*2.1. Surface Microstructure, Roughness and Chemical Composition of the Coatings*

The surface morphologies of the coatings prepared with different vol% of SiO2 in TiO2 are shown in Figure 2. The SiO2-TiO2 coating prepared with 7 vol% of SiO2 in TiO2 showed uniform, crack-free and smooth morphology (Figure 2a). However, significant cracks on the entire coating surface were observed with increase in SiO2 concentration in TiO2 (Figure 2b–d). Some small cracks were started to appear on the coating prepared from 10 vol% of SiO2 in TiO2 (Figure 2b), and these cracks goes bigger with increase in vol% of SiO2. For the coating prepared with 20 vol% of SiO2 in TiO2, the cracks on the surface were abundant and coating material was popped out detaching from the substrate (Figure 2d). In the case of sol-gel coated films, during drying process, the capillary forces might have generated which provides cracks on the surface [12]. The surface roughness of the coatings were also studied (Figure 3). The SiO2-TiO2 coating prepared with 7 vol% of SiO2 in TiO2 showed the surface roughness of 67 nm (Figure 3a), whereas the surface roughness increases drastically to 98, 191 and 287 nm for the coating prepared from 10, 15 and 20 vol% of SiO2 in TiO2 (Figure 3b–d), respectively. This drastic increase in surface roughness may due to the increased density of cracks on the coating surface.

**Figure 2.** Field Emission Scanning Electron Microscope (FE-SEM) images of the coatings prepared from (**a**) 7; (**b**) 10; (**c**) 15; and (**d**) 20 vol% of SiO2 in TiO2.

**Figure 3.** 3D Laser microscope images of the coatings prepared from (**a**) 7; (**b**) 10; (**c**) 15; and (**d**) 20 vol% of SiO2 in TiO2.

Figure 4 shows the FT-IR spectra of the coating prepared from 7 vol% of SiO2 in TiO2. The peaks observed in between 500 and 900 cm<sup>í</sup><sup>1</sup> can be attributed to the characteristic vibrational modes of TiO2 [13]. A peak observed near 954 cm<sup>í</sup><sup>1</sup> is associated with Si-O-Ti vibration [14]. The absorption peak at 1118 cm<sup>í</sup><sup>1</sup> confirms Si-O-Si linkage [14]. The absorption peaks near 3342 and 1630 cm<sup>í</sup><sup>1</sup> can be attributed to the presence of stretching and bending vibrations of hydroxyl groups, respectively [15].

**Figure 4.** FT-IR spectra of the coatings prepared from 7 vol% of SiO2 in TiO2.

*2.2. Superhydrophilic, Photocatalytic and Optical Properties of the Coatings* 

The wettability transition of the SiO2-TiO2 coating after UV illumination was studied. The prepared SiO2-TiO2 coatings were illuminated by UV light (365 nm, 2 mW/cm2 ). The UV illumination creates structural changes in TiO2 (transformation of Ti4+ sites to Ti3+ sites) [16] and oxidizes the organic contaminants present on the surface of TiO2, which effectively transforms the wettability of the TiO2 surface towards more hydrophilic [17]. This is called as photo-induced hydrophilicity on TiO2 surface [18]. The water contact angles (WCA) on the coatings before and after exposure to the UV light were measured. The water drop volume of 5 μL was used to measure the water contact angles on the coating surface. The effect of UV exposure time on the wettability of the coatings was also studied. Figure 5 shows the wettability of the coatings prepared from 7 and 20 vol% of SiO2 in TiO2, before and after UV irradiation. Before UV illumination, the coating prepared from 7 and 20 vol% of SiO2 in TiO2 showed the WCA of 23° and 28°, respectively. After UV illumination of 30 min, the water drop immediately spread on the surface and the WCA drastically decreased to 8° on the coating prepared from 7 vol% of SiO2 in TiO2 and even for longer UV illumination time (5 h), the WCA remained in the range of 7°. The coating prepared from 20 vol% of SiO2 in TiO2 also showed decrease in WCA in the range of 10°, after longer UV illumination time. This slightly higher WCA in case of the coating prepared from 20 vol% of SiO2 in TiO2 is due to relatively high surface roughness provided by significant density of cracks present on the surface. Even after placing the UV irradiated coatings in dark for 3 months, the coatings showed stable wetting properties with WCA measured well below 10°. Figure 6 shows the optical photograph of water drops on bare PC substrate and the coating prepared from 7 vol% of SiO2 in TiO2 after 30 min. UV irradiation. Some of the water drops were colored blue using Methylene Blue for better visualization of water drops. The water drops on bare PC substrate maintain the contact angle of ~84°, whereas the water drops spreads on UV irradiated SiO2-TiO2 coating, confirming high affinity towards the water. In the case of SiO2-TiO2 coatings prepared by Fateh *et al.* [7], it needed more than 700 h of UV (A) irradiation to switch the wettability of the coatings in the superhydrophilic range.

**Figure 5.** Wetting properties of the coatings prepared from 7 and 20 vol% of SiO2 in TiO2 (Insets shows the shape of water drops on coatings).

**Figure 6.** Optical photograph of water drops on bare PC substrate (**a**) and coating prepared from 7 vol% of SiO2 in TiO2 after 30 min. UV irradiation (**b**).

The TiO2 is famously known for its excellent photocatalytic property, as it can degrade organic contamination under the illumination of UV light. We studied the photocatalytic degradation of the ODS monolayers after UV light illumination (365 nm, 2 mW/cm2 ) by means of water contact angle measurements. At first, the bare PC substrate and coating prepared from 7 vol% of SiO2 in TiO2

were irradiated with UV light (365 nm, 2 mW/cm2 ) for 1 h to make them superhydrophilic and hydrophobic ODS self-assembled monolayers (SAMs) were applied on them through vapor phase. To employ ODS SAMs on the surface, 2 mL of ODS in beaker was kept in the closed metal box containing the samples at 100 °C for 24 h. After application of ODS SAMs on the surface, the WCA on bare PC substrate and coating prepared from 7 vol% of SiO2 in TiO2 showed 95° and 50°, respectively (Figure 7). The ODS treated bare PC showed almost no change in WCA after 120 min of UV illumination, whereas the coating prepared from 7 vol% of SiO2 in TiO2 showed significant decrease in WCA as a function of UV illumination time and the surface becomes superhydrophilic after 120 min of UV illumination (Figure 7) confirming photocatalytic degradation of the ODS monolayers.

Optical transparency is prerequisite for the application of self-cleaning coating on transparent glass or plastic surface. Figure 8 shows the optical transmission of SiO2-TiO2 coatings prepared on the PC substrate from different vol% of SiO2 in TiO2. All the coatings are highly transparent and showed the optical transmittance values above 85% over the entire visible wavelength range. The optical transmission was gradually decreased from 89% to 85% with an increase in SiO2 concentration in TiO2. The increased surface roughness is responsible for the slight loss in optical transmission in the visible range. The SiO2-TiO2 coatings prepared by Hwang *et al.* [8] showed an optical transmission of ~87%, whereas all the coatings prepared by Fateh *et al*. [7,10] showed an optical transmission of >92%.

**Figure 8.** Optical transmission spectra of the SiO2-TiO2 coatings.
