Structural Characteriaztion Of Graphitic Carbon Nitride And Its Derivatives And Its Structural Stability Under High Pressure

Graphitic carbon nitride(g–CN)has been considered as the most appropriate precursor for the high–density,supperhard CN phases.Recently,g–CN has also attracted great attention due to its promising photocatalytic properties.However,g–CN possesses very complicated structures,and exhibits very different properties that highly dependent on the specific synthetic conditions.In this thesis,the structures of g–CN and its derivatives have been characterized by combinatorial X–ray diffraction(XRD)and Fourier transform infrared(FTIR)spectroscopy analysis.A cluster model–based theoretical approach has been developed to simulate the FTIR spectra of the obtained products.Furthermore,the relationship between the structure and composition of g–CN and the different condensation pathways has been investigated,and the stability of triazine–based condensates has been studied under high–pressure conditions.The main results of thisthesis include the following four parts.Firstly,we started from the structure of melamine,which is simplest and representative molecular crystal in the g–CN related system.The crystal structure of melamine was optimized by using DMol~3 quantum chemistry module of Material Studio software package.The X–ray diffraction pattern was simulated by the Reflex module and compared with the experimental results.A conventional cluster model–based approach was developed to simulate the FTIR spectrum of melamine by using Gaussian software package.On the basis of the calculated potential energy distribution(PED),we further presented an accurate and visualized description on the characteristic vibrational modes of crystalline melamine.Secondly,melam,melem and g–CN samples were synthesized by a thermal condensation of melamine.Their crystal structures have been characterized.The cluster model–based approach proposed in the last chapter was applied to simulate the FTIR spectra of theese three structures,and a detailed assignment to the corresponding vibrational modes was presented.The simulated spectra provide complementary structural information for g–CN and its derivatives,particularly for the hydrogen–bonding interactions in the crystal structures;which paves the way for the accurate determination of the structural and bonding properties of g–CN related materials.Thirdly,the relationship between the composition of the g–CN condensates and their degree of polycondensation(DP)was calculated,which allow us to analyze the structures and condensation pathways of g–CN through the composition of the products.A series of heptazine–based g–CN samples were synthesized by thermal condensation of melamine under different temperatures.The changes of their composition,crystallinity,condensation pathways,and the average DP were analyzed and discussed in details.The possible components of the obtained g–CN condensates were discussed,which provides a better understanding on the structure of g–CN.Finally,triazine–based precursor is more prospective for the synthesis of supperhard CN materials.Therefore,we synthesized triazine–based melam condensates by multi–step thermal condensation of melamine.And the products were treated under a high pressure of5 GPa,at different temperatures.The results showed that the stability of the melam structure were obviously improved under high pressure.Whereas the residual melamine in the condensates were further polymerized,and thus the purity of the melam was improved.