*3.1. Development of Composite*

Ag/TiO2 was synthesized under UV technology [19], starting from a mixture of oxalic acid, AgNO3, and TiO2 NPs (Figure 2). The reaction color was found to change from colorless to a brownish shade under irradiation with ultraviolet light to verify the reduction of silver ions (Ag+) to silver metal (Ag0) and incorporating AgNPs onto TiO2 NPs. The color shift presented a visual verification for the photo-metallization process in the reaction system. Silver ions were initially subjected to cationic adsorption onto the surface of TiO2 NPs. When a suspension of TiO2 has a pH value <6, the main surface entities become TiOH2+, while the main surface entities becomes TiOH<sup>−</sup> for a suspension of TiO2 with a pH value higher than 6. Thus, NaOH(aq) was added for complete deposition of the adsorbed Ag<sup>+</sup> onto the surface of TiO2 NPs to result in the formation of silver(I) oxide (Ag2O), which were then reduced to Ag<sup>0</sup> by an ultraviolet supply. The ultraviolet irradiation has the ability to induce the transfer of free electrons from valence of TiO2 NPs to the other conduction band. TiO2 NPs comprises negative charges in the presence of Ti-OH<sup>−</sup>, facilitating deposition of Ag+ onto its surface. Thus, the photo-induced generated electrons function as reductive agents for Ag+ to provide Ag0. Production of tiny Ag0 crystals could occur by cathode-like reduction or by aggregation of Ag0. AgNPs has been known to show an absorbance band attributed to Plasmon effect owing to the interaction of the metallic NPs with UV, leading to oscillation of electrons. The color change of the solution to brown was attributed to the improved absorption at low wavelength owing to surface Plasmon. The reaction mechanism between AgNO3 and TiO2 is illustrated by the equations described below [30].

> Ag+ → Ag+ (adsorbed onto the surface of TiO2 NPs) 2Ag<sup>+</sup> (adsorbed) + 2OH<sup>−</sup> → Ag2O+H2O 2Ag2O + *<sup>h</sup><sup>υ</sup>* → 4Ag<sup>0</sup> + O2 TiO2 (e-/h+) + Ag+ → TiO2@Ag0

**Figure 2.** TEM graphs of TiO2 (**a**) and Ag0/TiO2 (**b**).

UV/Vis absorption spectra were studied to explore the influence of Ag<sup>0</sup> on the TiO2 optical activity, as illustrated in Figure 3. The absorption spectra of TiO2 and Ag0/TiO2 showed broad absorbance bands with a wavelength maxima <400 nm. This can be attributed to the electron transition in TiO2 depending on its energy band gap (~3.12 eV) owing to a charge transfer. The absorbance spectral curves of TiO2 were enhanced in Ag0/TiO2. Obvious variations in absorbance activity of Ag0/TiO2 were detected in the visible spectrum range as a result of the weak Plasmon effect owing to the low Ag<sup>0</sup> content

on TiO2 NPs. This could enhance both surface excitation and electron/hole separation. The absorbance band of Ag0/TiO2 demonstrated that Ag0/TiO2 exhibits properties similar to TiO2 NPs. The absorbance intensities were observed to exhibit a red shift for Ag0/TiO2, representing a decrease in TiO2 gap. The absorbance spectra showed maximum absorbance wavelengths at 383 and 388 nm for TiO2 and Ag0/TiO2, respectively. This monitored shift in the wavelength and the reduced band gap led to the increase in the light-induced catalytic activity of TiO2 NPs in the visible range.

**Figure 3.** UV/Vis absorption spectral curves of the prepared composites coated onto viscose fibers.

#### *3.2. Characterization of Viscose Fibers*

The morphology of the coated viscose before and after treatment with plasma, as well as plasma-pretreated viscose before and after coating, were studied by SEM as depicted in Figure 4. A surface of moderate smoothness was monitored for plasma-inactivated viscose. Plasma-cured viscose displayed etches on the fiber surface. Irregular nanoparticles were monitored on the surface of the plasma-treated fibers. Decreasing the thickness of the surface layers resulted in improving the rough surface in comparison with pristine fibers. Fibers loaded with TiO2 showed irregular and uneven clusters. Fibers coated with Ag0/TiO2 displayed a skinny film of inconsistent Ag0/TiO2. No cracking was detected and the small particles were monitored to cover the fibers. The changes in chemical compositions of samples due to plasma-curing and deposition of nanoparticles onto the surface of viscose were explored by EDX. The chemical compositions of blank and plasmauntreated fibers loaded with nanoparticles are summarized in Table 1. Both carbon and oxygen were detected as major contents due to the fabric, whereas Ti and/or Ag were detected as minor contents due to the deposition of TiO2 or Ag0/TiO2 onto the fabric surface. The plasma-cured sample showed a slight increase in the oxygen content due to generating oxygen-containing substituents onto the fiber by oxygen plasma treatment. Plasma curing by etching and oxidation has been employed to activate the fiber surface to induce the creation of substituents [31], such as carbonyl, alcohol, carboxyl, and ether, facilitating strong binding to nano-scaled particles.

**Figure 4.** SEM images of TiO2 NPs incorporated plasma-activated (**a**,**b**) and Ag0/TiO2 incorporated plasma-activated (**c**,**d**) fibers.



FT-IR spectra were explored for the coated viscose with and without plasma treatment, as shown in Figure 5. The main characteristic peaks were detected at 3339 cm−<sup>1</sup> for the hydroxyl group stretch vibration, as well as two peaks at 2932 and 1030 cm−<sup>1</sup> for the aliphatic C-H stretch and bend vibrations, respectively. No major shifts were detected in the absorbance bands; however, the intensity of the hydroxyl group was found to increase with the increasing deposition of the nanoparticles.
