Shape- and size-controlled synthesis of uniform anatase TiO₂ nanocuboids enclosed by active {100} and {001} facets

Uniform anatase TiO₂ nanocuboids enclosed by active {100} and {001} facets over a wide size range (60-830 nm in length) with controllable aspect ratios were solvothermally synthesized through hydrolysis of titanium tetraisopropoxide (TTIP) using acetic acid (HAc) as the solvent and the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]) as the capping agent. The size and aspect ratio of the anatase TiO₂ nanocuboids can be readily adjusted by changing the composition parameters including the contents of [bmim][BF₄], water, and HAc in the quaternary solution system. It was revealed that [bmim][BF₄] played an important role in stabilizing both the {100} and {001} facets of the anatase TiO₂ nanocuboids. On the one hand, [bmim][BF₄] acted as a fluoride source to release F^- ions for stabilizing the {001} facets; on the other hand, the [bmim]⁺ ions acted as effective capping ions to preferentially stabilize the {100} facets. The obtained near-monodisperse anatase TiO₂ nanocuboids exhibited an interesting self-assembly behavior during deposition. These single-crystalline anatase nanocuboids showed extremely high crystalline phase stability, retaining the pure phase of anatase as well as the morphology even after being calcined at 900 °C. Moreover, the anatase nanocuboids exhibited considerably enhanced photocatalytic activity owing the wholly exposed active {100} and {001} facets.