Enhanced Visible-light Photocatalytic Activity of g-C3N4/Nitrogen-doped Graphene Quantum Dots/TiO2 Ternary Heterojunctions for Ciprofloxacin Degradation with Narrow Band Gap and High Charge Carrier Mobility

Limited visible-light absorption and high recombination rate of photogenerated charges are two main drawbacks in g-C3N4-based photocatalysts. To solve these problems, g-C3N4/nitrogen-doped graphene quantum dots (NGQDs)/TiO2 ternary heterojunctions were facilely prepared via a one-step calcining method. The morphology, structure, optical and electrochemical properties of g-C3N4/NGQDs/TiO2 were characterized and explored. The optimal g-C3N4/NGQDs/TiO2 composite exhibits enhanced photocatalytic degradation performance of ciprofloxacin (CIP) compared with the as-prepared g-C3N4, TiO2(P25) and g-C3N4/TiO2 heterojunction under visible light irradiation. The apparent rate constant of the composite is around 6.43, 4.03 and 2.30 times higher than those of g-C3N4, TiO2 and g-C3N4/TiO2, respectively. The enhanced photocatalytic efficiency should be mainly attributed to the improvement of light absorption and charge separation and transfer efficiency, originating from the narrow band gap and high charge carrier mobility. The active species trapping experiments results showed that the h(+) and O-center dot(2)- were the main active species in the degradation process. A possible photocatalytic reaction mechanism of the g-C3N4/NGQDs/TiO2 composite for the enhanced degradation of CIP under visible light irradiation was also proposed.