Microwave-assisted NH4Cl gas-template engineering of hierarchically porous g-C3N4 nanosheets for enhanced photocatalytic antibiotic degradation

NH₄Cl gas template creates hierarchically porous g-C₃N₄ via one-step microwave synthesis. • 10 g-GCN achieves 2.35× increased surface area (26.09 m 2  g −1 ) and 88% CFS degradation. • Porous structure shortens charge migration paths and enhances mass transfer. • O₂ − dominates the degradation; ·OH plays a larger auxiliary role than in etching systems. • ECOSAR warns of high-toxicity intermediates requiring complete mineralisation. Graphitic carbon nitride (g-C 3 N 4 ) is a promising metal-free photocatalyst for water purification, but its low surface area and poor mass transfer in densely stacked layers limit practical performance. Herein, a microwave-assisted gas-template strategy using ammonium chloride (NH 4 Cl) was employed to prepare hierarchically porous g-C 3 N 4 nanosheets (GCN). During microwave-driven polycondensation of melamine, NH 4 Cl decomposed in situ to release NH 3 and HCl gases, which simultaneously disrupted the lamellar stacking and generated abundant mesopores without requiring a separate solid-template removal step. The optimised 10 g-GCN exhibited a specific surface area of 26.09 m 2  g −1 , 2.35 times higher than pristine g-C 3 N 4 , and a pore volume of 0.190 cm3 g-1. Under visible-light irradiation, 10 g-GCN degraded 88% of cefotaxime sodium within 60 min, with an apparent pseudo-first-order rate constant of 0.03242 min −1 , 4.16 times that of bulk g-C 3 N 4 . The enhanced activity is correlated with the porous nanosheet architecture, which is associated with shortened charge-migration distances, enhanced pollutant diffusion, and increased accessibility of active sites. Radical-trapping experiments confirmed superoxide radicals (·O 2 − ) as the dominant reactive species, while hydroxyl radicals (·OH) may be generated as a minor supplementary species through the electron-driven cascade reduction of O 2 . The degradation efficiency decreased from 88% to 79% after five cycles (approximately 90% of the initial efficiency retained), and the catalyst showed broad-spectrum degradation towards cephalosporin and non-cephalosporin antibiotics. LC-MS identified pathways including de-esterification, side-chain cleavage, beta-lactam ring opening, and fragmentation towards mineralisation. TOC confirmed 28.0% mineralisation, while ECOSAR indicated that some intermediates may still exhibit elevated aquatic toxicity, highlighting the need for sufficient mineralisation in practical treatment scenarios.