Hierarchical flower-like 0D/3D g-C3N4/TiO2 S-scheme heterojunction with enhanced photocatalytic activity

The wide band gap and high recombination rate of photogenerated electron-hole pairs greatly limit the photocatalytic efficiency of TiO2 under sunlight irradiation. We constructed a novel photocatalyst (TiO2/CNQDs-3) with the heterojunction of non-toxic, metal-free zero-dimensional carbon nitride quantum dots (CNQDs) and three-dimensional hierarchical flower-like TiO2 microspheres, which not only enhanced the visible-light absorption capability, but also inhibited the recombination of photogenerated electron-hole pairs. The TiO2/CNQDs-3 showed a high light absorption intensity, transient photocurrent density and photodegradation efficiency of methyl orange. And the reaction rate constant of the TiO2/CNQDs-3 for the first-order reaction kinetics was 5.4 times and 2.2 times that of the pure TiO2 and P25, respectively. With the increase of pH, the photo degradation efficiency of methyl orange by the TiO2/CNQDs-3 decreased due to electrostatic repulsion. Moreover, the main active species were center dot OH, h(+) and center dot O-2(-), and the Step-scheme (S-scheme) heterojunction enhanced the photodegradation of methyl orange. The TiO2/CNQDs composites are environmentally friendly, inexpensive, fast in the separation of electron-hole pairs, strong in redox capacity, large in specific surface area and pore volume, and have good application prospects in photodegradation of pollutants.