Quantum Dots Compete at the Acme of MXene Family for the Optimal Catalysis

Highlights All the synthesis routes and surfaced-modified strategy of MXene-derived quantum dots (MQDs), the synthesis of MQDs-based nanocomposites, and advanced characterization techniques of MQDs are fully covered. Catalytic application is classified and discussed by judging the roles of MQDs. Current challenge and prospect are proposed for promoting the development and catalytic application of MQDs.


Introduction
In 1836, the terms of catalyst and catalysis were firstly defined by the Swedish scientist Jöns Jakob Berzelius (Fig. 1a), describing "the catalyst was a new matter that can produce chemical activity," and Pt catalyzing the conversion of ethanol to acetic acid witnessed this magical discovery [1,2]. Thereafter, considerable various catalytic reactions were applied to an industrial production, largely promoting the development of chemistry and human society. Until 1910 [3], synthesis of ammonia promotes the development of agricultural cultivation, fuel production, and industrial manufacture, and thereof becomes a landmark in the history and development of catalytic technologies [4,5]. Furthermore, converting water, carbon monoxide (CO), and carbon dioxide (CO 2 ) electrocatalytically to clean fuels (e.g., H 2 , CH 3 OH) for replacing the limited fossil fuels is an important production route to confront the energy crisis. Currently, the catalytic reactions are mainly classified into electrocatalysis, photocatalysis, and photoelectrochemical reaction in dependence of various external energy devices.
Usually, MXenes can be prepared by selecting removal of "A" layers of MAX phases or with the similar compositions, and the forces include either mechanical or chemical exfoliations [50]. However, the wet-chemical etching method is most of the facile and high-production yield processes. Bulk MAX is referred to as a hexagonal layered ternary transition metal carbide, nitride, or carbonitride, where M is an early transition metal, A is a group IIIA or IVA element, X is C or/ and N, which can be described by the formula as M n+1 AX n (n = 1, 2 or 3). The stronger M-X bond is a mixture of covalent, metallic, and ionic ones, but the M-A metallic bond is weaker. Therefore, the M n+1 X n T x is usually prepared by etching "A" layer of the specific solvent, such as HF, NH 4 HF, and HCl/LiF [51].
Moreover, preparing fluorine-free (F-free) MXenes has attracted serious concern for meeting the requirements of specific functions and avoiding the corrosive reagents. [52][53][54] After etching, MXenes nanosheets were obtained by using intercalation agent such as dimethyl sulfoxide (DMSO), tetrabutylammonium hydroxide (TBAOH), tetramethylammonium hydroxide (TMAOH), alcohols, choline hydroxide, or n-butylamine through centrifugation or sonication method. However, when the lateral size of MXenes nanosheets is further reduced to nanometer size that is smaller than Bohr radius of the exciton (lateral size < 10 nm), shows strong photoluminescence, called zero-dimensional semiconductor nanomaterials-MXenes QDs (MQDs), which is an emerging branch of QDs ( Fig. 2) [55]. In 2017 [56], Wang et al. firstly reported the Ti 3 C 2 fluorescent ultrasmall monolayers MXene sheets by concurrent intralayer cutting and interlayer delamination. The method was also extended to prepare Ti 2 C and Nb 2 C ultrasmall sheets. The Ti 3 C 2 ultrasmall sheet has the lateral dimension of 2-8 nm and the average thickness of 1 nm. Furthermore, similar to the carbon dots, Ti 3 C 2 monolayers sheets showed the strong and tunable photoluminescence and excitation-dependent behavior with the change of pH. Although they were not defined as Ti 3 C 2 QDs, both their size and the fluorescence behavior are typical of QDs. MXene-derived MQDs not only inherit the merits of low toxicity, heavy metal-free ones, natural hydrophilicity, metallic conductivity, flexibility, and abundant active catalytic sites of the parental MXene [40, Fig. 2 The classification of quantum dots 57], but also afford the characteristics likes other QDs, such as best dispersibility, unique photoluminescence (PL) properties, quantum confinement effects, and small-size effects. Such diverse properties expand their applications for energy storage, catalysis, optoelectronic device, environmental monitoring, biomedical, and sensors.
To date, the publications of MQDs have been increasing dramatically, and the focused fields include optoelectronic device, sensors, catalysis, energy storage, and biomedical applications. In this review, we highlight systematically the research status of MQDs on catalysis, rather than covering all the respects of other promising application. Also, the research progress of MQDs, ranging from their synthesis and modification to advanced characterization techniques are summarized. Finally, their perspectives in catalytic field are discussed briefly (Fig. 3a). Expectedly, we hope this review will contribute to guide a rational design of highperformance MQDs-based catalysts in catalytic applications.

Development of MQDs
Since the MXene was discovered in 2011 by Gogotsi [7], achieving the soaring development from 3D layered bulk materials to 2D nanosheets, 1D nanowires, 0D QDs. Meanwhile, physical and chemical properties of materials change with the decrease in the lateral size, which can effectively enrich the surface areas, increase amounts of active sites that are particularly relevant to the homogeneous-heterogeneous catalysis. Especially, as the size of materials is reduced and smaller than its exciton Bohr radius, they show strong photoluminescence, endowing small-size effect and quantum confinement effect (Fig. 1b). However, the shortcoming is obvious since the required harsh synthesis condition and accompanied high surface energy leads to easy agglomeration. The photoluminescent Ti 3 C 2 MQDs first synthesized by a facile hydrothermal method for imaging in 2017. The average size of MQDs can be regulated by controlling the reaction temperature. However, the product is strictly dependent on the reaction conditions, when the reaction temperature surpasses 100 °C, the hybrid structures are obtained and even phase transition may occur [58,59]. Afterward, some researches involve in synthesis of the MQDs, but an increasing number of efforts were devoted toward preparing the multifunctional MQDs. We summarized the number of publications and application fields about MQDs. As described in Fig. 3b, over the past five years, the number of articles about MQDs has an obvious increase, and the application covers catalysis, energy storage, sensors, biomedical, and optoelectronic devices, especially in sensors, catalysis, and biomedical applications. Until now, the research enthusiasm of MQDs continues growing, and the application in the field of catalysis gradually becomes a hot topic.
Timeline recording the development of MQDs is listed for the fields of photocatalysis, electrocatalysis, and photoelectrochemical (Fig. 4). However, other applications such as electrocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) require to be mechanistically explored, and various catalysts of MQDs such as Ti 2 C, V 2 N, and Mo 2 C worth to be thereof prepared. For instance, the HER activities of 2D Ti 2 C, Ti 3 C 2 , Nb 2 C, Nb 3 C 4 , and V 2 C MXene with O* or/and OH* terminals calculated by using density functional theory [60]. Their result shows that Ti 2 CO 2 MXene has optimized Gibbs free energy of hydrogen adsorption (ΔG H* ), which is regarded as an ideal candidate of electrocatalysts. Furthermore, some literatures report that Ti 2 CT x MXene catalyst has achieved excellent HER performance under acidic conditions, whereas 570 mV@-10 mA cm −2 was afforded for the HER activity affords under alkaline conditions [54,61,62]. Additionally, the excellent HER activity of MQDs is also beneficial to achieve outstanding catalytic dehydrogenation ability, as most studies show that the strong H adsorption ability of MXene leads to easier dehydrogenation [63][64][65]. Anyhow, the ongoing efforts are needed toward the ultimate ideal commercial alkaline water splitting electrocatalyst. Thus, it is very necessary to optimize the performance of Ti 2 C MXene catalyst under alkaline conditions, and exploring the HER performance of 0D Ti 2 C MQDs or other analogues is worthy for pondering in the future.

Preparation of MQDs
Over the past decade, various methods have been adopted to synthesize two-dimensional inorganic derivatives such as graphene [80], phosphorene [81], 2D layered carbides-based QDs [82], and TMOs-based QDs [83]. Due to the similar layer structures (the strong covalent or ionic bonds in layers, the weak van der Waals forces in interlayers), the synthesis of 2D MXene-derived QDs is quite similar to other inorganic QDs. Up to now, there are different types of MQDs that have been prepared by top-down synthesis methods, such as Ti 3 C 2 QDs, V 2 C QDs, Nb 2 C QDs, Ti 2 N QDs, TiCN QDs, MXene-derived Ti n O 2n-1 and TiO 2 /C-QDs [84][85][86][87][88][89][90]. Furthermore, various surface modifications were used to improve the properties for further catalytic applications. A summary on the synthesis is classified into pure MQDs, and MQDs with surface modifications in the following.

Synthesis of Pure MQDs
Generally, the synthesis of MQDs consists of two steps, including the chemical exfoliations of 2D MXenes from 3D bulk MAX for the first step, or the homogeneous 2D MXene can be obtained by bottom-up route such as chemical vapor deposition (CVD) growth [91,92]. The next step is the preparation of MQDs mainly by top-down methods. Figure 5a shows all the synthesis methods in the current reports. Hydrothermal method with low energy consumption is regarded as the most common approach (Fig. 5b), and thereof MQDs have the advantages of morphology, size control, high crystallinity, high yield, etc. In the process, the formation mechanism assisted with high temperature and pressure enables 2D MXenes for easy cracking and assembling. The base or acid as medium with the controlled pH value 6-9, which reacts with metal hydroxides, accelerating the reaction process and promoting the formation of QDs [93][94][95]. Simultaneously, the inert gas such as argon (Ar) must be introduced into the reactors for avoiding oxidation of MQDs. However, considering the surface of MXenes covered by oxygen-containing groups (-OH, -O), higher reaction temperature or longer reaction time can lead to surface oxidation due to the dissolution of oxygen groups. Xue et al. [58] pointed the connection between reaction temperature and final product; when it is above 150 °C, the MXenederived carbon quantum dots (CQDs) was formed due to the dissolution of metallic Ti. Also, the crystallinity of MQDs decreases with the increasing temperature ( Fig. 5c-h), confirmed by X-ray diffraction (XRD) (Fig. 5i).
The mechanism of solvothermal reaction is the same as the aforementioned technique, but the solvent is organic, such as N, N-Dimethylformamide (DMF), Dimethyl sulfoxide (DMSO), and ethanol. Therefore, the formation of MQDs is related to the boiling point, and oxidation ability of solvents. The Ti 3 C 2 MQDs was prepared by using different organic solvent, showing that MQDs prepared by DMSO have a reduced quantum yield and photoluminescence properties, attributed to high boiling point and oxidation ability of DMSO ( Fig. 5j-k) [96]. Moreover, as the reaction temperature determines the size of MQDs due to the solubility difference of MXene in the solvent, MQDs usually show excellent solubility in both the water and ethanol [97]. Also, the strong quantum confinement of MQDs induced by size effect further affects photoluminescence (PL) behavior. Similar to other QDs, the PL undergoes blue shifts as the size of MQDs decrease, which is one of the viewpoints in the mechanism of luminescence. However, the products of MQDs vary with the reaction temperature. As a consequence, it results in different surface composition, further affecting fluorescence behavior. Currently, it remains a challenge for clarification of the fluorescence mechanisms of MQDs [98].
Moreover, the mechanics-assisted methods (e.g., sonic tip, bath sonication, and agitation) have become alternative to the hydrothermal or solvothermal methods. The process can bypass the necessity of the high temperature and high pressure, but an inert gas protection is required. The mechanism of mechanics-assisted preparation relies on the layer cutting and stacking cleavage. MXene materials are sensitive to surface functional groups. The functional groups (e.g., -O, -OH, -F, and -Cl) cannot avoid being introduced into the surface due to the intrinsic liquid-phase exfoliation process [99][100][101]. Many reports have confirmed the type of groups impact on the electrochemical performance of the MXene-based materials [40,102,103]. Furthermore, it is important for synthesis F-free MXenes, which contributes to improve the electrochemical activity [104]. Jang et al. prepared oxygen-functionalized MXenes by alkalized and heat process for hydrogen evolution reaction (HER), and the result is consistent with the previous calculation, oxygen sites as catalytic active sites provide ideal Gibbs free energy for hydrogen adsorption (△G H* ) [105,106]. Compared to 2D MXenes, the F-free 0D MQDs have the same property. The Ti 3 C 2 (OH) 2 MQDs with hydroxyl groups modification prepared by the alkalization treatment and mechanical agitation method for electrochemical N 2 reduction [70]. The experiment combines with computational findings confirmed that the -OH functional groups and abundant Ti edges contributed to the obtained outstand ammonia production performance. Such method expects to be extended to a wide range of MQDs-based catalytic systems.
However, the probe sonication depends on high power probe to break MXene nanosheet into small-sized MQDs (Fig. 6a) [108]. The probe of sonic tip is selective to the size and hardness of raw MXene materials. Thus, it is important for establishing the correlation to prepare our  [107]. Copyright @2021, American Chemical Society. c-h Morphology of MQDs at different reaction temperature of 100, 120, and 150 °C. The data was obtained by transmission electron microscopy; i XRD characterization of MQDs [58]. Copyright @2017, WILEY-VCH. j Scheme of synthesis MQDs at different solvents of DMSO, DMF, and ethanol; k XRD characterization of MQDs [96]. Copyright @2018, WILEY-VCH 1 3 expected MQDs. The discipline is yet to be explored. In addition, the bath sonication needs the protection of low temperature for preventing surface oxidation caused by overheating (Fig. 6b) [109]. The method of bath sonication is time-consuming. Therefore, the choice of the appropriate intercalation solvent affects the subsequent preparation, and the parameters of power and time affect the production yield and size of MQDs. Furthermore, the size and concentration of MQDs are also related to the amount of solution and final centrifugation speed due to the quality difference between MXene nanosheet and MQDs [110]. Sometimes, it is more convenient and safer to prepare MXene QDs by F-free probe sonication combined with bath sonication (Fig. 6c). Mechanical stirring has attracted extensive attention due to the advantages of simple and low cost (Fig. 6d) [111]. There are merely three articles that reported the method so far. In addition, there are some methods for the synthesis of MQDs. For example, the Ti 3 C 2 MQDs have been prepared through reflux [112], ball-milling, and microwave-assisted method [113,114]. The emerging technologies such as ultrafast shaped laser [115], micro-explosion [87], potential static and acoustomicrofluidic method remain in an exploratory stage [116,117].
Generally, the preparation of QDs is either top-down or bottom-up. Although the former has been wieldy used, the disadvantage of the complicated synthetic process, time-consuming and low yield requires to develop highly efficient methods. Compared to the top-down, the main synthesis mechanism of the latter lies at the cross-linking and polymerization through small molecules, thereof leading to controllable structure, size, composition, and morphology of QDs. So, the atomic utilization is maximized, thus obtaining the desired type of QDs [118]. However, there are seldom reports that MQDs were prepared by using bottom-up methods, mainly due to the issue that MQDs must simultaneously satisfy two points: (1) inheriting the structure of MXenes; (2) holding the physicochemical properties of QDs. It is worth mentioning that the composites of both Mo 2 C QDs/carbon nanosheets and Mo 2 C QDs/carbon polyhedron were prepared via bottom-up style, i.e., molten salt method and pyrolysis method, respectively [119,120]. Although they are not clearly defined as the MQDs, such simple, lowcost, and high-yield method is expected to a successful preparation of MQDs.
Currently, the production yield of MQDs is rarely referred, and the reported technique applied for further improving the yield remains a challenge. In addition, there is a key issue that the process of preparation produces small amount of metal oxidation in the surface of MQDs. Therefore, more efforts will be made and explored for preparing MQDs of the high purity.

Synthesis of Surface-modified MQDs
MQDs inherit abundant surface functional groups of the MXenes, including oxygen (-O), hydroxyl (-OH), chlorine (-Cl), or fluorine (-F) [121]. Gogotsi group reported the Ti 3 C 2 MXene containing the aforementioned groups was synthesized in water solution, which shows the ζ-potential of about −40 mV [90], indicating the groups are negatively charged. Therefore, various of organic/inorganic molecules, ions, and atoms were used as surface modification/functionalization [122][123][124][125] of the MQDs through the electrostatic interaction or physical adsorption to improve the stability, selectivity, conductivity, quantum yield, and photoluminescent properties [126][127][128]. Furthermore, 2D MXenes possess the excellent flexible, natural hydrophilicity, and the MQDs with the same structure are easily combined with other functional materials to form composites, producing a heterogeneous material by integrating their advantages. Also, MQDs possess strong quantum confine effect compared to 2D MXenes, and the MQDs can be as co-catalyst to control the energy band structure. Based on the different modifiers, the synthesis of surfaced-modified MQDs is summarized in Table 1. However, there are a few articles to address the surface chemical of MQDs on the catalytic research, so we only emphasize the common synthesis methods.

MQDs Modified by Single/Dual Heteroatoms
MQDs have been applied to biomedical [129,130], optical device [131][132][133], energy storage [134][135][136][137][138], and sensor Schematic illustration for the synthesis of MQDs by using different methods. a Probe ultrasound [108]. Copyright @2020, Wiley-VCH. b Bath sonication [109]. Copyright @2020, American Chemical Society. c A combination of probe sonication and bath sonication [110]. Copyright @2017, The Royal Society of Chemistry. d Mechanical stirring method [111]. Copyright @2021, Wiley-VCH ◂ fields [112,139,140] due to that they possess the advantages of non-toxicity, metal conductivity, excellent chemical stability and low cost. Currently, the research of pure MQDs cannot meet the development needs of practical application. The surface-modified MQDs by heteroatoms, still in its infancy stage, have intrigued great research enthusiasm. It can be seen from Table 1 that such heteroatoms are almost all non-metallic such as nitrogen (N) [139,141,142], phosphorus (P) [114], sulfur (S) [133] and chlorine (Cl) [117,143]. The common synthesis methods are hydrothermal. However, the improvement of properties of MQDs modified by using metal atoms is just on the beginning.
Generally, the strong electronegativity of non-metallic atoms is beneficial to passivate the active sites of MQDs, leading to the change of electronic structure, and thereof producing surface defects. Thus, some obvious changes will occur for the physicochemical properties of MQDs [86]. As shown in Fig. 7a, Guan et al. prepared N, P-doped Ti 3 C 2 MQDs with green fluorescence and size of 2.93 nm by hydrothermal method, and density functional theory (DFT) calculation reveals the electron transfer from P to N facilitates to improve the fluorescence (Fig. 7b-d). Compared to pure MQDs and single-atom-doped MQDs, the favorable electron transfer can enhance the photoluminescence quantum yield (PLQY) of 20.1% [144].
Likewise, The PLQY is an important parameter for judging the performance of QDs in the fields of biological, sensing, and optoelectronic devices. The S, N-Nb 2 C MQDs with an average size of 2.66 nm was synthesized, enhancing the QY of Nb 2 C and the stability by optimizing the PL properties. It is known the PL properties of QDs are related to size, surface composition, and pH. The nonmetal doping often occurs at carbon sites with a larger shrinkage of the defect-induced bond of MQDs, leading to a variety of fluorescence [130]. In addition, in 2019, the S, N-doped Ti 3 C 2 MQDs prepared by hydrothermal method (Fig. 7e), achieving the multiple-color emissive from blue to orange light ( Fig. 7f-h) [133], which will benefit for the mankind in the field of energy storage, photocatalysis, medicine and biology. MXenes are easily oxidized due to the dissolved oxygen and oxygen-contain groups, especially in the high temperature and pressure condition [58]. Therefore, this heteroatoms modification can also enhance the antioxidant capacity of MQDs. For example, the ethylenediamine (EDA) was introduced into the surface of MQDs as the additive, forming the surface electron-rich N-Ti 3 C 2 MQDs (Fig. 7i) [145]. The method not only avoids the surface oxidation of MQDs, and retains the intrinsic structure of MXenes, but also enhances the antioxidant ability, enabling N-Ti 3 C 2 MQDs as effective reductants (Fig. 7j-k). Apart from the above-mentioned issue, S, N co-doped Ti 3 C 2 MQDs [133], Cl, N co-doped Ti 3 C 2 MQDs [117], and N, B co-doped Ti 3 C 2 MQDs [142] have been prepared by using the same method to improve their physical-chemical properties. Furthermore, the Ti 3 C 2 MQDs modified by metal atom also contribute to enhance energy transfer process, leading to enhance the sensitive of detector [146].
Apart from non-metallic elements as the dopants to control the functional application, the metal atom modification is helpful to adjust the energy level structure of MQDs, thereby achieving highly catalytic active sites. Tang et al. synthesized Co-Ti 3 C 2 MQDs with a Janus-structured style by using Co ion thermal-anchoring reaction and ammoniaassisted hydrothermal method [72]. The introduction of Co constructs the Schottky junction, produces the rectifying effect, promoting effectively the photogenerated carrier separation/injection efficiency. It shows the excellent photoelectrochemical water oxidation capability.
Although the doped MQDs have made great progress, the application prospect in the catalytic field is still unknown, whether the MQDs make the breakthroughs like other inorganic QDs (e.g., CQDs, GQDs, and MoS 2 QDs) in the future is something worth investigating. Except for such finding, whether the modification of organic molecules also brings the considerable improvement of properties?

MQDs Modified by Organic Molecules
Organic molecules have been applied to modify the surface of QDs to improve the fluorescence responses in aqueous solution, enhanced the application in biological and optical fields [166][167][168]. However, such molecules modified MQDs have little application in catalysis. Compared to the nonmetal and metals, organic molecules have the advantages of low toxicity, low cost, easily biodegradability and better biocompatibility and so on. It was reported that such molecules are usually adsorbed to the surface of MQDs by physical absorptions or electrostatic interactions, contributing to improve the compatibility of MQDs, leading to the enhancement of dispersion, mechanical, and fire retarded  [67] properties [169,170]. For instance, the glutathione functionalized Ti 3 C 2 MQDs prepared by hydrothermal method, the MQDs of surface passivated by glutathione show outstanding fluorescence stability regardless of any pH value or time, which is a promising fluorescence probe [152]. Such surface modification facilitates to the stabilization of surface energy traps, leading to surface state luminescence with excitation independence [84]. Furthermore, MQDs have excellent stability of the PL intensity at different pH values, so MQDsbased nanomaterials are an ideal sensor [58]. Additionally, uric acid (UA) was used as ligand to enhance photophysical property. Wang et al. [112] prepared UA@Ti 3 C 2 MQDs by facile microwave-assisted strategy. 2D Ti 3 C 2 MXene was broken into 0D MQDs based on the acid etching and the high power. The UA as reaction solvent forming the large molecules that encase Ti 3 C 2 MQDs. The method is not only easily operation, but also enhanced the oxidation resistance and highly quantum yield of MQDs.  [133]. Copyright @2019, Elsevier. i Schematic illustration of the synthesis of N-Ti 2 C MQDs; j Antioxidants performance test at KMnO 4 solutions; k Mechanism of antioxidants [145]. Copyright @2021, American Chemical Society Apart from the above-mentioned molecules, the 2,3-diaminophenazine (DAP), ε-Poly-L-lysine (PLL), polyvinylpyrrolidone (PVP), and bovine serum albumin (BSA) were used to synthesis functionalized MQDs, thereby promoting the MQD in application to biomedical and physical fields [151,[153][154][155]. Furthermore, MQDs possess abundant hydrophilic functional groups. To avoid the self-aggregation of MQDs, improving the stability during the synthesis and reaction, the organic molecules were introduced onto the surface of MQDs, as possibly an effective method to increase the yields.

MQDs-based Heterostructures
Like single atoms and other 2D inorganic QDs, the MQDs with small-size affect easily the aggregation due to their high surface energy during synthesis and reaction process [96,171]. For catalysts, we not only pursuit excellent conductivity, low cost, environmentally friendly, and outstanding performance, but also the durability of operation. Constructing the MQDs/support heterostructure is an effective strategy. Such hierarchical heterostructures contributes to adjusting the band structure, achieving the excellent Catalytic activity. We will introduce the routine synthesis routes of heterostructure between 0D MQDs and different dimension support in the following section.

0D MQDs/0D Nanomaterials
The electronic coupling at the interface is essential for regulating the electronic structure and producing efficient charges transfer, which contributes to an improved electrochemical reaction process and device performance [172,173]. 0D nanomaterials has the lateral size range of 0.1 ~ 100 nm. Currently, there are few reports on the composite of MQD with other 0D nanomaterials, and such improving physical chemical properties is expected to be further explored. In 2020, the CsPbBr 3 QDs-Ti 3 C 2 T x MQD heterostructure was constructed by facile ultra-sonicating method (Fig. 8a) [156]. XRD of CPB-MXene QD/QDs composites retain the crystal structure of CsPbBr 3 QDs and MQDs, and no impurity phase was found (Fig. 8b). The morphology of nanocomposite is shown in Fig. 8c, and the corresponding high-resolution transmission electron microscopy (HRTEM) image is displayed in Fig. 8d. The local magnification of "U" and "V" represent the lattice fringes of CsPbBr 3 QDs and MQDs, respectively. The interface (orange dashed line) relies on the strong interaction between the functional groups on the surface of MQDs and Cs + , causing the photoluminescence (PL) quenching due to the charge transfer from Cs + to MQDs. However, when Cs + was introduced into the heterostructure again, the PL will recovery. Therefore, the 0D/0D heterostructure is expected to apply to ion detection and photodetector.
Currently, there are reports for the introduction of single atoms (SAs), nanoparticles into inorganic QDs such as cadmium-zinc sulfide quantum dots (ZCS QDs), carbon quantum dots (CQDs), graphene quantum dots (GQDs), and so on [174][175][176][177]. Compared to the bulk support, the exposure of specific crystalline planes can be precisely controlled, contributing to the synergistic effect between the SAs and coordinating elements, and the coordination environment of SAs can be regulated for increasing the selectivity of products [178]. Interestingly, MQDs possess the same properties as other inorganic QDs, but were endowed with the abundant surface groups (-OH, -O, -Cl, or -F). Therefore, MQDs are a promising support catalyst. The Ti 3 C 2 MQDs coated Ni nanoflowers were synthesized by using facile reduction reaction for wastewater treatment (Fig. 8e), the transmission electron microscopy (TEM) and HRTEM images confirmed the core-shell structure (the core: Ni flowers, the shell: MQDs), and the corresponding lattice fringes ( Fig. 8f-g) [77]. The elemental mapping shows that the Ti/C/O/Ni were uniformly distributed in the surface of nanocomposites (Fig. 8h). Such interfacial interaction not only avoids any aggregation of Ni nanoparticles, but also lowers the catalytic reaction activation energy of Cr (VI). It is of great interests to extend 0D/0D heterostructures to other catalytic fields.

0D
MQDs/1D Nanomaterials 1D nanomaterials allow electrons-dominating transfer, mainly including nanotubes, nanowires, nanorods, and nanobelts [10,179]. Transition metal oxides are commonly used as ideal photocatalyst due to their adequate optic bandgap. However, the regulation of bandgap facilitates to hinder the photogenerated carriers' recombination, achieving an efficient surface redox reaction. Zeng et al. [67] prepared Ti 3 C 2 MQDs/Cu 2 O nanowire composite by using electrostatic self-assembly strategy for highly efficient photocatalytic CO 2 conversion (Fig. 9a). TEM image confirmed the MQDs dispersed in the surface of Cu 2 O nanowires, and the corresponding HRTEM characterization confirmed the formation of heterogeneous interface (Fig. 9b-c), and 0.216 nm and 0.219 nm of lattice fringes are attributed to Ti 3 C 2 MQDs (0110) and Cu 2 O (200) , respectively. In addition, the energy dispersive X-ray (EDX) spectra displayed the Ti/C/O/Cu dispersed uniformly in the surface of Cu 2 O (Fig. 9d-g). Combined the DFT and experiment shows that the MQDs as co-catalyst to promote the separation of carriers and decrease the band bending edge, enhancing the light adsorption capability and the transport of carriers of Cu 2 O. Furthermore, the 1D nanowires not only provide long light adsorption path and short charge transport distance, but also enable quickly collecting the separated photogenerated carriers.
Likewise, the WO 3 /Ti 3 C 2 QDs/In 2 S 3 with Z-scheme heterostructure was fabricated by using facile solution method (Fig. 9h). TEM and HRTEM images confirmed that the In 2 S 3 nanosheet and MQDs with an average size of 1.66 ± 0.04 nm were uniformly dispersed on surface of WO 3 nanorods (Fig. 9i), and the WO 3 was coated by In 2 S 3 nanosheets (Fig. 9k), corresponding to the interface between the Ti 3 C 2 MQDs and WO 3 (or WO 3 and In 2 S 3 ) shown in Fig. 9j (Fig. 9l). This report shows that MQDs is an ideal co-catalyst to promote the separation of the photogeneration carriers, achieving efficient Cr (VI) reduction and photocatalytic oxidation of the BPA [74]. In addition, the MQDs were used as co-catalyst to promote photocatalytic water splitting due to their broader photoresponse and excellent conductivity. In 2021 [157], the Au nanorods/Ti 3 C 2 MQDs heterostructure was prepared via electrostatic interaction (Fig. 9m). TEM image showing the plasmonic gold nanorods (NRs) were distributed in the Ti 3 C 2 MXene QDs-interspersed Ti 3 C 2 nanosheet (TDTS) (Fig. 9n), and the corresponding  [156]. Copyright @2020, American Chemical Society. e Schematic of the formation of Ni@Ti 3 C 2 MQDs; f-g TEM image of Ni@Ti 3 C 2 MQDs. insets of A and B represent HRTEM image of Ni and MQDs, respectively; h EDS of Ni@ Ti 3 C 2 MQDs [77]. Copyright @2022, Elsevier HRTEM image of Au (200) is displayed in the inset of Fig. 9n. In addition, the Ti 3 C 2 MQDs @Au nanobones were also prepared by using the seed-mediated growth method and self-assembly for exploring the improving performance in the biomedical application.

0D MQDs/2D Nanomaterials Compared to 0D
and 1D support materials, 2D nanomaterials are referred to as that the electrons motion is unrestricted in two directions, which has lager planar size. As a result, it provides the abundant basal plane that is active with a number of anchored sites [128,180,181]. Currently, many reports address the preparation of 2D few-layer or monolayer nanosheets by using chemical vapor deposition, organic solvent interca-lation, liquid-phase exfoliation strategy and electrospinning, and so on [182][183][184], The introduction of intrinsic defects such as vacancies, lattice distortions and adatoms on the surface of graphene, g-C 3 N 4 and MoS [185][186][187], is beneficial to improve the physical properties of materials such as electronic conductivity. In 2020, the MQDs were used as co-catalyst to enhance the photocatalytic activity of 2D metal-organic framework (MOF). The Ti 3 C 2 MQD/ Ni-MOF catalyst was prepared by self-assembly strategy (Fig. 10a) [69]. SEM image revealed the 2D nanosheet morphology of Ni-MOF (Fig. 10b), and the HRTEM image confirmed the MQDs with an average size of 4.19 nm that were uniformly dispersed on the surface of Ni nanosheet (Fig. 10c). The presence of MQDs helps to enhance the light absorption and interface charge transfer ability, pro-  [67]. Copyright @2019, WILEY-VCH. h Schematic illustration of WO 3 /TQDs/In 2 S 3 heterostructure; i TEM image of WO 3 /TQDs; j HRTEM image of WO 3 /TQDs; k TEM image of WO 3 /TQDs/In 2 S 3 ; l HRTEM image of WO 3 /TQDs/In 2 S 3 [74]. Copyright @2021, Elsevier. m Schematic for the preparation of Au NRs/Ti 3 C 2 MQDs/Ti 3 C 2 nanosheets; n TEM image of Au NRs/TDTS. Inset illustration is HRTEM image of Ti 3 C 2 MQDs [157]. Copyright @2021, Elsevier 1 3 moting an efficient N 2 photoreduction reaction. Besides, the 0D/2D heterostructure is also applied to energy storage. Moreover, the defect-rich MQDs cluster/N-doped carbon nanosheet nanocomposites were prepared by using electrostatically self-assembly for Li-O 2 batteries (Fig. 10d) [137]. TEM image shows the uniform distribution of MQDs in the N-C nanosheets (Fig. 10e). However, as shown in inset of Fig. 10e, the quantum size effect MQDs contributes to the poor crystal quality of MQDs/N-C nanocomposites. This result can be proved by using HRTEM (Fig. 10f). Combining the experiment and DFT reveals the MQDs with abundant grain boundaries and edge defects as the active origins for increasing the adsorption of O 2 molecules and intermediates LiO 2 . Controlling the number of interfaces is of great significance for improving the structure of photocatalyst and enhancing the performance. The 2D/2D/0D (TiO 2 /C 3 N 4 / Ti 3 C 2 MQDs) hierarchical structure was engineered by van der Waals and electrostatic interactions (Fig. 10g) [159]. The TiO 2 /C 3 N 4 exhibits core-shell structure, and the MQDs decorated on the surface of C 3 N 4 nanosheet in three to four layers, and the corresponding HRTEM image is shown in Fig. 10h. Such ultrathin three-phase interface and the introduction of MQDs help to increase the transport channels of charges, providing the abundant photogenerated carries. In addition, g-C 3 N 4 nanosheets [160], nanofilms [138], SiC [71], and graphene [115] 2D nanomaterial have been prepared for loading 0D MQDs, improving the performance in electrocatalysis, photocatalysis, and supercapacitors.  [69]. Copyright @2020, American Chemical Society. d Synthesis process schematic of Ti 3 C 2 QDC/N-C nanocomposites and e TEM image of Ti 3 C 2 QDC/N-C, inset illustration is SAED pattern; f HRTEM image of Ti 3 C 2 QDC/N-C [137]. Copyright @2021, Wiley-VCH. g Schematic preparation of Ti 3 C 2 MQDs/TiO 2 /C 3 N 4 hierarchical structure; h HRTEM image of T-CN-TC heterostructure [159]. Copyright @2020, Elsevier

0D MQDs/3D Nanomaterials
Compared to other dimensional nanomaterials, 3D nanomaterials with porous structure provides the abundant gas diffusion channel and interface sites, and favorable for reactant diffusion direction [188,189]. MQDs have served as co-catalyst to avoid the recombination between the photogenerated electrons and photogenerated holes. Recently, constructing 0D/3D heterostructure has been reported to make MQDs as electron acceptor to promote surface redox reaction. In 2022, the Ti 3 C 2 MQDs with the surfaces detect-rich/3D mesoporous C 3 N 4 were prepared by electrostatically self-assembly strategy (Fig. 11a) [76]. TEM image confirmed that the MQDs were uniformly dispersed on the surface of hollow C 3 N 4 (Fig. 11b), and the HRTEM image gives the corresponding lattice spacing of 0.329 and 0.261 nm, attributed to the plane of C 3 N 4 (002) and Ti 3 C 2 (0110) , respectively, indicating the formation of Schottky junction (Fig. 11c). The XRD analysis for the weak signal of MQDs correlate with the low content (Fig. 11d). Meanwhile, such MQDs-induced Schottky junction catalyst was used to promote the photocatalytic H 2 O 2 production.
Lin et al. [73] prepared the Ti 3 C 2 MQDs decorated defective inverse opal g-C 3 N 4 (TC/CN) by using electrostatic self-assembly method (Fig. 11e). SEM image shows the microstructure of porous g-C 3 N 4 with a long-range order (Fig. 11f), and the corresponding HRTEM confirmed the formation of the interface between MQDs and g-C 3 N 4 (Fig. 11g). Such bonding contributes to achieve the carrier separation. In addition, the Ti 3 C 2 MQDs/ZnIn 2 S 4 /Ti Fig. 11 Schematic and morphological and structure characterizations of 0D/3D heterostructure. a The prepared process diagram of C 3 N 4 /r-Ti 3 C 2 QDs; b TEM image of C 3 N 4 /r-Ti 3 C 2 QDs; c HRTEM image of C 3 N 4 /r-Ti 3 C 2 QDs; d XRD pattern of C 3 N 4 , C 3 N 4 /r-Ti 3 C 2 QDs [76]. Copyright @2022, The Royal Society of Chemistry. e Schematic preparation of Ti 3 C 2 MQDs/3D Inverse Opal g-C 3 N 4 heterojunction; f SEM image of TC/CN-20 after adding 20 mL of MQDs solution and 20 mL water; g HRTEM image of TC/CN-20 [73]. Copyright @2020, American Chemical Society. h Schematic illustration of Ti 3 C 2 -QDs/ZnIn 2 S 4 /Ti(IV) heterostructure; i-k TEM image of Ti 3 C 2 -QDs/ZnIn 2 S 4 /Ti(IV) at different magnifications; l-p Elemental mappings of Ti 3 C 2 -QDs/ZnIn 2 S 4 /Ti(IV) [78]. Copyright @2022, MDPI and the authors 1 3 (IV) 3D hierarchical structure was constructed by impregnation and self-assembly methods (Fig. 11h) [78]. TEM and HRTEM images show that the Ti (IV) and MQDs are uniformly dispersed on the surface of 3D nanoflowers microspheres (Fig. 11i-k). The EDX elemental mapping confirmed the elements were uniformly dispersed in the surface of microsphere (Fig. 11l-p). The nanocomposites were used as co-catalyst to promote long-term stability. In addition, various of MQDs-based heterostructure has been designed such as MQDs/3D bio-aerogels [165], and the TiO 2 /MQDs [163] meet the growing needs of biomedical application, photoelectronic sensor, biosensor, and photocatalysis.
In summary, the properties difference of 0D MQDsbased heterostructure (0D/0D, 0D/1D, 0D/2D, 0D/3D) are mainly rooted from the variation of the support properties with different geometric structures. Whereas the coordination environment between the MQDs and their support is flexible and controllable, independent of the dimension of the support, and it determines the optimal performance to be achieved. Furthermore, the MQDs as catalyst offers abundant catalytic active sites. In order to maximize the utilization of active sites, achieve the fast electron transport channels, and ensure the efficient and stable working of catalysts, the support with different dimensions is often selected to optimize the overall performance. Also, the morphology of catalysts also needs to be considered to meet the application requirement.

Other MXene-Derived Inorganic QDs
According to the previous reports, the MXenes will expose inner carbon layer, or produce a small number of amorphous carbon due to the partial dissolution of external M metal atoms during etching process, which can be proved by using Raman spectrum [7,190,191]. In 2017, Sun et al. [192] prepared F-free Ti 2 CT x via electrochemical etching under the HCl aqueous solution. The result shows that such method enables easily exfoliating Ti layers, producing carbide-derived carbon (CDC), which is related to voltage, etching time, and electrolyte concentration. Furthermore, most of MXenes are sensitive to oxygen atmosphere, facilitating the formation of transition metal oxides in the surface [193,194]. Therefore, many inorganic QDs such as carbon dots (CDs), graphene quantum dots (GQDs), and transition metal oxide QDs can be prepared by using such materialderivatives, which provides facile, safe, and environmentally friendly method to prepare inorganic QDs. In 2020, the TiO 2 QDs supported on the surface of carbon layer were prepared by solvothermal method using small and fewer-layered Ti 3 C 2 MXene nanosheets (Fig. 12a) [88].
The condition of high temperature and high pressure induced the oxidization of MXene surface via the dissolved oxygen in solution. TEM and HRTEM images confirmed the formation of TiO 2 /C-QDs with an average of 5.23 ± 0.3 nm (Fig. 12b-c), and the UV-Vis adsorption spectrum shows that QDs have adsorption peak at 250 nm, and the inset of Fig. 12d corresponds to the optical photo-blue TiO 2 /C-QDs in daylight under 365 nm excitation wavelength. Moreover, the oxygen-vacancy-rich Ti n O 2n−1 QDs (OV-T n QDs) were prepared by H 2 O 2 oxidation and subsequent quenching in liquid nitrogen (Fig. 12e) [90]. The quench process makes TiO 2 nanoparticles fast crystallization and downsized to quantum size. After that, the annealing process with H 2 /Ar mixed gas promotes the generation of O vacancies. TEM image confirmed the uniform distribution of OV-T n QDs (Fig. 12f). The corresponding HRTEM image exhibit that the OV-T n QDs are made up of Ti 2 O 3 and Ti 3 O 5 (Fig. 12g-i).
We all know that the carbon dots are used in solar energy cell, optoelectronic, and biomedical applications due to low cost, environmentally friendly and non-toxic, and excellent biocompatibility. Currently, carbon dots are abundant in raw materials such as carbon nanotube [196], carbon-containing organic molecules [197], and biomass materials [198]. However, 2D MXene-derived CDs are rarely reported. In 2021, the 2D Ti 3 C 2 T x MXene-derived (CDs) was prepared by hydrothermally [199]. Also, the GQDs were reported through controlling the alkalized time and concentration of 2D MXenes treated by KOH (Fig. 12j) [195]. The previous articles reported that KOH has been used as activator to promote the formation of micropore in the carbon-based nanomaterials [99,200]. Therefore, the Ti-C covalent can be broken under the alkaline condition, producing the Ti and C-based nanoparticles and finally with formation of the CDs, or a small amount of amorphous C, and the metal oxides. As shown in Fig. 12k, the GQDs/TiO 2 nanoparticles with an average size of 1.5 nm. In addition, solvothermal strategy is also used to prepare GQDs [201]. Such carbon-based nanomaterials derived QDs possess low cost, excellent photoluminescence properties and stability, expect to apply in the fields of energy storage, devices, and imaging.

Characterization Techniques of MQDs
Generally, structure determines the performance of the materials, which is important for design of the catalysts with the specific functions. The MQDs derived from 2D MXene, changing the synthesis routes of the MXenes, which will produce different kinds of functional groups, or removing the groups by some post-processing, thereby impacting on various properties of MQDs (conductivity, adsorption, and magnetic applications) [99,105]. Furthermore, the MQDs with surface functional contribute to increase amounts of active sites, simultaneously, regulate the energy band structure. Such semiconductor engineering is challenging to hinder the recombination of electron and hole in the field of photocatalysis. Therefore, it is necessary to identify such materials by using basic characterization techniques, toward promoting their further development, the comprehensive characterization techniques of MQDs shown in Fig. 13.

Morphology Characterization
Compared to their 2D counterparts, the obvious difference of MQDs lies at a series of changes in physicochemical properties due to the small-size effect [85,202]. At present, the size of the reported MQDs is usually less than 10 nm. The morphology of MQDs is generally spherical. Since the spatial resolution of SEM usually insufficient to characterize the morphology. Therefore, TEM and atomic force microscopy (AFM) spectroscopy are often used to analyze the morphology of MQDs. They provide lateral size and height profile information, respectively. TEM images of Ti 3 C 2 MQDs, TiCN MQDs, Nb 2 C MQDs and Ti 2 N MQDs show a size range of 4.2 ± 0.6, 2.7 ± 0.2, 1.6-4.0, and 4.83 ± 2.69 nm [87,89,203]. Yu et al. prepared Ti 3 C 2 MQDs by using bath and probe sonication method [110]. TEM image shows the dotlike uniform distribution of Ti 3 C 2 MQDs, with the average size of 4.9 ± 1.6 nm (Fig. 14a-b). HRTEM image can display  [90]. Copyright @2021, Wiley-VCH. j Schematic illustration of the preparation of graphene quantum dots (GQDs); k TEM image of GQDs, and the inset illustration is size distribution of GQDs [195]. Copyright @2020, Elsevier 1 3 the lattice fringes with an inner plane spacing of 0.21 nm (Fig. 14c), and such clearly visible lattice fringes represent a good crystallinity of MQDs [109]. AFM image indicates that the thickness was 1.2 ± 0.3 nm (Fig. 14d-f). The size distribution combined with AFM image confirms their spherical structure. Furthermore, some reports show that the lateral size of MQDs exceeds 10 nm, related to the synthesis methods, the molecular weight cut-off of the dialysis bag, and the centrifugation speed.
Currently, the aberration-corrected scanning transmission electron microscopy (STEM) (AC-STEM) has been used to identify the single atom, defects such as vacancies, atomic doping, and lattice distortions based on the superresolution in both space and energy space [206,207]. For example, the atomically dispersed Ni was introduced into the cadmium-zinc sulfide QDs (ZCS QDs). AC-STEM can clearly distinguish the real position of atoms, and the corresponding fast Fourier transform (FFT) pattern further proves the favorable (111) plane of Ni atoms dispersion [174]. Analogously, the coordination of single Co with S edge and strain from lattice mismatch induced the phase transition from 2H-MoS 2 to 1T-MoS 2 . Such atomically visualizing technique directly show the different phase coordination environment and the presence of Co atoms at the 5 Å scale [208]. Currently, such spectroscopy and imaging technique has not been used to characterize 0D MQDs due to the limited development of MQDs. It is expected to be applied to the MQDs and MQDs-based nanocomposites in the future. However, it is noted that the highly electron irradiation will result in knock-on effect, as well as other electron-beam damages and changes [206,209].

Structure and Composition Identification
Identifying the composition and structure of matter through specific characterization techniques is essential for the development of materials science. XRD is basic characterization of phase composition and structure. However, XRD shows different peak shapes due to the high surface energy-induced aggregates [210,211]. For example, Fig. 14g-i mainly shows three XRD patterns of Ti 3 C 2 MQDs and Nb 2 C MQDs. Compared to 2D Ti 3 C 2 MXene, the (002) lattice spacing was further expanded due to the intercalation of TMA ions during preparation. However, the reduced intensities of ((10 l)), (004), and (110) diffraction peaks indicate a good dispersion of the Ti 3 C 2 MQDs [204]. Besides, Lu et al. prepared Ti 3 C 2 MQDs by hydrolyzing method; compared to the bulk Ti 3 C 2 , the weaker peak intensity and broad width of MQDs indicate  [110]. Copyright @2017, The Royal Society of Chemistry. g XRD patterns of Ti 3 AlC 2 , Ti 3 C 2 MXene, and Ti 3 C 2 MQDs [204]. Copyright @2019, The Royal Society of Chemistry. h XRD patterns of bulk Ti 3 C 2 MXene, Ti 3 C 2 nanosheet, and Ti 3 C 2 MQDs [59]. Copyright @2019, WILEY-VCH. i XRD patterns of Nb 2 AlC and Nb 2 C MQDs [205]. Copyright @2020, Elsevier 1 3 the grain refinement [59]. More importantly, in 2020, the Nb 2 C MQDs were prepared by high-intensity ultrasonication strategy. Under the dual action of mechanical force and intercalation solvent, the strong layering effect leads to much smaller-sized MQDs without any obvious peaks and only showing the broad spreading, indicating the layered structure was completely broken down [205]. Therefore, the above results show that the peak intensity and peak width are closely correlated to the layer number and the lateral size and crystallinity of MQDs, which is across-validated with their TEM and AFM results.
The unique surface chemical of MQDs can be detected by X-ray photoelectron spectroscopy (XPS) [52,212] and Fourier transform infrared spectroscopy (FTIR) [213,214]. They provide the surface composition, valence state, and functional groups information, respectively. As shown in Fig. 15a, the survey XPS spectrum of Ti 3 C 2 T x MQDs provides all the composition elements of Ti 2p (457 eV), C 1s (285 eV), O 1s (529 eV), and F 1s (684 eV) [215]. The detailed element valance state and coordination conditions can be identified by deconvolution of the constituent element in high-resolution XPS spectra. For example, fitting the high-resolution spectrum of Ti2p by using the multipeak Gaussian method (Fig. 15b), the binding energy peaks of 457. 38, 463.18, 455.68, 461.88, and 469.98 eV can be attributed to the bond of Ti-O, Ti-C, C-Ti-O, and Ti-F, respectively [204]. According to previous report, the Ti-O comes from surface oxidation, i.e., tetravalent. Whereas the C-Ti-O, C-Ti-OH, Ti-F belong to bivalent (II) and trivalent (III). In addition, the high-resolution spectrum of each element was fitted by using the multi-peak Gaussian method. The element proportion on the surface of MQDs and the percentage of bonding were qualitatively determined according to the peak area and element sensitivity.
FTIR is another important characterization techniques of functional groups. The surface of MQDs has oxygencontaining groups due to that the synthesis is mostly solution-oriented method. For example, the vibration peak of -OH group at 3410 cm −1 and 1488 cm −1 , is attributed to different vibration modes. Furthermore, the -F group has two different vibrations forms, Ti-F (830 cm −1 ) and C-F (1013 cm −1 ). The adsorption peaks of other groups such as Ti-O, Ti-C, C-C, and C = O at 701, 463, 917, and 1650 cm −1 (Fig. 15c) [215]. It is noted that the signal C-F Fig. 15 Composition and optical spectral characterization of MQDs. a XPS survey spectra of Ti 3 C 2 MQDs; b High-resolution spectra of Ti 2p [204]. Copyright @2019, The Royal Society of Chemistry. c FTIR spectra of Ti 3 C 2 MQDs [215]. Copyright @2018, The Royal Society of Chemistry. d Raman spectra of Ti 3 C 2 (OH) 2 MQDs [70]. Copyright @2020, WILEY-VCH. e UV-Vis adsorption, PL, and PLE spectra of Ti 3 C 2 MQDs; f PL spectra at different excitation wavenumbers [58]. Copyright @2017, WILEY-VCH and C = O originates from the breaking of the Ti-C bond during the MXene etching process, contributing to expose the inner carbon and adsorb the groups in solution [70].
Raman spectrum is also used to characterize the composition, layers, and defect intensity of QDs. For example, the 2D graphene-derived GQDs, the layers of GQDs can be judged by using characteristic G peak, the intensity, and shape of characteristic G´ peak, and the defect density of characteristic GQDs can be judged by the ratio of the D peak (1350 cm −1 ) to G peak (1580 cm −1 ) [176]. However, the present Raman characterization of MQDs elucidates the composition, while the defect states and amounts of layers remain to be further explained due to the uncertainty or the possibility of carbon exposure in the inner layer during the synthesis. Compared to 2D Ti 3 C 2 MXene, the characteristic Raman bands of 147 cm −1 , 260, 412, 609 cm −1 correspond to Ti-O and Ti-C (Fig. 15d) [54,190,216]. Furthermore, the D and G band comes from the exposure of inner C.
Apart from the above characterization techniques about structure and composition, other characterization of MQDs such as AC-STEM, X-ray synchrotron (XAS) [174,[217][218][219]. The synchrotron radiation provides a detailed ingredient analysis, including local coordination environments, valance states, and coordination number. Furthermore, the soft X-ray emission spectroscopy (SXES) based on electron microscopy can be applied to investigate the chemical bonding state of MQDs, especially in the form of nanocomposites based on the MQDs [220][221][222]. Besides, constructing in situ electrochemical reaction based on the synchrotron radiation means to monitor the dynamic changes of various substances in the catalytic reaction process. As a result, it reveals the catalytic reaction mechanism at the surface interface, which will help to promote the development of 0D MQDs in the field of catalysis.

Optical Characterization
Like other organic or inorganic QDs, optical spectroscopy characterization of MQDs is obviously strong evidence for the information of MQDs. Photoluminescence spectrum (PL), photoluminescence excitation spectrum (PLE), electrochemiluminescence (ECL), and UV-Vis spectra can be used to characterize the luminous behavior of the MQDs [89,223]. For example, the UV-Vis adsorption spectrum of Ti 3 C 2 MQDs shows the adsorption at 320 nm, corresponding to two luminescence peaks of 250 and 320 nm in the PLE spectrum (Fig. 15e). Such UV-Vis spectra represent the different electronic transition (σ → σ*, n → σ*, π → π*) of groups. Furthermore, according to the PL spectrum at different excitation wavelengths (340-440 nm), the strong excitation-dependent PL behavior is correlated to the size effect (Fig. 15f) [58]. More importantly, the PL properties of MQDs is important for improving photocatalytic performance. It determines the light absorption range of the photocatalyst from ultraviolet to near-infrared (NIR) regions, affecting the amount of photogenerated carriers. Such PL properties are related to size, surface composition, and the pH of solution of MQDs [224]. Ti 3 C 2 MQDs show white light, blue light in dimethyl sulfoxide (DMSO), N, N-Dimethylformamide (DMF), and ethanol, respectively, under the excitation wavelength of 365 nm [96]. Furthermore, other optical behavior should be concerned, which facilitate to understand the fluorescence mechanism of MQDs, and has great significance for promoting the development of MQDs and application in the fields of bioimaging [23,129,205], fluorescent probes [149,153,225], and optical devices [96,108,109].
In addition, the optical properties can also be further proved by using theoretical simulation. The size effect of MQDs enables the bandgap control, while the introduction of gap states after the surface modified by single/dual atom (N, P, S, etc.) can improve the free carrier lifetimes and promote charge separation ( Fig. 16a-b) [147]. Also, the density of states (DOS) demonstrates that the N defect increased the energy gap and work function of MQDs, contributing to fast electron migration (Fig. 16c-e). Simultaneously, the frontier orbitals are simulated by using DFT in the dual atoms modified MQDs (Fig. 16f-g) [86]. Compared to the pristine Nb 2 CO 2 MQDs, the weak electrons exchange interaction between Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) of S, N-Nb 2 CO 2 MQDs contributes to small △E, leading to enhance the emission of PL and improve the PLQY of MQDs. Such the combination of experiments and simulation enables clarification of the fluorescence mechanism, and further designing highly efficient photocatalyst.

3 5 Catalytic Applications
MQDs have been wildly applied to catalysis due to their unique physicochemical properties, especially quantum confinement effect. The way of catalysis can be classified into electrocatalysis, photocatalysis, and photoelectrochemical application. Currently, the application of MQDs mainly focused on photocatalysis due to their larger surface areas, tunable bandgap, and composition. The detailed application is summarized in Fig. 17a, including H 2 production, oxygen reduction reaction (ORR), pollutant degradation, CO 2 reduction, NH 3 production, and H 2 O 2 production.

Electrocatalysis
2D MXenes have been used to the field of electrochemical energy conversion, including hydrogen evolution reaction (HER) [10,103], oxygen evolution reaction (OER) [226,227], and nitrogen reduction reaction (NRR) [228,229] due to their highly tunable metal composition and surface functional groups, large specific surface area, good hydrophilicity, and excellent electrical conductivity. However, 2D MXene-derived MQDs are less reported in the field of electrocatalysis. Reducing the size of MXene to less than 10 nm is beneficial to increase abundant edge sites, decrease  [147]. Copyright @2018, The Royal Society of Chemistry. f-g DFT calculation of total and projected DOS of Nb 2 CO 2 QDs and S, N-doped Nb 2 CO 2 QDs [86]. Copyright @2020, The Royal Society of Chemistry electron diffusion length, expecting to become the high-performance electrocatalyst candidates.

Electrocatalytic Ammonia Synthesis
Ammonia, as an important chemical raw material, plays an indispensable role in the development of agriculture, industry and energy storage [230,231]. The traditional route of NH 3 production is Haber-Bosch process, but the conditions of high temperature and pressure increases the operating cost. Over the few years, electrocatalytic NRR has attracted attention due to the mild reaction conditions and abundant resources. However, the strong and stable N≡N bond, the sluggish adsorption of N 2 and competitive HER side reaction lead to low NRR selectivity and ammonia production rate.

The -OH Functional Groups of MQDs as Active
Sites MQDs as an emerging 0D nanomaterials, regarded as a promising NRR electrocatalyst due to their excellent conductivity, abundant surface catalytic active sites, and surface defects. For example, the Ti 3 C 2 OH MQDs were first prepared by agitate-assisting for NRR catalysts. Figure 17b shows the reason why Ti 3 C 2 OH acts as electrocatalyst for NRR with excellent performance: i) N 2 molecules adsorb at the edge of Ti with positive charge, which accelerates the N≡N bond length from 1.10 to 1.16 Å and activates nitrogen molecules; ii) Compared to both Ti 3 C 2 F 2 and Ti 3 C 2 , there occur no side reactions of HER when Ti 3 C 2 OH as catalyst to promote NRR, due to the free energy of the rate limiting step is 0.4 eV (the value of HER on the Ti edge of Ti 3 C 2 OH is 0.79 eV) [70]. Therefore, the Ti 3 C 2 OH MQDs with abundant Ti edge and -OH functional groups were prepared. Experiment result shows that the Ti 3 C 2 OH MQDs provides 62.94 µg h −1 mg −1 cat at − 0.50 V. Compared to 2D MXenes, it shows excellent NRR activity due to the offered more Reaction mechanism of electrocatalytic N 2 reduction and free energy calculation on the Ti edge of Ti 3 C 2 , Ti 3 C 2 F, Ti 3 C 2 OH MXene from the adsorption of N 2 to the reduction of NH 3 ; c The average NH 3 yield and faradaic efficiency of Ti 3 C 2 OH MQDs at different applied voltages [70]. Copyright @2020, WILEY-VCH. d Charge density difference of Ti 3 C 2 MQDs/Cu; e The NH 3 yield of Cu, MQDs, MQDs/Cu at applied voltage of -0.5 V, inset diagram is chronoamperometry test [79]. Copyright @2022, Zhengzhou University and Wiley 1 3 active sites, highlighting the unique advantages of size and surface functional groups of MQDs (Fig. 17c).

The Interface Design of MQDs-Based Composites
Apart from the surface terminal effect on the adsorption capability of N 2 , the interface engineering is of great significance for promoting the adsorption and activation of N 2 . Moreover, the synergistic catalysis has greater competitive merit compared with pure catalyst, such as enhanced conductivity and hydrophilicity. Consequently, it farcicalities improving the internal electron transport of catalyst or the catalyst-electrolyte interface, thereof further enhancing the catalytic activity, especially in semiconductor catalyst. Therefore, the porous Cu nanosheets with high conductivity were used as support to load Ti 3 C 2 MQDs for NRR, which were synthesized by chemical reduction. The electron coupling of MQDs-Cu promotes the electrons of MQDs are enriched in the interface (Fig. 17d) contributing to the improvement of electron conductivity [79]. The result shows that Ti 3 C 2 MQDs/Cu provides 78.5 µg h −1 mg −1 cat at − 0.50 V, better than the pure MQDs, Cu, and other reported analogues (Fig. 17e). The result is superior to the NRR activity of 2D Ti 3 C 2 nanosheet (4.7 µg h −1 mg −1 cat at − 0.20 V) under same condition [232].

Electrocatalytic Water Splitting
HER and NRR are a pair of competing reactions. The -OH functional groups offer favorable free energy with NRR for facilitating the cleavage of N≡N bonds, whereas the 2D Ti 3 C 2 MXene with -O group has been demonstrated to be HER active sites with the minimum Gibbs free energy (△G H *). Demonstrably, to reduce the size of MXenes to less than 10 nm and thereof increase the contact areas between the surface of MQDs and react environment, enable highlighting the unique surface properties. As a result, it will contribute to promote more active sites to participate in the reaction.
The surface -O groups of MQDs as active sites: The support should be introduced into the MQDs based on the high surface energy. For instance, the Ti 2 CT x MQDs/ Cu 2 O/Cu foam nanocomposite was prepared by selfassembly method (Fig. 18a), and the Ti 2 CT x MQDs with -Cl, -O, and -OH afford a spontaneous substantial process from -Cl groups to -O groups during HER [164]. The increase in oxygen-containing groups of active sites, the conductivity of the Cu-based support, as well as Cu 2 O nanoparticles as bridges providing the stability, contribute to the derived more excellent HER performance compared with 2D MXenes (Fig. 18b), attributed to the unique merit of MQDs. However, for the △G H * of O-terminated MQDs there is still a distance away from the theoretical value. Thus, some means such as the modification of transition metal atoms at the O sites enable weakening the binding energy of O-H bonding.

MQDs as Electronic Conductor
The MQDs also acting as co-catalyst to promote the ORR and methanol oxidation reaction (MOR) are of great significance for improving the commercial application of methanol fuel cells (DMFCs). However, there is only one report on the application MQDs in ORR and MOR. The internal electron transmission of electrocatalyst is enhanced due to the excellent electronic conductivity of MQDs. So, the MoS 2 QDs @ Ti 3 C 2 T x QDs@MWCNTs nanocomposites show excellent electrochemical performance due to the largest embedded area (Fig. 18c-d) [66]. Therefore, MQDs can be as electrons conductor to promote electrocatalytic reaction.

The Defect of MQDs as Active Sites
The presence of surface defects can promote local charge distribution of active sites, control intermediate adsorption behavior. So, it leads to reducing the redox energy barriers during Li 2 O 2 formation and decomposition, achieving enhanced electrocatalytic kinetics. Wang et al. [137] prepared Ti 3 C 2 MXene quantum dot clusters (Ti 3 C 2 QDC) with rich grain boundaries and edge defects through hydrothermal thermal-shearing reaction method. The defects were firstly characterized by using AC-STEM. Compared to perfect crystal of 2D MXene (Fig. 18e), the MQDs with defects show considerable grain boundary and unsaturated edge sites (Fig. 18f). Thus, MQDs exhibit better Li-O 2 catalytic activity with high capacity and cycling stability compared with 2D MXene (Fig. 18g-h). Such atomic-scale clarification of the catalytic reaction mechanism with multiple defect-dominated MQDs provides a strategy for designing highly active catalysts.

Photocatalysis
Photocatalysis is a redox reaction based on the photocatalyst surface under visible light, which has been regarded as one of the potential green technologies to solve the problems of energy shortage and environmental problems [233].
Broadening the spectral response range, increasing the carrier concentration, and reducing the recombination of photogenerated carriers are the keys to design efficient and stable photocatalysts [234]. MQDs have been considered as ideal co-catalysts due to their excellent conductivity, large surface areas, tunable bandgap, and strong quantum conferment effect, which has been successfully applied to photocatalytic hydrogen production, pollutant degradation (e.g., NO, heavy metal), CO 2 reduction, NH 3 production, and H 2 O 2 production.

Photocatalytic Water Splitting
Hydrogen (H 2 ) has been considered as one of the ideal fuel due to low densities, high calorific value, abundant raw materials, and non-polluting combustion products [235,236]. The light-driven water splitting to obtain H 2 is a promising conversion technology [237]. Over the past few years, transition metal oxides, transition chalcogenides, and organic semiconductors have been developed  [164]. Copyright @2022, Zhengzhou University and Wiley. c TEM image of MoS 2 QDs @ Ti 3 C 2 T x QDs@MWCNTs and d ORR performance of sample [66]. Copyright @2019, Elsevier. e AC-STEM image of Ti 3 C 2 nanosheet; f AC-STEM of Ti 3 C 2 MQDs; g Initial deep discharge-charge curves of the three samples at 200 mA g −1 ; h Cycling stability and terminal dischargecharge voltages of Ti 3 C 2 QDC/N-C electrode at 200 mA g −1 [137]. Copyright @2021, Wiley-VCH for photocatalytic hydrogen production due to their suitable energy band structures. The high-performance photocatalysts need to meet the following conditions: (i) the semiconductor possesses broad light adsorption range to generate more photogenerated carriers; (ii) the adequate energy band structure meets the thermodynamic requirements of water splitting; (iii) photogenerated electron and holes can be effectively separated. Therefore, designing the structure of semiconductors is of great significance for realizing an efficient photocatalytic water splitting reaction.

MQDs as Photoelectrons Acceptor
Compared to the traditional TiO 2 photocatalyst, the layered g-C 3 N 4 possesses narrow band gap (2.7 eV) and visible light activity, whereas low light response ranges from 450 to 460 nm, and high electron-hole pairs recombination rate results in poor photocatalyst performance [238]. Thus, the Ti 3 C 2 QDs with excellent conductivity were co-catalyst to improve it [160]. As shown in Fig. 19a, the conduction band of MQDs with abundant catalytic active sites is more positive than that of g-C 3 N 4 , which can capture electrons to facilitate surface redox reactions [160]. Such behavior of low recombination capability of photogenerated electron-holes pairs Fig. 19 Photocatalytic water splitting performances of MQDs-based heterostructure. a Schematic react mechanism of g-C 3 N 4 @Ti 3 C 2 QD; b Steady photoluminescence spectra of g-C 3 N 4, MQDs@g-C 3 N 4 ; c Time-resolved fluorescence decay spectra under the 325 nm excitation wavelength; d The transient photocurrent response; e Photocatalytic HER rate plot of catalyst [160]. Copyright @2019, American Chemical Society. f Schematic photocatalytic mechanism of BV@ZIS/TC QDs; g UV-visible diffuse reflectance spectra of BV@ZIS/TC QDs and control sample; h Photocatalytic gas production of BV@ZIS/TC QDs and control sample [68]. Copyright @2020, Elsevier induced a low PL intensity. The time-resolved fluorescence decay spectra show that lifetime of carriers increased to 10.1242 μs, further demonstrating the result (Fig. 19b-c). As a result, the photocurrent intensity of Ti 3 C 2 QDs/g-C 3 N 4 is higher than 2D g-C 3 N 4 nanosheets (Fig. 19d). Therefore, MQDs as electron acceptors to capture quickly the photogenerated electrons, facilitating the efficient carrier transfer. Finally, the H 2 production rate of Ti 3 C 2 QDs/g-C 3 N 4 (5111.8 μmol g −1 h −1 ) is far higher than g-C 3 N 4 (196.8 μmol g −1 h −1 ), Pt/g-C 3 N 4 (1896.4 μmol g −1 h −1 ) and 2D MXene/g-C 3 N 4 (524.3 μmol g −1 h −1 ) at the same conditions (Fig. 19e), highlighting the merit of MQDs.

Designing the Z-scheme structure of MQDsbased photocatalyst
For designing favorable energy band structure, the instruction of Schottky junction to increase the extraction of photoelectrons is an effective strategy. The MQDs as co-catalyst to construct Z-scheme structure of BiVO 4 @ZnIn 2 S 4 /Ti 3 C 2 (BV@ZIS/TC QDs) [68], forming the Schottky barrier at the interface. As described in Fig. 19f, the photogenerated electrons of conduction band in BV were injected into the valance band of ZIS. Then more abundant photogenerated electrons were injected into the conduction band of MQD, achieving an effective carrier separation. The presence of MQDs broadens the light response range from visible light to near-infrared region, helping to produce more photoelectrons, and thereof promoting efficient H 2 and O 2 production rates ( Fig. 19g-h). Also, the Ti 3 C 2 -QDs/ ZnIn 2 S 4 /Ti(IV) heterojunction photocatalyst was prepared for improving hydrogen production performance [78].

Photocatalytic CO 2 Reduction
The goal of carbon neutrality is an inevitable choice based on the high-quality development of China's economy and society. Photocatalytic CO 2 reduction is an efficient technology to achieve a low carbon economy [239]. Furthermore, methanol, the reduction product of CO 2 , can also be used as a fuel. It is worth noting that the photocatalytic CO 2 reduction activity is related to the light adsorption ability, photogenerated carriers' separation efficiency, and the activation ability of photocatalysts for CO 2 molecules [240]. Compared to 2D MXene, MQDs offer controllable bandgap due to size effect, abundant unsaturated sites to adsorb CO 2 molecules, contributing to the selectivity of reaction products.
Designing the S-scheme structure of MQDs-based photocatalyst: The semiconductor Cu 2 O has narrow bandgap of 2.2 eV, and the more negative potential of conduction band than reduction potential of CO 2 , which is unfavorable photocatalytic CO 2 reduction. Therefore, it is important for designing the energy band structure to suppress recombination of the carriers to promote effective reaction. In 2019 [67], Zeng et al. constructed S-scheme heterojunction by electrostatic self-assembly (Fig. 20a). The energy band structure shows that the photogenerated electrons of conduction band (CB) in Cu 2 O transfer into CB of MQDs due to the Fermi level of MQDs lower than CB of Cu 2 O, avoiding the recombination of carriers (Fig. 20b). The MQDs as co-catalyst to increase light adsorption capability of Cu 2 O nanowires, and reduce the charge transfer resistance (Fig. 20c-d). Therefore, compared to the 2D MXene, the Ti 3 C 2 MQDs/Cu 2 O/Cu contributes to high CH 3 OH yield (Fig. 20e). Furthermore, since designing core-shell MQDs-coupled nanosheet (TiO 2 /C 3 N 4 ) with S-scheme heterojunction (Fig. 20f), the formation of band edge bending and internal electric field at the interface enables balancing the Fermi level (Fig. 20g). Thus, the photoelectrons are accumulated on the conduction band of MQDs to achieve an efficient redox reaction for improving the selectivity of products (Fig. 20h). Such result shows the 0D MQDs as electron acceptor to accelerate spatial migration of electrons compared with 2D MXene, which is an effective strategy to control the type and amount of heterointerface [159].

Photocatalytic NH 3 and H 2 O 2 Production
Compared to the traditional complex Haber-Bosch process, photocatalytic NH 3 production is a feasible method due to economy, environmentally friendly, and facile conditions [241]. There are three conditions to achieve high ammonia yield: (i) the weak bond energy of N≡N; (ii) the broad-spectrum response range for increasing the concentration of carrier; (iii) the efficient carrier separation rate. Among them, the recombination of photogenerated carries is a main obstacle to hinder the activation of N 2 molecules. The MQDs with quantum confinement effect and good conductivity can promote the bandgap control and effective charge transfer.

Designing the Band Structure of MQDs-Based Photocatalyst
The MQDs as co-catalyst of Ni-MOF were anchored on the surface of 2D nanosheets Ni-MOF (Fig. 21a) [69]. The energy theory shows the excellent energy level matching hinders the recombination of photogenerated carrier under the simulated light irradiation, and more photogenerated electrons were accumulated on the CB of Ni-MOF (Fig. 21b), Compared to pure Ni-MOF photocatalyst, the MQDs help to enhance the light adsorption ability, increasing the carrier concentration (Fig. 21c). Leading to high-yield ammonia production (Fig. 21d).

Constructing Defects of MQDs for Reactant Adsorption
The defect sites with lower binding energy facilitate to promote the adsorption of N 2 molecules. In 2022, Chang et al. [76] engineered a photocatalyst with Schottky junction and defects. The Ti 3 C 2 MQDs with amount of oxygen vacancies (OV) and Ti 3+ sites were anchored on the surface of mesoporous C 3 N 4 hollow nanosphere (Fig. 21e). Such unique design has two advantages: 1) the vacancy defects are beneficial to increase the adsorption and activation of N 2 molecules; 2) the quantum confinement effect of MQDs promotes the light adsorption intensity of C 3 N 4 ( Fig. 21f-g), leading to high carrier concentration and excellent photocatalytic activity (Fig. 21h). This is of great significance for guiding the design of MQDs-based photocatalysts. In addition, the rich carbon vacancies of MQDs as bridge site induced the bonding MQDs with g-C 3 N 4 , forming Schottky junction  [67]. Copyright @2019, WILEY-VCH. f TEM of TiO 2 /C 3 N 4 with core-shell; g S-scheme heterojunction before and after contact, and light irradiation; h Schematic diagram of structure of TiO 2 /C 3 N 4 /Ti 3 C 2 MQDs [159]. Copyright @2020, Elsevier at the interface. Thus, it increases the work function of energy band, promoting the photoexcited carrier separation [73]. Such optimized photocatalytic activity of Ti 3 C 2 MQDs/g-C 3 N 4 achieves a high yield of 560.7 μmol L −1 h −1 (Fig. 21i-k).

Photocatalytic Pollutant Degradation
The introduction of MQDs could improve the photocatalytic activity of photocatalysts, which is also reflected in the field of wastewater treatment and purification of air  [69]. Copyright @2020, American Chemical Society. e TEM image of C 3 N 4 /r-Ti 3 C 2 MQDs Schematic diagram of photocatalytic N 2 fixation; f Photocurrent test in Ar and N 2 of samples; g UV-Vis DRS spectra of C 3 N 4 , C 3 N 4 /r-Ti 3 C 2 MQDs; h Photocatalyst NH 3 production rate and side reaction of H 2 production [76]. Copyright @2022, The Royal Society of Chemistry. i Transfer of photogenerated electrons and holes near the carbon vacancies of C 3 N 4 and C 3 N 4 / r-Ti 3 C 2 MQDs; j The energy band structure of C 3 N 4 / r-Ti 3 C 2 MQDs; k The photocatalytic reaction mechanism diagram of C 3 N 4 /r-Ti 3 C 2 MQDs [73]. Copyright @2021, American Chemical Society pollutants. In 2021, the Ti 3 C 2 MQDs acted as co-catalyst of Bi 2 O 3 (BiO/TiC) to boost photocatalytic tetracycline (TC) degradation in water [75]. Compared to pure Bi 2 O 3 , the MQDs/Bi 2 O 3 nanocomposites show a broad visible light adsorption due to excellent metal conductivity of the MQDs (Fig. 22a). Furthermore, the band gap is reduced from 2.91 to 2.71 eV due to the quantum confinement effect of the MQDs (Fig. 22b). It is favorable to promote the photogenerated carrier separation, and the photoexcited electrons were thereof accumulated on the CB of MQDs (Fig. 22c). Thus, it leads to excellent photocatalytic tetracycline degradation effect (Fig. 22d). Compared to pure Bi 2 O 3 photocatalyst, the MQDs as cocatalyst shows enhanced degradation efficiency by 5.85 times, and far surpass precious metal Au, Pt nanoparticles co-catalysts (1.75, 2.18 times). Likewise, such strategy of introducing of MQDs to balance the interface contact energy levels further leads to separation of photogenerated electron-hole pairs, which has been used to prepare other types of heterostructures to facilitate pollutant degradation. For example, the Ti 3 C 2 MQDs have also been used as co-catalyst to prepare Ti 3 C 2 MQDs/SiC nanocomposite [71] (Fig. 22e), all-solid-state WO 3 /TQDs/In 2 S 3 Z-scheme heterostructure [74] and Ni@MQDs [77] nanocomposite photocatalyst, achieving the goal of photocatalytic removal of NO purification and pollutants in water.

Photoelectrocatalysis
Compared to photocatalysis and electrocatalysis, photoelectrocatalysis is a stronger method to promote electrochemical reaction, which combines the advantages of both the catalysis methods [242]. Currently, MQDs have been used to research photoelectrochemical water splitting, and the progress was made.  [75]. Copyright @2021, Elsevier. e Energy band structure of MQDs/ SiC, SiC, MQDs [71]. Copyright @2020, American Chemical Society

MQDs as Photoanode
In 2020, the Janus-structure Co-Ti 3 C 2 MQDs were prepared by thermal-cutting method, which was used as photoanode for water oxidation [72]. The Co nanoparticles were coupled with Ti 3 C 2 MQDs forms Schottky junctions that increase the extraction of photogenerated carriers. Compared to pure MQDs, the introduction of Co triggers the concomitant surface plasmon effects, thus showing that enhanced light adsorption (200-600 nm) and additional adsorption peak (380-520 nm) (Fig. 23a), attributed to increase the amount of photogenerated carrier. Furthermore, the enhanced steady-state PL intensity indicates good carrier migration, while MQDs with high loadings as carrier recombination center reduces quantum yield, resulting in low PL intensity (Fig. 23b). As shown in Fig. 23c, the time-resolved Fig. 23 The photoelectrocatalytic performances of MQDs as co-catalyst. a UV-Vis adsorption spectra of Ti 3 C 2 MQDs, Co-MQDs with different rates of Co/Ti; b Photoelectric conversion efficiency of Ti 3 C 2 MQDs, Co-MQDs with different rates of Co/Ti; c Time-resolved photoluminescence (TRPL) spectra; d Photoelectrocatalytic water splitting mechanism under light irradiation; e Photocurrent test of Ti 3 C 2 MQDs, Co-MQDs with different rates of Co/Ti [72]. Copyright @2020, WILEY-VCH. f Schematic illustration of the charge transfer process for NiFeOOH/ MoOx/MQD/BiVO 4 photoanodes; g Photoelectrochemical water splitting device [161]. Copyright @2022, Wiley-VCH 1 3 photoluminescence (TRPL) spectra show that Co coupled with MQDs can increase the average carrier lifetime, indicating low carrier recombination rates, which is consistent with PL spectra result. Such Schottky hierarchical structure contributes to promote photogenerated electrons were extracted from the CB of Co to the CB of MQDs, promoting high photoelectrochemical water oxidation performance (Fig. 23d-e).

MQDs as Hole Transfer Layers
The stability and charge separation of photoanode is important for improving photoelectrochemical (PEC) water oxidation activity. The MQDs can be used as co-catalyst to construct the hole transfer layers of MoOx/MQDs for delaying carrier recombination (Fig. 23f), enhancing the light response range, leading to high activity and stability of PEC reaction (Fig. 23g) [161]. Such strategy broads the high-performance full-spectrum photoelectrochemical water splitting [157].

Summary and Perspectives
Since in 2017, 2D MXene-derived 0D MQDs have made great progress in proceeding into a variety of catalysis due to their improved and optimal physicochemical properties. Research has shown that the surface metal ions, functional groups, and abundant edge sites of MQDs can act as active sites to adsorb and activate the gas molecules, and the MQDs are also treated as co-catalyst to promote an efficient charge separation and enhance charge transfer kinetics. Consequently, it reveals that MQDs have promising potential in the field of catalysis. However, there are some problems upon MQDs remain to be resolved, such as low yield, easy aggregation, poor stability in preparation, and difficulty to precisely control the surface chemistry. Such disadvantages are not beneficial to the comprehensive development of MQDs in the field of catalysis.
In this review, we update the recent research progress in catalysis, including the research status, involved with the synthesis of pure MQDs and functional MQDs, and relevant characterization techniques, in order to design high-performance MQDs-based catalysts. Meanwhile, the challenges must be confronted on the basis of synthesis, formation mechanism, wide application, surface defects, and advanced characterization techniques of MQDs.

Synthesis Condition and Formation Mechanism of MQDs
Currently, the synthesis methods of MQDs are based on single-layer or multilayer MXene nanosheets as precursors, and MXene was prepared by F-containing etchant (e.g., HF, LiF + HCl). Many studies show that F terminal is unfavorable to electrochemical reaction process. Therefore, the preparation of F-free MQDs should be fully considered. Furthermore, since most of MQDs was prepared by hydrothermal strategy, the surface suffers from an easy oxidation in this process. So, it results in the difficulty to obtain high-purity MQDs. For such the irregularity of synthesis method, and the influence of impurities in the product, the formation mechanism of MQDs is not well clarified; it poses a challenge to precisely control the growth of MQDs. Also, for preparing high-quality MQDs (the desired structure, shape, size, distribution of functional groups, and types of surface defects), it is necessary to systematically study the composition of MQDs and various reaction conditions (solvent, temperature, reaction time, power, and pH) effect on the performance of MQDs. Moreover, the introduction of in situ characterization techniques can contribute to elucidate the formation mechanism and nanostructure. Furthermore, Bottom-up method has been used to synthesize other QDs due to the advantages of adjustable surface chemistry, morphology and size, and the precise controlling of synthesis conditions. Thus, such method can be considered for synthesizing MQDs with excellent crystallinity, monodisperses, and stability.

Synthesis of Novel MQDs
There are many kinds of MXenes reported so far experimentally and in theoretical prediction, while the application focuses mainly on Ti 3 C 2 MQDs. The structure and properties of MQDs are correlated with the type, quantity, and arrangement of metallic elements, which affect the energy band structure. Therefore, exploring the performance of different types of MQDs in catalysis not only is beneficial to building the relationship between the composition, structure, and properties of MQDs, but also to searching for low-cost, high-activity, and high-stability catalysts.

Role of MQDs in Electrochemical Reaction
Like other inorganic and organic QDs, MQDs tend to agglomerate due to surface effects during catalytic reactions. The choices of the support, and the study of the interaction between MQDs and the support, especially the chemical properties at the interface are of great significance for improving the electrochemical performance. Furthermore, apart from the performance of MQDs as catalysts, MQDs can also be considered as the support of nanomaterials such as single atoms and metal nanoparticles, benefited from the abundant functional groups on the surface of MQDs that can directly serve as anchoring sites, avoiding additional surface modification steps such as carbon-based materials.

Novel Application of MQDs
On the basis of the progresses, the application of MQDs in electrocatalysis is undoubtedly in the infancy stage. Especially, there are still lots of space in the field of HER and OER. Currently, some fluorescent QDs such as carbon dots (CDs), graphene QDs (GQDs), carbon QDs (CQDs) have been used to apply on catalysis. Their synthetic methods, surface-modified strategies, the roles recognition of catalytic reaction, characteristic techniques, and the exploration of catalytic mechanism have been widely investigated. As analogy, compared to carbon-based QDs, MQDs have controllable composition, complex internal structure. Thus, the exploration of MQDs for catalytic application can learn from the research style of carbon-based QDs, however, the challenges remain. For further guiding the design of highperformance catalysts, it is necessary to construct some theoretical models to predict the effect of surface state and external environment (temperature, pressure, and illumination) on the catalytic activity.

Surface Defects of MQDs
Defective sites with low binding energy are commonly considered as key sites for catalytic activity. They not only act as adsorption sites of reactants, but as the coupling site of metal nanoparticles. MXene QDs with surface defects can be obtained by atomic doping, electrochemical reduction, reducing agents, etc. The introduction of surface defects facilitates to improve the electronic structure of the active sites around the MQDs. So, the electron transfer process and the adsorption/desorption behavior of reactants can be well controlled during the electrocatalytic reaction. In addition, when MQDs are used as photocatalysts, the controllable size of MQDs enables a unique band gap structure, beneficial to photocatalytic, and photoelectrocatalytic reactions.
Generally, the photocatalytic reaction performance is related to the separation efficiency of photogenerated carriers and light absorption range. Constructing surface defect is an effective strategy. Furthermore, studies have shown that the passivation of surface defects induced by amino groups facilitates to improve the PL properties of MQDs, resulting in bright blue fluorescence and enhanced fluorescence lifetime [140]. The doping includes both non-metallic elements such as N, P, and S, and transition metal elements such as Ni, Co, and Cu. As a result, a variety of surface and subsurface defects are produced. Controlling the surface oxygen content of MQDs can also achieve surface defects, unfortunately yet to be realized. The effect of surface defects on the PL properties remains to be revealed.

In Situ Characterization Techniques
The atomic structure of MQDs was observed under ultrahigh-resolution electron microscopy to study the distribution of defects, the arrangement of atoms, and the special sample-supporting mesh was used to visualize the dynamic evolution of catalysts, which is something expected. Moreover, the advanced in situ techniques such as Raman, synchrotron radiation, FTIR expect to elucidate the nanostructure and formation mechanism of MQDs and MQDs-based catalyst, and contribute to reveal the structural evolution in catalytic process, toward designing high-performance MQDs-based catalysts. It is worth noting that the surface reconstruction of MQDs-based nanocomposites during the electrochemical reaction can be intuitively observed by using in situ characterization. It is thereof beneficial to clarify the real catalytic active sites, and confirm the morphology and structural changes after the reaction. On the basis of such foundation, it is expected to construct optimized performance catalysts.
In general, an increasing number of investigations concentrate on the synthesis, modification, application of MQDs in the past five years. Therefore, our review provides new insights into the recognition of MQDs, and illustrates recent progress in catalysis. As a result, it will provide guidance and reference for the preparation of novel MQDs and the design of high-performance MQDs-based catalysts.
Funding Open access funding provided by Shanghai Jiao Tong University.
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