Photocatalytic oxygen evolution from low-bandgap conjugated microporous polymer nanosheets: a combined first-principles calculation and experimental study

Multiscale modifications of carbon nitride to strengthen reaction kinetics and lower thermodynamic barriers for efficient photocatalytic oxygen evolution

Photocatalytic oxygen evolution reaction (OER) is pivotal for sustainable energy systems yet lacks high-performance catalysts capable of strong visible light absorption, robust charge dynamics, fast reaction kinetics, and high oxidation capability. Herein, we report the multiscale optimization of carbon nitride through the construction of porous curled carbon nitride nanosheets (CNA-B30) incorporating boron center/cyano group Lewis acid-base pairs (LABPs). The unique chemical and structural features of CNA-B30 extended the photoabsorption edges of π → π* and n → π* electronic transitions to 470 nm and 715 nm, respectively. Planar distortion and LABPs induced charge redistribution, enhancing the built-in electric field to promote efficient charge dissociation and transport. Moreover, boron atoms elevated the valence band of carbon nitride and served as active oxidation sites, effectively lowering the thermodynamic barrier for water oxidation. As a result, CNA-B30 demonstrated outstanding OER activity, achieving 586.5μmol g-1 h-1 (λ > 420 nm) without co-catalysts. With the addition of a Co co-catalyst, the oxygen evolution rate increased to 2085.5 μmol g-1 h-1 (λ > 420 nm), and an apparent quantum efficiency of 5.8 % at 420 nm, surpassing most state-of-the-art OER photocatalysts. This work offers valuable insights into designing advanced OER photocatalysts for efficient solar fuel production.

Enhanced photocatalytic hydrogen production via water splitting using cobalt-based organic nanofibers under visible light irradiation

This study focuses on the development of cobalt-based organic nanofibers as efficient photocatalysts for hydrogen production via water splitting under visible light irradiation. The depletion of fossil fuels necessitates the exploration of alternative energy sources, with hydrogen emerging as a promising candidate due to its clean and renewable nature. While conventional photocatalysts have shown potential, their limited activity under visible light and fast electron-hole recombination hinder their efficiency. In this work, cobalt acetate/poly(vinyl alcohol) (CoAc/PVA) nanofibers were electrospun and treated in a novel reactor design under water gas atmosphere at 160 °C to produce continuous, smooth, and stable nanobelts. The nanofibers displayed a band gap energy of 2.29 eV, indicating strong absorption in the visible light range. Detailed characterization using FTIR, XPS, SEM, and TGA confirmed the formation of organic-inorganic hybrid nanofibers with uniform cobalt distribution. Hydrogen production experiments showed that the proposed nanofibers significantly outperformed Co3O4 nanofibers, with an optimal hydrogen generation rate of 3.266 mmol gcat -1 s-1 at 70 vol% methanol. Furthermore, the treated nanofibers demonstrated good stability over multiple cycles, maintaining a constant hydrogen production rate after the third run. The study highlights the advantages of cobalt-based organic nanofibers in overcoming the limitations of traditional photocatalysts, providing a novel route for sustainable hydrogen production.

Biochemical hallmarks-targeting antineoplastic nanotherapeutics

Tumor microenvironments (TMEs) have received increasing attention in recent years as they play pivotal roles in tumorigenesis, progression, metastases, and resistance to the traditional modalities of cancer therapy like chemotherapy. With the rapid development of nanotechnology, effective antineoplastic nanotherapeutics targeting the aberrant hallmarks of TMEs have been proposed. The appropriate design and fabrication endow nanomedicines with the abilities for active targeting, TMEs-responsiveness, and optimization of physicochemical properties of tumors, thereby overcoming transport barriers and significantly improving antineoplastic therapeutic benefits. This review begins with the origins and characteristics of TMEs and discusses the latest strategies for modulating the TMEs by focusing on the regulation of biochemical microenvironments, such as tumor acidosis, hypoxia, and dysregulated metabolism. Finally, this review summarizes the challenges in the development of smart anti-cancer nanotherapeutics for TME modulation and examines the promising strategies for combination therapies with traditional treatments for further clinical translation.

Simple Functionalization of a Donor Monomer to Enhance Charge Transfer in Porous Polymer Networks for Photocatalytic Hydrogen Evolution

Porous polymer networks (PPNs) are promising candidates as photocatalysts for hydrogen production. Constructing a donor-acceptor structure is known to be an effective approach for improving photocatalytic activity. However, the process of how a functional group of a monomer can ensure photoexcited charges transfer and improve the hydrogen evolution rate (HER) has not yet been studied on the molecular level. Herein, we design and synthesize two kinds of triazatruxene (TAT)-based PPNs: TATR-PPN with a hexyl (R) group and TAT-PPN without the hexyl group, to understand the relationship between the presence of the functional group and charge transfer. The hexyl group on the TAT unit was found to ensure the transfer of photoexcited electrons from a donor unit to an acceptor unit and endowed the TATR-PPN with stable hydrogen production.

Recent advances in two-dimensional polymers: synthesis, assembly and energy-related applications

Two-dimensional polymers (2DPs) are a class of 2D crystalline polymer materials with definite structures, which have outstanding physical-chemical and electronic properties. They cleverly link organic building units through strong covalent bonds and can construct functional 2DPs through reasonable design and selection of different monomer units to meet various application requirements. As promising energy materials, 2DPs have developed rapidly in recent years. This review first introduces the basic overview of 2DPs, such as their historical development, inherent 2D characteristics and diversified topological advantages, followed by the summary of the typical 2DP synthesis methods recently (including "top-down" and "bottom-up" methods). The latest research progress in assembly and processing of 2DPs and the energy-related applications in energy storage and conversion are also discussed. Finally, we summarize and prospect the current research status, existing challenges, and future research directions of 2DPs.

A Ni-MOF as Fluorescent/Electrochemical Dual Probe for Ultrasensitive Detection of Picric Acid from Aqueous Media

A water-stable, microporous, luminescent Ni(II)-based metal-organic framework (MOF) (Ni-OBA-Bpy-18) with a 4-c uninodal sql topology was solvothermally synthesized using mixed N-, O-donor-directed π-conjugated co-ligands. The extraordinary performance of this MOF toward rapid monitoring of mutagenic explosive trinitrophenol (TNP) in aqueous and vapor phases by the fluorescence "Turn-off" technique with an ultralow detection limit of 66.43 ppb (K_sv: 3.45 × 10⁵ M^-1) was governed by a synchronous occurrence of photoinduced electron transfer-resonance energy transfer-intermolecular charge transfer (PET-RET-ICT) and non-covalent π···π weak interactions, as revealed from density functional theory studies. The recyclable nature of the MOF, detection from complex environmental matrices, and fabrication of a handy MOF@cotton-swab detection kit certainly escalated the on-field viability of the probe. Interestingly, the presence of electron-withdrawing TNP decisively facilitated the redox events of the reversible Ni^III/II and Ni^IV/III couples under an applied voltage based on which electrochemical recognition of TNP was realized by the Ni-OBA-Bpy-18 MOF/glassy carbon electrode, with an excellent detection limit of ∼0.6 ppm. Such detection of a specific analyte by MOF-based probe via two divergent yet coherent techniques is unprecedented and yet to be explored in relevant literature.

Impact of Interfaces, and Nanostructure on the Performance of Conjugated Polymer Photocatalysts for Hydrogen Production from Water

The direct conversion of sunlight into hydrogen through water splitting, and by converting carbon dioxide into useful chemical building blocks and fuels, has been an active area of research since early reports in the 1970s. Most of the semiconductors that drive these photocatalytic processes have been inorganic semiconductors, but since the first report of carbon nitride organic semiconductors have also been considered. Conjugated materials have been relatively extensively studied as photocatalysts for solar fuels generation over the last 5 years due to the synthetic control over composition and properties. The understanding of materials' properties, its impact on performance and underlying factors is still in its infancy. Here, we focus on the impact of interfaces, and nanostructure on fundamental processes which significantly contribute to performance in these organic photocatalysts. In particular, we focus on presenting explicit examples in understanding the interface of polymer photocatalysts with water and how it affects performance. Wetting has been shown to be a clear factor and we present strategies for increased wettability in conjugated polymer photocatalysts through modifications of the material. Furthermore, the limited exciton diffusion length in organic polymers has also been identified to affect the performance of these materials. Addressing this, we also discuss how increased internal and external surface areas increase the activity of organic polymer photocatalysts for hydrogen production from water.

Biomedical engineered nanomaterials to alleviate tumor hypoxia for enhanced photodynamic therapy

Photodynamic therapy (PDT), as a highly selective, widely applicable, and non-invasive therapeutic modality that is an alternative to radiotherapy and chemotherapy, is extensively applied to cancer therapy. Practically, the efficiency of PDT is severely hindered by the existence of hypoxia in tumor tissue. Hypoxia is a typical hallmark of malignant solid tumors, which remains an essential impediment to many current treatments, thereby leading to poor clinical prognosis after therapy. To address this issue, studies have been focused on modulating tumor hypoxia to augment the therapeutic efficacy. Although nanomaterials to relieve tumor hypoxia for enhanced PDT have been demonstrated in many research articles, a systematical summary of the role of nanomaterials in alleviating tumor hypoxia is scarce. In this review, we introduced the mechanism of PDT, and the involved therapeutic modality of PDT for ablation of tumor cells was specifically summarized. Moreover, current advances in nanomaterials-mediated tumor oxygenation via oxygen-carrying or oxygen-generation tactics to alleviate tumor hypoxia are emphasized. Based on these considerable summaries and analyses, we proposed some feasible perspectives on nanoparticle-based tumor oxygenation to ameliorate the therapeutic outcomes, which may provide some detailed information in designing new oxygenation nanomaterials in this burgeneous field.

Recent Developments in Heterogeneous Photocatalysts with Near-Infrared Response

Photocatalytic technology has been considered as an efficient protocol to drive chemical reactions in a sustainable and green way. With the assistance of semiconductor-based materials, heterogeneous photocatalysis converts solar energy directly into chemical energy that can be readily stored. It has been employed in several fields including CO2 reduction, H2O splitting, and organic synthesis. Given that near-infrared (NIR) light occupies 47% of sunlight, photocatalytic systems with a NIR response are gaining more and more attention. To enhance the solar-to-chemical conversion efficiency, precise regulation of the symmetric/asymmetric nanostructures and band structures of NIR-response photocatalysts is indispensable. Under the irradiation of NIR light, the symmetric nano-morphologies (e.g., rod-like core-shell shape), asymmetric electronic structures (e.g., defect levels in band gap) and asymmetric heterojunctions (e.g., PN junctions, semiconductor-metal or semiconductor-dye composites) of designed photocatalytic systems play key roles in promoting the light absorption, the separation of electron/hole pairs, the transport of charge carriers to the surface, or the rate of surface photocatalytic reactions. This review will comprehensively analyze the four main synthesis protocols for the fabrication of NIR-response photocatalysts with improved reaction performance. The design methods involve bandgap engineering for the direct utilization of NIR photoenergy, the up-conversion of NIR light into ultraviolet/visible light, and the photothermal effect by converting NIR photons into local heat. Additionally, challenges and perspectives for the further development of heterogeneous photocatalysts with NIR response are also discussed based on their potential applications.

Intramolecular hydroxyl nucleophilic attack pathway by a polymeric water oxidation catalyst with single cobalt sites

Exploration of efficient water oxidation catalysts (WOCs) is the primary challenge in conversion of renewable energy into fuels. Here we report a molecularly well-defined heterogeneous WOC with Aza-fused, π-conjugated, microporous polymer (Aza-CMP) coordinated single cobalt sites (Aza-CMP-Co). The single cobalt sites in Aza-CMP-Co exhibited superior activity under alkaline and near-neutral conditions. Moreover, the molecular nature of the isolated catalytic sites makes Aza-CMP-Co a reliable model for studying the heterogeneous water oxidation mechanism. By a combination of experimental and theoretical results, a pH-dependent nucleophilic attack pathway for O-O bond formation was proposed. Under alkaline conditions, the intramolecular hydroxyl nucleophilic attack (IHNA) process with which the adjacent -OH group nucleophilically attacks Co4+=O was identified as the rate-determining step. This process leads to lower activation energy and accelerated kinetics than those of the intermolecular water nucleophilic attack (WNA) pathway. This study provides significant insights into the crucial function of electrolyte pH in water oxidation catalysis and enhancement of water oxidation activity by regulation of the IHNA pathway.

Molecular Design of Two-Dimensional Covalent Heptazine Frameworks for Photocatalytic Overall Water Splitting under Visible Light

Photocatalytic water splitting sustainably offers clean hydrogen energy, but it is challenging to produce low-cost photocatalysts that split water stoichiometrically into H₂ and O₂ without sacrificial agents under visible light. Here, we designed 17 two-dimensional (2D) covalent heptazine frameworks (CHFs) by topologically assembling heptazine and benzene-containing molecular units that provide active sites for hydrogen and oxygen evolution reactions, respectively. Among them, 12 CHFs have band gap values of <3.0 eV with band margins straddling the chemical reaction potential of H₂/H⁺ and O₂/H₂O. In particular, a 2D H@DBTD CHF based on heptazine and 4,7-diphenyl-2,1,3-benzothiadiazole is a potential photocatalyst with a band gap of 2.47 eV for overall water splitting, which was confirmed with the calculated Gibbs free energy, non-adiabatic molecular dynamics, and preliminary experiment. This study presents an experimentally feasible molecular design of 2D CHFs as metal-free photocatalysts for overall water splitting under visible light.

Photocatalytic Water-Splitting by Organic Conjugated Polymers: Opportunities and Challenges

The future challenges associated with the shortage of fossil fuels and their current environmental impacts intrigued the researchers to look for alternative ways of generating green energy. Solar-driven water splitting into oxygen and hydrogen is one of those advanced strategies. Researchers have studied various semiconductor materials to achieve potential results. However, it encountered multiple challenges such as high cost, low photostability and efficiency, and required multistep modifications. The conjugated polymers (CPs) have emerged as promising alternatives for conventional inorganic semiconductors. The CPs offer low cost, sufficient light absorption efficiency, excellent photo and chemical stability, and molecular optoelectronic tunable characteristics. Furthermore, organic CPs also present higher flexibility to tune the basic framework of the backbone of the polymers, amendments in the sidechain to incorporate desired functionalities, and much-needed porosity to serve better for photocatalytic applications. This review article summarizes the recent advancements made in visible-light-driven water splitting covering the aspects of synthetic strategies and experimental parameters employed for water splitting reactions with special emphasis on conjugated polymers such as linear CPs, planarized CPs, graphitic carbon nitride (g-C₃ N₄ ), conjugated microporous polymers (CMPs), covalent organic frameworks (COFs), and conjugated polymer-based nanocomposites (CPNCs). The current challenges and future prospects have also been described briefly.

Semiconducting Polymers for Oxygen Evolution Reaction under Light Illumination

Sunlight-driven water splitting to produce hydrogen fuel has stimulated intensive scientific interest, as this technology has the potential to revolutionize fossil fuel-based energy systems in modern society. The oxygen evolution reaction (OER) determines the performance of overall water splitting owing to its sluggish kinetics with multielectron transfer processing. Polymeric photocatalysts have recently been developed for the OER, and substantial progress has been realized in this emerging research field. In this Review, the focus is on the photocatalytic technologies and materials of polymeric photocatalysts for the OER. Two practical systems, namely, particle suspension systems and film-based photoelectrochemical systems, form two main sections. The concept is reviewed in terms of thermodynamics and kinetics, and polymeric photocatalysts are discussed based on three key characteristics, namely, light absorption, charge separation and transfer, and surface oxidation reactions. A satisfactory OER performance by polymeric photocatalysts will eventually offer a platform to achieve overall water splitting and other advanced applications in a cost-effective, sustainable, and renewable manner using solar energy.

Nanoporous and nonporous conjugated donor-acceptor polymer semiconductors for photocatalytic hydrogen production

Conjugated polymers (CPs) as photocatalysts have evoked substantial interest. Their geometries and physical (e.g., chemical and thermal stability and solubility), optical (e.g., light absorption range), and electronic properties (e.g., charge carrier mobility, redox potential, and exciton binding energy) can be easily tuned via structural design. In addition, they are of light weight (i.e., mainly composed of C, N, O, and S). To improve the photocatalytic performance of CPs and better understand the catalytic mechanisms, many strategies with respect to material design have been proposed. These include tuning the bandgap, enlarging the surface area, enabling more efficient separation of electron-hole pairs, and enhancing the charge carrier mobility. In particular, donor-acceptor (D-A) polymers were demonstrated as a promising platform to develop high-performance photocatalysts due to their easily tunable bandgaps, high charge carrier mobility, and efficient intramolecular charge transfer. In this minireview, recent advances of D-A polymers in photocatalytic hydrogen evolution are summarized with a particular focus on modulating the optical and electronic properties of CPs by varying the acceptor units. The challenges and prospects associated with D-A polymer-based photocatalysts are described as well.

A Tetradentate Phosphonate Ligand-based Ni-MOF as a Support for Designing High-performance Proton-conducting Materials

Developing a robust metal-organic framework (MOF) which facilitates proton hopping along the pore channels is very demanding in the context of fabricating an efficient proton-conducting membrane for fuel cells. Herein, we report the synthesis of a novel tetradentate aromatic phosphonate ligand H₈ L (L=tetraphenylethylene tetraphosphonic acid) based Ni-MOF, whose crystal structure has been solved from single-crystal X-ray diffraction. Ni-MOF [Ni₂ (H₄ L)(H₂ O)₉ (C₂ H₇ SO)(C₂ H₇ NCO)] displays a monoclinic crystal structure with a space group of P 2₁ /c, a=11.887 Å, b=34.148 Å, c=11.131 Å, α=γ=90°, β=103.374°, where a nickel-hexahydrate moiety located inside the void space of the framework through several H-bonding interactions. Upon treatment of the Ni-MOF in different pH media as well as solvents, the framework remained unaltered, suggesting the presence of strong H-bonding interactions in the framework. High framework stability of Ni-MOF bearing H-bonding interactions motivated us to explore this metal-organic framework material as proton-conducting medium after external proton doping. Due to the presence of a large number of H-bonding interactions and the presence of water molecules in the framework we have carried out the doping of organic p-toluenesulfonic acid (PTSA) and inorganic sulphuric acid (SA) in this Ni-MOF and observed high proton conductivity of 5.28×10^-2  S cm^-1 at 90 °C and 98% relative humidity for the SA-doped material. Enhancement of proton conductivity by proton doping under humid conditions suggested a very promising feature of this Ni-MOF.

Nanostructured Lateral Boryl Substitution Conjugated Donor-Acceptor Oligomers for Visible-Light-Driven Hydrogen Production

Poor charge separation is the main factor that limits the photocatalytic hydrogen generation efficiency of organic conjugated polymers. In this work, a series of linear donor-acceptor (D-A) type oligomers are synthesized by a palladium-catalyzed Sonogashira-Hagihara coupling of electron-deficient diborane unit and different dihalide substitution sulfur functionalized monomers. Such diborane-based A unit exerts great impact on the resulting oligomers, including distinct semiconductor characters with isolated lowest unoccupied molecular orbital (LUMO) orbits locating in diborane-containing fragment, and elevated LUMO level higher than water reduction potential. Relative to A-A type counterpart, the enhanced dipole polarization effect in D-A oligomers facilitates separation of photogenerated charge carriers, as evidenced by notably prolonged electron lifetime. Owing to π-π stacking of rigid backbone, the oligomers can aggregate into an interesting 2D semicrystalline nanosheet (≈2.74 nm), which is rarely reported in linear polymeric photocatalysts prepared by similar carbon-carbon coupling reaction. Despite low surface area (30.3 m² g^-1 ), such ultrathin nanosheet D-A oligomer offers outstanding visible light (λ > 420 nm) hydrogen evolution rate of 833 µmol g^-1 h^-1 , 14 times greater than its A-A analogue (61 µmol g^-1 h^-1 ). The study highlights the great potential of using boron element to construct D-A type oligomers for efficient photocatalytic hydrogen generation.

Near-Infrared-Responsive Photocatalysts

Broadening the absorption of light to the near-infrared (NIR) region is important in photocatalysis to achieve efficient solar-to-fuel conversion. NIR-responsive photocatalysts that can utilize diffusive solar energy are attractive for alleviating the energy crisis and environmental pollution. Over the past few years, considerable progress on the component and structural design of NIR-responsive photocatalysts have been reported. This study aims to systematically summarize recent progress toward the material design and mechanism optimization of NIR-responsive photocatalysts in this area. Depending on the main strategies for harvesting NIR photons, NIR-responsive photocatalysts can be categorized as direct NIR-light photocatalysts, indirect NIR-light photocatalysts, and photothermal photocatalysts. Furthermore, the construction and application of different NIR-responsive photocatalytic systems are summarized. Conclusions and perspectives are presented to further explore the potential of NIR-responsive photocatalysts in this field.

Quinone-Enriched Conjugated Microporous Polymer as an Organic Cathode for Li-Ion Batteries

Among various organic cathode materials, C═O group-enriched structures have attracted wide attention worldwide. However, small organic molecules have long suffered from dissolving in electrolytes during charge-discharge cycles. π-Conjugated microporous polymers (CMPs) become one solution to address this issue. However, the synthesis strategy for CMPs with rich C═O groups and stable backbones remains a challenge. In this study, a novel CMP enriched with C═O units was synthesized through a highly efficient Diels-Alder reaction. The as-prepared CMP exhibited a fused carbon backbone and a semiconductive characteristic with a band gap of 1.4 eV. When used as an organic electrode material in LIBs, the insoluble and robust fused structure caused such CMPs to exhibit remarkable cycling stability (a 96.1% capacity retention at 0.2 A g^-1 after 200 cycles and a 94.8% capacity retention at 1 A g^-1 after 1500 cycles), superior lithium-ion diffusion coefficient (5.30 × 10^-11 cm² s^-1), and excellent rate capability (95.8 mAh g^-1 at 1 A g^-1). This study provided a novel synthetic method for fabricating quinone-enriched fused CMPs, which can be used as LIB cathode materials.

Conjugated nanomaterials for solar fuel production

Photocatalytic hydrogen production from water has the potential to fulfil future energy needs by producing a clean and storable fuel. In recent years polymer photocatalysts have attracted significant interest in an attempt to address these challenges. One reason organic photocatalysts have been considered an attractive target is their synthetic modularity, therefore, the ability to tune their opto-electronic properties by incorporating different building blocks. A wide range of factors has been investigated and in particular nano-sized particles have found to be highly efficient due to the size effect resulting from the ability of these to increase the number of charges reaching catalytic sites.

A novel crystalline nanoporous iron phosphonate based metal–organic framework as an efficient anode material for lithium ion batteries

A new Fe-MOF prepared by using a tetraphosphonic acid as a ligand is reported and it showed high specific capacity and excellent recycling efficiency in lithium-ion batteries.

The Influence of Disorder in the Synthesis, Characterization and Applications of a Modifiable Two-Dimensional Covalent Organic Framework

Two-dimensional covalent organic frameworks (2D-COFs) have been of increasing interest in the past decade due to their porous structures that ideally can be highly ordered. One of the most common routes to these polymers relies on Schiff-base chemistry, i.e., the condensation reaction between a carbonyl and an amine. In this report, we elaborate on the condensation of 3,6-dibromobenzene-1,2,4,5-tetraamine with hexaketocyclohexane (HKH) and the subsequent carbonylation of the resulting COF, along with the possibility that the condensation reaction on HKH can result in a trans configuration resulting in the formation of a disordered 2D-COF. This strategy enables modification of COFs via bromine substitution reactions to place functional groups within the pores of the materials. Ion-sieving measurements using membranes from this COF, reaction of small molecules with unreacted keto groups along with modeling studies indicate disorder in the COF polymerization process. We also present a Monte Carlo simulation that demonstrates the influence of even small amounts of disorder upon both the 2D and 3D structure of the resulting COF.

Water Oxidation with Cobalt-Loaded Linear Conjugated Polymer Photocatalysts

The first examples of linear conjugated organic polymer photocatalysts that produce oxygen from water after loading with cobalt and in the presence of an electron scavenger are reported. The oxygen evolution rates, which are higher than for related organic materials, can be rationalized by a combination of the thermodynamic driving force for water oxidation, the light absorption of the polymer, and the aqueous dispersibility of the relatively hydrophilic polymer particles. We also used transient absorption spectroscopy to study the best performing system and we found that fast oxidative quenching of the exciton occurs (picoseconds) in the presence of an electron scavenger, minimizing recombination.

A Highly Crystalline Perylene Imide Polymer with the Robust Built-In Electric Field for Efficient Photocatalytic Water Oxidation

A highly crystalline perylene imide polymer (Urea-PDI) photocatalyst is successfully constructed. The Urea-PDI presents a wide spectrum response owing to its large conjugated system. The Urea-PDI performs so far highest oxygen evolution rate (3223.9 µmol g^-1 h^-1 ) without cocatalysts under visible light. The performance is over 107.5 times higher than that of the conventional PDI supramolecular photocatalysts. The strong oxidizing ability comes from the deep valence band (+1.52 eV) which is contributed by the covalent-bonded conjugated molecules. Besides, the high crystallinity and the large molecular dipoles of the Urea-PDI contribute to a robust built-in electric field promoting the separation and transportation of photogenerated carriers. Moreover, the Urea-PDI is very stable and has no performance attenuation after 100 h continuous irradiation. The Urea-PDI polymer photocatalyst provides with a new platform for the use of photocatalytic water oxidation, which is expected to contribute to clean energy production.

Visible light-driven simultaneous water oxidation and quinone reduction by a nano-structured conjugated polymer without co-catalysts

In artificial photosynthesis, chemists are aiming to borrow principles from natural photosynthesis to develop photoelectrochemical cells (PEC) for water splitting. The water plastoquinone photo-oxidoreductase enzyme, also known as photosystem II, uses light to perform the four-electron, four-proton oxidation of water to dioxygen and stores reducing equivalents in reduced forms of quinones which are ultimately used in dark reactions for the synthesis of energy-rich molecules. We report a nano-structured semiconducting conjugated polymer based on poly(diphenylbutadiyne) (nano-PDPB) and its photocatalytic activities towards the water oxidation reaction under visible light irradiation when dispersed in water in the absence of any sacrificial agents or co-catalysts. Charge recovery at the nano-PDPB directly or delayed in time was exemplified by the reduction of quinone acting as a hydrogen reservoir. In the absence of quinones as electron acceptors H2O2 formation was detected, stemming from the partial reduction of O2.

Recent Advances in Conjugated Polymers for Visible-Light-Driven Water Splitting

With the ambition of solving the challenges of the shortage of fossil fuels and their associated environmental pollution, visible-light-driven splitting of water into hydrogen and oxygen using semiconductor photocatalysts has emerged as a promising technology to provide environmentally friendly energy vectors. Among the current library of developed photocatalysts, organic conjugated polymers present unique advantages of sufficient light-absorption efficiency, excellent stability, tunable electronic properties, and economic applicability. As a class of rising photocatalysts, organic conjugated polymers offer high flexibility in tuning the framework of the backbone and porosity to fulfill the requirements for photocatalytic applications. In the past decade, significant progress has been made in visible-light-driven water splitting employing organic conjugated polymers. The recent development of the structural design principles of organic conjugated polymers (including linear, crosslinked, and supramolecular self-assembled polymers) toward efficient photocatalytic hydrogen evolution, oxygen evolution, and overall water splitting is described, thus providing a comprehensive reference for the field. Finally, current challenges and perspectives are also discussed.

Sr2NiWO6 Double Perovskite Oxide as a Novel Visible-Light-Responsive Water Oxidation Photocatalyst

Screening of stable visible-light-responsive water oxidation semiconductor photocatalysts is highly desired for the development of photocatalytic water splitting systems. Herein, a visible-light-absorbing Sr₂NiWO₆ double perovskite oxide photocatalyst was successfully prepared via a conventional solid-state reaction method. The intrinsic Sr₂NiWO₆ shows photocatalytic oxygen evaluation reaction (OER) activity of 60 μmol h^-1 g^-1, even without loading any cocatalysts. The DFT calculation indicates that the Ni species on the surface is the active site for the OER. The photocatalytic OER activity was further improved by loading Pt and RuO₂ dual redox cocatalysts on the surface of Sr₂NiWO₆ to achieve a photocatalytic OER activity of 420 μmol h^-1 g^-1, which corresponds to a remarkable apparent quantum efficiency (AQE) of 8.6% (λ ≈ 420 nm). The result indicates that Sr₂NiWO₆ is one of the best double perovskite oxide-based photocatalysts for the photocatalytic OER, and the activity is even comparable to the benchmark BiVO₄-based photocatalyst. The improvement of the photocatalytic OER activity is due to the provision of more active redox sites as well as the synergetic effect of the dual redox cocatalysts in facilitating charge separation and transfer. This work demonstrates that double perovskite oxides may serve as a novel class of efficient and stable oxide-based semiconductor photocatalysts for water splitting.

Novel BP/BiOBr S-scheme nano-heterojunction for enhanced visible-light photocatalytic tetracycline removal and oxygen evolution activity

Designing heterojunction photocatalysts with strong interfacial interaction and matched band structure is an effective way to reduce the recombination of photogenerated carriers. Herein, the exfoliated black phosphorus (BP) nanosheets were coupled with BiOBr nanosheets having higher Fermi level, and thereby it constructed a novel layered BP/BiOBr nano-heterojunction with chemically bonding, larger contact interface and unique band structures. BiOBr nanosheets were self-assembled on the surface of BP nanosheets by a facile liquid-phase ultrasound combined with solvothermal method. The photocatalytic performance for tetracycline (TC) degradation, oxygen evolution and H₂O₂ production rate of Sol-10BP/BiOBr was 7.8, 7.0 and 2.6 times than that of pure BiOBr, respectively. The in-situ generated H₂O₂ and OH became the main active species of mineralization and decomposition of TC. The novel S-scheme two-dimensional BP/BiOBr nano-heterojunction for boosting spatial charge separation retained the useful holes-electrons with higher redox ability, which was very beneficial for producing more OH, H₂O₂ and O₂, and the photocatalytic activity was greatly improved.

A Simple Molecular Design Strategy for Two-Dimensional Covalent Organic Framework Capable of Visible-Light-Driven Water Splitting

Two-dimensional (2D) covalent organic frameworks (COFs) are promising metal-free materials for photocatalytic water splitting because of their high surface area and predictability to assemble various molecules with tunable electronic properties. Unfortunately, 2D COFs capable of visible-light-driven photocatalytic overall water splitting are rare, partly due to rigorous requirements to their band alignments and coexistence of catalytic sites for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, 12 2D nitrogen-linked COFs are designed based on first-principles calculations and topological assembly of molecular segments with catalytic activities toward either HER or OER, respectively. The electronic band structures calculated with HSE06 method indicate that 2D COFs are semiconductors with a widely tunable bandgap ranging from 1.92 to 3.23 eV. The positions of both conduction and valence band edges of nine 2D COFs match well with the chemical reaction potential of H₂/H⁺ and O₂/H₂O, which are capable of photocatalytic overall water splitting. Of particular importance is that three of them based on 2,4,6-tris(4-methylphenyl)-1,3,5-triazine (TST) can split water into hydrogen and oxygen under visible light. Our results agree with respect to the literature, with three of them having been studied for photocatalytic HER or CO₂ reduction. In addition, we further experimentally demonstrate that I-TST presents both HER and OER activity under visible light. Our findings present a route to design practical 2D COFs as metal-free and single-material photocatalysts for overall water splitting under visible light.

Development of BiOI as an effective photocatalyst for oxygen evolution reaction under simulated solar irradiation

In this study, crystalline BiOI powders were prepared for photocatalytic O₂ evolution in the presence of NaIO₃ as the electron mediator.

High-temperature 2D ferromagnetism in conjugated microporous porphyrin-type polymers

The need for magnetic 2D materials that are stable to the enviroment and have high Curie temperatures is very important for various electronic and spintronic applications. We have found that two-dimensional porphyrin-type aza-conjugated microporous polymer crystals are such a material (Fe-aza-CMPs). Fe-aza-CMPs are stable to CO, CO2, and O2 atmospheres and show unusual adsorption, electronic, and magnetic properties. Indeed, they are semiconductors with small energy band gaps ranging from 0.27 eV to 0.626 eV. CO, CO2, and O2 molecules can be attached in three different ways where single, double, or triple molecules are bound to iron atoms in Fe-aza-CMPs. For different attachment configurations we find that for CO and CO2 a uniform distribution of the molecules is most energetically favorable while for O2 molecules aggregation is most energetically preferable. The magnetic moments decrease from 4 to 2 to 0 for singly, doubly, triply occupied configurations for all gasses respectively. The most interesting magnetic properties are found for O2 molecules attached to the Fe-aza-CMP. For a single attachment configuration we find that an antiferromagnetic state is favorable. When two O2 molecules are attached, the calculations show the highest exchange integral with a value of J = 1071 μeV. This value has been verified by two independent methods where in the first method J is calculated by the energy difference between ferromagnetic and anitferromagnetic configurations. The second method is based on the frozen magnon approach where the magnon dispersion curve has been fitted by the Ising model. For the second method J has been estimated at J = 1100 μeV in excellent agreement with the first method.

An overview of photocatalysis facilitated by 2D heterojunctions

Two-dimensional (2D) photocatalysts have attracted considerable research interest in the past decades due to their unique optical, physical and chemical properties. Constructing 2D/2D heterojunctions with large interface area has been considered as an effective approach to enhance the transfer rate and the separation efficiency of the charge carriers, leading to dramatic increase in the photocatalytic performance of the photocatalysts. Here, the state-of-the-art progress on heterojunctions based on 2D materials is reviewed, including the photocatalysis principles using 2D heterojunctions, the categories of 2D heterojunctions and their application in different photocatalytic reactions, and the theoretical studies of the 2D heterojunctions. Moreover, the advantages and disadvantages of the 2D heterojunctions are also discussed. Finally, the ongoing challenges and opportunities for the future development of 2D photocatalysts with built-in heterojunctions are proposed.

Ferromagnetism in 2D organic iron hemoglobin crystals based on nitrogenated conjugated micropore materials

In this work we study a low-cost two-dimensional ferromagnetic semiconductor with possible applications in biomedicine, solar cells, spintronics, and energy and hydrogen storage. From first principle calculations we describe the unique electronic, transport, optical, and magnetic properties of a π-conjugated micropore polymer (CMP) with three iron atoms placed in the middle of an isolated pore locally resembling heme complexes. This material exhibits strong Fe-localized dz2 bands. The bandgap is direct and equal to 0.28 eV. The valence band is doubly degenerate at the Γ-point and for larger k-wavevectors the HOMO band becomes flat with low contribution to charge mobility. The absorption coefficient is roughly isotropic. The conductivity is also isotropic with the nonzero contribution in the energy range 0.3-8 eV. The xy-component of the imaginary part of the dielectric tensor determines the magneto-optical Faraday and Kerr rotation. Nonvanishing rotation is observed in the interval of 0.5-5.0 eV. This material is found to be a ferromagnet of an Ising type with long-range exchange interactions with a very high magnetic moment per unit cell, m = 6 μB. The exchange integral is calculated by two independent methods: (a) from the energy difference between ferromagnetic and antiferromagnetic states and (b) from a magnon dispersion curve. In the former case Jnn = 27 μeV. In the latter case the magnon dispersion is fitted by the Ising model with the nearest and next-nearest neighbor spin interactions. From these estimations we find that Jnn = 19.5 μeV and Jnnn = -3 μeV. Despite the different nature of the calculations, the exchange integrals are only within 28% difference.

Fully Conjugated Covalent Organic Polymer with Carbon-Encapsulated Ni2P for Highly Sustained Photocatalytic H2 Production from Seawater

Organic photocatalysts are widely used to mimic artificial photosynthesis for sustainable solar-driven hydrogen production from water splitting. However, few photocatalytic H₂ production is reported using seawater, which is a significantly important parameter for practical application, and most organic photocatalysts employed precious and scarce Pt as a cocatalyst. Herein, we report an organic hybridized photocatalyst (termed COP-TF@CNi₂P), carbon-encapsulated nickel phosphide, as a cocatalyst loaded on a fully conjugated organic polymer, which is applied for stable and efficient H₂ generation from seawater splitting. Both experiments and theory calculations suggest that the carbon layers covered around nickel phosphide not only can strengthen π-π interactions with the polymers but also can attract the photoinduced electrons from COP-TF to the surface of CNi₂P, which contributes to expedite exciton dissociation. As a result, the as-synthesized COP-TF@CNi₂P achieves a remarkable photocatalytic H₂ production efficiency up to 2500 μmol g^-1 h^-1 (λ ≥ 400 nm) from seawater and even maintains 92% of initial efficiency after 16 intermittent cycles, which lasts for half a month.

A Covalent Triazine-Based Framework Consisting of Donor-Acceptor Dyads for Visible-Light-Driven Photocatalytic CO2 Reduction

Photocatalytic conversion of CO₂ into value-added chemical fuels is a promising approach to address the depletion of fossil energy and environment-related concerns. Tailor-making the electronic properties and band structures of photocatalysts is pivotal to improve their efficiency and selectivity in photocatalytic CO₂ reduction. Herein, a covalent triazine-based framework was developed containing electron-donor triphenylamine and electron-acceptor triazine components (DA-CTF). The engineered π-conjugated electron donor-acceptor dyads in DA-CTF not only optimized the optical bandgap but also contributed to visible-light harvesting and migration of photoexcited charge carriers. The activity of photocatalytic CO₂ reduction under visible light was significantly improved compared with that of traditional g-C₃ N₄ and reported covalent triazine-based frameworks. This study provides molecular-level insights into the mechanism of photocatalytic CO₂ reduction.

Protonation-Assisted Exfoliation of N-Containing 2D Conjugated Polymers

Ultrathin 2D conjugated polymer nanosheets are an emerging class of photocatalysts for solar-to-chemical energy conversion. Until now, the majority of ultrathin 2D polymer photocatalysts are produced through exfoliation of layered polymers. Unfortunately, it still remains a great challenge to exfoliate layered polymers into ultrathin nanosheets with high yields. In this work, a liquid-phase protonation-assisted exfoliation is demonstrated to enable remarkably improved exfoliation yields of various 2D N-containing conjugated polymers such as g-C₃ N₄ , C₂ N, and aza-CMP. The exfoliation yields are only 2-15% in pure water whereas they can be substantially improved to 41-56% in 12 m HCl. The exfoliated ultrathin nanosheets possess average thicknesses less than 5 nm and can be easily dispersed in aqueous solutions. More importantly, the exfoliated nanosheets exhibit significantly enhanced photocatalytic activity toward photocatalytic water splitting compared to their bulk counterparts. Further characterizations and computational calculations reveal that protonation of the heterocyclic nitrogen sites in the conjugated polymer frameworks can lead to strong hydrogen bonding between the polymer surfaces and water molecules, resulting in facilitated exfoliation of polymers into the liquid phase. This study unveils an important protocol toward producing ultrathin 2D N-containing conjugated polymer nanosheets for future solar energy conversion.

Tumor Reoxygenation and Blood Perfusion Enhanced Photodynamic Therapy using Ultrathin Graphdiyne Oxide Nanosheets

Both diffusion-limited and perfusion-limited hypoxia are associated with tumor progression, metastasis, and the resistance to therapeutic modalities. A strategy that can efficiently overcome both types of hypoxia to enhance the efficacy of cancer treatment has not been reported yet. Here, it is shown that by using biomimetic ultrathin graphdiyne oxide (GDYO) nanosheets, both types of hypoxia can be simultaneously addressed toward an ideal photodynamic therapy (PDT). The GDYO nanosheets, which are oxidized and exfoliated from graphdiyne (GDY), are able to efficiently catalyze water oxidation to release O₂ and generate singlet oxygen (¹O₂) using near-infrared irradiation. Meanwhile, GDYO nanosheets also exhibit excellent light-to-heat conversion performance with a photothermal conversion efficiency of 60.8%. Thus, after the GDYO nanosheets are coated with iRGD peptide-modified red blood membrane (i-RBM) to achieve tumor targeting, the biomimetic GDYO@i-RBM nanosheets can simultaneously enhance tumor reoxygenation and blood perfusion for PDT. This study provides new insights into utilizing novel water-splitting materials to relieve both diffusion- and perfusion-limited hypoxia for the development of a novel therapeutic platform.

Layered Thiazolo[5,4- d] Thiazole-Linked Conjugated Microporous Polymers with Heteroatom Adoption for Efficient Photocatalysis Application

Conjugated microporous polymers (CMPs) with high surface areas, tunable building blocks, and fully conjugated structures have found important applications in optoelectronics. Here, we report a new series of CMPs with tunable band gaps by introducing thiazolo[5,4- d] thiazole as the linkage. Because they are synthetic polymers, the geometries and structures could be rationally designed. Their intrinsic wide visible-light absorption properties and layered architectures endow them with a promising photocatalytic performance. The role of geometries, surface areas, and morphologies of the CMPs in photocatalysis abilities is examined and discussed. The results indicate that geometries have a direct impact on the surface areas and morphologies of the CMPs and thus exert great influence on photocatalysis.

Carbon-Based Photocathode Materials for Solar Hydrogen Production

Hydrogen is considered a promising environmentally friendly energy carrier for replacing traditional fossil fuels. In this context, photoelectrochemical cells effectively convert solar energy directly to H₂ fuel by water photoelectrolysis, thereby monolitically combining the functions of both light harvesting and electrolysis. In such devices, photocathodes and photoanodes carry out the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively. Here, the focus is on photocathodes for HER, traditionally based on metal oxides, III-V group and II-VI group semiconductors, silicon, and copper-based chalcogenides as photoactive material. Recently, carbon-based materials have emerged as reliable alternatives to the aforementioned materials. A perspective on carbon-based photocathodes is provided here, critically analyzing recent research progress and outlining the major guidelines for the development of efficient and stable photocathode architectures. In particular, the functional role of charge-selective and protective layers, which enhance both the efficiency and the durability of the photocathodes, is discussed. An in-depth evaluation of the state-of-the-art fabrication of photocathodes through scalable, high-troughput, cost-effective methods is presented. The major aspects on the development of light-trapping nanostructured architectures are also addressed. Finally, the key challenges on future research directions in terms of potential performance and manufacturability of photocathodes are analyzed.

Hybrid materials based on conjugated polymers and inorganic semiconductors as photocatalysts: from environmental to energy applications

Hybrid materials photocatalysts based on conjugated polymers and inorganic semiconductors provide a sustainable way to remove pollutants or store energy in the form of solar fuels by processes such as water splitting, CO₂ photoreduction and NH₃ production (artificial photosynthesis).