S-scheme tubular g-C3N4/BiOI heterojunctions for boosting photodegradation of tetracycline and Cr(VI): Mechanism insight, degradation pathway and DFT calculation

A unique tubular g-C3N4 (TCN)/BiOI S-scheme heterojunction with BiOI microspheres grown on the surface of 1D TCN was successfully prepared by hydrothermal treatment and calcination. Herein, adjusting BiOI additive amount can effectively tune electronic band structures and photocatalytic properties of TCN/BiOI heterojunctions. Impressively, the TCN/BiOI composite (CB-2) with a TCN: Bi3+ molar ratio of 1:2 shows the best photocatalytic performance, where the photodegradation efficiencies for tetracycline (TC) and Cr(VI) ions can both achieve more than 77 % and 90 % within 10 and 30 min, respectively. Particularly, the photodegradation efficiencies for TC and Cr(VI) reach up to 90.1 % and 96.8 % at 30 and 40 min, respectively. Notably, the rate constants for photodegradation of TC and Cr(VI) reach 0.1687 and 0.0718 min−1, which are 27.2 and 8.5 times higher than those of bulk g-C3N4, respectively. The distinguished photocatalytic performance of CB-2 is attributed to the high-quality electron transport channels provided by its tubular structure and the effective separation of photogenerated carriers based on the S-scheme heterojunction. Further, O2−, 1O2 and h+ are manifested to be the main active species for the photodegradation of TC. DFT calculations and radical-related experiments fully confirm the formation of S-scheme heterojunctions. Several crucial factors (such as pH, inorganic salts) for wastewater purification are verified to be little effect on the photocatalytic performance. The degradation pathways of TC are proposed. This work establishes a new route to design and prepare high-efficiency S-scheme photocatalysts for the photodegradation of antibiotics and the photoreduction of heavy metal ions in wastewater.