Investigation of phase transitions for the hydrothermal formation of TiO2 in the presence of F− ions

The hydrothermal phase transition of ammonium titanate into TiO2 in aqueous ammonia media was investigated in the presence of F− ions by adding various kinds of electrolytes such as NaCl, NH4Cl, NaF, and NH4F. The phase composition and the morphologies of the products were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). With the assistance of F− ions, pure brookite TiO2 can be hydrothermally prepared with a wide range of Na+ ion concentrations from 0.25 M to 1.5 M in just 24 h, which has obvious advantages over the previous NaCl scheme [1] and [2]. Moreover, photocatalytic degradation of MO shows that both anatase and brookite prepared in the presence of F− ions show superior photocatalytic activities than those prepared in the absence of F− ions. The phase transition mechanism from ammonium titanate into TiO2 was also proposed. Besides the introduction of Na+ ions that benefits the phase transition from titanate to brookite, F− ions are suggested to play two key roles during the hydrothermal process: catalyzing the condensation reaction of TiOH to produce TiOTi linkages by forming TiF and regulating the intercalation and the de-intercalation of Na+ ions by counteracting the electrostatic interaction between the Na+ ions and the titanate layers.

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► Presence of F− ion affects the phase transition of ammonium titanate into TiO2.
► Pure brookite TiO2 was prepared in a wide Na+ concentration range with F−.
► F− ion catalyzes the condensation reaction of TiOH to produce TiOTi linkage.
► F− regulates the intercalation/de-intercalation of Na+ in the titanate layers.
► TiO2 prepared in the presence of F− showed superior photocatalytic activity.