**1. Introduction**

TiO2 is one of the most studied metal oxides: this is mainly due to its intrinsic properties, i.e. occurrence of different polymorphs and amorphous phases, low toxicity to humans, relatively low cost, good chemical and thermal stability and good electronic and optical properties [1]. Such characteristics make TiO2 the oxide of choice for application in (photo)catalysis,

adsorption/separation processes, sensing, energy storage/conversion and biotechnology/biomedicine [1].

Although several commercial products are available on the market, the intrinsic properties of TiO2 may be enhanced in the presence of porous samples that can be obtained by different synthesis procedures [2–9].

Besides increasing the specific surface, porosity facilitates some physico-chemical phenomena like adsorption and diffusion of chemical species, especially in the presence of mesopores (*Ø* = 2.0–50 nm). Conversely, micropores (*Ø* ≤ 2.0 nm) may have a detrimental effect on the diffusion of larger moieties.

For instance, mesoporous TiO<sup>2</sup> (M-TiO2 ) was found to be highly active in several photocatalytic processes, since mesopores promote the diffusion of chemical species, either reactants or products [5, 6], and simultaneously enhance M-TiO<sup>2</sup> photocatalytic activity by facilitating access to the reactive sites at the photocatalyst surface. Moreover, the possibility to synthesise nanoparticles (NPs) allows obtaining a material that is highly dispersible in solution: consequently, M-TiO<sup>2</sup> is not only considered for environmental photocatalytic processes (usually carried out in aqueous phase) but is attracting increasing attention also for applications in biomedicine [7], due to the possibility of controlling NP morphology and size during the synthesis.

Moreover, highly organised films of M-TiO<sup>2</sup> may be obtained by evaporation-induced selfassembly (EISA) process [8, 10, 11] in the presence of diblock and triblock copolymers [12–14]. The availability of both EISA process and Pluronic amphiphilic triblock copolymers allowed overcoming the main issue related to the synthesis of M-TiO2 , i.e. the high reactivity of Ti precursors, which are usually unstable when exposed to water or even to moisture [15].

Discovered and developed in the 1990s, the first type of ordered mesoporous oxide ever obtained was SiO2 , likely due to the high pliability of the Si–O–Si bond, which is a peculiarity of silicon [16, 17]. The synthesis methods leading to the production of ordered mesoporous SiO2 -based materials have developed much faster [18] with respect to other mesoporous oxides [19]. Reasons for that have been (i) the lack of suitable metal precursors for sol-gel synthesis and (ii) their lower stability with respect to Si-precursors. Nonetheless, the low thermal stability of some mesoporous oxides may induce formation of undesired crystalline phases and/or grain growth. Such processes are likely at the high temperatures reached during calcination, the process often adopted to remove organic moieties deriving from the template and/ or hydrolysis of the (metal alkoxide) precursor [20].

It is possible to obtain M-TiO2 materials with either disordered or ordered mesoporosity. Disordered mesostructures are obtained by template-free methods, whereas the use of a structure-directing agent and/or a template is necessary to obtain ordered mesostructures. Historically, the first attempts to obtain M-TiO<sup>2</sup> led to materials characterised by a disordered porosity, which were mainly studied for photocatalytic applications [5]. Ordered mesoporous frameworks are supposed to improve the photocatalytic performance of TiO2 , since an ordered porous structure favours diffusion of both reactants and products. The latter class of materials is therefore more interesting for photocatalytic applications in environmental remediation and is addressed in this chapter. Other types of ordered TiO2 structures, i.e. hollow fibres, nanotubes (not addressed here), are instead obtained by other methods like electrodeposition, electrospinning [21], etc.

adsorption/separation processes, sensing, energy storage/conversion and biotechnology/bio-

Although several commercial products are available on the market, the intrinsic properties

Besides increasing the specific surface, porosity facilitates some physico-chemical phenomena like adsorption and diffusion of chemical species, especially in the presence of mesopores (*Ø* = 2.0–50 nm). Conversely, micropores (*Ø* ≤ 2.0 nm) may have a detrimental effect on the

lytic processes, since mesopores promote the diffusion of chemical species, either reactants

access to the reactive sites at the photocatalyst surface. Moreover, the possibility to synthesise nanoparticles (NPs) allows obtaining a material that is highly dispersible in solution: conse-

carried out in aqueous phase) but is attracting increasing attention also for applications in biomedicine [7], due to the possibility of controlling NP morphology and size during the

assembly (EISA) process [8, 10, 11] in the presence of diblock and triblock copolymers [12–14]. The availability of both EISA process and Pluronic amphiphilic triblock copolymers allowed

Discovered and developed in the 1990s, the first type of ordered mesoporous oxide ever

ity of silicon [16, 17]. The synthesis methods leading to the production of ordered mesopo-

oxides [19]. Reasons for that have been (i) the lack of suitable metal precursors for sol-gel synthesis and (ii) their lower stability with respect to Si-precursors. Nonetheless, the low thermal stability of some mesoporous oxides may induce formation of undesired crystalline phases and/or grain growth. Such processes are likely at the high temperatures reached during calcination, the process often adopted to remove organic moieties deriving from the template and/

Disordered mesostructures are obtained by template-free methods, whereas the use of a structure-directing agent and/or a template is necessary to obtain ordered mesostructures.

porosity, which were mainly studied for photocatalytic applications [5]. Ordered mesopo-

ordered porous structure favours diffusion of both reactants and products. The latter class of materials is therefore more interesting for photocatalytic applications in environmental

rous frameworks are supposed to improve the photocatalytic performance of TiO2

cursors, which are usually unstable when exposed to water or even to moisture [15].

is not only considered for environmental photocatalytic processes (usually

, likely due to the high pliability of the Si–O–Si bond, which is a peculiar-

materials with either disordered or ordered mesoporosity.

led to materials characterised by a disordered

, since an


(M-TiO2

or products [5, 6], and simultaneously enhance M-TiO<sup>2</sup>

overcoming the main issue related to the synthesis of M-TiO2

or hydrolysis of the (metal alkoxide) precursor [20].

Historically, the first attempts to obtain M-TiO<sup>2</sup>

It is possible to obtain M-TiO2

Moreover, highly organised films of M-TiO<sup>2</sup>

may be enhanced in the presence of porous samples that can be obtained by different

) was found to be highly active in several photocata-

may be obtained by evaporation-induced self-

photocatalytic activity by facilitating

, i.e. the high reactivity of Ti pre-

medicine [1].

synthesis procedures [2–9].

diffusion of larger moieties.

quently, M-TiO<sup>2</sup>

obtained was SiO2

rous SiO2

synthesis.

For instance, mesoporous TiO<sup>2</sup>

of TiO2

120 Titanium Dioxide

Concerning the use of templates, both soft-template methods and hard-template methods are available [2]. The former imply the use of surfactants that form micelles (direct or inverse), whereas hard-template methods imply the use of preformed mesoporous solids like SiO2 or carbon, as well as (natural and/or synthetic) polymers (**Figure 1**).

The physico-chemical properties of the final product depend on several factors, like the chosen Ti precursor, which affects the hydrolysis and condensation reactions occurring during formation of the mesoporous network, and the calcination temperature. Calcination, usually necessary to remove the organic moieties, deriving by both the surfactants used in soft-template methods and the hydrolysis of Ti alkoxides used as Ti precursors, markedly affects the type of TiO<sup>2</sup> polymorph formed as well as the size of crystalline grains.

The physico-chemical properties of the final material are very important since they ultimately affect the performance of M-TiO<sup>2</sup> in the target application and may be responsible of a better (or worst) performance with respect to commercial products. Among the latter ones, so far, Degussa P25 is the most used and studied commercial TiO2 : due to its remarkable photocatalytic performance, it is used as benchmark material in most of the literature works.

Far from having the purpose of reviewing in detail the procedures for the preparation of ordered M-TiO2 , only some crucial aspects of the most acknowledged synthesis methods will be addressed in this chapter. Afterwards, the chapter will show some correlations between the peculiar physico-chemical properties of M-TiO2 and its performance in environmental photocatalysis, although the properties of M-TiO<sup>2</sup> -based systems are attracting considerable attention also for other applications, including H<sup>2</sup> production, fabrication of electrodes in Li-ion batteries, dye-sensitised solar cells and biomedicine [22].

**Figure 1.** Simplified scheme of the steps leading to the production of ordered M-TiO<sup>2</sup> by means of template-assisted syntheses.
