Enhanced light harvesting and charge separation of carbon and oxygen co-doped carbon nitride as excellent photocatalyst for hydrogen evolution reaction

Enhanced visible light photocatalytic performance of carbon and oxygen co-doped carbon nitride with a three-dimensional structure: Performance and mechanism study

As a cost-effective photocatalyst, carbon nitride (g-C3N4) holds tremendous promise for addressing energy shortages and environmental pollution. However, its application is limited by disadvantages such as low specific surface area and easy recombination of photogenerated electron-hole pairs. This study introduces C and O co-doped g-C3N4 with a three-dimensional (3D) structure achieved through a straightforward one-step calcination process, demonstrating excellent photocatalytic activity of hydrogen production and oxytetracycline degradation, with superoxide radicals as the primary active species. We propose a plausible enhanced mechanism based on systematic characterizations and density functional theory calculations. The 3D structure confers a substantial specific surface area, enhancing both the adsorption area and active sites of catalysts while bolstering structural stability. Co-doping optimizes the band structure and electric conductivity of the catalyst, facilitating rapid migration of photogenerated charges. The synergistic effects of these enhancements significantly elevate the photocatalytic performance. This study presents a convenient and feasible method for the preparation of dual-regulated photocatalysts with outstanding performance.

Facile fabrication of 3D hollow porous aminopyridine rings decorated polymeric carbon nitride for enhanced photocatalytic hydrogen evolution and dye elimination

In consideration of energy shortages and environmental pollution, there is a critical need to develop a photocatalyst with high catalytic performance for rapid hydrogen production and efficient pollutant degradation. We synthesized a photocatalytic composite catalyst with three-dimensional (3D) porous aminopyridine rings grafted on the edge of g-C3N4 (APCN) using melamine, cyanuric acid and 4-aminopyridine as raw materials. The composite catalyst exhibited excellent photocatalytic performance for H2 production (2.44 mmol g-1h-1) and RhB degradation (97.08%) under visible light. Subsequently, a possible enhanced mechanism of the catalyst was proposed on the basis of a series of characterization and photocatalytic experiments. The 3D porous structure not only enhanced the structural stability but also increased the surface area of the APCN catalysts, which generated more exposed active sites. Moreover, the aminopyridine ring embellishment was beneficial for achieving a narrowed bandgap and charge migration and separation, which decreased the occurrence of photogenerated carrier recombination. In summary, these two structural features showed a synergistic effect to enhance the photocatalytic performance of the APCN catalyst. Finally, an integrated feasible enhanced mechanism of photocatalytic activity was elucidated according to the results of active substance capture tests, showing that O2•- played an important role during RhB degradation.

Broad-bandgap porous graphitic carbon nitride with nitrogen vacancies and oxygen doping for efficient visible-light photocatalytic degradation of antibiotics

Effective degradation methods are required to address the issue of antibiotics as organic pollutants in water resources. Herein, a two-stage thermal treatment method was used to prepare porous graphitic carbon nitride (g-C3N4) modified with nitrogen vacancies and oxygen doping at the N-(C)3 position and deep in the g-C3N4 framework. Compared with bulk g-C3N4 (BCN) (7 ± 1 m2/g), the modified sample (RCN-2h) possesses a larger specific surface area (224 ± 1 m2/g), a larger bandgap (by 0.19 eV), and a mid-gap state. In addition, RCN-2h shows 15.4, 11.2, and 9.5 times higher photodegradation rates than BCN for the degradation of 100% ofloxacin (OFX) (within 15 min), tetracycline (within 15 min), and sulfadiazine (within 35 min), respectively. The RCN-2h catalyst also exhibits superior stability and reusability. Systematic characterization and density functional theory calculations demonstrate that the synergistic effect of the porous structure, nitrogen vacancies, and oxygen doping in RCN-2h provides additional reaction sites, improved charge separation efficiency, and shorter diffusion paths for reactants and photogenerated charge carriers. Trapping experiments reveal that •O2- is the main active species in OFX photodegradation, and a possible photodegradation pathway is identified using liquid chromatography-mass spectrometry. Benefiting from the simplicity of synthesis methods and the superiority of elemental doping, carbon nitride materials with functional synergy have great potential for environmental applications.

Liquid nitrogen quenching inducing lattice tensile strain to endow nitrogen/fluorine co-doping Fe3O4 nanocubes assembled on porous carbon with optimizing hydrogen evolution reaction

In this work, the lattice tensile strain of nitrogen/fluorine co-doping ferroferric oxide (Fe₃O₄) nanocubes assembled on chrysanthemum tea-derived porous carbon is induced through a novel liquid nitrogen quenching treatment (named as TS-NF-FO/PC_X-Y, TS: Tensile strain, NF: Nitrogen/Fluorine co-doping, FO: Fe₃O₄, PC: Porous carbon, X: The weight ratio of KOH/carbon, Y: The adding amount of porous carbon). Besides, the electrocatalytic activity influenced by the adding amount of porous carbon, the type of dopant, and the introduction of lattice tensile strain is systematically studied and explored. The interconnected porous carbon could improve electrical conductivity and prevent Fe₃O₄ nanocubes from aggregating. The induced nitrogen/fluorine could cause extrinsic defects and tailor the intrinsic electron state of the host materials. Lattice tensile strain could tailor the surface electronic structure of Fe₃O₄ via changing the dispersion of surface atoms and their bond lengths. Impressively, the designed TS-NF-FO/PC₅-0.25 delivers a low overpotential of 207.3 ± 0.4 mV at 10 mA/cm² and demonstrates desirable reaction dynamics. Density functional theory calculations illustrate that the electron structure and hydrogen adsorption free energy (ΔG_*H) are optimized by the synergistic effect among porous carbon, nitrogen/fluorine co-doping and lattice tensile strain, thus promoting hydrogen evolution reaction (HER) catalytic activity. Overall, this work paves the way to unravel the enhancement mechanism of HER on transition metal oxide-based materials by electronic structure and phase composition modulation strategy.

Lacunary polyoxometalate oriented construction of dispersed Ni3S2 confined in WO3 for electrocatalytic water splitting

Manufacturing low-cost, high-performance and earth-rich catalysts for hydrogen evolution (HER) and oxygen evolution reactions (OER) is critical to achieving sustainable green hydrogen production. Herein, we utilize lacunary Keggin-structure [PW₉O₃₄]^9- (PW₉) as a molecular pre-assembly platform to anchor Ni within a single PW₉ molecule by vacancy-directed and nucleophile-induced effects for the uniform dispersion of Ni at the atomic level. The chemical coordination of Ni with PW₉ can avoid the aggregation of Ni and favor the exposure of active sites. The Ni₃S₂ confined by WO₃ prepared from controlled sulfidation of Ni₆PW₉/Nickel Foam (Ni₆PW₉/NF) exhibited excellent catalytic activity in both 0.5 M H₂SO₄ and 1 M KOH solutions, which required only 86 mV and 107 mV overpotentials for HER at a current density of 10 mA∙cm^-2 and 370 mV for OER at 200 mA∙cm^-2. This is attributed to the good dispersion of Ni at the atomic level induced by trivacant PW₉ and the enhanced intrinsic activity by synergistic effect of Ni and W. Therefore, the construction of active phase from the atomic level is insightful to the rational design of dispersed and efficient electrolytic catalysts.

Non-Metal-Doped Porous Carbon Nitride Nanostructures for Photocatalytic Green Hydrogen Production

Photocatalytic green hydrogen (H₂) production through water electrolysis is deemed as green, efficient, and renewable fuel or energy carrier due to its great energy density and zero greenhouse emissions. However, developing efficient and low-cost noble-metal-free photocatalysts remains one of the daunting challenges in low-cost H₂ production. Porous graphitic carbon nitride (gCN) nanostructures have drawn broad multidisciplinary attention as metal-free photocatalysts in the arena of H₂ production and other environmental remediation. This is due to their impressive catalytic/photocatalytic properties (i.e., high surface area, narrow bandgap, and visible light absorption), unique physicochemical durability, tunable electronic properties, and feasibility to synthesize in high yield from inexpensive and earth-abundant resources. The physicochemical and photocatalytic properties of porous gCNs can be easily optimized via the integration of earth-abundant heteroatoms. Although there are various reviews on porous gCN-based photocatalysts for various applications, to the best of our knowledge, there are no reviews on heteroatom-doped porous gCN nanostructures for the photocatalytic H₂ evolution reaction (HER). It is essential to provide timely updates in this research area to highlight the research related to fabrication of novel gCNs for large-scale applications and address the current barriers in this field. This review emphasizes a panorama of recent advances in the rational design of heteroatom (i.e., P, O, S, N, and B)-doped porous gCN nanostructures including mono, binary, and ternary dopants for photocatalytic HERs and their optimized parameters. This is in addition to H₂ energy storage, non-metal configuration, HER fundamental, mechanism, and calculations. This review is expected to inspire a new research entryway to the fabrication of porous gCN-based photocatalysts with ameliorated activity and durability for practical H₂ production.