Three‐Dimensional Hierarchical Porous Carbon/Graphitic Carbon Nitride Composites for Efficient Photocatalytic Hydrogen Production

An updated review of few-layer black phosphorus serving as a promising photocatalyst: synthesis, modification and applications

Semiconductor photocatalysts represent a potential strategy to simultaneously solve the global energy shortage and environmental pollution, and black phosphorus (BP) has gained widespread applications in photocatalysis due to its high hole mobility, strong light trapping capabilities, and adjustable band gap. Nevertheless, the original material exhibits unsatisfactory photocatalytic activity in terms of low carrier separation efficiency, weak environmental stability, and difficult to control layer thickness. The following review briefly presents the fundamental characteristics and extensively discusses the synthesis methods and modification strategies for few-layer black phosphorus (FL-BP). Furthermore, various applications of composite photocatalysts derived from FL-BP such as water splitting, pollutant degradation, the carbon dioxide reduction reaction (CO2RR), phototherapy, bacterial disinfection, N2 fixation, and hydrogenation reactions are reviewed. Finally, the opportunities and challenges for the development and further investigation of advanced FL-BP-based photocatalysts are also presented.

Graphitic-carbon-nitride-hydrophilicity-dependent photocatalytic degradation of antibiotics with different log Kow

The surface hydrophilicity of a photocatalyst is an important factor that directly influences its interactions with organic pollutants and significantly impacts its degradation. In this study, we investigated the impact of increased hydrophilicity of g-C3N4 (CN) by alkaline solvothermal treatment on the degradations of three antibiotics (oxytetracycline (OTC), oxolinic acid (OA), and sulfamethoxazole (SMX)) with different log Kow values. Scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), and Fourier-transform infrared (FT-IR) spectroscopy showed no significant differences in the morphology, crystalline structure, and surface functional groups of CN after alkaline solvothermal treatment (Nv-HPCN). However, contact angle analysis revealed that Nv-HPCN (31.8°) was more hydrophilic than CN (61.1°). To assess the hydrophilicity of the antibiotics, the log Kow values of SMX (0.77), OA (0.43), and OTC (-0.34) were measured. Nv-HPCN showed faster OTC degradation than CN, whereas the opposite pattern was observed for the degradation of OA. Scavenger tests showed that O2•- and h+ mainly contributed to the degradation of these antibiotics. Furthermore, the influences of NOM and coexisting anions on antibiotic degradation were investigated. This study thus offers perspectives on the impact of surface hydrophilicity of photocatalysts on the degradation of antibiotics.

Carbon-Nitride Popcorn-A Novel Catalyst Prepared by Self-Propagating Combustion of Nitrogen-Rich Triazenes

The interest in development of non-graphitic polymeric carbon nitrides (PCNs), with various C-to-N ratios, having tunable electronic, optical, and chemical properties is rapidly increasing. Here the first self-propagating combustion synthesis methodology for the facile preparation of novel porous PCN materials (PCN3-PCN7) using new nitrogen-rich triazene-based precursors is reported. This methodology is found to be highly precursor dependent, where variations in the terminal functional groups in the newly designed precursors (compounds 3-7) lead to different combustion behaviors, and morphologies of the resulted PCNs. The foam-type highly porous PCN5, generated from self-propagating combustion of 5 is comprehensively characterized and shows a C-to-N ratio of 0.67 (C₃ N_4.45 ). Thermal analyses of PCN5 formulations with ammonium perchlorate (AP) reveal that PCN5 has an excellent catalytic activity in the thermal decomposition of AP. This catalytic activity of PCN5 is further evaluated in a closer-to-application scenario, showing an increase of 18% in the burn rate of AP-Al-HTPB (with 2 wt% of PCN5) solid composite propellant. The newly developed template- and additive-free self-propagating combustion synthetic methodology using specially designed nitrogen-rich precursors should provide a novel platform for the preparation of non-graphitic PCNs with a variety of building block chemistries, morphologies, and properties suitable for a broad range of technologies.

Supramolecular organization of melem for the synthesis of photoactive porous carbon nitride rods

Intrinsic defects and structural properties are two main factors influencing the photocatalytic performance of carbon nitride (CN) materials. Here, photoactive porous CN rods are fabricated through the thermal condensation of melem-based hexagonal supramolecular assemblies. To overcome the poor solubility of melem, we exfoliate the bulk melem using hydrochloric acid. The latter allows good dispersibility of the monomer in an aqueous medium, leading to the formation of H-bond bridged supramolecular assembly with good regularity in both size and rod-like morphology. After thermal condensation, a well-ordered structure of porous CN rods with fewer defects due to the high thermal stability of the melem-based supramolecular assembly is obtained. The new CN materials have a high specific surface area, good light-harvesting properties, and enhanced charge separation and migration. The optimal CN material exhibits excellent photocatalytic activity and durability towards hydrogen evolution reaction (HER) and CO₂ reduction reaction (CO₂RR, with good selectivity).

Urea-induced supramolecular self-assembly strategy to synthesize wrinkled porous carbon nitride nanosheets for highly-efficient visible-light photocatalytic degradation

Graphitic carbon nitride (g-C3N4) has attracted immense interest as a promising photocatalyst. To facilitate its versatile applications in many fields, new low-cost strategies to synthesize outstanding g-C3N4 need to be further developed. Although supramolecular preorganization has been considered as a promising candidate, the utilized supramolecules like melamine-cyanuric acid (MCA) are typically synthesized by expensive triazine derivatives. Herein, wrinkled porous g-C3N4 nanosheets were successfully fabricated by hydrothermal-annealing of supramolecular intermediate MCA synthesized by the cheap precursors dicyandiamide and urea. During the formation of MCA, urea could act as a facile agent to react with dicyandiamide to form melamine and cyanuric acid firstly and then assemble into MCA through hydrogen bonds. In addition, urea could serve as a porogen and decompose to generate bubbles for conducive formation of micro-size MCA self-templates and thus wrinkled porous g-C3N4 nanosheets could be obtained. The nanostructure and photocatalytic performance of g-C3N4 were optimized by modulating microstructures and physicochemical properties of MCA, which could be conveniently controlled by urea addition and hydrothermal duration. The obtained wrinkled porous g-C3N4 nanosheets exhibit highly-efficient visible-light photocatalytic degradation compared with traditional MCA-derived g-C3N4, which could remove 98.3% of the rhodamine B in 25 min. The superior photocatalytic activity is mainly attributed to the urea-induced larger specific surface area, better light harvesting ability, faster transfer and more advanced separation efficiency of the photogenerated electron-hole pairs. This research provides a new strategy for preparing high-performance porous g-C3N4 from the self-assembled supramolecule MCA synthesized by low-cost precursors.