Preparation And Thermoelectric Properties Of MXene And Its Composites With SWCNTs

In recent years,due to the increasingly serious problems of environmental pollution and energy shortage,the development and application of environmentally-friendly clean energy has attracted more and more attention from scientists.However,with the increase in the scope of human activities and the expansion of resources,phenomena such as energy shortages and the greenhouse effect of environmental pollution have become increasingly prominent.Therefore,the development of an environmentally-friendly clean energy is an urgent problem to be solved.Thermoelectric material is a kind of green energy conversion material,which can directly realize the mutual conversion between heat energy and electric energy,and has broad application prospects in solving the energy crisis and environmental pollution.Because of its unique electronic structure,two-dimensional（2D）nanomaterials have shown excellent potential in terms of electrical conductivity and Seebeck coefficient,and have been favored by researchers in recent years.Therefore,2D thermoelectric materials have very important potential research value.MXene was first assembled by Professor Yury Gogotsi from Drexel University in the United States.Compared with the bulk material MAX material,the Ti₃C₂T_xnanosheet structure has enhanced anisotropy,and has many improved properties in terms of electronic,optical,thermal and mechanical properties,especially the highest conductivity can reach the level of metal.However,compared to the currently popular inorganic thermoelectric materials,the Seebeck coefficient of Ti₃C₂T_x is low,which is not conducive to practical application.Researchers have made many efforts to study the properties of Ti₃C₂T_x semiconductors and found that the surface contains a large number of-F,-OH and-O functional groups,which can affect the band gap width of the material.The F and OH groups also affect the electronic structure of the original MXene system because each F and OH group can only get one electron from the surface.When the-F and-OH functional groups are adsorbed onto the surface of MXene,each F and OH group acquires one electron from MXene,and the Fermi energy moves downward,but MXene is still metallic after-F and-OH functionalization.In addition,an additional band occurs at approximately-7.0 e V through F-p and Ti-d.When the O group is adsorbed on the MXene surface,each O atom receives two electrons from MXene,causing the Fermi energy to move down to the center of the gap separating the Ti-d and C-p bands,which explains why MXene becomes a semiconductor after-O functionalization.Therefore,the optimization of thermoelectric performance through the regulation of functional groups is a very potential way,and theoretical calculations have found that it has a theoretical ZT value of about 1.On the other hand,composite materials are another way to improve thermoelectric performance.By combining MXene with a material with a large Seebeck coefficient,the advantages of the two materials can be combined to obtain a high-performance thermoelectric material.Therefore,it is particularly important to explore appropriate methods to regulate the functional groups on the surface of Ti3C2Tx and select suitable materials to improve the thermoelectric performance by optimizing the composite method.In this paper,LiF/HCl etching method is used to prepare single-layer or few-layer Ti₃C₂T_x nanosheets.The surface functional groups are adjusted and compounded with SWCNTs in different ways,and their thermoelectric properties are systematically tested and investigated.1.LiF/HCl etching method is used to prepare single-layer or few-layer MXene（Ti₃C₂）nanosheets and the method of adding alkali solution for hydrothermal reaction to adjust the functional groups on the surface of MXene（Ti₃C₂）.Finally,MXene（Ti₃C₂）thin-film was prepared by vacuum filtration,and its thermoelectric properties were studied in detail.The study found that the etched MXene（Ti₃C₂）can form a good dark green aqueous solution,and the resulting thin-film can obtain a high conductivity of about 2000 S/cm.Although the near-metal MXene（Ti₃C₂）has high conductivity,the Seebeck coefficient is only-5.5μV/K.Furthermore,the functional groups on the surface are adjusted by adding different alkaline solutions for hydrothermal reaction at different temperatures.Finally,by exploring the influence of alkaline environment and reaction temperature on the functional groups on the surface,the a-Ti₃C₂ composite film after KOH treatment was obtained.The conductivity of A-Ti₃C₂ still maintains 1652 S/cm,and the Seebeck coefficient is significantly improved to 16.5μV/K,and the power factor is as high as 44.98μW m^-1K^-2,which is 7 times that of Ti₃C₂ thin-film.2.LiF/HCl etching method is used to prepare single-layer or few-layer MXene（Ti₃C₂）nanosheets.By adding KOH for hydrothermal reaction,on the one hand,the functional groups on the surface of the composite material can be adjusted;on the other hand,the p-n type conversion of the composite material is used to improve the thermoelectric performance.Then MXene/SWCNTs（M/S）composite thin-film was prepared by vacuum filtration.Finally,the thermoelectric properties of composite materials were optimized by changing the different ratios of the two materials.The results show that the composite film（Hy-M/S）prepared by hydrothermal reaction treatment greatly improves the thermoelectric properties of the material.When the SWCNTs content is 50%,the power factor is as high as 56.89μW m^-1 K^-2,which is more than 8 times higher than that of the directly composite M/S composite film.The introduction of carbon tubes on the one hand reduces the stacking of MXenes nanosheets and helps maintain the high electrical conductivity of the composite thin-film.On the other hand,post-processing realizes the electron injection into the composite thin-film,which is beneficial to the increase of the Seebeck coefficient,and finally optimizes the thermoelectric performance.3.Prepare single-layer or few-layer MXene（Ti₃C₂）nanosheets by LiF/HCl etching.Directly mixed suction filtration and layered suction filtration were used to prepareMXene/SWCNTs（M/S）compositethin-filmsand SWCNTs-MXene-SWCNTs（S-M-S）and MXene-SWCNTs-MXene（M-S-M）composite thin-films with sandwich structures,and the thermoelectric properties of different composite methods were systematically studied.The results show that different composite methods have a great impact on the thermoelectric properties of the material.Because the two dispersions are directly mixed,the composite interface is disordered and disordered,and a good conductive network cannot be formed.The S-M-S composite thin-film with a sandwich structure has significantly improved thermoelectric properties compared to the M/S composite thin-film.MXene is in the middle and still maintains a highly ordered layered structure;however,SWCNTs on both sides become more disordered after post-processing.The disordered SWCNTs and the more ordered MXene interface will have a larger contact resistance during the contact process,and the electrical conductivity will remain at a lower level.The M-S-M composite thin-film with a sandwich structure not only maintains the high conductivity of the MXene material,but also obtains a large Seebeck coefficient.MXene is more orderly stacked on both sides of the film after suction filtration;SWCNTs are in the middle layer and are fixed by MXene on both sides,which will not affect its original order during the post-processing process.In the process of contacting two more ordered interfaces wit h each other,a smaller contact resistance will be formed to maintain the high electrical conductivity of the material.And the energy filtering effect will be generated at the interface.In the end,the composite material can obtain the Seebeck coefficient similar to SWCNTs,and the power factor is as high as 77.62μW m^-1 K^-2.