DUAN Ziyu, ZHANG Zhenzhen, SUN Jiajie, WANG Xiaotong, WANG Juan, WEI Zhishun

(School of Materials and Chemical Engineering, Hubei University of Technology, Wuhan 430068, Hubei, China)

Extended abstract:

[Background and purposes] With the acceleration of global industrialization and urbanization, water pollution has become an increasingly serious issue. In particular, the treatment of wastewater containing complex, hard-to-degrade dyes and antibiotics has emerged as a critical challenge. Semiconductor photocatalytic technology, with its high efficiency and safety, is regarded as an ideal strategy for addressing energy and environmental issues. Among numerous photocatalytic materials, bismuth-based halide oxides have garnered significant attention, due to their unique layered structure and electronic properties. BiOBr, a typical representative, possesses a narrow bandgap of 2.7 eV. Its crystal structure consists of alternately stacked [Bi₂O₂]²⁺ and Br− layers. The resulting built-in electric field effectively promotes the directional separation of photo-generated electron-hole pairs. Additionally, it readily forms hierarchical structures to increase specific surface area, exhibiting excellent visible-light catalytic activity. However, pure BiOBr still faces bottlenecks, such as limited light utilization and high charge recombination rates. Meanwhile, g-C₃N₄, as a non-metallic polymer semiconductor, possesses excellent thermal stability and chemical inertness, which can be synthesized at scale using inexpensive precursors. Its conduction band position endows it with strong reducing power. Since it does not adsorb contaminants, it is conducive to recycling. Although intrinsic g-C₃N₄ suffers from drawbacks, such as rapid charge recombination and poor conductivity, it can be combined with BiOBr to form a hybrid material without the shortcomings, serving as an effective approach to enhancing photocatalytic activity. In this work, a simple method was described for the controlled synthesis of a novel highly efficient composite photocatalyst. Specifically, spherical flower-like structures of bismuth bromide oxide (BiOBr) and g-C₃N₄ were prepared by using solvothermal method and high-temperature pyrolysis, respectively. By adjusting the composition, g-C₃N₄/BiOBr composite photocatalysts with different mixing ratios were further developed.

[Methods] To prepare g-C₃N₄/florette-shaped bismuth bromide oxide composite, 95 mg, 90 mg, 85 mg, 80 mg, 75 mg and 50 mg florette-shaped bismuth bromide oxide (Br:Bi=1.0:1.8) prepared above were placed them in 100 mL beakers (labeled C1–C6). Then, 5 mg, 10 mg, 15 mg, 20 mg, 25 mg and 50 mg g-C₃N₄ were added to each beaker, resulting in g-C₃N₄ mass fractions of 5%, 10%, 15%, 20%, 25% and 50%, respectively. 40 mL ethylene glycol was added to each beaker. After ultrasonic dispersion for 30 min, the mixtures were magnetically stirred for 1 h. The mixtures were then transferred to 100 mL reaction vessels and reacted at 140 ℃ for 8 h. After cooling to room temperature, the mixtures were washed 3–4 times with deionized water and dried at 60 ℃ for 12 h to obtain composite samples, designated as Bi/CN-5, Bi/CN-10, Bi/CN-15, Bi/CN-20, Bi/CN-25 and Bi/CN-50, respectively.

[Results] As the mass fraction of g-C₃N₄ increased, the spherical flower-like structure of BiOBr underwent an evolution from stability, through disruption and flaking, to a final state characterized by porous, loose and irregular agglomeration. The introduction of a small amount of g-C₃N₄ did not alter the crystal structure of BiOBr, but significantly enhanced its crystallinity through a secondary solvothermal process. According to the UV-Vis, DRS and M-S results, it is revealed that the composite not only broadened the photoresponse range and reduced the bandgap, but also through its matched band structure facilitated the transfer of photo-generated electrons from g-C₃N₄ to BiOBr, thereby effectively enhancing charge separation efficiency and photocatalytic activity. TC degradation experimental results confirmed that the Bi/CN-10 sample exhibited the highest degradation rate, because of the low adsorption properties of g-C₃N₄, while maintaining the integrity of the BiOBr flower-like structure. However, an excessive g-C₃N₄ led to a decline in performance, due to the structural disruption. DFT calculation results further confirmed that the bandgap is significantly reduced after the formation of the composite. This optimized band structure enhances photon absorption and electron-hole separation efficiency, thereby substantially improving photocatalytic activity.

[Conclusions] In this study, g-C₃N₄/BiOBr composite photocatalytic materials were successfully prepared using a combination of solvothermal and high-temperature decomposition methods. When the Br:Bi molar ratio was 1.0:1.8, the resulting flower-like BiOBr samples exhibited high dispersion and a relatively uniform spherical flower-like structure. The g-C₃N₄ loading significantly influenced the structure and performance of the composite material, with the Bi/CN-10 sample demonstrating the optimal photocatalytic performance, a degradation rate of 53.51%. The incorporation of g-C₃N₄ masks some of the adsorption sites on BiOBr, effectively reducing the dark adsorption rate of the composite toward the reactants. Meanwhile, the spherical flower-like structure of BiOBr provides a high specific surface area and active sites, contributing to the composite's excellent degradation performance and enhancing its overall photocatalytic performance. The combination of g-C₃N₄ and BiOBr forms an interlaced band structure, broadening the light-responsive range and suppressing carrier recombination, thereby enhancing photocatalytic activity and catalytic efficiency. In summary, a low-cost, highly active and easily recoverable g-C₃N₄/flower-like BiOBr composite photocatalyst was prepared via a simple solvothermal method, providing a new material and technical reference for water pollution control and treatment. In the future, the g-C₃N₄ preparation process can be further optimized and multi-component composite systems can be explored to enhance the material's practical application potential.

Key words: flower-like BiOBr; g-C₃N₄; composite materials; photocatalysis