*3.1. Characterization of Materials*

Sol-gel derived nanocrystalline TiO2 were subjected to the XRD analysis to determine crystalline phase and crystallite size. Titania exists in three crystalline polymorphs–anatase, rutile and brookite forms. Among these, anatase titania has been shown to exhibit higher antimicrobial activity than the other two and thus pure anatase phase content is a desirable feature [26]. The PXRD of titanias synthesized in this work had the peaks characteristic of anatase phase Figure 2a. (JCPDS No*.* 21-1272). From the X-ray diffraction patterns, the size of anatase TiO2 materials prepared were in the nanometric scale Table 1. The average crystallite size was determined from the (101) plane in the PXRD pattern using Scherer's formula. The calculated value of undoped TiO2 had bigger crystallite size while Ag-doped TiO2 showed a decrease in the crystallite size. A good correlation between the Raman and PXRD was also observed Figure 2b. The changes in the crystallite size of TiO2 nanocrystals upon Ag-doping are closely correlated to the broadening and shifts of the Raman bands with decreasing particle size [27]. Similar observations were made for the titania sysnthesised in the present work. During annealing process, silver nitrate thermally decomposes into silver. Bigger ionic radii of Ag+ (0.75 Å) compared to Ti4+ (0.605 Å) prevents it from entering the crystal lattice of anatase TiO2 because of a high energy barrier. Thus, it gets distributed uniformly on the surface of TiO2. However, the PXRD pattern of Ag-TiO2 did not reveal any Ag or Ag-containing phases. This may be due to the low concentration of Ag incorporated which is below the detection limit of the PXRD analysis.

Doping with Ag<sup>+</sup> ion also resulted in increase in the BET surface area of TiO2 (48 m2 /g), while that of undoped TiO2 showed BET surface area of 27 m2 /g. Thus, large surface area to volume ratio of Ag-doped TiO2 was advantageous for the release of Ag+ ion. From the energy dispersive X-ray (EDS) analysis at two locations (see Figure 3a), done during the SEM confirms silver is dispersed uniformly in TiO2 host. Figure 3b shows the changes in the absorbance of Ag-doped TiO2 in comparison to undoped TiO2 and Degussa P 25 titania. Ag doped TiO2 (calcined in ambient air at 500 °C) was found to have higher visible absorbance. In contrast, pure TiO2 prepared under similar experimental conditions, had its absorbance slightly shifted towards the visible region as compared to Degussa P25 titania (Figure 3b). The DRS spectra showed a characteristic absorption band at about 500 nm, due to the surface plasmon resonance of silver [28]. Using the different absorbance onsets, it was found that the Ag-TiO2 had a bandgap of ~2.8 eV while both of the undoped titania samples had wider band gaps estimated at ~3.1 eV for TiO2 and ~3.2 eV for the Degussa P25 TiO2 sample. Similar observations from previous studies can be confirmed [29].


**Table 1.** Physio-chemical properties of nanofiller, *T*g, weight of coated composite material and amount of silver ion released.

\* Concentration of silver in the exposure media as determined by Atomic Absorption Spectroscopy (AAS), after 48 h.

**Figure 2.** (**a**) Powder X-ray diffraction (XRD) and (**b**) Raman spectra of TiO2 and Ag-TiO2.

**Figure 3.** (**a**) Elemental analysis (EDS) of the silver doped TiO2 showing the presence of Ti and Ag species; (**b**) UV-Vis diffuse reflectance spectra (DRS) of Ag-doped TiO2, TiO2 and Degussa P25 titania.

The homogeneous distribution of nano-filler in a polymer matrix has major influence on the composite performance. The morphology of synthesized titania nanoparticles and their dispersion in epoxy matrix were examined by SEM analysis Figure 4. The primary particle size of undoped and silver doped titania are different, varying from nanometer to micron size for the same magnification as seen in SEM micrographs Figure 4a,b. The undoped sample exhibited a nanostructure consisting of spherical clusters with a diameter of 50–500 nm, which are extensively agglomerated with an average crystallite size of 36 nm. However, silver doped titania showed bigger aggregates and smaller segregated particles consisting of primary anatase nanocrystals of 18 nm size (Figure 4b). Dispersion is an important factor in determining a nanocomposite's properties. Composites with the same weight percent (1 wt%) of nanofiller showed different degree of dispersion Figure 4c,d. The unmodified TiO2 although thoroughly distributed in the matrix, yet particles agglomerated densely as shown in Figure 4c giving scattered hill lock like appearance on the surface of the composite. The size of these agglomerates varied from nanometers to micrometers. However, the Ag-TiO2 particles Figure 4d, showed a lesser degree of agglomeration; interparticle distance are clearly visible between the TiO2 particles. This indicates that the presence of silver enable good dispersion due to the interaction of oxidized silver ions with surface hydroxyl groups (titanol groups, Ti–OH) of TiO2 and increase its wettability in apolar media like epoxy (hydrophobic polymer matrix). While Figure 4e shows the fractured surface of the composite, dispersion in the bulk is similar to distance between agglomerates as on surface. This suggests that the doped nano-fillers have better dispersion due to surface modifications, which improve the interactions between particles and polymer matrix. Use of reactive diluant also significantly reduced viscosity of epoxy resin during preparation and optimized the dispersion along with sonication.

**Figure 4.** Scanning electron microscopic (SEM) characterization of (**a**) sol-gel synthesized TiO2; (**b**) 1.5 wt% silver doped TiO2; (**c**) 1 wt% epoxy/TiO2 composite; (**d**) 1 wt% epoxy/Ag-TiO2 composite; (**e**) Fractured surface of 1 wt% epoxy/Ag-TiO2 composite.

The glass transition temperature (*T*g) of the samples were determined from the tangents of DSC spectra as a function of temperature. The DSC curves of the neat epoxy and nanocomposites with 1 wt% of TiO2 and Ag-TiO2 nanofiller from the second run are shown in the Figure 5a. For thermosetting resin glass transition temperature (*T*g), values can shift due to reasons like cross-linking density, intermolecular interaction and chain length. The addition of nanometer sized TiO2 particles in epoxy resulted in increase in the *T*g from 93 °C for neat epoxy to 97 °C at 1 wt% loading of Ag-TiO2. Whereas, *T*g of composite shifts to lower temperature with undoped TiO2 (1 wt% loading) due to poor dispersion and agglomeration as evident in the SEM micrograph. Nanocomposites with Ag-TiO2 exhibited maximum *T*g value at 1.5 wt% loading (107 °C) (Figure 5b). A further increase in the nano-filler content to 2 wt% led to the drop in the *T*g value, this is due to their easy agglomeration arising from van der Waals attraction between particles.

**Figure 5.** (**a**) DSC thermograms of neat epoxy and nanocomposites with 1 wt% of TiO2 and Ag-TiO2; (**b**) Variations in *T*g values of neat resin and nanocomposites at different wt% of TiO2/Ag-TiO2 loading.

It can be seen from Figure 5b that the *T*g value increases steadily then value drops; this corroborates with the trend observed by other investigators [13,30]. With our study, the degree of dispersion and nanofiller loading affected the shifts in *T*g for epoxy/Ag-TiO2 composites. The size, loading and dispersion state of the nanofillers are the factors that impact the glass-transition temperature. The *T*g value increases due to polymer chain-filler (organic-inorganic interfacial contact) that are immobilized by cohesive interactions at the interface of nanofiller in the bulk of the material. On the other hand, higher loading of nanofiller or their agglomeration can result in mobile moieties within the matrix which significantly decrease the glass transition temperature. Very high *T*g values are not achievable by room temperature curing agents, and the composites reported here can find their applications at temperature conditions below their *T*g. These synthesized epoxy composites may be cross linked by means of any conventional hardener at room temperature, without the decomposition of incorporated biocides.
