Carbon Quantum Dot-Modifiedg-C3N5 and g-C3N4 Nanocompositesfor Sunlight-Driven Wastewater Remediation

Abstract Carbon quantum dot (CQD)-modified carbon nitride nanocomposites have emerged as promising sunlight photocatalysts for wastewater remediation owing to their tunable electronic structures and efficient charge-transfer properties. Herein, CQD-modified g-C3N4 (N4Q) and g-C3N5 (N5Q) were synthesized via a hydrothermal route and systematically compared to elucidate the influence of the carbon nitride framework on photocatalytic performance. Structural, morphological, optical, and electronic analyses confirmed successful CQD integration and the formation of nanoscale heterointerfaces. N5Q exhibited a narrower band gap (1.74 eV) than N4Q (2.41 eV), resulting in enhanced sunlight absorption and suppressed charge carrier recombination. Under sunlight irradiation, N5Q achieved 94.2% methylene blue degradation within 70 min, significantly outperforming N4Q (64%) and commercial TiO2–P25. Radical trapping, mineralization, and HRMS analyses revealed the dominant involvement of e– and •OH species and enabled the proposal of a degradation pathway. N5Q also demonstrated good stability, retaining 86% of its initial activity after six cycles. Density functional theory calculations further revealed favorable electronic structure modulation and charge-transfer characteristics following CQD incorporation, consistent with experimental observations. This comparative study demonstrates the superior photocatalytic potential of CQD-modified g-C3N5 and highlights nanoscale interface engineering as an effective strategy for designing high-performance photocatalysts for sustainable wastewater treatment.