**5. Applications of MTTFs**

Mesoporous TiO2 thin films have attracted researchers among various fields of applications spanning from sensors, self-cleaning coatings, lithium-ion batteries (LIBs), photocatalysis, and new-generation solar cells. For interested readers, three comprehensive reviews, on the photocatalytic applications of mesoporous TiO<sup>2</sup> -based materials [38], on the use of TiO<sup>2</sup> ordered materials for solar radiation applications [66], and on the self-cleaning applications of TiO<sup>2</sup> [67], have recently been published. The present section will therefore only cover the applications of mesoporous TiO<sup>2</sup> in sensor and LIB applications.

#### **5.1. Sensors**

**4.1. Evaluation of the photocatalyst activity**

70 Titanium Dioxide - Material for a Sustainable Environment

**Figure 7.** Small-angle XRD patterns of TiO<sup>2</sup>

film after irradiation with UV light (λ = 256 nm) [64].

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 miner-

thin films, as-synthesized (bottom) and calcined (top) films for (a) lamellar,

**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

alization and this process can be monitored as a function of time [63]:

(b) hexagonal, and (c) cubic titania mesostructures (figures adapted from Ref. 62).

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 mechanism includes three steps: initially, TiO<sup>2</sup> surface binds the analyte molecules; subsequently, a 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. TiO2 sensors can detect several gases, including either oxidative gas (O<sup>2</sup> , NO<sup>2</sup> ) or reductive gas (H<sup>2</sup> , CO, NH<sup>3</sup> ). The working principle of these sensors relies on the changes of the electronic resistance, due to the interaction of TiO<sup>2</sup> with the surrounding environment. The vacancies on TiO2 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 gas sensor based on MTTFs, first this molecule is physisorbed on TiO<sup>2</sup> surface through van der Waals forces and dipole interactions; immediately after, the gas molecule is chemisorbed via a strong chemical bond formed between the gas and the surface atoms of TiO<sup>2</sup> . In this step, 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 takes place:

$$\text{CO} + \text{O}^- \rightarrow \text{CO}\_2 + \text{e}^-$$

In this case, CO molecules react with adsorbed oxygen ions on the mesoporous film surface, which results, in turn, in an overall decrease of the electrical resistance of the metal oxide thin films. On the contrary, if a chemical sensor is exposed to an oxidation gas (e.g., NO<sup>2</sup> ), the following oxidizing reaction may take place:

widespread availability, good stability, and environmental benignity. Generally, the revers-

framework, with a specific percentage depending on the TiO<sup>2</sup>

and morphology. Specifically, anatase is probably the most electrochemically active form

for this purpose [8]. Moreover, it has been demonstrated that anatase exposing

presents many advantages in its usage as anode material for LIBS, such as the low-volume

expansion upon lithiation (<4%), good stability, and lack of lithium plating. However, TiO<sup>2</sup>

LIBs by many groups [73–75]. However, the preparation of nanostructured TiO<sup>2</sup>

low electrical conductivity [71]. A possibility to overcome these drawbacks is represented by

mesoporous thin films could offer some advantages, such as higher specific surface area and

titania thin films on titanium substrates, with a hexagonally ordered porous structure, and they tested these samples as anode materials for LIBs, reporting an improved electrochemical performance, without the necessity of additives to enhance the transport properties of the electrode [77]. The enhanced electrochemical activity was ascribed to the higher area and volumetric capacity of these films due to the presence of the 3D-ordered mesostructure.

Undeniable great progresses have been made in recent years in the design and synthesis of

as well as to further explore and enhance their applications. Nevertheless, challenges are remaining in developing cheap, low toxic, and reproducible synthetic approaches for achieving an easy and precise control over the pore size, wall thickness, surface area, morphology,

For optoelectronic applications, the main concern resides into the deposition of organized MTTFs onto semiconductive electrodes such as ITO or FTO keeping a homogenous disposition of the pores on the whole device electrode, and although some preliminary attempts have been made [52, 78], it remains a challenging issue owned to the wettability difference between ITO and Si wafers. Moreover, while small-sized devices have been tested, on large scale, difficulties are encountered to maintain such uniform orientation of the pores, especially in

thin films featuring novel and well-designed structures and morphologies

xLi<sup>+</sup> + TiO2 + xe− → Lix TiO2 (0 < x < 1)

also characterized by some limitations, including a limited Li<sup>+</sup>

can be expressed by the following equation:

/Li. Lithium ions reversibly insert/extract into/from the interstitial vacancies

. This redox reaction typically takes place at around

ion diffusion along this direction (c-axis) facilitating a fast

, which provides a higher specific surface area and shorter diffu-

nanotube array [76]. For example, Ortiz et al. prepared mesoporous

ions, compared to the corresponding bulk materials [72].

nanotubes have been exploited as anode materials for

crystalline form

in the form of

ion diffusion, low capacity, and

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

is

73

ible reaction between Li and TiO<sup>2</sup>

(001) facets exhibits efficient Li<sup>+</sup>

lithium insertion/extraction [70].

the nanostructuration of TiO<sup>2</sup>

thinner walls than a TiO<sup>2</sup>

mesoporous TiO<sup>2</sup>

and crystallinity.

sion pathways for electrons and Li<sup>+</sup>

In this context, vertically oriented TiO<sup>2</sup>

**6. Conclusions and perspectives**

1.7 V vs. Li<sup>+</sup>

of the TiO<sup>2</sup>

of TiO<sup>2</sup>

TiO2

where x is the mole fraction of Li in TiO<sup>2</sup>

$$\text{NO}\_2 + \text{e}^- \rightarrow \quad \text{NO}\_2^-$$

In this example, NO<sup>2</sup> molecules cause a depletion of electrons from the TiO<sup>2</sup> surface, which results in an increase of electrical resistance. The conductivity change can be easily transferred into resistance signal, which is the best-known sensor output signal.

Titania nanostructured materials are good candidates also for biosensing, because TiO<sup>2</sup> is able to form coordination bonds with the amine and the carboxylic groups of biomolecules, such as enzymes, while maintaining the enzyme's biocatalytic activity. Furthermore, TiO<sup>2</sup> is characterized by high stability and biocompatibility. A biosensor is an analytical device, which converts a biological response into readable or quantified signal. Biosensors can be applied to analyze a variety of samples including body fluids, food samples, and cell cultures [68]. The biosensing mechanism is based on a biochemical reaction. Typically in (bio-)electrochemistry, a measurable current (amperometric), a measurable potential, or a charge accumulation (potentiometric) will be generated upon the alteration of the conductive properties of a medium between electrodes when the sensing takes place.

Titania mesoporous thin films have been also used as sensors for *Escherichia coli*, an enterohemorrhagic bacterium whose infections have a low incidence rate but can have severe and sometimes fatal health consequences and thus represent some of the most serious diseases due to the contamination of water and food [69]. Titania films treated with APTES ((3-aminopropyl) triethoxysilane) and GA (glutaraldehyde) were functionalized with specific antibodies anti-*Escherichia coli* antibodies. In this case, FTIR spectroscopy has been used as an optical transduction method: the spectroscopic signals originated from the various functional groups related to proteins, lipid, and carbohydrates can be used for the identification and structural characterization of different pathogens and subspecies.
