Carbon Nitride Diversity
K.Takarabe
Okayama University of Science, Ridai, Okayama, 700-0005, Japan
Research in carbon nitride focuses in finding a catalyst or a hard material. This
diversity of material characteristics is resulted from chemical composition and bonding,
crystal structure, and electronic functions of carbon nitride. I will introduce the
scientific diversity of carbon nitride including our on-going study.
1. Catalyst of two dimensional or graphitic carbon nitride (2D-C3N4 or g-C3N4)
2D-C3N4 is a material with a stack of extended layer consisting of a heptazine unit,
C6N7. The crystalline or amorphous 2D-C3N4 is synthesized with a chemical or
physical method. In most cases it is nano material. The synthesis of a large single
crystal is difficult.
2D-C3N4 is a semiconductor with a band gap of ~ 2.7 eV. The optical absorption edge
is deep green or shallow purple, ~450 nm. The lack of single crystal can not determine
the precise band gap energy. The first hydrogen production by splitting water with this
catalyst and visible light was reported by Wang et al. in 20091). The improvement of
photocatalystic capability continues by doping.
We have synthesized nanoamorphous 2D-C3N4(na-2D-C3N4) using atomospheric
pressure plasma. We found that the annealed na-2D-C3N4, the composition already
defers from C3N4, shows a small overpotetial for the oxygen reduction close to a
platinum but the current is small at this stage. A virtual platinum is a dream for
searching non-precious catalyst.
2. A new three dimensional(3D) carbon nitride
The IV2V2VI class of compounds shows semiconducting properties with a wide band
gap. The crystal structure of this class consists of a four-coordinate IV element, threecoordinate V element, and two-coordinate VI element. Among the IV2V2VI
semiconductors, the Si2N2O compound, called silicon oxynitride or sinoite, has been
frequently studied with regard to its crystal structure at both ambient and high
pressures2). The building block of Si2N2O compound is a SiN3O tetrahedron.
The substitution of Si with C in Si2N2X(X=O, NH, CH2) introduces the C2N2X(X=O,
NH, CH2) family. Recently, the C2N2(NH) 3) and C2N2(CH2) 3) compounds have been
synthesized at extreme conditions, and their crystal structures have been determined
experimentally. The structural change of this family under pressure has been studied.
The theoretical study of the electronic structure of the C2N2X(X=O, NH, CH2) family
shows that they are very wide bandgap4). A new carbon nitride has a vacancy in its
crystal structure. The search for a hydrogen strage is a future plan.
These compounds were synthesized in the search for the predicted super-hard C3N4. If
the NH or CH2 atom unit in those compounds was replaced with the CN2 atom unit, the
composition at least would be the same as that of C3N4. However, a super-hard carbon
nitride has not yet been synthesized. It is a scientific challenge.
References
1. X.Wang et al., Nature Mat. 8, 76(2009).
2. A, Bischoff et al., Z. Naturforsch 60b, 1231(2005), S. R. Srinivasa et al., J. Appl. Cryst. 10,
167(1977), J. Haines et al., Phys. Rev. B 77, 144206(2008).
3. E. H-Bordon et al., Angew, Chem. Int. Ed. 26,1476(2007), A. Salamat et al., Phys. Rev. B 80,
104106(2009), M. Sougawa et al., Jpn. J. Appl. Phys. 50, 095503 (2011).
4. K.Takarabe et al., J. Appl. Phys.112, 013537 (2012).