**3.6. Thermal posttreatment**

After the aging phase, titania thin films are calcinated at high temperature. In this step, a sequence of transformations takes place: evaporation of the remaining solvent and acid (T < 200°C) which activates consolidation of the framework (this step is accompanied by an uniaxial shrinkage of the mesostructure along the z axis); template pyrolysis (T= 250–300°C), which generates the porosity; and crystallite nucleation and growth on walls (T > 300°C) [9].

Typically with mesoporous TiO<sup>2</sup> , the thermal posttreatment effectuated to convert the amorphous titania walls into crystalline walls usually results in the partial or total collapse of the mesoporous network [46, 47]. To circumvent such a drawback, several post-synthesis treatments have been developed mainly devoted to increase the thermal stability of the mesoporous TiO2 . In particular, Cassiers et al. [48] reported that the posttreatment of an uncalcinated mesoporous titania powder with ammonia resulted in the formation of mesoporous crystalline titania with thermal stability up to 600°C. Sanchez and coworkers [49] claimed that the mesoporous anatase network could be retained with a porosity of 35% above 650°C by applying a specific post-synthesis delayed rapid crystallization (DRC) treatment. Another study demonstrates that the thermal stability of these crystalline films can be enhanced, up to 850°C, by posttreatment of the film in supercritical carbon dioxide (sc-CO<sup>2</sup> ) with the presence of a small amount of precursor, such as tetramethoxysilane (TMOS) [50].

The thermal posttreatment has to be therefore carefully done and monitored especially if a specific TiO<sup>2</sup> polymorph is sought. Most of the reported literature data available are dedicated to the preferential formation of anatase vs. the rutile polymorph. Although through the literature a large inhomogeneity in temperature range over the control of a selected polymorph is present and reflects the influence of secondary important factors such as the Ti precursor, the template, the deposition technique, and the experiment conditions used to form the TiO<sup>2</sup> mesoporous material [51], most of the reports agree in the temperature range of 350–450°C to promote the exclusive crystallization of the anatase polymorph, while at higher temperature, the rutile phase transformation initiate [42, 52].
