Organic Framework (COF) Structures

Covalent organic frameworks (COFs) have emerged as a versatile class of materials owing to their well-defined crystalline structures and inherent porosity. In the realm of COFs, their appeal lies in their customizable nature, which can be further enhanced by incorporating diverse functionalities. Postsynthetic modifications (PSMs) emerge as a potent strategy, facilitating the introduction of desired functionalities postsynthesis. A significant challenge in PSM pertains to preserving the crystallinity and porosity of the COFs. In this study, we aim to investigate the intricate interplay between PSM strategies and the resulting crystalline and porous structures of the COFs. The investigation delves into the diverse methodologies employed in PSMs, to elucidate their distinct influences on the crystallinity and porosity of the COFs. Through a comprehensive analysis of recent advancements and case studies, the study highlights the intricate relationships among PSM parameters, including reaction conditions, precursor selection, and functional groups, and their impact on the structural features of COFs. By understanding how PSM strategies can fine-tune the crystalline and porous characteristics of COFs, researchers can harness this knowledge to design COFs with tailored properties for specific applications, contributing to the advancement of functional materials in diverse fields. This work not only deepens our understanding of COFs but also provides valuable insights into the broader realm of PSM strategies for other solid materials.

Enhanced Ionic Power Generation via Light-Driven Active Ion Transport Across 2D Semiconductor Heterostructures

2D semiconductor heterostructures exhibit broad application prospects. However, regular nanochannels of heterostructures rarely caught the researcher's attention. Herein, a metal-organic framework (i.e., Cu3(HHTP)2) and transition metal dichalcogenides (i.e., MoS2)-based multilayer van der Waals heterostructure (i.e., Cu3(HHTP)2/MoS2) realized band alignment-dominated light-driven ion transport and further light-enhanced ionic energy generation. High-density channels of the heterostructure provide high-speed pathways for ion transmembrane transport. Upon light illumination, a net ionic flow occurs at a symmetric concentration, suggesting a directional cationic transport from Cu3(HHTP)2 to MoS2. This is because Cu3(HHTP)2/MoS2 heterostructures containing type-II band alignment can generate photovoltaic motive force through light-induced efficient charge separation to drive ion transport. After introducing into the ionic power generation system, the maximum power density under illumination can achieve notable improvement under different concentration differences. In addition to the photovoltaic motive force, type-II band alignment and material defect capture-induced surface charge increase also raise ion selectivity and flux, greatly facilitating ionic energy generation. This work demonstrates that 2D semiconductor heterostructures with rational band alignment can not only be a potential platform for optimizing light-enhanced ionic energy harvesting but also provide a new thought for biomimetic iontronic devices.

Covalent Organic Frameworks Meet Titanium Oxide

In order to expand the applicability of materials and improve their performance, the combined use of different materials has increasingly been explored. Among these materials, inorganic-organic hybrid materials often exhibit properties superior to those of single materials. Covalent organic frameworks (COFs) are famous crystalline porous materials constructed by organic building blocks linked by covalent bonds. In recent years, the combination of COFs with other materials has shown interesting properties in diverse fields, and the composite materials of COFs and TiO2 have been investigated more and more. These two outstanding materials are combined through covalent bonding, physical mixing, and other methods and exhibit excellent performance in various fields, including photocatalysis, electrocatalysis, sensors, separation, and energy storage and conversion. In this Review, the current preparation methods and applications of COF-TiO2 hybrid materials are introduced in detail, and their future development and possible problems are discussed and prospected, which is of great significance for related research. It is believed that these interesting hybrid materials will show greater application value as research progresses.

Detection of Ascorbic Acid by Two-Dimensional Conductive Metal-Organic Framework-Based Electrochemical Sensors

The realization of efficient and accurate detection of biomolecules has become a key scientific issue in the field of life sciences. With the rapid development of nanotechnology, electrochemical sensors constructed from the superior physical and chemical properties of nanomaterials show faster and more accurate detection. Among nanomaterials, two-dimensional conductive MOF (2D cMOF) is considered to be a star material in electrochemical sensors due to its remarkable conductivity, high porosity, and stability. In this paper, a Cu3(HHTP)2/SPE electrochemical sensor for the detection of ascorbic acid (AA) was constructed by modifying 2D cMOF (Cu3(HHTP)2) on the surface of the screen-printed electrode (SPE). The sensor exhibited excellent catalytic activity in the detection of AA, with a lower detection limit of 2.4 μmol/L (S/N = 3) and a wide linear range of 25-1645 μmol/L. This high catalytic activity can be attributed to the abundant catalytic sites in Cu3(HHTP)2 and the rapid electron transfer between Cu+ and Cu2+, which accelerates the oxidation of AA. This work lays a foundation for the subsequent development of MOFs with special electrochemical catalytic properties and the integration of 2D cMOF into intelligent electrical analysis devices.

Enhanced photocatalysis of metal/covalent organic frameworks by plasmonic nanoparticles and homo/hetero-junctions

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have garnered attention in photocatalysis due to their unique features including extensive surface area, adjustable pores, and the ability to incorporate various functional groups. However, challenges such as limited visible light absorption and rapid electron-hole recombination often hinder their photocatalytic efficiency. Recent developments have introduced plasmonic nanoparticles (NPs) and junctions to enhance the photocatalytic performance of MOFs/COFs. This paper provides a comprehensive review of recent advancements in MOF/COF-based photocatalysts improved by integration of plasmonic NPs and junctions. We begin by examining the utilization of plasmonic NPs, known for absorbing longer-wavelength light compared to typical MOFs/COFs. These NPs exhibit localized surface plasmon resonance (LSPR) when excited, effectively enhancing the photocatalytic performance of MOFs/COFs. Moreover, we discuss the role of homo/hetero-junctions in facilitating charge separation, further boosting the photocatalytic performance of MOFs/COFs. The mechanisms behind the improved photocatalytic performance of these composites are discussed, along with an assessment of challenges and opportunities in the field, guiding future research directions.

Controlled synthesis of 2D-2D conductive metal-organic framework/g-C3N4 heterojunctions for efficient photocatalytic hydrogen evolution

Designing photocatalysts with efficient charge separation and electron transport capabilities to achieve efficient visible-driven hydrogen production remains a challenge. Herein, 2D-2D conductive metal-organic framework/g-C3N4 heterojunctions were successfully prepared by an in situ assembly. Compared to pristine g-C3N4, the ratio-optimized Ni-CAT-1/g-C3N4 exhibits approximately 3.6 times higher visible-light H2 production activity, reaching 14 mmol g-1. Through investigations using time-resolved photoluminescence, surface photovoltage, and wavelength-dependent photocurrent action spectroscopies, it is determined that the improved photocatalytic performance is attributed to enhanced charge transfer and separation, specifically the efficient transfer of excited high-energy-level electrons from g-C3N4 to Ni-CAT in the heterojunctions. Furthermore, the high electrical conductivity of Ni-CAT enables rapid electron transport, contributing to the overall enhanced performance. This work provides a feasible strategy to construct efficient dimension-matched g-C3N4-based heterojunction photocatalysts with high-efficiency charge separation for solar-driven H2 production.

A porous polyacrylonitrile (PAN)/covalent organic framework (COF) fibrous membrane photocatalyst for highly efficient and ultra-stable hydrogen evolution

Photocatalytic water splitting has been regarded as one of the most promising technologies to generate hydrogen as an ideal energy carrier in the future. However, most of the experience for such process are derived from the researches based on the suspension powder photocatalysts under a stirring condition and a practical scaling application is urgently calling for the high-efficient panel reactors based on the membrane photocatalysts. Herein, we develop a new series of flexible and ultrastable membrane photocatalysts through a controllable growth of covalent organic framework (COF) photocatalysts on the polyacrylonitrile (PAN) electrospun fiber membrane. Multiple characterization techniques verify the successful anchoring of the COF-photocatalysts on the PAN fibers, forming a three-dimensional porous PAN/COF membrane photocatalyst with excellent light absorption ability, high specific surface area, and good hydrophily. As a result, the optimized PAN/COF membrane photocatalyst exhibits excellent hydrogen evolution rate up to 1.25 mmol g-1h-1 under visible-light irradiation without stirring, which is even higher than that of the corresponding suspension COF-powder photocatalyst with stirring. In particular, the PAN/COF membrane photocatalyst demonstrates a much more superior hydrogen evolution stability and also a much better recyclability. This study gives some experience for the practical scaling application of solar-driven water splitting.

Superstructure-Induced Hierarchical Assemblies for Nanoconfined Photocatalysis

Most attempts to synthesize supramolecular nanosystems are limited to a single mechanism, often resulting in the formation of nanomaterials that lack diversity in properties. Herein, hierarchical assemblies with appropriate variety are fabricated in bulk via a superstructure-induced organic-inorganic hybrid strategy. The dynamic balance between substructures and superstructures is managed using covalent organic frameworks (COFs) and metal-organic frameworks (MOFs) as dual building blocks to regulate the performances of hierarchical assemblies. Significantly, the superstructures resulting from the controlled cascade between COFs and MOFs create highly active photocatalytic systems through multiple topologies. Our designed tandem photocatalysis can precisely and efficiently regulate the conversion rates of bioactive molecules (benzo[d]imidazoles) through competing redox pathways. Furthermore, benzo[d]imidazoles catalyzed by such supramolecular nanosystems can be isolated in yields ranging from 70 % to 93 % within tens of minutes. The multilayered structural states within the supramolecular systems demonstrate the importance of hierarchical assemblies in facilitating photocatalytic propagation and expanding the structural repertoire of supramolecular hybrids.

Highly Efficient Dual-Color Luminophores for Sensitive and Selective Detection of Diclazepam Based on MOF/COF Bi-Mesoporous Composites

Currently, studies on electrochemiluminescence (ECL) mainly focused on the single emission of luminophores while those on multi-color ECL were rarely reported. Here, a bi-mesoporous composite of the metal-organic framework (MOF)/covalent-organic framework (COF) with strong and stable dual-color ECL was prepared to construct a novel ECL sensor for sensitive detecting targets. A PTCA-COF with excellent ECL performance was loaded with a great amount of another ECL emitter Cu₃(HHTP)₂. Remarkably, the integrated composite had both ECL properties of PTCA-COF at 520 nm and Cu₃(HHTP)₂ at 600 nm wavelengths. Furthermore, Cu₃(HHTP)₂ with good electron transfer ability can greatly enhance the electrical conductivity and promote electrochemical activation. Thus, the simultaneous enhanced two-color ECL intensity and the catalytic properties of the conductive MOF exerted a dual enhancement effect on the ECL signal of the composite. Significantly, diclazepam can not only be adsorbed well on the multi-stage porous structure MOF/COF composite by π-π interactions but also selectively quench the ECL signal of the PTCA-COF, realizing the sensitive detection. The ECL sensor showed a wide detection range from 1.0 × 10^-13 to 1.0 × 10^-8 g/L, and the limit of detection (LOD) was as low as 2.6 × 10^-14 g/L (S/N = 3). The proposed ECL sensor preparation method was simple and sensitive, providing a new perspective for the potential application of multi-color ECL in the sensing field.

Partial Sulphidation to Regulate Coordination Structure of Single Nickel Atoms on Graphitic Carbon Nitride for Efficient Solar H2 Evolution

To develop a non-precious highly efficient cocatalyst to replace Pt on graphitic carbon nitride (g-C₃ N₄ ) for solar H₂ production is great significant, but still remains a huge challenge. The emerging single-atom catalyst presents a promising strategy for developing highly efficient non-precious cocatalyst owing to its unique adjustability of local coordination environment and electronic structure. Herein, this work presents a facile approach to achieve single Ni sites (Ni₁ -N₂ S) with unique local coordination structure featuring one Ni atom coordinated with two nitrogen atoms and one sulfur atom, confirmed by high-angle annular dark-field scanning transmission electron microscopy, X-ray absorption spectroscopy, and density functional theory calculation. Thanks to the unique electron structure of Ni₁ -N₂ S sites, the 1095 µmol g^-1 h^-1 of high H₂ evolution rate with 4.1% of apparent quantum yield at 420 nm are achieved. This work paves a pathway for designing a highly efficient non-precious transition metal cocatalyst for photocatalytic H₂ evolution.

Two Cu(i) coordination polymers based on a new benzimidazolyl-tetrazolyl heterotopic ligand for visible-light-driven photocatalytic dye degradation

Two Cu(i) CPs based on a new heterotopic tripodal ligand were constructed and their visible-light-driven photocatalytic performance were studied.

PdS Quantum Dots as a Hole Attractor Encapsulated into the MOF@Cd0.5Zn0.5S Heterostructure for Boosting Photocatalytic Hydrogen Evolution under Visible Light

Herein, a new photocatalyst PdS@UiOS@CZS is successfully synthesized, where thiol-functionalized UiO-66 (UiOS), a metal-organic framework (MOF) material, is used as a host to encapsulate PdS quantum dots (QDs) in its cages, and Cd_0.5Zn_0.5S (CZS) solid solution nanoparticles (NPs) are anchored on its outer surface. The resultant PdS@UiOS@CZS with an optimal ratio between components displays an excellent photocatalytic H₂ evolution rate of 46.1 mmol h^-1 g^-1 under visible light irradiation (420∼780 nm), which is 512.0, 9.2, and 5.9 times that of pure UiOS, CZS, and UiOS@CZS, respectively. The reason for the significantly enhanced performance is that the encapsulated PdS QDs strongly attract the photogenerated holes into the pores of UiOS, while the photogenerated electrons are effectively migrated to CZS due to the heterojunction effect, thereby effectively suppressing the recombination of charge carriers for further high-efficiency hydrogen production. This work provides an idea for developing efficient photocatalysts induced by hole attraction.

The Advanced Synthesis of MOFs-Based Materials in Photocatalytic HER in Recent Three Years

Since the advent of metal–organic frameworks (MOFs), researchers have paid extensive attention to MOFs due to their determined structural composition, controllable pore size, and diverse physical and chemical properties. Photocatalysis, as a significant application of MOFs catalysts, has developed rapidly in recent years and become a research hotspot continuously. Various methods and approaches to construct and modify MOFs and their derivatives can not only affect the structure and morphology, but also largely determine their properties. Herein, we summarize the advanced synthesis of MOFs-based materials in the field of the photocatalytic decomposition of water to produce hydrogen in the recent three years. The main contents include the overview of the novel synthesis strategies in four aspects: internal modification and structure optimization of MOFs materials, MOFs/semiconductor composites, MOFs/COFs-based hybrids, and MOFs-derived materials. In addition, the problems and challenges faced in this direction and the future development goals were also discussed. We hope this review will help deepen the reader’s understanding and promote continued high-quality development in this field.

Space-Confined Surface Layer in Superstructured Ni-N-C Catalyst for Enhanced Catalytic Degradation of m-Cresol by PMS Activation

The broad application of peroxymonosulfate (PMS)-assisted oxidation by heterogeneous catalysts for contaminant removal suffers from the limitation of low PMS decomposition efficiency and consequent excessive electrolyte residues. In this work, we report that a micrometer-scale superstructured Ni-N-C catalyst Ni-NCNT/CB with a nanotube-array surface layer exhibits ultrahigh m-cresol removal efficiency with low PMS input and possesses ∼17-fold higher catalytic specific activity (reaction rate constant normalized to per Ni-N_x site) compared to the traditional Ni-SAC catalyst. Electron paramagnetic resonance results indicate that ¹O₂ is the dominant oxygen species, and Ni-NCNT/CB with a space-confined layer exhibits high ¹O₂ utilization for m-cresol degradation. Electrochemical impedance spectroscopy and a normalized k value of Ni-NCNT/CB confirm the spatial confinement effect on the catalyst surface, which is beneficial for regulating the mass transfer and exerting the high activity of active sites. This study gives a new application for spatial confinement, and the configuration of Ni-NCNT/CB may guide a rational catalyst design for AOP wastewater treatment.

Metalated covalent organic frameworks: from synthetic strategies to diverse applications

Covalent organic frameworks (COFs) are a class of organic crystalline porous materials discovered in the early 21st century that have become an attractive class of emerging materials due to their high crystallinity, intrinsic porosity, structural regularity, diverse functionality, design flexibility, and outstanding stability. However, many chemical and physical properties strongly depend on the presence of metal ions in materials for advanced applications, but metal-free COFs do not have these properties and are therefore excluded from such applications. Metalated COFs formed by combining COFs with metal ions, while retaining the advantages of COFs, have additional intriguing properties and applications, and have attracted considerable attention over the past decade. This review presents all aspects of metalated COFs, from synthetic strategies to various applications, in the hope of promoting the continued development of this young field.

Porous organic polymers for drug delivery: hierarchical pore structures, variable morphologies, and biological properties

Porous organic polymers have received considerable attention in recent years because of their applicability as biomaterials. In particular, their hierarchical pore structures, variable morphologies, and tunable biological properties make them suitable as drug-delivery systems. In this review, the synthetic and post forming/control methods including templated methods, template-free methods, mechanical methods, electrospun methods, and 3D printing methods for controlling the hierarchical structures and morphologies of porous organic polymers are discussed, and the different methods affecting their specific surface areas, hierarchical structures, and unique morphologies are highlighted in detail. In addition, we discuss their applications in drug encapsulation and the development of stimuli (pH, heat, light, and dual-stimuli)-responsive materials, focusing on their use for targeted drug release and as therapeutic agents. Finally, we present an outlook concerning the research directions and applications of porous polymer-based drug delivery systems.

Boosting photocatalytic hydrogen evolution of covalent organic frameworks by introducing 2D conductive metal–organic frameworks as noble metal-free co-catalysts

A conductive MOF Cu3(HHTP)2 is introduced as a non-noble metal co-catalyst into a Tp-Pa-2-COF to construct an efficient hydrogen evolution system.