Graphitic Carbon Nitride/Zinc Oxide-Based Z-Scheme and S-Scheme Heterojunction Photocatalysts for the Photodegradation of Organic Pollutants

Graphitic carbon nitride (g-C3N4), a metal-free polymer semiconductor, has been recognized as an attractive photocatalytic material for environmental remediation because of its low band gap, high thermal and photostability, chemical inertness, non-toxicity, low cost, biocompatibility, and optical and electrical efficiency. However, g-C3N4 has been reported to suffer from many difficulties in photocatalytic applications, such as a low specific surface area, inadequate visible-light utilization, and a high charge recombination rate. To overcome these difficulties, the formation of g-C3N4 heterojunctions by coupling with metal oxides has triggered tremendous interest in recent years. In this regard, zinc oxide (ZnO) is being largely explored as a self-driven semiconductor photocatalyst to form heterojunctions with g-C3N4, as ZnO possesses unique and fascinating properties, including high quantum efficiency, high electron mobility, cost-effectiveness, environmental friendliness, and a simple synthetic procedure. The synergistic effect of its properties, such as adsorption and photogenerated charge separation, was found to enhance the photocatalytic activity of heterojunctions. Hence, this review aims to compile the strategies for fabricating g-C3N4/ZnO-based Z-scheme and S-scheme heterojunction photocatalytic systems with enhanced performance and overall stability for the photodegradation of organic pollutants. Furthermore, with reference to the reported system, the photocatalytic mechanism of g-C3N4/ZnO-based heterojunction photocatalysts and their charge-transfer pathways on the interface surface are highlighted.

Polymeric Carbon Nitrides for Photoelectrochemical Applications: Ring Opening-Induced Degradation

Active and stable materials that utilize solar radiation for promoting different reactions are critical for emerging technologies. Two of the most common polymeric carbon nitrides were prepared by the thermal polycondensation of melamine. The scope of this work is to investigate possible structural degradation before and after photoelectrochemical testing. The materials were characterized using synchrotron radiation and lab-based techniques, and subsequently degraded photoelectrochemically, followed by post-mortem analysis. Post-mortem investigations reveal: (1) carbon atoms bonded to three nitrogen atoms change into carbon atoms bonded to two nitrogen atoms and (2) the presence of methylene terminals in post-mortem materials. The study concludes that polymeric carbon nitrides are susceptible to photoelectrochemical degradation via ring opening.

Ternary photocatalysts based on MOF-derived TiO2 co-decorated with ZnIn2S4 nanosheets and CdS nanoparticles for effective visible light degradation of organic pollutants

A high-efficiency visible-light-responsive CdS/ZnIn2S4/TiO2 photocatalyst was prepared for the first time by the self-sacrificing template method followed by a two-step chemical bath process.

Biodegradation of Deoxynivalenol by Nocardioides sp. ZHH-013: 3-keto-Deoxynivalenol and 3-epi-Deoxynivalenol as Intermediate Products

Deoxynivalenol (DON) is one of the most devastating and notorious contaminants in food and animal feed worldwide. A novel DON-degrading strain, Nocardioides sp. ZHH-013, which exhibited complete mineralization of DON, was isolated from soil samples. The intermediate products of DON generated by this strain were identified by high-performance liquid chromatography and ultra-performance liquid chromatography tandem mass spectrometry analyses. It was shown that, on an experimental level, 3-keto-DON was a necessary intermediate product during the conversion from DON to 3-epi-DON. Furthermore, the ZHH-013 strain could also utilize 3-epi-DON. This DON degradation pathway is a safety concern for food and feed. The mechanism of DON and 3-epi-DON elimination will be further studied, so that new enzymes for DON degradation can be identified.

Photoelectrocatalytic degradation of deoxynivalenol on CuO-Cu2O/WO3 ternary film: Mechanism and reaction pathways

A ternary film of CuO-Cu₂O/WO₃ possessing high visible-light photoelectrocatalytic (PEC) performance was prepared for degradation of deoxynivalenol (DON). In such a ternary film, the introduction of CuO-Cu₂O significantly promoted the absorption of WO₃ in the visible region and reduced the recombination of photogenerated charge carriers. As a result, the CuO-Cu₂O/WO₃ film exhibited high photoelectrochemical activity under visible light illumination. The PEC performance of CuO-Cu₂O/WO₃ film was evaluated by the decoloration of Rhodamine B. The result indicated that the CuO-Cu₂O/WO₃ film exhibited higher PEC activity than WO₃ or CuO-Cu₂O film. When the CuO-Cu₂O/WO₃ film was applied to study the removal of DON, the degradation efficiency reached 87.6% after 180-min PEC treatment. According to reactive oxygen species detected by electron spin resonance analysis, a Z-scheme and type-II PEC mechanism was proposed for this ternary film. Furthermore, the intermediates formed during the PEC degradation process of DON were separated by high-performance liquid chromatography and identified with liquid chromatography-mass spectrometry. On the basis of sixteen intermediate products identified, we proposed a degradation pathway for DON in such a PEC system.

Carbon-Graphitic Carbon Nitride Hybrids for Heterogeneous Photocatalysis

Polymeric graphitic carbon nitride (g-C₃ N₄ ) and various carbon materials have experienced a renaissance as viable alternates in photocatalysis due to their captivating metal-free features, favorable photoelectric properties, and economic adaptabilities. Although numerous efforts have focused on the integration of both materials with optimized photocatalytic performance in recent years, the direct parameters for this emerging enhancement are not fully summarized yet. Fully understanding the synergistic effects between g-C₃ N₄ and carbon materials on photocatalytic action is vital to further development of metal-free semiconductors in future studies. Here, recent advances of carbon/g-C₃ N₄ hybrids on various photocatalytic applications are reviewed. The dominant governing factors by inducing carbon into g-C₃ N₄ photocatalysts with involving photocatalytic mechanism are highlighted. Five typical carbon-induced enhancement effects are mainly discussed here, i.e., local electric modification, band structure tailoring, multiple charge carrier activation, chemical group functionalization, and abundant surface-modified engineering. Photocatalytic performance of carbon-induced g-C₃ N₄ photocatalysts for addressing directly both the renewable energy storage and environmental remediation is also summarized. Finally, perspectives and ongoing challenges encountered in the development of metal-free carbon-induced g-C₃ N₄ photocatalysts are presented.

Surface defective g-C3N4-xClx with unique spongy structure by polarization effect for enhanced photocatalytic removal of organic pollutants

Natural sponge is an ancient marine organism with a single lamellar structure, on which there are abundant porous channels to compose full-fledged spatial veins. Illumined by the natural spongy system, herein, the Cl doped surface defective graphite carbon nitride (g-C₃N_4-xCl_x) was constructed through microwave etching. In this process, microwave with HCl was employed to produce surface defects and peel bulk g-C₃N₄ to form natural spongy structured g-C₃N_4-xCl_x with three-dimensional networks. The spongy structure of the photocatalyst could provide abundant and unobstructed pathways for the transfer and separation of electron-hole pairs, and it was beneficial for photocatalytic reaction. The spongy defective g-C₃N_4-xCl_x achieved excellent degradation of diclofenac sodium (100%), bisphenol A (88.2%), phenol (85.7%) and methylene blue (97%) solution under simulated solar irradiation, respectively. The chlorine atoms were introduced into the g-C₃N₄ skeleton in microwave field with HCl, forming C-Cl bonds and surface polarization field, which could significantly accelerate the separation of photogenerated electrons and holes. As an efficient and universal approach, microwave etching can be generally used to create surface defects for most photocatalysts, which may have potential applications in environmental purification, energy conversion and photodynamic therapy.

Synthesis of EDTA-bridged CdS/g-C3N4 heterostructure photocatalyst with enhanced performance for photoredox reactions

Photocatalytic reactions represent a kind of green and sustainable chemical processes for organic transformations, but the efficiency is limited by the severe recombination and/or inadequate redox potentials of photoinduced charge carriers in photocatalysts. To address these issues, herein, the CdS-EDTA/g-C₃N₄ heterostructures were designed according to Z-scheme photocatalytic mechanism and synthesized by the hydrothermal growth of CdS on g-C₃N₄ nanoflakes with assistance of EDTA chelating agent. EDTA played multiple roles in the formation of CdS-EDTA/g-C₃N₄ heterostructure photocatalysts, such as controlling the morphology of CdS nanostructures, linking CdS and g-C₃N₄ together, and boosting the charge transfer between two semiconductors. The optimized CdS-EDTA/g-C₃N₄(10%) photocatalyst exhibited much higher activities toward the selective reduction of nitrophenol and the selective oxidation of benzyl alcohol, than those of CdS/g-C₃N₄ heterostructures without EDTA. The enhanced photocatalysis of CdS-EDTA/g-C₃N₄ can be ascribed to the efficient separation and suitable photoredox potentials of photoexcited charge carriers in the EDTA-bridged Z-scheme system. This work provides the inspiration for exploring inexpensive organic electron mediators for constructing all-solid-state Z-scheme photocatalysts and demonstrates the enhanced performance of Z-scheme photocatalysts for photoredox reactions of organic transformations.

Graphitic carbon nitride nanotubes: a new material for emerging applications

We provide a critical review of the current state of the synthesis and applications of nano- and micro-tubes of layered graphitic carbon nitride. This emerging material has a huge potential for light-harvesting applications, including light sensing, artificial photosynthesis, selective photocatalysis, hydrogen storage, light-induced motion, membrane technologies, and can become a major competitor for such established materials as carbon and titania dioxide nanotubes. Graphitic carbon nitride tubes (GCNTs) combine visible-light sensitivity, high charge carrier mobility, and exceptional chemical/photochemical stability, imparting this material with unrivaled photocatalytic activities in photosynthetic processes, such as water splitting and carbon dioxide reduction. The unique geometric GCNT structure and versatility of possible chemical modifications allow new photocatalytic applications of GCNTs to be envisaged including selective photocatalysts of multi-electron processes as well as light-induced and light-directed motion of GCNT-based microswimmers. Closely-packed arrays of aligned GCNTs show great promise as multifunctional membrane materials for the light energy conversion and storage, light-driven pumping of liquids, selective adsorption, and electrochemical applications. These emerging applications require synthetic routes to GCNTs with highly controlled morphological parameters and composition to be available. We recognize three major strategies for the GCNT synthesis including templating, supramolecular assembling of precursors, and scrolling of nano-/microsheets, and outline promising routes for further progress of these approaches in the light of the most important emerging applications of GCNTs.