Porous materials as effective chemiresistive gas sensors

Chemiresistive gas sensors (CGSs) have revolutionized the field of gas sensing by providing a low-power, low-cost, and highly sensitive means of detecting harmful gases. This technology works by measuring changes in the conductivity of materials when they interact with a testing gas. While semiconducting metal oxides and two-dimensional (2D) materials have been used for CGSs, they suffer from poor selectivity to specific analytes in the presence of interfering gases and require high operating temperatures, resulting in high signal-to-noise ratios. However, nanoporous materials have emerged as a promising alternative for CGSs due to their high specific surface area, unsaturated metal actives, and density of three-dimensional inter-connected conductive and pendant functional groups. Porous materials have demonstrated excellent response and recovery times, remarkable selectivity, and the ability to detect gases at extremely low concentrations. Herein, our central emphasis is on all aspects of CGSs, with a primary focus on the use of porous materials. Further, we discuss the basic sensing mechanisms and parameters, different types of popular sensing materials, and the critical explanations of various mechanisms involved throughout the sensing process. We have provided examples of remarkable performance demonstrated by sensors using these materials. In addition to this, we compare the performance of porous materials with traditional metal-oxide semiconductors (MOSs) and 2D materials. Finally, we discussed future aspects, shortcomings, and scope for improvement in sensing performance, including the use of metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and porous organic polymers (POPs), as well as their hybrid counterparts. Overall, CGSs using porous materials have the potential to address a wide range of applications, including monitoring water quality, detecting harmful chemicals, improving surveillance, preventing natural disasters, and improving healthcare.

Graphene-Based Metal-Organic Framework Hybrids for Applications in Catalysis, Environmental, and Energy Technologies

Current energy and environmental challenges demand the development and design of multifunctional porous materials with tunable properties for catalysis, water purification, and energy conversion and storage. Because of their amenability to de novo reticular chemistry, metal-organic frameworks (MOFs) have become key materials in this area. However, their usefulness is often limited by low chemical stability, conductivity and inappropriate pore sizes. Conductive two-dimensional (2D) materials with robust structural skeletons and/or functionalized surfaces can form stabilizing interactions with MOF components, enabling the fabrication of MOF nanocomposites with tunable pore characteristics. Graphene and its functional derivatives are the largest class of 2D materials and possess remarkable compositional versatility, structural diversity, and controllable surface chemistry. Here, we critically review current knowledge concerning the growth, structure, and properties of graphene derivatives, MOFs, and their graphene@MOF composites as well as the associated structure-property-performance relationships. Synthetic strategies for preparing graphene@MOF composites and tuning their properties are also comprehensively reviewed together with their applications in gas storage/separation, water purification, catalysis (organo-, electro-, and photocatalysis), and electrochemical energy storage and conversion. Current challenges in the development of graphene@MOF hybrids and their practical applications are addressed, revealing areas for future investigation. We hope that this review will inspire further exploration of new graphene@MOF hybrids for energy, electronic, biomedical, and photocatalysis applications as well as studies on previously unreported properties of known hybrids to reveal potential "diamonds in the rough".

A facile synthesis of CeO2 from the GO@Ce-MOF precursor and its efficient performance in the oxygen evolution reaction

Electrochemical water splitting has enticed fascinating consideration as a key conduit for the advancement of renewable energy systems. Fabricating adequate electrocatalysts for water splitting is fervently preferred to curtail their overpotentials and hasten practical utilizations. In this work, a series of Ce-MOF, GO@Ce-MOF, calcinated Ce-MOF, and calcinated GO@Ce-MOF were synthesized and used as high-proficient electrocatalysts for the oxygen evolution reaction. The physicochemical characteristics of the prepared samples were measured by diverse analytical techniques including SEM, HRTEM, FTIR, BET, XPS, XRD, and EDX. All materials underwent cyclic voltammetry tests and were evaluated by electrochemical impedance spectroscopy and oxygen evolution reaction. Ce-MOF, GO@Ce-MOF, calcinated Ce-MOF, and calcinated GO@Ce-MOF have remarkable properties such as enhanced specific surface area, improved catalytic performance, and outstanding permanency in the alkaline solution (KOH). These factors upsurge ECSA and intensify the OER performance of the prepared materials. More exposed surface active-sites present in calcinated GO@Ce-MOF could be the logic for superior electrocatalytic activity. Chronoamperometry of the catalyst for 15°h divulges long-term stability of Ce-MOF during OER. Impedance measurements indicate higher conductivity of synthesized catalysts, facilitating the charge transfer reaction during electrochemical water splitting. This study will open up a new itinerary for conspiring highly ordered MOF-based surface active resources for distinct electrochemical energy applications.

Sulfur or nitrogen-doped rGO supported Fe-Mn bimetal - organic frameworks composite as an efficient heterogeneous catalyst for degradation of sulfamethazine via peroxydisulfate activation

In this work, sulfur/nitrogen modified reduced graphene oxide (S/N-rGO) was employed as both electron shuttle and support to fabricate Fe-Mn bimetallic organic framework@S/N-rGO hybrids (BOF@S/N-rGO) via a facile two-step solvothermal route. Compared with the transition metal ions (Fe²⁺/Mn²⁺), the classical metal oxide catalyst (Fe₂O₃ and Fe₃O₄) and nano zero-valent iron (nZVI), BOF@S/N-rGO catalyst can more effectively activate peroxydisulfate (PDS) with ultra-low concentration (0.05 mM) to degrade sulfamethazine (SMT). Quenching experiments, electron paramagnetic resonance (EPR) measurement and linear sweep voltammetry (LSV) showed that the activation pathways of PDS between the two catalysts were different. In BOF@N-rGO+PDS system, the degradation of SMT was mainly attributed to the oxidation of radicals including SO₄^•- and •OH, especially SO₄•- . However, in BOF@S-rGO+PDS system, in addition to the radical pathway, there are also non-radical pathways, namely ¹O₂ and direct electron transfer. Furthermore, the applicability of BOF@S/N-rGO used in the PDS-mediated advanced oxidation processes (AOPs) was systematically investigated in terms of the effects of operating parameters and coexisting substance (anions and humic acid (HA)), the degradation of other pollutants, as well as the stability and reusability of the catalyst. This study proved that BOF@S/N-rGO was a promising activator of PDS with ultra-low concentration for the degradation of SMT.

Functionality and design of Co-MOFs: unique opportunities in electrocatalysts for oxygen reduction reaction

The first report highlighting miraculous and intelligent electrocatalysts that can be tailored to form useful structures and morphologies with active sites for the oxygen reduction reaction.

Chemically Integrating a 2D Metal-Organic Framework with 2D Functionalized Graphene

Two-dimensional metal-organic frameworks (2D MOFs) are the next-generation 2D crystalline solids. Integrating 2D MOFs with conventional 2D materials like graphene is promising for a variety of applications, including energy or gas storage, catalysis, and sensing. However, unraveling the importance of chemical interaction over an additive effect is essential. Here, we present an unconventional chemistry to integrate a Cu-based 2D MOF, Cu-HHTP (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene), with 2D functionalized graphene, reduced graphene oxide (rGO), by an in situ oxidation-reduction reaction. Combined Raman spectroscopy, electron spin resonance (ESR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) measurements along with structural analysis evidenced the chemical interaction between Cu-HHTP and rGO, which was subsequently assigned to be the key for the manifestation of significantly modified physical properties. Of particular mention is the conversion of an n-type crystalline solid to a p-type crystalline solid upon the chemical integration of Cu-HHTP with rGO, as revealed by Seebeck coefficient. More importantly, the thermoelectric power factor exhibited an increasing trend with increasing temperature, unlike an opposite trend observed due to an additive effect. The results anticipate the ability of a redox reaction to chemically integrate other 2D MOFs with rGO and show how an in situ synthesis can trigger chemical interaction between two distinctive 2D materials.

Recent development of two-dimensional metal-organic framework derived electrocatalysts for hydrogen and oxygen electrocatalysis

Developing efficient and low-cost electrocatalysts with unique nanostructures is of great significance for improved electrocatalytic reactions, including the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Two-dimensional (2D) metal-organic frameworks (MOFs) have attracted recent attention because of their unique dimension-related properties, such as ultrathin thickness, large specific surface area, and abundant accessible active sites that can act as good precursors for the derivation of a variety of nanocomposites as active materials in electrocatalysis and energy-related devices. In this review, we present recent developments in 2D MOF-derived nanomaterials for hydrogen and oxygen reactions in overall water-splitting and rechargeable Zn-air batteries. The advantages of various synthetic strategies are summarized and discussed in detail. Finally, we discuss the main challenges and future perspectives of the development of 2D MOF-derived electrocatalysts.

2-Methylimidazole-assisted synthesis of a two-dimensional MOF-5 catalyst with enhanced catalytic activity for the Knoevenagel condensation reaction

A novel and facile strategy was developed for the synthesis of a 2D MOF-5 catalyst with 2-methyimidazole as regulation reagent and a Lewis basic site, which showed excellent catalytic activity in Knoevenagel condensation.

Two-dimensional metal–organic framework nanosheets: synthesis and applications

Synthesis and applications of two-dimensional metal–organic framework nanosheets and their composites are summarized.

A palladium complex immobilized onto a magnetic GO-MnFe2O4 surface as an effective and recyclable catalyst for the reduction of p-nitrophenol

A recyclable magnetic palladium complex catalyst immobilized onto a GO-MnFe₂O₄ surface was synthesized and characterized for the reduction of p-nitrophenol.

Covalent cross-linking as a strategy to generate novel materials based on layered (2D) and other low D structures

Covalent linking of 2D structures such as graphene, MoS₂and C₃N₄by employing coupling reactions provides a strategy to generate a variety of materials with new or improved properties.

Preparation and applications of novel composites composed of metal–organic frameworks and two-dimensional materials

The integration of metal–organic frameworks and two-dimensional materials has led to the improved performance in molecular absorption, separation and storage, and shown promise in selective catalysis and biosensing.

MOF-derived, N-doped porous carbon coated graphene sheets as high-performance anodes for lithium-ion batteries

N-doped porous carbon coated graphene sheet anode materials exhibit fascinating electrochemical performance with a capacity of 1040 mA h g⁻¹.

Ultrathin 2D Metal-Organic Framework Nanosheets

A facile surfactant-assisted bottom-up synthetic method to prepare a series of freestanding ultrathin 2D M-TCPP (M = Zn, Cu, Cd or Co, TCPP = tetrakis(4-carboxyphenyl)porphyrin) nanosheets with a thickness of sub-10 nm is developed. As a proof-of-concept application, some of them are successfully used as new platforms for DNA detection. The Cu-TCPP nanosheet-based sensor shows excellent fluorescent sensing performance and is used for the simultaneous detection of multiple DNA targets.

Tetraamino-zinc phthalocyanine covalently bound to benzoic acid-functionalized graphene composites for highly efficient visible light photocatalytic activities

Photodegradation of RhB over a composite of zinc phthalocyanine covalently bound to benzoic acid-functionalized graphene.

Controlled growth of dense and ordered metal–organic framework nanoparticles on graphene oxide

A facile synthetic strategy is developed to modulate the growth and distribution of MOF–graphene oxide (GO) nanohybrids, achieving dense and ordered MOFs featuring different sizes and morphologies dispersed on GO.

Coating two-dimensional nanomaterials with metal-organic frameworks

We demonstrate the coating of various 2D nanomaterials including MoS2 nanosheets, graphene oxide (GO), and reduced graphene oxide (rGO) with zeolitic imidazolate frameworks (i.e., ZIF-8) via a facile procedure. Additionally, ternary core-shell structures like Pt-MoS2@ZIF-8, Pt-GO@ZIF-8, and Pt-rGO@ZIF-8 have also been prepared. As a proof-of-concept application, a memory device based on MoS2@ZIF-8 hybrid was fabricated and it exhibited write-once-read-many-times (WORM) memory effect with high ON/OFF ratio and long operating lifetime. It is expected that MOF coated 2D nanomaterials may find wide applications in energy storage and conversion, catalysis, sensing, and information storage devices.