**4. Characteristics of TiO2 thin films deposited on natural fiber surfaces by DC reactive magnetron sputtering**

The surface is the boundary that separates the material and the external environment. All alterations that occur in the sense of changing the surface structure will condition and tailor the material for certain applications. A wide range of surface modification processes, such as reactive magnetron sputtering, thermal oxidation, thermal evaporation, molecular-beam-epitaxy, chemical vapor deposition, sol-gelassisted by dip-and/or-spin coating, spray pyrolysis, and electrodeposition have been used for used to produce thin film and tailor surfaces. However, in many of them, adhesion of the coating to the supports is still an important issue, which deserves special attention, especially in some types of substrates, such as natural fibers, which main composition is cellulose.

Cellulose is a polymeric chain with abundant hydroxyls (–OH) groups and other oxygen-containing functional groups –C=O, –C–O–C–, –CHO, and –COOH which makes the fiber surface potentially reactive [18]. These functional groups are available to bond to desired molecules and provide new properties and new applications for natural fibers [18]. The deposition of suitable coatings, such as TiO2 films, allows the optimization of natural fibers by creation of new tailored properties of their surfaces, such as the photocatalytic ones, which are independent of that exhibited by the bulk fiber. Recently, nanostructured TiO2 films successfully deposited on ginger lily fiber surfaces have been created by DC reactive magnetron sputtering [18].

The efficiency of the DC magnetron sputtering process to functionalize natural fibers (**Figure 11**) depends not only on the quantity cellulose reactive accessible

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

**Figure 11.**

*SEM images of ginger lily fibers: (a) pristine; (b) after TiO2 sputtered at 50% O2–1000 W [45].*

groups but also on sputtering conditions, such as the operating pressure, discharge power, O2 gas partial pressure, and deposition time.

The morphology of the films can be tailored by change of the partial pressure of the reactive gas (Ar/O2) and the sputtering power [10]. Moreover, different film typology, namely dense and porous, can be obtained, as well as amorphous or crystalline (anatase and/or rutile) nanofilms. The influence of the O2% in the discharge and the sputtering power on the amorphous/anatase phase transition, surface stoichiometry, and surface roughness of the films can be tailored.

### **4.1 Fibers preparation**

Ginger lily fibers are obtained by mechanical extraction from the stems of the plant, after removing the leaves, as shown in **Figure 12a–c**. Before TiO2 deposition by reactive magnetron sputtering (**Figure 12d**), fibers are cleaned successively in acetone, isopropanol, and deionized water to remove any organic contamination and further dried at low temperature (about 30°C).
