Influence of fluorine on the synthesis of anatase TiO2 for photocatalytic partial oxidation: are exposed facets the main actors?

Anatase TiO2 samples were synthesized in the presence of different amounts of HF acid with the aim of understanding the effect of the presence of fluorine on the photocatalytic activity. It is well known that the presence of fluoride ions influences the photocatalytic performance by adsorption on the surface of TiO2 and that they are capping agents in the control of specific exposed facets. The samples were characterized by X-ray diffraction (XRD), Raman spectroscopy, nitrogen adsorption-desorption measurements, scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis spectrophotometry, and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity was evaluated in gas-solid and liquid-solid systems by monitoring the oxidation of 2-propanol and 4-methoxybenzyl alcohol, respectively. Some differences in the physico-chemical properties of the different samples were noted; in particular, the presence of F- caused a net decrease of the specific surface area, an increase of the crystallinity degree and an increase of the conductivity, whilst only a slight difference was noted in the distribution of the exposed anatase facets. Moreover, an increase in the total oxidation of 2-propanol and in the selectivity to 4-methoxybenzaldehyde was observed.