Heptazine Imide Research Articles

Poly (heptazine imide) nanocrystal for hydrogen peroxide evolution in the dark by accumulating photo-generated electrons

Poly (heptazine imide) nanocrystal for hydrogen peroxide evolution in the dark by accumulating photo-generated electrons

Mixed Crystalline Covalent Heptazine Frameworks with Built-in Heterojunction Structures towards Efficient Photocatalytic Formic Acid Dehydrogenation.

Covalent heptazine frameworks (CHFs) are widely utilized in the recent years as potential photocatalysts. However, their limited conjugated structures, low crystallinity and small surface areas have restricted the practical photocatalysis performance. Along this line, we report herein the synthesis of a kind of mixed crystalline CHF (m-CHF-1) with built-in heterojunction structure, which can efficiently catalyze the formic acid dehydrogenation by visible light driven photocatalysis. The m-CHF-1 is synthesized from 2,5,8-triamino-heptazine and dicyanobenzene (DCB) in the molten salts, in which DCB plays as organic molten co-solvent to promote the rapid and ordered polymerization of 2,5,8-triamino-heptazine. The m-CHF-1 is formed by embedding phenyl-linked heptazine (CHF-Ph) units in the poly(heptazine imide) (PHI) network similar to doping. The CHF-Ph combined with PHI form an effective type II heterojunction structure, which promote the directional transfer of charge carriers. And the integration of CHF-Ph makes m-CHF-1 have smaller exciton binding energy than pure PHI, the charge carriers are more easily dissociated to form free electrons, resulting in higher utilization efficiency of the carriers. The largest hydrogen evolution rate reaches a value of 42.86 mmol h-1 g-1 with a high apparent quantum yield of 24.6 % at 420 nm, which surpasses the majority of other organic photocatalysts.

Atomically dispersed low-valent Au boosts photocatalytic hydroxyl radical production.

Providing affordable, safe drinking water and universal sanitation poses a grand societal challenge. Here we developed atomically dispersed Au on potassium-incorporated polymeric carbon nitride material that could simultaneously boost photocatalytic generation of ·OH and H2O2 with an apparent quantum efficiency over 85% at 420 nm. Potassium introduction into the poly(heptazine imide) matrix formed strong K-N bonds and rendered Au with an oxidation number close to 0. Extensive experimental characterization and computational simulations revealed that the low-valent Au altered the materials' band structure to trap highly localized holes produced under photoexcitation. These highly localized holes could boost the 1e- water oxidation reaction to form highly oxidative ·OH and simultaneously dissociate the hydrogen atom in H2O, which greatly promoted the reduction of oxygen to H2O2. The photogenerated ·OH led to an efficiency enhancement for visible-light-response superhydrophilicity. Furthermore, photo-illumination in an onsite fixed-bed reactor could disinfect water at a rate of 66 L H2O m-2 per day.

Polymeric triazine/heptazine imide heterostructures enable photocatalytic O2reduction to H2O2

Heptazine-based carbon nitrides are a class of transition metal free semiconductors, which are extensively studied in various photocatalytic processes. The localized nature of the excitons, incomplete exciton conversion into the charge-separated state and the recombination of the charge carriers are the factors that limit performance of the pristine heptazine-based carbon nitrides. Herein, we design heterojunctions composed of poly(triazine imide) intercalated by LiCl and poly(heptazine imide) phases. The donor-acceptor character of the heterojunctions improves the charge separation yield. The heterojunction with the optimal composition of the phases shows an apparent quantum yield of H2O2 formation of 14 % at 365 nm and 7 % at 465 nm, while the H2O2 production rate reaches 7.8 mol L−1 gphotocatalyst−1 h−1 (39 mmol L−1 h−1 produced by 5 mg of photocatalyst) in batch and 15.9 mol kgphotocatalyst−1 h−1 in flow using a packed-bed photoreactor. The heterojunction undergoes photocharging under anaerobic conditions. The charges stored in the material after irradiation enable O2 reduction to H2O2 in the dark with the rate 367 µmol gphotocatalyst−1 h−1 (1.8 µmol h−1 stored in 5 mg of photocatalyst).

Recent Advances in Poly‐Heptazine Imide and Based Materials for Photocatalytic Hydrogen Evolution

AbstractThis review presents an overview of recent developments and advancements in the field of photocatalytic hydrogen evolution using poly(heptazine imide) (PHI) and based materials. Photocatalytic hydrogen evolution (PHE) has received great amount of attention because of its potential as a clean and sustainable energy source and PHI based materials have emerged as promising candidates for this purpose due to their distinct structural properties, improved light absorption, charge separation and reduced recombination. This review discusses the essential concepts of PHE and the potential of poly(heptazine imide) materials and their derivatives as photocatalysts and also the limitations of these materials in photocatalytic activity. It addresses the synthetic routes and strategies adopted to enhance the photocatalytic activity of these materials including surface modifications, band engineering, doping, loading co‐catalysts and composites & heterojunction construction. Furthermore, the recent works in photocatalytic H2 evolution using PHI and based materials have been summarized and discussed. Additionally, we examined the challenges and future outlooks in this field, emphasizing the need for scalable and cost‐effective production methods.

Boosted scavenger-free photocatalytic H2O2 production over alkali-doped poly(heptazine imide) under controlled solution conditions

Photocatalysis renders appealing mechanisms for synthesizing hydrogen peroxide (H2O2) under ambient conditions; however, it still suffers from low efficiency due to the high kinetic barriers, low reaction selectivity, and severe recombination of photo-generated charge carriers. This study contributes to establishing a fundamental understanding of the influence of the solution environment, specifically the solution pH, on the photocatalytic performance of H2O2 production. By adjusting the solution environment from neutral to alkaline, the photocatalytic system, consisting of a prototype photocatalyst of alkali-doped poly(heptazine imide), exhibited a 9-fold increase in the production of H2O2, with an unprecedented rate of 6.78 gcat−1h−1. Mechanistic studies demonstrate that alkaline conditions can trigger the rarely occurring two-electron water oxidization, thus enabling H2O2 formation through both the oxygen reduction reaction and water oxidization pathways simultaneously. Moreover, the high pH condition also plays an important role in shifting the band edge of the photocatalyst and alleviating the photocatalytic decomposition of H2O2. These fundamental insights may provide guidelines for the development of superior photocatalytic systems.

Supercritical CH3OH-Triggered Isotype Heterojunction and Groups in g-C3N4 for Enhanced Photocatalytic H2 Evolution.

The structure tuning of bulk graphitic carbon nitride (g-C3N4) is a critical way to promote the charge carriers dynamics for enhancing photocatalytic H2-evolution activity. Exploring feasible post-treatment strategies can lead to effective structure tuning, but it still remains a great challenge. Herein, a supercritical CH3OH (ScMeOH) post-treatment strategy (250-300 °C, 8.1-11.8 MPa) is developed for the structure tuning of bulk g-C3N4. This strategy presented advantages of time-saving (less than 10 min), high yield (over 80%), and scalability due to the enhanced mass transfer and high reactivity of ScMeOH. During the ScMeOH post-treatment process, CH3OH molecules diffused into the interlayers of g-C3N4 and subsequently participated in N-methylation and hydroxylation reactions with the intralayers, resulting in a partial phase transformation from g-C3N4 into carbon nitride with a poly(heptazine imide)-like structure (Q-PHI) as well as abundant methyl and hydroxyl groups. The modified g-C3N4 showed enhanced photocatalytic activity with an H2-evolution rate 7.2 times that of pristine g-C3N4, which was attributed to the synergistic effects of the g-C3N4/Q-PHI isotype heterojunction construction, group modulation, and surface area increase. This work presents a post-treatment strategy for structure tuning of bulk g-C3N4 and serves as a case for the application of supercritical fluid technology in photocatalyst synthesis.

Metal Poly(heptazine imides) as Multifunctional Photocatalysts for Solar Fuel Production

AbstractSolar‐driven photocatalysis employing particulate semiconductors represents a promising approach for sustainable production of valuable chemical feedstock. Metal poly(heptazine imide) (MPHI), a novel 2D ionic carbon nitride, has been recognized as an emerging photocatalyst with distinctive properties. In this minireview, we first delineate the forefront innovations of MPHI photocatalysts, spanning from synthetic strategies and solving structures to the exploration of novel properties. We place special emphasis on the structural design principles aimed at developing high‐performance MPHI systems toward photocatalytic solar fuel production such as H2 evolution, H2O oxidation, H2O2 production and CO2 reduction. Finally, we discuss crucial insights and challenges in leveraging highly active MPHIs for efficient solar‐to‐chemical energy conversion.

Metal Poly(heptazine imides) as Multifunctional Photocatalysts for Solar Fuel Production.

Solar-driven photocatalysis employing particulate semiconductors represents a promising approach for sustainable production of valuable chemical feedstock. Metal poly(heptazine imide) (MPHI), a novel 2D ionic carbon nitride, has been recognized as an emerging photocatalyst with distinctive properties. In this minireview, we first delineate the forefront innovations of MPHI photocatalysts, spanning from synthetic strategies and solving structures to the exploration of novel properties. We place special emphasis on the structural design principles aimed at developing high-performance MPHI systems toward photocatalytic solar fuel production such as H2 evolution, H2O oxidation, H2O2 production and CO2 reduction. Finally, we discuss crucial insights and challenges in leveraging highly active MPHIs for efficient solar-to-chemical energy conversion.

Surface cyano groups optimize the charge transfer of poly heptazine imide for enhanced photocatalytic H2 evolution

Surface functionalization has been considered as an effective strategy to manipulate charge separation of carbon nitride and therefore to largely improve the photocatalytic H2 evolution efficiency. Poly heptazine imide (PHI) is a new class of crystalline carbon nitride frameworks that exhibits remarkable photocatalytic performance for hydrogen evolution. To further improve the H2 evolution performance of PHI and explore the reaction mechanism, ammonium thiocyanate was used as a precursor for the synthesis of poly heptazine imide at elevated temperatures under molten salt conditions. The optimized PHI with an abundance of surface cyano groups shows a significantly enhanced photocatalytic performance for H2 evolution, which is 4.3 times that on pristine PCN. Most importantly, the surface cyano group adjusts the electron intensity of the polymeric framework, enhances the light absorbance, reduces the bandgap, and improves the charge separation efficiency. The synthetic technique also could be applied to other sulfur-containing precursors for the synthesis of PHI frameworks with excellent hydrogen evolution production performance.

Understanding the effect of multiple configurations in polymeric carbon nitride for efficient solar-driven hydrogen peroxide photosynthesis

A oxygen and potassium co-doped polymeric carbon nitride (O/K-PCNs) was simply constructed by an acid gas cutting strategy for photocatalytic H2O2 generation. The effect of each accompanied configurations on photosynthesis of H2O2 was systematically analyzed. The C–O–C structure formed by O doping is capable of decreasing K ions incorporated energy barrier, thus remarkably improving the content of crucial K active sites. Dual heteroatom sites synergistically enhance O2 adsorption and surface ORR by a bridge K–O–O–C model. Moreover, the accompanied cyano defect and poly(heptazine imide) is responsible for improving in-plane and interlayer carriers separation, respectively. The optimal O/K-PCNs exhibits 60-fold higher photocatalytic H2O2 production rate than pure PCN. A positive correlation between the H2O2 production rate and the K heteratoms content was established. This work clarifies role of multiple configurations in PCN, especially O-containing groups, providing a new insight for design heteroatoms-doped materials for solar energy conversion.

Rational Design of TaON/Potassium Poly(Heptazine Imide) Heterostructure for Multifunctional Environmental Remediation

AbstractTantalum oxynitride (TaON) has recently attracted much attention due to its excellent performance as a semiconductor. However, its shortage in separation of photogenerated charges hinders the wide application, which, fortunately, can be made up by forming heterojunctions with other photocatalysts. In this study, the rational design of an inorganic‐organic TaON/potassium poly (heptazine imide) (KPHI) heterojunction is reported to separate the photo charges on the two sides of this dyadic structure. The incorporation of 20 wt.% TaON into KPHI resulted not only in H2 generation but also in a wider set of environmental remediation applications under visible light, including Cr(VI) reduction and degradation of otherwise resilient antibiotics. 20TaON/KPHI effectively reduced 17.5 ppm Cr(VI) solution in 35 min under low‐power blue light irradiation with a rate constant which is ≈6 and 30 times higher than that of pristine KPHI and TaON, respectively. Moreover, the composite demonstrated a 2‐ and 40‐fold increase in H2 evolution rate in comparison to pure KPHI and TaON, respectively. This remarkable enhancement in photocatalytic activity is ascribed to the efficient electron‐hole separation and formation of an effective Z‐scheme heterojunction. This work offers a new paradigm for designing efficient photocatalytic systems for environmental applications under low‐power LED and natural sunlight.

Salt‐melt synthesis of poly(heptazine imide) in binary alkali metal bromides for enhanced visible‐light photocatalytic hydrogen production

AbstractPoly(heptazine imide) (PHI), a semicrystalline version of carbon nitride photocatalyst based on heptazine units, has gained significant attention for solar H2 production benefiting from its advantages including molecular synthetic versatility, excellent physicochemical stability and suitable energy band structure to capture visible photons. Typically, PHI is obtained in salt‐melt synthesis in the presence of alkali metal chlorides. Herein, we examined the role of binary alkali metal bromides (LiBr/NaBr) with diverse compositions and melting points to rationally modulate the polymerization process, structure, and properties of PHI. Solid characterizations revealed that semicrystalline PHI with a condensed π‐conjugated system and rapid charge separation rates were obtained in the presence of LiBr/NaBr. Accordingly, the apparent quantum yield of hydrogen using the optimized PHI reaches up to 62.3% at 420 nm. The density functional theory calculation shows that the dehydrogenation of the ethylene glycol has a lower energy barrier than the dehydrogenation of the other alcohols from the thermodynamic point of view. This study holds great promise for rational modulation of the structure and properties of conjugated polymeric materials.

Artificially regulating the crystallinity for constructing poly(heptazine imide)-based S-scheme homojunction with boosted photocatalytic hydrogen evolution performance

Fabricating homojunction photocatalyst is a promising approach to accelerate the separation and transfer of photogenerated charge carriers, and to boost photocatalytic performance. Herein, a novel poly (heptazine imide) (PHI)-based S-scheme homojunction photocatalyst (U/T-LHPHI) is fabricated through an ionothermal synthesis route, which exhibits particular high and low crystallinity property, intimate interface combination, and locally N self-doping. The regulation of crystallinity contributes to the differentiated electronic structure in PHI, which leads to the establishment of internal electric field (IEF). The intense IEF and N doping level with electron extracting capacity synergistically promote the charge transfer from the high crystalline PHI (HPHI) to the low crystalline PHI (LPHI) following the S-scheme mechanism. Additionally, the strong interfacial interaction improves the interfacial charge transfer dynamics. As a consequence, photogenerated electrons with powerful reducing ability are maintained effectively. Upon light irradiation, the optimized U/T-LHPHI performs an H2 evolution rate of 4880, 2416, and 2375 µmol g−1 h−1 in deionized water, simulated seawater, and real seawater, respectively, which exceed that of many carefully designed noble metal Pt containing photocatalyst. This work provides an important verification that the rational design and construction of homojunction photocatalysts could effectively enhance photocatalytic activity.

In-situ one-step construction of poly(heptazine imide)/poly(triazine imide) heterojunctions for photocatalytic hydrogen evolution.

The construction of heterojunctions is challenging, requiring atomic-level contact and interface matching. Here, we have achieved atomic-level interfacial matching by constructing poly(heptazine imide)/poly(triazine imide) crystalline carbon nitride heterojunctions in an in-situ one-step method. The content of poly(triazine imide) in heterojunctions is positively related to the proportion of lithium chloride in potassium chloride and lithium chloride mixed-salts. The optimized heterojunction achieves an apparent quantum efficiency of 48.34 % for photocatalytic hydrogen production at 420 nm, which is at a good level in polymeric carbon nitride photocatalysts. The proposed ion-thermal assisted heterojunction construction strategy contributes to the development of polymeric carbon nitride photocatalysts with high crystallization and high charge separation efficiency.

Pt-Black-Modified (Hemi)spherical AFM Sensors: In Situ Imaging of Light-Driven Hydrogen Peroxide Evolution.

In this work, we present (hemi)spherical atomic force microscopy (AFM) sensors for the detection of hydrogen peroxide. Platinum-black (Pt-B) was electrodeposited onto conductive colloidal AFM probes or directly at recessed microelectrodes located at the end of a tipless cantilever, resulting in electrocatalytically active cantilever-based sensors that have a small geometric area but, due to the porosity of the films, exhibit a large electroactive surface area. Focused ion beam-scanning electron microscopy tomography revealed the porous 3D structure of the deposited Pt-B. Given the accurate positioning capability of AFM, these probes are suitable for local in situ sensing of hydrogen peroxide and at the same time can be used for (electrochemical) force spectroscopy measurements. Detection limits for hydrogen peroxide in the nanomolar range (LOD = 68 ± 7 nM) were obtained. Stability test and first in situ proof-of-principle experiments to achieve the electrochemical imaging of hydrogen peroxide generated at a microelectrode and at photocatalytically active structured poly(heptazine imide) films are demonstrated. Force spectroscopic data of the photocatalyst films were recorded in ambient conditions, in solution, and by applying a potential, which demonstrates the versatility of these novel Pt-B-modified spherical AFM probes.

Low-Temperature Synthesis of Cyano-Rich Modified Surface-Alkalinized Heterojunctions with Directional Charge Transfer for Photocatalytic In Situ Generation and Consumption of Peroxides.

The synthesis of low-temperature poly(heptazine imide) (PHI) presents a significant challenge. In this context, we have developed a novel low-temperature synthesis strategy for PHI in this work. This strategy involves the introduction of Na+ ions, which etch and disrupt the conjugated structure of carbon nitride (CN) during assisted thermal condensation. This disruption leads to the partial decomposition of the heptazine ring structure, resulting in the formation of C≡N functionalities on the CN surface, which are enriched with hydroxyl groups and undergo cyano modification. The formation of heterojunctions between CN and ZnO, which facilitate charge transfer along an immobilization pathway, accelerated charge transfer processes and improved reactant adsorption as well as electron utilization efficiency. The resulting catalyst was employed for the room temperature, atmospheric pressure, and solvent-free photocatalytic selective oxidation of cumene (CM), achieving a cumene conversion rate of 28.7% and a remarkable selectivity of 92.0% toward the desired product, cumene hydroperoxide (CHP). Furthermore, this CHP induced oxidative reactions, as demonstrated by the successful oxidation of benzylamine to imine and the oxidation of sulfide to sulfoxide, both yielding high product yields. Additionally, the utilization of a continuous-flow device significantly reduces the reaction time required for these oxidation processes. This work not only introduces an innovative approach to environmentally friendly, sustainable, clean, and efficient PHI synthesis but also underscores the promising potential and advantages of carbon nitride-based photocatalysts in the realm of sustainable and green organic transformations.

Highly Crystalline Poly(heptazine imide)-Based Carbonaceous Anodes for Ultralong Lifespan and Low-Temperature Sodium-Ion Batteries.

Carbon nitrides with layered structures and scalable syntheses have emerged as potential anode choices for the commercialization of sodium-ion batteries. However, the low crystallinity of materials synthesized through traditional thermal condensation leads to insufficient conductivity and poor cycling stability, which significantly hamper their practical applications. Herein, a facile salt-covering method was proposed for the synthesis of highly ordered crystalline C3N4-based all-carbon nanocomposites. The sealing environment created by this strategy leads to the formation of poly(heptazine imide) (PHI), the crystalline phase of C3N4, with extended π-conjugation and a fully condensed nanosheet structure. Meanwhile, theoretical calculations reveal the high crystallinity of C3N4 significantly reduces the energy barrier for electron transition and enables the generation of efficient charge transfer channels at the heterogeneous interface between carbon and C3N4. Accordingly, such nanocomposites present ultrastable cycling performances over 5000 cycles, with a high reversible capacity of 245.1 mAh g-1 at 2 A g-1 delivered. More importantly, they also exhibit an outstanding low-temperature capacity of 196.6 mAh g-1 at -20 °C. This work offers opportunities for the energy storage use of C3N4 and provides some clues for developing long-life and high-capacity anodes operated under extreme conditions.

Trap‐Assisted Exciton Dissociation Through Continuous Trapping/Detrapping in Lithium Ions Inserted Crystalline Carbon Nitride

AbstractHere, lithium ions inserted crystalline carbon nitrides (LCCN) with different defect concentrations are prepared by elaborately regulating the mass transfer process in thermal polymerization. It is revealed that there is no conventional exciton–polarons‐induced exciton dissociation mechanism in LCCN due to the weak coupling between excitons and phonons. On the contrary, excitons continue to trap/detrap through defect‐induced traps, which weakens the Coulomb force between electron–hole pairs, thereby accelerating exciton dissociation. It is worth emphasizing that this is the first systematic research on the exciton dissociation characteristics and photocatalytic activity of LCCN of poly(heptazine imide) structure. A strategy of assisting exciton dissociation through defect‐induced traps to address the inherently high exciton binding energy and low exciton dissociation efficiency issues in polymer photocatalysts is proposed.

Mechanism of charge accumulation in potassium poly(heptazine imide).

Dark photocatalysis is the ability of a photocatalyst to accumulate photocarriers during light irradiation and consume them for redox reactions under dark conditions. This phenomenon of photocatalysts storing photocarriers is known as charge accumulation. Dark photocatalysts can mitigate fluctuations in photocatalytic reaction efficiency in response to fluctuating solar irradiation. Potassium poly(heptazine imide) (K-PHI) has attracted considerable attention due to its high photocatalytic efficiency and ability to undergo dark photocatalysis. However, the detailed mechanism of charge accumulation in K-PHI remains unclear because photochromism, potassium ion desorption, and charge accumulation occur simultaneously triggered by light irradiation, limiting the comprehensive understanding of this mechanism. To elucidate the charge accumulation mechanism in K-PHI, highly oriented K-PHI thin films were prepared. Then, their fundamental physical properties and optical response of their electrical properties were investigated. We succeeded in separately observing photochromism, potassium ion desorption, and charge accumulation induced by light irradiation on K-PHI and proposed a comprehensive model to explain these phenomena. This study not only provides insights into the unique physical phenomena exhibited by K-PHI but also contributes to the development of solar energy-storage materials in the future.