A Review on Quantum Dots Modified g-C3N4-Based Photocatalysts with Improved Photocatalytic Activity

Abstract

In the 21st century, the development of sustainable energy and advanced technologies to cope with energy shortages and environmental pollution has become vital. Semiconductor photocatalysis is a promising technology that can directly convert solar energy to chemical energy and is extensively used for its environmentally-friendly properties. In the field of photocatalysis, graphitic carbon nitride (g-C₃N₄) has obtained increasing interest due to its unique physicochemical properties. Therefore, numerous researchers have attempted to integrate quantum dots (QDs) with g-C₃N₄ to optimize the photocatalytic activity. In this review, recent progress in combining g-C₃N₄ with QDs for synthesizing new photocatalysts was introduced. The methods of QDs/g-C₃N₄-based photocatalysts synthesis are summarized. Recent studies assessing the application of photocatalytic performance and mechanism of modification of g-C₃N₄ with carbon quantum dots (CQDs), graphene quantum dots (GQDs), and g-C₃N₄ QDs are herein discussed. Lastly, challenges and future perspectives of QDs modified g-C₃N₄-based photocatalysts in photocatalytic applications are discussed. We hope that this review will provide a valuable overview and insight for the promotion of applications of QDs modified g-C₃N₄ based-photocatalysts.

1. Introduction

With rapid urban and societal development, environmental pollution and energy demands have become two major global challenges, attracting increasing research attention [1,2,3]. Sunlight-driven photocatalytic technologies provide a promising method to solve these problems [4,5,6]. As a novel strategy to take advantage of solar energy, semiconductor photocatalysts are becoming a scientific research focus due to its environmentally-friendly and energy-efficient characteristics, as compared to biological and chemical methods [7,8,9,10]. In recent years, various semiconductor photocatalysts, including TiO₂ [11], boron nitride [12], black phosphorus [13], ZnO [14], and Bi₂WO₆ [15] have been synthetized and widely studied with the application in H₂ production [16,17,18,19,20,21], organic pollutants photodegradation [22,23,24,25,26], CO₂ reduction [27,28], and other fields.

As a metal-free semiconductor with a typical two-dimensional structure, graphitic carbon nitride (g-C₃N₄) has received high levels of attention since Wang et al. [29] first reported that g-C₃N₄ exhibited excellent photocatalytic performance in water splitting under visible light irradiation in 2009. These findings contributed to the evidence that g-C₃N₄ not only has relatively high thermal and chemical stability due to its tri-s-triazine ring structure and high degree of polymerization [30,31,32], but also possesses an appropriate band gap (2.7 eV) and conduction band (CB) position (1.07 eV) [33,34]. However, a high recombination rate of photogenerated charge carriers occurs because of the weak van der Waals interactions between neighboring CN layers [35], a small specific area [36,37,38,39], narrow visible-light absorption [40], and low quantum efficiency [25]. This greatly limits the photocatalytic activity of pure g-C₃N₄, which hinders its practical applications. To improve the photocatalytic activity of g-C₃N₄, various strategies have been developed, including nonmetal doping [41,42,43], passivation layer deposition [44,45,46], metal deposition [47], morphology control [48,49,50,51,52,53], constructing heterojunction structures [54,55,56,57,58,59,60], and modification by carbonaceous nanomaterials such as rGO, GO, carbon nanospheres, and carbon dots [61,62,63,64].

Because of their unique size, dependent properties, and well-defined electronic and optical properties, quantum dots (QDs) have become a key research focus [65,66,67]. Carbon quantum dots (CQDs) are an emerging zero-dimensional (0D) carbon-based nanomaterial with an average size below 10 nm. CQDs have attracted more research attention than traditional semiconductor QDs due to their unique and beneficial optical and electronic properties [68,69,70,71], such as aqueous solubility, up-conversion photoluminescence (PL) characteristics, photo-induced electron transfer properties, size effect, conductivity, photochemical stability, low cost, and non-toxicity [72,73,74,75,76]. Therefore, CQDs have been applied as an efficient component in the design of photocatalysts. Based on successful coupling with semiconductor photocatalysts (such as TiO₂ and ZnO), the catalytic activity of CQDs have been improved by inhibiting photogenerated carrier recombination [77,78,79,80]. CQDs also can broaden the visible light absorption range for other semiconductor photocatalysts (such as Si, Ag₃PO₄, CdS, and Bi₂WO₆) [81,82,83,84]. In addition, the conjugated π structure of CQDs can enhance their interaction with semiconductors to form stable composites [85]. Therefore, CQDs have been widely used in various fields of study, such as photocatalysis [86], biosensing [87], and chemiluminescence analysis [88].

Considering the unique properties of g-C₃N₄ and CQDs, an increasing body of research has attempted to couple g-C₃N₄ with CQDs to fabricate new photocatalysts to improve their photocatalytic performance. In recent years, CQDs modified g-C₃N₄ has been widely applied in H₂ evolution [89,90], the degradation of organic pollutants [91,92,93,94,95,96], CO₂ reduction [97], and disinfection [98], which exhibits excellent photocatalytic performance. As depicted in Figure 1, besides CQDs, graphene quantum dots (GQDs) and g-C₃N₄ QDs have also been used to modify g-C₃N₄. As shown in Figure 2, the number of papers related to QDs modified g-C₃N₄ photocatalysts has increased annually from 2010 to 2019, according to the statistics provided by Web of Science based on “g-C₃N₄,” “graphitic carbon nitride,” “quantum dots,” “CQDs,” and “GQDs” as keywords. However, a comprehensive summary of QDs modified g-C₃N₄-based photocatalysts remains absent.

The main purpose of this review was to summarize the recent progress in the combination of g-C₃N₄ with CQDs and other QDs, for the synthesis of new photocatalysts. We first summarize the latest fabrication strategies of QDs/g-C₃N₄-based photocatalysts and then elaborate on their application, photocatalytic performance, and the mechanism of formation of CQDs, GQDs, and g-C₃N₄ QDs modified g-C₃N₄ hybrids. Subsequently, other types of QDs modified g-C₃N₄-based photocatalysts are also presented. Lastly, the challenges, aspects requiring research focus, and the potential future development of QDs/g-C₃N₄-based photocatalysts in photocatalytic applications are discussed. This review enriches knowledge in the preparation methods, design, and application of QDs/g-C₃N₄-based photocatalysts and supports future research on the combination of g-C₃N₄ with other QDs for improved photocatalytic ability.

2. QDs/g-C₃N₄-Based Photocatalyst Synthesis Methods

Currently, various approaches have been developed to synthesize QDs/g-C₃N₄-based photocatalysts. A large body of research suggests that QDs modified g-C₃N₄ displays better photoactivity than pure g-C₃N₄. However, the synthesis methods have a great influence on the physical and chemical properties as well as on the photocatalytic performance of the produced photocatalysts. Table 1 shows various synthesis methods, applications, and the resulting photocatalytic efficiencies for QDs/g-C₃N₄-based photocatalysts.

2.1. Hydrothermal Method

The hydrothermal method is one of the most commonly used methods for synthesizing QDs/g-C₃N₄-based photocatalysts due to the low reaction temperature requirements. Ong et al. [75] used urea and glucose as precursors to synthesize metal-free 0D/2D carbon nanodot/g-C₃N₄ (CND/pCN) hybrid nanocomposites by a hydrothermal treatment at 120 °C. The CND/pCN-3 sample (3 wt.% of CND) showed the highest total evolutions of CH₄ (29.23 μmol g_catalyst⁻¹) and CO (58.82 μmol g_catalyst⁻¹) under visible light irradiation after 10 h of the reaction, which were 3.6 and 2.28 times higher than those generated with pCN, respectively. Overall, CND/pCN-3 demonstrated high stability and durability after four consecutive photoreaction cycles. Wang et al. [92] synthesized CDs/g-C₃N₄ via a facile hydrothermal approach (Figure 3). The generated CDs/g-C₃N₄ photocatalysts containing different amounts of carbon dots showed a red-shift in the absorption edge and an additional shoulder peak from 450 nm to 600 nm, compared with g-C₃N₄ (ca. 420 nm) (Figure 4a). The photocurrent of CDs/g-C₃N₄ (10 wt.%) was higher than that observed for g-C₃N₄, as shown in Figure 5a, which indicates that CDs can efficiently facilitate the interfacial charge transfer. The photocatalytic performance of CDs/g-C₃N₄ was evaluated and results suggested that 10 wt.% CDs/g-C₃N₄ showed the highest H₂ production rate (ca. 2.2 μmol h⁻¹), which was 4.4-fold more than that of g-C₃N₄ (ca. 0.5 μmol h⁻¹) under UV light irradiation. Liu et al. [99] prepared the 2D/0D ultrathin g-C₃N₄ nanosheets (UCN)/CQDs photocatalysts via a hydrothermal method using rapeseed flower bee pollens, dicyandiamide, and ammonium chloride as precursors. Results showed that coupling with CQDs extended the optical absorption of UCN to the near-infrared (NIR) region, which efficiently inhibited carrier recombination and enhanced the possibility of charge carriers participating in photocatalytic water splitting. The UCN/CQDs-0.2% composite exhibited optimal H₂ evolution of 88.1 μmol h⁻¹, 3.02-fold, and 1.91-fold greater than that of pristine UCN and bulk g-C₃N₄ (BCN)/CQDs-0.2%, respectively.

2.2. Thermal Polymerization Method

The thermal polymerization method proceeds via the conversion of monomeric precursors into polymers under high temperature conditions, which can enhance the connection between CQDs and g-C₃N₄ because of the hydrogen bonds formed between precursors. Wang et al. [90] synthesized N-doped carbon dots (NCD)/g-C₃N₄ composites through a facile hydrothermal-polymerized method using citric acid, urea, and dicyandiamide as precursors. The loading of NCD reduced the band gap and broadened the UV-vis absorption band. They applied the composite to the degradation of IDM and found that 1.0 wt.% NCD/g-C₃N₄ resulted in a 13.6-fold higher reaction rate than that of pristine g-C₃N₄ under visible light irradiation. Wang et al. [101] synthesized CQD-implanted g-C₃N₄ nanotubes (CCTs) by thermal polymerization of freeze-dried urea and the CQDs precursor, as illustrated in Figure 6. The HRTEM image (Figure 7a) exhibits that many small CQDs were distributed on the constitutive g-C₃N₄, which leads to micro-regional heterostructures between graphite carbon and g-C₃N₄. Additionally, the fast Fourier transform (FFT) pattern in Figure 7b displays a hexagonal crystalline structure of the CQDs. The CCTs could efficiently split water under visible light irradiation with an H₂ production rate up to 3538.3 mmol g⁻¹ h⁻¹ and a high quantum yield of 10.94% at 420 nm. Wang et al. [102] fabricated CDs/g-C₃N₄ hybrids via in situ thermal polymerization of precursors, urea, and glucose, which resulted in enhanced photocatalytic H₂ evolution activity under visible-light. CN/G0.5 hybrids exhibited good stability and an optimal H₂ evolution rate (2.34 mmol g⁻¹ h⁻¹), which was 4.55-fold higher than that of bulk g-C₃N₄ (0.51 mmol g⁻¹ h⁻¹). The stacking distance of the layer structure reduced from 0.322 nm to 0.318 nm due to the effect of CDs on the thermal polymerization of urea as depicted by XRD patterns (Figure 8a). The PL spectra showed that the intensity of CN/Gx hybrids were reduced as compared with BCN, which implies the interfacial interactions between CDs and g-C₃N₄ could suppress the recombination of charge carries.

2.3. Calcination Method

The calcination method usually occurs at 550 °C under ambient condition for the synthesis of QDs/g-C₃N₄ composites. Liu et al. [20] prepared CQDs/g-C₃N₄ composites by calcinating urea and ammoniated CQDs at 550 °C. CQDs/g-C₃N₄ had a large water-catalyst interface area of 97 m² g⁻¹. The Tauc plot curve of CQDs/g-C₃N₄ showed an apparent tail between 2.0 and 2.7 eV, which could improve the light absorbance. The performance of CQDs/g-C₃N₄ for photocatalytic water splitting was evaluated under visible light irradiation. The quantum efficiencies were measured at 16% for wavelength λ = 420 ± 20 nm, 6.29% for λ = 58 0 ± 15 nm, and 4.42% for λ = 60 0 ± 10 nm. An overall solar energy conversion efficiency of 2.0% was determined. Fang et al. [93] used a novel method to fabricate CDs/g-C₃N₄ hybrids by calcinating a mixture of CDs and dicyandiamide. The introduction of CDs caused the lattice distortion of the g-C₃N₄, but had little effect on the Brunauer-Emmett-Teller (BET) specific area of g-C₃N₄ and did not improve the light-harvesting ability of g-C₃N₄. The photocatalytic performance of Rhodamine B (Rh B) degradation under UV irradiation and hydrogen production under visible irradiation were studied. The results showed that CDs/g-C₃N₄ displayed excellent photocatalytic activity at 3-fold higher than pristine g-C₃N₄. Xie et al. [100] synthesized a highly efficient CDs/g-C₃N₄/MoO₃ (CCM3) photocatalyst using (NH₄)₆Mo₇O₂₄·4H₂O, dicyandiamide, citric acid, and urea through a facile calcination method. The SEM image (Figure 9a) shows that the crystalline MoO₃ particles were wrapped in thin amorphous g-C₃N₄ layers. The absorption abilities of 0.5 wt.% CDs/g-C₃N₄/MoO₃ (0.5CCM3) in the visible light region were enhanced significantly due to the presence of CDs, as illustrated in Figure 4b. PL analysis in Figure 10a shows that the emission peak of all samples occurred at ca. 450 nm under an excitation wavelength of 300 nm, and loading CQDs dramatically decreased the PL intensity, which suggested that CDs efficiently improved the separation of photoexcited charge carriers. The photocurrent density of 0.5CCM3 photocatalyst was about 1.5-fold to 2-fold higher than that of CM3, as shown in Figure 5b. Photocatalytic results showed that 0.5CCM3 resulted in 88.4% removal of tetracycline (TC) as compared to 43.2% of MoO₃/g-C₃N₄ composite under the same irradiation conditions.

2.4. Solvothermal Method

Using the solvothermal synthesis approach, the use of heat and solvents are combined, with various commonly available solvents employed, such as methanol and ethanol. This method has the advantages of mild reaction conditions, good uniformity, high efficiency, and easy control. Li et al. [103] deposited CQDs onto graphite-like carbon nitride nanosheets (CNNS) to form CNNS/CQDs composites via a facile solvothermal reaction in ethanol. CQDs showed strong optical absorption in the UV region, with a tail extending to the visible range. The PL emission intensity of CNNS/CQDs-7 prepared by adding 90 mg g-C₃N₄ nanosheets into 7 mL CQDs ethanol solution decreased clearly, which suggests that charge recombination was inhibited by loading CQDs. Compared to pure CNNS, light absorption in the UV-vis light region was greatly enhanced by increasing the content of CQDs. After introducing CQDs, the band gap of CNNS decreased from 2.83 eV to 2.80 eV, which enhanced the visible light absorption. The photocatalytic performance of CNNS/CQDs was evaluated based on the photocatalytic production of H₂ under visible light. The highest H₂ production rate was 116.1 μmol h⁻¹, which was two-fold higher than that of pure CNNS (37.8 μmol h⁻¹). Jiang et al. [105] prepared CDs/CdS QDs/g-C₃N₄ (CDs/CdS/GCN) composites through a combination of solvothermal and physical coprecipitation methods. The XRD pattern (Figure 8b) exhibits that the introduction of CDs would not affect the crystal phase and structure of CdS/GCN heterojunctions. The TEM image (Figure 7c) explains CdS QDs and CDs were uniformly loaded on the surface of the GCN nanosheets with less than 10 nm diameters. HRTEM image (Figure 7d) confirms the existence of GCN, CdS QDs, and CDs, and the close contact among them in 3%CDs/10%CdS/GCN. The PL intensities of the GCN nanosheets, PM CdS/GCN, 10%CdS/GCN, and 3%CDs/10%CdS/GCN decreased, as shown in Figure 10b, which indicated a more efficient separation rate of photogenerated electrons and holes. Photocatalytic activities of the obtained GCN nanosheets and CdS/GCN were evaluated by H₂ evolution reactions under visible-light irradiation in a p-nitrophenol solution. Results showed that 3%CDs/10%CdS/GCN exhibited optimal photocatalytic H₂ evolution efficiency.

2.5. Impregnation Method

Zhang et al. [96] synthesized a CDs/g-C₃N₄ photocatalyst via a facile impregnation-thermal method using cyanamide, citric acid, and ethylenimine as precursors. The PL emission spectra of CDs exhibited an excitation wavelength-dependent behavior due to the existence of different emission states of CDs. CDs enhanced the production of photogenerated electron-hole pairs by absorbing visible light with longer wavelength (550 nm) and then emitted photoluminescence at a shorter wavelength (400–500 nm) due to the upconverted photoluminescence properties. Under visible light irradiation, g-C₃N₄/CDs composites with 0.5 wt.% CDs resulted in a 3.7 times faster reaction rate for phenol photodegradation than pristine g-C₃N₄. Tang et al. [98] used an impregnation method to synthesize a metal-free CQDs/g-C₃N₄ photocatalyst, which significantly enhanced the photocatalytic inactivation of Staphylococcus aureus compared with pure g-C₃N₄ in vitro. CQDs/g-C₃N₄ caused a rapid increase in intracellular reactive oxygen species levels and the destruction of cell membranes under visible light, which eventually led to the death of bacteria, while the side effects of CQDs/g-C₃N₄ were negligible. CQDs reduced the band gap of g-C₃N₄ from 2.71 eV to 2.59 eV due to the narrow band gap of CQDs. Furthermore, CQDs/g-C₃N₄ exhibited significantly weakened emission peak intensity and higher photocurrent density as compared with g-C₃N₄, which implies that CQDs could remarkably inhibit the recombination of electron-hole pairs and promote charge-transfer properties.

3. The Property, Applications and Photocatalytic Mechanism of CQDs/g-C₃N₄ Based Photocatalysts

As depicted in Figure 11, many researchers have decorated g-C₃N₄ with CQDs to fabricate metal-free systems, which included binary and ternary photocatalysts and significantly enhanced the photocatalytic performance.

3.1. Binary CQDs/g-C₃N₄ Photocatalysts

3.1.1. CQDs/g-C₃N₄ Photocatalysts

Hong et al. [95] synthesized CQDs/g-C₃N₄ heterojunctions via a facile low temperature process. When CQDs were modified with g-C₃N₄, which possessed absorption wavelength from the UV to about 460 nm, the absorption edges of the CQDs/g-C₃N₄ heterostructures could extend to 400–600 nm. C0.50%/CN degraded 95.2% of Rh B and 78.6% of TC-HCl under visible-light within 3.5 h, and the reaction rate for the photodegradation of Rh B and TC-HCl were about 3.7-fold and 1.9-fold higher than that of bare g-C₃N₄, respectively. Moreover, CQDs/g-C₃N₄ also exhibited excellent photostability and recyclability for both the photodegradation of Rh B and TC-HCl. Feng et al. [97] doped nonpolar CQDs on g-C₃N₄ to construct a CQDs/g-C₃N₄ heterojunction photocatalyst. The narrow band gap of CQDs improved the visible-light absorption and even infrared light absorption. The red shift of the conduction band minimum resulted in a narrower band gap for CQDs/g-C₃N₄ (2.5 eV) compared with that of g-C₃N₄ (2.8 eV). PL spectra showed that the PL emission intensity of CQDs/g-C₃N₄ composites was significantly lower than that of g-C₃N₄, which indicated that CQDs improved the charge-carrier separation efficiency. It was found that CQDs could simultaneously improve the adsorption of nonpolar CO₂ and photo-induced H₂, which enhances reaction kinetics and alters CO₂ photoreduction pathways to generate CH₄. Consequently, in contrast to g-C₃N₄, which only generated CO and H₂, CQDs/g-C₃N₄ could generate 6-fold more CO and comparable levels of CH₄ without any detectable H₂ under the same conditions. No detectable performance deterioration was observed after five repeated cycles of use, which indicates sustained long-term stability of CQDs/g-C₃N₄. Jian et al. [106] developed a novel approach to obtain a CQDs/g-C₃N₄ (CQDs/HpCN) nanocomposite with effective interfacial contact by incorporating negatively charged CQDs and tailor-made proton-functionalized g-C₃N₄ via an electrostatic self-assembly strategy. The dominant peaks in CQDs/HpCN samples showed a slight shift to higher binding energies, which indicated that an electronic interaction between CQDs and HpCN was formed. The incomplete semicircular Nyquist spectrum of CQDs/HpCN modified photoelectrode was characterized with the lowest resistance. Thus, the CQDs/HpCN modified photoelectrode yielded the highest photocurrent, which was much larger than the photocurrent density of pure g-C₃N₄ and HpCN (as shown in Figure 5d). Results showed that CQDs/HpCN exhibited good photocatalytic activity for methylene blue (MB) degradation under visible light irradiation on account of the synergetic interactions between CQDs and HpCN.

In a CQDs/g-C₃N₄ binary system, the improved photocatalytic activity originated from the synergistic interactions between CQDs and g-C₃N₄. First, CQDs act as a photosensitizer, making the g-C₃N₄ absorb more visible light due to its up-converted photoluminescence property [103] and stimulating g-C₃N₄ to produce electron-hole pairs and more reactive ^•O₂⁻ and •OH radicals to improve its photocatalytic ability. Second, CQDs act as electron-sinks, which can trap photoexcited electrons and prevent their recombination with photogenerated holes at valence band (VB) in g-C₃N₄ [75]. Third, CQDs/g-C₃N₄ can form a type-II van der Waals heterojunction, which leads to a significant reduction in the band gap [122], and photoexcited electrons effectively migrate from g-C₃N₄ to CQDs via well contacted heterojunction interface, which retards charge recombination [75]. The band alignment in the CQDs/g-C₃N₄ junction can facilitate the separation of photogenerated electron-hole pairs [96].

3.1.2. Nonmetal-Doped CQDs/g-C₃N₄ Photocatalyst

Although CQDs have excellent properties, challenges still exist in the use of the CQDs/g-C₃N₄ photocatalyst, such as the high expense of precursors, rigorous reaction condition requirements, time-consuming purification processes, and low luminescent quantum yield (<10%) [123]. Similar to other nanomaterials, doping heteroatoms into CQDs could create defect levels in the form of recombination centers to alter charge transfer and improve performance [124]. In particular, N-doped CQDs (N-CQDs) have attracted considerable attention because N atoms can improve quantum yield [125], electron transfer, and extend the visible light adsorption region [126]. Currently, N-CQDs have exhibited unique properties such as electrocatalytic activity, tunable luminescence, and biocompatibility [127,128]. These properties make N-CQDs widely used in photocatalysis, photodetection, and bioimaging [129,130,131,132,133]. Wang et al. [90] synthesized N-doped carbon dots (NCD)/g-C₃N₄ via a polymerization method. Doping NCD remarkably decreased the PL intensity of NCD/g-C₃N₄ compared to g-C₃N₄ and the emission peak of NCD/g-C₃N₄ showed a clear red-shift from 444 nm to 455 nm. Results exhibited that the photocatalytic activity of 1.0 wt.% NCD/g-C₃N₄ was 13.6-fold higher than that of pristine g-C₃N₄ under visible light. Potential photocatalytic mechanisms were proposed in Figure 12. Under visible light irradiation, g-C₃N₄ absorbed <475 nm light to form electron-hole pairs. The >475 nm light was converted to <475 nm light by NCD and absorbed by g-C₃N₄. The CB-level of NCD is lower than that of g-C₃N₄, so the electrons in the CB of g-C₃N₄ can easily transfer to NCD via the heterojunction interface between g-C₃N₄ and NCD, which leads to efficient separation of electron-hole pairs. Molecular O₂ may capture the electrons in NCD to generate ^•O₂⁻. ^•O₂⁻ may react with H⁺ to generate •OH and react with h⁺ to generate O₂. When the VB position of g-C₃N₄ (+1.36 eV) is more negative than the standard redox potential of •OH/OH⁻ (+1.99 eV) and •OH/H₂O (+2.73 eV), the h⁺ from g-C₃N₄ cannot oxidize OH⁻ or H₂O to generate •OH. Then, the photodegradation of IDM in VB may due to the direct reaction with h⁺. Therefore, the efficient degradation of indomethacin (IDM) was contributed to the produced ^•O₂⁻, h⁺, O₂, and •OH.

Furthermore, a sulfur doped CQDs (S-CQDs)/hollow tubular g-C₃N₄ photocatalyst (HTCN-C) was fabricated via an ultrasonic assisted method by Wang et al. [104]. Characterization by UV-Vis diffuse reflectance spectra, photoluminescence, transient photocurrent responses, and electrochemical impedance spectroscopy demonstrated good optical and electrochemical properties in samples, which resulted in a significant improvement in photocatalytic efficiency. Doping of HTCN-C(2) with 2 mL S-CQDs solution exhibited optimized performance with a degradation rate of 0.0293 min⁻¹ for TC and 99.99% destruction of Escherichia coli under visible-light irradiation.

3.2. Ternary CQDs/g-C₃N₄ Based Photocatalysts

3.2.1. CQDs Mediated g-C₃N₄-Based Heterojunction Photocatalysts

It has been reported that combining g-C₃N₄ with other semiconductors to form hetero-structured photocatalysts would efficiently separate photogenerated electrons and holes, enhancing the photocatalytic activity of g-C₃N₄ under visible light irradiation [134,135,136]. Therefore, many researchers have attempted to introduce CQDs into g-C₃N₄ based heterojunction photocatalysts. Zhang et al. [91] constructed the Z-scheme 2D/2D BiOBr/CDs/g-C₃N₄ heterojunctions, using carbon dots as solid-state electron mediators. The BiOBr/CDs/g-C₃N₄ hybrids exhibited remarkable interfacial charge transfer abilities and a broadened solar light absorption range due to the short charge transport distance and the up-converted photoluminescence characteristics of CDs. Simultaneously, the enhanced specific surface area and nanosheet structure could provide more active sites to BiOBr/CDs/g-C₃N₄ composites. As a result, BiOBr/CDs/g-C₃N₄ composites significantly enhanced CIP photodegradation under visible light irradiation in comparison to pristine BiOBr nanosheets and g-C₃N₄ ultrathin nanosheets, as depicted in Figure 13a. Moreover, BiOBr/CDs/g-C₃N₄ composites showed excellent photostability and reusability for CIP degradation after 10 repeated cycles, as demonstrated in Figure 13b. Xie et al. [100] constructed a novel CDs modified Z-scheme photocatalyst (CDs/g-C₃N₄/MoO₃), which exhibited remarkably enhanced visible-light photocatalytic activity for the degradation of TC compared to pristine g-C₃N₄ and the MoO₃/g-C₃N₄ composite. Doping with 0.5% CDs showed the highest TC degradation rate, which was 3.5 and 46.2 times higher than that of MoO₃/g-C₃N₄ and g-C₃N₄, respectively. As illustrated in Figure 14, under visible light irradiation, MoO₃ and g-C₃N₄ can efficiently absorb light up-converted by CDs with wavelengths of less than 416 nm and 460 nm. Therefore, the electron-hole pairs were photogenerated on the surface of MoO₃ and g-C₃N₄, respectively. Electrons in the CB of MoO₃ transferred rapidly into the VB of g-C₃N₄ and recombined with holes due to the Z-scheme heterojunctions between MoO₃ and g-C₃N₄, which leads to the accumulation of electrons in the CB of g-C₃N₄ and holes in the VB of MoO₃. The CB of g-C₃N₄ (−1.16 eV) was more negative than O₂/^•O₂⁻ (−0.046 eV) and, therefore, electrons on the CB of g-C₃N₄ could react with O₂ to produce ^•O₂⁻. Similarly, the VB of MoO₃ (3.54 eV) was more positive than •OH/OH⁻ (+2.27 eV) and it was easy for holes to oxidize OH⁻/H₂O to product •OH. These active species could directly degrade TC.

Jiang et al. [105] prepared CDs/CdS QDs/g-C₃N₄ (CDs/CdS/GCN) photocatalysts. The UV-vis diffuse reflectance spectra for 3%CDs/CdS/GCN exhibited optimal absorption intensity in the visible region and marked red-shifts due to the good light absorption property of CDs and CdS QDs. As shown in Figure 5c, 3%CDs/10%CdS/GCN owned the highest photocurrent intensity, which indicated the best efficiency in terms of both interfacial charge transfer and separation of photogenerated electrons and holes. The optimum loading amount of CdS QDs is 10 wt.%, which shows the highest H₂ evolution rate of 9.4 μmol g⁻¹ h⁻¹ with a 98% 4-NP removal under visible-light irradiation due to the formation of interfaces between CdS QDs and GCN nanosheets, which leads to a high charge separation efficiency. Guo et al. [107] prepared the CDs/g-C₃N₄/ZnO nanocomposite using melamine, Zn(CH₃COO)₂·2H₂O, and graphite rods as precursors via a facile impregnation-thermal method, which exhibited outstanding photocatalytic activity for the photodegradation of TC. The BET specific surface area of 30 wt.% g-C₃N₄/ZnO (CZ30)-CDs4 was 9.56 m² g⁻¹ whereas it was 6.38 m² g⁻¹ for CZ30. The CZ30-CDs4 could degrade almost 100% of TC within 30 min under very weak visible irradiation (λ > 420 nm), which displays the highest activity among all assessed photocatalysts. The enhanced photocatalytic ability could be ascribed to CDs deposited on the surface of the binary g-C₃N₄/ZnO composite, which absorbed a wider spectrum of visible light and hindered the combination of electron-hole pairs through its excellent electron transfer property. Li et al. [108] prepared a novel ternary layered CDs/g-C₃N₄/TiO₂ nanosheets (CGT) composites by impregnation precipitation methods. TEM images exhibited that the size of CDs were mostly in the diameter range of 10–20 nm with lattice spacing of ca. 0.20 nm. Electron paramagnetic resonance (EPR) spectra showed that the loading of CDs distinctly enhanced peak intensity, which indicated a higher concentration of unpaired electrons for CGT2. Results suggested that the CGT composite revealed a high photocatalytic hydrogen evolution rate in triethanolamine aqueous solutions, which was five times higher than that of the optimal g-C₃N₄/TiO₂ composite.

3.2.2. Nobel Metal Nanoparticles and CQDs co-Loaded g-C₃N₄ Photocatalysts

Loading QDs can improve the photocatalytic performance of g-C₃N₄ in hydrogen production and the photodegradation of organic pollutant [137,138,139,140], but the recombination rate of photogenerated electrons and holes remains high. To further improve the quantum efficiency and photocatalytic activity of CDs/g-C₃N₄, it is necessary to find suitable semiconductors to couple with CDs/g-C₃N₄. As emerging and promising materials for exploiting visible light efficiently, noble metal atoms (such as gold, silver and platinum) function as suitable co-catalysts in photocatalytic systems, which mainly due to the surface plasmon resonance (SPR) effect of noble metals. The SPR effect can not only enhance the optical absorption in the visible light region [141], but can also promote the photocatalytic formation of electrons and holes. Moreover, noble metals can extend optical path lengths for photons in the photocatalyst by scattering resonant photons efficiently, and then improving the evolution rates of charge carriers [142]. Noble metals can also act as electron sinks similarly to CQDs and capture electrons, impeding the recombination of electron-hole pairs.

Qin and Zeng [89] designed Ag NPs/CQDs/g-C₃N₄ complex photocatalysts by a facile method using glucose, urea and AgNO₃ as precursors. The photocatalytic hydrogen production activity of the catalysts were evaluated, as depicted in Figure 15a, showing that the 3 wt.% Ag/CQDs/g-C₃N₄ hybrid system possessed the highest photocatalytic HER of up to 626.93 μmol g⁻¹ h⁻¹ under visible light. This was 6.7-fold and 2.8-fold higher than pure g-C₃N₄ and the optimal CQDs/g-C₃N₄ composite, respectively. The charge generation and transfer procedure of the Ag/CQDs/g-C₃N₄ composite combined with conjectural photocatalytic H₂ evolution mechanism are illustrated in Figure 16. Under sunlight irradiation, g-C₃N₄ could only absorb light at wavelengths of less than 460 nm, while photogenerated electrons help the Ag/CQDs/g-C₃N₄ system to capture photons with wavelengths shorter than 550 nm due to the SPR effect of Ag. At the same time, longer wavelengths of more than 550 nm were up-converted to shorter wavelengths by CQDs. All these factors increased the absorption of sunlight by Ag/CQDs/g-C₃N₄ and facilitated high-efficiency photoelectrons generation, transfer, and separation. In addition, the photoelectrons could rapidly migrate to CQDs and Ag NPs through the junction interface due to the matched band alignment [143,144]. Then, photoelectrons in the CB of g-C₃N₄ were available for H⁺ reduction to emit H₂ [145]. Furthermore, as shown in Figure 15b, there was no clear difference in catalytic effect after four cycles of reuse under visible light illumination. Wang et al. [100] prepared Pt-CDs/g-C₃N₄ photocatalysts using a Pt co-catalyst via a photoreduction approach. TEM images exhibit that Pt nanoparticles with a uniform diameter of ca. 2 nm were homogeneously distributed on the surface of carbon dots or g-C₃N₄. They evaluated the photocatalytic performance of photocatalysts and found that Pt-CDs/g-C₃N₄ (10 wt.%) showed the highest H₂ production, which was 2.1-fold more than for Pt/g-C₃N₄ under UV-light irradiation. Pt-CDs/g-C₃N₄ (10 wt.%) also exhibited the highest H₂ production rate under visible-light irradiation as well as being capable of splitting pure water to produce H₂.

3.2.3. Other Ternary CQDs/g-C₃N₄-Based Photocatalysts

Liu et al. [109] synthesized Fe(III)/CQDs/Fe-doped g-C₃N₄ (Fe-CN) binary composite via a facile impregnation method. The obtained composite exhibited an increased specific surface area of 17.51 m² g⁻¹, which was 2.06-fold greater than of pure CN, and the pore volume of Fe(III)/CQDs/Fe-CN was the largest. After incorporation of Fe(III) into Fe-CN, the absorption edge of Fe(III)/Fe-CN exhibited a distinct red shift and the light absorption intensity was enhanced over the whole visible light region. Results showed that photocatalytic activity increased with an increase in the nominal mass fraction of Fe from 2% to 8%, which exhibits that 8% Fe(III)/CQDs/Fe-CN could remove 93% of Methyl Orange (MO) within 60 min under visible light irradiation. This effect was ascribed to the largely promoted light absorption, the reduced transportation distance of photo-induced carriers by the interfacial charge transfer effect, and the generation of •OH active species by the photo-Fenton reaction originating from grafting of Fe(III). He et al. [110] synthesized a new type of three-dimensional (3D) ternary graphene-CQDs/g-C₃N₄ nanosheet (GA-CQDs/CNN) aerogel visible-light-driven photocatalyst via a two-step hydrothermal method. The FESEM image (Figure 9b) depicts that GA-CQDs/CNN-24% possessed a highly interconnected 3D porous structure formed by GO and the diameter ranged from 50 to 500 nm. Therefore, the BET surface areas of GA-CQDs/CNN-24% were larger than that of CNN and CQDs/CNN. The incorporation of CQDs and GA into CNN led to an enhancement of visible light absorption. GA-CQDs/CNN-24% showed the highest photocurrent intensity (0.8 μA cm⁻²), which was 1.78, 2.29, and 2.50 times higher than that of CQDs/CNN, CNN, and BCN, respectively. The MO removal ratio of GA-CQDs/CNN-24% reached 91.1%, which was 7.6-fold higher than that of bulk g-C₃N₄ under the same conditions. The enhanced photocatalytic performance was attributed to that both CQDs and rGO, which can act as a supporter for the 3D framework that could improve the visible light absorption and promote charge separation.

4. GQDs/g-C₃N₄ Based Photocatalysts

As a novel type of QDs with sizes usually less than 10 nm, GQDs comprise single or multiple layer graphene and have attracted attention in the fields of electrochemical biosensors, energy conversion, bioimaging, and drug delivery on account of its unique edge effects and quantum confinement [146,147,148,149]. Therefore, GQDs were widely used as light absorbers to couple with semiconductors [97].

Xu et al. [150] formed unique s-g-C₃N₄@GQD nanohybrids using a rationally designed strategy to assemble ox-GQDs onto s-g-C₃N₄ nanosheets by a one-step hydrothermal treatment. The TEM image of s-g-C₃N₄@GQDs revealed a nanojunction structure with nanosheets cross-linking together and a nanopore structure. The CV curve for s-g-C₃N₄@GQDs exhibited a typical capacitive behavior with an overall larger response current than that of GQDs and the original s-g-C₃N₄. This indicates the presence of more active sites as well as efficient charge separation and transfer channels within s-g-C₃N₄@GQDs. The ORR diffusion current density of s-g-C₃N₄@GQDs was the highest, which reached the potential of the Pt/C electrode. This indicates the high electrochemical activity. Therefore, nanohybrids exhibited remarkably enhanced catalytic activity in the oxygen reduction reaction compared to the original s-g-C₃N₄ and GQDs. Xu et al. [111] constructed ternary graphitic carbon nitride/zinc tetracarboxyphthalocyanine/graphene quantum dots (g-C₃N₄/ZnTcPc/GQDs) through chemical bonding and hydrothermal methods. The addition of GQDs in g-C₃N₄/ZnTcPc/0.1GQDs shifted the C-NH₂ peak from 286.0 eV to 285.7 eV and elevated the C-C peak in comparison with the C-NH₂ peak, as shown in XPS spectra (Figure 17a,b). The g-C₃N₄/ZnTcPc/0.1GQDs composites exhibited increased photocatalytic activity under solar light irradiation. Results showed that the removal rate of Rh B exceeded 98.2%, which was 2-fold higher than that of pure g-C₃N₄. Furthermore, g-C₃N₄/ZnTcPc/0.1GQDs also exhibited a better photocatalytic activity with removal of 63% sulfaquinoxaline sodium within 40 min and 59% carbamazepine within 180 min. Yuan et al. [112] fabricated a metal-free composite photocatalyst of graphene quantum dots modified graphitic carbon nitride nanorods (GQDs/g-CNNR). Characterizations of physicochemical properties indicated that GQDs/g-CNNR photocatalyst exhibited a high crystallization level, enhanced visible light absorption, and staggered band alignment, which improved the photocatalytic activity of GQDs/g-CNNR for the efficient removal of antibiotics. The photocatalytic reaction rate of GQDs/g-CNNR was 3.46-fold and 2.03-fold higher than that of pristine g-C₃N₄ and g-CNNR, respectively.

Meanwhile, it was reported that N atoms introduced into GQDs (N-GQDs) can serve as photosensitizers to capture visible and NIR light, efficiently promote optical and electron transfer properties, and improve the photocatalytic activity of other photocatalysts. Deng et al. [151] synthesized ternary Ag NPs/N-GQDs/g-C₃N₄ nanocomposites. After coating Ag on the surface of GCN-3 for the formation of AGCN-4, the SSA of AGCN-4 increased from 64.99 to 76.67 m²/g, while the pore volume decreased from 0.35 to 0.31 cm³/g. The band gap of AGCN-4 reduced from 2.79 to 2.74 eV, which might facilitate the absorption of more visible light and NIR light. Results indicated that 0.5% N-GQDs and 2.0% AGCN-4 could degrade 92.8% and 31.3% TC under full-spectrum light and NIR light irradiation, respectively, which was three-fold greater than that of pristine g-C₃N₄. The enhanced photocatalytic activity contributed to the synergistic effects among g-C₃N₄, N-GQDs, and Ag NPs. Feng et al. [152] fabricated the N-doped graphene QDs (N-GQDs) modified Ag₂CrO₄@g-C₃N₄ core-shell structured composite (AN@CN), which achieved a full-spectrum response from the UV to the NIR region. The XRD and EIS results confirmed the stable nature of AN@CN composites. In the AN@CN composites, Ag₂CrO₄ and g-C₃N₄ could be excited by UV and visible light, with N-GQDs greatly enhancing the utilization of solar light and effectively promoting photoelectron transfer from Ag₂CrO₄ to g-C₃N₄, which not only restrained charge combination, but also greatly inhibited the photo-corrosion of Ag₂CrO₄. Moreover, the core-shell structure provided a larger contact area between Ag₂CrO₄ and g-C₃N₄ compared with normal hybrid heterojunctions. Thus, AN@CN exhibited excellent photocatalytic degradation of doxycycline under full-spectrum light.

5. g-C₃N₄ QDs/g-C₃N₄-Based Photocatalysts

Constructing g-C₃N₄-based heterojunction photocatalysts has attracted more attention recently. However, the requirement for energy level matching of the two semiconductors limits its widespread development [153] and remarkable differences in physicochemical properties of two semiconductors may significantly influence the homogeneity, stability, and compatibility of heterojunctions. To overcome these shortcomings, a large effort has been made to construct isotype heterojunctions. Due to their similar molecular and crystalline structures, the hetero-interfaces between two kinds of g-C₃N₄ are natively compatible and enable an internal electric field, which promotes the efficient separation of charge carriers [154].

Wang et al. [113] used a hydrothermal method to construct g-C₃N₄/BCNQDs heterojunctions by modifying g-C₃N₄ with B doped g-C₃N₄ QDs (BCNQDs). A smaller resistance of the incomplete Nyquist plot of g-C₃N₄/BCNQDs formed with 7mL BCNQD (Figure 18a) indicated a more facilitated interfacial transfer of electrons and more efficient separation of photo-generated charge carriers. The g-C₃N₄/BCNQDs heterojunction exhibited an enhanced hydrogen evolution performance for water splitting under visible light irradiation, which contributed to the formation of heterojunctions between g-C₃N₄ and BCNQDs with well-matched band structures. Wang et al. [114] prepared a novel Sb₂S₃/ultrathin g-C₃N₄ sheets heterostructure embedded with g-C₃N₄ QDs (CNS) via a hydrothermal process. Characterization of physicochemical properties demonstrated that the composites exhibited fast electron transport and enhanced solar light absorption. Under NIR irradiation, the MO photodegradation rate of the optimal CNS was 0.0103 min⁻¹, which was 2.6 times higher than that of pure Sb₂S₃. The improved NIR photocatalytic activity may benefit from improved absorption in the NIR region, efficient electron-hole separation, and the up-converted PL property of g-C₃N₄ QDs. Zhang et al. [154] polymerized urea and DCDA into carbon nitrides nanosheets (U-CN) and carbon nitrides particles (D-CN) via different condensation processes to construct an isotype heterojunction. After coupling D-CN particles (containing 1% DCDA) with U-CN nanosheets, the HER gradually increased to a maximum value of 553 mmol h⁻¹ g⁻¹, which was 17 and 5 times more than individual D-CN and U-CN, respectively. Zhou et al. [155] reported the formation of g-C₃N₄ QDs (CNQDs) modified g-C₃N₄ (CNQDs/CN), prepared by a simple solvothermal method. HRTEM images show that the average diameter of CNQDs was 3 nm and that CNQDs were uniformly distributed on the surface of bulk g-C₃N₄, while no agglomeration occurred, which might provide more active sites. CNQDs/CN-20 exhibited the weakest PL intensity. Results confirmed that the CNQDs/CN-20 composite could almost completely degrade Rh B within 40 min. The enhanced photocatalytic performance of CNQDs/CN could be ascribed to the effective charge separation and transfer across the interfaces between g-C₃N₄ and CNQDs, which results in the excellent photoelectrochemical properties and prolonged lifetime of charge carriers.

6. Other QDs/g-C₃N₄ Based Photocatalysts

In addition to the previously mentioned photocatalysts, many other types of QDs/g-C₃N₄-based photocatalysts have been synthesized and utilized in various fields. CdS QDs have unique properties and adjustable band gaps, which makes them widely applicable to water pollution treatment and H₂ production [156,157]. However, it is difficult to control the shape of high quality homogeneous and stable semiconductor QDs using traditional solution chemistry [158,159]. One of the most efficient routes to solve this problem is to load CdS QDs onto g-C₃N₄ to form stable nanocomposites [160]. Zhao et al. [115] synthetized O-doped MoS₂ nanospheres/CdS QDs/g-C₃N₄ nanosheets (MoS₂/CdS/g-C₃N₄) through hydrothermal and chemical bath deposition-calcination processes. TEM image displayed that CdS QDs were attached to the surface of MoS₂ nanospheres and that MoS₂ nanospheres were coated with g-C₃N₄ nanosheets. UV-vis diffuse reflectance spectra exhibited that the MoS₂/CdS/g-C₃N₄ composite was capable of optical absorption in both the visible region and the infrared region. MoS₂/CdS/g-C₃N₄ photocatalysts showed the most efficient photocatalytic H₂ evolution. The photodegradation rate of Rh B and bisphenol A (BPA) for MoS₂/CdS/g-C₃N₄ was 99% and 95.2%, respectively. Zheng et al. [160] introduced CdS QDs onto the exterior surface of hollow carbon nitride spheres (HCNS) to synthesize hybrid nano-heterojunctions via an interface self-assembly method. CdS QDs increased the specific surface area of HCNS. The photocatalytic activities of different hetero-structural samples were evaluated for H₂ production. Results showed that 20 wt.% CdS QDs/HCNS exhibited an optimum HER of 601 μmol h⁻¹, which was five times faster than that of non-modified material. The enhanced photocatalytic performance of the heterostructure could be attributed to the unique 3D hollow architectural framework of HCNS, which could serve as a polymeric scaffold to form intimate interfacial contact with CdS QDs to facilitate the surface kinetics of charge separation and mass transfer.

Chen et al. [116] fabricated Bi₂WO₆ QDs/g-C₃N₄ binary heterojunction photocatalysts via a one-pot hydrothermal strategy. The N₂ adsorption-desorption experiment revealed a decreased surface area of BWCN10 (5.6 m² g⁻¹) compared to 11.2 m² g⁻¹ for Bi₂WO₆ QDs. The reduced surface area was attributed to the in-situ deposition of Bi₂WO₆ QDs into the porous structure of ultrathin g-C₃N₄ nanosheets and the formation of large mesopores and macropores. FT-IR spectra exhibited that the characteristic peak from the s-triazinering vibration of g-C₃N₄ nanosheets exhibited a clear blue shift after coupling with Bi₂WO₆ QDs, which indicated that some interactions occurred between g-C₃N₄ and Bi₂WO₆ QDs. The Bi₂WO₆ QDs/g-C₃N₄ composites exhibited clearly enhanced photocatalytic efficiency, which originated from the formation of an internal electric field induced by the enhanced separation efficiency of photogenerated electrons and holes because of the close heterogeneous interface and well-matched band structure. Chen et al. [117] used a novel one-step method to synthesize Ag QDs/g-C₃N₄ photocatalysts, which exhibited high photoactivity for hydrogen production under visible-light irradiation (λ > 420 nm). The hydrogen production efficiency of 0.5% Ag QDs/g-C₃N₄ reached 18.09 μmol g⁻¹ h⁻¹, which was enhanced 4.6 times compared to pure g-C₃N₄. XPS spectras (Figure 17c,d) show that the C 1s and N 1s binding energies had slightly shifted toward a higher binding energy compared to the pure g-C₃N₄, which was ascribed to the strong interfacial interaction between the Ag QDs and g-C₃N₄. UV-vis results demonstrated that the coupled Ag QDs enhanced light absorption in the visible region. In comparison with pure g-C₃N₄, the intensity of the PL signal for 0.5% Ag QDs/g-C₃N₄ decreased markedly, which indicated suppression in the recombination process of electron-holes and enhanced photocatalytic activity of Ag QDs/g-C₃N₄ photocatalysts. Han et al. [118] synthesized black phosphorus quantum dots (BP)@g-C₃N₄ composites via a simple electrostatic attraction approach via the interaction of BP QDs with g-C₃N₄ occurring via P-N coordinate bonding. After loading BP QDs on g-C₃N₄, the absorption band edge of g-C₃N₄ did not change significantly, which suggested that BP QDs did not improve the photon absorption efficiency. BP@g-C₃N₄ exhibited a higher photo-current response and smaller resistance of the incomplete Nyquist arc than g-C₃N₄, which indicated that BP QDs enhanced the light absorption, electron transfer, and carrier separation efficiency of g-C₃N₄. The BP@g-C₃N₄ composites displayed improved carrier separation efficiency and higher activities for photocatalytic CO₂ reduction to CO (6.54 μmol g⁻¹ h⁻¹ when loading 1 wt.% BP QDs) in comparison to pure g-C₃N₄ (2.65 μmol g⁻¹ h⁻¹) under UV-vis light.

Ye et al. [119] synthesized 0D/2D heterojunctions of vanadate (AgVO₃, BiVO₄, InVO₄, and CuV₂O₆) QDs/g-C₃N₄ NSs via an in-situ growth strategy. Vanadate QDs reduced the band gap of g-C₃N₄ NSs and exhibited a substantial absorption capacity in the visible-light region. The heterojunctions exhibited excellent visible light-driven catalytic activity in organic dye degradation due to the photoactive contribution, up-conversion absorption, and nitrogen coordinating sites of g-C₃N₄ NSs, which resulted in highly dispersed vanadate nanocrystals, strong coupling, and band alignment. Hao et al. [120] formed a 0D/2D Fe₂O₃ QDs/g-C₃N₄ composite using melamine and Fe(NO₃)₃·9H₂O by combining ultrasonic dispersion with low temperature calcination. The PL maximum emission peak spectrum exhibited a slight blue shift, which was attributed to a Fe₂O₃ QDs quantum-sized confinement effect. The 0.5% Fe₂O₃ QDs/2D-C₃N₄ photocatalyst has the lowest resistance of the incomplete Nyquist arc radius in Figure 18b, which suggested that it has a superior electron-transfer rate. The decreased arc radius of Nyquist plots for Fe₂O₃ QDs/2D-C₃N₄ composites suggested a smaller charge transfer resistance than that of 2D-C₃N₄. The 0.5% Fe₂O₃ QDs/2D-C₃N₄ composite exhibited the highest photocatalytic activity, which degrades about 75% MB within 180 min under visible light. Zhang et al. [121] used a facile annealing process to fabricate Co₃O₄ QDs/g-C₃N₄ nanosheets. The N₂ adsorption-desorption isotherms depicted specific BET surface areas to be 86.7 m² g⁻¹ for 0.8% Co₃O₄-C₃N₄-300 whereas 54.5 m² g⁻¹ for pristine g-C₃N₄ and an increased pore volume to be 0.31 cm⁻³ g⁻¹ for 0.8% Co₃O₄-C₃N₄-300 whereas 0.19 cm⁻³ g⁻¹ for pristine g-C₃N₄. The flat band potential values of g-C₃N₄ and 0.8% Co₃O₄-C₃N₄ (180, 250, 300) were −0.91, −0.92, −0.87 and −0.74V, respectively, which shows the weakest upward band alignment of g-C₃N₄ and demonstrates an electric field formed at the interface between g-C₃N₄ and Co₃O₄. This could elevate the efficiency of hole transport from g-C₃N₄ to Co₃O₄. Results showed that 0.8% Co₃O₄-C₃N₄-300 obtained the highest O₂ production rate, which was four times higher than that of pristine g-C₃N₄.

7. Summary and Perspectives

In conclusion, g-C₃N₄ has attracted considerable scientific attention due to its high thermal and chemical stabilities as well as unique electronic, optical, and photoelectric properties. Furthermore, as a stabilizer, g-C₃N₄ can improve the durability and stability of loaded materials. In order to optimize the photocatalytic activity of g-C₃N₄, more researchers have attempted to modify g-C₃N₄ with QDs, which can effectively capture photogenerated electrons in g-C₃N₄. However, research efforts in the area of QDs/g-C₃N₄-based photocatalysts remain in their infancy. Many challenges need to be solved in future studies. Therefore, the aspects that warrant further research for the effective application of QDs modified g-C₃N₄-based photocatalysts are proposed.

It is important to develop more effective, facile, economic, and environmentally-friendly synthetic methods to synthesize QDs/g-C₃N_4. In terms of choosing QDs, ideal QDs should have narrow particle size distributions, high fluorescence quantum yield, high stability, non-toxicity, and low cost. Developing efficient methods to load QDs on g-C₃N₄-based photocatalysts and to improve the recyclability of QDs/g-C₃N₄-based photocatalysts are also important. Novel insights are required to create highly effective g-C₃N₄ based photocatalysts to address energy and environmental issues. First, various morphologies such as 0D g-C₃N₄ QDs, nanofibers, nanotubes, nanorods, nanosheets, nanospheres, and mesoporous structures of g-C₃N₄ must be explored to achieve higher surface areas. In addition, improving the photocatalytic performance through hybridization (doping N, O, S), or changing the particle size and loading amount of QDs ought to be explored. Future work requires comprehensive and accurate understanding of the relationship between the structure and properties of QDs/g-C₃N₄-based photocatalysts as well as the dynamics and pathways of charge carrier transfer. Yet, more attention should be paid to the analysis of photodegradation products to avoid producing more harmful secondary pollutants. Attempts should be made to apply QDs/g-C₃N₄-based photocatalysts in more fields such as the preparation of solar cells and sensors. It is expected that this review will provide readers with a broad view of QDs applications and to promote their further use and development to cope with energy shortages and environmental pollution.

Author Contributions

Y.C. contributed to statistical analysis and manuscript preparation. X.B. contributed to manuscript design and revision. All authors have read and agreed to the published version of the manuscript.

Funding

The authors gratefully acknowledge the support provided by the National Natural Science Foundation of China (Grant No. 21876044), the Outstanding Youth Fund of Jiangsu province (Grant No. BK20170098), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

Conflicts of Interest

The authors declare no conflict of interest.

Figures and Table

Figure 1. Summary diagram of the synthetic methods, classification, and applications of quantum dots/g-C3N4 photocatalysts.

Figure 2. Number of publications of g-C3N4-related papers in terms of the keywords: “g-C3N4,” “graphitic carbon nitride,” and quantum dots/g-C3N4-related papers in terms of the keywords: “g-C3N4,” “graphitic carbon nitride,” “quantum dots,” “CQDs,” and “GQDs”. Data refers to indexed journal publications between 2011 and 2019, as reported on “Web of Science.”

Figure 3. A schematic of the synthesis of CDs/g-C3N4: (a) melamine, (b) bulk g-C3N4, (c) L-ascorbic acid/bulk g-C3N4 complexes and (d) CDs/g-C3N4 (reproduced with permission from [92]. Copyright Royal Society of Chemistry, 2017).

Figure 4. UV-vis diffuse reflectance absorbance spectra of (a) a–f: g-C3N4, CDs/g-C3N4 (1 wt.%), CDs/g-C3N4 (5 wt.%), CDs/g-C3N4 (10 wt.%), CDs/g-C3N4 (50 wt.%) and CDs/g-C3N4 (100 wt.%), respectively; (b) MoO3, g-C3N4, CM3 and 0.5CCM3 (reproduced with permission from [92,100]. Copyright Royal Society of Chemistry, 2017; Elsevier, 2018; respectively).

Figure 5. Transient photocurrent responses of (a) g-C3N4 and CDs/g-C3N4 (10 wt.%), (b) CM3 and 0.5CCM3, (c) GCN, GCN nanosheet, PM CdS/GCN, 10%CdS/GCN and 3%CDs/10%CdS/GCN, (d) g-C3N4, HpCN and CQDs/HpCN (reproduced with permission from [92,100,105,106]. Copyright Royal Society of Chemistry, 2017; Elsevier, 2018; American Chemical Society, 2018; Elsevier, 2016; respectively).

Figure 6. CCTs preparation procedure (reproduced with permission from [101]. Copyright Wiley-VCH, 2018).

Figure 7. (a) HRTEM image of CCTs and (b) FFT pattern of the crystallite in (a) (reproduced with permission from [101]. Copyright Wiley-VCH, 2018). TEM image (c) and HRTEM image (d) of 3%CDs/10% CdS/GCN (reproduced with permission from [105]. Copyright American Chemical Society, 2018).

Figure 8. XRD patterns of (a) BCN, CN/G0.25, CN/G0.5, CN/G1.0 and CN/C0.5, (b) pure GCN, GCN nanosheets, CdS QDs, 10%CdS/GCN and 3%CDs/10%CdS/GCN (reproduced with permission from [102,105]. Copyright American Chemical Society, 2018; Elsevier, 2016; respectively).

Figure 9. (a) SEM image of 0.5CCM3, (b) high-magnification FESEM image of GA-CQDs/CNN-24% (reproduced with permission from [100,110]. Copyright Elsevier, 2018; Elsevier, 2018; respectively).

Figure 10. Photoluminescence (PL) emission spectra of (a) g-C3N4, (1–10%) MoO3/C3N4 composites, and 0.5CCM3 at an excitation wavelength of 300 nm, (b) GCN, GCN nanosheets, PM CdS/GCN, 10%CdS/GCN and 3%CDs/10%CdS/GCN at an excitation wavelength of 325 nm (reproduced with permission from [100,105]. Copyright Elsevier, 2018; American Chemical Society, 2018; respectively).

Figure 11. The classification of CQDs/g-C3N4-based photocatalysts.

Figure 12. Schematic illustration of the possible photocatalytic processes in NCD/g-C3N4 composites (reproduced with permission from [90]. Copyright Elsevier, 2017).

Figure 13. Photocatalytic activity (a) and recycling performance (b) of BCCN-30 for ciprofloxacin (CIP) degradation under visible-light irradiation (reproduced with permission from [91]. Copyright Elsevier, 2018).

Figure 14. Proposed photocatalytic degradation mechanisms of TC under visible light irradiation with 0.5CCM3 composites (reproduced with permission from [100]. Copyright Elsevier, 2018).

Figure 15. (a) Photocatalytic activity of pure g-C3N4 and Ag NPs modified 6CCN (6 mL CQDs/g-C3N4) composites with different contents for photocatalytic H2 generation under visible-light irradiation, (b) Recycling performance of 3S6CCN towards photocatalytic H2 generation (reproduced with permission from [89]. Copyright Elsevier, 2017).

Figure 16. Schematic illustration of the charge generation and transfer procedure of the Ag/CQDs/g-C3N4 composite combined with a conjectural photocatalytic H2 evolution mechanism (reproduced with permission from [89]. Copyright Elsevier, 2017).

Figure 17. XPS spectra of (a) C 1s of g-C3N4/ZnTcPc, (b) C 1s of g-C3N4/ZnTcPc/0.1GQDs (reproduced with permission from [111]. Copyright Elsevier, 2019). XPS spectra of (c) C 1s of pure g-C3N4 and AX = 0.5% sample, (d) N 1s of pure g-C3N4 and AX = 0.5% sample (reproduced with permission from [117] Copyright Elsevier, 2016).

Figure 18. EIS Nyquist plots of (a) S0 and S7 samples, (b) 2D-C3N4, 0.2% Fe2O3 QDs/2D-C3N4, 0.5% Fe2O3 QDs/2D-C3N4 and 1% Fe2O3 QDs/2D-C3N4 (reproduced with permission from [113,120]. Copyright Elsevier, 2018; Elsevier, 2018; respectively).

Table 1

Synthetic methods, applications, and photocatalytic efficiency of QDs/g-C₃N₄ based photocatalysts.

Composites	Synthetic Methods	Applications	Light Source	Efficiency	Reference
CND/pCN	Hydrothermal	CO2 reduction	500 W Xe lamp (>400 nm)	CO evolution rate: 5.9 μmol g−1 h−1CH4 evolution rate: 2.9 μmol g−1 h−1	[75]
NCD/g-C3N4	Hydrothermal-polymerized	Degradation of IDM	Visible light	91.5% IDM in 90 min	[90]
BiOBr/CDs/g-C3N4	Hydrothermal	Degradation of CIP	300 W Xe lamp	82.7% CIP in 60 min	[91]
CDs/g-C3N4	Hydrothermal	H2 evolution	3 W LEDs at 365 nm	2.2 μmol h−1	[92]
Pt-CDs/g-C3N4	Photoreduction	H2 evolution	Visible light at 420 nm	183.0 μmol h−1	[92]
CQDs/g-C3N4	Facile low temperature process	Degradation of Rh B and TC-HCl	250 W Xe lamp at 420 nm	95.2% Rh B and 78.6% TC-HCl in 3.5 h	[95]
CDs/g-C3N4	Impregnation-thermal	Degradation of phenol	300W Xe lamp (<400 nm)	100% phenol in 200 min	[96]
Nonpolar CQDs/g-C3N4	Water vapor at room temperature	CO2 reduction	300 W Xe lamp at 400 nm	CO evolution rate: 23.4 μmol g−1 h−1CH4 evolution rate: 20.8 μmol g−1 h−1	[97]
CQDs/g-C3N4	Impregnation	Inactivation of Staphylococcus aureus	Visible light	-	[98]
g-C3N4 NSs/CQDs	Hydrothermal	H2 evolution	300 W Xe lamp at 420 nm	88.1 μmol h−1	[99]
CDs/g-C3N4/MoO3	Calcination	Degradation of TC	350 W Xe lamp at 420 nm	88.4% TC in 90 min	[100]
CQD-implanted g-C3N4 NTs	Thermal polymerization	H2 evolution	Visible light	3538.3 mmol g−1 h−1	[101]
CDs/g-C3N4	In situ thermal polymerization	H2 evolution	300 W Xe lamp (>420 nm)	2.34 mmol g−1 h−1	[102]
g-C3N4 NSs/CQDs	Solvothermal	H2 evolution	500 W Xe lamp (>420 nm)	116.1 μmol h−1	[103]
S-CQDs/HTCN-C	Ultrasonic assisted method	Inactivation of Escherichia coli	300W Xe lamp	99.99% Escherichia coli in 40 min	[104]
CDs/CdS QDs/g-C3N4	Solvothermal-coprecipitation	H2 evolution	300 W Xe lamp (>420 nm)	9.4 μmol g−1 h−1	[105]
CQDs/g-C3N4	electrostatic self-assembly strategy	Degradation of MB	Visible light	-	[106]
CDs/g-C3N4/ZnO	Impregnation-thermal	Degradation of TC	Xe lamp (<420 nm)	100% TC in 30 min	[107]
CDs/g-C3N4/TiO2	Impregnation precipitation	H2 evolution	350W Xe lamp	-	[108]
Fe(III)/CQDs/Fe-CN	Impregnation	Degradation of MO	300 W Xe lamp at 420 nm	93% MO in 60 min	[109]
GA-CQDs/CNN	Two-step hydrothermal	Degradation of MO	Visible light	91.1% MO in 4 h	[110]
g-C3N4/ZnTcPc/GQDs	Hydrothermal	Degradation of Rh B	Solar light	98.2% Rh B in 60 min	[111]
GQDs/g-CNNR	Hydrothermal	Removal of antibiotics	300 W Xe lamp	80% antibiotics in 120 min	[112]
g-C3N4/BCNQDs	Molten salt	H2 evolution	300 W Xe lamp	70.05 mmol h−1	[113]
Sb2S3/g-C3N4/CNS	Hydrothermal	Degradation of MO	300 W Xe lamp	0.0103 MO min−1	[114]
MoS2/CdS QDs/g-C3N4 NSs	Hydrothermal and chemical bath deposition-calcination	H2 evolution, Degradation of Rh B and BPA	Visible light	956 μmol h−1 g−1, 99% Rh B and 95.2% BPA in 20 min	[115]
Bi2WO6 QDs/g-C3N4	One-pot hydrothermal	Degradation of Rh B	Xe lamp	almost 100% Rh B in 30 min	[116]
Ag QDs/g-C3N4	One-step	H2 evolution	300 W Xe lamp (>420 nm)	18.09 μmol g−1 h−1	[117]
BP@g-C3N4	Electrostatic attraction	CO2 reduction	300 W Xe lamp	6.54 μmol g−1 h−1	[118]
Vanadate QDs/g-C3N4 NSs	In situ growth	Degradation of organic-dye	Visible light (>420 nm)	54% MO in 60 min	[119]
Fe2O3 QDs/g-C3N4	Calcination	Degradation of MB, MO, and Rh B	300 W Xe lamp	75% MB in 180 min	[120]
Co3O4 QDs/g-C3N4 NSs	Facile annealing process	Oxygen evolution reactions	300 W Xe lamp	20 μmol g−1 h−1	[121]

Abbreviations: Carbon nanodot/g-C₃N₄ (CND/pCN). N-doped carbon dots/g-C₃N₄ (NCD/g-C₃N₄). Carbon quantum dots (CQDs). g-C₃N₄ nanosheets (g-C₃N₄ NSs). Carbon dots (CDs). g-C₃N₄ nanotubes (g-C₃N₄ NTs). Sulfur doped CQDs/hollow tubular g-C₃N₄ (S-CQDs/HTCN-C). Fe(III)/CQDs/Fe-doped g-C₃N₄ (Fe(III)/CQDs/Fe-CN). Graphene-CQDs/g-C₃N₄ nanosheet (GA-CQDs/CNN). Graphitic carbon nitride/zinc tetracarboxyphthalocyanine/graphene quantum dots (g-C₃N₄/ZnTcPc/GQDs). Graphene quantum dots modified graphitic carbon nitride nanorods (GQDs/g-CNNR). g-C₃N₄/B doped g-C₃N₄ QDs (g-C₃N₄/BCNQDs). Sb₂S₃/ultrathin g-C₃N₄ sheets/g-C₃N₄ QDs (Sb₂S₃/g-C₃N₄/CNS). Indomethacin (IDM). Ciprofloxacin (CIP). Tetracycline (TC). Methyl Orange (MO). Rhodamine B (Rh B). Bisphenol A (BPA).