Facile synthesis of N vacancy g-C3N4 using Mg-induced defect on the amine groups for enhanced photocatalytic •OH generation

Photocatalysis offers opportunities to degrade recalcitrant organic pollutants without adding treatment chemicals. Nitrogen (N) vacancy is an effective point-defect engineering strategy to mitigate electron-hole recombination and facilitate hydroxyl radical (•OH) production via superoxide radical (O₂^•-) generation during photocatalytic application of graphitic carbon nitride (g-C₃N₄). Here, we report a novel strategy for fabrication of N-vacancy-rich g-C₃N₄ (NvrCN) via post-solvothermal treatment of Mg-doped g-C₃N₄. The addition of the Mg precursor during the polycondensation of urea created abundant amine sites in the g-C₃N₄ framework, which facilitates formation of N vacancies during post-solvothermal treatment. Elemental analysis and electron paramagnetic resonance spectra confirmed a higher abundance of N vacancies in the resultant NvrCN. Further optical and electronic analyses revealed the beneficial role of N vacancies in light-harvesting capacity, electron-hole separation, and charge transfer. N vacancies also provide specific reaction centers for O₂ molecules, promoting oxygen reduction reaction (ORR). Therefore, •OH generation increased via enhanced formation of H₂O₂ under visible light irradiation, and NvrCN photocatalytically degraded oxytetracycline 4-fold faster with degradation rate constant of 1.85 × 10^-2 min^-1 (light intensity = 1.03 mW/cm², catalyst concentration = 0.6 g/L, oxytetracycline concentration = 20 mg/L) than pristine g-C₃N₄. Overall, this study provides a facile method for synthesizing N-vacancy-rich g-C₃N₄ and elucidates the role of the defect structure in enhancing the photocatalytic activity of g-C₃N₄.

New insight into enhanced photocatalytic selectivity of g-C3N4 by nitrogen vacancy introduction: Experimental study and theoretical calculation

Constructing photocatalyst with both high efficiency and selectivity is highly desired in water treatment process. However, it is difficult to realize the selectivity of photocatalysis due to the non-selective oxidative species produced in this process. Herein, for the first time, the photocatalytic selectivity was achieved on g-C₃N₄ (CN) through N vacancy introduction for effective removal of organic pollutants, and the mechanism of vacancy induced selectivity enhancement was studied. The nitrogen vacancy modified CN (VCN) showed enhanced photocatalytic activity and unique selectivity towards phenolic compounds with electron-donating group, whose kinetic constant for p-aminophenol (p-NH₂) degradation was 5.95 times higher than that over CN. Moreover, VCN photocatalytic system also displayed similar selectivity in binary pollutant systems. Characteristics and theoretical calculation results confirmed the enhanced photocatalytic performance and selectivity of VCN was mainly attributed to the effect of N vacancy. On one hand, electron-deficient N vacancy enhanced the adsorption of the O₂ and phenolic compounds, which promoted the production of O₂^•- and strengthened the photocatalytic surface reaction. On the other hand, the N vacancy preferred to adsorb the electron-donating groups of phenolic compounds, which resulted in their selective removal.

Radical and non-radical cooperative degradation in metal-free electro-Fenton based on nitrogen self-doped biochar

To achieve sustainable metal-free electron-Fenton, N self-doped biochar air-cathode (BCAC) was prepared by pyrolyzing coffee residues. During the pyrolysis process, the endogenous N transformed from edge-doping to graphite-doping. Particularly, N vacancies started to evolve when the peak temperature exceeded 700 °C. A high Tetracycline removal rate of 70.42% was obtained on the BCAC at the current density of 4 mA cm^-2. Quenching tests incorporated with ESR spectroscopy were adopted to identify the specific oxidants produced on the cathode. The results showed that •OH (37.36%), •O₂^- (29.67%) and ¹O₂ (24.17%) played comparable role in the tetracycline removal, suggesting the coexist of radical and non-radical oxidants in our electro-Fenton system. According to the structure characterization and the DFT calculation, graphitic N was suggested as the critical site for H₂O₂ generation, and both graphitic N and pyridinic N were electroactive sites for H₂O₂ activation to •OH. Graphitic N and N vacancies with stronger capabilities in O₂ adsorption and electron-trapping were proposed as the electroactive sites for ¹O₂ and •O₂^- formation. This work predicts a novel electro-Fenton process with cooperative radical and non-radical degradation on N self-doped carbonaceous catalysts at a mild condition, which is extremely meaningful for boosting sustainable electro-Fenton technology.

Enhanced peroxymonosulfate activation on dual active sites of N vacancy modified g-C3N4 under visible-light assistance and its selective removal of organic pollutants

Constructing highly efficient metal-free material towards peroxymonosulfate (PMS) activation under photocatalytic assistance is a promising strategy for water decontamination. Herein, N vacancy modified g-C₃N₄ nanotube (VCN) was prepared to build a novel photo-assisted PMS activation system (PPAS), in which the unique electronic structure created by N vacancy could favor the PMS activation on VCN under visible-light irradiation. The role of N vacancy in PPAS was firstly studied through tuning its content in VCN. The results showed that the N vacancy greatly improved PMS activation on VCN PPAS towards organic pollutants removal. The VCN PPAS with moderate N vacancy modification performed best, whose kinetic constant for Rhodamine B degradation was 9.6 and 2.6 times higher than that of VCN/PMS system and pristine g-C₃N₄ PPAS, respectively. Moreover, the VCN PPAS performed well in wide pH range (3-12) and real water background. Selective removal of different organic pollutants was found on VCN PPAS, owing to the different interaction between pollutant and the catalyst surface with surface-bound radicals. The O₂^- and OH were major oxidants for pollutant removal in VCN PPAS, which were produced on dual active sites of VCN via two pathways: The N vacancy enhanced PMS adsorption and trapped photogenerated electrons for PMS reduction into OH, while the electron-deficient C atoms created by N loss promoted the PMS oxidation into O₂^-.