Microwave-assisted sol-gel synthesis of TiO2 in the presence of halogenhydric acids. Characterization and photocatalytic activity

The synthesis of mesoporous TiO2 nanosheets is reported using Ti(IV) Isopropoxide as Ti(IV) precursor. A sol-gel process combined with microwave activation is used. Three different halogenhydric acids (HX), were used to peptise the sol: HF(ac), HCl (ac) and HBr (ac). The three obtained TiO2-I(HX) samples were characterized by XRD, XRF, N-2-adsorption, SEM, TEM, DRS and XPS. The three synthesized samples have high values of specific surfaces (between 100 m(2)/g and 200 m(2)/g) and similar band gap values (3.2-3.3 eV). The analysis of the surface composition by XPS confirms the presence of the halogenated species (F, Cl or Br) on the surface of each ones of the samples. The nanometric size (ca 5 nm) of the particles for each of the three samples was confirmed by XRD and by TEM. On the other hand, the nature of the halogenated acid used plays a role in the composition of the phases. While the TiO2-I (HF) sample was 100 % anatase, the other samples turned out to be biphasic, showing anatase/rutile in the TiO2-I(HCl) sample and anatase/brookite in the TiO2-I(HBr) sample. The samples were tested under two illumination conditions (UV and visible light) using rhodamine B and caffeine. The indirect role of the halide agent on the photocatalytic activities thereof is discussed.