Carbon nitride-based Z-scheme heterojunctions for solar-driven advanced oxidation processes

Advances of carbon-based materials for activating peracetic acid in advanced oxidation processes: A review

In recent years, the peracetic acid (PAA)-based advanced oxidation process (AOPs) has garnered significant attention in the field of water treatment due to rapid response time and environmentally-friendliness. The activation of PAA systems by diverse carbon-based materials plays a crucial role in addressing emerging environmental contaminants, including various types, structures, and modified forms of carbon materials. However, the structural characteristics and structure-activity relationship of carbon-based materials in the activation of PAA are intricate, while the degradation pathways and dominant active species exhibit diversity. Therefore, it is imperative to elucidate the developmental process of the carbon-based materials/PAA system through resource integration and logical categorization, thereby indicating potential avenues for future research. The present paper comprehensively reviews the structural characteristics and action mechanism of carbon-based materials in PAA system, while also analyzing the development, properties, and activation mechanism of heteroatom-doped carbon-based materials in this system. In conclusion, this study has effectively organized the resources pertaining to prominent research direction of comprehensive remediation of environmental water pollution, thereby elucidating the underlying logic and thought process. Consequently, it establishes robust theoretical foundation for future investigations and applications involving carbon-based materials/PAA system.

Harnessing natural light: Novel nanoheterojunction photocatalyst NaGdF4:Yb,Tm@TiO2/Cu2(OH)2CO3 for actual wastewater remediation

This study introduces NaGdF4:Yb,Tm@TiO2/Cu2(OH)2CO3 (UTCu), an innovative nanophotocatalyst designed to address global energy crises and water contamination issues. Our developed photocatalyst, NaGdF4:Yb,Tm@TiO2/0.5mol%Cu2(OH)2CO3 (UTCu0.5), demonstrated exceptional efficiency, degrading 96.3% of malachite green (MG) within 2 h under Xenon lamp irradiation. The photocatalytic degradation rate of UTCu0.5 surpassed those of UT, Cu2(OH)2CO3, and P25 (Commercial TiO2) by 3.3, 9, and 2.8 times, respectively. The process effectively mineralized MG into less harmful compounds, marking its potential for eco-friendly wastewater treatment. Furthermore, UTCu0.5 exhibited robust degradation capabilities across various organic dyes and maintained its efficacy in mixed dye systems. Detailed mechanistic analysis revealed that the ·OH and ·O2- radicals play pivotal roles in the degradation process, facilitated by the formation of heterojunctions that enhance carrier separation and photocatalytic performance. Theoretical studies supported the significance of S-scheme heterojunctions in boosting the photocatalytic activity of UTCu0.5. Additionally, the catalyst was effective in degrading organic pollutants in different water matrices under both Xenon lamp irradiation and direct sunlight. Remarkably, it achieved a 77.4% removal rate of NH4⁺-N in real municipal wastewater under natural sunlight, with a selective conversion rate of 95.3% to N2, underscoring its practical applicability in environmental remediation. This research not only progresses photocatalysis technology but also provides vital insights for enhancing natural condition wastewater treatment strategies.

Performance, progress, and mechanism of g-C3N4-based photocatalysts in the degradation of pesticides: A systematic review

In the modern world, humans are exposed to an enormous number of pesticides discharged into the environment. Exposure to pesticides causes many health disorders, such as cancer, mental retardation, and endocrine disruption. Therefore, it is a priority to eliminate pesticides from contaminated water before discharge into aquatic environments. Conventional treatment systems do not efficiently accomplish pesticide remediation. Applying graphitic carbon nitride (g-C3N4; GCN)-based materials as highly efficient and low-cost catalysts can be one of the best methods for adequately removing pesticides. This study aims to review the most relevant studies on the use of GCN-based photocatalytic processes for degrading well-known pesticides in aqueous solutions. Thus, in the current state-of-the-art review, an overview is focused not only on how to use GCN-based photocatalysts towards the degradation of pesticides, but also discusses the impact of important operational factors like solution pH, mixture temperature, catalyst dosage, pesticide concentration, photocatalyst morphology, light intensity, reaction time, oxidant concentration, and coexisting anions. In this context, four common pesticides were reviewed, namely 2,4-dichlorophenoxyacetic acid (2,4-D), malathion (MTN), diazinon (DZN), and atrazine (ATZ). Following the screening procedure, 55 full-text papers were chosen, of which the most were published in 2023 (n = 10), and the most publications focused on the elimination of ATZ (n = 33). Among the GCN modification methods, integrating GCN with other photocatalysts showed the best performance in enhancing photocatalytic activity towards the degradation of pesticides. All GCN-based photocatalysts showed a degradation efficiency of > 90% for pesticides under optimum operating conditions. This review provides a detailed summary of different GCN modification methods to select the most promising and cost-effective photocatalyst degradation of pesticides.

Tandem microplastic degradation and hydrogen production by hierarchical carbon nitride-supported single-atom iron catalysts

Microplastic pollution, an emerging environmental issue, poses significant threats to aquatic ecosystems and human health. In tackling microplastic pollution and advancing green hydrogen production, this study reveals a tandem catalytic microplastic degradation-hydrogen evolution reaction (MPD-HER) process using hierarchical porous carbon nitride-supported single-atom iron catalysts (FeSA-hCN). Through hydrothermal-assisted Fenton-like reactions, we accomplish near-total ultrahigh-molecular-weight-polyethylene degradation into C3-C20 organics with 64% selectivity of carboxylic acid under neutral pH, a leap beyond current capabilities in efficiency, selectivity, eco-friendliness, and stability over six cycles. The system demonstrates versatility by degrading various daily-use plastics across different aquatic settings. The mixture of FeSA-hCN and plastic degradation products further achieves a hydrogen evolution of 42 μmol h‒1 under illumination, outperforming most existing plastic photoreforming methods. This tandem MPD-HER process not only provides a scalable and economically feasible strategy to combat plastic pollution but also contributes to the hydrogen economy, with far-reaching implications for global sustainability initiatives.

Engineering MOF/carbon nitride heterojunctions for effective dual photocatalytic CO2 conversion and oxygen evolution reactions

Photocatalysis appears as one of the most promising avenues to shift towards sustainable sources of energy, owing to its ability to transform solar light into chemical energy, e.g. production of chemical fuels via oxygen evolution (OER) and CO2 reduction (CO2RR) reactions. Ti metal-organic frameworks (MOFs) and graphitic carbon nitride derivatives, i.e. poly-heptazine imides (PHI) are appealing CO2RR and OER photo-catalysts respectively. Engineering of an innovative Z-scheme heterojunction by assembling a Ti-MOF and PHI offers an unparalleled opportunity to mimick an artificial photosynthesis device for dual CO2RR/OER catalysis. Along this path, understanding of the photophysical processes controlling the MOF/PHI interfacial charge recombination is vital to fine tune the electronic and chemical features of the two components and devise the optimum heterojunction. To address this challenge, we developed a modelling approach integrating force field Molecular Dynamics (MD), Time-Dependent Density Functional Theory (TD-DFT) and Non-Equilibrium Green Function DFT (NEGF-DFT) tools with the aim of systematically exploring the structuring, the opto-electronic and transport properties of MOF/PHI heterojunctions. We revealed that the nature of the MOF/PHI interactions, the interfacial charge transfer directionality and the absorption energy windows of the resulting heterojunctions can be fine tuned by incorporating Cu species in the MOF and/or doping PHI with mono- or divalent cations. Interestingly, we demonstrated that the interfacial charge transfer can be further boosted by engineering MOF/PHI device junctions and application of negative bias. Overall, our generalizable computational methodology unravelled that the performance of CO2RR/OER photoreactors can be optimized by chemical and electronic tuning of the components but also by device design based on reliable structure-property rules, paving the way towards practical exploitation.

Electronic Structure and Functions of Carbon Nitride in Frontier Green Catalysis

ConspectusGraphitic carbon nitride-based materials have emerged as promising photocatalysts for a variety of energy and environmental applications owing to their "earth-abundant" nature, structural versatility, tunable electronic and optical properties, and chemical stability. Optimizing carbon nitride's physicochemical properties encompasses a variety of approaches, including the regulation of inherent structural defects, morphology control, heterostructure construction, and heteroatom and metal-atom doping. These strategies are pivotal in ultimately enhancing their photocatalytic activities. Previous reviews with extensive examples have mainly focused on the synthesis, modification, and application of carbon nitride-based materials in photocatalysis. However, there has been a lack of straightforward and in-depth discussion to understand the electronic characteristics and functions of various engineered carbon nitrides as well as their precise tailoring strategies and ultimately to explain the regularity and specificity of their improved performance in targeted photocatalytic systems. In the past ten years, our group has conducted extensive investigations on carbon nitride-based materials and their application in photocatalysis. These studies demonstrate the close yet intricate relationship between the electronic structure of carbon nitride materials and their photocatalytic reactivity. Understanding the electronic structure and functions of carbon nitride, as well as different engineering strategies, is essential for the improvement of photocatalytic processes from fundamental study to practical applications.To this end, in this Account, we first delve into the nature of the electronic properties of carbon nitride, highlighting the electronic structures, including band structure, density of states, molecular orbitals, and band center, as well as its electronic functions, such as the charge distribution, internal electric field, and external electric force. Subsequently, based on recent research in our group, we present a detailed discussion of the strategic modifications of carbon nitride and the consequential impacts on the physicochemical properties, particularly the optical properties and intrinsic electronic characteristics, for enhancing the photocatalytic performance. These modifications are categorized as follows: (i) component changing, which involves intralayer and interface heterojunctions as well as homojunctions, to modulate the band-edge potentials and reactivity of photoinduced electrons and holes toward surface redox reactions; (ii) dimensional tuning, which engineers the dimensional structure of carbon nitride, to influence the electron transfer direction; (iii) defect and heteroatom modification, which introduces a symmetry break in the carbon nitride framework, to promote charge redistribution for altering the charge density and electronic structure; and (iv) anchoring of single-atom metals to facilitate orbital hybridization and charge transfer enhancement through the unique metal-N coordination configurations. Finally, we propose an appraisal of the prospects and challenges in the precise manipulation and characterization of the electronic structure and functions of carbon nitride. The integration of in situ electronic structure analysis, theoretical calculation based on machine learning, and precise mechanism study may propel its substantial development in the light-driven circular economy. We hope this Account aspires to offer novel insights and perspectives into the operational mechanisms and tailored structure of carbon nitride-based materials in photocatalysis.

Reusable isotype heterojunction g-C3N4/alginate hydrogel spheres for photocatalytic wastewater treatment

Various visible-light-driven photocatalysts have been studied for practical applications in photocatalytic wastewater treatment via solar irradiation. Among them, g-C3N4 has attractive features, including its metal-free and environmentally friendly nature; however, it is prone to charge recombination and has low photocatalytic activity. To solve these problems, isotype heterojunction g-C3N4 was recently developed; however, the methods employed for synthesis suffered from limited reproducibility and efficiency. In this study, isotype heterojunction g-C3N4 was synthesized from various combinations of precursor materials using a planetary ball mill. The isotype heterojunction g-C3N4 synthesized from urea and thiourea showed the highest photocatalytic activity and completely decolorized Rhodamine B (RhB; 10 ppm) in 15 min under visible-light irradiation. Furthermore, to improve recyclability, isotype heterojunction g-C3N4 was immobilized in alginate hydrogel spheres. The isotype heterojunction g-C3N4/alginate hydrogel beads were used in 10 repeated RhB degradation experiments and were able to maintain their initial photocatalytic activity and mechanical strength. These achievements represent an advance towards practical, sustainable photocatalytic wastewater treatment.

Enhancing the Activity of a Carbon Nitride Photocatalyst by Constructing a Triazine-Heptazine Homojunction

Establishing homojunctions at the molecular level between different but physicochemically similar phases belonging to the same family of materials is an effective approach to promoting the photocatalytic activity of polymeric carbon nitride (CN) materials. Here, we prepared a CN material with a uniform distribution of homojunctions by combining two synthetic strategies: supramolecular assemblies as the precursor and molten salt as the medium. We designed porous CN rods with triazine-heptazine homojunctions (THCNs) using a melem supramolecular aggregate (Me) and melamine as the precursors and a KCl/LiBr salt mixture as the liquid reaction medium. The triazine/heptazine ratio is controlled by varying the relative amounts of the chosen precursors, and the molten salt treatment enhances the structural order of the interplanar packing units for the THCN skeleton, leading to rapid charge migration. The resulting built-in electric field induced by the triazine-heptazine homojunction enhances photogenerated charge separation; the optimal THCN catalyst exhibits an excellent H2 evolution rate via photocatalytic water splitting, which is ∼24 times as high as that of reference bulk CN, with long-term stability.

Synergistic effect of Ag@CN with BiVO4 in a unique Z-type heterojunction for enhancing photoelectrochemical water splitting performance

In the realm of photoelectrochemical technology, the enhancement of photogenerated charge carrier separation is pivotal for the advancement of energy conversion performance. Carbon nitride (CN) is established as a photocatalytic material with significant potential and exhibits unique advantages in addressing the issue of rapid recombination of photogenerated carriers. This study utilized an efficient in situ doping method that combined Mo,W-doped BiVO4 (Mo,W:BVO) with silver-loaded CN (Ag@CN), yielding an all-solid-state Mo,W:BVO/Ag@CN heterostructure that effectively augments the separation efficiency of electron-hole pairs. Through the annealing process, Ag@CN was uniformly coated within the Mo,W:BVO thin film, significantly enlarging the interface contact area to enhance visible light absorption and photogenerated carrier movement. The results of the photoelectrochemical tests showed that the Mo,W:BVO/Ag@CN heterostructure had the highest photocurrent and charge transfer efficiency, which were 6.4 times and 3.6 times higher respectively than those of the unmodified Mo,W:BVO. Our research elucidates the interactions within all-solid-state Z-scheme heterojunctions, outlining strategic approaches for crafting innovative and superior photocatalytic systems.

Charge transfer in TiO2-based photocatalysis: fundamental mechanisms to material strategies

Semiconductor-based photocatalysis has attracted significant interest due to its capacity to directly exploit solar energy and generate solar fuels, including water splitting, CO2 reduction, pollutant degradation, and bacterial inactivation. However, achieving the maximum efficiency in photocatalytic processes remains a challenge owing to the speedy recombination of electron-hole pairs and the limited use of light. Therefore, significant endeavours have been devoted to addressing these issues. Specifically, well-designed heterojunction photocatalysts have been demonstrated to exhibit enhanced photocatalytic activity through the physical distancing of electron-hole pairs generated during the photocatalytic process. In this review, we provide a systematic discussion ranging from fundamental mechanisms to material strategies, focusing on TiO2-based heterojunction photocatalysts. Current efforts are focused on developing heterojunction photocatalysts based on TiO2 for a variety of photocatalytic applications, and these projects are explained and assessed. Finally, we offer a concise summary of the main insights and challenges in the utilization of TiO2-based heterojunction photocatalysts for photocatalysis. We expect that this review will serve as a valuable resource to improve the efficiency of TiO2-based heterojunctions for energy generation and environmental remediation.

Z-scheme WOx/Cu-g-C3N4 heterojunction nanoarchitectonics with promoted charge separation and transfer towards efficient full solar-spectrum photocatalysis

Construction of Z-scheme heterojunctions has been considered one superb method in promoting solar-assisted charge carrier separation of carbon-based materials to achieve efficient utilization of solar energy in hydrogen production and CO₂ reduction. One interesting concept in nanofabrication that has become trend recent years is nanoarchitectonics. A heterostructure photocatalyst constructed based on the idea of nanoarchitectonics using the combination of g-C₃N₄, metal and an additional semiconducting nanocomposite is investigated in this paper. Z-scheme tungsten oxide incorporated copper modified graphitic carbon nitride (WO_x/Cu-g-C₃N₄) heterostructures are fabricated via immobilization of WO_x on Cu nanoparticles modified superior thin g-C₃N₄ nanosheets. Mechano-chemical pre-reaction and a two-step high-temperature thermal polymerization process are the keys in attaining homogeneous distribution of Cu nanoparticles in g-C₃N₄ nanosheets. The horizontal growth of homogeneously distributed WO_x nanobelts on Cu modified g-C₃N₄ (Cu-g-C₃N₄) base via solvothermal synthesis is achieved. The photocatalytic performances of the heterostructures are evaluated through water splitting and CO₂ photoreduction measurements in full solar spectrum irradiation condition. The presence of Cu nanoparticles in the composite system improves charge transport between g-C₃N₄ and WO_x and thus enhances the photocatalytic performances (H₂ generation and CO₂ photoreduction) of the composite material, while the presence of WO_x nanocomposites enhances light absorption of the composite material in the near infrared range. The synthesized heterostructure with optimized WO_x to Cu-g-C₃N₄ ratio and in case of no co-catalyst addition exhibits enhanced photocatalytic H₂ evolution (4560 μmolg^-1h^-1) as well as excellent CO₂ reduction rate (5.89 μmolg^-1h^-1 for CO generation).

Modulating the electron structure of Co-3d in Co3O4-x/WO2.72 for boosting peroxymonosulfate activation and degradation of sulfamerazine: Roles of high-valence W and rich oxygen vacancies

Sulfate radical (SO₄^•-)-based heterogonous advanced oxidation processes (AOPs) show promising potential to degrade emerging contaminants, however, regulating the electron structure of a catalyst to promote its catalytic activity is challenging. Herein, a hybrid that consists of Co₃O_4-x nanocrystals decorated on urchin-like WO_2.72 (Co₃O_4-x/WO_2.72) with high-valence W and rich oxygen vacancies (OVs) used to modulate the electronic structure of Co-3d was prepared. The Co₃O_4-x/WO_2.72 that developed exhibited high catalytic activity, activating peroxymonosulfate (PMS), and degrading sulfamerazine (SMR). With the use of Co₃O_4-x/WO_2.72, 100 % degradation of SMR was achieved within 2 min, at a pH of 7, with the reaction rate constant k₁ = 3.09 min^-1. Both characterizations and density functional theory (DFT) calculations confirmed the formation of OVs and the promotion of catalytic activity. The introduction of WO_2.72 greatly regulated the electronic structure of Co₃O_4-x. Specifically, the introduction of high-valence W enabled the Co-3d band centre to be closer to the Fermi level and enhanced electrons (e^-) transfer ability, while the introduction of OVs-Co in Co₃O_4-x promoted the activity of electrons in the Co-3d orbital and the subsequent catalytic reaction. The reactive oxygen species (ROS) were identified as •OH, SO₄^•-, and singlet oxygen (¹O₂) by quenching experiments and electron spin resonance (EPR) analysis. The DFT calculation using the Fukui index indicated the reactive sites in SMR were available for an electrophilic attack, and three degradation pathways were proposed.

Facilely achieved enhancement of Fenton-like reactions by constructing electric microfields

In this work, we found that the presence of non-active ZnO crystals greatly accelerated the degradation of Bisphenol A (BPA) by 3.7 folds in the peroxymonosulfate (PMS, HSO₅^-)/Co₃O₄ system. Our mechanistic study revealed that the ZnO particles would create negative electric microfields around them, which are closely related with the zeta potentials (ζ) of ZnO and affected by solution pH. According to COMSOL simulation, the electrostatic repulsion between ZnO and PMS would drive HSO₅^- toward active Co₃O₄ surface, leading to the concentration increasing of HSO₅^- around active Co₃O₄ particles, which will then improve the degradation performance. The particle size of ZnO will also affect the promoting effect greatly by COMSOL simulation. Therefore, this study for the first time reveals synergy of electric microfields for enhanced heterogeneous Fenton-like reactions, providing a low-cost and effective strategy for enhanced persulfate catalysis.

G-C3N4 Dots Decorated with Hetaerolite: Visible-Light Photocatalyst for Degradation of Organic Contaminants

In this paper, a facile hydrothermal approach was used to integrate graphitic carbon nitride dots (CNDs) with hetaerolite (ZnMn2O4) at different weight percentages. The morphology, microstructure, texture, electronic, phase composition, and electrochemical properties were identified by field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Fourier transform-infrared (FT-IR), ultraviolet-visible diffuse reflectance (UV-vis DR), photoluminescence (PL), electrochemical impedance spectroscopy (EIS), Brunauer–Emmett–Teller (BET), Barrett–Joyner–Halenda (BJH), and photocurrent density. The results of XRD, FT-IR, EDX, and XPS analyses confirmed the synthesis of CNDs/ZnMn2O4 (20%) nanocomposite. As per PL, EIS, and photocurrent outcomes, the binary CNDs/ZnMn2O4 nanocomposite revealed superior features for interfacial transferring of charge carriers. The developed p–n heterojunction at the interface of CNDs and ZnMn2O4 nanoparticles partaken a significant role in the impressive charge segregation and migration. The binary nanocomposites were employed for the photodegradation of several dye pollutants, including rhodamine B (RhB), fuchsin, malachite green (MG), and methylene blue (MB) at visible wavelengths. Amongst the fabricated photocatalysts, the CNDs/ZnMn2O4 (20%) nanocomposite gave rise to about 98% RhB degradation efficiency within 45 min with the rate constant of 747 × 10−4 min−1, which was 66.5-, 3.44-, and 2.72-fold superior to the activities of CN, CNDs, and ZnMn2O4 photocatalysts, respectively. The impressive photodegradation performance of this nanocomposite was not only associated with the capacity for impressive visible-light absorption and boosted separation and transport of charge carriers, but also with its large surface area.

Enhanced photocatalytic organic pollutant degradation and H2 evolution reaction over carbon nitride nanosheets: N defects abundant materials

Post thermal treatment of bulk graphitic carbon nitride (g-C₃N₄) by ammonia gas acts as a significant structure regulation approach, while pure ammonia-assisted g-C₃N₄ synthesis from precursors like melamine is rarely investigated. Here we prove the synthesis of N-defects abundant carbon nitride nanosheets (ACN) through a one-pot thermal polymerization of melamine in pure ammonia gas, for photocatalytic organic pollutant removal in water and H₂ evolution applications. Compared to bulk g-C₃N₄ (BCN), ACN-550 (ACN prepared at 550 °C) exhibited thin-layered porous morphology with higher surface area and abundant N defects, resulting in wider distribution of active sites. Moreover, the abundant N defects in the heptazine heterocycle structure could change the electronic structure of g-C₃N₄, leading to more efficient transport of photogenerated charge carriers and enhanced photoreduction potential, which gives rise to notable improvement activities in photocatalytic reaction. With superoxide ion radical and photoinduced holes as the predominant reactive species, ACN-550 realized efficient photocatalytic bisphenol A (BPA) degradation, which is 1.6- and 4.7-fold high over commercial TiO₂ (P25) and BCN, respectively. ACN-550 exhibited excellent reusability and stability in five consecutive photocatalytic BPA degradation tests. In photo-reductive H₂ production system by ACN-550, 761.8 ± 4.3 μmol/h/g H₂ was produced, which was 11.6-fold as high as that by BCN.

Engineered inverse opal structured semiconductors for solar light-driven environmental catalysis

Inverse opal (IO) macroporous semiconductor materials with unique physicochemical advantages have been widely used in solar-related environmental areas. In this minireview, we first summarize the synthetic methods of IO materials, emphasizing the two-step and three-step approaches, with the typical physicochemical properties being compared where applicable. We subsequently discuss the application of IO semiconductors (e.g., TiO₂, ZnO, g-C₃N₄) in various photo-related environmental techniques, including photo- and photoelectro-catalytic organic pollutant degradation in water, optical sensors for environmental monitoring, and water disinfection. The engineering strategies of these hierarchical structures for optimizing the activities for different catalytic reactions are discussed, ranging from heterojunction construction, cocatalyst loading, and heteroatom doping, to surface defect construction. Structure-activity relationships are established correspondingly. With a systematic understanding of the unique properties and catalytic activities, this review is expected to orient the design and structure optimization of IO semiconductor materials for photo-related performance improvement in various environmental techniques. Finally, the challenges of emerging IO structured semiconductors and future development directions are proposed.

g-C3N4-Based Direct Z-Scheme Photocatalysts for Environmental Applications

Photocatalysis represents a promising technology that might alleviate the current environmental crisis. One of the most representative photocatalysts is graphitic carbon nitride (g-C3N4) due to its stability, cost-effectiveness, facile synthesis procedure, and absorption properties in visible light. Nevertheless, pristine g-C3N4 still exhibits low photoactivity due to the rapid recombination of photo-induced electron-hole (e−-h+) pairs. To solve this drawback, Z-scheme photocatalysts based on g-C3N4 are superior alternatives since these systems present the same band configuration but follow a different charge carrier recombination mechanism. To contextualize the topic, the main drawbacks of using g-C3N4 as a photocatalyst in environmental applications are mentioned in this review. Then, the basic concepts of the Z-scheme and the synthesis and characterization of the Z-scheme based on g-C3N4 are addressed to obtain novel systems with suitable photocatalytic activity in environmental applications (pollutant abatement, H2 production, and CO2 reduction). Focusing on the applications of the Z-scheme based on g-C3N4, the most representative examples of these systems are referred to, analyzed, and commented on in the main text. To conclude this review, an outlook of the future challenges and prospects of g-C3N4-based Z-scheme photocatalysts is addressed.

Designing oxygen vacancy mediated bismuth molybdate (Bi2MoO6)/N-rich carbon nitride (C3N5) S-scheme heterojunctions for boosted photocatalytic removal of tetracycline antibiotic and Cr(VI): Intermediate toxicity and mechanism insight

Polymeric N-rich carbon nitride of C₃N₅ is being utilized as a new visible-light-driven catalyst due to its narrower bandgap (∼2.0 eV). Building step-scheme (S-scheme) heterojunction by coupling with other semiconductors especially those own oxygen vacancies (OVs) can further upgrade the photocatalytic performance of C₃N₅-based photocatalysts. Herein, a novel S-scheme heterojunction of OVs mediated Bi₂MoO₆/C₃N₅ was fabricated by in-situ growing Bi₂MoO₆ nanoparticles with OVs on C₃N₅ nanosheets. Benefiting from the efficient separation and transfer of high energetic charge carriers by S-scheme charge migration, enriched structural defects, as well as the close contact by the in-situ growth, the heterojunction exhibited superior visible-light photocatalytic performance toward the removal of tetracycline (TC) and Cr(VI) than C₃N₅, Bi₂MoO₆, and their mechanical mixture under visible light. The TC degradation routes and the bio-toxicity evolution of TC were explored. Moreover, the photocatalytic mechanism for TC decomposition and Cr(VI) reduction over Bi₂MoO₆/C₃N₅ with OVs were elucidated. This work presents a newfangled vision for designing promising C₃N₅-based S-scheme heterojunction photocatalysts for pollution control.