Sub-level engineering strategy of nitrogen-induced Bi₂O₃/g-C₃N₄: a versatile photocatalyst for oxidation and reduction

Herein, the α-Bi₂O₃ nanocrystal decorated by nitrogen dopant and its heterojunction nanocomposite with g-C₃N₄ (N_0.1/Bi₂O₃/g-C₃N₄) is successfully fabricated for the first time, for photo-oxidation of RhB and photo-reduction of Cr(VI) to Cr(III). The resulting N_0.1/Bi₂O₃/g-C₃N₄ (3%) nanocomposite showed an optimal Cr(VI) photo-reduction and RhB photo-oxidation rates under visible-light irradiation, being 3–4 times higher than that of pure α-Bi₂O₃. The results from XPS confirmed the substitution of nitrogen with various oxidation states from N³⁺ to N^x+ (x < 5), due to the existence of different nitrogen oxides including N−O, O−N=O, and NO₃⁻ in the crystal structure. We investigated the reaction mechanism using catalytic tests, impedance spectroscopy, EPR technique, and density functional calculations. The DFT calculations presented the appearance of a new mid-gap hybrid of p states, comprised of N 2p, O 2p, and Bi 6P states, which enhance light absorption capacity and narrow band gap. The theoretical results were in excellent agreement with experimental UV-Vis data. The N_0.1/Bi₂O₃/g-C₃N₄ nanocomposite exhibited acceptable practical application value and recycling ability for removal of the contaminants. Such improved photocatalytic activity is originated from the modified band positions, new electron evolution pathway, introducing defects in α-Bi₂O₃ by insertion of N atoms into the Bi sites, and the enhanced charge carrier mobility between N_0.1/Bi₂O₃ and g-C₃N₄. The strategy to form nitrogen-doped bismuth-based nanocomposites may open a new opportunity to design atomic-level electronic defects by feasible methods to obtain a versatile photocatalyst material with simultaneous photo-reduction and photo-oxidation ability for removal of Cr(VI) and organic dyes from water. Graphical abstract: [Figure not available: see fulltext.].