A Review on Quantum Dots Modiﬁed g-C 3 N 4 -Based Photocatalysts with Improved Photocatalytic Activity

: 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-C 3 N 4 ) has obtained increasing interest due to its unique physicochemical properties. Therefore, numerous researchers have attempted to integrate quantum dots (QDs) with g-C 3 N 4 to optimize the photocatalytic activity. In this review, recent progress in combining g-C 3 N 4 with QDs for synthesizing new photocatalysts was introduced. The methods of QDs / g-C 3 N 4 -based photocatalysts synthesis are summarized. Recent studies assessing the application of photocatalytic performance and mechanism of modiﬁcation of g-C 3 N 4 with carbon quantum dots (CQDs), graphene quantum dots (GQDs), and g-C 3 N 4 QDs are herein discussed. Lastly, challenges and future perspectives of QDs modiﬁed g-C 3 N 4 -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-C 3 N 4 based-photocatalysts. 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-C 3 N 4 ) has obtained increasing interest due to its unique physicochemical properties. Therefore, numerous researchers have attempted to integrate quantum dots (QDs) with g-C 3 N 4 to optimize the photocatalytic activity. In this review, recent progress in combining g-C 3 N 4 with QDs for synthesizing new photocatalysts was introduced. The methods of QDs/g-C 3 N 4 -based photocatalysts synthesis are summarized. Recent studies assessing the application of photocatalytic performance and mechanism of modification of g-C 3 N 4 with carbon quantum dots (CQDs), graphene quantum dots (GQDs), and g-C 3 N 4 QDs are herein discussed. Lastly, challenges and future perspectives of QDs modified g-C 3 N 4 -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-C 3 N 4 based-photocatalysts.

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-C 3 N 4 with CQDs and other QDs, for the synthesis of new photocatalysts. We first summarize the latest fabrication strategies of QDs/g-C 3 N 4 -based photocatalysts and then elaborate on their application, photocatalytic performance, and the mechanism of formation of CQDs, GQDs, and g-C 3 N 4 QDs modified g-C 3 N 4 hybrids. Subsequently, other types of QDs modified g-C 3 N 4 -based photocatalysts are also presented. Lastly, the challenges, aspects requiring research focus, and the potential future development of QDs/g-C 3 N 4 -based photocatalysts in photocatalytic applications are discussed. This review enriches knowledge in the preparation methods, design, and application of QDs/g-C 3 N 4 -based photocatalysts and supports future research on the combination of g-C 3 N 4 with other QDs for improved photocatalytic ability.

Hydrothermal Method
The hydrothermal method is one of the most commonly used methods for synthesizing QDs/g-C 3 N 4 -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-C 3 N 4 (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 CH 4 (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-C 3 N 4 via a facile hydrothermal approach (Figure 3). The generated CDs/g-C 3 N 4 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-C 3 N 4 (ca. 420 nm) (Figure 4a). The photocurrent of CDs/g-C 3 N 4 (10 wt.%) was higher than that observed for g-C 3 N 4 , as shown in Figure 5a, which indicates that CDs can efficiently facilitate the interfacial charge transfer. The photocatalytic performance of CDs/g-C 3 N 4 was evaluated and results suggested that 10 wt.% CDs/g-C 3 N 4 showed the highest H 2 production rate (ca. 2.2 µmol h −1 ), which was 4.4-fold more than that of g-C 3 N 4 (ca. 0.5 µmol h −1 ) under UV light irradiation. Liu et al. [99] prepared the 2D/0D ultrathin g-C 3 N 4 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 H 2 evolution of 88.1 µmol h −1 , 3.02-fold, and 1.91-fold greater than that of pristine UCN and bulk g-C 3 N 4 (BCN)/CQDs-0.2%, respectively.

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 gcatalyst −1 ) and CO (58.82 μmol gcatalyst −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.

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 Ndoped 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 . UV-vis diffuse reflectance absorbance spectra of (a) a-f: g-C 3 N 4 , CDs/g-C 3 N 4 (1 wt.%), CDs/g-C 3 N 4 (5 wt.%), CDs/g-C 3 N 4 (10 wt.%), CDs/g-C 3 N 4 (50 wt.%) and CDs/g-C 3 N 4 (100 wt.%), respectively; (b) MoO 3 , g-C 3 N 4 , CM3 and 0.5CCM3 (reproduced with permission from [92,100]. Copyright Royal Society of Chemistry, 2017; Elsevier, 2018; respectively).

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 Ndoped 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

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-C 3 N 4 because of the hydrogen bonds formed between precursors. Wang et al. [90] synthesized N-doped carbon dots (NCD)/g-C 3 N 4 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-C 3 N 4 resulted in a 13.6-fold higher reaction rate than that of pristine g-C 3 N 4 under visible light irradiation. Wang et al. [101] synthesized CQD-implanted g-C 3 N 4 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-C 3 N 4 , which leads to micro-regional heterostructures between graphite carbon and g-C 3 N 4 . 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 H 2 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-C 3 N 4 hybrids via in situ thermal polymerization of precursors, urea, and glucose, which resulted in enhanced photocatalytic H 2 evolution activity under visible-light. CN/G0.5 hybrids exhibited good stability and an optimal H 2 evolution rate (2.34 mmol g −1 h −1 ), which was 4.55-fold higher than that of bulk g-C 3 N 4 (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-C 3 N 4 could suppress the recombination of charge carries.
Catalysts 2020, 10, x FOR PEER REVIEW 8 of 29 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.   Catalysts 2020, 10, x FOR PEER REVIEW 8 of 29 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.

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

Calcination Method
The calcination method usually occurs at 550 • C under ambient condition for the synthesis of QDs/g-C 3 N 4 composites. Liu et al. [20] prepared CQDs/g-C 3 N 4 composites by calcinating urea and ammoniated CQDs at 550 • C. CQDs/g-C 3 N 4 had a large water-catalyst interface area of 97 m 2 g −1 .
The Tauc plot curve of CQDs/g-C 3 N 4 showed an apparent tail between 2.0 and 2.7 eV, which could improve the light absorbance. The performance of CQDs/g-C 3 N 4 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-C 3 N 4 hybrids by calcinating a mixture of CDs and dicyandiamide. The introduction of CDs caused the lattice distortion of the g-C 3 N 4 , but had little effect on the Brunauer-Emmett-Teller (BET) specific area of g-C 3 N 4 and did not improve the light-harvesting ability of g-C 3 N 4 . 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-C 3 N 4 displayed excellent photocatalytic activity at 3-fold higher than pristine g-C 3 N 4 . Xie et al. [100] synthesized a highly efficient CDs/g-C 3 N 4 /MoO 3 (CCM3) photocatalyst using (NH 4 ) 6 Mo 7 O 24 ·4H 2 O, dicyandiamide, citric acid, and urea through a facile calcination method. The SEM image ( Figure 9a) shows that the crystalline MoO 3 particles were wrapped in thin amorphous g-C 3 N 4 layers. The absorption abilities of 0.5 wt.% CDs/g-C 3 N 4 /MoO 3 (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 MoO 3 /g-C 3 N 4 composite under the same irradiation conditions.

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

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

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-C 3 N 4 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 H 2 under visible light. The highest H 2 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-C 3 N 4 (CDs/CdS/GCN) composites through a combination of solvothermal and physical coprecipitation methods. The XRD pattern (Figure 8b 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 H 2 evolution reactions under visible-light irradiation in a p-nitrophenol solution. Results showed that 3%CDs/10%CdS/GCN exhibited optimal photocatalytic H 2 evolution efficiency.

Impregnation Method
Zhang et al. [96] synthesized a CDs/g-C 3 N 4 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-C 3 N 4 /CDs composites with 0.5 wt.% CDs resulted in a 3.7 times faster reaction rate for phenol photodegradation than pristine g-C 3 N 4 . Tang et al. [98] used an impregnation method to synthesize a metal-free CQDs/g-C 3 N 4 photocatalyst, which significantly enhanced the photocatalytic inactivation of Staphylococcus aureus compared with pure g-C 3 N 4 in vitro. CQDs/g-C 3 N 4 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-C 3 N 4 were negligible. CQDs reduced the band gap of g-C 3 N 4 from 2.71 eV to 2.59 eV due to the narrow band gap of CQDs. Furthermore, CQDs/g-C 3 N 4 exhibited significantly weakened emission peak intensity and higher photocurrent density as compared with g-C 3 N 4 , which implies that CQDs could remarkably inhibit the recombination of electron-hole pairs and promote charge-transfer properties.

The Property, Applications and Photocatalytic Mechanism of CQDs/g-C 3 N 4 Based Photocatalysts
As depicted in Figure 11, many researchers have decorated g-C 3 N 4 with CQDs to fabricate metal-free systems, which included binary and ternary photocatalysts and significantly enhanced the photocatalytic performance.
Catalysts 2020, 10, x FOR PEER REVIEW 11 of 29 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.

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.

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 metalfree systems, which included binary and ternary photocatalysts and significantly enhanced the photocatalytic performance. Figure 11. The classification of CQDs/g-C3N4-based photocatalysts. Figure 11. The classification of CQDs/g-C 3 N 4 -based photocatalysts.

CQDs/g-C 3 N 4 Photocatalysts
Hong et al. [95] synthesized CQDs/g-C 3 N 4 heterojunctions via a facile low temperature process. When CQDs were modified with g-C 3 N 4 , which possessed absorption wavelength from the UV to about 460 nm, the absorption edges of the CQDs/g-C 3 N 4 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-C 3 N 4 , respectively. Moreover, CQDs/g-C 3 N 4 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-C 3 N 4 to construct a CQDs/g-C 3 N 4 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-C 3 N 4 (2.5 eV) compared with that of g-C 3 N 4 (2.8 eV). PL spectra showed that the PL emission intensity of CQDs/g-C 3 N 4 composites was significantly lower than that of g-C 3 N 4 , which indicated that CQDs improved the charge-carrier separation efficiency. It was found that CQDs could simultaneously improve the adsorption of nonpolar CO 2 and photo-induced H 2 , which enhances reaction kinetics and alters CO 2 photoreduction pathways to generate CH 4 . Consequently, in contrast to g-C 3 N 4 , which only generated CO and H 2 , CQDs/g-C 3 N 4 could generate 6-fold more CO and comparable levels of CH 4 without any detectable H 2 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-C 3 N 4 . Jian et al. [106] developed a novel approach to obtain a CQDs/g-C 3 N 4 (CQDs/HpCN) nanocomposite with effective interfacial contact by incorporating negatively charged CQDs and tailor-made proton-functionalized g-C 3 N 4 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-C 3 N 4 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-C 3 N 4 binary system, the improved photocatalytic activity originated from the synergistic interactions between CQDs and g-C 3 N 4 . First, CQDs act as a photosensitizer, making the g-C 3 N 4 absorb more visible light due to its up-converted photoluminescence property [103] and stimulating g-C 3 N 4 to produce electron-hole pairs and more reactive • O 2 − 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-C 3 N 4 [75]. Third, CQDs/g-C 3 N 4 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-C 3 N 4 to CQDs via well contacted heterojunction interface, which retards charge recombination [75]. The band alignment in the CQDs/g-C 3 N 4 junction can facilitate the separation of photogenerated electron-hole pairs [96].

Nonmetal-Doped CQDs/g-C 3 N 4 Photocatalyst
Although CQDs have excellent properties, challenges still exist in the use of the CQDs/g-C 3 N 4 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-C 3 N 4 via a polymerization method. Doping NCD remarkably decreased the PL intensity of NCD/g-C 3 N 4 compared to g-C 3 N 4 and the emission peak of NCD/g-C 3 N 4 showed a clear red-shift from 444 nm to 455 nm. Results exhibited that the photocatalytic activity of 1.0 wt.% NCD/g-C 3 N 4 was 13.6-fold higher than that of pristine g-C 3 N 4 under visible light. Potential photocatalytic mechanisms were proposed in Figure 12. Under visible light irradiation, g-C 3 N 4 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-C 3 N 4 . The CB-level of NCD is lower than that of g-C 3 N 4 , so the electrons in the CB of g-C 3 N 4 can easily transfer to NCD via the heterojunction interface between g-C 3 N 4 and NCD, which leads to efficient separation of electron-hole pairs. Molecular O 2 may capture the electrons in NCD to 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.

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. Furthermore, a sulfur doped CQDs (S-CQDs)/hollow tubular g-C 3 N 4 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.

CQDs Mediated g-C 3 N 4 -Based Heterojunction Photocatalysts
It has been reported that combining g-C 3 N 4 with other semiconductors to form hetero-structured photocatalysts would efficiently separate photogenerated electrons and holes, enhancing the photocatalytic activity of g-C 3 N 4 under visible light irradiation [134][135][136]. Therefore, many researchers have attempted to introduce CQDs into g-C 3 N 4 based heterojunction photocatalysts. Zhang et al. [91] constructed the Z-scheme 2D/2D BiOBr/CDs/g-C 3 N 4 heterojunctions, using carbon dots as solid-state electron mediators. The BiOBr/CDs/g-C 3 N 4 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-C 3 N 4 composites. As a result, BiOBr/CDs/g-C 3 N 4 composites significantly enhanced CIP photodegradation under visible light irradiation in comparison to pristine BiOBr nanosheets and g-C 3 N 4 ultrathin nanosheets, as depicted in Figure 13a. Moreover, BiOBr/CDs/g-C 3 N 4 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-C 3 N 4 /MoO 3 ), which exhibited remarkably enhanced visible-light photocatalytic activity for the degradation of TC compared to pristine g-C 3 N 4 and the MoO 3 /g-C 3 N 4 composite. Doping with 0.5% CDs showed the highest TC degradation rate, which was 3.5 and 46.2 times higher than that of MoO 3 /g-C 3 N 4 and g-C 3 N 4 , respectively. As illustrated in Figure 14, under visible light irradiation, MoO 3 and g-C 3 N 4 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 MoO 3 and g-C 3 N 4 , respectively. Electrons in the CB of MoO 3 transferred rapidly into the VB of g-C 3 N 4 and recombined with holes due to the Z-scheme heterojunctions between MoO 3 and g-C 3 N 4 , which leads to the accumulation of electrons in the CB of g-C 3 N 4 and holes in the VB of MoO 3 . The CB of g-C 3 N 4 (−1.16 eV) was more negative than 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-C 3 N 4 (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 H 2 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-C 3 N 4 /ZnO nanocomposite using melamine, Zn(CH 3 COO) 2 ·2H 2 O, 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-C 3 N 4 /ZnO (CZ30)-CDs4 was 9.56 m 2 g −1 whereas it was 6.38 m 2 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-C 3 N 4 /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-C 3 N 4 /TiO 2 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-C 3 N 4 /TiO 2 composite.
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.

Nobel Metal Nanoparticles and CQDs co-Loaded g-C 3 N 4 Photocatalysts
Loading QDs can improve the photocatalytic performance of g-C 3 N 4 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-C 3 N 4 , it is necessary to find suitable semiconductors to couple with CDs/g-C 3 N 4 . 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-C 3 N 4 complex photocatalysts by a facile method using glucose, urea and AgNO 3 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-C 3 N 4 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-C 3 N 4 and the optimal CQDs/g-C 3 N 4 composite, respectively. The charge generation and transfer procedure of the Ag/CQDs/g-C 3 N 4 composite combined with conjectural photocatalytic H 2 evolution mechanism are illustrated in Figure 16. Under sunlight irradiation, g-C 3 N 4 could only absorb light at wavelengths of less than 460 nm, while photogenerated electrons help the Ag/CQDs/g-C 3 N 4 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-C 3 N 4 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-C 3 N 4 were available for H + reduction to emit H 2 [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-C 3 N 4 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-C 3 N 4 . They evaluated the photocatalytic performance of photocatalysts and found that Pt-CDs/g-C 3 N 4 (10 wt.%) showed the highest H 2 production, which was 2.1-fold more than for Pt/g-C 3 N 4 under UV-light irradiation. Pt-CDs/g-C 3 N 4 (10 wt.%) also exhibited the highest H 2 production rate under visible-light irradiation as well as being capable of splitting pure water to produce H 2 .
Catalysts 2020, 10, x FOR PEER REVIEW 16 of 29 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 cocatalyst 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. 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).

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 m 2 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  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 cocatalyst 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. 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).

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 m 2 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 Figure 16. Schematic illustration of the charge generation and transfer procedure of the Ag/CQDs/g-C 3 N 4 composite combined with a conjectural photocatalytic H 2 evolution mechanism (reproduced with permission from [89]. Copyright Elsevier, 2017).

Other Ternary CQDs/g-C 3 N 4 -Based Photocatalysts
Liu et al. [109] synthesized Fe(III)/CQDs/Fe-doped g-C 3 N 4 (Fe-CN) binary composite via a facile impregnation method. The obtained composite exhibited an increased specific surface area of 17.51 m 2 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-C 3 N 4 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-C 3 N 4 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.

GQDs/g-C 3 N 4 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-C 3 N 4 @GQD nanohybrids using a rationally designed strategy to assemble ox-GQDs onto s-g-C 3 N 4 nanosheets by a one-step hydrothermal treatment. The TEM image of s-g-C 3 N 4 @GQDs revealed a nanojunction structure with nanosheets cross-linking together and a nanopore structure. The CV curve for s-g-C 3 N 4 @GQDs exhibited a typical capacitive behavior with an overall larger response current than that of GQDs and the original s-g-C 3 N 4 . This indicates the presence of more active sites as well as efficient charge separation and transfer channels within s-g-C 3 N 4 @GQDs. The ORR diffusion current density of s-g-C 3 N 4 @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-C 3 N 4 and GQDs. Xu et al. [111] constructed ternary graphitic carbon nitride/zinc tetracarboxyphthalocyanine/graphene quantum dots (g-C 3 N 4 /ZnTcPc/GQDs) through chemical bonding and hydrothermal methods. The addition of GQDs in g-C 3 N 4 /ZnTcPc/0.1GQDs shifted the C-NH 2 peak from 286.0 eV to 285.7 eV and elevated the C-C peak in comparison with the C-NH 2 peak, as shown in XPS spectra (Figure 17a,b). The g-C 3 N 4 /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-C 3 N 4 . Furthermore, g-C 3 N 4 /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-C 3 N 4 and g-CNNR, respectively.
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. 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).

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].
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-C 3 N 4 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 m 2 /g, while the pore volume decreased from 0.35 to 0.31 cm 3 /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-C 3 N 4 . The enhanced photocatalytic activity contributed to the synergistic effects among g-C 3 N 4 , N-GQDs, and Ag NPs. Feng et al. [152] fabricated the N-doped graphene QDs (N-GQDs) modified Ag 2 CrO 4 @g-C 3 N 4 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, Ag 2 CrO 4 and g-C 3 N 4 could be excited by UV and visible light, with N-GQDs greatly enhancing the utilization of solar light and effectively promoting photoelectron transfer from Ag 2 CrO 4 to g-C 3 N 4 , which not only restrained charge combination, but also greatly inhibited the photo-corrosion of Ag 2 CrO 4 . Moreover, the core-shell structure provided a larger contact area between Ag 2 CrO 4 and g-C 3 N 4 compared with normal hybrid heterojunctions. Thus, AN@CN exhibited excellent photocatalytic degradation of doxycycline under full-spectrum light.

g-C 3 N 4 QDs/g-C 3 N 4 -Based Photocatalysts
Constructing g-C 3 N 4 -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-C 3 N 4 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-C 3 N 4 /BCNQDs heterojunctions by modifying g-C 3 N 4 with B doped g-C 3 N 4 QDs (BCNQDs). A smaller resistance of the incomplete Nyquist plot of g-C 3 N 4 /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-C 3 N 4 /BCNQDs heterojunction exhibited an enhanced hydrogen evolution performance for water splitting under visible light irradiation, which contributed to the formation of heterojunctions between g-C 3 N 4 and BCNQDs with well-matched band structures. Wang et al. [114] prepared a novel Sb 2 S 3 /ultrathin g-C 3 N 4 sheets heterostructure embedded with g-C 3 N 4 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 Sb 2 S 3 . 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-C 3 N 4 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-C 3 N 4 QDs (CNQDs) modified g-C 3 N 4 (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-C 3 N 4 , 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-C 3 N 4 and CNQDs, which results in the excellent photoelectrochemical properties and prolonged lifetime of charge carriers.
Catalysts 2020, 10, x FOR PEER REVIEW 19 of 29 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. 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).

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].  [113,120]. Copyright Elsevier, 2018; Elsevier, 2018; respectively).

Other QDs/g-C 3 N 4 Based Photocatalysts
In addition to the previously mentioned photocatalysts, many other types of QDs/g-C 3 N 4 -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 H 2 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-C 3 N 4 to form stable nanocomposites [160]. Zhao et al. [115] synthetized O-doped MoS 2 nanospheres/CdS QDs/g-C 3 N 4 nanosheets (MoS 2 /CdS/g-C 3 N 4 ) through hydrothermal and chemical bath deposition-calcination processes. TEM image displayed that CdS QDs were attached to the surface of MoS 2 nanospheres and that MoS 2 nanospheres were coated with g-C 3 N 4 nanosheets. UV-vis diffuse reflectance spectra exhibited that the MoS 2 /CdS/g-C 3 N 4 composite was capable of optical absorption in both the visible region and the infrared region. MoS 2 /CdS/g-C 3 N 4 photocatalysts showed the most efficient photocatalytic H 2 evolution. The photodegradation rate of Rh B and bisphenol A (BPA) for MoS 2 /CdS/g-C 3 N 4 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 H 2 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 Bi 2 WO 6 QDs/g-C 3 N 4 binary heterojunction photocatalysts via a one-pot hydrothermal strategy. The N 2 adsorption-desorption experiment revealed a decreased surface area of BWCN10 (5.6 m 2 g −1 ) compared to 11.2 m 2 g −1 for Bi 2 WO 6 QDs. The reduced surface area was attributed to the in-situ deposition of Bi 2 WO 6 QDs into the porous structure of ultrathin g-C 3 N 4 nanosheets and the formation of large mesopores and macropores. FT-IR spectra exhibited that the characteristic peak from the s-triazinering vibration of g-C 3 N 4 nanosheets exhibited a clear blue shift after coupling with Bi 2 WO 6 QDs, which indicated that some interactions occurred between g-C 3 N 4 and Bi 2 WO 6 QDs. The Bi 2 WO 6 QDs/g-C 3 N 4 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-C 3 N 4 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-C 3 N 4 reached 18.09 µmol g −1 h −1 , which was enhanced 4.6 times compared to pure g-C 3 N 4 . 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-C 3 N 4 , which was ascribed to the strong interfacial interaction between the Ag QDs and g-C 3 N 4 . UV-vis results demonstrated that the coupled Ag QDs enhanced light absorption in the visible region. In comparison with pure g-C 3 N 4 , the intensity of the PL signal for 0.5% Ag QDs/g-C 3 N 4 decreased markedly, which indicated suppression in the recombination process of electron-holes and enhanced photocatalytic activity of Ag QDs/g-C 3 N 4 photocatalysts. Han et al. [118] synthesized black phosphorus quantum dots (BP)@g-C 3 N 4 composites via a simple electrostatic attraction approach via the interaction of BP QDs with g-C 3 N 4 occurring via P-N coordinate bonding. After loading BP QDs on g-C 3 N 4 , the absorption band edge of g-C 3 N 4 did not change significantly, which suggested that BP QDs did not improve the photon absorption efficiency. BP@g-C 3 N 4 exhibited a higher photo-current response and smaller resistance of the incomplete Nyquist arc than g-C 3 N 4 , which indicated that BP QDs enhanced the light absorption, electron transfer, and carrier separation efficiency of g-C 3 N 4 . The BP@g-C 3 N 4 composites displayed improved carrier separation efficiency and higher activities for photocatalytic CO 2 reduction to CO (6.54 µmol g −1 h −1 when loading 1 wt.% BP QDs) in comparison to pure g-C 3 N 4 (2.65 µmol g −1 h −1 ) under UV-vis light.
Ye et al. [119] synthesized 0D/2D heterojunctions of vanadate (AgVO 3 , BiVO 4 , InVO 4 , and CuV 2 O 6 ) QDs/g-C 3 N 4 NSs via an in-situ growth strategy. Vanadate QDs reduced the band gap of g-C 3 N 4 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-C 3 N 4 NSs, which resulted in highly dispersed vanadate nanocrystals, strong coupling, and band alignment. Hao et al. [120] formed a 0D/2D Fe 2 O 3 QDs/g-C 3 N 4 composite using melamine and Fe(NO 3 ) 3 ·9H 2 O by combining ultrasonic dispersion with low temperature calcination. The PL maximum emission peak spectrum exhibited a slight blue shift, which was attributed to a Fe 2 O 3 QDs quantum-sized confinement effect. The 0.5% Fe 2 O 3 QDs/2D-C 3 N 4 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 Fe 2 O 3 QDs/2D-C 3 N 4 composites suggested a smaller charge transfer resistance than that of 2D-C 3 N 4 . The 0.5% Fe 2 O 3 QDs/2D-C 3 N 4 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 Co 3 O 4 QDs/g-C 3 N 4 nanosheets. The N 2 adsorption-desorption isotherms depicted specific BET surface areas to be 86.7 m 2 g −1 for 0.8% Co 3 O 4 -C 3 N 4 -300 whereas 54.5 m 2 g −1 for pristine g-C 3 N 4 and an increased pore volume to be 0.31 cm −3 g −1 for 0.8% Co 3 O 4 -C 3 N 4 -300 whereas 0.19 cm −3 g −1 for pristine g-C 3 N 4 . The flat band potential values of g-C 3 N 4 and 0.8% Co 3 O 4 -C 3 N 4 (180, 250, 300) were −0.91, −0.92, −0.87 and −0.74V, respectively, which shows the weakest upward band alignment of g-C 3 N 4 and demonstrates an electric field formed at the interface between g-C 3 N 4 and Co 3 O 4 . This could elevate the efficiency of hole transport from g-C 3 N 4 to Co 3 O 4 . Results showed that 0.8% Co 3 O 4 -C 3 N 4 -300 obtained the highest O 2 production rate, which was four times higher than that of pristine g-C 3 N 4 .

Summary and Perspectives
In conclusion, g-C 3 N 4 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-C 3 N 4 can improve the durability and stability of loaded materials. In order to optimize the photocatalytic activity of g-C 3 N 4 , more researchers have attempted to modify g-C 3 N 4 with QDs, which can effectively capture photogenerated electrons in g-C 3 N 4 . However, research efforts in the area of QDs/g-C 3 N 4 -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-C 3 N 4 -based photocatalysts are proposed.
It is important to develop more effective, facile, economic, and environmentally-friendly synthetic methods to synthesize QDs/g-C 3 N 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-C 3 N 4 -based photocatalysts and to improve the recyclability of QDs/g-C 3 N 4 -based photocatalysts are also important. Novel insights are required to create highly effective g-C 3 N 4 based photocatalysts to address energy and environmental issues. First, various morphologies such as 0D g-C 3 N 4 QDs, nanofibers, nanotubes, nanorods, nanosheets, nanospheres, and mesoporous structures of g-C 3 N 4 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-C 3 N 4 -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-C 3 N 4 -based photocatalysts in more fields such as the preparation of solar cells and sensors. It is expected that this review will provide