Effect of fluorination on the surface properties of TiO2 P25 powder: an FTIR study

A study was carried out on the consequences of the -OHsurf/F- exchange occurring at the surface of TiO2 P25 when suspended in HF/F- solutions. The maximum extent of fluorination was reached at pH 3.0, resulting in the fixation on the surface of ca. 2.5 F-/nm2. The surface features of fluorinated samples under two selected conditions were investigated by IR spectroscopy, in comparison with pristine TiO2. The collected data suggested that bridged -OHsurf, likely located on regular facets, was more resistant to exchange with F-. Combined high resolution transmission electron microscopy (HRTEM), inductively coupled plasma mass spectrometry (ICP-MS) and IR measurements indicated that the fluorination performed in the adopted condition did not induce any etching of TiO2 particles, and the -OHsurf/F- exchange appeared reversible by treatment in concentrated basic solutions. Furthermore, fluorination resulted in an increase of the Lewis acid strength of surface Ti4þ sites, which, as a consequence, retained adsorbed water molecules even after outgassing at 423 K. Such an effect involved the overwhelming majority of cations exposed on regular facets.