Enhanced photocatalytic activity of g-C₃N₄–n-p type flower-like ZnO/BiOBr heterojunction for hexavalent chromium and dye wastewater degradation

Construction of a composite g-C₃N₄, a 2D material with a layered structure, and flower-like ZnO/BiOBr heterojunction photocatalyst is an optimal strategy for the enhancement of photocatalytic performance to reduce the charge carrier recombination at the interface and increase the UV light absorption range to improve simultaneous oxidation of methyl orange (MO) and reduction of Cr(VI) using synthesized photocatalysts. Since the electron requirements for Cr(VI) reduction are theoretically higher than that of MO degradation, the amount of ZnO/BiOBr heterojunction catalyst and the different molar ratios of Bi/Zn for Cr(VI) degradation have been studied. The involved hydroxyl radicals (•OH) and superoxide anion radicals (•O₂⁻) were also investigated. With the initial concentration of C₀=20 mg/L of MO and C₀=10 mg/L of Cr(VI) at pH 2, the optimized amount of 0.05 g g-C₃N₄-0.2 g ZnO/BiOBr-0.5 (molar ratio of Bi/Zn=0.5) and 0.05 g g-C₃N₄-0.4 g ZnO/BiOBr-2 (molar ratio of Bi/Zn=2) were observed to achieve 99.3 % in 130 min of dye decolorization (rate constant=0.036 min ⁻¹) and 96 % of Cr(VI) removal in 180 min (rate constant=0.0105 min ⁻¹), respectively, indicating more electron requirements observed for Cr(VI) reduction are higher than that of MO degradation theoretically and experimentally. The rate constants were 20 % higher than those of the compared experiments, demonstrating excellent photocatalytic activity that is superior to that of pristine composite either g-C₃N₄-ZnO-BiOBr or ZnO-BiOBr alone. After four reuse cycles, the photoactivity of the photocatalyst was not significantly reduced, demonstrating the stability of the synthesized g-C₃N₄-ZnO/BiOBr. Therefore, the synthesized g-C₃N₄-n-p type flower-like ZnO/BiOBr heterojunction catalyst has outstanding potential for the Cr(VI) removal, and MO dye decolorization. The results showed that the flower-like structure can promote the desorption of high levels of active surface oxygen and enhance reducibility.