First-principles study of CN carbon nitride nanotubes.

The structural, electronic and optical properties of carbon nitride (CN) nanotubes (CNNTs) built from triazine units were investigated by using density-functional theory (DFT) within the generalized gradient approximation. The total energies per CN unit (relative values) of armchair CNNTs are smaller than those of zigzag CNNTs for the same tube index. This leads to unsmooth tubular surfaces for nitrogen-nitrogen lone pair repulsions and porous structures. The armchair CNNTs appear as polygons while zigzag CNNTs are circles. Both the armchair and zigzag CNNTs are direct band gap semiconductors and their band gaps are dependent on tube size and chirality. The calculated dielectric functions of CNNTs are dependent on tube size and the directions of polarization. Moreover, they may be used as photocatalysts to split water to produce hydrogen.