English

• 1. Introduction

  As society evolves and technology advances, the substantial consumption of fossil fuels has necessitated the confrontation of increasingly severe environmental pollution and energy crises [1]. Against this background, the imperative to develop renewable, pollution-free, and sustainable energy has become more critical and simultaneously more challenging. Of these, solar energy has become a renewable energy source of great interest because of its inherent safety, environmental friendliness, economy, inexhaustibility, etc. Photocatalysis, a pivotal technology for converting solar energy into chemical energy, demonstrates immense potential and prospects for the future [2]. By harnessing sunlight, photocatalytic reactions can convert abundant natural resources into many new forms of energy, for example, CO₂ reduction [3], biomass pyrolysis [4], photocatalytic water splitting for H₂ evolution [5], organic pollutant degradation [6]. These applications not only meet the urgent needs of current green development, but also attract extensive attention from many researchers [7]. Since 1972, when Fujishima and Honda groundbreakingly reported basic research on water decomposition by TiO₂ under ultraviolet light, semiconductor photocatalysts have been established as fruitful candidates for solving these emerging problems [8]. However, photocatalysis still faces many challenges in practical applications, such as limited visible light absorption, weak photogenerated carrier mobility, fast recombination of photogenerated electron-hole pairs, poor catalyst stability and photo corrosion [9]. To overcome these shortcomings, researchers have focused on developing more efficient and stable photocatalysts. So far, various types of photocatalysts have been successfully developed, such as metal oxides (TiO₂, Fe₂O₃, and WO₃) [10], carbon-based nanomaterials (graphene, g-C₃N₄) [11,12], and bismuth-based oxides [13,14]. Among them [15], GQDs as a novel type of carbon material, exhibit a strong quantum confinement effect [16], edge effect [17], optoelectronic conversion capability [18], electron mobility [19], and superior characteristics of broad absorption and narrow emission [20], thereby demonstrating their potential application value in the field of photocatalysis [21]. However, despite the numerous advantages of GQDs, issues such as low photoconversion efficiency, propensity for aggregation when used in isolation, and rapid recombination of photogenerated electron-hole pairs still limit their practical application in photocatalysis.

  To alleviate these problems, researchers have actively explored various methods, such as doping with heteroatoms to endow GQDs with more particular optical properties [22,23], utilizing surface functionalization techniques to enhance their stability [24], and decorating/compositing them with other semiconductors [25] to obtain nanocomposites that can adjust the material properties. Specifically, the narrow bandgap and distinct electron transfer capabilities of GQDs can enhance the visible light absorption of other semiconductors, promoting efficient electron-hole separation at the semiconductor (SC)/GQDs heterojunction. Additionally, GQDs can form P-N semiconductor heterostructures with other semiconductors, and the P-N built-in electric field can further enhance carriers separation efficiency, making them form semiconductor heterostructures with other semiconductors, and the P-N built-in electric field can further enhance carriers separation efficiency, making them widely applicable in the field of photocatalysis [26]. Through continuous research and optimization, GQDs-based catalysts in the field of photocatalysis is expected to achieve breakthroughs, providing new solutions to address energy crises and environmental pollution issues (Fig. 1).

  Figure 1

  

  Figure 1.  Advantages of GQDs-based photocatalysts in photocatalytic applications.

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  Metal-organic frameworks (MOFs) represent a novel class of materials, constructed through the coordination bonds between inorganic metal ions or clusters and organic ligands. They are distinguished by their substantial specific surface area, ordered porous structure, and adjustable framework architecture. These characteristics, coupled with the excellent physicochemical properties derived from metal clusters, render MOFs as versatile semiconductor catalysts that are highly effective in photocatalysis [27,28]. Research has shown that integrating GQDs with MOFs can extend the absorption spectrum and significantly enhance photocatalytic performance. Wei et al. [29] had ingeniously integrated GQDs into pores on the surface of Zn-MOFs nanoparticles with a unique design that produced CH₄ and CO at a rate of 20.9 and 3.7 µmol h⁻¹ g⁻¹ respectively, which are over 18 times higher than that of the pure GQDs. This research is anticipated to offer new opportunities for the utilization of carbon dot materials and porous MOFs crystals in photocatalytic applications. In addition to MOFs as the composites of advanced materials with GQDs, traditional photocatalysts, such as CdS [30], TiO₂, and g-C₃N₄ [31,32], can also be combined with GQDs to form GQDs-based composites for applications in the field of photocatalysis. GQDs-based photocatalysts have the potential to mitigate the shortcomings of traditional materials, such as insufficient light-harvesting capability, rapid recombination of electron-hole pairs, low porosity, and poor stability. Furthermore, by designing their crystal structure and morphology through various strategies (e.g., elemental doping, defect engineering, heterojunction structures, and loading of co-catalysts), the intrinsic optical properties, specific surface area, and charge transfer efficiency of the material can be enhanced, thereby improving the photocatalytic performance [33,34]. Lei et al. [35] used the hydrothermal method to firmly decorate the surface of CdS nanoparticles with GQDs, forming a "dot-on-particle" hetero-biopolymer structure to synthesize CdS/GQDs nanohybrid materials. This heterojunction structure enables the nanohybrid to have strong light absorption at wavelengths beyond the band edge of CdS, while the GQDs act as electron acceptors, remarkably improving the photocatalytic H₂ evolution performance of CdS/GQDs nanomaterials. Afterwards, bismuth-based composite oxides (Bi₂WO₆, Bi₂MoO₆, BiOCl, BiFeO₃, Bi₁₆CrO₂₇, etc.) stand out among many attractive semiconductor candidates due to their proper bandgap and good photostability. Notably, Bismuth-based composite catalysts modified with GQDs can overcome the shortcomings of pure bismuth-based materials, such as weak light-capturing ability and low electron-hole separation efficiency, which lead to poor solar photocatalytic activity, through strategies such as morphology control [36], construction of heterostructures [37] and introduction of oxygen vacancies (V_o) [38,39]. For example, Liu et al. [40] prepared B-GQDs/Bi₂MoO₆ heterojunction photocatalyst by in-situ growth method, which enabled B-GQDs to be successfully deposited on the surface of BMO microspheres. At the same time, B-GQDs has excellent hydrophilicity due to its oxygen-containing functional groups (such as carboxyl group and hydroxyl group), and the construction of heterojunction promotes the interfacial charge transfer and the separation of photogenerated electron-hole pairs, thus enhancing the photocatalytic nitrogen fixation activity. In general, GQDs-based catalysts are considered to be a promising promoter for constructing composite materials, the findings of these studies can provide insights for the design and development of new efficient GQDs-based photocatalysts.

  This review comprehensively summarizes the synthesis methods of GQDs, discussing both the top-down and bottom-up strategies, providing typical examples for each method and firstly summarizing the advantages and disadvantages of each. Then, the application of GQDs-based composite photocatalysts in the field of photocatalysis in recent years (photocatalytic reduction of CO₂, degradation of organic pollutants and water splitting for H₂ evolution) is systematically presented and the corresponding reaction mechanism is thoroughly discussed. Meanwhile, the number of papers on the research progress of GQDs-based photocatalysts has also been investigated and presented in detail (Fig. S1 in Supporting information). Finally, the problems, challenges and possible solutions for the application of GQDs and GQDs-modified composites in photocatalysis are overviewed and analyzed for the first time. This review aims to develop highly active photocatalysts using the electron acceptor or mediator role of GQDs, and provides new insights for solving key problems in the field of photocatalysis.

  2. Synthesis of graphene quantum dots (GQDs)

  In the past few years, with the deepening of research on GQDs, both domestic and international researchers have explored various methods for preparing GQDs and discovered numerous carbon source materials. These carbon source materials, through different synthesis pathways, produce GQDs with significant differences in particle size, surface functional groups and other aspects, thus endowing them with unique physicochemical properties. The synthesis methods of GQDs are mainly classified into two categories: "Top-down" strategy and "Bottom-up" strategy [41]. The Top-down method involves cutting and exfoliating large carbon materials (such as graphite, graphene) through physical or chemical means to obtain GQDs, while the Bottom-up method involves self-assembly and polymerization of small molecule precursors under specific conditions to generate GQDs. The classification of GQDs synthetic methods and their respective advantages and disadvantages are summarized in Table S1 (Supporting information).

  2.1 Top-down strategy

  The top-down strategy for synthesizing GQDs commonly employs graphite, carbon nanotubes, graphene oxide, coal [42], and other carbon materials with large sp² carbon domains as precursors. These materials are processed through methods such as oxidative decomposition, shearing or exfoliation to yield GQDs. Researchers favor the top-down approach due to its abundance of raw materials, simplicity of operation and the potential for large-scale production of GQDs. Moreover, GQDs synthesized through this method often exhibit abundant oxygen-containing functional groups at their edges, which enhances their solubility, and facilitates subsequent functionalization and passivation. In this section, several typical top-down methods will be specifically introduced, including oxidative/reductive cutting, hydrothermal/solvothermal method and electrochemical erosion, which are listed in Table S2 (Supporting information) [43–61].

  2.1.1   Oxidative/reductive cutting

  Oxidative/reductive cutting is one of the most common approaches for synthesizing GQDs using bulk graphitized carbon-based materials, such as graphite [62], carbon black, carbon fibers, graphene, graphene oxide, coal or carbon nanotubes (CNTs). In this method, oxidizing agents (e.g., HNO₃, K₂S₂O₈ and H₂O₂) or reducing agents (e.g. hydrazine, alkylamines, ammonia and N,N-dimethylformamide (DMF)) serve as scissors to cleave the bulk carbon-based precursors into small-sized GQDs. Oxidation methods typically employ strong oxidants or acids to cut C—C bonds in large-sized carbon source materials, resulting in the formation of small-sized GQDs. Initially, Peng et al. [43] utilized acid oxidation and pyrolysis of aluminum pitch-based carbon fibers to mass-produce GQDs (Fig. 2a). Through this method, GQDs with varying luminescent colors can be obtained at different reaction temperatures, as the reaction temperature affects the absorption properties of the synthesized GQDs, with lower temperatures resulting in GQDs that absorb at longer wavelengths. As shown in Figs. 2b and c, GQDs prepared at 80 ℃, 100 ℃ and 120 ℃ emit blue light, green light and yellow light, respectively, with particle sizes ranging between 1 nm and 4 nm. It is worth noting that GQDs produced by strong acids can consist of numerous oxygen-containing groups (such as carbonyl, carboxyl, hydroxyl and epoxy groups) and surface defects formed during the oxidation reaction. In addition to controlling the luminescent color and particle sizes of GQDs by temperature, the quantum dot size was also can be tuned by modulating the quantity of oxidizing agent added into the preparation process. For example, Zhao and colleagues [44] obtained two types of GQDs with average diameters of 3.38 nm and 2.03 nm using graphene oxide (GO) as a raw material after treatment with different concentrations of nitric acid (Fig. 2d). Chemical oxidation has introduced many disordered structures in graphene oxide, but the successful preparation of GQDs has been demonstrated by XPS characterization (Figs. 2e and f). Simultaneously, it reveals the possible mechanism for the formation of GQDs, which involves the initial oxidation of GO into O-GO using HNO₃, leading to the generation of a multitude of oxygen-containing functional groups and the introduction of nitrogen atoms. Subsequently, as some of the oxygen-containing groups are partially removed, the structure of O-GO is decomposed. Under sustained conditions of high temperature and pressure, a large number of nanosheets (R-O-GO) are formed. The collected R-O-GO is further oxidized by concentrated HNO₃ to yield GQDs. In summary, it can be understood that by altering the reaction temperature or the amount of oxidizing agent added, one can regulate the size, degree of oxidation, and functional groups of GQDs, thereby modulating their fluorescent properties or altering their hydrophilicity and hydrophobicity. These properties make GQDs suitable for applications such as cell imaging and tissue labeling. Although the method is cost-effective and suitable for mass production, its prolonged preparation time and the use of hazardous, polluting reagents are significant drawbacks. In addition, the subsequent treatment and discharge of a large amount of unspent acids also need to be addressed further.

  Figure 2

  

  Figure 2.  (a) Representation scheme of oxidation cutting of CF into GQDs. (b) UV–vis spectra of A, B, and C correspond to the synthesis reaction temperatures of GQDs at 120, 100, and 80 ℃, respectively. The illustration of (b) shows the corresponding GQDs with a 365 nm excited light gap under UV light. (c) XPS survey spectrum of CF and GQDs. Reprinted with permission [43]. Copyright 2012, American Chemical Society. (d) High resolution transmission electron microscopy (HR-TEM) images of GQDs and the right column shows the diameter distribution of GQDs. (e) High resolution XPS spectra of C 1s for GQDs. (f) High resolution XPS spectra of N 1s for GQDs. Reprinted with permission [44]. Copyright 2018, Multidisciplinary Digital Publishing Institute. (g) Representation of GQDs containing a surface passivator with oligo-PEG diamino. (h) A schematic illustration of various typical electronic transitions processes of GQDs. Normal PL mechanisms in GQDs for small size (A) and large size (B). Upconverted PL mechanisms in GQDs for large size (C) and small size (D). Reprinted with permission [45]. Copyright 2011, Royal Society of Chemistry. (i) TEM images of the GQDs with varied sizes of 2 nm. The text insets represent the concentration of OAm. Reprinted with permission [46]. Copyright 2014, American Chemical Society.

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  Besides oxidative cutting, current research has also focused on the reductive cutting techniques for graphitized carbon-based materials. Shen and coworkers [45] successfully synthesized surface-passivated GQDs from GO nanoparticles by employing hydrazine hydrate as a reducing agent combined with polyethylene glycol (PEG) diamine as a passivating agent (Fig. 2g). To elucidate the effects of PEG surface passivation on the formation, microstructure, and optical properties of GQDs, an energy level structure model of GQDs is established. Fig. 2h shows a schematic of various typical electron transition processes for GQDs. When a bunch of low-energy photons excite electrons in a π orbit, the π electrons transition to a higher energy state, such as LUMO, and then the electrons transition back to a lower energy state. Thus, when the electron transitions back to the σ orbit, it emits the up-converted photoluminescence (PL). It is shown that surface passivation can produce GQDs with higher fluorescence performance and up-conversion performance. Furthermore, research teams have utilized a mixture of oleyl amine, 1-octadecene, and hydrazine hydrate to perform amination cutting on fragmented graphite, leading to the synthesis of GQDs. In another remarkable work, researchers have successfully synthesized amino-functionalized GQDs by adopting a gentle amino-hydrothermal treatment of graphene oxide using an ammonia solution (Fig. 2i) [46]. While these GQDs contain a small amount of carboxyl and epoxy groups, these functional groups play a unique role in regulating electron-hole recombination processes, and they also exhibit considerable photoluminescence quantum yields (PLQY) ranging from 19% to 29%. These studies have enriched the preparation methods of GQDs and have also opened up more possibilities for the application of GQDs (biological imaging, biosensing, drug delivery, photothermal therapy, antibacterial agents, and catalysts, etc.) in various fields [47].

  The oxidative/reduction cutting pathway facilitates the acquisition of bulk precursors and is amenable to large-scale production. However, because of the cleavage of the bulk material occurring randomly at poorly intercalated sites dispersed throughout the carbon-based precursor structure, the oxidative/reduction cutting strategy cannot precisely control the edge and size states of the synthesized GQDs, thereby hindering the doping of heteroatoms. Despite the evident shortcomings of the oxidation/reduction cutting method, its potential for future applications cannot be overlooked. If future researches successfully develop a simple and efficient purification process that mitigates the environmental issues associated with this method, the oxidation/reduction cutting route may become a viable pathway for the large-scale production of GQDs.

  2.1.2   Hydrothermal/solvothermal method

  The hydrothermal/solvothermal method usually uses water as the solvent, and the precursors to be reacted are placed in a sealed high-temperature vessel (autoclave) to finally synthesize the reaction products. Firstly, carbon precursors undergo a meticulous oxidative pretreatment with oxidants like HNO₃ and ozone. This process introduces diverse oxygen-containing functional groups that enhance the materials' hydrophilicity and establish key reactive sites for further dissociation. Subsequently, the hydrothermal/solvothermal conditions are precisely modulated to facilitate reactions with NaOH and ammonia. These reactions selectively cleave the carbon precursors at the implanted sites, yielding finely-sized GQDs. Pan and coworkers [48] oxidized graphene obtained by cutting graphite with concentrated sulfuric acid having carboxyl groups at its edges and defect sites, while the basal positions contain both carboxyl and epoxy groups. These oxygen-containing functional groups are arranged approximately in a linear fashion along the lateral dimension of the carbon lattice and encase the surrounding sp² clusters, which directs the cleavage of graphene along straight lines during the hydrothermal reaction, resulting in the formation of GQDs with diameters of about 10 nm (Figs. S2a and b in Supporting information). These GQDs not only retain stable carboxyl groups, imparting them with water solubility, but also exhibit unique physical and chemical properties. Based on the reversible protonation mechanism of carbene-like zigzag sites and oxygen-containing functional groups in acid-based media (Fig. S2c in Supporting information), a structural model of GQDs in these environments is proposed. Notably, they exclude the possibility of protonation of electron-donor/acceptor complexes formed between delocalized π-electron systems and H, as the observed PL phenomenon is not directly related to the delocalized π-electron system. And the GQDs prepared by this method may extend the application of graphene-based materials to other fields such as optoelectronics and biomarkers. Whereafter, as shown in Fig. S2d (Supporting information), Wu and colleagues [49] synthesized nano-scale GO [63] based on the modified Hummers method and regulated the fluorescent properties and hydrophilicity/hydrophobicity of GQDs (Fig. S2e in Supporting information) through hydrothermal treatment using ammonia water of different concentrations at various temperatures. This innovation has achieved high yield and high photoluminescence quantum yield (PLQY), and also uncovers the immense potential and boundless possibilities of GQDs in cutting-edge fields such as luminescent materials and bioimaging.

  Beyond the traditional hydrothermal method, the ultrasonic-assisted hydrothermal method provides a more efficient and precise technique for the preparation of small-sized GQDs. This method ingeniously utilizes the alternating low-pressure and high-pressure waves generated by ultrasonic waves within the liquid, which induce the rapid formation, growth, contraction and eventual rupture of microbubble nuclei. During the instantaneous rupture of these microbubbles, intense collisions occur between surrounding liquid particles, accompanied by strong shock waves and liquid shear forces. These forces work together to achieve precise "top-down" cutting of graphene, effectively breaking C—C bonds and thus forming small-sized GQDs [64]. In scientific research, Xie and peers [50] successfully introduced abundant oxygen-containing functional groups onto carbon fibers by treating them with concentrated sulfuric acid and concentrated nitric acid under the assistance of ultrasound. With the aid of ultrasound, they further utilized sodium carbonate solution to adjust the pH value of the system to 8, ultimately preparing GQDs with a thickness of 1–2 layers of graphene (Figs. S2f and g in Supporting information). As shown in Fig. S2h, the PL emission spectrum of the solution can be further separated into blue and green PL GQDs using different dialysis bags. This method simplifies the preparation process and significantly improves the yield and quality of GQDs. Furthermore, the highly crystalline and ultrasmall GQDs would provide a novel nanoplatform for manipulating graphene-based electronics in a solution state. Meanwhile, Zhuo et al. [51] also employed an ultrasonic method to prepare GQDs under acidic conditions. By precisely controlling the ultrasonic parameters (wave velocity, power, and wavelength) and reaction conditions (temperature, time, and solvent type), they successfully prepared GQDs with a particle size distribution of 3–5 nm. These GQDs possess excellent physical and chemical properties, providing an important material basis for subsequent scientific research and practical applications.

  Hydrothermal/solvothermal methods have streamlined the synthesis process of GQDs and rendered their chemical passivation viable. With these technologies, GQDs with high PLQY can be prepared, laying the groundwork for their applications in fluorescence-based sensing and imaging technologies. However, a significant drawback of the hydrothermal method is the difficulty in separating the product GQDs generated in the process from the unreacted waste acid as well as the complexity of the purification process, which limits its application in large-scale production. Nonetheless, GQDs produced using this technique exhibit small particle sizes and low aggregation, which are crucial for applications requiring high dispersion and stability. Consequently, this technique is particularly suitable for biomedical field or other high-end applications that require small amounts of GQDs with high PLQY.

  2.1.3   Electrochemical erosion/exfoliation method

  Electrochemical erosion/exfoliation method is the destruction of a material surface by an electrochemical reaction with an ion-conducting medium. The advantages of this method for the preparation of GQDs lie in its controllability and high efficiency, which enables the generation of GQDs in high yields in a relatively short period of time. The method synthesizes nanoscale GQDs by applying an electrical potential to the precursor, allowing charged ions to enter the graphite layer of the bulk carbon material, and then these guest ions act like electrochemical scissors to sever carbon-carbon bonds [65,66]. For instance, Ananthanarayanan et al. [52] utilized an acetonitrile solution of 1‑butyl‑3-methylimidazolium hexafluorophosphate (PMIMPF₆) as the electrolyte and successfully prepared GQDs with fine sizes and moderate thicknesses from self-supported foam-like 3D graphene using electrolytic chemical vapor deposition (Fig. 3a). The obtained GQDs were used for the detection of iron ions (Fe³⁺). These GQDs have an average size of approximately 3 nm and a thickness of only 1.25 nm. Remarkably, as referenced to quinine sulfate, these GQDs emit blue fluorescence under 365 nm UV light, with a quantum yield of approximately 10% (Fig. 3b). The intrinsic mechanism of electrochemical cutting has been thoroughly dissected, highlighting that it is primarily attributed to the high electrical stress applied, the outstanding intercalation capability of PF₆ anions, and the interactions between the BMIM group and GQDs (including π-π electron cloud or cation-π interactions). It is also predicted that these 3D graphene frameworks, which have been electrochemically etched, could potentially become excellent electrochemical electrode materials due to their structural features (abundant defects, large surface area, and a continuous multi-path conduction network), thus the numerous defects and edges created by the exfoliation are able to facilitate electron transfer.

  Figure 3

  

  Figure 3.  (a) Schematic of the detection mechanism of GQDs and Fe³⁺ synthesized from 3D graphene. (b) UV–vis absorption (blue) and excitation (red) spectra of GQDs. The inset shows the photographs of aqueous dispersions of GQDs under day light (left) and UV (365 nm) illumination (right). Reprinted with permission [52]. Copyright 2014, Wiley-VCH. The inset of (c) is the photograph of the GQDs solution under day light and 365 nm illumination. Reprinted with permission [53]. Copyright 2015, Royal Society of Chemistry. (d) Schematic illustration of electrochemical exfoliation of defect-induced graphite rod. (e) Images of GQD1-GQD4 before (A-D) and after (E-H) electrochemical exfoliation and daylight/under 365 nm UV irradiation images for GQD1-GQD4 (I-L). Reprinted with permission [54]. Copyright 2017, American Chemical Society. (f) Schematic diagram of the preparation of GQDs in strong and weak electrolyte solutions. (g) TEM image. (h) Corresponding size distribution histogram with the orange Gaussian fitting curve based on the GQD sample (173 dots). Reprinted with permission [56]. Copyright 2018, American Chemical Society.

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  Because of the high cost of carbon materials such as graphene and multi-walled carbon nanotubes, recent research on the electrochemical synthesis of GQDs has gradually focused on exploring more economical and promising materials, including graphite, coke and carbon fibers [67]. Tan and colleagues [53] successfully prepared uniformly sized GQDs with red light emission properties by electrolyzing graphite rods in K₂S₂O₈ aqueous solution (Fig. 3c). The key to this synthesis lies in the strong oxidizing SO₄˙⁻ radicals generated under anodic potential, which effectively cleave the C—C bonds in graphite layers to achieve precise preparation of GQDs. Ahirwar and coworkers [54] employed a different strategy, utilizing graphite rods as raw materials and water solution of citric acid and alkali metal hydroxides as electrolytes for GQDs synthesis (Fig. 3d). Notably, pre-heating the graphite rods at high temperatures increased the defects in the graphite layers, thereby promoting the exfoliation process. Furthermore, it was found that fine tuning of the oxygen functional groups and optical properties of GQDs could be achieved by adjusting the concentration of the electrolyte, as shown in Fig. 3e. Reports also have shown that the electrolysis of graphite rods in various electrolytes such as NaOH aqueous solution, NaOH/ethanol solution and sodium methoxide aqueous solution [68] can also yield high-quality GQDs. These studies enrich the preparation methods for GQDs and provide more possibilities for optimizing their properties. While exploring more efficient and economical preparation methods, He et al. [55] successfully prepared tunable GQDs (green-GQDs (G-GQDs) of 3.02 nm, yellow-GQDs (Y-GQDs) of 4.15 nm, and orange-GQDs (O-GQDs) of 4.61 nm) from coke using a one-step electrochemical exfoliation method. And by carefully regulating the water content and current density in the electrolyte, precise control of the size and luminescence color of the quantum dots has been achieved. Most recently, Huang and colleagues [56] have developed an electrochemical method based on weak electrolytes (such as ammonia solutions), significantly improving the yield of GQDs (Fig. 3f). Efficient and controlled conversion from graphene layers to GQDs (range from 3 to 8 nm) of 1–4 layers (Fig. 3g) with an average diameter of 4.7 nm for these dots in ammonia solution with high yield (28%) (Fig. 3h). Their research reveals that electrolysis in weak electrolytes leads to an increased yield of GQDs compared to strong electrolytes on account of the prolonged oxidation reaction and inhibition of intercalation effects. This breakthrough has the potential to reduce the production cost of GQDs and lays a solid foundation for their large-scale applications.

  The application of electrochemical erosion method effectively alleviates the problem of poor crystallization of carbon nanostructures during the synthesis of GQDs. This method is straightforward, eliminating the need for extreme reaction conditions, making it highly efficient and feasible compared to other traditional approaches [56]. Additionally, electrochemical methods excel in scalability, economic viability, and operational simplicity [69]. However, achieving high-quality GQDs with uniform size and layer distribution using electrochemical exfoliation remains a challenge. The insertion of ions (OH^- from water electrolysis and depending on the specific composition of the electrolyte used) under anodic potentials poses a risk of graphite oxidation, potentially damaging the sp² hybridized carbon network. Another challenge associated with electrochemical methods is the enhancement of GQDs yield. During anodic exfoliation, two processes occur concurrently: intercalation-exfoliation and oxidative cutting effects. The intercalation-exfoliation process rapidly separates graphene flakes from the anode, but these exfoliated portions are not prone to oxidation or further cutting, resulting in a very low yield (< 1%) for GQDs synthesis [70]. Therefore, there is an imperative demand to explore new methods for high-yield synthesis of GQDs that can address these challenges.

  2.1.4   Other top-down methods

  Apart from the aforementioned oxidation/reduction cutting, hydrothermal/solvothermal, and electrochemical corrosion methods for synthesizing GQDs, several other top-down methods (electron beam lithography, liquid-phase exfoliation, potassium intercalation, and ruthenium catalyzed C60 cage opening) have also been developed for synthesizing nanoscale GQDs. Electron beam lithography is a technique that utilizes high-energy electron beams for fine processing on material surfaces. In this method, electron beams are used to etch specific patterns or shapes onto graphene sheets, thereby forming quantum dots. These quantum dots exhibit quantum confinement effects and unique optoelectronic properties. In 2008, Andre Geim et al. prepared GQDs from graphene using electron beam etching technology [57]. This method can etch graphene crystals into GQDs of any geometric shape or size on silicon chips, thereby obtaining quantum dots with specific sizes and shapes. The advantage of this method is that it can accurately control the size and shape of quantum dots, but it may require special equipment, complex steps, and low yields, so it is less commonly used. In addition, researchers have used high-resolution electron beam lithography technology to etch graphene crystals into arbitrary geometric shapes or sizes of GQDs on silicon chips. This method can prepare quantum dots with specific sizes and shapes.

  The liquid-phase exfoliation method involves the exfoliation of larger graphene structures into smaller quantum dots. Firstly, disperse graphene oxide (GO) or reduced graphene oxide (rGO) in a solvent. Secondly, GO or rGO is reduced by chemical reducing agents (such as hydroiodic acid) and stripped to form smaller graphene quantum dots. Finally, by adjusting the reaction conditions (such as time, temperature, and solvent type), the size and properties of GQDs can be controlled. Shi and his team [58] successfully prepared GQDs with good crystallinity and a diameter of 10–20 nm (Figs. S3a and b in Supporting information) using PGNFs as raw materials and DMSO solvent as stripping and dispersing agents. Afterwards, Lu and his team [59] improved this strategy by using ultrasound assisted liquid-phase graphite carbon raw materials for large-scale synthesis of GQDs with different sizes (2–6 nm), structures, and defect contents through exfoliation (Figs. S3c–e in Supporting information). The prepared GQD exhibits blue photoluminescence (Fig. S3f in Supporting information). This method has fast processing speed, environmental friendliness, and low cost, and can be used for large-scale production of industrial GQDs. Park et al. [60] synthesized high-quality GQDs with acceptable yield and relatively low oxidation degree using potassium intercalation method (Figs. S3g and h in Supporting information). In this method, potassium atoms are inserted into the graphite layer of MWCNTs and graphite, and then the bulk material is decomposed into GQDs under the energy generated by the violent reaction between potassium atoms and water. Ruthenium catalyst can also be used to promote the opening of fullerene C60 molecules, and then high-temperature treatment can be used to break the C60 molecules into carbon clusters, ultimately forming GQDs [61]. This method is more complex and yields lower, but can obtain GQDs with specific structures.

  The top-down strategy possesses significant advantages in the preparation of GQDs, mainly reflected in the easy availability of carbon source materials and the potential for large-scale synthesis of this method. However, this strategy also owns some limitations: (1) It requires specialized and expensive synthetic equipment, as well as strict operational procedures, thus resulting in high costs. (2) The preparation process is time-consuming and has a low yield, with only a small fraction of the carbon source material being converted into GQDs, which also tend to have a high number of defects and generally exhibit low fluorescence quantum yields. (3) There are some impurities in making water-soluble GQDs which are not easy to purify. (4) Improper use of strong acids, bases and organic solvents in the synthesis process can pollute the environment as well as endanger the health of organisms. (5) The "cut" sites of the carbon source material are not effortless to find, so it is sticky to control the particle size and morphology of GQDs and achieve accurate regulation of optical properties.

  2.2 Bottom-up strategy

  In recent years, the bottom-up strategy has emerged as a promising alternative to the traditional top-down strategy in the field of GQDs preparation. This strategy uses small molecules as the carbon source material and constructs GQDs through a series of intricate chemical reactions, providing significant controllability. The GQDs prepared by this approach exhibit superior characteristics in terms of size, morphology and properties. Consequently, the bottom-up approach has rapidly gained widespread attention and research. This strategy primarily includes carbonization method, hydrothermal method, microwave assisted method, soft-template method and stepwise organic synthesis [71], which will be introduced individually in this section. Their specific classifications and the corresponding typical samples are summarized in Table S3 (Supporting information) [72–84].

  2.2.1   Carbonization method

  Carbon nanomaterials are prepared by high-temperature pyrolysis of organic small molecules using carbonization method. This method is simple, efficient and has a broad selection of precursors. By heating small-molecule carbon source materials above their melting points, these molecules undergo condensation and nucleation processes through carbonization, ultimately leading to the formation of GQDs. Commonly used raw materials for GQDs synthesis through this method include citric acid, L-glutamic acid and various carbohydrates. Back in 2012, Dong et al. [72] pioneered the use of citric acid (CA) as the sole carbon source material to successfully synthesize GQDs with a particle size of approximately 15 nm and a thickness ranging from 0.5 nm to 2.0 nm. These GQDs exhibited a high quantum yield of 9.0% and displayed blue fluorescence properties. This method generates different products at varying stages of carbonization, with GQDs being the primary product at a low carbonization stage and luminescent graphene with lower quantum yield being the tendency at a higher carbonization level. As depicted in Fig. S4a (Supporting information), upon undergoing moderate pyrolysis, CA partially carbonizes and forms nanosheets enriched with sp² clusters, which are commonly referred to as GQDs. These sp² clusters are segregated within the incompletely carbonized CA, exhibiting uniform sizes and being well-passivated by the uncarbonized CA, thus resulting in the obtained GQDs displaying intense excitation-independent PL activity (Fig. S4b in Supporting information). However, as the pyrolysis process progresses further and CA becomes nearly fully carbonized, larger nanosheets, commonly known as graphene oxide (GO), are formed. This GO contains a significant number of small sp² clusters, but these clusters are isolated within a sp³ C—O matrix with uneven sizes and poor passivation, leading to their weaker and excitation-dependent PL activity (Fig. S4c in Supporting information). By adjusting the degree of carbonization, the ratio of GQDs and GO can be further modulated, based on their distinct photoluminescent characteristics (different bandgaps and emission wavelengths). This makes them suitable for a variety of optoelectronic devices, including light-emitting diodes, solar cells, and applications in bioimaging, photocatalysis, among others.

  In 2022, Rocha and colleagues [73] achieved a breakthrough in this research by carbonizing sucrose in a sulfuric acid environment, followed by treatment of the resulting carbonized material in dimethyl sulfoxide (DMSO), successfully preparing GQDs and N-GQDs. The nanoparticle sizes of the GQDs obtained by AFM/TEM ranged from 1 nm to 20 nm, and the nanoparticles of N-GQDs were in the range of 1–6 nm (Figs. S4d–g in Supporting information). Meanwhile, it is revealed that the photoluminescence of undoped GQDs primarily occurs through interbond transitions, initiated by photon excitation in the ultraviolet region, followed by non-radiative relaxation of energy states related to oxygen functional groups (Fig. S4h in Supporting information). Doping GQDs with nitrogen (nitrogen doping increases the density of states between the valence and conduction bands, thus decreasing the bandgap of the system) extends the luminescence range of GQDs and increases the PL intensity, mainly in the green to red region (Fig. S4i in Supporting information). This work demonstrates the feasibility of synthesizing GQDs using sucrose and also encourages the use of more low-toxicity and environmentally friendly precursor materials for the preparation of GQDs, which opens up new directions for future research.

  As an effective method for the preparation of GQDs, small-molecule carbonization greatly simplifies the synthesis and doping process. However, during large-scale applications, this method has encountered challenges in precisely controlling the degree of carbonization, potentially leading to the presence of a mixture of GO and GQDs in the product, which complicates and makes the separation process difficult. Hence, to enhance the productive efficiency of GQDs, it is imperative for researchers to engage in the innovation of more efficacious and advanced synthetic methodologies, thereby facilitating their integration into the realm of industrial manufacturing.

  2.2.2   Hydrothermal method

  In the hydrothermal reaction environment, under high-temperature and high-pressure conditions, small-molecule precursors undergo a crystallization process in an aqueous solution, ultimately leading to the formation of GQDs. This method is widely used because it is simple to operate and easy to control the reaction conditions. Qu and coworkers [74] utilized the straightforward hydrothermal synthesis strategy to fabricate N-GQDs and S, N-GQDs, using CA as the carbon source, and urea and thiourea serving as the nitrogen and sulfur sources, respectively (Fig. 4a). The resulting GQDs exhibited superior luminescent properties, with quantum yields of 78% and 71%, respectively. The growth mechanism of N-GQDs/S, N-GQDs is the self-assembly of CA into nanosheet structures due to intermolecular hydrogen bonding, followed by dehydrogenation reaction under hydrothermal conditions to form graphene nanoparticles with multiple carboxyl and hydroxyl groups. As the urea or thiourea existed in the reaction system, the -NH₂ and S groups reacted with the carboxyl or hydroxyl groups to form N-GQDs or S, N-GQDs with the extending reaction time. Further employing a simple impregnation, the N-GQDs/TiO₂ and S, N-GQDs/TiO₂ composite materials were prepared, exhibiting significantly enhanced photocatalytic activity for the degradation of Rhodamine B compared to pure TiO₂, it also demonstrated substantial potential for applications in environmental protection and energy conversion. Furthermore, other researchers have also adopted this strategy, using similar precursor materials, and explored the possibility of substituting nitrogen sources with different materials, such as triethanolamine, ethylenediamine, melamine, and hexamethylenetetramine [85,86]. These materials provide more options for the preparation of N-GQDs with specific properties. Moreover, thiophene and citric acid, as an ideal combination of raw materials, have been used to prepare S, N-GQDs [87]. This dual-doping structure endows GQDs with unique optical properties (enhancement or redshift of fluorescence emission characteristics).

  Figure 4

  

  Figure 4.  (a) The growth mechanism of N-GQDs and S,N-GQDs. Reprinted with permission [74]. Copyright 2013, Nanoscale. (b) TEM image of GQDs. (c) DLS diagram of GQDs. Reprinted with permission [75]. Copyright 2022, Elsevier. (d) The production process of GQDs. (e) Survey XPS spectrum. Reprinted with permission [76]. Copyright 2014, Springer Nature. (f) Schematic diagram of the formation of GQDs through the pyrolysis of L-glutamic acid. (g) The HRTEM image of GQDs with a scale bar of 5 nm. Inset: A typical single GQD with a lattice parameter of 0.246 nm. The scale bar is 1 nm. Reprinted with permission [77]. Copyright 2013, Royal Society of Chemistry.

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  Recently, Nxele et al. [75] obtained GQDs hydrolyzing citric acid and NaOH followed by autoclave heating. The GQDs exhibited excellent dispersibility, and the average size of the GQDs synthesized after 8 h was approximately 4.9 nm (Fig. 4b). To determine the hydrodynamic size of the synthesized quantum dots, researchers employed dynamic light scattering (DLS) technology. Initially, the quantum dot samples were dissolved in water and subjected to ultrasonic treatment to ensure the homogeneity of the solution. The DLS results indicate that the particle size gradually decreases with the extension of the reaction time. Furthermore, except for the GQDs synthesized for 8 h, the polydispersity index is relatively high (above 0.5), which may be related to some variables in the synthesis process (Fig. 4c). This work provides an effective method for synthesizing quantum dots with specific properties and discusses the effect of optimal synthesis time on the performance of GQDs. Apart from CA, other precursors such as polycyclic aromatic hydrocarbons and amino acids have also been utilized for the synthesis of GQDs through hydrothermal routes. In another study, researchers employed pyrene as a raw material to synthesize GQDs under mild and green hydrothermal conditions. This synthesis process involved the nitration of pyrene, followed by hydrothermal treatment of the resulting 1,3,6-trinitropyrene in an alkaline aqueous solution (Fig. 4d). The produced GQDs exhibited excellent optical properties, relatively high yield and an acceptable PLQY of 23% [76]. The XPS analysis demonstrates that 1,3,6-trinitropyrene is fused into OH-GQDs by total removal of the NO₂⁺ group under the strongly alkaline hydrothermal conditions. The hydroxyl group is bonded with the single-crystalline GQD lattice most likely at edge sites rather than at basal plane sites (Fig. 4e). Moreover, Wu and colleagues [77] reported a one-step pyrolysis method utilizing L-glutamic acid for the production of highly fluorescent GQDs (Fig. 4f). The GQDs (4.66 nm) emitted near-infrared (NIR) fluorescence in the range of 800–850 nm (Fig. 4g). This unique NIR behavior offers significant advantages for in vitro/in vivo imaging and sensing. These studies showcase the tremendous potential of hydrothermal methods in synthesizing high-quality GQDs and pave new ways for the application of GQDs in biomedical and other fields.

  While the chemical process of introducing heteroatom doping and passivation through hydrothermal methods can effectively enhance the PLQY of GQDs to exceed 25%, thus meeting the requirements of most imaging, photocatalysis and fluorescence-based sensing applications. The limitation of this method lies in its extended reaction duration and considerable energy expenditure. Additionally, the GQDs produced through this synthesis strategy sometimes exhibit small particle sizes and slight aggregation, potentially impacting their efficacy in optoelectronic, catalytic, and biosensing applications. Therefore, this synthesis strategy is primarily recommended for biomedical applications or applications that require only a small amount of GQDs with relatively high PLQY.

  2.2.3   Microwave assisted method

  Microwave assisted method provides a unique acceleration method for chemical reactions by generating a stable high-intensity energy flow. Compared to hydrothermal method, microwave assisted method significantly shortens the time required for precursor fusion and allows for simultaneous and uniform heating of reactants, ensuring uniform particle size distribution of GQDs. In 2012, Tang and colleagues [78] first reported the preparation of glucose-derived GQDs exhibiting purple fluorescence with a size of 3.5 nm using this method. The uniform and rapid energy supply provided by microwave heating enables glucose molecules to undergo dehydration condensation, forming the core of GQDs. Subsequently, the boundaries gradually grow, resulting in the formation of GQDs with controllable sizes (Figs. 5a and b). Studies have found that the properties of the prepared GQDs are influenced by various factors, including microwave oven power, heating time, glucose concentration, and solution volume (Fig. 5c). By adjusting these parameters, precise control over the size and properties of GQDs can be achieved. In fact, the optical properties of GQDs are attributed to the passivation layer, which leads to the formation of surface states between the π* band and π energy level. By coating GQDs on blue light-emitting diodes, it has been demonstrated that GQDs serves as an efficient light converter for converting blue light to white light and highlights its significant potential applications in the field of photonic devices. Additionally, similar results can be obtained when using sucrose and fructose as raw materials, further demonstrating the broad applicability and flexibility of microwave-assisted synthesis. This discovery provides new possibilities for the application of GQDs in biomedical, photoelectric materials and other fields.

  Figure 5

  

  Figure 5.  (a) GQDs are prepared by MAH method. (b) TEM images of the GQDs. (c) Diameter distribution of GQDs, red line is Gaussian fitting curve. Reprinted with permission [78]. Copyright 2012, American Chemical Society. (d) Schematic diagram of GQDs, N-GQDs and S-GQDs prepared by microwave assisted method. (e) Raman spectra of GQDs and N-GQDs samples at different temperatures. (f) Proposed energy band diagram and electron transitions of S-GQDs and the possible emission mechanism. Reprinted with permission [79]. Copyright 2022, Springer.

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  The doping of atoms such as N and S into GQDs is an effective method to modulate their electronic properties, surface and local chemical characteristics. This doping not only provides more active sites but also helps enhance the PLQY. Hung et al. [79] utilized microwave assisted method to prepare N-doped and S-doped GQDs from CA (Fig. 5d), providing a new approach for the modification of GQDs. This involves the thermal decomposition and carbonization of precursors containing nitrogen and sulfur doping elements using microwave energy, followed by hydrothermal treatment in NaOH to produce the doped GQDs. The Raman spectra of GQDs and N-GQDs indicate that the D and G bands commonly observed in graphene-based materials are present in the spectra (Fig. 5e). The samples prepared by the method produce N-GQDs with C–N bonds at the edges. In addition, the doping of S introduces additional energy levels as S-related defects between the π* and π states of carbon. Through intra-band and inter-band relaxations, excited electrons are able to emit photons at longer wavelengths. The absorption of photons with wavelengths of 290, 330, 360, 370, 380, and 420 nm (labeled as transitions 1–5 in Fig. 5f) results in the emergence of multiple electron transition pathways (labeled as transitions 6–12) in S-GQDs. Following inter-band crossing and vibrational relaxation, radiative recombination occurs, generating broadband photon emission at 457 nm and beyond. Consequently, the emitted photons possess less energy than the absorbed photons. These observations indicate that the introduction of sulfur provides numerous additional electron transition pathways in both absorption and PL emission. Therefore, the existence of sulfur-related energy levels is crucial for tuning and modifying the electronic structure and optical properties of S-GQDs. Subsequently, Hasan and his team utilized a simple one-step microwave-assisted hydrothermal method to synthesize N-GQDs and S, N-GQDs from the inexpensive glucose-based precursor material [80]. These quantum dots form a stable aqueous suspension and exhibit bright and stable fluorescence in the visible and near-infrared regions (with a quantum yield as high as 60%). This also demonstrates that these highly luminescent N-GQDs and S, N-GQDs are promising materials for new organic optoelectronics [81].

  Microwave assisted method serves as an efficient method for rapid heating and achieving uniform large-scale production of GQDs [82]. This technique grants researchers the ability to control reaction parameters, including temperature and time, for the swift and reproducible synthesis of quantum dots. Moreover, doping and functionalization of GQDs can be realized through microwave assisted synthesis [88]. Despite these considerations, the GQDs produced by this method may have a relatively small particle size, which can lead to aggregation, and further influence its crystallinity, consequently affecting their optical properties (PLQY, PL, and photostability). Moreover, microwave radiation often lacks deep penetration in most substrates, and it can restrict the size of the precursors. Furthermore, microwave heating can rapidly lead to energy concentration, and improper handling may result in explosions. To ensure the safe synthesis of GQDs with exceptional crystallinity, precise adjustment of microwave heating power and duration is essential. By optimizing these parameters, GQDs with higher crystallinity and more uniform sizes can be attained safely, thereby enhancing their suitability for applications across various fields.

  2.2.4   Soft-template method

  The soft-template method refers to the utilization of soft matter (such as polymers, biomacromolecules, and surfactants) as a template to form nanoscale reaction chambers, within which a limited number of precursor molecules undergo uniform integration to form the nanomaterial [89]. This method yields monodisperse GQDs without the need for complex separation and purification processes. Therefore, the soft-template method is ideal for low-cost industrial-scale synthesis of GQDs. Compared to hydrothermal and microwave assisted methods, the soft-template method exhibits significant control over the nucleation and growth of GQDs material, thereby achieving precise design of the material properties. Moreover, the method is carried out under milder conditions, which helps to reduce energy consumption and equipment requirements.

  Li and coworkers [83] synthesized N-GQDs through the soft-template method using 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) as the template and the sole precursor (Fig. S5a in Supporting information). To elucidate the main reaction pathways of TATB, molecular dynamics (MD) simulations (CASTEP code in Materials Studio) were performed to study the thermal decomposition of TATB. The simulation shows that the main gaseous products of TATB decomposition are H₂O, NO and NO₂, which is consistent with the experimental observation (Fig. S5b in Supporting information). First, TATB undergoes a thermal decomposition process, leading to the breakage of various chemical bonds and the generation of expansive gases. These gases then promote the rupture of the graphite-like TATB multilayer structure, resulting in the formation of single-layer graphene sheets. Finally, through an oxidative exfoliation process, N-GQDs are successfully prepared (Fig. S5c in Supporting information). In another groundbreaking study, Dong and his team [72] combined microwave assisted with soft-template method to develop a novel GQDs synthesis strategy. Glucose molecules were selected as the sole carbon source, while polyethylene glycol (PEG) with a molar mass of 20,000 g/mol was utilized as the soft template for GQD synthesis. Under microwave irradiation, a limited number of glucose molecules were precisely encapsulated in each template and underwent uniform carbonization, nucleation, crystallization and growth processes. Ultimately, by simply adjusting the heating duration, the diameter of the produced GQDs could be easily controlled, eliminating the need for any complex purification and separation procedures, thus offering a new perspective and methodology for subsequent GQD synthesis.

  2.2.5   Stepwise organic synthesis

  The stepwise organic synthesis utilizes organic molecules or polymers as precursors to construct the structure of carbon nanomaterials through a precisely controlled chemical process. With this method, the size, morphology and chemical composition of GQDs can be accurately adjusted, thus promoting the synthesis of GQDs with customized properties. Yan and colleagues [90] have employed this method where the oxidative coupling of aryl groups from polyphenylene dendritic precursors leads to the partial melting of graphene segments, culminating in the formation of GQDs containing 168, 132, and 170 conjugated carbon atoms, respectively. Specifically refer to start from small-molecule precursors, key intermediates are synthesized and then subjected to Suzuki coupling reactions to form polyphenylene dendritic precursors. These are oxidized using FeCl₃ in a mixed solvent to yield GQDs. Purification is achieved through silica gel chromatography, and the products are characterized using standard methods. The synthetic strategy is designed to prevent phenyl rearrangement and aggregation, ensuring high yields of the final GQDs. The method is an advancement of a previously reported route, allowing for tunable optoelectronics and molecular magnetism properties of the graphene through symmetry variation. Later, Zheng et al. [84] fully demonstrates the potential of polyphenylene dendritic precursors in aryl oxidative condensation reactions, further confirming that stepwise organic synthesis is an effective pathway for the preparation of GQDs with unique structures and sizes (Figs. S5d–i in Supporting information). For instance, the stepwise organic synthesis, through a carefully designed sequence of organic reactions, enables the fusion of small organic molecules, providing excellent controllability over the structure and photoluminescence properties of GQDs. However, the stepwise organic synthesis faces cumbersome and complex reaction steps, which significantly limit its potential for large-scale production and applications.

  3. Application of GQDs-based photocatalyst in photocatalytic reactions

  Since Andre Geim and Kostya Novoselov discover the graphene in 2004, the controllable synthesis and application of graphene-based two-dimensional materials in the field of photocatalysis have been extensively investigated [91]. Among these, GQDs as nano-derivatives of graphene, inherit many of the excellent properties of graphene, such as electrical conductivity, thermal conductivity, chemical stability, large specific surface area, and quantum effects. Furthermore, due to their exceptional capability for the separation of photogenerated electrons and holes, GQDs serve as efficient electron donors or acceptors in the field of photocatalysis, facilitating electron transfer. They act as photosensitizers, transferring electrons to the CB, and also function as catalysts accelerate the rapid interfacial charge transfer and enhanced interfacial photocatalytic reactions. However, their intrinsically low light conversion efficiency results in a relatively slow rate of photocatalytic reactions. To address this issue, modifications or doping with carriers are employed to improve their properties, and the formation of heterojunctions with semiconductor materials is utilized to boost electron transfer, thereby improving the photocatalytic performance [92]. GQDs-based photocatalysts have made significant progress in the photocatalysis owing to their large specific surface area, flexible and adjustable structural characteristics, and a large number of exposed active sites. In this section, the focus will be on the introduction of GQDs-based catalysts in three aspects: photocatalytic CO₂ reduction [93,94], degradation of organic pollutants, and H₂ evolution [95,96].

  3.1 Application of GQDs-based photocatalyst in photocatalytic CO₂ reduction

  Excessive emission of CO₂ is one of the primary causes of global warming [97,98]. Capturing CO₂ and converting it into value-added chemicals represents a significant challenge [99]. To mitigate the influence of CO₂ emission, photocatalytic technology has great significance for its ability to convert CO₂ into valuable fuels or chemicals (CO, CH₄, CH₃OH, HCOOH, C₂H₄, C₂H₆, etc.) using solar energy. The core of achieving efficient photocatalytic technology lies in the design and synthesis of high-performance photocatalysts. Compared to conventional photocatalysts [100,101], GQDs-based photocatalysts have the following advantages: (1) The edges of GQDs are rich in functional groups, which contribute to the separation of photogenerated electron-hole pairs, reduce their recombination rate, and further enhance photocatalytic CO₂ reduction activity. (2) GQDs can be encapsulated in situ in other materials, forming heterojunctions with semiconductor materials or serving as electron donors/acceptors to promote electron transfer, further improving charge separation efficiency and expanding the range of light absorption. (3) GQDs-based photocatalysts have more active sites, among which GQDs act as cocatalysts, synergistically promoting the adsorption and activation of CO₂ on the catalyst surface. Consequently, GQDs-based photocatalysts have been widely used in photocatalytic CO₂ reduction.

  MOFs have shown outstanding performance in multiple fields owing to their porosity, large surface area, and controllable structure [102,103]. They have potential in photocatalytic CO₂ conversion, but their efficiency is limited by issues such as electron hole recombination. The addition of GQDs can enhance the photocatalytic performance of MOFs, as they can absorb a wide spectrum and effectively transfer electrons, suppress recombination, and thus enhance CO₂ conversion efficiency [104]. Wei and his team [29] designed a composite that integrates GQDs into the pores on the surface of Zn-Bim-His-1 (MOFs) nanoparticles (Fig. 6a). The composite of Zn-Bim-His-1@GQDs integrates the advantages of MOFs and GQDs, effectively overcoming the limitations associated with bare MOFs, such as limited light absorption capacity, low efficiency in the separation of photogenerated charge carriers, and inferior chemical and thermal stability, resulting in a significant enhancement in photocatalytic activity. Under illumination, the production rates of CH₄ and CO (Fig. 6b) are 20.9 and 3.7 µmol h⁻¹ g⁻¹ respectively with a high CH₄ selectivity of up to 85%. The Zn-Bim-His-1@GQDs heterostructure also exhibits high photocatalytic stability and good reusability for CO₂ reduction, which can be attributed to the synergistic effect of the close contact between Zn-Bim-His-1 and GQDs and the abundant active sites on the defective MOFs surface. Additionally, this interface effect can effectively facilitate the spatial separation of photogenerated electron-hole pairs during the catalytic conversion process. The excited electrons are transferred to the Zn-Bi-His-1 catalyst, where they are captured by adsorbed CO₂ and H₂O, leading to the formation of CH₄ and CO (Fig. 6c). This study holds promising opportunities for utilizing carbon dot materials and porous MOFs crystals in photocatalytic applications. Subsequently, Yu et al. [105] encapsulated GQDs into the highly stable MOFs (PCN-222) via a solvothermal method, making GQDs@PCN-222 a highly efficient visible light absorber up to 800 nm. After the incorporation of GQDs, the shape of the GQDs@PCN-222 particles remained unchanged (Fig. 6d). The crystalline and porous structure of the obtained GQDs@PCN-222 was also preserved. Compared to PCN-222, GQDs@PCN-222 exhibited increased photocurrent and decreased PL emission intensity (Fig. 6e), indicating that the introduction of GQDs significantly enhances the separation of photogenerated carriers in PCN-222, thereby improving its visible-light photocatalytic CO₂ reduction activity. Concurrently, the enhanced activity of GQDs@PCN-222 is attributed to the result that the GQDs can be encapsulated in situ in other materials, forming heterojunctions with semiconductor materials or serving as electron donors/acceptors to promote electron transfer, further improving charge separation efficiency and expanding the range of light absorption. GQDs can act as electron traps, making them potential photocatalytic cocatalysts for accumulating photogenerated electrons from PCN-222. Consequently, the activity of GQDs@PCN-222 is nearly four times higher than that of pure PCN-222, achieving 147.8 µmol g⁻¹ h⁻¹ within 10 h under visible light irradiation (Fig. 6f). The photocatalytic reaction mechanism involves the generation of photoelectrons under visible light irradiation, which transition from the VB to the CB of PCN-222, and then transfer to the encapsulated GQDs within the MOFs (Fig. 6g). The electrons accumulate on the GQDs and reduce adsorbed CO₂ to CO. The remaining holes on PCN-222 oxidize organic amines (TEOA) to oxidized organic amines. indicating that the combination of GQDs and high light absorbing MOFs provides a new platform for photocatalytic CO₂ reduction.

  Figure 6

  

  Figure 6.  (a) Schematic illustration of the synthetic process of Zn-Bim-His-1@GQDs heterostructures. (b) CO₂ photoreduction performance. Generation of CH₄, CO and H₂ over different samples in the first 12 h under visible light (λ > 420 nm) irradiation under the same conditions. (c) Schematic energy-level diagram based on the bandgaps and Mott-Schottky plot. Reprinted with permission [29]. Copyright 2020, Royal Society of Chemistry. (d) SEM images of GQDs@PCN-222 with a scale bar of 1 µm. (e) PL emission with excitation wavelength at 365 nm of PCN-222 and GQDs@PCN-222. (f) Photocatalytic CO₂ reduction by photocatalysts. (g) Proposed photocatalytic mechanism of CO₂ reduction to CO by GQDs@PCN-222. Reprinted with permission [105]. Copyright 2023, Multidisciplinary Digital Publishing Institute. (h) CO yield per hour of BWO, GQDs/BWO, and GQDs/BWO_6−x. (i) Stability test of GQDs/BWO_6−x using four-run recycling experiments under visible light irradiation at 0 ℃. (j) Illustration of GQDs/BWO_6−x surface microstructure. (k) Illustration of a possible mechanism for photocatalytic CO₂ reduction by GQDs/BWO_6−x. Reprinted with permission [108]. Copyright 2022, Elsevier.

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  Besides MOFs, bismuth-based oxides such as Bi₂WO₆ stand out in semiconductors due to their appropriate bandgap (~2.7 eV) and excellent photostability [106]. However, pure Bi₂WO₆ has a relatively weak light absorption capacity and a low electron-hole separation efficiency, resulting in poor photocatalytic activity for CO₂ reduction. To address the challenge, researchers have used GQDs as co-catalysts, which can improve performance through morphological control, formation of heterojunctions, hybrid nanostructures, and construction of oxygen vacancy (V_o) [107]. Herein, Xiong and colleagues [108] prepared a GQDs and V_o co-modified Bi₂WO₆ composite (GQDs/BWO_6−x) for photocatalytic reduction of CO₂. The introduction of GQDs and V_o plays a positive role in promoting electron-hole separation, tuning the band structure, and altering the surface chemical state of the photocatalyst. Additionally, surface V_o construction can accelerate charge separation, while GQDs can facilitate electron transfer. The experimental results showed that GQDs/BWO_6−x exhibited excellent efficiency in the photocatalytic conversion of CO₂, with a yield of up to 43.9 µmol g⁻¹ h⁻¹, which was 1.7 times higher than that of bare BWO (Fig. 6h). Furthermore, through ¹³CO₂ labeled isotopic experiments, it has been confirmed that the photocatalytic reduction products indeed originated from the conversion of CO₂ rather than the decomposition of the catalyst itself, and verified the excellent stability of GQDs/BWO_6−x (Fig. 6i). Based on the aforementioned results, a possible mechanism for photocatalytic CO₂ reduction is proposed (Figs. 6j and k). With the stimulation of lights, BWO is capable of generating a significant number of electrons, while GQDs also contribute a small number of electrons. Most of the photogenerated electrons tend to migrate towards the atoms adjacent to V_o, facilitating charge separation. Furthermore, the GQDs lower the CB level of BWO, enhancing its visible light response. Concurrently, oxygen vacancies serve as adsorption and activation sites for CO₂, allowing CO₂ molecules to adsorb easily on V_o sites, forming *CO₂, which is then converted to *CO₂⁻ by capturing photogenerated electrons. Under the action of holes and H₂O radicals, H₂O dissociates to form H⁺, which combines to form other intermediate products such as *CHO and *COOH. Eventually, these intermediate products undergo further reduction processes, transforming into CO, CH₄ and other carbon-containing fuels. This work provides a novel and valuable insight for the design and construction of new and efficient photocatalytic systems. Here, we have summarized the research progress of GQDs-based photocatalysts in the field of photocatalytic CO₂ reduction in Table S4 (Supporting information) [29,105,108].

  In general, the utilization of photocatalysts to convert CO₂ into sustainable solar fuels represents an environmentally friendly and reliable energy solution for the future, which holds significant importance for mitigating global warming. The growth of quantum dots, particularly GQDs, as a zero-dimensional material with unique properties such as enhanced solar light absorption, surface reactivity, charge separation and migration efficiency, shows immense potential in the field of solar photocatalytic CO₂ reduction. Additionally, the controlled hydrogenation of CO₂ to produce formic acid and alcohols using sustainable hydrogen resources is also considered an attractive option for H₂ storage and CO₂ utilization. By combining CO₂ reduction functionalization with subsequent C-X (X = N, S, C, O) bond formation and indirect conversion strategies based on direct CO₂ hydrogenation, not only does it expand the diversity of CO₂-derived products, but it also propels CO₂ reduction and transformation technology into a new phase of development. Therefore, it is hoped that this perspective will spark significant interest and catalyze further innovation in the prospects for CO₂ utilization.

  3.2 Application of GQDs-based photocatalyst in photocatalytic degradation of organic pollutants

  In recent years, energy shortages and environmental pollution are becoming key components of ecological problems, with water pollution particularly drawing public attention [109]. Generally speaking, water pollution is primarily caused by heavy metal ions and organic pollutants, such as cadmium divalent (Cd²⁺), hexavalent chromium (Cr⁶⁺), Rhodamine B (RhB), phenol, thiophene, and antibiotic. Among these, organic pollutants in wastewater are highly toxic, carcinogenic and difficult to degrade, posing a significant threat to human health [110]. Photocatalytic water treatment is an efficient technology that utilizes solar energy to eliminate pollutants in water [111]. Photocatalysis encompasses photocatalytic oxidation and photocatalytic reduction [112]. In the oxidation process, the holes within the valence band exhibit potent oxidative properties, capable of oxidizing water molecules to produce hydroxyl radicals (^•OH), or directly oxidizing organic pollutants, thereby decomposing them into innocuous low molecular weight substances, such as CO₂ and H₂O. For the reduction process, electrons transfer to the CB to participate in the reaction, which can reduce O₂ or other oxidants dissolved in the solution, forming reactive oxygen species such as O²⁻ [113]. To enhance the charge separation efficiency of photocatalysts, modifying photocatalyst materials with carbon nanomaterials is one of the most popular strategies. Due to the quantum confinement effect of GQDs, GQDs-based photocatalysts have been applied in the photocatalytic degradation of organic pollutants.

  Inspired by the peculiar electronic and optical properties caused by GQDs quantum confinement and edge effects, Sarwar and coworkers [114] selected CuWO₄ as a medium bandgap oxide photocatalyst and attempted to develop GQDs/CuWO₄ (GCW) composite with high photocatalytic performance. The composite samples, 0.3GCW, 0.5GCW, and 0.7GCW, all exhibited similar characteristic peaks to the pure CuWO₄ sample, indicating that the composite photocatalysts possessed a triclinic crystal system structure (Fig. 7a). The composite samples exhibited very small graphite peaks in their respective XRD patterns due to the presence of GQDs, which could be attributed to the low graphite content or peak masking by the CuWO₄ peaks. The photocatalytic performance of the prepared photocatalysts was investigated using phenol-simulated wastewater as the target pollutant under visible light irradiation (5 W LED lamp). Under dark conditions (60 min), the phenol concentration decreased primarily due to the adsorption process, which was aimed at achieving adsorption-desorption equilibrium in the dark (Fig. 7b). Upon light irradiation, the phenol concentration further decreased, which was associated with the photocatalytic degradation of phenol on the photocatalyst surface. The study results showed a maximum efficiency of 53.41% for the photodegradation of phenol-simulated wastewater, compared to 19.08% efficiency exhibited by the pure CuWO₄ sample. The enhanced photocatalytic activity of the CuWO₄/GQDs samples compared to the pure CuWO₄ sample correlated with the optimized GQDs content, leading to the suppression of electron-hole recombination and enhanced absorbance in the visible light range. Where upon irradiation of the photocatalyst with incident light of an appropriate wavelength, photogenerated electrons transition in the conduction band, while holes are generated in the valence band. The photogenerated electrons interact with water and oxygen (adsorbed on the material surface), generating radicals such as OH^• and O²⁻. Among them, O²⁻ and H⁺ react to produce H₂O₂, which further generates OH^•. OH^• radicals have oxidizing properties and can degrade phenol into CO₂ and H₂O (Fig. 7c). Therefore, the composite material presents an effective and cost-effective method for improving wastewater treatment activity, providing an experimental basis for further exploring phenol degradation pathways and developing visible-light-active photocatalysts for environmental remediation.

  Figure 7

  

  Figure 7.  (a) XRD patterns of GQDs, pure CuWO₄, 0.3GCW, 0.5GCW, and 0.7GCW samples. (b) Photocatalytic degradation evaluation of phenol simulated wastewater demonstrated by change in concertation vs. irradiation time. (c) Mechanistic overview for photocatalytic degradation of phenol simulated wastewater. Reprinted with permission [114], Copyright 2023, Elsevier. (d) TEM images of GQDs/TCN-0.4. (e) HRTEM images of TCN-0.4. (f) photocurrent transient responses of the as-prepared samples. (g) The combination of degradation of 4-NP with H₂ evolution under simulated solar irradiation over the as-prepared GQDs/TCN-x composites with different loadings of CNS (1: 10%, 2: 20%, 3: 30%, 4: 40%, 5: 50%). (h) Possible photocatalytic mechanism of degradation of organic pollutants coupled with simultaneous photocatalytic H₂ evolution over GQDs/TCN-0.4 under simulated solar irradiation. Reprinted with permission [115]. Copyright 2018, Elsevier.

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  To develop a novel photocatalytic system for achieving simultaneous photodegradation of organic pollutants and photocatalytic H₂ production, Nie and coworkers [115] successfully prepared a GQDs/TCN composite catalyst by integrating GQDs and g-C₃N₄ nanosheets with Mn-N-TiO₂ for the first time. The catalyst effectively utilizes the synergistic effects of photocatalytic oxidation (photodegradation of organic pollutants) and photocatalytic reduction (photocatalytic H₂ production) by constructing a novel heterostructure. Mn-N-TiO₂ microspheres are composed of a large number of nanoparticles, and a thin layer of g-C₃N₄ is deposited on the surface of Mn-N-TiO₂ (TCN). GQDs are well dispersed on the surface of TCN (GQDs/TCN) with diameters of 10 nm (Figs. 7d and e). Specifically, GQDs, serving as graphene fragments with more "molecular-like" characteristics, are utilized to modify the semiconductor photocatalyst, significantly enhancing the photocatalytic performance. The GQDs/TCN catalyst exhibits efficient degradation of organic pollutants such as p-nitrophenol under simulated sunlight irradiation while achieving photocatalytic H₂ production. The transient photocurrent response measurements indicate that the photocurrent intensity follows the order of TiO₂ < Mn-N-TiO₂ < TCN-0.4 < GQDs/TCN-0.4, suggesting that GQDs/TCN-0.4 has the best separation rate of photogenerated electrons and holes (Fig. 7f). Moreover, the GQDs/TCN-0.4 catalyst demonstrates the optimal photocatalytic degradation performance of the organic pollutant (4-NP) and the highest H₂ production rate (Fig. 7g). Density functional theory calculations and liquid chromatography-mass spectrometry analysis reveal that a portion of the photogenerated electrons participate in the photodegradation process of 4-NP. Subsequent analyses revealed the photocatalytic enhancement mechanism of the GQDs/TCN-0.4 catalyst and the impact of organic pollutants on photocatalytic H₂ evolution (Fig. 7h). When the GQDs/TCN-0.4 catalyst is irradiated under simulated solar radiation, GQDs absorb long-wavelength light (600–800 nm) and then emit short-wavelength light (λ < 500 nm), which is absorbed by Mn-N-TiO₂ and CNS to generate electrons (e⁻) and holes (h⁺). Electrons from the CB of CNS transfer to the CB of Mn-N-TiO₂, subsequently reacting with water on the GQDs or Pt surface to produce H₂. During the photocatalytic degradation of 4-NP, the photocatalytic H₂ evolution rate in the 4-NP solution decreases due to the consumption of a small number of electrons by intermediate products of 4-NP. Correspondingly, holes from the VB of Mn-N-TiO₂ transfer to the VB of CNS, gradually decomposing 4-NP into CO₂ and H₂O. These organic pollutants are also mineralized through the oxidation of OH^- by holes to produce hydroxyl radicals (·OH). This mechanism elucidates the multiple roles of GQDs in the GQDs/TCN-0.4 composite and supports the coupling of photocatalytic photodegradation of organic pollutants with concurrent photocatalytic H₂ evolution on GQDs-based photocatalyst. Based on this, the research progress of GQDs-based photocatalysts in the field of photocatalytic degradation of organic pollutants is summarized in Table S5 (Supporting information) [114–122].

  3.3 Application of GQDs-based photocatalysts in photocatalytic H₂ production

  In the search for renewable, environmentally friendly, and sustainable energy sources, it has been found that semiconductor mediated proton exchange membrane for H₂ production through photocatalysis is a promising method [123]. Photocatalytic H₂ evolution involves the absorption of photons by photocatalysts under illumination, which excites electrons from the VB to the CB, generating electron-hole pairs [124]. The separated electrons are transported to the surface of the semiconductor and react with water molecules, reducing water to H₂ [125]. The holes remaining in the valence band can react with water molecules or other electron acceptors, producing oxygen or other oxidized products. In recent years, to augment the efficiency of photocatalytic H₂ generation, GQDs-based photocatalysts (which can be optimized through material doping, incorporation of cocatalysts, or formation of heterojunctions with semiconductors, etc.) have progressively gained prominence in the domain of H₂ evolution [126,127].

  CdS as a typical photo responsive semiconductor, has been extensively studied for photocatalytic water reduction because of to its narrow bandgap (≤2.4 eV), which maximizes the absorption of visible light from the solar spectrum, and its excellent photocatalytic activity [128]. However, the issues of recombination of photogenerated electron-hole pairs and stability caused by photo corrosion during the photocatalytic reaction still need to be addressed to ensure its practical application. One of the most widely used strategies is to combine CdS with GQDs to effectively promote the transfer of photogenerated electrons to the surface of the photocatalyst and mitigate the photo corrosion problem [129]. Here, Lei et al. [35] reported a strongly coupled nanohybrid formed by combining CdS and GQDs, which are used for efficient photocatalytic H₂ evolution under visible light irradiation (Fig. 8a). CdS/GQDs nanohybrids are synthesized via the hydrothermal method. where GQDs are decorated on the surface of CdS nanoparticles, forming a "dot-on-particle" heterodimer structure. The doping of GQDs has no influence on the microcrystalline structure of CdS but enables the nanohybrid to have strong light absorption at wavelengths beyond the band edge of CdS. Under visible light irradiation (≥420 nm), when the GQDs content is 1.0 wt%, the H₂ production rate of the CdS/GQDs nanohybrid (Fig. 8b) reaches a maximum of 95.4 µmol/h, which is 2.7 times higher than that of pure CdS nanoparticles. The high activity of CdS/GQDs is primarily attributed to the graphene nature of GQDs, which can act as an efficient electron acceptor to induce effective charge separation. Electrons are most likely to transfer from the CB of CdS to GQDs upon light irradiation at the interface. Furthermore, the graphene microdomains in GQDs will provide fast electron transfer pathways due to their graphene-like conductive properties, thus enabling efficient electron-hole separation and enhancing the photocatalytic H₂ evolution activity of the CdS/GQDs nanohybrid (Fig. 8c). This work clearly demonstrates that GQDs primarily function as electron acceptors rather than photosensitizers, providing new insights into the crucial role of GQDs in semiconductor/GQDs nanohybrid materials for efficient solar energy conversion applications.

  Figure 8

  

  Figure 8.  (a) Schematic illustration of the preparation process of GQDs (step (a)), CdS nanoparticles (step (b)), and CdS/GQDs nanohybrids (step (c)). (b) Average photocatalytic H₂ evolution rate over different samples. (c) Photocatalytic mechanism for enhanced H₂ evolution activity over CdS/GQDs nanohybrids. Reprinted with permission [35]. Copyright 2017, Elsevier. (d) Synthetic procedure of OH-functionalized GQDs. (e) TEM image of composite sample. (f) The rate of H₂ evolution over Ni₂P/OH-GQDs photocatalyst hybrid with different amounts of Ni₂P at room temperature under visible light (λ > 420 nm), and comparison of photocatalytic H₂ evolution with 1 wt% Pt. The system contains 6 mg photocatalyst, 1.2 mg cocatalyst and 2 mL TEOA in 18 mL deionized water. (g) Schematic illustration of the mechanisms for the photocatalytic H₂ evolution over Ni₂P/OH-GQDs under visible light irradiation. Reprinted with permission [130]. Copyright 2018, Elsevier. (h) TEM images along with particle size distribution (inset) of our synthesized Rho-GQDs material. (i) High resolution image showing lattice plane with d spacing of our synthesized Rho-GQDs material. (j) Transient photocurrent response of GQDs and Rho-GQDs under simulated solar light irradiation. (k) Energy profile diagram of dye-sensitized Rho-GQDs system for H₂ evolution under visible light irradiation. Reprinted with permission [131]. Copyright 2020, Elsevier.

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  In addition to functioning as an electron acceptor in photocatalytic H₂ evolution, GQDs can also serve as photosensitizers to broaden the absorption range of visible light, enhancing visible light response of photocatalysts. Zhu and colleagues [130] first reported the use of hydroxyl functionalized GQDs (OH-GQDs) as a photosensitizer coupled with Ni₂P nanoparticles (OH-GQDs are randomly distributed on the surface of Ni₂P) for photocatalytic H₂ production using λ > 420 nm light (Figs. 8d and e). The introduction of the non-noble metal Ni₂P as a cocatalyst has significantly enhanced the photocatalytic activity. Under optimal conditions, the photocatalyst exhibits a peak H₂ evolution rate of 1567 µmol h⁻¹ g⁻¹, which surpasses the bare OH-GQDs by approximately 94 times and is comparable to the catalytic performance of 1wt% Pt/OH-GQDs (1683 µmol h⁻¹ g⁻¹) (Fig. 8f). This superior performance is attributed to the semiconductor-cocatalyst interfacial interactions between Ni₂P and OH-GQDs, which facilitate an efficient charge transfer process and promote the separation of photogenerated charge carriers (Fig. 8g). This study underscores the potential of OH-GQDs as an effective metal-free catalyst for photocatalytic H₂ evolution when paired with suitable cocatalysts. However, despite their promising applications, the performance of GQDs is still constrained by their limited visible light absorption and p-type conductivity. To tackle these issues, Dinda and coworkers [131] synthesized Rho-GQDs (3 nm) by covalently functionalizing GQDs with the visible-light-active photosensitizer Rhodamine 123 (Figs. 8h and i). The findings reveal that the dye-sensitized GQDs can achieve stable photocatalytic H₂ evolution under visible light irradiation (with an initial rate of 488 µmol g⁻¹ h⁻¹ and 1360 µmol/g produced in 1 h), significantly surpassing the performance of control GQDs without the dye, highlighting the effect of covalent functionalization with Rho 123. Both the efficient photoinduced electron transfer from Rho 123 to GQDs and the transformation of p-type GQDs into n-p bipolar semiconductors, facilitated by the covalent bonds, contribute to the enhanced photocatalytic H₂ evolution reaction (HER) performance. Electrochemical and photoelectrochemical tests further reveal that the covalent functionalization with Rho 123 not only ensures efficient photoinduced electron transfer in GQDs but also passivates the electron trap sites in GQDs, restoring their n-type dielectric properties, thereby enabling Rho-GQDs to achieve remarkably high photocatalytic HER performance (Fig. 8j). Upon visible light irradiation, Rho 123 is first excited and TEOA molecules supply electrons to the excited Rho 123 molecules, forming radical anions (Rho*). These radicals then eliminate holes. Subsequently, these excited radical anions transfer electrons to the active catalyst GQDs, which are used to reduce water into H₂ (Fig. 8k). Here, we encapsulate the findings from recent studies on the utilization of GQDs-based photocatalysts in photocatalytic water splitting for H₂ generation (Table S6 in Supporting information) [35,130–140].

  4. Summary and prospect

  The majority of existing literature provides a broad overview of GQDs, highlighting their diverse applications in biomedical detection and electrochemical fields. In contrast, this review focus on summarizing the existing methods for the synthesis of GQDs, including both top-down and bottom-up strategies. It further delves into the burgeoning role of GQDs-based photocatalysts, which are composites formed with a variety of semiconductors such as MOFs, CdS, and bismuth-based oxides. These composite photocatalysts have demonstrated considerable promise in pivotal photocatalytic processes, including the reduction of CO₂, the degradation of organic pollutants, and the splitting of water for H₂ evolution. The review presents illustrative examples to clarify the underlying mechanisms of these GQDs-based photocatalysts during photocatalytic reactions. Subsequently, the current development progress of GQDs-based photocatalysts is outlined and their potential applications in environmental protection and energy conversion are proposed. While GQDs-based photocatalysts possess several intrinsic advantages (such as tunable band gaps, high charge separation efficiency, and enhanced photostability) for photocatalysis, there are still challenges yet to be addressed in photocatalytic reactions (Fig. 9).

  Figure 9

  

  Figure 9.  Current challenges, possible pathways, and prospect of GQDs-based photocatalysts in photocatalytic applications.

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  (1) The synthesis of GQDs-based photocatalysts is fraught with challenges that primarily stem from the intrinsic characteristics of GQDs and the complexities of the preparation process. The dimensions and shape of GQDs are pivotal in dictating their photocatalytic efficacy, yet current methodologies often struggle to precise control over these attributes. This limitation results in uneven product distribution, which can significantly impede the catalytic performance. Furthermore, the solubility and stability of GQDs in aqueous or organic media are poor during the preparation process, leading to a propensity for aggregation that undermines their photocatalytic potential. In addition, the current synthesis of GQDs-based photocatalysts is predominantly confined to laboratory scale, characterized by elevated production expenses and modest yields, thereby limiting their scalability and industrial viability. Consequently, there is an imperative to delve into innovative preparation technologies, optimize the synthesis protocols, curtail costs, and improve yields to make GQDs-based photocatalysts more accessible and economically feasible for broader applications. Meanwhile, the modification of GQDs-based photocatalysts and their applications in the field of photocatalysis will continue to expand in the future. The researchers will work to develop more environmentally friendly preparation techniques to improve the quality and quantum yield of GQDs. As technology advances and the market evolves, GQDs-based photocatalysts are expected to play a key role in future environmental and energy solutions.

  (2) The mechanism of photocatalytic reactions catalyzed by GQDs-based photocatalysts is not yet fully understood. When the photocatalysts absorb photons, they generate electron-hole pairs that are inherently susceptible to rapid recombination. Despite the advantageous two-dimensional geometry and exceptional electrical conductivity of GQDs, which should theoretically enhance electron transport, the challenge of efficiently promoting the separation of electron-hole pairs while inhibiting their recombination persists. Furthermore, the formation of heterojunctions by GQDs-based photocatalysts introduces and additional layer of complexity. Within these heterojunctions, charge generation and consumption can occur concurrently across various components, complicating the dynamics of charge transfer. The diversity of heterojunction types, including PN junctions, Type II, Schottky, and S-type, each with their distinct principles and efficiencies, adds to the complexity. Selecting the suitable type of heterojunction and optimizing it also presents a challenge. The current understanding of photocatalytic mechanisms involving GQDs-based photocatalysts remains at the stage of hypothesis and speculation, it is clear that there is a pressing need for more in-depth theoretical research for clarification. Additionally, employing in-situ characterization techniques is crucial for gaining real-time insights into the photocatalytic processes. These techniques can provide real-time information about the structural changes of catalysts and reaction intermediates, which is helpful for researchers to analyze the reaction mechanism. With the development of research, it is believed that the mechanism of photocatalytic reaction will become clearer and more accurate.

  (3) Many photocatalytic materials are prone to photodegradation under prolonged illumination conditions, which reduces the photocatalytic reaction performance. To enhance the efficacy of GQDs-based photocatalysts and increase the quantity of reactive surface sites, innovative modifications are necessary. The development of photocatalytic materials that exhibit microscale heterogeneity alongside macroscopic structural stability is crucial for the feasibility of large-scale preparation and application. It is imperative to consider the robustness of the macroscopic structure, particularly in the assembly of hierarchical structures that integrate components with microscale heterogeneity. These strategies are expected to facilitate the directional movement and spatial separation of photogenerated charge carriers within the semiconductor matrix, thus promoting the photocatalytic performance.

  This review aims to delineate the potential influence of GQDs within composite materials, highlighting their pivotal role in elevating photocatalytic performance. Hoping that this review can inspire further research and innovation, opening up more possibilities for the development of photocatalysis.

  Declaration of competing interest

  The authors declare that they have no competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Junqing Ye: Writing – original draft, Investigation. Mengyuan Ren: Visualization, Formal analysis. Junfeng Qian: Resources, Investigation. Xibao Li: Writing – review & editing, Funding acquisition, Conceptualization. Qun Chen: Writing – review & editing, Funding acquisition.

  Acknowledgments

  The authors gratefully acknowledge the financial support provided by National Natural Science Foundation of China (No. 22262024), research start-up funding from Changzhou University (No. ZMF23020031) and thank for the technical support from the Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Jiangxi Province Academic and Technical Leader of Major Disciplines (No. 20232BCJ22008), and Key Project of Natural Science Foundation of Jiangxi Province (No. 20232ACB204007).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.110857.

  
  
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  Figure 1  Advantages of GQDs-based photocatalysts in photocatalytic applications.

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  Figure 2  (a) Representation scheme of oxidation cutting of CF into GQDs. (b) UV–vis spectra of A, B, and C correspond to the synthesis reaction temperatures of GQDs at 120, 100, and 80 ℃, respectively. The illustration of (b) shows the corresponding GQDs with a 365 nm excited light gap under UV light. (c) XPS survey spectrum of CF and GQDs. Reprinted with permission [43]. Copyright 2012, American Chemical Society. (d) High resolution transmission electron microscopy (HR-TEM) images of GQDs and the right column shows the diameter distribution of GQDs. (e) High resolution XPS spectra of C 1s for GQDs. (f) High resolution XPS spectra of N 1s for GQDs. Reprinted with permission [44]. Copyright 2018, Multidisciplinary Digital Publishing Institute. (g) Representation of GQDs containing a surface passivator with oligo-PEG diamino. (h) A schematic illustration of various typical electronic transitions processes of GQDs. Normal PL mechanisms in GQDs for small size (A) and large size (B). Upconverted PL mechanisms in GQDs for large size (C) and small size (D). Reprinted with permission [45]. Copyright 2011, Royal Society of Chemistry. (i) TEM images of the GQDs with varied sizes of 2 nm. The text insets represent the concentration of OAm. Reprinted with permission [46]. Copyright 2014, American Chemical Society.

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  Figure 3  (a) Schematic of the detection mechanism of GQDs and Fe³⁺ synthesized from 3D graphene. (b) UV–vis absorption (blue) and excitation (red) spectra of GQDs. The inset shows the photographs of aqueous dispersions of GQDs under day light (left) and UV (365 nm) illumination (right). Reprinted with permission [52]. Copyright 2014, Wiley-VCH. The inset of (c) is the photograph of the GQDs solution under day light and 365 nm illumination. Reprinted with permission [53]. Copyright 2015, Royal Society of Chemistry. (d) Schematic illustration of electrochemical exfoliation of defect-induced graphite rod. (e) Images of GQD1-GQD4 before (A-D) and after (E-H) electrochemical exfoliation and daylight/under 365 nm UV irradiation images for GQD1-GQD4 (I-L). Reprinted with permission [54]. Copyright 2017, American Chemical Society. (f) Schematic diagram of the preparation of GQDs in strong and weak electrolyte solutions. (g) TEM image. (h) Corresponding size distribution histogram with the orange Gaussian fitting curve based on the GQD sample (173 dots). Reprinted with permission [56]. Copyright 2018, American Chemical Society.

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  Figure 4  (a) The growth mechanism of N-GQDs and S,N-GQDs. Reprinted with permission [74]. Copyright 2013, Nanoscale. (b) TEM image of GQDs. (c) DLS diagram of GQDs. Reprinted with permission [75]. Copyright 2022, Elsevier. (d) The production process of GQDs. (e) Survey XPS spectrum. Reprinted with permission [76]. Copyright 2014, Springer Nature. (f) Schematic diagram of the formation of GQDs through the pyrolysis of L-glutamic acid. (g) The HRTEM image of GQDs with a scale bar of 5 nm. Inset: A typical single GQD with a lattice parameter of 0.246 nm. The scale bar is 1 nm. Reprinted with permission [77]. Copyright 2013, Royal Society of Chemistry.

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  Figure 5  (a) GQDs are prepared by MAH method. (b) TEM images of the GQDs. (c) Diameter distribution of GQDs, red line is Gaussian fitting curve. Reprinted with permission [78]. Copyright 2012, American Chemical Society. (d) Schematic diagram of GQDs, N-GQDs and S-GQDs prepared by microwave assisted method. (e) Raman spectra of GQDs and N-GQDs samples at different temperatures. (f) Proposed energy band diagram and electron transitions of S-GQDs and the possible emission mechanism. Reprinted with permission [79]. Copyright 2022, Springer.

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  Figure 6  (a) Schematic illustration of the synthetic process of Zn-Bim-His-1@GQDs heterostructures. (b) CO₂ photoreduction performance. Generation of CH₄, CO and H₂ over different samples in the first 12 h under visible light (λ > 420 nm) irradiation under the same conditions. (c) Schematic energy-level diagram based on the bandgaps and Mott-Schottky plot. Reprinted with permission [29]. Copyright 2020, Royal Society of Chemistry. (d) SEM images of GQDs@PCN-222 with a scale bar of 1 µm. (e) PL emission with excitation wavelength at 365 nm of PCN-222 and GQDs@PCN-222. (f) Photocatalytic CO₂ reduction by photocatalysts. (g) Proposed photocatalytic mechanism of CO₂ reduction to CO by GQDs@PCN-222. Reprinted with permission [105]. Copyright 2023, Multidisciplinary Digital Publishing Institute. (h) CO yield per hour of BWO, GQDs/BWO, and GQDs/BWO_6−x. (i) Stability test of GQDs/BWO_6−x using four-run recycling experiments under visible light irradiation at 0 ℃. (j) Illustration of GQDs/BWO_6−x surface microstructure. (k) Illustration of a possible mechanism for photocatalytic CO₂ reduction by GQDs/BWO_6−x. Reprinted with permission [108]. Copyright 2022, Elsevier.

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  Figure 7  (a) XRD patterns of GQDs, pure CuWO₄, 0.3GCW, 0.5GCW, and 0.7GCW samples. (b) Photocatalytic degradation evaluation of phenol simulated wastewater demonstrated by change in concertation vs. irradiation time. (c) Mechanistic overview for photocatalytic degradation of phenol simulated wastewater. Reprinted with permission [114], Copyright 2023, Elsevier. (d) TEM images of GQDs/TCN-0.4. (e) HRTEM images of TCN-0.4. (f) photocurrent transient responses of the as-prepared samples. (g) The combination of degradation of 4-NP with H₂ evolution under simulated solar irradiation over the as-prepared GQDs/TCN-x composites with different loadings of CNS (1: 10%, 2: 20%, 3: 30%, 4: 40%, 5: 50%). (h) Possible photocatalytic mechanism of degradation of organic pollutants coupled with simultaneous photocatalytic H₂ evolution over GQDs/TCN-0.4 under simulated solar irradiation. Reprinted with permission [115]. Copyright 2018, Elsevier.

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  Figure 8  (a) Schematic illustration of the preparation process of GQDs (step (a)), CdS nanoparticles (step (b)), and CdS/GQDs nanohybrids (step (c)). (b) Average photocatalytic H₂ evolution rate over different samples. (c) Photocatalytic mechanism for enhanced H₂ evolution activity over CdS/GQDs nanohybrids. Reprinted with permission [35]. Copyright 2017, Elsevier. (d) Synthetic procedure of OH-functionalized GQDs. (e) TEM image of composite sample. (f) The rate of H₂ evolution over Ni₂P/OH-GQDs photocatalyst hybrid with different amounts of Ni₂P at room temperature under visible light (λ > 420 nm), and comparison of photocatalytic H₂ evolution with 1 wt% Pt. The system contains 6 mg photocatalyst, 1.2 mg cocatalyst and 2 mL TEOA in 18 mL deionized water. (g) Schematic illustration of the mechanisms for the photocatalytic H₂ evolution over Ni₂P/OH-GQDs under visible light irradiation. Reprinted with permission [130]. Copyright 2018, Elsevier. (h) TEM images along with particle size distribution (inset) of our synthesized Rho-GQDs material. (i) High resolution image showing lattice plane with d spacing of our synthesized Rho-GQDs material. (j) Transient photocurrent response of GQDs and Rho-GQDs under simulated solar light irradiation. (k) Energy profile diagram of dye-sensitized Rho-GQDs system for H₂ evolution under visible light irradiation. Reprinted with permission [131]. Copyright 2020, Elsevier.

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  Figure 9  Current challenges, possible pathways, and prospect of GQDs-based photocatalysts in photocatalytic applications.

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