Metal Oxide Quantum-Dot-g-C3N4 Nanocomposites

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1		Tejaswi Tanaji Salunkhe	--	4736	2023-11-15 03:58:15	\|
2	format correct	Catherine Yang	+ 3 word(s)	4739	2023-11-15 04:35:40	\|

Metal Oxide Quantum-Dot-g-C₃N₄ Nanocomposites

This entry is adapted from the peer-reviewed paper 10.3390/catal13111414

Quantum dots and graphitic carbon nitride (g-C₃N₄) are essential elements in this science. In contrast to quantum dots, which have enormous potential due to their size-dependent bandgap tunability and effective charge carrier production, g-C₃N₄ has properties like chemical stability and a configurable bandgap that make it a versatile material for photocatalysis.

quantum dots g-C3N4 photocatalytic nanocomposite

1. Introduction

Size and Quantum Confinement: the primary distinction lies in the size. Quantum dots are typically smaller than nanoparticles and are in the range of 1–10 nanometers (approximately 10–50 atoms in diameter). At this scale, quantum effects significantly influence the material’s properties, leading to phenomena like quantum confinement in semiconductor QDs, which is not observed in larger nanoparticles. This results in unique optical, electronic, and catalytic properties for QDs.

Optical Properties: due to quantum confinement, QDs exhibit size-dependent tunable photoluminescence, allowing them to absorb and emit light over a wide spectrum. This property is crucial for various applications, including photocatalysis, and is not prominently observed in larger nanoparticles.

Surface Properties: the high surface-to-volume ratio of QDs leads to a significant proportion of atoms being at the surface, which profoundly impacts their chemical reactivity and catalytic activity. While nanoparticles also have a high surface-to-volume ratio, the quantum effects in QDs enhance these surface-related properties.

Energy Band Structures: the discrete energy levels in QDs, a consequence of quantum confinement, differ substantially from the continuous band structure of bulk materials or larger nanoparticles. This affects their interaction with light, charge carrier generation, and transfer—critical factors in photocatalytic processes.

2. Wide-Bandgap Metal Oxide QD-g-C₃N₄ Nanocomposites

In recent half-decade research, TiO₂ quantum dots (QDs) have solidified their position as stalwarts in the realm of nanotechnology, largely attributed to their exceptional photocatalytic proficiencies ^[1]. These quantum entities hold immense promise in efficiently absorbing solar energy, paving the way for their integration into an array of environmental and energy-focused applications. Properties intrinsic to TiO₂ QDs set them apart in the vast quantum landscape. The quantum confinement effect empowers them with a modifiable bandgap, a boon for diversifying photocatalytic ventures. Their magnified surface-to-volume ratio augments their inherent reactivity, and their commendable photostability ensures longevity in demanding applications ^[2]. As for their real-world implications, these QDs shine in water purification, adeptly obliterating organic contaminants. Their prowess extends to hydrogen production, where they serve as linchpins in photoelectrochemical water splitting. Furthermore, their capabilities in air purification, specifically in annihilating noxious air pollutants, have been documented. In essence, the ongoing research narrative accentuates the transformative potential of TiO₂ QDs, suggesting a luminous path ahead in environmental rejuvenation and sustainable energy paradigms. According to Wang et al., creating a p-TiO₂ QDs@g-C₃N₄ p-n junction results in better photocatalytic performance than using pure g-C₃N₄. The improved performance is a result of the p-n heterojunction, strong interface interaction, and quantum-size impact ^[3]. Wang (2021) synthesized an F-doped TiO₂ quantum dot/g-C₃N₄ nanosheet Z-scheme photocatalyst through chemical bonding, resulting in improved oxidizability, reducibility, and interfacial charge transfer ability ^[4]. Lee (2023) created a 0D/2D heterojunction nanocomposite with TiO₂ quantum dots anchored on g-C₃N₄ nanosheets (Figure 1) that demonstrated accelerated solar-driven photocatalysis ^[5]. The integration of anatase/rutile homojunction quantum dots onto g-C₃N₄ nanosheets, which is intended to target the breakdown of antibiotics in saltwater matrices, was documented by Hu and colleagues. Their study delves into the combined mechanism of adsorption and photocatalysis, shedding light on its underlying intricacies. They further evaluated the ternary heterojunctions formed between anatase/rutile quantum dots (QDs) and g-C₃N₄, emphasizing their effectiveness in the removal of Oxytetracycline (OTC). Additionally, the research gauges the toxicity levels of the resultant intermediates detected post-process ^[6].

Figure 1. Synthetic process for fabricating TiO₂ QDs@g-C₃N₄ nanocomposite ^[5].

In recent years, the world of nanotechnology has seen an upswing in interest towards SnO₂ quantum dots (QDs), primarily owing to their potent capabilities in photocatalysis ^[7]. These quantum dots excel in efficiently harnessing light, thereby driving effective charge separation and curbing recombination—traits indispensable for successful photocatalysis. Delving into their synthesis, several cutting-edge methods have emerged over the past half-decade. The hydrothermal method, which revolves around reacting tin salts in water under specific temperature and pressure conditions, remains a favored choice ^[8]. However, the sol-gel approach, where a precursor solution transitions from a gel-like consistency to the desired quantum dots upon drying and calcination, is also prevalent ^[7]. Not to be overshadowed, the microwave-assisted synthesis leverages the power of uniform and rapid microwave heating, often resulting in SnO₂ QDs of superior crystallinity in a fraction of the conventional synthesis time ^[9]. What truly sets SnO₂ QDs apart are their intrinsic properties. The quantum confinement effect grants researchers the liberty to tweak their bandgap, ensuring adaptability for a range of light-driven reactions. Their nanoscale stature bestows upon them a vast surface area, ideal for fostering enhanced reactant interactions. Furthermore, they stand out in the quantum dot family for their remarkable chemical and thermal stability. On the application front, these QDs have been instrumental in several arenas, from water splitting, where they play a role in converting water into hydrogen fuel using sunlight, to the degradation of persistent organic pollutants in water ^[10]. Another noteworthy application is their potential in reducing CO₂, where they can transform atmospheric carbon dioxide into valuable fuels, presenting a promising avenue to combat escalating CO₂ levels ^[11]. Recent studies and trends hint that the true potential of SnO₂ QDs, especially when amalgamated with complementary materials, is yet to be fully unlocked, holding promises for advances in sustainable energy and environmental solutions.

SnO₂ quantum dots (QDs) with graphitic carbon nitride (g-C₃N₄) can improve photocatalytic activity. In 2018, Babu found that when exposed to sunlight, the mixture of SnO₂ QDs and g-C₃N₄ nanolayers displayed increased photocatalytic performance, effectively breaking down methyl orange. This increase in sunlight-driven photocatalytic activity is attributed to the cooperative interaction between the g-C₃N₄ nanolayers and SnO₂ quantum dots. These results highlight the potential of g-C₃N₄ nanolayers and SnO₂ QDs as powerful sunlight-responsive photocatalysts, particularly for the degradation of pollutants like methyl orange ^[12]. In 2019, Yousaf noted a marked increase in photocatalytic performance upon the embellishment of g-C₃N₄ with SnO₂ QDs, which led to the successful decomposition of Rhodamine B. The relative proportion of SnO₂ to g-C₃N₄ in these nanohybrids plays a pivotal role in determining their photocatalytic efficacy (Figure 2). Such findings highlight the potency of SnO₂/g-C₃N₄ nanocomposites, particularly in the domain of degrading contaminants like Rhodamine B (RhB) in solutions ^[13]. In 2017, Ji pioneered the synthesis of a composite photocatalyst combining SnO₂ with graphene-like g-C₃N₄. This composite exhibited superior visible-light-driven activities in degrading organic pollutants. Remarkably, its optimal photocatalytic efficiency under visible light exposure surpassed that of SnO₂ and g-C₃N₄ by almost 9 and 2.5 times, respectively ^[14]. The synergy between SnO₂ and graphene-like g-C₃N₄ is highlighted in this composite, which is represented as SnO₂/graphene-like g-C₃N₄, underlining its potential in photocatalytic processes. Its process in the degradation of Rhodamine B (RhB) with visible light in particular provides encouraging insights into its functional possibilities. All of these results point to the possibility that SnO₂ quantum dots and g-C₃N₄ work better together to accelerate photocatalytic breakdown of organic contaminants in water.

Figure 2. (a–f) Photocatalytic application of SnO₂ QD/g-C₃N₄ nanocomposite ^[13].

Zinc oxide (ZnO) quantum dots have shown significant promise for photocatalytic applications, driven by their unique physicochemical properties. ZnO quantum dots exhibit enhanced photocatalytic efficiency owing to their high surface area and quantum confinement effects ^[15]. Their ability to generate reactive oxygen species upon light irradiation makes them potent catalysts for degrading organic pollutants. Researchers have delved into surface modifications to improve the photocatalytic performance of ZnO quantum dots ^[16]. Techniques like doping, coating, or hybridizing with other materials have been shown to enhance their stability and photocatalytic activity. The advent of black ZnO quantum dots has opened up the possibility of utilizing visible light, significantly broadening the spectrum of light that can be used for photocatalytic applications. Innovations in the design of heterostructures with ZnO quantum dots have shown promise in promoting charge separation, which is crucial for efficient photocatalysis. ZnO quantum dots have found real-world applications in water treatment, air purification, and energy conversion, embodying the translation of academic research to practical solutions ^[17]. Studies have showcased the robustness of ZnO quantum dots in diverse environmental conditions, highlighting their potential for outdoor applications. The integration of ZnO quantum dots with other nanomaterials like graphitic carbon nitride has led to the creation of novel nanocomposites with superior photocatalytic properties ^[18]. Ren et al. created a composite by mixing graphitic carbon nitride (g-C₃N₄) and ZnO quantum dots (QDs) with the goal of enhancing the material’s photocatalytic properties. It was impressive to see how well the composite degraded Rhodamine B when exposed to visible light. A 96.8% degradation rate of Rhodamine B was attained under visible light within a 40 min window, which was a startling achievement. This heightened photocatalytic efficacy is believed to emanate from the synergistic interplay between ZnO QDs, GO, and g-C₃N₄. The separation of photogenerated electron–hole pairs was accelerated with this combination ^[19]. In conclusion, the ZnO QD/GO/g-C₃N₄ composite emerges as a potent contender for the remediation of organic pollutants in wastewater, presenting a workable solution for real-world wastewater treatment scenarios. This is due to its potent photocatalytic performance under visible light and its impressive durability. Investigating the visible-light-induced photocatalytic behavior of SnO₂-ZnO quantum dots attached to g-C₃N₄ nanosheets was the goal of Vattikuti et al. The two main areas of concern were the degradation of contaminants and the creation of H₂. They successfully anchored SnO₂-ZnO quantum dots onto g-C₃N₄ nanosheets with their efforts. The resultant composite manifested heightened photocatalytic prowess when exposed to visible light, especially evident in its commendable degradation rates for contaminants like RhB and phenol. When the data were analyzed, it was discovered that the composite’s RhB degradation rate was 3.5 times greater than that of pure g-C₃N₄. Similar to this, phenol’s rate of degradation was 2.8 times more rapid than that of g-C₃N₄. Additionally, it was found that the composite’s capacity to produce H₂ was astonishingly 4.6 times greater than that of pure g-C₃N₄. From these results, it is clear that the SnO₂-ZnO quantum dots, when attached to g-C₃N₄ nanosheets, significantly improve photocatalytic activity when exposed to visible light. This composite not only excels at degrading pollutants but also evinces significant potential in H₂ production. Such attributes earmark it as a viable solution for tasks ranging from environmental purification to fostering sustainable energy methodologies ^[20].

3. Bi-Based QD-g-C₃N₄ Nanocomposites

Bismuth-based quantum dots (QDs) have emerged as a captivating class of nanostructured semiconductors, drawing substantial interest because of their unique electronic, optical, and photocatalytic characteristics. Their size-dependent bandgaps offer specific tunability for photocatalytic reactions. BiVO₄ QDs ^[21], for instance, display improved light absorption due to quantum confinement effects, and they are recognized for their proficiency in visible-light-driven water splitting and pollutant degradation. Bismuth oxide (Bi₂O₃) QDs exhibit enhanced optical attributes and charge transfer characteristics, positioning them as formidable catalysts for UV and visible light pollutant degradation ^[22]^[23]. Bi₂WO₆ QDs, with their increased electron–hole separation at the quantum level, stand out in degrading diverse organic pollutants under visible light ^[24]. While Bi₂O₄ QDs are relatively less explored, they have demonstrated potential with amplified light interaction at the nanoscale and offer efficient photocatalytic reactions. Lastly, Bi₂MoO₆ quantum dots, known for their extended photogenerated charge carrier lifetimes, are potential frontrunners for organic compound degradation and hydrogen evolution tasks ^[25]. In a nutshell, the nano-dimensionality of bismuth-based QDs accentuates their photocatalytic performance by amplifying light absorption, optimizing charge transfer, and minimizing recombination, making them versatile contenders for an array of photocatalytic applications. The scientists wanted to create a ternary heterostructure comprising C60, g-C₃N₄, and BiVO₄ quantum dots as a photocatalyst. This was carried out to increase the photocatalytic activity when exposed to visible light. Under visible light irradiation, the synthesized ternary heterostructure shown improved photocatalytic activity. Compared to binary heterostructures and pure g-C₃N₄, the ternary heterostructure showed a greater photocatalytic degradation rate of Rhodamine B (RhB) ^[21]. The higher charge separation efficiency and expanded light absorption range were credited with the better photocatalytic performance. The potential mechanisms underlying the ternary heterostructure’s improved photocatalytic activity were discussed by the researchers. They emphasized how C60 and BiVO₄ quantum dots worked together to promote charge separation and lessen recombination. The enhanced photocatalytic efficiency of the ternary heterostructure was also attributed to a wider light absorption range. In comparison to other structures, the ternary heterostructure of BiVO₄ quantum dots/C60/g-C₃N₄ was effectively constructed and showed higher photocatalytic activity. The study offers suggestions for creating effective photocatalysts for a variety of uses, including environmental cleanup.

Liang et al. looked at how well Bi₂O₃ QDs/g-C₃N₄ performed as a photocatalyst for both organic and inorganic contaminants. Tetracycline (TC) and Cr (VI) were chosen as representative environmental pollutants to assess the effectiveness of the samples’ photocatalytic reduction and oxidation ^[23]. Under light illumination, the photocurrent density of the Bi₂O₃ QDs/g-C₃N₄ (ii) was noticeably higher than that of the g-C₃N₄, E-g-C₃N₄, and Bi₂O₃ QDs/g-C₃N₄, showing better charge transfer efficiency. Bi₂O₃ QDs/g-C₃N₄ had the shortest semicircular arc diameter, according to EIS Nyquist plots, indicating the lowest charge transfer resistance and quickest interfacial charge transport. Scavengers had an impact on the effectiveness of TC’s degradation, proving that certain radicals were involved in the process. Byproducts of TC were produced using photocatalytic mineralization processes, with some intermediates demonstrating decreased toxicity after photocatalytic degradation. Bi₂O₃ QDs/g-C₃N₄ underwent photoinduction to improve the separation and transfer of photogenerated charges. As model environmental pollutants, TC and Cr (VI) were used to assess the photocatalytic performance. For Bi₂O₃ QDs/g-C₃N₄, the results showed increased charge transfer effectiveness and quicker interfacial charge transport. The research showed that Bi₂O₃ QDs/g-C₃N₄ had the potential to be an efficient photocatalyst for the oxidation of both organic and inorganic contaminants. Its better photocatalytic performance was aided by its improved charge transfer efficiency and decreased charge transfer resistance. The research offers a potential method for creating highly scattered metal oxides on 2D lamella semiconductors, expanding the photocatalyst’s usefulness for removing a variety of environmental pollutants. Zeng et al. concentrated on the logical application of quantum dots (QDs) and graphitic carbon nitride (g-C₃N₄) semiconductors to increase their effectiveness as photocatalysts. The integration of g-C₃N₄ with QDs was intended to increase photogenerated electron transfer efficiency and produce significant photocatalytic activity. The researchers created brand-new Bi₂WO₆ QD/g-C₃N₄ nanocomposites with attapulgite (ATP) penetration ^[24]. The outcomes demonstrated that the ATP with a nanorod shape served as bridges to intercalate into the interlayers, thus enlarging the g-C₃N₄ inner space. With the use of an HPLC-MS system, the samples’ photocatalytic degradation activities were examined. Using a specified formula, the degrading effectiveness of MBT in the solution was determined. The MBT solution was broken down while being exposed to radiation to gauge the photocatalytic performance. The BCA5 sample exhibited favorable photoelectric characteristics, which indicated quick interfacial charge movement and low resistance for the production of charge carriers. As a potential strategy for future photocatalytic applications, the integration of g-C₃N₄ and QDs with the interpenetration of ATP led to improved photocatalytic performance. In order to create a 2D-0D g-C₃N₄/Bi₂WO₆-OV composite catalyst, Cheng et al. combined two-dimensional (2D) graphite carbon nitride (g-C₃N₄) nanosheets with oxygen-containing vacancy zero-dimensional (0D) Bi₂WO₆ (BWO-OV) quantum dots ^[22]. The goal was to improve the catalyst’s catalytic activity, increase the formation of photogenerated carriers, and improve light absorption. Utilizing Bi₂WO₆ with oxygen vacancies, which improved light absorption while simultaneously increasing the production of photogenerated carriers, was the novel method. The vacancy structure of Bi₂WO₆ and the heterojunction’s creation both contributed to the photogenerated carriers’ longer longevity. The composite of CN/BWO-OV-10 displayed the maximum intensity, indicating a greater capacity for NO degradation. The outcomes of the trapping tests revealed that superoxide radicals, holes, and electrons all contribute significantly to the photocatalytic reaction. Furthermore, hydroxyl is recognized as a less potent active free radical. By eliminating NO, the photocatalytic effectiveness was evaluated. The efficiency peaked at 61.2% when BWO-OV was 10% by mass of the total amount of CN. The CN/BWO composite demonstrated the superiority of CN/BWO-OV-10 with a rise in efficiency of 3.2%, achieving a degradation efficiency of 58%. At room temperature, the composite g-C₃N₄/Bi₂WO₆-OV structure removed nitric oxide (NO) at a rapid rate despite its low concentration. The composite catalyst’s efficiency was higher than that of g-C₃N₄ or BWO-OV and superior to that of g-C₃N₄/Bi₂WO₆ without oxygen vacancies. The best catalytic activity was demonstrated with the composite g-C₃N₄/Bi₂WO₆-OV-10, reaching up to 61.2%. The substance also demonstrated outstanding stability throughout several iterations of experimentation.

Ding and co. investigated Bi₂MoO₆ QDs/g-C₃N₄ with heterojunctions for their potential in the selective oxidation of aromatic alkanes into aldehydes under visible-light-driven catalysis ^[25]. Outstanding visible-light-driven catalytic performance was shown with the Bi₂MoO₆ QD/g-C₃N₄ heterojunction in the selective oxidation of aromatic alkanes into aldehydes. The heterojunction’s special structure, which facilitates the effective separation of charge carriers, is said to be responsible for the increased photocatalytic activity. Superior photocatalytic activity was demonstrated with the heterojunction of Bi₂MoO₆ QDs/g-C₃N₄, particularly in the selective oxidation of aromatic alkanes. The distinctive structure that enables greater charge carrier separation is responsible for this performance. Under visible light, the Bi₂MoO₆ QD/g-C₃N₄ heterojunction exhibits outstanding photocatalytic activity in the selective oxidation of aromatic alkanes to aldehydes. The distinctive structure, which improves charge carrier separation, is credited with the efficiency. The potential of 0D/2D photocatalysts with heterojunctions for the selective oxidation of C(sp³)-H bonds is crucial information provided in this study.

3. Other Metal Oxide QD-g-C₃N₄ Nanocomposites

Sun et al. sought to create CeO₂ quantum dots anchored on g-C₃N₄ (CeO₂/g-C₃N₄) and examine the photocatalytic performance of the material ^[26]. The creation of CeO₂ quantum dots anchored on g-C₃N₄—which are anticipated to have improved photocatalytic properties—represents the work’s originality. It is anticipated that the combination of these materials will enhance charge separation and increase the light absorption range. XRD, FTIR, SEM, TEM, XPS, and PL were used to characterize the synthesized CeO₂/g-C3N4. The outcomes demonstrated a homogeneous distribution of CeO₂ quantum dots on the g-C₃N₄ nanosheets. Better light absorption was discovered to be indicated with the bandgap of CeO₂/g-C₃N₄ being narrower than that of pure g-C₃N₄. By observing the degradation of Rhodamine B (RhB) under visible light irradiation, the photocatalytic performance was assessed. Compared to pure g-C₃N₄, the CeO₂/g-C₃N₄ demonstrated improved photocatalytic activity. The better charge separation and expanded light absorption range brought on with the presence of CeO₂ quantum dots are credited with the improved performance. Rhodamine B degradation under visible light showed higher photocatalytic performance from the CeO₂/g-C₃N₄ combination. This improved performance is a result of the cooperative action of g-C₃N₄ and CeO₂ quantum dots, which improves charge separation and light absorption. Although the exact findings section was not directly extracted, it may be deduced from the information given that the researchers were successful in creating CeO₂ quantum dots anchored on g-C₃N₄ with enhanced photocatalytic characteristics. Its improved Rhodamine B degradation under visible light made the composite material an attractive option for photocatalytic applications.

In order to create a novel photocatalyst, Zhu et al. combined g-C₃N₄ nanosheets and MoO₃ quantum dots (QDs). This mixture was created with the goal of improving photocatalytic activity for the reduction in U(VI) under visible light. The addition of MoO₃ QDs to g-C₃N₄ nanosheets is what makes this work novel ^[27]. Following their easy hydrothermal synthesis, the MoO₃ QDs were loaded onto g-C₃N₄ nanosheets using a straightforward ultrasonic dispersion technique. This mixture was predicted to facilitate photogenerated electron–hole pair separation and improve photocatalytic activity. The MoO₃ QD/g-C₃N₄ nanosheets showed remarkable photocatalytic performance for U(VI) reduction when exposed to visible light in their as-prepared state. It was discovered that MoO₃ QDs should be loaded at a rate of 2%. The efficient separation of photogenerated electron–hole pairs and the expanded light absorption range were credited with the improved photocatalytic activity. Various characterization approaches were used to support the postulated photocatalytic mechanism. By measuring the concentration of U(VI) in aqueous solutions while they were exposed to visible light, the photocatalytic performance was assessed. The outcomes demonstrated that the photocatalytic reduction rate for U(VI) was greater in the MoO₃ QD/g-C₃N₄ nanosheets than in pure g-C₃N₄. It can be concluded that g-C₃N₄ nanosheets in combination with MoO₃ QDs present a viable method for improving the photocatalytic reduction in U(VI) under visible light. The proposed mechanism and the innovative photocatalyst design offer important new perspectives for this field’s future study. The detailed description of recent research on the photocatalytic abilities of nanocomposites based on metal oxide QD’s-g-C₃N₄ were provided in Table 1.

Table 1. Detailed description of recent research on the photocatalytic abilities of nanocomposites based on metal oxide QDs-g-C₃N₄.

Photocatalyst	Pollutant	Dosage	Light Source	Efficiency	Ref.
SnO₂ QDs/g-C₃N₄	RhB	50 mg/L	Visible light	95% in 60 min	^[13]
TiO₂ QDs/g-C₃N₄	RhB	10 mg/L	Visible light	99% in 75 min	^[4]
TiO₂ QDs/g-C₃N₄	Phenol	10 mg/L	Visible light	98% in 100 min	^[4]
TiO₂ QDs/g-C₃N₄	Cr (VI)	20 mg/L	Visible light	99% in 60 min	^[4]
TiO₂ QDs/g-C₃N₄	MO	10 mg/L	Solar light	98% in 120 min	^[28]
CeO₂ QDs/g-C₃N₄	RhB	10 mg/L	Visible light	80% in 180 min	^[26]
CeO₂ QDs/g-C₃N₄	MO	10 mg/L	Visible light	82% in 180 min	^[26]
CeO₂ QDs/g-C₃N₄	MB	10 mg/L	Visible light	74% in 180 min	^[26]
BiVO₄ QDs-g-C₃N₄	RhB	10 mg/L	Visible light	81% in 90 min	^[21]
Bi₂O₃ QDs/g-C₃N₄	TC	10 mg/L	Visible light	83% in 120 min	^[23]
Bi₂O₃ QDs/g-C₃N₄	Cr (VI)	10 mg/L	Visible light	88% in 60 min	^[23]
Bi₂O₄ QDs/g-C₃N₄	RhB	10 mg/L	Visible light	78% in 160 min	^[29]
Bi₂WO₆ QDs/g-C₃N₄	MBT	20 mg/L	Visible light	99% in 80 min	^[24]
MoO₃ QDs/g-C₃N₄	U (VI)	50 mg/L	Visible light	96% in 150 min	^[27]
Co₃O₄ QDs-g-C₃N₄	MTZ	10 mg/L	Visible light	77% in 180 min	^[30]
Bi₂WO₆ QDs/g-C₃N₄	NO	0.5 ppm	Visible light	61%	^[22]
SnO₂ QDs/g-C₃N₄	NO	600 ppb	Visible light	32%	^[31]
SnO₂ NPs/g-C₃N₄	RhB	10 ppm	Visible light	97% in 50 min	^[32]
SnO₂ NPs/g-C₃N₄	MB	10 ppm	Visible light	99% in 90 min	^[33]
SnO₂ NPs/g-C₃N₄	CR	10 ppm	Visible light	96% in 90 min	^[33]
SnO₂ NPs/g-C₃N₄	NO	500 ppb	Visible light	40%	^[34]
SnO₂ NPs/g-C₃N₄	RhB	10 ppm	Solar light	86% in 240 min	^[35]
TiO₂ NPs/g-C₃N₄	TC	20 ppm	Visible light	99% in 120 min	^[36]
TiO₂ NPs/g-C₃N₄	TC	20 ppm	UV light	96% in 90 min	^[37]
TiO₂ NPs/g-C₃N₄	TC	20 ppm	Visible light	90% in 120 min	^[38]
TiO₂ NPs/g-C₃N₄	MB	20 ppm	Solar light	80% in 180 min	^[39]
TiO₂ NPs/g-C₃N₄	TC	100 ppm	Visible light	80% in 100 min	^[40]
ZnO NPs/g-C₃N₄	MB	10 ppm	Visible light	98% in 150 min	^[41]
ZnO NPs/g-C₃N₄	MB	50 ppm	Visible light	98% in 180 min	^[42]
ZnO NPs/g-C₃N₄	MB	10 ppm	Visible light	60% in 120 min	^[43]
ZnO NPs/g-C₃N₄	MB	10 ppm	Visible light	92% in 120 min	^[44]
ZnO NPs/g-C₃N₄	CR	10 ppm	Visible light	70% in 45 min	^[45]
CeO₂ NPs/g-C₃N₄	MO	10 ppm	Visible light	96% in 100 min	^[46]
CeO₂ NPs/g-C₃N₄	RhB	10 ppm	Visible light	96% in 60 min	^[47]
CeO₂ NPs/g-C₃N₄	Cr	20 ppm	Visible light	96% in 100 min	^[48]
CeO₂ QDs/g-C₃N₄	TC	10 ppm	Visible light	78% in 160 min	^[49]
CeO₂ NPs/g-C₃N₄	MB	10 ppm	Visible light	70% in 180 min	^[50]
BiVO₄ NPs/g-C₃N₄	4-CP	20 ppm	Visible light	95% in 100 min	^[51]
BiVO₄ NPs/g-C₃N₄	MO	20 ppm	Visible light	82% in 60 min	^[52]
BiVO₄ NPs/g-C₃N₄	MB	10 ppm	Visible light	88% in 120 min	^[53]
BiVO₄ NPs/g-C₃N₄	TC	10 ppm	Visible light	89% in 120 min	^[53]
Bi₂WO₆ NPs/g-C₃N₄	CIP	15 ppm	Visible light	98% in 120 min	^[54]
Bi₂WO₆ NPs/g-C₃N₄	Diuron	20 ppm	Visible light	75% in 120 min	^[55]
Bi₂WO₆ NPs/g-C₃N₄	ADN	10 ppm	Visible light	98% in 80 min	^[56]
Co₃O₄ NPs/g-C₃N₄	Atrazine	-	Visible light	78% in 35 min	^[57]
MoO₃ NPs/g-C₃N₄	TC	10 ppm	Visible light	86% in 100 min	^[58]
MoO₃ NPs/g-C₃N₄	RhB	10 ppm	Visible light	99% in 25 min	^[59]

Zhao et al. set out to create phosphorus-doped g-C₃N₄/Co₃O₄ quantum dots in a single step using vitamin B12. They sought to increase the synthesized substance’s visible-light photocatalytic activity for the destruction of metronidazole (MTZ) ^[30]. The innovative aspect of the process is the one-step production of phosphorus-doped g-C₃N₄/Co₃O₄ quantum dots using vitamin B12. This technique is distinctive because it enhances the photocatalytic performance of g-C₃N₄ by combining the traits of both phosphorus doping and Co₃O₄ quantum dots. Several approaches were used to characterize the synthesized composites. The photodegradation of MTZ under visible light irradiation was used to assess the photocatalytic activities of the composites. The explanation for the increased photocatalytic activity was determined to be the synergistic interaction between the produced Co₃O₄ quantum dots and P-doped g-C₃N₄. This interaction improved photo-induced electron and hole separation efficiency, prevented their recombination, and reduced band gap energy. The generated Co₃O₄ quantum dots and P-doped g-C₃N₄ worked together to enhance the photocatalytic performance of the created material. As a result of this synergy, photoinduced electrons and holes were separated and transferred more effectively, which increased the photocatalytic degradation of MTZ. The study successfully demonstrated the synthesis of g-C₃N₄/Co₃O₄ quantum dots doped with phosphorus utilizing a one-step procedure and vitamin B12. The created substance demonstrated improved visible-light photocatalytic activity for the breakdown of MTZ. The fundamental causes of the enhanced photocatalytic performance were determined to be the synergistic interactions between the produced Co₃O₄ quantum dots and P-doped g-C₃N₄.

The heterojunctions formed between traditional QDs like CdS/CdSe and g-C₃N₄ have demonstrated effective charge separation due to their staggered band alignment, which minimizes recombination and enhances photocatalytic performance. The researchers discussed how these strategies of interface engineering can be applied to metal oxide QDs to optimize their interactions with g-C₃N₄, focusing on creating synergistic band alignments that facilitate charge transfer and extend light absorption. In-depth analyses of the photocatalytic mechanisms in CdS/CdSe and Ag-In-Zn-S QDs combined with g-C3N4 have revealed critical factors such as quantum confinement effects, surface states, and the role of co-catalysts in improving photocatalytic activity. By integrating these insights, the researchers will elaborate on how similar principles might govern the activity of metal oxide QDs, and how understanding these mechanisms can guide the optimization of their photocatalytic performance. Drawing parallels between the successes of these traditional QDs and the subject metal oxide QDs, the researchers will discuss how strategies like precise size control, doping, or the introduction of defects, successful in traditional QDs, can be mirrored in metal oxides to modulate band structure, enhance light absorption, and improve charge carrier dynamics. While acknowledging the efficiency of traditional QDs, the researchers also recognize concerns regarding their toxicity and environmental impact, particularly for Cd-based QDs. This contrast presents an opportunity to highlight the relative environmental friendliness of metal oxide QDs and the importance of pursuing these materials for sustainable photocatalysis.

Carbon quantum dots (CQDs) have emerged as a captivating class of carbon nanomaterials, characterized by their unique optical, electrical, and physicochemical properties. These properties, combined with their aqueous stability, low toxicity, high surface area, economic feasibility, and tunable photoluminescence behavior, make them promising candidates for photocatalytic applications. On the other hand, graphitic carbon nitride (g-C₃N₄) has gained attention as a stable carbon-based polymer with potential applications in various fields. The combination of CQDs and g-C₃N₄ offers a synergistic effect, enhancing the adsorptive and photocatalytic activity of the resulting nanocomposite. This is attributed to the broader visible-light absorption, increased specific surface area, and enhanced electron–hole pair migration and separation efficiency of the composite. Comparatively, while metal oxide quantum dots also present potential in photocatalytic applications, the CQDs and g-C₃N₄ combination stands out due to its non-toxic nature, economic feasibility, and enhanced photoluminescence properties. The interaction within this multicomponent photocatalyst promotes photocatalytic performance, making it a superior choice for wastewater treatment and other environmental applications. In essence, the amalgamation of CQDs and g-C₃N₄ presents a novel and efficient approach to address the challenges of wastewater treatment, emphasizing the importance of continued research in this domain ^[60].

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Tejaswi Tanaji Salunkhe

Thirumala Rao Gurugubelli

Bathula Babu

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Table of Contents

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1. Introduction

2. Wide-Bandgap Metal Oxide QD-g-C₃N₄ Nanocomposites

3. Bi-Based QD-g-C₃N₄ Nanocomposites

3. Other Metal Oxide QD-g-C₃N₄ Nanocomposites

References

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1. Introduction

2. Wide-Bandgap Metal Oxide QD-g-C3N4 Nanocomposites

3. Bi-Based QD-g-C3N4 Nanocomposites

3. Other Metal Oxide QD-g-C3N4 Nanocomposites

References

2. Wide-Bandgap Metal Oxide QD-g-C₃N₄ Nanocomposites

3. Bi-Based QD-g-C₃N₄ Nanocomposites

3. Other Metal Oxide QD-g-C₃N₄ Nanocomposites