**3.1. Synthetic aspects**

**Figure 2.** Schematic mechanism of TiO<sup>2</sup>

photoactivation (adapted from Ref. [22]).

The PCO property is activated by absorption of UV photons of energy greater than TiO<sup>2</sup> bandgap energy. For anatase phase this energy is 3.2 eV; therefore, UV light (λ ≤ 387 nm) is required, while rutile phase has energy of 3.0 eV (λ = 400 nm). The absorption of a photon excites an electron to the conduction band (eCB −) generating a positive hole in the valence

The charge carriers can migrate to the catalyst surface and initiate redox reactions with adsorbates. The redox potential for hole in VB is +2.53 V, which is sufficiently oxidizing to overcome

radical can subsequently oxidize organic species with mineralization producing mineral salts,

oxide radical contributing to the degradation of organic molecules (**Figure 2**) [21]. The main drawback in the use of titania as photocatalyst is its high electron-hole recombination rate, but several methods to improve photocatalytic activity by promoting separation of the electron-

in the formation of a highly hydrophilic surface state. One possible explanation of the PSH property is that electrons reduce Ti(IV) cation sites in Ti(III) and holes oxidize O2− anions to

<sup>+</sup> + eCB −

O. Similarly, an electron in CB (E= −0.52 V) is sufficiently reductive to react with

to form a hydroxyl radical from water. The hydroxyl

molecules creates surface vacancies on which water

to produce hydroper-

wettability under UV irradiation and

band (hVB +), so that photoexcitation produces electron-hole pairs.

to form O2•− (superoxide radical anion), which can react with H<sup>+</sup>

hole pair have been developed, such as doping and heterojunction coupling.

TiO2 + h*v* → hVB

The PSH property consists in the alteration of the TiO<sup>2</sup>

the binding energy of electron in OH−

60 Titanium Dioxide - Material for a Sustainable Environment

molecular oxygen. The expulsion of O<sup>2</sup>

CO2

O2

, and H<sup>2</sup>

TiO2 nanomaterials can be classified according to their shape and dimension: 0D nanomaterials refer to nearly spherical nanoparticles (quantum dots); 1D refers to nanowires, nanobelts, and nanorods; and 2D materials correspond to thin films, while 3D is used to indicate porous nanostructures.

Mesoporous thin films have very peculiar features, particularly high surface area, controlled porosity, high flexibility in composition, and surface design [24]. For practical applications, mesoporous thin films must possess a number of general features: (i) they must be continuous and free of crack; (ii) crystalline walls are highly desirable in order to process into functional materials; and (iii) pores must be accessible, preferably from the film surface [25]. As a matter of fact, unlike powder, the internal space of a mesoporous thin film may not be accessible unless there are pores opening at the surfaces [26].

Mesoporous materials can form various pore structures including hexagonal (*p*6mm), cubic (*Ia*3*d*, *Im*3*m*, and *Pm*3*m*), and disorganized structures (**Figure 3**). Although the hexagonal structure is the most encountered one due to its easy synthetic procedure, this structure has a drawback in terms of pore accessibility when obtained as thin films, since the interfacial energy between the substrate surface and the film materials often lead to the pores laying parallel to the substrate surface [25]. For this reason, recent development of synthesis protocols of mesoporous titania thin films has been achieved to access to cubic structures or to hexagonal structures with aligned vertical pores.

**Figure 3.** A representation of the structures found in mesoporous materials: (a) hexagonal, (b) cubic (*Ia*3*d*), (c) cubic (*Im*3*m*), and (d) cubic (*Pm*3*m*) (from Ref. 25).

The first mesoporous thin films, which have been the object of extensive studies, were essentially silica-based and structure-directing agents, such as surfactants, were used for their synthesis. The formation mechanism of these materials involves the self-assembly of the templates in supramolecular structures, such as micelles, with the inorganic species associated to their hydrophilic portions. Simultaneously, the inorganic precursor undergoes hydrolysis and condensation reactions [25].

Inorganic precursors are salts that produce in water-solvated cations. The charge transfer from the bonding orbitals of water molecules to empty the orbitals of transition metals makes the water molecules more acidic. Different water complexes can form depending on the magnitude of this electron transfer. A condensation may take place via olation in which a hydroxyl bridge is formed between two metal centers. Olation occurs by a nucleophilic substitution (SN) where the hydroxyl group is the nucleophile and water is the leaving group; however, oxolation can also occur in which an oxo bridge is formed between two metal centers. Oxolation proceeds through two consecutive steps: the first step is a nucleophilic addition between

Metal-organic precursors are usually metal alkoxides which are more reactive due to the presence of highly electronegative OR groups that stabilize the metal center in the highest oxidation state and render it more susceptible to nucleophilic attack. Hydrolysis and condensations occur by nucleophilic substitution involving a nucleophilic addition followed by a proton

raisopropoxide or tetraethoxide), and tetrabutyl titanate (TBT) are often used as titanium sources. Handling titanium chloride requires careful attention since it reacts violently with water and high humidity atmosphere in fast exothermic hydrolysis reactions. In this case, alcoholysis or hydrolysis processes release protons that acidify the solution, a necessary step to promote polymerization. Titanium alkoxides are highly reactive and highly hygroscopic precursors requiring to be handled within a moisture-free environment. To induce the polymerization of the inorganic framework, in the case of titanium alkoxides, addition of strong acid such as HCl required, or in alternative, the use of specific chelating agents. HCl is, however, the preferred choice for the production of MTTFs since its high volatility does not impede the

A variety of templates, such as alkyl phosphate anionic surfactants [31], quaternary ammonium cationic templates [32, 33], primary amines [34, 35], and poly(ethylene oxide)-based surfactants [29, 36], have been used to manipulate the pore structures of titania. Nonetheless, to direct and control the morphology of the inorganic framework for the preparation of mesoporous thin films, block copolymers seem to be the more frequently chosen templating agents since their self-assembly properties are driven by evaporation. However, the choice and combination between the Ti source and the templating agent are the crucial steps for the

templating syntheses are the two most widely used methods to prepare these porous materials. A comprehensive description of both routes is given in the excellent book chapter written

The mostly employed soft-template-assisted route takes advantage of the self-assembly properties of organic ionic surfactants or neutral polymeric surfactants allowing to access to a diversity of supramolecular structures ranging from spherical micelles to hexagonal rods and lamellar liquid crystals. The formation of these supramolecular assemblies is governed by

), titanium alkoxides (mostly used tet-

Mesoporous TiO2 Thin Films: State of the Art http://dx.doi.org/ 10.5772/intechopen.74244 63

thin films [37]. Hard- and soft-

transfer and removal of protonated species (via olation or oxolation mechanisms).

hydroxy precursors, and the second step is water elimination [28].

In order to prepare MTTFs, titanium chloride (TiCl<sup>4</sup>

efficiency of the evaporation-assisted deposition [13].

successful preparation of highly organized mesoporous TiO<sup>2</sup>

by Bonelli et al. [38], to which interested readers are referred.

**3.3. The templating agents**

In general, the synthesis of mesoporous materials of transition metal oxides is more difficult than that of silica. A reason may be found in the fact that the hydrolysis and condensation reactions of transition metal ions are much faster (often hard to control due to the excessive rate) with respect to silicon-based precursors. Consequently, the inorganic precursor is prevented to effectively associate with the templates during condensation, and often mesoscopic disorder is obtained in the resulting material [27]. This high reactivity of transition metal precursors, compared to silicates, is attributed to the lower electronegativity of the metal and to its ability to exhibit several coordination states, so that the coordination expansion occurs spontaneously upon reaction with water or other nucleophilic reagents [28]. In order to minimize this problem, the precursor solution of Ti is often strongly acidic enough to suppress the hydrolysis and condensation reactions, so that the film material forms a liquid crystal-like state [29].

The synthetic approach adopted for mesoporous titania thin films (MTTFs) is often a combination between the sol-gel chemistry of an inorganic precursor and the self-assembly process of an organic template (evaporation-induced self-assembly (EISA)). The sol-gel process is a synthesis route which involves the preparation of a "sol" and the subsequent gelation upon the solvent removal. A "sol" consists of a liquid with colloidal particles which are not dissolved, but do not agglomerate or sediment. A gel consists of a three-dimensional continuous network, which includes a liquid phase. Sol-gel syntheses can use either a metal-organic precursor or an inorganic precursor. In both cases, a dilute, usually an acidic solution of precursor and an amphiphilic organic template, is introduced in a volatile solvent containing a specific amount of water. The solution is spin- or dip-coated onto virtually any type of substrate. Upon evaporation of the organic solvent, the system self-organizes to form a periodic inorganic-organic composite. As a matter of fact, surfactants can spontaneously self-assemble in micelles when their concentration in a given solvent is higher than the critical micellar concentration (CMC). At higher concentration, micelles can assemble in liquid crystalline arrays. A thermal posttreatment is often used to proceed to the cross-linking of the inorganic framework together with the removal of the organic template, leading, in turn, to the formation of the mesoporous thin film. The two more often encountered pore structures are either a cubic lattice displaying three-dimensionally interconnected pores or channel-like pores arranged in a hexagonal array [30]. Adjustment of the pore architecture of mesoporous materials strongly affects their properties, such as adsorption affinity toward guest molecules and photocatalytic properties of materials [16].

#### **3.2. The TiO2 precursor candidates**

Two classes of precursor for the preparation of mesoporous thin films can be used: (i) inorganic precursors and (ii) metal-organic precursors.

Inorganic precursors are salts that produce in water-solvated cations. The charge transfer from the bonding orbitals of water molecules to empty the orbitals of transition metals makes the water molecules more acidic. Different water complexes can form depending on the magnitude of this electron transfer. A condensation may take place via olation in which a hydroxyl bridge is formed between two metal centers. Olation occurs by a nucleophilic substitution (SN) where the hydroxyl group is the nucleophile and water is the leaving group; however, oxolation can also occur in which an oxo bridge is formed between two metal centers. Oxolation proceeds through two consecutive steps: the first step is a nucleophilic addition between hydroxy precursors, and the second step is water elimination [28].

Metal-organic precursors are usually metal alkoxides which are more reactive due to the presence of highly electronegative OR groups that stabilize the metal center in the highest oxidation state and render it more susceptible to nucleophilic attack. Hydrolysis and condensations occur by nucleophilic substitution involving a nucleophilic addition followed by a proton transfer and removal of protonated species (via olation or oxolation mechanisms).

In order to prepare MTTFs, titanium chloride (TiCl<sup>4</sup> ), titanium alkoxides (mostly used tetraisopropoxide or tetraethoxide), and tetrabutyl titanate (TBT) are often used as titanium sources. Handling titanium chloride requires careful attention since it reacts violently with water and high humidity atmosphere in fast exothermic hydrolysis reactions. In this case, alcoholysis or hydrolysis processes release protons that acidify the solution, a necessary step to promote polymerization. Titanium alkoxides are highly reactive and highly hygroscopic precursors requiring to be handled within a moisture-free environment. To induce the polymerization of the inorganic framework, in the case of titanium alkoxides, addition of strong acid such as HCl required, or in alternative, the use of specific chelating agents. HCl is, however, the preferred choice for the production of MTTFs since its high volatility does not impede the efficiency of the evaporation-assisted deposition [13].
