Electric field driven fast fabrication of 3D ordered Nano-micro hierarchical ammonium oxofluorotitanates towards mesocrystalline anatase TiO2

In this work, 3D ordered nano-micro hierarchical ammonium oxofluorotitanates are synthesized by in-situ dissolution of anodically grown TiO2 nanotubes and subsequent hydrolysis of (NH4)2TiF6 in an electrochemical cell via a nonclassical crystallization process by external electric field driven oriented particle attachment. The obtained ammonium oxofluorotitanate products are of mesocrystalline structure consisting of nanocrystalline building units oriented aligned at atomic level. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) characterizations are applied to reveal the morphology, structure and crystal phase of grown samples. By controlling the current density or/and content of ammonium fluoride, it is able to grow well-defined (NH4)3TiOF5 octahedrons and truncated octahedrons, (NH4)2TiOF4 nanorods and NH4TiOF3 disks. The transformation from (NH4)3TiOF5 octahedrons to truncated octahedrons is ascribed to the different ratios of growth rate for {111} planes and {100} planes. While the decrease of concentration of ammonium fluoride accounts for the crystal phase changes from (NH4)3TiOF5 to (NH4)2TiOF4 and finally to NH4TiOF3 disks. Upon thermal annealing, all these ammonium oxofluorotitanates could be converted to mesoporous hierarchical anatase TiO2 retaining their exterior morphology and keeping the orientation ordering of subunit nanoparticles, which could exhibit excellent photocatalytic activity when acting as photocatalyst in the photo-degradation of methylene blue.