**3. TiO2: brief structure and properties**

TiO2 naturally exists in three polymorphs such as rutile (tetragonal), anatase (tetragonal), and brookite (rhombohedral) [33]. **Figure 6** shows the three polymorphic structures of TiO2, which can be described based on their cell structure consisting of a TiO6 distorted octahedron. Each structure has a different degree of distortion of this octahedron, resulting in the characteristic differences observed between the polymorphs.

TiO2 is an n-type semiconductor material that exhibits a bandgap ranging from 3.0 eV (for rutile) to 3.2 eV (for anatase), which corresponds to a light absorption edge in the UV range. One of the advantages of TiO2 is that it is non-toxic, biocompatible, and has a high chemical stability. These properties, along with its exceptional electronic properties, make it an ideal material for use in photovoltaic applications, which convert solar energy into electricity. The discovery by Fujishima and Honda in 1972, which demonstrated the photocatalytic splitting of water in TiO2 electrodes [35], has led to a growing interest in using TiO2 as a photocatalyst for environmental purification [7], detoxification, and self-cleaning applications.

**Figure 6.** *TiO2 main polymorphs—anatase, rutile, and brookite [34].*

Anatase phase has proved to be a more efficient photocatalyst than rutile phase [36], that might be explained by the reduction ability of O2 higher that rutile, despite the larger band gap of anatase [36]. Recently, amorphous TiO2 has been considered very effective in antibacterial disinfection [22]. Is well known that TiO2 photoactivity is hampered by the narrow range of UV wavelengths for photoactivation. Its energy gap is only sensitive to radiation in the ultraviolet (UV) region of the solar spectrum, which represents only 4% of the global solar radiation [37].

Almost 20 years ago it has been reported that the TiO2 lattice doping with nonmetallic atoms like N [38] can shift the absorption edge from UV to lower energies and thus increase visible absorption. Recently, a photonic band gap of 3.18 eV (390 nm) was measured for amorphous TiO2 [22] which is slightly lower than the value reported to anatase (3.2 eV). TiO2 absorbs photons and acquires enough energy (*h*ν) to allow an electron in the valence band to jump to the conduction band.

This process (photocatalysis) gives rise to an electron (e�)-hole (h<sup>+</sup> ) pair, in accordance with the reaction TiO2 <sup>+</sup> *<sup>h</sup>*ν\$ <sup>h</sup><sup>+</sup> + e�, further responsible for the elimination of water toxic components by active species (•OH, •O2, and H2O2) generated by redox reactions on the TiO2 surface.

Rutile and anatase are stable phases at normal conditions an comprise identical TiO6 octahedron building unit but with diversely sharing corners and edges giving rise to different configurations [22]. The TiO6 in anatase are arranged in zigzag chains along {221}, sharing four edges, while in rutile, TiO6 share two edges and link up in linear chains along {001} [39]. These structural differences give rise to different densities and electronic band structures between these two phases [22].

Moreover, the number of shared edges is related to the "energy of the structure" (and thus its stability). Rutile is more stable that anatase (metastable) being the number of shared edges per octahedron, respectively, two and four [22]. The distance Ti-Ti between the center of edge-sharing octahedra being smaller with the decrease of the number of shared edges which provided shorter Ti-Ti distances and a more closely packed crystal structure of rutile. Thus, there is a strong interaction in the Ti-Ti bond of rutile which has only two Ti atoms at the shortest distance. On the other hand, in

### *TiO2 Nanocoatings on Natural Fibers by DC Reactive Magnetron Sputtering DOI: http://dx.doi.org/10.5772/intechopen.110673*

anatase, the Ti-Ti interaction, instead, depends on four Ti atoms, which allows for a Ti-Ti distance greater than that of rutile. Therefore, rutile exhibits less blockage around each TiO6 unit, leading to a more stable phase [40].

As in anatase there are four octahedrons, at a distance between them of 3.04 Å, while in rutile, despite its higher density, only two octahedrons are present at 2.96 Å [40], the distinct arrangement of TiO6 octahedrons gives rise to different structures packaging that will condition the anatase-rutile transition.

In many synthetic routes, amorphous TiO2 is often the first phase to form. The transformation of amorphous TiO2 into anatase and/or rutile, usually occurs by effect of temperature, both in wet chemical methods, such as sol-gel [41] and also in DC reactive magnetron sputtering [42].

X-ray diffraction (XRD) and Raman spectroscopy are commonly used techniques for analyzing the crystallization process in materials science.

X-ray diffraction is a technique that involves shining X-rays onto a crystalline material and observing the resulting diffraction pattern. The diffraction pattern is characteristic of the crystal structure and provides information about the crystal lattice parameters, the orientation of the crystal grains, and the degree of crystallinity.

The XRD patterns of anatase (JCPDS card No. 96-900-9087) and rutile (JCPDS card No. 96-900-9084) phases shown in **Figure 7** provide important information about the crystal structures of these two polymorphs of TiO2. In the XRD pattern of anatase, the main peak at 25.3° corresponds to the (101) plane of the crystal structure. This peak is relatively sharp and intense, indicating a high degree of crystallinity and a well-defined crystal structure. The presence of other peaks at lower angles also indicates the presence of other crystallographic planes in the anatase structure.

In the XRD pattern of rutile, the main peak at 27.4° corresponds to the (110) plane of the crystal structure. This peak is also relatively sharp and intense, indicating a

well-defined crystal structure and a high degree of crystallinity. The presence of other peaks at higher angles also indicates the presence of other crystallographic planes in the rutile structure.

Raman spectroscopy, on the other hand, involves shining laser light onto a material and measuring the scattered light as a function of wavelength. The scattered light provides information about the vibrational modes of the atoms in the material, which are characteristic of the crystal structure.

By combining XRD and Raman spectroscopy, a more comprehensive understanding of the crystallization process in a material is obtained. XRD provides information about the long-range order and crystal structure, while Raman spectroscopy provides information about the short-range order and local structure.

Anatase has a tetragonal crystal structure and is characterized by Raman peaks at around 144, 399, and 519 cm<sup>1</sup> . The peak at 144 cm<sup>1</sup> is due to the symmetric stretching vibration of the Ti-O bond, while the peaks at 399 and 519 cm<sup>1</sup> are due to the bending modes of the Ti-O-Ti bond. The Raman spectrum of anatase is also characterized by a broad peak at around 639 cm<sup>1</sup> , which is due to the lattice vibrations of the TiO6 octahedra. **Figure 8** shows the Raman spectrum of a pure anatase film.

Rutile, on the other hand, has a tetragonal crystal structure and is characterized by Raman peaks at around 143, 445, 610, and 880 cm<sup>1</sup> . The peak at 143 cm<sup>1</sup> is due to the symmetric stretching vibration of the Ti-O bond, while the peaks at 445, 610, and 880 cm<sup>1</sup> are due to the bending modes of the Ti-O-Ti bond. The Raman spectrum of rutile is also characterized by a sharp peak at around 237 cm<sup>1</sup> , which is due to the lattice vibrations of the TiO6 octahedra.

When a material undergoes a phase transformation from the amorphous or liquid phase to a crystalline phase, the specific crystalline phase that forms depend on a number of factors, including the thermodynamic stability of the different phases and

**Figure 8.** *Raman spectra of anatase TiO2 phase.*

### *TiO2 Nanocoatings on Natural Fibers by DC Reactive Magnetron Sputtering DOI: http://dx.doi.org/10.5772/intechopen.110673*

the kinetics of nucleation and growth. In the case of TiO2, the initial crystalline phase that forms is generally anatase, rather than rutile, because of its lower surface free energy compared to the rutile structure. Surface-free energy is a measure of the amount of energy required to create a unit area of a material's surface. Materials with lower surface free energy are typically more stable, because they have a lower tendency to form new surfaces or interfaces. In the case of TiO2, the anatase structure has a lower surface free energy than the rutile structure, which means that it is more thermodynamically stable.

This difference in surface free energy is due to the different crystal structures of anatase and rutile. The anatase structure has a higher percentage of exposed (001) surfaces, which have a lower surface free energy compared to the (110) and (100) surfaces that are more prevalent in the rutile structure. As a result, the anatase structure is more stable and more likely to form during the crystallization process.

The surface roughness and microstructure can significantly influence the performance and hence the purpose of TiO2 thin films. These characteristics depend on the deposition process, type of substrate, and chosen deposition parameters.

Liang et al. have produced TiO2 films by the sol-gel method [44], highly compacts, continuous and smooth (**Figure 9**), exhibiting excellent self-cleaning properties. **Figure 10** shows SEM images of TiO2 films (on glass substrates) prepared by reactive magnetron sputtering under different deposition conditions, namely plasma O2 concentration (50% and 75%) and used power (500 and 1000 W) [18]. It can be seen clearly the differences in the morphology of the surface of TiO2 coatings, as a function of different powers and concentrations of O2. In general, the morphology is typically constituted by several agglomerates of nanoparticles (or grains) in the shape of a cauliflower but of different sizes, which are distributed over the surface of the substrate [18] in accordance with what was reported by Sério et al. [42]. There is a variation in the size of the agglomerates in the morphology of the films dependent on the O2/(Ar + O2) ratio.

The value of the thickness (*th*) of the films allows estimating the deposition rate (*vd*), in nanometers per minute.

$$v\_d = \frac{t\_h}{t}$$

where *t* is the deposition time. The thickness measurement is performed on SEM images in cross-section (as exemplified in the insert of **Figure 10a**). Regardless of the geometry, the surface is covered evenly.

### **Figure 9.**

*SEM image of the surface morphology of TiO2 film deposited onto glass substrates by dipping-based sol-gel method. Adapted from [44].*

### **Figure 10.**

*SEM images of surface morphology of TiO2 films deposited onto glass substrates by reactive magnetron sputtering [18].*
