Boosting principles for the photocatalytic performance of Cr-doped Cu2O crystallites and mechanisms of photocatalytic oxidation for levofloxacin

Cr-doped Cu2O crystallites were synthesized by hydrothermal process. XRD, BET, SEM, TEM, UV-Vis and RSM were used to reveal the microstructure, optical characteristics and photocatalytic activity of the Cr-doped Cu2O crystallites. The main degradation intermediates and degradation reaction pathways of levofloxacin were distinguished based on HPLC-MS/MS. The potential photocatalytic reaction points of levofloxacin and the degradation intermediates were analysed using frontier electron densities. The results indicated that the synthesized Cr-doped Cu2O crystallites had higher specific surface and visible light absorptance than the pure Cu2O crystallites. The doping of Cr introduced impurity levels in the lattice of Cu2O and increased the forbidden band width of Cu2O crystallites, causing a blueshift of the absorption boundary. The prepared crystallites had high purity, and the morphologies of the prepared Cu2O, K-1-Cu2O, K-2-Cu2O and K-3-Cu2O crystallites were spheres, polyhedrons, octahedrons and cubes, respectively. The main active factors of the photocatalytic reaction for levofloxacin were photogenerated holes and hydroxyl radicals. The K-2-Cu2O sample had the strongest photocatalytic degradation efficiency and reusability for levofloxacin. The degradation rates of levofloxacin after the first and 8 cycling experiments reached 79.6% and 72.4%, respectively. Ten main intermediates in the degradation of levofloxacin were determined. The ring opening and decarboxylation of piperazine and quinolones might be the major paths of photocatalytic reactions. This research thoroughly revealed the mechanisms of photocatalytic reactions and performance improvements in Cr-doped Cu2O crystallites.