Fe-Free Photocatalysis-Self-Fenton System for Ultraefficient Water Purification Enabled by Carbon Nitride With Strong Built-in Electric Field

Abstract

Photocatalytic-self-Fenton system (PSFs) hold great promise for water purification through in situ generation-consumption of H₂O₂, yet is constrained by two main obstacles: (1) the scarcity of cost-effective and sustainable photocatalysts, which restricts the overall H₂O₂ production, and (2) the dependence on exogenous Fe²⁺, leading to issues such as Fe sludge formation and narrow pH operating ranges. Herein, a highly crystalline CN bearing K⁺, cyano groups and polyethyleneimine is synthesized through doping and molten-salt assistance calcination. The obtained catalyst exhibits a strong built-in electric field (KPFM and SPV) and efficient spatial charge separation (series photoelectric tests and DFT calculations). The exposed active sites (S_BET = 102.2 m²·g⁻¹) and abundant terminal -NH₂ groups create quasi-homogeneous system (SEM/TEM/AFM and free deposition experiment). The catalyst also exhibits high oxygen adsorption capacity and promotes the reaction pathway of O₂→·O₂⁻→H₂O₂→·OH, enabling photosynthesis H₂O₂ rate up to 14.90 mmol·g⁻¹ h⁻¹ (22.2 times that of CN). The constructed Fe-free PSFs achieves 100% degradation of high-concentration tetracycline (100 mg L⁻¹) within 10 min with a kinetic constant 3.46 times higher than that of common photodegradation system while overcoming the limitation of a narrow operational pH range. Furthermore, the Fe-free PSFs can also 100% degrade sulfamethoxazole, ofloxacin, and diclofenac sodium. At last, the improved degradation mechanisms, key reactive species, degradation pathways, and toxicity are systematically elucidated. This study overcomes key limitations of CN-based photocatalysts and provides novel insights into developing efficient Fe-free PSFs for pollutant photodegradation over a wide-pH.

Keywords

carbon nitride / Fe-free photocatalysis-self-Fenton / pH-universal / strong built-in electric field / water pollution remediation

References

[1] S. Meng, J. Li, S. Wang, S. Zhan, W. Hu, and Y. Li, “Homo-Hetero Double Junction Coupling Weakens Exciton Effects to Enhance Selective Photocatalytic O₂ Activation on Carbon Nitride,” Advanced Materials 37, no. 14 (2025): e2420080, https://doi.org/10.1002/adma.202420080.

[2] Z. Chen, J. Wang, B. Yang, et al., “Organic Carbon Transfer Process in Advanced Oxidation Systems for Water Clean-Up,” Nature Water 3, no. 3 (2025): 334–344, https://doi.org/10.1038/s44221-025-00399-7.

[3] F. Sun, Q. Xu, X. Wang, et al., “Multi-Electric Field-Enhanced CuInS₂-Modified TiO₂(Anatase)/TiO₂(Rutile)/PVDF Nanofiber Membrane for Multifunctional Piezo-Photocatalysis,” Advanced Functional Materials 35, no. 52 (2025): e05795, https://doi.org/10.1002/adfm.202505795.

[4] L. Yang, Z. Chen, Q. Cao, et al., “Structural Regulation of Photocatalyst to Optimize Hydroxyl Radical Production Pathways for Highly Efficient Photocatalytic Oxidation,” Advanced Materials 36, no. 3 (2024): e2306758, https://doi.org/10.1002/adma.202306758.

[5] H. Zhao, X. Xu, W. Cui, et al., “Synchronization Strategy for Activity and Stability in Fenton-Like Single-Atom Catalysis,” Advanced Materials 37, no. 30 (2025): e2503217, https://doi.org/10.1002/adma.202503217.

[6] H. Che, J. Wang, X. Wang, et al., “A Novel Solar-Driven Self-Fenton Reaction System With Spatially Separated H₂O₂ Generation and Activation Sites for Efficient Degradation of Refractory Pollutants,” Applied Catalysis B: Environmental 379, no. 125658 (2025): 125658, https://doi.org/10.1016/j.apcatb.2025.125658.

[7] Y. Wu, H. Che, B. Liu, and Y. Ao, “Promising Materials for Photocatalysis-Self-Fenton System: Properties, Modifications, and Applications,” Small Structures 4, no. 7 (2023): 2200371, https://doi.org/10.1002/sstr.202200371.

[8] J. Liu, B. An, B. Zhu, L. Liu, and F. Liu, “Recyclable Carbon Nitride Composite Catalyst for Degradation of Tetracycline Under Natural Light in a Novel Iron-Free Photo-Fenton System,” Separation and Purification Technology 354 (2025): 129177, https://doi.org/10.1016/j.seppur.2024.129177.

[9] T. Shan, Y. Wang, D. Luo, et al., “Extended H-Bonds/π-Bonds Networks for Boosting Electron Transfer Over Polydopamine-Covered Nanocellulose/g-C₃N₄ Toward Efficient Photocatalytic H₂O₂ Production,” Applied Catalysis B: Environmental 349 (2024): 123872, https://doi.org/10.1016/j.apcatb.2024.123872.

[10] J. Li, Y. Li, S. Wei, et al., “Asymmetric Framework Engineering of Carbon Nitride Nanotubes for Polarization Promoted H₂O₂ Photosynthesis,” Applied Catalysis B: Environmental 366 (2025): 125073, https://doi.org/10.1016/j.apcatb.2025.125073.

[11] D. Jiang, S. Hu, Y. Qu, et al., “Infrared Irradiation-Lattice Vibration Coupling-Initiated n-π* Electronic Transition in Carbon Nitride Nanosheets for Increased Photocatalysis,” Advanced Functional Materials 34, no. 16 (2024): 2311803, https://doi.org/10.1002/adfm.202311803.

[12] X. Ruan, C. Huang, H. Cheng, et al., “A Twin S-Scheme Artificial Photosynthetic System With Self-Assembled Heterojunctions Yields Superior Photocatalytic Hydrogen Evolution Rate,” Advanced Materials 35, no. 6 (2023): e2209141, https://doi.org/10.1002/adma.202209141.

[13] Q. Zhang, B. Wang, H. Miao, J. Fan, T. Sun, and E. Liu, “Efficient Photocatalytic H₂O₂ Production over K⁺-Intercalated Crystalline g-C₃N₄ With Regulated Oxygen Reduction Pathway,” Chemical Engineering Journal 482 (2024): 148844, https://doi.org/10.1016/j.cej.2024.148844.

[14] L. Jian, Y. Dong, H. Zhao, C. Pan, G. Wang, and Y. Zhu, “Highly Crystalline Carbon Nitrogen Polymer With a Strong Built-in Electric Fields for Ultra-High Photocatalytic H₂O₂ Production,” Applied Catalysis B: Environmental 342 (2024): 123340, https://doi.org/10.1016/j.apcatb.2023.123340.

[15] Y. Yan, T. Zhou, C. Liu, B. Hu, and G. Che, “Dual-Pathway Photosynthesis H₂O₂ Realized by Carbon Nitride With Strong Built-in Electric Field and Awakened n-π* Electron Transition: Based on Theoretical Calculation Guidance,” Chemical Engineering Journal 516 (2025): 164108, https://doi.org/10.1016/j.cej.2025.164108.

[16] M. Jing, H. Zhao, L. Jian, C. Pan, Y. Dong, and Y. Zhu, “Coral-Like B-Doped g-C₃N₄ With Enhanced Molecular Dipole to Boost Photocatalysis-Self-Fenton Removal of Persistent Organic Pollutants,” Journal of Hazardous Materials 449 (2023): 131017, https://doi.org/10.1016/j.jhazmat.2023.131017.

[17] Y. Shen, Y. Wang, P. Shan, et al., “Boosted Photo-Self-Fenton Degradation Activity by Fe-Doped Carbon Dots as dual-Function Active Sites for In-Situ H₂O₂ Generation and Activation,” Separation and Purification Technology 353 (2025): 12, https://doi.org/10.1016/j.seppur.2024.128529.

[18] Y. Zhang, H. Qin, X. Sun, et al., “Boosting Hydrogen Peroxide Production in Tubular Carbon Nitride With Hydroxy Modification for High-Efficient Photo-Self-Fenton Antibiotic Degradation,” Journal of Water Process Engineering 71 (2025): 11, https://doi.org/10.1016/j.jwpe.2025.107129.

[19] X. Jia, L. Xie, Z. Li, et al., “Photo-Electro-Fenton-Like Process for Rapid Ciprofloxacin Removal: The Indispensable Role of Polyvalent Manganese in Fe-Free System,” Science of the Total Environment 768 (2021): 144368, https://doi.org/10.1016/j.scitotenv.2020.144368.

[20] K. Mu, F. Chen, P. Wang, et al., “Enhanced Carriers Separation in Novel In-Plane Amorphous Carbon/g-C₃N₄ Nanosheets for Photocatalytic Environment Remediation,” Chemosphere 294 (2022): 133581, https://doi.org/10.1016/j.chemosphere.2022.133581.

[21] C. Chen, C. Zhang, Y. Zhang, et al., “Boosting H₂O₂ Generation by Shortening the Charge Migration Distance in BiPO₄ Nanocrystals,” Journal of Colloid and Interface Science 662 (2024): 1–10, https://doi.org/10.1016/j.jcis.2024.02.011.

[22] T. Shan, J. Li, S. Wu, et al., “Boosting H₂O₂ Production Over Carboxymethyl Cellulose Modified g-C₃N₄ via Hydrogen-Bonding-Assisted Charge Transfer,” Chemical Engineering Journal 478 (2023): 147509, https://doi.org/10.1016/j.cej.2023.147509.

[23] Y. Wu, Y. Bu, X. Li, et al., “A Robust Mn-Based Metal-Organic Framework Cathode With Long-Cycle-Life Enabled by Metal Valence Stability and Hydrogen Bond Networks for High-Performance Sodium-Ion Batteries,” Advanced Functional Materials 35, no. 33 (2025): 2500197, https://doi.org/10.1002/adfm.202500197.

[24] Y. Feng, S. Wang, Y. Li, et al., “Entanglement in Smart Hydrogels: Fast Response Time, Anti-Freezing and Anti-Drying,” Advanced Functional Materials 33, no. 21 (2023): 2211027, https://doi.org/10.1002/adfm.202211027.

[25] J. Yuan, N. Tian, Z. Zhu, et al., “P, K Doped Crystalline g-C₃N₄ Grafted With Cyano Groups for Efficient Visible-Light-Driven H₂O₂ Evolution,” Chemical Engineering Journal 467 (2023): 143379, https://doi.org/10.1016/j.cej.2023.143379.

[26] C. Wang, Y. Lu, Z. Wang, et al., “Salt-Assisted Construction of Hydrophilic Carbon Nitride Photocatalysts With Abundant Water Molecular Adsorption Sites for Efficient Hydrogen Production,” Applied Catalysis B: Environmental 350 (2024): 123902, https://doi.org/10.1016/j.apcatb.2024.123902.

[27] Y. Chen, J. Shi, Y. Xu, et al., “Polychloride and Benzene Ring Synergistically Boosting Photocatalytic Degradation and Nitrogen Fixation Performances of Carbon Nitride,” Separation and Purification Technology 354 (2025): 129231, https://doi.org/10.1016/j.seppur.2024.129231.

[28] Y. Wang, R. Zhao, Z. Zhou, Y. Xu, and P. Yu, “3D-Porous Carbon Nitride Through Proton Regulation and Photocatalytic Synergy for Efficient Uranium Extraction From Seawater,” Small 21, no. 5 (2025): e2408650, https://doi.org/10.1002/smll.202408650.

[29] W. Xing, H. Lin, W. Shao, et al., “Heteroatom- and Bonded Tailored Schottky Barrier in Ti₃C₂ MXene/Carbon Nitride Heterojunctions Toward Efficient Photocatalysis,” Chemical Engineering Journal 520 (2025): 166126, https://doi.org/10.1016/j.cej.2025.166126.

[30] H. Tong, J. Odutola, J. Song, et al., “Boosting the Quantum Efficiency of Ionic Carbon Nitrides in Photocatalytic H₂O₂ Evolution via Controllable n-π* Electronic Transition Activation,” Advanced Materials 36, no. 49 (2024): e2412753, https://doi.org/10.1002/adma.202412753.

[31] Z. Zhang, L. Ren, H. Li, et al., “π-Conjugated in-Plane Heterostructure Enables Long-Lived Shallow Trapping in Graphitic Carbon Nitride for Increased Photocatalytic Hydrogen Generation,” Small 19, no. 18 (2023): e2207173, https://doi.org/10.1002/smll.202207173.

[32] J. Shi, M. Tai, J. Hou, et al., “Intramolecular D-A Structure and n-π* Transition Co-Promoted Photodegradation Activity of Carbon Nitride: Performance, Mechanism and Toxicity Insight,” Chemical Engineering Journal 456 (2023): 141029, https://doi.org/10.1016/j.cej.2022.141029.

[33] Q. Li, Y. Jiao, Y. Tang, et al., “Shear Stress Triggers Ultrathin-Nanosheet Carbon Nitride Assembly for Photocatalytic H₂O₂ Production Coupled With Selective Alcohol Oxidation,” Journal of the American Chemical Society 145, no. 38 (2023): 20837–20848, https://doi.org/10.1021/jacs.3c05234.

[34] H. Wang, L. Cao, X. Tao, and G. Zhu, “Efficient Photosynthesis of Hydrogen Peroxide From Water and Air Over Water-Dispersible Anthraquinone-Based Porous Aromatic Frameworks,” Angewandte Chemie International Edition 64, no. 21 (2025): e202502943, https://doi.org/10.1002/anie.202502943.

[35] H. Che, X. Gao, J. Chen, J. Hou, Y. Ao, and P. Wang, “Iodide-Induced Fragmentation of Polymerized Hydrophilic Carbon Nitride for High-Performance Quasi-Homogeneous Photocatalytic H₂O₂ Production,” Angewandte Chemie International Edition 60, no. 48 (2021): 25546–25550, https://doi.org/10.1002/anie.202111769.

[36] T. Liu, Z. Pan, J. J. M. Vequizo, et al., “Overall Photosynthesis of H₂O₂ by an Inorganic Semiconductor,” Nature Communications 13, no. 1 (2022): 1034, https://doi.org/10.1038/s41467-022-28686-x.

[37] R. Tang, H. Zeng, Y. Deng, et al., “Dual Modulation on Peroxymonosulfate Activation Site and Photocarrier Separation in Carbon Nitride for Efficient Photocatalytic Organics Degradation: Efficacy and Mechanism Evaluation,” Applied Catalysis B: Environmental 336 (2023): 122918, https://doi.org/10.1016/j.apcatb.2023.122918.

[38] L. Ma, Y. Gao, B. Wei, et al., “Visible-Light Photocatalytic H₂O₂ Production Boosted by Frustrated Lewis Pairs in Defected Polymeric Carbon Nitride Nanosheets,” ACS Catalysis 14, no. 4 (2024): 2775–2786, https://doi.org/10.1021/acscatal.3c05360.

[39] X. Zhong, Y. Zhu, M. Jiang, Q. Sun, and J. Yao, “Dual Defective K-Doping and Cyano Group Sites on Carbon Nitride Nanotubes for Improved Hydrogen Photo-Production,” Energy & Fuels: An American Chemical Society Journal 37, no. 7 (2023): 5448–5456, https://doi.org/10.1021/acs.energyfuels.2c04371.

[40] J. Shao, H. Fei, H. Li, et al., “Synergizing n-π* Electronic Transition and Plasmonic Hot Electron Injection Enhances Carrier Generation of S-Doped Carbon Nitride Decorated With Au Nanoparticles for Cr(VI) Degradation,” Separation and Purification Technology 297 (2022): 121515, https://doi.org/10.1016/j.seppur.2022.121515.

[41] H. Wang, G. Huang, Z. Chen, and W. Li, “Carbon Self-Doped Carbon Nitride Nanosheets With Enhanced Visible-Light Photocatalytic Hydrogen Production,” Catalysts 8, no. 9 (2018): 366, https://doi.org/10.3390/catal8090366.

[42] S. Zhao, C. Zhang, Y. Wang, et al., “Facile Construction of Carbon Doped Carbon Nitride Tube With Increased π-Electron Density for Highly Efficient Hydrogen Production,” Colloids and Surfaces. A, Physicochemical and Engineering Aspects 660 (2023): 130872, https://doi.org/10.1016/j.colsurfa.2022.130872.

[43] X. Leng, K. Yang, L. Sun, J. Weng, and J. Xu, “Modulating the Band Structure of Two-Dimensional Black Phosphorus via Electronic Effects of Organic Functional Groups for Enhanced Hydrogen Production Activity,” Angewandte Chemie International Edition 64, no. 5 (2025): e202416992, https://doi.org/10.1002/ange.202416992.

[44] F. Sun, D. Xu, J. Xu, et al., “A Self-Standing Dual-Electric Field Synergistic Janus Nanofibre Piezoelectric Photocatalyst With Degradation of Antibiotics: Performances, DFT Calculation and Mechanism Unveiling,” Journal of Materials Science & Technology 208 (2025): 278–291, https://doi.org/10.1016/j.jmst.2024.05.007.

[45] T. Zhou, J. Shi, G. Li, et al., “Advancing n-π* Electron Transition of Carbon Nitride via Distorted Structure and Nitrogen Heterocycle for Efficient Photodegradation: Performance, Mechanism and Toxicity Insight,” Journal of Colloid and Interface Science 632 (2023): 285–298, https://doi.org/10.1016/j.jcis.2022.11.073.

[46] S. Zhang, Y. Zhang, Y. Lin, et al., “Donor-Acceptor Type of Triazine-Based Conjugated Organic Polymers With Excellent Charge Separation for Efficient Photocatalytic Production of Hydrogen Peroxide,” Separation and Purification Technology 354 (2025): 129388, https://doi.org/10.1016/j.seppur.2024.129388.

[47] G. Ba, H. Hu, F. Bi, et al., “Engineering Nitrogen Vacancies and Cyano Groups Into C₃N₄ Nanosheets for Highly Efficient Photocatalytic H₂O₂ Production,” Applied Catalysis B: Environmental 361 (2025): 124645, https://doi.org/10.1016/j.apcatb.2024.124645.

[48] Y. Wang, H. Zhang, D. Wei, L. Shangguan, Y. Guo, and J. Sun, “Molten-Salt-Induced Structural Heterogeneity of Carbon Nitride for Efficient Photocatalytic H₂O₂ Production,” Chemical Communications 60, no. 77 (2024): 10732–10735, https://doi.org/10.1039/d4cc04007d.

[49] N. Wen, Y. Huang, Y. Yang, et al., “Dynamic Bromine Vacancy-Mediated Photocatalytic Three-Step Three-Electron Oxygen Reduction to Hydroxyl Radicals,” ACS Catalysis 14, no. 15 (2024): 11153–11163, https://doi.org/10.1021/acscatal.4c02674.

[50] X. Zeng, Y. Liu, X. Hu, and X. Zhang, “Photoredox Catalysis Over Semiconductors for Light-Driven Hydrogen Peroxide Production,” Green Chemistry: An International Journal and Green Chemistry Resource GC (2021), https://doi.org/10.1039/D0GC04236F.

[51] Z. Weng, Y. Lin, S. Guo, et al., “Site Engineering of Covalent Organic Frameworks for Regulating Peroxymonosulfate Activation to Generate Singlet Oxygen With 100% Selectivity,” Angewandte Chemie International Edition 62, no. 43 (2023): e202310934, https://doi.org/10.1002/ange.202310934.

[52] K. Shi, J. Wang, L. Yin, et al., “Efficient Synergistic Degradation of Tetracycline Hydrochloride by Protonated g-C₃N₄ and Chlorella Pyrenoidosa: Kinetics and Mechanism,” Chemical Engineering Journal 462 (2023): 142331, https://doi.org/10.1016/j.cej.2023.142331.

[53] X. Zhan, Y. Zeng, Z. Zhang, et al., “g-C₃N₄ With Gradient Vacancies to Enhance Spatial Charge Carriers Transfer and Separation for Photodegrading Antibiotics Under Visible Light,” Chemical Engineering Journal 474 (2023): 145948, https://doi.org/10.1016/j.cej.2023.145948.

[54] R. Tang, D. Gong, Y. Deng, et al., “π-π Stacking Derived From Graphene-Like Biochar/g-C₃N₄ With Tunable Band Structure for Photocatalytic Antibiotics Degradation via Peroxymonosulfate Activation,” Journal of Hazardous Materials 423 (2022): 126944, https://doi.org/10.1016/j.jhazmat.2021.126944.

[55] Y. Li, C.-C. Wang, F.-X. Wang, et al., “Efficient Pollutant Degradation by Peroxymonosulfate Activated by a Co/Mn Metal-Organic Framework,” Dalton Transactions 53, no. 11 (2024): 5266–5273, https://doi.org/10.1039/d4dt00161c.

[56] J. Yang, J. Jing, W. Li, and Y. Zhu, “Electron Donor-Acceptor Interface of TPPS/PDI Boosting Charge Transfer for Efficient Photocatalytic Hydrogen Evolution,” Advanced Science (Weinheim) 9, no. 17 (2022): e2201134, https://doi.org/10.1002/advs.202201134.

[57] Z. Zhou, S. Zhao, Z. Li, P. Wang, S. Zhan, and M. Wang, “Activating Oxygen via the 3-Electron Pathway to Hydroxyl Radical by LaO₄ Single-Atom on WO₃ for Water Purification,” Angewandte Chemie International Edition 64, no. 6 (2025): e202418122, https://doi.org/10.1002/ange.202418122.

[58] P. Liu, T. Liang, Y. Li, et al., “Photocatalytic H₂O₂ Production Over Boron-Doped g-C₃N₄ Containing Coordinatively Unsaturated FeOOH Sites and CoOx Clusters,” Nature Communications 15, no. 1 (2024): 9224, https://doi.org/10.1038/s41467-024-53482-0.

[59] Q. Yi, J. Ji, B. Shen, et al., “Singlet Oxygen Triggered by Superoxide Radicals in a Molybdenum Cocatalytic Fenton Reaction With Enhanced REDOX Activity in the Environment,” Environmental Science & Technology 53, no. 16 (2019): 9725–9733, https://doi.org/10.1021/acs.est.9b01676.

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