Transparent , Adherent , and Photocatalytic SiO 2-TiO 2 Coatings on Polycarbonate for Self-Cleaning Applications

Photocatalytic TiO2 coatings are famously known for their excellent self-cleaning behavior, where very thin water layer formed on the superhydrophilic surface can easily wash-off the dirt particles while flowing. Here we report the preparation of the optically transparent, adherent, highly wettable towards water and photocatalytic SiO2-TiO2 coatings on polycarbonate (PC) substrate for self-cleaning applications. The silica barrier layer was applied on UV-treated PC substrate before spin coating the SiO2-TiO2 coatings. The effect of different vol% of SiO2 in TiO2 and its influence on the surface morphology, mechanical stability, wettability, and photocatalytic properties of the coatings were studied in detail. The coatings prepared from 7 vol% of SiO2 in TiO2 showed smooth, crack-free surface morphology and low surface roughness compared to the coatings prepared from the higher vol% of SiO2 in TiO2. The water drops on this coating acquires a contact angle less than 10° after UV irradiation for 30 min. All the coatings prepared from different vol% (7 to 20) of SiO2 in TiO2 showed high transparency in the visible range.


Introduction
TiO 2 is one of the most widely researched photocatalytic semiconductor materials to date.When irradiated with UV light, TiO 2 can decompose the organic pollutants present on its surface, and in addition the surface turns to be highly hydrophilic [1].Water spread out instantaneously by forming a thin layer on the superhydrophilic TiO 2 thin films and steadily carries away dust particles from the surface while flowing (Figure 1).Photocatalytic superhydrophilic thin films with excellent self-cleaning properties are receiving much research attention worldwide because such smart surfaces can save time and reduce maintenance costs [2].Today, the optically transparent, superhydrophilic and photocatalytic TiO 2 thin films are finding increasing industrial applications.Many reports on application of self-cleaning TiO 2 thin films on glass substrates are available in the literature [3][4][5][6].An attractive soft polymer material, polycarbonate (PC) has great demand in the optical industry.PC can replace heavy glass in the optical and electronic industries in the coming future due to its extremely low weight, durability, low-cost, and optical transparency [7].Many attempts have been made to improve the scratch resistance and UV degradation property of the PC by coating the surface by numerous photocatalytic materials with suitable binders.Hwang and coworkers [8] have prepared mechanically durable and optically transparent organic-inorganic SiO 2 -TiO 2 nanocomposite coatings on PC by simple spray coating.The coating solution was prepared by incorporating nanosized TiO 2 sol into the silica polymeric sol.This nanocomposite coating also protected PC from photo-degradation under UV illumination.Lam et al. [9] reported the comparative study on the influence of NaOH etching and UVC irradiation on the mechanical stability of the TiO 2 thin films prepared on PC.They observed adherent TiO 2 film on UVC irradiated PC than NaOH treated PC due to increase in -OH and -COOH groups on the UVC-treated PC.However, the strong hydrophilic, antifogging, photocatalytic and self-cleaning properties were observed on the TiO 2 film coated on NaOH treated PC.Recently, Razan Fateh et al. have prepared optically transparent, highly hydrophilic, mechanically stable and photocatalytic TiO 2 /SiO 2 [7,10], and TiO 2 /ZnO coating [11] on PC for self-cleaning applications.In the present research work, we prepared transparent, adherent, highly wettable and self-cleaning SiO 2 -TiO 2 coating on PC by simple spin coating.The coating showed faster wettability switching (hydrophilic to superhydrophilic) after 30 min. of UV irradiation.The effect of different vol% of SiO 2 sol in TiO 2 and its influence on the surface morphology, mechanical stability, wettability, and photocatalytic properties of the coatings were studied in detail.

Surface Microstructure, Roughness and Chemical Composition of the Coatings
The surface morphologies of the coatings prepared with different vol% of SiO 2 in TiO 2 are shown in Figure 2. The SiO 2 -TiO 2 coating prepared with 7 vol% of SiO 2 in TiO 2 showed uniform, crack-free and smooth morphology (Figure 2a).However, significant cracks on the entire coating surface were observed with increase in SiO 2 concentration in TiO 2 (Figure 2b-d).Some small cracks were started to appear on the coating prepared from 10 vol% of SiO 2 in TiO 2 (Figure 2b), and these cracks goes bigger with increase in vol% of SiO 2 .For the coating prepared with 20 vol% of SiO 2 in TiO 2 , 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 SiO 2 -TiO 2 coating prepared with 7 vol% of SiO 2 in TiO 2 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 SiO 2 in TiO 2 (Figure 3b-d), respectively.This drastic increase in surface roughness may due to the increased density of cracks on the coating surface.Figure 4 shows the FT-IR spectra of the coating prepared from 7 vol% of SiO 2 in TiO 2 .The peaks observed in between 500 and 900 cm −1 can be attributed to the characteristic vibrational modes of TiO 2 [13].A peak observed near 954 cm −1 is associated with Si-O-Ti vibration [14].The absorption peak at 1118 cm −1 confirms Si-O-Si linkage [14].The absorption peaks near 3342 and 1630 cm −1 can be attributed to the presence of stretching and bending vibrations of hydroxyl groups, respectively [15].

Superhydrophilic, Photocatalytic and Optical Properties of the Coatings
The wettability transition of the SiO 2 -TiO 2 coating after UV illumination was studied.The prepared SiO 2 -TiO 2 coatings were illuminated by UV light (365 nm, 2 mW/cm 2 ).The UV illumination creates structural changes in TiO 2 (transformation of Ti 4+ sites to Ti 3+ sites) [16] and oxidizes the organic contaminants present on the surface of TiO 2 , which effectively transforms the wettability of the TiO 2 surface towards more hydrophilic [17].This is called as photo-induced hydrophilicity on TiO 2 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 SiO 2 in TiO 2 , before and after UV irradiation.Before UV illumination, the coating prepared from 7 and 20 vol% of SiO 2 in TiO 2 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 SiO 2 in TiO 2 and even for longer UV illumination time (5 h), the WCA remained in the range of 7°.The coating prepared from 20 vol% of SiO 2 in TiO 2 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 SiO 2 in TiO 2 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 SiO 2 in TiO 2 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 SiO 2 -TiO 2 coating, confirming high affinity towards the water.In the case of SiO 2 -TiO 2 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.The TiO 2 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/cm 2 ) by means of water contact angle measurements.At first, the bare PC substrate and coating prepared from 7 vol% of SiO 2 in TiO 2 were irradiated with UV light (365 nm, 2 mW/cm 2 ) for 1 h to make them superhydrophilic and hydrophobic ODS self-assembled monolayers 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 SiO 2 in TiO 2 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 SiO 2 in TiO 2 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 SiO 2 -TiO 2 coatings prepared on the PC substrate from different vol% of SiO 2 in TiO 2 .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 SiO 2 concentration in TiO 2 .The increased surface roughness is responsible for the slight loss in optical transmission in the visible range.The SiO 2 -TiO 2 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%.

Mechanical Properties of the Coatings
The mechanical durability of the coatings is a very important criterion for industrial applications.The mechanical stability of the prepared SiO 2 -TiO 2 coatings is depicted in Figure 9.The bare PC substrates get easily scratched at the applied force of less than 4.2 mN.For the coating prepared from 7 vol% of SiO 2 in TiO 2 , the coating material was removed at the applied force of ~16.1 mN (Figure 9a), whereas this force was decreased to 12.2, 10.9 and 9.8 mN for the coating prepared from 10, 15 and 20 vol% of SiO 2 in TiO 2 (Figure 9b-d), respectively.The smooth, crack-free morphology and lower surface roughness on the coating resulted in the stable and adhesive coating on the PC substrate.Even rubbed by the fingers, the coating material was not easily removed.This is ascribed to the formation of an intermediate layer connecting the TiO 2 nanoparticles and the silica network.However, in the case of the coating prepared from higher concentration of SiO 2 in TiO 2 , due to significant cracks and increased surface roughness, the coating material could be removed at relatively lower applied force.

Self-Cleaning Coating on PC Substrates
PC substrates are known to be hydrophobic in nature and so the adhesion is not strong between the coating material and the PC substrate [19].The hydrophobic nature of PC substrates can be transformed to strongly hydrophilic by UV irradiation due to occurrence of photo-Fries reaction on the surface [20].At first, PC substrates were gently cleaned by using detergent and water and kept for ultrasonication in double distilled water for 30 min.After drying at room temperature, the PC substrates were illuminated by UV light (365 nm, 2 mW/cm 2 ) for 2 h.The PC substrates can photocatalytically degrade, if the TiO 2 coating is applied directly on the PC substrates.For this reason, the sol-gel processed silica layer was applied on the PC substrates prior to TiO 2 coating.A silica sol was prepared by adding 1.5 mL TEOS in 4 mL ethanol and this mixture was hydrolysed by adding 3 mL of double distilled water with 1 mL of nitric acid.This alcosol was stirred overnight and spin-coated on UV-treated PC substrates at 2000 rpm.This silica coating was annealed at 100 °C for 2 h.The above prepared silica sol was mixed at different vol% (7, 10, 15 and 20 vol%) in commercially available TiO 2 nanoparticle solution and spin-coated on SiO 2 pre-coated PC substrates with 2000 rpm and annealed in oven at 110 °C for 6 h.No change in PC substrate was observed after annealing at 110 °C, however for annealing above 120 °C, PC substrate start to bend in irregular shape due to softening.The schematic of coating preparation steps are shown in Figure 10.

Characterization Techniques
The surface morphology was studied by Field Emission Scanning Electron Microscope (FE-SEM, JEOL, JSM-7600F; Tokyo, Japan).The surface roughness of the prepared coatings was evaluated by laser microscopy (KEYENCE, VK-X200 series; Itasca, IL, USA).The chemical composition was studied by Fourier transform infrared spectroscopy (FT-IR, JASCO, FT/IR-6100; Tokyo, Japan).The water contact angles (WCA) were measured at six different locations on same sample by using contact angle meter (KYOWA, Drop Master; Saitama, Japan) and average value is reported as a final contact angle value.The optical transmission of the coatings was measured by UV-VIS spectrophotometer (JASCO, V-670; Tokyo, Japan).The coatings were illuminated by UV light (365 nm, 2 mW/cm 2 ).The UV lamps were purchased from TOSHIBA (FL10BLB; Tokyo, Japan) and assembled with proper electric supply inside the wooden box covered by thick black cloth.The adhesion of the coating material on PC substrate was checked at five different spots by using Scratch tester (Nano-Layer Scratch Tester, CSR-2000; Rhesca, Tokyo, Japan).The adhesion of the coatings on PC substrate and the maximum force required to remove the coating material was calculated.The cantilever was moved from right to left side in contact with the coating surface.The force was gradually increased while moving towards left side.The maximum force at which the coating material was removed from the PC substrate was noted as critical force to damage the coating.The optical photographs were recorded using Canon Digital Camera (G 15 series).

Conclusions
The mechanically durable, optically transparent, photocatalytically active and superhydrophilic SiO 2 -TiO 2 coatings are successfully prepared on PC substrates for self-cleaning applications.The uniform, crack-free coatings prepared from 7 vol% of SiO 2 in TiO 2 showed higher optical transmission in the visible range, strong superhydrophilicity and good scratch resistance properties.Such coatings can be applied on light-weight window and door polycarbonates for excellent self-cleaning applications.

Figure 1 .
Figure 1.Schematic showing the self-cleaning phenomena on superhydrophilic surface.

Figure 4 .
Figure 4. FT-IR spectra of the coatings prepared from 7 vol% of SiO 2 in TiO 2 .

Figure 5 .
Figure 5. Wetting properties of the coatings prepared from 7 and 20 vol% of SiO 2 in TiO 2 (Insets shows the shape of water drops on coatings).

Figure 6 .
Figure 6.Optical photograph of water drops on bare PC substrate (a) and coating prepared from 7 vol% of SiO 2 in TiO 2 after 30 min.UV irradiation (b).