Research progress of MOF/Bismuth-based semiconductor composites in photocatalytic technology

. Photocatalysis has significant potential for environmental remediation and energy conversion, with a focus on designing and developing highly efficient photocatalysts. Composite materials consisting of bismuth-based semiconductors and metal-organic frameworks (MOFs) exhibit outstanding photocatalytic activity, and have garnered significant attention from researchers as a highly sought-after material. A review is conducted on recent advances in MOF/bismuth-based semiconductor composites. On this basis, the synthesis methods of MBCs are described in detail, and then the applications of MBCs in organic pollutant degradation, Cr (VI) reduction, water (H2O) splitting, nitrogen (N2) fixation discussed. Finally, this paper highlights the current challenges in photocatalysis using MBCs and provides insights into the future development direction for MBCs photocatalysis technology. The preparation and modification methods of MBCs, the improvement of photocatalytic efficiency, the mechanism of oxidative degradation of organic matter and the mechanism of photocatalytic complete hydrolysis of water need further study.


Introduction
Since the photocatalytic process uses abundant and sustainable solar energy or light energy to catalyze the reaction, photocatalytic technology has developed rapidly in recent years and is widely used in environmental remediation and energy conversion. However, many photocatalytic materials face challenges such as wide band gap, rapid recombination of electron-hole pairs and low utilization of visible light. As a consequence, they exhibit low photocatalytic efficiency and hinder the practical application of photocatalytic technology in engineering.
Due to MOFs unique properties such as ultra-high specific surface area, stable porous structure, and changeable functionality, it has been widely used in the fields of gas adsorption and separation, sensing, biomedicine, proton conduction and catalysis. As a new visible light responsive photocatalyst, bismuth-based semiconductor has a band gap between 2.0eV and 3.0eV, which is considered to be suitable for removing pollutants from environmental media and catalyzing the synthesis of new energy source [1]. This paper reviews the various preparation methods of MBCs at home and abroad in recent years and the application progress in environmental remediation and energy conversion, in order to provide new ideas for the large-scale promotion and application of photocatalytic technology.

Preparation of MBCs
According to the preparation process of various MBCs photocatalysts reported in the current literature, the preparation methods can be mainly summarized into two categories. One method is to prepare MOFs materials by hydrothermal/solvothermal, electrochemical, ultrasonic, microwave and other pretreatment methods, and then to prepare MBCs by combining MOFs with bismuth-based semiconductors through hydrothermal/solvothermal processes. Another method is to make MOFs and bismuthbased semiconductor materials react at low temperature based on high-energy ball milling, and then form MBCs composites [2]. Figure 1 is a scanning electron microscope image of BiOBr/NH2-UiO-66 (BUN) hybrid composite prepared by hydrothermal method [3].

Application of MBCs photocatalysis
Metal-organic frameworks (MOFs) are a new type of crystalline porous materials formed by self-assembly of metal ions or clusters as nodes and organic ligands as linkers through the coordination of the two. It is one of the research hotspots in recent years. At present, in order to solve the shortage of fast recombination of photogenerated hole-electron pairs in intrinsic bismuthbased semiconductors, modification is usually carried out through strategies such as component adjustment, morphology control and construction of heterojunction structures. Based on the microstructure and optical properties of bismuth-based semiconductors and MOFs, MOFs/bismuth-based semiconductor composites (MBCs) can be designed to combine the advantages of the above two materials. In recent years, the photocatalysis application reports of MBCs have increased year by year, mainly for environmental restoration and energy conversion. The specific applications are shown in Table  1.

3.2.1
Photocatalytic decomposition of water. Photocatalytic decomposition of water technology for hydrogen production can transform solar energy into renewable and clean energy, thus effectively breaking the bottleneck of energy shortage [9]. The efficiency of photolysis water is mainly limited by the slow oxygen evolution kinetics. Han prepared a new composite material MIL-100(Fe) @ BiVO 4 by hydrothermal in situ growth of MIL-100 (Fe) nanoparticles on the surface of decahedral BiVO 4 . The experiment of photocatalytic decomposition of water shows that when the load of MIL-100(Fe) is 8%, MIL-100(Fe)@BiVO 4 composite exhibits the best photocatalytic oxygen evolution performance, which is 4.3 times the catalytic activity of pure BiVO 4 . The improvement of the photocatalytic performance is mainly due to the formation of a tight heterojunction interface between MIL-100(Fe) and BiVO 4 , which promotes the effective transfer and conduction of carriers between the bifunctional components.

Photocatalytic ammonia synthesis.
In order to solve global energy and environmental problems, an artificial photosynthesis system that can use solar energy is essential. Ammonia, as a raw material of chemical fertilizer and a potential hydrogen carrier and fuel, is an indispensable energy substance in modern industrial agriculture [10]. Traditional synthetic ammonia uses Haber -Bosch process. It not only requires a high temperature of 400°C -600°C and a high pressure environment of 20 MPa -40MPa, but also produces a large amount of carbon dioxide emissions in the process of ammonia synthesis [8].
It is a green and sustainable method to synthesize NH3 by using N 2 and H 2 O at normal temperature and pressure based on photocatalytic technology. Liu [9] The hydrophilic Bi 4 O 5 Br 2 deposited on a hydrophobic surface of ZIF-8, Bi 4 O 5 Br 2 / ZIF-8 composite photocatalyst having a gas-liquid-solid three-phase reaction interface. The photocatalytic synthesis rate of NH 3 is 327.338μ mol · L -1 · h -1 · g -1 , which is 3.6 times the synthesis efficiency of Bi 4 O 5 Br 2 . The source of this excellent activity is that the unique three-phase interface structure enables N 2 to be transported directly from the gas phase to the photocatalytic reaction interface, replacing the process of N 2 diffusion through the liquid phase. This rapid supply of N 2 can effectively capture photogenerated electrons, thereby improving the utilization efficiency of photogenerated electrons and nitrogen fixation activity. The mechanism diagram is shown in Figure. 4.

Conclusion
This paper reviews the two main preparation methods of MBCs and their applications in the fields of photocatalytic environmental remediation and energy conversion, mainly including the degradation of organic dyes, the degradation of PPCPs and Cr (VI) reduction, photodecomposition of water to produce hydrogen and synthesis of ammonia. In general, the construction of MBCs composite structure increases the activity check point of catalytic reaction, expands the spectral range and inhibits photogenerated carrier recombination, thereby improving the photocatalytic performance. However, further research on MOFs/ bismuth based composites is still needed. In particular, the preparation and modification methods of MBCs, the improvement of photocatalytic efficiency, the mechanism of oxidative degradation of organic matter and the mechanism of photocatalytic complete hydrolysis of water need further study.
• In the field of photocatalytic environment remediation and energy conversion, the research on the efficiency of photocatalysis, the efficiency of oxidative degradation of organic matter, and the speed of photocatalytic total hydrolysis all rely on the improvement of the photocatalytic performance of MBCs. • So far, the preparation methods of MBCs are still not perfect. Neither hydrothermal / solvothermal method nor ball milling method has its shortcomings. Hence, it is imperative to investigate novel approaches to enhance the manufacturing process of MBCs. Scholars should open up ideas, not rigidly adhere to the existing preparation methods, and boldly develop innovative preparation technologies. • Currently, MBCs are usually prepared with organic solvents and treated at high temperatures. Hence, it is imperative to assess the environmental ramifications of different preparation techniques for MBCs. If the organic solvent and high temperature are not treated well, it will have a negative impact on the environment.