Synthesis of g-C₃N₄/NiO p–n heterojunction materials with ball-flower morphology and enhanced photocatalytic performance for the removal of tetracycline and Cr⁶⁺

A novel three-dimensional ball-flower-like g-C₃N₄/NiO (GN) photocatalyst was firstly fabricated by a simple two-step calcination method for enhanced degradation capability. Our investigation focuses on the construction of p–n heterojunction and specific ball-flower-like structure to overcome limitations of single-component semiconductors, such as low surface area, poor light response and fast recombination of the photogenerated electrons and holes. Detailed photocatalytic experiments revealed that the novel ball-flower-like GN (50%) exhibited much higher activity for the removal of TC and Cr⁶⁺ than single g-C₃N₄, NiO and traditional layered GN. The trapping experiments proved that the superoxide radicals (\( ^{ \cdot } {\text{O}}_{2}^{ - } \)), holes (h⁺) and electrons (e⁻)were the main active species in the photodegradation process. Moreover, three different degradation pathways and fourteen intermediate products of TC were also determined by the LC/MS analysis and these intermediates could be further degraded completely into CO₂, H₂O, \( {\text{NH}}_{4}^{ + } \), carboxylic acids and any other inorganic intermediates. These results will benefit the fabrication of other 3D p–n heterojunction photocatalysts and their potential application in treatment of organic pollutants.