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

    时间:2022-12-10 10:50:03 来源:雅意学习网 本文已影响 雅意学习网手机站

    Yuhua Liu, Wei Zhang✉, Weitao Zheng✉

    ABSTRACT It is well known that two-dimensional (2D) MXene-derived quantum dots (MQDs) inherit the excellent physicochemical properties of the parental MXenes, as a Chinese proverb says, “Indigo blue is extracted from the indigo plant,but is bluer than the plant it comes from.” Therefore, 0D QDs harvest larger surfaceto-volume ratio, outstanding optical properties, and vigorous quantum confinement effect. Currently, MQDs trigger enormous research enthusiasm as an emerging star of functional materials applied to physics, chemistry, biology, energy conversion, and storage. Since the surface properties of small-sized MQDs include the type of surface functional groups, the functionalized surface directly determines their performance.As the Nobel Laureate Wolfgang Pauli says, “God made the bulk, but the surface was invented by the devil,” and it is just on the basis of the abundant surface functional groups, there is lots of space to be thereof excavated from MQDs. We are witnessing such excellence and even more promising to be expected. Nowadays, MQDs have been widely applied to catalysis, whereas the related reviews are rarely reported.Herein, we provide a state-of-the-art overview of MQDs in catalysis over the past five years, ranging from the origin and development of MQDs, synthetic routes of MQDs, and functionalized MQDs to advanced characterization techniques. To explore the diversity of catalytic application and perspectives of MQDs, our review will stimulate more efforts toward the synthesis of optimal MQDs and thereof designing high-performance MQDs-based catalysts.

    KEYWORDS MXene; Quantum dots; Catalysis; Surface groups; Structure

    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 (CO2) electrocatalytically to clean fuels (e.g., H2,CH3OH) 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.

    Nowadays, the precious metal catalysts are crucial, e.g.,Pt-based [11, 12], ruthenium oxide (RuO2) [13], and iridium oxide (IrO2) nanomaterials [14, 15] are listed as the most effective catalysts for driving CO oxidation, hydrogen evolution reaction (HER), NH3synthesis, and oxygen evolution reaction (OER). However, their high-cost and limited reserves hinder a large-scale utilization in industry. Therefore, many researchers focus on reducing the contents of noble-metal (e.g., Au, Pt, Ru, and Pd) loading on a support[16-18], or decreasing size for increasing amounts of active ingredients (single atoms, nanoclusters, and quantum dots(QDs)) [19-21], or coupling noble-metal with non-metal to regulate electron structure for preparing highly active catalysts (RhB, Pt3Ni, and Pd2B) [22-24], or searching for the replaceable non-precious metals (e.g., transition metal (TM)Fe, Co, and Ni) and their alloys [25, 26]. Presently, TM oxides (MoO, MnO2, Co3O4) [27, 28], Co, Ni, and Cu binary oxides (NiO/CuO, Co3O4/NiO, and CuO/Co3O4) [29, 30],and layered double hydroxide systems (FeNi-LDH, NiCo-LDH, and CoAl-LDH) have been widely investigated in the catalytic fields [31, 32]. Particularly, some TM-based catalysts (Bi, Cu, Mo, Cr, and W) have made great progress [33].Furthermore, TM and non-metallic single atoms, carbonbased hybrid, perovskite, and MOF-derived nanomaterials become also the research hotspots of catalytic fields. Therefore, it is witnessing the flourishing landscape to long-term explore low-cost, highly effective, and durable catalysts.

    Fig. 1 a Jöns Jacob Berzelius (1779-1848) [1]. Copyright @1948, American Chemical Society. b Schematic illustration of the energy band structure of materials with different sizes, and typical images of MAX and MXene from 3D multilayer to 2D nanosheets to 1D nanowires to 0D nanodots [6]. Copyright @2021, The American Association for the Advancement of Science. The morphology was obtained by field emission scanning electron microscopy [7-10], Copyright @2011, WILEY-VCH, @2019, WILEY-VCH, @2018, Elsevier and @2018, American Chemical Society

    In 2004, the exfoliation for graphene opened the door of low dimensional materials, trigging great enthusiasm for exploring a wide range of 2D layered materials, such as graphitic carbon nitride (g-C3N4) [34, 35], hexagonal boron nitride (h-BN) [36, 37], transition metal dichalcogenides (TMDs) and transition metal oxides (TMOs) [38,39]. Among a variety of 2D materials, graphene holds the highest flexibility, conductivity (106S cm-2), and transmittance (97.7%) so far [40]. In 2011, the 3D bulk Ti3AlC2was immersed in hydrofluoric (HF) acid solution by Gogotsi group. As a result, Ti3C2Tx(T represents functional groups such as hydroxyl (-OH), oxygen (-O), fluorine (-F) or chlorine (-Cl), and x is the contents of groups) was stripped,called as MXene with a layered structure similar to graphene, and the excellent conductivity (6000-8000 S cm-2)well comparable to graphene [41-43]. As a new 2D layered material, MXenes have the merits of other 2D materials;more importantly, surface functionalization renders them easily achieve the improved properties. Therefore, the application covers biomedical [44], energy storage devices (battery, supercapacitor) [45, 46], sensors [47], catalysis [48],and electromagnetic interference shielding [49].

    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 Mn+1AXn(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 Mn+1XnTxis usually prepared by etching “A” layer of the specific solvent, such as HF, NH4HF,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.[52-54] 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 Ti3C2fluorescent ultrasmall monolayers MXene sheets by concurrent intralayer cutting and interlayer delamination. The method was also extended to prepare Ti2C and Nb2C ultrasmall sheets. The Ti3C2ultrasmall sheet has the lateral dimension of 2-8 nm and the average thickness of 1 nm. Furthermore, similar to the carbon dots, Ti3C2monolayers sheets showed the strong and tunable photoluminescence and excitation-dependent behavior with the change of pH. Although they were not defined as Ti3C2QDs, 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 [40,57], 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.

    Fig. 2 The classification of quantum dots

    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.

    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 Ti3C2MQDs 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.

    Fig. 3 a State of the art and prospect of 0D MQDs. b Number of journal publications related to publication time and applications science 2017(Source: Web of Science)

    Fig. 4 The timeline showing the development of MQDs in catalysis in the past few years. Reproduced with permission from Refs. [66-79].Copyright @2019, Elsevier, @2019, WILEY-VCH, @2020, Elsevier, @2020, American Chemcial Society, @2020, WILEY-VCH, @2020,American Chemical Society, @2020, WILEY-VCH, @2021, American Chemical Society, @2021, Elsevier, @2021, Elsevier, @2022, The Royal Society of Chemistry, @2022, Elsevier, @2022. MDPI and the author, @2022, Zhengzhou University and Wiley

    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 Ti2C,V2N, and Mo2C worth to be thereof prepared. For instance,the HER activities of 2D Ti2C, Ti3C2, Nb2C, Nb3C4, and V2C MXene with O* or/and OH* terminals calculated by using density functional theory [60]. Their result shows that Ti2CO2MXene has optimized Gibbs free energy of hydrogen adsorption (ΔGH*), which is regarded as an ideal candidate of electrocatalysts. Furthermore, some literatures report that Ti2CTxMXene catalyst has achieved excellent HER performance under acidic conditions, whereas 570 mV@-10 mA cm-2was 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 [63-65]. 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 Ti2C MXene catalyst under alkaline conditions, and exploring the HER performance of 0D Ti2C MQDs or other analogues is worthy for pondering in the future.

    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 Ti3C2QDs, V2C QDs, Nb2C QDs, Ti2N QDs, TiCN QDs,MXene-derived TinO2n-1and TiO2/C-QDs [84-90]. 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.

    3.1 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[93-95]. 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 Ti3C2MQDs 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 [99-101]. 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 (△GH*) [105, 106]. Compared to 2D MXenes, the F-free 0D MQDs have the same property.The Ti3C2(OH)2MQDs with hydroxyl groups modification prepared by the alkalization treatment and mechanical agitation method for electrochemical N2reduction [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.

    Fig. 5 Development of synthesis methods and synthesis of MQDs. a Number of synthesis methods publications on MQDs. (Source: Web of science, 2017 to 2022s). b Scheme of hydrothermal synthesis method [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

    Fig. 6 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

    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 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 Ti3C2MQDs 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 Mo2C QDs/carbon nanosheets and Mo2C 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.

    3.2 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 Ti3C2MXene 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 [122-125] of the MQDs through the electrostatic interaction or physical adsorption to improve the stability,selectivity, conductivity, quantum yield, and photoluminescent properties [126-128]. 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.

    3.2.1 MQDs Modified by Single/Dual Heteroatoms

    MQDs have been applied to biomedical [129, 130], optical device [131-133], energy storage [134-138], and sensor 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 Ti3C2MQDs 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-Nb2C MQDs with an average size of 2.66 nm was synthesized,enhancing the QY of Nb2C 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 Ti3C2MQDs 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-Ti3C2MQDs (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-Ti3C2MQDs as effective reductants (Fig. 7j-k). Apart from the above-mentioned issue, S, N co-doped Ti3C2MQDs [133], Cl, N co-doped Ti3C2MQDs [117], and N, B co-doped Ti3C2MQDs [142]have been prepared by using the same method to improve their physical-chemical properties. Furthermore, the Ti3C2MQDs 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-Ti3C2MQDs 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 MoS2QDs) in the future is something worth investigating. Except for such finding,whether the modification of organic molecules also brings the considerable improvement of properties?

    3.2.2 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 [166-168]. 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 retardedproperties [169, 170]. For instance, the glutathione functionalized Ti3C2MQDs 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].

    Table 1 Synthesis routes of surface-modified MQDs

    Table 1 (continued)

    Fig. 7 Schematics, structural and optical behavior characterizations of MQDs modified by single/dual heteroatoms. a Schematic illustration of the synthesis of N, P-Ti3C2 MQDs; b Charge density difference of N, P functionalized Ti3C2 MQDs; c Fluorescence emission spectra of N,P-Ti3C2 MQDs; d Photoluminescence decay spectra of the N-Ti3C2 MQDs, P-Ti3C2 MQDs, N, P-Ti3C2 MQDs [144]. Copyright @2019, The Royal Society of Chemistry. e Schematic illustration of the synthesis of S-Ti3C2 MQDs, N-Ti3C2 MQDs, S, N-Ti3C2 MQDs; f-h UV-Vis adsorption spectra of S-Ti3C2 MQDs, N-Ti3C2 MQDs, S, N-Ti3C2 MQDs [133]. Copyright @2019, Elsevier. i Schematic illustration of the synthesis of N-Ti2C MQDs; j Antioxidants performance test at KMnO4 solutions; k Mechanism of antioxidants [145]. Copyright @2021, American Chemical Society

    Additionally, uric acid (UA) was used as ligand to enhance photophysical property. Wang et al. [112] prepared UA@Ti3C2MQDs by facile microwave-assisted strategy.2D Ti3C2MXene 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 Ti3C2MQDs. The method is not only easily operation, but also enhanced the oxidation resistance and highly quantum yield of MQDs.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, 153-155]. 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.

    3.2.3 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.

    3.2.3.1 0D MQDs/0D NanomaterialsThe 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 CsPbBr3QDs-Ti3C2TxMQD heterostructure was constructed by facile ultra-sonicating method (Fig. 8a) [156]. XRD of CPB-MXene QD/QDs composites retain the crystal structure of CsPbBr3QDs 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 CsPbBr3QDs 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 [174-177]. 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 Ti3C2MQDs 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.

    3.2.3.2 0D MQDs/1D Nanomaterials1D 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 Ti3C2MQDs/Cu2O nanowire composite by using electrostatic self-assembly strategy for highly efficient photocatalytic CO2conversion(Fig. 9a). TEM image confirmed the MQDs dispersed in the surface of Cu2O 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 Ti3C2MQDs(0110)and Cu2O(200),respectively. In addition, the energy dispersive X-ray (EDX)spectra displayed the Ti/C/O/Cu dispersed uniformly in the surface of Cu2O (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 Cu2O. 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.

    Fig. 8 Schematic and morphological and structural characterizations of 0D MQDs/0D nanocomposite. a Schematic illustration of the synthesis of CsPbBr3-Ti3C2Tx MQD/QD; b XRD patterns of CsPbBr3 QDs, Ti3C2Tx MQD, CsPbBr3-Ti3C2Tx MQD/QD; c TEM image of CsPbBr3-Ti3C2Tx MQD/QD; d HRTEM image of CsPbBr3-Ti3C2Tx MQD/QD [156]. Copyright @2020, American Chemical Society. e Schematic of the formation of Ni@Ti3C2 MQDs; f-g TEM image of Ni@Ti3C2 MQDs. insets of A and B represent HRTEM image of Ni and MQDs, respectively;h EDS of Ni@ Ti3C2 MQDs [77]. Copyright @2022, Elsevier

    Likewise, the WO3/Ti3C2QDs/In2S3with Z-scheme heterostructure was fabricated by using facile solution method (Fig. 9h). TEM and HRTEM images confirmed that the In2S3nanosheet and MQDs with an average size of 1.66 ± 0.04 nm were uniformly dispersed on surface of WO3nanorods (Fig. 9i), and the WO3was coated by In2S3nanosheets (Fig. 9k), corresponding to the interface between the Ti3C2MQDs and WO3(or WO3and In2S3) 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/Ti3C2MQDs heterostructure was prepared via electrostatic interaction (Fig. 9m).TEM image showing the plasmonic gold nanorods (NRs)were distributed in the Ti3C2MXene QDs-interspersed Ti3C2nanosheet (TDTS) (Fig. 9n), and the corresponding HRTEM image of Au(200)is displayed in the inset of Fig. 9n.In addition, the Ti3C2MQDs @Au nanobones were also prepared by using the seed-mediated growth method and self-assembly for exploring the improving performance in the biomedical application.

    Fig. 9 Schematic and morphological and structure characterizations of 0D MQDs/1D nanomaterials heterostructure. a Synthesis process of Ti3C2 QDs/Cu2O NWs/Cu heterostructure; b TEM image of the Ti3C2 QDs/Cu2O NWs heterojunction; c HRTEM image of the interface in Ti3C2 QDs and Cu2O; d-g EDX elemental mapping of Ti3C2 QDs/Cu2O NWs [67]. Copyright @2019, WILEY-VCH. h Schematic illustration of WO3/TQDs/In2S3 heterostructure; i TEM image of WO3/TQDs; j HRTEM image of WO3/TQDs; k TEM image of WO3/TQDs/In2S3;l HRTEM image of WO3/TQDs/In2S3 [74]. Copyright @2021, Elsevier. m Schematic for the preparation of Au NRs/Ti3C2 MQDs/Ti3C2 nanosheets; n TEM image of Au NRs/TDTS. Inset illustration is HRTEM image of Ti3C2 MQDs [157]. Copyright @2021, Elsevier

    Fig. 10 Schematic and morphological and structure characterizations of 0D MQDs/2D nanosheets heterostructure. a Schematic diagram of Ti3C2 MQDs/Ni-MOF; b SEM images of Ti3C2 MQDs/Ni-MOF; c TEM images of Ti3C2 MQDs/Ni-MOF. The inset illustration is the size distribution of Ti3C2 MQDs. Reproduced with permission [69]. Copyright @2020, American Chemical Society. d Synthesis process schematic of Ti3C2 QDC/N-C nanocomposites and e TEM image of Ti3C2 QDC/N-C, inset illustration is SAED pattern; f HRTEM image of Ti3C2 QDC/N-C[137]. Copyright @2021, Wiley-VCH. g Schematic preparation of Ti3C2 MQDs/TiO2/C3N4 hierarchical structure; h HRTEM image of T-CNTC heterostructure [159]. Copyright @2020, Elsevier

    3.2.3.3 0D MQDs/2D NanomaterialsCompared 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 intercalation, liquid-phase exfoliation strategy and electrospinning, and so on [182-184], The introduction of intrinsic defects such as vacancies, lattice distortions and adatoms on the surface of graphene, g-C3N4and MoS [185-187], 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 Ti3C2MQD/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, promoting an efficient N2photoreduction 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-O2batteries (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 O2molecules and intermediates LiO2. Controlling the number of interfaces is of great significance for improving the structure of photocatalyst and enhancing the performance. The 2D/2D/0D (TiO2/C3N4/Ti3C2MQDs) hierarchical structure was engineered by van der Waals and electrostatic interactions (Fig. 10g) [159].The TiO2/C3N4exhibits core-shell structure, and the MQDs decorated on the surface of C3N4nanosheet 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-C3N4nanosheets [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.

    3.2.3.4 0D MQDs/3D NanomaterialsCompared 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 Ti3C2MQDs with the surfaces detect-rich/3D mesoporous C3N4were prepared by electrostatically self-assembly strategy (Fig. 11a) [76]. TEM image confirmed that the MQDs were uniformly dispersed on the surface of hollow C3N4(Fig. 11b), and the HRTEM image gives the corresponding lattice spacing of 0.329 and 0.261 nm, attributed to the plane of C3N4(002)and Ti3C2(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 H2O2production.

    Fig. 11 Schematic and morphological and structure characterizations of 0D/3D heterostructure. a The prepared process diagram of C3N4/r-Ti3C2 QDs; b TEM image of C3N4/r-Ti3C2 QDs; c HRTEM image of C3N4/r-Ti3C2 QDs; d XRD pattern of C3N4, C3N4/r-Ti3C2 QDs [76].Copyright @2022, The Royal Society of Chemistry. e Schematic preparation of Ti3C2 MQDs/3D Inverse Opal g-C3N4 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 Ti3C2-QDs/ZnIn2S4/Ti(IV) heterostructure; i-k TEM image of Ti3C2-QDs/ZnIn2S4/Ti(IV) at different magnifications; l-p Elemental mappings of Ti3C2-QDs/ZnIn2S4/Ti(IV) [78]. Copyright @2022, MDPI and the authors

    Lin et al. [73] prepared the Ti3C2MQDs decorated defective inverse opal g-C3N4(TC/CN) by using electrostatic self-assembly method (Fig. 11e). SEM image shows the microstructure of porous g-C3N4with a long-range order (Fig. 11f), and the corresponding HRTEM confirmed the formation of the interface between MQDs and g-C3N4(Fig. 11g). Such bonding contributes to achieve the carrier separation. In addition, the Ti3C2MQDs/ZnIn2S4/Ti(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 TiO2/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.

    3.3 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 Ti2CTxvia 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 TiO2QDs supported on the surface of carbon layer were prepared by solvothermal method using small and fewer-layered Ti3C2MXene 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 TiO2/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 TiO2/C-QDs in daylight under 365 nm excitation wavelength. Moreover,the oxygen-vacancy-rich TinO2n-1QDs (OV-TnQDs) were prepared by H2O2oxidation and subsequent quenching in liquid nitrogen (Fig. 12e) [90]. The quench process makes TiO2nanoparticles fast crystallization and downsized to quantum size. After that, the annealing process with H2/Ar mixed gas promotes the generation of O vacancies. TEM image confirmed the uniform distribution of OV-TnQDs(Fig. 12f). The corresponding HRTEM image exhibit that the OV-TnQDs are made up of Ti2O3and Ti3O5(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 Ti3C2TxMXene-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/TiO2nanoparticles 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.

    Fig. 12 Schematic and morphological and structure characterizations of MXene-derived inorganic QDs. a Schematic of synthesis for TiO2/C-QDs; b TEM image of TiO2/C-QDs, the inset illustration is size distribution of TiO2/C-QDs; c HRTEM image of TiO2/C-QDs; d UVVis adsorption spectra of TiO2/C-QDs [88]. Copyright @2020, The Royal Society of Chemistry. e Illustration of preparation of oxygen-vacancyrich TinO2n-1 QDs @PCN; f-g TEM image of OV-Tn QDs @PCN at different magnification; h-i HRTEM image of TinO2n-1 QDs @PCN with different number of oxygen vacancies [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

    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.

    4.1 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 Ti3C2MQDs,TiCN MQDs, Nb2C MQDs and Ti2N 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 Ti3C2MQDs by using bath and probe sonication method [110]. TEM image shows the dotlike uniform distribution of Ti3C2MQDs, with the average size of 4.9 ± 1.6 nm (Fig. 14a-b). HRTEM image can display 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.

    Fig. 13 Characterization techniques of MQDs

    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-MoS2to 1T-MoS2. 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].

    Fig. 14 Morphology and structure characterization of MQDs. a TEM image of Ti3C2 MQDs; b Size distribution of Ti3C2 MQDs; c HRTEM image of Ti3C2 MQDs, the inset illustration is corresponded Fourier transform; d AFM image of Ti3C2 MQDs; e The height distribution based on AFM; f Height profiles of Ti3C2 MQDs along d image [110]. Copyright @2017, The Royal Society of Chemistry. g XRD patterns of Ti3AlC2, Ti3C2 MXene, and Ti3C2 MQDs [204]. Copyright @2019, The Royal Society of Chemistry. h XRD patterns of bulk Ti3C2 MXene,Ti3C2 nanosheet, and Ti3C2 MQDs [59]. Copyright @2019, WILEY-VCH. i XRD patterns of Nb2AlC and Nb2C MQDs [205]. Copyright@2020, Elsevier

    4.2 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 Ti3C2MQDs and Nb2C MQDs. Compared to 2D Ti3C2MXene, the (002) lattice spacing was further expanded due to the intercalation of TMA ions during preparation. However, the reduced intensities of ((10l)),(004), and (110) diffraction peaks indicate a good dispersion of the Ti3C2MQDs [204]. Besides, Lu et al. prepared Ti3C2MQDs by hydrolyzing method; compared to the bulk Ti3C2,the weaker peak intensity and broad width of MQDs indicate the grain refinement [59]. More importantly, in 2020, the Nb2C 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 Ti3C2TxMQDs 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-1and 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 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].

    Fig. 15 Composition and optical spectral characterization of MQDs. a XPS survey spectra of Ti3C2 MQDs; b High-resolution spectra of Ti 2p [204]. Copyright @2019, The Royal Society of Chemistry. c FTIR spectra of Ti3C2 MQDs [215]. Copyright @2018, The Royal Society of Chemistry. d Raman spectra of Ti3C2(OH)2 MQDs [70]. Copyright @2020, WILEY-VCH. e UV-Vis adsorption, PL, and PLE spectra of Ti3C2 MQDs; f PL spectra at different excitation wavenumbers [58]. Copyright @2017, WILEY-VCH

    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 Ti3C2MXene,the characteristic Raman bands of 147 cm-1, 260, 412,609 cm-1correspond 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) [174,217-219]. 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 [220-222]. 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.

    4.3 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 Ti3C2MQDs 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]. Ti3C2MQDs 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 Nb2CO2MQDs, the weak electrons exchange interaction between Highest Occupied Molecular Orbital (HOMO)and Lowest Unoccupied Molecular Orbital (LUMO) of S, N-Nb2CO2MQDs 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.

    Fig. 16 a-b Simulation diagram of energy levels and charge transfer processes in the MQDs and N-MQDs; c-d DOS calculation of MQDs and N-MQDs; e Work function of MQDs and N-MQDs [147]. Copyright @2018, The Royal Society of Chemistry. f-g DFT calculation of total and projected DOS of Nb2CO2 QDs and S, N-doped Nb2CO2 QDs [86]. Copyright @2020, The Royal Society of Chemistry

    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 H2production, oxygen reduction reaction (ORR), pollutant degradation, CO2reduction, NH3production, and H2O2production.

    5.1 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 electron diffusion length, expecting to become the high-performance electrocatalyst candidates.

    Fig. 17 Electrocatalytic performance of MQDs. a Number of journal publications on different catalytic aspects (Source: Web of Science). b Reaction mechanism of electrocatalytic N2 reduction and free energy calculation on the Ti edge of Ti3C2, Ti3C2F, Ti3C2OH MXene from the adsorption of N2 to the reduction of NH3; c The average NH3 yield and faradaic efficiency of Ti3C2OH MQDs at different applied voltages [70].Copyright @2020, WILEY-VCH. d Charge density difference of Ti3C2 MQDs/Cu; e The NH3 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

    5.1.1 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 NH3production 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 N2and competitive HER side reaction lead to low NRR selectivity and ammonia production rate.

    5.1.1.1 The-OH Functional Groups of MQDs as Active SitesMQDs 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 Ti3C2OH MQDs were first prepared by agitate-assisting for NRR catalysts. Figure 17b shows the reason why Ti3C2OH acts as electrocatalyst for NRR with excellent performance: i) N2molecules 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 Ti3C2F2and Ti3C2, there occur no side reactions of HER when Ti3C2OH 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 Ti3C2OH is 0.79 eV) [70]. Therefore, the Ti3C2OH MQDs with abundant Ti edge and -OH functional groups were prepared.Experiment result shows that the Ti3C2OH MQDs provides 62.94 μg h-1mg-1catat - 0.50 V. Compared to 2D MXenes,it shows excellent NRR activity due to the offered more active sites, highlighting the unique advantages of size and surface functional groups of MQDs (Fig. 17c).

    5.1.1.2 The Interface Design of MQDs-Based CompositesApart from the surface terminal effect on the adsorption capability of N2, the interface engineering is of great significance for promoting the adsorption and activation of N2. 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 Ti3C2MQDs 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 Ti3C2MQDs/Cu provides 78.5 μg h-1mg-1catat - 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 Ti3C2nanosheet (4.7 μg h-1mg-1catat - 0.20 V) under same condition [232].

    5.1.2 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 Ti3C2MXene with -O group has been demonstrated to be HER active sites with the minimum Gibbs free energy(△GH*). 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 Ti2CTxMQDs/Cu2O/Cu foam nanocomposite was prepared by selfassembly method (Fig. 18a), and the Ti2CTxMQDs 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 Cu2O 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 △GH* 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.

    5.1.3 Electrocatalytic OER, ORR, and MOR

    5.1.3.1 MQDs as Electronic ConductorThe 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 MoS2QDs @Ti3C2TxQDs@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.

    5.1.3.2 The Defect of MQDs as Active SitesThe 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 Li2O2formation and decomposition, achieving enhanced electrocatalytic kinetics. Wang et al. [137] prepared Ti3C2MXene quantum dot clusters (Ti3C2QDC) with rich grain boundaries and edge defects through hydrothermal thermal-shearing reaction method. The defects were firstly characterized by using ACSTEM. 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-O2catalytic 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.

    5.2 Photocatalysis

    Fig. 18 a Synthesis of Ti2CTx MQDs/Cu2O/Cu foam nanocomposite (top), the evolution process of functional groups and Soft X-ray emission spectrum (SXES) image; b The LSV image [164]. Copyright @2022, Zhengzhou University and Wiley. c TEM image of MoS2QDs @Ti3C2TxQDs@MWCNTs and d ORR performance of sample [66]. Copyright @2019, Elsevier. e AC-STEM image of Ti3C2 nanosheet; f ACSTEM of Ti3C2 MQDs; g Initial deep discharge-charge curves of the three samples at 200 mA g-1; h Cycling stability and terminal dischargecharge voltages of Ti3C2 QDC/N-C electrode at 200 mA g-1 [137]. Copyright @2021, Wiley-VCH

    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),CO2reduction, NH3production, and H2O2production.

    5.2.1 Photocatalytic Water Splitting

    Hydrogen (H2) 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 H2is a promising conversion technology [237]. Over the past few years, transition metal oxides, transition chalcogenides, and organic semiconductors have been developed 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.

    5.2.1.1 MQDs as Photoelectrons AcceptorCompared to the traditional TiO2photocatalyst, the layered g-C3N4possesses 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 Ti3C2QDs 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-C3N4, which can capture electrons to facilitate surface redox reactions [160]. Such behavior of low recombination capability of photogenerated electron-holes pairs 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 Ti3C2QDs/g-C3N4is higher than 2D g-C3N4nanosheets (Fig. 19d). Therefore, MQDs as electron acceptors to capture quickly the photogenerated electrons, facilitating the efficient carrier transfer. Finally, the H2production rate of Ti3C2QDs/g-C3N4(5111.8 μmol g-1h-1) is far higher than g-C3N4(196.8 μmol g-1h-1), Pt/g-C3N4(1896.4 μmol g-1h-1) and 2D MXene/g-C3N4(524.3 μmol g-1h-1) at the same conditions (Fig. 19e), highlighting the merit of MQDs.

    Fig. 19 Photocatalytic water splitting performances of MQDs-based heterostructure. a Schematic react mechanism of g-C3N4@Ti3C2 QD; b Steady photoluminescence spectra of g-C3N4, MQDs@g-C3N4; 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

    5.2.1.2 Designing the Z-scheme structure of MQDsbased photocatalystFor 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 BiVO4@ZnIn2S4/Ti3C2(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 H2and O2production rates (Fig. 19g-h). Also, the Ti3C2-QDs/ZnIn2S4/Ti(IV) heterojunction photocatalyst was prepared for improving hydrogen production performance[78].

    5.2.2 Photocatalytic CO2 Reduction

    The goal of carbon neutrality is an inevitable choice based on the high-quality development of China’s economy and society. Photocatalytic CO2reduction is an efficient technology to achieve a low carbon economy [239]. Furthermore, methanol, the reduction product of CO2, can also be used as a fuel. It is worth noting that the photocatalytic CO2reduction activity is related to the light adsorption ability, photogenerated carriers’ separation efficiency, and the activation ability of photocatalysts for CO2molecules[240]. Compared to 2D MXene, MQDs offer controllable bandgap due to size effect, abundant unsaturated sites to adsorb CO2molecules, contributing to the selectivity of reaction products.

    Designing the S-scheme structure of MQDs-based photocatalyst: The semiconductor Cu2O has narrow bandgap of 2.2 eV, and the more negative potential of conduction band than reduction potential of CO2, which is unfavorable photocatalytic CO2reduction. 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 Cu2O transfer into CB of MQDs due to the Fermi level of MQDs lower than CB of Cu2O, avoiding the recombination of carriers (Fig. 20b). The MQDs as co-catalyst to increase light adsorption capability of Cu2O nanowires, and reduce the charge transfer resistance (Fig. 20c-d). Therefore, compared to the 2D MXene, the Ti3C2MQDs/Cu2O/Cu contributes to high CH3OH yield (Fig. 20e). Furthermore, since designing core-shell MQDs-coupled nanosheet (TiO2/C3N4)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].

    5.2.3 Photocatalytic NH3 and H2O2 Production

    Compared to the traditional complex Haber-Bosch process,photocatalytic NH3production 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 N2molecules. The MQDs with quantum confinement effect and good conductivity can promote the bandgap control and effective charge transfer.

    Fig. 20 Photocatalytic CO2 reduction performances of MQDs-based photocatalysts. a TEM image of Ti3C2 MQDs/Cu2O/Cu; b Photocatalytic reaction mechanism of Ti3C2 MQDs/Cu2O/Cu; c UV-Vis diffuse reflectance spectra (DRS) of samples; d Nyquist plots of samples; e Methanol yield of samples [67]. Copyright @2019, WILEY-VCH. f TEM of TiO2/C3N4 with core-shell; g S-scheme heterojunction before and after contact, and light irradiation; h Schematic diagram of structure of TiO2/C3N4/Ti3C2 MQDs [159]. Copyright @2020, Elsevier

    5.2.3.1 Designing the Band Structure of MQDs-Based PhotocatalystThe 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).

    5.2.3.2 Constructing Defects of MQDs for Reactant AdsorptionThe defect sites with lower binding energy facilitate to promote the adsorption of N2molecules. In 2022, Chang et al. [76] engineered a photocatalyst with Schottky junction and defects. The Ti3C2MQDs with amount of oxygen vacancies (OV) and Ti3+sites were anchored on the surface of mesoporous C3N4hollow nanosphere (Fig. 21e). Such unique design has two advantages: 1) the vacancy defects are beneficial to increase the adsorption and activation of N2molecules; 2) the quantum confinement effect of MQDs promotes the light adsorption intensity of C3N4(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-C3N4, forming Schottky junction at the interface. Thus, it increases the work function of energy band, promoting the photoexcited carrier separation [73]. Such optimized photocatalytic activity of Ti3C2MQDs/g-C3N4achieves a high yield of 560.7 μmol L-1h-1(Fig. 21i-k).

    Fig. 21 Photocatalytic performances of NH3, H2O2 production. a SEM image of Ti3C2 MQDs/Ni-MOF; b Energy band structure of Ti3C2 MQDs/Ni-MOF; c UV-Vis diffuse reflectance spectra of Ti3C2 MQDs, Ni-MOF, Ti3C2 MQDs/Ni-MOF nanocomposites with different loads of MQDs; d The NH3 yield of Ti3C2 MQDs/Ni-MOF [69]. Copyright @2020, American Chemical Society. e TEM image of C3N4/r-Ti3C2 MQDs Schematic diagram of photocatalytic N2 fixation; f Photocurrent test in Ar and N2 of samples; g UV-Vis DRS spectra of C3N4, C3N4/r-Ti3C2 MQDs; h Photocatalyst NH3 production rate and side reaction of H2 production [76]. Copyright @2022, The Royal Society of Chemistry. i Transfer of photogenerated electrons and holes near the carbon vacancies of C3N4 and C3N4/ r-Ti3C2 MQDs; j The energy band structure of C3N4/ r-Ti3C2 MQDs; k The photocatalytic reaction mechanism diagram of C3N4/r-Ti3C2 MQDs [73]. Copyright @2021, American Chemical Society

    5.2.4 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 pollutants. In 2021, the Ti3C2MQDs acted as co-catalyst of Bi2O3(BiO/TiC) to boost photocatalytic tetracycline(TC) degradation in water [75]. Compared to pure Bi2O3,the MQDs/Bi2O3nanocomposites 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 Bi2O3photocatalyst, 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 Ti3C2MQDs have also been used as co-catalyst to prepare Ti3C2MQDs/SiC nanocomposite[71] (Fig. 22e), all-solid-state WO3/TQDs/In2S3Z-scheme heterostructure [74] and Ni@MQDs [77] nanocomposite photocatalyst, achieving the goal of photocatalytic removal of NO purification and pollutants in water.

    5.3 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.

    Fig. 22 Photocatalytic pollutant degradation performances of MQDs-based heterojunction. a UV-Vis diffuse reflectance spectra (DRS) of samples; b Calculation of band gap of Bi2O3, BiOTIC-75; c Photocatalytic reaction mechanism illustration of BiOTIC-75; d Photocatalytic degradation of TC by Bi2O3, Ti3C2 MQDs/Bi2O3 with different content of MQDs [75]. Copyright @2021, Elsevier. e Energy band structure of MQDs/SiC, SiC, MQDs [71]. Copyright @2020, American Chemical Society

    5.3.1 MQDs as Photoanode

    In 2020, the Janus-structure Co-Ti3C2MQDs were prepared by thermal-cutting method, which was used as photoanode for water oxidation [72]. The Co nanoparticles were coupled with Ti3C2MQDs 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 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).

    Fig. 23 The photoelectrocatalytic performances of MQDs as co-catalyst. a UV-Vis adsorption spectra of Ti3C2 MQDs, Co-MQDs with different rates of Co/Ti; b Photoelectric conversion efficiency of Ti3C2 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 Ti3C2 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/BiVO4 photoanodes; g Photoelectrochemical water splitting device [161]. Copyright @2022, Wiley-VCH

    5.3.2 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].

    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.

    6.1 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.

    6.2 Synthesis of Novel MQDs

    There are many kinds of MXenes reported so far experimentally and in theoretical prediction, while the application focuses mainly on Ti3C2MQDs. 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.

    6.3 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.

    6.4 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.

    6.5 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.

    6.6 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.

    AcknowledgementsThis research is supported by the National Natural Science Foundation of China (51872115, 51932003) and 2020 International Cooperation Project of the Department of Science and Technology of Jilin Province (20200801001GH).

    FundingOpen access funding provided by Shanghai Jiao Tong University.

    Open AccessThis article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing,adaptation, distribution and reproduction in any medium or format,as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.

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