Recent Advances in Carbon Dots-Based Photocatalysts for Water Treatment Applications

Abstract

Carbon dots (CDs), a rapidly emerging class of zero-dimensional (0-D) nanomaterials with small particle sizes (<10 nm), have garnered significant scientific interest owing to their exceptional physicochemical properties, non-toxicity, low-cost synthesis, and versatile applications. In recent years, the combination of various inorganic photocatalysts (e.g., metal oxides, metal chalcogenides, metal oxyhalides, MXenes, non-metallic semiconductors) with CDs has gained momentum as a promising strategy to enhance their photocatalytic efficiency. By incorporating CDs, researchers have addressed fundamental challenges in photocatalytic systems, including limited light absorption range, rapid electron–hole recombination rate, low quantum efficiency, etc. The present review is focused on the most recent developments in CDs-based heterostructures for advanced photocatalytic applications, particularly in the field of environmental remediation, providing a comprehensive overview of emerging strategies, synthesis approaches, and the resulting enhancements in photocatalytic water treatment applications.

1. Introduction

1.1. The Role of Photocatalysis in Water Treatment

Undoubtedly, water is a fundamental prerequisite for the existence and sustainability of life on Earth. Nevertheless, water quality encounters severe challenges, due to various factors, including natural processes (e.g., geochemical activity, forest fires, etc.), anthropogenic influences (e.g., agricultural and mining activities, industrial discharges, rapid population growth, etc.), and climate change, resulting in increased levels of pollutants and contaminants in water bodies [1]. The presence of emerging contaminants into the aqueous environment, such as heavy metals [2], pesticides [3], pharmaceuticals [4], dyes [5], polycyclic aromatic hydrocarbons [6], microplastics [7], etc., is a major concern for humans, aquatic species, animals, and vegetations. Therefore, their removal from water is vital for the protection of both human health and ecosystems. The scientific community has dedicated efforts to the successful elimination of water pollutants via several physicochemical methods, including photocatalysis [8], membrane separation [9], coagulation/flocculation [10], adsorption [11], biodegradation [12], etc.

Among these techniques, photocatalysis has emerged globally as a sustainable solution because of its high effectiveness, low-cost, and the ability to degrade a wide range of pollutants under mild conditions. Although the term “photocatalysis” has been a subject of substantial confusion and debate regarding its precise definition, it is officially restricted to reactions that occur in the presence of a semiconductor and illumination, according to the International Union of Pure and Applied Chemistry (IUPAC) [13]. Briefly, during a photocatalytic process (Figure 1), the semiconductor absorbs incident photons with energy equal to or greater than its band gap (hv ≥ E_g), leading to the excitation of electrons (e⁻) from the valence band (VB) to the conduction band (CB) and leaving behind holes (h⁺) in the VB. Electrons and holes can participate in various surface chemical reactions; however, these charge carriers may also undergo a recombination step followed by the production of phonons or heat. When the separated holes interact with H₂O, they oxidize donor molecules and form ·OH radicals, while the separated electrons react with dissolved O₂ to produce superoxide radicals (O₂·⁻), which can further react to generate ·OH. The above reduction/oxidation (redox) reactions are listed below (Equations (1)–(9)):

Semiconductor + hv → h⁺ + e⁻

(1)

e⁻ + O₂ → O₂^·−

(2)

O₂^·− + H⁺ → ^·OOH

(3)

2 ^·OOH → O₂ + H₂O₂

(4)

H₂O₂ + O₂^·− → ^·OH + OH⁻ + O₂

(5)

H₂O₂ + hv → 2 ^·OH

(6)

h⁺ + H₂O → ^·OH + H⁺

(7)

h⁺ + OH⁻ → ^·OH

(8)

Pollutants + (^·OH, h⁺, e⁻, ^·OOH, ^·O₂⁻) → Degradation Products

(9)

1.2. Limitations of Inorganic Photocatalysts

Over the years, various semiconductors have been employed as photocatalysts for the elimination of both organic and inorganic contaminants from water. Based on the type and composition, inorganic photocatalysts could be broadly clustered into the following categories: i. metal oxides (e.g., TiO₂, ZnO, Fe₂O₃, BiO₂, SrO₂, WO₃, etc.) [15], ii. metal chalcogenides, including sulfides (e.g., CdS, ZnS, Ag₂S, MoS₂, In₂S₃, etc.) [16], selenides (e.g., CdSe, ZnSe, MoSe₂, WSe₂, Ag₂Se, Bi₂Se₃, etc.) and tellurides (e.g., CdTe, ZnTe, etc.) [17], iii. metal halides/oxyhalides (e.g., AgCl, BiOBr, BiVO₄, etc.) [18], iv. 2-D metal carbides/nitrides (MXenes) (e.g., Ti₃C₂, V₂ZnC, Ti₃ZnC₂, Ti₂ZnN, etc.) [19], and v. metal-free materials (e.g., graphitic C₃N₄ (g–C₃N₄), etc.) [20]. Each class of materials exhibits distinct physicochemical properties, which make them suitable for various photocatalytic applications such as environmental remediation [21], antibacterial disinfection [22], self-cleaning [23], hydrogen evolution [24] and CO₂ reduction [25]. Metal oxides present unique electronic versatility, ranging from conductive to semiconductive and insulating, due to the wide band gap that influences their photocatalytic performance, while they are known for their high chemical and thermal stability [26]. Metal chalcogenides typically show narrower band gaps, compared to metal oxides and thus demonstrate remarkable light-harvesting capacity. In addition, they show effective charge carrier migration capacity [16]. Recently, 2-D MXenes have attracted significant attention due to their interesting properties, such as hydrophilicity, high specific surface area, tunable surface chemistry, and exceptional electrical conductivity, all of which contribute to superior charge carrier dynamics and reduced recombination rates [27]. Last but not least, g–C₃N₄, with exceptional chemical stability and a favorable electronic band structure, stands as a prominent and sustainable non-metal photocatalyst [20].

Despite their unique advantages, the aforementioned inorganic photocatalysts exhibit certain limitations that may hinder their practical applications. For instance, metal oxides, such as the well-studied TiO₂ [28] and ZnO [29], possess large band gaps (~3.2 eV and ~3.3 eV, respectively) and they could be activated under ultraviolet (UV) irradiation. This results in the inefficient exploitation of solar light since UV light only accounts for ~4–5% of the sunlight spectrum. Furthermore, metal oxides suffer from low quantum efficiency, arising from the high charge carrier recombination rate [30]. Metal chalcogenides also undergo rapid carrier recombination of e⁻–h⁺. In addition, they are susceptible to photo-corrosion and often contain toxic elements (e.g., Cd), limiting their environmental applicability [31]. Metal halides and oxyhalides, including perovskite materials, typically exhibit limited visible (Vis) light absorption and encounter stability issues in the aqueous environment [32]. Moreover, they may release toxic ions (e.g., Pb²⁺), which raises concerns regarding their environmental safety. Although MXenes have a number of attractive properties due to their unique 2-D structure, they are vulnerable to oxidative degradation reactions with water and/or oxygen because of their inherent defective sites [33]. They also face restacking and aggregation phenomena due to van der Waals interactions between nanosheets, which could hinder their photocatalytic activity [34]. Similarly, a 2-D metal-free g–C₃N₄ semiconductor faces aggregation tendencies. Furthermore, its modest visible light absorption and rapid charge carrier recombination lead to decreased efficiency [35].

In order to overcome these drawbacks, several strategies have been proposed, including the following: i. doping with metal or non-metal elements so as to decrease the band gap and enhance visible light absorption [36], ii. combination with carbon nanoallotropes [37] or conductive polymers [38] to improve the charge separation and reduce the recombination rates, or iii. surface modification to eliminate toxicity [39] and photo-corrosion [40], as well as agglomeration phenomena [41]. It is evident that the growing demand for enhanced photocatalytic properties has driven ongoing investigations into material combinations, with novel functionalities prompting the scientific community to shift emphasis toward hybrid materials [42].

Following this direction, the integration of inorganic photocatalysts with carbon dots (CDs)—the latest addition in the broad family of carbon-based nanomaterials—has emerged as a promising strategy and has garnered considerable attention over the past few years. CDs are currently at the forefront of research in nanomaterials due to their advantageous properties such as their fascinating optical properties, abundance of surface functional groups, non-toxicity, etc. The aim of this review is to highlight the most recent advances on the scientific area of CDs-based inorganic photocatalysts for water treatment applications, as reported in the literature.

2. The Combination of Inorganic Photocatalysts with Carbon Dots (CDs)

2.1. CDs (Structure, Properties, Synthesis)

In 2004, Xu et al. [43] serendipitously observed fluorescent carbon nanoparticles during the electrophoretic purification of single-walled carbon nanotubes. This observation, while not directly related to CDs, contributed to the early conceptualization of CDs as a unique class of carbon-based nanomaterials. They are typically small size (<10 nm) quasi-spherical nanoparticles, consisting of a graphitic, amorphous or semi-crystalline carbon core, surrounded by a large number of functional groups, as shown in Figure 2a. Due to the development of several top-down and bottom-up methods, as well as the abundance of precursor materials (e.g., graphite, polymers, small organic molecules, agricultural by-products, etc.), different structures of CDs could be obtained. Therefore, based on their structure, CDs are classified into the following groups: i. carbon quantum dots (CQDs), ii. graphene quantum dots (GQDs), iii. carbon nanodots (CNDs), and iv. carbon polymer dots (CPDs) (Figure 2b). Briefly, CQDs consist of a crystalline graphitic core surrounded by a multitude of functional groups. Similarly, GQDs are composed of segments of one or a few (<5) graphene (or graphene oxide) sheets encompassed by various surface functional groups. On the other hand, CNDs possess an amorphous carbon core. In contrast to the aforementioned categories, CPDs exhibit a low degree of carbonization and contain numerous functional groups and polymeric chains both in the core and the outer shell [44].

Due to their outstanding physicochemical properties, CDs have been utilized in numerous research domains, including photocatalysis. First and foremost, CDs are well known for their strong light absorption in the UV–Vis region (Figure 3a). This is attributed to the π–π* electron transitions associated with sp²-hybridized C=C bonds in the carbon core, as well as the n–π* electron transitions taking place in the shell, facilitated by the presence of abundant functional groups (e.g., -OH, -COOH, -NH₂, etc.) [46]. Furthermore, the photoluminescence (PL) is probably one of the most fascinating characteristics of CDs; however, its origin still remains a highly debated aspect. Studies have shown that both carbon core (Figure 3b) and surface state (Figure 3c) play a critical role in the PL mechanism. More specifically, sp² carbon cores could produce size-dependent fluorescence because of the quantum confinement effect, whereas surface functional groups could introduce localized energy levels, which trap excitons, leading to radiative emission [47]. Another important aspect in the optical performance of CDs is their resistance to photochemical degradation, which makes them ideal candidates for applications where materials are exposed to light for prolonged time and high photostability is required [48]. In addition to their remarkable optoelectronic properties, CDs are environmentally friendly and non-toxic nanomaterials [49], making them highly attractive to a wide range of technological fields. In addition, the ability to synthesize CDs using low-cost precursors, such as biomass waste and industrial or agricultural by-products, highlights their cost-effectiveness and sustainability [50]. Lastly, the rich surface functionality of CDs results not only in excellent water solubility, but also in the interactions with other molecules (e.g., water pollutants) or materials, forming novel heterostructures with enhanced properties.

CDs are synthesized via numerous techniques, which are classified into top-down and bottom-up methods (Figure 3d); the former approach involves the break-down of larger carbon-based structures (e.g., graphene), while the latter approach is based on the polymerization and carbonization of small organic molecules. The synthesis of CDs via top-down approaches involves techniques such as laser ablation [51], arc discharge, and chemical/electrochemical oxidation [52], which typically demand high cost and advanced equipment. On the other hand, bottom-up methods including hydrothermal/solvothermal synthesis [53,54,55] are facile and cost-effective. Notably, the synthesis method of CDs strongly influences their structural and physicochemical properties. CDs produced via a top-down approach typically present a wide size distribution and low quantum yield due to the introduction of surface defects during the harsh processing conditions. Nevertheless, the graphitic core is preserved, since the precursors are graphitic materials (e.g., graphite, graphene, etc.), resulting in enhanced conductivity. In addition, it has been suggested that the top-down methods enable superior control over structure and purity of the final products. In contrast, CDs prepared via bottom-up methods, such as hydrothermal synthesis, present improved control over size and surface passivation, leading to higher fluorescence quantum yields, as well as tunable optical properties. However, by-products and agglomerates could be formed as a concurrent process during the bottom-up synthesis of CDs, which may compromise their physicochemical characteristics. Therefore, further purification of the final product, via filtration, dialysis, electrophoresis, or chromatography is needed; this step is crucial to ensure the quality and reproducibility of the as-synthesized CDs, especially when aimed at optoelectronic or biomedical applications.

Figure 3. (a) Light absorption of CDs in the UV–Vis range [56]. The fluorescent properties of CDs are associated with (b) the quantum confinement effect and (c) the surface state. (Adopted from [47] with permission). (d) Top-down and bottom-up synthesis methods of CDs [57].

Figure 3. (a) Light absorption of CDs in the UV–Vis range [56]. The fluorescent properties of CDs are associated with (b) the quantum confinement effect and (c) the surface state. (Adopted from [47] with permission). (d) Top-down and bottom-up synthesis methods of CDs [57].

2.2. In Situ and Ex Situ Synthesis of CDs-Based Heterostructures

The integration of CDs with various inorganic materials, such as metal oxides, metal chalcogenides, MXenes, metal oxyhalides or non-metallic materials can be achieved through two main approaches: in situ and ex situ synthesis (Figure 4). The in situ synthesis involves three plausible routes: i. utilization of pre-formed CDs and inorganic precursors, ii. utilization of carbon-based precursors and pre-formed inorganic materials, or iii. employing the starting materials of both CDs and inorganic materials, simultaneously. By enabling the in situ synthesis of CDs alongside inorganic materials, either sequentially or simultaneously, strong interactions between the components and enhanced homogeneous dispersion are achieved, attracting the interest of materials scientists and engineers. Among various in situ strategies, hydrothermal or solvothermal synthesis is the most commonly employed and widely discussed in the literature; this is due to its low-cost, environmental friendliness, simplicity, control over reaction conditions and the ability to obtain products with high crystallinity and purity. During the hydrothermal/solvothermal method, the precursor mixture is placed into a Teflon-lined autoclave and heated under pressure, higher than atmospheric. The physicochemical properties of the as-prepared materials depend on the selected precursors and solvent, as well as the reaction conditions (e.g., temperature, time, pH, etc.). This review highlights a wide range of studies that rely on the in situ synthesis of CDs-based heterostructures using a hydrothermal/solvothermal process. On the other hand, the ex situ approach refers to the combination of CDs and inorganic materials that have been prepared separately in a prior step. Although the ex situ approach is more favorable for the large-scale production of hybrid materials, it presents some drawbacks, including low uniform dispersion and relatively weak interactions among the components. However, the nature of interactions depends on the synthesis method. For instance, when CDs and inorganic particles are mixed under ambient conditions via magnetic stirring, electrostatic interactions are more likely to dominate, whereas hydrothermal conditions may promote the formation of both covalent and non-covalent bonds.

2.3. CDs–Metal Oxide Heterostructures

Metal oxides constitute a versatile class of semiconductor materials with energy band gaps typically ranging from 1.0 to 4.0 eV (Figure 5a), which are widely employed in photocatalytic applications, including water treatment, due to their favorable electronic properties, chemical, and thermal stability, as well as their abundance [26]. Nevertheless, they face certain limitations, such as low exploitation of the full solar spectrum and low quantum efficiency due to the high charge carrier recombination rate, hindering their overall performance. Thus, various strategies have been proposed in order to enhance their photocatalytic activity. Among them, the modification of metal oxides with non-toxic and low-cost CDs has garnered considerable attention in the scientific community, reflected in the growing number of research articles on the above topic (Figure 5b).

Within the broad family of metal oxides, titanium dioxide (TiO₂) stands out as the most well-known photocatalyst. Due to its cost-effectiveness, chemical stability, and intrinsic non-toxicity, TiO₂ is still one of the most attractive photocatalytic materials. Yet, its wide band gap (≈3.2 eV for anatase, ≈3.0 eV for rutile) restricts absorption almost exclusively to UV light, which represents <5% of the solar spectrum. In addition, the fast recombination of photogenerated electrons and holes keeps the quantum yield low. By introducing dopants, tailoring nanostructures (nanotubes, mesoporous spheres, hierarchical films) to increase the active surface or creating heterojunctions with CDs, researchers have succeeded in extending its absorption into the visible range and enhancing charge separation, thereby unlocking its potential for solar-driven degradation of dyes, pesticides, and other contaminants in agro-industrial wastewaters.

To begin with, various CDs-TiO₂ hybrid photocatalysts have been used for the degradation of various compounds such as organic dyes and antibiotics, including [58] tetracycline (TC) and amoxicillin (AMX), as well as the reduction of hexavalent Cr (VI). For instance, Camilli et al. [59] investigated the role of different surface-functionalities of CDs in the enhancement of the visible-light photocatalytic performance of TiO₂ nanoparticles (NPs). More specifically, OH-functionalized, N-functionalized, and P-functionalized CDs were synthesized and used for the production of CDs-coated TiO₂ hybrid materials, followed by photocatalytic experiments in the degradation of glucose. In all cases, the addition of CDs to TiO₂ led to decrease of the band gap from 3.08 eV for the pristine TiO₂ down to 2.92 eV, 2.83 eV, and 2.89 eV for OH-CDs-TiO₂, N-CDs-TiO₂, and P-CDs-TiO₂, respectively, whereas the N-CDs heterostructure exhibited the best performance in glucose degradation. In addition, this work enhances the sustainability and green nature of the photocatalytic process by incorporating CDs obtained from waste materials. Recently, a CDs–TiO₂ hybrid material was developed by Sendao et al. [60], utilizing corn stover as a precursor of CDs, which is a major agricultural waste. The heterostructure was tested under visible light on multiple dyes, including Methylene Blue (MB), Rhodamine B (RhB), Orange II, and Methyl Orange (MO), achieving a degradation efficiency of 90.8%, 81.7%, 67.9%, and 37.0%, respectively. Furthermore, the performance of the photocatalyst in a mixed-dye system consisting of MB and RhB, was also examined, where CDs–TiO₂ showed a selective preference toward MB over RhB with a degradation efficiency of 89.1% and 43.7%, respectively. Ma et al. [61] developed, via hydrothermal treatment, a CDs–TiO₂ hybrid photocatalyst, using CDs derived from peanut shells. The hybrid material was tested in order to evaluate its photocatalytic performance in the reduction of Cr (VI), as it is presented in Figure 6. Various experimental factors, such as CDs-content (1–3%), photocatalyst dosage, and pH value were investigated. Regarding the CDs loading, the hybrid material containing 1% of CDs presented the best performance, whereas the increase of the photocatalyst concentration from 0.1 g/100 mL to 0.3 g/100 mL led to complete reduction of Cr (VI) within 60 min. Furthermore, the CDs–TiO₂ photocatalyst was tested in domestic sewage water achieving a reduction efficiency of 90.09%, whilst also exhibiting photocatalytic disinfection of E. coli. Herein, it is important to note that five regeneration cycles were conducted for the hybrid material presenting the best performance (CDs (1%)–TiO₂) in order to evaluate its practical applicability. Interestingly, even after the last cycle, the reduction efficiency was 96.83%. The characterization of the material before and after recycling with several techniques (e.g., FT-IR, XRD, XPS), showed that the chemical environment of the photocatalyst did not undergo significant alterations. Consequently, the combination of enhanced photocatalytic performance and excellent stability makes it well-suited for real-world photocatalytic applications.

Several research groups have engineered CDs–TiO₂-based photocatalysts with more complex morphologies to enhance visible-light harvesting and promote charge-carrier separation at heterojunctions. On this basis, Rawat et al. [62] reported a two-steps synthesis procedure of CDs–TiO₂ photocatalytic heterostructure (Figure 7a). Firstly, CDs were prepared via microwave irradiation of allylamine and ascorbic acid precursors. Following this, the CDs–TiO₂ mixture was converted into CDs-decorated TiO₂ nanotubes (NTs) through hydrothermal treatment. The CDs–TiO₂ NTs outperform both unmodified TiO₂ and CDs–TiO₂ NPs in the visible-light degradation of TC. In addition, the band gap shrinks from 3.02 eV for TiO₂ to 2.96 eV for CDs–TiO₂ NPs and 2.92 eV for CDs–TiO₂ NTs (Figure 7b). The results indicate that the morphological characteristics of TiO₂ have a significant effect in the photocatalytic performance.

Moreover, a CDs-decorated Ni-doped TiO₂ hybrid photocatalyst in which single Ni atoms are incorporated into the TiO₂ lattice and CDs onto the surface of TiO₂ was developed by Bui et al. [63]. The photocatalytic performance was evaluated by testing the degradation of ciprofloxacin (CPX). Integrating CDs onto Ni-doped TiO₂, results in a visible-light-responsive photocatalyst for antibiotic degradation, combining single-atom-induced oxygen vacancies (OVs) alongside CDs-driven up conversion and improved charge separation. Under visible-light irradiation for 150 min, the degradation efficiency of commercial TiO₂ was 27%, while sol-gel synthesized TiO₂ achieved 60% removal. Introducing Ni into the TiO₂ lattice increased degradation to 80%, achieving the same percentage of degradation as CDs-decorated TiO₂ (without Ni). However, the CDs–Ni-doped TiO₂ hybrid photocatalyst achieved 97% CPX removal from water. Furthermore, Taghiloo et al. [64] reported the design of a multifunctional CQD–TiO₂-based hydrogel, which combines dye adsorption and visible-light photocatalysis for wastewater treatment. Photocatalytic degradation studies of MB were performed evaluating the photocatalyst dose, initial pollutant concentration and pH value. Moderate pollutant concentration, an optimal photocatalyst dose (8 mg/20 mL), and mildly basic conditions (pH = 8–10) yielded the best combined adsorption–photodegradation performance. The CQDs–TiO₂–based hydrogel achieved 97% degradation of MB under visible-light irradiation, outperforming bare TiO₂ NPs. This degradation efficiency is attributed to the synergistic combination of dye adsorption by the hydrogel’s network and photocatalytic degradation at the CQDs–TiO₂ interface. Moreover, the photocatalytic-based hydrogel maintained its high performance over five consecutive degradation cycles, demonstrating good reusability.

Zinc oxide (ZnO) is also a well-studied semiconductor, chemically and physically stable, exhibiting high photosensitivity and biocompatibility, making it a promising photocatalytic material. However, its relatively wide bandgap energy of 3.37 eV limits its photocatalytic efficiency under visible light. Due to their high surface-to-volume ratio and nanoscale size, ZnO NPs serve as effective photocatalysts. To enhance their performance, N-doping, or/and the incorporation of nanocarbon-based supporting materials, such as CDs, have been shown to reduce the bandgap energy and suppress the recombination rate of electron–hole (e⁻–h⁺) pairs, thereby improving overall photocatalytic activity.

ZnO decorated with CQDs have been presented by several research groups for the photocatalytic degradation of dyes, especially MB [65,66,67,68], but also MO [69]. In most cases, the CDs–ZnO hybrid materials are synthesized via hydrothermal or solvothermal methods. In their study, Bhavsar et al. [70] achieved the increase of photodegradation of MB, MO, and RhB from 82.26%, 46.46%, and 62.87% assigned to pristine ZnO NPs, to 90.87%, 57.95%, and 86.70% for CDs-decorated ZnO, respectively. In general, various studies indicate that CDs introduce more active sites on the ZnO surface, while they increase the absorbance in the UV–Vis light range, thus improving the overall photocatalytic performance. In another work conducted by Hidayat et al. [66], a 91% degradation of the MB dye within 30 min is reported, where Ayu et al. [65] used CDs–N-doped ZnO photocatalyst and achieved decolorization of MB under basic conditions (90.1%), neutral pH (83.4%) and acidic pH (67.2%). The latter study also investigates the recyclability performance of the hybrid material after three reaction cycles in neutral pH (pH = 7.04). The photocatalytic efficiency was 83.4%, 70.6%, and 58.2% for cycle 1, cycle 2, and cycle 3, respectively. In addition, the structure of the photocatalytic material before and after its utilization for 3 cycles was characterized via XRD analysis and the results revealed that there was no difference in the crystal structure of the CDs–N-doped ZnO sample. Similarly, the morphology of the photocatalytic material remained unchanged, based on FE-SEM characterization. As a result, the hybrid material presented high photocatalytic performance and stability.

Experimental parameters, such as the pH of the solution, catalyst dosage, initial concentration of pollutants (Figure 8), and temperature affect the photocatalytic performance of the hybrid materials, as was investigated by Karaca et al. [71]. In their study, ZnO nanorods coated with Rheum ribes-derived N-doped CDs were synthesized via hydrothermal method for the photocatalytic degradation of TC. It was reported that the pH of the solution plays a critical role in the photocatalytic oxidation processes, as it could influence both the oxidative agents and the surface properties of the photocatalyst. Recently, in a similar work by Nugroho et al. [72], the performance of ZnO functionalized with coconut water-derived CDs in the photocatalytic degradation of ofloxacin (OFX), as well as reactive red azo dye (RR141), was investigated under the influence of several parameters (e.g., pH, photocatalyst dosage, pollutant concentration). Based on the optimum experimental conditions, the hybrid material achieved a degrading rate of 98% for RR141 and 96% for ofloxacin. Notably, Nugroho et al. [73] proceeded with a later study, investigating the degradation of other pharmaceutical compounds. The ZnO photocatalyst functionalized with Rosa indica petals-derived CDs achieved 99% degradation efficiency of both AMX and CPX in an acidic environment. The role of catalyst dosage and pollutant concentration was also examined, indicating that total degradation of amoxicillin and ciprofloxacin occurs at lower levels of concentration (5–10 ppm) and a higher dosage of catalyst (75 mg), within 40 min. Furthermore, as it was indicated, the photocatalytic activity of the hybrid material remained effective after five cycles. Based on XRD and SEM analysis, there were no significant alterations in the structural and morphological characteristics of the photocatalyst, before and after the photodegradation experiments, indicating its enhanced stability.

Table 1. Recent advances on photocatalytic water treatment using CDs–metal oxide heterostructures.

CDs–Metal Oxides	Water Pollutant	Light Source	Photocatalytic Degradation	References
N-doped CQDs–TiO2	Methylene Blue (10 ppm)	300 W Xenon lamp	93.10% after 60 min (10 mg/100 mL photocatalyst)	[74]
CQDs–TiO2	Methyl Orange (25 ppm)	300 W Xenon lamp >400 nm	85.00% after 130 min (20 mg/50 mL photocatalyst)	[75]
GQDs–ZnO	Metronidazole (100 ppm)	UV	>99.99% after 30 min (100 mg/100 mL photocatalyst)	[76]
CQDs–Zn2SnO4	Methylene Blue (3 ppm)	Natural Sunlight	95.00% after 150 min (200 mg/200 mL photocatalyst)	[77]
N-doped CDs–Cu2O	Methylene Blue (10 ppm)	500 W Xenon lamp	96.40% after 120 min *	[78]
CQDs–Cu2O	Tetracycline (10 ppm)	Xenon lamp Vis	92.49% after 100 min (100 mg/90 mL photocatalyst)	[79]
B-doped CQDs–Bi2MoO6	Bisphenol A (20 ppm)	300 W Xenon lamp	100% after 120 min (20 mg/60 mL photocatalyst)	[80]
GQDs–Bi2MoO6	Bisphenol A (20 ppm)	300 W Xenon lamp Vis	95.00% after 120 min (50 mg/60 mL photocatalyst)	[81]
Sargassum horneri-derived CDs–Bi2MoO6	Ciprofloxacin (20 ppm)	300 W Xenon lamp	97.70% after 180 min (50 mg/100 mL photocatalyst)	[82]
CQDs–BiVO4	Benzyl Paraben (10 ppm)	300 W Xenon lamp >420 nm	85.40% after 150 min (100 mg/100 mL photocatalyst)	[83]
CQDs–Co2VO4	Ciprofloxacin (20 ppm)	500 W Halogen lamp Vis	80.75% after 13 min (50 mg/L photocatalyst and peroxymonosulphate)	[84]
CDs–WO3	Methylene Blue (10 ppm) Malachite Green (10 ppm)	10 W Xenon lamp 400–800 nm	87.00% after 120 min 88.04% after 120 min (10 mg photocatalyst)	[85]
N-doped GQDs–CuWO4	Tetracycline (15 ppm)	300 W Xenon lamp 300–800 nm	99.00% after 90 min (10 mg photocatalyst)	[86]
GQDs–CuWO4	Phenol (100 ppm)	5 W LED light	53.41% after 90 min (200 mg/100 mL photocatalyst)	[87]

* not mentioned.

presents a selection of noteworthy studies published from 2023 up to now, reflecting the most recent developments in the combination of CDs with TiO₂ and ZnO, as well as other metal oxides, for effective water treatment via photocatalysis.

Lastly, the broad family of metal oxides includes magnetic materials, such as ferrites (MFe₂O₄, where M is a divalent metal cation). Ferrites (e.g., Fe₃O₄, CoFe₂O₄, CuFe₂O₄, etc.) have recently drawn the interest of the scientific community for various photocatalytic applications due to their narrow bang gap, strong absorption of visible light, and high chemical and thermal stability, as well as their magnetic properties which make them easily separable from the reaction mixture. Several studies have been devoted to developing ferrites with improved photocatalytic properties through different strategies, such as modification with noble metals, metal doping, and fabrication of heterostructures [88]. Among them, functionalization of ferrites with CDs is one of the most cost-effective and environmentally friendly strategies.

Table 2. Recent advances on photocatalytic water treatment using magnetic CDs–ferrites heterostructures.

CDs–Ferrites	Water Pollutant	Light Source	Photocatalytic Degradation	References
crocus cancellatus-derived CDs–Fe3O4	Fluorescein (45 μΜ)	Natural Sunlight	94.4% after 60 min (15 mg of photocatalyst and 0.05 M H2O2)	[89]
lemon peel-derived CDs–Fe3O4	Methylene Blue (50 ppm)		99.24% after 30 s (100 mg of photocatalyst and 0.045 M H2O2)	[90]
watermelon peel derived CDs–Fe3O4	Methylene Blue (7 ppm)	UV	98.00% after 30 min (50 mg of photocatalyst)	[91]
yerba mate derived CDs–Fe3O4	Methyl Orange (8.5 ppm)	>400 nm	97.70% after 7 h (100 ppm photocatalyst and 150 mM H2O2)	[92]
mushrooms derived CDs–ZnFe2O4	Methylene Blue (10 ppm) RhB (10 ppm)	200 W Xenon lamp Vis	94.25% after 30 min 96.10% after 60 min (10 mg/L photocatalyst)	[93]
N-doped CDs–Zn-doped CoFe2O4	Oxytetracycline (10 ppm)	250 W HPMVL lamp Vis	98.00% after 100 min (200 mg/L photocatalyst)	[94]
GQDs–CoFe2O4	Methylene Blue (10 ppm)	160 W bulb	~90% after 120 min (50 mg/100 mL photocatalyst)	[95]
Boerhavia diffusa derived CDs–CoFe2O4	Tetracycline (50 ppm)	150 W Xenon lamp	92% after 120 min (50 mg/100 mL photocatalyst and H2O2)	[96]
N-doped CDs–CoFe2O4	Naproxen (10 ppm)	Vis	89.50% after 100 min (20 mg/100 mL photocatalyst)	[97]
mango peel derived CDs–Co0,5Zn0,5Fe2O4	Reactive Blue 222 (50 ppm) Reactive Yellow 145 (50 ppm)	500 W Halogen lamp Vis	~95% after 25 min (1 g/L photocatalyst)	[98]
CDs–CuFe2O4	2-Nitroaniline (200 ppm) 4-Nitroaniline (200 ppm)	*	96.70% after 45 s 96.50% after 15 s (7 mg/5 mL photocatalyst and NaBH4)	[99]

* not mentioned.

summarizes the most recent publications (2023–present) on the field of magnetic CD–ferrites heterostructures for photocatalytic water treatment applications.

2.4. CDs–Metal Chalcogenide Heterostructures

Herein, the most recent literature on the combination of CDs with metal chalcogenides for photocatalytic degradation of water pollutants is discussed. Notably, the research in this technological field is focused on metal sulfides and there appears to be lack of publications on the integration of CDs with metal selenides or metal tellurides (in the past three years). Thus, the present section is dedicated to CDs–metal sulfide heterostructures and their enhanced photocatalytic activity. It is interesting to note that a basic search on Scopus using the phrase “carbon dots AND metal sulfides” generates a record of more than 400 papers published during the past decade (Figure 9), covering a wide range of applications, including environmental remediation, biomedicine, sensing, and energy-related processes. To begin with, metal sulfides are compounds in which a sulfur anion is combined with a cationic metal or semimetal to form M_xS_y with stoichiometric compositions such as MS, M₂S, M₃S₄, and MS₂. Based on their elemental composition, metal sulfides are categorized as binary (e.g., CdS, MoS₂, In₂S₃, etc.), ternary (e.g., ZnIn₂S₄), or multinary [100].

Among numerous binary metal sulfides, cadmium sulfide (CdS) stands out as a highly promising direct bandgap semiconductor. CdS has attracted considerable attention in the field of photocatalysis due to its narrow band gap (2.42 eV) and excellent light harvesting capability. However, its photocatalytic efficiency could be limited by several factors, such as photogenic charge–hole recombination, photo corrosion, and agglomeration phenomena. In order to overcome these issues, various strategies have been proposed, including structural and morphological alterations, incorporation of co-catalysts, surface sensitization, etc. Among them, the most promising is the functionalization of the CdS surface with CDs, which is attributed to the electron-accepting and transport characteristics of CDs [101,102]. In their study, Choudhary et al. [101] investigated the decoration of CdS with CDs prepared by neem leaves for the photocatalytic degradation of the pharmaceutical compound CPX (Figure 10a). More specifically, CDs–CdS hybrid materials containing two different amounts of CDs were synthesized via a hydrothermal method for 150 °C for 6 h. The CPX degradation performance of CDs, CdS, CDs (1 mL)–CdS, and CDs (2 mL)–CdS was observed to be 51%, 68%, 73%, and 74%, respectively (Figure 10b).

Copper sulfide (Cu_xS) is a p-type semiconducting material with a band gap varying from 1.2 eV to 2.2 eV, based on the percentage of copper [81]. Cheng et al. presented the modification of CuS with Cu-doped CDs and the utilization of the heterostructure as a Fenton-like photocatalyst for the degradation of the antibiotic TC with the assistance of H₂O₂ [103]. Based on the morphological characterization results, observed in Figure 10c,d, Cu-doped CDs have been successfully incorporated onto CuS nanospheres. The photocatalytic activity of Cu-doped CDs–CuS was determined by the photodegradation of TC. In each experiment, 100 mg of photocatalyst were added into 100 mL of 10 ppm TC aqueous solution, followed by the addition of 1 mL of H₂O₂ (30%) to initiate the Fenton-like reaction. The TC degradation efficiency was significantly enhanced (by 85%) in a Fenton-like system using the hybrid material (Figure 10e). The proposed mechanism of photocatalytic degradation of the TC is presented in Figure 10f, as well as Equations (10)–(16). More specifically, under illumination, CuS absorbed energy and produced photogenerated e⁻ and h⁺ on its CB. Following this, the photogenerated e⁻ transferred to the interface of the Cu-doped CDs–CuS (Equation (10)), which prompted the reduction of Cu²⁺ to Cu⁺ (Equation (11)). As a result, Cu⁺ reacted with O₂ to produce active radicals O₂·⁻ (Equation (12)). In addition, reduction of Cu²⁺ by H₂O₂ leads to the generation of Cu⁺ and O₂·⁻ (Equation (13)), while Cu⁺ could be oxidized by H₂O₂ to form Cu²⁺ and ·OH (Equation (14)). Electrons play a crucial role in enhancing the efficiency of Fenton-like photocatalytic reactions by accelerating the reduction of Cu²⁺ to Cu⁺, thereby facilitating the regeneration of Cu²⁺ active sites. The photogenerated holes (h⁺) are driven toward the Cu-doped CDs, where they react with surface hydroxyl groups to form ·OH radicals (Equation (15)). The reactive species O₂·⁻, h⁺, and ·OH generated on the Cu-doped CDs–CdS hybrid material diffuse into the solution, breaking down TC into smaller molecules (Equation (16)) and ultimately mineralizing it into CO₂ and H₂O. Furthermore, recyclability experiments were conducted for both pristine CuS and Cu-doped CDs–CuS. The results showed that after four cycles, the photocatalytic efficiency was decreased by 35% and 15%, respectively. Notably, XPS analysis of the hybrid material after its utilization in the photocatalytic degradation of TC demonstrated that Cu²⁺ ions were almost undetectable, indicating minimal leaching during the reaction cycles. Lastly, it is interesting to note that the modification of CdS with Cu-doped CDs resulted in excellent charge separation and photoresponse stability, as illustrated in Figure 10g.

CuS + hv → e⁻ + h⁺

(10)

Cu²⁺ + e⁻ → Cu⁺

(11)

Cu⁺ + O₂ → O₂^·− + Cu²⁺

(12)

Cu²⁺ + H₂O₂ → Cu⁺ + O₂^·− + 2H⁺

(13)

Cu⁺ + H₂O₂ → Cu²⁺ + ^·OH

(14)

Cu-doped CDs-OH + h⁺ → Cu-doped CDs + ^·OH

(15)

O₂⁻/h⁺/^·OH + TC → degradation products

(16)

Subsequently, the exceptional physicochemical properties of 2-D nanomaterials, defined by atomically thin layers, have enabled groundbreaking advancements. In this context, 2-D molybdenum disulfide (MoS₂) has gained considerable attention and widespread application in various fields. Bulk MoS₂ is an indirect-bandgap semiconductor (1.29 eV), which alters to a direct-bandgap semiconductor (1.90 eV) when reduced to monolayers. Although the narrow bandgaps of MoS₂ enhance its light absorption capability, the misaligned energy levels hinder the efficient photocatalytic oxidation and reduction processes. Thus, the functionalization of MoS2 with low-cost CDs with up-conversion fluorescent and photogenerated electron-transfer properties has been proposed by several groups.

Qu et al. [104] reported the synthesis of N and S co-doped CDs followed by their incorporation onto MoS₂ nanosheets via a hydrothermal method (200 °C, 24 h), as it is illustrated in Figure 11a. In this study, different mass ratios of N and S co-doped CDs to MoS₂ were tested to evaluate their effect on the photocatalytic activity. Regardless of the mass ratio, the as-prepared hybrid materials exhibited flower-like morphologies. MB and Malachite Green (MG) were dissolved in water samples collected from Tianjin Lake and photodegradation experiments were carried out following the addition of the heterostructure. As is observed in Figure 11b,c, the increasing N and S co-doped CDs concentration enhanced the photocatalytic degradation of organic dyes. However, at the highest mass ratio of CDs to MoS₂, a decline in the photocatalytic performance was observed. This may be attributed to possible aggregation resulting in active-site restriction. Herein, N and S co-doped CDs–MoS₂ hybrid material achieved degradation of 99.8% of MB and 95.1% of MG in an actual water system. The recyclability of the hybrid material was also assessed. As it was observed, MB degradation remained high, reaching 88.9% after five cycles. Simultaneously, XRD and SEM characterization of the used photocatalyst confirmed that its crystal structure and morphology remained unchanged, indicating excellent stability. In a similar work, Li et al. [105] synthesized N-doped CDs modified MoS₂ nanoflowers through a hydrothermal process, in an increased concentration of CDs and achieved 91.39% of MB dye photocatalytic degradation.

Recently, Zhang et al. [37] reported the utilization of CDs–MoS₂ hybrid material as a photo-piezoelectric synergistic catalyst. At this point, it is important to mention that when a piezoelectric material is subjected to external stress or strain, spontaneous polarization for piezocatalysis is caused and may also enhance the separation of photogenerated e⁻ and h⁺. The carriers can migrate to the surface due to the internal electric field of the piezocrystal. In other words, they could easily capture oxygen/water to produce reactive oxygen species (ROS) for pollutant degradation. Thus, the development of a synergistic photo-piezocatalyst shows great potential in enhancing the activation of peroxymonosulfate (PMS) to generate sulfate radicals (SO₄·⁻) for pollutant degradation. MoS₂ decoration with CDs could enhance its piezoelectric properties, due to the improved asymmetry, promote the electron transfer, and ensure the persistent PMS activation, as CDs possess a large π-electrons conjugated system. A CD–MoS₂ photo-piezocatalyst was utilized for the degradation of TC. More specifically, N-doped CDs were successfully attached onto a MoS₂ surface via a hydrothermal method (200 °C, 12 h), as it is illustrated by TEM (Figure 11d). Under visible light irradiation (Figure 11e), both MoS₂ and N-doped CDs–MoS₂, along with PMS, presented increased performance compared to only PMS, reaching TC degradation efficiency of 55.57% and 59.71%, respectively, after 60 min. Interestingly, under the synergistic visible light irradiation and ultrasonic vibration (Figure 11f) for 60 min in the presence of PMS, the N-doped CDs–MoS₂ photo-piezocatalyst presented 99.44% TC degradation. The possible mechanism is described by Equations (17)–(25). More specifically, under the synergistic stimuli, the internal electric field of photo-piezocatalyst generates a large number of charges at its surface (Equation (17)). Afterwards, surface charges and h⁺ attack the O–O bonds of PMS, resulting not only in the formation of SO₄·⁻ and ·OH (Equation (18)), but also the reduction of O₂ to O₂·⁻ (Equation (19)). In addition, h⁺ reacts with PMS and H₂O forming SO₅·⁻ (Equation (20)) and ·OH (Equation (21)). The two SO₅·⁻ molecules form SO₄·⁻ and ¹O₂ (Equation (22)) and then SO₄·⁻ could easily react with H₂O to produce ·OH (Equation (23)). Moreover, ·OH and O₂·⁻ could further react to ¹O₂ (Equation (24)). The reactive substances degrade TC into inorganic compounds (Equation (25)).

N-doped CDs-MoS₂ → e⁻ + h⁺

(17)

HSO₅⁻ + e⁻ → SO₄^·− + ·OH

(18)

O₂ + e⁻ → O₂^·−

(19)

HSO₅⁻ + h⁺ → SO₅^·− + H⁺

(20)

H₂O + h⁺ → ·OH + H⁺

(21)

SO₅^·− + SO₅^·− → 2SO₄^·− + ¹O₂

(22)

SO₄^·− + H₂O → H⁺ + SO₄²⁻ + ·OH

(23)

^·OH + O₂^·− → OH⁻ + ¹O₂

(24)

¹O₂/O₂^·−/·OH/SO₄^·− + TC → CO₂ + H₂O + inorganic

(25)

Tungsten disulfide (WS₂) is composed of an S–M–S multilayer structure, which contributes to extended light absorption and promotes efficient electron transfer in heterojunctions. Nevertheless, it still suffers from rapid charge carrier recombination, which limits its overall photocatalytic performance. Fatima et al. [106] reported the incorporation of CDs in a WS₂–polyaniline (PANI) heterojunction by ultrasonication for 3 h (Figure 12a,b). The photocatalytic performance of WS₂, WS₂–PANI and CDs–WS₂–PANI was evaluated for the degradation of estradiol (EST) and nitrofurantoin (NFT) pharmaceutical pollutants (Figure 12c,d). CDs–WS₂–PANI presented the highest degradation efficiency, 96.63% for EST and 95.76% for NFT in 60 min. In contrast, WS₂ and WS₂/PANI showed degradation of 18.64% and 58.06% for EST and 28.19% and 70.54% for NFT. The electrons present in the CB of WS₂ cannot be scavenged due to the lower oxidation potential, which results in rapid recombination of charge carriers. The conducting polymer PANI improves the degradation efficiency by reducing recombination and shifting the CB to favor ROS generation. In addition, CDs further improve the photocatalytic performance by extending light absorption, trapping electrons, and hindering e⁻–h⁺ recombination. To assess the recyclability of the CDs–WS₂–PANI hybrid material, photodegradation experiments were systematically performed under consistent conditions across each cycle. The results demonstrate that the photocatalyst maintained its degradation efficiency, showing no significant decline even after four cycles. Additionally, XRD analysis conducted before and after photocatalysis verified the structural stability of the hybrid material. These findings indicate that the synthesized photocatalyst is effective in the degradation of water pollutants and exhibits excellent stability.

Indium sulfide (In₂S₃) is a binary chalcogenide semiconductor with bandgap range of 2.0–2.5 eV. Moreover, it presents five different crystalline structures: a-phase defect cubic (α-In₂S₃), β-phase defect spinel (β-In₂S₃), γ-phase layered hexagonal (γ-In₂S₃), ϵ-phase rhombohedra structure (ϵ-In₂S₃) and Th₃P₄-phase defect cubic defect structure (Th₃P₄–In₂S₃). Due to its interesting photoelectric characteristics, stability, and diverse crystalline structure, In₂S₃ has become prevalent among researchers. However, the fast recombination of e⁻–h⁺ pairs, which is attributed to the relatively narrow bandgap, diminishes its photocatalytic efficiency; as little as 10% of the photogenerated electrons are available for photoreduction. Hence, heterojunction fabrication is an effective strategy for enhancing charge separation and redox capability [107]. On this basis, Yao et al. [108] prepared CDs–In₂S₃ hybrid materials in various mass ratios through a simple one-pot hydrothermal route. Briefly, InCl₃·4H₂O and thioacetamide were dissolved into distilled water followed by glucose addition. The homogeneous solution was transferred into a Teflon container and heated to 180 °C for 12 h. GQDs were obtained via the polymerization reaction of glucose during the process; therefore, the mass ratio of In₂S₃ to GQDs was controlled by adjusting the glucose amount. The final hybrids with different mass ratios demonstrated flower-like structures, where GQDs with a diameter of 3 nm are deposited onto In₂S₃ nanosheets. The photocatalytic activity of both In₂S₃ and GQDs–In₂S₃ heterostructures was evaluated by the degradation of MB. Based on the UV–Vis results, GQDs–In₂S₃ presented enhanced photocatalytic performance compared to pristine In₂S₃ after 30 min of irradiation. More specifically, In₂S₃ achieved 24% MB degradation and the GQDs–In₂S₃ heterostructures reached 72%, 90% and 75% as the GQDs loading was increased, indicating that their concentration may play a key role in photocatalysis. In addition, the photogenerated charge carrier behaviors of In₂S₃ and GQDs–In₂S₃ were also evaluated. The results showed that the photocurrent density of the heterostructure is significantly higher than In₂S₃, which suggests the enhanced charge transfer and separation rate.

Zinc indium sulfide (ZnIn₂S₄) belongs to the family of ternary chalcogenides and has been recently the subject of various studies in photocatalytic splitting of water, conversion of carbon dioxide, and removal of water pollutants [109]. Similar to other metal sulfides, ZnIn₂S₄ suffers from fast recombination of photogenerated e⁻–h⁺, while its CB and VB positions are not always sufficient for driving redox reactions. In the study of Li et al. [110], ZnIn₂S₄ modified with a N-doped CDs photocatalyst was developed for in situ H₂O₂ production and further combined with Fe²⁺ to assemble a photocatalysis-assisted self-Fenton system for degradation of antibiotics, such as levofloxacin (LEV) and AMX. The heterostructure was synthesized via mixing N-doped CDs and ZnIn₂S₄ precursors under heating at 80 °C for 2 h. The photocurrent intensity of N-doped CDs–ZnIn₂S₄ (0.34 μA/cm²) is higher than ZnIn₂S₄ (0.14 μA/cm²) indicating the larger amount of photogenerated carriers. Moreover, as it is discussed, N-doped CDs possess lower electrostatic potential value (4.762 eV) compared to ZnIn₂S₄ (5.978 eV); therefore, photogenerated electrons tend to migrate from ZnIn₂S₄ to N-doped CDs, promoting e⁻–h⁺ separation. The photocatalytic performance of H₂O₂ production by different samples was firstly evaluated (Figure 13a); the combination of ZnIn₂S₄ with N-doped CDs dramatically increased the H₂O₂ production rate. In addition, as the amount of N-doped CDs in the hybrid material was increased (from 100 mg to 250 mg), H₂O₂ production raised from 139.4 μM to 152.8 μM, respectively. However, further increase of the dosage hinders the efficiency, which may be assigned to the excessive accumulation of N-doped CDs on the active site. Due to the large amounts of H₂O₂ produced, a photocatalysis–self-Fenton system of Fe²⁺/N-doped CDs–ZnIn₂S₄ was conducted for the degradation of different antibiotics under pH = 7 with 1 mg/L Fe²⁺. Figure 13b,c shows that over 95% of LEV and 81% of AMX were decomposed within 30 min. It is also interesting to note that the recyclability of the N-doped CDs–ZnIn₂S₄ hybrid material was evaluated through its application in the degradation of LEV over five cycles. The photocatalytic material presented high degradation rate (>90%) towards the antibiotic pollutant even after the last cycle. In addition, no noticeable change in the crystalline structure was observed, confirming the excellent structural stability of the N-doped CDs–ZnIn₂S₄ hybrid material.

2.5. CDs–Mxene Heterostructures

Mxenes are a new class of 2-D nanomaterials, discovered in 2011 [111], and include transition metal carbides, nitrides, or carbonitrides. They are produced from 3-D MAX starting materials, represented by the general formula M_(n+1)AX_n, where M is an early transition metal, n = 1–3, A is an A-group element (primarily groups 13 and 14), and X is carbon or/and nitrogen. During chemical etching of MAX phases, A-layers are removed resulting in the formation of the Mxenes. The general formula of Mxenes is M_n+1X_nT_x, where T_x represents hydroxyl, oxygen, or fluorine termination groups are on their surface. The production of Mxenes involves several chemical and physical methods, including solution-based etching, such as HF or LiF/HCl (Figure 14), and electrochemical etching, and anhydrous etching, such as molten salt etching or halogen etching. Direct synthesis and various intercalation and delamination techniques have been thoroughly examined since their inception [112].

Mxenes have emerged as promising photocatalytic materials due to high metallic conductivity, excellent hydrophilicity, strong molecular adsorption, and efficient charge transfer [113] and can serve as an exceptional platform for the synthesis of monatomic catalysts, demonstrating commendable performance [114] in different applications [115], including biomedicine, energy storage devices (battery, supercapacitor), sensors, catalysts, etc. Although Mxenes demonstrate exceptional properties, they are constrained by drawbacks such as limited surface characteristics, risks for accumulation, and variable surface charges, which may influence their efficacy in water treatment processes [116]. Additionally, even though Mxenes cannot be utilized directly as photocatalysts as they are not semiconducting materials [112], the usage of Mxene-based photocatalysts (oxidized Mxene-precursors and surface modified Mxenes) in environmental remediation and artificial photosynthesis has been on the rise. The notable distinction between the two categories of photocatalysts is that some components in the latter originate from the oxidation of Mxene precursors. Moreover, various photocatalysts and surface reconstructions [117] exert distinct influences on photocatalytic efficacy [118,119]. Consequently, the synthetic procedure of Mxenes and the related catalysts are crucial for material design.

Figure 14. Synthesis and characterization of Ti₃C₂T_x Mxene nanosheets. (a) Schematic illustrating the synthesis of Ti₃C₂T_x Mxene nanosheets. (Adopted from [112] with permission). SEM images of exfoliated Ti₃AlC₂ by fluoride salts show a fully exfoliated grain [120]. (b) LiF with HCl at 50 °C for 48 h. (Adopted from [121] with permission). (c) NaF with HCl at 40 °C for 48 h. (Adopted from [122] with permission).

Figure 14. Synthesis and characterization of Ti₃C₂T_x Mxene nanosheets. (a) Schematic illustrating the synthesis of Ti₃C₂T_x Mxene nanosheets. (Adopted from [112] with permission). SEM images of exfoliated Ti₃AlC₂ by fluoride salts show a fully exfoliated grain [120]. (b) LiF with HCl at 50 °C for 48 h. (Adopted from [121] with permission). (c) NaF with HCl at 40 °C for 48 h. (Adopted from [122] with permission).

Different surface engineering methods have been applied, resulting in significant enhancements in the stability of Mxenes in aqueous environments and a reduction in their receptivity to oxidative damage and deterioration. The incorporation of hydrophilic functional groups, such as -OH and -O, improves the resilience and long-term use of Mxenes in the processing of wastewater [123]. The development of Mxenes containing nanocomposites, which integrate Mxenes with polymers, CNTs, CQDs or metal oxides, has resulted in materials exhibiting complementary characteristics not found in their respective parts [124].

Mxene-CDs nanocomposites are currently establishing themselves as a focal point of research, particularly for energy applications. Their remarkable capacity to enhance electron transport and ion migration, along with their specific capacitance, facilitates rapid redox reactions [125]. Guo et al. [126] recently reported the manufacturing of Mxene (Ti₃C₂T_x), MnO₂, and CDs-decorated lamellae through the substitution of CNTs in earlier nanocomposites with CDs. This resulted in an improved capacitance retention rate of the nanocomposite, which is essential for the development of flexible functional devices, including smart wearable electronics. Nguyen et al. [127] have prepared shell structured Mxene@carbon nanodots as highly active bifunctional catalysts both for electrochemical and photoelectrochemical solar-assisted water splitting.

Cao et al. [128] reported on the synergistic combination of a carbon matrix with a 2-D Mxene that offers significant potential for diverse applications of Mxene-based hybrid materials, particularly in areas such as electrochemical energy storage, photo- and electro-catalytic water splitting, electromagnetic wave absorption, and sensing [129]. The multidimensional carbon-based matrix, ranging from zero-dimensional (0-D) to three-dimensional (3-D) structures, facilitates the construction of diverse heterostructure configurations with 2-D Mxenes. This approach enables the optimization of an Mxene’s intrinsic strength, leveraging its layered structure and distinctive properties for various applications.

An interesting method to prepare CQDs and respective doped Mxenes has been reported by Chen and coworkers [130]. They used cationic polystyrene (CPS) covered with Ti₃C₂T_x–Mxene and pyrolyzed it at temperatures as low as 410 °C for 2 h in a nitrogen atmosphere. During the pyrolysis process, the Ti₃C₂T_x–Mxene shell served as a barrier, effectively retaining the release of CPS decomposition products and facilitating their penetration into the interlayers of the Mxene (Figure 15). The composite can be used as such but can also act as source for extraction of pure CQDs.

Modified Ti₃C₂ Mxenes have garnered significant interest due to their distinctive properties and are regarded as highly potential materials for pollutant removal (e.g., through active electro-adsorption processes) and water splitting [131]. Photo(electro)catalytic water decomposition for hydrogen production has become lots of attention in recent publications as Mxene-based and Mxene-derived photocatalysts have significant activity for environmental and renewable energy purposes, including the disintegration of water to generate H₂.

Hui et al. [132] have prepared MoIn₂S₄ and CQDs–MoIn₂S₄ composite photocatalysts by applying an in situ hydrothermal process, in which citric acid was both the carbon source and surfactant. The optimized CQDs–MoIn₂S₄ photocatalyst had the maximum photocatalytic efficacy, attaining MB elimination rate of 94.4% after 60 min of simulated solar exposure. The enhanced performance was chiefly ascribed to the up-conversion photoluminescence effect of CQDs, which improved the light absorption of MoIn₂S₄. The few-layer design of MoIn₂S₄ as a matrix contained multiple active sites that effectively prevented the clustering of CQDs. Moreover, the CQDs functioned as electron acceptors and transport hubs, mitigating the fast recombination of photogenerated carriers and enhancing electron transmission. Moreover, it has been shown that superoxide radicals (O₂·⁻) are the principal active species responsible for photocatalytic destruction.

Other applications of Mxenes based nanocomposites in the wastewater treatment are more focused on their excellent adsorptive properties and thus mostly used as adsorption agents or as separation membranes [133].

2.6. CDs–Metal Oxyhalide Heterostructures

In recent years, growing interest in the combination of metal oxyhalides (M_xO_yX_z, where X represents F, Cl, Br, or I) with CDs for photocatalytic water treatment applications and has been reflected in numerous studies. Among them, Bi-based oxyhalides (Bi_xO_yX_z) have attracted considerable attention because of their unique layered structure, tunable bandgap, high chemical stability, and low toxicity [134].

BiOCl is one of the most popular Bi-based oxyhalide photocatalyst for water pollutants degradation. However, due to its wide bandgap (3.6 eV), it exhibits limited activity under visible light. In order to improve its performance under irradiation in a wide-spectrum of light, CDs–BiOCl heterostructures have been developed and studied by several groups. The concentration of CDs appears to be a key factor in the photocatalytic performance; thus, various studies focused on optimizing the synthesis process by adjusting the amount of CDs loaded onto BiOCl. For instance, Li et al. [135] prepared CDs–modified 3-D flower-like BiOCl structures (Figure 16a–d) with various CDs concentrations and evaluated their performance in the degradation of RhB dye under visible light. After 30 min, the degradation rates of CDs–BiOCl as the loading of CDs increases are 59.3%, 89.1%, 99% and 95%, while for pristine BiOCl it is 54.3% (Figure 16e). In all cases, CDs enhance the degradation efficiency of BiOCl as they improve light absorption, charge transfer, and charge carrier separation. Although, beyond a certain loading level, the performance slightly declines, which may be attributed to CDs shielding the active sites of BiOCl. Moreover, the photocurrent density of CDs–BiOCl is increased compared to BiOCl, which indicates that the photogenerated carriers are separated efficiently (Figure 16f). The recyclability of the optimized CDs–BiOCl hybrid material was also investigated. It is worth noting that the photocatalyst maintained a degradation efficiency of 95% despite undergoing four cycles. Furthermore, XRD analysis conducted before and after the recycling process confirmed the high structural stability of the CDs–BiOCl photocatalyst. The removal of RhB dye using CDs–BiOCl heterostructures was also conducted by other research groups. Similarly, in the study of Shi et al. [136], the photocatalytic performance of the hybrid materials decorated with different amounts of GQDs was evaluated and the results showed that under visible-light irradiation the degradation efficiency of BiOCl was enhanced as the GQDs loading increased. (Figure 16g). Wang et al. [137] presented the photocatalytic performance of the optimized Bi-doped CDs–BiOCl sample under 190–1100 nm broadband light irradiation, which exhibited 2.3 times higher RhB degradation than BiOCl. Additionally, other organic compounds, such as tetracycline hydrochloride (TCH) and bisphenol A (BPA), have been successfully degraded using CDs–BiOCl photocatalysts. Lu et al. [138] presented an in-depth study about synergistic role of CDs and Ovs in the enhancement of BiOCl photocatalytic properties (Figure 16h). The hybrid materials were prepared via a high-temperature hydrothermal process, during which CDs are combined with BiOCl via C=O bonding. ESR spectroscopy was employed to verify that CDs promote the formation of additional Ovs within the material. As shown in the results (Figure 16i), a significant signal with a g-value of 2.003 is detected in pristine and modified BiOCl. Notably, the signal intensity of CDs–BiOCl is stronger, and combined with XPS analysis, these results indicated that CDs enhances Ovs. As a result, the photocatalytic performance of CDs in the degradation of TCH under visible light irradiation was highly improved from 19.48% to 79.50%. More specifically, the interactions of BiOCl with CDs via C=O bonding, leads to the increase in the number of Ovs, caused by local lattice defects and dislocations, which ultimately create new energy levels between the CB and VB bands of BiOCl. The Ovs serve as electron traps, promoting the capture of light-induced electrons at their respective energy levels; trapped electrons react with O₂ to form ·O₂⁻, while others are transferred to the CDs. Simultaneously, due to their up-conversion characteristics, the CDs extend absorption into visible light range, which results in the generation of more e⁻–h⁺ pairs. Therefore, CDs–BiOCl combination is beneficial for the degradation of TCH. Among various CDs–BiOCl hybrid materials prepared with different amounts of CDs and tested for their photocatalytic performance in TCH degradation, the optimum photocatalyst was further investigated in terms of recyclability and structural stability. Based on the results, the hybrid material demonstrated excellent photocatalytic stability, maintaining over 91% TCH degradation efficiency after five cycles, with XRD analysis confirming its structural integrity. In a similar work [139], the synergistic role of functionalization with CDs and heteroatom doping of BiOCl was investigated. N-doping could substitute Cl in the lattice structure of BiOCl or attach to Bi, forming an intermediate energy level and shorten the electron transfer distance. Nevertheless, the fast in situ recombination of charge carriers hinders their photocatalytic properties. Inspired by this issue, they modified N-doped BiOCl with CDs through a calcination process. Based on the EIS Nyquist plots (Figure 16j), CDs–N-doped BiOCl presented the smallest curve radius indicating that both incorporation of CDs and N-doping reduce the electron migration energy barrier, thereby enhancing interfacial charge transfer within the photocatalyst. The photocatalytic performance of BiOCl, N-doped BiOCl, and CD–N-doped BiOCl in the degradation of BPA was 31.88%, 48.39%, and 90.65% under visible light irradiation for 20 min, respectively.

The enhancement of photocatalytic activity In other metal oxyhalides through functionalization with CDs has also been explored. In a recent study [140], CDs–Cu₂Cl(OH)₃ heterostructures were synthesized via hydrothermal treatment, using different concentrations of CDs, for the photocatalytic degradation of MB dye from aqueous environment. Based on the UV–Vis spectra, Cu₂Cl(OH)₃ exhibits a broad but low absorbance at 200–500 nm. After modification with CDs, the absorbance is highly increased until overload and a blue-shift at 300–400 nm is also observed. (Figure 16k) The degradation of MB dye using Cu₂Cl(OH)₃ and the optimized CDs–Cu₂Cl(OH)₃ heterostructure was 37.40% and 88.20%, respectively, suggesting that CDs could greatly enhance the photocatalytic efficiency, with significant cycling stability (Figure 16l,m).

2.7. CDs–Metal-Free Heterostructures

During the past few decades, metal-free semiconductors have attracted significant attention as a green alternative class of photocatalytic materials due to their appealing characteristics and properties, making them suitable for various photocatalytic reactions [141]. Among them, one of the most promising candidates is the 2-D graphitic carbon nitride (g–C₃N₄), due to its environmentally friendly nature and facile and low-cost synthesis procedure, as well as chemical stability. Although g–C₃N₄ possesses a moderate bandgap (~2.7 eV) and favorable band edge positions, further improvements are still necessary to enhance its photocatalytic performance. Thus, researchers continue to investigate several strategies in order to optimize its capability, including the increase of specific surface area via morphology modulation [142], tuning the energy band structure via heteroatom doping [143], and enhancing charge carrier separation through the development of nanoarchitectures and interfacial heterostructures [144,145]. The combination of g–C₃N₄ with CDs has been considered as an appealing strategy for the development of metal-free photocatalysts with enhanced properties and eco-friendly characteristics. CDs could function as a photosensitizer, improving the visible light absorption of g–C₃N₄ through their up-converted photoluminescence properties [146,147], while also stimulating g–C₃N₄ to generate electron–hole pairs and produce more reactive O₂·⁻ and ·OH radicals, ultimately enhancing its photocatalytic ability. Additionally, CDs could serve as effective electron acceptors by capturing photoinduced electrons, thereby minimizing their recombination with holes in the valence band of g–C₃N₄. Herein, the most recent research efforts in the utilization of CDs–g–C₃N₄ binary systems in the degradation of water pollutants are presented.

To begin with, CD–g–C₃N₄ heterostructures have been successfully utilized in the removal of organic compounds, such as dyes and pharmaceuticals, from aqueous environment. As it is indicated in the study of Jourshabani et al. [148], N-doped CQDs, playing the role of electron acceptors, were anchored onto the surface of g–C₃N₄ nanosheets for RhB degradation. Briefly, g–C₃N₄ was synthesized via a two-step process, including hydrothermal treatment of cyanuric acid and a melamine mixture, followed by calcination, while N-doped CQDs were prepared by a micro-plasma technique. The above components were combined through ultrasonication, forming the final N-doped CQDs–g–C₃N₄ heterostructure. The effect of the N-doped CQDs concentration in the photocatalytic performance of the hybrid material was evaluated: 0.2 g of g–C₃N₄ were mixed with 1.0 mg, 2.0 mg and 3.0 mg of N-doped CQDs, respectively. Following this, RhB samples containing pristine g–C₃N₄ and N-doped CQDs–g–C₃N₄ hybrid materials, were exposed to simulated solar light for 2.5 h. Based on the results, g–C₃N₄ presented 65% degradation efficiency, while the heterostructure containing moderate amount of N-doped CQDs achieved a degradation efficiency of 91%. Further increase of N-doped CQDs in the heterostructure results in the decline of its photocatalytic performance, which may be attributed to the reduction of available active sites. In a similar study [149], N-doped GQDs–g–C₃N₄ hybrid material, which was successfully prepared via stirring for 12 h at room temperature (Figure 17a), presented excellent performance regarding the light-absorbing capacity, charge carrier separation, and visible-light driving. As a result, it degraded 91.2% of RhB within 90 min (Figure 17b) with a degradation rate 1.47 times higher than that of pristine g–C₃N₄ (Figure 17c). It is worth mentioning that the hybrid material retained its activity over 10 consecutive cycles with no significant decline in performance, highlighting its excellent photocatalytic stability. The structural stability of the N-doped GQD–g–C₃N₄ was also confirmed via XRD analysis of the recycled sample. In addition, Ishak et al. [150] presented the functionalization of g–C₃N₄ with microwave-synthesized CQDs for the efficient degradation of MB dye. In their noteworthy study, they address a critical issue that has attracted considerable attention from the scientific community: the presence of by-products in CDs synthesis. It is well known that during bottom-up synthesis of CDs, including methods such as microwave irradiation, a wide variety of by-products, agglomerates, or unreacted species may be formed. These components could potentially affect the photocatalytic activity of the resulting materials. To investigate this, the researchers isolated different fractions using column chromatography (Figure 17d). The first fraction contains 2-pyridone-based by-products, which are highly fluorescent molecules. These fluorophores were combined with g–C₃N₄ forming a hybrid material. Similarly, fluorophores-free CDs and as-prepared CDs were also attached onto g–C₃N₄ via a facile route involving 4 h of mixing, followed by 1 h of ultrasonication. The photocatalytic experiments showed that the best MB degradation performance was achieved by the purified CDs–g–C₃N₄ heterostructure, followed by as-prepared CDs–g–C₃N₄ > g–C₃N₄ > fluorophores-g–C₃N₄, indicating that by-products during CDs synthesis decrease the heterostructure’s photocatalytic activity.

In another study conducted by Awang et al. [151], the photocatalytic degradation of diclofenac by N-doped CQDs–g–C₃N₄ was evaluated. More specifically, the experiments were carried out for 180 min under visible light irradiation and the results showed that the hybrid material achieved higher photocatalytic efficiency (62%) in comparison with the bulk g–C₃N₄ (50%). N-doped CQDs improved visible light absorption, as well as charge carrier separation. Nevertheless, an excessive amount of N-doped CQDs could lead to a shield effect and hinder photocatalysis, which has been addressed in various publications. Recently, Yang et al. [152] investigated the crucial role of CDs surface functionality in the development of an effective CDs–g–C₃N₄ photocatalyst. Therefore, different CDs samples were produced, nominated as 1 N-CDs, 2 N-CDs and ca-CDs, which were assigned to CDs rich in pyrrolic N, graphitic N, and carboxyl groups (N-free), respectively (Figure 18). CDs were physically integrated with g–C₃N₄ to preserve their chemical functionalities. The efficiency of CDs–g–C₃N₄-catalyzed PMS in p-nitrophenol (PNP) degradation was further studied. The findings of this work prove that functional groups matter, as the pyrrolic-N and carboxyl groups promote electron trapping, improving charge separation and enhancing photocatalytic activity. On the other hand, the graphitic-N caused electron-rich regions that hinder charge separation, resulting in the reduction of the effectiveness. In particular, the 1 N-CDs–g–C₃N₄ photocatalyst, showed the greatest PNP degradation (95.9%), with the best charge separation and ROS generation. Furthermore, ca-CDs–g–C₃N₄ presented satisfactory performance (68.7%) due to surface carboxyl groups, whereas 2 N-CDs–g–C₃N₄ demonstrated poor performance (21.1%) due to electron accumulation barriers. The roles of reactive species involved in the PNP degradation process followed the order h⁺ > ¹O₂ > ·OH > O₂·⁻, while SO₄·⁻ played a minor role due to quenching by carboxyl groups. The possible reaction routes of photocatalytic processes have been described in Equations (26)–(31):

HSO₅⁻ + e ⁻ → ⋅OH + SO₄²⁻

(26)

O₂ + e ⁻ → O₂⋅⁻

(27)

O₂^·− + 2 H₂O → ¹O₂ + H₂O₂ + 2 OH⁻

(28)

O₂^·− + H⁺ → H₂O₂ + ¹O₂

(29)

H₂O₂ + e ⁻ → ⋅OH + OH⁻

(30)

h⁺/¹O₂/^·OH/O₂^·− + PNP → intermediates → CO₂ + H₂O

(31)

The CDs–g–C₃N₄ heterostructure has also been applied in water disinfection from bacteria. For instance, Yang et al. [153] synthesized a CDs–g–C₃N₄ photocatalyst through an electrostatic self-assembly method for the effective disinfection of water from methicillin-resistant Staphylococcus aureus (MRSA) (Figure 19a). In this work, O-doped g–C₃N₄ was prepared via hydrothermal treatment of bulk g–C₃N₄ in the presence of H₂O₂. The aim of O-doping was to hinder the recombination of photogenerated hole–electron pairs and promote the exposure of additional active centers. Simultaneously, CDs were synthesized via hydrothermal treatment of 2,5-diamino-phenyl sulfonic acid. The loading of CDs onto O-doped g–C₃N₄ enhances the generation of photogenerated electron–hole pairs by broadening the visible light absorption range and facilitates the separation of charge carriers through effective trapping of photogenerated electrons. Moreover, CDs incorporation alters the surface charge of the hybrid photocatalytic material from negative to positive, thereby improving the interaction between active species and bacterial cells. As it is presented in Figure 19b, the CDs–O-doped g–C₃N₄ heterostructure demonstrated a significantly enhanced ability to inactivate MRSA, achieving a 4.08-log reduction of cell density compared to 0.46-log and 1.13-log reduction of bulk g–C₃N₄ and O-doped g–C₃N₄, respectively. In addition, fluorescence-based live/dead staining assays were performed to verify the damaging effect of CDs–O-doped g–C₃N₄ towards MRSA (Figure 19c). In these assays, SYTO 9 dye penetrated the intact cell membrane and labelled live bacteria with green fluorescence, whereas PI dye selectively entered the damaged cell membrane and caused the dead bacteria to emit red fluorescence. As it was observed, the majority of bacteria exhibited red fluorescence, suggesting disruption of the cell membrane wall in the presence of the photocatalyst. The superior antibacterial performance of the heterostructure is primarily due to improved photoelectric conversion efficiency and modified surface charge characteristics. CDs not only extend the visible light absorption range but also facilitate the separation of photogenerated electron–hole pairs by acting as electron traps. Additionally, the positive surface charge of O-doped g–C₃N₄, after CDs incorporation, improves the electrostatic interactions among the photocatalyst and the bacteria, thereby enhancing the likelihood of active species coming into contact with bacterial cells. Reactive oxygen species, particularly superoxide radicals (O₂·⁻) and photogenerated holes, were identified as the dominant agents responsible for disinfection. These species compromise bacterial cells by damaging membranes, causing intracellular leakage, and ultimately leading to cell death.

The removal of heavy metal ions from water using CDs–g–C₃N₄ has been recently studied by Xu et al. [154]. Licorice was utilized as a carbon source for the green in situ formation of CQDs onto a g–C₃N₄ surface via a hydrothermal route. The as-prepared hybrid materials, containing various mass ratios of licorice:g–C₃N₄ were evaluated in the Cr (VI) photocatalytic reduction. Notably, after 20 min of visible light irradiation, the removal efficiency of Cr (VI) achieved by pristine g–C₃N₄ and the optimum sample of CQDs–g–C₃N₄ was 71.3% and 99.5%, respectively (Figure 19d). XRD analysis confirmed that the CQDs–g–C₃N₄ photocatalyst retains its original crystal structure after the Cr (VI) reduction, with no discernible changes compared to the initial sample. In addition, based on the pseudo-first-order kinetic model, the rate constant of hybrid material was 2.48 times higher than that of g–C₃N₄ (Figure 19e). It is also important to note that the main reactive species involved in Cr (VI) reduction were investigated via trapping experiments using benzoquinone, ammonium oxalate, and potassium bromate for the selective capture of superoxide radicals, photogenerated holes, and electrons, respectively. As it was noticed, the addition of benzoquinone had no significant effect in Cr (VI) removal. On the other hand, ammonium oxalate enhanced reduction, which could be ascribed to the fact that it traps holes, promoting the separation of photogenerated carriers and thereby producing more photogenerated electrons. In contrast, the addition of potassium bromate significantly suppressed Cr (VI) removal. Therefore, the results strongly indicate that the photogenerated electrons are the most crucial reactive species in the photocatalytic degradation of Cr (VI).

In addition, there are various noteworthy publications, from 2023 to the present, which address the photocatalytic degradation of water pollutants utilizing CDs–graphitic C₃N₄ heterostructures. A summary of these studies is provided in

Table 3. Recent advances on photocatalytic water treatment using CDs–graphitic C₃N₄ heterostructures.

CD–g–C3N4	Water Pollutant	Light Source	Photocatalytic Degradation	References
ginkgo leaves-derived CDs–g–C3N4	Rhodamine B (10 ppm)	Vis	>99.99% after 60 min (* photocatalyst powder/50 mL)	[155]
N-doped GQDs– P-doped g–C3N4	Tetracycline (20 ppm)	>420 nm	89.90% after 60 min (50 mg/100 mL photocatalyst)	[156]
Cl-doped CQDs– g–C3N4			91.70% after 120 min (40 mg/40 mL photocatalyst)	[157]
B-doped CQDs– g–C3N4	Rhodamine B (1 g/L) Methyl orange (1 g/L)	Vis	65.58% after 120 min 73.56% after 120 min (10 mg/100 mL photocatalyst)	[158]
CQDs–g–C3N4	Indigo Carmine (10 ppm) Carmoisine (10 ppm)	UV and Vis	96.00% after 60 min 93.50% after 60 min (1.0 mg/mL photocatalyst)	[159]
	E. coli (200 ppm) S. aureu (300 ppm)	300 W Xenon lamp >420 nm	** MIC of 12.5 ppm MIC of 40 ppm	[160]
	Meloxicam (10 ppm) Tetracycline (10 ppm)	Vis	99.90% after 30 min 95.97% after 45 min (20 mg photocatalyst and 20 mg peroxymonosulfate/50 mL)	[161]
	Sulfadiazine (40 µM)	300 W Xenon lamp >400 nm	98.00% after 30 min (20 mg photocatalyst and KMnO4/50 mL)	[162]

* not mentioned, ** MIC: minimum inhibitory concentration.

.

2.8. CDs-Based Ternary Heterostructures

Up to this point, the significance of hybrid materials has been well recognized. The growing interest reflects the potential of such materials to exhibit enhanced properties through the combination of distinct components. Concurrently, progress in materials science has led to the development of novel structures, often involving more than two constituents. These advanced multi-component systems demonstrate improved and synergistic functionalities, highlighting the evolving capabilities of modern material design. Therefore, the final section of this review presents CDs-based ternary heterostructures employed in the photocatalytic degradation of water pollutants.

For instance, in the recent study of Guan et al. [163], two different metal oxides were combined with CQDs, forming a CQDs–ZnO-TiO₂ ternary heterostructure. The novel photocatalytic material was utilized for the effective degradation of MB dye. Although ZnO is a promising material for photocatalytic applications, its efficiency is limited because of the rapid electron–hole recombination. On the other hand, TiO₂, as a n-type semiconductor, features numerous surface oxygen vacancies and highly polar Ti-O bonds, which facilitate water molecule dissociation and hydroxyl groups formation on its surface. As a result, combining ZnO with TiO₂ is an effective modification strategy to enhance photocatalytic degradation performance and hydrogen production. In order to improve the visible light utilization of ZnO–TiO₂ hybrid material, its functionalization with CQDs is an effective strategy. As it is presented in their work, the CQDs–ZnO–TiO₂ heterostructure achieved a 96.70% MB degradation efficiency. In another study, Hao et al. [164] combined Cu₂O and TiO₂ metal oxides with N-doped CDs using hydrothermal, as well as chemical precipitation, processes. The as-prepared N-doped CDs–TiO₂–Cu₂O ternary heterojunction was tested in the photocatalytic degradation of MO. Based on the results, the photocatalyst exhibited a 99% MO degradation efficiency within 20 min.

Moreover, metal oxide–metal sulfide heterojunctions combined with CDs have emerged in recent years as a subject of research due to their promising photocatalytic properties. A dendritic TiO₂–CdS heterostructure modified with CQDs was presented by Dou et al. [165]. The as-synthesized CQDs–TiO₂–CdS photocatalyst was utilized for RhB degradation. More specifically, the TiO₂–CdS hybrid material was prepared by a hydrothermal method, followed by CDs impregnation via soaking at room temperature (Figure 20a). Based on the photocatalytic experiments, it was found that CDs improved the efficiency of dendritic TiO₂–CdS, as the RhB degradation was increased from 69.00% to 84.50% within 50 min of irradiation. A CDs–TiO₂ heterojunction decorated with CDs lead to the increase of the absorption range in the visible region and improved the photo-generated electron–hole separation and carrier transfer rate when electrons are transferred from TiO₂ to CdS. As is observed in Figure 20b, photogenerated electrons in the CB of TiO₂ will recombine with holes in VB of CdS, resulting into the accumulation of electrons in CdS VB. Simultaneously, CDs act as electron trapping agents, preventing photogenerated electrons from recombination with holes. Under light, the excess holes in the VB of TiO₂ can participate in the oxidation reaction of RhB dye, which makes the photocatalyst having excellent degradation activity. Recently, Shi et al. [166] developed a CDs-decorated ZnFe₂O₄–ZnIn₂S₄ novel material and investigated the effect of CDs position (Figure 20c) on the charge transfer of the heterojunction. More specifically, two cases were studied, in which CDs were located either on the exterior or within the interior of the ZnFe₂O₄–ZnIn₂S₄ core–shell structure. The experimental results in the photocatalytic degradation of tetracycline revealed that when CDs were located at the contact interface of core–shell material (interior), the heterojunction presented enhanced performance. This indicates that CDs act as a high-speed and smooth pathway for electron migration, effectively facilitating the separation of photo-excited electrons and holes. Since the photocatalyst’s stability is a crucial factor for its practical application, CDs-decorated ZnFe₂O₄–ZnIn₂S₄ was further characterized by XRD and SEM analysis after undergoing multiple photocatalytic cycles. The results verified that both its crystal structure and morphology remained unchanged throughout the reaction process.

Among various mediator candidates, metals (e.g., Ag, Au, Pt, and Pd) are often used in ternary heterojunctions due to their high ability to form Schottky junctions and charge transfer carriers. Wang et al. [167] reported the preparation of a g–C₃N₄–N-doped CDs–Ag ternary structure and its application for organic pollution degradation in aqueous solutions. More specifically, the g–C₃N₄–N-doped CDs–Ag photocatalysts were constructed using an interfacial electronic control mechanism with double doping of N–CDs and Ag NPs. In that case, the N-doped CDs act as a bridge for electrons, guiding the flow of photogenerated charge carriers. The Ag NPs act as photosensitizers and electron acceptors, enhancing the electron transport in the material. The cooperative effect between N-doped CDs and Ag NPs could broaden the absorption range of the visible spectrum of the g–C₃N₄–N-doped CDs–Ag catalyst, enhance the excitation of electrons, and reduce the photogenerated electron–hole pairs recombination rate. The experimental results showed that the degradation of several contaminants (MO, RhB, TCH, and chrysin hydrochloride (CHC)) can be achieved within 100 min using a 300 W xenon lamp as the light source (λ > 400 nm). The reaction rates of g–C₃N₄–N-doped CDs–Ag photocatalysts for the degradation of MO and RhB were 13.5 and 12.9 times higher than those of the conventional g–C₃N₄, respectively. It is also important to mention that this study revealed that the g–C₃N₄–N-doped CDs–Ag heterostructures remained stable after six cycles of use and the degradation rate continued to be higher than 96%.

Ternary heterostructures based on CDs, Bi-based oxyhalides, and g–C₃N₄ components have also been investigated in water treatment applications. Interestingly, Bi-based oxyhalides exhibit lower bandgap as the atomic number of the halogen increases, which enhances their ability to absorb visible light. This property enables the strategic combination of different halogens in Bi-based oxyhalides structures to tailor their fundamental properties and enhance their photocatalytic efficiency. Recently, Abdurahman et al. [168] investigated the combination of g–C₃N₄ decorated with CQDs and BiOCl_xBr_1-x fabricated by calcination and hydrothermal methods. CQDs act as the electron reservoir, which could delay the photogenerated electron–hole pairs’ recombination rate, thus improving the overall photocatalytic performance. The obtained g–C₃N₄–CQDs–BiOCl_xBr_1-x was examined in the photodegradation of tetracycline under visible light illumination, along with the effects of the CQDs content, halogen loading, and pH value. The morphology characterization of the synthesized photocatalyst revealed that CQDs and BiOCl_xBr_1-x solid solutions were deposited onto a g–C₃N₄ surface. Based on the results, it was observed that synergistic effect of 1 wt% CQDs and BiOCl_0.75Br_0.25 significantly improved the interfacial charge transfer efficiency and light harvesting capacity of the hybrid material. The degradation rate of tetracycline (TC) over g–C₃N₄–CQDs–BiOCl_0.75Br_0.25 was 83.4% after 30 min and favorable stability with near-initial capacity under visible light irradiation. In this context, Zhu et al. [169] prepared a series of Bi₇O₉I₃–g–C₃N₄ heterostructures (Figure 21a) modified by different contents of lignin-derived CQDs (0.2 wt%, 0.5 wt% and 1 wt%) for synchronous photocatalytic reduction of Cr (VI) and levofloxacin (LEV) degradation, under simulated light irradiation, using a 300 W Xe lamp. As is observed in Figure 21b, pristine CQDs, Bi₇O₉I₃, and g–C₃N₄ photocatalysts exhibited limited Cr (VI) reduction efficiencies of 4.5%, 51.2%, and 36.8%, respectively, which may be due to the intensive photon scattering and rapid charge recombination. On the other hand, the binary structures of CQDs–Bi₇O₉I₃, CQDs–g–C₃N₄, and Bi₇O₉I₃/g–C₃N₄ presented higher Cr (VI) reduction performance of 64.1%, 41.6%, and 74.4%, due to the improved charge separation efficiency. Notably, the development of ternary CQDs–Bi₇O₉I₃–g–C₃N₄ heterojunctions could further enhance the photocatalytic performance. More specifically, the ternary heterojunction containing 0.5 wt% of CQDs demonstrates optimal performance through broadened UV–Vis light absorption, highly efficient charge carrier separation, as well as, enhanced redox properties. As a result, up to 100% of Cr (VI) photoreduction efficiency was achieved under 60 min of light irradiation, while its reaction rate (0.08725 min⁻¹) was about 4.8 times higher than that of Bi₇O₉I₃-g–C₃N₄ (0.01825 min⁻¹). Similarly, in Figure 21c the performance of the same photocatalysts in LEV degradation can be observed. CQDs-decorated Bi₇O₉I₃-g–C₃N₄ reaches 94.8% after 60 min, where the reaction rate (0.03863 min⁻¹) is 2.2 times higher than that of pristine Bi₇O₉I₃-g–C₃N₄ (0.0174 min⁻¹). Consequently, CQDs sandwiched with a Bi₇O₉I₃-g–C₃N₄ heterogeneous interface act as an electron reservoir, facilitating the efficient separation of photogenerated electron–hole pairs and accelerating the charge transfer.

The list of CDs-based ternary heterostructures is exceptionally extensive due to the vast variety of available materials and the numerous possible combinations among them. By integrating CDs with various binary systems, researchers have been able to significantly enhance photocatalytic performance, particularly in terms of light absorption and charge carrier mobility, resulting in the development of next-generation photocatalysts. Given this rapidly expanding research topic, Table 4 provides a concise summary of the most recent and noteworthy advances regarding the utilization of novel CDs-based ternary heterostructures in the field of photocatalytic water treatment.

Figure 21. (a) Schematic illustration of the preparation of CQDs–Bi₇O₉I₃–g–C₃N₄ hybrid material. Photoreduction of (b) Cr (VI) and (c) LEV by pristine CQDs, Bi₇O₉I₃, g–C₃N₄, binary heterostructures Bi₇O₉I₃–g–C₃N₄, CQDs–Bi₇O₉I₃, CQDs–g–C₃N₄, and ternary heterostructures CQDs–Bi₇O₉I₃–g–C₃N₄ containing different CQDs concentrations. (Adopted from [169] with permission).

Figure 21. (a) Schematic illustration of the preparation of CQDs–Bi₇O₉I₃–g–C₃N₄ hybrid material. Photoreduction of (b) Cr (VI) and (c) LEV by pristine CQDs, Bi₇O₉I₃, g–C₃N₄, binary heterostructures Bi₇O₉I₃–g–C₃N₄, CQDs–Bi₇O₉I₃, CQDs–g–C₃N₄, and ternary heterostructures CQDs–Bi₇O₉I₃–g–C₃N₄ containing different CQDs concentrations. (Adopted from [169] with permission).

Table 4. Recent advances in photocatalytic water treatment using CDs-based ternary heterostructures.

CD-Based Ternary Heterostructures	Water Pollutant	Light Source	Photocatalytic Degradation	References
N-doped GQDs–TiO2-Graphene Oxide	Methylene Blue (10 ppm) Crystal Violet (10 ppm) Basic Red 46 (10 ppm)	300 W Xenon lamp >420 nm	96.60% after 150 min 82.40% after 150 min >99.99% after 150 min (50 mg/100 mL photocatalyst)	[170]
GQDs–ZnO–NiO	Methylene Blue (30 ppm) Methyl Orange (30 ppm)	1000 W Halogen lamp UV and Vis	93.42% after 90 min 75.68% after 90 min (100 mg photocatalyst)	[171]
CQDs–CdS–Ta3N5	Levofloxacin (15 ppm)	300 W Xenon lamp >420 nm	91.90% after 60 min (25 mg/100 mL photocatalyst)	[172]
CDs–CdS–g–C3N4	Tetracycline (20 ppm)		98.00% after 45 min (40 mg/50 mL photocatalyst)	[173]
CDs–WO3–g–C3N4	Malachite Green (20 ppm)	Vis	96.30% after 80 min (50 mg/100 mL photocatalyst)	[174]
N-doped CDs–Co3O4–MoS2	RhB (30 ppm)	*	97.75% within 5 min (200 mg/L photocatalyst)	[175]
CQDs–Ag–MoS2	Tartrazine (20 ppm)	Vis	>99.9% after 30 min (300 mg/L photocatalyst)	[176]
CQDs–BiFeO3–BiOBr	Imidacloprid (10 ppm)	500 W Xenon lamp >400 nm	95.7% after 180 min	[177]
CQDs–BiOBr–g–C3N4	Tetracycline (10 ppm)	500 W Xenon lamp Vis	>99.9% after 60 min (40 mg/40 mL photocatalyst)	[178]
CQDs–BiOBr-Ti3C2	Moxifloxacin (10 ppm)	500 W Xenon lamp >420 nm	96.10% after 120 min (25 mg/50 mL photocatalyst)	[179]
CQDs–BiOBr–W18O49	Tetracycline (20 ppm)	300 W Xenon lamp	97.30% after 45 min (20 mg/50 mL photocatalyst)	[180]
CQDs–Bi2MoO6–CuS		300 W Xenon lamp >420 nm	96.98% after 60 min (20 mg/100 mL photocatalyst)	[181]
N-doped GQDs–TiO2–g–C3N4	Ciprofloxacin (10 ppm)	>420 nm	89.60% after 150 min (50 mg/100 mL photocatalyst)	[182]
F-doped CDs–TiO2–g–C3N4	RhB (3 ppm)	500 W Xenon lamp Vis	74.00% after 50 min (10 mg/100 mL photocatalyst)	[183]
P-doped GQD–TiO2–AgI	Methyl Orange (10 ppm)	300 W Xenon lamp >420 nm	78.20% after 60 min (100 mg/100 mL photocatalyst)	[184]
N-doped CD–CuFe2O4–g–C3N4	Tetracycline Hydrochloride (10 ppm)	300 W Xenon lamp	85.69% after 60 min (certain mass of photocatalyst in 100 mL of water pollutant)	[185]

* not mentioned.

Table 4. Recent advances in photocatalytic water treatment using CDs-based ternary heterostructures.

CD-Based Ternary Heterostructures	Water Pollutant	Light Source	Photocatalytic Degradation	References
N-doped GQDs–TiO2-Graphene Oxide	Methylene Blue (10 ppm) Crystal Violet (10 ppm) Basic Red 46 (10 ppm)	300 W Xenon lamp >420 nm	96.60% after 150 min 82.40% after 150 min >99.99% after 150 min (50 mg/100 mL photocatalyst)	[170]
GQDs–ZnO–NiO	Methylene Blue (30 ppm) Methyl Orange (30 ppm)	1000 W Halogen lamp UV and Vis	93.42% after 90 min 75.68% after 90 min (100 mg photocatalyst)	[171]
CQDs–CdS–Ta3N5	Levofloxacin (15 ppm)	300 W Xenon lamp >420 nm	91.90% after 60 min (25 mg/100 mL photocatalyst)	[172]
CDs–CdS–g–C3N4	Tetracycline (20 ppm)		98.00% after 45 min (40 mg/50 mL photocatalyst)	[173]
CDs–WO3–g–C3N4	Malachite Green (20 ppm)	Vis	96.30% after 80 min (50 mg/100 mL photocatalyst)	[174]
N-doped CDs–Co3O4–MoS2	RhB (30 ppm)	*	97.75% within 5 min (200 mg/L photocatalyst)	[175]
CQDs–Ag–MoS2	Tartrazine (20 ppm)	Vis	>99.9% after 30 min (300 mg/L photocatalyst)	[176]
CQDs–BiFeO3–BiOBr	Imidacloprid (10 ppm)	500 W Xenon lamp >400 nm	95.7% after 180 min	[177]
CQDs–BiOBr–g–C3N4	Tetracycline (10 ppm)	500 W Xenon lamp Vis	>99.9% after 60 min (40 mg/40 mL photocatalyst)	[178]
CQDs–BiOBr-Ti3C2	Moxifloxacin (10 ppm)	500 W Xenon lamp >420 nm	96.10% after 120 min (25 mg/50 mL photocatalyst)	[179]
CQDs–BiOBr–W18O49	Tetracycline (20 ppm)	300 W Xenon lamp	97.30% after 45 min (20 mg/50 mL photocatalyst)	[180]
CQDs–Bi2MoO6–CuS		300 W Xenon lamp >420 nm	96.98% after 60 min (20 mg/100 mL photocatalyst)	[181]
N-doped GQDs–TiO2–g–C3N4	Ciprofloxacin (10 ppm)	>420 nm	89.60% after 150 min (50 mg/100 mL photocatalyst)	[182]
F-doped CDs–TiO2–g–C3N4	RhB (3 ppm)	500 W Xenon lamp Vis	74.00% after 50 min (10 mg/100 mL photocatalyst)	[183]
P-doped GQD–TiO2–AgI	Methyl Orange (10 ppm)	300 W Xenon lamp >420 nm	78.20% after 60 min (100 mg/100 mL photocatalyst)	[184]
N-doped CD–CuFe2O4–g–C3N4	Tetracycline Hydrochloride (10 ppm)	300 W Xenon lamp	85.69% after 60 min (certain mass of photocatalyst in 100 mL of water pollutant)	[185]

* not mentioned.

2.9. From Conventional Photocatalysts to Novel Heterostructures: The Added Value of CDs

There is no doubt that the modern world has shifted its focus towards hybrid materials due to their advanced properties, which make them attractive for a wide range of technological applications.

Herein, the combination of conventional photocatalytic materials with CDs, forming either binary or ternary heterostructures with enhanced degradation efficiency of water pollutants was investigated. According to the most recent literature, CDs-based hybrid materials exhibit significantly improved photocatalytic performance compared to common inorganic semiconductors. As researchers indicate, the novel heterostructures have demonstrated superior efficiency in the removal of organic dyes, pharmaceuticals, or heavy metal ions from water, often achieving removal rates exceeding 90–95% within a short irradiation time. These enhancements are generally attributed to the unique characteristics of CDs, including the up-conversion efficiency and the ability to act as electron donors or acceptors. Thus, the final hybrid structures present enhanced light absorption (from UV to Vis) and decreased charge carrier recombination, respectively. Moreover, the abundant and tunable surface functionalities of CDs could enhance the active sites of the photocatalyst towards specific pollutants. In addition, CDs integration decreases agglomeration phenomena and plausible toxicity.

The enhancement in the overall photocatalytic performance of the hybrid materials also pave the way for more sustainable and efficient applications in various fields beyond environmental remediation, including hydrogen production, solar energy conversion, sensors, self-cleaning coatings, etc. As research continues to explore novel CDs-based heterostructures and study their physicochemical properties, their role in the development of next-generation functional nanomaterials becomes increasingly indispensable.

3. Outlook

The growing burden of water pollution—driven by urbanization, industrialization, and climate-induced stressors—presents a formidable global challenge. Photocatalysis has already proved its potential to degrade water pollutants via a sustainable and efficient approach. However, the transition from laboratory-scale innovations to real-world impact remains complex. Based on this view, there is an urgent need to develop next-generation photocatalysts with efficiency upon visible light irradiation, robustness, scalability, and diversity to several environmental conditions and pollutants. Conventional photocatalysts, including metal oxides, metal chalcogenides, metal oxyhalides, etc., suffer from various inherent limitations that hinder their practical application in water remediation.

In particular, most of the traditional photocatalysts are activated only upon UV light, impeding their solar utilization efficiency. High rates of electron–hole recombination also constitute a common issue, leading to reduced photocatalytic performance. Photochemical instability and limited water solubility have been reported in the use of photocatalysts, including metal chalcogenides. Among other factors, surface chemistry has been proven to be a crucial parameter for photocatalysis. Materials like MXenes underline the importance of surface charges and features in water treatment procedures. Apart from surface functionalization, photocatalyst synthesis routes should be taken into account given the fact that some synthetic protocols possess high demands on temperature or energy. These challenges collectively underscore the need for novel photocatalytic materials with improved light-harvesting capabilities, superior charge separation, and environmental compatibility, which are characteristics that emerging CD-based systems are increasingly being designed to address.

The development of heterostructures of inorganic photocatalysts with CDs has emerged as a promising strategy to address these intrinsic limitations. CDs provide a unique combination of superb physicochemical properties and cost- and environmental-efficient synthesis and functionalization. In particular, CDs can be synthesized with a variety of techniques and precursors offering multifunctional characteristics. Up-conversion photoluminescence ability and CDs’ dual role either as electron acceptors or donors can provide new perspective on their photocatalytic performance. CDs surfaces are abundant in various functional groups, such as -COOH, -OH and -NH₂, allowing the interactions with other semiconductor materials. The higher surface area of CDs, along with their porosity, provided abundant active surface for the adsorption of pollutants and thus increased photocatalytic efficiency. These features encourage the fabrication of CD-based heterostructures.

Given the fact that there is a rising demand for sustainable water treatment approaches, there has been a marked increase in research focused on CD-based photocatalytic heterostructures in recent years. Particularly, studies published from 2023 onward have demonstrated notable progress in the rational design, synthesis, and application of these hybrid materials for the degradation of common pollutants. Researchers are now exploring a wide range of inorganic hosts combined with CDs to construct tailored nanocomposites with enhanced photocatalytic efficiency and stability under visible light. These inorganic semiconductors, ranging from classic metal oxides (e.g., ZnO, TiO₂, ferrites) and metal sulfides (e.g., CuS, MoS₂, WS₂, In₂S₃, ZnIn₂S₄) to more novel systems, including MXenes and metal oxyhalides (e.g., BiOCl, Cu₂Cl(OH)₃), can serve as versatile platforms due to their tunable band structures, unique morphologies, and varying catalytic properties. In addition, CDs-based ternary heterostructures have gathered research interest aiming to exploit the synergistic effect among the components.

For instance, CDs have been successfully incorporated as co-catalysts or sensitizers into various metal oxide matrices. Among them, TiO₂ is a popular option in photocatalytic research and its modification with CDs further expands its application. Especially, functionalization of TiO₂ with CDs derived from natural sources provides a sustainable route to expand visible-light activity and improve charge separation. Future efforts should prioritize the rational design of these heterostructures with tailored morphologies, surface functionalities, and dopant engineering to further optimize efficiency in water treatment performance. Similar to TiO₂, ZnO photocatalysts have been modified with CDs overcoming common limitations of ZnO’s photocatalytic application, including wide bang gap and fast recombination of excitons. The tuning of physicochemical properties of CDs–ZnO photocatalysts could secure improved performance in the degradation of pollutants in complex water matrices. Moreover, CDs–ferrite heterostructures combine magnetic properties and improved photocatalytic action in the visible-light spectrum, offering a practical and sustainable approach for water remediation. Moving forward, the fabrication of this type of heterostructure, with optimum surface chemistry, magnetic recyclability and optical properties, holds strong promise for scalable and selective pollutant removal.

Metal chalcogenides, especially transition metal sulfides (e.g., MoS₂, CdS, ZnS, CuS), have aroused research interest in photocatalysis owing to their narrow band gaps, strong light-harvesting capability, and tunable electronic properties. However, their practical application is often hindered by rapid recombination of photogenerated charge carriers and photo-corrosion, particularly in the case of CdS under visible light. This review already underscored the potential of metal chalcogenides photocatalysts integrated CDs to address these limitations and provide new insights in water treatment. The addition of CDs can expand absorption properties to the visible region, as well as facilitate the separation of photogenerated electron–hole pairs, thereby suppressing recombination and improving photocatalytic efficiency. Moreover, the surface functional groups on CDs can form strong interfacial interactions with the metal sulfide matrix, promoting stable hybrid structures.

For instance, CDs addition to MoS₂ has been shown to increase the specific surface area and number of active sites while accelerating charge transfer at the interface, leading to significantly improved degradation rates of organic pollutants such as methylene blue and tetracycline. Similarly, CDs–CdS heterostructures showed superior photocatalytic activity under visible light, where CDs not only improve charge carrier dynamics but also alleviate photo-corrosion of CdS by scavenging photogenerated holes.

Additionally, CuS and MoS₂ nanosheets modified with doped CDs have demonstrated remarkable improvements in Fenton-like and piezo-photocatalytic degradation of tetracycline, owing to enhanced charge separation, radical generation, and internal electric field effects. Similarly, WS₂–PANI–CDs composites exploit the conducting nature of PANI and the electron-trapping ability of CDs to mitigate charge recombination and achieve nearly complete degradation of pharmaceutical pollutants. In ternary chalcogenides like ZnIn₂S₄, CDs facilitate not only enhanced photogenerated carrier separation but also in situ H₂O₂ production, further enabling advanced oxidation processes such as self-Fenton degradation of antibiotics. The synergistic effects between CDs and metal sulfides reinforce the generation of ROS, which are primarily responsible for the oxidative degradation of various organic contaminants in wastewater. This integration also enhances stability and reusability, making CDs–metal sulfide composites promising candidates for sustainable water purification technologies. Beyond chalcogenides, In₂S₃ heterostructures either with or without CDs exhibited flower-like morphologies, where the incorporation of CDs substantially boosts light absorption and suppresses carrier recombination.

Furthermore, the modification of MXenes with CDs not only improves their environmental stability and dispersion but also opens new avenues in photo(electro)catalytic water treatment and hydrogen evolution. In another promising direction, the fabrication of photocatalysts based on metal oxyhalides, especially BiOCl and CDs has garnered research interest, since extended light absorption and facile charge migration has been highlighted in the above presented studies. CDs also have a contributory role to these systems by triggering OVs in BiOCl and supporting ROS generation and therefore pollutant degradation.

Another promising strategy highlighted in this review was the development of metal-free heterostructures based on CDs, especially those with g–C₃N₄. Recent advancements clearly demonstrate that the incorporation of CDs into g–C₃N₄ architectures offers several improvements. CDs serve not only as photosensitizers, enhancing visible-light harvesting via up-conversion photoluminescence, but also as electron acceptors, significantly suppressing electron–hole recombination, therefore increasing ROS formation. Additionally, surface engineering of CDs has been shown to play a crucial role in tuning the photocatalytic behavior of the resulting heterostructures in terms of heteroatom doping or tuning with functional groups. Tailoring these surface functionalities can significantly influence charge separation efficiency, ROS generation, and pollutant-specific interactions, as demonstrated in several recent mechanistic studies. Among other parameters, the role of by products in the final performance has been investigated, pointing out that fluorescent by-products or unreacted precursor compounds can mitigate the photocatalyst’s efficiency. So, it is of the utmost importance to develop purification protocols that ensure purity and reproducibility.

The development of CDs-based ternary heterostructures has significantly advanced photocatalytic water purification by enabling improved charge separation, broader light absorption, and enhanced redox activity. However, despite these promising achievements, several challenges must be addressed to transition these materials from laboratory research to real-world applications. Stability and reusability of CDs-based ternary heterostructures under real, complex environmental matrices also require further examination. Photocatalysts often face challenges such as photo-corrosion, leaching of metal components, and performance loss over multiple cycles. Future research should aim to develop more robust and environmentally benign materials with long-term operational stability.

Independent from the choice of the inorganic host, research studies are focused on the implementation of doped CDs, which further highlights the importance of surface chemistry to optical properties and photocatalytic performance of the as-synthesized platforms. Other studies propose an eco-friendly approach for CDs synthesis by using natural materials as precursors, including crocus cancellatus, mushrooms, fruit waste, etc. Moreover, CDs content in the final heterostructures has been a critical factor for the evaluation of the proposed novel photocatalysts. A rise in CDs concentration led to improved action, but an excessive amount of CDs accounts for inhibition of photocatalysts’ active sites. However, a deeper understanding of how CDs properties such as size, surface chemistry, and doping configuration govern the photocatalytic mechanisms in heterostructures is needed.

Altogether, these examples highlight a growing research trend toward rational design of semiconductor-CDs hybrids, where the selection of the inorganic host is guided by its complementary interaction with CDs to enhance light harvesting, charge dynamics, and photocatalytic redox activity. The diversity of these hosts underscores the adaptability of CDs and their potential to act as universal enhancers across a broad spectrum of catalytic materials.

4. Conclusions

This review highlighted the recent advancements in CDs-based photocatalytic heterostructures for water purification, with a particular emphasis on their synthesis strategies, structural features, and mechanisms of action. CDs have emerged as versatile and efficient co-catalysts or sensitizers due to their unique optical and electronic properties, low toxicity, and ease of functionalization, as well as low cost and environmentally benign synthesis. Their ability to enhance charge carrier separation, extend light absorption into the visible region, and interact with a wide range of inorganic conventional photocatalysts positions them as key components in the development of innovative photocatalytic platforms.

Through a detailed examination of binary and ternary composites, the present review article underscored the influence of material design in the photocatalytic performance, since several factors, including pH, temperature, synthesis, and post-synthesis modifications affect their performance. Among them, CDs surface functionalization and loading amount have aroused research interest due to their influence on their interaction with the host photocatalyst and therefore their photocatalytic activity. These hybrid systems have demonstrated excellent performance in the degradation of various organic pollutants, including pharmaceuticals, dyes, and industrial chemicals, under visible light irradiation.

Apart from improved photocatalytic efficiency, CDs addition provides reinforced stability, environmental adaptability, and ROS generation pathways. There is growing alignment among photocatalytic application and sustainability driven by the utilization of biomass-derived precursors and green chemistry approaches for the fabrication of these heterostructures.

Collectively, CDs-based heterostructures represent a promising class of materials for addressing the global challenge of water contamination. Future research should prioritize a deeper mechanistic understanding, precise optimization of several parameters, potential for scale-up synthesis, and thorough evaluation of long-term stability under realistic environmental conditions to facilitate the practical application of CDs-based photocatalytic platforms.