Effect of annealing on the structure, morphology and photocatalytic activity of surface-fluorinated TiO2 with dominant {001} facets

The surface-fluorinated TiO2 crystal dominated with high energy {001} facets was synthesized by using hydrofluoric acid as capping agent which plays an exceedingly important role in controllable synthesis of 3D flower-like microstructures by self-assembling ultrathin nanosheets. The evolution of introduced fluoride species on the surface of TiO2 was studied when annealing temperature was continuously elevated in air atmosphere. X-ray diffraction results show that the as-prepared TiO2 is very stable and without phase transformation even calcination at 700 degrees C. Both Scanning Electron Microscope and Transmittance Electron Microscope techniques reveal that the TiO2 nanosheets are intact before calcination but are etched and form some square-like pores after calcination. Moreover, the change of fluoride species in TiO2 is discussed with the assistance of X-ray Photoelectron Spectroscopy, Thermogravimetry-Derivative Thermogravimetry, Fourier Transform Infrared Spectroscopy, and Ultraviolet-Visible spectroscopy. The results indicate that the Ti-F species is originally physical adsorbed on the TiO2 surface before calcination, then fluorine is interstitially doped in TiO2 lattice calcined at 400 - 500 degrees C, and finally fluoride species is totally removed over 600 degrees C. Furthermore, the photocatalytic results show that the TiO2 calcined at 500 degrees C has the highest photocatalytic activity and still maintain a stable durability after several recycles.