Recent Advances in Photocatalytic Degradation of Imidacloprid in Aqueous Solutions Using Solid Catalysts

Abstract

Imidacloprid (IMI), a widely used neonicotinoid pesticide, has led to significant water contamination due to excessive use. As a result, there is an urgent need for effective and straightforward methods to remove IMI residues from water. Photocatalytic technology, an integral part of advanced oxidation processes, is particularly promising due to its renewability, high catalytic efficiency, fast degradation ratio, and cost-effectiveness. This review systematically examines recent progress in the photocatalytic degradation of imidacloprid in aqueous solutions using various solid catalysts. It provides a comparative analysis of key factors affecting catalytic performance, such as catalyst synthesis methods, reaction times, catalyst loading, and IMI concentrations. Among the solid catalysts studied, nano-ZnO achieved a higher degradation rate of IMI in a shorter period and with a reduced catalyst dosage, reaching approximately 95% degradation efficiency within one hour. Additionally, this review explores the types of heterojunctions formed by the catalysts and elucidates the mechanisms involved in the photocatalytic degradation of IMI. In conclusion, this review offers a comprehensive evaluation of solid catalysts for the photocatalytic removal of IMI from water, serving as an important reference for developing innovative catalysts aimed at eliminating organic pollutants from aquatic environments.

1. Introduction

Imidacloprid (IMI) is a systemic insecticide from the nitroguanidine class, categorized as a neonicotinoid, which represents a new generation of nicotine-like insecticides. This compound is noted for its broad-spectrum activity, high efficacy, low toxicity, and minimal residual presence, leading to its extensive application in pest control, seed treatment, and the management of termites and fleas [1,2]. IMI works by disrupting normal signal transmission in the central nervous system of pests, causing paralysis and, eventually, death. Its effectiveness, combined with a relatively low risk of resistance development in target pests, has made IMI widely adopted in agricultural practices [3,4]. A United Nations report reveals that merely 1% of all pesticides used are effective against target pests, while the remaining 99% infiltrate the environment, causing detrimental effects on ecosystems [5]. The globally accepted safe concentration range for IMI emissions in the environment spans from 0.001 to 320 micrograms per liter. IMIs typically exhibit high persistence in water, leading to contamination of surface water and groundwater, as well as entry into the food chain. Global surveys of surface water have revealed that IMI is detected in 89% to 100% of cases, with concentrations reaching up to 4.50 μg/L, underscoring its pervasive presence [6,7]. Furthermore, studies show that IMI can persist in water for over 100 days [7]. As a result, when IMI enters water bodies through spray drift or rainfall runoff after application, it can pose significant threats to fish and other aquatic species, potentially disrupting entire aquaculture populations and ecosystems [6,7]. On the other hand, excessive use of IMI has been linked to the decline of bee populations, impairing female bees’ foraging abilities, increasing competition among males, and affecting overall numbers. The ecological consequences of neonicotinoids have prompted significant concerns, particularly regarding water contamination and the broader impacts on biodiversity [8,9]

The aquatic environment is crucial for human activities and daily living. Therefore, it is imperative to explore sustainable and efficient technologies for the removal of IMI residues from water. Various methods have been developed for the degradation of organic compounds, including wet oxidation, biological oxidation, electrochemical redox processes, and advanced oxidation processes (AOPs) [10]. AOPs are widely acknowledged as the most effective approach for degrading organic pollutants in water due to their high efficiency and manageable reaction conditions [11,12]. The primary types of AOPs include the Fenton reaction and its variants, electrochemical oxidation [13], ozone oxidation [14], and photocatalytic degradation [15]. Among this, photocatalytic technology is essential in the treatment of wastewater pollutants due to its significant advantages, which include remarkable reproducibility, high catalytic efficiency, rapid degradation rates, and cost-effectiveness [16,17].

Photocatalytic technology involves harnessing light energy to enhance the redox capabilities of a photocatalyst, making it effective under various illumination conditions. When exposed to light, the photocatalyst absorbs energy, prompting electrons to transition to elevated energy states and generating electron–hole pairs. These pairs subsequently interact with oxygen and water, leading to the formation of reactive species. Ultimately, these active species, in conjunction with the electron–hole pairs, facilitate the conversion of pollutants into water and carbon dioxide [11]. Consequently, the characteristics of the photocatalyst are crucial for the effective catalytic degradation of IMI in aqueous environments. As shown in Figure 1, this review investigates the effects of various solid photocatalysts on the degradation of IMI in aqueous environments. It provides comprehensive insights into the synthesis methods, reaction times, and degradation efficiencies of different solid catalysts. Additionally, it explores and summarizes the predominant reaction mechanisms that drive the photocatalytic decomposition of IMI.

2. Photocatalytic Degradation of Imidacloprid Using Metal Oxide Photocatalysts

Metal oxide catalysts are renowned for their remarkable stability and regenerative capabilities, allowing them to maintain high catalytic activity even after multiple uses. Furthermore, these metal oxides exhibit significant environmental compatibility, thereby reducing the risk of secondary pollution in practical applications [18]. Moreover, metal oxides possess advantageous light absorption characteristics that broaden the light response spectrum of the catalysts [19]. The photocatalytic degradation of IMI using metals and their oxides catalysts are outlined and summarized in

Table 1. Comparison of metal and metal oxide materials in photocatalytic degradation efficiency of IMI.

Photocatalyst	Light Source	Catalysis Time (min)	Catalyst Loading (g/L)	IMI Concentration (mg/L)	Efficiency (%)	Refs.
TiO2	UV light	360	0.6	20	90.0%	[20]
Black TiO2	visible light	360	1.0	20	90.0%	[21]
Cu-TiO2	fluorescent bulb	60	0.5	25	45.0%	[22]
HPW/TiO2-In2O3	Xenon lamp (225 W)	300	3.6	8	83.0%	[23]
ZnO	Xenon lamp	120	0.2	5	92.0%	[24]
nano-ZnO	visible light	30	2.0	50	96.6%	[25]
CuO	UV light	50	0.5	30	99.0%	[26]
Ag-ZnO	UV light	80	0.6	25	65.0%	[27]
Mg-ZnO/Nylon,6/PMMA	UV light	240	2.5	10	78.0%	[28]
N-MgO@Fe3O4	Xenon lamps	60	0.15	10	94.7%	[29]
Ag2S/Fe3O4@Ag3PO4	Xenon lamp (300 W)	90	0.5	2	73.0%	[30]
Ag/CuNb2O6/CuFe2O4	halogen lamp	240	0.5	10	96.0%	[31]
Co3O4/PMS	solar irradiation	120	0.4	2.5	99.0%	[32]
CeO2	light tubes (18 W)	360	0.15	20	30.0%	[33]
PWO/PI	Xenon lamp (225 W)	180	2.5	20	73.0%	[34]
Au-SnO2-CdS	LED bulb	180	0.03	1.5	95.0%	[35]
In2S3/AgI-300	Xenon lamp (300 W)	60	0.5	10	76.2%	[36]
WO3/SiO2	UV light	60	0.5	5	59.0%	[37]
Bi12.7Co0.3O19.35	visible light	240	1.0	10	96.0%	[38]
TiO2-Fe-HNT	UV light	300	0.5	8	41.0%	[39]

.

2.1. Titanium Dioxide (TiO₂)-Based Solid Photocatalysts

TiO₂ is a highly effective photocatalyst known for its non-toxic nature, chemical stability, and significant reactivity. These properties make it suitable for various applications, including water purification, removal of residual pesticides, and degradation of air pollutants [40]. For example, Luminita et al. [20] developed TiO₂ photocatalysts through an eco-friendly and sustainable sol–gel approach, demonstrating effective degradation of IMI under both UV and visible light exposure. In the process of IMI photodegradation, electrons are essential. The introduction of a hole (h⁺) scavenger resulted in an increase in the degradation ratio of IMI under UV light from 69% to 90% within a 6 h period. This improvement enhances the interaction between electrons and pollutants, thereby decreasing the recombination ratio of electron–hole pairs. Furthermore, the photocatalytic efficiency of TiO₂ is significantly influenced by the presence of defects and disorder within the material, including oxygen vacancies and Ti³⁺ defects. Consequently, TiO₂ nanoparticles that are rich in defects, such as black TiO₂, serve as optimal candidates for the fabrication of heterojunctions. Therefore, Luminita and co. [21] manipulated defect formation by adjusting the mass ratio of TiO₂ to NaBH₄ in an argon environment at 350 °C, which resulted in TiO₂ powders that ranged in color from gray to black. In comparison to the original white titanium dioxide, these modified powders exhibited a remarkable enhancement in photocatalytic activity, particularly in the degradation of IMI. When subjected to visible light irradiation, black TiO₂ demonstrated a photocatalytic degradation efficiency exceeding 90% for IMI. This improvement can be attributed to its increased specific surface area, reduced bandgap, and the prevalence of Ti³⁺ and oxygen vacancies on its surface, all of which promote effective charge carrier separation.

The ability of TiO₂ to effectively absorb visible light from the solar spectrum is limited by its wide band gap, which ranges from 3.0 to 3.2 eV, resulting in catalytic activity that is mainly restricted to ultraviolet radiation. Consequently, there is an urgent need to develop more efficient photocatalysts that maintain catalytic functionality under visible light to address the shortcomings of TiO₂ and enhance overall photocatalytic performance. Recent studies have utilized various metal oxide nanoclusters as cocatalysts to serve as hole trapping centers. Among these, copper (Cu) is an inexpensive alternative that can be used for the surface modification of TiO₂. It induces visible-light photocatalytic activity in a straightforward process without introducing impurities or vacancy levels into the crystal. The photocatalytic activity of Cu-modified TiO₂ is also influenced by the phase composition of TiO₂. Generally, rutile exhibits higher photocatalytic activity compared to anatase and brookite. Additionally, the highly dispersed Cu (I) and Cu (II) sites on the TiO₂ surface contribute to the enhancement of photocatalytic activity. More importantly, Cu (II) nanoclusters grafted onto the TiO₂ surface act as visible-light-sensitive cocatalysts and enhance electron–hole separation [41]. For instance, a nano-TiO₂ supported photocatalyst was prepared by Tihana et al. [22] through electrochemical anodic oxidation and subsequently modified with varying Cu concentrations (0.2–1 M) via wet impregnation. The degradation of IMI was evaluated using an unmodified reference catalyst (0 M CuTiO₂) and five Cu-modified catalysts (0.2 M, 0.4 M, 0.6 M, 0.8 M, and 1 M) under UV–visible irradiation in a continuous flow reactor, with 0.8 M CuTiO₂ exhibiting the highest photocatalytic activity. In the 0.8 M CuTiO₂ sample, the proportion of Ti–O–Cu bonds, which indicate Cu (II) cations directly bonded to the TiO₂ surface, versus Cu (II) present as CuO nanoclusters on the TiO₂ surface, is optimal for enhancing photocatalytic activity. Conversely, when the copper concentration is either too low (0.2 M CuTiO₂, 0.4 M CuTiO₂, and 0.6 M CuTiO₂) or too high (1 M CuTiO₂), the quantity of beneficial Ti–O–Cu bonds is not adequate to boost photocatalytic activity. Additionally, Cu–O–Cu bonds signify the formation of CuO nanoparticles, which can trap the holes necessary for the oxidation of IMI Therefore, a higher concentration of CuO on the TiO₂ surface (as in 1 M CuTiO₂) inhibits photocatalytic activity.

Although H₃PW₁₂O₄₀(HPW) demonstrates strong photocatalytic activity in homogeneous systems, its practical applications are significantly enhanced by immobilizing it on supports such as TiO₂. This immobilization leverages the substantial synergistic effect between HPW and TiO₂, thereby improving the catalyst’s photocatalytic performance. Moreover, indium oxide (In₂O₃) is another n-type semiconductor with an indirect band gap of 2.8 eV, making it responsive to visible light. Building on these findings, Liu et al. [21] developed an innovative type II heterojunction visible light catalyst, HPW/TiO₂-In₂O₃, characterized by a ternary composite structure using a sol–gel method. They investigated its catalytic efficiency for degrading IMI under visible light. The HPW/TiO₂-In₂O₃ catalyst demonstrated superior photocatalytic degradation performance for IMI, achieving efficiencies that are 5.6, 9.3, and 12.5 times greater than those of HPW/TiO₂, TiO₂-In₂O₃, and TiO₂, respectively. The enhanced photocatalytic activity of HPW/TiO₂-In₂O₃ under visible light is attributed to the efficient separation of photogenerated charge carriers facilitated by the type II heterojunction, along with the suppression of carrier recombination due to the ternary composite structure.

2.2. Zinc Oxide (ZnO)-Based Solid Photocatalysts

Zinc oxide (ZnO), recognized for its chemical stability and availability, is another extensively researched photocatalyst. It has a band gap similar to that of titanium dioxide (TiO₂) and demonstrates outstanding photocatalytic activity under ultraviolet light [42]. For instance, a study by Maria’s team [24] investigated the performance and efficiency of ZnO powder prepared by the sol–gel method for photocatalytic applications. Under simulated solar radiation, the study assessed the removal efficacy of IMI and found that, at an initial IMI concentration of 5 mg/L, the photocatalytic degradation efficiency reached 92% after 4 h of light treatment. Additionally, Dhiraj et al. [25] utilized three synthesis methods, including both template-assisted and non-template techniques, with xanthan gum (XG) serving as a sacrificial template. By modulating the growth conditions, they successfully fabricated ZnO nanostructures that exhibited high specific surface area and low bandgap energy, achieving a degradation efficiency of 96.09% for IMI under UV irradiation. Although ZnO possesses numerous advantages, its wide band gap limits its efficiency in the visible light spectrum, as it mainly absorbs ultraviolet light. Additionally, the rapid recombination of photogenerated electrons and holes adversely affects its photocatalytic performance. Therefore, targeted modifications and the development of composite materials have become essential strategies to overcome these limitations [43]. For example, on the basis of ZnO materials, Amna and colleagues [26] synthesized nano-copper oxide (CuO) and nano-zinc oxide (ZnO) from okra (Abelmoschus esculentus) using the sol–gel method. These nanoparticles demonstrated excellent photocatalytic activity, achieving a removal ratio of 99% for CuO and 81% for ZnO within 60 min of exposure to IMI. In another study, Mahwish et al. [27] synthesized Ag-ZnO composite materials via a hydrothermal technique to explore their efficacy in the complete mineralization of IMI. The findings revealed that the Ag-ZnO composite demonstrated superior photocatalytic activity in degrading IMI, achieving a degradation efficiency of approximately 65% within 80 min, outperforming pure ZnO. Similarly, Khalid et al. [28] prepared Mg-ZnO/Nylon 6,6/PMMA ternary nanocomposites via a solution casting method, investigating their performance in the photocatalytic degradation of IMI. The results showed a 78% increase in photodegradation rate compared to pure Nylon 6,6 and ZnO/Nylon 6,6/PMMA composites. The Mg-ZnO/Nylon 6,6/PMMA composite exhibited high degradation activity, primarily due to the presence of Mg ions, which suppress particle growth and agglomeration, resulting in smaller particle sizes and larger surface areas. This increased surface area provides more active sites for adsorption and catalytic reactions, thereby enhancing photocatalytic efficiency.

2.3. Magnetic Solid Photocatalysts

Magnetic catalysts present multiple advantages, such as straightforward separation and recovery, minimal waste generation, high catalytic performance, and wide-ranging applicability. These features allow them to overcome the limitations associated with traditional stationary catalysts, particularly regarding recycling and reusability. For example, Stefanos et al. [29] developed an innovative nitrogen-doped magnetite-based MgO nano-catalyst (N-MgO@Fe₃O₄) for the photocatalytic activation of persulfate (PMS) under visible light, achieving remarkable degradation of IMI. The degradation ratio for a concentration of 10 mg/L IMI reached approximately 95% within 60 min. In another research, Li et al. [30] utilized a solvothermal approach to deposit Ag₃PO₄ onto Fe₃O₄ nanoparticles, subsequently synthesizing Ag₂S-doped Fe₃O₄@Ag₃PO₄ nanocomposite films through an anion exchange reaction with Na₂S (Figure 2A). This process yielded a novel photocatalyst, Ag₂S/Fe₃O₄@Ag₃PO₄, which demonstrated remarkable catalytic activity under visible light irradiation, facilitating the rapid and efficient degradation of IMI. Notably, the leaching of silver was significantly minimized compared to both Fe₃O₄@Ag₃PO₄ and pure Ag₃PO₄. After four cycles, Ag₂S/Fe₃O₄@Ag₃PO₄ still achieved approximately 83.9% degradation of IMI. The superior photocatalytic activity and stability of the Ag₃PO₄ films can be primarily attributed to the presence of Ag₂S crystals, which effectively enhanced the separation of photogenerated charge carriers. Due to the rapid recombination of photogenerated electrons and holes, the photocatalytic activity of CuNb₂O₆ semiconductor material is relatively low. Consequently, introducing noble metal nanoparticles to construct metal-semiconductor composites is considered an effective solution. For instance, Zhang et al. [31] synthesized an Ag/CuNb₂O₆/CuFe₂O₄ ternary heterojunction composite photocatalyst (Figure 2B), which exhibits high efficiency and ease of recovery. The Ag/CuNb₂O₆/CuFe₂O₄ ternary heterojunction photocatalyst demonstrates superior photocatalytic activity, primarily attributed to its enhanced absorption in the visible light region and the rapid separation of photogenerated charge carriers. Compared to CuNb₂O₆, Ag/CuNb₂O₆, and CuNb₂O₆/CuFe₂O₄, the Ag/CuNb₂O₆/CuFe₂O₄ composite shows significantly improved photocatalytic degradation activity for IMI under visible light irradiation. Furthermore, the Ag/CuNb₂O₆/CuFe₂O₄ composite can be effortlessly separated and recycled using an external magnetic field. Consequently, its remarkable photoactivity and magnetic recoverability make the Ag/CuNb₂O₆/CuFe₂O₄ composite a promising candidate for applications in the environmental sector.

2.4. Other Metal Oxide Solid Photocatalysts

The activation of PMS is efficiently catalyzed by Co₃O₄, which also exhibits photocatalytic properties driven by visible light. Upon exposure to solar radiation, PMS is synergistically activated by the cobalt species and conduction band electrons within Co₃O₄, resulting in the production of sulfate radicals and reactive oxygen species (ROS). Roberta R.M. et al. [32] prepared Co₃O₄ coatings using precipitation and vacuum filtration methods and applied this photocatalytic material to degrade IMI under continuous flow conditions. Under optimal conditions, the photodegradation ratio of IMI reached 99% after 2 h of operation. The excellent performance of the Co₃O₄/PMS/solar radiation system is attributed to the synergistic interaction between the Co₃O₄ catalyst and the Co²⁺ and Co³⁺ species in the UV component of solar radiation. CeO₂ serves as an effective photocatalyst under visible light, and prior research has demonstrated the feasibility of utilizing CeO₂ nanocrystals for wastewater treatment. In their study, Luminita et al. [33] described the performance of transparent CeO₂ films applied to glass substrates, achieving a degradation ratio of 30% for IMI within 6 h. Furthermore, the degradation ratio exhibited a linear increase with extended light exposure, ultimately reaching complete degradation after 48 h. In another study, Liu et al. [34] developed a novel direct Z-type phosphorous tungsten trioxide/polyimide (PWO/PI) photocatalyst through an in situ solid-state polymerization method to enhance the visible light photocatalytic oxidation capability of polyimide (PI). The study investigated the effects of polymerization temperature and PWO content on the physicochemical properties of the PWO/PI composite and its performance in the photocatalytic degradation of IMI. The results indicated that the PWO/PI composite exhibited visible light photocatalytic degradation efficiency for IMI approximately 3.2 times greater than that of commercial P25, with the corresponding pseudo-first-order reaction rate constant being about 2.9 times that of pure PI. The n-n heterojunction formed between SnO₂ nanoparticles and CdS nanoparticles is an excellent material for visible light degradation. CdS, as a narrow bandgap material, effectively absorbs visible light, while its interface with SnO₂ helps suppress charge recombination caused by junction potential. Therefore, Mohanta and colleagues [35] synthesized a novel Au-SnO₂-CdS ternary nanocomposite catalyst and investigated its photocatalytic degradation activity against IMI under an LED light source. The absorption capabilities of Au and CdS for visible light, along with the heterojunction structure’s role in impeding charge recombination, facilitate the photocatalytic degradation of imidacloprid under LED light induction. The results indicated that the degradation efficiency of the Au-SnO₂-CdS nano-catalyst was 1.2, 1.4, and 2.1 times higher than that of the original Au, CdS, and SnO₂ nanomaterials, respectively, achieving approximately 95% degradation efficiency.

According to the data presented in Table 1, the catalysts nano-ZnO and N-MgO @ Fe₃O₄ demonstrated superior photocatalytic performance. They achieved a higher degradation ratio of IMI in a shorter time and with a lower catalyst dosage, reaching approximately 95% degradation efficiency within 1 h. However, their inherent drawbacks cannot be overlooked. For instance, their surface reaction activity is relatively low, necessitating modification or combination with other materials to enhance catalytic performance. Many metal oxides possess wide band gaps, limiting their light absorption within the visible spectrum. Additionally, photogenerated electrons and holes tend to recombine easily, thereby reducing photocatalytic efficiency.

In contrast, carbon materials (such as graphene and carbon nanotubes) and certain metal–organic frameworks (MOFs) exhibit significantly higher specific surface areas and effectively absorb visible light. Additionally, they offer excellent electrical conductivity, which facilitates the rapid transfer of photogenerated electrons and reduces electron–hole recombination. The structure of MOFs further facilitates electron separation and transport, thereby enhancing the photocatalytic performance. In the following, the application of high-efficiency photocatalysts prepared by metal-oxide-based carbon materials and MOFs in the removal of imidacloprid in water will be reviewed.

3. Photocatalytic Degradation of IMI Using Carbon-Based Photocatalysts

Carbon-based photocatalysts, such as graphitic carbon nitride, graphene oxide, and carbon quantum dots, can produce reactive species when exposed to light. These photocatalysts offer several benefits, including a high surface area, activity under visible light, stability, and recyclability. The following is a review of recent articles on the degradation of IMI in water using carbon-based photocatalysts, providing a reference for future research.

3.1. Graphitic Carbon Nitride (g-C₃N₄)-Based Solid Photocatalysts

Graphitic carbon nitride, characterized by its appropriate band structure, metal-free composition, facile synthesis, and outstanding chemical stability, stands out as a highly attractive photocatalyst for pesticide degradation [44,45,46,47]. However, g-C₃N₄ also has some drawbacks that need to be addressed, such as the rapid recombination of charge carriers, low surface area, and visible-light-harvesting efficiency, which hinder its application [48,49,50]. Currently, various modifications of pure g-C₃N₄ have been investigated; including alterations to its electronic structure, surface modifications through element doping, metal or metal oxide doping, the introduction of defects, and composite design, in order to enhance its photocatalytic application efficiency. Different g-C₃N₄-based solid photocatalysts for the IMI degradation are listed in

Table 2. Comparison of carbon-based and composite materials in photocatalytic degradation efficiency of IMI.

Photocatalyst	Light Source	Catalysis Time (min)	Catalyst Loading (g/L)	IMI Concentration (mg/L)	Highest Efficiency (%)	Ref.
SOCN8	xenon lamp (300 W)	300	1.0	20	91.4	[51]
CNT/PCN	LED light	540	0.6	10	93.0	[52]
Au @PPy-C/g-C3N4	visible light (250 W)	25	3.0	20	96.0	[53]
g-C3N4/ZnO	UV light	35	0.6	20	95.6	[54]
g-C3N4/TiO2	W lamp (300 W)	150	1.0	10	93.0	[55]
Ag2O/g-C3N4	halogen lamp (1000 W)	120	1.0	10	80.0	[56]
Ag-BO/GCN	LED light	600	0.5	10	93.0	[57]
g-C3N4@BiOCl	sunlight	180	0.3	10	73.4	[58]
g-C3N4/BiVO4	UV light	30	0.06	20	94.2	[59]
OCN	xenon lamp (500 W)	120	0.5	3.0	94.5	[60]
HPW/ACN	CEL-LAX500 xenon lamp	350	0.6	10	90.0	[61]
MCN450/HPW	CEL-LAX500 xenon lamp	180	0.7	10	96.0	[62]
g-C3N4/KPW-0.2	xenon lamp (300 W)	180	2.0	20	91.7	[63]
13TCN-390	xenon lamp (225 W)	180	2.0	20	~90.0	[64]
CN-PANI-CQDs	xenon lamp (500 W)	70	1.0	10	80.1	[65]

.

For example, a series of sulfur–oxygen co-doped carbon nitrides (SOCN₈) were synthesized by Ma et al. [51] using a one-step thermal polymerization method and, for the first time, were employed for the photodegradation of IMI. Notably, specific doping ratios of sulfur and oxygen led to the formation of a distinctive tubular structure, which significantly improved the mass transfer capabilities and specific surface area of g-C₃N₄. Under visible light irradiation, SOCN₈ exhibited a 57.6% increase in degradation efficiency of the target insecticide compared to pure g-C₃N₄, achieving a maximum degradation rate exceeding 90%. In another study, C60/PCN nanocomposites with varying C60 ratios were synthesized by coupling phosphorus-doped g-C₃N₄ (PCN) with fullerene (C60) through an ultrasonic method by Anita et al. [66], and these nanocomposites were subsequently applied to the degradation of IMI. In the C60/PCN catalyst, phosphorus doping extended the solar light response range, prolonged the lifetime of charge carriers, and facilitated charge separation and transfer. Photodegradation experiments demonstrated that the C60/PCN/H₂O₂ and C60/PCN nanocomposites achieved removal efficiencies of 95% and 91% for IMI, respectively, significantly outperforming the degradation efficiency of the individual material catalysts. Similar to this, Anita et al. [52] successfully synthesized PCN through thermal polycondensation, utilizing melamine, ammonium oxalate, and a phosphorus dopant as precursors. The resulting PCN was then combined with carbon nanotubes (CNT), resulting in CNT/PCN nanocomposites that demonstrated enhanced photodegradation activity. The findings indicated that the CNT/PCN photocatalyst demonstrated remarkable efficiency in the photodegradation of IMI, achieving a removal ratio of 97% with CNT/PCN/H₂O₂ and 93% with CNT/PCN nanocomposites. The fabrication of an ultrafast and highly efficient visible light-responsive ternary photocatalyst was investigated by Farid A et al. [53] using an efficient, simple, and straightforward methodology. The photocatalyst contained g-C₃N₄ nanostructures and was combined with doped polypyrrole carbon black (PPy-C) and Au nanoparticles. The Au@PPy-C/g-C₃N₄ ternary photocatalyst demonstrated a 96.0% removal ratio of the target analyte IMI, achieving approximately 2.91 times greater efficiency compared to bare g-C₃N₄. This study presents a promising and effective approach for the degradation of highly toxic pollutants, thereby addressing pressing environmental challenges.

The converter slag composed of Ca₂Fe₂O₅ exhibits considerable potential as a photocatalyst; therefore, Keiko et al. [67] synthesized a composite photocatalyst of graphitized carbon nitride and converter slag (g-C₃N₄/CS) using a co-calcination method, and conducted photocatalytic degradation experiments on IMI under visible light irradiation. The optimized g-C₃N₄/CS composite, which incorporated 11.58% converter slag, demonstrated a 2.5-fold increase in the degradation ratio of IMI over a 2 h period compared to pure g-C₃N₄. Due to the tunability of the band gap in ZnO catalysts through the incorporation of metals, non-metals, metal oxides, and other semiconductors, Bansal and co-workers [54] developed g-C₃N₄/ZnO hybrid nano-catalysts with varying compositions. The g-C₃N₄/ZnO semiconductor hybrid nano-catalysts demonstrated remarkable degradation efficiency, achieving a ratio of 95.6% within just 35 min, outperforming pure ZnO and g-C₃N₄ nano-catalysts. Furthermore, total organic carbon (TOC) analysis indicated that the mineralization ratio reached 95.5% after 90 min. Analogously, Puangrat’s team [55] developed a g-C₃N₄/TiO₂ composite material using a hydrothermal method and investigated its effectiveness in the photocatalytic degradation of the IMI pesticide. This composite creates a heterojunction between g-C₃N₄ and TiO₂, which leads to a reduction in the bandgap energy and inhibits the recombination of photogenerated carriers. The g-C₃N₄/TiO₂ composite demonstrates remarkable photocatalytic activity, achieving a removal ratio of 93% for IMI within 150 min, which is 2.2 times more effective than pure g-C₃N₄.

Ag₄V₂O₇ has garnered significant attention due to its unique photoelectric and chemical properties. Nevertheless, its photocatalytic performance remains relatively limited in its unmodified form. As a result, Ag₂VO₂PO₄/g-C₃N₄ composites were synthesized by Pu’s team [68] using a hydrothermal method, which led to the formation of a novel Ag₂VO₂PO₄/g-C₃N₄ heterojunction. This newly formed heterostructure demonstrated significantly enhanced photocatalytic degradation activity for IMI under visible light irradiation, achieving degradation efficiencies that were 24.3 times higher than those of the pure catalyst and the Ag₂VO₂PO₄/g-C₃N₄ composites. Similarly, Pu et al. [56] utilized a chemical precipitation method to synthesize a range of p-n junction Ag₂O/g-C₃N₄ photocatalysts with varying Ag₂O concentrations. The resulting Ag₂O/g-C₃N₄ photocatalysts demonstrated remarkable photocatalytic activity for the degradation of IMI under both visible and near-infrared light, achieving an IMI degradation efficiency of approximately 80% within 2 h. Through both thermal polymerization and a one-step hydrothermal method, g-C₃N₄ nanocomposites were synthesized, resulting in the formation of silver-supported Bi₂O₃/g-C₃N₄ (Ag-BO/g-C₃N₄) nanocomposites [57]. When exposed to visible light, the produced Ag-BO/g-C₃N₄ exhibited a remarkable degradation efficiency of up to 93% for IMI, significantly surpassing the performance of the individual Ag-BO and g-C₃N₄ components. Furthermore, the Ag-BO/g-C₃N₄ nanocomposite showed impressive stability, with the IMI degradation ratio reducing from 98% to merely 88% over ten consecutive catalytic cycles.

Due to the overlapping energy levels in the band structure of BiOCl and g-C₃N₄, they are well-suited for the preparation of visible-light-responsive heterojunction catalysts. Building on this premise, Soumen et al. [58] employed a wet chemical method to mix g-C₃N₄ nanosheets with BiOCl nanosheets, successfully synthesizing a series of g-C₃N₄@BiOCl composites with varying mass ratios, thereby enhancing their photocatalytic performance. The g-C₃N₄@BiOCl catalyst achieved a degradation efficiency of 73.4% for IMI within 1 h. Similarly, BiVO₄ has attracted significant attention as a photocatalyst due to its low bandgap energy and high efficiency in degrading organic and inorganic pollutants. Consequently, Ajay et al. [59] effectively synthesized BiVO₄/g-C₃N₄ composites by modifying the g-C₃N₄ composition using a hydrothermal technique. This composite exhibited a specific surface area 5.4 times greater than that of BiVO₄. Although its bandgap slightly increased, the photoluminescence intensity decreased, which enhanced its photocatalytic performance. Compared to pure BiVO₄ and g-C₃N4, the BiVO₄/g-C₃N₄ composite demonstrated superior catalytic ability, achieving a degradation efficiency of 94.2% for IMI within 30 min.

Oxygen-doped carbon nitride has been demonstrated to be an effective method for enhancing its photocatalytic or non-photochemical activation performance while maintaining its metal-free characteristics. For example, Zhang et al. [60] successfully synthesized a bifunctional oxygen-doped graphite carbon nitride (OCN) catalyst through a one-step thermal polymerization method, using oxalic acid as the oxygen source and urea as the precursor for g-C₃N₄. This catalyst was designed to activate PMS for the degradation of IMI. The modifications in the electronic structure of OCN promoted electron transfer and the formation of redox sites, which in turn improved its light absorption capacity, enhanced the separation efficiency of photogenerated charge carriers, and increased their migration speed. After 2.0 h, the removal ratio of IMI achieved 94.5% with the OCN-10/PMS system. Incorporating HPW or its salts into semiconductor photocatalysts serves as an effective strategy to diminish the recombination of electron–hole pairs [69]. HPW and acidified carbon nitride (ACN) were utilized by Liu et al. [61] as raw materials. By leveraging the hydrogen bonding between the oxygen-containing functional groups on the carbon nitride’s surface and the oxygen at the terminal of the HPW, HPW was successfully doped into the carbon nitride, resulting in the preparation of HPW/ACN composite materials. The hydrogen-bonding interaction between HPW and ACN enhances the photogenerated hole–electron separation efficiency of the composite material, thereby improving its photocatalytic activity. Under visible light irradiation, the degradation ratio of HPW/ACN reached 5.8 × 10⁻³ min⁻¹, which is 16 times that of ACN, and achieved a degradation ratio of over 90% within 6 h. Similarly, Liu et al. [62] synthesized nitrogen carbon (CN) rich in uncondensed amino and carbonyl groups using urea as a precursor through a low-temperature thermal condensation method. Building on this, they performed a reversible nucleophilic addition reaction between the carbonyl of formic acid and the amino groups of CN, resulting in the formation of modified carbon nitride (MCN). Subsequently, MCN was combined with HPW to create a novel photocatalyst, MCN/HPW. Research findings indicate that MCN450/HPW exhibits outstanding performance in the photocatalytic degradation of IMI, achieving a degradation ratio constant 6.4 times greater than that of CN, with a degradation efficiency of 96% within 3 h. In another study, a g-C₃N₄/KPW-0.2 composite photocatalyst was synthesized by Liu et al. [63] using potassium phosphotungstate (KPW) as the raw material through a post-impregnation activation method, as illustrated in Figure 3A. In photocatalytic processes, KPW serves as both an electron acceptor and mediator, thereby enhancing visible light absorption and minimizing the recombination of electrons and holes. The g-C₃N₄/KPW-0.2 catalyst demonstrates outstanding photocatalytic activity, achieving a degradation ratio of 91.72% for IMI under visible light irradiation, which is 2.8 times greater than that of g-C₃N₄ alone. In a separate study, Liu and his team [64] synthesized a series of carbonitride/tungstophosphate acid (TCN) photocatalysts by utilizing phosphotungstic melamine (MPW) as the precursor and employing an in situ solid-phase thermal conversion technique (Figure 3B). The melamine molecules within the MPW hybrids can participate in polymerization reactions, resulting in the formation of carbon nitrides characterized by heptazine structural defects. In comparison to the MPW hybrid, the TCN photocatalyst demonstrates remarkable visible light catalytic activity, achieving photocatalytic degradation efficiencies and rate constants (k) for IMI that are 6.38 and 13.50 times greater than those of CN-390, respectively.

3.2. Graphene-Oxide-Based (RGO) and Carbon Quantum Dots (CQD)-Based Solid Photocatalysts

Graphene is a two-dimensional material made up of a single layer of carbon atoms arranged in a hexagonal honeycomb lattice through sp² hybridization. This remarkable material exhibits a range of exceptional properties, including high mechanical strength, low production costs, excellent transparency, outstanding electrical conductivity, significant specific surface area, high adsorption capacity, and superior thermal conductivity (Figure 4A) [70]. Graphene oxide (GO) is a derivative of graphene, typically synthesized by reacting graphite with strong oxidants in an acidic environment. During this process, carbon atoms are modified with various oxygen-containing functional groups, such as hydroxyl, carboxyl, and epoxy groups [71]. GO possesses excellent properties as a catalyst material due to its large specific surface area and rich functionality, along with stable thermal conductivity and high electron transfer rates, making it suitable for the photocatalytic degradation of organic compounds (Figure 4B) [72]. By removing the oxygen-containing functional groups, GO can be reduced to reduced graphene oxide (RGO). RGO exhibits improved chemical stability and electronic conductivity compared to GO. Additionally, the hydrophobic nature of RGO facilitates its aggregation, which can alter its morphology and specific surface area. This transformation enhances its potential applications in various fields, including catalysis, energy storage, and environmental remediation [73]. The unique properties of both GO and RGO make them valuable materials for advanced photocatalytic processes and other technological applications [74].

Table 3. Comparison of photocatalytic degradation efficiency of IMI by RGO- and CQD-based composite materials.

Catalyst	Light Source	Catalysis Time (min)	Catalyst Loading (g/L)	IMI Concentration (mg/L)	Highest Efficiency (%)	Refs.
GO@TiO2	UV light	30	0.5	100	92.6	[75]
BiVO4/RGO-TNT	UV lamp (40 W)	30	1.4	80	73.0	[76]
GO@TiO2·ZnO·Ag HNM	visible light	115	0.5	10	50.0	[77]
Ce-TiO2/RGO	UV light	480	0.5	10	85.0	[78]
TiO2/RGO/Cu2O-CuS	Full-spectrum fluorescent bulb solar (160 W)	360	1.0	20	>95.0	[79]
GO@PdO@rGO.SrO	visible light	210	3.5	30	86.0	[80]
GO/Fe3O4/TiO2-NiO	visible light	30	2.5	10	97.3	[81]
CdS/MIPs	Hg light (400 W)	90	1.0	10	84.0	[82]

presents various solid photocatalysts based on GO and RGO that have been utilized for the degradation of IMI.

Based on the unique properties of GO and semiconductor TiO₂, Maged et al. [75] synthesized GO@TiO₂ nanocomposites and applied them for the photocatalytic degradation of IMI. In this composite, GO acts as an acceptor and transporter of the photogenerated electrons produced by the TiO₂ nanoparticles, effectively reducing the recombination of photogenerated electrons and holes within the TiO₂ nanoparticles. Compared to pure TiO₂ nanoparticles, the synthesized GO@TiO₂ nanocomposite exhibited higher efficiency in the photocatalytic degradation of IMI, achieving a degradation ratio of approximately 93% within 30 min. In another study, Teng et al. [76] synthesized BiVO₄/RGO-TNT (TiO₂ nanotubes) photocatalysts using a one-step hydrothermal method. The photocatalytic efficiency of the composite material is significantly improved, and the degradation ratio of IMI can reach 73% within 30 min. Similarly, hybrid nanomaterials consisting of GO@TiO₂, ZnO, and Ag composites were synthesized by Nagi M et al. [77], where graphene oxide (GO) was used to incorporate silver nanoparticles (Ag NPs), zinc oxide (ZnO NPs), and titanium dioxide (TiO₂ NPs). The carbon nanotubes and GO served as electron acceptors from the conduction band of the nanoparticles, thereby enhancing the photocatalytic efficiency, resulting in a degradation ratio of approximately 50% within about 2 h.

Recognizing the dual functional properties of cerium-doped titanium dioxide, the Sasmita team [78] employed a hydrothermal method to synthesize cerium-doped titanium dioxide nanoparticles (Ce-TiO₂) that exhibit excellent water dispersion. Using a deposition technique, they integrated Ce-TiO₂ with reduced graphene oxide (RGO) to create a novel ternary photocatalyst, Ce-TiO₂/RGO. Compared to conventional TiO₂ catalysts, this photocatalyst exhibited superior degradation efficiency for IMI under visible light exposure. In another study, Luminita et al. [79] investigated the performance of TiO₂@Cu₂O-CuS heterostructures enhanced by RGO in photocatalytic degradation of organic pollutants using IMI as the object. This heterojunction structure significantly enhances photocatalytic activity through the complementary properties of its constituent materials and RGO, surpassing the effect of each component used alone, exhibiting a significant photocatalytic degradation efficiency of over 95% for IMI. The composite photocatalyst PdO-GO-SrO-RGO was successfully synthesized by Nagi M et al. [80] through the integration of palladium oxide (PdO) nanoparticles with n-type semiconductor strontium oxide (SrO) nanoparticles, which function as electron carriers for RGO and GO nanosheets. When applied to the degradation of IMI using an adsorption–photocatalysis method, the degradation ratio was found to be 0.0086 min⁻¹, achieving a photocatalytic efficiency of 86%. To address the challenges of separation often faced by nanocomposites due to their extremely small dimensions, Ali et al. [81] synthesized TiO₂-NiO magnetic nanosheets on nanostructured graphene oxide using a sol–gel method. These nanosheets were then utilized for the visible-light-induced degradation of the IMI. Under optimal photocatalytic conditions, the catalyst of GO/Fe₃O₄/TiO₂-NiO achieved a degradation ratio of 97% for IMI within 30 min. Furthermore, even after at least four cycles of reuse, the catalyst maintained a degradation ratio of approximately 85% for IMI.

Carbon quantum dots (CQDs) derived from biomass are carbon-based materials characterized by remarkable optical and electrical properties. CQDs possess the ability to efficiently capture sunlight, adjust their photoluminescence, and transfer light-excited electrons effectively, making them ideal for luminescent applications [83,84]. Sourced from sustainable and renewable materials, CQDs exhibit significant potential for various applications in energy and environmental sectors, owing to their excellent water solubility and biocompatibility [85]. The addition of carbon quantum dots (CQDs) to graphitic carbon nitride with polyaniline (PANI) can improve its light absorption abilities and reduce the recombination of holes and electrons. Based on this, novel CQDs decorated on PANI with hollow porous graphitic carbon nitride (CN) were fabricated via an in situ polymerization followed by an ultra-sonication [65]. The nanocomposite of CN-PANI-CQDs exhibited the high visible light absorption with a high specific surface area. Under the visible light, the degradation was higher than 80.1% within 70 min, which can be attributed to its higher charge separation and destruction of recombination rate through the heterojunction of excited electrons among CN, PANI, and CQDs. In another study, Deepak et al. [82] utilized gingerol as a template molecule, ethylene glycol dimethacrylate (EGDMA) as a crosslinking agent, and azobisisobutyronitrile (AIBN) as an initiator to synthesize CdS quantum dots through a simple precipitation method. These quantum dots were then embedded into a molecularly imprinted polymer matrix, resulting in the novel nanocomposite CdS/MIPs. The study found that CdS/MIPs exhibited significant photocatalytic degradation of IMI under visible light irradiation, achieving a maximum degradation ratio of 84%.

Based on the above analysis and the data in Table 2 and Table 3, carbon-based catalysts exhibit a marked advantage in catalytic efficiency, with degradation ratio typically exceeding 80% or 90%. This superior performance is largely due to their high specific surface area, which offers a wealth of active sites. Additionally, carbon-based materials possess good stability, giving them a clear advantage in terms of recovery and repeated use. Consequently, utilizing carbon-based photocatalysts for the photocatalytic degradation of IMI represents a promising approach to reducing the environmental impact of this pesticide. Ongoing research and development in this area could optimize photocatalytic systems, thereby fostering more sustainable agricultural practices and promoting environmental protection.

4. Photocatalytic Degradation of IMI Using Metal–Organic-Framework-Based Solid Photocatalysts

Metal–organic frameworks (MOFs) represent a category of porous coordination polymers formed from metal nodes and organic ligands. These materials are characterized by tunable pore sizes, high surface areas, exceptional thermal stability, and strong capabilities for photocatalysis and adsorption. Recent research has demonstrated that MOFs can produce electrons (e⁻) and holes (h⁺) when exposed to light, indicating their significant potential and wide-ranging applications in the photocatalytic degradation and removal of various organic pollutants [86,87].

Table 4. Comparison of photocatalytic degradation efficiency of IMI by metal–organic framework-based solid photocatalysts.

Catalyst	Light Source	Catalysis Time (min)	Catalyst Loading (g/L)	IMI Concentration (mg/L)	Highest Efficiency (%)	Refs.
TiO2/ZIF-8	UV light	240	1.0	10	55.0	[88]
MIL-101(Fe)	Blue LED light	30	-	40	100.0	[89]
MOF/BWZTO/RGO	UV visible	300	0.6	15	90.0	[90]
Bi2WO6/NH2-MIL-88B(Fe)	xenon lamp	180	0.4	10	84.5	[91]
SAO/NH2-UiO-66	xenon lamp	120	0.6	20	97.0	[92]

summarizes the research progress on the photocatalytic degradation of IMI using metal–organic framework-based catalysts.

For example, Lucija et al. [88] synthesized a composite material of zeolitic imidazolate framework (ZIF-8) and isopropyl titanate using a mechanochemical method and then utilized a hydrothermal method to prepare TiO₂ composite photocatalysts (TiO₂/ZIF-8) with three different mass fractions. The study demonstrated that TiO₂/ZIF-8 exhibited approximately 55% degradation efficiency in the photocatalytic degradation of IMI. In another study, Meral’s research team [89] successfully synthesized iron-based metal–organic framework compounds, specifically MIL-101(Fe) and its amino-functionalized derivative NH₂-MIL-101(Fe), utilizing a conventional solvothermal approach. Additionally, they produced another significant iron-based framework, MIL-121(Fe), along with its amino-functionalized counterpart, NH₂-MIL-121(Fe). These metal–organic frameworks were employed in photocatalytic applications to facilitate the photodegradation of the pesticide IMI. Under optimal photocatalytic conditions, both catalysts achieved a complete IMI removal ratio of 100% within a span of 30 min. Moreover, the study assessed the adsorption capabilities of the catalysts, demonstrating that NH₂-MIL-101(Fe) displayed enhanced adsorption efficiency compared to MIL-101(Fe). In a similar manner, Chen et al. [93] investigated amino-functionalized metal–organic framework materials NH₂-MIL-88B(Fe) and NH₂-MIL-101(Fe). Both iron-based MOFs exhibited significant adsorption capacity and Fenton-like degradation ability for IMI, making them suitable for the removal of IMI in aqueous solutions. Under optimal experimental conditions, NH₂-MIL-88B(Fe) achieved a total removal ratio of 93% for IMI, while NH₂-MIL-101(Fe) reached a removal ratio of 97%. In another study, Taher et al. [90] successfully synthesized the ternary perovskite photocatalyst Bi₂WZnTiO₉ for the first time. To enhance its photocatalytic performance, they introduced metal–organic framework particles (PCN-222(Fe)) as a co-catalyst alongside the primary photocatalyst. Furthermore, they integrated carbon materials (CMs) as non-metal co-catalysts, leveraging their low-energy band gap and high surface area. This combination led to the development of a ternary composite nano-catalyst, designated as MOF/BWZTO/RGO. Under optimal experimental conditions, the MOF/BWZTO/RGO exhibited a photocatalytic degradation efficiency of 90% for the pesticide IMI and demonstrated reusability for up to five cycles. MOFs can enhance the light absorption and degradation efficiency of Bi₂WO₆ semiconductors. In a novel approach, Chen’s research group [91] exploited the synergistic interaction between Bi₂WO₆ and NH₂-MIL-88B(Fe) to develop a highly effective photocatalyst known as the Bi₂WO₆/NH₂-MIL-88B(Fe) (BNM) heterojunction. The optimized BNM catalyst demonstrated significant degradation rates under visible light due to the production of reactive hydroxyl radicals (·OH), achieving an impressive degradation efficiency of 84% for IMI within just 3 h.

Amino UiO-66 has gained significant attention in the field of photocatalytic degradation due to its superior adsorption characteristics. However, when utilized in isolation, NH₂-UiO-66 demonstrates limited degradation efficiency, primarily attributed to its sluggish electron transfer rate. To overcome this limitation, Li et al. [92] successfully developed a straightforward binary composite, SAO/NH₂-UiO-66, through a solvothermal approach. This composite exhibited remarkable photocatalytic performance in degrading the pesticide IMI. Importantly, the addition of the long afterglow material, SAO, effectively mitigated the challenge of inadequate illumination during photocatalysis, enabling the degradation of nearly 15% of IMI even five hours post removal of the light source. Additionally, mass spectrometry and zebrafish embryo experiments were performed to confirm the structures of potential photocatalytic degradation intermediates and assess their low toxicity (Figure 5).

In summary, solid photocatalysts excel in light absorption, multifunctionality, and catalytic efficiency. However, in practical applications, these catalysts also face some unavoidable limitations. The first issue is the high cost. Many efficient catalysts, especially noble metal catalysts (such as platinum and palladium), are expensive, which increases the economic burden for large-scale applications. Additionally, the synthesis processes of certain catalysts may consume significant resources and time, further driving up costs. The second issue is stability. During the reaction process, catalysts may degrade or become inactive, particularly under extreme reaction conditions (such as high temperatures, high pressures, or strong acidic or basic environments). The stability of the catalyst directly affects its lifespan and reaction efficiency; frequent deactivation can lead to decreased reaction efficiency and increased frequency and costs of catalyst replacement. Lastly, there is the issue of reusability. Although some catalysts can be recovered and reused after the reaction, their performance may decline over multiple uses. Particularly, in the presence of pollutants or by-products, the catalyst’s surface may become covered or poisoned, reducing its catalytic activity. Therefore, despite the important role of catalysts in enhancing the efficiency and selectivity of chemical reactions, challenges related to high costs, stability, and reusability still need to be addressed through further research and technological innovation.

5. Possible Mechanism of Photocatalytic Degradation of IMI

The photocatalytic efficiency of catalysts is primarily constrained by two key factors: (1) the energy of the incident photons (hv) must exceed the bandgap (Eg), suggesting that a smaller Eg enhances the effective utilization of solar energy; (2) the redox potential of the reaction should fall within the range defined by the top of the valence band (VB) and the bottom of the conduction band (CB) [94,95]. In the current literature on the photocatalytic degradation of IMI, the degradation mechanisms are predominantly classified into three categories: Type I, Type II, and Z-scheme. Type I photocatalysts utilize single-component systems, which often exhibit limited redox capabilities and suboptimal light absorption across a broad bandgap. Furthermore, these catalysts are susceptible to the recombination of photogenerated electrons (e⁻) and holes (h⁺) within the valence band (VB), as illustrated in Figure 6a. In Type II heterojunctions, excited electrons (e⁻) from the conduction band (CB) of catalyst S2 migrate to the CB of catalyst S1, while holes (h⁺) from the VB of S1 transfer to the VB of S2, as shown in Figure 6b. This mechanism enhances the separation of photogenerated charge carriers. However, the charge transfer in Type II heterojunctions may diminish both oxidation and reduction capabilities, inadvertently overlooking the electrostatic repulsion between like charges. Additionally, these systems often encounter difficulties in achieving efficient light absorption across a wide bandgap, effective charge carrier separation, and robust redox performance simultaneously [96].

Given the constraints of conventional Type I and Type II heterostructures, there is a pressing need to create a novel heterojunction that can effectively enhance light absorption, optimize charge carrier separation, and maintain robust redox capabilities from both theoretical and practical viewpoints. Unlike Type I and Type II heterojunctions, Z-scheme heterojunctions successfully reconcile the opposing demands of narrow bandgap and strong redox properties, thereby offering an efficient redox system. As a result, Z-scheme heterojunctions hold significant potential for applications in photocatalysis and various energy conversion processes [97,98]. The formation of Z-scheme heterojunctions is inspired by the process of photosynthesis in green plants. In nature, the unique structure of green plants enables them to efficiently harness solar energy to convert carbon dioxide and water into carbohydrates and oxygen (O₂). The generation of oxygen and reduction of cofactors occur in two distinct regions of the chloroplast, with these processes taking place simultaneously. The pathway of charge transfer in this mechanism resembles the letter “Z”, effectively facilitating the separation and utilization of charge carriers to enhance the overall efficiency of the photosynthetic process (Figure 6c) [95].

5.1. Type I and Type Ⅱ Heterojunction

For example, Figure 7A illustrates the photocatalytic degradation of IMI by Bi_12.7Co_0.3O_19.35 under varying pH conditions [38]. At a pH of 1.86, an abundance of free H⁺ ions in the wastewater attaches to the catalyst’s surface. The IMI molecule, being a nicotine derivative, engages in electrostatic interactions with these H⁺ ions, leading to its adsorption on the catalyst. In contrast, at a pH of 11.0, the accumulation of free OH⁻ groups on the catalyst’s surface results in the repulsion of IMI molecules, thereby explaining the absence of adsorption during the dark reaction. In acidic conditions, the photocatalytic degradation of IMI primarily involves oxidation, where the IMI molecules are oxidized to smaller organic compounds, which are subsequently mineralized into H₂O, CO₂, and inorganic salts. Conversely, in alkaline conditions, reduction reactions predominate. Figure 7B illustrates the photocatalytic reaction mechanism of the Ag/CNO/CFO ternary photocatalyst [31]. Under visible light irradiation, both CNO and CFO generate photogenerated electrons and holes. Due to the more negative CB energy level of CNO compared to that of CFO, the photogenerated electrons produced in CNO migrate to the CB of CFO and subsequently transfer to the Ag nanoparticles. Meanwhile, the photogenerated holes can transfer from the VB of CNO to the VB of CFO. Consequently, the electron transfer processes from CNO to Ag and/or from CNO to CFO and then to Ag significantly enhance the separation efficiency of photogenerated charge carriers in the Ag/CNO/CFO ternary heterojunction photocatalyst. In Figure 7C, RGO nanoparticles are stimulated by visible light, leading to the generation of e⁻ and h⁺ [90]. The electrons excited by light are transferred to the CB of the BWZTO triple perovskite, while dissolved oxygen is converted into reactive oxygen species (ROS). Simultaneously, the positively charged h⁺ that accumulate on the surface of the MOF particles facilitate the oxidation of water molecules, producing hydroxyl radicals. Ultimately, the generated hydroxyl (·OH) and superoxide (·O⁻₂) species contribute to the degradation of pollutants, such as IMI, into non-toxic byproducts. The photocatalytic degradation mechanism of IMI, as depicted in Figure 7D, involves the excitation of the Bi₂WO₆ semiconductor and NH₂-MIL-88B(Fe) under visible light irradiation, leading to the formation of electron–hole pairs through the promotion of electrons from the VB to the CB. Notably, the VB potential of NH₂-MIL-88B(Fe) (2.68 eV) exceeds that of OH⁻/·OH (2.38 eV), facilitating the generation of·OH by OH⁻ under visible light conditions [91].

5.2. Z-Scheme Photocatalytic Mechanism

Compared to traditional Type I and Type II heterojunction photocatalysts, Z-type heterojunction photocatalysts exhibit broader applications due to their wide light response range and excellent redox capabilities (Figure 8). To construct a visible-light-driven Z-type photocatalyst, the selected semiconductor components must be able to be excited by visible light, and their energy band potentials need to be well-matched to ensure effective recombination of photogenerated electrons (e⁻) in the conduction band (CB) of one semiconductor with holes (h⁺) in the valence band (VB) of the other semiconductor. For example, the photocatalytic efficiency of the Z-scheme g-C₃N₄/TiO₂ composite is markedly superior to that of TiO₂ or g-C₃N₄ when used independently, as illustrated in Figure 8A. When exposed to UV–Vis light, TiO₂ absorbs photon energy, which excites electrons from the valence band (VB) to the conduction band (CB). Concurrently, the holes generated in the valence band of TiO₂ remain there, while the electrons excited into the conduction band can swiftly transfer to the valence band of g-C₃N₄ due to their close proximity. This transfer allows for the electrons in the valence band of g-C₃N₄ to be further excited into its conduction band. This mechanism effectively enhances the separation of photo-induced electron–hole pairs, thereby improving their redox capabilities.

As illustrated in Figure 9A, electrons excited from the valence band (VB) of Ca₂Fe₂O₅ to the conduction band bottom (CB) can effectively migrate to the VB of g-CN through a well-constructed Z-scheme heterojunction between g-CN and Ca₂Fe₂O₅ [67]. This migration occurs via the CB of Ca₂Fe₂O₅, which hinders the recombination of electrons and holes. Consequently, the formation of interface heterojunctions within g-CN/CS composites enhances photocatalytic activity by facilitating the separation of generated electrons and holes. Additionally, the development of AgI nanoparticles (NPs) on In₂S₃ hollow tubes to create a stable Z-scheme heterojunction presents an innovative approach to enhance the intrinsic activity of In₂S₃ hollow tubes while significantly increasing the accessibility of active sites. When photoexcited, electrons from the conduction band of AgI will transfer to the valence band of In₂S₃, where they recombine with positive holes, thereby facilitating the effective separation of conduction band electrons from In₂S₃ and valence band holes from AgI (Figure 9B) [36]. In the Z-type heterojunction shown in Figure 9C, the electrons (e⁻) in the CB of Ag₃PO₄ transfer to the VB of Ag₂S, where they recombine with holes (h⁺) [30]. The electrons in the conduction band of Ag₂S react with oxygen (O₂) to generate superoxide anions (·O₂⁻), which subsequently form hydrogen peroxide (H₂O₂). H₂O₂ has a degradation effect on IMI, while the holes in the valence band of Ag₃PO₄ also possess the capability to directly oxidize IMI. In the Z-scheme photocatalytic system illustrated in Figure 9D, the photocatalytic degradation of IMI using the PWO/PI photocatalyst involves not only the direct oxidation of IMI by H+ but also the reaction of conduction band e⁻ from PI with O₂ to produce ·O₂⁻, which further contributes to the oxidation of IMI [34].

Through the analysis above, it is clear that in Type I heterojunctions, the conduction band and valence band of one material are entirely within the energy bands of another material. This structure leads to a high likelihood of photogenerated electrons and holes recombining, which reduces the number of effective photogenerated charge carriers and consequently lowers photocatalytic efficiency. In contrast, Type II heterojunctions feature staggered energy levels between the conduction and valence bands of the two semiconductor materials. In this arrangement, photogenerated electrons and holes migrate to different materials, which partially reduces carrier recombination. However, this can also result in the accumulation of electrons and holes within their respective materials, potentially leading to inefficiencies. Z-type heterojunctions connect the valence band of one semiconductor to the conduction band of another, allowing photogenerated electrons and holes to reside in materials with stronger reducing and oxidizing properties. This configuration maximizes the redox capabilities of the photogenerated electrons and holes while effectively suppressing carrier recombination, significantly enhancing photocatalytic efficiency. Overall, the design of the Z-type heterojunction plays a crucial role in optimizing the performance of photocatalytic materials.

6. Conclusions and Future Perspectives

This review offers a comprehensive examination of the research progress on various solid photocatalysts used for the photocatalytic removal of the pesticide imidacloprid (IMI) from water. It compares different photocatalysts based on their synthesis methods, light sources, and reaction times. Furthermore, this review discusses the mechanisms involved in the photocatalytic degradation of IMI, highlighting the effectiveness of various solid catalysts in this process. By addressing these aspects, this review aims to enhance our understanding of the degradation mechanisms, ultimately guiding future research and development in the field of photocatalytic remediation of water pollutants.

Building on this foundation, future research could prioritize the development of catalysts with enhanced stability to fulfill practical application requirements, improve cost-effectiveness for large-scale deployments, or investigate hybrid or modified catalysts that integrate the discussed materials’ properties. These directions will facilitate the advancement of photocatalytic technology in environmental remediation and practical applications. Moreover, future research should investigate the synergistic effects of various solid catalysts to improve degradation efficiency. Focus should be directed towards comprehensive studies of photocatalytic reaction mechanisms to clarify the distinct roles of different catalysts during the process. This will serve as a reference for optimizing catalyst design and enhancing their effectiveness in practical applications. Utilizing advanced characterization techniques can further deepen our understanding of the relationship between catalyst surface properties and reaction performance, providing critical insights for the development of future catalysts.