Bridging the g-C3N4 Interlayers for Enhanced Photocatalysis

Oxygen Doping in Graphitic Carbon Nitride for Enhanced Photocatalytic Hydrogen Evolution

The incorporation of oxygenic groups could remarkably enhance the light absorption and charge separation of graphitic carbon nitride (g-C₃ N₄ ). The intrinsic role of oxygenic species on photocatalytic activity in g-C₃ N₄ has been intensively studied, but it is still not fully explored. Herein, the essential relationships between oxygenic functionalities and the catalytic performance are revealed. Results demonstrate that C-O-C functionality as an electron trap could help to increase the resistance of conduction transfer (R_ct ) by limiting electrons transfer in CNx. In contrast, N-C-O functionality between different tri-s-triazine unites could promote the electrons transfer, leading to a reduced R_ct in CNx. The best H₂ production rate (3.70 mmol h^-1  g^-1 , 12.76-fold higher than that of CN) is obtained over CN3, because of the highest N-C-O ratio (r_N-C-O ). The apparent quantum efficiency (AQE) of CN3 at 405 nm, 420 nm, 450 nm, 500 nm and 550 nm is 33.90 %, 20.88 %, 8.25 %, 3.66 % and 1.01 %, respectively.

Phenyl-Bridged Graphitic Carbon Nitride with a Porous and Hollow Sphere Structure to Enhance Dissociation of Photogenerated Charge Carriers and Visible-Light-Driven H2 Generation

Graphitic carbon nitride (CN) suffers from rapid recombination of photoexcited charges due to the existing highly symmetrical tri-s-triazine ring and long charge diffusion path, resulting in moderate photocatalytic activity. The bridged phenyl embedded in the CN structure was used to reduce the symmetry of the tri-s-triazine ring. In addition, the CN material was constructed with a porous and hollow sphere structure to shorten the diffusion path of charge carriers. Herein, simple thermal polymerization of a trimesic acid-doped melamine-cyanuric acid (MCA) supramolecular was employed to construct phenyl-bridged graphitic carbon nitride (Ph-CN-MCA) with a hollow sphere structure composed of porous nanosheets for visible-light catalytic H₂ evolution. The porous and hollow sphere-structured Ph-CN-MCA possessed increased degree of polymerization, more negative conduction band potential, enlarged Brunauer-Emmett-Teller (BET) surface area, and shortened charge diffusion path. In addition, bridged phenyl embedded in the Ph-CN-MCA structure not only accelerated the dissociation of photogenerated carriers but also narrowed the band gap and extended the visible-light absorption. Further, the separated highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of Ph-CN-MCA facilitated the spatial dissociation of photogenerated charges, which was also confirmed by theoretical calculations. As a consequence, compared with the reference CN-MA catalyst prepared from melamine, Ph-CN-MCA showed approximately 48.42 times the photocatalytic H₂ evolution under visible-light irradiation. The developed synthetic method herein highlights that phenyl-bridged graphitic carbon nitride with a porous and hollow sphere structure could provide an efficient platform to boost the dissociation of photoexcited charge carriers and photocatalytic H₂ evolution.

Task-Specific Synthesis of 3D Porous Carbon Nitrides from the Cycloaddition Reaction and Sequential Self-Assembly Strategy toward Photocatalytic Hydrogen Evolution

Carbon nitride has drawn widespread attention as a low-cost alternative to metal-based materials in the field of photocatalysis. However, the traditionally synthesized carbon nitrides always suffer a bulky architecture, which limits their intrinsic activities. Here, a cycloaddition reaction is proposed to synthesize a triazine-based precursor with implanted sodium and cyano groups, which are mostly retained in the resulting carbon nitride after the following polymerization. Incorporated sodium and cyano defects can not only tune the band structure of the carbon nitride but also provide more additive active sites. The optimized properties enable it an adorable photocatalytic hydrogen evolution rate of 1070 μmol h^-1 g^-1, varying by almost an order of magnitude from the pristine carbon nitride (79 μmol h^-1 g^-1). Moreover, a sequential self-assembly strategy has been adopted to further improve its architecture. As a consequence, a three-dimensional (3D) porous carbon nitride microtube cluster is constructed, indicating abundant exposed active sites and the faster separation of charge carriers. The corresponding photocatalytic hydrogen evolution rate is 1681 μmol h^-1 g^-1, which is very competitive compared with the reported pure carbon nitride photocatalysts. Briefly, this new approach may offer opportunities to fabricate task-specific carbon- and nitrogen-based materials from the molecular level.

Synthesis and fabrication of g-C3N4-based materials and their application in elimination of pollutants

With the fast development of industrial and human activity, large amounts of persistent organic pollutants, heavy metal ions and radionuclides are released into the natural environment, which results in environmental pollution. The efficient elimination of the natural environment is crucial for the protection of environment to against the pollutants' toxicity to human beings and living organisms. Graphitic carbon nitride (g-C₃N₄) has drawn multidisciplinary attention especially in environmental pollutants' cleanup due to its special physicochemical properties. In this review, we summarized the recent works about the synthesis of g-C₃N₄, element-doping, structure modification of g-C₃N₄ and g-C₃N₄-based materials, and their application in the sorption, photocatalytic degradation and reduction-solidification of persistent organic pollutants and heavy metal ions. The interaction mechanisms were discussed from advanced spectroscopic analysis and computational approaches at molecular level. The challenges and future perspectives of g-C₃N₄-based materials' application in environmental pollution management are presented in the end. This review highlights the real applications of g-C₃N₄-based materials as adsorbents or photocatalysts in the adsorption-reduction-solidification of metal ions or photocatalytic degradation of organic pollutants. The contents are helpful for the undergraduate students to understand the recent works in the elimination of organic/inorganic pollutants in their pollution management.

Dual Functions of O-Atoms in the g-C3N4/BO0.2N0.8 Interface: Oriented Charge Flow In-Plane and Separation within the Interface To Collectively Promote Photocatalytic Molecular Oxygen Activation

The photocatalytic performance of two-dimensional materials is largely limited by the fast recombination of photogenerated charges. Herein, we design and fabricate a novel g-C₃N₄/BO_0.2N_0.8 van der Waals heterostructure to realize oriented charge flow in-plane and separation within the interface. On one hand, a B-C bond forms within the g-C₃N₄/BO_0.2N_0.8 interface after the introduction of O atoms. The B-C bond as the mediator bridges g-C₃N₄ and BO_0.2N_0.8 sides to enhance the effective separation of photogenerated charges. On the other hand, the existence of O atoms promotes the formation of a B-O-O-B intermediate to realize that molecular oxygen can directionally obtain electrons from the surface to generate •O₂^-. As a result, BO_0.2N_0.8 instead of g-C₃N₄ is considered to be the main reaction side, and the energy barrier of NO₃^- generation is significantly decreased. The NO removal performance of g-C₃N₄/BO_0.2N_0.8 is enhanced and the NO₂ generation is effectively controlled compared with that of g-C₃N and g-C₃N₄/BN. This work could provide an effective and facile strategy to tune oriented charge transfer.

Theoretical design and experimental investigation on highly selective Pd particles decorated C3N4 for safe photocatalytic NO purification

Rational design of highly active and selective photocatalyst for NO removal is significant for the commercial application of photocatalytic technology because the secondary byproduct caused by insufficient and non-selective pollutant oxidation process is a major challenge. In this work, Pd nanoparticles decorated C₃N₄ (PdCN) is designed by density functional theory (DFT) at first. The PdCN exhibits superiority to CN in terms of both kinetics and thermodynamics performances, as reflected in the lower activation barrier of rate-determining step and higher selectivity for the final product (nitrate) instead of toxic intermediate (NO₂). The as-designed highly selective and efficient photocatalyst is then fabricated by a facile method with an extremely low content of Pd particles supported on C₃N₄. Compared to bare CN, the synthesized PdCN exhibits highly enhanced purification of NO in air and strong inhibition of toxic NO₂ by-product as supported by in-situ DRIFTS investigation, which is consistent with the theoretical prediction. This work is a typical demonstration of setting up a bridge between theory and experiment to give a promising way to the rational design of advanced photocatalysts and atomic understanding of the reaction mechanism.

Graphitic carbon nitride with thermally-induced nitrogen defects: an efficient process to enhance photocatalytic H2 production performance

Graphitic carbon nitride (g-C3N4, CN) with nitrogen vacancies was synthesized by a controlled thermal etching method in a semi-closed air-conditioning system. The defect-modified g-C3N4 shows an excellent photocatalytic performance demonstrated by water splitting under visible light irradiation. With proper heat-treatment durations such as 2 h (CN2) and 4 h (CN4) at 550 °C, the hydrogen production rates significantly increase to 100 μmol h-1 and 72 μmol h-1, which are 11 times and 8 times the rate of the pristine CN (8.8 μmol h-1) respectively. The excellent hydrogen production performance of nitrogen defect modified CN2 is due to the synergy effect of the decreased band gap, enlarged specific surface area and increased separation/migration efficiency of photoinduced charge carriers. This simple defect engineering method provides a good paradigm to improve the photocatalytic performance by tailoring the electronic and physical structures of g-C3N4.

Manipulatable Interface Electric Field and Charge Transfer in a 2D/2D Heterojunction Photocatalyst via Oxygen Intercalation

Charge separation is the most important factor in determining the photocatalytic activity of a 2D/2D heterostructure. Despite the exclusive advantages of 2D/2D heterostructure semiconductor systems such as large surface/volume ratios, their use in photocatalysis is limited due to the low efficiency of charge separation and high recombination rates. As a remedy for the weak interlayer binding and low carrier transport efficiency in 2D/2D heterojunctioned semiconductors, we suggested an impurity intercalation method for the 2D/2D interface. PtS2/C3N4, as a prototype heterojunction material, was employed to investigate the effect of anion intercalation on the charge separation efficiency in a 2D/2D system using density functional theory. With oxygen intercalation at the PtS2/C3N4 interface, a reversed and stronger localized dipole moment and a built-in electric field were induced in the vertical direction of the PtS2/C3N4 interface. This theoretical work suggests that the anion intercalation method can be a way to control built-in electric fields and charge separation in designs of 2D/2D heterostructures that have high photocatalytic activity.

A Codoped Polymeric Photocatalyst with Prolonged Carrier Lifetime and Extended Spectral Response up to 600 nm for Enhanced Hydrogen Evolution

The intrinsic properties of photocatalysts, such as electron state and energy band structure, contribute significantly to their catalytic performance. However, it is difficult to alter these properties of semiconductors by conventional modifications. To adjust the intrinsic properties while preserving long-term conjugation of the polymeric photocatalysts, a post-thermal treatment is proposed to codope P and Na into polymeric carbon nitride in this work. After codoping, the absorption of visible irradiation is strongly extended up to 639 nm. Additionally, the lifetime of charge carriers is almost tripled from 1.09 to 2.93 ns. The photocatalytic hydrogen evolution performance under visible light is improved to 2032 μmol·h^-1 g^-1 in the optimized sample, corresponding to apparent quantum efficiencies of 6.79% and 0.09% at 420 and 600 nm, respectively. The enhanced catalytic activity is ascribed to the synergistic effects of prolonged lifetime and increased charge density that resulted from lattice distortion and extended visible utilization due to the formation of subgap state. Our work provides new pathways for the modification of polymeric catalysts toward high-performance and full-spectrum photocatalysis.

Plasmonic ternary hybrid photocatalyst based on polymeric g-C3N4 towards visible light hydrogen generation

Surface plasmon resonance (SPR) effect of noble metal nanoparticles (NPs) for photocatalysis has a significant enhancement. In this system, a plasmonic ternary hybrid photocatalyst of Ag/AgBr/g-C₃N₄ was synthetized and used in water splitting to generation H₂ under visible light irradiation. 18%Ag/AgBr/g-C₃N₄ showed the highest photoactivity, with the efficiency of hydrogen generation as high as 27-fold to that of pristine g-C₃N₄. Compared to simple mixture of Ag/AgBr and g-C₃N₄, hetero-composite Ag/AgBr/g-C₃N₄ showed a higher photoactivity, even though they contained same content of Ag/AgBr. We find that significant factors for enhancing properties were the synergistic effect between Ag/AgBr and g-C₃N₄, and the light absorption enhancing by SPR effect of Ag NPs. Ag/AgBr NPs firmly anchored on the surface of g-C₃N₄ and their high dispersion were also responsible for the improved activity and long-term recycling ability. The structure of Ag/AgBr/g-C₃N₄ hybrid materials and their enhancement to photocatalytic activity were discussed. Meanwhile, the possible reaction mechanism of this system was proposed.

Supramolecular self-assembly synthesis of noble-metal-free (C, Ce) co-doped g-C3N4 with porous structure for highly efficient photocatalytic degradation of organic pollutants

Developing inexpensive and stable photocatalysts without noble metals, yet remarkably enhancing the photocatalytic activities, is highly needed. Here, a novel carbon and cerium co-doped porous g-C₃N₄ (C/Ce-CN) has been successfully prepared through thermal polymerization of the supramolecular aggregation. The morphologies, chemical structures, optical and photoelectrochemical properties of the synthesized photocatalysts were analyzed via a series of characterization measurements. Experimental results indicated that C/Ce-CN showed remarkably enhanced photocatalytic activity of TC and RhB degradation, which is about 2.6 and 2.4 times higher than that of pristine CN, and it also exhibited a good stability. Compared with bare CN, the enhanced performance of C/Ce-CN is mainly attributed to the stronger utilization rate of visible light, the rapider charge transfer rate, the longer lifetime of carriers and the larger surface specific area. The main intermediates in degradation process of antibiotics were tested by the HPLC-MS. Finally, the possible photocatalytic degradation pathways and mechanisms were proposed.

A promising blue phosphorene/C2N van der Waals type-II heterojunction as a solar photocatalyst: a first-principles study

An appropriate band structure and effective carrier separation are very important for the performance of a solar photocatalyst. In this paper, based on first-principles calculations, it was predicted that blue phosphorene (BlueP) and a C2N monolayer can form a promising metal-free type-II heterojunction. The electronic structure of the BlueP/C2N heterojunction facilitated the overall water splitting reactions well. The projected band structure showed that the conduction band edge was contributed by C2N and the valence band edge was dominated by BlueP. Under the combination of the driving force of the band offset and the built-in electric field between the two layers, the photo-generated electrons and holes were transferred spontaneously to the conduction band of C2N and the valence band of BlueP, respectively. An effective carrier separation in the heterostructure was thus achieved. More notably, the obtained light absorption of the BlueP/C2N junction showed an obvious red-shift, which greatly extended the area of light adsorption to the visible light region. We further proposed that strain could also be used to modulate the band gap and the band edge positions of the heterojunction. Our results not only provide a theoretical design, but also reveal the fundamental separation mechanism of the photo-generated carriers in the BlueP/C2N heterojunction.

Nitrogen deficient carbon nitride for efficient visible light driven tetracycline degradation: a combination of experimental and DFT studies

The narrow visible-light absorption range and a high recombination rate of photo-excited electrons and holes are the main reasons for the confined photocatalytic performance of graphitic carbon nitride (g-C₃N₄).

Synthesis of narrow-band curled carbon nitride nanosheets with high specific surface area for hydrogen evolution from water splitting by low-temperature aqueous copolymerization to form copolymers

Carbon nitride has become a focus of photocatalytic materials research in recent years, but the low specific surface area, the bad separation efficiency of photocarriers, poor quantum efficiency, terrible photocatalytic activity hinder the development of carbon nitride in the field of photocatalysis. The preparation of carbon nitride nanosheets is one of the effective methods to improve the photocatalytic efficiency of carbon nitride, but the traditional top-down stripping process is time-consuming, complicated and expensive. Here we report a simple, cheap, non-toxic and environmentally friendly bottom-up method to prepare a curled g-C₃N₄ nanosheet (NS-C₃N₄), which is performed at low temperature and normal pressure. In the aqueous solution, melamine and cyanuric acid are copolymerized to form a copolymer. Glycerol is inserted between the molecular layers of the prepolymer by thermal diffusion. Finally, high-quality and high-yield curled g-C₃N₄ nanosheets (NS-C₃N₄) are obtained by thermal peeling and polycondensation. The NS-C₃N₄ has an highly efficient photocatalytic hydrogen production of 4061.8 μmol h⁻¹ g⁻¹, and the hydrogen evolution activity is 37.5 times that of bulk-C₃N₄ (B-C₃N₄). The specific surface area of NS-C₃N₄ is 60.962 m² g⁻¹. UV-vis absorption spectra, steady-state and time-resolved photoluminescence, and photoelectrochemical tests were used to study its photocatalytic mechanism.

Curled carbon nitride nanosheets with narrow-band gap for ultra-high hydrogen production efficiency.

OH/Na co-functionalized carbon nitride: directional charge transfer and enhanced photocatalytic oxidation ability

Graphitic carbon nitride (g-C₃N₄, CN for short) is a compelling visible-light responsive photocatalyst.

Fabricating Amorphous g-C3N4/ZrO2 Photocatalysts by One-Step Pyrolysis for Solar-Driven Ambient Ammonia Synthesis

Solar-driven nitrogen fixation remains a significant challenge. Graphitic carbon nitride (g-C₃N₄) is considered as a promising visible light photocatalyst. However, the photocatalytic performance of g-C₃N₄ is unsatisfactory because of the random transfer of charge carriers in the plane and the low activation efficiency of the reactants. Herein, amorphous ZrO₂ was used as a robust cocatalyst of g-C₃N₄ to increase the NH₃ production activity. The g-C₃N₄/ZrO₂ lamellar composites were constructed by a simple one-step pyrolysis of the deep eutectic solvent ZrOCl₂·8H₂O/urea. The optimum NH₄⁺ yield could reach as high as 1446 μmol·L^-1·h^-1 at 30 wt % ZrO₂ in the g-C₃N₄/ZrO₂ composites, with an apparent quantum efficiency over 2.14% at 400 nm. It is 7.9 times that of pristine g-C₃N₄ and 27.5 times that of ZrO₂. The introduction of amorphous ZrO₂ restrained the hydrogen generation, and the amorphous ZrO₂ and g-C₃N₄ together contribute to the rapid photoproduced electron transfer of less electron-hole pair recombination.

Bifunctional hydroxyl group over polymeric carbon nitride to achieve photocatalytic H2O2 production in ethanol aqueous solution with an apparent quantum yield of 52.8% at 420 nm

A photocatalytic system of carbon nitride grafted with hydroxyl groups working in ethanol aqueous solution under visible light irradiation was constructed to simultaneously achieve photocatalytic H2O2 production with an apparent quantum efficiency of 52.8% at around 420 nm and ethanol conversion to high-value acetaldehyde with a high selectivity of 99.95%.

Powerful combination of g-C3N4 and LDHs for enhanced photocatalytic performance: A review of strategy, synthesis, and applications

The utilization of solar energy with photocatalytic technology has been considered a good solution to alleviate environmental pollution and energy shortage. Constructing 2D/2D heterostructure photocatalysts with layered double hydroxide (LDH) and graphitic carbon nitride (g-C₃N₄) is an effective approach to attain high performance in solar photocatalysis. This paper provides a review of recent studies about 2D/2D LDH/g-C₃N₄ heterostructure photocatalysts. Main strategies for constructing the desired 2D/2D heterojunction are summarized. The planar structure of LDH and g-C₃N₄ offers a shorter transfer distance for charge carriers and reduces electron-hole recombination in the bulk phase. The face-to-face contact between the two materials can promote the charge transfer across the heterostructure interface, thus improving the electron-hole separation efficiency. The performance and mechanisms of LDH/g-C₃N₄ photocatalysts in hydrogen production, CO₂ reduction, and organic pollutant degradation are analyzed and discussed. Incorporating reduced graphene oxide or Ag nanoparticles into LDH/g-C₃N₄ heterojunction and fabricating calcined LDH/g-C₃N₄ composites are effective strategies to further facilitate charge transfer at the interface of LDH and g-C₃N₄ and improve the absorption capacity for visible light. This review is expected to provide basic insights into the design of 2D/2D LDH/g-C₃N₄ heterojunctions and their applications in solar photocatalysis.

Construction of a ternary g-C3N4/TiO2@polyaniline nanocomposite for the enhanced photocatalytic activity under solar light

The combination of two or more semiconductor materials for the synthesis of new hybrid photocatalyst could be a good approach to enhance the visible light absorption, electron-hole (e^-/h⁺) pair separation rate and photocatalytic decomposition of the organic contaminants. Herein, a facile in situ oxidative polymerization method has been used for the synthesis of visible light active g-C₃N₄/TiO₂@polyaniline (g-C₃N₄/TiO₂@PANI) nanocomposite for the decomposition of the congo red (CR) under the solar light irradiation. Prior to making the composite of TiO₂ (P25) with g-C₃N₄ and polyaniline, a lamellar structure was generated onto the TiO₂ brim by alkali hydrothermal treatment to enhance the surface area and adsorption properties. The PL and UV-visible analysis clearly showed the fast separation of the e^-/h⁺ pair, and reduction in the bandgap energy of the g-C₃N₄/TiO₂@PANI nanocomposite. The results revealed TiO₂, PANI and g-C₃N₄ showed the synergestic behavior in the g-C₃N₄/TiO₂@PANI nanocomposite and greatly enhanced the photocatalytic degradation of the CR. The photocatalytic decomposition of the CR was almost 100% for 20 mg/L at pH 5, 7 and 180 min. The reusability study of the spent catalyst showed the 90% degradation of CR after four consecutive cycles indicate that g-C₃N₄/TiO₂@PANI nanocomposite is a stable and efficient catalyst. The high efficiency and reusability of the g-C₃N₄/TiO₂@PANI nanocomposite could be attributed to the higher visible light absorption and sensitizing effect of the g-C₃N₄ and PANI.