Two-dimensional Hexagonal Boron Nitride With Regulated Structures Synthesized By Chemical Vapor Deposition And Their Growth Mechanism

Hexagonal boron nitride（h-BN）possesses excellent high temperature oxidation resistance,chemical stability,lubricating properties,high thermal conductivity,good neutron absorption,etc.,leading to a wide range of applications in machinery,metallurgy,electronics,aviation and other fields.As a structural analog of graphene,two-dimensional（2D）h-BN has attracted intensive attention in scientific community recently,due to the combination of the novel physics resulted from low dimensionality and these above mentioned exceptional physical and chemical properties.So far,2D h-BN nanomaterials,showing diversivity in structure and function,have been successfully synthesized by various methods.However,there remains challenging regarding the controllable synthesis and the corresponding growth mechanism.For instance,how to achieve low-cost,green and environmentally friendly preparation of 2D h-BN nanomaterials with desired properties?How to ensure the uniformity and controllability of its structure and properties?In addition,it still remains difficult to directly grow large-area,thickness-adjustable h-BN atomic layers on the surface of target substrates（especially dielectric substrates）.Based on this,this thesis focuses on the controlled synthesis of 2D h-BN using traditional chemical vapor deposition（CVD）method,starting from boron oxide（B₂O₃）and ammonia gas（NH₃）as precursors.The vertically grown 2D h-BN nanowalls have been successfully fabricated,showing selectively enhanced adsorption capability.Additionally,the large area,single crystalline h-BN films with controllable layer number have been directly grown on quartz and Si O₂/Si amorphous substrates.On this basis,the correlation between the structural characteristics,physicochemical properties and key growth methods was systematically studied.Combined with the nucleation,coalescence,growth and structural evolution of h-BN,the longitudinal and transverse growth mechanism of 2D h-BN nanostructures on r GO-assisted substrates was revealed.The details are as the followings:1.The solid precursors for the chemical reaction were prepared from B₂O₃ and r GO solution dispersed with SDS.The prepared hierarchical structure consists of large h-BN nanosheets at the bottom and h-BN nanowalls grown vertically on its surface,where the average thickness of the vertically grown nanowalls is 3.7 nm,corresponding to 11～12 atomic layers.The lateral size of the hierarchical structure is about 19μm,which strictly follows the size of the original r GO（about 20μm）,indicating the template role of r GO.XRD test results show the high purity and quality of the product.The product doesn’t contain carbon and other impurities with a yield of 12%.In the reaction without the participation of r GO,the product has no h-BN nanowall hierarchical structure and the content of h-BN in the product is low and has a poor quality.It shows that the participation of r GO plays a decisive role in the special hierarchical structure morphology of the product,which improves the quality and quantity of the product.2.The gas adsorption performance of h-BN nanowalls have been researched in detail.Two important gases（CH₄ and CO₂）were used to test the gas adsorption properties of the products,and make a comparison with commercial h-BN nanosheets with the same lamellae thickness.The results indicate that the adsorption capacity of the synthesized h-BN nanowalls for these two gases is much higher than that of commercial h-BN nanosheets,showing enhanced gas adsorption performance.Moreover,the adsorption capacity of the prepared h-BN nanowalls for CO₂ is much higher than that of CH₄,showing selective adsorption characteristics.Infrared test shows that the sample has a stretching vibration peak of-OH at 3420 cm^-1,indicating that functional modification is simultaneously achieved during the synthesis of the sample,which is the key to the improvement of the adsorption performance of the sample and the realization of selective adsorption.The above research results are of great significance for solving the excessive emission of greenhouse gases in daily life.3.Large area,single crystalline h-BN thin films with adjustable thickness were prepared by prefabricated r GO as seed layer on quartz and Si O₂/Si substrates by CVD process.The resulting films exhibit layered growth and are completely independent of the lattice orientation of the substrate.By controlling the amount of r GO in the reaction,the film thickness can be regulated from 2 nm to 20 nm.The grown films are uniform,continuous,good transmittance,and its band gap is closely related to the film thickness.Taking 0.25 mg/m L r GO dispersion as seed crystal layer as an example,the typical optical bandgap value of the prepared boron nitride film is 5.75e V.The high-density boundary generated by r GO layer stacking and its inherent defects act as active sites for nucleation,which are essential for large area h-BN growth.Oxygen-containing functional groups（such as C-O dangling bonds）on the surface of r GO facilitate the adsorption of B atoms to form O-B-N bonds for subsequent nucleation.Results Studies have shown that these characteristics of r GO can significantly increase the nucleation density of h-BN,which is beneficial to the direct growth of h-BN on dielectric substrates lacking catalysis.By changing the concentration of r GO on the surface of the dielectric substrate,that is,the density of r GO defects and edges,the growth rate and film thickness of the h-BN thin film will be affected,so that the number of h-BN layers on the substrate can be effectively controlled.This method provides a new idea for the preparation of high quality single crystal h-BN films at medium and low temperature and atmospheric pressure,and also provides inspiration for the preparation of other two-dimensional materials.