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[Display omitted] • Tunable heterojunctions of Cu x O nanoparticles anchored on CNNTs were prepared. • Annealing under H 2 /Ar (5% H 2) gases ensured a Cu 2 O@CuO/CNNTs Z-scheme heterojunction. • Treating under static air led to a CuO/CNNTs Type II heterojunction. • Cu 2 O@CuO/CNNTs exhibit improved photocatalytic nitrogen fixation efficiency. • Cu 2 O@CuO interface offers a high N 2 adsorption ability. Tunable Cu x O (core–shell structured Cu 2 O@CuO and CuO) nanoparticles (NPs) are finely loaded on carbon nitride nanotubes (CNNTs) using a facile one-pot method but annealing under different atmospheres. Calcining under H 2 /Ar (5% H 2) ensured a Z-scheme heterojunction of Cu 2 O@CuO/CNNTs, and core–shell nanostructured Cu 2 O@CuO NPs in particle size of 20–80 nm are firmly anchored along the nanochannels of CNNTs. Calcining under static air conditions led to a Type II heterojunction of CuO/CNNTs, and larger CuO NPs of ca. 200 nm are on the surface of CNNTs. The effect of the calcination temperature and loading content on the photocatalytic ammonia yield was studied. With the annealing temperature at 400 °C and 9 wt% copper percentage, the resulting Z-scheme Cu 2 O@CuO/CNNTs exhibit a nitrogen photofixation rate of 1.38 mM g cat –1h−1 with an apparent quantum efficiency of 6.28% at 420 nm, which is about 1.4 and 4.4 times higher than that of CuO/CNNTs and the bare CNNTs, respectively. Introducing Cu 2 O cores upshifts the band positions of the Cu x O NPs, resulting in the formation of a Z-scheme band structure. Comprehensive characterizations reveal that compared to Type II CuO/CNNTs, Z-scheme Cu 2 O@CuO/CNNTs offer higher N 2 chemisorption energy, accelerated charge carrier transfer and increased photoreduction capability. This study provides a reliable and promising route to engineer core–shell structured Z-scheme heterojunctions for enhanced photocatalytic nitrogen fixation. [ABSTRACT FROM AUTHOR]

• This work reports highly crystalline S-doped CNNTs with tunable geometry. • S-CNNTs-20 exhibit an enhanced visible-light photocatalytic N 2 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 g cat –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. [ABSTRACT FROM AUTHOR]