Fast growth of precursors for 3D ordered TiO2 mesocrystals: from (NH4)2TiOF4 plates to NH4TiOF3 disks

In this work, ammonium oxofluorotitanates, including (NH4)(2)TiOF4 and NH4TiOF3, are synthesized via the in situ hydrolysis of (NH4)(2)TiF6 produced by the field-assisted dissolution of freshly grown TiO2 during the high-field anodization of titanium in an NH4F-containing ethylene-glycol-based electrolyte. The obtained ammonium oxofluorotitanate products have a mesocrystalline structure consisting of 3D nanocrystalline building units aligned in an orderly fashion, which are self-assembled via a typical non-classical crystallization process, i.e., oriented particle attachment. (NH4)(2)TiOF4 with various morphologies, such as plates, rods and a unique cross-like structure, is obtained within a few minutes of reaction. As the reaction continues, an unexpected transformation process from (NH4)(2)TiOF4 crosses to NH4TiOF3 disks is recorded. NH4TiOF3 nucleates and grows in situ from the center of an (NH4)(2)TiOF4 cross with the continuous consumption of (NH4)(2)TiOF4, and a pure-phase NH4TiOF3 disk is finally obtained. Upon thermal annealing, all the obtained ammonium oxofluorotitanates, including the pure-phase (NH4)(2)TiOF4 crosses, (NH4)(2)TiOF4 crosses with an NH4TiOF3 disk at the center, and pure-phase NH4TiOF3 disks, could be converted to micrometer-sized mesocrystalline anatase TiO2 composed of nanocrystalline sub-units while maintaining their exterior morphologies. Physicochemical information related to the growth and transformation of the products is discussed.

Automated summary

In this study, ammonium oxofluorotitanates were synthesized through the in situ hydrolysis of (NH4)2TiF6 produced in the field-assisted dissolution of fresh TiO2 during the high-field anodization of titanium. The obtained products can be converted to mesocrystalline anatase TiO2 upon thermal annealing.