Dissolution and homogeneous photocatalysis of polymeric carbon nitride

Dissolution of g-C3N4 Using Zinc Chloride Molten Salt Hydrates for Nanobelt Fabrication and Photocatalytic H2O2 Production

Graphitic-carbon nitride (g-C3N4), a metal-free two-dimensional layered semiconductor material, holds great potential for energy conversion, environmental remediation, and sensing. However, the limited solubility of g-C3N4 in conventional solvents hinders its widespread application. Improving the dissolution of g-C3N4 in the liquid phase is highly desired but challenging. Herein, we report an innovative approach to dissolve g-C3N4 using ZnCl2 molten salt hydrates. The solubility of g-C3N4 in the solution reaches up to 200 mg mL-1. Density functional theory (DFT) results suggest that ZnCl+H2O is the key species that leads to charge redistribution on g-C3N4 surface and promotes the dissolution of carbon nitride in the solution. Furthermore, through dilution, the dissolved carbon nitride can be effectively recovered while maintaining its intrinsic chemical structure. The resultant regenerated C3N4 (r-C3N4) exhibits nanobelt morphology and demonstrates a substantially improved photocatalytic activity in H2O2 production. The rate of H2O2 production over the r-C3N4 reaches 20,228 μmol g-1 h-1, which is 6.2 times higher than that of pristine g-C3N4. This green and efficient dissolution route of g-C3N4 offers an effective approach for its diverse applications.

Photocatalytic degradation of fluoroquinolone antibiotics using chitosan biopolymer functionalized copper oxide nanoparticles prepared by facile sonochemical method

Photocatalytic degradation is an excellent method for removing pharmaceutical residues due to their simplicity, ecological benignity, high efficiency, and exceptional stability. Herein, we demonstrate the sonochemically synthesised chitosan biopolymer functionalized copper oxide nanoparticles as an efficient photocatalyst for the degradation of fluoroquinolone-based antibiotics. The X-ray diffraction Rietveld refinement revealed the formation of single-phase copper oxide (CuO) with a monoclinic structure. The presence of biopolymer functionalization was corroborated by Fourier Transform Infrared spectroscopy by observing the -NH2 and -OH functional groups. The high-resolution transmission electron microscopic images inferred that Chitosan functionalized copper oxide (C-CuO) particles are nano-sized with a smooth texture and aggregation-free particles. The strong absorbance and the broad photoluminescence emission in the ultraviolet-visible region confirm the suitability of CuO and C-CuO nanoparticles for photocatalytic applications. The catalytic activity was studied against fluoroquinolone-based antibiotics such as ciprofloxacin and norfloxacin under direct sunlight illumination. Interestingly, the C-CuO catalyst demonstrated 71.07 % (@140 min.) and 71.9 % (@60 min.) of degradation for ciprofloxacin and norfloxacin, respectively. The obtained photocatalytic activity of the prepared CuO and C-CuO catalysts was superior to the CuO particles prepared by the coprecipitation method (CC-CuO).

Regulation of Polymerization Kinetics to Improve Crystallinity of Carbon Nitride for Photocatalytic Reactions

Carbon nitride (CN) polymers exhibit tunable and fascinating physicochemical properties and are thus an essential class of photocatalytic materials with potential applications. Although significant progress has been made in the fabrication of CN, the preparation of metal-free crystalline CN via a straightforward method remains a considerable challenge. Herein, we describe a new attempt to synthesize crystalline carbon nitride (CCN) with a well-developed structure through regulation of the polymerization kinetics. The synthetic process involves the pre-polymerization of melamine to remove most of the ammonia and further calcination of the pre-heated melamine in the presence of copper oxide as an ammonia absorbent. Copper oxide can decompose the ammonia produced by the polymerization process, thereby promoting the reaction. These conditions facilitate the polycondensation process while avoiding carbonization of the polymeric backbone at high temperatures. Owing to the high crystallinity, nanosheet structure, and efficient charge-carrier transmission capacity, the as-prepared CCN catalyst shows much higher photocatalytic activity than its counterparts. Our study provides a novel strategy for the rational design and synthesis of high-performance carbon nitride photocatalysts by simultaneously optimizing polymerization kinetics and crystallographic structures.

Molecular assembly of carbon nitride-based composite membranes for photocatalytic sterilization and wound healing

A multifactorial mechanism for successful dissolution of polymeric carbon nitrides (pCN) was disclosed, enabling pCN to compound more advanced nanocomposites at the molecular level, beyond the traditional solar fuel applications in powders.

Sol-Gel Processing of Water-Soluble Carbon Nitride Enables High-Performance Photoanodes*

In spite of the enormous promise that polymeric carbon nitride (PCN) materials hold for various applications, the fabrication of high-quality, binder-free PCN films and electrodes has been a largely elusive goal to date. Here, we tackle this challenge by devising, for the first time, a water-based sol-gel approach that enables facile preparation of thin films based on poly(heptazine imide) (PHI), a polymer belonging to the PCN family. The sol-gel process capitalizes on the use of a water-soluble PHI precursor that allows formation of a non-covalent hydrogel. The hydrogel can be deposited on conductive substrates, resulting in formation of mechanically stable polymeric thin layers. The resulting photoanodes exhibit unprecedented photoelectrochemical (PEC) performance in alcohol reforming and highly selective (∼100 %) conversions with very high photocurrents (>0.25 mA cm-2 under 2 sun) down to <0 V vs. RHE. This enables even effective PEC operation under zero-bias conditions and represents the very first example of a 'soft matter'-based PEC system capable of bias-free photoreforming. The robust binder-free films derived from sol-gel processing of water-soluble PCN thus constitute a new paradigm for high-performance 'soft matter' photoelectrocatalytic systems and pave the way for further applications in which high-quality PCN films are required.

Impact of Bi Doping into Boron Nitride Nanosheets on Electronic and Optical Properties Using Theoretical Calculations and Experiments

In the present work, boron nitride (BN) nanosheets were prepared through bulk BN liquid phase exfoliation while various wt. ratios (2.5, 5, 7.5 and 10) of bismuth (Bi) were incorporated as dopant using hydrothermal technique. Our findings exhibit that the optical investigation showed absorption spectra in near UV region. Density functional theory calculations indicate that Bi doping has led to various modifications in the electronic structures of BN nanosheet by inducing new localized gap states around the Fermi level. It was found that bandgap energy decrease with the increase of Bi dopant concentrations. Therefore, in analysis of the calculated absorption spectra, a redshift has been observed in the absorption edges, which is consistent with the experimental observation. Additionally, host and Bi-doped BN nanosheets were assessed for their catalytic and antibacterial potential. Catalytic activity of doped free and doped BN nanosheets was evaluated by assessing their performance in dye reduction/degradation process. Bactericidal activity of Bi-doped BN nanosheets resulted in enhanced efficiency measured at 0-33.8% and 43.4-60% against S. aureus and 0-38.8% and 50.5-85.8% against E. coli, respectively. Furthermore, In silico molecular docking predictions were in good agreement with in-vitro bactericidal activity. Bi-doped BN nanosheets showed good binding score against DHFR of E. coli (- 11.971 kcal/mol) and S. aureus (- 8.526 kcal/mol) while binding score for DNA gyrase from E. coli (- 6.782 kcal/mol) and S. aureus (- 7.819 kcal/mol) suggested these selected enzymes as possible target.

Unraveling fundamental active units in carbon nitride for photocatalytic oxidation reactions

Covalently bonded carbon nitride (CN) has stimulated extensive attention as a metal-free semiconductor. However, because of the complexity of polymeric structures, the acquisition of critical roles of each molecular constituent in CN for photocatalysis remains elusive. Herein, we clarify the fundamental active units of CN in photocatalysis by synthesizing CN with more detailed molecular structures. Enabled by microwave synthesis, the as-prepared CN consists of distinguishable melem (M1) and its incomplete condensed form (M2). We disclose rather than the traditional opinion of being involved in the whole photocatalytic processes, M1 and M2 make primary contributions in light absorption and charge separation, respectively. Meanwhile, oxygen molecules are unusually observed to be activated by participating in the photoexcited processes via electronic coupling mainly to M2. As a result, such CN has a higher activity, which was up to 8 times that of traditional bulk CN for photocatalytic oxidation of tetracycline in water.

Metal-doped carbon nitrides: synthesis, structure and applications

This perspective provides a comprehensive overview of the latest progress of M–CN, which would promote further development, such as for single-atom catalysis and nanozymatic reactions.

Recent Advances on Metalloporphyrin-Based Materials for Visible-Light-Driven CO2 Reduction

Photocatalytic CO₂ reduction is a promising technology to mitigate environmental issue and the energy crisis. The four nitrogen atoms in the porphyrin ring can incorporate transition metals to form stable active sites for CO₂ activation and photoreduction. Nevertheless, the photocatalytic efficiency of metalloporphyrins is still low due to the insufficient photoelectron injection to drive CO₂ photoreduction upon visible light irradiation. To address this issue, considerable efforts have been made to introduce photosensitizers for constructing homogeneous or heterogeneous metalloporphyrin-based photocatalytic systems. In this Review, recent advances of metalloporphyrin-based materials for visible-light-driven CO₂ reduction were summarized. The methods for the modulation of photosensitizing process at molecular level were presented for the promotion of photocatalytic performance. The mechanism of CO₂ activation and photocatalytic conversion was illustrated. Better insight into the structure-activity relationship provides guidance to the design of metalloporphyrin-related photocatalytic systems.

C2N: A Class of Covalent Frameworks with Unique Properties

C2N is a unique member of the CnNm family (carbon nitrides), i.e., having a covalent structure that is ideally composed of carbon and nitrogen with only 33 mol% of nitrogen. C2N, with a stable composition, can easily be prepared using a number of precursors. Moreover, it is currently gaining extensive interest owing to its high polarity and good thermal and chemical stability, complementing carbon as well as classical carbon nitride (C3N4) in various applications, such as catalysis, environmental science, energy storage, and biotechnology. In this review, a comprehensive overview on C2N is provided; starting with its preparation methods, followed by a fundamental understanding of structure-property relationships, and finally introducing its application in gas sorption and separation technologies, as supercapacitor and battery electrodes, and in catalytic and biological processes. The review with an outlook on current research questions and future possibilities and extensions based on these material concepts is ended.

Rapid visible-light degradation of EE2 and its estrogenicity in hospital wastewater by crystalline promoted g-C3N4

Metal-free, chemically activated crystalline graphitic carbon nitride (g-C₃N₄) nanorods with enhanced visible-light photoactivity demonstrated rapid photodegradation of 17α-ethinylestradiol (EE2) in water and real hospital wastewater. Pure g-C₃N₄ and another three crystalline promoted g-C₃N₄ photocatalysts developed by hydrothermal method were characterized by, High-Resolution Transmission Electron Microscopy (HRTEM), X-ray diffraction (XRD), Fourier-Transform Infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET), Photoluminescence (PL), Electron spin resonance (ESR), X-ray Photoelectron Spectroscopy (XPS) and Diffuse Reflectance Spectroscopy (DRS). Hydrothermal-based chemical activation did not alter the crystal structure, functional group or surface morphology, but it enhanced the specific surface area of activated g-C₃N₄ due to intralayer delamination and depolymerization of g-C₃N₄. Compared to pure g-C₃N₄, the activated g-C₃N₄-3 demonstrated efficient degradation of EE2 (<30 min, 3 mg/l) by visible wavelengths of the solar spectrum. This work provides advanced insight into the construction of heterojunction visible-light photocatalysts and production of O₂^- via reduction of O₂ with photogenerated electrons. Proposed and derived mechanism for photodegradation of EE2 by g-C₃N₄-3 using gas chromatography-mass spectrometry (GCMS). Yeast Estrogen Screen (YES) was performed to evaluate the estrogenicity of treated water samples. Efficient removal of EE2 estrogenic activity (<45 min, 3 mg/l) was achieved using the visible light-activated g-C₃N₄. Estrogenicity removal rate corresponded well with EE2-degradation rate.

Efficient organic pollutants conversion and electricity generation for carbonate-containing wastewater based on carbonate radical reactions initiated by BiVO4-Au/PVC system

In this paper, we propose an efficient simultaneous refractory organics degradation and electricity generation method for carbonate-containing wastewater based on carbonate radical reactions initiated by a BiVO₄-Au/PVC (PVC: photovoltaic cell) system. In the system, nanoporous BiVO₄ film and Au modified PVC were used as photoanode and photocathode, respectively. HCO₃^- was used as the electrolyte. Carbonate radicals, which have lower recombination rates than hydroxyl radicals and strong oxidation abilities, can be generated easily by the capture reaction of hydroxyl radicals with HCO₃^-, which is one of the most abundant anions in the aquatic environment. The results show that the removal ratios of rhodamine b, methyl orange and methylene blue in the system increased sharply to 77.98 %, 89.15 % and 93.2 % from 18.23 %, 21 % and 23.14 % (BiVO₄-Pt/ SO₄^2-), respectively, after 120 min. Meanwhile, the short-circuit current density is up to 2.19-2.41 times larger than the traditional system. Other common ions in natural water minimally affected the properties of the new system. The excellent performance could be ascribed to large amounts of carbonate radicals in the system, which have great potential for efficient carbonate-containing wastewater treatment and energy recovery.

Water-Soluble Polymeric Carbon Nitride Colloidal Nanoparticles for Highly Selective Quasi-Homogeneous Photocatalysis

Heptazine-based polymeric carbon nitrides (PCN) are promising photocatalysts for light-driven redox transformations. However, their activity is hampered by low surface area resulting in low concentration of accessible active sites. Herein, we report a bottom-up preparation of PCN nanoparticles with a narrow size distribution (ca. 10±3 nm), which are fully soluble in water showing no gelation or precipitation over several months. They allow photocatalysis to be carried out under quasi-homogeneous conditions. The superior performance of water-soluble PCN, compared to conventional solid PCN, is shown in photocatalytic H2 O2 production via reduction of oxygen accompanied by highly selective photooxidation of 4-methoxybenzyl alcohol and benzyl alcohol or lignocellulose-derived feedstock (ethanol, glycerol, glucose). The dissolved photocatalyst can be easily recovered and re-dissolved by simple modulation of the ionic strength of the medium, without any loss of activity and selectivity.

A Facile and Green Combined Strategy for Improving Photocatalytic Activity of Carbon Nitride

We first report an efficient combined strategy that simultaneously integrates copolymerization, doping, and molecular self-assembly for the development of carbon-doping petal-like carbon nitride photocatalysts using melamine (MA), cyanuric acid (CA), and 2,4,6-triaminopyrimidine (TAP) as the starting precursors and water as the only green solvent. The morphology, textural, optical, and electronic properties of carbon nitride could be engineered by rationally manipulating the doping content of TAP. In the process of molecular self-assembly, TAP can insert the aggregate edge easily according to the results of density functional theory (DFT) calculations. The edge-termination effect of TAP made it easier for the modified carbon nitride materials to form petal-like nanosheets with porous structures and large BET surface areas. In addition, the incorporation of TAP also contributed to tuning the electronic band structures of carbon nitrides and enhancing the separation efficiency of photogenerated carriers. The as-prepared materials exhibited excellent photocatalytic activities in the degradation of tetracycline hydrochloride (TC-HCl) and rhodamine B (RhB). This work may not only offer universally powerful and stable photocatalysts for applications but also develop a new combined strategy to fabricate efficient photocatalysts in a facile and green way.