Z-scheme heterojunction of low conduction band potential MnO2 and biochar-based g-C3N4 for efficient formaldehyde degradation

The Z-scheme heterojunction-based photocatalyst often has excellent degradation activity for volatile organic compounds (VOCs). Herein, through unique reduction reaction, we developed a novel MnO2 photocatalyst that has a very low conduction band potential (-1.09 V), compared with those reported MnO2 in the literature. The key experimental evidence was from its XRD results showing much higher content of crystal face (1 1 0) in reference to that of standard α-MnO2 (No. 44–0144). Subsequently, a novel biochar/MnO2/g-C3N4 photocatalyst (C/CN/Mnx) was synthesized and displayed that the best formaldehyde degradation (91.78%) is 3.34 times that of g-C3N4 (27.51%) in air containing 0.5 mg L-1 formaldehyde at 3 h. Due to the favorable band potentials, the excited electrons of g-C3N4 combine with the holes of MnO2, thereby retaining low-potential electrons of MnO2, and these electrons from g-C3N4 and MnO2 can be effectively transferred to biochar so that their recombinations are minimized. The introduction of 5 mmol MnO2 can increase the singlet oxygen (1O2) concentration (2.78 × 1011 mm−3) of the photocatalyst by>4 times under visible light irradiation. This MnO2-based Z-scheme photocatalyst provides a platform technology for the photocatalytic degradation of VOCs.