The Exploration Of Low-dimensional Boron Materials

As an adjacent element of carbon,boron is the first one to possess a p electron in the periodic table,which lead to unique and complex characters,including electron deficiency,short covalent radius and flexibility to form multi-center bonds.Therefore,to explore the structure and physicochemical properties of low-dimensional boron and boron-based nanomaterials,which continues to widely attract scientific interests.In nanoclusters,finding highly stable boron-based clusters and synthesizing macroscopic quantities compounds has been the focus of intensive studies both experimentally and theoretically.However,as an electron-deficient element,boron clusters are easily oxidized and become less stable to move out of the vacuum.Transition-metal(TM)doped boron clusters is considered as an effective approach to stabilize pure boron clusters and change their geometries and electronic properties due to the abundant d electrons.Thus,we used our developed comprehensive genetic algorithm(CGA)combined with density functional theory(DFT)to search the ground state structures of different TM atoms doped boron clusters.In chapter 3,the structural evolution of the three Ru atoms doped boron clusters of Ru₃B_n(n=3?20)were explored under the high-precision optimization by Gaussian16 program.The results show that the structural transition of Ru₃B_n(n=3?20)clusters from planar or quasi-planar structure to the cage-like structure is n=8 and the size of cage becomes larger with the further increase of boron atoms.The perfect hollow cage appears in the Ru₃B₁₂ cluster with D_3hsymmetry,which is composed of three umbrella shaped structures of Ru@B₆ unit.However,combining binding energy per atom and second-order difference,we found that Ru₃B₁₃ cluster is most stable structure which has a HOMO-LUMO gap of 1.69 e V.In chapter 4,we report the global search for the lowest energy structures of the TM doped boron clusters,TM₄B₁₈^0/?(TM=Hf,Ta,W,Re,Os)and their electronic properties.The combination of the CGA method incorporated with DFT calculations shows that they are composed of the four planar TM@B₉ wheel units by sharing boron atoms except for Os₄B₁₈^0/?,which consists of the two types of planar molecular wheels of Os@B₇ and Os@B₈.Among these nanoclusters,it is found that the Ta₄B₁₈ cluster has a closed-shell with a large HOMO-LUMO gap of 2.61 e V.Adaptive natural density partitioning analysis(Ad NDP)reveals that Ta₄B₁₈ cluster has?antiaromaticity and?aromaticity,i.e.,a conflicting aromaticity.The simulated photoelectron spectra(PES)of all anionic clusters are also provided for future experimental investigations.In two-dimensional(2D)materials,borophene with native metallic conductivity,large elastic modulus,heavy anisotropy,high Fermi velocity(6.6×10⁵ m/s),excellent thermal and chemical stability is considered a nanomaterial comparable to graphene.However,most of the current studies on 2D borophene nanostructures is based on substrate-supported epitaxial techniques under UHV condition,which has severe concerns in their structure stability when exposed to the air or peeled off from the substrate.Alternatively,liquid exfoliation is another practical route for the large-scale production of freestanding 2D borophene,but the structural uniformity is a long-standing issue due to the minor difference in formation energy among various borophene 2D phases(?₃,?₁₂,and etc).Until now,the synthesis of freestanding single crystalline Borophene sheets is still of grand challenge and thus highly demanded for their appractical appliactions.In chapter 5,we cooperated with other experimental groups and they macro-synthesized few-layers borophene as efficient polysulfide electrocatalysts for Li-S batteries,which showed excellent catalytic performance and cycle stability than CNT.In this work,our HRTEM images showed that this borophene nanosheets has single crystalline(?₁₂ phase),few layers(less than15 layers),and large lateral area(?3?m).Furthermore,the DFT calculations revealed a lower surface diffusion barrier of Li ion and a stronger adsorption for Li₂S_n clusters on?₁₂-borophene that effectively inhibits the shuttle effect of polysulfides and accelerate their decomposition,leading to the extraordinary catalytic activity of?₁₂-borophene towards polysulfides.