The Structural Design,Synthesis And Properties Of MXene-derived Two-dimensional Carbon Based Nano-hybrids

Two-dimensional?2D?materials,such as graphene and two-dimensional transition metal carbides?MXenes?,have been extensively studied in energy storage&conversion fields due to their unique 2D structures.2D composite materials,on the one hand,can take fully advantages of the fast electron/ion transport properties of the 2D structure,on the other hand,it can also cover the disadvantes of the single component,which has attracted wide attention.However,the currently reported 2D composite materials suffer from the complicated preparation method,uncontrollable structure and weak interface,which affects the electron transport and structural stability of the interface.Graphene and MXene,which own excellent electron transport properties,mechanical properties and high specific surface areas,are two typical 2D materials.In this thesis,MXene is used as the precursor,and 2D carbon nanocomposite structure is taken as the research object.A series of high performance MXene-derived 2D-layered carbon nanocomposite structures are designed and synthesized.First,the simultaneous conversion of carbon and transition metal oxides was achieved by CO₂ oxidation of MXenes.The transformation mechanism and the influences of and oxidation conditions on their structures and properties were studied.After that,the defect control and photocatalytic mechanism of C/TiO₂ were studied.The nitridation process successfully realized the one-step synthesis of 2D carbon-coated transition metal nitride,and its synthesis mechanism and the influences of various conditions on performance were analyzed and studied.Finally,a MXene nanofiber structure was designed and its catalytic performance was studied.The main findings and contents are as follows:?1?The effects of CO₂ oxidation conditions on the structure and photocatalytic properties of 2D-layered C/TiO₂ were investigated.The results show that the complete conversion of Ti₃C₂MXenes to C/TiO₂ can be achieved via CO₂ oxidation,and its 2D layered structure is well preserved.As the oxidation temperature and CO₂ gas flow rate increase,the grain size gradually increases,the anatase TiO₂ will change to rutile TiO₂,and the oxidation of carbon will also increase.In the photocatalytic process,2D carbon can effectively promote electron-hole separation,and provide regular channels for the removal of H₂,thereby improving the photocatalytic activity of TiO₂.When the oxidation temperature is 700? and the CO₂ gas flow is 150 m L/min,the obtained C/TiO₂has the highest catalytic hydrogen production activity.?2?The influence mechanism of defects on the photocatalytic performance of C/TiO₂ were studied.The results show that C and TiO₂ phases can be successfully doped by S-melting impregnation of Ti₃C₂.The S doping amount can be controlled by the temperature and gas flow rate of CO₂ oxidation.When the temperature is 700? and the CO₂ gas flow rate is 150 m L/min,the S doping amount reaches the maximum.S-doped TiO₂?S-TiO₂?can effectively broaden the absorption wavelength range,while the defective carbon can reduce the activation energy of H⁺reduction,and can facilitate the transfer of electrons.The synergistic effects of S-TiO₂ and defect carbon can highly enhance the photocatalytic performance.The hydrogen production rate under visible light reaches 333?mol·g^-1·h^-1.The DFT theoretical calculation results also demonstrate the positive contribution of S doping to the reduction of the forbidden band width of TiO₂ and the co-catalytic effect of defective carbon.?3?One-step synthesis of 2D layered carbon-coated transition metal nitrides were successfully achieved and their supercapacitor properties were investigated.The results show that through the one-step nitridation of MXene,the preparation of TiN and the coating of carbon can be realized simultaneously,and the carbon-coated TiN?C@TiN?structure can be successfully synthesized.This method can also be used to prepare other carbon-coated nitride structures.Furthermore,the synthetic conversion mechanism was analyzed,and the possible reaction equations were proposed.The influence of nitriding temperature and nitriding time on its structure was analyzed.It was found that the 2D structure could be better maintained at750?.However,when the temperature was raised to 850?,the interlayer pores were severely blocked,and the specific surface area and pore volume were observed.The ratio of mesopores also decreases.At a temperature of 850 ?,the clogging of the tunnel will be more severe as the nitriding time increases.During the charge-discharge process,the outer carbon shell can effectively inhibit the oxidation of the nitrides,improve the electrical conductivity and prevent the mass loss of the active material,thereby improving the specific capacitance,rate performance and cycle stability.?4?The controlled synthesis and electrocatalytic properties of Ti₃C₂ nanofibers were investigated.The results show that Ti₃C₂ nanofibers can be successfully prepared by hydrolysis and a subsequent selective etching process in KOH solution.Compared to the Ti₃C₂ sheets,the specific surface area of Ti₃C₂ nanofibers is greatly improved,and the surface has more abundant functional groups.Owing to more catalytically active sites,shorter electron/ion migration pathways,and more abundant functional groups,the HER activity of Ti₃C₂ nanofibers is greatly enhanced.