Study On The Preparation Method Of Few-layer Borophene And Magnesium-deficient Few-layer Borophene Nanosheet

Borophene is a two-dimensional material that possesses strength,flexibility,toughness,and heat resistance.It overcomes the shortcomings of other two-dimensional materials that are more than tough but lacking in toughness.The excellent chemical and physical properties make this class of materials show great application potential in many fields.However,while boron is non-metallic,it has certain metallicity.Boron can not only form conventional strong covalent bond,but also form a series of multi-center chemical bonds,which make boron tend to form a complex polyhedral structure.Therefore,it is difficult to prepare borophene bound by weak van der Waals forces or the electrostatic attraction.At present,the borophene preparation with high quality is mainly realized by"bottom-up"molecular beam epitaxy（MBE）or chemical vapor deposition（CVD）methods.These preparation methods require high cost equipment and are mostly completed under ultra-high vacuum/high temperature.Also the prepared borophene needs to be transferred from the metal substrate,these limit its large-scale production and wide application.Although researchers mainly use bulk boron"top-down"liquid phase thinning technology to prepare few-layer borophene with different thicknesses,there are three main problems during the preparation of borophene by liquid phase thinning and exfoliation.At first,the mechanism of bulk boron liquid-phase thinning and exfoliation to prepare borophene is unclear due to the lack of pure boron precursor with a clear layered structure.Secondary,the liquid-phase thinning and exfoliation mainly adopt the ultrasonic technical route and followed by centrifugal separation,the processing environment and centrifugation rate have a great influence,making it challeng to prepare few-layer borophenes with uniform size and thickness;thirdly,although pure boron has good thinning and exfoliation dispersibility in polar protic solvents,the use of these media makes the subsequent product separation more difficult for preparing borophene in cheap and large scale.Therefore,it is of great significance to develop a new technology for large-scale and inexpensive liquid-phase preparation of borophene few-layer and borophene-based materials with uniform thickness and size in order to wide the applications of these borophene-based materials.In this study,the preparation method of few-layer borophene and Mg-deficient borophene nanosheets was developed by using bulk boron and layered MgB₂ as precursors,respectively,and the structure,morphology,and electrochemical properties of the prepared few-layer borophene and Mg-deficient few-layer borophene nanosheets were systematically characterized.It is expected to prepare few-layer borophene with uniform sheet thickness and large lateral size,and obtain Mg-deficient borophene-based materials with uniform lamella thickness,and also to solve the low Mg deintercalation rate and structural stabilization bottleneck during the preparation of the Mg-deficient borophene nanosheets by layered MgB₂.The full thesis consisted of six chapters.The first chapter was an introduction,in which the definition,classification,structure,property,and application of single element two-dimensional layered materials and borophene,as well as the preparation technology and challenges for borophene and borophene-based materials were systematically reviewed,also the significance and purpose of this research,the research content and innovation points were proposed.Chapters 2 to 5 were experiments,results and discussions,mainly including the preparation method of few-layer borophene with large size by using solvothermal assisted liquid phase exfoliation preparation technology in chapter 2,developing the preration technology for Mg-deficient borene-based materials by I₂ oxidation layered MgB₂ and followed by ultrasonic exfoliation in chapter 3,a vapor phase etching swelling and followed by ultrasonic exfoliation preparation method for Mg-deficient borene-based materials by using layered MgB₂ as precursor in chapter 4,and investigating the electrochemical property of the prepared few-layer borophene with large size and Mg-deficient borene-based materials in chapter 5.Chapter 6 was a summary of the full text.The main research contents were as follows:（1）The bulk boron powder was used as raw material,and it was initially thinned by ball milling and dispersed in acetone medium,then solvothermally treated at 200℃ for24 h and followed by ultrasonic treating,few-layer borophene with large size was prepared.The effects of solvent surface tension,ultrasonic power and time,and surfactant type on the morphology and size of few-layer borophene were systematically studied.Solvents with small surface tension and Hansen solubility parameters were more suitable for boron powder solvothermal assisted liquid phase thinning exfoliation to prepare few-layer borophene.The few-layer borophene prepared in acetone medium had a thickness of 3.5 nm and an average lateral dimension of 5.05nm,which was the largest lateral-sized borophene nanosheets obtained by liquid phase thinning and exfoliation.At the same time,acetone molecules were critical to the swelling process of bulk boron,and the swelling leaded to the weakening the interlayer force of bulk boron.The thinned and exfoliated borophene with few layers could be obtained by ultrasonic treating at 225 W for 4 h.It is an effective method to prepare few-layer borophene with large size,thereby avoiding the breakage of borophene nanosheets caused by long-term and high-power ultrasound.（2）By using MgB₂ with regular layered structure as raw material,CH₃CN as the reaction medium,and I₂ as oxidant,the layered MgB₂ was oxidized in a mixed medium of CH₃CN and HCl under N₂ protection for 14 days,and then ultrasonically exfoliated in CH₃CN for 2 h,Mg-deficient few-layer borophene nanosheets was prepared.The effects of oxidation time,p H of reaction medium,and ultrasonic conditions on the structure of FBN were systematically studied.Combined with density functional theory（DFT）calculations,the crystal structure change of the two-dimensional borophene-based materials during the deintercalation process of Mg atoms was also investigated.The stoichiometric ratio of the prepared FBN was Mg_0.22B₂,and the maximum deintercalation rate of Mg atoms was 78%.Meanwhile,the average thickness of FBN was 4.14 nm and the average lateral dimension was 500 nm,maintaining the hexagonal crystal structure of the B layer in MgB₂.DFT theoretical calculation analysis supported the experimental result that the deintercalation rate of Mg atoms was 78%.The electron-deficient nature of B atoms made it impossible to completely deintercalate Mg atoms in MgB₂ under maintaining the hexagonal crystal structure of the B layer.This preparation method can solve the bottleneck problems of low Mg atom deintercalation rate and unstable hexagonal boron structure during preparing FBN from precursor MgB₂.（3）Using layered MgB₂ as precursor,NH₄Cl as etching swelling agent,and acetone as the exfoliation medium,the layered MgB₂ precursor was uniformly mixed with NH₄Cl by ball milling,and then calcined at 450℃ in a vacuum glass tube for 1 h,the MgB₂precursor was etched and swelled.The etched and swelled samples were washed with ethanol,and ultrasonically treated for 2 h in acetone medium,Magnesium-deficient few-layer borophene nanosheets were prepared in a large-scale and inexpensive manner.The effects of calcination conditions,ultrasonic conditions,and the washing medium on the sample structure were systematically studied.The average thickness and lateral size of the prepared MBN were 3.58 nm and 410 nm,respectively,and the deintercalation rate of Mg atoms could reach 78%.The vapor-phase etching swelling-ultrasonic exfoliation method for preparing MBN has some advantages of simple experimental operation,few post-processing steps,low cost in scale,high deintercalation rate of Mg atoms,and effective exfoliation in aqueous system.（4）By using few-layer borophene with large size prepared by solvothermal assisted liquid phase exfoliation preparation technology and the Mg-deficient few-layer borophene nanosheets prepared by I₂ oxidation layered MgB₂ and followed by ultrasonic exfoliation as electrode materials,their electrochemical properties were systematiclly investigated by applicating the cyclic voltammetry curves,charge-discharge curves,the rate performance,and the energy storage mechanism in acidic,alkaline,and neutral electrolytes in a three-electrode system.The voltage windows of the two material electrodes in different electrolytes were not much different,both within 0-1 V.In KOH electrolyte,the material electrode could store energy in pseudocapacitance,while it storaged in electric double layer capacitacne in H₂SO₄ and Na₂SO₄ electrolytes.At a scan rate of 5 m V s^-1,these electrodes exhibited high mass specific capacitance in alkaline system,and the mass capacitances could reach 130 F g^-1 for few-layer borophene with large size electrode and 86.7 F g^-1 for Mg-deficient few-layer borophene nanosheets electrode.However,the rate performance of the two material electrodes in different electrolytes was not good,it is necessary to further improve the specific capacitance of borophene-based materials through strategies such as precise control of the product structure,conductivity,and the structural stability during the reaction process.