The Optoelectronic Properties And Applications Of Two-dimensional Ti₃C₂T_x Mxene-based Composites

As a new family of two-dimensional(2D)transition metal carbides,nitrides or carbonitrides,MXenes have strong hydrophilicity and high electrical conductivity.MXenes have been intensively investigated in supercapacitor,batteries,transparent conductive electrode,biomedical,sensors,photocatalysis and optoelectronic devices,since they were reported in 2011.Of all the MXenes synthesized to date,Ti₃C₂T_x MXene is widely investigated due to its relative ease of synthesis and high stability(T represents the multiple surface functional groups,such as=O,–F,and-OH terminated groups).Although there are many researches on the 2D layered material MXenes,they are mainly focused on the supercapacitors and batteries.It is far from enough for this new type of 2D material,especially in the fields of humidity sensor,photocatalysis and optoelectronic devices.The science researchers need to devote more energy to explore the application of MXene in depth.Based on the above considerations,this thesis starts with the inherent characteristics of the 2D layered material Ti₃C₂T_x MXene with good hydrophilicity,large work function and low Fermi level with surface group of=O.The optoelectronic properties and applications in the humidity sensor,photocatalysis and perovskite quantum dots light-emitting diode(LED)are explored.At last,improved humidity sensing response,photoinduced charge carriers separation and migration efficiency and external quantum efficiency(EQE)of perovskite quantum dot LED were achieved.The specific researches carried out are as follows:(1)The Ti₃AlC₂ ceramic material was etched with HF solution to prepare accordion-like Ti₃C₂T_x.The S-Ti₃C₂T_x/K2Ti4O9 composite was synthesized by ultrasonic and alkalization treatment of accordion-like Ti₃C₂T_x.In this composite material,the as-prepared K2Ti4O9 had an intertwined filiform nanostructure with a narrow width of 10?50 nm grown on the surface of small Ti₃C₂T_x MXene nanosheets.Both of the enlarged interlayer distance of Ti₃C₂T_x MXene and the formation of hydrophilic layered K2Ti4O9 nanowires promoted the adsorption,desorption and diffusion of water molecules throughout the entire porous framework.As a result,more than 8 times enhancement of the relative humidity response was achieved with S-Ti₃C₂T_x/K2Ti4O9 composite compared with accordion-like Ti₃C₂T_x.This work provides a promising new tactic to develop Ti₃C₂T_x MXene or other MXenes to construct high performance MXene-derived humidity sensors.(2)Ti₃C₂T_x MXene was synthesized through etching Ti₃AlC₂ by a mixed solution of HCl and Li F.Then,the Ti₃C₂T_x MXene was exfoliated into ultrathin Ti₃C₂T_x nanosheets by ultrasonic.Subsequently,the Ti₃C₂T_x/(001)Ti O2heterojunction nanosheets with highly exposed(001)facet of Ti O2 nanosheets were fabricated by an in-situ partial hydrothermal oxidation reaction with the raw material Ti₃C₂T_x MXene nanosheets and morphology-directing reagent Na BF4.In this heterostructure,the relative content of Ti₃C₂T_x and Ti O2 nanosheets can be controlled by the time of the hydrothermal oxidation reaction.The Ti₃C₂T_x is non-noble metal two-dimensional cocatalyst,while the Ti O2 nanosheets are classic semiconductor photocatalyst.The Ti₃C₂T_x MXene with surface group of=O has a lower Fermi energy level,which accelerates the separation of photoinduced charge carriers.Upon simulated sunlight irradiation,the optimal Ti₃C₂T_x/(001)Ti O2 heterojunction nanosheets exhibited about more than 3-fold enhancement on the photocatalytic H2evolution as compared to pure(001)Ti O2 nanosheets.(3)The 2D multi-hetero-interface was constructed in the well-designed nanosheet composite photocatalyst Ti₃C₂T_x/(001)Ti O2/C3N4 through an in-situ oxidation treatment combined with the electrostatic self-assembly strategy.In which,the spontaneous self-assembly behavior between the negatively-charged Ti₃C₂T_x/(001)Ti O2 heterojunction nanosheets and the positively-charged protonated g-C3N4 nanosheets can be achieved by electrostatic attraction.The 2D multi-hetero-interface offers broad electron-transfer channels,thereby accelerating the transfer of photoinduced charge carriers.Upon simulated sunlight irradiation,the optimal Ti₃C₂T_x/(001)Ti O2/C3N4 heterojunction nanosheets exhibited about 4-fold enhancement on the photocatalytic H2 evolution and 3-fold improvement on the photocatalytic CO2 reduction as compared to pure(001)Ti O2 or protonated g-C3N4nanosheets.Our work provides a promising new strategy to design high-quality noble metal-free 2D multi-hetero-interface composite photocatalyst.(4)The composite of ultrathin Ti₃C₂T_x nanosheets and ZnO quantum dots were utilized as the electron transport layer for perovskite quantum dots LED.The addition of Ti₃C₂T_x nanosheets decreases the potential well between electron transport layer and emitting layer,reduces the electron injection efficiency of ZnO/Ti₃C₂T_x electron transport layer and achieves the injection balance of charge carriers between electron transport layer and hole transport layer.In addition,compared to the pure ZnO electron transport layer,the ZnO/Ti₃C₂T_x electron transport layer also reduces the surface roughness,thereby ensuring the uniform distribution of the perovskite quantum dot layer and reducing the leakage current.As a result,a relatively high EQE of 15.6%was achieved by using the optimal ZnO/Ti₃C₂T_x electron transport layer,which is 18.2%improvement compared with the pristine ZnO-based device(13.2%EQE).In conclusion,this work started with the inherent characteristics of the 2D layered material Ti₃C₂T_x MXene.The optoelectronic properties and applications of Ti₃C₂T_x MXene in the directions of humidity sensors,photocatalysis and optoelectronic devices have been thoroughly studied and explored.At last,improved humidity sensing response,photoinduced charge carriers separation and migration efficiency and external quantum efficiency of perovskite quantum dot LED were achieved.