An S-scheme heterojunction between Mn/Mg co-doped BiFeO3 and g-C3N4 nanosheets for photodegradation of organic pollutants

Abstract

BiFe_1−2xMn_xMg_xO₃ (BFMM, x = 0−8%) was mixed with exfoliated g-C₃N₄ (GCN) to form a composite for establishing an S-scheme heterojunction for photodegradation. BFMM was synthesized by sol–gel method, and showed a decreased band gap from 2.24 eV to 1.75 eV as x increased from 0% to 7%, allowing a more efficient absorption of sunlight. GCN was prepared by thermal polymerization of melamine and then exfoliated to form nanosheets by sulfur acid in order to increase the specific surface area and thus increase reaction sites. A composite with a weight ratio of BFMM/GCN equal to 1 : 3 was prepared by sintering the powder mixture at 300 °C. Such a composite showed a greatly improved efficiency in photodegradation of methylene blue, which was over 6 times faster than pristine BiFeO₃, and the Mn/Mg co-doping improved the efficiency by 48%. The Mott–Schottky plots showed that both GCN and BFMM are n-type semiconductors with flat-band potentials of −0.79 and +0.11 V (vs. NHE), respectively. So, the band alignment allowed the S-scheme to work, leading to an efficient separation of photogenerated electrons and holes, which was confirmed by the greatly increased photocurrents measured with the composites.