Defect-engineering of mesoporous TiO2 microspheres with phase junctions for efficient visible-light driven fuel production

Highlights: • Defective mesoporous TiO₂ microspheres were synthesized by a facile and controllable confinement reduction method. • The resultant defective mesoporous TiO₂ microspheres can both promote the light adsorption and charge separation efficiency. • The defective microspheres exhibit a superior visible-light activity in terms of H₂ generation and CO₂ reduction. -- Abstract: Defect-engineering of TiO₂ materials is an effective way to enhance their light absorption. However, activities in the visible-light region are still far from satisfactory due to the uncontrollable defect location. Herein, we demonstrate a facile confinement reduction route to introduce controllable defects to mesoporous TiO₂ microspheres with phase junctions (denoted as M-TiO₂-PJs) by using sodium borohydride (NaBH₄) as the reducing agent. In this case, the confinement decomposition effect of mesopore channels over NaBH₄ enables the generation of defects more effectively at a mild reaction condition, enabling the well-retained mesoporous and phase junction structures of mesoporous TiO₂ microspheres. Moreover, by changing the reduction temperature, the defects are migrated from the nanocrystalline-exposed surfaces to phase junction interfaces, enabling that the location of the defects can be well-tuned. After the reduction at 300 °C, the resultant defective mesoporous TiO₂ microspheres show the well-retained mesostructure, high surface areas (~75.6 m² g^-1), large pore volumes (0.36 cm³ g^-1), slightly disordered surfaces and intimately contacted anatase-rutile interfaces, which exhibit the state-of-the-art activities for photocatalytic water splitting. The H₂ generation rate is as high as 42.6 μmol h^–1 (based on 50 mg of catalyst) under visible-light (λ > 400 nm) and the apparent quantum efficiencies are estimated to be 12.7% and 2.8% at 420 and 520 nm, respectively, which are the best values among TiO₂-based photocatalysts reported to date. We also show that the defective mesoporous TiO₂ microspheres possess a super CH₄ selectivity (57%) and yield (15 nmol h^-1) for CO₂ reduction under visible-light because of the activation and adsorption effect of defects for CO₂ molecules. This work provides new insight into rational design of high performance photocatalysts.