Construction Of Self-assembled MXene-based Nanocomposites And Study Of Their Photocatalytic Properties

With the rapid the development of social industrialization,environmental problems and energy depletion have become the focus of attention in the development of human society.Photocatalytic technology,which can convert solar energy into chemical energy,such as photolysis of water to produce hydrogen,degradation of organic pollutants,and photoreduction of carbon dioxide,has become a promising approach to improve the existing environmental and energy issues.Recently,two-dimensional transition metal carbides（MXene）,has been widely used in the fields of energy storage and conversion,environmental adsorption,and photocatalytic degradation of pollutants due to their excellent surface chemistry,high electrical conductivity,excellent ductility,and flexibility.However,the low photoconversion performance of MXene and the absence of active sites greatly limit the application of MXene in photocatalysis.In this study,a variety of metal nanoparticles/metal sulfides/oxides were combined with MXene phase based on a self-assembly strategy to construct MXene-based nanocomposite catalyst;the effects of the morphology,phase structure,elemental composition and elemental valence on the photocatalytic performance of MXene-based nanocomposite catalysts were investigated;and the photocatalytic cycle stability and photocatalytic mechanism of the composite catalyst were explored.This study solved the problems of easy aggregation,poor stability,low photoelectric conversion performance of metal particles and easy oxidation and difficult dispersion of MXene in the process of constructing MXene-based nano-composite catalyst;realizing the photocatalytic degradation of organic pollutants by MXene-based composite nanomaterials under visible light and the efficient hydrolysis of water to produce hydrogen under visible light.The innovative results can be seen as follows:（1）Based on the self-reduction properties of MXene,a novel MXene@Pd NPs nanocomposite was prepared by in-situ self-assembly method.Through the analysis of its morphology and element valence state,it is proved that the self-reduction property of MXene originated from the different valence states of titanium in MXene nanosheets.In addition,the photocatalytic activity and stability of MXene@Pd NPs nanocomposites with different morphologies and sizes were studied by adjusting the reaction time.The results show that the photocatalytic degradation efficiency of the composite was the highest when the self-reduction reaction time was 20 min.MXene as a carrier can not only provide a large specific surface area for the formation of metal particles,but also prevent the aggregation of metal particles during the reduction process,improving the degradation performance and cycle stability of the catalyst.At the same time,MXene acts as a cocatalyst in the catalytic reaction process,which plays the role of capturing and separating photogenerated holes,accelerating the reaction rate and further improving the photocatalytic activity.（2）Aiming at the problem of aggregation and oxidation of MXene nanosheets during the catalytic reaction process,the PVA/PAA/MXene composite nanofiber materials were prepared by electrospinning technology and self-assembly method.Further,the Pd NPs-supported composite fiber materials（PVA/PAA/MXene@Pd NPs）was finally obtained through self-reduction reaction,and their photocatalytic activity and stability under visible light were investigated.The results show that DMSO can enlarge the spacing between MXene layers,change the surface environment of MXene,and prevent the oxidation of MXene nanosheets.The MXene nanosheets were uniformly dispersed in the spun fibers by electrospinning technology,which prevented the aggregation and oxidation of MXene nanosheets during the catalytic process.In addition,MXene nanosheets can provide abundant active sites and sufficient space for the support of palladium particles,which can effectively prevent the aggregation of palladium particles during the reduction process,and thus the resulting PVA/PAA/MXene@Pd NPs shows excellent photocatalytic activity and photocatalytic stability.（3）Aiming at the problems of high cost and limited storage of noble metal materials,based on the unique advantages of ternary heterojunction structure and MXene-based composites in the field of photocatalysis,the ternary catalyst Cu₂O/Ti O₂/MXene was synthesized by in situ solvothermal method using hydrothermally synthesized Ti O₂/MXene as template and its photocatalytic performance was studied.The results show that the structure of Cu₂O/Ti O₂/MXene changes after the introduction of Cu₂O with smaller band gap width,forming a p-n type semiconductor,which can not only enhance the separation efficiency of photogenerated carriers,but also improve the lifetime of photogenerated electrons,thus increasing the photocatalytic activity.In addition,the use of MXene as substrates and other semiconductor materials instead of precious metals not only saves costs,but also improves the dispersion of heterojunction,solving the problem of poor catalytic stability,and providing more catalytic active sites for the catalytic reaction.（4）To expand the application of MXene-based composites in the field of photocatalysis,based on the self-reduction properties of MXene,a new type of ternary heterojunction composite catalyst MXene@Au@Cd S was synthesized by in-situ self-assembly method and hydrothermal method.Its structure,photocatalytic hydrogen production performance and charge transport mechanism were studied.The experimental results show that MXene@Au@Cd S exhibits excellent photocatalytic hydrogen production performance and high photocatalytic efficiency,and its excellent photocatalytic activity can be attributed to the fact that MXene provides more active adsorption sites for Au and Cd S nanoparticles.The dots and reaction centers inhibit the recombination of photogenerated electrons and holes,and the synergistic effect of Au strong surface plasmon resonance expands the optical response range of Cd S,enhances the utilization of sunlight and accelerates the rate of photocatalytic hydrogen production.