Study On Preparation And Properties Of Boron Nitride Nanosheets

Hexagonal boron nitride nanosheets (BNNSs) is the isoelectric analogue of graphene. Compared with graphene, BNNSs has many peculiar properties, including high temperature stability, broad bandgap, anti-oxidation ability, corrosion-resistant, and so on. These unique features lead to BNNSs having a promising application, such as semiconductor devices working at high temperature, composite materials with high thermal conductivity, optical and electronic materials and so on. However, compared to the preparation of graphene, the methods to prepare BNNSs are fewer and less effective. Therefore, it is hard to exfoliate hexagonal boron nitride by the traditional methods like graphite. This may attribute to the ionic characteristic of the bonding between neighboring boron nitride layers, which is stronger than the weak Van der Waals force between graphene layers. In addition, it is still a great challenge to improve the properities of BNNSs via functionalization or doping, which leads to extend their application fields.in this paper, the preparation and properties of BNNSs and fluorinated BNNSs (F-BNNSs) have been studied. In addition, the formation mechanisms of BNNSs and F-BNNSs have been proposed. The mainly research achievements are as follows:1. We have used H2SO4, KMnO4and H2O2to exfoliate the BNNSs from commercially available h-BN powders by a low costly and simple chemical exfoliation method. Through adjusting the ratio of the starting material and the reaction time, large size and few-layer BNNSs can be obtained. The size of the as-prepared BNNSs is about5μm and their thickness is1.44nm, which means the atomic layer number of nanosheets is about two layers; and their crystal structures were not damaged by this exfoliation process. Based on the experiment results analysis, we proposed an exfoliation mechanism in which the intercalation of hydrogen ions and MnO2nanoparticles in succession plays a key role in exfoliating BNNSs from bulk BN and the explosion effect of the generated oxygen gas may accelerate the complete exfoliation of BNNSs. 2. By using the cathodoluminescence (CL) spectra, the optical property of as-prepared BNNSs was investigated. The experiment results demonstrate that as-prepared BNNSs show stronger CL emission ability than BN powder, which will lead to important applications like UV light emitting devices.3. On the basis of the polyvinyl alcohol (PVA) and polyethylene glycol (PEG), we prepared BNNSs/PVA and BNNSs/PEG composites, and studied their thermal and optical properties. Through the characterization of TG/DTA, we confirmed BNNSs can improve the thermal stabilities of PVA and PEG. By using the UV-vis spectrophotometer to analyze, we find that as-prepared BNNSs/PVA and BNNSs/PEG composites have strong absorption in the violet-light range.4. We have developed a one-step route to exfoliate and fluorinate the BNNSs by NH4F from commercially available h-BN powder. Through adjusting the ratio of the starting material and the reaction temperature, single-layer and few-layer F-BNNSs can be obtained. The analysis results show that F atoms of F-BNNSs were covalently modified with some B atoms on the surface of BNNSs successfully without destroying their nanosheet structure. By means of studying the dispersibility of F-BNNSs, we find the concentration is0.032mg/mL in ethanol,0.165mg/mL in N, N-dimethylformamide,0.215mg/mL in N-methylpyrrolidinone,0.026mg/mL in tetrahydrofuran, and0.058mg/mL in chloroform. F-BNNSs have relatively large solubilities in N, N-dimethylformamide and N-methylpyrrolidinone. Through control experiments, we propose a fluorination-assisted exfoliation mechanism to explain the formation of the F-BNNSs. In this mechanism, F-and NH4+were key factors in exfoliating BNNSs from bulk h-BN.5. With a superconducting quantum interferometer device (SQUID) magnetometer, the magnetic properties of as-prepared F-BNNSs were studied. The experiment results demonstrate the as-prepared F-BNNSs exhibit obvious ferromagnetic characteristics, which is quite promising for applications in spintronic devices. In addition, the remanent magnetic moment (Mr) of the synthesized F-BNNSs decreases with the increase of temperature, and their ferromagnetic critical temperature (Tc) is580K. It is the first time direct experimental observation of the magnetic properties of F-BNNSs, which is consistent with previous theoretical predictions.