These days, because of rapid urbanisation, industry, and population increase, water contamination is a recognised problem that needs to be fixed. As a result, pollutants including pesticides, industrial chemicals, heavy metals, dyes, detergents, and so forth are created and end up in waterways, where they either directly or indirectly affect human health and cause a range of illnesses. Of these contaminants, organic dyes are the most notable since these dyes are mostly find its application in the plastic, cosmetics, textile, and pharmaceutical industries and adversely affect the water quality [1–2]. Congo Red (CR), among other colours, is widely used as pigments in the chemical, biological, textile, and pharmaceutical industries. Either directly or indirectly when ingested after being dissolved in water, azo dyes like CR can have harmful consequences on the body, including as neurological system diseases, inflammation of the respiratory tract, high BP levels, vomiting, anaemia, and nausea [3]. Degradation of these harmful dyes to a simpler, less toxic form is extremely difficult, and they are mostly resistant to chemicals, catalysis, and oxidising agents. Consequently, one of the essential steps in the treatment of waste water is to extract and/or allow these coloured materials to flow out of the water [4–8]. A variety of treatment techniques, such as exchange of ions, degradation by photosynthesis, coagulation, oxidation, ozonation, filtering, and electrochemical breakdown, are used to remove colours from water [9–13].
Photocatalysis is a popular and effective dye degradation technique that uses semiconductor materials' ability to induce chemical processes under light irradiation. It is also environmentally benign. The creation of electron-hole pairs as a result of photon absorption by the photocatalyst surface is the fundamental process of this mechanism. The adsorbed dye molecules are subsequently exposed to redox reactions by these active charge carriers, which break them down into smaller, less toxic components like carbon dioxide and water. Crucially, this degrading process occurs at ambient condition i.e., at normal pressure and room temperature, thereby making it more ecologically benign and energy-efficient as compared to other orthodox techniques like chemical oxidation or biological treatment [14, 15].
The utilisation efficiency of sunlight has significantly enhanced over the past few years due to the development of photocatalysts with visible light response [16]. Many attempts have been made to produce effective photocatalytic compounds that respond to visible light, including g-C3N4, BiFe2O4, and BiVO4 (BVO), additionally to altering the conventional substance TiO2 for photocatalysis to obtain reaction to visible light [17–21]. Bismuth vanadate (BVO) is one of the many types of photocatalysts available; its favourable band gap energy, inherent photocatalytic activity, stability, availability, low toxicity, and customisable surface features make it an extremely promising catalyst for dye degradation through photocatalysis [22–24]. BVO has a strong photocatalytic activity, but its broad industrial applicability is limited by its weak surface adsorption capacity. Numerous investigations on the alteration of BVO have been carried out in an effort to increase its photocatalytic activity. The doping of ions, composite substances, and the noble metal loading are examples of common modification techniques [25–27].
Metal-organic frameworks (MOFs) belong to a family of porous materials where metal ions or clusters are linked together by organic ligands using coordination bonds. Because of their distinctive structure, MOFs produce a highly porous network with a huge surface area, which makes them excellent choices for a variety of applications. Because of their high adsorption capabilities and adjustable pore diameters, MOFs are widely used in gas storage and separation processes. Furthermore, MOFs have demonstrated potential in sensing, environmental cleanup, drug delivery, energy harvesting and catalysis [28–30]. The unique features of metal-organic frameworks (MOFs) have led to increased exploration of MOFs as photocatalysts. These porous materials can be customised to improve light absorption, speed up charge separation and transfer processes thanks to their enormous surface area, configurable pore apertures, and adaptable chemical functionalities. To further increase their photocatalytic activity, MOF’s modular design also makes it possible to add photoactive elements like metal ions or organic ligands with light-absorbing qualities. Furthermore, the distinct structures of MOFs offer chances for exact control over reaction routes and product selectivity. Because of these characteristics, MOFs are appealing choices for photocatalytic applications, including organic synthesis, hydrolysis of water, and breakdown of pollutants, opening the door to effective and sustainable photocatalysis systems [31–33].
Zinc ions (Zn 2+) and imidazolate ligands combine to generate zeolitic imidazolate framework-8 (ZIF-8), a porous framework with a large surface area. Additionally, ZIF-8 possesses inherent stability under photocatalytic conditions, allowing for sustained catalytic activity. Furthermore, the tunability of ZIF-8 enables the incorporation of different metal ions or organic ligands to enhance its photocatalytic performance for specific reactions. Overall, these characteristics render ZIF-8 a promising photocatalyst for various applications, including pollutant degradation, hydrogen production, and organic synthesis [34–36]. In terms of photocatalytic uses, ZIF-8 is less advantageous because it has a wide band gap energy of approximately 4.9 eV, limiting its responsiveness to UV light activation [37]. The catalytic substance must react with both visible and infrared wavelengths of light, which make up the majority of solar power, in order to produce an effective photocatalyst. Through the reduction of the bandgap and facilitation of electron transfer, ZIF-8 can be coupled with a semiconductive material to maximise photocatalytic performance and minimise the rate of electron-hole recombination [38]. Reports on ZIF-8 and highly active photocatalyst in combination are still rare.
The synergistic combination of ZIF-8 and BVO in a composite structure offers several advantages for photocatalytic degradation applications. The intimate contact between the two components promotes transfer of charges at the interface, reducing recombination losses and enhancing photocatalytic efficiency. Furthermore, the composite material inherits the inherent stability of ZIF-8 and the photocatalytic activity of BVO, making it a robust and effective catalyst for dye degradation. In this study, we investigate the photocatalytic degradation of CR dye, utilizing a composite material composed of ZIF-8 and BVO.