Mesoporous black TiO₂ hollow shells with controlled cavity size for enhanced visible light photocatalysis

Black TiO₂ formed by introducing lattice disorder into pristine TiO₂ has a narrowed band gap and suppresses the recombination of charge carriers. This provides a potential strategy for visible light photocatalysis. However, the microstructural design of black TiO₂ for a higher optimization of visible light is still in high demand. In this work, we proposed the preparation of black TiO₂ hollow shells with controllable cavity diameters using silica spheres as templates for the cavities and the NaBH₄ reduction method. The decreased cavity size resulted in a hollow shell with an enhanced visible-light absorption and improved photocatalytic performance. Moreover, we demonstrated that this cavity can be combined with gold nanoparticles (AuNPs) to form AuNPs@black TiO₂ yolk-shells. The AuNPs provided additional visible light absorption and promoted the separation of photogenerated carriers in the yolk-shell structures. This further improved the photocatalysis, the degradation rate of Cr(VI) can reach 0.066 min^-1. Our work evaluated the effect of the cavity size on the photocatalytic performance of hollow and yolk-shell structures and provided concepts for the further enhancement of visible-light photocatalysis.