MOF-Derived Defective Co3O4 Nanosheets in Carbon Nitride Nanocomposites for CO2 Photoreduction and H2 Production

Crystalline carbon nitride derived self-assembly composite membranes with enhanced self-cleaning performance toward dye/salt separation

Membrane technology can realize the sustainable management of textile wastewater through dye/salt separation, but its practical implementation has long been hampered by membrane fouling. This work was to develop a novel carbon nitride based two-dimensional (2D) self-assembled membrane for efficient dye/salt separation with excellent photocatalytic self-cleaning performance toward azo dyes (e.g., methyl blue), anthraquinone dyes (e.g., reactive blue 19) and natural organic matters (e.g., humic acid). Specifically, conventional graphite carbon nitride (g-C3N4), poly triazine imide (PTI) and the composites of iron oxide doped PTI (PTI-Fe2O3) were comparatively investigated for manipulating the lamellar self-catalytic membrane through facile vacuum filtration. The considerable separation factors of sodium chloride to methyl blue (Ssalt/dye) of 76.97, 80.72, and 74.33 were obtained by g-C3N4, PTI, and PTI-Fe2O3, respectively. The generation of radicals and non-radicals were triggered by light irradiation and was accelerated by PTI compared to g-C3N4, resulting in the better flux recovery rate (FRR) of 95.7 % for PTI membrane than that of g-C3N4 membrane (74.4 %). Besides, the introduction of iron oxide enables the enhanced self-cleaning performance by coupling photocatalytic and Fenton-like process, leading to the highest FRR value of 99.4 %. Meanwhile, the fabricated membranes demonstrate remarkable stability (FRR ≥ 80 %) during prolonged operation (64 h) to both simulated and real textile wastewater, indicating that it can a promising alternative membrane material for textile wastewater management.

Plasmonic Nanocrystal-MOF Nanocomposites as Highly Active Photocatalysts and Highly Sensitive Sensors for CO2 Reduction over a Wide Range of Solar Wavelengths

Plasmonic nanocrystals have the potential to be widely used in green energy-related applications, due to their excellent optical properties and high reactivity over a wide range of solar wavelengths. Another benefit of using plasmonic nanocrystals for optical applications is that these nanocrystals strongly enhance Raman scattering and are therefore widely used in sensors. Recently, nanocomposites of porous materials deposited on plasmonic nanocrystals are demonstrated to enhance chemical reactivity by concentrating reactants on the surface of plasmonic nanocrystals. Here, three different plasmonic nanocrystals producing plasmonic responses within 400-900 nm are used as templates, and MOF-801 (Zr-based MOF) is produced on these nanocrystals as photocatalysts for the CO2 reduction reaction. Using nanocomposites as CO2 reduction reaction photocatalysts, the CO2 conversion rate can reach >50% within 30 min. The CO2 reduction reactivity of nanocomposites can be improved by the composition and morphology of plasmonic nanocrystals (increased by 40-50%), due to stronger synergistic effects and higher surface area to volume ratio. This report demonstrates that by controlling the plasmonic responses of nanocrystals, it is possible to realize photocatalysts that can be used for CO2 reduction reactions over a wide range of solar wavelengths.

Aeration-Free Photo-Fenton-Like Reaction Mediated by Heterojunction Photocatalyst toward Efficient Degradation of Organic Pollutants

The regulation of peroxymonosulfate (PMS) activation by photo-assisted heterogeneous catalysis is under in-depth investigation with potential as a replaceable advanced oxidation process in water purification, yet it remains a significant challenge. Herein, we demonstrate a strategy to construct polyethylene glycol (PEG) well-coupled dual-defect VO-M-Co3O4@CNx S-scheme heterojunction to degrade organic pollutants without aeration, which dramatically provides abundant active sites, excellent photo-thermal property, and distinct charge transport pathway for PMS activation. The degradation rate of VO-M-Co3O4@CNx in anaerobic conditions shows a higher efficient rate (4.58 min-1 g-2) than in aerobic conditions (1.67 min-1 g-2). Experimental evidence reveals that VO-M-Co3O4@CNx promotes more rapid redox conversion of photoexcited electrons induced by defects with PMS under anaerobic conditions compared to aerobic conditions. Additionally, in situ experiments and DFT provide mechanistic insights into the regulation pathway of PMS activation via synergistic defect-induced electron, revealing the competitive effect between O2 and PMS over VO-M-Co3O4@CNx during the reaction process. The continuous flow reactor and flow cytometry results demonstrated that the VO-M-Co3O4@CNx/PMS/Vis system has remarkably enhanced stability and purification capability for removing organic pollutants. This work provides valuable insights into regulating the heterologous catalysis oxidation process without aeration through the photoexcitation synergistic PMS activation.

Recent advances in the design and preparation of graphitic carbon nitride for photocatalysis

Graphitic carbon nitride (g-C3N4) has recently gained tremendous attention as a promising photocatalyst for environmental and energy-related applications owing to its high thermal and physicochemical stability as well as its suitable band structure. However, bulk g-C3N4, typically synthesized by directly heating N-rich small molecules, often suffers from severe aggregation, a low specific surface area for light harvesting and rapid recombination of photogenerated electron-hole pairs. These factors significantly hinder its photocatalytic efficiency. Consequently, considerable efforts have been devoted to the rational design and synthesis of g-C3N4 with tailored morphologies and controllable electronic and band structures, utilizing both top-down and bottom-up approaches. Thus far, in addition to the conventional and commonly used methods for carbon nitride preparation, new techniques and precursor families are continuously being developed. This review discusses the latest advancements in synthetic approaches for g-C3N4-based materials and provides valuable insights into utilizing these methods to enhance their photocatalytic performance. Finally, the review concludes by presenting an outlook on the future directions and challenges in the development of CN materials for photocatalysis.

MOF-Derived Hollow C, N-Doped Co3O4 Dodecahedral Nanostructure Enwrapped with MgIn2S4 Nanosheets for Enhanced Photocatalytic N2 Reduction

Design of hierarchical hollow nanoheterostructure materials through interfacial and defect engineering is an innovative approach for achieving optimal charge separation dynamics and photon harvesting efficiency. Herein, we have described a facile technique to fabricate hollow MOF-derived C, N-doped-Co3O4 (C, N-Co3O4) dodecahedral particles enwrapped with MgIn2S4 nanosheets for enhanced N2 reduction performance. ZIF-67 was initially used as a sacrificial template to prepare hollow C, N-Co3O4 using a carbonization route followed by low-temperature calcination treatment. The controlled synthetic protocol not only led to nonmetal doping but also produced an interwoven carbon matrix that improved the photoelectron mobility. Density functional theory calculations further substantiated the creation of atomic defects through substitution of C at tetrahedral Co2+ sites and N at lattice O2- sites of the Co3O4 structure. C, N-Co3O4 was subsequently coupled with MgIn2S4 nanosheets to prepare the C, N-Co3O4/MgIn2S4 [C, N-CM (X)] p-n heterojunctions. The photocatalytic study revealed that the NH4+ ion production activity of the optimal C, N-CM (1:1) material (334 μmol g-1 h-1) was significantly higher (4-10 times) than that of pure components. The enhanced activity of the composite was ascribed to its distinct topological features, superior charge carrier dynamics, and creation of atomic defects that afforded a large number of surface-active sites.

ZIF-derived oxygen vacancy-rich Co3O4 for constructing an efficient Z-scheme heterojunction to boost photocatalytic water splitting

With ZIF-67 as the precursor, oxygen vacancy-rich Co3O4 nanoparticles were derived and anchored on the surface of 2D polyimide (PI) to construct a Z-scheme hybrid heterojunction (20ZP) through a simultaneous solvothermal in situ crystallization and polymerization strategy. XRD, XPS and EPR confirmed that both Co(III) and oxygen vacancies are formed during the low temperature conversion of ZIF-67 to Co3O4 nanoparticles that in turn accelerate the polymerization of PI. Synchronous crystallization makes the interfacial architecture intermetal and compact, inducing a strong interfacial electronic interaction between Co3O4 nanoparticles and PI. UV-vis DRS spectra and transient photocurrent response demonstrate that the incorporation of Co3O4 on polyimide not only extends the light absorption in the visible range, but also enhances the charge transfer rate. EIS, TRPL techniques and DFT calculations have confirmed that the photoinduced interfacial charge transfer pathway of this hybrid heterojunction characterized the Z-scheme in which the photoinduced electrons transfer from the conduction band of Co3O4 to the valence band of PI, significantly inhibiting the recombination of electrons and holes within PI. More importantly, the oxygen vacancies located below the conductor band of Co3O4 can deepen the band bending, improve the charge separation efficiency and accelerate electron transfer between Co3O4 and PI. This Z-scheme hybrid heterojunction structure can not only maintain the high reducing capacity of photoinduced electrons on the conductor band of PI, but also enhance the oxidative capacity of the heterojunction composite material, thus promoting the overall progress of the photocatalytic hydrogen release reaction.

2D Atomic Layers for CO2 Photoreduction

Artificial photosynthesis can convert carbon dioxide into high value-added chemicals. However, due to the poor charge separation efficiency and CO2 activation ability, the conversion efficiency of photocatalytic CO2 reduction is greatly restricted. Ultrathin 2D photocatalyst emerges as an alternative to realize the higher CO2 reduction performance. In this review, the basic principle of CO2 photoreduction is introduced, and the types, advantages, and advances of 2D photocatalysts are reviewed in detail including metal oxides, metal chalcogenides, bismuth-based materials, MXene, metal-organic framework, and metal-free materials. Subsequently, the tactics for improving the performance of 2D photocatalysts are introduced in detail via the surface atomic configuration and electronic state tuning such as component tuning, crystal facet control, defect engineering, element doping, cocatalyst modification, polarization, and strain engineering. Finally, the concluding remarks and future development of 2D photocatalysts in CO2 reduction are prospected.

Synergistic Effect of Nickel Nanoparticles Dispersed on MOF-Derived Defective Co3O4 In Situ Grown over TiO2 Nanowires toward UV and Visible Light Driven Photothermal CO2 Methanation

Catalytic CO2 hydrogenation is an effective approach to producing clean fuels, but this process is expensive, in addition to the low efficiency of catalysts. Thus, photothermal CO2 hydrogenation can effectively utilize solar energy for CH4 production. Metal-organic framework (MOF) derived materials with a controlled structure and morphology are promising to give a high number of active sites and photostability in thermal catalytic reactions. For the first time, a novel heterostructure catalyst was synthesized using a facile approach to in situ grow MOF-derived 0D Co3O4 over 1D TiO2 nanowires (NWs). The original 3D dodecahedral structure of the MOF is engineered into novel 0D Co3O4 nanospheres, which were uniformly embedded over Ni-dispersed 1D TiO2 NWs. In situ prepared 10Ni-7Co3O4@TiO2 NWs-I achieved an excellent photothermal CH4 evolution rate of 8.28 mmol/h at 250 °C under low-intensity visible light, whereas UV light treatment further increased activity by 1.2-fold. UV irradiations promoted high CH4 production while improving the susceptibility of the catalyst to visible light irradiation. The photothermal effect is prominent at lower temperatures, due to the harmonization of both solar and thermal energy. By paralleling with mechanically assembled 10Ni-7Co3O4/TiO2 NWs-M, the catalytic performance of the in situ approach is far superior, attributing to the morphological transformation of 0D Co3O4, which induced intimate interfacial interactions, formation of oxygen vacancies and boosted photo-to-thermal effects. The co-existence of metallic/metal oxide Ni-Co provided beneficial synergies, enhanced photo-to-thermal effects, and improved charge transfer kinetics of the composite. This work uncovers a facile approach to engineering the morphology of MOF derivatives for efficient photothermal CO2 methanation.

Activation of 2D titanate nanosheet photocatalysts by nitrogen doping and solution plasma processing

Novel two-dimensional (2D) oxides are of great interest for photocatalysis because of their superlative physical features, namely, large surface areas, short charge diffusion pathways, high crystallinity and easy surface modification. However, most 2D oxides suffer from weak visible light absorption and severe photogenerated carrier recombination. Nitrogen doping can successfully narrow the bandgap of 2D oxides but can hardly improve the charge separation. In this work, we pre-dope nitrogen into 2D titanate nanosheets (HTiO), followed by surface processing with solution plasma. By dual modification of nitrogen doping and solution plasma processing (SPP), the modified 2D titanate nanosheets (N-HTiO-SPP) display broad absorption extending to the visible light region and the healing of oxygen vacancies brought about by nitrogen doping. Compared with HTiO and nitrogen doped titanate (N-HTiO), a higher removal rate and mineralization rate towards the photocatalytic degradation of acetaldehyde were achieved over N-HTiO-SPP under solar light. This work provides a powerful way to activate 2D wide bandgap semiconductors for enhanced photocatalytic activity.

Rod-Shaped Spinel Co3O4 and Carbon Nitride Heterostructure-Modified Fluorine-Doped Tin Oxide Electrode as an Electrochemical Transducer for Efficient Sensing of Hydrazine

Engineering low-cost and efficient materials for sensing hydrazine (HA) is critical given the adverse effects of high concentrations on humans. We report an efficient electrode made up of rod-shaped Co3O4/g-C3N4 (Co3O4/graphitic carbon nitride (GCN))-coated fluorine-doped tin oxide as a desirable electrode for the detection of HA. GCN is synthesized by the thermal decomposition of melamine, Co3O4, and the heterostructure is grown by a hydrothermal process. The as-prepared materials were characterized by using spectroscopic and microscopic techniques. The voltammetric studies showed that HA can be oxidized at a lower onset potential of 0.24 V vs reference Ag/AgCl, and the composite yielded a significantly enhanced oxidation peak current than the pure components because of the high electrocatalytic activity and the synergy between Co3O4 and GCN. By employing chronoamperometry, the proposed sensor can detect HA in a wide range with a high sensitivity of 819.52 μA mM-1 cm-2 and a detection limit of 3.14 μM. The high conductivity of Co3O4, enhanced electroactive surface area, the rich redox couples of Co2+/Co3+, and the additional catalytic sites from GCN are responsible for the high performance of the heterostructure.

Metal-organic frameworks as photocatalysts in energetic and environmental applications

Metal-organic frameworks (MOFs) are an exciting new class of porous materials with great potential for photocatalytic applications in the environmental and energy sectors. MOFs provide significant advantages over more traditional materials when used as photocatalysts due to their high surface area, adaptable topologies, and functional ability. In this article, we summarize current developments in the use of MOFs as photocatalysts for a variety of applications, such as CO2 reduction, water splitting, pollutant degradation, and hydrogen production. We discuss the fundamental properties of MOFs that make them ideal for photocatalytic applications, as well as strategies for improving their performance. The opportunities and challenges presented by this rapidly expanding field are also highlighted.