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

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.