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

Abstract

In this work, ammonium oxofluorotitanates, including (NH₄)₂TiOF₄ and NH₄TiOF₃, are synthesized via the in situ hydrolysis of (NH₄)₂TiF₆ 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)₂TiOF₄ 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)₂TiOF₄ crosses to NH₄TiOF₃ disks is recorded. NH₄TiOF₃ nucleates and grows in situ from the center of an (NH₄)₂TiOF₄ cross with the continuous consumption of (NH₄)₂TiOF₄, and a pure-phase NH₄TiOF₃ disk is finally obtained. Upon thermal annealing, all the obtained ammonium oxofluorotitanates, including the pure-phase (NH₄)₂TiOF₄ crosses, (NH₄)₂TiOF₄ 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.