Photocatalytic properties of specific facet-dominated anatase TiO2

The electronic and surface properties of anatase TiO2 greatly influence its efficiency in photocatalytic processes. Therefore the preparation of shape-controlled titania may help in understanding the role played by specific crystal facets in favoring selective photoreactions. In the present work a series of anatase TiO2 materials were prepared by employing the hydrothermal route in the presence of titanium isopropoxide as Ti precursor and HF as capping agent, which allows the preferential stabilization of the less stable but potentially more reactive {001} facets. In particular the attention was focused on the effect induced by varying the F/Ti molar ratio (in the 0.1 – 2 range) on the structural and surface properties of the so obtained TiO2 materials. A corresponding series of samples was also prepared by calcining the as-prepared nano materials in air at 500°C, in order to remove residual surface fluoride. All powders were characterized by XRPD, BET, SEM and HR-TEM analysis and their photocatalytic activity was studied in formic acid (FA) mineralization and Cr(VI) photoreduction in aqueous suspensions at pH 3.7 under UV-vis irradiation. All the employed physico-chemical techniques confirmed a progressive morphological evolution of TiO2 with increasing the HF agent concentration, without consistent variation in the resultant specific surface area (SSA). In particular, starting from the compact, spherically-shaped structure obtained in the presence of low fluoride amounts, thin nanosheet crystals were obtained. In particular the photocatalyst prepared with a F/Ti ratio equal to 1 was characterized by the highest percentage of exposed {001}facets. Concerning the photoactivity results, the presence of a larger amount of {001} crystal facets was found to favor FA photooxidation rather than Cr(VI) photoreduction, which was conversely promoted on samples with larger percentage of {101} facets, considered as the preferred reduction sites. These preliminary results seemed to confirm the specific reactivity of the main investigated TiO2 crystal facets, characterized by a suitable arrangement and coordination of surface atoms. Furthermore, the presence of residual surface fluoride was found to affect the photocatalytic trend of both test reactions, with an optimal F/Ti ratio for each of them. In particular, the thermal treatment applied to the as-synthesized materials produced a marked beneficial effect, at least in FA photodegradation, not only by removing surface fluoride but also by triggering morphological changes in the crystal aggregates.