Masaya Sougawa

Use of nitrogen atmospheric pressure plasma for synthesizing carbon nitride

Carbon nitride (CNx) with a high nitrogen content was synthesized using a nitrogen atmospheric pressure plasma. In this method, a reaction space with a high temperature and a high nitrogen content was generated. Under this condition, it was expected that a dense nitrogen radical reacting with a carbon radical would saturate carbon bonds, result in a simultaneous increase in the composition ratio (N/C), and produce a C–N single bond. The N/C ratio of the synthesized CNx reached 1.0, as determined by X-ray photoelectron spectroscopy (XPS) analysis. XPS and FT-IR analyses showed that the synthesized CNx consists of a dominant C–N bond with a C=N bond and a C≡N bond. The synthesized CNx appeared like a sphere with a diameter of about 100 nm. The optical emission of a CN radical was observed under the CNx growth conditions, and similarities and differences between a nitrogen atmospheric pressure plasma and plasma-enhanced chemical-vapor deposition were determined.

Crystal Structure of New Carbon–Nitride-Related Material C2N2(CH2)

A new carbon–nitride-related C2N2(CH2) nanoplatelet was synthesized by subjecting a precursor C3N4HxOy nanoparticle in a laser-heating diamond anvil cell to the pressure of 40 GPa and temperature of 1200–2000 K. The C and N composition of the quenched sample was determined to be C3N2 by using an energy dispersive X-ray spectroscope attached to a transmission electron microscope. The crystal structure and atomic positions of this C3N2 were obtained through Rietveld analysis of the X-ray diffraction pattern measured using synchrotron radiation. The hydrogen composition was difficult to determine experimentally because of the several-hundred-nanometer dimensions of the sample. First-principles calculation was alternatively used to discover the hydrogen composition. The synthesized C2N2(CH2) was accordingly found to be an orthorhombic unit cell of the space group Cmc21 with lattice constants a = 7.625 A, b = 4.490 A, and c = 4.047 A. If the CH2 atomic unit is replaced with the CN2 atomic unit and the bonding rearranged, the C2N2(CH2) becomes the expected superhard C3N4.

Electronic structure of C2N2X (X = O, NH, CH2): Wide band gap semiconductors

The electronic structure of IV2V2VI class semiconductors, C2N2X (X = O, NH, CH2), was investigated using first principles calculations. The crystal structures of C2N2X are isostructural with the Si2N2O compound, sinoite. The valence of the X atom is virtually two, and thus the substitution of X (X = O, NH, CH2) is isoelectronic. From the calculated density of states, the carbon 2 p orbital does not participate in the upper valence band (VB) (0 to –5 eV). The upper valence band is dominated by the N 2 p and X 2 p orbitals. The calculated optical absorption edge shifts to a lower energy as the substitution progresses from the O atom to the CH2 group. The calculated absorption edge is 7.76, 7.07, and 6.66 eV for C2N2O, C2N2(NH), and C2N2(CH2), respectively.

Synthesis of new carbon-nitride-related materials at high pressure and temperature

Low-compressive new carbon-nitride-related materials were synthesized with a high-pressure and high-temperature treatment of CNHO nanoparticle prepared with an atmospheric nitrogen plasma. New phases were recovered at ambient conditions and the most prominent new phase is tentatively assigned to be an orthorhombic unit cell with the lattice parameters; a = 7.635 A, b = 4.487 A, and c = 4.040 A.

Bond strengths of New Carbon-nitride-Related material C2N2(CH2)

A new carbon-nitride-related material C2N2(CH2) nanopletelet was synthesized by subjecting a precursor C3N4HxOy+Au in a laser-heating diamond anvil cell (LHDAC) to the pressure of 40 GPa and the temperature of 1200-2000 K. The synthesized C2N2(CH2) was accordingly found to be an orthorhombic unit cell of the space group Cmc21 with lattice constants a = 7.625A, b = 4.490A, and c = 4.047A. The bulk modulus B0 was determined to be B0 = 258 ± 3.4 GPa, only the 60 % that of the diamond. C2N2(CH2) consists of the tetrahedrally coordinated C with three C-N single bond and the one C-C single bond, and the bridging carbon with the C-CH2-C bond. The C-N single bond length of the tetrahedron ranges from 1.444 to 1.503 A. This bond length is close to the C-N single bond of 1.447 to 1.458 A in the superhard β-C3N4. The compressibility of the C-N and C-C single bond of C2N2(CH2) ranges from 0.976 to 0.982 with the pressure of 30 GPa. These values are very close to the compressibility of the C-N and C-C single bond of 0.978 to 0.982 in β-C3N4, cubic-C3N4, and diamond.

High-pressure and high-temperature synthesis of rhenium carbide using rhenium and nanoscale amorphous two-dimensional carbon nitride

Abstract Both Re2C and Re2N are ultra incompressible and have a bulk modulus of about 400 GPa. These materials are synthesized under high pressure and high temperature. The synthesis pressures are about 10 GPa or below for Re2C and 20–30 GPa for Re2N. If the synthesis pressure of Re2N was about 10 GPa or below, a large volume high-pressure cell like a multi-anvil apparatus can be used to synthesize Re2N. To realize this, a proper solid nitrogen source is needed instead of liquid or gas nitrogen. We used a precursor of a mixture of rhenium and home-made nanoscale amorphous two-dimensional carbon nitride as a solid nitrogen source. Consequently, the synthesis reaction produced Re2C but not Re2N. We characterized the synthesized Re2C by various techniques including high-pressure x-ray diffraction (XRD). The bulk modulus B0 of the synthesized Re2C under hydrostatic conditions was estimated to be 385.7 ± 18.0 GPa. This value is a little smaller than the previous data. When the pressure medium became non-hydrostatic, the peculiar compression behaviour occurred; the rate of broadening of XRD lines increased and the compression became negligible in the range of a few GPa. The reason for this peculiar behaviour is not known.

Bulk modulus and structural changes of carbon nitride C2N2(CH2) under pressure: The strength of C–N single bond

The experimental bulk modulus, B0, of C2N2(CH2) is determined to be 258 ± 3.4 GPa from the analysis of high-pressure (up to 30 GPa) X-ray diffraction patterns obtained using synchrotron radiation. This bulk modulus is 40% lower than that of diamond. At the level of a combined analysis of lattice constants determined experimentally and atomic positions obtained theoretically for the compression behavior of C2N2(CH2), the strength of the C–N single bond is determined to be the same as the C–C single bond in diamond. In other words, the tetrahedral frame of C2N2(CH2) which consists of CN3Cb, where Cb is a bridging carbon, is as hard as diamond. To account for the differing bulk moduli, we infer that the lower bulk modulus in C2N2(CH2) is due to the rotational freedom in the crystal at high pressures.