Enhanced visible-light Cr(VI) reduction over boron-doped mesoporous g-C3N4 nanorods

The rational design of g-C₃N₄ photocatalysts with tailored electronic structures and hierarchical morphologies has emerged as an effective strategy for improving visible-light-driven redox performance. Here, boron-doped one-dimensional (1D) g-C3N4 mesoporous nanorod photocatalysts (RBCNx) were synthesized via a supramolecular assembly route using melamine and boron tribromide (BBr₃) as both the boron source and structure directing agent. Structural and spectroscopic analyses, including TOF-SIMS, confirm the incorporation of boron into the g-C3N4 framework through B-N coordination, as evidenced by characteristic B-N-containing ionic fragments. This incorporation induces partial lattice distortion, increases the pore size (BET surface area of 52.8 m2/g), and enhances visible-light absorption. The optimized RBCN1.5 catalyst fully reduced Cr(VI) within 50 min, surpassing bulk CN and many reported g-C₃N₄-based photocatalysts. Density functional theory (DFT) calculations reveal that boron doping strengthens O₂ adsorption (with O2-adsorption -2.21 eV on RBCN1.5 vs -1.22 eV on bulk CN), narrows the bandgap, and induces asymmetric charge distribution, thereby enhancing charge separation and electron transfer, which collectively promote O₂ activation and accelerate Cr(VI) reduction. Kinetic fitting revealed a multi-step reduction process involving surface adsorption and pore diffusion. A marked enhancement in the diffusion rate constant (Kid) was observed upon boron incorporation and morphological transformation. Moreover, the effects of the added organic acid (SOAs) molecules, pH, and co-existing inorganic ions on the photocatalytic performance of RBCN1.5 were also investigated. Tartaric acid substantially enhanced Cr(VI) reduction (achieving 100% removal in 15 min) through charge-transfer-complex (CTC) formation and generation of COO·⁻ radicals. The catalyst maintained 86% efficiency after five cycles, demonstrating excellent structural stability. This work provides a facile, template-free approach to fabricating boron-engineered g-C3N4, offering a scalable route to advanced photocatalysts for environmental detoxification.