Fluorine-assisted structural engineering of colloidal anatase TiO ₂ hierarchical nanocrystals for enhanced photocatalytic hydrogen production

Anatase TiO2 materials are well-known for their photocatalytic properties and their structure-performance relationship has been intensively studied over the past few decades. In this study, we report a versatile strategy to control the geometric and electronic structure of hierarchical anatase TiO2 nanocrystals via a colloidal synthesis technique in order to optimize their photocatalytic performances. The synthesis is modified from a classical nonaqueous sol-gel approach in which titanium alkoxides and long carbon chain carboxylic acids are used as titanium sources and hydrolysis/capping agents, respectively. By introducing fluoride ions into the reaction as competitive capping agents and controlling other parameters, the geometric structure of TiO2 nanocrystals can be regulated from nanorods and nanobipyramids to their hierarchical assembly structures with controlled dimension and crystallinity. Meanwhile, it is confirmed that the fluoride capping agents also affect the surface structure of TiO2 by fluorine doping, which exerts an additional impact on the electronic structure of TiO2 nanocrystals apart from morphology variation. Further investigation of photocatalytic hydrogen production performances of TiO2 nanocrystals with different structures indicates that the catalytic efficiency is highly dependent on structural factors including hierarchical shape, surface area and doping status. Obvious improvement of photocatalytic performance is observed in the optimized hierarchical TiO2 nanocrystals (2033.6 μmol g-1 h-1) compared to that in commonly prepared TiO2 nanobipyramids (1135.5 μmol g-1 h-1) and other hierarchical TiO2 nanocrystals (1331.9 μmol g-1 h-1 or lower), which demonstrates the effectiveness of material optimization by the strategy developed in this study.