Triazine Units Research Articles

Fully Condensed Poly (Triazine Imide) Crystals: Extended π‐Conjugation and Structural Defects for Overall Water Splitting

AbstractConventional polymerization for the synthesis of carbon nitride usually generates amorphous heptazine‐based melon with an abundance of undesired structural defects, which function as charge carrier recombination centers to decrease the photocatalytic efficiency. Herein, a fully condensed poly (triazine imide) crystal with extended π‐conjugation and deficient structure defects was obtained by conducting the polycondensation in a mild molten salt of LiCl/NaCl. The melting point of the binary LiCl/NaCl system is around 550 °C, which substantially restrain the depolymerization of triazine units and extend the π‐conjugation. The optimized polymeric carbon nitride crystal exhibits a high apparent quantum efficiency of 12 % (λ=365 nm) for hydrogen production by one‐step excitation overall water splitting, owing to the efficient exciton dissociation and the subsequent fast transfer of charge carriers.

Intrareticular charge transfer regulated electrochemiluminescence of donor\u2013acceptor covalent organic frameworks

The control of charge transfer between radical anions and cations is a promising way for decoding the emission mechanism in electrochemiluminescence (ECL) systems. Herein, a type of donor-acceptor (D-A) covalent organic framework (COF) with triphenylamine and triazine units is designed as a highly efficient ECL emitter with tunable intrareticular charge transfer (IRCT). The D-A COF demonstrates 123 folds enhancement in ECL intensity compared with its benzene-based COF with small D-A contrast. Further, the COF’s crystallinity- and protonation-modulated ECL behaviors confirm ECL dependence on intrareticular charge transfer between donor and acceptor units, which is rationalized by density functional theory. Significantly, dual-peaked ECL patterns of COFs are achieved through an IRCT mediated competitive oxidation mechanism: the coreactant-mediated oxidation at lower potential and the direct oxidation at higher potential. This work provides a new fundamental and approach to improve the ECL efficiency for designing next-generation ECL devices.

Synergistic effect of KCl mixing and melamine/urea mixture in the synthesis of g-C3N4 for photocatalytic removal of tetracycline

To prepare g-C3N4 for photocatalytic tetracycline (TC) removal, KCl was employed to mix with or cover the top of the precursors, namely, melamine and urea. The mixing of KCl with the precursor will result in the incorporation of KCl within the layer structure of g-C3N4, whereas KCl covering the top might not have such an effect. Different precursor ratios contributed to the formation of heptazine-rich or triazine-rich units in the g-C3N4 structure. Melamine applied alone as a precursor will undergo a phase transformation into melam and triazine-rich g-C3N4, whereas with the addition of urea, the mixture will polymerize to form melem and heptazine-rich g-C3N4. The KCl-incorporated, heptazine-rich g-C3N4 (KCN80m) exhibited an improved photocatalytic activity for TC removal (greater than 80% during a 120-min period for 50 mL of a 20-ppm TC solution). The enhanced activity can be attributed to the improved charge separation through an electron and hole transfer through the K+ and Cl- sites, respectively; the formation of a nanojunction between the triazine and heptazine units of g-C3N4; an increased number of photoexcited electrons, indicated by the electron paramagnetic resonance spectroscopy results. We varied the mixing conditions of KCl and precursor ratio to synthesize different KCl-incorporated heptazine-rich g-C3N4 samples for effective removal of TC from water through photocatalysis.

Creating triazine units to bridge carbon nitride with titania for enhanced hydrogen evolution performance

In this work, a wealth of triazine units was created in carbon nitride through a facile molten salt method to bridge titania and carbon nitride for accelerating charge transportation and enhancing hydrogen production performance. The doping of triazine ring into C3N4 framework results in more exposure of − CN − and − CN bond and forms a homojunction (MCN), which favors photocatalysis by acting as photoresponse and active centers, respectively. Moreover, the triazine units can bridge the hybridized C3N4 and TiO2, forming a stable MCN/TiO2 homo-heterojunction. Attributed to the matched band energy structure of MCN and TiO2 and the structural characteristics of triazine/heptazine heterocyclic, the light response, charge separation and transfer as well as the lifetime of carriers on MCN/TiO2 hybrid are improved significantly. As a result, the MCN/TiO2 homo-heterojunction exhibits excellent activity and stability for photocatalytic hydrogen production performance, up to 2594 μmol∙g−1∙h−1 under simulated solar irradiation, which is 5.5 times higher than that of the bare g-C3N4.

Hybrid Inorganic-Organic Visible-Light-Driven Microrobots Based on Donor-Acceptor Organic Polymer for Degradation of Toxic Psychoactive Substances.

Light-driven microrobots based on organic semiconductors have received tremendous attention in the past few years due to their unique properties, such as ease of reactivity tunability, band-gap modulation, and low cost. However, their fabrication with defined morphologies is a very challenging task that results in amorphous microrobots with poor motion efficiencies. Herein, we present hybrid inorganic-organic photoactive microrobots with a tubular shape and based on the combination of a mesoporous silica template with an active polymer containing thiophene and triazine units (named as Tz-Th microrobots). Owing to their well-defined tubular structure, such Tz-Th microrobots showed efficient directional motion under fuel-free conditions. Depending on the accumulation of the polymer coating, these microdevices also exhibited stand-up and rotation motion. As a proof-of-concept, we use these hybrid microrobots for the capture and degradation of toxic psychoactive drugs commonly found in wastewater effluents such as methamphetamine derivatives. We found that the microrobots were able to decompose the drug into small organic fragments after 20 min of visible light irradiation, reaching total intermediates removal after 2 h. Therefore, this approach represents a versatile and low-cost strategy to fabricate structured organic microrobots with efficient directional motion by using inorganic materials as the robot chassis, thereby maintaining the superior photocatalytic performance usually associated with such organic polymers.

Oxygen and methyl co-modified carbon nitride for enhanced photocatalytic dagradation

Carbon nitride (C3N4) is a promising photocatalytic material to degrade various pollutants. However, the degradation activity is restricted by the limited light absorption and fast recombination of photoinduced carriers. Herein, a structure modification strategy by introducing a functional reagent during the polymerization process was adopted. The structure, composition and morphology of prepared materials were investigated by X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. Benefiting from the implantation of oxygen and methyl groups in triazine unit of C3N4, enhanced light absorption and effective carrier separation are achieved. As a result, the modified C3N4 exhibits a significant enhanced degradation activity and the optimal rate constant of modified C3N4 for Acid Red 9 degradation is 5.83 times that of pristine C3N4. The work demonstrates the effect of structure modification in C3N4 for enhancing degradation activity.

The Synthesis of a Covalent Organic Framework from Thiophene Armed Triazine and EDOT and Its Application as Anode Material in Lithium-Ion Battery.

As a class of redox active materials with some preferable properties, including rigid structure, insoluble characters, and large amounts of nitrogen atoms, covalent triazine frameworks (CTFs) have been frequently adopted as electrode materials in Lithium-ion batteries (LIBs). Herein, a triazine-based covalent organic framework employing 3,4-ethylenedioxythiophene (EDOT) as the bridging unit is synthesized by the presence of carbon powder through Stille coupling reaction. The carbon powder was added in an in-situ manner to overcome the low intrinsic conductivity of the polymer, which led to the formation of the polymer@C composite (PTT-O@C, PTT-O is a type of CTFs). The composite material is then employed in LIBs as anode material. The designed polymer shows a narrow band gap of 1.84 eV, proving the effectiveness of the nitrogen-enriched triazine unit in reducing the band gap of the resultant polymers. The CV results showed that the redox potential of the composite (vs. Li/Li+) is around 1.0 V, which makes it suitable to be used as the anode material in lithium-ion batteries. The composite material could exhibit the stable specific capacity of 645 mAh/g at 100 mA/g and 435 mAh/g at 500 mA/g, respectively, much higher than the pure carbon materials, indicating the good reversibility of the material. This work provides some additional information on electrochemical performance of the triazine and EDOT based CTFs, which is helpful for developing a deep understanding of the structure–performance correlations of the CTFs as anode materials.

Type-II surface heterojunction of bismuth-rich Bi4O5Br2 on nitrogen-rich g-C3N5 nanosheets for efficient photocatalytic degradation of antibiotics

A novel g-C3N5/Bi4O5Br2 surface heterojunction was developed via in-situ growth of Bi-rich Bi4O5Br2 on g-C3N5 nanosheets. The optimal composite achieved 3.6- and 16.0- times of sulfathiazole (STZ) degradation activity when compared with pristine Bi4O5Br2 and g-C3N5. The interlayer stacking morphology and extra nitrogen in triazine units significantly narrowed the conduction band of g-C3N5, which greatly promoted its visible utilization; while the bismuth-rich property of Bi4O5Br2 prolonged the excited charge carrier lifetime. Both photoluminescence and electrochemical impedance spectroscopy analysis demonstrated that the type-II surface heterojunction (g-C3N5/Bi4O5Br2) offered remarkable charge transfer and separation due to the matched energy band structure. The STZ degradation mechanism and pathways were proposed based on experiments and density functional theory calculation, and the contribution of reactive species for STZ degradation followed the order of O2∙- > h+ >OH. Moreover, the toxicity evolution of STZ was evaluated, suggesting that sufficient mineralization is required to ensure safe discharge. The Box-Behnken experimental design methodology study revealed that g-C3N5/Bi4O5Br2 exhibited high reactivity for antibiotics degradation under different water matrix. This study suggested that g-C3N5/Bi4O5Br2 has great application potential for cost-effective remediation of persistent organic contaminants by using solar light.

Benzonitirile modified N type host for exciplex host to enhance efficiency and lifetime in blue phosphorescent organic light-emitting diodes

Benzonitirile modified host materials, 3-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-(triphenylsilyl)benzonitrile (CNmSi-Trz), 3-(4-(dibenzo[b,d]furan-2-yl)-6-phenyl-1,3,5-triazin-2-yl)-5-(triphenylsilyl)benzonitrile (CNmSi-2DBF-Trz) and 3-(4-(dibenzo[b,d]furan-4-yl)-6-phenyl-1,3,5-triazin-2-yl)-5-(triphenylsilyl)benzonitrile (CNmSi-4DBF-Trz), were synthesized for improved efficiency and lifetime in blue phosphorescent organic light-emitting diodes (PhOLEDs). Triazine and benzonitrile units were introduced commonly to the three host materials for electron transporting properties. The three hosts were applied to mixed host as electron transport type hosts with a hole transport type 3,3-di(9H-carbazol-9-yl)biphenyl (mCBP) for blue PhOLEDs. They formed exciplex with mCBP, and the exciplex of CNmSi-2DBF-Trz with mCBP showed high triplet energy above 2.9 eV. As a result, the blue PhOLEDs fabricated with the CNmSi-2DBF-Trz host achieved high EQE over 23% and 650 h lifetime up to 70% of initial luminance at 200 cd/m2 using the exciplex host system with mCBP.

Ionothermal synthesis of phosphonitrilic-core covalent triazine frameworks for carbon dioxide capture

Covalent Triazine Frameworks (CTFs) are a class of porous organic frameworks with tunable properties obtainable through rational design targeting a wide range of applications. In the present work, we developed a series of CTFs with phosphazene core for CO2 adsorption. The CTFs were synthesized using building blocks of hexakis(oxy)hexabenzonitrile phosphazene (HCPz) under ionothermal conditions and ZnCl2 as catalyst. The effect of ZnCl2/monomer ratio and the ionothermal reaction temperature and synthesis procedure on the porosity of the Pz-CTFs were systematically explored. The Pz-CTF synthesized using a molar ratio of ZnCl2/monomer of 10 with gradient ionothermal reaction conditions (400 °C/25 h; 450 °C/13 h; 500 °C/1h; 600 °C/1h) yielded an amorphous, predominantly ultra-microporous material (Pz-CTF6) with high surface area (SBET of 1009 m2 g−1). The primary aspect of the applied gradient ionothermal reaction scheme is that it induces a simultaneous reversible and irreversible trimerization of nitriles, allowing restructuring of the triazine units, thus resulting in an extended microporous network. Besides, the highly crosslinked, electron-rich phosphazene core provided high structural and thermal stability (up to 500 °C) and enhanced CO2-philicity of the synthesized CTFs. As a result, Pz-CTF6 showed a CO2 uptake capacity of 4.19 and 2.47 mmol g−1 at 273 and 298 K, respectively, at 1 bar. In addition, it exhibited a high CO2/N2 selectivity of 147 for a feed containing 85% of CO2 at 50 mbar and 298 K, as determined by the IAST method. Furthermore, the developed adsorbents exhibited enhanced hydrophobicity, causing a rather mild reduction in CO2 capacity when humidity conditions were applied. These findings show that the synthesized Pz-CTFs are promising for CO2 capture and adsorptive separation.

Triazine-Based Conjugated Microporous Polymers for Efficient Hydrogen Production.

Developing visible-light-active porous organic polymers with high photocatalytic efficiency is highly desirable. Here, two triazine-based conjugated microporous polymers were synthesized. The structures were controllably adjusted to explore the structure–photocatalytic activity relationship. T-CMP-1 containing more triazine units exhibited a hydrogen evolution rate of 3214.3 μmol h–1 g–1, much higher than that of T-CMP-2 (242.1 μmol h–1 g–1). The increasing contents of triazine units bring better hydrogen evolution efficiency.

Enormous Promotion of Photocatalytic Activity through the Use of Near-Single Layer Covalent Organic Frameworks

Enormous Promotion of Photocatalytic Activity through the Use of Near-Single Layer Covalent Organic Frameworks

A novel conjugated porous polymer based on triazine and imide as cathodes for sodium storage

AbstractOrganic electrode materials have attracted more and more attention for sodium‐ion batteries (SIBs) that are regarded as one of the most promising alternatives of lithium‐ion batteries, because they can endure the storage of large sodium ions (with a larger radius than that of lithium ions) without obvious volume change. Herein, we report a novel conjugated porous polymer (TPIP) based on triazine and imide as cathodes material for SIBs. TPIP has abundant redox‐active sites, good thermal stability (400°C) and large specific surface area (306 m2 g−1). As a result, TPIP electrode delivered a specific capacity of 120 mAh g−1 after 50 cycles at a current density of 0.1 A g−1 and 85 mAh g−1 after 150 cycles at a current density of 1.0 A g−1. Ex‐situ X‐ray photoelectron spectra and Fourier transform infrared spectra showed that the TPIP electrodes reversibly stored three sodium ions per unit through the triazine rings and half of the carbonyl groups. These results deepen our understanding of charge storage mechanisms of polymers with triazine and imide units and will provide guidance for the future design of electrode materials for high‐performance SIBs.

Robust Barium Phosphonate Metal–Organic Frameworks Synthesized under Aqueous Conditions

The design and discovery of three-dimensional crystalline metal–organic frameworks (MOFs) from linkers with phosphonate coordinating groups and even alkaline earth metals is largely undeveloped. Herein, we report a strategy for realizing new, stable, and robust barium phosphonate MOFs, termed Empa-1 and Empa-2. The two-dimensional (2D) Empa-1 or three-dimensional (3D) Empa-2 could be realized by way of systematically modulating the ratio of Ba2+ with a tetratopic phosphonate-based linker that was crafted to incorporate nitrogen-rich triazine units bridged by a fixed piperazine core. In addition to this synthetic approach, temperature-dependent synchrotron-radiation powder X-ray diffraction analysis demonstrated that the 2D Empa-1 undergoes an irreversible phase transition upon heating and subsequent dehydration to form the 3D Empa-2. Given the presence of uncoordinated phosphonic acid moieties within the structure of 3D Empa-2, the CO2 sorption capabilities are reported. We believe our ability to link the alkaline earth metal barium with a novel tetratopic phosphonate linker, as evidenced by the robust structures of Empa-1 and -2, paves the way for further exploration and discovery of new crystalline, porous frameworks with greater structural diversity, stability, and wide-scale practical applicability.

Directional modulation of triazine and heptazine based carbon nitride for efficient photocatalytic H2 evolution

Hollow SPP-MS-CN nanotubes have been developed through a green one-step approach by sodium polyphosphate (SPP) in-situ mediated molten salt (MS) thermal condensation of melamine for the first time. In which, the structure of SPP-MS-CN with triazine and heptazine units can been directionally modulated by the SPP content. The molten salts at high-temperature function as green solvents to facilitate mass transfer, which tends to form triazine based CN. Addition of SPP mediates the fluidity of molten salt system, which in-situ modulates the thermal condensation process, boosting the formation of the extended π-conjugated system of heptazine based CN. The 0.5 wt% SPP-MS-CN exhibit photocatalytic H2 evolution rate up to 1785 µmol h−1 g−1, which is 2.16 times higher than that of molten salt synthesis of carbon nitride (MS-CN), even 14.5 times enhancement of photocatalytic H2 evolution rate over pure CN obtained by directly thermal condensation of melamine. It reveals that the hollow structures endow SPP-MS-CN with increased specific surface area, and the formed heptazine structure of SPP-MS-CN benefit for charge carrier separation and transfer and H2 reduction ability, which favors for the remarkable photocatalytic activity. Our work opens a new window for in-situ modulation of the structure of CN photocatalysts.

Influence of Phosphorus Doping on Triazole-Based g-C3N5 Nanosheets for Enhanced Photoelectrochemical and Photocatalytic Performance.

Triazole-based g-C3N5, a potential catalyst, has received little attention over the years. We prepared phosphorus-doped g-C3N5 with one triazole and two triazine units for the first time to investigate its photoelectrochemical (PEC) and photocatalytic properties. The doping states and crystalline structures of the samples were determined using X-ray techniques, namely, X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption fine structure analysis. Our results suggested that the phosphorus was substituted into carbon sites form P-N/P═N bonds with four coordination, which contribute P 2p level donor states in the band gap to enhance light absorption and reduce charge separation. Therefore, P-doped g-C3N5 exhibited higher PEC current density and better photocatalytic efficiency toward the degradation of rhodamine B dye or tetracycline under light irradiation compared to the undoped g-C3N5 sample. However, excess phosphorus doping resulted in the formation of impurities and disrupted the triazine and triazole units, reducing the PEC and photocatalytic efficiency. In summary, P-doped g-C3N5 was successfully prepared in the present study and represents a promising, facile, and effective catalyst for energy applications and environmental remediation.

Rational design of covalent organic frameworks as a groundbreaking uranium capture platform through three synergistic mechanisms

Herein, we report the first example of covalent organic framework (DHBD-TMT) linked by unsubstituted olefin-linkages for selective loading, chemical reduction and photocatalytic reduction of uranium. The unique structures of DHBD-TMT possess all the characteristics to be well suited as a capture platform for selective ligand complexation, efficient chemical reduction and photocatalytic reduction of uranium, thus exhibiting a groundbreaking uranium capture capacity (2640.8 mg g-1). In the dark, DHBD-TMT can effectively adsorb uranium through the hydroxyl groups laced on the skeleton and reduce UVI to UIV in situ, leading to a higher adsorption capacity and selectivity of uranium. At the same time, the synergistic effect of the hydroquinone and triazine units in the extended π-conjugated skeleton significantly improve the photocatalytic activity of DHBD-TMT, and an additional UVI photocatalytic reduction mechanism can occur under visible light irradiation, allowing significantly higher the capacity and faster adsorption kinetics.

Vinylene-linked covalent organic frameworks with enhanced uranium adsorption through three synergistic mechanisms

So far, it remains a challenge to synthesize uranium adsorbents with robust stability, high adsorption capacity, excellent photocatalytic activity and easy regeneration. Herein, we report the first example of vinylene-linked covalent organic framework (Tp-TMT) with enhanced uranium adsorption through combined selective ligand binding, chemical reduction and photocatalytic reduction. The unique structure and excellent photocatalytic activity of Tp-TMT make it very suitable for photo-enhanced uranium adsorption through three synergistic mechanisms, thus exhibiting an outstanding uranium adsorption capacity (2362.4 mg g−1). In the dark, a large number of hydroxyl groups in the Tp-TMT framework serve as selective binding sites for uranium, and reduce part of U(VI) to U(IV), thereby greatly improving the adsorption capacity. Meanwhile, the synergistic effect of the triazine units and hydroxyl groups in the highly conjugated framework greatly decreases the optical band gap of Tp-TMT, and an additional U(VI) photocatalytic reduction process can occur under light irradiation, further increasing the adsorption kinetics and capacity. This work explored the structural and functional design of covalent organic frameworks for the adsorption and reduction of uranium in nuclear industry wastewater.

Atomic-level localization of π-electrons in defect engineered tri-s-triazine units for increased photocatalytic hydrogen generation of polymeric carbon nitride

N defects in tri-s-triazine units can localize π electrons to reduce charge recombination and act as active sites, thus improving the efficiency of photocatalytic hydrogen production.

A new two-dimensional covalent organic framework with intralayer hydrogen bonding as supercapacitor electrode material

Abstract A new two-dimensional covalent organic framework (DBT-MA-COF) is designed and synthesized via Carbon–Nitrogen coupling reaction by using 2,5-dibromothiophene (DBT) and melamine (MA) as the monomers. The C−Hˑ N intralayer hydrogen bonding exists in the structure of DBT-MA-COF so that a plane rigid structure can be maintained. The material exhibits a quasi-reversible redox process caused by triazine units and the transformation of the quinone and aromatic structures to make the DBT-MA-COF possess pseudocapacitive characteristics. DBT-MA-COF has high heteroatom content for the existence of triazine and thiophene units, which can accelerate the transfer of electrolyte ions on the surface of DBT-MA-COF electrode and improve the solution conductivity of the material. The assembled asymmetric supercapacitor DBT-MA-COF//C-CTS ASC has a high energy density of 32.1 W h kg−1 with the power density of 800 W kg−1. The cycle life of the device is excellent which can reach 83% even after 30,000 continuous galvanostatic charge-discharge cycles. The strategy in this work provides a line of thinking to design and prepare new COFs used as long life and pseudocapacitive supercapacitor materials.