The structure of boron nitride (BN) nano-film materials is similar to that of carbon nano-materials. As a two-dimensional layered material with graphene-like structure, hexagonal boron nitride has become a current research hotspot. The boron nitride nano film has good high temperature resistance, oxidation resistance and good neutron radiation shielding performance. In addition, boron nitride also has excellent properties such as piezoelectricity, high thermal conductivity, super hydrophobicity, viscous friction between super high layers, catalysis and biocompatibility. Therefore, boron nitride nano film materials have broad application prospects in high temperature resistant, high-strength functional composite materials and biomedical fields.
I. Overview of Boron Nitride
1, boron nitride crystal structure
Various crystalline forms exist in the boron nitride structure. SP2 hybrid hexagonal boron nitride is a white polycrystalline material with lubricating properties, similar to the layered structure of graphite. This layered structure can also be stacked in the form of a cuboid. Other common structure SP3 hybrid cubic boron nitride has a structure similar to diamond and is the second hardest material known to date. Boron nitride nanomaterials also have a rare SP3 hybrid wurtzite structure similar to hexagonal carbon.
Crystal structure of boron nitride nanomaterials
2.Low-dimensional boron nitride nanostructures
Low-dimensional boron nitride nanostructures are diverse, including nano-films, nanotubes, nanoparticles, nano-ribbons, nano-fibers, and nano-wires. Among them, boron nitride nanofilms and nanotubes with a two-dimensional structure have attracted the most attention.
Schematic diagram of boron nitride nanofilm (single layer, left) and nanotube structure (right)
Boron nitride nanofilms and nanotubes have outstanding optical properties and are suitable for making deep ultraviolet emitters and various optoelectronic nanodevices. In addition, boron nitride nanofilms also have excellent properties such as piezoelectricity, high thermal conductivity, superhydrophobicity, viscous friction between superhigh layers, catalysis and biocompatibility. Therefore, boron nitride nanofilms have great application potential in high temperature resistant, high-strength functional composite materials and biomedical fields.
Second, the preparation of boron nitride nano film materials
The method of preparing boron nitride nano-film material is similar to that of synthetic graphene, mainly including mechanical peeling, chemical peeling, chemical vapor deposition, and high-energy electron irradiation.
1.Mechanical peeling method
The mechanical stripping method is used to prepare the boron nitride nano-film material. First, a wet ball mill is used to prepare boron nitride nano-sheets from the boron nitride powder, and the force of peeling the boron nitride film is the shear force. In this method, benzyl benzoate is added as a ball milling additive to reduce collision and damage to the boron nitride film during ball milling.
Schematic diagram of preparing boron nitride nano-film material by mechanical stripping method
2. Chemical stripping method
The chemical stripping method is to prepare single and several atomic layers of boron nitride nanoflakes from single crystal boron nitride using a chemical solution method. Single crystal boron nitride was placed in 5 ml of m-styrene and 2,5-styrene copolymer in a 1,2-dichloroethane solution (1.2 mg / 10 ml) for ultrasonic dispersion for 1 hour, and the boron nitride crystals were peeled off. Form flake boron nitride.
The chemical stripping method needs to add a strong polar solvent, such as N, N-dimethylformamide (DMF). The polar DMF molecules have a strong interaction with the surface of boron nitride, which helps to obtain boron nitride nanoflakes. . By chemical stripping method, pure boron nitride nanoflakes of milligram level can be obtained, and the thickness is between 2-10nm.
3.Chemical vapor deposition
Chemical vapor deposition of boron nitride nano-films is mainly divided into epitaxial growth and non-epitaxial growth.
(1) Epitaxial growth
The epitaxial growth of boron nitride nanofilms is performed by using binary system precursors (BF3-NH3, BCl3-NH3, B2H6-NH3), or pyrolysis using a single precursor, such as borazine (BN3H6), trichloro ring Borazane (B3N3H3Cl3) or hexachloroborazane (B3N3H3Cl6). Among them, boron nitride thin film with a stoichiometric ratio of 1: 1 can be deposited by pyrolysis of borazine.
Researchers at the University of Zurich, Switzerland, used rhodium as a matrix to construct a honeycomb-shaped boron nitride (also known as "white graphene") nanometer mesh, with a single-layer nanometer mesh thickness of 0.1 nanometers and a mesh spacing of 3.2 nanometers. By changing the atomic angle of a single layer of boron carbide, the transition from hydrophilic to hydrophobic can be achieved with or without power. Specifically, the material can change the static resistance of the atomic surface by changing the nanostructure (one state is a highly viscous hydrophilic state and the other state is a low viscous hydrophobic state), thereby changing its affinity / Hydrophobic state.
Structure of honeycomb boron nitride nanomesh (green spheres are nitrogen atoms, orange spheres are boron atoms, and gray spheres are rhodium atoms. The distance between nanosphere layers is 3.2 nm)
Researchers from Shanghai Institute of Microsystems have used chemical vapor deposition (CVD) to successfully prepare monoatomic layers of high-quality graphene / hexagonal boron nitride planar hetero-nano-film materials on copper-nickel alloy substrates, and successfully applied them to WSe2 / MoS2 two-dimensional photodetector device. This method utilizes the excellent catalytic ability of copper-nickel alloys, improves the crystalline quality of boron nitride single crystals, and eliminates random nucleation of graphene, so that the graphene crystal domains are only at the top corners of the triangular h-BN single crystal domains. Nucleate and grow along the h-BN side.
Schematic diagram of preparation of monoatomic layer high-quality graphene / hexagonal boron nitride planar hetero-nano film material
At present, the research of depositing monoatomic layer hexagonal boron nitride nano-films on the surface of metallic nickel is more popular. Studies show that there is a large degree of hybridization between the d orbital of nickel and the π orbital of hexagonal boron nitride, which indicates that there is a strong bond between hexagonal boron nitride and the metal substrate.
(2) Non-epitaxial growth
For non-epitaxial growth, boron oxide (B2O3) and melamine powder are used as precursors. By controlling different growth temperatures (1100-1300 ° C), the thickness of the boron nitride film can be controlled between 25-50nm. The number of layers of boron nitride nano film material is determined by the concentration of the reactants.
4.High energy electron irradiation method
The high-energy electron irradiation method for preparing boron nitride nano-films is based on the mechanical stripping method to prepare BN nano-sheets or nano-powders as raw materials, and the obtained flakes and powders are thinned by intensive electron beam irradiation. Through manual scanning of the electron beam, the BN nanosheets are thinned layer by layer until a monoatomic layer of BN nanofilm material is obtained.
5.Ion beam sputtering deposition method
Hexagonal boron nitride thin film was prepared by ion beam sputtering deposition method based on Ni foil with a purity of 99.5%. The Ni foil was pre-etched in situ from an auxiliary ion source, and then annealed at 1000 ° C for 10 minutes. Then, B atoms and N atoms are sputtered from the hexagonal boron nitride target by an argon ion beam emitted from the main ion source, and deposited on the pre-treated Ni foil to prepare a boron nitride nano-thin film material.
3. Application of boron nitride nano film material
1.Catalytic materials
Boron nitride nanofilm is a good support for silver nanoparticles, which can catalyze the reduction of nitrophenol to aminophenol. The silver iodide / boron nitride nanocomposite shows good photocatalytic activity, and has good application prospects in wastewater treatment and pollutant treatment.
2.Biomaterials
Boron nitride nano-film materials have better biocompatibility and are expected to be used in the fields of biological tissue engineering and medical. The University of Zurich, Switzerland, constructed a honeycomb-shaped boron nitride (also known as "white graphene") nanomesh on a rhodium matrix. When a voltage is applied to the material, the boron nitride nanomesh will be spread flat. The electrical control behavior has been applied biologically, and can be used for control and processing at the microscopic level of cells, and has a great role in promoting related scientific research such as creating new and complex artificial multi-cell arrangements. In addition, the research also provides a technical basis for the construction of microcapillary pumps, which can control the pressure and flow in nanoscale pipelines through electrical signals.
3.New high temperature resistant composite material
The boron nitride nano-film material is chemically and thermally stable, and free of dangling bonds and surface charge bands. Researchers at Pennsylvania State University have prepared hexagonal boron nitride / polyetherimide nano-film materials, which have significantly better performance than related competitive materials, and can be used at temperatures that are required for electric vehicle and aerospace power applications.
4, lithium battery materials
Boron nitride nano-film materials have high mechanical strength, thermal conductivity, electrochemical stability, and electrical insulation, and because boron atoms have empty pz orbitals, they are added to the gel polymer electrolyte as a multifunctional additive. It can fix the anions in the electrolyte and suppress the polarization, which can effectively inhibit the formation and growth of lithium dendrites and prolong the service life of lithium metal batteries.
5.Photoelectric / microelectronic materials
The boron nitride nano-film material is a wide bandgap semiconductor (5.0 ～ 6.0eV), good high-temperature chemical stability, and atomically flat surface, which makes it have broad application prospects in optoelectronics / microelectronics. The multilayer boron nitride nanofilm / graphene heterojunction with a few atomic layers has a high charge mobility, reaching 500,000 cm2.VS-1. Cubic boron nitride heterojunction structure is used to fabricate field effect tunneling transistor devices.