**1. Introduction**

In the last few years natural fibers of natural origin, especially those of lignocellulosic composition [1], attracted a great attention due its interesting properties as high tensile strength and rigidity, and to the advantages of being recyclable, lightweight, biocompatible, and extremely low costing, when compared to the synthetic ones. Among others, ramie, jute, kenaf, etc. reinforced composites have been highly emphasized [2]. The use of natural fibers fit into the concept of circular economy, which seeks to reduce, reuse, recover, and recycle products. In addition, there are economic and functional advantages in the use of natural fibers compared to the most common artificial fibers, made of carbon, glass, or polymeric resins [3], namely due to their low production cost and high abundance.

Sustainability is becoming a concern in the development of new materials, mainly due to problems related to the use of scarce resources and waste management. On the other hand, there is a kind of activation energy for the creation of new products and functionalities, enabling new commercial paradigms or complementing the existing ones.

Cellulose fibers can be obtained from many plants and represent one of the most abundant organic materials on earth. Invasive plants, such as ginger lily (*Hedychium gardnerianum*) [4], are abundant in various countries and are therefore an ecological source of fibers for scientific and industrial applications as an alternative to the traditional glass, polymer, or carbon fibers [5]. Invasive species are a threat to ecosystems and the survival of many endemic species, and their monitoring, control, or eradication is crucial, to prevent the modification of ecological processes and the loss of biodiversity. The ginger lily plant can be found in large quantities at Azores Islands (Portugal), and they are mainly considered as waste. However, its biological nature gives them specific characteristics less good for high-tech applications, namely the ease way since they absorb water. This causes dimensional changes and swelling on the fibers, mainly because its main composition of cellulose, which is structurally a linear polymeric chain with OH groups, and thus highly hydrophilic.

Nowadays, titanium dioxide (TiO2) is one of the most effective photocatalysis [6] demonstrating high efficiency of decomposition and detoxification of several toxins and pollutants [7]. However, there is a huge disadvantage that involves the removal of TiO2 catalysts after their applications, in the case of catalysts based on particles in suspension. In general, water purification reactors employ photocatalyst particles (powder type) that have higher photocatalytic activity due to less mass transfer limitations between the treated contaminants and the photocatalyst. However, powdered photocatalysts need to be filtered and separated after water treatment, which is a tedious and expensive process. So, to commercialize the process as a full-scale technology, it is critical to increase the photocatalytic activity of TiO2 and manufacture devices with TiO2 immobilized on a specific support. This strategy can bring a great benefit. Therefore, several attempts have been already used to immobilize TiO2 on different supporting materials [8] and shapes. From these, glass substrates [9, 10], glass spheres [11], fiberglass [12], activated carbon, zeolite, and ceramics [13] stainless steel [14] and polyamide fibers [15], can be emphasized. Moreover, photocatalytic fiber is an emerging solution to immobilize catalyst powders [16, 17]. The natural fibers are nowadays preferable due to their multiple advantages in terms of environmental sustainability. Inspired by these remarkable characteristics, fibers have found a great interest as supporting substrates [18]. Cotton and ginger lily fibers, have booth cellulose in its main composition and so have abundant hydroxyl groups (OH) [18, 19] to link photocatalysts through hydrogen bonds and van der Waals forces. Wool fiber, instead, possesses plenty of disulfide bonds (-S-S-), carboxyl groups (-COOH), and amino groups (-NH2) [16]. Moreover, natural fibers are considered desirable for TiO2 immobilization platforms on account of their intrinsic porous structure [20], large specific surface area, and flexibility [16]. Its flexible form can be adapted to different spaces and purification devices. In addition, they can be cut to any size, rolled up, etc., to meet the function's requirements. For example, cotton fibers were proved to be easily installed inside photoreactors [21].

Since TiO2 can impart antibacterial [22] and self-cleaning [23] properties to the fibers it becomes clear that is of great interest for the textile industry. The multiple fiber-related substrates involving fibers, yarns, and fabrics with different structures can be used as support substrates for photocatalyst proposes [16]. Important fiber properties are good adhesion of TiO2 which demands improvement of binding efficiency with fibers to keep the necessary high specific surface area to enhance the absorption affinity.

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

There are many methods used for the synthesis of TiO2 [24]. TiO2 nanoparticles/ films immobilization was prepared successfully in a variety of natural fibers, by the sol-gel method [25], microwave-assisted liquid phase deposition process [26], and DC-reactive magnetron sputtering [18].

A promising method to prepare photocatalytic immobilized TiO2-based thin films is by DC-reactive magnetron sputtering. This technique enables large-area deposition with high uniformity, yet it is essential to understand why the film properties exhibit after deposition. This is because the properties of the coatings obtained are highly dependent on the selected parameters, and so it is necessary to establish the ideal ones that satisfy any film application and understand the basic processes that control the properties of that films. These include, for example, the type of species deposited, their energy, and consequently the effects that their bombardment will have on the surface of a growing film, etc. In addition, the influence of substrate temperature on the nature of the film is also to be considered, among other factors, namely, for instance, the substrate position relative to the target, discharge pressure, and the gas mixture. In fact, bombardment can result in a series of surface effects, namely displacement of lattice atoms, creation of defects that can lead to increased atomic mobility, surface heating that can promote crystallization of nanoparticles, etc. These effects will consequently affect the internal stress, crystal size, morphology, and roughness of the deposited films. Quite often the physical structure of the thin film is directly responsible for the expected film property. For instance, in photocatalytic TiO2 films, deposition of crystalline or amorphous TiO2 is of crucial importance for their functionality [22]. So, understanding the relationship between deposition parameters that will affect film properties is therefore important for defining procedures.
