**3.1. Photocatalytic properties of pure M-TiO2**

Positive effects ascribable to the occurrence of an ordered mesoporous structure were observed in the photocatalytic degradation of methylene blue (MB), a dye commonly used in this type of studies as a model molecule of recalcitrant organic pollutants that are resistant to biodegradation and for which a photocatalytic treatment is necessary.

For instance, a M-TiO<sup>2</sup> occurring as pure anatase phase showed good photocatalytic activity towards MB removal [59]. The best photocatalyst was a hexagonal M-TiO2 obtained by sol-gel synthesis using CTAB as template and Ti isopropoxide as precursor. The material obtained through a 6-days sol-gel synthesis showed mesopores with 6.86 nm diameter and a specific surface area of 284 m2 g−1. Longer synthesis time led to a decrease of both values and to the simultaneous disappearance of the hexagonal mesophase. The sample showed indeed a sizeable MB degradation with respect to Degussa P25, especially after M-TiO<sup>2</sup> was recycled and contacted with a fresh MB solution. Such superior photocatalytic behaviour was assigned to both higher surface area and higher anatase content of M-TiO2 with respect to Degussa P25 [60].

Satisfactory results concerning the photobleaching of MB were obtained with a M-TiO2 using SBA-15 silica as hard template, the activity of the sample being again ascribed to a compromise between its high surface area and the percentage of anatase. The former parameter was likely responsible for a very efficient dye adsorption at the surface of M-TiO<sup>2</sup> , the latter likely reduced the electron/hole (e− /h<sup>+</sup> ) recombination, which is slow in crystalline and defect-free materials [28].

A M-TiO2 photocatalyst obtained through the EISA method by employing ethylene diamine as a stabiliser showed better photocatalytic activity than Degussa P25 towards the degradation of 2,4-dichlorophenol, a toxic chlorinated compound produced by environmental transformations of some chlorinated herbicides and/or antimicrobial agents [56]. Several M-TiO2 were studied and compared to Degussa P25: the most efficient degradation was obtained in the presence of a M-TiO2 calcined at 700°C, whereas the performance of M-TiO<sup>2</sup> calcined at higher temperatures decreased. Moreover, the performance of the M-TiO<sup>2</sup> photocatalyst resulted stable after recycling. The photocatalytic behaviour of the studied materials was explained not only on the basis of a higher surface area (122 m2 g−1) with respect to Degussa P25 (ca. 50 m2 g−1): according to the authors, a trade-off exists between the occurrence of pure anatase phase and a well-ordered mesoporous structure facilitating diffusion of reactants/ products. At higher calcination temperatures, i.e. 900°C rutile M-TiO<sup>2</sup> , formed, with a lower activity. Nonetheless, the sample calcined at 800°C, though occurring as pure anatase, showed both a partial collapse of mesoporous walls and the formation of larger particles, leading to a worst photocatalytic performance.

A performance comparable with Degussa P25 towards the degradation of dimethyl phthalate (a persistent antiparasite) was obtained with M-TiO2 prepared by using Pluronic P123 in weak acidic solution of acetic acid [60]. Similarly to what mentioned before, both the high surface area and the crystallinity of the anatase phase contribute to the catalytic activity of the sample.

As a whole, although different experimental conditions are adopted during photocatalytic tests carried out in different laboratories, the occurrence of pure anatase M-TiO<sup>2</sup> usually favours better photocatalytic performances under UV radiation. However, this is just a general rule, but several exceptions are observed. For instance, another M-TiO<sup>2</sup> obtained by soft-template route under high-intensity ultrasound irradiation showed better photocatalytic performance than P25 in the UV-assisted degradation of *n*-pentane in air (**Figure 6**). The studied M-TiO2 was not pure anatase, but a bi-crystalline material, containing ca. 20% brookite and 80% anatase, the formation of brookite being ascribed to the ultrasound treatment adopted during the synthesis [61]. The M-TiO2 samples after calcination were characterised by high surface area (112–128 m2 g−1) and the occurrence of both anatase and brookite phase (**Figure 7**).

The band gap of the as-prepared M-TiO2 materials (SM-1 and SM-2) was estimated through Tauc's plot since the band gap of brookite should be ca. 0.16 eV higher than that of anatase and could contribute to a more efficient UV light absorption, indicating that brookite could be a more powerful photocatalyst. According to the authors, composite materials of brookite and anatase can also suppress e<sup>−</sup> /h<sup>+</sup> recombination, similarly to what widely accepted for Degussa P25 (i.e. that its high photocatalytic activity is partially due the coexistence of 80% anatase and 20% rutile, which can inhibit the recombination of excited e− /h<sup>+</sup> [62]).

Other authors found a positive effect of the incorporation of Degussa P25 particles in M-TiO<sup>2</sup> thin films [63]: the films were tested in the photocatalytic degradation of Acid Black Dye, used as a model molecule of textile water pollutants. As expected, Degussa P25 incorporation led to a decrease of surface area in the composite with respect to M-TiO2 films: however, a 5.0 wt.% Degussa P25 content led to an increase in the dye degradation efficiency. The positive role of Degussa P25 was ascribed both to an increase of the M-TiO2 film thickness and to the presence of rutile in Degussa P25 leading to a band-gap change in the composite.

Another major use of TiO2 is in disinfection processes: to this respect, wormhole like anatase M-TiO2 was studied for the photocatalytic disinfection of *Escherichia coli*, showing better performances with respect to Degussa P25 [64].
