Nanomicro Lett.｜Competitive advantage of 2D MXene-derived MQDs in catalytic reactions

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Summary of the review
      It is well known that two-dimensional (2D) MXene-derived quantum dots (MQDs) inherit the excellent physicochemical properties of the parent phase MXenes, as the Chinese saying goes: "Blue is better than blue." Therefore, 0D quantum dots can obtain larger specific surface area, excellent optical properties and strong quantum confinement effect. At present, as a new star of functional materials, MQDs have aroused great research enthusiasm and have been applied in the fields of physics, chemistry, biology, energy conversion and storage. The surface properties of small-scale MQDs include the type of surface functional groups, and the functionalized surface directly determines their performance. As Nobel laureate Wolfgang Pauli said, "God created the body, but the surface was invented by the devil". It is precisely based on the abundant surface functional groups that MQDs still have a lot of room for exploration. We are witnessing this kind of excellence, and hopefully even more to be expected. Today, MQDs have been widely used in catalysis, but the related reviews are rarely reported. Here, we provide an up-to-date overview of MQDs in the field of catalysis over the past five years, from the origin and development of MQDs, synthetic routes for MQDs and functionalized MQDs, to advanced characterization techniques, and finally discuss the catalytic mechanism of MQDs, as well as the relationship between the two The competitive advantage of dimensional MXene. To explore the diversity and prospects of MQDs in catalytic applications, our review will inspire more efforts to synthesize optimal MQDs and design high-performance MQDs-based catalysts.

        Recently, the team of Zheng Weitao from the School of Materials Science and Engineering of Jilin University published a review paper entitled "Quantum Dots Compete at the Acme of MXene Family for the Optimal Catalysis" in the well-known domestic journal Nano-Micro Letters. Based on the research status of MQDs in the field of catalysis in the past five years at home and abroad, the latest progress of MQDs in the fields of electrocatalysis, photocatalysis and photoelectrocatalysis is systematically introduced, different catalytic mechanisms are expounded, and the role of MQDs in the catalytic reaction process is summarized. , which underscores the competitive advantage of MQDs and 2D MXenes in catalytic applications. In addition, this review summarizes the preparation methods and formation mechanism of MQDs, and based on the abundant surface groups of MQDs, the surface modification strategies of MQDs and the synthesis strategies of MQDs-based nanocomposites of different dimensions are summarized. It also summarizes the current advanced characterization techniques of MQDs and their limitations. Finally, the challenges and prospects for the development of MQDs in the field of catalysis are discussed.

Graphical guide

        In 1836, the Swedish scientist Jöns Jakob Berzelius first defined the terms of catalyst and catalysis, describing "a catalyst is a new substance that can produce chemical activity". Since then, various catalytic reactions have been widely used in industrial production, which has greatly promoted chemical and development of human society. The development of non-precious metal-based catalysts is of great significance for cost reduction. MXenes have attracted extensive attention due to their high electronic conductivity, abundant catalytic active sites, a large number of surface functional groups, and large specific surface area. The morphology of MXene can be flexibly controlled according to the requirements. Moreover, the semiconductor band gap of MXene increases with decreasing particle size, and this property plays an important role in promoting the spectral response range in photocatalytic reactions and improving the extraction of photogenerated electrons.

Figure 1. Catalysis and catalyst nomenclature and topography of MXenes with different dimensions, band structure diagram of the semiconductor bandgap as a function of size for zero-dimensional MQDs.

       Based on the application of MQDs in photocatalysis, electrocatalysis and photoelectric catalysis in the past five years, the timetable as shown in the figure below is summarized. The research scope mainly involves electrocatalytic water splitting, nitrogen reduction, methanol oxidation; photocatalytic water splitting, pollutant degradation , nitrogen reduction reaction; photoelectric catalytic water splitting reaction. In addition, the design of MQDs-based heterostructures and the corresponding catalytic reaction mechanism are discussed, and their unique advantages compared with 2D MXenes are also summarized.

Figure 2. Timeline of MQD development in catalysis over the past five years.

        Based on the current synthetic routes of MQDs, the following 10 methods are summarized, and the advantages and disadvantages of different methods are compared. The commonly used synthetic routes are hydrothermal, solvothermal and mechanical ultrasonic methods.

Figure 3. Summary of the existing synthetic methods of MQDs and the route, structure and morphology characterization of hydrothermal and solvothermal synthesis of MQDs.
Summary of advanced characterization techniques for MQDs.

Figure 4. Characterization techniques for MQDs.



Summarize

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       The diversity of synthesis, composition, and physicochemical properties of MQDs has made great progress in the field of catalysis. Compared with 2D MXenes, the surface metal ions, functional groups, and abundant edge sites of MQDs can all serve as adsorption and activation sites for gas molecules. Furthermore, the size effect of MQDs enables controllable band gaps, acting as cocatalysts to promote efficient charge separation and enhanced charge transfer kinetics. In order to further improve the low yield, easy aggregation, poor stability, and difficult to precisely control surface chemistry of MQDs, so as to develop high-performance MQDs-based catalysts, the following issues need to be considered:

   (1) New synthesis strategies of MQDs and controllable growth of MQDs:

     At present, most MQDs are prepared based on further processing after etching with fluorine-containing reagents. Green and safe synthetic routes for fluorine-free MQDs such as molten salt method and electrochemical method are worth exploring. In addition, in order to prepare high-quality MQDs (desired structure, shape, size, surface group arrangement, type of defects, etc.), precise control of reaction conditions (such as reaction temperature, time, pH, power, etc.) The formation mechanism and the realization of the controllable growth of MQDs are of great significance.

   (2) Diversity of MQDs composition:

     At present, most of the research on MQDs focuses on Ti3C2Tx MQDs. Other compositions such as Ti2CTx, Mo2CTx, Nb2CTx and other single metal MQDs, and double transition metal MQDs need to be synthesized, and their catalytic reaction mechanism will be further explored.

   (3) The role of MQDs in catalytic reactions:

       The high surface energy of MQDs can easily induce agglomeration, and the selection of supports and the exploration of the interaction mechanism between MQDs and supports are of great significance for improving the electrocatalytic performance. In addition, the interaction between the abundant functional groups on the surface of MQDs can form a conductive network, which can be used as a carrier for nanomaterials such as single atoms and nanoparticles to achieve the maximum utilization of catalytic activity.

    (4) Explore new catalytic applications of MQDs:

     Compared with carbon dots with simple internal structures, MQDs have more controllable compositions and complex internal structures, which pose greater challenges. To further guide the design of high-performance catalysts, it is necessary to construct some theoretical models to predict the effects of surface state and external environment (temperature, pressure, and illumination) on catalytic activity. In addition, the study of MQDs in electrocatalytic applications and catalytic reaction mechanism needs to be further enhanced.

    (5) Constructing surface defects of MQDs:

     Generally, defect sites with low binding energy are considered as key sites for catalytic activity, which can be obtained by atomic doping, electrochemical reduction, reducing agents, etc. The introduction of surface defects helps to improve the electronic junction of active sites around MQDs, thereby improving the electron transfer process and the adsorption/desorption behavior of reactants.

     (6) Introduction of in situ characterization technology:

    It is expected to observe the atomic structure of MQDs under ultra-high-resolution electron microscopy, study the distribution of defects, the arrangement of atoms, and use special sample-supported grids to visualize the dynamic evolution of catalysts. Advanced in situ techniques such as Raman, synchrotron radiation, and FTIR can explain the formation mechanism of MQDs-based catalysts, reveal the structural evolution during the catalytic process, and guide the design of high-performance MQDs-based catalysts. In addition, some in situ characterizations can visualize the surface reconstruction process of the composite during the electrochemical reaction, find out the real catalytic active sites, and compare the morphology and structure changes after the reaction, which is important for studying the stability of the catalyst. meaning.


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Literature link

https://doi.org/10.1007/s40820-022-00908-3

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