Sub-level engineering strategy of nitrogen-induced BiO/g-CN: a versatile photocatalyst for oxidation and reduction.

Herein, the α-BiO nanocrystal decorated by nitrogen dopant and its heterojunction nanocomposite with g-CN (N/BiO/g-CN) is successfully fabricated for the first time, for photo-oxidation of RhB and photo-reduction of Cr(VI) to Cr(III). The resulting N/BiO/g-CN (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-BiO. The results from XPS confirmed the substitution of nitrogen with various oxidation states from N to N (x < 5), due to the existence of different nitrogen oxides including N-O, O-N=O, and NO 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/BiO/g-CN 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-BiO by insertion of N atoms into the Bi sites, and the enhanced charge carrier mobility between N/BiO and g-CN. 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.