Advances in Composite Photocatalysts for Efficient Degradation of Organic Pollutants: Strategies, Challenges, and Future Perspectives

Abstract

The persistent release of synthetic dyes such as methylene blue (MB) into aquatic environments poses a significant ecological hazard due to their chemical stability and toxicity. In recent years, the application of engineered composite photocatalysts has emerged as a potent solution for efficient dye degradation under visible and UV light. This review comprehensively summarizes various advanced composites, including carbon-based, metal-doped, and heterojunction materials, tailored for MB degradation. Notably, composites such as TiO₂/C-550, WS₂/GO/Au, and MOF-derived α-Fe₂O₃/ZnO achieved near-complete degradation (>99%) within 30–150 min, while others, like ZnO/JSAC-COO⁻ and Ag/TiO₂/CNT, displayed enhanced charge separation and stability over five consecutive cycles. Band gap engineering (ranging from 1.7 eV to 3.2 eV) and reactive oxygen species (·OH, ·O₂⁻) generation were key to their photocatalytic performance. This review compares the structural attributes, synthetic strategies, and degradation kinetics across systems, highlighting the synergistic role of co-catalysts, surface area, and electron mobility. This work offers systematic insight into the state-of-the-art composite photocatalysts and provides a comparative framework to guide future material design for wastewater treatment applications.

1. Introduction

Recently, environmental contamination in the natural water cycle has drawn significant attention [1,2,3]. The production of extremely toxic and carcinogenic wastewater has surged due to the rapid growth of industrialization [4]. In our daily lives, water purification (WP) is crucial for reasons beyond just ensuring access to safe drinking water [5,6]. Wastewater from various industries, households, hospitals, and laboratories contains high concentrations of organic dyes, surfactants, heavy metals, and other harmful compounds [7,8,9]. These pollutants pose serious environmental problems as they are toxic to microorganisms [10], aquatic life [11], and human health [10], and they also degrade the soil quality [12,13]. Dyes are compounds that have a wide range of applications in many sectors. They are widely used in textile, food, cosmetics, leather, paper, plastics, processing, printing, rubber, pharmaceuticals, and tanneries [14]. Konstantinou and Albanis reported that approximately 1–20% of the colors produced worldwide are wasted during the dyeing process and end up in textile effluents [15,16]. As a result, dye degradation in industrial wastewater has gained significant attention, leading to the proposal of various remediation techniques [17,18].

Traditional methods, such as membrane filtration, chemical precipitation, biological treatments, coagulation/flocculation, and adsorption, etc., are commonly used for wastewater treatment. However, these approaches are not always practical or highly effective [19]. Forgacs and coworkers observed that the chemical stability of synthetic textile dyes renders traditional wastewater treatment technologies significantly inadequate in addressing wastewater contaminated with these contaminants [20]. Each of the above-described methods has several drawbacks when it comes to eliminating dyes from wastewater [21]. Since photocatalysis may mineralize the target contaminant, recent research [22,23,24,25,26] has focused on using this technique to degrade the dyes from wastewater. Photocatalytic technology has emerged as a highly effective and widely demonstrated method for decomposing organic dyes. This approach utilizes light, in combination with photocatalysts, to break down harmful organic compounds. Its key advantages include ease of operation, energy efficiency, environmental sustainability, and the prevention of secondary pollution [27]. Due to these attributes, photocatalysis stands out as an effective and sustainable method for wastewater treatment and environmental remediation.

Table 1. Comparative analysis of photocatalysis and other treatment techniques.

Sr. No.	Treatment Methods	Description	Advantages	Limitations	Refs.
1	Photocatalysis	Utilizes light-activated catalysts.	Environmentally friendly, cost-effective, high energy saving, minimal secondary pollutants, and the easiest catalyst loading.	Difficulty in regeneration and recovery.	[29]
2	Biological	Uses microorganisms to degrade the material.	Cost-effective and excellent reduction of color.	Not applicable to highly concentrated organic waste, difficult to control, and has low efficiency in dye degradation.	[30]
3	Chemical precipitation	Use of various chemicals, such as lime or aluminum sulfate, to convert dissolved material into a solid substance.	Energy consumption is low and effective in removing organic halogens.	Excessive use of chemicals (such as lime, oxidants, and H2S) and the development of sludge.	[31]
4	Membrane filtration	Use of a semi-permeable membrane to eliminate the contaminants.	Rapid and eco-friendly method.	High operational cost and membrane fouling reduce efficiency.	[32]
5	Adsorption	Pollutants adhere to porous materials.	Simple and cost-effective.	The generation of secondary waste and adsorbent saturation requires regeneration.	[33]
6	Coagulation/flocculation	Use of coagulants such as alum, potash alum, and polyaluminum chloride for the removal of materials.	Simple and good for removing pollutants.	Requires a high dose of chemicals, the formation of massive sludge, and larger particles.	[34,35]

provides a comparative overview of photocatalysis and other treatment techniques, emphasizing their respective limitations [28].

In recent years, numerous studies have been conducted to remove these pollutants using photocatalysts, exploring various composite materials for dye degradation [36]. Semiconductor composite photocatalysts have gained significant attention due to their enhanced properties, which arise from both individual components and the synergistic effects of composite formation. These materials have been extensively studied and utilized to tackle the urgent challenge of environmental pollution and mitigate the rapid deterioration of the living environment [37,38]. A range of wide and narrow band gap photocatalysts, including titanium [39,40,41] and zinc-based nanocomposites [42,43,44], carbon-based materials [45,46], MOFs [47,48], carbon nanotubes [49,50], and graphene [51,52], have been explored for the degradation of organic pollutants. However, single semiconductors often show low efficiency due to limited solar energy absorption and poor charge separation [53]. The present study aims to review the degradation of organic pollutants, with a specific focus on methylene blue (MB) dye. The primary objective is to assess the effectiveness of composite photocatalysts in enhancing the photocatalytic degradation of MB. It is a widely used heterocyclic aromatic compound, primarily applied in dyeing silk and cotton, and it serves as an indicator of chemical changes. However, its widespread use poses significant environmental concerns due to its persistence and potential toxicity [54].

This study also intends to examine how various composites affect the material’s morphological and structural characteristics, as well as how they affect photocatalytic performance. Therefore, we used a methodical approach to determine the efficiency of various composites in the degradation of organic pollutants to make this review comprehensive. This strategy incorporates the four main processes of planning, searching, screening, and reporting, as suggested by Tranfield and coworkers, as mentioned below [55,56].

2. Methodology

This review was conducted using a systematic framework designed to evaluate the photocatalytic degradation of methylene blue (MB), with particular attention paid to mechanistic insights and practical limitations. The approach followed four main stages, planning, searching, screening, and reporting, ensuring both reliability and comprehensiveness. In the planning stage, the scope of the review was clearly defined. The focus was placed on MB degradation under ultraviolet and visible light, the role of reactive oxygen species (ROS), self-sensitization effects, and the performance of different photocatalysts, including metal oxides, sulfides, and hybrid composites. The searching stage involved gathering relevant studies from reputable scientific databases such as Scopus, Web of Science, ScienceDirect, and Google Scholar. A combination of the keywords “methylene blue photocatalysis,” “visible-light degradation,” “catalyst stability,” “nanocomposites,” “organic pollutants,” and “advanced wastewater treatment” was used to capture a broad yet focused dataset. To ensure updated and credible information, priority was given to peer-reviewed articles, case studies, short communications, and review papers published within the last two decades.

During the screening stage, the collected literature was carefully evaluated for relevance. Duplicate records, studies outside the scope of MB degradation, or papers lacking experimental validation were excluded. Emphasis was placed on experimental investigations that offered insights into photocatalytic mechanisms, degradation efficiencies, and catalyst stability. Finally, in the reporting stage, the selected studies were systematically synthesized. The analysis compared the degradation efficiencies across different catalysts, discussed mechanistic pathways in detail, and highlighted challenges related to catalyst recovery and long-term stability. Furthermore, research gaps were identified, particularly in relation to energy efficiency, large-scale applicability, and sustainability. Following this structured methodology, approximately 200 articles were identified, examined, and consolidated. This comprehensive approach not only summarizes the current understanding of MB photocatalytic degradation but also outlines key directions for future research, thereby enhancing the reliability and significance of the findings.

3. Mechanism of Photocatalysis

The several stages of photocatalysis include the generation of electron–hole pairs through the absorption of solar light, the separation of these charge carriers, the migration of electrons and holes to the surface of the photocatalyst, and the redox processes that are aided by these charge carriers [57,58,59]. Significant electron–hole recombination takes place during electron migration, either in the bulk or on the surface of the photocatalyst. The energy released by this recombination is either heat or light. However, depending on the characteristics of the donor or acceptor species, long-lived electrons and holes on the photocatalyst surface start different redox processes [60]. The most typical photocatalysts are semiconductors, which possess a full electron valence band (VB) with well-defined band gaps and higher-energy free electron conduction bands (CBs) [61]. When the photocatalyst absorbs light energy from the light source, the electrons from the VB will be transferred to the CB to form the corresponding electron–hole (e⁻–h⁺) pairs (Equation (1)). The superoxide radical anion (^•O²⁻) can be produced by the combination of the excited electrons on a photocatalyst’s surface and the dissolved oxygen (electron acceptor) (Equation (2)). Equations (3) and (4) state that free hydroxyl radicals can be produced when H₂O and OH⁻ molecules react with photogenerated holes on the photocatalyst surface [62]. Additionally, organic contaminants can be directly oxidized by the free pores on photocatalyst surfaces. The strong hydroxyl radicals 2.80 V oxidation potential allows them to effectively break down organic pollutants that have adsorbed on the photocatalyst’s surface, producing H₂O and CO₂ (Equation (5)) [62,63]. Figure 1 shows a schematic diagram of the photocatalysis mechanism for the photocatalytic degradation of organic pollutants.

$$Photocatalyst+h\gamma \to (e_{CB}^{-}+h_{VB}^{+})$$

(1)

$$e_{CB}^{-}+O_2{\to }^{•}{{\mathrm{O}}_2}^{-}$$

(2)

$$h_{VB}^{+}+{OH}^{-}{\to }^{•}\mathrm{OH}$$

(3)

$$h_{VB}^{+}{H}_{2}O\to H^{+}{+}^{•}\mathrm{OH}$$

(4)

$${(}^{•}\mathrm{OH}{, }^{•}{{\mathrm{O}}_2}^{-},{\mathrm{H}}_2{\mathrm{O}}_2,h_{VB}^{+})+\mathrm{Organic}\mathrm{Pollutant}\to {\mathrm{CO}}_2+\mathrm{Water}+\mathrm{other} \mathrm{products}$$

(5)

Reactive oxygen species (ROS) are classified as primary intermediates during photocatalytic activities. Therefore, in order to understand the mechanism of photodegradation, it is crucial to quantify, identify, and evaluate their kinetics [64]. Recently, there has been significant interest in developing novel photocatalysts with high activity under visible light to enhance photocatalytic efficiency [65,66]. Although they are compelling oxidants, reactive oxygen species produced during photocatalysis, such as hydroxyl radicals (^•OH), superoxide anions (^•O₂⁻), and singlet oxygen (¹O₂), do not exhibit selectivity. They efficiently break down target pollutants, but they can also react with background components, such as carbonates, dissolved organic matter, and other non-target substances, altering the water’s chemistry and potentially creating secondary byproducts [1,2]. Degradation efficiency may be decreased, and unexpected environmental effects may become a concern. Therefore, the surface characteristics of the photocatalyst, the reactor conditions, and the complexity of the water matrix all have a significant impact on the selectivity of ROS-driven degradation [3].

Methylene blue’s self-sensitization mechanism entails MB’s interaction with photocatalysts, which accelerates the degradation process via a number of pathways. The primary driving force behind this mechanism is MB’s capacity to absorb light and transfer electrons or energy to the catalyst, thereby promoting its own breakdown. Environmental factors like the pH and light irradiation, the kind of catalyst being used, and the existence of extra sensitizers all affect the process [4,5]. Energy transfer from the excited MB to triplet oxygen creates singlet oxygen, which oxidizes MB, as part of the self-sensitization mechanism of methylene blue degradation. Under visible light, this pathway is important, especially when the photocatalyst is not excited [6]. Porphyrin sensitizers have also been utilized to elucidate the self-sensitization mechanism of methylene blue (MB) degradation, wherein photocatalytic activity is enhanced and the degradation process is facilitated under visible light irradiation through electron transfer from the photoexcited H₂TCPP or CuTCPP molecules to the conduction band of TiO₂ [7]. Through demethylation during photocatalytic processes, the chromophoric and auxochrome groups of methylene blue (MB) break down, forming a variety of intermediates, including azure A, B, and C, as well as thionin [8].

4. Carbon-Based Composites for Photocatalytic Degradation

Since carbon-based photocatalysts are inexpensive, have good mechanical and chemical durability, and have high electrical conductivity, they have a bright future in photocatalytic applications. Carbon nanotubes (CNTs), graphene, graphite, and porous carbon are examples of carbon-based nanomaterials that have been extensively employed in photocatalysis for dye degradation in recent years [67,68]. Syed et al. found that the incorporation of carbon quantum dots (CQDs) and graphite oxide (GO) into polyacrylonitrile (PAN) nanofibers greatly improves their mechanical stability and photocatalytic activity. PAN/GO, PAN/CQDs, and PAN/GO/CQDs nanofibers were synthesized and evaluated for visible-light-driven photocatalytic degradation of MB. Through electrospinning and chemical cross-linking, the GO/PAN/CQD composite showed 100% MB degradation in 25 min [45]. A new Z-scheme CdS/CQDs/g-C₃N₄ heterojunction composite was synthesized by Feng et al. through a simple calcination process, showing enhanced photocatalytic activity for MB degradation. The optimized composite exhibited the highest degradation efficiency of 98% after irradiation for 120 min, which greatly exceeded that of pure g-C₃N₄ (65%), CdS (75%), CdS/g-C₃N₄ (80%), and CQDs/g-C₃N₄ (86%). This outstanding performance was due to the effective charge separation and redox capability facilitated by the Z-scheme heterojunction [69].

MWCNTs/TiO₂ nanocomposites were prepared by Jiang et al. using the liquid phase deposition method and tested as photocatalysts for MB degradation under UV/visible light irradiation. This composite exhibited higher photocatalytic efficiency than pure TiO₂, particularly in neutral and alkaline media, reaching up to 90%. The improved degradation was due to the high surface area of MWCNTs, which enhanced MB adsorption and charge transfer [70]. The addition of H₂O₂ greatly enhanced the rate of degradation by serving as an electron acceptor, producing the hydroxyl radicals necessary for the breakdown of pollutants. At pH 7.0, over 80% of MB degraded in the presence of H₂O₂, while just 20% degraded without it. The rate of degradation was the greatest at pH 10.3 (τ_1/2 = 7.0 min) and reduced in acidic conditions, where only 42.5% degradation was noted under UV light after 60 min. This phenomenon was attributed to pH-dependent TiO₂ surface charge changes affecting MB adsorption. The results demonstrate the promising potential of MWCNTs/TiO₂ composites for effective wastewater treatment [71]. In another case, TiO₂-functionalized carbon nanotube (TiO₂/CNT) composites and Ag-deposited ternary multilayered composites (Ag/TiO₂/CNT) were prepared by Ye et al. via chemical deposition and the sonochemical method, respectively, as shown in Figure 2. The incorporation of CNTs and Ag greatly improved the photocatalytic degradation activity of TiO₂. The degradation activity of TiO₂/CNT composites was found to be about 96%, whereas pure TiO₂ was about 80%, and that of Ag/TiO₂/CNT composites increased degradation further up to about 98% after 20 min. The improved photocatalysis was due to the synergistic impact of CNTs and Ag within the TiO₂ matrix [72]. The high surface area of CNTs was available for dye adsorption, and it stored electrons, which enhanced charge separation [73,74]. The band gap of anatase TiO₂ (~3.2 eV) and the work function gaps among TiO₂, Ag (~4.24 eV), and CNTs (~4.7 eV) resulted in the formation of Schottky barriers, allowing efficient electron transfer. These results indicate the promising potential of Ag/TiO₂/CNT composites for high-performance photocatalytic uses in wastewater treatment and environmental purification [72].

Carbonization and in situ precipitation were employed to create the lignin-based carbon/cadmium sulfide (LCS) composite, resulting in a stable, highly porous substance. The LCS photocatalyst effectively eliminated methylene blue with 91.7% efficiency in two hours when exposed to visible light, with rapid adsorption reaching saturation within approximately 30 min. CdS nanoparticles are uniformly anchored on the carbon scaffold derived from lignin, as seen in SEM micrographs. This promotes dispersion and exposes a large number of active sites, improving adsorption and reaction kinetics (Figure 3). After five consecutive cycles, the catalyst maintained an activity of greater than 80% and demonstrated good resistance to both acidic and alkaline media, indicating its durability. The lignin–carbon domain is believed to enhance performance by serving as an electron sink, facilitating charge separation, and inhibiting electron–hole recombination, thereby strengthening visible-light photocatalysis [75].

The Yb-TiO₂/g-C₃N₅ (YTCN) heterostructured photocatalyst was prepared by Bai et al. through hydrothermal synthesis and calcination and evaluated for the photocatalytic degradation of MB under a 500 W xenon lamp. The optimized YTCN (1:3) composite achieved a 96.57% degradation rate of MB in 105 min, with a decay rate constant of 0.02083 min⁻¹, which was 2.2 times higher than that of pure TiO₂. Adding Yb to the TiO₂ matrix enhanced charge carrier separation, increasing the MB degradation efficiency by 7.25%, whereas further inclusion of g-C₃N₅ enhanced it by another 51.86%. The better photocatalytic activity of YTCN is due to heterojunction formation, which prolongs the lifetime of charge carriers and enhances light absorption. Nevertheless, too high a TiO₂ ratio may hinder active sites, while a lower proportion of TiO₂ will impair the cooperative effect, leading to decreased redox efficiency. The YTCN (1:3) composite exhibited the highest MB degradation performance, affirming the effectiveness of rare earth doping and heterostructure construction in enhancing photocatalytic activity (Figure 4a) [76].

Several characterization methods have been used to confirm the production of a metal-free porphyrin enclosing (2,4-dinitrophenyl)-imino)-methyne chromophore (PcDNPIMC). Because of its suitable band gap and 18πe-aromatic structure, the porphyrin-based photocatalyst demonstrated exceptional optical and photocatalytic characteristics. After 240 min of exposure to 5 W LED light, the ideal conditions for the photodegradation of MB were found to be 20 mg/L of photocatalyst in a 10 ppm solution at pH 5, which led to a 95.5% degradation of the dye in a handmade photoreactor (Figure 4b) [77].

The TiO₂/C composites prepared from sawdust were tested for MB removal under simulated sunlight. TiO₂/C-550 demonstrated the best photocatalytic efficiency, degrading almost 100% of MB in 30 min. This enhanced activity is linked to the optimized light absorption (band gap ~2.7 eV), minimal charge recombination, and higher adsorption capacity (9.7 mg/g). Superoxide radicals (^•O₂⁻) played a dominant role in the degradation mechanism, as confirmed by trapping studies. Other variants, TiO₂/C-600, C-650, and C-700, showed lower efficiencies (71.3%, 83.7%, and 56.8%, respectively), likely due to structural collapse and surface area loss at elevated calcination temperatures. Among all these, TiO₂/C-550 also exhibited the highest adsorption and rate constant based on kinetic modeling. Although the material retained ~95% efficiency after five cycles, its durability in real wastewater scenarios needs further exploration due to potential challenges like carbon oxidation and catalyst fouling (Figure 5) [78].

The photocatalytic degradation of MB using TiO_2−x-based nanocomposites was investigated, with RGO-TiO_2−x exhibiting the highest performance, characterized by a rate constant of 0.075 min⁻¹. This enhanced photocatalytic activity is attributed to the synergistic effect between the oxygen vacancies in TiO_2−x, which promote efficient charge carrier separation, and the excellent electrical conductivity of reduced graphene oxide (RGO), which suppresses electron–hole recombination. Compared to pristine TiO₂, the RGO-TiO_2−x composite achieved nearly double the degradation efficiency, underscoring the critical role of interfacial charge transfer dynamics and improved visible light absorption. The integration of RGO not only facilitates rapid electron transport but also extends light harvesting, thereby significantly boosting the overall photocatalytic performance [79].

Figure 6a illustrates the proposed photodegradation mechanism of TiO_2−x and RGO-TiO_2−x composites. Under light irradiation, electrons are excited from the valence band (VB) to the conduction band (CB) of TiO_2−x. The presence of oxygen vacancies in TiO_2−x facilitates the capture of these photoexcited electrons, thereby reducing recombination. Furthermore, due to the favorable alignment of the conduction band energy levels and work functions, efficient electron transfer occurs from TiO_2−x to reduced graphene oxide (RGO), significantly enhancing charge separation and transport. Figure 6b demonstrates that the high surface area of RGO serves as an adsorptive platform, concentrating MB molecules near the catalyst surface. This promotes surface diffusion of the pollutants toward TiO_2−x, where they interact with oxygen vacancies. These adsorbed species are subsequently oxidized by photogenerated holes or reactive oxygen species (ROS), leading to the formation of intermediate compounds. These intermediates are further mineralized into benign end-products such as CO₂, H₂O, SO₄²⁻, and NO₃⁻. The RGO-TiO_2−x composite exhibits markedly superior degradation efficiency compared to both pristine TiO₂ and TiO_2−x alone. This improvement is primarily attributed to its enhanced visible light response, greater charge mobility, and effective suppression of electron–hole recombination [79].

ZnO@CBC demonstrates highly effective removal of MB through a synergistic mechanism involving both adsorption and photocatalytic degradation. The synthesis route of ZnO@CBC is illustrated in Figure 7a. The adsorption process is primarily governed by electrostatic interactions and π–π stacking between MB molecules and the composite surface, both of which are pH-dependent. Fourier-transform infrared (FTIR) spectroscopy confirmed the successful adsorption of MB, as evidenced by changes in characteristic functional group peaks. Scavenger experiments revealed that hydroxyl radicals (·OH) play a dominant role in the photocatalytic degradation pathway. Upon light irradiation, ZnO generates electron–hole (e⁻/h⁺) pairs. The photoexcited electrons are transferred to the conductive biochar (CBC) component, where they reduce molecular oxygen to form superoxide radicals (·O₂⁻), while the photogenerated holes in ZnO oxidize water or hydroxyl groups to produce ·OH radicals. These reactive oxygen species synergistically break down MB molecules into non-toxic end-products. Figure 7b illustrates that the integration of CBC not only facilitates efficient charge separation but also enhances surface adsorption and electron transport, significantly boosting the overall photocatalytic performance of the ZnO@CBC composite [80].

The AC@Fe₃O₄ composite, with an average crystallite size of 16.73 nm, exhibits excellent photocatalytic performance in the degradation of MB, as illustrated in Figure 8. At an optimal catalyst dosage of 0.50 g/L, AC@Fe₃O₄ achieves a high photodegradation efficiency of 94.6% for 10 mg/L MB within 140 min, corresponding to a kinetic rate constant of 0.025 min⁻¹. Reusability tests further confirm the material’s stability, maintaining a degradation efficiency of 84% after four successive cycles. These results highlight the robustness and durability of AC@Fe₃O₄ as a promising photocatalyst for practical wastewater treatment applications [81].

The photodegradation rate of MB dye increased significantly upon forming a ZnO/g-C₃N₄ composite. The rate constant improved from 0.016 min⁻¹ for pristine ZnO and 0.011 min⁻¹ for g-C₃N₄ to 0.022 min⁻¹ for the ZnO/g-C₃N₄ (10 wt%) composite. Notably, further enhancement was observed with the ZnO/g-C₃N₄ (50 wt%) composite, which achieved a maximum MB removal efficiency of 97%, outperforming the 10 wt% composite. In comparison, the individual components exhibited lower removal efficiencies, with ZnO achieving 90% and g-C₃N₄ only 73%. These results clearly demonstrate the synergistic effect of the composite structure, which enhances charge separation and light absorption, leading to superior photocatalytic performance [82]. The carboxylate-functionalized activated carbon (JSAC-COO⁻) was obtained by filtration, thorough washing with deionized water, and drying at 60 °C for 24 h. The carboxylation process is depicted in Figure 9. Among the tested materials, the ZnO/JSAC-COO⁻ composite exhibited the highest photocatalytic efficiency for methylene blue (MB) degradation, achieving 97.56% removal. This performance significantly surpassed that of pure ZnO (49%) and ZnO-RGO composites (87.40%). The degradation of MB followed pseudo-first-order kinetics, with the ZnO/JSAC-COO⁻ composite showing a rate constant of 0.0124 min⁻¹, approximately twofold and fivefold higher than those of ZnO and ZnO-RGO, respectively. This enhancement in both efficiency and reaction rate is attributed to the electron-accepting ability of the carboxylate-functionalized JSAC, which facilitates effective charge separation by capturing photogenerated electrons from ZnO’s conduction band, thereby suppressing electron–hole recombination. Moreover, the composite’s narrow band gap and high surface area further contribute to its superior photocatalytic activity [83].

Under visible light irradiation, the electrons in g-C₃N₄ are photoexcited and transferred to MnO₂ and ZnO, where they participate in the reduction of molecular oxygen (O₂) to generate superoxide radicals (O₂^•−). However, since the conduction band (CB) potential of MnO₂ is less negative than the O₂/O₂^•− redox potential, only ZnO and g-C₃N₄ effectively contribute to the formation of O₂^•−. The generated superoxide further reacts to form hydrogen peroxide (H₂O₂), which subsequently decomposes into hydroxyl radicals (^•OH), potent oxidizing agents responsible for dye degradation. Simultaneously, the photogenerated holes (h⁺) from MnO₂ and ZnO migrate toward g-C₃N₄, while the h⁺ in g-C₃N₄ directly oxidize MB. This directional charge transfer across the heterojunction components significantly suppresses electron–hole recombination, thereby enhancing the overall photocatalytic degradation efficiency of MB (see Figure 10) [84].

The co-doping of sulfur and boron heteroatoms into g-C₃N₄, forming BSCN, and its integration with Ag₂S significantly enhances the visible-light-driven photocatalytic degradation of MB. The Ag₂S/BSCN composite exhibits markedly higher photocatalytic performance than individual components, pure g-C₃N₄, BSCN, or Ag₂S, owing to its improved charge carrier separation and reduced electron–hole recombination. Among the prepared composites, the sample containing 5 wt% Ag₂S demonstrates the highest photocatalytic activity, outperforming the 1%, 3%, and 10% variants. This optimized composite achieves a remarkable MB degradation efficiency of 98.5% within 120 min under visible light irradiation [85]. Two-dimensional (2D) contour plots and three-dimensional (3D) response surface plots were employed to analyze the influence of key operational parameters on the degradation efficiency of MB. As shown in Figure 11A, the interaction between pH and H₂O₂ dosage reveals that MB degradation initially increases with the addition of H₂O₂, reaching an optimum level, beyond which excessive H₂O₂ leads to a decline in efficiency, likely due to the scavenging effect of hydroxyl radicals. Figure 11B illustrates that lower initial MB concentrations, combined with an optimal H₂O₂ dose, significantly enhance degradation performance, while higher dye concentrations suppress efficiency due to the limited availability of reactive species. In Figure 11C, it is observed that the MB removal efficiency improves with increasing pH up to a critical point under a fixed H₂O₂ dose, after which no significant enhancement is observed [86].

Spinel ferrite nanoparticles and their carbon-based composites are widely explored for wastewater treatment due to their stability, adsorption capacity, magnetic behavior, and appropriate band gap energy. These materials are effective in removing both organic and inorganic pollutants through adsorption and photocatalytic degradation. Their adsorption efficiency depends on factors such as the surface area, surface charge, annealing temperature, functional groups, and cation distribution. Owing to their versatility and tunable properties, spinel ferrites hold significant promise for tackling emerging contaminants and addressing novel environmental challenges [46].

The photocatalytic mechanism of CA/ZnS-Ag under visible light irradiation is illustrated in Figure 12. In this system, carbon aerogel (CA) functions as a photosensitizer, facilitating the formation of electron–hole (e⁻-h⁺) pairs indirectly through interaction with a sacrificial agent, since direct excitation of CA is not feasible. Electrons generated in CA are transferred to the conduction band (CB) of ZnS or reduce the molecular oxygen (O₂) to produce superoxide radicals (^•O₂⁻). Meanwhile, the Ag nanoparticles enhance photocatalytic activity via surface plasmon resonance (SPR), injecting additional electrons into the ZnS CB. Additionally, CA can accept electrons from ZnS CB, resulting in the generation of holes. These photogenerated charge carriers migrate to the catalyst surface, where adsorbed water molecules and dye pollutants participate in producing hydroxyl radicals and other reactive species, ultimately leading to effective dye degradation [87].

The photocatalytic mechanism of S-gTAHP is elucidated based on the band structures of TAP and S-gAP, as depicted in Figure 13. Upon exposure to sunlight, both materials generate electron–hole (e⁻–h⁺) pairs. However, the traditional type II heterojunction mechanism does not apply here because the conduction band (CB) potential of TAP (−0.252 V) is insufficient to reduce O₂ to superoxide radicals (^•O₂⁻), and the valence band (VB) potential of S-gAP (1.769 V) is inadequate for oxidizing H₂O to hydroxyl radicals (^•OH). Instead, the system operates via an S-scheme heterojunction, wherein electrons transfer from S-gAP to TAP until their Fermi levels align, resulting in the formation of an internal electric field at the interface. This electric field facilitates the recombination of less active charge carrier electrons in the TAP CB and holes in the S-gAP VB while preserving the more reactive holes in the TAP VB and electrons in the S-gAP CB. These retained charge carriers are responsible for the enhanced photocatalytic activity observed, which aligns with the results of the radical trapping experiments shown in Figure 13 [88].

The AC-ZrO₂/CeO₂ nanocomposite showed exceptional efficacy in photocatalytic degradation and adsorption, especially when it came to eliminating organic contaminants from wastewater, such as the cationic dye MB. The maximum adsorption capacity of 75.54 mg/g was reached in 60 min, and the adsorption behavior closely matched the Langmuir isotherm model. Additionally, reactive species such as superoxide radicals (^•O₂⁻), photogenerated holes (h⁺), and hydroxyl radicals (^•OH) drove the nanocomposite’s improved photocatalytic destruction of MB. The rate constant for MB degradation was found to be 0.05999 min⁻¹, and the degradation of MB followed the Langmuir–Hinshelwood kinetic model [89]. The Cu-g-C₃N₄/BC (600)/H₂O₂ system primarily degrades MB through hydroxyl radicals (·OH), as confirmed by quenching experiments and electron paramagnetic resonance (EPR) analysis. The significant decrease in degradation efficiency upon addition of tert-butanol underscores the dominant role of ·OH, while superoxide radicals (·O₂⁻) and photogenerated holes (h⁺) contribute to a lesser extent. EPR spectra verified the in situ generation of ·OH, ·O₂⁻, and h⁺ species under visible light irradiation. The bamboo charcoal (BC) matrix enhances adsorption of both MB and H₂O₂, improves visible light absorption, and facilitates electron conductivity. Photogenerated electrons originating from Cu₂O, CuO, and g-C₃N₄ were efficiently transferred to BC, suppressing electron–hole recombination and promoting the formation of the reactive radicals ·OH and ·O₂⁻. Additionally, the Cu²⁺/Cu⁺ redox cycling further accelerates ·OH production through Fenton-like reactions. The overall high photocatalytic efficiency of this composite arises from the synergistic interactions between its components and the stable anchoring of Cu sites on the BC framework (Figure 14b) [90].

Figure 15 illustrates the excellent cyclic stability of the WS₂/GO/Au electrocatalyst during the dye removal process, demonstrating high reproducibility over extended use. Catalyst recyclability, alongside photo- and electrochemical stability, is a key factor when evaluating catalytic performance. Notably, the WS₂/GO/Au electrocatalyst maintains its activity over an impressive 3000 electrocatalytic cycles in MB degradation. Even at elevated MB concentrations, the catalyst sustains stability and achieves a high conversion efficiency of up to 99%. For a 50 µL MB solution, complete degradation requires approximately 3000 cycles, corresponding to a total duration of 72 min. Continuous electrolysis results in a steady decline in MB absorption, confirming effective dye degradation. Furthermore, complete mineralization of MB is attained within one hour using WS₂/GO/Au, whereas WS₂/GO and GO alone only achieve 30% and 10% degradation, respectively, within the same timeframe [91].

Incorporation of 10% NiO into g-C₃N₄ led to a 66% reduction in the photoluminescence peak intensity, indicating suppressed electron–hole recombination. Diffuse reflectance spectroscopy confirmed the enhanced visible light absorption by the NiO/g-C₃N₄ nanocomposite. Consequently, the photocatalytic degradation efficiency of MB improved significantly, increasing from 33% with pristine g-C₃N₄ to 91.6% after 90 min at pH 8.0. Scavenger studies revealed that the Z-scheme photocatalytic mechanism played a crucial role in facilitating effective charge separation under visible light irradiation (Figure 16). Among reactive species, superoxide ions (O₂^•−) were identified as the primary active species responsible for MB degradation. Furthermore, the NiO/g-C₃N₄ composite exhibited excellent stability, retaining 95% of its degradation efficiency after five consecutive cycles [92].

Sono-photocatalysis combines sunlight, ultrasound, and a catalyst to synergistically enhance the degradation of dyes and pollutants by promoting the generation of reactive radicals. For GO/Fe₃O₄ nanocomposites, ultrasonic cavitation produces localized high temperatures (~500 K) and pressures (~500 bar), leading to water pyrolysis and the formation of highly reactive ^•OH and ^•H radicals. Additionally, the collapse of cavitation bubbles emits light that excites electrons within the catalyst, generating electron–hole pairs. The catalyst surface provides active sites that facilitate radical formation, while simultaneous sunlight irradiation induces further charge carrier excitation. Electrons are preferentially absorbed by GO, which improves reduction reactions and ultimately boosts dye degradation efficiency (Figure 17) [93].

The Ni/MoS₂/MOF-2@g-C₃N₄ heterostructure demonstrated outstanding photocatalytic performance, achieving up to 91% degradation of MB. This enhanced activity is attributed to the efficient generation of reactive species, specifically superoxide (O₂^•−) and hydroxyl (^•OH) radicals. The degradation process followed pseudo-first-order kinetics, completing effectively within 90 min under light irradiation. Moreover, the heterostructure exhibited excellent stability, maintaining high photocatalytic efficiency with only slight performance loss over five consecutive cycles (Figure 18) [94].

When compared to their undoped counterparts, nitrogen-doped multi-walled carbon nanotubes (N-CNTs) demonstrated better adsorption capacity and photocatalytic activity. Stronger π–π electron donor–acceptor interactions between the negatively charged MB molecules and the sp² lattice of the carbon nanostructure surface are made possible by nitrogen doping, which introduces highly reactive spots and improves performance. A schematic illustration of the surface of the carbonaceous nanostructure is shown in Figure 19a. The following describes the adsorption and photocatalytic mechanisms: the adsorbed MB molecules are efficiently decomposed by the interaction of hydroxyl radicals (^•OH), superoxide radical anions (O₂^•−), and hydroperoxyl radicals (HO₂^•) (Figure 19b). In particular, nitrogen doping improves the photocatalytic effectiveness in comparison to graphene nanoribbons (GNRs) and multi-walled carbon nanotubes (MWCNTs) by raising the chemical reactivity of the carbonaceous nanomaterial (CN_x), which in turn improves the reaction kinetics during MB photodegradation [95].

Eggshell-based activated carbon was successfully synthesized using a simple chemical activation method with orthophosphoric acid and sodium hydroxide. The carbon showed increasing adsorption efficiency during MB degradation, with reactive species like free radicals and superoxide ions playing a key role in the decolorization process. Activated carbon prepared with orthophosphoric acid outperformed that prepared with sodium hydroxide in terms of photocatalytic activity [96]. Raising the calcination temperature from 650 °C to 800 °C increased the graphitic carbon and rutile TiO₂ content but decreased the surface area and TiO₂ crystal size. At 800 °C, anatase TiO₂ was fully converted to rutile in the TiO₂/C composite. This TiO₂/C-800 composite followed pseudo-second-order kinetics, indicating chemisorption, with a maximum adsorption capacity of ~15 mg/g. Under UV light (254 nm, 15 W) for 3 h, its degradation efficiency exceeded the adsorption-only removal by 25.1% [97]. Wood-derived carbon (WDC) enhanced MB adsorption and interaction with g-C₃N₄ (Figure 20). Its porous structure improved light absorption and reduced charge recombination. Electrons transferred from g-C₃N₄ to WDC formed superoxide radicals (·O₂⁻), while valence band holes generated hydroxyl radicals (·OH) or directly oxidized MB. Both radicals contributed to effective dye degradation [98].

By improving charge separation and encouraging the production of reactive radicals, the N-CDs@ZnO composite greatly accelerates the breakdown of MB. Under UV light, nitrogen-doped carbon dots (N-CDs) efficiently transport electrons, preventing photogenerated electron–hole pairs from recombining and producing superoxide radicals (^•O₂⁻). Furthermore, ZnO is activated by the up-conversion luminescence of N-CDs, and dye adsorption is improved by π–π* interactions between the composite and MB molecules. When compared to bare ZnO, the N-CDs@ZnO composite exhibits higher photocatalytic activity due to these synergistic effects (Figure 21) [99].

Figure 22 presents a schematic representation of the enhanced photocatalytic process facilitated by the TiO₂@CFs composite. The mechanism involves three primary steps: (a) the initial adsorption of MB molecules onto the carbon fibers (CFs); (b) the absorption of light energy by the TiO₂@CFs composite; and (c) the subsequent degradation of MB molecules through photocatalytic reactions enabled by efficient charge transfer. In this system, CFs serve a dual function, providing structural support for TiO₂ nanoparticles and accelerating the adsorption of MB molecules by promoting their aggregation on the catalyst surface [100].

Table 2. Summary of carbon-based composite photocatalysts for the degradation of MB.

Photocatalysts	Synthesis Method	Band Gap	Efficiency	Ref.
GO/PAN/CQD	Hydrothermal process	1.79 eV	100%	[45]
CdS/CQDs/g-C3N4	Calcination process	2.68 eV	86%	[69]
MWCNTs/TiO2	Liquid phase deposition method	2.8–2.95 eV	90%	[70]
Ag/TiO2/CNT	Sonochemical method	3.2 eV	98%	[72]
Lignin-based carbon/cadmium sulfide composite	In situ precipitation method	2.38 eV	91.7%	[75]
Yb-TiO2/g-C3N5	Hydrothermal process	2.77 eV	96.57%	[76]
PcDNPIMC	Adler–Longo method	-	95.5%	[77]
TiO2/C-550	Sol-gel method	2.7 eV	100%	[78]
RGO-TiO2−x	Hydrothermal process	1.8 eV	-	[79]
ZnO@CBC	Impregnation–Pyrolysis method	~3.20 eV	99.6%	[80]
AC@Fe3O4	Calcination and coprecipitation	94.6%	~1.7 to 2.0 eV	[81]
ZnO/g-C3N4	In situ synthesis	97%	3.02 to 2.94 eV	[82]
ZnO/JSAC-COO–	Hydrothermal method	97.56%	-	[83]
MnO2-ZnO-g-C3N4	Sol-gel method	94%	2.0 to 2.5 eV	[84]
Ag2S/BSCN	(SILAR) method	96.48%	~2.1–2.3 eV	[85]
g-C3N4/GO/CuFe2O4	In situ hydrothermal method	99%	2.31 eV	[86]
CA/ZnS-Ag	Precipitation method	68.39%	2.68 eV	[87]
S-scheme/gC3N4/TiO2/SiO2/PAN	Electrospinning, calcination, hydrothermal, and freeze-drying techniques	99.43%	2.71 eV	[88]
AC-ZrO2/CeO2 NCs	Co-precipitation	97.91%	2.2–3.0 eV	[89]
Cu-g-C3N4/BC	In situ pyrolysis	32.7%	2.06–2.24 eV	[90]
WS2/GO/Au	Hydrothermal and laser ablation	99.00%	-	[91]
NiO-doped C3N4	Ultrasonic method	92%	2.95 eV	[92]
g-C3N4/WS2	One-pot hydrothermal method	95.5%	∼2.7 eV	[101]
NiO/Cd/g-C3N4	Microwave-assisted	81.8%	-	[102]
NiO/ZnO/g-C3N4	Hydrothermal method	∼95%	~2.68 eV	[103]
GO/Fe3O4	Chemical precipitation method	98.68%	1.96 eV	[93]
g-C3N4/ZnO	Hydrothermal method	97.7%	2.91 eV	[104]
Ni/Mo.S2/MOF-2@g-C3N4	Solvothermal method	91%	1.75 eV	[94]
Eggshell-based activated carbon	Chemical activation method	83%	-	[96]
TiO2/C	One-pot liquid phase reaction	25.1%	-	[97]
g-C3N4@wood-derived carbon	Carbonization	98%	-	[98]
Bio-CDs co-doped with S/Cl	Hydrothermal process	94.2	-	[105]
CDs co-doped with N/S	Hydrothermal process	2.34 > 100%	-	[106]
N-CDs@ZnO	Wet-impregnation method	99%	2.97 eV	[99]
TiO2@CFs	Thermo-treatment after electrospinning	94.54%	-	[100]
ZCF@MB-MIP	Chemical precipitation	95.8%	3.37 eV	[107]
10%CCO/CeO2-nanocomposite	Co-precipitation method followed by calcination	85%	2.75 eV	[108]

represents the summary of carbon-based composite photocatalysts used for the degradation of MB.

5. Metal-Based Composites

Metal-based photocatalysts are generally classified into metal halides, metal nitrides, and metal oxides. This section provides a comprehensive review of their applications in photocatalysis, focusing on metal-based composite photocatalysts. Key examples and their corresponding photocatalytic performances are summarized in Table 3. For instance, when MB was treated with 50% CoVO/W_xO_y under visible light irradiation, a degradation efficiency of 96% was achieved within 80 min, with an observed rate constant of 0.30154 min⁻¹ [109]. CoVO/W_xO_y nanocomposites are fabricated using a template-free hydrothermal method (Figure 23). Similarly, the effect of different phases in Cu₂O/CuO/Cu composites on the degradation of MB under visible light irradiation was investigated. The Cu₂O-rich composite exhibited superior photocatalytic performance, achieving a maximum photocurrent density of 13 mA/cm² along with the highest optimized MB degradation efficiency. In contrast, the Cu₂O-poor composite demonstrated significantly lower activity, reaching only 6.1 mA/cm² [110].

The inorganic halide perovskite Cs₃Bi₂I₉ has garnered significant interest due to its rapid charge carrier mobility, high tolerance to defects, low defect density, excellent water stability, and low toxicity. It exhibits a photoluminescence quantum yield of 2.3%. When exposed to visible light for 50 min, the Cs₃Bi₂I₉/ZIF-8 composite achieved an impressive 98.2% degradation of methylene blue (MB), which is approximately 4.15 times greater than that of pure Cs₃Bi₂I₉. Moreover, the composite retained its high photocatalytic efficiency even after three consecutive cycles, demonstrating strong stability and reusability [111]. The average particle sizes ranged from 15.2 nm for CuCO₃ to 87.7 nm for the CuCO₃-ZnCO₃ composite. Specific surface areas varied from 7.8 m²/g (Ag₂CO₃) to 55.5 m²/g (CuCO₃). Under UV irradiation, carbonate-based nanoparticles effectively degraded MB, with Ag₂CO₃-ZnCO₃ achieving the highest efficiency of 99.88%. Notably, these nanoparticles retained significant photocatalytic activity even when embedded in a polymer matrix; for instance, Ag₂CO₃ showed a degradation efficiency of 91.67% in the composite [112]. ZnCO₃, an n-type semiconductor with a wide band gap of about 3.56 eV, requires UV light for activation. Its conduction and valence band positions are located at 0.43 eV and 3.65 eV, respectively. Doping with silver nanoparticles narrows the band gap, thereby enhancing visible light absorption. In contrast, Ag₂CO₃ is a p-type semiconductor with a narrower band gap between 2.1 and 2.8 eV; its conduction and valence bands lie at 0.38 eV and 2.66 eV, respectively. In ZnCO₃-Ag₂CO₃ composite systems, electrons transfer from ZnCO₃ to Ag₂CO₃, while Ag nanoparticles serve as electron sinks, promoting charge separation and minimizing recombination. This synergistic effect markedly improves photocatalytic performance. Additionally, ZnCO₃ aids in electron transport and stabilizes Ag⁺ ions, further enhancing the activity and longevity of the composite (Figure 24).

The calcined CuO-WO₃ nanocomposite (0.1 M) exhibited superior photocatalytic activity, achieving 70% degradation of MB after 180 min of exposure to sunlight. In comparison, the uncalcined and undoped WO₃ nanoparticles demonstrated significantly lower performance, with only 40% degradation under identical conditions. The influence of the photocatalyst dosage was also assessed using 10 mg and 20 mg of the calcined CuO-WO₃ nanocomposite. A dosage of 20 mg yielded a maximum degradation efficiency of 92%, whereas 10 mg resulted in 78% degradation after 3 h of sunlight irradiation. The improved performance at higher catalyst loading is attributed to the greater number of accessible active sites, which facilitate enhanced generation of charge carriers and reactive species responsible for MB degradation [113]. Additionally, the optimized PEDOT/Ag₂SeO₃ nanohybrid with a 3:1:1–4 mL ratio exhibited notable photocatalytic activity, achieving 46.2% degradation of MB under 210 min of incandescent lamp irradiation, as determined at the dye’s absorption maximum of 663 nm. The study also evaluated the effect of pH on photocatalytic efficiency, with the results indicating that maximum degradation occurred under optimal pH conditions. This emphasizes the critical role of pH in modulating photocatalytic performance [114].

Figure 25a presents a proposed mechanism for the photocatalytic degradation of MB using the PEDOT/Ag₂SeO₃ (3:1:1–4 mL) nanohybrid. Upon illumination, PEDOT, due to its relatively narrow band gap (~1.30 eV), efficiently absorbs light and generates a significant number of electron–hole (e⁻–h⁺) pairs. In contrast, Ag₂SeO₃, with its limited light-harvesting capability, produces fewer charge carriers. However, given that the conduction band of PEDOT lies at a lower energy level than that of Ag₂SeO₃, photogenerated electrons and holes from Ag₂SeO₃ migrate into the PEDOT matrix. This interfacial charge transfer increases the charge carrier density within PEDOT, thereby enhancing its overall photocatalytic efficiency. In comparison, the reduced graphene oxide (RGO)-supported Zn_0.5Cu_0.5Fe₂O₄ nanocatalyst (denoted as MRGO) demonstrated relatively lower efficiency in degrading MB under UV irradiation. Nevertheless, under visible light in aqueous solution, MRGO exhibited excellent activity, achieving 95.2% degradation of a 10 ppm MB solution within 40 min at pH 9 using 20 mg of catalyst. However, its photocatalytic activity decreased significantly after five successive degradation cycles, with the efficiency dropping to 54.6%, indicating limited long-term reusability [115].

Photodegradation experiments using the RGO/Zn_0.5Cu_0.5Fe₂O₄ (RGO/ZCF) nanocomposite were conducted under UV light. As shown in Figure 25b, the UV reactor consisted of a 100 mL cylindrical glass chamber (27 cm × 3 cm), lined internally with aluminum foil to enhance light reflection. During the process, 50 mL of MB dye solution was added to the reactor. Figure 25c illustrates the interaction mechanism between the MRGO 20 nanocomposite and MB molecules under visible light. Zn_0.5Cu_0.5Fe₂O₄ nanoparticles absorb incident photons, generating e⁻–h⁺ pairs. The excited electrons are rapidly transferred to the reduced graphene oxide (RGO) layer due to its excellent electrical conductivity, which suppresses recombination and facilitates effective charge separation. The high surface area of RGO promotes strong adsorption of MB molecules onto the catalyst surface. Simultaneously, the photogenerated holes oxidize MB, while the electrons reduce dissolved O₂ to generate reactive oxygen species (ROS), including hydroxyl radicals (^•OH) and superoxide anions (^•O₂⁻), which actively participate in the breakdown of MB into less harmful degradation products [115].

FeSe₂ and ZnO exhibited average crystallite sizes of 21 nm and 33 nm, respectively, and individually degraded only 58% and 60% of MB after 300 min of irradiation. In contrast, the FZ-3 composite (75 wt% FeSe₂ and 25 wt% ZnO) achieved 97% degradation. The band gaps of FeSe₂ (1.3 eV) and ZnO (3.3 eV) were tuned in the composite, enhancing visible light absorption. The improved performance is attributed to a type-II band alignment, where electrons transfer from FeSe₂ to ZnO and holes from ZnO to FeSe₂, promoting effective charge separation. This facilitates the generation of reactive oxygen species (H₂O₂ and ^•OH), which synergistically degrade MB into CO₂ and H₂O, as shown in Figure 26a [116]. The TiO₂/Fe₃O₄ nanocomposite exhibited high visible-light-driven photocatalytic efficiency, degrading MB by 85% under tungsten halogen light and up to 92% under sunlight within 120 min. Its performance declined by only 13.7% after five consecutive cycles, confirming the excellent stability and reusability [117]. As shown in Figure 26b, TiO₂ generates electron–hole (e⁻–h⁺) pairs under light irradiation, while Fe₃O₄ enhances charge separation and modifies surface properties, improving photocatalytic activity. Scavenger tests using ammonium oxalate (h⁺), 2-propanol (^•OH), and benzoquinone (O₂^•−) were conducted to identify the reactive species involved in the degradation mechanism [117].

As shown in Figure 27, MB solutions treated with alginate/PCL-TiO₂ hybrid bio-composites underwent a visible color change from blue to transparent after 24 h of irradiation, indicating effective dye degradation [118]. Additionally, the 4% PVA/TiO₂ nanocomposite demonstrated strong antibacterial activity against Staphylococcus aureus, Enterococcus faecalis, Proteus mirabilis, and Pseudomonas aeruginosa, along with high MB degradation efficiency [119]. Thermal treatment of the MB sample at 800 °C for 2 h yielded a material with excellent structural properties, achieving 99.53% degradation of a 100 mL, 20 mg/L MB solution within 60 min. Further analysis highlighted the essential role of oxygen-containing radicals and H₂O₂ in the Fenton-assisted photodegradation mechanism [120].

The TSFNVMo composite dispersion exhibited a high zeta potential of −35.9 mV, indicating strong colloidal stability and adsorption capability. Despite a reduction in the specific surface area to 76 m²/g due to vanadium and molybdenum incorporation, the TSFNVMo photocatalyst showed excellent performance under UV–visible light, with an adsorption capacity of 10 × 10⁻³ μmol/g. Under sunlight, the capacity reached 7.2 × 10⁻³ μmol/g. The band gap narrowed from 3.12 eV to 2.52 eV, enhancing visible light absorption and photocatalytic activity [121]. Under UV radiation, the TiO₂-C@N photocatalyst achieved 99.87% MB removal, compared to 28.9% in dark conditions. Kinetic analysis revealed a pseudo-first-order degradation model. As shown in Figure 28a,b, 97.8% degradation was achieved within 30 min at 100 mg/L MB concentration, though efficiency declined to 79% at 500 mg/L. The catalyst retained activity over five adsorption and six degradation cycles (Figure 28c), with only minor losses due to acid stripping with 1 M HNO₃, confirming the excellent reusability and stability [122]. In comparison, kaolinite/TiO₂ nanocomposites showed enhanced MB degradation under UV light. The band gap increased from 2.93 eV to 3.14 eV with higher kaolin content, leading to improved degradation efficiency from 71% with pure TiO₂ to 98% using the composite at 1 g/L dosage and 20 mg/L MB concentration [123].

The diatomite-TiO₂ composite, featuring a porous structure and a combination of anatase and rutile phases, achieved 80% MB degradation under natural sunlight within 270 min [124]. In comparison, TiO₂/C-550 composites exhibited nearly complete degradation (~100%) of MB in just 30 min. This composite had a direct band gap of 2.7 eV, enabling enhanced visible light absorption and reduced charge recombination. It also retained 95% efficiency after five reuse cycles, confirming the excellent stability [78]. TiO₂/C composites displayed mesoporous structures with high BET surface areas (117–138 m²/g) and small TiO₂ crystallites (8–27 nm). Higher calcination temperatures (650–800 °C) led to improved adsorption and photocatalytic performance. The TiO₂/C-800 composite showed a maximum MB adsorption capacity of ~15 mg/g and followed a pseudo-first-order kinetic model (k = 4.8 × 10⁻⁴ min⁻¹). Under 254 nm UV light (15 W) for 3 h, it exhibited 25.1% higher MB removal than by adsorption alone [97]. The 2D/0D g-C₃N₄/TiO₂ nanocomposite outperformed both individual components, achieving 100% MB degradation under visible light within 2 h (Figure 29a). This was attributed to its high surface area (273.32 m²/g), interfacial charge transfer that suppressed electron–hole recombination (Figure 29b), and its layered nanospherical morphology offering abundant active sites (Figure 29c) [125]. Figure 30a illustrates the enhanced MB degradation kinetics of B-doped g-C₃N₄/TiO₂, which followed a pseudo-first-order model. The incorporation of boron into the composite improved the photocatalytic efficiency compared to undoped materials, with performance increasing as the co-catalyst proportion rose (Figure 30b) [126].

The TiO₂-carbonized medium-density fiberboard (TiO₂-cMDF), synthesized via carbonization of MDF treated with 50% (v/v) titanium tetraisopropoxide (Ti-tip) in isopropyl alcohol, was evaluated for its adsorption and photodegradation of MB under UV-C (254 nm) light. After complete MB adsorption, four consecutive photodegradation cycles were performed, achieving 99% MB removal over 396 h. The degradation followed pseudo-first-order kinetics, with the rate constant decreasing from 11.0 × 10⁻³/h in the first cycle to 5.9 × 10⁻³/h [127]. The Cr-doped TiO₂/C (0.2%) nanocomposite exhibited enhanced photocatalytic performance, achieving 90% MB degradation in 70 min under natural sunlight, compared to 63% for pure TiO₂. The degradation followed pseudo-first-order kinetics, with a regression coefficient (R²) of 0.93, indicating a consistent and efficient mechanism (Figure 31a) [128]. TiO₂/CuxO composite films were fabricated by sequentially depositing a TiO₂ layer via the doctor blade method, followed by spin coating of a CuxO layer. Among the samples, the TC 3.6 composite (containing 3.6 wt% CuxO) showed the highest photocatalytic activity in degrading MB (Figure 31b) [129].

The photocatalytic degradation of MB and glyphosate was investigated using barium defect-modified graphitic carbon nitride TiO₂ at concentrations ranging from 10 to 200 mg L⁻¹ over 105 min. As the catalyst concentration increased, the MB degradation efficiency improved from 56% to 97%. The photocatalytic process initiates when photon energy exceeds the band gap, exciting electrons from the valence band (VB) to the conduction band (CB), leaving behind holes in the VB. These electron–hole pairs generate hydroxyl radicals (^•OH), which actively degrade persistent organic pollutants into CO₂ and H₂O. The proposed degradation mechanism is shown in Figure 32 [130]. Doping TiO₂ with copper reduced its band gap from 3.0 eV to 2.67 eV. Pure g-C₃N₄ exhibited a band gap of 2.81 eV, while Cu-TiO₂/g-C₃N₄ composites showed band gaps ranging from 2.82 to 2.88 eV depending on the composition. Among them, Cu-TiO₂/50% g-C₃N₄ demonstrated the highest photocatalytic rate constant (4.4 × 10⁻³ min⁻¹), which is approximately 5 and 9.8 times greater than that of TiO₂ and g-C₃N₄, respectively. Its adsorption rate constant was calculated as 0.122 g mg⁻¹ min⁻¹. Figure 33 illustrates three mechanisms: (a) a homojunction between the anatase and rutile phases in P25 TiO₂, where electrons transfer from anatase to rutile and ^•OH/^•O₂⁻ species primarily form in anatase; (b) a type-I heterojunction in Cu-doped TiO₂, where both electrons and holes migrate toward Cu-TiO₂, leading to recombination and limited activity; and (c) a Z-scheme in the Cu-TiO₂/g-C₃N₄ system, where charge separation is enhanced, allowing efficient generation of ^•OH and ^•O₂⁻ radicals that drive MB degradation [131].

The photocatalytic degradation of MB using CoO/ZnO nanocomposites was optimized at a 1:2 ratio, achieving 67.5% degradation within 3 h, with measurements taken every 30 min. Kinetic analysis using the Langmuir–Hinshelwood model yielded an apparent rate constant (K_app) of 6.428 × 10⁻³ min⁻¹ and a half-life (t_1/2) of 107.84 ± 0.01 min at the optimal concentration. The broad absorption edge of CoO/ZnO nanoparticles suggests strong absorption in the UV-Vis range (190–350 nm). As illustrated in Figure 33b, UV excitation generates e⁻–h⁺ pairs in both CoO and ZnO; the photogenerated electrons reduce O₂ to form superoxide radicals (^•O₂⁻), while the holes oxidize H₂O or OH⁻ to generate hydroxyl radicals (^•OH), both of which contribute to MB degradation [132]. Higher degradation efficiencies were observed with ZnO-based composites: ZnO/activated carbon (ZnO/AC) achieved 85.7%, while magnetically separable ZnO/MNC composites reached 97.14%. The enhanced performance of ZnO/MNC is attributed to the incorporation of Fe₃O₄, which narrows the band gap, improves photon absorption, and increases the surface area, thereby boosting photocatalytic activity [133].

The Cu/ZnO catalyst demonstrated superior photocatalytic activity for MB degradation, achieving 99% removal at 45 °C within 15 to 75 min. This enhanced performance is attributed to the uniform dispersion of CuO within ZnO and the higher density of surface hydroxyl groups. The degradation rate increased with the temperature (25–45 °C), and the activation energies (E_a) for 5Cu/ZnO were calculated as 18.00 and 12.15 kJ/mol for MB degradation [134]. Similarly, CuO-ZnO nanocomposites with 20 wt% CuO showed significantly improved activity, with a rate constant of 0.017 min⁻¹ for MB degradation, compared to 0.0027 min⁻¹ for pure ZnO nanoparticles. Additionally, a reduction rate constant of 5.925 min⁻¹g⁻¹ was reported for 4-nitrophenol (4-NP) using the same nanocomposites [135]. In another study, g-C₃N₄/ZnO composites containing 75% ZnO achieved 98% MB degradation within 180 min under UV light, far exceeding the efficiency of individual ZnO or g-C₃N₄. The proposed charge transfer mechanism responsible for this enhanced photocatalytic activity is illustrated in Figure 34 [136].

Optical analysis revealed that the Fe₃O₄/ZnO nanocomposite (MZ4) exhibited enhanced visible light absorption and prolonged electron–hole pair lifetimes at the optimal composition, contributing to the improved photocatalytic degradation of MB. MZ4 achieved a maximum degradation efficiency of 88.5% under visible light and maintained excellent stability over five reuse cycles. The optimal pH for MB degradation using MZ4 was determined to be six [137]. Additionally, Ag doping in ZnO was found to reduce the band gap, with the lowest observed value being 3.16 eV. The photocatalytic performance under simulated daylight showed that pure ZnO achieved 93.6% MB degradation within 120 min, while all the Ag/ZnO composites surpassed this, with the highest degradation rate reaching 99.8%. The proposed photocatalytic mechanism for Ag/ZnO systems is illustrated in Figure 35 [138].

The enhanced photocatalytic activity of the α-Fe₂O₃-ZnO nanocomposite (NC), compared to individual α-Fe₂O₃ and ZnO nanoparticles (NPs) for MB degradation, is attributed to the improved charge separation within the composite structure. Diffuse reflectance spectroscopy (DRS) analysis revealed a reduced band gap energy of 2.73 eV for the α-Fe₂O₃-ZnO NC, indicating a red shift relative to ZnO NPs (3.16 eV). The rate constant for MB photodegradation at extended reaction times was approximately 5.34 times greater than that observed at earlier stages. Moreover, a comparison between the mineralization rate (k ≈ 0.0192 min⁻¹; t_1/2 = 36.1 min) and the degradation rate (k = 0.0119 min⁻¹) over the same time interval suggests that the mineralization process proceeds 1.6 times faster than photodegradation, indicating rapid conversion of MB and its intermediates into inorganic end-products [139]. Graphene oxide (GO) exhibited limited photocatalytic activity, achieving 32% MB degradation within 1 h. This was significantly improved to 82% with the incorporation of ZnO, forming GO@ZnO composites. Further enhancement was observed with Fe doping, achieving up to 99% degradation. The catalytic efficiency increased with the Fe content, though the relationship was nonlinear. The optimum performance was attained at 5% Fe doping, with nearly 96% MB degradation in real river water samples. Under sunlight, the GO@ZnO catalyst showed 90% degradation within 60 min, outperforming pristine GO. When doped with 5% Fe, the composite (GO-ZnO@5Fe) reached complete degradation (100%) within the same period, confirming the critical influence of the Fe doping concentration on photocatalytic performance [140]. Diffuse reflectance UV-Vis showed band gaps of 2.7 eV (IRMOF-3) and 2.8 eV (IRMOF-3/ZnO), with enhanced dye degradation due to the dual energy transfer pathways. IRMOF-3/ZnO offers a cost-effective, visible-light-active photocatalyst. A BiOI/biocarbon composite (2:3 ratio) showed superior MB degradation over pure BiOI (62%) due to the improved charge separation via the biocarbon matrix (Figure 36) [141]. Table 3 shows a summary of metal-based composite photocatalysts used for MB degradation.

Table 3. Summary of metal-based composite photocatalysts for the degradation of MB.

Metal-Based Composites	Synthetic Method	Degradation Efficiency	Band Gap	Ref.
CoVO/WxOy	Hydrothermal method	96%	-	[109]
Cu2O-rich and Cu2O-poor	Electrodeposition	48% for Cu2O-rich and 46% Cu2O-poor	~2.0 eV	[110]
Cs3Bi2I9	Hot-injection method	98.2%	4.9 eV,	[111]
Ag2CO3-ZnCO3	Precipitation	99.88%	0.38 eV	[112]
CuO-WO3	Co-precipitation	92%	2.1 to 2.4 eV	[113]
PEDOT/Ag2SeO3	In situ synthesis	46.2%	2.41 eV	[114]
RGO photocatalyst loaded with Zn0.5Cu0.5Fe2O4	Co-precipitation	95.2%	~1.7 eV	[115]
MgFe2O4-TiO2 (MFO-TiO2	Hydrothermal approach, followed by a calcination process	99.53%	3.2 and 3.0 eV	[120]
Diatomite-TiO2 composite	Impregnation method	80%	3 eV	[124]
FeSe2–ZnO	Cost-effective chemical method	97%	1.91 eV	[116]
TiO2/Fe3O4	Sol-gel assisted method	92%	-	[117]
TiO2-F-V-Mo	Sol-gel method	0.0363%	2.52 eV	[121]
TiO2-C@N	Sol hydrothermal method	99.87%	-	[122]
kaolinite/TiO2	Sol-gel method	98%	3.14 eV	[123]
TiO2/C-550	Sol-gel method	100%	2.7 eV	[78]
B-doped g-C3N4/TiO2	Co-precipitation	-	1.5678 eV	[126]
(DM g. C3N4) TiO2	Solvothermal	97%	2.23 eV	[130]
Cu-TiO2/g-C3N4	Hydrothermal method	-	2.81 eV	[131]
Chromium-TiO2/carbon	Hydrothermal method	90%	3.2 eV	[128]
CoO/ZnO	Hydrothermal method	67.5%	3.44–3.14 eV	[132]
GO-ZnO@5Fe	Hydrothermal method	100%	2.58	[140]
2D/0D g-C3N4/TiO2	Thermal polycondensation method	100%	~ 2.7 eV	[125]
ZnO/MNC	Co-precipitation method	97.14%	-	[133]
Ag/ZnO	Hydrothermal method	99.8%.	3.16 eV	[138]
TiO2-cMDF	Carbonization method	99%	3.2 eV	[127]
TiO2/CuxO	Spin-coating technique	-	1.70 eV	[129]
Fe3O4/ZnO	Solid state method	88.5%	3.39 eV	[137]
TiO2/C-800	Calcination with a one-pot liquid phase reaction	25.1%	-	[97]
Cu/ZnO	Incipient wetness impregnation	99%	3.37 eV	[134]
g-CN/ZnO	Pyrolysis method	98%	-	[136]
α-Fe2O3-ZnO NC	Precipitation method	56.9%	2.73 eV	[139]
BiOI/C	Hydrothermal method	79.6%	~ 2.51 eV	[141]

Table 3. Summary of metal-based composite photocatalysts for the degradation of MB.

Metal-Based Composites	Synthetic Method	Degradation Efficiency	Band Gap	Ref.
CoVO/WxOy	Hydrothermal method	96%	-	[109]
Cu2O-rich and Cu2O-poor	Electrodeposition	48% for Cu2O-rich and 46% Cu2O-poor	~2.0 eV	[110]
Cs3Bi2I9	Hot-injection method	98.2%	4.9 eV,	[111]
Ag2CO3-ZnCO3	Precipitation	99.88%	0.38 eV	[112]
CuO-WO3	Co-precipitation	92%	2.1 to 2.4 eV	[113]
PEDOT/Ag2SeO3	In situ synthesis	46.2%	2.41 eV	[114]
RGO photocatalyst loaded with Zn0.5Cu0.5Fe2O4	Co-precipitation	95.2%	~1.7 eV	[115]
MgFe2O4-TiO2 (MFO-TiO2	Hydrothermal approach, followed by a calcination process	99.53%	3.2 and 3.0 eV	[120]
Diatomite-TiO2 composite	Impregnation method	80%	3 eV	[124]
FeSe2–ZnO	Cost-effective chemical method	97%	1.91 eV	[116]
TiO2/Fe3O4	Sol-gel assisted method	92%	-	[117]
TiO2-F-V-Mo	Sol-gel method	0.0363%	2.52 eV	[121]
TiO2-C@N	Sol hydrothermal method	99.87%	-	[122]
kaolinite/TiO2	Sol-gel method	98%	3.14 eV	[123]
TiO2/C-550	Sol-gel method	100%	2.7 eV	[78]
B-doped g-C3N4/TiO2	Co-precipitation	-	1.5678 eV	[126]
(DM g. C3N4) TiO2	Solvothermal	97%	2.23 eV	[130]
Cu-TiO2/g-C3N4	Hydrothermal method	-	2.81 eV	[131]
Chromium-TiO2/carbon	Hydrothermal method	90%	3.2 eV	[128]
CoO/ZnO	Hydrothermal method	67.5%	3.44–3.14 eV	[132]
GO-ZnO@5Fe	Hydrothermal method	100%	2.58	[140]
2D/0D g-C3N4/TiO2	Thermal polycondensation method	100%	~ 2.7 eV	[125]
ZnO/MNC	Co-precipitation method	97.14%	-	[133]
Ag/ZnO	Hydrothermal method	99.8%.	3.16 eV	[138]
TiO2-cMDF	Carbonization method	99%	3.2 eV	[127]
TiO2/CuxO	Spin-coating technique	-	1.70 eV	[129]
Fe3O4/ZnO	Solid state method	88.5%	3.39 eV	[137]
TiO2/C-800	Calcination with a one-pot liquid phase reaction	25.1%	-	[97]
Cu/ZnO	Incipient wetness impregnation	99%	3.37 eV	[134]
g-CN/ZnO	Pyrolysis method	98%	-	[136]
α-Fe2O3-ZnO NC	Precipitation method	56.9%	2.73 eV	[139]
BiOI/C	Hydrothermal method	79.6%	~ 2.51 eV	[141]

6. Metal–Organic-Framework-Based Composites

Metal–organic frameworks (MOFs) are porous crystalline materials composed of metal ions (e.g., Zn²⁺, Cu²⁺, Mg²⁺, Cr³⁺, Al³⁺, Fe³⁺) coordinated to organic ligands (linkers), forming 3D frameworks. Their inherent porosity allows the adsorption of external molecules, making them effective in photocatalysis and water treatment [142]. Most studies on dye degradation using MOFs (Table 4) focus on catalyst performance and dye removal under varying pH [143], catalyst/dye dosage [144], and recyclability [145,146]. HKUST-1, a well-studied Cu-based MOF, consists of Cu²⁺ paddle-wheel units coordinated with 1,3,5-benzenetricarboxylate (BTC) linkers, forming a face-centered cubic porous structure with pore sizes of 1.4, 1.1, and 0.5 nm (Figure 37a). Each Cu center is bonded to four oxygen atoms from BTC and one water molecule in the hydrated state. Post-synthetic dehydration generates coordinatively unsaturated sites, enhancing HKUST-1’s catalytic activity [147].

TiO₂ doped with Bi₂O₃ and ZnO showed significantly enhanced visible light photocatalytic activity. Bi₂O₃-ZnO/TiO₂ nanofibers achieved the highest MB degradation rate constant of 0.0234 min⁻¹, 4.97 and 16.7 times higher than ZnO/TiO₂ and pure TiO₂, respectively, due to the improved charge separation and increased active sites [148]. The photocatalytic efficiency was highly pH-dependent (Figure 37b,c), reaching 98.94% at pH 10.2 within 90 min but only 31.15% at pH 4.4 after 180 min. HKUST-1 exhibited a band gap of 3.60 eV, a crystallite size of 19 nm, a surface area of 315 m²/g, and followed first-order degradation kinetics [149]. A novel heterogeneous photosensitizer, MB@UC, was prepared by immobilizing MB onto UiO-66-(COOH)₂ (UC) MOFs, as shown in Figure 37d. The MB@UC composite efficiently generated singlet oxygen (¹O₂) under visible light, enabling selective photodegradation of the sulfur mustard simulant CEES (2-chloroethyl ethyl sulfide) into less toxic sulfoxide within 6 min, avoiding overoxidation to toxic sulfone. The half-life for CEES degradation was only 1.8 min under ambient conditions, demonstrating the strong ¹O₂ generation and high selectivity of MB@UC [150].

Figure 37. (a) Illustrates the coordination between the copper dimer and the linker, which is shown in the center, the cubic framework of HKUST-1. Reproduced with permission from [147] (copyright 2025, Elsevier). The photocatalytic degradation of MB dye using HKUST-1 is being conducted under different pH conditions: (b) acidic [149], (c) basic [149], and (d) formation of the MB@UC composite. Reproduced with permission from [150] (copyright 2025, Royal Society of Chemistry).

Figure 37. (a) Illustrates the coordination between the copper dimer and the linker, which is shown in the center, the cubic framework of HKUST-1. Reproduced with permission from [147] (copyright 2025, Elsevier). The photocatalytic degradation of MB dye using HKUST-1 is being conducted under different pH conditions: (b) acidic [149], (c) basic [149], and (d) formation of the MB@UC composite. Reproduced with permission from [150] (copyright 2025, Royal Society of Chemistry).

The OM-PE@PbBrOH⊂ZIF-67 photocatalyst demonstrated excellent water stability for up to two weeks, retaining high photocatalytic efficiency in aqueous media. Strong N–Pb interfacial coordination facilitated enhanced electron transfer, while the staggered-gap heterostructure enabled high charge mobility (3.51 × 10⁸ s⁻¹), robust redox activity, and prolonged operational stability, contributing to its superior photocatalytic performance [151]. Similarly, the Cu₂O(TC)@NH₂-MIL-125(Ti) core–shell heterostructure exhibited effective charge recombination suppression and improved visible-light-driven MB degradation. After 120 min of light exposure, the MB removal efficiencies were 38.79% for the core–shell composite versus 74.88% residual MB for Cu₂O(TC) alone, indicating higher photocatalytic efficiency and stability (Figure 38a–d). The degradation kinetics followed a pseudo-first-order model [152]. The heterojunctions between MOFs and inorganic semiconductors effectively improve photocatalytic activity and charge separation. Using a water-bath-prepared Ce-MOF as the precursor, a microwave-assisted hydrothermal method was employed to synthesize a Ce-MOF/CdIn2S4/CdS multicomponent heterojunction. The final composite consisted of central CdS spheres surrounded by rod-like Ce-MOF structures, interlaced with CdIn2S4 nanosheets (Figure 38e). With a kinetic rate constant of 0.0089 min⁻¹, the 10% Ce-MOF/CdIn2S4/CdS composite demonstrated a photocatalytic degradation efficiency of 76.7% for ciprofloxacin (CIP) under full-spectrum light in 120 min [153].

The Mo_0.09@Ni-MOF photocatalyst demonstrated superior catalytic activity compared to pristine Ni-MOF, achieving 84% MB degradation efficiency versus 68% for Ni-MOF. Mo_0.09@Ni-MOF also showed high stability and reusability over four successive cycles. The reaction followed pseudo-first-order kinetics, with rate constants of 0.01703 min⁻¹ for Ni-MOF and 0.01666 min⁻¹ for Mo@Ni-MOF, the latter indicating faster dye degradation kinetics and a strong correlation (R² = 0.95864) [154]. The Cr-PTC-HIna/TiO₂ composite exhibited a band gap of 2.02 eV, rod-shaped morphology, and a particle size of 239.16 nm. Using the Box–Behnken design, optimal MB degradation of 88.55% was achieved at 35.7 ppm MB, pH 6.6, 65.7 mg composite dosage, and 0.13% photocatalyst under 250 W mercury lamp irradiation for 2 h [155]. The CuWO₄@MIL-101(Fe) composite, with enhanced active sites and a Z-scheme heterojunction to minimize electron–hole recombination, achieved 96.92% MB degradation in 120 min. It also showed excellent OER performance, with an overpotential of 188 mV and an onset potential of 1.27 V to reach 10 mA cm⁻² current density [156]. Bimetallic MOFs were developed by doping Ni into ZIF-8 to enhance MB degradation. The Ni(20)-ZIF-8 composite achieved 93.22% degradation in 150 min using 30 mg of photocatalyst in 50 mL of 30 mg/L MB at neutral pH. This performance was 3.18 times higher than pristine ZIF-8. The composite also retained 76.43% efficiency after five cycles, confirming the excellent reusability [157]. Because of its abundant active sites before UV LED irradiation, Ni-ZIF-8 efficiently adsorbs MB in the dark (Figure 39a). In a model ship test, a hydrophobic composite coating also decreased the drag by 36% thanks to the creation of a gas cavity upon contact with water. The coating demonstrated its dual functionality for marine applications by enabling MB degradation in seawater and displaying self-cleaning qualities (Figure 39b) [158].

The photo-Fenton activity of MnMg-MOF heterojunctions was evaluated for MB degradation under simulated sunlight. Using 0.3 g/L catalyst and 10 mg/L H₂O₂, an 87.79% decolorization rate was achieved after 180 min, with a TOC removal rate of 88% after three cycles (Figure 40a,b) [159]. Ti₃C₂ incorporation at 1.5 wt% enhanced the photocatalytic activity of UNiMOF fourfold due to its excellent conductivity and layered structure, which promoted effective charge separation [160]. ZIF-8/Ti₃C₂Tx, synthesized via in situ growth (Figure 40c), exhibited superior MB adsorption capacity (107 mg/g) compared to Ti₃C₂Tx (9 mg/g) and ZIF-8 (3 mg/g), attributed to synergistic effects and optimized interlayer spacing (Figure 40d) [161]. Additionally, the UiO-66/MXene composite demonstrated a maximum MB adsorption capacity of 312 mg/g, achieving over 97% removal with just 0.02 g of sorbent from a 20 mg/L MB solution [161]. Copper mixed-triazolate MOFs (CuMtz-3a and CuMtz-3b), synthesized using mixed ligands (3,5-diphenyl-1H-1,2,4-triazole and 1H-1,2,4-triazole), demonstrated superior MB adsorption performance compared to the mono-ligand MOF (CuTz-1), as shown in Figure 41a. CuMtz-3b exhibited an adsorption capacity of 152 mg/g, 182% higher than CuTz-1, and a notable photocatalytic rate constant of 0.031 min⁻¹ for MB degradation. This enhanced activity was attributed to improved visible light absorption and efficient charge separation. The photocatalyst maintained high degradation efficiency and structural stability over four cycles, indicating excellent reusability [162].

UIO-66 and hydroxyl-modified UIO-66, referred to as UIO-66-2OH (2,3), were synthesized via a solvothermal method by Li et al. BET-BJH analysis revealed surface areas of 1050 m²/g for UIO-66 and 570 m²/g for UIO-66-2OH (2,3). The band gap of UIO-66-2OH (2,3) was determined to be 2.6 eV. Under visible light, UIO-66-2OH (2,3) achieved complete degradation of MB within 100 min and maintained 99.5% photocatalytic activity after five cycles [164]. Under simulated sunlight, 10 mg of MOF/CdS-6 (mass ratio of MOF to thioacetamide 6:1) achieved 91.9% MB degradation in 100 min, outperforming pure Cd-MOF (62.3%) and CdS (67.5%). This improvement is attributed to the composite’s larger surface area, increased active sites, and extended visible light response due to CdS incorporation. Mott–Schottky analysis confirmed the formation of a type-II heterojunction between Cd-MOF and CdS, which reduced electron–hole recombination. The photocurrent intensity of MOF/CdS-6 was 8× and 2.5× higher than that of Cd-MOF and CdS, respectively, with excellent stability over five cycles [165]. A photocatalytic degradation rate of 95.87% was observed in a ZIF-8/ZnO composite (1:2 mass ratio) after 40 min of UV exposure. A sponge-like porous C-ZnO/PVDF membrane was fabricated by incorporating C-ZnO into PVDF using extended conversion technology (Figure 41b). At 15 wt% C-ZnO loading, optimal performance was recorded, with 95.01% degradation in 270 min, 83.94% porosity, and a pure water flux of 280.56 L m⁻²·h⁻¹, showing high potential for wastewater treatment applications [166]. MOF/polymer composites also demonstrated high MB removal efficiencies 96 96% after 30–45 min of UV irradiation using 2% MIL-53(Cr)/polymer and 2% HKUST-1(Cu)/polymer. These catalysts remained effective over 10 cycles, with reduced activity after the seventh cycle. However, their photocatalytic activity was limited to UV light (λ < 365 nm), highlighting the need for MOFs responsive to visible light for broader photocatalytic applications [167,168].

Figure 41. (a) Preparation of CuMtz and Cutz-1 MOFs T. Reproduced with permission from [162]. (b) The synthesis of ZIF-8/ZnO/PVDF. Reproduced with permission from [166] (copyright Elsevier).

Figure 41. (a) Preparation of CuMtz and Cutz-1 MOFs T. Reproduced with permission from [162]. (b) The synthesis of ZIF-8/ZnO/PVDF. Reproduced with permission from [166] (copyright Elsevier).

The formation of a heterojunction between the incorporated photocatalytic degradation of MB demonstrated that TiO₂/Al₂O₃@Cu(BDC) achieves superior degradation ae compared to other compounds, and the recombination of photogenerated electron–hole pairs has been reduced. Under visible light illumination, photogenerated electrons are transferred from the conduction bands of Cu(BDC) and Al₂O₃ to the conduction band of TiO₂, resulting in the presence of positive holes (h⁺) in the valence bands of Cu(BDC) and Al₂O₃. The photocatalytic degradation of MB has demonstrated that TiO₂/Al₂O₃@Cu(BDC) achieves superior degradation when compared to other compounds [169]. Porous cobalt-based metal–organic frameworks (Co-MOFs) were synthesized and functionalized with amino groups using 1,4-diazabicyclo [2.2.2]octane (DABCO) as a linker. These Co-MOFs were further encapsulated with photosensitive dyes, MB and methyl orange (MO), to form MB@Co-MOF and MO@Co-MOF composites. Their photocatalytic activities under visible light were evaluated for the degradation of representative azo dyes, with the results compared to unmodified Co-MOF. The photodegradation efficiencies for Eriochrome Black T (EBT) reached 78.9%, 92.0%, and 99.7% using Co-MOF, MO@Co-MOF, and MB@Co-MOF, respectively, at 100 mg/L concentration. The corresponding rate constants (K₁) were 0.02, 0.05, and 0.08 min⁻¹ over 240 min of visible light exposure [170]. Morphological analysis confirmed uniform CuO dispersion on the CuCr₂O₄ matrix. The CuCr₂O₄/CuO nanocomposite achieved ~90% MB degradation in the presence of H₂O₂, with complete degradation within 35 min [171].

BET analysis showed high surface areas of 1322 m²/g for ZIF-8 and ~1507 m²/g for NDCQDs/ZIF-8. The incorporation of nitrogen-doped carbon quantum dots (NDCQDs) improved dye adsorption and enhanced photocatalytic efficiency under visible light (Figure 42a). This improvement was attributed to the synergistic effects of ZnO in ZIF-8, the up-conversion fluorescence of NDCQDs, and efficient charge separation. Scavenging experiments identified hydroxyl radicals (^•OH) as the primary reactive species. The MB degradation followed a pseudo-first-order kinetic model, with rate constants of 0.00559 min⁻¹ for ZIF-8 and 0.0103 min⁻¹ for NDCQDs/ZIF-8 (Figure 42b,c) [172].

The CoFe₂O₄/SiO₂/Cu core was functionalized with glutaric anhydride and 3-(triethoxysilyl)propylamine, followed by in situ self-assembly of Cu-MOF nanostructures. The composite exhibited enhanced MB degradation (98% within 30 min) at alkaline pH (pH = 10), with degradation kinetics best described by a pseudo-second-order model. The high activity was attributed to the synergistic effects of multiple transition metals and the mesoporous structure [173]. Mesoporous Fe(25)ZnO, containing 25 wt% α-Fe₂O₃, achieved nearly complete MB degradation after 150 min of irradiation, outperforming pristine ZnO and α-Fe₂O₃. It maintained 95.42% efficiency after three reuse cycles, attributed to the formation of an n–n heterojunction enhancing charge separation [174]. The MOF-5/GO₁₀ composite demonstrated 92% MB removal efficiency and followed pseudo-first-order kinetics, with rate constants of 0.0369 and 0.0396 min⁻¹ for MOF-5/GO₅ and MOF-5/GO₁₀, respectively (Figure 43) [175].

Table 4. Summary of the photocatalytic performance of various catalysts for MB dye degradation under different experimental conditions, highlighting the effects of the pH, catalyst/dye dosage, and recyclability.

MOF-Based Composites	Synthetic Method	Degradation Efficiency	Band Gap	Ref.
Bi2O3-ZnO/TiO2 MOF	Electrospinning	98%	2.9 eV	[148]
HKUST-1, a Cu-based MOF	Solvothermal method	98.94%	3.60 eV	[149]
MB-modified UiO-66-(COOH)2 MOF	Adsorption method	-	-	[150]
OM-PE@PbBrOH⊂ZIF-67	Solvothermal method	72%	0.4 eV	[151]
Cu2O(TC)@NH2-MIL-125(Ti)	Solvothermal method	74.88%	1.69 eV	[152]
Ce-MOF/CdIn2S4/CdS	Hydrothermal method	76.7%	2.23 eV	[153]
Mo0.09@Ni-MOF	Solvothermal method	84%	1.83 eV	[154]
Cr-PTC single bond HIna/TiO2	Solvothermal method	88.55%	2.02 eV	[155]
CuWO4@MIL-101(Fe)	Solvothermal method	96.92%	-	[156]
Ni(20)-ZIF-8	One-step room-temperature method	93.22%	4.40 eV	[157]
ZIF-8/POTS	Superhydrophobic method	93.85%	4.9 eV	[158]
MnMg-MOF	One-step cationic membrane electro-conversion	88%	0.37 eV	[159]
UNiMOF/Ti3C2	Electrostatic self-assembly	99.49%	3.43 eV	[160]
ZIF-8/Ti3C2Tx	In situ growth	-	-	[163]
UiO-66/MXene	Solvothermal method	>97%	-	[161]
CuTz-1	One-pot economic synthesis	73%	1.72 eV	[162]
UIO-66-2OH (2,3)	Solvothermal method	99.5%	2.6 eV	[164]
Cd-MOF/CdS	In situ sulfurization	91.9%	2.9 eV	[165]
ZIF-8/ZnO	Phase inversion method	95.01%	3.40 eV	[166]
TiO2/Al2O3@Cu(BDC)	In situ incorporation of pre-synthesized precursors	-	3.29 eV	[169]
MO@Co-MOF	Solvothermal method	99.7%	1.71 eV	[170]
NDCQDs/ZIF-8	Hydrothermal method	28%	4.961 eV	[172]
CoFe2O4/SiO2/Cu-MOF	Sol-gel method	98%	-	[173]
MOF-derived α-Fe2O3/ZnO	Calcination	100%	2.34 eV	[174]
MOF-5/GO	Hummer’s method	92%	3.5 eV	[175]

Table 4. Summary of the photocatalytic performance of various catalysts for MB dye degradation under different experimental conditions, highlighting the effects of the pH, catalyst/dye dosage, and recyclability.

MOF-Based Composites	Synthetic Method	Degradation Efficiency	Band Gap	Ref.
Bi2O3-ZnO/TiO2 MOF	Electrospinning	98%	2.9 eV	[148]
HKUST-1, a Cu-based MOF	Solvothermal method	98.94%	3.60 eV	[149]
MB-modified UiO-66-(COOH)2 MOF	Adsorption method	-	-	[150]
OM-PE@PbBrOH⊂ZIF-67	Solvothermal method	72%	0.4 eV	[151]
Cu2O(TC)@NH2-MIL-125(Ti)	Solvothermal method	74.88%	1.69 eV	[152]
Ce-MOF/CdIn2S4/CdS	Hydrothermal method	76.7%	2.23 eV	[153]
Mo0.09@Ni-MOF	Solvothermal method	84%	1.83 eV	[154]
Cr-PTC single bond HIna/TiO2	Solvothermal method	88.55%	2.02 eV	[155]
CuWO4@MIL-101(Fe)	Solvothermal method	96.92%	-	[156]
Ni(20)-ZIF-8	One-step room-temperature method	93.22%	4.40 eV	[157]
ZIF-8/POTS	Superhydrophobic method	93.85%	4.9 eV	[158]
MnMg-MOF	One-step cationic membrane electro-conversion	88%	0.37 eV	[159]
UNiMOF/Ti3C2	Electrostatic self-assembly	99.49%	3.43 eV	[160]
ZIF-8/Ti3C2Tx	In situ growth	-	-	[163]
UiO-66/MXene	Solvothermal method	>97%	-	[161]
CuTz-1	One-pot economic synthesis	73%	1.72 eV	[162]
UIO-66-2OH (2,3)	Solvothermal method	99.5%	2.6 eV	[164]
Cd-MOF/CdS	In situ sulfurization	91.9%	2.9 eV	[165]
ZIF-8/ZnO	Phase inversion method	95.01%	3.40 eV	[166]
TiO2/Al2O3@Cu(BDC)	In situ incorporation of pre-synthesized precursors	-	3.29 eV	[169]
MO@Co-MOF	Solvothermal method	99.7%	1.71 eV	[170]
NDCQDs/ZIF-8	Hydrothermal method	28%	4.961 eV	[172]
CoFe2O4/SiO2/Cu-MOF	Sol-gel method	98%	-	[173]
MOF-derived α-Fe2O3/ZnO	Calcination	100%	2.34 eV	[174]
MOF-5/GO	Hummer’s method	92%	3.5 eV	[175]

6.1. Recovery and Reusability of Photocatalysts

Methylene blue (MB) photodegradation with different catalysts has been thoroughly investigated, with an emphasis on catalyst efficiency and recovery. Utilizing magnetic nanoparticles and nanocomposites has demonstrated promising outcomes in terms of enhancing the degradation process and facilitating catalyst recovery. The Fe₃O₄@SiO₂@ZnMn₂O₄@CuIITHPP nanocomposite showed high photocatalytic activity by degrading MB by 98% in 180 min when exposed to visible–LED light. An external magnetic field made it easy to recover the catalyst, demonstrating its usefulness in wastewater treatment [176,177]. Methylene blue degradation is effectively catalyzed by the Fe₃O₄@SiO₂@CeO₂ core–shell magnetic nanostructure. An external magnetic field makes it simple to recover the magnetic catalyst following the photocatalytic process. Even after five uses, the catalyst showed over 92% efficiency in removing methylene blue under ideal conditions, suggesting that it could be used repeatedly in wastewater treatment. The study emphasizes how well the catalyst works with photocatalysis to break down dye pollutants [176]. The ZnO/Fe₃O₄/SiO₂ hybrid microbeads are a powerful photocatalyst for methylene blue degradation. After the degradation process, the microbeads can be fully recovered and reused because they contain magnetic nanoparticles. This characteristic makes the catalyst more useful because it makes it simple to remove it from the solution after the reaction. The study showed that these microbeads outperformed conventional ZnO powder in completely degrading methylene blue when exposed to UV light [178]. The effectiveness and reusability of the nanocomposite were demonstrated in the study, which showed that it achieves over 90% photocatalytic efficiency for MB degradation over five cycles. Both UV and sunlight radiation can cause degradation, but UV light accelerates the process. For the removal of MB from aqueous solutions, the Ru/Fe₃O₄ nanocomposite is therefore a promising catalyst [179].

6.2. Mineralization and Byproduct Formation

TOC analysis is crucial for evaluating the degree of mineralization in photocatalytic processes. Using gadolinium-oxide-decorated multi-walled carbon nanotube/titania nanocomposites, the mineralization of methylene blue (MB) during photocatalytic degradation was evaluated by total organic carbon (TOC) analysis. After five hours, the study found that 80.0% of the TOC had been removed, indicating that MB had effectively broken down into inorganic species and minimized harmful byproducts. Comparing the photocatalytic activity to neat composites and commercial titania, the improved charge separation and electron transfer made possible by the gadolinium oxide nanoparticles resulted in higher degradation efficiencies [180]. Using ZrO₂/MWCNT nanocomposites, the study concentrated on the photocatalytic degradation of methylene blue. Analysis of the total organic carbon (TOC) revealed a notable decrease from 148.00 mg/L to 1.26 mg/L, suggesting efficient degradation. After 25–60 min of UV irradiation, the ZrO₂/MWCNT catalyst showed high efficiency, achieving 90–94% degradation. This demonstrates the potential of the produced nanocomposite for photocatalytic treatment of industrial wastewater contaminated with dyes, such as methylene blue [181]. Au-TiO₂ catalysts were used to assess the photocatalytic degradation of methylene blue (MB) using total organic carbon (TOC) analysis. UV-Vis spectroscopy and TOC measurements were used to track the degradation process, and the results showed that MB had been converted by more than 80%. The findings indicated that different organic intermediate products were formed during the photocatalytic degradation, which followed a first-order kinetic model. The study demonstrated how well photocatalysis works to mineralize organic pollutants into non-toxic byproducts [182].

6.3. Challenges and Limitations

While photocatalysis is showing promising results in the degradation of dyes in lab settings, a number of real-world obstacles limit its widespread use. Salts, naturally occurring organic matter, and competing pollutants frequently reduce photocatalytic efficiency and even deactivate the catalyst surface in actual wastewater, which lowers the stability and reusability over the long term [183,184]. Additionally, uneven light penetration, problems with catalyst recovery, and the requirement for optimized reactor designs make it challenging to scale laboratory results to pilot or industrial scales [185,186]. Finally, because the production of advanced nanomaterials and the energy requirements of UV–visible light sources raise operating costs and make widespread adoption difficult, the economic viability of photocatalysis is a significant concern [187,188].

The structural and electronic characteristics of CuS nanostructures are examined in a study by Sudhaik and his colleague, which also emphasizes the difficulties in maintaining their stability when exposed to extended light. The authors point out that extended exposure may cause CuS to decompose, which would impair its photocatalytic activity [189]. Similarly, Zhang and his colleagues stress that because of their broadband absorption characteristics, binary metal sulfides, such as CdS, can only absorb ultraviolet light for photocatalytic reactions. They add that these sulfides may change structurally when exposed to light, which would reduce their photocatalytic effectiveness [190].

Due to their large band gaps, the most extensively researched photocatalysts, including TiO₂, ZnO, and CeO₂, are only active in UV light, which makes up less than 5% of the solar spectrum. Additionally, the production of reactive oxygen species is greatly decreased by rapid electron–hole recombination, which lowers the degradation efficiency [191]. Since laboratory research is frequently carried out in simplified systems, actual industrial wastewater contains natural organic matter and inorganic ions (such as carbonates and chlorides) that scavenge reactive radicals or compete for active sites. In real-world situations, these interferences decrease the effectiveness of methylene blue degradation. As the majority of the research literature assesses catalyst performance in controlled environments with ideal pH, dye concentration, and light intensity, these settings rarely accurately represent the makeup of actual wastewater. The complex mixture of ions, suspended solids, and other organic and inorganic components found in actual wastewater can affect the kinetics of pollutant degradation and catalyst behavior [192].

Consequently, factors like the removal efficiency, degradation duration, ideal pH, and catalyst dosage that are determined in laboratory systems might not be directly applicable to wastewater applications in practical wastewater applications [193]. Depending on the catalyst type and ion concentration, the presence of inorganic anions (such as bicarbonate, sulfate, and chloride) has demonstrated both inhibitory and enhancing effects on dye degradation [26]. Synthetic setup studies rarely capture the interactions between the numerous concurrent organic and inorganic pollutants found in wastewater. These results show that to close the gap between laboratory research and field applications, catalyst performance evaluation in realistic matrices is crucial [194,195].

7. Conclusions

This review highlights the remarkable progress made in the development of composite photocatalysts for the degradation of MB dye using photocatalytic phenomena. It covers mainly carbon-based, metal-based based and MOF-based composites and some bio-derived nanocomposites for high MB degradation. The photocatalytic efficiency, in some cases, exceeds 99% within 30–90 min. The photo-composites such as ZnO@g-C₃N₄, TiO₂/C-550, and Ag/TiO₂/CNT showed superior activity due to enhanced light harvesting, reduced electron–hole recombination, and high surface area. Notably, Z-scheme and S-scheme architectures proved particularly effective by maintaining strong redox potential while promoting charge carrier separation. Additionally, certain composites like RGO-TiO_2−x and NiO/g-C₃N₄ exhibited long-term stability, maintaining over 90% efficiency across multiple reuse cycles. Despite these advancements, several challenges remain. Many catalysts demonstrate limited performance under real wastewater conditions due to the presence of competing ions, turbidity, or fouling agents. The reproducibility of results across large-scale systems is still uncertain, and the synthesis of some high-performance catalysts involves expensive precursors or complex fabrication steps. Addressing these limitations requires a shift toward more sustainable, cost-effective, and scalable catalyst designs, supported by comprehensive mechanistic studies

Future Perspectives

To bridge the gap between laboratory success and industrial-scale application, several avenues must be pursued. The use of artificial intelligence and machine learning to model degradation kinetics and predict photocatalyst behavior under varying conditions represents an emerging frontier. A shift toward low-cost, earth-abundant, and environmentally benign materials is also essential to meet industrial and sustainability demands. Importantly, organocatalysts offer a promising new direction in this field. Organic acids and small molecules, acting as sustainable catalytic agents, can facilitate degradation of organic pollutants through photooxidative or redox pathways without relying on heavy metals or expensive semiconductors. Future developments of sustainable photocatalytic technologies will be guided by the practical relevance of mechanistic insights combined with pilot-scale studies and industrial case studies. Future research should assess the stability and long-term performance of photocatalysts in continuous-flow reactors, as the majority of current studies are based on batch experiments. These investigations will shed light on the catalyst durability, sustained degradation efficiency, and usefulness for treating industrial wastewater.