Size-controllable g-C₃N₄ quantum dots (QDs) were in-situ synthesized and grafted onto single-crystalline TiO₂ nanotube arrays (NTAs) via nanotube-confinement effect.

The g-C₃N₄ QDs and the multiple light reflections in TiO₂-NTAs promoted light harvesting owing to its narrow band gap and upconversion of QDs.

The single-crystal TiO₂ formed during the synthesis of g-C₃N₄ favored the photoelectron transfer, and g-C₃N₄/TiO₂ heterojunctions promoted the separation of photoelectrons from holes.

The strong g-C₃N₄/TiO₂ interaction and the confinement effect of TiO₂-NTAs inhibited self-gathering and leaching of g-C₃N₄ QDs, resulting excellent stability in photoelectrocatalysis.