Research On The Preparation Of Two-Dimensional Nanomaterials By Liqiud Phase Exfoliation In Deep Eutectic Solvent System And Its Application

Liquid phase exfoliation method is a significant method to realize the mass production of two-dimensional materials,but there are still problems such as low yield and long preparation time.The key to solving the above problems is the selection and optimization of exfoliative system.Traditional solvents such as aqueous solutions,organic solvents and ionic liquids are difficult to meet the requirements of low cost,high yield and green.Deep eutectic solvent（DES）is a new type of green solvent with low price,adjustable polarity,stable physical and chemical properties,and good conductivity,which is hopeful to be an excellent exfoliation system for preparing two-dimensional materials.Among many two-dimensional materials,boron nitride nanosheets（BNNSs）with hydrophobicity,narrow interlayer spacing,wide band gap and high chemical stability and transition metal carbon/nitride（MXene）with hydrophilicity,wide interlayer spacing and excellent conductivity are representative owing to their unique properties,and show great application prospects in energy storage,thermal conductivity,biomedicine and other fields.In this paper,DES was used as the exfoliation system,and hexagonal boron nitride（h-BN）and layered Ti₃C₂T_xwere selected as the research objects.Based on the intrinsic properties of DES and its interaction with h-BN and layered Ti₃C₂T_x,the technology and mechanism of large-scale preparation of BNNSs and Ti₃C₂T_xMXene by liquid phase exfoliation were studied.In addition,their applications in flexible solid-state supercapacitor（FSSC）and thermal conductivity were studied.The new technology and method of preparing two-dimensional materials by liquid phase exfoliation in DES system were formed through this study,and provided a theoretical basis for their wide application.The main contents of this paper are as below:1.The research of the intrinsic properties of DES and its interaction with two-dimensional materials（h-BN,layered Ti₃C₂T_x）.Firstly,four kinds of DES were prepared,and their physical properties were compared.The results show that the typeⅢDES has good viscosity and conductivity,strong physicochemical stability,toxic-free,no side effects,which is suitable as an exfoliation system.Three representative DESs,choline chloride-urea（Ch Cl-U）,choline chloride-ethylene glycol（Ch Cl-EG）and choline chloride-glucose-water（Ch Cl-Glu-H₂O）,were synthesized and their intrinsic characteristics were studied.The results show that with the increase of temperature,the density of three Ch Cl-based DESs changes little,the conductivity gradually increases,and the viscosity gradually decreases.In a certain temperature range（293～353 K）,the viscosity and conductivity conform to the Arrhenius law.Among them,Ch Cl-U has higher electrochemical activity and stronger thermal stability.The test and simulation results show that the interaction between Ch Cl-EG and h-BN or layered Ti₃C₂T_xis stronger.2.In DES system,high-quality BNNSs were prepared by ball milling liquid phase exfoliation.Using Ch Cl-U as exfoliation system,the effects of h-BN initial concentration and exfoliation time on ball milling liquid phase exfoliation were studied.Under the optimized conditions,the yield,average lateral size and average number of layers of the prepared BNNSs are about7.43%,0.78μm and 7,respectively.The high viscosity of DES makes it adhesive to the nanosheets,resulting in a shear force similar to’tape effect’’.The shear force is combined with the shear force generated by the collision of ball mill beads to achieve a simple and efficient exfoliation of h-BN.The above prepared BNNSs were subjected to controllable hydroxylation modification（OH-BNNSs）and used as fillers for thermal conductive encapsulating adhesives.Appropriate hydroxylated OH-BNNSs can significantly improve the mechanical and thermal properties of the encapsulating adhesives.When the addition amount is 10 wt%,the thermal conductivity of the composite encapsulant is increased by 287.3%.3.In DES and DES/water systems,high-quality BNNSs and OH-BNNSs were prepared by suspension electrochemical liquid phase exfoliation,respectively.Under the action of electric field,DES is rapidly intercalated into the interlayer of h-BN.Subsequently,the DES located between the layers is electrolyzed to produce gas for the expansion of the layers,which promotes the exfoliation of h-BN layers to obtain BNNSs.Using Ch Cl-U as the exfoliating system,the yield and average number of layers of BNNSs are about 19.01%and 6,respectively.By adding an appropriate amount of water to Ch Cl-U to adjust the electrolyte composition,OH-BNNSs can be prepared.The addition of water not only enhances the conductivity of the electrolyte to promote exfoliating,but also can in situ graft-OH groups on the surface of BNNSs by redox reaction.The yield and average number of layers of OH-BNNSs are about 21.63%and 6,respectively.Ch Cl-U and OH-BNNSs were added to the polyvinyl alcohol（PVA）to prepare a new composite gel electrolyte.The simulation calculations show that the-OH groups of OH-BNNSs can form a hydrogen bond network with the-OH groups on the PVA chain,which is beneficial to build a uniform ion transport pathway.The FSSC assembled by it exhibits a wide operating voltage window（2.3 V）,high specific capacitance(151.22F·g^-1),good cycle stability（96%capacitance retention after 1500 cycles）and flexibility.4.In DES/water system,high-quality Ti₃C₂T_xMXene was prepared by suspension electrochemical and ultrasonic composite liquid phase exfoliation.Under the action of electric field,DES is intercalated into the interlayer of multi-layer Ti₃C₂T_xand decomposes to expand the interlayer to prepare layered Ti₃C₂T_xwith large interlayer spacing.Then,layered Ti₃C₂T_xwas dispersed in an aqueous solution to obtain Ti₃C₂T_xMXene by ultrasonic exfoliation.DES is attached to the surface and interlayer of layered Ti₃C₂T_xto form a’DES protective cover’’,so that it can be directly exfoliated in air and water without oxidation.Using a mixed solution of Ch Cl-U and water as the electrolyte,the yield,average lateral size and average number of layers of obtained Ti₃C₂T_xMXene（MXene-DES）are about 9.52%,0.39μm and 5,respectively.Compared with the MXene prepared by traditional ultrasonic method in nitrogen atmosphere,the MXene-DES prepared by this method exhibits larger interlayer spacing,fewer layers and higher degree of hydroxylation.Ch Cl-U and MXene-DES were added to PVA to prepare a new composite gel electrolyte.The FSSC assembled by it exhibits good electrochemical and mechanical properties,indicating its great potential in practical applications.5.For electrochemical liquid phase exfoliation,the stronger the intercalation of DES and the stronger the interaction between DES and h-BN or layered Ti₃C₂T_x,the higher the exfoliation yield;the more intense the decomposition and expansion reaction of DES,the fewer layers of BNNSs or Ti₃C₂T_xMXene prepared.For ball milling liquid phase exfoliation,the higher the viscosity of DES,the closer the size of DES to the interlayer spacing of layered materials,the stronger the interaction between DES and layered materials,the higher the exfoliation yield,and the fewer the number of BNNSs prepared.The electrochemical liquid phase exfoliation method has the characteristics of high efficiency and good product controllability.The ball milling liquid phase exfoliation method has the advantages of easy large-scale preparation and large output,but the efficiency is low and the product controllability is poor.The exfoliation method can be selected according to the material characteristics and application fields.