β-Cyclodextrin as a Precursor to Holey C-Doped g-C3 N4 Nanosheets for Photocatalytic Hydrogen Generation

Fe-doped g-C3N4 with duel active sites for ultrafast degradation of organic pollutants via visible-light-driven photo-Fenton reaction: Insight into the performance, kinetics, and mechanism

The photo-Fenton process provides a sustainable and cost-effective strategy for removing refractory organic contaminants in wastewater. Herein, a high-efficient Fe-doped g-C3N4 photocatalyst (Fe@CN10) with a unique 3D porous mesh structure was prepared by one-pot thermal polymerization for ultrafast degradation of azo dyes, antibiotics, and phenolic acids in heterogeneous photo-Fenton systems under visible light irradiation. Fe@CN10 exhibited a synergy between adsorption-degradation processes due to the co-existence of Fe3C and Fe3N active sites. Specifically, Fe3C acted as an adsorption site for pollutant and H2O2 molecules, while Fe3N acted as a photocatalytic active site for the high-efficient degradation of MO. Resultingly, Fe@CN10 showed a photocatalytic degradation rate of MO up to 140.32 mg/L min-1. The dominant ROS contributed to the removal of MO in the photo-Fenton pathway was hydroxyl radical (•OH). Surprisingly, as the key reactive species, singlet oxygen (1O2) generated from superoxide radical (•O2-) also efficiently attacked MO in a photo-self-Fenton pathway. Additionally, sponge/Fe@CN10 was prepared and filled in the continuous flow reactors for nearly 100% degradation of MO over 150 h when treating artificial organic wastewater. This work provided a facile route to prepare highly-active Fe-doped photocatalysts and develop a green photocatalytic system for wastewater treatment in the future.

Improving U(VI) retention efficiency and cycling stability of GCN-supported calcined-LDH composite: Mechanism insight and real water system applications

The synthesis and characterization of graphitic carbon nitride (GCN) and its composites with calcined layered double hydroxide (CLDH) were examined in this investigation. The goal was to assess these composites' maximum adsorption capacity (qmax) for U(VI) ions in wastewater. Several different characterization methodologies were utilized to examine the fabricated substances. These methods encompass X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The GCN-CLDH composite displayed enhanced adsorption ability towards U(VI) ions due to its high surface functionality. Langmuir adsorption isotherm analysis showed that more than 99% of U(VI) ions were adsorbed, with a qmax of 196.69 mg/g. The kinetics data exhibited a good fit for a pseudo-second-order (PSO) model. Adsorption mechanisms involving precipitation and surface complexation via Lewis's acid-base interactions were proposed. The application of the GCN-CLDH composite in groundwater demonstrated adsorption below the maximum permissible limit established by USEPA, indicating improved cycling stability. These observations underscore the capacity of the GCN-CLDH composite's proficiency in adsorbing U(VI) aqueous solutions containing radioactive metals.

Porous cyclopentadiene unit-incorporated graphitic carbon nitride nanosheets for efficient photocatalytic oxidation of recalcitrant organic micropollutants in wastewater

For the purpose of searching for efficient photocatalysts to deal with recalcitrant organic micropollutants in wastewater, here an in-situ supramolecule self-assembly-thermal polymerization strategy is developed to prepare a series of porous cyclopentadiene (CPD) unit-incorporated g-C3N4 ultrathin nanosheets (CCPD-g-C3N4). The CCPD-g-C3N4 demonstrate CPD unit doping level-dependent and remarkably enhanced visible-light photocatalytic oxidation efficiency towards two organic micropollutants, acetaminophen and methylparaben, in which the optimized CCPD-g-C3N4-2 shows 6.1 and 3.5 times higher acetaminophen and methylparaben degradation rate than bulk g-C3N4; moreover, CCPD-g-C3N4-2 is still robust and efficient in the treatment of five mixed organic micropollutants in pharmaceutical wastewater, and the satisfactory micropollutant removal efficiency is obtained in a wide pH window and the presence of high concentrations of inorganic anions and cations as well as dissolved organic matters. Theoretical calculation combined with experimental test reveal that CCPD-g-C3N4 can significantly reduce ecological risk of the target pollutant after the photocatalytic degradation reaction. Such enhanced photocatalytic oxidation efficiency is dominated by the accelerated charge carrier separation dynamics and extended visible-light response region due to the incorporation of CPD units, which finally lead to the generation of abundant reactive oxygen species to degrade and mineralize target micropollutants efficiently.

Solar light photocatalytic transformation of heptachlorobiphenyl (PCB 180) using g-C3N4 based magnetic porous photocatalyst

A novel porous core-shell magnetic β-cyclodextrin/graphitic carbon nitride photocatalyst (Mβ-CD/GCN) was synthesized and employed in a solar light driven catalytic system for the degradation of polychlorinated biphenyls (PCBs). The Mβ-CD/GCN display superior photocatalytic performance on account of porous structure and ultrathin GCN nanosheets design, the former improves the utilization of visible light by multiple scattering and reflection of incident light, and the latter accelerates electron transfer. The ultrahigh specific surface area (1255 m² g^-1) of Mβ-CD/GCN provided a large number of active sites for adsorption and degradation of the target pollution. The pseudo-first order reaction rate constant (k_obs) for the degradation of PCB180 by Mβ-CD/GCN was 0.021 min^-1, which improved 3.23 times than the bulk GCN. Additionally, the effects of various reaction parameters and water matrices were studied on the degradation of PCB180. Three possible degradation pathways and mechanism of PCB180 were speculated according to the identification of reaction intermediates and detection of reactive species. The solar light driven Mβ-CD/GCN catalytic technology is a promising method not only for the control of persistent organic pollutants (POPs), but also the catalyst could be recovered and reused through simple magnetic separation.

Ion-Induced Synthesis of Crystalline Carbon Nitride Ultrathin Nanosheets from Mesoporous Melon for Efficient Photocatalytic Hydrogen Evolution with Synchronous Highly Selective Oxidation of Benzyl Alcohol

Crystalline carbon nitride (CCN) with a poly(heptazine imide) structure is efficient in photocatalytic hydrogen evolution (PHE), but synthesis of CCN ultrathin nanosheets (CCNuns) and their use in PHE with selective organic oxidation are still rare. Herein, CCNuns with Na⁺ doping are prepared using NaCl as the ion-induction and templating agent and mesoporous melon as the feedstock, exhibiting efficient synchronous PHE and benzyl alcohol oxidation to benzaldehyde, with an apparent quantum yield of 10.5% at 420 nm and a visible light PHE rate that is 94.3 times that of bulk polymeric carbon nitride (PCN). The selectivity of benzaldehyde formation (90.5%) is also much higher than that of PCN (40.7%). Interestingly, this selectivity increases gradually with increasing light wavelengths. The high photoactivity of CCNuns originates from their ultrathinness and Na⁺ doping, which considerably enhance the photogenerated charge separation. This work opens up an avenue for the synthesis of CCNuns and extends their application.

Synthesis of Vitamin B3 through a Heterogeneous Photocatalytic Approach Using Metal-Free Carbon Nitride-Based Catalysts

Vitamin B3 (nicotinic acid, VB3) was synthesized through the photocatalytic oxidation of 3-pyridinemethanol (3PM) under visible-light-emitting diode (LED) irradiation using metal-free graphitic carbon nitride (GCN) - based materials. A bulk (GCN) material was prepared by a simple thermal treatment using dicyandiamide as the precursor. A post-thermal treatment under static air and nitrogen flow was employed to obtain the GCN-T and GCN-T-N materials, respectively. The conditions adopted during the post-treatment revealed differences in the resulting materials’ morphological, electronic, and optical properties. The post-treated photocatalysts revealed an enhanced efficiency in the oxidation of 3PM into VB3, with the GCN-T-N photocatalyst being the best-performing material. The defective surface, reduced crystallinity, and superior photoabsorption of GCN-T-N account for this material’s improved performance in the production of VB3. Nevertheless, the presence of nitrogen vacancies in the carbon nitride structure and, consequently, the creation of mid-gap states also accounts to its highly oxidative ability. The immobilization of GCN-T-N in sodium alginate hydrogel was revealed as a promising strategy to produce VB3, avoiding the need for the photocatalyst separation step. Concerning the mechanism of synthesis of VB3 through the photocatalytic oxidation of 3PM, it was possible to identify the presence of 3-pyridinecarboxaldehyde (3PC) as the intermediary product.

β-Cyclodextrin-Derivative-Functionalized Graphene Oxide/Graphitic Carbon Nitride Composites with a Synergistic Effect for Rapid and Efficient Sterilization

The nonselectivity of phototherapy and the hydrophobicity of phototherapy agents limit their application in the treatment of antibiotic-resistant bacteria. In this work, β-cyclodextrin-derivative-functionalized graphene oxide (GO)/graphitic carbon nitride (g-C₃N₄) antibacterial materials (CDM/GO/CN) were designed and synthesized. CN is used as a photosensitizer for photodynamic therapy (PDT) and GO as a photothermal agent for photothermal therapy (PTT). In addition, the supramolecular host-guest complex on the substrate can not only increase the inherent water solubility of the substrate and reduce the aggregation of the photosensitizer/photothermal agent but also manipulate the interaction between the photosensitizer/photothermal agent and bacteria to capture specific bacteria. The hyperthermia caused by PTT denatures proteins on the cell membrane, allowing reactive oxygen species (ROS) to enter the cell better and kill bacteria. The specific capture of Escherichia coli CICC 20091 by mannose significantly improves the sterilization efficiency and reduces side effects. The synergistic antibacterial agent shows excellent antibacterial efficacy of over 99.25% against E. coli CICC 20091 after 10 min of 635 + 808 nm dual-light irradiation. Moreover, cell proliferation experiments show that the composite material has good biocompatibility, expected to have applications in bacterial infections.

Point-Defect Engineering: Leveraging Imperfections in Graphitic Carbon Nitride (g-C3 N4 ) Photocatalysts toward Artificial Photosynthesis

Graphitic carbon nitride (g-C₃ N₄ ) is a kind of ideal metal-free photocatalysts for artificial photosynthesis. At present, pristine g-C₃ N₄ suffers from small specific surface area, poor light absorption at longer wavelengths, low charge migration rate, and a high recombination rate of photogenerated electron-hole pairs, which significantly limit its performance. Among a myriad of modification strategies, point-defect engineering, namely tunable vacancies and dopant introduction, is capable of harnessing the superb structural, textural, optical, and electronic properties of g-C₃ N₄ to acquire an ameliorated photocatalytic activity. In view of the burgeoning development in this pacey field, a timely review on the state-of-the-art advancement of point-defect engineering of g-C₃ N₄ is of vital significance to advance the solar energy conversion. Particularly, insights into the intriguing roles of point defects, the synthesis, characterizations, and the systematic control of point defects, as well as the versatile application of defective g-C₃ N₄ -based nanomaterials toward photocatalytic water splitting, carbon dioxide reduction and nitrogen fixation will be presented in detail. Lastly, this review will conclude with a balanced perspective on the technical and scientific hindrances and future prospects. Overall, it is envisioned that this review will open a new frontier to uncover novel functionalities of defective g-C₃ N₄ -based nanostructures in energy catalysis.

Computational design of graphitic carbon nitride photocatalysts for water splitting

A series of structures based on graphitic carbon nitride (g-C₃N₄), a layered material composed of linked carbon-nitrogen heterocycles arranged in a plane, were investigated by density functional theory calculations. g-C₃N₄ is a semiconductor that absorbs UV light and visible light at the blue end of the visible spectrum, and is widely studied as a photocatalyst for water splitting; however, its photocatalytic efficiency is limited by its poor light-harvesting ability and low charge mobilities. Modifications to the g-C₃N₄ structure could greatly improve its optical and electronic properties and its photocatalytic efficiency. In this work, the g-C₃N₄ structure was modified by replacing the nitrogen linker with heteroatoms (phosphorus, boron) or aromatic groups (benzene, s-triazine and substituted benzenes). Two-dimensional (2D) sheets and three-dimensional (3D) multilayer structures with different stacking types were modelled. Several new structures were predicted to have electronic properties superior to g-C₃N₄ for use as water splitting photocatalysts. In particular, introduction of phosphorus, benzene and s-triazine groups led to band gaps smaller than in the standard g-C₃N₄ (down to 2.4 eV, corresponding to green light). Doping with boron in the linker positions dramatically reduced the band gap (to 1.6 eV, corresponding to red light); the doped material has the valence band position suitable for water oxidation. Our computational study shows that chemical modification of g-C₃N₄ is a powerful method to tune this material's electronic properties and improve its photocatalytic activity.

Synthesis of coralloid carbon nitride polymers and photocatalytic selective oxidation of benzyl alcohol

Polymeric carbon nitride (C₃N₄) is currently the most potential nonmetallic photocatalyst, but it suffers from low catalytic activity due to rapid electron-hole recombination behavior and low specific surface area. The morphology control of C₃N₄is one of the effective methods used to achieve higher photocatalytic performance. Here, bulk, lamellar and coralloid C₃N₄were synthesized using different chemical methods. The as-prepared coralloid C₃N₄has a higher specific surface area (123.7 m² · g^-1) than bulk (5.4 m² · g^-1) and lamellar C₃N₄(2.8 m² · g^-1), thus exhibiting a 3.15- and 2.59-fold higher photocatalytic efficiency for the selective oxidation of benzyl alcohol than bulk and lamellar C₃N₄, respectively. Optical characterizations of the photocatalysts suggest that coralloid C₃N₄can effectively capture electrons and accelerate carrier separation, which is caused by the presence of more nitrogen vacancies. Furthermore, it is demonstrated that superoxide radicals (·O₂^-) and holes (h⁺) play major roles in the photocatalytic selective oxidation of benzyl alcohol using C₃N₄as a photocatalyst.

Cellulose-assisted construction of high surface area Z-scheme C-doped g-C3N4/WO3 for improved tetracycline degradation

The preparation of heteroatom doping heterojunction photocatalysts with nontoxic carbonaceous materials and simple method still remains a challenge. Herein, ternary Z-scheme C-doped graphitic carbon nitride/tungsten oxide (C-doped g-C₃N₄/WO₃) was successfully fabricated via the hydrothermal impregnation with cellulose nanocrystal, high-temperature calcination, and electrostatic self-assembly with WO₃ nanocuboids in turns. Benefiting from the porous structure, high specific areas (57.20 m² g^-1), C-substitution, and the formation of Z-scheme heterojunction, the resulting photocatalyst exhibited narrower band-gap, enhanced visible-light absorption and separation of charge carrier, faster interfacial charge transfer, good oxidation/reduction capacities, and thus improved the photocatalytic activity performance. As such, this investigation will provide an effective route for not only incorporating semiconductors and heteroatoms into g-C₃N₄ but also developing more heterojunction with markedly improved photocatalytic performance.

Aqueous solution photocatalytic synthesis of p-anisaldehyde by using graphite-like carbon nitride photocatalysts obtained via the hard-templating route

Graphite-like carbon nitride (GCN)-based materials were developed via the hard-templating route, using dicyandiamide as the GCN precursor and silica templates. That resulted in urchin-like GCN (GCN-UL), 3D ordered macroporous GCN (GCN-OM) and mesoporous GCN (GCN-MP). The introduction of silica templates during GCN synthesis produced physical defects on its surface, as confirmed by SEM analysis, increasing their specific surface area. A high amount of nitrogen vacancies is present in modified catalysts (revealed by XPS measurements), which can be related to an increase in the reactive sites available to catalyse redox reactions. The textural and morphological modifications induced in GCN an enhanced light absorption capacity and reduced electron/hole recombination rate, contributing to its improved photocatalytic performance. In the photocatalytic conversion of p-anisyl alcohol to p-anisaldehyde in deoxygenated aqueous solutions under UV-LED irradiation, the GCN-UL was the best photocatalyst reaching 60% yield at 64% conversion for p-anisaldehyde production after 240 min of reaction. Under oxygenated conditions (air), the process efficiency was increased to 79% yield at 92% conversion only after 90 min reaction. The GCN-based photocatalyst kept its performance when using visible-LED radiation under air atmosphere. Trapping of photogenerated holes and radicals by selective scavengers showed that under deoxygenated conditions, holes played the primary role in the p-anisaldehyde synthesis. Under oxygenated conditions, the process is governed by the effect of reactive oxygen species, namely superoxide radicals, with a significant contribution from holes.

Efficient removal of parabens from real water matrices by a metal-free carbon nitride photocatalyst

Metal-free graphite-like carbon nitride (GCN-500) was obtained by thermal post-treatment of bulk polymeric carbon nitride at 500 °C. The catalyst was thoroughly characterized by morphological, optical and textural analysis techniques. The efficiency of GCN-500 was evaluated under visible (λ_exc = 417 nm) LED excitation for the photocatalytic degradation of methyl-, ethyl- and propyl-paraben in different water matrices either isolated or in a mixture of the three compounds. The GCN-500 proved to be more efficient than the benchmark TiO₂ P25, with complete conversion of the individual parabens within 20 min of irradiation, contrasting with 120 min needed for total degradation using TiO₂. Experiments in the presence of selected scavengers confirmed the high importance of superoxide radicals in the photocatalytic oxidation of parabens using GCN-500. The effect of the nature of the aqueous matrix in the kinetics of the photocatalytic process was assessed using ultrapure, tap and river waters spiked with a mixture of the three parabens. Although still very efficient, the complexity of the real water samples turned the degradation process slower due to the presence of other components such as ions and dissolved organic matter. GCN-500 proved to be stable in a continuous-flow system using GCN-500 coated glass rings (GCN-500-GR) to remove MP, EP and PP from real water matrices.

Improving the Visible-Light Photocatalytic Activity of Graphitic Carbon Nitride by Carbon Black Doping

Hydrogen production by water splitting and the removal of aqueous dyes by using a catalyst and solar energy are an ideal future energy source and useful for environmental protection. Graphitic carbon nitride can be used as the photocatalyst with visible light irradiation. However, it typically suffers from the high recombination of carriers and low electrical conductivity. Here, we have developed a facile mix-thermal strategy to prepare carbon black-modified graphitic carbon nitrides, which possess high electrical conductivity, a wide adsorption range of visible light, and a low recombination rate of carriers. With the help of carbon black, highly crystallized graphitic carbon nitrides with built-in triazine and heptazine heterojunctions are obtained. Improved photocatalytic activities have been achieved in carbon black-modified graphitic carbon nitride. The dye removal rate can be three times faster than that of pristine graphitic carbon nitride and the photocatalytic H₂ generation is 234 μmol h^-1 g^-1 under visible light irradiation.