Recent Advances in PDI-Based Heterojunction Photocatalysts for the Degradation of Organic Pollutants and Environmental Remediation

Abstract

With the rapid advancement of industrialization, the adverse impacts of organic pollutants on the water environment of aquatic ecosystems have become increasingly concerning. Consequently, the development of efficient and environmentally friendly photocatalytic degradation technologies has attracted considerable research attention. Perylene diimide (PDI)-based heterojunction photocatalysts have demonstrated remarkable potential in degrading organic pollutants, attributed to their broad spectral response, high charge separation efficiency, and exceptional stability. In recent years, substantial progress has been achieved in the field of PDI-based heterojunction photocatalysts. This paper provides an in-depth review of the existing research on PDI-based heterojunction photocatalysts. Specifically, it elucidates the principles and types of heterojunction construction, as well as the design and synthesis strategies for PDI-based heterojunction photocatalysts. Furthermore, this paper provides a comprehensive summary of the latest advancements in performance optimization and catalytic mechanisms. Finally, the existing challenges and future prospects of PDI-based heterojunction photocatalytic materials are discussed, with the aim of offering innovative solutions for the purification of resource-oriented wastewater.

1. Introduction

The rapid advancement of industrialization has resulted in the discharge of wastewater into natural waterways, thereby causing significant ecological and environmental damage [1,2,3,4,5]. These water bodies are often contaminated with a broad spectrum of organic pollutants, including dyes, pesticides, and pharmaceutical residues [6,7,8]. To mitigate the adverse effects of wastewater, numerous methods for removing organic pollutants from water have been developed. These methodologies can be broadly classified into physical, chemical, and biological approaches. Traditional dye removal techniques encompass physical methods (e.g., filtration and sedimentation), chemical methods (e.g., redox reactions and adsorption), and biological methods (e.g., microbial degradation). From the standpoint of wastewater treatment (such as adsorption or chemical precipitation), these conventional methods exhibit limitations in terms of their efficacy, cost-effectiveness, and environmental impact. Consequently, there is an increasing demand for environmentally friendly alternative technologies capable of effectively degrading organic pollutants [9,10]. Photocatalysis, as a highly promising “green” technology, demonstrates considerable potential for environmental remediation [11,12]. Semiconductor-based photocatalysis offers advantages such as high photosensitivity, environmental compatibility, and relatively low cost [13,14,15,16]. Photocatalysts can directly degrade organic pollutants into secondary products or mineralize them into carbon dioxide and water via the generation of reactive oxygen species (ROS), thereby enabling the treatment of refractory organic compounds [17,18]. However, most semiconductor photocatalysts encounter bottleneck issues, such as low separation efficiency of photogenerated carriers, limited surface area, narrow absorption spectrum, and inadequate cycling stability, which significantly impede their photocatalytic activity and practical application [19,20,21,22,23]. Therefore, designing and developing novel broad-spectrum photocatalysts plays a crucial role in promoting sustainable societal development.

2. Principles and Types of Heterojunctions

Photocatalytic heterojunctions are considered a highly effective and promising strategy for enhancing the separation efficiency of photogenerated charges and improving the oxidation capacity of holes [24,25,26]. In recent years, the development of low-cost heterojunctions with high photogenerated charge separation efficiency has become a significant trend in the field. Efficient charge separation is critically promoted by photocatalysts featuring heterojunction structures, thereby enabling their extensive application in wastewater treatment and related areas [27,28,29,30]. Semiconductor heterojunctions are constructed by combining two semiconductors with appropriately aligned conduction band (CB) and valence band (VB) potentials. This approach effectively accelerates the transfer and separation of photogenerated carriers, while simultaneously prolonging their lifetimes [31,32,33,34].

Researchers categorize various types of heterojunctions, including type-II, Z-scheme, and S-scheme heterojunctions, based on the relative positions of their band gaps [35,36,37]. The enhanced photocatalytic performance, characterized by a narrowed band gap, improved separation of photoinduced charge carriers, and reduced recombination rate, is attributed to the synergistic interaction between semiconductors [38,39]. However, in type-II heterojunctions, interfacial charge transfer is hindered by the inherent Coulomb attraction and electrostatic repulsion at the interface, leading to limitations in subsequent redox reactions and a negative impact on photocatalytic performance [40,41]. Compared to traditional heterojunctions, the separation of electron (e⁻)–hole (h⁺) pairs within different semiconductors can be effectively promoted by Z-scheme heterojunctions [42,43]. Nevertheless, issues such as the inefficient photogenerated charge transfer, weak redox capabilities of carriers, and inadequate light absorption characteristics have been identified as factors that impede the improvement of photocatalytic performance in this system [44]. Additionally, due to thermodynamic and kinetic limitations, the efficiency of traditional Z-scheme heterojunction photocatalysts is significantly compromised [45,46,47,48].

To address these challenges, the S-scheme (step-type) heterojunction theory was first proposed by Yu et al. based on direct Z-scheme heterojunctions, aiming to resolve the ambiguities between traditional type-II and Z-scheme heterojunctions and overcome their inherent limitations [49]. Typically, an S-scheme heterojunction is formed by coupling two semiconductors—one with a lower Fermi level functioning as an oxidative photocatalyst and the other with a higher Fermi level acting as a reductive photocatalyst [50,51,52,53]. In S-scheme heterojunctions, interfacial charge transfer is facilitated by the built-in electric field between semiconductors and the band bending at the interface, leading to a significant enhancement in photocatalytic activity [54,55].

3. Design and Construction of PDI-Based Heterojunction Photocatalysts

3.1. Properties of PDI

PDI is a derivative of polycyclic aromatic hydrocarbons [56,57], characterized by a molecular structure with a central perylene core connected to two imide groups (-CONHCO-) on either side [58]. PDI is known as a deep red organic dye [59] that is inherently non-toxic [60,61]. In its natural form, PDI exhibits a relatively low specific surface area; however, this limitation can be overcome through self-assembly processes, forming structures such as supramolecular nanofibers, layered assemblies, and three-dimensional frameworks [62,63,64]. Despite its poor water solubility caused by π–π stacking interactions between perylene units [65,66], researchers have successfully enhanced its solubility by introducing hydrophilic groups at specific positions, including the bay, imide, or ortho regions. For instance, ionic groups such as cationic ammonium salts [67,68,69], anionic carboxylic acids [70], sulfonic acids [71], and phosphonic acids [72] have been directly incorporated into the perylene core to produce water-soluble PDI derivatives. Additionally, nonionic substituents with multiple polar groups, such as polyethylene glycol (PEG), polyglycerol (PG) dendrimers, and dendritic carbohydrate derivatives, have been attached to the bay or imide sites of PDI [73]. The bandgap energy (E_g) of PDI typically ranges from approximately 1.5 to 2.5 eV, enabling it to absorb visible light with wavelengths up to around 750 nm [74]. For instance, Li et al. [75] reported that the valence band potential of PDI is +1.81 eV (vs. NHE), while the conduction band potential is –0.36 eV (vs. NHE), which meets the requirements for simultaneously photocatalytically decomposing water into hydrogen and oxygen, reducing carbon dioxide, and removing pollutants. PDI and its derivatives, as representative n-type semiconductors, exhibit superior electronic and optical properties and have been extensively investigated in the fields of solar cells and sensors [76,77]. Self-assembled supramolecular materials based on PDI demonstrate significant potential in photocatalytic applications owing to their exceptional charge transport characteristics, remarkable photothermal stability, and strong electron affinity, and have been widely applied in the fields of pollutant degradation, hydrogen evolution, oxygen evolution, and carbon dioxide reduction, among others [78,79,80]. Nevertheless, the practical application of PDI is hindered by issues such as the rapid recombination of photogenerated carriers, poor solubility, and insufficient adsorption capacity [81].

3.2. The Role of PDI in Heterojunctions and Construction Strategies

3.2.1. The Role of PDI in Heterojunctions

At present, the photocatalytic modification of perylene diimide primarily focuses on structural regulation and the development of composite materials [82,83,84]. Among these approaches, the construction of composite materials leverages the inherent advantages of PDI while mitigating its limitations, such as self-aggregation, thereby significantly enhancing the photocatalytic performance of the catalyst. The formation of heterojunctions is widely recognized as an effective strategy to improve the separation efficiency of photogenerated carriers [85,86]. In recent years, researchers have successfully developed heterojunction-based photocatalysts by combining PDI with other semiconductors, including rGO/PDI [87], Bi₂WO₆/PDI [88], PDI/g-C₃N₄ [89], and PANI/PDI [90]. These composites have demonstrated superior photocatalytic activity. The integration of PDI into heterojunction structures with other catalysts represents a highly promising approach to enhance photocatalytic performance, improve light responsiveness, and accelerate the transfer and separation of charge carriers.

3.2.2. The Interface Coupling of PDI Functional Groups with Heterogeneous Materials

PDI molecules can form coordination bonds with metal active sites on the surface of metal oxides. For example, Wang et al. [91] successfully connected PDI to MIL-125(Ti)-NH₂ heterojunctions using an acid-catalyzed method based on the blocked Lewis pair strategy. In this process, the amino groups in MIL-125(Ti)-NH₂ acted as nano-scaffolds via amidation reactions, enabling their interaction with PDI. Specifically, the unsaturated Ti-O clusters functioned as Lewis acid sites to accept photogenerated electrons, while PDI served as Lewis base sites for capturing these electrons. Furthermore, the edge substituents of PDI can establish stable coupling structures with other materials through covalent or hydrogen bonding. As an illustration, Chen et al. [92] introduced carboxyl-terminated organic supramolecular PDI during the synthesis of MIL-53(Fe) crystals, thereby constructing a PDI/MIL-53(Fe) (PM) Z-scheme heterojunction. In this structure, PDI nanofibers were uniformly dispersed and firmly anchored within MIL-53(Fe) via covalent bonds. In addition, composite materials can be prepared by imidization reactions involving the PDI precursor, perylene tetracarboxylic dianhydride (PTCDA). For instance, Wang et al. [93] fabricated a PDI/g-C₃N₄ heterojunction through a one-step imidization reaction between PTCDA and g-C₃N₄. During this process, PTCDA and g-C₃N₄ formed a hybrid structure via O=C-N-C=O covalent bonds at the heterojunction interface, significantly enhancing the material’s interfacial interactions and charge transport properties. In addition, PDI can form heterojunctions with other materials through π–π stacking interactions, thereby enhancing interfacial charge transfer and material performance. PDI molecules can self-assemble into supramolecular materials through π–π stacking interactions. Larger π–π stacking enhances π–electron delocalization and increases the overlap of π–electron clouds, which is beneficial for carrier migration and separation [94]. Based on the π–π stacking interactions in PDI supramolecular materials, researchers have attempted to combine PDI with other π–conjugated organic materials to construct advanced π–π composite systems. Leveraging the π–π stacking interactions between PDI molecules and organic materials, highly stable self-assembled composite materials can be obtained [87]. For instance, Dai et al. [90] constructed a heterojunction with well-matched energy levels through π–π interactions between PANI and PDI. This heterojunction exhibits enhanced π conjugation and stronger electron delocalization, which have been proven to facilitate carrier migration and separation. As a result, the photocatalytic performance and stability of the material were significantly improved.

3.2.3. Preparation Methods for PDI-Based Composite Photocatalysts

Various methods are currently employed for the preparation of PDI-based photocatalysts, including electrostatic adsorption, π–π stacking self-assembly, water bath heating, hydrothermal synthesis, and electrochemical approaches. The primary objective of optimizing these preparation methods is to fine-tune the band structure, particle size, specific surface area, morphology, and crystal planes of the photocatalyst, thereby improving its overall photocatalytic performance.

Electrostatic Adsorption Method: Miao et al. [95] successfully fabricated PDI@AuNPs (nanogold rod) composite materials via the electrostatic adsorption method. The visible light-induced degradation rate of phenol by PDI@AuNPs is 1.7 times higher than that of PDI nanowires. This enhanced photocatalytic activity of PDI@AuNPs can be attributed to the surface plasmon resonance (SPR) effect of AuNPs. Specifically, the SPR effect of AuNPs and the resonance energy transfer between AuNPs and PDI facilitate the utilization of visible light. Moreover, the lower Fermi level of Au quantum dots promotes the efficient transfer of photogenerated carriers from PDI to AuNPs. Yang et al. [96] synthesized GQD (carbon quantum dot)/PDI photocatalysts through electrostatic interactions. Under visible light irradiation, the rate constant for phenol photocatalytic degradation by GQD/PDI-14% (0.018 min⁻¹) was 4.73 times higher than that of nano-PDI. The superior photocatalytic activity of GQD/PDI is primarily due to the π–π interactions between GQDs and PDI, which enhance electron delocalization. Additionally, the quantum confinement effect of GQDs facilitates electron transfer from GQDs to PDI, shifting the conduction band of PDI to a more negative position and thereby enhancing its reduction capability. Wang et al. [97] developed an efficient Co-NG/PDI photocatalyst using an in situ electrostatic adsorption method. The Co-NG/PDI composite exhibited superior photocatalytic degradation performance for bisphenol A compared to pure Co-NG and self-assembled PDI. Notably, the 7% Co-NG/PDI composite demonstrated the best photocatalytic performance, with an apparent rate constant 6.64 times higher than self-assembled PDI and 7.6 times higher than Co-NG.

π–π Stacking Self-Assembly Method: Wei et al. [98] prepared PTCDI–C60 composite materials by self-assembling PTCDI (perylene diimide) and C60 (carbon 60) through π–π stacking interactions, enabling rapid transfer of photogenerated carriers. The π–π interactions effectively lowered the valence band position and narrowed the band gap, thereby enhancing redox capabilities and broad-spectrum responsiveness. The PTCDI–C60 organic photocatalyst achieved a phenol degradation rate constant of 0.216 h⁻¹, which is 8.24 times higher than that of PTCDI alone. Furthermore, the presence of C60 improved the stability of the composite material and reduced the accumulation of negative charges. Yang et al. [87] constructed 3D(three-dimensional) rGO/PDI composite materials via a simple water bath heating method. Self-assembled PDI nanofibers and rGO were effectively combined through non-covalent π–π stacking interactions. These π–π interactions enhanced long-range π–electron delocalization and electronic coupling effects, significantly improving carrier mobility and the separation efficiency of photogenerated electron–hole pairs. As a result, the 3D rGO/PDI composite material exhibited superior photocatalytic activity compared to pure PDI.

Water Bath Heating Method: Zhang et al. [88] successfully fabricated a Bi₂WO₆/PDI composite material by combining a Bi₂WO₆ photocatalyst with PDI through the water bath heating method. This composite material was utilized in the photocatalytic oxidation of phenol under simulated visible light irradiation. The results demonstrated that, under visible light irradiation, the degradation efficiency of phenol by the Bi₂WO₆/PDI composite material was significantly higher than that of the PDI self-assembled body and Bi₂WO₆ alone. In the photocatalytic process of the Bi₂WO₆/PDI composite material under visible light irradiation, the energy level alignment between the conduction and valence bands of Bi₂WO₆ and PDI facilitated the effective separation of photogenerated carriers.

Hydrothermal Method: Zhang et al. [99] synthesized PDI/TiO₂ photocatalysts via the hydrothermal method. The effects of introducing different structural forms of PDI into TiO₂ were evaluated by measuring the photocatalytic degradation rate of MB (methylene blue). The photocatalytic activities of the PDI-1/TiO₂ and PDI-2/TiO₂ catalysts were superior to that of PDI-3/TiO₂. This was attributed to the large surface areas of PDI-1 nanorods and PDI-2 nanobelts, which expanded one-dimensional photogenerated carrier channels, thereby promoting electron transfer to the TiO₂ surface and enhancing the photocatalytic activity of the composite material. Notably, the PDI-1/TiO₂ composite material exhibited the highest photocatalytic activity, maintaining 86.4% of its initial activity after four repeated uses. The extended π–π stacking of self-assembled PDI-1 and the strong interaction between self-assembled PDI-1 and TiO₂ played a critical role in accelerating charge transfer and reducing the recombination of photogenerated electron–hole pairs.

Electrochemical Co-deposition Method: Sheng et al. [100] prepared PDI/rGO composite materials using a simple electrodeposition–impregnation method. The removal rate of MB by the PDI/rGO catalyst in photoelectrocatalysis was 8.2 times higher than that of single photocatalysis and 4.5 times higher than that of single electrocatalysis. In dynamic photocatalytic degradation experiments, the degradation rate of MB by the PDI/rGO catalyst reached 88.5%, significantly surpassing that of PDI alone (56.3%). The PDI/rGO composite material exhibited exceptional stability, with no noticeable decrease in activity over 55 h of continuous photocatalytic degradation.

4. Application of PDI-Based Heterojunctions in Pollutant Degradation

4.1. Antibiotics and Drug Residues

The long-term environmental impacts of antibiotics and drug residues on water systems have emerged as a global environmental challenge. Their complex chemical structures and inherent resistance to degradation render traditional treatment technologies insufficient for achieving efficient removal. In recent years, PDI-based heterojunction photocatalytic technology has demonstrated significant advantages in antibiotic degradation due to its unique broad-spectrum light response capability and superior charge separation efficiency. Researchers have innovatively designed type-II, Z-scheme, and S-scheme heterojunction architectures by efficiently coupling PDI with semiconductors such as g-C₃N₄, metal–organic frameworks, and metal oxides. These designs not only broaden the light absorption range, but also markedly enhance photocatalytic activity through interfacial charge transfer mechanisms. This section systematically reviews the latest advancements in PDI-based heterojunctions for the degradation of antibiotics and drug residues, elucidates the underlying mechanisms driving performance enhancement, and explores potential directions for future technological optimization.

Wu et al. [101] successfully synthesized CNPDI (g-C₃N₄/PDI) type-II heterojunction catalysts via in situ self-assembly of PDI and g-C₃N₄ at controlled mass ratios. Under visible light irradiation for 60 min with potassium persulfate (PMS) activation, the CNPDI-7 photocatalyst achieved a degradation efficiency of 83.6% for tetracycline (TC), as shown in Table 1. This enhanced photocatalytic performance was primarily attributed to the formation of an effective type-II heterojunction between self-assembled PDI and g-C₃N₄, which significantly promoted the separation of electron–hole pairs and optimized charge density redistribution at the interface. In another study, Li et al. [102] developed a novel g-C₃N₄/PDI@NH₂-MIL-53(Fe) (CPM) type-II heterojunction through a combination of thermal polymerization, surface growth technology, and solvothermal synthesis. Structural characterization using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that spindle-shaped NH₂-MIL-53(Fe) particles were in intimate contact with the g-C₃N₄/PDI surface (Figure 1a). High-resolution TEM analysis of the CPM-2 interface (highlighted in Figure 1a) confirmed the establishment of efficient interfacial electron transfer pathways. The CPM heterojunction exhibited remarkable photocatalytic activity for the degradation of various aqueous organic pollutants under visible light irradiation in the presence of H₂O₂, achieving removal efficiencies of 90.0% for TC within 1 h, 78.0% for carbamazepine (CBZ) within 2.5 h, and complete degradation (100.0%) of both bisphenol A (BPA) and p-nitrophenol (PNP) within 10 and 30 min, respectively. This superior photocatalytic performance was ascribed to the formation of an optimized type-II heterojunction, facilitated by the close interfacial contact and well-aligned band structures between g-C₃N₄/PDI and NH₂-MIL-53(Fe), which enhanced charge separation efficiency and accelerated photodegradation kinetics. The photocatalytic degradation of organic pollutants in the CPM-2/visible light/H₂O₂ system is governed by a type-II heterojunction-mediated electron transfer mechanism. As illustrated in Figure 1b, this process involves the generation and subsequent migration of electron–hole pairs within the CPM-2 composite. Upon exposure to visible light irradiation, electron–hole pairs are initially formed on the CPM material. Subsequently, electrons from the conduction band of g-C₃N₄/PDI migrate to the conduction band of NH₂-MIL-53(Fe), while holes from the valence band of NH₂-MIL-53(Fe) transfer to the valence band of g-C₃N₄PDI. This charge transfer mechanism effectively suppresses electron–hole recombination and enhances the free electron density in the conduction band of NH₂-MIL-53(Fe). The Fe²⁺ ions generated via the reduction of Fe³⁺ by these electrons subsequently catalyze the decomposition of H₂O₂ to produce hydroxyl radicals (•OH), which play a critical role in the oxidative degradation of organic pollutants. The photocatalytic efficiency of this system is further enhanced by the high electron affinity of PDI, which positively modulates the redox potential of g-C₃N₄, thereby optimizing the band structure alignment between g-C₃N₄/PDI and NH₂-MIL-53(Fe). This optimized alignment facilitates the formation of an efficient type-II heterojunction, significantly improving the separation and transfer of photogenerated charge carriers. Notably, the activation of H₂O₂ by Fe²⁺ in NH₂-MIL-53(Fe) to generate •OH demonstrates substantially higher efficiency compared to the direct production of •OH through electron transfer from g-C₃N₄/PDI under visible light irradiation. Therefore, the establishment of an effective heterojunction between g-C₃N₄/PDI and NH₂-MIL-53(Fe) markedly enhances the Fenton-like photocatalytic degradation of organic pollutants under visible light conditions. Wu et al. [103] successfully achieved the modification of PDI on the surface of NH₂-MIL-88B(Fe, Mn) (FM88B) via a water bath heating process, leading to the formation of a type-II heterojunction between PDI and FM88B (PDI/FM88B). This modification was enabled by the formation of an amide bond between PDI and FM88B, as shown in Figure 1c. The amide bond not only enhanced the structural stability of the heterogeneous system but also provided an efficient pathway for interfacial electron transfer. In the Fenton system, under visible light irradiation for 30 min, the degradation efficiency of TC by 6.0% PDI/FM88B reached 89.0%. The improved photocatalytic performance was mainly ascribed to the combined effects of photocatalysis and the Fenton reaction. Moreover, the type-II heterojunction enabled the directional movement of photoinduced electrons from PDI (electron donor) to FM88B (electron acceptor). The built-in electric field (BIEF) existing at the PDI/FM88B interface significantly contributed to the separation of photoinduced e^––h⁺ pairs, which played a vital role in the TC degradation process within this photo–Fenton system, as depicted in Figure 1d. When exposed to visible light, the PDI/FM88B heterostructure, with its remarkable light absorption capacity, guided the transfer of photoinduced electrons from PDI to FM88B. The BIEF related to the type-II heterojunction further improved the separation and directional movement of photoinduced charge carriers. Furthermore, the amide bond served as a nanoscale rapid pathway, decreasing the transfer distance and energy barrier for photoinduced electrons moving from PDI to FM88B, thus substantially enhancing the efficiency.

Table 1. The photocatalytic degradation performance of antibiotics by a PDI-based heterojunction.

Photocatalyst	Synthesis Method	Antibiotics	Light Source	Time (min)	Efficiency (%)	Type	Reference
CNPDI	In situ self-assembly	TC	Visible	60	83.6	II	Wu et al. [101]
CPM	Surface growth, solvothermal	TC	Visible	60	90.0	II	Li et al. [102]
PDI/FM88B	Water bath heating	TC	Visible	30	89.0	II	Wu et al. [103]
I-PDI/PEDOT-M	Surface self-corrosion- assisted rapid spin- coating	TC	Visible	60	73.7	II	Lu et al. [104]
PDI/BiOCl-BiPO4	Ultrasonic, hydrothermal	TC	Simulated sunlight	150	81.0	Z	Zhuang et al. [105]
PDI/FePc	In situ self-assembly	TC	Visible	60	78.6	Z	Shi et al. [106]
WO3@Cu@PDI	Photo-deposition, water bath heating	TC	Visible	15	75.0	Z	Zeng et al. [107]
PDI/WO3/α-Fe2O3	PLAL	TC	254 nm	150	94.2	Z	Mao et al. [108]
PDI/MIL-53(Fe)	Solvent thermal	TC	Visible	30	94.1	Z	Chen et al. [92]
TMOP	Electrospinning	TC	Simulated sunlight	80	91.2	Z	Sun et al. [109]
Ag3PO4/PDIsm	Self-assembly	TC	Visible	8	82.8	Z	Cai et al. [110]
PANI/PDI	In situ growth	TC	Visible	120	70.0	Z	Dai et al. [90]
PDI-Ala/S-C3N4	In situ self-assembly	TC	Visible	90	90.0	S	Li et al. [111]
IM-NSH-PM	Imprint	TC	Simulated sunlight	60	71.9	S	Lu et al. [112]
PDIs/C, N, S-CeO2	Oil bath heating	TC	Visible	30	80.1	S	Jing et al. [113]
PDIs/Fe2O3@C	Hydrothermal calcination, oil bath heating	TC	Visible	8	78.9	S	Jing et al. [114]
PDI/ZnFe2O4	Ultrasonic	TC	Visible	60	66.7	S	Xu et al. [115]
Ag/PCN/UPDI	Self-assembly, photoreduction	OTC	Visible	150	97.5	S	Xiao et al. [116]
g-C3N4/PDI/Co-Fe	Co-precipitation	DOX	Visible	60	96.1	Z	Li et al. [117]
PDI-Urea/BiOBr	Solvent thermal, in situ growth	OFLO	Visible	150	93.0	S	Wang et al. [118]
Bis-PDI-T@TiO2	Double-solvent phase transfer	CBZ	Visible	30	100.0	Z	Yang et al. [119]
β-PDI/MIL-101(Fe)	Grinding	SMX	Visible	6	99.7	Z	Jia et al. [120]
MIL-101(Fe)-NH2/PDI	Grinding	SMX	Visible	5	99.2	Z	Jia et al. [121]

Table 1. The photocatalytic degradation performance of antibiotics by a PDI-based heterojunction.

Photocatalyst	Synthesis Method	Antibiotics	Light Source	Time (min)	Efficiency (%)	Type	Reference
CNPDI	In situ self-assembly	TC	Visible	60	83.6	II	Wu et al. [101]
CPM	Surface growth, solvothermal	TC	Visible	60	90.0	II	Li et al. [102]
PDI/FM88B	Water bath heating	TC	Visible	30	89.0	II	Wu et al. [103]
I-PDI/PEDOT-M	Surface self-corrosion- assisted rapid spin- coating	TC	Visible	60	73.7	II	Lu et al. [104]
PDI/BiOCl-BiPO4	Ultrasonic, hydrothermal	TC	Simulated sunlight	150	81.0	Z	Zhuang et al. [105]
PDI/FePc	In situ self-assembly	TC	Visible	60	78.6	Z	Shi et al. [106]
WO3@Cu@PDI	Photo-deposition, water bath heating	TC	Visible	15	75.0	Z	Zeng et al. [107]
PDI/WO3/α-Fe2O3	PLAL	TC	254 nm	150	94.2	Z	Mao et al. [108]
PDI/MIL-53(Fe)	Solvent thermal	TC	Visible	30	94.1	Z	Chen et al. [92]
TMOP	Electrospinning	TC	Simulated sunlight	80	91.2	Z	Sun et al. [109]
Ag3PO4/PDIsm	Self-assembly	TC	Visible	8	82.8	Z	Cai et al. [110]
PANI/PDI	In situ growth	TC	Visible	120	70.0	Z	Dai et al. [90]
PDI-Ala/S-C3N4	In situ self-assembly	TC	Visible	90	90.0	S	Li et al. [111]
IM-NSH-PM	Imprint	TC	Simulated sunlight	60	71.9	S	Lu et al. [112]
PDIs/C, N, S-CeO2	Oil bath heating	TC	Visible	30	80.1	S	Jing et al. [113]
PDIs/Fe2O3@C	Hydrothermal calcination, oil bath heating	TC	Visible	8	78.9	S	Jing et al. [114]
PDI/ZnFe2O4	Ultrasonic	TC	Visible	60	66.7	S	Xu et al. [115]
Ag/PCN/UPDI	Self-assembly, photoreduction	OTC	Visible	150	97.5	S	Xiao et al. [116]
g-C3N4/PDI/Co-Fe	Co-precipitation	DOX	Visible	60	96.1	Z	Li et al. [117]
PDI-Urea/BiOBr	Solvent thermal, in situ growth	OFLO	Visible	150	93.0	S	Wang et al. [118]
Bis-PDI-T@TiO2	Double-solvent phase transfer	CBZ	Visible	30	100.0	Z	Yang et al. [119]
β-PDI/MIL-101(Fe)	Grinding	SMX	Visible	6	99.7	Z	Jia et al. [120]
MIL-101(Fe)-NH2/PDI	Grinding	SMX	Visible	5	99.2	Z	Jia et al. [121]

Figure 1. (a) SEM images of CPM-2 (inset: enlarged image of the yellow frame). (b) Schematic illustration for the possible photocatalytic mechanism of the CPM-2 composite under visible light irradiation [102]. (c) Ammoniation reaction between PDI and FM88B. (d) Degradation mechanism of TC in the system of visible light/6% PDI/FM88B/H₂O₂ [103]. (e) TEM images of WO₃@Cu@PDI composite; (f) Z-scheme electron transfer mechanism [107].

Figure 1. (a) SEM images of CPM-2 (inset: enlarged image of the yellow frame). (b) Schematic illustration for the possible photocatalytic mechanism of the CPM-2 composite under visible light irradiation [102]. (c) Ammoniation reaction between PDI and FM88B. (d) Degradation mechanism of TC in the system of visible light/6% PDI/FM88B/H₂O₂ [103]. (e) TEM images of WO₃@Cu@PDI composite; (f) Z-scheme electron transfer mechanism [107].

In a related study, Lu et al. [104] developed an imprinted PDI/PEDOT type-II heterojunction photocatalyst anchoring film (I-PDI/PEDOT-M) using an N-methylpy- rrolidone (NMP)-induced surface self-corrosion-assisted rapid spin-coating method. Under visible light irradiation for 1 h, the degradation efficiencies of I-PDI/PEDOT for TC and ciprofloxacin (CIP) were 73.7% and 5.0%, respectively. Notably, the degradation efficiency of TC by I-PDI/PEDOT was 14.65 times higher than that of CIP. The construction of the type-II heterojunction between PDI and PEDOT effectively promoted the rapid separation of photogenerated electrons and holes, thereby maintaining the superior photocatalytic activity of I-PDI/PEDOT-M. Recent advancements in Z-scheme heterojunction photocatalysts have demonstrated significant improvements in photocatalytic performance through innovative material design and synthesis strategies. Zhuang et al. [105] developed a ternary Z-scheme PDI/BiOCl-BiPO₄ composite via ultrasonic dispersion and hydrothermal methods. The experimental results revealed that under 150 min of simulated solar irradiation, the PDI (5.0%)/BiOCl-BiPO₄ composite achieved remarkable degradation efficiencies of 98.0% for rhodamine B (RhB) and 81.0% for TC. The enhanced photocatalytic activity was primarily attributed to the Z-scheme heterojunction structure, which facilitated efficient separation and migration of photogenerated electron–hole pairs. In a parallel development, Shi et al. [106] engineered a novel PDI/iron phthalocyanine (FePc) Z-scheme heterojunction through a self-assembly approach, leveraging strong π–π interactions. This configuration demonstrated a tetracycline hydrochloride removal efficiency of 78.6% after 60 min of visible light exposure. The superior photocatalytic performance was ascribed to the robust π–π interactions between PDI and FePc, which effectively minimized the interlayer spacing of the supramolecular structure and enhanced charge carrier separation and transfer within the built-in electric field. A further innovation was demonstrated by Zeng et al. [107], who constructed a Z-scheme heterojunction by photo-depositing Cu onto WO₃, followed by coupling with PDI through a water bath method. The resulting 30% WO₃@Cu@PDI photocatalyst exhibited a 75.0% tetracycline removal efficiency within 15 min of visible light irradiation. Structural characterization (Figure 1e) revealed a uniform distribution of WO₃ nanoparticles on the PDI supramolecular surface, with dispersed Cu nanoparticles. The Z-scheme architecture enabled efficient electron transfer from WO₃ to PDI’s donor level, facilitating rapid recombination with h⁺. This process resulted in electron accumulation on PDI’s conduction band and hole accumulation on WO₃’s valence band, thereby enhancing redox activity. The electron transfer mechanism (Figure 1f) involved Cu-mediated electron transfer from WO₃’s conduction band to PDI’s valence band, followed by the generation of •O₂⁻ (superoxide radical) from PDI’s conduction band, confirming •O₂⁻ as the primary reactive species in the photocatalytic process. Mao et al. [108] successfully synthesized a perylene diimide/tungsten trioxide/hematite (PDI/WO₃/α-Fe₂O₃, PWF) composite photocatalyst with a dual Z-scheme heterojunction via the pulsed laser ablation in liquid (PLAL) technique. Photocatalytic performance evaluation revealed that the PWF composite achieved a 94.2% TC removal efficiency under 180 min of irradiation using a 15 W low-pressure mercury lamp (λ = 254 nm). The enhanced photocatalytic activity can be primarily attributed to two key factors: (1) the increased number of adsorption sites and strengthened surface charge interactions, which facilitated rapid TC adsorption; and (2) the engineered dual Z-scheme heterojunction, which optimized the band structure, thereby improving photovoltaic conversion efficiency, promoting efficient charge carrier separation, and ultimately enhancing photocatalytic performance. In a separate study, Chen et al. [92] developed a perylene diimide/metal–organic framework (PDI/MIL-53(Fe), PM) Z-scheme heterojunction composite photocatalyst through solvothermal synthesis. The 5PM composite demonstrated superior photocatalytic activity, achieving a 94.1% tetracycline degradation efficiency within 30 min of visible light irradiation. This performance represents a 4-fold enhancement compared to pristine PDI and a 33-fold improvement over MIL-53(Fe) alone. The exceptional photocatalytic efficiency can be ascribed to the synergistic effects of the Z-scheme heterojunction, which facilitates effective charge separation, and the intrinsic semiconductor properties that provide strong redox potential.

Sun et al. [109] successfully developed a novel self-supported tricolor microfiber directional heterojunction photocatalyst, designated as TMOP, via a three-axis parallel electrospinning technique. The TMOP structure consists of three distinct layers: [g-C₃N₄/poly(methyl methacrylate) (PMMA)]//[TiO₂/polyaniline (PANI)/PMMA]//[self- assembled 3,4,9,10-perylene tetracarboxylic diimide (PDI)/PMMA]. Under simulated solar light irradiation, the optimized TMOP exhibited exceptional photocatalytic degradation efficiencies: 91.2% for tetracycline hydrochloride (within 80 min), 89.0% for ciprofloxacin (CIP) (within 90 min), 77.6% for chlortetracycline (CTC) (within 150 min), 69.5% for levofloxacin (within 150 min), and 92.5% for methylene blue (within 50 min). The enhanced photocatalytic performance can be primarily attributed to three critical factors: (1) the unique morphology and heterojunction structure of the tricolor microfibers, which significantly increased active sites; (2) the broadband spectral characteristics of PDI, which expanded the solar spectrum absorption range; and (3) the synergistic effect between conductive polyaniline and the one-dimensional double Z-scheme directional heterojunction structure, which facilitated directional and rapid carrier transport through multiple pathways. This synergistic effect substantially reduced carrier recombination probability while maintaining strong redox capability. Furthermore, the self-supporting nature of TMOP ensured excellent recoverability and practical applicability. In a related study, Cai et al. [110] fabricated an Ag₃PO₄/PDI supramolecular Z-scheme photocatalyst using a two-step self-assembly strategy. As illustrated in Figure 2a, the synthesis process involved the initial formation of PDIsm (PDI supramolecular) rods through self-assembly, stabilized by π–π stacking and hydrogen bonding interactions. The polar nature of PDI molecules established an intramolecular electric field from the perylene core to the carboxyl groups, enabling the adsorption of silver ions (Ag⁺) onto the carboxyl groups of PDIsm rods. The subsequent reaction with NaH₂PO₄ resulted in the in situ loading of Ag₃PO₄ nanoparticles on the PDIsm surface. This spatial configuration facilitated the formation of an efficient Z-scheme system, achieving a degradation efficiency of 82.8% for tetracycline hydrochloride under visible light irradiation within eight minutes. The detailed photocatalytic mechanism and stability enhancement are further elaborated in the subsequent section. Comparative analysis of the photoluminescence (PL) spectra between the Ag₃PO₄ and Ag₃PO₄/PDIsm composites, as shown in Figure 2b, revealed a significant reduction in PL intensity for the Ag₃PO₄/PDIsm system. This observation indicates a marked improvement in charge carrier separation efficiency. Photocurrent response measurements (Figure 2c) further corroborated this finding, demonstrating that the Ag₃PO₄/PDIsm composite exhibited superior photocurrent density under visible light irradiation compared to pristine Ag₃PO₄ and PDIsm alone, thereby confirming more efficient migration of photogenerated charge carriers to the electrode interface. Electrochemical impedance spectroscopy (EIS) analysis, as depicted in the Nyquist plot (Figure 2d), revealed that the Ag₃PO₄/PDIsm composite possessed the lowest charge transfer resistance among the three samples, suggesting enhanced electrical conductivity and accelerated electron transfer kinetics following the incorporation of PDIsm, which collectively contribute to improved charge carrier separation efficiency. These experimental findings collectively demonstrate that the integration of PDIsm with Ag₃PO₄ significantly enhances charge carrier separation efficiency, thereby facilitating photocatalytic processes. The superior photocatalytic performance of the Ag₃PO₄/PDI composite can be mechanistically attributed to three primary factors: (1) The implementation of a Z-scheme charge transfer mechanism facilitates efficient spatial separation of charge carriers, wherein the negative conduction band potential of PDI promotes the reduction of molecular oxygen to generate •O₂⁻. (2) The inherent strong oxidative capacity of Ag₃PO₄ is preserved, enabling direct oxidation of tetracycline hydrochloride. (3) The establishment of an internal electric field effectively inhibits the reduction of Ag⁺, mitigates photochemical corrosion, and enhances the overall photostability of the composite system. Through in situ growth technology, Dai et al. [90] successfully fabricated a 3D polyaniline/perylene diimide (PANI/PDI) Z-scheme heterojunction photocatalyst. The experimental results demonstrated that after two hours of visible light irradiation, the 20% PANI/PDI composite material achieved a tetracycline degradation efficiency of 70.0%. The exceptional photocatalytic performance and structural stability of this system can be primarily attributed to three critical factors: first, the incorporation of the polyaniline polymer framework reinforced the PDI hydrogel structure, thereby enhancing the durability of the catalytic system; second, the three-dimensional network structure not only provided abundant active sites but also optimized the medium transport channels; and finally, the π–π interaction between polyaniline and perylene diimide formed an extensive delocalized π–electron conjugated system and constructed precisely aligned heterojunction structures, significantly improving the migration and separation efficiency of the photogenerated carriers. In another study, Li et al. [111] successfully developed an S-scheme heterojunction photocatalyst composed of the organic semiconductor PDI-Ala (N,N’-dipropionyl-perylene-3,4,9,10-tetracarboxylic diimide) and sulfur-doped g-C₃N₄ (S-C₃N₄) via an in situ self-assembly approach. Under visible light irradiation, the 30% PDI-Ala/S-C₃N₄ composite material achieved a TC removal efficiency of up to 90.0% within 90 min. This S-scheme heterojunction features ideal band alignment and robust interfacial bonding, which not only accelerates intermolecular electron transfer but also broadens the visible light absorption range of the heterojunction, thereby significantly enhancing its photocatalytic activity. Lu et al. [112] developed a novel photocatalytic membrane (IM-NSH-PM) through the combination of molecular imprinting technology and dopamine-assisted surface modification. Upon exposure to simulated sunlight for 60 min, the non-metallic S-scheme heterojunction photocatalytic membrane (IM-NSH-PM) demonstrated photocatalytic degradation efficiencies of 71.9% for TC and 25.5% for CIP. The membrane’s stability was ensured by the strong interfacial bonding between the polyvinylidene fluoride (PVDF) substrate and the imprinted non-metal S-scheme heterojunction (IM-NSH), achieved through dopamine cross-linking and polymerization. The construction of an S-scheme heterojunction between PDI and g-C₃N₄ significantly reduced electron–hole pair recombination, thereby boosting the photocatalytic activity. The proposed reaction mechanism of IM-NSH-PM is depicted in Figure 2e. The membrane’s selective TC adsorption capability stems from the synergistic combination of imprinted cavities and antibiotic molecular diversity. When TC molecules are specifically captured by the imprinted cavities, a series of redox reactions are triggered under simulated sunlight. The S-scheme heterojunction mechanism prevents e⁻ recombination on PDI and h⁺ recombination on g-C₃N₄. Due to the lower LUMO (lowest unoccupied molecular orbital) level of g-C₃N₄ compared to the standard reduction potential (E₀), electrons react with O₂ to produce •O₂⁻, which then convert to •OH. These reactive species, along with the holes in PDI’s HOMO (highest occupied molecular orbital), actively participate in pollutant degradation. The three-dimensional structural matching between TC and imprinted cavities promotes selective interaction, leading to the complete mineralization of TC into CO₂, H₂O, and other small molecules through radical-mediated reactions. In a separate study, Jing et al. [113] fabricated a 10% PDI/C, N, and S-CeO₂ S-scheme heterojunction by incorporating organic supramolecular self-assembled PDI (perylene diimide) semiconductors onto a three-dimensional metal oxide framework (C, N, and S-CeO₂) using oil bath heating. The material’s photocatalytic performance was assessed through organic pollutant degradation under visible light. Remarkably, the heterojunction achieved 80.1% TC degradation within 30 min of visible light exposure, with a reaction rate constant 15.83 times greater than pure C, N, and S-CeO₂. Upon 60 min of visible light irradiation, the 10% PDI/C, N, and S-CeO₂ composite material achieved degradation efficiencies of 72.9%, 83.5%, 44.9%, and 56.5% for CTC, CIP, RhB, and methylene blue, respectively. This enhanced performance resulted from optimized S-scheme band alignment, multidimensional charge transfer pathways, improved visible light absorption, and superior photothermal properties. In a subsequent study, Jing et al. [114] developed a novel composite photocatalytic material, a perylene diimide/iron oxide@carbon (PDI/Fe₂O₃@C) S-scheme heterojunction, through a facile hydrothermal calcination approach combined with an oil bath process. Within the PMS activation system, the 20% PDI/Fe₂O₃@C composite exhibited remarkable tetracycline degradation efficiency, reaching 78.9% within eight minutes of visible light irradiation. Upon 12 min of visible light irradiation, the 20% PDI/Fe₂O₃@C composite material achieved degradation efficiencies of 71.59%, 60.93%, and 45.61% for CTC, CIP, and sulfadiazine (SDZ), respectively. The enhanced photocatalytic performance was primarily ascribed to the synergistic integration of PDIs and Fe₂O₃@C, which effectively mitigated PDIs’ aggregation during self-assembly, increased the specific surface area of Fe₂O₃@C, exposed additional active sites, and facilitated the activation of the Fe²⁺/Fe³⁺ redox cycle in PMS. Furthermore, the hybrid organic–inorganic interface promoted efficient photogenerated electron transfer and electron–hole pair separation, establishing a novel S-scheme electron transport pathway that significantly augmented PMS activation efficiency. Xu et al. [115] employed an ultrasonic-assisted approach to fabricate a one-dimensional (1D) S-scheme heterojunction composed of PDI and ZnFe₂O₄. The composite’s photocatalytic efficiency was thoroughly assessed under visible light exposure for one hour, demonstrating a tetracycline degradation rate of 66.7%. This outcome indicates a remarkable improvement, surpassing the performance of pure PDI by 9.18-fold (7.3%) and ZnFe₂O₄ by 9.73-fold (6.9%). The enhanced photocatalytic activity is mainly due to the establishment of an S-scheme heterojunction between PDI and ZnFe₂O₄, which efficiently separates photogenerated electron–hole pairs and enhances redox potential. Furthermore, the rod-shaped structure of PDI and the nanoscale dimensions of ZnFe₂O₄ work in concert to promote charge carrier movement to the surface, thereby improving the overall photocatalytic efficacy.

In a related study, Xiao et al. [116] developed a ternary S-scheme heterojunction photocatalyst (Ag/PCN/UPDI) through a multi-step synthesis process. The fabrication involved electrostatic self-assembly of urea-modified perylene diimide (UPDI) with graphitic carbon nitride (PCN), followed by surface modification with silver nanoparticles (Ag) via photoreduction. The preparation methodology is schematically illustrated in Figure 3a. The photocatalytic efficiency of Ag/PCN/UPDI was comprehensively assessed through the degradation of oxytetracycline (OTC) under visible light irradiation for 150 min, achieving a remarkable degradation efficiency of 97.5%. The underlying photocatalytic mechanism for organic chloride degradation is comprehensively depicted in Figure 3b,c. The superior photocatalytic performance of Ag/PCN/UPDI is primarily ascribed to the integration of multiple electric fields (MEF, IEF, and PEF), which establish a continuous and efficient charge transfer pathway within the composite catalyst. Specifically, MEF enhances carrier separation in UPDI, while the generated free electrons migrate to PCN under the influence of IEF. Furthermore, PEF promotes charge separation on the PCN surface and increases the concentration of high-energy electrons. The enhanced reaction kinetics can be ascribed to two principal factors: (1) the spatial charge separation facilitated by the series electric field, which significantly prolongs carrier lifetime; and (2) the superior photothermal effect that accelerates near-field chemical reactions. Li et al. [117] successfully synthesized a ternary composite material, g-C₃N₄/PDI/Co-Fe, via a co-precipitation method, leading to the formation of a double Z-scheme heterojunction. Under visible light irradiation for 60 min in the presence of PMS, the degradation efficiency of doxycycline hydrochloride (DOX) by the g-C₃N₄/PDI/Co-Fe composite reached 96.1%. The enhanced degradation efficiency was primarily attributed to the double Z-scheme heterojunction, which significantly promoted the separation and migration of photogenerated charge carriers. The mechanism by which g-C₃N₄/PDI/Co-Fe PBA activates PMS for DOX degradation under visible light is illustrated in Figure 3d. Upon visible light irradiation, the composite catalyst generates photogenerated charge carriers and establishes a double Z-scheme heterojunction among g-C₃N₄, PDI, and Co-Fe PBA. This heterojunction facilitates the transfer of photogenerated electrons from the conduction band of PDI to the valence bands of g-C₃N₄ and Co-Fe PBA, thereby inhibiting the recombination of photogenerated electron–hole pairs and enhancing the photocatalytic activity. Furthermore, photogenerated h⁺ directly oxidizes DOX. Photogenerated electrons activate PMS to produce sulfate radicals (SO₄^•−), some of which react with hydroxide ions (OH⁻) to generate •OH and bisulfate ions (HSO₄⁻). Bisulfate ions subsequently react with photogenerated holes to regenerate SO₄^•−. Transition metals in Co-Fe PBA also contribute to the activation of PMS, generating additional SO₄^•− that can react with photogenerated electrons to form •O₂⁻ and O₂. •O₂⁻ then combines with photogenerated holes to form singlet oxygen (¹O₂). The synergistic action of ¹O₂, SO₄^•−, •O₂⁻, and photogenerated h⁺ collectively drives the degradation process. Additionally, •OH plays a pivotal role in the degradation and mineralization of DOX. A novel organic–inorganic stepped S-scheme heterojunction PDI-Urea/BiOBr composite photocatalyst was successfully fabricated by Wang et al. [118] through a simple solvothermal method coupled with an in situ growth technique. Under visible light irradiation, the 15% PDI-Urea/BiOBr composite demonstrated a degradation efficiency of 93.0% for ofloxacin (OFLO) within 150 min and achieved a removal rate of 58% for TC within 90 min. This significant improvement in photocatalytic efficiency mainly resulted from the construction of an S-scheme heterojunction, which effectively modified the band structure, broadened the spectral response range, and promoted the separation of photogenerated charge carriers. In a related study, Yang et al. [119] developed a 2,5-bis(tri-n-butylstannyl)thiophene-perylene diimide–TiO₂ (Bis-PDI-T @TiO₂) composite photocatalyst through a dual-solvent phase transfer method. This composite demonstrated exceptional peroxymonosulfate (PS) activation capability under visible light irradiation. Specifically, the 0.1% Bis-PDI-T@TiO₂ system, when combined with 1.0 mM PS at a catalyst concentration of 1.0 g/L, achieved complete degradation (100.0%) of carbamazepine (CBZ) within 30 min. The superior photocatalytic performance was ascribed to the formation of a Z-scheme heterojunction between Bis-PDI-T and TiO₂, which significantly enhanced the separation efficiency of photogenerated electron–hole pairs. Jia et al. [120] successfully developed a novel Z-scheme photocatalyst (MPx) through a mechanochemical synthesis approach, wherein β-alanine-modified perylene diimide derivatives (β-PDI) were integrated with MIL-101(Fe) via electrostatic interactions. The MP50/PS/visible light system demonstrated exceptional photocatalytic performance, achieving a 99.7% removal efficiency for sulfamethoxazole (SMX) within a remarkably short duration of six minutes. The reaction rate constant was determined to be 0.7532 min⁻¹, representing a 69.1-fold and 175.2-fold enhancement compared to the individual components, β-PDI and MIL-101(Fe), respectively. Mechanistic investigations revealed that PS played dual roles in the system: (1) as an activator generating SO₄^•−, and (2) as an electron scavenger, which effectively promoted the separation of photogenerated electron–hole pairs within the photocatalyst, thereby significantly enhancing the overall photocatalytic efficiency. In a subsequent study, Jia et al. [121] engineered MNPx(MIL-101(Fe)-NH₂/PDI) composites featuring Z-scheme heterojunctions through the mechanical grinding of MIL-101(Fe)-NH₂ and PDI. These composites were employed for the activation of peroxydisulfate (PDS) under visible light irradiation, demonstrating remarkable efficacy in the degradation of SMX. The MNP30/PDS/Vis system achieved a 99.2% degradation efficiency of SMX within five minutes. The Z-scheme heterojunction architecture in the MNPx photocatalysts facilitated efficient electron transfer, effectively suppressed the recombination of electron–hole pairs, and significantly enhanced the activation efficiency of PDS. Furthermore, the large specific surface area of MIL-101(Fe)-NH₂ provided abundant active sites for catalytic reactions. These synergistic effects collectively contributed to the rapid and efficient removal of SMX from the system.

In summary, PDI-based heterojunction photocatalysts have achieved remarkable technological advancements and demonstrated significant application potential in the degradation of antibiotics and pharmaceutical residues. Numerous researchers have successfully developed various types of PDI-based heterojunction photocatalysts using diverse preparation methods, including type-II, Z-scheme, S-scheme, and double Z-scheme heterojunctions. These photocatalysts exhibit superior degradation capabilities for a wide range of antibiotics and organic pollutants under visible light irradiation, achieving removal efficiencies of TC ranging from 70.0% to 99.7% within relatively short periods. The enhancement in photocatalytic performance is primarily attributed to the effective promotion of photogenerated carrier separation and migration by the heterojunction structure, which strengthens the oxidation–reduction ability of the photocatalyst, broadens the light absorption spectrum, and increases the number of active sites. These research findings offer critical insights and valuable references for the practical application of photocatalytic technology in addressing pollution from antibiotics and pharmaceutical residues.

4.2. Phenolic Compounds

In recent years, phenolic compounds—as representative refractory organic pollutants—have garnered significant attention in the development of efficient removal technologies. Among these materials, PDI-based heterojunctions have demonstrated remarkable advantages due to their distinctive photoelectric properties. Scholars have achieved a series of breakthroughs in enhancing photocatalytic performance through innovative heterojunction designs and advanced interface regulation strategies. Notably, researchers have realized directional carrier migration by modulating the heterojunction type (e.g., type II, Z-scheme, and S-scheme), combined with strategies such as plasma enhancement, multi-interface synergy, and three-dimensional structural design. These approaches not only significantly broaden the light response range, but also enhance the yield of reactive oxygen species by three to seven times. Such innovative systems exhibit universal applicability in the deep mineralization of bisphenol A and estrogenic pollutants, offering a novel solution for the efficient treatment of complex phenolic contaminants.

Chen et al. [122] fabricated a novel plasmonic Z-scheme heterojunction photocatalyst, Ag@AgCl/PDI, utilizing a stepwise approach that combines in situ deposition with photoreduction techniques. The fabrication process, as shown in Figure 4a, commenced with dissolving PDI in an alkaline triethanolamine (TEA) solution, subsequently acidified with HCl. Through π–π interactions and hydrogen bonding of carboxyl groups, self-assembled supramolecular PDI (SA-PDI) was formed. Silver chloride nanoparticles were then electrostatically assembled onto the SA-PDI surface through a precipitation reaction between Ag⁺ from silver nitrate and chloride ions (Cl⁻) from sodium chloride. The synthesis was completed by partially reducing Ag⁺ to metallic silver (Ag⁰) through in situ photoreduction, yielding the final Ag@AgCl/PDI composite. Figure 4b illustrates the electron transfer mechanism in the composite system. Electrons naturally flow from SA-PDI to AgCl through the silver component until Fermi level equilibrium is achieved among the three components. This electron migration decreases electron density in SA-PDI while increasing it in AgCl, creating positive charges on SA-PDI and negative charges on AgCl. Consequently, an internal electric field and band bending are established at the heterojunction interface. When exposed to light, this arrangement enables the movement of free electrons from the downward-bent conduction band of AgCl to the upward-bent conduction band of SA-PDI via the silver bridge. Figure 4c schematically presents this Ag-bridged Z-scheme heterojunction mechanism. The Ag@AgCl/PDI-3% composite exhibited remarkable photocatalytic performance, achieving 92.6% phenol removal efficiency after three hours of visible light exposure, as shown in Table 2. This enhanced photocatalytic oxidation capability stems from three key aspects: (1) The integration of Ag@AgCl nanoparticles substantially improves the light absorption of SA-PDI through surface plasmon resonance (SPR). (2) Silver nanoparticles act as efficient electron traps, capturing photogenerated electrons from SA-PDI’s conduction band. Additionally, the Schottky barrier formed by silver promotes the transfer of SPR-excited electrons from Ag to AgCl, boosting charge separation efficiency. (3) The composite generates increased concentrations of reactive oxygen species (•O₂⁻ and •OH) and h⁺, significantly contributing to its photocatalytic activity.

Table 2. The photocatalytic degradation performance of a phenolic by a PDI-based heterojunction.

Photocatalyst	Synthesis Method	Phenolic	Light Source	Time (min)	Efficiency (%)	Type	Reference
Ag@AgCl/PDI	In situ deposition, photoreduction	PhOH	Visible	180	92.6	Z	Chen et al. [122]
BiOCl/PDI	Water bath heating	PhOH	Simulated sunlight	180	87.0	Z	Gao et al. [123]
TOC-PDI-POSS/g-C3N4	Solvent exchange	PhOH	Simulated sunlight	60	97.0	S	Dai et al. [124]
BiOBr/Bi4O5Br2/PDI	Self-assembly	BPA	Visible	75	90.0	II	Wang et al. [125]
PDIBr/A10	Solvent exchange	BPA	Visible	60	71.0	Z	Zha et al. [126]
Bi12O15Cll6@W18O49 @g-C3N4/PDI	Solvent thermal, calcination	BPA	Simulated sunlight	30	100.0	Z	Zhang et al. [127]

Table 2. The photocatalytic degradation performance of a phenolic by a PDI-based heterojunction.

Photocatalyst	Synthesis Method	Phenolic	Light Source	Time (min)	Efficiency (%)	Type	Reference
Ag@AgCl/PDI	In situ deposition, photoreduction	PhOH	Visible	180	92.6	Z	Chen et al. [122]
BiOCl/PDI	Water bath heating	PhOH	Simulated sunlight	180	87.0	Z	Gao et al. [123]
TOC-PDI-POSS/g-C3N4	Solvent exchange	PhOH	Simulated sunlight	60	97.0	S	Dai et al. [124]
BiOBr/Bi4O5Br2/PDI	Self-assembly	BPA	Visible	75	90.0	II	Wang et al. [125]
PDIBr/A10	Solvent exchange	BPA	Visible	60	71.0	Z	Zha et al. [126]
Bi12O15Cll6@W18O49 @g-C3N4/PDI	Solvent thermal, calcination	BPA	Simulated sunlight	30	100.0	Z	Zhang et al. [127]

Figure 4. (a) Schematic illustration of synthesizing Ag@AgCl/PDI composite photocatalysts; (b,c) schematic diagram of the charge transfer process in Ag@AgCl/PDI [122]. (d,e) TEM images of BiOBr/Bi₄O₅Br₂/PDI; (f) UV–Vis DRS spectra of as-prepared samples; (g) photocatalytic degradation curves of BPA; (h) pseudo-first-order kinetic model fitting the kinetic degradation curves of BPA [125].

Figure 4. (a) Schematic illustration of synthesizing Ag@AgCl/PDI composite photocatalysts; (b,c) schematic diagram of the charge transfer process in Ag@AgCl/PDI [122]. (d,e) TEM images of BiOBr/Bi₄O₅Br₂/PDI; (f) UV–Vis DRS spectra of as-prepared samples; (g) photocatalytic degradation curves of BPA; (h) pseudo-first-order kinetic model fitting the kinetic degradation curves of BPA [125].

In a parallel investigation, Gao et al. [123] prepared PDI/BiOCl Z-scheme heterojunctions using a water bath heating technique. The 30% BiOCl/PDI composite demonstrated superior photocatalytic performance, achieving 87.0% phenol degradation efficiency after 180 min of simulated sunlight exposure. This represents a 2.2-fold enhancement compared to pure BiOCl and a 1.6-fold improvement over PDISA. The composite’s outstanding photocatalytic properties are ascribed to its enhanced generation of reactive species, comprehensive light absorption across the full spectrum, and efficient interfacial charge separation. Dai et al. [124] successfully synthesized a soluble hybrid material, titanium oxocluster–pyrene diimide–polyhedral oligomeric silsesquioxane (TOC-PDI-POSS), via the solvent exchange method. Exploiting π–π stacking interactions with g-C₃N₄, they successfully fabricated a novel TOC-PDI-POSS/g-C₃N₄ S-scheme heterojunction. Under simulated sunlight irradiation for 60 min, the TOC-PDI-POSS/g-C₃N₄ photocatalyst achieved a phenol removal efficiency of 97.0%. This exceptional photocatalytic performance is primarily attributed to the efficient separation of photogenerated carriers enabled by the S-scheme heterojunction in TOC-PDI-POSS/g-C₃N₄. Wang et al. [125] constructed a BiOBr/Bi₄O₅Br₂/PDI photocatalyst featuring a type-II double heterojunction through alkaline dehalogenation and PDI self-assembly techniques. As illustrated in Figure 4d, the thin belt-like structure of PDI is uniformly coated on the surface of BiOBr/Bi₄O₅Br₂, leveraging the superior photosensitivity of PDI to extend the visible light response range. High-resolution transmission electron microscopy (HRTEM) analysis (Figure 4e) revealed distinct lattice fringes of BiOBr and Bi₄O₅Br₂, with interplanar spacings of 0.276 nm for the (110) plane of BiOBr and 0.365 nm for the (310) plane of Bi₄O₅Br₂, while PDI exhibited an amorphous state. The crystalline structures of BiOBr and Bi₄O₅Br₂ in the BiOBr/Bi₄O₅Br₂/PDI composite remained intact post-solvothermal reaction and PDI self-assembly. Ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS) characterization (Figure 4f) demonstrated that BiOBr/Bi₄O₅Br₂/PDI exhibited a stronger visible light response compared to BiOBr/Bi₄O₅Br₂, with the light absorption wavelength extending from 479 to 721 nm upon PDI incorporation. The construction of the Bi₄O₅Br₂ heterojunction and the integration of PDI synergistically enhanced photocatalytic activity. Under visible light irradiation, BiOBr/Bi₄O₅Br₂/PDI degraded 90.0% of BPA within 75 min, completely degraded 17α-ethynylestradiol (EE₂) within 15 min, and fully removed 17β-estradiol (E₂) within the same duration. The BPA removal efficiency followed the order BiOBr/Bi₄O₅Br₂/PDI (20.0%) > Bi₄O₅Br₂/PDI > BiOBr/Bi₄O₅Br₂ > PDI > BiOBr/PDI > Bi₄O₅Br₂ > BiOBr (Figure 4g). Pseudo-first-order kinetic fitting of the degradation curves (Figure 4h) revealed that BiOBr/Bi₄O₅Br₂/PDI exhibited the highest reaction rate constant (K = 0.0376), significantly surpassing that of pure BiOBr/Bi₄O₅Br₂ (K = 0.0157) and pure BiOBr (K = 0.0056). The superior photocatalytic activity of the BiOBr/Bi₄O₅Br₂/PDI composite is ascribed to the matched band structure between Bi₄O₅Br₂ and BiOBr, which facilitates heterojunction formation and enhances spatial charge separation. Furthermore, the strong photosensitivity of PDI combined with BiOBr/Bi₄O₅Br₂ not only augments visible light photocatalytic activity but also broadens the light-harvesting range. Zha et al. [126] successfully demonstrated the synthesis of a novel Z-scheme heterojunction PDIBr/A10 composite material via a solvent exchange method, wherein perylene diimide derivatives (PDIs) were self-assembled on the surface of anatase TiO₂ (A10). The photocatalytic performance of the composite was systematically evaluated under visible light irradiation for one hour, achieving a degradation efficiency of 71.1% for BPA without a sacrificial agent and 71.7% with persulfate as a sacrificial agent. The enhanced photocatalytic activity was primarily attributed to the π–π orbital overlap in H-aggregates facilitated by PDIBr, which formed an interface layer on the TiO₂ surface. This interface layer promoted efficient charge transfer and suppressed electron–hole recombination. Additionally, the Z-scheme heterojunction mechanism was confirmed as the primary pathway for the photocatalytic degradation of BPA, using the PDIBr/A10 composite as a model system. In a related study, Zhang et al. [127] developed a plasmonic Bi₁₂O₁₅Cl₆@ W₁₈O₄₉@graphitic carbon nitride/poly(1,4-phenylene terephthalamide) (g-C₃N₄/PDI) double Z-scheme heterojunction photocatalyst featuring dual charge transfer pathways. The synthesis involved a solvothermal method, where W₁₈O₄₉ nanowires were grown on Bi₁₂O₁₅Cl₆ nanosheets, followed by a calcination process to load g-C₃N₄/PDI onto the Bi₁₂O₁₅Cl₆@ W₁₈O₄₉ structure. The resulting heterojunction exhibited exceptional photocatalytic performance for BPA degradation under simulated solar light irradiation, achieving complete removal efficiency within 30 min. The superior photocatalytic activity was ascribed to the synergistic effects of the double Z-scheme heterojunction and plasmonic resonance, which significantly enhanced the separation efficiency of photogenerated carriers and the overall photocatalytic activity of the composite material.

In a series of studies on the degradation of phenolic compounds, PDI-based heterojunction photocatalysts have exhibited outstanding performance. Through various preparation techniques, such as in situ precipitation, in situ deposition photoreduction, water bath heating, and solvent exchange, researchers have successfully fabricated diverse heterojunction structures, including type II, Z-scheme, and S-scheme. These heterojunctions not only enhance the light absorption capability of the photocatalysts but also significantly improve the separation and transfer efficiency of photogenerated carriers. For example, photocatalysts such as Ag@AgCl/PDI, PDI/BiOCl, and TOC-PDI-POSS/g-C₃N₄ demonstrate remarkable photocatalytic activity under visible light irradiation. These research achievements provide versatile strategies for photocatalyst development and offer robust technical support for the efficient removal of phenolic pollutants from water.

4.3. Industrial Dyes and Other Pollutants

With the rapid advancement of the printing and dyeing industry and modern agriculture, the treatment of complex pollutants, such as rhodamine B, atrazine (ATZ), malathion (MA), sodium lignosulfonate (SL), and iodinated organic halides (IOH), has emerged as a significant challenge in the environmental domain. To address the limitations of traditional photocatalytic materials, such as poor universality and low response efficiency for multiple pollutant types, researchers have achieved breakthroughs in co-degradation by precisely designing multi-component PDI-based heterojunction systems. These advancements indicate that through heterojunction interface engineering (e.g., π–π conjugation enhancement and stepwise bandgap modulation) and multi-mechanism coupling (e.g., plasmonic resonance and the piezoelectric effect), PDI-based composite materials are evolving into a new generation of environmental remediation materials with broad-spectrum degradation capabilities and substantial engineering application potential.

Through ultrasonic-assisted self-assembly technology, Zhang et al. [128] successfully fabricated PDI/BiO_2−x type-II heterojunctions. The experimental results demonstrated that the 20% self-assembled PDI/BiO_2−x composite materials exhibited remarkable photocatalytic performance under visible light irradiation, achieving a degradation efficiency of RhB up to 98.7% within 20 min, as shown in

Table 3. The photocatalytic degradation performance of other pollutants by a PDI-based heterojunction.

Photocatalyst	Synthesis Method	Pollutants	Light Source	Time (min)	Efficiency (%)	Type	Reference
PDI/BiO2-x	Ultrasonic, self-assembly	RhB	Visible	20	98.7	II	Zhang et al. [128]
PDISA/AgBr	Co-precipitation	RhB	Visible	20	97.8	II	Xu et al. [129]
PDI/Bi2O4	Water bath heating, ultrasonic	RhB	Visible	25	98.6	II	Zhang et al. [130]
PCN@UiO-66	Solvent thermal	RhB	Visible	140	99.0	II	Mardiroosi et al. [131]
SAPDI/Bi4O7	Self-assembly	RhB	Visible	20	87.6	II	Zhang et al. [132]
g-C3N4/PDI@ZnIn2S4	Oil bath heating	RhB	Visible	120	83.9	Z	Zhu et al. [133]
PDI/g-C3N4/TiO2@Ti3C2	Calcination	ATZ	Visible	60	75.0	S	Tang et al. [134]
H-PDI/NH2-MIL-101(Fe)	Hydrothermal	MA	Simulated sunlight	180	91.1	Z	Ren et al. [135]
In2O3/PDI/In2S3	Self-assembly	SL	420 nm	80	80.9	S	Chen et al. [136]
PDI/MIL-101(Cr)	Water bath heating	IOH	Visible	35	100.0	Z	Ji et al. [137]

. This enhanced performance is primarily attributed to the formation of type-II heterojunctions, wherein the built-in electric field effectively facilitates the spatial separation of photogenerated carriers. Xu et al. [129] developed PDISA/AgBr type-II heterojunction organic–inorganic hybrid photocatalysts via a chemical co-precipitation method. The experimental findings revealed that the PDISA/AgBr-40 composite materials achieved a degradation efficiency of RhB of 97.8% within 20 min under visible light irradiation. This superior photocatalytic performance can be ascribed to the synergistic effects of type-II heterojunctions, which not only enhance the separation of photogenerated electron–hole pairs but also broaden the light absorption spectrum. Furthermore, Zhang et al. [130] successfully synthesized PDI/Bi₂O₄ type-II heterojunction photocatalysts by integrating water bath heating and ultrasonic dispersion techniques. The 5% PDISA/Bi₂O₄ composites achieved degradation efficiencies of 98.6% for RhB and 97.0% for methylene blue within 25 min of visible light irradiation. The underlying mechanism involves the formation of an internal electric field at the PDI/Bi₂O₄ interface, which significantly promotes the separation and migration of photogenerated carriers, thereby enhancing the overall photocatalytic activity.

Mardiroosi et al. [131] developed an innovative type-II heterojunction material, PDI@g-C₃N₄@UiO-66 (PCN@UiO-66), through solvothermal synthesis. This composite exhibited exceptional photocatalytic properties when degrading rhodamine B under visible light exposure for 140 min, with an impressive removal rate of 99.0%. The outstanding performance of 30PCN@UiO-66 is mainly due to its improved visible light harvesting capability and the effective segregation and movement of photoinduced charge carriers, enabled by its type-II heterojunction configuration. Zhang et al. [132] constructed a novel SAPDI/Bi₄O₇ (PB) type-II heterojunction by integrating self-assembled perylene diimide (SAPDI) with Bi₄O₇. This catalytic material showed an 87.6% decomposition rate for rhodamine B when exposed to visible light for 20 min. The improved photocatalytic efficiency stems from the type-II photoinduced charge transfer occurring between Bi₄O₇ and SAPDI, which promotes electron transfer and increases the separation of photoinduced charge carriers. Through a combination of two-step heating and oil bath techniques, Zhu et al. [133] fabricated g-C₃N₄/PDI@ZnIn₂S₄ Z-scheme heterojunctions. The g-C₃N₄/PDI@ZnIn₂S₄-0.7 sample achieved an 83.9% removal rate for RhB after 120 min of visible light exposure, outperforming pure g-C₃N₄, g-C₃N₄/PDI, and ZnIn₂S₄ by factors of 7.0, 2.2, and 1.4, respectively. The superior photocatalytic activity results from the close interfacial connection and optimized band alignment between g-C₃N₄/PDI and ZnIn₂S₄, which enables the creation of an effective Z-scheme heterojunction, improving charge carrier separation and visible light utilization. Tang et al. [134] developed a novel π–π stacked PDI/g-C₃N₄/TiO₂@Ti₃C₂(PCT) photocatalyst featuring S-scheme heterojunction through a calcination process. As illustrated in Figure 5a, the photocatalytic degradation efficiency of ATZ was significantly enhanced through the activation of PMS. Under visible light irradiation for one hour, the PDI/g-C₃N₄/TiO₂@Ti₃C₂ photocatalyst achieved a 75.0% removal rate of ATZ. The pseudo-first-order kinetic curve in Figure 5b and the calculated reaction rate constants revealed that the initial reaction rate constant of ATZ was negligible (k₁ = 0.0002). The synergistic effect of g-C₃N₄ and PMS improved the degradation of ATZ, achieving a removal rate of 31.0% (k₂ = 0.0056). The incorporation of TiO₂@Ti₃C₂ or PDI into g-C₃N₄ further enhanced ATZ degradation, with no significant difference observed in the reaction constants between the PC(PDI/g-C₃N₄) and CT(g-C₃N₄/TiO₂@Ti₃C₂) samples (k₃ = 0.0089 and k₄ = 0.0100, respectively). This enhancement is ascribed to the intermolecular π–π interactions characteristic of the PC composites and the heterojunctions formed between TiO₂@Ti₃C₂ and g-C₃N₄. The combination of the intermolecular π–π interactions of PC with TiO₂@Ti₃C₂ to form an S-scheme heterojunction significantly increased the photocatalytic degradation rate of ATZ to 75.0% in the PCT/PMS/visible light system. Electron paramagnetic resonance (EPR) spectroscopy confirmed the presence of •OH and SO₄^•− radicals through the detection of the DMPO spectrum (Figure 5c). The results indicated that both •OH and SO₄^•− were suppressed in the dark, but strong •OH peaks (intensity ratio of 1:2:2:1) and weak SO₄^•− peaks were observed after visible light irradiation. This suggests that •OH and SO₄^•− may be activated during the photocatalytic process, but do not play a dominant role in ATZ degradation. The presence of ¹O₂ was also investigated using EPR spectroscopy (Figure 5d), with the DMPO-¹O₂ signal observed only after visible light irradiation. It is hypothesized that either a type-II or S-scheme heterojunction may have formed between PC and TiO₂@Ti₃C₂ (Figure 5e). In the case of a type-II heterojunction formation (illustrated on the left side of Figure 5e), visible light excitation would cause photogenerated electrons to transfer to the valence band of PC, while holes would move to the conduction band of TiO₂@Ti₃C₂. Nevertheless, the valence band energy level of PC (2.05 eV) is inadequate for •OH production, given the high oxidation–reduction potential of H₂O/•OH (2.37 eV). Consequently, the electron transfer process cannot be accounted for by a conventional type-II heterojunction. Considering the formation of •OH radicals, an S-scheme heterojunction offers a more convincing explanation for the electron transfer dynamics in the PCT photocatalyst (depicted on the right side of Figure 5e). The intimate interface between PC and TiO₂@Ti₃C₂ promotes the establishment of an S-scheme heterojunction. When exposed to visible light, photogenerated electrons emerge in the valence bands of both PC and TiO₂@Ti₃C₂, subsequently being excited to their respective conduction bands. Electrons from PC are then transferred to TiO₂@Ti₃C₂ until the Fermi levels of both materials achieve equilibrium. This charge redistribution induces bending of the valence band in PC and the conduction band edge in TiO₂@Ti₃C₂, thereby creating an internal electric field within the PCT composite. The S-scheme heterojunction mechanism elucidates the production of •OH and, in conjunction with the conduction band of PC, the formation of •O₂⁻ (with an oxidation–reduction potential of –0.33 eV). The π–π interactions within PDI/g-C₃N₄ facilitate the delocalization of photogenerated electrons, enhancing their mobility. Additionally, the close interfacial contact and staggered band alignment between PDI/g-C₃N₄ and TiO₂@Ti₃C₂ establish an S-scheme heterojunction, thereby minimizing the recombination of photogenerated charge carriers.

Through the hydrothermal synthesis method, Ren et al. [135] successfully fabricated an H-PDI supramolecular/NH₂-MIL-101(Fe) Z-scheme heterojunction. The experimental results demonstrated that the 40% H-PDI/NH₂-MIL-101(Fe) composite material achieved a degradation efficiency of 91.1% for malathion after 180 min of simulated sunlight exposure. This remarkable photocatalytic performance is primarily attributed to the formation of the Z-scheme heterojunction structure, which significantly enhances the spatial separation efficiency of photogenerated electron–hole pairs. Chen et al. [136] developed a novel organic–inorganic hybrid photocatalyst, In₂O₃/PDI/In₂S₃ (IO/PDI/IS), by integrating solvent-induced self-assembly and electrostatic interaction mechanisms. This catalyst features a unique double S-scheme structure. Under 420 nm LED light irradiation (5 W), the mineralization efficiency of sodium lignosulfonate reached 80.9% within 80 min. This superior photocatalytic performance can be ascribed to two critical factors: first, the incorporation of layered PDI optimizes the charge transfer pathway in the IO/IS type-II heterojunction, establishing a double S-scheme charge transfer mechanism; second, the extended π–π conjugation system in layered PDI significantly enhances the internal electric field strength, thereby improving the migration and separation efficiency of photogenerated carriers. Ji et al. [137] successfully synthesized a new Z-scheme perylene diimide/MIL-101(Cr) (PM) heterojunction via the water bath heating method. Figure 5f provides a detailed illustration of the preparation process of this hybrid material. The PM heterojunction was employed for visible light-assisted persulfate activation, constructing the PM/PS/Vis photocatalytic system for the degradation of IOH in aqueous environments. In the PM-7/PS/Vis system, IOH was nearly completely degraded within 35 min, achieving a removal efficiency close to 100.0%. Additionally, under conditions simulating actual wastewater treatment plant samples, the degradation rate of IOH reached 77.5% within the same time frame. Figure 5g illustrates the reaction mechanism of the PM-7/PS/Vis system. Unlike the traditional type-II heterojunction model, where photogenerated holes migrate from MIL-101(Cr)’s valence band to SA-PDI’s valence band and electrons transfer from SA-PDI’s conduction band to MIL-101(Cr)’s conduction band, this system exhibits unique characteristics. The conduction band potential of MIL-101(Cr) (–0.046 eV) is insufficiently negative to reduce O₂ to •O₂⁻, while SA-PDI’s valence band potential (1.54 eV) lacks the necessary positive potential to oxidize H₂O into •OH. These limitations led to the development of a Z-scheme heterojunction mechanism for the PM photocatalyst. In this configuration, excited electrons from MIL-101(Cr)’s conduction band interface with photogenerated holes in SA-PDI’s valence band, maintaining the robust reducing capacity of SA-PDI’s conduction band electrons and the potent oxidizing capability of MIL-101(Cr)’s valence band holes, ultimately boosting photocatalytic performance. PS integration serves as an efficient approach to minimize electron–hole recombination in SA-PDI. This compound demonstrates dual capabilities: directly accepting electrons from SA-PDI’s conduction band to form SO₄^•− and being activated by MIL-101(Cr) to produce reactive species. Within the PDI/PS/Vis framework, radical chain reactions occur, with SA-PDI’s conduction band potential (–0.046 eV vs. O₂/•O₂⁻) enabling •O₂⁻ formation. Conduction band electrons engage in various reduction processes, transforming persulfate (S₂O₈²⁻) into SO₄^•− and H₂O into •OH. Moreover, photogenerated electrons can directly break PS’s O-O bonds, generating additional SO₄^•−. These reactive species (SO₄^•−, •OH, •O₂⁻, and ¹O₂) work in concert with h⁺ to decompose IOH. The PM system’s Z-scheme heterojunction structure exhibits remarkable charge transfer efficiency, enabling electron movement from MIL-101(Cr)’s conduction band to SA-PDI’s valence band. This mechanism, combined with PS’s function as an electron acceptor, improves charge carrier mobility and stimulates reactive oxygen species production, thereby enhancing IOH oxidation. In a related development, Ren et al. [138] successfully synthesized NH₂-UiO-66(Zr)/PDI (NUPDI) Z-scheme heterojunction photocatalysts via a facile self-assembly approach. Under visible light irradiation for 120 min, the NUPDI photocatalyst demonstrated a remarkable hexavalent chromium (Cr(VI)) removal efficiency of 99.5%, representing 24.9-fold and 1.3-fold enhancements compared to pristine PDI and NH₂-UiO-66(Zr), respectively. The superior photocatalytic performance of the NUPDI heterojunction in Cr(VI) reduction can be primarily attributed to two key factors: (1) the π–π interactions between NH₂-UiO-66(Zr) and PDI, and (2) the energy-matched Z-scheme heterojunction structure. These both collectively contribute to enhanced charge separation and transfer efficiency.

Herein, we thoroughly investigated the application of PDI-based heterojunction photocatalysts in the degradation of industrial dyes and the treatment of various pollutants. Through diverse preparation methods, such as ultrasonic-assisted self-assembly, chemical co-precipitation, and water bath heating, researchers have successfully constructed type-II, S-scheme, and Z-scheme heterojunction structures. These heterojunction photocatalysts have markedly improved the degradation efficiency of pollutants, including rhodamine B, methylene blue, and hexavalent chromium, by promoting the separation of photogenerated carriers, extending the light absorption range, and enhancing the generation of reactive species. These studies not only highlight the superior performance of PDI-based heterojunction photocatalysts in degrading industrial dyes but also broaden their potential applications in heavy metal ion treatment and other fields, offering novel strategies and approaches for environmental pollution control.

5. Conclusions

This article provided a systematic review of the existing research on PDI-based heterojunction photocatalysts in the field of organic pollutant degradation. It highlighted that PDI-based heterojunction photocatalysts, characterized by their wide spectral response, high charge separation efficiency, and excellent stability, have been demonstrated to possess significant advantages for the efficient degradation of various pollutants, including antibiotics, phenolic compounds, and industrial dyes. By constructing heterojunction structures such as type II, Z-scheme, and S-scheme (e.g., PDI/g-C₃N₄, PDI/MOFs, and PDI/BiOCl), researchers have effectively mitigated the issues of high carrier recombination rates and narrow light absorption ranges commonly found in traditional photocatalysts. These heterojunctions significantly enhance photocatalytic activity through interfacial charge transfer mechanisms. For example, the degradation rate of tetracycline using PDI-based heterojunctions can reach 70.0% to 99.7%, while the degradation efficiency for pollutants such as bisphenol A and rhodamine B exceeds 90.0%. This article further summarized the core mechanisms underlying heterojunction design, including band alignment, optimization of built-in electric fields, and enhancement of π–π conjugation effects. Additionally, it emphasized the promoting roles of synergistic effects, such as plasmonic resonance and persulfate activation, in enhancing the generation of reactive oxygen species. Future research should focus on material functionalization strategies (e.g., group modification and supramolecular assembly), expanding the light absorption range (via narrow bandgap semiconductor composites), improving interface stability (through carbon coating or MOF encapsulation), and developing multi-pollutant co-degradation systems. These efforts will facilitate the transition of PDI-based heterojunction photocatalysts from laboratory-scale studies to large-scale environmental remediation applications.

6. Prospects

6.1. Functionalization and Modification of Photocatalytic Materials

In the context of molecular engineering regulation, the water dispersibility and pollutant adsorption capacity of PDI derivatives can be enhanced through functional group modification (e.g., sulfonic acid groups and amino groups). In terms of supramolecular self-assembly technology, ordered nanostructures such as nanofibers and micelles can be constructed via π–π stacking or hydrogen bonding interactions, thereby increasing the exposure of active sites. Integration of stimuli-responsive moieties (e.g., pH-sensitive or redox-active groups) could enable smart photocatalytic systems for adaptive environmental remediation under varying wastewater conditions.

6.2. Optimization of Light Absorption and Quantum Efficiency

Despite the wide spectral response capabilities of existing PDI-based materials, there remains potential for improvement in their utilization of the full solar spectrum, particularly in the visible and near-infrared regions. By incorporating narrow bandgap semiconductors (e.g., Ag₂CO₃ and CdS) to construct heterojunctions, the light absorption range can be expanded and the separation efficiency of photogenerated carriers can be significantly enhanced. Incorporating upconversion nanoparticles (e.g., NaYF₄:Yb³⁺/Tm³⁺) could extend light harvesting to the near-infrared region, enhancing energy utilization in turbid or shaded water bodies. Furthermore, element doping (e.g., non-metals or transition metals) and surface plasmon resonance effects (e.g., noble metal nanoparticles) can be employed to fine-tune the band structure and improve the quantum efficiency of light, thereby reducing the energy consumption of photocatalytic reactions.

6.3. Enhancement of Heterojunction Interfaces and Stability

The heterojunction interface plays a critical role in determining photocatalytic performance. Future research should focus on in-depth investigations of the interfacial electron transfer mechanisms between PDI and other semiconductors (e.g., TiO₂ and g-C₃N₄) to optimize interface contact and minimize carrier recombination. Additionally, addressing the stability issues of PDI-based materials, such as photo-corrosion and chemical degradation, is essential. This can be achieved through introducing protective layers (e.g., carbon coatings or MOF encapsulation) or developing self-healing material systems to extend the cyclic service life of the catalysts. Self-healing materials (e.g., dynamic covalent networks) could autonomously repair defects during operation, extending catalyst lifespan. For industrial viability, modular reactor designs incorporating flow-through catalytic membranes or immobilized PDI composites should be developed to ensure stability under continuous operation.

6.4. Development and Expansion of Synergistic Catalytic Technologies

Hybrid systems integrating PDI photocatalysis with complementary technologies (e.g., electrocatalysis, piezocatalysis, or bioaugmentation) could unlock multifunctional remediation platforms. In photoelectrocatalytic coupling, the synergistic interaction between an external electric field and light excitation enhances the generation efficiency of reactive species such as •OH and SO₄^•−, thereby improving the degradation efficiency of organic pollutants. Photoelectrocatalytic reactors combining PDI heterojunctions with conductive substrates (e.g., carbon cloth or Ti mesh) can enhance •OH yield via bias-assisted charge separation. Microbial fuel cell–PDI hybrids enable simultaneous organic pollutant degradation and bioelectricity generation. Plasmonic photothermal–PDI systems utilize localized heat to accelerate reaction kinetics in viscous or high-salinity wastewater.

6.5. Commercialization-Oriented Innovation

To accelerate the transition of photocatalytic technology from the laboratory to industrialization, future research should prioritize the following areas: In terms of low-cost raw materials, precious metals should be replaced with abundant alternatives (such as copper or iron-based cocatalysts) to reduce material costs and enhance sustainability. In the aspect of circular economy strategies, recyclable PDI composites (such as magnetic Fe₃O₄@PDI) should be developed or closed-loop recycling schemes should be established to improve resource utilization and environmental compatibility. In the field of artificial intelligence-driven process optimization, digital twin technology should be utilized to simulate large-scale photocatalytic reactors and predict their performance under dynamic operating conditions, thereby enhancing process efficiency and scalability.

The future advancement of PDI-based photocatalytic technology is expected to be centered on the improvement of material performance via molecular engineering approaches (e.g., functional group modification and supramolecular assembly), achieving full-spectrum light utilization through the design of narrow-bandgap heterostructures and upconversion materials, and overcoming stability limitations by incorporating interface engineering and self-healing mechanisms. Concurrently, efforts will be directed toward developing light–electromagnetic synergy catalytic systems to enable the simultaneous degradation of multiple pollutants. Furthermore, by integrating circular economy strategies (such as magnetic recovery and closed-loop regeneration) with artificial intelligence-driven digital twin technology, a cost-effective, high-efficiency industrial-scale photocatalytic solution may be established, thereby promoting the intelligent and sustainable development of environmental remediation technologies.