**2. Intrinsic properties of TiO2**

Titanium belongs to the IVA group of elements and, as many other metals, is able to form a wide range of oxides. Titanium(IV) dioxide (titania) exists in three common crystalline phases: rutile, which is the thermodynamically stable phase, and the metastable phases anatase and brookite. Anatase and rutile have more extensive applications because they are more stable than brookite (**Figure 1**). In all three forms, titanium (Ti4+) atoms are coordinated to six oxygen (O2−) atoms, forming "TiO<sup>6</sup> " octahedra. Anatase and rutile have a tetragonal

geometry, differently from brookite, which has an orthorhombic geometry. In rutile, the

polymorphs.

Titania is an n-type semiconductor since, like other metal transition oxides, the oxygen

ous structural defects; for example, point defects, which are empty sites (vacancies), where constituent atoms are missing within the structure and interstitial atoms occupy the space between the regular atomic sites. The point defects contribute to the electrical conductivity in two ways: they can provide ionization, and they can also move in response to an electric field producing an ionic current. An oxygen vacancy is formed by the transfer of an oxygen atom on a normal site to the gaseous state. These oxygen vacancies act as electron donors, so the material contains an excess of electrons resulting in an increase of the electri-

exhibits photocatalytic oxidation (PCO) and photoinduced superhydrophilicity (PSH)

" octahedron is slightly distorted, while anatase consists of strongly distorted octahedral units. In the rutile structure, each octahedron is surrounded by ten close octahedrons; instead, in the anatase polymorph, each octahedron is in contact with eight neighbors. Interatomic distances also differ between the polymorphs. For anatase, with respect to rutile, the Ti-Ti distances are lengthier and, instead, the Ti-O distances are shorter. These differences in lattice structures cause different mass densities and electronic band structures

. Due to its intrinsic properties, the anatase phase is the most

/ *mnm Pbca*

a = b = 4593 c = 2959

. Indeed, every crystal has vari-

a = 9184 b = 5447 c = 5145

Mesoporous TiO2 Thin Films: State of the Art http://dx.doi.org/ 10.5772/intechopen.74244 59

a good candidate for photocatalyst materials

"TiO<sup>6</sup>

Volume (Å<sup>3</sup>

Density (g cm<sup>3</sup>

between the two forms of TiO<sup>2</sup>

interesting one for applications.

and self-cleaning surfaces in air.

cal conductivity.

TiO2

vacancies represent the predominant defect type in TiO<sup>2</sup>

**Property TiO2** Molecular weight (g mol−1) 79.88 Melting point (°C) 1825 Boiling point (°C) 2500–3000

Dielectric constant 31 114

Space group *I*41

Lattice constant (Å) a = b = 3784

**Table 1.** Compared intrinsic properties of main TiO<sup>2</sup>

**Property Anatase Rutile Brookite** Refractive index [18] 2.52 2.72 2.63

Crystal structure [19, 20] Tetragonal Tetragonal Orthorhombic

c = 9515

Molecule/cell 4 2 8

/*amd P*42

) 136.25 62.07 257.38

) 3.79 4.13 3.99

when it is illuminated with UV light, making TiO<sup>2</sup>

**Figure 1.** Crystal structures of TiO<sup>2</sup> : (a) anatase, (b) rutile, and (c) brookite (adapted from Ref. [17]) (**Table 1**).


**Table 1.** Compared intrinsic properties of main TiO<sup>2</sup> polymorphs.

to antibacterial applications [12]. However, to design efficient systems and devices for the above-cited uses, stable materials with a well-defined crystalline structure, highly controlled crystallite size and shape, as well as high available surface area and accessible pore networks able to ensure contact with catalytic substrates, polymers, or nanospecies are required [13]. Although the early mesoporous materials were produced in the form of powders, some applications, such as membranes, low-dielectric-constant interlayers, optical sensors, and optoelectronic devices, require ultralow-k dielectrics and low-refractive-index materials with a good mechanical stability and of hydrophobic nature. These requirements led to the preparation of mesoporous thin films [14–16]. After a brief description of the different TiO<sup>2</sup> polymorphs and properties, the present chapter will therefore be dedicated to mesoporous

 thin films (MTTFs), reviewing the different preparation methods reported throughout the literature, the main characterization techniques employed to study the structure and the morphology of the prepared thin films, and finally the description of their most successful

Titanium belongs to the IVA group of elements and, as many other metals, is able to form a wide range of oxides. Titanium(IV) dioxide (titania) exists in three common crystalline phases: rutile, which is the thermodynamically stable phase, and the metastable phases anatase and brookite. Anatase and rutile have more extensive applications because they are more stable than brookite (**Figure 1**). In all three forms, titanium (Ti4+) atoms are coordinated

" octahedra. Anatase and rutile have a tetragonal

: (a) anatase, (b) rutile, and (c) brookite (adapted from Ref. [17]) (**Table 1**).

TiO2

applications.

**2. Intrinsic properties of TiO2**

58 Titanium Dioxide - Material for a Sustainable Environment

to six oxygen (O2−) atoms, forming "TiO<sup>6</sup>

**Figure 1.** Crystal structures of TiO<sup>2</sup>

geometry, differently from brookite, which has an orthorhombic geometry. In rutile, the "TiO<sup>6</sup> " octahedron is slightly distorted, while anatase consists of strongly distorted octahedral units. In the rutile structure, each octahedron is surrounded by ten close octahedrons; instead, in the anatase polymorph, each octahedron is in contact with eight neighbors. Interatomic distances also differ between the polymorphs. For anatase, with respect to rutile, the Ti-Ti distances are lengthier and, instead, the Ti-O distances are shorter. These differences in lattice structures cause different mass densities and electronic band structures between the two forms of TiO<sup>2</sup> . Due to its intrinsic properties, the anatase phase is the most interesting one for applications.

Titania is an n-type semiconductor since, like other metal transition oxides, the oxygen vacancies represent the predominant defect type in TiO<sup>2</sup> . Indeed, every crystal has various structural defects; for example, point defects, which are empty sites (vacancies), where constituent atoms are missing within the structure and interstitial atoms occupy the space between the regular atomic sites. The point defects contribute to the electrical conductivity in two ways: they can provide ionization, and they can also move in response to an electric field producing an ionic current. An oxygen vacancy is formed by the transfer of an oxygen atom on a normal site to the gaseous state. These oxygen vacancies act as electron donors, so the material contains an excess of electrons resulting in an increase of the electrical conductivity.

TiO2 exhibits photocatalytic oxidation (PCO) and photoinduced superhydrophilicity (PSH) when it is illuminated with UV light, making TiO<sup>2</sup> a good candidate for photocatalyst materials and self-cleaning surfaces in air.

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

can adsorb as OH groups, giving to the TiO<sup>2</sup>

unless there are pores opening at the surfaces [26].

tures with aligned vertical pores.

(*Im*3*m*), and (d) cubic (*Pm*3*m*) (from Ref. 25).

water contact angle for TiO<sup>2</sup>

**3. Preparation of MTTFs**

**3.1. Synthetic aspects**

ties of TiO<sup>2</sup>

TiO2

surface its superhydrophilic character. Indeed,


Mesoporous TiO2 Thin Films: State of the Art http://dx.doi.org/ 10.5772/intechopen.74244 61

was *ca.* 72° (hydrophobic), while UV illumination turned it close

when illuminated, titania surface exhibits amphiphilicity caused by the creation of alternating hydrophilic/hydrophobic domains. It was found that in the absence of UV illumination, the

to 1° (superhydrophilic) [23]. These photoinduced phenomena coupled with the PCO proper-

 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

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 struc-

**Figure 3.** A representation of the structures found in mesoporous materials: (a) hexagonal, (b) cubic (*Ia*3*d*), (c) cubic

are at the basis of the self-cleaning properties of TiO<sup>2</sup>

$$\text{TiO}\_2 + \text{h}v \rightarrow \text{h}\_{\text{VB}} "+ \text{e}\_{\text{CH}}\text{"} $$

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 the binding energy of electron in OH− to form a hydroxyl radical from water. The hydroxyl radical can subsequently oxidize organic species with mineralization producing mineral salts, CO2 , and H<sup>2</sup> O. Similarly, an electron in CB (E= −0.52 V) is sufficiently reductive to react with O2 to form O2•− (superoxide radical anion), which can react with H<sup>+</sup> to produce hydroperoxide 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 electronhole pair have been developed, such as doping and heterojunction coupling.

The PSH property consists in the alteration of the TiO<sup>2</sup> wettability under UV irradiation and 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 molecular oxygen. The expulsion of O<sup>2</sup> molecules creates surface vacancies on which water

**Figure 2.** Schematic mechanism of TiO<sup>2</sup> photoactivation (adapted from Ref. [22]).

can adsorb as OH groups, giving to the TiO<sup>2</sup> surface its superhydrophilic character. Indeed, when illuminated, titania surface exhibits amphiphilicity caused by the creation of alternating hydrophilic/hydrophobic domains. It was found that in the absence of UV illumination, the water contact angle for TiO<sup>2</sup> was *ca.* 72° (hydrophobic), while UV illumination turned it close to 1° (superhydrophilic) [23]. These photoinduced phenomena coupled with the PCO properties of TiO<sup>2</sup> are at the basis of the self-cleaning properties of TiO<sup>2</sup> -covered glasses and walls.
