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The graphitic carbon nitride(g-C3N4)nanotubes(CNNTs)were reported to have higher photocatalytic activity than normal g-C3N4 nanosheets.In this work,the structural,electronic,and chemical properties of single walled heptazine-based g-C3N4 nanotubes with two kinds of chirality and various diameters,as well as g-C3N4 nanosheets are investigated by first-principles calculations.Similar to the nanosheets,the nanotubes are also in corrugated porous structure.Unlike carbon nanotubes,the CNNTs show comparable stability with nanosheets,which may be due to the rolled geometry is good for release of the repulsion between the lone-pair elections of nitrogen atoms.With the diameter increasing,the work function of CNNTs decrease,while the band gap of the(n,0) CNNTs reduce but those of(n,n)CNNTs increase.It indicates that the(n,0)CNNTs would have more visible-light absorption and higher photocatalytic oxidation activity than g-C3N4 nanosheets,which is consistent with experiments.All the results manifest the mechanism of enhanced photocatalytic activity of CNNTs,and may propose strategies for designing potentially efficient photocatalysts.