**4. MTTF characterizations**

The aging temperature plays also an important role and is often chosen equal to 18°C, as

surfactants, the mesostructure could be tuned from cubic to hexagonal by aging at 18C or

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 tem-

After the aging phase, titania thin films are calcinated at high temperature. In this step, a sequence of transformations takes place: evaporation of the remaining solvent and acid (T < 200°C) which activates consolidation of the framework (this step is accompanied by an uniaxial shrinkage of the mesostructure along the z axis); template pyrolysis (T= 250–300°C), which generates the

phous titania walls into crystalline walls usually results in the partial or total collapse of the mesoporous network [46, 47]. To circumvent such a drawback, several post-synthesis treatments have been developed mainly devoted to increase the thermal stability of the mesopo-

mesoporous titania powder with ammonia resulted in the formation of mesoporous crystalline titania with thermal stability up to 600°C. Sanchez and coworkers [49] claimed that the mesoporous anatase network could be retained with a porosity of 35% above 650°C by applying a specific post-synthesis delayed rapid crystallization (DRC) treatment. Another study demonstrates that the thermal stability of these crystalline films can be enhanced, up to 850°C,

The thermal posttreatment has to be therefore carefully done and monitored especially if a

to the preferential formation of anatase vs. the rutile polymorph. Although through the literature a large inhomogeneity in temperature range over the control of a selected polymorph is present and reflects the influence of secondary important factors such as the Ti precursor, the template, the deposition technique, and the experiment conditions used to form the TiO<sup>2</sup> mesoporous material [51], most of the reports agree in the temperature range of 350–450°C to promote the exclusive crystallization of the anatase polymorph, while at higher temperature,

As previously mentioned, the pores of the thin films must be accessible. Indeed, many potential applications of mesoporous thin films are relying on the accessibility of the pores from

polymorph is sought. Most of the reported literature data available are dedicated

. In particular, Cassiers et al. [48] reported that the posttreatment of an uncalcinated

, the thermal posttreatment effectuated to convert the amor-

porosity; and crystallite nucleation and growth on walls (T > 300°C) [9].

by posttreatment of the film in supercritical carbon dioxide (sc-CO<sup>2</sup>

small amount of precursor, such as tetramethoxysilane (TMOS) [50].

the rutile phase transformation initiate [42, 52].

**3.7. Alignment of the pores**

and F127 as

) with the presence of a

reported in several studies [25, 27]. However, using ethanolic solutions of TiCl<sup>4</sup>

35°C, respectively [27].

**3.6. Thermal posttreatment**

Typically with mesoporous TiO<sup>2</sup>

rous TiO2

specific TiO<sup>2</sup>

perature (−20°C) and lower humidity (20% RH).

66 Titanium Dioxide - Material for a Sustainable Environment

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.

Electron microscopies SEM and TEM are local techniques and should be used as complementary techniques to diffraction. Scanning electron microscopy (SEM) is perhaps the most widely employed thin film and coating characterization instrument. It shows the surface pore arrangement giving information on the MTTF surface morphology. Atomic force microscopy (AFM) provides imaging topography of the sample surface, so it is a local but a very useful technique. The use of a very sharp probe that is scanned across the thin-film surface allows to produce very high-resolution topographic images at the sub-nanometer scale. Many different imaging modes are available in AFM, such as contact mode imaging which works in the repulsive regime because the probe remains in contact with the sample at all times, noncontact mode which works in the attractive regime because the probe is very close to the sample surface without touching it, and tapping mode which works in both the regimes. Tapping mode AFM is the most used mode in the case of titania thin films, whereby a sharp tip (typically, a silicon or silicon nitride crystal) is oscillated above the surface of a sample. Specifically, tapping mode overcomes major problems associated with friction, adhesion, electrostatic forces, and other tip-sample-related issues (**Figure 4**).

X-ray diffraction in Bragg-Brentano geometry is a noncontact and nondestructive technique that provides information about crystallinity, phase, crystallite size, and orientation. Comparison of the obtained XRD profile in the medium and wide-angle region (from 2θ = 10 to 80°) with reference patterns allows to determine the crystalline phase of the MTTFs as well as possible preferential orientational order through the increase in intensity of specific reflection peaks. The XRD patterns of the three main crystalline phases of TiO<sup>2</sup> are reported in **Figure 5**.

While there are no difficulties in discriminating between the anatase and the rutile phases since the first two reflection peaks are well separated (2θ = 25.28° for d101 of anatase vs. 2θ = 27.44° for d110 of rutile), spotting the difference between anatase and brookite might be more tricky. Only, the reflection peaks at 2θ = 30.81° relative to the d121 of the brookite phase or at 2θ = 62.57° relative to d204 of the anatase phase can help in making such a difference and/or

rutile (JCPDS card no. 21-1276), and (c) TiO<sup>2</sup>

Mesoporous TiO2 Thin Films: State of the Art http://dx.doi.org/ 10.5772/intechopen.74244 69

anatase (JCPDS card no. 21-1272), (b) TiO<sup>2</sup>

A small-angle diffraction pattern, instead, could reveal the eventual presence of an ordered array of mesopores within the MTTFs, allowing the determination of its geometry, i.e., cubic

thin film prepared by sol-gel and dip-coating techniques on a glass substrate

recognize the presence of both the polymorphic forms in an XRD pattern [61].

lamellar or hexagonal [44, 62], as shown in **Figure 7**.

**Figure 5.** XRD patterns of (a) TiO<sup>2</sup>

brookite (JCPDS card no. 29-1360).

**Figure 6.** XRD patterns of a dense TiO<sup>2</sup>

(from Ref. 60).

The analysis of XRD patterns must be carefully performed to correctly identify the crystalline phase of the MTTFs, especially when polymorphic forms are present within the film. This task is also often complicated by the presence of the wide halo of the amorphous substrate onto which the MTTF films have been deposited and/or the superposition of the reflection peaks relative to the eventual semicrystalline nature of the substrate used (e.g., when the TiO<sup>2</sup> thin film is deposited on ITO). In **Figure 6** an XRD pattern of a dense anatase TiO<sup>2</sup> thin film deposited on a glass substrate is reported [60], clearly showing the broadening of the background level due to the halo generated by the amorphous support.

**Figure 4.** (a) Top surface and (b) cross-sectional FESEM micrographs of the 3D hexagonal MTTF [7] and (c) AFM image of MTTF surface [27].

Electron microscopies SEM and TEM are local techniques and should be used as complementary techniques to diffraction. Scanning electron microscopy (SEM) is perhaps the most widely employed thin film and coating characterization instrument. It shows the surface pore arrangement giving information on the MTTF surface morphology. Atomic force microscopy (AFM) provides imaging topography of the sample surface, so it is a local but a very useful technique. The use of a very sharp probe that is scanned across the thin-film surface allows to produce very high-resolution topographic images at the sub-nanometer scale. Many different imaging modes are available in AFM, such as contact mode imaging which works in the repulsive regime because the probe remains in contact with the sample at all times, noncontact mode which works in the attractive regime because the probe is very close to the sample surface without touching it, and tapping mode which works in both the regimes. Tapping mode AFM is the most used mode in the case of titania thin films, whereby a sharp tip (typically, a silicon or silicon nitride crystal) is oscillated above the surface of a sample. Specifically, tapping mode overcomes major problems associated with friction, adhesion, electrostatic forces,

X-ray diffraction in Bragg-Brentano geometry is a noncontact and nondestructive technique that provides information about crystallinity, phase, crystallite size, and orientation. Comparison of the obtained XRD profile in the medium and wide-angle region (from 2θ = 10 to 80°) with reference patterns allows to determine the crystalline phase of the MTTFs as well as possible preferential orientational order through the increase in intensity of specific

The analysis of XRD patterns must be carefully performed to correctly identify the crystalline phase of the MTTFs, especially when polymorphic forms are present within the film. This task is also often complicated by the presence of the wide halo of the amorphous substrate onto which the MTTF films have been deposited and/or the superposition of the reflection peaks relative to the eventual semicrystalline nature of the substrate used (e.g., when the TiO<sup>2</sup>

ited on a glass substrate is reported [60], clearly showing the broadening of the background

**Figure 4.** (a) Top surface and (b) cross-sectional FESEM micrographs of the 3D hexagonal MTTF [7] and (c) AFM image

are reported

thin film depos-

thin

reflection peaks. The XRD patterns of the three main crystalline phases of TiO<sup>2</sup>

film is deposited on ITO). In **Figure 6** an XRD pattern of a dense anatase TiO<sup>2</sup>

level due to the halo generated by the amorphous support.

and other tip-sample-related issues (**Figure 4**).

68 Titanium Dioxide - Material for a Sustainable Environment

in **Figure 5**.

of MTTF surface [27].

**Figure 5.** XRD patterns of (a) TiO<sup>2</sup> anatase (JCPDS card no. 21-1272), (b) TiO<sup>2</sup> rutile (JCPDS card no. 21-1276), and (c) TiO<sup>2</sup> brookite (JCPDS card no. 29-1360).

While there are no difficulties in discriminating between the anatase and the rutile phases since the first two reflection peaks are well separated (2θ = 25.28° for d101 of anatase vs. 2θ = 27.44° for d110 of rutile), spotting the difference between anatase and brookite might be more tricky. Only, the reflection peaks at 2θ = 30.81° relative to the d121 of the brookite phase or at 2θ = 62.57° relative to d204 of the anatase phase can help in making such a difference and/or recognize the presence of both the polymorphic forms in an XRD pattern [61].

A small-angle diffraction pattern, instead, could reveal the eventual presence of an ordered array of mesopores within the MTTFs, allowing the determination of its geometry, i.e., cubic lamellar or hexagonal [44, 62], as shown in **Figure 7**.

**Figure 6.** XRD patterns of a dense TiO<sup>2</sup> thin film prepared by sol-gel and dip-coating techniques on a glass substrate (from Ref. 60).

C17 H<sup>35</sup> COOH + 26 O2 + *hv* → 18 CO2 + 18 H<sup>2</sup> O

monitoring of the asymmetric C-H stretching mode of the CH<sup>3</sup>

ily be detected in situ through spectrophotometric methods [65].

asymmetric and symmetric C-H stretching modes of the CH<sup>2</sup>

respectively (**Figure 8**) [64].

**5. Applications of MTTFs**

applications of mesoporous TiO<sup>2</sup>

nism includes three steps: initially, TiO<sup>2</sup>

resistance, due to the interaction of TiO<sup>2</sup>

photocatalytic applications of mesoporous TiO<sup>2</sup>

Mesoporous TiO2

of TiO<sup>2</sup>

TiO2

(H<sup>2</sup>

TiO2

takes place:

, CO, NH<sup>3</sup>

**5.1. Sensors**

SA decomposition can be demonstrated, for example, by FTIR spectroscopy through the

The photocatalyst activity of titania can also be evaluated via photocatalytic oxidation of methylene blue (MB) under UV illumination. MB is often used as model for recalcitrant azodye pollutant, and, being a highly colored organic molecule, its photodecomposition can eas-

spanning from sensors, self-cleaning coatings, lithium-ion batteries (LIBs), photocatalysis, and new-generation solar cells. For interested readers, three comprehensive reviews, on the

ordered materials for solar radiation applications [66], and on the self-cleaning applications

A good sensor requires high sensitivity, fast response, and good selectivity. Furthermore, low-cost materials and easy fabrication processes are important advantages for practical uses. Mesoporous titania thin films are excellent candidates for sensing applications because of the enhancement of the sensing signal due to the increased surface. Nevertheless, the MTTF sensitivity is also affected by the pore size and the carrier's diffusion length. The sensing mecha-

specific chemical or biochemical reaction takes place at the interface and gives rise to a chemical signal, converted, in the third step, into an electronic signal in turn amplified and detected.

 surface play an important role, since oxygen is adsorbed on these surface vacancies when the film is exposed to air forming anionic oxygen. When a gas molecule is in contact with a

der Waals forces and dipole interactions; immediately after, the gas molecule is chemisorbed

a charge transfer induced by the redox reaction between titania and the gas molecule occurs [68]. When a reducing gas (e.g., CO) is detected by a chemical sensor, the following reaction

). The working principle of these sensors relies on the changes of the electronic

sensors can detect several gases, including either oxidative gas (O<sup>2</sup>

gas sensor based on MTTFs, first this molecule is physisorbed on TiO<sup>2</sup>

CO + O<sup>−</sup> → CO2 + e<sup>−</sup>

via a strong chemical bond formed between the gas and the surface atoms of TiO<sup>2</sup>

[67], have recently been published. The present section will therefore only cover the

in sensor and LIB applications.

thin films have attracted researchers among various fields of applications

group at 2958 cm−1 and the

group at 2923 and 2853 cm−1,

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


surface binds the analyte molecules; subsequently, a

with the surrounding environment. The vacancies on

, NO<sup>2</sup>

) or reductive gas

surface through van

. In this step,


71

**Figure 7.** Small-angle XRD patterns of TiO<sup>2</sup> thin films, as-synthesized (bottom) and calcined (top) films for (a) lamellar, (b) hexagonal, and (c) cubic titania mesostructures (figures adapted from Ref. 62).

#### **4.1. Evaluation of the photocatalyst activity**

The activity of a photocatalytic titania mesoporous thin film can vary considerably depending on many factors, such as crystallinity, surface-to-volume ratio, pore accessibility, film thickness, and roughness. The photocatalytic activity of MTTFs can be determined evaluating the decomposition of stearic acid (SA), used as probe, under UV illumination (λ = 256 nm). This fatty acid is usually chosen for its high stability under UV illumination in the absence of suitable photocatalyst film. Furthermore, a thin layer of stearic acid can easily be deposited through dip- or spin-coating onto the film from a methanol or chloroform solution. SA provides a reasonably good model compound for solid films since it undergoes oxidative mineralization and this process can be monitored as a function of time [63]:

**Figure 8.** Disappearance of the stearic acid IR bands (C–H stretches at 2912 and 2847 cm−1) on the surface of a titania thin film after irradiation with UV light (λ = 256 nm) [64].

$$\text{C}\_{17}\text{H}\_{35}\text{COOH} + 26\text{O}\_2 + hv \rightarrow 18\text{CO}\_2 + 18\text{H}\_2\text{O}$$

SA decomposition can be demonstrated, for example, by FTIR spectroscopy through the monitoring of the asymmetric C-H stretching mode of the CH<sup>3</sup> group at 2958 cm−1 and the asymmetric and symmetric C-H stretching modes of the CH<sup>2</sup> group at 2923 and 2853 cm−1, respectively (**Figure 8**) [64].

The photocatalyst activity of titania can also be evaluated via photocatalytic oxidation of methylene blue (MB) under UV illumination. MB is often used as model for recalcitrant azodye pollutant, and, being a highly colored organic molecule, its photodecomposition can easily be detected in situ through spectrophotometric methods [65].
