**3. Photocatalytic applications of M-TiO2**

Ti alkoxides are more easy to handle with respect to TiCl4

with the Ti alkoxide. For instance, when Ti(OEt)<sup>4</sup>

Ti (OEt)

128 Titanium Dioxide

the Pluronic surfactants).

+

to a robust inorganic mesostructure.

curvature at the interface with the inorganic mesostructure [52].

grain growth, favoured at high temperatures [5].

it is possible to obtain cubic M-TiO2

not stable above 250°C.

M-TiO2

to ≡Ti─OH2

liser to control the hydrolysis: besides providing acidic conditions, HCl also forms complexes

form in the initial sol [49]. Such species are low-molecular-weight oligomers that, being resistant to hydrolysis, act as nanobuilding blocks (NBB) and cooperate with the hydrophilic portion of the copolymer micelles (for instance, by forming H-bonding with the PEO segment in

Concerning NBB, their surface is pH-dependent, due to the protonation of ≡Ti─OH groups

of ions present in the solution [50, 51], as depicted in **Figure 4b**, where the ionic template may be hexadecyltrimethylammonium bromide (CTAB). Formation of the hybrid composite depends on the CTAB/Ti ratio and pH of the solution [50, 51]. After self-assembly and condensation, NBB are likely located between the micelle and the inorganic framework: the subsequent hydrothermal treatment can promote the condensation of such NBB, finally leading

In order to obtain larger mesopores, it is possible to use a swelling agent, like, for instance, an *n*-alkyl alcohol (*n*BuOH), which solubilises the hydrophobic/hydrophilic interface of the micelle, thereby causing it to swell (**Figure 4a**) [52]. The degree of swelling is proportional to the amount of alcohol, which not only acts as a swelling agent but, being likely located at the hydrophilic/hydrophobic interface, also stabilises the liquid crystal phase and determines its

Calcination is the most effective process for the organic template removal, but it is also a crucial step, since the mesoporous network may collapse at high temperature, with consequent loss of specific surface area. Nonetheless, the thermal treatment also induces crystallisation of the initially amorphous material: the degree of crystallinity is of paramount importance for applications like photocatalysis; therefore, the temperature and time of the calcination have to be carefully controlled. A high degree of crystallinity is desirable, as it implies less surface defects, and therefore a better photocatalytic performance, as defects may act as recombination centres, lowering the photocatalyst performance. An opposite concomitant effect is the

In order to obtain thermally stable materials, with suitable grain size and few surface defects, different post-synthesis thermal treatments have been proposed in the literature. For instance,

calcination at 400°C (heating rate 1°C/min) [53]. This material was considered as a promising one for photocatalytic (and optoelectronic) applications, whereas the hexagonal M-TiO<sup>2</sup>

where synthesised by using Pluronic F127 (a triblock polymer with chemical composition:

thin films stable up to 600°C [54] due to the presence of rather thick walls (9.0–13 nm),

with anatase phase stable up to 400°C obtained after 4 h

, and in the presence of ionic surfactants, they can be stabilised by the charge

<sup>4</sup> + (4 − *x* − *y*)HCl + *y* H2 O → TiCl4−*x*−*<sup>y</sup>* (OEt)

Starting from different precursors, the same (partially hydrolysed) species TiCl4–*x*–*<sup>y</sup>*

, but they need some acid as a stabi-

*<sup>y</sup>* + (4 − *x*)EtOH (2)

(OEt)*<sup>x</sup>*

(OH)*<sup>y</sup>*

was

is used, the following reaction occurs:

*<sup>x</sup>* (OH)

Photocatalytic processes involving TiO2 are among the most common advanced oxidation processes (AOPs) proposed for the degradation of several inorganic and organic environmental pollutants [57, 58]. Ordered mesoporous structures are highly desirable for photocatalytic applications, since sufficiently large mesopores facilitate diffusion of reactants/products and high specific surface area improves adsorption, especially when bulky organic moieties are implied, like in wastewater and groundwater remediation.
