English

• 1. Introduction

  In the past few decades, the global industrialization process has accelerated, energy and environmental issues have become key factors restricting human survival and development [1-4]. Photocatalysis is one of the key methods to solve energy and environmental problems [5-7]. Since Fujishima and Honda first discovered in 1972 that TiO₂ electrodes can use sunlight to decompose water and produce H₂, photocatalysis technology has been widely studied [8]. Subsequently, the research on photocatalysts entered a rapid development stage, and various photocatalysts were reported, for example, g − C₃N₄, Bi₂O₃, MOFs, COFs [9-12]. However, due to low light utilization efficiency, fast charge recombination, and insufficient surface-active sites of photocatalysts, the catalytic activity of photocatalysts is far from meeting the requirements of practical applications. Therefore, researchers have constructed new photocatalysts through various methods to solve the above problems and improve photocatalytic activity [13-15].

  In 2011, Naguib et al. [16] discovered a new type of two–dimensional material, MXenes, which are mainly obtained by selectively etching the "A" layer of the MAX phase (the chemical formula of MAX is M_n+1AX_n, where "M" represents an early transition metal, "A" represents an IIIA or IVA group element, "X" represents C and/or N elements, and n = 1, 2, 3). Therefore, the chemical formula of MXenes is M_n+1X_nT_x ("T_x" represents surface functional groups of the "M" layer, such as –O, –OH, –F) [17-19]. So far, more than 30 kinds of MXenes have been synthesized and more than 100 kinds of MXenes have been predicted [20]. MXenes have good hydrophilicity, high surface area, and excellent conductivity, with a surface redox capacitance of up to 2400 S/cm, equivalent to multilayer graphene [21], which has attracted increasing attention in many fields [22]. Compared with the two−dimensional form, quantum dot form (MQDs) can not only inherit the advantages of MXenes but also have better dispersibility, high stability, larger specific surface area, and easier functionalization due to the quantum confinement effect [23-25]. Based on this, in 2017, Xue et al. [26] broke down 2D MXenes into MQDs (Fig. 1A, lateral size ≤ 10 nm) and applied them to cell imaging. The excellent and diverse characteristics of MQDs make them widely used in sensing, biomedical, optoelectronic devices, catalysis, and energy storage, among which they are particularly prominent in the field of photocatalysis [27-29]. Compared with other quantum dots, MQDs have better light absorption ability, which can absorb light in the range of ultraviolet to visible light, to achieve efficient catalytic reactions. The properties of MQDs can be adjusted by different preparation methods and surface modifications, so that they have better surface reaction performance and stability. Finally, MQDs have excellent electron transport properties, which can promote electron transport and shorten the time of electron recombination, and improve the photocatalytic efficiency. The above advantages make MQDs very suitable for photocatalytic application, and can effectively promote various photocatalytic reactions, such as water decomposition to produce H₂, photocatalytic reduction of CO₂ [30]. Therefore, MQDs have more application potential than MXenes (Fig. 1B). However, so far, research on the application of MQDs in photocatalysis is just beginning, and there is no comprehensive review on the application of MQDs in the field of photocatalysis. Therefore, this review summarizes the latest developments in MQD–based photocatalysts, focusing on the synthesis and modification methods of MQDs. The review also comprehensively introduces and analyzes the applications of MQD–based photocatalysts in recent years. Finally, the main challenges and development prospects of MQD–based photocatalysts are proposed (Fig. 1C). It is expected that this review can inspire for the development of MQD–based photocatalysts.

  Figure 1

  

  Figure 1.  (A) MQDs preparation diagram. Reproduced with permission [68]. Copyright 2022, Royal Society of Chemistry. (B) Number of MQDs published in journals. (Source: Web of Science, Keyword: MXene quantum dots and photocatalysis, Retrieval Date: December 30, 2022). (C) Overview of the synthesis, performance, and application of MQDs in the field of photocatalysis.

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  2. Synthesis and functionalization of MQDs

  The synthesis and functionalization methods of MQDs have a crucial impact on their properties. By changing the synthesis and functionalization methods, the material size can be adjusted, the morphology and structure can be controlled to obtain the desired QDs structure and function [31-33]. So far, various methods have been used to synthesize different QDs, such as black phosphorus, Ti₃C₂, graphene [34-36]. MXenes have similar morphology and structure to other 2D bulk materials. Therefore, similar methods can be used to synthesize MQDs, mainly divided into top–down and bottom–up methods [37,38].

  2.1 Top–down methods

  The term "top-down approach" describes a technique for gradually downsizing big sizes to the nanoscale size using physical, chemical, or electrochemical processes [39,40]. This technique is perfect for large-scale production since it is simple to use, has an abundance of raw materials, yet produces poor products. For example, Cai group used HF to remove the Al layer from Ti₃AlC₂, and then intercalated with a tetramethylammonium hydroxide solution under sonication, and finally collected Ti₃C₂ QDs through twice centrifugations [41]. V₂C QDs could be prepared through a hydrothermal procedure in which ammonium hydroxide was used to reduce the size of V₂C [42]. The top-down approach can also be classified as hydrothermal technique, solvent thermal method, ultrasonic method, electrochemical method, and so on.

  2.1.1   Hydrothermal method

  Hydrothermal method is to simulate the growth process of crystals in the natural mineralization process, under a certain temperature and pressure, with water as the solvent, in a closed system to complete the reaction. The process mainly involves crystal nucleation and further growth. Under high temperature and pressure, 2D MXene sheets are cracked and assembled to form the desired MQDs. The size, morphology, and properties of MQDs can be adjusted by controlling the temperature, pH value, reactant concentration, and other conditions [43-45]. For example, Xu et al. [46] prepared Nb₂C QDs with good optical stability by hydrothermal method, whose average transverse size is 2.4 nm (Fig. 2A). Moreover, the average lateral sizes (ALS), average thickness (AT), and properties of Ti₃C₂ QDs could also be regulated by the hydrothermal reaction temperature [26]. Arising the hydrothermal reaction temperature causes Ti₃C₂ oxidation. The higher the temperature, the more TiO₂ is generated, which is accompanied with the change in structure. The prepared V₂C QDs showed good performance in photothermal therapy [47]. The TEM study confirmed that the higher hydrothermal reaction temperature was beneficial to the smaller size of MXene QDs (Figs. S1A–C in Supporting information) [48]. Therefore, the yield of QDs can be controlled by increasing the hydrothermal temperature. In general, the hydrothermal method is a simple and inexpensive method that can adjust the particle size and morphology of QDs by changing the reaction conditions.

  Figure 2

  

  Figure 2.  (A) Schematic illustration of the hydrothermal synthesis of Nb₂C QDs. Reproduced with permission [46]. Copyright 2020, Royal Society of Chemistry. (B) Schematic diagram of the solvent thermal method for preparing MQDs. Reproduced with permission [50]. Copyright 2019, Royal Society of Chemistry. (C) Schematic diagram of the ultrasonic–solvent thermal method for preparing MQDs. Reproduced with permission [52]. Copyright 2021, Springer. (D) Schematic illustration of preparing MQDs by ultrasound method. Reproduced with permission [53]. Copyright 2017, Royal Society of Chemistry. (E) Schematic illustration of preparing MQDs by ultrasound–assisted liquid exfoliation method. Reproduced with permission [54]. Copyright 2018, Multidisciplinary Digital Publishing Institute. (F) Schematic illustration of preparing MQDs by electrochemical etching method. Reproduced with permission [58]. Copyright 2021, Elsevier. (G) Schematic illustration of preparing MQDs by intercalation method. Reproduced with permission [59]. Copyright 2022, Elsevier.

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  2.1.2   Solvothermal method

  The solvothermal method, using organic solvents as the reaction medium, was employed to fracture 2D MXenes into the desired MQDs. This method provides better control over the size and morphology of MQDs and is commonly utilized for MQD synthesis [49-51]. As shown in Fig. 2B, Zhang et al. [50] prepared Ti₃C₂ QDs with a size of 1.75 nm by stripping Ti₃C₂ in DMSO. Xu et al. [49] prepared different Ti₃C₂ QDs, named e–MQDs, f–MQDs, and s–MQDs, from Ti₃C₂ MXenes in ethanol, DMF, and DMSO solvents, respectively. The AT of all MQDs is between 1.0 and 2.5 nm, indicating a high degree of exfoliation and less layering. The size and quantum yield of MQDs are strongly influenced by the solvent's polarity, boiling temperature, and degree of oxidation. The boiling point of a solvent decreases as its polarity increases. Moreover, as the oxidative properties of the solvent decrease, the average lateral size of MQDs decreases, resulting in a higher quantum yield. In addition, the prepared dispersions display different photoluminescence (PL) colors under 365 nm excitation wavelength, with s–MQDs being white and e–MQDs and f–MQDs being blue. This indicates that the PL color of MQDs can be adjusted by changing the synthesis conditions, which is important for the study of MQDs' optical properties. Gao et al. [52] reported a ultrasonication–assisted solvothermal method to prepare Ti₃C₂ QDs. The average transverse size and thickness of Ti₃C₂ QDs were 2.75 nm and 1.02 nm, respectively (Fig. 2C). Ti₃C₂ MXene NSs can be more effectively fragmented into Ti₃C₂ QDs under ultrasonication. Compared with the hydrothermal method, the solvothermal method has no selection restrictions on the precursor materials, and it can be used to prepare QDs with different morphologies and properties. The as–prepared QDs have high crystallinity, which is conducive to their performance improvement.

  2.1.3   Ultrasound method

  Different from hydrothermal/solvothermal methods, the ultrasound method utilizes shockwaves and cavitation induced by ultrasound to convert both layered and non–layered materials (MAX/MXenes) into MQDs. As shown in Fig. 2D, Yu et al. [53] first used a probe–type ultrasound to break down large Ti₃AlC₂ materials into smaller sizes, exposing more edges and surfaces. Then, OH^– reacted with exposed Al to form Al(OH)₄^–, weakening the interlayer binding force of Ti₃C₂ and forming Ti₃C₂ NSs. Finally, thinner MQDs were prepared using Ti₃C₂ NSs as precursors in a tetrabutylammonium hydroxide aqueous solution with an ultrasound bath under fluorine–free conditions. Dai et al. [54] reported the preparation of Mo₂C QDs with fewer layers by dispersing Mo₂C powder in ultra–pure water and subjecting it to ultrasound treatment (Fig. 2E). The MXenes (cavitated MAX) can be used to prepare MQDs by ultrasound method, instead of using hazards HF (or LiF/HCl). However, the low power density of ultrasound makes it difficult for large–scale production. In addition, this method has low utilization efficiency for synthesizing QDs and may produce side effects such as mechanical vibration and thermal effects.

  2.1.4   Electrochemical etching method

  Electrochemical etching is a simple and effective method for synthesizing QDs that have been used for various QD materials such as SiC QDs and ZnO QDs [55-57]. The advantage of this method is that it can avoid irreversible defects due to re–oxidation and prolonged exposure to ultrasonic radiation. Zhao et al. [58] used Ti₃AlC₂ and Pt as the working electrodes, tetramethylammonium hydroxide and NH₄Cl as the electrolyte to etch Al, which destroyed the Ti–Al bond and formed AlCl₃ (Fig. 2F). Due to the interaction between the defects or active edges of the carbon layer and the nearby atoms, the carbon layer was broken, and Cl^– reacted with Ti to form Ti–Cl. N in NH₄Cl is inserted into MQDs by forming new chemical bonds, resulting in Cl, N–MQDs. This method can control the size and morphology of MQDs by adjusting reaction conditions such as potential, current, and electrolyte composition. It can be performed at room temperature and pressure without requiring a high vacuum environment or high–temperature conditions, making it widely applicable. However, this method produces by–products or introduces other substances during MQD preparation, and the cost is relatively high, making it unsuitable for large–scale production of MQDs.

  2.1.5   Other methods

  In addition to the above–mentioned methods, several other approaches were also developed for synthesizing MQDs. For example, Jiang et al. [59] utilized Li⁺ intercalation to cut 2D MXene NSs into 0D single–layer MQDs (Fig. 2G). Ti₃C₂ QDs, Nb₂C QDs, and V₂C QDs were prepared using this method. Zhang et al. [60] produced Ti₃C₂ QDs of different sizes by ball milling with various solid–state elements (C, S, P, Si). However, it is difficult to remove the solid–state elements from the final product, making it hard to synthesize composite materials. Moreover, Yuan et al. [61] focused laser beams on MXenes (dispersed in graphene) and induced plasma emission. By repeatedly burning particles with laser pulses, they reduced the particle size to make them more dispersed and uniform, ultimately resulting in transparent composite electrodes consisting of MQDs and reduced graphene oxide. Besides, Li et al. [62] injected liquid nitrogen into a Ti₃C₂T_x layer, followed by heat input to remove water. Utilizing the temperature difference between liquid N₂ and heat can cause microexplosion to damage the structure of Ti₃C₂T_x, creating N–MQDs. The ALS and AT sizes were 7.23 nm and 5 nm, respectively.

  Although top–down approaches have been widely used in MQDs synthesis, they inevitably suffer from low yield and poor reproducibility. Therefore, further research should be conducted to develop simple, cost–effective, high–yield, and reproducible methods to meet the demands of large–scale MQDs production in the future.

  2.2 Bottom–up methods

  The bottom–up method is another method of synthesizing nanomaterials, where small molecules or precursors are gradually assembled into nanostructures. Compared to the top–down approach, the bottom–up approach uses organic or inorganic precursors to achieve more precise control over the size, shape, and surface chemistry of QDs, and achieving higher precursor utilization and producing more structurally stable QDs [63-65]. The main methods for bottom–up preparation include molten salt synthesis and thermal decomposition.

  Molten salt synthesis was a method that used salt as a solvent. Generally, the salts with low melting point were added to precursors, and by heating the mixture above the melting point of the salt, the melted salt can act as solvent. For example, Cheng et al. [63] homogeneously mixed molybdenum acetylacetonate precursor, sucrose, and NaCl. Mo₂C/C composite were obtained through a restricted space formed by NaCl nanocubes during heat treatment. The size of the Mo₂C QDs was 2–3 nm using HRTEM.

  These studies provide insights into the bottom–up synthesis of MQDs. In order to meet the demand of mass production of MQDs in the future, it is necessary to further expand and improve the synthesis method to achieve the purpose of simple, mild, environmental protection and expanded production scale. Among them, the process of synthesizing MQDs by the precursor MAX is more critical: MXene material is obtained by etching the A layer of MAX materials. Then, MQDs is decomposed by hydrothermal, solvothermal, ultrasonic, electrochemical and other means to obtain MQDs (Table S1 in Supporting information). Optimizing these steps will help to improve the synthesis efficiency and quality of MQDs.

  2.3 Functionalization of MQDs

  Typically, MQDs have some inevitable drawbacks such as low production yield, tendency to aggregate, and susceptibility to oxidation [66]. These issues can be addressed through various energy–based methods, including surface modification, element doping, and coupling with other materials. During the synthesis of MQDs, surface groups such as –O, –F, and –OH can be generated, and the passivation of these surface groups is crucial for the performance and stability of MQDs [47]. Special functional groups can also be used to modify the surface of MQDs. For example, Lu et al. [67] modified Ti₃C₂ QDs with oleylamine as a surface passivating agent, improving their photoluminescence properties. Yuan et al. [68] used Sn²⁺ to modify the surface of Ti₃C₂ QDs, enhancing their light absorption and charge transfer capabilities. Besides, organic compounds can also be used to modify the surface of MQDs. Li et al. [69] used polyethyleneimine as a passivating agent to enhance the H₂ production ability of g–C₃N₄@Ti₃C₂ QDs. Zeng et al. [70] similarly used polyethyleneimine to modify Ti₃C₂ QDs, improving the CO₂ reduction ability of Ti₃C₂ QDs/Cu₂O nanowires/Cu. This indicates that surface modification of MQDs can improve their application performance in various fields, enabling them to play vital role in a wider range of applications. Element doping has many advantages and is commonly used for the energy–based fields. For example, Feng et al. [71] prepared N–doped Ti₃C₂ QDs using a solvothermal method. The doping of N reduced the particle size of MQDs and improved their fluorescence properties (Figs. 3A and B), resulting in a QY of 16.9%. Xu et al. [72] investigated S–doping, N–doping, and co–doping of Ti₃C₂ MQDs. Doped Ti₃C₂ QDs were connected to water molecules through hydrogen bonds, increasing the average lateral size of MQDs, a redshift in the emission spectrum, and a decrease in bandgap (E_g) with improved charge transfer ability. Zhao et al. [58] co–doped Cl and N elements into Ti₃C₂ QDs through electrochemical etching, resulting in Cl–N–Ti₃C₂ QDs with larger specific surface area and higher ^•OH scavenging activity. Element doping can also enhance the photoelectric properties of MQDs by reducing their particle size, increasing photoluminescence lifetime (PLLT), and improving photoluminescence quantum yield (PLQY), laying the foundation for their application in the field of photocatalysis.

  Figure 3

  

  Figure 3.  (A) Particle size distribution of three types of MQDs, (B) PL emission spectra. Reproduced with permission [71]. Copyright 2020, Elsevier. (C) WF of different MXenes with different terminations. Reproduced with permission [79]. Copyright 2015, American Physical Society. (D) Schematic diagram of the quenching ECL pathway. Reproduced with permission [83]. Copyright 2018, Royal Society of Chemistry.

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  Forming composite materials with other materials is also a common method to improve their performance. Currently, various materials have been used to composite with MQDs. For example, Cheng et al. [63] reported the embedding of Mo₂C QDs into C NSs to form Mo₂C/C, which exhibited superior electrocatalytic N₂ reduction reaction performance. Isotope labeling experiments confirmed that the N element in the synthesized NH₃ originated from supplied N₂. Guo et al. [73] used Ti₃C₂ QDs to modify Ni and formed a flower–shaped hybrid material (Figs. S2A and B in Supporting information), significantly enhancing the catalytic reduction performance of Cr(VI). Although many studies on energy–based methods for MQDs have been conducted, they are still far from sufficient. In summary, current research on energy–based methods for MQDs mainly focuses on surface modification, element doping, and composite with other materials. These methods can improve the application performance of MQDs in different fields by changing their surface structure, functional groups, composition, and other aspects. However, there are still some challenges in the research of energy–based methods for MQDs. Firstly, the energy–based methods for MQDs are relatively complex and require considering the influence of multiple factors comprehensively. Secondly, the energy–based methods for MQDs also need to consider environmental stability and biosafety issues, requiring strict evaluation and verification. Therefore, further research on the mechanism and application of energy–based methods for MQDs is needed in the future, aiming to achieve more extensive applications in materials science, biomedicine, optoelectronics, and other fields.

  3. The photoelectric characteristics of MQDs

  Due to their unique composition and structure, 2D MXenes exhibit excellent electrical and optical properties that can be altered by changing the surface functional groups [74]. MQDs not only inherit these characteristics of MXenes but also show even better performance due to the quantum confinement effect [72]. Their outstanding properties increase the potential for applications of MQDs in various fields, particularly in the field of photocatalysis.

  3.1 Electronic characteristic

  The electrical transport properties of most MXenes are similar to metals, with the electrical resistivity decreasing as temperature increases. The electrical properties of MXenes are determined by their structure, composition, layer number, and surface functional groups [75]. MQDs not only inherit the advantages of MXenes, but also have higher specific surface area and quantum confinement effect [76]. Therefore, compared to MXenes, the electrical properties of MQDs should have more attractive surface functional groups, providing more possibilities for their practical applications. The unique composition of MXenes not only gives them metallic conductivity but also results in high electron density near the Fermi level (E_F), leading to excellent electrical conductivity. MXenes have a dual–metal layer structure both internally and externally, with the outer transition metal layer having a greater impact on their electrical conductivity [31]. Furthermore, MXenes exhibit high electrical conductivity and work function (WF), even higher than graphene (conductivity: 2000 S/m, WF: 4.42 eV) [77]. This indicates that MXenes can effectively promote charge transfer and can undergo surface plasmon resonance effects and form Schottky barriers with semiconductors, thereby promoting photo–generated carrier separation. Recently, studies have shown that MQDs inheriting the advantages of MXenes can promote charge transfer more effectively. For instance, Xiao et al. [48] uniformly dispersed Ti₃C₂ QDs on the surface of nanosheets, forming a nano–dot/nano–sheet structure. Thus, the interfacial resistance and charge transfer were respectively reduced and promoted in Li–S batteries. Yu et al. [78] loaded Mo₂C QDs on N–doped graphene. These Mo₂C QDs have polarity and high conductivity, which can effectively inhibit shuttle effects in the material and improve its electrochemical and cycling performance.

  In a word, MXenes mainly enhance the performance of photocatalytic materials by providing adsorption sites, promoting charge transfer, and accelerating photo–generated carrier separation. In addition, changes in the surface functional groups of MXenes can lead to the formation of new energy bands below E_F, resulting in a decrease in E_F and a change in the WF of MXenes. Electrons transfer from the transition metal to the electronegative surface terminal, causing the d–band to shift above E_F, significantly reducing the density of states and producing an energy gap (E_g) [31]. For example, Khazaei et al. [79] calculated the WF of MXenes using DFT and found that MXenes with –O as the surface functional group have a higher WF than those with –F and –OH (Fig. 3C). This is due to the reduction in E_F during surface energy–based modification of MXenes, resulting in an increase in WF for MXenes with –O and –F surface functional groups.

  Kong et al. [80] studied the electronic structure of Ti₃C₂O₂ QDs loaded on monolayer graphene using DFT calculations and tested its H₂ evolution performance. The results showed that Ti₃C₂ QDs have a higher density of states near EF comparing to 2D MXenes NSs, and the –O end in Ti₃C₂ QDs can bring about strong local electric accumulation, which is more conducive to charge transfer. Zeng et al. [70] used DFT calculations to determine the electronic structure and E_F of Ti₃C₂ QDs with –O as the surface functional group and found that Ti₃C₂ QDs have rich electronic states near E_F, exhibiting excellent conductivity. These studies demonstrate that compared to MXenes, the more stable MQDs have superior electrical properties and can better promote photocatalytic reactions.

  In conclusion, MQDs possess excellent photocatalytic properties due to their unique electronic structure and surface functional groups, providing more possibilities for their practical applications.

  3.2 Optical characteristics

  The optical properties of MQDs are crucial for their photocatalytic applications. The optical properties of MQDs mainly include light absorption, photoluminescence (PL), and electrochemiluminescence (ECL) [81], which can be adjusted through methods such as changing solvents, synthesis temperature, reaction time, pH, and so on [67].

  Typically, the band structure of MQDs can be adjusted by changing their composition and surface functional groups, enabling significant light absorption from ultraviolet to near–infrared regions [82]. For example, Zeng et al. [70] loaded Ti₃C₂ QDs on Cu₂O nanowires to form a Ti₃C₂ QDs/Cu₂O NWs/Cu composite and analyzed their UV–visible diffuse reflectance spectra. The results show that loading Ti₃C₂ QDs on Cu₂O NWs/Cu can improve the optical absorption efficiency and broaden the optical absorption range. Different MQDs exhibit varying strengths of absorption in the near–infrared region, and can convert incident light energy into thermal, chemical, or other forms of energy, such as Ti₃C₂ QDs [53] and V₂C QDs [47] being used for cancer treatment or other applications. MQDs can exhibit strong PL emissions, but the corresponding reaction mechanism is not yet fully understood. Researchers usually study the fluorescence behavior of MQDs through PL color, fluorescence quantum yield, and fluorescence lifetime. Previous studies have reported that MQDs emit various colors of fluorescence (such as blue, green, red, and white) when excited at different wavelengths, with 365 nm being the most commonly used one. In addition, different MQDs can exhibit different colors under the same excitation wavelength due to their different compositions and preparation methods, and their PL colors can be controlled by changing the synthesis temperature, size, element doping, and other methods [83]. For example, Xue et al. [26] prepared Ti₃C₂ QDs using a hydrothermal method, and due to the influence of the core–shell structure, Ti₃C₂ QDs exhibited blue fluorescence at an excitation wavelength of 320 nm, and the emission spectrum of MQDs shifted from blue to red as the excitation wavelength increased within 340–440 nm, indicating that the bandwidth increased or energy level spacing decreased, making electron transitions easier. Feng et al. [71] prepared N–doped Ti₃C₂ QDs using a solvothermal method, and found that the charge transfer and separation ability of N–doped MQDs were much higher than those of undoped MQDs. Fig. S2C (Supporting information) shows the photoluminescence (PL) emission spectrum of N–MXene QDs at various excitation wavelengths. Increasing the excitation wavelength results in decreased PL intensity and a redshift in the emission spectrum of N–MQDs. This is because N doping can introduce more electronic energy levels, increase the probability of fluorescence emission, and thus improve the fluorescence lifetime. At the same time, it can also change the surface and edge states of MQDs, reduce the recombination of photo–generated carriers, and increase the fluorescence lifetime, indicating that N doping is an effective method to enhance the fluorescence performance of MQDs. Xu et al. [49] prepared three types of Ti₃C₂ QDs using DMSO, DMF, and ethanol as solvents, and found that MQDs exhibited a stronger quantum confinement effect in non–polar solvents, resulting in the maximum excitation and emission peaks shifting towards blue. Non–polar solvents do not interact with the surface groups of MQDs via hydrogen bond, reducing the screening effect of surface charges and increasing the surface energy of MQDs, thus altering their optical properties. This is of great significance for studying the properties and applications of MQDs in different environments.

  In addition to light absorption and PL, the electrochemiluminescence (ECL) property of MQDs is also an important optical characteristic. ECL is a technique that utilizes electrochemical reactions to generate luminescence. In ECL systems, excited species are generated on the electrode surface through electrochemical reactions, and these species can transfer energy to the emitter via energy transfer or charge transfer, thereby stimulating its luminescence. ECL can effectively eliminate background signals, and its high sensitivity, wide detection range, and ease of scalability make it widely applicable in the fields of clinical diagnosis and food testing [84]. Qin et al. [83] first studied the ECL properties of Ti₃C₂ QDs in detail, by using annihilation processes and co–reactant enhancement (Fig. 3D). The ECL signal were detected and the generation pathway of ECL was proposed: MQDs receive electrons to form negatively charged MQDs^•–, MQDs^•– oxidize and reduce MQDs to generate positively charged MQDs^•+, and then electron transfer occurs between MQDs^•– and MQDs^•+ to produce excited state of MQDs (MQDs*), which emits light and transfers to the ground state of MQDs, generating the ECL signal. In addition, Xu et al. [49] investigated the ECL performance of three types of Ti₃C₂ QDs with different co–reactants, including K₂S₂O₈, l–cysteine, citric acid, H₂O₂, and glutathione. The results showed that the ECL signal of MQDs was significantly enhanced when using K₂S₂O₈ as the co–reactant, indicating that the presence of co–reactants can enhance the ECL signal. MQDs with higher light absorption coefficients and stronger electron donor properties can better transfer light energy, improving the efficiency of photocatalytic reactions. However, research on the optical properties of MQDs has mostly focused on their PL properties, and further studies are needed to strengthen research on other optical properties such as light absorption, ECL, and photothermal conversion.

  4. Photocatalytic applications of MQDs

  In recent years, with the deepening research on MQDs, the application of MQD–based photocatalysts has gradually increased. Due to the interaction between the surface functional groups of MQDs and semiconductors, MQDs can be aggregated and adsorbed on the semiconductor surface. Moreover, its unique photoelectric properties not only enhance the visible light absorption of photocatalytic materials, but also receive photogenerated electrons from semiconductors, forming electron–hole pairs, improving the electron transfer rate, and promoting the progress of photocatalytic reactions. Therefore, MQDs have been applied in various fields, such as H₂ evolution, CO₂ reduction, N₂ fixation, and pollutant degradation [76].

  4.1 H₂ evolution

  H₂ is a well–known renewable energy source that can replace fossil fuels, thereby alleviating energy and environmental problems. Among them, the photocatalytic H₂ evolution reaction (HER) has received much attention since it can use solar energy to split water into H₂ [85]. Noble metals such as Au, Pd, and Pt are the most effective HER catalysts, which can promote the decomposition of water to H₂. However, due to the scarcity and expensive price of noble metals, their large–scale applications are limited. Therefore, seeking a low–cost and highly active material is of great significance. This alternative material should have HER catalytic activity similar to noble metals but at a lower price. Due to their unique photoelectric properties and high surface area, MQDs exhibit special catalytic activity for HER and are expected to become alternatives to noble metals [86]. MQDs are usually coupled with other semiconductors as electron acceptors or dielectrics to promote the separation of photo–generated carriers and enhance HER activity. For example, Yang et al. [87] prepared MQDs/ZIS/Ti(IV) by in–situ growth of Ti₃C₂ QDs on ZnIn₂S₄/Ti(IV) via electrostatic self–assembly. When Na₂S/Na₂SO₃ was used as a sacrificial agent, the H₂ production rate reached 7.52 mmol g^–1 h^–1, which was 4.02 times higher than that of ZIS/Ti(IV) (Fig. 4A). This is mainly due to the interface charge transfer between MQDs and ZIS, which promotes HER. Dong et al. [88] reported that combining Ti₃C₂ QDs with TiO₂/g–C₃N₄ heterojunctions can effectively promote light absorption, charge separation and transfer, forming a more efficient HER catalyst. As shown in Fig. S3A (Supporting information), compared with TiO₂ and TiO₂/g–C₃N₄, the composite system has a smaller E_g and exhibits stronger HER performance [89], with H₂ evolution rates 98 times and 1.6 times higher than TiO₂ and TiO₂/g–C₃N₄, respectively (Fig. 4B). Yuan et al. [68] used highly conductive Sn²⁺–modified Ti₃C₂ QDs (Sn–TQDs) as dielectrics, coupled with WO₃ and In₂S₃ to prepare WO₃/Sn–TQDs/In₂S₃, which was used for heavy metal reduction, organic degradation, and HER. As shown in the photocatalytic HER experiment (Fig. S3B in Supporting information), the H₂ evolution rate of WO₃/Sn–TQDs/In₂S₃ was significantly higher than WO₃ and In₂S₃, mainly because Sn–TQDs act as dielectrics, transferred electrons (e^–) from the conduction band (CB) of WO₃ to the valence band (VB) of In₂S₃ and combined with holes (h⁺), resulting in the accumulation of a large number of e^– on In₂S₃, thus driving HER (Fig. S3C in Supporting information).

  Figure 4

  

  Figure 4.  (A) Comparative study of photocatalytic hydrogen production performance of different samples under simulated solar light irradiation. Reproduced with permission [87]. Copyright 2022, Multidisciplinary Digital Publishing Institute. (B) corresponding photocatalytic hydrogen evolution rate. Reproduced with permission [88]. Copyright 2021, American Chemical Society. (C) Energy band structure of TiO₂, C₃N₄ and T₃C₂ QDs, (D) Transient photocurrent response of T, CN, T–CN, and T–CN–TC, (E) The photocatalytic CO and CH₄ production rates of different samples. Reproduced with permission [95]. Copyright 2020, Elsevier.

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  The above research shows that the introduction of MQDs can promote redox reactions and charge separation and transfer, significantly enhancing HER performance. However, research on the HER performance of MQDs is still insufficient, and the relevant mechanisms at the atomic layer level are not clear. Therefore, we need to invest more efforts to strengthen the research on MQDs.

  4.2 CO₂ reduction

  The removal and utilization of CO₂ have become a key issue for solving global warming. Currently, many methods have been used for CO₂ reduction, including photocatalysis, electrochemistry, photoelectrochemistry, and thermochemistry [90-93]. Among them, photocatalytic CO₂ reduction can convert CO₂ into hydrocarbons such as CO, CH₃OH, and CH₄ without consuming additional energy sources, which not only solves global warming but also alleviates energy shortages and achieves renewable energy.

  Currently, various photocatalysts have been used in CO₂ reduction research, but single–component photocatalysts usually have very low CO₂ reduction efficiency due to limited light absorption ability, poor adsorption, insufficient active sites, and rapid recombination of electron–hole pairs [94]. Therefore, researchers have developed multi–component photocatalysts by combining different photocatalysts to enhance the efficiency of CO₂ reduction.

  In recent years, researchers have found that MXenes with high conductivity can be used as electron acceptors or dielectrics to promote the separation of photoinduced charge carriers and provide more active sites for CO₂ adsorption, significantly enhancing CO₂ reduction efficiency. MQDs inherit all the features of 2D MXenes and exhibit unique properties due to size confinement effects. However, because the preparation technology is not mature, single MQDs were not used as photocatalyst. Actually, MQDs are usually used as co–catalysts coupled with other semiconductors for photocatalytic CO₂ reduction. For example, Zeng et al. [70] coupled Ti₃C₂ QDs with Cu₂O NWs/Cu via electrostatic self–assembly to improve CO₂ reduction. The experimental results showed that after loading Ti₃C₂ QDs onto Cu₂O NWs/Cu, the light absorption range, cyclic stability, as well as separation of photoinduced charge carriers were all improved. As a result, the CH₃OH production rate by Ti₃C₂ QDs/Cu₂O NWs/Cu photocatalyst reached 153.38 ppm/cm² in 6 h, which was 8.25 times and 2.15 times higher than that by Cu₂O NWs/Cu and Ti₃C₂ NSs/Cu₂O NWs/Cu, respectively. Under sunlight illumination, Cu₂O is excited to produce e^– and h⁺. However, because the CB energy level of Cu₂O is more negative than the E_F energy level of Ti₃C₂ QDs, Ti₃C₂ QDs act as electron acceptors, receiving e^– from Cu₂O and reducing CO₂ to CH₃OH.

  Besides, He et al. [95] used Ti₃C₂ QDs to modify a core–shell structured TiO₂/C₃N₄, forming a 2D/2D/0D TiO₂/C₃N₄/Ti₃C₂ photocatalyst for CO₂ reduction. As shown in Fig. 4C, the E_F of C₃N₄ is higher than TiO₂ and Ti₃C₂ QDs. The photogenerated e^– is transferred from C₃N₄ to TiO₂ and Ti₃C₂ QDs, forming an intrinsic electric field that suppresses the recombination of photoinduced charge carriers. Therefore, the introduction of Ti₃C₂ QDs effectively promotes the separation of photoinduced charge carriers and enhances the photocatalytic performance of CO₂ (Fig. 4D), resulting in maximum CO and CH₄ yields that are 1.3 and 2 times higher than those of TiO₂/C₃N₄ photocatalysts, respectively (Fig. 4E).

  Existing research is sufficient to demonstrate the important role of MQDs as co–catalysts in CO₂ reduction. However, current research is just beginning, and a more in–depth exploration is needed to understand the adsorption, activation, and charge transfer mechanisms, which will promote the development of MQDs in CO₂ reduction.

  4.3 Nitrogen fixation

  NH₃ is an important chemical raw material with wide applications in agriculture, aviation fuels and industrial production [96]. N₂ is considered the natural raw material for NH₃ production and the Haber–Bosch process is a common method in industrial production to fix N₂ into NH₃, but this reaction requires substantial fossil energy consumption and emits a lot of CO₂, which is not conducive to green chemistry [97]. Therefore, a low–energy–consumption and environmentally friendly N₂ fixation method is of great practical significance. Photocatalytic N₂ fixation has been widely studied due to its solar energy consumption and minimal environmental harm. Recently, research has shown that the unique properties of MQDs can be used to enhance photocatalytic N₂ fixation. For example, Qin et al. [98] synthesized Ti₃C₂ QDs/Ni–MOF for N₂ fixation by electrostatic self–assembly of Ti₃C₂ QDs and 2D nickel metal–organic frameworks (Ni–MOF). The Ni in the MOF material acts as an N₂ adsorption and activation site. Moreover, X–ray absorption near–edge structure test showed that the characteristic peaks A and B in Fig. 5A respectively originated from the π* excitation of carbon ring structures and the transition of oxygen–containing groups/surface contaminants to sp³ hybridized states. It can be seen from the figure that the characteristic peak A of 9MX–MOF is higher than that of Ni–MOF, while peak B is lower than that of Ni–MOF. This indicates that electrons may migrate from the carbon ring of Ti₃C₂ QDs to the external oxidized groups of Ni–MOF, causing a large accumulation of electrons on the CB of Ni–MOF, which helps to accelerate the conversion of N₂ to NH₃. Therefore, when Na₂SO₃ is used as a sacrificial agent, the photocatalytic N₂ fixation efficiency of Ti₃C₂ QDs/Ni–MOF can reach 88.79 µmol g^–1 h^–1. In addition, Chang et al. [99] synthesized C₃N₄/r–Ti₃C₂ QDs by loading partially reduced Ti₃C₂ QDs (r–Ti₃C₂ QDs) on the surface of mesoporous hollow C₃N₄ spheres, which have abundant surface defects. The surface chemical properties of Ti₃C₂ were adjusted by using l–ascorbic acid and introducing oxygen vacancies (OVs) under the Ar atmosphere, followed by hydrothermal treatment to form r–Ti₃C₂ QDs with surface defects. The X–ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) of Ti₃C₂ MXenes and r–Ti₃C₂ QDs confirmed the existence of Ti³⁺ and OVs (Figs. 5B and C). MQDs promote the transfer and separation of photo–generated carriers, accelerating the conversion of N₂ to NH₃. Therefore, the NH₃ generation rates of C₃N₄/r–Ti₃C₂ QDs under white light and visible light were 1.6 and 1.9 times that of C₃N₄, respectively.

  Figure 5

  

  Figure 5.  (A) C K–edge XANES spectra of Ni–MOF and Ni–MOF with 9 mL Ti₃C₂ QDs added to the supernatant. Reproduced with permission [98]. Copyright 2020, American Chemical Society. (B) Ti 2p XPS spectra of Ti₃C₂ MXenes and r–Ti₃C₂ QDs, (C) EPR spectra. Reproduced with permission [99]. Copyright 2022, Royal Society of Chemistry. (D) Kinetics of photocatalytic reduction of Cr(VI) using different materials, (E) Kinetics of photocatalytic degradation of BPA. Reproduced with permission [103]. Copyright 2021, Elsevier. (F) UV–vis diffuse reflectance spectra. Reproduced with permission [68]. Copyright 2022, Elsevier.

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  Overall, MQDs have great potential in the field of photocatalytic N₂ fixation, but related research is still in its early stages. Further exploration in this direction is necessary for achieving more efficient photocatalytic N₂ fixation.

  4.4 Pollutant removal

  In the past decades, photocatalysis has been widely applied in the removal of organic pollutants and toxic heavy metals [100,101]. Recently, MQD–based photocatalysts have also been developed due to their high conductivity, large specific surface area, and numerous surface functional groups. For example, Lai et al. [102] observed that Ti₃C₂ QDs can significantly enhance the photocatalytic performance of Bi₂O₃. The introduction of Ti₃C₂ QDs caused a red shift in the absorption edge of Bi₂O₃ and a decrease in E_g, thereby enhancing the light absorption of Bi₂O₃ and promoting the transfer and separation of photo–generated carriers. Consequently, after visible light irradiation for 90 min, the removal rate of tetracycline hydrochloride (TC) by Bi₂O₃/Ti₃C₂ QDs reached 82%, while the removal rate of TC by unmodified Bi₂O₃ was only 14%. In 2021, Yuan et al. [103] reported a new Z–type heterojunction WO₃/TQDs/In₂S₃, which employed Ti₃C₂ QDs as the dielectric layer between WO₃ and In₂S₃, and investigated its catalytic degradation activity towards Cr(VI) and bisphenol A (BPA). As shown in Figs. 5D and E, under visible light irradiation, WO₃/TQDs/In₂S₃ could reduce 99.8% of Cr(VI) within 12 min and degrade 97.6% of BPA within 120 min. Compared with WO₃/In₂S₃ without TQDs, WO₃/TQDs/In₂S₃ loaded with 3.9 wt% TQDs exhibited 4 times and 3 times higher degradation rates for Cr(VI) and BPA, respectively. The main mechanism is illustrated in Fig. S4A (Supporting information), where WO₃ and In₂S₃ are excited by light to generate e^– and h⁺. Herein, TQDs act as a dielectric layer to transfer e^– from the CB of WO₃ to In₂S₃, thereby promoting the separation of photo–generated carriers and accelerating the photocatalytic degradation process. In 2022, Yuan et al. [45] replaced the dielectric layer between WO₃ and In₂S₃ using Sn²⁺–modified Ti₃C₂ QDs (Sn–TQDs) with high conductivity. Compared with TQDs, Sn–TQDs not only enhanced the light absorption range of WO₃/Sn–TQDs/In₂S₃, but also further accelerated the transfer and separation rate of photo–generated carriers (Fig. 5F and Fig. S4B in Supporting information). Therefore, as shown in Fig. S4C (Supporting information), under visible light irradiation, WO₃/Sn−TQDs/In₂S₃ exhibited a 2.1 times higher rate towards Cr(VI) reduction than WO₃/TQDs/In₂S₃. In addition to organic pollutants and metal ions, MQDs are also used for NO removal. For example, Wang et al. [104] coupled SiC with Ti₃C₂ QDs to construct a novel heterojunction photocatalyst (Ti₃C₂ QDs/SiC). Under light conditions, Ti₃C₂ QDs/SiC can remove 74.6% of NO within 6 min, which is 3.1 times and 3.7 times higher than SiC and Ti₃C₂ QDs, respectively. This is mainly due to the strong interface contact between SiC and Ti₃C₂ QDs, which forms a heterojunction and accelerates the separation of photo generated charge carriers. In summary, MQD−based photocatalysts have significant advantages in organic pollutant degradation and heavy metal reduction, providing new pathways for photocatalytic applications.

  4.5 Other applications

  In addition, MQD−based photocatalysts have also been applied in other fields. For example, Lin et al. constructed Ti₃C₂ QDs−loaded defective g−C₃N₄ (TC/CN) via electrostatic self−assembly and applied it to produce H₂O₂. The highest H₂O₂ production rate by TC/CN reached 560.7 µmol/L, approximately 9.3 times that by bulk g−C₃N₄. The formation of numerous carbon vacancies in g−C₃N₄ leads to more delocalized electrons transferring to Ti₃C₂ QDs, causing the band edge to bend upward to form a Schottky barrier, thereby promoting the separation of photo−generated carriers [105]. In general, MQDs possess many surface-active sites, which can effectively adsorb and activate visible light. Additionally, MQDs exhibit excellent electron transport performance, which can promote the separation and transfer of photogenerated charges, thereby reducing their recombination rate and leading to enhanced performance.

  Although there have been some studies on MQD−based photocatalysts in the field of photocatalysis, they are mainly focused on Ti₃C₂ QDs, and research on QDs with other metal centers is relatively scarce, which is not enough to fully demonstrate the important role of MQDs-based photocatalysis. Moreover, for other areas of photocatalysis, such as desulfurization and denitrification of fuels, sterilization, and organic synthesis, only MXene−based photocatalysts have been studied, and the application of MQDs in these fields has not yet appeared. Therefore, further exploration of MQDs in the field of photocatalytic applications is still needed.

  5. Conclusions and outlook

  This review provides a systematic and comprehensive summary of the current synthesis methods and modification strategies of MQDs. At the same time, the applications of MQDs in photocatalytic H₂ production, CO₂ reduction, N₂ fixation, and pollutant removal were mainly introduced. However, as a new type of photocatalytic material, MQDs still face several challenges and need to be improved.

  5.1 Improvement of MQDs synthesis strategy

  Typically, 2D MXenes are obtained from MAX via HF (or LiF/HCl) etching, followed by a top−down approach to prepare MQDs by cleaving the 2D MXene precursors. However, this method often presents significant safety risks as well as environmental pollution issues. The fluoride−free method can replace the etching of the "A" layer in MAX with HF (or LiF/HCl), which is not only safer and simpler but also enables the synthesized MQDs to exhibit superior performance. However, there is currently limited research on using fluoride−free methods to prepare MQDs and more exploration of such methods is urgently needed. Compared with the top−down approach, MQDs prepared by the bottom−up approach can provide better control over their size, morphology, and structural stability. In addition, different preparation conditions can be used to regulate the surface functional groups of MQDs, affecting the activity of MQD−based photocatalysts. Moreover, due to the small size of MQDs, traditional methods have difficulty in achieving uniform size during preparation with MXenes as precursors. Therefore, more efforts are needed in the synthesis of MQDs to achieve uniform size and stability.

  5.2 Exploration of expanding non−Ti−based MQDs

  So far, over 100 types of MXenes have been theoretically predicted, while over 30 types of MXenes have been experimentally synthesized in laboratories, providing a foundation for the diversified application of MQDs in the field of photocatalysis. The composition, layer spacing and functional group types of metal elements in MQDs have significant effects on their photoelectric properties, but currently, most research on MQDs in the field of photocatalysis is focused on exploring Ti₃C₂ QDs. Therefore, it is of great significance to expand the exploration of non−Ti−based MQDs to extend their light absorption range and improve the efficiency of photocatalytic reactions.

  5.3 Expanding the application of MQDs photocatalysis

  Expanding the application scope of MQD−based photocatalysts to solve practical problems is of great significance. Many studies have shown that the excellent physical and chemical properties of MQDs make them attractive in the field of photocatalysis. However, most current research on MQD−based photocatalysts is focused on H₂ production and pollutant degradation, with less research in other areas of photocatalytic applications such as H₂O₂ production, organic synthesis, antibacterial and antiviral applications. Moreover, most of the research has been focused on single or dual pollutant treatment systems. To this end, more attention should be paid to the treatment effects of multiple coexisting pollutants, to better apply MQD−based photocatalysts to the treatment of actual wastewater.

  MQDs are a new type of nanomaterials, which have shown good application prospects in the field of photocatalysis. In the future, with the further research and development of MQDs, it will be possible to develop more efficient, flexible and environmentally friendly photocatalytic materials. In summary, there is still enormous potential to be explored for MQD−based photocatalysts. We should pay more attention to the above−mentioned issues, which are of great significance for promoting the rapid development of MQD−based photocatalysts. At the same time, we hope that this review can provide reference and inspiration for the research on MQD−based photocatalysts and promote their further development. With a deeper understanding of their mechanisms, we believe that more efficient and lower−cost MQD−based photocatalysts will be developed in the future.

  Declaration of competing interest

  The authors state that they have no known competing economic interests or personal relationships that could affect the work reported in this paper.

  Acknowledgment

  This work was supported by the National Natural Science Foundation of China (Nos. 22276168 and 21876154).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.109078.

  
  
• 1. [1]

     D.J. Chen, Y.L. Cheng, N. Zhou, et al., J. Clean. Prod. 268 (2020) 121725. doi: 10.1016/j.jclepro.2020.121725

  2. [2]

     X.Y. Li, B.R. Jie, H.D. Lin, et al., J. Environ. Manag. 308 (2022) 114664. doi: 10.1016/j.jenvman.2022.114664

  3. [3]

     N. Olama, M. Dehghani, M. Malakootian, Appl. Water Sci. 8 (2018) 1–12. doi: 10.1007/s13201-017-0639-9

  4. [4]

     A. Touati, T. Hammedi, W. Najjar, et al., J. Ind. Eng. Chem. 35 (2016) 36–44. doi: 10.1016/j.jiec.2015.12.008

  5. [5]

     S. Zheng, H. Du, L. Yang, et al., J. Hazard. Mater. 447 (2023) 130849. doi: 10.1016/j.jhazmat.2023.130849

  6. [6]

     B. Xue, Q. Li, L. Wang, et al., Chemosphere 332 (2023) 138829. doi: 10.1016/j.chemosphere.2023.138829

  7. [7]

     Q. Wang, L. Wang, S. Zheng, et al., J. Hazard. Mater. 451 (2023) 131149. doi: 10.1016/j.jhazmat.2023.131149

  8. [8]

     A. Fujishima, K. Honda, Nature 238 (1972) 37–38. doi: 10.1038/238037a0

  9. [9]

     Q. Guo, C.Y. Zhou, Z.B. Ma, et al., Adv. Mater. 31 (2019) 1901997. doi: 10.1002/adma.201901997

  10. [10]

      Q.H. Zhu, Z.H. Xu, B.C. Qiu, et al., Small 17 (2021) e2101070. doi: 10.1002/smll.202101070

  11. [11]

      Q. Wang, Q. Gao, A.M. Al-Enizi, et al., Inorg. Chem. Front. 7 (2020) 300–339. doi: 10.1039/C9QI01120J

  12. [12]

      J. Guo, D. Ma, F. Sun, et al., Sci. China Chem. 65 (2022) 1704–1709.

  13. [13]

      K.N. Van, H.T. Huu, V.N.N. Thi, et al., Chemosphere 289 (2022) 133120. doi: 10.1016/j.chemosphere.2021.133120

  14. [14]

      A. Malathi, J. Madhavan, M. Ashokkumar, et al., Appl. Catal. A: Gen. 555 (2018) 47–74. doi: 10.1016/j.apcata.2018.02.010

  15. [15]

      Y. Fu, Y. Xu, Y. Mao, et al., Sep. Purif. Technol. 317 (2023) 123922. doi: 10.1016/j.seppur.2023.123922

  16. [16]

      M. Naguib, M. Kurtoglu, V. Presser, et al., Adv. Mater. 23 (2011) 4248–4253. doi: 10.1002/adma.201102306

  17. [17]

      Y. Wei, P. Zhang, R.A. Soomro, et al., Adv. Mater. 33 (2021) e2103148. doi: 10.1002/adma.202103148

  18. [18]

      K. Rasool, M. Helal, A. Ali, et al., ACS Nano 10 (2016) 3674–3684. doi: 10.1021/acsnano.6b00181

  19. [19]

      P. Kuang, J.X. Low, B. Cheng, et al., J. Mater. Sci. Technol. 56 (2020) 18–44. doi: 10.1016/j.jmst.2020.02.037

  20. [20]

      A. VahidMohammadi, J. Rosen, Y. Gogotsi, Science 372 (2021) eabf1581. doi: 10.1126/science.abf1581

  21. [21]

      A. Raza, U. Qumar, A.A. Rafi, et al., Sustain. Mater. Technol. 33 (2022) e00462.

  22. [22]

      K.L. Huang, C.H. Li, H.Z. Li, et al., ACS Appl. Nano Mater. 3 (2020) 9581–9603. doi: 10.1021/acsanm.0c02481

  23. [23]

      K. Zhang, D.Q. Li, H.Y. Cao, et al., Chem. Eng. J. 424 (2021) 130340. doi: 10.1016/j.cej.2021.130340

  24. [24]

      F.M. Yang, Y.Q. Ge, T. Yin, et al., ACS Appl. Nano Mater. 3 (2020) 11850–11860. doi: 10.1021/acsanm.0c02369

  25. [25]

      G.H. Yang, J.L. Zhao, S.Z. Yi, et al., Sens. Actuator. B: Chem. 309 (2020) 127735. doi: 10.1016/j.snb.2020.127735

  26. [26]

      Q. Xue, H.J. Zhang, M.S. Zhu, et al., Adv. Mater. 29 (2017) 1604847. doi: 10.1002/adma.201604847

  27. [27]

      A. Rafieerad, W.A. Yan, A. Amiri, et al., Mater. Des. 196 (2020) 109091. doi: 10.1016/j.matdes.2020.109091

  28. [28]

      X.Y. Jiang, H.J. Wang, Y. Shen, et al., Sens. Actuator. B: Chem. 350 (2022) 130891. doi: 10.1016/j.snb.2021.130891

  29. [29]

      F.F. Liu, S. Jin, Q.X. Xia, et al., J. Energy Chem. 62 (2021) 220–242. doi: 10.1016/j.jechem.2021.03.017

  30. [30]

      Y.H. Park, G. Murali, J.K.R. Modigunta, I. In, S.I. In, Front. Chem. 9 (2021) 734108. doi: 10.3389/fchem.2021.734108

  31. [31]

      K. Hantanasirisakul, Y. Gogotsi, Adv. Mater. 30 (2018) e1804779. doi: 10.1002/adma.201804779

  32. [32]

      B. Bartolomei, J. Dosso, M. Prato, Trends Chem. 3 (2021) 943–953. doi: 10.1016/j.trechm.2021.09.003

  33. [33]

      W.F. Chen, G. Lv, W.M. Hu, et al., Nanotechnol. Rev. 7 (2018) 157–185. doi: 10.1515/ntrev-2017-0199

  34. [34]

      M.M. He, D. Wang, H. Shiigi, et al., Int. J. Hydrog. Energy 47 (2022) 17194–17203. doi: 10.1016/j.ijhydene.2022.01.228

  35. [35]

      J. Liu, Y.B. Shao, S.Y. Zuo, et al., Optoelectron. Adv. Mater. Rapid Commun. 16 (2022) 248–252.

  36. [36]

      T.A. Tabish, H. Hayat, A. Abbas, et al., Curr. Opin. Electrochem. 30 (2021) 100786. doi: 10.1016/j.coelec.2021.100786

  37. [37]

      Y.H. Xu, X.X. Wang, W.L. Zhang, et al., Chem. Soc. Rev. 47 (2018) 586–625. doi: 10.1039/C7CS00500H

  38. [38]

      C.H. Zhao, X.B. Song, Y. Liu, et al., Nanobiotechnology 18 (2020) 1–32. doi: 10.1186/s12951-019-0560-5

  39. [39]

      T.J. Pillar-Little, N. Wanninayake, L. Nease, et al., Carbon 140 (2018) 616–623. doi: 10.1016/j.carbon.2018.09.004

  40. [40]

      H. Kang, D.Y. Kim, J. Cho, ACS Omega 8 (2023) 5885–5892. doi: 10.1021/acsomega.2c07683

  41. [41]

      G.N. Cai, Z.Z. Yu, P. Tong, et al., Nanoscale 11 (2019) 15659–15667. doi: 10.1039/C9NR05797H

  42. [42]

      D.P. Huang, Y. Xie, D.Z. Lu, et al., Adv. Mater. 31 (2019) 1901117. doi: 10.1002/adma.201901117

  43. [43]

      A. Rafieerad, W.A. Yan, G.L. Sequiera, et al., Adv. Healthc. Mater. 8 (2019) 1900569. doi: 10.1002/adhm.201900569

  44. [44]

      D. Polsongkram, P. Chamninok, S. Pukird, et al., Phys. B: Condens. Matter 403 (2008) 3713–3717. doi: 10.1016/j.physb.2008.06.020

  45. [45]

      M.M. ElFaham, A.M. Mostafa, E.A. Mwafy, J. Phys. Chem. Solids 154 (2021) 110089. doi: 10.1016/j.jpcs.2021.110089

  46. [46]

      Q. Xu, J.F. Ma, W. Khan, et al., Chem. Comm. 56 (2020) 6648–6651. doi: 10.1039/D0CC02131H

  47. [47]

      Y. Cao, T.T. Wu, K. Zhang, et al., ACS Nano 13 (2019) 1499–1510.

  48. [48]

      Z.B. Xiao, Z.L. Li, P.Y. Li, et al., ACS Nano 13 (2019) 3608–3617. doi: 10.1021/acsnano.9b00177

  49. [49]

      G.F. Xu, Y.S. Niu, X.C. Yang, et al., Adv. Opt. Mater. 6 (2018) 1800951. doi: 10.1002/adom.201800951

  50. [50]

      Q.X. Zhang, Y. Sun, M.L. Liu, et al., Nanoscale 12 (2020) 1826–1832. doi: 10.1039/C9NR08794J

  51. [51]

      M.A.A.B. Abdul Rani, N.A. Karim, N.S. Shamsul, et al., Fuel 345 (2023) 128271. doi: 10.1016/j.fuel.2023.128271

  52. [52]

      X.H. Gao, X.C. Shao, L.L. Qin, et al., Nanoscale Res. Lett. 16 (2021) 1–8. doi: 10.1186/s11671-020-03464-0

  53. [53]

      X.H. Yu, X.K. Cai, H.D. Cui, et al., Nanoscale 9 (2017) 17859–17864. doi: 10.1039/C7NR05997C

  54. [54]

      W.H. Dai, H.F. Dong, X.J. Zhang, Materials 11 (2018) 1776. doi: 10.3390/ma11091776

  55. [55]

      J.Z. Chen, M.F. Chen, W.J. Zhou, et al., ACS Nano 16 (2022) 2461–2470. doi: 10.1021/acsnano.1c09004

  56. [56]

      M. Mwania, C. Janaky, K. Rajeshwar, et al., Electrochem. Commun. 37 (2013) 1–4. doi: 10.1016/j.elecom.2013.09.020

  57. [57]

      L.C. Chen, N.N. Xu, H.Y. Yang, et al., Electrochim. Acta 56 (2011) 1387–1391. doi: 10.1016/j.electacta.2010.10.050

  58. [58]

      L. Zhao, Z. Wang, Y. Li, et al., J. Mater. Sci. Technol. 78 (2021) 30–37. doi: 10.1016/j.jmst.2020.10.048

  59. [59]

      B.W. Jiang, T. Yang, T.T. Wang, et al., Chem. Eng. J. 422 (2022) 136119.

  60. [60]

      T.R. Zhang, X. Jiang, G.C. Li, et al., ChemNanoMat 4 (2018) 56–60. doi: 10.1002/cnma.201700232

  61. [61]

      Y.J. Yuan, L. Jiang, X. Li, et al., Adv. Mater. 34 (2022) e2110013. doi: 10.1002/adma.202110013

  62. [62]

      X.S. Li, F. Liu, D.P. Huang, et al., Adv. Funct. Mater. 30 (2020) 2000308. doi: 10.1002/adfm.202000308

  63. [63]

      H. Cheng, L.X. Ding, G.F. Chen, et al., Adv. Mater. 30 (2018) e1803694. doi: 10.1002/adma.201803694

  64. [64]

      K.A.S. Usman, J.W. Maina, S. Seyedin, et al., NPG Asia Mater. 12 (2020) 58. doi: 10.1038/s41427-020-00240-5

  65. [65]

      N. Abid, A.M. Khan, S. Shujait, et al., Adv. Colloid Interface Sci. 300 (2022) 102597. doi: 10.1016/j.cis.2021.102597

  66. [66]

      Q.W. Guan, J.F. Ma, W.J. Yang, et al., Nanoscale 11 (2019) 14123–14133. doi: 10.1039/C9NR04421C

  67. [67]

      S.Y. Lu, L.Z. Sui, Y. Liu, et al., Adv. Sci. 6 (2019) 1801470. doi: 10.1002/advs.201801470

  68. [68]

      Z.T. Yuan, W.R. Tian, T.T. Ren, et al., Environ. Sci. Nano 9 (2022) 2417–2426. doi: 10.1039/D2EN00317A

  69. [69]

      Y.J. Li, L. Ding, Y.C. Guo, et al., ACS Appl. Mater. Interfaces 11 (2019) 41440–41447. doi: 10.1021/acsami.9b14985

  70. [70]

      Z.P. Zeng, Y.B. Yan, J. Chen, et al., Adv. Funct. Mater. 29 (2019) 1806500. doi: 10.1002/adfm.201806500

  71. [71]

      Y.F. Feng, F.R. Zhou, Q.H. Deng, et al., Ceram. Int. 46 (2020) 8320–8327. doi: 10.1016/j.ceramint.2019.12.063

  72. [72]

      Q. Xu, W.J. Yang, Y.Y. Wen, et al., Appl. Mater. Today 16 (2019) 90–101. doi: 10.1016/j.apmt.2019.05.001

  73. [73]

      Y.L. Guo, Y.H. Cheng, X.C. Li, et al., J. Hazard. Mater. 423 (2022) 127053. doi: 10.1016/j.jhazmat.2021.127053

  74. [74]

      D. Dhamodharan, V. Dhinakaran, H.S. Byun, Carbon 192 (2022) 366–383. doi: 10.1016/j.carbon.2022.03.004

  75. [75]

      M. Naguib, V.N. Mochalin, M.W. Barsoum, et al., Adv. Mater. 26 (2014) 992–1005. doi: 10.1002/adma.201304138

  76. [76]

      T.P. Nguyen, D.M.T. Nguyen, D.L. Tran, et al., Mol. Catal. 486 (2020) 110850. doi: 10.1016/j.mcat.2020.110850

  77. [77]

      Z.Y. You, Y.L. Liao, X. Li, et al., Nanoscale 13 (2021) 9463–9504. doi: 10.1039/D1NR02224E

  78. [78]

      B. Yu, D.J. Chen, Z.G. Wang, et al., Chem. Eng. J. 399 (2020) 125837. doi: 10.1016/j.cej.2020.125837

  79. [79]

      M. Khazaei, M. Arai, T. Sasaki, et al., Phys. Rev. B 92 (2015) 075411. doi: 10.1103/PhysRevB.92.075411

  80. [80]

      Q.Q. Kong, X.G. An, L. Huang, et al., Front. Phys. 16 (2021) 53506. doi: 10.1007/s11467-021-1066-9

  81. [81]

      A.S. Sharbirin, S. Akhtar, J. Kim, Opto-Electron. Adv. 4 (2021) 200077. doi: 10.29026/oea.2021.200077

  82. [82]

      J.Y. Sui, X.F. Chen, Y. Li, et al., RSC. Adv. 11 (2021) 16065–16082. doi: 10.1039/D0RA10018H

  83. [83]

      Y.L. Qin, Z.Q. Wang, N.Y. Liu, et al., Nanoscale 10 (2018) 14000–14004. doi: 10.1039/C8NR03903H

  84. [84]

      S. Iravani, R.S. Varma, Nanomaterials 12 (2022) 1200. doi: 10.3390/nano12071200

  85. [85]

      X. Du, T.Y. Zhao, Z.Y. Xiu, et al., Appl. Mater. Today 20 (2020) 100719. doi: 10.1016/j.apmt.2020.100719

  86. [86]

      W.Y. Lei, T. Zhou, X. Pang, et al., J. Mater. Sci. Technol. 114 (2022) 143–164. doi: 10.1016/j.jmst.2021.10.029

  87. [87]

      L.Q. Yang, Z.H. Chen, X. Wang, et al., Nanomaterials 12 (2022) 542. doi: 10.3390/nano12030542

  88. [88]

      G.M. Dong, Y.W. Zhang, Y.Y. Wang, et al., ACS Appl. Energy Mater. 4 (2021) 14342–14351. doi: 10.1021/acsaem.1c03019

  89. [89]

      Z.Y. Wu, C.R. Li, Z. Li, et al., ACS Nano 15 (2021) 5696–5705. doi: 10.1021/acsnano.1c00990

  90. [90]

      F. Fang, Y.X. Liu, X. Sun, et al., Appl. Surf. Sci. 564 (2021) 150407. doi: 10.1016/j.apsusc.2021.150407

  91. [91]

      N.Q. Zhang, X.X. Zhang, Y.K. Kang, et al., Angew. Chem. Int. Ed. 60 (2021) 13500–13505. doi: 10.1002/anie.202103427

  92. [92]

      S.Y. Yu, N.J. Yang, S.T. Liu, et al., Carbon 175 (2021) 440–453. doi: 10.1016/j.carbon.2021.01.116

  93. [93]

      X.D. Li, S.M. Wang, L. Li, et al., J. Am. Chem. Soc. 142 (2020) 9567–9581.

  94. [94]

      S.C. Shit, I. Shown, R. Paul, et al., Nanoscale 12 (2020) 23301–23332. doi: 10.1039/D0NR05884J

  95. [95]

      F. He, B.C. Zhu, B. Cheng, et al., Appl. Catal. B 272 (2020) 119006. doi: 10.1016/j.apcatb.2020.119006

  96. [96]

      X.Z. Chen, N. Li, Z.Z. Kong, et al., Mater. Horiz. 5 (2018) 9–27. doi: 10.1039/C7MH00557A

  97. [97]

      L.X. Wang, Y. Xia, J.G. Yu, Chem 7 (2021) 1983–1985. doi: 10.1016/j.chempr.2021.07.009

  98. [98]

      J.Z. Qin, B.J. Liu, K.H. Lam, et al., ACS Sustain. Chem. Eng. 8 (2020) 17791–17799. doi: 10.1021/acssuschemeng.0c06388

  99. [99]

      B.B. Chang, Y.Z. Guo, H.L. Liu, et al., J. Mater. Chem. A 10 (2022) 3134–3145. doi: 10.1039/D1TA09941H

  100. [100]

       Y.J. Fu, M. Tan, Z.L. Guo, et al., Chem. Eng. J. 452 (2023) 139417. doi: 10.1016/j.cej.2022.139417

  101. [101]

       H. Du, N.Y. Li, L.X. Yang, et al., Sep. Purif. Technol. 304 (2023) 122204. doi: 10.1016/j.seppur.2022.122204

  102. [102]

       C. Lai, Z.W. An, H. Yi, et al., J. Colloid Interface Sci. 600 (2021) 161–173. doi: 10.1016/j.jcis.2021.05.022

  103. [103]

       Z.T. Yuan, H.S. Huang, N.J. Li, et al., J. Hazard. Mater. 409 (2021) 125027. doi: 10.1016/j.jhazmat.2020.125027

  104. [104]

       H.M. Wang, R. Zhao, H.X. Hu, et al., ACS Appl. Mater. Interfaces 12 (2020) 40176–40185. doi: 10.1021/acsami.0c01013

  105. [105]

       S.F. Lin, N. Zhang, F.C. Wang, et al., ACS Sustain. Chem. Eng. 9 (2020) 481–488.

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  Figure 1  (A) MQDs preparation diagram. Reproduced with permission [68]. Copyright 2022, Royal Society of Chemistry. (B) Number of MQDs published in journals. (Source: Web of Science, Keyword: MXene quantum dots and photocatalysis, Retrieval Date: December 30, 2022). (C) Overview of the synthesis, performance, and application of MQDs in the field of photocatalysis.

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  Figure 2  (A) Schematic illustration of the hydrothermal synthesis of Nb₂C QDs. Reproduced with permission [46]. Copyright 2020, Royal Society of Chemistry. (B) Schematic diagram of the solvent thermal method for preparing MQDs. Reproduced with permission [50]. Copyright 2019, Royal Society of Chemistry. (C) Schematic diagram of the ultrasonic–solvent thermal method for preparing MQDs. Reproduced with permission [52]. Copyright 2021, Springer. (D) Schematic illustration of preparing MQDs by ultrasound method. Reproduced with permission [53]. Copyright 2017, Royal Society of Chemistry. (E) Schematic illustration of preparing MQDs by ultrasound–assisted liquid exfoliation method. Reproduced with permission [54]. Copyright 2018, Multidisciplinary Digital Publishing Institute. (F) Schematic illustration of preparing MQDs by electrochemical etching method. Reproduced with permission [58]. Copyright 2021, Elsevier. (G) Schematic illustration of preparing MQDs by intercalation method. Reproduced with permission [59]. Copyright 2022, Elsevier.

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  Figure 3  (A) Particle size distribution of three types of MQDs, (B) PL emission spectra. Reproduced with permission [71]. Copyright 2020, Elsevier. (C) WF of different MXenes with different terminations. Reproduced with permission [79]. Copyright 2015, American Physical Society. (D) Schematic diagram of the quenching ECL pathway. Reproduced with permission [83]. Copyright 2018, Royal Society of Chemistry.

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  Figure 4  (A) Comparative study of photocatalytic hydrogen production performance of different samples under simulated solar light irradiation. Reproduced with permission [87]. Copyright 2022, Multidisciplinary Digital Publishing Institute. (B) corresponding photocatalytic hydrogen evolution rate. Reproduced with permission [88]. Copyright 2021, American Chemical Society. (C) Energy band structure of TiO₂, C₃N₄ and T₃C₂ QDs, (D) Transient photocurrent response of T, CN, T–CN, and T–CN–TC, (E) The photocatalytic CO and CH₄ production rates of different samples. Reproduced with permission [95]. Copyright 2020, Elsevier.

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  Figure 5  (A) C K–edge XANES spectra of Ni–MOF and Ni–MOF with 9 mL Ti₃C₂ QDs added to the supernatant. Reproduced with permission [98]. Copyright 2020, American Chemical Society. (B) Ti 2p XPS spectra of Ti₃C₂ MXenes and r–Ti₃C₂ QDs, (C) EPR spectra. Reproduced with permission [99]. Copyright 2022, Royal Society of Chemistry. (D) Kinetics of photocatalytic reduction of Cr(VI) using different materials, (E) Kinetics of photocatalytic degradation of BPA. Reproduced with permission [103]. Copyright 2021, Elsevier. (F) UV–vis diffuse reflectance spectra. Reproduced with permission [68]. Copyright 2022, Elsevier.

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