Defect engineering in polymeric carbon nitride photocatalyst: Synthesis, properties and characterizations

A state-of-art review on the redox activity of persistent free radicals in biochar

Biochar-bound persistent free radicals (biochar-PFRs) attract much attention because they can directly or indirectly mediate the transformation of contaminants in large-scale wastewater treatment processes. Despite this, a comprehensive top-down understanding of the redox activity of biochar-PFRs, particularly consumption and regeneration mechanisms, as well as challenges in redox activity assessment, is still lacking. To tackle this challenge, this review outlines the identification and determination methods of biochar-PFRs, which serve as a prerequisite for assessing the redox activity of biochar-PFRs. Recent developments concerning biochar-PFRs are discussed, with a main emphasis on the reaction mechanisms (both non-free radical and free radical pathways) and their effectiveness in removing contaminants. Importantly, the review delves into the mechanism of biochar-PFRs regeneration, triggered by metal cations, reactive oxygen species, and ultraviolet radiations. Furthermore, this review thoroughly explores the dilemma in appraising the redox activity of biochar-PFRs. Components with unpaired electrons (particular defects and metal ions) interfere with biochar-PFRs signals in electron paramagnetic resonance spectra. Scavengers and extractants of biochar-PFRs also inevitably modify the active ingredients of biochar. Based on these analyses, a practical strategy is proposed to precisely determine the redox activity of biochar-PFRs. Finally, the review concludes by presenting current gaps in knowledge and offering suggestions for future research. This comprehensive examination aims to provide new and significant insights into the redox activity of biochar-PFRs.

Amino-grafted Biochar as a Novel Photocatalyst for degradation of high concentration dye

Photocatalytic degradation of organic pollution by biochar was a sustainable strategy for waste water remediation, nevertheless, it still suffers drawbacks like low efficiency due to the poor photocatalytic properties of pristine biochar. Herein, amino groups were grafted on the edge sites/defects of biochar by Friedel-Crafts acylation to enhance the degradation of high concentration dye solutions. The results suggested that the amino groups played an important role in imparting photocatalytic properties to biochar. Owing to the strong Lewis basicity and electron-donating ability of amino groups, their interaction with oxygen-containing functional groups/aromatic structures in biochar was improved, which enhanced the electron exchange ability of biochar under visible light irradiation, resulting in excellent degradation performances of high concentration RhB (∼10 times faster than ungrafted biochar). In this work, amino-grafted garlic peel biochar delivered a new idea for the future direction of biochar-based photocatalysis in wastewater remediation.

Defective Potassium Poly(Heptazine Imide) Preventing Spin Delocalization and Hole Transfer Deactivation for Efficient Solar Energy Conversion and Storage

Anti-site defective potassium poly(heptazine imide) (KPHI) with the central nitrogen atoms partially replaced by graphitic carbon atoms in the flawed heptazine rings is prepared by direct ionothermal treatment of the rationally designed supramolecular complex in KSCN salt molten. Compared to the KPHIs without the anti-site defect, the anti-site defective KPHI demonstrates significantly improved photocatalytic and dark photocatalytic performances for H2 evolution reaction (HER). In the presence of the hole scavenger, the anti-site defective KPHI exhibits superior photocatalytic stability for HER lasting 20 h, whereas the deactivation is observed from the ordinary KHPIs after 3 h HER. Moreover, the H2 yield in the dark by the stored photoelectrons in the anti-site defective KPHI increases by more than an order of magnitude. Density functional theory calculations reveal that the anti-site defective unit in KPHI not only prevents spin delocalization but also inhibits the deactivation of hole transfer, which are beneficial to photoelectron storage and photocatalytic activity. The findings in this study provide insight into the photophysical and catalytic properties of KPHI, which conclude a strategy to improve the performances for solar energy conversion and storage by incorporating intrinsic anti-site defects in KPHI.

A comprehensive review on the boosted effects of anion vacancy in the heterogeneous photocatalytic degradation, part I: Focus on sulfur, nitrogen, carbon, and halogen vacancies

The greenest environmental remediation way is the photocatalytic degradation of organic pollutants. However, limited photocatalytic applications are due to poor sunlight absorption and photogenerated charge carriers' recombination. These limitations can be overcome by introducing anion vacancy (AV) (O, S, N, C, and Halogen) defects in semiconductors that enhance light harvesting, facilitate charge separation, modulate electronic structure, and produce reactive radicals. In continuing part A of this review, in this part, we summarized the recent AVs' research, including S, N, C, and halogen vacancies on the boosted photocatalytic features of semiconductor materials, like metal oxides/sulfides, oxyhalides, and nitrides in detail. Also, we outline the recently developed AV designs for the photocatalytic degradation of organic pollutants. The AV creating and analysis methods and the recent photocatalytic applications and mechanisms of AV-mediated photocatalysts are reviewed. AV engineering photocatalysts' challenges and development prospects are illustrated to get a promising research direction.

Alkali-assisted engineering of ultrathin graphite phase carbon nitride nanosheets with carbon vacancy and cyano group for significantly promoting photocatalytic hydrogen peroxide generation under visible light: fast electron transfer channel

Exfoliating bulk graphite phase carbon nitride (g-C₃N₄) into 2D nanosheets is considered to be an effective method to enhance its photocatalytic activity. However, optical absorption capacity of the exfoliated g-C₃N₄ nanosheets are lower than that of the original bulk g-C₃N₄ due to the quantum size effect. Here, the ultrathin graphite phase carbon nitride nanosheets containing both carbon vacancy and cyano group (UCNS₅₈₀) were prepared by two-step calcination in air with the assistance of KOH. The formation and position of carbon vacancy and cyano group were first investigated and determined. The simultaneous introduction of carbon vacancy and cyano group not only improved light absorption range and intensity of g-C₃N₄ nanosheets, but also more importantly constructed a fast transfer channel for photogenerated electrons, further enhancing the separation efficiency and migration ability of photogenerated carriers. The cyano group as the accumulation center of photogenerated electrons and the oxygen adsorption center increased the proportion of one-step two-electrons reaction path to efficiently generate H₂O₂. As a result, UCNS₅₈₀ exhibited highly boosted H₂O₂ generation activity, its H₂O₂ production yield for 6 h reached 939 µmol/L and the formation rate was up to 4167 µM h^-1 g^-1, which was in priority in the reported literature under the same conditions.

Recent advances in g-C3N4-based photo-enzyme catalysts for degrading organic pollutants

In recent years, photocatalytic reactions have shown great potential in degrading organic pollutants because of their simple operation and no secondary pollution. Graphitic carbon nitride (g-C₃N₄) is one of the most frequently used photocatalyst materials in the field of photocatalysis because it is a form of photocatalytic material with facile synthesis, no metal, visible light response, and strong stability. Enzyme-catalyzed degradation has received extensive attention due to its broad selectivity, high efficiency, and environmental friendliness. Horseradish peroxidase (HRP), one of several oxidoreductases utilized for pollutant degradation, has a wide range of applications due to its mild reaction conditions and high stability. Exploring efficient platforms for immobilizing g-C₃N₄ and HRP to develop photo-enzyme-coupled catalysis is an attractive practical topic. The coupling effect of g-C₃N₄ and HRP improves the carrier separation efficiency and generates more active species, which finally realize the solar-driven non-selective destruction of organic pollutants. We describe the alteration of g-C₃N₄ and the immobilization of HRP in detail in this study, and we outline recent developments in the photo-enzyme coupling of g-C₃N₄ and HRP.

Carbon vacancies enriched carbon nitride nanotubes for Pd coordination environment optimization: Highly efficient photocatalytic hydrodechlorination and CO2 cycloaddition

The use of easily available solar energy to achieve pollutants efficient degradation and waste carbon resource CO₂ utilization under mild conditions is highly desired. Herein, novel carbon vacancies enriched nanotubes graphitic carbon nitride (SCNT-500) has been successfully fabricated via melamine (MA) supramolecular hydrogen-bonded self-assembly in the presence of H₂SO₄. Pd NPs loaded carbon vacancies enriched carbon nitride nanotubes (Pd/SCNT-500) were used for photocatalytic chlorophenols hydrodechlorination and CO₂ cycloaddition with styrene oxide. Up to 6.93 s^-1 4-chlorophenol hydrodechlorination TOF and obviously improved CO₂ cycloaddition efficiency could be realized with Pd/SCNT-500. The improved photocatalytic efficiency should be related to the morphology and carbon vacancies based Pd coordination environment optimization. Such as, the surface area increased nanotubes structure promoted light harvesting along with photoelectrons and holes generation; the carbon vacancies improved excited electrons capture, photoinduced carriers recombination inhibition along with substrates adsorption with electron rich Pd NPs. Mechanism studies not only demonstrated the important role of atomic hydrogen and Pd coordination environment optimization in the chlorophenols hydrodechlorination, but also confirmed the promotion ability of photogenerated electrons on CO₂ cycloaddition.

Synthesis of vacant graphitic carbon nitride in argon atmosphere and its utilization for photocatalytic hydrogen generation

Graphitic carbon nitride (C₃N₄) was synthesised from melamine at 550 °C for 4 h in the argon atmosphere and then was reheated for 1–3 h at 500 °C in argon. Two band gaps of 2.04 eV and 2.47 eV were observed in all the synthetized materials. Based on the results of elemental and photoluminescence analyses, the lower band gap was found to be caused by the formation of vacancies. Specific surface areas of the synthetized materials were 15–18 m²g⁻¹ indicating that no thermal exfoliation occurred. The photocatalytic activity of these materials was tested for hydrogen generation. The best photocatalyst showed 3 times higher performance (1547 μmol/g) than bulk C₃N₄ synthetized in the air (547 μmol/g). This higher activity was explained by the presence of carbon (V_C) and nitrogen (V_N) vacancies grouped in their big complexes 2V_C + 2V_N (observed by positron annihilation spectroscopy). The effect of an inert gas on the synthesis of C₃N₄ was demonstrated using Graham´s law of ammonia diffusion. This study showed that the synthesis of C₃N₄ from nitrogen-rich precursors in the argon atmosphere led to the formation of vacancy complexes beneficial for hydrogen generation, which was not referred so far.

Adjusting the match-degree between electron library and surface-active sites and forming surface polarization in MOF-based photo-cocatalysts for accelerating electron transfer

The Ni-CoP supported by a carbon matrix as the cocatalyst is synthesized by precisely controlling the pyrolysis temperature for the metal–organic framework, then loaded onto the CdS host catalyst by means of self-assembly for photocatalytic hydrogen production.

Electrospun Ceramic Nanofibers for Photocatalysis

Ceramic fiber photocatalysts fabricated by electrospinning hold great potential in alleviating global environmental and energy issues. However, many challenges remain in improving their photocatalytic efficiencies, such as the limited carrier lifetime and solar energy utilization. To overcome these predicaments, various smart strategies have been invented and realized in ceramic fiber photocatalysts. This review firstly attempts to summarize the fundamental principles and bottlenecks of photocatalytic processes. Subsequently, the approaches of doping, surface plasmon resonance, and up-conversion fluorescent to enlarge the light absorption range realized by precursor composition design, electrospinning parameter control, and proper post heat-treatment process are systematically introduced. Furthermore, methods and achievements of prolonging the lifetime of photogenerated carriers in electrospun ceramic fiber photocatalysts by means of introducing heterostructure and defective composition are reviewed in this article. This review ends with a summary and some perspectives on the future directions of ceramic fiber photocatalysts.