**5. Mechanism of repellence moisture via photocatalysts nanoparticle**

Photocatalysis is a viable environmental protection process because it can oxidize small quantities of organic contaminants into benign compounds [37, 38]. Photocatalysis employs PHNPs to perform a photo-induced oxidation rection in order to degrade organic pollutants, inactivate microorganisms, and break down water molecules [39, 40]. **Figure 3** depicts the photocatalysis process. When photons with energies higher than the band gap energy of PHNPs are absorbed, the valence band (VB) electrons in PHNPs are excited to the conduction band, which opens up a variety of potential photoreactions. With sufficient photo energy, the photocatalytic surface produces a positive hole (h+) in the valence band and an electron (e) in the conduction band (CB). The positive hole may either directly oxidize organic contaminants or generate highly reactive hydroxyl radicals (OH•). The major oxidants in the photocatalytic system [41] are the hydroxyl radicals (OH•), which oxidize organics. The electron in the conduction band decreases the amount of oxygen adsorbed by PHNPs. TiO2, ZnO, and Fe2O3 are only a few of the semiconductors that could be used as PHNPs. The band gap energy plays a crucial role in the photocatalytic process. The bandgap is defined as the void area that extends from the uppermost portion of the filled valence band to the lowest part of the unoccupied conduction band and is determined to be around 3.3 eV for ZnO nanoparticles in its crystalline state [42]. Thus, the light of a specific wavelength (i.e., ultraviolet radiation (UV)) contains enough energy to promote electrons (e-) to the conduction band while leaving an

*Utilizing Photocatalysts in Reducing Moisture Absorption in Composites of Natural Fibers DOI: http://dx.doi.org/10.5772/intechopen.106543*

#### **Figure 3.**

*Schematic representation of the photocatalytic mechanism of ZnONPs (a) before exposure to UV, (b) after exposure to UV.*

electron–hole (h+) behind. Both the hole and electron can typically be recombined very quickly [43]. They can also migrate to the surface of the PHNPs as a result of their reaction with the adsorbed elements, initiating (i) the reaction between the electron and the oxygen and (ii) the reaction between the hole and the hydroxyl ion or H2O molecule to form superoxide and hydroxyl radicals, as shown in **Figure 3** the photocatalysis process of PHNPs.

The majority of experts concur that there are five distinct mechanisms involved in the moisture-repelling action via photocatalyst nanoparticles. The first mechanism involves PHNPs were able to penetrate through the cell wall of the NFr and deposited mainly in the cell lumens and partly in the cell walls of the NFr. It is expected that the physical and mechanical characteristics of the resultant NFr would be altered to varying degrees since the PHNPs are present in both cell walls and lumens [44, 45]. Hygroscopicity of the modified NFr was reduced because the inorganic PHNPs were integrated into the cell wall and were likely occupying empty space (micropores) inside the cell wall that would otherwise be accessible to water molecules [46, 47]. By establishing hydrogen bonds with inorganic PHNPs, the hydroxyl groups of NFr cell wall components, which are principally responsible for water absorption, may have been inhibited, hence lowering the hygroscopicity of NFr. As similar study conducted by Mohammed et al. [48] showed that integration of zinc nano particle is proposed; the water repellent capability is good enough to achieve the high performance of the composite with proper system formulation during the modification process.

The second mechanism involves the incorporation inorganic PHNPs into cell walls of the fiber decreases the capacity of the cell wall to swell owing to bulking, therefore increasing the dimensional stability of the impregnated NFr. Indeed, only inorganic PHNPs integrated into the cell wall are predicted to affect the hydrophilicity and dimensional stability of NFr significantly. In contrast, those deposited in the cell lumen are anticipated to have a negligible effect [49]. Moreover, by incorporating inorganic PHNPs into the NFr cell wall, the cell wall elements are expected to be coated with water-repellent inorganic PHNPs, which may stop water molecules

#### **Figure 4.**

*Schematic representation of electrostatic adhesion mechanisms between Fiber and matrix.*

and moisture from reaching them, thus slowing swelling of NFr and improving NFr dimensional stability [48, 50, 51].

Due to the presence of inorganic PHNPs in the cell wall and on the fiber surface, the third mechanism includes strong static electric forces that may attract the nonpolar polymer surface to the NFr. As a result of the compatibility between the NFr and the polymer matrix, the mechanical characteristics of the composites will be improved and their moisture absorption will be reduced. Electrostatic adhesion is a common bonding mechanism in which opposite charges (cathode and anode) are formed on the fiber and matrix surfaces. This results in two layers of opposite charges at the interface, which enhances the attraction between the fiber and matrix of the composite [52, 53]. Electrostatic discharge treatments were applied to the surfaces of polymer and electrostatic fibers to promote electrostatic adhesion at the interface region between matrix and NFr, thereby improving the transfer the stress from matrix to NFr leading to enhancing performance of the NFr composites [54]. In other words, the incorporation of the inorganic PHNPs into the micropore structure of the fiber cell walls can diminish the microvoid volumes in the fiber, as well as decrease air bubble formation during the composite manufacturing process [55]. **Figure 4** illustrates schematic representation of interfacial electrostatic bonding mechanisms between fiber and matrix.

According to Stark [56], moisture penetration increases as the number of moisture channels within NFr composites increases; consequently, the fourth mechanism refers to the addition of inorganic PHNPs as an additive to close these pathways and reduce moisture absorption. However, greater density NFr composites have lower moisture content and less swelling than untreated NFr composites [57]. This is because the inorganic PHNPs enhance the density of the composite. Therefore, by incorporating inorganic PHNPs, it is possible to lower the loading of NFr present in composites while simultaneously rising the tensile strength of the NFr composites [45, 58]. According to specific research, reducing the fiber loading improves the tensile strength of composites, whereas rising the fiber loading results in increased moisture absorption of composites depend on NFr as ingredients [59, 60].

PHNPs play two key roles in the water repellence mechanism of NFr composites via the fifth mechanism. **Figure 5** shows that when NFr absorbs a water molecule, the critical radius of PHNP atomic structures increases, pushing NFr molecules to press outward and be released to the composite surface, which is warmed by UV absorption [44]. Second,

*Utilizing Photocatalysts in Reducing Moisture Absorption in Composites of Natural Fibers DOI: http://dx.doi.org/10.5772/intechopen.106543*

**Figure 5.** *Illustration proposed reduction and oxidation reactions by PHNPs.*

suppose the water molecule is straight or near the composite surface. In that case, PHNPs will initiate an oxidation process with the assistance of radiated energy, resulting in the breakdown of the water molecule into H+ and OH- [44].

Raghupathi et al. [61] revealed that enhanced Active Oxygen Species (ROS) generation from PHNPs during UV exposure resulted in higher PHNPs activity, which regenerated the NFr water repellency mechanisms. These reactive species include anion of superoxide (O2), peroxide of hydrogen (H2O2), and hydroxide (OH-). The formation of ROS during the dark has been noticed by Hirota et al. [62] who tested ZnO-NPs toward E. Coli. They observed that activity might occur in the dark, which is consistent with the findings of Jones et al. [63] such unambiguous findings imply that new methods for producing reactive species in the absence of light and the dark are likely to be discovered in the future.

The activation of PHNPs by UV light can be represented by the following steps:

$$\text{PHNPSs} + h\nu \rightarrow e^- + h^\* \tag{1}$$

$$e^- + O\_2 \to O\_2^- \tag{2}$$

In this reaction, h+ and e- are powerful oxidizing and reductive agents respectively. The oxidative and reductive reaction steps are expressed as:

Oxidative reaction:

$$h^\* + \text{Organic} \to \text{CO}\_2\tag{3}$$

$$h^\* + H\_2O \to OH^- + H^+ \tag{4}$$

Reductive reaction:

$$\text{OH} + \text{Organic} \to \text{CO}\_2 \tag{5}$$
