**3.3. The templating agents**

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

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

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].

Two classes of precursor for the preparation of mesoporous thin films can be used: (i) inor-

 **precursor candidates**

ganic precursors and (ii) metal-organic precursors.

hydrolysis and condensation reactions [25].

62 Titanium Dioxide - Material for a Sustainable Environment

state [29].

**3.2. The TiO2**

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 successful preparation of highly organized mesoporous TiO<sup>2</sup> thin films [37]. Hard- and softtemplating 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 by Bonelli et al. [38], to which interested readers are referred.

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 non-covalent weak interactions in particular hydrogen bonding, van der Waals forces, and electrostatic interactions. These assemblies are in situ used as soft templates allowing the tuning of the pore size and organization within the resulting porous materials. PEO (polyethylene oxide)-based templates have been extensively used in soft-template synthesis of mesoporous thin films. A study by Stucky [36] suggested that non-hydrolytic reactions take place in their low-water-content systems, allowing the oxide network to be built in a more controlled way. The proposed mechanism suggested that, once the metal center is trapped by the PEO or PPO fragments, nucleophilic reactions occur between the cationic immobilized species, leading to the formation of oligomers attached to the polymer chains. These oligomers are anchoring points for the growing inorganic phase [39].

of the fluid to be deposited and the chosen ultimate rotation velocity are both parameters that mainly control the resulting thickness of the film. High angular speed produces thinner film. At a constant speed, the film thickness initially rapidly decreases, but this decrease slows at longer times. Typical coating thickness values are often below 1 μ when spin-coating is used. Dip-coating is a technique based on the deposition of a wet liquid film by withdrawal of a substrate from a liquid coating medium. This process is very simple, flexible, and economically advantageous. Nevertheless, it is necessary to make provisions for cleanliness in order to obtain a high-quality deposition. Dip-coating is one of the few techniques that allow a simultaneous double-sided coating which may be regarded as an advantage especially in production of optical filters. Typically, film thickness obtained with dip-coating ranges from a few nanometers to 200 nm for oxide coatings. The thickness of the liquid film depends mainly on two factors: (i) the viscosity of the solution and (ii) the speed rate used during the substrate

Mesoporous TiO2 Thin Films: State of the Art http://dx.doi.org/ 10.5772/intechopen.74244 65

In dip-coating, mesoscale ordering is achieved almost instantly after the covered substrate is withdrawn from the solution. Even if the self-assembled structure may form by spincoating, the degree of ordering cannot be as high as in the case of dip-coating. This is probably due to the faster solvent evaporation occurring during spin-coating leading to a more viscous film deposit, which, in turn, makes the rearrangement of the titania and surfactant species in solution that is too sluggish to achieve a high degree of ordering in a short period

Several parameters such as temperature, aging period, and relative humidity have to be taken into account during the aging phase. Indeed, they all have important effects onto the final mesoporous structure in titania-based system. The evaporation process during the aging period after the spin- or dip-coating deposition plays a critical role not only for the formation of ordered porosity but also to direct the symmetry displayed by the pore arrays [21]. During the last decade, many research groups have reported the effects of the aging conditions onto the mesostructure of titania thin films, and they confirmed that high humidity conditions are

During the aging phase, the titania species and the surfactant molecules are in a liquid crystallike state, and the mesoscopic ordering is achieved through the rearrangement of these species. The moisture inside the as-synthesized film materials plays a dual role. Kinetically, it is a lubricant to facilitate the rearrangement, and, thermodynamically, it is a structural ingredient to form the mesostructures. According to Lee et al. [27], it is important to keep the moisture of the as-synthesized film at a certain level (70%) until the full ordering is achieved. However, it has been observed that too prolonged treatments at humidity rates higher than 70% can be detrimental to the meso-ordering due to excessive water swelling [45]. Ozin et al. [15] have reported that a high humidity level (*ca*. 60%) during aging favors a cubic structure, while a lower humidity level (40%) favors the hexagonal structure. According to various studies, the aging time should not be longer than 72 h, because films may transform to other unidentified

a requisite to access highly ordered mesostructure in titania thin films [29, 43, 44].

withdrawn from the solution.

of time [26].

**3.5. Aging conditions**

structures of lower order [26, 27].

PEO containing block copolymers are often chosen as the preferred templates since they can easily be produced in variable lengths, allowing in turn to generate differently shaped templates. Furthermore, PEO blocks are highly hydrophilic and favorably interact with Ti-oxo hydrophilic species in solution stabilizing upon drying the inorganic/organic interface at the shell of the formed micelles [39]. Indeed, the relative size of the hydrophilic and hydrophobic domains is in great part responsible for the symmetry of the pore array. After mixing with surfactant followed by casting into thin films, this mixture turns into a liquid crystal-like state, which, in due course, self-assembles into mesostructures on aging under controlled temperature and humidity conditions [27].

The choice of the surfactant is also an influence on the type of mesostructure obtained. It was reported that while Brij 58 –(EO)20–C16H33– leads to an *Im*3*m* mesoporous structure, the Brij 56, characterized by a smaller hydrophilic domain, leads to hexagonally packed cylindrical micelles with a p6mm symmetry [29].

In a recent study, Lee et al. [27] investigated the influence of the surfactant concentration and reported that a concentration equal to 9% wt for Brij 58 led to an *Im*3*m* mesoporous structure. Ozin et al. [40, 41] indicated that mesoporous titania thin film having anatase walls can be prepared using triblock copolymers as template materials. According to these reports, the templating agent is also able to affect the crystallinity of the MTTF. Indeed, Innocenzi et al. [42] investigated the above aspect by a correlative analysis of a MTTF and a dense sol-gel titania film (without the block copolymer), prepared under the same conditions; they found that in mesoporous titania thin film (with surfactant) the crystallization to anatase is favored and occurs at lower temperatures.
