Study On Catalyst-assisted Growth Of Bn-based Nanomaterials And Thin Filmsand Their Properties

Boron nitride(BN) with a structure similar to graphite possesses many extraordinary properties such as high thermal conductivity, high oxidization resistance at high temperature, and high chemical stabiliy. Because of these properties, BN is of high potential for various applications such as optoelectronic devices, high frequency electronic devices, and composite materials, etc. However, BN materials suffer from the problem of poor electrical conductivity, which can be improved by introducing carbon to form ternary BCN compounds. Some attractive applications such as lithiuhym ion batteries, drogen storage, supercapacitors, electron field-emission, and electronic devices have been demonstrated for BCN materials.Currently, the research on preparation of BN materials suffers from the problems of high toxicity of boron containing precursors, aggregation of nanomaterials, and poor crystallinity of thin films. In order to promote the application of BN materials futher work is necessary to solve theses problems by decreasing the production cost and improving the performance of BN materials. In this dissertation, we have successfully prepared nanosheet-structured BN spheres(NSBNSs) and BCN spheres(NSBCNSs) from B powders through a simple catalyzing thermal evaporation approach. The NSBNSs for application as adsorbents for wastewater treatments and the NSBCNSs for application as electrocatalyst support for oxygen reduction reaction were investigated. Crystallinity improvement of hexagonal BN(h BN) films was achieved by introducing catalyzing effects during chemical vapor deposition(CVD), and high quality BN films were prepared by using Mo as catalysts. The main research work and results are as follows:Metal oxides were used as catalysts for preparing the NSBNSs. The B atoms were evaporated by heating the mixture of metal oxides and B powder at high temperatures, which react with the simultaneously introduced NH3 to form NSBNSs. The effects of catalyst, reaction temperature, the mass ratio of the B powders and the catalyst(RB/C) on the growth of the NSBNSs were investigated systematically. Co O/Co Fe2O4 and Ni O/Ni Fe2O4 mixtures were investigated as the catalysts to prepare the NSBNSs under appropriate conditions. The Co O/Co Fe2O4 catalyst exhibits excellent catalytic performance, which is much better than the Ni O/Ni Fe2O4 catalyst. The formation of the NSBNSs is strongly dependent on the reaction temperature and the RB/C. By appropriately changing the temperature and RB/C, the diameter and nanosheet thickness of the NSBNSs can be well-controlled. According to the research results, the NSBNSs possess excellent performance for application as adsorbents removing oil, dyes, and heavy metal ions from water. The oil uptake reaches 7.8 times its own weight. The adsorption capacities for malachite green( MG) and methylene blue(MB) are 324 and 233 mg/g, while those for Cu2+, Pb2+, and Cd2+ are 678.7, 536.7, and 107.0 mg/g, respectively. These results suggest that the present NSBNSs are promising for water treatment and cleaning.The NSBCNSs were prepared by heating the mixture of catalyst and B powder at high temperatures in the mixed gas of CH4 and NH3. The effects of the flow rate of CH4, reaction temperature, and RB/C on the growth of the NSBCNSs were investigated. According to the research results, the formation of the NSBCNSs is strongly dependent on the flow rate of CH4 and the reaction temperature, which were obtained at 1300 oC and CH4 flow rate of 10 sccm. The diameter of the NSBCNSs can be well controlled from 4 μm to 450 nm by appropriately changing the RB/C. The NSBCNSs possess unique structure and improved electrical conductivity for application as electrocatalyst support. The Pt nanoparticles were successfully loaded onto the NSBCNSs through the ethylene glycol reduction method, which were uniformly dispersed on the nanosheet surface of the NSBCNSs. The Pt/NSBCNSs catalysts possess excellent electrocatalytic activity for oxygen reduction reaction(ORR) and much better stability than commercial Pt/C catalyst in alkaline solution.High quality h BN films were prepared by microwave plasma chemical vapor deposition and post-annealing process using Mo as the catalyst. The effects of the flow rate of BF3, flow rate of H2, microwave power, and working pressure on the growth of the h BN film were investigated systematically. The effects of post-annealing process on morphology and crystallinity of the h BN film were investigated systematically. The research results indicate that the presence of the Mo catalyst can effectively increase the crystallinity of the h BN film during the CVD and post-annealing process. The catalyzing effect of Mo promoting the crystallization of the h BN films is particularly high. After annealing, the full width at half maximum of the Raman peaks of the h BN films after is as low as 9.3 cm-1. According to the experiment results, the growth mechanism of the BN film was discussed. It is believed that the catalyzing effect of Mo in improving the crystallinity of the h BN films is due to the dissolution and precipitation of N and B atoms into/out of Mo.