Studies On The Structures And Properties Of Chiral Borospherenes And Molybdenum Boride Films

Boron forms various complexes and nanomaterials with diversified structures,including boron bulk allotropes and metal boride nanomaterials.The theoretical and experimental research on these boron-based nanomaterials could help to further understand their phase structures and possible applications.Among them,the theoretical research of bare boron clusters and the exploration of the electrocatalytic properties of molybdenum boride materials have become research hotspots in recent years,and the theoretical analysis of the geometric structure and chemical bonding of bare boron clusters can help to understand the micro-physical structure and chemical properties of molybdenum boride materials,so as to provide a theoretical basis for exploring the electrocatalytic properties of molybdenum boride materials.Since the anionic boron nanoclusters B_n^-(n=3-42)and the small-sized cationic boron nanoclusters B_n⁺(n=3-27)were systematically characterized,our research group have made a detailed theoretical study on medium-sized cations or neutral boron clusters B_n^+/0(n=29-39).At the same time,based on previous researches on molybdenum boride materials in the field of electrochemical hydrogen evolution by different research groups,we also made detailed investigations on the experimental preparation and electrochemical hydrogen evolution reaction(HER)performance of molybdenum boride electrocatalyst materials.This thesis mainly focuses on the theoretical research on the structure and bonding of neutral B₃₄,monocation B⁺₃₅,B⁺₃₈and dication B₃₈²⁺,and the experimental exploration on the preparation and HER catalytic activity of molybdenum boride/oxide heterojunction nanoporous film.The main contents and results are as follows:1.Aromatic cage-like B₃₄ and B⁺₃₅:Gas phase experiments have confirmed that the B^-₃₄ and B^-₃₅ anion boron clusters are both quasi-planar structures,based on extensive global minimum searches and first-principles theory calculations,we present herein two new axially chiral members to the borospherene family:the aromatic cage-like C₂ B₃₄ and C₂ B⁺₃₅.Both C₂ B₃₄ and C₂ B⁺₃₅ feature one B₂₁ boron triple chain on the waist and two equivalent heptagons and hexagons on the cage surface,with the latter being obtained by the addition of B⁺into the former at the tetracoordinate defect site.Detailed bonding analyses show that they follow the universal bonding pattern of?+?double delocalization,with 11 delocalized?bonds over a?skeleton.Extensive molecular dynamics simulations show that these borospherenes are kinetically stable below 1000K and start to fluctuate at 1200 K and 1100 K,respectively.The IR,Raman,and UV-vis spectra of C₂ B₃₄ and C₂ B⁺₃₅ are computationally simulated to facilitate their experimental characterization.2.Axially Chiral borospherene B⁺₃₈ and B₃₈²⁺:Neutral B₃₈ has been proposed to have a cage-like structure with D_2h symmetry at various theoretical levels.Based on extensive global minimum searches and first-principles theory calculations,we predict herein the possible existence of the axially chiral cage-like C₂ B⁺₃₈ and C₂ B₃₈²⁺which are novel aromatic members of the borospherene family featuring a B₂₁ boron triple-chain on the waist and four B₆hexagonal holes on the cage surface.Detailed bonding analyses show that the C₂ B⁺₃₈and C₂ B₃₈²⁺possess 12 and 11 delocalized?bonds over a?-skeleton,respectively,following the universal bonding pattern of?+?double delocalization of the borospherene family.Extensive molecular dynamics simulations indicate that both C₂B⁺₃₈ and C₂ B₃₈²⁺are dynamically stable at 700 K.The IR,Raman,and UV–vis spectra of these cluster cations are computationally simulated to facilitate their future spectral characterizations.3.Experimental study on electrocatalytic hydrogen evolution reaction of molybdenum boride/oxide heterojunction nanoporous film:We have prepared molybdenum boron/oxide heterojunction nanoporous film materials on molybdenum foils using anodizing and CVD boronization methods,and systematically explored its electrocatalytic hydrogen evolution performance under acidic and alkaline conditions.The best thin film material prepared exhibits excellent HER catalytic activity in a wide p H electrolyte,and the onset overpotential(?_onset,defined as a potential required for reaching 1 m A cm^-2)in acidic and alkaline electrolytes are 45 m V and 55 m V,respectively,and both have small Tafel slopes of43 m V dec^-1 and 67 m V dec^-1.In addition,it can remain HER stability over 25 hours under acidic and alkaline conditions.The excellent HER performance of the film material is attributed to the highly self-supporting growth on the molybdenum foil and nanoporous characteristics of the film,which provides good conductivity and more exposed catalytic sites for the HER reaction,which is beneficial to the rapid transmission of electrons and the rapid desorption of hydrogen.In addition,the electron delocalization configuration at the interface of molybdenum boride and molybdenum oxide heterojunction makes the charge rich,optimizes the Gibbs free energy of H adsorption(?G(H*)),and improves the intrinsic activity of the catalyst,so as to accelerate the hydrogen evolution reaction.