**3.5. Aging conditions**

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

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

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

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

Two main deposition techniques have been widely used to prepare MTTFs, namely, spin-

Spin-coating is a simple process for a rapid deposition of thin films onto flat substrates. The substrate to be covered is held by some rotatable fixture (often using vacuum to clamp the substrate into place), and the coating solution is spread onto the surface; the action of spinning causes the liquid to spin radially outward by the centrifugal force until the thin film is formed. The initial volume of the fluid distributed onto the rotating substrate and the delivery rate have both a minor effect onto the final film thickness. On the other hand, the resulting viscosity

points for the growing inorganic phase [39].

64 Titanium Dioxide - Material for a Sustainable Environment

temperature and humidity conditions [27].

micelles with a p6mm symmetry [29].

and occurs at lower temperatures.

**3.4. Depositions techniques**

coating or dip-coating.

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

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 structures of lower order [26, 27].

The aging temperature plays also an important role and is often chosen equal to 18°C, as reported in several studies [25, 27]. However, using ethanolic solutions of TiCl<sup>4</sup> and F127 as surfactants, the mesostructure could be tuned from cubic to hexagonal by aging at 18C or 35°C, respectively [27].

the surface. Owe to interfacial orientation preferences between the substrate and the material, the 2D hexagonal mesoporous thin films present their pore channels parallel to the surface [25]. For this reason, many research groups have struggled to synthesize cubic mesoporous thin films, which present, regardless of the orientation, pore openings from their surfaces owe to their intrinsic geometry [44, 49–53]. Often, the solution resides in the optimization of the composition of the precursor solution performed by varying the ratios between quantity of surfactant and inorganic precursor [25, 54, 55]. In this context, Koganti et al. [56] showed that a modification of the substrate surface with a Pluronic block copolymer could lower the surface energy and induce the channels to tilt away from the substrate plane. Other groups have varied the aging temperature to form cubic structures [57]. Lee et al. [26] reported the

and by applying a specific heat treatment to direct the formation of the 3D network in a cubic phase. Zhou et al. [7] formed MTTFs with vertical pores, by calcination of the film at 450°C, so that the 3D hexagonal mesostructure was transformed to a grid-like mesostructure with quasi-perpendicular porosity through sintering-diffusion and pore merging along the c-axis. Richman et al. [30] used another approach: they synthesized mesoporous films through nanometer-scale epitaxy. They prepared a titania thin film with cubic symmetry and used it as a patterned substrate to direct a hexagonal top silica mesostructured with vertically oriented pore channels. Finally, Yamauchi et al. applied a very strong magnetic field (10T) in mesoporous silica films to align the channels, but the alignment was not homogeneously perfect, and

Beyond the synthesis of MTTFs, it is necessary to deeply characterize their structure, in particular the type of pore arrays, the film thicknesses, the porosity (pore volume, pore size distribution, interconnectivity, and specific surface area), the surface topography, the chemical composition, and the crystallinity grade. The most commonly used characterization techniques for MTTFs are microscopies such as SEM, TEM, or AFM; spectroscopic techniques such as UV-Visible spectroscopy or ellipsometry (E), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy; X-ray absorption techniques like XANES and EXAFS; and powder or small-angle X-ray diffraction (XRD). Spectroscopic ellipsometry (E) is an optical measurement technique based on the change in polarization occurring at many wavelengths. Ellipsometry is often highly sensitive to the properties of 1 nm to 10 μ thick films. The spectral response provides information about the sample properties, such as film thickness, surface conditions such as surface roughness (or the presence of surface contaminants), film thickness uniformity, anisotropy, and some important physical properties, such as refractive index [59]. Fourier transform infrared spectroscopy (FTIR) is a fast, contactless, and nondestructive technique used to control the complete removal of the template and the presence of organic groups. Raman spectroscopy is a useful technique used to determine titania crystalline phase by analyzing the Ti-O-Ti vibrations in the 200–600 cm−1 region. XANES and EXAFS had been used to accurately probe and

characterize the Ti(IV) environment in solution and within the final material.

as source of titanium, F127 as surfactant,

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preparation of MTTFs with vertical pores using TiCl<sup>4</sup>

such method remains rather impractical for large areas [58].

**4. MTTF characterizations**

In contrast to most studies, Oveisi et al. [16] recently reported that highly ordered mesostructures can be reached when the condensation reaction of metal alkoxides is occurring after the formation of the liquid crystal thin-film covering (after complete evaporation of solvents). This hypothesis was confirmed by working under non-usual aging conditions: lower temperature (−20°C) and lower humidity (20% RH).
