Efficient Degradation of Tetracycline <i>via</i> Coupling of Photocatalysis and Photo-Fenton Processes over a 2D/2D α-Fe₂O₃/g-C₃N₄ S-scheme Heterojunction Catalyst

Abstract

Graphitic carbon nitride (g-C3N4) has been widely used as a potential photocatalytic material for the removal of tetracycline from water. However, the poor visible light absorption ability and high recombination rate of the photogenerated charge significantly inhibit the catalytic activity of g-C3N4. Therefore, facile methods to improve the photocatalytic efficiency of g-C3N4 need to be developed. Hematite (α-Fe2O3), which has a good visible light absorption and corrosion resistance, is often used for photocatalysis and photo-Fenton reactions. Therefore, a two-dimension/two-dimension (2D/2D) S-scheme heterojunction constructed of g-C3N4 and α-Fe2O3 nanosheets could be expected to improve the degradation efficiency of tetracycline. In this study, 2D/2D Sscheme α-Fe2O3/g-C3N4 photo-Fenton catalysts were prepared using a hydrothermal strategy. The photo-Fenton catalytic activity of α-Fe2O3/g-C3N4 (α-Fe2O3 50% (w)) was significantly improved by the addition of a small amount of H2O2, removing 78% of tetracycline within 20 min, which was approximately 3.5 and 5.8 times the removal achieved using α-Fe2O3 and g-C3N4, respectively. The high catalytic activity was attributed to the synergy between the photocatalysis and Fenton reaction promoted by the continuous Fe 3+ /Fe 2+ conversion over the 2D/2D S-scheme heterojunction. The 2D/2D S-scheme heterojunction was crucial in the fabrication of the α-Fe2O3/g-C3N4 photocatalyst with a large surface area, adequate active sites, and strong oxidation-reduction capability. Furthermore, the photo-Fenton reaction provided additional hydroxyl radicals for the degradation of tetracycline with the aid of H2O2. The excess reaction product (Fe 3+ ) was reduced to Fe 2+ by the photogenerated electrons from the conduction band of α-Fe2O3. The resulting Fe 2+ could participate in the photo-Fenton reaction. The morphological structures of α-Fe2O3/g-C3N4 were analyzed using transmission electron microscopy to demonstrate the formation of a 2D/2D structure with face-to-face contact, and the optical properties of the composites were measured using ultraviolet-visible diffuse reflectance spectroscopy. α-Fe2O3/g-C3N4 possessed a significantly improved visible light absorption compared to g-C3N4. Five sequential cyclic degradation tests and X-ray diffraction (XRD) patterns obtained before and after the reaction showed that the α-Fe2O3/g-C3N4 composites possessed stable photo-Fenton catalytic activity and crystal structures. Transient photocurrent responses of α-Fe2O3/g-C3N4 demonstrated that the prepared composites exhibited a higher charge transfer efficiency compared to that of single α-Fe2O3 and g-C3N4. In addition, according to the photoluminescence analysis and active species trapping experiments, a possible S-scheme heterojunction charge transfer process in the photo-Fenton catalytic reaction was proposed. This study provided a promising method for the construction of a high-performance photo-Fenton catalytic system to remove antibiotics from wastewater.

Rational Design of an Efficient S-Scheme Heterojunction of CdS/Bi2WO6–S Nanocomposites for Photocatalytic CO2 Reduction

“…As a typical Aurivillius-phase material, Bi 2 WO 6 is built by alternating (Bi 2 O 2 ) n 2 n + layers and perovskite-like (WO 4 ) n 2 n – layers. With a band gap of 2.7 eV, Bi 2 WO 6 is a very promising photocatalyst benefiting from its unique layered structure, visible light response, high thermal and photochemical stabilities, and non-toxicity to the environment. − Especially, ultrathin Bi 2 WO 6 nanosheets have recently drawn widespread attention in photocatalysis, because a thin-layer-structured semiconductor can not only have more abundant active sites to promote its adsorption toward the reactants but also have a shorter transport distance for the photogenerated charge carriers, which, as a result, reduce their recombination. − In particular, a semiconductor with thin nanosheet morphology shows great advantages in fabricating a S-scheme heterojunction in which a good interface is required to ensure an effective charge transfer between the two semiconductor components. − …”

Simultaneous photocatalytic tetracycline oxidation and chromate reduction via a jointed synchronous pathway upon Z-scheme Bi12O17Cl2/AgBr: insight into intermediates and mechanism

“…11,28 In composite photocatalysis systems, according to the migration path of the photo-induced carriers, they could be divided into mainly type-II or Z-scheme heterojunctions. [30][31][32][33] However, their corresponding catalytic performances vary dramatically. The Z-scheme system can improve the separation rate of broad-band carriers and retain relatively high redox potentials 4,26,34 by driving electrons on the lower conduction band (CB) to combine with the holes on the lower valance band (VB) between the two semiconductors, 35 thus inhibiting the recombination of photo-induced charge carriers.…”

Construction of a Bi2MoO6/CoOx/Au system with a dual-channel charge transfer path for enhanced tetracycline degradation

“…Nevertheless, with the overuse of TC and the wanton discharge of pharmaceutical wastewater, TC released into aquatic environments gradually accumulates, which poses an adverse effect on the ecosystem and public health. [1][2][3][4][5] Fortunately, photocatalytic technology, a type of advanced oxidation process, offers a promising avenue for the elimination of residual TC in wastewater due to high efficiency, cost-effectiveness, and environmental friendliness. [6][7][8][9][10] Bismuth molybdate, which possesses a distinctive layered structure and suitable bandgap, has been reported as a promising two-dimensional (2D) photocatalyst for antibiotic degradation under visible light.…”