Research On Synthesis Methods And Crystal Structures Of Carbon Nitrides

In this thesis, we concentrate on the development in theoretical prediction and experimental synthesis of CN compounds in the recent 10 years. By means of ion beam sputtering, treating melamine at high temperature and pressure, pyrolyzing melamine and treating pyrolysis products of melamine by impulsive discharge and high-speed impact, we explore the routes to synthesize carbon nitrides. The chemical compositions, crystal structures and bonding states of the CN films and powders obtained from the experiment are studied by the use of EDX, XRD, TEM, XPS and FTIR.The experimental results show that the CNX film with an average nitrogen content of 20at% is obtained when N ion energy is from 800eV to 1200eV and substrate temperature is held at room temperature. Meanwhile, the maximal nitrogen content reaches 30% in the film. The nanometer-sized -C3N4 and cubic-C3N4 crystals are embedded in the amorphous CNX film. When substrate temperature varies from 200℃ to 850℃, dominant phase in the film becomes a-C3N4.When substrate temperature is below 650 C, the Ti2N crystals are dispersively distributed in the amorphous TiNy film deposited by ion beam sputtering. When the temperature is over 650C, TiN forms in the film. With the increment of substrate temperature, the TiN(l 11) texture is obviously enhanced.The ratios of [N]/[C] varies from 1.6 to 4.5 in the CNx/TiNy multilayers prepared by ion beam sputtering. According to the calculated structure factors of the predicted CN compounds, it is found that p-CaN*, graphite-C3N4 and cubic-C3N4 with size of 0.2 to 5 m are formed in the CNX layer. The chemical bonding schemes in CNX layer are C-N and C=N bonds. In CNx/TiNy bilayers, a local heteroepitaxial growth of cubic-C3N4 on Ti2N nuclei is observed, the epitaxial relationship of them is (1 1 2) cubic-Cs // (1 1 13) Ti2N, and [421] cubic-C3N4 // [1 10] Ti2N.The CN powder with composition of 44.8at%C, 52.1at%N and 3.1at%O is obtained after the pyrolysis of melamine under the condition of 5Gpa and 800C. It should be a monoclinic phase according to the XRD data, and its lattice parameters are a=8.5206±0.0006lA, b=9.1512±0.00066A? c=10.2048?.00074A,α=γ=90±0.004°, and P=96.051±0.04°. With the increase of treatment temperature, the emission peaks in photoluminescence spectra of CN powder obtained under the pressure of 5GPa shift to longer wavelength.Melamine becomes turbostratic CN powder after pyrolysis treatment. The H content in the CN powder is 50% less than that of melamine. After the treatment of pulsed discharge and high-speed impact, the crystal structure of the CN powder is still turbostratic, however, its structure order degree increases significantly.