Tunable Z-scheme and Type II heterojunction of CuxO nanoparticles on carbon nitride nanotubes for enhanced visible-light ammonia synthesis

Tunable CuxO (core-shell structured Cu2O@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 Cu2O@CuO/CNNTs, and core-shell nanostructured Cu2O@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 Cu2O@CuO/CNNTs exhibit a nitrogen photofixation rate of 1.38 mM gcat-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 Cu2O cores upshifts the band positions of the CuxO NPs, resulting in the formation of a Z-scheme band structure. Comprehensive characterizations reveal that compared to Type II CuO/CNNTs, Z-scheme Cu2O@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.