Self-assembled synthesis of defect-engineered graphitic carbon nitride nanotubes for efficient conversion of solar energy

High-yield and uniform-size graphitic carbon nitride nanotubes (g-C3N4 nanotubes) with abundant nitrogen defects are synthesized for the first time by a green and acid-alkali-free synthesis using a sole melamine precursor. This approach utilizes the slow in-situ conversion of part of melamine into cyanuric acid and consequent molecular self-assembly with the rest of melamine to form supramolecular intermediate. The following pyrolysis converts the supramolecular intermediate to g-C3N4 nanotubes with abundant nitrogen defects. The morphology thus resulted preferable performance than the traditional molecular self-assembly in which the mixture of melamine and cyanuric acid is used as precursors. The g-C3N4 nanotubes with orderly tubular morphology of length-diameter ratio of 30–70 exhibit excellent hydrogen evolution rate (118.5 μmol h−1), which is obviously superior to the bulk g-C3N4. The apparent quantum efficiency of g-C3N4 nanotubes under irradiation at 420 nm is achieved at 6.8%, which is among the top of one dimensional (1D) g-C3N4 structure, such as g-C3N4 nanotubes, nanowires and nanorods. The improved photocatalytic performance benefits from the tubular structure and the nitrogen defects, which lead to the improved optical absorption, more exposed active edges, nitrogen defects active sites, enhanced charge transfer and separation efficiency, higher surface area, fast and long-distance electron transport, and longer fluorescence lifetime. Beside hydrogen evolution reaction, the g-C3N4 nanotubes also have broad applications in environmental treatment and photoelectrochemical detection of organic dyes.