Preparation And Properties Of Ultrathin Boron Nanomaterials

Boron is the only non-metallic element in the third main group.Due to the electron-deficient nature of boron atoms,the atoms are distributed in the equilibrium system to form multi-center,two-electron bonds,thus forming a variety of allotropes.In recent years,the ultra-thin two-dimensional atomic crystal materials exhibited special physical and chemical properties different from bulk phase,and has shown great application potential for the fields of optoelectronic devices,memory devices,and flexible electronics.Among them,ultra-thin boron nanomaterials are theoretically predicted to have excellent mechanical properties and optical transparency,and can be metal or semiconductor.This property of boron makes it complementary to the semi-metallic two-dimensional graphene and the semiconducting silene.In addition,it probably has many special properties such as superconductivity and Dirac cone.It is worth noting that the two-dimensional boron material is a precursor for studying the structure and properties of zero-dimensional boron clusters and one-dimensional boron nano-particles,and has important research value.However,due to the complex structure and unstable chemical properties of 2D boron,although a large number of research advances have been made in theoretical calculations,there has been few experimental results on 2D boron.Therefore,this paper described the preparation and performance of ultra-thin boron nanomaterials and we have realized the first successful preparation of boron nanostructures on the tungsten metal.At the same time,we have conducted the first investigation into the hydrogen evolution properties of single-element boron nanomaterials.In addition,we investigated the growth of boron nanostructures on silicon substrates.The main research contents are as follows:(1)Deposition of boron nanosheets on the surface of tungsten metal by chemical vapor deposition.In the experiment,chemical vapor deposition was used with metal tungsten as the substrate and boron/boron oxide mixed powder as the precursor.Ultra-thin,metallic boron nanosheets were prepared on the tungsten metal surface by exploring different temperature and time conditions under a controlled flow of hydrogen(10%)and argon(90%).The bubbling separation method based on the electrochemical principle realized the separation of the boron nanosheets from the tungsten substrate.The boron nanosheets transferred to Si O₂/Si,Cu mesh and other substrates for performance characterization test.The surface morphology of boron was observed.It was found that micro-scale aggregates were formed by the random island-like nanosheets with the size of several hundred nanometers.The size of the micro-scale aggregates is in the range of several micrometers to several tens of micrometers.The prepared boron nanosheets have high single crystallinity and belong to the triclinic crystal of the P1 space group.This structure exactly matches the structure in previous theoretical calculations and is synthesized experimentally for the first time.(2)Exploring the hydrogen evolution reaction performance of boron nanosheets with different preparation conditions in acidic solution.According to the first principle calculation,the boron nanosheet of this structure exhibits metallic and the strong orbital occupancy of 2p orbiting electrons near the Fermi level results metallic state of the boron nanosheets.In addition,the free energy?G_H of hydrogen adsorption at two different positions on the B(1^-12)surface structure was calculated to be-0.58 eV and-0.60 eV,respectively,indicating that the structure should have relatively high intrinsic catalytic activity for HER.Subsequently,the results of our theoretical predictions were confirmed by hydrogen evolution reaction experiments.The boron nanosheets grown under the most optimal growth condition have a small initial overpotential of 199mV in current density of 1 m A/cm² and a Tafel slope of 64 mV/dec.The HER performance of boron nanosheets is much higher than W,WO_x,WC_x and so on.Comparing the HER performance of boron nanosheets with different preparation conditions indicates that the number of active sites and the charge transfer rate caused by the thickness directly affect the performance of electrocatalytic hydrogen evolution.(3)Growth of boron nanostructures was achieved on a silicon substrate by chemical vapor deposition.In the experiment,the clean silicon after hydrofluoric acid treatment was used as the substrate,and the pretreated sodium borohydride was used as the precursor.Under the condition of controlled flow of pure hydrogen,growth of boron nanostructures is successfully achieved.X-ray photoelectron spectroscopy analysis of the obtained samples revealed that Na BH₄ was not effectively decomposed at low temperature.The optimal deposition temperature was finally determined.The characterization results reveal that uniform,crystalline and large-sized boron nanosheets of several micrometers were obtained at the temperature of 550 ^oC and reaction time of20 minutes.This research work provides the possibility of constructing transfer-free boron nanodevice.