We demonstrate a rational fabrication of hierarchical treated rape pollen as metal-free catalyst for visible-light-driven photocatalytic CO₂ reduction.

Silver-loaded nitrogen-doped yolk–shell mesoporous TiO₂ hollow microspheres (Ag-N-TiO₂-YSM) were prepared by employing acetic acid as the hollowing controller and triethanolamine as the N source for the first time. Ag nanoparticles (NPs) were uniformly deposited by a simple in situ photo-reduction method, which can prevent the aggregation of Ag NPs. The efficiency of the as-prepared samples was investigated by monitoring the degradation of rhodamine B and ciprofloxacin under visible light irradiation. The experimental results indicate that N-doped yolk–shell mesoporous TiO₂ hollow microspheres show higher photocatalytic activity than P25 TiO₂ under visible light irradiation because of N doping and the unique yolk–shell structure. In addition, Ag-N-TiO₂-YSM shows enhanced activity compared with N-TiO₂-YSM due to the SPR absorption of silver NPs and the fast generation, separation and transportation of the photogenerated carriers. Moreover, the Ag contents can affect the photocatalytic activity of the Ag-N-TiO₂-YSM composite. A suitable amount of Ag deposition gives the highest photocatalytic activity. A higher loading does not improve the photocatalytic activity of N-TiO₂-YSM further. The active species generated in the photocatalytic system were also investigated. Based on our experimental results, a possible photocatalytic mechanism was proposed. The strategy presented here gives a promising route towards the development of delicate metal@hollow semiconductor composites for many applications in photocatalysis.