Geometry-tunable sulfur-doped carbon nitride nanotubes with high crystallinity for visible light nitrogen fixation

•This work reports highly crystalline S-doped CNNTs with tunable geometry.•S-CNNTs-20 exhibit an enhanced visible-light photocatalytic N2 fixation rate.•High crystallinity and S-N coordination firmly stabilize the surface N in S-CNNTs-20.•The synergistic effect is due to high crystallinity, S dopant and holey tubular form. Highly crystalline carbon nitride nanotubes doped with sulfur (S-CNNTs) were obtained from a facile co-polymerization of thiourea and melamine precursors. The geometry, crystallinity and the doped S content in CNNTs can be fine-tuned by varying the content of thiourea. Under the addition of 20 wt% thiourea, the resulting highly condensed S-CNNTs have a diameter of around 20 nm with mesopores on the tube wall, presenting an optimal visible light photocatalytic ammonia yield rate of 0.64 mM gcat–1h−1 with an apparent quantum efficiency of 5.65% at 420 nm. The surface N atoms in the optimal photocatalyst can be firmly stabilized due to the increased crystallinity and S-induced coordination. The high crystallinity and one-dimensional mesoporous structure promote the electron transfer along the longitudinal dimension, facilitate mass transport and extend visible light harvesting. The mesoporous nanotubes favor the exposure of S dopant active sites for the chemisorption and activation of nitrogen molecules. Moreover, the photoreduction ability is improved due to the upshifted conduction band level after S doping. The designed structure maximizes the utilization of the synergistic effect, thereby improving the photocatalytic activity. This work may provide a reliable and promising way to regulate the morphology and electronic structure of tubular CN for highly efficient photocatalysis via a nonmetal doping approach.