Novel Routes To Controlled Synthesis Of Boron Carbon Nitride

Based on the knowledge obtained from prepaing boron nitride by hydrothermal synthesis,together with the consideration of the characteristics of solvothermal process,a novel in-situ phase-selective synthesis route has been successfully introduced to the preparation of BN for the first time.Both the key factors affecting the preparation and the growth mechanism of BN crystals were preliminarily explored and discussed.Following these strategies,BN nanocrystals have been obtained under optimized conditions.And now we are able to control the growth process of BN to some extent.Furthermore,another new constant-pressure hydrothermal synthesis method has also been developed and used to synthesize carbon nitride.And products containing graphitic(g-)and beta(β-)carbon nitride have been attained.The detailed results are presented as follow:To control the solvothermal reaction process effectively,and to improve the yield of BN samples,the boron tribromide was replaced by the complex resulting from the reactions between boron tribromide and pyridine(C₅H₅N·BBr₃).On the basis of comprehensive characterization of this complex by FTIR,Raman,NMR and TG-DTA, the key experimental parameters were determined.Furthermore,BN nanocrystals were prepared by reacting lithium nitride(Li₃N)with the above complex in dry and oxygen-free pyridine under solvothermal conditions.The results indicate that the temperature,molar ratio of the reactants and pressure play important role in determining the phases and crystalline quality of BN products.Although the yield of BN samples have been obviously improved by using the complex C₅H₅N·BBr₃ as the boron source,it is still quite difficult to prepare BN nanocrystals in controllable way due to the intrinsic disadvantages of solvothermal synthesis method.In order to overcome this problem,the in-situ phase-selective solvothermal route is thus developed and employed.And the experimental results indicate that,the formation of hexagonal boron nitride(hBN)is favoured at comparatively low temperature,while the content of cubic boron nitride(cBN) increases with increasing temperature.And quite large cBN micro-crystals can be prepared when the temperature increased to 320℃.Hexagonal BN can be suppressed by adding excessive Li₃N into the reaction solution,at the same time,the formation of cBN and orthorhombic boron nitride(eBN)is promoted.The pressure also affects the phase and crystalline perfection of BN samples.If high purity nitrogen is impelled into the autoclave beforehand to make the reaction start with higher pressure,the crystalline quality of the obtained BN samples will become rather poor.Besides,the mixture of benzene and pyridine has also been used as the solvent during the in-situ phase-selective solvothermal syntheis of BN.Though reducing the crystalline quality of the samples,the mixture solvent favours the bulk crystal growth of BN;and large hBN micro-crystals have been obtained.Beta carbon nitride(β-C₃N₄)has a set of extreme properties similar to or even superior to cBN.In this study,another new constant-pressure hydrothermal synthesis route has been developed and used to explore the controlled synthesis of carbon nitride.As this method is employed,carbon nitride can be synthesized by adjusting the temperature and pressure independently.And the key factors influencing the phases and cyrstalline perfection of carbon nitride have been investigated.The results reveal that,the formation and stabilization of carbon nitride are facilitated at higher temperature(i.e.,above the decomposition temperature of NaN₃),and the impurities containing iron oxide are reduced,or even eliminated.Increasing the reaction pressure will inhibit the forming of nitrogen molecules to reduce the loss of nitrogen source, and promote the production of carbon nitride.By optimizing the preparation parameters,a sample with quite highβ-C₃N₄ content has been prepared at 300℃and 160 MPa.Generally,the potential application and practical value of both BN and C₃N₄ are determined by their phase structure,crystalline perfection and the particle size.To synthesize cBN andβ-C₃N₄ with specific crystalline perfection and particle size under moderate conditions,we have developed several new methods,e.g.in-situ phase -selective synthesis method,and constant-pressure hydrothermal synthesis method. And a series of experimental investigations have been carried out.Although these methods are still far from being perfect,they at least prove the possibilities of synthesizing cBN andβ-C₃N₄ in a moderate and controllable way.